CMOS Stress Sensor Circuits

Abstract

Two CMOS piezoresistive stress sensor circuits based on piezoresistive MOSFETs (PiFETs) are the focus of this dissertation. The first design is a multiplexed array of 512 piezoresistive sensors fabricated on a 2.2 by 2.2mm2 tiny chip. This array is composed of a PMOS array of 256 sensors and an NMOS array of 256 sensors, and an on-chip counter is used to scan the sensors in the array sequentially so that the sensors can be accessed with a limited number of I/O pins from the chip, allowing the data to be collected very efficiently. In the second design, a PMOS current mirror is used as a sensor cell and a delta-sigma modulator is used to detect the mismatch induced by stress. The output of the circuit is a modulated square wave that includes the sensor response information, and this can be either captured by digital equipment such as a counter, or by a radio receiver. The duty cycle of the output and DSBSC (Double side-band suppressed carrier) signal tone shift are proportional to the mismatch or applied stresses. MOSIS AMI_ABN 1.5um CMOS technology was used for the chip design and chip fabrication for this study. The calibration process was performed using a chip-on-beam technique, which utilized finite element analysis by ANSYS to determine the stress distribution on the die surface under load applied by a four-point-bending (4PB) fixture. The sensor array chip is used to measure the die stress for chip-on-beam under 4PB load, chip-on-beam encapsulated with ME525 underfill and DIP40 package encapsulated with ME525 underfill. The highest resolution die stress mapping thus far available was obtained using the sensor array constructed for this project. The delta-sigma modulation based stress sensor proposed here did not include the low pass filters normally used with delta-sigma ADC, as the spectra used here are located in the portion of the bandwidth which is normally filtered in an ADC. The frequency shift of the signal reflects the mismatch induced by stress. This delta-sigma modulation sensor offers an effective way to implement a sensor with a transmitter, which may be used for remote or embedded sensor applications in which it is difficult to contact the sensor.