Design of a Scanning Tunnelling MicroscopeIntegrated in a Transmission Electron

Microscope Sample Holder

Daniel Morrison

A report submitted to the Departments of Physics andMechanical and Vehicular Engineering

in partial fulfilment of the requirements for thedegree of Master of Science and Engineering.

Chalmers University of Technology and Gothenburg UniversityGothenburg, Sweden September, 1997

i

I . Abstract:

Quantized conductance has been investigated in semiconductors and metals during

the last few years. The theoretical and experimental treatments have been extensively

documented in the literature1,2,3,4. However, very little research has been carried out on

the physical geometry of the sample during the experiment. The actual shape of

nanowires during drawing and breakage of metallic samples is still a topic of debate. The

study of the physical phenomena occurring during quantized conductance experiments is

essential to the development of a coherent and accurate theory to explain and shed light on

the theoretical and experimental treatments carried out so far.

A novel transmission electron microscope (TEM) sample holder has been

designed, constructed and tested, for the purpose of aiding and facilitating the observation

of the physical conditions of metallic samples during conductance experiments. The

unique TEM sample holder incorporates a functioning scanning tunnelling microscope

(STM) into a sample holder body built for the purpose, and this should allow one to carry

out conductance measurements simultaneously as one carries out TEM observation of the

sample.

Etching procedures for scanning tunnelling microscope tips in gold were

developed and tested. 6M hydrochloric acid was used as etchant and an etching voltage

of 1.7 V dc was used. Etching time was determined by observation of the etching

process and removal of the gold sample from the etchant as soon as the immersed portion

was seen to fall off.

ii

I I . Acknowledgements:

I would first of all like to thank my supervisor Dr. Håkan Olin for his support and

consultation. I would also like to thank Dr. Eva Olsson for all her support and

information concerning the transmission electron microscope. I would also like to thank

the members of Dr. Olin’s research group, Erik Wahlström, Inger Ekvall and Karin

Andersson.

I would like to thank Juris Prikulis and Edgar Hürfeld, who assisted in the design

of the TEM holder and electronics.

I would like to thank Eduard Guttenberg and the staff of the physics workshop for

their help and good work.

I would lastly like to thank the department of physics at Gothenburg University

and Chalmers University of Technology (Swe. - Göteborgs Universitet and Chalmers

Tekniska Högskola) for offering the opportunity to perform my thesis work there.

iii

III. Table of Contents:

Section Page Number

I Abstract................................................................................... i

II Acknowledgements..... . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . ii

III Table of Contents.... . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . iii

IV List of Tables ...... . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . v

V List of Figures...... . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . vi

1. Background...... . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1

1.1 Brief History................................................................... 1

1.2 Quantized Conductance and Nanowires.... . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2

2. Experimental Methods..... . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3

2.1 Transmission Electron Microscope..... . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3

2.2 Scanning Tunnelling Microscope..... . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5

2.3 Sample Configuration..... . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 9

2.3.1 Scanning Tunnelling Microscope Technique .... . . . . . . . . . . . . . . . . . 9

2.3.2 Mechanically Controlled Break-Junction Technique... . . . . . . . . . 10

2.4 Sample Preparation..... . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 12

2.4.1 Materials' Choice.................................................... 12

2.4.2 Tip Preparation..... . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 13

2.5 TEM-STM Sample Holder.... . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 16

2.5.1 Introduction..... . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 16

2.5.2 Design..... . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 17

2.5.3 Assembly............................................................. 20

3. Results and Discussion..... . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 23

3.1 Initial STM Tip Shape..... . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 23

iv

3.2 Stepping Motor Control... . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 24

3.3 Sample Mounting..... . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 25

3.4 Sample Movement Testing..... . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 27

3.5 Vacuum Testing..... . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 31

3.6 Quantized Conductance...................................................... 32

3.7 Troubleshooting and Suggestions.......................................... 33

4. Conclusions............................................................................ 35

5. References...... . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 36

v

IV . List of Tables:

Table Page Number

1 Wires in the sample holder.... . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 20

2 Specifications of stepping motor and gearbox.... . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 22

vi

V . List of Figures:

Figure Page Number

1 Representation of STM operation..... . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6

2 Ability of tips to accommodate large height differences .... . . . . . . . . . . . . . . . . . . . . . . . 7

3 Schematic drawing of STM nanowire forming...................................... 9

4 Schematic drawing of MCB sample..... . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 10

5 Schematic drawing of MCB technique..... . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 10

6 Geometric problems for electron transparency.... . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 11

7 Schematic drawing of etching process.............................................. 13

8 Illustration of TEM-STM sample holder.... . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 17

9 Schematic drawing of tube piezoelectric ceramic.... . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 20

10 Schematic drawing of long and fragile tip.......................................... 23

11 Schematic drawing of short and rounded tip....................................... 23

12 Schematic drawing of appropriate tip.... . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 23

13 Optical micrograph of tips at a distance............................................. 28

14 Optical micrograph of tips somewhat closer .... . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 28

15 Optical micrograph of tips approaching..... . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 29

16 Optical micrograph of tips approaching further.................................... 29

17 Optical micrograph of tips nearly touching......................................... 30

18 Optical micrograph of tips in contact................................................ 30

1

1 . Background:

1 . 1 Brief History:

The imaging and manipulation of matter on the atomic scale was given a great

boost with the announcement of the scanning tunnelling microscope by Binnig et al. in

19825. The science of matter on the nanometer and smaller scale since then has proven to

be fascinating. Individual atoms have been imaged and they have even been manipulated

and made to write out, for example, IBM or The Gettysburg Address6. Nanotubes of

carbon7 and nanowires4 and their unique properties have made nanoscience more and

more interesting.

The interest in such phenomena are of more than purely scientific interest. In

today’s society computers are becoming more and more commonplace and it has become

of great interest for electronics manufacturers to increase the packing density of

transistors in their products. The future of miniturization is dependent on a thorough

understanding of quantum phenomena. As electronics within computers become smaller

and smaller these quantum effects become more and more important to consider.

2

1 . 2 Quantized Conductance & Nanowires:

Under normal conditions electrical conductance is a materials property. However

if the conductor diameter is reduced to less than the electron mean free path, scattering

will mainly take place at the boundaries. This is called ballistic transport and results in

material independent conductance, dependent only on the geometry and electron density

of the sample. Quantization is possible if the electrons are confined by the boundaries on

the length scale on the order of the Fermi wavelength. Experiments on semiconductors

have been carried out in recent years3.

In metals the mean free path and Fermi wavelength are about two orders of

magnitude less than in semiconductors, requiring the confining boundaries to be on the

order of nanometers. The invention of the scanning tunnelling microscope (STM),

allowing the manipulation of matter on the atomic scale, made it possible to meet these

requirements. Small nanowires of metal have been supposed to exist because research on

quantized conductance in metals in recent years indicates that quantized conductance does

occur in metals as well4,8.

Very little research has been carried out involving the actual observation of STM-

formed nanowires, and due to the very small sizes the only direct method of observation

is seemingly the use of a transmission electron microscope. Recent work in Japan9,10 has

attempted to make such direct TEM observations as has earlier work using a different

technique11.

3

2 . Experimental Methods:

2 . 1 Transmission Electron Microscope:

Electron optics are quite similar to light optics when one compares the two. The

resolution is wavelength limited. For example, light microscopes have a maximum

resolution of about 300 nm. Electrons display both particle and wave properties. Using

de Broglie's equation relating the wavelength of a particle to its momentum, putting it in

terms of the accelerating voltage and correcting for relativistic effects we get a wavelength

of 0.0037 nm for an accelerating voltage of 100 kV. This would seem more than ideal

for atomic resolution but because all magnetic lenses have large aberrations that can not be

corrected, shorter wavelengths have to be used.

The modern transmission electron microscope (TEM) resembles an optical

microscope in its basic construction, as well as its optics. As an optical microscope

consists of a light source, a condensor lens, an objective lens and an eyepiece, the TEM

consists of an electron source (illumination), a condensor lens system, an objective lens

and a projection system with a viewing screen.

The illuminating source in a TEM is an electron gun which emits and subsequently

gathers the electrons, and then causes them to accelerate. Some requirements are a high

intensity so that the final image is visible at high magnification, a small energy spread to

reduce chromatic aberration and a high beam density per unit solid angle to reduce the

effect of spherical aberration.

The condensor system normally consists of two lenses. The first condensor lens

is strong to collect electrons over a large solid angle and to get them as close to the optical

axis as possible. The weak second lens projects the beam a relatively long distance to get

it parallel to the optical axis. (Reduced beam divergence.) A condensor aperture is

present to limit the beam divergence and beam load on the specimen if necessary.

The objective lens is the most important lens in the system. It determines the

resolving power of the instrument. It is the first lens in the image forming system so any

errors present will be magnified by the projection system. To lessen the effect of

4

spherical aberration it is desired to design this lens as strong as possible, reducing the

focal length and thus the spherical aberration coefficient. An objective aperture is located

in the back focal plane to control image contrast. A selected area aperture is located in the

image plane to select small areas of the sample for diffraction patterns.

The rest of the image forming system, the projection lenses, simply magnifies the

image further and projects it onto a fluorescent screen where it can be viewed. Often,

camera facilities are underneath the fluorescent screen to record images.

5

2 . 2 Scanning Tunnelling Microscope:

Tunnelling is the physical principle behind the scanning tunnelling microscope

(STM)12. In classical physics an electron moving in a potential can not penetrate that

potential barrier. Quantum physics, however, describes the electron in terms of a wave

function and the Schrödinger equation. Solution of this equation leads to the observation

that the electron has a finite probability of penetrating the potential barrier. In a more

tangible way this phenomenon should occur when two electrodes are close, but not in

contact.

This phenomenon is put to use in the STM. The two electrodes are placed about 5

Å from each other, and a bias voltage is applied so that the tunnelling in one direction will

be greater than the tunnelling in the opposite direction. Thus a tunnelling current will be

set up. The tunnelling current is proportional to the distance between the electrodes.

Thus one can correlate the tunnelling current measured to the electrode to electrode

distance and thus achieve a topographic view of a nanoscale object. And since the

tunnelling current is exponentially dependent on the distance, a small distance change will

result in a large current change and the high resolution in STM is achieved. The tunnelling

tip is then scanned across the surface in the x and y directions to eventually form a

topographic image of the surface. A representation of STM operation is shown in Fig. 1

(Not recorded by us).

6

A

Feedbacksystem

Z

X

Y

Fig. 1 Representation of STM operation.

The construction of an STM is quite simple in principle. The most important part

of the STM apparatus is the STM tip. This is the key component of the whole instrument.

Proper STM tip preparation is the principal factor determining whether quality STM images

are attained or not. Ideally the tip should terminate in a single atom, if atomic resolution

is desired. However this statement should be modified to consider the nature of the

surface that is wished to be imaged. A smooth, flat surface is ideally imaged by such a

tip as described above. However a rough surface is not imaged as well by a tip ending in

a single atom because it is usually supported by a thick shank unable to accommodate

very large vertical differences as seen in Fig. 2.

7

Fig. 2 Ability of tips to accommodate large height differences.(Wiesendanger Fig. 1.54)

The other components all deal with the control of the STM tip. They consist of a

piezoelectric scanner for fine-scale scanning across the sample surface while keeping the

tunnelling current constant, a stepping motor for approaching the surface in large steps, a

feedback system for controlling the tunnelling current and a vibration isolation system

which reduces the sensitivity to external vibrations.

The piezoelectric scanner, or piezo, is commonly composed of a piezoelectric

ceramic. Piezoelectricity is a phenomenon where when such a material is deformed a

charge can be measured. Similarly, the inverse piezoelectric effect is when a voltage is

applied across a piezoelectric material a deformation is observed13. With typical

deformations of 10-100 Å/V, atomic-scale movements can be achieved.

The stepping motor is used as a coarse positioner. Since the piezo only has a

small range of movement, the stepping motor has to be used to bring the tip close to the

surface. It is done by computer control. The feedback loop is disabled, the z-piezo is

8

withdrawn, the coarse positioner is moved one step forward, the feedback loop is

activated and it is checked whether a tunnelling current is present or not. If it is not, the

procedure is repeated until one is detected.

The feedback loop is used to control the response of the tip to surface variations.

The tunnelling current is amplified and compared with a reference current. If the

measured tunnelling current is greater than the reference, the tip is withdrawn, and vice

versa. This ensures that the tip is kept at a constant height above the surface.

With no place for an internal vibration damping system the approach taken was to

make the STM as small as possible to create a small mechanical loop which has a natural

high resonance frequency. This imparts a resistance to the natural low frequency noise

present in most buildings. Also, the holder is intended for use inside a TEM which are

located on vibration isolation platforms for further protection.

9

2 . 3 Sample Configuration:

A method of affixing the sample within the confines of the TEM sample holder, yet

still leave room for the other components which were to be around the sample, had to be

chosen. In the past, quantized conductance experiments had been carried out in two

major ways. It was decided to try to adapt one or both of these for use inside the small

space of the TEM sample holder.

2.3.1 Scanning Tunnelling Microscope Technique:

One method of forming nanowires is to use an STM. Originally the STM, as a tool

was intended for use as a surface scanning microscope with the tip hovering above the

surface. It can also be used for nanowire forming. The tip is lowered is crashed into the

surface of the substrate. As the tip is retracted the substrate material adheres to the tip and

forms a thin neck. During retraction, the neck becomes thinner and thinner, fulfilling the

conditions for quantized conductance. Eventually the neck breaks and one can repeat the

procedure. The procedure is shown schematically in Fig. 3

Fig. 3 Schematic drawing of STM nanowire forming.

10

2.3.2 Mechanically Controlled Break Junction:

The mechanically controlled break junction (MCB) technique was developed

because of problems of vibrational stability with the STM technique. The sample in this

case is a thin wire (for example, 0.25 mm, gold) which is affixed to an elastic substrate,

typically a piece of flexible metal, at two points. To prevent electrical contact between the

sample and the flexible metal strip, a layer of PVC tape is placed between the wire and

metal strip. The whole sample placement is shown in Fig. 4.

Phosphorous bronze plate

Isolating PVC film

GlueDent

GlueCable Cable

Gold wire

Fig. 4 Schematic drawing of MCB sample.

The piece of wire between the two fixing points is thinned manually, commonly

by rolling it on a flat surface while pushing softly with a razor blade. The stretching

occurs by the action of a piezo pushing and retracting from behind, stretching, breaking

and relaxing (thus reconnecting) the wire. The decreased sensitivity to vibrations and

increased reproducibility is due to the displacement ratio of the piezo movement to the

actual wire movement14. The setup is shown in Fig. 5.

Micrometer screw

Macor

Piezo tube

Macor

Ball

Triangular waveform generator~

Sample

Sampleholder

Sampleholder

Fig. 5 Schematic diagram of MCB technique.

11

Both the above-mentioned techniques could lend themselves to modification to be

placed inside a TEM sample holder. However, the MCB technique would be more difficult

to scale down so that it could fit inside the sample holder. The STM technique was

chosen as a starting point because the amount of additional space needed to place an STM

tip inside is significantly less than attempting to fit even a scaled down MCB. If the STM

technique proved to be too mechanically noisy, the MCB technique would be more

suitable because of the smaller mechanical loop, giving a higher resonance frequency,

which renders it less sensitive to the high amplitude low frequency noise that is present in

all buildings.

A slight modification was made to the original STM method of nanowire forming.

The STM-TEM holder, as designed, allowed approximately 80 µm of lateral movement

and thus it was decided to use two STM tips and to fix one in position, line the two tips up

manually, as close as possible, and then allow the moveable tip to act as a normal STM

and to locate the fixed tip and then force the two tips together and retract them, creating

the neck. This modification on the usual flat substrate/STM tip scheme was necessary

because of the requirements of the TEM. For transmission electron microscopy to work it

is necessary to have electron transparency. This requirement is met at thicknesses less

than 100 nm. It can be seen in Fig. 6 that having a flat surface would present a difficult

alignment problem to maintain electron transparency, if one wished to view the neck

without interference. Because of the very small sizes involved, aligning the flat surface

perfectly is nearly impossible.

Electron BeamElectron Beam

Problem

Fig. 6 Geometric problems for electron transparency.

12

2 . 4 Sample Preparation:

2.4.1 Materials' Choice:

The most common materials for STM tips are tungsten and a platinum/iridium

alloy. Tungsten is very durable and hard and is therefore usually a good material to use

for normal STM work. However, in an oxygen atmosphere a layer of WO3 forms on the

surface, which is detrimental to STM work. Therefore, in air conditions, an inert metal is

more effective and has fewer drawbacks. The Pt/Ir alloy is often used for its chemical

nobility. For an easier comparison of previous results and future results, however, gold

was chosen as the material to be used, as well as for its inertness to oxidation.

In previous nanowire-forming experiments it was showed that the materials that

performed the best were soft materials. Tungsten is a very hard material and therefore it

is hard to form stable enough wires with it. Gold is soft and has showed good results.

13

2.4.2 Tip Preparation:

A multitude of tip forming methods are found in the literature15. Such methods as

diverse as simple mechanical cutting and ion milling to electrochemical etching and

ceramic whiskers have all been used. For simplicity of use and the small investment in

equipment necessary, electrochemical etching was chosen.

The principle behind the etching operation is quite simple. Part of a metal wire is

immersed in an etching solution, a potential is placed between the sample and a counter-

electrode and is electrochemically thinned. The thinning action is enhanced in the

meniscus region and thus a neck (a section thinner than its surroundings) is formed and

when the weight of the lower part becomes too much for the neck to support, it breaks

and falls off, leaving a pointed tip. Fig. 7 schematically shows the etching action.

KOH

V

WAu

HCl

Fig. 7 Schematic drawing of etching process.

The factors involved in STM tip etching and their relative effects are difficult to

deduce. The main factors to consider are solution choice and concentration, voltage level,

current type, immersion depth and time.

A survey of the literature revealed that some common etching solutions for gold

were concentrated hydrochloric acid16 and Aqua Regia17. Both of these acids are very

14

strong and dangerous. To avoid handling of numerous acids, pure HCl was chosen, but

to avoid the fuming odours of the concentrated HCl it was mixed with an equal volume of

water. The resulting etching solution was found to be very effective for etching the gold

samples, without having to be used in a fumehood.

Direct current with a voltage of approx. 1.5 - 2 V was suggested16. Other

alternatives such as alternative current would have been more difficult to carry out in

terms of equipment. Access to a direct current constant voltage source was available

(Oltronix power supply B6-10) and preliminary tests showed acceptable results. A more

automated method would be to have a power supply with an automatic voltage cut-off

once the current through the etched wire dropped below a certain level. A constant

voltage of around 1.7-1.8 V gave the best results with the chosen etching solution and

sample material.

The choice of immersion depth is a balance between having the lower portion be

too heavy and falling off too early leaving a blunt surface, and having it be too light and

allowing the etching process to go on for too long, leaving a long thin, possibly fragile,

point. The ideal immersion depth was arrived at through experimentation with a variety

of different depths and viewing the results with an optical microscope. The best results

were achieved with immersion depths between 1.25 and 1.5 mm below the surface of the

etching liquid.

The tip etching procedure was adapted from an etching procedure commonly used

for tungsten tips18. The tip etching procedure begins with the cutting of pieces of 0.25

mm diameter gold wire into 22 mm lengths. The cut lengths are then ultrasonically

cleaned in methanol for approximately 10 minutes. They are subsequently rinsed in

distilled water and dried in air. Then the specimen wire is immersed to 1.25 - 1.5 mm

below the surface of the etching solution, and a direct current potential of 1.7-1.8 V is

placed across the sample and the counter-electrode so that bubbles of hydrogen gas

appear at the counter-electrode. Immersion time was controlled by examining the

immersed portion and removing the wire from the etching solution once the immersed

portion was seen to break off and fall to the bottom. The etched portion was dipped

15

carefully in four different beakers of hot water and then in a beaker of methanol for

cleaning. The tips were then viewed with an optical microscope at 1000X to examine

them for proper shape and cleanliness. The prepared tips were stored in a box with

slotted foam for protection.

The etching proceeded by way of the electrochemical equation shown in Eq. 1.

Au Cl AuCl e E V

H e H E V

Au Cl H AuCl H

+ ⇔ + = −

+ ⇔ =

+ + ⇔ +

− − −

+ −

− + −

4 3 100

2 2 0 00

2 8 6 2 3

4

2

4 2

o

o

.

. Eq. 1

It could be distinctly observed that a stream of a deep yellow/orange substance

was flowing downwards from the submersed portion of the gold wire during etching.

This is the AuCl4- -rich solution which is denser than the surrounding HCl liquid. From

the counter-electrode gas bubbles are distinctly seen to be forming and rising to the

surface.

16

2 . 5 TEM -STM Sample Holder:

2.5.1 Introduction:

The transmission electron microscope as described above has been and is a very

useful piece of equipment in laboratories and industry. The basic holder for mounting a

sample to be viewed in the TEM is a long, thin rod especially designed for the electron

microscope. It is a design which allows a simple air-lock to operate inside the

microscope and the danger of damaging the pole-pieces of the magnetic lens is

minimised.

Over the years, however, the basic design of the TEM sample holder, which

allowed only stationary sample and static viewing, came to be seem as insufficient for the

needs of some researchers. Scientists wished to have time-resolved TEM, to see motion

and to utilise the power of the TEM for more advanced experiments. Many different

holders with new features were designed and created. Some allowed rotation of the

sample about two axes, rather than just one. Other holders allowed straining of samples

within the electron microscope19.

17

2.5.2 Design:

The design that was decided upon, was to incorporate a functioning STM within

the TEM sample holder. The ability to perform, for example, a nanowire drawing and

breaking experiment, while at the same time observing this experiment with the

transmission electron microscope would be very exciting. The design is not so limiting,

however, that this is the only task which could be performed. The design allows some

flexibility for development of different experiments in the future.

The necessity of maintaining the external dimensions of the basic TEM sample

holder unchanged required careful consideration of the placement of the additional

components within the sample holder, without leaving the holder weak or fragile. A two-

part design was agreed upon to provide the most flexibility. An outer shell would mimic

the shape of the basic TEM sample holder and allow usage in any microscope, but it

would be hollowed out to allow the insertion of a second, inner portion which would

contain all the necessary wiring and other components of the STM. Also, it would be free

to slide in and out, transferring the coarse movement provided by the stepping motor to

the sample site. The material was brass, chosen for its non-magnetic nature and good

workability. The design of the two components of the TEM sample holder will be

explained with the aid of Fig. 8.

Fig. 8 Illustration of TEM-STM sample holder. (Not to scale).

18

The outer shell has the same basic shape as a traditional sample holder, with the

necessary modifications made. The largest difference is that it has been made hollow to

accommodate the inner portion. The tip of the outer component has a flat, indented

section close to the normal sample location. This section has been drilled and tapped for a

M1.5 screw and cover plate which is used for fixing one sample. Then there is an indent

for containing the rubber O-ring necessary for sealing the holder tight within the

microscope to maintain vacuum. Approximately in the middle of the outer shell an oval

hole was drilled for the purpose of attaching a screw into the inner portion, which once

inserted, would prevent rotation of the inner portion during operation of the stepping

motor. At a distance of approximately 230 mm from the tip, the outer section was made

much larger than a traditional sample holder. At this distance the sample holder is well

out of the microscope so its width is no longer of immediate concern. The purpose of

this widened portion was to accommodate the workings of a micrometer screw attached to

the end of the inner portion. The very end of the outer shell was made into a wide plate

80 mm in radius and drilled for attachment to the electronics box.

The purpose of the inner portion was to hold the piezoelectric ceramic, contain the

wiring leading to the piezo and to act as a transfer rod for the stepping motor. The entire

inner component was drilled out to accommodate the wiring leading to the piezo. At the

wide end of the inner component a diagonal hole was drilled as an exit point for the wires

threaded through the middle of the inner component. The tip of the inner component is

where the piezo was attached. A flat 2 mm thick and 3 mm deep slot was cut into the tip

to leave the wires emerging from the inside a gap in which to be attached do the sides of

the piezo. The upper portion of the tip was flattened and two notches were cut into it to

hold a piece of elastic non-magnetic material. The purpose of this was to provide a small

measure of damping of vibrations. An indent for a rubber O-ring was made as well, to

provide vacuum sealing between the outer and inner sections of the sample holder. At a

location corresponding to the oval hole on the outer shell, a hole was drilled and tapped

on the inside for the fixing screw. The wide end of the inner component was drilled and

fitted for a micrometer screw. The micrometer screw was attached by various fittings to

19

the stepping motor assembly. The micrometer screw was be free to rotate, but by the

placement and fixing of a C-shaped washer attached to the screw, rotation of the screw

would cause the entire inner rod to move in and out. This provided the coarse adjustment

of the z movement.

20

2.5.3 Assembly:

The piezoelectric ceramic has metallized sides cut into quarters on the outside for

x, -x, y and -y movement. The inside of the piezo is metallized for the z fine movement.

See Fig. 9.

Fig. 9 Schematic drawing of tube piezoelectric ceramic.

The wires for the piezo movement were attached by electrically conducting epoxy glue

(CircuitWorks™). Pairs of wires were twisted together to provide a small measure of

protection against current being generated in the wires due to the magnetic fields present

due to the 50 Hz fields present in all electrical lines. Nine wires in total were threaded

through the centre of the inner component and their uses are outlined in Table 1.

Table 1 - Wires in the Sample Holder

Purpose Wires

Piezo x, -x, y, -y, z 5

Sample (outer tube) 1

Tip (second sample) 1

Extra (future purposes) 2

21

The piezo was attached to the brass inner portion by a non-conductive epoxy

(Araldit™). The hole in the inner brass rod where the wires emerged from the inside was

sealed with epoxy so that the air from the outside would not simply bypass the O-ring by

going through the middle of the inner rod.

It was necessary to thoroughly cleanse both components, to remove any residual

oils or greases which may have been left over from the workshop or simple manual

handling of the pieces. Any such contaminants would be volatilised in the vacuum of the

electron microscope and would contaminate the internal components of the microscope.

Proper and thorough cleaning of the internal and external surfaces of the

components is done by rubbing all exposed surfaces with a cotton swab wetted by

acetone. This is continued until the cotton swab shows no darkening when it is wiped

across the surface. Typically 7-10 wipings are necessary before the surface is considered

acceptably clean. The internal parts of the two components were cleaned by running

acetone through them and by swabbing the parts that were accessible from the outside. A

more important step is the cleaning of the grooves which will contain the rubber O-rings.

These are the only means by which the vacuum of the chamber is maintained, so it is vital

that the grooves be clean and free from any particulate matter which may prevent a proper

seal. The grooves are cleaned in much the same manner as the rest of the two pieces, but

in this case just the grooves themselves are wiped approximately 10 times with acetone,

and then they are wiped some additional times with a freon solvent which is much more

effective in removing unseen contaminants but is expensive and thus is saved for only the

last step of the cleaning process. The rubber O-rings themselves are inspected visually

with a microscope at 50X magnification to ensure that there were no contaminants or

particulates on them. A layer of vacuum grease was applied to the rubber O-rings.

Protective latex surgical gloves were worn in order to maintain the uncontaminated

surfaces clean. The prepared inner portion was carefully inserted into the outer shell.

The rotation prevention screw was inserted and the lock-ring which locked the inner

portion inside the outer shell was screwed shut. The tip section (the section which was to

be inserted into the electron microscope and which was to remain clean) was covered by a

22

protective tube which covered all the workings of the sample holder ahead of the rubber

O-ring.

Soldering of all the contacts of the stepping motor to wires and then to a 25-pin

connector was done. The 25-pin connector leads to the stepping motor controller which

controls the speed, direction and number of steps desired. The stepping motor was fitted

to a gearbox to gear down the motor. This assembly was attached to the inside of an

aluminium diecast box. The box, containing the stepping motor and gearbox was fitted to

the adapter on the inner rod and the box and outer shell were fastened securely. The

specifications of the stepping motor and the gearbox are shown in Table 2.

Table 2 - Specifications of Stepping motor20 and Gearbox21

Stepping Motor Gearbox

Four phase, unipolar Ratio - 250:1

Step angle - 7.5°

23

3 . Results and Discussion:

3 . 1 Initial STM Tip Shape

The STM tip shape was viewed through an optical microscope at 200X, 400X and

1000X magnification successively. As mentioned in the Sample Preparation section

(2.4), the tip shape was quite dependent on the immersion depth below the etching liquid

surface. If it were below the surface too little (ca. 0.75 mm), it would appear too long

and fragile, as in Fig. 10. If it were immersed too much (ca. 2 mm), it would appear

short, stunted and almost rounded, as in Fig. 11. If it were immersed at some

intermediate depth (ca. 1.25 - 1.5 mm), it would appear to have an ideal STM tip shape as

compared with other STM tips, as in Fig. 12.

Fig. 10 Schematic drawing of long and fragile tip.

Fig. 11 Schematic drawing of short and rounded tip.

Fig. 12 Schematic drawing of appropriate tip.

24

3 . 2 Stepping Motor Control:

The stepping motor controls the z-direction approach and withdrawal of the entire

piezo assembly, thus controlling the distance between the two STM tips in large steps.

Due to the limitations of unsteady hands and the very small distances involved, the

stepping motor is a necessary component of the assembly for getting the two tips close

enough for the piezo to commence its work.

Testing of the stepping motor and its controllability was performed. The control

electronics were put together by Juris Prikulis. It was set up to allow control of the

number of steps desired, their direction and two speed choices were available. If

necessary, these options could be expanded for further usage. In the current setup a

current flows through the stepping motor even when it is not operating. This causes the

motor body to heat up proportionately to the time that it is connected to the control board.

Further plans involve expanding the control board with the addition of serial 100KΩ

resistors to decrease the heating of the motor.

The stepping motor worked successfully for approaching and withdrawing the

piezo assembly from the stationary sample. A lag of about 2000 steps was noted when

reversing directions of the motor. This is most likely due to play in the screw mechanism

and play within the C-shaped washer which is the principal part keeping the screw fixed

and the inner rod free to move. It is simply necessary to acknowledge and compensate

for this lag whenever reversing directions is needed.

25

3 . 3 Sample Mounting:

It has already been mentioned in the TEM-STM sample holder design section

(2.5.2) that one STM tip was kept stationary and the other was mobile. Affixing the two

STM tips to the sample holder showed itself to be a very difficult and painstaking process

where one had to be extremely careful. This was because the STM tips , as etched, were

very fragile and simply dropping one, even from a height of 1 cm was sufficient to

damage the tip portion so that it was unusable.

As previously stated there are two positions to attach the STM tips. There is the

fixed position and the moving position. The fixed position holds the STM tip by the plate-

and-screw mechanism mentioned in the design section. The moveable position tip was

held in the brass tip which was fitted to the end of the piezo. The brass tip was threaded

to M1 and thus was too big for threading the STM sample wire diameter, so it was chosen

to bend the end of the sample into a V-shape and to insert it into the brass tip and it would

be held in place by the spring action of the metal pressing against the inside of the brass

tip.

In order to minimise the risk of ruining the etched tips, for example by dropping

them while handling or by accidentally knocking the two tips together in the sample stage

area, it was decided to insert the moveable tip first because it was found to be the more

difficult of the two. It was inserted by forming the non-tip end of the wire sample into a

V-shape and inserting it into the brass tip of the piezo. Once this was done the fixed

sample could be attached. It was slightly easier to do since it was relatively easy to form

the end of the wire into a square-bottomed U-shape and then simply lay it on the flat area

of the TEM sample holder and then carefully lay the plate on top of it and tighten the screw

loosely. Once the screw was tightened loosely it was possible to align the two tips as

closely as possible (in the x- and y-directions) by manipulating the loosely fixed wire

with a set of tweezers. Once the tips were aligned as close as was dared, the fixed STM

tip was simply immobilised by tightening the screw fully.

It is important to note that the x and y alignment is quite crucial while the z

26

alignment is more forgiving. The x and y alignment is crucial because the only

compensation mechanism for misalignment in those directions is the piezo ceramic itself.

There is no equivalent of the stepping motor in those directions. The piezo has an x and y

movement limit of about 80 µm and therefore the two tips must be at least be aligned this

close in the x and y directions. The z-direction has the larger freedom of movement

because the stepping motor is present. As well, it is important to bear in mind that as one

is aligning the two tips one must be careful that the two tips do not come into contact with

each other, because this will ruin one or both of them. After attachment and fixing of

both tips was completed, it was necessary to examine both the tips for any damage that

may have been incurred during the handling of either.

27

3 . 4 Sample Movement Testing:

An important step was to ensure that all the mechanisms present to control the

movement of the STM tip attached to the piezo were functional. The entire TEM-STM

sample holder was mounted so that the sample stage area could be viewed with an optical

microscope with 200X, 400X and 1000X magnification available. The microscope was

fitted with a closed-circuit camera which was attached to a personal computer for image

viewing, capturing and storing.

For the first test all the preliminary steps were taken and all the cables to the

stepping motor controller, the high-voltage power supply and controller were attached.

The stationary tip and the whole box/TEM holder assembly were grounded. The

moveable tip was brought as close as was dared to the stationary tip with the stepping

motor. The optical microscope with the closed-circuit camera was set up to view the

sample stage.

The z piezo segment was grounded. The z piezo movement controller was split

into four sections and each of these four wires was routed to its own 100KΩ resistor and

the onto each of the x, -x, y and -y sections on the piezo so that when the z knob on the

piezo controller was turned, the voltage was increased equally in each of the four piezo

metallized sections and the movement of the piezo was just as if the internal metallized

section had a voltage applied to it. It is perhaps better to apply a voltage to the inside

metallized section as well for increased range of movement.

The tests showed that the x, y and z movement of the piezo worked quite well.

Figs. 13, 14, 15, 16, 17 and 18 show optical microscope pictures taken of a test of the

piezo alignment and approach of the two tips as well as the two tips touching one another.

28

Fig. 13 Optical micrograph of tips at a distance. (400X)

Fig. 14 Optical micrograph of tips somewhat closer. (400X)

29

Fig. 15 Optical micrograph of tips approaching. (1000X)

Fig. 16 Optical micrograph of tips approaching further. (1000X)

30

Fig. 17 Optical micrograph of tips nearly touching. (1000X)

Fig. 18 Optical micrograph of tips in contact. (1000X)

31

3 . 5 Vacuum Testing:

As all electron microscopes operate in vacuum, the insertion point of samples is a

point of weakness were possible leaks are most likely to occur. With normal TEM sample

holders this is not so much of a problem since they are all one piece and a single O-ring

on the outside is sufficient to maintain the internal vacuum. With the new TEM-STM

sample holder and its internal and external sections and wires running through the inside,

the room for leaks was much greater.

The opportunity to test it under controlled conditions presented itself when the

electron microscopes were due for their regular maintenance and thus it was possible to

test the sample holder without worries because if something went amiss, the electron

microscope had to be disassembled in any case. The insertion test was carried out by the

maintenance technician and it was found that the entire TEM-STM sample holder, as

assembled, was vacuum-tight. However, upon extraction from the microscope it was

detected that the airlock control rod on the outer brass section of the holder had come

loose inside the electron microscope and the airlock was not shutting properly.

32

3 . 6 Quantized Conductance:

During the preliminary optical microscope testing of the sample holder a test

experiment was set up to test the conductance measuring equipment and its compatibility

with the new holder. The sample tip wire was connected to a Stanford Research Systems

model SR 570 low-noise current amplifier and further on to a Hewlett-Packard 54603B

digital oscilloscope fitted to communicate with a computer running a LabVIEW VI

program through a GPIB interface. The full conductance measurement setup and its use

is explained in more detail elsewhere22. Whenever operation on the small scale of the

piezo movements it was absolutely necessary to work in a vibration-free area. Even small

vibrations would cause the two STM tips to vibrate too much so that close approach was

not easily achieved. The preliminary testing was performed on a vibration isolation

platform. Using the above-mentioned conductance measurement setup and controlling

the z-piezo movement by the z-piezo position controller, it was possible to see quantized

conductance steps on the oscilloscope screen. This was an exciting and encouraging

finding.

33

3 . 7 Troubleshooting and Suggestions:

The variety of tests used revealed the successes and limitations of the TEM-STM

sample holder as it was designed and constructed. A number of suggestions for

improvement of the sample holder for future reference has been compiled.

The major problem, the problem which prevents further usage until it has been

corrected, is the low strength of the airlock pin. All TEM sample holders designed today

must include this airlock pin to work with currently designed electron microscopes. The

current design of the holder has the hardened steel pin soldered into a hole drilled in the

outer brass section. The brass is too soft to resist the strains on the pin. One option is

making the entire outer portion out of a harder metal. Another option which would

salvage the current sample holder, would be to make the airlock pin a part of a slightly

bigger section, that is, have the pin attached to or be integral with a small base of the same

hardened steel as the pin itself which could be fit to the current holder. Another

possibility would be to make a pin with a threaded support and to screw the new pin into

a hole drilled into the sample holder.

The z piezo element was grounded due to some internal event leaving the z piezo

wire and the wire leading to the brass tip of the piezo short circuited This could be due to

snagging an internal screw or other point-like object. A suggestion would be some

method to fix the wires internally so that they were not so loose inside the sample holder.

A possibly related problem is the use of conductive epoxies. The conductive epoxy

should be checked to ensure that it is properly conductive. As well it should be noted that

when working in small or confined spaces it may be difficult to limit the exposure of the

conductive epoxy only to the area that is desired and not to get any flow-over between

metallized areas of the piezo.

One possible and unconfirmed problem is that the STM tip which is affixed to the

end of the piezo may not be held firmly enough to resist the rush of air that happens when

the holder is inserted into the vacuum chamber. The hole at the end of the piezo is the

only escape route for the air that is inside the holder core to use. The solution is to affix

34

the sample very firmly within the brass tip at the end of the piezo ceramic or to drill a

small hole behind the piezo as an alternate air escape path. Also if the sample is

discovered to be held too loosely, a firmer sample holder can be designed from a small

screw with a hole drilled down its middle. The screw would be threaded into the brass

end portion of the piezo and the sample wire would be put snuggly into the drilled hole.

35

4 . Conclusions:

The task set out and presented in this thesis was to design, construct and

thoroughly test a new transmission electron microscope sample holder which also had the

capability to operate as a scanning tunnelling microscope. The specific purpose that was

kept in mind was that of performing quantized conductance measurements and

simultaneously viewing the contact areas of the two sample tips. The final goal would be

to investigate the physical phenomena occurring at the moment of separation of the two

tips and to form conclusions as to the reasons behind the quantized conductance

phenomenon.

The TEM-STM sample holder was designed and constructed. The testing revealed

both the capabilities and limitations of the sample holder. The stepping motor worked

successfully. The sample mounting positions were accessible and not difficult to operate.

After troubleshooting, the piezo ceramic’s movements were easily controlled and smooth.

The sample holder was both cleaned successfully and withstood the vacuum testing. The

airlock pin on the outer brass shell was not strong enough to hold against the force of the

vacuum or the twisting motion upon insertion and removal of the holder from the electron

microscope. This needs to be remedied and suggestions have been made. Preliminary

quantized conductance measurements show encouraging results and indicate that the TEM-

STM sample holder in nearly all respects is operational and functions as it had been

intended.

The gold STM tip etching procedures were modified from similar tungsten

procedures and successful results were achieved. The tips worked successfully during

the holder testing and the preliminary quantized conductance experiments attest to this.

The design of the sample holder, the sample preparation for use in the sample

holder and the testing of the sample holder was carried out. This work has shown that

such a sample holder and its samples are feasible to produce and are likely to produce

satisfactory results inside a transmission electron microscope. The full testing of the

holder inside a TEM is left for future work.

36

5 . References:

1 C.W.J. Beenaker and H. van Houten, Solid State Physics, 1991, 44, 1.

2 D.A. Wharam, T.J. Thornton, R. Newbury, M. Pepper, H. Ahmed, J.E.F. Frost,D.G. Hasko, D.C. Peacock, D.A. Ritchie and G.A.C. Jones, J. Phys. C, 1988, 21,L209.

3 B.J. van Wees, H. van Houten, C.W.J. Beenaker, J.G. Williamson, D. van derMarel and C.T. Foxton, Phys. Rev. Let., 1988, 60, 848.

4 M Brandebyge, J. Schiøtz, M.R. Sørensen, P. Stoltze, K.W. Jacobsen, J.K.Nørskov, L. Olesen, E. Lægsgaard, I. Stensgaard and F. Besenbacher, Phys. Rev.B, 1995, 52, 8499.

5 G. Binnig, H. Rohrer, C. Gerber and E. Weibel, Phys. Rev. Let., 1982, 49, 57.

6 R. Wiesendanger, J. Vac. Sci. Technol. B, 1994, 12, 515.

7 S. Iijima, Nature, 1991, 354, 56.

8 C.J. Muller, J.M. Krans, T.N. Todorov and M.A. Reed, Phys. Rev. B, 1996, 52,1022.

9 T. Kizuka, K. Yamada, S. Deguchi, M. Naruse and N. Tanaka, Phys. Rev. B, 1996,55, R7398.

10 Y. Naitoh, K. Takayanagi and M. Tomitori, Surf. Sci., 1996, 358, 208.

11 M. Kuwabara, W. Lo and J.C.H. Spence, J. Vac. Sci. Technol. A, 1989, 7, 2745.

12 C.J. Chen, Introduction to Scanning Tunnelling Microscopy, (New York: OxfordUniversity Press, 1993).

13 C.J. Chen, Introduction to Scanning Tunnelling Microscopy, (New York: OxfordUniversity Press, 1993) pp. 213.

14 J.M van Ruitenbeek, A. Alvarez, I. Piñero, C. Grahmann, P. Joyez, M.H. Devoret,D. Esteve and C. Urbina, Rev. Sci. Instrum., 1996, 67, 108.

15 M. Fotino, Rev. Sci. Instrum., 1993, 64, 159.

16 H.J. Mamin, P.H. Guethner and D. Rugar, Phys. Rev. Let., 1990, 65, 2418.

17 Q. Huang, J.F. Zasadzinski, K.E. Gray, D.R. Richards and D.G. Hinks, App. Phys.Let., 1990, 57, 2356.

18 I. Ekvall, Licentiate Thesis, Gothenburg University, 1997.

19 H-O. Andrén, B. Loberg and H. Nordén, J. Phys. E: Sci. Instrum., 1974, 7, 316.

20 Farnell Components Catalogue, 1995, p. 591

37

21 Farnell Components Catalogue, 1995, p. 592

22 J. Hogsved, Masters Thesis, Gothenburg University, 1997.