EP2764135A2 - Formaldehyde-free electroless copper plating solution - Google Patents

Abstract

The invention relates to an electroless aqueous copper plating solution, comprising a source of copper ions, a source of glyoxylic acid as reducing agent, and at least one polyamino disuccinic acid or at least one polyamino monosuccinic acid, or a mixture of at least one polyamino disuccinic acid and at least one polyamino monosuccinic acid as complexing agent, as well as to a method for electroless copper plating utilizing said solution and the use of the solution for the plating of substrates.

Description

Formaldehyde-free electroless copper plating solution

The present invention relates to an electroless copper plating solution, a method for electroless copper plating utilizing said solution and the use of the solution for the plating of substrates.

Electroless plating is the controlled autocatalytic deposition of a continuous film of metal without the assistance of an external supply of electrons. Non-metallic surfaces may be pretreated to make them receptive or catalytic for deposition. All or selected portions of a surface may suitably be pretreated. The main components of electroless copper baths are the copper salt, a complexing agent, a reducing agent, and, as optional ingredients, an alkaline, and additives, as for example stabilizers. Complexing agents are used to chelate the copper being deposited and prevent the copper from being precipitated from solution (i.e. as the hydroxide and the like). Chelating copper renders the copper available to the reducing agent which converts the copper ions to metallic form.

Common electroless copper baths used formaldehyde as reducing agent. Formaldehyde is the most important and established reducing agent of the common electroless copper plating process. In 1987 formaldehyde was classified as a probable human carcinogen by the U.S. Environmental Protection Agency. In June 2004 the International Agency for Research on Cancer (IARC) classified formaldehyde as a human carcinogen. Consequently,

formaldehyde-free electroless copper baths had been developed to fulfill safety and occupational health requirements. US 4,617,205 discloses a composition for electroless deposition of copper, comprising copper ions, glyoxylate as reducing agent, and a complexing agent, for example EDTA, which is capable of forming a complex with copper that is stronger than a copper oxalate complex. US 7,220,296 teaches an electroless plating bath comprising a water soluble copper compound, glyoxylic acid and a complexing agent which may be EDTA.

US 20020064592 discloses an electroless bath comprising a source of copper ions, glyoxylic acid or formaldehyde as reducing agent, and EDTA, tartrate or alkanol amine as complexing agent. US 20080223253 discloses an electroless copper plating solution including a copper salt, a reductant that may be selected from the group consisting of formaldehyde,

paraformaldehyde, glyoxylic acid, NaBH₄, KBH₄, NaH₂P0₂, hydrazine, formalin,

polysaccharide, such as glucose, and a mixture thereof, and a complexing agent which may be selected from the group consisting of ethylenediamine tetraacetic acid (EDTA), hydroxyethyl ethylene diamine triacetic acid (HEDTA), cyclohexanediamine tetraacetic acid, diethylenetriamine pentaacetic acid, and tetrakis (2-hydroxypropyl)ethylenediamine

(hereinafter also called "Quadrol", which is a trademark of BASF company). The drawback of EDTA, HEDTA, tetrakis(2-hydroxypropyl)ethylenediamine, and other related complexing agents is a lack of biodegradability.

Performance of a copper plating solution is generally not predictable and strongly depends on its constituents, especially the complexing agent and the reducing agent, and the molar ratio of its constituents.

An object of the present invention was to provide with an electroless copper plating solution that is formaldehyde-free. Another object was to provide with an electroless copper plating solution with improved performance, for example an improved copper deposition rate.

Another object of the invention was an electroless copper plating solution that employs a biodegradable complexing agent for copper.

Still another object was that a formaldehyde-free copper plating solution must live up to the standard of the formaldehyde electroless copper baths. It should be suitable for application in both horizontal and vertical processes, in which the final products are for example employed in high-end technology, like HDI (high density interconnect) PCBs and IC substrates

(IC = integrated circuit, PCBs = printed circuit boards). The solution should also be suitable in the manufacture of displays.

The present invention provides with an electroless copper plating solution, comprising

- a source of copper ions,

- a source of glyoxylic acid as reducing agent, and - at least one polyamino disuccinic acid or at least one polyamino monosuccinic acid, or a mixture of at least one polyamino disuccinic acid and at least one polyamino monosuccinic acid, as complexing agent,

wherein the molar ratio of the complexing agent to copper ions is in the range of 1 .1 :1 to 5:1 .

One or more of the above mentioned objects are achieved by the electroless copper plating solution (hereinafter abbreviated as the "solution") according to claim 1 , or by advantageous embodiments as described in dependent claims and the description. The solution of claim 1 is formaldehyde-free and shows an improved copper deposition rate. Deposition rates of 0.15 - 1 .0 μηι/10 min, 0.15 - 1 .5 μηι/10 min or even 0.15 - 2.0 μηι/10 min may be reached.

The advantages of this new formaldehyde-free copper bath are the good bath performance, bath stability, good coverage, high deposition speed and a low blistering tendency. The critical bath component formaldehyde is replaced by a non toxic reducing agent.

The molar ratio of the complexing agent polyamino disuccinic acid or polyamino

monosuccinic acid to copper ions leads to beneficial properties of the plating solution, which are suppressed copper hydroxide precipitation, bath stability and suppressed formation of blisters in copper plating process, as further explained below and in the working examples.

In one embodiment of the invention, the molar ratio of glyoxylic acid to complexing agent is <4.6 : 1 . It is shown in the present invention that such molar ratio has beneficial effects on the quality of a copper deposition on a substrate, as for example coverage, backlight and passivation. Further beneficial molar ratios of glyoxylic acid to complexing agent, particularly EDDS, are <4.5:1 , <4.2:1 , <4.0:1 , <3.8:1 , <3.6:1 . Preferable lower limits of molar ratios of glyoxylic acid to complexing agent, particularly EDDS, are 0.45:1 , or 0.7:1 , 1 :1 or 2:1 . Thus, preferable ranges of molar ratios of glyoxylic acid to complexing agent, particularly EDDS, are 0.45:1 to 4.5:1 , 0.45:1 to 4.2:1 , 0.45:1 to 4.0:1 , 0.45:1 to 3.8:1 , or 0.45:1 to 3.6:1 . Other preferable ranges are 1 :1 to 4.5:1 , 1 :1 to 4.2:1 , 1 :1 to 4.0:1 , 1 :1 to 3.8:1 , or 1 :1 to 3.6:1 . Still other preferable ranges of molar ratios of glyoxylic acid to complexing agent, particularly EDDS, are 2:1 to 4.5:1 , 2:1 to 4.2:1 , 2:1 to 4.0:1 , 2:1 to 3.8:1 , or 2:1 to 3.6:1 . The ratio is related to the molar amount of complexing agent, which means in this connection the total molar amount of complexing agents, if more than one complexing agent is used. The molar concentration of glyoxylic acid is preferably at least as high as the molar concentration of copper in the solution, more preferably higher. Thus, the molar ratio of glyoxylic acid to Cu is preferably≥1 :1 , preferably≥1 .5:1 , more preferably≥2:1 .

Polyamino disuccinic acids and polyamino monosuccinic acids show a very good or even high biodegradability. The plating solution of the present invention is free from

ethylenediamine tetraacetic acid (EDTA), N'-(2-Hydroxyethyl)-ethylenediamine-N,N,N'- triacetic acid (HEDTA), and tetrakis(2-hydroxypropyl)ethylenediamine.

The solution according to the invention and the process according to the invention are preferably used for the coating of printed circuit boards, chip carriers and semiconductor wafers or also of any other circuit carriers and interconnect devices. The solution is used in particular in printed circuit boards and chip carriers, but also in semiconductor wafers, to plate surfaces, trenches, blind micro vias, through hole vias (through holes) and similar structures with copper.

Particularly, the solution of the invention or the process of the invention can be used for deposition of copper on surfaces, in trenches, blind-micro-vias, through-hole-vias, and comparable structures in printed circuit boards, chips, carriers, wafers, and various other interconnect devices. The term "through hole vias" or "through holes", as used in the present invention, encompasses all kinds of through hole vias and includes so-called "through silicon vias" in silicon wafers.

Another application that is envisaged for the solution is metallization of display applications. In this regard, copper is deposited particularly on glass substrate, particularly flat glass surfaces. Wet electroless copper deposition on glass substrate is beneficial in comparison to metal sputtering processes that have been used so far. Benefits that can be reached with wet electroless deposition in comparison to sputtering techniques are, inter alia, reduced internal stress and reduced bending of a glass substrate, reduced equipment maintenance, effective use of metal, reduced material waste. Moreover, a high copper deposition rate on glass substrate is reached with the solution of the invention, especially on glass substrates that are pretreated with relatively few metal seeds.

The solution of the invention is an aqueous solution. The term "aqueous solution" means that the prevailing liquid medium, which is the solvent in the solution, is water. Further liquids, that are miscible with water, as for example alcohols and other polar organic liquids, that are miscible with water, may be added. The solution of the present invention may be prepared by dissolving all components in aqueous liquid medium, preferably in water.

The solution contains a copper ion source, which may for example be any water soluble copper salt. Copper may for example, and without limitation, be added as copper sulphate, copper chloride, copper nitrate, copper acetate, copper methane sulfonate ((CH₃0₃S)₂Cu), copper hydroxide; or hydrates thereof.

Electroless copper baths using reducing agents mentioned above preferably employ a relatively high pH, usually between 1 1 and 14, or 12.5 and 14, preferably between 12.5 and 13.5, or 12.8 and 13.3. The pH is adjusted generally by potassium hydroxide (KOH), sodium hydroxide (NaOH), lithium hydroxide (LiOH), ammonium hydroxide or tetramethylammonium hydroxide (TMAH). Thus, the solution may contain a source of hydroxide ions, as for example and without limitation one or more of the compounds listed above. A source of hydroxide is for example added if an alkaline pH of the solution is desired and if the pH is not already in the alkaline range by other constituents.

Especially preferred is the use of potassium hydroxide because the solubility of potassium oxalate is high. Oxalate anions are formed in the solution by the oxidation of the glyoxylic acid that is used as the reducing agent.

Glyoxylic acid is the reducing agent for the reduction of copper ions to elementary copper. As used herein the term "glyoxylic acid" includes non-dissociated glyoxylic acid as well as glyoxylate ions. In the solution, non-dissociated glyoxylic acid and glyoxylate-ions may be present. The exact nature of the species, acid or salt, present will depend in the pH of the solution. The same consideration applies to other weak acids and bases.

The term "source of glyoxylic acid" encompasses glyoxylic acid and all compounds that can be converted to glyoxylic acid in aqueous solution. In aqueous solution the aldehyde containing acid is in equilibrium with its hydrate. A suitable source of glyoxylic acid is dihaloacetic acid, such as dichloroacetic acid, which will hydrolyse in an aqueous medium to the hydrate of glyoxylic acid. An alternative source of glyoxylic acid is the bisulphite adduct as is a hydrolysable ester or other acid derivative. The bisulphite adduct may be added to the composition or formed in situ. The bisulphite adduct may be made from glyoxylate and either bisulphite, sulphite or metabisulphite. One or more further reducing agents may be added, if desired, as for example hypophosphoric acid, glycolic acid or formic acid, or salts of aforementioned acids. The solution of the invention does, however, not contain formaldehyde. Thus, the solution is free of formaldehyde. The further reducing agent is preferably an agent that acts as reducing agent but cannot be used as the sole reducing agent (cf. US 7,220,296, col. 4, 1. 20-43 and 54-62). A further reducing agent is in this sense also called an "enhancer".

Polyamino disuccinic acids are compounds having two or more nitrogen atoms, wherein 2 of the nitrogens are bonded to a succinic acid (or salt) group, preferably only two nitrogen atoms each have one succinic acid (or salt) group attached thereto. As used herein the term succinic acid includes salts thereof. The compound has at least 2 nitrogen atoms, and due to the commercial availability of the amine, preferably has no more than about 10 nitrogen atoms, more preferably no more than about 6, most preferably 2 nitrogen atoms. Nitrogen atoms which do not have a succinic acid moiety attached most preferably are substituted with hydrogen atoms. More preferably, the succinic acid groups are on terminal nitrogen atoms, most preferably each of which nitrogens also have a hydrogen substituent. By terminal it is meant the first or last nitrogen atom which is present in the compound, irrespective of other substituents. Another definition of a terminal nitrogen is a primary amine nitrogen, before a succinic acid moiety is attached. The terminal nitrogen is transferred to a secondary amine nitrogen after a succinic acid moiety was attached. Because of steric hindrance of two succinic groups on one nitrogen, it is preferred that each nitrogen having a succinic group has only one such group. Remaining bonds on nitrogens having a succinic acid group are preferably filled by hydrogens or alkyl or alkylene groups (linear, branched or cyclic including cyclic structures joining more than one nitrogen atom or more than one bond of a single nitrogen atom, preferably linear) or such groups having ether or thioether linkages, all of preferably from 1 to 10 carbon atoms, more preferably from 1 to 6, most preferably from 1 to 3 carbon atoms, but most preferably hydrogen. Preferred alkyl groups are methyl, ethyl and propyl groups. More preferably, the nitrogen atoms are linked by alkylene groups, preferably each of from 2 to12 carbon atoms, more preferably from 2 to 10 carbon atoms, even more preferably from 2 to 8, most preferably from 2 to 6 carbon atoms, namely ethylene, propylene, butylene, pentylene or hexylene. The polyamino disuccinic acid compound preferably has at least about 10 carbon atoms and preferably has at most about 50, more preferably at most about 40, most preferably at most about 30 carbon atoms. The term "succinic acid" is used herein for the acid and salts thereof; the salts include metal cation (e.g. potassium, sodium) and ammonium or amine salts. Polyamino disuccinic acids useful in the practice of the invention are unsubstituted

(preferably) or inertly substituted, that is substituted with groups that do not undesirably interfere with the activity of the polyamino disuccinic acid in a selected application. Such inert substituents include alkyl groups (preferably of from 1 to 6 carbon atoms); aryl groups including arylalkyl and alkylaryl groups (preferably of from 6 to 12 carbon atoms), with alkyl groups preferred among these and methyl and ethyl groups preferred among alkyl groups.

Inert substituents are suitably on any portion of the molecule, preferably on carbon atoms, more preferably on alkylene groups, for example alkylene groups between nitrogen atoms or between carboxylic acid groups, most preferably on alkylene groups between nitrogen groups.

Preferred polyamino disuccinic acids include ethylenediamine-N,N'-disuccinic acid (EDDS), diethylenetriamine-N,N"-disuccinic acid, triethylenetetraamine-N,N"'-disuccinic acid, 1 ,6 hexamethylenediamine Ν,Ν'-disuccinic acid, tetraethylenepentamine-N,N""-disuccinic acid, 2-hydroxypropylene-1 ,3-diamine-N,N'-disuccinic acid, 1 ,2 propylenediamine-N,N'-disuccinic acid, 1 ,3-propylenediamine-N,N"-disuccinic acid, cis-cyclohexanediamine-N,N'-disuccinic acid, transcyclohexanediamine-N,N'-disuccinic acid, and ethylenebis(oxyethylenenitrilo)- Ν,Ν'-disuccinic acid. The preferred polyamino disuccinic acid is ethylenediamine-N,N'- disuccinic acid.

Such polyamino disuccinic acids can be prepared, for instance, by the process disclosed by Kezerian et al. in U.S. Patent 3,158,635 which is incorporated herein by reference in its entirety. Kezerian et al disclose reacting maleic anhydride (or ester or salt) with a polyamine corresponding to the desired polyamino disuccinic acid under alkaline conditions. The reaction yields a number of optical isomers, for example, the reaction of ethylenediamine with maleic anhydride yields a mixture of three optical isomers [R,R], [S,S] and [S,R] ethylenediamine disuccinic acid (EDDS) because there are two asymmetric carbon atoms in ethylenediamine disuccinic acid. These mixtures are used as mixtures or alternatively separated by means within the state of the art to obtain the desired isomer(s). Alternatively, [S,S] isomers are prepared by reaction of such acids as L-aspartic acid with such

compounds as 1 ,2-dibromoethane as described by Neal and Rose, "Stereospecific Ligands and Their Complexes of Ethylenediaminedisuccinic Acid", Inorganic Chemistry, V. 7, (1968), pp. 2405-2412.

Polyamino monosuccinic acids are compounds having at least two nitrogen atoms to which a succinic acid (or salt) moiety is attached to one of the nitrogen atoms. Preferably the compound has at least 2 nitrogen atoms, and due to the commercial availability of the amine, preferably has no more than about 10 nitrogen atoms, more preferably no more than about 6, most preferably 2 nitrogen atoms. Remaining nitrogen atoms, those which do not have a succinic acid moiety attached, preferably are substituted with hydrogen atoms. Although the succinic acid moiety may be attached to any of the amines, preferably the succinic acid group is attached to a terminal nitrogen atom. By terminal it is meant the first or last amine which is present in the compound, irrespective of other substituents. Another definition of a terminal nitrogen is a primary amine nitrogen, before a succinic acid moiety is attached. The terminal nitrogen is transferred to a secondary amine nitrogen after a succinic acid moiety was attached. The remaining bonds on the nitrogen having a succinic acid group are preferably filled by hydrogens or alkyl or alkylene groups (linear, branched or cyclic including cyclic structures joining more than one nitrogen atom or more than one bond of a single nitrogen atom, preferably linear) or such groups having ether or thioether linkages, all of preferably from 1 to 10 carbon atoms, more preferably from 1 to 6, most preferably from 1 to 3 carbon atoms, but most preferably hydrogen. Preferred alkyl groups are methyl, ethyl and propyl groups. Generally the nitrogen atoms are linked by alkylene groups, each of from 2 to 12 carbon atoms, preferably from 2 to 10 carbon atoms, more preferably from 2 to 8, and most preferably from 2 to 6 carbon atoms, namely ethylene, propylene, butylene, pentylene or hexylene. The polyamino monosuccinic acid compound preferably has at least about 6 carbon atoms and preferably has at most about 50, more preferably at most about 40, and most preferably at most about 30 carbon atoms. Polyamino monosuccinic acids useful in the practice of the invention are unsubstituted (preferably) or inertly substituted as described above for polyamino disuccinic acid compounds. Preferred polyamino monosuccinic acids include ethylenediamine monosuccinic acid, diethylenetriamine monosuccinic acid, triethylenetetraamine monosuccinic acid, 1 ,6- hexamethylenediamine monosuccinic acid, tetraethylenepentamine monosuccinic acid, 2 hydroxypropylene-1 ,3-diamine monosuccinic acid, 1 ,2-propylenediamine monosuccinic acid, 1 ,3-propylenediamine monosuccinic acid, ciscyclohexanediamine monosuccinic acid, trans- cyclohexanediamine monosuccinic acid and ethylenebis(oxyethylenenitrilo) monosuccinic acid. The preferred polyamino monosuccinic acid is ethylenediamine monosuccinic acid.

Such polyamino monosuccinic acids can be prepared for instance, by the process of

Bersworth et al. in U.S. Patent 2,761 ,874, the disclosure of which is incorporated herein by reference, and as disclosed in Jpn. Kokai Tokkyo Koho JP 57,1 16,031 . In general, Bersworth et al. disclose reacting alkylene diamines and dialkylene triamines under mild conditions with maleic acid esters under mild conditions (in an alcohol) to yield amino derivatives of N-alkyl substituted aspartic acid. The reaction yields a mixture of the R and S isomers.

In one embodiment, when the solution contains a mixture of a polyamino disuccinic acid and a polyamino monosuccinic acid, it is preferred that the polyamino substituent of the polyamino disuccinic acid and the polyamino monosuccinic acid are the same. Thus by way of example, if the polyamino disuccinic acid is ethylenediamine-N,N'-disuccinic acid, the polyamine monosuccinic acid is ethylenediamine monosuccinic acid. In a preferred embodiment, ethylenediamine-N,N'-disuccinic acid (EDDS) is used as complexing agent. EDDS is a preferred complexing agent because of its high

biodegradability. Other electroless copper baths with biodegradable complexing agents, as tartrate, commonly use the toxic co-metal nickel. It has been found out that toxic co-metals can be avoided in the present invention. Thus, the solution of the present invention is free of toxic co-metals. The solution of the present invention is preferably free of nickel.

The present inventors have found out that a considerably improved copper deposition rate is obtained in solutions of the invention, comprising glyoxylic acid and EDDS. This is an unexpected result because in comparison examples employing formaldehyde, the copper deposition is not or only marginally improved when a formaldehyde-EDDS-combination is compared with formaldehyde-EDTA.

The term "EDDS" includes racemic EDDS or all optically active isomers thereof, such as (S,S)-EDDS, and salts and derivatives thereof. Preferably the term means (S,S)-EDDS or salts thereof. EDDS may be prepared by the process of PCT/GB94/02397. In the solution, ethylenediaminedisuccinic acid and ethylenediaminedisuccinate-ions may be present, depending on the pH of the solution.

The solution of the invention in one embodiment contains copper ions, complexing agent, which is preferably EDDS, and glyoxylic acid in following concentrations:

Cu-ions: 1 -5 g/l, corresponding to 0.016-0.079 mol/l

complexing agent: 5-50 g/l, corresponding to 0.034-0.171 mol/l

glyoxylic acid: 2-20 g/l corresponding to 0.027-0.270 mol/l The solution of the invention more preferably contains copper ions, complexing agent, which is preferably EDDS, and glyoxylic acid in following concentrations: Cu-ions: 2-3 g/l, corresponding to 0.031 -0.047 mol/l complexing agent: 20-40 g/l, corresponding to 0.068-0.137 mol/l

glyoxylic acid: 2-20 g/l corresponding to 0.027-0.270 mol/l In the present invention, the molar ratio of complexing agent, which means in this connection the total amount of complexing agent(s) (i.e. sum of moles of all complexing agents, if more than one complexing agent is used), to copper ions is in the range of 1 .1 :1 to 5:1 , more preferably 1 .5:1 to 5:1 . It has been shown that the solutions of the invention have a better performance when this molar ratio is used, i.e. when the complexing agent, particularly EDDS, is used in molar excess with respect to copper. In the present invention, when glyoxylic acid is used as reducing agent, it is shown that a molar ratio of complexing agent, particularly EDDS, to copper of at least 1 .1 :1 is necessary to complex the copper ions. A molar ratio <1 :1 leads to copper hydroxide precipitation and no copper plating is possible. On the other hand, a molar ratio of >5:1 leads to bath instability and high blister formation on the surface of a substrate in the copper plating process.

In a further embodiment, the molar ratio of complexing agent, which means in this connection the total amount of complexing agent(s) to copper ions is 2:1 to 5:1 , more preferably 3:1 to 5:1 .This embodiment is particularly advantageous if the copper bath is agitated during deposition, preferably agitated with air, and when a further reducing agent (also called "enhancer") is used in addition to glyoxylic acid, wherein the further reducing agent is preferably selected from glycolic acid, hypophosphoric acid, or formic acid, most preferably glycolic acid.

The solution of the present invention may comprise - and does not necessarily comprise - further components, as for example stabilizers, surfactants, additives, as rate controlling additives, grain refining additives, pH buffers, pH adjusters, and enhancers. Such further components are for example described in following documents, which are incorporated by reference in their entirety: US 4,617,205 (particularly disclosure in col. 6, 1. 17 - col. 7, 1. 25), US 7,220,296 (particularly col. 4, 1. 63 - col. 6, I. 26), US 2008/0223253 (cf. particularly paragraphs 0033 and 0038).

Stabilizers are compounds that stabilize the electroless plating solution against unwanted outplating in the bulk solution. The term "outplating" means unspecific and/or uncontrolled deposition of copper. Reduction of copper(ll) should only occur on the desired substrate surface and not unspecific in the whole bath. A Stabilizing function can for example be accomplished by substances acting as catalyst poison (for example sulfur or other chalcogenide containing compounds) or by compounds forming copper(l)-complexes, thus inhibiting the formation of copper(l)oxide.

Suitable stabilizers are, without limitation, dipyridyls (2,2'-dipyridyl, 4,4'dipyridyl),

phenanthroline, mercapto-benzothiazole, thio-urea or its derivatives like diethyl-thio-urea, cyanides like NaCN, KCN, K₄[Fe(CN)₆], thiocyanates, iodides, ethanolamines, mercapto- benzotriazole, Na₂S₂0₃, polymers like polyacrylamides, polyacrylates, polyethylene glycols, or polypropylene glycols and their co-polymers, wherein 2,2'-dipyridyl (abbreviated as "DP"), diethyl-thio-urea, K₄[Fe(CN)₆], NaCN and mercapto-benzothiazole are particularly suitable.

In one embodiment, the stabilizer is chosen, mainly for environmental and occupational health reasons, from a stabilizer that is free of cyanides. Thus, the solution of the present invention is preferably free of cyanides. In this connection, 2,2'-dipyridyl is a preferred stabilizer. Dipyridyl is prefably added in an amount of 1 - 10 mg/l.

European application EP1876262 discloses an electroless copper bath that contains as a necessary component one or more thiocarboxylic acids. The thio compounds mentioned in EP1876262 include compounds having a formula HS-(CX1 )r-(CHX2)s-COOH, wherein X1 is -H or -COOH; X2 is -H or -SH; r and s are positive integers where r is 0 to 2, or 0 or 1 ; and s is 1 or 2. Specific examples of thio compounds, mentioned in EP1876262, are thioglycolic acid, thiopropionic acid, thiomalic acid and dithiodisuccinic acid. According to EP1876262 such thiocarboxylic acids are compatible with glyoxylic acid and its salts and stabilize the electroless copper compositions by preventing the formation of copper oxide. The necessary minimum amount of a thio compound according to EP1876262 is 0.01 ppm. In the present invention it is shown that performance of an electroless copper bath is better when a thiocarboxylic acid component, as generically and specifically mentioned in EP1876262, is avoided or at least below the limit mentioned in EP1876262. Trace amounts of thiocarboxylic acid as generically and specifically mentioned in EP1876262 may be present, as long as the amount is below 0.01 ppm. It is however preferred that no thiocarboxylic acid is added to the solution of the invention, i.e. that the bath does not contain any thiocarboxylic acid as generically and specifically mentioned in EP1876262.

In another aspect, the present invention relates to a process for electroless copper plating, the process comprising contacting a substrate with an electroless copper plating solution as described above. For example, the substrate may be dipped or immersed in the solution of the invention. In the process a whole surface of a substrate may be plated with copper, or only selected portions.

It is preferred that the solution be agitated during use. In particular, work- and/or solution- agitation may be used. A preferred kind of agitation is air agitation of the solution. Air agitation may be achieved by bubbling air through the solution in use.

The process will be carried out for a sufficient time to yield a deposit of the thickness required, which in turn will depend on the particular application.

One application of the present invention that is envisaged will be particularly suitable for the preparation of printed circuit boards. The electroless deposition of copper according to the process of the invention can particularly be used for the through-plating of holes, surfaces, trenches, blind micro vias in printed circuit boards. Double sided or multilayer boards (rigid or flexible) may be plated by means of the present invention.

The process of the invention may be useful in providing electroless copper deposits with a thickness in the range of 0.1 to 25 μηι, preferably between 0.25 and 3 μηι. Substrates that are generally used for printed circuit board manufacture are most frequently epoxy resins or epoxy glass composites. But other substances, notably phenolic resins, polytetrafluoroethylene (PTFE), polyimides, polyphenyleneoxides, bismaleintriazine-resins (BT-resins), cyanate esters and polysulphones can be used. Aside from the application of the process in the production of printed circuit boards, it may be found to be useful in plating non-conductive substrates generally, including plastics, such as acrylonitrile butadiene styrene (ABS) and polycarbonate; ceramics and glass.

In one embodiment of the process of the present invention, the process is carried out at a temperature in the range of 20 - 60 °C, preferably 20 - 55^<€, more preferably 20 - 50 ^<€, even more preferably 20 - 45 °C, and most preferably 20 - 40 'Ό. This embodiment is very beneficial since with formaldehyde based solutions of the state of the art higher temperatures are necessary for a good plating performance, especially for a sufficient copper deposition rate.

The substrate, i.e. the surfaces of the substrate that are to be plated with copper, particularly non-metallic surfaces, may be pretreated by means within the skill in the art (as for example described in US 4,617,205, col 8) to make it/them more receptive or autocatalytic for copper deposition. All or selected portions of a surface may be pretreated. A pretreatment is, however, not necessary in every case and depends on the kind of substrate and surface. Within the pretreatment, it is possible to sensitise substrates prior to the deposition of electroless copper on them. This may be achieved by the adsorption of a catalysing metal (such as a noble metal, for example palladium) onto the surface of the substrate.

A pretreatment process strongly depends on parameters as the substrate, the desired application, and the desired properties of the copper surface

An exemplary and non-limiting pretreatment process, especially for printed circuit board laminates and other suitable substrates, may comprise following steps

a) contacting the substrate with an activator solution, that contains colloidal or ionic catalysing metal, such as a noble metal, preferably palladium, causing the substrate's surface to become catalytic,

and optionally, particularly if the activator contains ionic catalysing metal,

b) contacting the substrate with a reducer, wherein the metal ions of an ionic activator are reduced to elemental metal,

or, if the activator contains colloidal catalysing metal,

c) contacting the substrate with an accelerator, wherein the components of the colloid, for example a protective colloid, is removed from the catalysing metal.

Further steps, that can optionally be performed in any combination, preferably before above- mentioned step a), are:

i. Cleaning and conditioning the substrate to increase adsorption. With a cleaner,

organics and other residues are removed. The cleaner may also contain additional substances (conditioners) that prepare the surface for an activation step, i.e. enhance the adsorption of the catalyst and lead to a more uniformly activated surface.

ii. Etching the substrate, to remove oxides from the surface of the copper, especially from inner layers in holes. This may be done by persulphate or peroxide based etching systems.

iii. Contacting the substrate with a pre-dip solution, such as a hydrochloric acid solution or sulfuric acid solution, optionally with an alkali metal salt, such as sodium chloride, also in the pre-dip solution. The pre-dip serves to protect an activator from drag-in and contaminations. In another kind of pretreatment process a permanganate etching step is employed. The so- called Desmear process, using a permanganate etching step, is described in the appended examples. The Desmear process may be combined with the above described steps.

Particularly, the Desmear process may be performed before step a) of the above described pretreatment process, or before above-mentioned steps i)-iii) in case that one or more of steps i)-iii) is performed. The desmear process may also be performed instead of steps i) and ii).

In a pretreatment process which is particularly suitable in metallization for display

applications and in metallization of glass substrates, a surface is only contacted with a pre- dip solution and an activator solution and then with the solution of the invention. Contacting with a cleaning solution and an adhesion enhancer before the pre-dip step are optional steps that can be carried out in advance. Still another process, which is often used for glass substrates, may be carried out with following steps before copper plating: A glass surface that is to be plated exhibits metal particles as seeds. The metal particles may be brought onto the surface by sputtering techniques. Exemplary seeds are particles composed of copper, titanium, molybdenum, zirconium, aluminium, chromium, tungsten, niobium, tantalum or a mixture or alloy thereof. Another seed may be a metal oxide, or a mixed metal oxide, as for example indium tin oxide. This process can also be used for plastic substrates, as for example substrates made from polyethylene terephthalate.

Said pretreated glass surface is contacted with an activator solution that contains ionic catalysing metal, such as a noble metal, preferably palladium, causing the surface to become catalytic. The ionic catalysing metal is reduced onto the surface by the seed metal. In this process, addition of a further reducer may be omitted. This process is especially used in copper plating of glass substrates for display applications. The exemplary pretreatment processes, or single steps thereof, may be combined to alternative pretreatment processes, if found necessary.

In a further aspect, the present invention relates to the use of the electroless copper plating solution as described above for the plating of printed circuit boards, wafers, Integrated circuit substrates, MID (molded interconnect device) components, displays, such as liquid crystal displays, TFT-displays, plasma displays, electroluminescent displays (ELD), and

electrochromic displays (ECD), particularly displays for electronic devices or TVs, display components, flat sensors, such as X-ray imaging devices, or plastic parts, such as plastic parts for functional or decorative purposes.

The invention is now described in further detail by the following examples. These examples are set forth to illustrate the present invention, but should not be construed as limiting the present invention.

BRIEF DESCRIPTION OF THE FIGURES

Fig. 1 Steps of the Desmear multi-stage process for cleaning surfaces,

Fig. 2 Steps of the plating through hole process for activation, Fig. 3 Reference samples for backlight measurement, showing results from D1 to D10, Fig. 4 copper thickness on glass substrates, using baths with different complexing agents.

EXAMPLES Methods

Backlight method:

The coverage of the electroless copper plating into the through holes can be assessed using an industry standard Backlight Test, in which an electroless plated coupon is sectioned, so as to allow areas of incomplete coverage to be detected as bright spots when viewed over a strong light source [US 2008/0038450 A1 ].

The quality of the copper deposit is determined by the amount of light that is observed under a conventional optical microscope.

The results of the backlight measurement are given on a scale from D1 to D10, wherein D1 means the worst result and D10 the best result. Reference samples showing results from D1 to D10 are shown in Fig. 3. 1. Example

Bath composition

Make up table

Operation temperature: 38 - 50 °C

Deposition rate: ~ 0.6 μηι/10 min.

Work instruction

In this example, the test samples are treated in a common Desmear process to clean the hole wall surfaces and the inner layer copper surfaces. In addition the resin surface is roughened to achieve a good copper adhesion.

The Desmear process is a multi-stage process, whose steps are shown in Fig. 1 .

The Sweller is made of a mixture of organic solvents. During this step the drill smear and other impurities are removed. The high temperature of 60 - 80 °C promotes the infiltration of the Sweller which lead to a swelled surface. Therefore a stronger attack of the subsequently applied permanganate solution is possible. Afterwards the Reduction Solution (reaction conditioner) removes the manganese dioxides produced during the permanganate step from the surfaces.

In the PTH (plating through hole) process the non conductive material is prepared for the copper deposition. The activation steps of the PTH process are shown in Fig. 2.

The cleaner is used to remove organics and condition the surface for the next activation steps. The Etch Cleaner removes the oxides from the surface of the copper inner layers in the holes. Substances used as etch cleaner are selected from a mixture of sulfuric acid and hydrogen peroxide, or from peroxodisulfates, or from peroxomonosulfate. Etch cleaning solutions may, in addition to the etching component, also contain additives and/or stabilizers.

The activation of the panel and hole surfaces with palladium takes place in the activator that contains colloidal or ionic catalysing metal, such as a noble metal, preferably palladium, causing the surface to become catalytic. In one possible set-up the activator contains palladium ions complexed by an organic ligand. The prefixed Predip should protect the activator from drag-in and contaminations.

The last step of the activation process is the Reducer. There the palladium ions are reduced to elemental palladium with high catalytic activity. After the reducer-step, electroless copper deposition with a solution of the invention is carried out. The reducer is used in combination with ionic metal compounds as activators. It employs reducing agents like hypophosphites, boranates, aminoboranes.

For the electroless copper deposition the bath make up was made by adding the bath components in the order outlined in table 1 to the suited amount of water. Air agitation was used. The operation temperature was varied from 38 - 50 'C. Also the deposition time was set from 10 - 60 minutes to achieve the required copper thickness.

Bath characteristics

Deposition speed (measured on FR4 material e.g. Matsushita MC 100 EX): ~ 0.6 μηι/10 min. Blistering tendency (tested material: ABF GX-92 from Ajinomoto): low or none

Coverage (both FR4 and GX-92): good

Color (both FR4 and GX-92): salmon pink

Backlight is tested on copper cladded FR4 with through holes.

Plating example

The test panels run through the Desmear process (below table).

Desmear process Desmear step Immersion time [min]

Sweller 5

Permanganate 10

Reduction Conditioner 5

The activation process based on an ionic activator system:

Activation process

Activator step Immersion time [min]

Cleaner 4

Etch cleaner 1

Predip 1

Activator 4

Reducer 3

A make up example for the copper bath is described in the table below.

Example of bath make up

Operation temperature: 38^

Deposition rate: 0.6 μηι/10 min.

Blistering tendency: none

Coverage: good

Backlight D8

Color: Salmon pink 2. Comparison Examples: Glyoxylic acid with HEDTA or qlyoxylic acid with several biodegradable complexing agents

Comparison example 2.1 : HEDTA (N'-(2-Hvdroxyethyl)-ethylenediamine-N,N,N'-triacetic acid)

Bath composition

Operation temperature: 45 °C

Deposition rate: 0.5 μηι/10 min

Backlight: D7

Bath stability: good

Coverage: good

Color: salmon pink

Disadvantage: non-biodegradable complexing agent

Comparison example 2.2: biodegradable sorbitol

Bath composition

Operation temperature: 60 °C

Deposition rate: 0.3 μηι/10 min

Backlight: D8

Bath stability: low

Coverage: good

Color: salmon pink

Disadvantage: low deposition rate

Comparison example 2.3: biodegradable K-Na-tartrate

Bath composition

Operation temperature: 38 °C

Deposition rate: 0.3 μηι/10 min

Backlight: D8 - D9

Bath stability: low

Blistering tendency: low

Coverage: good

Color: salmon pink Disadvantage: high complexing agent concentration needed and low deposition rate Comparison example 2.4: Biodegradable gluconic acid Bath composition

Operation temperature: 50 °C

Deposition rate: 0.4 μηι/10 min

Backlight: D4 - D5

Bath stability: very low

Coverage: poor

Color: salmon pink, slightly dark

Disadvantage: poor copper deposition and very low bath activity

Summary:

When glyoxylic acid was used as reducing agent, the common biodegradable complexing agents, like tartrate, could not fulfill the bath requirements any longer. The comparative biodegradable complexing agents tested above with glyoxylic acid showed a low deposition rate and/or a high complexing agent concentration was needed. Further biodegradable complexing agents which were tested showed no copper deposition at all or poor results regarding coverage, deposition speed and blistering tendency. In contrast, (S,S)- Ethylenediamine-N,N'-disuccinic acid ((S,S)-EDDS) is biodegradable and fulfilled the requirements of the plating industry. The solution of the present invention containing EDDS was characterized by its good bath performance, good coverage, high deposition speed and a low blistering tendency. Due to the strong complexing properties of EDDS, the stability of the copper bath of the invention is much better than with other biodegradable complexing agents. 3. Example: Comparison EDDS vs. EDTA with different reducing agents

3.1 Examples with qlyoxylic acid as reducing agent

3.1 .1 Bath composition 1 : solution of the invention with glyoxylic acid/ EDDS

Operation temperature: 38 °C Deposition rate: 0.4 μηι/10 ηιίη

Backlight: D4

Bath stability: good

Coverage: poor, passivation can be observed

Color: salmon pink, slightly dark

Blistering: no

Results:

When comparing examples 3.1 .1 with 3.1 .2, the copper plating bath containing EDDS was twice as fast as the bath containing EDTA with respect to copper deposition rate under the same plating conditions. Moreover, a better coverage was reached with EDDS.

3.2 Comparative examples with formaldehyde as reducing agent

3.2.1 Bath composition 3: Comparative example with formaldehyde/ EDDS

Operation temperature: 38 °C

Deposition rate: 1 .1 μηι/10 min

Backlight: D7

Bath stability: good

Coverage: good

Color: salmon pink

Blistering: no blister 3.2.2 Bath composition 4: Comparative example with formaldehyde/ EDTA

Operation temperature: 38 °C

Deposition rate: 0.9 μηι/10 min

Backlight: D6

Bath stability: low

Coverage: poor, slightly passivation

Color: salmon pink, slightly dark

Blistering: yes

Results: The bath with EDDS/formaldehyde shows a slightly higher depositon rate than the bath with EDTA/formaldehyde. But the increase of deposition rate was much lower than with glyoxylic acid, when EDTA was replaced by EDDS (cf. Examples 3.1 .1 and 3.1 .2). Thus, in a bath with formaldehyde, EDDS is comparable or slightly better than EDTA. However, with glyoxylic acid as reductant EDDS shows much better results than EDTA (increase in deposition rate of about 100%), which could not have been foreseen in view of the prior art.

4. Example: Experiments with different concentrations of complexinq agent EDDS and copper

Variation of the molar ratio EDDS:Cu

Bath make-up

Copper 3.0 g/L

Complexing agent: EDDS 12-101 g/L

Alkali: KOH 7.2 g/L

Stabilizer: 2,2^'-dipyridyl 0.004 g/L

Reducing agent: glyoxylic acid 8 g/L

Operation temperature: 36 °C

Test results

Deposition rate: 0.6 μηι/10ηιίη

The molar ratio EDDS: Cu of 1 .1 :1 is at least necessary to complex the copper ions in alkaline solutions. The molar ratio < 1 .1 :1 leads to copper hydroxide precipitation. Thus, no copper plating is possible.

The molar ratio > 5:1 leads to bath instability and high blister tendency on PCB material. The deposition color is dark salmon pink and the backlight is under the required value of D7.

Uncontrolled copper deposition on the beaker can be found after electroless copper plating with bath 6 (molar ratio EDDS:Cu of 6:1 ). The bath stability is unsufficient.

5. Example: Plating solutions with different molar ratios of glyoxylic acid to EDDS

Different molar ratios of EDDS to glyoxylic acid in electroless copper baths are tested, as shown in the below tables.

Bath make-up:

Molar and mass amounts of Cu, complexing agent and reduction agent

The test panels run through the Desmear process (Tab. 2) and the activation process based on an ionic activator system (tab. 3) as described in Example 1 . The following parameters were applied for electroless plating in the electroless copper bath

- T = 38^<€

Dummy plating: 10-15min

Exposition time: 10min

- Tested material: In addition to GX-92 (short desmeared: 2^',4^',4^') and FR4 already described in Example 1 , test panels made of the following materials were used.

ABS (short desmeared: 2^',4^',2^') for testing coverage and passivation; R1755C (desmeared: 5^', 10^', 5^') for Backlight test.

In the below table, the results for the deposition tests are shown.

The bath stability decreases after one day in all baths. Next day, the precipitation of copper hydroxide occurs. For increasing bath stability, a ratio of EDDS to copper of at least 1 .1 : 1 is recommended. But the present experiments show influence of the molar ratio of glyoxylic acid:EDDS on the quality of the copper deposition on a substrate.

The tests show that the copper deposition quality decreases with an increased concentration of glyoxylic acid. With higher glyoxylic acid concentration in the electroless copper bath a copper-glyoxylic acid complex is likely formed in competition to Cu-EDDS complex and glyoxylic acid also takes the part of a complexing agent instead of the reducing agent.

Although the concentration of the glyoxylic acid is high enough to take on both parts

(complexing agent and reducing agent), the reduction process with the copper-glyoxylic acid complex seems to be clammed. The formation of a copper-glyoxylic acid complex causes bath instability together with passivation on copper clad material. Passivation means here the copper surface becomes inactive towards the electroless copper plating process; the electroless copper plating process terminates on a passive surface. The copper deposition quality decreases when the molar ratio of glyoxylic acid:EDDS is 4.6:1 or higher. The initial reactivity, the copper coverage and the passivation depend on the molar ratio of glyoxylic acid : EDDS. The complexing agent plays an important part in the electroless copper plating process. The reduction process is not possible with every complexing agent. The complexing agent EDDS forms with copper a complex that can be easily reduced. The deposition qualities (coverage, backlight) are very good at a ratio glyoxylic acid : EDDS of below 4.6:1 , particularly at 3.6 : 1 or below.

6. Example: Experiments with thiocarboxylic acid Comparison of EDTA and EDDS with/without thiocarboxylic acid Thioglycolic acid is used as thiocarboxylic acid in the experiments.

Operation temperature: 55 'Ό

Test results

Thioglycolic acid leads to a dark color of the electroless copper deposition. The deposition rate decreases with using thioglycolic acid in the electroless copper bath.

7. Example: Copper depositions on glass substrates, using copper baths with different complexing agents

Samples: Glass with sputtered Ti/Cu Seed

Pre-Treatment:

Temperature/

Chemistry °C Dwell Time

Alkaline

Cleaner 40 °C 1 min

Rinse RT not defined

Acid Pre Dip RT 20 sec

Ionic Pd

Activator RT 2 min

Rinse RT not defined

The results for obtained copper thickness for different complexing agents are shown in Fig. 4.

Claims

Claims

An electroless aqueous copper plating solution, comprising

- a source of copper ions,

- a source of glyoxylic acid, as reducing agent, and

at least one polyamino disuccinic acid, or at least one polyamino monosuccinic acid, or a mixture of at least one polyamino disuccinic acid and at least one polyamino monosuccinic acid, as complexing agent,

wherein the molar ratio of the complexing agent to copper ions is in the range of 1 .1 :1 to 5:1 .

The electroless aqueous copper plating solution according to claim 1 , wherein the molar ratio of glyoxylic acid to complexing agent is <4.6 : 1 .

The electroless aqueous copper plating solution according to one of the preceding claims, wherein the molar ratio of the complexing agent to copper ions is in the range of 1 .5:1 to 5:1 .

The electroless aqueous copper plating solution according to one of the preceding claims, which does contain less than 0.01 ppm of a thiocarboxylic acid.

The electroless aqueous copper plating solution according to one of the preceding claims, wherein the complexing agent is at least one polyamino disuccinic acid.

The electroless aqueous copper plating solution according to one of the preceding claims, wherein the complexing agent is ethylenediamine-N,N'-disuccinic acid (EDDS).

The electroless aqueous copper plating solution according to one of the preceding claims, wherein the solution further comprises one or more of a stabilizing agent.

The electroless aqueous copper plating solution according to claim 7, wherein the stabilizing agent is selected from dipyridyls, phenanthroline, mercapto-benzothiazole, thio-urea or its derivatives, cyanides, thiocyanates, iodides, ethanolamines, mercapto-benzotriazole, Na₂S₂0₃, polymers, like polyacrylamides, polyacrylates, polyethylene glycols or polypropylene glycols or their co-polymers.

9. The electroless aqueous copper plating solution according to one of the preceding claims, wherein the solution further comprises a source of hydroxide ions.

10. The electroless aqueous copper plating solution according to one of the preceding claims, wherein the solution comprises, besides glyoxylic acid, a second reducing agent.

1 1 . The electroless aqueous copper plating solution according to claim 10, wherein the second reducing agent is selected from the group of hypophosphoric acid, glycolic acid, formic acid, and a salt of these acids.

12. A method for electroless copper plating, the process comprising contacting a

substrate with an electroless aqueous copper plating solution according to one of claims 1 -1 1 .

13. The method according to claim 12, which is carried out at a temperature in the range of 20 - 60°C.

14. The use of an electroless aqueous copper plating solution according to one of claims 1 -1 1 for the plating of printed circuit boards, integrated circuit substrates, wafers, molded interconnect devices, displays or plastic parts.

15. The use of an electroless aqueous copper plating solution according to one of claims 1 -1 1 for the plating of glass substrates, particularly glass substrates for displays.