Instruction manual

SPM Fundamentals for the MultiMode
Feedback Gains
34 MultiMode SPM Instruction Manual Rev. B
permitted non-conducting surfaces to be imaged. Tunneling current was now used indirectly to
monitor a cantilevered tip as it profiled samples. Although this method allowed imaging of non-
conducting samples, it was unreliable due to adjustment difficulties with the STM probe and
cantilever flexion.
The next great leap in SPM design which presaged the present state of the art was the introduction
of light beam deflection. In one design, a laser is employed to configure a “light lever” similar to
those used with surface profilometers in industry. As the tip encounters surface features it flexes,
causing the incident laser beam to move across a photodiode detector. Another design relies upon
interferometric measurements; however, this design has been superseded by the light lever’s
simpler, more reliable design.
As new modes of SPM have been added to the field, the analysis of feedback signals has evolved to
keep up with changes. The next section discusses how signals are actually processed inside the
NanoScope to render images.
2.3.8 More about Feedback and Images
Veeco’s unique digital signal feedback architecture is described in numerous patents filed with the
US Patent Office. In summary, the basic feedback processes may be broken down as follows:
LookAhead Gain
In the example above, having a record of previous flights over terrain enabled three balloonists to
better anticipate the rises and falls of the terrain below them. Similarly, the feedback controller
relies upon data from the previous (immediately adjacent) scan line to anticipate local features. It is
easier to image samples which contain regular, periodic features (e.g., gratings) since scan lines
change relatively little from scan-to-scan. Consider, for example, scan lines tracing the surface of a
penny.
Although this scan is much larger than normally found in SPM, it illustrates how an adjacent,
lagging scan line can be used to determine local scan lines on regular surfaces. In most places (e.g.,
the forehead), each scan line changes little from the line next to it. In some local areas (such as
under the nose) there are small, sudden changes; however, these are relatively isolated. In contrast,