Shang-I Chou

Tunable diode-laser absorption measurements of methane at elevated temperatures

A diode-laser sensor system based on absorption spectroscopy techniques has been developed to monitor CH(4) nonintrusively in high-temperature environments. Fundamental spectroscopic parameters, including the line strengths of the transitions in the R(6) manifold of the 2ν(3) band near 1.646 μm, havebeen determined from high-resolution absorption measurements in a heated static cell. In addition, acorrected expression for the CH(4) partition function has been validated experimentally over thetemperature range from 400 to 915 K. Potential applications of the diode-laser sensor system includeprocess control, combustion measurements, and atmospheric monitoring.

Diode-laser measurements of temperature-dependent half-widths of H2O transitions in the 1.4 μm region

Abstract Two distributed-feedback InGaAsP diode lasers have been used to perform spectrally resolved measurements of H2O transitions near 1.4 μm. Line-shape analyses of the recorded absorption spectra yielded the temperature-dependent self-, N2-, and CO2-broadened half-widths. Room-temperature (296 K) half-widths for eight rovibrational transitions in the v1 + v3 and 2v1 vibrational bands were determined from measurements recorded in a variable pathlength (0.4–10 m) multi-pass cell. The temperature dependence of the half-widths for four of the transitions were determined from measurements conducted in a heated static cell over a temperature range of 400–1000 K. The room-temperature half-widths and temperature exponents were determined to an average accuracy of +8% − 6% and ± 9% , respectively, and are in good agreement with theoretical calculations and published half-widths for transitions with the same rotational quantum numbers.

Diode laser absorption measurements of CH 3 Cl and CH 4 near 1.65 µm

Two distributed-feedback (InGaAsP) diode lasers were used to record high-resolution absorption spectra of the parallel and the perpendicular components of the 2nu(4) band of methyl chloride (CH(3)Cl) and the 2nu(3) band of methane (CH(4)) near 1.65 mum. The room-temperature absorption measurements, which were conducted in a multipass cell with a variable path length (878-1020 cm), were used to determine the mole fractions of the constituent gases and thus demonstrate species-specific, nonintrusive concentration measurements of species with overlapping spectra.

HBr concentration and temperature measurements in a plasma etch reactor using diode laser absorption spectroscopy

In situ measurements of HBr concentrations and rotational temperatures were recorded in a 300 mm planar inductively coupled plasma (ICP) etch reactor using diode laser wavelength modulation spectroscopy. A pair of diode lasers operating near 1.95 and 2.00 μm were wavelength tuned over the R(7) and P(2) transitions of HBr (2–0 band), time-division multiplexed, and directed through an industrial wafer etch reactor. The rotational temperature (typically 435±8 K) was determined from the ratio of peak absorption signals and the HBr concentration was determined from the measured temperature and absorbance from a single line. The measured rotational temperature in the plasma was relatively independent of conditions studied. The estimated HBr dissociation fraction ranged from 25%–60%, depending on the ICP power applied, gas flow rate, and chamber pressure. Decreases in HBr concentration were detected 1 cm above the wafer surface during blank silicon wafer etching. The HBr dissociation fractions were measured befo...

Diode laser sensor to monitor HCL in a plasma etch reactor

Absorption measurements of HCl during plasma etching of poly-silicon are made using the P(4) transition in the first vibrational overtone band near 1.79 μm. Single path absorption provides a real-time HCl monitor during etching of six-inch wafers in a commercial Lam Research 9400SE reactor at the University of Michigan. Wavelength modulation at 10.7 MHz is used to distinguish the absorption signal from the strong plasma emission. The laser center frequency is ramp-tuned at 500 Hz providing an HCl measurement every 2ms. Direct absorption measurements without the plasma are used to calibrate the wavelength modulation signal. The minimum detectable absorbance was 5x(10)-6 with 50 ms averaging, leading to an HCl detection limit of ~(10)12cm-3. For a given ratio of the feedstock HBr/Cl2, the measured HCl concentration tracks the average etch rate. These measurements demonstrate the feasibility of a real-time diode laser-based etch rate sensor.

Multiplexed diode-laser sensor system for vapor-phase semiconductor wafer cleaning process monitoring

Vapor-phase wafer cleaning processes offer the potential to decrease chemical usage and water consumption significantly over current liquid cleaning methods. However, the mechanisms responsible for oxide etch processes and the precise role of each species are not completely understood. This project aims to develop a unique multiplexed diode-laser absorption sensor system capable of in situ measurements of gas temperature and the concentrations of HF and H/sub 2/O to enhance process modeling and control. The sensor system employs three narrow-linewidth DFB tunable diode lasers operating at IR wavelengths.