Experimental computational studies on transition metal silicides for novel applications in advance CMOS technologies

Chapter Chapter Introduction Introduction One of the most important inventions of the 20th Century is transistor, first made by John Bardeen, Walter Brattain, and William Shockley in Bell Laboratories in 1948. Integrated circuit (IC), which is a group of transistors manufactured from a single piece of material and connected together internally, is able to implement more complicated functions in a much easier way. Especially in the past two decades, the dramatic development of microcomputers has pushed the IC packing density double every couple of years. The latest Pentium-4 microprocessor packs 42 millions transistors within one chipset, which is called ultra-large-scale-integration (ULSI) technology. As a result of decreasing device dimension, many issues related to the scaling become critical and give rise to a series of hot research topics on designing new materials and technologies to meet requirements of the rapid development. 1.1 Evolution of CMOS Technologies The semiconductor revolution began in 1947 with bipolar devices fabricated on slabs of polycrystalline Ge. Single-crystalline materials were later introduced, making possible the fabrication of junction transistors. Development of Si-based devices was initially hindered by the easy oxidation of Si, necessitating a new generation of crystal pullers with improved environmental control to prevent SiO2 formation. Later the stable Si/SiO2 system with low interface-state density eventually enabled the migration from bipolar devices to field-effect devices in 1960. By 1968, complementary metal-oxide-semiconductor (CMOS) devices and polysilicon gate Chapter Introduction technology allowing self-alignment of gate to source/drain had been developed. 4, CMOS technology employs both n-channel (NMOS) and p-channel (PMOS) transistors to form logic elements. The cross-section of a CMOS device is illustrated in Figure 1.1. The advantage of CMOS technology is that very little current is needed to maintain the device state and substantial current is drawn only during the state transition, resulting in significant power saving. The innovations of CMOS technology permitted a significant reduction in power dissipation and device overlap capacitance, thus operation frequency was significantly improved to make them essential components in modern ICs. Figure 1.1 Cross section of modern CMOS transistor with an n-channel MOSFET and a p-channel MOSFET. 1.1.1 Scaling of CMOS Technology In 1965, Gordon E. Moore, the co-founder of Intel Corp., predicted that the number of transistors on an IC would double every eighteen months to two years, which has been acting as a guide for the IC industry ever since. Figure 1.2 plots how the processing power measured in millions of instructions per second (MIPS) Chapter Introduction follows the Moore’s Law by Intel CPUs. With increase of the number of the transistors integrated, the cost of implementing the same functions decreases. While silicon-based components and platform ingredients gaining in performance, they become exponentially cheaper to produce, and therefore more plentiful, more powerful, and more seamlessly integrated into our daily lives. Therefore, the driving force for each technological generation has always been the same: enhanced performance and functionality, improved reliability, increased throughput, decreased power dissipation, and reduced cost. Figure 1.2 Intel CPU processing power projection by Moore’s Law. (Cited from http://www.intel.com/technology/mooreslaw/) CMOS devices are now being scaled to gate lengths below 50 nm and Moore’s prediction suggests that the scaling is likely to continue for at least another Chapter Introduction decade. Significant challenges are unavoidable when the MOSFET gate lengths are reduced. In particular, the gate oxide thickness in state-of-the-art production devices is now below nm and increases quantum mechanical tunneling of charge through the gate insulator, resulting in the off-state current increase. A secondary effect of the reduction of gate insulator thickness and the required high channel doping is the reduction of electron and hole mobilities in the inversion layers of CMOS transistors through interactions with the charges in the gate. The degradation of the carrier mobility lowers the drain current and increases band-to-band tunneling across the junction and gate-induced drain leakage (GIDL). Moreover, statistical fluctuation of channel dopants causes increasing variation of the threshold voltage, posing difficulty in circuit design while scaling the supply voltage. Therefore enormous efforts and a number of technology solutions are being pursued to circumvent these problems. 1.1.2 Advanced CMOS Technologies Silicon is the material which has dominated the semiconductor industry for over 97% of microelectronic products over 30 years. In conventional Si technology, CMOS is one of the most popular devices because of low cost, simple processing as well as high input impedance. However, as a consequence of lower mobility of holes compared with electrons in Si, the p-channel devices are inferior to the n-channel ones in terms of current drive capability and speed performance. To match the current drive capability of NMOS devices, PMOS devices are designed about 2~3 times larger than NMOS, which adversely affects the level of integration and device speed. In order to improve the speed of ULSI circuits, new materials and device structures Chapter Introduction are being proposed. SiGe and Strained-Si heterostructure MOSFETs are two of the most favorable adventures. 9-11 SiGe Heterostructure MOSFET: SiGe technology has been driven to improve some of mediocre properties of silicon while retaining the current mature and cheap Si fabrication processes. With the advent of the first SiGe heterojunction bipolar transistor (HBT) in 1998, the market for SiGe devices in the radio-frequency (RF) market has increased over time and is predicted to increase at 30% per annum in the future. Being integrated into CMOS, SiGe devices are playing a more important role in semiconductor industry. 12, 13 The first study on SiGe can be traced back to 1955 on the magnetoresistance of silicon germanium alloys. 14 In the 1950s, conceptual SiGe device was patented as a bipolar transistor with a principle description in physics. 15 Such a transistor required epitaxial growth of SiGe heterostructures, and it was only demonstrated until 1975 by Kasper and colleagues using molecular beam epitaxy (MBE). 16 Substantially the field has rapidly expanded in the era of the 1980s by developing growth technology, the 1990s by HBT devices, and the start of the 2000s by strainedSi CMOS. Strain-induced modification of Si/SiGe films is found to have a significant impact on the band structure and carrier transport. Theoretically, a lattice- mismatched layer grown on a thick substrate is pseudomorphic. The lattice mismatch between Si and Ge is 4.2%, resulting in a very high misfit dislocation density. 17 If a thin Si1−xGex film is grown on top of a Si1−yGey film, the top layer is compressively Chapter Introduction strained when x > y , while tensile strained when x < y . For a Si1−xGex film grown on top of a Si substrate, the biaxial compressive strain results in the entire band offset in the valence band while the band offset in the conduction band is very small. This type of structure is favorable for hole confinement and has been exploited in several novel heterostructure devices, i.e. HBTs, buried channel p-MOSFETs, and p-channel modulation-doped field effect transistors (p-MODFETs). 18 The addition of a compressively strained pseudomorphic Si1−xGex channel for PMOS can produce a buried quantum well. By removing holes from a Si/SiO2 interface and placing them at a smoother heterointerface along the splitting of lighthole and heavy-hole bands, the reduced interface roughness scattering would increase the hole mobility. However, to confine carriers in the quantum well, a large discontinuity (Ge content x  0.2 ) is required at the heterointerface. The significant amounts of Ge in the quantum well increase alloy scattering therefore decreases the mobility of the holes. 19 Strained-Si Heterostructure FET: The advances in growth of strained-Si layers on relaxed Si1−xGex buffer layers have led to increased interest in Si-based heterojunction field effect transistors (HFETs) using conventional bulk Si technology. A smaller lattice constant silicon epilayer is under biaxial tension when grown on a larger lattice constant relaxed Si1−xGex substrate. For a strained-Si epilayer grown on relaxed Si1−xGex, a larger band offset is obtained in both the conduction and valence bands, relative to relaxed Si1−xGex layer. 20 Since strained-Si does not suffer from alloy scattering, a significant improvement in carrier mobility can be achieved. Chapter 21 Introduction It allows both electron and hole confinements in the strained-Si layer, making it useful for both n- and p-type devices for strained-Si based CMOS technology. The tensile strain splits conduction band valleys with the ∆ valleys being lowered in energy and the ∆ valleys being increased in energy to such an extent that only the lower ∆ valleys have any significant population of carriers. 22-24 It is the reduction of inter-valley scattering which has been demonstrated to be held responsibility for the significant increases in the NMOS mobility both at room and low temperatures. 25, 26 Strained-Si on insulator has also been used to enhance the mobility further by the reduction in capacitance through coupling to the substrate. These mobility enhancements are at all vertical effective electric fields, demonstrating that the enhancements can be achieved in deep sub-micrometer transistors. Strained Si is more difficult to grow than strained Si1−xGex. There are two major obstacles. The first one is that bulk Si1−xGex substrate is currently not available yet. The other reason is concentration of defects and dislocation due to large misfit. Recent studies show that with the incorporation of a small amount of C atoms to develop new types of buffer layers, misfit dislocations may be reduced in the Si/SiGe material system. 27 Anyway, the ability to achieve both NMOS and PMOS devices using strained Si provides a promising alternative for next-generation highperformance CMOS technology. 28 1.2 Transition-Metal Silicides No matter how the CMOS technology moving forward, as the link step between the front-end-of-line and back-end-of-line of the device manufacturing, self- Chapter Introduction aligned silicidation (salicidation) has always been a critical process for the ULSI CMOS fabrication. 29-31 Therefore, the fundamental mechanisms of their formation and stability are of great interests, and the solid-state reaction between a thin metal film and Si has been extensively and systematically analyzed. 32-35 1.2.1 Formation of Silicides In a salicidation process, silicides are simultaneously formed on the gate and source/drain areas without any additional masking. After the gate and source/drain junctions are fabricated, a thin metal film is blanket deposited by physical vapor deposition (usually sputtering) on top of the whole wafer. Silicide is formed with one or two rounds of rapid thermal annealing. The unreacted metal is thereafter removed by selective wet etch which etches metal much faster than silicides. The essence of the salicidation process is that only the metal in contact with the Si in source, drain, and polysilicon gate regions transforms to silicide, while the other part in contact with the SiO2 spacers remains as pure metal. The metal silicide self-aligned with Si becomes an indispensable component for IC fabrication due to its low resistivity, good adhesion, and self-passivation nature. It reduces the sheet, parasitic, contact, and interconnect resistance, thus shortening the RC delay time and consequently enhancing the performance speed of the device. Furthermore, the self-passivating nature of silicides in oxygen-rich environment and its perfect and stable adhesion to Si substrate make silicides the preferred materials over pure metals in CMOS processing. Last but not the least, the self-aligning property helps to save more spaces for routing and eliminates misalignment at all. Chapter Introduction Fundamental mechanisms related to silicide formation involve phase formation sequence, growth kinetics and microstructures. Two different mechanisms have been observed in metal/silicon reactions: diffusion-controlled and nucleationcontrolled mechanisms. 36, 37 It is important to note that thin film reactions differ from those on bulk reaction. Most of thin film formation involves both inter-diffusion and chemical reactions between phases at the interface. 38 Diffusion-Controlled Mechanism: For most of the metal/silicon thin film reactions, the square of formed silicide layer thickness varies linearly with time. The slope of the curve gives the rate of formation and the variation with temperature gives the activation energy for growth. The driving force for diffusion is the gradient of chemical potential expressed in the Nernst-Einstein equation as ⎛D J i = Ci ⎜ i ⎝ kT ⎞ d µi ⎟ ⎠ dxi (1.1) where J i , Ci , Di and µi are the flux, concentration, diffusion coefficient and chemical potential of the diffusing species, respectively, k is the Boltzmann constant and T is the temperature. It was found that the growth rate and the diffusion coefficient scale with ∆G , the free energy charge per moving atom when such an atom reacts to form a new phase. The kinetics of silicide growth is characterized by (1) formation rate which represents the effective diffusion coefficient of the most mobile species through the silicide layer and (2) activation energy which is related to the silicide melting temperature. The reaction kinetics of thin film is similar to that of bulk material, but the diffusivity is higher in thin film due to grain boundary diffusion enhancement. 39, 40 Previous experiments showed that lattice diffusion of Ni atom is Chapter Introduction slow with higher activation energy while grain boundary diffusion is rapid with lower activation energy in a Ni/Pt bilayer and Si diffusion couple. Nucleation-Controlled Mechanism: When two phases are in thermal equilibrium, at either melting or evaporation points, the free energy change ∆G is equal to zero. Any deviation from the equilibrium temperature can generate a driving force for the transition to another phase. Reduction of volume free energy is opposed by an increase in surface energy induced by the creation of nucleus. Some silicides such as NiSi2 form at relatively high temperature in an abrupt manner. The formation of a new phase implies the creation of an additional interface. The associated increase in the free energy is compensated by an equivalent or greater decrease of the surface energy due to the formation of the new phase. 1.2.2 Popular Silicides for CMOS Technology Self-aligned formation is the common feature of transition-metal silicides. Table 1.1 lists some key properties of transition-metal silicides. 41, 42 Among the silicides, the use of platinum silicide (PtSi) is mainly limited for bipolar transistors and IR detectors because of its low thermal stability and low Schottky barrier on n-Si. Titanium silicides (TiSi2) is the most commonly used material till 0.25µm generation. 43, 44 However, the sheet resistance of TiSi2 increases drastically with decreasing gate linewidth due to incomplete C49 TiSi2 to C54 TiSi2 transformation on narrow lines. Cobalt silicide (CoSi2) is a good substitute for the generation of 0.18µm down to 65nm technologies. 45 But the Si consumption of CoSi2 is much higher than other silicides. As a result, nickel silicide (NiSi) which 10 Chapter Ge Composition Effects on Formation of Germanosilicides phase is observed, with concurrent surface accumulation of Ge forming Ge-rich Si1xGex precipitates. 8, From the results of the total energy calculation, the energetically preferable phase can be easily determined. To simulate the formation of germanosilicide in a Si/Ge-rich environment, we consider a small system, with metal atoms, Si atoms, and Ge atoms, surrounded by silicide/silicon bulk. The system can have three different configurations: (1) one silicide MSi2 cube plus one Ge cube, (2) one germonasilicide M(Si0.5Ge0.5)2 cube plus one Si0.5Ge0.5, and (3) one germanide MGe2 cube plus one Si cube. If we neglect the interaction between silicide/germanide and Si/Ge, the total energy of the mimic system can be determined as E1 = EMSi2 + EGe = −7154.981eV E2 = EMSiGe + ESiGe = −7146.232eV E3 = EMGe2 + ESi = −7137.428eV Results of our calculation clearly show that the first case with MSi2 and separate Ge cluster combination gives the lowest energy compared to other mixtures. This can well explain why germanosilicides are thermally unstable compared to pure silicides from the energetic point of view. During the formation or under thermal stress, Ge atoms tend to agglomerate into small clusters to reduce the composition of Ge in neighboring area so that M(Si1-xGex)2 with smaller x is more stable with lower energy and can be formed. To further understand the relative stability of M(Si1-xGex)2 with different Ge composition, cohesive energy Ecoh , which is the energy required to break the atoms of 162 Chapter Ge Composition Effects on Formation of Germanosilicides the crystal into isolated atomic species, was calculated. Here, the cohesive energy is defined as Ecoh = −( Etot − ∑ N i Eiso ,i ) (7.3) i where, Etot is the total energy per unit cell of a crystal, Eiso ,i is the energy of an isolated atom of species i , and N i is the number of species i in one unit cell. To obtain Eiso , one atom of each species was placed at the center of a cubic supercell with the lattice constant of 10A and the single point energy of the isolated atom was calculated under the same condition as germanosilicides. Cohesive Energy (eV/atom) 5.4 Co(Si1-xGex)2 Ni(Si1-xGex)2 Fitting Curve 5.2 5.0 4.8 4.6 4.4 4.2 0.0 0.2 0.4 0.6 0.8 1.0 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. 163 Chapter Ge Composition Effects on Formation of Germanosilicides The calculated cohesive energy of MSi1-xGex as a function of the Ge composition x is shown in Figure 7.7. Higher cohesive energy of germanosilicides M(Si1-xGex)2 with smaller Ge composition x indicates that they are more energetically favorable and stable. The curve can be quantified by a parabola with a bowing parameter b , which describes the overall deviation from the linear interpolation between Eiso, MSi2 and Eiso, MGe2 . Ecoh ( x) = xEcoh, MGe2 + (1 − x) Ecoh, MSi2 + bx(1 − x) (7.4) The bowing parameter b was found negative and has a small value of -0.189 for CoSi-Ge and -0.148 for Ni-Si-Ge. The negative values indicate that M(Si1-xGex)2 is unfavorable in energetics compared with MSi2 and MGe2. To be noted, there is no existence of NiGe2 in the Ni-Ge binary phase diagram. The NiGe2 phase can only be formed under high pressure. 7.5 Ge Composition Effect on Electronic Properties Besides atomistic structure and thermal stability, electronic properties of silicides are also important. Band structures of CoSi2 and NiSi2 along high-symmetry lines in the Brillouin zone in a primitive cell with one M atom and two Si atoms are shown in Figure 7.8. The origin of the energy is taken to be the Fermi energy of the system. Band structure of MSi2 can be treated as a super-position of a simple cubic Si lattice with about a 15eV valance-band width combined with nearly isolated eightfold-coordinated Metal atoms. Although there is only one M atom per primitive cell, there are several levels at symmetry points in the Brillouin zone with large M d components. At Γ point, the splitting of the Co d − states centered at around -3eV is 164 Chapter Ge Composition Effects on Formation of Germanosilicides 2.107eV, and that of the Ni d − states centered at around -4eV is 1.641eV, respectively. The valence electron density-of-states (DOS) are also plotted in Figure 7.8. The valance band with the lowest energy mainly below -10.0eV is associated with the Si 3s orbitals and exists in nearly every kind of silicide without either hybridization of the Si sp or bonding between transition-metal and silicon. From -9.5eV up to 6.0eV, the energy states show a mixture of the Si 3s , p and the M 4s orbitals. With the increase of energy, the contribution of the Si 3s orbitals becomes weaker while the intensities of the Si p and M 4s orbitals are more dominant, indicating interference between them. The bonding and antibonding interactions between the M 3d and Si p orbitals generate similar structures on both sides of the primary M 3d orbitals, which give rise to the highest intensity in DOS at -1.9eV for CoSi2 and 3.4eV for NiSi2. The significant peak located at lower energy side and the relative weak one besides can be attributed to the bonding states and the features at higher energy side are identified as the antibonding states. Significant changes in the energy distribution of electronic states are induced due to the interaction in all transition metals with a high density of states at the Fermi energy EF : part of the d − states are detached from the main d − band to form antibonding states, and bonding states below the bottom of the d − band. From this respect the effect of the Si p /metal 3d coupling is to broaden the energy region where a nonvanishing density of d − states is present, although the nonbonding portion of the band is narrower than in the pure metal. The structures appearing on the two sides of the main d -band show significant contributions from the Si orbitals: 165 Chapter Ge Composition Effects on Formation of Germanosilicides they arise from bonding and antibonding combinations of Si p and M 3d states. For NiSi2, bonding states are located between -4eV and -7eV, while antibonding states are largely empty. As a consequence of hybridization, metal d − states are distributed over a large energy region. Although the high density nonbonding portion of the d-band narrows on passing from pure metals to silicides, the d − orbitals, which are involved in chemical combinations with Si states, occupy a wider region. Therefore their DOS is lower than in the central part of the band or in the pure metal. 10 Energy (eV) -5 -10 CoSi2 -15 10 Energy (eV) -5 -10 -15 NiSi2 W L G X Band Structure Figure 7.8 W K DOS (electrons/eV) Band structure and DOS of MSi2. Fermi level is set at the origin. 166 Chapter Ge Composition Effects on Formation of Germanosilicides Band structures of fully-relaxed MSi2 (solid line), MSiGe (dash line), and MGe2 (dot line) have been calculated and shown in Figure 7.9. As a result of the enlargement of the lattice constant induced by the larger Ge atoms, an upward shift of the valance bands and compression of the whole energy states (including both the valance and conduction bands) can be clearly observed with the increase of the Ge composition. For Co silicides, more significant difference is observed in conduction band area, while for Ni silicides, the variation is more localized in valence band area. However, there is an opposite trend around the gamma point near the Fermi level. For instance, the top of the sixth band of NiSi2 is flat around the gamma point with an average energy of -0.39eV; the band of NiSiGe shows a bowing with energy of 1.6eV at the gamma point although the rest of the band shifts upward like others; and the bowing is more severe for NiGe2 (-2.3eV) and even crosses the lower band. The bowing is supposed to be arisen from the difference between the interaction of the Si/Ge p / p and Ni 3d orbitals. Further investigation of the Ge composition effect on the electronic properties of the ternary compounds had been carried out and the total DOS are shown in Figure 7.10. With the increase of the Ge composition, significant shift and split can be observed within the range of the d − band to the Fermi level. The curves clearly show the entire d − band shift towards the Fermi level with higher intensity but narrower width. To understand the contribution of different orbitals, partial DOS projected from the total DOS to the M 3d , M 4s , Si/Ge 3s / s , and Si/Ge p / p orbitals are plotted in Figure 7.11, respectively. Here, the Si/Ge 3s / s orbitals not have much variation in valence band region, therefore are neglected in our discussion. PDOS of 167 Chapter Ge Composition Effects on Formation of Germanosilicides the M 3d , M 4s , and Si/Ge p / p orbitals show the similar trend of shifting towards the Fermi level with more Ge. The intensity of the M 4s is generally very small compared to others and neglected. With the shift, the major peak of the transition-metal d − band becomes stronger in intensity and narrower in width, showing more concentrated states. While the additional peak at the bottom of the d − band remains almost the same intensity after shifting. However due to the increase of the Si/Ge p / p orbitals, the overall change of the bonding states of the ternary compounds is still stronger and narrower. For transition metals, the main contribution to the cohesive energies is the formation of the d − band. 22 When metal atoms are brought together, the atomic d − orbitals form a band, whose width depends upon the hopping integrals between d − orbitals on neighboring atoms. The d − band is centered and broadened about a resonant level and lies higher with respect to the vacuum than in the free atom because of the conduction s − and p − electrons which are attracted into the WignerSeitz cell to make it neutrally charged and the substantial charge compression on forming the metal. 23 The strength of the d − bonding varies parabolically along a transition metal row, being largest near the centre of the row, i.e. for a half-filled band. When silicon atoms are inserted into the transition metal lattice, the atomic d − orbitals are driven farther from each other, the hopping integrals decrease, the d − band narrows, and the resonant levels move to higher binding energies. These lead to a weakening of the d − bonding strength and to a loss of cohesive energy. To offset the loss and stabilize the compounds, a substantial coupling between Si states and metal orbitals must take place, so that bonding states can be formed which are 168 Chapter Ge Composition Effects on Formation of Germanosilicides more tightly bound than either of the states from which they originate. When the Si atoms are replaced by Ge atoms, the atomic d − orbitals are driven in an opposite way to move back and make the d − band narrow, due to the larger atomic radius. Co(Si1-xGex)2 10 Ni(Si1-xGex)2 Energy (eV) -5 -10 -15 W Figure 7.9 L G X WK W G X WK Band structures of MSi2 (solid line), MSiGe (dash line), and MGe2 (dot line). Fermi level is set at the origin. Ni(Si1-xGex)2 Co(Si1-xGex)2 DOS (arbitrary unit) L x=1 x=1 x=0.875 x=0.875 x=0.75 x=0.75 x=0.625 x=0.625 x=0.5 x=0.5 x=0.375 x=0.375 x=0.25 x=0.25 x=0.125 x=0.125 x=0 x=0 -12 -8 -4 Energy (eV) -12 -8 -4 Energy (eV) Figure 7.10 M(Si1-xGex)2 DOS shifts with Ge composition x. Fermi level is set at the origin. 169 Chapter Ge Composition Effects on Formation of Germanosilicides 28 x=0 x=0.125 x=0.25 x=0.375 x=0.5 x=0.625 x=0.75 x=0.875 x=1 Ni-3d 21 14 PDOS (electron/eV) Ni-4s 0.9 0.6 0.3 0.0 Si-3s & Ge-4s Si-3p & Ge-4p -14 -12 -10 -8 -6 -4 -2 Energy (eV) 24 x=0 x=0.125 x=0.25 x=0.375 x=0.5 x=0.625 x=0.75 x=0.875 x=1 Co-3d 18 12 PDOS (electron/eV) Co-4s 0.9 0.6 0.3 0.0 Si-3s & Ge-4s Si-3p & Ge-4p -14 -12 -10 -8 -6 -4 -2 Energy (eV) Figure 7.11 M(Si1-xGex)2 projected DOS shifts with Ge composition x. Fermi level is set at the origin. 170 Chapter Ge Composition Effects on Formation of Germanosilicides 7.6 Conclusions In this chapter, the atomic structure, thermal stability, and electrical properties of transition-metal germanosilicides Co(Si1-xGex)2 and Ni(Si1-xGex)2 have been systematically investigated using first-principle pseudopotential method. According to different occupancies of the Si/Ge sites in the unit cell, the largest deviation of the randomness is more than 30meV/atom, demonstrating that Ge composition plays a dominant role to determine the configuration of the ternary compounds. The equilibrium volume of M(Si1-xGex)2 show a linear increase with the increase of the Ge composition x, following the Vegard’s law. Si/Ge-rich system tends to form the MSi2 plus Ge rather than MSiGe and SiGe or MGe2 and Si. And 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 shows effects on electronic properties of silicides. With increase of the Ge composition, upward shift of the valance bands and compression of the whole energy states can be observed. The M d − band becomes stronger and narrower with increase of Ge composition in the silicides. References: P. M. Mooney and J. O. Chu, Annu. Rev. Mater. Sci. 30, 335 (2000). B. S. Meyerson, D. Harame, J. Stork, et al., Int. J. High Speed Electron. Syst. 5, 473 (1994). U. Konig, Mater. Res. Soc. Symp. Proc. 533, (1998). A. Hokazono, K. Ohuchi, M. Takayanagi, et al., IEDM Tech. Dig., 639 (2002). 171 Chapter Ge Composition Effects on Formation of Germanosilicides S. Gannavaram, N. Pesovic, and M. C. Ozturk, IEDM Tech. Dig., 437 (2000). T. Jarmar, J. Seger, F. Ericson, et al., J. Appl. Phys. 92, 7193 (2002). K. L. 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B. Yao, S. Tripathy, and D. Z. Chi, Electrochem. Solid State Lett. 7, G323 (2004). 22 23 J. Friedel, The Physics of Metals (Cambridge University Press, London, 1969). L. Hodges, R. E. Watson, and H. Ehrenreich, Phys. Rev. B 5, 3953 (1972). 172 Chapter Conclusions and Future Work Chapter Conclusions and Future Work 8.1 Conclusions Novel applications of transition-metal silicides in advanced CMOS technologies have been systematically studied both experimentally and computationally in this work. Thickness shows remarkable effects on ultra-thin nickel silicide film formation process and thermal stability. The thermal stability of NiSi is degraded gradually with the decreasing of the as-deposited Ni film thickness, especially when the Ni film is thinner than 8nm. Both the NiSi film agglomeration and the NiSi2 phase transformation take place at lower temperatures as Ni film thickness decreases. The NiSi phase grows without any preferred orientation on (100) Si and is thermally stable independent of the initial thickness at 500°C, while the NiSi2 phase nucleates irregularly but epitaxially in certain areas when the annealing temperature approaches to around 750°C. We demonstrate that micro-Raman imaging (µMI) technique is feasible to image the interface morphology using the Si Raman band intensity attenuation at 520cm-1. The morphology of interfaces between metal or silicide thin films and silicon substrate can be effectively determined. The thickness of silicide films is correlated to the intensity of the Si Raman peak which is attenuated due to absorption. Compared with cross-section TEM and SEM, micro-Raman imaging does not require 173 Chapter Conclusions and Future Work special sample preparation and can easily resolve features of micrometer scale in a plane and nanometer scale in the normal direction (depth profile). Study on interface reconstruction of CoSi2/Si(001) and NiSi2/Si(001) heterostructures has been carried out using first-principle pseudopotential method. A new sevenfold-Z model with a zigzag Si dimer arrangement at the interface is proposed. The new model is energetically stable, and its valence electron charge density is more uniform and symmetric compared with the existing sixfold, eightfold and sevenfold-R models. The possible coexistence of this model and the sevenfold-R model can well explain the clear image in one direction and the blurred Si dimer image in perpendicular direction observed in the earlier STEM study. Furthermore, various interface structures of CoSi2/Si(001) and NiSi2/Si(001) system with different degrees of strain have been intensively investigated. Strain does show great impacts on atomistic, energetic, electronic and transport properties of the interfaces. With biaxially tensile strain induced by Si substrate, the atomic distance in the perpendicular direction is compressed accordingly. The compression is interfacedependent and thereafter the equilibrium volume of the interface supercell may not be the one without strain. In terms of energetics, the higher the in-plane strain, the smaller the interface formation energy generally. Strain in substrate also causes the compression on PDOS of Si atoms, but not much impact on metal atoms. Most importantly, Schottky barrier height of the MSi2/Si(001) heterojunction changes significantly with strain. Smaller barrier height difference in high strain condition suggests that coexistence of various interface configurations are allowed in applications. 174 Chapter Conclusions and Future Work Ge composition can affect formation and properties of germanosilicides Co(Si1-xGex)2 and Ni(Si1-xGex)2, ternary compounds formed on Si1-xGex substrates. According to different occupancies of the Si/Ge sites in the unit cell, the largest deviation of the randomness is more than 30meV/atom, demonstrating that Ge composition plays a dominant role to determine 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. And 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 shows effects on electronic properties of silicides. With increase of the Ge composition, upward shift of the valance bands and compression of the whole energy states can be observed. The M d − band becomes stronger and narrower with increase of Ge composition in the silicides. 8.2 Future Work Our computational results have predicted the strain effects on formation energy and electronic properties of silicide/Si heterojunction interfaces. Although interface formation becomes easier under higher strain, thermal process needs to be carefully tuned to avoid strain relaxation in strained-Si substrates. Moreover silicide film thickness has to be thin enough to consume less Si due to the critical thickness limit of strained-Si substrates. Schottky barrier height highly depends on interface structures. An energetically favourable configuration may not be the electrically 175 Chapter Conclusions and Future Work favourable one. How to form stable and desired interface structure will be challenge but meaningful for novel applications. It is well established that metal is the dominant diffusion species in the solidstate reaction during silicidation. The impact of different substrates (e.g. SiGe and strained-Si) on diffusion coefficients is another interesting topic. 176 Publication List F. F. Zhao, Y. P. Feng, and J. Z. Zheng, “Strain Effects on Interface of Silicide/Si(001) Heterostructure”, submitted to Phys. Rev. B. F. F. Zhao, Y. P. Feng, and J. Z. Zheng, “Ge Composition Effects on Formation Mechanism and Properties of Transition-Metal Germanosilicides M(Si1-xGex)2”, Phys. Rev. B, under revision. F. F. Zhao, Y. P. Feng, Y. F. Dong, and J. Z. Zheng, “Interface Reconstruction of MSi2/Si(001) (M=Co, Ni) from First Principles”, Phys. Rev. B 74, 033301 (2006). F. F. Zhao, W. X. Sun, Y. P. Feng, J. Z. Zheng, Z. X. Shen, C. H. Pang, and L. H. Chan, “Approach to Interface Roughness of Silicide Thin Films by MicroRaman Imaging”, J. Vac. Sci. Technol. B 23 (2), 468 (2005). F. F. Zhao, Z. X. Shen, J. Z. Zheng, T. Osipowicz, “Thermal Stability Study of NiSi and NiSi2 Thin Films”, Microelectron. Eng. 71, 104 (2004). F. F. Zhao, S. Y. Chen, Z. X. Shen, X. S. Gao, J. Z. Zheng, A. K. See, L. H. Chan, “Applications of Raman Microscopy in Salicide Characterization for Si Device Fabrication”, J. Vac. Sci. Technol. B 21 (2), 862 (2003). F. F. Zhao, Z. X. Shen, J. Z. Zheng, W. Z. Gao, T. Osipowicz, C. H. Pang, P. S. Lee, A. K. See, “Thickness Effect on Nickel Silicide Formation and Thermal Stability for Ultra Shallow Junction CMOS”, Mater. Res. Soc. Symp. Proc. 716, B1.8 (2002). 177 [...]... suitable for low-temperature processes Secondly, unlike the nucleation-controlled formation mechanism of TiSi2, formation of nickel silicides is dominated by the diffusion of Ni atoms during thermal treatment Therefore the problematic linewidth-dependence phenomenon is intrinsically eliminated in NiSi Also the silicon consumption to form NiSi is only a half of that for CoSi2 Lower silicon consumption is... Chapter 1 Introduction incorporating Ti either as a capping layer or a Co-Ti alloy at the deposition stage 56, 58 Ti or TiN capping layer that is in- situ deposited immediately following Co deposition can also effectively control the ambient contamination 59, 60 NiSi: Scaling of gate-to-gate spacing for higher packaging density and lower source/drain series resistances requires the corresponding scaling of... question of scalability due to the difficulty of achieving low sheet resistance on deep sub-micron structures The key obstacle for scaling Ti silicide is the dependence of the C49 to C54 transformation on lateral dimensions, called fine-line effect in IC industry Z Ma et al examined the C49-C54 phase transformation with in- situ transmission electron microscopy (TEM) and found that the transformation proceeded... widely adopted into 0.18µm down to 65nm generations Same as the C54 TiSi2, CoSi2 has a low resistivity of 14-20 (µΩ⋅cm), making it suitable for applications in salicide processes In contrast to the conventional Ti silicide processes, nucleation does not limit the formation or transformation of Co silicide phases, even for lateral dimensions in the 13 Chapter 1 Introduction deca–nano range As a consequence,... studied thoroughly for successful implementation of SiGe in advanced IC circuits This work is carried out with aim to fill in these technical gaps The thesis is devoted to studies on formation mechanisms and interface properties of transition- metal silicides through both experimental and theoretical approaches The shrinkage in lateral dimensions increases the demands to form ultrathin silicide films... isolation, mask, and passivation layers Polysilicon film can be used as conducting layer, semiconductor, or resistor by proper doping with different impurities And metal thin films basically act as local connection and interconnection from the basic constituent 24 Chapter 2 Experimental and Computational Methodologies devices, i.e transistors, up to the top layer of the whole circuit In brief, thin film technologies. .. imaging, to be an effective technique for determining the morphology of interfaces between metal or silicide thin films and silicon substrate, and evaluating the interface roughness In the theoretical part, formation of silicides on different substrates has been simulated and the corresponding properties have been explored In Chapter 5, the reported interface structures are reproduced and a new interface... and formation of smoother interface, because high annealing temperature promotes uniform lattice diffusion with a high activation energy and suppresses non-uniform grain boundary diffusion with a low activation energy 56, 57 Co contamination degrades gate oxide integrity due to the degradation of film and interface morphology at high temperature The high diffusivity and solubility of Co in Si also introduce... to trace impurities in the annealing ambient limit the application of furnace In contrast, RTA is a fast and single wafer process, minimizing the chance of particle contamination and deterioration of the properties of silicides 27 Chapter 2 Experimental and Computational Methodologies With RTP, a single wafer is heated in an extremely short period of time under atmospheric conditions or at low pressure... 2 Experimental and Computational Methodologies Chapter 2 Experimental and Computational Methodologies Thin films play very important roles in CMOS fabrication starting from the very first pad oxide and nitride to the final passivation layer deposition A great number of techniques have been developed to form uniform thin films and to characterize the quality of the films On the other hand, first-principles . problematic linewidth-dependence phenomenon is intrinsically eliminated in NiSi. Also the silicon consumption to form NiSi is only a half of that for CoSi 2 . Lower silicon consumption is desirable for. high- performance CMOS technology. 28 1.2 Transition- Metal Silicides No matter how the CMOS technology moving forward, as the link step between the front-end-of-line and back-end-of-line of. formation involves both inter-diffusion and chemical reactions between phases at the interface. 38 Diffusion-Controlled Mechanism: For most of the metal/ silicon thin film reactions, the