Field of the Invention

The present invention relates to a method for and of selective removal of palladium from a surface of a substrate and a treatment composition for said purpose and the use of the treatment composi tion. The method, the treatment composition and the use of the latter are particularly useful in the manufacturing of electronic devices, in particular when palladium is to be selectively removed in the presence of other materials such as metals or metal alloys which are not palladium, preferably in the presence of copper and copper alloys.

Background of the Invention

In the electronics industry, metals are often deposited by plating methods. For electroless plating, an activation of most surfaces is required if they are electrically nonconductive. This activation is usually accomplished by providing a very thin palladium activation layer onto the surface of the substrate which enables subsequent electroless depositions of other metals thereon. Flowever, the presence of such palladium activation layers in places where they do not belong is detrimental to the lifetime of electronic goods resulting for example in electrical shorts. To this end, various meth ods have been disclosed in the prior art of how to remove palladium layers from surfaces of sub strates.

US 2013/005 8062 A1 discloses various methods to remove palladium from substrate surfaces. These methods include the treatment with a palladium removal agent containing nitric acid and chloride ions, the treatment with a sulfur organic substance containing liquid, the treatment with a solution comprising potassium cyanide, the treatment with a desmear liquid containing permanga nate or the treatment with plasma.

EP 2 910 666 A1 teaches the removal of a noble metal activator using a solution comprising an acid, a source for halide ions, and thiourea derivative in a concentration of 1 to 200 mg/L. The doc ument teaches that concentrations of the thiourea derivative of 250 mg/L or greater result in unde sired effects such as skip plating (see comparative example 5 therein).

US 2014/091052 A1 relates to an iodine-based etching solution to etch a material in which a palla dium material and a metal material other than the palladium material coexist, said iodine-based etching solution comprising a water-compatible organic solvent and a water-soluble polymer com pound.

WO 2014/203649 A1 discloses a solution for preventing the occurrence of bridging of an electro less metal coat, preventing i.e., the deposition of an electroless metal coat on a resin in a wiring substrate after the formation of a circuit pattern, said solution being characterized by comprising a polythiol compound; and a method for manufacturing a printed wiring board using the solution.

The methods of the prior art mostly lack in efficiency of removing palladium from the surface of a substrate. Furthermore, the methods of the prior art lack selectivity. If palladium is present next to another metal or metal alloy, typically copper or an alloy thereof, most methods of the prior art re sult in the removal of palladium and the other metal or metal alloy. This removal of the other metal or metal alloy becomes a major concern when the metal or metal alloy forms e.g. small metal, e.g. copper, features which then can be damaged or unintentionally removed. In particular, isolated copper features are even more prone to such undesired chemical attacks of such methods. Thus, if structures having small copper features having a line and space (L/S) size (measured in microns) of 15 or even 10 or below are present on a substrate, typically, the methods of the prior art fail to deliver acceptable results. Mostly, the copper features are damaged to a degree that the overall device becomes dysfunctional.

One issue is still unresolved today is the selective removal of palladium, especially if very small amounts thereof are present, next to other materials, typically metals such as copper and copper alloys, and in particular if those other materials having large surface areas compared to that of the palladium layer which is to be removed.

Objective of the present Invention

It is therefore an objective of the present invention to overcome the shortcomings of the prior art. Moreover, it is an objective of the present invention to provide an improved method for the removal of palladium from a surface of a substrate and a treatment composition for this purpose.

It is a further objective of the present invention to provide a method for the selective removal of palladium from the surface of the substrate in the presence of another metal, preferably in the presence of copper or copper alloys and in particular if the copper or copper alloy features have a size of 15 pm or below and the treatment composition for said purpose.

It is still another objective of the present invention that there should be a reduced attack upon the other metal or metal alloy present, most notably on copper and copper alloys, during the method and by the use of the treatment composition compared to the methods and composition of the prior art.

It is yet a further objective of the present invention to provide a treatment composition for the selec tive removal of palladium from surface of a substrate which can be stored for a long period of time and a use of the treatment composition.

Summary of the Invention

Above-captioned objectives are solved by a method for selective removal of palladium from a sur face of a substrate having at least one structured surface with conductive features thereon made of one or more metal or metal alloys which are not palladium and at least one outer layer of palladium layer or parts thereof comprising the following method steps

(1 ) providing the substrate; and

(2) treating the surface of the substrate with a treatment composition comprising

at least one solvent; at least one hydroxycarboxylic acid or a salt thereof; and

at least one sulfur compound selected from compounds represented by formula (I)

wherein each of R ¹ , R ² , R ³ and R ⁴ are independently selected from the group consisting of hydrogen, alkyl group, alkenyl group, alkynyl group, aryl group, and combinations of the aforementioned;

compounds represented by formula (II)

wherein each of R ⁵ and R ⁶ are independently selected from the group consisting of hy drogen, alkyl group, alkenyl group, alkynyl group, aryl group and combinations of the aforementioned; and

Y is selected from the group consisting of C2-C3-alkanediyl group, C2-C3-alkenediyl group and C2-C3-alkynediyl group;

and mixtures of the aforementioned;

wherein the concentration of the at least one sulfur compound is at least 0.06 mol/L, wherein the at least one hydroxycarboxylic acid or a salt thereof is represented by the following formula (A)

wherein M is hydrogen ion, a metal ion or any other cation and z is an integer equal to the valency of M.

The method steps are carried out in the given order but not necessarily in immediate succession. Further steps may be included in the inventive method.

The objectives are further solved by the treatment composition comprising

i. at least one solvent;

ii. at least one hydroxycarboxylic acid or a salt thereof; and

iii. at least one sulfur compound selected from compounds represented by formula (I), compounds represented by formula (II) and mixtures of the aforementioned wherein the concentration of the at least one sulfur compound is at least 0.06 mol/L, wherein the at least one hydroxycarboxylic acid or a salt thereof is represented by the following formula (A)

wherein M is hydrogen ion, a metal ion or any other cation and z is an integer equal to the valency of M;

and its use to remove palladium selectively from a surface of a substrate.

It is an advantage of the method and the treatment composition for said purpose according to the present invention that palladium can be selectively removed from the surface of the substrate. Se- lective removal of palladium means that while palladium is (almost) entirely removed from the sur face of the substrate, other materials also being on the surface of the substrate and coming in con tact with the treatment composition during step (2) are not (substantially) being removed or dam aged. Such other materials include metals or metal alloys which are not palladium, in particular copper and copper alloys (see Example 3) or nickel and nickel alloys. This in particular avoids iso- lation defects between conductive lines made e.g. of copper or nickel. The undesired removal of such other metals, in particular copper or copper alloys, is advantageously reduced or entirely sup pressed compared to the methods provided by the prior art. In other words the other metal such as copper or copper alloys is not removed at all or at least to a far lower degree than palladium, also if the other metal layers such as copper and copper alloy layers have a much greater surface area compared to the surface area of palladium layers which are to be removed. It is still a further ad vantage that isolated metal features such as copper or copper alloy features are not significantly damaged when being subjected to the method according to the invention. Isolated features are in the context of the present invention understood as features which form the final circuit profile. Pref erably the surface of the substrate is formed by a Semi-Additive Process (SAP) route or a modified Semi-Additive Process (mSAP) route, wherein in principle a Pd catalyst seed layer is deposited onto a workpiece, e.g. a laminate, before electroless deposition of a metal, e.g. copper or copper alloy, forming a patterned layer, electrolytic metal deposition onto the patterned layer, e.g. electro lytic copper deposition, and removing the patterned layer and defferential etching the electroless metal layer.lt is another advantage of the method according to the present invention and the treat- ment composition for said purpose that palladium can be quickly removed from the surface of the substrate, therefore giving rise to an efficient and economically viable process.

It is a further advantage of the present invention that the redeposition of palladium (once it is re moved from the surface of a substrate) can be effectively prevented by the method according to the invention and the treatment composition for said purpose. In the majority of the methods of the prior art, the prior art solutions used to remove palladium from the surface of the substrate are not capable of preventing the redeposition of palladium. Palladium is then, in part or completely, rede posited on the substrate either on the same surface wherefrom palladium was removed or on one or more other surfaces forming one or more palladium layers thereon. This undesired redeposition potentially results in undesired catalytic spots for metal plating in later stages of the manufacturing process and/or electrical shorts.

It is thus another advantage of the method according to the present invention to treatment compo sition for said purpose that skip plating can be prevented. That is to say that the palladium remover process does not leave any material or component behind on the substrate surface which will pre vent application of, or inhibit a final solderable finishes (such as Immersion Tin, electroless nickel / immersion gold (ENIG), or electroless nickel / electroless palladium / immersion gold (ENEPIG)) either in part or for full coverage.

Brief description of the Figures

Figure 1 shows a schematic representation of a preferred embodiment of the present invention and the individual method steps are to be discussed hereinafter.

Detailed Description of the Invention

Percentages throughout this specification are weight-percentages (wt.-%) unless stated other-wise. Concentrations given in this specification refer to the volume or mass of the entire solutions / com positions unless stated otherwise. The terms“deposition” and“plating” are used interchangeably herein. Also,“layer” and“deposit” are also used synonymously in this specification. 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.

The term“alkyl group” according to the present invention comprises branched or unbranched alkyl groups 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 in this specifi cation and in the claims refers to 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, hexyl, heptyl and octyl. Substituted alkyl groups may theoretically be obtained by replacing at least one hydrogen by a functional group. Unless stated otherwise, alkyl groups are preferably selected from substituted or unsubstituted C1 -C8-alkyl groups, more preferably from substituted or unsubstituted C1 -C4-alkyl groups because of their improved solubility in water.

The term“alkenyl group” according to the present invention comprises branched or unbranched alkenyl groups comprising cyclic and/or non-cyclic structural elements, wherein cyclic structural elements of the alkenyl groups naturally require at least three carbon atoms. Alkenyl groups com prise at least one double bond between two adjacent carbon atoms and thus alkenyl groups gener ally require at least two carbon atoms to be present. C2-CX-alkenyl group in this specification and in the claims refers to alkenyl groups having 2 to X carbon atoms (X being an integer). Substituted alkenyl groups may theoretically be obtained by replacing at least one hydrogen by a functional group. Unless stated otherwise, alkenyl groups are preferably selected from substituted or unsub stituted C2-C8-alkenyl groups, more preferably from substituted or unsubstituted C2-C4-alkenyl groups because of their improved solubility in water.

The term“alkynyl group” according to the present invention comprises branched or unbranched alkyl groups comprising cyclic and/or non-cyclic structural elements, wherein cyclic structural ele ments of the alkynyl groups naturally require at least three carbon atoms. Alkenyl groups comprise at least one triple bond between two adjacent carbon atoms and thus alkynyl groups generally re quire at least two carbon atoms to be present. C2-CX-alkynyl group in this specification and in the claims refers to alkynyl groups having 2 to X carbon atoms (X being an integer). Substituted alkynyl groups may theoretically be obtained by replacing at least one hydrogen by a functional group. Unless stated otherwise, alkynyl groups are preferably selected from substituted or unsubstituted C2-C8-alkynyl groups, more preferably from substituted or unsubstituted C2-C4-alkynyl groups because of their improved solubility in water.

The term "aryl group" according to the present invention refers to ring-shaped aromatic hydro carbon residues, for example phenyl or naphtyl where individual ring carbon atoms can be re placed by N, O and/or S, for example benzothiazolyl. Furthermore, aryl groups are optionally sub stituted by replacing a hydrogen atom in each case by a functional group. The term C5-CX-aryl group refers to aryl groups having 5 to X carbon atoms (optionally replaced by N, O and/or S and X being an integer) in the ring-shaped aromatic group. Unless stated otherwise, aryl group are pref erably selected from substituted or unsubstituted C5-C10-aryl groups, more preferably from substi tuted or unsubstituted C5-C6-aryl groups because of their improved solubility in water.

Combinations of alkyl group, alkenyl group, alkynyl group, aryl group in this specification and in the claims are understood as the substituted derivatives of one of the members of the aforementioned by another member of said group. For example, a hydrogen atom of an alkyl group may be theoret ically be substituted by an aryl group forming an alkaryl group, e.g. a benzyl group. Such combina tions foremost include alkaryl, aralkyl, alkenylaryl, aralkenyl, alkynylaryl and aralkynyl.

The term“alkanediyl group” is the corresponding derivative of an alkyl group having two bonding sites. Sometimes alkanediyl groups are referred to as alkylene groups in the art. C1 -CX-alkanediyl group in this specification (X being an integer) and in the claims refers to alkanediyl groups having 1 to X carbon atoms, e.g. 1 to 12 or 2 to 6. The explanations and preferences outlined for alkyl groups apply to alkanediyl groups as well. Similarly,“alkenediyl group” and“alkynediyl group” are the corresponding derivatives of alkenyl group and alkynyl group, respectively, having two bonding sites each. The explanations and preferences outlined for alkenyl groups and alkynyl groups apply to alkenediyl groups and alkynediyl groups, respectively, as well.

Unless stated otherwise, above-defined groups are substituted or unsubstituted. Functional groups as substituents are preferably selected from the group consisting of hydroxyl, amino and carboxyl to improve the solubility of the relevant compounds in water. If more than one residue (substituent) is to be selected from a certain group, each of the residues is selected independently from each other unless stated otherwise herein.

In step (1 ) of the inventive method, the substrate is provided. The substrate comprises a surface whereon at least partly a palladium layer is present. Said palladium layer is to be removed. The palladium layer may consist of palladium or it only comprises palladium and other materials. The palladium layer to be selectively removed must be contactable by the treatment composition in subsequent step (2).

Preferably, the palladium layer (to be selectively removed) is of a palladium amount of 1 mg or less palladium per dm ² of surface area, more preferably 0.1 to 0.5 mg palladium per dm ² of surface area, even more preferably 0.01 to 0.05 mg palladium per dm ² of surface area.

Preferably, the palladium layer to be selectively removed is 100 nm or less in height (as measured by XRF), more preferably 50 nm or less in height, even more preferably 20 nm or less in height, and particularly preferably 10 nm or less in height.

The palladium layer on the surface of the substrate is continuous or discontinuous. Palladium lay ers resulting from activation of the surface of the substrate (denominated hereinafter as“palladium activation layers”) to render it catalytically active for subsequent electroless metal plating steps are preferred in the context of the present invention. Palladium activation layers often are discontinu ous and their structure depends inter alia on the mode of activation. While ionic activation palladi um layers often form small isolated thin layer fragments on a surface of a substrate, colloidal acti vation layers often are present as agglomerates resulting from the colloidal palladium particles.

Optionally, pretreatment steps can be included before and/or after step (1 ). Such pretreatment steps include forming recessed structures, e.g. by drilling vias, trenches and other recesses into the substrate. Mechanical or laser drilling are conventional methods to form such vias, trenches and other recesses into the surface of the substrate.

After the formation of recessed structures, e.g. by drilling, typically, a desmear treatment is advisa ble. Such desmear treatments usually include the treatment of the substrate or a workpiece with an aqueous solution comprising strong oxidizing agents to remove organic residues formed during drilling from the surface of the substrate or workpiece.

Further, usual pretreatment steps to be optionally included before and/or after step (1 ) are etching steps, swelling steps and cleaning steps. These steps are known in the art. Etching steps mostly use aqueous solutions with oxidizing agents in inorganic acids to roughen the surface of the sub strate, usually copper or copper alloy surfaces. Swelling steps use mostly (polar) organic solvents to treat the surface of the substrate with the goal of increasing the volume of the surface of the substrate. Cleaning steps typically employ aqueous solutions containing surfactants which are to remove undesired organic residues present on the surface of the substrate (e.g. grease).

In the present invention, a substrate having at least one structured surface is used. In particular, a substrate with said structured surface is used in the inventive method. According to the invention, a structured surface means that said surface has conductive features thereon made of one or more metal or metal alloys which are not palladium. Such conductive may form features likes lines, build up structures such as pillars as well as vias such as blind micro vias, through vias like through sili con vias and through glass vias. The substrate with said structured surface further comprises at least one (outer) layer of palladium layer or parts thereof. In particular copper or nickel and its al loys form said conductive features. In one embodiment the structured surface comprises or con sists of metal features which are not palladium, being e.g. circuit lines or vias; and at least parts of (activation) palladium layers.

Preferably, the conductive features of the structured surface of the substrate comprises metal or metal alloy, preferably copper or copper alloy, features thereon having a feature size of 15 pm or less, more preferably of 10 pm or less, even more preferably of 5 pm or less, yet even more pref erably of 2 pm or less, on the surface of the substrate. Features inter alia include in the context of the present invention lines, build-up structures such as pillars as well as vias such as blind micro vias, through vias like through silicon vias and through glass vias. The feature size relates to the size of the features in at least one dimension thereof, e.g. height, breadth, length or diameter. In one embodiment of the present invention, the breadth of lines, if present, has above described feature sizes and optionally the height thereof. In one embodiment of the present invention, the breadth and length of buildup structures such as pillars, if present and if having a rectangular basis has above described feature sizes. If the present buildup structures have a non-rectangular basis, in particular a round or oval basis, then the (maximal) diameter has above described feature sizes. In one embodiment of the present invention, the breadth and length of vias, if present and if having a rectangular basis has above described feature sizes. If the vias have a non-rectangular basis, in particular a round or oval basis, then the (maximal) diameter has above described feature sizes.

More preferably, the surface of the substrate has (preferably conductive) features as structures having a L/S (line/space) ratio of 15 pm / 15 pm, preferably 10 pm / 10 pm, more preferably 5 pm / 5 pm and even more preferably 2 pm / 2 pm. Flere, line means pattern widths, and space means the space between patterns, which is the distance between the centers of patterns.

Preferably, the substrate is selected from the group consisting of printed circuit boards, printed circuit foils, interposers, chip carriers, IC substrates, semiconductor wafers, circuit carriers, inter connect devices, copper clad laminates, Ajinomoto build-up films and precursors for any of the aforementioned. Precursors of the aforementioned inter alia include FR-1 , FR-2, FR-3, FR-4, FR-5, FR-6, CEM-1 , CEM-2, CEM-3, CEM-4, CEM-5, G-10, G-11 , ceramic materials like alumina, poly meric materials (optionally filled with ceramic and/or glass particles) like PTFE, polyimide, polyes ters (polyethylene terephthalate), composites of such polymers, and mixtures of the aforemen tioned. In step (2) of the inventive method, the surface of the substrate having at least one struc tured surface with conductive features thereon made of one or more metal or metal alloys which are not palladium and at least one outer layer of palladium layer or parts thereof is treated with a treatment composition comprising i. at least one solvent;

ii. at least one hydroxycarboxylic acid or a salt thereof; and

iii. at least one sulfur compound selected from compounds represented by formula (I)

wherein each of R ¹ , R ² , R ³ and R ⁴ are independently selected from the group consisting of hydrogen, alkyl group, alkenyl group, alkynyl group, aryl group, and combinations of the aforementioned;

compounds represented by formula (II)

wherein each of R ⁵ and R ⁶ are independently selected from the group consisting of hydro gen, alkyl group, alkenyl group, alkynyl group, aryl group and combinations of the aforemen tioned; and

Y is selected from the group consisting of C2-C3-alkanediyl, C2-C3-alkenediyl and C2-C3- alkynediyl;

and mixtures of the aforementioned;

wherein the concentration of the at least one sulfur compound is at least 0.06 mol/L.

By treating the surface of the substrate with the treatment composition, palladium is selectively removed from the surface of the substrate.

The treatment composition is usually a liquid treatment composition. Liquid treatment compositions include inter alia solutions, dispersions such as emulsions as well as suspensions and sols. Prefer ably, the liquid treatment composition is a solution.

Any solvent suitable to dissolve the at least one hydroxycarboxylic acid or a salt thereof and the at least one sulfur compound may be used. Preferably, the at least one solvent is water for it being ecologically benign. Further liquids, that are miscible with water, as for example alcohols and other polar organic liquids, that are miscible with water, may be added. It is preferred that at least 95 wt.-%, more preferably 99 wt.-%, of all solvents used are water. More preferably, the treatment composition is an aqueous solution because of its ecologically benign character and its general applicability in industrial processes.

The at least one hydroxycarboxylic acid or a salt thereof is preferably aliphatic. More preferably, at least one hydroxycarboxylic acid or a salt thereof is aliphatic and non-cyclic. Preferably, the at least one hydroxycarboxylic acid or a salt thereof comprises one carboxylic acid or carboxylate group. It is thus preferably a monocarboxylic acid or a salt thereof.

Even more preferably, the at least one hydroxycarboxylic acid or a salt thereof is represented by the following formula (A)

M

(A)

wherein M is hydrogen ion, a metal ion or any other cation and z is an integer equal to the valency of M. Depending on various factors known in the art such as the solvent in the treatment composi tion, it is also possible that the bond between the carboxylate group in formula (A) and M is not ionic but for example covalent or polar.

Such other cation is a suitable counter-ion for above depicted anion and can be for example am monium (NH ₄ ⁺ ) or a quaternary amine (e.g. tetraalkylammonium). In this case, z equals 1. Prefera ble metals ions are alkaline ions such as sodium ions and potassium ions (having a valency of 1 in which case z equals 1 ) and earth alkaline metal ions such as calcium ions and magnesium ions (having a valency of 2 in which case z equals 2). The person skilled in the art can select suitable counter-ions for M other than those described above.

Yet even more preferably, the at least one hydroxycarboxylic acid or a salt thereof is gluconic acid or a gluconate. Suitable gluconates are selected from the group consisting of alkaline gluconate (for example sodium gluconate), earth alkaline gluconate (for example magnesium gluconate) and ammonium gluconate.

Preferably, the treatment composition comprises the at least one hydroxycarboxylic acid or a salt thereof in a (total) concentration ranging from 0.1 to 3.0 mol/L, more preferably from 0.3 to 2.0 mol/L, even more preferably from 0.4 to 1.5 mol/L and yet even more preferably from 0.5 to 1.0 mol/L. The total concentration means that if more than one hydroxycarboxylic acid or a salt thereof is used in the treatment composition, the overall concentration of the all hydroxycarboxylic acids and salts thereof shall preferably lie in the concentration ranges defined above.

Concentrations outside above ranges may be used. However, when working below above-defined minimum thresholds, sometimes the treatments composition becomes inefficient and palladium removal becomes slow. Concentrations above the said maximum values might be possible in some cases but no further advantages were found when exceeding the concentrations beyond those values and thus, such higher concentrations only add to the cost.

Preferably, the at least one sulfur compound is selected to be a compound represented by formula (I). More preferably, the at least one sulfur compound is selected from the group consisting of thio urea, L/,L/'-dimethylthiourea, L/,L/,L/',L/'-tetramethylthiourea, L/,L/'-diethylthiourea and L/,L/,L/',L/'-tetraethylthiourea. Most preferably, the at least one sulfur compound is thiourea. The concentration of the at least one sulfur compound is at least 0.06 mol/L. This concentration is considered as the total concentration. That means, that in cases where two or more sulfur com pounds are used (by mixing them together), the total concentration of the two or more sulfur com pounds in the treatment composition is in total at least 0.06 mol/L. Below said concentration, the removal of palladium becomes inefficient and incomplete. This lack of efficiency cannot even be compensated for by prolonged treatment times. Preferably, the at least one sulfur compound is contained in the treatment composition in a (total) concentration ranging from 0.06 to 1 .4 mol/L, more preferably from 0.2 to 1 .0 mol/L and even more preferably from 0.35 to 0.7 mol/L. With other words the total concentration means that if more than one sulfur compound is used in the treatment composition, the overall concentration of all sulfur compounds shall preferably lie in the concentra tion ranges defined above.

Concentrations above the said preferred maximum values are possible but no further advantages were found when exceeding these concentrations beyond those values and thus, such higher con centrations only increase the cost.

Preferably, the treatment composition has a pH value ranging from 2 to 6, more preferably 3 to 5, even more preferably 3.5 to 4. These pH ranges allow for particularly selective removal of palladi um in the presence of another metal such as copper or copper alloys. The pH value can be adapted where necessary with acids, bases or buffers. Useful acids are those having a pK _a of at least 2.5, preferably of at least 3, for the reasons outlined below. The mentioned pK _a values refer in case of polyprotic acids to the first pK _a of the selected acid.

Preferably, the treatment composition comprises organosulfonic acids in a total concentration of 0.2 mol/L or less, preferably 0.1 mol/L or less, more preferably 0.02 mol/L or less, even more pref erably the treatment composition is (substantially) free of organic sulfonic acids. The term“substan tially free of” means that there is no such compound intentionally added to the treatment composi tion and ideally, the treatment composition does not contain any such compound. However, tech nical raw materials sometimes include such compounds as contaminations which shall preferably not be excluded. Organosulfonic acids include alkylsulfonic acids such as methanesulfonic acid or ethanesulfonic acid, alkanolsulfonic acid such as isethionic acid (2-hydroxyethanesulfonic acid), arylsulfonic acid such as phenylsulfonic acid or para-toluenesulfonic acid. Such organosulfonic acids may have a negative impact on copper or copper alloy features present on the surface of a substrate by chemically attacking those. This is highly undesired, in particular when small and/or isolated copper and copper alloy features are present, as they will be quickly removed from the surface of the substrate.

Preferably, the treatment composition comprises halide ions in a total concentration of 10 mg/L or less, preferably 5 mg/L or less, more preferably 1 mg/L or less, even more preferably 0.1 mg/L or less, most preferably the treatment composition is (substantially) free of halide ions. Halide ions also have the same negative impact on copper and copper alloy features present on the surface of the substrate as organosulfonic acids do. Preferably, the treatment composition comprises cyanide ions in a total concentration of 10 mg/L or less, preferably 5 mg/L or less, more preferably 1 mg/L or less, even more preferably 0.1 mg/L or less, most preferably the treatment composition is (substantially) free of cyanide ions. Cyanide ions were frequently used in the prior art but they are undesired due to their toxic and ecologically harm ful properties and might have the same negative impact outlined for organosulfonic acids.

Preferably, the treatment composition comprises oxidizing agents suitable to convert metallic cop per to copper oxide such as hydrogen peroxide, persulfate, peracids in a total concentration of less than 10 mmol/L or less, preferably 5 mmol/L or less, more preferably 1 mmol/L or less, even more preferably 0.1 mmol/L or less, most preferably the treatment composition is (substantially) free of oxidizing agents suitable to convert metallic copper to copper oxide. Such oxidizing agents may react violently with some potential sulfur compounds, e.g. thiourea, and also, they might disadvan tageous^ increase the attack on copper or copper alloys.

Preferably, the treatment composition comprises less than 0.1 mol/L nitrate ions, preferably less than 0.01 mol/L, more preferably less than 0.001 mol/L, most preferably it is (substantially) free of nitrate ions. This preference is based on the same reason as described for the organosulfonic ac ids.

Preferably, the treatment composition comprises less than 0.1 mol/L of acids having a pK _a of 1 or less. More preferably, the concentration of such acids is less than 0.01 mol/L, even more preferably less than 0.001 mol/L, most preferably it is (substantially) free of such acids.

More preferably, the treatment composition comprises acids having a pK _a of 1.5 or less in said amounts or less; even more preferably acids having a pK _a of 2 or less. This is irrespective of whether they are inorganic or organic acids. Typical examples of such acids are sulfuric acid, tri chloroacetic acid, hydrochloric acid, nitric acid, methanesulfonic acid, sulfamic acid, hydrobromic acid, phosphoric acid and so on. The presence of such strong acids in the treatment composition may sometimes result in an attack of copper or copper alloy features present on the surface of a substrate.

Preferably, the treatment composition comprises less than 0.1 mol/L of phosphate ions, hy- drogenphosphate ions and dihydrogenphosphate ions, preferably less than 0.01 mol/L, more pref erably less than 0.001 mol/L, most preferably it is (substantially) free of phosphate ions, hy- drogenphosphate ions and dihydrogenphosphate ions.

Preferably, the treatment composition is free of (intentionally added) metal ions that can be re duced under the given conditions to avoid the formation of undesired metallic layers on the surface of the substrate. Such undesired metallic layers can result in electrical shorts rendering an elec tronic device defective. Typically, such metal ions which can be reduced are those in groups IMA to VIIIA of the period table of elements. Free of such reducible metal ions means concentration of 0.01 mol/L or less, more preferably 0.001 mol/L, even more preferably 0.0001 mol/L, and the treatment composition is yet even more preferably (substantially) free of such reducible metal ions. Naturally, the treatment composition will contain some palladium ions (or other metal ions as well) while or after treating the surface of the substrate. The treatment composition is thus ideally not a plating bath, a plating bath being capable of and intended for forming metal layers on surfaces of substrates.

The treatment composition is preferably free of urea or its alkylderivatives such as N,N’-dialkylurea. Urea or its alkylderivatives are sometimes used to as coordination compounds for copper ions in solution in order to prevent precipitation of copper salts. This formation of coordination compounds with copper ions is not required for the treatment composition according to the invention as palladi um is selectively removed, also in the presence of copper or copper alloys, and no or very little amounts of copper, if present, are being removed from the surface of a substrate. Further, the treatment composition is surprisingly capable of keeping very high amounts of copper ions solubil ized, if required, which in turn advantageously increases the lifetime of the treatment composition, particularly when being in use.

The treatment composition can be prepared by dissolving all components in the at least one sol vent.

The palladium is selectively removed from the surface of the substrate by contacting the palladium layer to be selectively removed from the surface of the substrate with the treatment composition. Contact between said layer and the treatment composition may be accomplished by various estab lished means such as immersion the substrate (entirely, partially or only the surface whereon the palladium layer is present) into the treatment composition, by spraying or wiping the treatment composition onto the substrate (on its entirety, on it partially or only on the surface whereon the palladium layer is present).

The treatment time can be adapted to the amount of palladium to be removed. Typical treatment times range from 1 second to 1 hour, preferably from 15 seconds to 10 minutes, more preferably from 30 seconds to 5 minutes and even more preferably from 1 minute to 3 minutes.

The temperature during contact of the substrate and the treatment composition preferably ranges from 10 to 70°C, more preferably from 20 to 60°C, and even more preferably from 30 to 50°C.

A preferred embodiment of the present invention is described herein below with reference to Figure 1. First, as depicted in Figure 1 A, a workpiece (100) having a surface (100a) is provided. The workpiece is preferably selected from the group consisting of material suitable for the production of printed circuit boards, printed circuit foils, interposers, chip carriers, IC substrates, semiconductor wafers, circuit carriers, interconnect devices, copper clad laminates, build-up films and precursors for any of the aforementioned. The workpiece is optionally subjected to one or more pretreatment steps as those described above. In one embodiment of the present invention, the workpiece (con trary to the substrate) does not comprise any palladium layers (on its surface).

In Figure 1 B, a palladium activation layer (101 ) is deposited on the surface of the workpiece (100a). The palladium activation layer (101 ) can be formed by ionic activation or by colloidal activation. Both types of activation are known in the art. For ionic activation, the surface of the workpiece to be activated (100a) is first contacted with an aqueous solution of a palladium salt e.g. palladium sul fate or palladium chloride, optionally comprising complexing agents (also named chelating agents in the art) suitable to form coordination compounds with palladium ions, such as amines. Then, the surface of the workpiece (100a) is contacted with an aqueous solution containing a reducing agent suitable to reduce the palladium ions absorbed thereon to metallic palladium such as dimethyla- mino borane (DMAB). By this reduction, a palladium activation layer is formed on the surface (100a) of the workpiece.

Alternatively, the surface of the workpiece (100a) may be colloidally activated. This is typically ac complished by the treatment of the surface of the workpiece to be activated (100a) with an aque ous solution containing palladium particles. Said palladium particles are usually in the sub-micron (d ₅₀ £ 1 pm, measureable for example by SEM after deposition) or nanometer range (d ₅₀ £ 0.1 pm, measureable for example by SEM after deposition) and are typically covered by a shell made of organic or metallic material. Organic materials for this purpose are for example gelatin or polyvinyl alcohol, the preferred metallic material of choice is usually tin. The palladium particles can exem- plarily be prepared by in situ reduction of a palladium salt in water with tin(ll) salts as reducing agent. By treating the surface of the workpiece (100a) with a solution containing such palladium particles, a palladium activation layer is formed on the surface of the workpiece (100a).

Then, in Figure 1 C, an electroless copper or copper alloy layer (102) is deposited onto the activa tion palladium layer (101 ). The electroless copper or copper alloy layer (102) is deposited electro- lessly. The electroless deposition of copper or copper alloys is known in the art and various suita ble electroless copper or copper alloy plating baths are known. Typically, such plating baths com prise copper ions (provided for example as copper salt such as copper sulfate), one or more reduc ing agents suitable to reduce copper ions to metallic copper such as formaldehyde or hypophos- phite, one or more complexing agents for copper ions such as carboxylic acids and the respective hydroxy- and amino derivatives thereof, phosphonates, amines or polyamines, and optionally fur ther reducible metal ions such as nickel and cobalt ions (if a copper alloy is to deposited) and stabi lizing agents.

Thereafter, as shown in Figure 1 D, a patterned layer (103) is formed on the electroless copper or copper alloy layer (102). The patterned layer comprising gaps (103a) forming pattern structures (103b), wherein the gaps expose parts of the underlying electroless copper or copper alloy layer. Such a patterned layer can be formed by means known in the art, for example by placing a nega tive or positive photosensitive composition (e.g. a dry film) onto the electroless copper or copper alloy layer (102) and subsequently partly exposure to light and development. . Alternatively, a pho- toimageable solder mask can be placed on the electroless copper a copper alloy layer (102) to the same effect.

In the following step, as depicted in Figure 1 E, electrolytic copper (104) is deposited in the gaps (103a in Figure 1 D) of the pattern. The gaps (103a in Figure 1 D) can be filled partially or entirely with electrolytic copper or it is possible to overfill the gaps (103a in Figure 1 D) and then remove any undesired electrolytic copper overburden subsequently, for example by chemical mechanical planarization steps (CMP).

Electrolytic copper deposition is known widely in the art and also the filling of recessed structures such as the gaps shown in Figure 1 D. Useful electrolytic copper plating baths comprise copper ions (provided for example as copper salts like copper sulfate), one or more electrolytes (for example a strong acid or a salt thereof such as sulfuric acid or methanesulfonic acid) and optionally halide ions, one or more levelers (such as polyethylenimine), one or more accelerators (such as 3- mercapto-propylsulfonic acid or salts thereof) and one or more suppressors (such as polyethylene glycols).

After that and as depicted in Figure 1 F, the patterned layer with its pattern structures (103b) is re moved from the electroless copper or copper alloy layer (102). Thereby, those parts of the electro less copper or copper alloy layer become exposed on which no electrolytic copper layer was formed in the previous step (102a). Removal of such patterns is a known technique in the art and ready-made solutions are available for that purpose. For example, alkaline or amine containing solutions are employed to remove cured dry films.

Then, the parts of the electroless copper or copper alloy layer which became exposed in the previ ous step (102a) are removed. This is shown in Figure 1 G. Such removal of electroless copper or copper alloy layers can be accomplished by copper etching, e.g. differential copper etching. Useful copper etching solutions are typically aqueous and include one or more oxidizing agents suitable to oxidize metallic copper to copper ions such as sodium persulfate.

By removing the exposed electroless copper or copper alloy layer, the palladium activation layer becomes exposed (101 a) and available for further treatments. Thereby, a substrate having at least one surface is formed by above-described steps. On said at least one surface of the formed sub strate, at least a palladium layer (which is to be selectively removed) and deposits of electrolytic copper are present. This formed substrate is then used in the inventive method (corresponds to step (1 ) of the inventive method).

Finally in Figure 1 H a treated substrate is shown, wherein the substrate having at least one struc tured surface with conductive features thereon made of one or more metal or metal alloys which are not palladium and at least one outer layer of palladium layer or parts thereof (Figure 1 F) was treated with a treatment composition comprising

i. at least one solvent;

ii. at least one hydroxycarboxylic acid or a salt thereof; and

iii. at least one sulfur compound selected from compounds represented by formula (I), com pounds represented by formula (II) and mixtures of the aforementioned; and wherein the (total) concentration of the at least one sulfur compound is at least 0.06 mol/L. This step corresponds to step (2) of the inventive method. Preferences and details given above apply to this preferred embodiment mutatis mutandis. The outlined sequence of steps is sometimes referred to in the art as SAP (semi-additive process).

In a preferred embodiment of the present invention outlined above and in principle shown in Fig. 1 , the method according to the invention further comprises the following method steps (usually to be carried out in the given order and before method step (1 ))

(0.1 ) providing a workpiece having a surface;

(0.2) optionally, pretreating the workpiece;

(0.3) depositing an activation palladium layer onto the surface of the workpiece;

(0.4) depositing an electroless copper or a copper alloy layer onto the palladium activation layer;

(0.5) forming a patterned layer comprising gaps onto the electroless copper or copper alloy layer, wherein the gaps expose parts of the underlying electroless copper or copper alloy layer;

(0.6) depositing electrolytic copper into the gaps of the patterned layer;

(0.7) removing the patterned layer from the electroless copper or copper alloy layer; and thereby exposing those parts of the electroless copper or copper alloy layer on which no electrolytic copper layer was formed in the previous step;

(0.8) removing the parts of the electroless copper or copper alloy layer which became ex posed in the previous step

and thereby forming the substrate having at least one structured surface with conduc tive features thereon made of one or more metal or metal alloys which are not palladi um and at least one outer layer of palladium layer or parts thereof.

By carrying out step (0.8), the substrate becomes available and can be provided for use in the in- ventive method (corresponds to step (1 ) of the inventive method). Then, the method is concluded by further implementing step (2). Method steps (0.1 ) to (0.8) are one possible way to obtain a sub strate having a structured surface.

In one embodiment the structured surface comprises or consists of copper features, being e.g. circuit lines or vias, and at least parts of activation palladium layers.

Optionally, the inventive method further comprises the method step (3. a) after step (2)

(3. a) bonding the surface of the substrate to a further substrate.

This step is particularly useful if multilayer PCBs or the like are to be manufactured. To this end, the surface of the substrate is usually etch-treated using solutions comprising one or more oxidiz ing agent such as hydrogen peroxide, persulfate, ferric ions and cupric ions, optionally halide ions and strong acids such as sulfuric acid or nitric acid. After such an etch-treatment, a further sub strate can be laminated onto the substrate.

Optionally, the inventive method further comprises the method step (3.b) after step (2) (3.b) forming a final solderable finish on the surface of the substrate.

Such final solderable finishes include electroless nickel/immersion gold (ENIG), electroless nick el/electroless palladium/immersion gold (ENEPIG), electroless nickel/electroless palladi- um/autocatalytic gold (ENEPAG), organic solderable finishes (also referred to organic solderability preservative, OSP), electrolytic gold, immersion tin, immersion silver, electroless nickel/electroless palladium (ENEP) and so forth. Such final solderable finishes are widely used and methods for their deposition are known to the person skilled in the art.

The method according to the invention optionally comprises rinsing steps, preferably with water, and/or drying steps.

The present invention further concerns a treatment composition for selective removal of palladium from a surface of a substrate comprising

i. at least one solvent;

ii. at least one hydroxycarboxylic acid or a salt thereof;

iii. at least one sulfur compound selected from compounds represented by formula (I), compounds represented by formula (II) and mixtures of the aforementioned; wherein the concentration of the at least one sulfur compound is at least 0.06 mol/L.

The present invention is further directed to the use of the treatment composition according to the invention to selectively remove palladium from a surface of a substrate. Details and preferences outlined hereinbefore apply mutatis mutandis to the treatment composition and its use and are not recited again to avoid unnecessary repetition.

Industrial Applicability

The present invention can be used in various technical fields and industries. In particular, it is use ful in the manufacturing of electronic devices and semi-finished products such as those described hereinbefore.

The invention will now be illustrated by reference to the following non-limiting examples.

Examples

Commercial products were used as described in the technical datasheet available on the date of filing of this specification unless stated otherwise hereinafter. Nichem ^® 1100, Pro Select SF and Aurotech ^® CNN, Printoganth ^® PV are products available from Atotech Deutschland GmbH.

Standard FR4 dielectric materials were used as workpieces. Prior to using them as described in the in the following examples, they were treated as follows: Firstly, they were activated with palladium (corresponds to method step (0.3)), electroless copper was deposited thereon (corresponds to method step (0.4), Printoganth® PV) and a pattern was formed on the electroless copper layer (Ajinomoto build-up film GX13). Then, electrolytic copper was deposited on the substrate (corre sponds to method step (0.6)) followed by removal of the pattern (corresponds to method step (0.7)). Thereafter, the substrate was immersed into an aqueous solution of 100 g/L sodium persul fate, 50 mL/L 98 wt.-% sulfuric acid for 2 min at 25 °C to remove the accessible part of the electro- less copper layer (corresponds to method step (0.8). It is known in the art that above treatments, in particular the step to remove the accessible part of the electroless copper layer (corresponds to method step (0.8)), has no effect on the removal of palladium from a surface of a substrate.

The atomic absorption spectroscopy (AAS) was conducted on a Varian AA-240 spectrophotometer.

The total palladium amount on a surface of a substrate was measured as follows:

At least three substrates (having each a surface area of 1 dm ² , optionally cut into smaller pieces) were immersed into an aqua regia solution (375 mL/L of 37 wt.-% hydrochloric acid and 125 mL/L 65 wt.-% nitric acid were dissolved in 500 mL deionized water) at 25 °C for 2 min: Then, the total amount of dissolved palladium in said solution was measured by AAS.

It is a prerequisite in the electronics industry, for example to guarantee the non-occurrence of elec- trical shorts, that the amount of palladium removed is at least 95% of the total palladium amount on a surface of a substrate obtained from the test method described hereinbefore.

The microscopic characterisation of the copper features was performed using Carl Zeiss SMT Ltd. EV015-07.05 or a Helios NanoLab 650 scanning electron microscopes (SEM, both FEI Company). Alternatively, where applicable, a light microscope (Olympus Optical Ltd., BX 60F-3) was used. The metal layer thickness was measured at 10 points of each substrate by XRF using the XRF instrument Fischerscope XDV-SDD (Helmut Fischer GmbH, Germany). By assuming a layered structure of the deposit the layer thickness can be calculated from such XRF data.

Example 1 : Palladium removal

A treatment composition was prepared by dissolving 0.6 mol/L gluconic acid as the hydroxycarbox- ylic acid and 0.66 mol/L thiourea as the sulfur compound in water.

Substrates were immersed into the treatment composition using the parameters as given in Table 1 and the palladium removal was measured by AAS in relation to the total palladium amount on the surface of the substrate.

Table 1 : Palladium removal in dependency of time and temperature of the treatment composition.

A high palladium removal was found for all the temperatures and the immersion durations em ployed. The amount of removed palladium was in all cases sufficiently high to avoid negative re sults such as shorts caused by residual palladium on the surface of the substrate and often quanti- tative.

Example 2: Palladium removal with varying concentrations in the treatment composition

Example 1 was repeated whereby the concentrations of the hydroxycarboxylic acid and the sulfur compound were varied as given in Table 2.

Table 2: Palladium removal in dependency of concentrations in the treatment composition.

The amount of palladium removed was again always very high and sufficient to avoid negative results such as shorts caused by residual palladium on the surface of the substrate and often quan- titative.

Example 3: Copper line width reduction

Two solutions were prepared by dissolving the individual components in water:

a) an aqueous solution containing 175 g/L (0.9 mol/L) gluconic acid as the hydroxycarboxylic acid, 50 g/L (0.66-mol/L) thiourea as the sulfur compound (a treatment composition accord ing to the invention) b) an aqueous solution containing 100 g/L methanesulfonic acid, and 100 g/L urea to inhibit dissolved copper ions from precipitating (comparative)

Samples with copper lines of varying line spaces were each immersed into the solutions for 60 seconds at 50 °C. Before treatment, the copper lines had a rectangular shape and were 10 pm x 10 pm (abbreviated in the following table as 10/10) and 15 pm x 15 pm (abbreviated in the follow ing table as 15/15) in width and breadth, respectively. Then, the amount of line width reduction was measured microscopically (two-dimensional plan view) after immersion of the substrates into above-described solutions and compared to substrates not treated. These results are summarized in the following Table 3:

Table 3: Line width reduction of copper lines after treatment with the inventive treatment composi tion and a comparative solution.

A significant difference of the line width reduction of the copper lines was found between the com parative solution known from the prior art and the inventive treatment composition. While the line width was only reduced by 0.16 pm for the 15 pm x 15 pm copper lines (and thus within the meas urement error margin) for solution a), it was 100 times higher for the comparative solution.

Example 4: Long term reliability test

500 ml/L of a treatment composition containing 0.6 mol/L gluconic acid as the hydroxycarboxylic acid and 0.66 mol/L thiourea as the sulfur compound was prepared by dissolving the components in water.

Into the treatment composition, 10 substrates were successively immersed for 1 min each at 50 °C. Each substrate comprised 10 coupons of 1 dm ² each. In total, each substrate had a total palladium amount on a surface of a substrate of 0.76 mg/dm ² .

The substrates were prepared by using Printoganth® PV (product of Atotech Deutschland GmbH, applying standard processing conditions as described in the corresponding technical datasheet) and finally, the parts of the electroless copper layer which became exposed in the previous step were removed with a solution containing 100 g/L sodium persulfate.

The following table gives an overview of the palladium removal in dependency of the number of substrates immersed into the solution and the accumulated surface area of the substrates in rela tion to the volume of the solution (given as dm ² /L in the following table). Further, the copper line width reduction of substrates 5 and 10 (see Table 4), respectively was assessed as described in Example 3.

Table 4: Palladium removal long term reliability test.

The removal of palladium was consistently sufficient for the entire test series. There was no decline in the efficiency of the treatment composition in removing palladium from the surface of the sub strates. No significant copper line width reduction of 10 pm x 10 pm and 15 pm x 15 pm copper lines was found for the analysed substrates.

Other embodiments of the present invention will be apparent to those skilled in the art from a con sideration of this specification or practice of the invention disclosed herein. It is intended that the specification and examples be considered as exemplary only, with the true scope of the invention being defined by the following claims only.