Transcript Electrical Power Subsystems

SS/L-TRXXXX
October 19, 2009
20 April 2009
Capacitance Manometer Transducer
Bryn Asher
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Capacitance Manometer Transducer
 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

Transducer’s analog output is directly proportional to pressure

High Accuracy and Resolution

0-10VDC Output

Eliminates the need for reference pumps
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Capacitance Manometer Transducer-Physical Principle
 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

Thin diaphragms can measure down to 10^-5 torr
 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

The deflection changes the capacitance between the diaphragm and the
electrodes.

The measurement of the change in capacitance is a direct reading of the
pressure exerted by the gas on the diaphragm
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Capacitance Manometer Transducer-Implementation
 Powered by Power supply providing +/- 15VDC regulated at 30mA
 As the capacitance changes due to the pressure change, the bridge becomes
unstable and an AC signal related to the pressure is produced.
 The signal is fed into a buffer amplifier

Uses a linearizer to vary the feedback to obtain a signal of the applied pressure
 The amplified signal goes to a phase sensitive detector, which demodulates
the AC signal to a DC voltage
 Final stage output buffer amplifier generates a voltage level that is read by the
user
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Capacitance Manometer Transducer-Common Applications
 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.
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Capacitance Manometer Transducer-Challenges
 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.
 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.
 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.
 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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