REDUCTION OF SPECIMEN CHARGING

University Of Calif-Lawrenc Berkeley Lab, Berkeley CA

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Abstract

Images recorded in the electron microscope suffer a rapid fall-off in signal with increasing resolution. The strength of the computed Fourier transform of the specimen decreases faster than the corresponding electron diffraction, producing an image than may have less than five percent of the contrast that an ideal image would have at high resolution. The effect is particularly severe in many images of tilted specimens, in the direction perpendicular to the tilt axis, and with specimens that are poor conductors such as vitreous-ice embedded samples on holey films. Specimen charging is a likely candidate for the cause of much of this loss in signal, and the aim of this project is to develop methods that can routinely decrease the charging and thus improve image quality. In one promising approach, we will continue preliminary work that indicates significant benefit in coating the specimen with a thin conductive layer. Another possibility is to alternate illumination of the specimen area being recorded with illumination of an adjacent area; secondary electrons emitted from the adjacent area will discharge the first area. An extension of this approach is to enhance the production of secondary electrons from the objective aperture and to utilize them to discharge the specimen.