Electropolishing copper by using phosphoric acid

A. Hi, Miss Sae. According to the Electroplating Engineering Handbook [on AbeBooks or eBay or Amazon affil links] , commercial electropolishing of copper is done in phosphoric acid, or modified phosphoric acid (U.S. Patent 2,366,714) or phosphoric-chromic acid (U.S. Patent 2,347,039).

According to the Metals Handbook, phosphoric-chromic acid is used at 18V and 30-40 A/dm2 (much higher than you are using), 65 °C.

The Metal Finishing Guidebook lists three different approaches, only one of which contains sulfuric acid, and that solution also contains chromic acid and acetic acid ⇦this on eBay or Amazon [affil links] . I think a sulfuric acid bath is unsuitable for electropolishing copper. Good luck.

Q. More questions about copper electro-polishing: I use copper busbar (conductors); does it relate to the results?

A. Hi. I don't think the material of the busbar is effecting this process. I think your current density and the chemicals you are using are leading to copper plating onto the anodes instead of polishing the copper.

Still, electropolishing may not give as glossy a result as mechanical polishing. The reason for electropolishing is smoothness and freedom from microscopic burrs and clods of copper. A mechanically polished surface will always look like a plowed field at sufficient magnification, because that's what it is. A properly electropolished surface will resemble gentle ocean swells because the asperities will all have been dissolved away. Good luck.

Regards,

A. Hi Betty. Asking for feedback is fine, and I hope you get some. But still, I think you should retain an experienced plating consultant or industrial hygienist in your situation. Good luck.

Regards,

ASM Metals Handbook Vol. 5: "Surface Engineering"

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A. ASM's Metals Handbook, Vol. 5.
says 85 percent orthophosphoric acid plus 5 wt percent to saturation chromic acid at 55-65 C and 15-30 A/dm. That should be a start.

Q. Dear Sir,

I have the some question as yours. Have you started the polishing? Can you tell me what was going on?

Thanks,

A. I completed my MS in 2000. You do not need chromic acid to electropolish copper. You can also use lower or higher concentration of phosphoric acid. The trick is to locate where (what voltage region) electropolishing occurs.

Pure copper is 99.99% so most pure commercial coppers will work. Beware of not duplicating other theses since there are many.

Q. Hello all,

I want to know how we can Electropolish the copper in phosphoric acid without adding any chrome?

I will be glad very much for helping me on this topic.

A. There was a bit of work in Copper electropolishing integrated with chemical-mechanical-polishing in the dual damascene copper/low-k plated interconnect system. Applied Materials, Novellus, and Semitool all considered it during process infancy but I don't think it has ever been implemented as a process.

A. Dear Mr. Grujicic,

You may want to try a different electropolishing solution for your copper piece as follows;
DeIonized Water - 100 parts
Sulfuric Acid - 21 parts
o-Phosphoric Acid 85% - 78 parts

A stainless steel cathode is normally used. You may have to play with the current densities a bit and gently agitate the piece for about 30 seconds. You may also have to vary the time slightly for best results. Rinse thoroughly in deionized water followed by a rinse in methanol [affil links] or ethanol. Dry the piece carefully.

Good Luck,

"Electroplating Engineering Handbook"
by Larry Durney

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A. With the experimental work done, what results did you get, Joe?

You probably now need to do a computerized search of the published literature. You might try a literature search on Metal Finishing Information Service, which has abstracted most of the world's metal finishing periodicals. But I see on the net a reference to an article "Electrolytic polishing of copper in orthophosphoric acid", (Australia. Council for Scientific and Industrial Research. Division of Tribophysics. Physical metallurgy report) by R. W. K Honeycombe, and maybe you can track down a copy of that. There is a lot about electropolishing, but only a little about electropolishing of copper, in ASM Metals Handbook Vol. 5 "Surface Engineering" and the Electroplating Engineering Handbook.

Good luck!

A. See the book, Electrolytic & Chemical Polishing of Metals, by W. J. McG. Tegart (1956) and download 'The Electrolytic Polishing of metals. Application to Copper and Niobium' by V.Palmieri,
http://srf2003.desy.de/talks/Palmieri/Tutorial%20Palmieri.pdf

The figure on page 50 of Palmieri's tutorial is quite informative. Note also that during electropolishing, the anodic rate of oxide formation = rate of oxide dissolution by the acid.

A. What are the anode & cathode materials? What is the electrolyte?
Bubbles are normal -- hydrogen forms at the cathode, and oxygen at the anode. Trap a small amount in an inverted beaker and very carefully test with a match.

A wild guess re the observed colors: copper dissolves into sulfuric acid as blue copper sulphate, and plates out as reddish, metallic slime at the cathode.

Q. Hi, just to add on to the details of my question.The cathode is actually stainless steel, the anode is copper. The electrolyte used is H3PO4 in H2O. Therefore the chemical reaction behind the observations are? Thanks.

A. Cu (+2) in aqueous solutions forms a blue color.

Usual practice in electropolishing copper is to also use copper for the cathode.

The overall reaction is Cu (anode) --> Cu (cathode). The devil is in the details. My understanding of Cu electropolishing is limited, but goes as follows.
Cu metal in ambient air is covered by cuprite, Cu2O.
Acid solutions of < pH 4 dissolve cuprite, according to the Pourbaix diagram.
Phosphoric acid reacts with and adsorbs to the bare Cu, possibly as Cu3(PO4)2.3H2O, copper (II) phosphate trihydrate. When DC electricity is applied, the electrodes become polarized (charge accumulates on the surface), especially at high points on the surface (like tiny lightning rods into the solution) where anodic oxidation then occurs [2Cu --> 2Cu(+) + 2 e(-)], forming Cu2O. As an equal amount of current must flow through the solution and cathode; ergo, a cathodic reduction reaction. The initial reaction is 2 H(+) + 2 e(-) --> H2 (gas). After enough dissolved Cu(+2) is in solution, the cathode reaction becomes Cu(+2) + 2 e(-) --> Cu (metal).
The phosphoric acid chemically dissolves any Cu2O formed into solution, Cu2O + 2 H(+) --> 2 Cu(+2) + H2O, whilst new Cu2O continues to form electrochemically on the high points of the anode. This continued loss of Cu from the high points levels the surface of the anode, creating the polishing.

A more complete explanation would discuss diffusion through the boundary layer of chemical species more or less adsorbed to the Cu. Also, with excessive voltage ('overvoltage'), the diffusion and dissolution processes cannot keep up with the electrical current, and the excess current is expended in the dissociation of water, with 1/2 O2(gas) formed at the anode per H2(gas) formed at the cathode.

A. Agitate the solution vigorously to both insure fresh material at the interface, and to dislodge the gas bubbles, Johnny. Good luck.

A. Hi, Abbas. Phosphoric acid is probably the most commonly used acid for electropolishing copper. But electropolishing is too complicated to explain in a single paragraph. You can try to glean the info you need from this posting, but it would probably be best to get hold of a copy of the Electroplating Engineering Handbook [on AbeBooks or eBay or Amazon affil links] , which has an excellent chapter on electropolishing. Good luck.

Regards,

A. Hi Abhishek. It's certainly possible to electropolish copper, and we have appended your inquiry to a thread dealing with the subject. But electropolishing is rarely a one-step process to get you from raw copper to mirror finished copper, if that is what you are seeking. It's more a way to get from a polished surface to a surface with no microscopic asperities or "tooth".

Regards,

A. Hello Sudip. Ken Vlach's response already referred you to an on-line paper that exactly answers your questions. Good luck! Readers: please respect people's efforts by reading the page before asking people to re-post what they've already posted :-)

Regards,

A. You might try some ferric chloride ⇦this on eBay or Amazon [affil links] .

A. The common phosphoric/sulfuric electrpolishing bath will do a fine job on the stainless and will smooth and brighten the copper somewhat. You'll need to do some experimentation with times, temperatures and current densities.

But why electropolish? Certainly there are relatively simple cleaning methods for UHV use, no?