Silicide/SiGe Schottky barriers are of importance for applications in infrared detectors and SiGe contacts, as well as for fundamental studies of metal-semiconductor interfaces. We have fabricated silicide/SiGe Schottky diodes by the reaction of evaporated Pt and Ir films on p-SiGe alloys with a thin Si capping layer. The onset of metal-SiGe reactions was controlled by the deposited metal thickness. The Schottky barrier heights were determined from internal photoemission. Pt-SiGe and Ir-SiGe reacted diodes have barrier heights that are higher than the corresponding silicide/p-Si diodes. PtSi/Si/SiGe diodes, on the other hand, have lower “barrier heights” that decrease with increasing Ge concentration. The smaller barrier heights in such silicide/Si/SiGe diodes are due to tunneling through the unconsumed Si layer. Equations are derived accounting for this tunneling contribution, and lead to an extracted “barrier height” that is the Si barrier height reduced by the Si/SiGe band offset. Highly bias-tunable barrier heights are obtained (e.g. 0.30 eV to 0.12 eV) by allowing the SiGe/Si band offset to extend higher in energy than the Schottky barrier, leading to a cut-off-wavelength-tunable silicide/SiGe/Si Schottky diode infrared detector.

Thermal reaction of W with Si1−xGex films epitaxially grown by Rapid Thermal Chemical Vapor Deposition was investigated in the temperature range 500°C - 1000°C. The samples were annealed either in a Rapid Thermal Annealing (RTA) system or in a conventional furnace, both in flowing nitrogen. The reaction products were investigated by Rutherford Backscattering Spectroscopy (RBS), Energy Dispersive Spectrometry (EDS) and X-ray diffraction (XRD). Sheet resistance measurements were also performed to follow the progress of the reaction. The reaction of W with Si0.67Ge0.33 is similar to that of W with silicon. W reacts with silicon to form tetragonal WSi2. The Ge-content in the silicide is lower than that of the asdeposited alloy. It is shown that an oxygen contamination occurs during conventional annealing and leads to the formation of non homogeneous Si1−x Gex unreacted alloy below the silicide film. Rapid thermal annealing prevents this parasitic effect and the unreacted film remains homogeneous although a slight decrease in the Ge-content is observed. These results are correlated with Schottky barrier height measurements on p-Si0.83Ge0.17 partially strained films. We observed an increase of the barrier height with increasing the temperature for annealing from 500°C to 1000°C. This trend may be explained either by strain relaxation or (and) Ge-content decrease in the unreacted alloy.

The high temperature solid phase reaction of Ti with SixGe1−x produces a low resistivity titanium germanosilicide which is isomorphic with the C54 phase of TiSi2 and TiGe2. The composition of the final C54 Ti(SiyGe1−y)2 film is dependent on the composition of the initial Si-Ge alloy and on the annealing conditions. The intermediate phases of the Ti-Si and Ti-Ge reactions are C49 TiSi2 and Ti6Ge5 respectively. The reaction path of Ti - SixGe1−x shifts from that of Ti-Si to that of Ti-Ge as the SixGe1−x alloy composition changes (×=1→0). Phase separations were observed at low temperatures for Ti reactions with Si-Ge alloys and the C54 formation temperature was observed to decrease as the Si-Ge alloy composition approached Si.5Ge.5. Surface and interface morphologies were examined using SEM and TEM. The formation of smooth, large grain, low resistivity films has been observed for the reaction of Ti with low Ge content alloys (x≥0.7). As germanium content is increased the formation of faceted islands is observed. Reactions with high Ge content alloys (x≤0.3) produce films with morphologies similar to those of the Ti-Ge reaction.

In this paper we present results on solid state reactions between Ti and Si1−xGex alloys selectively deposited onto Si (100) substrates using rapid thermal annealing (RTA) for contact applications in novel device structures. Germanium concentrations of 0%, 30%, 50%, and 100% within the reacting Si1−xGex alloy are investigated. The Si1−xGex alloys (approximately 2500 ° thick) are deposited using rapid thermal chemical vapor deposition (RTCVD). Titanium is then deposited by evaporation. Sheet resistance measurements as a function of RTA temperature (10 second anneals) provide indications of various phases that occur during the reactions through the formation of constant sheet resistance plateaus. The RTA temperature required for the formation of a minimum resistivity phase is observed to increase for increasing Ge concentrations within the reacting Si1−xGex alloy. Using x-ray diffraction we have determined that for the reactions of Ti with Si the C49 TiSi2 metastable phase forms prior to the minimum resistivity C54 TiSi2 phase. For the reactions between Ti and Ge a minimum resistivity TiGe2 phase also with the C54 structure forms, however, this phase is preceeded not by a C49 TiGe2 structure, but by a Ti6Ge5 phase. The minimum resistivity phases for Ti reactions with 30% and 50% Ge Si1−xGex, alloy reactions also have a C54 structure with unit cell dimensions varying from that of TiSi2) to TiGe2 as the Ge concentration is increased. The grain structures for the reactions are investigated by cross-sectional transmission electron microscopy (XTEM). As the Ge concentration within the reacting alloy decreases the lateral grain size for the C54 structures increases. A self-aligned germanosilicide process is identified and used to fabricate raised, ultrashallow junctions with Ti(SiGe)2 (germanosilicide) contacts. Forward and reverse bias characterization of the junctions indicate that leakage current induced during silicidation can be eliminated using raised junctions with germanosilicide contacts.

Nonalloyed shallow ohmic contacts to n-Si have been fabricated by using an 80 Å thick strained Si0.5Ge0.5 buffer layer grown by molecular beam epitaxy. X-ray photoelectron spectroscopy was employed to investigate the Si 2p and Ge 3d core level binding energies of the strained and the relaxed Si0.5Ge0.5 and to determine their relative Fermi level positions. It was found that the surfaces of strained Si0.5Ge0.5 exhibit pinning very close to the conduction band. Rutherford backscattering and Auger depth profiling were employed to determine the contact reactions using Ti, W or Pt as contact metals. In the case of Pt, a 500 Å W diffusion barrier can protect the ohmic behavior up to 550 °C for 30 min. The specific contact resistance of the Pt/W/Si0.5Ge0.5/n-Si contact extracted from the D-type cross-bridge Kelvin resistor was 3.5x10-5 Ω·cm2.

Co(SiGe)x contacts have been formed on low defect density, relaxed Si0.7Ge0.3 layers by conventional self-aligned contact processing techniques. Test structures measuring the contact metal sheet resistance indicate that the resistivity is high for anneal temperatures from 450°C to 750°C. The lowest sheet resistivity was 100 Ω/□, about 10 times the resistivity of a comparable amount of CoSi2. Contact resistivities, measured by the transmission line method, were as low as 2 × 10−5 Ω cm2. There is a large discrepancy between contact resistivities measured by transmission line and 4 point Kelvin test structures that may be due to the fabricated contact sizes.

Silicide formation on strained GexSilx layers by the thermally-induced metal/GexSi1−x reaction results in enhanced relaxation of the underlying strained layer. Silicide formation on an amorphous layer is a potential means of decreasing processing temperatures, increasing reacted-layer/GexSi1−x interface planarity and hence, decreasing strained layer relaxation. For the present report, the thermally-induced Co/Ge.17Si.83 reaction on crystalline and amorphous substrates at temperatures of 500-600°C has been compared. Though the rate of silicide formation increased with amorphous substrates, strained layer relaxation was not inhibited.

Raman scattering measurements are used to characterize Co/Si, Co/Ge and CO/Si0.8Ge0.2 thin film reactions. For Co/Si samples, the phase transitions Co--CoSi--CoSi2 are identified by Raman spectroscopy. For Co/Ge samples, Raman features associated with Co5Ge 7 and CoGe2 phases were observed. For CO/Si0.8Ge0.2 samples, only CoSi was identified along with Ge enriched SiGe alloy peaks. No features associated with CoGe or Co(SiGe) were found.

Solid phase amorphization has been found to occur in all refractory metal and a number of rare—earth and platinum group metal thin film on silicon systems. For Ti/Si, Zr/Si, Hf/Si, V/Si, Nb/Si and Ta/Si systems, the growth of amorphous interlayer (a—interlayer) was found to follow a linear law in the initial stage. Si atom was found to be the dominant diffusing species in the solid phase amorphization in Ti/Si, Zr/Si and Hf/Si systems. For the Y/Si system, the stability of amorphous interlayer depends critically on the composition of the amorphous films.

Auto–correlation function analysis was utilized to determine the structure of the amorphous interlayers. HRTEM in conjunction with the fast Fourier transform were applied to determine the first nucleated crystalline phase. Simultaneous presence of multiphases was observed to occur in a number of refractory metal/Si systems. Interesting electrical properties of amorphous interlayers were found for Ti/Si, Zr/Si and Hf/Si systems.

Direct epitaxial CoSi2 formation from Co, which is contrary to the reported silicidation process : Co→Co2 Si→CoSi→CoSi2, has been found during anneal of Co / Ti / (100) Si system. 2 nm thick Ti and 15 nm thick Co films were sputter deposited, and then annealed for 30 min at temperatures between 375°C and 900°C. At room temperature, the 2 nrm Ti immediately forms an amorphous Ti-Si-Co layer between the Co and Si. Epitaxial CoSi2 begins to form at 400°C, while the amorphous layer continues to act as both a Co diffusion retardant and Si diffusion suppressant even at 900°C. This retarded diffusion of Co reduces the growth rate of the CoSi2 over the entire temperature range studied. Superiority of the epitaxial to polycrystalline silicide has been demonstrated. In self aligned structures, an epitaxial CoSi2 film is formed by a single-step anneal without any overgrowth onto adjacent field oxide areas utilizing the amorphous diffusion controlling layer. A p+/n junction of 40 nm depth with reduced leakage and low ideality factor has been obtained by impurity diffusion from epitaxial CoSi2.

The kinetics and mechanism of the C49 to C54 TiSi2 polymorphic transformation have been investigated in a temperature range from 660 to 720°C using in situ sheet resistance measurement and transmission electron microscopy. The kinetics results were correlated with the microstructural changes during the phase transformation. The main structural characteristics demonstrating the mechanism of the transformation were established by examining the nucleation and growth of the C54-TiSi2 in the polycrystalline C49-TiSi2 thin films. It was found that the C54 nuclei predominantly formed at grain edges (three-grain junctions) of the C49 phase and grew very fast by moving its incoherent interphase boundaries. Preliminary results have not revealed rigorous orientation relationships between the two phases. It is suggested that the C49 to C54 structural transition is massive in nature.

This study explores the local structures of TiSi2 and ZrSi2 films on Si. The strain effect on the surface morphologies and phase stability of the films is investigated. Titanium silicide and zirconium silicide films were fabricated on atomically clean Si(111) substrates in UHV using electron gun evaporation followed by in situ annealing. A quantitative X-ray absorption spectroscopy analysis was performed to study the formation of epitaxial TiSi2 and ZrSi2 films on the Si surfaces, as well as the phase transition of TiSi2 from the metastable C49 structure to the stable C54 structure. The surface morphologies and the surface roughness of the films were studied using atomic force microscopy (AFM). The strains of these films were determined by comparing the calculated bond lengths with the measured bond lengths using the extended X-ray absorption fine structure (EXAFS) technique. The final stable C54 structure of TiSi2 has less strain than the initially formed metastable C49 phase. While the surface and interface free energies and the bulk free energy are critical to the structural formations and phase changes of the films, the energy associated with the lattice strain is another important factor.

We have studied the CoSi and CoSi2 phase formation sequence in (001) bulk and SOI Si wafers, using Co/Ti bimetallic layers as source materials which are suitable for growing epitaxial CoSi2 films on (001) Si. In bulk Si, co-formation of polycrystalline CoSi and epitaxial CoSi2 phases at T>500°C have been observed. These phases form respectively at the metal and Si sides of the film. For very long times and/or at high temperatures, only epitaxial CoSi2 is observed, e.g., for samples annealed at 560°C for 30 min or at 900°C for 10 s. When using (001) SOI Si with inexhaustible Co supply, only polycrystalline CoSi has been formed for a 900°C 10 s annealing, which is in contrast to the bulk Si results. This phenomenon is understood on the basis of Gibbs free energy reduction in forming the two phases. In the CoSi2 formation temperature range, Gibbs free energy release in forming CoSi2 is only ∼10% more than that of forming CoSi. Consequently, after all Si atoms have been consumed, the formation of CoSi becomes energetically more favorable, since the free energy reduction due to formation of 2x mole of CoSi is much larger than that due to formation of lx mole of CoSi2, where x is the SOI Si mole number.

The structures of CoSi2 formed from the bilayer of Co/(refractory metal) have been investigated. For a layer reversal and epitaxial growth of CoSi2 on Si substrate, the silicidation temperature of interposed refractory metal should be higher than that of Co and the oxidation potential of the refractory should also be negatively higher than that of Si to remove the native oxide on Si substrate. The main role of the refractory metal layer in the epitaxial growth of CoSi2 is to limit the Co diffusion to Si substrate, controlling the Co-Si reaction rate. Thus Co atoms are able to locate their stable positions with the lowest energy in CoSi2 lattice.

Interactions of Cu with PtSi and TiSi2 films grown on Si (100) substrates have been studied in the temperature range of 250-500°C with Rutherfold backscattering spectrometry, Auger electron spectrometry, and x-ray diffraction. Upon annealing Cu migrates across the intermediate PtSi or TiSi2 layers and reacts with Si substrates to form Cu silicides. Unlike what were reported in the literature, no indications of the reaction of Cu with PtSi or TiSi2 have been observed. Both PtSi and TiSi2 retain their original thicknesses and compositions during Cu penetration. The structural integrity of the metal suicides in contact with Cu films is attributed to low formation energies of Cu-silicides. A 50-nm thick amorphous CoMo barrier impedes Cu diffusion up to 550°C, but fails as a diffusion barrier at higher temperatures due to CoMo-Si reaction.

We have investigated TiN/TiSi2 films as diffusion barriers between copper and silicon. The TiN/TiSi2 films were formed by annealing Ti/Si via rapid thermal process at a temperature of 850'C for 30 seconds in N2 ambient. The contact resistance and leakage current of the Cu/TiN/TiSi 2/n+-p Si contact system were measured after sintering at 350-600°C for 30 minutes in an N2 ambient. We found an abrupt increase of contact resistance occurred at a sintering temperature of 600°C. From the leakage current of n+-p diode, we observed that the thermally stable temperature was about 475ycC. Meanwhile, XTEM photographs showed small Cu3Si crystallites with size of about 0.25 μm precipitated in the n+Si substrate at a sintering temperature 500°C. The formation of Cu3Si increases the occupied volume, generates the gap between TiSi2 and n+Si, and gradually increases the specific contact resistance. SIMS profiles also showed that a certain amount of Cu atoms diffused into the n+-p Si junction and that Si atoms existed in the Cu film. The Cu3Si formation across the n+-p junction can explain the abrupt increase in the leakage current measurement at 500°C. So the failure mechanisms for contact resistance and leakage current are different.

Phase formations in Co thin films (200Å in thickness) reacting with atomically clean Si(100), Ge(100), and Si0.80Ge0.20 epitaxial layer (800Å in thickness on Si(100) substrates) in UHV have been investigated. For the Co/Si system, it is found that CoSi (FeSi structure) is formed at 375°C through a very disordered CoSi phase, and the final CoSi2 phase is formed at 575°C. On the other hand, the Co5Ge7 phase was identified for the Co/Ge samples annealed at 300°C and 450°C and the final CoGe2 phase is formed at 600°C. For the Co/Si0.8 Ge0.20 samples annealed from 400°C to 600°C, Co(Si1−yGey) phases with y∼0.10 were detected, and for annealing at 700°C, only the CoSi2 phase was formed. These results indicate a preferential Co- Si reaction when annealing the Co/SiGe structure. It was also found that the sheet resistance of the reacted thin films depend strongly on annealing temperature.

Micro structures and interface structures of epitaxially grown PtSi and over-capping Si films on Si(111) substrates prepared by MBE were studied by RHEED, HREM, SEM and XRD. An epitaxially grown Si layer on the PtSi layer, which was fabricated by coevaporation of Pt and Si with the stoichiometric ratio (Pt/Si=l/1), was obtained as a Si(111)/PtSi(010)/Si(111) double heterostructure at the substrate temperature of 400°C. On the other hand, it was found that the PtSi layer transformed into epitaxial columns and/or walls when a Si over-capping layer was grown on the PtSi layer at a substrate temperature of 600°C or higher. These columns and/or walls were surrounded by a Si matrix which showed epitaxial relations to the Si substrate with stacking faults.

Arsenic diffusion and segregation properties at the interface of the epitaxial CoSi2 and Si substrate have been studied. Samples have been prepared using Co-Ti bimetallic source materials and two types of (001) Si substrates: n+ (doped by As to ∼2}1019 cm−3) and p. For the n+ Si cases, the lower limit of the CoSi2 film formation temperature is increased by ∼200°C to ∼700°C. SIMS results showed As segregation into Si. For epitaxial CoSi2 film formation at 900°C, the As concentration has increased by a factor of ∼2 within a distance of ∼30nm from the interface, while the incorporated As in the film is ∼30-50 times less than that in Si. For p-type Si substrate cases, the epitaxial CoSi2 film was first grown and followed by As+ implantation (into the film) and drive-in processes. It is observed that As was segregated to the CoSi2-Si interface and diffused into Si. This is in qualitative agreement with our results obtained from the n+ substrate experiments and the results of other authors involving the use of polycrystalline CoSi2 films. In the present cases, all implanted As were conserved at a drive in-temperature of 1000°C for up to 100 s. This is in contrast to the polycrystalline CoSi2 film results which involve a substantial As loss to the film free surfaces. The physical reasons of this difference have been discussed.

Time resolved reflectivity curves (TRR) in combination with non-equilibrium thermal model are used to estimate the temperature interval of chemical reactions in the systems Ni-Si (100) and Ni-SiO2-Si (100) for a ∼ 500 nm thick Ni layer depending on the pulse energy density ranging from 0.5 J/cm 2 to 1.0 J/cm2. Numerical simulations are performed to calculate the temperature field within the system and to analyze the reactivity of the system.