Olivier Schneegans

Study of the local electrical properties of metal surfaces using an AFM with a conducting probe

The performances of coating materials for electrical contact elements are more and more often investigated through various means. We report here a new method we have developed for a few years in our lab, which consists in performing localized resistance measurements over a surface by means of an AFM with a conducting probe. This technique enables us to simultaneously obtain a cartography of the surface roughness and of the local conductance within a given microscopic area of a sample with nanometer scale resolution. Although the elaboration of suitable probes remains an open problem, some convincing images of metal surfaces have already been obtained, revealing occasionally surprising features. It can be observed for instance that the local resistance values can vary of several orders of magnitude between two adjacent grains. Calculations performed from the measurements allow to clarify the mechanical nature of the tip/surface nanocontact and hence to determine the most probable transport process according to the range of resistance considered.

Conducting probe atomic force microscopy applied to organic conducting blends

Atomic force microscopy (AFM) is used in contact mode with a conducting tip to probe the conducting network of the conductive polymer polyaniline blended in an insulating polymer matrix. The high resistance contrast and sharp boundaries between conductive and insulating phases is observed down to scales in the 10 nm range. The very low scale electric dispersion corresponds to the morphologic phase segregation known from conventional AFM or transmission electron microscopy measurements, which is responsible for the ultralow electrical percolation threshold previously demonstrated in this system.

Copper sample analyzed with an n-doped silicon tip using conducting probe atomic force microscopy

A study of the electrical properties of the nanocontact between a conducting tip made of n-doped silicon, and the surface of a cleaned copper sample has been carried out. Current–voltage (I–V) curves have been performed in order to clarify the electrical nature of the nanocontact. Apart from the breakdown phenomenon and the noticeable laser diode effect on the I–V curves, the nanocontact can be modeled by a Schottky diode, in series with a resistance of a few kΩ, which probably reflects the resistance of the semiconducting probe. This fact is partially confirmed by a simple calculation of the resistance of the tip/cantilever system using a rough model. Topographical and electrical images have also been recorded for a given tip/sample bias. For a positive sample–tip voltage, the copper surface appears uniformly conducting over 20×20 μm2, and remains stable during the whole period of study (three weeks). This study thus allows one to have a better understanding of the effects which must be kept in mind when...

Specific methodology for capacitance imaging by atomic force microscopy: A breakthrough towards an elimination of parasitic effects

On the basis of a home-made nanoscale impedance measurement device associated with a commercial atomic force microscope, a specific operating process is proposed in order to improve absolute (in sense of “nonrelative”) capacitance imaging by drastically reducing the parasitic effects due to stray capacitance, surface topography, and sample tilt. The method, combining a two-pass image acquisition with the exploitation of approach curves, has been validated on sets of calibration samples consisting in square parallel plate capacitors for which theoretical capacitance values were numerically calculated.

Multi-scale study of the electrical properties of organic layers grafted on gold surfaces

Many studies have been performed on the corrosion and fretting degradation mechanisms of gold plated contacts for low level applications. Reliability requirements constantly increase and finding new solutions able to postpone degradations is still a challenge. A large study has led to the elaboration of a fluorinated organic film grafted to the surface and allowing outstanding protection of the gold surfaces. These organic films involved new thiol functionalized perfluoro polyethers, deposited as mixed layers. Back-panel connector systems coated with various fluorinated organic films were submitted to a four gases corrosion test (Bellcore uncontrolled environment test). High values of the contact resistance Rc were found for the contacts coated with some of the films. The present study is aimed at understanding the electrical properties of these films. The electrical behavior of model contacts (dimple on flat) was investigated in static and in dynamic modes. Fretting experiments showed the combined influence of friction and of the surface roughness. Finally the electrical properties were investigated at a more microscopic scale. An experiment performed with a modified AFM microscope with a conducting probe (CP-AFM) showed the particular behavior of functionalized PFPE lubricant on gold. Results are discussed in order to have an insight into the conduction mechanisms involved.

Influence of temperature and pressure on the static contact resistance of vacuum heat-treated polyacrylonitrile films

Abstract The protection against corrosion of low-level electrical contacts is usually insured by metallic terminal coatings. However, they can be degraded by wear and corrosion, which finally causes the failure of the contacts. The field we have been investigating, consists of replacing the terminal metallic layer by an organic conducting film, derived from polyacrylonitrile (PAN) by heat-treatment. In this paper, we present results concerning the static contact resistance ( R c ) measured in a ball/plane configuration on metallic coupons over-layered with PAN films and heated under high vacuum at several temperatures: R c at room temperature under a constant normal load; R c at room temperature varying the normal load; R c at a constant normal load varying the temperature of the contact. These experiments reveal a transitional heat-treating temperature. Above this temperature, the film is very conducting and the behavior of the contact is ruled by the metallic electrodes. Below this temperature, the film is moderately conducting and then responsible for the contact resistance. In that case, pressure and temperature dependencies of the contact show that variable-range hopping is the most probable transport mechanism. At the transitional annealing temperature, the values of the contact resistance are scattered. Electrical investigations with a modified atomic force microscope (AFM) show that the film is covered by a very thin layer, non-adherent and poorly conducting.

First observations of YBaCuO thin films by atomic force microscopy with conducting tips

YBaCuO thin film surfaces have been studied with an original laboratory-made attachment associated with a commercial atomic force microscope. Using a doped silicon probe coated with doped diamond, we have obtained simultaneously topographical and local contact resistance surface images within a given area of the sample. YBaCuO films on various types of substrates were observed: polycrystalline yttria-doped zirconia (PYSZ), and MgO or SrTiO3 single crystals. For YBaCuO films grown on PYSZ, the electrical image has clearly revealed the presence of electrical disconnection zones between grains, which correspond to grain boundary areas observed on the topographical image. The presence of such defects can explain the modest critical current density (Jc approximately equals 3 X 104 A/cm2 at 77 K) measured on these granular films. On the opposite, for films grown on single-crystal substrates, the electrically connected areas between grains are visible on the electrical images, that can be correlated to better electrical transport properties of the films. Moreover, island-shaped grains exhibiting terraces of one unit cell vertical height could be seen, on close inspection of films grown on PYSZ and MgO substrates. For YBaCuO elaborated on SrTiO3 substrates, the observed grain structure rather exhibited a spiral shape.