In Situ Characterization of thin film Growth by Ftir Irras
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IN SITU CHARACTERIZATION OF THIN FILM GROWTH BY FTIR IRRAS J. E. BUTLER*, K. B. KOLLER*, AND W. A. SCHMIDT** *Naval Research Laboratory, Chemistry Division, Washington, DIC, 20375 "**Naval Research Laboratory, Electronics Science and Technology Division, Washington, DC, 20375. ABSTRACT In-situ analysis of low temperature Plasma Enhanced Chemical Vapor Deposition (PECVD) SiO 2 films deposited on HgCdTe, Silicon, and Aluminum substrates was performed by double modulation Fourier Transform Infrared Reflection Absorption spectroscopy (FT-IRRAS). The sensitivity and selectivity of this technique are sufficient for an in-situ assessment of the film quality and reaction conditions at any stage of film growth. An oblique angle of incidence of ca. 550 was chosen to yield maximum sensitivity for the 1260 cm-I LO mode of Si0 2 on Si. The peak frequency and shape of the LO mode absorption band varied with the quality of the Si0 2 films. This diagnostic technique can be applied readily to in-situ analysis of dielectric thin films formed under a variety of reaction conditions as long as the gaseous ambient is partially transmissive to the IR radiation. Introduction The formation of dielectric thin films of Si0 2 at low substrate temperatures by PECVD requires careful control of reaction conditions to yield high quality dielectric/semiconductor interfaces. The ability to monitor the quality of the thin dielectric film in-situ greatly facilitates the optimization process and avoids the possibility of sample corruption by the process of removal and ex-situ analysis. The authors have been interested in producing thin films of Si0 2 on the compound semiconductor HgCdTe (MCT) and have found that the quality of the samples is affected by multiple reaction variables (e.g., choice of reactants, flow rates, pressure, carrier gas dilution, substrate temperature, reactor configuration) [1]. Normal incident IR spectra of SiO 2 is a proven method for the nondestructive analysis of Si0 2 thin films [2-6]. The position and shape of the restrahlen band (1080 cm- 1) of Si0 2 are sensitive to SiOx stoichiometry, film density, and impurities [2-6]. Impurity absorption bands for compounds such as Si-OH, Si-H, NH, and H-OH are also observable. The restrahlen band arises from interaction of the IR photons with the transverse optical (TO) mode phonons and is most prominent at a normal angle of incidence. In-situ analysis of Si0 2 thin films by transmission spectroscopy on various substrates is limited by the severe restrictions placed upon reaction chamber configuration and the difficulty of transmission through the substrate. However, p-polarized light, at an appropriate non-normal angle of incidence, interacts strongly with the longitudinal optical (LO) mode of thin films of cubic crystals [7]. The LO absorption band is observable in either non-normal angle of incidence transmission spectroscopy or by external reflection spectroscopy when the thin films are on a metallic or dielectric substrate. The frequency of longitudinal optical phonons, Wl, is r
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