Electroplating method, electroplating apparatus

Abstract

To form a plating film that is more uniform than in the prior art.SOLUTION: A plating film is formed on at least a part of the surface of an object, using multiple anodes (12) which are individually current-controlled while facing an object (X) immersed in an electrolyte (E) in a plating tank (6) through the electrolyte. A control unit (26) controls the formation of the plating film onto the object by controlling an electric current applied to the anode. The control unit determines a distance D between an anode and another adjacent anode and a distance H between the anode and the object for each anode so that a distance W, which is the shortest distance between the film thickness maximum range regarding the film thickness of the plating film and the film thickness minimum range, whose film thickness is thinner than that of the film thickness maximum range, is the closest value to a target value.SELECTED DRAWING: Figure 1

Description

Technical Field

[0001] The present disclosure relates to an electroplating method and an electroplating apparatus.

Background Art

[0002] Patent Document 1 discloses an apparatus for electroplating, which includes a plurality of anodes.

Prior Art Documents

Patent Documents

[0003]

Patent Document 1

Summary of the Invention

Problems to be Solved by the Invention

[0004] Generally, a plating film formed by electroplating is required to have homogeneity.

Means for Solving the Problems

[0005] An electroplating method according to an aspect of the present disclosure is an electroplating method for forming a plating film on at least a part of a surface of an object by using a plurality of anodes that are individually current-controlled and face the object through an electrolytic solution in a plating bath, the method including: obtaining a target value of a distance W, which is the shortest distance from a thick film thickness maximum range in the plating film to a thin film thickness minimum range having a film thickness thinner than the film thickness in the film thickness maximum range; determining, for each of the anodes, a distance D between the anode and another adjacent anode and a distance H between the anode and the object such that the distance W in the plating film is closest to the target value; and forming the plating film by using the anodes that satisfy the distance D and the distance H.

[0006] Another aspect of the present disclosure relates to an electroplating apparatus including a plating bath, a plurality of anodes located inside the plating bath and individually current-controlled, and a control unit configured to control a current applied to the anodes to control formation of a plating film on at least a part of a surface of an object immersed in an electrolytic solution in the plating bath using the plurality of anodes facing the object through the electrolytic solution. The control unit further obtains a target value of a distance W, which is the shortest distance from a thick film thickness maximum range in the plating film to a thin film thickness minimum range having a film thickness smaller than that in the film thickness maximum range, and determines, for each anode, a distance D between the anode and another adjacent anode and a distance H between the anode and the object such that the distance W in the plating film is closest to the target value.

[0007] Another aspect of the present disclosure relates to an electroplating method for forming a plating film on at least a part of a surface of an object immersed in an electrolytic solution in a plating bath using a plurality of anodes facing the object through the electrolytic solution and individually current-controlled. For each anode, a distance D between the anode and another adjacent anode and a distance H between the anode and the object satisfy D ≦ 2H.

[0008] Another aspect of the present disclosure relates to an electroplating apparatus including a plating bath, a plurality of anodes located inside the plating bath and individually current-controlled, and a control unit configured to control a current applied to the anodes to control formation of a plating film on at least a part of a surface of an object immersed in an electrolytic solution in the plating bath using the plurality of anodes facing the object through the electrolytic solution. The control unit determines the distance D and the distance H such that D ≦ 2H for each anode. [Effect of the Invention]

[0009] A plating film more uniform than in the prior art can be formed. [Brief Description of the Drawings]

[0010]

Figure 1

Figure 2

Figure 3

Figure 4

Figure 5

Figure 6

Figure 7

Figure 8

Figure 9

Figure 10

Figure 11

Figure 12

Figure 13

Figure 14

Mode for Carrying Out the Invention

[0011]

Embodiment 1

[0012] The electroplating apparatus 2 shown in FIG. 1 includes a liquid tank 4. For example, the liquid tank 4 includes a plating tank 6 filled with the electrolytic solution E and a circulation tank 10 separated from the plating tank 6 by a partition wall 8. In the present embodiment, the liquid tank 4 is designed such that the electrolytic solution E exceeding the volume of the plating tank 6 overflows into the circulation tank 10 over the partition wall 8.

[0013] The electroplating apparatus 2 includes a plurality of anodes 12. For example, each of the anodes 12 has a flat plate shape and is two-dimensionally arranged on one surface of an anode support 14 located inside the plating tank 6. Therefore, when the plating tank 6 is filled with the electrolytic solution E, the anode 12 is immersed in the electrolytic solution E. For example, the electroplating apparatus 2 may include two anode supports 14 and may be arranged such that the surfaces on which the anodes 12 are formed face each other. In this case, during the formation of the plating film on the object X, the object X may be arranged between the two anode supports 14. For example, each anode 12 is arranged on the anode support 14 with a distance D between adjacent other anodes 12.

[0014] In this embodiment, there are no particular restrictions on the shape of each anode 12 and the method of arranging each anode 12 on the anode support 14. For example, in this embodiment, each rectangular anode 12 may be arranged on the anode support 14 at substantially equal intervals in the matrix direction. Alternatively, anodes 12 having different frame shapes or annular shapes may be arranged around the center of the anode support 14 so that their sizes gradually increase. Further, in this embodiment, the anode 12 may be in a mesh shape so that the liquid in the plating bath 6 containing the electrolytic solution E can pass through the anode 12. In addition, the anode support 14 may be, for example, plate-shaped, frame-shaped, or lattice-shaped.

[0015] The electroplating apparatus 2 includes a plurality of power supplies 16. Each power supply 16 is electrically connected to each of the anodes 12. Thereby, the electroplating apparatus 2 can individually control the current application to each anode 12 by individually controlling the output current value of each power supply 16. Further, each power supply 16 is electrically connected to the object X as an electrode on the ground side.

[0016] The electroplating apparatus 2 includes an object support 18 that supports the object X in the plating bath 6 by gripping the object X. The object support 18 supports the object X at a distance from each anode 12 on the anode support 14. For example, as shown in FIG. 1, the electroplating apparatus 2 may include a pair of object supports 18 that are located between two anode supports 14 and below and above the plating bath 6, respectively. In this case, the object support 18 may fix the object X in the plating bath 6 by gripping the object X in the vertical direction of the plating bath 6.

[0017] When the plating bath 6 is filled with the electrolytic solution E, each anode 12 on the anode support 14 and the object X supported by the object support 18 are immersed in the electrolytic solution E. Therefore, during the formation of the plating film on the object X by the electroplating apparatus 2, the object X and each anode 12 face each other through the electrolytic solution E.

[0018] As will be described in detail later, each anode 12 is fixed at a distance H from the object X by the support of each anode 12 by the anode support 14 and the support of the object X by the object support 18. The anode support 14 or the object support 18 may be movable. In this case, as the anode support 14 or the object support 18 moves, the distance H of each anode 12 from the object X changes.

[0019] The electroplating apparatus 2 includes a pump 20 that is a liquid pump. Further, the electroplating apparatus 2 includes a circulation tank pipe 22 that communicates the circulation tank 10 and the pump 20, and a plating tank pipe 24 that communicates the plating tank 6 and the pump 20, as hollow pipes through which the liquid can flow. The pump 20 takes in the electrolytic solution E in the circulation tank 10 through the circulation tank pipe 22 and sends the electrolytic solution E into the plating tank 6 through the plating tank pipe 24.

[0020] For example, the opening on the side where the electrolytic solution E is discharged of the plating tank pipe 24 is located near the bottom of the plating tank 6. With this configuration, the electroplating apparatus 2 sends the electrolytic solution E that has overflowed from the partition wall 8 to the circulation tank 10 in the upper part of the plating tank 6 to the lower part of the plating tank 6 through the circulation tank pipe 22, the pump 20, and the plating tank pipe 24 in sequence. Thereby, the electroplating apparatus 2 can stir the electrolytic solution E in the plating tank 6. In addition, in the present embodiment, the electrolytic solution E in the plating tank 6 may be replaced by taking out the electrolytic solution E in the circulation tank 10 and adding the electrolytic solution E to the circulation tank 10.

[0021] The plating tank pipe 24 may branch inside the plating tank 6, and openings may be formed at the respective tips of the branched plating tank pipes 24. By arranging the plurality of tips of the plating tank pipe 24 between the anode 12 and the object X, the plating tank pipe 24 may have a structure for sending the electrolytic solution E toward the object X.

[0022] For example, assume that the anode 12 is in a mesh shape and allows the electrolytic solution E to pass through, and the anode support 14 has a structure such as a frame shape or a lattice shape that allows the electrolytic solution E to pass through in a part thereof. In this case, the plating bath pipe 24 may be arranged such that the tip having the opening of the plating bath pipe 24 feeds the electrolytic solution E from the side opposite to the anode 12 of the anode support 14 toward the anode 12.

[0023] The electroplating apparatus 2 includes a control unit 26. The control unit 26 controls the operations of each part of the electroplating apparatus 2. For example, the control unit 26 may control the distance H of each anode 12 with respect to the object X by controlling the position of the anode support 14 or the object support 18. Further, the control unit 26 may control each power supply 16 to individually control the current applied to each anode 12. Further, the control unit 26 may control the circulation of the electrolytic solution E in the plating bath 6 by controlling the pump 20.

[0024] The control unit 26 may perform the control of each part based on the operation of an input interface (not shown) by the user or a program or the like recorded in a memory (not shown) or the like. The communication between the control unit 26 and each part of the electroplating apparatus 2 or with the outside may be implemented by signal transmission and reception via a signal line (not shown) or the like, or may be implemented by signal transmission and reception using a communication means (not shown).

[0025] (Power supply control method) FIG. 2 is an example of an equivalent circuit diagram of a part of the electroplating apparatus 2 for more detailed explanation of the control of the power supply 16 by the control unit 26 and the control of the current applied to each anode 12. In FIG. 2, the parts surrounded by the dotted line are located inside the plating bath 6 during the formation of the plating film on the object X.

[0026] For example, the control unit 26 may include a signal generation unit 28 and an arithmetic instruction unit 30. The signal generation unit 28 generates signals for controlling each power supply 16. The signals generated by the signal generation unit 28 are input to the arithmetic instruction unit 30. The arithmetic instruction unit 30 generates signals to be input to each power supply 16 based on the signals from the signal generation unit 28. The generation of the signals to be input to each power supply 16 by the arithmetic instruction unit 30 may be realized, for example, by transmitting the signals from the signal generation unit 28 to each power supply 16 while switching the power supply 16 to which the signals are input.

[0027] Each power supply 16 generates a signal to be applied to each anode 12 based on the signal from the arithmetic instruction unit 30, and applies a current to each anode 12. Here, a leakage resistor 12R may be formed between the plurality of anodes 12. In this case, the signal generation unit 28 may generate signals in consideration of the resistance values of the respective leakage resistors 12R.

[0028] The generation of the signals to be input to each power supply 16 by the arithmetic instruction unit 30 may be realized, for example, by transmitting the signals from the signal generation unit 28 to each power supply 16 while switching the power supply 16 to which the signals are input. Alternatively, the control unit 26 may calculate the information on the current to be generated by each of the power supplies 16 incorporating the signal generation unit 28, and the signal generation unit 28 incorporated in each power supply 16 may generate signals based on the calculation.

[0029] Each control by the control unit 26 may be executed by AI (Artificial Intelligence). In this case, the AI may operate in the control unit 26, or may operate in another device (for example, an edge computer or a cloud server, etc.).

[0030] (Specific method of electroplating) Referring to FIG. 3, the method of forming a plating film on the surface of the object X using the electroplating apparatus 2 according to the present embodiment will be described in more detail. FIG. 3 is an enlarged schematic view of a part of the electroplating apparatus 2 during the formation of the plating film on the surface of the object X, and in particular, it is an enlarged schematic view of the region A in FIG. 1.

[0031] FIG. 3 shows the extraction of the first anode 12A and the second anode 12B from among the anodes 12 of the electroplating apparatus 2. When the electroplating apparatus 2 applies current to each anode 12, an electric field is generated in each anode 12. For example, as shown in FIG. 3, when current is applied to each of the first anode 12A and the second anode 12B, an electric field is generated from each of the first anode 12A and the second anode 12B. Along with this, an electric force due to the electric field from the first anode 12A and an electric force due to the electric field from the second anode 12B act on the electrolyte in the electrolytic solution E. FIG. 3 shows examples of the first electric force lines EA and the second electric force lines EB formed by the electric fields from the first anode 12A and the second anode 12B, respectively.

[0032] For example, when the object X is a printed circuit board, the wiring located on the surface XS of the object X functions as a cathode corresponding to each anode 12. Therefore, in a state where the object X and each anode 12 are immersed in the electrolytic solution E, by controlling the current of each anode 12, the electrolyte in the electrolytic solution E flows toward the surface XS of the object X. Along with this, the flowing electrolyte approaches the surface XS, and by transferring electrons between the electrolyte and the object X, as shown in FIG. 3, a plating film P is formed on the surface XS.

[0033] Here, in the present embodiment, the distance H between each anode 12 and the object X is sufficiently longer than the distance D between adjacent anodes 12, specifically, D ≦ 2H. Therefore, for a portion of the surface XS facing a certain anode 12, a plating film P corresponding to the electric field formed by the anode 12 adjacent to the said anode 12 is formed. Specifically, for example, when forming the plating film P on the portion of the surface XS facing the first anode 12A, the electric field from the second anode 12B has an influence.

[0034] Thus, in the electroplating apparatus 2 according to the present embodiment, during the formation of the plating film P on the object X, one of the anodes 12 adjacent to each other is involved in the formation of the plating film P on the surface XS facing the other. In other words, in the electroplating apparatus 2 according to the present embodiment, an interaction occurs between the anodes 12 adjacent to each other. Specifically, for example, by controlling the current applied to the second anode 12B, it is possible to control the film thickness of the plating film P on the portion of the surface XS facing the first anode 12A. In FIG. 3, as an example, the case where the same voltage is applied to the first anode 12A and the second anode 12B to form the plating film P is shown. Details of the plating film P will be described later.

[0035] (Film thickness control of plating film) In the present embodiment, a resist R containing a photosensitive resist such as a dry film resist may be formed in advance on the surface XS at a position where the plating film P is not formed with respect to the position where the plating film P is not formed. Thereby, the plating film P can be formed only on a part of the surface XS. For example, the resist R is formed along the wiring pattern on the surface XS of the object X. For this reason, depending on the state of the surface XS, the density of the resist R on the surface XS may vary depending on the position of the surface XS.

[0036] Here, for example, in the present embodiment, in the vicinity of the position on the surface XS facing the first anode 12A, the density of the resist R on the surface XS is higher than that in the vicinity of the position facing the second anode 12B, while the density of the wiring pattern is lower. In this case, when the same current is applied to each anode 12, the electric field from the first anode 12A is concentrated on the wiring pattern in the vicinity of the position facing the first anode 12A as compared with the wiring pattern in the vicinity of the position facing the second anode 12B. For this reason, when the same current is applied to each anode 12, the plating film P formed in the vicinity of the position facing the first anode 12A has a thicker film thickness than the plating film P formed in the vicinity of the position facing the second anode 12B.

[0037] Referring to FIG. 4, the plating film P formed on the surface XS of the object X by the electroplating apparatus 2 according to the present embodiment, and the evaluation of the uniformity of the film thickness of the plating film P will be described in more detail. FIG. 4 is an enlarged schematic view of the vicinity of the surface of the plating film P. In particular, FIG. 4 shows only the object X shown in FIG. 3 and the plating film P formed on the surface XS of the object X. Further, in order to more clearly show the variation in the thickness of the plating film P on the surface XS, the scale of the plating film P shown in FIG. 4 is changed from the scale of the plating film P shown in FIG. 3.

[0038] As shown in FIG. 4, the film thickness of the plating film P may not be strictly uniform. Here, in this specification, the film thickness of the plating film P refers to the thickness of the plating film P along the film thickness direction DP perpendicular to the surface direction DX parallel to the surface XS, as shown in FIG. 4.

[0039] In the present embodiment, as shown in FIG. 4, in order to evaluate the uniformity of the plating film P formed on the surface XS, the film thickness of the plating film P was measured at six measurement points PA to PF of the plating film P within a preset region. As a result, it is assumed that the film thicknesses TA to TF of the plating film P were obtained at each measurement point. In the present embodiment, the film thickness at each of the measurement points where the film thickness was measured is compared, and a range where the film thickness is thick (hereinafter, the film thickness maximum range) and a range where the film thickness is thinner than that (the film thickness minimum range) are searched and determined. Thereby, the distance W, which is the shortest distance between the film thickness maximum range and the film thickness minimum range along the surface direction DX, is calculated. Here, the film thickness measurement may be performed by measuring the film thickness at five or more measurement points by various known methods such as an eddy current test method, a fluorescent X-ray test method, a stylus scanning method, and a scanning electron microscope test method.

[0040] For example, as a result of the film thickness search in FIG. 4, it is assumed that the measurement point PA having the film thickness TA is a point included in the film thickness maximum range A1, and the measurement point PB having the film thickness TB is a point included in the film thickness minimum range A2. Here, the film thickness TA is not necessarily the maximum value of the film thickness of the plating film P, and the film thickness TB is not necessarily the minimum value of the film thickness of the plating film P. For example, measurement points having substantially the same film thickness as the film thickness TA and the film thickness TB may be regarded as being included in the film thickness maximum range A1 and the film thickness minimum range A2, respectively. However, as will be described later, the "maximum" and "minimum" as used herein have different meanings from the maximum and minimum generally defined in mathematics.

[0041] For example, even when the film thickness TC of the plating film P at the measurement point PC is larger than the film thickness TA, if the difference between the film thickness TC and the film thickness TA is 10% or less of the film thickness TA, the measurement point PC is regarded as being included in the film thickness maximum range. Also, even when the film thickness TD of the plating film P at the measurement point PD is smaller than the film thickness TB, if the difference between the film thickness TD and the film thickness TB is 10% or less of the film thickness TB, the measurement point PD is regarded as being included in the film thickness minimum range A2. However, the measurement point PE having the film thickness TE, which is a film thickness between the film thickness TA and the film thickness TB, and the measurement point PF having the film thickness TF are excluded from consideration in the search for the film thickness maximum range A1 and the film thickness minimum range A2.

[0042] In the present embodiment, among the distances between the film thickness measurement points included in the film thickness maximum range A1 and the film thickness measurement points included in the film thickness minimum range A2, the shortest distance is adopted as the distance W. Thereby, the distance W can be regarded as the shortest distance from the film thickness maximum range A1 to the film thickness minimum range A2 of the plating film P. In this case, generally, the smaller the difference between the average film thickness value of the film thickness maximum range A1 and the average film thickness value of the film thickness minimum range A2, the better the uniformity of the plating film P.

[0043] In the present embodiment, the film thickness at each position of the plating film P can be controlled by controlling the current of each anode 12. In particular, when an interaction occurs between two adjacent anodes 12, the film thickness of the plating film P on the surface XS facing each of the anodes 12 can be more precisely controlled by controlling the current of the anode 12.

[0044] Therefore, the electroplating apparatus 2 according to the present embodiment can reduce the average film thickness difference between the maximum film thickness range and the minimum film thickness range of the plating film P by controlling the current of each anode 12.

[0045] Here, when current is applied to only one of the anodes 12 of the electroplating apparatus 2 and the plating film P is formed on the flat electrode, the shortest distance from the maximum film thickness range A1 to the minimum film thickness range A2 of the plating film P is defined as the distance WA. In this case, in the present embodiment, the control unit 26 may apply a common current to each anode 12 and use the distance W of the plating film P formed on the surface XS of the object X as a target value, and make the above-described distance WA approach this target value. More specifically, for example, the control unit 26 may determine the distance D and the distance H at each anode 12 so that the distance WA becomes the value closest to the distance W. Thereby, the difference between the average film thickness value of the maximum film thickness range A1 and the average film thickness value of the minimum film thickness range A2 can be more efficiently reduced by controlling the current of each anode 12.

[0046] In the present embodiment, the control unit 26 determines the distance D between the adjacent anodes 12 and the distance H between the anode 12 and the object X, and determines the above-described distance WA. Thereby, for the plating film P formed on the surface XS of the object X, the difference between the average film thickness value of the maximum film thickness range A1 and the average film thickness value of the minimum film thickness range A2 can be more efficiently reduced through the control of the distance D and the distance H.

[0047] With the above configuration, the electroplating apparatus 2 according to the present embodiment can form the plating film P more uniformly regardless of the state of the surface XS of the object X, for example, regardless of the density of the wiring of the printed circuit board. As a result, the characteristics of the plating film P are improved. In addition, the process of polishing the plating film P to make the film thickness uniform can be simplified. Along with this, since the portion where the plating film P is formed thick without necessity is reduced, the material cost of the plating film P is reduced. Furthermore, a shielding plate or the like between the anodes, which has been conventionally used to improve the uniformity of the plating film P, can be made unnecessary or simplified.

[0048] In this embodiment, as described above, for each anode 12, the distance D between the anode 12 and another adjacent anode 12 and the distance H between the object X satisfy D≤2H. Therefore, in the electroplating apparatus 2 according to this embodiment, during the formation of the plating film P, an interaction occurs between each anode 12 and another anode 12. Therefore, the electroplating apparatus 2 according to this embodiment can form a more effectively uniform plating film P.

[0049] In this embodiment, the distance W of the plating film P satisfies the following formula using the distance D between the anode 12 and another adjacent anode 12 and the distance H between the object X.

[0050]

Equation

[0051] Therefore, when the target distance W' is determined, the control unit 26 may determine the distance D and the distance H so as to satisfy the above formula. Thereby, using the electroplating apparatus 2, a plating film P having the distance W' can be efficiently formed. Further, according to the above configuration, when the current of the anode 12 is controlled, the flatness of the plating film P is improved until the film thickness maximum range A1 and the film thickness minimum range A2 that were originally observed can no longer be observed and the state where W' cannot be determined is reached. In other words, when the target distance W' is determined, the control unit 26 can more easily determine the distance D and the distance H by the above formula, and in addition, the flatness of the plating film P can be improved by controlling the current of the anode 12.

[0052] In this embodiment, when the resist R is formed on the surface XS of the object X, in the vicinity of the boundary between the resist R and the plating film P formed by the electroplating apparatus 2, the film thickness of the plating film P may increase or decrease compared to the surroundings. An example of the plating film P that can be formed by the electroplating apparatus 2 according to this embodiment will be described with reference to FIG. 5. FIG. 5 is a schematic diagram showing an enlargement of the vicinity of a part of the plating film P formed on the surface XS of the object X for an example of the plating film P.

[0053] As shown in plating film formation example 502 of FIG. 5, for example, the plating film P may be formed with a substantially constant film thickness regardless of the distance from the resist R. On the other hand, as shown in plating film formation example 504 of FIG. 5, the plating film P may have an edge portion E1 with a thinner film thickness than the surroundings formed in the vicinity of the boundary with the resist R. Alternatively, as shown in plating film formation example 506 of FIG. 5, the plating film P may have an edge portion E2 with a thicker film thickness than the surroundings formed in the vicinity of the boundary with the resist R.

[0054] The edge portion E1 or the edge portion E2 shown in FIG. 5 is formed by an increase or decrease in the electric field in the vicinity of the boundary between the plating film P and the resist R as compared with the electric field in the surroundings. When forming the plating film P by the electroplating apparatus 2 in the present embodiment, assuming that the edge portion E1 or the edge portion E2 is formed, the current of each anode 12 may be controlled, or the distances D and H at each anode 12 may be controlled.

[0055] (Trial plating and main plating) A method for forming the plating film P on the object X using the electroplating apparatus 2 according to the present embodiment will be described in more detail with reference to FIG. 6. FIG. 6 is a flowchart showing a method of electroplating using the electroplating apparatus 2 according to the present embodiment. In the present embodiment, prior to forming the plating film P on the object X, a method of performing trial plating to determine each condition necessary for forming the plating film P will be described as an example.

[0056] First, fill the plating bath 6 of the electroplating apparatus 2 with the electrolytic solution E, and immerse the test object in the electrolytic solution E by supporting the test object on the object support 18 (step S2). The test object may have the same shape and be made of the same material as the object X. When the object X is a printed circuit board, the test object may be a printed circuit board having the same wiring pattern as the object X. Further, when the object X has the resist R, the resist R having the same formation pattern as the object X may be formed on the test object.

[0057] Next, by controlling each power supply 16, current control of each anode 12 is performed to form a test plating film on the test object (step S4), and test plating is executed. Here, the test plating may be performed by applying the same current to all the anodes 12 by controlling each power supply 16. Here, in the test plating, it is assumed that for each anode 12, the distance D and the distance H satisfy D ≦ 2H. Thereby, each anode 12 functions as a common anode facing the test object through the electrolytic solution E. However, in order to further improve the uniformity of the electric field from each anode 12, H may be made as large as possible with respect to D. Alternatively, in the test plating, instead of the plurality of anodes 12, a single common anode formed on the anode support 14 may be used to form a test plating film on the test object.

[0058] After the formation of the test plating film on the test object, the test object is removed from the object support 18, and the test object is taken out from the plating bath 6 (step S6). Next, the characteristics of the test plating film formed on the test object are measured (step S8). For example, in the measurement of the characteristics of the test plating film, the maximum film thickness range A1 and the minimum film thickness range A2 at each position of the test plating film are measured, and the distance W', which is the shortest distance between the maximum film thickness range A1 and the minimum film thickness range A2 in the test plating film, is measured.

[0059] Next, based on the characteristics of the measured test plating film, plating conditions including the above-described conditions of the distance D and the distance H for each anode 12 used for forming the plating film P on the object X are determined (step S10). A specific method for determining the plating conditions will be described in more detail with reference to FIG. 7.

[0060] FIG. 7 is a flowchart showing a method for determining plating conditions including the above-described distance D and distance H for each anode 12 when forming the plating film P on the object X using the electroplating apparatus 2 according to the present embodiment.

[0061] First, a target value of the distance W of the plating film P formed by the electroplating apparatus 2 is determined (step S10-2). The target value of the distance W may be, for example, the distance W’ obtained in step S8. In the present embodiment, the distance W’ is obtained by actually forming a plating film in step S8 and measuring the film thickness of the plating film, but is not limited thereto. For example, the distance W’ may be determined in advance by simulation by the user of the electroplating apparatus 2. The target value of the distance W may be determined from the required specifications of the plating film P to be formed or the constraints of the apparatus design of the electroplating apparatus 2.

[0062] Next, constraint conditions of the distance D and the distance H for each anode 12 in the electroplating apparatus 2 are obtained (step S10-4). For example, in the electroplating apparatus 2 according to the present embodiment, since each anode 12 is directly formed on the anode support 14, the distance between two adjacent anodes 12 is fixed. Also, in the electroplating apparatus 2 according to the present embodiment, the distance between each anode 12 and the object X is constant regardless of the anode 12. Thus, the constraint conditions of the distance D and the distance H for each anode 12 are determined in advance by the manufacturer of the electroplating apparatus 2 according to the configuration of the electroplating apparatus 2.

[0063] Next, under the constraint conditions obtained in step S10-4, distances D and H at which distance W is closest to the target value obtained in step S10-2 are determined (step S10-6). In step S10-6, the control unit 26 determines, for example, based on the characteristics of the test plating film including the distance W' obtained in step S8, what electric field should be applied to the object X in the process of forming the plating film P on the object X. Next, the control unit 26 determines the configuration of each anode 12 capable of applying to the object X the electric field closest to the obtained electric field under the constraint conditions of distances D and H. As described above, as plating conditions when forming the plating film P on the object X using the electroplating apparatus 2, the distances D and H for each anode 12 are determined by the control unit 26. For example, the control unit 26 may determine, under the constraint conditions of distances D and H, distances D and H at which the distance W of the plating film P is closest to the distance W' obtained in step S8, according to a predetermined algorithm.

[0064] Returning to the reference of FIG. 6, next to step S10, an anode 12 that satisfies the changed plating conditions is prepared (step S11). Step S11 may be executed, for example, by immersing an anode support 14 in which each anode 12 is formed so as to satisfy the distance D obtained in step S10-6 in the electrolytic solution E. Also, step S11 may be realized by arranging the anode support 14 such that the distance H obtained in step S10-6 is satisfied in each anode 12.

[0065] Next, the object X is immersed in the electrolytic solution E by supporting the object X on the object support 18 (step S12). Next, by controlling each power supply 16, the current of each anode 12 is controlled to form a plating film P on the object X (step S14), and this main plating is executed. Here, this main plating may be carried out, for example, by controlling each power supply 16 with reference to the measurement results of the characteristics of the test plating film. After the formation of the plating film P on the object X, the object X is removed from the object support 18 and taken out of the plating tank 6 (step S16), whereby the formation of the plating film P on the object X is completed. During the above-described test plating and main plating, the electrolytic solution E in the plating tank 6 may be circulated by controlling the pump 20.

[0066] When adopting a method of forming a plating film P on the object X along the flowchart shown in FIG. 6, the control unit 26 can determine the plating conditions to be used for forming the plating film P on the object X from the characteristics of the test plating film formed on the test object. Therefore, by the above method, the electroplating apparatus 2 can form a plating film P with improved characteristics.

[0067] [Embodiment 2] (Masking plate) FIG. 8 is an enlarged schematic view of a part of the electroplating apparatus 2 during the formation of a plating film on the surface of the object X by the electroplating apparatus 2 according to the present embodiment, and in particular, it is an enlarged schematic view at the same position as the schematic view shown in FIG. 3. The electroplating apparatus 2 according to the present embodiment has the same configuration as the electroplating apparatus 2 according to the previous embodiment, except that it further includes a masking plate 32.

[0068] The shielding plate 32 has electrical insulation properties, and at least a part thereof is located between each anode 12. For example, the shielding plate 32 may include an insulator containing, for example, polyvinyl chloride. For example, at least a part of the shielding plate 32 extends in the direction from the object X to the anode 12. For example, the shielding plate 32 may be formed on the anode support 14 via the bonding portion 34 as shown in FIG. 8. The shielding plate 32 may be formed between each anode 12, but it suffices if it is formed between at least one set of adjacent anodes 12.

[0069] The shielding plate 32 may further include an auxiliary cathode 36 at a position overlapping the object X and the resist R via the electrolytic solution E. The auxiliary cathode 36 faces the anode 12 and the anode support 14 with an insulating material interposed therebetween, for example. The auxiliary cathode 36 may include, for example, stainless steel containing SUS. The auxiliary cathode 36 may be grounded or electrically floating, for example.

[0070] The formation of the plating film P on the object X using the electroplating apparatus 2 according to the present embodiment may be executed along the flowchart shown in FIG. 6. In this case, the presence or absence or shape of each shielding plate 32 may be determined by the control unit 26 based on the measurement results of the characteristics of the test plating film.

[0071] When the plating film P is formed using the electroplating apparatus 2 according to the present embodiment, the shielding plate 32 can reduce the interaction between the adjacent anodes 12. Thus, in the present embodiment, the interaction between the adjacent anodes 12 can be appropriately designed, and the uniformity of the plating film P is improved more efficiently.

[0072] Here, let the thickness of the shielding plate 32 in the planar direction of the surface XS of the object X be D', and the distance from the tip of the shielding plate 32 on the object X side to the object X be H'. In this case, the thickness D' and the distance H' may satisfy D'≤2H'. In this case, even when the distance D and the distance H described above do not satisfy D≤2H for any one of the anodes 12, it is possible to cause an interaction between the anode 12 and the adjacent anode 12.

[0073] Furthermore, in the present embodiment, the distance W of the plating film P may satisfy an equation in which the distance D in the above-described Equation 1 is read as the distance D' and the distance H is read as the distance H'. In this case, for any one of the anodes 12, even when the above-described distance D and distance H do not satisfy Equation 1, if the target distance W is determined, the plating film P having the distance W can be efficiently formed. Furthermore, by performing current control on each anode 12, the maximum film thickness range and the minimum film thickness range are not observed, and the flatness of the plating film P is improved until the state where the distance W is not observed is reached.

[0074] Also, when the shielding plate 32 includes the auxiliary cathode 36, electric lines of force are also generated in the direction from each anode 12 to the auxiliary cathode 36. For this reason, when forming the plating film P using the electroplating apparatus 2 according to the present embodiment, an excess plating film PR is formed on the auxiliary cathode 36. In this way, the electric field distribution at each anode 12 can be corrected by the auxiliary cathode 36, and the thickness of the plating film P formed on the surface XS of the object X can be more easily controlled.

[0075] The configurations of the shielding plate 32 and the auxiliary cathode 36 in the present embodiment are not limited to the configurations shown in FIG. 8. Other examples of the configurations of the shielding plate 32 and the auxiliary cathode 36 in the present embodiment will be described with reference to FIG. 9. FIG. 9 is an enlarged schematic view of a part of the electroplating apparatus 2 for explaining an example of the configurations of the shielding plate 32 and the auxiliary cathode 36 in the present embodiment, and is an enlarged schematic view at the same position as the schematic view shown in FIG. 3.

[0076] As shown in Configuration Example 802 and Configuration Example 804 of FIG. 9, the electroplating apparatus 2 includes the shielding plate 32, but may not include the auxiliary cathode 36. For example, as shown in Configuration Example 802 of FIG. 9, the electroplating apparatus 2 may include a shielding plate 32 extending from the anode support 14 in the direction of the object X between each anode 12. On the other hand, for example, as shown in Configuration Example 804 of FIG. 9, the electroplating apparatus 2 may include a shielding plate 32 that shields the ends of each anode 12 between the anode support 14 and the object X.

[0077] As shown in Configuration Example 806 of FIG. 9, the electroplating apparatus 2 includes the auxiliary cathode 36, and may not include the shielding plate 32. In particular, for example, as shown in Configuration Example 806 of FIG. 9, the electroplating apparatus 2 may include the auxiliary cathode 36 located at the end of the object X between the anode support 14 and the object X.

[0078] Alternatively, for example, as shown in Configuration Example 808 of FIG. 9, the electroplating apparatus 2 may include the shielding plate 32 located between the anodes 12 and the auxiliary cathode 36 between the anode support 14 and the object X. Here, as shown in Configuration Example 808 of FIG. 9, the shielding plate 32 may have a shielding portion that shields the ends of the anodes 12 in the direction from the anode support 14 to the object X. Further, as shown in Configuration Example 808 of FIG. 9, the auxiliary cathode 36 may be formed at a position overlapping the resist R in the direction from the anode support 14 to the object X in accordance with the formation pattern of the resist R.

[0079] 〔Embodiment 3〕 (Movable Anode) FIGS. 10 and 11 are schematic diagrams showing a part of the electroplating apparatus 2 enlarged during the formation of the plating film on the surface of the object X by the electroplating apparatus 2 according to the present embodiment. In particular, FIG. 10 is a schematic diagram enlarged at the same position as the schematic diagram shown in FIG. 3. FIG. 11 is a schematic diagram showing an enlargement of the first movable anode 38A shown in FIG. 10 and its surroundings in the direction from the upper side to the lower side of the liquid tank 4, which will be described later. In other words, FIG. 11 is a schematic diagram showing an enlargement of the first movable anode 38A shown in FIG. 10 and its surroundings when viewed from the upper side to the lower side toward the paper surface of FIG. 10. In FIGS. 10 and 11, the direction parallel to both the upper surface of the anode support 14 and the bottom surface of the liquid tank 4 is the X-axis direction, the normal direction of the bottom surface of the liquid tank 4 is the Y-axis direction, and the direction from the anode support 14 to the object X is the Z-axis direction.

[0080] The electroplating apparatus 2 according to the present embodiment has the same configuration as the electroplating apparatus 2 according to Embodiment 1, except that it includes a movable anode and an anode control unit instead of the anodes 12.

[0081] More specifically, for example, as shown in FIGS. 10 and 11, the electroplating apparatus 2 according to the present embodiment includes, instead of the first anode 12A, a first movable anode 38A, a first movable support portion 39A, and a first anode control portion 40A. The first movable support portion 39A supports the first movable anode 38A, and the first anode control portion 40A controls the position of the first movable support portion 39A. Further, as shown in FIG. 10, the electroplating apparatus 2 according to the present embodiment includes, instead of the second anode 12B, a second movable anode 38B, a second movable support portion 39B that supports the second movable anode 38B, and a second anode control portion 40B that controls the position of the second movable support portion 39B.

[0082] Each of the first movable anode 38A and the second movable anode 38B is, for example, a substantially spherical electrode, and is formed at the tip of each of the conductive first movable support portion 39A and the second movable support portion 39B. Each of the first movable anode 38A and the second movable anode 38B is individually current-controlled by the power source 16 via the first movable support portion 39A and the second movable support portion 39B.

[0083] Each of the first movable support portion 39A and the second movable support portion 39B is individually and three-dimensionally position-controlled in three directions of the X-axis, Y-axis, and Z-axis via the control by the first anode control portion 40A and the second anode control portion 40B. For example, each of the first movable support portion 39A and the second movable support portion 39B is controlled in position in the X-axis direction or the Y-axis direction by moving along the plane direction of the surface XS by each of the first anode control portion 40A and the second anode control portion 40B. Further, each of the first movable support portion 39A and the second movable support portion 39B is controlled in position along the Z-axis direction, which is substantially perpendicular to the plane direction of the surface XS, by each of the first anode control portion 40A and the second anode control portion 40B.

[0084] Therefore, with the control of the positions of the first movable support portion 39A and the second movable support portion 39B, each of the first movable anode 38A and the second movable anode 38B is three-dimensionally position-controlled. The first anode control unit 40A and the second anode control unit 40B may be, for example, a robot or a multi-axis actuator or the like.

[0085] For this reason, each of the first movable anode 38A and the second movable anode 38B is individually controlled in terms of its position in the X-axis direction or the Y-axis direction. In other words, the first movable anode 38A controls the distance D from the adjacent second movable anode 38B. Further, each of the first movable anode 38A and the second movable anode 38B may be individually controlled in terms of its position in the Z-axis direction, thereby individually controlling the distance H from the surface XS. The position control of each of the first movable anode 38A and the second movable anode 38B may be executed by the control of each of the first anode control unit 40A and the second anode control unit 40B by the control unit 26.

[0086] Furthermore, as shown in FIG. 11, the electroplating apparatus 2 according to the present embodiment includes a third movable anode 38C, a third movable support portion 39C that supports the third movable anode 38C, and a third anode control unit 40C that controls the position of the third movable support portion 39C. The third movable anode 38C, the third movable support portion 39C, and the third anode control unit 40C may have the same configuration as each of the first movable anode 38A, the first movable support portion 39A, and the first anode control unit 40A, except for the positions where they are arranged. For example, the third anode control unit 40C may be arranged at positions adjacent to each other in the X-axis direction with the first anode control unit 40A.

[0087] For this reason, also with respect to the third movable anode 38C, the position in the X-axis direction or the Y-axis direction is individually controlled. In other words, the third movable anode 38C controls, for example, the distance D from the adjacent first movable anode 38A. Further, the third movable anode 38C may be individually controlled in terms of its position in the Z-axis direction, thereby individually controlling the distance H from the surface XS. The position control of the third movable anode 38C may be executed by the same method as the position control of the first movable anode 38A or the second movable anode 38B.

[0088] The formation of the plating film P on the object X using the electroplating apparatus 2 according to this embodiment may be executed along the flowchart shown in FIG. 6. In this case, the position control of each movable anode may be executed based on the measurement results of the characteristics of the test plating film.

[0089] In this embodiment, the positions of the respective movable anodes can be controlled individually. For this reason, the electroplating apparatus 2 according to this embodiment can more precisely control the electric field distribution at each movable anode and can more precisely control the interaction between the movable anodes adjacent to each other. Therefore, the electroplating apparatus 2 according to this embodiment can form a more uniform plating film P.

[0090] 〔Embodiment 4〕 (Expansion and contraction part) FIG. 12 is an enlarged schematic view of a part of the electroplating apparatus 2 during the formation of the plating film on the surface of the object X by the electroplating apparatus 2 according to this embodiment, and in particular, is an enlarged schematic view at the same position as the schematic view shown in FIG. 3. The electroplating apparatus 2 according to this embodiment is different in configuration from the electroplating apparatus 2 according to Embodiment 1 in that it further includes a plurality of expansion and contraction parts. More specifically, for example, as shown in FIG. 12, the electroplating apparatus 2 according to this embodiment includes a first expansion and contraction part 42A and a second expansion and contraction part 42B.

[0091] Each of the first expansion and contraction part 42A and the second expansion and contraction part 42B is a member that expands and contracts in a direction substantially perpendicular to the plane direction of the surface XS of the object X, in other words, in a direction substantially the same as the film thickness direction of the plating film P. Each of the first expansion and contraction part 42A and the second expansion and contraction part 42B may be controlled by, for example, the control unit 26. Each of the first expansion and contraction part 42A and the second expansion and contraction part 42B may be, for example, an air cylinder.

[0092] A first anode 12A is formed at the tip of the first telescopic part 42A on the object X side, and a second anode 12B is formed at the tip of the second telescopic part 42B on the object X side. Therefore, as each of the first telescopic part 42A and the second telescopic part 42B expands and contracts, the distance H between each of the first anode 12A and the second anode 12B and the object X varies. By individually controlling the expansion and contraction of each of the first telescopic part 42A and the second telescopic part 42B, the distance H in each of the first anode 12A and the second anode 12B can be individually controlled.

[0093] Except for the above-described configuration, the electroplating apparatus 2 according to the present embodiment has the same configuration as the electroplating apparatus 2 according to the first embodiment. The formation of the plating film P on the object X using the electroplating apparatus 2 according to the present embodiment may be performed according to the flowchart shown in FIG. 6. In this case, the position control of each anode 12 by the expansion and contraction of each telescopic part may be performed based on the measurement results of the characteristics of the test plating film.

[0094] In the present embodiment, the distance H in each anode 12 can be individually controlled. Therefore, the electroplating apparatus 2 according to the present embodiment can more precisely control the electric field distribution in each anode 12 and can more precisely control the interaction between the anodes 12 adjacent to each other. Therefore, the electroplating apparatus 2 according to the present embodiment can form a more uniform plating film P.

[0095] 〔Embodiment 5〕 (Further divided anodes) FIG. 13 is an enlarged schematic view of a part of the electroplating apparatus 2 during the formation of the plating film on the surface of the object X by the electroplating apparatus 2 according to the present embodiment, and in particular, it is an enlarged schematic view at the same position as the schematic view shown in FIG. 3. The electroplating apparatus 2 according to the present embodiment has the same configuration as the electroplating apparatus 2 according to the first embodiment, except that each anode 12 is replaced with an anode obtained by further dividing the anode. More specifically, for example, as shown in FIG. 13, the electroplating apparatus 2 according to the present embodiment includes a plurality of first anodes 44A instead of the first anode 12A.

[0096] The first anode 44A is individually current-controlled independently of other anodes including the second anode 12B. Further, even between a plurality of first anodes 44A, they are individually current-controlled.

[0097] For example, the first anode 44A is formed at a position facing a position on the surface XS of the object X where the wiring pattern is denser or finer. Thereby, the electroplating apparatus 2 according to the present embodiment realizes more precise film thickness control of the plating film P for the fine wiring pattern.

[0098] With the above configuration, in the electroplating apparatus 2 according to the present embodiment, more appropriate film thickness control can be performed according to the wiring pattern on the surface XS of the object X. For this reason, the electroplating apparatus 2 according to the present embodiment can form a more uniform plating film P on the object X regardless of the wiring pattern on the surface XS of the object X.

[0099] 〔Embodiment 6〕 (Split pipe) FIG. 14 is a schematic diagram showing the electroplating apparatus according to the present embodiment and a state in which a plating film is formed using the electroplating apparatus, and in particular, is an enlarged schematic diagram at the same position as the schematic diagram shown in FIG. 1. The electroplating apparatus 46 according to the present embodiment is different in configuration only in that it includes a plating bath pipe 48 and a split pipe 50 instead of the plating bath pipe 24 as compared with the electroplating apparatus 2 shown in Embodiment 1.

[0100] In the present embodiment, the plating bath pipe 48 communicates from the pump 20 to the inside of the anode support 14. Further, in the anode support 14 in the present embodiment, split pipes 50 are respectively formed between the anodes 12. Each split pipe 50 communicates with the inside of the anode support 14 and has an opening facing the object X.

[0101] The liquid containing the electrolytic solution E, which is fed into the interior of the anode support 14 through the plating tank pipe 48 by the pump 20, is distributed to each split pipe 50 and sprayed from each split pipe 50 toward the object X. Inside the anode support 14, for example, a flow path designed to distribute the liquid from the plating tank pipe 48 to each split pipe 50 substantially uniformly may be formed.

[0102] The electroplating apparatus 46 according to the present embodiment stirs or replaces the electrolytic solution E in the plating tank 6 by the pump 20, and sprays the electrolytic solution E onto the surface of the object X facing each anode 12 by feeding the electrolytic solution E into each split pipe 50. Thereby, in the process of forming the plating film P on the object X, stirring or replacement of the electrolytic solution E between the anode 12 and the object X where the concentration change of the electrolytic solution E can be the largest can be carried out more efficiently. Therefore, the electroplating apparatus 2 according to the present embodiment can form a more uniform plating film P.

[0103] The formation of the plating film P on the object X using the electroplating apparatus 2 according to the present embodiment may be executed along the flowchart shown in FIG. 6. In the present embodiment, during the formation of the plating film P on the object X, spraying of the electrolytic solution E from the split pipe 50 onto the object X may be performed by the operation of the pump 20.

[0104] As described above, the invention according to the present disclosure has been described based on the drawings and examples. However, the invention according to the present disclosure is not limited to the above-described embodiments. That is, the invention according to the present disclosure can be variously modified within the scope shown in the present disclosure, and embodiments obtained by appropriately combining the technical means disclosed in different embodiments are also included in the technical scope of the invention according to the present disclosure. In other words, it should be noted that those skilled in the art can easily make various deformations or modifications based on the present disclosure. Also, it should be noted that these deformations or modifications are included in the scope of the present disclosure.

Explanation of reference numerals

[0105] 2, 46 Electroplating apparatus 6 plating bath 12 anodes 16 power supply 20 pump 22 circulation tank pipe 24, 48 plating bath pipes 26 control unit 32 masking plate 50 split pipe

Claims

1. An electroplating method for forming a plating film on at least a part of the surface of an object immersed in an electrolytic solution in a plating bath, using a plurality of anodes individually controlled in current, which face the object through the electrolytic solution, comprising: a step of obtaining a target value of a distance W, which is the shortest distance from a thick film thickness maximum range in the plating film to a thin film thickness minimum range thinner than the film thickness in the film thickness maximum range; a step of determining, for each of the anodes, a distance D between the anode and another adjacent anode and a distance H between the anode and the object such that the distance W in the plating film is closest to the target value; and a step of forming the plating film using the anodes that satisfy the distance D and the distance H. The electroplating method includes these steps.

2. Before forming the plating film, further comprising a step of immersing a test object in the electrolytic solution in the plating bath and forming a test plating film on at least a part of the surface of the test object using a common anode facing through the electrolytic solution, In the step of determining the distance D and the distance H, the electroplating method according to claim 1, wherein the distance D and the distance H are determined such that the distance W becomes a distance W', which is the shortest distance from the film thickness maximum range to the film thickness minimum range determined in the test plating film.

3. The electroplating method according to claim 1 or 2, wherein, for each of the anodes, the distance D and the distance H satisfy D≤2H.

4. In the step of determining the distance D and the distance H, for each of the anodes, the distance D and the distance H are such that, for the distance W at each position on the surface of the object, 【Number 1】 The electroplating method according to any one of claims 1 to 3, which is determined to satisfy.

5. In the step of forming the plating film, the plating film is formed in a state where a shielding plate having electrical insulation is disposed between the anode and the object and between at least one pair of two adjacent anodes. The electroplating method according to any one of claims 1 to 4.

6. In the step of forming the plating film, using a pump for circulating the electrolytic solution in the plating bath and a pipe for discharging the electrolytic solution sent out from the pump, while stirring or replacing the electrolytic solution between the anode and the object, and spraying the electrolytic solution onto the surface of the object facing the anode, the electroplating method according to any one of claims 1 to 5, wherein the plating film is formed.

7. A plating bath, a plurality of anodes located inside the plating bath and individually current-controlled, a control unit that controls the current applied to the anodes to control the formation of a plating film on at least a part of the surface of an object immersed in the electrolytic solution in the plating bath using the plurality of anodes facing each other through the electrolytic solution, and further obtains a target value of a distance W, which is the shortest distance from a thick film thickness maximum range in the plating film to a thin film thickness minimum range thinner than the film thickness in the film thickness maximum range, and at each anode, determines the distance D between each adjacent other anode and the distance H from the object so that the distance W in the plating film is closest to the target value. An electroplating apparatus comprising:

8. comprising a common anode located inside the plating bath, prior to forming the plating film, immersing a test object in the electrolytic solution in the plating bath, and forming a test plating film on at least a part of the surface of the test object using the common anode, The electroplating apparatus according to claim 7, wherein the control unit determines the distance D and the distance H so that the distance W becomes a distance W', which is the shortest distance from the film thickness maximum range to the film thickness minimum range determined in the test plating film.

9. The electroplating apparatus according to claim 7 or 8, wherein at each anode, the distance D and the distance H satisfy D ≦ 2H.

10. the control unit, for the distance D and the distance H at each anode, with respect to the distance W at each position on the surface of the object, 【Number 1】 The electroplating apparatus according to any one of claims 7 to 9, wherein the electroplating apparatus is determined so as to satisfy.

11. The electroplating apparatus according to any one of claims 7 to 10, further comprising a shielding plate having electrical insulation properties and located between the anode and the object and between at least one pair of two adjacent anodes.

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