HIGH-SPEED PURE GOLD ELECTROFORMING/ELECTROPLATING BATH

Abstract

“HIGH SPEED PURE GOLD ELECTROFORMING/ELECTROPLATING BATH” consisting of a. gold potassium sulphite and sodium gold sulphite of 1-40 g/l ; b. lithium sodium potassium rubidium or cesium salts of sulphurous acid of 2-100 g/l; c. alkali salts of phosphoric acid of 2-100 g/l; d. phosphonic acids in the form of 1-hydroxyl ethane-1, 1, diphosphonic acid, methylene phosphonic acid, ethylene diamine tetra methylene phosphonic acid of 2-50 g/l; e. bismuth as citrate, tartrate, oxalate, phosphonate or NTA complex of 5-1000 mg/l; and f. some amount of pyrophosphate, poly phosphates and other supporting electrolytes and wetting agents. The high speed pure gold electroforming/electroplating bath allows formation of wire structure in IC chips and/or dental/jewellery application with fine grain, high density, high uniformity, ability to withstand electrical stress and different hardness without using cyanide.

Description

FIELD OF INVENTION

The present invention relates to a high speed pure gold electroforming/electroplating bath that allows formation of wire structure in IC chips and jewelry and dental applications requiring deposit with different hardness while maintaining the fine grain, high density, high uniformity and has the ability to withstand electrical stress without using cyanide.

BACKGROUND OF INVENTION

Electroforming bath for gold is required for a variety of applications such as jewellery, dental applications like gold cap (used in root canal treatment) gold bridge, artificial tooth and electronic application like wire structures in IC chips.

Electronic application like forming wire structures in IC chips requires good electrical conductivity so deposit density should be high. Bath used for forming these structures should be able to yield deposits with maximum uniformity that is it should have high macro throwing power. Deposit should not change its conductivity when higher current flows through it. So deposit should be fine grained and should be free of alloying elements which may segregate on heating (by high current). All these characteristics namely, fine grain, high density, high uniformity and ability to stand electrical stress are obtained by the use of cyanide based baths with arsenic or thallium as grain refiner. Cyanide is a deadly poison and the grain refining agents are toxic.

Some of the cited prior arts are listed below:

• • EP026921A2 describes a cyanide based gold plating bath with bismuth as an addition agent for gold electroforming for dental application.
  • 1186/CHE/2005 describes a non-cyanide, non-ammonia based gold sulphite bath using gold potassium sulphite and bismuth for electro forming for dental applications. This bath produces deposits with hardness of 240-260 VHN (Vickers Hardness Number) suitability of this bath for IC industry is not established.
  • U.S. Pat. No. 4,396,471A deals about an all-purpose gold cyanide electroplating bath that utilizes a cobalt, nickel, or indium hardener as a chelate with the acid form of a methyl vinyl ether/maleic anhydride interpolymer. The bath is capable of producing high levels of hardness in deposits that are substantially pure gold.
  • U.S. Pat. No. 5,277,790A discloses a cyanide-free gold electroplating solution consisting of gold in the form of a soluble sulphite complex, an added source of sulphite and/or bisulphite ion, a supporting electrolyte, an organic polyamine or mixture of polyamines of molecular weight from about 60 to 50,000, and an aromatic organic nitro compound.
  • U.S. Pat. No. 3,057,789A discloses a gold plating bath containing potassium gold sulphite complex and disodium ethylene diamine tetra acetate.
  • WO2014054429A1 discloses a non-cyanide electrolytic gold plating solution comprising a gold source with an alkali gold sulphite or ammonium gold sulphite and a conductive salt comprising a sulphite and a sulphate, where the salt of at least one element is selected from iridium, ruthenium and rhodium in an amount of 1 to 3000 mg/L in terms of a metal concentration.

The present invention relates to a high speed pure gold electroforming/electroplating bath that allows formation of wire structure in IC chips with fine grain, high density, high uniformity, ability to withstand electrical stress and different hardness without using cyanide. The present invention can be utilized for dental/jewellery application, which requires deposits with different hardness. The present invention can also be used for coating electrical contacts (that is fingers) for improved life and for other applications like brazing or others requiring variable hardness.

OBJECT OF INVENTION

The main object of the present invention is to provide a high speed pure gold electroforming/electroplating bath that allows formation of wire structure in IC chips with fine grain, high density, high uniformity, ability to withstand electrical stress and different hardness without using cyanide.

Another object of the present invention is to provide a high speed pure gold electroforming/electroplating bath that can be utilized for dental/jewellery application, for coating electrical contacts (that is fingers) for improved life and for other applications like brazing or others requiring variable hardness.

Another object of the present invention is to provide a high speed pure gold electroforming/electroplating bath that prevents pollution and is free from toxic and carcinogenic additives.

Another object of the present invention is to provide a high speed pure gold electroforming/electroplating bath that reduces the problem of potassium sulphate built up in the bath by the utilization of some percentage of sodium gold sulphite in place of potassium gold sulphite, so that the need to constantly remove the potassium sulphate by freezing & filtering is eliminated.)

SUMMARY OF THE INVENTION

The high speed pure gold electroforming/electroplating bath of the present invention comprising of a bath electrolyte based on mixture of gold potassium sulphite and gold sodium sulphite containing one or more complexants from the group of phosphonate, citrate, tartrate, amino acetic acid and phosphate, besides one or more metal ions chosen from the group of antimony, indium, bismuth or gallium is used as additive to improve the properties of the deposit. Alkali metal ions metallic additives and wetting agents are optimally formulated to provide an improved bath because, they act synergistically. Metallic gold (as alkali gold sulphites) containing one or more alkali metal ions chosen from the group lithium, sodium, potassium, rubidium or cesium or mixtures thereof, will have a concentration in the range of 1-40 grm/l as metallic gold in the bath.

DETAILED DESCRIPTION OF INVENTION

The present invention discloses a high speed pure gold electroforming/electroplating bath that allows formation of wire structure in IC chips with fine grain, high density, high uniformity, ability to withstand electrical stress and different hardness without using cyanide.

The high speed pure gold electroforming/electroplating bath of the present invention, consisting of:

• • a. gold potassium sulphite and sodium gold sulphite of 1-40 g/l;
  • b. lithium, sodium, potassium, rubidium or cesium salts of sulphurous acid of 2-100 g/l;
  • c. alkali salts of phosphoric acid of 2-100 g/l;
  • d. phosphonic acids in the form of 1-hydroxyl ethane-1, 1, diphosphonic acid, methylene phosphonic acid, ethylene diamine tetra methylene phosphonic acid of 2-50 g/l;
  • e. bismuth as citrate, tartrate, oxalate, phosphonate or NTA complex of 5-1000 mg/l; and
  • f. some amount of pyrophosphate, poly phosphates and other supporting electrolytes and wetting agents.

A preferred composition of the gold electroforming bath is a mixture of sodium and potassium gold sulphite. By varying the proportion of sodium gold sulphite to potassium gold sulphite, hardness of the deposit is varied from 100 to 260 VHN. If all the gold is present as sodium gold sulphite, we can obtain lowest hardness of 100 VHN. If all the gold is present as potassium gold sulphite, we can obtain hardness of 260 VHN.

Intermediary hardness is obtained by adjusting the percentage of sodium gold sulphite. At 25% sodium gold sulphite & 75% potassium gold sulphite, hardness is 190 VHN/20 g load. At 50:50 of sodium & potassium gold sulphite, hardness is 160 VHN/20 g load. At 75% sodium gold sulphite and 25% potassium gold sulphite, hardness is 120 VHN/20 g load. All the deposits are ductile. Even the harder 260 VHN deposit is ductile.

Best electrical properties are obtained with baths containing<10% sodium gold sulphite. These baths produce deposit with resistivity of 2.34 micro ohm.cm, which compares well with a literature value of 2.35 micro ohm.cm.

In the absence of bismuth additive, deposit tends to become powdery at thicknesses greater than 2 micron and bismuth acts as a brightener and grain refiner. Bismuth can be added as citrate, tartrate, oxalate, phosphonate or nitrillo triacetic acid complex (NTA). For electronic application, phosphonate and NTA complexes are preferred. In the new electroforming bath, we can obtain a hardness of 260 VHN, maximum current density of 3 A/dm² and resistivity of 2.34 micro ohm cm.

The operating parameters of the gold electroforming/electroforming bath is given below.

ph                      7 to 8
Maximum current density 3 A/dm2
Temp.                   50-70° C.
Deposit thickness       1 micron or more
Anode                   Platinised titanium

The deposit properties of the gold electroforming/electroplating bath is given below.

Purity                     99.6-99.8%
Hardness at 20 g load      100-260 VHN
Resistivity                2.34 micro ohm · cm
Maximum rate of deposition 175 micron/hour

The high speed pure gold electroforming/electroplating bath can also be utilized for dental/jewellery application, for coating electrical contacts (that is fingers) for improved life and for other applications like brazing or others requiring variable hardness. The high speed pure gold electroforming/electroplating bath of the present invention is non polluting and free from toxic and carcinogenic additives. The utilization of some percentage of sodium gold sulphite in place of potassium gold sulphite also reduces the problem of potassium sulphate built up in the bath, so that the need to constantly remove the potassium sulphate by freezing & filtering is eliminated.

Claims

1. High speed pure gold electroforming/electroplating bath for use during fabrication of ICs, consisting of: a. gold potassium sulphite and sodium gold sulphite of 1-40 g/l;b. lithium sodium potassium rubidium or cesium salts of sulphurous acid of 2-100 g/l;c. alkali salts of phosphoric acid of 2-100 g/l;d. phosphonic acids in the form of 1-hydroxyl ethane-1, 1, diphosphonic acid, methylene phosphonic acid, ethylene diamine tetra methylene phosphonic acid of 2-50 g/l;e, bismuth as citrate, tartrate, oxalate, phosphonate or NTA complex of 5-1000 mg/l; andf. some amount of pyrophosphate, poly phosphates and other supporting electrolytes and wetting agents.

2. The bath as claimed in claim 1a, wherein the proportion of sodium gold sulphite to potassium gold sulphite is varied to obtain hardness of the deposit from 100 to 260 VHN.

3. The bath as claimed in claim 1, produces deposit with resistivity of 2.34 micro ohm.cm, hardness of 260 VHN and maximum current density of 3 A/dm2.

4. The bath as claimed in claim 1, can also be utilized for dental/jewelry application, for coating electrical contacts (that is fingers) for improved life and for other applications like brazing or others requiring variable hardness.