Characterization of interfacial mechanical properties using wedge indentation method 1

CHARACTERIZATION OF INTERFACIAL MECHANICAL PROPERTIES USING WEDGE INDENTATION METHOD YEAP KONG BOON NATIONAL UNIVERSITY OF SINGAPORE 2010... CHARACTERIZATION OF INTERFACIAL MECHANICAL

CHARACTERIZATION OF INTERFACIAL MECHANICAL PROPERTIES USING WEDGE

INDENTATION METHOD

YEAP KONG BOON

NATIONAL UNIVERSITY OF SINGAPORE

2010

CHARACTERIZATION OF INTERFACIAL MECHANICAL PROPERTIES USING WEDGE

INDENTATION METHOD

YEAP KONG BOON

(B Eng (Hons.), University Technology Malaysia)

A THESIS SUBMITTED

FOR THE DEGREE OF DOCTOR OF PHILOSOPHY

DEPARTMENT OF MECHANICAL ENGINEERING

NATIONAL UNIVERSITY OF SINGAPORE

2010

Preface

This dissertation is submitted for the degree of Doctor of Philosophy in the Department of Mechanical Engineering, National University of Singapore (NUS) under the supervision of Associate Professor, Dr Zeng Kaiyang Part of the research works have been conducted at Institute of Materials Research and Engineering, Singapore (IMRE) To the best knowledge of the author, all of the results presented in this dissertation are original, and references are provided to the works by other researchers The majority portions of this dissertation have been published or submitted to international journals or presented at various international conferences as listed below:

Journal Papers:

1 K B Yeap, K Zeng, H Jiang, L Shen and D Chi, Determining Interfacial

Properties of Submicron Low-k Films on Si Substrate by using Wedge

Indentation Technique, Journal of Applied Physics, 101, 123531 (2007)

2 K.B Yeap, K.Y Zeng and D.Z Chi, Determining the Interfacial Toughness of

Low-k Films on Si Substrate by Wedge Indentation: Further Studies, Acta Materialia, 56, p.977-984 (2008)

3 K.B Yeap, K.Y Zeng and D.Z Chi, Wedge Indentation Studies of Low-k

Films at Inert, Water and Ambient Environments, Materials Science and

Engineering A-Structural Materials Properties Microstructure and Processing,

518, p.132-138 (2009)

4 L Chen, K.B Yeap, K.Y Zeng and G.R Liu, Finite Element Simulation and

Experimental Determination of Interfacial Adhesion Properties by Wedge Indentation, 89, p 1395-1413 (2009)

Contributions: Providing nanoindentation experimental supports, involvement

in discussions and making comparison of the experimental and simulation results

5 J Zhu, K.B Yeap, K.Y Zeng and L Lu, Mechanical and Interfacial Properties

of Sputtered Ruo2 Thin Film on Si Substrate for Solid State Electronic Devices Submitted to Thin Solid Film for review

Contributions: Providing nanoindentation experimental supports and

involvement in discussions

Book Chapter:

1 K.Y Zeng, K.B Yeap, A Kumar, L Chen and H.Y Jiang, Fracture

Toughness and Interfacial Adhesion Strength of Thin Films: Indentation and Scratch Experiments and Analysis, to be published in CRC Handbook of Nano-structured Thin Films and Coatings (Three Volume Set), Vol 1, Chapter

3, Ed S.Zhang, CRC Press (2010)

Conference Presentations:

1 K B Yeap, K Zeng, L Shen and D Chi, Determining the Interfacial

Properties of Low-k Film by Wedge Nanoindentation, Materials

Tri-Conference: Thin Film 2006, Singapore (Presented by Kong Boon Yeap)

2 K.B Yeap, L Chen, K.Y Zeng, and D.Z Chi, A Simple Method to Quantify

Interfacial Mechanical Properties of Low-k /Si: Wedge Indentation Technique, MRS Spring 2009, California, USA (Presented by Kong Boon Yeap)

3 K.B Yeap and K.Y Zeng, A Simple Method to Quantify Interfacial

Mechanical Properties of Low-k /Si: Wedge Indentation Technique, ICMAT

2009, Singapore (Presented by Kong Boon Yeap)

4 K B Yeap and K.Y Zeng, Determination of Interfacial Mechanical and

Time-dependent Properties of Low-k Films by Wedge Indentation Method,

Advanced Materials Workshop 2009, Cottbus, Germany (Presented by Kong Boon Yeap)

5 K.B Yeap, K.Y Zeng, R Yvonne and E Zschech, Determining Cohesive

Toughness and Adhesion of Low-k Film by Nanoindentation, 11th Stress Workshop 2010, Dresden, Germany (Presented by Kong Boon Yeap)

Acknowledgements

First of all, I would like to express my sincere gratitude to my supervisors, Associate Prof Dr Zeng Kaiyang and Dr Chi Dongzhi, for their guidance, supervision, encouragement and advice during the course of my Ph.D study The scientific methods and research skills imparted by them are the most valuable gift for

my future research career in the field of “Nanomechanics of Materials”

Also, I would like to thank the research staffs in the Institute of Materials Research and Engineering (IMRE) and the National University of Singapore (NUS) I especially thank Ms Shen Lu (IMRE) for the help on conducting the nanoindentation experiments and Madam Zhong Xiangli (NUS) for the help on operating the focused-ion-beam system In addition, I would like to thank my room mates, laboratory colleagues and friends for their support and help

Finally, I owed many thanks to my girl friend and my family for their love,

patience, support and encouragement, without that I would not be able to complete this

Ph.D thesis

Chapter 1: Introduction 1

1.1 Overview of the Interfacial Toughness Characterization

Methods

2

1.3 Research Objectives and Significance 6

Chapter 2: Literature Review 14

2.1 Relationships between Interfacial Delaminations and

Nanoindentation Load-Penetration Curves

15

2.2 Indentation Methods Developed to Determine the Interfacial

Toughness of Thin Film/Substrate Structure

16

2.2.2 Microwedge Indentation of Line Structure 23

2.2.3 Wedge Indentation on the Systems with Strong

Interfaces

27

2.3 Area under Indentation Load-Penetration Curve and Work of

Fracture

31

2.4 Mechanical Aspects and Reliability of Low-k Films 33

2.4.1 Interfacial Fracture of Low-k Films 36

2.4.2 Time-Dependent Fracture of Low-k Films 39

3.3.1 Plane View Imaging of Indentation Impressions 58 3.3.2 Cross-Sectional Imaging of Indentation Impressions 59 3.3.3 Chemical Analysis of Fracture Surfaces 61

Chapter 4: Development of Wedge Indentation Method to

Characterize the Interfacial Toughness of Sub-Micron

Low-Dielectric (k) Thin Films

63

4.1 Correlations between the Nanoindentation P-h curves and

the Fracture Processes

64

4.1.1 The Correlation Studies on the MSQ/Si System 65 4.1.2 The Correlation Studies on the BD/Si System 75

4.4 Determination of Interfacial Adhesion 94

Chapter 5: Comparison of the Finite Element Simulation and

Experiments of Wedge Indentation Test

123

5.1 Elastic-Plastic Properties of BD and MSQ films 124

5.2 Interfacial Adhesion Energy and Strength 128

Chapter 6: Wedge Indentation Studies of Low-k Films at Inert,

Water and Ambient Environments

136

6.1 Time-Dependent Fracture during Wedge Indentation Tests 139

6.3 Influences of Test Environments on Fracture Processes 151

Reference 158

Chapter 7: Wedge Indentations on Hard-Film-Soft-Substrate System 160

7.1 Correlation Study on RuO2/Si System 161

7.2 Interfacial Toughness of RuO2/Si System 164

Appendix A: Schematic Diagram of a FIB Cutting on Wedge

Indentation Impression

183

Appendix B: Generalization of Indentation Induced Stress 184

Appendix C: EDX results 186

Appendix D: FEM simulation and model for Wedge Indentation

Induced Delamination

195

Summary

Shrinkage of device dimensions in microprocessors and mechanical systems to nanometer scale has brought major concerns on the device reliability and the material compatibility with existing fabrication processes A vast amount of work has been dedicated to understand and enhance the mechanical properties of this nanometer structure, e.g elastic modulus, cohesive toughness and interfacial adhesion However, the mechanics of interfacial adhesion is a rather complicated subject This thesis develops a simple way to measure the interfacial adhesion of thin film/substrate structure using the wedge indentation method We intend to simplify the existing indentation experimental procedures and analytical solutions, so that the measurement of interfacial adhesion is simple enough for scientists and engineers who may have little experience in this area

micro-electro-The development of a new wedge indentation experiment to characterize interfacial adhesion can be divided into three parts In the first part, the correlation between the cracking sequence and indentation load-penetration curve (indentation-fracture correlation) is established In the second part, based on the indentation-fracture correlation, a simple experiment and analysis procedure is developed to measure the interfacial adhesion of thin film/substrate structures In the third part, the

experiment is conducted on low-k/Si systems (e.g BlackDiamond™ (BD/Si) and

methyl-silsesquioxane (MSQ/Si)) and RuO2/Si system to verify the accuracy of the interfacial adhesion measured using the simple analysis procedure

To further confirm the validity of the analysis procedure, finite-element modeling (FEM) of the wedge indentation induced delamination is conducted in a collaborative study The results from wedge indentation experiments are compared with the predictions from FEM simulations in two aspects Firstly, the plastic properties (e.g yield strength and strain hardening exponent) of the thin film/substrate

structure are determined by matching the load-penetration (P-h) curves from

experiment and simulation Secondly, the interfacial energy and the interfacial strength

of the BD/Si and the MSQ/Si systems are determined by plotting the experimentally obtained critical indentation loads for interfacial separations into the contour plots derived from FEM simulations – the interfacial energy-strength contour

The understanding of the role of environment on the degradation of mechanical properties is crucial for long-term device reliability and fabrication processes

compatibility During the wedge indentation experiments on the low-k films,

time-dependent fractures can be observed, when a prolong holding at maximum load or a slow loading rate is applied Qualitative evaluations are therefore conducted to

examine the time-dependent fracture behavior of the low-k films under different

environmental conditions, e.g water, ambient and inert conditions

List of Tables

Table 1.1 The technology roadmap for interconnects in a microprocessor device

Table 2.1 Selected properties of the low-k dielectrics (MSQ and OSG) and the

conventional SiO2 dielectric

Table 3.1 The MSQ and BD films used in this study

Table 3.2 MTS Nanoindenter XP® system specifications

Table 3.3 UMIS-2000H® system specifications

Table 3.4 FIB cutting parameters for the MSQ, BD and RuO2 films

Table 4.1 Indentation load and plastic depth that are associated with the crack

profiles induced by the wedge and the Berkovich indentations on the MSQ/Si system

Table 4.2 Curvature of crack front and the dimensionless constant determined

for the BD Films with different thickness

Table 4.3 Elastic modulus and hardness for the MSQ and the BD films

Table 4.4 The calculation of interfacial toughness and key parameters for the

MSQ/Si system Note: (1) The average long axis lengths, 2b’ are 5.63

µm and 6.61 µm, for 90° and 120° wedge indentations, respectively

(2) Maximum Loads, P max for 90° and 120° wedge tip indentations are

9 mN and 15 mN, respectively

Table 4.5 The calculation of interfacial toughness for the BD/Si system Note:

(1) Data are averaged from 15 indentation impressions (2) The indentation plastic depth is determined from an indentation load-penetration curve averaged from 15 indentations

Table 4.6 The interfacial crack front’s stress-strain condition and the interfacial

toughness of the BD/Si systems Notes: (a) Errors represent the standard deviations from 15 data sets (b) The wedge indenter lengths,

l are 4.06 µm and 7.24 µm

Table 4.7 The fracture toughness (film and interfacial toughness) of the BD/Si

system and the interfacial toughness of the MSQ/Si system calculated following Malzbender’s method, Li’s method and modified Li’s method

Table 5.1 Elastic-plastic properties of the 500 nm thick BD film and the 400 nm

thick MSQ film Note: Yield strength and strain hardening in these given range could fit the simulation and the experimentally obtained

P-h curve very well

Table 6.1 The slopes and time for the stages of h-t curve of holding test done on

BD/Si system at (a) ambient environment, (b) inert environment, and (c) watered environment [9] Note: All the data above are averaged

from 40 sets of load-holding tests with P max = 6mN, or ε = 0.8 The

errors are standard deviations of the data

Table 7.1 Calculations of the interfacial toughness of the RuO2/Si system (90°

wedge indentation) Notes: (a) The average value of the long axis

length, 2b’ is 5.0μm (b) The critical buckling stress for the

straight-sided buckle is applied

Table 7.2 Calculations of the interfacial toughness of the RuO2/Si system (120°

wedge indentation) Notes: (a) The average value of the long axis

length, 2b’ is 6.6μm (b) The critical buckling stress for the

straight-sided buckle is applied

Table 7.3 Calculations of the interfacial toughness of the RuO2/Si system

(conical indentation) Note: The critical buckling stress for the circular buckle is applied

List of Figures

Fig 1.1 Mechanical flexure tests on thin film/substrate samples: (a)

Four-point-bending and (b) double-cantilever-cleavage Interface of interest

is sandwiched between two stiff substrates

Fig 1.2 A typical interconnects structure in a microprocessor device

Fig 1.3 Scanning electron microscopy (SEM) image of the low-k interfacial

delamination during chemical-mechanical-polishing process

Fig 2.1 Schematic representations of Swain and Menčík’s hypothesis on the

evolution of the nanoindentation load-penetration curves, when the interfacial adhesion changes from good to poor, for various film/substrate materials combinations during the spherical indentation experiments

Fig 2.2 Marshall and Evans hypothetical operations for quantification of the

interfacial crack driving force

Fig 2.3 Schematic diagram of microwedge indentation experiment setup The

film sample can be etched into a fine-line shape by lithography method

Fig 2.4 Schematic diagram of the wedge indentation test on the system with a

strong interface

Fig 2.5 Schematic diagrams of the indentation P-h curves and the energy

dissipated due to fracture: (a) Li’s method, and (b) Malzbender’s method

Fig 2.6 Schematic representation of the integration challenges associated with

the introduction of the low-k dielectric

Fig 2.7 Schematics of the molecular bridging within the pores inside MSQ

film: (a) porogen remnants after curing-process forming molecular bridgings and (b) the molecular bridgings are stretched and broken as the interfacial crack propagates

Fig 2.8 Schematic of molecular reaction mechanisms Reaction between

Si-O-Si bonds with (a) water molecule, (b) hydroxide ion, and (c) hydrogen peroxide molecule

Fig 2.9 Schematics of typical subcritical crack growth curve, (I) Reaction

controlled, (II) Transport controlled, and (III) Catastrophic fracture

Fig 2.10 Subcritical crack growth curve of a porous MSQ film (LKD-6103 JSR

Corp., Japan) in NaOH solutions and water

Fig 2.11 The interfacial toughness degradation of an OSG/SiNx system as a

function of the water-exposure time (water temperature at 95°C and 25°C)

Fig 3.1 Single loading/unloading curve of the BD film (500nm) by 90° wedge

indentation

Fig 4.1 Load vs penetration depth (P-h) curves for the MSQ/Si system: (a)

the 90° wedge indentation, (b) the 120° wedge indentation, and (c) the standard Berkovich indentation

Fig 4.2 Cross-sectional views of 90° wedge indentations on the MSQ/Si

system using FIB: (a) no interfacial crack; (b) - (d) interfacial crack propagation

Fig 4.3 Cross-sectional views of 120° wedge indentations on the MSQ/Si

system using FIB: (a) no interfacial crack; (b) - (d) interfacial crack propagation

Fig 4.4 Plane views of 90° wedge indentations on the MSQ/Si system using

FESEM: (a) central crack, (b) corner crack, and (c) - (d) interfacial crack kinked into the film and propagate to free surface

Fig 4.5 Plane views of 120° wedge indentations on the MSQ/Si system using

FESEM: (a) central crack, (b) corner crack, and (c) - (d) interfacial crack kinked into the film and propagate to free surface

Fig 4.6 (a) Cross sectional view of a Berkovich indentation on the MSQ/Si

system using FIB (b) Plane views of Berkovich indentations using FESEM: before cracks and (c) after the crack kinks to the film surface

Fig 4.7 Load-penetration depth (P-h) curves for the 500 nm BD film with the

maximum indentation loads varying from 7 mN to 10 mN

Fig 4.8 FESEM plane-view images of the 500 nm BD film with the maximum

indentation loads of (a) 7 mN: only plastic deformation; (b) 8.5 mN: film corner and central crack; and (c) 10 mN: complete spall off (d) FIB cross-sectional view image of the 500 nm BD film with a maximum load of 9 mN showing no interfacial crack, but the central crack has reached to the interface

Fig 4.9 Configuration of cracks during the steady state propagation of

interfacial crack Solid-line shows the elliptical-shape delamination

Normal T n and shear traction T s are acting on the crack front, while

there is no contact or no traction at the corner crack surface Dashed

line shows the displacement of the film above the interfacial crack, δ,

due to the indentation induced stress Bulge-out at the corner crack might happen during the indentation, causing a minor error in the calculation

Fig 4.10 Formation of an edge crack on the interface of a thin film/substrate

structure and the conventions for interfacial fracture mechanics analysis

Fig 4.11 Conventions for interfacial fracture mechanics analysis

Fig 4.12 Phase factor, ω(α, β) in Eq 4.8

Fig 4.13 Plane view of the interfacial crack pattern by SEM: (a) BD film with

thickness of 200 nm; (b) BD film with thickness of 500 nm Interfacial crack front is taken as before the crack kinks into the film and toward film surface

Fig 4.14 FESEM plane-view image of delaminated area on 500nm BD film A

ninth-order polynomial function is fitted to the crack front in order to determine the curvature of the crack front

Fig 4.15 Curvature of interfacial crack front determined for the BD films with

thickness ranging from 100 to 1000nm

Fig 4.16 FESEM plane-view images of the wedge indentation sites and the

associated interfacial crack front on (a) the 100 nm BD film: a straight crack front; (b) the 300 nm BD film: a slightly curved crack front; (c) the 500 nm BD film: a curved crack front, and (d) the 1000 nm BD film: almost circular delamination

Fig 4.17 Comparison between the indentation-induced stress and the critical

buckling stress to verify the delamination mode Open boxes represent the indentation-induced stress calculated based on Eq.(4.1) Closed boxes represent the critical buckling stress calculated using the

literature value of Y = 1.000 for 100 nm BD film, and 1.488 for 500

and 1000 nm BD films For intermediate film thickness, 300 nm BD film, the open triangle represents the critical buckling stress calculated

using the approximated value of Y = 1.390, while closed triangles represent that calculated by using the upper and lower bounds of Y

(1.000 and 1.488)

Fig 4.18 E f (unfilled boxes) and S2/P (filled boxes) obtained from

nanoindentation with the CSM option on the MSQ film at the 200 nm penetration depth

Fig 4.19 Cross-sectional views (MSQ film) by FIB to examine 2b’: (a) 90°

wedge tip indentation, (b) 120° wedge tip indentation, and (c) sectional view at 500 nm away from the end of the wedge indent for the 90° wedge tip indentation

cross-Fig 4.20 Plane view of the interfacial crack pattern by SEM: (a) BD film with

thickness of 200 nm; (b) BD film with thickness of 500 nm

Fig 4.21 Interfacial toughness for the BD films with thickness ranging from

100 nm to 1000 nm The interfacial toughness for the MSQ film measured in Section 4.4.2 is also included for comparison

Fig 4.22 Computed hardness, F/(a*l) versus wedge indenter length normalized

by a half width of the indentation l/a

Fig 4.23 The curvature of interfacial crack front, κ versus the ratio of wedge

indenter length and indentation half-width, l/a

Fig 4.24 The analysis of the P-h curve for a 500nm thick BD film to determine

the energy released during fracture (a) Film cracking and (b) delamination are observed at the pop-in, i.e sudden increase of the penetration depth (red arrows) The film crack and interfacial crack formations processes are overlapped in the region between the two arrows

Fig 4.25 The analysis of the P-h curve for a 400nm thick MSQ film to

determine the energy released during fracture Film cracks are observed at the pop-in, i.e sudden increase of the penetration depth, while interfacial cracks are observed at higher indentation loads (3mN

to 9mN)

Fig 5.1 P-h curves obtained from load controlled actual wedge indentation

experiments and a displacement controlled FEM simulation for the BD/Si system Open boxes represent the 90° wedge indentation experiment, closed boxes represent the 120° wedge indentation, and

open circles represent the simulation of 90° wedge indentation

Fig 5.2 P-h curves before interfacial delamination occurred for: (a) the BD/Si

system and (b) the MSQ/Si system While open and closed triangles represent the simulation and experiment curves of 120° wedge indentation, respectively, open and closed square boxes represent the simulation and experiment curves of 90° wedge indentation, respectively

Fig 5.3 The BD/Si system’s interfacial energy-strength contour for 90° and

120° wedge indentation showing the intersections of Pc90/(σ yf Δ o) = 5.16 – 5.18µm and Pc120/(σ yf Δ o) = 6.58 – 6.92µm Full lines represent the contour for Pc90/(σ yf Δ o), while dashed lines represent that for

Pc120/(σ yf Δ o)

Fig 5.4 The MSQ/Si system’s interfacial energy-strength contour for 90° and

120° wedge indentation showing the intersection of Pc90/(σ yf Δ o) = 4.52 – 6.78µm and Pc120/(σ yf Δ o) = 7.24 – 11.31µm Full lines represent the contour for Pc90/(σ yf Δ o), while dashed lines represent that for Pc120/(σ yf

Δ o)

Fig 6.1 The load-holding test results for the BD/Si system at ambient

environment: (a) load-penetration depth (P-h) curves for different maximum loads at holding, Pmax; (b) penetration depth-holding time

(h-t) curves for different Pmax, showing the consistent S-shaped curves, consisting of three stages

Fig 6.2 The load-holding test results for the MSQ/Si system at ambient

environment, showing the penetration depth - holding time (h-t) curves for different maximum loads, Pmax that assemble simple creep-like curves

Fig 6.3 The load-holding test results for the BD/Si systems with two different

film thickness (300 nm and 500 nm BD films) at ambient environment with indentation load of 5mN The penetration depth is normalized by film thickness

Fig 6.4 The varying-loading-rates test results for the MSQ/Si system at

ambient environment, showing the load-penetration (P-h) curves for different loading rates, dP/dt

Fig 6.5 The varying-loading-rates test results for the BD/Si system: (a)

load-penetration (P-h) curves for different loading rates, dP/dt, and (b) the plot of fracture-onset load, Ponset against loading rate, dP/dt

Fig 6.6 The relationship between the time-to-failure, tf and the maximum

load, Pmax for the BD/Si system at ambient, inert and watered environments

Fig 6.7 The invert time-to-failure, 1/tf vs the fracture-onset-load, Ponset

Fig 6.8 At stage 2C of the penetration depth–holding time (h–t) curve, film

crack connects with interfacial crack in a certain angle, α

Fig 7.1 Cross-sectional images of 90° wedge indentations on the RuO2 film

(thickness, t = 150 nm) (a) Interfacial crack is found at the pop-in

load (b) As the indentation load increases, minor film cracks can be found at the end of the wedge indentation impression

Fig 7.2 (a) Cross-sectional image of 120° wedge indentation on the RuO2 film

(thickness, t = 150 nm), showing the formation of interfacial crack at

the pop-in load (b) Plane-view image of 120° wedge indentation on the RuO2 film, showing no observable film crack

Fig 7.3 Load versus penetration depth (P-h) curves for the RuO2/Si system:

(a) the 90° wedge indentation, (b) the 120° wedge indentation, and (c) the conical indentation

Fig 7.4 Cross-sectional image of the conical indentation (P max = 6mN) on the

RuO2/Si system, showing the circular-shaped delamination and the

crack radius, a’

Fig 7.5 Cross-sectional image of the 90° wedge indentation on the RuO2 film

at P max = 40mN, showing the interfacial and substrate cracks as the indenter penetrates deeply into the substrate

Fig 8.1 FIB cross-sectional image of the wedge indentation on the SiN/BD/Si

system (SiN film at the top)

Fig 8.2 FIB cross-sectional image of the wedge indentation on the

TaN/SiN/BD/Si system (TaN film at the top)

Fig 8.3 FIB cross-sectional image of the wedge indentation on the

Cu/TaN/SiN/BD/Si system (Cu film at the top)

List of Symbols

a The half width of an indentation contact

a' The crack radius of a circular shaped delamination, the short

axis crack length of an elliptical shaped delamination or the crack length of a rectangular shaped delamination

b' The long axis crack length of an elliptical shaped

delamination or the width of the fine-line for microwedge indentation method

G Strain energy release rate

G TH The threshold strain energy release rate, below which there is

zero crack growth

h p Indentation plastic depth

l The length of wedge indenter tip

N The strain hardening exponential

P c Critical indentation load for interfacial crack initiation

P 90 c Critical indentation load for interfacial crack initiation for 90°

S1 The slopes of the curves at stage 1 of the penetration

depth-holding time (h-t) curves

S2A, S2B and S2C The slopes of the curves at stage 2 of the penetration

depth-holding time (h-t) curves

V c Interfacial crack volume

Y The dimensionless constant introduced in Eq.(4.10) to

determine the critical buckling stress

2φ The inclination angle of a wedge indenter tip

β The inclination angle between the film surface and the wedge

surface (90° minus φ)

σ o Indentation induced stress

σ c Critical buckling stress

Chapter 1: Introduction

Many technologically advanced devices, such as microelectronics, optoelectronics, biomedical and data storage devices, are constructed by depositing layers of nanometer thin film structures on a substrate In this thin film/substrate structure, it is common to see materials from all three basic classes – metals, ceramics and polymers Because of the differences in chemical composition and atomic structure, materials from each class have distinct characteristics and are used in a thin film/substrate structure for different purposes With regard to mechanical characteristics, ceramics are typically brittle and susceptible to fracture Ceramic thin films and their interfaces are usually the weakest part in a thin film/substrate structure

To ensure reliable device operations, it is important that the films and substrate materials not only fulfill their functional purposes, but also have desirable mechanical properties, e.g elastic modulus, hardness, interfacial toughness and time-dependent fracture properties Many mechanical measurement techniques have been developed for the purposes of quality control and materials development In this chapter, an overview of these topics will be briefly discussed Section 1.1 presents an overview of the experimental methods for thin film/substrate interfacial toughness characterization Section 1.2 presents a group of thin film materials (low dielectric constant (k) thin films) that has very weak interfacial toughness due to their inherent porous structure

Chapter 1

1.1 Overview of the Interfacial Toughness Characterization Methods

Fig 1.1: Mechanical flexure tests on thin film/substrate samples: (a) bending and (b) double-cantilever-cleavage Interface of interest is sandwiched between two stiff substrates

Four-point-Over the last few decades, several experimental methods have been developed

to characterize the interfacial toughness of thin film/substrate structures, including mechanical flexure tests [1-3] and indentation tests [4-16] The mechanical flexure or bending tests, such as four-point-bending and double-cantilever-cleavage, have been shown to provide accurate quantitative results of interfacial toughness Fig 1.1 shows the schematics of the four-point-bending and the double-cantilever-cleavage tests An important feature of the flexure tests is to sandwich the thin films between two stiff substrates by diffusion-bonding or epoxy-glued The sandwiched stack of films and substrates are then cut into well defined fracture mechanics sample geometry During film decohesion, the stiff substrates prevent the film residual stress from being relaxed and contributed to the crack driving force The only limitation of the flexure tests is the complicated and time-consuming experimental methodology, such as the preparations of stacked samples and the bending of the stacked samples one-by-one without process automation

Chapter 1

On the other hand, the indentation techniques have much simpler experimental procedures In a short time frame, large amount of indentations can be made on a small piece of sample The first indentation method was developed by Marshall et al [4,5] using a cone shaped indenter to characterize the interfacial toughness of a thin film/substrate system Based on the hypothetical operations of conical indentation [4,5], de Boer and Gerberich [6,7] developed the microwedge indentation technique

In their test, a wedge-shaped indenter was pressed into a fine-line shaped thin film sample As a result, a plane strain condition was created along the interfacial crack front, leading to a greater crack driving force as compared to that of the conical indentation However, the requirement to prepare the thin film sample in fine-line shaped has complicated this technique and limits its application Section 2.2 will review the indentation techniques that are significant to author’s PhD study, i.e., conical indentation [4,5], microwedge indentation [6,7] and wedge indentation [8-10]

1.2 Background of Low-k Thin Films

Since the introduction of the Moore’s Law* [17] about 40 years ago, research committed to semiconductor technology has successfully doubled the number of transistors in a microprocessor every two years The strategy employed to maximize transistor density emphasis on scaling the device feature size Fig.1.2 shows the cross section of a typical interconnect structure in a microprocessor device [18] According

to the international technology roadmap for semiconductors (ITRS, 2007) [18], from

* In 1965, Gordon E Moore, co-founder of Intel Corp, envisioned a long-term trend in the computing hardware, which later became the famous Moore’s Law

Chapter 1

2010 onward the transistor physical gate length will shrink to below 20nm, as a result, the interconnect metal-1 half-pitch* will be shorten to less than 50nm (Table 1.1) As the interconnect dimension reduces to nanometer scale, the distribution of the clock signals† that control the microprocessor’s operation will become a difficult task [19] Even with the Cu replacement (a common metallization with lower resistivity), the clock signal delay due to the wire resistance and the insulator capacitance (RC delay) still poses a challenge to develop new dielectric materials with low dielectric constant

Chapter 1 Table 1.1: The technology roadmap for interconnects in a microprocessor device [18]

Fig.1.3: Scanning electron microscopy (SEM) image of the low-k interfacial

delamination (upper-right of this image) during chemical-mechanical-polishing

process [20]

* RC delay is calculated by assuming no scattering and an effective ρ of 2.2 μΩ-cm

Chapter 1

A great number of low-k dielectric materials have been developed to replace

the conventional SiO2 They can be categorized into four groups: silsesquioxane (SSQ) based*, silica based, organic polymers and amorphous carbon [19,21,22] The value of

k reduces to as low as 2.5 by implementing the low-k dielectric materials, but to

achieve k value less than 2.5, meso or micro-pores have to be introduced into the low-k dielectric materials [18,21] The porous low-k dielectric materials are mechanically

weak and can be easily damaged during fabrication (chemical-mechanical-polishing) and assembly processes of the devices [20,23,24] Fig.1.3 shows the interfacial delaminations that occurred during the chemical-mechanical-polishing process [20] For successful integration, it is important to improve the mechanical properties and the

interfacial adhesion of the porous low-k dielectric films These integration challenges

have led to many theoretical and experimental studies on characterizing the

mechanical properties of the low-k dielectric films and their interfaces [16,25-36] Section 2.4 reviews the interfacial fracture of low-k films under instantaneous or fast loading, and the time-dependent fracture of low-k films due to reactive species in the

surrounding environment

1.3 Research Objectives and Significance

Nanoindentation and four-point-bending tests are the most popular

experimental methods to characterize the interfacial toughness of low-k films Volinsky et al showed that conical indentation alone is unable to delaminate the low-k

* Silsesquioxanes are organic-inorganic polymers with the empirical formula (R–SiO3/2)n, where R is usually a hydrogen atom or a methyl group

capable of accurately determining the interfacial toughness of low-k films [2]

Comparing to the nanoindentation methods, the sample preparation and experimental procedures of the four-point-bending test, however, are much more complicated and time consuming In the development of a new interfacial toughness characterization method, this thesis will focus on simplifying the existing nanoindentation methods and avoid the unnecessary complexities

The overall objective of this PhD study is to develop a simple experimental technique and a straight-forward analytical solution that can measure the interfacial mechanical properties of a thin film/substrate structure accurately For several reasons, this thesis proposes to use a wedge-shaped indenter to characterize the interfacial

toughness of the low-k thin films Firstly, the wedge indentation test can be performed

on an as-deposited thin film Because there is no sample preparation prior to the indentation test, the probability of sample damage/alteration is lower Secondly, the indentation raw data can be easily obtained from the test, and the interfacial toughness may be presented as a simple analytical formula Finally, due to the geometry of the wedge indenter, it is able to generate much higher crack driving force as compare to the conical or spherical shape indenters, allowing the test to be performed on samples with greater interfacial toughness

Chapter 1

In order to confirm the validity of this “wedge indentation method”, element method (FEM) is used to simulate the wedge indentation induced delamination process These FEM studies are part of a collaboration project with applied mechanics division, and the simulation works are beyond the scope of this thesis However, the experimental results of this thesis are important inputs for the

finite-FEM analysis The interfacial toughness values of the low-k/Si systems (organosilicate

(OSG) and methyl-silsesquioxane (MSQ) films) determined independently from the FEM analysis are compared to the results from the wedge indentation experiments to

evaluate the consistency between numerical and experimental results

Low-k films with silica network structure are known to fail prematurely in the

exposure to reactive chemical species To study the time-dependent fracture of OSG and MSQ films, this thesis proposes two indentation procedures: load-holding test and varying-loading-rate test When a long holding time or a slow loading rate is applied

during the wedge indentation, the interfacial delamination of the low-k/Si system may

occur at a very shallow indentation depth Qualitative analysis of the results from holding test and varying-loading-rate test may contribute to better understanding of the

load-time-dependent fracture of the silica based low-k film

After the wedge indentation method is proven to be reliable in quantifying the

interfacial toughness of soft-film-hard-substrate (SFHS) structures (low-k/Si systems),

the method is extended and applied on hard-film-soft-substrate (HFSS) structure The new material for microbattery electrode, ruthenium dioxide (RuO2) film, is selected in this thesis as the HFSS sample During the charge-discharge of the microbattery, the RuO2 electrode would experience structural and volume changes that may lead to

Chapter 1 reliability issues such interfacial delamination This exploratory study with the wedge indentation experiment may provide insights into the interfacial properties of the RuO2/Si system

More specifically, the objectives of this thesis are:

• to identify the most suitable indenter geometry for interfacial mechanical property characterization;

• to determine the fracture processes and the delamination crack shapes due to wedge indentation on a thin film/substrate structure;

• to develop an experimental methodology and possibly an analytical solution for characterizing the interfacial toughness and the time-dependent fracture properties of a thin film/substrate structure;

• to apply the wedge indentation method on SFHS structures (e.g OSG and MSQ films);

• to confirm the accuracy and the consistency of the wedge indentation method

in measuring the interfacial toughness of SFHS structures; and

• to apply the wedge indentation method on a HFSS structure (e.g RuO2 film)

Chapter 1

1.4 Thesis Outline

This PhD thesis is divided into eight chapters Chapter 1 includes the background information and the research objectives of this PhD thesis Chapter 2 reviews the literature findings on previous researches that are related to this PhD thesis Chapter 3 presents the sample materials and the experimental setup Chapters 4-7 present the results and discussions Chapter 4 develops the wedge indentation method

to characterize the interfacial toughness of low-k/Si systems Chapter 5 compares the

finite element simulations and the experiments of wedge indentation induced delamination Chapter 6 studies the time-dependent fracture behavior of the silica-

based low-k thin film under different environments Chapter 7 applies the wedge

indentation method on RuO2/Si system to quantify its interfacial toughness Chapter 8 concludes the contributions of this thesis and suggests some future research needed to improve the understanding on the interface mechanics of thin films

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Chapter 1

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Chapter 2: Literature Review

The crack systems (e.g radial crack and lateral crack) induced by an indentation test on a thin film/substrate structure are dependent on the mechanical properties of the structure (film, substrate and interface) and the indentation stress-strain field (e.g plane strain or plane stress) To quantify interfacial toughness, the formation of a lateral crack along the interface of a thin film/substrate structure is desired This chapter will therefore review the indentation experimental and analytical methodologies developed to characterize the interfacial delamination process and the interfacial adhesion properties Section 2.1 reviews the indentation load-penetration responses when interfacial fracture occurs in various thin film/substrate structures To determine the most suitable characterization method for a certain thin film system, it is important to evaluate the characteristics of each of the indentation tests Section 2.2 reviews the indentation methods developed to characterize the interfacial toughness of thin film/substrate structure, namely conical indentation [1,2] and wedge indentation [3-7], primarily focusing on the analytical solution and the experimental methodology Section 2.3 reviews the methodology to estimate work of indentation and interfacial toughness from the area under a load-penetration curve As the objective of this thesis

is to use the indentation experiments and analysis to study the mechanical aspects and

the reliability of the low-k dielectric films, the correlations between the fracture

properties (e.g interfacial fracture and time-dependent fracture) and the material

structure of the low-k films (MSQ and BD films) are reviewed in Section 2.4

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Chapter 2

The load-penetration (P-h) curve of an indentation test is the “fingerprint” of a

material, which is closely related to the elastic and plastic properties of the material and the indenter tip During indentation experiments, a nanoindentation system with a high depth-sensing resolution and a fast feed-back speed can sense even the smallest fluctuations of the properties in a material [9], such as plastic yielding and crack

initiation Fig 2.1 shows the nanoindentation P-h curves for some hypothetical cases

of different thin film/substrate systems that are well adhered and poorly adhered [8]

As indicated in the P-h curves (Fig 2.1), a more compliant indented area (the decrease

of the slope of the P-h curves) or a sharp increase of the penetration depth may be used

to predict the occurrence of interfacial delamination Nevertheless, microscopic observations (optical or electron microscopy) are always necessary to confirm the interfacial delaminations and to study the crack paths

2.2 Indentation Methods Developed to Determine the Interfacial Toughness of Thin Film/Substrate Structure

In this thesis, we are particularly interested in the application of the indentation methods to quantify the interfacial toughness of the thin films with thickness in the order of few hundred nanometers or even less Various indentation methods have been proposed to determine the interfacial toughness of thin film/substrate structures, such

as cross-sectional nanoindentation [10,11], wedge indentation [5-7], conical indentation [1,2] and superlayer nanoindentation [12] In general, the indentation methods have several advantages, such as: (a) the experiments are easy to conduct; (b) there is almost no sample preparation prior to the tests; and (c) the raw data are

Chapter 2 presented in the simple form of load and penetration depth Interpretation of the indentation data to obtain the interfacial toughness, however, requires advanced knowledge on the mechanics of interface and it may also require finite-element simulations Therefore, in this chapter, Sections 2.2.1-2.2.3 will discuss the details of the experimental methodology and the analytical solution of these indentation-based methods

In general, thin film/substrate systems can be divided into soft film on hard substrates (SFHS) systems, and hard film on soft substrate (HFSS) systems Both systems could have brittle or ductile interfaces depending on the nature of the films and substrates During an indentation test, an axisymmetric indenter (mainly Conical and Spherical indenter, and may include Vickers and Berkovich indenters to a certain extend) or a wedge indenter is pressed on the surface of the film For systems with brittle interfaces, the indenter tip may only need to penetrate into the film to cause an interfacial delamination by plastic deformation of the film material On the other hand, for the systems with strong or ductile interface, the indenter has to penetrate deeply into the substrate to cause an interfacial delamination by plastic deformation of the substrate material

The axisymmetric indentations are the most commonly-used methods to determine the elastic modulus and the hardness of thin films and bulk materials The method was further developed by Marshall et al to characterize interfacial toughness

of a brittle ceramic interface (ZnO/Si) [1,2] They have provided the important insights

of the hypothetical operations during the indentation induced delamination However, many studies have indicated the drawbacks in using the axisymmetric indentation [3,4]