WO2011000932A1

WO2011000932A1 - Method for producing a refractive optical element and refractive optical element

Info

Publication number: WO2011000932A1
Application number: PCT/EP2010/059411
Authority: WO
Inventors: Markus Simon
Original assignee: Baden-Wuerttemberg Stiftung Ggmbh
Priority date: 2009-07-01
Filing date: 2010-07-01
Publication date: 2011-01-06
Also published as: EP2449560A1; DE102009031476B4; DE102009031476A1; EP2449560B1

Abstract

The invention relates to a refractive optical element (5) comprising a plurality of optical partial elements (3), wherein the optical partial elements (3) are arranged such that the number of optical partial elements (3), which electromagnetic radiation shining on the optical element (5) in parallel to the optical axis of the optical element (5) passes, rises as the distance to the optical axis increases, wherein the optical partial elements (3) are designed to be curved about the optical axis. The invention further relates to a method for producing the optical element (5) according to the invention.

Description

Method for producing a refractive optical element and refractive optical element

The invention relates to a refractive optical element, in particular a refractive optical element for refraction or focusing of X-radiation. Furthermore, the invention relates to a method for producing a refractive optical element. For the refraction of X-radiation, ie electromagnetic radiation in a wavelength range of 5 pm to 50 nm, there is a particular problem in that the refractive power of the materials sufficiently transmissive for X-radiation is so low that, in the case of the use of solid lenses, comparatively extreme forms of lenses necessary, among other things, that the path of the radiation in the lens material is so long that no longer acceptable values for the absorption of the X-radiation in the lens material arise. According to the prior art, this problem is met by the fact that the optical element is formed from a plurality of partial optical elements with refractive surfaces, of which in the edge region of the optical element, ie in the further spaced from the optical axis of the optical element areas, more in the way of the radiation to be refracted and are passed by this than in the vicinity of the optical axis. In this way, a stronger refraction of the radiation is achieved in the edge region and the optical element achieves a focusing effect. For example, in Jark, W. et al., "Focusing X-rays with simple arrays of prism-like structures", Journal of Synchrotron Radiation, 2004, 11, 248-253, a so-called Clessidra lens is presented which comprises a plurality of total in The type of hourglass has arranged individual prisms by the already mentioned arrangement of the plurality of prisms in the areas further away from the optical axis. The disadvantage here, however, is the complex production of the optical element and the limitation to be able to produce only one line focus and no point focus with one of the optical elements. Although a point focus can be generated by the combination of several mutually rotated Clessidra lenses, this is at the expense of the transmittivity of the overall arrangement.

Based on the prior art, the invention has the object to provide a refractive optical element that is easy to manufacture, and further as good as possible Transmittivity with the option to realize a line focus and a large aperture has. Next to it is the

Invention the task of specifying a method for producing an optical element.

These objects are achieved by the method having the features specified in claim 1 and the optical element having the features specified in claim 10. The dependent claims relate to advantageous embodiments and variants of

Invention.

The refractive optical element according to the invention has a plurality of optical sub-elements, wherein the optical sub-elements are arranged such that the number of optical sub-elements, which are passed by electromagnetic radiation incident on the optical element parallel to the optical axis of the optical element, with increasing distance to the optical axis also increases. According to the invention are the optical sub-elements in particular rotationally symmetrical around the optical axis formed curved.

Thus, the optical sub-elements may, for example, be formed as annular elements with different radii and a triangular cross-section (ring prisms), which are arranged concentrically stacked on the optical axis and interconnected. In this case, an increasing number of such ring prisms is present with increasing radius, so that results in a corresponding stacking of the ring prisms as a result of a rotationally symmetric about the optical axis optical element in the form of an hourglass. In this way, it is possible to realize a refractive optical element with a point focus, with which, as a matter of principle, a large aperture and a comparatively high transmittivity can be achieved. Particularly suitable is the optical element described above for use in the X-ray region.

With appropriate choice of material and geometry, an application in a variety of other wavelength ranges is conceivable.

In a manufacturing technology particularly advantageous variant of the invention, the optical sub-elements are arranged on a carrier element which is wound around the optical axis. Here, too, the optical subelements may at least partially be prismatic bodies. The optical sub-elements and the carrier element may be integrally formed. This embodiment opens up the possibility, for the production of the optical element according to the invention, first of all of a foil, for example, of a polyimide, a thermoplastic see plastic or produce an epoxy resin on which the optical sub-elements are already formed. Thus, a carrier element is produced which has a multiplicity of, in particular, parallel prisms. This can be done by taking an impression of a method of photolithography, optionally in conjunction with other patterning methods, such as etching, negative mold; Other structuring methods are conceivable. An exemplary method is described with reference to the drawing. After producing the carrier body with the prisms as optical subelements, the carrier body is cut in such a way that it forms a trapezoidal or also pointed, preferably mirror-symmetrical structure with a comparatively short base side compared to its length. The tip of the structure can lie on the axis of symmetry and the prisms run parallel to the axis of symmetry in a direction perpendicular to the base side. Subsequently, the carrier element is rolled up with the prisms in the direction of the prisms from the top or the short side of the structure so that the desired shape of the optical element according to the invention results practically by itself. To support the Aufrollvorganges a cylindrical body, in particular a glass fiber, for example. Be used with a diameter of about 125 microns, around which the support member is wound. The advantage of using the glass fiber lies in its comparatively high mechanical stability. The diameter of the cylindrical body can be selected without significant deterioration of the optical properties of the optical element up to the range of the diameter of the desired focus. In addition, the maximum achievable aperture of the optical elements according to the invention is not restricted by the method described process-related. The arrangement of carrier element and optical sub-elements can before winding a total thickness of 5 .mu.m to 50 .mu.m preferably 8 .mu.m to 14 .mu.m, more preferably about 12 microns, wherein the carrier element itself has a thickness of 1 .mu.m to 5 .mu.m, preferably 1 μm to 1.5 microns, more preferably about 1.3 microns. Basically, the support element is as thin as to choose from stability aspects still acceptable. Overall, it is advantageous if, with the smallest possible optical sub-elements, the carrier element is made thin relative to the dimensions of the optical sub-elements. The carrier element acts as a plane-parallel plate and thus does not break for axis-parallel rays. Since a comparatively thin carrier element already provides the required mechanical stabilization, the total absorption by the carrier element can also be kept low.

The method according to the invention for producing a refractive optical element comprises the following steps:

- Making a negative mold from an anisotropically etched

Substrate, in particular a Si wafer,

Molding a structured plastic film from the negative mold,

- Generating the refractive optical element by winding the structured film.

As already mentioned, the wafer does not necessarily have to be a Si wafer; the use of substrates made from other materials is also conceivable. It is advantageous if the substrate has been produced from a single crystal and it is oriented in the crystal direction so that trapezoidal or triangular structures result from the etching process. The molding step may include the following steps:

Applying a plastic to the negative mold,

- curing the plastic,

Producing a structured film by peeling off the

Plastic.

Before molding, a release layer can be applied to the negative mold, which may contain, for example, gold, Teflon or HMDS.

A desired layer thickness can be adjusted by centrifuging, knife-coating or pressing a plate or a tool.

Before detaching the plastic from the negative mold, a carrier film can be applied to it.

Alternatively, the molding can also take place in that the desired structures are rolled into a thin layer of plastic by means of a structured roller.

Polymerization of a plastic precursor can take place during rolling.

It is also conceivable that the structures are impressed by rolling in a thermoplastic material. The invention will be described by way of example with reference to the drawing.

It shows:

1 shows an X-ray lens according to the prior art,

FIG. 2 shows a modification of the lens shown in FIG. 1 in which material regions which do not contribute to the refraction have been omitted, FIG.

3 is a detailed view of Figure 2,

Fig. 4 is an optically equivalent alternative to the in

Figure 3 shown element,

5 is a sectional view of a carrier body with optical sub-elements,

6 is a perspective view of the carrier body shown in the sectional view of Figure 5 with optical elements,

Fig. 7 is a sketch of an optical according to the invention

Element.

Fig. 8 shows an exemplary method for producing a

Negativform and a structured film as Intermediate for producing an optical element according to the invention; and

9 shows an overview of a selection of different

Process variants for producing the negative mold.

1 shows a sectional view of an X-ray lens 1 according to the prior art. Since the lens material, for example Si, B, Be, has a refractive index <1 for X-ray wavelengths, in this case the converging lens is designed as a concave lens. The optical effect of the lens shown in Fig. 1 may alternatively also - analogous to the known procedure for

Fresnel lenses - can be achieved by removing the non-refractive parts of the material from the lens and pushing the remaining curved edge portions of the lens together, as shown in FIG.

Fig. 2 shows in its upper portion the state after the stratified removal of the non-refractive contributing material. The lower part of Fig. 2 shows the case where the remaining pieces have been pushed together in the direction parallel to the optical axis. Good visible in Fig. 2 is that the resulting optical Fresnel lens elements 2, especially in the edge region of the X-ray lens due to the X-ray area required extreme curvature of the lenses show a very low ratio of height to length, which is the manufacture of the also As fins called Fresnel lens elements 2 difficult for the edge region. For illustration, such a Fresnel lens element from the edge region of the X-ray lens 1 is shown enlarged again in Figure 3. Fig. 4 now shows an arrangement which is optically similar

Effect as the Fresnel lens element shown in Fig. 3, but is much easier to manufacture. The solution shown by way of example in FIG. 4 likewise shows, like the preceding figures, in a cross-sectional illustration three similar optical sub-elements 3, which in the present case are designed as prisms. The sum of the optical effect of the prisms shown in FIG. 4 essentially corresponds to the optical effect of the Fresnel lens element 2 illustrated in FIG. 3.

5 likewise shows, in a cross-sectional illustration, a carrier body 4 for producing the optical element according to the invention, on which the optical subelements 3, which in the present case are designed as prisms having a triangular cross section, are arranged. In this case, the carrier body 4 with the optical sub-elements 3 in one piece in particular from

Polyimide be prepared. The carrier body 4 has a thickness of approximately 1.3 μm; the height of the optical sub-elements 3 can be about 10.7 microns.

FIG. 6 again shows the structure according to the invention in a perspective view. Good recognizable in Fig. 6 is the embodiment as a film, which forms a pointed, mirror-symmetrical structure with a comparatively short compared to their height base page. Although the illustration in Figure 6 gives the impression of an isosceles triangle, the shape of the sides and thus the contour of the film blank - depending on the desired surface function of the finished lens - deviate from the shape of a straight line.

In general, the relationship between the contour of the Film cut and the surface function of the finished lens in longitudinal section represent as follows:

With :

_ /. : Contour of the foil blank

fiκ (x): Desired surface function of the lens, x in the direction of the optical axis

d _j '. film thickness

If one now rolls this film on from the top of the structure, the x-ray roller lens 5 shown in FIG. 7 results as a wound, almost rotationally symmetrical body. Clearly recognizable in FIG. 7 is that the marginal rays are deflected more strongly by the X-ray roller lens 5 than the one which is in the

Area of the optical axis run, as in the edge region of the

X-ray lens 5 more optical sub-elements 3 are present than in the region of the optical axis of the X-ray roller lens 5. For use of the rolling lens 5 shown for wavelengths for which the refractive index of the material used is greater than 1, the film must be wound from the tip, the prisms do not point inwards as shown in FIG. 7, but rather outwards.

The X-ray lens 5 shown in Fig. 7, in contrast to the prior art Clessidra lenses, does not show a line focus, but a point focus, which opens up its utility for a variety of applications. The X-ray roller lens 5 shown in FIG. 7 moreover has the potential for a larger aperture than conventional lenses. Besides For example, the X-ray lens 5 shown is easy to manufacture, and due to the structure of the lens, the absorption in the lens material is significantly reduced compared to a solid lens.

8 shows, by way of example, in the subfigures 8.1 to 8.17, a method for producing the X-ray roller lens 5 according to the invention. The layer pattern is shown schematically in the subfigures in the left part of the respective subfigure, whereas the right part of the subfigure also shows the structure of the resulting intermediate product ,

In FIG. 8.1, an oxidized 100 silicon wafer is shown as the starting material. FIG. 8.2 shows the intermediate product after vapor deposition with a chromium layer; FIG. Fig. 8.3 shows the state after the spin-on of positive resist. As shown in FIG. 8.4, exposure is then carried out through a chromium mask and development of the exposed resist layer. The result is shown in Fig. 8.5. Below is the

Etched chromium layer by means of the resist layer as an etching mask, so that forms the structure shown in Fig. 8.6. Fig. 8.7 shows the state after the resist layer has been stripped by flood exposure and subsequent development. Now, the remaining chromium layer can be used as an etching mask, so that the oxide layer can be partially removed from the silicon wafer, which is shown in Fig. 8.8. The remaining chromium layer is subsequently removed or stripped by etching, so that the layer sequence shown in FIG. 8.9 results. Subsequently, the wafer is etched anisotropically so that trenches with a triangular profile are formed in the silicon layer, which can be seen in FIG. 8.10. In the next step, the remaining oxide layer can be removed by a further etching process, whereby the structure shown in Fig. 8.11 forms as a negative mold. The in the above-described

Steps produced negative mold can be used for the molding of the desired structure several times in the manner described below. First, as a release layer, a

Gold layer applied, as can be seen from Fig. 8.12; The gold layer can be vapor-deposited or sputtered on. To reduce the adhesion of the gold to the negative mold, the wafer may be dry or wet oxidized prior to application.

An alternative to gold as a release layer is the use of Teflon (polytetrafluoroethylene), which can be applied to the negative mold by means of a CVD process.

In addition, it is possible to take advantage of the fact that HMDS (hexamethyldisilazane) does not act as a coupling agent for certain substances but as a separating layer. By applying the HMDS to the non-oxidized wafer, a release layer can thus likewise be produced. As a next step, polyimide is dropped onto the structure and subsequently spiked portions of the polyimide are spun off, whereby the desired layer thickness can be adjusted (shown in Figs. 8.13 and 8.14). Instead of polyimide can - in particular for an application of the optical element in the X-ray range - also another X-ray-stable plastic such. B. SU-8 of the manufacturer Microchem Corp. be used.

In a variant of the invention, a thermoplastic material can also be embossed with the negative mold. The layer thickness can also be adjusted by doctoring or by pressing a plate or another tool. In this case, the layer should be chosen so thin that it still gives a sufficient stability with minimal optical influence of the overall structure. After a curing process of the polyimide layer by prebake, exposure and postbake (FIG. 8.15), in the case of using a gold separation layer, the composite of polyimide and gold can be peeled off (FIG. 8.16) and the gold separation layer can be removed by etching, so that the structure shown in FIG 8.17 intermediate, namely the

Polyimide film as a carrier element with the integrally formed prisms as optical sub-elements results.

Depending on the plastic used and the separating layer used (if necessary), alternative process steps are possible. So can when using a

Plastic without photoactive, but possibly with thermally activatable component dispensed with the exposure and only a thermal treatment (baking) are made.

For mechanical protection of the film during removal from the negative mold, a carrier film may additionally be adhered prior to detachment, and subsequently the composite of carrier film and structured film may be removed. Subsequently, the carrier film may then be detached from the structured film under the influence of heat.

The structure shown in Fig. 8.17 can now be wound, for example, after a blank, which results in the structure shown in Fig. 6, on a glass fiber with a Diameter of about 125 microns is wound up. A stabilization of the resulting rolling lens can be done by means of adhesive tape or other methods. For the use of rolling lenses for wavelength ranges other than the X-ray range, other materials come into question.

Alternatively, the structured film can also be produced by means of a so-called "RoIl-On" process. In this case, the desired structures are rolled into a thin layer of plastic or plastic precursor by means of a structured roller. The structured roller may, for example, consist of a very thin anisotropically etched Si wafer, which is glued onto a roller-shaped element. Polymerization of the plastic precursor may occur during rolling; Alternatively, the structure can also be embossed by rolling in thermoplastic material.

Figure 9 shows an overview of a selection of different

Process variants for producing the negative mold. The process illustrated in sub-figures 8.1 to 8.11 is included in Figure 9 as a possible example.

As starting material serve in the present example

Silicon wafer with an oxide or nitride layer. For the

Oxidation of Si wafers are available in different ways:

- dry chemical oxidation:

The wafers are oxidized at temperatures of about 1000 ^° C. under an oxygen atmosphere. Depending on the temperature, the oxide grows on the wafer (higher temperatures faster). The Growth rate is about 50 nm / h at 1000 ⁰ C. This method thus shows a low growth rate with a high density of the deposited oxide layer.

- Wet chemical oxidation:

In wet-chemical oxidation, oxygen is introduced through a

Water bath passed. The so obtained with water vapor

enriched atmosphere is subsequently passed over the wafer

directed. The growth rate here at 1000 ⁰ C is about 400 nm / h; however, the density of the deposited oxide layer is lower in this case than in the dry chemical oxidation.

- H2O2 combustion:

The required synthesis gases are passed separately to the wafer and burnt at the inlet. This variant allows the growth of thin and thick layers at lower temperatures and thus lower thermal

Load of the wafers.

For the Mitridschicht on the wafers there are

likewise a multiplicity of possibilities:

- nitridation of the Si (with N2 / NH4 / N2H4 at -1100 ° C),

Nitriding the SiQ ₂ (with NH ₂ at -HOO ⁰ C),

- CVD,

- Reactive sputtering,

- ion implantation.

For the preparation of the produced by anisotropic etching

Grave structures of the negative mold becomes the SiO ₂ or Si ₂ N ₄

Layer used as an etching mask. There are several ways to structure the shift: Wet-chemical structuring of the SiO 2 or Si 2 N 4 layer:

In order to structure the SiO 2 layer wet-chemically, an etching mask is required on the layer. There are in particular the following two possibilities:

- Mask of a photoresist:

In this case, a layer of a photoresist is applied to the wafer. Thereafter, the photoresist is patterned by exposure through a (chrome) mask. This is followed by a development step which, depending on the resist used (positive or negative) either the exposed or unexposed

Detach areas.

- Mask made of chrome:

Here, the chromium layer is first vapor-deposited on the wafer. A photoresist layer is then applied to this layer. This layer is then exposed and developed. The exposed chrome is then etched with chromium etchings. Thereafter, the remaining photoresist is dissolved. Hydrofluoric acid (HF) or buffered hydrofluoric acid (BOE) is used to structure the SiO 2 layer wet-chemically after the mask has been applied. The wafer is placed in the hydrofluoric acid for etching.

Dry-chemical structuring of the SiO 2 or Si ₂ N ₄ layer: If the SiO 2 layer is to be dry-chemically etched, a mask is first required on the layer. This consists of photoresist, which is applied to the layer, exposed and then developed. Thereafter, one of the dry etching methods listed below may be used, which are distinguished by the nature of the attack on the material to be etched in:

- chemical attack (plasma etching), for example under the action of CF ₄ , SF ₆ or NF _3,

- Physical attack (sputter etching), for example. Using ions such. B. Ar ions (high mass),

- Chemical + physical attack (reactive ion etching (RIE) and reactive ion beam etching (RIBE)) under the influence of a mixture of ions and reactive gas.

A wet-chemical isotropic structuring of the Si ₂ N ₄ layer can be carried out using hydrofluoric acid (HF), buffered hydrofluoric acid (BOE) or

Phosphoric acid (H ₃ PO ₄ ) are made. Alternatively, dry chemical structuring of the Si ₂ N ₄ layer can be carried out using CF ₄ or CHF ₃ .

To produce the desired trench structures in the

Negative form, the wafer is anisotropically etched, whereby

In particular, the required low surface roughness of the negative mold can be achieved. Usable bases are

Tetramethylammonium hydroxide (TMAH),

- Potassium, soda or lithium (KOH, NaOH, LiOH,)

- Mixture of water, pyrazine, catechol and ethylenediamine (EDP).

Finally, the etch mask is removed from the wafer by selective etching, which can be accomplished by either RF, BOE, or dry etching.

Claims

Patent claims

1. Method for producing a refractive optical

Element (5), with the following steps

Producing a negative mold from an anisotropically etched substrate, in particular a Si wafer,

- Molding a textured plastic film from the

Negative form,

- Generating the refractive optical element by

Winding up the structured film.

2. The method according to claim 1, characterized in that the molding step comprises the following steps:

Applying a plastic to the negative mold,

- curing the plastic,

Producing a structured film by peeling off the

Plastic.

3. The method according to any one of the preceding claims 1 or 2, characterized in that before molding a

Separating layer is applied to the negative mold.

4. The method according to claim 3, characterized in that the separating layer contains gold, Teflon or HMDS.

5. The method according to claim 2, characterized in that a desired layer thickness is set by centrifuging, doctoring or pressing a plate or a tool.

6. The method according to any one of the preceding claims, characterized in that prior to detachment of the plastic on these a carrier film is applied.

7. The method according to claim 1, characterized in that the molding takes place in that by means of a structured roller in a thin layer of plastic, the desired structures are rolled.

8. The method according to claim 7, characterized in that a polymerization of a Kunststoffpräkursors occurs during rolling.

9. The method according to claim 7, characterized in that the structures are impressed by rolling in a thermoplastic material.

10. Refractive optical element (5) with a plurality

optical sub-elements (3), wherein the optical sub-elements (3) are arranged such that the number of optical sub-elements (3) that of electromagnetic radiation which on the optical element (5) parallel to the optical axis of the optical element (5) is incident, happens, with increasing distance to the optical axis also increases, characterized in that the optical sub-elements (3) are formed curved around the optical axis.

11. Refractive optical element (5) according to claim 10, characterized in that the optical sub-elements (3)

rotationally symmetric with respect to the optical axis

are formed.

12. refractive optical element (5) according to claim 10, characterized in that the optical sub-elements (3) are arranged on a carrier element (4) which is wound around the optical axis.

13. refractive optical element (5) according to any one of the preceding claims 10-12, characterized in that the optical sub-elements (3) at least partially show a prismatic shape.

14. Refractive optical element (5) according to claim 12 or 13, characterized in that the optical sub-elements (3) and the carrier element (4) are integrally formed.

15. Refractive optical element (5) according to any one of claims 12-14, characterized in that the optical sub-elements (3) and the carrier element (4) are formed of a polyimide, a thermoplastic or an epoxy resin.

16. A refractive optical element (5) according to any one of claims 12-15, characterized in that the arrangement of

Carrier element (4) and optical sub-elements (3) a

Total thickness of 5 microns to 50 microns, preferably 8 microns to 14 microns.

17. Refractive optical element (5) according to claim 16, characterized in that the carrier element (4) has a thickness of

1 μm to 5 μm, preferably 1 μm to 1.5 μm.

18. Refractive optical element (5) according to any one of claims 12-17, characterized in that the carrier element (4) is wound on a cylindrical body, in particular a glass fiber.

19. A refractive optical element (5) according to claim 18, characterized in that the cylindrical body has a diameter of about 125 microns.