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SS/L-TRXXXX 20 April 2009 1 Capacitance Manometer Transducer Bryn Asher October 19, 2009.

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Presentation on theme: "SS/L-TRXXXX 20 April 2009 1 Capacitance Manometer Transducer Bryn Asher October 19, 2009."— Presentation transcript:

1 SS/L-TRXXXX 20 April 2009 1 Capacitance Manometer Transducer Bryn Asher October 19, 2009

2 2 Capacitance Manometer Transducer u Capacitance Manometer Transducer is an active sensor which makes direct vacuum/pressure measurements  Absolute pressure transducer  Pressure is sensed by measuring pressure induced displacement of a metal diaphragm l Transducer’s analog output is directly proportional to pressure l High Accuracy and Resolution l 0-10VDC Output l Eliminates the need for reference pumps

3 3 Capacitance Manometer Transducer-Physical Principle u Capacitance Manometer Transducer has a thin, radially tensioned metal diaphragm which separates two volumes  One volume is an electrode assembly placed in a sealed high vacuum reference cavity pumped continually to maintain the reference pressure  The other volume is exposed to the pressure to be measured l Thin diaphragms can measure down to 10^-5 torr u An electrode assembly in the reference vacuum along with a metal diaphragm make up the capacitor  As the pressure changes between the two volumes the metal diaphragm is proportionally deflected l The deflection changes the capacitance between the diaphragm and the electrodes. l The measurement of the change in capacitance is a direct reading of the pressure exerted by the gas on the diaphragm

4 4 Capacitance Manometer Transducer-Implementation u Powered by Power supply providing +/- 15VDC regulated at 30mA u As the capacitance changes due to the pressure change, the bridge becomes unstable and an AC signal related to the pressure is produced. u The signal is fed into a buffer amplifier  Uses a linearizer to vary the feedback to obtain a signal of the applied pressure u The amplified signal goes to a phase sensitive detector, which demodulates the AC signal to a DC voltage u Final stage output buffer amplifier generates a voltage level that is read by the user

5 5 Capacitance Manometer Transducer-Common Applications u Widely used in the semiconductor industry  Inconel body and diaphragm are suitable for the corrosive services of this industry  Favored because of their high accuracy and immunity to contamination  Measures the gas pressure for common semiconductor processes, such as Electron beam lithography as well as Plasma etching and Chemical vapor deposition tools.

6 6 Capacitance Manometer Transducer-Challenges u Sensor is not solid state; sensitive to particular contamination.  If the transducer is mounted upward, debris could fall into the port and cause deflection of the diaphragm or electrical shorts.  Mount transducer so port is facing downward. u Must be protected from transient sources of heat or cold; extreme temperature changes can cause a false pressure readings on the sensor.  Heating/cooling equipment often in use during high vacuum processes (e.g. vaporizers, cold fingers, diffusion pumps, etc).  Sensor must be far removed from such equipment. u Mechanical vibration sources near the sensor can cause noisy pressure readings.  To minimize the vibration, the transducer can be mounted to a padded support.  Non-rigid connections rated for high vacuum service remain a challenge. u Capacitance is affected by deposition of materials on the electrodes in vacuum.  Plastic or rubber connections to the sensor can introduce outgassing sources and cause false pressure readings.  These connections should be avoided, complicating vibration isolation.

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