How can a scanning tunneling microscope detect steps that are one atom high?

A scanning tunneling microscope (STM) detects steps that are one atom high by exploiting the extreme sensitivity of quantum mechanical tunneling to the distance between the probe tip and the sample surface.1

In this process, a sharp tungsten needle is positioned just a few angstroms from the surface, and a small voltage is applied to allow electrons to tunnel across the gap. The probability of these electrons tunneling decreases exponentially as the distance from the surface increases. Because of this exponential relationship, even a tiny change in the gap—such as the height of a single atom—results in a significant, measurable variation in the tunneling current. As the probe scans across the surface, the instrument registers these current fluctuations, which are then processed to create a topographical image that clearly reveals atomic-scale features like terraces and steps.1