Ralph Feder

Transmission microscopy of unmodified biological materials. Comparative radiation dosages with electrons and ultrasoft X-ray photons

The minimum radiation dosage in a specimen consistent with transmission microscopy at resolution d and specimen thickness t is calculated for model specimens resembling biological materials in their natural state. The calculations cover 10(4)-10(7) eV electrons and 1.3-90 A photons in a number of microscopy modes. The results indicate that over a considerable part of the (t,d)-plane transmission microscopy on such specimens can be carried out at lower dosage with photons than with electrons. Estimates of the maximum resolutions obtainable with electrons and photons, consistent with structural survival of the specimen, are obtained, as are data on optimal operating conditions for microscopy with the two particles.

Application of synchrotron radiation to x‐ray lithography

Synchrotron radiation from the German electron synchrotron DESY in Hamburg has been used for x‐ray lithography. Replications of different master patterns (for magnetic bubble devices, Fresnel zone plates, etc.) were made using various wavelengths and exposures. High‐quality lines down to 500 A wide have been reproduced using very soft x rays. The sensitivities of x‐ray resists have been evaluated over a wide range of exposures. Various critical factors (heating, radiation damage, etc.) involved with x‐ray lithography using synchrotron radiation have been studied. General considerations of storage ring sources designed as radiation sources for x‐ray lithography are discussed, together with a comparison with x‐ray tube sources. The general conclusion is that x‐ray lithography using synchrotron radiation offers considerable promise as a process for forming high‐quality‐submicron images with exposure times as short as a few seconds.

X-ray lithography

The advent of X-ray lithography as a natural compliment to electron beam pattern generation and photolithography seems to be filling a need in the fabrication of submicron devices. The X-ray technique, which is simple for single level devices, lags behind other lithographies in registration techniques. However, its proven high resolution capabilities is responsible for the increased interest in further development. At present a, variety of mask substrates are being evaluated with no one material exhibiting an overwhelming advantage. The type of substrate used is closely coupled to the permissable wavelength of the X-ray source. The X-rays used for lithography to date vary from Rh L (4A) up to CK (44A). Each wavelength shows a distinct advantage and disadvantage. For example, at short wavelengths substrates can be relatively massive but resists are less sensitive and high resolution masks have low contrasts. At longer wavelengths, resists are more sensitive and masks have higher contrast, but defects due to dust are more probable. The use of more than one X-ray source could fulfill the requirements imposed by mask making and device fabrication. High throughput for both masks and device require both foster resists and higher intensity X-ray sources.

Graded-index AR surfaces produced by ion implantation on plastic materials.

The damage tracks of high energy ions in dielectric materials can be etched until overlapping conical etch pits are obtained. If the depth of the pits is >lambda/2, an effective graded-index layer with very low reflectivity is obtained. Broadband antireflection treatment achieving reflectivities of <10(-4) has been applied to plastics like Lexan and Mylar.

Recent Results from the Stony Brook Scanning Microscope

We have recently completed our first run [21.1] with the Stony Brook scanning microscope at the National Synchrotron Light Source (NSLS), using a Fresnel zone plate fabricated at IBM as the principal focusing element. The brightness of this synchrotron radiation source coupled with our high — resolution optics have made a substantial improvement in resolution and throughput compared with our earlier work [21.2,3] which lacked these features.

Soft X-ray contact microscopy

Abstract Recent advances in X-ray contact microscopy (XCM) have improved our ability to understand and to characterize cellular structures. Most of this development has come from improvements in X-ray sources and in the introduction of high resolution image recording media, such as polymer resist. This paper offers general review of the technique and an introduction to the interpretation of images. Some significant physical phenomena which can limit the ultimate resolution of the imaging process, and which must be considered before quantitative microchemical analysis can be attempted, are discussed. These include penumbral blurring. Fresnel diffraction, and lateral dissolution of the X-ray resist. The resulting image contained in the surface relief of the resist can be examined using standard methods; optical, scanning electron and transmission electron microscopes.

Specimen replication for electron microscopy using x rays and x‐ray resist

We point out the advantages of the use of x‐ray resist as the recording medium in contact x‐ray micrography, with subsequent viewing under the scanning electron microscope. Untreated specimens may be replicated with a resolution better than 100 nm. Photographs of latex spheres obtained by this technique are presented.

Imaging unstained proteoglycan aggregates by soft X-ray contact microscopy

Soft X-ray contact microscopy is a relatively new form of ultrastructural imaging, having better than 6 nm resolution and being uniquely well suited for the examination of fragile, unstained biological specimens. The biological specimen placed on a layer of photoresist and exposed to soft X-rays (1-10 nm lambda) of a specific wavelength or broad band. After X-ray exposure, the specimen is removed from the photoresist and the latter chemically developed. When the developed replica is examined by high resolution scanning electron microscopy, the fine structure of the original biological specimen is faithfully reproduced. Since the soft X-ray replica is initially formed due to the differential absorption of the incident X-rays by the biological specimen, the resultant contact replica also reveals information about the elemental composition of the sample. This paper presents our application of this new technique for the study of the proteoglycans, the complex polyanionic macromolecules comprising the gel phase in the matrix of mammalian cartilage.

SOFT X‐RAY LITHOGRAPHIC STUDIES OF INTERPHASE CHROMOSOMES

Soft x-ray absorption lithograph patterns of purified interphase human nuclei and chromosome arrays, imaged on PMMA resists, were examined by scanning EM. The patterns obtained were compared to those utilizing more conventional sources, including transmission EM, scanning EM, high voltage EM, and various light microscopic techniques. The x-ray resist images revealed orderly arrays of absorption profiles in the 3-dimensional specimen with both mild and more extensive developments of the resist. Dense chromatin at the edge of interphase nuclei revealed aligned periodic peaks on the order of 2200 A diameter, with substructure. The periodicity and alignment of interphase chromosomes were entirely consistent with birefringent data on nuclei indicating a high degree of 3-dimensional order. This degree of 3-dimensional order was observed in nuclei containing essentially DNA and histones with only very few other minor (probably structural) proteins. Sonication and nuclease treatment to disperse interphase chromosomes revealed similar absorption periodicities in individual chromosome fibers. Analysis of x-ray absorption profiles thus appears to offer significant new insights into the ordered structure of these defined biological specimens.

A Determination of the Thermal Expansion of Pure Ge and a Measurement of the Differential Thermal Expansion of a Ge–GaAs Thin‐Layer Couple

The absolute thermal expansion of single‐crystal germanium was measured from 25 to 360°C. The fractional change in length determinations have a precision of 1 part in 105. The experimental data agree, within the limits of experimental error, with the results determined for polycrystalline germanium by Kirby. A technique was developed for measuring the differential thermal expansion of a two‐layer couple. An upper limit for the differential thermal expansion of an epitaxial germanium layer on a gallium arsenide substrate was determined and the result indicates that the thermal expansion coefficients agree to within 1 part in 108. This result indicates that thermal expansion effects do not contribute to the observed stress in epitaxial layers of germanium on gallium arsenide.