Ultra-miniature pressure sensors leading to a novel 3-terminal single element design

Development of ultra-miniaturized pressure sensors using two types of piezoresistive sensing techniques is reported. The two piezoresistive sensing techniques used in this research are (a) two piezoresistors in a half Wheatstone bridge configuration and (b) a novel 3-terminal single element design. The pressure sensors reported here are 0.69-French in size (1F= 333mum) and are designed and batch-fabricated using SOI (silicon on insulator) and DRIE (deep reactive ion etching) technologies. One of the major applications of this device is for blood pressure monitoring using a 1F catheter. The combination of SOI and DRIE technologies results in uniform diaphragm thickness and complete elimination of the post-processing dicing step by micromachining "die separation streets" during the DRIE process.;The second design has a single 3-terminal element similar to the X-ducer (Motorola, SPS Inc.), but with only three leads. The advantages of using a 3-terminal pressure sensor (3-TPS) over an X-ducer is smaller die size, application in less than or equal to 1F catheters where the catheter interiors can accommodate only three lead wires and also for use in catheter pressure monitoring where the maximum sensitivity required is 15 muV/V/mmHg [1, 2]. The optimal design of the 3-TPS element for a 180 mum diameter and 2.5 mum thick diaphragm is presented here. The optimum dimensions of the element were determined to be 40 mum length, 20 mum width, and an output terminal gap of 3 mum. Also, it was demonstrated that the 3-TPS element is sensitive to gradient stress across the width of the element regardless of its orientation. The 3-TPS has maximum sensitivity when the element is oriented at 45° to the <110> direction on a (100) wafer. These sensors had a sensitivity of 26--31 muV/V/mmHg or ppm/mmHg, with a NL% of +/-0.04--0.08%, TCO of 0.1--0.21 muA/°C and a drift of 0.05--0.4 mmHg/day. The performance characteristics of the half bridge design were compared to the 3-terminal design and it was determined that the 3-TPS had greater sensitivity, better non-linearity and lower drift.;The first design has a half Wheatstone bridge configuration with two piezoresistive elements oriented on the circular diaphragm. The piezoresistive gauges were formed using ion implantation and their bond pads are directly solderable using an aluminum/titanium/platinum metallization strategy. The sensors configured in a half Wheatstone bridge produced a sensitivity of 16.5--20 muV/V/mmHg or ppm/mmHg with a non-linearity (NL%) of +/-0.0.15--0.4%, temperature coefficient offset (TCO) of 150--190 muV/°C and a drift of 0.7--1.3 mmHg/day.