mechanical properties, fracture and water diffusion in...
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I
Mechanical Properties, Fracture and Water
Diffusion in Nanoporous Low Dielectric Constant
Materials
A dissertation presented
by
Han Li
to
The School of Engineering and Applied Sciences
in partial fulfillment of the requirements
for the degree of
Doctor of Philosophy
in the subject of
Applied Physics
Harvard University
Cambridge, Massachusetts
April, 2010
II
© 2010 − Han Li
All rights reserved.
III
Thesis Advisor Author
Professor Joost J. Vlassak Han Li
Mechanical Properties, Fracture and Water Diffusion in
Nanoporous Low Dielectric Constant Materials
Abstract
Extendibility of Cu/Low-k integration schemes beyond the 45 nm node requires
integration of nanoporous dielectrics with greatly reduced permittivity into the back-end
process of integrated circuits (IC). However, existing candidate materials suffer
insufficient mechanical integrity and fracture resistance to survive the harsh fabrication
flow. Their open pore structure is susceptible to the ingress of various detrimental
chemicals during processing. In this work, we investigate these critical challenges that the
industry confronts. In chapter 3, the intrinsic effect of porosity on the stiffness and
fracture toughness are modeled by first separating out effects caused by difference in the
matrix material at different levels of porosity, and by then comparing with finite element
calculations and physical models. It is demonstrated that the fracture energy of porous
organosilicate glasses (OSG) is largely determined by the porosity only. However, the
elastic stiffness depends on both porosity and the morphology of the porous structure.
Chapter 4 provides quantitative guidelines for the bottom-up design of new organosilicate
materials with high modulus and low dielectric constant. Atomistic simulations are
IV
conducted to model the strengthening effects of incorporating organic cross-links into the
glass network and the detrimental effects of terminal groups. For the first time, it is
demonstrated OSG can be made considerably stiffer than amorphous silica, while
maintaining a lower mass density, by engineering the network structure. In chapter 5, we
investigate the direct impact of water diffusion on the fracture behavior of film stacks that
contain nanoporous OSG. We show that exposure of the film stacks to water causes
significant degradation of the interfacial adhesion energy without affecting the cohesive
fracture energy of the nanoporous OSG layer. Isotope tracer diffusion experiments
confirm that water diffuses predominantly along the interfaces, and not through the
porous films due to the hydrophilic character of the interfaces.
It is anticipated that the findings of this work will contribute to assess and improve
the mechanical reliability of nanoporous low-k dielectrics for current and future IC
technologies.
V
Table of Content
Title page …………… ........................................................................................................ I
Copyright page.. .......................................................................................................... II
Abstract…………… .........................................................................................................III
Table of Content..........................................................................................................V
List of Figures…………………………………………………………………………………………………………….IX
List of Related Publications ................................................................................. XVII
Acknowledgements ..............................................................................................XVIII
Chapter 1 Introduction............................................................................................1
1.1 Low-k dielectric materials in advanced microelectronics .....................1
1.2 Integration challenges and reliability of porous low-k dielectrics.........5
1.3 Research objective and outline of the thesis ..........................................8
Chapter 2 Experimental techniques......................................................................11
2.1 Introduction..........................................................................................11
2.2 Bond structure characterization by FT-IR ...........................................11
2.3 Porosity characterization methods .......................................................13
2.3.1 Ellipsometry-based porosimetry (EP).........................................13
2.3.2 X-ray reflectivity porosimetry ....................................................17
2.3.3 Microscopy methods...................................................................21
2.4 Stiffness measurement by nanoindentation .........................................23
2.5 Cohesive fracture energy measurement ...............................................25
2.6 Interfacial adhesion measurement........................................................27
VI
Chapter 3 Effect of porogen loading on the stiffness and fracture energy of
nanoporous organosilicates .................................................................29
3.1 Introduction..........................................................................................29
3.2 Experiments .........................................................................................32
3.3 Finite element simulation.....................................................................40
3.4 Results and discussion .........................................................................42
3.4.1 Composition uniformity..............................................................42
3.4.2 Decoupling porosity and matrix effects......................................44
3.4.3 Porosity effect on stiffness..........................................................49
3.4.4 Porosity effect on fracture energy...............................................52
3.4.5 Further discussion on WNBD.....................................................58
3.5 Conclusions..........................................................................................60
Chapter 4 Stiffening of organosilicates by organic cross-linking ........................62
4.1 Introduction..........................................................................................62
4.2 Models construction.............................................................................65
4.3 Simulation protocol..............................................................................68
4.4 Type-I OSG: effect of organic bridging units......................................69
4.4.1 Structural and elastic properties at zero pressure........................69
4.4.2 Bond deformation under hydrostatic and shear loading .............74
4.5 Type-II OSG: effect of terminal groups...............................................79
4.6 Implication for synthesis of low-k dielectrics with improved rigidity 85
4.7 Conclusion ...........................................................................................86
VII
Chapter 5 Water diffusion and fracture behaviors in porous dielectric thin film
stacks ...................................................................................................88
5.1 Introduction..........................................................................................88
5.2 Experiments .........................................................................................90
5.2.1 Materials and sample fabrication ................................................90
5.2.2 Methods.......................................................................................92
5.3 Results and discussion .........................................................................96
5.3.1 Adhesion degradation of the OSG/SiCN interface .....................96
5.3.2 Cohesive fracture of OSG...........................................................97
5.3.3 Water diffusion .........................................................................102
5.4 Discussion..........................................................................................106
5.5 Conclusion .........................................................................................112
Chapter 6 Conclusions........................................................................................113
6.1 Summary of the thesis........................................................................113
6.2 Suggestions for future work...............................................................116
Appendix: New methods for analyzing nanoindentation of ultra-thin films on
substrate .............................................................................................118
7.1 Introduction and review of existing methods.....................................118
7.2 Theory ................................................................................................122
7.2.1 Yu’s analysis: elastic indentation problem and solution...........122
7.2.2 Some useful results from Yu’s solution....................................125
7.3 Application to elastic indentation of anisotropic thin films...............130
7.4 Application to elasto-plastic indentations with significant pile-up....136
VIII
7.5 Application to elasto-plastic indentations without material pile-up ..143
7.5.1 Materials ...................................................................................146
7.5.2 Experimental Methods ..............................................................147
7.5.3 Results and discussion ..............................................................149
7.5.4 A few additional considerations................................................163
7.6 Conclusion .........................................................................................166
Bibliography ......................................................................................................167
IX
List of Figures
Figure 1-1 Gate and interconnect delay time as a function of technology generation [1].. 2
Figure 1-2 Practical strategies to achieve a low dielectric constant materials for IC
application, after [3]........................................................................................ 3
Figure 1-3 Interconnect dielectric materials that have been implemented in volume
manufacturing of IBM CMOS microprocessors [6]. ...................................... 4
Figure 1-4 Chemical structure of precursors used in the deposition of low-k
organosilicates [6]. .......................................................................................... 5
Figure 1-5 Fracture failure of low-k dielectric material in the form of cohesive fracture
and interfacial delamination [3]. ..................................................................... 7
Figure 2-1 Schematic illustration of the ellipsometry-based porosimetry for porosity
characterization. ............................................................................................ 14
Figure 2-2 Determination of pore size distribution using ellipsometry-based porosimetry
and toluene probe gas.................................................................................... 16
Figure 2-3 Schematic illustration of the X-ray reflectivity setup (top) and a typical
reflectivity curve (bottom). Note the two sudden drops in the reflectivity
curve is due to the film (lower angle) and the silicon substrate, respectively.
....................................................................................................................... 18
Figure 2-4 X-ray porosimetry design (left) and the fabricated attachment without
Beryllium windows (right)............................................................................ 19
Figure 2-5 Schematic setup for the XRP measurement, excluding XRD system............. 20
X
Figure 2-6 Grazing incidence X-ray reflectivity measurements OSG films before and
after saturating with toluene vapor. .............................................................. 20
Figure 2-7 Cross-section images of the nanoporous OSG films using (a) SEM, (b) HIM
and (c) TEM.................................................................................................. 22
Figure 2-8 Illustration of a DCB specimen under testing. ................................................ 25
Figure 2-9 Illustration of a DCB specimen fabrication process. ...................................... 26
Figure 2-10 Illustration of a 4-PB specimen under testing. The bottom figure shows an
optical microscopic image of the crack initiation at the corner of the pre-
notch and propagation into the interface....................................................... 28
Figure 2-11 Illustration of a 4-PB specimen fabrication process. .................................... 28
Figure 3-1 Plane-strain elastic modulus as a function of indentation depth for a
representative subset of the porous OSG films............................................. 37
Figure 3-2 Typical XPS spectra of the fracture surfaces of an OSG film with a porosity of
14.8% after the DCB test. The near-identical spectra of both surfaces
indicate that fracture propagates within the OSG films................................ 39
Figure 3-3 (a) Depth profile of atomic composition in OSG film with k=2.43, determined
by XPS. Hydrogen content has been excluded from calculation. (b) Atomic
composition of OSG films as a function of relative dielectric constant. ...... 43
Figure 3-4 Analysis of the density of function groups and networking bonds in the porous
OSG films based on the infrared absorption cross-sections and FT-IR
absorption spectrum. ..................................................................................... 44
Figure 3-5(a) Typical FT-IR absorption spectra of the OSG films; spectra are offset for
comparison purpose. (b) Survey of the bond densities in porous OSG films
XI
with different levels of porosity, calculated from the FT-IR spectra using the
inverse infrared cross sections from reference [17]. ..................................... 46
Figure 3-6 Plane-strain modulus of a series of dense OSG films as a function of WNBD;
data taken from [16]...................................................................................... 48
Figure 3-7(a) Experimental values of the function ( )f p , compared with finite element
simulations of various pore microstructures. (b) Effect of the pore size
distribution on the effective modulus of a porous material with non-
overlapping pores. There is no significant difference between materials with
monodispersive or multidispersive pore distributions. ................................. 50
Figure 3-8 Fracture energy of OSG as a function of overall WNBD. Data for both dense
and porous OSG films are shown. ................................................................ 54
Figure 3-9(a) Effect of porosity on the fracture energy of OSG as expressed by g(p). (b)
The number of bonds that need to be fractured is proportional to (1-p) in the
planar through-pore fracture mechanism. ..................................................... 55
Figure 3-10 AFM micrographs of fracture surfaces (1 x 1 μ m2) for films with porosity
levels of (a) p = 44.5%, (b) p = 18.9%, (c) p = 7%. (d) RMS roughness as a
function of film porosity and scan size. ........................................................ 57
Figure 4-1 Flowchart for generating the type-I and type-II OSG models. ....................... 65
Figure 4-2 Unit cell of (a) amorphous silicon with 64 Si atoms, (b) silica derived from
(a), (c) type-I OSG with five methylene cross-links before relaxation, (d)
type-I OSG with five methylene cross-links after relaxation, and (e) type-II
OSG with ten methyl terminal groups after relaxation................................. 67
XII
Figure 4-3(a) Mass density of type-I OSG, (b) various bond angle, and (c) bond length as
a function of CH2/Si ratio. The error bars denote one standard variation..... 71
Figure 4-4 Bulk modulus and shear modulus, and (b) Young’s modulus of type-I OSG as
a function of CH2/Si ratio for different potentials. ....................................... 73
Figure 4-5 Evolution of bond length and angle distribution in SiO2 (left panels) and
Si(CH2)2 under hydrostatic loading. ............................................................. 75
Figure 4-6 The distribution of the relative change of the Si-O-Si angles in silica under (a)
a shear stress of 1GPa, (b) hydrostatic pressures of 1 GPa (compressive) and
(c) hydrostatic pressure of -1 GPa. ............................................................... 78
Figure 4-7 (a) The bulk and shear modulus, (b) Poisson’s ratio, and (c) density of type-II
OSG as a function of network connectivity.................................................. 81
Figure 4-8 (a) The plane-strain modulus and (b) Young’s modulus of type-II OSG as
functions of density, compared with similar materials and models from
experiment (a) and MD simulations (b)........................................................ 83
Figure 5-1 Schematic side view of the sample used for the SIMS measurements. The
SiCN layer is an effective barrier to water diffusion, forcing water to diffuse
into the film stack only from the edge. The width of the sample is 3cm...... 96
Figure 5-2 Adhesion energy of the OSG/SiCN interface as a function of water immersion
time for samples ULK-1 (square) and ULK-1-liner (circle). The solid curve
is the best fit by the diffusion model (D=9.8±0.7 x 10-11m2/s). The dotted
curve is the prediction of the diffusion model using the diffusion coefficient
obtained from the SIMS measurements (D=3.32 x 10-11m2/s). .................... 97
XIII
Figure 5-3 DCB result for ULK-1 after immersion in water for 7 hours with (a) showing
the original load-displacement data, and (b) the energy release rate as a
function of load-point displacement. The solid curve in (a) shows the
predictions by assuming a fracture resistance of 2.6 J/m2, as marked in (b).99
Figure 5-4 Cohesive fracture energy of various OSG films as a function of water
immersion duration. .................................................................................... 100
Figure 5-5 Crack velocity as a function of energy release rate for the cohesive fracture of
OSG film stacks ULK-1 and ULK-1-liner.................................................. 101
Figure 5-6 18O concentration profile for ULK-1 (a) before and (b) after subtracting the
signal obtained from the control sample..................................................... 103
Figure 5-7 18O concentration profile for ULK-1-pt (a) before and (b) after subtracting the
signal obtained from the control sample..................................................... 104
Figure 5-8 Normalized peak intensity of the 18O signal along the OSG/SiCN interface for
ULK-1 and ULK-1-pt after subtracting the reference signal. The diffusion
coefficient of water is calculated by fitting the experimental data with
complementary error function..................................................................... 106
Figure 7-1 The dimensionless correction factor ξ for an elastic indentation as a function
of normalized contact radius for (a) different elastic mismatch and a conical
indenter, and for (b) various indenter shapes.............................................. 127
Figure 7-2 Normalized contact stiffness versus contact radius calculated from Yu’s
solution for (a) different elastic mismatches, (b) various conical and
spherical punches, and (c) comparison of the effective indentation moduli
XIV
derived from Yu’s solution and from elastic finite element calculations [98,
99] ............................................................................................................... 129
Figure 7-3 Comparison of (a) indentation load, (b) contact stiffness, and (c) effective
indentation moduli as a function of indentation depth calculated from
anisotropic FEM simulations (markers) and from Yu’s elastic solution (solid
curves). Arrows indicate the theoretical indentation moduli of the film
materials...................................................................................................... 132
Figure 7-4 Comparison of the load-displacement curves for gold films of various
thicknesses on sodium chloride substrates. The circular markers are
experimental data from [106]; the solid curves are based on Yu’s elastic
solution. Origins of the data sets have been shifted for clarity. The inset
shows the deviation of the indentation response from the monolithic contact
model due to the substrate effect; film moduli are the same. Tip radius
(700nm) and elastic properties of the substrate (Ms = 44.5GPa, sν = 0.25) are
taken as experimentally determined in [106].............................................. 135
Figure 7-5 Comparison of the experimental and theoretical estimations of the S-a relation
for (a) finite element simulations (Film-1-2 and Film-2-2), and for (b)
indentation experiments on Cu films on silicon and fused silica (FS)
substrates..................................................................................................... 141
Figure 7-6 Experimental load-displacement curves for the Si3N4 and SiO2 films, and for
the silicon substrate..................................................................................... 150
Figure 7-7 Curves of the experimental contact stiffness versus indentation depth for the
Si3N4 film, the SiO2 film, and for the silicon substrate............................... 151
XV
Figure 7-8 Experimental (markers) and theoretical (solid curves) contact stiffness versus
contact radius for the Si3N4 and SiO2 samples. The inset presents the same
data in the form of S/2a versus a/t. ............................................................. 152
Figure 7-9 The pressure-displacement curve for a freestanding LPCVD silicon nitride
film obtained in the bulge test. The inset is the corresponding plane-strain
stress-strain curve, yielding a plane-strain modulus of 257.2±1.5 GPa...... 153
Figure 7-10 The indentation modulus obtained with the Oliver-Pharr method as a
function of contact radius normalized by film thickness, compared with the
results obtained using the new method. The shaded regions represent the
ranges of the SiO2 and Si3N4 indentation moduli reported in the literature.156
Figure 7-11 The hardness of the SiO2 film as a function of indentation depth calculated
using several methods. The hardness of bulk fused quartz is included for
comparison.................................................................................................. 157
Figure 7-12 The hardness of the Si3N4 film as a function of indentation depth calculated
using several methods. The hardness for the silicon substrate is included for
comparison.................................................................................................. 158
Figure 7-13 Load-displacement curves for the two OSG films of the same properties but
different thicknesses on silicon substrate, interfacial delamination at position
circled.......................................................................................................... 160
Figure 7-14 Experimental and theoretical contact stiffness as a function of contact radius
for the various OSG films. The inset presents the same data in the form of
S/2a versus a/t. ............................................................................................ 161
XVI
Figure 7-15 Contour plot of ( )210log χ as a function of f sM M and η for the SiO2/Si
sample, with minimum falling within the highlighted region. The unit of 2χ
is in 2nm . .................................................................................................... 164
Figure 7-16 Indentation moduli of the SiO2 and Si3N4 films as a function of the value of
Poisson's ratio assumed in the data analysis. .............................................. 165
XVII
List of Related Publications
1 Han Li, Jan M. Knaup, Efthimios Kaxiras and Joost J. Vlassak, "Stiffening of
organosilicate glasses by organic crosslinking", submitted, 2010.
2 Han Li, Nicholas X. Randall and Joost J. Vlassak, "New methods of analyzing
indentation experiments on very thin films", J. Mater. Res., 25(4), 2010.
3 Han Li, Ting Y.Tsui and Joost Vlassak, "Water diffusion and fracture behavior in
nano-porous low-k dielectric film stacks", J. Appl. Phys, 106, 2009.
4 Han Li and Joost J. Vlassak, "Determining the elastic modulus and hardness of an
ultra-thin film on a substrate using nanoindentation", J. Mater. Res., 24(3), 2009.
5 Han Li, Youbo Lin, Ting Y.Tsui and Joost Vlassak, "The effect of porosity on the
stiffness and fracture energy of brittle organosilicates", J. Mater. Res., 24(1),
2009.
6 Han Li and Joost. J. Vlassak, “A novel method to measure the elastic modulus of
thin film on elastically mismatched substrate by nanoindentation”, US patent,
series No. 12/506,648, 2009.
XVIII
Acknowledgements
I am deeply grateful to my thesis advisor, Professor Joost Vlassak, for his mentoring
throughout my PhD tenure at Harvard. It is his invaluable guidance that allows me to
reach this point of my academic career. He has generously provided everything I could
ever expect. Constantly, he shares his unusual wealth of knowledge and experience; he
gives objective advice I can count on; he encourages to motivate; and he criticizes to
perfect. His bar of research quality and his eye for important details effectively keep me
busy and improving. It is truly a privilege I can have him as my advisor.
I would also like to thank the other members of my thesis committee for reviewing
this work, and more importantly, for kindly supporting me over the years. Professor Frans
Spaepen literally restructured my knowledge of material sciences with his classic
‘AP282’. His care of my development is also delivered through his valuable
encouragement, questions and comments. Professor Zhigang Suo is a phenomenal teacher
and scientist. His lectures and talks are remarkably enjoyable, to the point, and easy to
follow. My favorite is his perceptive remarks in class, digested from years of reflection of
a brilliant mind. Also from Zhigang, I gained a sense how joyful a scientific career can
be. I am grateful to Professor Joanna Aizenberg for her interest in my research and for all
the constructive questions and suggestions she offered. I had the honor to invite her over
to the FAS student-faculty dinner at Dudley that turned out pleasant. I hope my
interactions with all of them far exceed my stay at Harvard.
During the five years at Harvard, I have the opportunity to learn from many other
distinguished scholars, including James Rice, Mike Aziz, John Hutchinson, Ken Crozier
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and many more. I could not list them all, but I thank them all. It is their efforts and
dedication that have made the SEAS curriculum truly unique and outstanding.
Both past and present members of the Vlassak group have provided generous help in
many ways. Yong Xiang, Youbo Lin, Xi Wang and Patrick McCluskey taught me hand
by hand how to use the lab equipments when I first got here. Their help never stop there.
I cannot forget to mention and thank those who have made my personal life at
Harvard easier and memorable. They are Wei Hong, Zhen Zhang, Nanshu Lu, Xuanhe
Zhao, Xiaodong Zhang, Gidong, Sheng Xu, Yizhuo Chu and more. They are amazing
friends. A special thank you goes to Michael Louise, who has been my Harvard host
since my first day in the States. He is a friend I can always turn for help and unofficial
advice. I enjoyed all the time with him, for beer, for pool and simply for being together.
I feel lucky to have the opportunity to collaborate with Dr. Jan Knaup and Prof.
Efthimios Kaxiras in the physics department at Harvard. I benefit tremendously from
their rich experience and unique insight in atomistic simulations. Professor Scot Martin
of SEAS generously provides access to the FT-IR equipment used throughout this work.
His kind help is sincerely appreciated.
I acknowledge my appreciation of the trainings and professional assistance from Dr.
Richard Schalek, David Lange, JD, Ling Xie and more at the CNS, Harvard. Without
their help, my research would have taken much longer.
Last but certainly not least, it is my family who has always been there for me with
unconditional love and support. This dissertation is dedicated to them. Xiaochun is truly a
blessing for me. Time and distance could not separate us. She is a wonderful wife and my
best friend, understanding me and accommodating me. To my parents, I simply could not
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owe more. They bring me to the world and make me who I am. I am proud of them as
they are proud of me, or maybe more.
1
Chapter 1 Introduction
1.1 Low-k dielectric materials in advanced microelectronics
The continued shrinking of feature size in integrated circuits (IC) has long been
driven by a simple idea: Smaller transistors work faster, and allow a higher integration
capacity to be achieved on a single chip. The down-scaling of device dimensions
necessitates a significant change in the on-chip interconnect system to distribute the clock
signals and electrical power. That is, a reduction in the wiring pitch and an increase in the
wiring levels. As the feature dimensions reduce to smaller than 250nm, signal delay
caused by the interconnect resistance and capacitance (‘RC’ delay) becomes increasingly
important in limiting the overall chip performance, as illustrated in Figure 1-1 [1].
Consequently, new materials and interconnect architectures need to be introduced to
minimize the ‘RC’ delay with device scaling.
Chapter 1: Introduction
2
Figure 1-1 Gate and interconnect delay time as a function of technology generation [1].
The RC delay time at a given metallization level depends on the wire resistivity and
on the dielectric constant of the insulator between the wires [2]. As such, copper has
replaced aluminum as the mainstream wiring material to decrease the wire resistivity.
However, the effort to reduce the interconnect capacitance has been hampered by
significant challenges in developing process-friendly low-k dielectric materials.
The relative dielectric constant, k , of a material is a measure of how strongly the
dipoles in the material respond to an external electrical field. Debye's equation establishes
the relation between k and the various polarization modes of the dielectric material
through
21 42 3 3e d
b
k Nk k T
π μα α⎛ ⎞− = + +⎜ ⎟+ ⎝ ⎠ (1.1)
Chapter 1: Introduction
3
where N is the number density of dipoles in the material, eα the electronic polarization,
dα the ionic polarization, μ the orientation dipole moment due to permanent dipoles,
bk the Boltzmann constant and T the absolute temperature. Depending on the frequency
of the external electrical field, the relative dominance of each polarization mechanism
varies. At typical processor clock speeds (MHz to GHz range), all three polarization
mechanisms are important. Hence, to achieve a lower dielectric constant, the material
structure needs to be engineered to reduce the dipole density and to lower the polarization
of the chemical bonds. In practice, these ideas have been realized by reducing film
density and by incorporating less polar bonds, as illustrated in Figure 1-2. One successful
example is the fluorine doped silicon dioxide as the first generation low-k materials used
in the 180 and 130 nm technology (See the timeline in Figure 1-3).
Figure 1-2 Practical strategies to achieve a low dielectric constant materials for IC application, after [3].
Chapter 1: Introduction
4
To attain a dielectric constant smaller than 3.0, organosilicate glasses (OSG) have
been developed. OSG are organic-inorganic hybrid materials that possess a silica-like
backbone structure. The low dielectric constant is achieved by introducing organic
terminal groups into the silica network. A variety of chemical precursors (Figure 1-4) can
be used for preparing OSG thin films by plasma-enhanced chemical vapor deposition
(PECVD) or by sol-gel chemistry. The network modification decreases the material
density and lowers the bond polarizability relative to that of silicon dioxide, and hence
leads to improved dielectric performance. So far, only PECVD OSG films have been
successfully implemented in volume production [4, 5]. In comparison, OSG films
synthesized using wet chemistry suffer inferior mechanical reliability at equivalent
dielectric constant, and need to be further improved for device application.
Figure 1-3 Interconnect dielectric materials that have been implemented in volume manufacturing of IBM CMOS microprocessors [6].
Chapter 1: Introduction
5
Organosilicates are also widely regarded as the most promising low-k dielectrics for
future integrated circuit technologies. To further lower the dielectric constant, porous
versions of these materials will be required. However, implementation of porous OSG
low-k dielectrics becomes increasingly more difficult with decreasing device scale. In the
next section, we briefly review the integration challenges and reliability issues associated
with porous OSG.
1.2 Integration challenges and reliability of porous low-k dielectrics
Interlayer dielectrics must meet a number of stringent requirements in order to be
successfully integrated into the interconnect structure, including requirements on their
electrical properties, mechanical properties, thermo-chemical properties, and
compatibility with other materials. While the addition of porosity is effective in reducing
Figure 1-4 Chemical structure of precursors used in the deposition of low-k organosilicates [6].
Chapter 1: Introduction
6
the film dielectric constant, it also poses numerous integration challenges. First of all,
sufficient mechanical integrity is required for the porous OSG to withstand the rigors of
the fabrication process, and to minimize the susceptibility of interconnects to electro-
migration failure during service. The compliance of the dielectric material is also an
important material parameter for the optimization of chemical mechanical polishing
(CMP) process. This is because the pressure distribution between the slurry and the
multilayered interconnect film stack, and thus the polishing rate, depends directly on the
elastic properties of the dielectrics. Compared to their dense counter parts, porous
dielectrics possess a much reduced stiffness that leads to amplified local deformation and
that promotes delamination of the capping layer from the underlying dielectrics.
The fabrication of multilevel interconnect structures subject the dielectric materials to
multiple thermal cycles that may reach 400ºC or above, and cause thermal-mechanical
stresses in the multilayer stacks due to thermal expansion mismatch between different
materials. Stresses can also be generated during the CMP process. Such stresses can lead
to fracture of OSG in the form of cohesive cracking and/or interfacial delamination (cf.
Figure 1-5 ). Indeed, dielectric fracture failure is a major reliability issue in current back-
end technology, and is anticipated to become significantly worse as the degree of porosity
increases. Hence, high fracture resistance and good adhesion to adjacent layers are
important criteria when screening the next-generation low-k materials. Considerations are
also warranted on how the dielectric materials interact with the process flow and
integration scheme.
Chapter 1: Introduction
7
Figure 1-5 Fracture failure of low-k dielectric material in the form of cohesive fracture and interfacial delamination [3].
With respect to chemical and electrical reliability issues, the porous structure makes
the dielectric coatings vulnerable to the penetration of water and other reactive chemicals
during device fabrication. It has been reported that water can diffuse quite effectively into
film stacks containing dielectrics layer, even though the dielectric materials are usually
hydrophobic [7-10]. The ingress of water into the dielectrics stacks negatively impacts
both the electrical performance of the devices and their mechanical integrity. On the
electrical side, water has a relative dielectric constant of approximately 80 owing to the
polar O-H bonds, so that even a small amount of water uptake can be fatal to the overall
dielectric characteristics. Water also influences the leakage current of the dielectrics
adversely. From the mechanical standpoint, the presence of water decreases the resistance
of the dielectric materials to various forms of fracture caused by stress corrosion (i.e.,
subcritical crack growth) [9, 11-13]. Small cracks can grow over time even when the
mechanical driving force is well below the material’s intrinsic fracture resistance, causing
reliability issues and yield loss.
Chapter 1: Introduction
8
Other integration challenges concern the material compatibility in the interconnect
structures, such as thermal expansion mismatch, chemical stability at elevated
temperature as well as heat dissipation. Sensitivity to process-induced damage during
plasma treatment, etching, and ashing is yet another important limiting factor for
developing next generation low-k materials.
1.3 Research objective and outline of the thesis
As reviewed in the previous section, poor mechanical properties and fracture
resistance of porous low-k dielectrics are important reliability issues that are anticipated
to become even worse as the degree of porosity increases. Moreover, the fracture
behavior of a dielectric in a multilayered stack couples with the diffusion of water and
other reactive species. How water is transported and how it affects fracture behavior is of
great concern because of the technological impact on the reliability of interconnect
fabrication as well as its scientific significance. In this work, we investigate these critical
challenges that the semiconductor industry confronts, and aim at achieving a better
understanding of the underlying mechanics and physics that can facilitate the
development and implementation of porous low-k dielectrics for future generation of IC
application.
In chapter 2, selected experimental techniques used in this study will be briefly
reviewed, with focus targeted on the general working principles.
In chapter 3, intrinsic porosity effect on the stiffness and fracture toughness are
modeled by first separating out effects caused by matrix difference at different levels of
porosity, and then comparing with finite element calculation and physical models. Such a
Chapter 1: Introduction
9
separation relies on the capability of characterizing the density of various networking
bonds in the dielectric film, and on the experimentally established correlation between
bond density and material properties. It was demonstrated that the fracture energy of
porous OSG is largely determined by the porosity only, however elastic stiffness depends
on both porosity and the morphology of the porous structure.
In chapter 4, atomistic simulations are utilized to understand the fundamental
relationship between molecular network structure and resulting physical and mechanical
properties of an emerging class of organosilicates. Molecular models with well-controlled
network structure can be generated and simulated using molecular dynamics to quantify
the effect of network connectivity on material properties, especially the stiffening of the
network by organic crosslinking. For the first time, it is demonstrated OSG can be made
considerably stiffer than amorphous silica while maintaining a lower mass density by
engineering the glass network structure.
In chapter 5, we investigate the direct impact of water diffusion on the fracture
behavior of film stacks that contain nanoporous organosilicate. We show that exposure of
the film stacks to water causes significant degradation of the interfacial adhesion energy
without affecting the cohesive fracture energy of the nanoporous OSG layer. The
adhesion degradation behavior can be well described by an analytical 1D diffusion
model. The result is further corroborated by isotope tracer diffusion experiments and
contact angle measurements, which consistently indicate that water diffuses
predominantly along the interfaces, and not through the porous films due to the
hydrophilic character of the interfaces.
Chapter 6 summarizes the major results and presents an outlook for future research.
Chapter 1: Introduction
10
As a side project related to the thesis work, we have developed new methods of
analyzing indentation experiments on very thin films to extract intrinsic mechanical
properties of the films. These methods are not limited to low-k dielectrics but are for
general elastic and elasto-plastic indentations. This work is presented at the end of the
thesis as an appendix.