Elliptic Diaphragm Capacitive Pressure Sensor and Signal Conditioning Circuit Fabricated in SiGe CMOS Integrated MEMS

This paper presents a novel CMOS integrated capacitive pressure sensor, fabricated in silicon-germanium microelectromechanical systems (SiGeMEMS) process, with the sensor held and linked to the CMOS beneath. The CMOS process houses the on-chip signal conditioning circuit. The superior stress-strain behavior of polycrystalline silicon- germanium (poly-SiGe) is effectively utilized to develop and characterize the structure of the pressure sensor diaphragm element. The edge-clamped elliptic structured diaphragm employs semimajor axis L-shaped clamp springs to yield high sensitivity, wide dynamic range, and good linearity. To maximize the center deflection of the diaphragm, a stem structure is designed to transfer the entire stress on to the diaphragm center. The signal conditioning circuit is fabricated in a 0.18 μm TSMC CMOS process (forming the host substrate for the SiGe-MEMS sensor) and achieves a high overall gain of 100 dB for the sensor readout. Experimental results indicate a high sensitivity of ~0.12 mV/hPa (at 1.4 V supply), along with a nonlinearity of ~1% for the full-scale range of applied pressure load. The diaphragm with a wide dynamic range of 100-1000 hPa is stacked on top of the CMOS circuitry. The piggyback structure reduces the combined sensor and conditioning circuit implementation area of the intelligent sensor chip to ~750 μm × 750 μm. The major and minor axis dimensions of the sensor were 485 μm and 280 μm, respectively. The device achieved wider low-pressure sensing range at lower supply voltage compared with commercial pressure sensors.