EP4680782A1

EP4680782A1 - A method for the pre-treatment of a surface of a substrate

Info

Publication number: EP4680782A1
Application number: EP24722006.4A
Authority: EP
Inventors: Sebastian ZARWELL; Jennifer RAASE; Don Jang; Tobias Bernhard
Original assignee: Atotech Deutschland GmbH and Co KG
Current assignee: Atotech Deutschland GmbH and Co KG
Priority date: 2023-04-26
Filing date: 2024-04-25
Publication date: 2026-01-21
Also published as: TW202449229A; CN121002223A; WO2024223735A1; KR20260003107A

Abstract

A method for the pre-treatment of a surface of a substrate prior to subsequent electroless copper deposition, the method comprising: a) providing a substrate comprising an activated surface onto a nonconductive material wherein the activated surface having an activator wherein the activator comprises a catalyzing metal; b) contacting the activated surface with an aqueous pre-treatment solution for pre-treating, the aqueous pre-treatment solution comprising or consisting of copper ions, an inorganic acid, and a reducing agent wherein the pH of the aqueous pre-treatment solution is below 6 to obtain a pre- treated surface; and with the proviso that the reducing agent is capable to reduce copper ions in an alkaline pH, but not capable to reduce copper ions within the pH below 6; an aqueous stock composition and its use for preparing a pre-treatment solution for a pre-treatment of a substrate, comprising an activated surface onto a nonconductive material wherein the activated surface having an activator wherein the activator comprises a catalyzing metal, to be used for a subsequent electroless deposition of copper on the pre-treated surface.

Description

A method for the pre-treatment of a surface of a substrate

Description

Field of the Invention

The present invention relates to a method for the pre-treatment of a surface of a substrate, an aqueous stock composition and its use for preparing a pre-treatment solution for a pretreatment of a surface. The method is in particular suitable to be used for a subsequent electroless deposition of copper on the pre-treated surface.

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 the production of printed wiring boards (PCBs), electroless copper plating baths are used to deposit e.g. copper onto activated surfaces having structures as through-holes and circuit paths as a base for subsequent electrolytic copper plating. The activation can be done by e.g. a catalyzing metal in form of e.g. palladium particles, or palladium-based colloids. The activation of the surface is needed to make the nonconductive surface susceptible by the catalyzing metal for the subsequent electroless metal plating. Said electroless metal plating is used for the deposition of e.g. copper as a base for further electroless or electrolytic plating of copper, nickel, gold, silver and other metals as required. Typical electroless copper plating baths contain copper compounds, complexing agents for the copper ions, reducing agents, and various additional compounds to make the bath more stable, adjust the plating rate and/or brighten the copper deposit.

JP2015147987A relates to an electroless copper plating to a non-electroconductive board substrate, in particular it relates to the water-system copper colloidal-catalyst liquid and the said electroless copper plating method for carrying out catalyst provision as pre-processing instead of a normally used palladium activation. After this pre-processing a further electroless copper plating liquid is applied to the first copper catalyst layer is conducted.

EP 3 578 683 B1 concerns an electroless copper plating bath for depositing at least a copper or copper alloy layer on a surface of a substrate, a method for depositing at least a copper or copper alloy layer on a surface of a substrate utilizing said electroless plating bath, and a kit-of- parts for providing the inventive electroless copper plating bath.

Although many of such electroless plating compositions are successful used, the metallization industry has still a demand to improve electroless copper plating to provide uniform plating layers for subsequent plating processes and avoids the formation of voids within through-holes and circuit paths.

Object of the Invention

It is therefore an object of the present invention to provide means to improve the surface of a substrate to use it for a subsequent improved electroless copper plating which provides uniform plating results onto the surface.

It is further an object of the present invention to improve the surface of a substrate to use it for a subsequent electroless copper deposition after activation of the surface to be treated in terms of homogeneity and/or efficiency.

It is still another object of the present invention to provide means for a pre-treatment of a surface prior to subsequent electroless copper deposition. Thus, when the surface is treated with an electroless metal plating bath, the metal or metal alloy coverage does not remain incomplete and thus unsatisfactory.

It is still another object to provide a method for a pre-treatment of a surface to improve subsequent electroless copper plating, in particular by avoid a depletion of copper ions in the beginning of said subsequent electroless copper deposition, and to allow for an improved copper coverage of said subsequent electroless deposited copper layer, particularly for obtaining a reduced number of voids when copper is deposited on the surface of the substrate, in particular in surface structures as through-holes and circuit paths after the pre-treatment.

Summary of the Invention The invention solves the problems of the prior art and provides according to the independent claims a method for the pre-treatment of a surface of a substrate which is in particular suitable to be used for subsequent electroless copper deposition on a pre-treated surface, an aqueous stock composition and the use of an aqueous stock composition for preparing the pre-treatment solution for preparing a pre-treatment solution for the pre-treatment of a surface prior to a subsequent electroless deposition of copper on the pre-treated surface.

In this regard, the invention provides, in a first aspect, a method for the pre-treatment of a surface of a substrate preferably to be used for subsequent electroless copper deposition, the method comprising: a) providing a substrate comprising an activated surface onto a nonconductive material wherein the activated surface having an activator wherein the activator comprises a catalyzing metal; b) contacting the activated surface with an aqueous pre-treatment solution for pre- treating, the aqueous pre-treatment solution comprising or consisting of

- copper ions, preferably Cu²⁺ ions,

- an inorganic acid, preferably sulfuric acid or hydrochloric acid, and

- a reducing agent, preferably formaldehyde, wherein the pH of the aqueous pre-treatment solution is below 6, preferably from 1 to 5, to obtain a pre-treated surface; and with the proviso that the reducing agent is capable to reduce copper ions in an alkaline pH, but not capable to reduce copper ions within the pH below 6.

In a further aspect, the invention provides an aqueous stock composition for preparing the pretreatment solution for the pre-treatment of a substrate, comprising an activated surface onto a nonconductive material wherein the activated surface having an activator wherein the activator comprises a catalyzing metal, preferably to be used for a subsequent electroless copper deposition, comprising or consisting of

- copper ions, preferably Cu²⁺ ions, having a concentration from 700 to 900 mmol/L,

- an inorganic acid, which is preferably selected from sulfuric acid or hydrochloric acid, having a concentration from 100 to 600 mmol/L, preferably from 200 to 400 mmol/L

- optionally a reducing agent, preferably formaldehyde, having a concentration 700-3300 mmol/L, and wherein the pH of the aqueous stock composition is below 6; and with the proviso that the optional reducing agent is capable to reduce copper ions in an alkaline pH, but not capable to reduce copper ions within the pH below 6.

In still a further aspect, the invention relates to the use of the aqueous stock composition above for preparing a pre-treatment solution for the pre-treatment of a substrate comprising an activated surface onto a nonconductive material wherein the activated surface having an activator wherein the activator comprises a catalyzing metal, preferably to be used for a subsequent electroless deposition of copper on the pre-treated surface.

Brief Description of the Drawings

Features will become apparent to those of ordinary skill in the art by describing in detail exemplary embodiments with reference to the attached drawings in which:

Fig. 1 illustrates results of copper plating experiments after a pre-treatment of the present invention

Detailed Description of the Invention

The terms “electroless copper deposition bath”, “electroless copper plating bath” and “aqueous electroless copper deposition bath” have the same meaning if not otherwise stated.

In the following description of embodiments of the present invention, the terms of a singular form may include plural forms unless the context clearly indicates otherwise.

The aqueous pre-treatment solution is intended to pre-treat an activated surface of substrate before applying a subsequent electroless copper deposition step in order to enhance the formation of the subsequent plated copper layer which is deposited from an electroless copper deposition bath. The aqueous pre-treatment solution is therefore also called as “enhancer solution”. Both terms will be used in the description.

The method of the present invention allows for an improved copper coverage of a subsequent electroless deposited copper layer onto the activated and pre-treated surface of the substrate and reduces the number of voids when copper is deposited on the surface, in particular in surface structures as trough-holes and circuit paths after the pre-treatment. Without wish to be bound by theory, it is believed that during usual electroless copper deposition without pre-treatment, the amount of the to be reduced copper ions at the surface and/or direct vicinity of the surface of the substrate is depleted at the beginning of the electroless copper deposition. This leads to uneven plating layers and voids within the surface structures, in particular within trough-holes and circuit paths.

It could surprisingly be found that by contacting the activated surface with the aqueous pretreatment solution of the present invention, the obtained pre-treated surface is deposited with at least copper ions and reducing agent. Due to the acidic pH, the reduction of copper ions will not start at this point and the pre-treated surface is prepared for further processing. With other words, the reducing agent can reduce copper ions in an alkaline environment, but is not capable to reduce copper ions within the pH below 6, preferably below 5, more preferred in the pH range from 1 - 5, most preferably below 2 or in the pH range from 1 to 2, if this is the main or only difference in the solution composition. Thus, once the obtained pre-treated surface is brought into contact with a subsequent applied alkaline electroless copper plating bath the before mentioned problems can be overcome and the reducing agent onto the pre-treated surface will become active.

The aqueous pre-treatment solution preferably does not comprise or contain intentionally added nickel ions or cobalt ions as functional compounds. Intentionally added compounds in this context mean, that the compounds are not added as needed compounds of the solution and that the compounds if present from other source as drag-in don't have a functional impact of the pretreatment solution.

The aqueous pre-treatment solution preferably also does not comprise any further organic additives as wetting agents, surfactants, or stabilizer in addition to the mentioned reducing agent, which may be an organic reducing agent, and in addition to an optional complexing agent which may be an organic complexing agent.

In a preferred embodiment of the invention the aqueous pre-treatment solution used with the method consisting of copper ions, an inorganic acid for pH adjustment, and a reducing agent, wherein the pH of the aqueous pre-treatment solution is below 6 to obtain a pre-treated surface; and with the proviso that the reducing agent is capable to reduce copper ions in an alkaline pH, but not capable to reduce copper ions within the pH below 6. The substrate is preferably a substrate that is intended to be metallized with a copper layer on the surface or on at least a part of the surface having structures as trough-holes and circuit paths. The substrate is preferably a substrate for the manufacturing of an integrated circuit (IC substrate) or a substrate for the manufacturing of a printed circuit board (printed circuit board substrate).

The surface of the substrate to be treated with the pre-treatment solution of the present invention comprises an activated surface onto a nonconductive material having an activator onto said surface wherein the activator comprises or is a catalyzing metal such as copper, silver, gold, palladium, platinum, rhodium, cobalt, ruthenium, iridium in form of pure metal or mixed metal colloids thereof, preferably the activator comprises palladium colloids. The activated surface after activation with a catalyzing metal normally does not result in a discrete metal layer but in an islandlike structure of metallic spots on the surface of the substrate. Within the activation, it is possible to sensitize substrates prior to the deposition of the metal or metal alloy thereon. This may be achieved by the adsorption of a catalyzing metal onto the surface of the substrate.

“Pure” in this context means only one metal forms a metal colloid. Alternative "pure” further means that the metal content of the catalyzing metal is of at least 98 wt.-%, even more preferably of at least 99 wt.-%.

The substrate comprises the nonconductive (dielectric) material, preferably a non-metallic material, more preferably a plastic and/or a resin. Resins and plastics include dielectric materials typically used in the electronics industry which are to be metallized. Resins and plastics are preferably selected from epoxy as epoxy resin, isocyanate resin, bismaleimide triazine resin, and phenylene resin; polyester such as polyethylene terephthalate (PET), polyimide (PI), polytetrafluorethylene, acrylonitrile-butadiene-styrene (ABS) copolymer, polyamide (PA), polycarbonate (PC) as well as mixtures and blends of the aforementioned.

The organic polymers more preferably comprise polyimide resins or epoxy resins wherein the polyimide resins can be modified by the addition of polysiloxane, polycarbonate, polyester or the like. The epoxy resins can be glass filler epoxy board material comprising a combination of the epoxy resin and glass filler, or the same modified to have a low thermal expansion and a high glass-transition temperature, constituting a high glass-transition temperature glass filler epoxy board material. The organic polymer can comprise a filler, such as glass filler or a silica filler. A preferred glass filler is preferably selected from borosilicate glass, quartz glass, silica glass, fluorinated glass. The size of different filler has a range from 0.01 pm to 5 pm in diameter with preferably an average of 0.5 pm in diameter.

The method can be used in vertical and horizontal plating equipment. Preferably the method is used in vertical plating equipment, wherein the substrate is conveyed by a transport device to be processed through the treatment modules of the plating equipment.

The copper ions, preferably Cu²⁺ ions, of the present invention can be obtained from any suitable, water-soluble copper salt. Non-limiting examples are copper sulfate or copper chloride.

The concentration of the copper ions in the pre-treatment solution is preferably from 20 to 270 mmol/L, more preferably 30 to 135 mmol/L, most preferably 55 to 75 mmol/L.

The concentration of the inorganic acid is preferably from 10 to 100 mmol/L, more preferably 20 to 50 mmol/L.

The inorganic acid is preferably selected from sulfuric acid or hydrochloric acid, more preferably sulfuric acid.

In a preferred embodiment inorganic acid is sulfuric acid having a concentration from 10 to 100 mmol/L (basing on water-diluted sulfuric acid of 50 weight-% of the total diluted solution).

The pH of aqueous pre-treatment solution is preferably from 1 to 5, more preferably 1 to 4, most preferably 1 to 2.

In one embodiment an organic acid, preferably an alkanoic acid, more preferably formic acid, can be additionally added to the aqueous pre-treatment solution. The organic acid has preferably a concentration from 10 to 100 mmol/L.

In one embodiment of the used pre-treatment solution, the reducing agent which is capable to reduce copper ions in an alkaline pH, but not capable to reduce copper ions within the pH below 6, is selected from glyoxylic acid and/or formaldehyde. Preferably the reducing agent is formaldehyde. Preferably if formaldehyde is used, the pH is from 1 to 5, more preferred from 1 to 2.

The concentration of the reducing agent in the pre-treatment solution is preferably from 80 to 320 mmol/L, more preferably from 120 to 280 mmol/L.

The pre-treatment solution preferably comprises a complexing agent that is suitable to complex the copper ions, preferably suitable to complex Cu²⁺ ions in the pre-treatment solution.

The complexing agent of the pre-treatment solution is not needed but can improve plating performance and efficiency. The complexing agent has preferably a concentration from 4 to 60 mmol/L.

In one embodiment, the complexing agent is preferably selected from ethylenediaminetetraacetic acid (EDTA), hydroxyethylethylenediaminetriacetic acid (HEDTA), (Ethylendinitril)-tetra-2- propanol (also called Quadrol) and (2R,3R,4S)-Pentane-1 ,2,3,4,5-pentol (also called Xylitol) and tartaric acid. In a preferred embodiment, the complexing agent is tartaric acid.

Following specific embodiments of the pre-treatment solution have proven to be particularly effective in avoiding voids in a subsequently deposited copper layer.

In one embodiment, the pre-treatment solution has

- the concentration of the Cu²⁺ is 20 to 270 mmol/L;

- the concentration of the reducing agent is 80 to 320 mmol/L; and

- if present: the concentration of the optionally used complexing agent, preferably tartaric acid, is 4 to 60 mmol/L, and the pH is adjusted to be below 6 by the inorganic acid.

Preferred concentrations of the inorganic acid, preferably sulfuric acid, that can be combined with the aforementioned concentration ranges are 10 to 100 mmol/L, more preferred 20 to 50 mmol/L.

Preferred concentrations of hydrochloric acid that can be combined with these concentration ranges are 10 to 100 mmol/L. In a preferred embodiment, the pre-treatment solution has

- the concentration of the Cu²⁺ is 30 to 135 mmol/L;

- the concentration of the reducing agent is 80 to 320 mmol/L; and

- if present: the concentration of the optionally used complexing agent, preferably tartaric acid, is 7 to 30 mmol/L, and the pH is adjusted to be below 6 by the inorganic acid.

Preferred concentrations of sulfuric acid that can be combined with these concentration ranges above are preferred 20 to 50 mmol/L.

In one embodiment of the pre-treatment solution, the pH of the pre-treatment solution is adjusted to 1 to 5, preferably 1 to 4, more preferably to 1 to 2.

Pre-treating the surface with the pre-treatment solution can be done in different ways, such as, without limitation, immersion or rinsing. Pre-treating the surface with the pre-treatment solution is preferably done by immersion of the surface into the pre-treatment solution. A preferable immersion time is 0.5 to 3 min. The temperature of the pre-treatment solution is preferably in a range of from 20 to 35°C.

The method of the present invention is preferably suitable to be used for subsequent electroless copper deposition with an alkaline electroless copper plating bath, preferably the electroless copper plating bath is directly applied after obtaining the pre-treated surface in step b).

In one embodiment, the inventive method therefore further comprises an electroless copper deposition step onto the pre-treated surface after pre-treating the surface with a pre-treatment solution in order to obtain an electroless deposited copper surface. Preferably, the electroless deposition is done after the pre-treatment of the surface without any intermediate rinsing step. It has turned out that avoiding an intermediate rinsing step reduces the number of voids in the metallic layer that is deposited.

After the electroless copper deposition of the pre-treated surface to obtain a copper surface, the method preferably further comprises an electrolytic deposited copper deposition. If the method comprises the pre-treatment of the surface and a following copper deposition, it can also be called a method for pre-treatment and copper deposition, or simply a method for copper deposition.

In one embodiment, after the pre-treatment is done, electroless metallization is conducted with an aqueous electroless copper deposition bath for electroless deposition of copper, the bath comprising copper ions, preferably Cu²⁺ ions, a complexing agent as tartaric acid, a reducing agent as formaldehyde, and wherein the pH of the bath is 9 to 14, preferably > 10, more preferred from 11 to 14, most preferred from 13 to 14. A pH in this range can be reached by presence of a suitable amount of NaOH or KOH. A suitable amount means an amount that is suitable to reach the desired pH. The electroless copper deposition bath may or may not contain nickel ions. Suitable electroless copper plating bath compositions are Printoganth MV TP2 und MV Plus and can be purchased from Atotech Deutschland GmbH & Co. KG. The concentration of the copper ions in this electroless copper bath composition preferably ranges from 1 g/l to 10 g/L, more preferably from 2 g/l to 5 g/l. Said at least one reducing agent is thus capable of converting copper(l)-ions and/or copper(ll)-ions present in this composition to elemental copper. The concentration of the at least one reducing agent in electroless copper bath used preferably ranges from 0.02 mol/l to 0.3 mol/l, more preferably from 0.054 mol/l to 0.2 mol/l, even more preferably from 0.1 mol/l to 0.2 mol/l. Preferably, the electroless copper bath comprises at least one complexing agent for copper ions, e.g. hydroxycarboxylic acids such as tartaric acid. Preferably, the concentration of the complexing agent ranges from 0.004 mol/l to 1.5 mol/l, more preferably from 0.02 mol/l to 0.6 mol/l, even more preferably from 0.04 mol/l to 0.4 mol/l.

In this embodiment the pre-treatment solution (enhancer solution) that is used for pre-treatment according to the method of the invention comprises same ingredients, Cu²⁺, an inorganic acid, and a reducing agent, as the following bath for electroless deposition of copper. If the pretreatment solution comprises a complexing agent, then preferably the same complexing agent is used as in the bath for electroless deposition of copper. A preferred complexing agent is tartaric acid. The difference between the pre-treatment solution and the bath for electroless deposition of copper (also called electroless copper plating bath) are: i) the bath for electroless deposition of copper can comprise further organic additives that are known from such baths and ii) the pretreatment solution has a lower (acidic) pH than the bath for electroless deposition, which is alkaline. Own experiments have shown that not all reducing agents are capable to reduce copper ions to metallic copper in a solution having a pH below 6. With other words, copper ions are not reduced in such a solution having a pH below 6 if the reducing agent is e.g. formaldehyde and/or glyoxylic acid.

Generally, the used electroless alkaline copper plating bath comprises further optional components, which are stabilizing agents, co-solvents, wetting agents and functional additives such as brighteners, accelerators, suppressors, anti-tarnish agents. Such baths and components are known in the art.

The electroless copper plating bath may further comprise sources of nickel ions, sources of cobalt ions and mixtures thereof, wherein the pre-treatment solution does not contain any nickel ions, cobalt ions and mixtures thereof.

In one embodiment, the method of the invention comprises further pre-treatment steps prior to the pre-treating of the substrate comprising an activated surface onto a nonconductive material, wherein the activated surface having an activator wherein the activator comprises a catalyzing metal with the pre-treatment solution, to obtain said activated surface, wherein the said pretreatment steps comprise in this order:

- optional step (i) treating the surface with an aqueous alkaline cleaner solution comprising ethanolamine;

- step (ii) treating the surface of step (i) with an aqueous etch cleaner solution comprising persulfate;

- optional step (iii) treating the surface of step (ii) with a pre-dip solution;

- step (iv) treating the surface of step (iii) with an activator solution, preferably comprising catalyzing metal such as copper, silver, gold, palladium, platinum, rhodium, cobalt, ruthenium, iridium in form of pure metal or mixed metal colloids thereof; and

- step (v) treating the surface of step (iv) with a reducer solution if the activator solution comprises metal ions.

Step (i) is preferably performed at 50° to 70 °C for 3 to 7 min. In a preferred embodiment, the cleaner comprises 10 to 20 g/L sodium hydroxide and 10 to 16 g/L ethanolamine (MEA). Step (ii) is preferably performed at 25° to 40 °C for 1 to 2 min. In a preferred embodiment, the cleaner comprises 100 to 150 g/L sodium persulfate and 30 to 45 g/L sulfuric acid.

Optional step (iii) treating the surface of step (ii) with a pre-dip solution. The pre-dip solution preferably comprises a pH regulator, preferably as sulfuric acid and/or sodium hydrogen sulfate and a non-ionic surfactant, preferably as a polyethylene glycol (PEG) compound, more preferably PEG 1500 MW or PEG 10,000 MW.

The pre-dip solution can be an acidic or alkaline pre-dip solution.

Step (iv) treating the surface of step (iii) with an activator solution, preferably comprising metal, metal colloids or metal ions - containing compound. The activator solution is applied onto the surfaces of the nonconductive layer, preferably treating the surface with a palladium activator solution wherein a palladium ion layer is deposited onto the surfaces of the nonconductive layer of step (iv). Other useful activator solution known in the art may comprise metal colloids containing e.g. copper, nickel and alloys as palladium-tin.

The palladium activator solution comprises at least one source of palladium ions. Additionally, the solution may comprise other sources of metal ions, as sources of ruthenium ions, sources of rhodium ions, sources of palladium ions, sources of osmium ions, sources of iridium ions, sources of platinum ions, sources of copper ions, sources of silver ions, sources of nickel ions, sources of cobalt ions, sources of gold ions and mixtures thereof. The palladium ions and said additional metal ions are being adsorbed on the surface of said substrate.

(v) treating the substrate of step (iv), in case a palladium activator solution was used, is done with a palladium reduction solution wherein the deposited palladium ion layer in step (iv) is transformed into a metallic palladium layer.

Treating the surface of substrate comprising at least the palladium ions is conducted with a solution comprising at least one reducing agent suitable to reduce the metal ions to metallic state (at least the palladium ions) adsorbed on the surface of said substrate selected from the group consisting of boron based reducing agents, sources of hypophosphite ions, hydrazine and hydrazine derivatives, ascorbic acid, iso-ascorbic acid, sources of formaldehyde, glyoxylic acid, sources of glyoxylic acid, glycolic acid, formic acid, sugars, and salts of aforementioned acids. Further optional steps of the method of the invention is/are one or more cleaning steps, particularly a desmear process, which is known from the prior art.

Following embodiments relate to the aqueous stock composition of the invention. The aqueous stock composition is intended to be used for preparing the pre-treatment solution for the pretreatment of a substrate comprising an activated surface onto a nonconductive material wherein the activated surface having an activator wherein the activator comprises a catalyzing metal.

In one embodiment, the aqueous stock composition is prepared having the listed amounts as follows.

The aqueous stock composition comprises, or consists of:

15-50 g/L (calculated for H2SO4 96%weight-%), preferably 20-35 g/L, 100-400 g/L CuSO₄ x 5H₂O, and optionally 13-55 g/L complexing agent, preferably tartaric acid, and wherein the pH of the aqueous stock composition is below 6.

The reducing agent, that is capable to reduce copper ions in an alkaline pH, but not capable to reduce copper ions within the pH below 6, can be added to the aqueous stock composition or can be provided separately for preparing the pre-treatment solution for the pre-treatment of a surface of a substrate prior to subsequent electroless copper deposition. If the reducing agent is added directly to said aqueous stock composition, the added reducing agent, preferably formaldehyde and/or glyoxylic acid, more preferred formaldehyde, has a final concentration of 25-120 g/L within the aqueous stock composition. If the reducing agent is added separately to the pre-treatment solution for the pre-treatment of a surface of a substrate prior to subsequent electroless copper deposition, the concentration of the reducing agent in a reducing agent stock solution is 25-120 g/L and will be diluted in the same way as the aqueous stock composition for preparing the pretreatment solution for the pre-treatment of a surface of a substrate prior to subsequent electroless copper deposition.

Following embodiments relate to the use of the aqueous stock composition of the invention for preparing a pre-treatment solution for the pre-treatment of an activated surface of a substrate to be used for a subsequent electroless deposition of copper on the (activated and) pre-treated surface. The aqueous stock composition of the invention is also called “enhancer stock solution”.

In one embodiment, the aqueous stock solution is diluted with water when preparing the pretreatment solution. A preferred factor of dilution is in the range of 6 to 24, preferably 6 to 12. The pH is controlled to be below 6 preferable from 1 to 5, most preferred from 1 to 2.

In one embodiment, a reducing agent, preferably formaldehyde, is added when preparing the pretreatment solution. Preferably the reducing agent is provided in a reducing agent-stock composition. Both, the aqueous stock composition and the reducing agent stock composition can be mixed and then diluted. The resulting dilution factor of the stock composition(s) is the quotient of the resulting end volume (volume of the aqueous stock composition, optionally plus volume of the reducing agent stock composition, plus volume of added water) and the used volume of the aqueous stock composition and optionally the volume of the reducing agent stock composition. For example, if the end volume is 1000 ml and the used volume of aqueous stock composition (comprising the reducing agent) is 85 ml, the dilution factor is 1000/85 = 11.8.

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

Examples

1. Preparation of a stock solution

An aqueous stock composition with following components and concentrations was prepared. The components were diluted in deionized water.

Aqueous stock composition

Reducing agent stock composition

28 g/L formaldehyde

The final pH was below 2. 2. Preparation of pre-treatment solutions

The above stock composition was diluted in deionized water in order to produce pre-treatment solutions. pH was below 2.

2.1 Pre-treatment solution 1:

85 ml aqueous stock composition and 15 ml formaldehyde stock solution were diluted with water to a final volume of 1000 ml. pH was about 1.

2.2 Pre-treatment solution 2:

170 ml aqueous stock solution and 30 ml formaldehyde stock solution were diluted with water to a final volume of 1000 ml. pH was about 1.

2.2 Pre-treatment solution 3:

42.5 ml aqueous stock solution and 7.5 ml formaldehyde stock solution were diluted with water to a final volume of 1000 ml. pH was about 2.5.

The concentrations in mmol/L are for each component half of the concentration in comparison to pre-treatment solution 1.

3. Platina experiments

First, a substrate surface of each substrate was treated, in this order, with cleaner solution, etch cleaner solution, pre-dip solution, activator solution and reducer solution, and with intermediate rinsing steps. Details are shown in the below table. The substrate was immersed in each of the solutions.

Then, the surfaces were treated with the different pre-treatment solutions 1/2/3, as prepared in example 2, and without pre-treatment solution. The substrate was immersed in each of the pretreatment solutions.

After pre-treatment and without pre-treatment, the surface was metallized with copper by immersion in an electroless copper bath, without intermediate rinsing step.

Following Substrates were used a) Samples “60” (60pmx60pm VIA) b) Samples “75” (75pmx75pm VIA)

Fig. 1 shows the results of the plating experiments with different substrates and different pretreatment solutions. For both of the two substrates, the best results, in terms of minimization of voids after copper plating, were obtained for pre-treatment solution 1, wherein 85 ml stock composition of example 1 and 15 ml formaldehyde stock composition (28 wt.%) were diluted with water to a final volume of 1000 ml. The results for pre-treatment solution 1 are shown in col. 1/ “POR enhancer” for each substrate.

Without pre-treatment (col. 2 /”w/o enhancer”) and with pre-treatment solution 3 (col. 3/ “half cone, of enhancer”) leads to a significantly higher density of voids. With substrate a) (”60”), good results are also reached with pre- treatment solution 2 (col. 4/ “double amount of enhancer”), which may be caused by a smoother surface in substrate b) (’’Sample 75”) and other dimensions of the VIAs (vertical interconnection access) that may cause a better solution exchange.

Claims

1. A method for the pre-treatment of a surface of a substrate, the method comprising: a) providing a substrate comprising an activated surface onto a nonconductive material wherein the activated surface having an activator wherein the activator comprises a catalyzing metal; b) contacting the activated surface with an aqueous pre-treatment solution for pre- treating, the aqueous pre-treatment solution comprising or consisting of

- copper ions,

- an inorganic acid, and

- a reducing agent, wherein the pH of the aqueous pre-treatment solution is below 6 to obtain a pre-treated surface; and with the proviso that the reducing agent is capable to reduce copper ions in an alkaline pH, but not capable to reduce copper ions within the pH below 6.

2. The method of claim 1 , wherein the pre-treatment solution comprises a complexing agent that is suitable to complex the copper ions in the pre-treatment solution.

3. The method of claim 2, wherein the complexing agent has a concentration from 4 to 60 mmol/L.

4. The method of claim 1 or 2, wherein in the pre-treatment solution

- the concentration of the copper ions is from 20 to 270 mmol/L, preferably 30 to 135 mmol/L, more preferably 55 to 75 mmol/L,

- the concentration of the reducing agent is 80 to 320 mmol/L.

5. The method of one of the preceding claims, wherein the inorganic acid has a concentration from 10 to 100 mmol/L, more preferably 20 to 50 mmol/L.

6. The method of one of the preceding claims, wherein the method further comprises an electroless copper deposition of the pre-treated surface to obtain an electroless deposited copper surface.

7. The method of claim 5, wherein electroless copper deposition is done directly after the pre-treatment of the surface without any intermediate step as rinsing or drying the pretreated surface.

8. The method of claim 5 or 6, wherein the electroless copper deposition is done with an aqueous electroless copper deposition bath, the bath comprising copper ions, preferably Cu²⁺ ions, a complexing agent, a reducing agent, and wherein the pH of the bath is 9 to 14, preferably 13 to 14.

9. The method of one of the preceding claims, wherein the catalyzing metal is copper, silver, gold, palladium, platinum, rhodium, cobalt, ruthenium, iridium in form of pure metal or mixed metal colloids thereof.

10. The method of one of the preceding claims 1 - 9, wherein the method comprises further pre-treatment steps prior to the pre-treating of the surface, preferably a nonconductive surface, with the pre-treatment solution, wherein the said further pre-treatment steps comprise, or consist of, in this order:

- step (iv) treating the surface of step (iii) with an activator solution, preferably comprising catalyzing metal, mixed metal colloids or metal ions ; and

- step (v) treating the surface of step (iv) with a reducer solution if the activator solution comprises precious metal ions.

11. The method of one of the preceding claims, wherein the pre-treatment solution does not contain intentionally added nickel ions and cobalt ions.

12. An aqueous stock composition for preparing the pre-treatment solution for the pretreatment of a substrate comprising an activated surface onto a nonconductive material wherein the activated surface having an activator wherein the activator comprises a catalyzing metal, comprising or consisting of - copper ions, preferably Cu²⁺ ions, having a concentration from 700 to 900 mmol/L,

13. The aqueous stock composition of claim 12, further comprising 160 to 200 mmol/L of a complexing agent, preferably tartaric acid, that is suitable to complex copper ions, preferably Cu²⁺ ions, in the liquid composition.

14. Use of the aqueous stock composition of claim 12 or 13 for preparing a pre-treatment solution for the pre-treatment of a substrate comprising an activated surface onto a nonconductive material wherein the activated surface having an activator wherein the activator comprises a catalyzing metal, according to one of the claims 1 - 11.

15. The use of one of the preceding claims 12 to 14, wherein the reducing agent, is preferably formaldehyde and/or glyoxylic acid, is added when preparing the pre-treatment solution.