Si multilayers

Classified abstracts 6671 ~6678

Jes6s A del Alamo and Takashi Mizutani, J appl Phys, 62, 1987, 34563458. 34 6671. Crystallization of amorphous silicon during thin-film gold reaction The crystallization of a-Si in a-Si (50-nm) and Au (5-nm) thin-film bilayers has been investigated during heat treatment in a transmission electron microscope. When crystallization of a-Si first begins at 130°C, the A ~ Si alloy (Au and a precursor phase) reflections observed at lower temperatures vanish, and several new reflections from metastable A ~ S i compounds occur. Dendritically growing islands of poly-Si are observed after heating at 175°C. If the samples are held at a constant temperature of 175°C for 10 rain, the poly-Si islands coalesce. The formation of poly-Si depends on the diffusion of Au into a-Si and the formation of metastable Au-Si compounds, which act as transport phases for both Si and Au. After crystallization Au segregates to the front and back surfaces of the poly-Si film. The result of this work and earlier diffraction investigations are interpreted in terms of superlattices based on a sublattice. A fundamental body-centered-cubic structure with a = 5.52 A and composition Au4Si is suggested for the Au-Si compounds. L Hultman et al, J appl Phys, 62, 1987, 3647 3655. 34 6672. Investigation of thin-film Nilsingle-crystai SiC interface reaction Interface reaction between Ni thin film and bulk SiC during heat treatment was investigated by MeV ion backscattering spectrometry using resonance scattering of helium~arbon, x-ray diffraction, and Auger electron spectroscopy (AES). Polycrystalline nickel-silicide, Ni2Si, was formed by heat treatment at 600°C in forming gas. Carbon compounds were not detected in the reaction products. Carbon was distributed uniformly with a concentration of about 25 at. % in the reacted film, and the C KLL line shape of AES in the reaction products as similar to that of elementary carbon. I Ohdomari et al, J appl Phys, 62, 1987, 3747-3750. 34 6673. A correlation between the enthalpy of mixing and the internal strain energy in the III-V alloy semiconductor system The enthalpy o f mixing calculated using the delta lattice parameter model has been compared with the internal strain energy estimated by the elastic sphere approximation. The composition dependencies of both energies are very similar for the quarternary III-V alloy semiconductors. Substantial origin of increase of the enthalpy in the alloy is concluded to be the internal strain caused by the difference in the bond length of constituent compounds. H Sonomura, J appl Phys, 62, 1987, 41424145. 34 6674. Orientation control of the silicon film on insulator by laser recrystallization We have studied the influence of the growth direction and the solidification speed on crystal quality of the silicon-on-insulator (SOI) film by laser recrystallization. In a (100) direction on a {100} Si substrate, lateral epitaxial growth of single-crystal regions from a seed extended as much as 1 mm. It was found that the crystalline orientation of the SO1 film changes continuously from {100} toward {110}. These results indicated that the quality of the SOI film is strongly affected by the crystallographic arrangement of the growth front relative to the composition of { 111 } faceted planes. A new recrystallization method for large area SOl films was developed by stabilizing the growth front. K Sugahara et al, J appl Phys, 62, 1987, 41784181. 34 6675. Oxygen behavior during titanium silicide formation by rapid thermal annealing Sputtered titanium thin films on (100) Si wafers were exposed to rapid thermal annealing (RTA) in argon at temperatures of 60~900°C. Auger depth profiling shows that when exposed to air the Ti films takes in 3 x 10 ~6 oxygen atoms cm 2 independently of Ti thickness. During RTA the oxygen tends to redistribute uniformly and a snowplow effect, due to the O solubility difference between Ti and TiSi2, confines it to the unreacted titanium layer. Early on, silicon is observed at the Ti surface and a second silicide layer starts growing. All of the oxygen is finally fixed in the C54 TiSi2 where the two silicide fronts meet (at around 50 nm from the surface). This reproducible effect is associated with RTA and is not observed in classical furnace annealing. R Pantel et al, Jappl Phys, 62, 1987, 43194321.

34 6676. The microstructure and electrical properties of contacts formed in the Ni/AI/Si system due to rapid processing The microstructure and electrical properties of the contacts formed in the Ni (30 nm)/A1 (10 nm)/(100) n-Si system due to rapid thermal processing were studied at temperatures between 300 and 900°C. A melting at the intermediate A1 layer was observed already at about 580°C after 2-s heat treatments. This rapid eutectic melting, assumed to initiate at the A1-Si interface, resulted in the formation of a unique contact composed of the Ni(AlosSio.5)/A13Ni/NixSiy/n-Si structure with fairly smooth interfaces between the layers. The sheet resistance of the layers and the Schottky barrier height of the contact were measured as a function of the rapid thermal processing temperatures. As a result of the eutectic melting reaction at 580°C the sheet resistance of the formed layers decreased from 3.2 to 2.6 f~/[], the Schottky barrier height between the layers and Si increased from 0.614).76 eV, and the effective electrically active area of the contact increased. These electrical properties are discussed in correlation with the microstructure formed in the Ni/A1/Si system due to the rapid thermal processing. A Katz and Y Komem, J appl Phys, 63, 1988, 552(~5533 34 6677. Growth and structure of aluminum films on (001) silicon carbide The formation and the physical and electronic structure of the interface between A1 and SiC films, grown epitaxially on Si(001), are studied using x-ray photoelectron spectroscopy (XPS), low-energy electron diffraction (LEED), and energy-loss spectra (ELS). Zr M-zeta excitation (hv = 151.4 eV) is employed to obtain high surface sensitivity in the Si and A1 2p and valence-band photoemission. The first few monolayers of A1 grow as layers, with A1 island formation at higher coverage. A1Si interaction is apparent as a shift of the A1 2p (Si 2p) to higher (lower) binding energy (BE) for 0 ~< 1. A Schottky barrier height of ~ 1.4 eV is estimated. At higher 0 the A1 2p assumes the BE and shape characteristic of bulk A1, and the Si 2p shows satellite structure to lower BE suggesting both Si bonded to A1 and C as well as Si interacting mainly with A1. Annealing (350 ~< T ~< 1050°C) leads to a reduction in A1 coverage and reversal of the trends observed during sequential deposition. Before annealing LEED shows only a weak (1 × 1) pattern. Annealing at successively higher temperatures leads to a sharper (1 × 1), followed by two-domain (4 x 1) and two-domain c(8 × 2) patterns. No clear indication orAl carbide formation is found in Auger electron spectra or in the A1 2p XPS unless the SiC, prior to A1 deposition, is first treated at high temperature to generate a C-rich surface. V M Bermudez, J appl Phys, 63, 1988, 49514959. 34 6678. Composition, microstructure, and properties of crystalline molybdenum silicide thin films produced by annealing of amorphous Mo/Si multilayers Amorphous Mo/Si multilayers were prepared by alternately sputtering Mo and Si onto silicon single-crystal substrates covered with SiO2 and onto substrates covered with polycrystalline Si. The multilayer thickness was about 200 nm and the composition modulation period was about 0.8 nm. The Mo/Si atomic ratio averaged over the multilayer was about 0.6. These specimens were isochrnnally annealed for 1 h at temperatures up to 1000°C. In order to analyze annealing-induced variations in composition, microstructure, resistivity, and internal stresses, the specimens were investigated by x-ray diffractometry, Rutherford backscattering spectroscopy, and resistometry. Although the multilayer remained amorphous during annealing at temperatures below 350°C, interdiffusion of Mo and Si occurred. Above 350°C the layered structure disappeared and crystalline phases, viz., hexagonal MoSi2, MosSi3, and tetragonal MoSi2, appeared successively for increasing temperatures. According to the (equilibrium) phase diagram both hexagonal MoSi2 and crystalline Mo 5Si3 were expected to occur simultaneously. It appeared, however, that hexagonal MoSi2 formed first, probably because of difficult nucleation of crystalline MosSi 3. The hexagonal MoSi2 nucleated homogeneously, whereas MosSi3 nucleated heterogeneously. In MoSix layers on SiO2 the MosSi 3 grew at the outer surface and at the MoSiJsubstrate interface. In MoSix layers on polycrystalline Si the MosSi~ reacted with Si to form hexagonal MoSi2 at temperatures above 700°C Finally, the hexagonal MoSi2 phase transformed into tetragonal MoSi 2. The resistivity of the MoSix layer decreased distinctly as soon as hexagonal MoSi2 was formed and an even larger decrease occurred when hexagonal MoSi2 transformed into tetragonal MoSi2. The latter resistivity decrease was 515

Classified abstracts 6679-6686 accompanied by a considerable improvement of overall crystalline perfection of the MoSix layer. The lowest resistivity (58/K2 cm) was obtained after annealing at 1000°C. The internal stress in the MoSix layer can be explained by the difference in thermal contraction between the MoSi, layer and the Si substrate. After annealing at 1000°C the internal stress equaled about 2.0 GPa. O B Loopstra et al, Jappl Phys, 63, 1988, 49604969. 34 6679. The intercalation and exfofiation of tungsten disulfide The exfoliation of WS2, the separation of this layer c o m p o u n d into single molecular layers suspended in solution, was found more difficult than the exfoliation of MoS2 reported earlier. The difficulty was found to be the resistance of the WS2 to intercalation. By ultrasonic treatments while exposed to hexane plus n-butylithium, the lithium was found to intercalate, and exfoliation by immersion in water became possible. Restacking the WS2 by drying in a basic solution led to m u c h larger crystallites than the as-received material, while flocculating by decreasing the pH led to small crystallites with a high density of edge planes. Nickel and a l u m i n u m inclusions lead to poor restacking, with no regular c spacing between WS2 basal planes. The more vigorous exfoliation procedure applied to MoS2 also leads to loss of regular c spacing (the X-ray diffraction pattern is essentially that of single molecular layers). B K Miremadi and S R Morrison, Jappl Phys, 63, 1988, 49704974. 34 6680. Stresses in sputtered Ti-Si multilayers and polycrystalline silicide films Titanium silicon multilayers have been produced by alternating sputter deposition. The stress in the as-deposited layers is a function of the period of the multilayer structure. The multilayers were subsequently annealed to form silicide films. F r o m stress and strain measurements on these films Poisson's ratio is determined. After annealing, the films exhibit a tensile stress which can be attributed to the difference in thermal expansion coefficient between substrate and silicide film. P J J Wessels et al, Jappl Phys, 63, 1988, 49794982. 34 6681. Advanced layer material constitution The increasing knowledge and understanding o f the interrelations between the constitution, microstructure and properties of materials are the basis for the development of so-called advanced materials. In particular, ceramic bulk materials, metastable structures and special coatings for various applications are material groups of high interest. The background for developments in these areas is the constitution of multic o m p o n e n t systems and the adjustment of special microstructures. Critical assessments and compilations of phase diagrams, the so-called maps of materials, are undertaken with strong emphasis worldwide. In m a n y cases, however, the conditions employed in low temperature coating techniques do not allow the formation of equilibrium phases. Metastable structures and a m o r p h o u s solid solutions are frequently observed. These are frozen in up to temperatures of about 0.3 Tm (where T~ is the melting temperature). As a consequence new metastable materials can be used in hard material coating systems (melting points of about 2500-3300°C) up to 700~1000°C. The possibility of predicting the constitution of multicomponent layer materials is a subject of special interest. This would support finally a specific layer material development. It should be pointed out that the ideas discussed are not restricted to hard coatings but have to be considered for all kinds of functional coatings. H Hoileek and H Sehulz, Thin Solid Films, 153, 1987, 11 17. 34 6682. Optical properties of the thermal oxide-GaAs rough interface The surface roughness of G a A s originating during thermal oxidation at a temperature of 450°C was studied using multiple-angle-of-incidence ellipsometry at 632.8 n m and reflectometry on samples after the dissolution o f the oxide film. The rough surface is represented by an effective layer with optical constants determined in the Bruggeman effective medium approximation. Experimental data were used to determine the thickness of the effective layer and the volume fraction of voids. The properties of the natural oxide film on the surface of the samples are also discussed. Y Gaillyovfi, Thin Solid Films, 155, 1987, 217 -225.

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34 6683. lnterfacial reactions in bimetallic Ag-Sn thin film couples Interracial reactions in bimetallic A ~ S n thin film couples have been investigated by measurement of electrical resistance and contact resistance as a function of time and temperature in order to understand kinetic behaviour in the above system where the intermetallic phase y-Ag3Sn is formed. Since the reaction is found to start at room temperature, the conventional v a c u u m coating unit has been modified for preparing such films and conducting subsequent measurements without breaking the vacuum. The results from the above different methods of resistance measurement indicate that interracial reactions are characterized by a mean diffusion coefficient of 10 ~3 cm 2 s t at room temperature. Xray diffraction indicates growth of the y-Ag3Sn phase immediately after deposition. Scanning electron microscopy confirms the diffusion of tin into silver by grain boundary diffusion rather than by bulk diffusion. The results from transmission electron microscopy confirm the presence of a y-Ag3Sn phase. S K Sen, Thin SolidFilms, 155, 1987, 243 253. 34 6684. Transport in refractory metals and their interaction with SiO 2 • comparison of tungsten and molybdenum We have investigated sputtered and electron-beam-evaporated thin films of tungsten and molybdenum deposited onto 100 A. SiO2. These two refractory metals have comparable room temperature resistivities (5~5/zf~ cm) and a work function at the midgap between n ÷ and p+ polycrystalline silicon. Therefore both can be considered for application as a gate electrode in submicron very large scale integration technology. We have probed their interaction with SiO2 using Auger spectroscopy, cross-sectional transmission electron microscopy (CTEM) and with current-voltage ( ~ V ) characteristics of molybdenum and tungsten metal/oxide/ semiconductor (MOS) capacitors as a function of the annealing temperature. We have also measured electrical resistivity in the 4.2-300 K temperature range to ascertain the differences and the similarities between the two. We find that, in terms of resistivity, the difference between the two metals is slight. Although at room temperature, PMo is 10% higher than Pw, at liquid nitrogen, molybdenum is around 30% more conductive after a 9 0 ~ 1 0 0 0 ° C anneal. The interface with SiO2 appears in C T E M to be very s m o o t h for both. Auger depth profiling, however, reveals some interface interaction for molybdenum, but not tungsten. This is confirmed with leakage and breakdown measurements on an MOS capacitor structure 32 m m in diameter. After anneals at and above 900°C, molybdenum capacitors on 100/~ oxide are all shorted. The same annealing schedule produces a reasonable distribution of breakdown voltages for tungsten capacitors. L Krusin-Elbaum et al, Thin Solid Films, 153, 1987, 349 358. 34 6685. Wafer back metallization for semiconductor packaging Thin film metallization has found numerous uses in the semiconductor industry since the very beginning. One of these applications, wafer back metallization, has evolved to meet the increasingly demanding requirements of high power, greater speed, higher reliability, and improved circuit performance. Optimization of the electrical, mechanical, and thermal properties of a semiconductor package is necessary to meet the individual device design goals. A m o n g the candidate metallurgical schemes, gold is still the material of choice in m a n y applications although a n u m b e r of multilayer metallization processes are being used or developed to meet special application requirements. Material requirements are described in this paper and material selection for wafer back metallization and its impacts on the performance of microelectronic devices are discussed. The significance of Au Si contact is also discussed and a new observation on the low temperature behavior of g o l d ~ - t y p e silicon is reported. Namsoo P Kim and Richard F Cooley, Thin Solid Films, 153, 1987, 447 457. 34 6686. Metastable phases and thermodynamic equilibrium The hardening of steel is one of the oldest and best-known processes in which a metastable crystalline phase occurs. A supersaturated solution of carbon in iron is formed, and the associated crystalline structure is called martensite. Metastable a m o r p h o u s phases in metal alloys have been the subject of keen interest in recent years because of their interesting properties, which arise because they have no regular atomic order and no grain boundaries. The occurrence of metastable phases, either