Advanced layer material constitution

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