PLATING BATH FOR THE ELECTROLESS PLATING OF A SUBSTRATE

Abstract

A plating bath for electroless plating of a substrate with nickel. The plating bath includes a nickel ion source and a stabilizing system comprising an iodate ion source and a heavy metal ion source. The substrate can be a copper or aluminum substrate.

Claims

1. A plating bath for electroless plating of a substrate with nickel, the plating bath comprising a nickel ion source and a stabilizing system comprising an iodate ion source and a heavy metal ion source.

2. The plating bath according to claim 1, wherein the heavy metal ion source is a copper salt.

3. The plating bath according to claim 1, wherein the iodate ion source is potassium iodate.

4. The plating bath according to claim 1, further comprising at least one reducing agent.

5. The plating bath according to claim 1, wherein the nickel ion source is nickel sulfate.

6. The plating bath according to claim 1, wherein the iodate ion source has a concentration of approximately 100 μl of a 0.05 molar solution/l to approximately 400 μl of a 0.05 molar solution/l and the heavy metal ion source has a concentration of approximately 20 μl of a 0.1 molar solution/l to approximately 80 μl of a 0.1 molar solution/l.

7. The plating bath according to claim 1, wherein the plating bath has a pH of approximately 3 to 5 and a temperature of approximately 80° C. to 90° C.

8. A use of an iodate ion source and a heavy metal ion source, in particular a copper ion source, for stabilizing a nickel plating bath.

9. A method for depositing nickel on a substrate, the method comprising the following steps: a) treating the substrate to be plated with an acid; b) activating the substrate surface using palladium; c) contacting the activated substrate with a plating bath according to claim 1.

10. The plating bath of claim 1, wherein the substrate is a copper or aluminum substrate.

11. The plating bath according to claim 1, wherein the heavy metal ion source is a copper sulfate.

12. The plating bath according to claim 4, wherein the reducing agent comprises sodium hypophosphite and/or DMAB (dimethylaminoborane).

13. The plating bath according to claim 4, wherein the reducing agent comprises at least one complexing agent and at least one pH adjuster.

14. The plating bath according to claim 6, wherein the iodate ion source is potassium iodate.

15. The plating bath according to claim 6, wherein the heavy metal ion source is CuSO.sub.4.5H.sub.2O.

16. The plating bath according to claim 6, wherein the iodate ion source-has a concentration of approximately 200 μl of a 0.05 molar solution/l, and the heavy metal ion source has a concentration of approximately 40 μl of a 0.1 molar solution/l.

17. The plating bath according to claim 1, wherein the plating bath has a pH of approximately 4.4, and a temperature of approximately.

18. The method of claim 9, wherein the step of treating the substrate to be plated comprises treating the substrate with sulfuric acid.

Description

[0036] Furthermore, it has been found that the plating bath according to the disclosure, which contains the stabilizing system, does not exhibit a lower deposition rate of nickel on copper substrates than a comparable plating bath without said stabilizing system. In this regard, a comparative test was carried out, in which the nickel deposition rate was run with a plating bath according to the disclosure and with a plating bath without a stabilizing system. The results are illustrated in FIG. 1. Samples were run on a small scale (bath volume 1.61) and on a large scale (bath volume 1001). Both copper test chips and copper wafers having different test structures and sizes were used for this evaluation.

[0037] FIG. 1 shows: deposition rate of a Ni—P layer on copper surfaces using a plating bath with and without a stabilizing system. A deposition time of 15 minutes was selected for each sample.

[0038] As shown in FIG. 1, none of the stabilization components (copper ions and iodate ions) act as a catalytic poison. For most of the known stabilizing agents, a certain concentration must not be exceeded since an excess of a certain concentration causes a deterioration of the plating process. For instance, an average concentration of lead salts or thiourea not having a negative effect on the plating process is very low (approx. 1 ppm). In the stabilizing system of the plating bath according to the disclosure, the concentration of the components of the stabilizing system can be significantly higher.

[0039] Since no negative impact on the plating rate is observed when using the stabilizing system of the plating bath according to the disclosure, it can be assumed that there is no co-deposition of the used stabilizing components on a substrate to be plated. After all, any co-deposition of copper ions or iodate ions would have an impact on the plating rate.

[0040] Properties of the Deposited Nickel Plating:

[0041] The surface topographies of the platings were examined using an optical microscope and a scanning electron microscope. No significant differences of the surface qualities of the Ni—P layers deposited using a plating bath with and without a stabilizing system were observed. The platings have a homogenous appearance in both cases. Physical and chemical properties of the electroless nickel platings vary depending on the phosphor content in the deposited layer. An EDX analysis showed that the phosphor content in the Ni—P plating is in the range of 6% to 7%. This range is known to provide good solderability and corrosion resistance if gold is applied to the plating. The corrosion resistance is known to increase with an increasing phosphor content in the plating.

[0042] FIG. 2 shows: FIB cross sections of copper pads plated with nickel in an electroless manner, copper sulfate and potassium iodate having been used as a stabilizing system for one pad (illustration on the right) and no stabilizing agent having been used for the comparative pad (illustration on the left). The bath compositions were identical except for the stabilizing components (copper sulfate and potassium iodate). The photos show that the structural morphology of the two samples is nearly identical.

[0043] The two copper pads showed no significant differences in the two interfaces of the platings to the copper substrate. Moreover, it is to be noted that an increase in gloss and smoothness of the layer would have to be expected in the event of a co-deposition of copper on the pad to be plated. However, such effects are not found in the case at hand, which means that a co-deposition of copper can be virtually excluded.

[0044] Ultimately, the Bath Stability was Examined.

[0045] The stability of the plating baths was examined by intentionally compromising the plating baths with a PdCl.sub.2 solution (titration method). A certain amount of PdCl.sub.2 solution (1 ml of a 50 mg/l solution) was admixed to the plating baths during a period of 60 seconds, and the added amount was monitored throughout said period. Table 2 shows the amount of titration solution required in order to decompose the plating bath in the presence of a stabilizing system (bath no. 2) and in the absence of a stabilizing system (bath no. 1). A combination of copper sulfate and potassium iodate was used as the stabilizing system. Baths of a volume of 1.6 liters were used. As shown in Table 2, bath no. 2 requires four times the amount of PdCl.sub.2 in order to decompose the bath.

TABLE-US-00002 TABLE 2 Plating bath Amount of PDCl.sub.2 solution required (bath volume 1.6 l) for decomposition (ml) #1 (without stabilizing system) 2.5 #2 (with stabilizing system) 10

[0046] The stability tests showed high repeatability. It could be proven that the stability of electroless nickel plating baths could be significantly increased if a stabilizing system (iodate ions and copper ions in this case) is admixed, this plating system having no impact on the plating rate and the plating quality.

[0047] Examination of the Bath Tank:

[0048] For this purpose, a plating bath according to the disclosure was left in a bath tank for approx. 1 month. A visual inspection of the tank revealed that no contaminations or deposits are deposited on the tank interior or on the bottom of the tank. The same observations could be made on smaller scales (e.g., in a beaker).

[0049] Subsequently, bath samples were collected after the plating process. Thereafter, the bath tank was emptied and filled with water. Thereafter, the water was removed from the tank and what is referred to as a stripping process was performed using nitric acid. Thereafter, the nitric acid was removed from the tank, whereupon the latter was again filled with water in order to determine possible residue of stabilizers. An ICP elementary analysis of the collected bath samples revealed that no contaminating residue resulting from the components of the stabilizing system was present in the bath samples.