FR2500012A1 - ELECTROLYTIC GOLD COATING BATH AND DEPOSIT METHOD - Google Patents

Abstract

THE INVENTION CONCERNS ELECTROLYTES FOR ELECTROLYTIC GOLD COATING ON SUBSTRATES CONTAINING GOLD AND AMMONIUM CYANIDES OR ALKALINE METAL, A CONDUCTING AGENT, OPTIONAL A BUFFER AND AT LEAST 30 GRAMS PER LITER OF AN AGENT OF GRAIN REFINING CHOSEN FROM THE GROUP CONSISTS OF OXALIC ACID, FORMIC ACID AND THE AMMONIUM OR ALKALINE METAL SALTS OF THESE. THE ELECTROLYTIC COATING BATHS OF THE INVENTION OPERATE AT A PH BETWEEN 5 AND 7.5, A TEMPERATURE BETWEEN 50 AND 80C, AND A CURRENT DENSITY BETWEEN 0.1 AND 2ADM. THE PROCESS FOR USING THE GOLD ELECTROLYTIC COATING BATHS OF THE INVENTION IS ALSO DESCRIBED.

Description

1. The present invention relates to compositions and methods

  of lemon yellow gold electrolytic coating on sub-

  trats from baths with a pH between about 5 and 7.5 o the gold source consists of cyanides of alkali metals or gold and ammonium.

  Many electrolytic baths have been proposed in recent years

  Ticks for the electrolytic coating of gold on substrates. Patents concerning this problem are, for example, the following patents: Patents of the United States of America: n 2,905,601 - Rinker and others n 3. 104,212 - Rinker and others n 3,156,634 - Duva and others n 3,156,635 Foulke n 3.367.853 - Schumpelt n 3.373.094 - Foulke n 3.423.295 Greenspan n 3.776.821 - Baker n 3.776.822 - Baker n 3.833.487 Reinheimer n 3.878 .066 - Dettke English patent: n 1.065.308 - Nobel and others It appears in these patents that an appeal must be made, in order to obtain the desired deposits of gold by electrolytic coating,

  special processes and additives when the electrolytes work

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  2. operate at a pH between about 5 and 7.5 instead of a pH in the lower range with limits around 3 to 5. So, for example, for higher pH, it has been common to use grain refining agents such as hydrazine, arsenic, thallium and the like. When using such electrolyte baths without grain refining agents, the deposits tend to have a brownish appearance, and the electrolytes only work in a very limited density range. Unfortunately, conventional grain refiners

  used in the prior art have been found in certain cases to be

  rigid and / or harmful to the integrity of gold, i.e. they

  degrade the gold plate due to their occlusion in the deposits.

  The possibility of using gold coating gains at a pH between about 5 and 7.5, that is to say at a substantially neutral pH, is advantageous from several points of view. With operation in such a range and not in higher or lower ranges,

  the quantity of dissolved gold coming from the coated parts is significant

  badly reduced. In addition, there is a reduction in the amount of conta-

  minants of the metal in the electroplating bath and any codeposition

  any such contaminants that may be present is minimized.

  According to the present invention, it has been found that the use

  special grain refining agents, and more particularly when

  that these are found at least in certain prescribed quantities, in

  electrolytic gold coating baths formulated to function

  work with a pH between about 5.0 and 7.5, allow to obtain

  to deposit deposits of lemon yellow gold without the problems mentioned above

  be asked. More particularly, the problems inherent in the use of conventional grain refining agents are avoided in the present invention by the use of formic acid, oxalic acid, as well as salts of alkali metals or ammonium of these acids, in a concentration of at least 30 grams per liter, calculated

  in acid. The electrolytic coating baths of the present invention

  tion generally operate at current densities between about 0.1 and 2 A / dm

  The gold electroplating baths of this in-

  vention operate between slightly acidic conditions and con-

  substantially neutral editions, that is to say with a pH between

  about 5 and 7.5, and use an alkali or gold cyanide and

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  3. ammonium as a source of gold. The gold content of the electrolyte will be at least sufficient to allow the deposition of gold on the substrate when the bath is electrolysed and can reach the maximum solubility of gold in the bath. However, in general, the use of very dilute or very concentrated gold solutions presents no particular disadvantage and can, in certain cases, lead to an increase in the overall cost of the process. Therefore, it is normally desirable for the metal gold content of the electrolyte to be between about 2 and 10 grams.

  per liter, and preferably between about 4 and 8 grams per liter.

  The use of potassium cyanides and gold and amrnium is particularly

  highly recommended, but it will be understood that neither the quantity of metal gold

  neither the particular gold cyanide complex employed is critical.

  The electrolytic coating bath will also contain a

  conventional conduction agent such as alkali metal or am-

  monium of citrates, phosphates, sulfamates, acetates or the like. In general, the conduction agents recommended in the present invention

  include potassium diacid phosphate, tripotas phosphate-

  sium and ammonium citrate. The quantity of conduction agent used will be at least that which will give sufficient conductivity to the bath.

  to carry out the electrolytic deposition of gold, up to the ma-

  maximum of agent in the bath. Typically, the conduction agent will be present in an amount between about 2.5 and 200 grams

  per liter and preferably between about 5 and 50 grams per liter.

  In many cases it is necessary to adjust the pH in order to obtain the desired value in the bath, which is

  between approximately 5 and 7.5. Compounds such as phospho- acid

  rique, potassium hydroxide, sulfamic acid, ammonia, mixtures thereof and the like can be used for adjusting the pH to a

  value between 5 and 7.5, preferably at a value between-

  be 6.5 and 7.5. Alkali metal hydroxides, other than hydroxide

  potassium can also be used for this purpose.

  As will be appreciated by those skilled in the art, a buffering agent can

  also be used in the formulation of electrolytic coating baths

  gold tick of the present invention. The same materials as indicated above

  above as conduction salts can also be used as buffering agents. The amount of the buffering agent (s) used will be

  less sufficient to give the desired buffer effect to the coating bath-

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  4. electrolytic until the maximum solubility of the agent in the bath. It is desirable that the amount of buffering agent is between about 2.5 and 200 grams per liter, and it will preferably be

  between about 5 and 50 grams per liter. We will note naturally-

  The same material can be used to provide both the desired conductivity and the buffering effect. Alternatively, different

  materials can be used to perform each of these functions.

  In each case, the quantity used will correspond to the quantity per-

  putting to provide the necessary buffer effect and / or conductivity.

  According to the preceding discussion, it will be understood that the characteristic

  essential tick of the present invention is the choice of the agent for

  grain finishing. So the disadvantages of using agents

  previously employed are eliminated by the use of business agents

  fining of specific grains, i.e. oxalic acid, form acid

  mique, and alkali or ammonium salts of these caric acids

  special boxylics. The agents more particularly recommended are oxalic acid as well as its potassium and ammonium salts. In general, the quantity of grain refining agents will be at least

  grams per liter, since smaller quantities do not

  will not achieve the desired results. Although quantities

  up to the maximum solubility in the bath can be used

  In most cases, the amount of grain refining agent will be between about 30 and 200 grams per liter, calculated as acid. The recommended quantities are between approximately 35 and 80

grams per liter.

  It will be understood, on the other hand, that the electrolytic coating process of the present invention will generally be applied with current densities between about 0.1 and 2 A / dm and at temperatures between about 500 and 900C, preferably between 60 and 700C. Although bath temperatures above 900C, for example reaching the boiling point of the bath, may be employed,

  there is often an excessive evaporation of the bath at temperatures

  res as high, so the use of these high temperatures is generally not recommended. These conditions, plus the pH range between 5 and 7.5, constitute the operating parameters

  gold electroplating baths of the present invention.

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  Another feature of the present invention is the use of

  use of specific brightening agents (brighteners) which

  tront to easily obtain deposits of lemon yellow gold, while avoiding the difficulties or the limitations indicated previously which are related to the use of the conventional brighteners used until now

  in electroplating baths.

  Although the electrolytic gold baths of the present invention can effectively be used for the deposition of gold on various substrates, their use for electroplating integrated circuits has been found to be of primary importance. In this regard, the circuits

  integrated require that the gold deposit be of very high purity and con-

  hold a minimum amount of impurities of metallic elements codepo-

  his. The electrolytic gold plating baths of the present invention are unique in their ability to meet this condition, without raising the difficulties and problems associated with gold plating baths

of the prior art.

  The present invention will be understood more clearly with reference to the following embodiments given by way of illustration:

EXAMPLE 1

G / L

  Gold in the form of gold and potassium cyanide 6 Potassium diacid phosphate 100 Oxalic acid 60 The pH of the bath is adjusted to 7.0 with phosphoric acid or potassium hydrate. A deposit of lemon yellow gold, with a refined grain,

  is obtained on a nickel coated substrate. The bath is used at a tem-

  temperature of 651C and at a current density reaching 1 A / dm

EXAMPLE 2

  The bath of Example 1 is used, except, however, that oxalic acid is not used. By using this bath at current densities reaching 0.3 A / dm, a yellow gold deposit is obtained, but at a density

  current of about 0.3 A / dm the deposit has an undesirable brown color.

EXAMPLE 3

  Another bath is prepared which has the following composition

G / L

  Gold in the form of gold cyanide and potassium 8 Potassium sulfamate 42

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  6. Tripotassium phosphate 16 Oxalic acid, added in the form of potassium oxalate 120 The bath is adjusted with sulfamic acid to a pH of 6. At an operating temperature of 60 C, a deposit of lemon yellow gold with refined grain, up to a current density of 1 A / dm; without potassium oxalate the deposit has a brown color

  undesirable for a current density of 0.6 A / dm or more.

EXAMPLE 4

  io0 The electrolytic gold coating bath has the following composition: G / L Gold in the form of gold cyanide and ammonium 4 Ammonium citrate 100 Oxalic acid added in the form of ammonium oxalate 80 Ammonia up to at pH 7.5

  At a bath operating temperature of 65 C, one obtains

  holds deposits of lemon yellow gold, with refined grain, up to a den-

current site of 1 A / dm.

EXAMPLE 5

  Another bath is used, the composition of which is as follows: G / L Gold in the form of gold and potassium cyanide 8 Potassium diacid phosphate 25 Formic acid added in the form of potassium formate 60

  The pH is adjusted to 7.5 with orthophosphoric acid.

  Lemon yellow gold deposits are obtained with this coating bath

  electrolytic under the conditions described above.

  The present invention is not limited to the exemplary embodiments.

  sation which have just been described, it is on the contrary likely to

  modifications and variants which will appear to those skilled in the art.

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7.

Claims (5)

  1 - Electrolytic coating bath in metallic gold,   characterized in that it comprises an alkali metal or gold cyanide and   of ammonium in an amount at least sufficient to allow the de-   pSt gold on a substrate when the bath is subjected to electrolysis,   a conduction agent in an amount at least sufficient to   monitor the conductivity of the bath, and a selected grain refining agent   tiated from the group consisting of oxalic acid, formic acid   that, and alkali or ammonium salts of these acids, in an amount of at least about 30 grams per liter of acid, bath   which has a pH between about 5 and 7.5.   2 - Bath according to claim 1, characterized in that   the conduction agent is an ammonium citrate or alkali metal salt   flax, phosphate, sulfamate and acetate.   3 - Bath according to claim 1, characterized in that the   pH is between about 6.5 and 7.0.   4 - Bath according to claim 1, characterized in that   grain refining agent is oxalic acid.   - Bath according to claim 1, characterized in that the grain refining agent is an alkali metal salt of oxalic acid. 6 - Bath according to claim 5, characterized in that the   alkali metal salt is potassium oxalate.   7 - Bath according to claim 1, characterized in that   the grain refining agent is ammonium oxalate.