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Electric Techniques

EFM Troubleshooting/Pointers

Rev. D Dimension 3100 Manual 295

16.3 EFM Troubleshooting/Pointers

16.3.1 Use Low Setpoint When Tapping in Electric Field

If a voltage is applied during the main TappingMode line (as well as the interleave line) either

through Analog 2 or an external source, be careful to ensure proper operation. It is possible to enter

a mode in which the tip is never actually touching the surface, even though the amplitude setpoint is

being met. As the feedback moves the tip closer to the sample surface, the resonant frequency of the

tip is shifting (see Figure 16.2q). The closer the tip is to the surface, the larger the force gradient

and the more the resonant frequency shifts. It is possible for the resonant frequency to shift enough

so that the amplitude setpoint is met without the tip touching the surface. The tip can be a

significant distance away from the sample. The height image has comparatively poor resolution as

does the corresponding electric field gradient image. A significantly lower amplitude setpoint is

needed to really tap the surface in the presence of a strong electric field.

16.3.2 Verify Electric Field at Surface

If applying Analog 2 voltage to tip or sample to generate an E-ﬁeld, set the phase channel Realtime

planeﬁt to None. While in Scope Mode, vary the interleave Analog 2 value and verify that phase

signal shifts accordingly.

16.3.3 Fine Tune Lift Scan Height

Set the lift height to be as small as possible without hitting the surface, because lateral resolution of

EFM improves with decreased tip/sample separation. The minimum lift height depends on the

roughness of the sample, the difference between the amplitude setpoint and free air amplitude, and

the quality of the height image. Hitting the surface usually produces phase data with extremely high

contrast (either black or white pixels).

16.3.4 Fine Tune Interleave Drive Amplitude

Decrease the drive amplitude in the interleave line to further minimize the lift height. Be aware that

the S/N ratio also decreases with decreasing drive, so there are diminishing returns after a point.

16.3.5 Optimize Tune in Vicinity of Surface

Select View > Sweep > Cantilever Tune to open tune panel while engaged. A tip offset of 30nm

usually works well for smooth surfaces (if the tune has a noisy straight area in the middle instead of

a smooth peak, the tip is hitting the surface and a larger tip offset is needed typically for rough

surfaces). Adjust drive frequency to center of peak or just to the left. If phase data is to be collected

during the main line, zero the phase at the drive frequency.

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