Formation Of C54 TiSi2 Research Articles

Mechanism of enhanced formation of C54–TiSi2 in high-temperature deposited Ti thin films on preamorphized (001)Si

Enhanced formation of C54-TiSi2 in high-temperature deposited Ti thin films on preamorphized (001)Si has been investigated by high-resolution transmission electron microscopy in conjunction with autocorrelation function analysis. The increase in the thickness of the amorphous TiSix layer is due to the preamorphization implantation for the most part. The dominant effect of high-temperature sputtering is to increase the density of crystallites in the amorphous TiSix layer. The enhanced formation of C54–TiSi2 in high-temperature deposited samples is attributed to the more extensive presence of silicide crystallites, which serve as nucleation sites, in the amorphous TiSix layer than that in samples deposited at room temperature.

The Formation of C54 TiSi2 in The Presence of Implanted or Deposited Molybdenum

AbstractThe presence of Mo, either implanted in Si substrate prior to Ti deposition or deposited at the interface between Ti and Si, leads to the formation of C54 TiSi2 at 600 °C. Without Mo, the C54 TiSi2 does not form below 700 °C. It is shown that the C54 TiSi2 formed with the implanted Mo of a nominal dose of 5x1014 at./cm2 and that formed with the deposited Mo of 0.09 nm average thickness, display similar microstructure properties. The preferential orientation of the C54 TiSi2 is <110> in the samples with implanted Mo or with 0.09 nm Mo interlayer, as well as in the reference sample without any Mo. It becomes <010> when the Mo interlayer is 0.73 nm thick. The silicide surface and Si/silicide interface are appreciably rougher for the silicides formed with 0.09 nm Mo interlayer, with implanted Mo or in the absence of Mo, than for the silicide formed with 0.73 nm Mo interlayer. The experimental results indicate that the enhanced formation of C54 TiSi2 is caused by the same effect, i.e. template mechanism, irrespective of the means of Mo addition to the Ti-Si system.

Silicide Contacts for Sub-0.25 μm Devices

AbstractLow resistivity TiSi2, CoSi2 and NiSi are the three primary candidates for metal contacts in sub-0.25 μ m devices. In the present paper, we review recent progress in the investigations of lowresistivity contacts, which include enhanced formation of C54-TiSi2 on (001)Si by tensile stress, high temperature sputtering, and interposing Mo or TiN layer, improved thermal stability of C54-TiSi2 by the addition of N2 during Ti sputtering or N implantation in (001)Si, self-aligned formation of CoSi2 on the selective epitaxial growth silicon layer on (001)Si, effects of stress on the epitaxial growth of CoSi2 on (001 )Si, improvement of thermal stability of CoSi2 by nitrogen ion implantation or high temperature sputtering, and improvement of thermal stability of NiSi by nitrogen ion implantation or compressive stress.

The Influence Of Stress on The Growth of Titanium Silicide Thin Films on (001) Silicon Substrates

AbstractThe influence of stress on the enhanced formation of C54-TiSi2 phase has been investigated. Tensile stress induced by backside CoSi2 film on the silicon substrate has been found to enhance the growth of C54-TiSi2 on (001)Si. The thickness of amorphous interlayers (a-interlayers) between Ti films and silicon substrates was found to be thicker and thinner in the tensilly and compressively stressed samples, respectively. From auto-correlation function analysis, the thicker a-interlayer was found to consist of a higher density of crystallites. The crystallites provide nucleation sites for C49-TiSi2 and facilitate the formation of C49-TiSi2 of small size. The larger total area of C49-TiSi2 grain boundaries supplies more nucleation sites for the phase transformation of C49- to C54-TiSi2. Therefore, the tensile stress present in the silicon substrate promotes the formation of a-interlayer and decreases the grain size of C49- TiSi2, which increases the nucleation density of the C54-TiSi2 phase. As a result, the transformation of C49- to C54-TiSi2 phase is enhanced.

Advanced salicides for 0.10 μm CMOS: Co salicide processes with low diode leakage and Ti salicide processes with direct formation of low resistivity C54 TiSi2

The scaling of CMOS technologies to 0.10 μm and beyond imposes increasingly demanding constraints to self-aligned silicide (salicide) processes. For high performance devices, it is essential that salicide processes achieve low gate and source-drain sheet resistance as well as low silicide to source-drain diffusion contact resistance, and maintain low junction leakage. This becomes increasingly difficult as junction depths and linewidths are scaled. In this paper we present an overview of the development of advanced Ti and Co salicide processes, with implementations into a high performance 0.10 μm complementary metal-oxide-semiconductor (CMOS) technology. For Co salicide, the main scaling issue is diode leakage on shallow junctions. We show that the use of pre-amorphization implants or a pre-Co deposition sputter etch improves diode leakage distributions, but fails to eliminate high leakage outliers. Co deposition temperature and rapid thermal processing (RTP) variables were found to have a strong effect on diode leakage, with optimization of either one resulting in tight low leakage distributions on shallow junctions. For conventional Ti salicide processes, the main scaling issue is sheet resistance on narrow lines due to incomplete high resistivity C49 TiSi2 to low resistivity C54 TiSi2 transformation. We present X-ray diffraction (XRD) and high resolution transmission electron microscopy (HRTEM) studies that indicate that direct growth of C54 TiSi2 bypassing the C49 phase is achieved at low temperatures on polycrystalline or amorphous Si with the addition of Mo impurities, eliminating the narrow line effect. The mechanism is demonstrated to be nucleation of MoSi2 and an unidentified phase at the Ti/Si interface, followed by epitaxial growth of C54 TiSi2 on these templates. Ti salicide processes with Mo impurities were evaluated, demonstrating an optimized one-step RTP process combining Mo and pre-amorphization implants that maintains low sheet resistance to 0.06 μm gate lengths. Successful implementations into a 0.10 μm flow were achieved both for optimized Co salicide or Ti salicide processes.

Enhanced thermal stability of C49 TiSi2 in the presence of aluminum

The introduction of a thin layer of Al at the interface between Ti films and Si substrates enhances the formation of C49 TiSi2 and retards the transition from C49 to C54. An Al interlayer, 0.64 nm thick, reduces the time required to form C49 TiSi2 isothermally at 500 °C from 14 to 7 min. The C49–C54 transformation temperature is increased from 767 to 853 °C, when heating the samples at a constant ramp rate of 3 K/s. Most of the Al is found toward the interface between a Ti-rich silicide at the surface and TiSi2, rather than at the interface between TiSi2 and the Si substrate. The grain size of the C49 TiSi2 formed in the presence of Al is about five times smaller than that formed on a control sample with pure Ti, indicating that the increased density of grain boundaries in C49 TiSi2 in the presence of Al does not help the C49–C54 transformation. Therefore, the improved thermal stability of C49 TiSi2 is likely to be caused by other factors such as a reduced electron/atom ratio when replacing Si with Al in the disilicide.

Effect of Boron Doping on the C49 TO C54 Phase Transformation in Ti/Si (100) Bilayers

AbstractWe have demonstrated that the formation of C54 TiSi2 on Boron-doped single crystal silicon substrates, under RTA annealing conditions in a Nitrogen ambient, leads to a thicker TiN capping surface layer, thinner silicide layer, higher C49 to C54 transformation temperature and greater interface roughness compared to C54 TiSi 2 formation on undoped single crystal silicon substrates. Titanium films 32 nm thick were deposited on undoped and boron-doped single crystal silicon substrates. The films were annealed at 3 /C/isn nitrogen to final quenching temperatures between 500 °C and 900 TC. Ex-situ four point probe sheet resistance, cross sectional transmission electron microscopy (XTEM), high resolution transmission electron microscopy (HRTEM) and x-ray diffraction (XRD) were used to analyze the resulting TiN on TiSi2 bilayer. The C49 to C54 transformation occurs circa 760 TC and 810 TC for the undoped and boron-doped cases respectively. HRTEM observations reveal a thick 20 nm TIN layer on the C54 TiSi2 film in the boron-doped case but only fine dispersed TiN particles embedded on the top of the silicide in the undoped case. It was observed that the resultant silicide in the boron-doped case was thinner and the TiSi2 /Si(100) interface is rougher. XRD and TEM analysis show that in the boron doped case, there is a preferred C54 (040) orientation compared to a random orientation for the undoped case.

In-Situ x-Ray Diffraction Analysis of TiSi2 Phase Formation from a Titanium-Molybdenum Bilayer

AbstractWe demonstrate that the addition of a molybdenum interlayer between titanium and silicon enhances the formation of C54 TiSi2, without bypassing the formation of the C49 TiSi2 phase. In situ x-ray diffraction analysis during rapid thermal annealing, at a rate of 3 °C/s, was used to study the phase formation sequence of TiSi2 starting from a blanket bilayer of Ti on Mo on a polycrystalline Si substrate. It was shown, as in the case without the Mo layer, that the C49 TiSi2 phase forms first, followed by the C54 TiSi2 phase. The results were similar for undoped or arsenic, boron, and phosphorous doped polycrystalline silicon substrates. The temperature range over which the C49 phase is stable is reduced, on average, by 80 °C. The lower end of the range (appearance of C49) is increased by approximately 60 °C and the upper end of the range (disappearance of C49) is decreased by about 20 0C. The orientation of the C49 phase differs in that both the C49(131) and C49(060) orientations are observed, compared to the case without the Mo layer where only the C49(131) orientation is observed.

Titanium Germanosilicide Phase Formation During The Ti-Si1-xGex Solid Phase Reactions

AbstractThe effect of Si1−xGex alloy composition on the titanium germanosilicide phase formation sequence during the Ti-Si1−xGex solid phase reaction was examined. For the Ti-Si reaction the initial formation of C49 TiSi2 is followed, at higher temperatures, by the formation of C54 TiSi2. For the Ti-Ge reaction the initial formation of Ti6Ge5 is followed, at higher temperatures, by the formation of C54 TiGe2. It was determined that the Ti-Si1−xGex reaction follows three different reaction paths depending on the composition of the initial Si1−xGex alloy. For Si rich Si1−xGex alloys the Ti-Si1−xGex reaction follows a “Ti-Si like” reaction path (Ti+M ↠ C49 TiM2 ↠ C54 TiM2, where M = Si1−xGex). For Ge rich Si1−xGex alloys the reaction follows a “Ti-Ge like” reaction path (Ti+M ↠ Ti6M5 ↠ C54 TiM2). Both Ti6M5 and C49 TiM2 form during the reaction of titanium with Si1−xGex alloys in an intermediate composition range. Properties of the final C54 phase were observed to be strongly dependent on the phase formation sequence. Smooth continuous C54 titanium germanosilicide forms during the “Ti-Si like” reaction and discontinuous islanded C54 titanium germanosilicide forms during the “Ti-Ge like” reaction. An optimum Si1−xGex alloy composition range of 0.00 ≤ x ≤ 0.36 was determined for the formation of continuous- low-resistivity- C54 titanium germanosilicide films from the solid phase reaction of Ti and Si1−xGex alloy.

Activation energy for C94 and C54 TiSi2 formation measured during rapid thermal annealing

The activation energies Ea for C49 and C54 TiSi2 formation were determined using in situ resistance measurements during rapid thermal annealing. Ti films were evaporated on undoped polycrystalline Si (poly-Si) and single-crystal Si on sapphire (SOS) substrates. The resistance was monitored for heating rates from 1 to 95 °C/s up to 1000 °C. The Ea’s determined from Kissinger plots were 1.86±0.23 and 1.65±0.31 eV for C49 TiSi2 formation and 3.30±0.16 and 3.67±0.13 eV for C54 TiSi2 formation for Ti/poly-Si and Ti/SOS samples, respectively. These are the first reported measurements of Ea’s for C49 and C54 TiSi2 formation at such high heating rates. The formation sequence remained the same for the range of heating rates examined.

Stress Evolution During the Formation and Transformation of Titanium Silicide

AbstractThe evolution of stress vs. temperature was measured during the formation of TiSi2 in the reaction of Ti with (100) Si and with polycrystalline Si, and during the phase transformation from C49 to C54 TiSi2. The formation of C49 TiSi2 causes an increase in compressive stress, followed by relaxation before the transformation to C54 TiSi2, which causes no significant stress change. C54 TiSi2 is shown to be elastic in the temperature range of 750–860 °C. This difference in the deformation mechanisms of C49 and C54 TiSi2 affects the morphological stability of TiSi2 in fine line structures.

Kinetics of the formation of C49 TiSi2 from Ti-Si multilayers as observed by in situ stress measurements

The formation of C49 TiSi2 from sputter-deposited amorphous Ti-Si multilayers on Si(100) substrates was studied during isochronal and isothermal heat treatment. The formation process was analyzed for multilayer periodicities (i.e., the thickness of one Ti and one Si layer) of 1, 5, and 12 nm, respectively, by in situ stress measurements and differential scanning calorimetry (DSC). A strong correlation between the evolution of the stress and the formation of the C49 TiSi2 phase was found. During heat treatment an abrupt change of the stress in the film was observed. DSC and x-ray diffraction experiments revealed that this change of the stress coincides with the formation process of the C49 TiSi2 phase. The transition temperature of the Ti-Si multilayer to the C49 TiSi2 phase shifts from ∼295 to ∼550 °C with increasing multilayer periodicity. From kinetic analysis of the experiments the activation energy of the C49 TiSi2 phase formation process was acquired. For the smallest periodicity of 1 nm an effective activation energy of 170±25 kJ/mol was found, for multilayers with 12 nm periodicity the activation energy was 280±40 kJ/mol. The difference in activation energy is explained by different rate-limiting steps during silicide formation as a function of the multilayer periodicity. DSC measurements revealed two processes during the formation of the C49 TiSi2 phase: mixing and crystallization. The enthalpy for mixing for Ti-Si multilayers with periodicities of 1 and 12 nm was −38 and −33 kJ/mol, respectively. The crystallization enthalpy was comparable for both periodicities: −5 kJ/mol.

Nucleation and growth in the initial stage of metastable titanium disilicide formation

Initial stage of the C49–TiSi2 formation was investigated at 530 °C and at a rate of 10 °C/m using transmission electron microscopy. Morphological studies reveal that the C49 phase first separately nucleates at the interface between amorphous silicide and crystalline silicon, then followed by simultaneous lateral and vertical growth. The growth proceeds very fast until the formation of a continuous layer of C49–TiSi2. Local chemical analysis shows that the composition range of the amorphous silicide is narrowed due to the C49 formation. For isothermal annealing, a linear density of the C49 nuclei is about 6.7×10−3/Å, and remains the same upon prolonged annealing. In the case of annealing at 10 °C/m, the linear density depends on temperature, reaching a maximum of 7.2×10−3/Å at around 575 °C.

C54-TiSi2 formed by direct high current Ti-ion implantation

We report, in this letter, the formation of TiSi2 by direct Ti-ion implantation into silicon wafers using a metal vapor vacuum arc ion source. Implantation was conducted by 80 KeV Ti ions to a dose of 5×1017/cm2 with various ion current densities. When the ion current density exceeded 100 μA/cm2, the equilibrium TiSi2 of the C54 structure was uniquely formed. Additional evidence of the formation of C54-TiSi2 was given by the resistivity measurements, i.e., the sheet resistivity was below 3.0 Ω/⧠. The formation mechanism is also discussed in terms of the beam heating effect during implantation.

Kinetic analysis of C49-TiSi2 and C54-TiSi2 formation at rapid thermal annealing rates

We have used in situ resistance versus temperature measurements to demonstrate that a 60 nm titanium thin film on polycrystalline silicon heated at rates up to 3000 °C/min always forms high-resistivity base-centered orthorhombic C49-TiSi2 before the low-resistivity face-centered orthorhombic C54-TiSi2 phase. Kinetic analysis of the shift in transformation temperatures with heating rate indicates that the activation energies for the formation of C49-TiSi2 and C54-TiSi2 are 2.1±0.2 and 3.8±0.5 eV, respectively, when formed during the same annealing cycle. The higher activation energy of formation of C54-TiSi2 as compared to C49-TiSi2 suggests that under very high heating rates and annealing temperatures, the formation of C49-TiSi2 before C54-TiSi2 might be completely or partially bypassed.

Quantitative investigation of titanium/amorphous-silicon multilayer thin film reactions

Growth of amorphous-titanium-silicidc and crystalline C49 TiSi2 in titanium/amorphous-silicon multilayer films was investigated using a combination of differential scanning calorimetry (DSC), thin film x-ray diffraction, Auger depth profiling, and cross-sectional transmission electron microscopy. The multilayer films had an atomic concentration ratio of 1Ti to 2Si and a modulation period of 30 nm. In the as-deposited condition, a thin amorphous-titanium-silicide layer was found to exist between the titanium and silicon layers. Heating the multilayer film from room temperature to 700 K caused the release of an exothermic heat over a broad temperature range and an endothermic heat over a narrow range. The exothermic hump was attributed to thickening of the amorphous-titanium silicide layer, and the endothermic step was attributed to the homogenization and/or densification of the amorphous-silicon and amorphous-titanium-silicide layers. An interpretation of previously reported data for growth of amorphous-titanium-silicide indicates an activation energy of 1.0 ± 0.1 eV and a pre-exponential coefficient of 1.9 × 10−7 cm2/s. Annealing at high temperatures caused formation of C49 TiSi2 at the amorphous-titanium-silicide/amorphous-silicon interfaces with an activation energy of 3.1 ± 0.1 eV. This activation energy was attributed to both the nucleation and the early stages of growth of C49 TiSi2. The heat of formation of C49 TiSi2 from a reaction of amorphous-titanium-silicide and crystalline titanium was found to be –25.8 ± 8.8 kJ/mol and the heat of formation of amorphous-titanium-silicide was estimated to be –130.6 kJ/mol.

Structural Transitions in Titanium/Amorphous-Silicon Multilayers

We report a quantitative investigation of silicidation in Ti/amorphous-Si thin-films using Differential Scanning Calorimetry (DSC), thin-film X-ray diffraction and Cross-sectional Transmission Electron Microscopy (XTEM). Multilayered thin films were used to facilitate calorimetric observation of the heat released or absorbed at many reacting interfaces. It is shown that calorimetric analysis, combined with structural analysis using X-ray diffraction and XTEM, is effective in providing both kinetic and thermodynamic information about interdiffusion reactions in thin films. The present paper describes experimental results for multilayers with an atomic concentration ratio of 1 Ti to 2 Si and modulation periods ranging from 10 to 60 nm. A thin amorphous titanium suicide layer was found to exist between the as-deposited Ti and a-Si layers. Heating the multilayer film caused the amorphous Ti-silicide to grow over a broad temperature range by an exothermic reaction. An endothermic relaxation occurs during the late stage of amorphous suicide growth. Heating to temperatures over 800K causes C49-TiSi2 to form at the a-si1icide/a-Si interface. Temperatures at which all the above structural transitions occur vary with modulation period. Analysis of the DSC data indicates an activation energy of 3.1 eV for the formation of C49-TiSio, which is attributed to both the nucleation and the early growth of the suicide. The heat of formation for C49-TiSi2 from a reaction of a-silicide and a-Si was found to be -30±5KJ/mol. Nucleation appears to be the controlling step in C49-TiSi2 formation.