When automotive ignition and carburetor systems initially required inputs from the engine to control spark and fuel and lower vehicle emissions, a capacitive pressure sensor was designed to specifically measure manifold absolute pressure (MAP) and barometric absolute pressure (BAP). Called a silicon capacitive absolute pressure (SCAP) sensor, it consisted of a micromachined diaphragm etched into a silicon top wafer anodically bonded to a glass substrate. The capacitance occurred between a movable metalized electrode in the top that deflected under pressure and the fixed metal layer deposited on the glass.
Image Source: Understanding Smart Sensors, Artech House, 1995.
After attaching the top silicon wafer to the glass substrate, the drilled holes were solder-sealed under vacuum creating the internal reference for absolute measurements. With solder bumps for direct mounting to a circuit board or ceramic substrate, the flip chip type package was possibly the first high volume use of General Electric’s controlled collapse chip connection (C4) attachment technology developed for military applications. The value of the capacitor changed linearly from approximately 32 to 39 pF with applied pressure from 17 to 105 kPa. The 6.5 mm x 6.5 mm x 0.9-mm capacitive element had a low temperature coefficient of capacitance (-30 to 80 ppm/°C), good linearity (≈ 1.4%), fast response time (≈ 1 ms), and no exposed bond wires.
While capacitive pressure sensors require more complex signal conditioning circuits and calibration algorithms than piezoresistive pressure sensors, they offer many advantages. In addition to higher accuracy and lower total error band, capacitive pressure sensors have low power consumption since no DC current flows through the capacitive sensing element. As a result, capacitive pressure sensors using surface micromachining with CMOS circuity are designed for many portable consumer applications today.
For example, Infineon Technologies’ DPS310 is a miniaturized Digital Barometric Pressure Sensor in an 8-pin land grid array (LGA), 2.0 mm x 2.5 mm x 1.0-mm package. In addition to its low current draw of 3 μA (1 measurement/s) and stand-by current of <1 μA, it has:
- Ultra-high precision ±006 hPa (±-5cm)
- Relative accuracy ±06 hPa (±0.5m)
- Operating range of 300 – 1200 hPa at -40 – 85C°