Capacitance Manometer Physics

Capacitance manometers are electro-mechanical gauges that measure both pressure and vacuum. The capacitance gauge translates a pressure-modulated movement in a thin diaphragm into an electrical signal proportional to the pressure. Figure 1 shows the major components of a commercial capacitance manometer. The pressure sensor is the thin diaphragm that is exposed to the pressure or vacuum being measured via the inlet tube. An electrode, typically a ceramic disk with conductive pathways, is mounted in the reference cavity behind this diaphragm and is powered with an electrical signal (either AC or DC). Pressure differences between the process and the reference cavity deflect the diaphragm slightly, changing the distance between it and the electrode. Changes in this distance produce variations in the capacitance between the diaphragm and the electrode and this variation in electrical signal is proportional to the pressure change. This signal is amplified by the on-board electronics and output to the device's electrical connector for transmission to pressure indicators and process controllers.

Capacitance manometer schematic showing the internal components and functional zones
Figure 1. Capacitance manometer schematic showing the internal components and functional zones.

Since the diaphragm deflection is proportional to force (pressure), pressure measurements by the capacitance manometer are independent of the composition of the gas being measured. Thus, a capacitance manometer's output doesn't change if the gas composition in the process changes. In many chemical and thin film processes the chemistry can change on a second-by-second basis; capacitance manometers provide a critical advantage in such processes. Capacitance manometers typically read pressure or vacuum over a five-decade range; i.e., from 1000 Torr to 0.01 Torr. There are two basic types of capacitance manometer sensors:

  • Absolute capacitance manometers in which the reference cavity is evacuated to high vacuum so that the pressure measurements will always be referenced to vacuum
  • Differential capacitance manometers that do not have a reference cavity - just a tube or passage that can be connected to any pressure or vacuum source. These manometers read the difference in pressure between the inlet tube and the reference cavity at the backside of the electrode. They are routinely used as safety switches and for airflow pressure drop measurements

Capacitance manometers are frequently used as reference devices for other types of gauges (i.e., they are the calibration standard against which other gauge types are measured). An unheated 1000 Torr capacitance manometer has an accuracy specification (which includes repeatability) of about 0.25% of reading. This can be compared with an accuracy of 5-25% of reading for a Pirani or thermocouple gauge of the same range - a ratio of 100X.

NOTE: Thermal or mechanical gauges should NEVER be used to calibrate a manometer Ð the manometer is far more accurate.

Capacitance manometers are vacuum measurement workhorses in the semiconductor industry. Because of their insensitivity to gas composition absolute capacitance manometers are found in almost every semiconductor process tool where they are used to monitor in-process pressures. Differential capacitance manometers find broad application in areas requiring pressure-based switching and control such as chamber load locks.


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