Development of surface micromachined Aluminum Nitride air-bridges for piezoelectric MEMS/NEMS applications by Metal Organic Vapor Phase Epitaxy techniques

Group III-nitrides have attracted considerable attention for piezoelectric Micro/Nano electromechanical (MEMS/NEMS) applications due to their excellent bio compatibility, well developed growth techniques for high quality thin films and structural stability at high temperatures when compared to the commonly used piezoelectric metal oxides. Among the group III-nitrides Aluminum Nitride (AlN) possess superior material properties such as highest piezoelectric coefficient and good mechanical properties. Growth techniques for fabricating group III-nitride MEMS/NEMS by metal organic vapor phase epitaxy (MOVPE) techniques have involved sacrificial layers such as epitaxial group III-nitrides/ alloys, nanocrystalline films and porous interlayers. However, the material properties of the MOVPE grown films on the amorphous sacrificial layers such as silicon oxide have not been adequately investigated to evaluate potential MEMS/NEMS devices such as piezoelectric micro/ nanofluidic channels.;This work demonstrates a process for the fabrication of Aluminum nitride (AlN) thin film air-bridges using MOVPE techniques on silicon templates. Micro-FTIR techniques were used to study the crystallographic orientation of the AlN thin film air-bridges with lateral dimensions as low as 100 mum. FTIR results also show that the wet etching process to remove the underlying sacrificial layer also improves the material properties of the AlN films on SiOx. The study indicates that AlN air-bridges are polycrystalline in nature and are preferentially c-axis oriented after wet etching. Lateral field excitation of the piezoelectric films and laser Doppler vibrometer techniques were combined to investigate the piezoelectric response of the AlN films on the sacrificial layer. Lateral field excitation of the AlN films grown on the amorphous sacrificial layer shows that the AlN films exhibit piezoelectric properties. The displacement of the AlN air-bridges obtained by lateral field actuation is around 1 nm over an air-gap of 130 nm after the removal of the sacrificial layer. However, the mismatch in the coefficient of thermal expansion between the substrate and thin films induces significant residual stress in the heterostructure. The AlN air-bridges on silicon substrate exhibit fracture due to the tensile residual stress exceeding the fracture limit.