Abstract
Tunable diode laser absorption spectroscopy is used to monitor hydrogen chloride(HCl) concentration in a commercial, high-density, low-pressure plasma reactorduring plasma etching. A near-infrared diode laser is used to scan the P(4)transition in the first overtone of HCl near 1.79 µmto measure changes in HCl levels. A variety of HBr and Cl2feedstock recipes are investigated at a process pressure of 10 mTorr asa function of rf power transformer coupled plasma, bias power andthe total flow rate. Using 50 ms averaging and a signal modulationtechnique, we estimate a minimum detectivity of 4 × 10−6in peak absorbance, which corresponds to an HCl numberdensity of ∼2 × 1011 cm−3.The diode-laser based HCl sensor is sufficiently sensitive to detect smallconcentration variations and HCl concentration correlates with poly-Si etch ratefor the conditions studied. These measurements demonstrate the feasibility of areal-time diode laser-based sensor for etch rate monitoring and the potential forprocess control.
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