CA1135903A - Electroless copper deposition process having faster plating rates - Google Patents

Abstract

ABSTRACT OF THE DISCLOSURE
There is provided a method for increasing the useful effective plating rate of an electroless copper deposition solution which comprises copper ion, a complexing agent for copper ion, 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. In accordance with this invention, the plating rate of such a solution may be significantly increased by operation thereof in, the presence of an accelerating or depolar-izing agent at a pH to achieve a plating rate above the plating rate of the solution without such an agent at the same pH. The accelerating or depolarizing agents for use herein 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.

Description

B~CK~,~OUWD OF Tlll':LNV~NTION
.. . . .
Electroles.s, i.e., autocata:Lytic, metal deposition - solutions for the formation of metal layers on non me-tallic or metallic substrates are well known in the art. Thcse are characterized by -the capacity to deposit metal in virtually any desired thickness on a wide variety of surfaces without the need for an external supply of electrons. Such solutions differ from electropla-ting 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 thic]cness. Electroless metal deposition solutions are especially suitable for forming metal layers on the surface of non-metallic or resinous articles wh:ich have been pretreated to render the surface catalytic to the electroless reception of metal.
Special mention is made of the use of electroless metallizing procedures in the plating of plastics generally and the manufacture of printed circuit boards particularly. In the plating of plastics, a thin layer of copper is electrolessly deposited on the sensitized surface of a resinous article, e.g., an insulatin~ material, to produce a metallized or metal plastic ~; part of use, e.g., in the automobile lndustry, as grills, door knobs and the like. In the manufacture of printed circuit boards, a thin layer~of copper is electrolessly deposited on a sensitized surface of an insulating substratum, selected areas .
of the.surface of the electroless deposit are masked, the initial layer o unmasked copper is then built up by electroplating, and .
the masked areas of copper are etched awa~ after removal of the :
masking layer to Leave the~desired conducting pattern of copper on the surface. In another procedure, selected areas of the ~m~ 2 -S~03 ~ urface of the insulatirl~ substratum are sensitized in the ~orm of a desired printed circuit pattern and copper is electrolessly deposited on the sensitized areas to form khe desired circuit pattern. In the manufacture of printed circuit boards, electro-less metal deposition techniques are also often used to plate the sensitized walls of through-holes formed in the insulating article in order to, e.g., produce electrically conductive connections, so-called plated through holes, between circuit patterns formed on opposite sides of the article surface.
10A shortcoming of early processes for the electroless 'I deposition of copper was that the deposi-tion 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 whi.ch deposition was taking place. To overcome such shortcomings, the art has proposed a number of compounds as stabilizing agents for prolonglng the useful llfe of electro]ess metal deposition solutions and for improving the quality of the copper deposit.
These include 2-mercaptobenzothiazole, in Pearlstein, U.S.
;3,222,195~; 2,5-dimercapto-1,3,4-thiodiazole and 8-mercaptopurine, in Jackson, U.S. 3,436,233; o-phenanthroline, in Stone, U. S.
: ~ :
3,615,735; l-phenyl 5-mercaptotetrazole, in Jonker et al, U. S.
3,804,638; 2,2' dipyridyl and 2-(2-pyridyl)-benzimidazole, in ~` Hirohata et al, U. S. 4,002,i86; and benzothiazole-thioether-polyethyleneglycol, in ~lolenaac et al, U. S. 3,~43,373.
Still other stabilizing aqents are dlsclosed in Schneble et al, U. S. 3,257,215, ~for example, thiazoles, lsothiazoles and thiozines, Maguire, U. S. 3,~93,038, for example, benzo-~; ~30 triazole, diazole, imidazole, guanidlne, pyrimidine, and :
dm: v - 3 -~3S9~3 "
others and Tori.yal et al , ~. S. 3,377, 174, for example, 2,2'-biquinoline, 2,9-dlmethylphenanthroline and 4,7~diphenyl-1,10-phenanthro:l. ine .
1 Schoenherg, U. S. 3,708,329, discloses that the addition of a heterocyclic aromatic nitrogen compound having up to 3 rings ; with a hydroxy group honded to one of the rings, results in amarked increase in the stability of electroless copper platiny baths without adversely affecting the plating rate. See also Schoenberg, the Journal of the Electrochemical Society, ~18, 157I
(1971). Although Schoenberg in V. S. 3,708,320 talks about il improving plating rate, 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 4 ~ 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 solutlons at a low rate, i.e., less than about 6 microns per hour so as to produce a~copper deposit of good quality, i.e., a coherent, structurally stable, thin film of , ~ ~ copper adherent to the surface being coated. The experience in ;: :
the art has further~been that plating rates above about 6 microns per hour resulted in the production of a copper deposit of poor quality, i.e., one wh~ich flakes off or tends to flake off ~he j ~ . . , ~ ~ surface or which was non-coherent.

.
~ dm:JO ~ -~ 4 - ~

- ~L3~9.~3 A.s used herein, the phrase adheren~ copper deposlt refers to an electrolessly formed copper deposit which can be strippe~icl from a platcd insulating suhstratum in the form of a thin, integral film such that when stri:pped, retains its structural integrity or cohesiveness as a film without crumbling.
As used herein, the phrase non-adherent copper deposit referes to an e].ectrolessly formed copper deposi-t which flakes or tends to flake off the coated substrat~um. Such a deposit lacks cohesiveness and cannot he stripped from the insulatiny substratum in the form of a thin, stable, structurally integral film.
It is one object of this invention to increase the rate at which copper can he electrolessly formed.
It is a further object of this invention to provide procedures and compositions for increasing tne rate for electrolessly forming an adherent copper deposit.
Another object of this invention is to provide electro-less 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 hlgh 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 operating a given . . -~ , dm~

~: :

13S~03 lec-trol~.ss copper solution of t:he ty~e disclosed in the presence of an accelerator or depolarizing agent at a pH greater than the peak plating rate p~I of the solution without such an agent. 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 capab^le of accelerating by at least 20% and up to 100% or between about 35 and 90%, the cathodic partial reaction or the anod:ic 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 ranqe depending upon the formulation used and the quality of copper desired. In general, rate increase$ achieved by practice of this invention will be at least up to 300% br more depending upon solution ~; formulation. However, rate increases of up to 1 or 1 1/2 orders of magnitude, i.e., 10 times (1000%) or even 50 times (5000%) are possible. Achlevement 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 practlce 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 char~acterized~by a room te~perature plating - , ~ ~ rate above 7 microns per hour, and qenerally above 9 microns per :: : ~ ~, ~ dm:~d ~ ~ - 6 .

~L13S903 hour, or between about 9 and 25 mlcrons per hour and hi~her and are characterized by the ab;lity to electrolessly form copper at a rate of up to at least 30 micxons per hour for a period of at least 15 minutes. Elevated tem~perature rates of up to 70 microns per hour or even higher are however possible.
Here again, achievement of such rates are unexpected and sur-prising. Moreover, such rates may be achieved for periods of time ranging from one or several minutes up to prolonged pe~iods up to eight hours or more. Typical are operating times of about 5 minutes to about 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 plating periods unnecessary.
Electroless formation of copper in accordance with this invention-will result in many operating advantages, including shorter platlng times and, concomitantly, increased production capacity. Compared to commercial practices now available, the procedures and compositions of this invention require less equip-~ ment, lower capital lnvestment costs and lower e~ergy requirements.
20i~ Unlike the current commercial practices, the procedures herein ;~ taught are particularly suitable for use in automatic plating .
systems with relatively short dwell times.
DESCRIPTION OF THE INVENTION -`
This invention provides a method for operating an electro-less copper deposition solution to increase the plating rate. The ,!
solution comprises copper ions, a complexing agent for copper ions, a reducing agent and a pH~adjustor and is characterized by a plat-ing rate which first increases and passes through a peak plating rate and then`decreases as a function of pH above 10. -B ~
~ 7 ~ ~

, ..... :, ;i~ , ~:. : ' ''i' 1~3S9(~3 The method of the invention relates to improveménts for depositing at a rate greater than about 7 micrometers of electroless copper per hour in a bath composition operated at a temperature of about 25C to about 35C to a rate greater than 19 micrometers of electroless copper per hour in a bath composition operated at a temperature above 35C, a coherent, structurally stable thin film of electroless copper adherent to a substratum , comprising: tA) incLuding within the electroless copper deposition solution an accelerating 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; (B) contacting the electroless copper deposition solution with a substratum sen-sitive to the deposition of electroless copper; and (C) while ;
, operating the electroless copper deposition solution at a pH above10, regulating the pH thereabove and the amount of the accelerating agent therein to maintain a deposition within ~he rate, to there-by achieve a coherent structurally stable thln film of electroless ;~
copper adhered to the surface of the substratum.
The terms "depolarizing agent" and "accelerating agent"are used interchangeably herein.
The preferred depolarizing or accelerating agents of this invention have a free~electron palr~of the nitrogén atom ad~acent to a pi-bond.
~;~ By way of illustration, the heterocyclic aromatic nitro-gen compound, (A)(a), is selected from among pyridines, e.g., pyridine, cyanopyridine,~chl~oropyridine, vinylpyridine, - ~13~i~03 ,ninopyrid:ine, 2-pyrazolo-(~,3-c)--pyri.dince, 3~v-triazolo-(4,5-b)pyridine, 2,~'~dipyridyl, picolines, ~and thc like;
pyridazi.ne; pyr:Lmidines, e.g., In-diazine, 2-hydroxypyrimidine,

2-oxy-6-aminopyrimicline (cytosine), and the like; pyrazines;
triazine; tetrazine; indoles, e.g., indo:Le, tryptamine, trypto-phan, 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-pyrazole, 1,3-imidazole, and the like; triazoles, e.g., pyrro-diazole, benzotriazole, diphenyltriazole, isotriazoles, and the like; tetrazoles, and benzodiazoles, e.g., indazole, benzimidazole and the like.
Also included are mercapto-derivatives and thio-derivatives of any of the foregoing, such as mercaptopyridines, mercaptopyrimidines, thiazoles, thiazoline, thiazolidine, mercaptothiazoles, imidazolethiols, mercaptoimidazole, mercapto-purines, mercaptoquinazolinones, thiodiazoles, mercaptothio-diazoles, mercaptotriazoles, mercaptoquinolines, and the like.
20 ~ Illustratively, the non-aromatic nitrogen compound, (A)(b), is selected from among ureas, guanidines and derivatives thereof.
Preferably, the aromatic amine, (A)(c), is selected , ~
- from among p-nitrobenzylamine, anilines, phenylenediamines and mixtures thereof.
Prefer~ably, the~depolarizing or accelerating agent will : . :

~:;: . : , ~ ' .

~ ~ dm~ g :

S9~)3 ' `':
be present in a small effective amount, i.e., generally at least about 0.0001 to abou-t 2.5 grams per llter, more specifically ahout o . obos to 1.5 grams per liter and preferably from about 0.001 to about 0.5 grarns per liter. In general, the amount of depolarizing or accelerating a~ent used wil] vary depending upon the par-ticular 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 ion, an ion of a metal or metals selected from among the transi~
tion metals, preferablv 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 .005 to about 30~, by weight of the Group VIII metal, based on the weight of the copper salt, are used.
The copper ion is normally supplied in the form of a water soluble copper salt. The choice of the salt is chieEly a ; matter of economics. Copper sulfate is frequently preferred, but copper ha]ides, 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 complextn,? agent for copper~lons is selected from - ~ compounds conventionally employed for this purpose, includin~ but ;

:
:
-; ~ d~:J~ 10 ~

.

`` ~L135~3 not llrnited to ~ochelle salts, the sod:i.um (mono-, di-, tri ancl tetrasodium~ salts of ethylenediaminetetraacetic acid (hereinafter sometimes referred to as "EDTA"), diethylene~iaminepentaacetlc acid, nitriloacetic acid and its alkal:i salts, gluconic acid, ~luconates, triethanolamine, diethylaminoethanol and glucono-~-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 ~1, N, M',N'-tetrakis-(2-hydroxypropyl)-ethylenediamine (hereinafter sometimes referred to as "Quadrol*"), triethanolamine, ethylenenitrilotetraethanol, nitrilotri-2-propanol, tetrahydroxyethylenediamine and N-hydroxyethyl-N,N'-M'-(trihydroxypropyl) ethylenediamine. These are commercially available or can be Prepared by following procedures described in tlle literature.
The reducing a~ent lS selected from among, illustratively, formaldehyde an~ formaldehyde precursors or derivatives, e.g., paraformaldehyde, trioxane, dimethylhydantoin, ~lyoxal, and the like; boranes; borohydride; hydroxylamines; hydrazines and hypo-~; phosphite.
The pH may be re~ulated by the use of a pH adjustor,preferably a water soluble alkali metaI or alkaline earth;metal hydroxide, e.g., magnesium hydroxide, calcium hydroxide, potassium hydroxide, sodium hydroxide, or the like. Among these sodlum ;~
hydroxide is preferred, chiefly for reasons o `economy. During .
operation, the pH lS monitored and raised or lowered, as needed, by the addition of suitable amounts of the pl~ adjustor.
*Trade mark dm~

~135~()3 Other ingredients can also be added. For instancer it may be desirable to employ a minor, effective amount of a we~ting agent or agents, preferably i~n amounts of less than 5 grams per liter. Examples of such commercially available sur-factants include PLURONIC* P85, BASF-Wyandotte Corp., a non~
ionic block copolymer of ethylene oxide and propylene oxide and G~FAC* 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 lQ sub~ect to wide variation within certain ranges which may be defined as follows:
Copper salt 0.002 to 1.20 mol~
Reducing agent 0.03 to 3 moles Cupric ion complexing 0.5` to 20 times the moles agent of copper Alkali metal hydroxide sufficient to give a pH of 10.0 to I4~0 and preferably of 11.0 to 14.0 as measured `~
at room temperat~re ~i , Water ~ sufficient to make l liter `
When non-aqueous~solvents are used instead of water, pre-0 ferably they are selec~ted from among, for example, dimethylformamide, dimethylsulfoxide and;acetyl acetate.
More preferably, the plating baths of the present inven~
tion are compounded withln~more narrow llmi~ts than set forth immed-ately above/ and the~preferred embodlments comprlse~

*Trademark . ~

~ 3~i~3(~3 A soluble cupric sall, 0.002 to 0.~ mole preferal)Jy cupric .sulfate Alkali metal hydroxide, pll 11.2 to 13.7, as preferably sodium hydroxide, measured at room to give temperature Formaldehyde (reducing agent) 0.06 to 0.50 mole Cupric ion complexing agent 0.002 to 2.0 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.
In considering the general formula and the specific working formulae which are set forth below, it should be under stood 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 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 0.000], preferably at least 0.0005, up to about 2.5 gram/liter. With the depolarizing compound present, the pH of the solution will be adjusted as desired to achieve a faster plating rate in comparision with the so].ution without the accelerating agent at the same pH. Preferably, but not necessarily, the pH of the solutlon is adjusted to be the greater than the peak plating rate pll of the solution without the depolari~ing agent.
In using the baths, the surface to be plated should be ~ree of grease and other contaminating material.
~ . ' dm~ 13 -~L3S~3 ~ here ~ n~n-metal.l.i.c surface is to be plated, the sur~ace areas to receive the deposit should first be treated, as in conventional processes, wlth a conventional sensitizing and seeding solution, such as stannous chloride (SnC12~, followed by treatment with a dilute solution of palladium chloride (PdCl2).
Alternatively, extremely good sensitiæation is achieved by using an acidlc solution prepared from stannous chloride and precious metal chlorlde, such as palladium ch].oride, the stannous chloride being present in stoichiometric excess, based on the amount of precious metal chloride. These are well known in the art.
Where a metal surface, such as copper foil, is to be treated, it should be degreased, and then treated with acid, such as hydrochloric or phosphoric acid, 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 solution.
Following pre-treatment and/or sensi.tization, 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 practice, 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 heing plated can be continuously X
dm:Jc .

~13~3~
sonveyed ~hrou~h a tank or other reservoir conta:ining the pLating solution or a spra~ curtain of the platincJ solution.
In general, the electroless meta] deposition solution is prepared by addiny the complexing agent to an aqueous s~lution 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 derivative. 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 15 and boiling, e.g., 100C., can be used, and tempera-tures of between 20 and 80 C. are preferred. It is noteworthy that bright adherent copper deposits are obtained at good rates even at room temperature, e.g., about 25C.
The process of this invention is employed to electro-lessly deposit copper on non-metallic or insulating surfaces, such as paper, glass, ceramics, synthetic resins and plastics, e.g., silicones, phenolics, alkyds, epoxies, styrenes, acrylics, vinyl chlorides, nylon, mylar, acrylonitrile-butadiene-styrene, and the like.
Applications of the invention include the 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 ~lass for copper mirroring.

dm J~ - 15 -:,' ` . ' . ;;. :' . : ' :, - ~13~i~03 The ~agl: deposil:ion rates achievable J~y the use o~ this lnvention make possible the formation o metal layers by electro-less deposition at rates which are comparable to those obtainecl hy conventional electroforming copper techniques and electroless nickel techniques.
This invention is especially useful in the manufacture of printed circuit boards and the me-tallizing of plastic articles.
By way of illustration, whole or sele~cted por-tions of the surface of an insulating article, e.g., phenolic paper, epoxy-glass laminate, molded acrylonitrile-butadiene-styrene terpolymer or platable nylon or polysulfone surfaces, are pretreated -to sensitize the surface to the electroless deposition of copper.
After sensitization, the article is immersed in an electroless copper deposition solution, such as described herein, and permitted to remain there until a layer of copper is deposited on the surface. The copper layer can be built up to a desired thickness by further electroless metal deposition or by electro-plating with copper or combinations of metals such as copper, nickel and chromium.
In the case of printed circuit board manufacture, if desired, interconnections between opposite surfaces of the insulating article can be provided by drilllng or punching holes therethrough, and sensitizing the walls of the through-holes prior to exposure to an electroless metal deposition bath. Copper builds up on the walls of the holes to form interconnections.
When formaldehyde is the reducing agent, the electroless copper deposition reaction can be represented as being divided into partia] reactions:
, ~, dm~ 16 -;

, ~1359~)3 A. CH2O -~ 2 OH ~ > HCOO ~ 1/2 H2 ~ H2O -~ le C. Cu~ ~ 2e ~ > Cu ~ ithout wishing to be bouncl by any theory, in analogy to electroplating, the "A" partial reac-tion is the anodic reac-tion 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 in-creases, the potential or polarization of the surface becomes more positive. When the electroless copper deposition solution is modified by adding an accelerating or depolariæing agent according to this invention, -the positive potential or polariza-tion resulting from a given current density is less than the poten-tial, or polarization, obtained from the deposition solution with-out the accelerating agen-t. This difference in potential or de-polarization is a measure of the acceleration of the anodic re-action. -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 cath-ode. 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 differenae 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.
DESCRIPTION OF THE DRAWINGS `
The instant invention will be more fully understood from the following description taken with the appended drawings, in which pg ~ - 17 -35~03 Figure 1 is a graph in which cUrrent density and potential are plotted Eor a solution without axl accele.ra-tor and for the same solution with an accelerator to show the effect on polarization according to the invention;
Figure 2 is a graph in which plating rate and pH are plotted to show the effect on platiny rate by one accelerator according to the invention;
. Figure 3 is a graph similar to Figure 2 but showing the effect on plating rate by a different accelerator;
Figure 4 is a graph similar to Figures 2 and 3 but showing the effect on platiny rate of a still differen-t accelera-.
tor; and, Figure 5 is a graph similar to Figures 2, 3 and 4 but showing the plating rate effect o a still different acceler- .
ator.
, With reference to FIG. 1 depolarization D measures the Pg/~ - 17A -~ ' .

';303 ~ crease of -the polarization P, at ~he current denc;ity :i, ofiected by the presence o an accelerating ayent accordincJ to this invention. The percent depolar:izatioll exprescies -tlle same ef~ect in terms of percen-t. If D is zero, there is no acceleration 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 reac-tion. In a similar manner, the depolarization of the cathodic reaction by accelera-ting 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 w:ith the ligand or complexing agent for the copper ion.
Using electroless deposition solutions having the formulations stated below, the percent depolarization effected by a num~er of the accelerating agents taught herein was measured.
BATH FORMULATIONS FOR TABLES I AND II

.
TARTRATE LIGAND BATH

Rochelle salt 54.3 g/1 Formaldehyde (37% soln.) 10 ml/l CuSO.,.5H20 18.0 g/l Rochelle salt:Copper (Molar ratio) 5.0:1 pH 12.8 Temyerature 25 C ~ 1 dm:~ - 18 -' 3L~3S.~ )3 Atmosphere l~rcJon purged Accelerating aCJent 0.001 g/l QUADROL l,IG~ND B~TH
_. .
[N,N,N',N'-tetrakis-(2-hydroxypropyl)ethylene-diamine] 34 g/l Formaldehyde (37% Soln.) 10 ml/l CUsOl,.5H2O 18.0 g/l Quadrol:Copper (Molar ratio) 1.6:1 pH 12.8 Temperature 25 C + 1 Atmosphere Argon purged Accelerating ayent 0.001 g/l EDTA LIGAND BATH
EDTA, disodium salt 43.3 g~l Formaldehyde (37% soln.) 10 ml/l CuSOL,.5H2O 18.0 g/l Na2EDTA:Copper (Molar ratio) 1.6:1 pH 12.8 Temperature 25 C -~ 1 Atmosphere Argon purgec, Accelerating agent 0.001 g/l In measuring percent depolarization, the galvanostatic current was supplied by a Hewlett-Packard HP hl77C constant current DC power supply and the resulting polarization potential recorded on a Hewlett-Packard 7004A X, Y recorder. The test results are summarized in Table I.

dm:~, - i9 -~ .
~ . . ' '. ' .

3.59~
TABI,E I
Anod:ic and Cathodic Percent Dcpolari~ation .
Percent Depolarization Ligand AcceleratorAnodicCathodic N,N,N',N'-tetrakis-(2-hydroxypropyl(ethylene diamine Cytosine 79 28 Adenine 82 31 Benzotriazole 72 27 Sodium 2-mercapto-benzothiazole 79 37 Pyridine 70 20 Guanidine o ~9 EDTA Cytosine 78 56 Guanidine 0 52 Tartrate Cytosine 0 35 Guanidine o 35 As shown in Table I, the agents of this invention can selectively accelerate the cathodic partial reaction, or simultaneously accelerate the anodic and the cathodic partial reactions, to the same or a different extent.
Cathodic and anodic depolarizations 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 o the addi-tive. The precent depolarization measurements in Table II were made using the same electroless metal depostion solutions and the same equipment as were used in obtaining the data of Table I.

dm~ 20 -.

~L~3l3S9 r ~
.IJ r~) o r~) O r~ r~l E~

~:
.0 .1~ r .IJ ~ ~
~ N O Ul t~ ~D
(U rl ,~ ~1 t~
~> h d ,~ o O
a rl O o . t5~ t~
~¢ ~
~1 O O
~) rl r t~ S~ -IJ
s~
o a) ~ o t~o t~
. r t~ ~ rl r-l C) I¢ r-l N N
~:: ,~ ~> a) a~
~rl N (~ 4 H h , ~ f C . O
H a) r-l a:l ,~ O h tr, P~ O ~ ~ O
a) ~ ,~1o ~ N
a r~ .
o ~ ~_ ~
rl ~ ~ O
~ O r-l . rl E~ ~
Q) O ~ O . ' .
u~ ~\
~1 ~ a) u~
~1~ IJ O ~1 ~ 0 ~1 1~ ~ ~1 > o ~ a) ~1 tlo o O ~ 1 ~7 ~1 ~1 o ~1 a) o N r-l ~1 ~ rl ~ ~) o ~ .
h :~ o ~ a) a) :
,. ~ .~
,~ O O O
: C~
t) o C~

N Q~ rl -I h,~ ' :
rl ~ `
h z ~ O c ~ ~) ~ h .~ h Z
~1 E~: Z ~ .C a ~ ~' ' ".
: .
- . -dm ~ 21 -- ~13~ 3 As shown in 'IJabl~ II, inclusion o~ cy-tosine with appropriate~ pH regulatlons as taughk herein caused an increase in plating rate of from 1~0 to 250 per cent, depending upon the ligand present in the electroless copper solutlon. Such results were surprising and unpredictable.
In addition to the classes o~ 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.
DESCRIPTION OF TE-IE SPECIFIC EMBODIMENTS
. _ . .. . .....
The process of this invention is 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, a 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 15 minutes, rinsed and dried at 100C 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-sopper clad through holes having an outsi.de diameter of 0.040 inches, was cleaned with an aqueous solution of ALTREX*, B~SF-Wyandotte Corp., an alkaline cleaning agent, *Trade mark dm~ 22 -~ ~3~j~0;~

at a concentration of 45 yrams per liter in water and a temper-ature of 50C to remove surface dirt and thereafter rinsed with water~ The copper clad surface was then cleaned with a 10 per-cent aqueous solution of sodium persulfate and rinsed with water. Following this, the laminate was sequentially contacted with 10 percent sulfuric acid, rinsed with water and contacted with 30 percent hydrochloric acid. The non-copper clad through holes were then sensitized to the electroless deposition of copper by contacting for 5 minutes at room -temperature with OXYTRONE* ACTIVATOR 316, a palladium chloride/tin chloride sensitizing solution commercially ayailable from Sel-Rex.Co., a division of O.M.F. Corp., Nutley, New Jersey. After contacting with the sensitizing solution, the laminate was rinsed with water and contacted with a 5 percent fluoboric acid solution by volume also containing 4 g/l. of N-~2-hydroxyethyl)- ethylenediamine triacetic acid, to remove excess tin salt and, again, rinsed with water. The laminate was then immersed in an electroless copper plating solution, as described hereinafter, for 15-30 minutes, to deposit from 2 to 4 microns of copper. More specifically~ the laminate was immersed in the plating solution for 15 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 in co-pending Canadian Application Serial No. 309~.781, filed August 22, 1978, which has a common assignee to this application~ Briefly, current is appl.ied across one or more of the copper plated through holes in the lamin-. .
a-te at a constant increasing rate of 3 amperes per second starting * Trademark ` ~ / - 23 -.. ~

3~V3 from zero, until the maximum curren-t carrying capacity of -the conductive copper in the through hole is reached. ~t this point, the copper in the through hole fuses and burns out and the current value at burn out is determined by means of an ammeter. The value of the burn out corresponds to the copper thickness in the through hole, by the relationship;

copper current = 0.2 x burn out thickness hole diameter The plating rate is determined in microns per hour from the copper thic]cness 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.

Pg/~ ~ 24 -~.: , .: . .

~13~ 3 1~_MP~JE
Th:Ls example illu6trates the use of pyricline, a he-terocyclic aromatic nitrogen cornpouncl, as an agcnt to accelerate the copper plating rate in a bath haviny the following compos-tion.
BATEI _ ~,N,~ N'-tetrakis (2-hydroxy-propyl)ethylenediamine 34 g/1 CuSOI,.5J-12~ 18 y/l Formaldehyde ~37% Soln~) 20 ml/l Wetting Agent (PLURONIC P-85, BASF-~yandotte Co.) 0.001 g/1 Sodium hyclroxide to desired pll Bath A, to which 0.1 g/l (100 mg/]) of pyridine was added, was run at 25C. The effect of the presence of pyridine and the inter-regulating thereof with pM 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 tha-t data is also sumnarized in the table below and in FIG. 2.
BATH A* BATH A ~ Pyridine Plating rate, Plating rate, pH microns/hr. pH microns/hr.
12.4 9.5** (BO) 12.4 10.7 (BO) 13.1 6.3 13.1 14.2**

* comparison experiment ** peak plating rate dm J~ - 25 .. ~ . .

;, : , : , i .

~3~9~3 ~X~MPr,U 2 The procedure of l~xample!] is rcpeated, except that 14.3 g coppcr ace-tate is substituted for CuS0l,.5H20 and 0.005 g/l of 2-mercapto~yridine (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 mercaptop~ridine Plating rate, Plating ra-te, 10 ~ microns/hr. ~ microns/hr.
12.4 9.5** (B0~ 12.4 12.5 (B0) 12.8 6.7 12.8 14.0 . _ _ . .
* comparison experiment ** peak plating rate dm )~ - 26 -,, . ., .. , ... ~ ... - .. ~ . . .. .. , - - -~X~MP ~,r~ 3 'I'hls example illustrates thc effec-t o combln:ing two plating rate accelerating agents accordinq 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 ~xample 1 is repeated, and the results are summarized as follows:

2-mercaptobenzothiazole sodium salt, g/l 0* 0.002** 0** 0** 0.002 0.002 2-hydroxypyridine, g/l 0 0 0.001 0.005 0.001 0.005 pH 13.3 ]3.3 13.0 13.0 13.3 ]3.3 plating rate, microns/hr. 5.8 11.7(BO) 7.9 11.5 12.3 :13.3 * control experiment in the sense that no accelerating a~ent is present ** control experiment in the sense that only one of the two accelerating agents is present It is shown that the combination of 2-hydroxypyridine and 2-mercaptobenzothiazole provides à 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 deposi-ted copper is not as briqht and shiny as desirable. On the other hand, the use of 2-hydroxypyridine, by itself, results i.n a copper deposit which is bright and shiny in comparison with the control bath having only 2-mercaptobenzothiazole present or the control without either of the two compounds.

, dm~ 27 -~. , .. :

3L13~03 FXAM~ 4 Tlle procedure of Examplc 1 i5 repeated, except that p-nitrobenzylamine hydrochloride, an aromatic arnine, is used as the platin~ rate accelerating agent in bath ~, in an amoun-t of 0.1 g/l. The results are s~lmmarized as follows:

B~TH A ~
BATH A* p-nitrobenzylamine HCI.

Plating rate, Plating r~-te, pH microns/hr. ~ 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 ra-te EXAMPLE S
_ The procedure of 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, microns/hr. ~ microns/hr.
12.4 9.5** (BO) 12.4 10.3 (BO) 12.7 7.0 12.7 11.0** (BO) * comparison experiment ~; ** peak plating rate dm~ 28 -`' .

35~j3 3XAMPL~ 6 Thi.s example illustra-tes the effect of increasiny the tempe~ature on the p].atin~ rate in a process accord:ing to this invention.
~ sing the procedure in Example 1, the plating rate of copper in bath A also containing 2-mercaptobenzothiazole is measured at 26 C, 38 C and 70 C. 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, C 26 38 70 Plating rate, microns/hr. 13.0 (BO) 19.3 65 It is shown that, all other conditions being sub-stantially the same, the plating rate undergoes an increase as the temperature is raised. Also, it is observed that the copper deposit has reduced internal stress. At 70 C, the bath was modified by lowering the formaldehyde concentration to 12 ml/l.
Mention should be made of the fact that the 65 micron/hr. plating rate achieved with the 70 C. bath is extraordinary. Also considerably noteworthy is 19.3 microns/hr. plating rate achieved with the bath when operated at 38C.

~, .
dm:~b - 29 -~3~03 ..
EXAMPI,E_7 This exarnple i].l.ustrates the effect of using a Group VC:CI
metal in combination wi.th a plating rate accelerating aqent in accordance with this invention.
The procedure of Example 1 is repeated, usinq electroless copper deposition baths having the cornposi.tion stated in the table below. As shown by the data in the Table, the presence of a ~roup VIII metal further enhances the^plat:ing rate of the electroless copper plating solutions of this invention.

dm~ 3 35~03 ,. ..

d' ~0 0 0 0 ~ ~ O
,, ~ o o o o o o o o o ,, ~ ~ ~
~ *
d ' CO O O O N O
r~ ~ ~ O O o O O O O
O O ' ~1 $~ ~
o o o ~ ~ o ~ ~ ~ O O O O ~ O ~ ~ ~
O O

i $ * ~ ~
o o ~ o r~ ,1 ~ o ~ o o o o o O O
~1 ~1 U~
~ N ~;~
e:r OD O O ~ O ~ O O
O O ' ~ O ~ ~
o H

r ~ o 0~ 0 0 0 ~1 ~ O O O O O O O ~ O
O O

~1 ~ iC ~ ~

h ~p ~ ~ ~ F
~I~ a) Q)h ~ n) ~ U~ n~
i~) F. ~aO ~I Q rd ~ ~3 I oO ~ tJl:~;Q Ul O O ~ ~ o td r-lN ,~ `JN ~ O ~) ~ ~_1 z ~ 5 ~ ~ , ~ . F, ~1 O r~ -P F~
2 a) ~ I'g~rl P~ ~ rU _t N N ~_ ~ rl h Z., ~ O ~ ~ ~Q~
N. Z ~ 14 ~;
~ . .
f~ ' dm~ 31. -, . . . ~ . .. . . .. .. .

~L~L3S~
:I:n ~xamples 1-7, it wlll be seen tha-t operation in the presence of th- addi~ive(s) as tauc~h-t herein resul-ts in a marked increase on the plating rates of the electroless deposition solutions, compared wi.th the control bath. In addition, the addi-tive(s) containing solutions of ~xamples 1-7 produce an adherent, substantially non-stressed copper deposit, whereas the control bath wi-thout the additive(s~ produced a non-adherent copper deposit which tended to flake off the substratum.

This example illustrates the use of cytosine, a plating rate accelerating agent accordiny to this invention, to accelerate the rate of copper deposition in a hath havillg the following composition:
BATH B

Tetrasodium ethylenediamine tetraacetate dihydrate 138 ~/1 CuSOI,~5H20 14.7 ~/1 Formaldehyde (37% Soln.) 30 ml/l NaO~ to pH
Using th~ procedure for determining the plating ra-te described above, a stainless steel foil having the dimensions 3cm x 5cm is catalyzed Eor electroless metal deposition and electrolessly plated with copper at 25 C in ba-th B, to which 0.004 g/l (4 mg/l) of cytosine has been added.

,:
I

dm:~c ~35903 The ~fE~cl: of the presence of cytos:ine and the change in pH on the p]atillg ra-te of copper is shown in the table and FIG. 3. For pur~oscs of comparison, the eEfect of the changc ln pH on the copper plating rate in bath B without cytosine is also shown.
BATH B* BATM B + cytosine Plating rate, Plating rate microns/hr._ ~ microns/hr.
12.4 5.3** 12.4 9.3 12.75 4.5 12.75 10.4**

* control experiment ** peak plating rate 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 +
BATII B* 2-mercaptobenzothiazole Plating rate, Plating rate microns/hr. pll microns/hr 12.4 5.3 12.411.0**

13.1 3.5 13.17.3 * control experiment ** peak plating rate dm ! ~, 33 X~ .

.. ,. ,. . , - ,, .. . - .. ., - ...

3~3~3 EX~MPLI~ 10 The procedure of Example 8 is repea-ted, except that 2-mercaptopyrimidine, in the amount o~ 0.003 y/l, is used as the accelerating agent. The results are shown in ~IG. 4 and summarized as follows:

BATH B ~
BATH s* 2-mercapto~yrimidine Plating rate, ^ Plating rate, ~ microns/hr. ~ microns/hr.
12.~ 5 3** 12.4 5.3 13.0 3.5 13.0 8.8**

.
* control experimen-t ** peak plating rate EXAMPLE ll .
The procedure of Example 8 is repeated, except that guanidine hydrochloride, a non-aromatic nitrogen compound, is used as the plating rate accelerating ayent, in the amount of 0.005 g/l (5 mg/l). The results are shown in FIG. 5 and summarized in the ~ollowing table.

dm~ 34 -~1 , ' ' ~3S~

.

BATH B* BAT~I B -~ guanidine HCl , _ . . . .
Plating rate, Plating rate~
pH microns/hr. pH microns/hr.
_ _ _ 1~.4 5.3** 12.4 8.0 12.72 4.4 12.72 10.5**

. .
*control experimen~
**peak platlng rate With rëspect 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 non-additive con-taining control.

l'his example illustrates a particularly effec-tive com-position ~or practicing the invention and the results achieved therewit'h.
Copper sulfate 18 g/l Quadrol* 36 g/l "
Pluronic* P-85 wetting agent , 1 mg/l 2-mercaptobenzothiazole 1.5 mg/l

4.6H2O 0.61 g/l Rochelle salt 1 g/l Formaldehyde (376 soln.) , 12 ml/l NaOH 37 g/l 4-hydroxypyridine 40 mg/l pH 13.15 (measured at 25c) :
Temperature 70C

*Trademark p ~ - 35 - ' ~3~ 303 \
Ra-te 32 microns~hr.
Ductility 2 bends Bath Stability ~ery good.
It will be notec1 that in addition to having a fast rate, the bath of Example 12 produced copper of great ductility.
EXAMPI.E 13 This example further-illustrates the electrolessly fast plating rate achieveable by practice~ of the in~ention.
Copper sulfate 18 g/l Quadrol* 34 g/l Formaldehyde (37% soln.) 15 ml/l Pluronic* P-85 wetting 1 mg/l agent 2-mercaptobenzothiazole 1.5mg/1 pH 13.2 4-hydroxypyridine 40 mg/l Polyox* coagulant, Union Carbide Corp. 1 mg/l Rate 72 microns/hr.
Tempera-ture 70C

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. ~-*Trademark ,:,-.

Pg/L~ - 36 -' ~3S~O~
.
B~T1-1 C

N,~l,N',N'-tetrakis (2-hydroxy- 65.4 g/l (,22 mole/1.) propyl)ethylenediamine CuSO,,.51120 50 g/l (.20 mole/l) Formaldehyde (37% soln.) 20 ml/l (.27 mole/l) Wetting agent (PLURONIC* P-85 0.00]. g/l BASF-Wyandotte Co.) Sodium hydroxide 3.9 g/l (9.l mole/l) pH 13.2 Temperature 25 C
In Example 14, the gravimetric test for platiny rate was done using a copper rather than a stainless steel plate, For comparison, dilute Bath A of Example l was run using the same type of copper plates as the deposition substratum. The results are tabulated below.
' BathCytosine (mg/l) Plating rate (microns/hr.

A 0 3.6 C 0 4.0 C 5 7.9 .
C l0 9.8 C 15 l0.5 C 20 ~.l.3 C 40 9.l Given the concentration 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 platin~ solutions.

Heretofore the practice in the art has been to use dilute *T d k ra e mar dm ~b _ 37 ~
b , ~ lutions, i.e., solutions conta:i.nlng less than 0.1 mole/l of copper salt, ancl yenerally about 0.06 mole/l. By practice of the teachings herein, electroless copper solutions o 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 pre-sent, increasing the copper concentration in the bath (18 g/l of CuSOI,.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 emhodiments, special mention is made of electroless copper deposition processes according to this invention wherein the accelerating agent consists of 2-mercaptobenzothiazole in combination with imidazole or 4- .:
hydroxypyridine, which leads to brighter deposits of copper in :~ comparison with no accelerating agent or 2-mercaptobenzothi.azole : alone; and processes wherein the accelerating agent consists of pyridine in combination with 2-mercaptobenzothiazole, which leads to enhancements in stability in comparison with pyridine aIone, : as well as brighter déposits of copper in comparison with ~ -2-mercaptobenzothiazole alone.
Especially preferably, the plating rate accelerating ;agent is selected ~from among 2-mercaptobenzothiazole, ~

`: :: :
hydroxypyridine, 2-mercaptopyridine, aminopyrazine, pyrido (2l3,b) : pYraZine~cytosine, guanldine hydrochloride, pyrldine, 2-hydroxy- .
pyridine, para-nitrobenzylamine hydrochloride, imidazole and ; :
: ~ : -:mixtures thereof.

~ ecause of the fast rate of copper deposition from the :

-: ; 30 ~ solutions~made:in accordance with this lnvention,~ frequent : dm~ 38 - ~ .

~3S~0~3 pl^nishment may be necessary i.~ diJ.u-te .solutlons are used.
Surprisingly, it is possLble to practice this inventlon using hiqhly concentrated plating so]utions. See, e.y., Example 14.
Heretofore, the practice in the art has been to use dilute solutions.
In general, there may be used as the depo]arizing a~ent any a~ent which, when added to the solution, produces at least a 20 per cent and preferably at least 30 per cent 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-~lass lamina*e is drilled to provide multiple through holes. The surface and the holes are cleaned with an alkaline cleaning solution, e.g., ALTREX , * BASF-Wyandotte Corp., at a concentration of 45 grams per liter and a temperature of 50C, and thereafter rinsed with water. The copper : ~
clad surface is then cleaned with a lO per cent aqueous solution of sodium persulfate and the surface is~rinsed with water. The ~20 laminate is sequentially~contacted with 10 per cent sulfuric acid, rinsed with water and contacted with 30 per cent hydrochloric acid.
;~ After the pre-treatment, the non-copper clad hole ~ ~ barrels are catalyzed ~or electroles;s copper deposition in the :
standard manner using a palladium/t~ln salt catalyst, rinsed briefly with water, treated with 5 per cent fluoroboric acid solution to remove excess tin salt,~ and a~ain rinsed with water. ~ `
~::
The~epoxy-glass lamlnate is now ready for treatment by a process according to this invention.
* Trade mark .

dm~ 39 -:

13~03 I'he catalyzed epoxy~cJJass larn:l.nate is immcrse-d in an electroless copper deposition bath ~any o~ the above-descri.bed~' to depo.si.t 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 maskiny material, e.g., RISTON* 310, a dry film photoresist sold by E.I. DuPont DeNemours Co., Inc., copper is built up on the unmasked areas by conventional electro-plating, and followed by electroplating tin-lead alloy (an etc~
resist). The masking is stripped off using a mild alkali, e.~., 4-15 percent solution of NaOH, and the background copper in the previously masked areas is etched away, e.g., using ammoniacal CuCl2. The product is an epoxy-~lass laminate having a pattern of copper conductor lines on the surEace, and copper inter-connections in the through-holes, all coated with tin-lead.
It will be clear from the examples that the complexing ~ -agent preferred for use herein is N,N,N'-N'tetrakis (2-hydroxy-propyl)ethylenediamine (i.e., ~uadrol*). Good results are also obtained using ethylenediamine tetraacetic acid and its salts.
~20 The least preferred complexing agent are tartrate salts, e.g., -RocheIle salts.
Other modificati;ons and variations of the present invention are posslble in the light of the above disclosure. It is therefore to be understood that changes may be made in the particular embodiments described which are within the full intended scope of the invention as defined by~the appended claims.
* Trade mark , ; dm~ ' 40 L3S9~)3 ~ he inventi.on in :I.ts broader asl?ects is not limited to the speci.fi.c steps, processes and composi.tions shown and described hut departures may be made therefrom w:lthin the scope of the accompariying claims without departing ~rom the principles of the invention and without sacrificing its chief advantages.

.:

~ : dm~ 41 - ~

:

. :

Claims (22)

1. THE EMBODIMENTS OF THE INVENTION IN WHICH AN EXCLUSIVE
   PROPERTY OR PRIVILEGE IS CLAIMED ARE DEFINED AS FOLLOWS:
   l. In a method for electrolessly depositing copper from 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 pH above 10, the improvement for de-positing at a rate greater than about 7 micrometers of electro-less copper per hour in a bath composition operated at a temper-ature of about 25°C to about 35°C to a rate greater than 19 micro-meters of electroless copper per hour in a bath composition oper-ated at a temperature above 35°C, a coherent, structurally stable thin film of electroless copper adherent to a substratum, com-prising:
   (A) including within the electroless copper deposition solution an accelerating 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;
   (B) contacting the electroless copper deposition solution with a substratum sensitive to the deposition of electroless copper;
   and (C) while operating the electroless copper deposition solution at a pH above 10, regulating the pH thereabove and the amount of said accelerating agent therein to maintain a deposition within said rate, to thereby achieve a coherent structurally stable thin film of electroless copper adhered to the surface of said substratum.
2. 2. The method of claim 1 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, para-nitrobenzylamine hydrochloride, imidazole and mixtures thereof.
3. 3. The method of claim 1 wherein the accelerating agent is present in an amount of at least about 0.0001 gram per liter of the electroless metal deposition solution.
4. 4. The method of claim 2 wherein the accelerating agent is present in an amount of from about 0.0001 to about 2.5 grams per liter.
5. 5. The method of claim 1 wherein the accelerating agent has a free electron pair on a nitrogen atom adjacent to a pi-bond.
6. 6. The method of claim 1 wherein the electroless metal deposition solution includes an ion of at least one metal select-ed from Group VIII of the Periodic Table of the Elements.
7. 7. The method of claim 6 wherein said copper ion is supplied as a salt and said metal ion is present in an amount of from about 0.005 to about 30% by weight, based on the weight of the copper salt.
8. 8. The method of claim 1 wherein the reducing agent is selected from among formaldehyde and precursors or derivatives thereof, boranes, borohydrides, hydroxylamines, hydrazines and hypophosphite.
9. 9. The method of claim 1 wherein the pH adjustor is an alkali metal hydroxide or alkaline earth metal hydroxide.
10. 10. The method for depositing a coherent, structurally stable thin film of copper from an electroless copper deposition solution having a pH greater than 10 at a rate of between about 9 micrometers and 25 micrometers of electroless copper per hour in a bath composition operating at about 25°C to about 35°C to a rate greater than 19 micrometers of electroless copper per hour in a bath composition operated at a temperature above 35°C, which comprises including within the deposition 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 while operating the solution at a pH above 10, regulating the pH thereabove and the amount of said agent so as to maintain the deposition at said rate.
11. 11. The electroless deposition method of claim 10 wherein the agent causes at least a 20% and up to 100% depolarization of the anodic partial reaction of the solution.
12. 12. The electroless deposition method of claim 11 wherein the agent causes at least a 20% and up to 100% depolarization of the cathodic partial reaction of the solution.
13. 13. The electroless deposition method of claim 11 wherein the agent causes at least a 20% and up to 100% depolarization of both the anodic and cathodic partial reactions of the solution.
14. 14. The method of claim 11 which further comprises including in the electroless copper deposition solution a nonionic block co-polymer of ethylene oxide and propylene oxide.
15. 15. The method of claim 10 which further comprises including in the electroless copper deposition solution a nonionic block copolymer of ethylene oxide and propylene oxide.
16. 16. The method of claim 1 in which the electroless copper deposition solution is capable of electrolessly depositing copper at a rate of not less than 7 and up to at least 30 microns of electroless copper per hour for a period of at least 15 minutes when measured at room temperature.
17. 17. The method of claim 10 in which the electroless copper deposition solution is capable of electrolessly depositing copper at a rate of not less than 9 and up to at least 25 microns of electroless copper per hour for a period of at least 15 minutes.
18. 18. The method of claim 1, in which the deposition solution is operated at a temperature between 20 and 70°C.
19. 19. The method of claim 1, in which the deposition solution is operated at a temperature of about 25°C.
20. 20. The method of claim 10, in which the deposition solution is operated at a temperature of about 25°C.
21. 21. The method of claim 6, in which the Group VIII metal is cobalt or nickel or both.
22. 22. The method of claim 10, in which the depolarizing agent 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.