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Feedback In SPM Instrumentation

Background

One of the first mechanical profilers to use a feedback loop to maintain a fixed relation- ship between a probe and a surface was the Topografiner(1). In the Topografiner, the field emission current between a probe and surface is used to establish feedback. Because a current between the probe and surface is required, the Topografiner required electrically conductive samples for operation.
Figure 2. Block diagram of the topografiner developed by R. Young in 1972. A current between the probe and surface is used for feedback. Piezoelectric ceramics scan the probe over the surface in the x,y and z axis. An image of the surface is obtained by measuring multiple line scans as the probe is raster scanned across a surface.
Figure 2. Block diagram of the topografiner developed by R. Young in 1972. A current between the probe and surface is used for feedback. Piezoelectric ceramics scan the probe over the surface in the x,y and z axis. An image of the surface is obtained by measuring multiple line scans as the probe is raster scanned across a surface.
Figure 3 illustrates a mechanical profiling instrument having a constant probe-sample force that was capable of making profiles on electrically insulating surfaces. This profiling instrument, developed in 1979 by Schnell et.al.(2), used a piezoelectric bimorph for measuring the force between the probe and surface, and a feedback loop for keeping the force constant during scanning.
Figure 3: This illustration from U.S. Patent # 4,359,892 is of a mechanical profiler that uses a feedback electronic circuit to maintain a constant force between the probe and surface during a scan. The force between the probe and surface is adjusted using the set-point voltage, Us.
Figure 3: This illustration from U.S. Patent # 4,359,892 is of a mechanical profiler that uses a feedback electronic circuit to maintain a constant force between the probe and surface during a scan. The force between the probe and surface is adjusted using the set-point voltage, Us.
In their pioneering work on surface structures, Binnig and Rohrer(3) used a feedback loop in their Scanning Tunneling Microscope(STM). The STM monitors the tunneling current between a probe and surface to establish a fixed relationship while scanning. Later, in 1986, a feedback electronic circuit was used in the atomic force microscope(4). In the atomic force microscope, the forces between a probe and surface were established with a probe at the mounted at the end of a cantilever. Using the cantilever method permitted scanning with a force of less than a nanoNewton between the probe and surface.

 
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