Magnetic Force Microscopy
Advanced MFM Operation
264
Dimension 3100 Manual
Rev. D
Table 15.2a
Frequency Modulation Gains Initial Settings
As with topography gains, the scan can be optimized by increasing the gains to maximize feedback
response, but not so high that oscillation sets in.
15.3 Advanced MFM Operation
15.3.1 Lift Scan Height and Magnetic Imaging Resolution
The most important parameter affecting imaging resolution is
Lift scan height
. The range of 10–
200nm is most useful. In general, MFM resolution is roughly equal to the lift height. Smaller
Lift
scan heights
give better resolution; magnetic features smaller than the
Lift scan height
may not be
resolved. The tip also experiences stronger fields close to the surface, giving improved signal-to-
noise ratios.
For example, the image of metal-evaporated tape in
has a resolution limited by the 100
nm
Lift scan height
. To improve the resolution, try reducing the
Lift scan height
to ~
25nm
.
Ensure that the tip does not strike the surface on the low point of its swing in the Lift image. Tip
strikes appear as black or white spots, or even noisy, high-contrast streaks crossing the image. If the
tip begins to strike the surface, reduce the
Interleave
Drive Amplitude
. (In general, MFM tips are
not damaged by intermittent tip strikes in
LiftMode
, except in extreme cases of very large
amplitude and small lift heights.) An example of an image of the metal-evaporated tape taken with
a
Lift scan height
of
30nm
is shown in
. Note the fine magnetic structure that is not
. When imaging a sample for the first time, begin with moderate
Lift scan
heights
(
50nm or greater
), then adjust downward. On relatively smooth samples (e.g., hard disks),
lift heights down to 0nm can be used, as long as the drive amplitude is adjusted accordingly. (Lift
scan heights of 0nm still correspond to a non-zero mean tip-sample distance. See the section on
Setpoint
below.) It is usually not beneficial to use
Lift scan heights
much smaller than the surface
roughness. Users are encouraged to experiment for the best images on their samples.
The ultimate lateral resolution of MFM is near 20nm. Resolution is affected by properties of the tip,
including mechanical sharpness and magnetic structure. When in good condition, magnetically-
coated tips routinely give 50nm resolution, and many achieve 30nm or better.
SPM Controller
Integral Gain
Proportional Gain
Phase Extender
50
50
Quadrex Extender
0.6
0.6
NanoScope IV
0.6
0.6
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