Production of nanowires for mass production

Abstract

The invention relates to a method of producing a multitude of nanowires (1) on an electrically conductive surface (2), comprising: a) applying a masking layer (3) to the surface (2), wherein the masking layer (3) has multiple cut-outs (4) in which the surface (2) is left uncovered by the masking layer (3), b) applying, in each of the cut-outs (4), respective pieces of film (5) to the surface (2), wherein the pieces of film (5) each have a multitude of pores (6) which extend continuously over a thickness (dF) of the corresponding piece of film (5), c) growing the nanowires (1) onto the surface (2) into the pores (6) of the pieces of film (5).

Description

[0001] Production of nanowires for mass production

[0002] The invention relates to a method for producing a plurality of nanowires on an electrically conductive surface.

[0003] For example, from DE 102017 104922 A1, it is known to connect components using a multitude of nanowires. For this purpose, a multitude of nanowires are created on each of the surfaces to be joined. When the surfaces are brought together, the nanowires interlock. Alternatively, the nanowires are created on only one surface. The connection between the components can then be formed by the nanowires bonding to the surface of the other component under relatively low temperature conditions.

[0004] In any case, a nanowire connection can be formed with minimal effort and, in particular, without significant temperature exposure. The resulting connection exhibits excellent mechanical stability, excellent electrical conductivity, and excellent thermal conductivity. Such connections are therefore superior to conventional connections like solder joints in many respects, especially in the manufacture of electronic components.

[0005] There is therefore a need to be able to coat the surfaces of components to be joined with nanowires. Various methods for coating surfaces with nanowires are known. One example is described in DE 102017 104 906 A1. Excellent results can be achieved with this method.

[0006] In order to make nanowire technology accessible for the mass production of electronic components, it is also desirable to be able to produce nanowires at the lowest possible cost. In this respect, all known methods have potential for improvement. The object of the present invention is to provide a way to produce a large number of nanowires on a surface with particularly low effort.

[0007] This problem is solved by the method according to the independent claim. Further advantageous embodiments are specified in the dependent claims. The features described in the claims and in the description can be combined with one another in any technologically meaningful way.

[0008] According to the invention, a method for producing a plurality of nanowires on an electrically conductive surface is described, comprising: a) applying a masking layer to the surface, wherein the masking layer has several recesses in which the surface is uncovered by the masking layer, b) applying a respective piece of foil to the surface in each of the recesses, wherein the foil pieces each have a plurality of pores which extend continuously over a thickness of the respective foil piece, c) growing the nanowires onto the surface into the pores of the foil pieces.

[0009] The described method allows the fabrication of nanowires on an electrically conductive surface. Here, a nanowire is defined as any material body with a wire-like shape and a size ranging from a few nanometers to a few micrometers. A nanowire can, for example, have a circular, oval, or polygonal base. In particular, a nanowire can have a hexagonal base.

[0010] Preferably, the nanowires at the end of step c) have a length in the range of 100 nm [nanometers] to 100 µm [micrometers], particularly in the range of 500 nm to 60 µm. Most preferably, the nanowires have a length in the range of 3 to 45 µm. Furthermore, the nanowires preferably have a diameter in the range of 10 nm to 10 µm, particularly in the range of 30 nm to 4 µm. Most preferably, the nanowires have a diameter in the range of 50 to 1500 nm, particularly between 100 nm and 1000 nm. The term "diameter" refers to a circular base; however, if the base is not circular, a comparable definition of diameter must be used.

[0011] It is particularly preferred that all nanowires used have the same length and diameter. However, this is not required. It is also particularly preferred that the nanowires are of the same length within a tolerance of 10%, preferably 5%. In this case, the length of the nanowires does not vary by more than 10% or 5%, respectively. However, it is also particularly acceptable if individual nanowires are significantly longer or shorter than the other nanowires. It is preferred that 80% of the nanowires have a length that is the same within a tolerance of 10%, preferably 5%.

[0012] The described method is applicable to a wide variety of nanowire materials. Electrically conductive materials, especially metals such as copper, silver, gold, nickel, tin, and platinum, are preferred for the nanowires. However, non-conductive materials such as metal oxides are also preferred. Preferably, all nanowires are made of the same material.

[0013] The surface onto which the nanowires are to be grown is electrically conductive. If the surface is part of an otherwise non-conductive body (such as a substrate), electrical conductivity can be achieved, for example, through metallization. A non-conductive substrate can be coated with a thin layer of metal. Metallization can be used to create an electrode layer. Depending on the material of the surface and / or the electrode layer, it may be advantageous to include an adhesive layer between the surface and the electrode layer to ensure adhesion. Generally, the surface can be formed on a component such as a wafer, a substrate, an electrical component, or a tape.

[0014] For the basic functionality of the described method, the depth to which the surface is electrically conductive is generally irrelevant. It is sufficient that the surface is reliably electrically conductive. A metallization as thin as 20 nm on an otherwise electrically insulating body can be sufficient for this purpose. Even thinner metallizations are conceivable in principle. Preferably, the metallization is between 50 and 400 nm thick.

[0015] Due to its electrical conductivity, the surface can be used as an electrode for the galvanic growth of nanowires. The substrate can be, in particular, a silicon substrate. The surface can be, in particular, the surface of a body equipped with electrically conductive structures.

[0016] The component can, in particular, have an electrically insulating surface on which several spaced-apart contact pads are formed. The component with the contact pads is preferably a printed circuit board, especially for power electronics. The component can, in particular, be an element of a power module. The component with the contact pads can, in particular, be a so-called DBC substrate (Direct Bonded Copper substrate), AM B substrate (Active Metal Brazed substrate), FPC substrate (Flexible Printed Circuit substrate), PCB (Printed Circuit Board), or a leadframe. The electrically conductive surface of the component preferably has an area in the range of 1 to 50 cm². 2 , especially in the range of 5 to 2000 cm 2 The electrically conductive surface can be, for example, 3 cm x 4 cm in size.

[0017] The described method allows nanowires to be selectively grown on specific areas of the surface. In step a), a masking layer is applied to the surface. This layer covers the areas where nanowires are not to be grown. The masking layer has several cutouts where the surface is exposed. The nanowires are grown in these areas. The masking layer can also be described as a template.

[0018] The masking layer can be obtained in various ways. In particular, methods known from semiconductor technology, such as photolithography, can be used to form the masking layer. The masking layer can be formed using a photoresist. The photoresist locally prevents the growth of the nanowires. This implies that the masking layer is electrically insulated from the electrically conductive surface. In the simplest case, the masking layer is made of an electrically insulating material.

[0019] The masking layer can be formed, for example, by negative photoresist. First, the entire surface can be coated with a layer of negative photoresist. This can be done, in particular, by laminating. The negative photoresist can be supplied as a roll-on coating. Using such a negative photoresist is particularly simple and inexpensive. Furthermore, negative photoresist is generally non-flammable. After the entire surface has been coated with a layer of negative photoresist, the negative photoresist can be exposed through a mask, so that only the cutouts remain unexposed. The negative photoresist hardens in the exposed areas, i.e., outside the cutouts. Subsequently, the negative photoresist can be removed from the unexposed areas, leaving the masking layer with the cutouts.

[0020] The masking layer can also be formed using positive photoresist. First, the entire surface can be coated with a layer of positive photoresist, primarily through lamination. Then, the positive photoresist is exposed via a mask, so that only the cutouts are exposed. The positive photoresist hardens in the unexposed areas, i.e., outside the cutouts. Finally, the positive photoresist can be removed from the exposed areas, leaving the masking layer with the cutouts.

[0021] The masking layer can also be in the form of adhesive tape. The cutouts can, for example, be punched out of the tape. When the tape is then applied to the surface, it forms the masking layer.

[0022] The basic functionality of the described method is unaffected by how the masking layer is formed. It is sufficient that, after step a), the masking layer is applied to the surface and that the masking layer has several recesses in which the surface is uncovered by the masking layer.

[0023] In the next step of the process, the nanowires are grown onto the surface within the recesses. For this purpose, in step b), a piece of foil is applied to the surface in each of the recesses.

[0024] The film pieces are preferably each formed from a plastic material, in particular a polymer material. It is especially preferred that the film pieces are bonded to the surface in such a way that they do not slip. This could impair the quality of the grown nanowires.

[0025] It is preferred that the thickness of the film pieces is in the range of 10 to 100 µm, particularly in the range of 25 to 75 µm. For example, the film pieces can each have a thickness of 50 µm.

[0026] The foil pieces each have a multitude of pores extending continuously across the thickness of the respective foil piece. The nanowires can be grown into these pores. The continuous pores of the foil pieces are preferably realized such that they form continuous channels from the top side of the foil piece to the bottom side. In particular, it is preferred that the pores are cylindrical. However, it is also possible for the pores to be curved channels. A pore can, for example, have a circular, oval, or polygonal base. In particular, a pore can have a hexagonal base. Preferably, the pores are uniform (i.e., the pores preferably do not differ in size, shape, arrangement, and / or distance to neighboring pores).When the nanowires are grown in step c), the pores are preferably (and especially completely) filled with the electroplated material. This gives the nanowires the size, shape, and arrangement of the pores. By selecting the foil pieces or the pores within them, the properties of the nanowires to be grown can be determined or influenced. The foil pieces can therefore also be referred to as "templates," "template foils," or "stencils." The pores can be introduced into the foil pieces in various ways. Since a separate pore is provided for each nanowire, the number of pores to be created is correspondingly large. Regardless of the technique used to create the pores, the production of the foil pieces is therefore correspondingly complex. The described method addresses this issue and makes it possible to use as little foil material with pores as possible.Instead of applying a single, continuous film to the surface, a piece of film is applied to each of the recesses – that is, to the areas where nanowires are actually to grow.

[0027] In step b), applying a piece of film to the surface in each of the recesses means that each piece of film comes into contact with at least part of the surface within its corresponding recess. The film pieces can cover the entire recess. It is also conceivable that the film pieces extend beyond the recess and partially contact the top surface of the masking layer. Generally, the film pieces can be smaller than the corresponding recess, exactly the same size as the recess, or larger than the recess.

[0028] The only requirement is that at least one piece of film is provided for each of the cutouts, preferably exactly one. Since there are multiple cutouts, there are correspondingly multiple pieces of film. This generally allows for particularly efficient use of the porous film material. This also applies if the film pieces extend beyond the cutout and partially come into contact with the top surface of the masking layer. Even in this case, the film pieces can be smaller than would be the case if a single film were used.

[0029] However, the less the film pieces extend beyond the recesses, the more of the porous film material can be saved. It is therefore preferred that the film pieces each have a size that is at most 20% larger than the size of the corresponding recess in the masking layer. It is particularly preferred that the film pieces each have a size that is at most 10% larger than the size of the corresponding recess in the masking layer. It is even more preferred that the film pieces each have a size that is at most 10% larger than the size of the corresponding recess in the masking layer.

[0030] It is particularly preferred that the film pieces are confined to the corresponding recess. In this case, the film pieces do not protrude beyond the edge of the corresponding recess at any point. However, this is generally not necessary to achieve the advantages described herein. In particular, it is harmless if a film piece, for example, protrudes beyond the edge of the corresponding recess on one side.

[0031] The cutouts preferably have a rectangular shape. In this case, the film pieces can be produced with particularly little waste. Furthermore, it is preferred that all the cutouts are the same size. This can also contribute to producing film pieces with particularly little waste.

[0032] Furthermore, the masking layer can facilitate the positioning of the film pieces. This is particularly true wherever a film piece does not extend beyond the edge of the corresponding cutout. This is especially relevant when the film pieces are confined to the corresponding cutout.

[0033] The cutouts in the masking layer form pockets into which the pieces of film can be inserted. The edges of the cutouts act as stops, allowing each piece of film to be easily positioned precisely within the cutout.

[0034] It is preferred that the masking layer thickness be in the range of 2 to 80 µm, particularly in the range of 5 to 40 µm. For example, the masking layer can have a thickness of 20 µm. Typically, a thinner masking layer is preferred in the relevant technical field. Surprisingly, however, it has been found that a comparatively large masking layer thickness is preferable in the described process. This is explained by the advantage described above: the film pieces can be positioned particularly well due to the masking layer. This advantage is especially pronounced with a comparatively large masking layer thickness.

[0035] In step c), the nanowires are grown onto the surface. For this purpose, an electrical voltage can be applied between the electrically conductive surface to be grown and a counter electrode. Both the electrically conductive surface and the electrode are in contact with an electrolyte. In this way, the nanowires grow from the electrolyte onto the surface and into the pores of the foil pieces.

[0036] At least during part of the nanowire growth process, an elastic element, similar to a sponge, can be attached to the foil pieces. This prevents the foil pieces from slipping. This is particularly advantageous because the described method uses several individual foil pieces instead of one large foil. Once the nanowires have grown to the point where the foil pieces are held in place by the nanowires on the surface, the elastic element can be removed. Without the elastic element, the electrolyte can distribute particularly well, allowing the nanowires to grow especially uniformly.

[0037] In a preferred embodiment of the method, the film pieces are produced from one or more films using a separation process before step b).

[0038] The separation process can involve cutting or punching. Specifically, the separation process can include laser cutting, metal punching, or ceramic punching.

[0039] The fundamental operating principle of the described method depends primarily on the fact that the film pieces are produced from one or more films. The film pieces are preferably produced with minimal waste. Instead of, for example, using a single large film, the porous film material can thus be used particularly efficiently. In a further preferred embodiment of the method, the film pieces are positioned on the surface in step b) using a positioning device.

[0040] The foil pieces can be precisely positioned where the nanowires are to be grown. The positioning device can be, for example, a gripper. The positioning device can pick up the foil pieces, for instance, by applying pressure. After each foil piece has been positioned in the corresponding recess of the masking layer, the pressure can be released, leaving the foil piece on the surface. Alternatively, the foil pieces can be picked up electrostatically, for example.

[0041] In another preferred embodiment of the method, the thickness of the foil pieces is each greater by a factor in the range of 0.5 to 5 than the thickness of the masking layer.

[0042] In this embodiment, the thickness of the film pieces is defined relative to the thickness of the masking layer. It has been found that a ratio within the specified range is particularly advantageous with regard to the benefit described above, namely that the masking layer facilitates the positioning of the film pieces.

[0043] In a further preferred embodiment of the method, the foil pieces each have a size that is a maximum of 20%, preferably a maximum of 5%, smaller than the size of the corresponding recess in the masking layer.

[0044] The foil pieces are therefore each sized to match the size of the corresponding recess, or to be up to 20% or 5% smaller, respectively. These sizes are defined as the area in a cross-section parallel to the surface. This allows the foil pieces to be inserted into the corresponding recess without protruding beyond its edge. In this configuration, the masking layer particularly facilitates the positioning of the foil pieces.

[0045] Preferably, the film pieces are each 1 to 20% smaller than the size of the corresponding recess in the masking layer. In this case, the film pieces are smaller than the corresponding recesses. This allows the film pieces to be inserted into the recesses with sufficient clearance.

[0046] In a further preferred embodiment of the method, after step b) a circumferential gap is formed between the foil pieces and an edge of the corresponding recess of the masking layer, which has a width that at no point exceeds five times the thickness of the foil pieces.

[0047] The width of the gap is defined here relative to the thickness of the foil pieces. Preferably, the width of the gap does not exceed the thickness of the foil pieces at any point.

[0048] Preferably, the width of the gap is no greater than 250 µm at any point, and particularly preferably no greater than 50 µm at any point.

[0049] In this embodiment as well, the foil pieces are each smaller than the corresponding recess in the masking layer. As described in the previous embodiment, this facilitates the positioning of the foil pieces.

[0050] In a further preferred embodiment, the method further comprises: d) removing the masking layer after step c) such that the foil pieces with the nanowires grown therein remain on the surface.

[0051] The masking layer can be removed by a stripping process to which the film pieces are resistant. This is made possible by the choice of materials for both the masking layer and the film pieces, as well as by the choice of the stripping process. For example, a negative photoresist can be used as the masking layer, which can be stripped using sodium hydroxide (NaOH). The film pieces are preferably made of a polymer material, in particular polycarbonate, which is not attacked by sodium hydroxide. This highlights another significant advantage of the described method. By using the particularly cost-effective negative photoresist with its preferably large layer thickness, it is possible to exploit the fact that the masking layer can be removed without damaging the pore layer.If, on the other hand, the masking layer were made of the generally more expensive positive photoresist, a chemical would typically have to be used for the stripping process, which could potentially damage the film pieces. Therefore, the use of a negative photoresist is preferable to a positive one in this case. However, if the advantage of protecting the nanowires discussed here is not required, a positive photoresist can also be used.

[0052] If the masking layer is removed in such a way that the film pieces with the embedded nanowires remain on the surface, the film pieces can protect the nanowires during subsequent processing steps. After these further processing steps, the film pieces can be removed in a further stripping step. Since the masking layer can be removed by a stripping process to which the film pieces are resistant, this can be described as sequential stripping.

[0053] In a further preferred embodiment of the method, the surface is formed at the beginning of the method by an electrically conductive coating of a body, the method further comprising: e) after step d) removing the coating in at least one area in which no nanowires were grown in step c), f) after step e) removing the foil pieces so that the nanowires remain on the surface.

[0054] This embodiment exemplifies how the foil pieces can protect the nanowires in a further process step. This step involves removing the coating initially present on the component in at least one area where no nanowires were grown in step c). The coating can, in particular, be the metallization mentioned above, which provides an otherwise electrically insulating body with an electrically conductive surface. Accordingly, the body is electrically insulating in this embodiment. Since the coating is only temporarily required for nanowire growth, it can be completely or partially removed after the nanowires have grown, at least in those areas where no nanowires were grown. The coating can be removed, in particular, by etching. The foil pieces protect the nanowires during this process.This is particularly advantageous because it ensures that the shape and size of the nanowires remain unchanged after etching. The shape and size of the nanowires can thus be directly determined by the shape and size of the pores in the foil pieces. If the nanowires were subjected to etching, this would, in particular, reduce their diameter. While the pores of the foil pieces could be made correspondingly larger to accommodate this reduction in the size of the nanowires, it is difficult to accurately predict the extent of the etching's effect on the nanowires. Even if this prediction is successful, the result is generally only valid for an etching process with precisely defined parameters. If these parameters are not adhered to precisely, the nanowires will not achieve the desired size. This problem is eliminated in the present embodiment of the described method.The only possible consequence is that the nanowires may be attacked at their tips. However, this only reduces the length of the nanowires. This is far less critical than a change in the diameter of the nanowires.

[0055] After step e), the foil pieces are removed, leaving the nanowires on the surface. This exposes the nanowires. The surface can then be connected to another surface, for example, via the nanowires.

[0056] In another preferred embodiment of the method, the masking layer can be removed by a stripping process against which the film pieces are resistant.

[0057] The invention is explained in more detail below with reference to the figures. The figures show a particularly preferred embodiment, to which, however, the invention is not limited. The figures and the size relationships shown therein are only schematic. They show:

[0058] Figs. 1a to 1g: a method according to the invention for producing a plurality of nanowires on an electrically conductive surface, Fig. 2: a top view of the result of the method according to the invention shown in side view in Fig. 1g,

[0059] Fig. 3a: a top view of a film from which the film pieces for the process shown in Figs. 1a to 1g can be produced,

[0060] Fig. 3b: a top view of the situation from Fig. 1b.

[0061] Figures 1a to 1g illustrate a method for producing a plurality of nanowires 1 (shown in Figures 1d to 1g) on ​​an electrically conductive surface 2.

[0062] Fig. 1a shows that at the beginning of the process, the surface 2 is formed by an electrically conductive coating 11 of a body 12. As a first process step, a masking layer 3 is applied to the surface 2.

[0063] Subsequently, several recesses 4 are formed in the masking layer 3, in which the surface 2 is uncovered by the masking layer 3. The result is shown in Fig. 1b. The masking layer 3 is only still present over an area 13 of the coating 11. The respective edge 9 of the recesses 4 is shown.

[0064] Subsequently, a positioning device 8 is used to apply a respective film piece 5 to the surface 2 in each of the recesses 4. This is shown in Fig. 1c as an example for one of the film pieces 5. The film pieces 5 each have a plurality of pores 6 which extend continuously over a thickness d. Fextend to the corresponding foil piece 5.

[0065] Subsequently, the nanowires 1 are grown onto the surface 2 into the pores 6 of the foil pieces 5. The result is shown in Fig. 1d. It can also be seen that a circumferential gap 10 is formed between the foil pieces 5 and the edge 9 of the corresponding recess 4 of the masking layer 3, which has a width b. The width b is never more than five times the thickness d. F of the foil pieces 5. The foil pieces 5 each have a size that is a maximum of 20% smaller than the size of the corresponding recess 4 of the masking layer 3. In addition, a thickness d M the masking layer is shown. The thickness d F In the example shown, the foil pieces 5 are each 3 times larger than the thickness d. M the masking layer 3.

[0066] The masking layer 3 is then removed in such a way that the film pieces 5 with the nanowires 1 grown within them remain on the surface 2. This is possible in particular because the masking layer 3 can be removed by a stripping process to which the film pieces 5 are resistant. The result is shown in Fig. 1e.

[0067] The coating 11 is then removed in the area 13 where no nanowires 1 have grown. This can be done, in particular, by etching. The nanowires 1 remain protected by the remaining foil pieces 5. The result is shown in Fig. 1f.

[0068] The foil pieces 5 are then removed, leaving the nanowires 1 on the surface 2. The result is shown in Fig. 1g. This figure shows the final result of the process.

[0069] Fig. 2 shows a top view of the final result of the process shown in side view in Fig. 1g. It can be seen that where the masking layer 3 had the recesses 4, a plurality of nanowires 1 were grown onto the surface 2 formed by the coating 11. The remaining part of the body 12 is formed by the area 13 in which the masking layer 3 covered the surface 2 during nanowire growth, so that no nanowires grew in the area 13.

[0070] Fig. 3a shows a top view of a film 7, from which the film pieces 5 for the process shown in Figs. 1a to 1g can be produced. The film pieces 5 are cut from a corner of the film 7 in a space-saving manner. This can be done using a cutting process.

[0071] Fig. 3b shows a top view of the situation from Fig. 1b. The four recesses 4, in which the masking layer 3 does not cover the surface 2, are visible. A comparison of Figs. 3a and 3b shows that by generating the film pieces 5 from the film 7, the film material is used particularly efficiently. If a continuous film were laid over the entire masking layer 4, this would be less efficient. In particular, the film would also be located between the recesses 4 of the masking layer 3, where, however, no nanowires are grown anyway. The described method is therefore particularly resource-efficient and correspondingly cost-effective.

[0072] Reference symbol list

[0073] 1 nanowire

[0074] 2 Surface

[0075] 3 masking layer

[0076] 4 Exclusion

[0077] 5 pieces of foil

[0078] 6 pores

[0079] Slide 7

[0080] 8 Positioning device

[0081] 9 Rand

[0082] 10 columns

[0083] 11 Coating

[0084] 12 bodies

[0085] 13 Area d F Thickness of the foil pieces d M Thickness of the masking layer b Width of the gap

Claims

Claims 1. A method for producing a plurality of nanowires (1) on an electrically conductive surface (2), comprising: a) applying a masking layer (3) to the surface (2), wherein the masking layer (3) has several recesses (4) in which the surface (2) is uncovered by the masking layer (3), b) in each of the recesses (4) applying a respective film piece (5) to the surface (2), wherein the film pieces (5) each have a plurality of pores (6) extending continuously over a thickness (d F ) of the corresponding foil piece (5), c) growth of the nanowires (1) onto the surface (2) into the pores (6) of the foil pieces (5).

2. Method according to claim 1, wherein the film pieces (5) are produced from one or more films (7) by a separation process prior to step b).

3. Method according to one of the preceding claims, wherein the foil pieces (5) are positioned on the surface (2) in step b) using a positioning device (8).

4. Method according to any one of the preceding claims, wherein the thickness (d F ) of the foil pieces (5) is each by a factor in the range of 0.5 to 5 greater than a thickness (d M ) the masking layer (3).

5. Method according to one of the preceding claims, wherein the foil pieces (5) each have a size which is a maximum of 20% smaller than a size of the corresponding recess (4) of the masking layer (3).

6. Method according to one of the preceding claims, wherein a circumferential gap (10) is formed between the foil pieces (5) and an edge (9) of the corresponding recess (4) of the masking layer (3), the gap having a width (b) which at no point exceeds five times the thickness (d) F) of the foil pieces (5).

7. A method according to any one of the preceding claims, further comprising: d) removing the masking layer (3) after step c) such that the foil pieces (5) with the nanowires (1) grown therein remain on the surface (2).

8. A method according to claim 7, wherein the surface (2) is formed at the beginning of the method by an electrically conductive coating (11) of a body (12), and wherein the method further comprises: e) after step d) removing the coating (11) in at least one region (13) in which no nanowires (1) were grown in step e), f) after step e) removing the foil pieces (5) so that the nanowires (1) remain on the surface (2).

9. Method according to one of the preceding claims, wherein the masking layer (3) is removable by a stripping process against which the film pieces (5) are resistant.

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