HK1148561B - Mould for galvanoplasty and method of fabricating the same - Google Patents

Description

Mold for electroforming and method of manufacturing the same

Technical Field

The present invention relates to a mold for manufacturing a micromechanical part using electroforming (electroplating), and a method of manufacturing the mold.

Background

Electroforming has been used for a long time and is well known. LIGA-type methods (known in german under the acronym "rontgenlithrograph," Galvanoformung & abenforming ") are the latest. The method includes forming a mold by photolithography using a photosensitive resin and then growing a metal deposit such as nickel therein by electroforming. The precision of the LIGA technique is much better than that of conventional dies, obtained for example by machining. This precision thus allows the manufacture of micromechanical parts, in particular for timepiece movements, which was previously unthinkable.

However, these methods are not suitable for micromechanical components with a high length-to-diameter ratio, such as coaxial escape wheels made of nickel-phosphorus containing, for example, 12% of phosphorus. The electrolytic deposit of such components delaminates during electroplating due to internal stresses in the electroplated nickel-phosphorus, which causes it to separate at the interface with the substrate.

Disclosure of Invention

An object of the present invention is to overcome the above drawbacks, wholly or partly, by proposing an alternative mould that can provide at least the same manufacturing precision and allows the manufacture of parts with several levels and/or high aspect ratios.

The invention therefore relates to a method for manufacturing a mould, comprising the steps of:

a) providing a substrate having a top layer and a bottom layer, said bottom and top layers being made of a micro-machinable electrically conductive material and being interconnected (fixed) by an electrically insulating intermediate layer;

b) etching at least one pattern in said top layer until reaching the intermediate layer so as to form at least one cavity in said mould;

c) coating a top portion of the substrate with an electrically insulating coating;

d) the coating and the intermediate layer are directionally etched so as to limit the presence of the coating and the intermediate layer exclusively at each vertical wall formed in the top layer.

According to other advantageous features of the invention:

-etching the second pattern in step b) to form at least one recess communicating with the at least one cavity and providing the top layer with a second level;

-mounting a component after step d) to form at least one recess communicating with said at least one cavity and providing said mould with a second level;

-the method comprises a final step e): installing a rod in the at least one cavity to form a hole in a part to be made in the mold;

-step b) comprises a stage f): constructing at least one protective mask on the conductive top layer, stage g): performing an anisotropic etch of the top layer on portions not covered by the at least one protective mask; and stage h): removing the protective mask;

after the preceding step, the method comprises a step a'): depositing a conductive material at the bottom of the at least one cavity; b'): etching a pattern on the bottom layer until the deposit of conductive material is reached to form at least one cavity in the mold; and c'): coating the assembly with a second electrically insulating coating;

after step c '), the method comprises a step d'): directionally etching the second coating layer to restrict the presence of the second coating layer exclusively at each vertical wall formed in the bottom layer;

-etching the second pattern during step b') to form at least one recess communicating with said at least one cavity and providing said bottom layer with a second level;

-mounting a component after step d') so as to form at least one recess communicating with said at least one cavity and providing said mould with a second level;

the method comprises a final step e'): mounting a rod in the at least one cavity in the bottom layer to form a hole in a part to be made in the mold;

-step b ') comprises a phase f'): structuring at least one protective mask, g'), on the conductive top layer: performing an anisotropic etch of the top layer on portions not covered by the at least one protective mask; and a phase h'): removing the protective mask;

-manufacturing several moulds on the same substrate;

the conductive layer comprises a doped silicon-based material.

The invention also relates to a method for manufacturing a micromechanical part by electroforming, characterized in that it comprises the following steps:

i) manufacturing a mould according to the method of one of the preceding variants;

j) performing electrolytic deposition by connecting electrodes to the conductive bottom layer of the substrate to form the component in the mold;

k) releasing the part from the mold.

Finally, the invention relates to a mold for manufacturing a micromechanical part by electroforming, characterized in that it comprises a substrate having a top layer and a bottom layer, which are electrically conductive and are fixed to each other by an electrically insulating intermediate layer, wherein the top layer comprises at least one cavity that exposes a portion of the bottom layer of the substrate and comprises electrically insulating walls, such that an electrolytic deposit can grow in the at least one cavity.

According to other advantageous aspects of the invention:

-the top layer further has at least one recess communicating with said at least one cavity and having electrically insulating walls for continuing electrolytic deposition in said at least one recess after said at least one cavity has been filled;

-the bottom layer comprises at least one cavity revealing part of the electrically insulating layer of the substrate and having electrically insulating walls enabling electrolytic deposition to grow in said at least one cavity of the bottom layer;

the bottom layer further comprises at least one recess communicating with the at least one cavity of the bottom layer and having electrically insulating walls for continuing the electrolytic deposition in the at least one recess after the at least one cavity in the bottom layer has been filled.

Drawings

Further features and advantages will become more apparent from the following description, given purely by way of non-limiting illustration, with reference to the accompanying drawings, in which:

figures 1-7 are sequential step diagrams of a method of manufacturing a micromechanical component according to a first embodiment of the present invention;

figures 8-12 are sequential step diagrams of a method of manufacturing a micromechanical component according to a second embodiment of the present invention;

figure 13 is a flow chart of a method of manufacturing a micromechanical component according to the present invention;

figures 14-19 are diagrams of successive steps of a method of manufacturing a micromechanical component according to a variant of the present invention.

Detailed Description

As shown in fig. 13, the invention relates to a method 1 for manufacturing a micromechanical component 41, 41', 41 ″ using an electroforming process. The method 1 preferably comprises a method 3 of manufacturing a mould 39, 39 ', 39 ", followed by an electroforming step 5 and a step 7 of releasing the part 41, 41', 41" from said mould.

The mould manufacturing method 3 comprises a series of steps for manufacturing a mould 39, 39', 39 ", preferably comprising a silicon-based material.

The first step 10 of the method 3 consists in taking a substrate 9, 9 'comprising a top layer 21, 21' and a bottom layer 23, 23 'made of conductive, micromachinable material and fixed to each other by a conductive intermediate layer 22, 22', as shown in figures 1-8.

Preferably, the base material 9, 9' is SOI (silicon on insulator). Furthermore, the top layer 21, 21 'and the bottom layer 23, 23' are made of crystalline silicon and are sufficiently doped to be electrically conductive, the intermediate layer being made of silicon dioxide.

According to the invention, method 3 comprises, after step 11, two different embodiments, which are represented in fig. 13 by triple and single lines, respectively.

According to a first embodiment, in step 11, as shown in fig. 2, a protective mask 15 is constructed on the conductive top layer 21, and then a mask 24 is constructed. As also shown in fig. 2, the mask 15 has at least one pattern 27 that does not cover the top layer 21. Furthermore, the mask 24, which preferably completely covers the mask 15, has at least one pattern 26 that does not cover the top layer 21.

As an example, the mask 15 may be made by depositing a silicon dioxide layer so as to form a predetermined depth. Next, the mask 24 may be obtained by photolithography, for example, by covering the mask 15 with a photosensitive resin.

According to a first embodiment, illustrated by triple lines in fig. 13, in a third step 2 the top layer 21 is etched to reveal the intermediate layer 22. According to the invention, the etching step 2 preferably comprises an anisotropic dry etch of the Deep Reactive Ion Etching (DRIE) type.

In step 2, first, an anisotropic etch is performed in the top layer 21 with the pattern 26 of the mask 24. This etch is the start of an etch in the top layer 21 that is performed for at least one cavity 25 in a portion of its thickness. Next, the mask 24 is removed and then a second anisotropic etch is performed with the pattern 27 of the mask 15 still present on the top layer 21. The second etching continues the etching of the at least one cavity 25 and also starts the etching of at least one recess 28, which communicates with the at least one cavity 25 but has a larger cross-section.

In a fourth step 4, the mask 15 is removed. Thus, at the end of the fourth step 4, the top layer 21 is etched with at least one cavity 25 over its entire thickness and with said at least one recess 28 over a portion of its thickness, as shown in fig. 3.

In a fifth step 6, as shown in fig. 4, an electrically insulating coating 30 is deposited, which coating covers the entire top of the substrate 9. The coating 30 is preferably obtained by oxidizing the top of the etched top layer 21 and intermediate layer 22.

In a sixth step 8, a directional etching of the coating 30 and the intermediate layer 22 is performed. Step 8 serves to limit the presence of the insulating layer exclusively at each vertical wall formed in the top layer 21, i.e. the wall 31 of said at least one cavity 25 and the wall 32 of said at least one recess 28. According to the present invention, during directional or anisotropic etching, the vertical component of the etching phenomenon is weighted with respect to the horizontal component by modulating the chamber pressure (very low operating pressure) in, for example, a RIE reactor. This etching may be, for example, ion milling or sputter etching.

By performing step 8, as shown in fig. 5, it can be clearly seen that the bottom of the cavity 25 reveals the bottom layer 23, which is electrically conductive, and the bottom of the recess 28 reveals the top layer 21, which is also electrically conductive.

To improve adhesion for future electroforming, an adhesive layer may be provided at the bottom of each cavity 25 and/or at the bottom of the recess 28. The adhesive layer can thus consist of a metal, for example a CrAu alloy.

Preferably, during a sixth step 8, as shown in fig. 5, immediately during electroforming step 5, a stem 29 is mounted to form a shaft hole 42 for micromechanical part 41. This not only has the advantage of avoiding machining of the part 41 when electroforming is complete, but also means that an internal cross-section of any shape can be achieved over the entire height of the hole 42, whether or not the hole is uniform. Preferably, the rod 29 is obtained by photolithography, for example, using a photosensitive resin.

In the first embodiment, after step 8, method 3 of manufacturing mold 39 ends, followed by micro-mechanical component manufacturing method 1, with electroforming step 5 and step 7 of releasing component 41 from the mold.

The electroforming step 5 is realized by: the deposition electrode is connected to the bottom layer 23 of the mould 39 so as to grow the electrolytic deposition first in the cavity 25 of said mould and then in the second phase only in the recess 28, as shown in figure 6.

Indeed, advantageously, according to the invention, when the electrolytic deposit is flush with the top portion of cavity 25, it can be in electrical contact with top layer 21 through the adhesive layer, which enables the deposit to continue to grow beyond the whole of recess 28. Advantageously, the invention makes it possible to manufacture a component 41 having a high aspect ratio, i.e. in which the cross section of the cavity 25 is much smaller than the cross section of the recess 28. In this way, delamination problems can be avoided even for nickel-phosphorus materials containing, for example, 12% phosphorus.

Since silicon is used for the conductive layers 21, 23 and possibly for their adhesion layers, delamination phenomena at the interface are reduced, which avoids splitting caused by internal stresses in the electrodeposited material.

According to the first embodiment, the manufacturing method 1 ends with step 7, in which the component 41 formed in the cavity 25 and in the recess 28 is released from the mould 39. The release step 7 may be achieved, for example, by etching the layers 23 and 21. According to this first embodiment, as shown in fig. 7, it is clear that a micromechanical component 41 is obtained having two layers 43, 45-each having a different shape and completely independent thickness, and comprising a single axial hole 42.

This micromechanical part 41 can be, for example, a coaxial escape wheel or escape wheel 43-pinion 45 assembly, which not only has a geometrical precision on the order of microns, but also has an ideal reference, i.e. perfect positioning, between the levels.

According to a second embodiment of the invention, the method 3 has a second step 11 comprising the structuring of at least one protective mask 24 'on the conductive top layer 21', as shown in fig. 8. Fig. 8 also shows that the mask 24 ' comprises at least one pattern 26 ' which does not cover the top portion 21 '. The mask 24' can be obtained by photolithography using a photosensitive resin, for example.

In a third step 12, the top layer 21 'is etched until the intermediate layer 22' is revealed. In accordance with the present invention, the etching step 12 preferably comprises an anisotropic dry etch of the Deep Reactive Ion Etch (DRIE) type. An anisotropic etch is performed in the top layer 21 ' with the pattern 26 ' of the mask 24 '.

In a fourth step 14, the mask 24' is removed. Thus, at the end of fourth step 14, at least one cavity 25 'is etched through the entire thickness of top layer 21', as shown in fig. 9.

In a fifth step 16, an electrically insulating coating 30 'is deposited, which covers the entire top of the substrate 9', as shown in fig. 10. The coating 30 ' is preferably obtained by oxidizing the top of the etched top layer 21 ' and intermediate layer 22 '.

According to the sixth step 18, the coating 30 'and the intermediate layer 22' are directionally etched. Step 18 serves to limit the presence of the insulating layer exclusively at the vertical walls formed in the top layer 21 ', i.e. the walls 31 ' -of said at least one cavity 25 '. By performing step 18 and as shown in fig. 11, it is clear that the bottom of the cavity 25 ' reveals the top of the bottom conductive layer 23 ' and the top layer 21 ' which is also conductive.

As in the first embodiment, to improve adhesion for future electroforming, an adhesive layer may be provided at the bottom of each cavity 25 'and/or at the top of top layer 21'. The adhesion layer may be composed of a metal such as a CrAu alloy.

During a sixth step 18, as explained for the first embodiment of fig. 1-7, a rod can be mounted immediately (directly) in the electroforming step 5 in order to form a shaft hole for the micromechanical component, which has the same advantages as the aforementioned ones.

In the second embodiment, after step 18, method 3 of manufacturing mold 39 'ends and micromechanical component manufacturing method 1 continues with electroforming step 5 and step 7 of releasing the micromechanical component from mold 39'.

The electroforming step 5 is realized by: the deposition electrode is connected to the bottom layer 23 'of the mould 39' to grow an electrolytic deposit in the cavity 25 'of said mould 39'.

According to a second embodiment, the manufacturing method 1 ends with a step 7, which step 7 is similar to that described in embodiment 1, and in which the part formed in the cavity 25 'is released from the mould 39'. According to this second embodiment, it is clear that the micromechanical component obtained has a single layer of the same shape throughout its entire thickness and may contain an axial hole.

This micromechanical component may for example be an escape wheel, an escape pawl or even a pinion with a geometrical precision of the order of microns.

According to an alternative of the second embodiment, shown by double lines in fig. 13, after step 18, the method 3 of manufacturing a mould 39 'comprises an additional step 20 of forming at least a second level in the mould 39' as shown in fig. 12. The second level is thus formed by mounting a component 27 ' on the top layer 21 ', which component 27 ' comprises an electrically insulating wall 32 ', which top layer 21 ' is not removed during step 12.

Preferably, the added part 27 ' forms at least one recess 28 ' having a larger cross section than the removed part 25 ' by photolithography, for example, by using a photosensitive resin. However, the component 27 'may also comprise an insulating silicon-based material that is pre-etched and then fixed to the conductive layer 21'.

Therefore, according to an alternative of the second embodiment, after step 20, method 3 of manufacturing mold 39 ' ends and micromechanical component manufacturing method 1 continues with electroforming step 5 and step 7 of releasing component 41 ' from mold 39 '.

The electroforming step 5 is realized by: the deposition electrode is connected to the bottom layer 23 'of the mould 39' so as to grow the electrolytic deposition first in the cavity 25 'of said mould and then in the second phase only in the recess 28', as shown in figure 12.

Indeed, advantageously, according to the invention, when the electrolytic deposit is flush with the top portion of cavity 25 ', it can be electrically connected to top layer 21 ' by means of its adhesive layer, which enables the deposit to continue to grow up to the whole of recess 28 '. Advantageously, the invention makes it possible to manufacture a component 41 ' with a high aspect ratio, i.e. the cavity 25 ' has a cross section much smaller than that of the recess 28 '. In this way, delamination problems can be avoided even for nickel-phosphorus materials containing, for example, 12% phosphorus.

Since silicon is used for the conductive layers 21 ', 23' and possibly for their adhesion layers, delamination phenomena at the interfaces are reduced, which avoids splitting caused by internal stresses in the electrodeposited material.

According to a second embodiment alternative, the manufacturing method 1 ends with step 7, in which the part 41 'in the mould 39' is released, as explained in the first embodiment. As shown in fig. 12, it is clear that the micromechanical part 41' obtained has two layers, each having a different shape and totally independent thickness, and that they may comprise a single axial hole 42. This micromechanical component 41 'may thus have the same shape as the component 41 obtained by the first embodiment, and therefore this micromechanical component 41' not only has a geometrical precision on the order of microns, but also has an ideal reference, i.e. perfect positioning, between the levels.

According to a variant of the two embodiments of the method 1 visible in fig. 14-19 (indicated by the double-dashed line in fig. 13), the method 3 can also be applied to the bottom layer 23, 23 ', adding one or two other levels to the mould 39, 39'. To avoid overcomplicating the drawings, an example is described in detail below, but it is clear that the bottom layer 23, 23' can also be modified according to the first and second embodiments described above (with or without variants).

Depending on the embodiment used, this variant is identical to method 1 described above before steps 8, 18 or 20. In the examples shown in fig. 14-19, the example of the first embodiment shown by the triple line in fig. 13 will be taken as the starting point of method 1.

Preferably, according to this variant, the bottom layer 23 will be etched to form at least one second cavity 35 in the mold 39 ". As can be seen preferably between fig. 5 and 14, a deposit 33 has been formed in a portion of the first cavity 25 to provide an electroforming start layer. Preferably, this deposit 33 starts in step 5 until a predetermined depth is reached. However, this deposition may be performed according to different methods.

As shown by the double-dashed lines in fig. 13 and 14-19, a variation of method 1 applies steps 11, 12, 14, 16 and 18 of the second embodiment of method 3 to the bottom layer 23.

Thus, according to this variant, the method 3 comprises a new step 11 consisting in structuring at least one mask 34 on the conductive underlayer 23, as shown in fig. 15. As further shown in fig. 15, mask 34 includes at least one pattern 36 that does not cover underlayer 23. This mask 34 can be obtained by photolithography using a photosensitive resin, for example.

Next, in a new step 12, the layer 23 is etched in a pattern 36 until the conductive deposit 33 is revealed. The protective mask 34 is then removed in a new step 14. Thus, as shown in fig. 16, at the end of step 14, bottom layer 23 is etched through its thickness with at least one cavity 35.

In a new step 16, an electrically insulating coating 38 is deposited, as shown in fig. 17, which coating 38 covers the entire bottom of the substrate 9 ". The coating 38 is preferably obtained by depositing silicon dioxide on top of the bottom layer 23, for example by using vapour deposition.

If a single layer is added to the mold 39 ", then preferably no new step 18 is required. Otherwise, a directional etch of the coating 38 is performed. The new step 18 can be used to limit the presence of the insulating layer exclusively at the vertical walls 39 formed in the bottom layer 23, i.e. the walls of said at least one cavity 35. In the example of fig. 14-19, only the new step 18 is performed to remove the oxide layer in the bottom of the at least one cavity 35.

In a new step 18, as previously described, a stem 37 can be mounted so as to form a shaft hole 42 "in micromechanical component 41' immediately during electroforming step 5, which has the advantages previously described.

In a variant of method 1, after step 18, method 3 of manufacturing mold 39 "ends and micromechanical component manufacturing method 1 continues with electroforming step 5 and step 7 of releasing component 41" from mold 39 ". Preferably, rods 29 and 37 are aligned if formed in cavities 25 and 35, respectively. The rod 37 is preferably obtained by photolithography, for example, by using a photosensitive resin.

After the new step 8, 18 or 20, the electroforming step 5 is carried out by connecting the deposition electrode to the bottom layer 23, so as to grow an electrolytic deposition in the cavity 35 and continue the growth of the deposition in the cavity 25, as shown in fig. 18, and then in the second phase, to grow an electrolytic deposition only in the recess 28. The manufacturing method 1 ends with step 7, in which the part 41 is "released" from the mould 39, as described above.

According to this variant, it is clear that, as shown in fig. 19, a micromechanical component 41 "is obtained having at least three layers 43", 45 "and 47", each having a different shape and completely independent thickness, and having a single hole 42 ".

This micromechanical component can be, for example, a coaxial escape wheel 43 ", 45" with a pinion 47 "or a wheel set with three tooth levels 43", 45 ", 47", which not only has a geometrical precision on the order of microns but also has an ideal reference, i.e. perfect positioning, between said levels.

Of course, the invention is not limited to the examples shown, but may comprise various alternatives and modifications apparent to the person skilled in the art. Thus, several dies 39, 39 ', 39 "can be manufactured on the same substrate 9, 9 ', 9" to achieve a continuous manufacturing of micromechanical components 41, 41 ', 41 "that are not necessarily identical to each other. In addition, it is conceivable to exchange the silicon-based material for crystalline alumina or crystalline silica or silicon carbide.

Claims (17)

1. Method (3) for manufacturing a mould (39, 39 ') for manufacturing a micromechanical component (41, 41') by electroforming, comprising the steps of:

a) providing (10) a substrate (9, 9 ') having a top layer (21, 21') and a bottom layer (23, 23 '), which are made of electrically conductive doped crystalline silicon and are connected to each other by an electrically insulating intermediate layer (22, 22');

b) etching (11, 12, 14, 2, 4) at least one pattern (26, 26 ', 27 ') in the top layer (21, 21 ') up to the intermediate layer (22, 22 ') in order to form at least one first cavity (25, 25 ') in the mould;

c) coating (6, 16) the top portion of the substrate with an electrically insulating coating (30, 30');

d) the coating and the intermediate layer are directionally etched so as to limit the presence of the coating and the intermediate layer exclusively at vertical walls (31, 31') formed in the top layer.

2. Method (3) according to claim 1, characterized in that in step b) the second pattern is etched to form at least one recess (28) communicating with said at least one first cavity and providing said top layer with a second level.

3. A method (3) according to claim 1, wherein a component is mounted after step d) to form at least one recess (28') communicating with the at least one first cavity and providing the mould with a second level.

4. A method (3) according to claim 1, characterized in that it comprises the following final steps:

e) -forming a stem (29, 29 ") in said at least one first cavity by means of photolithography so as to form a hole (42, 42 ') in a part (41, 41') to be made in said mould.

5. Method (3) according to claim 1, characterized in that it comprises, after the preceding step, the following steps:

a') depositing a conductive material at the bottom of said at least one first cavity;

b ') etching (11, 12, 14, 2, 4) a pattern (26, 26', 27 ') on said bottom layer (23, 23') until reaching the deposit of conductive material so as to form at least one second cavity (35) in said mould;

c') coating (16) the entire base layer with a second electrically insulating coating (38).

6. Method (3) according to claim 5, characterized in that, after step c'), it comprises the following steps:

d') directionally etching said second electrically insulating coating (38) so as to limit its presence exclusively at vertical walls (39) formed in said bottom layer.

7. Method (3) according to claim 6, characterized in that in step b') the second pattern is etched to form at least one recess communicating with said at least one second cavity and providing said bottom layer with a second level.

8. Method (3) according to claim 6, wherein after step d') a component is mounted so as to form at least one recess communicating with said at least one second cavity and providing said mould with a second level.

9. A method (3) according to claim 5, characterized in that it comprises the following final steps:

e ') forming a stem (37) in said at least one second cavity of said bottom layer (23, 23') by photolithography so as to form a hole (42 ") in a part (41") to be made in said mould.

10. Method (3) according to claim 1, characterized in that several moulds (39, 39 ', 39 ") are made on the same substrate (9, 9', 9").

11. The method (3) according to claim 1, characterized in that step c) is carried out by oxidizing the top portion of the substrate.

12. A method (3) according to claim 11, characterized in that the electrically insulating coating (30, 30') is formed of silicon dioxide.

13. Method (1) for manufacturing a micromechanical component (41, 41') by electroforming, characterized in that it comprises the steps of:

i) -manufacturing a mould (39, 39', 39 ") according to the method (3) of one of the preceding claims;

j) performing (5) electrodeposition by connecting electrodes to an electrically conductive bottom layer (23, 23 ') of a substrate (9, 9') so as to form said component in said mould;

k) releasing the part (41, 41') from the mould.

14. Mould (39, 39 ', 39 ") for manufacturing a micromechanical component (41, 41', 41") by electroforming, characterized in that it comprises a substrate (9, 9 ', 9 ") having a top layer (21, 21') and a bottom layer (23, 23 ') of doped crystalline silicon, which are electrically conductive and interconnected by an electrically insulating intermediate layer (22, 22'), wherein the top layer (21, 21 ') comprises at least one first cavity (25, 25') that reveals a portion of the bottom layer (23, 23 ') of the substrate and comprises electrically insulating vertical walls (31, 31') such that an electrolytic deposition can grow in the at least one first cavity.

15. A mould (39, 39 ', 39 ") according to claim 14, wherein the top layer (21, 21') further has at least one recess (28, 28 ') communicating with said at least one first cavity and having electrically insulating walls (32, 32') so as to continue electrodeposition in said at least one recess after said at least one first cavity has been filled.

16. A mould (39, 39 ', 39 ") according to claim 14 or 15, wherein the bottom layer (23, 23 ') has at least one second cavity (35) revealing a portion of the electrically insulating layer of the substrate and having electrically insulating silicon dioxide walls (40) so that an electrolytic deposition can be grown in said at least one second cavity of the bottom layer (23, 23 ').

17. Mould according to claim 16, wherein said bottom layer (23, 23 ') further comprises at least one recess communicating with at least one second cavity of said bottom layer (23, 23 ') and having electrically insulating walls, so as to continue electrolytic deposition in said at least one recess after said at least one second cavity in said bottom layer (23, 23 ') has been filled.