Mechanical sensorsTwo different types of mechanical sensors will be discussed here. The first uses physical mechanisms to directly sense the parameter of interest (e.g. distance, strain). The second type uses microstructures to enable the mechanical sensors to detect parameters of interest that cannot be measured directly with the first type of sensor (e.g. acceleration).
PiezoresistorsThe change in resistance of a material with applied strain is termed the piezoresistive effect. Piezoresistors are relatively easy to fabricate in silicon; being just a small volume of silicon doped with impurities to make it n-type or p-type.
Piezoelectric sensorsWhen a force is applied to a piezoelectric material, a charge is induced on the surface which is proportional to the applied force. The applied force can thus be deduced by measuring the electrical potential that appears across the crystal. Common piezoelectric crystals used for microengineered devices include zinc oxide and PZT (PbZrTiO3 - lead zirconate titanate), which can be deposited on microstructures, and patterned.
Capacitive sensorsFor two parallel conducting plates, separated by an insulating material, the capacitance between the plates is given by equation 3; where A is the area of the plates, d the distance between them, and e is a constant depending on the material between the plates (this assumes the circumference of the plates is much larger than the distance between them, so what happens at the edges of the plates can be neglected).
C = eA / d (For air, e is approximately 8.9E-12 F/m). From this, it can be seen that the measured capacitance is inversely proportional to the distance between the two plates. It is possible to use this technique to measure small displacements (microns - tens of microns) with high accuracy (sub-nanometer); however the instrumentation required to measure capacitance changes can be a little complex.
Optical sensorsSilicon is a reflective material, as are other materials used in semiconductor device fabrication (e.g. aluminium). Thus optical means may be used to sense displacement or deformation of microengineered beams, membranes, etc. A laser is directed at the surface to be monitored in such a way that interference fringes are set-up. By analysing these fringes, displacement or deformation may be detected and quantified. One area where this is often employed in in atomic force microscopy, to monitor the deflection of the beam upon which the sensing 'tip' is mounted.
Resonant sensorsThese are based on micromachined beams or bridges which are driven to oscillate at their resonant frequency. Changes in the resonant frequency of the device would typically be monitored using implanted piezoresistors, or optical techniques.Figure 10a shows a bridge, driven to resonance, on a thin membrane. The resonant frequency of the bridge is related to the force applied to it (between anchor points), its length, thickness, width, its mass, and the modulus of elasticity of the material from which it has been fabricated. If the membrane that the bridge is mounted on is deformed (figure 10b), for instance there is greater pressure on one side than the other, then the force applied to the bridge changes, and hence the resonant frequency changes.
 Figure 10. Alternatively, a resonant device may be used as a biosensor, by coating it with a material that binds to the substance of interest. As more of the substance binds to the device, its mass will be increased, again altering the resonant frequency.
AccelerometersMicroengineered acceleration sensors, accelerometers, consist of a mass suspended from thin beams (figure 11). As the device is accelerated, a force (force = mass x acceleration) is developed which bends the suspending beams. Piezoresistors situated in the device where the beams meet the support (where strain is greatest) can be used to detect acceleration. Another alternative is to capacitively sense displacement of the mass.
 Figure 11.
Pressure sensorsMicroengineered pressure sensors are usually based around thin membranes. On one side is an evacuated cavity (for absolute pressure measurement), and the other side is exposed to the pressure to be measured. The deformation of the membrane is usually monitored using piezoresistors, or capacitive techniques. |