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 mum 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 before and after SF6 plasma clean with various focus rings made of quartz, alumina, and silicon carbide. The HBr dissociation fraction increased 17% with the quartz focus ring after the chamber was seasoned. The silicon carbide focus ring had essentially no influence on HBr concentration before and after chamber clean. The success of this work demonstrates the potential of diode lasers for in situ monitoring of the plasma etch process for real time control applications.