Images of dopant profiles in low-energy scanning transmission electron microscopy

A scanning electron microscope is used in transmission mode. The image is formed with secondary electrons, collected by the standard detector, resulting from the conversion of transmitted electrons on a circular disk, covered with MgO smoke, located below the thinned specimen, and centered on the optical axis. Operating in this mode, bright-field images of As dopant profiles in Si, having a peak concentrations of 5 and 2.5 at. % and a spatial extension of about 40 nm, have been observed in cross sectioned specimens. The description of the dopant profiles has a resolution of 6 nm as defined by the spot size of the microscope, equipped with a LaB6 tip, and operating at 30 keV.