Deposition Profile Simulation: Topological Effects
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ABSTRACT There is a considerable amount of ex idence for the dependence of microstructure and surface morphology of films obtained by conventional thermally activated chemical vapor deposition on mass transport phenomena. This paper addresses the simulation of the influence of convectivediffusive transport phenomena as well as homogeneous reactions on the deposition profile of silicon films in the case of some characteristic topological effects. For that purpose, the Si-H chemical system including both homogeneous and heterogeneous reactions is used. The evolution of the deposition profile during vapxor growth is simulated for various surface defects such as a step or a trench. The influence of basic processing parameters such as temperature or flowrate is discussed. INTRODUCTION Surface cov'erage is an important feature of the deposits processed by chemical vapor depxosition (CVD). Many papers address the step coverage and/or trench filling of microelectronic deN ices [1-81. In these studies, the typical diameter of the trench is smaller than the mean freepath and Knudsen diffusion is the dominating mechanism for mass transfer. Surface coverage is also an important feature for the ceramic coating of parts presenting complex geometry or surface defects. In such cases the typical feature dimension is much greater than the mean free-path and the \ iscous fluid theory applies. Van den Brekel 19] first stressed the influence of the competition between mass transfer and surface kinetics limitations on the surface morphology of thin films. As a matter of fact, the gas phase transport limiting process enhances surface defects, while the surface kinetics limitation tends to smooth out the surface. An explanation based on the boundary layer theory without considering the flow phenomenon and gas-phase reactions was provided by the author. This paper is intended to look at the influence of both flowv and gas-phase reactions on topological effects simulated by a trench or a step. MODEL DESCRIPTION Topological effects were modelled in a simple axisymmetrical tubular reactor with a gas mixture flowing upwards. The surface discontinuity was simulated by an annular rectangular trench. The governing equations including the continuity, momentum, energy balance, and mass balance (diluted system) equations were solved in a 2D domain 1101. The silicon-hydrogen chemical system was considered, with silicon deposit provided by SiFT 4 diluted in N2. The gasphase mechanism and kinetic constants were those proposed by Coltrin et a]. 1I]. Among the 16 reactant gaseous species included in this model, a kinetically closed sub-system composed of 8species was used according to a previous work [121]. The discrepancy between the two models was lower than 1%for the typical deposition conditions of this study. The silicon carrier species are decomposed into silicon and hydrogen on solid surface according to the reactive sticking model [12-141. The influence of the geometrical variations during deposition was not taken into account. To co
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