Design of a MEMS Force Sensor for Quantitative Measurement in the Nano- to Pico-Newton Range

We describe the design and fabrication of a MEMS nano- to pico-Newton force sensor with SI traceability. There has been much recent interest in developing instrumentation for the quantitative measurement of forces in the nano- to pico-Newton range. Forces in this range are frequently encountered when investigating mechanical properties of nanomaterials, in nanobiotechnology, and in single-molecule biophysics. Various methods of measuring forces at these levels include using AFM cantilevers, scanning probe microscopy, and nanoindentation. However, such measurements are relative, and in order to obtain precise quantitative measurements, it is necessary to be able to calibrate such sensors in a manner that is traceable to fundamental SI units. One such method of calibration is using an Electrostatic Force Balance (EFB) that has been established at NIST. We thus describe the design and fabrication of a MEMS-based force sensor that may be directly calibrated with the EFB and thus has the potential to measure nano- to pico-Newtons of force with SI traceability. The sensor consists of a silicon rigid arm supported on silicon tethers and which are attached to capacitive electrodes. The bar, tethers and electrodes are made from the device layer of a double side SOI wafer. A glass wafer with patterned metal electrodes is anodically bonded on both the top and bottom of the wafer to form symmetrical capacitive electrodes. An external force moves the silicon arm and the resulting capacitive force gradient of the electrodes is measured with the EFB. The mechanical structure and electrodes are designed for force sensitivity in the nano- to pico-Newton ranges and for operation in UHV to reduce thermomechanical noise. We discuss the design, initial fabrication and testing of this force sensor as a step toward the ultimate goals of quantitative nanomechanical testing of materials, NEMS, and engineered surfaces at the nanoscale.