GB2093069A - Gold electroplating bath and process - Google Patents

Abstract

Electrolytes for the electrodeposition of gold metal on substrates containing alkali metal or ammonium gold cyanides, a conducting agent, optionally a buffering agent, and at least 30 grams per litre of a grain refiner consisting of oxalic acid or formic acid or an alkali metal or ammonium salt thereof. The electroplating baths of the invention are operated at a pH of from 5 to 7.5, a temperature of 50 DEG to 80 DEG C, and a current density ranging from 1 to 20 ASF (0.11 to 2.2 ASD).

Description

SPECIFICATION Gold electroplating bath and process The present invention relates to compositions and methods for the electrodeposition of lemon yellow coloured gold metal on substrates from baths having a pH of from about 5 to 7.5 wherein the source of the gold is an alkali metal or ammonium gold cyanide.

Numerous electrolyte baths have been proposed in recent years for the electrodeposition of gold on to substrates. Representative patents include the following U.S. Patents: 2,905,601 -- Rinker et al.

3,104,212 -Rinkeret al.

3,1 56,634 -- Duva et al.

3,1 56,635 - Foulke 3,367,853 - Schumpelt 3,373,094 -- Foulke 3,423,295 -Greenspan 3,776,821-Baker 3,776,822 - Baker 3,833,487 -- Reinheimer 3,878,066 -- Dettke and British Patent: 1 065,308 - Nobel et al.

These disclosures indicate that in order to obtain the desired gold electroplating deposits special procedures and additives have to be employed when the electrolytes operate at a pH of from about 5 to 7.5 rather than at the lower pH range of from about 3 to 6. Thus, for example, at the higher pHs it has been customary to use grain refiners such as hydrazine, arsenic, or thallium.

When such electrolyte baths are operated without these grain refiners the deposits tend to have a brownish appearance, and the electrolytes only operate over a very limited current density range.

Unfortunately, the conventional grain refiners employed heretofore have been found in some instances to be either harmful to the integrity of the gold, i.e. to cause degradation of the gold plate, because of their occlusion in the deposits, or to be carcinogenic or both.

The ability to operate gold plating baths at a pH within the range of about 5-7.5, i.e. at a substantially neutral pH, is advantageous for several reasons. When operated within such range, rather than lower or higher ranges, the amount of gold dissolved from the plated parts is significantly reduced. Additionally, there is a reduction in the amount of metal contaminants in the plating bath and codeposition of any such contaminants that may be present is minimized.

In accordance with the present invention it has now been found that the use of special grain refiners, and especially in at least certain prescribed amounts, in gold electrolyte baths formulated to operate at a pH ranging from about 5.0 to 7.5, results in lemon yellow gold deposits without the problems noted above. More particularly, the problems attendant upon the use of conventional grain refiners are avoided by the present invention through the use of formic acid or oxalic acid, or an alkali metal or ammonium salt of such acids, in a concentration of at least 30 grams per litre, calculated as the acid. The electroplating baths of this invention are generally operated at current densities of from about 1 to 20 ASF (0.11 to 2.2 ASD).

The gold electrolytes or electroplating baths of the present invention will be operated under slightly acid to substantially neutral conditions, i.e.

from about 5 to 7.5 pH, and will utilize an alkali metal or ammonium gold cyanide as source of the gold metal. The gold metal content of the electrolyte will be at least sufficient to deposit gold on the substrate when the bath is electrolyzed and may be up to the maximum solubility of the gold in the bath. In general, however, the use of very dilute or very concentrated gold solutions provide no particular advantages and may, in some instances, increase the overall cost of the process. Accordingly, it is normally desirable that the gold metal content of the electrolyte range from about 2 to 10 g/l, and preferably from about 4 to 8 g/l. The use of potassium and ammonium gold cyanides, is especially preferred, but it will be understood that neither the amount of gold metal nor the particular gold cyanide complex employed is critical.

The electroplating bath will also contain a conventional conducting agent such as an alkali metal or ammonium salt of a citrate, phosphate, sulphamate, or acetate. In general, the preferred conducting agents for the present purposes include dibasic potassium phosphate, tripotassium phosphate, and ammonium citrate. The amount of conducting agent employed will be at least that which will provide sufficient conductivity to the bath to effect the gold electrodeposition, up to the maximum solubility of the conducting agent in the bath. Typically, the conducting agent will be present in an amount from about 2.5 to 200 g/l, and preferably from about 5 to 50 or 100 g/l.

In many instances pH adjustment is necessary to achieve the desired value in the bath of about 5 to 7.5. Such compounds as phosphoric acid, potassium hydroxide, sulphamic acid, ammonium hydroxide, or mixtures thereof, may be utilized for adjusting the pH to 5 to 7.5, preferably 6.5 to 7.5.

Alkali metal hydroxides, other than potassium hydroxide, may also be used for this purpose.

As will be understood by those skilled in the art, a buffering agent may also be employed in formulating the gold electroplating baths of the present invention. The same materials listed above as being suitable conducting salts may also be used as buffering agents. The amount of the buffering agent or agents used will be at least sufficient to provide the desired buffering of the plating bath, up to the maximum solubility of the buffer in the bath. Desirably, the amount of buffering agent will range from about 2.5 to 200 g/l, and will preferably be from about 5 to 50 g/l. It will, of course, be appreciated that the same material may be used to provide both the desired conductivity and buffering functions.

Alternatively, different materials may be used to achieve each of these functions. In either case, the amount used will be that which is required to provide the necessary buffering and/or conductivity.

In accordance with the foregoing discussion, it will be understood that the essential feature of the present invention is the selection of the grain refiner agent. Thus, disadvantages associated with the use of previously employed agents have been overcome by utilizing specific grain refiners; namely, oxalic acid or formic acid, or an alkali metal or ammonium salt of these particular carboxylic acids. The especially preferred agents are oxalic acid as well as its potassium and ammonium salts. In general, the amount of grain refiner will be at least 30 gIl, since lower amounts will not achieve the desired results. Although amounts up to the maximum solubility in the bath may be used, for most purposes, the amount of grain refiner will range from about 30 to 200 g/l, calculated as the acid.Preferred amounts will range from about 35 to 80 or 120 g/l.

It will be further understood that the electroplating process of this invention will generally be operated at current densities ranging from about 1 to 20 ASF (0.11 to 2.2 ASD) and temperatures of from about 50" to 900C, preferably 60 to 700C. Although bath temperatures above 900C, e.g., up to the boiling point of the bath, can be used, at such higher temperatures, excessive evaporation of the bath often occurs, so that use of these higher temperatures are generally not preferred. These conditions, plus the pH range of from 5 to 7.5, describe the operating parameters used with the present gold electroplating baths.

It is a further feature of the present invention to use specific brighteners which will readily achieve lemon yellow gold deposits, while avoiding the difficulties or the limitations as described above, attendant upon the use of the conventional brighteners heretofore employed in gold electroplating baths.

Although the gold plating baths of this invention may be used effectively for the deposition of gold on many different substrates, its use for plating integrated circuits has been found to be especially outstanding. In this regard, such integrated circuits require that the gold deposits be of very high purity and contain a minimum amount of codeposited metallic element impurities. The gold plating baths of the present invention are very advantageous in their ability to fulfill this requirement without encountering the difficulties and problems attendant with the prior art gold plating baths.

The invention may be put into practice in various ways and a number of specific embodiments will be described to illustrate the invention with reference to the accompanying examples.

EXAMPLE 1 - Gold as Potassium 6 Gold Cyanide Dibasic Potassium 100 Phosphate Oxalic Acid 60 The pH of the bath was adjusted to pH 7.0 with phosphoric acid or potassium hydroxide. A grain refined, lemon yellow gold deposit was obtained on a nickel plated substrate immersed as the cathode in the bath which was operated at a temperature of 650C and a current density of up to 10 ASF(1.1 ASD).

EXAMPLE 2 Example 1 was repeated except that the oxalic acid was excluded. When operating this bath at current densities of up to 3 ASF (0.32 ASD) a yellow gold deposit was obtained, but above 3 ASF (0.32 ASD) the deposit had an undesirable brown colour.

EXAMPLES 3A and 3B Another bath was prepared from the following components: g/l Gold as Potassium 8 Gold Cyanide Potassium Sulphamate 42 Tripotassium Phosphate 1 6 Oxalic Acid, added as 120 Potassium Oxalate The bath was adjusted with sulphamic acid to a pH of 6. At an operating temperature of 600C a grain refined, lemon yellow gold deposit (Example 3A) was obtained at current densities of up to 10 ASF; without the potassium oxalate the deposit (Example 3B) had an undesirable brown colour at 6 ASF (0.65 ASD) or higher.

EXAMPLE 4 A gold electroplating bath was formulated as follows: gIl Gold as Ammonium 4 Gold Cyanide Ammonium Citrate 100 Oxalic Acid as 80 Ammonium Oxalate Ammonium Hydroxide to pH 7.5 At a bath operating temperature of 650C grain refined, lemon yellow gold deposits were obtained at current densities of up to 10 ASF (1.1 ASD).

EXAMPLE 5 Another plating bath was formulated from the following ingredients: g/l Gold as Potassium 8 Gold Cyanide Dibasic Potassium 25 Phosphate Formic Acid added as 60 Potassium Form ate The pH was adjusted to 7.5 with orthophosphoric acid. Lemon gold deposits were obtained with this electroplating bath using the conditions described above.

Claims (14)

1. An electroplating bath for the electrodeposition of gold metal deposits, which comprises an alkali metal or ammonium gold cyanide in an amount at least sufficient to deposit gold on a substrate when the bath is electrolyzed, a conducting agent in an amount at least sufficient to provide conductivity to the bath, and as a grain refiner oxalic acid or formic acid, or an alkali metal or ammonium salt of oxalic acid orformic acid, in an amount of at least 30 grams per litre calculated as the acid, the bath having a pH in the range of from about 5 to 7.5.

2. An electroplating bath as claimed in Claim 1 in which the said conducting agent is an alkali metal or ammonium citrate, phosphate, sulphamate or acetate salt.

3. An electroplating bath as claimed in Claim 1 or Claim 2 in which the pH is in the range of from about 6.5 to 7.0.

4. An electroplating bath as claimed in Claim 2 or Claim 3 in which the grain refiner is oxalic acid.

5. An electroplating bath as claimed in Claim 2 or Claim 3 in which the grain refiner is an alkali metal salt of oxalic acid.

6. An electroplating bath as claimed in Claim 5 in which the alkali metal salt is potassium oxalate.

7. An electroplating bath as claimed in Claim 2 or Claim 3 in which the grain refiner is ammonium oxalate.

8. An electroplating bath for the electrodeposition of gold metal deposits which comprises as a source of gold ions, an alkali metal or ammonium gold cyanide in an amount of 2 to 10 g/l, as a conducting agent, dibasic potassium phosphate, tripotassium phosphate or ammonium citrate in an amount of 2.5 to 200 g/l, as a grain refiner, oxalic acid or formic acid or an alkali metal or ammonium salt thereof in an amount of 30 to 200 g/l calculated as the free acid, the bath having a pH of 5 to 7.5.

9. An electroplating bath as claimed in Claim 8 in which the gold is present in an amount of 4 to 8 g/l.

10. An electroplating bath as claimed in Claim 8 or Claim 9 in which the conducting agent is present in an amount 5 to 100 g/l.

11. An electroplating bath as claimed in Claim 8, 9 or 10 in which the grain refiner is present in an amount of 35 to 120 g/l.

1 2. An electroplating bath as claimed in Claim 1 substantially as specifically described herein with reference to any one of Examples 1, 2, 3A, 4 or 5.

1 3. A method of depositing gold on a substrate which comprises passing an electric current through an electroplating bath as claimed in any one of Claims 1 to 1 2 between the substrate as the cathode and an anode, for a period of time sufficient to produce the desired gold deposit on the substrate.

14. A method as claimed in Claim 13 substantially as specifically described herein with reference to any one of Examples 1, 2, 3A, 4 or 5.

4 5. A substrate whenever provided with a gold electrodeposit made by a method as claimed in Claim 13 or Claim 14.