A micromachined strain sensor with differential capacitive readout

This paper describes a laterally deflecting micromachined device that can be used to electronically monitor residual strain and Young´s modulus of microstructural materials. Residual strain is indicated by a change in the differential capacitance change, whereas the Young´s modulus is provided by the slope of a simple CV (capacitance vs. voltage) test. The device is suitable for automated wafer level as well as post-packaging readout. Typical strain sensitivities are 0.1-1 fF/MPa, and the CV slope essentially doubles (e.g. from 0.23 fF/V to 0.47 fF/V at 70 V bias) as the Young´s modulus changes from 220 to 130 GPa. Nickel plated and polysilicon strain sensors were fabricated by surface micromachining techniques and coated with self-assembled monolayers using an ODS-based process. Both stress and Young´s modulus measurements from these structures were found to closely match theoretical models.