Infrared laser spectroscopy of BCl3 in an rf discharge

Abstract

Infrared laser measurements of the extinction coefficient of a gas in a discharge can be used to determine, in a nonintrusive manner, the rotational and translational temperature and degree of dissociation of the gas, thus enlarging the spectral regions wherein diagnostic measurements of plasma properties may be performed. This is accomplished by obtaining extinction coefficients at 14 CO2 laser lines near 10 μm in a BCl3 rf discharge at two different power levels, as well as under quiescent conditions. All data points are fit to a band model calculation which yields values for upper state rotational constants, band center position, and temperature in the cell as well as total band strength at each power level. The rotational temperature of the gas is found to agree with the cell wall temperature, verifying equilibrium of rotational and translational temperatures. Since these are absolute measurements of band strengths, it was possible to determine a degree of dissociation of ≲15%. The results obtained from this technique are shown to complement those obtained from other plasma diagnostic methods, such as laser induced fluorescence. Since BCl3 is a gas commonly used as a component in the plasma etching of aluminium and aluminium oxide, a more detailed understanding of the chemistry of this reactive plasma system is pertinent to the improved fabrication of integrated circuits.