High-Accuracy Atomic-Force-Microscope Head For Dimensional Metrology

We present the design and experimental results of a high-accuracy atomic force microscope head (HAFM) to be used for dimensional metrology. HAFM uses monolithic flexures which are designed for minimum error motion. A piezoelectric stack drives the head over a range of 20 /m. HAFM uses a self-sensing AFM probe, which is operated in constant-amplitude self-resonance, for frequency-measuring microscopy. A discrete-time surface-tracking controller is implemented on a real-time FPGA board. The controller tracks the surface by maintaining a constant self-resonance period. To avoid spurious mixing, the controller's sampling is made synchronous to the self-resonance oscillations. Three capacitive displacement sensors directly measure the surface tracking motion. We have experimentally demonstrated surface tracking control with 100 Hz unity-crossover frequency, 70 degrees phase margin, and 0.12 nm RMS noise in a 100 Hz measurement bandwidth. We have also used the HAFM to measure calibration gratings and the surface of freshly cleaved sanded Mica sample.