2. SiC MEMS and NEMS

SiC is an excellent structural material for MEMS and NEMS applications due to its outstanding mechanical, chemical, and electrical properties combined with its compatibility with Si micromachining techniques. Nowhere is the limitation to silicon-based MEMS more apparent than in applications characterized by high temperature environments. The electronic properties of silicon place an upper limit on the operating temperature of electronic devices at roughly 250 ℃ for devices fabricated on conventional bulk silicon substrates and around 300 °C for devices built on silicon-on-insulator substrates. As for the mechanical properties, the upper limit for silicon-based micromechanical structures is around 500 °C, as seen in the plastic deformation of Si membranes subjected to deflecting loads at that temperature. The silicon surface is chemically active and will appreciably oxidize at temperatures above 800 °C. These material limitations require that silicon-based MEMS structures be enclosed in protective packaging to make them suitable for use in these conditions. In many cases, the packaging is so extensive that the benefits of using a silicon based MEMS device (i.e., low cost and small device size) are completely negated by the package. The desire to capitalize on MEMS technology for applications where the use of silicon is impractical has motivated the development of alternative semiconductors whose material properties are better suited for such applications [2].

Mostly, efforts to develop SiC for MEMS have focused on developing device technologies for harsh environment applications where Si, the dominant material in MEMS, is not well suited. Such environments include high temperature (>600 °C), high mechanical wear, high radiation, high oxidation, and harsh chemicals. Properties that make SiC particularly well suited for harsh environments include a wide electronic bandgap (ranging from 2.9 eV for 3C-SiC to 3.2 eV for 4H-SiC), high hardness (2,480 kg/mm2 d\_z\$%#\$z.!/%/0\*!z0+z\$!)%(z!0\$¥ ing in acids and bases, slow oxidation rates, and very strong covalent Si–C bonds. SiC is of particular interest for use in MEMS-based microactuators, where its inert surface resists the deleterious effects of stiction and its high Young's modulus (~400 GPa) enables fabrication of mechanical resonators that can operate over a very wide frequency range, including the GHz range. Its chemical inertness, favorable mechanical properties, and biocompatibility make SiC particularly attractive for bioMEMS applications. Several comprehensive reviews of SiC MEMS technology, including material properties, processing techniques, and device examples have recently been published [2, 3].

(z/+z%0z)%#\$0z\*+0z(35/z!z/1""%%!\*0z0+z0('z+10z0\$!z%++),0%%(%05z+"zz/,!%"%z)0!.%¥ al. Cell-semiconductor hybrid systems represent an emerging topic of research in the %+0!\$\*+(+#%(z .!z3%0\$z%\*0.%#1%\*#z ,+//%(!z ,,(%0%+\*/^z z +),.!\$!\*/%2!z 1\* !./0\* ¥ %\*#z +"z 0\$!z%\*0!.0%+\*/z #+2!.\*%\*#z /1\$z /5/0!)/z%/z 0\$!z /%/z +"z,.!/!\*0z \* z "101.!z !2!(+,¥ ment of biologically interfaced device performance. To date, very little is known about the main processes that govern the communication between cells and the surfaces they adhere to. When cells adhere to an external surface an eterophilic binding is generated between the cell adhesion proteins and the surface molecules. After they adhere, the interface between them and the substrate becomes a dynamic environment where surface chemistry, topology, \* z!(!0.+\*%z,.+,!.0%!/z\$2!z!!\*z/\$+3\*z0+z,(5z%),+.0\*0z.+(!/^z\$!z%++),0%%(%05z/%\*¥ gle-crystal SiC was determined by culturing mammalian cells directly on SiC substrates and 5z!2(10%\*#z0\$!z.!/1(0%\*#z!((z \$!/%+\*z-1(%05z\* z,.+(%"!.0%+\*^z\$!z.5/0((%\*!z%z%/z%\*¥ !! zz2!.5z,.+)%/%\*#z)0!.%(z"+.z%+w,,(%0%+\*/\_z3%0\$z!00!.z%+w,!."+.)\*!z0\$\*z.5/¥ talline Si. 3C-SiC, which can be directly grown on Si substrates, appears to be an especially promising bio-material: the Si substrate used for the epi-growth would in fact allow for costeffective and straightforward electronic integration, while the SiC surface would constitute a more biocompatible and versatile interface between the electronic and biological world. The main factors that have been shown to define SiC biocompatibility are its hydrophilicity and surface chemistry. SiC surface morphology is shown to influence cell adhesion only when macropatterned, while SiC polytypism and doping concentration seem to have no influence on cell proliferation. The identification of the organic chemical groups that bind to the SiC surface, together with the calculation of SiC zeta potential in media, could be used to better understand the electronic interaction between cell and SiC surfaces. Using an appropriate cleaning procedure for the SiC samples before their use as substrates for cell cultures is also %),+.0\*0^z \$!z (!\*%\*#z \$!)%/0.5z)5z ""!0z !((z ,.+(%"!.0%+\*z \* z !),\$/%6!z 0\$!z%),+.¥ tance of the selection of an appropriate cleaning procedure for biosubstrates. SiC has been /\$+3\*z 0+z!z/%#\*%"%\*0(5z!00!.z 0\$\*z%z/zz/1/0.0!z"+.z!((z1(01.!\_z3%0\$zz\*+0%!(5z.!¥ duced toxic effect and enhanced cell proliferation. One of the possible drawbacks that may !z//+%0! z3%0\$z0\$!z1/!z+"z%z%\*z2%2+z%/z.!(0! z0+z0\$!z1\*(!.z\* z\$%#\$(5z !0! z50+¥ toxic level of SiC particles. Nonetheless, the potential cytotoxicity of SiC particles does not represent a dramatic issue as much as it does for Si, since the great tribological properties of

Silicon Carbide: A Biocompatible Semiconductor Used in Advanced Biosensors and BioMEMS/NEMS

http://dx.doi.org/10.5772/51811

353

Several papers have discussed testing silicon SiC in vitro. In one study the researchers tested SiC deposited from radiofrequency sputtering using alveolar bone osteoblasts and gingival fibroblasts for 27 days. The investigators reported that ''Silicon carbide looks cytocompatible +0\$z +\*z /(z \* z /,!%"%z 50++),0%%(%05z(!2!(/^z+3!2!.\_z "%.+(/0z \* z +/0!+(/0z 0¥ 0\$)!\*0z%/z\*+0z\$%#\$(5z/0%/"0+.5\_z\* z 1.%\*#z0\$!z/!+\* z,\$/!z+"z+/0!+(/0z#.+30\$\_z+/0!+¥ blast proliferation is very significantly reduced by 30%''. According to another paper, in a 48 \$z/01 5z1/%\*#z\$1)\*z)+\*+50!/\_z%z\$ zz/0%)1(0+.5z!""!0z+),.(!z0+z,+(5)!0\$.5¥ late. Cytotoxicity and mutagenicity has been performed on SiC-coated tantalum stents. )+.,\$+1/z %z % z \*+0z /\$+3z \*5z 50+0+4%z .!0%+\*z 1/%\*#z)%!z "%.+(/0/z LELz !((z 1(¥ tures when incubated for 24 h or mutagenic potential when investigated using Salmonella

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