Ultrasonic flowmeter for the accurate metering of gases for semiconductor processing

Abstract

Semiconductor production processes such as chemical vapor deposition rely upon accurate control of the flow rate (in the range of 1–1000 standard cm3/min) of several ultrapure, yet highly toxic and/or reactive gases. Currently, thermal mass flow meters (TMFMs) are widely used in industry for these processes. Complications with the use of TMFMs arise because the flow must be divided (due to limited dynamic ranges) and calibration is normally performed with inert gases. In the present research, an ultrasonic Doppler shift flowmeter has been developed to measure gas flows. Measurements of the flow-induced phase shift of an acoustic plane wave propagating in an unobstructed tube, combined with the speed of sound of the gas, are used to determine the mean gas velocity. The combination of the mean gas velocity with measurements of the pressure and temperature allow calculation of the mass flow rate. The experimental system uses stainless steel tubes of diameter 0.05–0.1 cm, and frequencies near 100 kHz. No division of the flow is necessary. Prototype meters indicate that sensitivities on the order of 0.75°/standard cm3/min are possible. Experimental data of two prototype systems using different geometries and receivers will be presented. [Work supported by NIST.]