Intermodulation Conductive Atomic Force Microscopy

Introduction to ImCFM

Hardware setup

Transimpedance amplifier

ImCFM is compatible with any transimpedance amplifier, but we recommend Femto DLPCA-200. Settings on the amplifier should be: Input shield grounded (AC bias is applied externally to the shield), full bandwidth, output DC-coupled. Suitable gain can differ very much depending on conductivity of sample and tip.

Cantilever

Soft cantilever, suitable for contact mode AFM, with conductive coating on the probe side.

Setting up the measurement

• On AFM: Set AFM to operate in contact mode. This measurement is truly a contact mode measurement. The AFM should perform feedback on the static deflection. Therefor make sure to turn off any signal access module or script that replaces the true deflection signal which you might otherwise use for intance in ImAFM, ImEFM or ImCFM Tapping.

• On AFM: Approach the cantilever to the surface.

• On AFM: Lift away from the surface

  Note

  In this chapter “lift away” refers to moving few nanometer - few microns away from the surface so that there is no ohmic contact between the tip and sample by retracting the z-piezo. In some AFMs the best way to do this is to manually adjust the feedback setpoint so that the cantilever retracts while having a limited z-range / z-limit.

• Click the ImCFM icon in the tool bar. If the icon is not visible click menu Advanced->ImCFM. This requires licence key to include ImCFM mode.

  

• Choose pixel rate (df), oscillation voltage (Vac) and if needed DC-bias point (Vdc). Set compensation voltage (Vcomp) to zero inially to run without compensation.

• Set an initial gain on the transimpedance amplifier.

  Note

  There is no automatic connection between the actual gain setting on the amplifier and the Gain setting in the GUI, it is up to the user to ensure that the GUI is manually updated to reflect the actual gain.

• Click “Setup drive”. This will apply your selected oscillation voltage to Out2 and measure the parasitic current.

• Ensure that the amplifier is not in overload due to parasitic capacitance (red LED on the transimpedance amplifier), if so lower the gain.

  Note

  Any time the gain is changed on the transimpedance amplifier you must remeasure the parasitic current (current while lifted away from the surface) since the program assumes it is measured at the same gain setting as the remainder of the image.

• Optional: Apply compensation voltage to nullify parasitic.

  • Click “Auto compensation” to automatically measure the parasitic and apply a counter voltage on the shield.

  • It may now be possible to adjust the gain to higher values for better signal-to-noise ratio, if you do change the gain remember to also run “Remeasure parasitic”.

• Click “Start”.

• On AFM: Choose scan rate as close as possible to the Scan Rate or Half Scan Time displayed in the bottom of the control panel.

• On AFM: Engage to the surface and start to scan.

• Ensure again that the amplifier is not in overload due to high conductance, if so reduce gain until the overload LED turns off, lift away from the surface and remeasure parasitic before re-engaging.

Analyzing and viewing the data

Images

While scanning or when viewing a perviously scanned file one image, corresponding to some parameter calculated from the I/V or capacitance curve will be displayed. The View group controls which parameter is plotted in the main figure. The Analyze contol can create additional images to plot, but the default images are:

• G linear conductance the linear component (fundamental Fourier component) of the galvanic current converted to conductance. For a purely linear I/V curve it corresponds to the conductance.

• C linear capacitance the linear compeonent of the displacement current converted to capacitance. For a flat C/V curve it corresponds to the capacitance.

• IG average the average galvanic current (DC component).

• IG at 0V slice of galvanic current-volume at bias = 0V.

• IG at max V slice of galvanic current-volume at the maximum bias in the measurement (Vac + Vdc).

• IG at min V slice of galvanic current-volume at the minimum bias in the measurement (-Vac + Vdc).

I/V curves

Select the Inspect single pixel tool (blue cross) in the image toolbar and click on any pixel to view the I/V and C/V curves at that pixel. Right-click to remove the cross closest to the mouse cursor. The Smooth pixel tool (blue circle) creates curves calculated from a Gaussian average of the neighboring pixels to reduce noise, while the Average tool (lasso) can be used to draw an area and display the curves for the average in the drawn region.

Analysis settings

The Analyze group is used to set parameters which affect how or what analysis is performed. The following settings control how the raw lockin spectrum is used to calculate galvanic and displacement currents used in all analysis:

• Gain 10^x - must be set to reflect the gain of the transimpedance amplifier. Note that the software does not physically set the gain on the amplifier, the user has to manually ensure the gain in the GUI matches the actual gain on the amplifier for correct values. If the gain was incorrect when the image was stored, the correct gain value can still be used when performing analysis in postprocessing (provided the user made notes of the correct gain).

• Nr harmonics - selects the number of harmonics used in the analysis. With too few harmonics sharp changes in I/V curve can not be reconstructed, however with too many harmonics the curves can become noisy. Consider making use of the Spectrum tab in the Signal inspector to determine a suitable number.

• Oversampling - the amount of interpolation that is performed to display a smooth I/V curve. The number of independent datapoints in one I/V curve is equal to the the Nr harmonics, however additional oversampling can be used to interpolate a contineous curve.

The following controls are used to create new images:

• Recalculate standard images - recalculates all the standard images (see below), useful for instance if any of the settings above were changed after the scan was done. Only possible on complete images.

• Slice at voltage - selects the voltage for slices below.

• Slice IG - create an image of the galvanic current at the selected slice bias.

• Slice C - create an image of the capacitance at the selected slice bias.

• Voc - create an image of the open-circuit voltage (bias at which galvanic current crosses zero).

Troubleshooting

• Disconnect “tip coax” from “Active guard adapter”

  • Check that the tip coax shield (pink) is not connected to the tip (green).

  • Check that the tip coax shield (pink) is not connected to signal ground (yellow).

  • Check that there is a low ohm connection between the coax core (green) and the tip-side of the cantilever chip.

• Disconnect also the “sample coax” from the MLA

  • Check that the core of the sample coax (red) is not connected to the shield of the same coax (yellow).

  • Check that the core of the sample coax is connectred to the sample.