STM relies on “tunneling current” between the probe and the sample to sense the topography of the sample. The STM probe, a sharp metal tip (in the best case, atomically sharp), is positioned a few atomic diameters above a conducting sample which is electrically biased with respect to the tip. At a distance under 1 nanometer, a tunneling current will flow from sample to tip. In operation, the bias voltages typically range from 10 to 1000 mV while the tunneling currents vary from 0.1 to10 nA.
The tunneling current changes exponentially with the tip-sample separation, typically decreasing by a factor of two as the separation is increased 0.2 nm. The exponential relationship between the tip separation and the tunneling current makes the tunneling current an excellent parameter for sensing the tip-to-sample separation. In essence, a reproduction of the sample surface is produced by scanning the tip over the sample surface and sensing the tunneling current.
STM relies on a precise scanning technique to produce very high-resolution, three-dimensional images of sample surfaces. The STM scans the sample surface beneath the tip in a raster pattern while sensing and outputting the tunneling current to the SPM Controller. The digital signal processor (DSP) in the Controller controls the Z position of the Piezo Scanner based on the tunneling current error signal. The STM operates in both “constant height” and “constant current” data modes, depending on the Feedback Gain settings. The DSP always adjusts the height of the tip based on the tunneling current error signal, but if the feedback gains are set extremely low (e.g., Integral Gain < 15 and Proportional Gain < 15), the piezo remains at a nearly constant height while tunneling current data is collected. With the Feedback Gains high (e.g., Integral Gain >15 and Proportional Gain >15), the Scanners Piezo height changes to keep the tunneling current nearly constant, and changes in piezo height are used to construct the image. The exponential relationship between tip-sample separation and tunneling current allows the tip height to be controlled very precisely.
