Scanned Probe Microscope Dimensional Metrology

Scanning tunneling microscopes (STM) were the first types of SPM to be widely used (1-3). In this technique, a metallic probe is brought to within the electron tunneling distance of a conductive sample surface (see Fig., 2), such that a small tunneling current flows when a voltage bias is applied. The tunneling current (typically 1 nA) flows to or from a single atomic cluster at the apex of the probe. Tunneling current density decays exponentially with distance between the two electrodes (tip and sample). Essentially, this can be thought of as a form of "near-field" SEM. There is no need for vacuum or electron lenses because the source and sample are brought to within the near-field distance from each other and scanned while tunneling occurs. Tip to sample separation is adjusted by moving either the sample of tip with a piezoelectic transducer that is controlled by a feedback system which maintains constant current, and hence distance. In this way, the tip is made to track the height of the surface. An image is obtained by rastering the probe across the field of view and plotting the change n tip height position as it tracks the surface. It is this exponential dependence of the current on tip to sample separation that give the STM its extremely high vertical resolution (sub-angstrom). Lateral resolution on the atomic scale is achieved if the current eminates from a single cluster (the ideal situation) near the apex if a tip. Unfortunately, the types of samples that can usually be imaged with the SM are limited to only those having highly conductive surfaces.