Step Coverage Modeling of Thin Films Deposited by CVD Using Finite Element Method
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STEP COVERAGE MODELING OF THIN FILMS DEPOSITED BY CVD USING FINITE ELEMENT METHOD
CHING-YI TSAI* and SESHU B. DESU** *Department of Engineering Science and Mechanics **Department of Materials Engineering Virginia Polytechnic Institute and State University, Blacksburg, VA, 24061 ABSTRACT A two--dimensional finite element model was developed to study the step coverage of submicron trenches with arbitrary shape under chemical vapor deposition processes. Parameters that characterize the step coverage were found to be the surface Damkohler number, ratio of diffusion coefficients inside and outside of the trench, and aspect ratio of the trench geometry. Efforts were concentrated on studying the step coverage of SiO 2 film deposited from SiH 4/O2 precursors within rectangular shape trenches. The model predictions were found to be in good agreement with reported experimental results. INTRODUCTION Although chemical vapor deposition processes (CVD) are known to have better film conformity than those of physical vapor deposition processes (PVD), as aspect ratio (width/depth) of the trenches becomes smaller, conformity of the deposition materials may be lost, resulting in creation of voids inside the trenches. This problem is known as the step coverage problem. Poor step coverage leads to non-planar surfaces and thus non-uniform electrical resistance. Poor step coverage also leads to significant problems in lithographic processes. Some efforts have been spent in analyzing the deposition profiles of the CVD processes inside the trenches using the fundamental Boltzmann equations. Due to the difficulties associated with obtaining numerical solutions of the Boltzmann equations directly, simulation techniques, such as Monte Carlo Simulations, have been widely used to obtain solutions of the Boltzmann equations statistically [1-2]. Although Monte Carlo simulations were successful in simulating the film step coverage of the CVD processes, they suffer from expensive computational costs [3]. In 1939, Thiele [4] proposed a one-dimensional (1-D) model for simulating simultaneous diffusion and chemical reactions in a single pore. This 1-D model was recently used by McConica et al. (1990) [5] to study the step coverage of CVD tungsten process in cylindrical contact holes. Using this 1-D model, McConica et al. [5] obtained qualitative agreements between the model's predictions and experimental results. In general, 1-D step coverage models require lumping the trench geometry and, thus, necessitate the assumption that the film thickness is uniform over the bottom of the trench. Furthermore, the film thickness at the entrance side wall is also assumed to be identical with the film thickness along the flat surface adjacent to the trench mouth in these 1-D models. In other words, obtaining quantitative predictions of film thickness profiles over the trench surface is very difficult using these 1-D models [6-7]. A two-dimensional (2-D) model would alleviate the problem of geometric lumping for simulating the film step coverage. In this paper, a
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