Dynamics of SiF4 desorption during etching of silicon by XeF2

Time-of-flight distributions have been obtained for SiF4, the principal product of the spontaneous etching reaction between silicon and XeF2. The data were extracted from modulated beam mass spectrometric measurements of SiF+4 intensity as a function of time. Comparison of the velocity and energy distributions derived from the data to the corresponding Maxwell–Boltzmann distributions shows clearly that although the most probable values of the distributions are very close to those expected when the desorbed species are in thermal equilibrium with the surface, the average values are not. The etch product velocity and energy distributions are not characterized by a single temperature, but comprise both hot and cool populations. These results show that the etch product desorption process cannot be described as simple evaporation of weakly bound species in equilibrium with the surface. The nature of the relationship between formation and volatilization is discussed.