High-sensitivity diode laser absorption spectroscopy.

摘要

Recent advances in the vapor phase materials processing have lead to more stringent requirements for diagnostics tools. A new diagnostic tool aimed at meeting some of the new requirements is developed by utilizing high sensitivity diode laser absorption spectroscopy.; By suppressing sensitivity limiting etalon fringes while preserving the true absorption signal, the fundamental while noise limited sensitivity of laser absorption spectroscopy is restored. The enhancement of absorption signal to background fringe ratio is obtained by proper control of the operational parameters of a dual modulation scheme dubbed Combined Wavelength and Frequency Modulation Spectroscopy (CWFMS). This dual modulation scheme combines the spectral selectivity of wavelength modulation spectroscopy and the high sensitivity of frequency modulation spectroscopy into one.; In addition to the high sensitivity obtained, the scheme provides multiple orders of signals. Any two of those consecutive orders can form a pair of signals for measuring the absorbance and locking the laser frequency simultaneously. One of these two orders has an absolute maximum at the absorption peak while the other is zero crossing and antisymmetric about the absorption peak.; The sensitivity of CWFMS has been measured by using several lead-salt diode lasers and an AlGaAs diode laser and white noise limited detection was achieved. CWFMS was applied to two different processing environments namely plasma processing and metalorganic chemical vapor deposition. For plasma processing, characteristics of SF{dollar}sb6{dollar} and {dollar}rm Csb2Fsb6{dollar} plasmas were studied. The detection of plasma etching end-point was demonstrated. For metalorganic chemical vapor deposition. The high resolution spectra of methylsilazane during pyrolysis were obtained and analysed to identify a vibrational excited state and methane as a by-product.