A UHV diffraction camera has been built which can be used in either transmission or reflection mode. Fig. 1 shows a schematic view of the instrument. The optical axis of the instrument is vertical, with gun below and a downward-looking CCD camera on top. The pressure with gun operating is 1×10−10 Torr. An ion pump and oil-free turbo system is used. The electron gun is a fully bakeable custom UHV RHEED gun with LaB emitter and two magnetic lenses. This allows the beam to be focused to an area of about 1 micron on the surface of the sample. The gun works in the energy range of l-50keV. A double tilt stage, with sample exchange, has been built which allows azimuth rotation by 360° and tilt by ° 15°. The sample can be resistively heated on the stage up to 1300°C. The sample holder can also be used with 3mm diameter TEM grids for transmission diffraction. An external lens-coupled liquid-nitrogen cooled Princeton CCD camera with 1024×1024 pixels is used for image acquisition, viewing a transmission phosphor through a lead-glass window.

The energy filter consists of three spherical grids serving as a retarding field energy filter. The design is similar to that of Horio [1]. The first and last grids are grounded, while the center grid holds the retarding voltage. High voltage stability up to 10kV has been achieved with this filter. Fig. 2 shows an energy loss spectrum obtained.from the specular spot of the Si(111) surface. The spectrum was obtained from the measured integrated intensities by numerical differentiation.