Micromachined Frequency-Output Force Probes

This letter presents a new class of microelectromechanical systems-based frequency output force and displacement probes with sub-nanometer displacement resolution. The force probe is a single layer monolithic crystalline silicon micro-structure comprised of a thermal-piezoresistive resonator embedded within a micro-cantilever. Deflection of the cantilever due to the applied force mechanically distorts the resonator modulating its resonance frequency. Such devices can be used as atomic force microscope probes or high-resolution surface profilometers with fully electrical operation eliminating the bulky and complex optical detectors typically used in such systems. A prototype sensor, operating at 7.5MHz, shows a displacement sensitivity of 2.5 Hz/nm. With a typical Allen deviation of 0.1-0.2 ppm for similar resonators operated as self-sustained oscillators, frequency resolution in the order of 1 Hz is expected to be achievable. This translates to 0.4 nm of displacement and 11 nN of force resolution for such sensors.