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2-18 MEMS: Design and Fabrication

C H . Typical epitaxial growth temperatures range from 1500 to 1700°C. Epitaxial films with p- or n-type
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conductivity can be grown using such dopants as Al and B for p-type films and N or P for n-type films.
In fact, doping with N is so effective at modifying the conductivity that growth of undoped SiC is virtu-
ally impossible because the concentrations of residual N in these deposition systems can be quite high. At
these temperatures, the crystal quality of the epilayers is sufficient for the fabrication of electronic device
structures.
Both APCVD and LPCVD can be used to deposit the only known polytype to grow epitaxially on a
non-SiC substrate, namely 3C-SiC on Si. Heteroepitaxy is possible because 3C-SiC and Si have similar lattice
structures. The growth process involves two key steps. The first step, called carbonization, involves con-
verting the near-surface region of the Si substrate to 3C-SiC by simply exposing it to a propane/hydrogen
mixture at a substrate temperature of about 1300°C. The carbonized layer forms a crystalline template on
which a 3C-SiC film is grown by adding silane to the hydrogen/propane mix. A 20% lattice mismatch
between Si and 3C-SiC results in the formation of crystalline defects in the 3C-SiC film. The density is
highest in the carbonization layer, but it decreases with increasing thickness, although not to a level com-
parable with epitaxial 6H- and 4H-SiC films. Regardless, the fact that 3C-SiC does grow on Si substrates
enables the use of Si bulk micromachining techniques to fabricating a host of SiC-based MEMS
structures such as pressure sensors and resonant structures.
Polycrystalline SiC, hereafter referred to as poly-SiC, has proven to be a very versatile material for SiC
MEMS. Unlike single-crystal versions of SiC, poly-SiC can be deposited on a variety of substrate types,
including common surface micromachining materials such as polysilicon, SiO and Si N . Moreover,
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poly-SiC can be deposited using a much wider set of processes than epitaxial films; LPCVD, APCVD,
PECVD, and reactive sputtering have all been used to deposit poly-SiC films. The deposition of poly-SiC
requires much lower substrate temperatures than epitaxial films, ranging from roughly 500 to 1200°C.
The microstructure of poly-SiC films is temperature and substrate dependent [Wu et al., 1999]. In gen-
eral, grain size increases with increasing temperature. For amorphous substrates such as SiO and Si N ,
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poly-SiC films tend to be randomly oriented with equiaxed grains, with larger grains deposited on SiO
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substrates. In contrast, for oriented substrates such as polysilicon, the texture of the poly-SiC film
matches that of the substrate as a result of grain-to-grain epitaxy [Zorman et al., 1996]. This variation in
microstructure suggests that device performance can be tailored by selecting the proper substrate and
deposition conditions.
Direct bulk micromachining of SiC is very difficult due to its outstanding chemical durability.
Conventional wet chemical techniques are not effective; however, several electrochemical etch processes
have been demonstrated. These techniques are selective to certain doping types, so dimensional control of
the etched structures depends on the ability to form doped layers, which can only be formed by in situ or
ion implantation processes, as solid source diffusion is not possible at reasonable processing temperatures.
This constraint limits the geometrical complexity of fabricated devices. To fabricate thick (hundreds of
microns), three-dimensional, high-aspect-ratio SiC structures, a molding technique has been developed
[Rajan et al., 1999]. The molds are fabricated from Si substrates using deep reactive ion etching, a dry etch
process that has revolutionized Si bulk micromachining. The micromachined Si molds are then filed with
SiC using a combination of thin epitaxial and thick polycrystalline film CVD processes. The thin-film
process, which produces 3C-SiC films with a featureless SiC/Si interface, is used to ensure that the molded
structure has smooth outer surfaces. The mold-filling process coats all surfaces of the mold with a very thick
SiC film. To remove the mold and free the SiC structure, the substrate is first mechanically polished to
expose sections of the mold and then the substrate is immersed in a Si etchant to completely dissolve the
mold. Because SiC is not attacked by Si etchants, the final SiC structure is released without the need of
any special procedures. This process has been successful in the fabrication of solid SiC fuel atomizers, and
a variant has been used to fabricate SiC structures in Si-based micro-gas-turbines [Lohner et al., 1999].
In both cases, the process capitalizes on the chemical inertness of SiC in conjunction with the reactivity
of Si to create structures that could otherwise not be fabricated with existing technologies.
Although SiC cannot be etched using conventional wet etch techniques, thin SiC films can be patterned
using conventional dry etching techniques. RIE processes using fluorinated compounds such as CHF and
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