GB2032462A - Electroless Copper Deposition at Faster Rates - Google Patents

Abstract

The plating rate of an electroless copper deposition solution which comprises copper ions, a complexing agent for copper ions, a reducing agent and a pH adjustor and which is characterized by a plating rate which first increases and passes through a peak plating rate and then decreases as a function of a pH above 10 is increased by the addition of an accelerating or depolarizing agent. The accelerating or depolarizing agents include compounds containing a delocalized pi-bond, such as heterocyclic aromatic nitrogen and sulfur compounds, non-aromatic nitrogen compounds having at least one delocalized pi-bond, and aromatic amines. The plating rate is at least 7 microns/hour and the accelerating compound content of the solution is 0.0001-2.5 g/l.

Description

SPECIFICATION Electroless Copper Deposition Processes Having Faster Plating Rates Electroless, e.g., autocatalytic, metal deposition solutions for the formation of metal layers on non-metallic or metallic substrates without the need for an external supply of electrons are well known in the art. Such solutions differ from electroplating baths which require an externally supplied source of electrons, and they also differ from displacement metal plating and metal mirroring methods where the metal deposited is only a few millionths of an inch in thickness.

A shortcoming of early processes for the electroless deposition of copper was that the deposition solution was unstable initially or became unstable after a relatively brief operating period and then had to be dumped. Such solutions also tended to produce electrolessly formed copper deposits which were dark in color and which tended to flake off the substratum on which deposition was taking place.

To overcome such shortcomings, the art has proposed a number of compounds as stabilizing agents for prolonging the useful life of electroless metal deposition solutions and for improving the quality of the copper deposit. These include 2-mercaptobenzothiazole in U.S. 3,222,195; 2,5 dimercapto-1 ,3,4-thiodiazole and 8-mercaptopurine in U.S. 3,436,233; o-phenanthroline in U.S.

3,615,735; 1 -phenyl-5-mercaptotetrazole in U.S. 3,804,638; 2,2'-dipyridyl and 2-(2-pyridyl)benzimidazole in U.S. 4,002,786; and benzothiazolethioether-polyethyleneglycol in U.S. 3,843,373.

Still other stabilizing agents are disclosed in U.S. 3,257,21 5, e.g., thiazoles, isothiazoles and thiozines; in U.S. 3,793,038 e.g., benzotriazole, diazole, imidazole, pyrimidine, and others; and in U.S.

3,377,174, e.g., 2,2'-biquinoline, 2,9-dimethylphenanthroline and 4,7-diphenyl-1 , 1 0-phenanthroline.

U.S. 3,708,329 disclosed that the addition of a heterocyclic aromatic nitrogen compound having up to 3 rings with a hydroxy group bonded to one of the rings, results in a marked increase in the stability of electroless copper plating solutions without adversely affecting the plating rate. See also Schoenberg, The Journal of the Electrochemical Society, 118, p. 1571(1971). Although Schoenberg in U.S. 3,708,329 talks about improving plating rates, the fastest bath described by Schoenberg has a room temperature plating rate of only 3.1 microns per hour. Even that slow rate, however, is higher than any long term rate mentioned in any of the other prior art references identified above.The fastest reported long term rate for electroless copper plating solutions currently available commercially, e.g., Dynachem DC-920 and MacDermid 9027, is 5 microns per hour. U.S. 3,377,174 reports a short term plating rate of 0.5 microns in a five minute period.

Heretofore, it was considered necessary to operate electroless copper plating solutions at a slow rate, e.g., less than about 6 microns per hour so as to produce a copper deposit of good quality, e.g., a coherent, structurally stable, thin film of Cu adherent to the surface being coated. The experience in the art has further been that higher plating rates resulted in the production of a copper deposit of poor quality, e.g., one which flakes off or tends to flake off the surface or which was noncoherent.

As used herein, the phrase adherent copper deposit refers to an electrolessly formed copper deposit which can be stripped from a plated insulating substratum in the form of a thin, integral film such that when stripped, retains its structural integrity or cohesiveness as a film without crumbling.

As used herein, the phrase non-adherent copper deposit refers to an electrolessly formed copper deposit which flakes or tends to flake off the coated substratum.

It is one object of this invention to increase the rate at which copper can be deposited electrolessly.

It is a further object of this invention to provide procedures and compositions for increasing the rate for electrolessly forming an adherent copper deposit.

Another object of this invention is to provide electroless copper deposition solutions having high plating rates.

Still another object of this invention is to provide compositions and procedures for electrolessly forming adherent copper deposits at high rates heretofore considered unachieveable.

Other and further objects of this invention will be clear from the description which follows and from the examples.

In accordance with the invention, it has been found that these and other objects may be achieved by the method for increasing the rate at which adherent copper may be deposited from an electroless copper deposition solution having a pH greater than 10, which comprises including in the solution an agent which produces depolarization of the anodic partial reaction of the solution or the cathodic partial reaction of the solution or both reactions, and operating the solution at a pH greater than the peak plating rate pH of the solution without the depolarizing agent present so as to electrolessly deposit copper at a rate greater than the plating rate of the solution without said agent at the same pH and the same temperature.

In general, the depolarizing agent should be capable of achieving at least 20% and up to 100% or between about 35% and 90% depolarization of the anodic partial reaction or the cathodic partial reaction of the solution, or both. Stated differently, the depolarizing agent should be capable of accelerating by at least 20% and up to 100% or between about 35% and 90%, the cathodic partial reaction or the anodic partial reaction of the solution, or both.

The increase in the rate at which adherent copper may be deposited from a given electroless copper solution by practice of this invention will vary over a wide range depending upon the formulation used and the quality of copper desired. In general, rate increases achieved by practice of this invention will be at least up to 300% or more depending upon solution formulation. However, rate increases of up to 1 or 1 1/2 orders of magnitude, e.g., 10 times (1000%) or even 50 times (5000%) are possible. Achievement of such rate increases was unexpected and surprising.

Similarly, the rate at which adherent copper may be deposited for a prolonged period of time from a given electroless copper solution by practice of this invention will vary over a wide range, again depending upon the formulation used and the quality of copper desired. With additive present, the solutions covered herein are characterized by a room temperature plating rate above 7 microns per hour, and generally above 9 microns per hour, or between about 9 microns and 25 microns per hour and higher. Elevated temperature rates of up to 70 microns per hour or even higher are, however, possible. Here again, achievement of such rates was unexpected and surprising. Moreover, such rates may be achieved for periods of time ranging up to eight hours and more. Typical are operating times of, e.g., 5 minutes to, e.g., 8 hours.With proper replenishment, the solutions may continue in use for extended periods of time, e.g., weeks. It should be noted that the fast rates of the solutions generally make prolonged continuous plating periods unnecessary.

Electroless formation of copper in accordance with this invention will result in many operating advantages, including shorter plating times and, concomitantly, increased production capacity.

Compared to commercial practices now available, the procedures and compositions of this invention require less equipment, lower capital investment costs and iower energy requirements. Unlike the current commercial practices, the procedures herein taught are particularly suitable for use in automatic plating systems with relatively short dwell times.

The electroless copper deposition solution of the present invention comprises copper ions, a complexing agent for copper ions, a reducing agent and a pH adjustor and is characterized by a plating rate which first increases and passes through a peak plating rate and then decreases as a function of pH, the said plating rate increase occurring above pH 10 and usually above pH 11.The method of the invention comprises: (A) operating the electroless copper deposition solution in the presence of at least one acceleration or depolarizing agent; and (B) regulating the pH of the electroless copper deposition solution in the presence of the accelerating or depolarizing agent at a pH greater than the peak plating rate pH of the solution without said agent present so as to electrolessly deposit copper at a rate greater than the plating rate of the solution without the accelerating agent at the same pH and at the same temperature.

Preferably, the accelerating or depolarizing agent is selected from among compounds containing a delocalized pi-bond, including (a) heterocyclic aromatic nitrogen and sulfur compounds; (b) non-aromatic nitrogen compounds having at least one delocalized pi-bond; (c) aromatic amines; and (d) mixtures of any of the foregoing.

The terms "depolarizing agent" and "accelerating agent" are used interchangeably herein.

The preferred depolarizing or accelerating agents of this invention have a free electrofipair on the nitrogen atom adjacent to a pi-bond.

By way of illustration, the heterocyclic aromatic nitrogen compound, {A)(a), is selected from among pyridines, e.g., pyridine, cyanopyridine, chloropyridine, vinylpyridine, aminopyridine, 2-pyrazolo- (4,3-c)-pyridine, 3-v-triazolo-(4,5-b)-pyridine, 2,27-dipyridyl, picolinesi and the like; pyridazine; pyrimidines, e.g., m-diazine, 2-hydroxypyrimidine, 2-oxy-6-aminopydrnidine (cytosine), and the like; pryzines; triazine; tetrazine; indoles, e.g., indole, tryptamine, tryptophan, 2,3-indolinedione, indoline, and the like; purines, e.g., 6-aminopurine (adenine); phenanthrolines, e.g., o-phenanthroline; quinolines, e.g., 8-hydroxyquinoline; azoles, e.g., pyrrole, dibenzopyrrole, pyrroline, and the like; diazoles, e.g., 1 ,2-pyrzole, 1 ,3-imidazole, and the like; triazoles, e.g., pyrrodiazole, benzotriazole, diphenyltriazole, isotriazole, and the like; tetrazoles, and benzodiazoles, e.g., indazole, benzimidazole and the like.

Also included are mercapto-derivatives and thio-derivates of any of the foregoing, such as mercaptopyridines, mercaptopyrimidines, thi-azoles, thiazoline, thiazolidine, mercaptothi-azoles, imidazolethiols, mercaptoimidazole, mercaptopurines, mercaptoquinazolinones, thiodiazoles, mercaptothiodiazoles, mercaptotriazoles, mercaptoquinolines, and the like.

Illustratively, the non-aromatic nitrogen compound, (A)(b), is selected from among ureas, guanidines and derivates thereof.

Preferably, the aromatic amine, (A)(c), is selected from among p-nitrobenzylamine, anilines, phenylenediamines and mixtures thereof.

Preferably, the polarizing or accelerating agent will be present in a small effective amount, e.g., generally at least about 0.0001 to about 2.5 grams per liter, more specifically about 0.0005 to 1.5 grams per liter and preferably from about 0.001 to about 0.5 grams per liter. In general, the amount of depolarizing or accelerating agent used will vary depending upon the particular agent employed and the formulation of the solution.

In another aspect of this invention, the electroless metal deposition solution can also include, in addition to copper ions, ions of a metal or metals selected from among the transition metals, preferably Group VIII, and especially preferably cobalt and/or nickel. These may be added in the form of metal salts, e.g., halides or sulfates, optionally with a suitable complexing agent, e.g., a tartrate. In general, amounts of from about 0.005 to about 30%, by weight of the Group VIII metal, based on the weight of the copper salt, are used.

The copper ions are normally supplied in the form of a water soluble copper salt. The choice of the salt is chiefly a matter of economics. Copper sulfate is frequently preferred, but copper halides, e.g., chloride and bromide, copper nitrate, copper acetate, as well as other commercially available organic and inorganic acid salts of copper can also be used. Although water soluble metal salts are preferred, normally water insoluble compounds, such as copper oxide or copper hydroxide, can be used since these are rendered soluble by the complexing agent or agents in the deposition solution.

The complexing agent for copper ions is selected from compounds conventionally employed for this purpose, including, but not limited to, Rochelle salts, the sodium (mono-, di-, tri- and tetrasodium) salts of ethylenediaminetetraacetic acid (hereinafter sometimes referred to as "EDTA"), diethylenediaminepentaacetic acid, nitriloacetic acid and its alkali salts, gluconic acid, gluconates, triethanol-amine, diethylaminoethanol and glucono-b-lactone, as well as modified ethylenediamineacetates, e.g., N-hydroxyethylethylenediaminetriacetate, phosphonates, e.g., ethylenediaminetetra(methylene phosphonic acid) and hexamethylenediaminetetra (methylene phosphonic acid).

Preferably, the complexing agent is of the alkanolamine type. Examples include N,N,N',N'tetrakis-(2-hydroxypropyl)-ethylenediamine (hereinafter sometimes referred to as "Quadrol"), triethanolamine, ethylenenitrilotetraethanol, nitrilitri-2-propanol, tetrahydroxyethylenediamine and N hydroxyethyl-N,N',-N'-(trihydroxypropyl)ethylenediamine. These are commercially available or can be prepared by following procedures described in the literature.

The reducing agent is selected from among, illustratively, formaldehyde and formaldehyde precursors or derivates, e.g., paraformaldehyde, trioxane, dimethylhydantoin, glyoxal, and the like; boranes; borohydride; hydroxylamines; hydrazines and hypophosphite.

The pH may be regulated by the use of a pH adjustor, preferably a water soluble alkali metal or alkaline earth metal hydroxide, e.g., magnesium hydroxide, calcium hydroxide, potassium hydroxide, sodium hydroxide, or the like. Among these, sodium hydroxide is preferred, chiefly for reasons of economy. During operation, the pH is monitored and raised or lowered, as needed, by the addition of suitable amounts of the pH adjustor.

It may be desirable to employ a minor, effective amount of a wetting agent or agents, preferably in amounts of less than 5 grams per liter. Examples of such commercially available surfactants include Pluronic(RTM) P85, BASF-Wyandotte Corp., a non-ionic block copolymer of ethylene oxide and propylene oxide, and GAFAC(RTM RE 610, GAF Corp., an anionic phosphate ester.

The concentrations of the various ingredients in the basic electroless copper deposition solution for use herein are subject to wide variation within certain ranges with may be defined as follows: Copper salt 0.002 to 1.20moles Reducing agent 0.03 to 3.00 moles Cupric ion complexing agent 0.5 to 20 times the moles of Cu Alkali metal hydroxide sufficient to give a pH of 10 to 14 and preferably of 11 to 14, as measured at room temperature Water sufficient to make 1 liter.

When non-aqueous solvents are used instead of water, preferably they are selected from among, e.g., dimethylformamide, dimethylsulfoxide and acetyl acetate.

More preferably, the plating baths of the present invention comprise: A soluble cupric salt, preferably 0.002 to 0.4 mole cupric sulfate Alkali metal hydroxide, preferably pH 11.2 to 13.7, as measured at room sodium hydroxide, to give temperature Formaldehyde (reducing agent) 0.06 to 0.50 mole Cupric ion complexing agent 0.002 to 2.00 mole Water sufficient to make 1 liter In practice, concentrated solutions or compositions can be manufactured for subsequent dilution to operating compositions as described herein.

It should be understood that as the baths are used up in plating, the cupric salt, the reducing agent and the cupric ion complexing agent and the depolarizing compound and other compounds present in the fresh plating bath solution may be replenished from time to time.

In operation, the pH of the solution and the presence of depolarizing compound in the solution will be monitored and adjusted as taught herein. The depolarizing compound will be supplied in an amount of at least 0.0001, preferably at least 0.0005 and up to 2.5 grams per liter. With the depolarizing compound present, the pH of the solution will be adjusted as desired to achieve a faster plating rate in comparison with the solution without the accelerating agent at the same pH.

In using the baths, the surface to be plated should be free of grease and other contaminating material.

Where a non-metallic surface is to be plated, the surface areas to receive the deposit should first be treated, as in conventional processes, to render them receptive to the electroless deposition of copper.

Where a metal surface, such as copper foil, is to be treated, it should be degreased and treated to free the surface of any oxide.

For inert metals, e.g., stainless steel, improved deposition is achieved if the metal foil is immersed in a palladium chloride/hydrochloric acid solution for about 1 minute prior to exposure to the plating bath solution.

Following pre-treatment and/or sensitization, the surface to be plated is immersed in or otherwise exposed to, as by spraying or slurry, the autocatalytic copper baths, and permitted to remain in the bath until a copper deposit of the desired thickness has been built up. In pactice, the substratum or article or part being coated can be stationary and the solution moved into contact therewith, or, alternatively, the solution or offset or part being plated can be continuously conveyed through a tank or rather reservoir containing the plating solution or a spray curtain of the plating solution.

In general, the electroless metal deposition solution is prepared by adding the complexing agent to an aqueous solution of the copper salt or salts to form a water-soluble complex or chelate of the copper cation. The complexing agent can be added as a base, salt or other water-soluble derivate. The other ingredients are thereafter dissolved in the solution in any desired order.

The process of this invention can be conducted over a broad range of temperatures. For example, temperatures of between 1 50 and boiling, e.g., 1 000C, can be used, and temperatures of between 200 and 800C are preferred. It is noteworthy that bright adherent copper deposits are obtained at good rates even at room temperature, e.g., at 250C.

Applications of the invention include high speed application of conductive metal layers on normally non-conductors for purposes of static elimination, or insulated cable for coaxial cable formation or on glass for copper mirroring.

The fast deposition rates achievable by the use of this invention make possible the formation of metal layers by electroless deposition at rates which are comparable to those obtained by conventional electroforming copper techniques and electroless nickel techniques.

The present invention is especially useful in the manufacture of printed circuit boards and the metallizing of plastic articles.

The copper layer can be built up to a desired thickness by electroless metal deposition or by electroplating with copper or combinations of metals such as copper, nickel and chromium.

When formaldehyde is the reducing agent, the electroless copper deposition reaction can be represented as being divided into partial reactions: A. CH2O+2OH- < HCOO-+ l /2H2+H2O+ 1 e C. Cu+++2e-oCu Without wishing to be bound by any theory, in analogy to electroplating, the "A" partial reaction is the anodic reaction and the "C" partial reaction is a cathodic reaction. If the surface being electrolessly plated with copper is made anodic in an electrolytic cell, the rate of anodic reaction will increase with an increase in current density. As the current density increases, the potential or polarization of the surface becomes more positive.When the electroless copper deposition solution is modified by adding an accelerating or depolarizing agent according to the present invention, the polarization resulting from a given current density is less then the polarization obtained from the deposition solution without the accelerating agent. This difference in potential or depolarization is a measure of the acceleration of the anodic reaction.

Polarization measurements may be performed by standard galvanostatic electrochemical techniques in which a predetermined current is passed through the solution from the anode to the cathode. When the anode is the test electrode, the current passing between the anode and the cathode will induce a polarization of the test electrode, the anode. The polarization is the difference of the potential between the test electrode and a reference electrode, e.g., saturated calomel electrode, when current is passing and when no current is passed, e.g., at equilibrium.

With reference to Fig. 1, depolarization D measures the decrease of the polarization P, at the current density i, effected by the presence of an accelerating agent according to this invention. The percent depolarization expresses the same effect in terms of percent. If D is zero, there is no accleration based upon depolarization. Larger values of D correspond to greater accelerations.

Similarly, with respect to cathodic polarization, if a surface being plated in an electroless copper solution is made the negative electrode of an electrolytic cell, it will provide the means to measure the cathodic reaction. In a similar manner, the depolarization of the cathodic reaction by an accelerating agent is a measure of the acceleration of the cathodic reaction.

The accelerating effects of the agents on the anodic or cathodic reactions have been found to vary with the ligand or complexing agent for the copper ions.

Using electroless deposition solutions having the formulations stated below, the percent depolarization effecteduby a number of the accelerating agents taught herein was measured.

Bath Formulations for Tables I and II Tartrate Ligand Bath Rochelle salt 54.3 g/l Formaldehyde (37% soln.) 10 ml/l CuSO4. 5H2O 18.0 g/l Rochelle salt:copper (Molar ratio) 5.0:1 pH 12.8 Temperature 250C+1 Atmosphere Argon purged Accelerating agent 0.001 g/I Quadrol Ligand Bath [N,N,N',N'-tetrakis-(2-hydroxypropyl)- 34 g/I ethylene-diamine] Formaldehyde (37% soln.) 10 ml/l CuSO45H2O 18.0 g/l Quadrol:copper (Molar ratio) 1.6:1 pH 12.8 Temperature 250C+10 Atmosphere Argon purged Accelerating agent 0.001 g/l EDTA Ligand Bath EDTA, disodium salt 43.3 gIl Formaldehyde (37% soln.) 10 ml/l CuSO4. 5H2O 18.0 g/l Na2EDTA:copper (Molar ratio) 1.6::1 pH 12.8 Temperature 250C+10 Atmosphere Argon purged Accelerating agent 0.001 g/l In measuring percent depolarization, the galvanostatic current was supplied by a constant current DC power supply and the resulting polarization potential recorded on a X, Y recorder. The test results are summarized in Table As shown in Table I, these agents can selectively accelerate the cathodic partial reaction, or simultaneously accelerate the anodic and the cathodic partial reactions.

Table I Anodic and Cathodic Percent Depolarization Percent depolarization Ligand Accelerator Anodic Cathodic N,N,N',N'-tetrakis-(2- Cytosine 79 28 hydroxypropyl)ethylene diamine Adenine 82 31 Benzotriazole 72 27 Sodium 2-mercapto- 79 37 benzothiazole Pyridine 70 20 Guanidine 0 49 ETDA Cytosine 78 56 Guanidine 0 52 Tartrate Cytosine 0 35 Guanidine 0 35 Cathodic and anodic depolarization caused by the presence of an accelerating agent can be additive, as shown in Table II. The gravimetric accelerating factor A is defined as the ratio between the rate of electroless metal plating in the presence of the additive and the rate in the absence of the additive. The percent depolarization measurements in Table II were made using the same electroless metal deposition solutions and the same equipment as were used in obtaining the data of Table I.

Table II Gravimetric Accelerating Factor A and Total Depolarization Rate of electroless Gravimetric plating (gravimetric) Accelerating Percent microns/hr Factor A depolarization Without With Ligand Accelerator accelerator accelerator Anodic Cathodic Total Tartrate Cytosine 0.5 0.9 1.8 0 35 35 N,N,N',N'-tetra- Cytosine 2.8 6.4 2.3 79 28 107 kis-(2-hydroxy propyl)-ethyl enediamine ETDA Cytosine 1.0 2.5 2.5 78 56 134 As shown in Table II, inclusion of cytosine with appropriate pH regulations as taught herein caused an increase in plating rate of from 1 80 to 250%, depending upon the ligand present in the electroless copper solution. Such results were surprising and unpredictable.

In addition to the classes of compounds specifically mentioned herein, many other classes of depolarizing compounds are known in the electrochemical arts. It is to be understood that such compounds are also contemplated for use in this invention.

The process of this invention is further illustrated by the following examples, which are not to be construed as limiting.

In the examples, plating rates were determined by using either a "gravimetric" or a "burn-out" test.

In the gravimetric technique, 9 stainless steel foil, 5 centimeters in length and 3 centimeters in width, was first cleaned and then sensitized by immersing in a palladium chloride/hydrochloric acid solution for about 1 minute, followed by a water rinse. The foil was then immersed in the plating bath for about 1 5 minutes, rinsed and dried at 1 000C for about 20 minutes, weighed and then treated with nitric acid to etch off all of the deposited copper. The foil was then rinsed, dried and re-weighed. The thickness of the copper deposit was computed from the weight of copper plated and the known surface dimensions of the foil.

In the burn-out test, a copper clad epoxy-glass insulating laminate having a thickness of 0.062 inches and multiple non-copper clad through holes having a diameter of 0.040 inches, was cleaned with an aqueous solution of an alkaline cleaning agent at a concentration of 45 grams per liter in water and a temperature of 500C to remove surface dirt and thereafter rinsed with water. The copper clad surface was then cleaned with a 10% aqueous solution of sodium persulfate and rinsed with water.

Following this, the laminate was sequentially contacted with 10% sulfuric acid, rinsed with water and contacted with 30% hydrochloric acid. The hole walls were then sensitized to the electroless deposition of copper by contacting for 5 minutes at room temperature with a palladium chloride/tin chloride sensitizing solution commercially available. After contacting with the sensitizing solution, the laminate was rinsed with water and contacted with a 5% fluoboric acid solution by volume also containing 4 g/l of N-(2-hydroxyethyl)-ethylenediamine triacetic acid, to remove excess tin sat and, again, rinsed with water. The laminate was then immersed in an electroless copper plating solution, as described hereinafter, for 1 5 to 30 minutes, to deposit from 2 to 4 microns of copper.More specifically, the laminate was immersed in the plating solution for 1 5 minutes in the case of Bath A, or 30 minutes in the case of Bath B and Bath C. After plating, rinsing and drying, the maximum electrical current carrying capacity of the copper following deposition was then measured using the burn-out test described hereinafter. Briefly, current is applied across one or more of the copper plated through holes in the laminate at a constant, increasing rate of 3 amperes per second starting from zero, until the maximum current carrying capacity of the conductive copper in the through holes is reached. At this point, the integrity of the copper in the through holes is destroyed, "burns out", and the current value at burn-out is determined by means of an ammeter. The value of the "burn-out" current corresponds to the copper thickness on the through hole wall by the relationship burn out thickness copper thickness=0.2 x hole diameter The plating rate is determined in microns per hour from the copper thickness and the immersion time. In the examples, "burn-out" test data are identified by the designation "BO". All data not so identified in the examples were obtained using the "gravimetric" technique.

Example 1 This example illustrates the use of pyridine, a heterocyclic aromatic nitrogen compound, as an agent to accelerate the copper plating rate in a bath having the following composition: Bath A N,N,N'-N'-tetrakis (2-hydroxypropyl)ethylenediamine 34 g/l CuSO4. 5H20 18 g/l Formaldehyde (37% soln.) 20 ml/l Wetting agent (PluronicRTM P-85, BASF-Wyandotte Co.) 0.001 g/I Sodium hydroxide to desired pH Bath Aj-to which 0.1 g/l (100 mg/l) of pyridine was added, was operated at 25 OC. The effect of the presence of pyridine and the inter-regulating thereof with pH on the copper plating rate as taught herein is shown by the plating rate data in the table and Fig. 2.For purposes of comparison, plating rate data was also taken for Bath A without pyridine and that data is also summarized in the table below and in Fig. 2.

Bath A* Bath A+Pyridine Plating rate Plating rate pH microns/hour pH microns/hour 12.4 9,5** (BO) 12.4 10.7 (BO) 13.1 6.3 13.1 14.2** *comparison experiment **peak plating rate.

Example 2 The procedure of Example 1 is repeated, except that 14.3 g copper acetate is substituted for CuSO4. .5H2O and 0.005 g/l of 2-mercaptopyridine (a heterocyclic aromatic nitrogen compound) is used as the plating rate accelerating agent in the bath. The results are summarized as follows: Bath A* Bath A+2-mercaptopyridine Plating rate Plating rate pH microns/hour pH microns/hour 12.4 9.5** (BO) 12.4 12.5 (BO) 12.8 6.7 12.8 14.0 *comparison experiment **peak plating rate.

Example 3 This example illustrates the effect of combining two plating rate accelerating agents according to this invention, 2-mercaptobenzothiazole sodium salt and 2-hydroxypyridine, which are heterocyclic aromatic nitrogen compounds. Using these two agents in combination in Bath A, the plating procedure of Example 1 is repeated, and the results are summarized as follows: 2-mercaptobenzothiazole sodium 0* 0.002** 0** 0** 0.002 0.002 salt (g/l) 2-hydroxypyridine (g/l) 0 0 0.001 0.005 0.001 0.005 pH 13.3 13.3 13.0 13.0 13.3 13.3 plating rate (microns/hour) 5.8 11.7 7.9 11.5 12.3 13.3 (BO) *control experiment in the sense that no accelerating agent is present **control experiment in the sense that only one of the two accelerating agents is present.

The combination of 2-hydroxypyridine and 2-mercaptobenzothiazole provides a faster plating rate than either of the two compounds alone and a copper deposit which is bright and shiny. When used alone, 2-mercaptobenzothiazole provides a more stable bath in comparison with the control without either of the two compounds present, but the deposited copper is not as bright and shiny as desirable. On the other hand, the use of 2-hydroxypyridine, by itself, results in a copper deposit which is bright and shiny in comparismn with the control bath having on 2-mercaptobenzothiazole present or the control without either of the two compounds.

Example 4 The procedure of Example 1 is repeated, except that p-nitrobenzylamine hydrochloride, an aromatic amine, is used as the plating rate accelerating agent in bath A, in an amount of 0.1 g/l. The results are summarized as follows: Bath A* Bath A+ p-nitrobenzylamine HCI Plating rate Plating rate pH microns/hr pH microns/hr 12.4 9.5** (BO) 12.4 10.5 (BO) 12.9 6.3 12.9 11.8 (BO) *comparison experiment **peak plating rate.

Example 5 The procedure in Example 1 is repeated, except that 2,2'-dipyridyl, in the amount of 0.005 g/l, is used as the plating rate accelerating agent in bath A. The results are summarized as follows: Bath A* Bath A+2,2'-dipyridyl plating rate Plating rate pH microns/hr pH microns/hr 12.4 9.5** (BO) 12.4 10.3 (BO) 12.7 7.0 12.7 11 .O** (BO) *comparison experiment **peak plating rate.

Example 6 This example illustrates the effect of increasing the temperature on the plating rate in a process according to this invention.

Using the procedure of Example 1, the plating rate of copper in Bath A also containing 2mercaptobenzothiazole is measured at 260C, 380C and 700C. The results are summarized as follows: 2-mercaptobenzothiazole sodium salt (g/l) 0.002 0.002 0.002 pH (measured at room temperature) 13.2 13.2 13.2 Temperature (OC) 26 38 70 Plating rate (microns/hr) 13.0 19.3 65 (BO) The copper deposit formed at elevated temperatures has reduced intemal stress. At 700C, the bath was modified by lowering the formaldehyde concentration to 12 ml/l. Mention should be made of the fact that the 65 microns/hour plating rate achieved with the 700C bath is extraordinary. Also considerably noteworthy is 1 9.3 microns/hour plating rate achieved with the bath when operated at 38"C.

Example 7 This example illustrates the effect of using a group VIII metal in combination with a plating rate accelerator agent in accordance with this invention.

The procedure of Example 1 is repeated, using electroless copper deposition baths having the composition stated on Table III below. As shown by the data in the Table, the presence of a Group VIII metal further enhances the plating rate of the electroless copper plating solutions of this invention.

Bath operation in the presence of the additive(s) as taught herein results in a marked increase on the plating rates compared with the control bath. In addition, the additive(s) containing solutions of Examples 1 to 7 produce an adherent, substantially non-stressed copper deposit, whereas the control bath without the additive(s) produced a copper deposit of lesser quality.

Table Ill N,N,N',N'-tetra- 34 g/; 34 g/I 34 g/l 34 g/l 34 g/l 34 g/l kis-(2-hydroxy propyl)-ethyl enediamine CuSO4.5H20 18g/l 18 gel 18g/l 18g/l 18g/l 18 g/l Formaldehyde 20 ml/l 20 ml/l 20 ml/l 20 ml/l 20 ml/l 20 ml/l (37%) Wetting agent 0.001 g/l 0.001 g/l 0.001 g/l 0.001 g/l 0.001 g/l 0.001 g/l (PluronicRTM P-85) NaOH topH topH topH topH topH topH 2-mercaptobenzo- 0.002*gel 0.002 g/l 0.002*g/ 0.002 g/l 0.0015*gel 0.0015 g/l thiazole sodium salt Table II (contd.) NiSO4. 6H2O 0 g/l 1 g/l 0 g/l 0 g/l 0 g/l 0 g/l CoCI2.2H2O o g/l O g/l 0 g/l 4.5 g/l o g/l o g/l PdCI2 os/l os/l Og/l og/l os/l 0.01 g/l Sodium potassium 0 g/l 1.6 g/l 0g/l 4.5g/l 0g/l 0g/l tartrate pH 13.4 13.4 13.2 13.2 13.2 13.2 Plating rate 10.4 19 12.8 15 9 12 (microns/hour) * control experiment in the sense that a Group VIII metal is not present.

Example 8 This example illustrates the use of cytosine, a plating rate accelerating agent according to this invention, to accelerate the rate of copper deposition in a bath having the following composition: Bath B Tetrasodium ethylenediamine tetraacetate dihydrate 138 g/l CuSOfl .5H2O 14.7 g/l Formaldehyde (37% solution) 30 ml/l NaOH to pH Using the procedure for determining the plating rate described above, a stainless steel foil having the dimensions 3x5 cm is catalyzed for electroless metal deposition and electrolessly plated with copper at 250C in Bath B, to which 0.004 g/l (4 mg/l) of cytosine has been added.

The effect of the presence of cytosine and the change in pH on the plating rate of copper is shown in the table and Fig. 3. For purposes of comparison, the effect of the change in pH on the copper plating rate in Bath B without cytosine is also shown.

Bath B* Bath B+cytosine Plating rate Plating rate pH microns/hour pH microns/hour 12.4 5.3** 12.4 9.3 12.75 4.5 12.75 10.4** *control experiment **peak plating rate.

Example 9 The procedure of Example 8 is repeated, except that 2-mercaptobenzothiazole, in the amount of 0.005 g/l, is used as the plating rate accelerating agent. The results are summarized as follows: Bath B* Bath B+2-mercaptobenzothiazole Plating rate Plating rate pH microns/hour pH microns/hour 12.4 5.3 12.4 11.0** 13.1 3.5 13.1 7.3 *control experiment **peak plating rate.

Example 10 The procedure of Example 8 is repeated, except that 2-mercaptopyrimidine, in the amount of 0.003 g/l, is used as the accelerating agent. The results are shown in Fig. 4 and summarized as follows: Bath B* Bath B+2-mercaptopyrimidine Plating rate Plating rate pH microns/hour pH microns/hour 12.4 5.3** 12.4 5.3 13.0 3.5 13.0 *control experiment **peak plating rate.

Example 11 The procedure of Example 8 is repeated, except that guanidine hydrochloride, a non-aromatic nitrogen compound, is used as the plating rate accelerating agent, in the amount of 0.005 g/l (5 mg/l).

The results are shown in Fig. 5 and summarized in the following table.

Bath B* Bath B+guanidine HCI Plating rate Plating rate pH microns/hour pH microns/hour 12.4 5,3** 12.4 8.0 12.72 4.4 12.72 10.5** *control experiment **peak plating rate.

With respect to Examples 8 to 11, it will be noted that operation in the presence of the additives as taught herein leads to a marked increase in the plating rate of the solution, compared with the nonadditive containing control.

Example 12 This example illustrates a particlarly effective composition for practicing the invention and the results achieved therewith.

Copper sulfate 18 g/l QuadrolRTM 36 g/l PluronicRTM P-85 wetting agent 1 mg/l 2-mercaptobenzothiazole 1.5 mg/l NiSO4. 6H2O 0.61 g/l Rochelle salt 1.0 gel Formaldehyde 12.0 ml/l NaOH 37.0 g/l 4-hydroxypyridine 40.0 mg/l pH 13.15 (measured at 250C) Temperature 700C Rate 32 microns/hour Ductility 2 bends Bath stability very good It will be noted that in addition to having a fast rate, the bath of Example 12 produced copper of great ductility.

Example 13 This example further illustrates the electrolessly fast plating rate achieveable by practice of the invention.

Copper sulfate 18 g/l QuadrolRTM 34 g/l Formaldehyde 15 ml/l PluronicRTM P-85 (wetting agent) 1 mg/l 2-mercaptobenzothiazole 1.5 mg/l pH 13.2 4-hydroxypyridine 40 mg/l Polyox coagulant 1 mg/l Rate 72 microns/hour Temperature 700C Example 14 This example illustrates the practice of the invention using a highly concentrated solution. With such highly concentrated bath, the need for frequent batch wise or continuous replenishment is reduced or eliminated.

Bath C N,N,N',N'-tetrakis (2-hydroxypropyl)ethylenediamine 65.4 g/l (.22 mole/l) CuSO4. 5H,O 50 g/l (.20 mole/l) Formaldehyde (37% soln.) 20 ml/l (.27 mole/l) Wetting agent (PluronicRTM P-85, BASF-Wyandotte Co.) 0.001 g/l Sodium hydroxide 3.9 g/l (9.3 mole/i) pH 13.2 Temperature 250C In Example 14, the gravimetric test for plating rate was done using a copper rather than a stainless steel plate. For comparison, dilute Bath A of Example 1 was run using the same type of copper plates as the deposition substratum. The results are tabulated below.

Plating rate Bath Cytosine (mg/I) microns/hour A 0 3.6 C 0 4.0 C 5 7.9 C 10 9.8 C 15 10.5 C 20 11.3 C 40 9.1 Given the concentrate of the plating solution, the plating rates achieved with the cytosine present were unexpected. These rates achieved in this example illustrate the efficacy of the teachings herein to very concentrated plating solutions. Heretofore, the practice in the art has been to use dilute solutions, e.g., solutions containing less than 0.1 mole/l of copper salt, and generally about 0.06 mole/l. By practice of the teachings herein, electroless copper solutions of greater than 0.1 mole of copper salt can be used to achieve plating rates of greater than 7 microns per hour.A comparison of Baths A and C also shows that in these baths without the cytosine present, increasing the copper concentration in the bath (18 g/l of CuSO4. 5H2O in Bath A versus 50 g/l of the same salt in Bath C) has no significant effect in the plating rate. Rather, it is the presence of the cytosine, interregulated with the pH, which results in the plating rate increases.

In addition to the above embodiments, special mention is made of electroless copper deposition processes according to this invention wherein the accelerating agent consists of 2mercapto-benzothiazole in combination with imidazole or 4-hydroxypyridine, which leads to brighter deposits of copper in comparison with no accelerating agent or 2-mercaptobenzothiazole alone, and processes wherein the accelerating agent consists of pyridine in combination with 2mercaptobenzothiazole, which leads to enhancements in stability in comparison with pyridine alone, as well as brighter deposits of copper in comparison with 2-mercaptobenzothiazole alone.

Especially preferably, the plating rate accelerating agent is selected from among 2 mercaptobenzothiazole, 4-hydroxypyridine, 2-mercaptopyridine, aminopyrazine, pyrido (2,3,b) pyrazine, cytosine, guanidine hydrochloride, pyridine, 2-hydroxypyridine, para-nitrobenzylamine hydrochloride, imidiazole and mixtures thereof.

Because of the fast rate of copper deposition from the solutions made in accordance with this invention, frequent replenishment may be necessary if dilute solutions are used. Surprisingly, it is possible to practice this invention using highly concentrated plating solutions. See, e.g., Example 14.

In general, there may be used as the depolarizing agent any agent which, when added to the solution, produces at least a 20% and preferably at least a 30% depolarization of the anodic partial reaction or the cathodic partial reaction of the solution, or both.

By way of illustrating the use of this invention in the manufacture of printed circuit boards, prior to electroless metal deposition a copper clad epoxy-glass laminate is drilled to provide multiple through holes. The surface and the holes are cleaned with an alkaline cleaning solution at a concentration of 45 g/l and a temperature of 5O0C, and thereafter rinsed with water. The copper clad surface is then cleaned with a 10% aqueous solution of sodium persulfate and the surface is rinsed with water. The laminate is sequentially contacted with 10% sulfuric acid, rinsed with water and contacted with 30% hydrochloric acid.

After the pre-treatment, the non-copper clad hole barrels are catalyzed for electroless deposition in the standard manner using a palladium/tin salt catalyst, rinsed briefly with water, treated with 5% fluoroboric acid solution to remove excess tin salt, and again rinsed with water. The epoxy-glass laminate is now ready for treatment by a process according to the present invention.

The catalyzed epoxy-glass laminate is immersed in an electroless copper deposition bath (any of the above described) to deposit 2-4 microns of copper, typically.

After an initial deposit of copper in the hole barrels is obtained, e.g., 2-4 microns, portions of the copper clad surface are covered with a masking material, e.g., RistonRTM3 1 0, a dry film photoresist commercially available, copper is built up on the unmasked areas by conventional electroplating, and followed by electroplating tin-lead alloy (an etch resist). The masking is stripped off using a mild alkali, e.g., 4-1 5% solution of NaOH, and the background copper in the previously masked areas is etched away, e.g., using ammonical CuCI2. The product is an epoxy-glass laminate having a pattern of copper conductor lines on the surface, and copper interconnections in the through-holes, all coated with tinlead.

Claims (20)

1. A method for increasing the rate at which adherent copper may be deposited from an electroless cooper deposition solution having a pH greater than 10, which comprises including in the solution an agent which produces depolarization of the anodic partial reaction of the solution or the cathodic partial reaction of the solution or both reactions, and operating the solution at a pH greater than the peak plating rate pH of the solution without the depolarizating agent present so as to electrolessly deposit copper at a rate greater than the plating rate of the solution without said agent at the same pH and the same temperature.

2. The method of Claim 1 wherein the agent causes at least a 20 percent depolarization of the anodic partial reaction of the solution.

3. The method of Claim 1 wherein the agent causes at least a 20 percent depolarization of the cathodic partial reaction of the solution.

4. The method of Claim 1 wherein the presence of the agents causes at least a 20 percent depolarization of both the anodic and cathodic partial reactions of the solution.

5. The method of Claim 1 whereby an adherent, non-stressed deposit of copper may be produced from an electroless copper deposition solution comprising copper ions, a complexing agent for copper ions, a reducing agent and a pH adjustor and displaying a plating rate which first increases and passes through a peak plating rate and then decreases as a function of a pH above 10, said method comprising: (A) including in the electroless copper deposition solution an accelerating agent containing a delocalized pi-bond selected from among (a) heterocyclic aromatic nitrogen and sulfur compounds; (b) non-aromatic nitrogen compounds having at least one delocalized pi-bond; (c) aromatic amines; and (d) mixtures of any of the foregoing; and (B) operating the electroless copper deposition solution in the presence of the accelerating agent at a pH greater than the peak plating rate pH of the solution without said agent present, so as to electrolessly deposit copper at a rate greater than the plating rate of the solution without the agent at the same pH and the same temperature.

6. The method as claimed in any preceding claim wherein the accelerating agent is selected from among 2-mercaptobenzothiazole, 4-hydroxypyridine, 2-mercaptopyridine, aminopyrazine, pyrido(2,3,b)pyrazine, cytosine, guanidine hydrochloride, pyridine, 2-hydroxypyridine, paranitrobenzylamine hydrochloride, imidazole and mixtures thereof.

7. The method of Claim 5 wherein the accelerating agent is present in an amount of at least 0.0001 gram per liter of the electroless metal deposition solution.

8. The method of Claim 5 wherein the accelerating agent is present in an amount of up to about 2.5 grams per liter.

9. The method as claimed in any preceding claim wherein the accelerating agent has a free electron pair on a nitrogan atom adjacent to a pi-bond.

10. The method as claimed in any preceding claim wherein the electroless metal deposition solution includes ions of at least one metal selected from Group VIII of the Periodic Table of the Elements.

11. The method of Claim 10 wherein said metal ions are present in an amount of from about 0.005% to about 30% by weight based on the weight of the copper salt.

12. The method as claimed in any preceding claims wherein the reducing agent is selected from among formaldehyde and precursors or derivates thereof, boranes, borohydrides, hydroxylamines, hydrazines and hypophosphite.

13. The method as claimed in any one or more of Claims 1 to 11 wherein the pH adjustor is an alkali metal hydroxide or alkaline earth metal hydroxide.

14. The electroless copper deposition solution which comprises copper ions, a complexing agent for copper ions, a reducing agent and at least one depolarizing agent and a pH adjustor to maintain such solution above pH 11 and which is characterized by a plating rate of at least 7 microns per hour and higher than the plating rate of the solution without the depolarizing agent.

1 5. The electroless copper deposition solution of Claim 14 wherein the solution includes a depolarizing agent which contains a delocalized pi-bond and is selected from among (a) heterocyclic aromatic nitrogen and sulfur compounds; (b) non-aromatic nitrogen compounds having at least one delocalized pi-bond; (c) aromatic amines; and (d) mixtures of any of the foregoing.

1 6. The electroless copper deposition solution of Claims 1 3 or 14 characterized by the ability to electrolessly form an adherent copper deposit at a room temperature rate of at least about 9 microns per hour.

1 7. The solution of Claim 1 6 wherein the depolarizing agent accelerates the anodic partial reaction of the solution by at least 20 per cent.

1 8. The solution of Claim 1 6 wherein the depolarizing agent accelerates the cathodic partial reaction of the solution by at least 20 percent.

1 9. The solution of Claim 1 6 wherein the depolarizing agent accelerates both the cathodic and anodic partial reactions of the solutions by at least 20 percent.

20. The electroless copper deposition solution of Claim 14 having a pH above 10 and a concentration of copper salt above 0.1 mole per liter.