CA1331420C - Method of consistently producing a copper deposit on a substrate by electroless deposition which deposit is essentially free of fissures - Google Patents

Abstract

ABSTRACT OF THE DISCLOSURE

Electroless metal plating solutions are formulated and controlled to provide high quality metal deposits by establishing the intrinsic cathodic reaction rate of the solution less than 110% of the intrinsic anodic reaction rate. Electroless copper plating solutions con-taining plating rate accelerators containing delocalized pi bonds and having the concentration of the reducing ager no greater than 1.2 times the concentration of t copper ion can deposit copper on printed wiring ?ards of quality sufficient to pass a thermal stress of 10 seconds contact with molten solder at 288 °C
without cracking the copper deposits on the surface of the printed wiring boards or in the holes. As the plating solution ages by build up of plating reaction by-products or by-products and contamination, the quality of the copper deposited can be maintained by increasing the copper ion concentration and pH of the solution, while reducing or maintaining substantially constant the concentration of the reducing agent.

Description

-` 133~420 Method of Consistently Producing a Copper Deposit on a Substrate by Electroless Deposition Which Deposit is Essentially Free of Fissures BACKGROUND OF THE INVENTION
Electroless copper deposition solutions comprise copper ions and a reducing agent for the copper ions. The reducing agent oxidizes on a catalytic surface, and provides electrons to the surface.
The general equations written for a system with copper ions and alkaline formaldehyde are:
2HCHO + 40H- = 2HCOO- + 2H2O + H2 + 2e-, and CuLn+2 + 2e- = cu + Ln where e- designates an electron, L designates the ligand necessary to prevent precipitation of basic copper compounds in alkaline solution and n refers to the valence of the ligand ion.
Copper deposits on substrates produced by electroless deposition or electroless deposition reinforced by electroplating are an important part of many processes used for the manufacture of printed circuits. Additive or fully additive printed wiring boards are made with a process which uses 100%
electrolessly formed copper.
A specification, Mil Spec. P-55110-D, describes tests which measure the performance of printed circuits when subjected to conditions and environments the printed circuits will be exposed to during manufacture and use. In order to provide reliable printed circuits, the criteria for printed circuits in military and some commercial applications are based on the ability to meet the requirements of this specification.
Heretofore, electroless copper deposits on FR-~
epoxy glass material using the fully-additive method of making printed circuits have not been able to pass the `
35 Mil. Spec. P~55110-D thermal stress test. When exposed - ~
ycc/in ~-2,~

to this test, plated-through holes would fracture during a 10-second exposure to molten solder, the fracture usually occurring at the intersection of the hole wall with the surface, the corners of the holes.
The build-up of by-products and trace contaminants in electroless copper deposition solutions has had major detrimental effects on the quality of copper deposits. In addition to the normal by-products formed during operation, chemical contamination can enter the plating solution through chemical additions, water supplies, air or from the work placed in the electroless copper bath. Many of the inorganic contaminants, such as iron, cuprous ions, silver, gold, antimony, arsenic and many other metals and their compounds, as well as many organic contaminants, can cause deleterious results for both bath operation and the quality of the copper deposits, even when only present in parts per million concentration. ~-SUMMARY OF THE INVE~TION
Definitions:
By the term anodic reaction rate is meant the rate of oxidation of the reducing agents on a metal surface in an electroless metal deposition solution.
By cathodic reaction rate is meant the rate of 25 reduction of metallic ions to metal on a metallic ~ ;
surface in an electroless deposition solution.
By the intrinsic anodic reaction rate, r~', is meant the anodic reaction rate as measured on a metallic surface in an electroless plating solution by imposing a potential slightly more positive than the mixed potential on the metallic surface.
By the intrinsic cathodic reaction rate, rc', is meant the cathodic rate as measured on metallic surface in an electroless plating solution by imposing a ycc/in 3 1 33~42~
potential slightly more negative than the mixed potential on the metallic surface.
By the mixed potential, Emp~ is meant the potential difference between a reference electrode and a metallic surface on which both the anodic and the cathodic reactions are proceeding, and metal is being electrolessly deposited. Unless otherwise stated, the reference electrode is a saturated calomel electrode, SCE.
By the term thermal stress test is meant a test of printed circuit specimens containing plated through -holes wherein the specimens are conditioned at 120C to 150C for a period of 2 hours minimum to remove moisture; after conditioning, the specimens are placed in a dessicator on a ceramic plate to cool; the specimens are then fluxed (type RMA of MIL F-14256) and floated in a solder bath (Sn 63 + 5%), and maintained at 288 + 5C for a period of 10 seconds; after stressing, the specimens are placed on a piece of insulator to .:, :
cool; then the specimens are microsectioned in a vertical plane at the center of the hole and examined for cracks at 50 to 100 magnifications. A minimum of one microsection containing at least three holes is made for each sample tested. Any cracks forming in the copper deposit on the specimens will indicate thermal stress failure.
By an electroless plating reaction being under cathodic control is meant that the cathodic reaction : , controls the overall plating rate, i.e., the plating ;-~
rate depends on the concentration of the cathodic reactants, the concentration of the metal ions, or the concentration of depolarizers for the half reaction involving the metal ions.
By an electroless plating reaction being under anodic control is meant that the anodic reaction controls the overall plating rate, i.e., the plating ycc/in ;~

4 ~33:~420 rate depends on the concentration of the anodic reactants, the concentration of the reducing agents for the metal ions, or depolarizers for the half reaction involving the reducing agents.
By the term high quality copper is meant copper that has small crystals with a grain size less than 10 micrometers and low frequency of crystal dislocations, defects and twinning. High quality copper on printed circuit boards will pass the thermal stress test.
When referring to electrolessly deposited copper, by the term satisfactory copper quality is meant ~ -also high quality copper. ~ -By fissure free copper deposits is meant electroless copper deposits free from internal cracks or fissures or internal defects capable of causing cracks or fissures when the copper deposit is th~rmally ;
stressed. Fissure resistant copper means copper deposits that will not form fissures or cracks when ~-exposed to thermal stress, thermal cycling or in use.

20 Objects of the Invention: -It is an object of this invention to provide -copper metal deposits with good physical properties from electroless plating solutions. -It is also an object of this invention to provide electrolessly deposited copper for printed circuit boards which is resistant to crack formation under thermal stress testing at 288C.
It is an object of this invention to provide highly reliable printed wiring boards.
It is a further object of this invention to provide a method of operating and maintaining an ;~
electroless copper plating solution which ensures the deposition of copper having good physical properties and being free of fissures.

ycc/in . ~, ~ ,. .

33~20 4a It is an object of this invention to provide a method of formulating electroless copper plating solutions that are capable ~:

~., .,:

~ .-. ::-.

' ;~'' , .

'..,~:: , :' ` 1 '~"'';~ ~, .. ....

::
,: ~ ,. ..
.~
, ~", ".
ycc/in ':~ .' '-.: ~. : ;;; `,. ,~ ,.
'' '~'~"', ; 133~2~
5of depositing copper free of fissures and resistant to cracking under thermal stress.
It is an object of the present invention to provide a method of formulating and operating an electroless plating solution for forming copper deposits being substantially free of fissures, the solution comprising copper ions, a complexing ligand for copper ions, a pH adjustor, a reducing agent, and at least one of a stabilizer and ductility promoter and having a desired 10 initial ratio of the intrinsic anodic reaction rate to the :-intrinsic cathodic reaction rate, characterized in that the influence of build-up of by-products being formed during bath operation is compensated for by raising the copper ion concentration and the pH value or decreasing the concentration of the reducing agent or by raising the copper ion concentration and the pH value and decreasing the concentration of the reducing agent, and thereby maintaining the ratio of intrinsic anodic reaction rate to intrinsic cathodic reaction rate at less than about 1:1 and the original deposition rate as well as depositing substantially fissure free copper.
Brief Description of the Invention:
This invention is based upon the discovery that, ~
in order to produce satisfactory copper the constituents .
comprising the electroless copper deposition solution are ~ -~
present in the solution in concentrations and under operating conditions such that, at the operating ycc/jj !~jJ ~ , .

5a 133142~) :
temperature of the solution, the intrinsic anodic reaction rate is not greater than the intrinsic cathodic reaction rate.
In one aspect, this invention comprises a method of monitoring and controlling electroless plating solutions to obtain electrolessly formed me1:al deposits of high quality, characterized in that the ratio of the intrinsic reaction rates is maintained less than 1.1 during copper deposition. In another embodiment, the invention comprises a method of monitoring the ratio of the intrinsic anodic and cathodic reaction rates of the electroless deposition solution, and adjusting the solution composition and/or operating conditions to maintain the intrinsic anodic reaction rate less than 110% of the intrinsic cathodic reaction rate.
In yet another embodiment, the invention ~
comprises methods of selecting an electroless copper ~- `
plat;ng baths which will operate under anodic control. -~ ;
Alkaline electroless copper plating baths comprise copper 20 ions, ligands to solubilize the copper ions, a reducing ~ -agent capable of reducing the copper ions to metal, a pH
adjusting compound, and additives such as stabilizers, accelerators, ductility promoters and surfactants.~`; f . ,~; . i Solutions under anodic control can be achieved by 25 maintaining the ratio of the mole concentration of the ~'~
reducing agent t:o the mole concentration of copper ions ;
,,.-:: ., ;
ycc/~c : .. ~
",~
,:, ~", i~ ".,''~,'', ,':

5b 13 3 ~ 4 2 0 less than about 1.2. These embodiments include methods of maintaining constant plating rates, and methods for increasing plating rates.
Broadly, the present invention provides a method of formulating and operating an electroless plating solution for forming copper deposits which are substantially free of fissures. This solution comprises copper ions, a complexing ligand for copper ions, a pH
adjustor, a reducing agent, and at least one of a stabilizer and ductility promoter and has a desired initial ratio of the intrinsic anodic reaction rate to intrinsic cathodic reaction rate. The method is characterized in that the influence of build-up of by-products that are formed during bath operation is compensated for by raising the cooper ion concentration and the pH value or decreasing the concentration of the reducing agent, or by raising the copper ion concentration and the pH value and decreasing ~
the concentration of the reducing agent, so as to maintain -the ratio of intrinsic anodic reaction rate to intrinsic `
cathodic reaction rate and the original deposition rate as well as to deposit substantially fissure free copper.
The present invention also provides an ~-electroless copper plating solution which comprises copper ions, a complexing ligand for copper, a pH adjustor, a reducing agent, a stabilizer or ductility promoter and an accelerator which contains a delocalized pi bond. The : .
yc¢/jo ` 5c ~.33~20 ~ ~
accelerator is selected from (i) heterocyclic aromatic nitrogen and sulfur compounds, (ii) nonaromatic nitrogen ~:
compounds having at least one delocalized pi bond, and (iii) aromatic amines, and mixtures of the foregoing. The plating solution is characterized in that the mole concentration of the reducing agent is no greater than the mole concentration of the copper ions.

~, , ... ...

',' ;; ~' " '~
,,,.-'.'.',.,' ' ';
,: .~ ~", ,, '( ~;., .~ :
.:, . :
.,'' ,-, '' '-. :, ycctjc ~';~':

: ' ',: ' -6 1331~20 BRIEF DESCRIPTION OF THE DRAWINGS
Fig. 1 is a schematic diagram of apparatus suitable for carrying out electrochemical measurements according to this invention.
Fig. 2 is the plot of the potential applied in making the measurements vs. time as described in Example 1.
Fig. 3 is the plot of the current produced vs.
the potential applied as described in Example 1.

DETAILED DESCRIPTION OF THE INVENTION
While the invention will be described in thecontext of alkaline electroless plating bath solutions, its scope is not limited to such solutions.
There are many ele~troless copper plating solution formulations which will initially deposit high quality copper. However, experience has shown that as the plating baths age, the quality of the deposit ~ -deteriorates, and the bath must be replaced in whole or in part. The age of an electroless plating solution is determined by build-up of plating reaction by-products and by build-up of contaminants. The build-up of by-products can be measured by the change in solution ~ ;~
density or specific gravity. Under fixed operating conditions, the contaminants also will build-up in -proportion to the change in solution density~ The teaching of this invention allows the extension of the useful life of such solutions by controlling the mole ratio of formaldehyde to copper, and increasing the copper concentrations and the pH of such solutions to 30 obtain adequate plating rates at the desired mole ratio. ~ ; -Aqueous electroless copper plating solutions for use in the processes of this invention contain copper compounds which serve as the source of copper ions to form the copper metal deposits; reducing agents which are themselves oxidized and provide the electrons ycc/in .- : .,.

`~~` 7 1331420 necessary to reduce the copper ions to copper metal deposits; pH adjusting compounds which provide a pH
suitable for reduction of the copper ions by the reducing agents; complexing agents to solubilize the copper ions at the pH of the solutions; and additives to stabilize the solution, brighten the deposits, reduce surface tension, inhibit hydrogen inclusion in and improve ductility of the copper metal deposits. -Among copper compounds that are suitable as sources of copper ions are copper sulfates, copper nitrates, copper halides, copper acetates, copper -phosphates, copper oxides, copper hydroxides, basic copper sulfates, halides and carbonates and soluble copper complexes. Copper(II) compounds are preferred, -15 and copper(II) sulfate and copper(II) chloride are~; ~
commonly used. Another source of copper ions is -metallic copper which may be electrochemically dissolved ~
into the electroless plating solution, or -electrochemically dissolved into an electrolyte and 20 diffused through a membrane into the electroless plating ;~;~
solution.
The lower limit for the concentration of the copper compound in the electroless plating solution should be high enough to maintain the intrinsic cathodic ~5 reaction rate greater than 90~ of the intrinsic anodic reaction rate. The upper limit is the concentration where copper metal precipitates homogeneously throughout the solution instead of only forming copper deposits on preselected catalytic surfaces. The upper limit also depends on the stabilizer additive used to control homogeneous precipitation. For most electroless copper plating bath formulations, the concentration will be above 0.01 molar and below 0.2 molar, and increases as the bath ages by build-up of plating by-products and/or contamination.

ycc/in ;

~ ~ .

1331~20 In one embodiment of the invention, the copper concentration and the pH of the electroless plating solution are increased as the by-products and contaminants build-up in the solution. In this embodiment, in order to obtain fissure free copper deposits when contaminants and/or by-products build-up in the solution the copper concentration is increased 20 to 200% preferably 40 to 100% while the pH also is increased.
Among the reducing agents that are suitable for the reduction of copper ions are formaldehyde reducing agents. Formaldehyde reducing agents include compounds such as formaldehyde, formaldehyde bisulfite, paraformaldehyde, dimethylhydantoin, and trioxane.
Other suitable reducing agents are boron hydrides such as boranes and borohydrides such as alkali metal borohydrideR.
The upper limit for the reducing agent in the electroless plating bath is the concentration at which the intrinsic anodic reaction rate is 110% the intrinsic cathodic reaction rate. The lower limit is the concentration at which copper plating on a clean copper surface doesn't occur, i.e., the plating solution is passive. Preferably, the lower limit is the concentration at which the intrinsic anodic reaction rate is 75% to 85% of the intrinsic cathodic reaction rate. For formaldehyde reducing agents, the limits depend on additives, pH and very strongly on the temperature. In solutions where the intrinsic anodic ~0 and cathodic reaction rates have not been determined, the concentration of formaldehyde will preferably be set above 0.01 molar and below 1.2 times the molar concentration of copper ions and more preferably maintained at or below the molar concentration of the copper ions.

ycc/in ~ .
. ;:

9 ~33~420 Suitable pH adjusting compounds include the alkali metal hydroxides and copper oxide. In the operation of an alkaline, electroless copper plating solution, the pH usually drops during plating, and hydroxides are added to raise or maintain pH. If the pH
needs to be lowered, an acidic compound would be used as a pH adjusting ion. When the reclucing agent is a formaldehyde reducing agent, the activity of the reducing agent depends on the pH as well as the concentration of the reducing agent. Therefore to increase the activity of the reducing agent and thus increase the intrinsic anodic reaction rate, as described herein below, either the concentration of the formaldehyde reducing agent or the concentration of the hydroxide compound ~i.e., pH) may be increased. In operating an electroless copper solution when the intrinsic anodic reaction is to be increased, preferably ~;-pH is increased and formaldehyde concentration is held substantially constant or even decreased.
In one embodiment of this invention as the solution ages, the intrinsic cathodic reaction rate is -~
increased by raising the copper concentration by 40 to 100% and the anodic reaction rate is increased less than the cathodic reaction rate by raising the pH 0.1 to 1 pH ~ ~
25 unit, more preferably by 0.2 to 0.6 pH unit. ~ ;
For formulations with formaldehyde type ;
reducing agents, the pH (measured at room temperature) ;~ ~;
is usually set between 9.5 and 14. When the ratio of the mole concentration of the reducing agent to the mole ~ ~
30 concentration of the metal ion is less than about 1.2 ~! ;,'~., ;~`
the pH is preferably greater than 11.9, more preferably greater than 12.2.
Suitable complexing agents for electroless `
copper plating solutions are well known to those skilled in the art. Among the complexing agents useful for electroless copper plating solutions are ~

ycc/ln - -- ::
~::, ., ~

- ~33~2~

ethylenedinitrilotetra-acetic acid (EDTA), hydroxyethyl-ethylenediaminetriacetic acid (HEDTA), diethylenetrinitrilo-pentaacetic acid (DTPA), nitrilotriacetic acid (NTA), triethanol-amine, tetrakis(2-hydroxypropyl)ethylenediamine (THPED), penta-hydroxypropyldiethylenetriamine, and tartaric acid and its salts (Rochelle salts). Copper deposits without fissures, and plated through hole printed circuits capable of withstanding a thermal stress of 288OC for 10 seconds may be plated from solutions comprising these complexing agents or mixtures thereof by the methods and procedures of this invention.
Many additives have been proposed for use in electroless copper plating solutions. The additives which have been proposed may be classified by function into different groups. Most additives have more than a single effect on the electroless copper plating solutions, so classification of additives into groups may be somewhat arbitrary. There is some overlap between the additive groups, and almost all the additives affect the rate of the oxidation of the reducing agent (the anodic reaction) or the reduction of the copper ion to metal (the cathodic reaction).
One group of additives are surfactants or wetting agents to control surface tension. Anionic, nonionic, amphoteric or cationic surfactants may be used. The choice of surfactants may vary depending on `~
the operating temperature and the ionic strength of the electroless plating solution employed. Preferably, the surfactant is used at solution temperatures and ionic strengths below its cloud point. Surfactants containing polyethyoxy groups or fluorinated surfactants are preferred. Among the preferred surfactants are alkylphenoxypolyethoxy phosphates, polyethoxypolypropoxy block copolymers, anionic perfluoroalkyl sulfonates and ycc/in ~ ~
.:
~ " .. .

~331~20 carboxylates, nonionic fluorinated alkyl alkoxylates and cationic fluorinated quatenary ammonium compounds.
A second group of additives are stabilizers which prevent the spontaneous decomposition of the plating solution and/or the formation of undesired copper deposits outside of, or extraneous to, the desired deposit, so called "extraneous copper". Among the additives that have found use as stabilizers and to inhibit extraneous copper are oxygen (e.g., oxygen added to the plating solution by stirring or air agitation of the solution), divalent sulfur compounds (e.g., thiols, mercaptans, and thioethers), selenium compounds (e.g., selenocyanates), covalent mercury compounds (e.g., `~
mercuric chloride and phenylmercury), and copper (I) 15 complexing agents (e.g., cyanides, 2,2'-dipyridyl and -1,10-phenanthrolines). , A third group of additives may be classified as ductility promoters and/or additives to retard hydrogen inclusion in the deposit. This group would include polyalkylene ethers, cyanides, nitriles, compounds of vanadium, arsenic, antimony and bismuth, nickel salts, 2,2'-dipyridyl, 1,10-phenanthrolines and some organic ;
silicones.
The ductility promoters also act as stabilizers and are used alone or in combination with other stabilizers. Suitable concentrations for various stabilizers and ductility promoters have been described by Zeblisky et al., U.S. Patent 3,095,309; Schneble et ~ -al., U.S. Patent Nos. 3,257,213, 3,310,430 and -i 3,361,580; Zeblisky et al., U.S. Patent No. 3,485,643;
Schneble, U.S. Patent No. 3,607,317; Underkofler et al., ~
U.S. Patent No. 3,844,799; Heymann et al., U.S. Patent -No. 3,454,416; Clauss, U.S. Patent No. 3,492,135; Gulla et al., U.S. Patent No. 3,663,242; Shipley et al., U.S. -Patent No. 3,615,732; Jonker et al., U.S. Patent No.
3,804,638; Molenaar et al., U.S. Patent No. 3,843,373;

ycclin 12 1331~2~ ~
Morishita et al., U.S. Patent No. 4,099,974; Nakaso et al., U.S. Patent No. 4,548,644; and Nakaso et al., U.S.
Patent No. 4,557,762.
The amount of stabilizPr and/or ductility promoter in the electroless copper plating solution depends on the stabilizers or ductility promoters selected and on the concentration of copper ions, reducing agent and pH. The concentrations range between 0.01 and 100 mg/l. Jackson, U.S. Patent No. 3,436,233 describes some stabilizers that are used in even larger quantities up to 2 g/l. In general, stabilizers and/or ductility promoters should be present in the electroless plating solution in an amount sufficient to prevent extraneous plating, i.e., plating on masks or resists, and substantially less than the amount that would cause passivation of metal surfaces being plated or that would -stop the plating reaction.
A fourth class of additives is the group of plating rate accelerators (also known as depolarizers) as taught by McCormack et al., U.S. Patent No.
4,301,196. These accelerating agents are compounds containing delocalized pi bonds such as heterocyclic ;
aromatic nitrogen and sulfur compounds, aromatic amines and non-aromatic nitrogen compounds having at least one delocalized pi bond. Among such compounds are purines, pyrimidines, pyridines, thiazines, triazines and thiol derivatives.
Preferably, the depolarizing or accelerating ~-~
agent will be present in a small effective amount, i.e., ~-30 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.

ycc/in .'.. ~ .... :

-: ~331~20 :
13 ~
, Electrolessly deposited copper in plated through holes must be thick enough to pass thermal stress and thermal cycling tests. The minimum thickness is about 10 micrometers, and preferably is at least 15%
of the height of the plated through hole. Preferably, the copper thickness is at least 17~ of the height of the plated through hole and more preferably, at least 20%. ~
Although electrolessly deposited copper has ! .~', been known for many years to be inferior to electrolytically deposited copper in resistance to thermal stress, ductility and other physical properties, surprisingly it has been found that if electroless copper deposition solutions are formulated and controlled to have an intrinsic anodic reaction rate less than 110% of the intrinsic cathodic reaction rate, copper deposits with superior physical properties, including resistance to thermal stress, may be obtained.
The intrinsic rate ratio can b~ determined by measuring the reaction rates for the two half reactions in the neighborhood of the mixed potential, e.g., at +lOmV for the one and at -lOmV for the other half reaction; or by sweeping the potential on the one and the other side of the mixed potential and measuring the -~
current.
In one method, the intrinsic anodic reaction rate at the mixed potential is estimated from the current required to vary the potential on a working electrode which is electrolessly depositing copper. The potential between the working electrode and a reference electrode is varied in a potential ramp between En1p and +30 mV from Emp by passing current between the working electrode and a counter electrode and simultaneously measuring the potential and the anodic current as the potential changes. Alternatively, if the counter electrode is at Emp and very much larger than the working -ycc/in . ~ '': ' 331~20 electrode, it can also serve as a reference electrode since the current passed between it and the working electrode would be too small to shift the counter electrode potential. The intrinsic anodic reaction rate at Emp may be determined from the slope of a current vs.
voltage plot as it approaches E~p.
Similarly, the intrinsic cathodic reaction rate may be determined from the slope of the current vs.
voltage plot between -3OmV from h~mp and En1p.
When the intrinsic cathodic deposition rate is maintained greater than the intrinsic anodic deposition rate, or when the ratio of the intrinsic anodic deposition rate to the intrinsic cathodic deposition rate, r'~/r'c, is less than 1.1, preferably is less than 1.05, and more preferably is less than 1.0, it had been found that copper with superior physical properties is deposited. In order to maintain the desired ratio, it -may be desirable to increase the rate of the intrinsic ~ ~
cathodic reaction, or increase the intrinsic cathodic ~-rate more than an increase in the rate of the intrinsic anodic reaction.
Among the methods for increasing the rate of the intrinsic cathodic reaction are (1) raising the concentration of the cathodic constituent, i.e., the metal ion concentration; (2) addition of a catalyst or depolari~er to accelerate the cathodic reaction; and (3) increasing the surface area available for the cathodic reaction (e.g., by reducing the contaminants or the i~ `~
stabilizer concentration and the surface area blocked by contaminants or stabilizer; this may be accomplished by diluting the solution with fresh solution or by carbon treatment of the solution to remove contaminants blocking the surface area available for the cathodic reaction). When the metal ion concentration becomes too 35 high, extraneous metal deposition in the bulk of the ;
solution or outside the desired metal pattern may be ycc/in ,_, ! , 15 ~331~20 :
observed. For many electroless copper plating solutions, this occurs at copper ion concentrations above the range of 0.08-0.12 moles per liter.
As electroless plating solutions build-up by-products and contamination, the Ratio usually will increase. The Ratio, r~/rC~ may k,e maintained less than 1 while increasing both the anodic and cathodic reaction rates, by increasing the rate of the intrinsic anodic reaction less than an increase in the rate of the lo cathodic reaction. The rate of the intrinsic anodic reaction may be increased by (1) decreasing the concentration of the reducing agent (i.e., lower formaldehyde) while increasing the pH; or (2) increasing the concentration of anodic depolarizers such as heterocyclic aromatic nitrogen or sulfur compounds. If the concentration of the formaldehyde is lowered too much, the Emp of the solution may rise by 50-200 mV and the solution becomes passive, i.e., there is no electroless deposition. Frequently, the solution will become active again at a higher temperature. It has been found that to increase the concentration of the anodic reactants, the product of the formaldehyde concentration and the square root of the hydroxide ion concentration, [CH20][0H-]~s, must be increased.
Although the formaldehyde concentration may be decreased, held constant, or even increased, the product [CH2O][OH-]s, is increased to maintain the intrinsic anodic reaction rate less than the cathodic rate as the cathodic rate is increased.
For plating solutions operating above room temperature, the square root of the hydroxide ion concentration [oH-]05 may be conveniently estimated using the room temperature (25C)pH of the solutions.
In the event, that bath contaminants cause 35 reduction of deposition rate and inadequate copper -quality because of temporary, localized passivation of ,; ~
ycc/in : .

16 1 3 3 1~12 ~
the plating surface, the condition must be compensated for by increasing the plating current produced by the anodic half-reaction, i.e., by increasing pH. Since this will increase intrinsic anodic reaction rate, the copper concentration must be increased to bring the Ratio of r9/rC to the original value before the solution became contaminated, or a value below 1.1 and adequate for the resulting plating rate.

Measurement of the intrinsic rate of the partial lo reactions We have determined the ratio of the intrinsic rate of the partial anodic and cathodic reactions from measurements of current-potential relationships in a narrow potential range (e.g., from -30 to +30 mV from the mixed potential, Emp)~ This relationship is used in two ways. Both methods give similar conclusions ~
regarding conditions for producing copper of preferred ~ -qualities.
In one method, the cathodic current, ic, at the potential which is 10 mV negative with respect to the Emp (i.e., the overpotential, Eta = -10 mV vs. El~p) is taken ~;~
as the rate of the cathodic partial reaction, (rC)l~lvl or simplified rc; the anodic current i~ at the potential which is 10 mV positive with respect to the mixed potential, Empr (i.e., the overpotential, Eta = +lOmV vs.
Emp) is taken as the rate of the anodic partial reaction, (r9~+l~Vl or simplified r9.
Alternatively, in a computerized method, the intrinsic rates of the partial reactions is determined 0 using the rate expression n n r' = ~ [ijEj]/ ~ [(Ej)2]
j=l j=l ,.,~ ', ~ ,~

ycc/in ' ' '",:'.'' - ~33142~

where r' is the partial rate, ij is the current density -at an overpotential, ~j(Eta), referenced to the mixed potential, Emp~ and Ej is calculated from the overpotential vs. Emp~ ~j (Eta), according to the equation Ej = lO(~j/ba~ - 10(~ c) where b~ and bc are the Tafel slopes. For an electrochemical reaction, a plot of the overpotential, ~, from the thermodynamic equilibrium potential vs.
logarithim of the current, log i, was found by Tafel to be of the form ~ = a - b(log i).
For many electroless solutions, the anodic reaction, CH20 + 20H- =HCoo- + H20 + ~H2 + e-the constant b~ has the value 940 mV/decade, and for the cathodic reaction, CuLn+2 + 2e- = Cu + Ln bc has the value 310 mV/decade.
The rate of the cathodic partial reaction, rc', is obtained, in this invention, by applying the above equation to a set of pairs of experimental values (ij~Ej) from the cathodic potential range which is, e.g., from -3 0 mV vs. Emp to E~p . The rate of the partial anodic reaction, ra', is obtained by applying the above equation to a set of pairs of experimental values obtained from the anodic potential range which is, e.g., from El"p to E=+3 0 mV vs. Emp~
The currents used to calculate intrinsic reaction rates are measured at potentials near Emp~ e.g., 10 ' ~ 50 mV from Emp~ which may introduce some errors in 3 0 the determination of the intrinsic reaction rates. The equations strictly apply only close to the mixed potential. If one examines both positive and negative overpotentials and currents for a particular solution, one will find near the mixed potential, the over-35 potential departs from the Tafel (semilogarithmic)ycc/in , ~

133~20 relationship. The current measurements for determination of the intrinsic anodic and cathodic reaction rates must be in the range where the semi-logarithmic relationship is non-]inear. This range is often within + 40 mV of the Emp/ hut can be larger or smaller depending on the electroless plating solution formulation. The admissable error depends on the set point of the ratio of the intrinsic anodic and cathodic reaction rates and thus on the formulation of the ~-electroless plating solution.

Procedure An experimental setup for carrying out electrochemical measurements of r~, r~', rc and r'c, according to this invention, is shown in Fig. 1. The setup shown in Fig. 1 is composed of an electrochemical cell (110), a potentiostat with function generator (120) and a recorder (130).
In a typical test, an all-glass, single compartment cell with three electrodes was used. The test electrode was a platinum wire, 3.8mm2 in area (length 2.0 mm, diameter 0.6mm), and the auxiliary electrode a platinum cylinder (about lOmm2 in area), both electroplated with copper. Plating was done in an acid copper solution (CuS04.5H20 - 188 g/l, H2S0~ - 74g/l) at 25 10 mA/cm2 for 1-5 min. A saturated calomel electrode (SCE) was used as a reference electrode.
The current-potential curves were obtained with an IBM Instruments Inc. EC/225 Voltammetric Analyzer~
(120 in Fig. 1) and recorded on an IBM Instruments Inc. ~ -`
30 7424 X-Y-T Recorder~` (130).
The test electrode, (111) in Fig. 1, an auxiliary electrode, (112), and a reference electrode (113) are connected to the potentiostat, (120). The potentiostat with function generator was used in a DC
operating mode, for linear sweep voltammetry (LSV). The ycc/in . ~

-` 19 ~331~20 sweep waveform as shown in Fig. 2 is a linear rampJ the current is continuously sampled; when the potential reached a final value it is left at this value for a short period of time and then reset to the initial value, or an automatic scan reversal to the initial value can be used.

Example 1 :
An electroless copper plating solution was :
prepared with a high copper concentration and a correspondingly high specific gravity. The ratio of the mole concentration of the formaldehyde reducing agent to the mole concentration of the copper was 0.67. The formulation was as follows:

Copper sulfate 0.12 moles/1 15 Ethylenedinitrilotetraacetic acid 0.20 moles/1 Formaldehyde 0.08 moles/1 pH (25C) 11.9 [CH2o][oH-]5 0.007 (m/1)~5 Cyanide (Orion electrode) 110 mV vs. SCE ~
20 Vanadium pentoxide 5 mg/1 ~ ~;
Specific gravity 1.124 Operating Temperature 75~C
rA 0.14 mA/cm2 ~:.
rc 0.16 mA/cm2 ;.~
25 Ratio (r~/rC) 0.88 . : .
r~' 1.13 mA/cm2 ;:~ :
rc. 1.96 mA/cm2 . ~.
Ratio' (r~'/r0') 0.58 Additive printed circuit boards were plated in 30 this solution and after plating, tested by the thermal `.
stress test at 288C for 10 seconds. There were no cracks formed in the copper by the thermal stress test ycc/in "
. . ~ .

~ ~33l~2a which confirmed the results from the ratio of the intrinsic anodic and cathodic reaction rates.

Example 2 A solution from a working, production, electroless copper, plating bath was operated to the formulation below as far as its formulated bath constituents are concerned. The formulation was known to be able to produce high quality copper. However the ratio of formaldehyde to copper was greater than 1.2 so 10 the solution would not consistently deposit high quality ~
copper as the by-products and contaminents build-up and , the ratio changed. Electrochemical analysis of the solution gave a Ratio of 1.1 and a Ratio' of 1.05, indicating borderline performance. The deviation of the electrochemical Ratio results from the good Ratio results indicate the presence of an unknown contaminant.
Fully additive printed wiring boards were prepared on adhesive coated, epoxy glass laminates in this electroless copper plating bath. Thermal stress testing 20 showed cracks in 20% of the copper hole walls. `~
'' ' ` ,~
'`

;' ''''','.''.' ycc/in ~ ~

~" ~ ~S

~33142~

The solution had the following formulation:
Copper Sulfate moles/1 0.028 EDTA mo:Les/1 0.076 Formaldehyde mo:Les/1 0.049 pH (at 25C) 11.6 [HC~o][oH-]05 (moles/1/)~5 0.0031 Sodium Cyanide mV vs. SCE -110 (Orion electrode) Vanadium Pentoxide grams/l 0.0012 Specific Gravity grams/ml 1.094 (at 25C) Temperature C 75 r~ mA/cm2 0.33 rc mA/cm2 0.30 Ratio 1.10 r~' mA/cm2 2.87 I
rc' mA/cm2 2.74 : :
Ratio' 1.05 Thermal Stress cracks 20%
:

ycc/in ..
, ~ :

.

22 ~331~2~
In order to deposit copper that would pass the thermal stress test, a similar solution was prepared with a pH of 11.9 and a ratio of formaldehyde to copper ::
of 0.84. The solution had the following formulation: :

Copper sulfate 0.056 moles/l ~ :~
EDTA 0.110 moles/1 Formaldehyde 0.047 moles/l :; ::~
pH ~at 25C) 11.9 [CH2o~[oH-]5 0.0042 (moles/l)~5 ~p Sodium Cyanide -100 mV vs SCE
(by Orion electrode) Vanadium Pentoxide 0.004 grams/l :
Specific Gravity 1.066 (at 25C) Temperature 75C
ra 0.33 mA/cm2 rc 0.40 mA/cm~
Ratio 0.83 r~' 1.69 mA/cm2 ~ ~ :
rcl 1.98 mA/cm2 Ratio' 0.85 : Thermal Stress no cracks : ;~
. ,, ~.
Because the solution was under anodic control and the anodic rate was only slightly increased, the increase in the copper ion concentration to twice the ~ ;
concentration did not cause a corresponding increase in the plating rate. The copper metal was deposited at ~
approximately the same rate, and it required 17 hours to .:~:
deposit copper 35 micrometers thick.

, ~,.
. . ~. ., ~: j ., , ycc/in -. ' ''~' , ,~'`':.' ' 23 13~1420 In order to accelerate the plating rate, since the concentration of the cathodic reactant had already been doubled, the concentration of the anodic reactants was increased by increasing the pH to 12.2. The changes in the formulation are shown below:
pH (at 25C) 12.2 [CH20][0H-]s 0.006 (moles/l) 1'5 Sodium Cyanide -110 mV vs SCE
Specific Gravity 1.070 (at 25C) rn 0.47 mA/cm2 rc 0.49 mA/cm2 Ratio 0.96 `~
r~' 5.02 mA/cm2 -~
rc~ 5.30 mA/cm2 Ratio' 0.95 Thermal Stress no cracks This solution deposited copper 35 micrometers thick in less than 8 hours. This example illustrates how the principles of this invention may he used to obtain copper with superior physical properties at fast plating rates.

Example 3 In this example, a test solution was deliberately contaminated to show how the teaching of this invention may be used to adjust the formulation, or reset the control parameters, to obtain fissure free copper deposits from a solution in which contaminants ~
have build-up over a period of time as the solution i9 ~, used.
An electroless copper test solution was prepared with a stabilizer system using both vanadium and cyanide additions agents. In the table below, this solution is marked A. The electrochemical analysis of the solution gave a ratio of the intrinsic anodic ycc/in . . ~ :

--- 133l ~2~
2~
reaction rate to the intrinsic cathodic reaction rate, Ratio' = r'~/r'c, o~ less than 1.1 indicating the solution would deposit fissure free copper.
As a deliberate contaminant, 1 mg/1 of 2-mercaptobenzothiazole (2-MBT), was added to the test solution. The addition of the contaminant turned the solution passive, i.e., stopped the electroless plating reaction, and the mixed potential of the copper electrode in the test solution was shiEted outside the electroless plating range.
The conventional practice in the prior art was to increase the formaldehyde and the pH in order to regain a mixed potential sufficient for electroless copper plating. Following this conventional procedure, formaldehyde was added to the solution to triple the concentration and enough sodium hydroxide was added to increase the pH by one pH unit. In addition, copper was added to increase the cathodic reaction rate. The -modified formulation is listed in the table as solution 20 B. While these adjustments overcame the passivation and -~
increased the rate of deposition, the ratio of formaldehyde to copper was 2.4, which is greater than 1.2. As expected the electrochemical analysis of the -;-;
intrinsic anodic and cathodic reaction rates gave a ~5 Ratio' greater than 1.1 indicating the copper deposits ;;i would be subject to fissures.
To lower the intrinsic anodic reaction rate relative to the intrinsic cathodic reaction rate the `~
solution was reformulated with the original formaldehyde 30 concentration and a formaldehyde to copper ratio of 0.7; , -~
this is solution C. The Ratio' was reduced to less than , 1.1, so the solution would deposit copper resistant to ' fissures.
To achieve a preferred Ratio' of the intrinsic anodic reaction rate to the intrinsic cathodic reaction rate, the concentration of the anodic reactant, -~

ycc/in formaldehyde, was further reduced. The formulation is listed as solution D. The Ratio' of the intrinsic anodic reaction rate to the intrinsic cathodic reaction for this solution is less than 1.0, and thus the solution can provide a high quality, fissure free copper deposits.

. ,'., ~.

~. ' ycc/in :

'' .:.

26 ~331~2~

~D ~ Ul ' ~':
,~, "., ,~, o o ~1 ou~ O oo . . .. . o ~ o ~ I
O O O ~ O ~ N ~ `J O
.

~o r~ I` .
In U~ ~t' O O O O ~ U~ , ~) O rl O In o ~ O O
. . . . O ~1 0 r~ Ul 0~ ' O O O ,~ O ~ ~ ~ I ~ ~ ~I d' ' :'~ ' ' ' ; '`

`. ': `

O ~ O ~ O U~ O ~ : ,--':
m o ~ O
. . . . o ~ o ~ ~ 0~
o o o ~ o ~ ~ r~ I ~ ~ ,1 ~ ::,~ :: : : :
~, .,., . ~
.:.:..
i', ,.`".',, I ~ o ~ O o ~ ~, ~ .....
~ O rl O ~ O ~ ~r ~ ~ I`
~, . O ,~ ~ o ~ i 0 ,~

'. . `'~ .; ' ~, ~, ~, O ,~ ~;
:. ,. .

U~ O a~

1~ o7 ~ O O
,~ U~ ~C ~ X
O ~ ~ ~ ~ U
u~ ~ ~ Z ~ 4 4 IS) o Ln .

This example shows that with mercaptobenzothiazole as an accelerator or depolarizing agent, increased copper concentration and pH with the same or decreased formaldehyde concentration leads to faster plating rates and high quality copper deposits.
The plating rate of solution A without the accelerator or depolarizing agent was 1.4 micrometers per hour, The plating rates of solutions C and l) with the accelerator or depolarizing agent were 4.0 and 3.3 micrometers per hour, respectively.

Example 4 The procedure of Example 3 was repeated using a plating tank for 70 liters of the solution. The plating tank was equipped with an electroless copper plating bath controller which continuously measured the solution parameters such as the copper and formaldehyde concentrations, the pH, the cyanide ion activity and the temperature. The plating bath controller automatically compared the measured parameters to the set points and made additions to the solution to maintain the solution within the preset operating limits.
The plating solution was operated to deposit approximately 6 turnovers. (A turnover is replacing the copper salt content of the solution once). This raised the specific gravity of the solution due to the formation of by-product sodium sulfate and sodium formate. The intrinsic anodic and cathodic reaction rates were measured by electrochemical analysis, and the -Ratio' of the intrinsic anodic reaction rate to the intrinsic cathodic reaction rate was less than 1.1 which indicates the copper deposit is resistant to fissures.
The solution was used to make additive printed circuits ;
by the electroless deposition of copper to form surface conductors and plated through holes. The printed ~ -~
circuits were thermally stressed by contact with molten ycc/in 1 331~2~
2~
solder at 288C for 10 seconds. After thermal stress, the plated through holes were microsectioned and i examined for cracks in the deposited copper. There was no evidence of cracks or fissures in the copper 5 conductors or plated through holes. The formulation `
tested is shown in the table below.
The operating solution, found to deposit fissure free copper, was then treated with 0.5 mg of 2-mercaptcbenzothiazole (2-MBT) as a deliberate contaminant to simulate the effect of contamination of the plating solution by organic compounds. Organic! ' contamination is a frequent problem in electroless copper plating, especially in solutions operated for ~ -five or more turnovers. Sources of contamination ; -include leaching from plastic substrates being electrolessly plated, from the plating resist or from fortuitous contamination.
After the addition of the contamination, the plating solution became substantially passive. The plating rate was about 0.03 micrometers of copper per hour and the solution would no longer deposit copper on the hole walls of the insulating base material to make `~
plated through holes. The Ratio' of the intrinsic anodic and cathodic reaction rates was greater than 1.1, -25 so even if copper would have deposited on the hole ~-walls, the formed deposit, and thus the plated through -~
holes, would fail the thermal stress test. This solution is more fully described below. ~;
Following the procedures of Example 3 in a ~
30 sample of the solution, the pH was raised to provide a ; ;;
more active platiny solution, and the copper concentration was increased to adjust the Ratio' of the intrinsic anodic and cathodic reaction rates to less than 1.1. The increase in the copper concentration reduced the ratio of formaldehyde to copper from 1.7 to 0.85. When the Ratio' of less than 1.1 was achieved ~ ~

ycct in ,,: . -' : ~` .

33~2~

with the sample solukion, the set points on the electroless plating bath controller for copper concentration and pH were reset. Additive printed circuit boards were plated in the contaminated electroless plating solution using the new set points.
The copper deposited on these printed circuit boards was tested by thermal stress with molten solder at 288C for ten seconds and was found free of cracks or fissures.
The formulation, set points and test data for this solution are also given below.

Original Solution Good with Solution Reset Controls CuS04 mol/1 0.028 0.040 EDTA mol/1 0.087 0.100 CH2O mol/1 0.047 0.047 pH 25C 11.75 12.40 [CH2o][oH-]o5 (m/1)~5 0.0035 0.007 Gafac RE-610 mg/l 40 40 NaCN (Orion elec-trode vs. SCE) mV -130 -130 v2os mg/1 1 1 ~ `~
Specific gravity g/cm3 1.066 1.066 25 Temperature C 75 75 Emp vs. SCE mV -764 -687 Plating Rate m/hr 1.7 2.9 r~a mA/cm2 1.44 2.57 mA/cm2 1.39 2.40 Ratio' 1.04 0.93 Thermal stress pass pass In this example, a passive, contaminated solution was restored to active plating, and then by adjustment of the formulation, according to the ycc/in 1331~2~

teachings of this invention, the intrinsic anodic and intrinsic cathodic reaction rates of the conkaminated solution were adjusted to deposit high quality copper.
The addition of 2-mercaptobenzothiazole, a heterocylic nitrogen and sulfur compound, and increasing the copper concentration and pH resulted in a 70% increase in the plating rate.

Example 5 In this example, fissure resistant copper was deposited from an electroless copper deposition solution operating at low temperature. A first electroless copper plating solution was formulated to operate at 300C. The formaldehyde concentration was higher than -similar solutions at 75C as is the common practice in 15 electroless copper solutions operating near room ;
temperature. The ratio of the formaldehyde ~ -concentration to the copper concentration was 2.4. The solution plated slowly, depositing 25 micrometers of copper in three days. This first solution composition is given in the table below. As reported in the table, the ratio of the intrinsic anodic reaction rate to the intrinsic cathodic reaction rate is greater than 1.1, -~ ~-and the additive printed circuit boards prepared in the solution failed the thermal stress test. ~

.; ~ ' ,:'' ~,' ' ycc/in , ;,''' ;~'~
. . ~ "::

-- 1 3 ~ 2 0 Following the teachings of this invention, the concentration of the formaldehyde reducing agent was reduced to lower the anodic reaction rate relative to the cathodic reaction rate. The ratio of the formaldehyde concentration to the copper concentration was reduced to 0~5. The resulting solution is the 2nd solution in the table below.

Solution -1st 2nd 0 CUSO4 mol/1 0.02B 0.028 EDTA mol/1 0.087 0.087 Formaldehyde mol/1 0~067 0.013 pH 25C 12.5 12.5 CcH2o]~oH-]~s (m/1)~5 0.012 0.002 15 NaCN mg/l 20 20 v2os mg/l 3 3 Temperature C 30 30 Emp vs. SCE mV -783 -750 ra' mA/cm2 0.341 0.323 rc~ mA/cm2 0.280 0.304 Ratio' 1.22 1.06 ~ ~-The second solution is used to plate additive printed circuit boards with copper 25 micrometers thick.
It is difficult to initiate electroless plating on catalytic adhesive and catalytic base materials at low temperatures and low formaldehyde concentration.
Therefore, before plating the additive circuit boards, the conductive~pattern including the plated through holes is covered with a thin layer of copper about 0.2 micrometers thick in an electroless strike solution which has a formaldehyde concentration of 0.13 -~
moles/liters.

ycc/in ~ ;~

,". ~.

~` 1331~2~

These additive printed circuit boards from the second solution pass the thermal stress test, demonstrating that maintaining a :Eormaldehyde to copper ratio in an electroless plating solution less than 1.2 : .:
can provide fissure free copper dleposits.

- .

ycc/in : ~
... .

Claims (8)

1. A method of formulating and operating an electroless plating solution for forming copper deposits being substantially free of fissures, said solution comprising copper ions, a complexing ligand for copper ions, a pH adjustor, a reducing agent, and at least one of a stabilizer and ductility promoter and having a desired initial ratio of the intrinsic anodic reaction rate to the intrinsic cathodic reaction rate, characterized in that the influence of build-up of by-products being formed during bath operation is compensated for by raising the copper ion concentration and the pH value or decreasing the concentration of the reducing agent or by raising the copper ion concentration and the pH value and decreasing the concentration of the reducing agent, and thereby maintaining the ratio of intrinsic anodic reaction rate to intrinsic cathodic reaction rate at less than about 1:1 and the original deposition rate as well as depositing substantially fissure free copper.

2. The method of claim 1 characterized in that the copper ion concentration or the pH value, or the copper ion concentration and the pH value, are sufficiently increased or decreased for maintaining the ratio of the intrinsic anodic reaction rate and the intrinsic cathodic reaction rate at or below the ratio originally selected.

3. The method of claim 1 characterized in that the mole concentration of the reducing agent is no greater than 1.2 times the mole concentration of the copper ions.

4. The method of claim 2 characterized in that the mole concentration of the reducing agent is no greater than 1.2 times the mole concentration of the copper ions.

5. The method of any one of claims 1, 2, 3 or 4 characterized in that the reducing agent is formaldehyde.

6. The method of any one of claims 1, 2, 3, or 4 characterized in that the bath solution further comprises an accelerator selected from (i) heterocyclic aromatic nitrogen compounds, (ii) nonaromatic nitrogen compounds having at least one delocalized pi bond, and (iii) aromatic amines and mixtures of the foregoing, and that the pH is adjusted to at least 11.9 and preferably to 12.5 measured at 25°C.

7. The method of either claims 1 or 3 characterized in that either the copper ion concentration and the pH are increased sufficiently to maintain the ratio of the intrinsic reaction rates at or below the initially selected value, or that the reducing agent concentration is sufficiently decreased to maintain the initially selected and established plating rate.

8. An electroless copper plating solution comprising copper ions, a complexing ligand for copper ions, a pH adjustor, a reducing agent, a stabilizer or ductility promoter and an accelerator containing a delocalized pi bond selected from (i) heterocyclic aromatic nitrogen and sulfur compounds, (ii) nonaromatic nitrogen compounds having at least one delocalized pi bond, and (iii) aromatic amines and mixtures of the foregoing, characterized in that the mole concentration of the reducing agent is no greater than the mole concentration of the copper ions.