Low temperature formation of insulating layers on silicides by anodic oxidation

The anodic oxidation of the silicides CoSi, CoSi₂, CrSi₂, Ni₂Si, NiSi, NiSi₂, Pd₂Si, PtSi, TiSi₂ and ZrSi₂ was studied by using Rutherford backscattering of 2MeV alpha particles. The room temperature oxidation was carried out at a constant current density of 8.9 mA cm⁻² using n-methylacetamide (2% H₂0 and 1% KNO₃) as electrolyte. No oxidation of Pd₂Si and PtSi was detected. Pure SiO₂ layers were grown on CoSi, CoSi₂, Ni₂Si, NiSi and NiSi₂ at a much lower rate than on Si〈100〉 and with a thickness increase per volt of 0.6 ± 0.03 nm V⁻¹. Mixed layers of SiO₂/metal oxide were grown on CrSi₂, TiSi₂ and CrSi₂. All oxidations occurred at the expense of the silicide layer. It is also shown how the purity of the SiO₂ layer formed can be predicted from thermodynamic considerations.     

Electrical characteristics of palladium silicide

Resistivity and Hall mobility measurements were performed on Pd₂Si films grown on <100〉 and <111〉 oriented silicon substrates as a function of temperature and thickness of the films. The results show that Pd₂Si has metallic character. The Debye temperature of Pd₂Si was found to be 120±20°K and the concentration of the charge carriers, which are electrons, is 4 × 10²¹ cm^?3. The bulk value of the resistivity at room temperature is 25–30 ωΩcm and the Hall mobility is 50–60 cm² V^?1 sec^?1, both depending on the structure of Pd₂Si which is known to be epitaxial on <111〉 substrate but not on <100〉 substrate. This structural difference is clearly reflected by the electrical characteristics.     

The effects of doping impurities and substrate crystallin on the formation of nickel suicides on silicon at 200–280° C

Transmission electron microscopy and Auger electron spectroscopy have been applied to investigate the effects of doping impurities and substrate crystallinity on the formation of nickel suicides at 200–280° C in nickel thin films on silicon. The systems investigated included samples with as-implanted BF₂, B, F, As, and P and recrystallized (001) Si as well as P-doped low pressure chemical vapor deposited (LP-P) and B-doped plasma enhanced chemical vapor deposited (PE-B) amorphous silicon substrates. In samples annealed at 220–280° C, substantial amounts of epitaxial NiSi₂ were found to form on crystalline structure of BF₂, B and F implanted samples to various extents at different temperatures. High resolution lattice imagings of cross-sectional samples showed that the epitaxial NiSi₂/Si interfaces are coherent. No NiSi₂ was detected in all nickel thin films deposited on implantation-amorphous specimens. NiSi₂ epitaxy was found to be a sensitive function of annealing temperature. Good correlation was found between the atomic size factor and resulting stress and NiSi₂ epitaxy at low temperature. The formation of Ni₂Si and NiSi was observed to be influenced by the dopant species and crystallinity of the substrates. The vast difference in inducing the formation of nickel suicides in implantation-amorphous and recrystallized samples is likely due to variations in initial structure and/or dopant distribution. The finding that bothn-type andp-type dopants influenced the formation of Ni₂Si and NiSi suggested that they may be related to the electrical activity of the doping species in recrystallized samples. NiSi, possessing one of the lowest resistivity among all metal silicides, was found to be the only phase formed in all implantation-amorphous as well as LP-P and PE-B amorphous silicon samples annealed at 280° C. Nickel thin film appears to be an attractive candidate for the metallization of amorphous silicon devices.     

Ni and Ti silicide oxidation for CMOS applications investigated by XRD,XPS and FPP

Although silicide oxidation was studied 20 years ago, the interest of obtaining a robust process for new application appears significant today. Indeed, for the new architectural development process are required dense and narrow spaces. This paper focuses to bury a silicide layer under a protective layer such as silica in order to keep constant the physical and electrical properties of silicide after oxidation. Earlier works show the possibility to oxidize preferably the silicon (Si) in metal contained silicide rather than a pure crystalline Si at high temperatures. Thus, we first tried to reproduce and study these conditions and once acquired, targeted to decrease the oxidation temperature in order to fit with industrial requirements. Titanium (Ti) and Nickel (Ni) are chosen for their metallurgical interest and their integration capability in devices. Thus, four different group/phases (TiSi, TiSi₂, Ni₂Si, NiSi) of silicide were targeted by adjusting the temperature. In situ X-ray diffraction (XRD), photoelectron spectroscopy and sheet resistance (four point probe) measurements were carried out simultaneously before and after oxidation of silicide to characterize the phase and chemical composition. After silicide formation last three phases (TiSi₂, Ni₂Si, NiSi) were confirmed by XRD and G1(Ti/Si) was unknown, where only for NiSi was observed the low sheet resistance (≈7.3 Ω/□) and resistivity (18 μΩ·cm). After (dry, wet and plasma) oxidation, the phases of TiSi₂ and Ni₂Si changed and only NiSi was observed the constant phase, even pure SiO₂ was noted on NiSi after wet oxidation.     

The improvement of thermal stability of nickel silicide by adding a thin Zr interlayer

This is the first report of a technique for inserting a thin Zr interlayer into a nickel film to improve the thermal stability of the silicide formed from this film. The sheet resistance of resulting Ni(Zr)Si film was lower than 2 Ω/□. X-ray diffraction and Raman spectral analysis showed that only the silicides low resistance phase (NiSi), rather than high resistance phase (NiSi₂), was present in the sandwich structure. This proves that the incorporation of a thin Zr interlayer into NiSi delayed the occurrence of NiSi₂ phase and widened the upper boundary of silicide formation window by about 100 °C. These experimental results could be explained by Gibbs free energy theory. Furthermore, Ni(Zr)Si/Si Schottky diodes were fabricated by rapid thermal annealing at 650, 700, 750 and 800 °C in order to study the I–V characteristics of the SBD diodes. The barrier height generally fixed at 0.63 eV, and the ideality factor was close to 1. These results show that Ni(Zr)Si film is a favorable local interconnection and contact silicide material.     

Effects of prolonged annealing on NiSi at low temperature (500°C)

The effects of prolonged annealing (10 h) at low temperature (500°C) have been studied in 20-nm Ni/Si (100) thin films using Rutherford backscattering spectroscopy (RBS), x-ray diffraction (XRD), scanning electron microscopy (SEM) in conjunction with energy-dispersive spectrometry (EDS), and four-point probe techniques. We observe that nickel monosilicide (NiSi) is stable up to 4 h annealing at 500°C. It is also found that, after 6 h and 10 h annealing, severe agglomeration sets in and NiSi thin films tear off and separate into different clusters of regions of NiSi and Si on the surface. Due to this severe agglomeration and tearing off of the NiSi films, sheet resistance is increased by a factor of 2 despite the fact that no NiSi to NiSi₂ transition occurs. It is also observed that, with increasing annealing time, the interface between NiSi and Si becomes rougher.     

Crystallization investigation of NiSi2 thin films

Recrystallizations of ion-irradiated single crystalline and polycrystalline NiSi₂ films are investigated. In the single crystalline case, the irradiated surface portion of the NiSi₂ film reorders epitaxially in a layer-by-layer manner, initiating from the remaining undamaged single crystalline NiSi₂ seed near the NiSi₂/Si interface. The irradiated polycrystal-line NiSi₂ layer recrystallizes via growth from a fixed number of existing nuclei in the layer. In both cases, the recrystallization occurs with a relatively high velocity at very low temperatures (~ 100°C) and the activation energy of the growth rate is similar (1.2 to 1.4 eV). These results reflect the metallic bonding nature of NiSi₂ and the fact that nucleation and mass transport are not required for growth from an amorphous mixture with a stoichiometric atomic composition and existing nuclei.     

Atom probe tomography of Ni silicides: First stages of reaction and redistribution of Pt

The silicide formation and the redistribution of Pt after deposition and after a heat treatment at 290 °C of Ni_1−xPt_x films on Si have been analysed by atom probe tomography assisted by femtosecond laser pulses. Two phases with different composition were found to form during deposition at room temperature: a NiSi layer with a relatively constant thickness of approximately 2 nm and a particle of Ni₂Si. The shape of the Ni₂Si particle is in accordance with nucleation followed by lateral growth formation. After heat treatment, two silicide phases Ni₂Si and NiSi were found together with the Ni_1−xPt_x solid solution. The redistribution of Pt at the Ni_1−xPt_x/Ni₂Si interface is a clear illustration of the snowplow effect. A segregation of Pt at the Ni₂Si/NiSi interface has been observed and is attributed to interfacial segregation. The effect of the redistribution of Pt on the silicide formation is discussed.     

The reaction characteristics of ultra-thin Ni films on undoped and doped Si (100)

Reaction characteristics of ultra-thin Ni films (5 nm and 10 nm) on undoped and highly doped (As-doped and B-doped) Si (100) substrates are investigated in this work. The sheet resistance (Rs) measurements confirm the existence of a NiSi salicidation process window with low Rs values within a certain annealing temperature range for all the samples except the one of Ni(5 nm) on P⁺-Si(100) substrate (abnormal sample). The experimental results also show that the transition reaction to low resistivity phase NiSi is retarded on highly doped Si substrates regardless of the initial Ni film thickness. Micro-Raman and x-ray diffraction (XRD) measurement show that NiSi forms in the process window and NiSi₂ forms in a higher temperature annealing process for all normal substrates. Auger electron spectroscopy (AES) results for the abnormal sample show that the high resistivity of the formation film is due to the formation of NiSi₂.     

Improvement of thermal stability of Ni silicide on N-Si by direct deposition of group III element (Al, B) thin film at Ni/Si interface

The direct deposition of a thin Al or B layer at Ni/Si interface was proposed as a new method to solve a problem of degraded thermal stability of Ni silicide on heavily doped N⁺-Si substrates. Significant improvement of thermal stability evaluated by the sheet resistance vs. silicidation temperature properties was observed. The improvement is attributed to suppression of agglomeration of the silicide layers. The Al layer was effective only when it was located at the Ni/Si interface before the silicidation process. The deposited Al and B layers under Ni layer segregated at the surface after the silicidation process. The use of B layer was preferable to control the phase transition from NiSi to NiSi₂.     

High-resolution transmission electron microscopy of silicide formation and morphology development of Ni/Si and Ni/Si<Subscript>1−x</Subscript>Ge<Subscript>x</Subscript>

Nickel-silicide phase formation in the Ni/Si and Ni/Si_1−xGe_x (x=0.20) systems and its correlation with variations in sheet resistance have been studied using high-resolution transmission electron microscopy (HRTEM) and related techniques. Following a 500°C anneal, uniform and low-resistivity NiSi and NiSi_1−xGe_x (x<0.20) crystalline films were formed in the respective systems. Annealed at 900°C, NiSi₂, in the form of pyramidal or trapezoidal islands, is found to replace the NiSi in the Ni/Si system. After a 700°C anneal, threading dislocations were observed for the first time in the Ni/Si_1−xGe_x system to serve as heterogeneous nucleation sites for rapid lateral NiSi_1−xGe_x growth.     

Thermal stability of Ni silicide films on heavily doped n and p Si substrates

Electrical and structural properties of Ni silicide films formed at various temperatures ranged from 200 °C to 950 °C on both heavily doped n⁺ and p⁺ Si substrates were studied. It was found that surface morphology as well as the sheet resistance properties of the Ni silicide films formed on n⁺ and p⁺ Si substrates at the temperatures higher than 600 °C were very different. Agglomerations of Ni silicide films on n⁺ Si substrates begin to occur at around 600 °C while there is no agglomeration observed in Ni silicide films on p⁺ Si substrates up to a forming temperature of 700 °C. It was also found that the phase transition temperature from NiSi phase to NiSi₂ phase depend on substrate types; 900 °C for NiSi film on n⁺ Si substrate and 750 °C for NiSi film on p⁺ Si substrate, respectively. Our results show that the agglomeration is, especially, important factor in the process temperature dependency of the sheet resistance of Ni silicides formed on n⁺ Si substrates.     

A Hafnium interlayer method to improve the thermal stability of NiSi film

The effect of a thin Hafnium interlayer on the thermal stability of NiSi film has been investigated. Both X-ray diffraction and Raman spectra show that no high resistivity NiSi₂ appears in the Hf-additioned films which were post-annealed at temperatures ranging from 600 °C to 800 °C. Auger electron spectroscopy and Rutherford back scattering show that the Hf interlayer has moved to the top of the film after rapid thermal annealing, working as the diffusion barrier for upper Ni atoms. The three-dimensional surface morphology by atom force microscopy shows that the agglomeration of NiSi is effectively suppressed, which is attributed to the barrier effect of the Hf interlayer. The fabricated Ni(Hf)Si/Si Schottky diodes still displays good current-voltage characteristics even after annealed at temperatures varied from 650 °C to 800 °C, which further show that the Hf interlayer can improve the thermal stability of NiSi.     

Improved thermal stability of Ni-silicides on Si:C epitaxial layers

The thermal stability of Ni-silicides on tensily strained in situ P doped Si:C epitaxial layers was evaluated. The baseline Ni silicidation process was shown to be compatible with Si:C Recessed Source-Drain (RSD) stressors for NMOS strain engineering while the thermal stability of NiSi:C contacts was significantly improved compared to NiSi ones. Dominant degradation mechanism was shown to be the transition to the NiSi₂:C phase. It was demonstrated that the Si:C strain level affects the silicide formation but has no significant effect on the NiSi:C thermal stability. A mechanism responsible for the improved thermal stability of NiSi:C silicides is discussed.     

Nickel silicide formation on Si(100) and Poly-Si with a presilicide N2 + implantation

The key feature of this study is to incorporate N₂ ⁺ implant prior to Ni sputtering on the poly-Si gate and source/drain regions. The results show that the incorporation of the presilicide N₂ ⁺ implant is able to suppress agglomeration in the Ni silicide films up to 900°C and enhance the phase stability of NiSi on Si(100) up to 750°C. Stable and low sheet resistance was achieved on the silicided undoped poly-Si up to 700°C due to reduced layer inversion, which is driven by grain boundary energy and the surface energy of the poly-Si.     

Structure and property of magnetron sputtered ternary cobalt-nickel silicide films

Ternary cobalt-nickel silicide films were prepared using magnetron sputtering from an equiatomic cobalt-nickel alloy target on Si substrate. The effect of post-deposition annealing on the phase formation, structural properties and resistivity of the resultant films has been studied. The results of XRD show that the annealing temperature and impurity level of oxygen play a crucial role in controlling the phase transformation of ternary silicide. Silicide phases are absent in the as-deposited film due to the amorphous nature. At relatively low annealing temperature, the phase of CoNi₃Si (2 2 0) and CoNiSi (2 2 0) coexist. With the increase of annealing temperature, the phase of CoNi₃Si (2 2 0) begins to transform into CoNiSi (2 2 0). At high annealing temperature (800 °C), only the phase of CoNiSi₂ (2 2 2) is formed. For Co-Ni silicide film annealed in pure argon gas ambient, two Raman peaks at 1357 cm⁻¹ and 1591 cm⁻¹ are attributed to the vibrational mode of CoSi₂ and NiSi₂ compounds. For ternary silicide annealed in atmosphere ambient, two Raman peaks located at 538 cm⁻¹ and 690 cm⁻¹ were observed and may be related to Si oxide or Co-Ni oxide. The 3D views of AFM images show that the surface roughness is relatively low when the silicidation temperature is smaller than 550 °C. After silicidation in 800 °C, the surface roughness increases abruptly. The resistance initially decreases with the increase of annealing temperature, and achieves minimum value (19 μΩ cm) in temperature ranges 500-550 °C. When the annealing temperature increases from 600 °C to 800 °C, the resistivity was found to increase slightly to 26 μΩ cm. The ternary silicide shows a temperature window for low resistivity as compared to binary NiSi.     

Electrical properties of proton-irradiated CdSnAs2

Electrical properties and isochronal annealing of proton-irradiated (5 MeV, 2 × 10¹⁶ cm^–2) n-and p-type CdSnAs₂ crystals have been studied. The limiting electrical parameters of the irradiated material were determined: Hall constant 〈R _H〉 ≈ ?1.2 cm³/C, electrical conductivity 〈σ〉 ≈ 1350 Ω^?1 cm^?1, Hall mobility 〈|R _H|〉〈σ〉 ≈ 1500 cm²/(V s), and Fermi level position F _lim ≈ 0.43–0.45 eV above the valence-band top. The energy position of the “neutral” point for the CdSnAs₂ compound was calculated.     

Effect of a thin W, Pt, Mo, and Zr interlayer on the thermal stability and electrical characteristics of NiSi

It is reported that the thermal stability of NiSi is improved by employing respectively the addition of a thin interlayer metal (W, Pt, Mo, Zr) within the nickel film. The results show that after rapid thermal annealing (RTA) at temperatures ranging from 650 °C to 800 °C, the sheet resistance of formed ternary silicide Ni(M)Si was less than 3 Ω/□, and its value is also lower than that of pure nickel monosilicide. X-ray diffraction (XRD) and raman spectra results both reveal that only the Ni(M)Si phase exists in these samples, but the high resistance NiSi₂ phase does not. Fabricated Ni(M)Si/Si Schottky barrier devices displayed good I-V electrical characteristics, with the barrier height being located generally between 0.65 eV and 0.71 eV, and the reverse breakdown voltage exceeding to 40 V. It shows that four kinds of Ni(M)Si film can be considered as the satisfactory local connection and contact material.     

The effect of Si addition and Ta diffusion barrier on growth and thermal stability of NiSi nanolayer

Formation and thermal stability of nanothickness NiSi layer in Ni(Pt 4 at.%)/Si(1 0 0) and Ni_0.6Si_0.4(Pt 4 at.%)/Si(1 0 0) structures have been investigated using magnetron co-sputtering deposition method. Moreover, to study the effect of Si substrate in formation of NiSi and its thermal stability, we have used Ta diffusion barrier between the Ni_0.6Si_0.4 layer and the Si substrate. Post annealing treatment of the samples was performed in an N₂ environment in a temperature range from 200 to 900 °C for 2 min. The samples were analyzed by four point probe sheet resistance (R_s) measurement, X-ray diffraction (XRD) and atomic force microscopy (AFM) techniques. It was found that the annealing process resulted in an agglomeration of the nanothickness Ni(Pt) layer, and consequently, phase formation of discontinuous NiSi grains at the temperatures greater than 700 °C. Instead, for the Ni_0.6Si_0.4(Pt)/Si structure, 100 °C excess temperature in both NiSi formation and agglomeration indicated that it can be considered as a more thermally stable structure as compared with the Ni(Pt 4 at.%)/Si(1 0 0) structure. XRD, AFM and R_s analyses confirmed formation of a continuous NiSi film with R_s value of 5 Ω/□ in a temperature range of 700−800 °C. Use of Ta diffusion barrier showed that the role of diffusion of Ni atoms into the Si substrate is essential in complete silicidation of a NiSi layer.     

High-density NiSi nanocrystals embedded in Al2O3/SiO2 double-barrier for robust retention of nonvolatile memory

NiSi nanocrystals of high density and good uniformity were synthesized by vapor–solid–solid growth in a gas source molecular beam epitaxy system using Si₂H₆ as Si precursor and Ni as catalyst. A metal–oxide–semiconductor memory device with NiSi nanocrystal–Al₂O₃/SiO₂ double-barrier structure was fabricated. Large memory window and excellent retention at both room temperature and high temperature of 85 °C were demonstrated.