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Transfer doublet and an elaborated phase plate holder for 120 kV electron-phase microscope.

A Zernike-type phase plate, which consisted of an amorphous carbon film, was developed for a 120 kV transmission electron microscope (TEM). The thickness of the film causes the scattered electrons to experience a pi/2 phase shift. The axial electrons pass through the central hole of the phase plate. The TEM was modified with an additional lens doublet, which allowed more flexibility in applying the phase plates. The doublet transfers the image from the back focal plane to a plane below the objective lens where a special phase plate holder, anticontaminator, and other necessary devices could be employed. The diameter of the central hole in the phase plate was 1 microm. The same mechanical system as in the specimen stage is used to align the axial beam into the hole. The use of a heating holder to protect the phase plate from contamination and consequential charging was investigated. Comparative diffractograms of amorphous carbon are shown with and without using a phase plate, demonstrating the change in phase contrast modulation from a sine to a cosine function when a phase plate is used. Images of biological specimens acquired using a phase plate are presented and the contrast enhancement due to the cosine transfer function is clearly demonstrated.