A METROLOGICAL CONSTANT FORCE STYLUS PROFILER

A second-generation constant force profiler is described combining a capacitance based stylus force probe with interferometric optical metrology to monitor position of the specimen carriage. The probe has a vertical range of 15 mum with a resolution of 0.23 nm and a flat bandwidth response with a -3 dB cutoff at 228 Hz. Passing the output signal through a single pole 200 Hz low-pass filter, an overall system peak-to-peak broadband noise of less than 5 nm is observed. Manufactured primarily from inexpensive materials, the instrument demonstrates a stability of around 1 nm min-1 under controlled laboratory conditions. Using an inexpensive motor/gearbox/micrometer open-loop drive with velocity variations of up to 20%, profiles showing nanometer repeatability over 15 mm traces are demonstrated. Also shown is a profile of a soft contact lens in the hydrated state. The effect of the contact stiffness on the stylus probe/specimen interaction is illustrated by examining the dynamics of the force probe system while operating in the repulsive mode.