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