Conductive atomic force microscopy
[1] The fact that the CAFM uses two different detection systems (optical for the topography and preamplifier for the current) is a strong advantage compared to scanning tunneling microscopy (STM).
Using this setup, when a potential difference is imposed between tip and sample an electrical field is generated, which results in a net current flowing from tip-to-sample or vice versa.
[5][6] Aeff has been defined as:"the sum of all those infinitesimal spatial locations on the surface of the sample that are electrically connected to the CAFM tip (the potential difference is negligible).
As such, Aeff is a virtual entity that summarizes all electrically relevant effects within the tip/sample contact system into a single value, over which the current density is assumed to be constant.
[7][8][9][10][11][12][13] The value of Aeff can fluctuate depending on the environmental conditions, and it can range from 1 nm2 in ultra high vacuum (UHV) to 300 nm2 in very humid environments.
Following works by Olbrich[21][22][23] and Ebersberger[24] reported that, in SiO2 films thinner than 5 nm, the tunneling current increases exponentially with thickness reductions.
In general, the CAFM can monitor the effect of any process that introduces local changes in the structure of the dielectric, including thermal annealing,[34][35][12][36][37][16][38] dopping[39] and irradiation,[40][41][42] among others.
The CAFM also helped to confirm the percolation theory of the BD by experimentally proving that this is a very local phenomenon that occurs in small areas typically below 100 nm2.
[15][43][44] The severity of the BD event can also be studied from the dielectric breakdown induced epitaxy,[27][45][46][47] which can be observed from subsequent topographic images collected with the CAFM after the voltage ramp.
A new approach to resolve these issues is the Soft ResiScope mode which combines fast point contacts and constant force.
[3][7][69] The varnish should be thick enough to withstand the large current densities and frictions, and at the same time thin enough to not increase significantly the radius of the tip apex, maintaining its sharpness and ensuring a high lateral resolution of the CAFM technique.
[69] The main problems of diamond-coapted CAFM tips are: i) they are much more expensive, and ii) they are very stiff and can damage (scratch) the surface of the samples under tests.
A cheap and effective methodology to protect CAFM tips from degrading is to coat them with graphene, which can withstand well the high current densities and mechanical friction.
The analogical current signals flowing through the tip/sample nanojunction are sent to the preamplifier, which transforms them into digital voltages that can be read by the data acquisition (DAQ) card of the computer.
Many manufacturers integrate the preamplifier in the so-called "CAFM application module", which is a removable component that can be fixed to the AFM (usually very near to the tip to minimize the electrical noise) to perform conductivity measurements.
The company FEMTO, one of the world leading manufacturers of preamplifiers compatible with CAFMs, can provide devices with electrical noise as low as 3 fA and a gain up to 1013 V/A.
[72] Nevertheless, the main limitation of CAFM preamplifiers is their narrow current dynamic range, which usually allows collecting electrical signals only within three or four orders of magnitude (or even less).
[72] A more sophisticated solution for this problem is to combine the CAFM with a sourcemeter,[73][74] semiconductor parameter analyzer or with a logarithmic preamplifier,[75] which can capture the currents flowing through the tip/sample system at any range and with a high resolution.
