Photoelectron X-Ray Microscopy: Recent Developments

Imaging using focussing gratings like zone plates suffers from a very strong chromatic aberration, thus focus and magnification are altered by wavelength tuning [26.21. The efficiency and the numerical aperture of these lenses also steeply decrease at shorter wavelength. With scanning x-ray mi­ croscopes, fluorescence analysis can be done around 1 nm wavelength, but should be difficult to exploit for the low Z elements [26.31. Presently, resolutions in the order of 10 nm only are expected for such instruments [26.3-51. On the contrary, contact microscopy has already produced analytical re­ sults, using photoresists as image detectors [26.11. The resolution can be as good as a few nanometers and the quantum efficiency is excellent. The process mostly suffers from the detector saturation and nonlinearity which strongly limit the sensitivity. This point can be overcome by replacing the photochemical detector by a photocathode layer used in transmission mode and followed by an emission electron microscope. A few attempts have been made in this area [26.6,7,3], and the former instrument of this type has been con­ structed by HUANG and MOLLENSTEDT, who, in 1955, using a special built - in x-ray tube,obtained a 120 nm resolution. Synchrotron radiation has now opened the way to new developments. We ex­ amine here what characteristics can be expected from photoelectron x-ray microscopy, and discuss the means of improving the resolution and efficien­ cy. We describe the instrument which is presently under realization at the Institut d'Optique (Orsay) and make a few comments on the solutions given to the main technical problems.