WO2000022194A2

WO2000022194A2 - Method of, and apparatus for, electro-plating a structure

Info

Publication number: WO2000022194A2
Application number: PCT/GB1999/003448
Authority: WO
Inventors: Anthony Robert Corless; Richard Lynn Hedgley
Original assignee: Central Research Laboratories Limited
Priority date: 1998-10-15
Filing date: 1999-10-15
Publication date: 2000-04-20
Also published as: WO2000022194A3; GB9822457D0

Abstract

The invention relates to techniques for selective electro-plating. Previously electro-plating structures with edges sometime resulted in abrupt changes in electric field. Consequently non-uniform deposition occurred and it was difficult to overcome this. More significantly it is sometimes desired to selectively electro-plate a non-uniform layer on a substrate. The present invention provides an apparatus and method for selectively electro-plating a substrate (16) by occluding portions of the substrate to be electro-plated.

Description

Method of, and apparatus for, electro-plating a structure

The present invention relates to a method of, and apparatus for, electro-plating a structure.

In certain circumstances it is desirable to introduce a controlled non-uniformity into the thickness of an electro-plated structure or layer. An example is in the fabrication of an interconnection means used to connect a sensor element to its peripheral or support electronic components. In such situations there may be a conflict between a need for thin metal, in the region of the sensor element, to prevent excess interference to the sensing function by the metal, and for thicker metal^'in other regions, for example to allow good electrical conductivity; or to provide an adequate mechanical support; or a sufficient thickness to allow solder joining processes.

A particular example is the fabrication of flexible circuit elements which are used to connect micro-fabricated piezo-electric sensing elements to peripheral or support electronic components. In such cases it is desirable to maintain a thin layer of metal in the region at which the piezo-electric element is attached, while also providing a low resistance electrical pathway to electronic components in order to allow the device to operate efficiently at high frequencies.

Another example is the use of flexible circuits which may, for example, be used in ultrasound imagers. Here, plated copper thickness should be high across an interconnect region, but reduced in the area of the ultrasound transducer in order to prevent interference with the ultrasound propagation by the metal.

Other examples arise in the fabrication of thermal detection elements, where a high thermal resistance may be required in the vicinity of a sensing element, whereas at more remote locations from the element, a reduction of electrical resistance may be essential.

Published Japanese Patent Application JP-A-2225694 discloses a technique of electroplating. The thickness of the electro-plated material varies.

In an existing method of fabrication a substrate (comprising a base layer and a thin overall deposited metal layer), has a seed layer, spin coated on it, together with a layer of photoresist, for example AZ4562 or AZ5214E (both manufactured by the Hoechst Corporation). Spin coating exposure and development achieves apertures corresponding to loci in which pads, tracks etc were fabricated. The photoresist thickness commonly used is in the range lμm to 30μm, with track dimensions ranging from 3μm to 30000μm (or still larger). Following photoresist coating, imaging and development the substrate undergoes a first electro-plating stage, in order to build a first thickness of material. Materials which may be electro-plated include copper, nickel and gold, with a first plating thickness ranging between 0.5μm and 5μm, although other thickness were achieved. Following this plating stage the substrate is washed and dried and a second photoresist coat is applied selectively to cover regions of track, which are intended to remain "thin". For this process a simple painting technique is employed. A range of materials may be used as the selectively applied photoresist but it has generally been found convenient to use the AZ4562 photoresist.

The newly painted photoresist is then dried and a second plating step carried out in order to increase the thickness of the plate in regions which remain exposed to the solution. Following this second electroplating stage the photoresist is removed and the seed layer etched. The resulting track profile is similar to that shown in Figure 1 (Prior Art). However, the transition between the thicker and thinner plated regions is abrupt.

It will be seen that the previous fabrication process may be difficult to achieve in that the application of the second photoresist layer, to define the thickness transition point, requires considerable skill and patience if repeatable results are to be achieved. Moreover, the abrupt thickness transition can be disadvantageous. For example, if the substrate is a flexible material, flexure of the substrate in service can lead to cracking of the tracks particularly at the abrupt thickness transition. Hence a more gradual thickness transition is desirable for reliable service. Moreover, in this process it is possible for plating thickness to build up adjacent the painted photoresist to a greater degree than in other areas of the substrate. This is because ions flowing in the plating solution, in the vicinity of the selectively applied photoresist, may migrate to the nearest available contact point thereby causing an excess level of plating at the edge of, and in the vicinity of, the selectively applied photoresist.

An aim of the present invention is to produce an electro-plated structure with varying thickness, without the stages of drying the structure and selectively applying a second photoresist. A further aim of the present invention is to provide a less abrupt transition between thick and thin regions of electro-plated substrate, preferably by providing a degree of control over the width and profile of the transition region.

According to a first aspect of the present invention, there is provided a method of electro-plating a structure, comprising the steps of: providing an electrolyte which in use contacts the surface of the structure; connecting an electric current source to the structure, and to the electrolyte, so that, in use, ions migrate from the electrolyte onto the surface of the structure; and occluding a portion of the surface of the structure, thereby causing migration of ions to occur at a different rate in the vicinity of the occluded portion of the structure.

Occluding a portion of the surface of the structure to be electro-plated may be achieved by using a conductive member, which is hereinafter referred to as a clip. The clip is advantageously in electrical connection with the surface to be electro-plated.

Previously variation in thickness of an electro-plated layer has been achieved by utilising a pattern plating technique. In this technique, copper is plated into apertures in a photoresist coating. In a first plating stage, a thin layer of copper is plated across the whole area of a circuit, including, for example, sensing elements. At this point plating is halted, the circuit is dried, and a second photoresist is applied preventing further plating in unwanted areas. The thicker regions of copper required for tracks were then completed in a second plating stage. The drying of the circuit and the application of the second photoresist were time-consuming processes and sometimes damaged the photoresist.

An impoπant advantage with the present invention is that it overcomes the build-up of edges or corners in electro-plated devices. Also by selective use of the clip there is a tendency for stepped regions to be removed and instead smooth, graduated sloped regions to result. The present invention also overcomes the aforementioned problems of applying second photoresists.

One way in which the present invention removes so-called steps or shelves, is achieved by placing the clip at a distance between lOOμm to 5mm from the surface to be electro-plated.

According to a second aspect the invention provides an apparatus for electro -plating a structure comprises: a reservoir, which in use receives an electrolyte; first and second electrodes, which in use connect to the structure and to the electrolyte so as to provide an electrical pathway therebetween; a source of electrical current which, in use, causes ions to migrate from the electrolyte and deposit at a surface of the structure; and a clip for occluding a portion of the surface of the structure, so that, in use, ions are caused to deposit on a substrate, in a predetermined manner in the vicinity of the clip.

The clip may be constructed so as to permit the transport of ions therethrough. For example, the clip may have slots or holes formed in it. The size, shape and packing density of the holes or slots may vary at different locations so as to enhance graduated deposition at the occluded surface.

In an alternative embodiment the clip may comprise a foraminous substance, which selectively permits flow of ions from the electrolyte to the surface to be electro-plated. Preferably the foraminous substance is selectively coated with material serving to reduced its local porosity.

According to another aspect of the invention, there is provided a method of electroplating a layer on a substrate supporting electrically conductive pathways, the layer having varying thickness, by selectively impeding the ion migration to the surface.

The plating process is varied when a thin layer of metal has been deposited on a desired region. The clip is then apphed across, or close to, the substrate. As a result of this clip ions (which would previously have been deposited in the unwanted areas) are now deposited on the clip.

Most preferably the clip comprises an insulated main member and carries areas of conductive regions, which are similar in disposition and area to the open area of the features which are to receive minimal further deposit. For example the clip may conveniently incorporate a printed circuit board material bearing a conventionally fabricated conductor pattern. The insulated portion may have holes or slots to allow selective passage ions.

Preferably the clip and substrate share a common circuit at least to the extent that ion flow originating from an anode may have its circuit completed either by arrival at the clip or at the substrate.

Examples of the invention are now described, with reference to the Figures, in which: Figure 1 is a section through a structure electro-plated using prior art methods and apparatuses;

Figure 2 is a section through a structure electro-plated using the present invention; and

Figure 3 is a diagrammatical section showing an embodiment of the present invention.

Referring to Figure 3 there is shown a reservoir 10 containing an electrolyte 12, which includes copper ions. A direct current source 14 is connected, at a cathode, to substrate 16 and its anode is placed in contact with the electrolyte 12. A controller 18 controls current flow and an agitator (not shown) displaces the substrate 16 and clip 20. Clip 20 is perforated with small holes and slots. Clip 20 is made from a conductive material so shaped and disposed in relation to and connected to the substrate 16 as locally to intercept the flow of ions within reservoir 10 substantially preventing further deposition of material onto selected areas of the substrate 16 and reducing the degree of deposition into adjacent areas.

When current is apphed a layer of copper 22 is selectively applied to substrate 16 as permitted by clip 20. A section of the substrate is shown in Figure 2.

In an alternative method, the substrate 16 is secured together with a semi-permeable clip 20 (not shown) which restricts the flow of copper ions to unwanted areas, and thus reduces the effective deposition rate in those area-.. Together with this approach, substrate and copper source anode are displaced so that the copper ion path to circuit area 22b is shorter than the path to a surface of the substrate 22a. This serves to reinforce the depletion of copper ions in unwanted areas 22b relative to the copper deposit 22a. The semi-permeable clip 20 may be a porous material, a sheet having an array of holes, or a slotted member.

An alternative method provides for a first plating stage in which the substrate is coated with a thin layer of metal. There then follows a second plating stage. During the second plating stage the ions (which would have deposited into apertures located in the "thin" regions of the tracking) instead copper is deposited onto the conductive portions of the clip 20. An approximately equal conductor area is provided on the clip 20, to that portion of substrate not to be plated. This ensures that the perturbation to the ion flow in the electrolyte 12 is minimised and that there is no requirement to adjust electrode sizes, positions or currents between the two plating passes. It will be apparent that by judicious adjustment of the extent of the clip 20 and its spacing (d) from the substrate 16 the width and the thickness of the transition region can be tailored to suit a myriad applications. It is this feature of the method that allows for example a thin layer of copper to be deposited and to be overcoated with a layer of nickel of selectively varying thickness.

The substrate is secured at the first plating stage by a retainer which incorporates a selectively permeable member. Said selectively permeable member serves to restrict the passage of ions to the substrate dependent on the local permeability of the member to ions and its spacing from the substrate. The control of the local ion flow results in control of local plating thickness.^" In conjunction with this approach the source anodes may be positioned so as to promote a favourable ion flow density pattern.

In a third method clip 20 may be employed which has both the features of controlled permeability and conductive members electrically connected to the substrate and receptive of ions from the plating solution. This method being of particular application when the range of thickness variation required becomes large.

An alternative embodiment of apparatus comprises a clip 20 is made of an insulating material (not shown) and so shaped and disposed in relation to and connected to a substrate 16, as locally to intercept the flow of ions within an electro-plating reservoir 10. The insulating material bearing conductive elements having an area and disposition similar to the elements onto which plating is to be impeded and electrically connected to the substrate 16. A conventional printed circuit board bearing suitably patterned conductors and equipped with simple mechanical mounting components facilitating placement in registration to the substrate 16 is an exemplary embodiment of a clip 20 of this type.

A further embodiment apparatus comprises a clip 20 including a material having locally controlled permeability so as to locally modulate the passage of ions between anode and the substrate 16. An array of conductors (not shown) suitably sized and disposed further modulate the local flow of ions to the substrate 16. An example of such a clip is a conventional printed circuit bearing suitably patterned conductors, having mechanical mounting components facilitating placement in registration to the substrate, and also being drilled, milled or slotted so as locally to permit the flow of ions through the board material.

It will be apparent that variations on the principal methods described are possible, for example the chp may be employed at the first plating stage and removed during the second plating stage permitting the deposition of a layer of approximately uniform thickness over a layer having a controlled thickness variation.

The invention has been described by way of examples only and variation may be made to the examples without departing from the scope of invention.

Claims

1. A method of electro-plating a structure, comprising the steps of: providing an electrolyte which in use contacts the surface of the structure; connecting an electric current source to the structure, and to the electrolyte, so that, in use, ions migrate from the electrolyte onto the surface of the structure; and occluding a portion of the surface of the structure, thereby causing migration of ions to occur at a different rate in the vicinity of the occluded portion of the structure.

2. A method according to Claim 1 wherein a conductive chp is used to occlude a portion of the surface to be plated.

3. A method according to any preceding Claim wherein the chp is displaced relative to the surface to be plated.

4. A method according to either of Claims 2 or 3 wherein the clip is placed at a distance between lOOμm, to 5mm from the surface to be plated.

5. Apparatus for electro-plating a structure comprises: a reservoir, which in use receives an electrolyte; first and second electrodes, which in use connect to the structure and to the electrolyte so as to provide an electrical pathway therebetween; a source of electrical current which, in use, causes ions to migrate from the electrolyte and deposit at a surface of the structure; and a chp for occluding a portion of the surface of the structure, so that, in use, ions are caused to deposit on a substrate, in a predetermined manner in the vicinity of the clip.

6. An apparatus according to Claim 5 wherein the chp is formed from a conductive material.

7. Apparatus according to Claim 6 wherein the conductive material is different from the material of the substrate.

8. Apparatus according to any of Claims 5 to 7 wherein the chp includes slots or holes.

9. Apparatus according to Claim 8 wherein the packing density of the slots or holes varies.

10. Apparatus according to any of Claims 5 to 9 wherein the chp includes an insulated member and conductive regions.

11. A method substantially as herein described with reference to Figures 2 and 3.

12. Apparatus substantially as herein described with reference to Figures 2 and 3.