Research on 4H-SiC Based MEMS Pressure Sensors in Extreme Environments

Research on 4H-SiC Based MEMS Pressure Sensors in Extreme Environments

In the fields of aerospace engines, geothermal development, and automotive electronics, there is a widespread demand for pressure measurement under extreme high temperature conditions. For a long time, silicon-based microelectromechanical systems (MEMS) pressure sensors have been widely used, with the advantages of miniaturization and high accuracy. However, the inherent properties of materials limit their ability to operate stably under high temperature conditions exceeding 150 ° C. SiC has material properties such as wide bandgap, high thermal conductivity, radiation resistance, and acid alkali corrosion resistance, making it an excellent semiconductor material for producing high-temperature sensing devices. The research on pressure sensors based on SiC substrate initially focused on epitaxial growth of 3C-SiC on Si, but there were issues with substrate intolerance to high temperatures and failure caused by thermal stress mismatch. With the development of SiC single crystal substrate growth and epitaxial technology, pressure sensors on SiC substrates are considered to have greater application potential and research value. PAM-XIAMEN can offer SiC substrate and epitaxial wafer for researches on pressure sensors, additional specifications please refer to https://www.powerwaywafer.com/sic-wafer.

There have been relevant research reports on pressure sensors on SiC substrates. The main challenges and issues focus on the following points. Firstly, due to its high hardness and resistance to acid and alkali corrosion, SiC processing is difficult, and SiC sensor chips with complex structures are difficult to process. Therefore, current research mainly focuses on piezoresistive sensitive components with relatively simple structures. Secondly, due to the high complexity of crystal structure, there is currently a lack of systematic and comprehensive research on the piezoresistive effect of SiC, which leads to the inability of sensors to ensure higher sensitivity and better performance from the structural design stage; In addition, the lack of reliable high-temperature signal transmission and packaging testing solutions has limited the testing and application of SiC based sensors at present.

 1. Nonlinear Piezoresistive Effect of 4H-SiC

Based on the above difficulties, the researchers first studied the piezoresistive effect of N-type 4H SiC in a wide temperature range (-50 ° C-500 ° C), analyzed the internal scattering mechanism of its nonlinear piezoresistive effect, and obtained a minimum resistance temperature coefficient value of 60.5 ppm/° C, verifying that its piezoresistive effect can be further developed and applied, further improving the relevant research on the piezoresistive effect of 4H-SiC.

Then, through experiments and simulation analysis, the influence of geometric size effect on the piezoresistive coefficient of N-type 4H SiC was studied, and the size of the piezoresistive strip with the maximum piezoresistive coefficient value was obtained, ensuring the sensitivity of the sensor chip from the perspective of piezoresistive design.

Fig. 1 SiC Sensor chip design

Fig. 1 SiC Sensor chip design: (a) 4H-SiC epitaxial wafer; (b) Schematic diagram of sensor chip structure; (c) Cross sectional view of chip A-A ‘; (d) On chip Wheatstone bridge connection.

Fig. 2 Nonlinear temperature effect of 4H-SiC piezoresistance

Fig. 2 Nonlinear temperature effect of 4H-SiC piezoresistance: (a) Changes in varistor resistance values of four metal pads on the sensor chip at different temperatures; (b) Temperature coefficient of resistance (TCR); (c) Microscopic scattering mechanism of charge carriers at different temperatures

It can be seen that when the temperature is less than 400 ℃, the resistance temperature coefficient (TCR) changes more uniformly, and the TCR value is within a small range, indicating that the prepared piezoresistive and Ohmic contacts themselves are less affected by temperature.

2. Output Characteristics of 4H-SiC Based Pressure Sensor in Extreme Environment

Considering the anisotropic Young’s modulus of 4H-SiC, an NPN type piezoresistive 4H-SiC pressure sensor chip was designed using multi physics field simulation optimization, and the chip structure was comprehensively designed and optimized to ensure the sensitivity of the chip. Subsequently, by designing a high-temperature packaging scheme, real-time repeatability testing of the sensor’s performance was achieved in a wide temperature range (-50 ° C to 300 ° C) and extreme environments (strong corrosion, strong radiation).

The test results show that the 4H-SiC sensor has stable output in extreme environments, with a low sensitivity temperature coefficient (-0.067% FS/° C), excellent output sensitivity (3.38 mV/V/MPa), and comprehensive accuracy (0.56% FS), providing a reference for extreme environment testing of MEMS high-temperature pressure sensors.

Fig. 3 Real time repeatability test and pressure relief repeatability test of 4H-SiC pressure sensor full range output

Fig. 3 Real time repeatability test and pressure relief repeatability test of 4H-SiC pressure sensor full range output: (a) Full range output; (For the cyclic measurement curve of each temperature point, in order to observe subtle changes in the output, an enlarged image of the full range steady-state output is shown in the upper right corner when the pressure reaches 2MPa. Similarly, the enlarged image of the steady-state output after pressure relief is shown in the lower right corner.) (b) The sensor output is gradually pressurized in the forward direction and gradually depressurized in the reverse direction.

For more information, please contact us email at victorchan@powerwaywafer.com and powerwaymaterial@gmail.com.

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