Ultra-small pressure sensors fabricated using a scar-free microholes inter-etch and sealing (MIS) process

This paper presents a tiny 0.4 mm × 0.4 mm piezoresistive absolute pressure sensor chip with a fabrication cost as low as 0.01 US$/die. With the thin-film under bulk-silicon technique, a very thin but uniform poly-silicon pressure-sensing diaphragm is formed beneath a bulk-silicon beam-island structure to accommodate piezoresistors. The thin diaphragm exhibits a high sensitivity, and the beam-island-reinforced structure helps reduce deflection and improve linearity. The sensor is fabricated using a novel scar-free micro-hole inter-etch and sealing (MIS) process. In this newly developed process, seals are removed from the diaphragm area and placed on both the single-crystalline silicon island and single-crystalline silicon frame surrounding the poly-silicon diaphragm. Thus, the thin diaphragm is kept flat and smooth, thereby enhancing the sensing performance and fabrication yield. More importantly, the diaphragm-beam-island structure is small enough to fabricate sensor chips as small as 0.4 mm × 0.4 mm. With the high-yield process, a very low fabrication cost of 0.01 US$/die is realised owing to a high throughput of 90 000 die per 6 inch wafer. The ultra-small and low-cost pressure sensor exhibits sensitivity of 0.88 mV kPa−1/3.3 V, hysteresis of 0.15% full scale (FS), repeatability error of 0.04% FS, and non-linearity of 0.10% FS. Without any additional thermal compensation method, the sensor exhibits a low temperature coefficient of zero-point offset of −0.064%/°C FS and a temperature coefficient of sensitivity (TCS) of −0.22%/°C within the temperature range from −25 °C to +85 °C, at a full measurement range (i.e. full scale) of 100 kPa. With the new scar-free MIS process, the presented pressure sensor looks promising for smartphones, drones, and other consumer-electronic applications.