A capacitance diaphragm gauge, commonly referred to as a CDG, measures vacuum gas pressure by direct measurement of the applied force on the surface of a thin diaphragm. A traditional capacitance vacuum transducer measures absolute pressure where one side of the diaphragm is exposed to a permanently sealed vacuum cavity integrated in the sensor element and the other side of the diaphragm is exposed to the process media.
The mechanical deflection of the elastic sensor diaphragm is a function of the applied pressure. The diaphragm constitutes an electrode and together with an integrated auxiliary electrode it forms a pressure dependent capacitor that in an electrical measurement circuit converts the applied pressure to an electrical signal.
The sensitivity and low range measurement capability of a capacitance diaphragm gauge is primarily determined by the thickness of the diaphragm. A thin membrane provides the highest sensitivity and lowest pressure measurement capability.
A capacitance diaphragm gauge has a dynamic pressure measurement range of up to 4 decades and a 0.1 Torr full-scale transducer offers an useful measurement down to 1.0E-4 Torr (1.33E-5 mbar). Typically, three individual capacitance gauge transducers are needed to cover the measurement range from atmospheric pressure to 1E-4 mbar, due to the limited dynamic range of a single sensor.
The preferred sensor diaphragm material of a capacitance manometer is either a ceramic material like Alumina (Al2O3) or a metal like Inconel. Both Inconel and ceramic diaphragms have a high level of corrosion resistance to many aggressive gases.
Capacitance diaphragm vacuum gauges measures the direct force on the diaphragm and consequently it measures independently of gas type and concentration. The gas type independent measurement is a significant advantage of the capacitance vacuum gauge and critically important in applications where different gas types or mixtures are used.
The measurement accuracy specification of the capacitance diaphragm gauge is provided as accuracy deviation of reading and is typically 0.1 to 0.5% of the measurement value. The accuracy specification is applicable for a certain range of the gauge (typically 2-3 decades) which can vary between different product types and manufacturers.
A capacitance vacuum gauge is sensitive to temperature variations and variation can result in a measurement offset error. The offset drift can result in significant measurement errors in the low range.
Temperature controlled capacitance manometers have an integrated heater that stabilizes the capacitance diaphragm sensor element and improve the zero error as a function of ambient temperature changes.
The mounting orientation of the gauge can cause measurement error due to gravity forces on the diaphragm for high sensitivity low-range capacitance manometers. Mounting position for the gauge is provided by the gauge manufacturer.
The thin diaphragm of the capacitance gauge sensor is sensitive to contamination and corrosion. Heated capacitance diaphragm gauges are used to prevent deposition of material on the sensor diaphragm. Sensor temperatures of 45 to 200 °C are commonly used. Contamination or depositions on the sensor diaphragm will typically result in a measurement offset error that can be zero-adjusted by the user until a certain level where the gauge malfunctions.
Contamination can also be reduced by protective baffle solutions that prevent particulates from reaching the sensor diaphragm and cause material deposition.
The semiconductor industry has historically been the primary application for capacitance diaphragm gauges for measurement and control of vacuum gas pressure during semiconductor wafer processing. The corrosion resistance sensor and gas independent measurement has been the driver for its widespread use in semiconductor application where aggressive gases are common.
In metrology applications, precision capacitance diaphragm gauges are the preferred gauge for calibration of other vacuum gauges.
In physical vapor deposition (PVD) applications, different gas mixtures are used to achieve different coating properties and capacitance gauges are often used as pressure control for coating processes.
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