Electroless gold plating bath
The present invention concerns an electroless gold plating bath comprising a) gold ions; b) sulfite ions; c) iodide ions; d) at least one phosphonate compound according to formula (1) wherein each X is independently an alkanediyl group; R 1 , R 2 , R 3 and each R 4 are independently alkanediyl groups; M is independently hydrogen, a metal atom or a cation forming radical; each n is a rational number and selected in accordance with the valency of the respective M; and b is an integer ranging from 1 to 10. The invention further is directed to the use of the bath and a method for depositing a gold layer on a surface of a substrate. The bath is particularly suitable in the manufacture of printed circuit boards, IC substrates, semiconducting devices, interposers made of glass and the like.
1. An electroless gold plating bath, comprising
a) gold ions;
b) sulfite ions;
c) iodide ions; and
d) at least one phosphonate compound according to formula (1)
wherein
each X is an alkanediyl group;
R 1 , R 2 , R 3 and each R 4 are independently alkanediyl groups;
M is independently hydrogen, a metal atom or a cation forming radical;
each n is a rational number and selected in accordance with the valency of the respective M; and
b is an integer ranging from 1 to 10, and
wherein a molar ratio of gold ions to sulfite ions ranges from ⅓ to 1/20.
2. The electroless gold plating bath of claim 1 wherein the pH value of the electroless gold plating bath ranges from 5.5 to 8.5.
3. The electroless gold plating bath of claim 1 wherein the concentration of the sulfite ions ranges from 10 to 150 mmol/L.
4. The electroless gold plating bath of claim 1 wherein the concentration of the iodide ions ranges from 4 to 100 mmol/L.
5. The electroless gold plating bath of claim 1 wherein R 1 , R 2 , R 3 and each R 4 are independently C1-C6-alkanediyl groups.
6. The electroless gold plating bath of claim 1 wherein b ranges from 1 to 6.
7. The electroless gold plating bath of claim 1 wherein the electroless gold plating bath comprises at least one hydroxycarboxylic acid.
8. The electroless gold plating bath of claim 7 wherein the at least one hydroxycarboxylic acid is aliphatic.
9. The electroless gold plating bath of claim 7 wherein the at least one hydroxycarboxylic acid is a C1-C12-hydroxycarboxylic acid.
10. The electroless gold plating bath of claim 1 wherein the electroless gold plating bath is free of thiosulfate ions.
11. The electroless gold plating bath of claim 1 wherein the electroless gold plating bath is free of cyanide ions.
12. The electroless gold plating bath of claim 8 wherein the at least one hydroxycarboxylic acid is a C1-C12-hydroxycarboxylic acid.
13. The electroless gold plating bath of claim 1 wherein molar ratio of gold ions to sulfite ions ranges from ¼ to 1/16.
14. The electroless gold plating bath of claim 1 wherein the electroless gold plating bath is free of thiosulfate, is free of cyanide ions, and is free of thiourea.
15. The electroless gold plating bath of claim 1 wherein the electroless gold plating bath is free of thiourea.