Implementation of a Sinusoidal Raster Scan for High-Speed Atomic Force Microscopy
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Implementation of a Sinusoidal Raster Scan for High-Speed Atomic Force Microscopy Luke Oduor Otieno and Yong Joong Lee∗ School of Mechanical Engineering, Kyungpook National University, Daegu 41566, Korea
Bernard Ouma Alunda School of Mines and Engineering, Taita Taveta University, P.O. Box 635-80300, Voi, Kenya (Received 13 May 2020; revised 18 June 2020; accepted 18 June 2020) To improve the speed of an atomic force microscope (AFM), one must improve the bandwidth of its components, and the lateral XY scanner is no exception. Sinusoidal raster scans provide a simple way of improving lateral scan rates without the need for additional hardware and/or complex control algorithms. However, a raster scan using a sinusoidal waveform leads to a non-uniform probesample velocity. Uniform spatial sampling of scan data can be achieved in this case by varying the sampling rate as the probe sample velocity varies. In this work, we present a field-programmable gate array (FPGA)-based implementation of a sinusoidal raster scan with uniform spatial sampling for a high-speed atomic force microscope (HS-AFM). Using a home-made HS-AFM scanner and a custom controller, we demonstrate the performance of our approach by imaging Blu-ray disk data tracks in the contact mode. While the results show images comparable to those acquired using the traditional triangular raster scans, mirroring effects are better suppressed in high-speed imaging with sinusoidal scan signals. Keywords: High-speed atomic force microscopy, Sinusoidal raster scan, FPGA DOI: 10.3938/jkps.77.605
I. INTRODUCTION
The past two and a half decades have seen much progress in the development of high-speed atomic force microscopy (HS-AFM). State-of-the-art HS-AFMs are now capable of acquiring 100 pixels × 100 pixels images at a rate of 10 to 20 frames per second [1]. The imaging rate of an atomic force microscope is generally limited by the bandwidth of its individual components. Among these components are the scanning probe, the lateral and vertical scanners, the vertical feedback controller, and the data acquisition system. The limitations have been detailed by Fleming [2] and the references therein. As a way to improve the imaging rate of AFMs, the bandwidth of at least one of the speed-limiting components is commonly increased. One such component is the XY scanner. Conventionally, an AFM acquires images by using triangular positioning signals to scan a sample one line at a time in a raster scan pattern. The lateral XY scanner has to track the positioning signal within acceptable error limits, and as such, the scanning rates are limited to a maximum of between 1% and 10% of the lateral scanner bandwidth when using triangular ∗ E-mail:
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signals [3]. Limiting the line rate in this manner has two effects: the likelihood of exciting the resonant modes of a scanner is reduced and sufficient harmonics of the triangular positioning signal are accommodated to reduce the tracking error to within acceptable limits for most scanning app
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