Selective nucleation of silicon clusters in CVD
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A nucleation model is developed that includes chemical etching of atoms as an additional loss process besides thermal dissociation that competes with the process of atom addition in forming a cluster. The model has the proper qualitative features to describe observations of the evolution of cluster formation on amorphous silicon substrates in the low pressure CVD of a mixture of SiH 2 Cl 2 /HCl/H 2 .
I. INTRODUCTION Nucleation is generally treated as a thermal fluctuation-induced barrier-crossing process, in which the energy barrier results from the competition between atom or molecule addition and thermal dissociation in forming a cluster. Such a nucleation theory based solely on thermodynamic considerations has successfully been employed to obtain qualitative results for many complex problems including 3-D growth of epilayers,1 bubble formation in liquid helium at negative pressure,2 displacement transformation in metallic and nonmetallic materials,3 formation of voids in nuclear reactor materials,4 and gas phase nucleation.5 The same thermodynamics-based approach has also been used to examine gas phase and surface nucleation and growth problems in chemically reacting systems such as in CVD reactors. In a chemical reacting system, however, chemical processes may participate in the nucleus formation process directly besides providing the source for monomer. For example, Katz and Donohue6 investigated the rate of nucleation when the arrival rate of condensing molecules and rate of surface chemical reaction are of similar magnitude. It is the goal of the present study to develop a nucleation model in chemically reacting systems where chemical etching of atoms from a cluster acts in addition to the loss due to thermal dissociation of atoms from the nucleus. By studying a particular system, a general framework for investigating complex nucleation problems in chemical reacting systems will be suggested. Such a framework enables one to gain a qualitative understanding of the nucleation process in many complex problems in chemical reacting systems and can be useful in the improvement and design of processes to tailor nucleation to specific requirements. The motivation for this study arises from the inability to interpret recent observations of the dynamic evolution of cluster formation on amorphous silicon surfaces in low pressure CVD systems in terms of any thermodynamics-based nucleation theory.7'8 By meaJ. Mater. Res., Vol. 7, No. 7, Jul 1992 http://journals.cambridge.org
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suring the cluster density and size distribution as a function of time, it is found that in the CVD of a mixture of SiH 2 Cl 2 /HCl/H 2 on SiOx, x < 2, all the fine clusters of Si that are formed in the early stage of deposition do not survive in spite of the fact that they are much larger than the critical size for nucleation. It is observed, instead, that large crystalline clusters emerge when the fine ones suddenly disappear from the surface.7-8 Using artificial nucleation sites of Si3N4 (4 /im x 4 /xm) on SiO 2 , the sa
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