EP3262658A1

EP3262658A1 - Layer structure for multilayer laue lenses or circular multilayer zone plates

Info

Publication number: EP3262658A1
Application number: EP16707082.0A
Authority: EP
Inventors: Stefan Braun; Adam KUBEC; Andreas Leson; Maik MENZEL
Original assignee: Fraunhofer Gesellschaft zur Forderung der Angewandten Forschung eV
Current assignee: Fraunhofer Gesellschaft zur Forderung der Angewandten Forschung eV
Priority date: 2015-02-27
Filing date: 2016-02-26
Publication date: 2018-01-03
Also published as: WO2016135279A1; DE102015203604A1; DE102015203604B4

Abstract

The invention relates to a layer structure for multilayer Laue lenses or circular multilayer zone plates for x-ray radiation, comprising layers which are arranged on a substrate in an alternating manner and each of which is made of a material with a high refractive index and a material with a low refractive index. The layer structure is made of layers which are arranged in an alternating manner within periods consisting of a high-refractive layer, a low-refractive layer, and at least one third material for intermediate layers. Low-refractive layers have internal pressure stress and high-refractive layers have internal tension stress, and the product of the layer thickness and the internal stress is equal, or at least approximately equal, in magnitude. An intermediate layer is arranged between each of the high- and low-refractive layers.

Description

Layer structure for multilayer Laue lenses or circular multi-layer

zone plates

The invention relates to a layer structure for multilayer Laue lenses or circular multi-layer zone plates.

Multilayer Laue lenses are usually of planar geometry, circular multilayer zone plates usually have a curved geometry, e.g. in the form of wire coatings, on. Common to these lenses is that they should focus or image X-rays. For simplicity, both embodiments will be referred to below as MLL.

The structures for these lenses are produced in known thin-film processes, in which at least two different materials are alternately applied to an extremely smooth starting surface of a substrate, the sum of the layer thicknesses of a period corresponding to the zone plate law increasing with distance from the optical axis decreases.

An important goal of current research is the increase of the total thickness of the MLL coatings, as this determines the numerical aperture of the lens and thus the resolution limit or the focusing limits.

A technical problem is present, that by the well-known

Coating process in the production of multilayer Laue lenses usually compressive stresses occur. These are detectable by measuring the wafer curvature before and after the coating and the application of the so-called Stoney equation. In particular, with increasing overall MLL thicknesses, this can lead to deformation or even destruction of the substrates to be coated (usually silicon wafers). In addition, cracks may form within the multilayer system or the layer may delaminate.

All previously published works on multilayer Laue lenses are based on the sequence of layers with two different materials. The

Problem of residual stresses is well known and will

currently solved by various approaches:

• by using substrates of higher thickness to reduce deformation through the MLL coating,

• by heat treatment of the MLL after coating

stress relaxation,

• by changing the layer thickness ratios between z. B.

Silicon and tungsten silicide in favor of tungsten silicide to the influence of the introduced through the Si coating

To reduce residual compressive stresses.

However, the solutions used so far only partially succeed. In particular, they are not suitable for an increase in the MLL coating thickness.

In the case of using thick substrates, the

Leakages in the layer lead to the flaking off of the layers. In most cases, the weakest point forms at the interface between substrate and layer so that the layer completely delaminates. Ion pretreatment can reduce but not completely solve this problem.

Furthermore, the layer stresses can also lead to microcracks within the multilayer. Areas with such cracks are for the

Can not be used as a lens.

The subsequent heat treatment is always possible in principle. However, this does not solve the problems of cracking or delamination during coating. A heat treatment can only be used if

sufficiently low residual stresses are present and there are no cracks in the layer. If high temperatures are required for the relaxation of the residual stresses, this can adversely affect the interfaces of the multilayers, since in this case diffusion is stimulated which leads to a deterioration of the optical properties of the lens.

Even the third approach - the change in the layer thickness ratio - can only be used sensibly within certain limits. Too large a deviation from the ideal thickness ratio of 1: 1 between low- and high-index layers leads to significant efficiency losses. Therefore, with this

Method only a reduction in residual stresses can be achieved, but does not allow any increase in the layer thicknesses.

Since the lenses are to be used in the beam path of intense X-ray radiation at the synchrotron, material combinations must be used which have a low Interdiffusionsneigung and high thermal stability. Usually, in the literature so far

Material combinations such as WSi ₂ / Si, MoSi ₂ / Si, Ti / Zr0 ₂ or Al ₂ 0 ₃ / Ta0 ₂ described.

In-situ investigations of WSi ₂ / Si multi-layer systems have shown that residual compressive stresses mainly occur during deposition of the silicon layers.

An obvious approach would therefore be to replace silicon with an alternative material with lower residual compressive stresses. There as However, the boundary condition must be taken into account that both a high X-ray optical contrast to the second layer material, as well as a low X-ray absorption is given, only elements of low atomic number or their compounds come into question. Examples are Be, B, C, Mg, Al, Si, Ca, Sc, Ti, V and their oxides, carbides or nitrides.

Among these materials, however, there is only one aluminum with which low-stress layers are known. However, aluminum has extremely unfavorable growth and forms very rough layers. It is expected that if aluminum is used as the layer material in MLL, there would be an unacceptable increase in the roughness from layer to layer, which leads to poor diffraction properties of the lens.

It is therefore an object of the invention to provide a multilayer structure for thermally stable and low-stress nanometer multilayers, so that these multilayers are better suited for use in MLL. With the present invention, a solution is proposed, which at the same time to high diffraction efficiencies in the

X-ray focusing leads.

According to the invention, this object is achieved with the features of claim 1. Advantageous embodiments and further developments of the invention can be realized with features described in the subordinate claims.

The approach taken in this invention for the layer construction of an MLL is to accept the compressive stresses of the low-refractive layers and to provide for immediate compensation in the choice of the other involved layer partners.

In an inventive layer structure, the compressive stresses of the lower refractive layers 0) by residual tensile stresses of the higher-refractive-index layers _(σ Α, ί> 0) completely, at least almost completely compensated for, so that d _A, iC _A, i + dsj rsj = 0. Thus it is known in that Mo / Si multilayers can be produced completely free from inherent stress with a suitable choice of the layer thicknesses. Higher-index layers are formed of a material in which at the interface to the vacuum, a stronger deflection of the

original propagation direction of the radiation takes place. Such a high refractive index material has a smaller refractive index (n _A = 1 - δ _Α ). Accordingly, low-index layers have a larger one

Refractive index (n _s = 1 - 5 _S ). The distinction between higher refractive and lower refractive is made on the difference of the respective

Refractive index to the refractive index = 1.

In general, the residual stress can not at a

Layer thickness ratio of d _A : d _s = 1: 1 can be achieved. The magnitude higher compressive stresses of the lower-refractive layers can have a higher thickness d _{A of} the tensile residual stresses

higher refractive layers are counteracted.

In order nevertheless to obtain a correct ratio between lower and higher refractive index materials, according to the invention at least one third layer material for intermediate layers (ZS) is in the stack

included in the sense of the invention additionally meet the following requirements:

• It should be the formation of a nearly stress-free layer ~ 0), ie any residual stresses are max. at 20% of the permissible compressive stress of the respective lower-index layers, or max. 20% of the permissible tensile stresses of the respective higher refractive layers,

• The material should have the lowest possible absorption for

X-ray radiation, i. the degree of absorption should be close to zero (ßzs ~ ^ min) and lower than the absorption coefficient of the higher refractive layer,

• The refractive index n _Z s = 1-5 _Z s of the intermediate layers should be as similar as possible to that of the low-refractive index material, ie max. by 10% of this, and the material of the intermediate layers should be lower refractive than the material of the high-index layers (5s <5 _ZS <δ _Α ). According to the invention, the intermediate layers should be arranged between lower and higher refractive index layers. The interlayers may advantageously act as a diffusion barrier between the lower and higher refractive index layers to increase the temperature stability of the overall system.

The coating systems hitherto available according to the prior art have high residual compressive stresses. These lead to a deformation of the components to be coated, which on the one hand adversely affect the optical performance and on the other hand mechanical problems, such. Cracks or even fractures, can cause. The crucial point is the force exerted by the layer on the substrate. In order to reduce this, every single shift should still be taken for granted

Have residual stresses. If, however, opposite

Voltage states, i. Compressive and tensile stresses are present, can be achieved in equal amounts of the products from the layer's own stress and layer thickness that the resulting force of the multilayer to the substrate goes to zero.

In the case of the combination of molybdenum for higher refractive index layers and silicon for lower refractive index layers, MoSi ₂ is particularly suitable for intermediate layers. In addition, however, other materials, such. As Mo ₂ C, aC, B ₄ C, BN, Al ₂ 0 ₃ or other chemically stable compounds of elements of low atomic number conceivable, especially if they can be deposited low in natural light.

Based on the layer structure described, a sequence of periods whose thicknesses follow the zone plate law is produced by coating (dp, i_i <dp, i <dp, _{i +} i).

With regard to the layer thicknesses apply in an inventive

Layer structure prefers the following specifications:

• d _A , i = (0.5 ± 0.2) -d _<i , with d _P thickness of the layer period and d _A thickness of higher refractive layer, and

• d _ZS i, i + d _ZS 2, i = d _P , i - d _A , i - d _s , i, with dzsij and d _ZS 2, i = thicknesses of

Interlayers 1 and 2 in the period i, d _s as the thickness of the lower refractive layer.

With a downstream structuring of the coated substrate, the desired MLL can be obtained, which can be used in transmission for the focusing of X-radiation. The deposition of the multilayer system according to the invention can be effected both on planar surfaces (eg silicon wafers), but also on concavely or convexly curved surfaces. In the first case, this is the classical MLL (linear type), in which two one-dimensional focusing or imaging optics must be crossed. In the second case, the term usually refers to multi-layer zone plates

(circular type) used. Here already a single MLL is sufficient for the point focusing or two-dimensional mapping.

The coated structures can be either directly at the

Be provided coating with a lateral Dickengradienten or subsequently, z. B. be converted by lateral coating in the so-called wedged geometry. This is favorable in high aspect ratios, as they are commonly used in MLL, to high

To obtain diffraction efficiencies and the best possible focusing properties of the lenses.

The use of the thermally stable and

Low-stress nanometer multilayer systems open up this

Potential to produce coatings for multilayer Laue lenses with significantly greater overall thicknesses (> 50 μm) without the risk of deformation,

Delamination or cracking exists. With the higher layer thicknesses, larger numerical apertures can be realized, which in turn are a prerequisite for the focusing of X-radiation in nanometer dimensions. In particular, real microscopy and tomography experiments require focal lengths in the centimeter range, which for focal spot sizes in the

Nanometer range are possible only with significantly increased layer thicknesses. The technical application potential includes above all the use of the MLL according to the invention as optics for X-ray microscopy and

-tomography. The improvement of the resolution with MLL and reduced measurement times result in considerable potential for nanoanalytics. A high-resolution X-ray tomography for error analysis in semiconductor technology would be conceivable, for the investigation of

Replacement processes in energy storage (batteries, fuel cells) and for the examination of biological samples.

In the following, the layer structure according to the invention will be explained in more detail by means of examples.

Showing:

Figure 1 shows the layer structure according to the invention for almost

residual stress-free MLL layers,

2 shows schematically the manufacturing process of a MLL with the

Layer structure according to the invention.

FIG. 1 schematically shows the layer structure for MLL layers with almost no residual stress. 1 denotes the higher refractive index layers, 2 the lower refractive index layers and 3 the intermediate layers. After each higher-refractive-index layer 1 and each lower-refractive-index layer 2, there follows an intermediate layer 3. In each case a higher refractive index layer 1, a lower refractive index layer 2 and the associated intermediate layers 3 form a period. The layer thicknesses are different in each period, where dp _j -i <d _<i <d _{<i +} i. The thickness of the higher-refractive-index layer 1 is given by d _A , i = (0.5 ± 0.2) d _{p <i} . The thickness of the intermediate layers 3 is d _zs i _j or d _ZS 2, i and together with the thickness of the higher refractive index layer 1 and the thickness of the lower refractive layer 2 the total thickness of one period (d _<i = d _A , i + d _s , i + d _Z si, i + d _Z s2, i) - The thickness of the low-refraction layer 2 is chosen such that d _A , i σ _Α , _ί + d _s , i σ _Α , = 0. FIG. 2 is a schematic representation of the process steps for producing a one-dimensionally focusing multi-layer Laue lens (linear type) from a wafer coating. From a wafer, on the one

Was applied according to the invention MLL layer structure, a strip 4 is cut out with appropriate dimensions. By means of structuring, the desired MLL element 5, similar to the production of a

Cross-sectional preparations for transmission electron microscopy, exposed by removing the applied MLL layer structure according to the invention in the areas 6. The thickness of the exposed MLL element 5 is typically a few micrometers. The size of the exposed MLL element 5 depends on the MLL coating thickness. In the case of a 50 μιη thick MLL coating this z. B. as a width x height of 55 μιη x 55 μιη be. It is important that no damage occurs within the multilayer due to the structuring and that the length of the exposed MLL element 5 is homogeneous parallel to the optical axis over the above-mentioned MLL aperture of 55 μm x 55 μm.

In one example of the inventive construction of multilayer Laue lenses, an MLL layer structure having a total thickness of approximately 50 μm was selected and applied to a substrate.

A total of 12800 individual layers were alternately arranged, forming the zones numbered 600 to 7000. In this case, there is alternately one zone each of the higher refractive index layer and the other zone of the low refractive index layer and the intermediate layers. The

The higher refractive index layer 1 was formed of tungsten (W), the lower refractive index layer 2 of silicon (Si), and the intermediate layers 3 of tungsten disilicide (WSi ₂ ).

The smallest layer thickness of the higher refractive layers 1 in the period i = 1 was 5.30 nm, belonging to the same period i = 1

Low-refraction layer 2 had a layer thickness of 3.30 nm, and the intermediate layers 3 also belonging to this period i = 1 had layer thicknesses of 1.0 nm and 1.0 nm.

The largest layer thickness of the higher refractive layers 1 in the period i = 3200 was 18.08 nm, the low-refraction layer 2 belonging to the same period i = 3200 had a layer thickness of 11.26 nm and the intermediate layers 3 also belonging to this period i = 3200 had layer thicknesses of 3.41 nm and 3, 41 nm.

The decrease in the total thickness of the periods i starting from the optical axis thus follows the zone plate law.

After the layers were applied to the substrate, a strip 4 approximately 10 mm x 0.05 mm in size was sawn out of the substrate. By structuring the MLL element 5 was exposed. For this purpose, a thickness of μιη of a width of 55 μm was selected as the thickness of the layer stack formed from alternately arranged layers. Such a lens has a focal length of 9.5 mm at a photon energy of E = 15 keV.

Claims

claims

Layer structure for multilayer Laue lenses or circular multi-layer zone plates for X-radiation with layers (1, 2) arranged alternately on a substrate, each of a material with a higher refractive index and a material with a lower refractive index, characterized in that this layer structure alternately arranged layers within periods, consisting of a high-refractive layer (1), a low-refractive layer (2) and at least a third material for intermediate layers (3) is formed, wherein lower-index layers (2) compressive stress and higher-refractive layers (1) tensile residual stress and the product of layer thickness and residual stress is equal in size, at least approximately equal, and between higher and lower refractive layers (1, 2) is arranged in each case an intermediate layer (3).

Layer structure according to claim 1, characterized in that the residual stress of the intermediate layers (3) at least nearly zero (azs, i * 0).

Layer structure according to one of the preceding claims, characterized in that the material or the materials of intermediate layers (3) have a lower absorption for the X-radiation than the high-index layer (1).

4. Layer structure according to one of the preceding claims, characterized in that the refractive index of the intermediate layers (3) is in the range between that of the lower refractive index material and the higher refractive index material.

5. Layer structure according to one of the preceding claims, characterized in that the intermediate layers (3) form a diffusion barrier between the layers with higher and lower refractive index (1, 2).

6. Layer structure according to one of the preceding claims, characterized in that the sum of the layer thicknesses of a period decreases with increasing distance from the optical axis of the X-ray radiation.

7. Layer structure according to one of the preceding claims, characterized in that the thickness of the layer with the higher refractive index (1) d _A , i the (0.5 ± 0.2) times the period thickness d _{p <i} corresponds, wherein the Period thickness is the sum of the thicknesses of the higher refractive layer (1) and the lower refractive layer (2) and the associated two intermediate layers (3) (d _A , i = (0.5 ± 0.2) d _p , i).

8. Layer structure according to one of the preceding claims, characterized in that the material of the intermediate layers (3) is selected from MoSi ₂ , Mo ₂ C, aC, B ₄ C, BN, Al ₂ 0 ₃ and other chemically stable compounds with elements with low atomic number.

9. Layer structure according to one of the preceding claims, characterized in that the layer structure is applied to concave or convex curved surfaces of a substrate.

10. Layer structure according to one of the preceding claims, characterized in that the layers have a lateral thickness gradient.