Nathan Shemonski
University:
Gradation Date: May 2009
Hometown:
My Project: Building a Scanning Kelvin Force Microscope Feedback
Loop
Simple methods of Scanning Kelvin Force Microscopy (SKFM) can be performed using a standard Atomic Force Microscope (AFM). These methods include open loop and amplitude modulation (AM) SKFM. Although these methods are useful, the resolutions of their images are less than desirable. Open loop SKFM does not give the cantilever the sensitivity required for high resolution imaging, and AM SKFM results in a smeared image due to its dependence on capacitance and not the gradient of capacitance. Other more advanced methods, such as frequency modulation, may not be possible with the standard equipment due to some of the signals being fully internal to the system.
Using a cheap (~$2,500) digital signal processing (DSP) board, a function generator, a lock-in amplifier, and a summing op-amp, a feedback loop can be built to perform all the signal processing external to the AFM. By tapping into the raw cantilever response signal, and applying the AC and DC biases between the AFM tip and the sample directly, full control over the signals can be achieved and higher resolution imaging should be possible.
Other possibilities include taking advantage of the onboard FPGA chip. A standard hardware image for the FPGA comes with the DSP board, but with some modifications it may be possible to put some of the external devices such as the lock-in amplifier and the summing op-amp inside the FPGA.
