Fabrication and characterization of folded SU-8 suspensions for MEMS applications

Abstract

A novel wafer-level fabrication technique for polymeric springs combined with a silicon proof mass is presented for applications in MEMS force sensors or scanning micromirrors. Furthermore, passive test structures for the determination of mechanical parameters of the polymer are integrated on the wafer during device fabrication. The test structures allow for chip scale process monitoring in respect to the material parameters. The large difference of material stiffness and thickness between the springs and the proof mass concentrates the entire mechanical deformation into the springs. Furthermore, the polymeric springs can be used as electrically isolated suspension for electrostatically levitated devices. Devices have been fabricated and characterized by means of frequency response analysis. Thereby, a polymeric suspension with a spring constant of 8.7 N/m is presented, while the suspended structure itself features a spring constant of 140 kN/m. Material properties of the spring material (SU-8 2025) were measured to be 2.45 ± 0.6 GPa, 27.2 ± 2.2 MPa and 13.8 ± 3.2 MPa/m, for Young's modulus, the residual stress and the residual stress gradient, respectively.