Aluminum/nickel silicide contacts on silicon

The results of a study of the interaction occurring at elevated temperatures between nickel silicide contacts on n-type 〈111〉 silicon and a thin aluminum overlayer are presented. The electrical and structural characteristics of the Al-nickel silicide interaction were investigated using Schottky barrier diodes, Auger electron spectroscopy, X-ray diffraction and scanning electron microscopy. As-grown diodes were found to consist mainly of NiSi and the NiSi-Si interface exhibited a Schottky barrier energy φ_Bn of 0.62 eV. Upon heat treatment the NiSi layer was transformed to the intermetallic NiAl₃, and the barrier energy for the resulting NiAl₃-Si interface was found to be 0.76 eV. The electrical characteristics of the NiAl₃ layer were stable up to 500°C and no evidence of aluminum penetration into the silicon substrate was found.     

Stability of amorphous Fe-W alloys in multilayer metallizations on silicon

Co-sputtered amorphous Fe_0.37 W_0.63 alloys were investigated for applications as diffusion barriers in multilayer metallizations on silicon. Interface reactions and recrystallization during thermal annealing at 400–800°C were studied by back-scattering spectrometry and X-ray diffraction. On SiO₂ substrates the recrystallization of these films occurs at approximately 700°C. On silicon the recrystallization is accompanied by the formation of a silicide layer containing FeSi₂ and WSi₂ phases. No detectable reaction is observed when the alloy film is amorphous. In contact with an overlay metal such as aluminum, copper, nickel or platinum the amorphous Fe_0.37 W_0.63 layer prevents direct interaction between the silicon substrate and an overlay metal film 1000 Å thick during thermal annealing for 30 min at 650°C. The lifetime of the barrier is limited by dissolution and compound formation at the interface and at grain boundaries of the overlay metal.     

Au/Ni bilayers on n-type silicon: Metallurgical and electrical behaviour

Diffusion effects during the formation of silicides in the Ni-Au-Si system were investigated by means of ⁴He⁺ MeV Rutherford backscattering spectrometry, Auger electron spectroscopy coupled with Ar⁺ ion sputtering and X-ray diffraction as a function of the heat treatment temperature (280–350°C) and time (10–1000 min). Schottky barrier heights were used to identify the type of metal present at the silicon surface. Au/Ni/Si and Ni/Au/Si structures were prepared by electron gun deposition of thin gold and nickel films onto n-type Si〈111〉 single crystals. After thermal treatment only Ni₂Si and NiSi compounds were observed and their formation follows the phase order confirmed by previous investigations on the Ni/Si system, with a growth controlled by a lattice diffusion process. In the Ni/Au/Si〈111〉 structure the diffusion of the silicon through the gold film was detected during the formation of nickel silicide and the kinetics of growth of Ni₂Si and NiSi were similar to those studied in the Ni/Si〈100〉 system. A diffusion of gold towards the Si-NiSi interface was observed during the growth of NiSi in the Au/Ni/Si〈111〉 structure. The Schottky barrier height measurements confirm these findings.     

Preparation and characterization of ultra-thin cobalt silicide for VLSI applications

Ultra-thin cobalt silicide (CoSi₂) was formed from 10 nm cobalt film by solid phase reaction of Co and Si by use of rapid thermal annealing (RTA). The Ge⁺ ion implantation through Co film caused the interface mixing of the cobalt film with the silicon substrate and resulted in a homogeneous silicide layer. XRD was used to identify the silicide phases that were present in the film. The metallurgical analysis was performed by RBS. XRD and RBS investigations showed that final RTA temperature should not exceed 800°C for thin (< 50 nm) CoSi₂ formation.     

Composition profiles of CVD platinum and platinum silicide by Auger electron spectroscopy and secondary ion mass spectrometry

Phosphorus, platinum, silicon and oxygen profiles have been studied in thin film Pt formed on Si by chemical vapor deposition (CVD) from Pt(PF₃)₄, and in the platinum silicide formed by interdiffusion at 450° and 625°C. Low voltage sputtered Pt and its silicide have also been studied. Two profiling techniques have been used: inert ion sputtering with sequential Auger electron spectroscopy (AES), and secondary ion mass spectrometry (SIMS).Phosphorus in as-deposited (225°C) Pt formed by chemical vapor deposition (CVD Pt) was found to be non-uniformly distributed, indicating that some diffusion had already occurred even under these mild conditions. There was a high concentration (10–20 at.%) at the surface, decreasing rapidly to the 0.1% range at a depth of about 50 Å. At the interface with the silicon substrate there was a broad peak with a P concentration of about 1 at.%. When platinum silicide was formed, most of the phosphorus from the interior migrated to the surface, resulting in an enhanced P zone now 150–200 Å thick. The phosphorus did not appear to impede silicide formation in any way. The same $\text{Pt}\text{Si}$ ratio was found in platinum silicide formed from CVD Pt at either 450° or 625°C, in silicide from sputtered Pt and in single-crystal PtSi. The composition of the silicide was essentially constant through most of the film. A strong oxygen signal combined with an Si peak shift was present within 100–200 Å of the surface of the silicide. This is the layer of SiO₂ which has been detected by independent methods earlier, and which protects PtSi from attack by the aqua regia used to remove unreacted Pt from insulator areas. The surface silica coincided with the enhanced phosphorus zone when CVD Pt was used, and thus in this case should be considered as a phosphosilicate glass.     

Effects of stress on the interfacial reactions of metal thin films on (0 0 1)Si

The influences of stress on the interfacial reactions of Ti and Ni metal thin films on (0 0 1)Si have been investigated. Compressive stress present in the silicon substrate was found to retard significantly the growth of Ti and Ni silicide thin films. On the other hand, the tensile stress present in the silicon substrate was found to enhance the formation of Ti and Ni silicides. For Ti and Ni on stressed (0 0 1)Si substrates after rapid thermal annealing, the thicknesses of TiSi₂ and NiSi films were found to decrease and increase with the compressive and tensile stress level, respectively. The results clearly indicated that the compressive stress hinders the interdiffusion of atoms through the metal/Si interface, so that the formation of metal silicide films was retarded. In contrast, tensile stress facilitates the interdiffusion of atoms. As a result, the growth of Ti and Ni silicide is promoted.     

Interfacial reaction of Mo-W alloys with silicon

Solid state contact reactions of thin films of Mo_xW_100−x (x =85, 57, 22) with silicon have been studied using Rutherford backscattering spectrometry, X-ray diffraction and scanning electron microscopy. Reactions and silicide formation were obtained by thermal annealing at temperatures between 450 and 900°C for all the alloys. No layer-by-layer phase separation was observed during silicide growth for the alloy-silicon reaction at these temperatures. The reaction temperature increased with increasing tungsten concentration in the alloys. Ion implantation of 4×10¹⁵ cm⁻² of silicon at 250 keV into Mo₅₇W₄₃/Si and Mo₂₂W₇₈/Si reduced their reaction temperatures and also made the reaction products more uniform.     

Observations of Al2O3 and free silicon at the interface between aluminum films and SiO2

Auger electron spectroscopy in conjunction with ion sputter profiling was used to study the physical distribution and chemistry of aluminum, silicon and oxygen at the interface between sputter-deposited aluminum films and SiO₂ substrates. It is expected from thermodynamic considerations that the aluminum will reduce the SiO₂ wherever the two are in direct contact, leaving Al₂O₃ and free silicon. We describe the capabilities of this experimental technique in the analysis of the reaction. Our observations of the atomic percentages of the solid state reaction products and their variation with distance through the interface show that the reduction does occur, that it results in regions with up to approximately 10% free silicon and Al₂O₃ and that the reaction products are distributed over a layer approximately 400 Å thick.     

Effect of Co,Pd and Pt ultra-thin films on the Ni-silicide formation: investigating the sandwich configuration

The effect of Co, Pd and Pt ultrathin films on the kinetics of the formation of Ni-silicide by reactive diffusion is investigated. 50 nm Ni/1 nm X/ 50 nm Ni (X?=?Co, Pd, Pt) deposited on Si(100) substrates are studied using in-situ and ex-situ measurements by X-ray diffraction (XRD). The presence of Co, Pd or Pt thin films in between the Ni layers delays the formation of the metal rich phase compared to the pure Ni/Si system and thus these films act as diffusion barriers. A simultaneous silicide formation (δ-Ni₂Si and NiSi phases) different from the classic sequential formation is found during the consumption of the top Ni layer for which Ni has to diffuse through the barrier. A model for the simultaneous growth in the presence of a barrier is developed, and simulation of the kinetics measured by XRD is used to determine the permeability of the different barriers. Atom probe tomography (APT) of the Ni/Pd/Ni system shows that the Pd layer is located between the Ni top layer and δ-Ni₂Si during the silicide growth, in accordance with a silicide formation controlled by Ni diffusion through the Pd layer. The effect of the barrier on the silicide formation and properties is discussed.

     

Effect of annealing on magnetic properties and silicide formation at Co/Si interface

The interaction of Co (30 nm) thin films on Si (100) substrate in UHV using solid state mixing technique has been studied. Cobalt was deposited on silicon substrate using electron beam evaporation at a vacuum of 4×10^?8 Torr having a deposition rate of about 0·1 Å/s. Reactivity at Co/Si interface is important for the understanding of silicide formation in thin film system. In the present paper, cobalt silicide films were characterized by atomic force microscopy (AFM) and secondary ion mass spectroscopy (SIMS) in terms of the surface and interface morphologies and depth profile, respectively. The roughness of the samples was found to increase up to temperature, 300°C and then decreased with further rise in temperature, which was due to the formation of crystalline CoSi₂ phase. The effect of mixing on magnetic properties such as coercivity, remanence etc at interface has been studied using magneto optic Kerr effect (MOKE) techniques at different temperatures. The value of coercivity of pristine sample and 300°C annealed sample was found to be 66 Oe and 40 Oe, respectively, while at high temperature i.e. 748°C, the hysteresis disappears which indicates the formation of CoSi₂ compound.     

Improving the stability of amorphous silicon ultraviolet sensors

In this paper we present an improved structure of an amorphous silicon/amorphous silicon carbide ultraviolet sensor, previously presented in literature, whose overall performances have been enhanced by growing a very thin layer of chromium silicide film on the top of the sensor. The sensor is a n-type amorphous silicon/intrinsic amorphous silicon/p-type amorphous silicon carbide stacked structure deposited on a glass substrate. The substrate is covered with a chromium film that acts as back metal contact. The top metal contact is a grid shaped chromium/aluminum/chromium metal stack that allows the incident radiation to reach the active p-type layer.The responses of two sets of sensors fabricated with and without the alloy film under ultraviolet radiation have been studied. The role of the very thin chromium silicide layer is to increase the conductivity of the top surface without attenuating the UV radiation absorbed in the device active layer. The increased top-surface conductivity ensures a better collection of the photogenerated carriers and hides the resistivity variation of the underlying p-doped layer under ultraviolet light caused by dopant activation, leading to a stable and linear behavior. Comparing the photocurrent values obtained on sensors with different area and distance between the grid electrodes, we found that the presence of the chromium silicide film extends the charge collection length by a factor of 10, allowing a better device photoresponse.     

The diffusion barrier effect of a vanadium layer in the formation of nickel silicides

The formation of nickel silicides and vanadium silicides and the diffusion barrier effect of a vanadium layer in the formation of nickel silicides were studied for annealed metal films deposited on silicon substrates by depth profiling using secondary ion mass spectrometry and X-ray diffraction. Nickel films more than 2500 Å thick react with silicon after annealing at 400 °C for 30 min in a vacuum. A vanadium layer 250 Å thick between nickel and silicon shows a barrier effect in the nickel silicide formation after annealing at 400 °C for 30 min. The barrier effect of a vanadium layer 250 Å thick becomes imperfect after annealing at 500 °C for 30 min.     

Back-scattering investigation of the low temperature stability of the aluminum-silver system

The large angle elastic scattering of 2.9 MeV ⁴He₂ has been used to investigate the Al-Ag system. This system has important optical applications and affords an opportunity to study directly chemical kinetics. Heretofore, only indirect measurements of the system have been made. The samples consisted of thick Si substrates covered by a 5000 Å thermal SiO₂ layer. Then 2000 Å Al and 3000 Å Ag films were deposited in both sequences without raising the system vacuum above 2 × 10^-5 torr. The back-scattering measurements were performed after each step of a sequence of isothermal annealing treatments at 135°C for times up to 6 h. This study shows that the Si-SiO₂-Al-Ag system is stable after such a treatment. Thus, for Al first depositions, no intermediate metal is necessary for a diffusion barrier at room temperature. Indirect evidence is presented which suggests that the acting diffusion barrier in the Al first depositions is probably the thin natural aluminum oxide layer expected to be formed for the deposition conditions used. However, the adherence of these films to one another is shown to require further characterization if hifh bonding values are required. By comparison in Ag first depositions, extensive intermixing of the Al and Ag has occurred after as little as 105 min at 135°C. The back-scattering measurement correlated with the previous reflectivity measurements suggest that the interdiffusion of Ag and Al without significant compound formation plays a significant role in at least the initial changes in the reflectivity of thin films of Ag and Al. Finally, this study shows that a 500 Å Cr deposition between the Ag and Al depositions serves as an adequate diffusion barrier after an anneal treatment of 60 min at 300°C.     

Selective Formation of Titanium Silicide by Chemical Vapor Deposition Using Titanium Halides and Silicon Wafer as the Precursors

Titanium silicide thin films were formed on Si substrate by reaction of TiX ₄ (X=C1, Br) with Si under different experimental conditions. The Si consumption and titanium silicide obtained were calculated by the film thickness. In some reactions, titanium silicide thin film was found not only on the Si substrates but also on the SiO₂ wall at the outlet of the reaction chamber. The quantity of Si consumption and the quantity of silicon-containing materials obtained on the wall of the deposition chamber varied as the reaction conditions changed. The minimum Si consumption and maximum titanium silicide obtained on silicon were the most favorable result, found in the reaction of TiBr₄ with Si at 1000°C. The metallization reactions were studied in detail and the reaction pathway is proposed.     

Silicide formation with nickel and platinum double layers on silicon

Diffusion effects and silicide formation in double layers of electron-gun-evaporated thin films of nickel and platinum on 〈100〉 and 〈111〉 silicon substrates were studied by megaelectronvolt backscattering spectrometry, transmission electron microscopy and glancing angle X-ray diffraction as a function of heat treatment (200–900 °C) for both sequences of thin films. It was found for the Si/Ni/Pt(Si/Pt/Ni) system that Ni₂Si(Pt₂Si) starts growing first. When all the nickel (platinum) has been consumed by this compound growth, platinum (nickel) diffuses through the Ni₂Si(Pt₂Si) layer and accumulates at the SiNi₂Si(SiPt₂Si) interface. This platinum (nickel) diffusion seems to be a grain boundary diffusion.For 〈100〉 Si/Ni/Pt samples with thin platinum layers it has been shown that platinum acts as a marker for the moving species in the transition from Ni₂Si to NiSi. For thick platinum layers it was observed that similar processes occur, leading to essentially a four-layered silicide where the layers are alternately rich in nickel and rich in platinum (450 °C, 20 min). In the silicide for the 〈100〉 Si/Pt/Ni system the distribution of nickel and platinum is approximately the reverse of the asdeposited distribution (about 450 °C, 20 min). In the further evolution of the profiles the elemental distribution becomes smooth and flat for both sequences of the layers (750 °C, 20 min). We suggest the existence of a ternary of the type SiNi_1?xPt_x.     

A thermally robust Ni-FUSI process using in 65 nm CMOS technology

The interest in the low resistivity fully silicided (FUSI) gate increased significantly because of promising in use as contact to the source, drain, and gate for sub−65 nm/45 nm CMOS devices. NiSi is potentially an attractive material due to its capability to maintain low resistivity even for channel length down to 100 nm. The Formation of thermally stable silicide gates is important for improving the devices fabrication processes. In order to obtain a thermally stable Ni-FUSI gate electrode, we introduced a two-step annealing process associated with properly tuned thickness of the initial Ni film and additional of implantation of BF₂ during the poly-gate formation to push the transformation of NiSi₂ to higher temperatures at about 900°C and retard agglomeration. A mixed-phase of nickel silicide layer was commonly observed during phase transformation. For the first time, we established an effective way to identify the phase transformations by some nondestructive techniques such as X-ray diffraction, sheet resistance measurement and AFM analysis. The correlations between its electrical and morphological changes during Ni–Si phase transformation were presented. Furthermore, the effect with addition amount of BF₂ impurities into NiSi was investigated. F-incorporation demonstrated some improvements in both morphology and phase stability of the NiSi films at high processing temperatures.     

Structural and electrical properties of swift heavy ion beam irradiated Fe/Si interface

The present work deals with the mixing of iron and silicon by swift heavy ions in high-energy range. The thin film was deposited on a n-Si (111) substrate at 10⁻⁶ torr and at room temperature. Irradiations were undertaken at room temperature using 120 MeV Au⁺⁹ ions at the Fe/Si interface to investigate ion beam mixing at various doses: 5 × 10¹² and 5 × 10¹³ ions/cm². Formation of different phases of iron silicide has been investigated by X-ray diffraction (XRD) technique, which shows enhancement of intermixing and silicide formation as a result of irradiation. I-V measurements for both pristine and irradiated samples have been carried out at room temperature, series resistance and barrier heights for both as deposited and irradiated samples were extracted. The barrier height was found to vary from 0·73–0·54 eV. The series resistance varied from 102·04–38·61 kΩ.     

A study on the penetration of platinum into silicon in Ti/Pt/Au beam lead metallization systems

The penetration of platinum into silicon in the Ti/Pt/Au beam lead metallization system was investigated using scanning electron microscopy, electron probe microanalysis and X-ray diffraction. Specimens with different thicknesses of titanium and platinum deposited onto polycrystalline silicon (polysilicon) substrates by vacuum evaporation or r.f. sputtering were heat treated at 360 °C in 1 atm N₂ for up to 2000 h. The platinum penetration was examined by counting the black spot defects under a metallurgical microscope. An increase in the thickness of the titanium layer was shown to be more effective in reducing the formation of defects than an increase in the thickness of the platinum layer. This indicates the role of titanium as a diffusion barrier. The most striking fact is the necessity of the presence of gold for defect formation. The defects were revealed by etching the silicon substrate to be spherical lumps and they consisted of platinum and silicon. X-ray diffraction spectra taken for specimens having a Pt/Ti/polysilicon structure with or without a gold layer on the platinum indicated the effect of gold in promoting the breakdown of the titanium layer as a diffusion barrier and on the formation of platinum silicide. A model for the mechanism is proposed in which the localized formation of platinum silicide is enhanced by gold at a defect in the titanium layer.     

Thin films of rare earth metal silicides

The phase composition, conductance and surface morphology of thin films of silicides of rare earth metals (of the yttrium subgroup) were studied. The silicides were formed by annealing thin film structures of the rare earth metal (Ln) and silicon at 473–673 K for 1–120 min in vacuum. X-ray analysis revealed the formation of crystalline silicide phases of composition LnSi_2-x and of the AlB₂ structural type for all the metals concerned except scandium, gadolinium and lutetium.It was established that the formation of the crystalline silicide phase is determined by the relation between the crystallographic parameters of a rare earth metal hexagonal lattice and silicon; the critical value of the lattice mismatch a is ± 1.3%. The kinetics of formation of a silicide phase were determined by measuring the conductance of thin film structures. A model for the formation of rare earth metal silicides in thin film structures is proposed, which serves as a basis for establishing conditions for the formation of quasi-amorphous, polycrystalline or large-block rare earth metal silicide layers, with a perfect silicide-silicon interface, taking into account the crystallographic orientation and parameter relationship of the substrate and the silicide.     

Thermal oxidation of transition metal silicides: The role of mass transport

Thin films of almost all transition metal silicides on a silicon substrate oxidize and form SiO₂ on their surface when they are annealed in an oxidizing ambient atmosphere. In applications of these silicides as interconnects in integrated circuits, the oxidation characteristics of the silicide and the SiO₂ growth rate are very important.We review the four major steps controlling silicide oxidation: (1) oxidant transport through the oxide; (2) reaction at the silicide-oxide interface: (3) net transport of silicon atoms with respect to metal atoms in the silicide; (4) reaction at the silicide-silicon interface. The oxidant transport is shown to be the same for all silicides. The reaction at the silicide-oxide interface is explored using equilibrium thermodynamic arguments. The transport through the silicide is discussed and experimental results of inert marker experiments are presented.The diffusing species during the oxidation of PdSi, Pd₂Si, NiSi₂, CoSi₂, PtSi, CrSi₂ and TiSi₂ are discussed. The diffusing species during oxidation correlate with the moving species in silicide formation. A discussion of a mechanism that explains why the oxidation rate of some silicides on a silicon substrate is faster than that of the bare substrate is presented.