Electrolysis: processes – compositions used therein – and methods – Electrolytic coating – Depositing predominantly single metal coating
Reexamination Certificate
2002-03-18
2004-01-13
Wong, Edna (Department: 1741)
Electrolysis: processes, compositions used therein, and methods
Electrolytic coating
Depositing predominantly single metal coating
C106S001260
Reexamination Certificate
active
06676823
ABSTRACT:
FIELD OF THE INVENTION
The present invention generally relates to plating copper on a substrate. In particular, the present invention relates to efficiently plating copper having desirable characteristics from acid sulfate plating baths with decreased power.
BACKGROUND OF THE INVENTION
Copper plating is used extensively in a variety of manufacturing settings. For instance, copper plating is used to prevent corrosion on various surfaces (such as on iron surfaces), copper plating is used as a binding layer for additional metal layers, copper plating is used to increase electrical or thermal conductivity, and copper plating is used to provide conducting paths in many electrical applications. Much attention for copper electroplating is directed to manufacture of electrical devices, such as circuit boards, integrated circuits, electrical contact surfaces, and the like. In fact, copper plating is indispensable in the manufacture and processing of printed circuit boards.
A particular difficulty in all copper electroplating is obtaining copper films of sufficient quality for a particular application. For example, in electrical applications, it is desirable to have ductile, smooth bright copper films with high electrical conductivity. Smooth films of uniform thickness are also highly desirable in a general sense.
The desirable characteristics of electroplated copper are impacted by the amount and source of copper in the plating baths, the amount and identity of acids in the plating baths, the amount and identity of other ions in the plating baths, current densities employed, the use of additives in the plating baths, and the like. While the use of additives can improve brightness and ductility of the copper plating, often it is desirable to simplify the composition of the plating bath. This is because additives might decompose rapidly or the use of additives might be inconvenient under certain circumstances.
Nevertheless, one concern when using copper plating baths is that specific brighteners are effective for particular plating bath chemistries. For example, the brighteners effective in a sulfuric acid-copper sulfate plating bath are typically different from the brighteners used in a boric acid-fluoboric acid-copper fluoborate plating bath. As a result, specific brighteners must be identified after a particular plating bath chemistry is selected. Such identification can be time consuming and cumbersome.
While copper plating has many attractive attributes, it has several negative characteristics. Copper plating involves enormous power consumption. Consequently, energy costs associated with copper plating operations are high. Attempts have therefore been made to perform copper plating processes at faster rates or decrease the amount of energy required. However, high speed plating requires high current densities and thus high amperage. Another problem with high current densities is that the anode tends to become inert and very high over potentials are common. This is known as polarization. Yet another problem is that it is difficult to obtain commercially acceptable copper deposits when employing high current densities, such as over 40 amperes per square foot (ASF).
In copper electroplating, copper concentration in the plating bath impacts threshold electrode current densities. Plating at increased speeds is facilitated by increasing copper solubility in the plating bath. However, it is difficult to electrolytically force copper and current from an anode into a saturated solution. Moreover, it is increasingly difficult to obtain high quality copper layers when increasing the plating speed.
In circuit board applications, it is difficult to efficiently plate copper within through holes. Higher conductivity is generally required to effectively electroplate copper in through holes. But as the conductivity is increased, the levels of copper decrease while the amount of acid increases. The lower levels of copper in solution undesirably lead to slower plating speed.
SUMMARY OF THE INVENTION
The present invention involves plating copper on substrates from a plating bath containing sulfate ions and a supplemental acid. The combination of sulfate ions and a supplemental acid in the plating bath permits the employment of increased current without anode polarization leading to increased plating speed. The combination of sulfate ions and a supplemental acid may also lead to decreased power consumption by way of enhanced solution conductivity. Increased conductivity also facilitates the effective plating within circuit board through holes. Despite the presence of a supplemental acid, such as fluoboric acid, standard sulfate plating bath brighteners may be employed in the present invention. The resultant copper layer formed in accordance with the present invention has many desirable characteristics including one or more of uniform thickness, excellent leveling, excellent ductility, lack of pinholes, bright finish, effective plating within circuit board through holes, and controllable thickness.
One aspect of the invention relates to an aqueous copper plating bath containing sulfuric acid with a specific ratio to at least one supplemental acid selected from the group consisting of fluoboric acid, alkane sulfonic acids, and alkanol sulfonic acids; a copper salt; chloride ions; and at least one sulfate bath brightener. Another aspect of the invention relates to aqueous copper plating bath containing fluoboric acid and/or methane sulfonic acid but no sulfuric acid, copper sulfate, chloride ions, and at least one sulfate bath brightener.
Yet another aspect of the invention relates to methods of plating copper from the aforementioned copper plating baths. Still yet another aspect of the invention relates to methods of plating copper at high speed using relatively high current densities.
DETAILED DESCRIPTION OF THE INVENTION
The present invention can be employed to plate copper on a substrate. The substrates that can be copper plated include metal structures and non-metal structures. Metal structures, or structures with a metal surface contain surfaces of one or more of aluminum, bismuth, cadmium, chromium, copper, gallium, germanium, gold, indium, iridium, iron, lead, magnesium, nickel, palladium, platinum, silver, tin, titanium, tungsten, zinc, and the like. Non-metal structures include plastics, circuit board prepregs (including materials such as glass, epoxy resins, polyimide resins, Kevlar®, Nylon®, Teflon®, etc.), metal oxides, and the like. With specific regard to circuit board substrates, the copper plating baths and methods of plating copper are particularly effective to plate copper within through holes present in the circuit board.
The copper plating bath is an aqueous solution. In this connection, the copper plating bath contains water. However, the copper plating bath may optionally contain one or more co-solvents. Such co-solvents include water-miscible solvents such as alcohols, glycols, alkoxy alkanols, ketones, and various other aprotic solvents. Specific examples of co-solvents include methanol, ethanol, propanol, ethylene glycol, 2-ethoxy ethanol, acetone, dimethyl formamide, dimethyl sulfoxide, acetonitrile, and the like.
The copper plating bath contains copper. Copper is generally present in an ionic state (Cu
2+
). The copper is obtained by adding a suitable copper source, such as one or more copper salts, to the plating bath. For example, copper may be obtained from a copper salt such as copper sulfate, copper polyphosphate, copper sulfamate, copper chloride, copper formate, copper fluoride, copper nitrate, copper oxide, copper tetrafluoroborate, copper trifluoromethanesulfonate, copper trifluoroacetate, or hydrates thereof, such as copper sulfate pentahydrate.
In one embodiment, the plating bath contains about 1 g/l or more and about 150 g/l or less of a copper salt (as Cu
2+
). In another embodiment, the plating bath contains about 10 g/l or more and about 125 g/l or less of a copper salt. In yet another embodiment, the plating bath contains about 15 g/l or more and about
Amin & Turocy LLP
Taskem Inc.
Wong Edna
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