Estimating pre-exponential factors for desorption from semiconductors: consequences for a priori process modeling

Quantitative estimation of gas desorption rates from semiconductor surfaces is playing an increasingly important role in designing and optimizing devi ce manufacturing processes. Typical rate expressions use a simple Arrhenius form for the rate constant with an activation energy E-d and pre-exponenti al factor nu. In the absence of experimental data, E-d can often be estimat ed via standard quantum methods. This approach does not provide an estimate of nu, however, so modelers often simply set nu equal to a typical vibrati onal frequency near 10(13) s(-1). The present work surveys the available ex perimental literature to assess the likely validity of this procedure. We s how that, like metals, semiconductors commonly give prefactors differing fr om 10(13) s(-1) by many orders of magnitude. A brief survey of theoretical treatments of desorption shows that numerous factors can account for this v ariability, although the factors operating in any specific case cannot usua lly be identified ahead of time. The resulting uncertainties in estimating nu significantly reduce the reliability of a priori process modeling for op erations involving gas-solid reactions, and the results of such modeling sh ould be viewed with proportionate circumspection. (C) 2001 Elsevier Science B.V. All rights reserved.