Phosphorus-containing chemical solution suitable for polishing copper and alloys thereof by plasma electropolishing

Abstract

The invention relates to a process for electrochemical polishing of metal surfaces known as plasma electropolishing electropolishing (PEP), which uses a salt-based solution. The present invention provides ideal compositions of the polishing solution and optimal process parameters to attain better results over traditional electropolishing techniques.

Description

[0001] PHOSPHORUS-CONTAINING CHEMICAL SOLUTION SUITABLE FOR POLISHING COPPER AND ALLOYS

[0002] THEREOF BY PLASMA ELECTROPOLISHING

[0003] Technical field of the invention

[0004] The invention relates to a composition for plasma electropolishing of metal surfaces and to an electropolishing process using said composition. In particular, the invention concerns a composition consisting of an aqueous solution of a salt containing phosphorus in +5 oxidation state, a second salt and phosphoric acid, and an associated plasma electropolishing electropolishing process.

[0005] Prior art

[0006] The present invention relates to a process for plasma electropolishing of metal surfaces. More specifically, the present invention relates to the composition of the polishing solution and the optimal process parameters to be applied to the plasma electropolishing electropolishing process for metal surfaces such as copper (Cu), cupronickel, constantan, brass, copper-zinc-nickel alloys, and copper-based alloys.

[0007] Typically, the polishing of metal surfaces is carried out through chemical or electrochemical processes.

[0008] In chemical polishing, the workpiece to be treated is immersed in a concentrated aqueous solution of several acids, the action of which removes a surface layer of metal, acting preferentially upon the irregularities of the surface and reducing the roughness thereof.

[0009] In the electrochemical process, also referred to as electropolishing or electrolytic polishing, the workpiece to be treated is immersed in a concentrated aqueous solution of several acids and connected to the positive pole of an electric circuit, thus acting as the anode of an electrolytic cell. A second metal electrode, immersed in the same solution and facing the workpiece from a suitable distance (usually about ten centimeters), acts as the cathode. The application of a potential difference across the anode and the cathode triggers an oxidation reaction on the metal of the workpiece, thereby partially dissolving it into the solution. Such oxidation reaction occurs preferentially on the irregularities of the metal surface, which are thus dissolved to a greater extent and smoothed out. The overall effect is a reduction in the workpiece's roughness.

[0010] With reference to copper or copper alloy surfaces, the solutions used for chemical polishing consist of concentrated solutions of acids (e.g., acetic acid, phosphoric acid, nitric acid, hydrochloric acid) or mixtures of sulphamic acid, ammonium citrate, n-butanol and hydrogen peroxide. On the other hand, the solutions most commonly used for electropolishing consist of phosphoric acid (H3PO4) in aqueous solution with the addition of moderators such as butanol, ethylene glycol and glycerol; the operating voltage varies between 0.2V and 2.2V.

[0011] An alternative approach is the so-called plasma electropolishing, or plasma electrolytic polishing (PEP), which differs from traditional electropolishing for the higher voltages employed, between 200 V and 400 V, which ensure faster processing times and permit the use of diluted solutions. In fact, PEP uses diluted aqueous solutions of salts, which permit the creation of a uniform layer of vapor and gas, in which the surface smoothing process can take place. This process is well known in the industry, and is applied to various metals and alloys, such as chrome-cobalt-molybdenum and stainless steel, for which polishing recipes are available. As concerns copper and its alloys, some compositions are known in the literature for plasma electropolishing [I. A. Kostogrud, E. V. Boyko, P. E. Matochkin, and D. V. Sorokin, 'Comparing the methods of copper substrate polishing for CVD graphene synthesis', J. Phys.: Conf. Ser., vol. 2057, no. 1, p. 012121, Oct. 2021, doi: 10.1088 / 1742- 6596 / 2057 / 1 / 012121; United States patent US20100200424A1; Belarussian patent BY 11410; Russian patent RU 2127334; Belarussian patent BY 8424; Russian patent RU2013129493A], which, however, do not guarantee a sufficient roughness reduction to obtain a mirror finish of the surface. In particular, the above-mentioned patent US20100200424A1 claims a process that requires a significantly lower voltage than described herein, thus not ensuring good polishing results. Patent RU 2127334 describes a process consisting of two distinct steps requiring different compositions and process conditions, thus being more complex and time-consuming than the one disclosed in the present invention, which only requires one step.

[0012] Notwithstanding the advantages offered by plasma electropolishing over the other polishing methods described above, it is still remains challengingto identify optimal electrolyte composition parameters and ideal time and current conditions for processing other metals.

[0013] It is the object of the present invention to provide a polishing solution composition and optimal process parameters to be applied to a plasma electropolishing process for processing metal surfaces of copper (Cu), cupronickel, constantan, brass, copper-zinc-nickel alloys, and copper-based alloys, such as to give better results than traditional electropolishing techniques.

[0014] Summary of the invention

[0015] The present invention relates, in general, to a composition of a solution to be used in a plasma electropolishing process for metal surfaces. More specifically, the invention relates to an aqueous solution containing: A) a salt containing phosphorus, wherein phosphorus exhibits the +5 oxidation state; B) a second salt; C) phosphoric acid. Furthermore, the present invention defines a plasma electropolishing process for metal surfaces that uses said solution.

[0016] The object of the invention is the composition of an aqueous solution of phosphoric acid for use in a plasma electropolishing process for metal surfaces; said solution comprises at least one compound selected, respectively, from the following groups:

[0017] A. a salt containing phosphorus in +5 oxidation state, selected from: potassium pyrophosphate, K4P2O7, sodium pyrophosphate, Na4P2O7, ammonium pyrophosphate, (NH4)4P207, disodium pyrophosphate, Na2H2P2O7, diammonium phosphate, (NH4)2HPO4, monoammonium phosphate, NH4H2PO4, ammonium phosphate, (NH4)3PO4, disodium phosphate, Na2HPO4, dipotassium phosphate, K2HPO4, sodium phosphate, Na3PO4, potassium phosphate, K3PO4, monosodium phosphate, NaH2PO4, monopotassium phosphate, KH2PO4;

[0018] B. sodium and potassium tartrate, C4H4KNaO6, tribasic ammonium citrate, (NH4)3-Cit, dibasic ammonium citrate, (NH4)2H-Cit, monobasic ammonium citrate, (NH4)H2-Cit, citric acid, ammonium sulfamate, ([NH4]SO3NH2), sodium sulfamate, NaSO3NH2, potassium sulfamate, KSO3NH2, sulphamic acid, H2NSO3H, EDTA disodium, EDTA-Na2, ethylenediaminetetraacetic acid, EDTA;

[0019] C. Phosphoric acid in a concentration to achieve a pH between 2 and 5.

[0020] It is a further object of the present invention to provide a plasma electropolishing process that uses said composition. In particular, said plasma electropolishing process relates tometai surfaces made of any of the following metals: copper (Cu), zinc (Zn), nickel (Ni), or alloys containing such metals.

[0021] Brief description of the drawings

[0022] Figure 1: Schematic description of the plasma electropolishing process.

[0023] Key:

[0024] 1 - surface to be treated, 2 - composition for plasma electropolishing, 3 - second metal element, 4 - positive pole, 5 - negative pole, 6 - voltage generator.

[0025] Detailed description of the invention

[0026] In the description of the present invention, "surface to be polished" means any object having an exposed metal surface whose roughness has to be reduced; "anode" means the electrode of an electrochemical cell undergoing an oxidation reaction, and "cathode" means the electrode where a reduction reaction occurs. According to the present invention, a composition of soluble salts in aqueous solution is provided for use in a plasma electropolishing process for metal surfaces as set out in claim 1.

[0027] For a better understanding of the present invention, a preferred embodiment thereof will be described below merely by way of non-limiting example.

[0028] The present invention concerns an aqueous solution of phosphoric acid for use in a plasma electropolishing process for metal surfaces; said solution comprises at least one compound selected, respectively, from the following groups:

[0029] A. a salt containing phosphorus in +5 oxidation state, selected from: potassium pyrophosphate, K4P2O7, sodium pyrophosphate, Na4P2O7, ammonium pyrophosphate, (NH4)4P2O7, disodium pyrophosphate, Na2H2P2O7, diammonium phosphate, (NH4)2HPO4, monoammonium phosphate, NH4H2PO4, ammonium phosphate, (NH4)3PO4, disodium phosphate, Na2HPO4, dipotassium phosphate, K2HPO4, sodium phosphate, Na3PO4, potassium phosphate, K3PO4, monosodium phosphate, NaH2PO4, monopotassium phosphate, KH2PO4;

[0030] B. sodium and potassium tartrate, C4H4KNaO6, tribasic ammonium citrate, (NH4)3-Cit, dibasic ammonium citrate, (NH4)H-Cit, monobasic ammonium citrate, (NH4)H2-Cit, citric acid, ammonium sulfamate, ([NH4]SO3NF2), sodium sulfamate, NaSO3NH2, potassium sulfamate, KSO3NH2, sulphamic acid, H2NSO3H, EDTA disodium, EDTA-Na2., ethylenediaminetetraacetic acid, EDTA;

[0031] C. Phosphoric acid in a concentration to achievea pH between 2 and 5.

[0032] According to one aspect of the invention, the mass concentration of the salt selected from group A is comprised between 2.0% and 6.0%, the mass concentration of the salt selected from group B is comprised between 0.1% and 2.0%, and the concentration of phosphoric acid is such as to reach a pH value of the solution comprised between 2 and 5.

[0033] In a preferred embodiment of the invention, the salt selected from group A is potassium pyrophosphate (K4P2O7) with a mass concentration of 3.0% to 5.0%, the salt selected from group B is sodium and potassium tartrate (C4H4KNaOe) with a mass concentration between 0.5% to 1.0%, and the concentration of phosphoric acid is such as to give a pH value of the solution comprised between 2 and 5. Phosphoric acid concentration should thus be comprised between 12 ml / l and 30 ml / l.

[0034] In a second preferred embodiment of the invention, the salt selected from group A is diammonium phosphate ((NH4)2HPO4) with a mass concentration of between 2.0% to 6.0%, the salt selected from group B is tribasic ammonium citrate ((NH4)3-Cit) with a mass concentration of between 0.3% to 1.5%, and the concentration of phosphoric acid is such as to give a pH value of the solution comprised between 3 and 4. Phosphoric acid concentration should thus be comprised between 17 ml / l and 50 ml / l.

[0035] In a third preferred embodiment of the invention, the salt selected from group A is potassium pyrophosphate (K4P2O7) with a mass concentration of 4.0%, the salt selected from group B is sodium and potassium tartrate (C4H4KNaO6) with a mass concentration of 1.0%, and the concentration of phosphoric acid is such as to give a pH value of the solution of 4. Phosphoric acid concentration should thus be comprised between 17 ml / l and 24 ml / l.

[0036] In a fourth preferred embodiment of the invention, the salt selected from group A is diammonium phosphate ((NH4)2HPO4) with a mass concentration of 3.0%, the salt selected from group B is tribasic ammonium citrate ((NH4)3-Cit) with a mass concentration of 0.5%, and the concentration of phosphoric acid is such as to give a pH value of the solution of 4. Phosphoric acid concentration should thus be comprised between 22 ml / l and 30 ml / l.

[0037] Advantageously, according to the invention, the solution composition described herein is used in a plasma electropolishing process for metal surfaces comprising one of the following metals: copper (Cu), zinc (Zn), nickel (Ni), or alloys containing such metals.

[0038] With reference to Figure 1, according to another aspect of the invention, the plasma electropolishing process defined by the present invention comprises the following steps: a) Placing the metal surface to be polished 1 in contact with the composition 2; b) Placing at least one further metal element 3 in contact with said composition 2, so that electrical contact between the metal surfaces 1 and 3 is ensured through said composition 2; c) Connecting the metal surface to be polished 1 to the positive pole 4 of a voltage generator 6 and the further metal element 3 to the negative pole 5 of said voltage generator 6, so that the metal surface to be polished 1 acts as an anode of an electrochemical cell; d) Increasing the temperature of the composition so that it is comprised between 40°C and 95°C; e) Applying a continuous potential difference comprised between 200 V and 400 V across the two electrodes. Advantageously, according to the invention, the potential difference applied across the two electrodes is 300 V, while the temperature of the solution is maintained at 75°C.

[0039] It is an objectof the present invention to provide a composition for plasma electropolishing of metal surfaces, as well as an electropolishing process using said composition to overcome the limitations of the prior art.

[0040] One advantage of the present invention over the prior art is the speed at which the desired treatment is accomplished; as a matter of fact, the process according to the invention has proven to be 10 times faster than conventional electropolishing, and approximately 3.5 times faster than chemical polishing.

[0041] A further advantage of the present invention over the prior art is its higher efficiency; in fact, the polishing effect is achieved on the treated surface by removing less material than required by the processes currently known in the art.

[0042] Another advantage relates to the compositions of the present invention which, being made up of solutions with lower concentrations, are less harmful for the environment and the health of operators than those used in the prior art.

[0043] A further advantage of the invention relates to the capability of removing contaminants from the processed surface due to the higher power used per area unit, thus avoiding the need for pretreatments as commonly required by conventional electropolishing and chemical polishing processes: (degreasing, ultrasonic cleaning, activation, polishing).

Claims

Claims1. Aqueous solution of phosphoric acid for use in a plasma electropolishing process for metal surfaces, comprising at least one salt selected from the following group A:A. a salt containing phosphorus in +5 oxidation state, selected from: potassium pyrophosphate, K4P2O7, sodium pyrophosphate, Na4P2O7, ammonium pyrophosphate, (NH4)4P207, disodium pyrophosphate, Na2H2P2O7, diammonium phosphate, (NH4)2HPO4, monoammonium phosphate, NH4H2PO4, ammonium phosphate, (NH4)3PO4, disodium phosphate, Na2HPO4, dipotassium phosphate, K2HPO4, sodium phosphate, Na3PO4, potassium phosphate, K3PO4, monosodium phosphate, NaH2PO4, monopotassium phosphate, KH2PO4; and at least one salt selected from the following group B:B. sodium and potassium tartrate, C4H4KNaO6, tribasic ammonium citrate, (NH4)3-Cit, dibasic ammonium citrate, (NH4)2H-Cit, monobasic ammonium citrate, (NH4)Hz-Cit, citric acid, ammonium sulfamate, ([NH4]SO3NH2), sodium sulfamate, NaSO3NH2, potassium sulfamate, KSO3NH2, sulphamic acid, H2NSO3H, EDTA disodium, EDTA-Na2, ethylenediaminetetraacetic acid, EDTA.

2. Composition according to the preceding claim, wherein the mass concentration of the salt selected from group A is comprised between 2.0% and 6.0%, the mass concentration of the salt selected from group B is comprised between 0.1% and 2.0%, and the concentration of phosphoric acid is such as to reach a pH value of the solution comprised between 2 and 5.

3. Composition according to the preceding claims, wherein the salt selected from group A is potassium pyrophosphate (K4P2O7) with a mass concentration of 4.0%, the salt selected from group B is sodium and potassium tartrate (C4H4KNaO6) with a mass concentration of 1.0%, and the concentration of phosphoric acid is such as to give a pH value of the solution equal to 4.

4. Composition according to the preceding claims, wherein the salt selected from group A is diammonium phosphate ((NH4)2HPO4) with a mass concentration of 3.0%, the salt selected from group B is tribasic ammonium citrate ((NH4)3-Cit) with a mass concentration of 0.5%, and the concentration of phosphoric acid is such as to give a pH value of the solution of 4.

5. Plasma electropolishing process for metal surfaces using the composition according to one of the preceding claims, comprising the steps of: a) Placing the metal surface to be polished (1) in contact with the composition (2);b) Placing at least one further metal element (3) in contact with the same composition (2), so that electrical contact between the metal surfaces (1) and (3) is ensured through said composition (2); c) Connecting the metal surface to be polished (1) to the positive pole (4) and the further metal element (3) to the negative pole (5) of a voltage generator (6), so that the metal surface to be polished (1) acts as an anode of an electrochemical cell; d) Increasing the temperature of the composition so that it is comprised between 40°C and 95°C; e) Applying a continuous potential difference comprised between 200 V and 400 V across the two electrodes.

6. Process according to the preceding claim, wherein the metal surface to be polished (1) comprises at least one of: copper (Cu), zinc (Zn), nickel (Ni), or an alloy of said metals.

7. Process according to the preceding claim, wherein the potential difference applied across the two electrodes is 300 V.

8. Process according to the preceding claim, wherein the temperature of the solution is maintained at 75°C.

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