Smooth Silicide Formation by Ion Beam Mixing of Ti/Si-Layers
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SMOOTH SILICIDE FORMATION BY ION BEAM MIXING OF Ti/Si-LAYERS K. Kohlhof, S. Mantl and B. Stritzker Institut fUr Festkbrperforschung Kernforschungsanlage JUlich, P.O. Box 1913, D-5170 JUlich, Fed. Rep. Germany ABSTRACT Ion beam mixing experiments of Ti-Si layers have been performed with 2 Kr ions of 250 keV energy and doses ranging from 7 * 1015 to 7 * 1016 cmat temperatures between liquid nitrogen temperature and 450 0 C. At substrate temperatures below 120°C no silicide formation could be detected. Only weak mixing at the Ti-Si interface is observed. At temperatures above 1200 C the formation of TiSi 2 could be verified by Rutherford backscattering and X-ray diffractometry. Layers of TiSi 2 produced by ion beam mixing show smooth surfaces in contrast to those prepared by conventional furnace annealing. Those display rough surfaces and interfaces. INTRODUCTION Transition metal silicides are of great interest for contacts and interconnects in VLSI applications. As device dimensions are scaled down high conductivity materials are required to obtain low-ohmic interconnnects. TiSi 2 is one of the favoured materials because it has the lowest resistivity (16 p9cm in the C-54 structure) among transition metal silicides [1 ]. The conventional method of preparing TiSi is a solid state reaction in a furnace at about 600*C [21 . For device fabrication one is interested in low temperature processing steps to minimize unwanted reactions e.g. thermal destruction, diffusion processes etc. Ion beam mixing offers the possibility to form silicides at low temperatures. In some cases transition metal silicides may be formed by implanting heavy ions at room temperature [3 ]. For VLSI-devices smooth layers are required to prevent short circuits or spiking effects. However, thermally reacted silicides often show rough surfaces and interfaces. Again ion beam mixing is a promising tool to grow silicides of higher uniformity [4 1. In this work we present first results of our investigations on low temperature formation of smooth TiSi 2 films by ion beam mixing. EXPERIMENTAL Ti films of approximately 60 nm thickness were deposited by electrongun evaporation on (111)-silicon wafers at a base pressure of 2 * 10 -8 Torr. The wafers were cleaned in acetone and isopropanol and dipped in a dilute HF-solution to remove the native oxide. After evaporation the wafers were stored in an argon atmosphere to reduce further oxidation of thi titanium. Ion befg mixigg was performed with Kr ions of doses from 7 * 10 5 to 7* 10 cm - . The ion energy of 250 keV was chosen to achieve a projected range of the Kr-ions just beyond the Ti/Si interface. During irradiation the substrates were held at temperatures ranging from liquid nitrogen temperature to 450 0 C. The dose rate of 5 * 10 -12ions/sec was sufficiently small to ensure that beam heating could be neglected. Layer mixing was investigated by Rutherford backscattering spectroscopy (RBS). The incident He beam of 2.2 MeV energy yielded well resolved signals from Ti and Kr, respectively. The samples were tilted
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