#1.3.1.1 Material properties

Silicon Carbide (SiC) is a compound of covalently bonded silicon and carbon atoms. It is a hard ceramic, originally used as an abrasive powder by Acheson (1893)⁷⁰ to polish diamond stones, under the name of carborundrum. The crystal structure of SiC exhibits polytypism, which is a form of polymorphism. All polytypes are composed of the same planar structure, a pavement of silicon tetrahedra with central carbon atoms. Different SiC polytypes are different vertical stacking patterns of this base plane. Although there are more than 250 identified SiC polytypes, the only commonly used in the industry are 3C–SiC (\beta-phase, the only known cubic phase), 4H–SiC and 6H–SiC (\alpha-phase), due to the ability to synthetize them in wafers or thin films (Mehregany et al. 2000)⁶⁶.

Table 1.1 displays some mechanical, electrical and thermal properties of 3C and 6H–SiC, compared to those of silicon and diamond. SiC generally shows excellent mechanical robustness thanks to its high Young’s modulus, as well as exceptional thermal and chemical stability. At ambient temperature, 3C–SiC is stable, then transforms to 6H–SiC at 1700°C, which is stable up to the sublimation point at 2830 °C. Above 1200 °C, SiC oxidizes in air in presence of water vapor, as follows: \text{SiC} + 2\text{O}_2 \longrightarrow \text{SiO}_2 + \text{CO}_2. However, the oxidation rate is very low relative to Si, and it results in the development of a thin, stable SiO₂ surface layer.

#1.3.1.2 SiC coatings for hot and oxidative environments

Inside jet engines, turbine blades face the harshest conditions. As their function is to power the drive shaft, they are subjected to the hot exhaust gases expanding from the combustion chamber, as well as extreme centrifugal forces due to rotational speeds. Typical requirements involve 1700 °C temperatures, 1.25 Mach speed, 10 bar pressure and particle erosion/corrosion (Spitsberg & Steibel 2004)⁷². Material scientists have engineered high-performance alloys and single-crystal grow processes for the bulk of the turbine blades. However, the conditions being hotter than the phase transition of the alloy, and as debris corrosion would wear out the part, a surface treatment is applied to produce a hard insulating shell. Silicon carbide is often used as the outer coating layer, in the form of weaved fibers which are fixed to the surface by a pyrolysis treatment (Zhu 2018)⁷³.

To give another example of SiC usage as a thermal protection outer shell, the wing leading edges of the space shuttles were tiled with a composite material called RCC. It is a stacking of different graphite layers, which are coated with SiC to provide a hard shell as well as thermal oxidation protection.

#1.3.1.3 SiC electronic properties

#1.3.2.1 Microscale SiC pieces for gas turbine applications

Mehran Mehregany, who worked at MIT at the time of the first WSS sensor developments, pioneered the technological developments for SiC MEMS aimed at harsh environment applications (Mehregany & Zorman 1999)⁷⁵. After his thesis work on micromachined silicon mechanisms, he transposed a commercial accelerometer process to make a WSS balance, at CWRU (Pan et al. 1999)⁶⁵. In the late 1990s, his team partnered with NASA Glenn Research Center in an effort to develop a technological field around smart jet engines (Mehregany et al. 2000)⁷⁶, which means implementing flow control strategies in jet combustion chambers. They successfully fabricated and tested SiC atomizers for fuel injection (Rajan et al. 1999)⁷⁴.

#1.3.2.2 Poly-SiC resonant structures

The CWRU / Glenn Research Center collaboration enabled the developments of a vertical APCVD reactor for SiC film growth (DeAnna et al. 1998)⁷⁸ on 4-inch wafers, laying the groundwork for SiC MEMS structures. In the following, Mehregany’s team successfully created a resonant floating mass by depositing APCVD poly-SiC on a SoI wafer, defining an Al etch mask for RIE patterning of the SiC layer (Roy et al. 2002)⁷⁷. A capacitive transduction scheme was used, similarly to MIT-like WSS balances. The device was characterized in resonant frequency and SiC/Si thermal expansion coefficients, up to 1000 °C. Ni bond pads and wire-bonding were used to withstand the high temperatures, as its 1455 °C melting point far exceeds that of gold (1064 °C). Iterating on this work, Rajgopal et al. (2009)⁷⁹ fabricated a poly-SiC accelerometer, characterized for a range of 5000 g and 18 kHz bandwidth.

In the same period, Kulite Semiconductors Products started working on pressure diaphragms for high temperature applications. Robert Okojie’s team presented a 6H–SiC design withstanding 500 °C conditions (Okojie et al. 1998)⁸⁰. Foreseeing the technological potential, the company patented several SiC-based pressure transducer designs in the 2000s (Kurtz & Ned 2001)⁸¹, (Kurtz 2009)⁸². Robert Okojie joined the silicon carbide research group at Glenn Research Center in 1999, and has considerably developed the sensor and electronics technologies since then. In 2006, he patented a complete packaged system comprising a SiC piezoresistive cantilever anemometer (similar working principle than fences or pillars), including the sensing elements, fabrication process and packaging solution (Okojie et al. 2003)⁸³. In the following years, he publishes reports on SiC device manufacturing methods at Glenn Research Center, called “CLASSSiC” (Okojie 2005)⁸⁴, which enabled to batch manufacture SiC pressure sensors, accelerometers and cantilever anemometers. Simultaneously, he reports the full fabrication, packaging and characterization at 600 °C of the patented anemometer (Okojie et al. 2004)⁸⁵ (figure 1.18).

As the CLASSiC process could not be implemented to \alpha phases of SiC, Okojie presented an uncooled 4H–SiC piezoresistive pressure sensor functioning up to 800 °C, aimed at closer insertion to jet engines combustion chambers to quantitatively monitor the combustion dynamics (Okojie et al. 2015)⁸⁶.

#1.3.2.3 SiC-based CMUTS

-> (Portail et al. 2022)⁸⁷

#1.3.2.4 SiC as a CMOS alternative for solid-state electronics

Because of the 125 °C thermal limit of standard CMOS, due to the resistivity of silicon significantly dropping, electrical engineers looked for alternative solid-state electronics solutions to accompany the MEMS developments for harsh environments.