The present invention relates to an electroforming process for forming a metal structure.

Electroforming is an electrodeposition process for forming a product of one or more metal layers on a removable substrate. The substrate is placed in an electrolytic bath in electrical contact with a cathode. At the anode ions of the metal to be deposited are solved into the electrolyte. These ions flow to the substrate where they are replenished with electrons and deposit on the substrate surface as a layer of neutral metal molecules. Once the metal product has a desired thickness, it is removed from the substrate.

Typically, a pattern of a non-conductive coating is applied on parts of the substrate defining the geometrical outline of the structure to be electroformed. Such a non- conductive coating is usually applied on the substrate by means of a photolithographic process using a UV-sensitive photoresist coating material. The photoresist can for example be applied by spin coating to obtain a very smooth layer of uniform thickness. Parts of the photoresist are selectively exposed to UV-light, e.g., by using laser direct imaging (LDI), or by using a photomask. If the photoresist is a positive photoresist, the exposed parts are removed. If the photoresist is a negative photoresist, the non-exposed parts are removed. A patterned coating of the photoresist will remain on the substrate. The electrically conductive substrate with the photoresist pattern forms a mandrel. Electroforming will only take place on sections of the substrate not shielded by the photoresist pattern.

US 2005/0253606 discloses an electrodeposition process for forming complex multi-layer structures with staggered layers and recesses. Each layer must be planarized before a next layer can be applied. NL 1031259 discloses an electroforming process involving forming a metal layer on a substrate and a sacrificial layer supporting the structure after removal of the substrate.

It is an object of the invention to provide an electroforming process which can be used for a broader range of metal products, in particular for products with one or more inner layers protruding from outer layers.

The object of the invention is achieved with a process of electroforming a metal structure, the process comprising the following steps:

- a first layer is deposited on a substrate followed by one or more next layers partially overlapping the first layer to form an intermediate structure having a substrate surface facing the substrate;

- in a next step, the intermediate structure is removed from the substrate and one or more further layers are deposited on said substrate surface of the intermediate structure .

After removal of the substrate the uncovered surface of the intermediate structure is just as flat as the substrate it was made on. Therefore, further layers can be applied on this side of the intermediate structure without the need of first planarizing it.

The first layer itself can comprise one or more layers. Also the partially overlapping layer can comprise one or more layers. The layers can be formed by electrodeposition.

Due to the partial overlap of the next layer, the first layer protrudes relative to this next layer. In a specific embodiment, the first layer will also protrude relative to the one or more layers on the substrate surface of the intermediate structure. This way, the first layer will protrude from the outer layers at both sides, e.g., to form a tip. The layers at both sides of the first layer may be symmetrical or asymmetrical. Optionally, a layer of a sacrificial material is formed over the intermediate structure before the intermediate structure is removed from the substrate. In a final step the layer of sacrificial material can removed, for example by selective etching. Suitable sacrificial materials include for example copper, silver or a polymeric material. The body of sacrificial material forms an auxiliary substrate, supporting the intermediate structure after removal of the substrate when the further layers are applied on the substrate side of the intermediate structure. The substrate side is the side of the intermediate structure directly contacted by the substrate before removal of the substrate.

In a specific embodiment, the substrate is a mandrel with a non-conductive coating pattern, such as a photoresist pattern, defining an outline of at least the first layer.

Using electrodeposition, the first layer can be deposited very accurately without overgrowing the respective photoresist.

Before forming the overlapping layer, the photoresist can be at least partially removed so a new photoresist pattern can be applied to confine the overlapping layer. Optionally, the complete photoresist coating can completely be removed and replaced with a fresh photoresist coating of the desired patter.

Optionally, the first layer can be of a different material than the overlapping layer and/or the layers deposited on the opposite side. This is particularly suitable for making a structure with a multilayer body of a first material, such as nickel or a nickel alloy, and a protruding tip of a second material, such as rhodium. Such a structure can for example be used as a test probe of a probe card for testing semiconductors on a wafer. In a particular specific embodiment, the process may for example comprise the following steps to manufacture a structure with a tip:

- after electroforming the first layer, the first layer is covered with a fresh non-conductive coating, e.g., a photoresist;

- in a next step parts of the non-conductive coating are removed to expose a part of the substrate, adjacent one side of the first layer;

- in a next step a second layer is electroformed on the exposed parts of the substrate surface;

- in a next step parts of the non-conductive coating on top of the first layer are removed and a third layer is electroformed on top of the exposed parts of the second layer and on top of an adjacent part of the first layer;

- in a next step the remaining parts of the non-conductive coating are removed and the body of sacrificial material is applied on top of the first and third layers;

- in a next step the substrate is removed to expose a substrate side of the intermediate structure;

- in a next step a further non-conductive coating is applied covering the part of the first layer projecting from the third layer and covering uncovered parts of the sacrificial layer adjacent the first layer;

- in a next step a further layer is electroformed where the non-conductive coating is not present;

- in next steps the non-conductive coating and the sacrificial material are removed.

Substrates for electroforming are made of conductive material, e.g., a metal, passivated to allow subsequent separation of the finished electroformed structure. Non- conductive substrates can for example be made of glass, silicon, or plastic polymeric material, and require the deposition of a conductive layer prior to electrodeposition. The sacrificial material can be any material allowing selective etching without affecting the materials of the final structure. Suitable sacrificial materials include for instance copper, silver or polymeric materials.

The non-conductive coating is typically a photoresist material, e.g., a positive or negative photoresist, although other types of non-conductive coatings can also be used, if so desired. The process according to the present invention allows producing multilayer metal structures with projecting inner layers on any scale, also on a very small scale, e.g., micrometer-scale, in a very accurate and reliable manner.

The thickness of the photoresist layer can for example be in the range of 10 to 100 micrometer, but can also be outside this range, if so desired.

The thickness of the electroformed layers can for example be in the range of 10 to 100 micrometer per layer, but can also be outside this range, if so desired.

Examples of suitable metals include, inter alia, nickel, nickel alloys, such as nickel-palladium alloy; chromium, rhodium, copper or copper alloys.

Examples of suitable electroplating baths for nickel include, i.a., a Watts bath (NiS04), a sulphamate bath, and examples of suitable electroplating baths for copper include a copper sulphate bath.

The invention is further explained with reference to the accompanying drawings, Figures 1A - N, showing consecutive steps of an exemplary embodiment of a process according to the invention.

Figure 1A shows a substrate 2 partly covered by a non- conductive coating of a photoresist 3. The photoresist 3 is applied, e.g., by means of spin coating, to form a UV- sensitive coating of a uniform thickness. Parts of the photoresist 3 are removed after selective exposure to UV- light, e.g., by means of laser direct imaging. As a result, the top surface 2 of the substrate 1 has conductive bare sections 4 and non-conductive sections 3 coated with the photoresist, as shown in Figure 1A.

The mandrel 1 is then placed in an electrolytic bath and electro-conductively connected to a cathode. By supplying rhodium cations, a rhodium layer 7 is deposited on the conductive sections 4 of the mandrel 1 (Figure IB). The thickness of the rhodium layer 7 does not exceed the thickness of the photoresist 3.

The mandrel 1 is then taken from the electrolytic bath and spin coated with a second photoresist layer 8, which covers the first photoresist 3 and the rhodium layer 7.

Alternatively, the first photoresist may be removed and complete replaced by the fresh second photoresist. Parts of the second photoresist 8 are selectively exposed to UV for curing and the uncured parts are washed away. In Figure ID, the first and second photoresist layers directly adjacent the rhodium layer 7 are removed leaving part of the substrate 2 uncovered (Figure ID).

The mandrel 1 is then placed in a second electrolytic bath and connected to the cathode, the anode being configured to release nickel cations. A nickel layer 9 is deposited on the uncovered electro-conductive section of the mandrel's top surface 2. The nickel layer 9 has the same thickness as the rhodium layer 7 (Figure IE).

In a next step, the mandrel 1 is removed from the electrolytic bath and to allow removal of a part of the photoresist 8 on top of the rhodium layer 7 adjacent the nickel layer 9 (Figure IF). The mandrel 1 is then returned to the same electrolytic bath and a further nickel layer 10 is electroformed on top of the uncovered part of the rhodium layer 7 and the first nickel layer 9 (Figure 1G). The resulting intermediate structure has a rhodium tip projecting from a nickel body. The rest of the photoresist is subsequently removed (Figure 1H).

A layer 11 of a sacrificial material, in this case copper, is then applied to cover the complete mandrel 1 and the nickel and rhodium layers 7, 10 (Figure II). The copper can be applied by means of electroforming or by any other suitable deposition process. The thickness of the copper layer is about the same as the thickness of the mandrel 1, but can be more if so desired.

The copper, nickel and rhodium layers 7, 10, 11 are jointly removed from the mandrel 1 (Figure 1J). The mandrel side 12 of the rhodium layer, i.e., the side facing the mandrel before its removal, is now uncovered. This side 12 is just as flat as the mandrel surface 2 and therefore a very suitable substrate for electroforming further layers, without the need of a planarization step.

A new photoresist layer 13 is applied and cured on top of the projecting part of the rhodium layer 7 (Figure IK). The rest of the rhodium layer 7 remains uncovered, just as the nickel layer 10.

The structure is then returned into the second electrolytic bath and again connected to the cathode. A further layer 14 of nickel is deposited on the uncovered parts of the rhodium layer 7 and the adjacent surface of the nickel part 10 (Figure 1L).

The last photoresist 13 and the copper 11 can now be removed, e.g., by selective etching. The remaining final structure 15 comprises a nickel body 16 with one end sandwiching a projecting rhodium tip. In an alternative embodiment, the layer in line with the rhodium tip can be a different material, e.g., copper, e.g., sandwiched by layers of nickel or nickel alloys.