Advanced SPM Modes

Conductive AFM

Probing the Local Electronic Structure of a Sample’s Surface

Although bulk property measurements once provided accurate information for material processing, in recent years, technological advances have placed many demands on materials such that bulk property measurements no longer adequately describe them. This is especially true in the semiconductor, solar, and hard drive industries where device and critical dimensions continue to decrease year after year. In these cases, measuring local properties can help to increase the accuracy of the measurement over bulk property values. Atomic Force Microscopy plays a major role in the measurement of local surface properties. Using Conductive AFM, one can simultaneously measure topography and current distribution (conductivity) over the sample surface by using the AFM tip as a nanometer-scale electric probe.

The operation of the Conductive AFM is similar to Scanning Tunneling Microscopy (STM), in which a bias voltage is applied between tip and sample and the tunneling current between the two is measured. The main difference between the two modes is the type of probe used. STM uses asharpened metallic wire/tip whereas Conductive AFM uses a conductive cantilever. In Conductive AFM, during imaging, a conductive cantilever scans the sample surface in contact to produce a topography map, the electric current between the cantilever and the sample is measured simultaneously (Figure 1). While the topography is acquired using the deflection signal of the cantilever, the electric conductivity is measured through an electric current amplifier (Figure 2). Since the electric current through the cantilever can be as small as a pico-ampere, the current amplifier is chosen so that the electrical noise is suppressed on the order of a femto-ampere.

To achieve the low electric noise level required for Conductive AFM, Park Systems utilizes current amplifiers. Three Conductive AFM sub-modes are available and named according to the current amplifier being used. These submodes are: ‘Internal Conductive AFM’, ‘Variable-Enhanced Conductive AFM (VECA)’, and ‘Ultimate Low-noise Conductive AFM (ULCA).’ Internal Conductive AFM, which uses an embedded amplifier, measures general samples with a current range up to 10 μA and 1 pA noise level. VECA, with its variable gain setting, is adaptable to wide range of conductivity of up to 10 μA with the noise level of 0.3 pA[1] (Figure 3). ULCA, from its ultra low noise circuit design, achieves less than 0.1 pA noise level, which is effective in observing variations on highly insulating samples.

For detailed investigations of the sample properties, Conductive AFM can be used to measure the I-V relationship of a surface. Since the contact size of the AFM cantilever tip can be as small as a few nanometers, the local variation of the electrical property, which is unseen in the macroscopic level, can be observed by the I-V curve (Figure 4).

Specifications