CH704955B1 - A method of manufacturing a metal microstructure and microstructure obtained using this prodédé. - Google Patents

Abstract

The present invention relates to a method for manufacturing a metal microstructure characterized in that it comprises the steps of forming, by a LIGA-UV type process, a photosensitive resin mold (4) and alternately electro-galvanically depositing layers a first metal (5 1, 5 2) and at least one second metal (6 1, 6 2) from said conductive layer to form a block (7) substantially reaching the upper surface of the photoresist, said block being formed of a stack of layers (5 1, 2, 6 1, 6 2) of the first and second metals.

Description

The present invention relates to a method of manufacturing a metal microstructure by LIGA type technology. In particular, the invention relates to such a process for the manufacture of such a microstructure using at least two metals in order to confer on the microstructure properties that adapt optimally to the application for which it is intended. The invention also relates to such a metal part obtained by this method.

[0002] The LIGA technology (Lithography Galvanik Abformung) developed by W. Ehrfeld of the Karlsruhe Nuclear Research Center, Germany, in the 1980s has proved to be of interest for the manufacture of high precision metal microstructures.

In principle, the LIGA technique consists of depositing on a conductive substrate or coated with a conductive layer, a layer of a photosensitive resin, to be carried out through a mask corresponding to the contour of the desired microstructure irradiation X by means of a synchrotron; to develop, that is to say to remove by physical or chemical means the portions of the non-irradiated photoresist layer to define a mold having the contour of the microstructure, to electro-galvanically deposit a metal typically nickel in the photosensitive resin mold and then remove the mold to release the microstructure.

The quality of the microstructures obtained is not critical, but the need to implement expensive equipment (synchrotron) makes this technique little compatible with a mass production of microstructures to have a low unit cost.

This is why on the basis of this LIGA process have been developed analogous processes but using UV photosensitive resins. Such a method is for example described in A.B. Frazier et al., Entitled "Metallic Microstructures Fabricated Using Photosensitive Polyimide Electroplating Molds", Journal of Microelectromechanical Systems, Vol. 2, N deg. 2, June 1993 for the manufacture of metal structures by electrodeposition of metal in polyimide-based photosensitive resin molds. This process comprises the following steps: creating on a substrate a sacrificial metal layer and a priming layer for a subsequent electroplating step, apply a layer of photosensitive polyimide, UV irradiating the polyimide layer through a mask corresponding to the contour of the desired microstructure, develop by dissolving the non-irradiated parts of the polyimide layer so as to obtain a polyimide mold, electro-galvanically depositing nickel in the open part of the mold up to the height thereof, and eliminate the sacrificial layer and separate the metal structure obtained from the substrate and remove the polyimide mold.

The microstructures obtained according to the processes of the prior art are metal microstructures made of a single metal generally nickel, copper nickel-phosphorus which is not always optimal depending on the application for which they are intended . Indeed, there are in particular applications for which one or other of these materials does not have optimal properties both mechanically and tribologically. Typically a toothed wheel must be rigid enough to withstand breaking under strong stress but must also have teeth with a low coefficient of friction to facilitate meshing. The choice of nickel is therefore very interesting from the point of view of its mechanical strength, on the other hand nickel has tribological properties less interesting since it has a relatively high coefficient of friction. There is therefore a need for a method to overcome these disadvantages.

The present invention aims to overcome the aforementioned drawbacks as well as others by providing a method for manufacturing microstructures adapted, from the point of view of their composition optimally to the application to which they are intended.

The present invention also aims to provide such a method that is simple and inexpensive to implement.

For this purpose the invention relates to a method for manufacturing a metal microstructure, characterized in that it comprises the steps of: <tb> a) <sep> provide a substrate covered with a conducting conductive layer. <tb> b) <sep> applying on the conductive part of the substrate surface a layer of photoresist, <tb> c) <sep> irradiating the resin layer through a mask defining the contour of the desired microstructure; <tb> d) <sep> dissolve the non-insolated areas of the photoresist layer to expose the conductive surface of the substrate in places, <tb> e) <sep> alternately electro-galvanically depositing layers of a first metal and a second metal from said conductive layer to form a block substantially reaching the top surface of the photosensitive resin, said block being formed a stack of layers of the first and second metals, <tb> g) <sep> delamination separating the resin layer and the electro-deposition block from the substrate, and <tb> h) <sep> remove the photosensitive resin layers from the delaminated structure to release the microstructure thus formed.

This method therefore allows the production of finished parts having a section formed of a plurality of alternating layers of a first and a second metal. A judicious choice of the two metals forming this stack of layers makes it possible to better adapt the mechanical properties of the part to a given application. For example, in the case of the production of a gear wheel, it is possible to imagine an alternating stack of layers of nickel and nickel-phosphorus, the nickel conferring on the workpiece the mechanical resistance to the workpiece while the nickel-phosphorus favors a lowering the coefficient of friction of the part.

According to a preferred embodiment of the invention the first and second metals have different mechanical properties and preferably, the first metal has a higher mechanical strength than the second metal and the second metal has a higher coefficient of friction. weak than the first metal. The first metal is, for example, nickel and the second metal is a nickel-phosphorus alloy.

Preferably, the layers of the first and second metals have a substantially equal thickness, but it will be understood that according to the application envisaged for the microstructure differentiated thicknesses for the layers of the two metals can be envisaged.

Typically the priming layer is formed of a stack of layers of chromium and gold.

This method makes it possible to produce several micromechanical structures on the same substrate.

According to another embodiment of the invention, the method further comprises, before step g) a step of depositing an armor layer and a repetition of steps b) to f) with a second mask defining a second contour for a second level of the microstructure, for example in view of producing a toothed wheel having two teeth of different diameters.

The method of the invention finds a particularly advantageous application for the manufacture of micromechanical parts of watch movements. In particular the parts may be selected from the group consisting of gears, escape wheels, anchors, pivoted parts, jumper springs, spirals, cams, and passive parts.

Other features and advantages of the present invention will emerge more clearly from the following detailed description of an exemplary embodiment of a method according to the invention, this example being given for purely illustrative and non-limiting purposes only, in conjunction with the accompanying drawing in which: - figs. 1 to 7 illustrate the process steps of an embodiment of the invention for the production of a toothed wheel.

The substrate 1 used in step a) of the process according to the invention is for example formed by a wafer of silicon, glass or ceramic, on which a priming layer has been deposited by evaporation. that is to say a layer able to start an electroforming reaction.

Typically the priming layer is formed of an underlayer of chromium 2 and a gold layer 3 (FIG 1).

Alternatively, the substrate may consist of stainless steel or another metal capable of starting the electroforming reaction. In the case of a stainless steel substrate, the latter will be degreased before use.

The photosensitive resin 4 used in step b) of the process according to the invention is preferably an octofunctional epoxy-based resin available from Shell Chemical under the reference SU-8 and a photoinitiator chosen from the salts triarylsulfonium such as those described in US Patent 4,058,401. This resin is likely to be photo-polymerized under the action of UV radiation. It will be appreciated that a solvent which has been found suitable for this resin is gammabutyrolactone (GBL) (Fig. 2).

Alternatively, a Novolac-type phenolformaldehyde-based resin in the presence of a DNQ photoinitiator (DiazoNaphtoQuinone) may also be used.

The resin 4 is deposited on the substrate 1 by any means typically suitable for spinning to the desired thickness. Typically the resin thickness is between 150 mu and 1 mm. Depending on the desired thickness and the deposition technique used the resin 4 will be deposited in one or more times.

The resin 4 is then heated between 90 and 95 ° C for a duration depending on the thickness deposited to remove the solvent.

The following step c) illustrated in FIG. It is possible to irradiate the resin layer by means of UV radiation through a mask defining the contour of the desired microstructure and thus the insolated zone 4a and non-insolated zones 4b. Typically this UV irradiation is 200 to 1000 mJ.cm <-> <2>, measured at a wavelength of 365 nm depending on the thickness of the layer. If necessary, a step of annealing the layer may be necessary to complete the photopolymerization induced by the UV irradiation. This annealing step is preferably carried out at 90 ° C to 95 ° C for 15 to 30 minutes. The exposed areas (light-cured) become insensitive to a large majority of solvents. On the other hand, non-insolated zones may subsequently be dissolved by a solvent.

The following step d) illustrated in FIG. 4comproves to develop the non-insolated zones 4b of the photosensitive resin layer in order to reveal, in places, the conductive layer 3 of the substrate 1. This operation is carried out by dissolving the non-insolated zones 4b using a solvent chosen from GBL (gammabutyrolactone) ) and PGMEA (propylene glycol methyl ethyl acetate). An insolated photosensitive resin mold 4a having the contours of a metal structure is thus produced.

The following step e) illustrated in FIG. It is a matter of electro-galvanically depositing in the mold a layer 51 of a first metal from said conductive layer, ensuring that the first layer extends only over part of the depth of the mold. A layer 61 of a second metal, which is different from the first one, is then electro-galvanically deposited in the mold while ensuring that the second layer extends only over part of the depth of the mold. These operations are repeated several times according to the number of layers 5n of the first and 6n of the second metal which it is desired to obtain in the stack until forming a block 5 substantially reaching the upper surface of the photosensitive resin, the block 7 being formed a stack of layers of the first and second metals 51, 52 ... 5n and 61, 62 ... 6n. By metal in this context are of course included metal alloys. Typically the first and second metals will be selected from the group consisting of nickel, copper, gold or silver, and as the alloy gold-copper, nickel-cobalt, nickel-iron, and nickel- phosphorus.

The thickness of the layers of each metal and the thickness ratio of a layer of the first metal to a layer of the second metal may vary depending on the use of the desired microstructure. Typically, the thickness of each metal layer may vary between 1 to 100 microns and the thickness ratio between a layer of the first metal to a layer of the second metal may vary from 1 to 10. Moreover, the number of layers of the metal Stacking is at least two or one layer of each metal. In a particular application such as a cam, for example, it will be possible to produce a microstructure comprising a stack of three layers, namely a thicker layer forming a core of a first metal having good tribological qualities typically made of nickel-phosphorus, interposed between two less thick layers of a second mechanically resistant metal, typically nickel. It will also be possible, if desired, to obtain a microstructure having good mechanical properties and an attractive aesthetic appearance replacing in the preceding example the nickel with gold or a gold alloy.

The electroforming conditions, in particular the composition of the baths, the geometry of the system, the voltages and current densities, are chosen for each metal or alloy to be electro-deposited according to the techniques well known in the art of the art. electroforming (see for example Di Bari GA "electroforming" Electroplating Engineering Handbook 4th Edition written by LJ Durney, published by Van Nostrand Reinhold Company Inc., NY USA 1984).

In a subsequent step f) illustrated in FIG. 6 the electro-formed block is leveled with the resin layer. This step can be done by abrasion and polishing in order to directly obtain microstructures having a flat upper surface having in particular a surface state compatible with the requirements of the watch industry for the production of high-end movement.

The following step g) illustrated in FIG. It is necessary to separate by delamination the resin layer and the electro-deposited block from the substrate. Once this delamination operation has been carried out, the photosensitive resin layer is removed from the delaminated structure in order to release the microstructure thus formed. To do this, the photopolymerized resin is dissolved in a step h) by N-methylpyrrolidone (NMP) or this resin is removed by plasma etching.

The microstructure thus released can either be used directly or, if appropriate, after appropriate machining.

Claims (15)

A method of manufacturing a metal microstructure, characterized in that it comprises the steps of: a) fabricating a substrate covered with a conductive firing layer. b) applying on the conductive part of the substrate surface a layer of photoresist, c) irradiating the resin layer through a mask defining the contour of the desired microstructure; d) dissolving the non-insolated areas of the photoresist layer to expose the conductive surface of the substrate in places, e) depositing alternately electro-galvanically layers of a first metal and at least a second metal from said conductive layer to form a block substantially reaching the upper surface of the photosensitive resin, said block being formed of a stack layers of the first and second metals, g) separating by delamination the resin layer and the electro-deposited block from the substrate, and h) removing the photosensitive resin layer from the delaminated structure to release the microstructure thus formed. 2. Method according to claim 1, characterized in that the first and second metals have different mechanical properties. 3. Method according to claim 1 or 2, characterized in that the first metal has a higher mechanical strength than the second metal and in that the second metal has a lower coefficient of friction than the first metal. 4. Method according to one of claims 1 to 3, characterized in that the first metal is nickel and the second metal is a nickel-phosphorus alloy. 5. Method according to one of claims 1 to 4, characterized in that the layers of the first and second metals have a substantially equal thickness. 6. Method according to one of the preceding claims, characterized in that the priming layer is formed of a stack of layers of chromium and gold. 7. Method according to one of the preceding claims, characterized in that several micromechanical structures are manufactured on the same substrate. 8. Method according to one of the preceding claims, characterized in that it further comprises, before step g), a step of depositing a second conductive layer of armor and a repetition of steps b) to e) with a second mask defining a second contour for a second level of the microstructure. 9. Method according to one of the preceding claims, characterized in that it comprises, after step e), a step f) of flattening the resin and the deposited metal to bring the resin and the electro-deposited block to same level. 10. Metal microstructure obtained according to a method according to one of the preceding claims, characterized in that its thickness comprises at least one electroformed layer of a first metal and at least one electroformed layer of a second metal allowing the peripheral wall of said microstructure to benefit from a compromise of the mechanical characteristics of each of said metals. 11. Metal microstructure according to claim 10, characterized in that the thickness of each of said at least one layer is substantially equal in order to obtain a peripheral wall whose characteristics are a balance between the characteristics of each metal. 12. Microstructure metal according to claim 10, characterized in that the thickness of each of said at least one layer is different in order to obtain a peripheral wall whose characteristics of a metal are preferred. 13. Microstructure metal according to one of claims 10 to 12, characterized in that said peripheral wall has a toothing to form a meshing means. 14. Microstructure metal according to one of claims 10 to 12, characterized in that said peripheral wall is substantially planar to form a contact surface. 15. A watch movement, characterized in that it comprises a microstructure according to one of claims 10 to 14.