5-6-5 SiC MicroElectromechanical Systems (MEMS) and Sensors
As described in Hesketh’s chapter on micromachining in this book, the development and use of siliconbased MEMS continues to expand. While the previous sections of this chapter have centered on the use of SiC for traditional semiconductor electronic devices, SiC is also expected to play a significant role in emerging MEMS applications . SiC has excellent mechanical properties that address some shortcomings of silicon-based MEMS such as extreme hardness and low friction reducing mechanical wear-out as well as excellent chemical inertness to corrosive atmospheres. For example, SiCs excellent durability is being examined as enabling for long-duration operation of electric micromotors and micro jet-engine power generation sources where the mechanical properties of silicon appear to be insufficient .
Unfortunately, the same properties that make SiC more durable than silicon also make SiC more difficult to micromachine. Approaches to fabricating harsh-environment MEMS structures in SiC and prototype SiC-MEMS results obtained to date are reviewed in References 124 and 190. The inability to perform fine-patterned etching of single-crystal 4H- and 6H-SiC with wet chemicals (Section 5.5.4) makes micromachining of this electronic-grade SiC more difficult. Therefore, the majority of SiC micromachining to date has been implemented in electrically inferior heteroepitaxial 3C-SiC and polycrystalline SiC deposited on silicon wafers. Variations of bulk micromachining, surface micromachining, and micromolding techniques have been used to fabricate a wide variety of micromechanical structures, including resonators and micromotors. A standardized SiC on silicon wafer micromechanical fabrication process foundry service, which enables users to realize their own application-specific SiC micromachined devices while sharing wafer space and cost with other users, is commercially available .
For applications requiring high temperature, low-leakage SiC electronics not possible with SiC layers deposited on silicon (including high-temperature transistors, as discussed in Section 5.6.2), concepts for integrating much more capable electronics with MEMS on 4H/6H SiC wafers with epilayers have also been proposed. For example, pressure sensors being developed for use in higher temperature regions of jet engines are implemented in 6H-SiC, largely owing to the fact that low junction leakage is required to achieve proper sensor operation . On-chip 4H/6H integrated transistor electronics that beneficially enable signal conditioning at the high-temperature sensing site are also being developed . With all micromechanical-based sensors, it is vital to package the sensor in a manner that minimizes the imposition of thermomechanical induced stresses (which arise owing to thermal expansion coefficient mismatches over much larger temperature spans enabled by SiC) onto the sensing elements. Therefore (as mentioned previously in Section 5.5.6), advanced packaging is almost as critical as the use of SiC toward usefully expanding the operational envelope of MEMS in harsh environments.
As discussed in Section 5.3.1, a primary application of SiC harsh-environment sensors is to enable active monitoring and control of combustion engine systems to improve fuel efficiency while reducing pollution. Toward this end, SiC’s high-temperature capabilities have enabled the realization of catalytic metal–SiC and metal-insulator–SiC prototype gas sensor structures with great promise for emission monitoring applications and fuel system leak detection . High-temperature operation of these structures, not possible with silicon, enables rapid detection of changes in hydrogen and hydrocarbon content to sensitivities of parts per million in very small-sized sensors that could easily be placed unobtrusively on an engine without the need for cooling. However, further improvements to the reliability, reproducibility, and cost of SiC-based gas sensors are needed before these systems will be ready for widespread use in consumer automobiles and aircraft. In general, the same can be said for most SiC MEMS, which will not achieve widespread beneficial system insertion until high reliability in harsh environments is assured via further technology development.