EP2633528B1 - Resist structure for producing an x-ray optical grating structure - Google Patents

Description

The present invention relates to a resist pattern for producing an X-ray optical grating structure.

For many applications, such as in medical diagnostics or material analysis, X-rays are used. Here, optical gratings are used whose structure and in particular their aspect ratio special requirements are made, since the quality of the imaging technique referred to in the jargon as Differential Phase Contrast Imaging (DPCI) with X-ray depends decisively on the height of the optical grating structures.

In order to achieve a high contrast relevant for medical applications, the absorption of the material deposited in the struc- ture gaps in the resist structure (usually gold) should be in the range greater than 80%. Depending on the application, X-ray energy, for which the tomographic structure is optimally designed and referred to as design energies, ranges from 20 keV to 60 keV, with the polychromatic radiation of the x-ray tube also allowing energies up to about 10 keV above the design energy are present. This means that the thickness of the absorbing gold must be at least 100 microns and thus the height of the resist structure over 100 microns.

Methods are known in the art for producing resist structures with heights of several hundred microns. In Pfeiffer et al., Nature Physics, 2006, Advanced Online Publication, p.1 , the possibilities of phase-contrast X-ray imaging with non-coherent X-ray sources are described. For the realization of these imaging systems, the production of lattice structures with a high aspect ratio is necessary. These dimensions requirements However, the absorbent structures, as well as their mechanical stability raise process engineering problems.

In E. Reznikova et al., Soft X-ray lithography of high aspect ratio SU 8 submicron structures, Micro Syst. Techn., 14: 1863-1688, 2008 , a method is described which in principle allows the production of such structures. In this case, however, it is also clear that at structural heights of greater than 60 .mu.m to a bending of the differently long selected webs comes and thus the aspect ratio is limited.

wobei I_max der maximale Intensitätswert ist und I_min der minimale Intensitätswert im erzeugten Röntgenbild ist.In J.Knowner, et al., Front and backside structuring of gratings for phase contrast imaging with x-ray tubes, Proc. SPIE, Vol. 7804, p. 780408, pp. 1-10, 2010 , Resist structures are shown with which it was attempted to prevent the problem of bending the grid bars by connecting the grid bars with filling bars. The disadvantage of this solution is that the gold bars are interrupted again and again by quasi transparent areas (filling bars). This results in fluctuating visibility values when analyzing the visibility V with a detector having a pixel size in the range of fewer grating periods and less. The visibility V is defined as follows $$V=\left(I_{\mathit{Max}}-I_{\mathit{min}}\right)/\left(I_{\mathit{Max}}+I_{\mathit{min}}\right)$$

where I _{max is} the maximum intensity value and I _{min is} the minimum intensity value in the generated X-ray image.

The DE 10 2009 019 595 A1 discloses a resist pattern for making an X-ray optical grating structure comprising a plurality of lands and beams stabilizing the lands, the lands being perpendicular to the substrate and beams being interposed between the lands.

Out G. Feiertag et al., Sloped Irradiation Techniques in Deep X-Ray Lithography for 3-D Shaping of Microstructures, Proc. SPIE 3048, Pp. 126-145, 1997 ), various three-dimensional resist structures are known which are prepared by consecutive exposures of a resist along + 45 ° and -45 ° directions. W. Becker et al., Fabrication of microstructures with high aspect ratios and great structural heights by synchrotron radiation lithography, electroforming, and plastic molding (LIGA process), Microelectr. Closely. 4, pp. 35-56, 1986 , describe high aspect ratio resist structures that are designed without stabilizing bars. In JA van Kann et al., Resist material for proton micromachining, Nucl. Instr. & Meth. In Phys. Res. B, 158, p. 179-184, 1999 , structures are written by means of high-energy protons in a direct writing process in a resist of polymethyl methacrylate (PMMA).

The WO 2009/116956 A1 discloses a metamaterial having an array of s-shaped resonators whose ends are supported by a frame. The preparation of the metamaterial is carried out by means of a 3-level UV / X-ray lithography within the framework of the LIGA process. Proceeding from this, the object of the invention is to propose a resist structure for producing a roentgen optical lattice structure which avoids the disadvantages and limitations listed. In particular, the production of X-ray optical grating structures with aspect ratios of over 500 is to be made possible without the grid bars bending or the stabilizing structure having an adverse effect on the visibility.

This object is achieved by a resist structure having the features of claim 1. The dependent claims describe advantageous embodiments.

The invention is based on the idea that stabilizing beams are introduced into the resist structure. The webs of the Resisstruktur are attached at a first angle α on the substrate and the stabilizing beam with a second angle β . In a preferred embodiment, between α and β at least a distance of 20 ° and at most a distance of 70 °, preferably from 40 ° to 50 °, in order to obtain the best possible stabilizing effect.

In a preferred embodiment, the ratio of the height h of the webs to the width b 'of the web gaps has a value of 10 to 500.

In a preferred embodiment, the distance between two adjacent stabilizing beams is in the range between twice and 20 times the slot width, and the perimeter diameter d of each beam is between 1 μm and 10 μm, preferably from 2 μm to 5 μm. According to the invention, each stabilizing beam penetrates at least two webs.

Such an arrangement and dimensioning of the beams as well as the choice of the angle causes a web gap of the resist structure and thus in the later webs of the lattice structure, the total height of the stabilizing beam is at most 20% of the lattice height and preferably at most 10% of the lattice height and thus has only a slight influence on the visibility.

ergibt, wobei d der Umkreisdurchmesser des Balkens ist. Die stabilisierende Wirkung der Balken bleibt vollständig erhalten, da sich über die Anordnung der Balken eine Stabilisierung der gesamten Fläche ergibt.Such resist structures are suitable for the production of X-ray optical grating structures. In the area of the stabilizing beam, the height of the material deposited in the web gaps is reduced by a maximum of the value d ' resulting from $$d\text{'}=d*1/\mathit{cos\beta }$$

where d is the perimeter diameter of the beam. The stabilizing effect of the bars is completely preserved, since the arrangement of the bars results in a stabilization of the entire area.

Another advantage is that the height of the stabilizing beams can be repeated at will and thus significantly higher structures than previously possible. For N structures arranged one above another, there is a height change of N xd, which corresponds to a few% of the total height and has only a marginal effect on the visibility.

In a particular embodiment, in addition to the stabilizing beams arranged at an angle β , the present resist structure has further stabilizing beams arranged at an angle β 'and wherein the angle β ' does not have the same value as one of the angles α or β β .

In a preferred embodiment, the resist structure consists of a negative resist material.

With such structures, gratings for phase-contrast X-ray imaging at any height with approximately constant visibility be realized over the entire surface of the grid structure. This makes it possible to realize structures with energies of more than 40 keV, which have an absorption of 80% and more. This and the uniformity of the absorption allows a much better resolution in the phase contrast image.

Due to their high aspect ratios, the described resist structures are also suitable for the production of gratings for neutron imaging.

The invention will be explained in more detail below with reference to an example and the figure.

Fig. 1 schematically shows an embodiment of a resist structure, the webs 1 are arranged at an angle α = 90 ° on a substrate 2 and a field of cross-sectionally round bars 3, which stabilizes the webs 1 . The stabilizing beams 3 are mounted at an angle β = 45 ° on the substrate 2 and intersect the webs 1 in the embodiment shown at an angle of 45 °.

The resist structures shown in the example are particularly suitable for the production of X-ray optical grating structures made of gold.

Claims (11)

1. Resist structure for producing an X-ray optical grating structure, comprising a substrate (2), a plurality of webs (1) with a height h and a width b, as well as web gaps with a width b', and the bars (3) stabilising the webs (1), with a circumcircle diameter d, wherein the webs (1) and the bars (3) stabilising the webs (1) are arranged on the substrate (2), characterised in that
   the webs (1) are arranged at an angle α on the substrate (2), and the bars (3) stabilising the webs (1) are arranged at an angle β, wherein the angles α and β are not the same, and in that
   in a web gap of the resist structure, and therefore in the later webs of the X-ray optical grating structure, which exhibit a grating height which corresponds to the height of a material to be cut off in the web gaps, the total height of the stabilising beams amounts to maximum 20% of the grating height, and in that each stabilising bar penetrates through at least two webs.
2. Resist structure according to claim 1, characterised in that the total height of the stabilising bars in a web gap amounts to a maximum of 10% of the grating height.
3. Resist structure according to claim 1 or 2, characterised in that the angle α has a value of 90°
4. Resist structure according to any one of claims 1 to 2, characterised in that the angles α and β exhibit a difference of at least 20° and up to a maximum of 70°
5. Resist structure according to claim 4, characterised in that the angles α and β exhibit a difference of 40° to 50°.
6. Resist structure according to any one of claims 1 to 5, wherein the height h of the webs (1) exhibits a ratio to the width b' of the web gaps of 10 to 500.
7. Resist structure according to any one of claims 1 to 6, characterised in that the spacing between two stabilising bars (3) amounts to at least double the width b' and a maximum of 20 times the width b'.
8. Resist structure according to any one of claims 1 to 7, characterised in that the circumcircle diameter d of the stabilising bars (3) is between 1 µm and 10 µm.
9. Resist structure according to claim 8, characterised in that the circumcircle diameter d of the stabilising bars (3) is from 2 µm to 5 µm.
10. Resist structure according to any one of claims 1 to 9, characterised in that the resist structure consists of a negative resist material.
11. Resist structure according to any one of claims 1 to 10, characterised in that, in addition to the stabilising bars (3), which are arranged at an angle β, further stabilising bars are arranged at an angle β', and wherein the angle β' does not have the same value as one of the angles α or β.

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