MOLECULAR-DYNAMICS SIMULATIONS OF SILICON-FLUORINE ETCHING

Molecular dynamics simulations of the reactions between gaseous fluori ne atoms and (SiFx)(n) adsorbates on the Si{100} - (2 x I) surface are performed using the SW potential and compared to simulations with the WWC reparameterization of the SW potential. Theoretical and experimen tal work has demonstrated that the reactive fluorosilyl layer during s ilicon fluorine etching is composed of tower-like adspecies of SiF, Si F2, and SiF3 groups. The objective of the simulations is to determine how the chemical composition, mechanism of formation, and energy distr ibution of the etched gas-phase products depend on the identity of the reacting adsorbate, the incident kinetic energy, and the parameteriza tion of The potential energy function. Three reactions are simulated: F(g) + SiF3(a), F(g) + SiF2-SiF3(a), and F(g) + SiF2-SiF2-SiF3(a), SiF 4 is the major product and Si2F6 and Si3F8 are minor products. In Si2F 6 and Si3F8, the silicon-fluorine bond that is formed is stronger than the silicon-silicon bond in the molecule and therefore, the majority of these products have enough energy to dissociate and will fragment b efore reaching the detector. An S(N)2-like mechanism is the primary me chanism responsible for the formation of SiF4, Si2F6 and Si3F8. In add ition at higher energies, the simulations have discovered a previously unknown mechanism for the formation of SiF4, which involves an insert ion between a silicon-silicon bond The results of the simulations with the two potentials differ quite substantially in their prediction of the reactivity of the adsorbates. The SW potential predicts a 2- to 3- eV lower energy threshold for reaction and a much higher reaction cros s-section, especially for the SiF4 product. These results are explaine d in terms of the differences in the potential energy functions used t o describe the silicon-fluorine interactions. In addition, the results are compared to experimental data on silicon-fluorine etching. (C) 19 96 by Elsevier Science Inc.