Semefab MEMS Sensor Meets Low Pressure Challenge
12 Aug 2011
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.
Copyright 2010 MEMS Investor Journal
June 10, 2010 at 06:02 PM | Permalink
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