A high-sensitivity and quasi-linear capacitive sensor for nanomechanical testing applications

The design, modeling, fabrication and characterization of triplate differential capacitive sensors employed in novel on-chip tensile and compression material testing systems are reported, where the capacitive sensors are integrated to measure the load on the specimen or the specimen deformation. Analytical expressions for studying stability, linearity and sensitivity, including the effect of the electrostatic force generated by the excitation signal on the sensing electrodes, are derived for the first time and discussed for quasi-static applications. The possible influence of the electron beam of an electron microscope on the capacitance measurement is also analyzed. The in-plane suspension stiffness of the fabricated device is determined by a resonance method performed inside a scanning electron microscope and used for pull-in voltage prediction. Sensitivity and linearity are extracted from capacitance-to-displacement measurements and agree well with analytical and finite element analysis results. The fabricated capacitive sensors show a high sensitivity of 0.61 fF/nm within a quasi-linear moving range of 2250 nm, which yields a displacement resolution of 1 nm and a load resolution of 34 nN.