An acoustic sensor, the Leybold Inficon ComposerTM, was implemented downstream to a production-scale tungsten chemical vapor deposition (CVD) cluster tool for in-situ process sensing. Process gases were sampled at the outlet of the reactor chamber and compressed with a turbo-molecular pump and mechanical pump from the sub-Torr process pressure regime to above 50 Torr as required for gas sound velocity measurements in the acoustic cavity. The high molecular weight gas WF6 mixed with H2 provides a substantial molecular weight contrast so that the acoustic sensing method appears especially sensitive to WF6 concentration.

By monitoring the resonant frequency of exhaust process gases, the depletion of WF6 resulting from the reduction by H2 was readily observed in the 0.5 Torr process for wafer temperatures ranging from 300 to 350 C. Despite WF6 depletion rates as low as 3-5%, in-situ wafer-state metrology was achieved with an error less than 6% over 17 processed wafers.

This in-situ metrology capability combined with accurate sensor response modeling suggests an effective approach for acoustic process sensing in order to achieve run-to-run process control of the deposited tungsten film thickness.