EP0270968A2 - Roentgen microscope - Google Patents

Abstract

In an X-ray microscope, the object (3) is illuminated coherently or partially coherently with quasi monochromatic X-ray radiation (1) via a condenser (2) and is magnified in the image plane (6) using a high-resolution X-ray objective (5). In order to achieve the highest possible image contrast, an element (8) is arranged in the Fourier plane (7) of the x-ray objective, which element gives a preselected diffraction order of the radiation a phase shift. The element extends over the surface area in the Fourier plane, which is acted upon here by the diffracted radiation to be influenced. The use of the phase shift of a preselected diffraction order of the radiation with respect to the uninfluenced radiation makes it possible to carry out investigations, in particular biological structures with a low radiation dose, and yet to produce a high image contrast. In addition, it will be possible to shift the wavelength range of the X-ray radiation to be used for shorter wavelengths at which X-ray microscopy has so far not been possible due to the low absorption.

Description

The present invention relates to an X-ray microscope in which the object is illuminated coherently or partially coherently with quasi monochromatic X-ray radiation via a condenser and is magnified in the image plane by means of a high-resolution X-ray objective.

Such X-ray microscopes are described, for example, in Part IV of the book "X-Ray Microscopy" by Schmahl and Rudolph, Springer-Verlag 1984. On pages 192/202 of this book you will find a description of an X-ray microscope in which each imaging element, ie condenser and X-ray lens, is designed as a zone plate. Such a zone plate consists of a large number of very thin rings, for example made of gold, which are applied to a thin carrier film (for example made of polyimide). These rings form a circular grid with a radially increasing line density. The zone plates diffract the incident monochromatic X-ray radiation of the wavelength and thus cause an image. Quasi monochromatic radiation is understood here to mean radiation of a certain bandwidth Δλ, this bandwidth being given in connection with zone plates by the relationship ^λ / Δλχ p. m (p = number of lines, m = number of the diffraction order still to be determined).

In such known X-ray microscopes, the contrast in the image is mediated by photoelectric absorption in the object, i.e. structures are imaged which effect an amplitude modulation of the X-rays passing through.

The wavelength range of the X-rays, which is between 2.4 nm and 4.5 nm, ie between the oxygen K edge and the carbon K edge, is particularly suitable. This area is also known as the water window, since water has a transmission that is about ten times higher than that of organic materials. It can be used in this wavelength range examine organic materials and thus cells and cell organelles in living condition.

The resolution achieved so far in X-ray microscopy is about a factor of 10 better than in light microscopy, with a further increase in X-ray microscope resolution by about an order of magnitude being still possible. The limit resolution in X-ray microscopy of amplitude structures will be given by the radiation exposure of the objects to be examined.

It is now the object of the present invention to provide an X-ray microscope which enables examinations, in particular of biological structures, to be carried out with a radiation dose which leads to a lower radiation exposure of the objects than the previously usual methods, without having to accept a deterioration in the image contrast must become.

This object is achieved, starting from an X-ray microscope according to the invention, in that an element is arranged in the Fourier plane of the X-ray objective, which element extends over the area affected by the zero or a preselectable other order of the radiation deflected by the object, and gives the radiation passing through a phase shift.

In the X-ray microscope according to the invention, phase-shifting properties of object structures are used to form contrast. The phase-shifting element arranged in the beam path gives the preselected order of the X-ray radiation coming from the object a phase shift with respect to the other radiation coming from the object that does not pass through the element. The phase-shifted and the unaffected radiation components interfere in the image plane and create a high-contrast, enlarged picture of the object.

It has proven to be particularly advantageous to give the zero-order X-ray radiation of the radiation coming from the object a phase shift of 90 ° with respect to the orders refracted by the object structures. This can be done particularly easily since the zero-order radiation illuminates a central circular disk in the Fourier plane of the X-ray objective. A suitable design of the phase-shifting element is described in claims 3 and 4.

The invention is based on the knowledge that the refractive index n of an element in the X-ray range is composed of two differently acting variables, which can be expressed schematically by the relationship n = 1 - δ - i β. The quantity β describes the absorption, which becomes smaller as the wavelength λ of the X-ray radiation becomes shorter. The size δ is decisive for the phase shift which is given to the continuous X-ray radiation. The size δ generally varies very slowly with the wavelength. For this reason, if the phase shift is used by the object, a significant improvement in the contrast in the image can be achieved.

In particular, images can also be generated with a lower radiation exposure to the object, the contrast of which is no worse than when the amplitude contrast is used with a higher radiation exposure.

This consideration also gives the further essential advantage of the X-ray microscope according to the invention. Since the size δ changes only slightly with the wavelength λ, when using the phase shift, the wavelength range of the X-rays can be shifted to shorter wavelengths at which X-ray microscopy has so far not been possible due to the low absorption, ie small β, because of the low contrast levels that can be achieved in the image was sensibly possible.

Under certain circumstances, it may also be possible not to influence the zero-order X-ray radiation in the phase, but rather higher orders of the radiation refracted by the object. These orders form rings in the Fourier plane of the X-ray objective, so that the phase-shifting element is designed according to claim 5.

As the formula for the refractive index n in the X-ray range, namely n = 1 - δ - i β shows, a phase shift is always associated with an absorbing effect. Of course, this also applies to the phase-shifting element used in the X-ray microscope according to the invention. It may therefore be necessary to adjust the intensities of the interfering orders in the image plane of the radiation coming from the object. For this purpose, the phase-shifting and the absorbing effect of the phase-shifting element is advantageously distributed over different corresponding surfaces in the Fourier plane of the X-ray objective. The radiation passing through these corresponding surfaces is influenced independently of one another in phase and amplitude, specifically in such a way that the intensities of the radiation interferences in the image plane are matched to one another.

• Fig. 1 ein Ausführungsbeispiel für den prinzipiellen Aufbau eines Röntgen-Mikroskops nach der Erfindung;
• Fig. 2 die Draufsicht auf eine als abbildendes Element ver­wendete Zonenplatte;
• Fig. 3 das im Mikroskop der Fig. 1 enthaltene phasenschie­bende Element in Draufsicht;
• Fig. 4 eine Draufsicht eines anderen Ausführungsbeispiels für ein phasenschiebendes Element.

The invention is explained below with reference to Figures 1-4 of the accompanying drawings. In detail show:

• Figure 1 shows an embodiment of the basic structure of an X-ray microscope according to the invention.
• 2 shows the top view of a zone plate used as an imaging element;
• 3 shows the phase-shifting element contained in the microscope of FIG. 1 in a top view;
• Fig. 4 is a plan view of another embodiment for a phase shifting element.

In Fig. 1, the radiation coming from an X-ray source is designated by (1). For example, a synchrotron or another source described in Part 1 of the book "X-Ray Microscopy" by Schmahl and Rudolph, Springer-Verlag 1984 can be used as the X-ray source.

The x-ray radiation passes through an x-ray condenser (2) and is guided by this to the object (3) to be observed, which is arranged on a central diaphragm (4). The X-ray radiation deflected by the object (3) passes through a high-resolution X-ray lens (5) and is imaged by the latter into the image plane (6).

With (7) the Fourier plane of the lens (5) is designated, in which the decomposition of the radiation passing through the object (3) is found in harmonic Fourier components. This distribution is represented again in the image plane (6) by a Fourier inverse transformation as a real image.

Zone plates such as are shown, for example, in FIG. 2 are advantageously used as imaging elements (2) and (5). This zone plate consists of a large number of rings which are placed on a very thin carrier foil, e.g. are applied from polyimide. The rings are usually made of gold or chrome and have a low layer thickness of approx. 0.1 µm. The rings form a circular grid with a radially increasing line density.

A phase-shifting and / or absorbing element (8) is arranged in the Fourier plane (7) of the objective (5). This consists, as shown in FIG. 3, of a thin carrier film (9) which is contained in a ring (10) and on which a thin layer of phase-shifting material, for example chrome in the form a central circular disc (11) is applied.

As can be seen from FIG. 1, the zero-order X-ray radiation (1) coming from the object (3) penetrates the central circular disk (11). This radiation is given a phase shift of 90 ° with respect to the orders diffracted by the object structures. In the image plane (6) there is interference between the phase-shifted radiation and the uninfluenced radiation and thus a high-contrast, enlarged image of the object (3) is created, which can be captured directly on a photosensitive layer, for example.

If, for example, X-ray radiation with a wavelength λ = 4.5 nm is used and the central circular disk (11) of the element (8) consists of a 0.09 μm thick chrome layer, then a protein structure of 10 nm thickness in water results in the X-ray microscope of FIG. 1 an approximately 20 times better contrast than the usual image in the amplitude contrast.

Fig. 4 shows an embodiment of an element (8) serving for phase shift and / or absorption, in which a ring (12) made of the appropriate material, for example chrome, is attached to the carrier film (9). This ring gives higher orders of the radiation deflected by the object a phase shift. Which order is to be influenced is determined by the diameter and the width of the ring (12).

Claims (5)

1. X-ray microscope in which the object is coherently or partially coherently illuminated with a condenser with quasi-monochromatic X-radiation and enlarged in the image plane by means of a high-resolution X-ray lens, characterized in that an element in the Fourier plane (7) of the X-ray lens (5) (8) is arranged, which extends over the area affected by the zeroth or a preselectable other order of the radiation deflected by the object (3) and gives the radiation passing through a phase shift. 2. X-ray microscope according to claim 1, characterized in that the phase-shifting and the absorbing effect of the element (8) to compensate for the intensities of the different orders, independently of one another on the different corresponding surfaces in the Fourier plane (7) of the X-ray objective (5) is distributed. 3. X-ray microscope according to claim 1 and 2, characterized in that the element (8) from a in the form of a central circular disc (11) on a carrier film (9) applied layer of such a thickness that the zero-order X-ray radiation passing through it Phase shift of 90 ° and a possibly amplitude-adapting absorption. 4. X-ray microscope according to claim 3, characterized in that the central circle (11) of the element (8) at an X-ray wavelength λ = 4.5 nm consists of a 0.09 µm thick chrome layer. 5. X-ray microscope according to claim 1 and 2, characterized in that the element (8) consists of an annular layer (12) of the diffracted by the object (3) Radiation n ^th order (| n | ≧ 1) a phase shift and given if an amplitude matching absorption is given.

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