Competitive halogenation of silicon surfaces in HBr/Cl2 plasmas studied with x-ray photoelectron spectroscopy and <i>in situ</i>, real-time, pulsed laser-induced thermal desorption

Abstract

Using x-ray photoelectron spectroscopy (XPS) and real-time, laser-induced thermal desorption–laser-induced fluorescence (LD–LIF), we have determined the coverage of Br and Cl on Si(100) surfaces that are etched in mixed HBr/Cl2 plasmas. Halogen coverages measured by XPS after etching are directly proportional to the fraction of the respective halogen in the feed gas. LD–LIF was detected from SiCl(g) and SiBr(g) products with intensities that are a semiquantitative measure of instantaneous Cl and Br coverages. Saturated coverages during etching in Cl2 and HBr plasmas are 1.0×1015 Cl/cm2 and 6.0×1014 Br/cm2, respectively. Etch rates at these two extremes are 2170 and 1330 Å/min, and therefore are proportional to the respective halogen coverages. It therefore appears that the rate of formation of volatile Si–halides, stimulated by ion bombardment, is lower for HBr mainly because less halogen is available on the surface at saturated coverage. Langmuir probe measurements indicate that the ion flux is 17% lower in pure HBr plasmas, compared with Cl2 plasmas at the same power. The plasma potential, direct current bias voltage, and hence ion energy were nearly constant, however, over the range of gas mixtures. The slightly different ion fluxes suggest that the ion bombardment-stimulated process, although similar for Cl2 and HBr plasmas, is actually slightly more efficient on a per halogen basis for Br versus Cl. Positive photoresist surfaces that were also exposed to the plasma differ from Si in that less Br adsorbs on the surface both in pure HBr and mixed HBr/Cl2 plasmas. Apparently the lower reactivity of photoresist with Br (compared with Cl) is at least partly responsible for the widely observed slower etch rate of photoresist in HBr plasmas, and hence enhanced selectivity when etching Si.