US3475186A - Electroless copper plating - Google Patents

Description

United States Patent 3,475,186 ELECTROLESS COPPER PLATING Oleh B. Dutkewych, Newton, Mass., assigno'r to Shipley Company, Incorporated, Newton, Mass., a corporation of Massachusetts No Drawing. Filed Jan. 5, 1968, Ser. No. 695,860

Int. Cl. C09d /10 US. Cl. 1061 14 Claims ABSTRACT OF THE DISCLOSURE BACKGROUND OF THE INVENTION This invention relates to electroless deposition of copper and has for its principal objects the provision of improved copper plating solutions and deposition methods employing the same.

Electroless copper deposition refers to the chemical plating of copper over active surfaces by chemical means and in the absence of external electric current. Such processes and compositions useful therewith are known and are in substantial commercial use. They are disclosed in a number of prior art patents, for example US. Patent No. 3,001,920, issued Dec. 16, 1961.

Known electroless copper deposition solutions generally comprise four ingredients dissolved in water. These are (l) a source of cupric ions, usually copper sulphate, (2) reducing agent such as formaldehyde, (3) free hydroxide, generally an alkali metal hydroxide, and usually sodium hydroxide, sufficient to provide the required alkaline solution in which said compositions are effective, and (4) a complexing agent for the copper sufficient to prevent its precipitation in alkaline solution. A large number of suitable complexing agents are known and are described for example in the above-cited patent, and also in US. Patents Nos. 2,874,072; 3,075,586; 3,119,709 and 3,075,855 all incorporated herein by reference. Known electroless plating solutions of the above type usually provide a plate which, if mechanically dense and strong, is somewhat brittle such that it can withstand only limited bending or thermal stresses without fracture. This is not a substantial disadvantage where the electroless plate is of the order of millionths of an inch in thickness and is overplated with ductile electrolytic copper. However, where the entire desired thickness, typically 1 to 3 mils in electrical applications, is provided by electroless plating, limited ductility can be a serious limitation.

One means of improving the ductility, or bending characteristics of electroless copper plating is described in US. Patent No. 3,257,215, which discloses the use of cyanide and sulphur compounds in plating solution. An additional means is described in United States Patent No. 3,310,430, which discloses the use of a water soluble compound of vanadium, molybdenum, niobium, tungsten, rhenium, arsenic, antimony, bismuth, rare earth of the actinium series, and rare earths of the lanthanum series. The instant invention provides additional additives that act to improve ductility and strength and in a preferred embodiment, provide the additional advantage of improved appearances as more fully described hereinafter.

DESCRIPTION OF THE INVENTION The instant invention is predicated upon the discovery "ice that electroless plating solution of the above type can be substantially improved by the inclusion therein of a small but noncritical quantity of a silicon compound in which silicon is believed to be the effective agent. It is used in an amount of at least 1 part per million of solution by weight, preferably between about 5 and 2500 parts per million and most preferably between about 25 and 500 parts per million. Generally, the higher the concentration of copper in solution, the larger the amount of silicon compound required to obtain the same benefit. Excessive silicon compound retards copper deposition.

Exemplary of the silicon compounds within the scope of the invention are the substituted silanes, such as ethyltriethoxy silane, amyltriethoxysilane, ethyltrichlorosilane, amyltrichlorosilane, vinyltrichlorosilane, vinyltriethoxysilane, phenyltriethoxysilane, etc., and low to high molecular weight polysiloxanes such as silicone fluids, gums and resins substituted with methyl, ethyl, vinyl, phenyl, chloro, bromo, methoxy, hydroxy, etc.

The polysiloxanes are the least soluble silicon compounds in basic copper solutions, but are preferred because they provide the greatest increase in ductility and also enhance appearance by yielding a finer grained, more highly reflective copper deposit. Of the polysiloxanes, the silicone fluids are most preferred. The silanes must be handled with caution as they are reactive with water, and in some cases, violently reactive.

The mechanism by which the silicon compound performs is not understood, but is believed to be effective by suppressing side reactions in the bath which result in instability, poor grain structure, and inclusion of impurities such as hydrogen in the metal deposit. Maximum benefits from its inclusion depend upon the quality and balance of the basic copper solution, but the addition of a silicon compound will provide measurable benefit in any useable bath.

The present invention is further illustrated by the following examples:

1 Brittle (fractures).

Z Ductile (no fracture).

In the above examples, agitation of the solution is provided to maintain the silicon compound at least partially dispersed in the bath. Copper sulphate is the source of cupric ion, NaOI-I provides the desired free hydroxide, and HCHO acts as the reducing agent. As is well known, other suitable materials may be substituted. The EDTA.4Na is the tetrasodium salt of ethylenediaminetetracetic acid and is the complexing agent for the cupric ions rendering them soluble in alkaline solution. Other well known complexing agents for this purpose, for example those disclosed in the foregoing patents, can be employed.

Concentration of the above ingredients is not critical and is generally within the ranges previously known to be useful in electroless coppering solutions. Sufiicient copper must be employed to provide a useful deposit and sutficient alkali to provide the high pH required, generally from about 10 to 14. Sufiicient formaldehyde must be employed to serve as a reducing agent for the copper in the presence of a catalytic surface and sufficient complexing agent used to prevent precipitation of copper in alkaline solution during storage and use. Generally, for

purposes of illustration, preferred limits of concentration for these ingredients is given in the following Table 1:

TABLE I.CONCENTRATIONS Cu++ mols/liter 0.02-0.12 Free hydroxide mols/liter 0.10-1.25 HCHO mols/m'ol Cu++ 2.020.0 Complexers mols/mol Cu++ 1.0-8.0

Strength is measured by removing a copper plate, having an area measuring 1 inch by 6 inches, from the substrate, and clamping the plate to a rigid support over a rectangular opening therein 1 inch by 4 inches in area, the opening being covered by a rubber diaphragm. Air pressure is applied outwardly against the diaphragm and the copper plate slowly in stepwise increments of onehalf p.s.i., each step being held from 1 to 2 seconds, until rupture occurs. Such test is believed to measure a combination of shear and tension and to include a measure of the ductility of the sample. Due to difl-iculty of obtaining standardized samples for conventional measurement, ductility is additionally measured empirically by bending the sample upon itself 180 and creasing the resulting apex of bend. If the plate can be creased without fracture one or more times, it is considered to be excellent in ductility. A combination of both strength and ductility is normally required for applications subjected to mechanical or thermal stress.

Example 2 is repeated with substitution of various silicone fluids with results as follows:

Silicon Amount Ductility Example No. Fluid (p.p.m (180 Bend) 150 No fracture. 250 Do. 100 D0. 250 Do. 50 Do. 250 Do. 250 Do. 250 Do. 250 Do. 250 D0. 250 Do. 250 Do.

Amount Ductility (p.p.m.) (180 Bend) 250 No fracture. 250 Do.

Silicon No. Compound 15. Silane A455. Methyldichlorosilane. 16. Silane A157 Methylv myldiehlorosilaue. 17- Silanc A-173- Bis-triehlorosilyl- Description 250 Do. ethane. 18. 1 -1-3514 Silicone glycol 250 Do.

polymer. 10. F-1-1049 do 250 Do. 20. Antifoam-GO- Polysiloxane emuls1ou 250 Do.

The following examples illustrate the use of a silicon compound with a copper solution of a different composition:

The advantages obtained from the use of silicon compounds in electroless copper bath include improvement in grain structure, minimizing gas entrapment and observable blistering in the copper plate, especially in plate obtained with relatively rapid plating baths; plating at a more constant rate during use, e.g., up to 60% of the copper originally present can be plated at a rate which is within 10 to 15% of the rate originally obtained with the copper at concentration; stability 'of the electroless solution is improved, permitting greater variations in operating temperature and concentration of active ingredients; strength and ductility of the products are substantially improved; and with polysiloxanes there is substantial improvement in appearance, the deposit being finer grained with improved light reflection.

It should be understood that the foregoing description is for the purpose of illustration only and that the invention includes all modifications within the scope of the appended claims.

I claim:

1. In a basic aqueous electroless copper plating composition containing a source of cupric ions, free hydroxide suflicient to provide required pH, reducing agent to reduce copper ions in the presence of a catalytic surface, and sufficient complexing agent to render cupric ion soluble in alkaline solution the improvement comprising the addition of an organic silicon compound in an amount suflicient to improve the copper plate obtained from the bath and insufficient to prevent deposition.

2. A composition according to claim 1 where the silicon compound is present in an amount of at least 1 part per million parts of solution.

3. A composition according to claim 1 where the silicon compound is present in an amount between 5 and 2500 parts per million parts of solution.

4. A composition according to claim 1 where the silicon compound is present in an amount between 25 and 500 parts per million parts of solution.

5. A composition according to claim 1 of the following composition:

Cu++ mols/liter 0.02-0.12 Free hydroxide mols/liter 0.10-1.25 Reducing agent mols/mol Cu++ 2.0-20.0 Compexer(s) mols/mol Cu++ 1.0-8.0 Silicon compound (parts per million parts of solution) 5-2500 where the sili- 5 6 comprises contacting a catalyzed substrate with the plat- References Cited ing claim UNITED STATES PATENTS 12. The method of electroless plating of copper which comprises contacting a catalyzed substrate with the 3,361,580 1/1968 schnqble at plating composition of claim 3 5 3,415,666 12/1968 Nagal et a1.

13. The method of electroless plating of copper which comprises contacting a catalyzed substrate with the plat- DONALD Pnmary Exammer ing composition of claim 5. L. HAYES, Assistant Examiner 14. An article of manufacture comprising a substrate and a bright, ductile, electroless copper plate deposited US. Cl. X.R.

from a copper solution containing silicon. 10 117-130, 160