PFM Near Contact Resonance

Contact resonance is the frequency at which a system consisting of the AFM tip with the cantilever in contact with a piezoresponse sample (at a particular deflection setpoint) achieves resonance. The determination of the contact resonance is very simple. One needs to use the Generic Lock-In interface and sweep the drive frequency while monitoring the amplitude and the phase of the PR signal. Using the PPLN sample discussed in previous sections, determine the contact resonance and perform measurements near resonance.

1. Set up the microscope to operate with the high-speed lock-in at 80 kHz drive frequency, 10 V drive amplitude, about 100 degrees drive phase with the Vertical 16X Gain Enabled.
2. Capture an image with the above parameters to compare the results with the microscope operating at contact resonance. Figure 2 shows such an image collected with the high-speed lock-in operation.

Figure 1: HS PR Amplitude and HS PR Phase Near Contact Resonance

Figure 2: Data collected on a PPLN sample with SCM-PIT cantilever operating at 80 kHz, 10 V AC drive frequency with a lock-in bandwidth of 500 Hz using the High-Speed Lock-In on the main line.

3. To determine the contact resonance frequency, select Generic Sweep from the RealTime menu and set the tip offset to zero.
4. Set Channel 1 to HS PR Amplitude and Channel 2 to HS PR Phase and the Sweep Output to HS PR Drive Frequency. Change the Pause Between Steps parameter to 20000 μs.

5. If you are using a (recommended) MESP-RC probe, change the Sweep Center to about 800 kHz and increase the Sweep Width to about 500 kHz. The choice of these parameters will vary depending on the cantilever substrate combination. For the PPLN sample and the SCM-PIT cantilever, try 350 kHz and 200 kHz, respectively.
6. The PR amplitude and the PR phase channels should already be showing the contact resonance curve as illustrated here:

Figure 3: HS PR Amplitude (top) and HS PR Phase (bottom) near contact resonance using an SCM-PIT probe

7. Finally, use Offset to center the frequency at the peak of the resonance and determine the contact resonance frequency. It also helps to reduce the sweep width to center the sweep at contact resonance.

Figure 4: Contact resonance frequency of the system with an SCM-PIT probe determined to be 338.75 kHz

The contact resonance of the PPLN sample with SCM-PIT probe, shown in Figures 3 and 4 was determined to be at 338.75 kHz. Note that this frequency will differ from one cantilever to another, and from one substrate to another. Each cantilever, substrate and system combination will have a different contact resonance. For an MESP-RC probe, the contact resonance is expected to be around 900 kHz.

In the Generic Sweep window, Offset the drive frequency to select a frequency to the left of the contact resonance for operation. In the example below (SCM-PIT on a PPLN sample), a frequency of 337.1 kHz was chosen for capturing the PR data.

Figure 5: Selecting a frequency for sub-contact resonance Piezoresponse measurements

Figure 6 shows Piezoresponse data captured at 337.1 kHz with 10 V AC drive amplitude, 500 Hz lock-in bandwidth and 100 degrees drive phase. By operating near or at the contact resonance, the amplitude of the Piezoresponse measurement becomes significantly larger than at other frequencies. From Figure 1, one can estimate that the base amplitude of oscillation was about 10 mV. This is also evident from captured data shown in Figure 2. However, by operating at a frequency near contact resonance, an amplitude of about 80 mV was measured. Amplification of the Piezoresponse signals near resonance is one of the main reasons why the microscope is used in this mode.

Figure 6: Piezoresponse data captured sub-contact resonance at 337.1 kHz

The next section presents a discussion on operating the microscope super-contact resonance.

In the Generic Sweep window, Offset the drive frequency to select a frequency to the right of the contact resonance for operation. In Figure 7, a frequency of 340.38 kHz (with an SCM-PIT probe on a PPLN sample) was chosen to operate on the right side of the contact resonance. The Zero Phase button in the Generic Sweep view can be used to set the drive phase offset make the measured phase difference zero.

Figure 7: Selecting a frequency for super-contact resonance Piezoresponse measurements

Figure 8 shows data collected at super-contact resonance with a drive frequency of 340.38 kHz, 10 V AC sample drive, 500 Hz lock-in bandwidth and -59.57 degrees drive phase. Note the reversal of phase contrast from Figure 6 to Figure 8. This is because there is a phase change of 180 degrees while going over a resonance. Other observations about the amplification of the amplitude signal remain the same.

Figure 8: Piezoresponse data captured super-contact resonance at 340.38 kHz

Although operating at contact resonance has advantages in terms of the signal-to-noise ratio, the interpretation of the data obtained can be difficult. Hence, most users first operate at frequencies away from contact resonance. Then, having understood the material properties away from resonance, they try to correlate those to measurements near the resonance. The NanoScope V Controller and NanoScope software provide a vast number of options for you to explore your sample piezoresponse characteristics. In exploring one such option, shown in Figure 9, the microscope was operated with the High-Speed lock-in on both the main and the interleave line. The frequencies of operation were 337.72 kHz for the main line and 341.52 kHz for the interleave line. These were to the left and to the right of the contact resonance frequency. The applied AC bias was 10 V and the lock-in bandwidth was 500 Hz. The data clearly shows a reversal in phase contrast.

Figure 9: Data obtained with simultaneous main (top) and interleave (bottom) operation near contact resonance using the high-speed lock-in