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7-2 MEMS: Design and Fabrication

packaging schemes (e.g., plumbing for air or water cooling) to help maintain continuous and reliable
operation at high temperature. Added to the complexity and cost is the increase in size and weight.
Therefore, to meet the increasing need for sensors to operate reliably at higher temperature and at rela-
tively lower cost, new and innovative devices made from materials more robust than silicon are needed.
Technological advancement in the growth of wide-bandgap semiconductor crystals such as silicon car-
bide (SiC) has made it possible to extend the operation of solid-state devices and MEMS beyond 500°C.
SiC has long been viewed as a potentially useful semiconductor material for high-temperature applica-
tions [Neudeck et al., 2002]. Its excellent electrical characteristics — wide-bandgap, high-breakdown
electric field, and low intrinsic carrier concentration — make it a superior candidate for high-temperature
electronic applications [Pearson et al., 1957].
Table 7.1 shows the comparison of relevant electrical and mechanical properties between 6H-SiC and
silicon. The fact that SiC exhibits excellent thermal and mechanical properties at high temperature, com-
bined with its fairly large piezoresistive coefficients, makes it well-suited for use in the fabrication of high
temperature electromechanical sensors. There already are efforts to develop pressure sensors based on SiC
[Okojie et al., 1996, Berg et al., 1998, Mehregany et al., 1998].
SiC appears in various crystal structures called polytypes. The polytypes most frequently available are
the hexagonal 4H-SiC and 6H-SiC and the cubic-SiC (also referred to as 3C-SiC or β-SiC). The promi-
nent physical differences between the 4H- and the 6H-SiC are the stacking sequences of the Si-C atomic
bi-layers, the number of atoms per unit cell, and the lattice constants [Park, 1998]. The hexagonal crys-
tals are grown in large boules, mostly by the sublimation (Acheson) process, and then sliced into wafers;
currently, the largest commercially available single crystal size is 76mm in diameter. Homoepitaxial
growth by chemical vapor deposition (CVD) can be performed on the hexagonal single crystal substrate
to obtain epilayers of various thicknesses and doping levels (typically nitrogen doping for n-type and alu-
minum or boron for p-type), as desired for various device applications.
Electronically, 4H- and 6H-SiC have bandgaps of 3.2eV and 2.9eV, respectively. The bulk-grown
3C-SiC polytype of device quality is not readily commercially available because of the nonexistent effec-
tive growth technology. Aggressive research is ongoing in this area; the payoff is the harnessing of its rela-
tively high electron mobility to produce high frequency device switching. Heteroepitaxial CVD of 3C-SiC
on silicon substrate is generally applied to grow epilayers a few microns thick. However, the 20% lattice
mismatch that exists at the 3C-SiC/Si heterojunction induces dislocation defects in the 3C-SiC het-
eroepilayer, which greatly degrades the performance of electronic devices fabricated in it. Such defects in
3C-SiC and its relatively low bandgap (2.3 eV) have largely limited its attractiveness for broader, high-
temperature mechanical sensor applications.
The discussions in this chapter will focus on the technological progress made in recent years in terms of
implementing MEMS-based SiC piezoresistive pressure sensors, as representative of accelerometers and
flow sensors fabricated in SiC that apply the same sensing principles. These SiC sensing devices are criti-
cally needed for closer proximity sensing in the applicable harsh environment. This chapter’s five sections
will provide readers with in-depth understanding of the present and future challenges that confront this
emerging technology, and of the efforts being made to surmount these challenges.

TABLE 7.1 Comparison of Properties of α-(6H)-SiC with Silicon
Properties Silicon 6H-SiC

Bandgap (eV) 1.12 2.9
Melting Point (°C) 1420 1800
6
1
Breakdown Voltage ( 10 Vcm ) 0.3 2.5
Young’s Modulus of Elasticity (GPa) 165 448
1
Thermal Conductivity [W(cm-C) ] 1.5 5.0
1
7
Electron Saturation Velocity ( 10 cms ) 1 2
Maximum Operating Temperature (°C) 300 1240

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