Method and apparatus for non-immersive plating of a surface of a workpiece

Abstract

An apparatus for non-immersive wet-chemical treatment of a surface of a workpiece (2) comprises: a holding device (7) for holding the workpiece (2) with the surface in a predominantly upright orientation; a liquid application system (5) for flowing liquid (6) across the surface of the workpiece (2), when held by the holding device (7), as a film flowing in a predominantly downwards direction; a receptacle (4) for collecting liquid (9) that has flowed across and detached itself from the surface; and a system (11,12,16,18-20,22,24) for recirculating liquid collected in the receptacle (4) to the liquid application system (5). The system (11,12,16,18-20,22,24) for recirculating liquid comprises a device (22,23) for introducing sparging gas (21) into the liquid.

Description

Technical Field

[0001] The invention relates to an apparatus for non-immersive wet-chemical treatment of a surface of a workpiece, comprising: a holding device for holding the workpiece with the surface in a predominantly upright orientation; a liquid application system for flowing liquid across the surface of the workpiece, when held by the holding device, as a film flowing in a predominantly downwards direction; a receptacle for collecting liquid that has flowed across and detached itself from the surface; and a system for recirculating liquid collected in the receptacle to the liquid application system.

[0002] The invention also relates to a method of plating a surface of a workpiece, comprising: holding the workpiece with the surface in a predominantly upright orientation; applying liquid comprising a plating solution to the workpiece by flowing the liquid across the surface of the workpiece as a film flowing in a predominantly downwards direction; collecting liquid that has flowed across and detached itself from the surface; and recirculating the collected liquid for renewed application to a next workpiece. Background Art

[0003] WO 97 / 22733 A1 discloses an apparatus for deposition of a metal film onto a substrate. The plating solution is sprayed onto an activated substrate, which will initiate the autocatalytic deposition of the plating solution metal. The activated substrate may be any conducting material which will initiate the autocatalytic deposition of the deposition metal from the electroless plating solution. The plating solution may be sprayed in a manner which forms very fine droplets and may be carried in an inert gas. In the disclosed apparatus, a first reservoir contains a metal stock solution. The metal stock solution is connected via a line to a manifold. A metering valve allows precise control of the flow of metal stock solution to the manifold. A second reservoir contains a reducing solution and is connected via a line and metering valve to the manifold. The manifold serves as a mixing chamber in which the electroless plating solution is prepared. From the manifold, the prepared electroless plating solution is carried via a supply line to a process chamber into which the article to be plated is placed. A nitrogen source is connected via a line and valve to the process chamber. It is preferred that pressurised inert gas be used to force flow when a valve is opened. Pressurised connections between the nitrogen source and the reservoirs may be used for this purpose. The process chamber is sealed from the ambient environment, and it contains a turntable and a central spray post containing a plurality of vertically disposed spray orifices. Wafer cassettes are loaded onto the turntable and rotated around the spray post. It is disclosed that it is also possible to spray the plating solution using nozzles which form generally continuous blade or cone streams, rather than atomised droplets, but not how this would be done. A problem of the apparatus is that the sealed chamber is a requirement. This slows down the processing of the wafers and leads to a large use of inert gas or the need to provide some sort of gas lock to remove the wafers. Processing these in batches leads to non-uniform plating, as does the use of a spray. Unless the sealed inert gas atmosphere is maintained very effectively, it will be difficult to prevent oxygen from entering into the plating solution, in particular if the latter is recycled. This leads to instability of the plating solution.

[0004] EP 0 236 718 B1 discloses an apparatus comprising a conventional plating tank and associated external piping. The plating tank has an input port for supplying plating solution, an output port for discharging plating solution and a chemical feed controller. A pump pumps the plating solution from the output port through a heat exchanger, a filter and a static mixer. A chemical feed controller senses the concentration of various chemicals in the tank and introduces plating solution to maintain each of the chemicals at the desired level. One level of oxygen is maintained in the plating tank and a significantly higher level of oxygen is maintained in the external piping to reduce or eliminate plating in the external piping of the electroless plating system. The desired operating point for the oxygen in the tank is maintained by balancing an oxygen feed controller and a nitrogen feed controller that introduces nitrogen into the plating solution as the plating solution passes through the heat exchanger. The plating tank is filled with plating solution. The tank must be kept closed to keep the oxygen level at the intended value, since most of the solution remains in the tank. This complicates handling of the workpieces to be plated. Furthermore, it is difficult to generate a uniform fluid flow across the workpiece and achieve uniform plating results when the workpiece is immersed in the plating solution.

[0005] US 2014 / 0116334 A1 discloses a surface treating apparatus. The surface treating apparatus includes a transport hanger for transporting a plate-like workpiece in a horizontal direction, which is clamped by clamps and held vertically. An electroless copper plating tank includes a tank body mounted on a frame and a circulation pump for circulating electroless plating processing solution accumulated on the bottom in the tank body by supplying a squirting part with the liquid. The processing solution is squirted from a squirt port of the liquid squirting part toward the plate-like workpiece obliquely upwards to a horizontal plane. The processing solution is attached to the upper side of the plate-like workpiece that is clamped by the transport hanger inside of the tank body. Accordingly, it becomes possible to attach the processing solution to the surface of the plate-like work while the processing solution is running down the plate-like workpiece. Thus, a system is employed in which circulated processing solution runs down the plate-like workpiece without dipping the plate-like workpiece into stored processing solution. Therefore, it becomes possible to reduce the total amount of the processing solution used for the surface treating apparatus in whole as compared with a dipping type. The occurrence of bubbles when striking the work can be suppressed by squirting the processing solution obliquely upwards to the horizontal plane. This makes it possible to prevent from increasing the amount of dissolved oxygen in the processing solution. The existence of dissolved oxygen is described as causing a problem because it is stated to lead to low precipitation of plating by the effect of restraining reaction between the electroless plating solution and a catalyst attached to the workpiece. A problem of this type of apparatus is that preventing bubbles is not sufficient to achieve a high level of uniformity in thickness of the layer plated onto the workpiece.Summary of Invention

[0006] It is an object of the invention to provide an apparatus and method of the types mentioned above in the opening paragraphs that are suitable for plating the surface of the workpiece with a layer having a relatively uniform thickness whilst still making efficient use of the treatment solution.

[0007] This object is achieved according to a first aspect by the apparatus according to the invention, which is characterised in that the system for recirculating liquid comprises a device for introducing sparging gas in the liquid.

[0008] The sparging gas has the effect of reducing the oxygen content in the liquid prior to application by the liquid application system and of inhibiting the entry of any oxygen in the environment of the film into the solution as the liquid film flows across the surface. The processing liquid is preferably an aqueous electrolyte, more preferred an electroless aqueous electrolyte which is used as a plating solution (also mentioned as bath). An electroless aqueous electrolyte will generally comprise metal ions to be plated onto the surface of the workpiece and a reducing agent capable of reducing the metal ions. It is believed that the reduction of any oxygen present in the liquid competes with the plating reaction. This competing reaction can give rise to a phenomenon known as skip plating. The sparging gas helps ensure that this competing reaction is limited. In turn, this makes it possible to use a thinner liquid film, which flows more slowly. This in turn allows the flow to remain laminar over a larger section of the surface, which in turn further inhibits the entry of oxygen into the liquid and limits the transport of dissolved oxygen to the surface of workpiece by slow diffusion processes only. Although the apparatus may still comprise a sealable processing chamber and even means to fill such a sealed processing chamber with an inert gas, this is not required to the same degree. Quite the opposite, the liquid may pick up oxygen as the liquid becomes detached from the workpiece surface and is collected in the receptacle, because this helps keep the plating solution stable.

[0009] Compared to systems that make use of a processing bath into which the workpiece is immersed, the apparatus does not need to compromise between bath stability and suppressing the competing redox reactions.

[0010] The apparatus may be comprised in a larger system for non-immersive treatment of the workpiece. Such a system will include multiple apparatus and a system for transporting the workpiece from station to station, including into and from the apparatus that is the focus of the present disclosure.

[0011] The apparatus may be configured for treatment of only one surface of the workpiece, in particular a planar surface. Alternatively, the apparatus may be configured for treatment of both opposing major surfaces of a planar workpiece, e.g. a planar workpiece in the form of a panel or foil.

[0012] The apparatus comprises a holding device for holding the workpiece with the surface in a predominantly upright orientation. This means that the apparatus is configured to hold and treat the workpiece with the surface of the workpiece in a vertical position in the apparatus whilst the treatment method is applied. Thus, the liquid flow across the surface is gravity-driven.

[0013] The apparatus comprises a liquid application system for flowing liquid across the surface of the workpiece, when held by the holding device, as a film flowing in a predominantly downwards direction. Such a system may comprise one or more flooding devices for generating a sheet-shaped jet that is projected onto the workpiece surface or a flow guide surface of the holding device. Alternatively, the holding device may comprise one or more flooding devices for guiding the liquid onto the surface. Here, the term jet is used in the sense of referring to a coherent stream of liquid projected into a surrounding medium in this case the atmosphere in the apparatus, usually from some kind of nozzle or aperture. The stream is unguided on entering the medium.

[0014] The apparatus comprises a receptacle for collecting liquid that has flowed across and detached itself from the surface. The receptacle may be just the bottom of a processing chamber in which the holding device is arranged to hold the workpiece. The receptacle need not be closed on all sides, but may be shaped like a gutter instead.

[0015] In an embodiment, the system for recirculating liquid further comprises a filter for filtering particles from the liquid.

[0016] This prevents such particles from flowing across the surface of the workpiece. In the case of sensitive workpieces, the risk of damage to the surface or the metal layer being applied is reduced. In any case, such particles could otherwise act as seeds that lead to non-uniform treatment or even disrupt the film flow or prevent the film flow from being established.

[0017] In a particular example of this embodiment, the device for introducing sparging gas into the liquid is arranged upstream of the filter.

[0018] A filter with a pore size sufficiently small to filter out the types of particle that might arise in a plating solution is also suitable for reducing sparging gas bubbles to micro- or nano-bubbles. This enhances the effect of the sparging gas. A larger amount and with this a larger surface area of the sparging gas can cover the liquid and dissolve into the liquid and remove oxygen in a shorter period of time.

[0019] In an embodiment of the apparatus, the system for recirculating liquid further comprises a container, e.g. a sump, for storing the liquid, and the container is located in flow direction between the receptacle and the device for introducing sparging gas into the liquid.

[0020] Thus, the oxygen content is only reduced after intermediate storage. Intermediate storage is of a liquid with a higher oxygen concentration. In the case of a plating solution, in particular a copper plating solution, this means that the liquid has a relatively high stability whilst in intermediate storage. The higher oxygen concentration enhances stability by inhibiting or even reversing the formation of monovalent metal ions. Such monovalent ions are generally not reduced further to the metal to be plated onto the surface of the workpiece. Rather, they give rise to undesired precipitates. The higher oxygen concentration counters this.

[0021] In a particular example of this embodiment, the system for recirculating liquid further includes at least one diffuser, located in the container, for introducing at least one gas, e.g. at least oxygen, into liquid in the container.

[0022] The diffuser may be comprised in an aerator, or only oxygen may be diffused into liquid in the container. In either case, stability of the liquid held in intermediate storage is enhanced further.

[0023] In an embodiment of the apparatus, the liquid application system is arranged to supply the liquid to the holding device, and the holding device comprises at least one flow guide surface for directing liquid towards, e.g. onto, the surface of the workpiece.

[0024] This facilitates the generation and maintenance of a laminar film flow across a relatively large section of the surface of the workpiece. By contrast, a curtain coater directing a sheet-shaped jet directly onto the surface will generally do so at a distance from an (upper) edge of the workpiece and give rise to splashes and turbulence.

[0025] In an embodiment of the apparatus the holding device is configured to hold a planar workpiece.

[0026] Such a workpiece can be easily clamped in an edge region, e.g. an upper edge region. Furthermore, this embodiment allows to treat two surfaces simultaneously by means of a liquid application system configured to apply liquid to both major surfaces of the planar workpiece.

[0027] In an example of any embodiment in which the holding device is configured to hold a planar workpiece, the holding device comprises a clamping device for engaging major surfaces of the planar workpiece in an edge region along an edge of the planar workpiece, e.g. an edge region along an in use upper edge of the planar workpiece.

[0028] This allows the workpiece to be held with most of the surface of the workpiece exposed and uninterrupted. This also functions to control the thickness and velocity of the liquid film flowing across the workpiece. This helps reduce the risk of turbulence. That in turn ensures the plated metal layer is relatively uniform in thickness.

[0029] In an example of any embodiment of the apparatus in which the liquid application system is arranged to supply the liquid to the holding device, the holding device comprises at least one flow guide surface for directing liquid towards, e.g. onto, the surface of the workpiece, the holding device is configured to hold a planar workpiece and the holding device comprises a clamping device for engaging major surfaces of the planar workpiece in an edge region along an edge of the planar workpiece, e.g. an edge region along an in use upper edge of the planar workpiece, the holding device comprises a pair of flow guide surfaces on opposite sides of a space for accommodating the edge region, each extending towards a respective edge bounding a gap for insertion of the edge region into the space, and at least respective opposing sections of the flow guide surfaces converge towards each other in a direction of the edges.

[0030] This helps promote the establishment of a laminar film flow from close to the inserted edge of the workpiece downwards. Therefore each film emerges from a gap between the inserted section of the workpiece and one of the two flow guide surfaces. The opposing converging sections help ensure that the edge region is fully immersed in the processing liquid. It is observed that the gap need not be bounded on all sides. That is to say that the gap need not be a slot. This is also the case for the space, although it can be helpful if the space is bounded also at opposite longitudinal ends of the space. Here, the longitudinal direction corresponds to the direction in which the edges of the flow guide surfaces extend. The edges towards which the flow guide surfaces extend may be straight edges for enhanced uniformity of flow.

[0031] In a particular example of this embodiment, the holding device comprises at least one further flow guide arranged to introduce liquid supplied to the holding device into the space at a location upstream of the gap.

[0032] This helps avoid eddies at the edges of the flow guide surfaces that bound the gap, because the liquid is supplied to the space between the flow guide surfaces at a location that is in between the flow guide surfaces. The further flow guide may comprise a conduit or a flow guide surface onto which a stream of liquid is directed, in use, or both.

[0033] In an example of any embodiment in which the holding device is configured to hold a planar workpiece and the holding device comprises a clamping device for engaging major surfaces of the planar workpiece in an edge region along an edge of the planar workpiece, e.g. an edge region along an in use upper edge of the planar workpiece, the clamping device comprises at least one first protruding engagement feature protruding from a first engagement feature support and at least one second protruding engagement feature protruding from a second engagement feature support, wherein the first engagement feature support and the second engagement feature support are at least partially movable with respect to each other to adjust a distance between the first and second protruding engagement features in a direction in which they protrude.

[0034] Thus, only the surface of the protruding engagement features at the free ends of the protruding engagement features engage the surface of the workpiece.

[0035] The contact area is thereby kept small. Processing liquid can flow around the protruding engagement features.

[0036] In an example of this embodiment, the at least one first protruding engagement features comprise an array of first protruding engagement features spaced apart in a direction transverse to the direction in which they protrude, and the at least one second protruding engagement features comprise an array of second protruding engagement features spaced apart in a direction transverse to the direction in which they protrude.

[0037] This minimises disruption of the flow field by the protruding engagement features. Processing liquid can flow around and between the protruding engagement features of each array. Each individual engagement feature can be relatively slender, so that the plated layer is only slightly thinner immediately downstream of the engagement feature. There are also fewer eddies, minimising the risk of turbulent flow across the remainder of the workpiece surface.

[0038] In an example of any embodiment in which the liquid application system is arranged to supply the liquid to the holding device, the holding device comprises at least one flow guide surface for directing liquid towards, e.g. onto, the surface of the workpiece, the holding device is configured to hold a planar workpiece, the holding device comprises a clamping device for engaging major surfaces of the planar workpiece in an edge region along an edge of the planar workpiece, e.g. an edge region along an in use upper edge of the planar workpiece, the holding device comprises a pair of flow guide surfaces on opposite sides of a space for accommodating the edge region, each extending towards a respective edge bounding a gap for insertion of the edge region into the space at least respective opposing sections of the flow guide surfaces converge towards each other in a direction of the edges, the clamping device comprises at least one first protruding engagement feature protruding from a first engagement feature support and at least one second protruding engagement feature protruding from a second engagement feature support, wherein the first engagement feature support and the second engagement feature support are at least partially movable with respect to each other to adjust a distance between the first and second protruding engagement features in a direction in which they protrude, the first and second protruding engagement features are located in the space.

[0039] Because the liquid backs up in the space due to the converging surface sections, there is in effect a reservoir of liquid in which both the protruding engagement features and the edge region of the workpiece are immersed. Compared with the alternative of arranging the protruding engagement features further downstream, the flow past the protruding engagement features is less likely to swirl. Uniformity of treatment of the surface in the edge region is improved further.

[0040] In an example of any embodiment in which the holding device is configured to hold a planar workpiece and the holding device comprises a clamping device for engaging major surfaces of the planar workpiece in an edge region along an edge of the planar workpiece, e.g. an edge region along an in use upper edge of the planar workpiece, the holding device comprises a frame supporting the clamping device for engaging opposite major surfaces of the planar workpiece in the edge region along the edge of the planar workpiece, and the holding device comprises at least one further clamping device, supported by the frame, for engaging the major surfaces of the planar workpiece in a further edge region along a further edge of the planar workpiece opposite the edge, e.g. an edge region along an in use lower edge of the planar workpiece.

[0041] The workpiece can be relatively thin, e.g. in the shape of a foil or flat panel, in this embodiment. The further clamping device helps avoid that the workpiece starts to flutter as the liquid film flows across the surface. Thus, a relatively uniform, e.g. laminar, flow is maintained across a relatively large section of the surface.

[0042] According to another aspect, the method of plating a surface of a workpiece according to the invention is characterised by introducing sparging gas in the collected liquid prior to the renewed application to the next workpiece.

[0043] Thus, even though the liquid flowing across the surface and collected after having flowed across the surface may have a somewhat elevated oxygen concentration, the liquid is rendered suitable for renewed application by the introduction of sparging gas. The sparging gas also helps avoid skip plating. This is achieved without having to provide a sealed processing chamber with a conditioned atmosphere.

[0044] The sparging gas or gases may comprise a non-oxidising gas, e.g. an inert gas.

[0045] Suitable gases include nitrogen, argon forming gas (a mixture of hydrogen and nitrogen), hydrogen, helium, methane and neon, for example. Thus, the sparging gas may be, but is not limited to be an inert gas. The sparging gas or gases may be introduced into the liquid as bubbles.

[0046] In an embodiment, recirculating the collected liquid comprises intermediate storage of the collected liquid in a container, sparging gas is introduced into liquid removed from the container after the intermediate storage, and the method includes introducing at least oxygen into the collected liquid prior to removal of liquid from the container.

[0047] The oxygen may be introduced into the collected liquid in the container or prior to entry into the container, or both. The stability of the plating solution is improved by the introduction of oxygen. The adverse effect on the uniformity of the layer plated onto the surface of the workpiece is avoided by the introduction of sparging gas into the liquid subsequent to removal from the container.

[0048] An embodiment of the method further comprises activating the surface prior to applying the liquid comprising the plating solution.

[0049] This entails producing a catalytic metallic surface by techniques known to a skilled person in order to prepare the surface for subsequent metal plating as electroless metal plating. The activation process may be wet-chemical or comprise vapour deposition (physical or chemical). Chemical activation generates catalytic metallic nuclei of metal, e.g. palladium, palladium / tin or copper, on the surface. Vapour deposition may be used to deposit a relatively highly conductive layer onto the surface of the workpiece.

[0050] An embodiment of the method further comprises filtering the liquid by passing the liquid through a filter prior to applying the liquid to the workpiece.

[0051] This filters out particles that could have a negative impact on the quality of the layer of metal plated onto the surface of the workpiece. It is, for example, prevented that such particles block or disrupt the flow of liquid through orifices of the device used to apply the liquid to the surface of the workpiece or that they act as seeds on which metal is deposited in preference to the surface of the workpiece. The filter may be a screen filter, depth filter or membrane filter. The filter may be rated to filter out particles having a diameter above a value in the micrometre range, e.g. 1 µm.

[0052] In an example of such an embodiment, sparging gas is introduced into the liquid in the form of bubbles, sparging gas is introduced upstream of the filter, and the bubbles are reduced in size by passing the liquid through the filter.

[0053] This enables removal of oxygen from the liquid and more of the sparging gas to dissolve into to the liquid at a relatively short time, as governed by Fick's laws of diffusion.

[0054] In an embodiment of the method, applying liquid comprises: filling a space between a flow guide surface and an upper section of the surface extending to an upper edge of the workpiece with liquid; and flowing the liquid onto a section of the surface lower than the upper section through a gap between the upper section of the surface and the flow guide surface.

[0055] The upper section is thus immersed in the liquid. The only exception may be any areas engaged by a device holding the workpiece, if such a device is used. Thus, this section is plated relatively uniformly. In addition, the liquid flowing out through the gap onto the lower section of the workpiece surface will generally do so in the form of a laminar film flow. That in turn helps keep the oxygen content in the boundary layer close to the surface low, improving plating uniformity.

[0056] In an embodiment of the method, the method is carried out using an apparatus according to the invention. Similarly, the apparatus of the invention is suitable for carrying out any embodiment of the method according to the invention.Brief Description of Drawings

[0057] The invention will be explained in further detail with reference to the accompanying drawings, in which: Fig. 1is a schematic diagram of an apparatus for plating surfaces of a planar workpiece; Fig. 2is a perspective view of a holding device for holding the planar workpiece in the apparatus; Fig. 3is a detailed perspective view of an, in use, upper section of the holding device of Fig. 2; Fig. 4is a detailed perspective view of an, in use, lower section of the holding device of Figs. 2 and 3; Fig. 5is a perspective view of part of the upper section of the holding device of Figs. 2-4; Fig. 6is a view into a space for accommodating an edge region of the planar workpiece in the holding device; Fig. 7is an enlarged view into the space shown in Fig. 6; Fig. 8is a perspective view of parts of an upper section of a modified version of the holding device of Figs. 2-7; Fig. 9is a diagram showing the development in horizontal and vertical direction of the average thickness of a layer plated onto a front major surface of a planar workpiece held by a holding device of the type shown in Figs. 2-8 without the introduction of a sparging gas into the liquid comprising the plating solution; Fig. 10is a diagram corresponding to that of Fig. 9 showing the development in horizontal and vertical direction of the average thickness of a layer plated onto a back major surface of the workpiece; Fig. 11is a diagram showing the development in horizontal and vertical direction of the average thickness of a layer plated onto a front major surface of a planar workpiece held by a holding device of the type shown in Figs. 2-8 when a sparging gas is introduced into the liquid comprising the plating solution; and Fig. 12is a diagram corresponding to that of Fig. 11 showing the development in horizontal and vertical direction of the average thickness of a layer plated onto a back major surface of the workpiece. Description of Embodiments

[0058] An apparatus 1 for electroless plating a surface of a workpiece 2 (Fig. 1) is configured to plate both opposite major surfaces of a planar workpiece 2 in the illustrated example. In a variant, the apparatus may be configured to plate only one major surface.

[0059] Copper plating, preferably electroless copper plating, will be used as an example here, in the understanding that different chemistry can be used to plate, preferably by electroless plating methods, the surfaces with a different metal, e.g. nickel, tin or gold.

[0060] The deposited layer or layers will have a thickness of at least 20 nm, e.g. at least 50 nm. A maximum thickness may be 50 µm or less, e.g. 500 nm.

[0061] The apparatus 1 will generally be comprised in a larger system for wet-chemical treatment of the workpiece 2, comprising a series of processing stations. These may include processing stations for rinsing, de-smearing and pre-treatment. Pre-treatment will in particular include activating the surface to be plated. Activating can comprise metalising the surface by means of a cationic palladium treatment followed by reduction of the palladium ions to metal, a colloidal palladium-tin activator, a colloidal copper activator, a colloidal silver activator, a silver-palladium activator or a lead-tin activator, for example. Activating renders the surface suitable for electroless plating.

[0062] The apparatus 1 comprises a processing chamber 3. The processing chamber 3 may be sealable and contain an atmosphere comprising at least one inert gas and substantially only inert gases. However, the processing chamber 3 need not necessarily be sealable and may contain just air, optionally filtered to remove particles. Here, it will be assumed that the processing chamber 3 contains air.

[0063] A bottom section of the processing chamber 3 constitutes a receptacle 4 for collecting processing liquid. In an alternative embodiment, a separate receptacle (not shown) is arranged in a lower section of the processing chamber 3.

[0064] A holding device (not shown in Fig. 1) is arranged to hold the workpiece 2 with the major surfaces in a predominantly upright orientation, meaning that normals to these surfaces extend in a predominantly horizontal direction. If only one surface is to be plated, the workpiece 2 may be held at an angle to the vertical, but still in a predominantly upright orientation and thus at an angle smaller than 45°, e.g. parallel to the vertical to within 5°.

[0065] In the illustrated example, the workpiece 2 is a planar workpiece 2. This includes workpieces that are flexible, e.g. in the form of a foil. The workpiece 2 is then at least planar when mounted to the holding device. Suitable workpieces include substrates such as printed circuit boards, semiconductor dies, integrated circuit substrates and the like. In the examples to be discussed below, the workpiece 2 is quadrilateral, e.g. square, seen looking onto the major surfaces of the planar workpiece 2. Other polygonal or even round shapes are also possible. A shape defining a straight edge at which the workpiece 2 can be held, in particular clamped, is most suitable.

[0066] A liquid (aqueous liquid) application system 5 is arranged to apply processing liquid in the form of an active electrolyte 6 to the major surfaces of the planar workpiece 2.

[0067] The holding device and the liquid application system 5 may comprise a holding device and a pair of flooding devices of the types disclosed in detail in WO 2024 / 046870 A1 and WO 2024 / 046871 A1, for example, the contents of which are hereby incorporated by reference. Two versions of an alternative holding device 7 comprising an integrated flooding device (Figs. 2-8) will be described in more detail below. Both versions of the alternative holding device 7 are optimised to create a laminar film flow across the surfaces of the workpiece 2.

[0068] In use, the active electrolyte 6 will flow across the surface of the workpiece 2 as a film flowing in downwards direction. The flow will be laminar at the top of the surface and, assuming the thickness of the liquid film is thin and flow liquid film is undisturbed and remains at controlled velocity, and the workpiece 2 does not have dimensions in the direction of flow excessively large, the flow will remain laminar. Otherwise the flow will transition to turbulent. Oxygen diffuses into the active electrolyte at a higher rate when the flow is turbulent than when the flow is laminar. Thus, at least detached electrolyte 8 will have a higher concentration of dissolved oxygen than the active electrolyte 6 near a top edge of the workpiece 2. Collected electrolyte 9 will have a higher concentration still.

[0069] In the illustrated embodiment, an optional first oxygen sensor 10 is arranged to measure the oxygen concentration for process control purposes. The collected electrolyte 9 leaves the processing chamber 3 through a conduit in which an (optional) valve 11 is arranged. The collected electrolyte 9 then enters a container 12 for storing deactivated electrolyte 13. In this example, the container 12 is a sump, so that no pump is required.

[0070] In the illustrated embodiment, an optional second oxygen sensor 14 is arranged to measure the oxygen concentration in the deactivated electrolyte 13 for process control purposes.

[0071] In the illustrated embodiment, the collected electrolyte 9 is turned into deactivated electrolyte 13 by aerating the liquid in the container 12. To this end, an air supply 15 and diffuser 16 are arranged to introduce bubbles 17 into the liquid contained in the container 12. In an alternative embodiment, pure oxygen may be used instead of air.

[0072] This process increases the stability of the aqueous electrolyte. The aqueous electrolyte (plating bath) is preferably an autocatalytic type solution comprising preferably copper ions and a reducing agent capable of reducing copper ions to metallic copper so that a copper layer is deposited on the surface of the workpiece 2. The aqueous electrolyte will generally comprise the following components: at least one metal salt, e.g. selected from the group consisting of copper sulphate, copper chloride, copper nitrate, copper acetate, copper methane sulfonate, copper hydroxide, copper formate and hydrates thereof. The concentration of Cu(ll) ions in the electroless plating bath preferably ranges from 0.05 to 10 g / l, e.g. 1-5 g / l, corresponding to 0.016-0.079 mol / l, preferably 2.0 3.0 g / l; at least one reducing agent capable to reduce metal ions selected from the group consisting of formaldehyde, hypophosphite, dimethylamine borane, glyoxylic acid and 2,2'-dialkoxyacetaldehydes. The formaldehyde is preferred and own experiments showed best results with copper ions. The concentration of the reducing agent preferably ranges from 0.1 to 50 g / l, e.g. 0.027-0.270 mol / l, preferably formaldehyde: 0.8 to 8.5 g / l; at least one complexing agent for metal ions e.g. selected from the group consisting of carboxylic acids, hydroxycarboxylic acids, aminocarboxylic acids, alkanolamines, polyols and mixtures thereof, such as EDTA, triethanolamine (TEA), quadrol, tartrate, citrate. The concentration of the complexing agent preferably ranges from 5-50 g / l, more preferably 20 - 40 g / l. Preferably the molar ratio of the complexing agents, related to the total molar amount of all complexing agents, to copper ions is in the range of 1:1 to 10:1, preferably 1:1 to 8:1, more preferably 2:1 to 5:1; optionally a compound for adjusting pH, e.g. sodium hydroxide, sulphuric acid, wherein a pH value is adjusted from 10 to 14; optionally a buffer; optionally a surfactant; and optionally at least one stabilising additive and / or at least one sulphur-containing additive.

[0073] In same case, it might be useful to also add a source of nickel ions, as nickel sulphate, in order to prevent blistering above 200 nm Cu layer thickness. The concentration of the nickel ions preferably ranges from 50 to 300 mg / l.

[0074] Add Ni as an option to prevent blistering above 200 nm Cu thicknesses

[0075] The aqueous electrolyte, or at least the active electrolyte 6, is preferably held at a temperature in the range of 15 to 60°C. The workpiece 2 is contacted with the processing liquid for electroless plating for 5 s to 30 min, depending on the desired thickness of the copper layer to be plated and on the used plating rate.

[0076] In principle, electroless copper plating solutions are known to the skilled person. E.g. Printoganth ®< MV Plus Electroless Copper can be used as electroless copper plating bath.

[0077] The reducing agent will reduce some bivalent copper to monovalent copper. The monovalent copper is not readily reduced further in the presence of bivalent copper. It will instead exist as a precipitate or disproportionate to produce randomly dispersed copper particles in the electrolyte. This is one cause of instability. The aeration carried out in the container 12 re-oxidises the monovalent copper to bivalent copper and thus improves the stability of the electrolyte. The non-immersive treatment in the processing chamber 3 allows for less intensive stability-enhancing treatment in the container 12. Furthermore, the formation of precipitates or disperse particles in the processing chamber 3 is avoided to at least a certain extent as oxygen diffuses into the detached electrolyte 8 and the collected electrolyte 9. Thus, there is a further effect if the processing chamber 3 is in communication with ambient atmosphere.

[0078] The deactivated electrolyte 13 is removed from the container 12 through a pipe inlet 18 and the action of a pump 19.

[0079] In the illustrated embodiment, an (optional) deaerator 20 is comprised in the recirculating system. The deaerator 20 may comprise an ultrasonic or megasonic device to generate sound waves, for example. It is also possible to use a thermal device or a device arranged to generate at least a partial vacuum. The deaerator 20, if present, reduces the oxygen content of the deactivated electrolyte 13 after the deactivated electrolyte 13 has left the container 12.

[0080] Sparging gas bubbles 21 are introduced into the electrolyte through a sparging gas inlet 22 positioned downstream of the deaerator 20.

[0081] The sparging gas is supplied from a sparging gas source 23. A flow meter may be provided at the sparging gas inlet 22 to control the rate at which the sparging gas is introduced. Used flow rates are from 5 to 20 l / min. The sparging gas bubbles comprise at least one sparging gas, meaning in the present context a non-oxidising gas. Suitable are nitrogen, argon, forming gas (a mixture of hydrogen and nitrogen), hydrogen, helium, methane and neon, for example. Thus, the sparging gas may be, but need not be, an inert gas.

[0082] The effect of the raised nitrogen concentration is that, when the active electrolyte 6 is applied to the surface of the workpiece, the exchange of gas into the solution is limited. In the electrolyte, the reaction by which oxygen is reduced, the Oxygen Reduction Reaction (ORR), competes with the reactions by which copper ions are reduced, the plating reactions. A lower oxygen concentration favours the latter and thus speeds up the deposition rate. In addition, if the ORR occurs at a relatively high rate, this gives rise to a phenomenon known as skip plating, whereby parts of the surface of the workpiece 2 are not plated at all or coated with a much thinner layer of metal. The sparging gas inhibits exchange of gas, specifically the entry of oxygen into the active electrolyte 6.

[0083] Introduction of sparging gas bubbles 21 is followed by filtration in a filter 24 to remove particles. The filter 24 may be a cartridge filter, for example. A further effect is that the sparging gas bubbles 21 are reduced in size to microbubbles 25.

[0084] In the illustrated embodiment, the thus activated active electrolyte 6 passes through an optional flow meter 26 and past an optional third oxygen sensor 27.

[0085] As explained, it is desirable to prevent the solution of oxygen into the active electrolyte 6 and subsequent diffusion towards the surface of the workpiece 2 when the active electrolyte 6 flows across the surface of the workpiece 2 in the processing chamber 3. The rate of diffusion is lower when the flow is laminar than when the flow is turbulent. A lower velocity favours laminar flow (the Reynolds number is lower). A thin film of liquid flowing downwards has a lower velocity (the boundary layer is relatively large compared to the total film thickness).

[0086] The holding device 7 (Figs. 2-7) is designed to increase the likelihood of laminar flow. This holding device 7 and variants providing the same effect are disclosed in more detail in European patent application No. 24188389.1, filed on 12 July 2024.

[0087] The holding device 7 comprises a support structure for engaging an external support to position the holding device 7 in the apparatus 1. In this example, the support structure comprises a frame arranged to frame at least a central section of a major surface of the workpiece 2, seen looking on to the major surface. In the illustrated holding device 7, the frame frames the complete major surface of the workpiece 2.

[0088] The frame comprises at least lateral members 28a,b, lower members 29a,b and an upper member 30. The upper member 30 comprises laterally projecting arms 31a,b for engaging supports (not shown), so as to suspend the holding device 7 in the processing chamber 3 of the apparatus 1. A claw 32 for engaging a horizontally extending pin of the support is defined at a distal end of at least one of the arms 31a,b. Examples of a suitable support are disclosed in WO 2020 / 260389 A1.

[0089] The configuration of the support structure is such as to allow the holding device 7 to be conveyed from station to station in the treatment system comprising the apparatus 1. To this end, the external support may either be comprised in a transporter of a conveying system (not shown) or the conveying station may be configured to transfer the holding device 7 between supports provided in respective treatment stations. At least one of the lower members 29a-d may be provided with one of at least one skid and at least one caster (not shown) for movably supporting the holding device 7 on a support surface.

[0090] It is convenient to define a co-ordinate system (Fig. 2) with reference to the planar workpiece 2 when mounted to the holding device 7 with an orientation permitting the non-immersive wet-chemical treatment to be carried out. In that situation, the major surfaces of the planar workpiece 2 are in a plane parallel to a first direction -z and a second direction y. They face in directions aligned with a third direction x. In the orientation permitting non-immersive wet-chemical treatment to be carried out, the first direction -z is essentially parallel to the vertical, i.e. the planar workpiece 2 has an upright orientation, so that liquid can flow down the major surfaces of the workpiece 2 as a gravity-driven film flow.

[0091] The holding device 7 comprises an upper clamping device (described in more detail below) and a lower clamping device 33 (Fig. 4).

[0092] The lower clamping device 33 comprises a pair of support bars 34a,b on which protruding lower engagement features 35a-f are mounted. The lower engagement features 35a-f provided on one of the support bars 34a,b protrude towards the other one of the support bars 34a,b and vice versa. The lower engagement features 35a-f are configured to engage the opposite major surfaces of the planar workpiece 2 in an edge region at an (in use lower) edge of the planar workpiece 2. To this end, the support bars 34a,b are mounted on pivotable lower support levers 36a-d. Biasing devices comprising resilient elements, in this case lower coil springs 37a-d are provided to urge the support bars 34a,b towards each other in the third direction x. Cams (not shown in detail) or other features may be defined on the lower support levers 36a-d to apply a force moving the support bars 34 apart in the third direction x to release the workpiece 2. Other types of biasing device may be used, e.g. other types of elastic element, gas springs or magnets.

[0093] The upper clamping device for clamping the workpiece 2 in an edge region close to an, in use upper, edge of the workpiece 2 comprise a first array 38 of engagement features and a second array 39 of engagement features (Fig. 6). The first array 38 and second array 39 extend in the second direction y. The engagement features of each array 38,39 are discrete features spaced apart in the second direction y. They protrude in directions corresponding to the third direction x, so that liquid can pass in the first direction -z between adjacent engagement features.

[0094] In the illustrated holding device 7, the engagement features of the first and second arrays 38,39 are in each case arranged in a single straight line. Alternatively, one or both of the arrays 38,39 may comprise multiple rows of engagement features, each row extending in the second direction y, with the rows spaced apart in the first direction z. The arrays, in the case of the alternative example the rows, may extend in non-straight, e.g. curved lines, e.g. if the edge region adjoins a curved workpiece edge.

[0095] In the illustrated holding device 7, the engagement features of the first array 38 are formed by first pins 40 (Fig. 7) mounted to a first body 41. The engagement features of the second array 39 are formed by second pins 42 mounted to a second body 43. In the illustrated holding device 7, the first and second pins 40,42 are provided with threads, mounted in respective bores or other through-going passages in the first and second body 41,43 and secured by first and second nuts. Other means of securing the first and second pins 40,42 may be used instead.

[0096] The first and second bodies 41,43 are mounted to pivotable upper support levers 44a-d, which are pivotably mounted to the support structure of the holding device 7. In this example, they are pivotably mounted to the upper member 30 of the frame. The pivot axes extend in the second direction y. Biasing devices comprising resilient elements, in this example upper coil springs 45a,b, are provided to urge the first and second arrays 38,39 towards each other in a direction corresponding to the third direction x. These resilient elements thus provide the clamping force. In a variant, other types of biasing device may be used, e.g. comprising at least one gas spring or at least one magnet.

[0097] In a variant, only one of the first and second bodies 41,43 is mounted to pivotable upper support levers 44 and the other is fixed to the support structure of the holding device 7. In another variant, the first and second bodies 41,43 or alternative supports carrying the first and second arrays 38,39, are themselves pivotably mounted to the support structure. In that case, they are thus only partially movable with respect each other, because their respective pivot axes are fixed in position.

[0098] The first body 41 presents an inward-facing surface comprising a first flow guide surface 46. The second body 43 presents an inward-facing surface comprising a second flow guide surface 47. These surfaces are non-planar, but they are oriented parallel to the second direction y everywhere and parallel to or at an angle smaller than 90° to the first direction -z. Thus, the first flow guide surface 46 and the second flow guide surface 47 each progress in the first direction -z up to a first edge 48 and a second edge 49, respectively. There is a gap 50 between the first edge 48 and the second edge 49 through which an edge region of the planar workpiece 2 can pass into a space 51 for accommodating the edge region. This space 51 is defined between at least sections of the inward-facing surfaces of the first and second bodies 41,43, such that the first flow guide surface 46 and the second flow guide surface 47 are located on opposite sides of the space 51.

[0099] A series of baffle plates 52 partially sub-divides the space between the inward-facing surfaces of the bodies 41,43.

[0100] Going along the first flow guide surface 46 and the second flow guide surface 47 in the first direction -z, the dimension in the third direction x of the space 51 decreases at least over an interval in the first direction -z. A gravitational flow of liquid flowing the first direction -z thus faces a constriction there. The liquid will back up.

[0101] The engagement features of the first and second arrays 38,39 are located in the space 51 at a distance in opposite direction z to the first direction -z from the edges 48,49. In particular, the engagement features of the first and second arrays 38,39 are located in the space 51 at a distance in opposite direction z to the first direction -z from where the dimension of the space 51 in third direction x has decreased to an at least local minimum. Generally, this minimum will correspond to a dimension in the third direction xof the gap 50 between the edges 48,49, as in the illustrated example.

[0102] In the illustrated holding device 7, the space 51 is accessible to liquid at a distance from the edges 48,49 in opposite direction z to the first direction -z at least equal to the distance in opposite direction z to the first direction -z between the first and second arrays 38,39 and the edges 48,49. In an alternative holding device, liquid may enter the space 51 at a location closer to the edges 48,49, but at a distance in opposite direction zto the first direction -z from where the dimension of the space 51 in third direction x has decreased to an at least local minimum, since a gravitational flow of liquid will in any case then back up.

[0103] In the illustrated holding device 7, a gravity-driven flow of liquid can simply flow down between the first and second bodies 41,43.

[0104] Although not required, the space 51 and gap 50 are also bounded at at least one of two opposite ends in the second direction y. In the illustrated holding device 7, end plates 53a-d are provided to this end. This helps keep the first and second arrays 38,39 submerged, in use. As a consequence, the direction of flow is uniformly in the first direction -z. Furthermore, the speed of flow is lower, because liquid can only flow out through the gap 50. Due to these effects, there are fewer or no eddies in the region of the first and second arrays 38,39. In other word, the space 51 is closed laterally to form a tub, achieving uniformity of flow in the second direction y. The fill level in the tub is relatively uniform, which promotes the formation of a laminar flow out of the gap 50.

[0105] The edges 48,49 are relatively sharply defined, in order to ensure that the liquid flows out of the gap 50 and onto the major surfaces of the workpiece 2 with relatively few eddies. The first flow guide surface 46 meets a first exterior surface 54 at an angle at the first edge 48. The second flow guide surface 47 meets a second exterior surface 55 at an angle at the second edge 49. The angle may be equal to or smaller than 90°, for example.

[0106] The holding device 7 is configured to be conveyed through the apparatus for wet-chemical treatment of the workpiece 2. To facilitate this, systems for supplying processing liquid are stationary in the apparatus and configured to direct one or more jets (coherent streams of liquid projected into a surrounding medium, usually from some kind of a nozzle or aperture) onto or into the holding device 7. In the illustrated example, sheet-shaped jets of liquid can be projected from either side onto outward-facing surface sections of a flow guidance part, in this case in the form of a flow guidance plate 56. The outward-facing surface sections extend in the first direction -z to respective, in use lower, edges proximal to the space 51. First and second dam wall sections 57,58 are defined by the first and second bodies 41,43 and bound a collecting space sub-divided by the baffle plates 52 and in communication with the space 51.

[0107] To supply the sheet-shaped jets of processing liquid, the stations of the apparatus for non-immersive wet-chemical treatment of the planar workpiece 2 may comprise flooding devices of the type disclosed in WO 2024 / 046870 A1. These are configured to direct a downwardly-directed stream of liquid onto the outward-facing surface sections of the flow guidance plate 56. The dimension of the flow guidance plate 56 in the first direction -z is sufficient to establish a laminar flow at the lower edges of the flow guidance plate 56. Due to the fact that the liquid backs up from the first and second edges 48,49, this is not strictly required. However, it helps to limit splashing so that more or less all of the processing liquid also flows across the major surfaces of the workpiece 2.

[0108] In an alternative embodiment, the collecting space is filled directly from a nozzle that is movable into position close to the first and second bodies 41,43. Alternatively, it is also possible to attach a flexible conduit to a port formed in one of the bodies 41,43 or end plates 53a-d.

[0109] In a modified version of the holding device 7 (Fig. 8), the baffle plates 52 are replaced by a flow distributor 59. The flow distributor 59 separates a collecting space 60 bounded by the first and second dam wall sections 57',58' defined by the first and second bodies 41',43' from the space 51' for accommodating the edge region of the workpiece 2. The flow distributor 59 is provided with apertures 61 forming channels extending in the first direction -z. These channels open into the space 51' for accommodating the edge region of the workpiece 2. In the illustrated example, the flow distributor apertures 61 are slit-shaped, having a longitudinal axis extending in the second direction y. Other shapes are possible.

[0110] In all variants, a laminar flow emerges through a gap between the first edge 48;48' and one major surface of the workpiece 2 and through a gap between the second edge 49; 49' and the other major surface of the workpiece 2.

[0111] The effect on plating uniformity of the introduction of sparging gas and of the enhanced laminar flow due to the immersion of the upper edge region of the planar workpiece 2 in the activated electrolyte in the holding device 7 separately and in combination have been investigated.

[0112] Four panels were plated in the respective constellations indicated in Table 1. Table 1 Panel #SpargingHolding device optimised for laminar flow1NoNo2NoYes3YesNo4YesYes

[0113] The holding device optimised for laminar flow was of the configuration described above. In the alternative, a conventional holding device was used. Each was used once in combination with the introduction of sparging gas and once without the introduction of sparging gas. Pure nitrogen was used as the sparging gas.

[0114] The electrolyte composition was Ni 2+< ions at a concentration in the range of 130-160 mg / l; Cu 2+< at a concentration of 4.3 g / l; Tartrate at a concentration of 0.16 mol / l; NaOH at a concentration of 3 g / l; stabilising additives at a concentration in the range of 5-20 ppm and sulphur-containing additives at a ppb level; and surfactants at a concentration of 200 ppm (all values are approximate values). The temperature was 34° C and the deposition time 6 min. The inlet flow rate of nitrogen gas was set at 10 L / min and the measured dissolved oxygen concentration within the active electrolyte prior delivery to the workpiece was < 1 ppm during plating of both Panel 3 and Panel 4.

[0115] Results are shown in Table 2 and graphically in Figs. 9-12 for panels No. 1 (Figs. 9), No. 2 (Figs.10), No. 3 (Figs. 11), and No. 4 (Figs. 12). Table 2 Average Thickness [nm]Standard Deviation [nm]Relative standard deviation [%]Panel #FrontBackBoth sidesFrontBackBoth sidesFrontBackBoth sides139838138939.135.2379.89.309.51236336336325.921.321.45.875.905.89334134234110.810.110.53.112.913.143863903886.36.36.61.641.631.71

[0116] In Table 2, the average thickness across the surface of the panel concerned, as measured by measuring the conductivity, is listed for each of the panel surfaces (two sides). In addition, the average over both surfaces is indicated. The standard deviation indicates the uniformity of the thickness across the surface. In the drawings, the central curve in each two-dimensional diagram represents the average thickness, and the two other curves represent the sum of the average and the standard deviation and the difference between the two, respectively.

[0117] It can be seen from Table 2 that the holding device 7 and the introduction of sparging gas each produce an effect, and that the combined effect on the plating uniformity is even greater. In addition, the curves in the two-dimensional diagrams for panel No. 4 are flatter and the standard deviation is relatively small at each abscissa value and each ordinate value, compared with the curves for panel No. 2. This shows that increased uniformity is also achieved in edge regions by introducing sparging gas.

[0118] The invention is not limited to the embodiments described above, which can be varied within the scope of the accompanying claims. For example, although a planar workpiece 2 has been used here as an example, non-immersive plating of a surface of a differently-shaped workpiece, possibly with a flat surface to be plated, is equally possible. Alternative suitable plating liquid compositions to the one used here to demonstrate the effect of the sparging gas and holding device can be found, for example, in Schlesinger, M. and Paunovic, M. (eds.), "Modern Electroplating", 5th Ed. Hoboken, Wiley, 2010, p. 444. One or more of the oxygen sensors 10, 14, 27, when present, may be in-line sensors. Such sensors can be used to control the rate at which the sparging gas is introduced and / or the rate at which air or oxygen is introduced in the container 12.List of reference numerals

[0119] 1- Apparatus 2- Workpiece 3- Processing chamber 4- Receptacle 5- Liquid application system 6- Active electrolyte 7- Holding device 8- Detached electrolyte 9- Collected electrolyte 10- 1 st< oxygen sensor 11- Valve 12- Container 13- Deactivated electrolyte 14- 2 nd< oxygen sensor 15- Air supply 16- Diffuser 17- Air bubbles 18- Pipe inlet 19- Pump 20- Deaerator 21- Sparging gas bubbles 22- Sparging gas inlet 23- Sparging gas source 24- Filter 25- Microbubbles 26- Flow meter 27- 3 rd< oxygen sensor 28a,b- Lateral members 29a,b- Lower members 30- Upper member 31a,b- Projecting arms 32- Claw 33- Lower clamping device 34a,b- Support bar 35a-f- Lower engagement features 36a-d- Lower support lever 37a-d- Lower coil spring 38- 1 st< array of engagement features 39- 2 nd< array 40- 1 st< pin 41;41'- 1 st< body 42- 2 nd< pin 43;43'- 2 nd< body 44a-d- Upper support lever 45a,b- Upper coil springs 46- 1 st< flow guide surface 47- 2 nd< flow guide surface 48;48'- 1 st< edge 49;49'- 2 nd< edge 50;50'- Gap 51;51'- Space 52- Baffle plate 53a-d;53c,d'- End plates 54- 1 st< exterior surface 55- 2 nd< exterior surface 56- Flow guidance plate 57;57'- 1 st< dam wall section 58;58'- 2 nd< dam wall section 59- Flow distributor 60- Collecting space 61- Flow distributor aperture

Examples

Embodiment Construction

[0058]An apparatus 1 for electroless plating a surface of a workpiece 2 (Fig. 1) is configured to plate both opposite major surfaces of a planar workpiece 2 in the illustrated example. In a variant, the apparatus may be configured to plate only one major surface.

[0059]Copper plating, preferably electroless copper plating, will be used as an example here, in the understanding that different chemistry can be used to plate, preferably by electroless plating methods, the surfaces with a different metal, e.g. nickel, tin or gold.

[0060]The deposited layer or layers will have a thickness of at least 20 nm, e.g. at least 50 nm. A maximum thickness may be 50 µm or less, e.g. 500 nm.

[0061]The apparatus 1 will generally be comprised in a larger system for wet-chemical treatment of the workpiece 2, comprising a series of processing stations. These may include processing stations for rinsing, de-smearing and pre-treatment. Pre-treatment will in particular include activating the surface to be pla...

Technical Field

[0001] The invention relates to an apparatus for non-immersive wet-chemical treatment of a surface of a workpiece, comprising: a holding device for holding the workpiece with the surface in a predominantly upright orientation; a liquid application system for flowing liquid across the surface of the workpiece, when held by the holding device, as a film flowing in a predominantly downwards direction; a receptacle for collecting liquid that has flowed across and detached itself from the surface; and a system for recirculating liquid collected in the receptacle to the liquid application system.

[0002] The invention also relates to a method of plating a surface of a workpiece, comprising: holding the workpiece with the surface in a predominantly upright orientation; applying liquid comprising a plating solution to the workpiece by flowing the liquid across the surface of the workpiece as a film flowing in a predominantly downwards direction; collecting liquid that has flowed across and detached itself from the surface; and recirculating the collected liquid for renewed application to a next workpiece. Background Art

[0003] WO 97 / 22733 A1 discloses an apparatus for deposition of a metal film onto a substrate. The plating solution is sprayed onto an activated substrate, which will initiate the autocatalytic deposition of the plating solution metal. The activated substrate may be any conducting material which will initiate the autocatalytic deposition of the deposition metal from the electroless plating solution. The plating solution may be sprayed in a manner which forms very fine droplets and may be carried in an inert gas. In the disclosed apparatus, a first reservoir contains a metal stock solution. The metal stock solution is connected via a line to a manifold. A metering valve allows precise control of the flow of metal stock solution to the manifold. A second reservoir contains a reducing solution and is connected via a line and metering valve to the manifold. The manifold serves as a mixing chamber in which the electroless plating solution is prepared. From the manifold, the prepared electroless plating solution is carried via a supply line to a process chamber into which the article to be plated is placed. A nitrogen source is connected via a line and valve to the process chamber. It is preferred that pressurised inert gas be used to force flow when a valve is opened. Pressurised connections between the nitrogen source and the reservoirs may be used for this purpose. The process chamber is sealed from the ambient environment, and it contains a turntable and a central spray post containing a plurality of vertically disposed spray orifices. Wafer cassettes are loaded onto the turntable and rotated around the spray post. It is disclosed that it is also possible to spray the plating solution using nozzles which form generally continuous blade or cone streams, rather than atomised droplets, but not how this would be done. A problem of the apparatus is that the sealed chamber is a requirement. This slows down the processing of the wafers and leads to a large use of inert gas or the need to provide some sort of gas lock to remove the wafers. Processing these in batches leads to non-uniform plating, as does the use of a spray. Unless the sealed inert gas atmosphere is maintained very effectively, it will be difficult to prevent oxygen from entering into the plating solution, in particular if the latter is recycled. This leads to instability of the plating solution.

[0004] EP 0 236 718 B1 discloses an apparatus comprising a conventional plating tank and associated external piping. The plating tank has an input port for supplying plating solution, an output port for discharging plating solution and a chemical feed controller. A pump pumps the plating solution from the output port through a heat exchanger, a filter and a static mixer. A chemical feed controller senses the concentration of various chemicals in the tank and introduces plating solution to maintain each of the chemicals at the desired level. One level of oxygen is maintained in the plating tank and a significantly higher level of oxygen is maintained in the external piping to reduce or eliminate plating in the external piping of the electroless plating system. The desired operating point for the oxygen in the tank is maintained by balancing an oxygen feed controller and a nitrogen feed controller that introduces nitrogen into the plating solution as the plating solution passes through the heat exchanger. The plating tank is filled with plating solution. The tank must be kept closed to keep the oxygen level at the intended value, since most of the solution remains in the tank. This complicates handling of the workpieces to be plated. Furthermore, it is difficult to generate a uniform fluid flow across the workpiece and achieve uniform plating results when the workpiece is immersed in the plating solution.

[0005] US 2014 / 0116334 A1 discloses a surface treating apparatus. The surface treating apparatus includes a transport hanger for transporting a plate-like workpiece in a horizontal direction, which is clamped by clamps and held vertically. An electroless copper plating tank includes a tank body mounted on a frame and a circulation pump for circulating electroless plating processing solution accumulated on the bottom in the tank body by supplying a squirting part with the liquid. The processing solution is squirted from a squirt port of the liquid squirting part toward the plate-like workpiece obliquely upwards to a horizontal plane. The processing solution is attached to the upper side of the plate-like workpiece that is clamped by the transport hanger inside of the tank body. Accordingly, it becomes possible to attach the processing solution to the surface of the plate-like work while the processing solution is running down the plate-like workpiece. Thus, a system is employed in which circulated processing solution runs down the plate-like workpiece without dipping the plate-like workpiece into stored processing solution. Therefore, it becomes possible to reduce the total amount of the processing solution used for the surface treating apparatus in whole as compared with a dipping type. The occurrence of bubbles when striking the work can be suppressed by squirting the processing solution obliquely upwards to the horizontal plane. This makes it possible to prevent from increasing the amount of dissolved oxygen in the processing solution. The existence of dissolved oxygen is described as causing a problem because it is stated to lead to low precipitation of plating by the effect of restraining reaction between the electroless plating solution and a catalyst attached to the workpiece. A problem of this type of apparatus is that preventing bubbles is not sufficient to achieve a high level of uniformity in thickness of the layer plated onto the workpiece.Summary of Invention

[0006] It is an object of the invention to provide an apparatus and method of the types mentioned above in the opening paragraphs that are suitable for plating the surface of the workpiece with a layer having a relatively uniform thickness whilst still making efficient use of the treatment solution.

[0007] This object is achieved according to a first aspect by the apparatus according to the invention, which is characterised in that the system for recirculating liquid comprises a device for introducing sparging gas in the liquid.

[0008] The sparging gas has the effect of reducing the oxygen content in the liquid prior to application by the liquid application system and of inhibiting the entry of any oxygen in the environment of the film into the solution as the liquid film flows across the surface. The processing liquid is preferably an aqueous electrolyte, more preferred an electroless aqueous electrolyte which is used as a plating solution (also mentioned as bath). An electroless aqueous electrolyte will generally comprise metal ions to be plated onto the surface of the workpiece and a reducing agent capable of reducing the metal ions. It is believed that the reduction of any oxygen present in the liquid competes with the plating reaction. This competing reaction can give rise to a phenomenon known as skip plating. The sparging gas helps ensure that this competing reaction is limited. In turn, this makes it possible to use a thinner liquid film, which flows more slowly. This in turn allows the flow to remain laminar over a larger section of the surface, which in turn further inhibits the entry of oxygen into the liquid and limits the transport of dissolved oxygen to the surface of workpiece by slow diffusion processes only. Although the apparatus may still comprise a sealable processing chamber and even means to fill such a sealed processing chamber with an inert gas, this is not required to the same degree. Quite the opposite, the liquid may pick up oxygen as the liquid becomes detached from the workpiece surface and is collected in the receptacle, because this helps keep the plating solution stable.

[0009] Compared to systems that make use of a processing bath into which the workpiece is immersed, the apparatus does not need to compromise between bath stability and suppressing the competing redox reactions.

[0010] The apparatus may be comprised in a larger system for non-immersive treatment of the workpiece. Such a system will include multiple apparatus and a system for transporting the workpiece from station to station, including into and from the apparatus that is the focus of the present disclosure.

[0011] The apparatus may be configured for treatment of only one surface of the workpiece, in particular a planar surface. Alternatively, the apparatus may be configured for treatment of both opposing major surfaces of a planar workpiece, e.g. a planar workpiece in the form of a panel or foil.

[0012] The apparatus comprises a holding device for holding the workpiece with the surface in a predominantly upright orientation. This means that the apparatus is configured to hold and treat the workpiece with the surface of the workpiece in a vertical position in the apparatus whilst the treatment method is applied. Thus, the liquid flow across the surface is gravity-driven.

[0013] The apparatus comprises a liquid application system for flowing liquid across the surface of the workpiece, when held by the holding device, as a film flowing in a predominantly downwards direction. Such a system may comprise one or more flooding devices for generating a sheet-shaped jet that is projected onto the workpiece surface or a flow guide surface of the holding device. Alternatively, the holding device may comprise one or more flooding devices for guiding the liquid onto the surface. Here, the term jet is used in the sense of referring to a coherent stream of liquid projected into a surrounding medium in this case the atmosphere in the apparatus, usually from some kind of nozzle or aperture. The stream is unguided on entering the medium.

[0014] The apparatus comprises a receptacle for collecting liquid that has flowed across and detached itself from the surface. The receptacle may be just the bottom of a processing chamber in which the holding device is arranged to hold the workpiece. The receptacle need not be closed on all sides, but may be shaped like a gutter instead.

[0015] In an embodiment, the system for recirculating liquid further comprises a filter for filtering particles from the liquid.

[0016] This prevents such particles from flowing across the surface of the workpiece. In the case of sensitive workpieces, the risk of damage to the surface or the metal layer being applied is reduced. In any case, such particles could otherwise act as seeds that lead to non-uniform treatment or even disrupt the film flow or prevent the film flow from being established.

[0017] In a particular example of this embodiment, the device for introducing sparging gas into the liquid is arranged upstream of the filter.

[0018] A filter with a pore size sufficiently small to filter out the types of particle that might arise in a plating solution is also suitable for reducing sparging gas bubbles to micro- or nano-bubbles. This enhances the effect of the sparging gas. A larger amount and with this a larger surface area of the sparging gas can cover the liquid and dissolve into the liquid and remove oxygen in a shorter period of time.

[0019] In an embodiment of the apparatus, the system for recirculating liquid further comprises a container, e.g. a sump, for storing the liquid, and the container is located in flow direction between the receptacle and the device for introducing sparging gas into the liquid.

[0020] Thus, the oxygen content is only reduced after intermediate storage. Intermediate storage is of a liquid with a higher oxygen concentration. In the case of a plating solution, in particular a copper plating solution, this means that the liquid has a relatively high stability whilst in intermediate storage. The higher oxygen concentration enhances stability by inhibiting or even reversing the formation of monovalent metal ions. Such monovalent ions are generally not reduced further to the metal to be plated onto the surface of the workpiece. Rather, they give rise to undesired precipitates. The higher oxygen concentration counters this.

[0021] In a particular example of this embodiment, the system for recirculating liquid further includes at least one diffuser, located in the container, for introducing at least one gas, e.g. at least oxygen, into liquid in the container.

[0022] The diffuser may be comprised in an aerator, or only oxygen may be diffused into liquid in the container. In either case, stability of the liquid held in intermediate storage is enhanced further.

[0023] In an embodiment of the apparatus, the liquid application system is arranged to supply the liquid to the holding device, and the holding device comprises at least one flow guide surface for directing liquid towards, e.g. onto, the surface of the workpiece.

[0024] This facilitates the generation and maintenance of a laminar film flow across a relatively large section of the surface of the workpiece. By contrast, a curtain coater directing a sheet-shaped jet directly onto the surface will generally do so at a distance from an (upper) edge of the workpiece and give rise to splashes and turbulence.

[0025] In an embodiment of the apparatus the holding device is configured to hold a planar workpiece.

[0026] Such a workpiece can be easily clamped in an edge region, e.g. an upper edge region. Furthermore, this embodiment allows to treat two surfaces simultaneously by means of a liquid application system configured to apply liquid to both major surfaces of the planar workpiece.

[0027] In an example of any embodiment in which the holding device is configured to hold a planar workpiece, the holding device comprises a clamping device for engaging major surfaces of the planar workpiece in an edge region along an edge of the planar workpiece, e.g. an edge region along an in use upper edge of the planar workpiece.

[0028] This allows the workpiece to be held with most of the surface of the workpiece exposed and uninterrupted. This also functions to control the thickness and velocity of the liquid film flowing across the workpiece. This helps reduce the risk of turbulence. That in turn ensures the plated metal layer is relatively uniform in thickness.

[0029] In an example of any embodiment of the apparatus in which the liquid application system is arranged to supply the liquid to the holding device, the holding device comprises at least one flow guide surface for directing liquid towards, e.g. onto, the surface of the workpiece, the holding device is configured to hold a planar workpiece and the holding device comprises a clamping device for engaging major surfaces of the planar workpiece in an edge region along an edge of the planar workpiece, e.g. an edge region along an in use upper edge of the planar workpiece, the holding device comprises a pair of flow guide surfaces on opposite sides of a space for accommodating the edge region, each extending towards a respective edge bounding a gap for insertion of the edge region into the space, and at least respective opposing sections of the flow guide surfaces converge towards each other in a direction of the edges.

[0030] This helps promote the establishment of a laminar film flow from close to the inserted edge of the workpiece downwards. Therefore each film emerges from a gap between the inserted section of the workpiece and one of the two flow guide surfaces. The opposing converging sections help ensure that the edge region is fully immersed in the processing liquid. It is observed that the gap need not be bounded on all sides. That is to say that the gap need not be a slot. This is also the case for the space, although it can be helpful if the space is bounded also at opposite longitudinal ends of the space. Here, the longitudinal direction corresponds to the direction in which the edges of the flow guide surfaces extend. The edges towards which the flow guide surfaces extend may be straight edges for enhanced uniformity of flow.

[0031] In a particular example of this embodiment, the holding device comprises at least one further flow guide arranged to introduce liquid supplied to the holding device into the space at a location upstream of the gap.

[0032] This helps avoid eddies at the edges of the flow guide surfaces that bound the gap, because the liquid is supplied to the space between the flow guide surfaces at a location that is in between the flow guide surfaces. The further flow guide may comprise a conduit or a flow guide surface onto which a stream of liquid is directed, in use, or both.

[0033] In an example of any embodiment in which the holding device is configured to hold a planar workpiece and the holding device comprises a clamping device for engaging major surfaces of the planar workpiece in an edge region along an edge of the planar workpiece, e.g. an edge region along an in use upper edge of the planar workpiece, the clamping device comprises at least one first protruding engagement feature protruding from a first engagement feature support and at least one second protruding engagement feature protruding from a second engagement feature support, wherein the first engagement feature support and the second engagement feature support are at least partially movable with respect to each other to adjust a distance between the first and second protruding engagement features in a direction in which they protrude.

[0034] Thus, only the surface of the protruding engagement features at the free ends of the protruding engagement features engage the surface of the workpiece.

[0035] The contact area is thereby kept small. Processing liquid can flow around the protruding engagement features.

[0036] In an example of this embodiment, the at least one first protruding engagement features comprise an array of first protruding engagement features spaced apart in a direction transverse to the direction in which they protrude, and the at least one second protruding engagement features comprise an array of second protruding engagement features spaced apart in a direction transverse to the direction in which they protrude.

[0037] This minimises disruption of the flow field by the protruding engagement features. Processing liquid can flow around and between the protruding engagement features of each array. Each individual engagement feature can be relatively slender, so that the plated layer is only slightly thinner immediately downstream of the engagement feature. There are also fewer eddies, minimising the risk of turbulent flow across the remainder of the workpiece surface.

[0038] In an example of any embodiment in which the liquid application system is arranged to supply the liquid to the holding device, the holding device comprises at least one flow guide surface for directing liquid towards, e.g. onto, the surface of the workpiece, the holding device is configured to hold a planar workpiece, the holding device comprises a clamping device for engaging major surfaces of the planar workpiece in an edge region along an edge of the planar workpiece, e.g. an edge region along an in use upper edge of the planar workpiece, the holding device comprises a pair of flow guide surfaces on opposite sides of a space for accommodating the edge region, each extending towards a respective edge bounding a gap for insertion of the edge region into the space at least respective opposing sections of the flow guide surfaces converge towards each other in a direction of the edges, the clamping device comprises at least one first protruding engagement feature protruding from a first engagement feature support and at least one second protruding engagement feature protruding from a second engagement feature support, wherein the first engagement feature support and the second engagement feature support are at least partially movable with respect to each other to adjust a distance between the first and second protruding engagement features in a direction in which they protrude, the first and second protruding engagement features are located in the space.

[0039] Because the liquid backs up in the space due to the converging surface sections, there is in effect a reservoir of liquid in which both the protruding engagement features and the edge region of the workpiece are immersed. Compared with the alternative of arranging the protruding engagement features further downstream, the flow past the protruding engagement features is less likely to swirl. Uniformity of treatment of the surface in the edge region is improved further.

[0040] In an example of any embodiment in which the holding device is configured to hold a planar workpiece and the holding device comprises a clamping device for engaging major surfaces of the planar workpiece in an edge region along an edge of the planar workpiece, e.g. an edge region along an in use upper edge of the planar workpiece, the holding device comprises a frame supporting the clamping device for engaging opposite major surfaces of the planar workpiece in the edge region along the edge of the planar workpiece, and the holding device comprises at least one further clamping device, supported by the frame, for engaging the major surfaces of the planar workpiece in a further edge region along a further edge of the planar workpiece opposite the edge, e.g. an edge region along an in use lower edge of the planar workpiece.

[0041] The workpiece can be relatively thin, e.g. in the shape of a foil or flat panel, in this embodiment. The further clamping device helps avoid that the workpiece starts to flutter as the liquid film flows across the surface. Thus, a relatively uniform, e.g. laminar, flow is maintained across a relatively large section of the surface.

[0042] According to another aspect, the method of plating a surface of a workpiece according to the invention is characterised by introducing sparging gas in the collected liquid prior to the renewed application to the next workpiece.

[0043] Thus, even though the liquid flowing across the surface and collected after having flowed across the surface may have a somewhat elevated oxygen concentration, the liquid is rendered suitable for renewed application by the introduction of sparging gas. The sparging gas also helps avoid skip plating. This is achieved without having to provide a sealed processing chamber with a conditioned atmosphere.

[0044] The sparging gas or gases may comprise a non-oxidising gas, e.g. an inert gas.

[0045] Suitable gases include nitrogen, argon forming gas (a mixture of hydrogen and nitrogen), hydrogen, helium, methane and neon, for example. Thus, the sparging gas may be, but is not limited to be an inert gas. The sparging gas or gases may be introduced into the liquid as bubbles.

[0046] In an embodiment, recirculating the collected liquid comprises intermediate storage of the collected liquid in a container, sparging gas is introduced into liquid removed from the container after the intermediate storage, and the method includes introducing at least oxygen into the collected liquid prior to removal of liquid from the container.

[0047] The oxygen may be introduced into the collected liquid in the container or prior to entry into the container, or both. The stability of the plating solution is improved by the introduction of oxygen. The adverse effect on the uniformity of the layer plated onto the surface of the workpiece is avoided by the introduction of sparging gas into the liquid subsequent to removal from the container.

[0048] An embodiment of the method further comprises activating the surface prior to applying the liquid comprising the plating solution.

[0049] This entails producing a catalytic metallic surface by techniques known to a skilled person in order to prepare the surface for subsequent metal plating as electroless metal plating. The activation process may be wet-chemical or comprise vapour deposition (physical or chemical). Chemical activation generates catalytic metallic nuclei of metal, e.g. palladium, palladium / tin or copper, on the surface. Vapour deposition may be used to deposit a relatively highly conductive layer onto the surface of the workpiece.

[0050] An embodiment of the method further comprises filtering the liquid by passing the liquid through a filter prior to applying the liquid to the workpiece.

[0051] This filters out particles that could have a negative impact on the quality of the layer of metal plated onto the surface of the workpiece. It is, for example, prevented that such particles block or disrupt the flow of liquid through orifices of the device used to apply the liquid to the surface of the workpiece or that they act as seeds on which metal is deposited in preference to the surface of the workpiece. The filter may be a screen filter, depth filter or membrane filter. The filter may be rated to filter out particles having a diameter above a value in the micrometre range, e.g. 1 µm.

[0052] In an example of such an embodiment, sparging gas is introduced into the liquid in the form of bubbles, sparging gas is introduced upstream of the filter, and the bubbles are reduced in size by passing the liquid through the filter.

[0053] This enables removal of oxygen from the liquid and more of the sparging gas to dissolve into to the liquid at a relatively short time, as governed by Fick's laws of diffusion.

[0054] In an embodiment of the method, applying liquid comprises: filling a space between a flow guide surface and an upper section of the surface extending to an upper edge of the workpiece with liquid; and flowing the liquid onto a section of the surface lower than the upper section through a gap between the upper section of the surface and the flow guide surface.

[0055] The upper section is thus immersed in the liquid. The only exception may be any areas engaged by a device holding the workpiece, if such a device is used. Thus, this section is plated relatively uniformly. In addition, the liquid flowing out through the gap onto the lower section of the workpiece surface will generally do so in the form of a laminar film flow. That in turn helps keep the oxygen content in the boundary layer close to the surface low, improving plating uniformity.

[0056] In an embodiment of the method, the method is carried out using an apparatus according to the invention. Similarly, the apparatus of the invention is suitable for carrying out any embodiment of the method according to the invention.Brief Description of Drawings

[0057] The invention will be explained in further detail with reference to the accompanying drawings, in which: Fig. 1is a schematic diagram of an apparatus for plating surfaces of a planar workpiece; Fig. 2is a perspective view of a holding device for holding the planar workpiece in the apparatus; Fig. 3is a detailed perspective view of an, in use, upper section of the holding device of Fig. 2; Fig. 4is a detailed perspective view of an, in use, lower section of the holding device of Figs. 2 and 3; Fig. 5is a perspective view of part of the upper section of the holding device of Figs. 2-4; Fig. 6is a view into a space for accommodating an edge region of the planar workpiece in the holding device; Fig. 7is an enlarged view into the space shown in Fig. 6; Fig. 8is a perspective view of parts of an upper section of a modified version of the holding device of Figs. 2-7; Fig. 9is a diagram showing the development in horizontal and vertical direction of the average thickness of a layer plated onto a front major surface of a planar workpiece held by a holding device of the type shown in Figs. 2-8 without the introduction of a sparging gas into the liquid comprising the plating solution; Fig. 10is a diagram corresponding to that of Fig. 9 showing the development in horizontal and vertical direction of the average thickness of a layer plated onto a back major surface of the workpiece; Fig. 11is a diagram showing the development in horizontal and vertical direction of the average thickness of a layer plated onto a front major surface of a planar workpiece held by a holding device of the type shown in Figs. 2-8 when a sparging gas is introduced into the liquid comprising the plating solution; and Fig. 12is a diagram corresponding to that of Fig. 11 showing the development in horizontal and vertical direction of the average thickness of a layer plated onto a back major surface of the workpiece. Description of Embodiments

[0058] An apparatus 1 for electroless plating a surface of a workpiece 2 (Fig. 1) is configured to plate both opposite major surfaces of a planar workpiece 2 in the illustrated example. In a variant, the apparatus may be configured to plate only one major surface.

[0059] Copper plating, preferably electroless copper plating, will be used as an example here, in the understanding that different chemistry can be used to plate, preferably by electroless plating methods, the surfaces with a different metal, e.g. nickel, tin or gold.

[0060] The deposited layer or layers will have a thickness of at least 20 nm, e.g. at least 50 nm. A maximum thickness may be 50 µm or less, e.g. 500 nm.

[0061] The apparatus 1 will generally be comprised in a larger system for wet-chemical treatment of the workpiece 2, comprising a series of processing stations. These may include processing stations for rinsing, de-smearing and pre-treatment. Pre-treatment will in particular include activating the surface to be plated. Activating can comprise metalising the surface by means of a cationic palladium treatment followed by reduction of the palladium ions to metal, a colloidal palladium-tin activator, a colloidal copper activator, a colloidal silver activator, a silver-palladium activator or a lead-tin activator, for example. Activating renders the surface suitable for electroless plating.

[0062] The apparatus 1 comprises a processing chamber 3. The processing chamber 3 may be sealable and contain an atmosphere comprising at least one inert gas and substantially only inert gases. However, the processing chamber 3 need not necessarily be sealable and may contain just air, optionally filtered to remove particles. Here, it will be assumed that the processing chamber 3 contains air.

[0063] A bottom section of the processing chamber 3 constitutes a receptacle 4 for collecting processing liquid. In an alternative embodiment, a separate receptacle (not shown) is arranged in a lower section of the processing chamber 3.

[0064] A holding device (not shown in Fig. 1) is arranged to hold the workpiece 2 with the major surfaces in a predominantly upright orientation, meaning that normals to these surfaces extend in a predominantly horizontal direction. If only one surface is to be plated, the workpiece 2 may be held at an angle to the vertical, but still in a predominantly upright orientation and thus at an angle smaller than 45°, e.g. parallel to the vertical to within 5°.

[0065] In the illustrated example, the workpiece 2 is a planar workpiece 2. This includes workpieces that are flexible, e.g. in the form of a foil. The workpiece 2 is then at least planar when mounted to the holding device. Suitable workpieces include substrates such as printed circuit boards, semiconductor dies, integrated circuit substrates and the like. In the examples to be discussed below, the workpiece 2 is quadrilateral, e.g. square, seen looking onto the major surfaces of the planar workpiece 2. Other polygonal or even round shapes are also possible. A shape defining a straight edge at which the workpiece 2 can be held, in particular clamped, is most suitable.

[0066] A liquid (aqueous liquid) application system 5 is arranged to apply processing liquid in the form of an active electrolyte 6 to the major surfaces of the planar workpiece 2.

[0067] The holding device and the liquid application system 5 may comprise a holding device and a pair of flooding devices of the types disclosed in detail in WO 2024 / 046870 A1 and WO 2024 / 046871 A1, for example, the contents of which are hereby incorporated by reference. Two versions of an alternative holding device 7 comprising an integrated flooding device (Figs. 2-8) will be described in more detail below. Both versions of the alternative holding device 7 are optimised to create a laminar film flow across the surfaces of the workpiece 2.

[0068] In use, the active electrolyte 6 will flow across the surface of the workpiece 2 as a film flowing in downwards direction. The flow will be laminar at the top of the surface and, assuming the thickness of the liquid film is thin and flow liquid film is undisturbed and remains at controlled velocity, and the workpiece 2 does not have dimensions in the direction of flow excessively large, the flow will remain laminar. Otherwise the flow will transition to turbulent. Oxygen diffuses into the active electrolyte at a higher rate when the flow is turbulent than when the flow is laminar. Thus, at least detached electrolyte 8 will have a higher concentration of dissolved oxygen than the active electrolyte 6 near a top edge of the workpiece 2. Collected electrolyte 9 will have a higher concentration still.

[0069] In the illustrated embodiment, an optional first oxygen sensor 10 is arranged to measure the oxygen concentration for process control purposes. The collected electrolyte 9 leaves the processing chamber 3 through a conduit in which an (optional) valve 11 is arranged. The collected electrolyte 9 then enters a container 12 for storing deactivated electrolyte 13. In this example, the container 12 is a sump, so that no pump is required.

[0070] In the illustrated embodiment, an optional second oxygen sensor 14 is arranged to measure the oxygen concentration in the deactivated electrolyte 13 for process control purposes.

[0071] In the illustrated embodiment, the collected electrolyte 9 is turned into deactivated electrolyte 13 by aerating the liquid in the container 12. To this end, an air supply 15 and diffuser 16 are arranged to introduce bubbles 17 into the liquid contained in the container 12. In an alternative embodiment, pure oxygen may be used instead of air.

[0072] This process increases the stability of the aqueous electrolyte. The aqueous electrolyte (plating bath) is preferably an autocatalytic type solution comprising preferably copper ions and a reducing agent capable of reducing copper ions to metallic copper so that a copper layer is deposited on the surface of the workpiece 2. The aqueous electrolyte will generally comprise the following components: at least one metal salt, e.g. selected from the group consisting of copper sulphate, copper chloride, copper nitrate, copper acetate, copper methane sulfonate, copper hydroxide, copper formate and hydrates thereof. The concentration of Cu(ll) ions in the electroless plating bath preferably ranges from 0.05 to 10 g / l, e.g. 1-5 g / l, corresponding to 0.016-0.079 mol / l, preferably 2.0 3.0 g / l; at least one reducing agent capable to reduce metal ions selected from the group consisting of formaldehyde, hypophosphite, dimethylamine borane, glyoxylic acid and 2,2'-dialkoxyacetaldehydes. The formaldehyde is preferred and own experiments showed best results with copper ions. The concentration of the reducing agent preferably ranges from 0.1 to 50 g / l, e.g. 0.027-0.270 mol / l, preferably formaldehyde: 0.8 to 8.5 g / l; at least one complexing agent for metal ions e.g. selected from the group consisting of carboxylic acids, hydroxycarboxylic acids, aminocarboxylic acids, alkanolamines, polyols and mixtures thereof, such as EDTA, triethanolamine (TEA), quadrol, tartrate, citrate. The concentration of the complexing agent preferably ranges from 5-50 g / l, more preferably 20 - 40 g / l. Preferably the molar ratio of the complexing agents, related to the total molar amount of all complexing agents, to copper ions is in the range of 1:1 to 10:1, preferably 1:1 to 8:1, more preferably 2:1 to 5:1; optionally a compound for adjusting pH, e.g. sodium hydroxide, sulphuric acid, wherein a pH value is adjusted from 10 to 14; optionally a buffer; optionally a surfactant; and optionally at least one stabilising additive and / or at least one sulphur-containing additive.

[0073] In same case, it might be useful to also add a source of nickel ions, as nickel sulphate, in order to prevent blistering above 200 nm Cu layer thickness. The concentration of the nickel ions preferably ranges from 50 to 300 mg / l.

[0074] Add Ni as an option to prevent blistering above 200 nm Cu thicknesses

[0075] The aqueous electrolyte, or at least the active electrolyte 6, is preferably held at a temperature in the range of 15 to 60°C. The workpiece 2 is contacted with the processing liquid for electroless plating for 5 s to 30 min, depending on the desired thickness of the copper layer to be plated and on the used plating rate.

[0076] In principle, electroless copper plating solutions are known to the skilled person. E.g. Printoganth ®< MV Plus Electroless Copper can be used as electroless copper plating bath.

[0077] The reducing agent will reduce some bivalent copper to monovalent copper. The monovalent copper is not readily reduced further in the presence of bivalent copper. It will instead exist as a precipitate or disproportionate to produce randomly dispersed copper particles in the electrolyte. This is one cause of instability. The aeration carried out in the container 12 re-oxidises the monovalent copper to bivalent copper and thus improves the stability of the electrolyte. The non-immersive treatment in the processing chamber 3 allows for less intensive stability-enhancing treatment in the container 12. Furthermore, the formation of precipitates or disperse particles in the processing chamber 3 is avoided to at least a certain extent as oxygen diffuses into the detached electrolyte 8 and the collected electrolyte 9. Thus, there is a further effect if the processing chamber 3 is in communication with ambient atmosphere.

[0078] The deactivated electrolyte 13 is removed from the container 12 through a pipe inlet 18 and the action of a pump 19.

[0079] In the illustrated embodiment, an (optional) deaerator 20 is comprised in the recirculating system. The deaerator 20 may comprise an ultrasonic or megasonic device to generate sound waves, for example. It is also possible to use a thermal device or a device arranged to generate at least a partial vacuum. The deaerator 20, if present, reduces the oxygen content of the deactivated electrolyte 13 after the deactivated electrolyte 13 has left the container 12.

[0080] Sparging gas bubbles 21 are introduced into the electrolyte through a sparging gas inlet 22 positioned downstream of the deaerator 20.

[0081] The sparging gas is supplied from a sparging gas source 23. A flow meter may be provided at the sparging gas inlet 22 to control the rate at which the sparging gas is introduced. Used flow rates are from 5 to 20 l / min. The sparging gas bubbles comprise at least one sparging gas, meaning in the present context a non-oxidising gas. Suitable are nitrogen, argon, forming gas (a mixture of hydrogen and nitrogen), hydrogen, helium, methane and neon, for example. Thus, the sparging gas may be, but need not be, an inert gas.

[0082] The effect of the raised nitrogen concentration is that, when the active electrolyte 6 is applied to the surface of the workpiece, the exchange of gas into the solution is limited. In the electrolyte, the reaction by which oxygen is reduced, the Oxygen Reduction Reaction (ORR), competes with the reactions by which copper ions are reduced, the plating reactions. A lower oxygen concentration favours the latter and thus speeds up the deposition rate. In addition, if the ORR occurs at a relatively high rate, this gives rise to a phenomenon known as skip plating, whereby parts of the surface of the workpiece 2 are not plated at all or coated with a much thinner layer of metal. The sparging gas inhibits exchange of gas, specifically the entry of oxygen into the active electrolyte 6.

[0083] Introduction of sparging gas bubbles 21 is followed by filtration in a filter 24 to remove particles. The filter 24 may be a cartridge filter, for example. A further effect is that the sparging gas bubbles 21 are reduced in size to microbubbles 25.

[0084] In the illustrated embodiment, the thus activated active electrolyte 6 passes through an optional flow meter 26 and past an optional third oxygen sensor 27.

[0085] As explained, it is desirable to prevent the solution of oxygen into the active electrolyte 6 and subsequent diffusion towards the surface of the workpiece 2 when the active electrolyte 6 flows across the surface of the workpiece 2 in the processing chamber 3. The rate of diffusion is lower when the flow is laminar than when the flow is turbulent. A lower velocity favours laminar flow (the Reynolds number is lower). A thin film of liquid flowing downwards has a lower velocity (the boundary layer is relatively large compared to the total film thickness).

[0086] The holding device 7 (Figs. 2-7) is designed to increase the likelihood of laminar flow. This holding device 7 and variants providing the same effect are disclosed in more detail in European patent application No. 24188389.1, filed on 12 July 2024.

[0087] The holding device 7 comprises a support structure for engaging an external support to position the holding device 7 in the apparatus 1. In this example, the support structure comprises a frame arranged to frame at least a central section of a major surface of the workpiece 2, seen looking on to the major surface. In the illustrated holding device 7, the frame frames the complete major surface of the workpiece 2.

[0088] The frame comprises at least lateral members 28a,b, lower members 29a,b and an upper member 30. The upper member 30 comprises laterally projecting arms 31a,b for engaging supports (not shown), so as to suspend the holding device 7 in the processing chamber 3 of the apparatus 1. A claw 32 for engaging a horizontally extending pin of the support is defined at a distal end of at least one of the arms 31a,b. Examples of a suitable support are disclosed in WO 2020 / 260389 A1.

[0089] The configuration of the support structure is such as to allow the holding device 7 to be conveyed from station to station in the treatment system comprising the apparatus 1. To this end, the external support may either be comprised in a transporter of a conveying system (not shown) or the conveying station may be configured to transfer the holding device 7 between supports provided in respective treatment stations. At least one of the lower members 29a-d may be provided with one of at least one skid and at least one caster (not shown) for movably supporting the holding device 7 on a support surface.

[0090] It is convenient to define a co-ordinate system (Fig. 2) with reference to the planar workpiece 2 when mounted to the holding device 7 with an orientation permitting the non-immersive wet-chemical treatment to be carried out. In that situation, the major surfaces of the planar workpiece 2 are in a plane parallel to a first direction -z and a second direction y. They face in directions aligned with a third direction x. In the orientation permitting non-immersive wet-chemical treatment to be carried out, the first direction -z is essentially parallel to the vertical, i.e. the planar workpiece 2 has an upright orientation, so that liquid can flow down the major surfaces of the workpiece 2 as a gravity-driven film flow.

[0091] The holding device 7 comprises an upper clamping device (described in more detail below) and a lower clamping device 33 (Fig. 4).

[0092] The lower clamping device 33 comprises a pair of support bars 34a,b on which protruding lower engagement features 35a-f are mounted. The lower engagement features 35a-f provided on one of the support bars 34a,b protrude towards the other one of the support bars 34a,b and vice versa. The lower engagement features 35a-f are configured to engage the opposite major surfaces of the planar workpiece 2 in an edge region at an (in use lower) edge of the planar workpiece 2. To this end, the support bars 34a,b are mounted on pivotable lower support levers 36a-d. Biasing devices comprising resilient elements, in this case lower coil springs 37a-d are provided to urge the support bars 34a,b towards each other in the third direction x. Cams (not shown in detail) or other features may be defined on the lower support levers 36a-d to apply a force moving the support bars 34 apart in the third direction x to release the workpiece 2. Other types of biasing device may be used, e.g. other types of elastic element, gas springs or magnets.

[0093] The upper clamping device for clamping the workpiece 2 in an edge region close to an, in use upper, edge of the workpiece 2 comprise a first array 38 of engagement features and a second array 39 of engagement features (Fig. 6). The first array 38 and second array 39 extend in the second direction y. The engagement features of each array 38,39 are discrete features spaced apart in the second direction y. They protrude in directions corresponding to the third direction x, so that liquid can pass in the first direction -z between adjacent engagement features.

[0094] In the illustrated holding device 7, the engagement features of the first and second arrays 38,39 are in each case arranged in a single straight line. Alternatively, one or both of the arrays 38,39 may comprise multiple rows of engagement features, each row extending in the second direction y, with the rows spaced apart in the first direction z. The arrays, in the case of the alternative example the rows, may extend in non-straight, e.g. curved lines, e.g. if the edge region adjoins a curved workpiece edge.

[0095] In the illustrated holding device 7, the engagement features of the first array 38 are formed by first pins 40 (Fig. 7) mounted to a first body 41. The engagement features of the second array 39 are formed by second pins 42 mounted to a second body 43. In the illustrated holding device 7, the first and second pins 40,42 are provided with threads, mounted in respective bores or other through-going passages in the first and second body 41,43 and secured by first and second nuts. Other means of securing the first and second pins 40,42 may be used instead.

[0096] The first and second bodies 41,43 are mounted to pivotable upper support levers 44a-d, which are pivotably mounted to the support structure of the holding device 7. In this example, they are pivotably mounted to the upper member 30 of the frame. The pivot axes extend in the second direction y. Biasing devices comprising resilient elements, in this example upper coil springs 45a,b, are provided to urge the first and second arrays 38,39 towards each other in a direction corresponding to the third direction x. These resilient elements thus provide the clamping force. In a variant, other types of biasing device may be used, e.g. comprising at least one gas spring or at least one magnet.

[0097] In a variant, only one of the first and second bodies 41,43 is mounted to pivotable upper support levers 44 and the other is fixed to the support structure of the holding device 7. In another variant, the first and second bodies 41,43 or alternative supports carrying the first and second arrays 38,39, are themselves pivotably mounted to the support structure. In that case, they are thus only partially movable with respect each other, because their respective pivot axes are fixed in position.

[0098] The first body 41 presents an inward-facing surface comprising a first flow guide surface 46. The second body 43 presents an inward-facing surface comprising a second flow guide surface 47. These surfaces are non-planar, but they are oriented parallel to the second direction y everywhere and parallel to or at an angle smaller than 90° to the first direction -z. Thus, the first flow guide surface 46 and the second flow guide surface 47 each progress in the first direction -z up to a first edge 48 and a second edge 49, respectively. There is a gap 50 between the first edge 48 and the second edge 49 through which an edge region of the planar workpiece 2 can pass into a space 51 for accommodating the edge region. This space 51 is defined between at least sections of the inward-facing surfaces of the first and second bodies 41,43, such that the first flow guide surface 46 and the second flow guide surface 47 are located on opposite sides of the space 51.

[0099] A series of baffle plates 52 partially sub-divides the space between the inward-facing surfaces of the bodies 41,43.

[0100] Going along the first flow guide surface 46 and the second flow guide surface 47 in the first direction -z, the dimension in the third direction x of the space 51 decreases at least over an interval in the first direction -z. A gravitational flow of liquid flowing the first direction -z thus faces a constriction there. The liquid will back up.

[0101] The engagement features of the first and second arrays 38,39 are located in the space 51 at a distance in opposite direction z to the first direction -z from the edges 48,49. In particular, the engagement features of the first and second arrays 38,39 are located in the space 51 at a distance in opposite direction z to the first direction -z from where the dimension of the space 51 in third direction x has decreased to an at least local minimum. Generally, this minimum will correspond to a dimension in the third direction xof the gap 50 between the edges 48,49, as in the illustrated example.

[0102] In the illustrated holding device 7, the space 51 is accessible to liquid at a distance from the edges 48,49 in opposite direction z to the first direction -z at least equal to the distance in opposite direction z to the first direction -z between the first and second arrays 38,39 and the edges 48,49. In an alternative holding device, liquid may enter the space 51 at a location closer to the edges 48,49, but at a distance in opposite direction zto the first direction -z from where the dimension of the space 51 in third direction x has decreased to an at least local minimum, since a gravitational flow of liquid will in any case then back up.

[0103] In the illustrated holding device 7, a gravity-driven flow of liquid can simply flow down between the first and second bodies 41,43.

[0104] Although not required, the space 51 and gap 50 are also bounded at at least one of two opposite ends in the second direction y. In the illustrated holding device 7, end plates 53a-d are provided to this end. This helps keep the first and second arrays 38,39 submerged, in use. As a consequence, the direction of flow is uniformly in the first direction -z. Furthermore, the speed of flow is lower, because liquid can only flow out through the gap 50. Due to these effects, there are fewer or no eddies in the region of the first and second arrays 38,39. In other word, the space 51 is closed laterally to form a tub, achieving uniformity of flow in the second direction y. The fill level in the tub is relatively uniform, which promotes the formation of a laminar flow out of the gap 50.

[0105] The edges 48,49 are relatively sharply defined, in order to ensure that the liquid flows out of the gap 50 and onto the major surfaces of the workpiece 2 with relatively few eddies. The first flow guide surface 46 meets a first exterior surface 54 at an angle at the first edge 48. The second flow guide surface 47 meets a second exterior surface 55 at an angle at the second edge 49. The angle may be equal to or smaller than 90°, for example.

[0106] The holding device 7 is configured to be conveyed through the apparatus for wet-chemical treatment of the workpiece 2. To facilitate this, systems for supplying processing liquid are stationary in the apparatus and configured to direct one or more jets (coherent streams of liquid projected into a surrounding medium, usually from some kind of a nozzle or aperture) onto or into the holding device 7. In the illustrated example, sheet-shaped jets of liquid can be projected from either side onto outward-facing surface sections of a flow guidance part, in this case in the form of a flow guidance plate 56. The outward-facing surface sections extend in the first direction -z to respective, in use lower, edges proximal to the space 51. First and second dam wall sections 57,58 are defined by the first and second bodies 41,43 and bound a collecting space sub-divided by the baffle plates 52 and in communication with the space 51.

[0107] To supply the sheet-shaped jets of processing liquid, the stations of the apparatus for non-immersive wet-chemical treatment of the planar workpiece 2 may comprise flooding devices of the type disclosed in WO 2024 / 046870 A1. These are configured to direct a downwardly-directed stream of liquid onto the outward-facing surface sections of the flow guidance plate 56. The dimension of the flow guidance plate 56 in the first direction -z is sufficient to establish a laminar flow at the lower edges of the flow guidance plate 56. Due to the fact that the liquid backs up from the first and second edges 48,49, this is not strictly required. However, it helps to limit splashing so that more or less all of the processing liquid also flows across the major surfaces of the workpiece 2.

[0108] In an alternative embodiment, the collecting space is filled directly from a nozzle that is movable into position close to the first and second bodies 41,43. Alternatively, it is also possible to attach a flexible conduit to a port formed in one of the bodies 41,43 or end plates 53a-d.

[0109] In a modified version of the holding device 7 (Fig. 8), the baffle plates 52 are replaced by a flow distributor 59. The flow distributor 59 separates a collecting space 60 bounded by the first and second dam wall sections 57',58' defined by the first and second bodies 41',43' from the space 51' for accommodating the edge region of the workpiece 2. The flow distributor 59 is provided with apertures 61 forming channels extending in the first direction -z. These channels open into the space 51' for accommodating the edge region of the workpiece 2. In the illustrated example, the flow distributor apertures 61 are slit-shaped, having a longitudinal axis extending in the second direction y. Other shapes are possible.

[0110] In all variants, a laminar flow emerges through a gap between the first edge 48;48' and one major surface of the workpiece 2 and through a gap between the second edge 49; 49' and the other major surface of the workpiece 2.

[0111] The effect on plating uniformity of the introduction of sparging gas and of the enhanced laminar flow due to the immersion of the upper edge region of the planar workpiece 2 in the activated electrolyte in the holding device 7 separately and in combination have been investigated.

[0112] Four panels were plated in the respective constellations indicated in Table 1. Table 1 Panel #SpargingHolding device optimised for laminar flow1NoNo2NoYes3YesNo4YesYes

[0113] The holding device optimised for laminar flow was of the configuration described above. In the alternative, a conventional holding device was used. Each was used once in combination with the introduction of sparging gas and once without the introduction of sparging gas. Pure nitrogen was used as the sparging gas.

[0114] The electrolyte composition was Ni 2+< ions at a concentration in the range of 130-160 mg / l; Cu 2+< at a concentration of 4.3 g / l; Tartrate at a concentration of 0.16 mol / l; NaOH at a concentration of 3 g / l; stabilising additives at a concentration in the range of 5-20 ppm and sulphur-containing additives at a ppb level; and surfactants at a concentration of 200 ppm (all values are approximate values). The temperature was 34° C and the deposition time 6 min. The inlet flow rate of nitrogen gas was set at 10 L / min and the measured dissolved oxygen concentration within the active electrolyte prior delivery to the workpiece was < 1 ppm during plating of both Panel 3 and Panel 4.

[0115] Results are shown in Table 2 and graphically in Figs. 9-12 for panels No. 1 (Figs. 9), No. 2 (Figs.10), No. 3 (Figs. 11), and No. 4 (Figs. 12). Table 2 Average Thickness [nm]Standard Deviation [nm]Relative standard deviation [%]Panel #FrontBackBoth sidesFrontBackBoth sidesFrontBackBoth sides139838138939.135.2379.89.309.51236336336325.921.321.45.875.905.89334134234110.810.110.53.112.913.143863903886.36.36.61.641.631.71

[0116] In Table 2, the average thickness across the surface of the panel concerned, as measured by measuring the conductivity, is listed for each of the panel surfaces (two sides). In addition, the average over both surfaces is indicated. The standard deviation indicates the uniformity of the thickness across the surface. In the drawings, the central curve in each two-dimensional diagram represents the average thickness, and the two other curves represent the sum of the average and the standard deviation and the difference between the two, respectively.

[0117] It can be seen from Table 2 that the holding device 7 and the introduction of sparging gas each produce an effect, and that the combined effect on the plating uniformity is even greater. In addition, the curves in the two-dimensional diagrams for panel No. 4 are flatter and the standard deviation is relatively small at each abscissa value and each ordinate value, compared with the curves for panel No. 2. This shows that increased uniformity is also achieved in edge regions by introducing sparging gas.

[0118] The invention is not limited to the embodiments described above, which can be varied within the scope of the accompanying claims. For example, although a planar workpiece 2 has been used here as an example, non-immersive plating of a surface of a differently-shaped workpiece, possibly with a flat surface to be plated, is equally possible. Alternative suitable plating liquid compositions to the one used here to demonstrate the effect of the sparging gas and holding device can be found, for example, in Schlesinger, M. and Paunovic, M. (eds.), "Modern Electroplating", 5th Ed. Hoboken, Wiley, 2010, p. 444. One or more of the oxygen sensors 10, 14, 27, when present, may be in-line sensors. Such sensors can be used to control the rate at which the sparging gas is introduced and / or the rate at which air or oxygen is introduced in the container 12.List of reference numerals

[0119] 1- Apparatus 2- Workpiece 3- Processing chamber 4- Receptacle 5- Liquid application system 6- Active electrolyte 7- Holding device 8- Detached electrolyte 9- Collected electrolyte 10- 1 st< oxygen sensor 11- Valve 12- Container 13- Deactivated electrolyte 14- 2 nd< oxygen sensor 15- Air supply 16- Diffuser 17- Air bubbles 18- Pipe inlet 19- Pump 20- Deaerator 21- Sparging gas bubbles 22- Sparging gas inlet 23- Sparging gas source 24- Filter 25- Microbubbles 26- Flow meter 27- 3 rd< oxygen sensor 28a,b- Lateral members 29a,b- Lower members 30- Upper member 31a,b- Projecting arms 32- Claw 33- Lower clamping device 34a,b- Support bar 35a-f- Lower engagement features 36a-d- Lower support lever 37a-d- Lower coil spring 38- 1 st< array of engagement features 39- 2 nd< array 40- 1 st< pin 41;41'- 1 st< body 42- 2 nd< pin 43;43'- 2 nd< body 44a-d- Upper support lever 45a,b- Upper coil springs 46- 1 st< flow guide surface 47- 2 nd< flow guide surface 48;48'- 1 st< edge 49;49'- 2 nd< edge 50;50'- Gap 51;51'- Space 52- Baffle plate 53a-d;53c,d'- End plates 54- 1 st< exterior surface 55- 2 nd< exterior surface 56- Flow guidance plate 57;57'- 1 st< dam wall section 58;58'- 2 nd< dam wall section 59- Flow distributor 60- Collecting space 61- Flow distributor aperture

Examples

Embodiment Construction

[0058]An apparatus 1 for electroless plating a surface of a workpiece 2 (Fig. 1) is configured to plate both opposite major surfaces of a planar workpiece 2 in the illustrated example. In a variant, the apparatus may be configured to plate only one major surface.

[0059]Copper plating, preferably electroless copper plating, will be used as an example here, in the understanding that different chemistry can be used to plate, preferably by electroless plating methods, the surfaces with a different metal, e.g. nickel, tin or gold.

[0060]The deposited layer or layers will have a thickness of at least 20 nm, e.g. at least 50 nm. A maximum thickness may be 50 µm or less, e.g. 500 nm.

[0061]The apparatus 1 will generally be comprised in a larger system for wet-chemical treatment of the workpiece 2, comprising a series of processing stations. These may include processing stations for rinsing, de-smearing and pre-treatment. Pre-treatment will in particular include activating the surface to be pla...

Claims

1. Apparatus for non-immersive wet-chemical treatment of a surface of a workpiece (2), comprising: a holding device (7) for holding the workpiece (2) with the surface in a predominantly upright orientation; a liquid application system (5) for flowing liquid (6) across the surface of the workpiece (2), when held by the holding device (7), as a film flowing in a predominantly downwards direction; a receptacle (4) for collecting liquid (9) that has flowed across and detached itself from the surface; and a system (11,12,16,18-20,22,24) for recirculating liquid collected in the receptacle (4) to the liquid application system (5), characterised in that the system (11,12,16,18-20,22,24) for recirculating liquid comprises a device (22,23) for introducing sparging gas (21) into the liquid.

2. Apparatus according to claim 1, wherein the system (11,12,16,18-20,22,24) for recirculating liquid further comprises a filter (24) for filtering particles from the liquid.

3. Apparatus according to claim 2, wherein the device (22,23) for introducing sparging gas (21) into the liquid is arranged upstream of the filter (24).

4. Apparatus according to any one of the preceding claims, wherein the system (11,12,16,18-20,22,24) for recirculating liquid further comprising a container (12), e.g. a sump, for storing the liquid, and wherein the container (12) is located in flow direction between the receptacle (4) and the device (22,23) for introducing sparging gas (21) into the liquid.

5. Apparatus according to any one of the preceding claims, wherein the liquid application system (5) is arranged to supply the liquid (6) to the holding device (7), and wherein the holding device (7) comprises at least one flow guide surface (46,47) for directing liquid (6) towards, e.g. onto, the surface of the workpiece (2).

6. Apparatus according to any one of the preceding claims, wherein the holding device (7) is configured to hold a planar workpiece (2).

7. Apparatus according to claim 6, wherein the holding device (7) comprises a clamping device for engaging major surfaces of the planar workpiece (2) in an edge region along an edge of the planar workpiece (2), e.g. an edge region along an in use upper edge of the planar workpiece (2).

8. Apparatus according to claims 5 and 7, wherein the holding device (7) comprises a pair of flow guide surfaces (46,47) on opposite sides of a space (51;51') for accommodating the edge region, each extending towards a respective edge (48,49;48',49') bounding a gap (50;50') for insertion of the edge region into the space (51;51'), and wherein at least respective opposing sections of the flow guide surfaces (46,47) converge towards each other in a direction (-z) of the edges (48,49;48',49').

9. Apparatus according to claim 7 or 8, wherein the clamping device comprises at least one first protruding engagement feature (40) protruding from a first engagement feature support (41;41') and at least one second protruding engagement feature (42) protruding from a second engagement feature support (43;43'), wherein the first engagement feature support (41;41') and the second engagement feature support (43;43') are at least partially movable with respect to each other to adjust a distance between the first and second protruding engagement features (40,42) in a direction (x,-x) in which they protrude.

10. Apparatus according to claim 9, wherein the at least one first protruding engagement features (40) comprise an array (38) of first protruding engagement features (40) spaced apart in a direction (y) transverse to the direction (x,-x) in which they protrude, and wherein the at least one second protruding engagement features (42) comprise an array (39) of second protruding engagement features (42) spaced apart in a direction (y) transverse to the direction (x,-x) in which they protrude.

11. Method of plating a surface of a workpiece (2), comprising: holding the workpiece (2) with the surface in a predominantly upright orientation; applying liquid (6) comprising a plating solution to the workpiece (2) by flowing the liquid (6) across the surface of the workpiece (2) as a film flowing in a predominantly downwards direction; collecting liquid (9) that has flowed across and detached itself from the surface; and recirculating the collected liquid (9) for renewed application to a next workpiece (2), characterised by introducing sparging gas (21) in the collected liquid (9) prior to the renewed application to the next workpiece (2).

12. Method according to claim 11, wherein recirculating the collected liquid (9) comprises intermediate storage of the collected liquid (9) in a container (12), wherein sparging gas (21) is introduced into liquid (13) removed from the container (12) after the intermediate storage, and wherein the method includes introducing at least oxygen into the collected liquid (9) prior to removal of liquid (13) from the container (12).

13. Method according to claim 11 or 12, further comprising activating the surface prior to applying the liquid (6) comprising the plating solution.

14. Method according to any one of claims 11-13, wherein applying liquid (6) comprises: filling a space between a flow guide surface (46,47) and an upper section of the surface extending to an upper edge of the workpiece (2) with liquid (6); and flowing the liquid (6) onto a section of the surface lower than the upper section through a gap between the upper section of the surface and the flow guide surface (46,47).

15. Method according to any one of claims 11-14, wherein the method is carried out using an apparatus (1) according to any one of claims 1-10.

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