Stress Distribution in Ultra Thin SiO 2 Film/Si Substrate System Measured by a Low Level Birefringence Detection Techniq
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Stress Distribution in Ultra Thin SiO2 Film/Si Substrate System Measured by a Low Level Birefringence Detection Technique X. H. Liu*, H. J. Peng*, S.P. Wong*1, Shounan Zhao**, *Department. of Electronic Engineering and Materials Science and Technology Research Centre, The Chinese University of Hong Kong, Hong Kong, China ** Department of Applied Physics, South China University of Technology, Guangzhou, China ABSTRACT In this work, a low level birefringence detection system was employed to study the stress distribution in Si substrate induced by thermally grown ultra-thin SiO2 film. According to traditional bi-metallic strip theory, it is expected that the stress should show a linear dependence on depth with the zero stress plane located at the position of two third of the thickness of the substrate from the SiO2/Si interface. The linear dependence of stress on depth in accordance with the bi-metallic strip theory was observed only in part of the substrate. For the region below the SiO2/Si interface extending to a depth of about 1/5 of the thickness of the substrate, the magnitude of the stress was found to be significantly smaller than expected. The position of the zero stress plane was found to depend on the thickness of the SiO2 film and the oxidation conditions. The zero stress plane seemed to move towards the bottom of the Si substrate as the thickness of the SiO2 film became thinner and no zero stress plane was observed in the Si substrate when the SiO2 films became sufficiently thin. INTRODUCTION The oxidation of silicon is an important industrial process, which has been studied intensively for tens of years. Though several models have been proposed and successfully fit the case of thick oxidation, there are still some arguments on the initial oxidation or ultra-thin oxidation. For example, “initial enhanced oxidation" and "pattern dependent oxidation" phenomena cannot be explained by the conventional thermal oxidation theory satisfactorily [1]. One of the difficulties in addressing this issue is the lack of tools to characterize the thin oxidation process. Stress measurement in the oxide layer and silicon substrate is a popular method helping us to understand the oxidation mechanism. Some stress measurement techniques have been used to characterize the stress in the silicon substrate and oxide film. Curvature method is widely adopted in the film stress measurement [2]. But this method could not suit the need of the ultra thin oxidation stress characterization due to its stress sensitivity limitation. Another popularly used method is the infrared photoelastic (PE) technique which is non-destructive [3]. The traditional full-field imaging method determines the stress state using a plane or a circular polariscope. However, its sensitivity is quite low since it provides phase information by counting 1
Contacting Author: S. P. Wong
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the fringes which are proportional to the magnitude of the birefringence. Thus, the resulting minimum measurable phase retardation is in general a fraction of 180° or
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