An improved determination of calibration gas corrections for a typical thermal mass flowmeter design

The mass flow controller, or MFC, is a thermal flow device used extensively for the measurement and control of hazardous and corrosive process gases in the semiconductor industry. Calibration of the mass flow meter is conventionally performed using benign, non-hazardous surrogate gases, such as nitrogen, with correction of the calibration data to actual process gas indications by the use of linear correction factors. This conventional method is responsible for significant errors in the process gas measurement and control in the MFC's. This project, which focuses on the mass flow meter (MFM) component of the MFC, was undertaken to develop a better understanding of gas correction factors of the MFM, which are actually not constant but functions dependent on the total flowrate of gas through the MFM. A finite element and analytical model was developed for a specific MFM and used to help determine better gas corrections for both helium and argon. The results of this modeling led to identifying some of the sources of non-linear functional dependence of the correction factors. The methodology for using the model is outlined for further studies using actual toxic and corrosive process gases.