Radical recombination sensor based on dual probe thermopile heat flux sensors

Abstract

Hydrogen radicals play an important role in, e.g., the cleaning of extreme ultraviolet reflective mirrors. Therefore, there is a need to quantify the surface radical flux in the various (plasma) setups where these effects are studied. In this paper, a catalytic radical sensor is presented, based on the measurement of the recombination heat of radicals on a surface, using dual probe thermopile heat flux sensors (HFSs). The first HFS1 has a high recombination (probability) coefficient coating, e.g., Pt. The second HFS2 has a low recombination coefficient coating, e.g., Al2O3. Signal subtraction largely eliminates common mode heat losses/gains such as conduction/convection and IR radiation, the net result representing the radical recombination heat. The signal can be improved by switching the radical source on/off at regular intervals. Radical recombination rates were measured in a remote microwave plasma chamber (38 Pa H2) over the range 1018−1021 atH/(m2 s), with nearly linear response as a function of plasma power setting. The sensor full scale limit is ∼1023 atH/(m2 s) and is dictated by the maximum allowable sensor surface temperature (<250 °C).