Sources of Current Error in Nanoscale Measurements

Measuring low currents requires an understanding of the various potential error sources that can cause undesirable measurement errors. Two very common sources of error that affect measurements of many types of nanoelectronic devices are triboelectric effects and electrochemical effects (Figure 1).

The charge generated by friction between conductors and insulators will cause so-called triboelectric currents. Friction removes free electrons from the conductors, creating a charge imbalance, which in turn generates current. An example of this is the friction between the insulator and conductor in a coaxial cable. Error currents can also be caused by electrochemical effects, in which ionic chemicals form a weak battery between two conductors. Etching solutions, solder flux, or other contaminants on printed circuit boards can cause currents of several nanoamperes between conductors.

Triboelectric currents are caused by the friction between conductors and insulators. Friction removes free electrons from the conductors, creating a charge imbalance and a corresponding current. A typical example is the current generated by the insulator and conductor rubbing together in a coaxial cable as shown in the figure. "Low-noise" cables greatly reduce this effect. Under its outer shield, the inner insulator is often polyethylene coated with graphite. The graphite acts as a lubricant and forms a conductive equipotential cylinder to maintain charge balance and minimize friction-induced charges caused by cable movement. However, even low-noise cables will generate some noise when subjected to vibration, stretching and contraction, so all connecting wires should be kept as short as possible and temperature changes (which will cause thermal expansion forces) should be avoided. It is best to tape or tie the cables to a vibration-free surface such as a wall, workbench or rigid structure.