Signal Processing

The figures to the right show a 300 x 400 µm2 section of an operational transistor imaged using a 0.1 mm diameter Pt tip. The bias potential changes are clearly visible through the native oxide layer above a single pn junction, together with the associated topographic image.
One major problem associated with the null method is obviously the low signal to noise level at balance. Together with co-workers at Twente University in the Netherlands I developed an 'off-null' approach. Here Vb is derived from a computer steered digital-to-analogue converter which is set to a range of potentials around the balance point. At each value Vptp is determined: a plot of Vptrp versus Vb is thus a straight line, the intersection with the Vb axis producing Vc. In this fashion changes in Vc can be determined to sub-mV resolution.
An advantage of this 'off-null' method is that measurements are performed on high signal levels and the balance point is determined via extrapolation. A further benefit is that the gradient of the Vptp versus Vb line is very sensitive to changes in mean spacing and can be used to maintain a constant spacing either during an experiment or throughout a scan. In this fashion the KP can map out surface potential changes simultaneously with sub- micron scale sample topography.