Thermochemistry of Silicon Lpcvd Revisited with Kinetic Data
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ABSTRACT Partial equilibrium calculations simulating the kinetically controlled low pressure chemical vapor deposition of silicon from silane are presented. Firstly, the sticking coefficient of each reactive species (a) is evaluated by means of homogeneous calculations in the gas phase. Then, in a series of heterogeneous calculations, the influence of a on the formation of the film is considered a) by reducing the quantities of the reactive species in the input gas, and b) by modifying their thermodynamic data. The obtained composition of the gas phase is used to estimate its supersaturation and the driving force for the deposition of silicon. The results are correlated with the morphology of the films deposited in equivalent conditions and, in view of this correlation, the validity of the presented approach is discussed. INTRODUCTION A thermochemical study of a chemical vapor deposition (CVD) process, performed by minimizing the total Gibbs energy of the involved chemical system, can yield reliable results when deposition occurs within the diffusion controlled regime [1, 2]. When the simulated process is kinetically controlled a misfit is observed, due to a) the infinite reaction time assumed in the calculations, b) the considered composition of the gas phase which is not necessarily the same as the one at, or in the vicinity of, the growing surface. Both of these limitations can be faced if appropriate experimental information on the composition of gas phase in the operating conditions is available. In such a case, partial or modified equilibrium calculations can reliably illustrate the process. The CVD of polycrystalline silicon (polySi) from silane SiH 4 is a kinetically controlled process. Consequently, the thermodynamic behavior of the Si-H system do not illustrate the mechanisms which are responsible for the deposition. However, as for example was investigated by Tao and Hunt [3], by excluding the presence of condensed silicon, this approach reveals the trends of the evolution of the gas phase as a function of processing conditions. In the case of deposition from pure silane, it was shown that the gas phase is mainly composed of SiH 4 at low temperature, and Si oligomers at higher temperature. In fact, intermediate species, produced by the partial decomposition of SiH 4 (ex. silylene SiH 2 ) are much more efficient towards deposition of polySi than SiH 4 itself. The efficiency of a gaseous precursor towards the deposition of polySi can be quantified by its reactive sticking coefficient (or ), defined as the probability that this gas will be incorporated into the silicon upon collision with the surface [4]. Taking into account this parameter in the thermodynamic calculations for species which, due to their concentration or their reactivity with the surface are expected to play an important role in the deposition, should further improve the reliability of the simulation of the process. 951
Mat. Res. Soc. Symp. Proc. Vol. 507 ©1998 Materials Research Society
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