Electrode and device for electrolytic treatment of workpieces, assembly for forming a unit of said device, method and computer program

Abstract

The invention provides an electrode for an apparatus (1) for electrolytically treating a workpiece (3), the apparatus (1) being of a type arranged to deliver the workpiece (3) having a surface to be treated past and directed towards a surface of the electrode, the electrode being divided into segments (23a to e) at least at this surface of the electrode. The segments (23a to e) are arranged adjacent to one another in a first direction (x). Adjacent segments (23a to e) are separated from one another along respective segment edges (24a to f) so as to allow adjacent segments (23a to e) to be maintained at different respective voltages. In use, the segment edges (24a to f) extend at least partially in a second direction (y) from a common value (y0) of a coordinate in the second direction (y) to an edge (25, 26) of at least a conductive portion of the electrode surface, the second direction (y) being transverse to the first direction (x) and corresponding to the direction of movement of the workpiece. The segment edges (24a to f) between at least one pair of adjacent segments (23a to e) extend along respective paths, the angles formed by the respective paths with the electrode surface edges (25, 26) decreasing from the common value (y0) of the coordinate to the electrode surface edges (25, 26).

Description

Technical Field

[0001] The invention relates to an electrode for an apparatus for electrolytically treating a workpiece, said apparatus being of the type arranged to convey said workpiece having a surface to be treated past and directed towards a surface of said electrode,

[0002] wherein the electrode is divided into segments at at least this surface of the electrode,

[0003] wherein the segments are arranged next to each other in a first direction,

[0004] wherein adjacent segments are separated from one another along respective segment edges so as to allow the adjacent segments to be maintained at different respective voltages, and

[0005] wherein in use, the segment edge extends at least partially in a second direction from a common value of coordinates in the second direction to an edge of at least the conductive portion of the electrode surface, the second direction being transverse to the first direction and corresponding to a direction of movement of the workpiece.

[0006] The invention also relates to an assembly for forming a unit of an electrolytic treatment plant.

[0007] The invention also relates to an electrolytic treatment plant comprising at least one treatment unit.

[0008] The invention also relates to a method comprising at least a computer-implemented step of designing an electrode of the above-mentioned type.

[0009] The invention also relates to a computer program. Background Art

[0010] From the Internet: <URL: https: / / iopscience.iop.org / article / 10.1149 / MA2013-02 / 29 / 2089 / pdf "Copper plating uniformity on resistive substrates with segmented anodes" by Yang, L. et al., "Copper plating uniformity on resistive substrates with segmented anodes," retrieved November 1, 2013, ECS Meeting Abstracts, 224th Session, Abstract #2089, relates to a method for improving wafer-scale copper plating uniformity on resistive substrates using segmented anodes. In this plating cell setup, instead of a circular anode, multiple annular segments are used to control the input current. Specifically, an anode configuration with three concentric segments is disclosed.

[0011] US Pat. No. 6,919,010 B1 discloses an anode assembly comprising a primary, azimuthally asymmetric anode and a plurality of secondary anode segments. A workpiece is positioned above the anode assembly and rotates about an axis substantially aligned with the anode assembly's central axis. In a typical embodiment, the footprint of the workpiece corresponds (at least approximately) to the periphery of the anode assembly. Initially, to provide a large proportion of the ion current to the central region of the workpiece (near the axis of rotation), only the asymmetric anode is energized and provides current. During this initial phase of the electroplating process, when terminal effects are most severe, the area of ​​the assembly occupied by the segments does not provide any significant current. Thus, at any given moment, a relatively large section of the workpiece periphery is not positioned above the top of the anode (or otherwise aligned with any portion of the anode). The electroplating cell has a container for holding an electrolyte. A wafer holder holds a wafer having a seed layer thereon. Circuitry variably distributes the electroplating current to each of the two anodes (the primary asymmetric anode and the secondary asymmetric anode).

[0012] EP 1 419 290 B1 discloses a horizontal electroplating system for circuit boards, comprising upper and lower anodes arranged one behind the other in the conveying direction of the circuit boards. The workpiece (in this case, the circuit board) is held by at least one fixture, electrically contacted, and conveyed from one anode to the next. Current is fed to the circuit boards via the contacts and fixtures. The anodes are divided into individual electrically isolated anode segments that are divided transversely to the conveying direction. The anode segments, together with the substrate on the circuit board, form electrolytic partial units. Each partial unit is fed with current from a separate current source (e.g., its own segment rectifier). The circuit board to be treated forms the cathode of the partial unit to be metallized on its upper layer. In an embodiment, the separating line delineating the anode segments extends at an angle α>0 to the conveying direction of the workpiece. Given a sufficiently large inclination of the separating line and, therefore, a sufficiently large inclination of the segmentation of the anodes and the insulator, almost all areas of the circuit board to be produced extend briefly above or below the insulating area of ​​each anode. In this way, the influence of the insulator on the layer thickness is balanced. In a preferred embodiment, the angle α relative to the conveying direction in the side edge area of ​​the circuit board, in particular in the area close to the fixture, should be selected to be smaller than the angle in the remote (contact member remote) area, because the voltage drop per unit length in the base layer due to the large current generated in the area close to the fixture is substantially greater than the voltage drop in the area removed from the fixture.

[0013] By increasing the number of segments, the current density across the workpiece can be made more uniform, but this is limited by the fact that the insulators between the segments also occupy some surface area. Furthermore, the associated increase in the number of rectifiers required to maintain the segments at their respective voltage levels increases the complexity and cost of the electroplating system. In practice, the achievable variation in the thickness of the coating on the workpiece is no more than 13%.

[0014] Further improvements can be achieved by influencing the electrolyte flow. In current systems with segmented anodes (in which a shielding device in the form of a perforated plate is provided between the anode and the workpiece), plugs are inserted into certain holes. However, determining which holes to plug is complex, and the actual insertion is time-consuming. The plug pattern depends on the distance between the anode and the workpiece surface. Therefore, this pattern must be determined individually for each anode through which the workpiece is passed, and if workpieces with different initial thicknesses are to be processed, a new pattern must be determined and set. Even so, the thickness variation is still not much better than 7%. Summary of the Invention

[0015] It is an object of the present invention to provide an electrode, an assembly, an electrolytic treatment apparatus, a method and a computer program which allow obtaining an improved uniformity of the current density across at least a large part of the extent of a workpiece in a first direction.

[0016] According to a first aspect, this object is achieved by an electrode according to the invention, characterized in that the segment edges between at least one pair of adjacent segments extend along respective paths, the angles of which decrease with the electrode surface edge from a common value of the coordinates to the electrode surface edge.

[0017] The electrode can be used as an anode in a cell of an electroless plating system, for example, for plating a planar workpiece in the form of a panel or foil. The electrode can also be used as a cathode in an etching system. The effect of the electrode is explained here using the example of an electroless plating system.

[0018] In this apparatus, a workpiece is fed vertically or horizontally through the electrolyte. The workpiece, having the surface to be treated, is fed past and directed toward the electrode surface, the two surfaces being substantially parallel. Non-conductive structures, such as shielding structures, may be present between the surface to be treated and the electrode surface.

[0019] At the beginning of the electroplating process, there is only a very thin conductive layer on the surface of the workpiece, for example, deposited by vapor deposition or electroless plating. The fixture is in electrical contact with the workpiece only at one or two edges (seen in a first direction transverse to the direction of movement). The resistance of the thin conductive layer is relatively large compared to the resistance of the electrolyte. Therefore, the voltage at the surface of that layer drops relatively sharply in the first direction. In the case of an unsegmented electrode (used as an anode in the electroplating example), there will be a large current density near the fixture or several fixtures. Since the current density from the workpiece surface through the electrolyte bath to the anode determines the rate at which the layer thickness increases, the non-uniformity of the current density leads to the non-uniformity of the thickness of the deposited layer of the electroplated material.

[0020] The proposed electrode is divided into segments at least at the electrode surface facing the surface to be plated. These segments are electrically isolated or weakly coupled from one another, allowing them to be maintained at different voltages by respective rectifiers. The current delivered from each segment to the workpiece surface can be individually controlled. Because the segments are arranged in close proximity in a first direction (the direction along which the voltage at the workpiece surface drops), a more uniform voltage difference across the electrolyte bath can be maintained.

[0021] Adjacent segments are separated from each other along respective segment edges.In use, the segment edges extend partially in a second direction, which is transverse to the first direction and corresponds to the direction of movement of the workpiece.

[0022] The segment edges extend from a common value of coordinates in the second direction to an edge of at least the conductive portion of the electrode surface. The common value of the coordinates may correspond to opposite edges in the second direction. In the case where the path of the segment edges consists of two segments that are mirror images of each other, they may alternatively correspond to the middle of the electrode. The segment edges will typically extend to respective endpoints at the edge of the conductive portion of the electrode. The values ​​of the coordinates at the respective endpoints in the second direction will typically deviate from the average value of the coordinates at those endpoints by less than 10%, for example, less than 5%. In most embodiments, the values ​​of the coordinates at the respective endpoints in the second direction will be identical. Thus, the edges will be substantially straight. This is typically the case with electrodes in apparatus for electrolytically treating workpieces, where the apparatus is of a type arranged to pass the workpiece across the surface of the electrodes. Otherwise, the workpiece would not be treated evenly across its width (corresponding to the first direction). Furthermore, in use, multiple electrodes of this type can then be arranged in a row in the second direction corresponding to the direction of movement of the workpiece without large, uneven gaps between consecutive electrodes.

[0023] If the segment edges extend only in the second direction—meaning they would be straight lines—the result would be lines on the workpiece surface where the gaps separating the edges of adjacent segments prevent current from flowing through the electrolyte bath. Furthermore, there would still be non-uniformity in the current density in the first direction between the coordinates of the segment edges, i.e., within the segments corresponding to the electrode segments.

[0024] The latter effect is counteracted by the fact that the segment edge between at least one pair of adjacent segments follows a corresponding path whose angle with the electrode surface edge decreases from the common coordinate value to the electrode surface edge. Because of the decreasing angle, the path is not a straight line, but rather a curve or piecewise linear curve. At each coordinate in the first direction, the workpiece traverses two segments for different corresponding durations, where the ratio between the durations varies nonlinearly to compensate for the nonlinear voltage drop in the conductive layer on the workpiece surface. As a result, the average current density is relatively uniform in the first direction. This is at least the case in the central region away from the edge, as edge effects due to electrolyte flow and workpiece contact can be a further cause of nonuniformity.

[0025] The segment edges between any pair of adjacent segments will generally have the same shape.The opposing segment edges of each segment may extend along respective paths having different shapes.

[0026] In an embodiment, at least in each half of the electrode seen in the first direction, the paths extend from the common value of the coordinate to an edge of the electrode surface along the same orientation in the first direction.

[0027] That is, the paths are all inclined in the same direction, at least within each half of the electrode as viewed in the first direction. The direction of movement of each path of an imaginary observer traveling in the first direction along the path from a point at which the second coordinate has a common value to the edge of the electrode surface has the same sign. The sign is the same along the extent of each path, that is, it is constant along the path, and this is also true for all relevant paths (all paths or all paths within each corresponding half). For applications in which the workpiece is contacted at two opposing edges, the paths extend from the common value of the coordinate to the edge of the electrode surface in the same direction in the first direction only within each half of the electrode. For applications in which the workpiece is contacted at only one edge, all paths extend from the common value of the coordinate to the edge of the electrode surface in the same direction in the first direction.

[0028] In an embodiment, the path from the common value of the coordinates to the edge of the electrode surface is a curve.

[0029] Compared to the piecewise linear curve, this embodiment achieves a more uniform average value of the current density.

[0030] In an embodiment, at least the conductive portion of the electrode surface, seen in the second direction, comprises two halves, wherein respective sections of the segment edges in one half are mirror images of respective sections of the segment edges in the other half with respect to a line of symmetry located at a common value of the coordinate.

[0031] This allows the path to have a higher inclination. That in turn helps to avoid the above-mentioned spalling effect, which is due to segments of the workpiece surface passing only or almost only through the non-conductive spaces between segments.

[0032] In an embodiment, points at the electrode surface edge on the path of a first one of the segment edges of each segment are removed from points at common values ​​of coordinates on the path of another one of the segment edges of that segment at the same coordinate value or in a first direction.

[0033] If we label the coordinates in the first direction as x-coordinates and the coordinates in the second direction as y-coordinates, then the first edge of a segment extends from point (x1, y0) at a common y-coordinate value (y0) to point (x2, y1) at the edge of the electrode surface. The second edge of the segment extends from point (x3, y0) at a common y-coordinate value y0 to point (x4, y1) at the edge of the electrode surface. In this embodiment, x3 ≥ x2. Therefore, there are no values ​​of coordinate x in the first direction where the workpiece surface passes under or over the insulation barrier between adjacent segments more than once or twice. Furthermore, each point on the workpiece surface is exposed to at most two segment voltages, thereby simplifying the configuration of the device unit including the electrode.

[0034] In an embodiment, at least within each half of the electrode seen in the first direction, the width of the segments corresponding to the distance between the edges of the segments at common values ​​of the coordinates increases from segment to segment in the first direction.

[0035] This further takes into account the fact that the voltage at the workpiece surface drops most sharply at the edge where it contacts the workpiece. If the electrode is intended for an application where the workpiece is held at two edges by a clamp that determines its voltage, then the above conditions will apply within each half of the electrode as seen in the first direction, with the width being smallest where the two halves join.

[0036] This effect is also achieved in an embodiment where, at least within each half of the electrode seen in the first direction, the angle made at the electrode surface edge of the path of a pair of segment edges between a pair of adjacent segments with the surface edge increases from one pair to the other in the first direction.

[0037] The segment edge closest to the electrode edge where electrical contact is made with the workpiece in the first direction has a relatively small inclination, while the segment edges further removed from that electrode edge have a relatively large inclination. Similarly, if the electrode is intended for an application in which a workpiece is held at two edges by a clamp that determines its voltage, then the above conditions will apply within each half of the electrode as seen in the first direction, with the angle being smallest for the pair closest to where the two halves meet.

[0038] In an embodiment, the electrode comprises a mesh electrode.

[0039] In particular, the electrode surface, and therefore the segment surface, can be formed from a mesh. This allows electrolyte to flow through the electrode. This allows for relatively uniform replenishment of the electrolyte between the electrode and the workpiece surface. This uniform replenishment can be achieved without the need for conduits or the like between the electrode and the workpiece. This, in turn, allows for a relatively uniform average current density.

[0040] In an embodiment, the electrodes are at least according to a design obtainable by performing the method according to the invention, if not obtainable by performing the method according to the invention.

[0041] According to another aspect, an assembly according to the invention for forming a cell of an electrolytic treatment plant comprises at least one electrode according to the invention.

[0042] For example, two such electrodes may be present in a cell for simultaneously processing both sides of a planar workpiece. The cell further comprises at least one device for filling the space between the workpiece surface and the electrode with electrolyte. The at least one device may be configured to circulate the electrolyte so that the electrolyte flows out of the space between the workpiece surface and the electrode through windows located at the edge of the electrode in a first direction.

[0043] An embodiment of the unit further comprises at least one shielding device extending in the first and second direction in front of an electrode surface of one of the at least one electrode.

[0044] This embodiment helps prevent contact between the electrode and the workpiece, particularly in the case of relatively thin workpieces that are supported only at one or more of their edges. The shielding device can also be used to influence the electric field between the surface of the workpiece to be processed and the surface of the electrode. The shielding device can be used, in particular, to improve the uniformity of the average current density, for example by compensating for edge effects.

[0045] In an example of this embodiment, the shielding device comprises a plate provided with a plurality of through-going channels that are permeable to liquid and distributed in the first and second directions.

[0046] The effect is that the electrolyte can flow through the shielding device. Therefore, the electrolyte between the electrode and the workpiece surface can be replenished relatively evenly without arranging a conduit or the like between the electrode and the workpiece. The distribution and / or size of the channels can be locally non-uniform in order to compensate for other non-uniformities. Locally allowing more electrolyte to pass through will reduce the bath resistance and increase the current density, thus compensating for the distortion of the electric field due to other structures or edge effects. Regular flow can be achieved when the channels are uniformly and regularly distributed at a certain pitch according to the grid. A local increase in permeability can be achieved by locally interconnecting adjacent channels. A local reduction in permeability can be achieved by locally omitting channels at certain locations on the grid.

[0047] In a particular version of this example, all paths extend in the same orientation in a first direction from a common value of the coordinate to an edge of the electrode surface, and in a strip of the plate, the entire liquid permeability area of ​​the through-channels extending in the second direction along the edge of the plate in front of the edge of the electrode surface approached by the paths as they proceed from the common value of the coordinate to the edge of the electrode surface is lower than the liquid permeability area in an adjacent parallel strip of the plate of the same width.

[0048] The total liquid permeability of a through-channel extending in the second direction along the edge of the plate, in front of the edge of the electrode surface approached by the path as the path progresses from the common coordinate value to the electrode surface edge, can be lower than the average of all parallel strips of the same width. When all paths extend in the same orientation from the common coordinate to the electrode surface edge in the first direction, the electrode is configured for use with a workpiece that is electrically contacted at only one edge. The strip at the edge of the shielding plate opposite the workpiece is relatively impermeable to liquid. At this edge, a window exists through which electrolyte can flow out of the space between the shielding device and the workpiece surface. Without relatively closed strips, a relatively high average current density would be achieved at their edges. This would result in a locally increased thickness of the layer formed in an electroplating apparatus including the assembly. In other words, the imaginary strip at the edge of the shielding plate, furthest from the edge positioned at the location where the electrode is clamped, is relatively impermeable to liquid.

[0049] In an example of an embodiment of the assembly (wherein the unit further comprises at least one shielding device extending in the first and second directions in front of the electrode surface of one of the at least one electrode, and the shielding device comprises a plate having a plurality of through-channels permeable to liquid and distributed in the first and second directions), for each segment, at least one electrical contact is arranged at a corresponding position having a coordinate in the first direction, and the total liquid-permeable area of ​​the through-channels extending in the second direction at the coordinate in the first direction in the strip of the plate is lower than the liquid-permeable area in an adjacent parallel strip of the same width.

[0050] The strips passing less electrolyte compensate for the increased average value of the current density that would otherwise be established at the coordinates in the first direction of the electrical contact.

[0051] In an example of an embodiment of the assembly (wherein the unit further comprises at least one shielding device extending in first and second directions in front of an electrode surface of one of the at least one electrodes, and the shielding device comprises a plate having a plurality of through-channels permeable to liquid and distributed in the first and second directions), the plate is secured by at least one fastener extending in a direction transverse to the plate and positioned at an associated position having a coordinate in the first direction, the fastener having a cross-section having a certain width at a surface of the plate distal to the electrode, wherein the total liquid permeability area of ​​the through-channels extending in the second direction at the coordinate in the first direction in a section of a strip of the plate having a certain width is higher than the liquid permeability area in an adjacent section of an adjacent parallel strip of the same width.

[0052] The fastener prevents the flow of electric current. This is because, even though it is made of a conductive material, it acts as an electrically insulating element. This effect is compensated by the fact that the rest of the strip, where the fastener is located, has a higher permeability to the electrolyte.

[0053] In an example of an embodiment of the assembly (wherein the unit further comprises at least one shielding device extending in the first and second directions in front of the electrode surface of one of the at least one electrodes, and the shielding device comprises a plate having a plurality of through-channels permeable to liquid and distributed in the first and second directions), the entire liquid permeable area in a strip of the plate extending in the second direction along the edge of the plate in front of the edge of the electrode (from which the paths diverge as they proceed from the common value of the coordinates to the electrode surface edge) is higher than the liquid permeable area in an adjacent parallel strip of the plate of the same width.

[0054] The total liquid permeability area of ​​the through-channel extending in the second direction along the edge of the plate in the strip of the plate in front of the edge of the electrode surface approached by the path as the path progresses from the common value of the coordinate to the edge of the electrode surface may be lower than the average of all parallel strips of the plate of the same width.

[0055] In the strip of the plate, the total liquid permeability of the through-channels extending in the second direction along the edge of the plate ahead of the edge of the electrode (from which the paths diverge as they proceed from the common coordinate value to the electrode surface edge) can be particularly higher than the average of all parallel strips of the plate of the same width. This allows more electrolyte to pass through the plate at the edge of the shielding device ahead of the edge of the electrode where the workpiece is electrically contacted. This promotes current flow through the workpiece surface, as opposed to current flow directly from the electrode to a fixture or similar device that electrically contacts the workpiece.

[0056] In an example of an embodiment of the assembly (wherein the unit further comprises at least one shielding device, the at least one shielding device extending in the first and second directions in front of the electrode surface of one of the at least one electrode, and the shielding device comprising a plate having a plurality of through-channels permeable to liquid and distributed in the first and second directions), the plate is made of an electrically insulating material.

[0057] This simplifies, among other things, the mounting of the plate. The fasteners used to mount the plate will generally have to extend at least between the plate and the electrode at a position remote from the edge. The fasteners may be made of an electrically conductive material.

[0058] Embodiments of the assembly further comprise at least one further electrode extending in the second direction along an edge of one of the at least one electrode and in a third direction transverse to the first and second directions.

[0059] The further electrode extends in a second direction and transversely to the first and second directions. This further electrode can, in particular, be positioned at an edge of a segmented electrode facing an edge of a workpiece, such as where the workpiece is electrically contacted by one or more clamps. In the case where the segmented electrode functions as an anode, the further electrode is also arranged to function as an anode and is also referred to herein as a clamp anode. A clamp anode is a structure whose dimensions in the third and second directions are an order of magnitude (ten or even a hundred times) larger than its dimensions in the first direction. In use, the clamp anode carries a controlled current to influence metal deposition on the workpiece near the edge where the clamp contacts the workpiece. Because the clamp may not be fully shielded, some current from the segmented anode will otherwise flow to the clamp, as opposed to the workpiece. On the one hand, the clamp anode prevents current from flowing from the segmented anode to the edge strip of the clamp and workpiece. On the other hand, since current flows from the segmented anode to the clamp rather than the workpiece, the clamp anode compensates for any reduction in metal deposition. A similar effect is achieved in embodiments where the segmented electrode and the further electrode function as cathodes and the workpiece is contacted at the edge to function as an anode.

[0060] In a specific example of this embodiment, an electrically insulating shield is provided between the further electrode and the segmented electrode.This electrically insulating shield may take the form of a surface layer on the surface of the further electrode facing the segmented electrode.

[0061] According to another aspect, the electrolytic treatment plant according to the invention comprises at least one treatment unit comprising at least one assembly according to the invention.

[0062] As mentioned, the electrolytic treatment apparatus may be used for plating or etching, ie for building up or destroying a layer of conductive material on the surface of the workpiece.

[0063] Embodiments of the electrolytic processing apparatus include a plurality of processing cells and a delivery system for delivering workpieces through and between the cells.

[0064] The delivery system may be a vertical delivery system in which the surface of the workpiece extends generally vertically, or a horizontal delivery system in which the surface of the workpiece extends generally horizontally as the workpiece moves through and between the units.

[0065] In an example of this embodiment, the delivery system includes at least one clamp for releasably holding a planar workpiece at an edge of the planar workpiece while delivering the workpiece through and between the units.

[0066] The workpiece can remain unsupported in an area removed from the edge where the workpiece is held by at least one fixture. This helps prevent wear on the workpiece surface. Processing uniformity is also improved because there is no supporting structure between the workpiece and the electrode, except at the edge or edges of the workpiece where the workpiece is held by at least one fixture. Therefore, even if the delivery system is a horizontal delivery system, the workpiece can be processed on both sides in one unit. The delivery system can use more than one fixture per workpiece. At least one fixture can be mounted on an endless chain or belt. Each fixture can be automatically closed at the first unit and automatically disengaged from the workpiece at the last of a series of units through which the workpiece is delivered. The delivery system may include fixtures for holding the workpiece at two opposite edges in a first direction (i.e., a direction transverse to the direction of movement). In that case, the feed point to which the current is applied can be a mirror image relative to the line of symmetry of the workpiece.

[0067] In a particular version of this example, at least one of the at least one clamp includes an arm including a conductive portion for electrically contacting the workpiece when pressed against the major surface of the workpiece such that the workpiece can serve as an electrode.

[0068] The workpiece is thus both held at a specific electrical potential and conveyed through the apparatus.Because a clamp is used, the workpiece is electrically contacted at the surface to be treated.

[0069] In a particular example, at least an end section of an arm for engaging the workpiece includes, in addition to a surface section for engaging a major surface of the workpiece, a conductive core portion covered by an electrically insulating shield.

[0070] This helps avoid coating or flaking of the arms and facilitates processing of the workpiece surface by forcing the current to flow through the conductive layer on the workpiece surface, where this surface is held by the clamp.

[0071] According to another aspect, the method according to the invention comprises at least a step of designing an electrode according to the invention, wherein said designing step comprises determining the shape of said path.

[0072] The electrode can thus be adapted to the configuration of the processing unit in which it is to be used. For example, in a multi-unit apparatus, the path shape can differ between electrodes used in different units. The path shape can be determined, inter alia, based on at least one of the following: the number of segments, the extent of the electrode surface in a first direction, the resistivity of the electrolyte, the distance between the workpiece and the electrode surface, the resistivity and thickness of the conductive layer at the workpiece surface, and the extent of the electrode surface in a second direction.

[0073] In an embodiment of the method, determining the shape of the path comprises determining coefficients of a polynomial (eg, a quadratic polynomial) for the coordinates in the first direction, the polynomial representing the coordinates in the second direction.

[0074] In a plan view onto the electrode surface, the path from the common value of the coordinate in the second direction to the edge of the electrode surface will therefore be based on at least a polynomial, such as a segment of a parabola. A further step of the design step may include superimposing the deviations onto the parabola or high-degree polynomial. The process of determining the coefficients may be an iterative process.

[0075] In an embodiment, the coefficient is obtained by calculating a voltage drop function that is a function of a coordinate in the first direction and represents a voltage variation along the surface of the workpiece in the first direction.

[0076] The voltage drop function may be a quadratic polynomial function. The coefficients may correspond to coefficients of the voltage drop function scaled by the dimension of the electrode surface in the first direction and divided by the voltage difference between adjacent segments whose edges extend along the path whose shape is to be determined.

[0077] Embodiments of the method further include fabricating the electrode according to the design.

[0078] According to another aspect, a computer program according to the invention comprises instructions which, when said program is executed by a computer, cause said computer to carry out the steps of the method according to the invention.

[0079] The computer program may be embodied in one or more computer-readable non-transitory storage media. BRIEF DESCRIPTION OF THE DRAWINGS

[0080] The present invention will be described in further detail with reference to the accompanying drawings, in which:

[0081] Figure 1 is a typical schematic top plan view of an electrolytic treatment apparatus;

[0082] Figure 2 is a cross-sectional detail of a gripper arm for contacting a workpiece conveyed through an electrolytic processing apparatus;

[0083] Figure 3 is a schematic plan view of the surface of an anode of a unit for an electrolytic treatment apparatus;

[0084] Figure 4 corresponds to Figure 3 But to the plan view on the opposite side of the anode;

[0085] Figure 5 is a plan view of a shielding device for placement between the anode and the workpiece;

[0086] Figure 6 is a detailed view of a section of the shielding device;

[0087] Figure 7 is a diagram illustrating steps in a method for obtaining an anode;

[0088] Figure 8 It is an explanation Figure 7 a diagram of an embodiment of one of the steps of;

[0089] Figure 9 is a graph illustrating the voltage difference between the anode and the segments of the workpiece, the voltage drop in the electrolyte bath between the workpiece and the anode, and the voltage drop in the conductive layer on the surface of the workpiece facing the anode;

[0090] Figure 10 is a schematic plan view of one half of an anode to illustrate how the segment edge shape is determined;

[0091] Figure 11 is a diagram showing the first stage of determining the average value of the target current density of the segment;

[0092] Figure 12 is a graph showing the results of determining the average value of the target current density; and

[0093] Figure 13 Is demonstrated through simulation including Figure 3 、 4 and anodes of the type described in 10 and as Figure 5 and 6 A graph showing the percentage deviation of the current density at locations along the length of the workpiece from the average current density calculated using the shielding device unit shown. DETAILED DESCRIPTION

[0094] Electroplating apparatus 1 includes several processing units 2a to d for electroplating planar workpieces 3a to f. Planar workpieces 3a to f may be, for example, foils or panels made primarily of dielectric material. Surfaces parallel to the plane of the workpieces are referred to herein as major surfaces. At least one of these major surfaces is to be plated by apparatus 1. This includes plating the sidewalls of any vias through the workpieces 3a to f or the sidewalls of grooves in the workpieces 3a to f.

[0095] Here only the electrolytic plating apparatus 1 is described and illustrated. This apparatus 1 will previously typically be an apparatus for carrying out preliminary processing steps (comprising ablation, desmearing, ion activation and electroless deposition) to form a conductive precursor layer on the workpieces 3a to f.

[0096] It is convenient to define a first direction x, the dimension of the workpieces 3a to f also referred to herein as width. A second direction y transverse to the first direction x corresponds to the direction of movement of the workpieces 3a to f through the apparatus 1 .

[0097] Apparatus 1 includes a housing 4 that defines a circulating electrolyte bath. Rollers 5a-c support workpieces 3a-f until an entry point into housing 4, where rollers 5a-c are engaged by a delivery system 6, schematically shown as comprising a series of clamps 7 for engaging the major surfaces of workpieces 3a-f at proximal edges 8a-f. Distal edges 9a-d are positioned at what is herein referred to as the window of each cell 2a-d, where the electrolyte flows out of the cells 2a-d. In the illustrated embodiment, workpieces 3a-f are not held at distal edges 9a-d. Workpieces 3a-f are also not supported by any solid structure between edges 8, 9. However, workpieces 3a-f are immersed in the electrolyte. In alternative embodiments, support elements may be provided. Workpieces 3a-f may also be clamped on both sides, as viewed in a first direction x.

[0098] The clamps 7 automatically engage the workpieces 3a to f as they enter the enclosure 4 and disengage as they leave the enclosure 4. The clamps 7 are supported on an endless belt 10, which may be a belt or chain having a toothed profile, for example driven by one or more drums 11a, b around which the endless belt 10 is arranged and supported by the drums 11a, b.

[0099] It should be noted that Figure 1 In a practical embodiment, the clamp 7 will extend into the cells 2a to d so that the workpieces 3a to f protrude only slightly or not at all at their proximal edges 8a to f.

[0100] The clamp 7 comprises an arm 12 ( Figure 2 The arm 12 comprises an electrically conductive core portion 13 covered (except for a surface section 15 for engaging a major surface of the workpiece 3) by an electrically insulating shield 14. The or each clamp 7 engaging the workpiece 3 forms part of an electrical circuit comprising the workpiece 3 acting as a cathode and an anode 16.

[0101] The arrangement is mirrored in units 2 for electroplating both major surfaces of a workpiece 3. The present discussion will focus only on those components for electroplating one major surface of a workpiece 3, which in the illustrated embodiment faces upwards.

[0102] The anode 16 of the example comprises two layers 17, 18 ( Figure 2 In alternative embodiments, there may be one layer or even multiple layers. At least the lower layer 18, located proximal to the workpiece 3, comprises a mesh section. The mesh is permeable to the electrolyte. The upper layer 17 may also be a mesh layer or, as in the illustrated example, a layer made of perforated plate sections. The electrolyte can thus flow through the anode 16 toward the workpiece 3.

[0103] A shielding device, including a shielding plate 19 made of an electrically insulating material and having a passage therethrough, is positioned between the anode 16 and the workpiece 3. The shielding plate 19 serves to prevent short circuits caused by contact between the workpiece 3 and the anode 16. In some embodiments, the shielding plate 19 may be omitted. The shielding plate 19 extends in a first direction x and a second direction y in front of the anode surface facing the workpiece 3. The shielding plate 19 may be substantially coextensive with the anode 16. In the illustrated embodiment, minor deviations exist, as will be explained.

[0104] In the illustrated embodiment, the fixture anode 20 ( Figure 2 ) along the edge 22 close to the clamp 7 in the second direction y ( Figure 3 and 4 ) and extends in a third direction z, transverse to the first direction x and the second direction y. The fixture anode 20 is provided with a separately controlled current supply (not shown in detail). The fixture anode 20 is arranged to prevent current from flowing from the anode 16 to the fixture 7 or to a region of the workpiece 3 located at the edge of the workpiece 3 where the fixture 7 contacts the workpiece 3. Furthermore, the fixture anode 20 compensates for metal deposition on the fixture 7 rather than on the workpiece 3 by providing additional current flow to the workpiece 3. This further contributes to the uniformity of the layer formed on the workpiece 3. If the workpiece 3 is a printed circuit board, the fixture anode 20 provides a plated edge region, typically up to 25 mm wide, which is required for contacting in subsequent processing stages.

[0105] In an embodiment, the surface 21 of the clamp anode 20 that faces the anode 16 is covered with an electrically insulating material. This is useful because the current from the clamp anode 20 is controlled independently of the current from the anode 16 so that a potential difference can exist between the two currents.

[0106] At least the layer 18 of the anode 16 whose surface faces the workpiece 3 is divided into segments 23a to e. Adjacent segments 23a to e are separated from each other along respective segment edges 24a to h ( Figure 3 ). Segment edges 24a to e between adjacent segments 23 form pairs. The pairs may be separated by gaps or by electrically insulating material. The width of the gaps or separating strips of electrically insulating material imposes a limit on the number of segments 23a to e that can be provided, but does not necessarily define a maximum number of segments 23a to e.

[0107] In any case, the spacing means that the segments 23a to e are electrically isolated from each other. Since the electrolyte between the workpiece 3 and the anode 16 and the conductive starting layer on the surface of the workpiece 3 are conductive, there is a small coupling. The segments 23a to e are separated in such a way as to allow adjacent segments 23a to e to be maintained at different respective voltages. The coupling is below the range required to be controlled to apply an adjustable current to each individual segment 23a to e. The voltage difference between each segment and the clamp 7 can be independently controlled by an associated respective rectifier (not shown). This voltage difference will be referred to as the anode clamp voltage Uc i , where i is the number of segments 23 counted starting from the segment 23 a close to the clamp 7 in the first direction x.

[0108] Segment edges 24a-e extend partially in the second direction y from a common y-coordinate value y0 to a first electrode edge 25 extending in the first direction x. In the illustrated embodiment, segment edges 24a-e also extend partially in the second direction y in opposite directions from a common y-coordinate value y0 to a second electrode edge 26 extending in the first direction x. Thus, first and second electrode edges 25, 26 are opposing edges. Line of symmetry 27 is located at the common y-coordinate value y0. As viewed in the second direction y, anode 16 can be considered to include two halves 28, 29.

[0109] Segment edges 24a to e extend along respective paths whose angles with first electrode edge 25 decrease from a common y-coordinate value y0 to first electrode edge 25. Also, their angles with second electrode edge 26 decrease from a common y-coordinate value y0 to second electrode edge 26.

[0110] The segments of the segment edges 24a to e in the first half 28 of the first and second halves 28, 29 extend from a point at a common value y0 of the y-coordinate to the first electrode edge 25 along the same orientation in the first direction x, i.e., the value of the x-coordinate increases along the path toward the first electrode edge 25. The segments of the segment edges 24a to e in the second half 29 extend from a point at a common value y0 of the y-coordinate to the second electrode edge 26 along the same orientation in the first direction x, i.e., the value of the x-coordinate increases along the path toward the second electrode edge 26.

[0111] In the illustrated embodiment, the path of the segment edges 24a to e is a curve. In other embodiments, it can be a piecewise linear curve.

[0112] In the illustrated embodiment, a point at the first electrode edge 25 on the path of a first one of the segment edges 24a to h of each segment 23a to e has the same x-coordinate as or a smaller x-coordinate value than a point at the common value y0 on the path of another one of the segment edges 24a to h of that segment 23a to e. Taking the third segment 23c as an example ( Figure 3), first edge 24d extends from point (x1, y0) to point (x2, y1). Second edge 24e extends from point (x3, y0) to point (x4, y1), where x4 ≥ x3. Thus, each point on the surface of workpiece 3 faces at most two electrode segments 23a to e.

[0113] Counting segments 23a through e starting from proximal electrode edge 22, the width of segments 23a through e, corresponding to the distance between segment edges 24a through 24h at a common y-coordinate value y0, increases from segment to segment in the x-direction. The gradual widening of segments 23a through e reflects the fact that, when contact is made only at proximal electrode edge 22, the voltage at the surface of workpiece 3 changes most steeply in the x-direction at that edge 22.

[0114] The segment edges 24a to e also become progressively more curved in the x-direction. In other words, the angle at the first electrode edge 25 of the path of a pair of segment edges 24a to h between a pair of adjacent segments 23a to e increases in the x-direction from one pair to the other (the shape of the path of the segment edges 24a to h forming this pair is substantially the same). This applies mutatis mutandis to the angle with the second electrode edge 26.

[0115] The shielding plate 19 has a plurality of through-going channels that are generally regularly distributed, wherein some adjacent channels are interconnected to form a single channel having a larger cross-sectional area and channels are omitted at certain locations (see Figure 6 ).

[0116] from Figure 4 As will be apparent from the top view of the anode 16, electrical contacts 30a through f extend into the lower layer 18 to contact segments 23a through e. Electrical contacts 30a through f are positioned at corresponding locations with corresponding x-coordinates. The total channel area in the strip of shielding plate 19 extending in the second direction y at the corresponding x-coordinate is smaller than the total channel area in the adjacent parallel strip of the same width. This width will generally approximate the width of electrical contacts 30a through f. Thus, the tendency for current to flow directly to the locations of electrical contacts 30a through f is counteracted.

[0117] In a similar manner, the shield plate 19 is secured by at least one fastener 31a to g (for clarity, in FIG. Figure 5Only some fasteners are shown in the figure. Fasteners 31 have a cross-section with a certain width at the surface of shielding plate 19 located distally from anode 16. The total cross-sectional area of ​​the channel extending at the x-coordinate in the second direction y in a section of a strip of shielding plate 19 having a certain width is higher than the total cross-sectional area of ​​the channel in an adjacent section of an adjacent parallel strip of the same width. In other words, permeability increases in the strip sections on either side of the location where fasteners 31 are attached to shielding plate 19 to compensate for the fact that fasteners 31 behave as non-conductive elements (despite being made of a conductive material).

[0118] The shielding plate 19 is also configured to compensate for edge effects.

[0119] In use, the proximal shield edge 32 ( Figure 5 ) has an irregular shape. This is to increase the total liquid-permeable area in the strip of plate extending along that proximal shield plate edge 32 in the second direction y relative to the corresponding total liquid-permeable area in adjacent parallel strips of the same width. Otherwise, the current density along the edge of the workpiece 3 would decrease. While this decrease is not a problem in principle, a localized decrease causes an increase in the adjacent strips of the workpiece 3. This is avoided by increasing the permeability at the proximal shield plate edge 32. Because the channels are of equal size and regularly distributed (with the same pitch), the result is an irregular proximal shield plate edge 32.

[0120] The distal shield edge 33 is configured to counteract the sharp decrease in current density, particularly if the workpiece 3 has a smaller extent in the first direction x than the anode 16 and the shield 19. The total liquid permeability of the channels extending along the distal shield edge 33 in the strip of shield 19 in the second direction y is lower than the total liquid permeability of the channels in the adjacent parallel strip of the same width. This helps avoid the formation of ribs of electroplated material along the corresponding distal edge 9 of the workpiece 3.

[0121] In the method of obtaining the anode 16, the intervals between adjacent segments 23a to e are ignored, as shown in FIG. Figure 9 and 10 Each edge segment 24a to h is a quadratic polynomial. As seen in the first direction x, the point at the first electrode edge 25 of each edge segment 24a to h (except the last edge) is at the same coordinate value x as the point at the common value y0 of the next edge segment 24a to h. The number of segments 23a to e and the size of the anode 16 are also fixed. Within these constraints, it remains to find the coefficients of the quadratic polynomial defining the edge segments 24a to h, as well as the potential difference Uc relative to the fixture. i , where i indicates the number of segments 23a to e counted starting from the proximal segment 23a in the first direction x.

[0122] The potential difference across the bath at the center of the ith segment, as seen in a first direction x, is Umb i The voltage difference between the corresponding position in the surface layer on the workpiece 3 and the position of the fixture is Um i , assuming the fixture is at the origin, that is, x = 0. Figure 9 , we obtain the following equation:

[0123] Umb=Uc-Um (1),

[0124]

[0125] Dashed line graph ( Figure 9 ) shows the voltage target distribution. It should be noted that Um is simply the average voltage in the segment of the surface layer on the workpiece 3 that is opposite to a specific one of the segments 23a to e. The voltage drop in the surface layer is a quadratic polynomial.

[0126] In the first step 34 of the design process ( Figure 7 ), design parameters are obtained. These parameters include the thickness of the conductive material layer on the workpiece 3, the dimensions of the workpiece 3 in the first direction x and the second direction y, the resistivity of the electrolyte, the distance between the surface of the workpiece 3 and the surface of the anode 16, and the resistivity of the conductive material on the workpiece 3. A further requirement is the nominal current density average value, which is the average value across the area of ​​the anode 16. Based on this result, the target current density average value for each segment 23a to e is calculated according to the formula:

[0127] CDA[i]=m·i p +n (3),

[0128] where i is the number of segments, p is a fixed value determined empirically and the values ​​of m and n are determined by trial and error with the nominal current density averaged for the final segment (e.g., i=5 in the illustrated embodiment) and a specific value for the first segment (i=1). Figure 11 and 12 This process is described in . Figure 11 Shows the results of taking an excessively large value for the average current density in the first section of CDA [1]. Figure 12 The results of adjusting this value downward to an appropriate value are shown. The value of the average value of the current density of all other segments 23a to e is obtained using equation (3).

[0129] In a next step 35 , the shape of the path of the segment edge is determined.

[0130] like Figure 8 As explained in , this step 35 involves initializing (step 36) and calculating (step 37) the target current density average value for each segment 23a to e according to equation (3).

[0131] This is followed by a series of iterative steps.

[0132] First (step 38), the current density average is calculated for each segment 23a to e. This involves dividing the first half 28 into narrow strips extending from the proximal electrode edge 22 to the opposite edge in the first direction x, each strip having a relatively small dimension in the second direction y. Using the voltage drop function and the segment voltage Uc i The current contribution of each segment 23a to e can be calculated for that narrow strip. The contributions of all narrow strips are then summed to find the current of each segment 23a to e, which is divided by the area of ​​that segment. The resulting value is compared with the target value and the value Uc is adjusted. i The calculations (steps 39, 38) are repeated to bring the average current density of segments 23a to e closer to the target value or until another stopping criterion is met (eg, a certain number of iterations).

[0133] Next (step 40), the segment edges 24a to h are adjusted.

[0134] Figure 10 The first half 28 of the anode 16 is shown. The dashed line corresponds to the path that a point on the workpiece 3 faces when the workpiece 3 moves in the second direction y. During the electroplating process, the amount of deposited metal is proportional to the charge Q. The charge Q is defined by the current I multiplied by the time t:

[0135] Q=I·t (3).

[0136] Assume that the velocity v of workpiece 3 is constant:

[0137] v=LIt, (4)

[0138] where L is the dimension in the second direction y of the first half 28 of the anode 16. At each position in the first direction x, the time t is the same, so that the charge is the product of the current I and the length L of each point on the workpiece 3 that moves through only one segment 23a to e.

[0139] In order to achieve equal metal deposition at each location x[i] in the first direction x, the collected charge Q must be the same. This leads to the following constraints:

[0140] L S5，x[i] I S5，x[i] +L S4，x[i] I S4，x[i] =Q[i]·v,

[0141] L S5，x[i+1] I S5，x[i+1] +L S4，x[i+1] I S4，x[i + 1]=Q[i+1]·v,

[0142] L S5，x[i+2] I S5，x[i+2] +L S4，x[i+2] I S4，x[i+2] =Q[i+2]·v,

[0143] Where v and Q are constants.

[0144] Anode segments 23a to e are divided into narrow strips of equal size extending in the y-direction. Each strip extends through two adjacent segments 23a to e. Because segments 23a to e are at different voltages, the local currents entering the workpiece 3 are also different. The currents along the strips are summed, reflecting the fact that the workpiece 3 passes in front of the entire anode 16.

[0145] The conductive layer on the workpiece 3 is modeled as a one-dimensional resistor chain, each resistor having a length in the x-direction corresponding to the distance between one strip and the next. This allows us to model the current entering at the nodes of the resistor chain. From this result, the voltage drop allows the calculation of a new voltage drop function. This function is a quadratic polynomial, as mentioned. The coefficients of the polynomial determine the shape of the segment edges 24a to h, which is a corresponding quadratic polynomial. Using the new shape of the segment edges 24a to h obtained in the second step 40, the method returns to the segment voltage Uc i Calculation.

[0146] The iterations are repeated until a discontinuation criterion is met (e.g., a fixed number of iterations, a specified maximum deviation of the current density average from a target value, etc.). In one particular embodiment, the discontinuation criterion is that the respective current contributions of the strips defined in step 40 of adjusting the segment edges 24a to h are equal (or differ by less than a predetermined maximum allowable deviation).

[0147] In the optional further step 41 ( Figure 7 ), the current density is calculated by simulation across the surface of the workpiece 3. Next, the permeability of the shielding plate 19 is locally adjusted (optional step 42) to reduce the deviation of the current density from the average value. This takes into account the spacing between adjacent segments 23a to e, which is ignored when calculating the shape of the segment edges 24a to h. Both steps 41 and 42 are iteratively performed to achieve the optimal pore distribution of the shielding plate 19.

[0148] Finally (step 43), the anode 16 is manufactured according to the design.

[0149] Simulations of the anode 16 designed in this process show that the deviation from the average current density remains within 5% across the range of the workpiece 3 in the first direction x ( Figure 13 ), except for small strips at the edges 8 and 9.

[0150] The invention is not limited to the embodiments discussed above, which may vary within the scope of the appended claims. For example, an improvement in the uniformity of the current density may be achieved without the shielding plate 19 discussed above.

[0151] Component Symbol List

[0152] 1 Equipment

[0153] Units 2a-d

[0154] 3a-f Workpiece

[0155] 4. Cladding

[0156] 5a-c Rollers

[0157] 6 Delivery System

[0158] 7. Fixture

[0159] 8a-f Near side workpiece edge

[0160] 9a-d Far side workpiece edge

[0161] 10 belts

[0162] 11a,b drums

[0163] 12 arms

[0164] 13 core part

[0165] 14 core partial shield

[0166] 15 Core surface section

[0167] 16 Anode

[0168] 17 Upper

[0169] 18 Lower Level

[0170] 19 Shielding plate

[0171] 20 fixture anode

[0172] 21. Anode surface of fixture

[0173] 22 Proximal electrode edge

[0174] Paragraphs 23a-e

[0175] 24a-h segment edge

[0176] 25 First electrode edge

[0177] 26 Second electrode edge

[0178] 27 Symmetry Line

[0179] 28 First Half

[0180] 29 Second Half

[0181] 30a-f electrical contacts

[0182] 31a-g Fasteners

[0183] 32 Near shield edge

[0184] 33 Far shield edge

[0185] 34 Steps (Obtaining Design Parameters)

[0186] 35 steps (determine the path shape)

[0187] 36 steps (initialization)

[0188] 37 steps (calculate the average target current density for each segment)

[0189] 38 Steps (Determine the actual average current density of each section)

[0190] 39 steps (adjusting the segment voltage)

[0191] 40 steps (determine the edge shape of the new segment)

[0192] 41 steps (run simulation)

[0193] 42 steps (shield optimization)

[0194] 43 steps (manufacture the anode according to the design).

Claims

1. An electrode of an apparatus (1) for electrolytically treating a workpiece (3), said apparatus (1) being of a type arranged to convey said workpiece (3) having a surface to be treated past and directed towards a surface of said electrode, wherein the electrode is divided into segments (23a to e) at at least this surface of the electrode, wherein the segments (23a to e) are arranged next to each other in a first direction (x), wherein adjacent segments (23a to e) are separated from one another along respective segment edges (24a to f) so as to allow adjacent segments (23a to e) to be maintained at different respective voltages, and wherein in use, the segment edges (24a to f) extend at least partially in a second direction (y) from a common value (y0) of a coordinate in the second direction (y) to an edge (25, 26) of at least a conductive portion of the electrode surface, the second direction (y) being transverse to the first direction (x) and corresponding to a direction of movement of the workpiece, wherein the width of the segments (23a to e) corresponding to the distance between the edges (24a to h) of the segments (23a to e) at the common value (y0) of the coordinate increases from segment (23a to e) to another segment (23a to e) so that the segments (23a to e) gradually widen with distance from the electrode edge (22) in the first direction (x), or Wherein this condition applies to each half of the electrode as seen in the first direction (x), it is characterized in that The segment edges (24a to f) between at least one pair of adjacent segments (23a to e) extend along respective paths, the angles formed by the respective paths with the electrode surface edges (25, 26) decreasing from the common value (y0) of the coordinate to the electrode surface edges (25, 26).

2. The electrode according to claim 1, wherein at least within each half of the electrode as seen in the first direction (x), the path extends from the common value of the coordinate (y0) to the electrode surface edge (25, 26) along the same one of the opposite directions in the first direction (x), so that the paths are all inclined in the same direction, at least within each half of the electrode as seen in the first direction (x).

3. The electrode according to claim 1 or 2, wherein the path from the common value of the coordinates (y0) to the electrode surface edge (25, 26) is a curve.

4. The electrode according to claim 1 or 2, wherein at least the conductive portion of the electrode surface comprises two halves (28, 29) as seen in the second direction (y), The respective sections of the segment edges (24a to f) in one half (28, 29) are mirror images of the respective sections of the segment edges (24a to f) in the other half (28, 29) relative to a line of symmetry (27) located at the common value (y0) of the coordinates.

5. The electrode according to claim 1 or 2, wherein a point at the electrode surface edge (25, 26) on the path of a first of the segment edges (24a to f) of each segment (23a to e) is at the same coordinate value in the first direction (x) as or removed from a point at the common value (y0) of the coordinates on the path of another of the segment edges (24a to f) of that segment (23a to e).

6. The electrode according to claim 1 or 2, wherein the angles made at the electrode surface edges (25, 26) of the path of a pair of segment edges (24a to f) between a pair of adjacent segments (23a to e) with the surface edges increase with distance from the electrode edge (22) in the first direction (x) from one pair to the other, or This condition applies within each half of the electrode, as seen in the first direction (x).

7. An assembly of cells (2a to e) for forming an electrolytic treatment apparatus (1), wherein the assembly comprises at least one electrode (16) according to any one of the preceding claims.

8. The assembly according to claim 7, further comprising at least one shielding device extending in the first and second directions (x, y) in front of the electrode surface of one of the at least one electrode (16).

9. The assembly according to claim 8, The shielding device comprises a plate (19) having a plurality of through-channels which are permeable to liquid and distributed in the first and second directions (x, y).

10. An electrolytic treatment plant comprising at least one treatment unit (2a to e) comprising at least one assembly according to any one of claims 7 to 9.

11. A method comprising at least the computer-implemented step of designing an electrode (16) according to any one of claims 1 to 6, The designing step comprises determining a shape of the path (35).

12. The method of claim 11, wherein the designing step comprises: Obtain design parameters (34); determining a shape of the path (35); Initialize(36); Calculate the average target current density of each segment (37); Determine the actual average current density of each section (38); Adjust segment voltage (39); Determine the edge shape of the new segment (40); executing a simulation (41); and Optimize the shielding plate (42).

13. The method according to claim 11, wherein determining the shape (35) of the path comprises determining respective coefficients of a polynomial representing the coordinates in the first direction (x), the polynomial representing the coordinates in the second direction (y), and wherein each path from the common value (y0) of the coordinate in the second direction, as seen in a plan view on the electrode surface, corresponds to the polynomial with a corresponding set of coefficients, optionally with a superposition bias. The method of claim 13 , wherein the polynomial is a quadratic polynomial.

15. The method according to claim 13, The coefficient is obtained by calculating a voltage drop function, which is a function of the coordinate in the first direction (x) and represents a voltage change along the surface of the workpiece (3) in the first direction (x).

16. A computer program comprising instructions which, when executed by a computer, cause the computer to carry out the design steps of the method according to any one of claims 11 to 15.

17. The computer program of claim 16, wherein the designing step comprises: Obtain design parameters (34); determining a shape of the path (35); Initialize(36); Calculate the average target current density of each segment (37); Determine the actual average current density of each section (38); Adjust segment voltage (39); Determine the edge shape of the new segment (40); executing a simulation (41); and Optimize the shielding plate (42).

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