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Electroplating of thin silver films





September 16, 2010

For my masters project I have to do electroplating of thin and smooth films of silver (around 500 nm in thickness) and electrodeposition of silver in a AAO template, and in trenches. I found a lot of articles, and in almost all of these articles they use a potentiostat. I know what their purpose is but I'm not sure when they are really necessary.

My first question is: when exactly do you need a potentiostat? Is it required for a controlled growth of very thin smooth layers? And if so, do I also need a standard electrochemical cell, or will an improvised set up with a beaker and three electrodes also do?

Secondly, how should I clean the cathode? Again I found a lot about this, but there are many different ways to do it. Is a piranha solution suitable? Are you familiar with the ASTM B322 [affil. link] "Standard guide for cleaning metals", and would it be useful in my case? Do I also have to clean the beaker and the anode?

My third question is: what water I should use for my electrolyte? In the articles I found they often use twice distilled water or milliQ water: do I have to use that, or would demineralized water also be suitable?

Also, what should you do with the surface after electroplating? I've read about rinsing it with deionized water, and drying it in air. Is that the standard way to do it?

Another thing I'm not sure about is what electrolyte to use. Most articles describe a solution with silver nitrate, and some additives such as boric acid, citric acid, magnesium sulfate, sulfuric acid,ammonium thiosulfate and many more. Also silver bromide or silver sulfate based electrolytes can be used. In addition, it is possible to buy a commercial plating bath. Do you have any ideas on this? I assumed that a commercial ready to use plating bath would be the easiest,as in that case I don't have to think about concentrations and PH, but maybe there are some disadvantages to using a commercial plating bath?

Another question I have is the role of pH. As I understand it at a wrong pH the metal may not stay in solution, the anode may dissolve, excess acidity or alkalinity can harm the brighteners and other organics and pH out of range can neutralize the solvent. So the required pH depends on the specific electrolyte you use? How do I determine the necessary pH, is it just a matter of trial and error or is there a more intelligent approach?

Are you familiar with adding gelatin to the solution?

Finally, what temperature would be required? I know that for silver, room temperature is most often used, but I also found an article in which they used 40 degrees, as that would improve the silver quality. Given that deposition still works at 40 degrees, I guess that what temperature you use is not that crucial? How sensitive do you think electrodeposition is to temperature fluctuations?

I'm sorry for my long questions, but I'm totally lost and really need some help. I would already be very happy if you could just answer a few of my questions.
Kind regards,

Hinke Schokkermaster student
student - Oegstgeest, Netherlands



First of two simultaneous responses -- September 18, 2010

Dear Hinke.

-You nearly ask for a manual on electroplating.
Electroplating is a separate specialisation, with a lot of things to care about. Therefore, although it is already a long answer, we only scratched the surface of the subject. Please find my answers below, but continue reading in the references I suggested to you if you want to do this yourself.

My first question is: when exactly do you need a potentiostat? Is it required for a controlled growth of very thin smooth layers? And if so, do I also need a standard electrochemical cell, or will an improvised set up with a beaker and three electrodes also do?

-Like you said, if you want to do investigations under controlled conditions, if you only want to make test specimens with pure Ag of 0,5u thickness, there is no need to apply this potentiostat, although it can give you additional information.

Secondly, how should I clean the cathode? Again I found a lot about this, but there are many different ways to do it. Is a piranha solution suitable? Are you familiar with the "ASTM B322 [affil. link] standard guide for cleaning metals", and would it be useful in my case? Do I also have to clean the beaker and the anode?

-From strictly theoretical point of view, a piranha solution should do the job, but I don't like to work with it, as it is a very aggressive, explosive mixture which will severely attack your base material as well, again depending on the type of base. So just don't do it!.
The standard is a informative paper, If I remember well it tells you something about the "water break test" (After the acid dip, a water film on the surface should remain "unbroken" over the surface for some time, otherwise it could indicate the presence of organics on the surface).

You better go to a supply house and use a proprietary electrolytic cleaner solution from such a supply house. They will also supply you with a manual and if you use this cleaner and afterwards you rinse and dip the cathode in a 10% by vol. sulphuric acid solution, you have a relative good start, Keep in mind that I don't know the composition of your base material and a pre-treatment should be adapted to the type of base-material as well. If you have e.g. brass with lead in the alloy, you need to do adaptations to get an smooth surface and avoid porosity in your silver plating.

Plasma cleaning, followed by a short acid dip and good rinsing before plating is also an option.

My third question is: what water I should use for my electrolyte? In the articles I found they often use twice distilled water or milliQ water: do I have to use that, or would demineralized water also be suitable?

-Demineralized water is OK for practical purposes to set up an electrolyte.

Also, what should you do with the surface after electroplating? I've read about rinsing it with deionized water, and drying it in air. Is that the standard way to do it?

-In your situation, I would thoroughly rinse the parts with DI-water and as a last step, before drying, dip them in clean alcohol before drying them. The point is what you intent to do with your specimen. As soon as you have them dried, the will start to sulfidise (Clean Silver discolourates after a period of time due to this). Supply houses deliver solutions to prevent this, but these so called inhibitors or passivators leave either metallo-organic or metallic films on your silver surface.

Another thing I'm not sure about is what electrolyte to use. Most articles describe a solution with silver nitrate, and some additives such as boric acid, citric acid, magnesium sulfate, sulfuric acid,ammonium thiosulfate and many more. Also silver bromide or silver sulfate based electrolytes can be used. In addition, it is possible to buy a commercial plating bath. Do you have any ideas on this? I assumed that a commercial ready to use plating bath would be the easiest,as in that case I don't have to think about concentrations and PH, but maybe there are some disadvantages to using a commercial plating bath?

-In your situation, I would go (If you have all safety regulations in place and are sure that you can handle it, have the right fume hoods and are sure that you know what you're doing) for either an "old fashioned" reliable cyanide Silver electrolyte system or buy a cyanide free silver plating system from a supplier with all the prescriptions. Building your own, will only give you frustration and you'll have to spend a lot of your time on something which is probably not significant for your study.
In all case, note that you probably need a "strike" solution first (Low silver app. 1g/l. And high cyanide app 75g/l. ) in order to assure good adhesion of the Silver (Silver will form a non-adhering immersion deposit on many base materials)
Again, the pre-treatment will also depend on the base material you use.
Big advantage of the Ag-cyanide electrolyte is that the current density range is relatively large and that, by adding a few milligrams of Sb or Se you will have a quite wide bright plating range. Use a temperature of 50หล° for the actual silver plating and room temperature for the strike. You can easily calculate with 100% efficiency, and this means that you will plate 0,64u/Amin/dm2. (If you apply 1 A on a square decimetre and you plate 1 minute and all silver would be equally distributed (which is not the case), you will have an average thickness of 640nm (0,64u).
I don't know why you aim for 500 nm, be aware that you still have a possibility that you will have pores in this thickness, if you increase your thickness to 2000nm (2u) you will greatly reduce that risk, assuming your pre-treatment is suited for your base-material.

Another question I have is the role of pH. As I understand it at a wrong pH the metal may not stay in solution, the anode may dissolve, excess acidity or alkalinity can harm the brighteners and other organics and pH out of range can neutralize the solvent. So the required pH depends on the specific electrolyte you use? How do I determine the necessary pH, is it just a matter of trial and error or is there a more intelligent approach?

-Yes, required pH depends on the electrolyte. Often the electrolyte doesn't function well outside the pH-limits. For the cyanidic Ag you can forget about it's significance, as the pH will be nearby 12-13 and is uncritical For the non-cyanidic electrolytes, a pH-meter (preferably with temperature will be OK to

Are you familiar with adding gelatin to the solution?

-I guess you would like to add gelatin for brightening purposes but you better buy a "ready-to-use" solution from a supplier. Gelatin is a bit old-fashioned nowadays.

Finally, what temperature would be required? I know that for silver, room temperature is most often used, but I also found an article in which they used 40 degrees, as that would improve the silver quality. Given that deposition still works at 40 degrees, I guess that what temperature you use is not that crucial? How sensitive do you think electrodeposition is to temperature fluctuations?

-Higher temperature gives the possibility to apply higher cathodic CD. Also effectiveness of the plating electrolyte could be affected by the temperature and yes, this can be critical, although a few degrees difference will not be a problem. In commercial use higher temperature often means higher applicable CD and this means shorter plating times, In your situation you could go low lower CD at lower temperatures as well if talking about the Ag-cyanide electrolyte. For a commercial electrolyte, a higher temperature could also be needed to avoid crystallisation of the basic components. However the supply house will tell you the temperature range.

I'm sorry for my long questions, but I'm totally lost and really need some help. I would already be very happy if you could just answer a few of my questions.

-It's not for nothing that a vast amount of literature exists for electroplating, I don't know what you want to do with your silver surfaces, but I just touched the subject here, maybe you should consider to outsource the electroplating of your specimen to a specialised electroplating company in the Netherlands (I know there still exist many of them). You can look in Eindhoven or Voorburg as a start.

It could well be that the cost of this, is lower than the time and cost you have to spent to set up your own "plating department"
If for whatever reason you decide to go for your own plating, read a few good reference books, the Library of Delft university probably could help you to get "Handbuch der Galvanotechniek ๕  Dettner Elze" One of the best references in German, or "Electroplating" by Frederick A. Lowenheim. Here you will find a wealth of information.
Op zijn nederlands: Ik wens je veel succes (In Dutch: I wish you lots of success)

Kind regards,

Harry van der Zanden
Harry van der Zanden
- Budapest, Hungary



Second of two simultaneous responses -- September 20, 2010

Hi Hinke, I will try to help.

First you mention potentiostats. This tells me that the papers you have read are research papers and have very little connection with the real world of plating. Use a lab power supply with good control of current in the range you are using.

Silver plating has been a large scale industrial process since the 1880s and is a very mature technology.
With very few exceptions, silver, either for decorative or technical purposes, is plated from a cyanide solution and for a bright deposit, proprietary additives are common.

Cleaning. a hot alkaline soak clean is normal to remove grease followed by an acid dip to deoxidise the surface. Good rinsing between steps in tap water. Use hot deionised water for the final rinse to speed drying and avoid drying marks. Cleaning depends to some extent on the base material and its starting condition. Ignore papers that advise degreasing in acetone. It is a great solvent but evaporates so fast that it puts the grease right back. They then follow up with an abrasive paper - because they are now desperate to achieve some adhesion!

A good standard text on plating is "Modern Electroplating" [affil. link to book on Amazon], Pub Wiley, Ed Lowenheim

Gelatin was once used as a grain refiner with some plating processes. It went out of fashion when chemists stopped wearing pointy hats!

I think that your easiest way would be to contact your plating suppliers. I believe that Metalor and Macdermid both serve the Netherlands. They can supply a suitable solution and much more importantly the operating parameters. You should also ask their advice on the water break test, and both macro and micro throwing power of the process if you need to plate into trenches.

Have you considered how to measure the thickness?

Please note this is the much simplified answer. Come back if we can help more.

geoff smith
Geoff Smith
Hampshire, England



September 27, 2010


Harry van der Zanden: dank je wel! Thanks for the extensive answer. Your response was more useful to me than the past 4 weeks of self study.

Good to know that a potentiostat is not really required. Also, I will look for a ready to use electrolytic cleaner solution.
I started my experiments and I used ammonia to degrease, and after that I etched the substrate with a sulfuric acid solution (by putting the substrate on a cathode, and apply a voltage so that the current density was around 20 mA/cm2). When I used that, the substrate appeared to dissolve a bit when I put it back in the electrolyte. Do you know why that is? The pH of my electrolyte was around 3.

I already assumed, from what I have read, that it would be better to buy a ready to use plating bath, and preferably one based on silver cyanide. The pH of these plating baths is around 12/13 as you said, which would be a problem if I wanted to do template electrodeposition as the required pH for that should be between 4 and 8. Do you think I can adjust the pH by adding a buffer without changing the deposition process?

The 500 nm thickness is actually an upper limit. But you think that may be a problem? Do you know why I will get these pores?

Thank you for the reference books! Not sure if I'm able to read the German book but I'll definitely try to find the book of Lowenheim.
Geoff Smith: Thanks for your response! Yes, the papers I read were research papers. Thank you for the reference book, I'll also try to find that one.

About the gelatin: it was used in some article on deposition in a template, but the authors were not sure about the effect of it. But apparently it is not a normally used additive.

I haven't thought about how to measure the thickness, but I know that it should be possible. Do you know an easy way to measure it?
Now that I've started the experiments I only have more questions.
Last week I started doing electrodeposition with silver nitrate and boric acid, where I'm using the concentrations that I found in an article. The only thing I've achieved up until now is a whitish layer that can be wiped of the ITO easily. Also, a dark layer is deposited on the anode. More strangely, when I increase the voltage to around 3 V, the dark deposition on the platinum anode starts to grow 'spikes' in the direction of the cathode. Also, at this voltage, I don't get a flat layer on the substrate anymore, but the substrate starts growing white 'silver warts' of about 1 cm in thickness, in the direction of the anode, and when I wait five minutes, the spikes and the warts even start to connect. Do you have any idea what is happening here?

Does any of you know when it is necessary to stir, and when it is not required? And is this related to the bath stability? What would determine the plating bath stability? Does the fact that I get a dark deposit on the anode mean that the plating bath is not stable?

I'm also having some problems with my setup. I'm using an improvised setup where I attach clips to the anode and cathode directly and hang them in the beaker. The distance between anode and cathode is not fixed in this way, and also I can only hang them just underneath the electrolyte-air surface. Also, for small or breakable substrates it will not be possible anymore to hold the substrate with the clips. Do you have an idea of how to connect the substrate to the power supply in a different way? Are there standardized set ups that I can buy for doing this in a more controlled way?

Obviously I already tried to find answers to these questions myself but some professional advice would be more useful.

Hinke Schokker
- Oegstgeest, Netherlands



October 4, 2010

Dear Hinke,

-Ask Rohm and Haas, Metalor, TECNIC or Enthone-OMI for their help. They are commercial companies, so be prepared that you have to pay or have to explain at least to them what you intent to do and have to share technical details in order to make it interesting for them as well.

I started my experiments and I used ammonia to degrease, and after that I etched the substrate with a sulfuric acid solution (by putting the substrate on a cathode, and apply a voltage so that the current density was around 20 mA/cm2). When I used that, the substrate appeared to dissolve a bit when I put it back in the electrolyte. Do you know why that is? The pH of my electrolyte was around 3.

-As soon as you switch off the current, the sulphuric acid will dissolve the substrate. By nature your substrate is amphoteric. It is common practice to use the sulphuric acid to remove the oxides formed and to neutralize the hydroxide on the surface, as acids are easier rinsed off from the surface. On the other hand, as soon as you talk about ITO, oxides are formed faster than you could transport the parts from process to process.

I already assumed, from what I have read, that it would be better to buy a ready to use plating bath, and preferably one based on silver cyanide. The pH of these plating baths is around 12/13 as you said, which would be a problem if I wanted to do template electrodeposition as the required pH for that should be between 4 and 8. Do you think I can adjust the pH by adding a buffer without changing the deposition process?
-No! Forget about that! The cyanidic electrolyte is containing free KCN or NaCN and you will form poisonous hydrogen cyanide which is really a very poisonous gas. Apart from that, the KAg(CN)2 salt isn't stable and will also form an insoluble silver salt and hydrogen cyanide gas! I understand that pH 4 ๕  8 is a must. Now I understand why you can't use the cyanide electrolytes. I know companies like EPI (Electrochemical Products Inc.) [a finishing.com supporting advertiser], New Berlin, WI, Metalor, Technic and Enthone-OMI and Rohm and Haas and some others as well have cyanide free electrolytes which may work for you. (Still most of them pH 8,5 ๕  10).
Trying to make your own composition will make it a lengthy exercise and a study topic in itself. You can try to use the strong crown ethers as complexers and MSA to keep the Ag dissolved and Silver nitrate as metal source , and add your samples life into the electrolyte (use a voltage stabilised rectifier) don't let the maximum current be more then what you investigated was needed to get a smooth, non burned deposit, Add some (5 mg/l.) Se(II). to your electrolyte for brightening purposes . Forget the ammonium, it can give explosive mixtures with silver salts! However, it is a lengthy way to find a good solution, you can look on the internet for "Cyanide Free Silver Electrolytes", but as earlier said, it's a subject in itself! I really don't recommend you to do this, unless you like to do it and have lots of time and are prepared for many disappointments and a few successes.

The 500 nm thickness is actually an upper limit. But you think that may be a problem? Do you know why I will get these pores?
-Many base materials have small irregularities in the surface (oxides, silicates) sometimes even non-conductive, these places need to be overplated and the deposit will laterally close the tiny pores at higher thickness. If you have a very pure smooth, monometallic, even surface, this will not be a problem. I don't know what your application is, so I cannot predict what your problems will be.

I haven't thought about how to measure the thickness, but I know that it should be possible. Do you know an easy way to measure it?

-The cheapest way is cross sectioning; after silver plating, put a copper or nickel layer over the Silver, make a cross section, etch the specimen lightly and examine under a microscope. Not very accurate for a 0,5u maximum Silver thickness, as the optical microscope reaches it maximum somewhere at 1000 ๕  2000X magnification (and the last only with special wavelengths. But if you have a calibrated SEM, this is a good method. Easy said, but look for extended info on the internet by typing in "materialographic preparation equipment" or "metallographic preparation equipment". Some companies have very nice informative glossy brochures.
-Second best is Couloscopic method. The silver is electrolytical dissolved and if the instrument detects a potential difference once it has reached the base material. This instrument is costing you app 10.000 Euro.
-A very accurate method could be X-ray fluorescence. However these instruments are from 30.000 euro. Maybe you should call a supplier and ask them if they could/would provide you with this service, or look for electroplating companies which serve the industry with precious metal plating and ask them for their help.
Now that I've started the experiments I only have more questions.
Last week I started doing electrodeposition with silver nitrate and boric acid, where I'm using the concentrations that I found in an article. The only thing I've achieved up until now is a whitish layer that can be wiped of the ITO easily. Also, a dark layer is deposited on the anode. More strangely, when I increase the voltage to around 3 V, the dark deposition on the platinum anode starts to grow 'spikes' in the direction of the cathode. Also, at this voltage, I don't get a flat layer on the substrate anymore, but the substrate starts growing white 'silver warts' of about 1 cm in thickness, in the direction of the anode, and when I wait five minutes, the spikes and the warts even start to connect. Do you have any idea what is happening here?

-Yes, you unfortunately have a composition which is absolutely unsuited for your purpose. A good electrolyte contains conducting salts, complexers, grain refiners, stabilizers. The purpose of all these components is to prevent immersion deposits, keep the material to be plated in solution, prevent spontaneous oxidation or reduction of the components and the metal and assure a good, even deposit which is evenly distributed over the cathode and has a certain dense structure as well. Therefore the research the big companies do is considerable and in their patents they will never tell you the exact composition they use, as they want to have some profit afterwards as well, which seems to be fair to me. Concerning your situation, Silver is a much more noble material as your base material (I assume the base is Indium Tin Oxide to which you refer) and will immediately start to form immersion deposits (The Sn and ln are exchanged for Ag-ions.) These deposits are by nature non-adhering and normally you cannot get a good adhesion on oxides. The clue will be to reduce these oxides and at the same time deposit some metal with good properties, making your task even a greater challenge, as this probably means you need to evolve hydrogen gas at your cathode and at the same time deposit a good adhering, crystalline Silver layer . Read e.g. something about the use of a Nickel strike on Stainless Steel substrates as comparison. I don't know if you're aware of the work of some people who claim they had to R.F sputter 1u of Ag on the ITO, in order to get a good adhesion of the Silver, but it doesn't seem to be an easy job.
-You probably form insoluble salts at your anode and dissolve some of the platinum and if the potential difference is exceeding a critical limit, this Pt-"rich" anode film is attracted by the strongly negative cathode. (If there's e.g. chloride in your electrolyte, Pt dissolves.) An electrolyte without the proper additives or complexers will form uneven deposits, as the local formed crystallisation sites will give preferential growth immediately and this gives you the unwanted effects you face.

Does any of you know when it is necessary to stir, and when it is not required? And is this related to the bath stability? What would determine the plating bath stability? Does the fact that I get a dark deposit on the anode mean that the plating bath is not stable?
-In general, stirring reduces the thickness of the so called diffusion layers nearby the anode and cathode and make the application of higher current densities possible and is recommended.

I'm also having some problems with my setup. I'm using an improvised setup where I attach clips to the anode and cathode directly and hang them in the beaker. The distance between anode and cathode is not fixed in this way, and also I can only hang them just underneath the electrolyte-air surface. Also, for small or breakable substrates it will not be possible anymore to hold the substrate with the clips. Do you have an idea of how to connect the substrate to the power supply in a different way? Are there standardized set ups that I can buy for doing this in a more controlled way?

-Difficult topic, for sure, you cannot rack your ITO samples, so I think basically the set-up you use is is correct, although you should try to maintain a fixed distance between anode and cathode. I don't know if you want to plate both sides or only one side and I don't know how fragile your ITO material is, I also don't know if you could plate a larger coupon and cut it to a smaller one afterwards. Due to the primary current distribution the edges and the corners will have higher layer thickness. You can compare the potential field lines (and so the primary current distribution) with magnetic field lines for your imagination. Here you need to do your own improvisation I guess.

Obviously I already tried to find answers to these questions myself but some professional advice would be more useful.
-I've done my best, I've been away for some time, but now I'm back, so feel free to ask some more. I hope you have enough time to investigate all this. It looks like a nice, interesting challenge to me. Good luck.

Harry van der Zanden
Harry van der Zanden
- Budapest, Hungary



October 5, 2010

thumbs up signWow! I haven't done an actual word count, but I think this is the most detailed and exhaustive reply in the history of finishing.com, Harry. Thanks!

Regards,

Ted Mooney, finishing.com
Ted Mooney, P.E.
Striving to live Aloha
finishing.com - Pine Beach, New Jersey



October 7, 2010

Hi Ted!

Do you call the people from the Guiness book of records, or do I have to do it myself?

Best regards,

Harry.

Harry van der Zanden
Harry van der Zanden
- Budapest, Hungary



October , 2010

I'm waiting for their followup; they told me "don't call us, we'll call you".

Regards,

Ted Mooney, finishing.com
Ted Mooney, P.E.
Striving to live Aloha
finishing.com - Pine Beach, New Jersey


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