EP1632113A1

EP1632113A1 - Plasma-based generation of x-ray radiation with a stratiform target material

Info

Publication number: EP1632113A1
Application number: EP04739766A
Authority: EP
Inventors: Manfred Faubel; Bernd Abel; Ales Charvat; Eugene Lougovoi; Jens Assmann; Jürgen TROE
Original assignee: Max Planck Gesellschaft zur Foerderung der Wissenschaften eV; Georg August Universitaet Goettingen
Current assignee: Max Planck Gesellschaft zur Foerderung der Wissenschaften eV; Georg August Universitaet Goettingen
Priority date: 2003-06-11
Filing date: 2004-06-09
Publication date: 2006-03-08
Also published as: JP2006527469A; DE10326279A1; WO2004110112A1; US20110116604A1

Abstract

The invention relates to methods for effecting the plasma-based generation of X-ray radiation involving the following steps: providing a target material (50) in the form of a free flow pattern (51) in a vacuum chamber (20), and; irradiating the target material (50) in order to create a plasma state in which the X-ray radiation is emitted. The flow pattern (51) is shaped in such a manner that the target material has, at least at the site of irradiation, a surface (52) with a local curvature minimum. The invention also relates to devices for carrying out these methods and, in particular, to X-ray sources for effecting the plasma-based generation of X-ray radiation.

Description

Plasma-based generation of X-rays with a layer-shaped target material

The invention relates to methods for plasma-based generation of x-rays with the features of the preamble of claim 1, x-ray sources for plasma-based generation of x-rays with the features of the preamble of claim 22 and methods for injecting a liquid target material into a vacuum chamber.

It is known to generate X-rays with X-ray sources in which a target material is brought into a plasma state by high-energy radiation (for example laser radiation), in which material-specific X-ray fluorescence radiation is emitted. The first developments were made with solid, layered target materials. However, solid target materials have a relatively high mass density, so that a relatively large amount of material is also released during plasma excitation, which is disadvantageous for practical applications. An improvement was achieved by using liquid, drop-shaped target materials. For example. According to EP 186 491, a sequence of liquid drops is generated in an evacuated chamber with a piezoelectric drop generator, each of which is converted into a plasma state by laser radiation. The emission of soft X-rays occurs from the plasma state, which emerges through a window in the chamber or is collected with an optical system. Progress has been made through the use of liquid target materials. So far, however, these X-ray sources have a number of disadvantages which, depending on the application, are tolerated or compensated for by special measures. The X-ray source according to EP 186 401 is limited to the use of mercury as a liquid target material. Accordingly, the x-ray radiation that can be generated is limited to certain spectral lines. Another disadvantage of mercury is its relatively high vapor pressure, which causes problems with mercury trapping and contaminants in the chamber. Liquid metals are generally incompatible with the sensitive and extremely cost-intensive X-ray optics. So on gold optics z. B. are standard in Fresnel zone X-ray microscopy, damage caused by mercury amalgam compounds. In order to avoid contamination, US Pat. No. 5,459,771 proposes using frozen water crystals as the target material. However, this technique has the disadvantage of a large expenditure on equipment in the production of the crystals and in the collection of the target material.

Other liquid target materials have been proposed in particular for applications in X-ray lithography. By L. Rymell et al. is featured in "Rev. Sei. Instrum." Volume 66, 1995, page 4916-4920 describes the use of ethanol as a liquid target material. However, ethanol or other monomeric liquids have the disadvantage that the plasma excitation brings target molecules into the gas phase and deposits on the surfaces of sensitive components. The deposited molecules are decomposed by the generated X-rays, whereby in the case of alcohols, tar-like decomposition products are formed which are reflected as undesirable impurities in the X-ray source and in particular on optical components. To reduce this radiation-induced decomposition, shielding with a gas jet is provided, which, however, disadvantageously complicates the structure. In addition to ethanol, ammonia, water or fluorine-containing liquids are used as target in accordance with WO 97/40650. material used. In order to counter a further general disadvantage of conventional liquid target materials, namely the more difficult drop formation due to low viscosity, it is proposed in WO 97/40650 to introduce the target material in the form of a thin beam into the chamber of the X-ray source. However, monomeric target material is also used in this technique, so that the above-mentioned problems caused by radiation-induced decomposition of precipitation occur. The use of water as a target material is also known from US 6 377 651. In US 6 324 255 it is proposed to use nitrogen, carbon dioxide, krypton or xenon as the target material.

By L. Malmqvist et al. the use of fluorinated hydrocarbon compounds (C _n F _m ) is proposed in "Appl. Phys. Lett." Volume 68, 1996, pages 2627-2629. Although these are well adapted to the generation of fluorine lines (λ ~ 1 to 2 nm), they also have several disadvantages. Firstly, the so-called perfluorocarbons have a high vapor pressure, which makes it difficult to form a liquid jet and to catch the target material after plasma excitation. For example. the vapor pressure of perfluoropentane at 0 ° C is already 0.3 bar. Furthermore, especially in applications in the field of X-ray spectroscopy, the generation of further, longer-wave lines, such as. B. the generation of carbon emissions of interest. So far, however, alcohols have been used as a target for this (Rymell et al., See above).

A general disadvantage of the conventional plasma-based generation of X-rays is the low conversion effectiveness in the irradiation of the target material in order to generate the plasma state. With an increasing atomic mass of the target material, the conversion effectiveness may be increased, but at the same time it becomes more difficult with the increasing atomic mass to provide the target material in the liquid state. The efficiency of the conversion of, in particular, liquid target material into the plasma state, that is to say the ratio of the atoms or molecules of the target material excited in the plasma state to the irradiated energy of the laser light, is therefore relatively low. For example, BAM Hansson et al. ("Proceedings of SPIE", Vol. 4688, 2002, pp. 102-109) indicated an efficiency of only 0.75% for the generation of EUV light.

So far, efforts have been made to improve the focusing of the incident laser light in order to increase the efficiency. However, focusing is a considerable problem under practical conditions, since the target material has hitherto been in the form of a beam or in the form of drops with typical diameters in the range of, for. B. 10 microns to 40 microns is provided. An increase in the beam diameter, which would make focusing easier, would be associated with a greater load on the vacuum in the vacuum chamber. The previously practiced effort to keep the material input into the vacuum chamber as small as possible, that is to say the smallest possible diameter of the steel or droplets, also makes it more difficult to focus the laser light for plasma generation.

The object of the invention is to provide improved methods, in particular for the plasma-based generation of x-rays, with which the disadvantages of conventional techniques are overcome and which are characterized in particular by an increased efficiency in the generation of plasma and thus the generation of x-rays and a simplified fo kissability of the external radiation to generate the Mark the plasma state with constant or reduced material input into the vacuum chamber. The object of the invention is also to provide improved target materials for plasma-based X-ray generation (in particular soft X-rays or extreme UV radiation), with which the disadvantages of conventional target materials are overcome and which are suitable for implementing the methods according to the invention. The target materials are intended in particular to solve the conventional problems in collecting the target material and to avoid the generation of impurities. Finally, it is also an object of the invention to provide an improved x-ray source which is suitable for carrying out the improved methods for plasma-based x-ray steel production.

These tasks are solved by methods and X-ray sources with the features according to patent claims 1 and 22. Advantageous embodiments and applications of the invention result from the dependent claims.

In terms of the process, the invention is based on the general technical teaching, a process for the plasma-based generation of X-rays, in which target material in the form of a free flow structure is irradiated in a vacuum chamber to generate a plasma state, in which the X-rays are emitted, so that white - To further develop that the flow structure is formed with a surface that has different radii of curvature, the target material having a surface with a local minimum of curvature (local

Maximum of the radius of curvature). The target material is therefore irradiated at a point where the flow structure is less curved than along the surrounding surface, or even relative to other parts of the surface is oppositely (negatively) curved. This means that the cross-sectional area of the flow structure is deformed, deviating from the conventionally realized circular shape, into an elongated shape or, if necessary, at least concave on one side.

Under a free flow structure is generally a flowing liquid with a defined surface, z. B. understood in the form of a jet or a liquid layer flowing apart. The liquid flows freely, i.e. with a surface that is free on all sides and without binding to a carrier through the vacuum chamber. The flow structure has a fixed spatial shape, which is therefore essentially unchangeable over time.

The advantages of the flow structure shaped according to the invention result from the following findings of the inventors. The inventors have found that increasing the diameter of a conventionally formed beam of the target material results in a significant improvement in the efficiency of the plasma generation. It was found that the improvement is not due solely to a larger amount of substance in the focus of the external radiation, but to the following effect. A beam of the target material with an enlarged diameter has a less curved surface, which is more favorable for the coupling in of the external radiation energy. On a less curved surface, a larger proportion of the focused radiation can strike the target material with a steeper angle of incidence, so that reflection losses are reduced.

However, an increase in the beam diameter of the target material is undesirable because of the associated increased material input into the vacuum chamber. The invention solves this contradiction in that the flow structure has a non-circular cylindrical shape completely or at least at the location of the irradiation. The radius of curvature of the target surface can thus be maximized at least locally while the material input remains the same. The inventors have found that, surprisingly, free liquid structures can be created in vacuum which, contrary to the striving caused by the surface tension to form a cylindrical or spherical shape to minimize the surface area, are sufficiently stable to achieve the desired flow structure or pattern to form.

Irradiation of the flow structure at a local minimum surface curvature has a number of advantages. First, the angle of incidence of the radiation can be optimized. Reflection losses are reduced. The efficiency of plasma generation can be increased significantly. Furthermore, the target material can offer a larger, free area for irradiation while the material input remains the same. This simplifies the focusing of laser light on the target material and enables the structure of the X-ray source to be simplified. On the other hand, due to the increased efficiency of the plasma generation, a target material with a relatively high vapor pressure with smaller beam dimensions can be introduced without the X-ray intensity being greatly reduced.

Another particular advantage is that, in contrast to the conventional cylindrical or spherical target material from which the X-ray radiation originated with an isotropic distribution, an anisotropic X-ray emission takes place in the method according to the invention. This can be used to further increase efficiency in the generation of X-rays. Furthermore is the anisotropy the emitted x-ray radiation can be measured in relation to the target surface and can also be set by a predetermined rotation of the target surface.

According to a preferred embodiment of the invention, the flow structure which is formed in a free-standing manner in the vacuum chamber is provided with an elongated cross-sectional area. The cross-sectional area given perpendicular to the main flow direction of the flow structure has a greater extent in a main axis direction than in a different, e.g. B. 90 ° on the major axis direction minor axis direction. The local minimum of curvature is thus given on at least one side of the flow structure, which corresponds to the minimum transverse extent of the cross-sectional area. This embodiment of the invention has the particular advantage that the target material provides a particularly large area for external radiation in accordance with the minor axis direction. The cross-sectional area preferably has an oval, e.g. B. elliptical or a rounded, rectangular shape. These variants can have advantages with regard to the provision of the flow structure with one or more nozzles and the handling of the target material in the vacuum chamber.

It is particularly advantageous if the flow structure forms a free-standing liquid layer or liquid lamella at least at the location of the external radiation. The surface of the liquid layer can locally form a flat or vanishingly slightly curved surface, into which externally irradiated laser light can be coupled in particularly effectively.

If the external radiation, in particular with laser light on the target material is essentially perpendicular to the surface before the local minimum of curvature, e.g. For example, on the surface of the free liquid layer, reflection losses during the irradiation can advantageously best be reduced and the efficiency of the plasma generation increased accordingly.

The inventors have developed various methods by which the flow structure can be formed with the desired flattened surface. According to a first variant, the flow structure is generated with a target source which has a nozzle with a non-circular cross section. It has surprisingly been found that the flow shape, which is impressed on the flow structure with a flattened nozzle, for example, is retained in the vacuum chamber over sufficiently long flow lengths. Special advantages for the generation of a liquid layer can result if a nozzle with a slit-shaped cross section is used.

If, according to a variant of the invention, the flow structure has a concave surface at least on one side or preferably on both sides, i. H. has a surface with a negative radius of curvature, the thickness of the flow structure can advantageously be reduced, in particular at the site of the irradiation. The material released in the vacuum chamber during the irradiation can thus be reduced.

According to a further embodiment of the invention, the use of a rotatable nozzle with a non-circular cross section enables the setting of a predetermined orientation of the nozzle and thus of the target material relative to the direction of the irradiation of the target material. The nozzle can be adjusted around an axis in accordance with the main flow direction of the flow structure so that the irradiation development of the target material takes place substantially perpendicular to the surface of the flow structure.

According to a second variant, it is provided that the flow structure is generated with two primary rays of the target material brought together at an angle. At the point where the primary rays meet, an all-round spreading occurs upon collision, in which an essentially layered flow structure is generated. This variant can have advantages with regard to the flexibility in setting the flow structure by varying the flow properties of the primary jets involved.

The creation of baffles between confluent

Liquids are described by G. Taylor in "Proceedings of the Royal Society A", Vol. 259, 1960, pp. 1 to 17. However, the earlier findings of G. Taylor were based on macroscopic systems (nozzle diameter: a few centimeters) at Nor- The inventors have found that, surprisingly, the desired flow structures can also be realized under negative pressure and with microscopic liquid jets (microjets).

If the primary jets are brought together in opposite directions at an angle of 180 °, an axially symmetrical flow structure can advantageously be generated. If the primary beams are brought together at a smaller angle, there may be advantages for the structure of the X-ray source. Intersection angles of the primary beams are preferably chosen to be less than or equal to 180 ° (such as 120 °), in particular less than or equal to 90 °. Another particular advantage of the invention is that the generation of the flattened flow structure with the known target materials for generating X-rays, such as. As water, glycerol, alcohol, liquefied gas, in particular liquefied noble gas, such as. B. Xenon or liquid metal can be realized. However, a target material consisting of at least one hydrocarbon compound which comprises at least one polymer which is liquid at room temperature is preferred. The use of liquid, polymeric hydrocarbon compounds has one

A number of advantages with regard to the provision of the target material in an X-ray source, the avoidance of contamination and the construction of the X-ray source, as shown below.

First, the liquid, polymeric target material is not volatile. Hardly volatile substances can be removed particularly easily from a vacuum chamber in which the plasma is excited to generate radiation. The substances can be caught directly as a liquid in a trap and separated there under their own vapor pressure. A further vacuum system for evacuating the trap is not absolutely necessary, so that the structure of the X-ray source is considerably simplified.

Secondly, the desired spatial shape of the flow structure can be generated with liquid polymers with a particularly high spatial stability. This means that the flattened surface of each flow structure can be provided with a comparatively large distance from the nozzle of the target source, for example up to 100 mm, which considerably simplifies the focusing of the external radiation. Thirdly, the polymers used according to the invention reduce erosion damage in the vacuum chamber. The inventors have found that erosion damage can occur due to the interaction of the gas atmosphere, which is always formed by the vapor pressure of a liquid target, and the generated X-rays. Target molecules present in the gas atmosphere are ionized by the radiation. The deposition of the ions on surfaces in the vacuum chamber, e.g. B. on nozzles for introducing the target material, cause a plasma etching through which the respective material is eroded. The target material which is polymeric according to the invention is not volatile, so that the particle concentration in the gas atmosphere and possible erosion damage are minimized.

Fourth, the precipitation of polymeric target material in the vacuum chamber is not critical. In the case of radiation-induced decomposition, the polymers give rise to readily volatile products which can easily be pumped out of the vacuum chamber. According to the invention, a target material precipitate can even act as a protective film on components of the vacuum chamber, which prevents high-energy polymer fragments from reaching the components directly and, if necessary, can be easily removed during cleaning.

According to a preferred embodiment of the invention, the liquid polymer has at least one ether bond between carbon atoms. The use of a hydrocarbon with at least one ether bond (or oxygen bridge) achieves advantages which also have a positive effect on all phases of the plasma-based generation of X-rays. The oxygen bridge connections between carbon atoms result in high molecular flexibility. This results in a high molecular flexibility (or: low viscosity) of the polymeric target material. The low viscosity has an advantageous effect both on the generation of the flattened flow structure and on the disintegration into low molecular weight components after the plasma excitation. Furthermore, the composition of the target material, in particular of fluorine, carbon and oxygen, leads to an expanded area of use for the target material. A universal target for various applications is provided.

It is particularly advantageous if a polymer which is liquid at room temperature (around 20 ° C.) and comprises at least one partially fluorinated or perfluorinated, polymeric hydrocarbon ether is used as the target material. The partial or complete fluorination of the polymer promotes the formation of volatile decomposition products under X-ray radiation.

A perfluoropolyether (PFPE) or a mixture of several perfluoropolyethers is preferably used as the target material. PFPE compounds are of high molecular weight, which further promotes the formation of the flow structure. Furthermore, by breaking oxygen bridges when energy is supplied, they can decompose into volatile compounds that can be easily pumped off. This prevents deposits and contamination, especially on optical components in the X-ray source. The invention advantageously protects the expensive and sensitive x-ray optics. Undecomposed residues of the target material can be collected particularly easily in a vacuum without special precautions for condensation.

According to preferred embodiments of the invention, the polymeric target material has a vapor pressure which is less than 10 mbar, preferably less than 1 mbar, at room temperature, z. B. 10 ^δ mbar, is a molecular weight greater than 100 g / mol, preferably greater than 300 g / mol, e.g. B. in the range 400 to 8000 g / mol, and / or at room temperature a viscosity which is selected in the range of 1 to 1800 cS. The mass density of the target material is preferably in the range from 1.5 to 2.5 g / mol, e.g. B. 1.8 to 1.9 g / mol. These parameters, which may be provided in combination, improve the shaping of the target material and the collection of material residues after the plasma excitation.

According to a preferred embodiment of the invention, the target material, in particular the liquid polymeric target material, is irradiated in an environment at a pressure which is greater than the gas pressure of the material released during the irradiation. By increasing the vapor pressure of the target material in the vacuum chamber, local supersaturation during plasma generation and thus droplet formation in the vacuum chamber are avoided. In this case, the released gas mostly remains in the gas phase. The evacuation from the vacuum chamber is done by pumps. Advantageously, reduced requirements are thus placed on the vacuum conditions in the chamber of an X-ray source, so that the method can be carried out with less expenditure on equipment.

In terms of the device, the above-mentioned object is achieved by providing an X-ray source for plasma-based generation of X-radiation, which has a target source for providing the target material in the form of a free flow structure in a vacuum chamber and an irradiation device for high-energy irradiation of the target material and is further developed according to the invention for this purpose that the target source is set up to impress a flow shape on the target material, so that a flow structure is formed which has a local minimum of curvature in at least one surface area.

According to a preferred embodiment of the invention, the target source has a nozzle with a non-circular cross section, with which the desired flow pattern is impressed on the target material. A nozzle with a slot-shaped mouth is particularly preferred, since it can be used to form an essentially layered flow structure.

According to a particularly preferred embodiment of the invention, the nozzle, in particular at its outlet opening, has a cross-sectional area that tapers inwards at least on one side. With this design, the concave flow structure described above is advantageously formed. If the nozzle is rotatably arranged in the vacuum chamber, there can be advantages for the alignment of the flow structure for optimal external radiation.

According to an alternative embodiment of the invention, the target source is equipped with two nozzles which are set up to generate primary jets which meet in the vacuum chamber at a predetermined angle. If the nozzles are directed towards each other at an angle of 180 °, there can be advantages for a uniform formation of the flow structure. If the nozzles are directed towards one another at an angle of less than or equal to 90 °, there may be advantages for the structure of the X-ray source and the flexibility in shaping the flow structure.

According to a further embodiment of the invention, the x-ray source has at least one heating device with which at least parts of the vacuum chamber can be tempered. The provision of the at least one heating device results in Particular advantages when using the above-mentioned polymeric target material, since the vapor pressure of the target material can be set higher than the pressure of the gas released by the irradiation of the target material with the heating device. By increasing the temperature, the vapor pressure can be increased, which provides advantages for the construction of the vacuum device and the reduction of precipitation.

If the x-ray source is equipped with radiation optics arranged in the vacuum chamber for irradiating the target material, it can be advantageous to connect a heating device to the radiation optics, so that precipitation of the target material is avoided thereon.

By increasing the effectiveness of the radiation and plasma generation, the efficiency of the X-ray source increases. If the radiation optics are arranged outside the vacuum chamber, it is advantageously possible to dispense with a separate heating device on the radiation optics. The structure of the x-ray source is simplified.

According to a further preferred embodiment of the invention, the x-ray source is equipped with a collecting device for collecting coolant-free target material residues. The X-ray source according to the invention has the advantage of a simplified structure. The stability of the flow structure of the target material simplifies the adjustment of an irradiation device to excite the plasma state. Thanks to the use of a simple vacuum system and the avoidance of a complex cooling device, the X-ray source is suitable as a mobile device for an extended area of application in laboratories and in industry. According to a preferred application of the invention, the X-ray source with an X-ray lithography device, for. B. combined for structuring semiconductor surfaces. The X-ray lithography device can be arranged in the vacuum chamber in the immediate vicinity of the location of the X-ray radiation generation. In contrast to the conventional systems, this is possible for the first time because of the low droplet formation and reduced precipitation of the target material used according to the invention. Conversely, the x-ray source can be integrated directly into an x-ray lithography device. The X-ray lithography device is preferably equipped with its own heating device, so that any residual precipitation that may occur can be easily converted into the gas phase and pumped out.

According to a modified embodiment of the invention, the vacuum chamber of the X-ray source can be combined with an additional vacuum chamber which contains the X-ray lithography device. Due to the simplified construction of the X-ray source according to the invention, both vacuum chambers can be arranged in a small space.

The x-ray source according to the invention has the particular advantage that x-ray radiation (or correspondingly radiation in the far UV range) can be generated during continuous operation. The system can work practically continuously (e.g. over days), which is particularly important for industrial applications of the X-ray source.

Further objects of the invention, which can be implemented analogously to the embodiments described below, but independently of the generation of X-rays, are a vacuum chamber with a nozzle with a slot-shaped outlet opening for the injection of liquid target material. rial into the vacuum chamber and method for injecting a liquid target material in the form of a free flow structure into a vacuum chamber, the flow structure being shaped so that the target material has a surface with a local curvature minimum and preferably forms a free, lamellar layer.

Further details and advantages of the invention are described below with reference to the accompanying drawings. Show it:

1: a schematic illustration of the irradiation of a non-cylindrical flow structure,

FIGS. 2 and 3: illustrations of the beam shaping with a slit-shaped nozzle,

4: a schematic illustration of the cross-sectional area of a concave flow structure,

FIGS. 5 and 6: illustrations of a slit-shaped nozzle,

FIGS. 7 and 8: illustrations for generating a surface target from two primary beams,

FIGS. 9 and 10: Structural formulas for characterizing the target material used according to the invention, and

FIGS. 11 to 14: schematic representations of embodiments of an X-ray source according to the invention. FIG. 1 illustrates the generation and irradiation according to the invention of a liquid target material 50 which is freely standing in a vacuum under vacuum conditions and which has a surface which is at least slightly curved on one side. The target material 50 is shaped as a flow structure, the cross-sectional area of which is exemplarily illustrated perpendicular to the flow direction. The target material 50 is irradiated with an irradiation device 30 (see below). The radiation is directed onto the surface 52 of the flow structure 51, at which the local

Radius of curvature is maximum and the curvature is minimal. As a result, the external irradiation with the entire focusing cross section can take place essentially perpendicularly on the surface 52.

In the example shown, the flow structure 51 has an elongated, in particular elliptical cross section. The y direction forms a main axis direction in which the flow structure has the longitudinal dimension Δy. The x direction with the smaller transverse dimension Δx forms the minor axis direction in which the irradiation also takes place. The target material 50 has, for example, the following geometric parameters: longitudinal dimension Δy: 100 μm to 20 mm, transverse dimension at the location of the irradiation Δx: 2 μm to 2 mm, perpendicular distance of the illustrated cross-sectional area from the nozzle of a target source: 0.1 mm to 10 cm.

Figures 2 and 3 show the creation of non-cylindrical liquid shapes using a nozzle with a slot-shaped outlet opening. FIG. 2 shows the end of the nozzle 13 with the slot-shaped outlet opening 14, which projects into the vacuum chamber (see below). The geometric structure of the nozzle as a slot nozzle is selected in accordance with the desired shape of the flow structure 51 (see also FIG. 5, 6). However, the outlet opening 14 is formed for producing microjets according to the invention with correspondingly smaller dimensions. The slot has a width of 0.1 mm and a length of 3 mm, for example.

Target material passes from the nozzle 13 through the slit-shaped outlet opening 14 into the vacuum chamber of the X-ray source. The exit speed is set so that the target material does not freeze in the vacuum chamber and is, for example, approx. 20 to 100 m / s.

With increasing distance from the nozzle 13, non-cylindrical jets can have a variable jet shape, which is dependent on the viscosity, the surface tension and the flow rate of the emerging liquid. The non-cylindrical shape of the flow initially remains only over a finite range of a few millimeters. In an effort to minimize the surface area, a constriction 53 (FIG. 2) with an essentially circular cross section of the liquid target material is initially formed. Due to the inertia of the liquid moving in the jet, however, the liquid target material 50 is subsequently expanded 54 again.

The alternation of constrictions and widenings forms an oscillating structure, which was theoretically already described mathematically by Rayleigh in "Proceedings of the Royal Society" Volume 29, 1879, pp. 71 to 97 and is illustrated in FIG. 3. Constrictions 53 and widening are alternating - Formed 54, the orientation of the widenings 54 alternatingly perpendicular and parallel to the plane of the drawing. Advantageously, the irradiation of the target material at the location of an expansion 54 after several periods of the luring structure take place where there is a relatively large distance from the nozzle 13. Setting the largest possible distance of the plasma generated in the target material from the nozzle has the particular advantage that the outlet opening of the nozzle before erosion by the radiation released or by charged particles which arise from the plasma or by plasma-induced radiation is protected.

The shape of the oscillating structure, in particular the number of widenings 54 that are implemented and their distance from the nozzle, can be adjusted in particular by a suitable choice of the viscosity of the liquid target material. The target material can thus advantageously be selected for optimal focusing of the external radiation. If the target material is a highly viscous liquid, the oscillations shown do not form. In this case, the flow structure with an elliptical cross section remains relatively far after the outlet opening 14 and changes into the cylindrical shape without back-oscillation. In this

In this case, the irradiation takes place in the area of the primary expansion in accordance with the slot-shaped embossing, the flow structure.

FIG. 4 illustrates, in a schematic, enlarged view, the cross-sectional area of a concave flow structure 51 that is curved inwards on both sides. The surface 52 has a radius of curvature or radius of curvature that is negative relative to the center of the flow structure 51, so that the thickness Δx is reduced towards the center. The thickness can be reduced by up to 99% from the edge to the center, for example, and can be selected in the range from 500 nm to 500 μm. Deviating from the illustration in FIG. 4, a shape that is only concavely curved on one side can be provided. The irradiation of the flow structure 51 is preferably carried out perpendicular to the surface 52 at the location of the minimum transverse extent Δx. Depending on the material or the geometry of the radiation, it may be advantageous, alternatively the

Irradiate surface 52 outside the location of the smallest transverse extent.

The cross-sectional shape of the flow structure is determined in particular by the design of the nozzle of the target source.

Surprisingly, it has been found that in particular the concave or dumbbell-shaped flow shape according to FIG. 4 can be impressed on the flow structure by a suitable nozzle shape and remains stable over a sufficiently large distance when exiting into a room with negative pressure (in particular vacuum).

In principle, the nozzle 13 can be formed by a slot-shaped opening 14 at the end of a line for the target material (FIG. 2). Particular advantages for a stable, non-cylindrical jet result when using a nozzle structure, which is illustrated in FIGS. 5 and 6. FIG. 5 shows the mouth or outlet opening of a nozzle 13 in the flow direction (from the inside, left partial image) and counter to the flow direction (from the outside, right partial image). A nozzle slot 14a is provided on the inside, which extends over the entire width of the outlet opening 14 and whose slot width decreases in the direction of flow (see the right partial image in FIG. 6). A conical mouth 14b is provided in the flow direction to the nozzle slot 14a, through which the target material 50 exits into the vacuum chamber (see FIG. 6). The flowing target material is first pressed through the nozzle slot 14a, where it converges. Then the tar- Get material apart at the edges of the cone opening 14b, so that the desired lamella shape of the flow structure results. The first oscillation of the flow structure (see FIG. 3) is influenced by the cone opening 14b.

A particular advantage of the nozzle 13 according to FIG. 5 is that the concave shape of the flow structure according to FIG. 4 is formed by the interaction of the nozzle slot 14a and the cone opening 14b. The thickness of the flow structure 51 increases towards the edges (see dashed line in FIG. 6).

According to a preferred embodiment of the invention, the nozzle for generating the flattened flow structure is rotatably arranged. The rotatability refers to the axis of the

Direction of exit or direction of injection or flow of the target material through the nozzle. The rotatability can be achieved, for example, by using a rotary holder for the nozzle and a twistable liquid line from the target source. Alternatively, a rigid liquid line can be connected to the nozzle via a rotary coupling. In order to set a specific orientation of the nozzle, in particular relative to the direction of irradiation, the nozzle is equipped with an adjusting device which comprises, for example, a stepper motor or a piezoelectric drive.

FIGS. 7 and 8 illustrate the formation of the flow structure 51 on the impact surface between two primary jets 55, 56 of the target material, which are directed towards one another with two separate nozzles 15, 16 in the vacuum chamber. FIGS. 7A to 7C are based on drawings from the publication by G. Taylor mentioned. According to the figures 7A and 8A, two primary beams with a diameter of z. B. 30 microns at an angle of z. B. 60 ° brought together, so that the flow structure 51 with a thickness of less than 30 microns (z. B. 3 microns) and an expansion of z. B. forms 1 to 2 mm. If, according to FIG. 7B, the meeting of the primary beams 55, '56 at an enlarged cutting angle of z. B. 90 °, the flow structure 51 is also formed above the impact surface, there is a greater expansion of the layer of the flow structure 51. If the nozzles 15, 16 are oriented opposite to each other by 180 ° according to Figure 8B or 8C, a flow structure results 51 according to FIG. 7C, which can be irradiated laterally horizontally (FIG. 8B) or vertically via a deflecting mirror (FIG. 8C).

In general, the location of the combination of the primary jets is selected so that the primary jets have not yet decayed into drops (the distance from the nozzles is less than the drop decay distance). The nozzles 15, 16 can have circular or slot-shaped, in particular elliptical or rectangular cross-sectional areas.

The combination of two jets (primary jets) has the advantage that the shape of the flat flow structure in the room is variable. In this case too, the flow structure can be provided at an increased distance from the nozzle 13.

The target material preferably used according to the invention in a plasma X-ray source is based on a polymeric hydrocarbon compound which is liquid at room temperature, in particular with at least one ether bond. A building block of such a hydrocarbon compound is illustrated by way of example in FIG. 9. It is emphasized that the implementation of the invention is not limited to the illustrated examples. As an alternative to fluorinated polyethers, according to generally also non-fluorinated polymers, mixtures of fluorinated and non-fluorinated polymers or polymers with a low solvent content (less than 20% by volume) can be used. Furthermore, the fluorination can at least partially be replaced by another halogenation, in particular chlorination.

The target material shown by way of example in FIG. 9 consists of a large number of building blocks constructed in this way or correspondingly from C, F, 0 and possibly H, so that a low-volatility polymer is formed. The use of the low volatility polymer advantageously reduces the demands on the vacuum system of an X-ray source.

The target material forms in particular a partially or perfluorinated polyether (PFPE) or a mixture of several partially fluorinated or perfluorinated polyethers. A perfluoropolyether is illustrated by way of example in FIG. 10. The PFPE compounds FOMBLIN (registered trademark) and GALDEN (registered trademark) also belong to this class of substances.

An example of an X-ray source according to the invention is schematically illustrated in FIG. The x-ray source comprises a target source 10, which is connected to a temperature-controllable vacuum chamber 20, an irradiation device 30 and one

Collecting device 40. The target source 10 comprises a reservoir 11 for the target material, a feed line 12 and a nozzle 13. With an actuating device (not shown), which comprises, for example, a pump or a piezoelectric conveying device, target material is fed to the nozzle 13 and from there dispensed in the form of a liquid jet 50 and injected into the vacuum chamber 20. For example, the liquid jet 50 is vertically injected into the vacuum chamber 20 as shown. Alternatively, another beam direction, such as a horizontal injection or an injection at a different angle relative to the horizontal, can be provided to implement the invention.

The radiation device 30 comprises a radiation source 31 and a radiation optics 32 with which radiation from the radiation source 31 can be focused onto the target material 50. The radiation source 31 is, for example, a laser, the light of which is possibly directed towards the target material with the aid of deflecting mirrors (not shown). Alternatively, an ion source or an electron source can be provided as the radiation device, which is also arranged in the chamber 20.

The collecting device 40 comprises a sensor 41 z. B. in the form of a funnel or a capillary, the target material, which has not evaporated under the influence of the radiation, removed from the vacuum chamber and passed into a collecting container 42. Because the liquid polymer is used as the target material, the collected liquid can advantageously be collected in the collecting container 42 without further measures. In order to avoid the risk of backflow of collected target material into the vacuum chamber 20, if necessary, cooling of the collecting container 42 can be provided with a cooling device (not shown) and / or a vacuum pump (not shown).

The vacuum chamber 20 comprises a housing 21 with at least one first window 22, through which the target material 50 can be irradiated, and at least one second window 23, through which the generated X-ray radiation emerges. The two te window 23 is optionally provided to decouple the generated X-rays from the vacuum chamber 20 for a specific application. If this is not necessary, the second window 23 can be omitted (see below). The vacuum chamber 20 is also connected to a vacuum device 24, with which a negative pressure is generated in the chamber 20. This negative pressure is preferably below 10 ^{~ 4} mbar. The radiation optics 32 are also arranged in the vacuum chamber 20.

The vacuum chamber 20 is equipped with a heating device 60, which comprises one or more thermostats 61 to 63. The housing 21, the sensor 41 and / or the radiation optics 32 can be temperature-controlled with the thermostats. Possibly. the target source 10 can also be tempered. A thermostat includes, for example, a resistance heater known per se.

The set with the heater 60 temperature is chosen so that the vapor pressure of, ^'which is formed by irradiation of the target material 50 with the irradiation device 30 and in particular of polymeric target material exceeds the gas pressure. This avoids oversaturation of the gas phase in the vacuum chamber. The released polymer remains gaseous and can be pumped out almost quantitatively with the vacuum device 24.

The second window 23 consists of a window material transparent to soft X-rays, e.g. B. from beryllium. If the second window 23 is provided, an evacuable processing chamber 26 can be connected, which is connected to a further vacuum device 27. In the processing chamber 26, the x-ray radiation for material processing can be imaged on an object. For example, it is one X-ray lithography device 70 is provided, with which the surface of a semiconductor substrate is irradiated. The spatial separation ^'of the X-ray source in the vacuum chamber 20 and the X-ray lithography device 70 in the machining chamber 26 has the advantage that the deposits not exposed of evaporated target material to be machined material.

The x-ray lithography device 70 comprises, for example, a filter 71 for selecting the desired x-ray wavelength, a mask 72 and the substrate 73 to be irradiated. In addition, imaging optics (for example mirrors) can be provided in order to direct the x-ray radiation onto the device 70.

In the modified embodiment of the invention according to FIG. 12, the X-ray lithography device 70 is arranged in the vacuum chamber 20. In order to avoid precipitation, the device 70 is also connected to a thermostat 64. FIG. 12 further illustrates the use of a double nozzle 15, 16 (see FIG. 8) for generating flow structures according to FIG. 7.

If the radiation optics 32 according to FIG. 13 are arranged outside the vacuum chamber 20, a separate temperature control can advantageously be dispensed with. In this

In this case, however, the window 22 must be sufficiently stable with respect to the at least partially focused and possibly highly repetitive radiation from the radiation source 31. Furthermore, in this embodiment, the target material 50 is guided past the window 22 in a relatively tight manner (for example at a distance of a few cm). In this embodiment too, a double nozzle can be used instead of the illustrated nozzle 13. If liquid polymers are used as target material, the vapor pressure of which is so high that temperature control of the housing 21 is not required, sensitive components of the vacuum chamber 20, such as, for example, B. the imaging optics 32 or the device 70 are heated. This embodiment of the invention is illustrated in FIG. 14. The local heating advantageously ensures that the target material released during the irradiation is preferably deposited on the colder walls of the housing 21. The sensitive components that are important for the respective application are protected.

For the generation of x-rays according to the invention, a beam or drop of the target material 50 in the form of the flow structure according to the invention is generated with the target source 10. The flow structure 50 is irradiated with the irradiation device 30 in a manner known per se. The radiation is focused with such an intensity that the target material is converted into a plasma state. For example, an energy supply of 100 mJ per irradiation pulse (e.g. per laser shot) is provided. An output power of up to 50 W is achieved at a pulse rate of 10 kHz. In the plasma state, soft x-ray radiation is emitted and coupled out through the second window 23 for the respective application.

The X-ray radiation has a wavelength range of up to approximately 15 nm. In particular, the Kα line with λ = 3.37 nm, F lines with λ = 0.7 nm to 1.7 nm and 12.6 nm and the O line with λ = 13 nm are advantageously emitted , It is particularly advantageous that when using perfluoropolyether, the carbon Kα line can be generated while avoiding annoying graphite deposits. In X-ray microscopy, the Kα line is of great interest, since this falls into the so-called "water window", in which no X-ray absorption by water occurs. Due to the permanent avoidance of erosions and deposits, the X-ray source according to the invention is excellently suitable for X-ray microscopic and lithographic applications. Another advantage is the miniaturization of the structure. The device 70 (see FIG. 12) can be arranged in the immediate vicinity of the focus of the irradiation device 30.

Because of the low volatility of the material used according to the invention, the collecting device 40 can advantageously be operated without a coolant and without a cooling device. In particular, it is not necessary to provide a so-called cryoprobe or a separator for condensing residual materials. The sensor 41 and the collecting container 42 are connected directly to one another.

The residual materials not captured by the collecting device 40 are advantageously volatile components which can be removed from the chamber 20 with the vacuum device 24. The vacuum devices 24, 27 include, for example, rotary vane oil pumps.

Preferred applications of the x-ray source according to the invention are in analytical chemistry, in x-ray microscopy, in x-ray lithography and in combination with other spectroscopic measurement methods, such as, for. B. fs spectroscopy.

Further applications of the invention exist wherever an interest in the study or use of free liquids under vacuum conditions is of interest. For example, liquid samples for photoelectronic or Photo absorption spectroscopic examinations or corresponding scattering experiments are introduced into the respective examination chamber in accordance with the technique according to the invention. High-energy radiation or particle bombardment can be provided.

An alternative application of layered target materials according to the invention is given in possibly time-resolved X-ray absorption experiments with synchrotron radiation (see KR Wilson et al. In “J. Phys. Chem. B ^λλ , Vol. 105, 2001, pp. 3346-3349). The increased longitudinal extent of the layer flow is also advantageous in these applications, since it makes it easier to focus the radiation onto the target.

Finally, the liquid layer formed according to the invention can be used as a source for droplets or macro clusters (spray). After a finite distance from the nozzle, the flow structure disintegrates into individual droplets, which are irradiated to generate X-rays.

The features of the invention disclosed in the above description, the drawings and the claims can be significant both individually and in combination for the implementation of the invention in its various configurations.

Claims

1. Method for the plasma-based generation of X-rays, with the steps:

- Provision of a ^' target material (50) in the form of a free flow structure (51) in a vacuum chamber (20), and

- Irradiation of the target material (50) to produce an impure plasma state in which the X-radiation is emitted, characterized in that

- The flow structure (51) is shaped so that the target material has a surface (52) with a local minimum curvature at least at the location of the radiation.

2. The method according to claim 1, wherein the flow structure (51) at least at the site of the irradiation has a cross-sectional area which has a longitudinal dimension Δy in a main axis direction (y) which is greater than a transverse dimension Δx in one of the main axis direction ( y) deviating minor axis direction (x).

3. The method of claim 2, wherein the flow structure (51) has an oval cross-sectional area or a rounded, rectangular cross-sectional area at least at the location of the radiation.

4. The method according to claim 2 or 3, wherein the flow structure (51) forms a free, lamellar layer at least at the location of the irradiation.

5. The method according to claim 3 or 4, wherein the flow structure (51) has a concave surface at least on one side at least at the location of the radiation.

6. The method according to at least one of the preceding claims, wherein the flow structure (51) of the target material is generated with a target source which has a nozzle with a non-circular outlet opening (14).

7. The method according to claim 6, wherein the flow structure (51) of the target material is generated with a dispenser having a nozzle with a slot-shaped outlet opening (14).

8. The method according to claim 6 or 7, wherein the nozzle for

Setting a predetermined orientation relative to the direction of irradiation of the target material (50) is rotated.

9. The method according to at least one of the preceding claims 1 to 4, in which the flow structure (51) of the target material is generated with two primary jets which are brought together at a predetermined angle to form a free-standing liquid layer.

10. The method of claim 9, wherein the primary beams are brought together at an angle which is less than or equal to 180 °.

11. The method of claim 9, wherein the primary rays are brought together at an angle that is less than or equal to 90 °.

12. The method according to at least one of the preceding claims, wherein the flow structure (51) of the target material on the surface (52) is irradiated with the local minimum curvature substantially perpendicular.

13. The method according to at least one of the preceding claims, in which one of the following materials is used as the target material: at least one hydrocarbon compound which comprises at least one polymer which is liquid at room temperature, water, glycerol, alcohol, liquefied gas or liquid metal.

14. The method of claim 13, wherein the hydrocarbon compound used as the target material has at least one ether bond between carbon atoms.

15. The method according to claim 14, wherein the hydrocarbon compound used as the target material has at least one partially fluorinated or perfluorinated, polymeric carbon-hydrogen ether.

16. The method according to claim 15, wherein the hydrocarbon compound used as the target material comprises a perfluoropolyether or a mixture of perfluoropolyethers.

17. The method according to at least one of claims 13 to 16, wherein the hydrocarbon compound used as target material has a vapor pressure at room temperature less than 10 mbar, a molecular weight greater than 100 g / mol and / or a viscosity in the range of 1 cS to 1800 cS ,

18. The method according to any one of the preceding claims 13 to 17, in which the irradiation of the target material (50) takes place in a vacuum chamber (20) which is at least locally heated in this way is that the vapor pressure of the target material (50) is higher than the pressure of the gas released by the irradiation of the target material (50).

19. The method according to any one of the preceding claims 13 to 18, in which the target material (50) after the irradiation is collected in a collecting device (40) at room temperature.

20. Use of polymeric hydrocarbon compounds which are liquid at room temperature to provide target material in the form of a flow structure (51), the target material having a surface with a local minimum of curvature at least at the point of irradiation for generating soft X-rays.

21. Use of partially fluorinated or perfluorinated, polymeric hydrocarbon ethers to provide target material in the form of a free flow structure (51), the target material having a surface with a local minimum of curvature at least at the point of irradiation to produce soft X-rays.

22. X-ray source for the plasma-based generation of X-rays by high-energy irradiation of a target material (50) in the form of a free flow structure (51), which comprises:

- A target source (10), which provides the target material (50) in a vacuum chamber (20), and

- An irradiation device (30) for irradiating the target material (50) in the vacuum chamber, characterized in that:

- The target source is set up to shape the target material so that the target material in the flow structure (51) has a surface with a local minimum of curvature at least at the location of the irradiation.

23. X-ray source according to claim 22, wherein the target source is a nozzle (13) with a non-circular outlet opening

(14) has.

24. X-ray source according to claim 23, wherein the target source has a nozzle (13) with a slot-shaped outlet opening (14).

25. X-ray source according to claim 24, in which the target source has a nozzle (13) with an elliptical, rectangular or convexly tapering outlet opening (14).

26. X-ray source according to claim 24, wherein the nozzle (13) has an outlet opening (14) with a nozzle slot (14a) and a cone opening (14b).

27. X-ray source according to at least one of claims 23 to 26, wherein the nozzle (13) in the vacuum chamber (20) is rotatably arranged.

28. X-ray source according to claim 22, in which the target source has two nozzles (15, 16) for generating primary rays which are brought together at a predetermined angle to form a free-standing liquid layer (51).

29. X-ray source according to claim 28, wherein the nozzles (15, 16) are aligned so that the primary rays are brought together at an angle of 180 °.

30. X-ray source according to claim 28, wherein the nozzles (15, 16) are aligned so that the primary rays are brought together at an angle which is less than or equal to 90 °.

31. X-ray source according to at least one of claims 22 to 30, in which at least one heating device (60) is provided, with which at least parts of the vacuum chamber (20) can be tempered.

32. X-ray source according to claim 31, wherein the heating device (60) comprises a plurality of thermostats (61-64) which are connected to components on and / or in the vacuum chamber (20).

33. X-ray source according to claim 32, in which the radiation device has radiation optics which are arranged in the vacuum chamber (20) and connected to a thermostat (63).

34. X-ray source according to at least one of claims 22 to 32, in which the irradiation device has an irradiation optics which is arranged outside the vacuum chamber (20).

35. X-ray source according to at least one of claims 22 to

34, in which a collecting device (40) for collecting the target material (50) after the irradiation is provided, which is set up for coolant-free operation.

36. X-ray source according to at least one of claims 22 to

35, in which an X-ray lithography device (70) is arranged in the vacuum chamber (20).

37. X-ray source according to claim 36, wherein the X-ray lithography device (70) is connected to a thermostat (64).

38. X-ray source according to at least one of Claims 22 to 37, in which the vacuum chamber (20) is connected to a processing chamber (26) in which an X-ray lithography device (70) is arranged.

39. Vacuum chamber with a nozzle (13) with a slot-shaped outlet opening (14) for injecting liquid target material into the vacuum chamber.

40. Vacuum chamber according to claim 39, wherein the nozzle (13) is rotatably arranged about an axis which is parallel to the direction of injection of the liquid target material.

41. Method for injecting a liquid target material

(50) in the form of a free flow structure (51) in a vacuum chamber (20), characterized in that the flow structure (51) is shaped in such a way that the target material has a surface (52) with a local curvature minimum.

42. The method of claim 41, wherein the flow structure

(51) forms a free, lamellar layer.

43. The method of claim 41 or 42, wherein the flow structure (51) has a concave surface (52) on at least one side.