Atomic force microscopy (AFM) has become a useful tool across many areas of science to determine the atomic structure (and the properties under certain scenarios) of the surface of a bulk material or the structure of an ultra-thin material. Aside from the conventional modes, AFM can be adapted to measure the electrical conductivity of a sample. This is known as conductive AFM (C-AFM). In this article, we look at this AFM variant mode and how it works.
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There are many different types of AFM available today. In these different modes, the basic operating principles are often the same, but the type of tip used is generally the differentiating factor. A different tip can retrieve various types of information from a sample (compared to standard AFM) and can be used with numerous types of materials. Here, we look at how C-AFM works and the applications it can be used in.
What is Conductive AFM (C-AFM)
C-AFM is a type of AFM mode that uses a sharp conductive tip (often a conductive diamond-coated silicon tip). Unlike standard AFM, which backs out the topography by (most commonly) tapping the surface of a material to determine the relative position of atoms using van der Waals interactions, C-AFM applies a ramped voltage between the tip and the sample. This enables a topographic map to be constructed based on the localized conductivity of atoms.
Whilst it is a technique used for mainly measuring conductive samples, the applied voltage does measure whether the area being analyzed is conductive or not, and this can be used to distinguish between conductive and non-conductive regions of a material- something which is very useful for measuring the properties of material, rather than determining its molecular structure.
As such, it is a method that is often applied to quality control and property analysis rather than structural/elemental analysis. This differs from standard AFM which is mostly concerned with determining the atomic structure of a surface (unless a colloidal probe is used).
How C-AFM Works
The operational mode of C-AFM is more reminiscent of scanning tunneling microscopy (STM) than it is of standard AFM, although it does apply principles from both methods. STM employs a voltage bias between the tip and the surface, much in the way that C-AFM does.
With relevance to standard AFM, this is an entirely different way of interacting with the surface, however, where AFM and C-AFM are similar is in the post-interaction mechanisms and relative position deduction, i.e., both AFM and C-AFM determine the relative position of the atoms by the laser deflecting off a cantilever beam onto a position-sensitive photodiode (PSPD).
Both the tip and the sample start in an electronically ground state. The tip is placed above the surface of the sample, and as it scans the surface, a voltage bias is applied through the tip to the sample. The relative topography of the sample is deduced in the same way as standard AFM.
However, the current flowing between the tip and the surface is also measured using a current-to-voltage amplifier, and this enables the measurement of the electrical conductivity to take place. A constant force is maintained between the tip and the sample, and this allows both the topography and electrical map to be determined simultaneously. Current amplifiers can also be employed to reduce the electrical noise from the sample.
This approach is often considered to be better than STM because a rough topography can sometimes cause current fluctuations – and STM cannot account for whether these fluctuations are due to an actual change in the current or whether the topography influences them. The dual measuring/imaging mode of C-AFM does not suffer from these issues.
Applications of C-AFM
C-AFM has been around for a long time now. As such, it has found its way into a few applications over the years. One of the oldest uses is in the monitoring of thin-film dielectric materials in electronics and nanoelectronics.
However, it is now also used to determine the thickness of solid-state oxide materials, monitor local phenomena such as charge trapping, trap-assisted tunneling, and stress-induced leakage currents, as well as monitoring the effect of any locally induced changes to a material such as thermal annealing, doping, and irradiation.
Outside of its monitoring capabilities, C-AFM can also be used to alter the properties of a material by applying a localized electric field. This application is useful to examine areas of a sample are susceptible to dielectric breakdown. There also more specialized applications of C-AFM which only apply to this specific mode, such as acting as a type of localized photolithographic instrument by applying a bias-assisted local anodic oxidation (LAO) process to a material.