Scanning probe microscopes go video rate and beyond
In this article we introduce a, video-rate, control system that can be used with any type of scanning probe microscope, and that allows frame rates up to 200 images/s. These electronics are capable of measuring in a fast, completely analog mode as well as in the more conventional digital mode. The latter allows measurements at low speeds and options, such as, e.g., atom manipulation, current–voltage spectroscopy, or force–distance curves. For scanning tunneling microscope (STM) application we implemented a hybrid mode between the well-known constant-height and constant-current modes. This hybrid mode not only increases the maximum speed at which the surface can be imaged, but also improves the resolution at lower speeds. Acceptable image quality at high speeds could only be obtained by pushing the performance of each individual part of the electronics to its limit: we developed a preamplifier with a bandwidth of 600kHz, a feedback electronics with a bandwidth of 1MHz, a home-built bus structure for the fast data transfer, fast analog to digital converters, and low-noise drivers. Future improvements and extensions to the control electronics can be realized easily and quickly, because of its open architecture with its modular plug-in units. In the second part of this article we show our high-speed results. The ultrahigh vacuum application of these control electronics on our (UHV)-STM enabled imaging speeds up to 0.3 mm/s, while still obtaining atomic step resolution. At high frame rates, the images suffered from noticeable distortions, which we have been able to analyze by virtue of the unique access to the error (dZ) signal. The distortions have all been associated with mechanical resonances in the scan head of the UHV-STM. In order to reduce such resonance effects, we have designed and built a scan head with high resonance frequencies (⩾64kHz), especially for the purpose of testing the fast electronics. Using this scanner we have reached video-rate imaging speeds up to 200frames/s (5ms/image), while still obtaining atomically resolved structures.