handbook of modern sensors

534 18 Sensor Materials and Technologies
Table 18.1. Stimuli of Silicon-Based Sensors
Stimuli
Effects
Radiant Photovoltaic effect, photoelectric effect, photoconductivity,
photo-magneto-electric effect
Mechanical Piezoresistivity, lateral photoelectric effect,
lateral photovoltaic effect
Thermal Seebeck effect, temperature dependence of conductivity
and junction, Nernst effect
Magnetic Hall effect, magnetoresistance, Suhi effect
Chemical Ion sensitivity
Source: Ref. [1].
Unfortunately, silicon does not posses the piezoelectric effect. Most effects of
silicon such as the Hall effect, the Seebeck effect, piezoresistance, and so forth are
quite large; however, a major problem with silicon is that its responses to many stimuli
show substantial temperature sensitivity. For instance: strain, light, and magnetic
field responses are temperature dependent. When silicon does not display the proper
effect, it is possible to deposit layers of materials with the desired sensitivity on top
of the silicon substrate. For instance, sputtering of ZnO thin films is used to form
piezoelectric transducers which are useful for the fabrication of SAW (surface acoustic
waves) devices and accelerometers. In the later case, the strain at the support end
of the an etched micromechanical cantilever is detected by a ZnO overlay.
Silicon itself exhibits very useful mechanical properties which currently are
widely used to fabricate such devices as pressure transducers, temperature sensors,
force and tactile detectors by employing the MEMS technologies. Thin film and
photolithographic fabrication procedures make it possible to realize a great variety
of extremely small, high-precision mechanical structures using the same processes
that have been developed for electronic circuits. High-volume batch-fabrication techniques
can be utilized in the manufacture of complex, miniaturized mechanical components
which may not be possible with other methods. Table A.14 in the Appendix
presents a comparative list of mechanical characteristics of silicon and other popular
crystalline materials.
Although single-crystal silicon (SCS) is a brittle material, yielding catastrophically
(not unlike most oxide-based glasses) rather than deforming plastically (like
most metals), it certainly is not as fragile as is often believed. Young’s modulus of silicon
(1.9 × 10 12 dyn/cm or 27 × 10 6 psi), for example, has a value of that approaching
stainless steel and is well above that of quartz and of most glasses. The misconception
that silicon is extremely fragile is based on the fact that it is often obtained in thin
slices (5–13-cm-diameter wafers) which are only 250–500 µm thick. Even stainless
steel at these dimensions is very easy to deform inelastically.
As mentioned earlier, many of the structural and mechanical disadvantages of
SCS can be alleviated by the deposition of thin films. Sputtered quartz, for example,
is utilized routinely by industry to passivate integrated circuit chips against airborne
impurities and mild atmospheric corrosion effects. Another example is a deposition of
silicon nitrate (TableA.14) which has a hardness second only to diamond.Anisotropic