Interpretation of electron beam induced charging of oxide layers in a transistor studied using electron holography

Off-axis electron holography has been used to characterize a linear array of transistors, which was prepared for examination in cross-sectional geometry in the transmission electron microscope using focused ion beam milling. In reconstructed phase images, regions of silicon oxide that are located between metal contacts show unexpected elliptical phase contours centered several hundreds of nm from the specimen edge. The experimental images are compared with simulations performed using three-dimensional calculations of the electrostatic potential inside and outside the specimen, which take into account the mean inner potential of the specimen and the perturbed vacuum reference wave. The simulations suggest that the oxide layers contain a uniform volume density of positive charge and that the elliptical contours result from the combined effect of the electrostatic potential in the specimen and the external electrostatic fringing field.