A PRECISION, LOW-FORCE BALANCE AND ITS APPLICATION TO ATOMIC-FORCE MICROSCOPE PROBE CALIBRATION

We present the design and evaluation of a novel precision balance cons isting of a thin silica rod suspended on a beryllium copper spring fle xure. At opposing ends of the rod are a capacitance sensor and a solen iod/magnet force actuator. When an external force is applied to the si lica rod, the, resultant deflection of the springs is detected and fed back through a servo controller and current amplifier to the force ac tuator, thus maintaining a null signal. Consequently, the coil current , which, for relatively small fields, can be considered proportional t o its applied force, is monitored as the balance output. Three class I dead weight, masses of 2, 5, and 100 mg were used for calibration wit h six less accurate weights used to verify the linearity of the balanc e to better than 1%. Using standard vibration isolation techniques and an inexpensive environmental chamber, the present design will measure loading forces up to 1 mN (100 mg) and it has a resolution of better than 70 nN Hz1/2 (7 mug Hz1/2). Presently measuring 40 mm in diameter and 40 mm height this compact design is constructed from low expansivi ty materials giving a thermal sensitivity of better than 0.03 muN K-1. Results from a calibration of an atomic force probe are also presente d and discussed.