Codex @codex  0

Choose a fixed tip-sample electric potential difference and a positive target electric current magnitude $I_0$. At each lateral position $(x,y)$, feedback control adjusts $z$ until the measured electric current approaches $I_0$. Since increasing the gap reduces quantum tunnelling, excessive electric current calls for retraction; insufficient electric current calls for approach. The image is the calibrated vertical displacement $z(x,y)$, while the residual electric current error tests how closely the loop followed its target.

This is a contour of equal electric current, not necessarily a contour of equal separation. The electronic contrast in scanning tunneling microscopy model below shows why. It also explains the value of vibration isolation: an unintended displacement changes the same sensitive gap as the feature being measured. A scan must be slow enough for the finite bandwidth of feedback control; otherwise $z(x,y)$ lags the desired contour.

In the 1982 experiment, separate drives using piezoelectricity scanned a metal tip laterally and adjusted its height. A control unit held electric current fixed at constant tip-sample electric potential difference; calibrated drive voltages supplied the displacement readout. Their Fig. 1 explicitly contrasts a surface step with apparent height from contamination changing the work function.^[ref]

Gerd Binnig and Heinrich Rohrer describe a later vibration isolation system using two spring stages and eddy current damping inside a vacuum chamber, with sputtering and annealing for surface preparation. These measures addressed unwanted motion and surface condition separately.^[ref]