Spatially resolved optical emission study of sputtering in reactive plasmas

Abstract

The study of material sputtering under low-pressure reactive ion etching conditions in various gases (Cl2, SiCl4, O2) was performed using optical emission spectroscopy with high spatial resolution. Sputtering-induced secondary photon emission (atomic and molecular) from the processed materials (Si, Al2O3, GaAs) was found to be strongly localized near the target surface. A spatial distribution of atomic line emission intensity was shown to be essentially nonmonotonical with distance from the surface. This effect was explained by a cascade feeding from the upper lying atomic levels, which is enhanced in plasma (collisional) environment. A simplified model accounting for the cascading has been developed, and velocities of sputtered excited atoms (in the range of 2–7×106 cm/s) and molecules (about 2–5×105 cm/s) have been evaluated from the emission spatial decay parameters. The excited sputtered atoms and molecules are produced in different types of collisions. Fast excited atoms can be produced only in the first few collisions of the incident ion in the surface top layers, whereas excited molecules are knocked off by secondary (slow) atoms originated from a collision cascade inside the solid. Based on this concept of the process, simple expressions for atomic and molecular excitation yields as functions of the incident ion flux and surface coverage were deduced. The technique can be used for in situ surface probing during plasma processing.