The Lack of Trapping Centers for Positrons at the Interface of W/Si System and the Investigation of the Depletion Layer
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THE LACK OF TRAPPING CENTERS FOR POSITRONS AT THE INTERFACE OF W/Si SYSTEM AND THE INVESTIGATION OF THE DEPLETION LAYER IN THE SCHOTTKY BARRIER BY POSITRONS AS TEST CHARGE SIMULATING HOLES S. TANIGAWA', Y. TABUKI', L WEI', K. HINODE", N. KOBAYASHI*', T. ONAI" and N. OWADA*." "Institute of Materials Science, University of Tsukuba, Tsukuba, Ibaraki 305, Japan "*Central Research Laboratory, Hitachi, Ltd., Kokubunji, Tokyo 185, Japan 'Device Development Center, Hitachi, Ltd., Oume-shi, Tokyo 198, Japan ABSTRACT Interfacial phenomena in the W-Si systems were studied by a monoenergetic positron beam. Doppler broadening profiles of the positron annihilation were measured as a function of incident positron energy. Tungsten thin films of 100 nn in thickness were deposited on p-type, 10 flcm Cz-grown Si wafers with (100) orientation by the DC magnetron sputtering method. Specimens were annealed at various temperatures in order to form silicides. It was found that the position of the interface of both W/Si and W silicide/Si is very different from the position expected from a simple situation neglecting the effect of electric field on the diffusion of positrons. This fact arouse the utility of positrons as a test charge to probe directly the electric field gradient in the Schottky barrier. INTRODUCTION In recent years, refractory metals are widely studied for their use in the VLSI technology for interconnects and gate metallization schemes as a replacement for the polycrystalline silicon and aluminum. As the increasing of the silicon integrated circuits (SIC) packing density, the line width gets narrower and the sheet resistance contribution to the RC delay increases. The usefulness of refractory metal silicides for SIC are the low resistivity and high temperature stabilities. Such refractory metals have been studied in SIC as Schottky barriers, ohmic contacts and low resistivity gates and interconnect[l]. The properties of refractory metal silicides have been studied by many investigators using Auger electron spectroscopy (AES), secondary ion mass spectroscopy (SIMS) and Ratherford backscattering spectroscopy (RBS) and so on. The formation of silicides transforms automatically metal/Si interfaces to clean silicides/Si interfaces. The use of such a clean silicide/Si contact as the Schottky barrier has been expected to realize the reliable and reproducible Schottky barriers. However, the properties of the silicide/Si interfaces have not yet been sufficiently well characterized. One of the silicides which meet these requirements is tungsten disilicides (WSi2 )[2, 3]. The WSi2 layers grown on Si substrates have low resistivities ranging from 30-40 Cicm[4, 5]. In the present experiment, we investigated the behavior of positrons for the specimens which have tungsten-silicon Schottky barrier and the behavior of the transition from tungsten to tungsten disilicide as a function of annealing temperature using a variable-energy positron beam. EXPERIMENTAL The specimens used in this experiment were based on the p-type, 10 0cm Czochralski
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