Bulk Micromachined Pressure Sensor

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INTRODUCTION
pressure sensors have a wide-range of applications in
various fields, from automotive industry to medical
equipments, such as airbag system and respiratory devices. A
typical piezoresistive pressure sensor consists of two main
components: a diaphragm and resistors. They are two methods
of fabricating these parts: bulk (BM) and surface (SM)
micromachined. In the later one, the diaphragm is built on top
of the substrate surface. It offers several advantages in
comparison to BM pressure sensor: smaller size, better
dimensional control, and compatibility to CMOS technology.
BM pressure sensor, on the other hand, utilizes the substrate
as the diaphragm. There are also some advantages to this
method: better mechanical properties and well-developed
technology.

II. DESIGN AND ANALYSIS
The design utilizes a thin square diaphragm made by etching a
hole at the backside of a Si-substrate to almost all the way
through the front surface, leaving only 20-30 mm thin layer of
silicon. Polysilicon resistors are built on top of the diaphragm.
Any deflection due to pressure differences on the two sides of
the diaphragm will induced either compressive or tensile stress
Manuscript received May 1, 2003.


II DESIGN EXAMPLE
Lets look closely on one particular sensor design. The
backside opening was designed to be 2500 mm by 2500 mm. The
desired diaphragm thickness is 30 Å. According to equation
(1), the actual diaphragm is 1800 mm by 1800 mm.
Poly resistors are built on top of the diaphragm. R1 and R4
have dimension of L = 700 mm and W = 100 mm. For R2 and R3,
each consists of two resistors L= 350 mm and W = 100 mm in
series. Based on the actual measurement in the lab, Rs of the
doped polysilicon is 60.7 W/sq. Theoretically, R1 to R4 should
have the same resistor value of 424.9 W when there is no
pressure applied. Assuming that VS is 5 volts, both V+ and V_
will be exactly 2.5 volts. Vout should be zero


V. CONCLUSIONS
Bulk micromachined piezoresistive pressure sensors were
designed. Calculations for expected output voltage were made.
Masks were made. A fabrication process was designed.
Devices were fabricated. Devices were tested at the wafer level.
Wafers were diced and chips packaged for testing. Reasons for
some device failures were identified. Test results agreed with
predictions. In conclusion, the laboratory project was
successful in providing a platform for the students to learn
different aspects involved in designing MEMs devices