Plating bath composition and method for electroless plating of palladium

Abstract

The present invention relates to a plating bath composition and a method for depositing a palladium layer by electroless plating onto a substrate. The aqueous acidic plating bath according to the present invention comprises a source for palladium ions, a reducing agent, a nitrogenated complexing agent for palladium ions and a water-soluble stabilizing agent selected from the group consisting of aromatic compounds comprising at least two residues wherein at least one residue is a hydrophilic residue and at least one residue has a negative mesomeric effect. The plating bath has an increased stability against undesired decomposition while maintaining a sufficient plating rate.

Claims

1. An aqueous acidic plating bath composition for electroless deposition of palladium, comprising (i) a source for palladium ions, (ii) a nitrogenated complexing agent for palladium ions, (iii) a reducing agent selected from the group consisting of formic acid, derivatives and salts thereof, (iv) a water-soluble stabilizing agent selected from the group consisting of aromatic compounds comprising at least two residues wherein at least one residue is a hydrophilic residue and at least one residue has a negative mesomeric effect, and wherein the at least one hydrophilic residue is selected from the group consisting of hydroxyl, carboxyl, sulfonate and salts thereof; and wherein the at least one residue having a negative mesomeric effect is selected from the group consisting of nitro, nitrile, acetyl, carboxyl and sulfonate.

2. The aqueous acidic plating bath composition according to claim 1 wherein the source for palladium ions is selected from the group consisting of palladium chloride, palladium nitrate, palladium acetate, palladium sulfate, palladium perchlorate, di-chlorodiethylenediamine palladium, dinitrodiethylenediamine palladium and diacetatodiethylenediamine palladium.

3. The aqueous acidic plating bath composition according to claim 1 wherein the concentration of palladium ions in the plating bath ranges from 0.5 to 500 mmol/l.

4. The aqueous acidic plating bath composition according to claim 1 wherein the nitrogenated complexing agent for palladium ions is selected from the group consisting of primary amines, secondary amines and tertiary amines.

5. The aqueous acidic plating bath composition according to claim 1 wherein a mole ratio of the nitrogenated complexing agent for palladium ions and palladium ions in the electroless plating bath ranges from 2:1 to 50:1.

6. The aqueous acidic plating bath composition according to claim 1 wherein the formic acid derivative is selected from the group consisting of esters of formic acid and substituted and unsubstituted amides of formic acid.

7. The aqueous acidic plating bath composition according to claim 1 wherein the concentration of the reducing agent ranges from 10 to 1000 mmol/l.

8. The aqueous acidic plating bath composition according to claim 1 wherein the pH-value ranges from 4 to 7.

9. The aqueous acidic plating bath composition according to claim 1 wherein the water-soluble stabilizing agent is selected from the group consisting of 2-nitrophenol, 3-nitrophenol, 4-nitrophenol, 3,5-dinitrophenol, 2,4-dinitrophenol, 2,4,6-trinitrophenol, 2-nitrobenzoic acid, 3-nitrobenzoic acid, 4-nitrobenzoic acid, 3,5-dinitrobenzoic acid, 2,4-dinitrobenzoic acid, 2,4,6-trinitrobenzoic acid, 2-hydroxy-3, 5-dinitrobenzoic acid, 2-nitrobenzosulfonic acid, 3-nitrobenzosulfonic acid, 4-nitrobenzosulfonic acid, 3,5-dinitrobenzosulfonic acid, 2,4-dinitrobenzosulfonic acid, 2,4,6-trinitrobenzosulfonic acid, 2-acetylbenzoic acid, 3-acetylbenzoic acid, 4-acetylbenzoic acid, 3,5-diacetylbenzoic acid, 2,4-diacetylbenzoic acid, 2,4,6-triacetylbenzoic acid, 2-acetylphenol,3-acetylphenol, 4-acetylphenol, 3,5-diacetylphenol, 2,4-diacetylphenol, 2,4,6-triacetylphenol, 2-acetylbenzosulfonic acid, 3-acetylbenzosulfonic acid, 4-acetyl-benzosulfonic acid, 3,5-diacetylbenzosulfonic acid, 2,4-diacetylbenzosulfonic acid, 2,4,6-triacetylbenzosulfonic acid, 2-cyanobenzoic acid, 3-cyanobenzoic acid, 4-cyanobenzoic acid, 2-hydroxybenzonitrile, 3-hydroxybenzonitrile, 4-hydroxybenzonitrile, and their corresponding ammonium, sodium and potassium salts.

10. The aqueous acidic plating bath composition according to claim 1 wherein the water-soluble stabilizing agent is selected from the group consisting of 2-nitrobenzoic acid, 3-nitrobenzoic acid, 4-nitrobenzoic acid, 3,5-dinitrobenzoic acid, 2,4-dinitrobenzoic acid, 2,4,6-trinitrobenzoic acid, 2-hydroxy-3,5-dinitrobenzoic acid and their corresponding ammonium, sodium and potassium salts.

11. The aqueous acidic plating bath composition according to claim 1 wherein the concentration of the stabilizing agent ranges from 1 to 200 mg/l.

12. A method for electroless palladium plating comprising, in this order, the steps of a) providing a substrate having a metal surface, b) providing an aqueous acidic plating bath composition according to claim 1, c) contacting said substrate with said aqueous acidic plating bath composition and thereby depositing a layer of palladium onto the metal surface of the substrate.

13. The method for electroless palladium plating according to claim 12 wherein the substrate is contacted with the aqueous acidic plating bath composition at a temperature of 35 to 95 C. for 1 to 60 min in step c).

Description

EXAMPLES

(1) The present invention is further explained by the following non-limiting examples.

(2) General Procedure

(3) Plating Bath Matrix:

(4) A plating bath matrix having a pH-value in the range of 5 to 6 and comprising water, palladium ions, sodium formate and ethylenediamine as the nitrogenated complexing agent for palladium ions was used throughout all examples. The stability against undesired bath decomposition for the plating bath matrix without any stabilizing agents was tested in Example 1. Different amounts of stabilizing agents were added to said plating bath matrix throughout examples 2 to 9 and the stability against undesired decomposition was tested accordingly.

(5) Test for Determining the Stability Against Undesired Decomposition:

(6) 250 ml of the aqueous acidic palladium plating bath were heated up to the desired test temperature of 60 C. while stirring the plating bath. Next, 1 ml of a palladium colloid solution (60 mg/l palladium) was added to the aqueous acidic plating bath every 60 s until a grey precipitate was formed in the aqueous acidic plating bath indicating the undesired decomposition of a given plating bath. The stability number corresponds to the amount of test solution added (in ml of palladium colloid solution) until formation of a grey precipitate, i.e. undesired decomposition of the plating bath. Accordingly, an amount of 6 ml palladium colloid solution added to a given plating bath until formation of the grey precipitate corresponds to a stability number of 6.

(7) The stability against undesired decomposition of palladium plating baths is expressed in terms of the stability number.

Example 1 (Comparative)

(8) The plating bath matrix was tested for stability against undesired decomposition without addition of a stabilizing agent. The stability number of such a plating bath matrix was 10.

Example 2 (Comparative)

(9) 30 mg/l of 1,3,5-trihydroxybenzene (an aromatic molecule with three hydrophilic residues but without a residue having a negative mesomeric effect) were added to the plating bath matrix. The stability number of such a plating bath was 10.

Example 3 (Comparative)

(10) 30 mg/l of nitrobenzene (an aromatic molecule with one residue having a negative mesomeric effect but without a hydrophilic residue) were added to the plating bath matrix but remained as solid particles on top of the plating bath surface. Accordingly, nitrobenzene was not water-soluble in a required amount in the plating bath matrix.

Example 4

(11) 30 mg/l of 4-nitrophenol were added to the plating bath matrix. The stability number of such a plating bath was 13. Accordingly, the stability of said plating bath against undesired decomposition of the plating bath was considered sufficient.

Example 5

(12) 40 mg/l of 4-nitrobenzoic acid were added to the plating bath matrix. The stability number of such a plating bath was 14. Accordingly, the stability of said plating bath against undesired decomposition of the plating bath was considered sufficient.

Example 6

(13) 50 mg/l of 3,5-dinitrobenzoic acid were added to the plating bath matrix. The stability number of such a plating bath was 17. Accordingly, the stability of said plating bath against undesired decomposition of the plating bath was considered sufficient.

Example 7

(14) 50 mg/l of 2,4-dinitrobenzoic acid were added to the plating bath matrix. The stability number of such a plating bath was 15. Accordingly, the stability of said plating bath against undesired decomposition of the plating bath was considered sufficient.

Example 8

(15) 54 mg/l of 2-hydroxy-3,5-dinitrobenzoic acid were added to the plating bath matrix. The stability number of such a plating bath was 15. Accordingly, the stability of said plating bath against undesired decomposition of the plating bath was considered sufficient.

Example 9

(16) 42.5 mg/l of 2-acetylbenzoic acid were added to the plating bath matrix. The stability number of such a plating bath was 12. Accordingly, the stability of said plating bath against undesired decomposition of the plating bath was considered sufficient.

Example 10 (Comparative)

(17) In order to simulate a real life plating situation, an already used plating bath was used throughout examples 10 to 14. The already used plating bath had been prepared according to the composition of the plating bath matrix outlined in the General procedure. Throughout examples 10 to 14, the stability against undesired decomposition was determined according to the test outlined in the General procedure.

(18) The stability against undesired bath decomposition for the already used plating bath without any stabilizing agents was tested in example 10. The stability number of such a plating bath was 6.

Example 11

(19) 40 mg/l of 4-nitrobenzoic acid were added to the already used plating bath according to example 10. The stability number of such a plating bath was 11. Accordingly, the stability of said plating bath against undesired decomposition of the plating bath was considered sufficient.

Example 12

(20) 40 mg/l of 4-nitrophenol were added to the already used plating bath according to example 10. The stability number of such a plating bath was 10. Accordingly, the stability of said plating bath against undesired decomposition of the plating bath was considered sufficient.

Example 13 (Comparative)

(21) 40 mg/l of sodium saccharin dihydrate were added to the already used plating bath according to example 10. The stability number of such a plating bath was 9.

Example 14

(22) 40 mg/l of 3-nitro benzene sulfonic acid were added to the already used plating bath according to example 10. The stability number of such a plating bath was 8.