Fabrication technologies - Micromixers Fundamentals Design and Fabrication - page 131

polysilicon surface micromachining. Diamond-based technologies also allow the fabrication of basic
electronic components, such as diodes and transistors. Electronic components make this technology
suitable for more complex diamond-based devices. A recent review on diamond-based semiconductor
technology was given by Gurbuz et al. [35] .
4.1.4.2 Silicon carbide
Silicon carbide SiC poses excellent electrical, mechanical, and chemical properties. Thus, devices
based on silicon carbide can be used in harsh environments at high temperature and pressure. Silicon
carbide sublimes at around 2000 C, which is much higher than the melting temperature of silicon
(1410 C). Silicon carbide is therefore suitable for making micromixers, which are used as micro-
reactor with extremely high operation temperatures.
Silicon carbide wafers are commercially available in both single-crystalline and polycrystalline
forms. However, similar to polysilicon-based and diamond-based MEMS, the growth of a thin silicon
carbide film is important for making SiC devices. Epitaxial SiC can be deposited in a CVD process
on a SiC wafer or on a silicon wafer with a SiC seeding layer. Micromixers may not need the high
quality of epitaxial SiC, and amorphous and polycrystalline SiC film can be the economical choice
for the fabrication. There are a wide range of deposition techniques for polycrystalline and amor-
phous SiC, such as sputtering, reactive sputtering, reactive evaporation, CVD, LPCVD, APCVD, and
PECVD [36] .
With the availability of SiC wafer and SiC film, both bulk micromachining and surface micro-
machining are possible. The extremely high temperature required for etching of SiC makes chemical
etching impractical for bulk micromachining of SiC. The only etching method available for room
temperature is photoelectrochemical etching (PEC) for n-type SiC and dark electrochemical etching
for p-type SiC [37] . Combining the deposition of thick-film SiC and silicon micromachining, a bulk
SiC microcomponent can be fabricated. First, a mold is etched in the silicon substrate using DRIE.
Next, SiC is deposited to fill the mold. After polishing away excess SiC, the mold is dissolved in
a silicon etchant such as KOH, releasing the SiC component [38] .
Silicon carbide surface micromachining can be realized with polysilicon as sacrificial layer and
RIE for etching the SiC functional layer. Plasma chemistries with fluorinated compounds, such as
CHF 3 ,SF 6 ,CF 4 , CBrF 3 and NF 3 , and oxygen, are often used. Due to the high oxygen content,
conventional photoresist cannot be used for masking purpose. A hard mask made of a metal, such as Al
or Ni, is needed to withstand the oxygen plasma.
For application in life sciences, biocompatibility is an issue for selecting the right material for
a micromixer. The biocompatibility of the materials used in silicon-based devices, such as single-
crystalline silicon, polysilicon, silicon dioxide, silicone nitride, and silicon carbide, was evaluated
according to ISO 10993 standards by Kotzar et al. [39] . Using mouse fibroblasts in the tests, none
of the materials were found to be cytotoxic. An in vivo test based on implantation in rabbit muscle
showed no sign of irritation. Only silicone nitride and SU-8 showed detectable nonvolatile resi-
dues. Further in vivo studies using stainless-steel cages [40] and Teflon cages [41] reveal that
silicon, silicon nitride, silicon dioxide, gold, and SU-8 are biocompatible. However, silicon and
SU-8 have shown increased biofouling. For more details on technologies and biocompatibility
issues, the reader is referred to a recent review by Grayson et al. [42] . The good biocompatibility of
devices made with common micromachining technologies allows the exploration of these tech-
nologies [43] .