Interferometric thermometry measurements of silicon wafer temperatures during plasma processing

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Abstract
This article reports the application of interferometric thermometry to the measurement of silicon wafer temperatures during exposure to low-pressure (1 mTorr), high-density (ne= 1 × 1011 cm3) helical resonator plasmas. This technique measures the temperature-dependent optical pathlength from the front to the back of a wafer polished on both sides. The measurement does not perturb the plasma, nor does it suffer from interference from the plasma, problems which often plague other techniques. Nonetching N2 plasmas heat the wafer to 140 °C in the first 3 min, and 200 °C after 10 min when the wafer is simply placed on the platen with no provisions for heat dissipation. When the wafer is clamped to an o-ring seal on the platen and a small gap between the wafer and platen is pressurized with helium (so-called helium backside cooling), the wafer temperature rise is reduced. With a helium pressure of 10 Torr between the wafer and platen, the temperature was 40 °C after 2 min and 70 °C after 10 min. When silicon wafers are etched in a Cl2 plasma, the interferometric signal contains contributions from both heating and etching, which respectively increase and decrease the optical pathlength within the wafer. A deconvolution of this signal using the known etch rate indicates that plasma-induced heating in a Cl2 plasma is approximately the same as in a N2 plasma of about the same power, indicating that any contributions from heating due to chemical reactions in the etching process are negligible compared to heating from the plasma.

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