ACOUSTIC FLOWMETER FOR THE ACCURATE METERING OF GASES FOR SEMICONDUCTOR
PROCESSING. Adam M. Calabrese, Building 220, Room A-55, NIST, Gaithersburg,
MD 20899 (301- 975-4836, email: firstname.lastname@example.org)
Semiconductor 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 ultra-pure, yet highly toxic and/or reactive gases. Currently, thermal mass flow meters (TMFMs) are widely used in industry for these processes. TMFMs are usually calibrated with inert gases, and because the heat transfer in TMFMs depends on a complex combination of the gases properties, their use with semiconductor gases has been suspected by the semiconductor industry as a liability concern. Further complications with TMFMs arise because the flow must be divided in order to make a measurement due to limited dynamic ranges of the TMFMs. In the present research, an ultrasonic Doppler shift flowmeter has been developed in order to measure gas flows in the range of interest. Measurements of the flow-induced phase shift of an acoustic plane wave propagating in an unobstructed tube are used to determine the velocity of the gas. Combination of the 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 of 100 kHz. No division of the flow is necessary, and the meters use materials that are compatible with the relevant process gases. Prototype meters using a piezoelectric transducer as a source and a condenser microphone as a receiver indicate that sensitivities on the order of 1 degree/sccm are possible in the range of 1-1000 sccm. Meters built using a piezoelectric transducer as both source and receiver give similar results, although noise (in the form of mechanical coupling) presents problems that require further refinement.