Video‐rate scanning probe control challenges: setting the stage for a microscopy revolution

Abstract

Scanning probe microscopy is at the verge of revolutionizing microscopy once again. Video-rate scanning tunneling microscope (STM) and video-rate atomic force microscope (AFM) technology will enable the direct observation of many dynamic processes that are impossible to observe today, such as atom or molecule diffusion, real time film growth, or catalytic reactions. In this paper we discuss the critical aspects that have to be taken into account when working on increasing the imaging speed of scanning probe microscopes. We highlight the state-of-the-art developments in the control of the piezoelectric scanning elements and describe the latest innovations regarding the design and construction of the whole mechanical loop including new scanner geometries. We identify critical aspects for which no obvious solution exists and aspects where advanced control engineering can help, like piezo non-linearities, the acceleration limit and the challenging technical requirements for the preamplifiers that are needed for measuring a tunneling current. Finally, we provide an overview of a number of new directions that are being pursued to solve the problems currently encountered in scanning probe technology.

Improving the accuracy of walking piezo motors

“…The rigidity of the motors is reflected in high resonance frequencies of the microscope and, therefore, its high-speed performance: we easily obtain video-rate imaging. [45][46][47] It is to mention here that we had to apply a special DAC card (Measurement Computing PCI-DIO24H) to reach sufficiently high speed for setting the voltages as well as to build very lownoise piezo drivers for the motors. As the position of the motors and, therefore, also of the sample holder is determined by the voltage applied to the motors, the noise of the drivers determines the height stability.…”

“…Using our new microscope 60,61 in combination with our special SPM control electronics, [45][46][47] which is also commercially available via, 64 we have a (theoretical) height resolution of only 0.24 pm. If we do not reach this limit, the reason is our particular choice for a tunnel-current preamplifier with a large bandwidth or the existence of mechanical/acoustical noise, due to, e.g., still running turbomolecular pumps or a setup that is not mechanically isolated from its surroundings via air legs.…”

“…If the line frequency, or a higher harmonics of it, hits a resonance frequency of the mechanical loop, imaging gets not only strongly distorted, but it also often leads to a tip crash. [45][46][47] As the pushing and holding force of a motor is directly related to its rigidity, the higher these forces are the better this motor is suited for an application as coarse approach motor in a high-speed SPM.…”

Force-feedback high-speed atomic force microscope for studying large biological systems

“…A definitive version was subsequently published in Micron, Volume 43, Issue 12, 2012. DOI: 10.1016/j.micron.2012 of the force-feedback HSAFM will be improved by using the piezo scanner with high-resonance frequencies such as conical piezo tubes (Rost et al, 2009). …”

A modified positive velocity and position feedback scheme with delay compensation for improved nanopositioning performance

“…Due to the recently increased demand for highspeed AFM in the areas of imaging of fast biological processes [6], and video-rate AFM [7,8,9], piezoelectric tube scanners have been replaced by flexure-based piezoelectric stack driven nanopositioners. Among their advantages are low cross-coupling between motion axes, robust mechanical construction, large motion range and high mechanical bandwidth.…”