Page 278 - Microsensors, MEMS and Smart Devices - Gardner Varadhan and Awadelkarim

258 MICROSENSORS

Piezoresistors Glass cap

Polysilicon
diaphragm

OvXXXXXXXXXXXXXXXI
Anodic bonding

(a) Glass support
L Inlet hole

Reference Sensing
capacitors capacitors

Pressure

Figure 8.27 Basic types of silicon pressure sensors based on a vertical deflection: (a) piezo-
resistive (polysilicon) and (b) capacitive (single-crystal silicon)

The two most common methods to fabricate pressure microsensors are bulk and surface
micromachining of polysilicon. Silicon diaphragms can be made using either technique
as described earlier. Figure 8.27 illustrates the basic principles of a piezoresistive sensor
and a capacitive pressure sensor.
The deflection in the diaphragm can be measured using piezoresistive strain gauges
located in the appropriate region of maximum strain, as shown in Figure 8.27(a). The
strain gauges are usually made from doped silicon and are designed in pairs with a read-
out circuit such as a Wheatstone bridge. The change in strain can be related to the applied
pressure (P — P 0) and stored in a lookup table. The precise relationship depends on the
relevant piezoresistive coefficient n of the diaphragm material.

A
V out oc /?ocn(/>-/> 0 ) (8.32)
A single crystal of silicon is a desirable material to use for the diaphragm because neither
creep nor hysteresis occurs. The piezoresistive constant (044) is typically +1–138.1 pC/N
and that makes measuring pressure in the range of 0 to 1 MPa relatively straightforward.
Figure 8.27(b) shows the general arrangement of a single-crystal silicon pressure sensor
with capacitive pickup. In this case, a capacitive bridge can be formed with two reference