Acknowledgements It is my great pleasure to have the opportunity here to record my gratitude to people and institutions that made this thesis possible. Foremost, I would like to express my utmost gratitude and appreciation to my supervisor, Professor FENG Yuan Ping for his invaluable guidance, kindness and encouragement throughout my research. And it is my co-supervisor Dr. ZHENG Jia Zhen who guides me into the field on CMOS and transition-metal silicides. I would like to thank Dr. Lap CHAN and Dr. Alex SEE in Chartered Semiconductor Manufacturing Ltd for their education on the advanced technologies in semiconductor industry. I also thank Assoc. Prof. SHEN Ze Xiang, Assoc. Prof. Thomas OSIPOWICZ, Dr. LIU Jin Ping, and Dr. CHI Dong Zhi for their constructive suggestions and great support on my experimental work. My gratitude is also conveyed to all my colleagues and friends in Theoretical and Computational Condensed Matter Physics group, Laser Spectroscopy Laboratory group, and Special Project group, for their continuous co-operations, fruitful discussions, and warm care during my PhD study. The award of Research Scholarship by National University of Singapore and the financial support from Chartered Semiconductor Manufacturing Ltd. throughout this project are highly appreciated. Finally, I owe my beloved family for their consistent love and support. i Table of Contents Acknowledgements……………………………………………………………………i Table of Contents……………………………………………….…………………ii Summary……………………………………………………………………………v List of Tables………………………………………………………………………viii List of Figures………………………………………………………………………ix Chapter Introduction…………………………………………………………1 1.1 Evolution of CMOS Technologies……………………………………1 1.2 Transition-Metal Silicides……………………………………………7 1.3 Objective and Scope…………………………………………………17 References……………………………………………………………19 Chapter Experimental and Computational Methodologies………………24 2.1 Thin Film Growth………………………………….…………………24 2.2 Thin Film Characterization Techniques……………………………29 2.3 First-Principles Calculation………….………………………………37 References……………………………………………………………52 Chapter Growth and Stability of Ultra-Thin Nickel Silicide Films………55 3.1 Introduction…………………………………………………………55 3.2 Ultra-Thin Nickel Silicide Film Growth……………………………56 3.3 Electrical Property of Nickel Silicides………………………………57 ii 3.4 Phase Identification and Transformation……………………………60 3.5 Thermal Stability……………………………………………………65 3.6 Conclusions…………………………………………………………70 References………………………………………………………… 71 Chapter Surface and Interface Roughness of Silicide Thin Films…………72 4.1 Introduction…………………………………………………………72 4.2 New Approach by Micro-Raman Imaging…………………………74 4.3 Modeling of Interface Roughness……………………………………77 4.4 Surface and Interface Roughness……………………………………81 4.5 Agglomeration Mechanism…………………………………………95 4.6 Conclusions…………………………………………………………99 References……………………………………………………………99 Chapter Interface Reconstruction of Silicide/Si(001) Heterostructure…101 5.1 Introduction…………………………………………………………101 5.2 Interfacial Structures of Silicide/Si(001)……………………………102 5.3 Models and Calculation Details……….……………………………105 5.4 Results and Discussion…………………………….………………109 5.5 Conclusions…………………………………………………………117 References………………………………………………………… 118 Chapter Strain Effects on Interface of Silicide/Si(001) Heterostructure .120 6.1 Introduction…………………………………………………………120 6.2 Total Energy Calculations…………………………………………122 iii 6.3 Strain Effect on Interface Atomistic Configuration………………127 6.4 Strain Effect on Interface Formation Energetics……………………129 6.5 Strain Effect on Electronic Properties………………………………131 6.6 Strain Effect on Schottky Barrier Height…………………………135 6.7 Conclusions…………………………………………………………146 References…………………………………………………………147 Chapter Ge Composition Effects on Formation of Germanosilicides……149 7.1 Introduction…………………………………………………………149 7.2 Total Energy Calculations…………………………………………151 7.3 Ge Composition Effect on Atomistic Structure……………………160 7.4 Ge Composition Effect on Energetics………………………………161 7.5 Ge Composition Effect on Electronic Properties…………………164 7.6 Conclusions .………………………………………………………171 References…………………………………………………………171 Chapter Conclusions and Future Work……………………………………173 8.1 Conclusions…………………………………………………………173 8.2 Future Work………………………………………………………175 Publication List……………………………………………………………………177 iv Summary Transition-metal silicides are self-aligned to Si with low resistivity, good adhesion, and self-passivation characters. Silicides have been widely used to decrease contact resistance and thus increase speed of devices in complementary metal-oxidesemiconductor (CMOS) ultra-large-scale integration (ULSI) technologies. Among various silicides, titanium, cobalt, and nickel silicides are the most popular materials used in Si fabrication process. Rapid pace of development leads semiconductor industry exciting but facing a lot of challenges. As device feature dimensions shrink to nanoscale, new architectures and materials have to be employed. Silicides have been extensively studied for a few decades both experimentally and theoretically. However, most of the studies have been carried out on the conventional bulk Si substrate. Silicides for advanced CMOS technologies are still not well-studied due to the lack of effective characterization tools and their formation mechanisms are still far from deep understanding. The goal of this research is to fill in some of the technical gaps through both experimental and theoretical approaches. Ultra-shallow junction is a necessity of shrinkage in lateral dimensions of CMOS devices. To avoid excessive consumption of silicon, ultra-thin silicide films must be adopted. The formation process and stability of ultra-thin nickel silicide films with as-deposited metal thickness down to 4nm have been systematically studied. It was found that thermal stability of NiSi degrades gradually with the v decrease in the as-deposited Ni film thickness, especially when thinner than 8nm. Both the NiSi film agglomeration and the NiSi2 phase transformation take place at lower temperatures as Ni film thickness decreases. Interface condition is crucial for applications of ultra-thin films. To effectively determine the morphology of interfaces between silicide thin films and silicon substrate, a new technique, microRaman imaging (µMI), has been successfully developed in our study based on modeling and experiments. The thickness of silicide films is determined by the attenuation of Si Raman peak at 520cm-1. Compared with cross-section TEM and SEM, micro-Raman imaging does not require special sample preparation and can easily resolve features of micrometer scale in a plane and nanometer scale in the normal direction. Strained-Si is a promising replacement of conventional Si substrate due to the enhanced carrier mobility of both electrons and holes. Various interface structures of CoSi2/Si(001) and NiSi2/Si(001) system with different degrees of strain have been intensively investigated. To understand the effects of strain/stress, first-principles calculation was used to expand the system to extreme conditions (from fully-relaxed to fully-strained) which is difficult to reach via experimental approaches. A new model for the MSi2/Si interface, the sevenfold-Z model with a zigzag Si dimer arrangement at the interface, has been proposed. The proposed model is equally possible as the existing model with comparable interface formation energy and atomic plane space. The possible coexistence of this model and the existing one can well explain the ambiguity in the earlier STEM study. Strain does show great impact on atomistic, energetic, electronic and transport properties of the interfaces. With vi biaxially tensile strain induced by Si substrate, the atomic distance in the perpendicular axis is compressed accordingly. In terms of energetics, the higher the in-plane strain, the smaller the interface formation energy generally. Strain in substrate also causes the compression in PDOS of Si atoms, but has less impact on metal atoms. Most importantly, Schottky barrier height of the MSi2/Si(001) heterojunction changes significantly with strain, which has direct impact on device performance. As new materials are being introduced into advanced CMOS technologies, such as SiGe, chemical composition of the substrates will also influence the solidstate reaction process in silicidation. xGex)2 Germanosilicides Co(Si1-xGex)2 and Ni(Si1- with different Ge compositions, ternary compounds formed on Si1-xGex substrates, have been systematically investigated using first-principle method. Simulation results show that Ge composition dominates the configuration of the ternary compounds. The equilibrium volume of M(Si1-xGex)2 shows a linear increase with the increase of the Ge composition x, following Vegard’s law. Si/Ge-rich system tends to form the MSi2 plus Ge rather than MSiGe and SiGe or MGe2 and Si. The lower cohesive energy of M(Si1-xGex)2 with larger x explains the reason for the tendency of Ge agglomeration and cluster formation in the compounds under thermal stress. Ge composition also affects electronic properties of silicides. With increase of the Ge composition, upward shift of the valance bands and compression of the whole energy states are observed. The M d − band becomes stronger and narrower with the increase of Ge composition in the silicides. vii List of Tables Table 1.1 Important properties of common self-aligned silicides. Table 3.1 Silicide film thickness and relative ratios of silicide to Ni films for different as-deposited metal films. Table 5.1 Optimized lattice parameter c (Å) and calculated interface formation energy. ∆Eform is the difference in formation energy of a given structure and that of the sevenfold-R model. Table 5.2 Bond length (Å) between interface metal atoms and their neighboring Si atoms for sevenfold-R and Z configurations. Refer to Figure 5.6 for labels of the bonds. Table 5.3 Calculated inter-atomic-plane distance (Å) for the sevenfold-R and Z models of NiSi2/Si(001). Refer to Figure 5.6 for labeling of atomic planes. Table 6.1 Calculated lattice constants of Si, CoSi2, and NiSi2 with strain varying from 0% (fully-relaxed as aSi ) to 100% (fully-strained as aGe ). Table 6.2 Equilibrium lattice constant c0 and volume V0 of various MSi2/Si(001) interface structures with substrate strain varying from 0% to 100%. Table 6.3 Minimum total energy and interface formation energy of various MSi2/Si(001) interface structures with substrate strain varying from 0% to 100%. Table 6.4 Lineup and p-type SBH Φ p of various MSi2/Si(001) interface structures with substrate strain varying from 0% to 100%. Table 7.1 Lattice constants of Co, Ni, Si, CoSi2 and NiSi2 obtained by LDA and GGA. Table 7.2 Calculated equilibrium volume, total energy, and bulk modulus of M(Si1-xGex)2 with Ge composition x variation. viii List of Figures Figure 1.1 Cross section of modern CMOS transistor with an n-channel MOSFET and a p-channel MOSFET. Figure 1.2 Intel CPU processing power projection by Moore’s Law. (Cited from http://www.intel.com/technology/mooreslaw/) Figure 1.3 Phase diagram of Ni-Si phase binary system. (Cited from SGTE alloy databases 2004) Figure 2.1 Schematic illustration of a RF sputtering chamber. Figure 2.2 Schematic diagram of a rapid thermal processing system. Figure 2.3 Energy level diagram of Raman scattering. Figure 2.4 Schematic diagram of a four-point probe. Figure 2.5 Principle of Rutherford Back-scattering Spectrometry. Figure 2.6 Schematic diagram of an atomic force microscope (cited from Vecco application notes). Figure 3.1 Sheet resistance of nickel silicide (NiSi and NiSi2) films as a function of initial Ni thickness and annealing temperature. Figure 3.2 Silicide formation rate as a function of the initial Ni film thickness. To calculate the film thickness, the resistivity of NiSi and NiSi2 are taken as 13 ohm/sq and 50 ohm/sq respectively. Figure 3.3 Raman spectra of NiSi formed at 450°C with different initial Ni thickness. Figure 3.4 Raman spectra of NiSi2 formed at 800°C with different initial Ni thickness. Figure 3.5 Raman spectra of NiSi formed at 450°C and NiSi2 formed at 800°C on (100) Si substrates. The Raman spectrum with co-existence of two phases indicates the phase transition process from NiSi to NiSi2 at the critical temperature of 750°C. ix Figure 3.6 RBS spectra of 16nm as-deposited Ni films under various thermal treatments, (a) uniform NiSi film formed at 500°C, (b) agglomerated NiSi film formed at 750°C, and (c) NiSi2 film formed at 850°C. Figure 3.7 AFM images of Ni/Si samples with 16nm as-deposited Ni on top of Si substrates annealed at (a) 500°C, (b) 600°C, (c) 700°C, (d) 750°C, (e) 800°C, and (f) 850°C. RMS surface roughness increases with temperature dramatically when temperature higher than 750°C. Figure 3.8 Comparison of sheet resistance and surface roughness of 16nm Ni/Si samples as a function of annealing temperatures. Figure 4.1 Schematic diagram of the scanning Raman system: 1-XYZ scanning stage, 2-tip, 3-driving piezo, 4-laser diode, 5-microscope objective, 6laser, 7-Raman signal, 8-quadrant detector, 9-sample, 10-controllor, 11spectrometer, 12-computer. 1, 2, 3, and constitute the scanning head, which is put on the mechanical stage of the microscope. Laser is delivered by microscope objective 5, which is also used to collect Raman signal in this embodiment. Figure 4.2 Schematic diagram of film thickness measurement by micro-Raman spectroscopy. The laser and Raman signal are exponentially attenuated by the optical absorption thin film. Figure 4.3 Schematic diagram of thin film with (a) two-step interface and (b) sinusoidal interface configurations. Figure 4.4 Calculation results of Si Raman peak intensity attenuation at 520cm-1 as a function of (a) uniformity variable x for two-step configuration; and (b) amplitude A for sinusoidal configuration. Figure 4.5 Si Raman peak intensity at 520cm–1 attenuated by NiSi thin films. The inset shows the calculated NiSi film thickness from 45nm down to 10nm, which is verified by RBS, where the Si peak intensity of the film with thickness of 10nm is chosen as the reference intensity I0. Figure 4.6 Standard deviation of Si Raman peak intensity and derived film thickness. The 30nm NiSi film forms under 500°C RTA. Figure 4.7 Raman spectra of the C49 phase, C54 phase and C40 phase TiSi2 on (100) Si substrates. The C49 phase is annealed by RTA, while the C40 phase is annealed by PLA. The C54 phase can be obtained based on both of them. Figure 4.8 Micro-Raman images of (a) C49 TiSi2 phase formed at 600°C and (b) C54 TiSi2 phase formed at 800°C plotted by intensity of Si Raman peak. x Figure 4.9 Micro-Raman image of as-deposited 16nm pure Ni film plotted by (a) the intensity of Si Raman peak at 520cm-1 and (b) the resulting calculated thickness of the film. Figure 4.10 Cross-section SEM image of the interface between NiSi film and Si substrate. Figure 4.11 Micro-Raman images of NiSi film formed in N2 ambient at 500°C for 30s, plotted by (a) the intensity of Si Raman peak at 520cm-1 and (b) the resulting calculated thickness of the film. Figure 4.12 Micro-Raman images of NiSi film formed in N2 ambient at 500°C for 30s, plotted by (a) the intensity of Si Raman peak at 520cm-1 and (b) the intensity of NiSi Raman peak at 214cm-1. Figure 4.13 AFM and Micro-Raman morphologies of NiSi films after annealing at (a) 500°C, (b) 600°C, (c) 700°C, and (d) 750°C. Figure 5.1 Conventional MSi2/Si(001) interface models: (a) sixfold, (b) eightfold, (c) LJY, (d) BJV, (e) sevenfold unconstructed, and (f) sevenfold reconstructed. Figure 5.2 Interface structures, (a) sixfold; (b) eightfold; (c) sevenfold-R; and (d) sevenfold-Z, of MSi2/Si interface in < 110 > (top) and < 10 > (bottom) directions. Si atoms are shown by the yellow balls while metal atoms are represented by the blue balls. Figure 5.3 (a) Side view and (b) top view of the sevenfold-R and sevenfold-Z interface structures. For easy reference, metal atoms at the layer beneath are projected to the interfacial layer. Si atoms are shown by the yellow balls while metal atoms are represented by the blue balls. Figure 5.4 Calculated total energy as a function of the lattice parameter c , for the four interface models with 13 atomic layers of CoSi2 and 15 atomic layers of Si each. Figure 5.5 Calculated total energy as a function of the lattice parameter c , for the four interface models with 13 atomic layers of NiSi2 and 15 atomic layers of Si each. Figure 5.6 Bond and atom layer of sevenfold-Z interface structures in (a) < 110 > and (b) < 10 > directions. Si atoms are shown by the yellow/light balls while metal atoms are represented by the blue/dark balls. Figure 5.7 Contour plots of valence electron charge density (electron/A3) at the Sidimer interface plane for (a) sevenfold-R model and (b) sevenfold-Z model, overlaid by atomic structures at the interface plane with Si atoms (light sphere) and projected M atoms (dark ball) for easy reference. xi Figure 6.1 Stacking structure of a thin strained-Si layer grown on relaxed Si1-xGex. Figure 6.2 Total energy per supercell versus c axis length of various CoSi2/Si(001) interface supercells on free (solid symbol) and 20% strained (open symbol) Si substrate as example. Figure 6.3 Total energy per supercell of CoSi2/Si(001) and NiSi2/Si(001) interface supercells as a function of Si substrate strain. Figure 6.4 and V0 of CoSi2/Si(001) sixfold and sevenfold-R interface supercell as a function of Si substrate strain. Figure 6.5 Interface formation energy of CoSi2/Si(001) and NiSi2/Si(001) interface supercells as a function of Si substrate strain. Figure 6.6 Projected density of states of Si at atomic layers from Si bulk to interface to CoSi2 bulk in CoSi2/Si(001) sixfold interface supercell with free Si substrate. Fermi energy of the system is 5.797 eV. Figure 6.7 Projected density of states of Si at atomic layers from Si bulk to interface to NiSi2 bulk in NiSi2/Si(001) sixfold interface supercell with free Si substrate. Fermi energy of the system is 5.886 eV. Figure 6.8 PDOS of Si atoms at interfacial layer in CoSi2/Si(001). Figure 6.9 PDOS of Co atoms at interfacial layer in CoSi2/Si(001). c0 Figure 6.10 Schematic band diagrams for metal/Si heterojunction. Schottky barrier heights ( Φ n and Φ p ) is shown. Definition of Figure 6.11 Planar and macroscopic average of electrostatic potentials of various CoSi2/Si(001) interfaces structures with free Si substrate. Figure 6.12 Planar and macroscopic average of electrostatic potentials of various NiSi2/Si(001) interfaces structures with free Si substrate. Figure 6.13 Macroscopic average of electrostatic potentials of various CoSi2/Si(001) interfaces structures. Figure 6.14 Macroscopic average of electrostatic potentials of various NiSi2/Si(001) interfaces structures. Figure 6.15 p-type SBH of CoSi2/Si(001) and NiSi2/Si(001) interface supercells as a function of Si substrate strain. . Figure 7.1 Crystal structure of (a) face-centred cubic Co/Ni, (b) diamond Si, and (c) cubic MSi2. xii Figure 7.2 Crystal structure of cubic M(Si0.5Ge0.5)2 (Grey – Metal, White – Si, Black – Ge). Figure 7.3 Possible configurations of Si atoms substituting by Ge atoms for M(Si0.5Ge0.5)2 (White – Si, Black – Ge). Figure 7.4 Total energy per unit cell versus volume change of Co(Si1-xGex)2 as a function of Ge composition x. Figure 7.5 Total energy per unit cell versus volume change of Ni(Si1-xGex)2 as a function of Ge composition x. Figure 7.6 Unit cell volume of M(Si1-xGex)2 change with Ge composition x. Figure 7.7 Cohesive energy Ecoh of M(Si1-xGex)2 change with Ge composition x. Both of them have a negative bowing parameter. Figure 7.8 Band structure and DOS of MSi2. Fermi level is set at the origin. Figure 7.9 Band structures of MSi2 (solid line), MSiGe (dash line), and MGe2 (dot line). Fermi level is set at the origin. Figure 7.10 M(Si1-xGex)2 DOS shifts with Ge composition x. Fermi level is set at the origin. Figure 7.11 M(Si1-xGex)2 projected DOS shifts with Ge composition x. Fermi level is set at the origin. xiii [...]... 6.3 Total energy per supercell of CoSi2/Si(001) and NiSi2/Si(001) interface supercells as a function of Si substrate strain Figure 6.4 and V0 of CoSi2/Si(001) sixfold and sevenfold-R interface supercell as a function of Si substrate strain Figure 6.5 Interface formation energy of CoSi2/Si(001) and NiSi2/Si(001) interface supercells as a function of Si substrate strain Figure 6.6 Projected density of states... Ni(Si1-xGex )2 as a function of Ge composition x Figure 7.6 Unit cell volume of M(Si1-xGex )2 change with Ge composition x Figure 7.7 Cohesive energy Ecoh of M(Si1-xGex )2 change with Ge composition x Both of them have a negative bowing parameter Figure 7.8 Band structure and DOS of MSi2 Fermi level is set at the origin Figure 7.9 Band structures of MSi2 (solid line), MSiGe (dash line), and MGe2 (dot line) Fermi... film formed in N2 ambient at 500°C for 30s, plotted by (a) the intensity of Si Raman peak at 520 cm-1 and (b) the intensity of NiSi Raman peak at 21 4cm-1 Figure 4.13 AFM and Micro-Raman morphologies of NiSi films after annealing at (a) 500°C, (b) 600°C, (c) 700°C, and (d) 750°C Figure 5.1 Conventional MSi2/Si(001) interface models: (a) sixfold, (b) eightfold, (c) LJY, (d) BJV, (e) sevenfold unconstructed,... to interface to CoSi2 bulk in CoSi2/Si(001) sixfold interface supercell with free Si substrate Fermi energy of the system is 5.797 eV Figure 6.7 Projected density of states of Si at atomic layers from Si bulk to interface to NiSi2 bulk in NiSi2/Si(001) sixfold interface supercell with free Si substrate Fermi energy of the system is 5.886 eV Figure 6.8 PDOS of Si atoms at interfacial layer in CoSi2/Si(001)... face-centred cubic Co/Ni, (b) diamond Si, and (c) cubic MSi2 xii Figure 7 .2 Crystal structure of cubic M(Si0.5Ge0.5 )2 (Grey – Metal, White – Si, Black – Ge) Figure 7.3 Possible configurations of Si atoms substituting by Ge atoms for M(Si0.5Ge0.5 )2 (White – Si, Black – Ge) Figure 7.4 Total energy per unit cell versus volume change of Co(Si1-xGex )2 as a function of Ge composition x Figure 7.5 Total energy... (a) the intensity of Si Raman peak at 520 cm-1 and (b) the resulting calculated thickness of the film Figure 4.10 Cross-section SEM image of the interface between NiSi film and Si substrate Figure 4.11 Micro-Raman images of NiSi film formed in N2 ambient at 500°C for 30s, plotted by (a) the intensity of Si Raman peak at 520 cm-1 and (b) the resulting calculated thickness of the film Figure 4. 12 Micro-Raman... model, overlaid by atomic structures at the interface plane with Si atoms (light sphere) and projected M atoms (dark ball) for easy reference xi Figure 6.1 Stacking structure of a thin strained-Si layer grown on relaxed Si1-xGex Figure 6 .2 Total energy per supercell versus c axis length of various CoSi2/Si(001) interface supercells on free (solid symbol) and 20 % strained (open symbol) Si substrate as example... atoms at interfacial layer in CoSi2/Si(001) c0 Figure 6.10 Schematic band diagrams for metal/ Si heterojunction Schottky barrier heights ( Φ n and Φ p ) is shown Definition of Figure 6.11 Planar and macroscopic average of electrostatic potentials of various CoSi2/Si(001) interfaces structures with free Si substrate Figure 6. 12 Planar and macroscopic average of electrostatic potentials of various NiSi2/Si(001)... sevenfold reconstructed Figure 5 .2 Interface structures, (a) sixfold; (b) eightfold; (c) sevenfold-R; and (d) sevenfold-Z, of MSi2/Si interface in < 110 > (top) and < 1 10 > (bottom) directions Si atoms are shown by the yellow balls while metal atoms are represented by the blue balls Figure 5.3 (a) Side view and (b) top view of the sevenfold-R and sevenfold-Z interface structures For easy reference, metal. .. with 13 atomic layers of NiSi2 and 15 atomic layers of Si each Figure 5.6 Bond and atom layer of sevenfold-Z interface structures in (a) < 110 > and (b) < 1 10 > directions Si atoms are shown by the yellow/light balls while metal atoms are represented by the blue/dark balls Figure 5.7 Contour plots of valence electron charge density (electron/A3) at the Sidimer interface plane for (a) sevenfold-R model . Introduction………………………………………………………… 120 6 .2 Total Energy Calculations………………………………………… 122 iv 6.3 Strain Effect on Interface Atomistic Configuration……………… 127 6.4 Strain Effect on Interface Formation. Chapter 2 Experimental and Computational Methodologies…………… 24 2. 1 Thin Film Growth………………………………….……………… 24 2. 2 Thin Film Characterization Techniques………………………… 29 2. 3 First-Principles Calculation………….………………………………37. attenuation at 520 cm -1 as a function of (a) uniformity variable x for two-step configuration; and (b) amplitude A for sinusoidal configuration. Figure 4.5 Si Raman peak intensity at 520 cm –1