Polarization Raman microprobe analysis of laser melting and etching in silicon

Abstract

Polarization Raman microprobe spectroscopy is used to study crystalline silicon heated to the melting point by a tightly focused cw laser beam, which is either fixed or scanned across the surface. By examining optical phonons in solid silicon, the real-time Raman spectrum monitors the progress of silicon flow during melting and the trench depth during melt-assisted etching. Raman peaks lie between 482 cm−1, which is the Raman shift for silicon uniformly heated to the melting point (1690 K), and ∼510 cm−1, which is the Raman shift for c-Si heated just to the melting point and probed by the same beam. During laser melting with a static laser, the Raman spectrum of scattered light with z(x,y)z̄ polarization has two peaks, while the z(x,x)z̄ spectrum has one peak. This shows that at the beginning of melting in vacuum by a static laser there is a central region with solid silicon floating in the melt, which is surrounded by hot solid material. Because of the flow of the molten semiconductor, the temperature profile changes, causing the Raman spectrum to change rapidly. Laser melting of c-Ge and thin films of silicon in vacuum is also studied, as is the melting of c-Si by a static laser in the presence of an inert buffer gas. The presence of an inert buffer is shown to affect the temperature profile very strongly during melting and also during laser heating at lower laser powers when no melting occurs. During scanned laser melting and etching, the Raman spectrum has a single peak using either polarization configuration. Raman analysis during melting of silicon by a scanning laser shows that the average temperature in the probed region is much higher when there is a gas-phase argon buffer present (and no etching) than when there is an etching mixture of argon/chlorine gas (and etching). Along with these real-time Raman measurements, the reflection of the incident laser was monitored, and post-process Raman analysis and profilometry were also performed to characterize surface changes due to melting and etching.