Site Balance Models in Plasma Processing: A Comparison to Molecular Dynamics Simulations
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M.E. Barone and D.B. Graves Department of Chemical Engineering University of California Berkeley, California 94720
ABSTRACT Molecular dynamics (MD) simulations were conducted of CI+ impact (at 10, 25 and 50 eV) of an initially bare silicon surface, leading to steady state coverage of Cl in a mixed chlorosilyl layer. Our main goal in this study was to compare the MD predictions to models of ion-assisted etching involving the concept of a site balance. For the case of 50 eV C1+ etching silicon, the coverage vs. exposure results in the simulation could be reasonably well reproduced in a site balance model, but only if the correct parameters in the model were taken from the simulation. The results of the comparison suggest that MD simulations can be helpful in the development of physically sound phenomenological models of ion-assisted etching. INTRODUCTION Plasma etching, plasma-enhanced chemical vapor deposition, sputtering, and a host of related processes are widely used in the microelectronics industry. However, plasmasurface chemical and physical interactions are perhaps the least well understood part of
plasma-assisted materials processing technology. In particular, there is a need for the development of phenomenological models of ion-assisted etching to be used in plasma tool simulations as boundary conditions, and for feature profile evolution codes.1-4 In this paper, we report on an initial attempt to develop a physically based phenomenological model of a relatively simple situation involving direct reactive ion beam etching in the absence of thermal neutral species. We have employed molecular dynamics to simulate the processes of silicon surface chlorination and etching from Cl+ ions (assumed to be neutralized by Auger processes before impacting the surface 5 ) impacting the surface at three separate energies (10, 25 and 50 eV) and at normal incidence. After briefly describing the results from the MD simulation, we use the simulation predictions to construct a simple 'site balance' model of etching. DESCRIPTION OF THE SIMULATION Details of the simulation can be found elsewhere. 6' 7 The potentials we have used are based on the Stillinger-Weber (SW) potentials originally developed for Si-Si, Si-F and F-F.8 We have employed the modifications to the SW potentials reported by Feil et al. to model Si-Cl and C1-Cl. 7 The simulation procedure for direct reactive ion etching of silicon proceeds as follows. Starting with an initially crystalline silicon surface (1008 atoms, with 29 Mat. Res. Soc. Symp. Proc. Vol. 389 0 1995 Materials Research Society
periodic lateral boundaries, and two bottom fixed layers), the layer is repeatedly hit with Cl+ at the desired energy and at normal incidence. The layer is brought back to 300 K. The surface of the layer is searched for the presence of SiCl2 species that would desorb thermally in the approximately 10-4-100 s between ion impacts. Statistics for each impact and subsequent trajectory are recorded. The next energetic ion at a new random position is brought at normal incide
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