Unique blood pressure sensor is highly accurate, less costly

June 08, 1998

San Sebastian, Spain--Un-afraid to take on the established players in a mature market, CEIT, a Spanish technology research center, believes it has found a better way to skin the disposable blood pressure transducer (DPT) cat. The firm's micromachined single-use pressure sensor is claimed to be accurate, biocompatible, and well suited to low-cost batch production.

Based in San Sebastian, engineers at the center developed the microsensor in conjunction with local company Oiarso that specializes in disposable medical supplies. The unit is based on a Wheatstone Bridge comprising four piezoresistive strain gages created within a silicon wafer, using conventional semiconductor manufacturing techniques. The underside of the wafer, beneath the four resistors, is then etched away to leave a diaphragm just 15 microns thick.

This diaphragm acts as the pressure-measuring element. When blood flows through the sensor, pressure causes the diaphragm to deform, creating tensile and compressive stress zones within the silicon. The location of the resistors causes two of them to come under tensile stress while the other two suffer compression. Their resistance increases and decreases respectively from the unstressed value of 1k (delta). The small output signal's magnitude is directly proportional to the applied pressure.

Diaphragm thickness is critical in determining the characteristics of the sensor. Make it too thick and the sensor's sensitivity drops to unusable levels; make it too thin and there is a risk that excess blood pressure will burst the membranes. "You have to find a compromise between sensitivity and mechanical resistance," says Javier Gracia who heads microelectronics research at CEIT.

CEIT settled on a thickness of 15 microns for its design, providing a sensitivity of 20 (mu)V/V/mm Hg. Designed to operate from -100 to +300 mm Hg, the sensor can handle an overpressure of five times the maximum rating.

The thin membrane is created with electrochemical etch-stop etching. A thin epitaxial layer whose thickness corresponds to that of the final membrane is grown on the surface of a silicon wafer. Then, four piezoresistors are created inside the layer and electrical contacts created on the top of the wafer.

A 1.5- X 1.5-mm window is opened in the oxide layer beneath the wafer, and through this window the silicon substrate is etched away. When the etchant reaches the boundary between the epitaxial layer and the substrate, the etching stops automatically because the exposed epitaxial layer is rapidly oxidized by the applied voltage.

Multiple sensors can be created simultaneously on a single wafer, which is cut into individual 5- X 5-mm dies. The sensor is then glued to a ceramic substrate using a biomedical-grade adhesive, and connection wires are attached to the metal pads on the die. The whole device is then mounted inside a polycarbonate body and sealed with biomedical-grade silicone gel.

CEIT's design is said to be extremely versatile. Already, engineers have applied the technology to a pressure sensor for washing machines, providing a continuous reading of water pressure for greater washing control.

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