1. Field of the Invention
The present invention relates to a method for regenerating a plating composition which is suitable for depositing at least one a first metal on a substrate as well as to a regeneration apparatus for regenerating said composition which is suitable for depositing said at least one a first metal on said substrate. Such methods and apparatus are used to regenerate compositions which are suitable for the generation of a metal film such as a nickel, cobalt, or tin film on a substrate, like a plastic, ceramic, glass, and/or metallic part by electroless, i.e., autocatalytic plating of metal.
2. Brief Description of the Related Art
Metal deposition is well-known since decades and has first been used to plate metallic parts like tubings, fittings, valves, and the like. These metal deposits were formed using electrolytic deposition employing an external current source and providing the electric current to the parts and to a counter electrode being in contact with a plating composition.
To plate metal on plastics and on other electrically non-conducting substrates as well as plating metal on parts having isolated metallic regions thereon which cannot be electrically contacted individually, electroless plating was developed. In this case a plating composition is used which contains ions of the metal to be plated and a reducing agent which is capable of reducing the metal to be plated. Such electroless plating compositions have extensively been investigated and used in industry. Electroless plating compositions suitable to plate copper contain, in addition to a copper salt and complexing agents for copper ions, formaldehyde as the reducing agent. These solutions are highly alkaline. Electroless plating compositions suitable to plate nickel contain, in addition to a nickel salt and complexing agents for nickel ions, a hypophosphite salt or the acid thereof, dimethylamine borane, a borohydride, or a hydrazinium salt as the reducing agent. When a hypophosphite salt or the acid thereof is used as the reducing agent, phosphorous will be incorporated into the nickel deposit which might be as much as 12 at.-% of the deposit. When dimethylamine borane or a borohydride salt is used as the reducing agent, boron will be incorporated into the nickel deposit, which might be as much as 5 at.-% of the deposit. When a hydrazinium salt is used as the reducing agent, the nickel deposit may essentially be made of pure nickel, eventually containing a small amount of nitrogen (S. Yagi, K. Murase, S. Tsukimoto, T. Hirato, Y. Awakura: “Electroless Nickel Plating onto Minute Patterns of Copper Using Ti(IV)/Ti(III) Redox Couple”, J. Electrochem. Soc., 152(9), C588-C592 (2005)).
For electroless plating of nickel which is virtually free of any impurities, a nickel plating composition containing, in addition to nickel sulfate, titanium chloride (TiCl3) as a reducing agent has been proposed (M. Majima, S. Inazawa, K. Koyama, Y. Tani, S. Nakayama, S. Nakao, D.-H. Kim, K. Obata: “Development of Titanium Redox Electroless Plating Method”, Sei Technical Review, 54, 67-70 (2002); S. Nakao, D.-H. Kim, K. Obata, S. Inazawa, M. Majima, K. Koyama, Y. Tani: “Electroless pure nickel plating process with continuous electrolytic regeneration system”, Surface and Coatings Technology, 169-170, 132-134 (2003); S. Yagi et al., ibid.).
M. Majima et al., ibid. report that the electroless nickel plating compositions contain nickel sulfate, trivalent titanium chloride, trisodium citrate, nitrilotriacetic acid and an amino acid. The pH of the composition is 8-9 and is adjusted using ammonium hydroxide. Bath temperature is 50° C. The deposition rate is reported to be in a range of from about 0.1 to about 0.2 μm/h. The experiments to show feasibility of nickel deposition were performed using a urethane foam. This resulted in a porous nickel (Celmet) that can be used as a current collector for batteries. The urethane foam was pretreated prior to electroless nickel deposition by contacting the foam with Pd which was absorbed as a catalyst by the sensitizer-activator process.
S. Yagi et al., ibid. report performing nickel deposition on minute patterns on silicon semiconductor devices which have lines and spaces which are as small as 160 nm. The plating composition is similar to that of M. Majima et al.
S. Nakao et al., ibid. additionally report that the deposition rate decreased with increasing the plating time when the concentration of trivalent titanium ions is not controlled. Such decrease would be attributed to a trivalent titanium ion concentration decrease with time because of, in addition to consumption due to the nickel deposition, spontaneous oxidation with dissolved oxygen in the solution. In order to keep the deposition rate constant by keeping the concentration of trivalent titanium ions constant, the deposition solution was subjected to electrolytic regeneration. An apparatus for such regeneration was shown to comprise the plating bath as a catholyte and a sodium sulfate solution as an anolyte and a liquid connection therebetween comprising an ion-exchange membrane.
U.S. Pat. No. 6,338,787 B1 further mentions that tin, cobalt, and lead could also be deposited and that, apart from trivalent titanium, also cobalt, tin, vanadium, iron, and chromium could be used as the reducing agents. This document specifies the ion-exchange membrane of a preparation tank to be an anion exchange membrane. Furthermore, U.S. Pat. No. 6,338,787 B1 reports that an activation process is used to prepare the plating bath which comprises using an electrode as an anode which may be made from the same metal as that of the metal which is deposited. Since the metal ions can be supplied to the plating bath by an anode dissolving reaction in the anode chamber simultaneous with activation of the plating bath by a cathode reaction in the cathode chamber, the composition of the bath can be easily regenerated. A first apparatus is shown which comprises the cathode and anode, wherein the cathode is made from platinum-coated titanium and the anode is made from nickel. In order to suppress nickel deposition on the cathode, its area is kept low so that the electrical current density at the cathode is set greater than the limit electrical current density of nickel electrodeposition. U.S. Pat. No. 6,338,787 B1 also reports using a carbon electrode which is activated with an oxidative process thus more securely preventing deposition of the deposition metal on this electrode during the activation step. A second apparatus is also shown which comprises a cathode chamber with a cathode and an anode chamber with an anode, these two chambers being separated from each other by an anion exchange membrane. The cathode chamber is connected to a plating tank and the anode chamber is connected to an anode liquid tank. The anode liquid is dilute sulfuric acid. In this case, both cathode and anode are made from carbon felt. If a nickel foil was used as the cathode instead, much less efficiency was achieved. Further, U.S. Pat. No. 6,338,787 B1 reports that nickel being deposited on the cathode can be dissolved into the plating bath if this electrode is used as an anode in the next process of activation of the bath.
It has turned out that the plating rate of the plating bath of U.S. Pat. No. 6,338,787 B1 is very low. For example 0.6 μm of nickel are deposited on a Pd-activated ABS resin plate within 2 hours. Such plating rate is too low for most industrial purposes such as manufacture of printed circuit boards, IC substrates, and the like. Furthermore, it also turned out that metal concentration in the plating bath steadily increases due to the use of an anode which is made from the metal to be deposited. Therefore, steady-state conditions cannot be achieved easily. Furthermore, it also turned out that plating out of the metal to be deposited in the regeneration cell occurs easily, if the plating bath is tuned to fast plating. This behavior is detrimental because the ion selective membrane separating the anode and cathode compartments can easily be destroyed.