A micromachined nanoindentation force sensor

A capacitive force sensor for in situ nanoindentation experiments in TEM has been designed, manufactured and evaluated. The confined space of the TEM specimen holder restricts the size of the fabricated sensor to 2 mm × 1.5 mm × 2 mm to allow mounting. A unique feature of the sensor is an integrated fixture for interchangeable tips, e.g. diamond tips. The sensor is fabricated in silicon anodically bonded to glass and the device is formed by DRIE. To improve the control of spring thickness and circumvent problems during fabrication a SOI wafer and slightly altered design was used in conjunction to an improved process, which resulted in a yield near 100%. The sensor is characterized by a force application using a piezoelectric positioning system, an electrostatic evaluation and a resonance frequency test using a scanning laser doppler vibrometer. The capacitance is measured with an off-chip read-out circuit. The resonance frequency test yielded a spring constant of 750 N/m, which results in a sensitivity of 0.27 pF/0.1 μN for small deflections. The evaluation shows that the force sensor is suitable for in situ nanoindentation for measurements in the range of 0-100 μN.