Diagnosis of Gas-Phase Reactions by Laser Spectroscopy

Abstract

New diagnostic means based on laser spectroscopy are described. These include laser-induced fluorescence, Raman spectroscoy and coherent anti-Stokes Raman spectroscopy (CARS). These new techniques are required to clarify gas-phase processes involved in various CVD methods, including plasma processes. There are specific requirements for the diagnostic means to be useful. Each of the spectroscopic means has been examined to see how these requirements are satisfied, and any shortcomings are explained. CARS is described most in detail, because of a strong interest in this technique recently. The experimental setup is based on either a Nd : YAG laser or an excimer laser in combination with dye lasers. It has been applied to a diagnostic study of CO₂ laser CVD of silane, where the internal excitation of material gas molecules is examined and the rise in gas temperature following light absorption is measured as a function of a distance from a substrate.