Kinetics of the reaction of silicon with gaseous bromine

Abstract

The chemical etching of intrinsic polycrystalline silicon has been studied at Br₂ pressures between 0.05 and 30 Torr, and at temperatures ranging from 540 to 600 °C. In this temperature range, and at a pressure of 1 Torr and above, the etch rate is given by: R_etch= 10^{10.2 ± 0.4} nm min^–1 Torr^–1/2 exp-[(132 ± 7 kJ mol^–1)/RT]×P^1/2– 10^{9.8 ± 2.4} nm min^–1 exp-[(140 ± 40 kJ mol^–1)/RT]. Deviations from this rate law at low pressures were found to be consistent with a reaction mechanism which involves the dissociative adsorption of Br₂ on the silicon surface. A kinetic analysis of the pressure and temperature dependences of the etch rate yields a value of 10^{8.9 ± 1.3} nm min^–1 Torr^–1 exp-[(108 ± 22 kJ mol^–1)/RT] for the first-order rate-controlling rate constant at low pressures and 10^{10.2 ± 0.4} nm min^–1 Torr^–1/2 exp-[(132 ± 7 kJ mol^–1)/RT] for the half-order ‘composite’ rate constant which is observed at high pressures. The possibility that bromine atoms are produced in the gas phase, and that these species are responsible for the observed etch rates is also considered. The atom reaction and the molecule reaction proceed through the same intermediate, and the relative contribution from the two processes depends on how far the concentrations of these species are from their equilibrium values.