Page 40 - An Introduction to Microelectromechanical Systems Engineering

Silicon-Compatible Material System 19

Table 2.2 Temperature Dependence of Some Material Properties of Crystalline Silicon
300K 400K 500K 600K 700K
Coefficient of linear –0,002.616 –0,003.253 –0,003.614 –93.842 –94.016
−1
expansion (10 −6 K )
Specific heat (J/g·K) –0,000.713 –0,000.785 –0,000.832 –90.849 –90.866
Thermal conductivity –0,001.56 –0,001.05 –0,000.8 –90.64 –90.52
(W/cm·K)
Temperature coefficient –0,–90 –0,–90 –0,–90 –90 –90
−1
of Young’s modulus (10 −6 K )
Temperature coefficient –2,500 –2,500 –2,500 — —
−1
of piezoresistance (10 −6 K )
−3
18
(doping <10 cm )
Temperature coefficient –1,000 –2,5— –2,5— — —
−1
of permittivity (10 −6 K )
(Source: [5].)

applications. For example, experiments have shown that silicon remains intact in
the presence of Freon™ gases as well as automotive fluids such as brake fluids.
Silicon has also proven to be a suitable material for applications such as valves
involving the delivery of ultra-high-purity gases. In medicine and biology, studies
are ongoing to evaluate silicon for medical implants. Preliminary medical evidence
indicates that silicon is benign in the body and does not release toxic sub-
stances when in contact with biological fluids; however, it appears from recent
experiments that bare silicon surfaces may not be suitable for high-performance
polymerase chain reactions (PCR) intended for the amplification of genetic DNA
material.

Silicon Oxide and Nitride
It is often argued that silicon is such a successful material because it has a stable
oxide that is electrically insulating—unlike germanium, whose oxide is soluble in
water, or gallium arsenide, whose oxide cannot be grown appreciably. Various
forms of silicon oxides (SiO , SiO , silicate glass) are widely used in micromachin-
2 x
ing due to their excellent electrical and thermal insulating properties. They are also
used as sacrificial layers in surface micromachining processes because they can be
preferentially etched in hydrofluoric acid (HF) with high selectivity to silicon. Sili-
con dioxide (SiO ) is thermally grown by oxidizing silicon at temperatures above
2
800°C, whereas the other forms of oxides and glass are deposited by chemical
vapor deposition, sputtering, or even spin-on (the various deposition methods will
be described in the next chapter). Silicon oxides and glass layers are known to sof-
ten and flow when subjected to temperatures above 700°C. A drawback of silicon
oxides is their relatively large intrinsic stresses, which are difficult to control. This
has limited their use as materials for large suspended beams or membranes.
Silicon nitride (Si N ) is also a widely used insulating thin film and is effective as
x y
a barrier against mobile ion diffusion—in particular, sodium and potassium ions
found in biological environments. Its Young’s modulus is higher than that of silicon
and its intrinsic stress can be controlled by the specifics of the deposition process.
Silicon nitride is an effective masking material in many alkaline etch solutions.