Atomic force microscope for accurate dimensional metrology

Our prototype atomic force microscope (AFM) uses a piezoelectric tube to scan a probe tip over a sample surface, while a PC-based digital controller maintains a constant tip–sample separation based on feedback from a quartz tuning fork proximity sensor. We have successfully run the AFM in both the shear and tapping modes, using optical fibers as probe tips. The AFM utilizes a set of capacitance sensors with a spherical target for direct measurement of probe tip displacements. We have used this system to image localized surface topography of a square wave silicon calibration grating with localized step height accuracy of 1 nm in the vertical direction and vertical RMS noise less than 4 nm. The lateral accuracy is on the order of 100 nm, and our largest lateral measurement scans have been performed on 20μm×20μm regions with step heights of 26.5 nm and a modest scan speed of 0.8μm/s.