Page 35 - An Introduction to Microelectromechanical Systems Engineering

14 Materials for MEMS

Table 2.1 Properties of Selected Materials
Property a Si SiO Si N Quartz SiC Diamond GaAs AlN 92% PolyimidePMMA
2 3 4
Al O
2 3
Relative 11.7 3.9 4–8 3.75 9.7 5.7 13.1 8.5 9 — —
permittivity (ε )
r
Dielectric 0.3 5–10 5–10 25–40 4 10 0.35 13 11.6 1.5–3 0.17
strength
6
(V/cm ×10 )
Electron 1,500 — — — 1,000 2,200 8,800 — — — —
mobility
2
(cm /V·s)
Hole mobility 400 — — — 40 1,600 400 — — — —
2
(cm /V·s)
Bandgap (eV) 1.12 8-9 — — 2.3–3.2 5.5 1.42 — — — —
Young’s 160 73 323 107 450 1,035 75 340 275 2.5 3
modulus (GPa)
Yield/fracture 7 8.4 14 9 21 >1.2 3 16 15.4 0.23 0.06
strength (GPa)
Poisson’s ratio 0.22 0.17 0.25 0.16 0.14 0.10 0.31 0.31 0.34 —
3
Density (g/cm ) 2.4 2.2 3.1 2.65 3.2 3.5 5.3 3.26 3.62 1.42 1.3
Coefficient of 2.6 0.55 2.8 0.55 4.2 1.0 5.9 4.0 6.57 20 70
thermal
expansion
−6
(10 /ºC)
Thermal 157 1.4 19 1.4 500 990–2,000 0.46 160 36 0.12 0.2
conductivity
at 300K
(W/m·K)
Specific heat 0.7 1.0 0.7 0.787 0.8 0.6 0.35 0.71 0.8 1.09 1.5
(J/g·K)
Melting 1,415 1,700 1,800 1,610 1,800 b 3,652 b 1,237 2,470 1,800 380 c 90 c
temperature (ºC)
a
Properties can vary with crystal direction, crystal structure, and grain size.
b
Sublimates before melting.
c
Glass transition temperature given for polymers.

(approximately $10 for a 100-mm-diameter wafer and $15 for a 150-mm wafer)
makes them attractive for the fabrication of micromechanical components and
systems.
Silicon as an element exists with three different microstructures: crystalline,
polycrystalline,or amorphous. Polycrystalline, or simply “polysilicon,” and amor-
phous silicon are usually deposited as thin films with typical thicknesses below 5
µm. Crystalline silicon substrates are commercially available as circular wafers with
100-mm (4-in) and 150-mm (6-in) diameters. Larger-diameter (200-mm and
300-mm) wafers, used by the integrated circuit industry, are currently economically
unjustified for MEMS. Standard 100-mm wafers are nominally 525 µm thick, and
150-mm wafers are typically 650 µm thick. Double-side-polished wafers commonly
used for micromachining on both sides of the wafer are approximately 100 µm thin-
ner than standard thickness substrates.
Visualization of crystallographic planes is key to understanding the dependence
of material properties on crystal orientation and the effects of plane-selective etch
solutions (see Figure 2.1). Silicon has a diamond-cubic crystal structure that can be