New Analytical Method of SiO 2 Structure by Infrared Reflection Absorption Spectroscopy (Ir-Ras)
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K. ISHIKAWA, H. OGAWA, C. INOMATA, S. FUJIMURA, AND H. MORI Fujitsu Ltd., 1015 Kamikodanaka, Nakahara-ku, Kawasaki 211, Japan
ABSTRACT Simulation of a thin oxide RAS spectrum obtained by using the dielectric function extracted from thick oxide RAS spectra is shown to be a viable method for the comparison of oxide films of differing thickness. Bulk and near-interfacial features of thermally grown SiO 2 thin films were studied by this method and it was found that the LO phonon peak at about 1255 cm- 1 reflects bulk SiO 2 structure and a higher reflectance between 1100 and 1200 cm- 1 reflects SiO 2 /Si interface structure.
INTRODUCTION The SiO 2 /Si interface structure has been reported to affect the performance of silicon semiconductor devices [1] and, in this study, infrared absorption reflection spectroscopy (IR-RAS) was applied to the analysis of SiO 2 /Si interface features. Conventional IR transmittance methods have been used for the study of SiO 2 [2] where the absorbance trough due to the transverse optical phonon polariton arising from Si-O stretching vibration (hereinafter referred to as TO) can be observed. The shape and position of the TO reflects sensitively oxide structure but shows only thickness-averaged structural changes, making it difficult to analyze interface structure. In contrast, IR spectroscopy methods using p-polarized light at oblique angles of incidence such as grazing internal reflection (GIR) [3] or RAS [4,5] detect the longitudinal optical phonon polariton arising from Si-O stretching vibration (hereinafter referred to as LO). These methods are probably more suitable for analysis of interface structure and, additionally, RAS can be applied on industrial wafers with lapped back surfaces. RAS was thus used in this study.
CONCEPTION Oxide structure in the vicinity of the SiO 2 /Si interface has been reported to differ from bulk SiO 2 .[1] If an oxide layer is characterized into two parts, bulk and interface structures, then a thick 425 Mat. Res. Soc. Symp. Proc. Vol. 318. c1994 Materials Research Society
oxide contains more bulk structural components and a thin oxide contains more interface structural components. Thus by comparing thick oxide and thin oxide spectra, bulk and interface structures can probably be separated. However, it is difficult to compare samples of different thicknesses since the measured reflectance is the summation of the reflectance and transmittance of the oxide after multiple reflections in the Si substrate (Fig. 1). RAS spectra include both reflection absorption by the oxide and, in practice, structure and thickness affect RAS spectra. To overcome this problem, the dielectric function was extracted from RAS spectra of a thick oxide taken at different incidence angles and was used to simulate a thin oxide spectrum. Simulated and actual spectra were then compared. The extraction of the dielectric function involves two RAS spectra since the function is complex: composed of two unknown real Re(e) and imaginary Im(e) parts. The RAS spectra were taken at 700 and 80'
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