An electron beam of diameter as small as 100 Å may be employed to study MOS structures, providing information about the surface topography (secondary electron imaging), surface chemical composition (Auger electron spectroscopy), bulk chemical composition (x-ray fluorescence), as well as the structure of interface regions with high spatial resolution. The penetration depth of the beam and therefore of the excited volume can be altered by adjusting the accelerating voltage and the incident angle. Images of the internal surfaces of MOS structures are formed due to variatons in the metal-insulator barrier height, the semiconductor-insulator barrier height, defects within the oxide or at the interfaces, and topography causing localizing high-field conditions. These mechanisms give rise to small ac signals in the current induced across the structure by the electron beam. These signals may then be amplified and used to modulate the intensity on a CRT. Several different structures have been observed at the interfaces in MOS systems using this technique. Electrical measurements and bias thermal stressing experiments have been used to determine the nature of the structures responsible for observed images.