computational studies of stress and structure development

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Computational studies of stress and structure development resulting from the coalescence of metallic islands by Andrew Rikio Takahashi B.S. Materials Science and Engineering Massachusetts Institute of Technology, 1999 SUBMITTED TO THE DEPARTMENT OF MATERIALS SCIENCE AND ENGINEERING IN PARTIAL FULFILLMENT OF THE REQUIREMENTS FOR THE DEGREE OF MASTER OF SCIENCE IN MATERIALS SCIENCE AND ENGINEERING AT THE MASSACHUSETTS INSTITUTE OF TECHNOLOGY SEPTEMBER 2007 COPYRIGHT 2007 MASSACHUSETTS INSTITUTE OF TECHNOLOGY. ALL RIGHTS RESERVED Signature of Author .......................... :...-...;.... .. ................... Department of Materials Science and Engineering August 3rd, 2007 Certified by ............................ ....... ...- . . ............ Carl V. Thompson Stavros Salapatas Professor of Materials Science and Engineering Thesis Supervisor /7 Accepted by.............. S.................. ........ ........... ............. Samuel M. Allen POSCO Professor of Physical Metallurgy Chair, Departmental Committee on Graduate Students ARCHIVES ASAOCHUSETTS IN8TflhtE OF TECHNOLOGY SEP 2 4 2007 LIBRARIES

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Page 1: Computational studies of stress and structure development

Computational studies of stress andstructure development resulting from

the coalescence of metallic islandsby

Andrew Rikio Takahashi

B.S. Materials Science and Engineering

Massachusetts Institute of Technology, 1999

SUBMITTED TO THE DEPARTMENT OF MATERIALS SCIENCE AND ENGINEERING IN PARTIALFULFILLMENT OF THE REQUIREMENTS FOR THE DEGREE OF

MASTER OF SCIENCE IN MATERIALS SCIENCE AND ENGINEERING AT THE MASSACHUSETTSINSTITUTE OF TECHNOLOGY

SEPTEMBER 2007

COPYRIGHT 2007 MASSACHUSETTS INSTITUTE OF TECHNOLOGY. ALL RIGHTS RESERVED

Signature of Author .......................... :...-...;.... .. ...................Department of Materials Science and Engineering

August 3rd, 2007

Certified by ............................ ....... ...- . . ............Carl V. Thompson

Stavros Salapatas Professor of Materials Science and EngineeringThesis Supervisor

/7

Accepted by.............. S.................. ........ ........... .............Samuel M. Allen

POSCO Professor of Physical MetallurgyChair, Departmental Committee on Graduate Students

ARCHIVES

ASAOCHUSETTS IN8TflhtEOF TECHNOLOGY

SEP 2 4 2007

LIBRARIES

Page 2: Computational studies of stress and structure development

Abstract

Thin film component properties are critical design elements in almost all industries. These filmsare particularly important in the performance of micro- and nano-electromechanical systems(MEMS and NEMS). Residual stress in thin film components is often treated as an unavoidableside effect of processing steps and the degree of residual stress can drastically affect theperformance and properties of the final product. While high levels of residual stress are oftendetrimental to performance, control of the stress and stress gradients can also be used toenhance performance and even generate new capabilities.

The work presented in this thesis examines the role of island coalescence in the development ofstructure and stress in thin films. The primary methods of investigation are molecular dynamics(MD) and finite element analysis (FEA). The semi-empirical MD calculations show thatcoalescence is a very rapid process for unconstrained spheres and for hemispheres allowed toslide on a frictionless substrate. Particle rotations are commonly observed during thecoalescence calculations. The extent of neck formation between 2 particles is consistent withcontinuum models even down to length scales which would normally be outside the range inwhich the models might be expected to be applicable. The MD calculations also show thatinternal island defects may be induced by the island coalescence process, but only for aparticular range of island sizes. We present an energetic model for the existence of such a sizerange and have located experimental evidence in the literature for such defects. Our FEA workextends an earlier study on the effects of contact angle on island coalescence. Our FEA study ofislands with greater than 90 degree contact angle coalescence shows that neck formation occursvery similarly to the free sphere coalescence case.

We conclude that MD and FEA calculations are useful tools in analyzing the island coalescenceprocess and can provide mechanistic insight beyond what is available from the more generalcontinuum models.

Page 3: Computational studies of stress and structure development

Table of ContentsA bstract .......................................................................................... ..................................... 2Table of Figures ..................................................................................................................... . . ...... .. 51 Acknow ledgm ents ........................................................ ............................................... 62 M otivatio n ............................................................................... ........................................... 8

2.1 Applications of metallic thin films................................................. ......................... 82.2 Stress engineering .................................................................................... ................... 102.3 Reliability and stress................................................. ................................................ 112.4 Fundamentals of film formation .................................................................................... 13

3 Backgro und ......................................................................................... ............................. 153.1 Stresses in thin film s....................................................... .......................................... 15

3.1.1 Therm al stress .................................................................... .............................. 153.1.2 G row th stress ......................................................................................................... 17

3.2 Particle sintering and coalescence .................................................... 193.2.1 Continuum-based modeling ............................................................................. 203.2.2 Com putational studies .................................................................... ................. 26

3.3 Mechanical deformation of islands .......................................................................... 273.4 Stress relief mechanisms of supported atomic clusters ....................................... 27

4 -M etho ds ........................................................................................... ................................ 324.1 Molecular dynamics .................................................. 32

4.1.1 Embedded atom method ........................................................... ................. 324.1.2 Predictor-corrector iterators............................................................................... 344.1.3 Lennard-Jones potential ....................................................................................... 344.1.4 Thermodynamic averages .......................................................................... 354.1.5 Metrics for local atomic environments ........................................ ......... 36

4.2 Finite elem ent m ethod..................................................... ........................................ 375 Calculations of time required for coalescence.......................... ................ 39

5.1 Initial structure setup ...................................................... ......................................... 395.2 Structure equilibration ................................................................................................... 395.3 Structure placem ent ........................................................................... ..................... 405.4 Evolution over tim e ............................................................................ ...................... 40

6 Calculations of length scales during coalescence ............................................................ 426.1 Analysis of neck form ation .......................................................................................... 42

6.1.1 Binning algorithm for neck size determination ................................................... 426.1.2 Computed neck size via MD ............................................................................. 43

7 Defects induced by boundary formation ..................................... ........ ....... 447.1 Internal structure of isolated clusters............................................ ................. 447.2 Internal structure of islands on substrates ......................................... ...... 447.3 Structural analysis of coalesced structures................ ...... ....... 45

7.3.1 Types of stacking faults .................................................................... ................ 457.3.2 Size window for stacking fault formation ........................................ .... 467.3.3 Model for the size dependence of defect formation............................. ... 47

8 FEA of coalescence with a greater than 90 degree contact angle ..................................... 539 Conclusions ................................................................................................... . ... ............. 5910 Future w ork ......................................................................................................... .... ............ 61

Page 4: Computational studies of stress and structure development

11 Bibliography ...................................................................................... ................................ 63

Page 5: Computational studies of stress and structure development

Table of Figures

Figure 2.1: The structures shown in this micrograph are self-assembled interconnect arrayprobes generated using the StressedMetal process from Xerox-PARC. Image taken from theXerox-PA RC w ebsite........................................................................................... ................................... 11Figure 3.1: Typical behavior of the stress thickness product during the deposition of a metal onan amorphous substrate. This particular case is Au on silicon nitride at room temperature and adeposition rate of 1 A/s ................................................................................................................................... 17Figure 3.2: Schematic drawing of coalescence for various contact angles .............................................. 19Figure 3.3: Neck growth behavior predicted by Cabrera for a 1 nm sphere of material withdifferent values of B ................................................................................................................. . . .... 21Figure 3.4: Bulk diffusion enhancement from reduction in particle size for Au ......................................... 30Figure 5.1: Evolution of center of mass distances over time. The coalescence process iscom plete in a m anner of nanoseconds....................................................... .......................................... 40Figure 6.1: Comparison of the atomistic calculation data with continuum models. The diamondsdenote the atomic calculation data and the lines denote existing continuum models. The 2-DFreund and Chason model and the Seel FEM model produce very similar results for the zippingheight ....................................................................................................................................................................... 43Figure 7.1: Representation of a coalescing cluster structure. The lighter atoms denote locallyhcp atom s w hich com prise a stacking fault. ........................................................................ .................. 45Figure 7.2: Histogram showing the number of calculations which included stacking faults for thedifferent sizes. These calculations were tabulated for the free space coalescence case............... 46Figure 7.3: Illustration of the unfaulted and faulted sphere configurations .............................................. 47Figure 7.4: Projection of the fault plane in the sphere showing the relevant distances used in themodel. ........................................................................................................................................................................ 48Figure 7.5: Critical stress for generation of an induced stacking fault for Ag. The predictedcoalescence stresses for the most applicable continuum models are also shown. The insetshows the small thickness strengthening effect ..................................................................................... . 51Figure 8.1: FEA mesh used to analyze the strains required for island zipping. ..................................... 54Figure 8.2: Strain energy mapping using up and down zipping distances as parameters ....................... 55Figure 8.3: Contour plot showing the strain energy required to zip in each direction for a contactangle of 110 degrees. ................................................................................................................................... 57Figure 8.4: Plot of the energy minimizing zipping distances for a 5 nm island with a contact angleof 110 degrees................................................................ .......................................... ............. ..... 58Figure 8.5: Zipping stress as a function of contact angle ................................................................. 59

Page 6: Computational studies of stress and structure development

1. Acknowledgments

First of all, I am deeply indebted to the Singapore-MIT Alliance and to the NSF for financial

support of this project over the years.

I would also like to express my appreciation to the entire Materials Science and Engineering

Department at MIT. Nearly everything I know about science and engineering was drilled into me

during my undergraduate years. Everything else, I learned during my year at Stanford and in the

subsequent years in graduate education at MIT.

Upon my return to MIT, I was very fortunate to find immediate challenges that enabled me to

become engaged with the work. For computational assistance for the early portion of the work,

I am particularly grateful to Prof. Carter and Prof. Yip. Of course, I owe a huge debt of gratitude

to my advisor, Prof. Carl Thompson, who gave me the freedom to pursue the challenges posed

by the project in my own way.

For both technical and moral support, I would not have progressed very far without the

assistance of the post-docs and fellow graduate students in the research group. From the old

guard, Mauro Kobrinsky, Steve Seel, Rob Bernstein, and Cody Friesen provided a tremendous

resource of information regarding thin film stress. From the 'middle' guard, I drew a great deal

of motivation from the discussions I had with Zung-sun Choi, Frank Wei, Amanda Giermann, and

Reiner Moenig. When I started to hit the doldrums in the latter part of the project, the new

students arriving in the group reenergized my interest. I feel particularly fortunate to have

gotten to know Steve Boles and Jeff Leib, both through scientific discussion and Wii parties.

My proudest non-academic achievement at the Institute was the foundation of the MIT

Micropoker Club. Even during my hardest weeks at the Institute, the club always provided a

Page 7: Computational studies of stress and structure development

stress relief valve for me and I expect the friendships formed during the club 'meetings' will last

me the rest of my lifetime.

For the rare occasion when the Micropoker Club counselors were not sufficient in reducing my

stress level, I found the trained professionals at MIT Mental Health to be quite capable. I am

particularly grateful to Dr. Hsi for getting me through a rough patch right before my wedding.

Although I did not really appreciate it much until the very end, my family's support is the main

reason I have made it this far and is also a huge influence in my life moving forward. My core

values are based largely on my parents' teachings and much of my development as a person

comes from chasing after my brother's accomplishments and exploits.

Lastly and most importantly, I want to thank my wife, Melody, who has been a dominant

influence in my life for nearly 10 years now. While my return to MIT from Stanford was driven

mostly by the desire for new academic challenges, my desire to share geography with my closest

friend was probably just as important in my decision to return to Cambridge. Her love and

support have kept me going even during my darkest moments. Now that we no longer need to

be separated, I am very much looking forward to joining her and starting our new life together

in North Carolina.

Page 8: Computational studies of stress and structure development

2 Motivation

The primary motivation for this project is the universal need for metallic thin film components

and the ability to tailor the performance of those components through processing conditions.

While metallic thin film components are typically employed as passive devices, these thin films

are essential to the operation of critical active devices such as the transistors in

microprocessors. Even in microprocessors, the limiting factor in circuit performance is now the

RC delay in the thin film interconnects. In mechanical applications, the performance

characteristics of thin film components are heavily dependent on the processing conditions of

the thin film, while in electrical applications the reliability characteristics are more affected.

Typically, the important thin film properties altered by processing are the microstructure and

internal stress. The ability to engineer the internal stress through control of processing

parameters has lead to the development of novel devices and has the potential to introduce

technological breakthroughs while using standard thin film deposition equipment. This project

is aimed at understanding the fundamentals of thin film formation through computation in

order to enable thin film engineers to more effectively control stress and structure in the pursuit

of new technology.

2.1 Applications of metallic thin films

Thin metallic films are an essential component for a wide variety of products. From applications

such as reflective coatings in food packaging up to performance critical interconnects in

microprocessors, ultrathin metal films are ubiquitous. In many cases, thin films have properties

which are fundamentally different from their bulk counterparts. In other cases, thin films are

used simply because the geometry of the application precludes the use of bulk material. Thin

Page 9: Computational studies of stress and structure development

films are also used in many applications where the material required is prohibitively expensive

so economics drives product designers to use the minimum amount of material required.

One of the major driving industries for the commercialization of thin films is the computer

microprocessor market. Ever since the first integrated circuit was developed to replace vacuum

tube processors, the industry has been driven to high density, low cost devices. With over 300

million transistors and miles of connecting wire on a typical chip, planar thin film processing is

nearly synonymous with semiconductor microfabrication (1).

In a similar vein, magnetic storage companies are constantly trying to improve the bit density on

hard drive platters or find an alternative technology which provides improved storage capacity

(2). Careful engineering of the grain size distribution has allowed for the creation of magnetic

media with densities near the super-paramagnetic limit.

The media used for optical data recording is also comprised of thin films. While the phase

change materials are chacogenide glasses, new schemes for reading and writing information to

the media involve nanoheaters manufactured from purely metallic Pt thin films (3).

Another interesting application for metallic thin films is their use as thermal barrier coatings. As

an example, silver-based multilayers are often used to coat large windows which are in direct

sunlight. The optical properties of the thin silver films allow optical frequencies of light to pass

through the window while reflecting away infrared frequencies which would lead to undesirable

heating of the building interior (4). Earlier work highlights the different requirements for

energy-saving windows in both warm and cold climates and outlines the theory for generating

selectively reflective windows (5).

Page 10: Computational studies of stress and structure development

Typically, hard coatings for tools are fashioned out of ceramics but these brittle thin films are

often in danger of cracking. In order to improve the crack resistance, the hard ceramic coatings

are often layered with ductile metals. Inelastic yielding in the metal thin films increases the

fracture toughness of the hard coating without having noticeable deleterious effects on the

hardness (6).

On the more exotic side of applications, much research has been devoted to the use of metal

particles as plasmon waveguides. One route for generating these metal particles would be from

the dewetting of a thin film. The potential for generating ordered arrays of particles could have

implications for energy storage and transfer (7)(8).

2.2 Stress engineering

The properties of thin films depend significantly on the manner in which those films are

processed. One of the most critical characteristics of a thin film is the amount of internal stress.

Different processing conditions can lead to different levels of stress within the film. A deeper

understanding of how this stress develops allows process engineers to tailor thin film structures

by manipulating the internal stress.

Researchers at Xerox have managed to form robust interconnect systems and on chip inductors

by engineering stress gradients (9). Adjustments of the sputtering power can lead to

development of large stress gradients through the thickness of the film. The binding of the film

to the substrate restricts the amount of curvature that the film can experience. However, if the

film is partially released from the substrate via a chemical etch, the resulting curvatures can be

quite large. Figure 2.1 shows an example of a structure generated using the above mentioned

chemical release process.

Page 11: Computational studies of stress and structure development

The intrinsic stress level of a metal thin film can also be controlled via thermal processing.

Different annealing temperatures can lead to precipitate formation which will alter the level of

stress within a thin film. The thermal mismatch between film and substrate can also lead to

inelastic deformation yielding of the film and repeated thermal cycles can be used to set the

appropriate level of stress within a film.

The reliability of thin film components depends strongly on the level of internal stress within the

film. Component failure can be the result of standard fatigue processes in the case of dynamic

loading. In the case of static loading, electromigration or plastic yielding can also lead to

component failure. On the more optimistic side, tailoring of the internal stress can also result in

beneficial effects on the component reliability.

Page 12: Computational studies of stress and structure development

For dynamically loaded thin films, accumulated fatigue damage occurs as the result of surface

cracks, voids and extrusions (10). This behavior differs from the bulk behavior since the thin film

geometry inhibits the formation of dislocations and persistent slip bands. For thin film fatigue,

the formation of free surface and grain boundary defects is more energetically favorable than

the generation of dislocation cells. The reason for the change in mechanism is that the thin film

geometry alters the localized stress state.

For microprocessor interconnects subjected to DC electrical loads, component failure occurs due

to electromigration. The impact of electromigration voiding in interconnects can be greatly

amplified due to the thermal stress voiding which occurs during interconnect processing (11).

The existence of stress gradients within a line is an important factor in determining whether the

critical stress required for void nucleation will be reached or if the steady state will result in an

immortal line(12)(13).

One of the most dramatic demonstrations of reliability enhancement due to stress engineering

is the Prince Rupert's drop experiment. The Prince Rupert drop is essentially a thermally

engineered glass bead. The thermal processing of a Rupert's drop results in a highly

compressive stress near the surface of the drop-head. Internally, the drop is under extreme

tension. The compressive nature of the drop head makes the glass highly resistant to impact

damage, but releasing the tensile stress by cracking the tail of the drop results in explosive

decomposition of the glass(14). While the behavior will likely be quite different in ductile

materials, a metallic analog of the Prince Rupert's drop could be quite useful in enhancing the

reliability of thin film coatings.

Page 13: Computational studies of stress and structure development

2,A Fundamentals of film formation

The formation of thin films on substrates depends strongly on the surface energy of the material

being deposited and of the substrate. In cases where full wetting occurs, growth occurs by a

layer by layer process and is known as the Frank-van der Merwe growth mode. For the case of

partially wetting systems, three dimensional islands are formed on the substrate and full

continuity is achieved after island impingement occurs. This type of growth mode is called

Volmer-Weber. A third growth mode occurs for systems which are initially wetting but

transition to island growth. This type of growth mode is known as Stranski-Krastanow

(15)(16)(17).

The deposition of metals on amorphous substrates typically occurs via the Volmer-Weber

growth mode. Well documented cases of this growth mode are seen for low magnitude

cohesive energy metals such as Au, Ag, Cu, Al, Ni, Pt, Fe, Pd, Co, and Pb on higher magnitude

cohesive energy solids such as ionic crystals and stable oxides and nitrides (15).

Deposition via electron beam evaporation is well described by the Langmuir equation. The flux

of material out of the source is given by

N PO

AT 27rmkBT

Here N is the number of arriving atoms, A is an area, i is a time interval. PO is the equilibrium

vapor pressure at the source temperature, m is the atomic mass, kB is Boltzmann's constant and

T is the source temperature. The arrival flux on the substrate depends on both the source to

substrate distance and the source temperature. In practice, the arrival flux is most commonly

Page 14: Computational studies of stress and structure development

altered through changes in the source temperature. In most e-beam systems, the input power

of the electron beam is easier to adjust than the sample to source distance.

The theory behind the nucleation behavior of Volmer-Weber thin films has been developed in

some detail with a rate equation approach (18). Recent modifications of the rate equation

approach have allowed for the treatment of surfaces which have defect sites or mobile clusters

(19). Still, film formation via Volmer-Weber growth is a complex process and the island

structures are often quite complex. In-situ TEM experiments of metals on ionic crystals have

been performed by Pashley and co-workers (20)(21)(22). The observed films started as isotropic

nuclei with equiaxed lateral dimensions, but continued growth quickly led to anisotropic island

shapes due to island coalescence and the formation of channels between the larger islands.

These channels can exist for much longer than would be expected from isotropic growth

conditions. Eventually, the channels fill and full continuity is achieved.

Page 15: Computational studies of stress and structure development

3 Background

This computational effort deals mainly with the stress and structural changes induced during

island coalescence. This chapter will start with a brief review of the general stress sources and

characteristics of high atomic mobility metallic thin films on amorphous substrates. This

overview will be followed by a more detailed description of the stress and structural models

which are particular to the island coalescence process. We will also review computational

efforts similar to ours in order to highlight the extensions and new developments that the

current work provides. Towards the end of the chapter, we discuss the current understanding

of mechanical deformation of nanoscale islands and some of the stress relief mechanisms which

are also expected to be active during island coalescence.

3.1 Stresses in thin films

3.1. Thermal strless

One of the major sources of stress in thin films is the stress generated by differences in the

linear coefficient of thermal expansion. A dissimilarity between the thin film and the thick

substrate leads to an additional stress if the system is allowed to change temperature.

Conventionally, we assume that the thin film accommodates the thermal strain in the substrate

layer.

athermal = AaAT

Here, A a is the difference between the expansion coefficients and A T is the temperature

change from the initially strain free deposition condition. Typically the thermal expansion

coefficient for the metal is 3-4 times the value for the amorphous insulator. If the substrate is

Page 16: Computational studies of stress and structure development

composed of multiple layers such as nitrided or oxidized silicon, the thermal expansion will be

largely dictated by the thicker component. Some representative values of thermal expansion

coefficients are given in Table 1.

Table 1.: Thermal coefficient of expansion for selected materials. Values for metals are obtained fromWebElements, Values for compounds and glasses are obtained from the ASM Engineered Materials Handbook,

Material a (10 - 6 K- 1 )

Ag 18.9

Au 14.2

Cu 16.5

Si 2.6

AI203 8.0

SiO2 8.6

Si3N4 2.9

Borosilicate glass 3.5

Almost any type of manufacturing process will involve large magnitude thermal cycles. This

statement is particularly true for the microelectronics industry where high temperature anneals

to control dopant concentration are often required. These thermal cycles must be accounted

for in design processes and the effects on the internal structure and performance of the film can

be dramatic.

Consideration must also be given to the operating conditions of the thin film. For applications

such as laser-based actuators and fast switching MEMS, the local temperature of the film during

Page 17: Computational studies of stress and structure development

operation conditions can cause large thermal stresses. If these stresses are not accounted for

appropriately, positioning and deflection errors can occur.

3.,2 Growth stress

The development of intrinsic stress within a growing film is a complex process. The general

behavior is shown below in Figure 3.1.

·z

CO

F-~ZCOCo~

C)

CoL~

-1'0 100 200 300 400

Thickness (Angstrom)

Figure 3.1: Typical behavior of the stress thickness product during the deposition of a metal on an amorphoussubstrate, This particular case is Au on silicon nitride at room temperature and a deposition rate of I A/s.

500

Page 18: Computational studies of stress and structure development

3.1.2.1 P recoalescence stress

In the earliest stages of film formation, many researchers have noted a precoalescence

compressive stress for fcc metals on amorphous substrates. Most notably, Au and Al on SiO2

show a clear compressive dip at the beginning of deposition. This dip is absent in observations

of Ag on SiO2. The mechanism most often referred to for the precoalescence stress phenomena

is a capillary pressure argument developed by Cammarata et al (23). Other explanations have

included the effects of step density and the force dipole induced by adatoms on the substrate

(24). Our studies of Au on nitride and Ag on nitride have not shown this behavior to be

reproducible and as indicated in the figure above, we often find the initial transient to be tensile

rather than compressive.

3.:.2.2 Stress arising during coalescence

Previous studies have shown that the tensile rise during film deposition is due to island

coalescence (25). The existing models for stress generation due to island coalescence will be

discussed in detail in the next section.

To some extent tensile behavior during the period of island impingement and coalescence is

also expected due to grain growth and densification. Any process which leads to densification of

the thin film will also result in tensile strain.

3.1.2.3 Sre.ss behavior durinq continued deposition

After a high mobility metal thin film reaches the point of continuity, the growth stress becomes

more compressive. The source of this later stage compressive stress is still under debate.

Possible explanations for the asymptotic compressive stress have included the incorporation of

excess adatoms at surface ledges (26), the compressive nature of adatom force-dipoles(27), and

Page 19: Computational studies of stress and structure development

an excess atomic concentration within grain boundaries (28). However, recent atomistic

calculations show that the adatom concentrations necessary for the force-dipole model are

unlikely to be achieved under normal deposition conditions (29).

3.2 Particle sintering and coalescence

Island coalescence has a close analog in the ceramics sintering literature. In its own right, the

behavior of islands coalescing on a substrate has been considered within a purely mechanical

framework as well.

Figure 3.2: Schematic drawing of coalescence for various contact angles.

Figure 3.2 is a schematic representation of how island coalescence is typically modeled. The

first case shows coalescence of islands with 90 degree contact angles. This case is the most

common case used in models since the equations describing a semi-circular or hemispherical

island have convenient mathematical forms. However, the strain field arising from the point of

Zf--

Page 20: Computational studies of stress and structure development

impingement may be heavily dependent on the local geometry near the contact. The second

case in the illustration shows coalescence between islands with less than 90 degree contact

angles. This type of coalescence is expected for materials which have a high degree of

wettability on the substrate. The third case in the illustration shows the coalescence for islands

with a greater than 90 degree contact angle. The greater than 90 degree contact angle case is

expected when the deposited material wets the substrate poorly. Most high mobility metals on

amorphous substrates will fall into the third case.

3.2.1 Continuuim-based modeling

The behavior of spherical particles in contact at a single point has been analyzed by many

researchers in an attempt to understand sintering. Kuczynski found that the neck radius

between spherical particles could be fit to a power law in the early regime where the neck size is

much smaller than the particle radius. Kuczynski and later Cabrera studied the phenomena with

fairly large particles and Cabrera arrived at a slightly different power law than Kuczynski (30).

For sphere-sphere coalescence events, the Cabrera model shows

-- 25Bt.a

2

Here, x is the neck radius while a is the particle radius. t is time and B is the constant defined as

Dsyvf 2

B=kBT

where Ds is the surface diffusivity, y is the surface tension, v is the number of atoms per area,

and 0 is the atomic volume.

Page 21: Computational studies of stress and structure development

The extracted value for B obtained from sintering experiments on Cu is approximately 10-29

cm4 /s (31). Mullins used known chemical data in his earlier paper to arrive at an estimate for B

of 10-18 cm 4/s (32).

0.4

0.3

Ct/-ItC

0.2

0.1

n0 le-09 2e-09

time (s)3e-09 4e-09

Figure 3.3: Neck growth behavior predicted by Cabrera for a I nm sphere of material with different values of B.

While considering more general surface diffusion and surface morphology issues, Mullins

developed a comprehensive mathematical theory of morphological evolution based on the

different mechanisms of surface diffusion, bulk diffusion, grain boundary diffusion, and

evaporation/condensation.

B=le-29 cm4/s

B=le-27 cm 4/s

B=le-24 cm 4/s

- -

1

a

ir. .

*.

. .. . . . . .

m

. -.

dlmo~w l m lW i •~ l

)

Page 22: Computational studies of stress and structure development

Nichols went on to implement the derived theory within a computational framework and using

finite difference methods to solve the morphological diffusion equation. Nichols's numerical

results produced yet another power law for the neck radius between equal sized particles. The

form used by Nichols was

an B B aK- = y -as)at y as asHere n is the surface normal, c s is the differential surface segment and K is the curvature (33).

In the sintering literature, the energetics of particle arrays has been reviewed by Kellett and

Lange (34). In their consideration of the energetics, particles were assumed to retain their

energy minimizing surface curvature during interpenetration. The energetic model does not

accurately capture the transient behavior during neck formation but does provide a reasonable

estimate of the energy minimizing particle configurations.

Stress in the Kellett and Lange model is based on the dihedral angle and size of spherical

particles in a compact. The sintering stress defined in the ceramics literature is not a true stress

but is really an energetic parameter which captures the capillarity driving force for sintering.

The sintered compact is assumed to be stress free since the mechanisms for sintering should all

lead to a reduction in stress. For a linear array of spherical particles, the sintering stress is

Similarly, the limiting shrinkage strain of a linear array of spherical particles is given in terms of

the equilibrium dihedral angle as

1(2cos2() 3

Page 23: Computational studies of stress and structure development

Within the context of thin films, Hoffman was the first to propose that islands coalescence

would lead to tensile film stresses (35). Hoffman's original argument was presented as a one

dimensional model where the total system energy could be reduced by closing the gap between

two islands. In order to close this gap, each island would have to generate a spontaneous

tensile strain and form a grain boundary. For block-like islands, the resulting tensile stress was

given by

EAa'Hoffman -

Here E is the Young's modulus of the island, L is the island size and A a is the size of the gap

between islands. The gap size is conventionally set to the atomic dimension of the element

comprising the island.

Nix and Clemens chose to refine Hoffman's original argument by considering more realistic

island geometries (36). The Nix and Clemens analysis modeled two-dimensional elliptical islands

at the point of impingement. By altering the starting geometry of Hoffman's original model, Nix

and Clemens were able to eliminate the use of the atomic gap parameter. By performing an

energy balance between strain energy and surface energy, the resulting expression for the

zipping height is

zo [36(1 - v)(1 + v) 3 (2ysv - Ygb

b Ea

where z o is the zipping height, b is the y-axis length of the ellipse, v is the Poisson ratio, Ysv is

the island-vapor surface energy, Ygb is the grain boundary energy and a is the x-axis length of

the ellipse. The average stress within the island can be expressed as

Page 24: Computational studies of stress and structure development

[(1 + v) E(2ysv - Ygb)21=--v) a

Or alternatively in terms of the zipping height,

< U > E 1 1 ZO 2 b arcsin Y_1V2 2 b (2zo)

While the Nix and Clemens model(36) captures the essentials of the balance between surface

and strain energy, the energetic criteria used by those authors gives an upper bound on the

zipping height and average stress in the film. Experimental evidence showed that typical metal

films on amorphous substrates have an order of magnitude less stress than predicted by the Nix

and Clemens model.

In order to improve the predictions of the model, Seel et al (37) used FEA and a different energy

minimization criteria to find the equilibrium island configuration in terms of zipping height and

stress. The Seel model excludes the effects of surface and bulk diffusion since on an elastic time

scale under the assumption that these processes should be slow.

In a further extension to the numerical model, Seel and Thompson (38) also considered the

coalescence of islands with a less than 90 degree contact angle. In this case, the elastic strains

for zipping are increased greatly over the 90 degree contact angle case. The critical parameter is

recast as the maximum in-plane strain and the results for any contact angle can be mapped onto

a single curve using this parameter.

In an analytical approach starting from Hertzian contact theory, Freund and Chason examined

the impingement of periodic arrays of islands (39). The consideration of cohesion during the

impingement of islands requires invocation of the Johnson-Kendall-Roberts(JKR) theory (40).

Page 25: Computational studies of stress and structure development

For the impingement of 3 dimensional islands, Freund and Chason found that the stress of

impingement is unrelated to the mechanical properties of the island and only depends on the

island geometry. However, in a realistic coalescence scenario, mixing of 2-D and 3-D

coalescence events will occur and the elastic properties of the solid will again become relevant.

The general expression derived for the stress at impingement is

< > N YSV 2Y gb

Note that y is defined differently here so I should either alter the equation or make a note of the

notation change. R is the particle radius. N refers to the dimensionality of the coalescence

events. c1=1/2, c2=2/3 and c3=1. A1=0.82, A2=0.44, and A3=4. For the Freund and Chason

model, the dimensionless neck size at zero load is constant at 4.46 for 3 dimensional

coalescence and constant at 2.2 for 2 dimensional coalescence.

Rajamani et al developed a model which includes the effects of continuous tensile stress

development during film growth(41). This model may be applicable to metals with low mobility

but does not accurately capture the phenomenology for the high mobility metals such as Ag, Au,

and Cu. The FE model presented is most applicable for faceted semiconductor growth. The

model captures the local stresses at the grain boundary triple junction and in the absence of

stress relaxation and diffusion, Rajamani et al predict that the majority of tensile stress

generation occurs continuously during boundary formation rather than instantaneously as

predicted by the other continuum models. While the instantaneous effect at island

impingement is still present, Rajamani et al claim that the more significant contribution to the

tensile stress development is the continuous advancement of the island faces during growth.

Page 26: Computational studies of stress and structure development

3,2.2 Computational studies

The sintering of copper nanoparticles was previously studied by Zhu and Averback (42)(43).

Their research showed that the particles exhibited extremely high shear forces during

coalescence and that these forces were often large enough to induce defects. They did not

however examine the dependence of defect formation on the size of the particle.

A more comprehensive study of Au particles was undertaken by Lewis et al (44). These

researchers examined the effects of melting, freezing and island coalescence for Au particles.

The study incorporated clusters of different sizes but fewer calculations were performed on the

large clusters. The coalescence portion of the study focused on the differences between liquid-

liquid, liquid-solid, and solid-solid coalescence events. Surprisingly, the solid-solid coalescence

events occurred on timescales which were just as rapid as the liquid-liquid and liquid-solid

coalescence events.

Mazzone's study of island coalescence used the most detailed potential model and many

rigorous metrics for cluster structure (45). However, Mazzone restricted his study to cluster

sizes'in the 50-300 atom range of Ag, Cu, and Pb. For the size ranges studied, Mazzone

concluded that the continuum theories for sintering were inadequate in determining the extent

of neck formation and the timescales for sintering. The simulated clusters formed necks at rates

orders of magnitude faster than would be predicted using a Cabrera model and standard values

for surface diffusivity. Mazzone noted that the failure of continuum sintering theories involving

diffusion suggests that nanoparticle sintering occurs by a different mechanism. Athermal

sintering was not explicitly defined in Mazzone's work, but an athermal sintering mechanism is a

possible alternative to the diffusive mechanisms which were identified as inadequate.

Page 27: Computational studies of stress and structure development

3.3 Mechanical deformation of islands

For bulk materials, the dependence of the yield strength on grain size is well-known. The Hall-

Petch relation is used to justify the dependence on yield stress. The basic argument for the

Hall-Petch effect in the bulk is that the grain boundaries serve as barriers to dislocation motion.

This impedes plastic deformation and results in increased yield strength. As we approach thin

film dimensions, the situation changes. Due to the reduced dimensionality present in a thin film

1 1the scaling law follows a dependence rather than a 1 dependence. Thompson used an

energetic model to explain why the dependence for thin films would differ from that of a bulk

material (46). If we view nanocrystals as an even further reduced dimensionality, we notice that

the dependence of yield strength on grain size changes drastically. In the case of nanocrystalline

materials, the yield strength starts to again decrease as grain size is decreased. This inverse

Hall-Petch effect has been attributed to grain boundary sliding or other deformation processes

not involving the standard dislocation cell approach(47)(48).

3.4 Stress relief mechanisms of supported atomic clusters

In our discussion of the coalescence stress, we shall be referring to the instantaneous stress

which elastic models such as the NC and FC models predict will result from boundary formation.

This elastic stress will be an upper bound on the actual measured stress since several stress

relief mechanisms will occur simultaneously with the boundary formation stress. These

mechanisms may include any combination of surface diffusion, bulk diffusion, grain boundary

creep, island-substrate interfacial creep, viscous and plastic flow, dislocation creep, or other

complex processes.

Page 28: Computational studies of stress and structure development

Surface diffusion is likely to be the dominant relaxation path for high curvature islands. The

governing differential equation for surface diffusion is 4th order. Data on surface diffusion is

scarce and different surface diffusion mechanisms can lead to estimates of surface diffusion

which vary over many orders of magnitude. However, redistribution of the material at the

surface of the clusters does not greatly reduce the bulk stress in the film.

In order for surface diffusion to be an effective stress relief pathway, grain boundary diffusion

must also occur and will possibly be rate limiting. Typically grain boundary diffusion is studied

under the A-,B-, and C-kinetic regimes based on the Fisher model for a grain boundary (49). The

C-kinetic regime refers to the situation where the enhanced diffusion region is completely

limited to the grain boundary width. The C-regime is most commonly observed when the grain

size is much larger than the grain boundary width. The B-kinetic regime occurs when the

enhanced diffusion region extends deeper into the grain but the profile is still identifiable. In

the A-kinetic regime, the grain size is small enough such that the enhanced diffusion regions of

each grain boundary interact and the grain boundary diffusion becomes inseparable from the

bulk and surface diffusive behavior. To our knowledge, all recorded values of grain boundary

diffusivity are in the B- or C-kinetic regimes while we expect the small island sizes at coalescence

to be dominated by the A-kinetic regime.

Normally, atomic diffusion through the bulk of the island would be a relatively slow process.

However, the coupling of bulk diffusion to surface and grain boundary diffusion along with the

high levels of stress in the coalesced islands may lead to atomic creep in the bulk. Bulk diffusion

coefficients are also known to scale with the melting temperature of the solid in question and

the decreasing melting temperature with decreasing particle size could lead to homologous

Page 29: Computational studies of stress and structure development

temperatures high enough to make bulk atomic transport a significant stress relaxation

mechanism. Brown and Ashby (50) have shown that a good estimate for the bulk diffusion

constant in fcc metals is

D - 5.4 x 10- s exp (-18.4 ) in units of m2/s.

Buffat and Borel (51) have shown that the melting temperature as a function of particle radius is

given by

2

Tm = Ti 1 - sysv - 1ý

where T° is the bulk melting point, L is the latent heat, and Ps and pi are the specific gravities of

the solid and liquid phase, respectively. Using material parameters for Au,

Tm = 1336 x (1- 4.24710-10)

where r is input in units of m and melting point is returned in K. Combining the above equations

gives

D(r, T) = 5.4 x 10-5 exp (2452.4 1 - 4.247 ))

The magnitude of the bulk diffusion enhancement for Au is plotted in Figure 3.4.

Page 30: Computational studies of stress and structure development

X-1

2 4 0 I)

radius (nm)

Figure 3.4: Bulk diffusion enhancement from reduction in particle size for Au.

Diffusion enhancement at room temperature begins to have a significant effect for sizes below

4nm. While the average particle size at coalescence is usually larger than 4nm, collisions of

larger islands with smaller islands which may be present due to secondary nucleation may be

affected by enhanced diffusion in the smaller particle. Also, the diffusion enhancement for very

small sizes should be considered for early stage growth and nucleation models as the

assumption of a constant diffusion coefficient will not apply to small nuclei.

Stress relaxation is not limited to the above mentioned diffusive mechanisms. The generation of

1-D and 2-D defects within the coalescing islands can also lead to a reduction of stress.

Localization of the strain field by the formation of dislocations can result in a net reduction of

Page 31: Computational studies of stress and structure development

energy and a reduction of the bulk stress. Movement or dissociation of these dislocations can

further reduce the stress in the islands. If the dislocation moves through the bulk of the island,

the process is similar to dislocation creep in bulk materials. If the dislocation moves along the

island-substrate interface, interfacial slip will occur, which could result in rigid body translations

or rotations of the islands.

Page 32: Computational studies of stress and structure development

4 Methods

4.1 Molecular dynamics

Molecular dynamics refers to the computational method of solving the real-space trajectory of

molecular units. In classical MD, the force law that governs the trajectory of the molecules is

simply a matter of applying Newton's second law, F = md. For any reasonably sized system,

the number of molecules involved in the calculation is quite large and computer automated

methods are required to solve the equations of motion. Typically the equations of motion are

solved using iterative methods and standard algorithms from the area of applied mathematics.

The most accurate method of calculating the forces acting on the molecules is to use density

functional theory (DFT). However, DFT calculations are prohibitively time consuming for large

systems.

4.1.1 Embedded atomr method

The embedded atom method (EAM) was developed by Daw, Foiles and Baskes at Sandia(52).

The core principle of the EAM is that the binding energy of an atom is a simple function of the

electron density provided by the surrounding atoms in the matrix. While the physical derivation

of the EAM is unique, the final derived form for the energy functional is not unique. Other

researchers have used alternate physical reasoning to arrive at a potential function which is

identical in practice to the EAM. Among these approaches are the 'glue' potentials of Ercolessi

and the second moment approximation to tight binding potentials provide by Rosato, Guillope,

and Legrand (53).

In the original formulation of EAM

Page 33: Computational studies of stress and structure development

Ec Gi (I pa (rij)) + 2 U1 (rqj)i j*i i,j(j*i)

Here Ec is the cohesive energy, Gi is the embedding energy, p7a is the spherically averaged

electron density around atom j. rij is the distance between atom i and j. Uij is the electrostatic

interaction between atom i and atom j.

The key to effective use of the EAM lies in the selection of the appropriate interaction term Uij

and the exact shape of the embedding functional. Typically, the interaction term requires at

least two adjustable parameters and the embedding functional can use any number of fitting

parameters. It is not uncommon to see EAM calculations which use upwards of 13 parameters

in order to capture the real behavior of a material.

4-..111 RGL potential

This study uses the potential form devised by Rosato, Guillope, and Legrand.

1

Ec=[[- - 2 exp [-2q (r'j- 1)] + A exp [-p ( 1)]]i j

k is the effective hopping integral, q is an effective distance parameter for the attractive portion

of the potential, ro is the atomic radius, A is a parameter describing the strength of the Born-

Mayer repulsion term and p is a parameter related to the distance interaction of the repulsive

term.

In practice this potential has 4 parameters and for the particular case of silver, the parameters

have been fit to the elastic constants and cohesive energy of the bulk solid. The calculated

surface energy using these parameters results in values which are close to the experimentally

observed values.

Page 34: Computational studies of stress and structure development

In order to compare with the form of Equation 3.1, the term UUj has been replaced with the

Born-Mayer repulsive potential. The first term on the RHS of Equation 3.2 represents the

embedding functional.

The key elements in an MD calculation are the equations of motion, the interatomic potential

and the time integrator which will be used to solve the equations of motion(54). Some of the

simplest integrators use numerical derivatives and fixed time steps to advance the equations of

motion. The direct Verlet and Verlet leapfrog algorithms fall into this category. More complex

algorithms which make use of higher-order derivatives have been shown to produce faster

convergence and greater accuracy for certain classes of problems. The Gear predictor-corrector

is a common integrator in MD calculations which takes advantage of higher-order derivative

information(55).

In the MD literature, the algorithm referred to as the Gear predictor-corrector is actually a

special case of a more general predictor-corrector algorithm. The general method of solution is

known as the Adams-Bashforth predictor-corrector but Gear was able to optimize the algorithm

coefficients for solving Newtonian equations of motion. Unfortunately, the optimization of the

algorithm depends on the particular type of motion being analyzed and two sets of Gear

coefficients are commonly used. For atomistic calculations, the value of the second order

coefficient is set to 3/20 instead of 3/16.

As a model for the substrate, the Lennard-Jones potential provides the necessary characteristics.

Initially, the Lennard-Jones potential was found to be an adequate model for the interaction of

Page 35: Computational studies of stress and structure development

gases via Van der Waals forces (56). The form of the potential is physically justified for the

treatment of noble gases and the convenient mathematical form makes it popular with the

study of other systems as well. The general form of the potential is given by

ULJ=- 4 c1 6 U) -

where co is a characteristic distance at which the potential takes on a zero value. The actual

energy minimum occurs at rij = 21/6a o.The U potential is a pairwise potential which is quite

successful in predicting the properties of a non-interacting gas like Argon. For fcc solids, the

potential reproduces the packing correctly but the surface relaxations and mechanical

properties derived from the potential often run contrary to experiment. Nevertheless, this

potential is quite useful due to its mathematical simplicity and is effective as a boundary

condition for atomistic calculations with a rigid wall.

4.L4 Thermodynamic averages

In simple terms, the surface energy is the excess free energy due to the fact that the crystal is

not infinite and terminates at a free boundary. For particular crystal facets, this quantity can be

evaluated computationally. One of the key considerations when performing this calculation is

that the initial slab thickness should be chosen such that the two free surfaces do not interact

with each other. Typically the length of convergence is determined by running the calculation

for various thicknesses until the energy per area becomes constant with some tolerance for

error.

Page 36: Computational studies of stress and structure development

These types of calculations allow for the location of the free surface in the sense of a Gibbs

dividing surface. The Gibbs interface does not necessarily coincide with the physical termination

of the crystal.

The situation becomes far more complex when considering a general surface of the crystal or in

considering a crystal which is not semi-infinite. The analogous quantity for a liquid is the surface

tension, which is measurable both experimentally and computationally. Definition of the

surface energy requires that the total free energy can be partitioned into a surface component

and a bulk component. This type of partitioning is equivalent to saying that the free energy of a

planar defect is localized to a region surrounding the defect and that the characteristic length

scale of the localization is much smaller than the thickness of the bulk region.

While the true energy that drives any physical process is the free energy, we can gain some

insight into the energetic changes that occur in the system by studying the total internal energy.

Strain is an inherently continuum concept but analogous measures have been developed for the

analysis of atomistic data. Typically, a Langrangian strain is defined in terms of a reference

crystal structure. The properties of the Langrangian strain are not entirely compatible with the

continuum mechanics definition of strain.

In order to interpret the wealth of data available in a fully atomistic calculation, we must define

appropriate metrics which reduce the amount of information to a manageable level. Often we

will be primarily interested in the characteristics of the local atomic environment around a

specific location in the system. For instance, the location of a free surface seems like an easy

thing to visualize or to quantify in a macroscopic system but given a list of unsorted atomic

Page 37: Computational studies of stress and structure development

coordinates, the problem of locating that surface becomes a much more difficult problem.

Similarly, a broad view of a 2-D crystal will often draw our attention to areas which are dissimilar

or defective in some way. When our system is fully 3-D, we can identify similar locations by

taking slices through the crystal at each atomic plane but this process becomes cumbersome.

Also, the definition of the defective area using only atomic coordinate data is problematic.

With this and similar problems in mind, practitioners of computational materials have

developed a toolbox of metrics for examining the structure in solids. One of the primary metrics

is to examine the local coordination number around any atom. This metric is useful in

identifying certain types of defective areas such as free surfaces or grain boundaries. However,

this metric will fail if there are non-equivalent local environments which have the same

coordination number. This problem arises often in the case of fcc crystals since the local atomic

coordination number for an fcc atom is degenerate with the local coordination number for a hcp

crystal.

The next higher order consideration for local atomic environment takes into account the bond

angles present around a particular atom. Our study takes a simple binning approach to the

analysis of bond angle coordination since we are primarily using the metric to distinguish

between two very different local atomic structures.

4.2 Finite element method

For our FE calculations, we use a commercial code developed by Prof. K.J. Bathe and his

research group at MIT. The ADINA program implements the methods described in Prof. Bathe's

book(57). In essence, the FE method requires partitioning the system into pieces which can

each. be solved as a coupled linear elasticity problem. The solution of the coupled problem is

Page 38: Computational studies of stress and structure development

achieved through a variational energy principle. The nodes in each element are displaced

according to an energy criterion and the calculation is considered complete when the global

energy is minimized to within a set tolerance.

Page 39: Computational studies of stress and structure development

5 Calculations of time required for coalescence

Within the MD framework, we were able to calculate the time required for island coalescence.

In all of the cases considered, coalescence was completed in a matter of nanoseconds. The time

required is only weakly dependent on the size of the islands. The size range of islands studied

varied from 1.5nm in diameter to 6.5 nm in diameter. Both coalescence in free space and on

flat frictionless surfaces was considered.

5.1. Initial structure setup

The initial cluster structures were generated by extracting a spherical section of atoms from a

perfect fcc crystal. The fcc crystal was generated using a simple utility written in the C

programming language (may need to include the code in an appendix). The algorithm for

extracting the spherical cluster utilized a simple radial distance cutoff to delete any atoms which

were too far away from the intended center of mass.

5.2 Structure equilibration

Prior to use in an MD calculation, each structure had to be equilibrated to the desired

temperature. Practically, the equilibration is achieved by assigning random velocities consistent

with the desired temperature and then evolving the structure via the equations of motion.

Since the initial structure is not in equilibrium, evolving the system microcanonically would

result in a large conversion of thermal energy to stored potential energy and an undesirable

decrease in kinetic energy. To avoid this problem, the velocities of the atoms are periodically

rescaled via a Gaussian thermostat to maintain the desired temperature. This Gaussian

thermostat is only applied during the equilibration process since the actual coalescence process

that we are modeling will occur microcanonically.

Page 40: Computational studies of stress and structure development

For free space structure, the structures were placed with 1 to 2 atomic separation distances.

This spacing assured that the interaction forces of the potential would be active and that the

particles would be attracted to each other. Variation of the initial separation did not drastically

impact the coalescence behavior in either time or length scale.

5.4 Evolution over time

09

E0

__JZ5-j

0 500 1000 1500 2000Time (picoseconds)

2500

Figure 5.1: Evolution of center of mass distances over time. The coalescence process is complete in a manner ofnanoseconds.

5.3 Structure placement

Page 41: Computational studies of stress and structure development

Figure 5.1 shows the typical evolution of the center of mass distance between two particles

during a coalescence process. The approach velocities vary from 1 m/s to 3 m/s which is a quite

modest variation considering that the limiting elastic velocity would be on the order of 1000

m/s. The microcanonical nature of the calculation shows up in the recoil effects seen after the

initial island collision. The oscillation in the center of mass distance is expected since the latent

energy of boundary formation is converted directly into kinetic energy. Over time, the

oscillations are damped as the kinetic energy is scattered more randomly over the coalesced

structure.

Page 42: Computational studies of stress and structure development

6 Calculations of length scales during coalescence

6.1 Analysis of neck formation

6.1. Binuing algorithm for neck size determination

For a discrete calculation, the radius and location of the coalescence neck is not completely

defined. For our purposes, we will select a definition and remain consistent with our

terminology for the treatment of neck analysis. We begin by noting that the neck should form

at the perpendicular bisector of the segment connecting the two particle centers of mass. We

also note that the neck is defined as the minimum radial distance measured along this segment.

Since the radial distances we consider in an atomic calculation are discrete instead of

continuous, we divide the segment into several bins and consider the radius to be the maximum

radial distance of all atoms within that bin. We find that this method accurately predicts the

location and thickness of the neck.

Page 43: Computational studies of stress and structure development

6.1.2 Computed neck size via MD

nrt

E 20

D 18

16cD 14

cz 1210

c 8

Fl 64

220 25 30 35 40 45 50

Diameter (Angstrom)

I Simulated 0 NC v FC3D , Seel et al. - FC2D

Figure 6.1: Comparison of the atomistic calculation data with continuum models. The diamonds denote the atomiccalculation data and the lines denote existing continuum models. The 2-D Freund and Chason model and the SeelFEM model produce very similar results for the zipping height.

Figure 6.1 shows the atomistic calculation data along with the predictions from the continuum

models. The atomistic data matches well with best with the 2-dimensional FC model and the

Seel model. Geometrically, the setup for the MD calculations is more compatible with the

assumptions of the 3-dimensional FC model. The FC models and Seel model all predict a zipping

height which scales with particle radius to the 0.67 power. The extracted power from fitting the

atomistic data is 0.57.

Page 44: Computational studies of stress and structure development

7 Defects induced by boundary formation

Stacking faults are formed during the boundary formation process. The energetics of these

stacking faults is analyzed using a simple energetic model.

Experimentally, the coalescence process is known to generate dislocations and stacking faults

(20).

7,1 Internal structure of isolated clusters

Clusters in free space do not show much deviation from the original fcc structure that they were

assigned. Surface atoms experience short distance oscillations as well as longer scale

migrations, but the internal structure of the islands is unaffected. At the temperatures used in

the study, point defects such as vacancies and interstitials were limited to the surface layers of

the clusters.

7.2 Internal structure of islands on substrates

Islands generated on Lennard-Jones substrates will adjust their structure in the region nearest

to the substrate in response to the substrate potential. Typically, this results in 2-3 atomic

layers becoming disordered while in contact with the substrate. Some wetting behavior is

observed. The atomic layers closest to the substrate will often rearrange into a close packed

structure if they were not already in that arrangement prior to interacting with the substrate.

This rearrangement process can drive defects through the internal structure of the island. The

adjustment of the shape of the island increases the number of surface/interface atoms relative

to the bulk atoms so at times the distinction between bulk atomic structure and interface

atomic structure becomes blurred.

Page 45: Computational studies of stress and structure development

7.3 Structural analysis of coalesced structures

7.3.1 Types of stacking faults

Figure 7A.: Representation of a coalescing cluster structure, The lighter atoms denote locally hcp atoms whichcomprise a stacking fault.

The stacking fault illustrated in Figure 7.1 was induced by the shear stress generated during the

coalescence process. The particles in Figure 7.1 are not constrained to a substrate but the

behavior for particles on frictionless substrates was seen to be similar. The initial

misorientations of the coalescing particles did not have a discernible effect on the defect

formation behavior. Other types of defects can form just due to the misalignment of the

clusters prior to coalescence. A stacking fault generated due to misfit can act as a source of

stress but does not require any additional material transport upon its formation (i.e. motion of a

partial dislocation is not required to generate a misfit fault).

Page 46: Computational studies of stress and structure development

7.3.2 Size vvindow for stacking fault formation

# with stackingfault

U # without stack-ing fault

3.4 3.9diameter (nm)

5.9

Figure 7.2: Histogram showing the number of calculations which included stacking faults for the different sizes.These calculations were tabulated for the free space coalescence case.

Our examination of the post coalescence island structures showed that the occurrence of

induced stacking faults is restricted to a particular size window. These observations are

summarized in Figure 7.2. Our initial expectation was that the larger initial clusters would

experience less faulting since the stress generated during coalescence is much less than for

smaller clusters. The absence of defects in the smallest coalescing islands is a bit more

complicated and we will elaborate on this in the next section.

8

7-

6-

C 5-0

> 4-L.

: 3-0

2-

1-

0A I 5.9

Page 47: Computational studies of stress and structure development

7,3.3 Model for the size dependence of defect formation

Figure 7.3: Illustration of the unfaulted and faulted sphere configurations.

In order to understand the apparent size window for defect formation we have developed an

energetic model for the creation of defects within an initially defect-free sphere. The absence of

defects for the larger island sizes is easily explainable by the fact that all of the continuum stress

models predict a decreasing coalescence stress with increasing size. The larger sized islands lack

the necessary stress to nucleate a planar defect. The strengthening of the islands at smaller

sizes is more complex and our model addresses this issue specifically.

Figure 7.3 shows the unfaulted sphere and the expected configuration if a planar defect is

allowed to form within the sphere. The location of the defect plane is illustrated as crossing

through the equator but in reality the defect plane will be projected along the most accessible

slip plane near the newly formed boundary. This complication alters the model only slightly

since the circular geometry of the defect is still preserved.

If we assume an initial hydrostatic stress, a, in a reference sphere, the strain energy is

Page 48: Computational studies of stress and structure development

Estrain,unfaulted = - (7.))

where B is the bulk modulus. The sum of the strain and surface energy is then

22 R3 1 2y.Etot,unfaulted 3B + 4 2y. (7.2)

Figure 7.4: Projection of the fault plane in the sphere showing the relevant distances used in the model.

Figure 7.4 shows the projected area of the defect in the sphere. After the defect has been

allowed to pass through the sphere, an atomic shift will occur between the upper and lower

halves of the sphere. We represent the atomic shift by the Greek symbol A. The excess surface

energy generated by the fault process is given by

Esurf,faulted = YsfAoverlap + YAledg e . (7.3)

Here Aoverlap is the overlapping area of the two intersecting circles, Aledge is the area of the

non-intersecting ledges. Simple geometry leads us to

Aoverlap = 2 Rarccos2R) 2 - 4

(7.1)

(7.4)

Page 49: Computational studies of stress and structure development

and

Aledge = 2 (R2 - 2 R2 arccos 2 -f) (7.5)

We expect Ysf to be roughly one order of magnitude less than y. Generally, the values of

surface energies are close to 1 J/m 2 in metals and the stacking fault energies are close to 0.1

J/m2. Some representative values of these energies are given in Table 2. Additionally, we can

calculate the amount of strain energy relieved by the formation of the stacking fault by

analyzing the energy required to move a partial dislocation through the fault. The strain energy

relief is given as

Estrain relief = TbAoverlap (7.6)

T is the shear stress in the cluster prior to the passage of the partial dislocation and b is the

Burger's vector of the dislocation. The magnitude of the Tb term should be on the order of 0.1

J/m 2. The strain energy of the faulted sphere is

2 bA (7.7) 3Estrain,faulted = 3 overlap (7.7)

and the total energy of the faulted structure is

2 U2tf le3 2 y.tot,faulted 3B - bAoverlap Ysf Aoverlap + YAledge + 47R2 (7.8)

The total energy change from the unperturbed structure to the perturbed structure is achieved

by taking the difference of Equation 7.8 and Equation 7.2.

AEsf = (Ysf - T-b)Aoverlap + YAledge (7.9)

Page 50: Computational studies of stress and structure development

Table 2: Values of stacking fault and surface energy from Hirth and Lothe, Theory of Distocations.

Metal Ysf (J/m2 ) y (J/m2)

Ag .016 1.14

Al .166 0.98

Au .032 1.49

Cu .045 1.73

We can use an energy equivalence criteria between the faulted and unfaulted structure to

determine the critical stress for a given particle size. We set the energy change described in

Equation 7.9 to zero and solve for -.

Tcrit = YsfAoverlap+yAledge = Ysf ( 1 + YAledgebAoverlap b YsfAoverlap/

(7.10)

Using the stacking fault and surface energies for Ag from Table 2 and a Burger's vector length of

0.5nm, the behavior of rcrit is shown in Figure 7.5. As particle size decreases, the critical stress

Page 51: Computational studies of stress and structure development

for which the faulted structure is energetically favorable increases dramatically.

Vt)

Co

"O 100 200 300 400 500 600Radius (nm)

Figure 7.5: Critical stress for generation of an induced stacking fault for Ag. The predicted coalescence stresses forthe most applicable continuum models are also shown. The inset shows the small thickness strengthening effect.

The values of coalescence stress predicted by the continuum models are less than the critical

stress for the smallest island sizes. For larger islands, coalescence stress is greater than the

critical value and the occurrence of induced stacking faults is expected. The limiting value of the

critical stress is , which would be 32 MPa for Ag. The Seel model predicts the upper limit for

induced defect formation at 100nm while the FC2D model predicts that the transition will occur

at 450nm. Both of these predicted upper limits are well above the limit we observe from the

• •J',

Page 52: Computational studies of stress and structure development

MD calculations of around 5nm but the existence of the defect size window is qualitatively

justified.

The discrepancy between our atomistic calculation data and the upper range predicted by the

continuum models is not fully understood. As an energetic model, our explanation for the

existence of the defect size window does not include kinetic effects. Our atomistic calculations

may include additional activation barriers to the defect formation which are not accounted for

in the continuum model.

Page 53: Computational studies of stress and structure development

8 FEA of coalescence with a greater than 90 degree contact angle

By expanding on the earlier work of Seel and Thompson with islands of less than 90 degree

contact angle (38), we find that when the point of calescence occurs above the substrate, the

mechanical process is similar to the coalescence of clusters in free space. In these cases, the

contact and deformation behavior is similar to the 90 degree contact angle calculations except

for the extreme cases when the fixed motion boundary point interferes with the deformation.

Seel and Thompson (38) used the maximum in plane zipping distance,y o, rather than zo in order

to quantify the effects of zipping for contact angles less than 90 degrees. Irrespective of contact

angle, the normalized strain energy of the zipped configuration is given by

Er 2 4

Here, r is the radius of the contact area between the particle and substrate. The average stress

within an island was found to be

) E LYo,eq1-v2 Y w

where YO,eq was obtained numerically.

Page 54: Computational studies of stress and structure development

!:i!:uv U ,I- t'E~i mesh used to •ima/ze the stte.ins r eq.,niwed for island %ipping,

Figure 8.1 shows the general setup for the FE problem formulation. By symmetry, we only need

to consider one half of the impinging island geometry. The initial structure is a plane strain

cylinder which is set to have perfect traction at the cylinder-substrate interface. By applying

nodal displacements to form different contact surfaces, we obtain the system strain energy as a

function of two spatial variables. The spatial variables used in this study are the distance zipped

up from the point of initial contact and the distance zipped down from the initial contact. The

initial contact angles we have examined are 110, 120, 130, 150, and 180 degrees.

Page 55: Computational studies of stress and structure development

Strain Energy

6e-10

5e-10

4e-10

3e-10

2e-10

le-10

0

1.2

Figure 8.2: Strain energy mapping using up and down zipping distances as parameters.

Figure 8.2 shows a strain energy map generated from a FE island coalescence calculation. The

energy surface is well represented by

Estrain . E A(R, 6)(coz% + cZdozwn),

where zup and Zdown are the up and down zipping distances normalized to island radius,

respectively, and

A(R, 0) (-' - sin(6)(1 + cos(S)) - (1+cos(O)))R22 3 8sin(8)

The constants, co and c1, are extracted from the FE calculation and characterize the strain

energy penalty for upward and downward zipping, respectively. The extracted values for co and

cl are given in Table 3.

Page 56: Computational studies of stress and structure development

Table 3: Extracted fitting parameters for the strain energy surface

Contact angle (degrees) co cl

110 0.047 0.049

120 0.044 0.044

130 0.040 0.040

150 0.034 0.034

180 0.027 0.027

By minimizing the sum of the island strain energy and the newly formed boundary energy, we

can obtain both the normalized up and down zipping distances.

1 1

z = (z ( Iucp = 4EA(R, O)co ; down 4EA(R, 0)cl

2

Since the area function scales with R2 , the total zipping distance will scale with R3. This scaling

matches well with the earlier derived scaling for coalescence of islands with 90 degree contact

angles.

Page 57: Computational studies of stress and structure development

0.6

0.4

0,2

ۥ

0 0.2 0.4 0.6 0.8Z downi nm)

Figure 8.3: Contour plot showing the strain energy required to zip in each direction for a contact angle of 110degrees.

Figure 8.3 shows a contour plot of the strain energy surface for the case of a 110 degree contact

angle and allows for a more visual interpretation of zipping distances. The contours are nearly

symmetric in both the up and down distances. This symmetry is only broken when the position

of the cylinder-substrate contact restricts the range of the down zipping parameter.

th8

Page 58: Computational studies of stress and structure development

0,25

0.2

0 15

cl

A1

0 0.5 1 1.5 2

Surface Energy (JI/m)

Figure 8.4: Plot of the energy minimizing zipping distances for a 5 nm island with a contact angle of 110 degrees.

Figure 8.4 shows the energy minimizing zipping distances in both directions for a variety of

different surface energies. For large surface energy driving forces, the actual energy minimizing

configuration shows some asymmetry but this deviation is quite small. Under the boundary

conditions used in our calculations, the islands will have only a slight preference to zip away

from the constraint of the substrate.

Page 59: Computational studies of stress and structure development

250

Figure 8.5: Zipping stress as a function of contact angle.

Figure 8.5 shows our results for the average island stress along with the results from Seel and

Thompson(38). The Seel and Thompson results show that the zipping stress increases with

increasing contact angle up to the case of 90 degree contact angles. Our results show that the

90 degree contact angle case represents the maximum stress that can be generated in the island

through zipping.

The lack of strong asymmetry in the zipping behavior of greater than 90 degree contact angle

islands suggests that the contact process occurs in a manner similar to the coalescence of free

space spheres.

9 Conclusions

In conclusion, computational techniques such as MD and FEA are valuable tools in studying the

stress and structural evolution of films formed via island coalescence. The timescales for island

coalescence are tractable by the MD method and the resulting structures are in agreement with

200

150

o 100

50

0

--0-- Seel 10nm

S--Seel 20nm

-- &--Takahashi 10nm

***X*** Takahashi 20nm

0 50 100 150 200

0 (degrees)

1_^______X_____~II__________1_~111111111

Page 60: Computational studies of stress and structure development

the continuum models. The fact that the fundamental energetic continuum models can be used

to describe the behavior of clusters which consist of discrete collections of atoms is a bit

surprising but provides some relief to device designers who can continue using the existing

models.

The presence of defects induced by island coalescence is another interesting result for MEMS

and NEMS designers. The elevated defect density in vacuum deposited thin films is well known,

and the population of coalescence-induced defects may be an explanation. From calculations

and our energetic model, the quantity of these induced defects will be lessened for larger

islands at coalescence and will be absent for the smallest islands. Control over the island

structure prior to coalescence should allow thin film engineers to tailor the defect density for

their particular application.

Coalescence of islands with greater than 90 degree contact angle can be studied using FEA. For

all practical purposes, the fact that initial impingement occurs above the plane of the substrate

results in coalescence behavior which is quite similar to the coalescence of free spheres. Zipping

from the impingement point is symmetric in the up and down directions unless the contact

angle is very near 90 degrees. The importance of the boundary condition definition becomes

increasingly important as the contact angle approaches 90 degrees.

Page 61: Computational studies of stress and structure development

10 Future work

From a computational perspective, the most interesting extension of this work would be to

consider the effects of partial and complete traction on the coalescence behavior. The limiting

case of complete traction could be investigated by freezing the positions of the first layer of

atoms in each island. The dynamic layers in each island would still be allowed to coalesce but

the atomic configuration of the layers in contact with the substrate would be fixed. Special care

would have to be taken with atoms which are initially dynamic but random walk into an atomic

position in the first layer which should be frozen. The optimal number of layers to freeze in

order to represent the effect of traction is also not known a priori.

The effects of partial traction can be treated for both the isotropic case and the case for

preferred orientations. The most appropriate treatment for the isotropic case would be to

introduce a damping term on the motion of atoms in the first layer of the island. The form of

this damping term is not known explicitly but the strength of the term should correlate with the

dynamic coefficient of friction for the island material on the substrate material.

Another possible extension to the work presented here is to repeat the analysis for a wider

range of material models and potentials. This study has focused on Ag, but extension to other

metals and more advanced interatomic potentials should be straightforward. Ideally, a similar

approach could also be taken with a DFT model of the solid clusters.

Also, now that the presence of induced defects has been confirmed in both computational and

experimental studies, the incorporation of these defects into a continuum stress model is

desirable. The defect generation model that we presented can be merged with the continuum

island zipping models to provide improved estimates of the actual stress generated in

Page 62: Computational studies of stress and structure development

experiments. Before the effects of induced defects can be quantitatively added into the

continuum models, further experimental characterization of the defects is necessary. In-situ

TEM characterization of the actual size window in which defects occur should allow for the

extraction of the critical stress required for defect formation and the minimum critical island

radius.

Ultimately, it may be possible to quantitatively simulate the entire stress-thickness evolution

during thin film growth. Seel et al have already shown that qualitative agreement can be

achieved with a kinetic island growth model utilizing FE estimates of stress(37). Modification of

the kinetic model to include defect formation and other stress relief mechanisms would be a

significant step in taking the model from qualitative to quantitative.

Page 63: Computational studies of stress and structure development

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