Block Diagram of SPM-M Kit. In the experiment discussed below, the actual AFM configuration consisted of an XYZ probe nanopositioner with a separate sample nanopositioner.

Mad City Labs manufactures high-resolution AFM instruments consisting of nanopositioning systems and the MadPLL^® controller that can be used with a variety of resonant probes such as a tuning fork with an etched Tungsten tip or an Akiyama probe. As an example, see the SPM-M kit.

The AFM probe tip is positioned in proximity to the sample surface using the Z-axis of a nanopositioner. The distance between the probe tip and the sample surface is directly related to the magnitude of the force between the tip and sample. The nanopositioner is moved to maintain constant force between the tip and the surface. In a typical experiment, the tip is scanned along the X and Y axes. In this experiment, the Z-axis position is measured directly to provide a height measurement of the sample.

Mad City Labs nanopositioners include integrated, direct measurement position sensors with PicoQ^® sensor technology, already calibrated using NIST-traceable interferometric methods. Any displacement of the moving portion of the nanopositioner will yield a change in position sensor output, with the resolution ultimately limited by the noise of the nanopositioning system. Total AFM noise must be sufficiently low in order to externally measure sub-nanometer motion. Measuring the output of the nanopositioner's absolute position sensors assures accurate scan size and Z-axis position of the probe as it scans over the surface.

A Silicon (111) substrate of single atomic layer steps was mounted to a Nano-LP100 with PicoQ^® sensor technology, operating in closed loop, which was placed underneath the AFM described in the Test Setup section, above.

The Z-axis of the Nano-LP100 was commanded with a 1nm peak-to-peak sine wave at approximately 0.8 Hz using a 20 bit digital interface, causing the Silicon substrate to move in a 1nm peak-to-peak sine wave in the Z direction. No XY scanning was involved in this test while the sample nanopositioner (Nano-LP100) and the AFM maintained constant XY position. The sine-wave motion of the Nano-LP100 Z-axis carrying the Silicon substrate caused the MadPLL^® to adjust the Z-axis position of the probe in order to maintain a constant force between the probe and Silicon substrate, and the probe's Z-axis position was measured and recorded. The test was performed over 600 iterations. The data result in a two dimensional colored image, with the X-axis representing time, Y-axis representing the iteration number, and the colors representing the displacement of the AFM probe [Figure 1].

Figure 1. 2D representation of 1 nm peak-to-peak sine wave motion measured by the AFM over 600 iterations. X-axis represents time, each Y-axis line represents one measurement iteration, and color represents measured probe displacement in nanometers.

The experimental question is: can the Nano-LP100 perform a 1 nm peak to peak motion? The data confirm that it can, as externally detected and measured by the AFM.

Figure 2 shows a single test cycle of the 1 nm sine wave probe displacement measurements:

Figure 2. 1D representation of a single test iteration of measured probe displacements. X-axis represents time and the Y-axis represents nanopositioner displacement of 1 nm peak-to-peak.