EP0425012B1 - Baths and process for chemical polishing of copper and copper alloy surfaces - Google Patents

Abstract

Baths for chemical polishing of copper or copper alloy surfaces, comprising, in aqueous solution, hydrogen peroxide, phosphoric acid and tetraborate ions.

Description

The present invention relates to the composition of baths for the chemical polishing of copper or copper alloy surfaces.

Chemical polishing of metal surfaces is a well-known technique (electrolytic and chemical polishing of metals - W.J. Mc G. TEGART - Dunod - 1960 - p. 122 et seq.); it consists of treating the metal surfaces to be polished with oxidizing baths.

For the chemical polishing of copper and its alloys, aqueous baths of orthophosphoric acid, nitric acid and acetic acid were used (dito: pages 135 and 136). These baths require high working temperatures, of the order of 50 to 80 ° C and intense mechanical stirring. They also attack the metal at high speed, limiting the duration of polishing to less than five minutes. These particularities of these known baths constitute disadvantages: on the one hand, their use is accompanied by toxic gaseous emissions, on the other hand, their high speed of action and the need to subject them to mechanical agitation makes control difficult and random polishing. To overcome these disadvantages, aqueous baths have been proposed comprising hydrogen peroxide and a mixture of nitric, phosphoric and hydrochloric acids. These aqueous baths allow lower working temperatures, of the order of 25 to 35 ° C and their metal attack speed is between 2.5 and 5 microns per minute [Electroplating - October 1953 - 6 - pages 360 to 367 (pages 363 and 364)]. The speed of action of these known baths on metal is nevertheless still excessive for certain applications. In particular, it makes them unusable for polishing the internal face of the walls of large tanks, such as boilers, autoclaves or crystallizers. The time required for the filling and emptying of such tanks being generally much greater than the duration of the optimum chemical polishing treatment, it indeed becomes impossible to obtain a uniform polish of the wall, certain zones of the latter being insufficiently polished, others being deeply corroded. These known baths are also ineffective for polishing surfaces in contact with which the renewal of the bath is difficult, since this results in abrupt modifications to the local compositions of the bath.

Baths have also been proposed comprising, in aqueous solution, phosphoric acid, hydrogen peroxide, hydrochloric acid and 2,6-di-tert-butyl-4-N, N-dimethylaminomethylphenol [SU -A-1211338 (ORG. PHYS. CHEM. INST.)]. These known baths would have the property of better stability, but they imply working temperatures of at least 50 ° C., their speed of action is too fast and they do not allow regular quality polishing.

In document EP-A-309031 (SOLVAY & CIE), chemical polishing baths are proposed which remedy the abovementioned drawbacks, these baths comprising, in aqueous solution, hydrogen peroxide, chloride ions and an acid mixture phosphoric, phosphate and hydrogen phosphate ions.

The invention aims to provide another composition for polishing baths which also overcomes the drawbacks listed above and which produce good quality polishes with a slow speed of action and moderate loss of metal.

The invention therefore relates to baths for the chemical polishing of copper or copper alloy surfaces, comprising, in aqueous solution, hydrogen peroxide and phosphoric acid according to which they also include tetraborate ions and according to which the amounts of hydrogen peroxide, phosphoric acid and tetraborate ions are respectively between 1 and 5 moles / l, between 0.01 and 1 mole / l and between 0.01 and 0.5 mole / 1, the quantities of phosphoric acid and tetraborate ions being further adjusted to measure in the bath a pH value of less than 3.

In the baths according to the invention, the hydrogen peroxide serves as an oxidizing agent for the metal to be polished.

The phosphoric acid and the tetraborate ions serve as a buffer mixture for the aqueous solution, the pH of which must be acidic. The tetraborate ions can be used in the form of all water-soluble inorganic compounds, such as acid tetraboric or alkali metal salts, for example anhydrous or hydrated borax.

As a general rule, it is recommended that the phosphoric acid and the tetraborate ions be present in the aqueous solution in respective amounts adjusted to give the latter a pH value preferably between 1 and 2.8, the optimum values being between 1.4 and 2.5. These pH values are those actually measured in the aqueous solution of the baths according to the invention (apparent pH); they generally differ from the theoretical values obtained by mathematical calculation from the contents of phosphoric acid and tetraborate ions in the aqueous solution.

By achieving the above-mentioned pH value in the aqueous solution of the polishing bath, the respective contents of hydrogen peroxide, phosphoric acid and tetraborate ions are chosen according to the nature of the metal treated and the working temperature. and the desired duration for the polishing treatment. According to the invention, the baths comprise, per liter of the aqueous solution, from 1 to 5 moles of hydrogen peroxide, from 0.01 to 1 mole of phosphoric acid and from 0.01 to 0.5 mole of tetraborate ions. These polishing baths in accordance with the invention are well suited to polishing at low speed, requiring more than one hour of contacting the surface with the bath, the temperature of which can moreover be moderate, generally less than 70 °. C, for example between 20 and 50 ° C.

The aqueous solution of the baths according to the invention may optionally contain, in usual proportions, additives commonly present in aqueous baths for the chemical polishing of metals, for example surfactants, viscosity regulators and stabilizers of peroxide. hydrogen.

The chemical polishing baths according to the invention make it possible to produce surface polishes of good quality, in particular greater than that of the polishes obtained with the polishing baths. described in document SU-A-1211338. A great advantage of the baths according to the invention lies in their ability to carry out polishing at moderate speed of action, which can be distributed over several hours, so as to allow uniform polishing of large surfaces or surfaces that are difficult to access. The baths according to the invention have the advantageous characteristic of limiting to a moderate value the loss of metal from the metal surfaces subjected to polishing, while producing a polish of quality at least equal and generally superior to the quality of the polish obtained with the baths of known polishing. This feature of the baths according to the invention is intended especially for the treatment of thin metal parts; it also mitigates the loss in mechanical strength of the metal parts subjected to polishing.

The baths according to the invention are suitable for polishing all surfaces of copper or copper alloy, for example brass.

The invention therefore also relates to a method for polishing the surface of a copper or copper alloy object, according to which the surface to be polished is brought into contact with a polishing bath according to the invention.

In the process according to the invention, the polishing bath can be used at all temperatures and pressures for which there is no risk of degrading its constituents. However, it has proved advantageous to use the bath at atmospheric pressure, at a temperature above 20 ° C and below 80 ° C, temperatures between 30 and 60 ° C being preferred.

The contact of the metal surface with the bath can be carried out in any suitable manner, for example by immersion.

In the method according to the invention, the contact time of the surface to be polished with the bath must be sufficient to achieve effective polishing of the surface; however, it cannot exceed a critical value beyond which local corrosions may appear on the surface. Time to optimum contact depends on many parameters such as the metal or alloy constituting the surface to be polished, the configuration and initial roughness thereof, the composition of the bath, the working temperature, the possible turbulence of the bath in contact with the surface, the ratio between the area of the metal surface to be polished and the volume of the bath used; it must be determined in each particular case by routine laboratory work.

In a preferred embodiment of the method according to the invention, the surface to be polished is kept in contact with the bath for a time sufficient to carry out an attack on the metal to a depth at least equal to 5 microns, preferably less than 50 microns, for example between 5 and 30 microns and, preferably between 10 and 30 microns. The duration of the treatment of the surface with the bath is thus, in most cases, between 1 and 6 hours.

In another particular embodiment of the method according to the invention, the metal surface to be polished is brought into contact successively with a first polishing bath in accordance with the invention and with a second chemical polishing bath comprising, in aqueous solution, hydrogen peroxide, chloride ions and a mixture of phosphate ions and hydrogen phosphate ions in, respective amounts adjusted to give the aqueous solution a pH value between 1.25 and 3. Details of the composition of the second polishing bath used in this embodiment of the method according to the invention are accessible in document EP-A-309031 (SOLVAY & CIE). This particular embodiment of the method according to the invention makes it possible to reconcile an optimum quality polish and a minimum metal loss.

The advantage of the invention will be highlighted on reading the application examples given below.

Example 1 (according to the invention)

• . 3,5 moles de peroxyde d'hydrogène,
• . 0,48 mole d'acide phosphorique,
• . 0,2 mole de borax (Na₂B₄O₇.10H₂O).

A copper plate of 10 cm² of area was immersed in 500 cm³ of a bath kept in stirring, containing, per liter:

• . 3.5 moles of hydrogen peroxide,
• . 0.48 mole of phosphoric acid,
• . 0.2 mole of borax (Na₂B₄O₇.10H₂O).

The pH value of the bath thus prepared was measured: 2.2.

• avant et après l'immersion, la rugosité moyenne arithmétique R_a, qui est la déviation moyenne par rapport à la surface moyenne de la plaque [Encyclopedia of Materials Science and Engineering, Michael B. Bever, Vol. 6, 1986, Pergamon Press, pages 4806 à 4808 (page 4806)] :
  
• la profondeur d'attaque du métal, définie par la relation $$\text{Δe =}\frac{\text{10⁴}}{\text{S.d}}\text{.ΔP}$$
  
  où

  S

  désigne l'aire de la plaque (en cm²),

  d

  désigne la masse spécifique du métal (en g/cm³),

  ΔP

  désigne la perte en poids (en g) de la plaque pendant l'immersion dans le bain,

  Δe

  désigne la profondeur d'attaque (en µm).

On a obtenu les résultats suivants :
   R_a (avant immersion) = 0,52 µm,
   R_a (après immersion) = 0,07 µm,
   Δe = 18 µm.The bath temperature was maintained at about 40 ° C for the duration of the immersion, which was 6 hours. At the end of this, the plate was removed from the bath, rinsed with demineralized water and dried. We measured

• before and after immersion, the arithmetic mean roughness R _a , which is the mean deviation from the mean surface of the plate [Encyclopedia of Materials Science and Engineering, Michael B. Bever, Vol. 6, 1986, Pergamon Press, pages 4806 to 4808 (page 4806)]:
  
• the depth of attack of the metal, defined by the relation $$\text{Δe =}\frac{\text{10⁴}}{\text{Nd}}\text{.ΔP}$$
  
  or

  S

  designates the area of the plate (in cm²),

  d

  denotes the specific mass of the metal (in g / cm³),

  ΔP

  designates the weight loss (in g) of the plate during immersion in the bath,

  Δe

  denotes the depth of attack (in µm).

The following results were obtained:
R _a (before immersion) = 0.52 µm,
R _a (after immersion) = 0.07 µm,
Δe = 18 µm.

Example 2 (reference)

• Composition du bain de polissage :
  • . 9,4 moles de peroxyde d'hydrogène par litre,
  • . 0,584 mole d'acide phosphorique par litre,
  • . 0,047 mole d'acide chlorhydrique par litre,
  • . 0,04 g de 2,6-di-tert-butyl-4-N,N-diméthylaminométhylphénol par litre,
  • . pH = 1,05 ;
• Température de travail : 50° C ;
• Durée du traitement : 15 minutes.

On a obtenu les résultats suivants :
   R_a (avant immersion) = 0,40 µm,
   R_a (après immersion) = 0,08 µm,
   Δe = 60 µm.The test of Example 1 was repeated under operating conditions in accordance with those described in Example 7 of document SU-A-1211388:

• Composition of the polishing bath:
  • . 9.4 moles of hydrogen peroxide per liter,
  • . 0.584 mole of phosphoric acid per liter,
  • . 0.047 mole of hydrochloric acid per liter,
  • . 0.04 g of 2,6-di-tert-butyl-4-N, N-dimethylaminomethylphenol per liter,
  • . pH = 1.05;
• Working temperature: 50 ° C;
• Duration of treatment: 15 minutes.

The following results were obtained:
R _a (before immersion) = 0.40 µm,
R _a (after immersion) = 0.08 µm,
Δe = 60 µm.

A comparison of the two tests shows that the bath according to the invention produced a higher quality polish, while causing less loss of metal (depth of attack). It also shows that the use of the polishing bath according to the invention has considerably lengthened the duration of the polishing, which constitutes another objective of the invention.

Claims (6)

1. Baths for chemical polishing of copper or copper alloy surfaces, comprising, in aqueous solution, hydrogen peroxide and phosphoric acid characterised in that they also comprise tetraborate ions and in that the amounts of hydrogen peroxide, phosphoric acid and tetraborate ions are respectively, between 1 and 5 moles/l, between 0.01 and 1 mole/l and between 0.01 and 0.5 mole/l, the respective amounts of phosphoric acid and tetraborate ions additionally being controlled so as to measure a pH value of less than 3 in the bath.
2. Baths according to Claim 1, characterised in that the tetraborate ions are used in the aqueous solution in the form of alkali metal tetraborate.
3. Baths according to Claim 1 or 2, characterised in that they contain the phosphoric acid and the tetraborate ions in amounts controlled so as to measure a pH value of between 1 and 2.8 in the bath.
4. Process for polishing a copper or copper alloy surface, according to which process the surface is brought into contact with a chemical polishing bath according to any one of Claims 1 to 3.
5. Process according to Claim 4, characterised in that the surface is kept in contact with the bath for a time sufficient to achieve an attack on the metal to a depth of between 5 and 30 microns.
6. Process according to Claim 4 or 5, characterised in that the surface is brought into contact successively with a first chemical polishing bath according to any one of Claims 1 to 3 and with a second chemical polishing bath comprising, in aqueous solution, hydrogen peroxide, chloride ions and a mixture of phosphoric acid, phosphate ions and hydrogen phosphate ions in respective amounts controlled so as to confer a pH value of between 1.25 and 3 on the aqueous solution.