Conductive AFM is a powerful current sensing technique for characterizing conductivity variations in resistive samples. It allows current measurements in the range of hundreds of femtoamps to nearly a microamp. Conductive AFM can simultaneously map the topography and current distribution of a sample. It is a measurement useful in a wide variety of material characterization applications including thin dielectric films, ferro-electric films, nanotubes, conductive polymers, etc.
How The Conductive AFM Works
The ORCA module consists of a specially designed cantilever holder that includes a transimpedance amplifier. The gain of the amplifier can be chosen by the user. Standard values range from 5x107 to 5x109 volts/amp. The cantilever holder is used with conductive AFM probes to make the measurement. The easiest imaging mode for measuring the localized conductivity of a sample is to combine the current measurements with contact mode AFM imaging. All images in this application note were acquired using contact mode with a PtIr coated Electri-Lever (Olympus), with a nominal spring constant of 1-2N/m and good wear characteristics. Coated cantilevers are vulnerable to imaging artefacts associated with irreversible changes in the tip shape or coating. This is an important consideration when interpreting ORCA measurements.
Current AFM Measurements
Data in this application note was made using a gain of 5 x 108 volts/amp on the initial stage (see ORCA-58 in Figure 1). On the MFP-3D, the output of the ORCA was digitized with one of the auxiliary 100kHz ADCs and then digitally filtered at 1kHz. The measured RMS noise for these settings was 0.5pA, consistent with the Johnson Noise performance predicted in Figure 1. Figure 2 shows an example image made at a 1.5 volt bias. The sample is a 10nm thick film of Europium doped ZnO. This is a relatively high resistivity sample, particularly challenging for 2μm scan.
Figure 1. Gain selection chart. Johnson Noise and the relevant current ranges for a transimpedance amplifier digitized at 16 bits. At a gain of nearly 1010 volts/amp, Johnson noise is equivalent to the best resolution of a 16-bit ADC. At smaller gains, the main limitation is the resolution of the ADC.
Figure 2. Topography (A) and current (B) image of a Europium-doped ZnO sample at a bias of 1.5 volts, 2μm scan. Corresponding IV curves (C) recorded at three specific positions from those indicated in B. The curves are consistent with the current contrast observed in 2B. Specifically, the conductance is highest at the black location, in between at the red, and lowest at the blue. Sample courtesy of the Krishnan Lab, Univ. of Washington.
Conductance AFM Measurements
The contact mode topographic image “A” shows a relatively uniform grainy structure. The current image “B”, however, shows patches of high conductivity surrounded by very low conductivity regions (see Figure 3 for a higher resolution scan). The NPS™ Nanopositioning closed loop sensors on the MFP-3D make it possible to reproducibly position the cantilever at a point of interest as shown by the colored circles in Figure 2B. The tip was positioned in the center of the colored circles using the MFP-3D’s “pick a point” force curve interface. The bias voltage was then swept from -5 to 5 volts and the response current measured. Figure 2C shows the resulting current-voltage (IV) curves. The conductivity curves are consistent with the contrast observed in Figure 2B. Specifically, the conductivity is highest at the position marked with the black circle, in between at the red, and lowest at the blue.
Figure 3. High resolution topography (l) and current (r) at a bias of 1.5 volts, 50pA scale, 2μm scan. Sample courtesy K. Krishnan Lab, University of Washington.
Current as a Function of Loading Force
Using the flexible IGOR Pro software interface and the MFP-3D Controller, ORCA can also be used for a variety of transport measurements. Figure 4 shows the effects of loading force on the IV curves. For this experiment, the cantilever was positioned at an XY point on the sample. The cantilever deflection was maintained at a constant value with a feedback loop. At the same time, the tip bias was swept sinusoidally at an amplitude of 1 volt and a frequency of 10Hz. The measured current was then plotted vs. the drive voltage, see Figure 4. Not surprisingly, as the loading force is increased, the conductance increases. There is also more noise in the signal at smaller loading forces.
Figure 4. IV curves as a function of loading force on a Europium doped ZnO sample. Each curve was recorded at the same X-Y position on the sample as a function of cantilever load.
Combined Force and Current Measurements
Figure 5 shows another measurement where the current flow is a function of the load at two fixed biases. There is an asymmetry in the responses at the different biases consistent with the IV curve measurements in Figures 2C and 4. There are also some additional features in these curves. First, the conductance seems to be highest at a point where there is a step in the contact portion of the force curve. There is also a difference between the extension and retraction curves. These features are consistent with the contact resistance of the sample varying as a function of load.
Figure 5. Current (above) and force (below) as the cantilever extends towards the ZnO sample surface and retracts away. The blue curves show the positively biased response while the red shows the negatively biased response.
The ORCA conductive AFM option for the MFP-3D provides low-noise, flexible transport measurements at the nanoscale. The flexible software environment enables a variety of standard measurements to be made as well as allowing the researcher to define their own experiments.
• The standard ORCA cantilever holder has a gain of 5 x 108 volts/amp (~1pA to 10nA).
• Optional gain is 5 x 107 volts/amp (~10pA to 100nA). Other values are available upon request.
• Includes module and cantilever holder.