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.

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; and f. 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/dm.sup.2.

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.

Description

DETAILED DESCRIPTION OF INVENTION

[0017] 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.

[0018] The high speed pure gold electroforming/electroplating bath of the present invention, consisting of: [0019] a. gold potassium sulphite and sodium gold sulphite of 1-40 g/l; [0020] b. lithium, sodium, potassium, rubidium or cesium salts of sulphurous acid of 2-100 g/l; [0021] c. alkali salts of phosphoric acid of 2-100 g/l; [0022] 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; [0023] e. bismuth as citrate, tartrate, oxalate, phosphonate or NTA complex of 5-1000 mg/l; and [0024] f. some amount of pyrophosphate, poly phosphates and other supporting electrolytes and wetting agents.

[0025] 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.

[0026] 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.

[0027] 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.

[0028] 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/dm.sup.2 and resistivity of 2.34 micro ohm cm.

[0029] The operating parameters of the gold electroforming/electroforming bath is given below.

TABLE-US-00001 ph 7 to 8 Maximum current density 3 A/dm2 Temp. 50-70? C. Deposit thickness 1 micron or more Anode Platinised titanium

[0030] The deposit properties of the gold electroforming/electroplating bath is given below.

TABLE-US-00002 Purity 99.6-99.8% Hardness at 20 g load 100-260 VHN Resistivity 2.34 micro ohm .Math. cm Maximum rate of deposition 175 micron/hour

[0031] 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.