UltraminiaturizedPressure Sensor for Catheter Based Applications

by Melvås, Patrik

The trend of miniaturization driven by the medical minimally-invasive surgery and the ability to process large amounts of data at high speed have increased the demand for miniaturized sensors for biomedical applications with high sensitivity and biocompatible designs at a low price. The silicon based technology of microelectromechanical systems (MEMS) offers the tools for batch fabricating miniaturized sensors with high accuracy and with biocompatible materials at a reduced cost per sensor. This thesis presents miniaturized pressure sensors, designed for catheter based applications. The sensors use a force transducing beam to achieve a leverage effect and thus higher sensitivity, media isolation of the detecting strain-gauges, higher temperature compensation accuracy than traditional sensors. The beam design also allows the pressure induced diaphragm deflection to be detected using resonant as well as nonresonant techniques. The thesis also presents a novel diode-based detection technique that reduces the number of electrical leads needed for temperature compensated, piezoresistively detected, pressure sensors and thus reduces of the overall cost of the packaged sensor. Further design development of the force-transducing beam resulted in the new H-shaped design, which integrates the sensing piezoresistor in the beam without any current leakage between the legs of the piezoresistor. The leverage beam pressure sensors with a strain-gauge on the beam, results in a nonresonant pressure sensitivity between 0.8 µV/V/mmHg and 0.9 µV/V/mmHg and the sensors with the piezoresistor integrated in the beam is 5 µV/V/mmHg. A dual-beam configuration decreases the relative temperature dependence mismatch from 6 % to 3 % compared to a commercialized traditional piezoresistive pressure sensor. The miniaturized resonant pressure sensors use the force-transducing beam, located in the reference vacuum cavity, as a resonator. The resonance frequency is determined by sensing the amplitude of the beam vibration either electrically, using a piezoresistor, or optically. The resonant pressure sensitivity measures approximately 3 %/bar. It has a compensated and an uncompensated temperature dependence of –5 ppm/°C and –40 ppm/°C, respectively.