Hybrid control system for scanning probe microscopes

Abstract

A scanning probe microscope controller includes a digital signal processor (DSP) and an analog feedback control loop. The DSP serves to process the output of the scanning probe in the digital realm after conversion of the signal to digital form. After processing, the signal is restored to analog form. The height correction signal to be applied to a transducer controlling the distance between the scanning probe and a sample surface is then generated by an analog feedback control circuit, at least one parameter of which is under computer control. At the end of each scan-line, a variance may be calculated for the data and the inverse of this quantity is used to adjust the gain with which digitization of the data is carried out. Linearization of the data to correct for non-linearities in the scanning transducers may be carried out by the DSP after the data is acquired. This permits the scanning ramps applied to the transducers to be linear but the final displayed data do not show the effects of non-linearities. Adjustment of the feedback control loop gain may be carried out by monitoring the signal level from the scanning probe as each line is scanned. A computer determines if slow variations in this signal level have occurred across a line-scan. If so, the gain is increased. This process is continued until signals characteristic of excess gain (oscillation) are detected. At that point, the gain is reduced to a level slightly below the onset of the oscillation.