Novel fabrication processes for thin film vapour deposited ...€¦ · involved the sputter...
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Novel Fabrication Processes for Thin Film Vapour Deposited Strain Gauges on Mild Steel by Richard Djugum Thesis Submitted for the Degree Doctor of Philosophy December 2006 School of Engineering and Science Swinburne University of Technology Australia
Transcript of Novel fabrication processes for thin film vapour deposited ...€¦ · involved the sputter...
Novel Fabrication Processes for Thin Film Vapour Deposited
Strain Gauges on Mild Steel
by
Richard Djugum
Thesis Submitted for the Degree Doctor of Philosophy
December 2006
School of Engineering and Science Swinburne University of Technology
Australia
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ABSTRACT
Pressure measurement using a strain gauge bonded with epoxy adhesive to a metallic
mechanical support has been, and still is, extensively employed, however, for some
applications the use of an epoxy is inadequate, especially when temperatures exceed
120ºC. There is therefore particular interest in the use of thin film techniques to
vacuum deposit strain gauges directly on metallic substrates. Such devices are highly
cost effective when produced in large quantities due to the manufacturing techniques
involved. This makes them ideally suited for use in large-volume products such as
electronic weighing scales and pressure transducers. In this thesis, new techniques for
fabricating thin film vapour deposited strain gauge transducers on metal substrates for
application as novel pressure sensors in the fastener industry are developed.
Clearly, for a vapour deposited strain gauge to function correctly, it is essential that it be
deposited on a defect free, high quality electrically insulating film. This was a
significant challenge in the present study since all available physical vapour deposition
(PVD) equipment was direct current (DC) and insulators of around 4 µm thick were
needed to electrically isolate the strain gauges from metal. As a result, several methods
of depositing insulators using DC were developed. The first involved the use of DC
magnetron sputtering from an aluminium target to reactively deposit up to 4 µm thick
AlN. DC magnetron discharges suffer arc instability as the AlN forms on the target and
this limits the maximum thickness that can be deposited. Consequently, the arc
instability was suppressed manually by increasing argon gas flow at the onset of arcing.
Although the deposited AlN showed a high insulating resistance, it was found that the
breakdown voltage could significantly increase by (a) utilising a metallic interlayer
between the thin film insulator and the metallic substrate and (b) annealing in air at
300ºC. A second deposition method involved the use of DC magnetron sputtering to
deposit modulated thin film insulators in which an aluminium target was used to
reactively deposit alternating layers of aluminium nitride and aluminium oxide. These
films showed significant increases in average breakdown voltage when the number of
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layers within the composite film was increased. The third method involved the
deposition of AlN thin film insulators using partially filtered cathodic arc evaporation
with shielding. Initially, AlN was deposited under partially filtered conditions to obtain
a relatively thick (~ 4 µm) coating then, while still depositing under partially filtered
conditions, a smooth top coating was deposited by using a shielding technique. The
deposition of metal macroparticles is an inherent problem with cathodic arc deposition
and shielding is one form of macroparticle filtering. Such particles are highly
undesirable in this study as they are electrically conductive. A fourth coating technique
for depositing insulators on steel was based on thermal spray technology. Insulating
films of Al2O3 were plasma sprayed and then polished to thereby fabricate viable
electrical insulators for vapour deposited strain gauges.
With respect to depositing strain gauges two methods were employed. The first
involved the sputter deposition of chromium through a shadow mask to form a strain
gauge with gauge factor sensitivity of around 2. The second used cathodic arc
evaporation to fabricate a multi-layered strain gauge composed of alternating CrN and
TiAlN layers that yielded a gauge factor of around 3.5. The technique achieves better
compatibility between gauge and insulator by allowing a wider selection of materials to
form the gauge composition. Finally, a novel pressure sensor in the form of a load cell
was developed that consisted of a chromium strain gauge on a steel washer electrically
insulated with AlN thin film. The load cell showed good performance when tested under
compressive load.
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ACKNOLEDGEMENTS
It is a pleasure to acknowledge the generous help of the people who have assisted in the research for and preparation of this thesis.
Special thanks must be given to my academic supervisors, Professor Derry Doyle, Professor Erol Harvey, Professor Steven Prawer, Dr Karlo Jolic and my industry
supervisor from Ajax Fasteners Dr Saman Fernando. I would also like to acknowledge the generous support of Mr Hans Brinkies and the fellow postgraduate students working
in the surface engineering group at Swinburne University. Thanks must also be extended to Dr. John Long from Deakin University and Dr Johan Du Plessis from RMIT for their assistance in carrying out part of the research. The assistance and
support from Ajax Fasteners, Surface Technology Coatings and MiniFab Pty Ltd was invaluable. I would also like to acknowledge the support of family and friends.
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STATEMENT OF ORIGINAL AUTHORSHIP
This thesis contains no material which has been accepted for the award to the Candidate of any other degree or diploma, except where due reference
is made in the text of the thesis.
This thesis, to the best of my knowledge, contains no material previously published or written by another person except where due reference is
made in the text of the thesis.
Signed _______________________
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TABLE OF CONTENTS
CHAPTER 1 ................................................................................................... 1
INTRODUCTION.......................................................................................... 1
1.1 Vapour Deposited Thin-Film Strain Gauges...................................1
1.2 Electrical Insulation Barrier.............................................................3
1.3 Scope of Work....................................................................................4
CHAPTER 2 ................................................................................................... 8
LITERATURE REVIEW.............................................................................. 8
2.1 Thin Film Strain Gauge Transducers..............................................8
2.1.1 Vapour Deposited Strain Gauges...................................................................8
2.1.2 Shadow Masking............................................................................................10
2.1.3 Load Cell Fabrication ...................................................................................11
2.2 Thin Film Interfaces........................................................................13
2.2.1 Substrate Surface ..........................................................................................13
2.2.2 Interlayers ......................................................................................................15
2.2.3 Interdiffusion .................................................................................................16
2.3 Thin Film Insulation........................................................................18
2.3.1 Sputtered Aluminium Oxide (Al2O3)...........................................................18
2.3.2 Sputtered Aluminium Nitride (AlN)............................................................19
2.3.3 Cathodic Arc Evaporated Aluminium Nitride (AlN).................................21
2.4 Thin Film Multi-layers....................................................................22
2.4.1 Fabrication Techniques ................................................................................22
2.4.2 Electrical Properties......................................................................................25
2.4.3 Electrical Applications ..................................................................................27
CHAPTER 3 ................................................................................................. 29
FABRICATION & MEASUREMENT TECHNIQUES ........................... 29
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3.1 Introduction .....................................................................................29
3.2 Strain Gauges...................................................................................29
3.2.1 Introduction ...................................................................................................29
3.2.2 Shadow Masking............................................................................................30
3.2.3 Gauge Factor..................................................................................................32
3.2.4 Temperature Effects......................................................................................33
3.2.5 Surface Roughness ........................................................................................35
3.3 Thin Film Electrical Insulation ......................................................36
3.3.1 Electrical Conduction....................................................................................36
3.3.2 Breakdown Field Strength............................................................................38
3.3.3 Porosity and Pinhole Formation ..................................................................39
3.4 Thin Film Deposition.......................................................................40
3.4.1 Magnetron Sputtering...................................................................................40
3.4.2 Cathodic Arc Evaporation............................................................................43
3.4.3 Thermal Spraying..........................................................................................44
3.5 Thin Film Analysis ..........................................................................46
3.5.1 X-Ray Diffraction ..........................................................................................46
3.5.2 X-Ray Photoelectron Spectroscopy .............................................................48
3.5.3 Glow Discharge Optical Emission Spectrometry .......................................49
CHAPTER 4 ................................................................................................. 53
EXPERIMENTAL METHODOLOGY ..................................................... 53
4.1 Introduction .....................................................................................53
4.2 Substrate Preparation .....................................................................53
4.3 Mask Preparation............................................................................54
4.4 Insulator Coatings ...........................................................................56
4.4.1 DC Sputtered AlN .........................................................................................56
4.4.2 DC Sputtered AlNx/AlxOy .............................................................................57
4.4.3 Filtered Cathodic Arc Evaporated AlN.......................................................58
4.4.4 Plasma Sprayed Al2O3...................................................................................60
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4.5 Strain Gauge Coatings ....................................................................61
4.5.1 Sputtered Chromium Gauges.......................................................................61
4.5.2 Cathodic Arc Evaporated CrN/TiAlN Gauges ...........................................62
4.6 Transducer Measurements .............................................................64
4.6.1 Electrical Insulation ......................................................................................64
4.6.2 Strain Gauge and Signal Conditioning........................................................64
CHAPTER 5 ................................................................................................. 68
RESULTS & DISCUSSION........................................................................ 68
THIN FILM INSULATION ........................................................................ 68
5.1 Introduction .....................................................................................68
5.2 DC Sputtered AlN............................................................................68
5.2.1 Surface Coating Evaluation..........................................................................68
5.2.2 Structure and Characterisation ...................................................................72
5.2.3 Insulator Evaluation......................................................................................78
5.3 DC Sputtered AlNx/AlxOy................................................................81
5.3.1 Structure and Characterisation ...................................................................81
5.3.2 Insulator Evaluation......................................................................................87
5.4 Filtered Cathodic Arc Evaporated AlN .........................................87
5.5 Thermal Sprayed Al2O3 ..................................................................93
CHAPTER 6 ................................................................................................. 95
RESULTS & DISCUSSION........................................................................ 95
THIN FILM STRAIN GAUGES ................................................................ 95
6.1 Introduction .....................................................................................95
6.2 Shadow Masking..............................................................................95
6.3 Single Layered Chromium Gauge..................................................98
6.4 Multi-layered CrN/TiAlN Gauges................................................109
6.5 Load Cell Washer ..........................................................................115
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CHAPTER 7 ............................................................................................... 120
CONCLUSIONS ........................................................................................ 120
7.1 Introduction ...................................................................................120
7.2 DC Magnetron Sputtered AlN......................................................120
7.3 DC Magnetron Sputtered AlNx/AlxOy ..........................................121
7.4 Filtered Cathodic Arc Evaporated AlN .......................................121
7.5 Strain Gauges on Plasma Sprayed Al2O3.....................................122
7.6 Shadow Masking............................................................................122
7.7 Multi-layered CrN/TiAlN Gauges................................................123
7.8 Load Cell Washer ..........................................................................123
CHAPTER 8 ............................................................................................... 125
REFERENCES........................................................................................... 125
LIST OF PUBLICATIONS....................................................................... 139
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LIST OF FIGURES
Figure 1: Photographs showing (a) commercial strain gauge on polymer backing bonded
to a steel member using cement and (b) chromium strain gauge deposited through a
shadow mask onto a steel member electrically insulated with Al2O3. .......................2
Figure 2: SEM image cross-section of a deposited chromium strain gauge on AlN
insulation showing titanium interlayer on steel..........................................................4
Figure 3: Mild steel washer showing meandering strain gauge pattern and electrical
contact pads. ...............................................................................................................7
Figure 4: Schematic cross-section of a bilayered film configuration showing
interdiffusion zones above and below the bilayer interface. ....................................26
Figure 5: Schematic image of a shadow mask showing the transport of atomic species
through the mask aperture. .......................................................................................30
Figure 6: Optical image of chromium strain gauge deposited through a mask showing
the effects of shadowing...........................................................................................31
Figure 7: Schematic diagram of a Wheatstone bridge circuit showing strain-gauge
temperature compensation........................................................................................34
Figure 8: Diagram of pinhole defects showing (a) hole defect (b) current flow across
grain boundary region...............................................................................................40
Figure 9: Diagram of a vacuum chamber showing the sputtering process. ....................41
Figure 10: A schematic diagram of the filtered cathodic arc PVD system showing the
full-filtration configuration (right) and the partial-filtration (left). ..........................44
Figure 11: Schematic cross-section of a DC arc plasma spray torch showing an internal
particle feed injector. ................................................................................................45
Figure 12: Schematic of diffraction of X-rays from a set of crystal planes. From [96].47
Figure 13: XPS process showing an incident X-ray photon absorbed and a photoelectron
emitted. .....................................................................................................................49
Figure 14: A schematic diagram of the glow-discharge showing the sputtering of sample
atoms by ionized argon atoms, where λ is the wavelength of the photon of light
emitted from energized atoms as they return to a stable energy state. .....................50
x
Figure 15: Image of a GD-OES sample showing a crater that forms after cathodic
sputtering bombardment...........................................................................................51
Figure 16: Schematic cross-section of a load cell washer showing dimensions.............54
Figure 17: Schematic diagram of a mask/mild steel substrate assembly showing brackets
used to hold the apparatus together. .........................................................................55
Figure 18: Schematic of a mild steel dog-bone shaped substrate showing the allocated
area of Al2O3 deposited by plasma spraying. ...........................................................60
Figure 19: A schematic diagram of the multi-source random cathodic arc PVD coating
system showing the relative position of the arc cathode sources in relation to the
substrates. .................................................................................................................63
Figure 20: Schematic of cantilever testing method showing Wheatstone bridge circuit
(left) that was connected to strain gauges attached to cantilevers under load (right).
..................................................................................................................................65
Figure 21: Optical image showing a ZWICK Z010/TN2S extensometer used to test
samples under load. ..................................................................................................66
Figure 22: Optical micrograph of AlN on silicon wafer showing no delamination after
preparing the silicon surface by simply washing in acetone and alcohol.................69
Figure 23: Optical micrograph of AlN on mild steel showing delamination after
preparing the mild steel surface by simply washing in acetone and alcohol............70
Figure 24: SEM fractographic cross-sectional image of Cr/AlN/Ti on mild steel. ........72
Figure 25: XRD spectra of AlN coatings showing wurtzite hexagonal structures for flow
rates of (a) 12 sccm and (b) 20 sccm of nitrogen. ....................................................73
Figure 26: XPS image of sputtered AlN on silicon substrate showing thin film survey
spectrum. ..................................................................................................................75
Figure 27: XPS spectrum for the N1s photoelectron peak showing components N-N and
N-Al..........................................................................................................................76
Figure 28: GD-OES images of DC sputtered coatings showing depth profiles of (a)
Cr/AlN/Ti and (b) AlN/Ti on mild steel...................................................................77
Figure 29: GD-OES depth profile from the sample used to produce Figure 28 (b)
showing the effects after annealing at 300°C for two hours in air. ..........................78
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Figure 30: Graph of breakdown voltage versus film thickness showing a linear
relationship with a gradient of around 1.68 x 106 V cm-1. .......................................79
Figure 31: SEM fractographic cross-section of sputtered AlNx/AlxOy on silicon showing
two separate AlNx and AlxOy layers. ........................................................................82
Figure 32: GD-OES profile of atomic percent versus thickness for modulated AlNx and
AlxOy on mild steel substrate showing the effects of using (a) shielding and (b) no
shielding during the change over of nitrogen and oxygen gas flow.........................84
Figure 33: XRD spectrum for AlNx/AlxOy modulated layers (diffraction peaks α and β
are assigned to the silicon substrate). .......................................................................85
Figure 34: XPS survey spectrum of sputtered AlNx/AlxOy modulated layers on silicon
substrate showing no evidence of AlNx at the surface. ............................................86
Figure 35: SEM surface images of filtered cathodic arc evaporated AlN showing (a)
unshielded AlN and (b) shielded AlN deposits. .......................................................89
Figure 36: SEM surface image of filtered cathodic arc evaporated AlN showing a
shielded AlN top coating deposited over unshielded AlN. ......................................90
Figure 37: GD-OES profile of atomic % versus thickness showing AlN from filtered
cathodic arc evaporation...........................................................................................91
Figure 38: XRD spectra showing wurtzite hexagonal structures for (a) unshielded and
(b) shielded filtered cathodic arc evaporated AlN (diffraction peaks α and β are
assigned to the silicon substrates). ...........................................................................92
Figure 39: SEM image showing the polished surface from thermally sprayed Al2O3
insulator coating. ......................................................................................................93
Figure 40: The XRD angle as a function of intensity for thermally sprayed Al2O3 on
mild steel showing both α-Al2O3 and γ-Al2O3 phases. ............................................94
Figure 41: Photograph showing a chromium strain gauge on glass and a stainless steel
mechanical mask. .....................................................................................................96
Figure 42: SEM images from deposited chromium on glass substrate showing (a)
electrical contact pad and (b) strain gauge meander. ...............................................97
Figure 43: SEM fractographic cross-sectional images showing chromium strain gauges
on (a) AlN and (b) thermally sprayed Al2O3 base..................................................100
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Figure 44: Reproducibility plot of a chromium strain gauge on sputtered AlN base
showing the relationship between fractional resistance change and applied strain.
................................................................................................................................101
Figure 45: Reproducibility plot of chromium strain gauge on thermal sprayed Al2O3
base showing the relationship between fractional resistance change and applied
strain. ......................................................................................................................102
Figure 46: Load cycles from Figures 44 and 45 showing least squares fit, linear
regression and linearity...........................................................................................106
Figure 47: Hysteresis plot showing the response of a chromium gauge on AlN/Ti/mild
steel using the cantilever testing method................................................................107
Figure 48: Hysteresis plot showing the response of a chromium gauge on Al2O3/mild
steel using the extensometer testing method. .........................................................108
Figure 49: Chromium strain gauges showing TCR values over a range of temperatures.
................................................................................................................................109
Figure 50: TEM cross-sectional image of CrN/TiAlN film showing the multi-layered
structure that was formed using cathodic arc evaporation. ....................................110
Figure 51: Reproducibility plot of CrN/TiAlN multi-layered strain gauge showing the
relationship between fractional resistance change and applied strain. ...................111
Figure 52: Load cycles from Figure 51 showing least squares fit, linear regression and
linearity...................................................................................................................113
Figure 53: Plot of a multi-layered CrN/TiAlN strain gauge showing hysteresis..........114
Figure 54: CrN/TiAlN strain gauge showing the TCR over a range of temperatures. .115
Figure 55: Diagram of a load cell washer showing a meandering strain gauge pattern
(left) and cross-sectional profile with dimensions (right). .....................................116
Figure 56: Diagram showing (a) explicit test situation of the washer under load and (b)
practical application for the washer........................................................................117
Figure 57: Plot from a load cell washer showing voltage versus applied force............119
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LIST OF TABLES
Table 1: DC sputtering conditions for AlN/Ti coatings..................................................57
Table 2: DC sputtering conditions for AlNx/AlxOy coatings...........................................58
Table 3: Filtered cathodic arc evaporation conditions for AlN coatings ........................59
Table 4: Plasma spraying conditions for Al2O3 coatings ................................................61
Table 5: DC magnetron sputtering conditions for Cr coatings .......................................62
Table 6: Random cathodic arc evaporation conditions for CrN/TiAlN coatings............64
Table 7: Average breakdown voltages for AlN thin film after annealing in air for two
hours at 300°C. .........................................................................................................81
Table 8: Breakdown voltages for AlNx/AlxOy film with modulated layers ....................87
1
Chapter 1
INTRODUCTION
1.1 Vapour Deposited Thin-Film Strain Gauges
Transducing elements enabling conversion of mechanical stress into electrical signals can
be represented by strain gauges. In general, strain gauges are sensors that are used to
measure physical quantities such as force and pressure. The force to be measured causes a
change in length or height of a piece of material. The strain gauge subsequently measures
this change in dimensions when the gauge is stretched elastically, as well as any change in
the basic resistivity of the material. If the maximum strain direction coincides with a major
geometrical dimension of the strain gauge (usually the length), then the change in resistance
can be related to the change in length of the strain gauge. As a result, a gauge factor can be
determined for the strain gauge that defines its sensitivity [1, 2, 3].
Strain gauges are usually bonded onto a strainable member of the transducer by cement or
glue. The use of glue has a number of disadvantages such as creep, relaxation hysteresis
and a limited range of operational temperatures. The use of thin film technology is seen as a
way of obviating these disadvantages [4]. In its simplest form, a deposited strain gauge
consists of a thin film vapour-deposited directly onto the surface of a strainable member.
An attraction of using thin film technology for strain gauges is that it can be used in mass
production, with considerable cost reduction [5]. Figure 1 shows (a) a glued and (b) a
deposited strain gauge attached to steel members.
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Figure 1: Photographs showing (a) commercial strain gauge on polymer backing bonded to
a steel member using cement and (b) chromium strain gauge deposited through a shadow
mask onto a steel member electrically insulated with Al2O3.
The direct deposition of thin film patterns can be accomplished using a suitably constructed
aperture commonly referred to as a mask [6]. The mask can be made of metal, graphite, or
glass plate with the desired pattern cut or etched into it. The mask is placed in close
proximity to the substrate, thereby allowing vapour from the target material to deposit on
the exposed substrate areas. When depositing in vacuum (< 6 x 10-3 mbar) at moderate
substrate temperatures (< 150°C), and with the mask in physical contact with the substrate,
strain gauge patterns can be formed directly on insulated steel substrates. One of the main
drawbacks is that the mechanical masks are restricted to very simple structures because of
dimensional changes caused by heating effects during deposition. Also, the edges of the
pattern can sometimes be poorly defined since some of the material scatters under the mask
during deposition [7]. Techniques were investigated to limit the impact of dimensional
change and material scattering. Photolithography is often the preferred technique for
fabricating strain gauges because high resolution gauge patterns are formed when using a
photo-mask [5, 8]. Photolithography is, however, labour intensive and involves spin
coating, baking, exposure, development and rinsing. The advantage of using shadow
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masking over photo-masking lies in the fact that post-deposition processing is far less
involved than that in photolithography.
1.2 Electrical Insulation Barrier
For a strain gauge to function correctly on a metal substrate, one has to deposit a defect
free, high quality electrically insulating film on the metal as a base for the strain gauge.
This requirement is the single greatest challenge for fabricating thin film strain gauges on
metallic substrate surfaces [2, 4]. Good quality insulation containing few pinholes and
voids must be bonded to the substrate which, in turn, requires that the substrate be polished
to a mirror surface finish. The deposited insulating layer must be sufficiently thin to have a
negligible contribution to the stiffness of the metal substrate and, the insulation must be
sufficiently thick in order to offer reasonable breakdown voltages (~ 400 volts DC). As a
result, when forming effective insulation barriers for strain gauges, an insulation layer up to
several microns thick is required.
A potential limitation of the use or exploitation of PVD for the generation of insulator thin
films on metal substrates is that of pinhole formation. The latter can give rise to a short
circuit or deterioration of resistivity. Therefore a thick deposit of up to several micrometers
is often used to restrict the number of through-hole pinholes. An alternative method is to
exploit the use of sandwich structures with alternating layers of thin film insulator. A
multi-layered structure can reduce the number of pinholes that penetrate throughout the
thickness of the layered thin film insulators [5, 9]. For example, a multi-layered structure
contains fewer pinholes owing to the fact that the interfaces between the insulation layers
interrupt the vertical pores thereby restricting the flow of current. Also, multi-layers can
accommodate strain since at interfaces between the layers crack arrest results [10, 11].
When thin films are vapour deposited onto metal substrates they inevitably contain high
levels of internal stress. The latter are due in part to differences between the thermal
expansion coefficients of the film and the substrate. The thermal expansion coefficient of a
4
material in the form of a thin film can be controlled by varying the deposition parameters.
These parameters determine the structure and composition of the film, and hence it’s
thermal properties. Another way to better match thermal expansion coefficients between a
steel substrate and a thin film insulator is to use a thin interlayer of metal having an
expansion coefficient value somewhere between the insulator and metal substrate.
Deposited interlayers also act to increase adhesion between layers [12], reduce wear [13]
and act as metal diffusion barriers [14]. A clear example from a fractographic cross-
sectional image of Cr/AlN/Ti on mild steel is shown in Figure 2.
Figure 2: SEM image cross-section of a deposited chromium strain gauge on AlN
insulation showing titanium interlayer on steel.
1.3 Scope of Work
The objective of the present study was to develop new and improved processes and
techniques for fabricating thin film transducers on metal substrates with potential for
5
application as pressure sensors for the fastener industry. To detect large forces on metal
fasteners, robust sensing mechanisms must be used, which must incorporate high insulating
barriers between the fastener and the sensing mechanism. Consequently, novel techniques
were developed for fabricating thin film insulators based on AlN and AlNx/AlxOy*, strain
gauges electrically insulated using thermal spray technology and multi-layered strain
gauges of CrN and TiAlN. These techniques were utilised in the fabrication of a load cell
washer for the fastener industry. It should be noted that, initially, it was intended to
fabricate thin film insulators using a newly constructed pulse laser deposition (PLD)
system. However, delays in the delivery and commissioning of the PLD system meant that
a large part of the study was committed to depositing insulating thin films using DC PVD
systems. The achievements are summarized in the following.
Process control difficulties can be experienced due to arcing when reactively depositing
AlN using DC magnetron sputtering. These arc instabilities are caused by insulating layers
forming on the target surface thereby producing electric fields inside the layers that exceed
the breakdown strength of AlN [15, 16]. In this study, a strategy was employed to maintain
stability of the arc for deposition times of up to four hours during sputtering. Within this
strategy the argon gas pressure was increased manually at the onset of arcing and then
readjusted back to a more stable pressure. During the deposition process the pressure in the
chamber was kept at a critical gas pressure between 6 and 7 x 10-3 mbar while the nitrogen
flow rate remained constant at 12 sccm. This procedure enables AlN films with thicknesses
≥ 4 µm to be reactively deposited using DC power.
In cathodic arc evaporated films of AlN, macroparticles of metals are incorporated into the
AlN resulting in rough surfaces and potential conductive regions giving rise to short
circuits. It has been reported [17] that if a shield is placed between the substrate and target,
then smooth, transparent and highly oriented AlN film can be produced but with a
reduction in the deposition rate of approximately 70%. In this work, partially filtered
cathodic arc evaporation in combination with shielding was used to deposit thin film AlN
insulators. The objective was to produce insulating coating thicknesses of around 4 µm
* The subscripts x in AlNx/AlxOy refer to unknown film stoichiometry due to the presence of oxynitride.
6
under DC deposition conditions in order to accommodate vapour deposited strain gauge top
coatings. This was achieved by initially depositing AlN without a shield, thereby
producing thick insulating layers then, using the shield to deposit a smooth AlN top surface
coating.
A novel method used in this study was to generate modulated insulators by DC sputter
depositing exclusively from a single metal target and changing the reactive gas in the
chamber from nitrogen to oxygen such that AlNx was deposited followed by AlxOy. The
process was repeated until a modulated insulation barrier formed. This deposition
technique has the potential to embed many alternating AlNx and AlxOy layers within one
micrometer, which could lead to many potential applications in the microelectronics
fabrication industry. The viability of changing the reactive gas to vary composition, with
this film has been demonstrated in other applications [18]. However, it has not been used
for modulated insulator materials.
In the manufacture of thin film strain gauge sensors for industrial applications, attempts
have been made previously to produce thermally sprayed zinc strain gauges shadow
masked onto steel substrates thermally sprayed with Al2O3, however, the results have so far
shown poor gauge performance due to high porosity in the coatings [19]. In this study,
insulating films of Al2O3 were deposited using thermal spray technology followed by the
sputter deposition of a chromium strain gauge using shadow masking techniques. The
gauge factor value for the chromium film was successfully measured and found to be
around 2.
In a further aspect of this study, a multi-layered thin film strain gauge was fabricated using
a random cathodic arc evaporation system and shadow masking technique. Multiple targets
of dissimilar materials were used to deposit and generate a multi-layered thin film gauge.
Multi-layered thin films, with approximately 10 nm alternating layers, were formed.
Chromium and Ti0.5Al0.5 targets were set up to run simultaneously at opposite ends of the
chamber while the substrate was rotated. Argon and nitrogen gas was introduced into the
chamber.
7
The final aspect of this study involved DC magnetron sputtering to deposit an AlN thin film
insulator with an overlying shadow-masked chromium strain gauge on a mild steel washer.
The washer was tested under compression up to 1000 N and found to show good quality
performance. An optical image of the washer is shown in Figure 3.
Figure 3: Mild steel washer showing meandering strain gauge pattern and electrical contact
pads.
8
Chapter 2
LITERATURE REVIEW
2.1 Thin Film Strain Gauge Transducers
2.1.1 Vapour Deposited Strain Gauges
As indicated in section 1.1, strain gauges can be fabricated onto the surface of a strainable
member or spring element using PVD techniques. Advantages for using vapour deposited
over glued or cement strain gauges are the absence of adhesive material, flexibility to tailor
the properties of the sensing film and the opportunity to batch process the transducer
device. PVD systems enable a variety of different transducers to be deposited for strain
gauge sensing applications; accordingly many different strain gauge materials have been
developed with various gauge factor sensitivities (up to around 15) that include Bi2O3-V2O5
[20], manganese [21], CrN [22] and Invar [23] to name a few. The gauge factor sensitivity
values obtained in the literature not only depend upon the type of strain gauge material used
but also on conditions of deposition, gauge thickness, surface imperfections and
temperature.
Gouault et al. [24] used electron beam evaporation to deposit constantan (60 % copper and
40 % nickel) strain gauges on a polished steel substrate. The gauges were insulated with
ceramic layers of Al2O3 and SiOx comprised of a 1 μm thick Al2O3 layer and a 0.5 μm thick
SiOx layer deposited by electron beam evaporation. A mask was then used to define the
geometry of the constantan deposit in the form of a thin rectangular strain gauge element.
The latter had a gauge factor of approximately 2.2 with good stability consistent with the
characteristics required for good quality vapour deposited strain gauges [2].
9
In vapour deposited metallic strain gauges ultra-thin or discontinuous metal films result in
large gauge factor values that lack the stability and reproducibility required for industry
based strain gauge applications [25]. Meiksin et al. [26] investigated the possibility of
vapour depositing relatively drift-free Cr-SiO strain gauge structures that were more stable
than metallic films and possessed large gauge factor values. A Thin film (< 0.08 μm thick)
Cr-SiO was deposited on glass using flash evaporation to produce a chromium “island
structure” within a SiO matrix. They obtained strain gauge factors in the range 12-50
(compared with about 3.5 for bulk metals) along with good stability with no measurable
change over a period of several months.
Meiksin et al. [26] also deposited low density tantalum films (12.2 g/cm3) and compared
them with higher density tantalum films (16 g/cm3). They found that the lower the
sputtering voltage and the thinner the films the more the tendency towards island-like
structure formation and lower density. The tantalum gauge factor sensitivities were found
to range from 2 to 16 for thicknesses between 0.26 and 0.02 μm respectively. They also
sputter deposited thin film Au-SiO strain gauges on glass with film thicknesses ranging
from 0.07 to 0.236 μm and found that an optimal gauge composition of Au 80 % and SiO
20 % produced a gauge factor of 38.
When comparing SiOx and polyimide as potential dielectric layers on stainless steel for thin
film strain gauge applications, Bravo et al. [4] highlighted the fact that setting up of the
insulating barrier between the steel and the Wheatstone bridge of the strain gauge circuit
was the most difficult fabrication stage. This is due to the large insulator thicknesses that
are required (~ 3 to 7 µm) to electrically isolate the strain gauge sensor from the steel.
Consequently, when preparing vapour deposited strain gauges researchers use readily
available insulators such as glass plate as the load-reacting spring element [20, 27]. This
eliminates some of the effort required when fabricating vapour deposited strain gauges and
thereby enables more extensive investigations of the strain gauge sensing mechanism. The
main advantage of using an insulated steel substrate, as opposed to glass, is the fact that
larger strains can be measured.
10
Arshak et al. [27] used thermal evaporation to deposit oxide thin films of Bi2O3-V2O5 strain
gauge material through mechanical masks onto stainless steel and glass substrates. To
isolate the thin film strain gauges from the steel, dielectric layers were screen-printed using
thick film techniques. Strain gauge thicknesses of around 0.6 µm deposited on glass were
compared with strain gauges deposited on steel. They found that the Bi2O3-V2O5 gauge
factor values on glass were 10-15 in contrast to 2-4 found for the steel. They concluded
that the reduction in sensitivity for strain gauges on steel was primarily due to non-
uniformity in the film that was attributed to the surface roughness of the thick film
dielectric layer used (Heaeus IP211 glass). It was recognized that PVD deposition methods
for fabricating the dielectric insulating films produce smoother surfaces and favour less
variable gauge sensitivities in contrast to the thick film screen-printing techniques used.
2.1.2 Shadow Masking
The fabrication process for vapour deposited strain gauges relies on the ability to distribute
the vapour deposited material in well-defined patterns [6, 20, 21]. One of the most
effective ways of doing this is to place a physical barrier, which has contained within it the
required pattern, in close contact with the substrate. This physical barrier or mask
intercepts and prevents condensation on the corresponding covered areas of the substrate
allowing for a variety of thin film patterns. This process is referred to as shadow masking
and has been used to form strain gauge patterns of deposited thin films onto insulated
substrates [9, 28, 29]. The shadow masks are usually made from stainless steel and cut
using electro-discharge machining [6].
Rajanna and Mohan [30], in their work on vapour deposited manganese strain gauges,
recognized that the production of strain gauges using the shadow masking technique was
much more efficient than conventional photolithography because it has fewer processing
steps. However, shadow masking is limited to strain gauge widths ≥ 100 µm due to
shadowing effects around the mask edge [31]. Also, when gaps form between substrate and
mask, primarily due to bending caused by heat produced during deposition, the patterned
11
edges can be poorly defined since the mask does not maintain intimate contact with the
substrate at every point and some of the material may scatter under the mask.
In order to resolve the problem of precision or sharpness of a deposited film pattern through
a shadow mask, Yamazaki et al. [7] used DC magnetron sputtered Ag films through glass
masks onto glass substrates and measured the thickness distributions near the pattern edges
using a surface roughness tester. They found that the thicknesses of patterned films
abruptly changed along their widths by an amount equal to half the thickness of the mask.
Also, they found that the gap between mask and substrate had caused additional shadowing
at the patterned edge that was equal in width to the size of the gap.
When depositing PVD thin films defects may be generated in the form of cracks, porosity
and pinholes which can limit their use as reliable coatings for the manufacture of strain
gauges. Iwabuchi et al. [32] developed an effective method of reducing porosity in pulsed
laser deposited (PLD) SrTiO3 thin films. They placed a shield in front of the substrate to
block the line of sight of particles travelling at high velocity from the target to the substrate.
This technique is known as the Eclipse Method. They found that species deposited in the
shadow zone of the shield resulted in smoother surfaces and a reduced deposition rate. This
“shadow zone effect” has been used to account for the smooth surfaces encountered in
strain gauge patterns arc evaporated through shadow masks [33].
2.1.3 Load Cell Fabrication
A vapour deposited strain gauge can be mass-produced with considerable cost reduction
when compared with photolithography fabrication methods [2, 8]. The simplicity of
construction, the ease of manufacture and low production costs allow for devices such as
load cells to be fabricated. The electrical signals generated from load cells result from the
physical deformation of strain gauges that are bonded onto a load cell spring element and
wired into a Wheatstone bridge configuration. Force applied to the load cell either through
compression or tension produces a deflection of the spring element, which introduces strain
12
to the gauges [3]. White and Bringnell [34] concluded that the main considerations for load
cell design are simplicity of construction, ease of manufacture, effects of load eccentricity
and cost. Other important factors include long-term stability, linearity, hysteresis, thermal
considerations and sensitivity to applied load.
Load cells are typically made using high quality stainless steel spring elements in the form
of circular plates or diaphragms. Bethe and Schon [5] developed a load cell consisting of a
centrally loaded circular steel plate clamped at the circumference edge. In the fabrication
of the transducer the plate surface was electropolished, cleaned, and then a thin interlayer of
CrNi alloy was sputter deposited; this was followed by an electrically insulating sandwich
structure consisting of a 2 µm thin film of Al2O3/MgO/Al2O3 – the method for producing
the sandwich structure was not disclosed in the journal article. Finally, CrNi thin film
strain gauge material was DC magnetron sputter deposited onto the insulator followed by
gold film electrical contacts that were then connected to a Wheatstone bridge configuration.
The strain gauge and electrical contact patterns were formed using photolithography
techniques. They concluded that the load cell design was capable of measuring forces in
the range 1 to 100 kN.
Nayak et al. [9] fabricated a transducer diaphragm with a meandering-path thin film strain
gauge to sense fluid pressure. They electron beam evaporated manganese thin film strain
gauges (thickness 0.1 µm) onto steel diaphragms that were insulated from the substrate by
alternate layers of Al2O3 and SiO2. The insulation layer was deposited by reactive electron
beam evaporation of aluminium and silicon in the presence of oxygen and the total layer
thickness was 1 µm. Copper electrode pads were then deposited by keeping a mechanical
shadow mask in close contact with the insulating layers. This mask was then replaced by a
second mask, containing an electro discharge machined strain gauge meander pattern
through which the manganese film was deposited onto the copper pads and insulation.
Finally, to protect the strain gauge an overlayer of SiO2 (0.2µm) was deposited without
removing the mask. Their experimental results showed a gauge factor of around 3 at room
temperature, full scale output hysteresis values ≤ 0.16 % and insulation resistance between
the gauge and the body of the transducer were 100 MΩ at 45 V DC.
13
2.2 Thin Film Interfaces
2.2.1 Substrate Surface
It is well accepted [6] that the surface condition of the substrate can significantly influence
the performance of vapour deposited thin films. For example, the adhesion of PVD thin
films is critically influenced by substrate roughness [35, 36], cleanliness [37, 38] and the
type of substrate material used. In the case of glass or silicon wafer substrates, smooth
surface finishes are generally acceptable for the deposition of thin films and further
polishing processes can therefore be avoided. In the case of metal surfaces, it is necessary
to produce a surface finish compatible with the required thickness of the deposited thin
film, since irregularities such as polishing scratches, porosity and grain boundaries can
occur on the surface of a metal substrate [6].
Liu et al. [36] used a DC magnetron sputtering system to deposit thin film aluminium onto
a relatively rough (Ra 0.004 µm) titanium surface. They used atomic force microscopy to
study the effect of the substrate roughness on the evolution of surface roughness from the
deposited aluminium. They found that the roughness of titanium played a critical role in
determining the roughness of the deposited aluminium at the initial stage of film growth.
Aluminium was deposited for between 15 and 750 seconds and this corresponded to
roughness values ranging from 0.0096 to 0.4791 µm respectively. They concluded that the
surface roughnesses from aluminium film thicknesses smaller than 0.095 µm were
significantly affected by the titanium substrate surface roughness.
E. E. Mitchell et al. [35] investigated the effect of surface roughness when depositing thin
films of YBa2Cu3O7-δ onto single crystal MgO substrates obtained from a range of
commercial suppliers. They also showed, as did Liu et al. [36], that substrate roughness
directly affects the roughness of epitaxially grown thin films. They used X-ray diffraction
(XRD) on 0.25 µm thick YBa2Cu3O7-δ films to show that (00l) peaks corresponded to
14
highly crystalline film that was preferentially orientated with the c-axis normal to the MgO
substrate surface. They found that rough substrates gave rise to more randomly orientated
grain growth. They used a rocking curve analysis to determine the full width half
maximum (FWHM) from (006) peaks and found that smaller FWHM values corresponded
to more c-axis grain growth aligned normal to the substrate. They concluded that scratches
in the MgO substrate surface produced a range of crystal orientations that propagated
through the YBa2Cu3O7-δ film.
The cleanliness of a substrate surface can have a significant influence on the adhesion of a
thin film. For example, Koski et al. [37] investigated the connection between adhesion and
sputter ion-etch cleaning when DC magnetron sputtering carbon, aluminium, chromium and
tungsten thin films of approximately 100 nm thicknesses on stainless steel and titanium
substrates. Adhesion strength was observed using cantilever and three-point bending tests
under compressive and tensile stresses respectively. They found a clear correlation
between adhesion and sputter argon-ion-etch time in which a minimum of 10 minutes in-
situ ion-etching was required for best adhesion. They concluded that it was necessary to
execute at least one in-situ sputter ion-etching step before the deposition of a thin film and
attributed the main cause of poor adhesion to oxygen and carbon contaminants at the
interface.
Differences in the microstructure and grain growth of a thin film deposited onto different
substrates can also be related to the properties of substrate structure [39, 40]. Chae et al.
[39] chemically vapour deposited diamond thin films on SiC-30TiC-10Cr3C2 substrates in
order to examine the effect of microstructural morphology of substrate on the diamond-
substrate adhesion strength. Two types of diamond-coated specimens, one with small and
equiaxial β-SiC grains and the other with large elongated α-SiC grains, were indented
using a Vickers diamond indentor and the resulting surface cracks formed were observed
using an SEM. They found that better diamond-substrate adhesion was obtained when the
substrate morphology was composed of the larger SiC elongated grains and concluded that
mechanical interlocking with the diamond films had resulted in the increased adhesion
strength. The diamond-substrate bonding was not quantitatively determined by their
15
experiment, yet they did show experimentally that the bond strength is critically dependent
on the microstructure of the substrate surface. To highlight further the thin film growth
behaviours influenced by substrate surfaces, Jung et al. [40] reactively sputter deposited
CrN films on glass, AISI 1040 steel and Si(110) and found that differences in the structure
and grain growth of the CrN films were predominantly related to the properties of the
different substrate structures used. They observed that XRD spectra of CrN thin films
deposited on Si(110) had a highly preferred (200) orientation. When CrN films were
deposited on glass and steel, preferred orientations of (200) and some (111), (311) and
(220) orientations were observed indicating that the growth structure of the thin film was
influenced by the surface to which it was deposited. These results highlight the need for a
thin film interlayer to be deposited prior to coating so that better control over grain growth
and microstructure of a deposited thin film can be achieved.
2.2.2 Interlayers
Between a substrate surface and deposited thin film coating, intrinsic and extrinsic stress
can accumulate and lead to poor adhesion and delamination problems [6]. The intrinsic
stress is due to shrinkage and constrained densification during growth. Extrinsic stresses
occur upon cooling due to the mismatch between the thermoelastic properties of the film
with those of the substrate, while further extrinsic stress originates from the lattice
parameter mismatch between the film and the substrate. To alleviate some of the extrinsic
stress accumulated, a thin film interlayer can be used to allow for a gradual transition
between the different lattice structures thereby promoting adhesion and therefore avoiding
delamination problems. The effects of interlayer insertion on the adhesion of various
coatings have been studied in the literature [13, 41, 42].
Tang et al. [13] studied the effects of different metallic interlayers on friction, wear and
adhesion. Magnetron sputtering was used to grow TiC coatings on steel that contained
different interlayers of titanium, chromium and molybdenum of varying thicknesses. Pin-
on-disk tests in conjunction with metallographic analysis revealed that the resistance to
wear increased after chromium or titanium interlayers were deposited between TiC coatings
16
and steel substrate having less delamination than molybdenum. TiC coatings without an
interlayer where shown to have poor adhesion between the TiC coating and steel substrate
while coatings containing either chromium or titanium interlayers performed best. SEM
images of the former showed wear tracks indicating that the coating surface without the
interlayer was severely grooved during the wear process, in comparison to those containing
Cr or Ti interlayers. A variation in chromium thicknesses did not affect the TiC coating
performance, however, increased titanium thickness, from 50 to 500 nm, revealed lower
hardness and poor adhesion for the same TiC coatings. The cause of the poor performance
from 500 nm thick titanium interlayers was not investigated.
Glazman et al. [14] used chemical vapour deposition to deposit diamond films on steel
substrates containing CrN interlayers. They established that thermal stress, resulting from
a mismatch between the thermal expansion coefficient of diamond and steel substrate was
the main contributor to residual stresses observed in the deposited film. The CrN interlayer
deposited between the diamond film and the substrate had influenced the adhesion strength
of the film to the substrate. No delamination occurred in their 1000 N load indentation
testing. SEM and auger electron spectroscopy (AES) analysis revealed surface micro-pores
that consisted of diamond/chromium carbide (and/or nitride) composites. The chemical
composition of the interface was complicated by the diffusion of nitrogen and carbon atoms
within the interfacial region.
2.2.3 Interdiffusion
Interdiffusion and phase formation processes between two thin films often results from
thermal effects [43]. Contaminants from the surrounding environment can influence the
interdiffusion process depending on the temperature during deposition [44, 45]. One
attempt to gain better control has been to use interlayers as diffusion barriers of various
species and thicknesses [46, 47, 48].
17
Wang and Chen [44] used electron beam evaporation to deposit titanium interlayers (~ 250
nm thick) on AlN substrates at room temperature. They showed that interdiffusion and
reaction occurs at the AlN and titanium interfaces when post-deposition annealing
temperatures of 950 and 1150°C are applied for 30 seconds in nitrogen gas.
Characterisation of the films using XRD revealed TiO2 peaks in the spectrum, which
formed due to residual oxygen contamination when heated at 950°C. At 1150°C, AES
spectra revealed a layer consisting mainly of Al2O3 and an inner TiN layer formed beneath
a TiO2 layer. They concluded that at 1150°C, oxygen had diffused from the surface and
nitrogen from the substrate through the interface, which then reacted with aluminium and
titanium to produce the Al2O3 and TiN. Paransky et al. [45] also observed interdiffusion
and reaction effects in Ti-AlN interfaces when investigating a mixture of AlN powder (50-
100 µm sized particles of 99.9 % purity) and titanium powder (< 30 µm sized particles of
99.7 % purity) wrapped in titanium foil. These were heated in a quartz tube between 900
and 1100°C for periods ranging from 1 to 40 hours. They used electron probe X-ray
microanalysis (EPMA) to investigate the phase composition and found that TiN, Ti3AlN
and Ti3Al phases were formed at the interface, however, in this study there was no
contamination.
Terblanche et al. [46] also used XRD and AES characterisation techniques as did Wang
and Chen [44] in order to study interdiffusion processes of consecutive layers consisting of
platinum and chromium electron beam evaporated onto iron substrates with titanium
interlayers. In contrast to Wang and Chen [44] they used much lower substrate
temperatures and found that two phases namely PtCr and PtCr2 were formed at 500°C and a
Pt3Cr phase at 600°C with no interdiffusion observed at 400°C. Of particular interest was
the fact that at 400°C the titanium layer did limit the amount of nitrogen, oxygen and
carbon impurity diffusion between platinum and chromium layers, however, it did not
influence the interdiffusion between platinum and chromium. They concluded that the
titanium layers had formed TiN and TiO2 compounds via gettering which in turn limited
the diffusion of contaminates through to the Pt/Cr layers.
18
Diffusion effects were also investigated by Wang and Feng [48] when attempting to
improve the epitaxial quality of β-FeSi2 coatings. Electron beam evaporation was used to
deposit 50 nm thick iron films on (001) silicon substrates with 20 nm thick titanium
interlayers. These coatings were then annealed at 780°C in vacuum (2 x 10-6 Torr) for 5
minutes. Depth profiling characterisation using AES showed that iron atoms had diffused
through the titanium and reacted with silicon atoms, resulting in β-FeSi2 film after
annealing. In addition, SEM images of the β-FeSi2 films, with and without titanium
interlayers, showed an improvement in surface roughness when employing titanium. They
concluded that during the preparation of β-FeSi2 films the titanium interlayer acted as a
diffusion barrier to limit the supply of iron and this resulted in improved epitaxial β-FeSi2
film quality.
Interdiffusion between thin films can also occur at relatively low temperatures (at or above
150°C). Heller et al. [47] electron beam evaporated consecutive layers of palladium (94
nm thick) and FeTi (110 nm thick) films on silicon wafers then heated them in vacuum, air
and oxygen. They used AES, X-ray photoelectron spectroscopy (XPS) and Rutherford
back scattering to characterise the films. In vacuum, at temperatures between 150 and
300°C, they found traces of iron and some titanium diffusion in the palladium layer. Up to
150°C in oxygen or air, the composition and layered structure of the samples was
unaffected, however, at temperatures between 200 and 300°C iron and titanium oxidized
forming Fe2O3 and TiO2 at the coating surface. They concluded that a palladium film
thickness of 4 nm and 20 nm was sufficient to protect against oxidation for temperatures of
150 and 200°C respectively.
2.3 Thin Film Insulation
2.3.1 Sputtered Aluminium Oxide (Al2O3)
Sputter deposited thin films of Al2O3 are often used as thin film insulators because they
exhibit high-quality dielectric properties with low leakage current and high breakdown
19
strength [49]. In microelectronic applications, it has been shown that to electrically insulate
metal conductors it is necessary to have thin film Al2O3 insulators between 3 and 7 μm
thick [2]. However, insulator performance as a function of coating thickness is due largely
to the sputter deposition conditions [50].
Cremer et al. [50] investigated the structure of thin film Al2O3 insulators as a function of
thickness between 1 and 8 µm after reactively depositing on mild steel substrates using RF,
DC and pulsed DC magnetron sputtering over a range of deposition conditions.
Characterisation of the films using XRD revealed that the structures were a mixture of
amorphous Al2O3 and γ-Al2O3 with the degree of crystallinity increasing with deposition
temperature (from 300 to 600°C). They established that crystalline γ-Al2O3 phase was
formed at 3 sccm oxygen flow rate, while above that amorphous Al2O3 coatings formed.
They concluded that the decrease in crystallinity was probably attributed to an increase in
lattice parameter from 0.790 nm at low oxygen flow rates to 0.801 nm at higher flow rates
for the γ-Al2O3 phase.
Mantyla et al. [49] RF sputter deposited 5 μm thick Al2O3 film on copper substrates with 1
μm titanium interlayers in order to produce electrical insulators for magneto-hydrodynamic
generators. They found breakdown strength and conductivity at room temperature to be (5-
20) x 105 V cm-1 and 10-15 Ω-1 cm-1 respectively. Of particular significance was a large
variability in their results when breakdown strengths from a number of different samples
were measured. Specifically, open pores, pinholes and loose nodules were identified and
thought to cause short circuits between electrode and substrate. They concluded that
changes in contact pressure between sample and electrode, variations in coating thickness
and local defects in the coating caused scatter in their results.
2.3.2 Sputtered Aluminium Nitride (AlN)
Thin film AlN is a useful material because it is extremely hard, has a high melting point,
has high thermal conductivity and is chemically stable [51]. With reference to this study
20
sputtered AlN was of interest because it is an extremely good electrical insulator with
breakdown strength of the order of 106 V cm-1 [52]. However, only a small number of
papers have been published on the electrical properties of sputter deposited AlN thin film
[53, 54] and this means that the full potential of thin film AlN insulators has not been
successfully demonstrated.
Mortet et al. [55] used DC pulsed magnetron sputtering from an aluminium target to
reactively deposit AlN films on (100) silicon substrates for the purpose of developing
surface acoustic wave filters. They characterised the films using XRD and cross-sectional
TEM images of the films and showed that the film consisted of wurtzite AlN, strongly
002 orientated with a columnar grain structure normal to the substrate surface. They
addressed the issue of electrical stability and performed measurements using a four-point
probe that showed a resistance of 1014 Ω and resistivity of 1010 Ω cm for AlN deposited at
300°C. For temperatures lower than 300°C their measuring equipment was out of range
and an extrapolation of the data indicated higher than 1011 Ω cm resistivity values at room
temperature, which clearly makes these films ideal candidates for insulators.
Gregory et al. [53] demonstrated that the resistivity for AlNx strain gauge material can vary
depending on the deposition conditions used. They reactively RF sputter deposited AlNx
thin films and adjusted the nitrogen from 0 to 100 % by volume with the balance being
argon. When increasing the partial pressure of nitrogen three distinct regions of
conductivity were observed: a low resistive metallic-like region where the nitrogen dropped
below a threshold value of 14 % and some phase separation of aluminium and AlN
occurred. A highly sensitive semiconductor region in the range of 18 to 25 % nitrogen with
rapidly increasing resistivity for relatively small changes in nitrogen content. A highly
resistive region above 25 % nitrogen where the electrical resistivity was independent of
nitrogen content.
Dimitrova et al. [54] investigated the nitrogen gas concentration for reactive DC magnetron
sputtered AlN thin film when studying dielectric properties. Circular electrical contacts of
aluminium were sputter deposited through a mask onto ~ 350 nm thick AlN film on glass
21
and aluminium foil substrates. Dielectric constants, obtained from capacitance
measurements at 300 kHz, were found to be 7.1 and 6.9 when nitrogen partial pressures of
1 x 10-4 and 5 x 10-4 mbar were used respectively. They concluded that the dielectric
constant values obtained were relatively low and that this was due to incorporation of
excess argon and nitrogen in the AlN lattice during sputtering.
2.3.3 Cathodic Arc Evaporated Aluminium Nitride (AlN)
Cathodic arc evaporation is a PVD technique that uses a vaporisation source where the
vaporised material originates from a high current density arc on the cathode surface [56].
Cathodic arc systems have a high level of ionization compared to sputtering, however, one
limitation is the presence of macroparticles [57] which degrade the quality of the coating
because they generate voids and increase the surface roughness. The latter has largely been
overcome through control of the arc velocity by magnetically steering the cathode spot on
the surface of the cathode [58] and/or filtering techniques [57, 59] with the most successful
method being based on the use of a curved plasma duct filter enclosed by an axial magnetic
field. The aforementioned techniques enable the deposition of high quality thin films,
however, one can simply place a shield between the cathode and the substrate to further
prevent the deposition of macroparticles although at some reduction in deposition rate.
This shielding technique has been used to produce exceptionally high quality AlN thin
films [17, 60, 61].
Takikawa et al. [17] employed this simple shielding technique to produce AlN thin film at
room temperature using a cathodic aluminium arc in nitrogen gas at pressures in the range 3
x 10-3 to 3 x 10-2 mbar. The films were characterised in terms of deposition rate, surface
morphology and crystalline structure. The deposition rate from shielded AlN film was
found to be approximately a third of the rate of unshielded film. The shielded films were
smooth with the minimum detectable droplets in the submicron range while the unshielded
films produced rough surfaces containing numerous macroparticles approximately 10 μm
in diameter. Of particular significance was the fact that XRD spectra from unshielded
22
deposits revealed AlN (002) and (100) peaks with the (002) peak much stronger than the
(100) peak, however, for films deposited by shielded arc only AlN (100) was detected.
Manova et al. [61] successfully used the shielding technique to reduce the number of
macroparticles reaching (100) silicon and (0001) sapphire substrates when reactively
vacuum arc depositing AlN films at room temperature. Moreover, a number of samples
were subjected to different pulsed bias voltages between 0 and 10 kV during coating and
these were later characterised using XRD and atomic force microscopy techniques. Their
XRD spectra revealed an AlN (002) preferential orientation that was independent of the
bias voltage. They found that the Rrms surface roughness without pulse bias was 0.3 nm
and this increased to about 5 nm at intermediate voltages then decreased to approximately 2
nm at the higher voltage of 10 kV. They concluded that higher energetic atoms incident on
the substrate surface after applying the intermediate voltages led to higher radiation damage
and therefore increasing surface roughness. At higher voltages a second affect, most
probably an increased temperature at the substrate surface lead to a better film quality.
Dixit et al. [62] used a filtered cathodic arc evaporation system to reactively deposit AlN
film on silicon and sapphire substrates in order to study the grain size of AlN crystallites in
the temperature range 200 to 800°C. Micrographs from thin film deposited at low
temperature showed that crystallites were in the range 2 to 10 nm, whereas those deposited
at higher temperature showed a larger average crystallite size between 25 and 35 nm. They
concluded that the larger crystallite sizes produced at 800°C were attributed to the presence
of oxide contamination in the grain boundary regions.
2.4 Thin Film Multi-layers
2.4.1 Fabrication Techniques
PVD can be used to deposit multi-layered thin films that provide higher hardness and
improved adhesion than single layered films [63, 64, 65]. One technique [66] used to
deposit a multi-layered film was to adjust periodically the gas flow to the deposition
23
chamber so that alternate reactive and non-reactive layers form. Multi-layer combinations
of Al/AlN [66], Cr/CrN [67] and Al/Al2O3 [68] thin film have been deposited using this
technique. Other techniques include periodically adjusting and readjusting substrate bias to
deposit TiN [11], rotating substrates under different targets to deposit AlN/TiN and
AlN/stainless steel [69, 70, 71] and modulating the flux impinging on a fixed substrate to
deposit AlOx [72]. The following is a review of multi-layer techniques relevant to the
present study.
Bull and Jones [11] deposited multi-layered coatings on stainless steel substrates at a
temperature of 300ºC using DC sputtering. They deposited alternating layers of titanium
and TiN by periodically stopping the nitrogen gas flow into the chamber. They also
modulated the bias voltage in order to deposit TiN multi-layers with different levels of
residual stress. Indentation and scratch testing characterisation techniques were used to
compare multi-layers with conventional single-layer TiN coatings. They found that their
multi-layer films had higher hardness and improved adhesion than the single-layered films
and concluded that discrete layers approximately 7 nm thick for structural and 50 nm thick
for compositional multi-layered film achieved the best performance. Kusano et al. [64]
also sputter deposited Ti/TiN thin film multi-layers by adjusting periodically the nitrogen
gas flow in the chamber. They too used indentation techniques and found that layer
thicknesses of 10 nm produced a maximum hardness 1.3 times monolithic TiN film, which
is consistent with results obtained by Bull and Jones [11].
Lee et al. [66] investigated the mechanical properties of multi-layers that were deposited by
modulating the nitrogen gas flow during deposition. They used RF magnetron sputtering to
deposit multi-layered Al/AlN films with layer thicknesses ranging from 10 to 200 nm on
(100) silicon substrates. They used indentation and scratch testing methods to evaluate film
hardness and adhesion, however, in contrast to Bull and Jones [11] and Kusano et al. [64]
they found the hardness values from multi-layered films to be lower than monolithic AlN
film. They found that a large (~ 25 %) aluminium volume fraction in the Al/AlN film
resulted in a decrease in measured hardness due to the softer aluminium layers. In addition,
when comparing their scratch test data they found that 20 nm bilayers of Al/AlN could
24
sustain almost twice the critical load of monolithic AlN. They concluded that the
alternating layers between soft metallic and hard ceramic layers prevented crack arrest
which in turn lead to an increase in critical load.
Wang et al. [69] deposited multi-layered AlN/TiN coatings from aluminium and titanium
targets in argon and nitrogen gas using a dual-cathode unbalanced DC magnetron sputtering
system with a pulsed DC power supply. Silicon and stainless steel substrates were rotated
between the two targets and, as they passed each target, thin film composed mainly of
adjacent target material was deposited. They concluded that a critical bilayer thickness of
5 nm was required to increase the film hardness in their samples twofold. Zhang [70] also
used DC magnetron sputtering to deposit alternating layers of stainless steel and AlN film
on glass substrates by rotating them over targets of stainless steel (type 316) and aluminium
that ran simultaneously in a mixture of argon and nitrogen gas. AlN/stainless steel film
was deposited as the substrates rotated around the chamber while briefly stopping over each
target for a few seconds. Characteristic curves of cathode voltage versus nitrogen partial
pressure showed rapid transition from the metal mode to the dielectric mode at a low
pressure of 1 x 10-4 mbar. Consequently, by utilising this low nitrogen gas flow they were
able to produce AlN film nearly free of the metal phase that resulted in approximately 4 nm
thick alternating stainless steel and AlN films.
Tokumaru and Hashimoto [72] used a two-step RF magnetron sputtering process to deposit
alternating layers of AlOx thin film insulators on stainless steel from an Al2O3 target. Of
particular interest was the fact that they modulated the impinging flux on the substrates by
adjusting the position of the substrate normal and oblique angle off normal to target with
respect to the plasma source. A similar fabrication method was used by Lintymer et al.
[73] when varying the substrate-to-target angle of incidence for the purpose of producing
multi-layered microstructures of oriented chromium. They DC sputter deposited from a
chromium target in argon gas on (100) silicon and glass substrates and found that the
columnar direction of the coatings had changed when the direction of the incident flux of
the species impinging on the substrate surface changed. SEM cross-sectional images
25
showed a columnar microstructure with a “zigzag” geometry that was attributed to a
periodical change of the particles flux direction.
2.4.2 Electrical Properties
To avoid electrical breakdown between a metal substrate and a thin film insulator, the
insulator should contain few pinholes, voids and impurities and, preferably should be
several micrometers thick [2]. To further increase the resistance in thin film insulators a
multi-layered structure can be utilised to provide even higher electrical quality in contrast
to single layered coatings of the same thickness [72, 74].
Segda et al. [74] deposited thin film multi-layered insulators of Al2O3/GeO2 using RF
sputtering techniques at various deposition temperatures. Using a capacitance bridge they
measured the breakdown strength of 0.3 μm thick multi-layers. They found that with
increasing numbers of layers the breakdown strength increased to 2.03 MVcm-1 for Al2O3
/GeO2 composed of five layers while single layered films of Al2O3 and GeO2 were found to
be 1.17 and 1.27 MVcm-1 respectively. The breakdown strength was not affected by
temperature variations in the range 20 to 200°C.
An improvement in thin film electrical quality was also found by Tokumaru and Hashimoto
[72] when RF sputtering a bilayer of AlOx film on stainless steel substrate. The total
thickness of the bilayer was 500 nm and to achieve the best insulating quality nominal
thicknesses were approximately 20 and 480 nm for the first and second layers respectively.
They found that after immersing single and bilayered AlOx coatings in chemical solution
using a copper decoration technique the pinhole density in the bilayered thin film had
reduced by more than half in contrast to the single-layered film. Leakage currents were
found to be low at voltages over 20 V for bilayered film, whereas breakdown occurred in
their single layered films at voltages below 20 V.
26
Bruckner et al. [75] investigated the resistivity of sputter deposited CuNi/NiCr multi-layers
and developed a model in which electrical parameters were modified within an
interdiffusion zone at the bilayer interfaces. In their model the lateral current flow to the
substrate surface is summed via the conductance of each layer (or reciprocal sheet
resistances Rs) such that for two layers Equation (1) is applicable
dRdd
dR is
111)11(1
2
1
21++−=
ρρρ (Equation 1)
where, ρ1 and ρ2 is the resistivity of each layer, d is the total thickness of the two layers and
d1 is the first layer as shown in Figure 4.
Figure 4: Schematic cross-section of a bilayered film configuration showing interdiffusion
zones above and below the bilayer interface.
27
The interface resistances Ri are found using Equation (2)
)11()11(1
2,2,2
1,1,1 ρρρρ
−+−=i
ii
ii
ddR
(Equation 2)
where, ρ1,i and ρ2,i are modified resistivities in the interface zones with thicknesses d1,i and
d2,i in materials 1 and 2. Relative to a single layer, a triple layer of (100 nm NiCr)/(1000
nm CuNi)/(100 nm NiCr) was deposited and, using the model, they found a reduction in
resistance of 8.0 % and 9.4 % for as-deposited and 300°C annealed states respectively and
attributed this to a more conductive CuNi concentration shift.
Barzen et al. [76] deposited multi-layers of Al2O3/Cr3C2 thin film using RF magnetron
sputtering and ion plating in a dual-source apparatus. Their objective was to measure the
stress using XRD in order to find optimal bilayer thicknesses and correlate these with
electrical resistance of the film. They found that the stress in Al2O3/Cr3C2 films rapidly
decreased below 20 nm bilayer thicknesses. In addition, the Cr3C2 film resistance was
found to rapidly decrease from 500 Ω to 200 Ω for 10 nm and 100 nm bilayer thicknesses
respectively.
2.4.3 Electrical Applications
Electrically insulating thin film multi-layers have been utilised in the development of strain
gauge devices [5, 24, 77, 78]. The following is a review of multi-layered thin film
electrical insulator applications relative to the present study.
Bethe and Schon [5] RF magnetron sputter deposited MgO between Al2O3 on steel
substrates for the purpose of electrically insulating strain gauges. They found that fewer
pinholes penetrate through the insulator coating when implementing the multi-layered
28
structure and concluded that at the interfaces the vertical pore propagation developed by the
columnar growth of the crystals are interrupted due to altered nucleation conditions.
Rajanna et al. [77] also used thin film multi-layered insulators to electrically insulate strain
gauges from steel substrates. They electron beam evaporated 2 μm thick film composed of
SiO2 layers between Al2O3 and found that the multi-layered insulator had a resistance of
300 MΩ when measured at 10 V DC using a high-voltage probe. Of particular significance
was the fact that a post deposition heat treatment, in air for one hour at 120°C, resulted in a
large electrical resistance increase of 300 to 40,000 MΩ. They concluded that enhanced
oxidation of the individual oxide layers within the film was the main contributor to this
increase. Jung et al. [79] also found increases in film resistance after low temperature
annealing in air and they too attributed this to oxidation at the interfaces, however, in
contrast to Rajanna et al. [77] they measured the sheet resistance from DC magnetron
sputtered In3O3SnO2/Ag/In3O3SnO2 multi-layers and found only small changes in
resistance for a temperature range of 25 to 400°C. They did, however, find a relatively
large increase of 1.2 to 5.0 Ω/sq over a temperature range of 400 to 500°C. They
concluded that the sharp increase in sheet resistance was due to oxygen diffusing through
the In3O3SnO2 layers and oxidizing with the Ag layers.
Lei and Will [78] deposited bilayers of Al2O3 and SiO2 thin film insulators for strain gauge
applications at high temperature (1100°C). They used electron beam evaporation to deposit
Al2O3 or SiO2 as the initial layer then sputter deposited Al2O3 as a second layer.
Thicknesses of 5-8 μm for Al2O3 and 2-3 μm for SiO2 were deposited on Al2O3 and
“superalloy” substrates and, in order to minimize thermal stress during deposition, these
were heated from 800 to 900°C. They concluded that for strain gauges to be usable to
1100°C, an Al2O3 coating with a thickness of 7 μm or higher was needed to provide at least
0.1 MΩ resistance.
29
Chapter 3
FABRICATION & MEASUREMENT TECHNIQUES
3.1 Introduction
This chapter describes a set of fabrication tools and measurement techniques that were used
in the development of vapour deposited strain gauges on insulated mild steel substrates.
Shadow masking, strain gauge sensitivity, temperature effects and surface roughness is
examined. To identify the electrical properties and gain better understanding of the as-
deposited insulator coatings, analytical methods of conduction and breakdown field
strength are investigated. Deposition techniques including sputtering, cathodic arc and
thermal spraying are also investigated and characterisation techniques for the as-deposited
coatings include XRD, XPS and GD-OES.
3.2 Strain Gauges
3.2.1 Introduction
The fabrication of vapour deposited strain gauges requires the deposition of patterned
conductive layers on an insulated substrate. This can be done by using a shadow mask
during deposition. The properties of the conductive layer determine the strain gauge factor
which relates strain to a change in electrical resistance. The gauge factor measurements are
affected by temperature changes in the surrounding environment and these can be
compensated for by using Wheatstone bridge signal conditioning. In addition, smooth
insulator surfaces are required to preserve the strain gauge continuity.
30
3.2.2 Shadow Masking
A shadow mask is typically a stainless steel shim in which a suitably patterned aperture has
been cut [6]. When this is placed in direct contact with the substrate it allows vapour to
condense only on the exposed substrate areas thereby allowing the direct deposition of
strain gauge patterns. Collisions between the depositing atoms and background gas give
rise to a range of deposition trajectories that ultimately limits the resolution of the deposited
pattern by forming shadow zones adjacent to the exposed substrate edges [31]. Figure 5
shows atomic species being transported through a shadow mask.
Figure 5: Schematic image of a shadow mask showing the transport of atomic species
through the mask aperture.
31
As high velocity ions travel through the mask and onto the substrate their numbers are
reduced by sidewall shadowing resulting from the narrow collimated path cut through the
mechanical mask [33]. As a result, the deposition rate through the slot is reduced, however,
a smooth surface is formed. Air gaps between the mask and substrate can cause material to
scatter under the mask that gives rise to patterns larger than the slot in the mask. The
patterned strain gauge films are limited to a minimum gauge width of approximately 100
μm, which limits the use of evaporation masks in many of the present microcircuit
manufacturing processes [8]. However, vapour deposited strain gauges formed by shadow
masking can be used for larger scale, more durable, applications such as load cells. Figure
6 shows a chromium strain gauge pattern that was formed by depositing through a shadow
mask.
Figure 6: Optical image of chromium strain gauge deposited through a mask showing the
effects of shadowing.
32
3.2.3 Gauge Factor
The gauge factor (or sensitivity) of a strain gauge is the ratio of the relative strain and the
resulting relative change in resistance [1, 80]. The value of this factor is based on
ALR ρ
= (Equation 3)
where R is resistance ρ is the resistivity of the gauge material, A is the cross-sectional area
and L the length of the gauge.
It follows that a change dR in R depends on these basic parameters; therefore differentiating
Equation (3) gives
AdL
AdLdR ρνρ ⋅
++⋅
=)21(
(Equation 4)
where ν is Poisson’s ratio.
The gauge factor )(L
dLR
dR is then given by Equation (5)
( )
LdL
d
LdL
RdR ρ
ρ
ν ++= 21 (Equation 5)
33
The first part on the right hand side of Equation (5) represents a resistance change in length
and cross-sectional area of the gauge. For relatively thick (2 to 3 µm) continuous metal
films the gauge factor is dominated by such geometric effects and is usually found to be
between 1 and 2, since Poisson’s ratio for most metals is approximately 0.4. Reducing the
strain gauge thickness below 0.1µm will result in film discontinuities that produce higher
gauge factor values with large instability [25].
The second part on the right hand side of Equation (5) represents resistance changes due to
stress-induced modulation of the electric field charge transport [3]. For metallic strain
gauges this value is relatively small and can be neglected [2]. Consequently, gauge factors
higher than about 2 are attributed to changes in the specific resistivity of the film, which
occurs due to the number of free electrons and the variation of their mobility with applied
strain. Combined geometric effects and the electric field charge transport result in the
piezoresistive effect [81].
Piezoresistivity is defined as the change in resistance of a material due to an applied stress
and this term is commonly used in connection with semi-conducting materials [1]. For
example, doped semiconductor strain gauges based on p-type or n-type silicon can have
gauge factors as high as 150 [82]. When a strain is applied and the doped silicon crystal
structure deforms, the energy band structure redistributes [81] resulting in mobility and
carrier density changes in resistivity of the material. The effect has been utilised to make
single-crystal silicon strain gauges that are sensitive only to the relative direction of stress
and electric field. As a result, longitudinal and transverse gauge factors can be obtained.
3.2.4 Temperature Effects
Temperature changes can contribute to variations in strain gauge sensitivity [2] and with
metallic strain gauges the effect is usually quite small for temperatures between 0 and
100ºC. Semiconductor strain gauges, however, are highly temperature sensitive and must
therefore be temperature compensated using Wheatstone bridge configurations [1] that
utilise “dummy” gauges. These dummy gauges are attached to the same substrate as the
34
active gauge and placed in adjacent legs of the Wheatstone bridge. When a load force is
applied to the active gauge the output voltage from the bridge will shift with temperature
unless the differential temperature coefficient of resistance and thermal expansion of each
arm is identical. Figure 7 shows a Wheatstone bridge configuration used to compensate for
temperature changes.
Figure 7: Schematic diagram of a Wheatstone bridge circuit showing strain-gauge
temperature compensation.
The temperature coefficient of resistance (TCR) is defined as the relative variation of the
gauge resistance ΔR per degree of temperature variation ΔT and its value is determined by
Equation (6)
35
TR
RTCRΔΔ
= (Equation 6)
where, R is the conductor resistance at the conductor temperature in ºC.
The coefficient of linear expansion, α, is described by Equation (7)
)273( −Δ
=Toλ
λα (Equation 7)
where Δλ is the change in length of the body relative to the length of the body λo at 0°C,
and T is the temperature in Kelvin.
3.2.5 Surface Roughness
One method for measuring the roughness of a surface is to use a contact stylus instrument.
Stylus instruments measure roughness properties characterised numerically by the average
deviation from an arbitrary mean position of a tracer arm drawn across the surface [83].
Roughness values can be derived by dividing the peaks and valleys into narrow segments of
height and then averaging under a square root form, yielding a roughness Rrms value defined
by Equation (8).
nyyyyR n
rms
223
22
21 +⋅⋅⋅+++
= (Equation 8)
36
where, y1, y2…..,yn are equal to the heights of component segments of the trace.
Similar average roughness values can be achieved [83] when using an arithmetic average
Ra to measure roughness. To derive this value, a trace is drawn over an assigned length of
surface so that the areas under the curve above and below the centreline are equal. It is
calculated as the area of the absolute value of roughness profile height over the evaluated
length, or the area between the roughness profile and its mean line. The Ra value is defined
by
MLdcbaRa
⋅⋅⋅++++= (Equation 9)
where a,b,c,d…. are the areas under the peaks and valleys of the trace, L is the assigned
length of the stylus and M is the vertical magnification used.
In the present study, an Ambios XP-2 stylus profilometer was used to carry out surface
roughness and film thickness measurements. Ra roughness was measured and the range for
the scan distance was 2 mm. Qualitative analysis of surface roughness was also carried out
using a Joel JSM-840 scanning electron microscope (SEM).
3.3 Thin Film Electrical Insulation
3.3.1 Electrical Conduction
A single conduction process does not adequately describe the electrical properties of thin
insulating films since different field-strength ranges exhibit different electrical phenomena
[84]. There are five possible separate conducting mechanisms through thin insulating films
and these are summarised below [85]:
37
• Ionic Conduction: This occurs due to the drift of defects under the influence of an
applied electric field such that ions or vacancies cross over a potential barrier from
one defect site to the next.
• Space-charge-limited Flow: This occurs when ohmic contact is made to the
insulator and charge in a given region of space is injected into the conduction band
of the insulator where it is capable of carrying current.
• Internal Field Emission: This is a form of tunnelling through a classically-forbidden
(or quantum) energy state in which electrons pass through a barrier in the presence
of a high electric field.
• Schottky Emission: This corresponds to the thermal activation of electrons over the
metal-insulator interface barrier with the added effect that the applied field reduces
the height of this barrier.
• Impurity Conduction: This occurs when an electron occupying an isolated donor
level in an impure insulator has a wave function localized about the impurity and
energy slightly below the conduction band minimum. Since there is a small but
finite overlap of the wave function of an electron of one donor with neighbouring
donors, a conduction process is possible in which the electron moves between
centres without activation into the conduction band.
Insulators contain very few volume-generated carriers, in many instances considerably less
than one per cm3 [6], and therefore have virtually no conductivity. For thin film insulators,
however, the electrical properties are determined not by the intrinsic properties of the
insulator but by other properties, such as the nature of the electrode-insulator contact. For
example, consider the intrinsic current density I carried by an insulator, defined by
Equation (10).
)2
exp(kT
EFNeI g
c −= μ (Equation 10)
38
where e is the electronic charge, μ is the mobility, F is the potential field in the insulator, Nc
is the effective density of states in the insulator, Eg is the insulator energy gap, k is
Boltzmann’s constant and T is the absolute temperature. At room temperature Nc = 2.5 x
1019 cm-3, and assuming Eg = 3 eV and μ = 100 cm2 V-1 s-1, then even for F = 106 V cm-1 the
current density is only of the order 10-18 A cm-2, which is more than several orders of
magnitude smaller than the current densities encountered for insulators with band gaps
greater than 3 eV [6].
The source of the conductivity in thin films is thought to be extrinsic [6, 86] and is due to
the inherent defect nature of evaporated compound films. Stoichiometric films of
compound insulators are notoriously difficult to prepare by PVD and, this being the case,
the most dominant conducting mechanisms for thin film insulators deposited in this way are
impurity conduction and ionic conduction.
3.3.2 Breakdown Field Strength
The breakdown field strength of an insulating material is the maximum potential gradient
that can be applied without causing a sudden passage of current through the insulator [6].
This is usually expressed in volts per centimetre (V cm-1). The physical breakdown of
films at a critical voltage is due to a large increase in electrons (electron “avalanche” effect)
within the insulation [85]. The breakdown occurs at localized defects where the intensity
of the local electric field is increased to a level in excess of the intrinsic breakdown field
strength. The breakdown field strength of a thin film insulator is usually greater than that
of bulk materials [86]. The best insulating films have breakdown field strengths near 107 V
cm-1, whereas the best thick insulators break down near 106 V cm-1 [84].
39
3.3.3 Porosity and Pinhole Formation
Porosity in thin film coatings is associated with the open spaces between grains generated
during the deposition process [87]. The size and distribution of these pores are important
parameters of the material quality and will vary depending upon the deposition conditions.
As the pores form, gases or impurities become trapped and when transported through the
pores these impurities react with the pore walls. Pinholes in the film form along columnar
grain boundaries during the deposition process. This is illustrated in Figure 8. These
pinholes can sometimes extend through the film from the surface to the interface such that
when a top electrode is deposited it is conductively coupled to the bottom electrode via the
pinhole. A more complex type of pinhole is a defective region in a crystalline insulator
material, which breaks down and conducts along the grain boundaries when a voltage is
applied. Such a region is indicated in Figure 8 (b).
40
Figure 8: Diagram of pinhole defects showing (a) hole defect (b) current flow across grain
boundary region.
Thin films can have a larger number of pinholes per unit area, which in turn directly
influences the breakdown field strength of an insulator material. For example, starting
with relatively thick films (~ 50 μm), the breakdown strength increases steadily with
decreasing thickness until reaching a maximum at around 5 μm [84]. Below this the
breakdown strength begins to decrease again due to a greater tendency for pinhole
formation and local imperfections.
3.4 Thin Film Deposition
3.4.1 Magnetron Sputtering
Sputter deposition involves the impact of high-energy particles onto the surface of a target
material. The use of plasma to generate the high-energy particles is the most common
method [56]. During sputtering positive argon ions in the plasma are accelerated towards
41
the cathode target by an applied voltage and strike it with a kinetic energy of several
hundred electron volts. This powerful impact ejects material from the cathode target. As
part of this collision secondary electrons are emitted from the surface. These electrons
accelerate away from the cathode and gain significant energy to ionize the argon gas
through collisions, thus sustaining the discharge. The process is schematically represented
in Figure 9.
Figure 9: Diagram of a vacuum chamber showing the sputtering process.
In magnetron sputtering, magnets are placed behind the target resulting in a denser plasma
below the target, which in turn improves the efficiency of sputtering. Magnetron sputtering
42
sources operate at direct current (DC), pulsed DC or radio-frequency (RF) modes. It is
essential in DC sputtering that the target material is electrically conductive. If the target is
not sufficiently conductive then a positive charge builds up at the cathode surface and
sputtering ceases to occur. RF sputtering can be used to overcome this problem [56]. For
example, during part of each half-cycle, the potential is such that ions are accelerated to the
surface with enough energy to cause sputtering while during alternate half-cycles, electrons
reach the surface and hence prevent charge build-up. This sputtering process is mainly
used to deposit insulators from ceramic targets. One disadvantage of this process is that the
poor thermal conductivity of the ceramic target does not allow it to cool effectively [88],
which results in local hot spots and consequent cracking of the material. As a result,
optimum RF sputtering rates are generally limited to targets with a low coefficient of
thermal expansion that are not very susceptible to cracking. In addition, RF sputtering rates
from ceramic targets can be quite low (0.1-0.2 µm h-1) due to the low voltages (40-160 for
RF-powers of 10-100 W) that are generated at the target [89].
An alternative method for depositing insulators is to use a metal target and reactively form
the insulator. This method is termed “reactive sputtering” and results in faster deposition
rates typically for insulators 0.8-1.5 µm h-1. Reactively depositing insulators can be
achieved using DC power. This deposition process can provide superior sputter yields and
deposition rates compared with RF sputtering, however, two main problems can occur,
namely arcing and a phenomenon known as “the disappearing anode” [15, 16, 90]. If the
insulating layer charges up by ionic bombardment to a point at which the dielectric strength
is exceeded, electrical breakdown will occur leading to arc discharges. The “disappearing
anode” occurs when the electron path to the anode is blocked by the electrically insulating
layer being deposited. This results in the anode losing its ability to conduct current over
time.
43
3.4.2 Cathodic Arc Evaporation
In the cathodic arc evaporation process, an arc discharge is initiated on a cathode surface
using a mechanical igniter or trigger that is momentarily brought into contact with the
cathode. The residence time of the trigger and consequently the arc discharge at any
location on the cathode surface is sufficiently brief that no molten pool is generated [91].
The high current density at the arc spot causes the cathode to emit metal plasma containing
a high density of metal ions (~ 1023 m-3) as well as electrons, neutral atoms and spherical
agglomerates called ‘macroparticles’ [92] which vary in size from tens of micrometers to
sub-micrometers. These macroparticles are formed by evaporation of the molten or solid
target material [56] and their number and size increase with lower melting point materials,
higher cathode currents and higher cathode temperatures.
The majority of macroparticles can be filtered from the arc by using a duct containing
focusing magnetic coils as shown in Figure 10. The application of a magnetic field induces
the acceleration of electrons through the field. In order to balance charge, positive ions are
attracted to the electrons and in doing so are accelerated. The ions follow the electrons
around a bend, whereas the neutrals and macroparticles travel straight and deposit onto the
wall of the filter duct.
44
Figure 10: A schematic diagram of the filtered cathodic arc PVD system showing the full-
filtration configuration (right) and the partial-filtration (left).
Cathodic arc evaporation can be used to deposit insulators by evaporating from metal
cathodes in a reacting gas. This method has perhaps the highest deposition rate of all PVD
methods for oxide materials (~ 6 µm h-1) [92]. However, over time the anode losses its
ability to conduct current at which point arc evaporation ceases to occur. Nonetheless, high
quality insulating films of AlN can be deposited at rates of the order of 2-4 mm h-1at room
temperature [93] presumably before the disappearing anode takes effect.
3.4.3 Thermal Spraying
Thermal spraying is a process in which coating materials are either partially or totally
molten as they exit a spray torch (see Figure 11) and are deposited onto a surface forming a
coating [94]. One major category of the thermal spray process is plasma spraying. In this
process the energy source is a DC electric arc that is initiated with a high frequency spark.
A gas is passed through the arc resulting in a plasma jet that emerges from a nozzle. The
45
materials to be deposited are introduced into the jet in the form of a powder that melts and
strikes the surface to be coated at high speed. For ceramic processing this can generally be
operated in air. The arc burns between a cylindrical tungsten cathode and radial, concentric
copper anode, both of which are internally water-cooled. The plasma flame is sustained by
an inert gas such as argon that enters through the cathode. As the plasma flame exits,
ionised gas recombines and becomes neutral in the surrounding area of the exit opening
[95], yielding a high level of enthalpy. It is into this recombination region (beyond the
plasma core) that feedstock powder is introduced, carried out by the flame, melted, and
accelerated to the substrate, where it impacts and undergoes rapid solidification.
Figure 11: Schematic cross-section of a DC arc plasma spray torch showing an internal
particle feed injector.
46
In the plasma spraying process preparation of the substrate surface is a relatively simple
task since only grit blasting prior to coating is required. One distinct advantage plasma
spraying has over PVD is that much higher deposition rates (~ 1 mm h-1) can be achieved,
however, deposited layers suffer poor adhesion and high porosity.
3.5 Thin Film Analysis
3.5.1 X-Ray Diffraction
X-ray diffraction (XRD) is a tool for investigating the fine structure of matter [96] and
identification of materials. The wavelengths of X-rays are comparable in size to the
distances between atoms in most crystals, and the repeated pattern of the crystal lattice acts
as a diffraction grating for X-rays such that they are reflected if the Bragg condition is
satisfied:
θλ sin2dn = (Equation 11)
where λ is the wavelength, d is the interplanar spacing, θ is the grazing angle of incident
(and outgoing ) radiation and n is an integer. The diffracted X-rays are detected at an
angle of 2θ, typically called the diffraction angle. Figure 12 shows a schematic of
diffraction of X-rays from a set of crystal planes.
47
Figure 12: Schematic of diffraction of X-rays from a set of crystal planes. From [96].
One of the benefits in using XRD is the fact that different crystallographic directions can be
identified in polycrystalline structures. This is achieved by studying isolated single crystals
[96], since such studies permit measurement of the properties of the individual building
blocks in the composite mass. For example, in a polycrystalline structure each grain
normally has a crystallographic orientation different from that of its neighbours.
Considered as a whole, the orientations of all the grains may be randomly distributed in
relation to some selected frame of reference, or they may tend to cluster, to a greater or
lesser degree, about some particular orientation or orientations. Any aggregate
characterised by the latter condition is said to have a preferred orientation, or texture, which
may be defined simply as a condition in which the distribution of crystal orientations is
non-random.
48
3.5.2 X-Ray Photoelectron Spectroscopy
X-ray photoelectron spectroscopy (XPS) is a well-established tool for characterising the
molecular structure at the outer surface of a thin film [97, 98]. The method uses an
electron spectrometer to measure the energy distribution of photo-emitted electrons from a
surface irradiated by X-ray photons. When these X-rays penetrate into the material X-ray
absorption by an atom in the solid leads to the ejection of an electron either from one of the
tightly bound core levels or from the more weakly bound valence levels. Some fraction of
these electrons emerges from the surface into a partial vacuum and, overall, this is known
as the photoelectric effect. The energy of the emitted photoelectrons is then analysed by
the electron spectrometer to produce a spectrum of electron intensity (counts) as a function
of kinetic energy in electron volts. The XPS survey spectrum can then be used to confirm
the presence of the desired elements in the film and to evaluate its atomic composition.
The X-ray sources in common use are aluminium and magnesium non-monochromatic Kα
X-rays at 1486.6 and 1253.6 eV respectively. In the case of a conducting sample in
electrical contact with the spectrometer, conservation of energy leads to
WEhE BK −−= ν (Equation 12)
where EK is the kinetic energy of the emitted electron, hν is the photon energy, EB is the
binding energy in the conducting solid and W is the spectrometer work function (~5 eV).
The process of photoemission is shown in Figure 13.
49
Figure 13: XPS process showing an incident X-ray photon absorbed and a photoelectron
emitted.
One disadvantage of the XPS technique is the fact that it is inherently surface sensitive only
to depths of approximately 10 nm since the electrons analysed do not have enough energy
to escape from beyond these depths. However, much information can be provided from
thin film coatings using XPS that include: identification of elements, determination of
surface composition and information about oxidation states and the bonding atoms.
3.5.3 Glow Discharge Optical Emission Spectrometry
In Glow Discharge Optical Emission Spectrometry (GD-OES), sputtering and atomic
emission are combined to provide a rapid technique for depth profiling and analysis of
surface coatings and interfaces [99]. GD-OES relies on the fact that when a material is
heated sufficiently it will emit visible light in a discrete spectrum characteristic of the
elements in the sample. The intensity of emitted light from a particular element is directly
proportional to its concentration in the glow discharge (or plasma) and hence in the sample
50
surface. Qualitative analysis is obtained from the profile of intensity versus sputter time.
The measured light intensities for all elements of interest can then be calibrated
quantitatively using a spectral database [100] to produce atomic % versus film thickness.
GD-OES techniques employ DC or RF discharge sources for analysing conducting or non-
conducting samples respectively. The sample specimen forms the discharge cathode, and is
bombarded by ionized argon gas molecules to produce a glow discharge. The excited
argon ions cause sputtering of the sample surface. The sputtered atoms from the sample
enter the plasma, and they are excited by collisions with electrons or other argon atoms.
The resulting optical emission forms the atomic spectrum from the sample. Figure 14
shows the GD-OES process.
λλ
λ
Figure 14: A schematic diagram of the glow-discharge showing the sputtering of sample
atoms by ionized argon atoms, where λ is the wavelength of the photon of light emitted
from energized atoms as they return to a stable energy state.
51
During sputtering a non-uniform crater forms on the sample that produces a general
decrease in depth resolution with depth. This is one disadvantage with GD-OES. Figure
15 is an image of a steel sample coated with AlN film and shows a typical crater formed
after cathodic sputtering with the GD-OES system.
Figure 15: Image of a GD-OES sample showing a crater that forms after cathodic sputtering
bombardment.
Since there is no ultra high vacuum involved (~ 1 to 1.5 mbar), the main advantage of using
GD-OES as a depth profiling technique is the fact that very fast measurements can be
achieved. For example, a depth profile of 500 nm using GD-OES would take
approximately 6 seconds in contrast to a similar profile generated using XPS which would
take approximately 24 hours. Consequently, thin films having thicknesses several tens of
micrometers can be quickly analysed. Another feature of GD-OES is that large-area
samples with diameters greater than 20 mm are needed. This may be considered an
52
advantage since it averages results over areas of approximately 12 mm2 in comparison to
XPS measurements (~ 0.5 mm2) [101].
GD-OES measurements were carried out using a Leco GDS-850A spectrometer [100, 102].
The copper anode had a diameter of 4 mm, for a sampling area of 12.5 mm2. The
spectrometer was equipped with dual Rowland circles, having curved, holographic
diffraction gratings of 1800 lines/mm and 3600 lines/mm, respectively, for a spectral range
of 120–800 nm. There were 32 detectors, giving the ability to detect 29 elements, including
most standard metals, nitrogen, oxygen and carbon. An RF glow-discharge was used as the
sputter source since AlN is electrically non-conductive. The RF source was operated in
power-voltage mode, where the RF power was fixed at 14 watts. A variable argon pressure
held the lamp voltage fixed at 700 volts. The resulting plasma current was 20 mA. The
instrument was calibrated by the manufacturer against a large number of certified reference
materials, for both elemental concentrations and sputter depth into the sample surface.
53
Chapter 4
EXPERIMENTAL METHODOLOGY
4.1 Introduction
This chapter presents the experimental methodology that was used to fabricate thin film
transducers on mild steel substrates. To prepare the metal substrate surfaces for coating a
process of grinding, polishing and cleaning was used. PVD coatings of AlN and
AlNx/AlxOy thin film insulators were deposited by DC magnetron sputtering and cathodic
arc evaporation. Thick insulators of plasma sprayed Al2O3 were also fabricated and a
high-voltage probe was used to assess the quality of all insulator coatings. Strain gauges
were fabricated by vapour depositing chromium and CrN/TiAlN film through a shadow
mask made from stainless steel sheet. The strain gauge sensitivities were obtained using
cantilever and extensometer testing methods.
4.2 Substrate Preparation
Mild steel substrate samples with dimensions 30 x 6 x 2 mm and 65 x 6 x 2 mm were cut
using an abrasive wheel, ground and then mechanically polished. The latter was carried out
on SiC abrasive papers with increasingly finer grit size particles down to 1200 followed by
polishing to a 1 μm diamond finish. In addition, load cell washers with dimensions shown
in Figure 16 were machined from mild steel tube using a MACSON 70-10 lathe.
54
Figure 16: Schematic cross-section of a load cell washer showing dimensions.
Test samples of (100) silicon wafer and quartz glass slides were included in several of the
coating runs where the objective was to characterise the coatings in terms of thickness and
surface texture using profilometry and SEM. All samples were cleaned in an ultrasonic
bath of acetone for 10 minutes, dried using hot air then wiped with pure alcohol prior to
PVD coating. After coating, fractographic cross-sections from mild steel and silicon
samples were prepared for SEM analysis by breaking them in two.
4.3 Mask Preparation
Mechanical masks with strain gauge pattern apertures were cut from 0.2 mm thick stainless
steel shim using a Coherent AVIA 3ω Nd:YAG laser with two galvo mirror beam scanners.
The laser parameters used were: wavelength 355 nm, scan speed 0.11 ms-1, pulse energy
approximately 250 μJ, pulse frequency 10 kHz, 380 passes. The focus spot diameter was
approximately 30 µm. The masks were then cleaned in acetone in an ultrasonic bath, dried
55
using hot air and then wiped with high purity alcohol and dried. The masks were then
placed over the substrate surfaces, while leaving the gauge pattern exposed, and fixed using
stainless steel brackets. These were then placed into the deposition chamber on fixtures
that allowed two degrees of rotation. Figure 17 shows a typical mask/substrate assembly
system that was used.
Figure 17: Schematic diagram of a mask/mild steel substrate assembly showing brackets
used to hold the apparatus together.
Additional stainless steel mechanical masks with thicknesses of 1.5 mm were used to form
meandering patterns of chromium onto glass substrates. These masks were prepared using
electro discharge machining (EDM) in which 0.2 mm diameter brass wire was used to cut
56
out the meandering strain gauge patterns. The mechanical masks were kept in close contact
with the glass substrates by attaching fasteners at either end.
4.4 Insulator Coatings
4.4.1 DC Sputtered AlN
Thin films of AlN were reactively deposited using DC balanced magnetrons with targets of
aluminium (99.99 %). Initially, the chamber was pumped down to a base pressure of 2 x
10-5 mbar. Prior to coating, the substrates were argon ion etched for ten minutes at 1 x 10-2
mbar followed by sputter cleaning of the shielded target for a further ten minutes. The
shield was then moved away from the target surface and an interlayer of titanium up to 1
µm in thickness was sputter deposited onto the substrates from a titanium (99.99 %) target.
The titanium interlayer was deposited at an argon gas pressure of 5 x 10-3 mbar using a DC
voltage of 420 V and a source current of 0.5 A. The substrates were then positioned over
the aluminium target and AlN was reactively deposited using nitrogen gas flow of 12 sccm
in which the target was sputtered at 240 V DC and current 1.0 A. The gas pressure during
the AlN coating was around 6 x 10-5 mbar. The AlN films were deposited for up to 3
hours, left to cool overnight and then annealed in air for two hours at 300oC. It should be
noted that substrates were not preheated prior to deposition and were effectively self-biased
during coating. All substrate-to-target distances were 10 cm. Process parameters are given
in Table 1.
57
Table 1: DC sputtering conditions for AlN/Ti coatings
Parameters AlN film Ti film
Base pressure (mbar) 2 x 10-5 2 x 10-5
Target-substrate distance (cm) 10 10
DC power (W) 240 210
Working pressure (mbar) 6-7 x 10-3 5.5 x 10-3
Ar flow rate (sccm) 25 40
N2 flow rate (sccm) 12
Process control difficulties were experienced due to arcing when using the DC power
supply to reactively deposit AlN. The arc instability was caused by insulating layers
forming on the target surface [16] thereby producing electric fields inside the layers that
exceed the breakdown strength of the AlN. Arc stability was achieved by manual
intervention, that is, at the onset of arcing the argon gas pressure was increased and then
readjusted back to a more stable pressure. It was demonstrated empirically that control
could be achieved by having a nitrogen flow rate of 12 sccm combined with a flow rate of
argon sufficient to have a chamber pressure of around 6 x 10-5 mbar. At the onset of arcing,
the argon flow rate was increased such that the chamber pressure was 7 x 10-5 mbar at
which time the arcing ceased. The argon flow rate was then readjusted to return the
chamber pressure to its original value of around 6 x 10-5 mbar. The low level of nitrogen in
the chamber was sufficient to form AlN but not too great to cause undue poisoning of the
target, as previously reported by Sproul et al. [16].
4.4.2 DC Sputtered AlNx/AlxOy
DC magnetron sputtering was used to reactively deposit AlNx/AlxOy thin film insulators on
mild steel and (100) silicon substrates. After pumping the deposition chamber down to a
base pressure of 1 x 10-5 mbar, the substrates were argon ion etched at a pressure of 1 x 10-2
58
mbar. During coating AlNx was first reactively deposited using the system parameters
described in section 4.4.1, however, after ten minutes, the nitrogen flow rate was reduced to
zero and oxygen was introduced at a flow rate of 12 sccm. This sequence was repeated for
a total deposition time of two hours resulting in a total of 12 modulated layers. The
exercise was repeated for deposition intervals of 20 minutes (6 layers) and 60 minutes (2
layers). In an attempt to produce sharper interfaces between modulated layers, a second set
of AlNx/AlxOy samples were deposited using the same sequences of deposition intervals but
this time a shield was placed over the sputter target during the change over from nitrogen to
oxygen and vice-versa. The composite coating thicknesses of the insulator thin films
formed in this way were around 2 µm thick. The surface roughness was measured to be
between 0.01 and 0.02 µm Ra. Process parameters are given in Table 2.
Table 2: DC sputtering conditions for AlNx/AlxOy coatings
Parameters AlNx/AlxOy film
Base pressure (mbar) 2 x 10-5
Target-substrate distance (cm) 10
DC power (W) 240
Working pressure (mbar) 6-7 x 10-3
Ar flow rate (sccm) 25
N2 flow rate (sccm) 12
O2 flow rate (sccm) 12
4.4.3 Filtered Cathodic Arc Evaporated AlN
AlN thin films were reactively deposited on (100) silicon wafers and mild steel substrates
using a cathodic arc evaporation system under partially filtered conditions with a bias
voltage of -150 V and no substrate heating employed. Initially, the chamber was pumped
down to a base pressure of 8 x 10-5 mbar. Prior to coating, the substrates were nitrogen ion
59
etched for ten minutes at 1 x 10-3 mbar using a dedicated hot anode/cold cathode type ion
gun that was operated with a discharge voltage of 2.0 kV and a discharge current of 100
mA. During coating, a 99.99% purity aluminium cathode of 5.8 cm diameter was used to
reactively deposit the AlN in a mixture of nitrogen gas flow rate of 12 sccm with the
balance being argon. The cathode current used was 90 A and the gas pressure during the
AlN coating was around 6 x 10-5 mbar. The AlN films were deposited for 1-1.5 hours and
left to cool overnight. All substrate-to-cathode distances were 57 cm. Process parameters
are given in Table 3.
Table 3: Filtered cathodic arc evaporation conditions for AlN coatings
Parameters AlN film
Base pressure (mbar) 8 x 10-5
Cathode-substrate distance (cm) 57
Cathode arc current (A) 90
Working pressure (mbar) 6 x 10-3
Ar flow rate (sccm) 18
N2 flow rate (sccm) 12
Substrate bias (V) -150
In the present study shielding was used to fabricate AlN insulating films in which the
substrate-to-shield distance was 22 cm. It was found that the AlN could be deposited for
only 1 to 1.5 hours before uncontrollable arcing occurred in the chamber. This was due to
the “disappearing anode” effect [15]. As a result, shielded AlN films could only yield
thicknesses of around 1 µm whereas unshielded AlN films were able to yield thicknesses ≥
4 µm. Therefore, AlN films were fabricated by initially depositing without a shield to
thereby produce optimal (≥ 4 µm) insulating thicknesses then, by placing the shield
between the cathode and substrate, smooth top surface coatings were deposited in order to
accommodate thin film (≤ 1 µm) strain gauge material. Surface roughnesses for shielded
60
and unshielded films were measured using a profilometer and found to be Ra 0.015-0.018
μm and Ra 0.096-0.132 μm respectively.
4.4.4 Plasma Sprayed Al2O3
Al2O3 powder (~ 5 µm diameter) was plasma sprayed on 5 mm thick dog-bone shaped mild
steel substrates using a Praxair Miller plasma arc spray unit. The Al2O3 coatings were
approximately 100 µm thick (surface dimensions are given in Figure 18). To remove any
surface contamination and obtain good adhesion the substrate surfaces were de-greased and
blasted with G70 grit prior to plasma spraying. Parameters for the plasma sprayed coatings
are shown in Table 4. After coating, the Al2O3 surfaces were ground using SiC abrasive
paper and polished to a 1 µm diamond finish then washed using acetone and ethanol.
Figure 18: Schematic of a mild steel dog-bone shaped substrate showing the allocated area
of Al2O3 deposited by plasma spraying.
61
Table 4: Plasma spraying conditions for Al2O3 coatings
Parameters Al2O3 film
Spraying distance (cm) 7.5
DC power source (kW) 36
Power particle size (µm) 5
Air pressure (kPa) 490
4.5 Strain Gauge Coatings
4.5.1 Sputtered Chromium Gauges
Chromium strain gauges were DC magnetron sputter deposited onto sputtered AlN and
plasma sprayed Al2O3 coated mild steel substrates through shadow masks. Initially, the
chamber was pumped down to a base pressure of 2 x 10-5 mbar. Prior to coating, the
substrates were argon ion etched for ten minutes at 1 x 10-2 mbar followed by sputter
cleaning of the shielded target for a further ten minutes. The shield was then moved away
from the target surface and chromium layers 0.5 to 3 µm thick were sputter deposited onto
the substrates from a chromium (99.99 %) target. The chromium films were deposited for
periods between 15 seconds and 4 minutes at an argon gas pressure of 5.5 x 10-3 mbar using
a DC voltage of 210 V and a source current of 1 A. The mask/substrate assemblies were
not preheated prior to deposition and were effectively self-biased during coating. The
chromium strain gauges were later stress relieved by annealing in a partial vacuum (< 1 x
10-4 mbar) for two hours at 200°C. Process parameters are given in Table 5.
62
Table 5: DC magnetron sputtering conditions for Cr coatings
Parameters Cr film
Base pressure (mbar) 2 x 10-5
Target-substrate distance (cm) 10
DC power (W) 210
Working pressure (mbar) 5.5 x 10-3
Ar flow rate (sccm) 40
4.5.2 Cathodic Arc Evaporated CrN/TiAlN Gauges
To deposit CrN/TiAlN strain gauges a multi-source random cathodic arc system was used.
In these experiments four cathodes (each 120 mm in diameter) were used, as shown in a
schematic of the deposition system in Figure 19. The stainless steel vacuum chamber had a
volume of approximately 1.0 m3 (1,200 mm diameter × 920 mm height) that housed six
spindle substrate holders, which rotated about centrally located heating elements on a
diameter of 840 mm. Mask/substrate assemblies were mounted on one of the substrate
holders and, during the deposition process, alternate thin film layers of CrN and TiAlN
were deposited as the substrates rotated past each of the cathodes.
63
Figure 19: A schematic diagram of the multi-source random cathodic arc PVD coating
system showing the relative position of the arc cathode sources in relation to the substrates.
The CrN and TiAlN thin films were reactively deposited onto cathodic arc evaporated AlN
coated mild steel substrates through shadow masks. Initially, the chamber was pumped
down to a base pressure of 6 x 10-5 mbar. Prior to coating, the mask/substrate assemblies
were argon ion etched for ten minutes at 2 x 10-2 mbar at a bias voltage of -1000 V. During
coating, 99.99% purity Cr and Ti0.5Al0.5 cathodes were used to reactively deposit
CrN/TiAlN in a mixture of nitrogen gas flow rate of 60 sccm with the balance being argon.
The cathode currents used were 120 and 85 A for Cr and Ti0.5Al0.5 respectively and the gas
pressure during coating was around 8 x 10-3 mbar. The CrN/TiAlN films were deposited
for 20 minutes with a substrate bias voltage of -100 V and a chamber temperature of around
250°C. All substrate-to-cathode distances were 360 mm. Process parameters are given in
Table 6.
64
Table 6: Random cathodic arc evaporation conditions for CrN/TiAlN coatings
Parameters CrN/TiAlN film
Base pressure (mbar) 6 x 10-5
Cathode-substrate distance (cm) 180
Cr arc current (A) 120
Ti0.5Al0.5 arc current (A) 85
Working pressure (mbar) 8 x 10-3
N2 flow rate (sccm) 60
Ar flow rate (sccm) 15
Substrate temperature (°C) 250
Substrate bias (V) -100
4.6 Transducer Measurements
4.6.1 Electrical Insulation
A SATURN ISO high-voltage probe was used to assess insulator quality. Breakdown
voltages were measured by placing equally spaced spring probes over the surface of each
insulator sample and applying increments of 10 volts (starting at 50 volts) until breakdown
occurred. This was then repeated for several measurements around the surface in order to
obtain average breakdown voltages. Also, breakdown field strength was obtained from the
gradient of the plot of average breakdown voltage versus film thickness for sputtered AlN.
4.6.2 Strain Gauge and Signal Conditioning
Gauge factors were calculated from a change in electrical resistance in response to a
longitudinal strain using two different techniques. The first technique was based on the
cantilever method [1], in which forces were applied via the application of weights, and the
65
second, used a ZWICK Z010/TN2S extensometer with type KAP-Z 10 kN load cell.
Experimental arrangements for the cantilever and extensometer testing methods are shown
in Figure 20 and Figure 21 respectively.
Figure 20: Schematic of cantilever testing method showing Wheatstone bridge circuit (left)
that was connected to strain gauges attached to cantilevers under load (right).
66
Figure 21: Optical image showing a ZWICK Z010/TN2S extensometer used to test samples
under load.
Leads were bonded directly to the ends of each strain gauge with quick-drying silver paint
and the Wheatstone bridge configuration shown in Figure 20 was used to obtain voltage
signals, which were then amplified using an AD623AN operational amplifier.
For the cantilever method, strain values ε were calculated using the relation
2
6
Ebd
WL=ε (Equation 13)
67
where E is Young’s modulus for the mild steel beam, b is the beam width, d is the beam
thickness and L is the distance from the centre of the gauge to the point of application of the
load W.
Gauge factors were calculated using the relation
εRR
GFΔ
= (Equation 14)
where RRΔ is the fractional change in resistance of the strain gauge. All resistance
measurements for the strain gauges were made using a Tektronix DM2510G programmable
digital multi-meter and substituted into Equation (14). In addition, instant and short term
repeatability measurements from the strain gauges were obtained using hysteresis and creep
values respectively. The hysteresis was found by loading the strain gauges twice with some
specific force and the creep error was found as a percentage of the difference in output at
maximum load after 30 minutes. The temperature coefficients of resistance were also
found by measuring the resistance of the gauges over a range of temperatures. Second
order effects from the thermal expansion coefficient of the system were assumed to be
negligible. A FSE Selsius hotplate and Digitech QM-1320 multi-meter were used.
When using the extensometer, strain and force measurements were calculated via ZWICK
test software. Dog-bone shaped specimens were gripped with clamping jaws while tensile
forces were applied. The corresponding voltage measurements were recorded at 20-second
intervals. Data from plots of force versus strain were used to assess strain gauge
performance.
68
Chapter 5
RESULTS & DISCUSSION THIN FILM INSULATION
5.1 Introduction
This chapter presents results obtained from thin film insulation deposited onto substrates of
mild steel and silicon using DC techniques. The thin films investigated were AlN and
AlNx/AlxOy deposited by sputter deposition, Al2O3 deposited by thermal spraying and AlN
deposited by filtered cathodic arc evaporation. The films were characterised using the
following techniques: SEM, XRD, XPS and GD-OES. Electrical breakdown voltages
were obtained from the films and used to assess insulator quality.
5.2 DC Sputtered AlN
5.2.1 Surface Coating Evaluation
With a view to establishing fabrication conditions for the deposition of AlN thin film
insulation, initial trials were conducted in which DC magnetrons were used to reactively
sputter AlN from aluminium targets onto mild steel and silicon substrates. The latter were
selected because they are reliable substrate materials for characterising thin film coatings
and have matching thermal expansion coefficients with AlN. Additionally, silicon is easy
to prepare requiring only minimal pre-cleaning in acetone and alcohol. In the present
study, it was found that this level of pre-cleaning without any ion-etching prior to
deposition was sufficient to obtain good adhesion between the AlN thin films and silicon
substrates (see Figure 22).
69
Figure 22: Optical micrograph of AlN on silicon wafer showing no delamination after
preparing the silicon surface by simply washing in acetone and alcohol.
To obtain a surface finish comparable to the silicon wafers, mild steel substrates were
ground, polished and then cleaned (see section 4.2) prior to being coated with AlN. In
contrast to the silicon substrates, this pre-cleaning procedure resulted in delamination of
AlN coatings from the mild steel (see Figure 23). In order to prevent the delamination of
AlN thin films from mild steel substrates, it was essential that in-situ argon ion etch
cleaning be performed for at least 10 minutes prior to depositing the AlN films.
70
Figure 23: Optical micrograph of AlN on mild steel showing delamination after preparing
the mild steel surface by simply washing in acetone and alcohol.
For conditions in the present study, it is suggested that the cause of delamination of AlN
from mild steel substrates was probably related to the extrinsic residual stress component in
the coating [6]. At room temperature the thermal expansion coefficients of silicon and mild
steel are 2.6 x 10-6 and 15 x 10-6 K-1 respectively while AlN has a value of approximately
4.2 x 10-6 K-1, which is comparable to silicon. Therefore, delamination of AlN from mild
steel could result from high extrinsic residual stress produced during cooling due to the
poor match of thermal expansion coefficients between AlN and mild steel. Typically,
problems of delamination are overcome in PVD by at least one of two techniques. The first
is ion bombardment by in-situ argon ion etching which heats the substrate surface due to
the impingement of the charged argon ions [6]. The second is to utilise a thin film
interlayer [13]. Titanium interlayers deposited between the AlN and mild steel substrate
can reduce some of the residual stress accumulated by allowing for a gradual transition
between the AlN and mild steel. It should be noted that, when thin films of AlN were
71
sputtered on glass using a DC triode system, Vacandio et al. [103, 104] were not initially
able to obtain well adhered AlN film above film thickness values of 0.15 µm and later
solved this adhesion problem by one of two ways. They first increased the substrate
temperature during coating to 250ºC and secondly they deposited a nitrogen deficient AlN
interlayer.
In the present study, sufficient adhesion was obtained by argon ion etching and the
deposition of a thin titanium interlayer before starting the vapour deposition cycle for
AlN. At room temperature titanium has a thermal expansion coefficient value of
approximately 8.6 x 10-6 K-1. This value lies between that of AlN and mild steel and
enables the AlN to be less susceptible to cracking and delamination during temperature
changes.
Chromium has a thermal expansion coefficient of approximately 4.9 x 10-6 K-1 at room
temperature and was selected as strain gauge material since its thermal expansion
coefficient closely matches that of AlN and would be expected to remain adhered to the
AlN even under strain conditions. Thus, the complete coating assembly consisted of
Cr/AlN/Ti on mild steel. Figure 24 shows a SEM image from a fractographic cross-
section of sputter deposited Cr/AlN/Ti on a polished mild steel substrate. The profile of
the AlN layer is suggestive of a columnar microstructure orientated normal to the substrate
surface. Columnar growth of the AlN is in agreement with XRD results presented in
section 5.2.2, which show that the AlN is textured.
72
Figure 24: SEM fractographic cross-sectional image of Cr/AlN/Ti on mild steel.
5.2.2 Structure and Characterisation
In order to identify the crystallographic structure of the AlN films, XRD analysis was
performed using a Bruker AXS D8 ADVANCE X-Ray diffractometer with Cu-Kα
radiation in the Bragg-Brentano θ-2θ scan mode. XRD patterns from two different
nitrogen gas flow rates of 12 and 20 sccm are shown in Figure 25. It should be noted that a
third sample with a nitrogen flow rate of 16 sccm produced an identical spectrum to that of
Figure 25 (b). The X-ray results reveal a wurtzite hexagonal AlN structure as indicated by
the sharp and predominant (100) and (103) AlN peaks. Diffraction peaks α and β were
assigned to the silicon substrate.
73
Figure 25: XRD spectra of AlN coatings showing wurtzite hexagonal structures for flow
rates of (a) 12 sccm and (b) 20 sccm of nitrogen.
74
Of particular interest was the fact that extra (101) and (110) AlN peaks can be seen for the
lower nitrogen flow rate of 12 sccm. It was reported by Cheng et al. [105] that the
formation of preferred 101 surface planes occur when reactively sputter depositing AlN
thin films under low nitrogen concentration conditions with high argon arrival rates. The
present XRD results support this interpretation. Nevertheless, it is evident that the film
structure in the range 12 to 20 sccm of nitrogen was composed mostly of AlN phase shown
by dominate (103) and (100) AlN peaks.
XPS analysis was used to identify the chemical state at the surface. This was done using a
VG MicroLab 310-F XPS analyser operated in CAE =100 eV mode at 1 eV steps for 140
seconds and the X-ray source was aluminium non-monochromatic Kα X-rays at 1486.6 eV.
XPS uses X-ray photons of a constant energy to ionise the surface atoms. The chemical
state is identified by measuring the energy of the emitted photoelectrons. The mean free
path of these photoelectrons is very short, typically a few nanometres, consequently the
XPS signal originates in the first few nanometres of the surface.
Figure 26 shows a typical XPS spectrum obtained from the same sample to that used for
XRD in Figure 25 (a). The spectrum clearly reveals the photoelectron peaks Al2p, Al2s
and N1s, indicating the formation of an AlN thin film. The peaks of Al2p and Al2s are
located at 78.8 and 119.8 eV respectively. There is also a weak carbon peak at 284.5 eV,
which is probably due to residual surface contamination, and a sharp O1s peak at 532.6 eV
which probably comes from Al2O3 forming on the outer surface. A detailed binding energy
scan, using peak deconvolution [98, 106], was performed on the N1s photoelectron peak to
identify the bonding environment and Figure 27 shows the N1s photoelectron signal where
two components are resolved. The two N1s peaks (indicating two types of bonding) are
located at 397.8 and 399.9 eV. The high binding energy component is due to nitrogen
atoms bonded to nitrogen atoms and the lower binding energy component is due to nitrogen
atoms bonded to aluminium atoms.
It should be noted that when Manova et al. [107] used XPS to produce a detailed spectrum
of the N1s photoelectron peak for reactively DC magnetron sputtered AlN thin films on
75
(100) silicon wafer, an additional N1s peak was observed at an energy value of 395.9 eV.
They suggested that this peak was attributed to N-O bonding within the growing oxide
layer in the AlN film surface. The N-O bonds had apparently formed because the samples
were stored in air for several weeks before analysis.
In the present study detailed binding energy scans were performed on the Al2p and Al2s
peaks. However, these peaks were relatively symmetrical and therefore the presence of
oxynitride (AlNxOy) could not be confirmed. Relative surface compositions up to a depth
of approximately 5 nm were found to be 27 % oxygen, 24 % nitrogen, 46 % aluminium.
The conclusion drawn is that that AlN was successfully deposited under DC conditions.
Figure 26: XPS image of sputtered AlN on silicon substrate showing thin film survey
spectrum.
76
N-Al
N-N11000
16000
21000
26000
31000
36000
41000
393394395396397398399400401402403
Binding Energy (eV)
Inte
nsity
(arb
itrar
y un
its)
Figure 27: XPS spectrum for the N1s photoelectron peak showing components N-N and N-
Al.
In order to investigate depth profiles from the deposited coatings, GD-OES analysis was
used. Figure 28 shows GD-OES quantitative depth profile images obtained from the
Cr/AlN/Ti coating in Figure 24 and an AlN/Ti coating (2 μm thick) on mild steel. The
multi-matrix calibration technique by Bengston et al. [108, 109] was used to convert the
raw data of elemental line intensities versus time to quantitative elemental concentrations
versus depth. It is evident from Figure 28 that oxygen is present at a concentration of
around 10 at. % throughout both films with an increase at the AlN/Ti interfaces. The films
reveal a consistent stoichiometry of approximately 30 at. % aluminium and 60 at. %
nitrogen.
N1s
77
Figure 28: GD-OES images of DC sputtered coatings showing depth profiles of (a)
Cr/AlN/Ti and (b) AlN/Ti on mild steel.
78
After the GD-OES analysis, the same specimen in Figure 28 (b) was annealed in air at
300°C and analysed again using GD-OES. Figure 29 shows that the oxygen peak is
slightly more skewed to the left at the titanium interface in contrast to Figure 28 (b). This
result suggests some possible oxidation of the titanium layer had taken place after
annealing the sample.
Figure 29: GD-OES depth profile from the sample used to produce Figure 28 (b) showing
the effects after annealing at 300°C for two hours in air.
5.2.3 Insulator Evaluation
A key attribute for an insulator used in strain gauge applications is electrical resistance,
since it is this property that determines the maximum detectable field strength that the
insulator can withstand under DC operating conditions. For the present study, the DC
electrical breakdown field strength for thin film AlN deposited onto polished mild steel
79
substrates was measured using the method described below and the results are presented in
Figure 30.
Figure 30: Graph of breakdown voltage versus film thickness showing a linear relationship
with a gradient of around 1.68 x 106 V cm-1.
The breakdown voltage was measured in steps of 10 volts, starting at 50 volts using a high-
voltage probe. The voltage was applied across AlN films of different thicknesses using
equally spaced spring probes placed on the surface. Voltages were then measured until
breakdown occurred. Each sample point on the plot represents the average breakdown
voltage measured at a given film thickness and these values show little scatter about the
gradient, which relates to a breakdown field strength of 1.68 x 106 V cm-1. This is a
80
relatively large breakdown field strength but consistent with similar values reported for
physical vapour deposited AlN [110, 111].
In the present study, sputtering was used to deposit 1 μm thick chromium electrode pads of
area 0.2 cm2 through a shadow mask on a 2.8 μm thick AlN coated mild steel substrate. An
average AlN breakdown voltage of 396 ± 2 V was found when using the voltage probe to
measure across the deposited electrodes. Of particular interest was the fact that this value is
approximately 19 % less than the average breakdown voltage value found in Figure 30 for
2.8 μm thick AlN. The discrepancy in average breakdown voltage for AlN was probably
due to the deposited chromium penetrating through porosity and pinholes in the AlN
coating [6].
Interestingly, it has been reported [6] that many thin films, when heated in air, undergo
increases in resistance because of oxidation. For example, Nayak et al. [9] used electron
beam evaporation to reactively deposit Al2O3/SiO2/Al2O3 multi-layered thin film insulators
and then subjected them to low temperature annealing treatments (120°C) under ambient
conditions. They observed an increase in resistance from 100 MΩ at 45 volts DC to 160-
15000 MΩ at 45 volts DC and suggested that the reason for the increase was oxidation
during annealing. In the present study, the effect of low temperature annealing on
breakdown voltage was investigated for films of AlN deposited onto mild steel and AlN
deposited onto mild steel with an interfacial layer of titanium. The films of AlN were
deposited in the one deposition cycle and hence had the same thicknesses. The annealing
treatments were carried out in air for two hours at 300°C. Table 7 shows the results
obtained.
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Table 7: Average breakdown voltages for AlN thin film after annealing in air for two hours
at 300°C.
It is evident from Table 7 that there was a significant change in average breakdown voltage
found for the AlN/Ti/mild steel sample compared with the AlN/mild steel sample after
annealing. Of particular interest was the fact that no significant difference in breakdown
voltage was detected for the AlN/mild steel sample. An increase in resistance was only
observed when a titanium interfacial layer was present. It is speculated that the columnar
grain boundaries within the AlN film represented a possible diffusion pathway for the
transfer of oxygen. In the presence of a titanium interfacial layer this resulted in the
formation of TiOx, thereby increasing the oxidation resistance of the thin film. The GD-
OES results from section 5.2.2 supports this interpretation. Clearly, the oxidation of the
mild steel substrate would have only limited capacity for improving the oxidation resistance
in contrast to the high quality TiOx insulator.
5.3 DC Sputtered AlNx/AlxOy
5.3.1 Structure and Characterisation
A feature of physical vapour deposited thin films is the inevitable presence of pin holes [6].
Such a feature can be problematic when seeking to deposit thin film AlN insulators, due to
short circuits or deterioration of the insulation resistivity along columnar grain boundaries.
It is possible that this problem can be overcome through the exploitation of a multi-layered
thin film [5]. The concept is simply one in which the growth of pin holes is interrupted by
the deposition of a second insulator material and this pattern may be repeated. In such a
system, the pin holes are confined to individual layers and are not pathways to the
AlN/(mild steel) AlN/Ti/(mild steel)
Before annealing 160 ± 2 V 158 ± 3 V
After annealing 155 ± 2 V 240 ± 4 V
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substrate. In the present study, the method described in section 4.4.2 was used to deposit a
layer of AlNx followed by a layer of AlxOy. Figure 31 is a fractographic cross-sectional
SEM image of the as-deposited thin film on a silicon substrate showing the two well-
defined uniform layers with thicknesses of approximately 0.6 µm each. The bright band
between the films is a consequence of charging from an interface step or ledge.
Figure 31: SEM fractographic cross-section of sputtered AlNx/AlxOy on silicon showing
two separate AlNx and AlxOy layers.
In an effort to deposit multi-layers the technique was extended by varying or modulating
the gas flow of oxygen and nitrogen into the vacuum chamber. The effect of such a
modulation on the composition of the deposited coating is shown in the GD-OES image
Figure 32 (a). It is evident that although the oxygen and nitrogen compositions vary as the
flow rate of each gas is increased or decreased using the mass flow controllers, at no stage
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does either gas fall to zero. This was possibly due to inefficient zeroing of the mass flow
controllers when switching between oxygen and nitrogen gas flows. In addition, the
aluminium composition is falling as the thickness of the coating increases and it would
appear that the level of aluminium was influenced by the levels of oxygen and nitrogen
displayed in the compositional profiles.
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Figure 32: GD-OES profile of atomic percent versus thickness for modulated AlNx and
AlxOy on mild steel substrate showing the effects of using (a) shielding and (b) no shielding
during the change over of nitrogen and oxygen gas flow.
Figure 32 (b) shows a GD-OES image from a modulated AlNx/AlxOy coating that was
deposited using intermittent shielding described in section 4.4.2. For example, a shield was
placed between the aluminium target and substrate during the change over of nitrogen and
oxygen gas flow. Figure 32 (b) shows a similar spectrum to Figure 32 (a), however, a
sharper, more abrupt transition between the two outermost layers is evident. The sharp
transition is preferred but is one that was too difficult to achieve repeatably due to the poor
zeroing function of the mass flow controllers used.
In order to resolve the structure of the modulated coating, samples were analysed using
XRD. A silicon substrate, coated in-situ with the GD-OES sample from Figure 32 (b), was
measured and the XRD spectrum is shown in Figure 33.
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Figure 33: XRD spectrum for AlNx/AlxOy modulated layers (diffraction peaks α and β are
assigned to the silicon substrate).
The spectrum shows only AlNx peaks from the modulated layer and does not show any
AlxOy or AlNxOy peaks, which suggests that the oxide present is amorphous. Ianno et al.
[112] found a similar result when DC magnetron sputtering AlN and AlNxOy film on silicon
substrates. They found that when reactively sputtering in a gas mixture of 12.5 sccm
nitrogen, 25 sccm argon and less than 0.5 sccm of oxygen, only XRD peaks of AlN were
observed, regardless of substrate temperatures up to 300°C. Interestingly, for films
deposited above 1 sccm oxygen gas flow rates, their XRD results did not show any AlN,
Al2O3 or AlNxOy peaks. In contrast to these results, Figure 33 clearly shows that AlNx
peaks are found in the composite film and suggests that crystalline AlNx and amorphous
AlxOy layers had formed within the modulated structure.
XPS analysis was performed on the outer surface of the AlNx/AlxOy coating in Figure 32
(b) and Figure 34 shows a typical XPS spectrum that reveals photoelectron peaks of Al2p,
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Al2s and O1s, located at 75.6, 119.8 and 532.6 eV respectively. Of particular interest was
the fact that no nitrogen peaks were detected, indicating the presence of an Al2O3 layer at
the surface. To identify the bonding environment, detailed binding energy scans of the
Al2p and Al2s photoelectron peaks were performed, however, since the photoelectron
signals were highly symmetrical, the presence of oxynitride at the surface was not evident.
The relative surface composition was found to be approximately 59 % oxygen and 41 %
aluminium, which is expected for Al2O3.
Figure 34: XPS survey spectrum of sputtered AlNx/AlxOy modulated layers on silicon
substrate showing no evidence of AlNx at the surface.
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5.3.2 Insulator Evaluation
To assess the resistive quality from AlNx/AlxOy coatings, samples prepared with 2, 6 and 12
modulated layers of AlNx and AlxOy were fabricated and average breakdown voltage values
were measured using a high-voltage probe. The results in Table 8 show that greater
average breakdown voltages are obtained when increasing the number of modulations in
the coatings.
Table 8: Breakdown voltages for AlNx/AlxOy film with modulated layers
Number of
modulated layers
Average breakdown
voltage (V)
Thickness (µm)
2 140 ± 4 0.53
6 170 ± 3 0.44
12 340 ± 3 0.54
It is evident that there is a correlation between the degree of modulation and the breakdown
voltage implying that modulation of the layers had affected the resistivity in the coatings.
The exact structure would need more detailed investigations from a further study.
5.4 Filtered Cathodic Arc Evaporated AlN
The possibility of depositing insulator thin films in the DC mode using cathodic arc
evaporation was investigated. It is well acknowledged [59] that one of the major concerns
of cathodic arc evaporation is the generation of a large number of macroparticles which are
ejected from the cathode surface and deposited onto the growing coating. It was considered
in this investigation that such defects could compromise the performance of the insulator
and the thin film strain gauge. Many methods have been proposed to eliminate or reduce
the number of macroparticles in cathodic arc evaporation [92, 113]. In the present study
two methods were employed. The first included the use of partially filtered cathodic arc
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evaporation in which a duct containing magnetic coils reduces the number of
macroparticles reaching the substrate (see section 3.4.2). The second involved the use of a
shield placed directly between the partially filtered cathode and the substrate. The shield
effectively acts as a line of sight filter preventing the macroparticles from reaching the
substrate while the positively charged aluminium ions are attracted to the negatively biased
substrate. There is of course a significant reduction in the deposition rate.
Figure 35 (a) and 36 (b) are SEM micrographs of unshielded and shielded AlN thin films
deposited on mild steel from a partially filtered cathode of aluminium. It is evident from
Figure 35 (a) that notwithstanding the use of partial filtration there is still a relatively large
number of macroparticles in the coating. The surface roughness of this film measured
using a profilometer was Ra 0.132 μm. Figure 35 (b) shows that the shield has almost
completely eliminated the macroparticles. The surface roughness was measured at Ra
0.015 μm.
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Figure 35: SEM surface images of filtered cathodic arc evaporated AlN showing (a)
unshielded AlN and (b) shielded AlN deposits.
In order to achieve a reasonably thick coating with adequate surface finish a procedure was
adopted in which AlN was first deposited without a shield from a partially filtered cathodic
arc and then the shield was used to give a smooth top coating. This procedure was used to
deposit the AlN film shown in Figure 36. The surface roughness was measured at Ra 0.061
µm. In addition, a high-voltage probe was applied across the AlN sample and this resulted
in breakdown voltages of around 200 V.
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Figure 36: SEM surface image of filtered cathodic arc evaporated AlN showing a shielded
AlN top coating deposited over unshielded AlN.
The AlN coating in Figure 36 was characterised using GD-OES (see Figure 37). It is
evident that the total coating thickness was 1.2 to 1.3 µm. The film reveals a consistent
stoichiometry of approximately 30 at. % aluminium and 60 at. % nitrogen with the balance
being oxygen.
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Figure 37: GD-OES profile of atomic % versus thickness showing AlN from filtered
cathodic arc evaporation.
Figure 38 shows the XRD spectrum for shielded and unshielded AlN film on silicon
substrate. The results show a wurtzite hexagonal structure with dominant (100) and (103)
AlN peaks in which the orientation for both shielded and unshielded AlN are similar.
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Figure 38: XRD spectra showing wurtzite hexagonal structures for (a) unshielded and (b)
shielded filtered cathodic arc evaporated AlN (diffraction peaks α and β are assigned to the
silicon substrates).
93
5.5 Thermal Sprayed Al2O3
The electrical insulation properties of Al2O3 are widely appreciated and have a significant
role as plasma sprayed topcoats for insulated metal substrates [95]. In the search for
developing thin film insulation for deposition on mild steel substrates the possibility of
thermal spraying Al2O3 was explored. The benefit of using thermally sprayed insulators is
that thick films can be obtained at high deposition rates. One well known limitation of
thermal spraying [95], however, is the presence of porosity in the coatings which can give
rise to high surface roughness which can, in turn, directly influence the growth and
performance of vapour deposited strain gauge thin film. Consequently, mild steel samples
were thermally sprayed (in this case plasma sprayed) with Al2O3 to a thickness of 100 μm
and polished to a 1 μm diamond finish. Figure 39 shows an SEM image of thermal sprayed
Al2O3 in which the surface roughness was measured at Ra 0.4 µm using a profilometer.
Figure 39: SEM image showing the polished surface from thermally sprayed Al2O3
insulator coating.
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The structure of the thermally sprayed Al2O3 was investigated using XRD (see Figure 40).
The spectrum shows that the coating consists mainly of γ-Al2O3 with some α-Al2O3 also
present. The γ-Al2O3 is more abundant because it nucleates in preference to α-Al2O3 in
high-energy plasma since the cooling rate after solidification is sufficiently rapid to prevent
the γ-Al2O3 to α-Al2O3 transformation [114].
Figure 40: The XRD angle as a function of intensity for thermally sprayed Al2O3 on mild
steel showing both α-Al2O3 and γ-Al2O3 phases.
Several insulation tests were performed on the thermal sprayed Al2O3 coatings using a
high-voltage probe. Resistances of 3 GΩ at voltages greater than 2000 volts were
measured for the 100 µm thick Al2O3 insulator coatings. These breakdown voltage
measurements are outstanding. As a comparison, Bravo et al. [4] deposited 3 µm thick
SiOx layers and found breakdown voltages of 6 GΩ at 100 volts DC and 1.2 GΩ at 250
volts DC.
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Chapter 6
RESULTS & DISCUSSION THIN FILM STRAIN GAUGES
6.1 Introduction
In this chapter gauge factor values are reported for two types of vacuum deposited strain
gauges, namely, chromium and CrN/TiAlN. The gauges were deposited through shadow
masks onto glass and a range of insulator thin film bases deposited onto mild steel
substrates. Three types of insulator thin films were investigated, namely, reactively DC
magnetron sputtered AlN, thermally sprayed Al2O3 and filtered cathodic arc evaporated
AlN. Cantilever and extensometer methods were used to test the gauges. Strain gauge
fractographic SEM images, hysteresis, creep and TCR results are presented and discussed.
6.2 Shadow Masking
In the present study partially filtered cathodic arc evaporation was used to deposit
chromium strain gauges onto glass substrates via shadow masking. The mechanical mask
used and strain gauge pattern on glass is shown in Figure 41.
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Figure 41: Photograph showing a chromium strain gauge on glass and a stainless steel
mechanical mask.
By mounting a collimating mask (with an appropriate gauge pattern and contact pads) onto
the glass substrate between the plasma plume and the cathode source, variation of surface
structure and smoothness of the deposited gauge was possible. Comparisons between thin
film deposits of meandering strain gauges and electrical contact pad layers were analysed
using microscopy. Figure 42 (a) and (b) are SEM images from a thin film chromium
contact pad and strain gauge meander pattern respectively.
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Figure 42: SEM images from deposited chromium on glass substrate showing (a) electrical
contact pad and (b) strain gauge meander.
One interesting consequence of interposing a shadow mask behind the cathode source and
the substrate was the difference in surface finish between the contact pad and the strain
gauge meander pattern. Figure 42 (a) shows that there is a larger number of macroparticles
at the surface. Macroparticles are an inevitable consequence of the high energy vapour
emission from the cathode source in arc deposition and can be the direct cause of pinhole
leakage current [59]. Figure 42 (b) shows almost complete elimination of macroparticles.
This reduction in the number of macroparticles is attributed to the shadow mask’s inherent
geometric design. As macroparticles travel through the mask and onto the substrate their
numbers are reduced by the sidewall shadowing from the narrow collimated meandering
path cut out of the mechanical mask. High velocity chromium ions have a greater
probability of reaching the substrate than the slower moving macroparticles which tend to
stick to the sidewalls of the mask. As a result, the deposition rate through the meandering
slots is reduced and a smoother surface with virtually no macroparticles is formed. As for
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the electrical contact pad, a rougher and thicker surface is produced since a larger opening
in the mask allows more of the macroparticles to reach the substrate.
Considerable reduction in the deposition rate was found for the thin film deposited through
the meander gauge pattern of the mask compared to the circular contact pads. The strain
gauge and electrical contact pads average thicknesses were 0.149 μm and 0.7025 μm
respectively – a thickness difference of approximately 79 %. The decreased deposition rate
through the meander can be attributed to the extra shadowing effect of the sidewall of the
mask edge. To increase the deposition rate through the meandering pattern, as opposed to
the deposition rate through the circular contact pads of the mask, the mask thickness would
need to be reduced [7]. A reduction in mask thickness can however cause excessive
heating of the mask and substrate during the deposition process and lead to increased
macroparticle content. This will affect the quality of the gauge deposited since thermal
distortion of the mask can occur. Consequently, when fabricating strain gauges using
shadow masks a compromise between mask thickness and deposition rate is necessary.
Compromises between mask thickness and deposition rate will depend on specific strain
gauge applications.
6.3 Single Layered Chromium Gauge
Generally, the selection of a strain gauge material is based on the material’s ability to
respond to strain. Chromium has a gauge factor value of around two, which is consistent
with most metallic strain gauges, however, this value is relatively small in comparison to
current sensitivities for the alloys used in industrial load cell devices today (~ 10). In spite
of this, chromium thin film is a convenient choice of strain gauge material for testing and
development purposes since it has (a) sufficiently large resistivity thereby allowing more
accurate readings to be measured from resistor bridges and (b) a thermal expansion
coefficient closely matching ceramic insulator coatings such as AlN and Al2O3.
99
Chromium strain gauge sensors were fabricated in the form of single layered thin films by
DC magnetron sputtering through shadow masks on to two different types of substrates.
The first substrate was vapour deposited AlN/Ti on mild steel and the second, was
thermally spayed Al2O3 on mild steel. The AlN insulator base was relatively smooth (Ra
0.01-0.02 μm) consequently the thickness of the chromium gauge was kept to around 2 ±
0.5 μm. Whereas, the thermally sprayed Al2O3 was relatively rough (Ra ≥ 0.4 μm).
Therefore, chromium strain gauges of thickness around 3 μm were deposited in order to
preserve gauge continuity. Figure 43 shows fractographic SEM cross-sections of
chromium strain gauges that were sputter deposited on their respective insulator bases.
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Figure 43: SEM fractographic cross-sectional images showing chromium strain gauges on
(a) AlN and (b) thermally sprayed Al2O3 base.
It is well recognised [6] that vapour deposited thin films can contain a number of internal
defects, which can lead to inconsistencies in conductivity measurements which in turn lead
to unstable strain gauge sensitivities. It has been shown [2] that these defects can be
annealed out on the application of a relatively mild heat treatment. In the present study
chromium strain gauge coatings were stress relieved by annealing in vacuum (< 1x10-4
mbar) for two hours at 200°C and then slowly cooled to room temperature overnight.
When comparing the resistance of the gauges before then after the annealing treatment, an
average 6.8 % decrease in gauge resistance was found. Others have found similar
decreases in resistance after annealing metallic thin film strain gauges [9, 27] and have
attributed this decrease to annealing out of point defects produced during the deposition
process.
The use of glass as a substrate material for vapour deposited strain gauges is often
employed since glass is cost effective, readily available and has a smooth surface. When
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testing vapour deposited strain gauges researchers limit the strain to a full-scale range of
around 250 micro strain or less because of the spring element’s susceptibility to failure by
cracking [27]. However, for strain gauges fabricated on mild steel larger strains can be
sustained. In the present study the sensitivity of vapour deposited strain gauges is
represented by the gauge factor value [1] which was calculated from a change in electrical
resistance in response to a longitudinal strain using the cantilever and extensometer
methods (see section 4.6.2). The fractional resistance change versus applied strain for
chromium strain gauges with corresponding AlN and Al2O3 insulator bases on mild steel
are shown in Figure 44 and Figure 45 respectively.
Figure 44: Reproducibility plot of a chromium strain gauge on sputtered AlN base showing
the relationship between fractional resistance change and applied strain.
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Figure 45: Reproducibility plot of chromium strain gauge on thermal sprayed Al2O3 base
showing the relationship between fractional resistance change and applied strain.
In contrast to the response in Figure 44, Figure 45 shows evidence of slipping between
specimen grips that occurred initially during the extensometer testing. This slipping can be
attributed to the initial non-linear response observed in Figure 45. Figure 46 shows each of
the load cycles from Figure 44 and Figure 45 with least squares fit, linear regression and
linearity values. The linearity was measured as the maximum deviation from the straight
line, expressed as a percentage of the full scale.
103
104
105
106
Figure 46: Load cycles from Figures 44 and 45 showing least squares fit, linear regression
and linearity.
The results show that all linear regression line values are above 0.95 and the linearity
errors, relative to full scale of the gauge, are between 3.3 and 12.9 %. The non-linearity
between trials can be attributed to linear error in the application circuit. The longitudinal
gauge factor values for chromium, calculated from the slope of each graph, are between 1.9
and 2.4. This is in agreement with predicted gauge sensitivity values for metal thin films
[1, 2]. The difference in the observed gauge factor values can be attributed to coating
thickness and/or the spring element, which have a considerable affect on gauge sensitivity
[25].
Figure 47 and Figure 48 show the effect of load cycling on the respective chromium gauges
on AlN and Al2O3. At the relatively large strains of 1100 and 1650 micro strain for AlN
and Al2O3 insulator bases respectively, typical hysteresis values were found to be lower
107
than 10 % of full-scale output. Additionally, after applying a maximum load for 30
minutes and measuring the percentage difference in output, the creep for both strain gauges
was found to be less than 3 %.
Figure 47: Hysteresis plot showing the response of a chromium gauge on AlN/Ti/mild steel
using the cantilever testing method.
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Figure 48: Hysteresis plot showing the response of a chromium gauge on Al2O3/mild steel
using the extensometer testing method.
The gauges were also subjected to a thermal cycle (+ 20ºC to + 150ºC) and the temperature
coefficient of resistance (TCR) was found using Equation 6 from section 3.2.4 as shown in
Figure 49. The TCR for both strain gauges show positive values indicating the metallic
nature of the deposited film. The slight decrease in the TCR values with increasing
temperature suggests that some metallic conductivity in the film had increased. In addition,
the TCR values are lower with respect to bulk (~ 0.005 ºC-1) and this is due to the large
instability that is often associated with thin metallic films [115].
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Figure 49: Chromium strain gauges showing TCR values over a range of temperatures.
6.4 Multi-layered CrN/TiAlN Gauges
It has been reported [116] that multi-layered thin films have a low TCR and a large
resistivity, which makes them interesting candidate materials suitable for the development
of thin film strain gauges. It has also been reported [11] that multi-layered thin films
exhibit less intrinsic residual stress than single-layered coatings, this again suggests that
multi-layering of thin film should be useful for developing vapour deposited strain gauges
on mild steel substrates.
In the present study, multi-layered strain gauges with alternating layers of CrN and TiAlN
were deposited by cathodic arc evaporation on an AlN insulator base which in turn was
cathodic arc evaporated on a mild steel substrate. The CrN/TiAlN coatings were deposited
reactively in nitrogen gas with chromium and Ti0.5Al0.5 cathodes running simultaneously at
opposite ends of the chamber while the substrate holder was rotated (see section 4.5.2).
110
The strain gauges had thicknesses of around 1 μm with a surface roughness of Ra 0.01 μm.
A cross-sectional TEM image of a multi-layered CrN/TiAlN coating is shown in Figure 50.
It is evident that the coating consists of two main layers, namely CrN and TiAlN. This
layered structure can be attributed to the rotation of the substrate holder through the emitted
arc evaporated plasma at a frequency equal to the number of layers in the coating. At the
interfaces smooth transitions are apparent and the individual layers of CrN and TiAlN are
approximately 10 nm thick, which implies that the two sets of cathodes operating at
opposite ends of the chamber were depositing at similar rates.
Figure 50: TEM cross-sectional image of CrN/TiAlN film showing the multi-layered
structure that was formed using cathodic arc evaporation.
The cantilever method (see section 4.6.2) was used to measure the response of CrN/TiAlN
strain gauges. Figure 51 shows the change in fractional resistance (ΔR/R) with micro
strain (με), showing the reproducibility response with tensile stress. A longitudinal gauge
factor of around 3.5 was determined from the gradients. The thickness of the gauge was
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measured at 0.7 µm. Figure 52 shows each of the load cycles from Figure 51 with least
squares fit, linear regression and linearity values.
Figure 51: Reproducibility plot of CrN/TiAlN multi-layered strain gauge showing the
relationship between fractional resistance change and applied strain.
112
113
Figure 52: Load cycles from Figure 51 showing least squares fit, linear regression and
linearity.
Hysteresis was detected and calculated as 8 % of full-scale output for the CrN/TiAlN
gauge, which is shown in Figure 53. On load cycling the sample twice, the gauge exhibited
good linear characteristics even at relatively large strains in the order of 1880 micro strain.
In addition, creep from the CrN/TiAlN strain gauge was found to be lower than 1.2 % after
30 minutes at full-scale output.
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Figure 53: Plot of a multi-layered CrN/TiAlN strain gauge showing hysteresis.
Figure 54 shows the TCR for the CrN/TiAlN strain gauge. In contrast to the chromium
gauges shown in Figure 49 the values here are negative indicating the non-metallic nature
of the deposited film. The slight increase in the TCR values with increasing temperature is
due to a tunnelling-type of conduction [56] associated with the ceramic nature of the film.
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Figure 54: CrN/TiAlN strain gauge showing the TCR over a range of temperatures.
6.5 Load Cell Washer
The fabrication of industrial load cells is challenging because of the need to tailor the
desired transducer/sensor performance with some of the main design considerations such
as, ease of manufacture, cost, sensor installation and effects of unconventional load
behaviour [117]. The more intrinsic design factors of the gauge itself include linearity,
creep, hysteresis and temperature effects. Vapour deposited strain gauges provide the
means for developing a low-cost sensing element, which can be easily fabricated onto an
elastic load cell material.
The performance of a metal fastener is critical to the integrity of joined components in
many industries that include automotive, construction, mining and machinery. The cost of
failure can range from inconvenience and cost through to life threatening. Consequently,
there is a need for tension sensing in fastener components. Today’s tension-sensing
fastener manufactures are limited to large sized fasteners (> 16 mm diameter) and carry a
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piece cost that limits them to high value critical applications in the heavy construction
industry. In particular, a tension-sensing bolt system is not suitable for studs, where a
considerable market demand exists. The current work offers a novel solution to this long-
standing problem by using strain gauge sensors deposited around the circumference of a
steel washer.
Figure 55 shows a prototype for a load cell washer transducer in which a deposited
chromium strain gauge was used as the sensing element. The transducer itself was
fabricated using sputtering and shadow masking methods as described in previous chapters
to deposit Cr/AlN/Ti thin films on polished mild steel. A flat cylindrical steel spacer, with
diameter 3 mm larger than the diameter of the washer hole, was placed over the washer and
compressive load tests of up to 1000 N were applied.
Figure 55: Diagram of a load cell washer showing a meandering strain gauge pattern (left)
and cross-sectional profile with dimensions (right).
The outer rim on the washer perimeter was designed to facilitate flexural movement around
its upper surface under load. For example, as a load-induced radial movement on the
washer surface occurs, negative change in the local meander strain gauge results in the
measured sensitivity of the support mechanism. Figure 56 shows the explicit test situation
used and a practical application for the washer.
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Figure 56: Diagram showing (a) explicit test situation of the washer under load and (b)
practical application for the washer.
Figure 56(a) shows that the rim, due to its finite torsional stiffness, undergoes considerable
deformation under load such that positive and negative change of the local flange radius (r)
occurs with the negative strain occurring at the loaded (upper) side of the washer. Of
particular interest is the fact that at some certain height (z) the radius deformation will stop
118
as the rim tilts around the inner edge of the washer and becomes almost identical with the
inner rim height (h).
Figure 57 shows a plot of voltage output versus load with a negative gradient since the
strain gauge is designed to work in compression. It is evident from the results that the
change in voltage increases linearly with the increase in applied force since a linearity of
0.8 % and a linear regression of 0.999 were found.
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Figure 57: Plot from a load cell washer showing voltage versus applied force.
Hysteresis was examined in Figure 57 (b) by observing the maximum deviation when load
cycling back down to the initial force and was found to be around 2 % of full-scale output.
In addition, the creep was found to be less than 1.4 % of maximum load after 30 minutes.
These values are relatively large in comparison with commercially available load cells,
which have been reported to be around 0.003 % and 0.05% for hysteresis and creep
respectively [118]. Nevertheless, based on previous work by Bethe [5, 117] and Ciureanu
et al. [2], this procedure can be improved by appropriate mechanical and heat treatments.
Therefore, much work needs to be done before this load cell can be optimised. However,
the results demonstrate a viable means of detecting strain on a washer. Moreover, the
fabrication processes used to develop this load cell has set a benchmark for future
investigations.
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Chapter 7
CONCLUSIONS
7.1 Introduction
The aim of the present investigation was to explore the possibility of using PVD
technology to fabricate a load sensing metal washer. This necessitated the deposition of a
thin insulating layer on the metal washer prior to the deposition of the load sensor. The
original intention was to exploit the technology of PLD to deposit the insulating film. In
the event, delays in the delivery and commissioning of the PLD system meant that a large
part of the study was committed to depositing insulating thin films using DC PVD systems.
With respect to the load sensing transducer it was decided, after some preliminary
experimental work, that a strain gauge would be the preferred option for investigation.
Given the scope of the study, the main conclusions are summarised as follows:
7.2 DC Magnetron Sputtered AlN
Notwithstanding the problems of arcing due to target poisoning, it was found that thin film
insulators of AlN could be deposited by manually controlling the gas flow. The techniques
involved holding the nitrogen flow rate constant at 12 sccm while varying the argon
pressure between 6 and 7 x 10-3 mbar in response to any incidence of arcing. Adjustment
of the nitrogen gas in the same pressure range (6-7 x 10-3 mbar) was not found to be
effective in arc suppression.
In agreement with Sproul et al. [16], it is concluded that target poisoning can be maintained
for low nitrogen gas flow rates which means that high deposition rates and optimal film
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properties are maintained. It was found that, the performance of vapour deposited thin
films in terms of the dielectric breakdown strength for film thicknesses of 4 µm and more,
resistances greater than 3 GΩ can be achieved at applied DC voltages of 450 V. This value
can be increased by around 50 % by vapour depositing a metallic interlayer prior to
depositing the AlN and annealing in air at 300ºC. It appears that this increase is due to
oxidation of the interlayer.
7.3 DC Magnetron Sputtered AlNx/AlxOy
It was found that the breakdown voltage of thin film insulators can be increased by
alternating the reactive gas flows of oxygen and nitrogen such that a modulated thin film of
AlNx/AlxOy is produced. It is postulated that the paths of the electrical discharge tend to
break at the layered interfaces thereby reducing leakage current. This effect was addressed
previously by Bethe and Schon [5] and Segda et al. [74]. In the present study it was found
that AlNx/AlxOy film structures with 12 modulated layers had more than twice the average
breakdown voltage of a 2 layer coating deposited under the same deposition conditions.
7.4 Filtered Cathodic Arc Evaporated AlN
Partially filtered cathodic arc evaporation was found to be initially quite efficient at
depositing AlN electrical insulators with rates of up to 4 µm h-1. However, after
approximately 1 to 1.5 hours of deposition, severe arcing occurred which could not be
alleviated by adjusting the inert or reactive gas flow rates. The effect was due to the
“disappearing anode”.
The deposited AlN coatings contained a number of macroparticles that increased the
surface roughness. It was found, in order to achieve continuity, that the gauge thickness
needed to be of the order of 1 µm or more in thickness. If strain gauge thicknesses less
than 0.5 µm are required then it was found that shielding techniques can be employed in the
122
final stage of the AlN coating process to effectively reduce surface macroparticles and
thereby produce smooth surface finishes (~ Ra 0.06 µm).
7.5 Strain Gauges on Plasma Sprayed Al2O3
The benefit of using plasma spraying in the development of strain gauges is that thick
insulator coatings can be obtained at high deposition rates (~ 1 mm h-1) thereby reducing
fabrication costs. Previous studies [19] of plasma sprayed strain gauges on plasma sprayed
insulators have not been successful in producing a gauge factor result [19]. In the present
study an insulating film was made by plasma spraying Al2O3, and found to be a suitable
insulator for strain gauges. This layer had excellent electrical resistance of around 3 GΩ at
2000 volts for a 100 µm thick film. It was demonstrated that chromium strain gauges
sputter deposited on plasma sprayed Al2O3 insulators result in gauge factors of around 2.
The successful fabrication of sputter deposited chromium strain gauges on plasma sprayed
Al2O3 was possible when (a) Al2O3 insulator coatings were polished to a 1 µm diamond
finish and (b) chromium gauge thicknesses were around 3 µm. The fabrication process
used is not limited to only chromium and Al2O3. Other material combinations are possible
depending on the compatibility requirements of the process.
7.6 Shadow Masking
A study of the surface morphology of chromium strain gauge patterns that were cathodic
arc evaporated through mechanical masks was carried out. An interesting result, not
previously reported, showed a better surface quality finish for the strain gauge pattern in
contrast to the electrical contact pads. These results demonstrated that strain gauge patterns
deposited through shadow masks produce smoother film with very few macroparticles
depending on the dimensions of the shadow mask. Sidewall shadowing encountered from
the meandering slots of the mask causes a reduction in deposition rate which in turn
produces a smoother surface finish. The latter was a function of aperture width, that is, the
123
finer the aperture the smoother the coatings. The principal for this improvement in surface
finish was a reduction in deposition rate. Hence, the effectiveness of the mask may be
optimised for particular deposition requirements.
7.7 Multi-layered CrN/TiAlN Gauges
In this study a new multi-layered thin film CrN/TiAlN strain gauge material was developed
using cathodic arc evaporation. It was demonstrated that CrN/TiAlN films have gauge
factors of around 3.5, which are similar to single layered ceramics and high in comparison
to metallic gauge factor values. Of particular interest is the fact that cathodic arc
evaporation, combined with shadow masking techniques, provides a novel strategy for
fabricating multi-layered strain gauges. Within this strategy it was demonstrated that
multiple layers of strain gauge material (~ 10 nm thick) can be deposited without
interrupting the deposition process. As a result, this fabrication process has the potential to
engineer and test a variety of different multi-layered strain gauge materials with the key
advantage being cost since a number of processing steps are eliminated.
7.8 Load Cell Washer
In this study a novel load sensing washer mechanism was demonstrated that functions as a
traditional load cell and offers a solution to the long-standing problem of size and cost for
tension-sensing fasteners in the heavy construction industry. The washer device consists of
a strain gauge meander pattern formed using a shadow mask during the evaporation of
chromium and is electrically insulated with sputter deposited AlN/Ti thin film. Its
sensitivity performance is based on (a) the chromium gauge sensitivity and (b) a support
mechanism in which an outer rim on the washer perimeter is designed to facilitate flexural
movement around its upper surface under load. The bending motion of the washer surface
is designed to work in compression and relaxation states.
124
Other strain gauge material combinations are possible for the washer device depending on
the compatibility requirements of the deposition process. Semiconductor silicon strain
gauge material for instance, should increase the sensitivity of the device, however,
depositing this would require AC or DC-pulsed power supplies. Nonetheless, most
important in the future development of this load cell washer is material selection, which is
driven largely by high gauge factor materials and manufacturing costs.
125
Chapter 8
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LIST OF PUBLICATIONS
1. R. Djugum, D. Doyle, “Deposition of Chromium Thin Film Strain Gauges
Through Collimating Shadow Masks”, 3rd Asia Pacific Forum, PSFDT March
2003, Melbourne, Australia, pages 157-161.
2. R. Djugum, K. I. Jolic, “Fabrication Process for Vacuum Deposited Strain
Gauges on Thermally Sprayed Al2O3”, Journal of Micromechanics and
Microengineering, 16, (2006) 457-462.
3. R. Djugum, E. C. Harvey, E. D. Doyle, “Fabrication Process for CrN/TiAlN
Multilayered Strain Gauges on Mild Steel”, Journal of Micromechanics and
Microengineering, 16, (2006) 1475-1479.
4. R. Djugum, D. Doyle, J. M. Long, J. Du Plessis, “Study of Thin Film Electrical
Insulators Sputter Deposited with Ti Interlayers”, paper submitted to the
Journal of Material Science & Engineering B, 2006.
