Page 166 - MEMS Mechanical Sensors

7.2 Silicon-Based Devices 155

temperature change. Other gauge arrangements are also used. Semiconductor strip
strain gauges can be very small, ranging in length from 0.7 to 7 mm, and having
width typically a tenth of the element length; thus, they are useful in the measure-
ment of highly localized strains.
In a diffused semiconductor strain gauge (Figure 7.2), an n-Si base has a p-Si
diffused layer, and this layer works as a stress-sensitive conductor when its resis-
tance is measured between leads attached to deposited metallizations. A cantilever
with four C-shaped diffused gauges is stretched and compressed at its upper and
lower surfaces, respectively, when the cantilever undergoes bending deformation
under force F. All the gauges are identical since they are made on the same die and
during the same technological cycle.
MEMS technology makes use of silicon as a mechanical structural material
because of its excellent mechanical properties and the relative ease of fabricating in
high volumes small mechanical devices by the process of micromachining [7, 8].
Silicon is an excellent piezoresistive material, with good mechanical properties.
Amorphous silicon can be deposited directly on a mechanical part, for example,
glass or plastics. The basic structure of such a sensor is shown in Figure 7.3 [9]. A
thin amorphous silicon layer (n-, p-, or micro-compensated) acts as the sensitive
area, with size 300 × 300 µm, and four metallic contacts. Two of these contacts are
used to apply a fixed current to the sensing element, while the other two, orthogonal
to the previous ones, provide as output a voltage proportional to the mechani-
cal stress. When a mechanical stress is applied, an anisotropic modification of
resistivity occurs.

3
2
F 1

5 4
Figure 7.2 Cantilever integrated strain gauge element. F = force, 1 = cantilever, 2–5 = C-shaped
strain gauge.

V out
I in
V in

Flexible support

a-Si
Figure 7.3 Structure of the sensing element. (From: [9]. © 2003 IEEE. Reprinted with
permission.)