Measurement of CH 4-concentration in HMDSO-containing process plasmas by quantum cascade laser absorption spectroscopy

Abstract

For thin film deposition or plasma etching often organic precursors are used in the process plasma and related transient species are formed. In general it is not possible to measure the converted quantity of these precursors directly. In the present work we have used a special laser absorption spectroscopy to investigate characteristic molecular lines in the plasma to determine the concentration of stable organic molecules. Quantum cascade laser absorption spectroscopy (QCLAS) is a rather new technique for the precise measurement of absolute molecule concentrations. QCL’s can be operated at room temperature. They emit light within the mid infrared and have similar spectroscopic characteristics to Tunable Diode Lasers (TDL). The commercially available system Q-MACS (Quantum Cascade Laser Measurement and Control System) offers a solid platform for the measurement of absolute molecule concentrations in plasmas and gas mixtures. The used Q-MACS is due to its laser characteristics particularly well suitable for determination of the concentrations of acetylene and methane. Molecular concentrations of methane were measured in hexamethyldisiloxane (HMDSO) containing plasmas, too. The methane concentration was found to depend on rf power and HMDSO flow.