Long range constant force profiling for measurement of engineering surfaces

A new instrument bridging the gap between atomic force microscopes (AFMs) and stylus profiling instruments is described. The constant force profiler is capable of subnanometer resolution over a 15-μm vertical range with a horizontal traverse length of 50 mm. This long traverse length, coupled with the possibilities of utilizing standard radius, diamond measurement styli, make the force profiler more compatible with existing profiling instrument standards. The forces between the specimen and a diamond stylus tipped cantilever spring are sensed as displacements using a capacitance bridge. This displacement signal is then fed through a proportional plus integral controller to a high stability piezoelectric actuator to maintain a constant tip-to-sample force of approximately 100 nN. Much of the sensor head and traverse mechanism is made of Zerodur glass-ceramic to provide the thermal stability needed for long travel measurements. Profiles of a 30-nm silica step height standard and an 8.5-μm step etched on Zerodur are presented.