The FAST Module: An Add-On Unit Based on FlexRIO for Driving Scanning Tunneling Microscopes to Video Rate and Beyond

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"The flexibility of the FlexRIO platform together with the performance of the NI 5781 module and NI 6221 board helped us meet the stringent requirements on both the waveform generation and the signal acquisition sides."

- Carlo Dri, ELETTRA IOM-CNR TASC Laboratory

The Challenge:
Overcoming the time resolution limitation of commercial scanning tunneling microscopes (STM), which is imposed by the control electronics and mechanical response of the STM scanners and makes it impossible to study many dynamical processes of fundamental and technological interest at surfaces.

The Solution:
Developoing an add-on control unit, based on the FlexRIO platform, to increase both the STM probe motion speed and the signal acquisition speed, and tailoring the shape, properties, and phase relationships between the scan waveforms to deal with the complex mechanical behavior of the STM scanner.

Carlo Dri - ELETTRA IOM-CNR TASC Laboratory

Scanning tunneling microscopes (STM) can acquire images of surfaces with the ultimate spatial resolution at the nanoscale to observe single atoms and molecules in real space. However, commercial STM instruments are limited by slow acquisition speeds. It can take tens of seconds to scan a complete image. We partially or totally lose the essential details of atomic scale dynamical processes occurring at surfaces, which are of fundamental and technological interest. Surface diffusion, phase transitions, self-assembly phenomena, film growth and etching, chemical reactions, and conformational changes of molecules are some examples among the infinite variety of processes that one would like to investigate with high-temporal resolution.

We faced two issues in increasing the scan speed of STMs: the mechanical resonances of the STM scanner head (including the tip and the piezoelectric actuator and its supporting structures) and the limited bandwidth of the components of the control electronics. Years of careful experiments with a custom-built early prototype highlighted the importance of generating extremely clean scan waves for driving the scanning element, as well as ensuring well-defined and stable synchronization between them. 

We needed to do this to avoid the unwanted excitation of mechanical resonances of the scanner and to avoid distortions in the video frames due to uncontrolled tip motion. The cleanness and the synchronization are particularly important for the waves in the fast scan direction and in the direction orthogonal to the surface. We needed to precisely control the tip in the latter direction to follow, during each scan line, the inevitable tilting of the sample with respect to the tip axis and synchronize this motion with the lateral fast motion of the tip. The slow scanning wave is instead less critical in terms of cleanness (its fundamental frequency lies well below the scanner resonances), whereas synchronization with the acquisition is still essential to guarantee correct image reconstruction.
Concerning the signal acquisition side, we must enforce a stable synchronization between the probe motion and the acquisition since the data of each pixel needs to be uniquely reconstructed in the video frames. Common current-to-voltage converters exhibit a low bandwidth (limited to one hundred kilohertz or less), and since the pixel frequencies can easily exceed the megahertz range, we required an A/D converter  with strong oversampling capabilities to improve the signal-to-noise ratio.

The NI 5781 module is an ideal solution both for generating the two fast scanning signals (the lateral and the vertical scan) and for acquiring the tunneling current. The two 16-bit D/A converters and the 14-bit A/D converter, combined with the 100 MS/s sampling rate, yield superior cleanness of the scan waves while guaranteeing stable and reliable synchronization between motion and acquisition. An NI 6221  board that features two analog outputs with 16-bit resolution and a 833 kS/s sampling frequency handles the slow probe motion. Both products are part of the FlexRIO platform which, advantageously, means we can use intuitive LabVIEW virtual instruments to program the NI 5781 FPGA. This also means that nonexperienced HDL programmers can modify the code. Moreover, we can easily control these boards through a single LabVIEW project although they use different software libraries (the LabVIEW FPGA Module  for the NI 5781 and NI-DAQmx for the NI 6221).

The project developed for this application using these solutions can control both boards to drive the STM tip movements and acquire the tunneling current. Moreover, it provides a straightforward user interface for scientists to view and analyze the images acquired by the system.

The flexibility of the FlexRIO platform together with the performance of the NI 5781 module and NI 6221  board helped us meet the stringent requirements on both the waveform generation and the signal acquisition sides so we could avoid the expense and time necessary to develop custom-made boards.

Author Information:
Carlo Dri

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