Instruction manual

Magnetic Force (MFM) Imaging
Advanced Topics
Rev. B MultiMode SPM Instruction Manual 235
13.5.2 Saturation in Amplitude Detection
If using amplitude detection, the magnetic force image can saturate (appear completely featureless)
if the Interleave Drive Amplitude is significantly different than the Drive Amplitude in the main
scan. Adjust the Interleave Setpoint to restore the image. (Note that the Interleave Setpoint has
no physical effect in LiftMode since there is no surface feedback during the lift pass.
13.5.3 Optical Interference
When using Amplitude Detection, optical interference may sometimes appear in the Lift (magnetic
force gradient) image when imaging highly reflective samples. Optical interference appears as
evenly spaced, sometimes wavy lines with ~1–2µm spacing superimposed on the lift image. This
occurs when ambient laser light (i.e., light passing around or through the cantilever, then reflecting
off the sample) interferes with laser light reflecting from the cantilever. Interference can be
alleviated by moving the beam spot up the cantilever away from the tip; about one-third of the
cantilever length works well. The adjustment can be refined by carefully moving the beam spot
laterally a small distance on the cantilever while scanning until interference fringes are minimized.
Be careful not to move the beam off the cantilever or feedback may be lost.
Note: Optical interference is essentially eliminated by using phase detection
or
frequency modulation.)
13.6 Advanced Topics
13.6.1 Lift Scan Height and Magnetic Imaging Resolution
The most important parameter affecting imaging resolution is the 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; conversely, 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 Figure 13.2e has a resolution limited by the
100nm 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, 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 Figure 13.6a. Note the fine magnetic structure that is not visible in Figure 13.2e.
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 “Setpoint” on page 238 below). It