Characteristics of Silver and Germanium Sulfide Films
Studied by Nanoindentation Methods
Rongzhu Wang
A thesis submitted to the Department of Chemistry in
conformity with the requirement for the degree o f
Master of Science
Queen's University
Kingston, Ontario, Canada
September 200 1
Copyright
Rongzhu Wang, 2001
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To my family
Abstract
Si lver photo-di ffùsed arnorphous germanium suIfide films are known to be
promising in microfabrication technology.
Detailed study on silver and germanium
sulfide film deposition parameters was carried out and it was found that the roughness of
the film surface was controlled by the deposition rate and the best deposition conditions
for obtaining smooth, homogeneous films have been determined. Mechanical properties
of silver and germanium sulfide on Si (1 1 1) substrate were studied using nanoindentation
techniques. Load-displacement curves were obtained and the effect of the substrate on
the composite mechanical properties was observed. The film thickness c m be estimated
from the load-displacement curves and the values were consistent with those obtained
from Rutherford Backscattenng Spectroscopy (RBS). The Young's moduli of silver film
and germanium sulfide were extracted fiom the composite Young's modulus of both the
film and the substrate underneath. Finally, the charactenstics of germanium su1fide fi lm
under 24 hours W illumination in air was also addressed.
Acknowledgrnents
First of ali, 1 owe a great deal of gratitude to my supervisor Dr. J. Hugh Horton for
his constant encouragement, continued assistance, advice and much other help. Financial
support for this project from NSERC is much appreciated.
1 also extend sincere thanks to Jingxin Li and Gillian Gonng for their advice on
AFM technique and knowledge on computer software.
Particular thanks are due to Dr. Loock and Dr. Natansohn who were my supervisory
cornmittee members, provided precious advice and suggestions in the middle of the
research.
1 gratefully acknowledge the heIp of Zhaoguo Tong, Xiongwei Cai, Qizhi Cui,
Kathy Lin and others, who were the source of much useful advice.
Finally, my family and fnends have my everlasting gratitude for their continuous
love and support.
Table of Contents
Abstract
iii
Acknowledgements
iv
Table of Contents
v
List of-Tables
viii
List of Figures
ix
....................................................... 1
Literature Review ................................................
3
Introduction
1
2
................m...e....
3
2.1
Introduction of Photodiffusion Phenornenon
2.2
Applications of Photodiffusion Phenomenon
.........................4
2.2.1
In
producing
Technology
Photoresist
Material
In
Microlithography
...........................................................................4
2.2.2
Io Higb-Density Rewritable Hologrnphic aod/or Bit-by-Bit Optical
2.2.3
.................................................................5
Otber Applications..................................................................5
Recording
2.3
a
Devices
Measurement of the Kinetics of the Effect
............................ 6
........m.....................................
6
Reflectivity Measurements.........................................................8
lntensity of Ag [ I l 1) X - Ray Reflection Measurement....................10
Diffraction Efficiency Measurement ...........................................11
Other Methods......................................................................11
Electrical Resistaoce Measurement
....a..m................................
12
2.4
Photodiffusion Kinetic Process
.................................................................. 12
~ ~Efïicient
2
Photodiffusion Period ................................................15
2.4.2.1.1
Wavelength Dependence................................................ 15
2 ~ ~ 2 Light
~ 2 Intensity Dependence.............................................17
2.4.2.3
Temperature Dependence...............................................18
2.4.1
2
.
Induction Period
2.4.2.4
Pressure Dependence..................................................... 18
................................................20
2.4.2.6
Summary....................................................................21
2.4.3
Final Slowing Down Period.................. ......
21
24.2.5
Composition Dependence
S..........
Nanoindentation Technique
2.5
............................................23
.................................23
Indentation Test and Basic Quantities........................................24
.
Analysis of Indentation Test Data..............................................26
2.5.3.1
Theoretical Background .................................................26
2.53.2
Methods o f Aaalysis......................................................27
Introduction to Nanoindentation Technique
2.5.3.3
Determination of Contact Area
.......................................31
................................................34
Experimental Setup
3
......................................................34
3.1.1 Sources for thin Films .............................................................34
3.1 .2 Physhal Thermal Vapor Deposition...........................................34
3.1.3 Thin Film Simples Preparation .................................................36
3.1
3.2
Sample Preparation
Atomic Force Microscope (AFM) imaging
........................37
List of Tables
4.1
The fi tting parameters for three di fferent thick silver films on a germanium sulfide
substrate. . ............... .................................................. ...............85
viii
List of Figures
2.1
Schematic illustration of the photodiffbsion of a metal in a chalcogenide g l a s
...................................................................................................3
2.2
Measured thickness dependence of the resistivity of as-evaporated Ag films. The
measured resistivity values with are notmalized with respect to the bulk value
f . . ..............................................................................................7
2.3
A schematic cross-section through a sample during the photodi ffûsion process
...................................................................................................
2.4
2.5
8
A typical plot of reflectivity as a function of exposure time obtained for Ag
photodiffusion into an As3,S,, film using 633nm He-Ne laser light ..................9
htensity dependence of the Ag ( 11 1) diEaction peak on silver film thickness
................................................................................................ 10
2.6
Kinetics of photodiffision as monitored by the change in the electncal resistance
of silver
. . films as a function of light exposure time for different illumination
intensities 1,. ............................................................................... -13
2.7
Spectral photosensitivity of the Ag-A+,
photodiffusion system, when
illuminated through As,S,layers of thickness d . ....................................... 15
2.8
Shift of photosensitivity maximum (arrow) in the Ag-As,S, system with exposure
time. Sensitivity scale for the lower two spectrograms are reduced.. ..............16
2.9
Inhibition of the photodiffision of silver in amorphous As2 S
during illumination
. 3
by light with a high intensity (120mWcm-2) following previous exposure by lowintensity light (5.6mWcm-2) for various durations.. .................................-17
2.10
The pressure dependence of the optical band-gap energies.. ........................ 19
2.1 1
Change in the silver concentration profile of a photo-diffised amorphous
chalcogenide film with illumination time afler exhaustion of the metallic source
............................................................................................... -22
2.12
A schematic representation of a section through an indentation showing various
quantities used in the analysis.. ......................................................... .24
2.13
A schematic representation of load versus indenter displacement showing
quantities used in the analysis as well as a graphical interpretation of the contact
depth.. .......................................................................................-25
2-14
The geornetry used by Sneddon in his derivation of the load-displacement
relations for a rigid punch of arbitrary profile .......................................... 28
3.1
Schematic illustration of physical thermal vapor deposition process ...............34
3.2
Schematic diagram of the experimental vacuum chamber used in film deposition
............................................................................................... -35
3.3
The operation principle of tapping mode ................................................ 37
3.4
Schematic diagram of tapping mode AFM system .................................... 38
3.5
Transducer module .......................................................................... 39
3.6
Our AFM based nanoindentation system ................................................ 40
3.7
A typical load-time plot for an indentation ............................................. 41
3.8
Schematic diagram of the Cube Corner tip and the projected contact area of the tip
............................................................................................... -42
3.9
A sample calculated area vs displacement (area fùnction) plot .......................43
4.1
M M images (3-D view4eft. plane view-right) of 5Onm GeSi.68films on Si (1 1 1)
substrate .................................................................................... -47
4.2
A plot of deposition rate vs RMS surface roughness of
4.3
Film growth modes ........................................................................ -49
4.4
AFM images of 50nm Ag films on Si (1 11) substrate................................. 53
4.5
A plot of deposition rate vs RMS surface roughness of silver films ................. 53
4.6
Load-displacement curve for sample GeSi.6$Si (1 11) of 2000 MI film thickness
up to a load of 458 pN and displacement to 230 nm ..................................54
4.7
Experimental data and the fitting curve of loading portion of 2000nm G e s 1 . 6film
~
4.8
Experimental data and the fitting curve of unloading portion of 2000nm Ges1.6~
film .......................................................................................... -57
4.9
Multi load-displacement curves for sample GeS1.6a/Si (1 1 1) ......................... 58
4.1O
A plot of loading curve slope vs indenter displacement for 2 17nm GeSl film on
Si (1 11) ....................................................................................... 59
.
films ............... 48
Multi load-displacement curves for sample GeSi.6s/Si (1 11) of 217 nm film
thickness with di fferent peak loads ...................................................... 60
AFM image (plane view) of nanoindentation on 2 17nm GeS 1-68film on Si ( 111)
.................................................................................................
62
Reduced Young's modulus for the 47nm germanium sulfide film .................-63
Reduced Young's modulus for the 2 17nm germanium sulfide film .................63
Reduced Young's modulus for the 2000nm germanium sulfide film ...............64
The reciprocal of measured Young's modulus of 217nm thick GeS1.68film as a
fiinction o f thrn............................................................................ 67
Young's modulus of three different film thickness vs. h.,/t
........................ 69
Load-displacement curve for sample Ag/% (1 I I ) of 2000 nm film thickness up to
a load of 1400pN and displacement to 523 nrn ........................................ 70
Experimental data and the fitting curve of loading portion of 2000nm silver film
Experimental data and the fitting curve of unloading portion of 2000nm silver
film .......................................................................................... -72
Multi load-displacement curves for sample Ag/Si (1 11) of 274.5 nrn film
thickness with different peak loads ...................................................... 73
Multi load-displacement curves for samples Ag/Si(l11) of different film thickness
............................................................................................... -74
A plot of loading curve slope vs . indenter displacement for the 274.5nrn Ag film
on Si (1 11).................................................................................. 75
AFM image (plane view) of nanoindentation on the 223nm silver film on Si(ll1)
............................................................................................... -75
Reduced Young's modulus for 39nm silver film on Si (1 12) ........................ 76
Reduced Young's modulus for 223nm silver film on Si (1 1 1)....................... 76
Reduced Young's modulus for 2000nm silver film on Si (1 1 1).....................77
The reciprocal of measured Young's modulus of 223nm thick Silver film as a
function of.
.
h
t ........................................................................... 78
4.29
Young's modulus of three different silver film thickness vs. hm&.
...............79
Load versus displacement curves for the silver films deposited ont0 2000nm thick
germanium sulfide substrate.. ............................................................ 80
The values of n in the equation [4.3] versus the thickness of silver film.. ..........8 1
Young's moduli of three different silver film thicknesses on 2000nm germanium
sulfide substrate.. ......................................................................... -82
Measured Young's modulus as a function of relative penetration hm&.
...........83
The reciprocal of measured Young's modulus of 5 0 0 m thick silver film on
2000nm GeSIe68substrate as a fùnction of th,,. ...................................... 84
The reciprocal of measured Young's modulus of 250m thick silver film on
2000nrn GeS .68 substrate as a fünction of t
h
,
, (t is the silver film thickness) ...84
The reciprocal of measured Young's modulus of lOOnm thick silver film on
2000nm GeS 1.68 substrate as a function of th,,. ......................................85
Load versus displacement curves for the 2000nm thick germanium sulfide films
pnor to UV illumination and afler 24 hours UV illumination in air.. ...............87
Reduced Young's modulus for 2000nm germanium sulfide film on Si (111)
before and after photochernical oxidation.. ........................................... ..88
xii
Chapter 1 Introduction
Chapter 1 Introduction
Silver photodifhsed amorphous Ge-S films are known to be promising for
applications in nanoli thography and holography.
Their advantages as a type- of
photoresist material over conventional organic resists have been realized in such
properties as high resolution, high contrast, and strong optical absorption.
Although the photodiffùsion process has been extensively studied from the early
1970s' detailed research on the characterization of the kinetic process is still needed in
order to improve the performance of the matenal.
Nanoindentation techniques have become widely used in determining mechanical
properties of homogeneous and layered material such as thin films on substrates. Since
the Young's modulus essentially reflects the property of the material and the properties of
reactants and products of the photoinduced reaction are different, it is possible to
characterize the reaction by measunng the Young's modulus of the sarnple.
A
commercial nanoindenter can be mounted on the Atomic Force Microscope (AFM)
scanner and the advantage of this combination is to allow the researcher to image the
sarnple and choose the location or feature that they are interested in, thus detennining the
properties of thin films at the nanometer scale.
This work studied the deposition parameters for the minimum film roughness and
the mechanical properties (elasticity, Young's modulus) of silver and germanium sulfide
films and the performance of multi-layer (Ag/GeSl.sa) films of different silver film
thickness. The thicknesses of germanium sulfide or silver overlayers on silicon substrate
were estimated fiom the load-displacement curves. The application of this method has
Chapter 1 Introduction
not been found so far in the literature. The measured Young's modulus varies with the
indentation depth. The influence of the substrate depends on both the indentation depth
and the thickness of film overlayers. The effect of W illumination on germanium
su1fide films in air was also illustrated by the indentation results.
Chapter 2
Literature Review
2.1 Introduction o f Photodiffusion Phenornenon
In the middle 19603, Kostyshin, Mikhailovskaya and Romanenko
['
found the
phenornenon of the illumination of a double layer sarnple consisting of a chalcogenide
layer and silver film by light leading to rapid dissolution of the metal in the
semiconductor.
Figure 2.1
Schematic illustration of the photodiffusion of a metal in a chalcogenide glass.
Most of the investigations have been carried out on arsenic-based chalcogenides or
Ge based glasses. The Ag source c m be a silver metal layer or silver compound. Other
metals such as zinc and cadmium or even some kinds of alloy c m also be dissolved in
Chapter 2 Literature Review
amorphous chalcogenides. In this case, however, the illumination has to be performed at
elevated temperatures [21.
2.2 Applications of Photodiffusion Phenornenon
2.2.1
In producing a Photoresist Material In Microlithography Technology
This photostimulated silver diffusion phenornenon has been reported for a wide
range of compositions in arnorphous As and Ge chalcogenides 131 which maybe useful in
silicon integrated circuit fabrication as a high-resotution photoresist. To begin with, a
thin film of a chalcogenide glass is vacuum deposited on a substrate. A silver layer is
formed over the glass either by vacuum deposition or by dipping into a silver nitrate
solution. Parts of the bilayer structure are then exposed to light and consequently silver
penetrates into the chalcogenide glass. The remaining silver is then removed by etching
in an acid solution, in which the photodifhsed regions are essentially insoluble. Thus a
negative resist pattern is delineated in the chalcogenide glass film.
This kind of
photoresist based on photodiffusion has advantages over conventional organic resists:
dunng illumination, lateral migration of silver fiom the non-illuminated region into the
illuminated region takes place. This process which was called "edge-sharpening" is very
beneficial as high-resolution (30nm)features have been realized
IJ1and
the resolution
limit is estimated to be 10A [? High contrast, strong optical absorption in the ultraviolet
region and an increase in sensitivity with decreasing atomic number of the element can
al1 be realized using this method
[61.
Chapter 2 Literature Review
2.2.2
In High-Density Rewritable Holograpbic and/or Bit-by-Bit Optical Recording
Devices
For its application to new optical recording devices, direct positive Ag patteming
with high-contrast has been attempted using amorphous Ag-rich Ag-Ge-S films (55-65
at. %Ag). The attractive features of the Ag patteming are as follows ['?
with respect to
the resolution o f Ag patterns, Ag-rich films are supenor to conventional photographic
films, since the photodeposited Ag particles are much smaller in size (less than 0.2 Fm in
diameter) than Ag particles of photographic films and ultrahigh resolution (-500A width)
has been obtained using soft x ray synchrotron radiation l9]. The Ag pattern c m be
completely fixed by overiaying a semitransparent metallic film and the Ag pattem written
by the photoinduced process is erasable by annealing at a temperature lower than the
glass transition temperature of the films or by exposure to intense illumination. We can
thus expect high-density rewritable holographie andlor bit-by-bit optical recording
devices that can be written using blue light and read using red or infrared probing light
using this method.
2.2.3
Other Applications
The Ge-S-Ag compounds formed by photoillumination have conductivities several
orden of magnitude larger than the original, and the ionic conductivity in silvercontaining glasses is usually much greater than the electronic conductivity, these
materials may be regarded as solid electrolytes ['O1.
This photo-induced difision process was also used by Kostyshin et al.t111
to
produce ciiffiaction gratings for visible light with symmetric and asymmetnc lines. The
Chapter 2 Literature Review
diffiaction efficiency of such gratings exceeded 50%.
The high transparency of
chalcogenide in the infrared region is the basis for the fabrication of infiared diffraction
gratings.
2.3 Measurernent of the Kinetics of the Effect
The knowledge of the kinetics of the effect is a prerequisite not only for
understanding the basic mechanism but also for exploiting its technological potential,
since a number of factors govem the exposure necessary to produce a useful image in the
chalcogenide.
2.3.1
Electrical Resistan ce Measuremea t
Goldschmidt and Rudman
[12'
measured the electrical resistance of the metallic film
and then, using a calibration curve, they determined the thickness of the metallic layer.
Measurement of the resistance of an Ag film as a function of time during the
photodiffusion process is directly related to the time-dependent thickness, d ( ' , of the Ag
layer and hence to the photodiffision rate itself, only if the resistivity p of the film is
independent of the thickness; in this case, d and R are related via the well-known
expression
where I and w are the length and width, respectively, of the film.
However, for thicknesses of as-deposited Ag films less than lpm, the resistivity is
no longer a constant but becomes dependent on the sample geometry, specifically the
Chapter 2
--
film thickness.
Literature Review
-
For thicknesses down to about
-
-- -
-
~ o o A ,the resistivity,
p(d), increases
gradually with decreasing thickness, d, due to electronic boundary scattering kom the
film surfaces (or possibly crystallite grain boundaries). However, the resistivity sharply
increases with decreasing film thickness for d a 200A as shown in Figure 2.2.
A g Film Thick nec. I A
Figure 2.2 Measured thickness dependence of the resistivity of as-evaporated Ag films. The
measured resistivity values with p are normalized with respect to the bulk value p" ''".
The abrupt increase in resistivity at very smail film thickness is due to the onset of
islanding in the deposited films, i.e. the films are no longer continuous. The existence of
such discontinuous islanded Ag films for thickness less than d
- 60A was confirmed
directly by transmission electron micrographs. Even if the thickness dependence of the
resistivity of Ag films has been known, there is no reason to assume that the change in
resistivity with varying thickness of as-deposited Ag films should be the sarne as that of
Ag films during the photodiffision process itself', in which the film thickness changes due
Chapter 2 Literature Review
~~~~~
to diffusion of the metal in the chalcogenide. In other words, the microstructure of Ag
films, which is a dominate factor in detemining the electrical resistivity, is not
necessarily the sarne at a given film thickness for a film deposited with that thickness and
for a film photo-difhsed down to that particular thickness.
evidence
Indeed, experimental
has shown that during the photodifhsion process of Ag in amorphous
GeS2, the microstructure of the silver film does change dunng the reaction.
For instance, scanning electron microscope micrographs clearly show the onset of
islanding during the photodiffusion process, indicating that the Ag dissolves into the
chalcogenide in an inhomogeneous fashion (at grain boundaries).
Thus, it is not
sufficient to take into account only the thickness dependence of the resistivity of asdeposited films in order to obtain estimates for the time-dependent Ag thickness and
hence information on the photodifhsion kinetics as assumed previously.
2.3.2
Reflectivity Measurements
Chalcogenide g l a s
Photodiffused layer
Metal
Figure 2.3 A schematic cross-section through a sample during the photodiffusion process.
Chapter 2 Literature Review
A method o f optical monitoring of photodiffision kinetics has been proposed by
Ewen
el
al.
['jl
They present an optical reflectivity technique used to examine how the
kinetics is affected by the composition of the chalcogenide glass and metal source.
This technique is based on the periodic variation in the reflectivity of a thin weakly
absorbing film with its thickness due to interference between light reflected fiom the top
and bottom surfaces of the film. The photodifised region is generally highly absorbing.
As the diffùsion front propagates through the film, the interference conditions in the
multi-layer structure change, giving rise to oscillations on the reflectivity curve. In other
words, reflectivity changes will be observed due to the decreasing thickness of the
undiffused layer, giving rise to oscillations in the reflectivity curve shown in Figure 2.4.
60
1O 0
160
Exposure Time Cs)
200
Figure 2.4 A typical plot of reflectivity as a function of exposure time obtained for Ag
photodiffusion into an
I m usiag 633nm He-Nelaser light l"'.
A cornparison of the experimental resuks with the results of cornputer simulation
enabled these researchers to investigate the diffusion kinetics. However, the reflectivity-
Chapter 2 Literature Review
time trace has the simple form shown above only when the photodimised layer is highly
absorbing, othenvise the light c m reach the silver interface and be reflected, and in this
case the reflectivity oscillations arise from the interference of light reflected fiom three
interfaces rather than two. Besides, the photodimision rate varies with a number of
factors such as chalcogenide composition, the thickness of the silver layer etc. Modeling
the optics of the rnultilayer structure is complicated and the analysis becornes even more
difficult.
2.3.3
Intensity of Ag 1111) X - Ray Reflection Measurement
An absolute method for the study of photodiffusion has been proposed by Rennie
and Elliot et al.
[')'
These workers made use of the fact that the evaporated crystalline
silver layer has a preferred orientation (1 1 1).
Figure 2.5 Intensity dependence of the Ag (1 11) diffraction peak on silver film thickness
'"'.
Using the diffiactometer, the height of the silver (1 11) Bragg difiaction peak was
recorded during the course of the reaction. Provided that the thickness of the silver is
.
Chapter 2 Literature Review
much less than the X-ray absorption length, the area of the silver peak is directly
proportional to the quantity of silver present. It was determined that the width of the
peak does not change during the reaction so the thickness of undissolved silver was
monitored during photodifision by measunng the silver (1 1 1) peak height. The linearity
between the intensity of the Ag [ I l l ] reflection peak and the Ag thickness (O < d < 120
nm) was checked separately as seen in Figure 2.5.
2.3.4 Diffraction EfMency Measuremeot
The method by diffraction efficiency was also developed by some workers. Silver
was diffused into a semiconductor in the interference field of two laser bearns (a
diffraction grating was actually obtained). The illuminated sample was composed of a
series of stripes (doped and undoped) with different refiactive indices. When such a
sarnple was later illuminated by a single laser, silver in the previously non-illuminated
regions diffused into the chalcogenide, resulting in a decrease in the diffraction
efficiency. The analysis of the diffraction efficiency kinetics yields information about the
dissolution kinetics.
2.3.5 Other Methods
A method for the study of the kinetics based on a detailed analysis of the optical
transmission spectra has been proposed Kawaguchi and Masui
["?
Some researchers
utilize the fact that photodoped and undoped chalcogenide semiconductors have different
dissolution rates in various solvents. The etching of one of the regions (usually undoped)
allowed the thickness of the doped layer to be measured directly.
Chapter 2 fiterature Review
Methods for the determination of the metal profile such as the use of radioactive
tracers, Rutherford backscattering spectroscopy, secondary-ion mass spectrometry and
electron spectroscopy for chernical analysis have also been applied.
2.3 Photodiffusion Kinetic Process
Previous work on the photodifision process, parîicularly the factors that have
effects on the process is very helpful in order to undentand and further explore the
mechanisms underlying this phenornenon. Generally the photodiffhsion of silver in
chalcogenide may be divided into three stages: an initial stage, which was often called
an induction penod, when the diffùsion rate is negligibly small or equal to zero,
acceleration stage with fast difision rate and a final step when the diffusion rate starts
to decrease.
2.4.1
Induction Period
The induction period was determined b)y Goldschrnidt and Rudman [ ' 2 1 by measuring
the electrical resistance of the silver Iayer and hence the quantity of the silver
photodiffused as a function of the exposure (exposure
=
light intensity
x
time). Figure
2.6 shows the initial stage where the difision rate is zero. The induction period was
explained by Goldschrnidt and Rudman as the accumulation of radiation damage in the
amorphous chalcogenide.
Chapter 2 Literature Review
I
O.?
-
-
1.O
10
-tm
IllumlnriUon tlmm (min)
Figure 2.6 Kinetics of photodiffusion as monitored by the change in the electrical
resistance of silver films as a functiou of light exposure time for different illumination
intensities 1 ,['*l.
The fact that the photodiffusion exhibits an induction time was considered
that
there is a certain minimum darnage required before the silver can difhse into the
chalcogenide. Thus the darnage in the chalcogenide required for photodiffusion requires
a minimum critical exposure in order to build up either c o ~ e c t i v i t yor agglomeration of
elementary damage sites. This can even be accomplished by pre-illumination of the
chalcogenide prior to deposition of the silver layer.
However, Ishikawa [''I observed that the presence or absence of the induction period
is related to the sequence of deposition of the layers. The period is observed when
amorphous chalcogenide is evaporated on silver. From their photovoltaic study, they
concluded that the induction stage could be explained as the time required for the
' diffision into the chalcogenide films. If
disappearance of the contact b h e r for A ~ ion
Chapter 2 Literature Review
the evaporation of silver follows the evaporation of the chalcogenide, the barrier is lost
because during sample preparation, evaporated hot silver difises into the amorphous
chalcogenide films, an induction period will be absent.
In addition, Yoshikawa and other workers
1'81 found
that the photodi f i s i o n process
is stopped if the samples are prepared at liquid nitrogen temperature: in effect this is
equivalent to an extremely long induction penod.
If the sample prepared at low
temperature is annealed for a certain tirne, then photodiffusion becomes possible.
An effect of the sequence of the deposition of the layers has also been reported by
other workers
who have demonstrated clearly that the artificial introduction of a
difhsed layer between silver metal and chalcogenide resulted in the disappearance of the
induction period, impiying that this incubation period is associated with the formation of
a continuous doped layer under the metal before photodiftùsion starts.
Another suggestion provided by Buroff Iml was that the induction period arises from
the formation of a bamer layer between the silver and the chalcogenide during the
evaporation procedure. He stressed that the induction period could only be observed if
there is a break in the vacuum dunng the evaporation of the two layers. If both layers are
evaporated during the same vacuum cycle, the induction period cannot be observed. This
result is explained by the formation of a thin oxide layer, inhibiting the diffusion.
Most workers now believe that the presence of the induction penod is determined by
the preparation procedure. If this procedure allows the formation of a thin layer of the
reaction product between chalcogenide and silver, then there is no induction period. The
formation of a thin (about IO& reaction product layer during the sample preparation has
been reported
[2'1.
If the preparation conditions are such that the formation of the reaction
Chapter 2
Literature Review
product layer is inhibited, for exarnple, if the films are evaporated at low temperature or
if a thin oxide layer is formed prior to silver deposition, a certain time is needed to
overcome the barrier (e.g. oxide) layer.
2.4.2 Efficient Photodiffusion Period
The end of the induction penod marks the beginning of the stage when efficient
photodiffusion occurs. In this part, the photodiffusion rate depends on such factors as the
intensity and the wavelength of light, temperature, pressure, composition of
chalcogenide.
2.4.2.1
Wavelength Dependence
Kokado et al IZ2' claimed that in order that a distinct arnount of silver photodiffùsion
is observed, the band-gap excitation is required at the metal-chalcogenide interface.
Results for the spectral sensitivity in a silver-chalcogenide glass system are s h o w in
figure 2.7.
9
340
440
540
Wavelength (nm)
Figure 2.7 Spectral photosensitivity of the Ag-As2S6photodiffusion system, when illuminated
through As2S61ayersof thickness d Iz2'.
Chapter 2 Literature Review
When samples were illuminated fiom the chalcogenide layer side, the shorter
wavelength limit of sensitivity varied with the thickness of the chalcogenide layer.
As photodiffusion proceeds, a layer of silver-diffused chalcogenide grows, and the
absorption by the whole layer tums out to be extended toward longer wavelengths as seen
in figure 2.8.
It was suggested, therefore, that photons absorbed by silver-diffbsed
chalcogenide contribute at least partly to photodiffusion.
15 min
1
I
180 min
Wavelength (nm)
Figure 2.8 Shift of photosensitivity maximum (arrow) in the Ag-As2& system with exposure
time. Sensitivity scale for the lower two spectrograms are reduced
"''.
However, other workers observed that photodiffusion is still efficient at longer
wavelength. Coupled with the fact that at this wavelength the photodifision rate is
independent of the chalcogenide layer thickness, they deduced that, in the longwavelength range, the photodiffusion rate is not due to light absorption in the
chalcogenide layer. The light is absorbed in the silver layer and the photodiffision is
presurnably due to photoelectrons ejected fiom the silver into the chalcogenide.
Chapter 2 Literature Review
2.4.2.2
Light Iotensity Dependence
The influence of the light intensity on the photodiffusion rate has been reported as a
linear dependence
sysiem
['ji
['-'!.
A sublinear dependence was also observed for the Ag/GeSel
and superlinear dependence for As,S3 at low intensitiec
["j.
A blockin= effect of low intensity light was o b s e r ~ e d l":.
A preliminary
illumination of the chalcogenide/silver structure by low intensity light (1 < 20 m ~ l c m ' )
resulted in a substantial decrease in the rate of silver photodiffusion for subsequent
illumination by higher intensity (>50mw/cm2) light. This effect is illustrated in figure
2.9, which shows the dependence of the photodiffusion rate as a function of the previous
exposure time by low intensity light.
"
--
2.0
- -
4.0
6.0
OB
1 (mm)
Figure 2.9 Inhibition of tbe photodiffusion of silver in amorphous As& during illumination
by light with a high intensity (ItOrnW~m'~)
following previous exposure by low-intensity
light (5.6mWcmJ) for various durations Isi.
Chapter 2 Literature Review
-
-
-
It was also found that the blocking effect may or may not occur depending on the
preparation technique of the sarnple and on the illumination conditions
[261,
e.g. it
disappears if the illumination is performed in vacuum. This phenornenon was explained
1261
that the blocking effect arises fiom the forming of different reaction products under
intensity light that would block the continuation of the dissolution.
low
2.4.2.3 Temperature Dependence
Generally speaking, the photodiffusion rate increases with increasing temperature
following the Arrhenius law:
k = Ae -%r
r2-21
whereas T is the reaction temperature and E is the activation energy of the reaction that
varies depending on the composition, the light wavelength, and the temperature ranges.
In an expenment with the Ag-As2S9.6 system, it was found that the photodiffusion
rate was very fast at the initial stage but then slows down
['21.
The activation energy for
the initial reaction was as low as 0.033 eV in the temperature range of -200 to +20°C.
Kostyshin et al.
IO 0.4
1'7w28f
found the activation energies for Ag-As's3 system Vary fiom 0.01
eV depending on the light wavelength and the temperature range.
2.4.2.4 Pressure Dependence
Investigations have been camed out on the effect of the application of pressure on
si lver photodi ffbsion in amorphous chalcogenide films.
Takaka
[291
studied the
photodifision process in vacuurn-evaporated bilayer films consisting of a 250A-thick
Chapter 2 Literature Review
layer of silver and 1Spm-thick layer of chalcogenide. The bilayer film was peeled off
the substrates and then pressurized in a gasketed diamond-anvil cell.
Upon light
illumination, the optical transmittance of the bilayer film increases in response to a
decrease in the silver Iayer thickness, and the photodiffusion eficiencjr was evaluated
through the changing rate of the transmittance. An increase in the efficiency followed by
a drastic decrease with increasing pressure was found. However, the authors mention
that these characteristics depend on which surface is irradiated and the photon energy of
excitation.
Figure 2.10. The pressure dependence of the optical band-gap energies .'nl
The pressure dependence of the band-gap energies in the photodiffused and
undiffised chalcogenide films was obtained as seen in figure 2.10. The undifised g l a s
exhibits a drastic decrease in the band-gap energy, which was explained due to the
Chapter 2 Literature Review
broadening of the valence band. The broadening is ascnbed to an enhanced overlap of
wave fùnctions of lone-pair electrons in chalcogen atorns as a consequence of the reduced
intemolecular distance.
However, in silver-photodifised chalcogenide glass, the
pressure effects on the energy gap are smaller, probably because silver ions are inserted
between the molecular clusters.
2.4.2.5 Composition Dependence
The photodiffusion rate is also affected by the composition of the chalcogenide. It
was found that the photodiffision rate in a series of As2& films is maximized at n = IO.
Tsuchihashi and Kawarnoto
['O1
have found that the concentration of S-S bonds in the
giass network becomes highest near these compositions, since the excess sulfur tends to
separate into Ss rings. Weakly bound sulhir atoms such as those forming S-S bonds may
provide a good base for stabilization of diffused silver and therefore play a significant
role in the mechanism of photodiffusion. Similady, it was explained on the basis of an
analysis of electron energy loss spectra
[3'1
, in which the differences between the
photosensitivities of glasses with diffenng arsenic contents are due to the network
structure and not to the differences between the electronic structures.
It was also
suggested that the dependence of the photodiffiision rate on As-S composition would be
affected by changes in the absorption coefficients of the photodifhised and undiffused
layers as a function of composition.
It was determined that an increase in the chalcogen content in Ge-Se system leads
to an increase in the photodifision rate, and the rate is a maximum for the composition
GeSe4d 13'' .
Chapter 2 Literature Review
2.4.2.6 Summary
Different researchers have reported different tirne dependences for the efficient stage
o i the photodifision process. Most investigators believe that photodifision is a lightenhanced chemical reaction between silver and the chalcogenide.
The reacting
components are separated by the reaction product layer and, for the reaction to proceed,
this product must be penetrable by at least one of the reagents.
In this case, together with the illumination connected with the interaction of the
components, a limitation anses because of the necessity to diffùse through the reaction
product. Depending on the importance of each of these factors, either a linear (if the
reaction rate is more important) or a square-root (if the diffksion factor is rate lirniting)
time dependence is observed experimentally. This makes it clear that for thin silver
layers the linear time dependence is expected. In this case, the reaction product layer is
also thin and the reagents c m easily penetrate through it, since the reaction rate is small
(for a light intensity of several milliwatts per square centimeter). If a high light intensity
is employed ( - 7 5 m ~ c m - ~ )then
, diffusion becomes the limiting step and the square-root
dependence should be observed.
2.4.3
Final Slowing Dowa Period
Most workers believe that, for the photodiffusion to proceed, a metallic layer is
needed
r33"51.
The slowing down of the difision rate is believed to be comected with
the exhaustion of one of the components of the reaction (more often, silver). Although
most researchers have maintained that the photodiffùsion stops after the silver source is
exhausted, it was reported by Kasyarum and Kudryavtsev
[361
that photodiffusion
Chapter 2 Literature Review
continues even when there is no metallic silver lefl but that the diffusion rate is much
slower. The diffision front becomes less sharp and the silver is redistibuted in the
chalcogenide under the influence o f the light intensity gradient as shown in figure 2.1 1 ,
resulting in a complete change in the concentration profile.
Figure 2.1 1 Change in the silver concentration profile of a photo-diffused amorplmus
chalcogenide film with illumination time after exhaustion of the metalüc source: (1) Oh; (2)
0Sh; (3) 2h; (4) 4h; (5) 8h
'".
Chapter 2 Literature Review
2.5 Nanoindentation Technique
In order to improve the performance of photodiffused silver into arnorphous
chalcogenide semiconductors, not only the optical and electrical properties of the
material, but also its mechanical properties such as Young's modulus and hardness
should be addressed.
The application of the nanoindentation method to the
photodiffusion phenornenon is that the Young's modulus obtained fiom this technique
allows us to characterize thin films and hence to determine the kinetics of the diffusion
process. A commercial nanoindenter can be mounted on the Atomic Force Microscope
(AFM) scanner and the advantage of this combination is to allow the researcher to image
the sample and choose the location or feature that they are interested in, thus determining
the properties of thin films at the nanometer scale.
2.5.1
Introduction to Nanoindentation Technique
There has been considerable interest in the last decade in the mechanical
characterization of thin film systems or materials in small volumes using sub-micron or
nano-scale indentation tests carried out with square, spherical or pyramidal indenters.
Since conventional microhardness testers require direct imaging of the indentations to
obtain hardness and modulus, large errors are introduced due to the measurement,
especially when the indentations are small. The advent of depth-sensing nanoindentation
testing instruments has enabled the depth of penetration to be measured on the order of
nanometers. This makes it possible to obtain reliable values of Young's modulus and
hardness of thin films and other finely structured material.
Chapter 2 Literature Review
Research on nanoindentation has shown that elastic moduli, hardness, and timedependent deformation efiects can be measured. Elastic moduli arid hardness of the
specimen material are extracted fiom experirnental readings of indenter load and depth of
penetration. The validity of the results for hardness and modulus depends largely upon
the analysis procedure used to process the raw data.
2.5.2 Indentation Test and Basic Quantities
In a typical test, a load is applied to an indenter that is in contact with the surface of
a sample. Load and depth of penetration are recorded as load is applied fiom zero to
some maximum and then fiom the maximum load back to zero.
It is necessary to describe some of the basic quantities and analytical relations that
are used in nanoindentation to detemine the Young's modulus fiom the measured ioads
and displacements.
WRFACE FRORLE A R E R
LOAD REUOVAL
lSDaSTER
\
+
Figure 2.12 A schematic representation of a section through an indentation sbowing
various quantities used in the analysis.
Chapter 2 Literature Review
LOADING 1
/
DISPLACEMENT, h
Figure 2.13 A schematic represeotation of load versus indenter displacement showing
quantities used in the analysis as well as a graphical interpretation of the contact depth.
Figure 2.12 shows a cross section of an indentation and in figure 2.12 and figure
2.13, the parameters used in the analysis are identified. h, is called the contact depth and
h, is the displacement of the surface at the perimeter of the contact. At peak load, the
load and displacement are P,,,, and h,,,,,
respectively, and the radius of the contact circle
is a. Upon unloading, the elastic displacement is recovered, and when the indenter is
fully withdrawn, the final depth of the residual hardness impression is hf
unfoading slope is the contact stiffhess
.
The initial
Chapter 2 Literature Review
---
--
-
-
Analysis of Indentation Test Data
2.5.3
2.5.3.1
Theoretical Background
In a commonly used method, data are obtained from one complete cycle of loading
and wiloading. Even though the specimen undergoes elastic-plastic deformation dunng
loading, it is presumed that the initial unloading is an elastic event [371 . The unloading
data are then analyzed according to a mode1 for the deformation of an elastic half space
by an elastic punch that relates the contact area at peak load to the elastic modulus.
Assuming that the contact area remains constant dunng initial unloading, the
relationship between the unloading curve and the elastic modulus of the material being
tested can be described by elastic contact theory. The elastic contact problem, which
plays a key role in the analysis procedure, was originally considered in the late 191h
century by Boussinesq and Hertz. Boussinesq ["1
developed a method based on potential
theory for computing the stresses and displacements in an elastic body loaded by a rigid,
axisymmetric indenter. His method has subsequently been used to derive solutions for a
number of important geometnes such as cylindncal and conical indenters. Hertz [39i
analped the problems of the elastic contact between two spherical surfaces with different
radii and elastic constants.
His now classic solutions forrn the basis of much
expenmental and theoretical work in the field of contact mechanics and provide a
h e w o r k by which the effects of non-rigid indenters can be included in the analysis.
Another major contribution was made by Sneddon, who denved general
relationships among the load, displacement, and contact area for any punch that c m be
described as a solid of revolution of a smooth fùnction
. His results show that the
Chapter 2 Literature Review
-
-
-
load-displacement relationships for many simple punch geometries can conveniently be
written as
P = ahm
[2-41
where P. is the indenter load, h is the elastic displacement of the indenter, and a and rn are
constants. Values of the exponent m for some common punch geometries are m
flat cylinders, nr
=2
=1
for
for cones, m = 1.5 for spheres in the limit of small displacement, and
m = 1.5 for paraboloids of revolution.
The earliest expenrnents in which load and displacement sensing indentation
methods were used to measure mechanical properties were perfonned by Tabor [421 - The
author studied the indentation of a number of metals defomed by hardened spherical
indenters. Subsequently the behavior of conical indenters was examined by Stillwell and
Tabor
[431 .
The importance of these expenments is that since elastic contact solutions
exist for each of these geometries (i.e., a spherical indenter in a spherical hole and a
conical indenter in a conical hole), the ways in which plasticity affects the interpretation
of elastic unloading data can be dealt with by taking into account the shape of the
perturbed surface in the elastic analysis. Tabor used these results to show that the shape
of the entire unloading curve and the total amount of recovered displacement can be
accurately related to the elastic modulus and the size of the contact impression for both
spherical and conical indenters.
2.5.3.2
Methods of Analysis
There is a popular method for relating the modulus, E, tu the initial unloading
stifhess S. The basic assumption is that during the initial withdrawal of the indenter, the
Chapter 2 Literature Review
contact area between the indenter and the specimen remains constant. in such a case the
analysis of Sneddon
['O1
for the indentation of an elastic half space by a flat, cylindrical
punch approximates the behavior. Pharr, Oliver, et al.
'371
followed Sneddon's analysis
for the indentation of linear elastic half spaces by rigid punches. Figure 14 shows the
geometry of the punch.
Figure 2.14 The geometry used by Sneddon in his derivation of the load-displacemeat
relations for a rigid punch of arbitrary profile.
The punch is described by function z =f @) which is rotated about the z-axis so as to
produce a solid of revolution. The function is chosen so that f (0) = O, with the only
restriction being that the function be smooth.
When a load P is applied to the punch, it is elastically displaced into the half space
by an amount h producing a circle of contact at the surface with radius a.
A
dimensionless variable x = p ln is defined such that in the region of contact, O < x < 1, the
indenter shape is described by the function z =f (x).
Chapter 2 Literature Review
Sneddon has derived expressions for both h and P in ternis of simple integrals of the
shape function.
AAer a series of mathematical derivations, a simple expression of the contact
stiffhess is obtained:
a is the radius of the cylindrical indenter, p is the shear modulus, and vis Poisson's
ratio.
Noting that the area of the contact circle, A = za', is the projected area of the
elastic contact and that the shear modulus c m be related to the elastic modulus through
Pz
E
, equation [ M l becomes
2(1+ v )
In these experiments, the effects of non-rigid indenters on the load-displacement
behavior can be effectively accounted for by defming a reduced modulus, Er, through the
equation
where E and v are the Young's modulus and Poisson's ratio for the specimen and Ei and
v , are the same parameters for the indenter.
If we substitute reduced modulus Er for E, then equation 12.61 becomes
Chapter 2 Literature Review
This equation was originally limited to those instances for which the indenter
behaves as a flat ~ylii~dricat
punch and it has been shown that it holds well for any
indenter that cm be described as a body of revolution. Nevertheiess, the indenters used
most commonly in load-displacement sensing indentation techniques such as squarebased, triangular-based (cube corner, Berkovich) and Vickers indenters cannot be
described as bodies of revolution, but it appears that even this does not place severe
restriction on the use of equation [2.8].
King performed finite element calculations of the load-displacement characteristics
of elastic half spaces deformed by flat-ended punches with circular, triangular, and square
cross sections ["I
.
It has been found that for al1 three geometnes, the unloading stifhess
is given by
where the values of the constant ,û are
circular:
p= 1.O00
triangular:
P = 1.O34
square :
/?= 1.012
The nurnerical values of p resulted fiom the solution procedures. For the triangular
and square geornetries, the stiffness deviates fiom the circular one by only 3.4% and
1.2%. It is apparent that equation [2.8] can be used without great error even when the
indenter is not a true body of revolution.
2.5.3.3 Determination of Contact Area
Chapter 2 Literature Review
2.5.3.3
Determination of Contact Area
From above equations, it can be seen that to make accurate measurernent by
indentation expenments, the contact areas of the indentations must be precisely known.
The standard practice is to assume that the area of contact for an indentation of a
specified depth is the cross-sectional area of the indenter tip at that depth. In this method,
the geometry of the tip is described by an area function A (It,), and this function is used to
determine the contact area of an indentation made to a prescribed depth.
One method for detennining the area function of a tip involves imaging the
indentations. The calibration is made by fitting the imaged areas at known depths to an
area function.
However, optical imaging of sub-micron indentations is difficult and
direct imaging of indentations by Scanning Electron Spectroscopy (SEM) does not
provide sufficient contrast to discern the area of contact for shalIow indentations.
Doerner and Nix described a calibration method using Transmission Electron
Spectroscopy (TEM) images of indentation replicas ["I.
For this study, a series of
indentations of varying size were produced in an annealed a-brass. Cellulose acetate
replicating tape was applied to the sample. Platinum with 20% palladium was used as a
shadowing agent. Following shadowing, a carbon film was evaporated ont0 the replica
and the cellulose acetate removed by dissolving in acetone. The prepared replicas were
then imaged in the TEM. The areas of the indentations were measured by using a
graphics tablet and software for calculating the area of closed curve. Obviously, this
method is somewhat inconvenient, being both expensive and time-consuming.
Another method was introduced by Oliver and Phan P71 , which is based on the
measurement of contact stifbess and requires no imaging of the indentations.
This
Chapter 2 Literature Review
method assumes that the elastic modulus is independent of indentation depth. The total
measured compliance C is the sum of the compliance of the specimen and the compliance
of the machine.
C=C,+C,
Since the specimen compliance dunng elastic i
[2.1 O]
ven by the in7
e contact
stiffness
If the modulus is constant, a plot of C vs A-''' is linear for a given matenal, and the
intercept of the slope is a direct measure of the machine compliance.
To find the area hnction and the machine compliance, the authors take advantage of
the fact that relatively large indentations can be made in aluminum because of its low
hardness. The area fùnction for an ideal Berkovich indenter is given
A ( h , ) = 24.5h:
[2.12]
which c m be used to provide a first estimate of the contact area. Initial estimates of Cf
and Er were thus obtained by plotting C vs A-''
for the two largest indentations in
aluminum. Using these values, contact areas were then computed for several other
indentation sizes by rewriting equation 13.91 as
fiorn which an initial guess at the area hnction was made by fitting the A vs h, data to the
relationship
Chapter 2 Literature Review
where CI through Cgare constants. The first terni describes a perfect Berkovich indenter;
the othen descnbe deviations fiom the Berkovich geometry due to blunting at the tip.
Since the form of the area function influences the values of C',and E,., the procedure
was applied again using the new area function and iterated several times until
convergence was achieved. To determine the tip geometry at shallower depth, a similar
method involving indentations in fused quartz is used, assuming the sarne machine
compliance as measured in the aluminum experiments. This area function is assumed to
descnbe the tip geometry completely, and thus it yields the contact area of any
indentation made to a known depth. In this approach the cross-sectional area of the tip
and the contact area of the indenter and the specimen, at a given depth, are treated
implicitly as if they are the sarne.
Chapter 3 Experirnental Setup
Chapter 3 Experimental Setup
3.1 Sample Preparation
3.1.1
Sourcesforthinfilms
The chalcogenide starting material was glassy Ge&, made by reacting together
stoichiometric amounts of powdered 99.999% Ge and S at 1200°C for 24 hours in an
evacuated quartz tube
Torr) pnor to cooling down to room temperature in oven
ISlf,
and then the germanium sulfide was pulverized into fine powder. 99.999% silver wire
(Advent Research Materials) was used to prepare the Ag film. No oxygen fiom air was
found in RBS.
3.1.2
Physical Thermal Vapor Deposition
Thin-film samples were prepared by thermal evaporation in a vacuum charnber. The
deposition process is illustrated in figure 3.1
Figure 3.1 Scbematic illustration of physical thermal vapor deposition process.
34
Chapter 3 Experimental Setup
In the process of deposition, contamination and the supply rate are the major issues.
The high vacuum atmosphere (see section 3.1.3) provides an important advantage of clear
access to the deposition surface. The deposition rate was detemined by the temperature
of source material that was controlled by a constant current source. The deposition rate is
directly related to the roughness of sarnple surface. In addition, the uniformity of thin
films on the substrate is determined by the geometry of source container as well as the
geometry of the substrate. Our homemade Ta container has dimensions of 2cm (L)
lcm (W)
x
0.8cm (H) and the substrate was cut into lcm (L)
x
k m (W) pieces. Figure
3.2 shows the experimental setup used in film deposition.
Out
Film Sample or
Crystal Monitor
x
Sample Source in
Fa
B.at
-
To Vacuum Pumps (Rotary
Pump & Diffusion Pump)
Figure 3.2 Schematic diagram of the experimental vacuum chamber used in
film deposition.
Chapter 3 Experimental Setup
3.13 Film Samples Preparation
The chamber was pumped down by rotary pump and difision pump and a liquid
nitrogen cold trap was used to furiher lower the pressure (< 10" mbar) before the
deposition.
Sample sources were contained in Ta boats that were heated resistively.
Films
were deposited on polished Si (1 11) wafers (Virginia Semiconductor Ltd.). The silicon
substrate was nnsed in methanol and distilled water and dried in a flow of high purity
nitrogen before mounting in the vacuum chamber.
Silver wire was rinsed in organic
solvent (e-g. acetone or methanol).
We installed a tumtable in the chamber that can hold four source containers
simultaneously. This setup is easy to operate and make it unnecessary to break the
vacuum during the deposition of multilayer sample and hence avoids the problem of
fonning an oxide or other contamination layer between different materials.
When
performing multilayer depositions, different sources can be placed at the same position,
directly below the sample substrate and in this way the non-uniformity problem can be
avoided.
A constant current source makes it possible to prevent large power fluctuations and
to obtain a stable temperature and deposition rate.
The deposition rates varied fiom 0.2 A/s to 6 A/s for GeSz layer and 0.2 A/s to 10
A/s for the silver layer, these were monitored by a quartz oscillator.
The GeSz film sample could also be exposed to W radiation for 24 hours in air
using a 4500 pW Hg arc lamp, with maximum intensity at 254nm tu examine the oxide
layer formation.
Chapter 3 Experhental Setiip
-
-
3.2 Atomic Force Microscope (AFM) imaging
Al1 AFM images of our sample surface were acquired using a PicoSPM (Molecular
Imaging, Tempe, AZ) and a Nanoscope IIE controller (Digital Instruments, Santa
AFM operates by measunng attractive or
Barbara. CA) under ambient conditions.
repulsive forces between the tip and the surface of the sarnple. In our experiments, the
thin film surfaces were irnaged by AFM tapping mode. Tapping mode AFM images the
sarnple by alternately placing the tip in contact with the surfaçe and then lifting the tip off
the surface to avoid dragging the tip across the surface. The tip is vertically oscillated at
its resonance frequency (here, about 70 kHz) with fiee amplitude, Ao, when the tip is not
in contact with the surface and a reduced amplitude, A, when tapping the surface as seen
in figure 3.3. The probe is shown in figure 3.4. The cantilever is magnetically coated
and is dnven by an extemal oscillating magnetic field.
Sarnple surface
'Tapping"
7
/
1
\
/
\
,
Amditude reduced
Figure 3.3 The operation principle of tapping mode.
During tapping mode operation, the cantilever oscillation amplitude is maintained
constant by a feedback loop. When the tip scans over a high point on the surface, the
cantilever has less room to oscillate and the amplitude of oscillation decreases.
Chapter 3 Experimental Setup
Conversely, when the tip scans over a depression, the cantilever has more room to
oscillate and the amplitude increases (approaching the maximum fiee air amplitude). The
oscillation amplitude of the tip is measured by the detector and input to the controiler
electronics. The digital feedback loop then adjusts the tip-sarnple separation to maintain
a constant amplitude over the sample. Tapping mode imaging inherently prevents the tip
from sticking to the surface and causing damage during scanning. As shear forces are
reduced since the applied force is always in the vertical direction. Figure 3.4 shows the
configuration of our tapping mode AFM system.
Film Sarnple
Mignetic Alternating
Current (MA) Mode
magnetic field
Figure 3.4 Scbematic diagram of tapping mode AFM system.
38
Chapter 3 Experimentd Setup
3.3 Young's Modulus Measurement
3.3.1 Nanoindentation Apparatus
Our indentation experiments were conducted using a Hysitron Triboscope, a low
'
load test system manufactured by Hysitron Inc. Other necessary equipment includes
mounted diarnond indenter tip and the hardness and modulus analysis software. The
heart of the instrument for depth-sensing nanoindentation is a transducer, which is a
parallel geometry sandwiching of a plate held by leaf springs between two rigid, parallel
plates. A diarnond indenter tip, attached to a shafi, is mounted to the middle plate, and is
accomrnodated by a small hole in the center of the lowest plate. The mode1 is s h o w in
figure 3.5.
Diarnond Tip
Figure 3.5
Transducer module.
A force, F, is generated by applying a voltage between the middle plate and the
bottom plate through electrostatic actuation, and a capacitive sensor is used to measure
the resulting displacement by change in capacitance between the middle plate and the
outer plates.
Chapter 3 Experimental Setup
We mount the system on our molecular imaging AFM, which gives the instrument
in siltc imaging capabilities. The transducer replaces the AFM head such that irnaging is
still controlled with the AFM software but the cube corner diarnond indenter tip is now
perfoming the imaging rather than the AFM cantilever probe. The exact location to
perform an indentation can now be chosen and an image of the indentation site can be
obtained within minutes of the indentation taking place.
Triboscope
Nanoscope Controller
œ
-
Sample
Transducer and Probe
Figure 3.6 Our AFM based nanoindentation system.
40
Chapter 3 Experimental Setup
Our measurement systern is shown in figure 3.6. The instrument (AFM Scanner and
nanoindenter) was located in an insulated box on a heavy block mounted with bangee
cords so that extemal vibration noise and thermal drift were reduced.
A typical load-time plot for an indentation is shown in figure 3.7. The maximum
load vanes from 20 p N üp to 7000 pN.
Figure 3.7 A typical load-time plot for an indentation.
The load on the indenter was controlled so as to maintain the indenter velocity until
the programmed maximum load was reached. The load was held constant for 0.2 seconds
until the indenter velocity dropped low enough to allow for time-dependent plastic
deformation. After the hold, the Ioad was removed at a constant rate. The high loading
and unloading rates help to limit the measurement error caused by thermal drifi arising
fiom changes in laboratory temperature.
Chapter 3 Experimental Setup
3.3.2
Tip Sbape Celibration
Tip shape calibration is based on determining the area huiction according to the
method provided by Oliver and Pham l3'1
. A detailed explanation of the theory has been
reviewed in chapter 2. The method is based on the assumption that Young's modulus of
elasticity is independent of indentation depth.
in our expenment, fused quartz with
Young's modulus of 72 GPa is used as a standard sample for calibration purposes. The
tip shape calibration was carried out every one or two samples.
As mentioned before, a reduced Young's modulus is defined considenng the effects
of non-rigid indenters on the load-displacement behavior. The elastic modulus of the
diarnond indenter is 1140 GPa, and the Poisson's ratio is 0.07. The Poisson's ratio of
fused quartz is 0.170. According to equation [2.7], the reduced modulus for Our standard
sarnple is then 69.6 GPa.
For an ideal cube corner (90") tip, the projected contact area to depth relationship
can be denved as fol lowing:
Figure 3.8 (a) Schematic diagram of the Cube Corner tip and (b) the projected contact area
of Qhetip.
Chapter 3 Experimental Çetup
-
p
p
-
A 0 is the contact depth that is perpendicular to the surface MST.
--
--
The area of
equilateral triangle MST is the projected contact area. If we assume AS = AT = AM = a,
then the area of the triangle MST is A = d3
2
a
and the contact depth A 0 is h, = -
JI-S"
we have
To determine the area function of Our not very sharp tip, a senes of indents at
various contact depths (From 20nm to 500n.m)are performed on fused quartz specimen
assuming an ideal area hnction. The real contact area of each indent can be calculated
using equation [2.13] since we know the reduced modulus of the sarnple and can get the
stiffhess (S) fiom each load-displacement curve.
A plot of the computed area as a
function of contact depth is obtained and a typicai area hnction plot is given in figure
3.9.
Figure 3.9 A sample calculated area vs displacement (area function) plot.
Chapter 3 Experimental Setup
A fitting procedure is employed to fit the area (A) versus
(h)to
a fifth order
polynomial of equation [2.14], which is
The constants for this equation were then inputted into the Data h a l y s i s section of the
Hysitron Program for use on indents on our samples.
Chapter 4
Results and Discussion
Chapter 4. Results and Discussion
4.1 Depositioo of thia films
4.1.1
Ces,,
films
Amorphous GeS,.,, films were deposited under a variety of deposition rates with the
substrate held at roorn temperature. The deposition rate was varied fiom 0.2 A/s to 6&s
as detemined by a quartz thin film monitor. When heated, glassy GeS, decomposes to
form gas-phase GeS and S2. The relative arnounts of the two species will be dependent
on source temperature. The composition of the film produced, however, wi Il dependent
not only on the composition of the vapor, but also on the relative sticking probabilities of
the GeS and S2 species on the surface. It has been detennined by previous workers lS21
that the composition remained roughly constant regardless of deposition rate at GeS,
-
.
The result was consistent with our own which was GeS,.,, averagely determined by
Rutherford Backscattenng Spectroscopy (RBS) located at the University of Western
Ontario.
While deposition rate had little effect on the composition of the films, it had
important effects on the surface roughness. In Our expenments, preparing minimum
roughness films is cntical which makes subsequent interpretation of silver deposition and
oxidation o f the film more straightforward.
Figure 4.1 shows an AFM image of GeS,,* films deposited at different rates on Si
(1 1 1) substrate.
Chapter 4
Results and Discussion
Chapter 4
Results and Discussion
nrn
Figure 4.1 A F M images (3-D view-left, plane view-right) of 50 nm GeS,, films
on Si (1 11) substrate deposited at (a) 0.2 A/s @) 1 A/s (c) 6 A/s
Image (a) was taken at setpoint y,, (=A/&, where A and A. were illurstrated in
Figure 3.3 ) of 0.9 and z range (the amplitute in Z mis) of 15 nm, and (b) and (c) were
obtained at setpoint of 0.9 and z range of 5 nm.
The roughness difference can be seen fiom the above images. The most common
and general measure of roughness is RMS roughness, which is defined as
O ,
=
,/q
[s(x)
- s ( x ) ~
i4.11
x = O
where
,ais the surface height at point,
height of the surface profile.
in the surface profile and, ,(,)
is the average
Chapter 4 Results and Discussion
The RMS roughness of images obtained c m be quantified by the AFM analysis
program, the relationship of deposition rate and roughness is plotted in Figure 4.2. The
RMS roughness shown in Figure 4.2 was the average values obtained fiom more than
three measurements and the error of the data was less than 1%.
0.2
O
0.6
0.4
0.8
Deposition Rate (nrnls)
.
.
--
.
- --
-
-
-
-
Figure 4.2 A plot of deposition rate vs R M S surface roughness of GeS,, films.
The roughness decreases fiom 0.850 nm to 0.471 nrn as the deposition rate increases
from 0.2 A/s to about 1 A/s, and increases slightly to 0.499 nm at 6
us. This result can
be explained from thin film growth theory. Generally the deposition process c m be
classified into several modes depending on the relative strengths of the interactions
between film and substrate.
fa) Frank-Van der Merwe (layer)
Chapter 4 Results and Discussion
( b ) Stranski-Krastanov
-
- - - --
- -- --
( c ) Volmer-Weber (island)
Figure 4.3 Film growth modes.
(1) Frank-Van der Merwe mode: this layer growth mode is observed when there are
strong interactions between the deposited matenal and the substrate. This comrnonly
occurs when metals are deposited ont0 metallic substrates, for exarnple, silver on gold.
(2) Stranski-Krastanov growth: this type of growth is charactenzed by mixed growth of
layers and isolated islands on top of existing layers. In other words, three-dimensional
growth takes place locally once a monolayer has been formed. Some cornmon systems
that exhibit this type of growth are the deposition of silver on tungsten, silver on silicon,
and lead on tungsten. In the initial stages of film growth, the substrate's surface tension
exceeds that of the deposit and interface, leading to Iayer growth. Subsequent deposition
then takes place on the initial monolayer, rather than on the substrate, so the effective
"interfacial" surface energy becomes the sarne as the deposit's surface energy. As a
result of this change, the surface tension balance is altered and the growth mode shifts
h m layer to island growth. (3) Volmer-Weber island growth: essentially the opposite of
Chapter 4 Results and Discussion
hyer growth, occurs when the deposited material is highly cohesive and interacts only
weakly with the substrate. This type of growth commonly occurs when metals are
deposited on akali halides.
Island growth is observed when the substrate's surface
tension is less than the surn of the surface tensions of deposit and interface.
For our sample of germanium sulfide on a silicon substrate, previous work has
demonstrated that the film showed a layered structure for 10 nrn thickness deposited at
low rates of 0.02 n~n/s.[~'lCombined with the observation from AFM that agglomerated
particles formed on the thicker layers (50 m),it can be detennined that Ge&,, growth
on the silicon substrate proceeds via the second mechanism - the Stranski-Krastanov
mode, in which clusters f o m on an initial monolayer.
The deposited germanium sulfide film's roughness afier the first monolayer depends
on the relative strength of the cluster growth and nucleation processes. Cluster growth
c m occur by two means: single adatoms may be 4'captured" when they diffùse into the
vicinity of an existing cluster, or arriving adatoms from the gas strearn may contribute to
the cluster in what is known as "direct impingement". A small cluster results when the
nucleation density is high, Le. high deposition rate. When the deposition rate is high,
which means the flux of incoming free atoms is large, the adatom formation rate is
correspondingly increased. As a result of the large nucleation rate, less time is available
for surface diffusion of the adatoms, and the nuclei tend to form localized clusters. This
generally results in a small-grained structure with good comectivity between grains.
Although the high deposition rate can facilitate the formation of a flat surface, it is
actually controlled by the source temperature.
It is energetically favorable for the
adsorbed atoms to join into clusters, because this reduces their total surface area and
Chapter 4 Resulb and Discussion
.surface energy. High source temperature leads to a high kinetic eriergy of the gas phase
atoms and increases the difïùsivity of adatoms so the number of adatoms passing within a
cluster's capture range is large and thereby increases the probability of adatom captured
by existing clusters.
The two opposite effects (flux of free atoms and diffusivity of
adatoms) caused by the temperature had to be balanced to get the optimum conditions.
From the above analysis and the results obtained fiom the experiments, it c m be
detemined that a low deposition rate (0.2&s) is not suitable in film preparation which
can result in large RMS surface roughness; the medium rate ( l k s ) is good for obtaining
flat surface of germanium sulfide films; the high deposition rate is not preferable for two
reasons: one is roughness increases at high rate, the second is that the overheated g l a s
source undergoes a process very like sintering and produces a layer of crust on top of the
source. So the following deposition rate will be changed. Therefore lA/s is optimal for
germanium sulfide deposition.
4.1.2
Silver films
Silver films were deposited under a variety of deposition rates varying fiorn 0.2 k s
to 10&s on either the silicon wafer or germanium sulfide layer. The AFM images of
silver layer on Si (1 I l ) wafer at diRerent rates are shown in figure 4.4. The z range is
25nrn for al1 three sarnples and the setpoint is 0.9. The corresponding roughness of silver
fiIms obtained at different deposition rates is shown in figure 4.5.
As mentioned before, it is well known that the silver layer on silicon substrate shows
a Stranski-Krastanov growth mode.
For the same reason as in germanium sulfide
deposition, the medium deposition rate ( 5 h ) in Our test range is best to rninimize
Chapter 4 Results and Discussion
surface roughness (0.73hm). Particularly, the deposition rate of silver on GeS,.68 cannot
be too high because the hot silver atoms can react thermally with germanium sulfide
undemeath which could make the subsequent results of the examination of the
photodi ffision phenornena ambiguous.
Chapter 4
Results and Discussion
Figure 4.4 AFM images of 50nm Ag films on Si (1 11) substrate deposited at (a) 0.2 A/s (b)
2 A/s (c) 10 A/s
O
--
0.2
0.4
- .
0.6
0.8
Deposition Rate (nmls)
_ _
_
-
1
---
-
-
1.2
-
--
--
Figure 4.5 A plot of deposition rate vs RMS surface roughness of silver films.
Chapter 4. Results and Discussion
4.2
Nanoindentation tests and data analysis
4.2.1. Ces,,
.
films on Si (1 11) substrate
In order to characterize the silver photodifision process using the nanoindentation
technique, the performance and properties of pure GeS, .68,0.i
fully understood.
and d v e r iayers should be
Nanoindentation tests were carried out on GeS, .,8
films with
thicknesses of roughly 47 nm, 217 n m and 2000 nm as indicated by RBS measurements.
in every GeS,.,,/Si sarnple, over 20 indentations were made, with the minimum load
being 20 pN and the maximum load in the range of 2000 pN to 4000 pN, depending on
the thickness of the films.
Figure 4.6 Loaci-displacement curve for sample GeS,,/Si (11 1) of 2000 am film
thickness up to a load of 458 &,W and displacement to 230 am.
Chapter 4. Results and Discussion
Figure 4.6 shows a selected load-displacement c w e for a sample with film
thickness of approximately 2000nrn; the shape of the curve is typical of a thick GeS,.,*
layer. This figure was obtained with indenter maximum penetration depth of 230 m.
If the loading cuwe is fitted using the published equation [461
P = ahn
14-21
we found the function did not fit very well at low loads. The reason probably is that
mechanical or thermal drift has a significant effect at low loads, adding a constant
background to the load value. If we add a constant c in the equation
!
I
O
I
I
50
I
1
1
100
150
I
I
200
250
Displacement (nm)
Figure 4.7 Experimental data and the fitti~gcurve of loading portion of 2000nm Ces,.,
film.
Chapter 4. Results and Discussion
Then the function gave a satisfactory fitting result for the low load/displacement region.
The experimental data and the fitting curve overlap well in the whole loading range. We
have a = 0.070 I0.001, n
=
1.618
+ 0.003 and c = 1 1.1 + 0.3, so the loading c u v e of
GeS1.68film with our cube corner tip can be expressed by
P = I1.1+0.070h1.618
P.41
For an ideal Hertzian [471 loading curve where the indenter is a sphere on a flat plane,
and the substrate behaves cornpletely elastically
where n is 1.5, R is the tip radius of curvature, and E' is the reduced modulus of the
system. Of course, we cannot expect this equation to fit well here because the system we
tested is clearly not etastic. The efTect of plasticity is to increase n, and the constant in
£?ont is no longer easily analyzed.
In some experimental observations the value of n has been reported to be 1.74 in the
literature L481. Here we get 1.62 as n value. This discrepancy has also been considered by
Hainsworth et al 1491 . They believe that a value of n so different results fiom the method
of curve fitting. We found a similar result in our experiment. If we use equation (4.21 as
a fitting function, a value of 1.52 is obtained. Besides the fitting method, we should take
account the influence of tip shape that differs fiom a perfectly formed point, and the
specimen surface that is not ideally smooth.
From the unloading part of the curve, the onset of plasticity can be observed and it
has been found to fit an expression given below [461
Chapter 4. Results and Discussion
-
P = p(h 4,)"
f4.63
where p is another constant, hfis the final displacement which is 170nm for this curve as
shown in figure 4.6 and m is dependent on the test material. The parameters obtained
h m the fitting are: ,û= 0.204 + 0.002; m =1 . S E kO.003.
Displacement (nm)
Figure 4.8 Experirnentsl data and the fitting curve of unloading portion of 2000nm Ceslwa
film.
The unloading part can be expressed as
P = 0.21 ( h - h ,
)'.88
Chapter 4. Results and Discussion
For thimer layer samples (47nm and 21 7nm), the initial part of the loading curves
was found to be consistent with the above parameters for the curve frorn the 2000nrn
thick sarnple. In al1 cases the indenter load versus true indenter displacement curves
dunng the whole procedure of loading and unloading are continuous, suggesting that no
film debonding or cracking occurred. However, a change in the slope of the loading
curves can be observed for the thinner GeS,.68films on the d i c o n substrate if sufficient
load is placed on the indenter.
Figure 4.9 Multi load-displacement corves for sample GeS,dSi (1 11) of different film
thickness (1) 47 nm (2) 217 nm (3) 2000 am.
Chapter 4. Results and Discussion
Displacement (nm)
Figure 4.10 A plot of loading curve dope vs. iidenter displacement for 217nm Ces,., film
on Si (111).
Figure 4.9 shows the load-displacement curves of GeS,.,, films on Si for different
film thickness. As we c m see, the displacement at which the dope changes differ on the
various samples and these changes are dependent on the thickness of the overlayen. If we
choose one of curves (2 17 nm thick) and plot the slope change of loading curve vs. the
indenter displacement in figure 4.10, the dope difference can be seen clearly.
The slope change is always at a displacement of
= 215 nm for different loading
curves for the sarne sarnple (2 17 nrn) as shown in figure 4.1 1.
Chapter 4. Results and Discussion
Figure 4.1 1 Multi load-displacemest curves for sample GeS, JSi (1 11) of 217 nm film
thickness witb different peak loads.
The measured displacements at slope change points were 43nm and 215nm
approximately using the method shown in figure 4.10.
As determined by RBS the
thickness of films (1) and (2) are 47.ûnm and 217.On.m respectively based on the density
of GeSz of 2.94 gcm". Obviously, the displacement at slope change point is nearly equal
to the thickness of films and this can be explained by the large difference in hardness of
the bulk silicon and GeSIe6,films. The hardness of the silicon substrate is much higher
than that of the germanium sulfide film. When we apply a high enough load on the
indenter, it c m push fiom the germanium sulfide layer directly into the bulk silicon
substrate. Once the indenter encounters the silicon, a much larger load is needed in order
Chapter 4. Results and Discussion
to get the same displacement as in the film layer; hence the slope becomes significant
larger.
Consequently, a simple method is provided to determine the thin film thickness on a
silicon substrate by measuring the position of dope change. Meanwhile, it also indicates
that film thickness prescnbes a limit to the indentation load-displacement range that can
be used to characterize a sample. In order to obtain film properties, the applied load must
be low enough to avoid the influence of the substrate, yet large enough to avoid surface
topography effects. The critical depth at which the substrate begins to signiticantly affect
the measured mechanical properties depends on the properties of the film, the substrate
and the interface between them. For our germanium sulfide sample, if we compare the
load-displacement curves with film thickness of (1) 2 17 nm and (2) 2000 nm, we found
that at a displacement of = 100 nm (1/2 thickness of film), the loading curve of 2 17 nrn
film began to deviate from that of the thick 2000 nm film. Since the 2000 nrn film is very
thick compared to the maximum indenter displacement in any experiment, the substrate
effect can be neglected in this case. The deviation on the 21 7 nm sample arises fiom the
substrate effect. From the beginning of the deviation, the mechanical properties obtained
fiom the test are no longer the properties of film layer itself, but a combination of the
performance of both film and the substrate.
An examination of the indentation imaged by AFM using the diarnond tip is shown
in figure 4.12.
Chapter 4. Results and Discussion
un
Figure 4.12 AFM image (plane view) of onnoindentation on 217nm GeS,., film on Si (1 11).
I f the tip is perpendicular to the sarnple surface, the projected area of an equilateral
triangle can be observed. Otherwise the triangles will be distorted and the data fiom such
tests are no longer reliable.
Chapter 4. Results and Discussion
4tnm GeS1.68 film on SNI 11)
Figure 4.13
Reduced Young's modulus for the 47nm germanium sulfide film.
100
1SO
200
250
300
Contact Oepth (nrn)
Figure 4.14
Reduced Young's modulus for the 217nm germanium sulfide film.
Chapter 4. Results and Discussion
Figure 4.15
Reduced Young's modulus for the 2OOOnm germanium sulfide film.
The reduced Young's moduli of 47 nm,217 nm and 2000 nm germanium sulfide
layers were calculated fiom the measured partial unloading data as shown in figure
4.13-4.15.
Calculations were based on the assumption of contact area equal to the cross
section of the indenter.
The area function of our cube corner tip calibrated using fused quartz as a standard
sarnple (reduced Young's modulus of 69.6 GPa) is
Due to the influences induced by the morphology of the sample surface, the
truncation or roundness of the indenter tip and the drift of applied loads dunng
indentation tests on the thin films, Young's moduli obtained are somewhat scattered,
Chapter 4. Results and Discussion
especially at small penetration depths. The values of reduced Young's moduli are a
fwiction of the penetration depth. In the case of the two thinner films, Young's moduli
increase with indentation depth. For the 2000 nm film, a decrease in Young's modulus is
obtained with indentation depth.
For the 47nm thick film in figure 4.13, the moduli measured in the expenments
starts near SOGPa, and increases up to above 8OGPa when the contact depth is more than
20nm and thereafier remains constant. The 217nrn thick film was also tested and the
Young's modulus is shown in figure 4.14. As shown in this tigure, the Young's modulus
at initial low load is around 28 GPa and it increases gradually as the contact depth
increases. At the contact depth of more than 2 0 h , the value reaches 45GPa. Such
depth dependence can be attributed by the presence of the harder silicon substrate. As the
indentation depth increases, the substrate mechanical properties begin to dominate the
composite modulus of film overlayer and the substrate. For thimer films, the substrate
contributes more to the composite mechanical properties of the system.
in order to evaluate the intrinsic mechanical property of a thin film layered on a
substrate, the penetration depth should be no more than one half of the film thickness as
we discussed before. Some workers even suggest that a penetration depth of less than
one tenth of the film thickness is necessary.
However, the precise indentation
measurement on such small thicknesses is difficult due to the influence of thermal and
mechanical drift. Here, we have adopted an empincal relationship proposed by Doemer
and Nix [j51
Chapter 4. Results and Discussion
-
-
-
--
where /? is an arbitrary parameter unique for the ~amplek i n g tested and the indenter
k i n g used, r the tilm thickness, h the penetration depth and E' the composite Young's
modulus. Subscriptsf and s refer to the film and substrate properties, respectively.
If we rearrange equation C4.81,
Here, we substitute the Young's modulus Es of silicon (1 11) wafer of 160 GPa into
equation [4.9] and it becornes
Successfùl fits of equation i4.1 O] were achieved on the 2 1 7 nm thick film but not on
the other two films. For the 47 nm film, the data at low loads are scattered and the fitting
results are not convincing. For the very thick 2000 nm film, the substrate has almost no
effect on the film modulus so that equation [4.8] is not suitable.
Figure 4.16 shows the 217nm film data replotted using
axes vaiues, and the data fit using equation [4. IO].
f/.,,
and 1/E* as x and y
if we have the fitting done
automatically by the cornputer and with two parameters variable, the fiaing curve does
not follow the trend of Our experimental data and the standard errors are large. This
problem has been noted by J. MenCik et al. ['O1 who suggested an optimization procedure
for determination of the optimum combination of P and Ex As a criterion of suitability of
individual combinations, it is possible to use the sum of squared differences between the
measured values and the calculated one or to use the standard error of approximation.
Chapter 4. Results and Discussion
The optimization can use special programs, but can also be "hand-made": a starting value
of Ef is chosen and the other variable p is changed step-by-step as long as the sum of
squared difference decreases; then P is held constant and Ef is changed, and so on. The
standard error decreased in our case fiom around 10% to 5%.
Figure 4.16 The reciprocal of measured Young's modulus of 2 l7nm thick GeS1.68film as a
function of î/h,,,
( t is the film thickness).
The parameters were obtained using the above optimization procedure with l / Ef =
0 . 0 3 3 ~ ~ a "I /P
, = 1.18. Therefore El =30.3GPa, /3
and equation [4.1 O] becomes
=
0.85 for our sample were derived
Chapter 4. Results and Discussion
The Young's modulus of germanium sulfide derived above is 30.3GPa which is in
the range of 20-35 GPa obtained from nanoindentation tests on 2000 nm thick film. The
variation of the modulus of 2000 nm GeS,.,, film depends on the penetration depth in
figure 4.15. At low loads and small penetration depth, the moduli are higher than those at
hi& loads and large penetration depth. There are two possible reasons for the changes o f
E* with indentation depth on the 2000 nrn germanium sulfide film. One reason may be
due to poor calibration of the tip. However, we have carefully calibrated our indenter and
checked it using the standard sample, so the area h c t i o n of Our indenter should not be a
problem. A more likely reason is that the properties of this film change somewhat with
depth as it took such a long time to deposit. Homogeneity of the film and the effect o f
the roughness over the thickness range were not known and it needs to be exarnined by
other analytical technique.
For comparison, we have plotted al1 three modulus curves vs. h,Jt
When h,Jt
in figure 4. 17.
is less than 0.5, which means the indentation depth is less than half the
thickness of the films, the influence of the substrate to the composite is not significant for
thick films. The Young's moduli of 217 nm and 2000 nrn thick films are close to 30
GPa, which is in agreement with the result obtained from the fitting method. But for the
47 nm thick film, the observed rnodulus is higher. Since this film is so thin, this result
may be due to difision at the Si/GeS,.68interface, or due to inhomogeneities in the film.
Chapter 4. Results and Discussion
GeSl.68 film on Si (111)
,t=47nm ;t=217nm
- ---
-A
-
0.2
0.1
O
--
t=2000nm
-
0.3
- .
0.4
0.5
hmaxlt (t film thickness)
- -
-
.
-
- -- ..
-
-
-
-
--
--
Figure 4.17 Young's modulus of three different film thickness vs. hma!t (t the film
tbickness).
4.2.2
Silver films on silicon substrate
The Young's moduli of silver films of different thickness on Si (1 11) substrate were
also measured by the nanoindentation technique. The thicknesses of the two thinner
films were detemined by RBS, as 51.2 nm and 275 nm respectively.
Over 20
indentations were made on every Ag/Si sample, with the minimum load being 20 pN and
the maximum load in the range of 2000 pN to 4000 pN, depending on the thickness of
the films.
Figure 4.18 shows a selected load-displacement curve for a sample with film
thickness of 2000 nm approximately; this is a typical curve for the thick silver films.
Chapter 4. Results and Discussion
-
This data was obtained with maximum load of 1400 pN and indenter maximum
penetration depth of 524 nrn.
Figure 4.18 Load-displacement c u w e for sample Ag/Si (1 11) of 2000 nm film
thickaess up to a load of 1400 pN and displacement to 523 am.
The unloading behavior of silver films observed is considerately less reversible than
that of germanium sulfide sample in figure 4.6, which suggests that deformation in these
matenals after the initial loading has a considerately larger plastic contribution.
As in the case of fitting the loading curve on the 2000 nm germanium sulfide film, a
constant load background has to be considered when we fit the loading cuve on the
2000nm silver film in figure 4.18. Equation [4.3] was used and the parameters obtained
Chapter 4. Results and Discussion
-
were: a = 0.028 f 0.001; n
=
--
-
-
-
-
--
1.732 f 0.005; c = 22 f 1. Hence the fünction of the fittîng
curve is
O isplac eme nt (nm)
Figure 4.19
-
Experimental data and the fitting curve of loading portion of 2OOOnm silver
film in Fig. 4.18.
A larger background load was obtained which could be caused by the rougher
surface of silver sample compared to that of the germanium sulfide sample. n for this
system is 1.73. As we discussed before, n reflects the elastic/plastic performance of the
sample surface. The effect of plasticity tends to increase n, which is consistent with our
results. It is obvious Fom figures 4.18 and 4.6 that the deformation on silver film is more
Chapter 4. Results and Discussion
plastic if we compare the unloading curves of silver films and germanium sulfide films.
The value of n was 1.62 for the GeS1,3sample; smaller than that for the silver film.
The unloadïng curve was fit using equation [4.5],the final displacement hf as shown
in figure 4.1 8 is approximately 493nm.
D isp alceme nt (nm)
Figure 4.20
Experimental data and the fitting curve of unloading portion of 2000nm
silver film.
The parameters obtained from the fitting are: P =3.760 f 0.426; m =1.80 1 f. 0.030.
Therefore the function for the unloading curve [4.5] can be expressed as
P = 3.76(h -
[4.13]
For the thinner silver samples (5 1.2nm and 274.5m),the initial parts of the loading
curves are consistent with the above parameters for the 2000nm thick silver sarnple. The
Chapter 4. Resdts and Discussion
loading curves at different applied loads exhibited poorer reproducibility than those for
the germanium sulfide sample, which was probably caused by the large roughness of the
silver film surface. A change in the slope of the loading curves for the thinner silver
films on the silicon substrate is, however, very evident in figure 4.21 and 4.23.
Figure 4.21 Multi load-displacement curves for sample AgISi (1 11) of 274.5 nm film
thickness with different peak loads.
The two thin films were analyzed by Rutherford Backscattering Spectroscopy
(RBS). Based on the density of the silver film of 10.47 gcmJ, the thicknesses are 5 1.2nm
and 274.5nm respectively.
The displacements in the loading curve at which the dope changes occur are smaller
than the thickness measured by RBS: 39 nrn and 230 n m respectively. This discrepancy
may be caused by the different measurement methods.
In RBS measurements, the
Chapter 4. Results and Discussion
average thickness was obtained across the region of measurement which could be as large
as 0.2 mm in diameter. However, using nanoindentation method, the local thickness of
the film was obtained which may be different from one location to another. Another
reason causing the discrepancy might be the thermal diffision of some amount of silver
into the substrate which can be detected by RBS method but not the nanoindentation
met hod.
Figure 4.22 Multi load-displacement cuwes for samples Ag/Si(lll) of different film
thicbess as determined by RBS (1) 51.2 am (2) 274.5 nm (3) 2000 am.
One of the loading curves (274.5n.m)
was chosen to illustrate relationship of the
dope of the loading curve with the displacements as shown in figure 4.23.
A top view of an indentation image by AFM is shown below in figure 4.24.
Chapter 4. Results and Discussion
1
O
50 100 150 200 250 300
Displacement (nm)
~ i ~ u 4.23
r e A plot of loading curve slope vs. indenter displacement for the 274.5nm Ag film
on Si (1 11).
Figure 4.24 AFM image (plane view) of nanoindentation on the 223nm silver film on
Chapter 4. Results and Discussion
Contact Depth (nm)
Figure 4.25
Figure 4.26
Reduced Young's modulus for 39nm silver film on Si (1 1 1).
Reduced Young's modulus for 230 nm silver film on Si (1 11).
Chapter 4. Results and Discussion
Contact Oepth (nm)
Figure 4.27
Reduced Young's modulus for 2000nm silver film on Si (1 11).
The reduced Young's rnoduli of 39 nm, 230 nrn and 2000 nrn germanium sulfide
layers were obtained based on the indenter area function of
In the case of the two thimer films, Young's moduli increase with penetration
depth. For the 2000nm film, a decrease in Young's modulus is observed with penetration
depth. Such behavior is quite similar to the GeS,.,, films and has been discussed in the
previous section.
We have also analyzed our expenmental data to determine
for the silver film
using the same fitting approach as for germanium sulfide film and applying equation
[4.10] to the 230 nm thick silver film.
Chapter 4. Results and Discussion
Figure 4.28 The reciprocal of measured Young's modulus of 230 nm thick Silver film as a
function of th,,,
(t is the film thickness).
The parameters obtained were: 1 / Ef = 0.0 1 2 6 4 ~ ~ a -1/P
' , = 1.26. Therefore EJ
=79.1 GPa, P = 0.79 for Our silver system and the equation [4.1O] becomes
The result of Young's modulus of silver film is 79.132.9 GPa, which is slightly
higher than the literature value of 7 1 GPa for bulk silver.
Chapter 4. Resdts and Discussion
Silver fikns on Si (111)
120
.
-
.-
a
100
b
s2
hl
-a
B
s
a
O
a
80
A
*
t
(16
ge
60
40
a
39nm
20
230nm
A 2000nm
O
O
0.1
0.2
0.3
0.4
0.5
0.6
hmmlt (t thickness)
Figure 4.29 Young's modulus of tbree different silver film thickness vs. hm,Jt (t the film
thickness).
Figure 4.29 is the plot of all three modulus curves versus h,,/t.
As we can see,
when hm& is less than 0.6, the Young's moduli of al1 three different thick silver films
are very consistent, and close to the value of 79 f 3 GPa obtained using equation L4.101.
4.2.3
Silver films on GeS,,
substrate
In order to study the silver photodifision process under illumination, the
nanomechanical performance of silver film on germanium sulfide substrate before
exposure to the UV light should be addressed. To eliminate the influence of the very
Iowest silicon substrate layer, we deposited silver layers of different thickness on the very
Chapter 4. Results and Discussion
thick 2000nm germanium sulfide layer. Three samples with silver thickness of 1O O m ,
250n.m and 5 0 0 m were prepared and the nanoindentation tests were performed.
Figure 4.30 Load versus displacernent curves for the (a) 100, (b) 250 and (c) 500nm thick
silver films deposited onto 2OOOnm thick germanium sulfide substrate.
in al1 the indentation experiments on the silver films of various thicknesses, both
loading and unloading were found to be continuous and there did not appear to be any
interfacial debonding or cracking of the film during this process. Unlike the thin silver or
germanium sulfide layer on the silicon substrate, an obvious change of the slope in the
Ioading curves was not observed. However, the diflerent thicknesses of silver layers c m
be distinguished by comparing the load-displacement c w e s .
Chapter 4. Results and Discussion
The loading c w e s shown in figure 4.30 have been fitted by the equation [4.3], and
the values of n for the 100, 250 and 5 0 h m thick films were found to be 1.88, 1.93 and
Silver film thickness (nm)
-
.
-
Figure 4.31 The values of n in the equation 14.3) versus the thickness of silver film.
The value of n increases with the silver film thickness, exhibiting a nearly straight
line. It is reasonable if we relate the plastic defonnation to the magnitude of n. As we
discussed before, a large plastic deformation leads to a large n. Silver films have been
observed to be less elastic than germanium sulfide film. Therefore the thicker silver
films should have a larger n. From figure 4.3 1, the thickness of a silver overlayer can be
estimated approximately.
If we compare three unloading curves, we can see that from the maximum load point
to the minimum load point, curve (a) goes back 150 nrn roughly, curve (b) a little bit
Chapter 4. Results and Discussion
more than 100 nrn and curve (c) only around 80 nrn. S o curve (a) for the thimest silver
overlayer is the most elastic and (c) for the thickest silver overlayer is the most plastic.
This is consistent with the results discussed before: the silver layer is less elastic than
germanium sulfide layer. The more silver in the sarnple, the less elastic the unloading
curve.
The reduced Young's rnoduli have been determined and shown in figure 4.32.
a&
--
nm
-
~ ~ 2o0 0nGeS
--
- -
A 250 nm Ag on 2000 G&
-
-
é 0 0 nm
on 2000 0.5
Figure 4.32 Young's moduli of three different silver film thickness on 2000nm germanium
sulfide substrate.
The Young's moduli of samples with three different silver thicknesses certainly
appear quite different. The thickest one has the largest moduIus and the thinnest one has
the smallest modulus. The modulus for a11 sarnples starts fiom a higher value and then
Chapter 4. Results and Discussion
- -
-
-
decreases to a plateau. The modulus of 500 nm thick silver film varies fiom around 100
to 40 GPa and remains at 40 GPa afier the maximum penetration depth is larger than
500nm; as for 250 nrn thick silver film, the starting point is approximately 70GPa and
also ends up at 40GPa; the final modulus value of 100 nrn silver film is lower than other
two thicker films, varying between 40 to 30 GPa. The data can be nomalized for film
thickness by replotting figure 4.31 using E* versus h,,/t.
Here, we limit the scale of the
x-axis h m O to 1, which means the penetration depth is less than the film thickness and
the indenter has not reached the germanium suifide substrate.
--. - --
-
500 nm Ag on 2000 GeS
-
Figure 4.33
-
-
-
/ 250m
- -
.-
-
1100 nm Ag on 2000 GeS
--
---
Measured Young's modulus as a function of relative penetration b&t (t the
thickness of silver overlayer).
Figure 4.33 shows the variation of measured Young's moduli with h-/t.
interesting to note that for h,dt
thickness are consistent.
it is
of up to 1, Young's moduli of three films with different
Chapter 4. Results and Discussion
Figure 434 The reciprocal of measured Young's modulus of 500nm thick silver film on
2000nm Ces,., substrate as a function of Ubwx(t is the silver film thickness).
Figure 435 The reciprocal of measured Young's modulus of 2SOnm thick silver film on
2000nm GeS,, substrate as a function of tm_ (t is the silver film thickness).
Chapter 4. Results and Discussion
Figure 4.36 The reciprocal of measured Young's modulus of 100nm thick silver tilm on
2OOOnm Ces,., substrate as a function of tRmx(t is the silver film thickness).
Fits of equation [4.9] were canied out three samples with different silver film
thickness.
Es W P a )
Efm ' a .
P
25.6
53.5
1.16
32.9
86.9
0.4 1
lOOnm silver film
on 2OOOnm GeS,,
m o m sitver film
on 2000nm GeS,.,
500nms
on 2OOOnm Ces,,
~
35.3
93.2
0.29
Table 4.1 The fitting parameters for three different thick silver films on germanium sulfide
substrate.
Chapter 4. Results and Discussion
ï'here are three parameters in this equation: Efi Es and
fl . For this system, the
germanium sulfide film is the substrate and the silver film is the overlayer.
The
parameters obtained on al1 three samples were listed in table 4.1. Here we may compare
the values with some results obtained fiom indentations in silver or germanium sulfide
film on silicon substrate. n i e Young's modulus of germanium sulfide film was derived
previously as 30.3GPa and silver film 79.1GPa respectively. In table 4.1, the Young's
modulus of germanium sulfide film varies fiom 25.6 to 35.3GPa as the film thickness
increases and that of silver film fiom 53.5 to 93.2GPa. For 250nm and 500nrn thick
films, the moduli of silver films are 86.9 and 93.2GPa that are closer to the value of
79.1GPa than that of thinner lOOnm film. As discussed before, both the thermal and
mechanical drift have more effect at srnaIl penetration depth and so it again appears on
thin lOOnm silver film modulus measurement. The modulus of germanium sulfide film
increases as the thickness of silver film increases but is still reasonably consistent with
the value of 30.3 derived using Doerner and Nix empirical relation.
Although the
substrate was held at room temperature when doing the film deposition, thermal difision
can still occur at the interface of silver and germanium sulfide since the fiee silver atoms
were fiom fbsed rnetal and very hot. So one possible reason for the discrepancy of the
results is the formation of such a reaction product. Another reason is the experimental
enor that is inevitable in such small scale. As for the value of O
, , we might expect that P
does not change for the same Ag/GeSi.68 system; however, it decreases h m 1.16 to 0.29
as the silver film thickness increases.
The thicknesses of silver films here were not
accurately measured but estimated using the thickness monitor during the deposition.
Further work needs to be done in the future on the thickness measurement.
Chapter 4. Resdts and Discussion
4.2.4
Photochernical oxidatioo of Ces,.,
film
.Upon exposure to UV illumination in the presence of oxygen, a number of changes
were observed in the surface morphology of the germanium sulfide film in the previous
work. Chen and Horton 15'1 demonstrated using the combination of AFM and nuclear
reaction analysis (NRA) that when exposed to UV radiation the films undergo a photoinduced oxidation process, characterized by slow initiation at defect sites followed by
rapid oxide growth and subsequent control by adsorption limited kinetics.
Nanoindentation tests were camed out on the 2000 nrn germanium sulfide film afier
24 hours UV illumination and the changes in mechanical performance can be o b s e ~ e din
the load-displacement curve.
Figure 4.37 Load versos displacement curves for the 2000nm tbick germanium sulfide
films (a) prior to W illumination @) after 24 hours W illumination in air.
Chapter 4. R e s d b and Discussion
-
The loading curve for the GeS
and the parameter
,.
film after 24 hours illumination in air was fitted
, in the power law equation [4.3] becomes 1.92.
This value is larger
than that of as-deposited films and so the film after photochemical oxidation is l e s
elastic. This phenornenon can also be seen by comparing the unloading portion of two
curves. The measured Young's Moduli of both films are shown in figure 4.38.
--
-.
-
--
After illumination . ~ e f o r e iilusnation -
O
100
200
300
400
500
600
700
Contact Oepth (nm)
-
Figure 4.38
-
-
-
Reduced Young's modulus for 2OOOnm germanium sulfide film on Si (1 11)
before and after photocbemical oxidation.
AAer 24 hours exposure to UV light in air, the Young's modulus of the film
increased, ranging fiom 50 to 30GPa at contact depth from O to around 300nm. This
result indicates that oxidation occurred on the surface of the film and has a significant
Chapter 4. Results and Discussion
effect on the modulus down to an indent depth of 300nm. Further reaction is either
limited by the diffùsion of oxygen or the power of the illumination source.
Chapter 5
Conclusions
Chapter 5
Conclusions
5.1 Summary
The application of the nanoindentation technique on the study of silver and
germanium sulfide thin films has been explored, with the goal of applying this technique
to the study of solid-state difision reactions and thin film deposition. In this thesis a
num ber of resu lts are descnbed that dernonstrate progress tow ards achieving this goal :
(1) The roughness of the film surface is controlled by the deposition rate and the
best deposition conditions have been determined. The best deposition rate for silver films
is 5A/s and l&s for germanium sulfide films. The roughnesses under such deposition
conditions are 0.47 1 and 0.73 1nm for GeS
,
-68
and Ag films respectively.
(2) It has also been possible to observe the effects of the substrate on the mechanical
performance of film overlayers. In particular, a sudden slope change was obtained in the
loading curve during the indentation of a silver or germanium sulfide overlayer directly
on Si (1 11) substrate. The displacements in the slope change were found to be consistent
with the film thickness.
(3) An empirical relation proposed by Doerner and Nix was used to extract Young's
modulus of film overlayers for our samples has been applied. The Young's moduli were
derived as 30.3 and 79.1GPa for germanium sulfide and silver film, respectively. We
found that the silver film is less elastic than the germanium sulfide film by cornparhg the
unloading curves and the parameter
elasticity of the material.
in fitting the loading curves since
reflects the
Chapter 5
Conclusions
(4) The mechanical performance of silver overlayers of different thicknesses on a
thick 2000 nrn germanium sulfide substrate was investigated.
The elasticity of the
sarnple decreases with the increase of the silver film thickness. The silver film thickness
can be estimated from the parameter
in the fitting equation for the loading c w e . The
Young's modulus of silver film obtained fiom this system is somewhat different fiom the
value obtained in Ag/Si (1 11) system. This may be explained as some combination of
product formation due to interfacial thermal reaction dunng deposition and possible
experimental error.
(5) Preliminary results were obtained on the germanium sulfide films exposure to
UV illumination in air for 24 hours. The film becarne less elastic after the photochernical
oxidation and the Young's modulus larger than for as-deposited film.
5.2 Future work
In the area of experimental work, silver photodiffusion in amorphous germanium
sulfide has not been carried out. This photodiffision process depends on a number of
factors and each of them can have a significant effect on the final mechanical properties
of the sample.
With reference to the present method being used, there are some aspects that could
be fùrther improved. The fitting equation proposed by Doemer and Nix is not an ideal
one, particularly using the step-by-step optimization with minimizing the standard error.
Althougb the indenter was carefùlly calibrated, the area fûnction couldn't be exactly the
same for the same tip in different matenal (standard sarnple and our real sarnple). Further
Chapter 5
Conclusions
calibration of the projected contact area should be investigated especially when the
piling-up phenornenon occurs.
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