Pressure sensors that contain thin diaphragms made from the 6H polytype of silicon carbide (6H-SiC) have been developed. These are prototypes of pressure sensors for use at high temperatures in engines, power plants, material-processing systems, and numerous other applications.
The wide band gap (3.0 V), high breakdown electric field (2.5 MV/cm), and high electron saturation speed (2 × 107 cm/s) of 6H-SiC make it a superior candidate material for high-temperature electronic devices. In addition, SiC exhibits excellent thermal and mechanical
properties at high temperatures and large coefficients of piezoresistance — a combination of properties that makes this material suitable for high-temperature electromechanical sensors.
The prototype SiC pressure sensors were batch-fabricated by micromachining and demonstrated to work at temperatures from room temperature up to 500 °C. The 6H-SiC starting material (i.e., wafers) have micropipes in them. The processing conditions applied in this work plugged the micropipes, thereby making the 6H-SiC material useable.
The SiC pressure sensors offer the following five major advantages in addition to those mentioned above:
- Junction leakage, which renders silicon semiconductor devices inoperative at high temperatures, is insignificant because of the wide band gap of 6H-SiC.
- The low hole mobility [50 cm2/(V•s)] of a lightly-p-doped diaphragm makes the diaphragm highly resistive to planar electric current.
- Because the results of bulk micromachining are reproducible, batch production offers advantages of low cost, short processing time, and high yield.
- Because of the homogeneity of bulk micromachined 6H-SiC, these pressure sensors are not subject to the adverse effects of thermal-expansion mismatches, which can be problematic in devices made from heterogeneous materials.
- Plastic deformation of SiC is not known to occur at the upper end of the temperature range of interest. Consequently, the diaphragms in these sensors remain effective force collectors, even at this high temperature.
This work was done by Anthony A. Ned, Anthony D. Kurtz, and Robert S. Okojie of Kulite Semiconductor Products, Inc., for Glenn Research Center. For further information, access the Technical Support Package (TSP) free on-line at www.nasatech.com/tsp under the Physical Sciences category.
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