A Batch-fabricated High-performance Piezoresistive Pressure Sensor.

Abstract

Efficient, low-cost pressure sensors are a critically needed element for many next-generation electronic systems. Silicon piezoresistive pressure sensors are a promising candidate for many of these applications but in the past have suffered from low yield and excessive temperature sensitivity. This thesis addresses both of these problems. The possible configurations for silicon piezoresistive sensors are first reviewed. A full-bridge configuration formed by <110>-oriented p-type diffused resistors in a (100)-oriented silicon wafer is shown to offer the highest pressure sensitivity and greatest compatibility with integrated-circuit formation are evaluated. The various techniques for thin-diaphragm formation are evaluated, and a new technique, the electrochemical p-n junction etch-stop, is shown to provide significantly improved diaphram thickness control, uniformity, reproducibility, and yield. This thesis represents the first application of this technique to a solid-state sensor. Six sources of temperature sensitivity are identified for peizoresistive devices, and the significance of each mechanism is evaluated over the temperature range from -40(DEGREES)C to +180(DEGREES)C. New data on the thermal expansion coefficient mismatch between silicon and silicon dioxide is presented. For lightly doped n-type samples the mismatch is 2.15 x 10('-6)/(DEGREES)C with a plasticity limit of 800(DEGREES)C. For heavily boron-doped samples, the effective mismatch is 3.9 x 10('-6)/(DEGREES)C with a plasticity limit equal to the oxidation temperature. It is shown that for a piezoresistive pressure sensor having a diaphragm measuring 1 mm x 1 mm x 10 (mu)m, fabricated using a high-performance ion-implanted resistor process and electrostatically sealed to a 7740-glass substrate, it should be possible to achieve a pressure sensitivity of 78 ppm/mmHg with an untrimmed offset equivalent to (+OR-)6.4 mmHg. The temperature sensitivity of offset is about -0.06 mmHg/(DEGREES)C. While the temperature coefficient of sensitivity is about -1330 ppm/(DEGREES)C, it should be highly reproducible. Thus, it appears possible to reduce the temperature sensitivity in piezoresistive sensors as much as an order of magnitude below present levels, eliminating the expensive temperature trims in many applications.