EP3839092A1

EP3839092A1 - Method for activating at least one surface, activation composition, and use of the activation composition to activate a surface for electroless plating

Info

Publication number: EP3839092A1
Application number: EP19219076.7A
Authority: EP
Inventors: Sebastian ZARWELL; Bexy Dosse; Michael MERSCHKY; Lutz Stamp; Regina Czezcka
Original assignee: Atotech Deutschland GmbH and Co KG
Current assignee: Atotech Deutschland GmbH and Co KG
Priority date: 2019-12-20
Filing date: 2019-12-20
Publication date: 2021-06-23

Abstract

The present invention is directed to a method for activating at least one surface of a substrate, the method comprising the steps
a) providing the substrate having the at least one surface;
b) optionally, pretreating the at least one surface of the substrate with a pretreating composition;
c) optionally, conditioning the at least one surface of the substrate with a conditioning composition;
d) contacting the at least one surface of the substrate with an activation composition comprising
i) copper ions; and
ii) at least one unsubstituted or substituted N-heteroaromatic compound;

e) contacting the at least one surface of the substrate obtained after step d) with a reducing agent containing composition such that copper ions are reduced to metallic copper on said surface
and thereby activating the at least one surface of the substrate.

Description

Field of the Invention

The present invention concerns a method for activating at least one surface of a substrate. In a further aspect, the present invention is directed at an activation composition for activating at least one surface of a substrate. In another aspect, the present invention relates to a use of an activation composition for activating at least one surface of a substrate for subsequent electroless plating.

Background of the Invention

The wet-chemical deposition of metal or metal alloy layers onto surfaces has a long tradition in the art. This wet-chemical deposition can be achieved by means of electrolytic or electroless plating of metals. Electroless plating is the controlled autocatalytic deposition of a continuous film of metal without the assistance of an external supply of electrons. Contrary to that, electrolytic plating requires such an external supply of electrons. These methods are of high importance in the electronics industry and, among other applications, are used in the manufacturing of printed circuit boards, semiconductor devices and similar goods.
In particular for electroless plating, the surfaces to be plated require a certain catalytic activity to allow a uniform and homogeneous metal deposition thereon. While metallic surfaces in many cases are catalytic enough for electroless plating, non-metallic surfaces are traditionally treated with palladium to make them receptive or catalytically active for subsequent metal deposition. This treatment is referred to as activation.
Palladium is however very expensive and only available in limited quantities. Electrical shorts are another issue known from palladium-based activation methods. These shorts are often caused by involuntary dissolution and re-deposition of palladium during subsequent steps such as during electroless plating. These shorts are highly undesired as they result in scrap production as the entire good becomes dysfunctional.
For these reasons, there exist various attempts in the prior art to substitute palladium for other metals such as silver and in particular copper. The latter is of paramount interest as it is widely used in the electronics industry for the build-up of conductive structures, is available in sufficient amounts, highly conductive and very cost-effective.
Although, there are in the art various method to substitute palladium as metal in the activation such as using copper nanoparticles and certain copper salts such as Cu₂I₂ for surface activation, the methods of the prior art lack in practicability, reliability and stability. Many compositions of the prior art using metals different from palladium are of poor stability requiring frequent replacements of the employed compositions. This, however, is unacceptable both in terms of economic and ecological points of view. Another requirement the prior art solutions rarely meet is a sufficiently strong activation. Typically, the activation of surfaces remains insufficient in terms of homogeneity and/or efficiency. Thus, when the substrates are treated after activation with an electroless metal plating bath, the metal or metal alloy coverage remains incomplete and thus unsatisfactory.

Objective of the present Invention

It is therefore the objective of the present invention to overcome the shortcomings of the prior art.
It is another objective of the present invention to provide a method for activating at least one surface of a substrate which does not require the use of expensive noble metals such as palladium and silver.
It is still another objective of the present invention to provide a method for activating at least one surface of a substrate and allow for an improved metal or metal alloy coverage.
It is a further objective of the present invention to provide an activation composition having a sufficient stability (i.e. in particular a sufficient lifetime).

Summary of the Invention

Above objectives are solved by the inventive method for activating at least one surface of a substrate, the method comprising the steps

a) providing the substrate having the at least one surface;
b) optionally, pretreating the at least one surface of the substrate with a pretreating composition;
c) optionally, conditioning the at least one surface of the substrate (preferably obtained after step a) and/or step b)) with a conditioning composition;
d) contacting the at least one surface of the substrate (preferably obtained after step b) and/or step c)) with an activation composition comprising
1. i) copper ions; and
2. ii) at least one unsubstituted or substituted N-heteroaromatic compound;
e) contacting the at least one surface of the substrate obtained after step d) with a reducing agent containing composition such that copper ions are reduced to metallic copper on said surface

Preferred embodiments of the present invention are outlined in the dependent claims and in the specification hereinafter. The named steps are preferably carried out in the given order but more preferably not necessarily in immediate succession. In some cases a method of the present invention is preferred, wherein the order of steps b) and c) is reversed.
In addition to steps b) and c), further steps are optionally included in between the named steps.
In some cases a method of the present invention is preferred, wherein the at least one surface obtained after step d) is contacted with the conditioning composition, preferably prior to step e). In some cases it is preferred that this is the only contacting with the conditioning composition. In other cases it is preferred that the contacting with the conditioning composition is carried out prior to step d) and after step d) (preferably prior to step e)).
Own experiments have shown that a surface of a substrate activated with the method of the present invention is excellently and homogeneously covered with a metal or metal alloy layer in a subsequent electroless metallization step carried out after step e). Said electroless metallization is comparatively fast and an increased metal or metal alloy layer coverage is obtained compared to prior art methods.
Advantageously, no palladium ions or silver ions are required for the purpose of activating the at least one surface of the substrate. Thus, a significant cost advantage can be achieved. Preferred is a method of the present invention, wherein the activation composition is substantially free of, preferably does not comprise, palladium ions and/or palladium compounds. Preferred is a method of the present invention, wherein the activation composition is substantially free of, preferably does not comprise, silver ions and/or silver compounds.
It is a further advantage of the present invention that no palladium incurred shorts can occur because there is no need to utilize palladium.
It is another advantage of the present invention that the surface obtained after step e) provides an improved stability. This means that contacting the activated surface obtained after step e) with a subsequently used bath for metal or metal alloy plating, in particular for electroless copper plating, does not significantly contaminates the subsequently used plating bath. This in turn enhances the lifetime of subsequently used plating baths.
It is still another advantage of the present invention that also very smooth substrate surfaces can be sufficiently activated.

Detailed Description of the Invention

Activating a surface of a substrate means in the context of the present invention that said surface is rendered suitable, i.e. is being receptive, in particular for electroless metal or metal alloy deposition. Without limitation, the inventors believe that the method of the present invention results in a catalytically active surface ready for subsequent electroless plating.
The "copper coverage" is a possibility of assessing the effectiveness of a method for activating. After activation, an electroless metal plating step is performed on the activated surface and the surface coverage with the deposited metal/metal alloy layer is measured and compared to other activated surfaces with a deposited metal/metal layer thereon.
In the context of the present invention, the term "alkyl group" comprises branched or unbranched alkyl groups, each comprising cyclic and/or non-cyclic structural elements, wherein cyclic structural elements of the alkyl groups naturally require at least three carbon atoms. C1-CX-alkyl group denotes alkyl groups having 1 to X carbon atoms (X being an integer). C1-C8-alkyl group for example includes, among others, methyl, ethyl, n-propyl, iso-propyl, n-butyl, iso-butyl, sec-butyl, tert-butyl, n-pentyl, iso-pentyl, sec-pentyl, tert-pentyl, neo-pentyl, n-hexyl, n-heptyl, and n-octyl. Substituted alkyl groups are typically obtained by replacing at least one hydrogen by a substituent. X is preferably 12, more preferably 10, even more preferably 8, most preferably 6, if not stated otherwise. Unless stated otherwise, a substituted or unsubstituted alkyl group is preferably selected from a substituted or unsubstituted C1-C8-alkyl group, more preferably from a substituted or unsubstituted C1-C4-alkyl group. Such alkyl groups typically have an increased water-solubility. Preferably, above described groups are substituted with one or more than one substituent or unsubstituted. Unless stated otherwise in this specification, preferably the substituents are independently selected from the group consisting of hydroxy group (-OH) and carboxyl group (-CO₂H) including its salts. Such substituents typically significantly increase water-solubility.
If more than one group is to be selected from a given group, each of the groups is selected independently from each other unless stated otherwise hereinafter, meaning they can be selected to be the same members or different members of said group.
It is understood that embodiments of the present invention described in this specification can be combined unless this is technically not feasible or specifically excluded. Preferred embodiments thus described for one aspect of the present invention are applicable mutatis mutandis to all the other aspects of the present invention unless stated otherwise herein. Where appropriate in a chemical formulae given in this specification, bonding sites are highlighted as asterisks.
Generally, in order to pretreat, condition, and activate, respectively, the at least one surface of the substrate, said surface is contacted with the respective composition, preferably by entirely or at least partially immersing said surface into the respective composition. Alternatively, said surface is sprayed or wiped.
In step a) of the method of the present invention the substrate having the at least one surface is provided. The at least one surface is preferably a non-conductive surface. The beneficial effects of the present invention are particularly pronounced when using such a surface. Preferred is a method of the present invention characterized in that the at least one surface is or comprises a non-metallic surface, preferably is or comprises an organic polymer and/or an oxide.
A preferred oxide is silica (i.e. silicon dioxide).
A preferred organic polymer is selected from the group consisting of plastic and silicones. Preferred silicones are polysiloxanes.
Preferably, said substrate is or comprises a material selected from the group consisting of glass, plastic, silicon, ceramic, and combinations of the aforementioned.
Glass preferably comprises one or more of silica glass (i.e. comprising amorphous silicon dioxide), soda-lime glass, float glass, fluoride glass (also referred to as fluorinated glass), aluminosilicates, phosphate glass, borate glass, quartz glass, borosilicate glass, chalcogenide glass, and aluminum oxide.
Plastic preferably comprises one or more of epoxy-based resins, acrylonitrile-butadiene-styrene copolymer (ABS copolymer), polyamide (PA), polycarbonate (PC), polyimide (PI), polyethylene terephthalate (PET), liquid-crystal polymers (LCPs), and cyclic olefin copolymer (COC). More preferably, plastic comprises one or more of epoxy-based resins, polyimide (PI), liquid-crystal polymers (LCPs), cyclic olefin copolymer (COC), and polyethylene terephthalate (PET).
A preferred epoxy-based resin is FR4.
Silicon preferably comprises one or more of polysilicon (preferably comprising doped polysilicon, most preferably comprising one or more of p-doped polysilicon and n-doped polysilicon), monocrystalline silicon, silica, silicon nitride, and silicon oxynitride.
Ceramics preferably comprise one or more of oxide-based ceramics, non-oxide based ceramics, and mixed oxide/non-oxide ceramics.
Preferred oxide-based ceramics comprise one or more of aluminum oxide, beryllium oxide, magnesium oxide, cerium oxides, yttrium oxides, titanium oxide, zirconium oxide, aluminum titanium oxide, and barium titanium oxide.
Preferred non-oxide based ceramics comprise one or more carbide (preferably silicon carbide), boride, nitride (preferably boron nitride and aluminum nitride), and silicides.
Preferred mixed oxide/non-oxide ceramics comprise one or more of silicon oxide nitride and silicon aluminum oxide nitride.
In some cases a method of the present invention is preferred, wherein the substrate is selected from the group consisting of printed circuit boards, printed circuit foils, interposers, chip carriers, IC substrates, semiconductor wafers, circuit carriers, interconnect devices, copper clad laminates, and build-up films.
In optional step b) of the method of the present invention, the at least one surface of the substrate is pretreated with a pretreating composition. Preferably, the pretreating composition is aqueous. In some cases it is preferred that in step b) more than one pretreating composition is applied (i.e. pretreating the at least one surface of the substrate with one or more than one pretreating composition) each having a different purpose (e.g. desmearing, cleaning, etching, reducing).
Step b) is preferably carried out between steps a) and d) and is herein referred to as "pretreatment step". Preferably, the one or more than one pretreating composition is individually acidic or alkaline.
Preferably, the one or more than one pretreating composition individually comprises one or more of an oxidizing agent, a surfactant, a reducing agent.
A preferred oxidizing agent is hydrogen peroxide.
A preferred reducing agent is NaBH₄.
In some cases a method of the present invention is preferred, wherein between all method steps of the method of the present invention a pretreatment step is carried out.
In optional step c) of the method of the present invention, the at least one surface of the substrate is conditioned with a conditioning composition. Preferably, the conditioning composition is an aqueous conditioning composition .
Step c) is preferably included in the method of the present invention after step a) and before step d). If optional step b) is also included, it is preferred to carry out optional step c) after step b). Step c) advantageously results in an improved metal or metal alloy coverage. Step c) is herein referred to as "conditioning step".
Preferably, the conditioning composition comprises one or more of a nitrogen containing polymer, preferably a nitrogen containing polymer having a high positive surface charge. A preferred nitrogen containing polymer is selected from polyquaterniums. Most preferably, the nitrogen containing polymer is different from the at least one unsubstituted or substituted N-heteroaromatic compound utilized in the activation composition.
Preferred is a method of the present invention, wherein the nitrogen containing polymer is selected from a group consisting of polyethylenimines and nitrogen containing polymers comprising one or more cationic building block. Preferred polymers comprising one or more cationic building block are selected from the group consisting of nitrogen containing polymers comprising one or more quaternized amines, preferably selected from the group consisting of polyimidazole, polydial-lyldimethylammonium, and polyepichlorhydrin-dimethylamine and mixtures of the aforementioned.
The one or more cationic building block increases the positive surface charge of the respective polymers. Preferred is a nitrogen containing polymer having a MW of 1000 g/mol or more, preferably determined by gel permeation chromatography (GPC).
Preferably, the nitrogen containing polymer has a total concentration in a range from 0.01 wt.-% to 15 wt.-%, based on the total weight of the conditioning composition, more preferably from 0.1 wt.- % to 5 wt.-%, and even more preferably from 0.3 wt.-% to 2 wt.-%.
Preferably, step c) is carried out at a temperature ranging from 25°C to 80°C, more preferably from 40°C to 75°C, and even more preferably from 45°C to 70 °C.
Preferably, step c) is carried out for a time period ranging from 10 sec to 15 min, preferably from 30 sec to 10 min, and more preferably from 1 min to 5 min.
Preferably, the pH of the conditioning composition is alkaline, preferably being in a range from 7.5 to 12, more preferably from 8 to 11, and even more preferably from 8.5 to 10.
In step d) of the method of the present invention, the at least one surface of the substrate is contacted with an activation composition (i.e. the activation composition of the present invention). This step is herein referred to as "activation step".
Preferably, in step d) the activation composition has a temperature in a range from 25°C to 70 °C, more preferably from 35°C to 60 °C, and even more preferably from 40°C to 60 °C.
Preferably, step d) is carried out for a time period ranging from 30 sec to 25 min, preferably from 1 min to 15 min, and more preferably from 2 min to 10 min. Typically, shorter time periods were found not to be sufficient for activation, while longer time periods did not improve further the activation.
In step d), copper ions are placed on the at least one surface of the substrate. Thus, the activation composition comprises copper ions, preferably by utilizing at least one water-soluble copper salt or copper complex. Preferred water-soluble copper salts and copper complexes, respectively, are selected from the group consisting of copper sulfate, copper alkyl sulfonates (preferably copper methane sulfonate), copper aryl sulfonates (preferably copper p-toluene sulfonate and copper phenyl sulfonate), copper halides (preferably copper chloride), copper carboxylates (preferably copper acetate and copper citrate), copper fluoroborate, copper oxide, copper carbonate, hydrates and mixtures of the aforementioned. More preferably the water-soluble copper salts and copper complexes are selected from the group consisting of copper acetate, copper sulfate, copper alkyl sulfonates (preferably as defined above), and mixtures of the aforementioned.
Preferably, in the activation composition the copper ions have a concentration ranging from 0.1 g/L to 30 g/L, more preferably from 0.5 g/L to 10 g/L, and even more preferably from 2 g/L to 7 g/L.
The activation composition comprises at least one unsubstituted or substituted N-heteroaromatic compound. In the context of the present invention, "N-heteroaromatic compound" denotes a compound comprising at least one aromatic ring with at least one ring nitrogen atom (i.e. at least one ring carbon atom is substituted with a nitrogen atom).
Preferred is a method of the present invention characterized in that the at least one unsubstituted or substituted N-heteroaromatic compound is a monocyclic compound.
Preferred is a method of the present invention characterized in that the at least one unsubstituted or substituted N-heteroaromatic compound is selected from the group consisting of unsubstituted five-membered N-heteroaromatic compounds, substituted five-membered N-heteroaromatic compounds, unsubstituted six-membered N-heteroaromatic compounds, and substituted six-membered N-heteroaromatic compounds.
Preferred is a method of the present invention characterized in that the at least one unsubstituted or substituted N-heteroaromatic compound is selected from the group consisting of unsubstituted and substituted pyrroles, unsubstituted and substituted pyrazoles, unsubstituted and substituted imidazoles, unsubstituted and substituted 1,2,3-triazoles, unsubstituted and substituted 1,2,4-triazoles, unsubstituted and substituted tetrazoles, unsubstituted and substituted oxazoles, unsubstituted and substituted isoxazoles, unsubstituted and substituted isothiazoles, unsubstituted and substituted thiazoles, unsubstituted and substituted 1,2,3-oxadiazoles, unsubstituted and substituted 1,2,5-oxadiazoles, unsubstituted and substituted 1,3,4-thiadiazoles, unsubstituted and substituted 1,2,5-thiadiazoles, unsubstituted and substituted pyridines, unsubstituted and substituted pyridazines, unsubstituted and substituted pyrimidines, unsubstituted and substituted pyrazines, unsubstituted and substituted 1,2,4-triazines, unsubstituted and substituted 1,3,5-triazines, unsubstituted and substituted 1,2,3,4-tetrazines, unsubstituted and substituted 1,2,3,5-tetrazines, and unsubstituted and substituted 1,2,4,5-tetrazines.
More preferred is a method of the present invention characterized in that the at least one unsubstituted or substituted N-heteroaromatic compound is selected from the group consisting of unsubstituted and substituted pyrroles, unsubstituted and substituted pyrazoles, unsubstituted and substituted imidazoles, unsubstituted and substituted oxazoles, unsubstituted and substituted isoxazoles, unsubstituted and substituted isothiazoles, unsubstituted and substituted thiazoles, unsubstituted and substituted pyridines, unsubstituted and substituted pyridazines, unsubstituted and substituted pyrimidines, and unsubstituted and substituted pyrazines.
Preferred is a method of the present invention characterized in that the at least one unsubstituted or substituted N-heteroaromatic compound is represented by the following formula (A):
wherein

X is selected from the group consisting of -N-, -N(R¹)-, -O-, -S-, and -C(R¹)-;
each R is independently selected from the group consisting of hydrogen, unsubstituted and substituted alkyl group, unsubstituted and substituted amidoxime group, hydroxamate group, hydroxy group, amino group, carboxyl group, oxycarbonyl group, carbamoyl group, formyl group, cyano group and isocyano group;
R¹ is selected from the group consisting of hydrogen, unsubstituted and substituted alkyl group;
b is 1, 2, 3 or 4; and
c is 0, 1 or 2;
with the proviso that the sum of b and c is 3 or 4.

X is selected as defined above in view of the bonding requirements present in the at least one unsubstituted or substituted N-heteroaromatic compound. If the sum of b and c is 4, preferably, X is not - N(R¹)-. Thus, if the sum of b and c is 4, X is preferably selected from the group consisting of -N-, - O-, -S-, and -C(R¹)-.
If the sum of b and c is 3, preferably, X is not -N-. Thus, if the sum of b and c is 3, X is preferably selected from the group consisting of -N(R¹)-, -O-, -S-, and -C(R¹)-. Also, where applicable, X may be bonded by single and/or double bonds depending on the necessities to fulfill the underlying Hückel requirement.
Preferred is a method of the present invention, wherein at least one R is other than hydrogen. Preferably, the R being other than hydrogen is bonded to a carbon atom adjacent to the nitrogen atom in the N-heteroaromatic compound (i.e. adjacent to the N-atom in the ring).
Preferred is a method of the present invention, wherein, each R is independently selected from the group consisting of hydrogen, unsubstituted and substituted alkyl group, hydroxy group, amino group, carboxyl group, oxycarbonyl group, and carbamoyl group, more preferably substituted alkyl group and carbamoyl group.
A preferred substituted alkyl group independently comprises an alkyl group substituted with one or more than one hydroxy group, an alkyl group substituted with one or more than one amino group, and/or an alkyl group substituted with one or more than one carboxyl group.
Preferably, the unsubstituted and substituted amidoxime group is represented as:
, wherein R^A1 and R^A2 are independently selected from the group consisting of hydrogen and alkyl group. A substituted amidoxime group preferably comprises at least one of R^A1 and R^A2 not being hydrogen. However, an unsubstituted amidoxime group preferably comprises R^A1 and R^A2 being hydrogen.
Preferred is a method of the present invention characterized in that the at least one unsubstituted or substituted N-heteroaromatic compound as defined in formula (A), X is -C(R¹)- and the sum of b and c is 4. Thus, the unsubstituted or substituted N-heteroaromatic compound is a six-membered unsubstituted or substituted N-heteroaromatic compound.
In some cases a method of the present invention is preferred, characterized in that the substituted N-heteroaromatic compound comprises at least a compound represented by the following formula (E):
wherein

R^E1 and R^E2 are independently selected from the group consisting of hydrogen, substituted and unsubstituted alkyl group, carboxyl group, and carbamoyl group
with the proviso that at least one of R^E1 and R^E2 is other than hydrogen.

Preferably, R^E1 and R^E2 are independently selected from the group consisting of hydrogen, alkyl group substituted with one or more (preferably one) hydroxy group, carboxyl group, and carbamoyl group (preferably as defined throughout the text). A preferred alkyl group substituted with one or more hydroxy group is a methylol group (-CH₂OH) and an ethylol group (-CH₂-CH₂OH).
The substituted N-heteroaromatic compound represented by formula (E) was found to be particularly effective (even compared to the compound represented by formula (L); see text below) in the method of the present invention, in particular by enhancing the copper coverage (see also examples below).
In some other cases a method of the present invention is preferred, wherein the sum of b and c is 3. In such cases a substituted five-membered N-heteroaromatic compound is very preferred. Thus, preferred is a method of the present invention characterized in that the substituted N-heteroaromatic compound comprises at least a compound represented by the following formula (L):
wherein
R^L is an alkyl group independently substituted with one or more than one substituent selected from the group consisting of amino group and carboxyl group.
Preferably, R^L is a C1-C4-alkyl group independently substituted with one or more than one substituent selected from the group consisting of amino group and carboxyl group.
Particularly preferred is a method of the present invention characterized in that the substituted N-heteroaromatic compound comprises one or more than one compound selected from the group consisting of pyridinedimethanol, picolinamide, picolinic acid, nicotinic acid, nicotineamide, histidine, and salts of the aforementioned,
preferably selected from the group consisting of 2,6-pyridinedimethanol, 2-picolinamide, 2-picolinic acid, nicotinic acid, nicotineamide, histidine, and salts of the aforementioned, most preferably selected from the group consisting of 2,6-pyridinedimethanol, 2-picolinamide, 2-picolinic acid, and salts of the aforementioned.
Preferred salts are alkaline salts, earth alkaline salts, and ammonium salts.
Preferred is a method of the present invention, wherein the at least one unsubstituted or substituted N-heteroaromatic compound (in its total amount including more than one compound, substituted and unsubstituted) and the copper ions are present in the activation composition in a molar ratio ranging from 0.1:1 to 3:1, preferably from 0.3:1 to 1.5:1, and more preferably from 0.5:1 to 1:1.
Preferred is a method of the present invention characterized in that the molar ratio of the copper ions to all of the N-heteroaromatic compounds in the activation composition ranges from 0.5eq to 10eq, preferably from 0.75eq to 5eq, more preferably from 1eq to 3eq, and even more preferably from 1eq to 2eq.
In some cases it is preferred that the activation composition comprise further compounds.
Preferred is a method of the present invention characterized in that the activation composition comprises
iii) at least one enhancer compound different from ii) selected from the group consisting of sulfuric acid, sulfonic acids, carboxylic acids, polyols, salts and mixtures thereof.
Preferred salts are alkaline salts, earth alkaline salts, and ammonium salts. Although the activation composition utilized in the method of the present invention provides an acceptable stability, stability is preferably further increased by utilizing said at least one enhancer compound.
Preferably, sulfuric acid is added as a respective salt, i.e. as a sulfate salt. Preferred salts comprise alkaline sulfates, earth alkaline sulfates, and sulfates of main group III metals of the period table of the elements (preferably aluminum sulfate).
Preferred sulfonic acids are selected from the group consisting of alkylsulfonic acids (preferably methanesulfonic acid and ethanesulfonic acid) and arylsulfonic acids (preferably phenylsulfonic acid, p-toluenesulfonic acid, and 5-sulfosalicylic acid (also known as 2-hydroxy-5-sulfobenzoic acid)). Very preferred sulfonic acids are arylsulfonic acid.
Preferred carboxylic acids are selected from the group consisting of aliphatic hydroxycarboxylic acids (preferably citric acid, tartaric acid, and malic acid), aliphatic aminocarboxylic acids (preferably glycine and alanine), and unsubstituted aliphatic carboxylic acids (preferably acetic acid and malonic acid). Preferred carboxylic acids are aliphatic hydroxycarboxylic acids.
Preferred polyols (i.e. having at least two hydroxy groups) are selected from the group consisting of glycols, sugar alcohols, and polyvinyl alcohols. Preferably, the polyols are aliphatic, even more preferably non-cyclic. The polyols are preferably selected from the group consisting of C2-C10-polyols, more preferably from C2-C6-polyols. Very preferred polyols are selected from the group consisting of ethane-1,2-diol (ethylene glycol), propane-1,2-diol (propylene glycol), propane-1,2,3-triol (glycerol) and (2S,3R,4R,5R)-hexane-1,2,3,4,5,6-hexol (sorbitol).
Most preferably, the at least one enhancer compound different from ii) is selected from the group consisting of sulfuric acid, sulfonic acids, polyols, salts and mixtures thereof; even more preferably sulfonic acids and salts thereof, and yet even more preferably the at least one enhancer compound different from ii) is an arylsulfonic acid or a salt thereof, most preferably is sulfosalicylic acid or a salt thereof.
In the activation composition, the at least one enhancer compound different from ii) (in its total concentration) and the copper ions are preferably present in a molar ratio ranging from 0.05:1 to 5:1, more preferably from 0.2:1 to 2:1, and even more preferably from 0.5:1 to 1.5:1.
Own experiments have shown that the at least one enhancer compound does not only improves the stability of the activation composition but furthermore allows for improved metal or metal alloy coverage and furthermore synergistically works together with the at least one unsubstituted or substituted N-heteroaromatic compound (see examples below). In contrast, own experiments show that the enhancer compound alone (i.e. without the at least one unsubstituted or substituted N-heteroaromatic compound) does not allow for the benefits of the present invention to be obtained (see examples below).
Preferred is a method of the present invention, wherein the activation composition is substantially free of, preferably does not comprise, palladium ions.
"Substantially free of' preferably means a concentration of 50 mg/L or less, more preferably of 10 mg/L or less, even more preferably 1 mg/L or less.
Preferred is a method of the present invention, wherein the activation composition is substantially free of, preferably does not comprise, silver ions.
Very preferred is a method of the present invention, wherein the activation composition is a clear solution (i.e. without particles).
Preferred is a method of the present invention, wherein the activation composition is substantially free of, preferably does not comprise, surfactants. Own experiments showed that surfactants in some cases lowered the adhesion of subsequently deposited metal or metal alloy layers on the at least one activated surface.
Generally, the pH of the activation composition is not particularly limited and is preferably adapted in such a way that the substrate having the at least one surface is not damaged in step d) of the method of the present invention. Preferred is a method of the present invention, wherein the pH is at least 7, preferably the pH ranges from 8 to 14, more preferably from 8.5 to 12, and even more preferably from 9 to 10. Said pH values are well suited for activating basically most of the substrates preferably used in electronics industry.
Preferably, the method of the present invention comprises further between step d) and step e), step
d1) rinsing the at least one surface of the substrate obtained after step d) with an alkaline aqueous rinsing composition.
Step d1) is herein referred to as "alkaline rinsing step".
Preferred is a method of the present invention, wherein the alkaline aqueous rinsing composition has a pH of 8 or more, preferably is in a range from 9 to 14, even more preferably from 10 to 13.5, and yet even more preferably from 11 to 13. Preferably, the pH is adjusted with one or more than one hydroxide, preferably one or more of NaOH, KOH, ammonia, and hydroxylamine.
Step d1), which is preferably optional, typically reduces the risk of drag-in of copper ions from the activation composition into a subsequently used compositions. Such copper ions may for example reduce the lifetime of a subsequently used compositions such as of a reducing agent containing composition used in step e).
In step e) of the method of the present invention, the at least one surface of the substrate obtained after step d) (or preferably obtained after step d1)) is contacted with a reducing agent containing composition in order to chemically reduce the copper ions to metallic copper. Preferably, the reducing agent containing composition is an aqueous reducing agent containing composition, most preferably without particles.
By carrying out step e), the at least one surface of the substrate is finally activated. Step e) is herein referred to as "reducing step". The reducing agent containing composition preferably comprises at least one reducing agent.
Preferably, the at least one reducing agent is selected from the group consisting of hypophosphoric acid, formaldehyde, paraformaldehyde, glyoxylic acid, sources of glyoxylic acid (preferably dihalo-acetic acid, dichloroacetic acid (which liberates glyoxylic acid in aqueous media)), amino boranes (preferably dimethylamino borane), borohydrides (preferably alkaline borohydrides, more preferably NaBH₄ and KBH₄), hydrazine, polysaccharides, sugars (preferably glucose), glycolic acid, formic acid, and salts and mixtures thereof. Preferred salts are ammonium, alkaline and earth alkaline salts.
More preferably, the at least one reducing agent is selected from the group consisting of boranes, preferably dimethylamino borane and borohydrides, preferably alkali borohydrides. Even more preferably, the at least one reducing agent is at least one borohydride, most preferably selected from the group consisting of sodium borohydride and potassium borohydride. The latter have been found to be particularly effective in the context of the present invention.
Preferably, the at least one reducing agent has a total concentration ranging from 0.05 g/L to 20 g/L, based on the total volume of the reducing agent containing composition, preferably from 0.1 g/L to 15 g/L, and more preferably from 0.3 g/L to 15 g/L. Generally, the concentration of the at least one reducing agent preferably depends on the reducing strength of the particular reducing agent. While strong reducing agents such as sodium borohydride may be used in basically lower concentrations, weaker reducing agents such as dimethylamino borane are typically used in comparatively higher concentrations. The person skilled in the art can determine useful concentrations for that purpose by routine experiments.
Preferred is a method of the present invention, wherein in step e) the reducing agent containing composition has a temperature in a range from 20°C to 60°C, more preferably from 30°C to 50 °C. The Preferred is a method of the present invention, wherein step e) is carried out for a time period ranging from 10 sec to 30 min, more preferably from 20 sec to 15 min.
As a result of step e), the at least one surface of the substrate is activated.
Preferably, the method of the present invention comprises further after step e), step
f) contacting the at least one surface of the substrate obtained after step e) with an oxidation preventing rinsing composition.
Step f) is optional, but additionally improves the metal or metal alloy coverage. Optional step f) is herein referred to as "anti-oxidative treatment step". The oxidation preventing rinsing composition is preferably aqueous, most preferably an aqueous solution.
The oxidation preventing rinsing composition preferably comprises at least one anti-oxidative compounds, preferably selected from the group consisting of ascorbic acid, thiosulfate, hydrazine, hy-drazinecarbonat, carbohydrazide, methyleneglycolate, Ti(III) ions, formaldehyde, and borohydrides.
Preferably, the at least one anti-oxidative compound is ascorbic acid and/or a salt thereof. A preferred salt is sodium ascorbate and potassium ascorbate.
Preferably, the at least one anti-oxidative compound has a concentration ranging from 1 mg/L to 10 g/L, based on the total volume of the oxidation preventing rinsing composition, more preferably from 100 mg/L to 5 g/L, and even more preferably from 500 mg/L to 2 g/L.
Preferably, in step f) the oxidation preventing rinsing composition has a temperature in a arrange from 10°C to 50°C, preferably from 15°C to 40°C, and more preferably from 20°C to 30°C. Preferably, step f) is carried out for a time period ranging from 5 sec to 5 min, more preferably from 20 sec to 1 min.
Preferred is a method of the present invention, wherein in step f) the oxidation preventing rinsing composition is kept an inert atmosphere while step f) is carried out, preferably under nitrogen and/or argon.
Preferably, in step f) the oxidation preventing rinsing composition has a pH in a range from
4 to 7, more preferably from 5 to 6.5, most preferably if the oxidation preventing rinsing composition is kept under an inert atmosphere,
or
7 to 13, more preferably from 8 to 12, most preferably if the oxidation preventing rinsing composition comprises formaldehyde.
Thus, general preferred is a pH ranging from 4 to 14.
Preferably, the method of the present invention comprises further after step e) or after step f), step
g) contacting the at least one surface of the substrate with at least one electroless metal or metal alloy plating bath.
As a result of step g), a metal or metal alloy layer is deposited on the activated at least one surface of the substrate.
A preferred metal or metal alloy, respectively, to be deposited in step g) is selected from the group consisting of copper, copper alloy, nickel, nickel alloy, cobalt and cobalt alloy, most preferred being coper and copper alloy. A copper and copper alloy is of paramount interest in the electronics industry and therefore particularly preferred in the context of the present invention. Suitable electroless metal or metal alloy plating baths are generally known in the art and commercially available. By adding optional (but preferred) step g), the method of the present invention for activating at least one surface of a substrate is preferably converted into a method for depositing a metal or metal alloy layer on at least one activated surface of a substrate. Step g) is herein referred to as "electroless plating step".
Very preferred is a method of the present invention comprising the steps:

a) providing the substrate having the at least one surface;
b) optionally, pretreating the at least one surface of the substrate with a pretreating composition;
c) optionally, conditioning the at least one surface of the substrate with a conditioning composition;
d) contacting the at least one surface of the substrate with an activation composition comprising
1. i) copper ions
2. ii) at least one unsubstituted or substituted N-heteroaromatic compound;
d1) optionally, rinsing the at least one surface of the substrate obtained after step d) with an alkaline aqueous rinsing composition;
e) contacting the at least one surface of the substrate obtained after step d) with a reducing agent containing composition such that copper ions are reduced to metallic copper on said surface (and thereby activating the at least one surface of the substrate);
f) optionally, contacting the at least one surface of the substrate obtained after step e) with an oxidation preventing rinsing composition;
g) optionally, contacting the at least one surface of the substrate with at least one electroless metal or metal alloy plating bath (and thereby depositing a metal or metal alloy layer on the at least one activated surface of the substrate).

The method of the present invention preferably comprises further steps, most preferably drying and/or rinsing steps.
The present invention also refers to an activation composition for activating at least one surface of a substrate, preferably an activation composition as defined above and utilized in the method of the present invention (most preferably as defined as being preferred). The activation composition is preferably for activating a non-metallic surface.
Thus, the aforementioned regarding the method of the present invention (including its preferred variants) preferably applies likewise to the activation composition of the present invention.
Very preferred is an activation composition of the present invention characterized in that the activation composition is an aqueous solution comprising:

i) copper ions;
ii) at least one compound selected from the group consisting of pyridinedimethanol, picolinamide, picolinic acid, nicotinic acid, nicotineamide, histidine, and salts of the aforementioned; and
iii) at least one enhancer compound different from ii) selected from the group consisting of sulfuric acid, sulfonic acids, carboxylic acids, polyols, salts and mixtures thereof.

In particular regarding this very preferred activation composition, the aforementioned regarding the method of the present invention most preferably applies likewise.
Own experiments have shown that in particular above very preferred activation composition of the present invention is very stable and can be used for a prolonged period of time for activating the at least one surface of the substrate for electroless metal or metal alloy plating (compared to many prior art solutions).
Preferably, the activation composition of the present invention is prepared by dissolving all components in a solvent, preferably water.
In a further aspect, the present invention relates to a use of an activation composition according to the present invention for activating at least one surface of a substrate for subsequent electroless plating.
The aforementioned regarding the method of the present invention (including its preferred variants) and regarding the activation composition of the present invention (including its preferred variants) preferably applies likewise to the use of the present invention.
In a further aspect, the present text relates to a kit-of-parts suitable to formulate the activation composition of the present invention, preferably utilized in the method of the present invention. The kit-of-parts of the present invention comprises at least the following individual parts:

A) an aqueous solution comprising copper ions; and
B) an aqueous solution comprising at least one unsubstituted or substituted N-heteroaromatic compound.

Optionally (but preferably), the kit-of parts comprises further individual part:
C) an aqueous solution comprising at least one enhancer compound different from ii) selected from the group consisting of sulfuric acid, sulfonic acids, carboxylic acids, polyols, salts and mixtures thereof.
Regarding each individual part, the aforementioned regarding the method of the present invention (including its preferred variants) preferably applies likewise.
Alternatively, individual part C) is comprised in part B) (i.e. in the aqueous solution comprising at least one unsubstituted or substituted N-heteroaromatic compound).
The kit-of-parts as defined above easily facilitates the preparation of the activation composition of the present invention as the individual parts can be mixed easily and, if necessary, diluted further with a solvent, preferably water. Additional components as described throughout the text for the activation composition are preferably added as further individual parts (of the kit-of-parts, preferably as aqueous solutions thereof) or already included in one or more of parts A), B), and C). When using the kit-of-parts, no solids have to be handled which is advantageous from environmental and occupational health and safety aspects.

Industrial Applicability

The present invention is particularly well suited to be used in the electronics and semiconductor industry, for example in the manufacturing of printed circuit boards, IC substrates and the like.
The invention will now be illustrated by reference to the following non-limiting examples.

Examples

Commercial products were used as described in the technical datasheets available on the date of filing of this specification unless stated otherwise hereinafter.

Copper coverage measurement after activation:

The copper coverage of the activated surface was evaluated by measuring the copper concentration in the activation composition before and after the activation was carried out. The determined concentration difference corresponds to the amount of metallic copper formed on the surface.

Backlight test: copper or copper alloy coverage of surfaces in recessed structures

After activation and subsequent deposition of a copper or copper alloy layer, the coverage of such a layer in recessed structures is typically evaluated using an industry standard Backlight Test. For that, the substrate is sectioned, so as to allow areas of incomplete coverage to be detected as bright spots when viewed over a strong light source [see for further information US 2008/0038450 A1 , incorporated herein by reference in its entirety]. The coverage of the copper or copper alloy layer is correlated to the amount of light that is observed under a conventional optical microscope.
The results of the backlight test are given on a scale from D1 to D10, wherein D1 means the worst result (very low coverage) and D10 the best result (high coverage). Reference samples showing results from D1 to D10 are shown in Fig. 3 of WO 2013/050332 A1 .

A) Activating at least one surface of silica substrates

For the following examples, the substrates were amorphous silica substrates.
The following aqueous activation compositions were prepared by dissolving in deionized water (for further details see Table 1):

i) copper acetate; and
ii) at least one substituted N-heteroaromatic compound.

If needed, each activation composition was adjusted to the pH value given in Table 1.

Then, the substrates were subjected to the respective activation composition and subsequently to a reducing agent containing composition (see B) below) to activate them.

Table 1: Activation compositions and copper coverage

#	N-heteroaromatic compound	c (N-heteroaromatic compound) [mmol/L]	c (Cu²⁺) [mmol/L]	pH	increase in copper coverage relative to #a
a	none (comparative)	0	1.8	3-4	0
b	histidine	0.96	0.9	10.7	81.7%
c	2,6-pyridinedimethanol	1.91	1.8	11.7	98.6%
d	2,6-pyridinedimethanol	2.40	1.8	13.4	97.0%
e	2,6-pyridinedimethanol	1.80	0.9	10.9	95.4%
f	2-picolinic acid	1.91	1.8	11.0	87.7%
g	2-picolinic acid	1.20	0.9	11.1	92.8%
h	2-picolinamide	1.20	0.9	7.8	93.5%
i	2-picolinamide	1.91	1.8	11.7	93.4%
j	2-picolinamide	3.60	1.8	12.5	84.4%

Obviously, the copper coverage increased significantly when the activation composition comprised a substituted N-heteroaromatic compound. The copper coverage was increased by more than 80% compared to comparative example #a, which did not use any N-heteroaromatic compound in the activation composition. Interestingly, the N-heteroaromatic compounds utilized in #c to #j showed an even improved copper coverage compared to #b.

As further comparative examples, activation compositions containing additives being different from ii) (i.e. being not an unsubstituted or substituted N-heteroaromatic compound as defined for the present invention), were tested on silica substrates too. The results are provided in Table 2:

Table 2: Activation compositions (comparative) and copper coverage

#	additive	c (additive) [mmol/L]	c (Cu²⁺) [mmol/L]	pH	change in copper coverage relative to #a
a	none	0	1.8	3-4	0
k	citric acid	0.96	0.9	11.0	-2.0%
1	citric acid	1.91	1.8	11.7	-12.4%
m	citric acid	1.20	0.9	11.4	0.2%
n	1,2-diaminocyclohexane				*

* No copper deposition on the substrates was detectable but significant reduction of copper ions in the activation composition was observed due to copper precipitation.

Comparative examples #k to #n did not show any increased copper coverage compared to comparative example #a, and are therefore ineffective.

B) Activating at least one surface of FR-4 and ABS (acrylobutadiene-styrene-copolymer) substrates

Respective aqueous activation compositions were prepared by dissolving in deionized water:

i) 3.8 g/L copper ions (added as copper sulfate);
ii) 3 g/L picolinic acid; and
iii) 17 g/L sulfosalicylic acid

Each activation composition was adjusted to a pH value of 10.
Then, FR-4 substrates (Isola IS 410) and ABS substrates, respectively, were subjected to the following steps to activate them:
First, after providing the respective substrates, they were pretreated with a pretreating composition (Securiganth Cleaner 902, Atotech) at neutral pH in order to clean the at least one surface.
Second, after rinsing with water, an etch cleaning as additional pretreating was carried out (Securiganth etch cleaner SPS, Atotech) with subsequent rinsing.
Third, the obtained substrates were conditioned with a conditioning composition (Nano cleaner V, Atotech) with subsequent rinsing.
Fourth, the pretreated and conditioned substrates were contacted with the respective activation composition at 50°C for 5 minutes. Subsequently, the substrates were rinsed with an alkaline solution under an inert atmosphere.
Fifth, the obtained substrates were contacted with a reducing agent containing composition (Neoganth Reducer WA, Atotech) at 35°C for 20 seconds. Afterwards an anti-oxidative post-treatment composition was applied under an inert atmosphere.
Sixth, electroless copper plating was successfully carried out for approximately 10 minutes (Printoganth PV plus).
The substrates were fully covered with copper after the electroless copper plating step. The thickness of the copper layer on the surface of the substrate was 1 µm on FR-4, and 0.6 µm on ABS, respectively, determined by measuring 5 points on each substrate with XRF using the XRF instrument Fischerscope XDV-SDD (Helmut Fischer GmbH, Germany).
The backlight test for FR-4 was D8. A comparative example basically carried out as described above except that a palladium comprising activation composition (Neoganth, Atotech) was used instead of the above described activation composition according to the invention resulted in a backlight test of D7.
It was further observed that the activation composition according to the present invention (i.e. comprising copper ions, at least one unsubstituted or substituted N-heteroaromatic compound, and an enhancer compound) was considerably more stable compared to activation compositions according to the present invention but without an enhancer compound. It was observed that the tendency of forming agglomerates and even precipitates is significantly reduced in the presence of the enhancer compound.

Claims

A method for activating at least one surface of a substrate, the method comprising the steps
a) providing the substrate having the at least one surface;

b) optionally, pretreating the at least one surface of the substrate with a pretreating composition;

c) optionally, conditioning the at least one surface of the substrate with a conditioning composition;

d) contacting the at least one surface of the substrate with an activation composition comprising
i) copper ions; and

ii) at least one unsubstituted or substituted N-heteroaromatic compound;

e) contacting the at least one surface of the substrate obtained after step d) with a reducing agent containing composition such that copper ions are reduced to metallic copper on said surface
and thereby activating the at least one surface of the substrate.
The method according to claim 1 characterized in that the at least one unsubstituted or substituted N-heteroaromatic compound is a monocyclic compound.
The method according to claim 1 or 2 characterized in that the at least one unsubstituted or substituted N-heteroaromatic compound is selected from the group consisting of unsubstituted five-membered N-heteroaromatic compounds, substituted five-membered N-heteroaromatic compounds, unsubstituted six-membered N-heteroaromatic compounds, and substituted six-membered N-heteroaromatic compounds.
The method according to any one of the preceding claims characterized in that the at least one unsubstituted or substituted N-heteroaromatic compound is selected from the group consisting of unsubstituted and substituted pyrroles, unsubstituted and substituted pyrazoles, unsubstituted and substituted imidazoles, unsubstituted and substituted 1,2,3-triazoles, unsubstituted and substituted 1,2,4-triazoles, unsubstituted and substituted tetrazoles, unsubstituted and substituted oxazoles, unsubstituted and substituted isoxazoles, unsubstituted and substituted isothiazoles, unsubstituted and substituted thiazoles, unsubstituted and substituted 1,2,3-oxadiazoles, unsubstituted and substituted 1,2,5-oxadiazoles, unsubstituted and substituted 1,3,4-thiadiazoles, unsubstituted and substituted 1,2,5-thiadiazoles, unsubstituted and substituted pyridines, unsubstituted and substituted pyridazines, unsubstituted and substituted pyrimidines, unsubstituted and substituted pyrazines, unsubstituted and substituted 1,2,4-triazines, unsubstituted and substituted 1,3,5-triazines, unsubstituted and substituted 1,2,3,4-tetrazines, unsubstituted and substituted 1,2,3,5-tetrazines, and unsubstituted and substituted 1,2,4,5-tetrazines.
The method according to any one of the preceding claims characterized in that the at least one unsubstituted or substituted N-heteroaromatic compound is represented by the following formula (A):
wherein
X is selected from the group consisting of -N-, -N(R¹)-, -O-, -S-, and -C(R¹)-;

each R is independently selected from the group consisting of hydrogen, unsubstituted and substituted alkyl group, unsubstituted and substituted amidoxime group, hydroxamate group, hydroxy group, amino group, carboxyl group, oxycarbonyl group, carbamoyl group, formyl group, cyano group and isocyano group;

R¹ is selected from the group consisting of hydrogen, unsubstituted and substituted alkyl group;

b is 1, 2, 3 or 4; and

c is 0, 1 or 2;
with the proviso that the sum of b and c is 3 or 4.
The method according to claim 5 characterized in that X is -C(R¹)- and the sum of b and c is 4.
The method according to claim 6 characterized in that the substituted N-heteroaromatic compound comprises at least a compound represented by the following formula (E):
wherein
R^E1 and R^E2 are independently selected from the group consisting of hydrogen, substituted and unsubstituted alkyl group, carboxyl group, and carbamoyl group
with the proviso that at least one of R^E1 and R^E2 is other than hydrogen.
The method according to claim 5 characterized in that the substituted N-heteroaromatic compound comprises at least a compound represented by the following formula (L):
wherein
R^L is an alkyl group independently substituted with one or more than one substituent selected from the group consisting of amino group and carboxyl group.
The method according to any one of claims 1 to 4 characterized in that the substituted N-heteroaromatic compound comprises one or more than one compound selected from the group consisting of pyridinedimethanol, picolinamide, picolinic acid, nicotinic acid, nicotineamide, histidine, and salts of the aforementioned,
preferably selected from the group consisting of 2,6-pyridinedimethanol, 2-picolinamide, 2-picolinic acid, nicotinic acid, nicotineamide, histidine, and salts of the aforementioned, most preferably selected from the group consisting of 2,6-pyridinedimethanol, 2-picolinamide, 2-picolinic acid, and salts of the aforementioned.
The method according to any one of the preceding claims characterized in that the activation composition comprises
iii) at least one enhancer compound different from ii) selected from the group consisting of sulfuric acid, sulfonic acids, carboxylic acids, polyols, salts and mixtures thereof.
The method according to any one of the preceding claims characterized in that the molar ratio of the copper ions to all of the N-heteroaromatic compounds in the activation composition ranges from 0.5eq to 10eq, preferably from 0.75eq to 5eq, more preferably from 1eq to 3eq, and even more preferably from 1eq to 2eq.
The method according to any one of the preceding claims characterized in that the at least one surface is or comprises a non-metallic surface, preferably is or comprises an organic polymer and/or an oxoide.
An activation composition as defined in any one of claims 1 to 12 for activating at least one surface of a substrate, preferably of a non-metallic surface.
The activation composition of claim 13 characterized in that the activation composition is an aqueous solution comprising:
i) copper ions;

ii) at least one compound selected from the group consisting of pyridinedimethanol, picolinamide, picolinic acid, nicotinic acid, nicotineamide, histidine, and salts of the aforementioned; and

iii) at least one enhancer compound different from ii) selected from the group consisting of sulfuric acid, sulfonic acids, carboxylic acids, polyols, salts and mixtures thereof.
Use of an activation composition according to claim 13 or 14 for activating at least one surface of a substrate for subsequent electroless plating.