Dual-Stage Vertical Feedback for High-Speed Scanning Probe Microscopy

Many popular modes of scanning probe microscopy require a vertical feedback system to regulate the tip-sample interaction. Examples include constant-current scanning tunneling microscopy and constant-force atomic force microscopy. Due to the control of tip-sample interaction, these modes of microscopy provide precise topographic information and result in drastically reduced sample damage, hence their popularity. Unfortunately the vertical feedback controller also imposes a severe limit on the scan-speed of scanning probe microscopes. In this paper, the foremost bandwidth limitation is identified to be the low-frequency mechanical resonances of the scanner. To overcome this limitation, a dual-stage vertical positioner is proposed. This comprises the original scanner, plus an additional high-speed stage. The improved bandwidth provided by the high-speed stage allows a vast improvement in feedback gain and bandwidth. In this work, the bandwidth is increased from 83 Hz to 2.7 kHz. This improvement allows image quality to be retained with a speed increase of 33 times, or alternatively, feedback error can be reduced by 33 times if scan speed is not increased. The techniques proposed are mechanically and electrically simple and can be retrofitted to any scanning probe microscope.