Conductive AFM is a powerful current sensing technique for electrical characterization of conductivity variations in resistive samples. It allows current measurements in the range of hundereds 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 It 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 artifacts associated with irreversible changes in the tip shape or coating. This is an important consideration when interpreting ORCA measurements.
Figure 1. ORCA cantilever holder.
Figure 2. ORCA sample mount.
Data in this application note was made using a gain of 5.15x108 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 the Gain Selection Chart. The chart illustrates Johnson Noise and the relevant current ranges for a transimpedance amplifier that is 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, at higher gains, Johnson noise dominates. Practical applications will also involve some other noise sources including mains. The size of this contribution will depend on details of the sample connections.
Figure 3. Gain Selection Chart
The figure to the left 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 conductance AFM measurements. The contact mode topographic image on top shows a relatively uniform grainy structure. The current image in the middle, however, shows patches of high conductivity surrounded by very low conductivity regions. 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 the current image. The tip was positioned in the center of the colored circles using the MFP-3D’s “pick a point” force curve interface. With the tip in position, the bias voltage was swept from -5 to 5 volts and the response current measured. The bottom graph shows the resulting current-voltage (IV) curves. The conductivity curves in this figure are consistent with the contrast observed in the current image. Specifically, the conductivity is highest at the position marked with the black circle, in between at the red, and lowest at the blue. This is just one example measurement for ORCA. For additional examples, you may download the complete ORCA application note.
Figure 4. Topography (top), current image (middle), and corresponding IV curves (bottom) of Europium-doped ZnO sample at a bias of 1.5 volts, 2µm scan sample courtesy of the Krishnan Lab, Univ. of Washington.
This information has been sourced, reviewed and adapted from materials provided by Asylum Research - An Oxford Instruments Company.
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