There is now an increasing use of the electrical characterisation capabilities of AFM in nanotechnology-based fields such as energy harvesting, organic/polymer-based electronics, semiconductors etc. Flexible electronics based on organic compounds are gaining popularity as soft electrical materials.
Currently, two different methods are used for Conductive AFM (C-AFM) measurements of soft materials such as conductive polymers.
Sinusoidal Regime Method
In the Sinusoidal Regime method, the cantilever is mechanically excited in the range of 100–2000 Hz i.e. well below its natural resonance frequency. The tip of the cantilever interacts with the substrate periodically during the bottom part of its sinusoidal displacement. This method allows easy imaging of soft samples by controlling the amplitude of the movement of the tip. However, some quantitative measurements cannot be performed (electrical, thermal, etc…) because the force exerted by the tip on the sample is variable.
Linear Regime Method
The Linear Regime Method is based on the force versus distance spectroscopy curves. In this quasi-static approach, the cantilever follows an approach-retract cycle towards the sample with constant velocity. Hook’s Law is used to select the force to be exerted. F=k*z, where F is the applied force, k is the cantilever constant and z is the cantilever deflection relative to the rest deflection position. However, this method is slow with an approach-retract cycle of 1 sec. At this rate, it can take up to 3 days to measure a standard 512 x 512 image.
A new approach to resolve these issues is the Soft ResiScope mode, introduced by AFM manufacturer Concept Scientific Instruments, which combines fast point contacts and constant force. In a previous article, the most advanced module for C-AFM, the ResiScope was discussed. The ResiScope incorporates high dynamic range and smart electronics to prevent undesirable effects such as the Joule Effect, Probe-induced Local Oxidation and Bimetallic Effect. However, it poses a risk of damage to soft samples such as conductive polymers due to contact mode imaging. This is overcome by the Soft ResiScope that combines advanced capabilities of the ResiScope with an intermittent contact mode.
The Soft ResiScope uses a constant force control to stay in contact with the sample for a short period of time to measure resistance and current under ideal conditions. The tip is then retracted and moved to the next measurement point. The Soft ResiScope mode can be operated regularly at scan rates of 1 line/s with the same type of cantilevers as those used for standard C-AFM or ResiScope mode (1–40 N/m spring constant range, 50–300 kHz frequency range).
In any intermittent contact mode, the deflection feedback must be fast and robust to guarantee a quantitative and reliable electrical measurement. One way to test the reliability of the results is to analyse a well-known sample using both the contact and intermittent contact methods of electrical measurements. This recently published book by Mario Lanza constitutes a comparison of ResiScope and Soft ResiScope techniques using Static Random Access Memory (SRAM) sample (Chapter 12, Pacheco and Martinez. 2017).
The figure above shows topography (left) and resistance (right) from the SRAM sample imaged by Soft ResiScope (top half) and ResiScope (bottom half). No significant difference between the electrical measurements is visible.
In addition, electric behaviour can be accurately determined by plotting I-V spectroscopy curves for the regions of interest.
The higher dynamic range in real time of ResiScope and greater tip and sample protection offered by Soft ResiScope offer significant advantages over the conventional C-AFM to measure current and resistance with nanoscale resolution. Currently, Soft ResiScope is the best choice to perform conductive measurements on soft samples.