Platinum silicide formation under ultrahigh vacuum and controlled impurity ambients

Abstract

The thin-film Pt-Si silicide system has been investigated under clean, well controlled conditions. A (UHV) ultrahigh vacuum apparatus for this study is described which allows preparation of atomically clean Si surfaces and subsequent evaporation of Pt films in vacuo 10−9 Torr. Samples are annealed and characterized with in situ (RBS) Rutherford backscattering and (AES) Auger electron spectroscopy and later examined with (TEM) transmission electron microscopy and (SEM) scanning electron microscopy techniques. Impurity-free Pt films deposited on clean, room-temperature Si substrates react initially to form Pt2Si and then PtSi with diffusivities one to three orders of magnitude higher than previously reported in the 200–325 °C temperature range. However, the observed activation energies of 1.3±0.2 eV and 1.5±0.2 eV are in reasonable agreement with previous reports. No differences in PtSi-Si interface width, PtSi surface character, growth rates, and phase growth progression are observed between extremely clean, well controlled UHV processed samples and samples prepared with normal wet and dry device processing cleanliness levels provided that the Pt films are impurity-free. The presence of oxygen in the deposited Pt film is crucial in determining reaction rates and the phase growth sequence. Increasing impurity concentrations in the initial metal film or at the Pt-Si interface cause macroscopically nonplanar PtSi-Si interfaces and PtSi surfaces. Diffusivities for both Pt2Si and PtSi formation are observed to be strongly dependent upon oxygen contamination in the initial Pt film over a wide range; excessive contamination alters the phase growth sequence. This behavior would tend to indicate that Pt-silicide growth occurs by grain boundary diffusion at these temperatures.