Kinetics of Pt Silicide Formation Studied by Spectral Ellipsometry
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ABSTRACT Silicide formation during thermal annealing of thin Pt layers deposited by evaporation onto crystalline silicon substrates was studied by in-situ spectral ellipsometry. As was shown in an earlier study, Pt silicide is formed in a two-step process with intermediate stages of Pt 2 Si and PtSi at temperatures of about 190 and 240 °C, respectively. We observed a shift of about 15 °C of the di- and monosilicide formation, when the anneal rate was lowered from 3 to I K/min. The analysis of the reaction kinetics using the normalized ellipsometric angle A yields a good fit to the data for different anneal rates with an activation energy of (1.6 ± 0.2) eV. The underlying model of silicide formation through a multilayer system was checked with depth profiles and compositional information obtained from Rutherford Backscattering. 1. INTRODUCTION The low Schottky barrier height of about 230 meV of Pt silicide on p-doped crystalline silicon and the compatibility with silicon integrated circuit technology makes PtSi an attractive material for IR detector applications up to a wavelength of about 5 ýtm [1,2]. Even lower barrier heights near 160 meV are obtained with Ir silicides on p-Si. With the aim to find an acceptable trade-off between detector responsivity and noise introduced by leakage currents, work is currently under way on the preparation of mixed Pt-Ir silicides [3]. The silicide formation during thermal annealing of the ultrathin metal layers is difficult to control since the silicidation temperatures of Pt and Ir are different and since PtSi is formed via an intermediate step of disilicide formation. Also the barrier height might show spatial non-uniformity in actual devices [4]. The reaction from Pt 2Si to PtSi was observed in the range from 250 to 600 oC with an activation energy of 1.47 eV [5]. In the case of Ir the silicide is directly formed from Ir and Si near 500 oC with an activation energy of 1.9 eV [3]. For these studies, a number or researchers have employed Rutherford Backscattering Spectrometry (RBS) to analyze the silicide formation process [6]. For precise material characterization and for the detailed study of silicide formation a number of researchers have employed spectral ellipsometry (SE) as an elegant tool for nondestructive analysis of optical film properties both before and after silicide formation and, as an in-situ technique, during thermal annealing of silicide samples. For example, the complex refractive index h = n - ik, with n and k being the usual index of refraction and the extinction coefficient, respectively, in polycrystalline IrSi was measured between 0.6 and 5.5 eV by ellipsometry and between 4.1 and 24.5 eV by synchrotron radiation experiments [7]. Recently, the trajectories of the ellipsometric angles (TA) were monitored in-situ during PtSi formation in 259 0 Mat. Res. Soc. Symp. Proc. Vol. 470 1997 Materials Research Society
order to study intermediate steps via Pt 2 Si for different initial Pt thicknesses [8]. A detailed picture of substrate effects and of the composit
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