High-Accuracy Atomic Force Microscope

Abstract

We have designed, built, and tested a high-accuracy atomic force microscope (HAFM) to be used for dimensional metrology. The HAFM is specialized for use in conjunction with our Sub-Atomic Measuring Machine (SAMM), serving as the surface measurement probe with 0.1 nm resolution over the SAMM travel range of 25 mm by 25 mm by 100 μm. In this configuration, all lateral scanning is provided by the SAMM, and so the HAFM is designed to move its probe with a single degree of freedom motion normal to the sample of interest. This sample-normal probe motion (Z-axis) is guided in the HAFM by symmetric monolithic flexures which are designed for high thermal stability and minimum lateral error motion. A piezoelectric stack drives the HAFMZ-axis over a range of 20 μm. The HAFM uses a commercially available self-sensing quartz-tuning-fork-based AFM probe, which is operated in constant-amplitude self-resonance via electronics described herein. In this configuration, the sample-probe separation is sensed via frequency shift of the probe resonance. The probe-sample separation is controlled using a discretetime surface-tracking controller implemented on a field-programmable gate array (FPGA). The controller tracks the surface by actuating the piezo to maintain 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 trackingmotion, providing a high-accuracymeasurement of surface height. We have experimentally demonstrated surface tracking control with 100 Hz unity-gain 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 a freshly cleaved sanded Mica sample. The HAFM probe has recently been installed on the SAMM stage; we show a preliminary image taken using this combination.