ELECTROLESS NICKEL OR COBALT PLATING SOLUTION

Abstract

An electroless nickel or cobalt plating solution, comprising nickel ions or cobalt ions, Ti.sup.3+ ions as reducing agent for reducing said nickel ions and cobalt ions, at least one accelerator selected from the group consisting of sulfites, dithionites, thiosulfates, tetrathionates, polythionates, disulfites, sulfides, disulfide, polysulfide, elemental sulfur and mixtures thereof; and one or more than one complexing agent,
wherein the pH value of the plating solution is from 5 to 10.5.

Claims

1. An electroless nickel or cobalt plating solution, comprising nickel ions or cobalt ions, Ti.sup.3+ ions as reducing agent for reducing said nickel ions and cobalt ions, at least one accelerator selected from the group consisting of sulfites, dithionites, thiosulfates, tetrathionates, polythionates, disulfites, sulfides, disulfide, polysulfide, elemental sulfur and mixtures thereof; and one or more than one complexing agent, wherein the pH value of the plating solution is from 5 to 10.5.

2. The plating solution according to claim 1, wherein the accelerator(s) is/are selected from the group consisting of alkaline metal sulfites, alkaline metal hydrogen sulfites, alkaline earth metal sulfites, alkaline earth metal hydrogen sulfites, ammonium sulfite, ammonium hydrogen sulfite, alkaline metal dithionites, alkaline metal hydrogen dithionites, alkaline earth metal dithionites, alkaline earth metal hydrogen dithionites, alkaline metal thiosulfates, alkaline metal hydrogen thiosulfates, alkaline earth metal thiosulfates, alkaline earth metal hydrogen thiosulfates, ammonium thiosulfate, ammonium hydrogen thiosulfate, alkaline metal tetrathionates, alkaline metal hydrogen tetrathionates, alkaline earth metal tetrathionates, alkaline earth metal hydrogen tetrathionates, ammonium tetrathionates, ammonium hydrogen tetrathionate, alkaline metal polythionates, alkaline metal hydrogen polythionates, alkaline earth metal polythionates, alkaline earth metal hydrogen polythionates, ammonium polythionates, ammonium hydrogen polythionate, alkaline metal disulfites, alkaline metal hydrogen disulfites, alkaline earth metal disulfites, alkaline earth metal hydrogen disulfites, ammonium disulfites, ammonium hydrogen disulfite, alkaline metal sulfide, alkaline metal disulfide, alkaline metal polysulfide, ammonium sulfide, and cyclo-octasulfur (S.sub.8).

3. The plating solution according to claim 1, wherein the total amount by weight of the accelerator(s) in the plating solution ranges from 0.01 to 300 ppm.

4. The plating solution of claim 1, wherein the one or more than one complexing agent is/are independently selected from the group consisting of an organic phosphonic acid compound, its salts and esters, an organic polyphosphoric acid compound, its salts and esters, an inorganic polyphosphoric acid compound, its salts and esters, and an organic carboxylic acid compound, its salts and esters.

5. The plating solution of claim 1, wherein the one or more than one complexing agent is independently selected from the group consisting of an organic phosphonic acid compound, its salts and esters.

6. The plating solution of claim 1, wherein the organic and/or inorganic polyphosphoric acid compound, its salts and esters, comprise 2 to 10 phospho-building units linked together.

7. The plating solution of claim 1, with the proviso that the plating solution does not comprise pyrophosphate.

8. The plating solution of claim 1, wherein a molar ratio of the complexing agent to the Ti.sup.3+ ions is 1.5:1 or higher.

9. The plating solution of claim 1, wherein a concentration of the Ti.sup.3+ ions is in a range from 0.03-0.2 mol/L.

10. The plating solution of claim 1, wherein a concentration of the one or more than one complexing agent is in a range from 0.03-2.0 mol/L.

11. The plating solution of claim 1, having a pH in a range from 4.0-9.5.

12. A method for electroless plating of a nickel or a cobalt deposit (4) on a substrate (2, 3), the method comprising contacting the substrate with the electroless plating solution according to claim 1 such that the nickel or cobalt deposit (4) is electrolessly plated on the substrate (2, 3).

13. The method of claim 12, wherein the electroless plating solution has a temperature in a range from 30-80° C.

14. The method of claim 12, wherein the substrate (2, 3) comprises a copper layer (3), wherein the nickel or cobalt deposit (4) is plated on the copper layer (3).

15. (canceled)

16. The plating solution according to claim 1 wherein the one or more than one complexing agent is independently selected from the group consisting of an organic phosphonic acid compound, its salts and esters and the organic phosphonic acid compound, its salts and esters is the only complexing agent in the plating solution.

Description

BRIEF DESCRIPTION OF THE FIGURE

[0192] The FIGURE shows an electronic article of the invention.

EXAMPLES

Example 1—Plating of Nickel

[0193] Autocatalytic (electroless) nickel plating with Ti.sup.3+ as reducing agent and pyrophosphate as complexing agent is carried out. Nickel sulfate (hexahydrate) is used as nickel salt. The reducing agent is synthesized in a regeneration cell as described in WO 2013/182478 A2.

[0194] Preparation of a Solution Comprising Ti.sup.4+:

[0195] In a beaker, 1M or 330.34 g/L sodium pyrophosphate and 0.4 M or 39.17 g/L 85% ortho-phosphoric acid are dissolved in deionized water and the solution is heated to 85° C. Then, 0.1 M or 28.42 g/L Titanium(IV)iso-propoxide is slowly added. The pH is 7.8-7.9. The solution becomes turbid (including a white precipitate) and is heated until the white precipitate is dissolved and iso-propanol is completely evaporated. Subsequently, the solution is filtered and transferred into the regeneration cell.

[0196] Preparation of a Reducing Agent Solution Comprising Ti.sup.3+:

[0197] In the regeneration cell a constant cathodic current is applied (current I=20 A) and in the solution comprising Ti.sup.4+ ions the Ti.sup.4+ ions are reduced to Ti.sup.3+ ions. The finally obtained reducing agent solution comprises 0.8 M Ti.sup.3+ and 0.2 M Ti.sup.4+.

[0198] Preparation of the Electroless Plating Solution:

[0199] The electroless plating solution comprises among others the above-mentioned nickel salt and the reducing agent solution and has the following final composition: [0200] Ni.sup.2+=0.05 M [0201] Ti.sup.3+=0.060 M [0202] Ti.sup.4+=0.011 M [0203] pyrophosphate=0.7 M [0204] ammonium sulfide=see Table 1 [0205] pH=8 [0206] T=65° C. [0207] Pb.sup.2+=1 mg/L

TABLE-US-00001 TABLE 1 Dependence of deposition rate on ammonium sulfide c (ammonium deposition sulfide) V Dosing rate Solution [mg/L] [mL/L] [mL/(h*L) [nm/h] 1a Comparative 0 0 0 144 1b Inventive 0.4 6 4 480 1c Inventive 0.6 9 4 600

[0208] As substrates, printed circuit boards comprising a non-conductive base material (FR4, a resin) and patterned copper are used. Plating was carried out for 60 minutes. Thereafter a nickel layer with a thickness of approximately 300 nm selectively on copper was obtained (FR4 was not affected).

[0209] The thickness of the nickel layer was measured with XRF throughout all examples.

[0210] Similar experiments with a plating solution having a significantly lower molar ratio of Ti.sup.3+ ions to pyrophosphate resulted in an undesired instability and precipitation (data not shown). This shows that a significant excess of pyrophosphate is necessary in order to stabilize the Ti.sup.3+ ions.

[0211] Furthermore, experiments with a significantly reduced concentration of nickel resulted in a very low deposition rate of far below 40 nm/h. In some cases no plating was observed at all (data not shown).

Example 2—Plating of Cobalt

[0212] Auto catalytic (electroless) cobalt plating with Ti.sup.3+ as reducing agent and pyrophosphate as complexing agent is carried out. Cobalt(II)chloride (hexahydrate) is used as cobalt salt. The reducing agent solution is prepared as defined in Example 1.

[0213] Preparation of the Electroless Plating Solution:

[0214] The electroless plating solution comprises among others the above-mentioned cobalt salt and the reducing agent solution and has the following final composition: [0215] Co.sup.2+=0.05 M [0216] Ti.sup.3+=0.06 M [0217] Ti.sup.4+=0.01 M [0218] pyrophosphate=0.7 M [0219] ammonium sulfide=see Table 2 [0220] pH=7.6 [0221] T=70° C.

TABLE-US-00002 TABLE 2 Dependence of deposition rate on ammonium sulfide c (ammonium deposition sulfide) V Dosing rate Solution [mg/L] [mL/L] [mL/(h*L) [nm/h] 2a Comparative 0 0 0 2b Inventive 0.4 6 4 2c Inventive 0.6 9 4

[0222] Substrates as used in Example 1 are also utilized in Example 2. Plating is carried out for 30 minutes, wherein after 30 minutes a selectively plated cobalt layer with a thickness of approximately 160 nm to 180 nm is obtained, corresponding to a plating rate of approximately 320 nm/h.

[0223] Similar experiments with a plating solution having a significantly lower molar ratio of Ti.sup.3+ ions to pyrophosphate resulted again in an undesired instability and precipitation (data not shown).

[0224] Furthermore, experiments with a significantly reduced concentration of cobalt also resulted in a very low deposition rate of far below 50 nm/h. Again, in some cases no plating was observed at all (data not shown).

Example 3—Electronic Article

[0225] The FIGURE shows (not true to scale) an electronic article 1, for example a printed circuit board or a wafer, comprising a nickel layer 4 or a cobalt layer 4. The nickel layer 4 or the cobalt layer 4 was produced by contacting a substrate 2, 3 with a plating solution of the invention. The substrate 2, 3 comprises a carrier body 2, for example a wafer, and a copper layer 3 which is arranged on a surface of the carrier body 2.

[0226] When contacting the copper layer 3 of the substrate with the plating solution of the invention, a cobalt layer 4 or a nickel layer 4 is plated on the copper layer 3 without further activation of the copper layer 3. In a further method step, a gold layer 5 or a tin layer 5 is plated on the nickel layer 4 or the cobalt layer 4.

[0227] In the shown stack of layers, the cobalt layer 4 or nickel layer 4 serves as a barrier layer between the copper layer 3 and the gold layer 5 or tin layer 5.