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Pulsed-Laser Deposition of Silicon Dioxide Thin-Films Using the Molecular Flourine Laser Brian Douglas Jackson A thesis submitted in conforrnity with the requirements for the degree of Master of Applied Science Graduate Department of Electrical and Computer Engineering University of Toronto O Copyright by Brian Douglas Jackson 1997
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Pulsed-Laser Deposition of Silicon Dioxide
Thin-Films Using the Molecular Flourine Laser
Brian Douglas Jackson
A thesis submitted in conforrnity with the requirements for the degree of Master of Applied Science
Graduate Department of Electrical and Computer Engineering University of Toronto
O Copyright by Brian Douglas Jackson 1997
395 Wellington Street 395, rue Wellington Ottawa ON K I A ON4 Ottawa ON K1A ON4 Canada Canada
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Thin-Films Using ;he Molecular Fluorine Laser
Brian Douglas Jackson Master of Applied Science Degree, 1997
The Department of Electrical and Computer Engineering The University of Toronto
Abstract
The short-wavelength extension of pulsed-laser deposition (PLD) to the 157-nm
F2 laser may enable low-temperature growth of silica films for electronics and photonics
applications.
This thesis examines the effects of laser fluence, background gas, and substrate
temperature on the properties of SiOz films grown for the first time using the F2-laser.
The deposited films were characterized by atomic force microscopy, x-ray photoelectron
spectroscopy and Fourier-transform infrared spectroscopy.
The strong absorption of 157-nm radiation in fused silica enabled the growth of
virtually particulate-free Si02 films by F2-PLD, in contrast to results with longer
wavelength lasers. Stoichiometric films were produced in ambient oxygen (4x10'' Ton),
which compensated for oxygen loss due to Sion dissociation by the 7.9-eV laser photons.
Akhough the process parameters were not fully optimized, F2-PLD produced
films with comparable properties to the best-previous PLD-grown SiO2 films, but inferior
to films deposited by VUV-assisted CVD.
1 would first like to thank my thesis supervisor, Professor Peter Herman, for his
ongoing advice during the coarse of this project. Additionally, 1 would like to recognize
the financial contributions of NSERC and the Ontario Laser and Lightwave Research
Centre.
Several individuals provided assistance through the use of diagnostics
instruments: Professor Stefan Zukotynski, of the Department of Electrical and Cornputer
Engineering, allowed me the use of his FTIR spectrometer and mechanical profilorneter;
Dr. Rana Sohdi, of the Surface Science Lab in the Centre for Biornaterials, provided
ongoing advice regarding the use of the x-ray photoelectron spectrometer in his lab; and
finally, Prof. Cynthia Goh, of the Department of Chemistry, provided the use of her
atomic force microscope.
1 would also like to thank my fellow graduate students in the lab: Keith Beckley,
Knsten Coupland, Jianhao Yang, David Moore, and Sola Ness. They helped in a variety
of ways. Additionally, a high school CO-op student under my supervision, Tony Yoo, was
a great help in designing and building Our deposition system.
Finally, 1 would like to thank my fiancée, Trish, for her ongoing patience and
encouragement in the face of adversity.
* *
Abstract ........................................................................................................................... ii
... ............................................................................................................ Acknowledgements iii
. . List of TabIes ................................................................................................................... vil
... List of Figures .................................................................................................................. viii
1 . Introduction ............................................................................................................. 1
1.1 Project Motivation ............................................................................................. 1
1.2 Thesis Outline .................................................................................................. 2
2 . Background Information .............................................................................................. 4
2.1 Pulsed-Laser Deposition .................................................................................... 4
2.1.1 Basic Mechanisms .................................................................................. 4
2.1.2 Advantages and Disadvantages of PLD ................................................. 6
2.1.3 Applications of PLD .............................................................................. 8
.................................................................................................. 2.2 Laser Ablation 10
.............................................................................................. 2.2.1 Overview 10
......................................................................... 2.2.2 Photothermal Ablation 10
....................................................................... 2.2.3 Photochernical Ablation 13
2.2.4 Particul @e Generation During Laser Ablation ..................................... 15
............................................... 2.2.5 Minimization of Particulate Generation 17
.............................................................................. 2.3 Silicon Dioxide Thin-Films 18
......................................................................................... 2.3.1 Applications 18
.............................................................................. 2.3.2 Growth Techniques 20
.................................................... 2.4 Pulsed-Laser Deposition of Silicon Dioxide 21
............................................... 2.4.1 Motivation for PLD of Silicon Dioxide 21
................................................... 2.4.2 Past Results of SiOz Growth by PLD 22
................. 2.4.3 Vacuum-Ultraviolet F2-Laser Ablation of Silicon Dioxide 25
........................................................................................... 3 . Experimental Procedures 30
.......................................................................................................... 3.1 Overview 30
3.2 Film Deposition ................................................................................................ 31
.............................................................. 3.2.1 The Molecular Fluorine Laser 31
................................................ 3.2.1.1 Basic Principles of Operation 31
................................................................. 3.2.1.2 Ruorine Operation 33
...................................................... 3.2.1.3 Argon Fluoride Operation 34
...................................................... 3.2.2 Initial Experimental Configuration 34
............................................... 3.2.3 Follow-up Experimental Configuration 37
3.2.3.1 F2-PLD Deposition Chamber Design ..................................... 37
......................................................... 3.2.3.2 Experimental Procedure 39
....................................................................................... 3.3 Film Characterization 41
............................................................................. 3.3.1 Surface Roughness 4 1
................................................... . 3.3.1 1 Atornic Force Microscopy 4 2
................................................ 3.3.2 Chernical Composition and Structure 4 2
...................................... 3.3.3.1 X-ray Photoelectron Spectroscopy 4 2
............................. 3.3.3.2 . Fourier-Transfomi Infrared Spectroscopy 47
..................................................................................... 3.3.3 Film Thickness 5 0
....................................................... 3.3.3.1 Mechanical Profilometry 5 0
............................. 3.3.3.2 Fourier-Transform Infrared Spectroscopy 5 0
3.3.3.3 X-ray Photoelectron Spectroscopy ...................................... 51
......................................................................................... 3.3.4 Film Density 51
4 . ExperimentalResults ................................................................................................ 52
.......................................................................................................... 4.1 Overview 52
........................................................................................... 4.2 Surface Roughness 53
4.4 Chernical Structure .......................................................................................... 62
................. 4.4.1 Wide-Range FïIR Transmission Spectra (450-5200 cm-') 62
4.4.2 Si-O-Si Asymmetric Stretching Mode Peak Spectra ........................... 65
................................................ 4.5 Film Thickness, Deposition Rate. and Density 69
5 . Discussion .................................................................................................................. 72
.......................................................................................................... 5.1 Overview 72
................................................................................................ 5.2 Deposition Rate 72
5.3 Particulate Reduction in F2-PLD versus ArF-PLD .......................................... 75
5.4 Chernical Composition and Structure .............................................................. 78
...................................................... 5.4.1 Correlation of XPS and FTIR Data 78
.................................. 5.4.2 Carbon Contamination of the Deposited Films 80
.... 5.4.3 Oxygen-Deficiency of SiO, Films Deposited in Vacuum or Argon 82
5.4.4 Surface Oxygen-Deficiency of Films Deposited in Dry Air or O2 ....... 84
........... 5.4.5 Effect of Deposition Parameters on IR ASM Peak Parameters 86
6 . Scientific and Commercial Significance .................................................................... 89
.......................................................................................................... 6.1 Overview 89
6.2 Cornparison With Previous Results ................................................................. 89
....................................................................................... 6.3 Scientific Importance 92
6.4 Cormnercialization of F2-PLD of SiOz ........................................................... 94
7 . Conclusions ............................................................................................................ 96
................................................................................................................ 8 . References 100
Appendix A - F2-PLD Charnber Optical System ...................................................... 108
................................................................................. 2- 1 Controllable Parameters in PLD 6
2-2 Previous Examples of SiO2 Thin-Film Growth by PLD .......................................... 23
................................................................. 3- 1 Fluorine Laser Operating Characteristics 33
3-2 Argon Fluoride Laser Operating Characteristics ...................................................... 34
3-3 Initial Experimental Parameters ............................................................................... 37
........................................................................ 3-4 Follow-up Experimental Parameters 41
3-5 XPS Peak Energies and Sensitivity Factors .............................................................. 43
.............................................................. 3-6 Significant Infrared Features in Si02 Films 48
4-1 Deposition Conditions for Analyzed Samples .......................................................... 52
......................................................... 4-2 EIemental Ratios From 0" and 60" XPS Scans 57
..................................................... 4-3 XPS O 1 s and Si2p Peak Widths and Separations 61
4-4 Relative % Absorption of FTIR Spectral Features in PLD Si02 Films .................... 64
................................................. 4-5 Si-O-Si ASM Peak Parameters of PLD Sioz Films 68
.................................................................... 4-6 PLD SiOs Film Thickness and Density 70
.................................. 5- 1 157-nm and 193-nm Radiation Interaction with Fused Silica 76
5-2 Summary of XPS and FTIR Data for F2- and ArF-PLD of Si02 .............................. 79 .............................. 5-3 Range of Possible Oxygen-Deficient Surface Layer Parameters 85
6-1 Cornparison of Si-O-Si ASM Peak Parameters With Previous Results ................... 89
A-1 Effect of Lens Birefringence on the Optical Imaging System ................................ 109
vii
Pulsed-Laser Deposition Geometry .......................................................................... 4
Ablation-Rate Dependence on Laser Fluence for a Typical Insulator ..................... 12
F2- and ArF-Laser-Ablated UV-Grade Fused Silica ............................................... 26
Optical Absorption Spectmm of UV-Grade Fused Silica ....................................... 26
F2- and ArF-Laser Ablation Rates in UV-Grade Fused Silica ................................ 27
Schematic of the Initial Deposition Chamber ......................................................... 35
Schematic of the Very-High Vacuum F2-PLD Deposition Chamber ...................... 38
Typical XPS Survey Spectrum of a Carbon-Contaminated SiO. Film ................... 44
Angle-Resolved XPS of a Carbon-Contarninated Silica Film ................................ 45
Sample Si-O-Si ASM Peak Spectrum in an SiOz Thin-Film .................................. 49
AFM Image of an ArF-PLD Sioz Film ................................................................... 54
AFM Image of an F2-PLD SiOa Film ..................................................................... 54
0" and 60" XPS Survey Scans of Si02 Film Sample A ............................................ 55
O" XPS Survey Scans of Si02 Film Samples F. G. and H ....................................... 56
........................ 01s Photoelectron Peaks for F2-PLD SiOî Film Sarnples K and L 60
....................... Si2p Photoelectron Peaks for F2-PLD SiOz Film Samples K and L 60
450-5200 cm-' IR Transmission Spectra of PLD Si02 Samples F and N ............... 63
............................ 700- 1300 cm-' Nomalized Transmission Spectra for Sample L 66
....................... 900-1300 cm4 Transmission Spectra for Sarnples H. K. L. and M 67
Deposition Rates for F2-PLD of SiOz ...................................................................... 73
viii
1.1 Project Motivation
Pulsed-laser deposition (PLD) is a highly flexible thin-film growth technique
which has been successfully applied to a wide range of materials [l]. The energetic
nature of the depositing species [2] enhances the growth process [3], potentially enabling
the deposition of high quality films on low-temperature substrates. Additionally,
sequential ablation of multiple target materials allows accurate control of the film
stoichiometry, enabling the growth of heterostructures and the deposition of films with
well-defined doping profiles. These two characteristics of PLD motivate the application
of this technique to the growth of SiO2 thin-films. In particular, low-temperature
(c 450°C) growth of high quality SiOÎ is required for applications in the semiconductor
electronics industry [4]. Likewise, the fabrication of planar optical waveguides and active
gain media for future optical integrated circuits will be enabled by the growth of high
quality silica films with independently controlled concentration profiles of two, or more,
dopant ions.
Past research efforts [5,6,7,8] have exarnined the application of a variety of
conventional lasers to the deposition of silica films, with only iimited success. In
particular, the best previous SiOl films grown by PLD [5], obtained by 193-nm ArF-laser
ablation of silicon or silicon monoxide targets in an 0 2 background, were of significantly
lower quality than films produced by VUV-lamp-assisted chernical vapour deposition [9].
produced films contaminated by 50- 100 nm particulates [5 ] .
The observed generation of particulates dunng ArF-laser ablation of fused silica
onginates with the weak optical absorption of the 6.3-eV photons by silicon dioxide,
which has a bandgap of - 9-eV. In contrast, the 7.9-eV photons of the F2-laser are
strongly absorbed by permanent defects within the bandgap of fused silica and transient
defects generated during laser-irradiation of the silica surface. Previous work in this lab
has shown [IO] that this strong radiation-matenal interaction causes F2-laser ablation of
fused silica to produce smooth etch patterns, with significantly less cracking and debris
generation than is observed for ablation at 193-nm.
The significant reduction in debris-generation observed in F2-laser ablation of
fused silica, relative to ArF-laser ablation, motivates the extension of pulsed-laser
deposition to the 157-nm wavelength to enable the deposition of particulate-free silica
films from a bulk silica target. Furtherrnore, the 7.9-eV photons of the F2-laser are
expected to interact strongly with the laser-generated vapour plume, creating the highly
excited species needed for low-temperature growth of high quaIity silica films.
1.2 Thesis Outline
This thesis describes the application of the F2-laser to PLD of Si02 films:
Chapter 2 presents a review of pulsed-laser deposition, laser ablation, the applications
of SiOz thin-films, existing Si02 thin-film growth techniques, and past results of PLD
a 1 1 u MSC;I ~ I U I ~ L I W I I UI ~ U ~ C U S I ~ L C ; ~ . 1111s 11iait;n5~ G A ~ I I U S U ~ U I I LILE; ~ L U J C ; L ; L I w m v a i i u I l
presented above, and provides a background to later discussions of the observed
results.
The experimental setup and procedures for F2-PLD of Si02 films are described in
Chapter 3, as are the diagnostic techniques used to characterize the deposited films.
Chapter 4 presents the measured properties of F2-PLD Si02 films for a range of
deposition pararneters. The particular film properties which have been measured are
surface roughness, chernical composition and structure, thickness, and density .
The observed correlation between the deposition pararneters and the resulting film
properties are discussed in Chapter 5. In particular, the observed reduction in
particulate contamination in F2-PLD is explained, as is the observed improvement in
film quality resulting from increases in the oxygen background pressure, reductions in
the laser fluence, and mcreases in the substrate temperature.
Chapter 6 compares the F2-PLD results with previously published silica film
properties, demonstrating that the results presented here are comparable to the best
previous results of SiOa film-growth by PLD. Additionally, the scientific and
commercial significance of this work is discussed in light of the observed results.
Finally, Chapter 7 summarizes the key points of this thesis and discusses the wide
range of potential future work leading from this project.
2.1 Pulsed-Laser Deposition
2.1.1 Basic Mechanisms
In its simplest fom, pulsed-laser deposition (PLD) c m be described in terms of
three stages: Iaser ablation, vapour-plume transport, and film growth. Figure 2-1, below,
presents the basic geometry of the PLD process.
In the laser ablation process, high-power laser pulses are focused ont0 the surface
of a bulk target, vaporizing a thin (- 10-1000 nm) layer of material in 10-50 ns. As the
-1 Substrate
/" Film
Figure 2-1 - Pulsed-Laser Deposition Geometry A focused laser pulse vaporizes the surface of a bulk target, creating a plume which explosively expands towards the deposition substrate. The rapid plume expansion compresses the background gas, creating a shock-wave which slows the plume expansion, reducing the energy of the depositing species.
highly energetic atoms, ions, electrons, and molecules. The ablation process, and its
dependence upon material and laser properties, are described in more detail in
Section 2.2.
Once formed, the initially-dense vapour plume expands into the surrounding
background gas according to the laws of hydrodynamics. This expansion produces a
highly forward-peaked velocity distribution, with typical on-axis kinetic energies of
10-400 eV per atom or ion [Il]. As the plume expands, it compresses the background
gas, creating a shock-wave which slows the plume expansion [12,13,14]. Depending
upon the target-to-substrate distance and the background gas pressure, the expanding
vapour reaches the substrate with typical energies of 0.1-100 eV per atom [2], depositing
sub-monolayer thicknesses of material per pulse. The kinetic energy of the condensing
vapour is transferred to the target surface, supplying thermal and kinetic energy to the
deposition process. This energy encourages the formation of smooth, high-quality films
by enhancing the mobility of atoms on the surface of the film and discouraging the
growth of islands on the film surface [3].
Each stage of the pulsed-Iaser deposition process is a highly complex process
which is critically dependent upon a number of controllable parameters, the rnost
significant of which are outlined in Table 2-1, on the following page. As a whole, the
pulsed-laser deposition process is highly tunable, allowing high quality deposition of a
wide range of materials.
'l'able 2-1 - Controllable Yarameters in YLU
Stage
Laser Ablation
Plume Expansion
Film Growth
Controllable Parameter
Laser (wavelength, pulse-length, pulse- energy, fluence (energyhea), and repetition rate)
Target (composition, density, and rotation) Dual-Bearn Ablation
Ambient Gas (species and pressure) Target-to-Substrate Distance Particulate Filtering Plasma Excitation
Substrate (material, temperature, and translationlrotation)
Energetic-Bearn Assist
2.1.2 Advantages and Disadvantages of PLD
Pulsed-laser deposition has a number of attractive features:
Tunability - The quality of P m - g r o n films is critically dependent upon a number of
controllable parameters, permitting the process to be tuned to suit a wide range of
applications.
Low Substrate-Temperature Growth - A significant portion of the energy required for
smooth, crystalline film growth may be supplied by the kinetic energy of the condensing
vapour, reducing the need for elevated substrate temperatures.
O - - - - - - - - 1-------- - -- - L Y
grow films of multi-component alloys from elemental targets, and to grow complex
heterostructures.
Reactive Deposition - Ablation in the presence of a reactive background gas or ion-beam
permits the deposition of complex oxides and nitrides which are difficult to grow using
conventional ultra-high v&uum techniques (i.e. SiO,N,, and Carbon Nitride).
Non-equilibrium Process - The energetic nature of the condensing vapour and the
pulsed growth process enables the growth of material phases which are not
therrnodynamically allowed [15,16], as is evidenced by recent examples of carbon nitride
growth by ion-beam-assisted PLD [17].
Although
remain:
these advantages are undisputed, three distinct disadvantages of PLD
Deposition Area - The highly forward-directed plume expansion causes the deposited
film area to be significantly smaller than in conventional film-growth techniques such as
molecular beam epitaxy and chemical vapour deposition. However, this drawback c m be
overcome by scanning the laser andor substrate relative to the target. For exarnple,
recent work [18j has demonstrated the deposition of yttria films with thickness
uniforrnities of +/- 3.4% over 8" substrates,.
Particulate Generation - As described in more detail in Section 2.2.4, the laser ablation
process often produces particulates, ranging in size from 10-nm to several prn in
diameter, which can contaminate the growing film. Although interesting applications
exist for the controlled deposition of particulates, their presence represents a major
contamination problern for most applications. Ongoing research shows that the amount
of debris deposited on the film surface can be significantly reduced by a combination of
mechanical filtering and process parameter tuning [l9,20,2 1,221.
Defect Generation - Highly-energetic condensing atoms and ions may penetrate the
surface of the deposited film creating defects in the substrate and film 131. In electronics
applications, in particular, these may limit the attainable film quality [23]. Adjusting the
pressure of the background gas controls the average velocity of the incident particles, thus
controlling defect generation.
2.1.3 Applications of PLD
Pulsed-laser deposition has been used to deposit an extraordinarily wide range of
materials, as evidenced b; a recent bibliography of PLD references [24].
Historically, the most significant application of PLD has been in the area of high
temperature superconducting thin films. The demonstration that PLD could be used to
deposit YBCO films with zero resistivity at - 85 K [25] sparked a significant amount of
high temperature superconductivity research over the past decade, and has stirnulated
research in PLD in general.
h addition to superconducting films, PLD of cornplex ceramic oxides has also
been shown to produce high quality ferroelectric 126,271, magnetoresistive [28,29], and
- - -
recent demonstration of colossal magnetoresistivity in films of Lao.67C~.33Mn0, [28].
The most interesting aspect of these developments is that it is now possible to use PLD to
combine two or more types of ceramic oxide in heterostructures, enabIing novel device
applications [3 1,32,33].
In the area of semieonducting films, PLD has been used to deposit a wide range of
materials, including SiGe- [34] and GaAs-based [35] alloys, II-VI compounds [36], and
Group IiI-Nitrides [37,38,39]. Again, the ability to tailor device properties by the growth
of heterostmctures is interesting, as is the possibility of introducing dopants during
growth through the use of controlled pressures of reactive background gas [23].
PLD has also been used to deposit a number of optical materials, including: Zn0
as a piezo-electric, piezo-optical, transparent conductor [40]; TiOl for use as
antireflection coatings on Silicon 1411; rare-earth-doped phosphate glasses for optical
waveguide applications [42]; and hafnia, yttria, and zirconia for optical multilayer
structures [43].
The final area of significant PLD research is in the field of hard coatings, such as
boron nitride [38], carbon nitride [17,44], and diamond-like carbon [45]. In these cases,
the energetic nature of the depositing vapour appears to be responsible for enabling the
growth of the desired hard phases, which are not thermodynamically stable under the
growth conditions of conventional techniques [16,17].
2.2.1 Overview
In the ablation process, a high-power laser is directed ont0 the surface of a target,
depositing energy by a combination of photothermal and photochernical processes. Given
a sufficient intensity of incident laser light, a combination of thermal vaponzation and
electronic bond-breaking &es place, leading to the removal of nm- to pm-thick layers of
material per laser pulse.
In the sections that follow, the basic mechanisms of photothermal ablation
(thermal vaporization) and photochernical ablation (electronic bond-breaking) are
explained in more detail. Additionally, the generation of particulates during the laser
ablation process is discussed, as are techniques for rninimizing the particulate
contamination of pulsed-laser-deposited films.
2.2.2 Photothermal Ablation
When high-power laser pulses are focused ont0 the surface of a material, optical
absorption leads to localized heating of the surface layer of the target. In a thermal mode1
of ablation, this absorption of energy leads to the melting and vaporization of the target
surface.
Two parameters control the depth of energy-deposition in the target and, thus, the
thickness of the ablated layer - the optical absorption coefficient and the thermal
diffusivity of the target material. Assuming linear . absorption, the optical pulse energy
-
to the inverse of the opticaI absorption coefficient:
Additionally, as the surface layer heats up, energy is carried into the bulk of the target by
thermal diffusion, with a characteristic depîh of:
where D is the thermal diffusivity, is the laser pulse length, and z, is the carrier lifetime
in the target material [46].
For transparent materials, such as fused silica, thermal diffusion is negligible
relative to the optical absorption depth, and the absorbed energy per unit volume can be
approximated by:
where Uabs(z) is the density of absorbed energy, Fo is the incident laser fluence (energy
per pulse per unit area), and R is the reflectivity of the target material.
Assuming that the abIated depth is defined by the depth at which the absorbed
energy is equal to the energy required to vaporize the target material, UVp, Equation 2-3
can be inverted to give the ablated depth per laser pulse, dvap. This equation then takes
the form of the Beer-Lambert Law shown in Figure 2-2 and Equation 2-4:
Assumed Piameters: ' .
, .
1 10
Fluence (~/crn')
Figure 2-2 - Ablation-Rate Dependence on Laser Fhence for a Typical Insulator The ablation rate for insulating materials is logarithrnically dependent upon the laser fluence. Thus, there is a fluence threshold, below which ablation does not occur. Above threshold, the ablation rate is inversely proportional to the absorption coefficient of the target material.
where, Fth, the threshold fluence for ablation, is defined as:
Thus, the ablation depth per pulse varies as the natural logarithm of the laser fluence,
with a slope qua1 to the inverse of the optical absorption coefficient.
mode1 for the laser ablation process:
@ Thermal diffusion increases both the threshold fluence and the etch-rate.
* The optical and thermal properties of the target material change as it is heated to the
melting and boiling temperatures.
At high pulse-rates, the surface of the target does not cool to room-temperature
between pulses, reducing the threshold fluence and increasing the etch-rate.
* Photochernical effects can significantly alter the laser-material interaction through
defect formation and photo-decomposition in the bulk target and the vapour plume
(see section 2.2.3).
2.2.3 PhotochemicaI Ablation
In a wide range of laser ablation processes, the thermal mode1 presented in the
previous section does not fully explain the observed effects. In particular, when laser
photon energies are comparable to the energies of specific bonds andlor defects in the
target material, direct photo-chernical process may take place. Examples of this type of
process include: laser-induced desorption, the formation of defects on the surface or in
the bulk, and photo-dissociation andor photo-ionization of the gas phase.
Laser-induced desorption describes the process by which individual atoms, ions,
or molecules are rernoved from a surface by laser fluences significantly below the
ablation threshold. Examples of laser-induced desorption include desorption from
surface defects and photo-dissociation at the surface of an oxide or nitride.
For example, the work of Dickinson et al. [47,48] has shown that photons with
energies below the bandgap of the bulk target may induce ion andior neutral atom or
molecule emission frorn pre-existing surface defects. In the case of ArF-laser irradiation
of Mg0 [48], it is believed that the incident radiation photo-ionizes low-energy defects on
the target surface, creating positive ions which are ejected from the surface by Coulomb
forces.
Alternatively, the work of Kurosawa et al., [49] has shown that above bandgap
irradiation of Si3N4 by 193-nm radiation creates silicon-rich surfaces in which silicon
crystallites form. In these cases, the high energy photons directly dissociate the Si-N
bonds, leading to the observed desorption of nitrogen.
Defect and Electron-Hole Pair Formation
When "high" energy photons irradiate the surface of a bulk target, coupling of
radiation into the material may cause bonds to be modified or broken, forrning defects or
electron-hole pairs. These defects may be semi-permanent or permanent, as in the case of
the duration of the laser pulse, as discussed below.
An example of the effect that defect and electron-hole pair formation can have on
the laser ablation process is discussed in recent work by Sugioka et al. [50]. In this
experiment, Raman scattering of a 266-nm laser bearn was used to create a bearn
consisting of - 45% 266-nm radiation plus a coincident spectrum of Stokes and anti-
Stokes shifted beams with wavelengths ranging from 594-nm to 133-nm. When this
multi-wavelength bearn was focused ont0 a fused silica target, the VUV components of
the beam were found to create both metastable oxygen-deficient defect sites and short-
lived electron-hole pairs in the silica target. The electron-hole pairs, in particular,
strongly absorbed the 266-nm fundamental bearn, ailowing high quaIity ablation to occur
at fluences significantly below the threshold for ablation with 266-nm radiation alone.
2.2.4 Particdate Generation During Laser Ablation
As mentioned in Section 2.1.2, film contamination by particulates generated
during the laser ablation process is one of the prirnary disadvantages of pulsed-laser
deposi tion. A number of possible mechanisms have been proposed for particulate
generation, including: explosive-boiling, subsurface super-heating, shock-induced
droplet emission, target erosion, and clustering.
Explosive Boiling - The phenomena of explosive boiling describes a rapid liquid-to-gas
phase transition during the absorption of the laser pulse. The sudden rise in pressure
carry liquid droplets with it [5 1,521.
Subsurface Super-Heating - Subsurface absorption by defects in the bulk ancilor
evaporative cooling of the surface can lead to a situation in which a buried layer
vaporizes before the surface. The explosive expansion of this trapped vapour could eject
liquid droplets a d o r solid particles from the target surface 1531.
Shock-Induced Droplet Emission - The explosive force with which the vapour plume
expands from the target surface generates shock-waves within the target. These shock-
waves may eject droplets from the liquid surface of the target 1211.
Target Erosion - It is well documented that repeated ablation at the sarne target site can
lead to the formation of surface ripples and cones [54]. Although the mechanism for cone
formation is not definitively known, one possibility is that inhomogeneous etching results
from the interference of the incident bearn with waves scattered from defects. Once
cones and ripples are formed, thermal andfor mechanical shock rnay eject droplets and/or
solid particles from the sharp edges of the structure.
Clustering - If the vapour plume expands into a background gas of sufficiently high
pressure, collisions may cause atoms and ions in the plume to coalesce into nm-scale
clusters, as is observed in the formation of C60, or buckeyballs, when graphite is ablated
in a high-pressure helium jet [55].
Particulate generation, and the resulting contamination of films by the
particulates, can be minirnized by a number of techniques which can be grouped into 4
main categories: target optimization, laser optimization, velocity filtering, and diffusive
deposition.
Target Optimization - Particulate generation is minirnized by using ablation targets of
high density and opticaf quality [19]. Alternatively, recent experiments have shown that
ablation from a Iiquid target produces significantly fewer particulates than ablation from a
solid target [56].
Laser Optirnization - Near-threshold ablation fluences have been shown to produce
significantly fewer particulates than high fluences [20]. AdditionalIy, the laser
wavelength can be tuned to reduce particulate contamination of the film - strong optical
absorption in the bulk minimizes the generation of particulates [16,19], while strong
plume absorption may result in vaporization of particulates within the plume [57,58].
VeIocity Filtering - The generated particulates travel with characteristic velocities which
are orders-of-magnitude lower than the plasma expansion velocity [59]. Thus, properly-
timed mechanical shutters may be used to intercept the particulates without greatly
affecting the plasma expansion [2 11.
Diffusive Deposition - Due to collisions with the background gas, the vapour plume can
diffuse around objects, while the heavier particulates tend to travel in straight lines.
absorbs the partides, while a significant portion of the vapour diffuses around the mask
to reach the substrate 1221. Alternatively, orienting the substrate plane perpendicular to
the target plane causes the particulates to drift past the substrate and deposit on the
chamber walls. The disadvantages of these techniques are reduced deposition rates and a
significant reduction in the kinetic energy of the depositing vapour [60].
Silicon Dioxide Thin-Films
2.3.1 Applications
Silicon dioxide films are used in a variety of applications in the electronics and
photonics industries, with a corresponding range of desired properties.
For electronics applications, the siIicon dioxide insulating Iayer is only 7-nm
thick in 0.25-pm silicon-based MOS structures [4]. These very thin layers require tight
control of the film thickness (I0.7-nm) and the absence of mounds, particdates, or
pinholes on the film surface (0.5 defects/cm2). Additionally, low deposition temperatures
(< 450°C) are required to minimize dopant migration between already-deposited layers
[61] and to permit deposition on temperature-sensitive aluminum interconnects [4].
Other requirements placed upon these films include ultra-high punty, low oxide-charge
densities and uniforrn deposition over large areas.
A second application of silica films which is rapidly growing in importance, is as
the basis for optical waveguides in optical integrated circuits. In this case, a planar
index-of-refraction-modifying ions. Co-doping of this structure with optically-active
ions, such as erbium oxide, allows the formation of an optically-active waveguide - the
basis for optical devices such as amplifiers and lasers for telecommunications
applications.
A third application of silicon dioxide films is in the formation of rnultilayer
dielectric stacks for use as wavelength-selective optical filters and mirrors. Because silica
has a relatively Iow index-of-refraction (n-1.46) compared with other transparent oxides,
such as titanium oxide (nL2.4) and yttria (Y203, n-1.8), it is useful as the low index-of-
refraction material in these structures.
Silicon dioxide films for optics and photonics applications must be of high purity
and have low surface roughness. In particular, although pure fused silica is transparent
over a wide spectral range (0.17-8.0 pm), impurities produce absorption bands which may
interfere with desired applications. For example, OH' impurities in otherwise-pure fused
silica produce an absorption band at - 1.4-prn which interferes with the 1.55-pm
telecommunications wavelength band [62]. Additionally, particulates, pinholes, and
surface roughness scatter light. For example, according to (631, the surface scattering
coefficient (s) scales with the ratio of surface roughness (O) to optical wavelength (h):
Silicon dioxide layers are grown by a wide variety of techniques:
Oxidation - Oxide layers in silicon electronics have historically been grown by the high-
temperature oxidation of the surface of a silicon wafer, or a previously-grown silicon
film. Due to the maturity of silicon processing technology, high purity and precise
thicknesses are routinely obtained by this technique. Unfortunately, the production of
high quality films at reasonable rates requires temperatures of 800-1200°C, leading to
significant problems with dopant diffusion [61]. Rapid thermal oxidation (RTO) is used
to minimize the thermal load on the silicon wafer, however, temperatures of - 950°C are
needed for up to 2 minutes, which remains too long for some applications [4]. UV larnps
have been used to assist RTO through the creation of highly reactive atomic oxygen.
However, the deposition temperatures needed (450-550°C) for this process remain higher
than would be desired, especially for deposition on aluminum interconnect layers [4J.
Chernical Vapour Deposition - CVD is a technique commonly used in the growth of
silica-based layers for electronics and optical applications. Precursor gases (Le. SiC12H2
and N20 for SiOz growth) are supplied to a heated surface, where they react to produce a
film of the desired material. The temperatures required for thermally-activated CVD of
high quality silica films reach 700-900 OC, depending upon the particular precursor gases
used [4,6 1,641. Plasma-enhanced CVD allows the deposition temperature to be reduced
to 250-400 OC, however, ion-bombardment has been found to produce defects in the
deposited films which limit the film quality [64]. Photon-assisted CVD has been used to
are lower than with other CVD techniques [64,9].
Sputtering - Electric-arc-discharge-, ion-beam-, and plasma-discharge-sputtering are
used to deposit many types of optical layers, including SiOz [65,66]. Typically, energetic
electrons andor ions bombard the surface of a bulk target, leading to the ernission of
atoms, ions, and electrons from the target surface. The sputtered species travel through
the surrounding background and deposit on a nearby substrate. As in the case of pulsed-
laser deposition, the sputtered atoms and ions are energetic, allowing low-temperature
deposition and the use o f a reactive background gas. However, unlike in the case of the .
photon beam in pulsed-laser deposition, the beams of sputtering electrons and ions
originate frorn a source interna1 to the deposition chamber, potentially leading to
contamination of the deposited film.
2.4 Pulsed-Laser Deposition of Silicon Dioxide
2.4.1 Motivation for Pulsed-Laser Deposition of Silicon Dioxide
A need remains for a low-temperature growth technique which can produce Sion
films of comparable quality to high-temperature-grown films, at commercially-viable
growth-rates. The potential for pulsed-laser deposition to produce high quality films at
low temperatures raises the prospect that an optimized PLD process may fil1 this void.
The relative ease with which PLD may produce silica films with several
independently-controlled doping profiles raises the prospect for the growth of high quality
control of the index-of-refraction to define a planar optical waveguide, while additional
erbium oxide doping will create a waveguide which is optically-active in the 1550-nm
telecommunications wavelength band. The additional possibility for low-temperature
growth may be of benefit for applications requiring growth on temperature-sensitive
substrates such as previously-fabricated electronic or photonic circuits.
The third motivation for studying PLD of Si02 films is that silicon dioxide, due to
its wide bandgap, forms a test case for the deposition of other optical materials. As will
be discussed in the following section, the high optical transparency of silicon dioxide at
most conventional laser wavelengths makes pulsed-laser deposition from a bulk fused
silica target difficult. Thus, information gained in studying the deposition of silica films
should be useful in future work studying the deposition of other wide-bandgap materials.
2.4.2 Past Results of SiOz Growth by PLD
Table 2-2 summarizes the published examples of silicon dioxide film growth by
pulsed-laser deposition. Lasers ranging from the IO-pm CO2 laser to the 193-nm Argon
Fluoride laser have been-used, producing Si02 films by both direct deposition from a
silicon dioxide target and reactive deposition from a silicon or silicon monoxide target.
For each study, Table 2-2 lists the laser used, the ablation target material, and the range of
particulate sizes found on the surfaces of the deposited films, where this information is
available.
Table 2-2 - Previous Examples of SiOz Thin-Film Growth b y PLD
---
SiOs 1 unknown
Reference '
Ban and Kramer, 1970 [67]
Bykovskii et al., 1978 [68]
Wolf, 1992 [6]
Slaoui et al., 1991 1691 Slaoui et al., 1992 [70] Fogarassy et al., 1992 [SI
Chen et al., 1993 [8]
Baeri et al., 1995 [7] .
Si02 1 unknown
Ablating Laser
Ruby (480-nm)
TEA CO2 (10-pm)
Nd:YAG (1 .O6-pm)
Argon Fluoride (1 93-nm)
2 0 Nd:YAG (532-nm) plus Oz-plasma discharge
Xenon Chloride (308-nm)
Si 1 negligible
Si02,
Si, S i0
To date, the best SiOz thin-films grown by PLD were produced by Fogarassy et al.
[5,69,70]. In this work, an Argon Fluoride laser was used to ablate bulk silicon and
silicon monoxide targets in a background of O2 (- 100-mTorr), producing stoichiometric
Si02 films 151. Optimization of the deposition parameters produced films with
0.1 - 1 .O pm
negligible
reasonably good electrical and optical properties, however, this required substrate
temperatures of - 450°C andor a post-deposition anneal at - 800°C [69,70]. These high
temperatures preclude thè application of this process to several potential electronics
applications [4].
The quality of films deposited at low temperatures is expected to be improved by
increasing the kinetic energy of the depositing species, thus reducing the need for
additional thermal excitation in the growth process. This increase in kinetic energy may
between the plume species and the background gas.
For the case of ablation of a silicon or silicon monoxide target in a background of
0 2 gas, reduced ambient pressures (below 100-mTorr) were found [SI to produce
substoichiometric SiO, films. Reduction of the oxygen pressure below 100-mTorr
requires the presence of highly reactive oxygen species such as atomic oxygen. For
exampIe, Chen et al. [8] used an RF-driven oxygen plasma to assist 532-nm PLD from a
silicon target, reducing the pressure required for stoichiometric growth to 1-mTorr (a
lOOx reduction relative to growth in 0 2 gas). However, this plasma-assisted technique
increased the systern corn;>lexity, and the films produced were of lower quality than those
produced by Fogarassy et al. using ArF-PLD without a plasma-assist [5,69,70].
The use of a fused silica target offers a significant potential advantage over the use
of a Si or S i0 target by enabling the growth of stoichiometric films in low background
pressures. For exarnple, Fogarassy et al. obtained stoichiometric SiOa films by ArF-laser
ablation of fused silica in a 10'~ Torr vacuum. However, the films grown to-date by PLD
from a fused silica target have been contarninated by significant quantities of 0.1- 10 Fm
particulates [5,6,7]. This -particulate contamination is the result of the high transparency
of bulk fused silica at the IR and UV wavelengths used - a situation which is known to
lead to particulate generation during the laser ablation process 181. These previous
studies also demonstrated a significant trend towards reduced particulate size and density
as the ablating laser wavelength was reduced from 308-nm to 193-nm. A further short-
vacuum-ultraviolet absorption bands in bulk fused silica.
2.4.3 Vacuum-Ultraviolet F2-Laser Ablation of Silicon Dioxide
The general trend of previous results towards reduced particulate contamination at
shorter wavelengths indicates that a vacuum-ultraviolet laser is an attractive candidate for
PLD of fused silica films from a bulk target. Of the lasers available which emit radiation
in the vacuum-ultraviolet (VUV), only the 157-nm molecular fluorine laser produces
sufficient pulse energies (10's of mJ) at sufficient repetition rates (1-100 Hz) for
application to PLD.
Past work by Herman et al. [10,71] has shown that F2-laser ablation produces high
quality etch patterns in ultraviolet-grade fused silica, while ArF-laser ablation generates
cracking and debris around the etch sites, as is demonstrated in Figure 2-3. This effect
was attributed to an increase in the strength of the laser-material interaction in fused silica
in moving from the 193-nrn wavelength of the ArF laser to the 157-nm wavelength of the
F2 laser.
Although the bandgap of SiOz is - 9-eV, defects and blurring of the bandedge in
fused silica result in significant linear absorption of the 7.9-eV F2-laser photons, as is
shown in Figure 2-4. In contrast, the 6.3-eV energy of the ArF-laser photons lies well
within the bandgap of fused silica, and the 193-nm radiation-material interaction occurs
through a weak two-photon process.
ArF-Laser Ablation F2-Laser Ablation
Figure 2-3 - ArF- and F2-Laser-Ablated UV-Grade Fused Silica Optical microscope images of ArF- and F2-laser ablated holes in fused silica. show a significant quantity of debris surrounding the ArF-laser-ablated hole (3.5-~/cm~, 62 shots). This debris is not present around the F2-laser-ablated hole (2-~/cm~, 50 shots). Both holes are -100x200-pm2. Adapted from [IO].
Absorption Coefficient Abs. Cocf. (157-nrn) = 14 cmA- 1 - % Abs. of Irnm of UV-Grade Silica
8 Abs. (157-nrn) = 75%
0.1 0.11 0.12 0.13 0.14 0.15 0.16 0.17
Wavelengtli (pni)
Figure 2-4 - Optical Absorption Spectrum of UV-Grade Fused Silica The optical absorption coefficient and the resulting optical absorption of 1-mm of UV-grade fused silica are shown. Due to defects and bandedge blurring, fused silica is absorbing at 157-nrn (ais7 - 14-cm-') [72]. In contrast, fused silica is highly transparent at the 193-nm (no absorption data availabie at 193-nm).
ine nign transparency or msea siiica ro ~YJ-nrn raaiarion causes mr-iswt;~
abIation of Si02 to be difficult to control, especially near the fluence threshold, where
incubation effects and a sudden onset of ablation (see Figure 2-5) have been found to
make the process unpredictable [71]. These effects result in poor ablated-surface
morphologies and, in some cases, the generation of particulates [IO]. In contrast, the
relatively strong Iinear absorption of 157-nm radiation in fused silica causes the F2-laser
ablation process to be characterized by a srnooth onset of ablation, smoother ablated
surfaces, and the absence of particulate debris [7 11.
Figure 2-5 - F2- and ArF-Laser Ablation Rates in UV-Grade Fused Silica Ablation rate as a function of laser fluence for ArF- and IFz-laser ablation of UV-grade fused silica [71]. Note the sharp ablation onset at the 4-5 ~ / c m ~ threshold for ArF-laser ablation. In contrast, F2-laser ablation has a smooth onset at a threshold fluence of l.o-~/crn~.
PLD is expected to enable the low-temperature growth of high quality silicon
dioxide films with precisely controlled dopant concentration profiles for applications in
the electronics, optics, and photonics industries (see Section 2.4.1). However, despite a
number of studies using a range of lasers [5,6,7,8], the low-temperature growth of high-
quality silica films by PLD has yet to be demonstrated (see Section 2.4.2).
Deposition of silicon dioxide from a fused silica target is expected to produce
stoichiometric films in relatively low ambient pressures, enhancing low-temperature
growth, as compared with reactive deposition in high background pressures of oxygen.
However, to-date, silica films grown by PLD from a fused silica target have been found to
be contaminated by 0.1-10 pm particulates generated during the ablation process [5,6,7].
This particulate generation can be directly linked to the weak optical absorption of
fused silica at the laser wavelengths used. In contrast, high-energy photons from the F2
Iaser have been shown to enable well-controlled laser ablation of fused silica, with
minimal debris generation [10,71]. Thus, Fz-laser ablation is expected to enable the
deposition of particulate-free silicon dioxide films from a fused silica target. Further, the
high photon energy is also expected to result in a strong absorption of the F2-laser
radiation by the laser-ablated products, producing a highly excited vapour plume. The
combination of this strong plume-heating and growth in low ambient pressures will
produce the energetic species needed for low-temperature growth of high-quality films.
& . S v W.--= vi * L .. YY V- ".---Y.* UIY..iUI 1" ...Y.. Y 4 -Il..----- --- -- ---- -------------
information which may be learned about the 157-nm radiation-material interaction by
examining the effects of laser parameters on the properties of the deposited films. This
basic knowledge may be applied in ongoing work studying F2-laser photosensitivity and
ablative micromachining of silica-based materials [7 11.
Further, this study is only the second published example of PLD using a vacuum-
ultraviolet laser. Thus, this short-wavelength extension of the PLD process will lay the
groundwork for future studies examining the deposition of other high bandgap materials
from appropriate bulk targets. For example, the one previous F2-PLD study, by Fujii et
al. (731, examined the growth of fluoropolyrner thin-films by F2-laser ablation of a
TeflonTM target.
3.1 Overview
The deposition experiments described Section 3.2 have proceeded in two stages:
In the initial experiments, films were deposited in a moderate-vacuum ablation
chamber, with the goal of demonstrating the deposition of debris-free silica films
using the fluorine laser. For this purpose, films were deposited using the fluorine and
the argon fluoride lasers, allowing the 157-nm results to be contrasted with results
obtained using the longer 193-nrn wavelength.
As a follow-up to this experiment, a new, very-high-vacuum deposition chamber was
built to allow the determination of the effects of a range of deposition parameters on
the properties of the deposited films.
Both stages of the experiment were performed using the lab-built molecular
fluorine laser described in Section 3.2.1. The deposition charnbers and the pararneter
ranges studied in the two stages of the experiments are described in Sections 3.2.2 and
3.2.3.
The deposited films where characterized by several techniques:
Atomic force microscopy (AFM), described in Section 3.3.1.1, provided two-
dimensional maps of the surface of the deposited films.
measure the chemical composition of the deposited films.
Fourier-transform infrared (FTIR) spectroscopy was used to analyze the chernical
structure of the films, as described in Section 3.3.2.2.
Film thickness and density were determined by combinations of FïIR, ion-milling
during XPS, and mechanical profilometry, as is outlined in Sections 3.3.3 and 3.3.4.
3.2 Film Deposition
3.2.1 The Molecular Fluorine Laser
3.2.1.1 Basic Principles of Operation
The molecular fluorine laser produces pulsed vacuum-ultraviolet (VUV) radiation
at 157-nm. This corresponds to photon energies of 7.9-eV, which is above the bandgap
of most common materials, enabling novel material processing applications.
The design of the laser used in these experiments [74] is based upon standard
excimer laser technology, in which a high-voltage pulsed-power supply is used to drive a
transverse electric discharge in the laser cavity. This discharge excites the laser gas
mixture (-0.2% fluorine in 9.5-atm. of helium), forming an excited fluorine molecular
state which radiatively de-excites to the lower laser state by emitting a 7.9-eV photon.
Unlike standard excimer laser transitions, the lower laser state is weakly bound, and the
Iaser is self-terminating, producing 15-ns pulses. To overcome the effects of lower-state
pressures (9.5-atm.) with a small electrode gap spacing to provide intense electron-impact
pumping of the laser gas. .
The resulting high laser gain permits the use of a single-pass optical resonator, in
which the front laser-optic is a magnesium-fluoride window providing only 4%
reflection. The back laser optic is a concave, aluminum-coated (second surface),
magnesium-fluoride mirror with a radius of curvature of 10-rn to partially offset the
divergence of the laser beam.
Unfortunately, the interactions of fluorine gas and ultraviolet radiation with the
intemal components of the laser vesse1 causes the gradual build-up of impurities in the
gas mixture. Due to the short wavelength of the F2-laser, even relatively srnaIl
concentrations of these impurities in the laser gas may strongly absorb the beam energy,
leading to the gradual decay of the laser energy over time. This effect is counteracted by
slowly cycling the gas mixture through a cryogenic gas purifier (API C-2000-HP) which
removes the impurities, extending the gas fil1 lifetimes by a factor of 3-4.
The operational characteristics of the fluorine laser used in these experiments are
summarized in Table 3- 1, -below:
Table 3-1 - Fluorine Laser Operating Characteristics [74]
1 Pulse Energy 1 40-50 ml
1 Wavelength 1 157-nm
1 Pulse Rate 1 1-2 Hz
Pulse Length
Spectral Width
1. BeamArea 1 -3x12mm2
Gas-Fil1 Lifetirne - 10,000 shots
15-11s
0.002-nm
The pulse rate of 1-2 Hz is limited by the lack of interna1 gas circulation in the
laser vesse1 and the capacity of the high-voltage power supply system - in particular, the
spark gap switch. Additionally, as discussed in the previous section, the gas-fil1 lifetime
is limited by the sensitivity of the fluorine laser to contaminants which strongly absorb
the 157-nm radiation. Without a gas purifier, fil1 Iifetimes of - 3000 shots are typical.
The pulse energy, pulse rate, and gas-fil1 lifetimes of this laser are al1 significantly
lower than for commercially-available UV-excimer lasers (- 500-mJ, 200-Hz, 108 shots),
making the application of this F2 laser to PLD particularly challenging.
By f i h g the laser vessel with different gas mixtures, different wavelength
radiation can be producedl In particular, a 6-atm. gas fil1 of 0.2% fluorine, 4% argon, and
96% helium was used for an argon fluoride laser, producing radiation at 193-nm. The
characteristics of the ArF laser used in these experiments are outlined in Table 3-2.
Table 3-2 - Argon Fluoride Laser Operating Characteristics
1 Pulse Energy 1 90-100 ml 1
1 Pulse Length 1 15-ns 1 Pulse Rate 1-2 Hz
Gas-Fil1 Lifetime I I I 3.2.2 Initial Experimental Configuration
Because 157-nm radiation is absorbed in air, al1 experiments were done with
evacuated beam-tubes connecting the laser vessel and the deposition charnber.
Additionally, in order to maximize the energy transmitted to the target, a single
magnesium-fluoride focusing lens was used for al1 experiments.
f--- Argon or Dry Air 157-nm or 193-nm
I Supply Line Laser Beam 4----- Evacuated Beamline
Star Tech w Energy Detector Rotating Beamsplitter . MgF2 Lens Turbopump on 2-Stage - *.
8
Silicon Substrate
Ablation Plume UV-Grade Fused
Silica Target Exhaust to
Rotary-Vane id X-Y Stage Pump
Figure 3-1 - Schematie of the Initial Deposition Chamber
The original deposition chamber, depicted in Figure 3-1, is a general-purpose
vacuum chamber evacuated by a 50-Ys turbomolecular pump (Turbovac 50) backed by a
3-CFM dual-stage rotary-vane mechanical pump (Trivac D4A). With this combination,
the base pressure obtained during these experiments was - 6x10-~ Torr. During
deposition, a micrometering valve was used to control the ambient pressure in the range
of 16'- 1 o - ~ Torr of argon or dry air.
An aperture (- 3.0x7.5-mm2) was placed in the bearn tube to select a unifonn
portion of the beam. This aperture was imaged by an 8.6-cm magnesium-fluoride
focusing lens, producing typical image sizes of -220~480-~m~ on the fused silica target.
to be controlled with 10-20% accuracy. The target was slowly scanned relative to the
laser beam (- 10-pdpulse) to allow fresh material to be exposed during ablation, a
technique which is generally known to reduce particulate generation resulting from target
texturing.
The substrates used for îhese experiments were - 1x1-cmZ pieces of a sjlicon
(100) wafer which was polished on both sides to make infrared transmission
measurements possible. The substrates were positioned 1.5-2.5 cm from the target,
along the line of the target normal, with four substrates mounted symmetrically on the
holder for each experiment. In some cases, a narrow (- 1-mm) strip of aluminum was
stretched over one of the substrates to create a trough in the deposited film which could
be profiled to determine the film thickness (see sub-section 3.3.3.1).
Early results showed hydrocarbon contamination to be a significant problem, and
the chamber was therefore baked before some deposition trials, producing base pressures
as low as 2.5x10-~ Torr. Unfortunately, bake-out was complicated by the long g l a s beam
tubes (- 1.5-m), large O-ring surfaces, and external welds in the vacuum chamber, and
contamination remained a problem.
The deposition conditions
Table 3-3, on the following page:
studied in these initial experiments are summarized in
1 Base Pressure 1 2 . 5 ~ 1 0 ' ~ - 8x10-~ Torr 1
Energy on Target
Fluence on ~arget
Laser Repetition-Rate
1 Substrate Temperature 1 25°C 1
5-10 mT
3-5 ~ / c r n ~
Ambient Pressure
Target-to-Substrate Distance
3.2.3 Follow-up Experimental Configuration
20-mJ
8- 10 ~ / c r n ~
1 oe5 - 4x 1 0' Torr argon or dry air
1.5-2.5 cm
Based upon the results of the initial experiments, a new very-high-vacuum
1-1.5 Hz
deposition chamber was built to eliminate film contamination and to allow a wider range
of deposition parameters to be controlled. Using this new deposition charnber, a set of
follow-up experirnents were perforrned to study the effects of laser fluence, ambient gas
pressure and species, and substrate temperature on the quality of F2-PLD silica films.
3.2.3.1 F2-PLD Deposition Chamber Design
A new
Figure 3-2, on
deposition chamber was designed and built for this project, as shown in
the following page. In general, the chamber was designed to produce as
low a vacuum as possible, within the time and price constraints of this project. Al1 large
flanges were sealed with copper gaskets, with a small number of VitonTM O-ring and
TeflonTM-taped pipe-thread seals confined to feedthroughs (dl-rnetal-sealed parts can be
Turbopump C
s mcon suDsrrare M . -"...-.-- - on Heated Holder Beamtube
Welded Bellows
Vacuum-Sealed
Substrate Shield
Figure 3-2 - Schematic of the Very-High Vacuum F2-PLD Deposition Chamber
extremely expensive). Al1 interna1 metal parts were stainless steel, with TeflonTM sleeves
used to prevent the binding of several metal-on-metal rotating contacts.
The magnesium-fluoride focusing lens aiso served as the entrance window to the
chamber, separating the deposition-chamber vacuum from the lower-grade beamtube
vacuum, while also minimizing the number of optical elements to maximize energy
throughput. This lens was mounted on the end of a welded bellows, allowing a
feedthroughs allowed the user to: shield the substrate during pre-deposition laser-
cleaning of the target, position one of two targets in the path of the laser, and rotate the
targets dunng ablation. The substrate was heated to a maximum tested temperature of
450°C by a tungsten-wire resistive heater, and the temperature was monitored by a type-K
thennocouple mechanically attached to the substrate holder. Viewports were included to
allow visual observation of the ablation plume during deposition.
3.2.3.2 Experimental Procedure
The new deposition charnber was connected by a sealed beamtube to the existing
ablation chamber, which was, in turn, connected to the laser. Thus, the entire bearnpath
was a sealed system which was evacuated during deposition by a dual-stage rotary-vane
pump (Trivac D433, attached to the ablation chamber) to maintain optical transparency.
After initial evacuation, argon flow was used to maintain a pressure of - 1-Torr in the
beamtubes to minimize oil backstreaming from the mechanical pump.
The deposition charnber was evacuated with a turbomolecular pump (Turbovac 50
- 50-Vs or Turbo-V250 - 250-Vs) backed by a liquid-nitrogen-trapped, 3-CFM dual-stage
rotary-vane pump (Trivac D4A), producing base pressures of - 2 . 5 ~ 1 ~ ~ or 1 . 4 ~ 1 ~ ~ Torr.
Prior to al1 but one deposition experiment, the charnber was baked to - 80-90°C for
16-24 hrs to accelerate the outgassing process and reduce pump-down tirnes, allowing
samples to be produced every other day. A micrometering valve was used to allow argon,
dry air, or oxygen to flow through the chamber, with the turbopump running, producing
after initial assembly, and then re-evacuated pior to most deposition experiments to
minirnize potential contamination from oil or water vapour in the supply lines..
A 5.0-cm focal length magnesium-fluoride lens focused the bearn ont0 the silicon
dioxide target. Unfortunately, the lens used for this experiment was birefringent, causing
the horizontal and vertical polarizations to have focal lengths differing by - 0.15-cm. This birefringence made it difficult to accurately define the laser fluence, as is described
in more detail in Appendix A. However, two image positions were defined and used in
this experiment - one corresponding to a highly peaked fluence profile, with a
maximum fiuence s I O - J / C ~ ~ , and the second corresponding to a flatter profile, with a
fluence of - 3-4 ~/cm*. .
The target was mounted on a stainless-steel holder which waç slowly rotated
(- 0.2-RPM) during the experiment to reduce target texturing resulting from repeated
ablation at the same site. Immediately prior to deposition, an annular region of the target
was laser-cleaned using a defocused laser fluence of - 1-1.5 Ucm2 (appropriate lens
position defined by visual observation of the reduction of the ablation plume size near
threshold). The distance from the target to the substrate was set at 2.5-cm for al1
experimen ts.
One 1 x 1 -cm2 substrate plus one 2x 1 -cm2 substrate were used for each trial. Pnor
to being loaded into the deposition chamber, the substrates were ultrasonically cleaned
with acetone, followed by methanol. Additionally, in most cases, after chamber bakeout,
---- ---------- -.--- ----- -- --- - . - - - - - - - - - - - - - - - - - - - -
contaminant~ from the substrate surface. During deposition, the temperature of the
silicon substrates was allowed to cool to 25°C or maintained at 400°C.
The range of pamheters studied in this stage of the experiment are summarized in
Table 3-4, below:
Table 3-4 - Follow-up Experimental Parameters
Energy on Target
Peak FIuence on Target
Laser Repetition-Rate
Base Pressure
Ambient Pressure
Target-to-Substrate Distance
Substrate Temperature
2 . 5 ~ 1 O-' or 1 . 4 ~ 1 o ' ~ Torr
vacuum to 1 x 1 v3 Ton argon, air, O2
3.3 Film Characterization
3.3.1 Surface Roughness
The surface roughness of the deposited films was studied using atomic force
microscopy .
Atomic Force Microscopy (AF'M) was used to measure the RMS surface
roughness of the deposited films, and to measure and count the particulates on the surface
of the deposited films. AFM is a high-resolution scanning-probe rnicroscopy technique
which produces a two-dimensional map of surface height down to the atomic-scale. The
scanning-probe is mounted on a cantilever which flexes due to the inter-atomic forces
between the tip of the probe and the sarnple surface. Monitoring the deflection of the
cantilever with a laser beam permits the height of the surface to be mapped as the probe-
tip is scanned over a 1-100 pm range in the x- and y-directions. The vertical height-
resolution of this technique ranges from several microns down to a fraction of a
nanometer. The primary advantages of AFM over scanning electron microscopy are: the
ability to study insulating surfaces and the production of quantitative height data.
3.3.2 Chernical Composition
The chemical composition of the deposited films has been analyzed using x-ray
photoelectron spectroscopy (XPS) and Fourier-transform infrared spectroscopy (FTIR):
3.3.2.1 X-ray Photoelectron Spectroscopy
X-ray photoelectron spectroscopy (XPS) is a common chemical analysis technique
which provides quantitati~e measurements of the elemental ratios in the surface-region
(to a 5-10 nm depth) of a deposited film, plus information about the local chemical state.
X-rays directed ont0 the sample cause core electrons to be ejected from atoms near the
film surface. As shown in Table 3-5, the ejected electrons have energies which are
energy analyzer to record a spectrum with peaks corresponding to the each element in the
film, as shown in Figure 3-3 for a carbon-contaminated silica film.
Table 3-5 - XPS Peak Energies and Sensitivity Factors [75]
--
carbon 1 s ( C (graphite) 1 284.5 1 0.34
Peak
Silicon 2p
The sensitivity factors given in Table 3-5, together with the areas under the
spectral peaks of the analyzed elements, are used to calculate the relative elemental
concentrations within - 5% accuracy. For example, the ratio of oxygen to silicon in a
sarnple can be determined using the equation:
where 1, is the measured peak area, Fx is the sensitivity factor, and C, is the atomic
concentration of element x.
S tmcture
si (si4 si'+
siZ+
Energy (eV)
99.3
100.3 [76]
101.1 [76]
Sensitivity Factor
0.40
-1000 -800 -600 -400 -200 O
Binding Energy (eV)
Figure 3-3 - Typical XPS Survey Spectrum of a Carbon-Contarninated SiO, Film The identification of the peak-energies in this survey spectmrn allows the elements present in the sample to be identified as O, C, and Si. Additional higher-resolution scans of the Ols, Cl s and Si2p peaks are used to determine the chemical composition of the film from the relative peak areas.
Also, as shown in Table 3-5, the energy levels of the core electrons shift by up to
5%, depending upon the binding-state of the atom. Therefore, the energy of a
photoelectron emitted from a particular atom is dependent upon the local chemical state.
For example, the Si2p peak-energy shifts from 99.3-eV to 103.5-eV in moving from bulk
silicon (sio) to SiOz (si4*), with three intermediate States having intermediate energies.
Thus, a substoichiometric oxide will likely have a mixture of oxidation States, allowing
the position and width of the Si2p peak to be used as measures of the stoichiometry of the
deposited films. Unfortunately, sample-charging during x-ray irradiation makes
measurement of the absolute Si2p peak-energy difficult. However, past work by Tao et
al. [77] has shown that the energy separation between the 01s and Si2p peaks is also a
sample charging, and thus, is easily measured.
The depth of sensitivity of XPS is - 10-nm, due to an exponential decay of the
ejected photoelectrons with a range of h - 2.7-nm [78]. Thus. tilting the sample relative
to the energy analyzer adjusts the depth-of-sensitivity, permitting the measured elemental
concentrations to be corrected for thin layers of surface contamination which can
accumulate during sarnple storage and transport to the analysis chamber.
For example, in the film structure shown in Figure 34, a carbon-contaminated
film of SiO, is covered by a carbon overlayer of thickness dc. In this case, the predicted
intensities of the oxygen, carbon, and silicon peaks as a function of the angle to the
Detected Surface
Electrons Normal
Incident
/ l Film: (1-(2"3;x)'z~:C + z SiO, 1 1
- ._._____.._-_._-____. ._-,
Figure 3-4 - Angle-Resolved XPS of a Carbon-Contaminated Silica Film Tilting the sample changes the angle, 8, between the detected photoelectron trajectories and the sample normal. Combining measurements at two, or more, angles, allows the effect of a thin overlayer of contamination to be accounted for in determining the elemental concentrations in the bulk of the film.
where K is a geometrical factor. Thus, measurements of the oxygen, carbon and silicon
peak intensities at 0' and 60' to the surface normal may be used to calculate the thickness
of the carbon overlayer and the concentration of carbon in the underlying film:
And, knowing z, the corrected concentrations of O, C, and Si in the film are:
{co = XZ, cc = l - ( l+x)z , Cs = )corn.tc,, (3-5)
Ion-milling of the sample surface enables depth-profiling of the elernental ratios in
the sample film. This also allows surface contamination to be removed prior to analysis.
However, accurate analysis of a sputtered depth-profile requires that the measured
- - - - - - - - - - - - - - - A Y
another. In particular, the sputtering of oxides, such as SiO2, often produces an oxygen-
deficient surface [79]. Because ion milling is a destructive technique, it was only used on
a limited number of sarnples. Additionally, due to the difficulty in correcting for the
effect of preferential sputtering, the measured depth-profiles were only used for the
detennination of film thickness, as will be discussed in sub-section 3.3.3.3.
3.3.2.2 Fourier-Transform Infrared Spectroscopy
Fourier-transforrn infrared spectroscopy (FTIR) probes the vibrational structure of
a sarnple material, perrnitting the analysis of the chemical structure of a film which has
been deposited on an infrared-transparent substrate, such as polished silicon. The
transmission of 2-20 pm radiation through the sample was referenced to the transmission
of a blank silicon substrate, providing the transmission spectrum of the deposited film.
Absorption peaks in the analyzed film can be assigned according to published
peak positions [6,9,70,80,8 1,821, allowing the chemical composition of the deposited
films to be determined. Table 3-6 summarizes the primary spectral features of relevance
to the transmission of silica films on silicon substrates. Of the features listed, only the
1080-cm-', 800-cm-', and 450-cm-' peaks appear in high quality fused silica. The SiOH,
H20, and OH features are indicators of contamination during deposition and/or film
porosity, which can trap airborne water vapour. Additionally, the 880-cm-' Si203 feature
is an indicator of oxygen-deficiency in the film.
The strongest silicon dioxide peak, at 1050-1090 cm-', shown in Figure 3-5,
corresponds to the asymmetric stretching mode (ASM) of the Si-O-Si structure. Shifts in
the peak-wavenumber and width of this peak are indicators of film quality. In particular,
shifts in the peak to lower wavenurnber have been attributed to combinations o c
Wavenumber (cm-')
3600
3350
3000
1630
1150-1200
1050- 1090
940
880
800
450
substoichiometry, contamination, porosity, and stress [6,70,80,8 11. Likewise, these
effects also cause the peak to broaden from the - 70-75 cm-' FWHM for a high-quality
oxide [9,83]. As an extreme example, Fogarassy et al. have shown that in thermally-
evaporated SiO, the main peak is centred at 980cm", with a width of - 175-cm-' [82].
Wavelength (ilml
2.8
3.0
3.3
6.1
8.3-8.7
9.3
10.6
11.4
12.5
22.2
Structure
SiOH
SiOH
OH
H20
Si02
Si02
SiOH
Si20î
Si02
Si02
Notes
porosity
porosity
porosity
porosity
disorder
Si-O-Si asymmetric stretching mode (ASM)
porosity
substoichiometry or SiH
Si-O bending mode
Si-O rocking mode
Ref.
1701
[ ~ O I
[O]
~701
WI
[6,70,80, 8 1,821
vol
[9,8 11
vol
~701
1300 1200 1100 Io00 900 800 700
Wavenumber (cm-')
Figure 3-5 - Sample Si-O-Si ASM Peak Spectrum in an SiOz Thin-Film The position and width of this peak are indicators of film quality, as is the strength of the high-wavenumber shoulder on the peak, defined here as the ratio of the absorption at the knee in the spectrum (B) to the peak absorption (A).
The high-wavenumber shoulder on the SiOz ASM peak (see Figure 3-5) has been
related to disorder in the Si-O-Si lattice structure [70,80], and is weak relative to the
ASM peak for a high-quality oxide layer. For example, VUV photochernical deposition
has been shown [9,83] to produce films in which the absorption in the shoulder region is
less than 20% of the main peak absorption. For the purpose of this thesis, the strength of
this feature has been defined as the absorption at the knee in the peak spectrum (B),
referenced to the ASM peak absorption (A), as seen in Figure 3-5. This definition is
reasonable, provided that the ASM peak absorption is not too great (< 30%). Othenvise,
deviations from the linear representation of the exponential absorption cause this
definition to overestimate the relative strength of this feature.
Mechanical profilometry, FTIR, and XPS have al1 been used to estimate the
thickness of the deposited films.
3.3.3.1 Mechanical Profilometry
Profilometry was used to measure the thickness of some films deposited in the
initial stage of the experiment. Masking a l-Zmm-wide strip of the substrate before
deposition produced a trough which was scanned with a profilometer, providing thickness
measurements down to - 10-nm. The disadvantage of this method was that the mask may
have introduced changes to the local environment during deposition, potentially altering
the results. For this reason, this technique was not employed in the case of the samples
discussed in this thesis.
As one alternative, it was found that in some cases, carefully scratching the film
surface produced a sharp trough which could be scanned with the profilometer.
Unfortunately, this technique was only effective for films which were highly
contaminated with carbon, and thus, of low quaiity.
3.3.3.2 Fourier-Transform Infrared Spectroscopy
FTIR was used to estimate film thickness by comparing the integrated area of the
Si-O-Si ASM peak in the deposited films with areas calculated from tabulated infrared
absorption data [72]. However, this technique measured the mass-density of the
deposited SiOî, requiring assumptions of the film density to infer the film thickness.
effective thickness of the Si02 in the film, assurning that the contamination did not
introduce strong absorption features in the region of the SiO2 ASM peak. Thus, porosity
andor contamination may have caused this technique to underestimate the film thickness.
3.3.3.3 X-ray Photoelectron Spectroscopy
Ion-mil1 depth-profiling during XPS was used to measure the thickness of the
deposited films by measuring the time required to mil1 through the film, to the underlying
substrate. The measured ion-milling time was proportional to the film thickness. After
ion-milling, a mechanical profiler was used to measure the depth of the ion-milled holes,
producing another estimate of film thickness.
3.3.4 Film Density .
Film density was calculated by comparing film thickness measurement
techniques. In particular, the thickness measured by profiling the XPS ion-milled holes
was compared to the thickness calculated from FTIR. Because the latter calculation was
linearly dependent upon the film density, the ratio of the rneasured and calculated
thicknesses was used to calcufate the film density, p ~ h , from the bulk density, pbulk:
PROFILER
where ~ F T I R was the thickness deterrnined from the Si02 ASM peak area, and ~ P R O F ~ L E R was
the measured depth of the XPS ion-milled holes.
4.1
presen t
Overview
The following sections the combined experimental results from both
deposi tion chambers. Table 4- 1 summarizes the deposition conditions for each analyzed
low-grade vacuum chamber, whiIe sarnples sample. Samples A-E were deposited in the
F-N were deposited in the very-high vacuum
sarnples were deposited with the F2-laser, at a
chamber. Unless otherwise specified, the
2.5-cm distance, and at room temperature.
Table 4-1 - Deposition Conditions for Analyzed Samples
Energy Fiuence #of Rate Vacuum (mJ) (~/cm*) Shots (Hz) (Torr)
(&O%) (&O%)
Ambient Tar. Subst. (Torr) CIean Bake
vacuum
lx10"air Y -325°C
vacuum Y - 320°C
1 :ILC 1 400"
2x10~ air 1 4x10'~ o2
energy during deposition. The "FIuence" is the estimated on-target fluence based on the
measured beam-energy, while the "Rate" is the total number of laser shots divided by the
deposition time. The "Vacuum" column lists the pre-deposition base pressure, while the
"Ambient" column lists the background gas pressure and species during deposition. The
"Tar. Clean" clean column identifies whether a pre-deposition laser-cleaning of the target
was employed (Yes or No). Finally, the "Subst. Bake" column identifies whether the
substrates were baked at high temperature before deposition, with the temperature listed
corresponding to the temperature measured during baking.
4.2 Surface Roughness
Figures 4-1 and 4-2 show atornic force microscope (AFM) images of ArF- and
F2-PLD Si02 films, respectively. The area of each image is 2.23~2.23-~m~. The
ArF-deposited film (sample F) is seen to be contarninated by - 50-100 nm diameter
particulates covering - 1% of the deposited film. In contrast, the F2-deposited film
(sample D) is virtually particulate-free. Discounting particulate contamination, the RMS
surface roughness of these two films are similar - 0.23-nm for the ArF- and 0.30-nm for
the F2-deposited films, as compared to 0.16-nm for an uncoated substrate (not shown).
Thus, both film samples are extremely smooth (- monolayer surface roughness). Both
films were deposited in 4 x 1 0 ~ Torr of argon, at laser fluences of - 10-~/cm~ and
- 5 - ~ / c m ~ for the ArF- and F2-PLD films, respectively. The thickness of each film is
- 5 n m , as determined frbm R I R absorption spectra (see section 4.4.3 and 4.4.4).
Figure 4-1 - AFM Image of an Ad?-PLD Si02 Film ArF-PLD film grown by 1 0 - ~ / c m ~ ablation in 4 x 1 0 ~ Torr of argon, at 25°C. Note the - 1% surface-coverage by - 50-100 nm particulates. The RMS roughness in the boxed area is 0.23-nm. The image area is 2.23~2.23-pm2.
Figure 4-2 - AFM Image of an F2-PLD Sioz Film F2-PLD film grown by 5-Ucm2 ablation in 4x10-~ Torr of argon, at 25°C. Note the absence of particulate contamination. The RMS roughness in the boxed area is 0.30-nm. The image area is 2.23x2.23-pm2.
The chernical composition of the deposited films was analyzed by x-ray
photoelectron spectroscopy. For example, Figure 4-3, below, shows survey scans taken at
0" and 60" to the sample normal, for sample A, an F2-PLD film deposited in an ambient of
4x lo4 Torr of argon, with a background pressure of 7x 10-~ Torr (unbaked). As indicated,
the rneasured peaks correspond to oxygen, carbon and silicon photoelectrons, with the
relative intensity of the carbon peak increasing in the 60" scan, indicating the presence of
a carbon overlayer on the silica film.
- 1000.00 -800.00 -600.00 -400.00 -200.00 0.00
Binding Energy (eV)
Figure 4-3 - 0" and 60" XPS Survey Scans of SiOa Film Sample A XPS survey scans of F2-PLD film grown in 4 x 1 0 ~ Torr of argon, with a base pressure of 7 x 1 ~ ~ Torr (no chamber bakeout). Note the increase in the intensity of the carbon peak relative to the oxygen and silicon peaks for the 60" scan, indicating that there is-an overlayer of carbon on the film surface.
very-high vacuum chamber. These scans illustrate the effects of the base vacuum, and
pre-deposition substrate baking and target cleaning on the measured carbon photoelectron
signal. In particular, sample G, deposited after a 80-90°C chamber bakeout, was found to
have a significantly lower carbon peak intensity than sample F. Additionally, sample H
was produced after baking the substrate and laser-cleaning the target, resulting in a further
reduction of the carbon peak intensity relative to sample 1".
- 1000.00 -800.00 -600.00 -400.00 -200.00 0.00 Binding Energy (eV)
Figure 4-4 - O" XPS Survey Scans of SiOz Film Sarnples F, G, and H XPS survey scans of three F2-PLD film samples grown by high fluence ablation (> 10-~/crn~ peak fluence) in 2x10~ Torr of argon (F,G) and vacuum (H). Note the significant reduction in the carbon peak intensity observed as a result of chamber bakeout (G and H grown after 80-90°C chamber bakeouts) and target cleaning and substrate baking (H grown after laser cleaning the target and baking the substrates at - 320°C).
- and Si2p peaks (after subtracting a local background) for 0" and 60" scans of each film.
These resuIts are summarized in Table 4-2. For each sample, the peak-area ratios were
used to calculate the carbon content of the film, using Equations 3-4 and 3-5 in Section
3.3.2.1.
Table 4-2 - Elemental Ratios From 0" and 60" XPS Scans
H (high F. vacuum)
1 (high F, 2x 1 o4 air) l
J (high F, 1 x 1 ~ ~ air) **
K (Iow F, vacuum) ** 1
1 M (L.F., air, 400°C)
NIA. NIA. - 50% 3.0 25
2.4 3
1.1 1.5
0.7 O
** - note that samples J, K, and L were analyzed 1.5-2.5 weeks after growth, while al1 other samples were analyzed within 1 week of growth
Several ooservanons can De maae regardmg the data in .l'able 4-2.
without bakeout, the films were highly contaminated with carbon (40-56 at. %)
(samples A, B, and C, grown in the original charnber, and sample F, grown in the new
chamber)
in the new deposition-chamber, combining a predeposition bakeout, target clean and
substrate bake reduced carbon contamination to - 1%, or less (samples 1 to N) - target
cleaning and substrate bakeout were not possible in the original ablation chamber
films deposited in vacuum (sarnples H and K) or argon (sarnples D and G) arnbients
were significantly oxygen-deficient (Si01.55 to Si01.7S)
in vacuum, high fluence ablation (sample H) appeared to produce a more oxygen-
deficient film than low fluence ablation (sample K) (SiOi.65 versus Si01.75).
However, this observation is inconclusive due to the delayed analysis of sarnple K
deposition in dry air (samples 1, J, L, and M) or O2 (sample N) produced films with
- 10-20 % higher oxygencontent than deposition in vacuum or argon (sarnples D, G,
and H) (SiO1 .9-SiOi,95 in air or O2 versus Si01 .55-Si01 in vacuum or argon)
no significant difference was observed between films deposited in 2 x 1 0 ~ Tom or
1 x IO-^ Torr of dry air, or 4x lû5 Torr of O2 (sarnples 1 versus J and L versus N)
no statistically-significant difference was observed between films deposited using
high or low fluence ablation in a dry air background (sample 1 versus L)
intensity ratios in O" scans than in 60" scans
In addition to the relative peak intensities, the higher resolution peak spectra were
used to compare the peak widths and the energy separation of the 01s and Si2p peaks for
the PLD SiOz samples. For exarnple, Figures 4-5 and 4-6, on the following page, compare
the Ols and Si2p photoelectron peak spectra for film samples K and L. The Si2p peak
for the film deposited in a dry air arnbient (K) was observed to be 0.3-eV narrower than
for the film deposited in vacuum (L). In addition, the separation of the Si2p and 01s
peaks was - 0.1-eV srnaller for the sample deposited in dry air. To account for sample
charging, the energy scale for each sample was calibrated by setting the energy of the Cls
peak (not shown) to 285.0-eV.
Table 4-3 summarizes the 01s and Si2p peaks widths and the 01s-Si2p peak
separations for the 14 PLD-grown Si02 samples and a bulk fused silica reference. From
this data, several observations can be made:
the Si2p peak width and the 01s-Si2p peak separations for carbon-contaminated films
deposited in argon (films A, B, C, F, and G) were significantly larger than in bulk
silica (0.4-0.6 eV wider Si2p peak, 0.4-0.9 eV larger separation)
reducing carbon contamination (films D, E, and 1 to N) reduced both the 01s and Si2p
peak widths and the peak energy-separation (- 0.1-eV narrower 0 1 s peak,
- 0.1-0.2 eV narrower Si2p peak, 0.4-0.8 eV reduced separation)
-538.00 -536.00 -534.00 -532.00 -530.00 -528.00
Binding Energy (eV)
Figure 4-5 - 0 1 s PhotoeEectron Peaks for F2-PLD Si02 Film Samples K and L Ols peak spectra of F2-PLD Sioz films grown by 3-4 ~ / c r n ~ ablation in vacuum (K) and 2 x 1 0 ~ Torr of air (L). Note that the peak width was - equal for deposition in air or vacuum (- 2.1-2.2 eV)
-108.0 - 106.0 - 104.0 - 102.0 -100.0 -98.0
Binding Energy (eV)
Figure 4-6 - Si2p Photoelectron Peaks for F2-PLD SiOz Film Samples K and L Si2p peak spectra of F2-PLD Si02 films grown by 3-4 ~ l c r n ~ ablation in vacuum (K) and 2x10~ Torr of air (L). Note the reduced peak width for deposition in air versus deposition in vacuum (2.1-eV versus 2.4-eV).
0 1 s Peak F W H M (eV) (4.1-eV)
Si2p Peak FWHM (eV)
(&O. &eV)
01s-Si2p Separation (eV)
(IO. 1-eV)
1 B (1.5-cm distance)
C (dry air ambient)
D (50°C bakeout)
F (no chamber bake)
1 G (no pre dep. clean)
H (high F, vacuum) 1 1 1 (high F, 2x 1 0 ~ air) 1 1 J (high F, air) ** 1
K (low F, vacuum) **
1 L (low F, 2x10~ air) ** 1 1 M (L.F., air, 400°C) 1 N (low F, 4x10-' 02)
1 Bulk Fused Silica ( ** - note that sarnples J, K, and L were andyzed 1.5-2.5 weeks after growth, as
compared with < 1 week for al1 other sarnples.
- - . .
01s peak-widths were generally comparable to that for the fused silica reference
(2.2-2.3 eV for A, B, C, and F; - 2.0-2.1 eV for al1 others)
films deposited in vacuum (samples H and K) had - 0.2-0.4 eV broader Si2p peaks
than films deposited in air (samples 1, J, L, and M) or O2 (sample N)
the Si2p peak widths and 01s-Si2p peak separations for films deposited with low
fluences in dry air (sample L and M) and 0 2 (sample N) were similar to those for the
bulk silica reference (peak-width - 2.0-2.1 eV, peak separation - 429.3-429.4 eV)
4.4 Chemical Structure
4.4.1 Wide-Range F'TIR Transmission Spectra (450-5200 cm-')
The chemical structure of the deposited films was analyzed using FTR
transmission spectroscopy. Figure 4-7, on the following page, shows the transmission
spectra for two deposited samples (F and N). In addition to the characteristic Si-O-Si
features at - 1050-cm" and - 800-cm", several other features are observed. The
oscillatory structures in the 1400-1900 cm-' and 3400-4000 cm-' ranges, and peaks at
- 2400-cm-' and 4700-cm" decrease in magnitude as the spectrometer is allowed to
purge, and have been excluded frorn this analysis as an artifact of air. However,
underlying film features are seen at - 3000-3600 cm-', 2900-cm-', and 1400-2200 cm".
Additionally, it is seen that the background is neither flat, nor constant, from scan to scan.
This is believed to be due to absorption by contarninants on the backside of the sample, or
Sample ,
6000 5000 4000 3000 2000 1000 O
Wavenumber (cm")
Figure 4-7 - 450-5200 cm" IR Transmission Spectra of PLD S i 9 Samples F and N IR transmission spectra of a contarninated F2-PLD film deposited in argon (F) and a non- contarninated film deposited in 4 x 1 0 ~ Torr of Ot (N). Note the strong 3400-cm' absorption in the contaminated film. The strong, sharp absorption peak at - 1050-cm" in both samples in the Si-O-Si ASM peak.
on the dicon reference. In several cases, scans have been repeated one or more tirnes
after careful cleaning to obtain more accurate results.
Table 4-4, on the following page, lists, for each sample, the peak absorption of
each feature, relative to a local linear background.
Table 4-4 - Relative % Absorption of FTIR Spectral Features in PLD SiOz Films
From the data in Table 4-4, several observations can be made:
Sample
C (dry air)
D (bakeout)
E ( A m
F (no bakeout)
G (no clean)
H (high F, vac.)
1 (high F, 2x 104 air)
J (high F, 1 x 10-~ air)
K (low F, vacuum)
the features at 1050-cm'' and 800-crn-~ correspond to the Si-O-Si asymmetric
stretching mode [6,9,70,80,8 11 and Si-O bending mode [70,81], respectively
3400 cm-' (%)
(*20%)
. 1.1
O. 1
0.4
0.4
O. 1
O. 1
0.3
0.3
- < 0.1
L (low F, 2x10~ air) < O. 1
M (L.F., air, 400°C) < 0.05
N (low F, 4x 10" 0 2 ) 0.05
2900 cmd (%)
(120%)
0.8
0.05
0.1
0.2
0.05
0.1
0.3
0.2
0.2
O. 1
0.05
O. 1
2000 cm-' (%)
(*20%)
0.7
noise
O. 1
noise
noise
0.2
0.3
0.2
0.4
0.2
0.3
0.3
1050 cm-' (W
(~10%)
2.5
4.4
5.4
1.8
7.2
5.6
5.1
13.4
8.4 ------ 4.4
6.7
4.1
880 cm-" (W
( ~ 2 0 % )
0.7
0.2
-
-0
-0
-0
0.2
0.6
-0
800 cm-' (%)
(&20%) 1
-
0.7
-
O. 3
1
0.5
0.5
1 .O
0.8
0.2
O. 1
0.3
0.5
0.7
0.5
features previously observed at 3350-cm-' [70], 3000-cm" [6], and - 1630-cm-' [70]
the peak at 880-cm" has been separately attrïbuted to Si203 [81] and SiH [9]
the 3400-cm" feature was strongest in samples C and F (44% and 22% of the ASM
peak strength, respectively), corresponding to films produced without a chamber
bakeout. In contrat, the strength of this feature was - 6-7% of the ASM peak height
in samples D and 1, and < 2% of the ASM peak height in al1 other samples.
the absorption of the 800-cm" peak correlates strongly with the absorption of the
l050-cm-' peak:
A , , = ~ x A , ~ ~ ~ , rn=O.llSf 0.005 (4-1
no other sets of peaks are significantly correlated, and, other than the 800-cm-' and
1050-cm-' peaks, no sét of peaks correlates with the various deposition parameters.
4.4.2 Si-O-Si Asymmetric Stretching Mode Peak Spectra
The measured IR transmission spectra have been analyzed to determine the peak
absorption strength, wavenumber, width, and high-wavenumber shoulder height of the
SiOt ASM absorption peak at 1027-1060 cm". This process has been complicated by the
previously-mentioned variation in the background of the measured spectra. For exarnple,
Figure 4-8, on the following page, shows three spectra collected for sample L, in which
each spectmm has been normalized to the transmission at 1300-cm-'.
1300 1200 1100 1 O00 900 800 700
Wavenumber (cm")
Figure 4-8 - 700-1300 cm-' Normalized Transmission Spectra for Sample L Cornparison of three ASM peak spectra for F2-PLD SiOa film sarnple L, showing the effect of contamination in altering the shape of the spectrum backgrounds, causing the measured peak heights to Vary by - 8% (4.40-4.76%) These curves have been normalized to the transmission at 1300-cm"'.
Figure 4-9, on the following page, compares the 900-1300 c r i ' transmission
spectra for sarnples H, K, L, and M, demonstrating the effects of high versus low ablation
fluences (H versus K), vacuum versus air ambient (K versus L), and 25°C versus 400°C
deposition temperatures (L versus M). A linear background has been subtracted from
each spectrum to simplify the determination of the ASM peak parameters.
1200 1100 1 O00 900
Wavenumber (cm-')
Figure 4-9 - 900-1300 cm1 Transmission Spectra for Samples H, K, L, and M FIIR ASM peaks for F2-PLD Sioz samples grown by: > I O - J I C ~ ~ ablation in vacuum (H), 3-4 ~/cm* ablation in vacuum (K), 3-4 ~/crn' ablation in 2x10~ Torr of dry air at 25°C (L), and 2.5-4 Ucm2 ablation in 2 x 1 0 ~ Torr of dry air at 400°C (M). Note the increase in the peak wavenumber, and the reduction in the peak width and the relative strength of the high-wavenumber shoulder for reduced fluence, deposition in air vs. vacuum, and increased substrate temperature.
Table 4-5, on the following page, surnmarizes the ASM peak features for each of
the analyzed samples. The shoulder strengths presented here are the ratio of the
absorption at the high wavenumber inflection point in the ASM peak spectrum to the
ASM peak area, as previously described in Figure 3-5 of Chapter 3.
-A ame 4-3 - 31-u-31 ASM reaK raramerers or rLu mu2 r( iims
Referring to Figure 4-9 and Table 4-5, several observations can be made:
Sample
C (dry air)
D (bakeout)
E (ArF)
F (no bakeout)
G (no clean)
H (high F, vac.)
1 (high F, 2x104 air)
J (h ighF, lx l~*~air )
K (low F, vacuum)
L (low F, 2x10~ air)
M (LF, air, 400°C)
N (low F, 4x10-~ 02)
films deposited without a charnber bakeout (samples C, and F) had very broad peaks
(FWHM = 136-161 cm-') and very strong high-wavenumber shoulders (42-58 %)
Peak Position (cm-')
(&-cm*')
1060
1030
1 027
1037
1030
1030
1042
1047
1035
1047
1053
1046
Peak Strength
(W (110 %)
2.6
4.4
5.4
1.8
4.0
7.2
5.1
13.4
8.4
4.4
6.7
4.1
Peak FWHM (cm")
(&-cm-')
140
112
131
161
136
122
95
90
102
91
84
88
Peak Area
(115 % )
3.4
6.1
8.1
3.0
6.1
10.5
6.0
15.0
10.5
4.9
6.7
4.3
S houlder Strength
(%) (&20%)
58
32
46 I
61
42
38
27
25
26
18
15
20
84-122 cm', shoulder strength = 15-38 %)
deposition in increasing pressures of dry air produced films with higher wavenumber
peak positions, and reduced peak widths and high-wavenumber shoulder heights,
relative to deposition in vacuum (high fluence: 1 and J vs. H, low fluence: K vs. L)
low fluence ablation produced films with 5-cm-' higher wavenumber peak positions,
11-27 cm-' narrower widths, and 9-12% lower shoulder heights, relative to high
fhence ablation (samples K vs. H, and sarnples L vs. 1)
deposition at 400°C (sarnple M) produced a 6-cm-' higher wavenumber peak position,
7-cm-' narrower width, and - 3% lower shoulder height, relative to room-temperature
deposition (sample L)
no significant difference was observed between the ASM peaks of films deposited in
2x 1 o4 Torr of dry air (sample L) and 4x 10'' Torr of a (samples N)
4.5 Film Thickness, Deposition Rate, and Density
The thickness of several deposited films was measured by two techniques
previously described in Section 3.3.4: calibration of the IR transmission peak-areas to
tabulated absorption data [72], and profiling holes which were ion-milled through the
films during XPS. Unfortunately, ion-milling was found to produce extremely
nonuniform etch patterns; making accurate depth determination difficult. The density
1 F (no balceout)
G (no dean)
H (H.F., vac.)
1 J (H.F., 1 x 1 o - ~ air)
L (L.F., 2x10~ air)
M (LF, air, 400°C)
1 N (L.F., 4x 10.' 09
was calculated by comparing the IR-calculated thickness and the profiled depth, while the
deposition rate was calculated using the IR data. Table 4-6 summarizes the IR-calculated
thickness, ion-milling times, and ion-rnilled depths for the analyzed films.
Although the large uncertainties in the data in Table 4-6 preclude any specific
observations from being made about the effects of the deposition parameters on the
# of Laser Shots
10,000
7300
5000
5000
4300
7500
7800
8200
8300
8700
7500
8000
deposition rate and film density, a few general observations can be made:
Mill Time
(4 (15s)
-
-
400
360
110
150
1 20
IR Peak Area
(*IO%)
3.4
6.1
8.1
3 .O
6.1
10.5
6.0
15.0
10.5
4.9
6.7
4.3
IR Thicknes
s(nm) (*IO%)
7
13
17
6
13
22
12
32
22
10
14
9
Mill Depth (nm)
( ~ 2 0 % )
-
Density (% of Bdk)
(&O%)
-
Dep. Rate
( h h o t ) (*IO%)
0.007
0.01 8
0.034
0.012
0.030
0.030
0.0 17
0.039
0.027
0.01 1
0.018
0.01 1
-
-
-
-
-
- 47
20
20
14
11
-
-
-
- -
- 70
110
50
100
80
G-K and N (- 50% less), corresponding to a low laser energy (- 15-25% less) used
for the deposition of samples F, L, and N
the deposition rates for the ArF-PLD sample (E) and the F2-PLD sample deposited in
1 x 1 0 ~ ~ Torr of dry air (J) were significantly higher than for al1 other samples
the average measured film density was - 80% of the bulk density of fused silica.
However, no clear correlation can be made between the film density and the
deposition parameters, with the available data set and the large errors
the time required to ion mil1 through the film samples is linearly related to the IR-
measured film thickness:
milling time {s} = 13.3 x IR thickness {nm}, R' = 0.91 (4-2)
5.1 Overview
The following sections analyze the results presented in the Chapter 4, with the
goal of explaining the major effects which have been observed. This discussion focuses
on the film deposition rate, the elimination of particulates in F2-PLD of SiO2, and the
dependence of the obsewed film properties on the deposition parameters.
5.2 Deposition Rate
The measured deposition rates of 0.010-0.040 A per laser shot can be explained
by a brief examination of the known ablation rates of fused silica and typical film
thickness profiles for PLD. From previous work, the etch rate for F2-laser ablation of
fused silica is known to be [71]:
where F is measured in Ucm2. Likewise, it is known that PLD film profiles are highly
forward-peaked, with typical distributions of the f o m [84]:
where, r is the radial position in the target plane and L is the target-to-substrate distance.
(Le. no redeposition occurs), and that al1 of the material which hits the substrate sticks
(Le. the sticking coefficient = l), then equations 5-1 and 5-3 can be used to calculate the
deposited film thickness per pulse, for a given set of deposition parameters, and an
assumed power factor of n. For example, Figure 5-1, shows the predicted deposition rates
for n = 8 and 14, for parameters corresponding to the high and low fluence conditions
used in film deposition trials F to J and K to L, respectively. From this graph, it is seen
that the measured deposition rate is less than the predicted rates. However, the
discrepancy is less than a factor of two, which is reasonably small, given the simplicity of
the model used.
O 5 10 15 20
On-Target Laser Energy (rd)
Figure 5-1 - Deposition Rates for F2-PLD of Si02 Predicted F2-PLD déposition rates versus on-target energy for a radial position of 5-mm on the target, a target-to-substrate distance of 2.5-cm, fluence distributions equivalent to the high and low fluence cases used in this work (F* > 10-~/cm~ or = 3-4 ~ / c r n ~ ) , and power factors of n = 8 and 14. Note that the experimental data lies within a factor of 2 of this simple model.
here can be grouped into three primary categories: a reduced volume of ablated material,
non-unitary transport efficiency of the ablated plume, and a non-unitary sticking
coefficient. A smaller-than-predicted ablation volume may be the result of non-
uniformities in the fluence distribution on the target. In particular, a significant portion of
the on-target energy may lie in a low-fluence tail which contributes little towards
ablation. Additionally, not al1 of the ablated material will reach the substrate - a portion
will be reflected back ont0 the target surface due to collisions within the plume [85], and
between the plume and the shock-wave generated in the background gas [12]. Finally,
not al1 of the material that reaches the substrate will be incorporated into the growing film
(Le. the sticking coefficient is less than 1).
The non-uniformities in the bearn image may also account, at least in part, for the
- 40-50% drop in the deposition rate observed for three samples deposited with laser
energies - l5-25% below the average. If a significant percentage of the on-target energy
lies in the low-fluence portion of the distribution, then the logarithrnic dependence of the
etch-rate on laser fluence will cause relatively small changes in the laser energy to resuIt
in large changes in the etch-rate.
The sharply peaked film thickness distribution may also be partially responsible
for the observed discrepancy between thickness measurement techniques, and between
the predicted and measured thicknesses. Referring to Equation 5-2, if one assumes an
intermediate value of n=l 1, it is found that the norrnalized film thickness drops from 1 .O
positions may result in significantly different estimates of the film thickness.
Finally, the sarne equations used to calculate the curves in Figure 5-1 c m also be
used to estimate the deposition efficiency (i.e. the percentage of the ablated volume which
is deposited on the substrate). For exarnple, for a 2.5-cm diameter substrate, the predicted
eff~ciency is - 55 and 80% for n = 8 to 14. hcreasing the substrate diameter tu 5.0-cm
increases the predicted efficiency to 90-99%. In practice, the previously discussed effects
of non-unitary plume transport efficient and a non-unitary sticking coefficient will reduce
the deposition efficiency. Based upon the relationship between the observed and
predicted deposition rates, these effects rnight be expected to lead to efficiencies of
- 30-50 % for a 2.5-cm diameter substrate.
5.3 Particulate Reduction in F2-PLD versus ArF-PLD
As was shown, in Section 4.2 and in previous work by Fogarassy et al. [SI,
ArF-laser ablation of fused silica produces - 1% surface-coverage of 50-100 nm
particulates which contaminate the surface of the deposited SiOz films. In contrast,
F2-PLD has been shown (Section 4.2) to produce films which are virtually particulate-
free.
The fundamental reason for the observed reduction of particulate generation is the
increased absorption of the 7.9-eV F2-laser photons relative to the 6.4-eV ArF-laser
photons. Table 5-1, on the following page, presents the optical absorption coefficient for
1"--', a"'- -a---- -- ". a---- -1-- .a--- - 7 ---- -
absorption coefficients and threshold fluences determined from ablation data [7 11.
Table 5-1 - 157-nm and 193-nm Radiation Interaction with Fused Silica
** sharp ablation onset observed at a threshold fluence 3-4x greater than the extrapolated threshold (- 1.6-J/cm2)
ArF (1 93-nm, 6.4-eV)
F2 (157-nm, 7.9-eV)
From Table 5-1, it is seen that the bulk optical absorption coefficient, which
controls the initial absorption of energy in the target, and thus, the heat-affected depth, is
orders-of-magnitude larger for 157-nm radiation than for 193-nm radiation. Additionally,
the effective absorption coefficient, which controls the deposition of energy in the target
during the latter stages of the laser pulse and the volume of ablated material, is 70%
larger for 157-nm radiation than for 193-nm radiation.
F2-laser ablation of fused silica is described by a logarithmic-dependence of the
etch-rate on laser fluence, with a well-defined threshold fluence of 1 . O - J / C ~ ~ . In contrast,
ArF-laser ablation has a very sharp ablation-onset at a threshold fluence which is 3-4
times larger than the extrapolated threshold fluence (> 5-J/cm2 versus - 1.6-~/cm~), as
1721 ( c d )
transparent
14
was previously illustrated in Figure 2-5. Further, incubation, cracking and surface
swelling were found to degrade the quality of ArF-laser-ablated sites [7 11.
G~~ 17 1 1 Fth C711 (J/cm2)
I .OX los
1 . 7 ~ 1 0'
> 5-J/cm2 **
1 .O
to particulate generation through several mechanisms which were previously described in
Section 2.2.4: explosive boiling, subsurface super-heating, shock-induced droplet
emission, and target texturing. Additionally, for near-threshold ArF-ablation of fused
silica, the steep ablation onset will cause relatively small spatial nonuniforrnities in the
laser beam to produce lzirge nonuniformities in the etch-rate and thermal expansions.
These nonuniforrnities in etch-rate will generate large lateral stress gradients in the target
which may directly eject droplets andor particles from the surface 185 1. Additionally, the
nonuniformities in etch-rate will accelerate the process of target texturing, leading to
additional particulate formation.
In contrat with the 193-nm case, the stronger absorption of 157-nm radiation in
the fused silica target will reduce the generation of particulates caused by explosive
boiling, subsurface super-heating, shock-induced droplet emission, and target texturing.
Additionally, the smooth onset of ablation at the threshold fluence will reduce particulate
generation caused by nonuniformities in the spatiai distribution of the beam.
In addition to reducing the initial generation of particulates, it is possible that the
high energy of the Fz-laser photons may lead to vaporization of particulates within the
ablation plume. In particular, 157-nm radiation is known to be strongly absorbed by
molecular oxygen (a - 0.2-cm-' at 1-Torr), which may be generated by the dissociation of
SiOz during ablation. 193-nm radiation, on the other hand, is not strongly absorbed by
O*. Absorption of the trailing edge of the laser pulse by molecular oxygen within the
than the 193-nm ablation plume. This heating could cause droplets and particulates
which are generated during ablation to be vaporized within the plume.
The elimination of particulates in F2-PLD SiOl films grown by ablation from a
fused silica target is a major advantage of F2-PD, potentially enabling the growth of
high-quality silica-based rnaterials.
5.4 Chernical Composition and Structure
5.4.1 Correlation of XPS and FTIR Data
Referring to Tables 4-2, 4-3, and 4-5, the information obtained from XPS and
FTJR is summarized in Table 5-2, on the following page. From this data, it is seen that
there is a strong correlation between the XPS and FTIR data. For example, comparing
samples A-C and F with samples D, E, and H-N, it is seen that the oxygentontent, the
01s-Si2p peak separation, the ASM peak width, and the ASM shoulder height al1
increase with high levels of carbon contamination. Additionally, comparing sarnples 1, J,
L, M, and N with samples D, E, O, H, and F, it is seen that the Si2p peak width, 01s-Si2p
peak separation, ASM peak wavenurnber, ASM peak width and ASM shoulder height al1
correlate with the XPS-measured oxygen-content of the deposited films. Increased
oxygen-content reduces the Si2p peak width, the 01s-Si2p peak separation, the ASM
peak-width, and the ASM shoulder height, and increases the ASM peak wavenumber.
Table 5-2 - Summary of XPS and FTIR Data for Fs- and ArF-PLD of Si02
XPS Data FTIR ASM Peak Data 1 I
O ls-Si2p ASM ASM Separation Position FWHM Shoulder
(eV) I (cm") (cm-') ml
C (6x10~ air)
D (bakeout, Ar)
F (no bakeout, Ar)
G (no ctean, Ar)
H (H.F., vacuum)
1 (H.F., 2x10" air)
J (H.F., 1 x 1 0 ~ ~ air)
1 L (L.F., 2 x 1 0 ~ air)
M (LF, air, 400°C) I N (L.F., 4x IO-' 02) I
The strong correIation observed within the combined XPS and FîR data set
increases the confidence in conclusions which are drawn with respect to the effect of
--= ------- =- -"""" -" "'- r"-r""-- -a "" -- --- -- - ------- -- - - - ' Y
delay between sarnple deposition and analysis varied from 2-15 days for XPS analysis
and frorn 2 days to 2 months for FïR analysis, for each sarnple, at least one of the two
anaIysis techniques was performed within 1 week of deposition. Thus, the observed
correlation within the data indicates that the variation of the deposition parameters had a
more significant impact on the rneasured film properties than the effect of sample ageing.
5.4.2 Carbon Contamination of the Deposited FiIms
As was discussed in Section 4.3, films grown without a predeposition bakeout of
the deposition chamber were found to be highly contaminated with carbon (- 50 at. %).
It is believed that the source of this carbon was oil contamination of the vacuum system.
The vacuum chamber used in the initial deposition experiments was constructed from flat
stainless-steel plates joined by external welds. These extemal welds leave nmow cracks
between the plates on the vacuum-side of the welds. These cracks, in turn, trapped oil
which backstreamed from the mechanical pump during several years of medium-to-low
vacuum operation without a liquid-nitrogen trap on the pump line.
The sensitivity of these experiments to contamination is explained by the low
deposition rates measured in these experiments, and a brief examination of gas kinetics.
It is known that the time required for a monolayer to form on an initially-bare
surface, in a background gas of pressure P and average molecular size 6, is [86j:
where S is the sticking coefficient. Thus, assuming a sticking coefficient of 1, air
(d, = 3.72 A) forrns a monolayer in:
Additionally, because the mass of a molecule is approximately proportional to d3, the
monolayer formation time can be expected to scale with d? Thus, the contaminating oil
molecules, which are larger than air molecules, are expected to deposit at a faster rate.
However, this is offset by the fact that, in practice, the sticking coefficient is c 1.
As a general rule-of-thumb, it has been suggested [2] that to produce pure films,
the following inequality should be maintained:
base pressure s < IO-' ~ o r r -o . depostion rate A
For example, in the experiments performed in the original ablation chamber, the base
pressure was - 5x10.~ Torr, resulting in a base-pressure/deposition-rate quotient of
1 .7x104 TOK-S/A for the deposition rates observed (- 0.03-as). Experimentally, these
conditions were observed to result in significant levels of contamination in the deposited
films, as would be expected. In cornparison, in the experiments performed in the very-
high vacuum PLD chamber, the base pressure was - 10-~ Torr, resulting in a base-
recommendation of Equation 5-5, explaining the observed purity of the deposited films..
5.4.3 Oxygen-Deficiency of SiO, Films Deposited in Vacuum or Argon
Films grown by high fluence F2-laser ablation of fused silica in vacuum or argon
were shown in Sections 4.3 and 4.4 to be significantly oxygen-deficient. This
substoichiometry is likely the result of photo-dissociation of silicon dioxide by the high
energy F2-photons. The atomic oxygen generated by this dissociation process would be
much more volatile than silicon monoxide or atomic silicon. For this reason the oxygen
rnay be consumed by interactions with, for example, background gas constituents or the
walls of the deposition chamber.
The bandgap of silicon dioxide is - 9-eV, which is significantly larger than the
7.9-eV F2-photon energy. However, this laser photon energy is comparable to the energy
levels of permanent defects which cause the band-edge of fused silica to blur to - 8eV.
In addition to the defects present in the bulk phase, it is also known that transient defects
are generated during the laser ablation process. These defects cause the effective
absorption coefficient determined from ablation data to be a factor of -104 larger than the
optical absorption coefficient at 157-nm [7 11. Thus, photo-dissociation in the bulk phase
may occur as the result of the high-energy F2-laser photons coupling into both the
permanent and the transient defect sites.
In addition to the photo-dissociation of the bulk, it also possible that strong
absorption of the incident laser energy by the vapour plume may cause further
reach 1000's of O C [59]. These high temperatures will result in the population of excited
states from which 7.9-eV photons may photo-dissociate the vapour molecules. For
example, if the excited states are uniformly distributed in energy, then at a temperature of
3000°C, a Boltzmann distribution will result in 1.4% of the electrons occupying states
with excitation energies larger than 1.1-eV. Obviously this mechanism is dependent upon
the existence of an appropriate range of excited states from which photon-excited
transitions to the vacuum level are allowed.
Section 4.3 showed that the oxygen-content of films deposited in vacuum is
increased by reducing the ablation fluences (SiOi.6s for > 10-J/cm2 versus Si01.75 for
3-4 J/cm2). This relative improvement in stoichiometry apparent in moving from high
fluence to low fluence can be explained by both bulk and plume photo-dissociation
mechanisms. In the case of bulk photo-dissociation, non-linearities in the ablation
process may increase the degree of dissociation with increases in the laser intensity.
Likewise, higher fluence~ will lead to longer laser-plume interaction times and higher
plume temperatures which will enhance photo-dissociation within the plume.
The observation that ArF-PLD in argon produced an oxygen-deficient film is
contrary to the results presented by Fogarassy et al. [5]. In that work, it was shown that
ArF-PLD in vacuum produced stoichiometric films. However, the base pressure in those
experiments was relatively high (- IO-' Torr), and oxygen may have been incorporated
into the film from the background gas.
for the deposition of superconducting YBCO films by KrF- and ArF-PLD [88]. In both
cases, it was found that an O2 ambient was required to produce stoichiometric films,
however, higher oxygen pressures were required in the case of ArF-PLD relative to
KrF-PLD. This result was attributed to increased photo-dissociation of Cu0 by the 6.4-
eV photons produced by the ArF-laser. Similariy, the work of Takigawa et al. [88] has
shown that above-bandgap irradiation of Si02 by low fluence 126-nm radiation causes
oxygen to desorb from the target surface, producing a silicon-rich layer on the target
surface. This work also showed that low fluence irradiation by 146-nm radiation did not
produce this effect. Thus, if photo-dissociation during 157-nm ablation does occur from
the bulk phase, it may be the result of a non-linear effect due to the higher laser intensities
used in the present experiments.
5Ae4 Surface Oxygen-Deficiency of Films Deposited in Dry Air or 4
As described briefly in Section 2.3, 0" XPS scans of films deposited in dry air or
oxygen produced calculated stoichiometries of - Si01.9, while 60" scans produced
stoichiometry calculations of - SiOi.s. This leads to the conclusion that the surfaces of
the deposited films must be oxygen-deficient, since the calibration of the energy analyzer
is not dependent upon the measurement angle. Although the mechanism explaining this
effect is not known, a quick calculation can be used to predict the range of possible film
structures which could lead to this observation.
of thickness d and composition SiO,, then one predicts that the measured oxygen and
silicon photoelectron intensities will be:
where h-2.7-nm [80], Fo=0.78, and Fsi=0.4, as discussed in Section 3.3.2.1. Thus, it is
possible to calculate a range of points (d,x) for which the oxygen:silicon ratios calculated
from the photoelectron intensity ratios are -1.9 and -1.8 for 0" and 60" scans,
respectively. Table 5-3, below, summarizes this range, complete with the calculated
oxygen:silicon concentration ratios. As can be seen, based on the data available, is not
possible to determine the exact structure of the oxygen-deficient layer, other than to Say
that it is likely < 1.5-nm thick. A more definitive conclusion than this would require the
use of a more accurate analysis technique.
Table 5-3 - Range of Possible Oxygen-Deficient Surface Layer Parameters
In Section 4.2.2, four deposition parameters were shown to strongly affect the
quality of F2-PLD Si02 films, as evidenced by changes in the position and shape of the
ASM peak in the IR transmission spectrum: vacuum quality, oxygen partial pressure
during deposition, laser fluence, and substrate temperature.
The vacuum quality affects film properties through the effects of carbon
contamination. As would be expected, reducing the contamination levels by improving
the vacuum quality leads to improved film quality, as is demonstrated by reductions in the
peak-width (1 6 1 -cm-' to 122-cm-') and the high-wavenumber shoulder strength
(61% to 38%).
Laser fluence has also been shown to strongly affect the film quality. For the
cases of both deposition in vacuum and deposition in 2 x 1 0 ~ Torr of dry air, the low
fluence case was found to produce significantly higher quality films than the low fluence
case. In particular, for deposition in vacuum, the low fluence condition was found to
produce a film with a higher peak-wavenumber (1035-cm1 versus 1030 cm-'), a smaller
peak-width (102-cm-' versus 122-cm-'), and a reduced shoulder-height (26% versus
38%), relative to the high fluence condition. For deposition in vacuum, the improvement
in film quality c m definitively be linked to the XPS-measured improvement in the
oxygen-content of the film deposited with a reduced laser fluence. However, in the case
of deposition in air, if there was an improvement in film stoichiometry, it was at a level
below the sensitivity of the XPS analysis used. Another cause of the improvement in film
plume species. It is possible that high energy species in the plume, generated by an
increase in laser-plume heating at high laser fluence, were responsible for creating defects
in the film, reducing the film quality.
Increasing the partial-pressure of O2 in the ambient gas from vacuum to
4x105 Torr has been sho& to improve the film quality, as indicated by increased ASM
peak-wavenumbers, reduced peak-widths, and reduced shoulder-heights. However, it
was found that deposition in 2 x 1 0 ~ Torr of dry air and 4x10~' Torr of O2 produced
statistically-equivalent results. This observation indicates that the primary effect of the
ambient was to improve the film stoichiornetry by supplying oxygen to the depositing
film. Although the ambient gas may, in some cases, affect the film quality by slowing the
expansion of the ablated plume, this effect was not observed in this pressure range.
A further increase'in the ambient pressure to 1x10-~ Torr of dry air was seen to
further improve film quality. In this case, it is not possible to definitively determine
which mechanism was responsible for the improvement. It is possible that the oxygen
content of the film may have been improved by the increase in oxygen partial pressure.
However, it is also possible that an increase in the number of collisions between the
plume and the background acted to retard highly energetic plume species which would
otherwise have generated defects in the deposited film.
Finally, for deposition in 2x10~ Torr of dry air, increasing the substrate
temperature to 400°C was found to significantly improve the film quality. Possible
depositing film and a therrnally-driven increase in the reaction rate for a key step in the
incorporation of oxygen from the background gas. For example, an increase in the
surface mobility of oxygen adatoms could be responsible for an improvement in film
stoichiometry by encouraging the diffusion of these atoms into oxygen vacancies in the
lattice. Additionally, an .enhancement of the surface and volume mobility of the film
atoms would help to heal defects created by the impact of energetic plume species with
the film surface.
6.1 Overview
This section discusses the significance of the work presented in this thesis, in the
context of previous PLD studies and alternate approaches to Si02 film deposition.
Additionally, the scientific importance and the potentid commercialization of F2-PLD of
Si02 are discussed.
6.2 Comparison With Previous ResnIts
The width and .wavenumber position of the asyrnmetric stretching mode
absorption peak at - 1050-1090 cm-' are strong indicators of the quality of a Si02 film
[9,69,80]. Table 6-1, below, compares the IR ASM peak parameters of the highest
quality F2-PLD SiOz films grown in this work witb previous results obtained by ArF-PLD
Table 6-1 - Comparison of Si-O-Si ASM Peak Parameters With Previous ResuIts
Shoulder Height (cm-')
18
15
35
35
20
Deposition
Technique
F2-PLD (25°C)
F2-PLD (400")
ArF-PLD [5,69,8 1 ]
RF-assisted PLD (25°C) [8]
VUV-CVD (300°C) [9,83]
Peak Position (cm-')
1047
1053
1080
f 072
1 065
Peak FWHM (cm-')
91
84
78-80
101
70-75
are for low fluence ablation in 2x10~ Torr of dry air, at temperatures of 25°C and 400°C.
Likewise, the ArF-PLD result corresponds to - 10-~/crn~ ablation of silicon or silicon
monoxide in 100-mTorr of O2 at 450°C, and the RF-assisted PLD result corresponds to
0 .4-~/cm~ 532-nm
Additionally, these
ablation of silicon in a 1-mTorr oxygen plasma at 40°C.
PLD results are compared with the films produced by W V photo-
assisted CVD [9,83], a low-temperature (300°C) technique which has been shown to
produce films comparable in quality to those grown by high-temperature oxidation.
The study of F2-PLD of Si02 films was based on a coarse optirnization of the
process parameters, leaving considerable room for future improvement in film quality.
Nevertheless, Table 6-1 shows that the best F2-PLD SiO2 films are comparable with the
best previous results for PLD of Si02 15,811. In particular, the ASM peak of the best
F2-PLD film is marginally wider (- 5-cm-') than in the best ArF-PLD films. However,
the high-wavenumber shoulder on the ASM peak of the F2-PLD film, which was linked
to structural disorder in the films [69,80], is significantly weaker (- 50%) than in the
ArF-PLD film, indicating a higher quality.
Relative to previous work studying PLI3 of silica films from a bulk fused silica
target, the F2-PLD process has been shown to produce particulate-free films, while results
of ArF-PLD (this work and [5]), XeCl-PLD [7], and Nd:YAG PLD [ 6 ] ) were
contaminated by 0.1 - 10 prn particulates. These particulates limited the quality which
- - - - - - - - - - - - J ---- O-- - * - - - - - - - a.-- - - - - - - - P r a - --O-----
improvement in moving to the Fz laser.
The need for further optimization of the F2-PLD process is a remaining issue,
given that the F2-PLD films are of lower quality than those produced by VUV-assisted
CVD [9,83]. This evidence is provided by the larger (10-14 cm") ASM peak-widths in
the F2-PLD films, relative to the WV-assisted CVD films. These CVD films were
shown to have peak-widths comparable to the generally accepted width of 70-cm-' for a
high quality thermal oxide [9,83].
Significant room remains for optimization of the deposition parameters. In
particular, increased partial-pressures of O2 may further improve the stoichiometry of the
F2-PLD films by the incorporation of background oxygen to fil1 oxygen vacancies in the .
film structure. These higher arnbient pressures may also slow down energetic plume
species which can create defects in the growing film. Beyond the effects of increased
oxygen pressures, a wide range of growth temperatures and laser fluences remain to be
tested. Additionally, the uniformity of the on-target fluence distribution may be
improved, and the target-substrate geometry may be adjusted during the optimization
process. Thus, it is expected that further work should result in the production of higher
quality films.
The work presented here is of scientific interest for three general reasons: the
potential for high quality Si02 film growth, the uniqueness of F2-laser PLD, and the
fundamental information which may be learned about the interaction of 157-nm radiation
with fused silica.
Despite the wide range of materials which have been successfully grown by PLD,
no group has successfully demonstrated PLD of extremely high quality silica films (see
section 2.4.2). The work of Fogarassy et al. 15,811 produced films of reasonably good
quality by ablation of a silicon or silicon monoxide target in 100-mTorr of O2 at 450°C.
However, a 20-s 800°C post-deposition rapid thermal annealing step, was required to
produce optimal-quality films 1811, limiting the potential applications of this technique to
growth on temperature-insensitive substrates. This study has shown that that, after only a
coarse optimization, F2-PLD produces films of similar quaiity to the best previous
ArF-PLD results [5,8 11, without a high-temperature anneding step.
Due to the relatively small number of F2-lasers currently in use in labs world-
wide, the work desci-ibed in this thesis is only the second-known demonstration of
F2-PLD. However, F2-PLD is of general interest to the scientific community due to the
novel applications that may be enabled by the strong interaction of the 7.9-eV F2-laser
photons with most materials. This strong laser-material interaction will allow PLD to be
extended to the ablation of targets which are transparent to radiation from conventional
lasers. For example, in the work of Fujii et al. [73], the deposition of fluoropolymer thin-
A L L L i i U TV- W h h U U i W U UJ A ;L-lC&JWi U W A U b L W i i U A A W r i U i L . I lUUiLiWLlUliJ, O L i W r l 6 A L A L V A U Y C L V A L U
between the short wavelength radiation and the laser-ablated plume will allow the
formation of highly excited plume species. An exarnple of one such potential application
of F2-PLD which has not yet been explored is in the area of hard coatings, such as
diarnond-Iike carbon. In that case, a strong interaction of the 157-nm radiation with the
laser-ablated plume should produce the highly-excited plume species which are needed
for the growth of the metastable phases in hard coatings 1173.
F2-laser processing of fused silica and related wide-bandgap materials may lead to
novel applications in the production of photonic and electronic devices. Current areas of
interest include: fibre and planar-waveguide photosensitivity, laser rnicromachining of
fused silica for the fabrication of optical waveguide devices, and laser micromachining of
crystal quartz for applications in rnicrowave devices [7 11.
In order to optimize these processes, the interaction of the 157-nm radiation with
fused silica needs to be thoroughly understood. Studying the fiIms deposited by F2-laser
ablation of fused silica is one technique for studying the laser-material interaction. In
particular, this work has shown that Silicon dioxide dissociates during F2-laser ablation,
leading to the questions of where this dissociation occurs, and whether it is due to a
purely photochemical process or a combination of thermal and photochernical effects.
Two major issues must be addressed before F2-PLD of Si02 may be used in a
commercial product or process: deposition rate and process optimization.
The deposition rates measured in these experiments (- 0.02-0.04 A/s over an area
- 2.5-cm in diameter) are much too low to be cornrnercially viable. Commercially
available excimer lasers operate at up to 300 Hz, which represents a factor of 200-300
improvement over the pulse-rates used in these experiments. Additionally, optimization
of the optical system and the laser design may increase the on-target-energy by a factor of
- 5, leading to a total deposition rate of - 150-200 n m h , averaged over a 20-cm
diameter wafer. This rate remains relatively small, when compared with rapid thermal
CVD, for which deposition rates larger than 6.0 Clm/hr may be achieved [4].
Due to the low deposition rates predicted for F2-PLD of Sion, relative to more
conventional techniques such as rapid thermal CVD, an optirnized PLD process must
produce unique films not producible by the lower-cost conventional techniques. In
particular, the areas in which F2-PLD of SiO2 may have an advantage over techniques
such as CVD are: deposition at low temperatures (c 450°C), growth of films with
complicated doping profiles, and the incorporation of SiO2 films into heterostnictures
with other PLD-grown rnaterials.
The required SiOl film thicknesses for electronics dielectnc layers are only 7-nm.
Thus, the relatively modest deposition rates predicted for F2-PLD may be sufficient for
..a**" - r y A * - U I I V . . . A*".." .VI, L..V A V y Y I . V Y I V 1 ,,111-- C i r i - - - ---- - --------a- --J --- --
very high. Although the quality of films presented in this thesis are comparable with the
best results produced by PLD [5,8 11, they are of lower structura1 quality than films
produced by WV-lamp-assisted CVD [9,83]. Thus, further process optimization,
focussing upon the electronic properties of the deposited films, is needed before F2-PLD
may be applied to the growth of Si02 for electronics applications. The work presented
here provides direction for this effort by presenting an overview of the effects of the
primary deposition parameters on the structural quality of the deposited films.
In the area of optical waveguide applications, the required films are much thicker
(several pm) than those produced here (10-30 nm). However, the required structural
quality of the deposited films is lower than that dernanded in electronics. Thus, it is
possible that the present F2-PLD process rnay already produce films of sufficient quality
for these applications, al'though fuaher analysis is needed to characterize the optical
properties of the deposited films. Additionally, the slow deposition rates predicted for
F2-PLD, relative to CVD techniques, may be offset by the relative simplicity of
introducing controlled concentrations of dopants in the PLD-grown films.
Pulsed-laser deposition of Si02 films has been extended, for the first tirne, to the
vacuum-ultraviolet 157-nm wavelength of the F2-laser. This short-wavelength extension
is expected to enable the low-temperature growth of high quality silica films with
controlled doping profiles-for applications in the electronics andior photonics industries.
The work presented here has exarnined the effects of the laser fluence,
background gas, and substrate temperature on the
very-high-vacuum deposition chamber h a been
properties of the deposited films. A
built and implemented to meet the
specific challenges of working with the F2-laser. AFM, XPS, and FTIR have been used to
determine the surface roughness, chernical composition, and structural quality,
respectively, of the deposited films. Particular attention has been paid to the IR
absorption peak corresponding to the asyrnmetric stretching mode (ASM) of the Si-O-Si
bond, which is known to be a strong indicator of the structural quality of the deposited
films 19,801.
AFM has shown that the relatively strong absorption of 157-nm radiation by fused
silica enables the growth of virtually particulate-free 15-nm-thick SiOz films from a bulk
fused silica target, in sharp contrast to results obtained with conventional, longer-
wavelength lasers (this work and
silica target is expected to enable
temperatures.
[5,6,7]). This particulate-free deposition from a fused
the growth of high quality silica films at low substrate-
silicon dioxide during ablation, causing films deposited in vacuum or argon to be
significantly oxygen-deficient (Si01.55-1.75). This oxygen-deficiency has been observed to
be reduced by the use of a low laser fluence (3-4 ~ / c r n ~ versus > 10-~/cm~).
The effect of dissociation during ablation was counteracted by deposition in a
relatively low partial-pressure of oxygen (4x10" Torr), which significantly increased the
oxygen-content of the deposited films (to SiOl.g.2.0). This resulted in a corresponding
improvement in the film structure (- 20-25% reduction in the ASM peak width).
Additionally, increasing the substrate temperature from 25°C to 400°C produced a further
8% reduction in the ASM peak width.
The coarse optimization process described in this thesis has produced F2-PLD
grown silica films (for ablation by 3-4 k m 2 in 2 x 1 0 ~ Torr of dry air, at a,temperature of
400°C) which are comparable to the best previous Si02 films grown by PLD [5,81].
Although these best F2-PLD grown SiO2 films are structurally inferior to the
highest quality oxides deposited by techniques such as VUV photo-assisted CVD [9,83],
significant room remains for optimization of the F2-PLD process. Thus, it is expected
that fine tuning of F2-PLD may yet produce very high quality, particulate-free silica films,
at low substrate-temperatures (c 450°C). Additionally, it is possible that films of less-
than-optimal structural quality will be acceptable for use in some applications, such as
optical structures. ~ o w e v e r , these applications may require optimization of other process
characteristics, such as the deposition rate.
deposition parameters, depositing thicker films, examining additional film properties,
growing doped-silica films, examining other materials systems, and studying the
fundamental aspects of the interaction of 157-nm radiation with fused silica.
Additional experiments are required to optimize the deposition process. The
effects of higher partial-pressures of oxygen (> 2 x 1 0 ~ TOIT of 02) should be explored and
a detailed examination of the effects of substrate temperature is needed. The on-target
fluence should also be optimized, and made more uniforrn with a new lens that eliminates
birefringence.
Optimization of the F2-PLD process will also require the characterization of
application-specific film properties, including: electrical properties (i.e. dielectric
constant and breakdown voltage), optical properties (i.e. absorption and refractive index),
and mechanical properties (i.e. density, stress, chemical resistance, and adhesion to the
substrate).
Thick films (- 1-10 pm) are required for optical applications. The growth of
these films should be facilitated by a new 20-50 Hz F2 laser which is currently being
tested in the lab. The deposition of thicker films will also improve the accuracy of the
FTIR analysis of the deposited films by reducing the sensitivity of the technique to thin
layers of surface contamination.
With the application of a high repetition-rate laser to the deposition process, thick
films for optical applications may be grown. At this stage, it will be necessary to study
oxide, in the deposited film. This will allow the formation of passive and active optical
waveguiding regions which may be examined for their application to integrated optical
circuits.
As was mentioned in Section 6.3.2, there is interest in the application of F2-PLD
to materials other than pure and doped silica. In particular in the growth of hard coatings
such as diarnond-like carbon, a strong interaction between the high energy F2-laser
photons and the ablated plume may create highly excited plume species, enabling the
growth of high quality films of this important class of material. Additionally, F2-PLD
may be applied to other wide bandgap materials, such as laser host crystals (YAG and
YLF) and fluorides (MgF2, CS2, and LiF) for potentially novel applications.
Finally, the work presented here provides
energy F2-photons with silica-based materials.
insight into the
However, it
interaction of the high
also raises additional
questions. Of particular interest is the determination of whether the dissociation of
silicon dioxide during ablation occurs in the bulk, the plume, or both. The answer to this
question is of importance, not only in optimizing the F 2 - P D process, but also in the
application of the F2-laser to photosensitivity and laser-ablative micromachining of silica-
based materiah.
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a I Y Y
In order to maxirnize the on-target laser energy, a single 5.0-cm focal length,
1.5"-O.D., magnesium-fluoride lens was used to focus the laser beam without the use of
an imaging aperture. Due to the relatively large size of the laser beam by the time it
reaches the PLD charnber, approximately 50% of the beam energy is lost before the light
reaches the lens. A coarse estimate for the lens transmission predicts that another 33% of
the beam energy is lost in passing through the lens, producing an estimated on-target
energy of 15-mJ for a typical 45-ml laser pulse.
The 5.0-cm focal length of the lens allows large optical demagnifications to be
obtained, yielding high laser fluences on target. However, the close proximity of the lens
to the target causes it to be coated during regular P D operation, making periodic
cleaning necessary (every 5- 10 experiments).
Unforhinately, this lens is birefiingent, with the optical axis in the plane of the
lens, causing the horizontally and vertically polarized light to see different indices of
refraction. In particular, the ordinary and extraordinary indices of refraction of MgF2 at
157-nm are 1.464 and 1.478, causing the lens to have focal lengths of 5.0-cm and 4.85-
cm for the two polarizations. This leads to a corresponding separation between the image
planes, as outlined in Table 5-1 on the following page, making precise definition of laser
fluence difficult.
The optical imaging problem is further complicated by the beam divergent, which
causes the energy density at the object plane to be dependent upon the distance of the
WJGU ~ W G llulll LUC I ~ C I . I IIG 0t;am uivergence ais0 increases me slze or m e rocus spot
relative to the diffraction Iimit for a collimated beam, and causes the focus plane to shift
away from the lens to - 5.05-cm for the ordinary polarization. The measured divergence
of the beam is - 4-rnrad in the vertical plane and - Zmrad in the horizontal plane.
Table A-1 - Effect of Lens Birefringence on the Optical Imaging System
I Relative Lens Position
2.0-mm
3.5-mm
Ordinary Beam
4.0-mm
The result of diffraction and birefringence is that the image and focal planes of the
two polarizations overlap. For example, Table 5-1, shows that in moving the lens doser
to the lens, the focus plane for the ordinary polarization is reached before the image plane
for the extraordinary polarization. Thus, choosing to position the lens at 5.5-mm
produces a relatively well-defined fluence of - 3.5-Jlcm2, however, the rnajority of the
energy in the extraordinary polarization is not used in ablation. Alternatively, positioning
the lens at 3.5-mm ensures that the rnajority of the beam energy contributes to ablation,
however, the fluence distribution on target is extrernely non-uniform.
Extraordinary Beam I F, - 1 -J/cm2
F, - 15-3/cm2
5.5-mm
focused, F. > 25-~ /cm~
imaged, F, = 2.84/cm2
focused, F. > 2 5 - ~ / c m ~ Fo - 1-J/cm2
imaged, F. = 2.8-J/cm2 F. - 0.6-Ucm2
APPLIED A IMAGE, lnc - = 1653 East Main Street - -. , Rochester, NY 14609 USA -- -- - - Phone: 7161482-0300 -- -- - - Fax: 716/288-5989
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