Development of A 350-KV Holography Electron Microscope and its Applications

Abstract

The 350 kV field-emission electron microscope shown in Fig.1 has been developed to widen the applications of electron holography. A field emission beam is used because it is very bright at first and monochromatic. However, its brightness deteriorates while passing through accelerating electrodes and condenser lenses because of their spherical and chromatic aberrations. A magnetic lens is installed just below a (310)-oriented tungsten tip. A magnetic lens is used so that the electron source image can be located at the most favorable position between the accelerating tube and the first condenser lens to minimize the aberrations and to increase brightness. The measured brightness (probe current) ranges from 1.4x109A/cm2/sr (0.37 nA) to 6.7x108A/cm2/sr (2.2 nA) with 10 μA total emission current at 300 kV.These increased brightness and narrow energy spread of the electron beam enable observing fine spacing lattice fringes in a gold thin film. Lattice fringes of 0.065 nm spacing were actually observed in the electron micrograph shown in Fig. 2. The incident electron beam was along the [001] axis, and the (400) andreflected beams were used to form the fringes. A 0.055 nm spacing lattice image is shown in Fig. 3. These fringes resulted from the interference of the electron beam, with an incident axis from the [111] direction into the gold thin film, by theanddiffracted beams. This spacing is the shortest observed to date.