WO2022043889A1

WO2022043889A1 - Method for electroless nickel deposition onto copper without activation with palladium

Info

Publication number: WO2022043889A1
Application number: PCT/IB2021/057786
Authority: WO
Inventors: Aldona Jagminiene; Ina Stankeviciene; Karolis RATAUTAS; Eugenijus Norkus; Gediminas Raciukaitis
Original assignee: Valstybinis Moksliniu Tyrimu Institutas Fiziniu Ir Technologijos Mokslu Centras
Priority date: 2020-08-27
Filing date: 2021-08-25
Publication date: 2022-03-03
Also published as: LT6899B; EP4204599A1; KR20230056751A; US20220064801A1; WO2022043889A4; CN116324032A; JP2023538951A; LT2020542A

Abstract

The invention relates to selective deposition of a nickel layer on a copper surface. The invention may be used in the production of electrically conductive areas for electronic circuits. Method for nickel deposition on the surface of copper comprises immersing an item, which surface is to be deposited with the nickel layer, into one or more baths, of which at least one contains a reducing agent and of which at least one is adapted for (electroless) plating of nickel. In order to extend the field of application and to obtain practically pure nickel coatings, said reducing agent comprises boronic or phosphoric compounds, comprising morpholine borane (C₄H₉BNO), or dimethylamine borane (C₂H₇BN), or sodium tetrahydroborate (NaBH₄), or sodium hypophosphite (NaH₂PO₂) and said reducing agent directly or indirectly reduces insoluble copper (I) or copper (II) compounds on the copper surface.

Description

Method for electroless nickel deposition onto copper without activation with palladium

Field of invention

The invention relates to the field of chemical methods of metal deposition, which are used in electronics, automotive and other areas. In particular, the invention is for selective deposition of a nickel layer on a copper surface. The invention is essential in the production of electrically conductive areas for electronic circuits, where a barrier layer of nickel with excellent adhesion is formed on an electrically conductive copper track to prevent undesired diffusion of copper into other materials in contact with electrically conductive areas and elsewhere where copper is required for final or intermediate coating.

Background of invention

The patent US6180523B1 of Chwan-Ying Lee (13/10/1998) discloses three options for forming Cu I Au contacts and bonds using electroless metal deposition with a three- layer system: an adhesive layer, a barrier layer and a finishing layer. The three versions have different barrier layers for electroless deposition of Cu or Au. The adhesive layer is formed of Ni, Al, polycrystalline silicon or PdSi_x on electrically non- conductive material. The first barrier layer is electroless deposited on the adhesive layer. The first barrier layer consists of Ni, Pd, Co or Ni, Pd and Co alloys. The first barrier layer is activated using a solution containing PdCI₂. Finally, the top layer of Cu or Au is electroless coated.

The disadvantage of the known method is that a complex multilayer deposition system is used. Ionic activation with palladium is performed using an expensive and rare material. Palladium is a very active catalyst that adsorbs perfectly not only on the metal; thus, the selectivity of the metal deposition can be lost, and the coated metal can be deposited not only on the desired metal structure but also next to it on the surface of the dielectric substrate.

Japanese Patent Application No. 9-307234 by Yo. Funada et al. (20/05/1996) (US Patent No. 5,830,563) described a method used to make conductive areas on printed circuit boards where palladium is deposited on a copper surface by immersion into palladium containing bath: by galvanic replacement reactions of copper with palladium. Palladium attached to the surface acts as a catalyst for the electroless deposition of metals. On the other hand, activation with palladium for the chemical (electroless) deposition of metals in which palladium ions are reduced by the oxidation reaction of tin ions has been known in the past.

The disadvantage of the known method is that activation is made by palladium, which uses expensive and rare material. Palladium is a highly active catalyst that adsorbs perfectly not only on the metal, thus losing the areal selectivity of the metal deposition, and the coating metal can be deposited not only on the desired metal structure but also near it on the surface of the dielectric substrate.

EP2233608B1 Elisabeth Zettelmeyer et al. (23/03/2009) invention relates to a final layer coating process for the production of printed circuit boards (PCBs). The invention describes a method of electroless coating of nickel on a copper surface. The process comprises i) activating the copper surface with palladium ions; (ii) removal of excess palladium ions or their precipitate by a special solution containing at least two different types of acids, wherein one type being an organic amino carboxylic acid, and (iii) deposition of nickel without flow.

The disadvantage of the known method is that palladium activation is used, which uses expensive and rare substances such as palladium. The acid washing procedure is incompatible with many semiconductors, thus limiting the application of the technology. Washing palladium with an acidic solution consumes the expensive and rare palladium even more.

The patent KR101883249B1 (13/11/2013) described by Tae-Hyeon Lee and Tae- Kwon Lee relates to surface preparation for the electroless deposition of nickel on copper, comprising the step of etching in the liquid etchant for cleaning and roughening the copper surface; after the etching procedure, the copper surface is treated with a preparation liquid containing acid, water and a reducing agent; after this procedure, the prepared surface is activated with an ionic palladium solution.

The disadvantage of the known method is that the etching procedure severely limits the substrate (dielectric) materials. Moreover, the non-conductive surface can be roughened during etching so that active palladium ions can be adsorbed on surfaces that should not be coated. Also, palladium is expensive and rare.

US20110051387A1 (10/08/2009) K. Tachibana, T. Ito and Y. Mitsui describes a method for electroless nickel-palladium-gold coating on a substrate where a delicate metal structure is formed on an organic resin; the method comprises the steps of treating the metallic structure with a palladium catalyst, followed by the formation of nickel-palladium-gold layers on the structure. The method in which at least one surface treatment selected from treatment with a solution of pH 10 to 14 and a plasma treatment is performed at an optional step after the step of providing a palladium catalyst and before the step of performing electroless palladium plating.

The disadvantage of the known method is that palladium is used not only for activation but also for the formation of a barrier layer by electroless deposition. This method uses a lot of expensive palladium.

A common disadvantage of all the above methods (using palladium catalysis) is that palladium forms an island-like structure on the surface to be catalysed, which can lead to a non-uniform coating containing undercoated areas. Therefore, the growing of a barrier layer using such a nucleus of palladium catalyst will produce a non-uniform and heterogeneous barrier coating. In addition, to form a barrier layer as a continuous uniform coating over the entire metal surface, it is necessary to increase the thickness of the barrier layer, the thickness of which depends on the density of formed palladium islands. Palladium is also known to have very strong adsorption on any surface, which reduces the selectivity of the metal layer deposition. Such problems make it difficult to control the process.

In US patent No. 4,002,778, by H. Bellis et al. (15/08/1978), an electroless process for the deposition of nickel or cobalt on electrically non-conductive substrates without the use of palladium as a catalytic layer is described. The electrically non-conductive layer is activated by a solution consisting of water and a reducing agent. Alkali metal borohydrides, alkali metal cyanoborohydrides and amino boranes can be used as reducing agents in a concentration of at least 0.5 g/l of reducing agent in said solution. The coated article is then immersed in an electroless plating bath in order the chemical reduction of the nickel would take place, provided that the reducing agent in the activation solution is an alkali metal borohydride which is also used as a reducing agent in the plating bath. It should be noted that an additional process step for electroless deposition of copper - oxidation of the copper surface with an aggressive oxidiser - persulfate is used.

In addition, the solution proposed in US pat. No. 4,002,778 contains alkali metal salts, which are particularly undesirable in semiconductor processing. The deposition process is slow.

The Patent application US2008/0254205A1 (13/04/2007) claims electroless deposition of cobalt or nickel alloys from solutions containing two stabilizers, namely hypophosphite and Mo(VI) compound.

In the case of the present invention, no stabilizers are necessary. In some implementations, hypophosphite acts as a reducing agent but not as a stabilizer. Concerning boron containing reducing agents, we use morpholine borane or sodium borohydride, which are not used in above mentioned Application. In addition, according to Patent application US2008/0254205A1 , only nickel alloys could be platted, while the present application allows obtaining pure nickel coatings.

US patent No. 5,695,810 issued to V. Dubin et al. (09/12/1997) describes a method in which the copper surface is coated with a complex cobalt-tungsten-phosphorus (Co-W- P) coating. The technology attempts to create a barrier layer on the copper surface to prevent copper diffusion by forming layers and I or structures on a semiconductor wafer. In this method, the cobalt-tungsten-phosphor coating is deposited by electroless deposition, activating the copper surface before the coating process.

Such a process is characterised by a low deposition rate, which can be improved by catalytic activation.

US patent No. 6794288B1 Kolics et al. (05/05/2003) describe a method for selectively depositing Co-W-P structural coatings on copper without palladium activation by forming hydrogen-enriched complexes on a metal surface followed by deposition of the metal. More specifically, this method involves the formation of said complexes on copper surfaces before the Co-W-P system coatings are chemically deposited. This is achieved by short-time contacting the copper surface with reducers at high temperatures. Such reducing agents include hypophosphitic acid or boron-containing reducing agents such as dimethylamine borane. Reduction with hypophosphitic acid is preferred because it is more compatible with the chemical precipitation solution. The method is applicable for coatings containing phosphorus, deposited by electroless plating procedure.

The disadvantage of this method lies in the fact that the authors wash the activated copper surface with water, which is a significant drawback, as hydrolysis reactions can occur during the water wash, the products of which can adsorb and contaminate the coated surface. This is especially an issue when hypophosphitic acid is used. In addition, the chemical plating process is carried out at high temperatures, which is not attractive from an industrial point of view.

Another disadvantage of the known method is that the activation is used only for chemically deposited coatings that contain phosphorus, which significantly narrows the application of the barrier layer formation method.

The US patent application 2005/194255A1 , Tiwari Chandra S. (04/03/2004) describes a method of metal plating to form interconnect on a semiconductor assembly. The patent application US2005/194255A1 teaches about the self-activating process of Ni electroless deposition on a copper substrate, using hypophosphite or dimethylamine borane as a reducing agent. The electroless Ni deposition proceeds from alkaline solutions (pH>7) and temperatures > 85°C (in the case of hypophosphite) or > 55°C (in the case of dimethylamine borane). The disadvantage of the invention is that initiation of the nickel plating on copper surface is obtained only by reducing the copper oxides without binding them to complexes, thus reduced but still active copper can pollute the plating bath by forming copper contaminants like -hydroxy or -oxy compounds (again) back on the surface or in the bath solution, therefore reducing the quality of the plated layer and limiting the deposition rate. Another disadvantage is that the bath solution is limited to an alkaline environment only. Alkaline pH for electroless Ni deposition bath with hypophosphite as reducing agent: pH > 8, temperature limit higher than 85°C. The alkaline solution is not preferent in metal plating since surface contaminants such as oxides and hydroxides are formed on the metal surface. Since the time of filing of the patent application US2005/194255A1 , the understanding of electro-chemical processes has changed, and hence statements like at [0024] “... Ni⁺ ions displacing Cu⁺⁺ ions, which causes the monolayer of Ni.. are now know to be erroneous because there are no Ni⁺ ions (only Ni⁺⁺ ions) in the bath nor are Cu⁺⁺ ions displaced. Moreover, the explanation [0030-0049] on the mechanism of self-initiating therein is not in line with the present knowledge indicated above. To design methods for nickel deposition on a surface of an item that is produced from copper or has a copper layer on it, a good understanding of the chemical and/or thermodynamically behaviour is required, especially in relation to the self-initiating mechanism. It is known today that in the case of spontaneous displacement reactions, only metals having more positive standard reduction potentials can be deposited during the redox reaction. E.g. Ni (E⁰ = -0.25 V) + Cu²⁺( E⁰ = 0.34 V) ® Cu + Ni²⁺ (cell electromotive force EMF is 0.34 -(-0.25) = 0.59 V). The opposite reaction thermodynamically is not able. Therefore, no Cu displacement by Ni from Ni²⁺ solutions is possible, as alleged in the patent application US2005/194255A1.

In EP2177646A1 , Boyle Mike et al. (17/10/2008) describe the method of electroless plating of triple metal coating on Al or Cu surface: (a) a Ni-P layer, (b) a Pd layer and, optionally, (c) an Au layer. The first Ni-P layer is deposited by means of the electroless Ni deposition method, using hypophosphite as a reducing agent.

The disadvantage of the invention is that the mentioned process is not self-activating one, since Pd activation is required prior to the Ni-P deposition.

Technical problem to be solved

The aim of the invention is to create economically viable and rare materials, such as palladium-saving technology for nickel barrier layer on copper surface formation for electrically conductive traces, and contacts production, expand its field of application, enabling nickel barrier layer to be formed directly on the copper surface without auxiliary activation of palladium or the similar layer formation, therefore, reducing the number of process steps. Practically pure nickel coatings can be obtained by this method, in contrast to the methods described in the analogues. Disclosure of the invention The essence of the solution according to proposed invention is that in a method for nickel deposition on a surface of an item that is produced from copper or has a copper layer on the item comprising: immersing said item into one or more baths, at least one bath containing a reducing agent; at least one bath being adapted for electroless plating of nickel wherein, said reducing agent comprises boronic or phosphoric compounds, comprising morpholine borane (C₄H₉BNO), or dimethylamine borane (C₂H₇BN), or sodium tetrahydroborate (NaBH₄), or sodium hypophosphite (NaH₂PO₂) and said reducing agent directly or indirectly reduces insoluble copper (I) or copper (II) compounds on the surface of the item which is produced from copper or on the copper layer of the item; and at least one of the mentioned baths comprises a ligand or mixture thereof, the ligand in the bath facilitating the dissolution of incompletely reduced insoluble copper compounds - copper oxides - by binding them to soluble complexes, such that a substantially pure copper surface is present where nickel is deposited. Said ligand or mixture thereof consists of any water-soluble chemical compounds capable of forming sufficiently stable complexes with copper ions, comprising amino acetic acid (C₂H₅NO₂), nitrilotriacetic acid (C₆H₉NO₆), diethylenetriaminepentaacetic acid (C₁₄H₂₃N₃O₁₀) and their salts, tartaric acid (C₄H₆O₆); citric acid (C₆H₈O₇) and their salts; ammonia (NH₃), ethylenediamine (C₂H₈N₂), diethylenetriamine (C₄H₁₃N₃), N,N,N′,N′-tetrakis (2-hydroxypropyl) ethylenediamine (C14H32N2O4). Said method further comprises: (i) immersing said item in the activation bath, which comprises said reducing agent and said ligand wherein electrons which appear after an anodic oxidation reaction of said reducing agent from the activation bath activate the copper surface by reducing Cu(I) and Cu(II) oxides and/or oxy-hydroxy compounds on the surface, at the same time, anodic catalytic or thermal decomposition reactions occur, releasing hydrogen, also reacting as an active reducing agent with the Cu(I) and Cu (II) compounds on the copper surface, leaving the substantially pure copper surface where nickel is then deposited; and; (ii) immersing said item with the activated copper surface into a second, electroless nickel plating bath, in which nickel is electrolessly deposited on the copper surface, wherein the immersing said item into the second electroless nickel plating bath occurs after immersing said item in the activation bath without any intermediate step.

Said method comprises the single step of immersing said item directly into the electroless nickel plating bath comprising a reducing agent selected from the group consisting of said boronic or phosphoric reducing compounds, which concentrations are selected to reduce the insoluble copper (I) and copper (II) compounds present on the surface of the item produced from copper or having a copper layer; and the a ligand or mixture of ligands selected from a group of said chemical compounds, selecting concentrations of said chemical compounds of said ligand or mixture of ligands such that said chemical compound dissolves the incompletely reduced insoluble copper compounds by binding them to soluble complexes so that substantially pure copper surfaces, on which nickel is deposited, remain.

Said activation bath consists of sodium hypophosphite (NaH₂PO₂) solution with a concentration from 0.5 M up to the limit of solubility and said ligands or mixtures thereof in a concentration from 0.001 M to the solubility limit, and the immersion time ranges from 1 to 15 min at a temperature of 80-96 °C.

Said activation bath consists of morpholine borane (C₄H₉BNO) solution with a concentration from 0.01 M up to the limit of solubility and said ligands or mixtures thereof in a concentration from 0.001 M to the solubility limit, and the immersion time ranges from 1 to 15 min at a temperature of 18-50 °C.

Said activation bath consists of dimethylamine borane (C₂H₇BN) solution with a concentration from 0.01 M up to the limit of solubility and said ligands or mixtures thereof in a concentration from 0.001 M to the solubility limit, and the immersion time ranges from 1 to 15 min at a temperature of 18-50 °C.

Said activation bath consists of sodium tetrahydroborate (NaBH₄) solution with a concentration from 0.01 M up to the limit of solubility and said ligands or mixtures thereof in a concentration from 0.001 M to the solubility limit and the immersion time ranges from 1 to 15 min at a temperature of 18-50 °C. Said electroless nickel plating bath consists of nickel sulphate (NiSO₄) at a concentration of 0.05-0.5 M; the reducing agent of sodium hypophosphite (NaH₂PO₂) at a concentration of 0.25-3 M; amino acetic acid (C₂H₅NO₂) at a concentration of 0.25- 1 M; and sodium hydroxide (NaOH) at a concentration sufficient for required pH adjustment.

Said surface of said item, which is produced by copper, is plated by nickel using a sequence of two different nickel plating baths, the first nickel plating bath containing sodium hypophosphite as the reducing agent, initially, an item which is necessary to metalize is immersed in an alkaline electroless nickel plating bath with pH in the range of 8.5-10.0, and later said item is immersed in the second nickel plating bath, which is an acidic electroless nickel plating bath with pH in the range of 4.0-6.0.

Said electroless nickel plating bath consists of nickel sulphate (NiSO₄) at a concentration of 0.05-0.5 M; the reducing agent of morpholine borane (C₄H₉BNO) at a concentration of 0.01-1 M; the ligand of diethylenetriamine (C₄H₁₃N₃) at a concentration of 0.001 -0.5 M; sodium hydroxide (NaOH) at a concentration sufficient for pH adjustment to pH 5.0-7.8; and plating at a temperature of 18-35 °C.

Said electroless nickel plating bath consists of nickel sulphate (NiSO₄) at a concentration of 0.05-0.5 M; the reducing agent of dimethylamine borane (C₂H₇BN) at a concentration of 0.01-1 M; the ligand of diethylenetriamine (C₄H₁₃N₃) at a concentration of 0.001 -0.5 M; sodium hydroxide (NaOH) at a concentration sufficient for pH adjustment to pH 5.0-7.8; plating at a temperature of 18-35 °C.

Said electroless nickel plating bath consists of nickel sulphate (NiSO₄) at a concentration of 0.05-0.5 M; the reducing agent of sodium tetrahydroborane (NaBH₄) at a concentration of 0.01-0.5 M; the ligand of ethylenediamine (C₂H₈N₂) at a concentration of 0.001-0.5 M; the ligand of potassium sodium tartrate (KNaC₄H₄O₆) at a concentration of 0.05-0.2 M; disodium thiosulphate (Na₂S₂O₃) at a concentration of 0.001-0.01 M; sodium hydroxide (NaOH) at a concentration sufficient for pH adjustment to pH 12.0-13.0; platting at a temperature of 18-35 °C. Said nickel electroless plating bath consists of nickel sulphate (NiSO₄); sodium hydroxide (NaOH); the ligand of amino acetic acid (C₂H₅NO₂), and the reducing agent of sodium hypophosphite (NaH₂PO₂) which concentration exceeds 0.8 M.

Said electroless nickel plating bath consists of nickel sulphate (NiSO₄) at a concentration of 0.05-0.5 M; the reducing agent of sodium hypophosphite (NaH₂PO₂) at a concentration of 0.8-3 M; the ligand of amino acetic acid (C₂H₅NO₂) at a concentration of 0.25-1 M; sodium hydroxide (NaOH) at a concentration sufficient for pH adjustment to pH 4.0-6.0; and plating at a temperature of 80-96 °C.

Said electroless nickel plating bath consists of nickel sulphate (NiSO₄) at a concentration of 0.05-0.5 M; the reducing agent of morpholine borane (C₄H₉BNO) at a concentration of 0.1-1 M; the ligand of diethylenetriamine (C₄H₁₃N₃) at a concentration of 0.001 -0.5 M sodium hydroxide (NaOH) at a concentration sufficient for pH adjustment to pH 5.0-7.8; and plating at a temperature of 18-35 °C.

Said electroless nickel plating bath consists of nickel sulphate (NiSO₄) at a concentration of 0.05-0.5 M; the reducing agent of dimethylamine borane (C₂H₇BN) at a concentration of 0.1-1 M; the ligand of diethylenetriamine (C₄H₁₃N₃) at a concentration of 0.001 -0.5 M, sodium hydroxide (NaOH) at a concentration sufficient for pH adjustment to pH 5.0-7.8; and platting at a temperature of 18-35 °C.

Said electroless nickel plating bath consists of nickel sulphate (NiSO₄) at a concentration of 0.05-0.5 M; the reducing agent of sodium tetrahydroborane (NaBH₄) at a concentration of 0.02-0.5 M; a ligand of ethylenediamine (C₂H₈N₂) at a concentration of 0.001-0.5 M; the ligand of potassium sodium tartrate (KNaC₄H₄O₆) at a concentration 0.05-0.2 M; disodium thiosulphate (Na₂S₂O₃) at a concentration of 0.001-0.01 M; sodium hydroxide (NaOH) at a concentration sufficient for pH adjustment to pH 12.0-13.0; and platting at a temperature of 18-35 °C.

Said method comprises determination of the concentration and/or process timing experimentally or by computer-implemented or semi-automatic means according to read and loaded the information on the quality of the deposited nickel layer of the item and the concentrations of the reagents in the baths, and on-site computing the concentration of the baths required for the process and/or the timing of the chemical process.

Advantages of the invention

The present invention is related to the formation of a nickel barrier layer on a copper surface for microelectronics or a decorative coating without the use of palladium activation. The practical advantage of the present invention is that the use of precious and rare metals such as palladium is eliminated. The proposed method significantly improves the quality of the barrier layer formation and the spatial selectivity resolution of the deposited electrically conductive areas, as palladium activation is not used. Palladium is a very active metal, so it absorbs and activates not only the desired regions but also the substrate next to it. As a result, nickel can later be deposited on undesired areas and create short circuits in electronic circuits. This method also allows the activation of copper surfaces for nickel plating faster than the other methods mentioned, because a ligand is used together with the reducing agent. Another essential advantage of this method is that nickel is formed directly on the copper surface and thus shortens the number of process steps and thus the duration as well. This method, unlike the alternatives for palladium-free copper surface activation mentioned in the analogues, allows the deposition of pure (not contaminated by side- products) nickel coatings.

Description of the invention and examples

Electrically conductive areas in electronic devices are mainly formed from copper, as copper has one of the highest electrical conductivity (higher than that of gold), is relatively inexpensive, and can be easily deposited on a dielectric substrate such as fibre-reinforced resins, polymers, or even ceramics. However, copper atoms diffuse into other materials such as solder, which cause degradation of the copper layer. Moreover, copper tends to oxidise easily. For these reasons, a barrier layer is deposited. The most convenient is to form a barrier layer of nickel into which no diffusion of copper atoms takes place, and a thin oxide-protective and electrically conductive layer of gold or platinum can be easily deposited on this layer by immersion method. Electroless deposition of nickel on a copper surface using electroless nickel- plating baths containing phosphorus compounds is practically impossible or a prolonged process without additional activation or sensitisation, and, therefore, a surface activation step is introduced. Activation is usually carried out with palladium, but the process is expensive and difficult to control the spatial selectivity of the deposition.

The present invention is intended for the activation of a copper surface for electroless deposition of nickel from a bath. The process can be implemented in two ways. First, by activation of the intended metallised areas with an activation solution (bath) consisting of reducing agents containing boron or phosphorus and comprising copper ion ligands or mixtures thereof, followed by immersion of the activated copper surface in an electroless nickel plating bath in which a nickel layer is selectively formed directly on the copper surface to be coated. Another option: Nickel can be deposited directly on the copper surface without additional surface activation step, using morpholine borane (C₄H₉BNO) or dimethylamine borane (C₂H₇BN) or sodium tetraborane (NaBH₄) or higher (over 0.8 M) concentration of sodium hypophosphate (NaH₂PO₂) as a reducing agent in electroless nickel plating baths.

Initially, the surface is washed or wiped with sulfuric acid H₂SO₄, concentration: 0.5- 4 M for 3-15 minutes. This step is performed if the surface is strongly oxidized. This step can be omitted if all other multilayer coating steps are performed immediately (within 5 minutes) after the chemical deposition of copper. The activation of the surface by an activation bath follows, optionally using one of the following activation processes by dipping in: a) sodium hypophosphite (NaH₂PO₂) solution with a concentration from 0.5 M up to the limit of solubility and which contains one or more copper ions ligand, which concentration is from 0.01 M up to the limit of solubility and the immersion time ranges from 1 to 15 minutes at 80-96°C temperature. After the activation, the surface is washed with 0.5 M NaH₂PO₂ solution at 80-96°C for 1 second. b) morpholine borane solution with a concentration from 0.01 M up to the limit of solubility and which contains one or more copper ions ligands, which concentration is from 0.01 M up to the limit of solubility and the immersion time ranges from 1 to 15 minutes at 18-50°C temperature. After activation, the surface is washed with water at 80-96°C for 1 second. c) dimethylamine borane solution with a concentration from 0.01 M up to the limit of solubility and which contains one or more copper ions ligands, and the immersion time ranges from 1 to 15 minutes at 18-50°C temperature. After activation, the surface is washed with water at 80-96°C for 1 second. d) with sodium tetrahydroborate (NaBH₄) at a concentration of 0.01 M to the solubility limit and one or more copper ions ligands at a concentration of 0.01 M to the solubility limit; process time 1-15 minutes at 18-50°C; after activation, the surface is washed with deionized water at 80-96°C for 1 second.

Examples of activation baths:

Bath no. 1 composition and coating conditions:

• 0.5 M - sodium hypophosphite (NaH₂PO₂);

• 0.01 M - N, N, N ', N'-tetrakis (2-hydroxypropyl) ethylenediamine;

• sodium hydroxide (NaOH) in a concentration sufficient to adjust the pH of the solution to 9.2;

• at 88°C for 5 min.

Bath no. 2 composition and coating conditions:

• 0.5 M - morpholino borane (C₄H₉BNO);

• 0.01 M - diethylenetriaminepentaacetic acid (C₁₄H₂₃N₃O₁₀);

• sodium hydroxide (NaOH) in a concentration sufficient to adjust the pH of the solution to 7;

• at 30°C for 5 min.

Further, it follows the chemical electroless deposition of nickel by immersing the article in chemical metallisation bath No. 3 (deposition rate ~9.2 pm/h), which is alkaline to initiate the deposition process, the coating process is performed for 1-10 minutes, after that, the article is immersed in bath No. 4 (acidic), in which the nickel layer is thickened, deposition rate ~3.7 pm/h.

Bath No. 3 composition and conditions:

• 0.1 M nickel sulphate (NiSO₄);

® 0.5 M sodium hypophosphite (NaH₂PO₂);

• 0.6 M amino acetic acid (C₂H₅NO₂);

• Add sodium hydroxide (NaOH) for pH adjusting to pH 9.2;

• plating at 85-92°C temperature.

Bath No. 4 composition and conditions:

• 0.1 M nickel sulphate (NiSO₄);

• 0.25 M sodium hypophosphite (NaH₂PO₂);

• 0.4 M amino acetic acid (C₂H₅NO₂);

• Add sodium hydroxide (NaOH) for pH adjusting to pH 5.2;

• plating at 85-95°C temperature.

Activation route a). Advantage: after the activation, there is no need to wash the part; it can be immersed directly in a nickel-plating bath containing the same reducing agent - sodium hypophosphite.

Activation routes b), c), and d). Advantage: the process takes place at room temperature, low reagent concentrations are used; can be platted in a phosphorus-free electroless nickel-plating bath using morpholine borane or dimethylamine borane as reducing agents; practically pure nickel coatings are deposited.

The advantage of described baths is that the activation can be performed in any of the described baths, and then the nickel plating can also be performed in any of the described ones. Moreover, acidic (not alkaline) bath for electroless Ni deposition solutions with hypophosphite as a reducing agent is used with pH 4-6 at a lower temperature of 80°C. The acidic solution is preferred since no surface contaminants such as oxides and hydroxides are formed on the metal surface due to their dissolution

Example: NiO + 2H⁺ Ni²⁺ + H₂O

Ni(OH)₂ + 2H⁺ — >• Ni²⁺ + 2H₂O

Another advantage is that pure nickel coatings without phosphorus impurities can be deposited using baths in which the reducing agents are boron compounds.

Principle of operation: the electrons which formed during the anodic oxidation of the reducing agent in solution activate the copper surface by reducing copper (I) and copper (II) oxides and oxyhydroxy compounds on the surface to metallic copper. At the same time as the thermal and I or catalytic decomposition (dehydrogenation) of the reducing agent takes place, the atomic hydrogen formed during the release (in statu nascendi - Latin) is adsorbed on the surface. It is known that the hydrogen formed during the release is a very active reducing agent and thus further cleans the surface of the copper, giving it catalytic properties. In addition, an added ligand helps to dissolve incompletely reduced insoluble copper compounds by binding them to soluble complexes. When the activated item is immersed in the electroless nickel plating bath, the electroless nickel plating process on the pure copper surface easily begins.

Reactions during copper surface activation: with sodium hypophosphite

2H₂PO₂- + 2OH- 2H₂PO₃ - + H₂f + 2e

Cu²⁺ + 2e- Cu⁰

Cu⁺ + e- → Cu⁰

H₂PO₂ + H₂O → H₂PO₃ + 2H_(ads)

Cu²⁺ + 2H_(ads) Cu⁰ + 2H⁺ Cu⁺ + H_(ads) → Cu⁰ + H⁺ with morpholine borane

2C₄H₈O-NH BH₃ + 8OH' 2BO₂- + 2C₄H₈O-NH + 4H₂O +3H₂↑ + 6e-

C₄H₁₂BNO

Cu²⁺ + 2e- → Cu⁰

Cu⁺ + e- → Cu⁰

C₄H₈O-NH BH₃ + 4OH' BO₂- + C₄H₈O-NH + 2H₂O + 3H_(adS) + 3e

Cu²⁺ + 2H_(adS) Cu⁰ + 2H⁺

Cu⁺ + H_(ads) CU° + H⁺ with dimethylamine borane

2(CH₃)₂NH BH₃+ 8OH' 2BO₂- + 2(CH₃)₂NH + 4H₂O +3H₂↑ + 6e

Cu²⁺ + 2e- → Cu⁰

Cu⁺ + e- → Cu⁰

(CH₃)₂NH BH₃+ 4OH' BO₂- + (CH₃)₂NH + 2H₂O + 3H_(ads) + 3e’

Cu²⁺ + 2H_(ads) Cu⁰ + 2H⁺

ClI + H_(ads) → Cu⁰ +H⁺ with tetra hydroborate

BH₄- + 4OH- BO₂- + 2H₂O + 2H₂↑ + 4e

Cu²⁺ + 2e- → Cu⁰

Cu⁺ +e- → Cu⁰

BH₄" + 3OH" BO₂ + H₂O + 5H_(ads) ⁺ 3e"

Cu²⁺ + 2H_(ads) Cu⁰ + 2H⁺ Cu⁺ + H_(ads) → Cu⁰ + H⁺

Advantages compared to other activation baths: only copper surfaces are activated and later is nickel-plated while using classic activation solutions with Pd (II) salts. The entire surface of the part is activated, and nickel is deposited not only on copper but also on a plastic surface.

In one of the methods, a nickel coating can be deposited on a copper surface by skipping treatment with an activating bath. If a chemical nickel-plating solution containing at least one of the following reducing agents is used: morpholine borane (C₄H₉BNO), dimethylamine borane (C₂H₇BN), sodium tetrahydroborate (NaBH₄) or sodium hypophosphite (NaH₂PO₂) in higher concentrations (more than 0.8 M), copper may be coated with nickel or its alloys without additional activation and washing steps.

Initially, the surface is washed or wiped with sulfuric acid H₂SO₄ with a concentration of 0.5-4 M for 3-15 minutes. This step is performed if the surface is strongly oxidised. This step can be omitted if all other multilayer coating steps are performed immediately (within 5 minutes) after step by step. This is followed by electroless deposition of nickel by immersing the item into electroless metallization baths No. 5 or No. 6, or No. 7.

Bath No. 5 composition and conditions:

• 0.05 M nickel sulphate (NiSO₄);

• 0.1 M morpholine borane (C₄H₉BNO);

• 0.015 M diethylenetriamine (C₄H₁₃N₃);

• Add sodium hydroxide (NaOH) for pH adjusting to pH 7.0;

• platting at 30°C temperature.

Bath No. 6 composition and conditions:

• 0.05 M nickel sulphate (NiSO₄);

• 0.1 M dimethylamine borane (C₂H₇BN);

• 0.015 M diethylenetriamine (C₄H₁₃N₃);

• Add sodium hydroxide (NaOH) for pH adjusting to pH 7.0;

• platting at 30°C temperature. Bath No. 7 composition and conditions:

• 0.125 M nickel sulphate (NiSO₄);

• 0.125 M sodium tetrahydroborane (NaBH₄);

• 0.25 M ethylenediamine (C₂H₆N₂);

• 0.15 M potassium sodium tartrate (KNaC₄H₄O₆);

• 0.008 M disodium thiosulphate (Na₂S₂O₃);

• Add sodium hydroxide (NaOH) for pH adjusting to pH 12.5;

• platting at 30°C temperature.

Another example with no activation step can be given for electroless nickel bath where sodium hypophosphite is used as a reducing agent, and the activation step can be skipped if the concentration of NaH₂PO₂ exceeds 0.8 M in the nickel electroless plating bath, which consists of nickel sulphate (NiSO₄); sodium hydroxide (NaOH); amino acetic acid (C₂H₅NO₂), and sodium hypophosphite (NaH₂PO₂).

Bath No. 8 composition and conditions:

• 0.1 M nickel sulphate (NiSO₄);

• 1 M sodium hypophosphite (NaOH);

• 0.4 M amino acetic acid (C₂H₅NO₂);

• Add sodium hydroxide NaOH for pH adjusting to pH 5.2;

• platting at 92°C temperature.

In this type of chemical nickel plating bath (No. 5-8), no additional chemical activation is required because the concentration of the reducing agent already in the solution is sufficient for the preparation and activation of the copper surface. Ni²⁺ ions in the chemical nickel plating bath do not interfere with the copper activation process, and diethylenetriamine (C₄H₁₃N₃), ethylenediamine (C₂H₈N2), amino ethanoic acid (C₂H₅NO₂) and potassium sodium tartrate (KNaC₄H₄O₆)) may also be involved in the transfer of copper (dissolution) as Cu(ll) ligands, for example for copper oxide (CuO) and amino acetic acid (C₂H₅NO₂):

CuO + 2H₂NCH₂COOH CU(H₂NCH₂COO-)2 +H₂O The ligand in a bath facilitates the dissolution of incompletely reduced, insoluble copper compounds by binding them to soluble complexes, leaving a substantially pure copper surface where nickel is then deposited. The ligand produces a complex with insoluble copper (from copper oxides, thus obtaining completely different function relative to the state of the art process.

The ligands can be chosen but not limited to the following Cu(ll) ligand groups: amines, polyhydroxilic alcohols, polycarboxylic acids, hydroxy-polycarboxylic acids, polyamine-polycarboxylic acids. The ligand or their mixture thereof consist of any water-soluble chemical compounds capable of forming sufficiently stable complexes with copper ions at a high rate, comprising, but are not limited to, amino acetic acid (C₂H₅NO₂), nitrilotriacetic acid (CeHgNOe), ethylenediaminetetraacetic acid (C₁₀H₁₆N₂O₈), diethylenetriaminepentaacetic acid (C₁₄H₂₃N₃O₁₀) and their salts, tartaric acid (C₄H₆0e); citric acid (C₆H₈O₇) and their salts; ammonia (NH₃), N, N, N ', N'-tetrakis (2-hydroxypropyl) ethylenediamine (C₁₄H₂₃N₃O₄).

Optimal concentrations of reagents in the baths and process durations can be determined using an automated method for determining solution concentrations and I or process durations, using equipment that indicates and loads the information about the surface of the item and condition of the bath to a computer, and calculates concentrations of the baths and I or chemical process data in situ based on the loaded information. Optimization by iterations (cycles) can be performed in this way.

As a final consideration, it is worth emphasising that the invention described a method for nickel deposition on a surface of an item that is produced from copper or has a copper layer on it comprising steps of reducing, dissolving, and plating.

In one embodiment of the invention, these steps can happen one after another in a sequential way and possibly such order is enforced by changing the baths between the steps.

However, in an alternative embodiment of the invention, these steps can also happen in parallel or substantially simultaneously. While the chemistry, of course, demands that at least some reduction has to happen before the solving can take place, it is the reduction of insoluble copper compounds and dissolution of such insoluble compounds by the formation of soluble complex compounds that occur actually rather simultaneously because the reduction reaction rate is much higher (preferably at least two or more orders in magnitude), comparing with that of dissolution reaction. The advantage of this embodiment is that all the processes appear in a single bath (single step) because the electroless nickel plating bath contains every mentioned compound: reducer and ligand (hence a suitable mixture of all required components). The invention has been described in terms of specific embodiments, which should be considered as examples only and not limiting the practical scope of the invention. Therefore, any changes and modifications to the technological processes, materials and reactions are possible provided that the changes and modifications do not depart from the definition of the patent invention.

Claims

1. A method for nickel deposition on a surface of an item that is produced from copper or has a copper layer on the item comprising:

• immersing said item into one or more baths, o at least one bath containing a reducing agent; o at least one bath being adapted for electroless plating of nickel wherein

• said reducing agent comprises boronic or phosphoric compounds, comprising morpholine borane (C₄H₉BNO), or dimethylamine borane (C₂H₇BN), or sodium tetrahydroborate (NaBH₄), or sodium hypophosphite (NaH₂PO₂) and said reducing agent directly or indirectly reduces insoluble copper (I) or copper (II) compounds on the surface of the item which is produced from copper or on the copper layer of the item; and

• at least one of the mentioned baths comprises a ligand or mixture thereof, the ligand in the bath facilitating the dissolution of incompletely reduced insoluble copper compounds - copper oxides - by binding them to soluble complexes, such that a substantially pure copper surface is present where nickel is deposited.

2. The method of claim 1 , wherein said ligand or mixture thereof consists of any water- soluble chemical compounds capable of forming sufficiently stable complexes with copper ions, comprising amino acetic acid (C₂H₅NO₂), nitrilotriacetic acid (C₆H₉NO₆), diethylenetriaminepentaacetic acid (C₁₄H₂₃N₃O₁₀₎ and their salts, tartaric acid (C₄H₆O₆); citric acid (C₆H₈O₇) and their salts; ammonia (NH₃), ethylenediamine (C₂H₈N₂), diethylenetriamine (C₄H₁₃N₃), N,N,N',N'-tetrakis (2-hydroxypropyl) ethylenediamine (Ci4H₃₂N₂O₄).

3. The method of claim 1 , wherein said method further comprises: (i) immersing said item in the activation bath, which comprises said reducing agent and said ligand wherein electrons which appear after an anodic oxidation reaction of said reducing agent from the activation bath activate the copper surface by reducing Cu(l) and Cu(ll) oxides and/or oxy-hydroxy compounds on the surface, at the same time, anodic catalytic or thermal decomposition reactions occur, releasing hydrogen, also reacting as an active reducing agent with the Cu(l) and Cu (II) compounds on the copper surface, leaving the substantially pure copper surface where nickel is then deposited; and;

(ii) immersing said item with the activated copper surface into a second, electroless nickel plating bath, in which nickel is electrolessly deposited on the copper surface, wherein the immersing said item into the second electroless nickel plating bath occurs after immersing said item in the activation bath without any intermediate step.

4. The method of claim 1 , wherein said method comprises the single step of immersing said item directly into the electroless nickel plating bath comprising a reducing agent selected from the group consisting of said boronic or phosphoric reducing compounds, which concentrations are selected to reduce the insoluble copper (I) and copper (II) compounds present on the surface of the item produced from copper or having a copper layer; and the a ligand or mixture of ligands selected from a group of said chemical compounds, selecting concentrations of said chemical compounds of said ligand or mixture of ligands such that said chemical compound dissolves the incompletely reduced insoluble copper compounds by binding them to soluble complexes so that substantially pure copper surfaces, on which nickel is deposited, remain.

5. The method of claim 3, wherein said activation bath consists of sodium hypophosphite (NaH₂PO₂) solution with a concentration from 0.5 M up to the limit of solubility and said ligands or mixtures thereof in a concentration from 0.001 M to the solubility limit, and the immersion time ranges from 1 to 15 min at a temperature of 80- 96 °C.

6. The method of claim 3, wherein said activation bath consists of morpholine borane (C₄H₉BNO) solution with a concentration from 0.01 M up to the limit of solubility and said ligands or mixtures thereof in a concentration from 0.001 M to the solubility limit, and the immersion time ranges from 1 to 15 min at a temperature of 18-50 °C.

7. The method of claim 3, wherein said activation bath consists of dimethylamine borane (C₂H₇BN) solution with a concentration from 0.01 M up to the limit of solubility and said ligands or mixtures thereof in a concentration from 0.001 M to the solubility limit, and the immersion time ranges from 1 to 15 min at a temperature of 18-50 °C.

8. The method of claim 3, wherein said activation bath consists of sodium tetrahydroborate (NaBH₄) solution with a concentration from 0.01 M up to the limit of solubility and said ligands or mixtures thereof in a concentration from 0.001 M to the solubility limit and the immersion time ranges from 1 to 15 min at a temperature of 18- 50 °C.

9. The method of claim 1 , wherein said electroless nickel plating bath consists of nickel sulphate (NiSO₄) at a concentration of 0.05-0.5 M; the reducing agent of sodium hypophosphite (NaH₂PO₂) at a concentration of 0.25-3 M; amino acetic acid (C₂H₅NO₂) at a concentration of 0.25-1 M; and sodium hydroxide (NaOH) at a concentration sufficient for required pH adjustment.

10. The method of claim 1 , wherein said surface of said item, which is produced by copper, is plated by nickel using a sequence of two different nickel plating baths, the first nickel plating bath containing sodium hypophosphite as the reducing agent, initially, an item which is necessary to metalize is immersed in an alkaline electroless nickel plating bath with pH in the range of 8.5-10.0, and later said item is immersed in the second nickel plating bath, which is an acidic electroless nickel plating bath with pH in the range of 4.0-6.0.

11 . The method of claim 1 , wherein said electroless nickel plating bath consists of nickel sulphate (NiSO₄) at a concentration of 0.05-0.5 M; the reducing agent of morpholine borane (C₄H₉BNO) at a concentration of 0.01 -1 M; the ligand of diethylenetriamine (C₄H₁₃N₃) at a concentration of 0.001 -0.5 M; sodium hydroxide (NaOH) at a concentration sufficient for pH adjustment to pH 5.0-7.8; and plating at a temperature of 18-35 °C.

12. The method of claim 1 , wherein said electroless nickel plating bath consists of nickel sulphate (NiSO₄) at a concentration of 0.05-0.5 M; the reducing agent of dimethylamine borane (C₂H₇BN) at a concentration of 0.01-1 M; the ligand of diethylenetriamine (C₄H₁₃N₃) at a concentration of 0.001 -0.5 M; sodium hydroxide (NaOH) at a concentration sufficient for pH adjustment to pH 5.0-7.8; plating at a temperature of 18-35 °C.

13. The method of claim 1 , wherein said electroless nickel plating bath consists of nickel sulphate (NiSO₄) at a concentration of 0.05-0.5 M; the reducing agent of sodium tetrahydroborane (NaBH₄) at a concentration of 0.01 -0.5 M; the ligand of ethylenediamine (C₂H₈N₂) at a concentration of 0.001 -0.5 M; the ligand of potassium sodium tartrate (KNaC₄H₄O₆) at a concentration of 0.05-0.2 M; disodium thiosulphate (Na₂S₂O₃) at a concentration of 0.001-0.01 M; sodium hydroxide (NaOH) at a concentration sufficient for pH adjustment to pH 12.0-13.0; platting at a temperature of 18-35 °C.

14. The method of claim 4, wherein said nickel electroless plating bath consists of nickel sulphate (NiSO₄); sodium hydroxide (NaOH); the ligand of amino acetic acid (C₂H₅NO₂), and the reducing agent of sodium hypophosphite (NaH₂PO₂) which concentration exceeds 0.8 M.

15. The method of any claim of 4 and 14, wherein said electroless nickel plating bath consists of nickel sulphate (NiSO₄) at a concentration of 0.05-0.5 M; the reducing agent of sodium hypophosphite (NaH₂PO₂) at a concentration of 0.8-3 M; the ligand of amino acetic acid (C₂H₅NO₂) at a concentration of 0.25-1 M; sodium hydroxide (NaOH) at a concentration sufficient for pH adjustment to pH 4.0-6.0; and plating at a temperature of 80-96 °C.

16. The method of claim 4, wherein said electroless nickel plating bath consists of nickel sulphate (NiSO₄) at a concentration of 0.05-0.5 M; the reducing agent of morpholine borane (C₄H₉BNO) at a concentration of 0.1-1 M; the ligand of diethylenetriamine (C₄H₁₃N₃) at a concentration of 0.001-0.5 M sodium hydroxide (NaOH) at a concentration sufficient for pH adjustment to pH 5.0-7.8; and plating at a temperature of 18-35 °C.

17. The method of claim 4, wherein said electroless nickel plating bath consists of nickel sulphate (NiSO₄) at a concentration of 0.05-0.5 M; the reducing agent of dimethylamine borane (C₂H₇BN) at a concentration of 0.1 -1 M; the ligand of diethylenetriamine (C₄H₁₃N₃) at a concentration of 0.001 -0.5 M, sodium hydroxide (NaOH) at a concentration sufficient for pH adjustment to pH 5.0-7.8; and platting at a temperature of 18-35 °C.

18. The method of claim 4, wherein said electroless nickel plating bath consists of nickel sulphate (NiSO₄) at a concentration of 0.05-0.5 M; the reducing agent of sodium tetrahydroborane (NaBH₄) at a concentration of 0.02-0.5 M; a ligand of ethylenediamine (C₂H₈N₂) at a concentration of 0.001 -0.5 M; the ligand of potassium sodium tartrate (KNaC₄H₄O₆) at a concentration 0.05-0.2 M; disodium thiosulphate (Na₂S₂O₃) at a concentration of 0.001-0.01 M; sodium hydroxide (NaOH) at a concentration sufficient for pH adjustment to pH 12.0-13.0; and platting at a temperature of 18-35 °C.

19. The method of any one of claims 1 to 18, wherein said method comprises determination of the concentration and/or process timing experimentally or by computer-implemented or semi-automatic means according to read and loaded the information on the quality of the deposited nickel layer of the item and the concentrations of the reagents in the baths, and on-site computing the concentration of the baths required for the process and/or the timing of the chemical process.