ELECTROLESS NICKEL PLATING SOLUTION

Abstract

An electroless nickel plating solution, including a source of nickel ions, a source of molybdenum ions, a source of tungsten ions, a source of hypophosphite ions at least one complexing agent, at least one organic sulphur containing compound in a concentration of 0.38-38.00 μmol/L, and at least one amino acid in a concentration of 0.67-40.13 mmol/L, and
a method for electroless plating of a nickel alloy layer on a substrate, a nickel alloy layer, and
an article comprising the a nickel alloy layer.

Claims

1. An electroless nickel plating solution, comprising a source of nickel ions, a source of molybdenum ions, a source of tungsten ions, a source of hypophosphite ions at least one complexing agent, at least one organic sulphur containing compound in a concentration of 0.38-38.00 μmol/L, and at least one amino acid in a concentration of 0.67-40.13 mmol/L.

2. The plating solution of claim 1, wherein the molar ratio of amino acid to organic sulphur containing compound is from 282:1 to 14,079:1.

3. The plating solution of claim 1, wherein a concentration of hypophosphite ions is 0.09-0.27 mol/L.

4. The plating solution of claim 1, wherein a concentration of nickel ions is 0.067-0.133 mol/L.

5. The plating solution of claim 1, wherein a concentration of molybdenum ions is 1.05-4.18 mmol/L.

6. The plating solution of claim 1, wherein a concentration of tungsten ions is 12.1-109.2 mmol/L.

7. The plating solution of claim 1, wherein the amino acid is a non-sulphur containing amino acid.

8. The plating solution of claim 1, wherein the organic sulphur containing compound is selected from the group consisting of N,N-dimethyl-dithiocarbamyl propyl sulfonic acid, 3-Mercaptopropane sulfonic acid, 3,3-Dithiobis-1-propane sulfonic acid, 3-(2-Benzthiazolylmercapto)propane sulfonic acid, 3-[(Ethoxy-thioxomethyl)thio]-1-propane sulfonic acid, 3-S-Isothiuroniumpropane sulfonate, sodium diethlydithiocarbamate, thiodiacetic acid, dithiodiacetic acid, thiodiglycolic acid, dithiodiglycolic acid, thiosulfate, thiourea, thiocyanate, cysteine and cystine.

9. The plating solution of claim 1, wherein the complexing agent is selected from the group consisting of citric acid, isocitric acid, EDTA, EDTMP, HDEP and Pyrophosphate.

10. A method for electroless plating of a nickel alloy layer (5) on a substrate (2), particularly a wafer (3), the method comprising contacting said substrate with an electroless nickel plating solution according to claim 1.

11. The method of claim 10, wherein the substrate comprises a copper layer (4) or aluminium layer, wherein the nickel alloy layer (5) is plated on the copper layer (4) or the aluminium layer.

12. A nickel alloy layer, comprising 81.5 to 98.4 wt % nickel 1 to 10 wt % molybdenum 0.1 to 4 wt % tungsten 0.5 to 4.5 wt % phosphor.

13. The nickel alloy layer of claim 12, having a normal stress in the range of −40 to +120 N/mm.sup.2, measured by following Bent Strip Method: Deposition on Cu stress stripes (Copper-Iron Alloy PN: 1194) with subsequent stress determination by use of the Deposit Stress Analyzer (A STM Standard B975).

14. The nickel alloy layer of claim 12, having a thickness in the range of 0.1-5 μm.

15. (canceled)

16. (canceled)

17. (canceled)

18. The plating solution of claim 1, wherein the amino acid is selected from the group consisting of glycine, alanine, valine, leucine and isoleucine.

19. The plating solution of claim 1, wherein a concentration of hypophosphite ions is 0.09-0.27 mol/L; wherein a concentration of nickel ions is 0.067-0.133 mol/L; wherein a concentration of molybdenum ions is 1.05-4.18 mmol/L; and wherein a concentration of tungsten ions is 12.1-109.2 mmol/L.

20. The plating solution of claim 1, wherein the plating solution does not comprise any reducing agent comprising boron.

Description

BRIEF DESCRIPTION OF THE FIGURES

[0154] FIG. 1 shows an article of the invention.

EXAMPLES

[0155] Methods

[0156] Stress Measurement

[0157] The normal stress is measured by the Bent Strip Method: Deposition on Cu stress stripes (Copper-Iron Alloy PN: 1194) with subsequent stress determination by use of the Deposit Stress Analyzer (A STM Standard B975).

[0158] Fracture Toughness

[0159] The indentation test was performed with a Fischerscope H100C using a Vickers. The nickel layer to be tested is annealed to a temperature of 400° C. during a period of 10 minutes (excluding heating-up and cooling-down). Then, a Vickers indenter (a specimen as used for determining Vickers hardness) is pressed into the layer with the maximum load of 1N, a holding time of 4 seconds and a loading/removal rate of dF/dt=+/−0.5 N/s. The indentations are then inspected by optical microscopy.

Example 1: Composition of a Plating Solution of the Invention

[0160] Following ingredients are mixed in water

TABLE-US-00001 Technical Concentration Concentration Compound Function in g/L in mol/L NiSO.sub.4 × 6 H.sub.2O Ni Source 28.00 0.11 Corresponding to Ni 6.4 (Sodium) hypophosphite × H.sub.2O Reducing agent 18.2 0.16 Citric Acid × H.sub.2O (C.sub.6H.sub.8O.sub.7 × H.sub.2O) Complexing agent 28.8 0.14 Sodium molybdate × 2 H.sub.2O Mo Source 0.4 1.66 mmol/L Corresponding to Mo 0.16 Sodium tungstate × 2 H.sub.2O W Source 20.00 60.5 mmol/L Corresponding to W 11.15 Sulphuric acid or sodium hydroxide pH adjustor In an amount to obtain a pH of 9.0 Lead nitrate Stabilizer 1.6 mg/L 4.9 μmol/L Corresponding to Pb 1.00 mg/L sulphur containing compound - Organic Stabilizer 1.0 mg/L 3.8 μmol/L Cystine amino acid (Glycine/Alanine) Stress-reducing 1.0 g/L 13.3 mmol/L additive

Example 2: Layer Composition

[0161] Example 2 shows the compositions of layers from six different depositions by using the solution of example 1. Plating time was 10 minutes for each deposition.

TABLE-US-00002 Ni Mo W P [wt %] [wt %] [wt %] [wt %] 1 89.71 4.94 1.79 2.60 2 89.96 5.12 1.86 2.37 3 90.67 5.59 1.08 1.82 4 89.38 6.98 1.19 2.18 5 91.36 5.29 0.56 1.65 6 89.13 6.33 1.34 2.43 Average wt % 90.04 5.71 1.30 2.18 Minimum wt % 89.13 4.94 0.56 1.65 Maximum wt % 91.36 6.98 1.86 2.43 Average at % 92.7 3.6 0.4 4.2 (atom percent) Minimum at % 91.7 3.1 0.2 3.2 Maximum at % 94.0 4.5 0.6 4.7

Example 3: Addition of Reducing Agent Based on Boron

[0162] Example 3 relates to comparative examples using all compounds of the solution of the invention but wherein the reduction agent Hypophosphite is mixed together with increasing amounts of DMAB (dimethylamine borane).

[0163] Following plating solution is used in Example 3: 2 L Bath from Example 1, stirring at 250 rpm, 88° C. Plating time is reduced successively to obtain comparable Ni thicknesses.

[0164] Example 2a is an example according to the invention. Increasing amounts of DMAB are added to the plating solution indicated above, as further reducing agent (2c-e).

TABLE-US-00003 Layer Plating thickness Deflection Material constants DMAB Time Ni U K Tens. K Comp. Stress Sample [g/L] [min] [μm] [−/+−] [—] [—] [N/mm.sup.2] 2a 0.0 10.00 1.385 4.0 0.2757 0.2416 46.5 2c 1.0 3.50 1.282 20.0 0.2757 0.2416 251.0 2d 2.0 2.50 1.113 21.0 0.2757 0.2416 303.5 2e 3.0 2.50 1.055 21.0 0.2757 0.2416 320.3

[0165] With the experiment it is shown, that even if the combination and concentration of N,N-dimethyldithiocarbamyl propyl sulfonic acid and glycine in the solution of the invention is used in combination with DMAB, this will not lead to a stress reduction, in contrast to the present invention.

[0166] Addition of a boron based reducing agent deteriorates the stress properties of the layer drastically.

Example 4: Variation of Amino Acid

[0167] Example 4 relates examples using a solution of the invention with different amounts of the amino acid alanine.

[0168] Following plating solution is used: Bath from Example 1, stirring at 250 rpm, 88° C. but with following additive combination:

TABLE-US-00004 Sodium Layer thickness Deflection Material constants Alanine Diethyldithiocarbamate Ni U K Tens. K Comp. Stress Sample [g/L] [mg/L] [μm] [−/+−] [—] [—] [N/mm.sup.2] 3a 0.0 0.0 1.804 13.5 0.2757 0.2416 120.4 3b 0.0 2.0 1.758 14.5 0.2757 0.2416 132.7 3c 0.5 2.0 1.834 6.0 0.2757 0.2416 52.6 3d 1.0 2.0 1.620 0.7 0.2757 0.2416 7.0 3e 1.5 2.0 1.576 −0.3 0.2757 0.2416 −2.7 3f 2.5 2.0 1.456 −1.5 0.2757 0.2416 −14.5 3g 2.5 4.0 1.487 −1.7 0.2757 0.2416 −16.1

[0169] Plating time is 8 min in all experiments. The experiments show that increasing amounts of alanine improves (lowers) the stress, and makes possible to reach the region of compressive stress (negative values) which may be beneficial because when the layer is heated, the stress increases and may result in a value of about zero or in the region around zero.

Example 5: Variation of Amounts of Sulphur Containing Compound and Amounts and Amino Acid Glycine

[0170] Bath from Example 1, stirring at 250 rpm, 88° C., pH=9.5, 10 minutes plating time but with following additive combination:

[0171] 5.1 Combination of N,N-Dimethyl-Dithiocarbamyl Propyl Sulfonic Acid and Glycine

TABLE-US-00005 N,N-dimethyl- Stress in dithiocarbamyl Ni U± Stress propyl sulfonic Glycine Thickness (+tensile; in acid in mg/L in g/L in μm −compressive) N/mm.sup.2 0 0.5 3.9 10 49.2 2.5 0 3.5 20 109.4 2.5 0.5 3.3 −1.5 −7.4

[0172] The first two experiments are comparative examples, the plating bath here only comprising one of N,N-dimethyl-dithiocarbamyl propyl sulfonic acid or glycine.

[0173] It is shown that the combination of both improves the stress properties of the layer in synergistic manner.

[0174] 5.2 Combination of Cystine and Glycine

TABLE-US-00006 Stress in Ni U± Stress Cystine Glycine Thickness (+tensile; in in mg/L in g/L in μm −compressive) N/mm.sup.2 3 0 3.3 20 116 3 0.1 3.5 16.5 90 3 0.2 3.5 11.5 63 3 0.3 3.5 7.5 41 3 0.4 3.5 4.5 24 3 0.5 3.5 2.0 11 3 0.6 3.1 −0.5 −3 3 0.7 3.1 −1 −5 3 0.8 3.1 −0.5 −3 3 0.9 3.0 −0.5 −3 3 1.0 3.0 −1.5 −8 4 0.5 3.4 −0.5 −2 5 0.5 0.9 0 0 6 0.0 3.0 21 133 6 0.1 3.6 17.5 94 6 0.2 3.5 12 66 6 0.3 3.5 5.5 30 6 0.4 2.7 0 0 6 0.5 0.7 −0.5 −12

[0175] The two experiments without glycine are comparative examples.

[0176] It is shown that increasing amounts of glycine, at different amounts of cystine, improves the stress properties of the layer in synergistic manner.

Example 6: Fracture Toughness

[0177] All samples have been annealed at 400° C. for 10 minutes.

TABLE-US-00007 Comp. Comp. Comp. Ex. 1 Ex. 2 Ex. 3 Example 4 Example 5 Example 6 Example 7 Example 8 Example 9 Thickness 1.59 2.34 1.82 1.90 1.80 1.50 1.83 1.86 1.76 [μm] P-content 8.20 2.26 4.85 2.60 3.66 1.65 2.85 1.43 1.18 [wt. %] Ni-content 91.90 97.13 88.53 89.71 88.70 91.36 91.75 88.82 91.31 [wt. %] W-content — — 1.15 1.79 1.15 0.56 0.85 0.68 0.18 [wt. %] Mo-content — — 4.27 4.94 5.66 5.29 3.89 8.17 6.33 [wt. %] Bath 84° C., 84° C., 88° C., 88° C., 88° C., 88° C., 88° C., 88° C., 88° C., parameter pH 4.6 pH 7.3 pH 8.5 pH 8.5 pH 8.5 pH 8.5 pH 8.2 pH 8.8 pH 8.2 Cracks in Yes Yes Yes but None None None None None None Fracture less than toughness in Comp. test Ex. 1 and 2

[0178] Fracture toughness test is done as described in the methods above. Layers of the invention turn out to be significant more tough and show no cracking after indentation.

[0179] In comparative examples 1 and 2, Mo and W are missing wherein example 1 represents a common standard binary NIP layer with a medium P content of about 8.2 wt. %. This layer suffers from cracks after indentation. Example 2 is also a binary NIP layer and shows even the same P content like the layer of the invention but in contrast it shows cracks after indentation.

[0180] Comparative example 3 is a comparative example because the P content is higher than in the present invention.

[0181] Example 5 is an example with relatively high amount of phosphor, but in the range of the present invention.

[0182] Example 6 is an example with less phosphor in terms of the invention in the presence of relatively low amount of tungsten.

[0183] Example 7 is an example with medium P content in terms of the invention also in presence of both the low amounts of tungsten and molybdenum.

[0184] FIG. 1 shows (not true to scale) an electronic article 1 of the invention, comprising a nickel alloy layer 5. The nickel alloy layer 5 is produced by contacting a substrate 2, comprising a wafer 3 and the copper layer 4, with a plating solution of the invention.

[0185] When contacting the copper layer 4 of the substrate 2 with the plating solution of the invention, a nickel alloy layer 5 is plated on the copper layer 4. In a further method step, a palladium layer 6 is optionally plated on the nickel alloy layer 5.

[0186] In the shown stack of layers, the nickel alloy layer 5 serves as a barrier layer between the copper layer 4 and the palladium layer 6.

[0187] A gold layer 7 is optionally plated on the by palladium layer 6 and a solder ball 8 is placed on the gold layer 7 which is part of a ball grid array. Only a part of the wafer 3 is shown here. The wafer 3 comprises several layer stacks 4,5,6,7 at different positions, each layer stack comprising a solder ball 8). The wafer can comprise individual compartments of the wafer surface (“pads”) and each compartment may comprise a layer stack 4,5,6,7 as shown here in one example.