Film thickness measurement of amorphous silicon

Abstract

Amorphous silicon has been used extensively in electro-optical applications. Its use as a gate electrode material for advanced CMOS devices is currently being developed, as it offers certain desirable characteristics compared to the commonly used polycrystalline silicon. The properties of amorphous silicon, including optical and electronic, are highly variable depending on process conditions, i.e. deposition temperature, etching conditions, and chemistry. The variable optical properties present a challenge for broad-band reflected intensity optical thickness measurement techniques. This is because the constant angle reflection interference spectroscopy (CARTS) technique requires the knowledge of a film's dispersion (the change in refractive index with wavelength) before the film thickness can be determined'. An updated feature in Prometrix CARIS-based spectrophotometers allows the user to easily determine or verify dispersion for both transparent and absorbing films.As a demonstration of this technique, we have studied a series of amorphous silicon films deposited at varying temperatures. The deposition temperature ranged from 54(Y C to 570' C. The nominal thickness was approximately 1575A to 3170A. Due to the differences in deposition temperature, one would expect the optical properties to vary slightly. Using software that allows analysis of the spectral information, the dispersion was examined for each sample. With this knowledge, the film thicknesses could be reliably measured.