Pacific Nanotechnology Inc.
PID feedback controllers used in scanning probe microscopes can be constructed from either analog or digital control circuits. There are advantages and disadvantages to each of the designs.
The primary advantage of digital control is that it is very flexible. The flexibility is a result of the fact that the digitized sensor output is processed with software algorithms. Changing feedback conditions such as response time, and probe motions are easily done with different software algorithms.
With the advent of Digital Signal Processing(DSP) chips digital feedback became a practical alternative to analog feedback methods. In the early 1980's, Texas Instruments, a pioneer in the development of DSP chips, suggested that all analog feedback circuits should be replace by digital control circuits. The primary advantage of digital feedback is that it is very flexible.
There are two early examples of the flexibility of digital feedback circuits. In the first, 1986, Hartoonian et.al. used digital feedback to control the motion of a NSOM probe as it was scanned across the surface, see Figure 6.
Figure 6: Digital feedback circuit used by Hartoonian et. Al. In this microscope, the probe was lowered to the surface until a tunneling current was measured, and then the probe was removed from the surface, moved horizontally, and then lowered to the surface. This jumping motion was repeated until an image of the surface could be constructed.
A second example demonstrating the flexibility of digital feedback was made by Bard et. Al. in 1987. Bard demonstrated that it was possible to use digital feedback to scan a probe at a fixed distance above a surface. He outlined the following steps for scanning a probe at a fixed distance above a surface:
From the paper
"...Before the photo-etching experiment it is necessary to gauge the roughness and any tilt of the substrate material to avoid crashing the tip into surface features of the substrate when it is scanned at close spacing from the substrate.."
First:
"...The substrate was first scanned via the tunneling mode with the tip moved along a predefined path and the positional data, including roughness and tilt, were stored on a floppy disk.."
Then:
"...The spacing between tip and substrate was about 1 micron and this adjustment was maintained by the computer based on the data found in the preliminary scan.."
This method of scanning a probe over a surface and storing the topography data and then using the data to lift a probe above a surface can be applied to many other types of scanned probe techniques such as the magnetic force microscope(MFM) and the electric force microscope(EFM).
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