5. SiC thin film based MEMS devices: sensors, RF MEMS and BioMEMS

#### 5.1. Piezoresistive and capacitive sensors

It is well known that silicon piezoresistive sensors can not be used for high-temperature applications because of the p-n insulation of the piezoresistors. Several studies have demonstrated that the use SiC thin film piezoresistors is a good alternative for these applications due to their high gauge factor together with thermal stability. Ziermann et al. developed a piezoresistive ß-SiC-on-silicon on insulator (SOI) pressure sensor with an on chip polycrystalline SiC thermistor for high operating temperatures. The test results from room temperature to 573 K demonstrated the capability of this sensor to monitor the cylinder pressure of combustion engines. We have studied the piezoresistive properties of amorphous SiC (a-SiC) films produced at low temperatures by PECVD and magnetron sputtering (Fraga, 2011d). Figure 7 (a) illustrates a piezoresistive pressure sensor with a-SiC piezoresistors developed by us.

In 2004, Young et al. proposed single-crystal 3C-SiC capacitive pressure sensors (see schematic illustration shown in Figure 7(b)) for sensing capabilities up to 400℃. The fabrication of polycrystalline 3C-SiC capacitive pressure sensors was reported by Du et al. More recently, Chen and Mehregany reported the first all-SiC capacitive pressure sensor, incorporating a SiC diaphragm on a SiC substrate. Measurements of pressures up to 700 psi and temperatures up to 574°C were demonstrated. This shows that thin film-based technology has a lot to be developed to achieve the performance of sensors based on bulk materials.

Figure 7. Examples of SiC film sensors shown in the literature: (a) piezoresistive and (b) capacitive.

#### 5.2. RF MEMS

SiC films have been shown as a good alternative to the metal films in radiofrequency microelectromechanical systems (RF MEMS) applications, especially microbridge-based RF MEMS switches (Parro, 2008; Mishra, 2009) and MEMS resonators (Chang and Zorman, 2008).

Mishra et al. proposed a MEMS switch with low actuation voltage as illustrated in Figure 8 (a). This model uses a beam that is made of two materials: the SiC film to give mechanical stability and the Au to provide the conducting path to the ground. A process using four masks was employed to fabricate it: a high-resistivity p-type (100) Si wafer with 1.0 um thermal SiO2 was used as substrate, 800 nm of Au was deposited and patterned using lift-off to define the coplanar waveguide (CPW), to form the switch, a 1.5 µm of polyimide sacrificial layer was spun, soft-baked, and patterned to define the anchors. The anchors and the beam are made by depositing a 0.3 um and 0.9 µm layers of SiC and Au, subsequent etching of the polyimide sacrificial layer. The switch exhibited an isolation of -40 db at 10 GHz and pull down voltage of 3 V.

The single-crystal and polycrystalline 3C-SiC lateral resonators were developed by Chang and Zorman. An illustration schematic of the cross-section of this resonator is shown in Figure 8 (b). The single crystalline (100) SiC film was produced by APCVD whereas the polycrystalline (111) SiC film by LPCVD. Both films were deposited on SiO2/Si substrates. The experimental results showed that the 3C-SiC lateral resonators exhibit a resonant frequency similar to polysilicon devices and temperature coefficient of 22 ppm/℃ comparable to quartz oscillators (from 14 to 100 ppm/℃), which confirm the potential of SiC films for RF MEMS applications.

Figure 8. Examples of SiC thin film RF MEMS shown in the literature:

(a) switch and (b) lateral resonators.

#### 5.3. BioMEMS

Biomedical or Biological Micro-Electro-Mechanical Systems (BioMEMS) are defined as systems or devices, which are constructed using micro/nanofabrication technology, for the analysis, delivery, processing, or for the development and construction of chemical and biological entities (Bashir, 2004). The first efforts in this field were directed to study the biocompatibility of common MEMS materials such as Si, SiO2, SigNy, polysilicon, SiC and SU-8. Kotzar et al. performed comparative studies among these materials and interesting conclusions were reached: (a) all materials were classified as non-irritants based on 1- and 12-week rabbit muscle implantations; (b) none of the materials were found to be cytotoxic in vitro using mouse fibroblasts; (c) only silicon nitride and SU-8 leached detectable non-volatile residues in aqueous physiochemical tests and (e) only SU-8 leached detectable non-volatile residues in isopropyl alcohol.

The biocompatibility of c-SiC and a-SiC films have been widely studied and promising results were reported (Santavirta, 1998; Kalnıns, 2002; Coletti, 2007).

3C-5iC films grown on silicon substrates have been shown as a potential material for Bio-MEMS applications, especially for biosensing. Due to the mechanical strength, surface areato-volume ratio, and extreme low mass, 3C-SiC BioMEMS structures have the potential to be mass sensors and resonators that are able to detect individual protein adsorption events (Zorman, 2012).

On the other hand, a-SiC based BioMEMS has been extensively developed and tested. Among the various bio-applications of a-SiC films can be mentioned: (i) coating material for implantable microsystems requiring hermetic sealing, owing to the fact that a-SiC is a excellent diffusion barrier material; (ii) membranes for microfluidics and Lab-on-a-Chip applications due to good a-SiC film chemical inertness property (Zorman, 2012).
