What is the reason for the temperature drift of the diffused silicon pressure sensor?

- Dec 30, 2020-

Drift is an inevitable characteristic of the pressure sensor. One of the essential reasons is the fatigue problem of the elastic material of the pressure sensor. No matter which material it is, there will be elastic fatigue after each elastic deformation recovery. The drift of pressure sensors using different elastic materials varies according to different materials. As long as it is a qualified product, the drift will be controlled within an acceptable range.

   In addition to the drift caused by the material itself, there is also temperature drift. Temperature drift refers to the change in the output of the pressure sensor caused by temperature changes. Temperature drift is actually caused by the characteristics of the material, and many materials are also very sensitive to temperature. Therefore, pressure sensors usually perform temperature compensation. The method of temperature compensation is to use another material with the opposite temperature characteristic to offset the temperature-induced changes or use digital compensation technology.

  In the early days, the diffused silicon pressure sensor was sealed with glass powder between the diffused silicon chip and the metal base. The disadvantage is that there is large stress around the pressure chip. Even after annealing, the stress cannot be completely eliminated. When the temperature changes, due to the different thermal expansion coefficients of metal, glass and diffused silicon chips, thermal stress will be generated, which will cause the zero point of the sensor to drift. This is why the zero thermal drift of the sensor is much larger than the zero thermal drift of the chip. Silver paste and terminal posts are used for welding, which is not easy to handle and will easily cause unstable contact resistance. Especially when the temperature changes, the contact resistance is more likely to change. These factors are the reasons for the large drift and temperature drift of the sensor zero.

The later gold-silicon eutectic welding method can reduce the drift to a large extent. The gold-silicon eutectic sealing between the diffused silicon and the base is used because the gold is relatively soft and the stress is small. The pressure tube is a glass tube. The glass tube is sintered to the silicon ring, and the glass tube and the base are bonded with high-temperature glue. In order to measure the gauge pressure, a metal tube is bonded to the outside of the glass tube, which is open to the atmosphere. Diffused silicon resistance strips form a Wheatstone bridge, and a conductive book is formed by a highly doped method, which reliably connects the bridge to the aluminium electrodes distributed on the periphery, instead of the usual method of steaming aluminium and engraving the aluminium strip This will help reduce the hysteresis of the sensor. The aluminium electrode and the terminal are welded with gold wire and ultrasonic welding to make the resistance at the contact more stable.