WO1993011862A1

WO1993011862A1 - Micromechanical filter and method for the manufacture thereof

Info

Publication number: WO1993011862A1
Application number: PCT/DK1992/000364
Authority: WO
Inventors: Jens Anders Branebjerg; Peter Gravesen; Ole Søndergaard JENSEN
Original assignee: Danfoss A/S
Priority date: 1991-12-12
Filing date: 1992-12-03
Publication date: 1993-06-24
Also published as: DE4140922C1; EP0643616A1; AU3255293A

Abstract

A micromechanical filter is provided with a top layer (10) provided with openings (13), a bottom layer (12), and an intermediate layer (11) provided in predetermined first regions (A) between the top layer (10) and the bottom layer (12), which intermediate layer substantially determines the spacing between the top layer and the bottom layer in predetermined intermediate layer-free second regions (B). It is desirable for a filter of that kind to be operable with as small a pressure difference as possible. For that purpose, third regions (C) are provided in which the spacing between the top layer and the bottom layer is larger than in the first and second regions.

Description

Micro echanical filter and method for the manufacture thereof.

The invention relates to a micromechanical filter with a top layer provided with openings, a bottom layer, and an intermediate layer provided in predetermined first regions between the top layer and bottom layer, which intermediate layer substantially determines the spacing between the top layer and bottom layer in predetermined intermediate layer-free second regions, and a method for the manufacture of such a filter, in which the top layer and the bottom layer are joined together by means of the intermediate layer, first and second regions being formed by removal of parts of the intermediate layer.

A filter of that kind is known from WO 89/08489. Here, the top layer and bottom layer have openings which are staggered relative to one another. These openings are joined to one another by intermediate layer-free second regions so that a fluid path from an opening in the top layer through the intermediate space between the top layer and the bottom layer to the opening in the bottom layer can be formed. The degree of permeability of the filter or the filter opening is in that case determined by the spacing between the top layer and the bottom layer. This spacing is in its turn determined by the thickness of the intermediate layer, which supports the top layer and the bottom layer against one another. The terms "top layer" and "bottom layer" serve here merely to differentiate two layers, they do not determine the orientation of the filter. In the known filter, a relatively large pressure difference is necessary to propel the fluid through the filter. This leads to a correspondingly high pressure drop across the filter.

The invention is based on the problem of providing a filter which creates a lower pressure drop during filtering.

This problem is solved in a filter of the kind mentioned in the introduction by provision of third regions in which the spacing between the top layer and the bottom layer is greater than in the first and second regions.

In the third regions the flow resistance for the fluid is therefore quite considerably reduced. The filtering effect is not impaired thereby, since the filtering characteristic is still determined by the second regions. Depending on the spacing of the top layer from the bottom layer in the second regions, this filter allows only corresponding particle sizes to pass through and holds others back. In the third regions there is admittedly a larger dead volume, but this is far more easily controllable than in the known case. For example, a fluid is able to flow substantially more easily out of the third regions, with the result that emptying of the filter is improved. Despite the larger spacing in the third regions, it is possible to support the top layer adequately relative to the bottom layer in the first regions, so that a robust mechanical construction can be obtained. The degree of support and the number of supporting points depend on the desired intended use.

In an especially preferred embodiment, provision is made for a second region to be provided between a third region and an opening. The effect of this is that only filtered fluid flows in this third region. The filter is thereby easily controllable. It is also preferable for the area of the third regions to be substantially larger than that of the second regions. The second regions, which form the main flow resistance, then still suffice to hold back the desired particles, but there is no need for a larger width, with the result that the flow resistance of the filter can be kept small to the required degree.

Advantageously, the third regions form ducts. The fluid flowing through the filter can be collected in these ducts and guided in a controlled manner. Control of the fluid flow is thereby already effective in the filter.

An advantageous construction furthermore provides for a plurality of ducts to be connected in parallel. The flow conductance values of the individual ducts are added together as a result, which leads to a further reduction in flow resistance.

It is also preferable for the third regions to be in connection with at least one inlet and at least one outlet. A flow through the third regions can thereby be achieved which renders the filter particularly suitable for use as a dialysis filter. In this case, particles or, in an extreme case, even elemental particles, such as ions, are able to pass from the openings through the filtering slot formed by the second regions into the third regions, and can be taken up by an acceptor medium flowing from the inlet to the outlet.

It is an advantage herein for the inlet and/or the outlet to flow intoy a collecting space. The acceptor medium can be distributed in the collecting space to the ducts or can be collected from there again. This enables the acceptor medium to be controlled relatively easily.

The openings are preferably longer than they are wide and their width corresponds substantially to the width of the ducts, a transition from an opening into a duct being effected substantially at the long sides thereof. The flow behaviour through the openings can thereby be matched to the flow behaviour through the ducts. The donor medium and the acceptor medium are able to flow substantially parallel to one another, the desired particles being able to change over from the donor medium into the acceptor medium through the filtering slot into the second regions.

It is here preferable for the ducts to be arranged in groups substantially parallel with one another, adjacent groups being arranged so that the ends of the respective ducts face towards one another and just one outlet region or just one inlet region is formed between two adjacent groups. The inlet region is then in connection with the inlet and the outlet region is in connection with the outlet. Within a group the ducts are arranged parallel with one another. Two adjacent groups have a common inlet region or common outlet region. This results in the individual groups in turn being arranged parallel with one another.

Advantageously, two adjacent groups together form an angle; in the apex region of the angle there is a separation between the inlet and outlet region. Because the groups themselves separate the inlet region from the outlet region, the angle ensures that a separation between the inlet region and the outlet region where there are no groups of ducts is confined to a narrow space. The measures for separating the inlet region from the outlet region therefore involve very little expense.

Advantageously, a plurality of groups is arranged in a meander, which separates the inlet region from the outlet region. Because both the individual groups and also the ducts within a group are arranged in parallel, this produces a parallel arrangement of very many ducts, which contributes to a considerable reduction in the flow resistance of the filter, in particular for the acceptor medium, and at the same time provides a large transitional area between the donor medium and the acceptor medium. The overall size of the filter is kept small, however. In principle, only the lengths of the individual groups determined by the lengths of the ducts are added, and not the total width of all the ducts determined by the sum of the widths of the ducts.

It is also preferred to provide a plurality of meanders, with either just one inlet region or just one outlet region being provided between adjacent meanders. This features enables a large number of ducts to be provided, in particular when the ducts are relatively short. By that means, 1,500 ducts to an area of 1 cm² can be achieved without difficulty.

The bottom layer is preferably formed from boroεilicate glass. This material has proved reliable in microtechnology.

In a further preferred embodiment, the filter can be covered on the side having the openings by a protective layer, which leaves free at least a subregion of the openings. The openings can thereby be supplied with medium, but at the same time the top layer is largely protected against mechanical influences and attendant damage.

The top layer is preferably inclined in relation to the bottom layer at the transition from a first or a second region to a third region. This facilitates manufacture and improves the flow behaviour.

It is also preferable for a heating means to be provided, in particular a heating means formed by two electrodes. By way of the electrodes a current can be produced across the filter. Because the filter has a certain electrical resistance, this current leads to the formation of heat, which can easily lead to a temperature of 100° C in air. Biological contamination of the filter, such as accumulations of bacteria, can thereby be rendered harmless.

Preferably, different combinations of materials can be used for the individual layers. Thus, the top layer can consist of silicon, doped silicon, or boron-doped silicon. The intermediate layer can be formed from quartz, with the bottom layer being formed from silicon or glass. Finally, the intermediate layer can consist of metal, in particular aluminium, while the top layer is formed from silicon and the bottom layer from glass.

In a preferred embodiment, the filter has an integrated conductivity meter, which is arranged in particular in the collecting space. Using such a conductivity meter the electrical conductivity of the media, especially the acceptor medium, can be ascertained. On the basis of these values the inflow to the filter can be controlled. With the conductivity meter arranged in the collecting space, the small dead volume of the filter can be exploited well, and a very rapid response is achieved.

The conductivity meter is preferably formed by a thin-film disposed on the bottom layer or top layer, in which pairs of electrodes are formed. The co-operating pairs of electrodes enable a current to be passed through the medium to be examined, by means of which the conductivity of the medium can be ascertained.

In order to obtain a relatively large electrode area, provision is preferably made for each electrode to have prongs projecting from a main lead, the prongs of one electrode projecting into the gaps of the other electrode.

The invention also relates to the use of such a filter for chemical analysis, in particular gas analysis. In the method of the kind mentioned in the introduction, provision is made for third regions to be formed in the top layer before the top layer is joined to the bottom layer. Because the third regions are still freely accessible as they are being formed, they can be readily shaped and matched to the governing requirements.

The bottom layer is preferably joined to the intermediate layer by bonding. Bonding is a very suitable method for microtechnology, and is also known from semiconductor technology. It is in this case an anodic joining technique.

It is also preferable for those parts of the intermediate layer that are to be removed to be removed prior to joining to the bottom layer. That allows more rapid manufacture.

The third regions are advantageously etched from a substrate, and the resulting recesses are lined with the top layer. Later on, the substrate is then removed. This "negative moulding" allows a high precision when producing the top layer.

Before removal of the substrate the intermediate layer is advantageously applied. This facilitates manufacture.

The top layer can also be formed by mechanically processed metal or plastics material of small thickness, in particular by punching, stamping or boring.

Application by means of thin-film technology on a substrate of, for example, silicon, is also possible.

It is then preferable for the intermediate layer to be applied by vacuum-evaporation. Aluminium is preferably used in this method. Techniques of that kind are known from semiconductor technology.

The invention is described hereinafter with reference to preferred embodiments and in^*co junction with the drawings, in which shows a diagrammatic view of the filter in section, shows several steps in the method for manufacturing the filter, shows a perspective view of a filter, shows an example for the use of the filter as a dialysis filter, shows a further embodiment of a dialysis filter, and

shows an enlarged fragmentary view from

Fig. 5.

A filter 1 has a top layer 10, an intermediate layer 11 and a bottom layer 12. The top layer 10 has openings 13. The top layer 10 and the bottom layer 12 are^' joined in predetermined first regions A to the intermediate layer 11. Furthermore, there are second regions B free from intermediate layer, in which the spacing between the top layer 10 and the bottom layer 12 corresponds to the thickness of the intermediate layer 11. These second regions B are arranged adjacent to the openings 13. Furthermore, third regions C are provided, in which the spacing between top layer 10 and bottom layer 12 is larger than in the first regions A and second regions B. The space between the top layer 10 and the bottom layer 12 in the second regions B forms a filtering slot 14, through which a fluid entering through the openings 13 must pass in order to reach the third regions C. The third regions are of duct-like construction, so that the fluid passing through the filtering slot 14 can also be drained off again at a point not illustrated in detail. As the duct height is substantially larger than the height of the slots 14, the flow resistance which is Altogether, the flow resistance of the filter can therefore be kept low.

Fig. 2 shows by way of example the manufacture of such a filter with three ducts 15. For this purpose, standard methods known from micromechanics can be used, such as oxidation, anisotropic etching or doped- selective etching, as also known from WO 89/08489. Etching can be influenced by earlier diffusion steps, for example using boron.

Fig. 2a shows a silicon substrate 16 which carries an oxide layer 17, 18 on a top side and on its bottom side respectively. The oxide layer 18 on the bottom side of the substrate 16 has already been selectively etched in regions which will later correspond to the third regions C.

Fig. 2b shows the result of etching in KOH (potassium hydroxide) or of an anisotropic etching, in which recesses 19 have been created. In this step, the remainder 20 of the oxide layer 18 still illustrated in Fig. 2a was etched away on the underside of the substrate 16, for example using hydrofluoric acid. A first region A forms here later on.

Fig. 2c shows the creation of the top layer 10 by diffusion of boron into the regions that will later correspond to the first, second and third regions A, B, C. The top layer 10 is here formed by the area of the silicon substrate 16 enriched with boron, the area also following the recesses 19. In the transition from Fig. 2c to Fig. 2d, the remaining oxide layer 18 on the underside of the substrate 16 is removed by etching, the top layer 10 produced by boron diffusion presenting an increased resistance to the etching and thus remaining. Fig. 2d shows the result produced after the intermediate layer 11 has been applied and the bottom layer 12 attached. The bottom layer 12 can consist, for example, of Pyrex glass, which is electrostatically joined by means of bonding, that is, - lo ¬ an anodic joining technique, to the intermediate layer 11.

In the next process step, the result of which is illustrated in Fig. 2 , the substrate 16 is removed selectively by etching, so that only left-hand and right-hand walls 21, 22 remain. For the rest, above the bottom layer 12 there remains merely the top layer 10 , which is interrupted by openings 13, and the intermediate layer 11. The intermediate layer 11 too can now be removed through the openings 13 down to a residual part in the first region A, so that the same characteristic features of the filter occur as those illustrated in Fig. 1. The right-hand duct of the • three ducts 15 is of course not just suspended in air. The region A, by way of which it is connected to the bottom layer 12, does not lie in the plane of the drawing illustrated. This is especially apparent from Fig. 3, where the section line II-II that illustrates the section course from Fig. 2 is drawn in.

The illustrated manufacturing process with etching and boron diffusion is merely an example. In another construction both the top layer and the bottom layer can be manufactured from silicon. It is also possible to use thin-film technology on a substrate of silicon or a similar material. The top layer 10 can also be ^' formed by punching, stamping or boring a metal, for example stainless steel, or a plastics material, onto which a different material is applied by vacuum-evaporation, for example aluminium. A glass substrate can then be bonded to the intermediate layer, whereupon the intermediate layer can again be selectively etched.

It is also possible to machine the intermediate layer even before the bottom layer 12 is applied in Fig. 2d, such that, for example, the intermediate layer is removed in the second and third regions B and C and in the regions of the openings 13. This accelerates - li ¬ the process steps illustrated in Figs 2e and 2f because not so much material then has to be removed during these steps.

Fig. 3 shows a perspective drawing of a filter with a construction that corresponds to that in Fig. 2f. Here, the parts of the top layer 10 forming the ducts 15 are joined to the bottom layer 12 only in the region A. Should it be necessary, however, the join can be effected also at other points. No filtering slot 14 is then provided at these points.

As also visible from Fig. 3, all three ducts 15 flow into a collecting space 23 which is provided in the substrate 16. A similar collecting space can also be provided at the other end of the ducts.

A conductivity meter is integrated into the collecting space 23. It comprises two electrodes 40, 41 with connecting contacts 47, 48. The electrodes comprise main leads 44, 45 respectively. Prongs 42, between which there are gaps 43, project from each main lead. The prongs 42 of the one electrode 41 project into the gaps 43 of the other electrode 40. When an electrical voltage is applied between the two connecting contacts 47, 48, a current that is dependent on the conductivity of the medium to be examined flows. The conductivity of the medium can be ascertained in this manner. Control of the medium, for example, of the acceptor medium, through the filter can be effected, for example, in dependence on the electrical conductivity. Because the dead volume of the collecting space 23 is relatively small, a very rapid response to changes in conductivity is achieved. On the other hand, there is sufficient room available to accommodate sufficient medium. This enables a relatively trouble-free measurement of the conductivity to be effected.

The conductivity meter 46 is constructed in thin-film technology. It is applied to the bottom layer 12. Alternatively, it can be applied to the top layer 10. The thin-film can be applied to the bottom layer 12, for example, by vacuum-evaporation.

The fluid to be filtered can now be introduced from above into the aperture formed between the walls 21, 22 into the openings 13. From there it is able to pass through the filtering slots 14 into the ducts 15 and finally into the collecting space 23, from where it can be removed again by means of a drain, not illustrated in Fig. 3.

It is also possible, however, to use such a filter as a dialysis filter. Such an embodiment is illustrated diagrammatically in Fig. 4. Here, the openings 13 are masked by a protective layer 24, which leaves the openings 13 free at least in a subregion, namely, at a feed opening 25 and at a removal opening 26. A flow of a donor medium denoted by arrows 27 can be set up through the feed opening 25 and the removal opening 26. This donor medium flows through the elongate openings 13 past the outside of the ducts 15 formed by the top layer 10.

The collecting space 23 is connected to an inlet 28. A further collecting space 29 at the other end of the ducts 15 is connected to a outlet 30. By this means it is possible to produce a flow of an acceptor medium, illustrated by arrows 31, through the ducts 15. The donor medium and the acceptor medium now flow parallel to one another, separated by the top layer 10. A cross-over from one medium to the other is possible only through the filtering slot 14. Only particles of a desired maximum size are therefore able to pass over from the donor medium into the acceptor medium. If, for example, clean water is being used as the acceptor medium, at the output side this water can be examined to find out what contaminating particles or what elemental particles have passed over from the donor medium through the filtering slots 14. A dialysis filter of that kind is especially suitable for examining waste water. If the filtering slots 14 are selected to be sufficiently small, then such a dialysis filter can also be used to investigate the ion charge of the waste water, in that after passing through the filter the acceptor medium is examined for the ions in question.

In the embodiment illustrated in Fig. 4, the medium to be examined, the donor medium, flows past the filtering slots 14. This may, however, be disadvantageous in the case of donor media which are loaded with contaminants because the flow can then lead to the filtering slots 14 becoming blocked.

In Fig. 5 another embodiment is therefore shown, ^• in which the filter can be immersed directly into the donor medium. Because in this embodiment there is no flow of donor medium past the filtering slots 14, it is necessary to provide the filtering slots 14 with sufficient length to ensure a satisfactory passage of the desired particles into the acceptor medium. But because such a large filtering slot length involves the risk that the acceptor medium will also cross over into the donor medium, which involves a certain loss, it is necessary for the acceptor medium to be transported at a very low pressure through the filter. This is achieved by a large number of short ducts that are arranged parallel with one another. Fig. 5 shows a plan view of such a filter 31. The openings 13 in the top layer 10 are indicated by black strokes, which are arranged substantially parallel with one another. Eight inlet openings 28 and three outlet openings 30 for the acceptor medium are also illustrated diagrammatically. Arrows 32 indicate the flow of the acceptor medium.

The ducts through which the acceptor medium flows, which are not visible because they are masked by the top layer 10, are arranged between the openings 13. The filtering slots already mentioned are arranged between the ducts and the openings 13. The ducts are arranged in groups, the ducts of each group 33 being parallel with one another. Adjacent groups are arranged so that the respective ends of the ducts face one another. The inlet openings 28 flow into an inlet region 34. The outlet openings 30 are connected to an outlet region 35. The ends of the ducts 15 of adjacent groups 33 facing one another enclose between them either just one inlet region 34 or just one outlet region 35. In other words, the ducts of two adjacent groups are arranged parallel with one another in the flow direction. Two adjacent groups form with each other an angle, there being a separation in the apex region 36 between the inlet region 34 and the outlet region 35. The acceptor medium therefore always has to flow through the groups 33 of ducts in order to get from the inlet 28 to the outlet 30.

The individual groups 33 are alternately mutually inclined, so that they are connected in the manner of a meander 37, that is, for example in the form of a zig-zag curve. Several of these meanders 37 are arranged parallel with one another so that a large number of parallel ducts 15 is provided. In the embodiment illustrated this number is about 1,500. The dialysis filter illustrated in Fig. 5 has an area of about 1 cm².

The filter is furthermore provided with two electrodes 38, 39 to which an electrical voltage can be applied in order to drive a current through the filter. This current leads to heating of the filter. For example, at a voltage of 6 V and a current of 0.3 A the filter can be heated in air to temperature values of about 100° C. This is, in particular, of great value when the filter is used in an environment laden with bacteria and for that reason has to be cleaned periodically. Fig. 6 shows an enlarged fragmentary view from Fig. 5. Here, the openings 13 are indicated by hatching from bottom left to top right, while the top layer 10 is indicated by hatching from top left to bottom right. The areas that correspond to the first regions A have no marking, that is to say, are left blank. At these points the top layer 10 is connected by way of the intermediate layer 11 to the bottom layer 12. It is clear that the top layer can be secured to the bottom layer reliably and with an adequate number of supporting points. The filtering slot 14 and the duct 15 are located beneath the narrow webs that are arranged between two openings 13; the width ratio can, for example, be as illustrated in Fig. 1.

Various materials or combinations of materials can be used for the individual layers. Thus, the top layer 10 can consist, for example, of silicon, doped silicon or boron-doped silicon. The intermediate layer 11 can be formed from quartz. It is then advantageously combined with a bottom layer 12 of silicon or glass. The intermediate layer 11 can also be formed from metal, for example, aluminium, and can then be combined with a top layer 10 of silicon and a bottom layer 12 of glass.

Such a filter is also especially suitable for chemical analyses, for example for a gas analysis.

Claims

Patent Claims

1. A micromechanical filter with a top layer provided with openings, a bottom layer, and an intermediate layer provided in predetermined first regions between the top layer and bottom layer, -which intermediate layer substantially determines the spacing between the top layer and bottom layer in predetermined intermediate layer-free second regions, characterized in that third regions (C) are provided, in which the spacing between the top layer (10) and the bottom layer (12) is greater than in the first (A) and second regions (B).

2. A filter according to claim 1, characterized in that a second region (B) is provided between a third region (C) and an opening (13).

3. A filter according to claim 1 or 2, characterized in that the area of the third regions (C) is substantially larger than that of the second regions (B).

4. A filter according to one of claims 1 to 3, characterized in that the third regions (C) form ducts (15).

5. A filter according to claim 4, characterized in that a plurality of ducts (15) is connected in parallel.

6. A filter according to one of claims 1 to 5, characterized in that the third regions (C) are in connection with at least one inlet (28) and at least one outlet (30) .

7. A filter according to claim 6, characterized in that the inlet (28) and/or the outlet (30) open into a collecting space (23, 29).

8. A filter according to one of claims 4 to 7, characterized in that the openings (13) are longer than they are wide and their width corresponds substantially to the width of the ducts (15), a transition from an opening (13) into a duct (15) being effected substantially at the long sides thereof.

9. A filter according to one of claims 4 to 8, characterized in that the ducts (15) are arranged in groups substantially parallel with one another, adjacent groups (33) being arranged so that the ends of the respective ducts face towards one another and just one outlet region (35) or just one inlet region (34) is formed between two adjacent groups (33).

10. A filter according to claim 9, characterized in that two adjacent groups together form an angle, in the apex region (36) of which angle there is a separation * between the inlet and outlet region (34, 35).

11. A filter according to claim 10, characterized in that a plurality of groups (35) is arranged in a meander (37), which separates the inlet region (34) from the outlet region (35).

12. A filter according to claim 11, characterized in that a plurality of meanders (37) is provided, with either just one inlet region (34) or just one outlet region (35) being provided between adjacent meanders (37).

13. A filter according to one of claims 1 to 12, characterized in that the bottom layer (12) is formed from borosilicate glass.

14. A filter according to one of claims 1 to 13, characterized in that the filter is covered on the side having the openings (13) by a protective layer (24), which leaves free at least a subregion of the openings.

15. A filter according to one of claims 1 to 14, characterized in that the top layer (10) is inclined in relation to the bottom layer (12) at the transition from a first (A) or a second region (B) to a third region (C) .

16. A filter according to one of claims 1 to 15, characterized in that a heating means is provided,- in particular a heating means formed by two electrodes (38, 39).

17. A filter according to one of claims 1 to 16, characterized in that the top layer (10) is formed from silicon, doped silicon, or boron-doped silicon.

18. A filter according to one of claims 1 to 17, characterized in that the intermediate layer is formed from quartz, with the bottom layer (12) being formed from silicon or glass.

19. A filter according to one of claims 1 to 18, characterized in that the intermediate layer (11) consists of metal, in particular aluminium, while the top layer (10) is formed from silicon and the bottom layer (12) from glass.

20. A filter according to one of claims 1 to 19, characterized by an integrated conductivity meter (46), which is arranged in particular in the collecting space (23, 29).

21. A filter according to claim 20, characterized in that the conductivity meter (46) is formed by a thin- film disposed on the bottom layer (12) or top layer (10), in which pairs of electrodes (40, 41) are formed.

22. A filter according to claim 21, characterized in that each electrode (40, 41) has prongs (42) projecting from a main lead (44, 45), the prongs (42) of one electrode (41) projecting into the gaps (43) of the. other electrode (40).

23. Use of a filter according to one of claims 1 to 22 for chemical analysis.

24. Use of a filter according to one of claims 1 to 23 for gas analysis.

25. A method for the manufacture of such a filter according to one of claims 1 to 22, in which the top layer and the bottom layer are joined together by means of the intermediate layer, first and second regions being formed by removal of parts of the intermediate layer, characterized in that third regions are formed in the top layer (10) prior to joining the top layer (10) and the bottom layer (12).

26. A method according to claim 25, characterized in that the bottom layer (12) is joined to the intermediate layer (11) by bonding.

27. A method according to claim 25 or 26, characterized in that those parts of the intermediate layer (11) to be removed are removed prior to joining to the bottom layer (12).

28. A method according to one of claims 25 to 27, characterized in that the third regions (C) are advantageously etched from a substrate (16) , and the resulting recesses (19) are lined with the top layer (10) and the substrate (16) is later removed.

29. A method according to claim 28, characterized in that before removal of the substrate (16) the intermediate layer (11) is applied.

30. A method according to one of claims 25 to 27, characterized in that the top layer (10) is formed by mechanically processed metal or plastics material of small thickness, in particular by punching, stamping, or boring.

31. A method according to claim 30, characterized in that the intermediate layer (11) is applied by vacuum-evaporation and is formed in particular from aluminiu .