TW201910565A - Electrolytic copper plating anode and electrolytic copper plating device using same - Google Patents

Abstract

The purpose of the present invention is to provide: a positive electrode for electrolytic copper plating with which plating characteristics such as plating facilitation and via-filling properties can be improved without increasing the complexity of the apparatus structure; and an electrolytic copper plating apparatus using the same. To achieve said purpose, a positive electrode for electrolytic copper plating that is to be disposed in an electrolysis tank in which an electrolytic copper plating solution is stored, is characterized in that: said electrolytic copper plating solution is an acidic electrolytic copper plating solution containing a disulfide compound; and said positive electrode is provided such that a soluble copper positive electrode and an insoluble positive electrode are electrically connected.

Description

Anode for electrolytic copper plating and electrolytic copper plating device using same

The invention relates to an anode for electrolytic copper plating and an electrolytic copper plating device using the same.

Conventionally, when a conductor is formed on a printed wiring board, electrolytic copper plating is performed. In the case of electrolytic copper plating, the anode includes a method using a soluble copper anode made of a copper element and a method using an insoluble anode made of platinum, titanium, iridium oxide, or the like. In addition, in the electrolytic copper plating solution, additives such as a brightener and a leveling agent are added in order to improve plating characteristics such as plating promotion properties and through-hole filling properties.

Here, when electrolytic copper plating is performed, the use of a soluble copper anode has become mainstream. The reason is that, compared with insoluble anodes, the advantages of soluble copper anodes include that the equipment can be simplified, maintenance costs are not required, the anode itself is relatively cheap, and the cost can be reduced.

However, if a soluble copper anode is used, it is generally known that bis (3-sulfopropyl) disulfide is added as a brightener in the electrolytic copper plating solution with the reaction of the copper electrode chemically dissolved in the copper plating solution. (Hereinafter simply referred to as "SPS") will be reduced to 3-mercaptopropane-1-sulfonic acid (hereinafter simply referred to as "MPS"). If the MPS is present in a certain amount or more in the electrolytic copper plating solution, there is a problem that the desired plating characteristics cannot be obtained.

In order to solve the above problems, as described in Japanese Patent Application Laid-Open No. 2004-143478, attempts have been made to increase the dissolved oxygen concentration in the copper plating solution by blowing air into the copper plating solution. Specifically, in Japanese Patent Application (Japanese Patent Application Laid-Open No. 2004-143478), a "plating device for acid copper" is disclosed, which is characterized in that: a plating tank having a parallel overflow tank and a plating liquid provided below the plating tank; In the spout discharge unit, copper anodes, and acid copper plating equipment for rods used for coatings, air stirring and oxygen stirring devices are installed in the overflow tank. "

However, in the plating device disclosed in Japanese Patent Application Laid-Open No. 2004-143478, an overflow groove needs to be separately provided, and the structure of the plating device becomes complicated. Therefore, there is a problem that the equipment cost becomes high.

Based on the above reasons, an object of the present invention is to provide an anode for electrolytic copper plating and an electrolytic copper plating device using the same, which can improve plating characteristics such as plating promotion properties and through-hole filling properties without complicating the device structure. .

After careful study, the inventor of this case achieved the above-mentioned object by adopting the following methods.

The anode for electrolytic copper plating of the present invention is an anode arranged in an electrolytic treatment tank having an electrolytic copper plating solution stored therein, and is characterized in that the electrolytic copper plating solution is an acidic electrolytic copper plating solution containing a disulfide, and the anode In an electronically connected state, a soluble copper anode and an insoluble anode are included.

The electrolytic copper plating device of the present invention: The electrolytic copper plating device of the present invention is characterized by including the above-mentioned anode for electrolytic copper plating. [Inventive effect]

According to the anode for electrolytic copper plating and the electrolytic copper plating device of the present invention, it is possible to improve plating characteristics such as plating promotion properties and through-hole filling properties without complicating the device structure.

Hereinafter, the anode for electrolytic copper plating and the electrolytic copper plating apparatus using the same will be described using drawings. FIG. 1 is a schematic cross-sectional view illustrating a case where the soluble copper anode of the present invention is used in an electrolytic copper plating apparatus.

The electrolytic copper plating device 10 of the present invention includes the anode 1 for electrolytic copper plating of the present invention. The anode 1 for electrolytic copper plating is installed in an electrolytic treatment tank 20 in which an electrolytic copper plating solution 21 is stored. The electrolytic copper plating solution 21 is an acidic electrolytic copper plating solution containing a disulfide (for example, SPS), and the anode 1 includes a soluble copper anode 2 and an insoluble anode 3 in an electronically connected state. These configurations will be described below.

The electrolytic copper plating device 10 of the present invention is a device in which the element W to be plated (cathode) W and the anode 1 are immersed in an electrolytic treatment tank 20 in which an electrolytic copper plating solution 21 is stored. Power is supplied from time to time, and the processed surface of the element W to be plated is subjected to electrolytic treatment. Here, the element W to be plated of the present invention can be used as a printed wiring board or a wafer formed by laminating circuit wiring with an insulating material such as epoxy resin. These printed wiring boards or wafers may be printed circuit boards or wafers having through holes and / or through holes. The through hole and / or through hole is generally a hole with such a small diameter of 10 μm to 1000 μm, and the electronic connection between signal layers is performed through this hole. According to the electrolytic copper plating device 10 of the present invention, it is possible to perform an electrolytic treatment for filling the inside of these through holes and / or through holes with copper. In addition, in the electrolytic copper plating device 10 of the present invention, to improve characteristics such as uniform electrical conductivity, a structure may be adopted. On the one hand, bubbles are diffused in the electrolytic copper plating solution 21, and on the other hand, they are connected to the circulation pipe 5. The nozzle 6 ejects a high-pressure gas or the like to stir the electrolytic copper plating solution 21.

The electrolytic copper plating solution 21 of the present invention is an acid electrolytic copper plating solution containing sulfur dioxide. Generally, the acidic electrolytic copper plating solution 21 is composed of copper sulfate pentahydrate, sulfuric acid, chloride ions, and additives. For example, the composition of the acidic electrolytic copper plating solution 21 can be used in the ranges of copper sulfate pentahydrate 30 g / L to 250 g / L, sulfuric acid 30 g / L to 250 g / L, and chloride ion 30 mg / L to 75 mg / L. The temperature of the acidic electrolytic copper plating solution 21 can be generally used in the range of 15 ° C to 60 ° C, and more preferably 20 ° C to 35 ° C. With the increase in the concentration of copper sulfate pentahydrate or the increase in the concentration of sulfuric acid, the crystals of copper sulfate pentahydrate sometimes precipitate on the copper anode. Therefore, attention must be paid to the concentration management of the two. Here, the sulfuric acid concentration in the acidic electrolytic copper plating solution 21 is preferably 30 g to 400 g / L. When the sulfuric acid concentration is less than 30 g / L, the conductivity of the acidic electrolytic copper plating solution 21 decreases, and it becomes difficult to energize the acidic electrolytic copper plating solution 21. On the other hand, when the sulfuric acid concentration exceeds 400 g / L, copper sulfate easily precipitates in the acidic electrolytic copper plating solution, which adversely affects the characteristics of the coating.

In the electrolytic copper plating apparatus 10 of the present invention, as described above, the soluble copper anode 2 and the insoluble anode 3 arranged in the electrolytic treatment tank 20 function as the anode 1 by being electronically connected. In the case of electrolytic treatment, if the electrolytic copper plating solution 21 containing SPS is used, the SPS will be changed to MPS. Due to the generation of this MPS, the plating uniformity ability in the through-hole bath will be reduced or the appearance of the coating will be poor. Problems such as a decrease in the filling rate in the through-hole filling bath or poor appearance of the plating layer occur. Here, even when electrolysis is stopped and only the electrolytic copper plating solution 21 is left, it can be confirmed that SPS is reduced near the anode 1 and MPS is generated. The generation of this MPS is also the reason for the anode sludge composed of MPS-Cu ⁺ composite. This anode sludge may cause degradation of plating characteristics such as fillability of via holes and uniform plating properties. However, the anode 1 for electrolytic copper plating of the present invention includes a soluble anode 2 and an insoluble anode 3 in the electrolytic treatment tank 20 in an electronic connection state, and the insoluble anode 3 can supply oxygen to the electrolytic copper plating solution 21. Oxygen generated from the insoluble anode 3 can oxidize MPS to SPS, suppress the increase of MPS concentration in the electrolytic copper plating solution 21, and eliminate the adverse effects of MPS. Therefore, according to the electrolytic copper plating device 10 of the present invention, even if the electrolytic copper plating solution 21 contains SPS as a brightener, desired plating characteristics can be obtained.

The soluble copper anode 2 constituting the anode 1 for electrolytic copper plating of the present invention is used to maintain the concentration of copper ions consumed in the electrolytic copper plating solution 21 at a predetermined concentration during electrolysis. The shape of the soluble copper anode 2 is not limited. By adopting a shape with a surface area as large as possible, more copper ions can be generated during electrolysis, and the plating efficiency can be improved.

The soluble copper anode 2 of the present invention is preferably composed of a phosphor-containing copper member. By making the soluble copper anode 2 composed of a phosphorus-containing copper element, a compound film called a Cu ₂ P black film can be formed on the surface of the phosphorus-containing copper element during electrolysis, which suppresses the production of monovalent copper ions and Effectively suppress the generation of anode sludge and prevent the degradation of plating properties. In addition to further suppressing the anode sludge generation of the phosphorus-containing copper element, the phosphorus content should preferably be between 0.02% and 0.06%. The use of a phosphorus-containing copper element on the soluble copper anode 2 facilitates the smooth dissolution of copper during electrolysis.

Regarding the insoluble anode 3 constituting the anode 1 for electrolytic copper plating of the present invention, an anode made of any material can be used as long as it is a material that does not elute the metal in the electrolytic copper plating solution 21. For example, anodes made of iridium oxide, platinized titanium, platinum, graphite, ferrous salts, lead dioxide, and titanium, stainless steel, lead alloy coated with platinum group element oxides can be used, but the present invention is not limited thereto . The insoluble anode 3 may be configured by coating a substrate on a substrate. In this case, the entire surface of the substrate may be covered, but only a part of the substrate may be covered within a range functioning as the insoluble anode 3. In this case, the thickness of the coating is not particularly limited, but it is preferably 0.1 µm to 10 µm from the viewpoints of durability and cost.

The shape of the insoluble anode 3 of the present invention is not limited. The insoluble anode 3 adopts a shape and size that can efficiently generate oxygen without hindering the dissolution of the soluble anode 2 during electrolysis, thereby rapidly oxidizing and reducing MPS existing in the electrolytic copper plating solution 21 to SPS, and suppressing MPS. It accumulates in this electrolytic copper plating solution 21, and prevents a decrease in plating characteristics.

In the anode 1 for electrolytic copper plating of the present invention, from the viewpoint of suppressing the generation of MPS, the area ratio of the surface of the soluble copper anode 2 and the insoluble anode 3 immersed in the electrolytic copper plating solution 21 is preferably 10: 1 to 1:10. If the area ratio of the surface of the soluble copper anode 2 and the insoluble anode 3 immersed in the electrolytic copper plating solution 21 is less than 10: 1, the generation of oxygen from the surface of the insoluble anode (for example, an iridium oxide element) 3 is extremely small, so the electrolysis cannot be sufficiently suppressed The MPS concentration in the copper plating solution 21 is increased, and the desired plating characteristics cannot be obtained. In addition, if the area ratio exceeds 1:10, the generation of oxygen from the surface of the insoluble anode (for example, an iridium oxide element) 3 significantly increases. Therefore, the additives contained in the electrolytic copper plating solution 21 are oxidatively decomposed, which increases the consumption of the additives. the amount. Furthermore, in this case, the copper supplied from the soluble copper anode 2 is insufficient, and in order to maintain the copper concentration in the electrolytic copper plating solution 21 at a predetermined concentration, it is necessary to supply copper from another source. Here, the area ratio of the surface of the soluble copper anode 2 and the insoluble anode 3 immersed in the electrolytic copper plating solution 21 is 5: 1 to 1: 5, and the above effects are more easily obtained.

Further, in the electrolytic copper plating device 10 of the present invention, the applicable cathode current density is preferably within a range in which a phosphorus-containing copper element used in an electrolytic copper plating process for a printed wiring board is used. Specifically, the cathode current density of ^{0.1A / dm 2 ~ 10A / dm} 2, preferably from ^{0.5A / dm 2 ~ 6A / dm} 2, 1A / dm 2 ~ 5A / dm 2 is better. The anode current density can usually be 0.1A / dm ² to 3A / dm ² , and 1A / dm ² to 3A / dm ² is more preferable. The copper concentration in the electrolytic copper plating solution 21 tends to increase when the anode current density is too low, and it tends to decrease when the anode current density is too high. Therefore, the anode area needs to be adjusted according to the cathode current density used.

Here, the effect obtained when the anode 1 for electrolytic copper plating of the present invention is used when electrolysis is stopped and electrolysis is stopped will be described more specifically. Generally, during the electrolysis and when the electrolysis is stopped, the soluble copper anode 2 is dissolved as shown in Formula (1) of the following Chemical Formula 1. In addition, during the electrolysis, the cathode generates a reaction represented by Formula (2) of the following Chemical Formula 1, and copper is precipitated. In addition, when the electrolytic copper plating solution 21 contains a disulfide, the electrons released when the soluble copper anode 2 is dissolved, as shown in formula (3) of the following chemical formula 1, SPS is reduced and MPS is generated . The generated MPS is shown in formula (4) of the following chemical formula 1, and a part of it is oxidized and converted into SPS, but Cu (I) MPS bound to a monovalent copper ion is shown in formula (5) of the following chemical formula 1, It becomes MPS. ^{Cu → Cu 2 + + 2e -} ... (1) Cu 2 + + 2e - → Cu ... (2) SPS + 2H + + 2e - → 2MPS ... (3) 4MPS + 2Cu 2 + → 2Cu (I) MPS + SPS ^{+ 4H + ... (4) 2Cu} (I) MPS + H + + e - → 2Cu + 2MPS ... (5) 2H 2 O → 4H + + O 2 + 4e - ... (6) [ chemical formula 1]

(1), (3) to (5) in the above Chemical Formula 1 show the process of generating MPS which causes the degradation of the plating characteristics. However, the anode 1 for electrolytic copper plating of the present invention is used in combination with the soluble copper anode 2 By including the insoluble anode 3 in the electronically connected state, the increase in the MPS concentration in the electrolytic copper plating solution 21 can be suppressed. In other words, during the electrolysis, the insoluble anode 3 performs the electronic decomposition of the water in the electrolytic copper plating solution 21 as shown in (6) of the above chemical formula 1. At this time, the generated oxygen causes the MPS to be oxidized and converted into SPS. , Can reduce the MPS that has been generated.

By including the above structure, the electrolytic copper plating apparatus 10 of the present invention can suppress the increase in the MPS concentration in the electrolytic copper plating solution 21. Therefore, according to the anode 1 for electrolytic copper plating and the electrolytic copper plating device 10 using the same, even if the electrolytic copper plating solution 21 left for a long period of time is directly used to start electrolysis, it is difficult to cause a poor appearance of the plating layer, and it can be achieved without Ideal for maintenance.

The soluble copper anode of the present invention and the storage method of the electrolytic copper plating solution using the same have been described above. Examples of the present invention will be described below and the present invention will be described in more detail. In addition, this invention is not limited at all by these Examples.

[Example 1] In Example 1, a test was performed to confirm the effect of using a soluble copper anode and an insoluble anode as anodes for electrolytic copper plating in an electronically connected state.

In this embodiment 1, first, a slag removal treatment is performed on a plated element (printed substrate) having a plate thickness of 1.0 mm, a through hole diameter of 100 μm, and a depth of 80 μm by a Melplate MLB-6001 process (manufactured by Meltex Inc.). Next, electroless copper plating was performed by a Melplate CU-390 process (manufactured by Meltex Inc.). Then, this printed circuit board was subjected to acid degreasing, water washing, and sulfuric acid treatment with Melplate PC-316 (manufactured by Meltex Inc.), and then electrolytic copper plating was performed under the conditions shown below.

In the acidic electrolytic copper plating solution used in Example 1, a 3 L through-hole filling bath was used, which was added to a plating solution containing 200 g / L of copper sulfate pentahydrate, 100 g / L of concentrated sulfuric acid, and 50 mg / L of chloride ions. Lucent Copper SVF-A (manufactured by Meltex Inc., disulfide-based) 0.4mL / L, Lucent Copper SVF-B (manufactured by Meltex Inc., disulfide-based) 20mL / L, Lucent Copper SVF-L (Meltex Inc. Manufacturing, disulfide system) 15mL / L adjusted after. The temperature of the acidic electrolytic copper plating solution was 25 ° C.

Then, an anode for electrolytic copper plating was provided in the electrolytic treatment tank in a state of being immersed in the accommodated through-hole filling bath. The anode for electrolytic copper plating is electrically connected in a state where a soluble copper anode (a phosphorous copper plate of 50 mm x 120 nn) and an insoluble anode (a 50 mm x 120 mm iridium oxide-coated plate) are arranged in an electrolytically connected state. In Example 1, the electrolytic treatment was performed in the electrolytic treatment tank while circulating the plating solution using a pump.

In Example 1, the area ratio of the surface of a soluble copper anode and an insoluble anode immersed in an electrolytic copper plating solution was 1: 1. As a cathode, a printed substrate to which electroless copper plating of 50 mm × 130 mm was applied was immersed in an electrolytic copper plating solution. Then, the electrolytic treatment was performed at a current density of 2A / dm ² for 45 minutes under the conditions that the current of the electrolytic copper plating solution was 0AH / L (new bath), 10AH / L, 50AH / L, and 100AH / L. Then, the cross-section method was used to observe the filling conditions of the plating layer in the through-hole under each condition. FIG. 2 is a cross-sectional photograph showing the filling state of the plating layer in the through hole in Example 1. FIG.

[Example 2] In Example 2, a test was performed in the same manner as in Example 1 to confirm the effect when the dissolved copper anode and the insoluble anode were used as anodes for electrolytic copper plating in an electronic connection state.

In this second embodiment, the same element to be plated is used as in the first embodiment. In Example 2, the same electrolytic pretreatment conditions and electrolytic treatment conditions were used as in Example 1, except that Lucent Copper SVF-A (0.4 mL / L) was changed to MPS (1 mg / L). Therefore, the processing conditions used in Embodiment 2 are not described here.

In Example 2, with respect to the anode for electrolytic copper plating, the area ratios of the soluble copper anode and the insoluble anode immersed in the electrolytic copper plating solution were "10: 1", "5: 1", "1: 1", "1: 1 (insoluble anode using platinum-plated titanium)", "1: 5", "1:10". Then, under the conditions that the energization amount of the electrolytic copper plating solution was 0AH / L, 0.5AH / L, 1AH / L, 4AH / L, and 10AH / L, as in Example 1, 50 mm × was applied as a cathode. A 130 mm electroless copper-plated printed substrate was immersed in an electrolytic copper plating solution, and subjected to an electrolytic treatment at a current density of 2 A / dm ² for 45 minutes. Then, the cross-section method was used to observe the filling conditions of the plating layer in the through-hole under each condition. FIG. 3 is a sectional photograph showing the filling state of the plating layer in the through hole in Example 2. FIG.

[Comparative Example 1] In Comparative Example 1, a test was performed to confirm the effect when only a soluble copper anode was used as the anode for electrolytic copper plating.

In Comparative Example 1, the same electrolytic pretreatment conditions and electrolytic treatment conditions were used as in Example 1, except that only a soluble copper anode was used as the anode for electrolytic copper plating. Therefore, regarding those conditions used in Comparative Example 1, explanations are omitted here.

In Comparative Example 1, the same test as in Example 1 was performed. In FIG. 2, in order to be comparable with Example 1, a cross-sectional photograph showing the filling condition of the plating layer in the through hole in Comparative Example 1 is shown.

[Comparative Example 2] In Comparative Example 2, as in Comparative Example 1, a test was performed to confirm the effect when only a soluble copper anode was used as the anode for electrolytic copper plating.

In Comparative Example 2, except for the point that only a soluble copper anode was used as the anode for electrolytic copper plating and the point that Lucent Copper SVF-A (0.4 mL / L) was changed to MPS (1 mg / L), it was adopted as in Examples. 1Same conditions for electrolytic pre-treatment and electrolytic treatment. Therefore, the processing conditions used in Comparative Example 2 are not described here.

In Comparative Example 2, the same test as in Example 2 was performed. In FIG. 3, for comparison with Example 2, a cross-sectional photograph showing the filling condition of the plating layer in the through hole in Comparative Example 2 is shown.

From the results shown in FIG. 2, it can be seen that, as the anode disposed in the electrolytic treatment tank containing the electrolytic copper plating solution, if an anode including a soluble copper anode and an insoluble anode in an electronically connected state is used, and only the soluble copper is used, The situation of the anode is different. Even if the amount of current reaches 100AH / L, there is almost no difference in the filling conditions of the through holes, and stable and excellent filling properties can be obtained.

From the results shown in FIG. 3, it can be seen that, as the anode disposed in the electrolytic treatment tank in which the electrolytic copper plating solution is stored, if an anode including a soluble copper anode and an insoluble anode in an electronically connected state is used, compared with the case where only the anode is dissolved In the case of a copper anode, even if the electrolytic copper plating solution contains 1 mg / L of MPS, the filling property can be restored in a short period of time. At this time, when the area ratio of the soluble copper anode and the insoluble anode immersed in the electrolytic copper plating solution in Example 2 was "10: 1" and the area ratios other than this were compared, it was found that the soluble copper anode The larger the ratio, the more difficult it is to restore filling properties. On the other hand, the smaller the ratio of the soluble copper anode, the less copper is supplied by the soluble copper anode in the electrolytic copper plating solution. In order to maintain the copper concentration in the electrolytic copper plating solution, it is conceivable that Sources supply copper. From the above viewpoints, it is understood that the area ratio of the soluble copper anode and the insoluble anode immersed in the electrolytic copper plating solution is preferably 5: 1 to 1: 5.

From the above, it can be seen that by using the anode for electrolytic copper plating of the present invention and the electrolytic copper plating device using the same, it is possible to improve plating characteristics such as plating promotion properties and via hole filling properties without complicating the device structure. From this, it can be understood that when an electrolytic treatment is performed using the anode for electrolytic copper plating of the present invention, the adverse effects caused by the increase in the concentration of MPS in the electrolytic copper plating solution can be effectively eliminated.

[Industrial advantage] According to the anode for electrolytic copper plating and the electrolytic copper plating device using the same, when the acid electrolytic copper plating solution containing disulfide is used, the increase in the concentration of MPS can be effectively suppressed. The desired plating characteristics are obtained stably. In addition, by using the anode for electrolytic copper plating of the present invention, the structure of the electrolytic copper plating device can be simplified, and the equipment cost can be reduced. Therefore, the anode for electrolytic copper plating of the present invention and the electrolytic copper plating apparatus using the same are particularly suitable for applying an electrolytic plating process to a printed wiring board, a wafer, and the like having through holes and / or through holes.

1‧‧‧Anode for electrolytic copper plating

2‧‧‧Soluble copper anode

3‧‧‧ insoluble anode

5‧‧‧Circular piping

6‧‧‧ Nozzle

10‧‧‧ electrolytic copper plating device

20‧‧‧ Electrolytic treatment tank

21‧‧‧electrolytic copper plating bath

[Fig. 1] A schematic cross-sectional view illustrating a case where the soluble copper anode of the present invention is used in an electrolytic copper plating apparatus. [Fig. 2] A cross-sectional photograph showing a filled state of a through hole in Example 1 and Comparative Example 1. [Fig. [Fig. 3] A cross-sectional photograph showing a filled state of a through hole in Example 2 and Comparative Example 2. [Fig.

Claims (5)

An anode for electrolytic copper plating, which is arranged in an electrolytic treatment tank in which an electrolytic copper plating solution is stored, is characterized in that the electrolytic copper plating solution is an acidic electrolytic copper plating solution containing a disulfide, and the anode includes an electronic connection state and includes Dissolved copper anode and insoluble anode. For example, the anode for electrolytic copper plating of item 1 of the patent application scope, wherein the area ratio of the surface of the electrolytic copper plating solution immersed in the soluble copper anode and the insoluble anode is 10: 1 to 1:10. For example, the anode for electrolytic copper plating of item 2 of the patent application scope, wherein the area ratio of the surface of the electrolytic copper plating solution immersed in the soluble copper anode and the insoluble anode is 5: 1 to 1: 5. An electrolytic copper plating device, characterized in that it comprises the anode for electrolytic copper plating as in any one of the application scope items 1 to 3. For example, the electrolytic copper plating device according to item 4 of the application, wherein the element to be plated is a printed wiring board or wafer having a through hole and / or a through hole, and the inside of the through hole and / or the through hole is filled. Electrolytic treatment of copper.