Usha Gowrishetty

Single element 3-terminal pressure sensors: A new approach to pressure sensing and its comparison to the half bridge sensors

We report the development of a novel 3-terminal single element piezoresistor for ultra-miniature pressure sensor applications and compare its performance to that of a traditional half Wheatstone bridge design. The pressure sensors reported here are 0.69-French in size (1F= 333µm) 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 ultra-miniature 1F catheters. 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 novel 3-terminal single element design and half Wheatstone bridge sensors were optimized using finite element analysis (FEA). Performance characteristics of the half bridge and 3-terminal sensors, i.e. sensitivity, nonlinearity (NL%), temperature coefficient offset (TCO) and drift were measured and compared. It was determined that the 3-terminal pressure sensors (3-TPS) had greater sensitivity, better non-linearity and lower drift compared to half bridge design sensors. The 3-TPS devices were also less sensitive to alignment errors.

Development of Ultra-Miniaturized Piezoresistive Pressure Sensors for Biomedical Applications

Ultra miniaturized 0.69-French piezoresistive pressure transducers are designed and fabricated for biomedical applications. Silicon on insulator (SOI) and deep reactive ion etching (DRIE) technologies are used for the fabrication of the pressure sensors. A combination of SOI and DRIE technologies eliminates the dicing step and results in uniform diaphragm thickness. The dimensions of the final fabricated sensor die are 650 mum times 230 mum times 150 mum (length, width, thickness) with 2.5 mum thick diaphragms. Sensitivity of the sensors with half Wheatstone bridge configuration is determined to be 27-31 muV/V/mmHg.

Development of ultra miniature 1-french sensors for wireless radial arterial pressure monitoring

Miniaturized pressure sensors attached to catheters have numerous applications in the biomedical and life science fields including the monitoring of arterial, ventricular, intracranial and intraocular pressures. Recent advances in MEMS (microelectromechanical systems) technology and its associated fabrication processes, such as DRIE (deep reactive ion etching), have allowed for further miniaturization of these devices, expanding the application landscape to include small animal models and new human applications. One such human application is the wireless measurement of radial arterial blood pressure (BP). The goal of this research program is to produce a truly portable wireless radial arterial monitoring system using a custom-designed ultra-miniature pressure sensor appropriately sized to fit within the tip of a 1-French catheter (333 micron), so that convenient in-vivo measurements of BP are possible. The proposed system consists of two assemblies. The sensor, catheter, and wristband containing the battery and telemetry circuit form one assembly and the portable wireless PDA-sized transceiver module forms the other. This paper reports on the custom miniature MEMS pressure sensor developed for this application.

Mechanics of Buckled Structure MEMS for Actuation and Energy Harvesting Applications

Buckled structures offer many great benefits to microelectromechanical systems (MEMS), using naturally occurring residual stresses to provide structures with switchable stable states capable of large transverse deflections. In this work a simple, circular bi-layer diaphragm style of buckled MEMS devices is discussed. The buckling behavior of the system, including buckling height and switching criteria, is modeled and analyzed in ANSYS, then compared with theoretical equations and experimental measurements. Results of this work will help to yield optimal design parameters for both energy harvesting and actuation MEMS applications.Copyright

No-power vacuum actuated bi-stable MEMS SPDT switch

A “No-power” MEMS based vacuum/pressure actuated single pole double throw (SPDT) switch is presented. Compressive stresses in thermal oxide and TiW layer provide pre-stress in the polyimide mechanical films that initiates diaphragm buckling upon release. The diaphragms are bi-stable in nature with a buckling height greater than 28 µm. These bi-stable diaphragms were incorporated into a MEMS based no-power SPDT switch which comprises of 2 series single pole single throw (SPST) switches. Pressure/vacuum was used to actuate the diaphragms and OPEN/CLOSE the series SPST switches. The actuation or switching pressure/vacuum of the SPST switches was determined to be greater than 40 KPa which compares favorably with the analytical model predictions. The contact resistance of the MEMS SPST switches was determined to be less than 200 Ω. The proposed no-power switches are passive, programmable, monolithic, and small and can find several applications in the field of MEMS. Also, the polyimide diaphragms can be used as the fundamental building blocks in micro-pumps, micro-valves, optical devices and energy harvesting devices which require large cyclic displacement. Because of their bi-stable nature, the diaphragms can also be used for applications in mechanical memory storage.