Effects of ion bombardment in plasma etching on the fluorinated silicon surface layer: Real-time and postplasma surface studies

Abstract

The silicon-fluoride reaction layer formed on silicon surfaces when etched in low pressure discharges of CF4 and SF6 has been characterized using ellipsometry in real time and x-ray photoemission spectroscopy (XPS) after sample transfer in vacuum. By comparing reactive ion etching (sheath voltage Vs= 50–500 eV) and plasma etching (F5—30 eV) over the pressure range 10–250 mTorr and by using different rf power levels, the effects of ion bombardment on the SiF, layer were examined. The thickness of the SiFi layer depends primarily on the sheath voltage of the discharge, which determines the maximum energy of the bombarding ions. For low ion bombardment etching the apparent thickness of the SiFi layer is —0.5 nm. It increases up to — 1.7 nm as the sheath voltage is increased. Nondestructive profiling of the SiFx layer was performed by angle-resolved photoemission to determine the variation of the various fluorosilyls (SiF, SiF2, SiF3, and SiF4) as a function of depT. SiF3 has the highest intensity at the surface of the SiF, layer and drops off rapidly in relative importance as the reacted layer is probed deeper. At the largest depth, SiF becomes the dominant species. SiF2 and SiF4 are also peaked at the surface, but their relative importance is always smaller than that of SiF and SiF3. The initial stages of fluorination of the Si surface were monitored in real time using ellipsometry. Within the first seconds of exposure to the plasma, a damaged Si layer is introduced which is subsequently fluorinated and partly consumed. The steady-state SiFx layer thus formed and present on the crystal during etching is stable when the discharge is terminated. This finding validates the use of postplasma XPS analysis for the study of the SiFx layer. The ellipsometric measurements demonstrate that an increase in the sheath voltage of the discharge results in increased surface damage and deeper fluorination of the Si surface. These results are in conflict with the predictions of several models of ion-enhanced etching and indicate that ion-induced mixing and generation of damage may play an important role in ion-enhanced etching reactions. © 1993, American Vacuum Society. All rights reserved.