Contributions of Gas Phase Reactions to CVD Processes
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CONTRIBUTIONS OF GAS PHASE REACTIONS TO CVD PROCESSES S. B. Desu* and Surya R. Kalidindi** * Department of Materials Engineering, Virginia Polytechnic Institute, Blacksburg, VA 24061; "**Department of Mechanical Engineering, MIT, Cambridge MA 02139.
Abstract An analytical model to account for the effects of gas phase reactions in CVD processes is presented. In this model a system with only two reactions, gas phase production of an intermediate and reaction of the intermediate on the surface of the substrate to form the film is considered. The reaction kinetics, convective and diffusive transport mechanisms are coupled to analyse the thickness distribution over the length of the reactor. Theoretical deposition rate profiles in the direction of gas flow are shown with varying deposition temperatures and gas flow velocities for both surface and gas phase reaction controlled mechanisms. It has been shown that by analyzing the nature of the deposition rate profiles, the rate limiting step in a CVD process can be identified.
Introduction Chemical vapor deposition is a very versatile process for deposition of metals, semiconductors and dielectrics and is being used commercially in growing single crystalline and polycrystalline silicon, silicon carbide, II-VI and III-V semiconductors, binary metal oxide films for microelectronic device applications. In all these processes, besides the surface reactions, some sort of gas phase reactions may take place in the heated gas layer in contact with the deposition surface. In majority of these cases, the gas phase reactions produce activated species which then diffuse to the surface and decompose to produce the desired film. In many cases, the production of these intermediate species in the gas phase is the rate limiting step. Thus, understanding the gas phase reactions will enable us to obtain films with the desired quality. The formation of SiH 2 in the gas phase for the CVD processes using silane was proposed in the literature [1-3]. This was further exploited in developing HOMOCVD concept for depositing films on substrates at low temperatures [4]. Similarly, gas phase formation of SiCl 2 in CVD reactions using silicon chlorides was also proposed [5]. Gas phase complex formation was shown in the deposition of III-V compounds by MOCVD [6-8]. Recently it was shown that the gas phase reactions control the deposition rate of Si0 2 in the pyrolysis of silicon alkoxides [9]. Despite all this evidence, most published models for CVD either neglect gas phase reactions or take them into consideration through a lumped overall rate equation [10]. Besides affecting the overall rate, gas phase reactions can significantly effect the step coverage and thickness nonuniformity. To understand these effects, the individual reactions (gas phase as well as surface reactions) should be explicitly considered in the analytical model.
Analytical Model A typical CVD process comprising of a gas phase reaction and a surface reaction is considered for simplicity. In the gas phase reaction the reagent (R) d
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