WO2008016541A1

WO2008016541A1 - Constituent maintenance of a copper sulfate bath through chemical dissolution of copper metal

Info

Publication number: WO2008016541A1
Application number: PCT/US2007/016844
Authority: WO
Inventors: Wayne Charles Walker
Original assignee: Wayne Charles Walker
Priority date: 2006-07-31
Filing date: 2007-07-27
Publication date: 2008-02-07
Also published as: WO2008016541B1

Abstract

A copper electroplating bath is provided with a constant source of copper ions by monitoring the copper concentration in the bath. Copper metal is dissolved with sulfuric acid in the separate system which comprises one or more reactors that are electrically isolated from the bath. The reactors are pressurized with oxygen gas. Plating bath that is depleted of copper is transferred to a mixing tank where it is mixed with copper solution from the reactor(s). The replenished plating solution is filtered and is returned to the plating bath. The dissolution rate of the copper is determined by the rate of oxygen addition to the reactors which in turn is controlled by the pressure of the oxygen in the reactors and the use of a controller such as an ampere-time indicator which monitors the amount of electrical current applied to the plating tank during operation.

Description

Constituent Maintenance of a Copper Sulfate Bath through Chemical

Dissolution of Copper Metal

Technical Field

This invention relates to the field of continuous electrodeposition of copper onto a substrate from an acid copper sulfate plating bath, and the regulation of the copper ion concentration in the bath.

Background Art

Copper sulfate baths are widely used for electroplating copper metal in such diverse applications as printed circuits, electronics, rotogravure, electroforming, decorative plating, and plating on plastics. Such baths are economical to prepare, simple to use, and easy to waste treat. The bath chemistry is simple and comprises a highly conductive electrolyte composed of between about 0.2 M/ Liter and about 1M/Liter of copper sulfate and between about 3.7 M/Liter and about 0.4 M/Liter of sulfuric acid. Typically, a traditional bath contains a high concentration of copper and low acid concentration. Conversely, if a high throwing power bath is needed, high acid and low copper concentrations are used. The bath can be operated at room temperature. Depending on the end use application, the bath generally contains other additives such as brighteners, levelers, anti-pitting compounds and the like. Copper anodes typically have been used to supply the copper ions to the plating bath where the sulfuric acid ionizes the copper in the anode. Copper is plated onto an electrolyzed cathodic electrode. The copper that is consumed during plating must be replenished to allow the bath to continue to function with high efficiency.

The conventional production of copper sulfate is well known and has been adequately described in many publications and claimed in numerous patents. One such method is described in United States patent 6,294,146 Bl issued to Gabriel Benet on September 25, 2001. This method utilizes a continuous chemical reaction for manufacturing copper sulfate. The contents of this patent are included herein by reference. The copper and acid content of a typical copper electroplating bath are maintained by dissolving copper from an electrically active anode while simultaneously depositing the metal on a cathode. This method has been used commercially for many years and is successful due to the high efficiency of copper being transferred from the anode to the cathode. While efficient and the technology well known, there are problems with particles dislodging from the anode. These particles can either deposit on the pristine cathode surface as bumps, or can chemically dissolve into the bath solution and force the copper concentration to rise. Both alternatives are undesirable.

The problem of particle deposition on the cathode can be reduced by placing the anode in a bag that served to filter the particles. This means the metallic particles are not electrolytically bound to the rest of the anode, and tend to chemically oxidize in the bag and coat the anode holder or the remaining anode metal. This buildup required the anode bag to be cleaned frequently. If left alone, the buildup changes the current density characteristics of the anode and leads to burning as well as other problems.

Another very effective way to reduce particle generation is by the incorporation of phosphorus into the copper anode matrix. As the copper is removed from the anode, the phosphorus is left behind on the metal surface and acts as a gel that retards the metal particles from leaving the anode. Even with mild agitation, the gel holds the particle in electrical contact with the main body of the anode. This contact allows the copper to dissolve with high efficiency through electrolytic conditions. While the phosphorus has an insulating effect on the anode, its weak physical bonding strength keeps too large a quantity from building on the anode by simply falling off during the stress of increased voltage or mechanical agitation. Once detached from the anode, the phosphorus drops down in the filter bag where it gradually builds up. The low concentration of phosphorus required to form an effective gel requires cleaning of the filter bag once every six to twelve months instead of monthly when compared to a non-phosphorus containing anode.

Various methods of electroplating have found that the electrolytic dissolution of a copper anode may not be effective when compared to a filtered solution of copper sulfate in acidic media. Using a polish filtering step to remove particles leads to a clean plating medium. Also, the use of high agitation sweeps fresh solution into pores, recesses, and other low current density areas that normally are difficult to plate uniformly. While effective at keeping the cathode clean, this high agitation is detrimental to the typical anode. The bag material is resistant to the corrosive nature of the plating solution, but the bag will decompose thereby contributing to the particles in the bath directly as lint or allowing an increasing amount of anode particles to pass into the bath. This can be overcome by the use of an insoluble anode while replenishing the copper ions in a separate part of the system. By being segregated from the deposition area, the copper ion source can be controlled before coming into contact with the sensitive areas to be coated with copper metal. No particles can come in contact with the cathode.

Some early efforts to add copper have used various forms of copper and copper oxide. Generally, a grade of material is used that consists of a mixture of mainly cuprous oxide with some cupric oxide and a residue of copper metal fines. Either the sulfuric acid added to the make up of the bath or the sulfuric acid generated during the plating cycle acts to neutralize the alkaline copper oxide and create cupric sulfate. Over a period of time the copper oxide dissolves and neutralizes the electrolytically formed sulfuric acid. This method can be difficult to control due to the greatly different reactivities of copper in the three forms found in copper oxide. Cupric oxide dissolves readily in sulfuric acid, but cuprous oxide requires an oxidative agent to avoid disproportionation into copper metal and cupric ion. The copper metal, whatever the source, must be oxidized to form cupric ions. It is fairly simple to oxide copper metal. A very common method is to bubble air into the solution. The oxygen component will partially dissolve into the solution and will react with copper metal to be converted into cupric ions. The use of air is considered inexpensive, but the total cost of its use should be taken into consideration. The air may be free, but must be compressed to force it into the solution. Also, the air must be filtered to remove any oil, dust, or other contamination to avoid transferring dirt into the solution. These costs may not be major, but do contribute to operation expenses. It should also be noted that 78% by volume of the air comprises nitrogen which is unreactive and can generate a misting problem when it bubbles through the solution. Cupric oxide has been considered as a source to replenish the plating solution. The major problem with this material is that of pricing. While cupric oxide is relatively pure, its preparation requires several more steps than the procedure for cuprous oxide. Due to the additional effort required to make it, the cost of cupric oxide is higher per unit of copper than the spot market price of copper metal.

Copper metal is the cheapest source of copper which is why it has been used to make anodes. The use of copper anodes is an efficient source of copper ions. By the judicial use of proper power, appropriate bag filtering, and phosphorus entrapment of blown off metal particles, the efficiency is greater than 99 percent for the transfer of copper from the anode to the cathode. A new method must approach this transfer efficiency while maintaining a semi- or fully automatic control of copper transfer.

DISCLOSURE OF THE INVENTION

One object of the present invention is a method to prepare soluble copper from copper metal. It does so while maintaining solution control such that the copper and sulfuric acid concentrations do not vary by more than 5 percent from what the user considers optimum.

The present invention addresses the difficulties of existing processes and uses the cost benefits of lower priced materials. The invention removes the source of particles by using an insoluble anode while replenishing copper ions and removing excess acid in an isolated dissolution module. This module is controlled by monitoring the current in the plating tank through a device such as an ampere-hour meter that triggers the release of a controlled amount of reagent to dissolve the copper. The metered reagent is inexpensive and harmless to the plating operation. It is easily removed when used in excess of a stoichiometric value required to maintain the solution.

The invention also relates to an apparatus, system and process for electroplating copper on a substrate. The apparatus includes an insoluble anode, and an electrical connection that connects to an object to be plated, and applies a potential allowing the object to become a cathode. At least one reactor that is electrically isolated from the plating chamber is used for the chemical dissolution of copper, and is maintained at a pressure of between about 40 psi ( 2.8 kg/cm² ) and about 80 psi (5.6 kg/cm² ). A monitoring device such as an ampere-time indicator is used to control the amount of oxygen added to the dissolution reactor. By measuring the electrical current consumption, the ampere-time indicator directs oxygen at a pressure between about 5 psi (0.35 kg/cm² ) and about 10 psi (0.7 kg/cm² ) higher than the reactor pressure to be added to plating solution that is deficient in copper before contact with the copper in the dissolution reactor. The oxygen containing acid solution is maintained at a temperature of between about 25° and about 80° Celsius before addition to the dissolution reactors. The apparatus also includes a mixing tank where copper depleted solution from the plating tank is returned and is replenished with the higher copper containing solution generated by the chemical oxidation of metallic copper in the reactor. This replenished solution preferably is filtered before being returned to the plating tank.

BRIEF DESCRIPTION OF THE DRAWING

The drawings as described herein are presented for the purpose of illustrating the invention, and its environment, and are not intended to serve as a limitation on the invention.

Fig. 1 shows the overall apparatus used for the practice of the present invention;

Fig. 2 is a perspective view of a pair of copper dissolution reactors; and

Fig. 3 is a partially sectioned elevation view of one of the dissolution reactors

MODES FOR CARRYING OUT THE INVENTION

Referring to the drawings, Fig. 1 shows a plating tank 10 containing an object 12 that is to be cathodically plated with copper deposited by the electromotive potential of a rectifier 14, and insoluble anodes 16 capable of completing the electrical connection of the plating operation. The object 12 is hung from a stringer 18 suspended from a rack (not shown) mounted over the plating tank 10. The plating tank 10 is physically and electrically isolated from a source of copper metal that serves to replenish the ionic copper in solution. An excess of high purity copper is supplied by packing the metal in one or more pressurized dissolution vessels such as vertical reactors, two of which are shown connected in series in the Figs 1 and 2 as 20a, 20b. The reactors typically are mounted on a platform 44 as shown. The reactors may be any size that is practical to manufacture, but appropriate replenishment rates may be obtained from reactors having diameters between about 2 inches (5.1 cm) and about 12 inches (30.5 cm) and lengths between about 2 feet (51 cm) and about 12 feet (3.65 meters). The reactors can easily be taken offline for replenishment with fresh copper when a quantity has been consumed. Removal of the dissolution reactors from the system for a period of 1 to 2 hours will not have a serious effect on the plating operation. Although one reactor is sufficient for carrying out the teachings of the present invention, multistage reactors in series are preferred.

Each dissolution reactor has a top inlet 24a, 24b where a stream of depleted plating solution is introduced. The solution is transferred by pump 42 and is mixed with an oxidizer, preferably oxygen gas from an oxygen source 28 such as an oxygen generator or a pressurized tank. A throttling valve 30 serves to control the pressure of the oxidizer before introduction into the reactor. It should be understood that the streams may be mixed before or after entering the reactor. When multiple reactors are used, it is desirable to add the oxidizing gas only in the first reactor 20a.

A partial cross sectional view of the interior of the first reactor is shown in Fig. 3. As previously described, the reactor 20a may be mounted on a platform 44. A plurality of small rods 38 of pure copper are shown stacked in the reactor. All of the rods are positioned so that they will be wetted by the liquid 36 in the reactor.

The dissolution reactors preferably are fabricated from a suitable material such as stainless steel or plastic. To minimize the possibility of the acid and oxygen dissolving part of the metal from which the reactor is constructed, thereby contaminating the bath, the level of copper metal in the reactors should not become too low. It has been found that about 2/3 of the copper is dissolved in the first reactor, when using multiple reactors. Therefore, the first reactor should be kept at least about 50% full of copper. Because of the relatively low temperatures at which the oxidation of the copper occurs within the reactors, they can be constructed from plasties such as HDPE or CPVC that will resist 10% sulfuric acid and oxygen. The use of these polymeric materials can serve to substantially reduce the construction costs and the overall weight of the reactors. However, it should be observed that such a reactor might not withstand high pressures in the event of a control or process malfunction, resulting in a build up of pressure in the reactor.

Enriched plating solution is collected at the bottom of the reactor. The solution contains copper ions at a level that will allow the desired deposition of copper metal on the cathode. The solution may contain from about 5 to about 80 grams per liter, and preferably between about 50 and about 70 grams per liter of copper. The bath solution contains acid, preferably sulfuric acid, at a concentration that allows adequate conductivity of the bath while permitting copper to deposit on the cathode, and retaining plating additives and bath impurities in solution. The sulfuric acid concentration is between about 30 and about 250 grams per liter, preferably between about 50 and about 70 grams per liter.

The bath solution in the reactors is maintained at an appropriate temperature to allow the copper to be oxidized to cupric ions. The temperature may range from about 25° to about 80° Celsius, preferably at about 25° to about 35° Celsius. The liquid flow through the dissolution reactors 20a, 20b is sufficient to provide adequate acid to dissolve the copper and the pressure is able to dissolve the oxygen while not substantially changing the ratio of copper to acid in the solution. The pressure of the liquid in the reactors is from about 20 to about 100 psi, preferably between about 50 psi (3.5 kg/cm² ) and about 80 psi (5.6 kg/cm² ) as measured by the pressure gauge 46. Due to the high efficiency of the oxygen in converting copper metal to cupric salt, the addition of oxygen is controlled through a meter 40 that monitors the ampere-hours of current used to plate out copper on the cathode. The meter also opens a valve 30 to allow the flow of oxygen from the source 28 to the inlet 24a of the first reactor 20a to permit the chemical oxidation of copper metal. No copper is dissolved unless oxygen remains in contact with the metal. Oxygen is added to the reactor at a pressure of up to 20 psi greater than the liquid pressure in the dissolution reactor, preferably 5 to 10 psi greater.

The copper enriched solution is transferred through outlet 32a at the bottom of the reactor 20a to the top inlet 24b of the second reactor 20b. The solution, which is further enriched in reactor 20b is then transferred from the bottom discharge 32b through a control valve 34 to a mixing-venting tank 50. There it is blended with depleted plating solution that because of its reduced copper content is slightly less dense that the copper enriched solution and rises to the top of the plating tank 10, The depleted plating solution is transferred through the weir 56 from the plating tank 10. While the enriched solution is in the mixing-venting tank, oxygen bubbles are allowed to escape the liquid so as to not be transferred to the plating tank where the presence of entrained oxygen could cause pitting on the objects being plated.

A stream of the replenished copper sulfate/ sulfuric acid solution free of entrained oxygen is transferred by pump 54 from the mixing tank 50 through a polishing filter 52 to the plating tank 10 where the insoluble anodes 16 and active cathode (object 12 to be plated) are located and copper is deposited on the cathode. The filter 52 can be any type of filtering agent that has a resistance to sulfuric acid and that has been washed free of any water soluble agents. Plating solution depleted of copper is returned from the plating tank 10 through weir 56 to the mixing tank 50 where it is rebalanced. The plating tank solution is exchanged with the mixing tank at a rate of 1 to 6 plating tank volumes per hour, preferably 2 to 3 plating tank volumes per hour.

The following conditions are typical of what is required to carry out the invention. Numerous minor modifications may be made to adjust the system by those familiar with the technology while remaining within the general scope of the invention.

A plating solution is prepared from copper sulfate and sulfuric acid at a ratio of 1.043 copper to sulfuric acid in grams per liter. The initial concentration of copper is 58.0 g/L and the sulfuric acid is 55.6 g/L. The specific gravity is determined to be 1.1712 g/cc at 20° Celsius. The solution is added to the plating system shown in Figure 1. The filter cartridge is a nominal 10 microns of woven polypropylene fiber. The dissolution reactors are packed with 1 inch (2.54 cm) long by 5/16 inch (0.8 cm) diameter copper nuggets. An immersion heater (not shown) is added to the mixing tank and is attached to a temperature control system set at a maximum of 32° Celsius. No other outside heating is applied to the liquid to be pumped to the dissolution reactors. A suitable pump 42 for pressurizing the reactors is an air powered diaphragm pump. A pump 54 useful for filtering the plating tank solution is a magnetic drive pump. The anode is non-copper containing and is insoluble in the solution. The cathode is whatever is desired to be plated. The electrolyzing power is through a rectifier capable of providing direct current up to 50 amperes at various voltages.

Pump 54 is started to establish flow into the plating tank and to allow solution to return to the mixing tank at a rate of 2-3 bath turn-overs per hour. Pump 42 is then started and valve 34 is partially closed to raise the pressure in the reactors to 40 psi. A throttling valve 34 is adjusted to the oxygen inlet such that the oxygen pressure is 45 psi when oxygen is added to the reactor inlet as measured by gauge 46.

Power is provided from the rectifier to begin plating copper on the cathode. The voltage varies as the bath temperature changes, but is typically at 3.5 to 4.1 volts for the system while the bath temperature is maintained between 25° and 35° Celsius. During a typical run of 6 to 8 hours, the bath is analyzed by titration at the end of each testing period for copper content and sulfuric acid concentration. The Specific Gravity of the solution is measured to the nearest 0.0001 g/cc using an Anton Paar DMA 60 Density Meter equipped with a DMA 602 Density Measuring Cell and temperature controlled with a constant temperature bath set at 20.0° Celsius. The cathode is removed, rinsed with deionized water, air dried before weighing, then compared to the weight of the cathode before the run.

Table I

Relationships between Copper and Sulfuric Acid in Examples 1-17

Examples 1-3 show the high efficiency of copper deposition based on electrical current flow rates although the reaction temperature is too low to allow copper to dissolve at the low range of pressure. During this period copper was removed from solution by electrodeposition, and was not replenished by chemical dissolution.

Example 4 was run to show that the chemical oxidation of copper occurs once the minimum temperature is reached for the lower end of the pressure range.

Examples 5-6 were run to show that the copper can be removed from the plating bath through electroplating without having a major effect on the dissolution of copper in the reactors. Examples 7-10 demonstrate the stability of the system over an extended period of operation. The copper and acid concentrations can be adjusted during operation by adjusting the amount of oxygen added to the reactors.

Examples 11-13 show that the dissolution rate of copper is increased by a small increase in the pressure of oxygen in the reactors while maintaining a constant bath temperature.

Examples 14-17 show that the system is stable during a major time period if the dissolution rate of copper is controlled by adjusting the oxygen flow rate in the dissolution reactors.

Example 1

Power was on for 7 hours and 6 minutes. The average current was 44 amperes. Oxygen was added to the first dissolution reactor at 40 psi (2.8 kg/cm²) during the run. The bath temperature ranged from 20 to 25° Celsius. No heat was added through the immersion heater. The pressure in the reactors was 32 psi (2.24 kg/cm² ). At the end of the run, the weight of the cathode had increased by 0.82 pounds (100% efficiency). The ratio of copper to sulfuric acid decreased from 1.043 to 0.932 and the Specific Gravity decreased from 1.1712 to 1.1707.

Example 2

Power was on for 8 hours. The average current was 45 amperes. Temperature of the bath ranged from 19.5° to 25° Celsius. No heat was added through the immersion heater. Oxygen flow to the first dissolution reactor was continued for 4 hours and 11 minutes. At the end of the run, the weight of the cathode had increased by 0.94 pounds or 0.43 kg (100% efficiency). The ratio of copper to sulfuric acid decreased from 0.932 to 0.841 and the Specific Gravity decreased from 1.1707 to 1.1685.

Example 3

Power was on for 7 hours and 50 minutes. The average current was 45 amperes. Temperature of the plating bath ranged from 19.5° to 25° Celsius. No heat was added through the immersion heater. Oxygen flow to the first dissolution reactor continued for 8 hours. At the end of the run, the weight of the cathode had increased by 0.92 pounds or 0.42 kg (100% efficiency). The ratio of copper to sulfuric acid decreased from 0.841 to 0.814 and the Specific Gravity decreased from 1.1685 to 1.1667.

Example 4

No power was on during this run. Temperature of the bath ranged from 21 ° to 31.5° Celsius. The immersion heater was turned on at the beginning of the run and the bath reached a temperature of 30° after two hours. A pressure of 30 psi (2.1 kg/cm² )was applied to the liquid in the reactors and oxygen applied for 8 hours. The ratio of copper to sulfuric acid increased from 0.814 to 0.949 and the Specific Gravity increased from 1.1667 to 1.1710.

Example 5

Power was on for 3 hours and 27 minutes. The average current was 45 amperes. Temperature of the bath ranged from 17° to 32° Celsius. The immersion heater was turned on at the beginning of the run and the bath reached a temperature of 32° after 90 minutes. The liquid pressure in the reactors was 30 psi (2.1 kg/cm² ). The oxygen was added to the reactors over a period of 8 hours at a pressure of 37 psi (2.6 kg/cm² ). The ratio of copper to sulfuric acid increased from 0.949 to 1.006 and the Specific Gravity increased from 1.1710 to 1.1728.

Example 6

Power was on for 3 hours and 44 minutes. The average current was 45 amperes. Temperature of the bath ranged from 22.5° to 34° Celsius. The immersion heater was turned on at the beginning of the run and the bath reached a temperature of 30° after 90 minutes. The liquid pressure in the reactors was 30 psi (2.1 kg/cm² ). The oxygen was added to the reactors over a period of 7 hours and 33 minutes at a pressure of 37 psi (2.6 kg/cm ). The ratio of copper to sulfuric acid increased from 1.006 to 1.122 and the Specific Gravity increased from 1.1728 to 1.1776. Example 7

Power was on for 8 hours. The average current was 45 amperes. Temperature of the bath ranged from 22° to 35° Celsius. The immersion heater was turned on at the beginning of the run and the bath reached a temperature of 31 °' after 90 minutes. The liquid pressure in the reactors was 30 psi (2.1 kg/cm² ). The oxygen was added to the reactors over a period of 8 hours at a pressure of 37 psi (2.6 kg/cm² ). The ratio of copper to sulfuric acid remained the same at 1.122 and the Specific Gravity decreased from 1.1776Jo 1.1767.

Example 8

Power was on for 5 hours and 45 minutes. The average current was 45 amperes. Temperature of the bath ranged from 27 to 34° Celsius. The immersion heater was turned on at the beginning of the run. The liquid pressure in the reactors was 30 psi (2.1 kg/cm² ). The oxygen was added to the reactors over a period of 1 hour and 33 minutes at a pressure of 37 psi (2.6 kg/cm² ). The ratio of copper to sulfuric acid decreased from 1.122 to 1.042 and the Specific Gravity decreased from 1.1776 to 1.1736.

Example 9

Power was on for 7 hours. The average current was 45 amperes. Temperature of the bath ranged from 25° to 34° Celsius. The immersion heater was turned on at the beginning of the run. The liquid pressure in the reactors was 30 psi. The oxygen was added to the reactors over a period of 7 hours at a pressure of 37 psi. The ratio of copper to sulfuric acid decreased from 1.042 to 1.013 and the Specific Gravity decreased from 1.1736 to 1.1723.

Example 10

Power was on for 7 hours and 45 minutes. The average current was 42 amperes. Temperature of the bath ranged from 27 to 35° Celsius. The immersion heater was turned on at the beginning of the run. The liquid pressure in the reactors was 30 psi (2.1 kg/cm² ). The oxygen was added to the reactors over a period of 7 hours and 45 minutes at a pressure of 37 psi (2.6 kg/cm² ) The ratio of copper to sulfuric acid decreased from 1.013 to 1.005 and the Specific Gravity decreased from 1.1723 to 1.1719.

Example 11

During this run, the power was on for 7 hours and 45 minutes. The average current was 42 amperes. Temperature of the bath ranged from 26° to 35° Celsius. The immersion heater was turned on at the beginning of the run. The liquid pressure in the reactors was 34 psi (2.4 kg/cm² ). The oxygen was added to the reactors over a period of 7 hours and 45 minutes at a pressure of 39 psi (2.7 kg/cm² ) The ratio of copper to sulfuric acid decreased from 0.957 to 0.904 and the Specific Gravity decreased from 1.1750 to 1.1732.

Example 12

Power was on for 6 hours and 45 minutes. The average current was 42 amperes. Temperature of the bath ranged from 27° to 35° Celsius. The immersion heater was turned on at the beginning of the run. The liquid pressure in the reactors was 37 psi (2.6 kg/cm² ). The oxygen was added to the reactors over a period of 6 hours and 45 minutes at a pressure of 43 psi (3.0 kg/cm² ). The ratio of copper to sulfuric acid increased from 0.904 to 0.913 and the Specific Gravity decreased from 1.1732 to 1.1724.

Example 13

Power was on for 8 hours and 15 minutes. The average current was 42 amperes. Temperature of the bath ranged from 21° to 34° Celsius. The immersion heater was turned on at the beginning of the run. The liquid pressure in the reactors was 38 psi (2. kg/cm² ) The oxygen was added to the reactors over a period of 8 hours and 15 minutes at a pressure of 43 psi (3.0 kg/cm² ). The ratio of copper to sulfuric acid decreased from 0.913 to 0.900 and the Specific Gravity remained at 1.1724.

Example 14

Power was on for 7 hours. The average current was 42 amperes. Temperature of the bath ranged from 21° to 34° Celsius. The immersion heater was turned on at the beginning of the run. The liquid pressure in the reactors was 38 psi. The oxygen was added to the reactors over a period of 7 hours at a pressure of 43 psi. The ratio of copper to sulfuric acid decreased from 0.900 to 0.891 and the Specific Gravity decreased from 1.1724 to 1.1717.

Example 15

The power was on for 7 hours and 10 minutes. The average current was 42 amperes. Temperature of the bath ranged from 26° to 34° Celsius. The immersion heater was turned on at the beginning of the run. The liquid pressure in the reactors was 38 psi (2.7 kg/cm² ). The oxygen was added to the reactors over a period of 7 hours and 10 minutes at a pressure of 43 psi (3.0 kg/cm² ). The ratio of copper to sulfuric acid decreased from 0.891 to 0.887 and the Specific Gravity increased from 1.1717 to 1.1727.

Example 16

Power was on for 6 hours and 50 minutes. The average current was 42 amperes. Temperature of the bath ranged from 26° to 34° Celsius. The immersion heater was turned on at the beginning of the run. The liquid pressure in the reactors was 38 psi. The oxygen was added to the reactors over a period of 6 hours and 50 minutes at a pressure of 43 psi. The ratio of copper to sulfuric acid increased from 0.887 to 0.945 and the Specific Gravity decreased from 1.1727 to 1.1723.

Example 17

Power was on for 7 hours and 50 minutes. The average current was 42 amperes. Temperature of the bath ranged from 27° to 34.5° Celsius. The immersion heater was turned on at the beginning of the run. The liquid pressure in the reactors was 38 psi (2.7 kg/cm² ). The oxygen was added to the reactors over a period of 7 hours and 50 minutes at a pressure of 43 psi (3.0 kg/cm² ) The ratio of copper to sulfuric acid decreased from 0.945 to 0.937 and the Specific Gravity increased from 1.1723 to 1.1734. The concentration of copper ions in the plating tank is the driving force that determines if and when copper-enriched solution should be added to the bath. One method of determining this ion concentration is the use of an ampere-hour meter to determine the depletion rate of copper from the bath and to add an oxidizer to the dissolution tower system to replenish dissolved copper. This method has the advantage that it determines the total depletion of copper ions in the total system and is not concerned with localized mixing problems. This method is simple to operate and has been used in numerous situations to replenish consumed components in baths. This type of control is relatively insensitive to interferences from plating additives or changes in concentration due to evaporation, dilution, and drag out. The following example shows the use of an ampere-hour meter to monitor and control the operation of the dissolution reactors.

Example 18

The addition of oxygen to the inlet 24a of reactor 20a is controlled by an ampere- hour meter such that valve 30 is open from the oxygen source through a flow meter after the feed pressure is regulated to the desired value above the pressure measured on gauge 46. Oxygen is released to reactor 20a for a predetermined period when a preset amount of current has been used by the plating operation. Running the system to consume 1.0 ampere-hour will remove 1.2 grams of copper from the solution and require only 0.3 grams of oxygen to replenish the solution. Using STP conditions for the conversion of mass to volume, the amount of oxygen required for each ampere-hour is 210 mLs. The plating of copper is 100% efficient, but the oxygen consumption is lower due to the poor solubility of the gas in a salty solution. The elevated pressure of the dissolution reactors improves the solubility of oxygen in the solution such that the oxygen consumption efficiency approaches 100%. A slight excess of oxygen is very inexpensive while allowing reasonable oxidation rate of the copper in the reactors. The excess of oxygen can be reduced by visual monitoring of the liquid discharge into the mixing-venting tank 50 after valve 34. Such control is not available when air is used as the oxidation medium due to the presence of non reacting nitrogen. Out gassing at valve 34 discharges into tank 50 and is minimal when oxygen is used and monitored to determine consumption rates. Ampere-hour meters useful in the teachings of the present invention are available from numerous suppliers such as Dynapower Corporation, Enthone-OMI and Atotech GmbH.

There are various other methods of monitoring and controlling bath constituents for the purpose of regulating the oxidation of copper in the reactors. The system can be monitored by colorimetric measurements, conductivity monitoring, potentiometric means, density determination, or cyclic voltammetric stripping (CVS).

• Colorimetric: An Ultraviolet-Visible Spectrophotometer (UV-Vis) can be used to follow copper concentration and to add oxygen when the value starts to decrease. This method can be used to follow the amount of copper in solution and use defined upper and lower limits to set the period of time that an oxidizer is added to the dissolution tower system. Since the copper ion is brightly colored in solution an ultraviolet-visible spectrophotometric instrument may be used to follow copper concentration changes and control the addition of oxidizer. Addition will stop when a predetermined upper concentration is reached. This method will lag the removal of copper during the plating operation but can be adjusted to reduce the time required to replenish copper ions. The solution control is affected by poor mixing issues and may lead to regions of high and low copper salts in the plating tank. Also, various plating additives may interfere with this type of monitoring.

• Conductivity: Follows the ionic character of the plating solution. Due to differences in conductivity-resistivity between acid and copper salt solutions, a conductivity meter or a device using a modified Wheatstone bridge can be used to determine consumption of soluble copper with a corresponding increase in acid. Acids are typically more conductive than copper ions and will lead to an increase in solution conductivity as copper is plated out of the bath and replaced with acid generated at the insoluble anode. The conductivity meter is attached to the control valve which releases oxidizer to the dissolution tower system as required. This method has similar drawbacks to those encountered when using colorimetric analysis, and is sensitive to temperature fluctuations in the system.

• Potentiometric: This monitors the concentration of copper in solution by its ease of plating. The electromotive potential of the plating bath shifts with the reduction of ionic copper, and monitoring the redox potential is used to follow changes in the bath. Again, plating additives may interfere with control of the copper to acid ratio using this method.

• Density: Since there are only two principal components in the bath solution, monitoring the density will show any changes in concentration. Due to the relative weight difference between copper and hydrogen it is possible to monitor the ratio of copper to acid by determining the specific gravity of the bath. A portable densitometer can be used to control large changes while a more sensitive device can be used with an automatic sampler and portions stabilized with constant temperature baths to reduce temperature variations.

• CVS This system of cyclic voltammetric stripping is available from ECI Technology, Inc. in Wayne, New Jersey.

It should be understood that these other monitoring procedures likewise fall within the scope and spirit of the present invention.

Typically, the bath components are monitored by independent methods in an analytical laboratory. These laboratory methods may be more time consuming than the in situ methods but will confirm the composition of the bath and correct potential problems before the bath can drastically shift out of balance in the event the direct bath monitor should fail.

Control using pH measurements would be unsatisfactory because the bath is too acidic INDUSTRIAL APPLICABILITY

The invention is applicable to acid copper plating processes for many diversified uses such as printed wiring board plating, electronic hardware, rotogravure cylinder plating, electroforms, decorative items, and plating on plastics.

While the invention has been described in combination with specific embodiments thereof, there are many alternatives, modifications, and variations that are likewise deemed to be within the scope thereof. Accordingly, the invention is intended to embrace all such alternatives, modifications and variations as fall within the spirit and scope of the appended claims.

Claims

What is claimed is:

1. An apparatus for electroplating copper on a receptive substrate from an acid copper plating bath, the apparatus comprising a plating tank containing

a. a cathode electrical connection that connects to the substrate and applies a potential allowing the substrate to become a cathode; and

b. an insoluble anode;

the apparatus further characterized by

c. at least one pressurized vessel for the chemical oxidation of metallic copper, said vessel being electrically isolated from the plating bath; and

d. a monitoring device for controlling the amount of oxidizing agent added to the at least one pressurized vessel, based upon the concentration of copper ions in said plating bath.

2. The apparatus according to claim 1, further characterized by a mixing tank in which copper depleted solution from the plating tank is replenished with copper solution containing a higher concentration of copper generated by the chemical oxidation of metallic copper in said at least one pressurized vessel.

3. The apparatus according to claim 2 including a filter to remove particulates from the replenished solution before being returned from the mixing tank to the plating tank.

4. The apparatus according to claim 1 wherein the at least one pressurized vessel comprises a reactor system consisting of between 1 and 4 dissolution reactors connected in series and maintained at a pressure of between about 40 psi (2.7 kg/cm² ) and about 80 psi (5.4 kg/cm² ).

5. The apparatus according to claim 4 wherein each dissolution reactor comprises a tower having a diameter between about 2" (5.1 cm.) and 12 inches (30.48 cm) and a length of between about 2 feet (61 cm.) and about 12 feet (365 .8 cm).

6. The apparatus according to claim 2 wherein the monitoring device is an ampere- time indicator which controls the addition of oxidizing agent to an inlet of said at least one pressurized vessel based on the monitored usage of copper ions in the plating bath.

7. The apparatus according to claim 6 wherein the oxidizing agent is oxygen, and the ampere-time indicator directs oxygen at a pressure that is 5 psi to 10 psi higher than the pressure of the pressurized vessel to be added to the depleted copper solution before contact with the copper in the first of the dissolution reactors.

8The apparatus according to claim 7, wherein the containing acid solution is maintained at a temperature of between about 25° and about 80° Celsius before addition to the pressurized reactor system.

9.The apparatus according to claim 8, wherein the reactor system serves to replenish the copper solution by the dissolving action of sulfuric acid and a controlling amount of oxygen on copper metal before this solution is returned to the mixing tank.

10. A process for electroplating copper metal on to a receptive substrate from an acid copper plating bath, comprising the steps of:a) maintaining the plating bath comprising an electrolyte and a source of copper ions in solution or suspension; and

b) passing a rectified DC current through the plating bath; the process further characterized by c) generating copper sulfate as the source of the copper ions for the bath; and d) controlling the demand for the addition of the generated copper sulfate based upon the concentration of copper ions in the plating bath.

11. The process according to claim 10 wherein the copper sulfate is generated in a pressurized system that is electrically isolated from the plating bath.

12. The process according to claim 11 wherein copper metal is oxidized to copper sulfate in the pressurized system utilizing an oxidizer in the presence of sulfuric acid.

13. The process according to claim 12 wherein the copper metal is oxidized to copper sulfate utilizing oxygen gas as the oxidizer at a temperature above ambient temperature.

14. The process according to claim 14 wherein the pressurized system consists of at least one dissolution reactor that is maintained at least 50% full of copper metal.

15. The process according to claim 13 wherein the concentration of copper is enriched to between about 5 and about 80 grams/ liter of dissolved copper.

16. The process according to claim 14 wherein the generated copper sulfate is sent to a mixing tank prior to being introducing into the plating bath.

17. The process according to claim 16 wherein copper depleted solution flows from a plating tank into said mixing tank where it is replenished with the dissolved copper sulfate pumped into the mixing tank.

18. The process according to claim 17 wherein the replenished solution is pumped through a filter before being transferred from the mixing tank to the plating tank.

19. The process according to claim 18 wherein between about 1 and about 6 volumes of solution in the plating tank are circulated between the plating tank and the mixing tank each hour.

20. The process according to claim 15 wherein depleted plating solution in the mixing tank is pumped to the pressurized system that is maintained at a pressure of between about 40 psi ((2.7 kg/cm² ) and about 80 psi (5.4 kg/cm² ).

21. The process according to claim 15 wherein the amount of electrical current applied to the plating tank is monitored by an ampere-time indicator, and the consumption of copper that is plated is used to control the addition of oxidizer to the pressurized system.

22. The process according to claim 11 including maintaining the concentrations of copper and sulfuric acid in the plating solution to within about 5% of predetermined optimum limits.