Capacitive thin film vacuum gauges are known as "absolute pressure measurement" due to their high precision, wide range, and independence from gas composition. They occupy a core position in semiconductor manufacturing, vacuum coating, scientific research experiments, and other fields. The core principle can be broken down into three key steps:
1、 Pressure driven mechanical deformation
The core component of a capacitive thin film vacuum gauge is an ultra-thin metal film (usually made of Inconel chromium nickel iron alloy or ceramic material, with a thickness of only a few tens of microns), which is connected to the vacuum chamber and reference chamber on both sides. When the pressure inside the vacuum chamber changes, the diaphragm undergoes elastic deformation under the action of pressure difference: if the vacuum chamber pressure is lower than the reference chamber, the diaphragm protrudes towards the vacuum side; Otherwise, it will be concave towards the reference side. This deformation directly reflects the value of absolute pressure and is independent of the type of gas, only depending on the magnitude of the pressure difference.
2、 Precise detection of capacitance changes
The membrane and fixed electrode form a parallel plate capacitor. When the membrane deforms, there is a slight change in the electrode spacing (such as on the order of 0.1 micrometers), resulting in significant fluctuations in capacitance (about 0.3 pF). To improve measurement accuracy, modern vacuum gauges often adopt a differential dual capacitor structure: electrodes are symmetrically arranged on both sides of the diaphragm to form two independent capacitors. When the membrane is offset, one capacitance increases and the other decreases. The difference in capacitance is converted into a voltage signal through a bridge circuit, effectively eliminating environmental interference.
3、 Signal conversion and calibration output
The backend circuit converts the capacitance change into a standard electrical signal (such as voltage or current) and calculates the true pressure value through a preset formula. For example, a certain model of vacuum gauge achieves wide range coverage from 10 ⁻ Pa to 10 ⁵ Pa by measuring the relationship between compensation voltage and pressure difference (P=KU, K is the gauge constant). To eliminate the influence of temperature drift, some devices use polycrystalline silicon temperature control substrates to maintain a constant working environment (such as 28 ℃), ensuring long-term stability.
The linearity and repeatability of the capacitive thin film vacuum gauge are both better than 0.1%, and there is no need for a reference pressure. Even if suddenly exposed to the atmosphere, it will not be damaged, making it an ideal tool for absolute pressure measurement.