EP0873566B1 - X-ray microscope with zone plates - Google Patents

Abstract

Light-intensive zone plates (4) are disclosed which are useful as condensers and X-ray objectives for high resolution X-ray microscopes. They have high refraction effectiveness in a high refraction order thanks to a high aspect ratio (H/P) and a suitably adjusted line-slot ratio (P1/P2) lower than 1. Additional improvements may be obtained by zones (6, 7) inclined relative to the optical axis (3). The zone plates (4) may also be operated in Bragg reflection. They thus provide efficient optics with a high numeric aperture and make X-ray microscopes with 10 nm resolution possible. The zone plates (4) may have a relatively coarse structure, and thus they are easy to produce in a relatively short time. The zone plates (4) with high numerical aperture may be used in a particularly advantageous manner as small condensers in laboratory X-ray microscopes, as they can capture light from a microplasma X-ray radiation source in a particularly wide solid angle and focus it on an object.

Description

The invention relates to zone plates with diffraction structures for X-rays and an X-ray microscope with these zone plates for one Condenser monochromator and for a micro lens.

From US 5 199 057 an X-ray microscope is known, in which for the Diffraction of the X-rays using a phase zone plate negligible thickness, i.e. a negligible small Aspect ratio is used.

In J. Vac. Sci. Technol. B 13 (6), Nov./Dec 1995, pages 2809 - 2812, G. Schneider et al. is the production of a zone plate with a line / gap ratio 1: 1 described.

Also in optics 91, no. 3 (1992), September, pp. 95-99, C. David et al. becomes described the manufacture of a zone plate in which the Diffraction efficiency decreases with the square of the diffraction order as it does corresponds to the known theory of thin zone plates. The dash / gap ratio is 1: 1.

In recent years, there have been significant advances in X-ray microscopy made in the wavelength range of about 0.2 - 5 nm. There were X-ray microscopes developed that operate on brilliant X-ray sources become. Electron storage rings emit highly concentrated X-rays. Compact are also under development X-ray sources intended for the use of X-ray microscopes in the laboratory are. Such x-ray sources can be obtained from hot microplasmas (diameter of the radiating area: typically 10 - 50 µm) with help generated by pulsed laser beams. They shine their X-rays on everyone Spatial directions.

Nowadays only come as high-resolution lenses in X-ray microscopes Micro zone plates for use. Microzone plates are rotationally symmetrical Transmission circuit grating with lattice constants decreasing towards the outside, typically have a diameter of up to 0.1 mm and several hundred zones. The numerical aperture of a zone plate is generally through the Diffraction angle determined under which the outer and therefore finest zones Bend vertically incident X-rays. The achievable spatial The resolution of a zone plate is determined by its numerical aperture. The numerical aperture of the used X-ray lenses could in the last Years increased significantly, so that their resolution improved. This trend towards higher resolution will continue.

From the theory of microscopy it is known that the numerical aperture of the illuminating condenser of a transmitted light microscope always adapted approximately should be at the numerical aperture of the microscope objective in order to incoherently radiating light sources also include incoherent object lighting and thus an almost linear relationship between object intensity and Get image intensity. If the condenser aperture is smaller than that of the microscope objective, there is a partially coherent image and the linear transformation between object intensity and image intensity goes for that important high determining the resolution of the microscope Spatial frequencies lost.

So that the X-ray sources can be used in a simple and adapted manner for the brightfield, Phase contrast and in particular darkfield microscopy can be used, a bright condenser must be used. Usually, too diffractive optics as condensers, e.g. Zone plates used because with this allows the X-ray radiation to be monochromatized at the same time. Such Zone plates should have the highest possible diffraction efficiency, to focus as much of the captured radiation on the object as possible.

Such "condenser zone plates" are usually used in the first Diffraction order used, in which all realized so far Condenser zone plates have their highest diffraction efficiency. there it is difficult to adjust the numerical aperture of the Condenser zone plates to that of the micro zone plate (X-ray lens) to reach. In order to realize the adjustment, the condenser zone plate have the same fine zones on the outside as the microzone plate itself The brightest built microzone plates now have zone widths of only 19 nm (corresponding to 38 nm period of the zone structures), So far, zone plates with such fine zone structures can only be used Methods of electron beam lithography, in which the zones are in succession generated, manufactured. Holographic methods that match the pattern create a zone plate "in parallel" in one step and thus in a short time, are out of the question because there is no suitable short-wave UV holography. Accordingly, condenser zone plates could also be adapted numerical aperture only with methods of electron beam lithography, which is to be described as a serial and thus slow process become. However, such condenser zone plates have not yet been manufactured been.

For darkfield X-ray microscopy, they are even more powerful Condenser monochromator arrangements with an annular hollow cone aperture necessary, unless a very precisely adjustable absorbent ring in the rear focal plane of the micro lens is placed. The periods of Zone structures of suitable condenser zone plates would need less than that 38 nm.

A condenser monochromator is used for phase contrast X-ray microscopy Arrangement of advantage, as much as possible of the jet pipe available posed X-ray light in an annular hollow cone aperture large Aperture angle to the object.

Generally, for X-ray microscopes, the zone plates are used as X-ray lenses use a hollow cone-shaped object lighting. Otherwise it would the radiation from the 0th and 1st Overlay the diffraction order of the condenser zone plate. This is because the vast majority of radiation that is parallel or almost parallel to optical axis falls on the object, this and the following micro zone plate (the x-ray lens) penetrates without diffraction and is a more general diffuser Background in a straight line, i.e. noticeable in the center of the image field makes. For this reason, all use transmission X-ray microscopes ring-shaped condensers and the usable, not diffusely outshone area of the The larger the inner, radiation-free field of view, the larger the image field Solid angle range of the condenser is.

In order to improve the resolution of the X-ray microscope to 10 nm is currently working on developing micro zone plates, the one have the smallest zone width of only about 10 nm. This increases the apertures of the micro zone plates and accordingly the necessary numerical ones Apertures of the condensers to provide incoherent object lighting ensure. This increases the difficulties already mentioned further.

Such high-resolution microzone plates would have zones with approximately 10 nm Possess structure width. So far, however, it has neither been successful nor clarified whether such exposed zone structures supported by a support film, which in the Usually consist of a metal like germanium or nickel, still with the help of electron beam lithography and have it transferred into metal. Also for "sputtered-sliced" zone plates is for such small structure widths not yet shown that with the sputtering method sufficiently stable and not Zone rings disturbed by material diffusion can be produced finally process into a zone plate using the thinning process let, in particular the zones of material with low scattering capacity should preferably be etched out so that the profile of a Laminar structure arises.

From the diffraction theory of optics it is generally known that with higher Diffraction orders in principle higher apertures and thus by a factor the diffraction order m higher spatial resolution can be achieved. Amounts the finest structure width, for example, 30 nm, which is easy to manufacture, so a resolution of 10 nm would theoretically be possible in the 3rd order of diffraction. A diffraction efficiency would also have to be achieved, however far exceeds that of the other diffraction orders.

The diffraction efficiency of zone plates as X-ray optics has previously been calculated using a geometrical optical approximation. It was assumed that the aspect ratio of the zone structures, i.e. the ratio of the zone height and the length of the zone period, is significantly less than 10: 1. According to this approach, high diffraction efficiencies in high diffraction orders cannot be expected in principle. On the contrary, the maximum possible diffraction efficiencies scale with 1 / m ² for the diffraction orders m = 1,3,5 ..., so that only a few percent are possible according to this model. Correspondingly, the diffraction efficiency for the 3rd diffraction order has decreased at least by the factor ∼1 / m ² = (1/3) ² = 1/9, so that hardly any light is available in the higher diffraction order. The contrast of an image is therefore greatly weakened by the radiation of the other, much more effective diffraction orders. In practice, therefore, zone plates in higher diffraction orders could not be used so far.

Also from the theory of coupled waves, applied to zone plates, it is known that zone structures, if they have an aspect ratio> 1, a particularly high diffraction efficiency only in its first order can accept (up to about 50% for X-ray optically suitable and realistic, i.e. technologically processable materials). The prerequisite for this is that the zone structures run along the areas of constant phase that one for an object point on the optical axis and the associated image point can construct. These surfaces run parallel and concentric to optical axis, the zone structures act like the network levels of one Crystal used in Bragg reflection and hence the Bragg condition Fulfills. In general there is Bragg reflection if the zone structures are like this are inclined to be parallel to the bisector ("Braggwinkel") of incident and diffracted beam direction. The following is therefore for such a case of "zone plates in Bragg reflection" spoken.

Furthermore, a was used to calculate the diffraction efficiency theoretical description based on the wave equation (theory Coupled Waves) applied to more accurate data for the efficiency of the 1st order also for higher aspect ratios. In the Wave equation became a Fourier representation with a line / gap ratio of 1: 1 to describe the lattice structures of the zone plate used. The dash / gap ratio gives the ratio of the structure widths X-ray scattering and weak scattering Zone material. The resulting system of differential equations has been numerically integrated, which can also be used on fast computers (e.g. IBM RS-6000) needed many hours for a calculation, even with layer thicknesses less than 1 µm. However, only the 1st order considered as an imaging order. The theoretical results for the diffraction efficiency was very good approximation with that geometric optical approach with aspect ratios up to a maximum of about 5: 1 match. Only with higher aspect ratios and when the Zone structures could have higher efficiencies than after the geometrical optical model can be calculated. Both after the geometrically optical It appeared as a model and as well from the theory of coupled waves So far impossible, even for higher diffraction orders (m = 2.3, .....) high ones To obtain diffraction efficiencies. Also from experimental experience there was no evidence of this.

It is the object of the invention to use an X-ray microscope with a resolution of to display at least 10 nm and to indicate zone plates, which in higher diffraction orders can be operated, being in the higher Diffraction orders achieved at least similarly high diffraction efficiencies should be like the known ones in the first diffraction order own operated zone plates, and their zone structures are significantly coarser can be as 10 nm and which are suitable for use in condenser monochromator arrangements and suitable as a micro lens.

This object is achieved by the characterizing part of the claim 11 specified features solved. The claim 1 relates to the application of the invention Zone plates in an X-ray microscope. Advantageous configurations and Further developments of the invention result from the subclaims.

If the specified zone plates are in an X-ray microscope Condenser monochromator and used as a micro lens, so one can Achieve a resolution of 10 nm. Through a suitably adjusted line / gap ratio smaller than 1: 1 and achieved by a high aspect ratio Diffraction efficiency of these zone plates is at a maximum in a higher one Diffraction order. This provides efficient X-ray optics with the necessary high numerical aperture. They are also with them X-ray microscopes with 10 nm resolution possible without the extremely small and technologically extremely difficult to manufacture zone structures must be used for zone plates with the same resolution Would be necessary to use the first diffraction order. At the same time, in this higher diffraction order reached a diffraction efficiency that so far only could be obtained in the first diffraction order. Such zone plates with high diffraction efficiency and with high numerical aperture are particularly advantageous in laboratory X-ray microscopes as small condensers Use light from a particularly high solid angle from a microplasma x-ray source catch and focus on the object.

The way to solve the given task could only be done by a comprehensive conduct an analytical description of the diffraction behavior of zone plates, the overview of all diffraction orders, different line / gap Conditions and much larger zone heights. This task was with the previous numerical iterative calculation methods because of the enormous increasing demand for computing time excluded.

There were two problems to be overcome. For one, another had to Mathematical procedures can be found to make the computing time clear to lower it, even in large aspect ratios, quickly enough to be able to calculate. On the other hand, another parameter in the Wave equation the dash / gap ratio, which the Fourier representation of the grating and thus the wave equation clearly gets more complicated. The result was a system of differential equations, which was solved as a complex-value eigenvalue problem, whereby complex matrices up to the dimension of 100 X 100 elements occur. This solution method reduced the computing times by about 1000 times. Each diffraction order can function as a function of Zone height can be displayed. It turned out that the Diffraction efficiency can be increased drastically in high orders (e.g. m = 6), if the line / gap ratio is chosen to be less than 1: 1, the zones are high Have aspect ratio and, in addition, the zone structures are analogously smaller Mirrors can be arranged in Bragg reflection.

So far this is not known and only by one that has not yet been drawn Understand how multilayers work. In practice it can this effect can be used to achieve high diffraction efficiency to realize high apertures in X-ray optics without simultaneously focusing on the Generation of extremely narrow zone structures to rely on, as it is for operation in the 1st diffraction order would be necessary.

It has been found that a zone plate with a high aspect ratio (typical value: greater than 10) a comparable high Diffraction efficiency in one of their high diffraction orders like one in zone plate with the high aspect ratio used in the first diffraction order has if the mentioned line / gap ratio is significantly less than one. Since such a zone plate is used in a high diffraction order, it has one compared to applications in the first diffraction order greatly enlarged aperture. For example, a zone plate with high Aspect ratio (about 20) and small line / gap ratio (about 0.25), if they are in the 6th diffraction order and in Bragg reflection at 2.4 nm Wavelength is used to have up to 45% diffraction efficiency. For this X-ray optically suitable and technologically processable materials used. In general, the parameters of the zone plate, e.g. Materials, aspect ratio and dash / gap ratio are different for each have the desired higher diffraction order optimized.

Advantageous for zone plates with a large aspect ratio and a small line gap Ratio - when using a higher diffraction order and the Bragg reflection - is that with the same numerical aperture one in high Diffraction order used zone plate only relatively rough zone structures needed compared to one used in the first diffraction order Zone plate of the same numerical aperture. For the example above one X-ray microscopes with 10 nm resolution result for the finest Zone structure to be produced has a width of approximately 30 nm with a period of 120 nm if the zone plate is operated in the 6th diffraction order should. Such structural widths are nowadays by means of To produce electron beam lithography well. Add to that 6 times less Zones are to be written, which is significantly faster. For one electron-beam written zone plate condenser, this means that drastically reduce writing times.

A zone plate for Bragg reflection can be made using known evaporation techniques be produced, e.g. according to the known method for producing so-called "sputtered sliced zone plates" by sputter coating a polished wire rotating in vacuum, alternating the materials suitable for X-ray optics are applied. The wire with the applied materials are then embedded in a substrate and cut in slices perpendicular to its wire axis. This creates Zone plates, the inner area of which is absorbent, i.e. X-ray optical What is ineffective for the condenser outlined in the introduction Reasons is desired.

As an alternative manufacturing method for a zone plate, instead of one Wire an optically polished metal or glass ball can be used. The Sphere becomes - rotating - in vacuum with a multilayer system occupied and then on their circumference except for a spherical zone of a few thinned by width near its equator. If the thinned ball zone is not exactly on the equator of the sphere, so is the remaining layer sequence inclined. If the inclination is half the required beam deflection and coinciding with the angle bisector mentioned above, then the Layer sequence under the Bragg angle. The layer sequence acts like an Multiple mirrors so that the diffraction efficiency reaches a maximum.

In the following, schematically illustrated embodiments of the Invention explained with reference to the drawing.

1 shows a zone plate according to the invention.

Fig. 2 shows an X-ray microscope with a condenser and micro zone plate both are operated in Bragg reflection.

3 shows an X-ray microscope with a condenser and micro zone plate both have inclined zones and are operated in Bragg reflection.

4 shows an X-ray microscope with a focuser with a ring focus and one downstream annular zone plate and a micro zone plate.

In Figure 1, an embodiment of a zone plate 4 according to the invention is shown schematically in cross section. The diffractive properties of the zone plate 4 are determined by the line / gap ratio P ₁ / P ₂ , the aspect ratio H / P and by the inclination of the zones 6, 7 with respect to the optical axis 3. Of course, the X-ray optically effective materials of zones 6,7 also play a role. The line / gap ratio P ₁ / P ₂ indicates the ratio of the structure width of the material of the zones 6, which strongly scatters the incident X-ray radiation 1, and the structure width of the weakly scattering material of the zones 7. The line / gap ratio P ₁ / P ₂ is constant over the entire zone plate 4. The aspect ratio indicates the ratio of the zone height H and the length P of the zone period and in this embodiment increases from the optical axis 3 to the edge of the zone plate 4.

According to the invention, a high diffraction efficiency is achieved in a higher diffraction order if the line / gap ratio P ₁ / P _{2 is} less than 1, as is shown, for example, with 0.5 to scale in FIG. 1, and if a large aspect ratio, for example greater than 10 is realized, which is however not shown to scale in FIG. 1.

A further increase in diffraction efficiency in a higher one Diffraction order can be achieved in certain applications with zones 6.7 be inclined to the optical axis 3. The embodiment 1 shows zones 6, 7 which are close to the optical axis 3 parallel to this run. With increasing distance of the zones 6, 7 from the optical axis 3 the inclination of the zones 6, 7 with respect to the optical axis 3 also increases. A further improvement can be achieved if the zone plate 4 with their zones 6,7 is used in Bragg reflection.

The X-ray radiation 1 incident on the zone plate 4 is included different intensities diffracted into different diffraction orders. 1 shows the directions of propagation for the diffraction of the zero order 8, first order 9a, second order 9b and third order 9c. The diffraction angle increases with the higher diffraction orders. Therefore can with a high diffraction order when using the zone plate 4 as a condenser and / or as an objective in an X-ray microscope Aperture and thus a high resolution of the X-ray microscope can be achieved. In this case, easily and in a relatively short time are advantageously sufficient Rough structures to be produced as zones 6, 7 of zone plate 4.

Figures 2-4 show schematically zone plates 4 in arrangements as Condensers and microzone plates for X-ray microscopes with special high resolution, which are operated with different radiation sources.

In Fig.2 the optics of an X-ray microscope is shown, in which as Radiation source is an isotropically radiating micro-plasma X-ray source 17 is used.

An annular zone plate 14 with is not suitable as a condenser for this inclined zones 6,7, which is advantageously operated in Bragg reflection. The Zone plate 14 focuses the x-ray radiation 1 of the microplasma x-ray source 17 via a hollow radiation cone 10 in focus 13 on the optical axis 3. The object thus illuminated is located there. At this A monochromator pinhole 11 is also arranged, which the unwanted diffraction orders and wavelengths of the X-ray light for hides the further beam path. The zone plate 14 thereby acts together with the monochromator pinhole 11 as a condenser monochromator, the general for illuminating objects in X-ray microscope is used.

A microzone plate 12 with inclined zones 6, 7 serves as the X-ray objective and with Bragg reflection. It creates an image of the object in the image plane 18.

The zone plate 14 and the micro zone plate 12 have a central one X-ray absorbing zone plate area 19 in order - as in the Introduction already mentioned - the undiffracted X-rays as diffuse To eliminate underground.

In Figure 3, the optics of an X-ray microscope is shown, the optical Elements a condenser zone plate 15 with Bragg reflection and inclined Zones and a micro zone plate 12 with Bragg reflection and inclined zones 6.7 used. The almost parallel incident X-rays 1 Undulator or a deflection magnet on an electron storage ring into a object at a high aperture angle and with high diffraction efficiency focused in the plane of the monochromator pinhole 11. To at this Zones 6,7 of the application have to be effective in Bragg reflection Condenser zone plate 15 be inclined. The central one, the X-rays Absorbent zone plate area 20 consists of a spherical one Carrier.

4 shows an X-ray microscope with a focuser 21 Ring focus and an annular zone plate arranged downstream in the beam path 16 with Bragg reflection and inclined zones 6.7. The focuser 21 and the Zone plate 16 together with a monochromator aperture 11 form one Condenser monochromator. The focuser 21 with ring focus focuses the incident parallel bundled x-ray radiation 1 from an undulator or one Deflection magnets of an electron storage ring in the form of a ring. The Zone plate 16 is arranged close to the ring focus of the focuser 21. The Zones 6, 7 of the zone plate 16 are modified so that they are out of the ring focus of the focuser 21 by diffraction a point-like focus 13 on the generate optical axis 3. The advantage of this arrangement is that the Zone plate 16 does not need to have a large area, since it is close to Ring focus of the focuser 21 can be located. So there are only a few structures to produce on the zone plate 16. The light-collecting surface becomes alone determined by the focuser 21. It has only rough zone structures and can therefore be easily produced using methods of electron beam lithography. This arrangement can be used particularly advantageously for well-collimated ones X-rays 1, e.g. from an undulator.

This condenser-monochromator arrangement also serves as X-ray lens a micro zone plate 12 with Bragg reflection and inclined Zones 6.7.

Claims (21)

1. X-ray microscope with zone plates for a condenser-monochromator and for a micro-objective, characterised in that at least one zone plate (4, 12, 14, 15, 16) arranged on the optical axis (3) of the X-ray microscope is provided with a high aspect ratio (H/P) and a line/slot ratio (P₁/P₂) smaller than 1.
2. X-ray microscope according to claim 1, characterised in that the aspect ratio (H/P) increases towards the edge of the zone plate (4, 12, 14, 15, 16).
3. X-ray microscope according to claim 1 or 2, characterised in that the central region (19, 20) of the zone plate (4, 12, 14, 15, 16) is absorbent for X-ray radiation.
4. X-ray microscope according to one of the preceding claims, characterised in that the zones (6, 7) of the zone plate (4, 12, 14, 15, 16) are aligned parallelly or obliquely to the optical axis (3).
5. X-ray microscope according to one of the preceding claims, characterised in that the zones (6, 7) of the zone plate (4, 12, 14, 15, 16) are aligned parallelly to the optical axis (3) in the region near this, and towards the edge of the zone plate (4, 12, 14, 15, 16) are increasingly inclined relative to the optical axis (3).
6. X-ray microscope according to one of the preceding claims, characterised in that a zone plate (4, 12, 14, 15, 16) operated in Bragg reflection is provided.
7. X-ray microscope according to one of claims 1 to 6, characterised in that for a condenser-monochromator the zone plate (14, 15, 16) is constructed to be annular and a monochromator aperture plate (11) is arranged in its focus (13).
8. X-ray microscope according to one of claims 1 to 6, characterised in that for a condenser-monochromator a focussator (21) with a ring focus and a zone plate (16) constructed to be annular and arranged in the beam path are provided, wherein a monochromator aperture plate (11) is arranged in the focus (13) of the zone plate (16).
9. X-ray microscope according to one of claims 1 to 6, characterised in that the zone plate (12) is used as a micro-objective.
10. X-ray microscope according to one of the preceding claims, characterised in that zone plates (4, 12, 14, 15, 16) are provided, the zones (6, 7) of which are formed on a wire or a polished sphere.
11. Zone plate (4, 12, 14, 15, 16) with diffraction structures (6, 7) for X-ray radiation, characterised in that the diffraction structures (6, 7) have a high aspect ratio (H/P) and a line/slot ratio (P₁/P₂) smaller than 1.
12. Zone plate (4, 12, 14, 15, 16) according to claim 11, characterised in that the aspect ratio (H/P) increases towards the edge of the zone plate (4, 12, 14, 15, 16).
13. Zone plate (4, 12, 14, 15, 16) according to claim 11 or 12, characterised in that the central region (19, 20) of the zone plate (4, 12, 14, 15, 16) is absorbent for X-ray radiation.
14. Zone plate (4, 12, 14, 15, 16) according to one of claims 11 to 13, characterised in that it is operated in Bragg reflection.
15. Zone plate (4, 12, 14, 15, 16) according to one of claims 11 to 14, characterised in that its diffraction structures (6, 7) are formed on a wire or a polished sphere.
16. Zone plate (4, 12, 14, 15, 16) according to one of claims 11 to 15, characterised in that it is used in an X-ray microscope.
17. Zone plate (4, 12, 14, 15, 16) according to claim 16, characterised in that its diffraction structures (6, 7) are aligned parallelly or obliquely to the optical axis (3) of the X-ray microscope.
18. Zone plate (4, 12, 14, 15, 16) according to claim 16, characterised in that its diffraction structures (6, 7) are aligned parallelly to the optical axis (3) of the X-ray microscope in the region near this axis, and towards the edge of the zone plate (4, 12, 14, 15, 16) are increasingly inclined relative to the optical axis (3).
19. Zone plate (4, 12, 14, 15, 16) according to one of claims 16 to 18, characterised in that for a condenser-monochromator it is constructed to be annular and a monochromator aperture plate (11) is arranged in its focus (13).
20. Zone plate (4, 12, 14, 15, 16) according to one of claims 16 to 18, characterised in that for a condenser-monochromator it is constructed to be annular and a focussator (21) with a ring focus is arranged downstream in the beam path, wherein a monochromator aperture plate (11) is arranged in the focus (13) of the zone plate (16).
21. Zone plate (4, 12, 14, 15, 16) according to one of claims 16 to 18, characterised in that it is used as a micro-objective (12) in the X-ray microscope.

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