MEMS sensors meet low pressure challenge

by R. Colin Johnson
Contributing Editor, MEMS Investor Journal

Ultra low pressure sensing got a microsystem solution recently when fabless MEMS pressure sensor specialist Acuity Inc. (Fremont, Calif.) decreased its full-scale reading to 10 mbar -- suitable for a wide range of low pressure applications like medical ventilation and respiration, industrial pressure and flow applications, as well as heating, ventilating, and air conditioning (HVAC) systems.

Piezoresistive sensors measure the changing electrical resistance of a material due to applied mechanical stress.  Acuity claims the very low mass of its pressure-sensing diaphragm overcomes the g-force effects of vibration and shock without complex correction schemes.  Piezoresistive MEMS sensor die are smaller in size than traditional low pressure die, yet have better zero-stability, reduced sensitivity to g-forces as well as reduced sensitivity to humidity.  The small 1.9 millimeter square die from Acuity lowers the part's cost while reducing the chance of package stress effecting low pressure readings.

Acuity's low pressure sensor die was modeled on its previous 20-to-100 mbar full-scale MEMS pressure sensor.  Both use a piezoresistive sensor that features very low zero-drift, enabling it to be used for even lower pressures by simply adding amplification.  The company claims that the die's accuracy and stability enables amplification to extend its full-scale readings down to 2.5 mbar.

Acuity's wafers are processed at the MEMS foundry Semefab Ltd. (Glenrothes, Scotland) which has a 6500 square foot class 100 fab for four- and six-inch wafers using 800-nanometer design rules.  The company's MEMS fab makes use of Deep Reactive Ion Etching (DRIE) -- a silicon etch process, licensed from Bosch.  Semefab is capable of through-wafer etching for membrane structures at very high etch rates, double-sided silicon-on-insulator (SOI) processing as well as trench etching with varying wall smoothness and aspect ratios.

Semefab can process both MEMS wafers and CMOS wafers in a nearby but isolated facility.  Besides its business relationship with Acuity for pressure sensors that both companies brand, Semefab also has designs in other MEMS areas, including energy harvesting, medical sensors and defense applications.

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