One of the standard measurement modes of an AFM has been the analysis of the electrical conductivity of a sample surface (conductive AFM or C-AFM). Until now, the limiting factor has been the larger radius of the specially coated, conductive cantilever tips (20-30 nm) compared with normal, uncoated tips (< 10 nm). The result has been much poorer lateral resolution both of the topography and the conductivity signal. Our partner GETec is taking a new path by using self-sensing and conductive cantilevers which feature a different design.
Fig. 1 shows a cantilever with platinum tip, applied with the FEBIP (focused electron beam induced processing) technique. Typically, these have a radius < 20 nm. In the image magnification the tip is seen as a little needle pointing upwards. During FEBIP, a nanogranular film, consisting of platinum nanoparticles embedded in a carbon matrix (Pt(C) film) is modified by using the electron beam of an SEM. Under the beam radiation, carbon is oxidized by the oxygen contained in the water that remains on the surface and dissolved out of the matrix. This compresses the matrix and shortens the distance between the nanoparticles.
Furthermore, the remaining carbon changes its structure towards a conductive, graphene-like morphology. These two processes (shortening of the distance between Pt particles and changes in the carbon structure) significantly increase the conductivity of the entire Pt(C) film. This increase can easily be monitored and controlled directly in the SEM by using the AFSEM. Contaminations are basically ruled out, as the entire process takes place in the SEM vacuum. In the end, the platinum tip (produced with FEBIP) is connected with the current amplifier by golden conductors.