Paper MN-TuM9
The Effect of Substrate Material and Metallization Layers on the Mechanical Properties of Micromachined Amorphous Silicon Carbide Structures

Tuesday, October 21, 2008, 10:40 am, Room 206

Amorphous SiC (a-SiC) films are attractive for micromachined structures requiring the properties of SiC but whose substrates cannot tolerate the high deposition temperatures associated with the conventional CVD methods used to deposit the crystalline forms. The preponderance of data in the literature focuses on the properties of a-SiC films deposited directly onto Si; much less is known about the properties of these films when deposited onto silicon dioxide sacrificial layers. Even less is known about how metallization (required for electrostatic actuation) affects the mechanical behavior of a-SiC micromachined structures. This study examines the roles that the substrate and metallization layers play on the mechanical properties of a-SiC structures. Test specimens were fabricated from 300 nm-thick a-SiC films deposited by PECVD on bare (100) Si wafers and (100) Si wafers coated with a 3.2 um-thick PECVD silicon dioxide film. After deposition, the wafers were annealed at 450C for 30 min to convert the as-deposited compressive film stress to a low tensile stress. Suspended membranes with areas of 750x750 um2 were bulk micromachined by anisotropic etching in KOH at 50C. For the oxide-coated wafers, a 35 min immersion in BOE at 25C was performed to remove the silicon dioxide beneath the SiC membranes. The membranes were subjected to load-deflection testing at differential air pressures between 0 and 138 kPa. From the resulting pressure versus deflection data, it was found that the average Young’s modulus for a-SiC films deposited on Si was 129 GPa with a residual stress of 162 MPa, while films deposited on silicon dioxide had a Young’s modulus of 116 GPa with a residual stress of 154 MPa. For films deposited on oxide-coated wafers, the effect of metallization on the residual stress of the membranes was characterized. Membranes were first subjected to load-deflection testing, then coated with a Cr adhesive layer and Au structural layer by e-beam evaporation and again subjected to load-deflection testing. For a total metal thickness of 60 nm (10 nm/50 nm Cr/Au), the average residual stress increased by 52 MPa. For a total metal thickness of 125 nm (25 nm/100 nm Cr/Au), the average residual stress increased by 43 MPa, and for 250 nm (50 nm/200 nm Cr/Au) the average residual stress increased by 35 MPa.