US 3708329 A
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3 Jan. 2, 1973 CHANGE IN PLATING RATE (MlCRONS/HR) CHANGE IN PLATING RATE (MlCRONS/HR) L. N. SCHOENBERG 3,708,329
ELECTROLESS COPPER PLATING Filed Sept. 10, 1971 CONCENTRATION OF HIQSA IN mg/l FIG 2 1 l 1 l J o 100 200 CONCENTRATION OF P IN mg/I United States Patent Office 33%;85329 Patented Jan. 2, 1973 3,703,329 ELECTROLESS COPPER PLATING Leonard Norman Sehonenberg, Livingston, N.J., assignor to Bell Telephone Laboratories, Incorporated, Murray Hill and Berkeley Heights, NJ.
Filed Sept. 10, 1971, Ser. No. 179,421 Int. Cl. 344d 1/092; C23c 3/02 U.S. Cl. 117-47 R 12 Claims ABSTRACT OF THE DISCLOSURE Certain heterocyclic aromatic nitrogen compounds, when added to conventional electroless copper plating baths in small amounts, stabilize these baths against spontaneous decomposition into metallic copper. A particular advantage of these stabilizing agents is that they do not adversely affect the plating rate. Indeed, with some compounds the plating rate is significantly increased by the addition of the stabilizing agent.
BACKGROUND OF THE INVENTION (1) Field of the invention The invention is concerned with electroless copper plating baths. Interest in electroless copper plating has increased recently because of its use in the manufacture of printed circuits.
(2) Description of the prior art Electroless copper plating processes have found extensive use in the manufacture of electronic circuits and devices. A persistent problem in the use of electroless copper plating baths is the tendency of these baths to spontaneously decompose either on standing or while in use. This spontaneous decomposition involves premature initiation of the plating reaction at places other than one the surface to be plated. After initiation, the reaction continues on the metallic copper already liberated until all the dissolved copper salt is reduced.
Numerous methods have been described in the literature to stabilize electroless copper plating baths. These methods have the undesirable effect of decreasing the plating rate of the bath. In many commercial operations this decrease in plating rate is a distinct disadvantage because of the increased time required to carry out the plating operation. Some stabilization methods which have only a moderate effect on the plating rate often require large amounts of stabilizer which alter the basic properties of the bath in ways which are sometimes undesirable.
Because of growing interest in electroless copper plating baths for the fabrication of printed circuits and other type electronic circuits, there is considerable need for a stable bath suitable for commercial applications.
SUMMARY OF THE INVENTION The addition of certain heterocyclic aromatic nitrogen compounds to conventional electroless copper plating bath results in a marked increase in the stability of these baths wtihout adverse effects on the plating rate of the bath. The inclusion of as little as 1 mg. per liter markedly increases the stability of the plating bath. Also, in the case of some compounds, the plating rate is significantly increased.
The heterocyclic aromatic nitrogen compounds have up to three rings with a hydroxy group bonded directly to one of the rings. Such compounds with more than 3 rings are not likely to be significantly soluble in copper plating solutions. An example is Z-hydroxy pyridine. Also, iodine substitution on the rings, such as in the compound 2-iodo-3 pyridinol, leads to further stabilization. Plating rate is enhanced by the substitution of a sulfonic acid group directly on the aromatic ring. Also, sulfonic acid substitution increases the solubility of the compound in the bath. Examples are 8-hydroXy-7-iodo-5 quinoline sulifonic acid and S-hydroxy-S quinoline sulfonic acid.
Except for the stabilizer, the composition of the plating bath is conventional. The copper is included in the bath in the form of a salt such as copper sulfate. A complexing agent such as disodium ethylenediaminetetracetate and a reducing agent such as formaldehyde are also included in the solution as well as sufiicient alkaline agent to make the pH of the solution at least 11. Other substances conventionally used in electroless copper baths may also be included as, for example, to improve the properties of the plating or increase the wettability of the surface being plated.
BRIEF DESCRIPTION OF THE DRAWING DETAILED DESCRIPTION (1) Glossary of compounds For convenient reference, some of the compounds referred to in the disclosure are given below. The compounds are identified by their structural formulas and chemical names, as well as by an abbreviated name which is used in the disclosure.
8-hydroxy-7-iodo-5 quinoline sull'onic acid 2-iod0-3 pyridinol I H P OH Q-hydroxy pyridine 8-hydroxy-5 quinoline sultonic acid S 0 3H PSA 3-pyridine sultonie acid QSA 8-quinoline sultonic acid (2) Plating mechanism An understanding of the invention is aided by a brief description of the plating mechanism and the mechanism by which the plating baths spontaneously decompose. The essential contents of an electroless copper plating bath are soluble cupric salt such as copper sulfate, a complexing agent such as disodium ethylenediaminetetracetate, an alkaline metal hydroxide or other agent to adjust the pH of the aqueous solution and a reducing agent such as formaldehyde. The overall plating reaction has been established to be This is a heterogeneous reaction and proceeds only on catalytic surfaces. Metallic copper is known to be a good cotalyst for this reaction and therefore this plating reaction is said to be autocatalytic. This is in contradistinction to displacement plating in which the surface material reacts with the plating solution.
Essentially two types of mechanisms are responsible for initiating spontaneous decomposition of electroless copper baths. The first involves the introduction or presence in the bath of ions of catalytic metals which are reduced by the bath to the metallic state and then catalyzes the plating reaction. For example, Pd++ is often used to sensitize surfaces to be copper plated. Some of the Pd++ ion is inevitably introduced into the copper plating bath. This ion is then reduced to metallic Pd which in turn catalyzes the copper plating reactions. After initiation of the reaction, sufiicient metallic copper is produced by the plating reaction to catalyze subsequent decomposition of the plating solution.
The second mechanism for spontaneous decomposition involves the formation in the solution during electroless plating of small amounts of metallic copper or cuprous oxide particles which catalyze the plating reaction in the solution rather than on the surface to be plated.
(3) Stabilization measurements Measurements were carried out to show the stabilizing eifects of certain heterocyclic aromatic nitrogen compounds on electroless copper plating solutions. For purposes of comparison, a particular bath composition was used throughout these experiments. The composition of the bath was 0.02 M CuSO -5H O, 0.1 M disodium ethylenediaminetetracetate dihydrate, 0.4 M formaldehyde and sufficient sodium hydroxide to obtain a pH of 12.8
Electroless copper plating solutions vary somewhat erratically in their stability. This is due largely to the fact that initiation of the spontaneous decomposition of the bath is often due to very small amounts of impurities or dust particles which are difficult to control or eliminate. In order to obtain results which reflect the stabilizing influence of the added stabilizer, the decomposition of the plating solution was deliberately initiated by adding small amounts of a solution of palladium chloride in dilute hydrochloric acid. The procedure was as follows. Samples containing 45 ml. of the plating solution were placed in a 75 C. constant temperature water bath. After 5 minutes, 3 drops of an initiator solution were added. This initiator solution was made by dissolving 1 gram of palladium chloride in mls. of concentrated hydrochloric acid and diluting to 1 liter. Under these conditions the plating solution without added stabilizer decomposed instantaneously. The results where various stabilizers were added are shown in Table I. The importance of the table is to show that certain organic additives markedly improve 4 bath stability under rather drastic conditions. Under less drastic conditions usually encountered in ordinary use, much longer decomposition times are found. The table shows the decomposition times for the electroless copper plating bath as a function of stabilizer concentration for various organic stabilizers.
cyclic aromatic nitrogen compound with a hydroxy group substituted to the ring such as in the case of HP enhances the stability of the plating solution. Unfortunately, HQ forms a precipitate under conditions of the experiment so that its stabilizing effect cannot be measured. However, the sulfonic acid derivative of this compound, namely HQSA, is soluble and has a stabilizing effect as shown in Table I. The substitution of iodine onto one of the aromatic rings enhances the stabilizing effect of a hydroxy substituted heterocyclic nitrogen compound, as shown by the decomposition times of IP shown in the table. The addition of a sulfonic acid group to the aromatic ring does not remove the stabilizing effect of the compound, as can be seen from the results for HIQSA shown in Table I. The addition of the sulfonic acid group increases the solubility of these compounds in the copper plating bath and also has an enhancing effect on the plating rate, as will be described below. The presence of a sulfonic acid group alone on the ring of a heterocyclic aromatic nitrogen compound has little stabilizing effect. This is shown by the results given in Table I for QSA and PSA.
(4) Plating rate studies A particular advantage of the use of these stabilizing compounds is that the plating rate is not significantly decreased in the process of obtaining bath stabilization. To show this result, plating rate measurements were made using various concentrations of stabilizer. Two stabilizers were investigated, namely HIQSA and IP. The measurements were carried out on 2 mil thick copper rectangles measuring approximately 1" x /2" on a side. These copper rectangles were etched for 15 seconds in 20 percent nitric acid, rinsed and activated for one minute in a 0.1 percent palladium chloride solution and rinsed again. Plating was carried out in 9 0 mil solutions of the copper plating bath with varying amounts of stabilizer added. The thickness of the copper deposits was determined by Weight gain measurements. The results for HIQSA are shown in FIG. 1. The graph shows the change in plating rate (from a bath with no stabilizer added) as a function of stabilizer concentration. Plating solution without added stabilizer has a change in plating rate of zero. As stabilizer is added, the plating rate of the bath increases to a maximum of approximately 40-50 percent over that of the plating bath without stabilizer. On continued addition of stabilizer, the plating rate decreases and eventually becomes slower than the bath without stabilizer. Similar results were obtained using an electroless copper bath with composition 0.06 M CuSO -5H O, 0.3 M disodium ethylenediaminetetracetate dihydrate, 0.4 M formaldehyde and sufiicient sodium hydroxide to obtain a pH of 12.8. Here the maximum plating rate was obtained with a stabilizer concentration of about mg./l. This maximum plating rate was about 3.1 microns per hour.
FIG. 2 shows the results of plating rate studies using IP as the stabilizer. The coordinates are the same as in FIG. 1. In the case of IF, the stabilizing effect is much greater than with HIQSA, but the plating rate is enhanced little if at all. However, IP does provide significant stabilization without impairing plating rate.
() Composition The invention has generally been described in terms of the use of certain heterocyclic aromatic nitrogen compounds as stabilizers in otherwise conventional electroless copper plating baths. The copper is included in the solution in the form of a copper salt. The bath also includes a complexing agent for the copper ions and a reducing agent. Sufiicient alkaline agent is added to make the pH of the bath at least 11. While such a general description of the invention is justified, certain specific compositional ranges of particular ingredients are preferred for certain purposes. These preferred compositions are given below.
(1) Insoluble cupric salt (e.g., sulfate.
nitrate, chloride 0.002 M to (2) Complexing agent, e.g., disodium 0.15 M.
ethylenediaminetetracetate dihydrate (corresponding acid or tetrasodium salt), Rochelle salt, citric acid or alkali metal salt of citric acid 0.003 M to (3) Alkaline agent such as alkali 1M.
metal hydroxide (e.g., NaOH,
KOH) The range is sufiicient to produce a pH of at (4) Formaldehyde (or other suitable least 11.
reducing agent) 0.02 M to 2 M. (5) Stabilizing agent 1 mg. to 500 mgs./liter.
In addition to the above, certain other ingredients may be added to improve bath properties or the quality of copper plating. These additives are Well known to those skilled in the art. For example, small amounts of cyanide ion may be added to improve the properties of the plating. Other additives such as organic phosphate esters may be added to increase the wettability of the surface being plated. Generally, these additives should not exceed 5 percent.
The compositional limits will now be discussed. Below the minimum cupric salt concentration the plating rate is unreasonably slow; above the maximum, the salt is no longer completely soluble. The concentration range of the complexing agent is determined largely by the cupric ion concentration and need only be sufiicient to complex the cupric ion present in the solution. The pH should be at least 11 to insure reasonable plating rates and to inhibit formation of a cuprous compound. Minimum formaldehyde concentration is determined by reasonable plating rates; maximum concentration is limited by uncontrollable spontaneous deposition by the bath.
Minimum stabilizer concentration is determined by the desired stability of the bath and by the concentrations of cupric salt and reducing agent. In the case where baths are to be used only over a short period of time, i.e., one hour, a concentration of 1 mg./liter is adequate. Where long shelf life is desired, concentration as high as 500 mgs./liter is desirable. The upper limit is determined by the fact that too high a stabilizer concentration will inhibit plating completely. In practical applications, the stabilizer concentration will usually be between 25 and 400 rug/liter. A preferred concentration is determined by the fact that for each plating composition there is a stabilizer concentration which maximizes plating rate and still provides bath stabilization. For the bath composition used in obtaining the data in FIG. 1, this preferred stabilizer concentration is between 180 and 300 mgs./liter. For the 0.06 M copper sulfate given above, the preferred concentration of HIQSA is between and 200 mgs./l.
(6) Other considerations Bath compositions of this invention may be used on all substrates under all conditions in which electroless copper baths are considered usable. Plating may be carried out on conductive surfaces such as copper, nickel, palladium, as well as on insulating surfaces. For insulating surfaces, sensitizing and activating procedures well known in the art should be followed.
Plating procedures may be carried out under a variety of temperature conditions within the range that the bath remains liquid. Increased temperature generally increases the plating rate and the rate of decomposition.
What is claimed is:
1. An electroless plating bath in which at least 95% of the soluble constituents consists essentially of a soluble copper salt, a complexing agent, a reducing agent, an alkaline agent sufiicient to obtain a pH of at least 11, and an organic stabilizing agent characterized in that the organic stabilizing agent is a heterocycic aromatic nitrogen compound having up to 3 rings with at least one ring-substituted hydroxyl group.
2. The bath of claim 1 in which the ring atoms other than nitrogen atoms in the organic stabilizing agent are carbon atoms.
3. The bath of claim 1 in which the organic stabilizing agent has at least one iodine substituent attached to at least one of the rings.
4. The bath of claim 1 in which the copper salt is contained within the range from 0.002 M to 0.15 M and is selected from a group consisting of nitrates, sulfates, chlorides, bromides and fluorides and in which the complexing agent is selected from a group consisting of an ethylenediaminetetracetate, the disodium salt of ethylenediaminetetracetate and tetrasodiu'm salt of ethylenediaminetetracetate, Rochelle salt, citric acid and the alkali metal salts of citric acid.
5. The bath of claim 4 where an alkali metal hydroxide is included to adjust pH.
6. The bath of claim 1 in which the concentration of the organic stabilizing agent is between 1 mg. and 500 mgs./ liter.
7. The bath of claim 6 in which the concentration of organic stabilizing agent is between 25 and 400 mg./liter.
8. The bath of claim 7 in which the concentration of the organic stabilizing agent is between 180 and 300 mg./ liter.
9. The bath of claim 1 in which the organic stabilizing agent is 2-iodo-3-pyn'dinol.
10. The bath of claim 1 in which the organic stabilizing agent is 8-hydroxy-7-iodo-5 quinoline sulfonic acid.
11. A method of electroless plating of copper on an insulating surface comprising the steps of sensitizing and activating the surface to be plated and thereafter wetting said surface with the electroless bath of claim 5.
12. An electroless copper plating bath stabilized against spontaneous decomposition by presence of an organic stabilizing agent characterized in that the stabilizing agent is a heteorcyclic aromatic nitrogen compound containing up to 3 rings and having at least one ring-substituted hydroxyl group.
No references cited.
ALFRED L. LEAVITTI, Primary Examiner I. A. BELL, Assistant Examiner US. Cl. X.R.
106-1; 117-47 A, E, 47 R