GB2083080A - Electroless deposition of copper alloy layers - Google Patents

Abstract

Solutions for the electroless deposition of copper alloy layers exhibiting improved ductility catalytic substrates have a pH of from 11.5 to 13.5 and contain cupric ions, < 0.20M formaldehyde, <0.25M alkali metal hydroxide, one or more salts of alloying components, and one or more complex-forming compounds for ions of the alloying components, which may be one or more of Ni, Co, Fe, Pt, Pd, Rh, Ru and Ir. When Ni, Co or Fe is used, the complex-forming compound binds cupric ions more strongly than the alloying ions. When Pt, Pd, Rh, Ru or Ir is used, the complex-forming compound binds the alloying ions more strongly than cupric ions. In examples, complexing agents which bind Cu more strongly than Ni, Co, or Fe include triethanolamine and nitrilo-tri-2- propanol, and where Cu is applied with a precious metal (eg Pd) a mixture of EDTA and cyanide may be used. The substrates are glass plates and may be prepared by contacting with HF, SnCl2, AgNO3 and PdCl2, with intermediate rinsing.

Description

SPECIFICATION Solution for electroless deposition of copper alloys The invention relates to solutions for electroless deposition of copper alloys, to a method of produc ing copper alloy layers or patterns on substrates which cataiyse the electroless deposition of copper and to products produced by such a method.

In accordance with such a method, nuclei which are catalytically active for the electroless deposition .of copper are first deposited on a substrate, if neces sary. This may be effected in different manners, which are described in the literature. United States Patent Specification 3,011,920, for example, dis closes a method in which a colloidal solution of metal which is catalytic to the deposition of winter alia copper is contacted, together with a protective col loid, with the substrate. United States Patent Specifi cation 3,674,485 describes a light-sensitive resin which can be used as a substrate or a surface layer on an other substrate and which contains a dis persed light-sensitive semiconducting oxide.After having been exposed to light, this light-sensitive oxide is capable of reducing a salt of a metal which is catalytic to the deposition of inter alia copper to that metal in the form of nuclei.

United Kingdom Patent Specification 1,283,715 discloses an electroless copper-plating solution which deposits copper having an improved appear ance and improved bending and elongation proper ties. This solution is an aqueous solution comprising a source of cupric ions, hydroxy radicals, a source of formaldehyde, sufficient complexing agent to render said cupric ions soluble in alkaline solution, and at least two additives selected from a formaldehyde addition agent, a solution soluble salt of a Group VIII metal, an organic silicon compound and a hydrogen inclusion retarding agent, excepting any combina tion including both an organic silicon compound and a hydrogen inclusion retarding agent.The ductility of the deposit was determined by peeling a copper deposit from the substrate, and bending itthrough 1 80C in one direction, creasing at the fold, then returning itto its original flat position and pressing ing component(s) the complex-forming compound binds cupric ions more strongly than the ions of this (these) metal(s), and when Pt, Pd, Rh, Ru or Ir is (are) used as the alloying component(s), the complexforming compound binds ions or this (these) metals more strongly than cupric ions.

A copper alloy layer or pattern is produced on a substrate which is catalytic for electroless deposition of copper, by contacting the substrate with a solution according to the invention. If necessary, a substrate is made catalytic for electroless deposition of copper by depositing nuclei which are catalytically active, on the substrate.

The invention is based on the appreciation of the fact that formaldehyde isdehydrogenated art a metal surface and chemisorption occurs of the atomic hydrogen produced. Depending on the metal at whose surface the reaction occurs, these hydrogen atoms can react in two manners:

along the crease to flatten it. This cycle constitutes one bend. The procedure is repeated until the sample breaks at the crease.

The objects of the invention are to provide an electroless copper-plating solution from which copper alloys are deposited with a low formation of hydrogen gas so that a small quantity of hydrogen is trapped in the deposit and high rates of deposition can be used without impairing the bending or elongation properties of the deposit.

The invention provides a solution for the electroless deposition of a copper alloy on a catalytic substrate, the solution having a pH of from 11.5 to 13.5, and containing cupric ions, < 0.20M formaldehyde, < 0.25M alkali metal hydroxide, one or more salts of the alloying component(s), and one or more complex-forming compounds for ions of the alloying component(s), wherein the alloying component(s) is (are) one or more of Ni, Co, Fe, Pt, Pd, Rh, Ru and Ir, wherein when Ni, Co or Fe is (are) used as the alloy These reactions result in two different overall metal lizing reactions:

Reaction A occurs only at a copper surface, as there the rate of reaction (1) is much higher than the rate of reaction (2).By depositing a metal at whose surface reaction (2) proceeds much faster than reaction (1), the separation of hydrogen and conse quentlythe degree to which it is built into the deposit is considerably reduced, the conversion of formaldehyde remaining the same. The rate at which the alloy can be deposited is therefore considerably higherthan the rate at which pure copper can be deposited without impairing the bending and elongation properties of the deposit.

It is first of all necessary, in order to obtain a solution which is stable to such an extent that thick alloy deposits (at least 25 ,u ) can be deposited, for the concentration of alkali metal hydroxideto remain below 0.25M and the concentration of the formal dehydeto remain below 0.20 M.

It was also found during the investigations which led to the present invention, that the choice of the complexing compound used in the electroless bath plays an essential part.

If Ni, Co or Fe are used as the alloying component, a complex forming compound must be chosen which binds cupric ions more strongly than the alloy ions. However, if Pt, Pd, Ir, Rh or Ru are used, then a complex-forming compound or a combination of complex-forming compounds must be chosen such that it is precisely the ions of the last-mentioned metals which are bound more strongly than cupric ions.

The complexing constant K which is defined by the equation K = [MLXln-xm)+] /[Mn+ Lm-] is decisive therefor. The relevant literature contains Tables showing the values of this constant for a large number of complex-forming compounds; for example Critical Stability Constants, by A. E. Martell and R. M. Smith, as in Table I, in which log K is indicated for several metals and complex-forming compounds.

TABLE I

F Fe2+ C+ N Ct?+ Pd2+ T water-soluble cyanide 35,4 30,2 Unstable 45,3 triethanolamine 1,7 3,1 6,0 ethylenediamine 9,7 14,1 18,4 20,2 18,4 36,5 nitrilotriaceticacid 12,8 14,4 16,4 17,4 19,3 glycine 7,6 10,8 14,0 15,0 1 27,5 ethylenediamine-N,N'-diacetic acid 11,2 13,6 16,2 ethylenediaminetetra-acetic acid 14,3 16,3 18,6 18,8 18,5 tetrahydroxy-propylethylene diamine 10,2 11,2 9,0 ethylenediamine-isoproply phosphonic acid 11,4 11,1 20,3 ethylenediamine-N,N'-dimethyl phosphonic acid 10,2 11,7 17,5 ethylenediaminetetramethyl phosphonic acid 17,4 17,0 23,0 cyclohexane-1 ,2-diaminetetra methylphosphonic acid 3,3 3,9 9,7 It has been found by means of experiments that commonly used complex-forming compounds, such as ethylene diaminetetra-acetic acid and tetrahydroxyproplylethylenediamine do not improve the ductility in the presence of Ni and Co-salts in the copper plating solution, which can be explained on the basis of the complexing constants, as then no Ni and Co, respectively, are present in the deposit.

Triethanol amine and nitrilo-tri-2-propanol are suitable for the deposition of copper together with Ni or Co, while ethylenediamine-diacetic acid and the phosphonic compounds in accordance with United Kingdom Patent Specification 1,425,298 can be used.

In addition, it appears that if the cupric ions are complexed by, for example, ethylenediamine tetraacetic acid, cyanide is a particularly suitable complex-forming agent for palladium ions, as the complex of cupric ions with cyanide is very unstable and the complexing constant of ethylenediaminetetra-acetic acid with Pd is much less than that of cyanide with palladium.

Some embodiments of the invention will now be described with reference to the following Examples.

Example 1 Glass plates measuring 5 x 1 cm2 are subjected to the following treatments.

a) both sides of the plates are roughened with carborundum until a roughness (Ra) of 0.8-1.0 Ccm is reached, b) rinse in cold water, c) the plates are immersed for 10 seconds in a 4% solution of HF at 20"C, d) rinse in cold water, e) the plates are degreased by keeping them immersed for at least 24 hours in a 5% solution of "Decon 90" at 20"C, f) rinse in cold deionized water, g) the plates are kept immersed for 1 minute in an aqueous solution which comprising 100 mg SnCl2 and 0.1 ml concentrated MCI per litre, which solution is at 20"C, h) immerse for 1 minute in deionized water at 20"C, i) immerse for 1 minute in an aqueous solution which contains 1 g of AgNO3 per litre, which solution is at 20"C, j) immerse for 1 minute in deionized water at 20"C, k) immerse for 1 minute in a solution which contains 100 mg of PdCl2 and 3.5 mls of concentrated HCI per litre, which solution is at 20 C, I) immerse for 1 minute in deionized water at 20 C, m) immerse for 1 minute in deionized water at 90,C, n) the plates are kept in an electroless metallizing solution for such a period of time until a copper alloy layer of the desired thickness has been obtained.

The temperature of the solution is maintained at a constant value.

o) the plates are dried and the rate of deposition is determined by weighing the deposited metal and measuring the duration of metallizing. The ductility from the glass plate, bending it in one direction through 1804 pressing the fold, returning itto its original position and smoothing the fold. This cycle represents one bending operation. This test is repeated until the sample fractures at the fold, p) during step (n) the quantity of hydrogen formed is measured for a number of samples, in order to determine the ratio between the quantity of deposited metal and the quantity of released hydrogen, q) the percentage of the built-in alloying component is determined analytically for a number of layers.

Bath composition CuSO4.5H20 0,08 mole PdCI2 0,006 mole ethylene diamine tetra-acetic acid tetra-Na-salt 0,096 mole NaOH 0,11 mole HCHO 0,10 mole KCN X mole water up to 1 litre Bath temperature 50or.

In these copperplating baths the palladium ions are complexed by reacting a quantity for 10 minutes of 0.02 mol. PdCl2 per litre at 50"C with the required quantity of a solution of 0.30 mollI KCN. The quantity of KCN is varied, so that a series of ratios KCN : PdCl2 is obtained between 3 and 6. The following Table II shows the results.

TABLE II

ratio stability rate of % Pd bath KCN/ of the deposition number in the H2/M no. PdC12 bath mg.cm-2.hr-t of bends layer ratio 1 3,0 < 1 min. - - - 2 4,0 55 min. 3,4 > 5 0,3 3 4,25 75 min. 3,4 > 5 0,3 0,69 4 4,5 > 2 hrs 3,3 5 0,15 0,82 5 5,0 > 2 hrs 3,1 1 0 1,00 6 6,0 > 2hrs 0,0 - - - From the above range of compositions it appears that if palladium is built-in the formation of hydrogen is clearly inhibited and that the ductility of the deposit is therefore considerably better. In these cases the rate at which the metal is deposited is kept constant. It also appears from Table II that the improvement cannot be attributed to the presence of cyanide in the bath: according as the cyanide concentration increases the, P8" -ions are complexed to a greater extent.At a ratio CN-/Pd2+ over 4.5 the incorporation of Pd in the deposition decreases considerably. As a resu It thereof the hydrogen formation increases again, which results in a poor bendability. At a ratio CN-/Pd2+ over 5, the copper plating bath is poisoned, as then no complex bound cyanide is present.

Example 2 Glass plates are roughened and cleaned as described in Example 1. Nucleation is done by performing steps (g) to (m) (inclusive) of Example 1 twice. The nucleated glass plates are metallized by means of an aqueous solution which contains per litre CuSO4.5H2O 0,02 mol NiSO4.6H2O 0,0008 mol triethanol amine 0,065 mol NaOH 0,20 mol formaldehyde 0,10 mol The metallizing solutions are renewed several times to prevent exhaustion.

At a bath temperature of 25"C, the rate of deposition is 1,0 mg/hour and after 46 hours the ductility is 1 bend. The H2/M ratio is 0,67, while the metal layer contains 3,80iso nickel. If metallization is performed at a bath temperature of 45"C, the rate of deposition becomes 2,0 mg/cm2 hour. After 3 hours a layer has been obtained which has a ductility of 2 to 3 bends.

The deposit contains approximately 2% of nickel.

If a metal layer is deposited from a solution containing no nickel ions, but which has in all other respect the same composition as described above, then no compact metal layer is obtained either at 25 or at 45"C, but only a pulverulent powder is deposited.

Example 3 Glass plates were roughened and nucleated as described in Example 1. Metallization was done at 45"C in one of the solutions of the following composition: a) CuSO4.5H2O 0,02 mol nitrilo-tri-2-propanol 0,65 mol formaldehyde 0,10 mol NaOH 0,20 mol water up to 1 litre b) as (a) + 0,0004 mol CoSO4.7H2O c) as (a) + 0,0004 mol NiS04.6H20.

The results are shown in Table Ill.

TABLE III

ductility dM/dt number of %M (mg/cm2 hour) bends H2/M built-in a porous metal layer 1,00 b 2,0 24'2 0,59 0,04 c 1,9 24 0,47 1,5 The complex constants for the Cu and Nicomplexes, respectively, of nitrilo-tri-2-propanol were determined in an alkaline medium as being 10-29 and 1016 respectively.

Claims (7)

1. A solution for the electroless deposition of a copper alloy on a catalytic substrate, the solution having a pH of from 11.5 to 13.5, and containing cupric ions, < 0.20M formaldehyde, < 0.25M alkali metal hydroxide, one or more salts of the alloying component(s), and one or more complex-forming compounds for ions of the alloying component(s), wherein the alloying component(s) is (are) one or more of Ni, Co, Fe, Pt, Pd, Rh, Ru and Ir, wherein when Ni, Co or Fe is (are) used as the alloying component(s) the complex-forming compound binds cupric ions more strongly than the ions of this (these) metal(s), and when Pt, Pd, Rh, Ru or Ir is (are) used as the alloying component(s), the complexforming compound binds ions of this (these) metals(s) more strongly than cupric ions.

2. A solution as claimed in Claim 1, wherein the alloying component(s) is (are) Ni and/or Co, and the complex forming compound istriethanolamine.

3. A solution as claimed in Claim 1, wherein the alloying component(s) is (are) Ni and/or Co, and the complex-forming compound is nitrilo-tri-2-propanol.

4. A solution as claimed in Claim 1, containing one or more of the metals Pt, Pd, Rh, Ru and Ir, and a combination of a cyanide and ethylenediaminetetra-acetic acid as complex-forming compounds.

5. A solution for the electroless deposition of a copper alloy on a catalytic substrate as claimed in Claim 1, substantially as herein described with reference to any of Examples 1 to 3.

6. A method of producing a copper alloy layer or pattern on a substrate which is catalytic for electroless deposition of copper, by contacting the substrate with a solution as claimed in any preceding Claim.

7. A substrate bearing a copper alloy layer or pattern produced by a method as claimed in Claim 6.