A reinvestigation of the etch products of silicon and XeF₂: Doping and pressure effects

Abstract

The gas-phase etch products of silicon and XeF2 have been studied as a function of doping and XeF2 flux using molecular beam mass-phase spectrometry. In this experiment the product flux, not the incident reactant beam, is modulated, providing direct information on product identity and velocity, and indirect information on surface reaction kinetics. The data show that although the product ion distributions exhibit little change over the range of experimental conditions used, the neutral products from which the ions are formed vary significantly. More SiF4 desorbs from n-type than p-type silicon at constant flux. The balance of the products mainly consists of Si2F6 and SiF3, the relative amounts of which are flux dependent. These species have not been identified previously as etch products. Average product translational temperatures have also been determined, providing important information on surface interactions during etching. The stable molecules SiF4, Si2F6, and reflected (unreacted) XeF2 all have translational temperatures of approximately 270 K. The free radical SiF3 is much colder, having a temperature of about 200 K. Although etching of the two types of silicon is qualitatively similar, a quantitative comparison reveals intrinsic differences in their reactivity which are not attributable to field effects. The possible origin of these differences is discussed.