IN-SITU FORCE CALIBRATION OF HIGH FORCE-CONSTANT ATOMIC-FORCE MICROSCOPE CANTILEVERS

A miniature capacitive force sensor which fits in the sample position of an atomic force microscope (AFM) has been used to calibrate the for ce applied by the scanning tip during nanohardness measurements. The s ensor is simple, physically robust, and easily fabricated from readily available materials. It can be operated with commonly available elect ronic instrumentation. The device described here is optimized for tip- sample forces between 10(-4) and 10(-2) N, a range useful for micromet er-scale mechanical modification of surfaces. With minor design change s, sensors of this general type should be capable of much greater sens itivity. The sensor is calibrated outside the AFM by the application o f free weights. Calculated estimates of AFM cantilever force constants are difficult for all but the simplest geometries and may rely on unc ertain values for certain critical dimensions and material properties. These estimates can thereby be replaced by a simple and direct measur ement.