Fabrication and characterization of 4H-SiC MEMS for harsh environments and MoS2 for chemical sensing

Allen Vincent Lim

In this thesis, the fabrication and characterization of single crystalline 4H-silicon carbide (4H-SiC) MEMS for harsh environments applications and a Molybdenum Disulfide (MoS2) chemiresitor has been reported. Current MEMS devices made of conventional materials such as silicon is not applicable for harsh environment applications due to the limitations of its material properties. This has led to development of other materials such as wide band gap semiconductors. Among these wide band gap semiconductors, 4H-SiC has shown to have the most promise due to its superior intrinsic material properties. However, releasing of single crystalline 4H-SiC microstructures has been a challenge due to its high chemical inertness. Using a newly developed surface micromachining technique known as photoelectrochemical etching, single crystalline 4H-SiC MEMS in-plane and out-of-plane resonators have been released and reported. Micrometer thin film SiC MEMS material properties may not be the same as bulk properties. The resonant frequency of thin film single crystalline 4H-SiC out-of-plane resonators ranging from 10 [mu]m to 100 [mu]m have been reported, from the resonant frequencies young's modulus was derived. The resonant frequency of a single crystalline 4H-SiC in-plane resonator has also been reported. Chemical and biological sensing is critical in medical diagnosis, environmental monitoring and pharmaceutics. 2D layered transition metal dichalcogenides such as Molybdenum Disulfide (MoS2) have been rapidly researched due to its unique physical, chemical and mechanical properties. MoS2 also contains a thickness depended band gap which makes it an interesting material for semiconductor devices. MoS2 has the potential to provide high sensitivity and its planar nature allows for monolithic integrations which makes it very promising for sensor material. Due to the weak out of plane bonds and strong in plane bonds of MoS2, monolayer and bilayer MoS2 can be obtained by mechanical exfoliation. Using this technique and optical photolithography, MoS2 chemiresistors have been fabricated. Crystal thicknesses' ranging from 5 nm to 12 nm has been reported and has shown to have a change in conductance for a wide range of pH buffer solutions. The conductance of the chemiresistors decreases with the increase of pH values which demonstrates p-type behavior.

Fabrication and characterization of 4H-SiC MEMS for harsh environments and MoS2 for chemical sensing

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