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Electric Techniques

Surface Potential Detection—Theory

296 Dimension 3100 Manual Rev. D

16.3.6 Optimize Tune in Interleave

Find optimal lift height. Use View > Sweep > Cantilever Tune to open the tune window while

engaged. Choose a Tip Offset equal to the Lift Height. Make any changes to the Drive frequency

and Drive phase to adjust the main tune (tip #3). In the Sweep Controls panel, select the

Interleave Controls button. Activate (green button) the Drive frequency and Phase, and Analog

2. With Analog 2 set to operating voltage, set the frequency to the center of the peak and zero the

phase.

16.3.7 If Voltage is Needed, Use Analog 2 When Possible

It is often very difﬁcult to get a high quality TappingMode image with an electric ﬁeld present.

Analog 2 has an important advantage over an external power supply – it can be used to create an

E ﬁeld only in interleave and be set to 0V in the main line (> v4.31). Its range is limited to +/- 12V.

16.3.8 Try Uncoated Si Tip

EFM has been successful with a variety of tips, including standard Si tips with no metal coating

(fesp, tesp, ltesp). The Si is highly n-doped and is often conductive enough for EFM measurements.

Metal coatings make the tip less sharp, decreasing lateral resolution. The standard tips also seem to

give cleaner images over longer periods of time. There is likely a reduction in sensitivity to small

E ﬁelds with the decreased conductivity.

16.4 Surface Potential Detection—Theory

Note: Surface potential detection EFM is only possible using the one of the extender

modules or the NanoScope IV controller. This section does not apply to

microscopes which are not equipped with the Basic or Quadrex Modules, or the

NanoScope IV controller.

• The Basic Extender Module allows measurement of local sample surface potential. This

is similar to techniques called Scanning Maxwell Stress Microscopy and Kelvin Probe

Microscopy. Surface potential detection is a two-pass system where the surface

topography is obtained in the ﬁrst pass and the surface potential is measured on the

second pass. The two measurements are interleaved, that is, they are each measured one

line at a time with both images displayed on the screen simultaneously.

A block diagram of the surface potential measurement system is shown in Figure 16.4a. On the ﬁrst

pass, the sample topography is measured by standard TappingMode. In TappingMode the cantilever

is physically vibrated near its resonant frequency by a small piezoelectric element. On the second

pass, the piezo that normally vibrates the cantilever is turned off. Instead, to measure the surface

potential, an oscillating voltage is applied directly to the cantilever tip. This creates an

ωtcos

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