Design of a new sensor for mass flow controller using thin-film technology based on an analytical thermal model

Thermal mass flow controllers are widely used in the semiconductor industry to deliver accurate process gas compositions and concentrations to the processing chamber. The sensor, a stainless-steel capillary tube, is the most critical part of the mass flow controller. We have designed a new sensor using thin-him technology in order to improve its reliability and to decrease its response time. The two temperature sensors are deposited platinum on an electrical insulating layer directly upon the capillary tube (without exposure to the gas stream) whereas the sensor is constantly heated by a resistive wire. The so-obtained sensors are very promising and the new design and technology could give us improvements which the old design and technology (two resistive coils) would not have been able to give us. The physical mechanisms of this new sensor are described by a third-order steady-state analytical thermal model which gives good results when compared with experiments. This fundamental study also includes the evolution of the correction factors as a function of the flow. We present here the thermal model, the new structure of the sensor, and its advantages. (C) 1998 American Vacuum Society. [S0734-2101(98)02306-9].