Electrical Measurements on Soft Sample Using AFM Intermittent Contact

Many AFM applications require conductive and resistance measurements at the nanoscale. The first conductive techniques based on contact mode were established with limited dynamic range. However, two additional capabilities were required by researchers:

  • large dynamic of measurement (8-10 decades).
  • Soft interaction to handle delicate samples such as biological samples and polymers.

This article discusses an innovative technique for current/resistance measurements on all types of samples, including delicate samples.

ResiScope II

The key point is the resistance/current range of measurements. The ResiScope II has been addressing the dynamic issue for the past several years, providing resistance/ current measurements over a range of 10 decades (102 to1012Ù). The principle of the ResiScope II is illustrated in Figure 1. Performing measurements in contact mode is suitable for hard samples such as semiconductors. However, contact mode techniques could cause damage to delicate samples such as biological samples or conductive polymers.

ResiScope II principle

Figure 1. ResiScope II principle

Instrumentation

CSInstruments developed a method involving a Nano-Observer AFM microscope equipped with a unique "Soft ResiScope" module to perform measurements on a soft sample. The soft alternative mode allows the AFM probe to make contact with the sample for a short period of time under a constant force, enabling the ResiScope II to perform current/resistance measurements under the best possible conditions. This is followed by retraction of the tip to perform the next step (Figure 2).

Soft ResiScope principle

Figure 2. Soft ResiScope principle

Experimental Procedure and Results

The new "Soft ResiScope" non-destructive method was validated by comparing it with traditional oscillating mode on a polymer blend sample, which is fragile in nature. The results were in good agreement with each other (Figure 3). As can be seen in Figure 3, the traditional oscillating mode caused damage to the sample surface (blue square), which obtained a scratch by the tip.

Areas of a soft sample (polymer blend) measured in contact mode (blue area), then oscillating mode (green area), and finally in "Soft ResiScope" mode (red area).

Figure 3. Areas of a soft sample (polymer blend) measured in contact mode (blue area), then oscillating mode (green area), and finally in "Soft ResiScope" mode (red area).

To evaluate the quantitative measurements performed by the "Soft ResiScope", they were compared with the contact mode ResiScope measurements on a standard electrical sample such as SRAM (Figure 4). The two methods provided the same results for topography and resistance signals. In addition, identical results were obtained for both Standard (contact) and "Soft ResiScope" modes while performing a cross section analysis on resistance signal (Figure 5).

Resistance measurements with "Soft ResiScope" mode (red area) compare with contact mode (blue area) on SRAM sample.

Figure 4. Resistance measurements with "Soft ResiScope" mode (red area) compare with contact mode (blue area) on SRAM sample.

Cross section analysis on resistance measurements shows identical results between Soft and Standard ResiScope modes.

Figure 5. Cross section analysis on resistance measurements shows identical results between Soft and Standard ResiScope modes.

To conclude this study, an organic solar cell sample (P3HT) was measured. Figure 6 shows the characterizations made on the solar cell sample. Performing these measurements in contact mode is challenging task. The sample was free from any surface damage as indicated by the topography signal. However, there was some variation in resistance observed on different areas with high spatial resolution as indicated by the resistance signal.

Soft ResiScope, Intermittent Mode, Organic solar cell sample (P3HT), 3µm

Figure 6. Soft ResiScope, Intermittent Mode, Organic solar cell sample (P3HT), 3µm

Conclusion

The unique combination of ResiScope large dynamic range and the intermittent contact mode (Soft ResiScope mode) enables electrical measurements on soft samples with a large dynamic range. The breakthrough "Soft ResiScope" mode capabilities have been fully demonstrated on soft samples. The validation of quantitative resistance/current measurements performed by the new method paves the way for electrical characterization on types of samples, including delicate samples.

This information has been sourced, reviewed and adapted from materials provided by CSInstruments.

For more information on this source, please visit CSInstruments.

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