An integrated high resolution barometric pressure sensing system.

Abstract

The rapid recent growth of silicon-based microelectromechanical (MEMS) devices has been driven by the need for miniaturized low-cost high-resolution sensors. Pressure sensors, in particular, have numerous applications in industry, and tremendous growth has been projected for the coming decade. This thesis has developed a barometric pressure sensing system, which for the first time demonstrates a 25 mTorr (equivalent to one foot of air at sea level) pressure resolution over a range from 500 to 800Torr and a temperature span of -20 to 85°C. Factors which affect the performance of the transducer, its interface circuitry, and the calibration/compensation procedures for the overall system, are discussed in detail. Two versions of the system have been realized: one integrates the transducer and its interface circuit separately, while a second fabricates them as one monolithic device. The sensor consists of multiple capacitive pressure transducers, each covering a different pressure range. The silicon devices are anodically bonded to a glass substrate at wafer level and are later etched back to realize the diaphragms. The 3.7mum-thick bossed diaphragm is formed from heavily boron-doped single-crystal silicon and provides a typical sensitivity of 3800ppm/Torr over a 70Torr dynamic range. Single- and double-polysilicon lead transfers provide wafer-level vacuum sealing that is applicable to a variety of MEMS devices. The uncompensated devices have been stable to less than 300mTorr/year. The readout circuitry consists of a three-stage switched-capacitor circuit with a fully-differential front-end integrator. The differential configuration along with the use of correlated double sampling eliminates many offset and noise effects. The circuit is electronically trimmed to accommodate transducer capacitances from 16fF to 40pF and circuit gain from 0.02mV/fF to 1.25mV/fF. DC self-test is provided by a novel scheme which uses a VCO-controlled charge-pump to generate 8--30V on-chip for seamless electrostatic actuation of the diaphragm. Temperature compensation is done digitally in software. The monolithic sensor demonstrates for the first time, batch-processed vacuum encapsulated integration of a switched-capacitor circuit with five transducers and a reference capacitor on a single die using a new micromachined BiCMOS (HBiCMOS) process. The signal is amplified within the vacuum-sealed reference cavity of the transducer, eliminating external environmental sensitivity.