AFM is capable of mapping different electric properties of materials to topography images. These data can be used for analysis of the structure and composition of heterogeneous samples, as well as for quantitative characterization of individual grains or defects on surface.

Electric properties of a sample can be mapped using probes with conducting coatings, when AC or DC bias is applied between the tip and the sample. Contact mode or lift mode can be used for this purpose.

Electric Measurements in Contact Mode

A number of specific AFM experiments such as studies of piezoresponse (Piezoresponse Force Microscopy), capacitance (Scanning Capacitance Microscopy) and measurements of DC/AC current (conducting AFM, tunneling AFM, current-sensing AFM, etc) are performed in the contact mode AFM.

Contact electric techniques generally require the absence of a gap between the tip and the sample and sufficient area for ohmic or capacitive contact, which is related to higher tip-sample forces and cantilever spring constants (up to 50 N/m) than in regular contact mode. Relatively soft (0.03 - 0.3 N/m) conducting cantilevers are optimal for high lateral resolution Cantilevers with higher spring constant are better for quantitative measurements.

The measured currents can lie in the range of 100 fA for Tunneling AFM to 1A for SSRM. However, it is generally not recommended to exceed the 200-nA current limit for coated probe tips. The Pt coating offers the highest resistance to wear and electromigration firmness, as well as minimum tip radius of 25 nm.

Electric Measurements in Oscillatory Modes

The examination of electric properties is performed in the single-pass or two-pass (lift-mode) techniques. Traditionally, the probes of stiffness ~ 5 N/m with conducting coatings are used for such measurements. In some cases, doping of the Si wafer, which was used for microfabrication of the probes, is sufficient to provide reasonable force response to electrostatic interactions with a sample.

Lift mode measures topography during the first pass and another electric or magnetic property of the sample during the second pass. This minimizes interference between the two kinds of data. The spring constant and resonant frequency of the cantilever should be chosen to provide stability in tapping mode and high sensitivity to weaker forces on the second pass.

In most cases the NSC18 series is a good place to start. Among the conducting coatings, the Pt coating offers the minimum tip radius of 20 - 25 nm and the highest resistance to wear. The coating is applied to tip side only; the cantilever backside may have additional Al coating as an option for better reflectance.
 

Among the serious limitations of Electrical Force Microscopy are the sensitivity and signal to noise ratio. Enlargement of the probe tip radius can increase the area of electrical interaction and improve these parameters, but this improvement leads to a loss of resolution in both topography and EFM imaging. The novel DPE probe consists of a special structure of conductive layers, which provides a more stable electrical signal and less noise with little impact on resolution.

Figure 1 compares the topography and EFM scans of a semiconductor structure imaged by a standard NSC14 probe with Ti-Pt coating and a new DPE probe. The signal to noise ratio for the scans taken with the NSC14 is about 3-4, while it is more than 10 for the scans using the DPE probe.