In this review we propose to address the question: for the life-science researcher, what does X-ray microscopy have to offer that is not otherwise easily available?We will see that the answer depends on a combination of resolution, penetrating power, analytical sensitivity, compatibility with wet specimens, and the ease of image interpretation.

Soft X-ray microscopes and their biological applications

Photoelectron microscopy and applications in surface and materials science

Energy‐dispersive x‐ray diffraction studies were carried out on the III‐V compound aluminum nitride to 65 GPa using a synchrotron x‐ray source. A pressure‐induced first‐order phase transition from a wurtzite structure to a rocksalt structure was observed. The first peaks of the high‐pressure phase appeared at 14 GPa. On further loading of the diamond anvil cell, the wurtzite peaks disappeared by 20 GPa. After unloading the cell to atmospheric pressure, the rocksalt structure persisted. The equilibrium transformation pressure lies on the interval 0–14 GPa.

Pressure‐induced rocksalt phase of aluminum nitride: A metastable structure at ambient condition

Recent advances in sources, optics, and detectors for soft x rays have led to rapid developments in applications to microscopy. The history of the field and the advantages of soft x rays for the examination of biological specimens are briefly described. Limitations in resolution and radiation induced damage are mentioned before embarking on a survey of the various approaches in use. Contact microscopy is the highest resolution technique available today. It is particularly powerful when used with flash sources for stop‐action imaging. Imaging with Fresnel zone plates provides direct magnification, high resolution, and short exposure times. Scanning microscopy leads naturally to digitally recorded images, with minimal radiation exposure to the specimen. The focused beam for scanning can be generated with multilayer mirrors, or with zone plates. This last approach is treated in greater detail. Extensions of soft x‐ray imaging to three dimensions, and to elemental mapping, are considered as important areas under active development.

Soft x‐ray microscopes

Real‐time images produced by photoelectrons from metal and semiconductor samples have been generated using synchrotron radiation. A single electrostatic objective lens coupled to a two‐stage image intensifier has been used to evaluate the feasibility of combined imaging and spectroscopy with photoelectrons in the ultraviolet and soft x‐ray regions. Images were generated using photon energies ranging from 9 to 160 eV. The use of monochromatic synchrotron radiation provides a unique capability for generating image contrast by digital subtraction of images formed with photon energies above and below a core‐level binding energy. A simple electron‐optical objective lens produces images with a few microns resolution, and adequate sensitivity to image 45‐eV kinetic energy electrons using bending magnet radiation. Calculations based on these experiments show that comparable resolutions are possible for combined microscopy and spectroscopy with electrons of order 100‐eV kinetic energy using currently available bending magnet synchrotron radiation. Calculations for expected performance from undulator sources are presented.

Photoelectron microscopy with synchrotron radiation

Synchrotron radiation has recently contributed to the development of high‐resolution x‐ray microscopy either for biological investigations or for chemical microanalysis. Contact microscopy for this end appears to be particularly useful, but is mainly limited by the detectors that are used. In order to overcome these limitations—i.e., to obtain real time imaging and quantitative information—a study of a photoelectron x‐ray microscope is made. Contact configuration is used, but the usual detector is replaced by a thin photocathode. The emitted secondary photoelectrons are then imaged by a suitable emission electron microscope. Problems of photoemission, resolution, and D.Q.E. limitation are discussed. The particular nature of electron lens aberrations, due to photoemission characteristics, is taken into account. The conclusions are that resolutions down to 500 A can be obtained in certain conditions. A preliminary low‐ magnification experiment is described and results are shown.

S. Lowenthal

Papers

Photoelectron microscope for x‐ray microscopy and microanalysis