E. Knapek

Improvements in electron microscopy by application of superconductivity

Resolution tests on amorphous carbon foils were carried out in an electron microscope with a superconducting system containing 4 lenses including a shielding lens at 200 kV beam voltage. Due to the mechanical and electrical stability of the system and the absence of contamination of the specimen the highest space frequencies transferred at vertically incident beam were 6 nm-1 corresponding to a resolution of 0.17 nm, a value which approaches the theoretical resolving power of the electron optical system. It should also be feasible to apply such a lens system for microprobe analysis without strongly reducing the theoretical resolution limit, if the construction of the shielding lens is slightly changed.

Grid polarizer for the visible spectral region

We report on the fabrication of a grid polarizer for the visible spectral region, based on metallic grids with periods below 100 nm. Theoretical calculations of the degree of polarization predict useful values for the visible region, if the period is in the range of 100 nm. The properties of the polarizer are shown to be strongly dependent on the optical constants of the metal and the substrate. For the fabrication of the grids, direct e-beam writing in combination with a dry etching process has been used. Metallic grids with 50 nm lines and a period of 100 nm were fabricated on a glass substrate. Measurements of the degree polarization were made at a wavelength of 670 nm. The results are compared to theoretical calculations made in the framework of a rigorous diffraction theory.

Superconducting lens design

Abstract A survey of various types of superconducting lenses is given. Cryogenic problems, particularly material problems, are discussed which must be solved in order to construct lens systems applicable for electron microscopy. The results obtained up to now with the few superconducting lens systems in operation prove a better resolution than is achievable with conventional lenses. The specimen has a temperature of approximately 4 K since it is completely surrounded by walls directly cooled by liquid helium, and the stability of the superconducting lens system permits long exposure times of the micrographs which make it possible to minimize the heating of the object by the beam. For this reason due to cryoprotection one can take advantage of a drastic reduction of radiation damage by the beam.

Electron radiation damage of a thin protein crystal at 4 K

Thin crystals of crotoxin complex embedded in glucose were used to investigate the radiation damage effect at 4 K in an electron microscope equipped with a superconducting lens. It was found that the high resolution (

Properties of organic specimens and their supports at 4 K under irradiation in an electron microscope

Radiation damage is one of the most severe problems in the electron microscopy of organic materials. Observations which were made in an electron microscope with a superconducting lens system have shown that the damage caused by the electron beam is considerably reduced by one to two orders of magnitude if the specimen is cooled to liquid helium temperature (4.2 K). This cryoprotection is even higher in favorable cases, particularly when embedded specimens are used. The cryoprotection decreases for specimens mounted on plastic carrier films due to the low thermal and electrical conductivity of these materials at low temperatures.

Radiation damage due to knock-on processes on carbon foils cooled to liquid helium temperature

Radiation damage on a holey carbon foil was investigated in an electron microscope with a superconducting lens system, where the temperature of the specimen and its environment initially was 4 K. Due to an electron dose of 2 X 10(4) As/cm2 the diameter of a hole increased 5 nm. Rough calculations show that this increase can be ascribed to knock-on processes. Estimates of the rise in specimen temperature during the irradiation are given.

Electron microscopical results on cryoprotection of organic materials obtained with cold stages

Abstract Electron microscopy work on radiation damage performed with cold stages as described in the literature is reported. The gain in tolerable exposure dose due to cooling is of the order of five. There are indications that this gain will increase with decreasing temperature. The advantages of cold stages are discussed.

The interpretation of radiation damage measurements with electron diffraction of organic materials at very low temperatures

Abstract At very low object temperatures organic specimens suffer not only radiation damage but, to a higher extent, charging and heating during electron irradiation due to very low electrical and thermal conductivities of organic materials. The fading of electron diffraction patterns is seriously influenced by the superposition of both effects. For this reason an exact determination of the progress in radiation damage is nearly impossible by direct observations of the diffraction patterns, particularly for obtaining the cryoprotection factors of organics. Charging and subsequently heating cause intrinsic motions of the irradiated areas which also seem to lead to a destruction of the crystallinity. With a suitable preparation of organic specimens, charging and heating effects at very low temperatures could be avoided, and it could be demonstrated that the true cryoprotection was relatively good.

Superconducting lenses in electron microscopy

Abstract Various types of superconducting electron microscope lenses and lens-systems have been designed and tested in the last ten years. A resolution of 0.4 nm was achieved at several laboratories. Future potentialities for the application of superconducting lenses including microwave lenses, for example, in high voltage electron microscopy are discussed.

Computer aided proximity correction for direct write E-beam lithography

Abstract First results of an evolutionary software project are presented. It aims at improving high resolution of many different e-beam writers and will be available on workstations. Selecting critical structure details and preserving hierarchy are key issues to cut down computer time. An “ASIC” and an optical grid were written with CAPROX. Critical dimensions are in good agreement with nominal values as opposed to uncorrected patterns.