In-situ plasma chamber monitoring for feedforward process control

This paper examines the effects of polymer buildup in plasma etching systems and describes a micromachined sensor for in-situ polymer thickness measurement. Using gas flows of 45 sccm CHF3CHF3 and 15 sccm CF4CF4 at 50 mTorr and 1000 W, the oxide:polysilicon selectivity ranges from 2.6 to 8.5 as the polymer thickness on the tool walls varies from 0 to 240 nm. The polymer sensor is based on an electrothermal oscillator that measures the thermal mass change as polymer builds up on a stress-compensated dielectric window. The change in the thermal mass of the window can be detected as a variation in the pulse width (cooling time) of the oscillation. The device operates with a typical cooling time of 2.7 msec and has a measurement resolution of better than 1 nm. The device is flush-mounted in the chamber wall with the exposed window area protected by a thin film of iridium against damage by the plasma. © 1998 American Institute of Physics