Hi Garen KWwn,

Most logical explanation is that your silver is oxidizing in ambient air. This depends a lot on air humidity and contamination levels (expecially sulfur, SO2, SO3). An easy way to check this by putting a thin oxide layer on top of the silver. If you don't see a change, the hazing is caused by ordinary oxidation.

There also might me something else happening: It is possible you have initial physical bonding between SiO2 and the Ag layer but after a number of hours the adhesion gets worse and you see this as haze. You can simple test this but scotch tape test. Sometimes you can also see bubbles under the microscope). If you pull off the Ag, you do not have good adhesion. Normally, sputtered materials with adhesion properties like Mo, Ti, Cr, V, Nb, cannot be pulled off by such test. I have seen similar behaviour with gold layers on SiO2. After a while it comes off. There is no good intrinsic bonding between noble metals and oxides.

There also might be a more complicated mechanism at hand between the PECVD SiO2 and the silver. I assume the silver was put down with a PVD technique? If so, during the first steps during the deposition process some SiO2 might be dissociated and can create some AgO at the interface or after a while Ag forms AgO with the SiO2.

Hope these ideas help you to understand the haze?

kind regards,

Peter

Hi Peter,

Really appreciate the ideas. I have seen the Ag2S/AgCl2 explanation but am not sure of how to determine its validity. I will attempt the method you suggested to see if it helps.

As for debonding, that is not possible as hazing does not appear to change the bonding properties. In fact, the haze appears sometimes after a thermal step which was performed to improve the bonding between Ag and SiO2. The haze appeared despite the good bonding properties between Ag and SiO2.

In fact, I have attempted an experiment where a bonding layer was inserted between the SiO2 and Ag and still the hazing appeared.

Thanks for the final idea, it is something that I have not considered. However, from my prior experience of heating AgNW in air at elevated temperatures, I am quite sure that AgO does not form readily, if at all.

It is really quite frustrating at the moment as I suspect the haze may affect the properties of my devices.

Thanks again for your suggestions, appreciate your thoughts.

Regards,

Garen

Hi Garen,

Reading your reaction on tests you already have done, the most logical explanation is the surface reaction with contaminants. Ag is really very sensitive to sulfur in the environment and covering the layer before heating steps might help to stop it or understand this is the root cause. Did you ever leave the sample for a long time? Did they get brownish? This is a strong indication of formation of Ag2S compounds.

kind regards,

Peter

The haze remains whitish over long periods of time (weeks), so I am suspecting it to be AgCl2. I am trying means and ways to 'create' the haze now to see if I can pinpoint the source.

Hi Garen,

Do you use/have a chlorine environment?

The acids are in a separate room from the sputter tool, so I do not think there is any effect. To confirm, I walked around the lab with samples of silver on silicon trying to 'catch' chlorine molecules. I am waiting for the reaction to take place (if any) and will know by tomorrow, though I highly doubt.

I am now wondering if the haze could have been caused by handling/accidental contact with bodily fluids. I have placed thumbprints on a separate silver sample and will see if bodily fluids can react with silver to cause this haze. Actually I won't be surprised if it does, but want to see if the haze signature appears.

Hi Garen,

I would just place a sample in a vacuum for a while. You exclude all environmental influences and the sample should stay shiny. Accidental handling and touching samples could be a problem when handling Ag samples. It is known some people can't wear silver jewelry because it will turn hazy and eventually brown due to the influence of sweat (NaCl). So thumbprints could also cause this.

This tarnishing can be eliminated by grooving the substate with features below 0.5 microns. The oxide layers will then adhere nad protect

Thanks for the suggestion. I have begun to suspect that it could be some physical effect that could cause the hazing.

How do you effect this grooving? etching with mask or some other physical means?

@Peter Knapen, I have concluded the human influence check, it appears to be a possible cause. At the same time, I notice that evaporated Ag samples seem to be far more susceptible to this problem than sputtered samples.

That is, for the same contamination source (thumbprints), the degree of hazing was seen to be far more severe on the evaporated samples than the sputtered ones. Attached images below, the scratches were introduced because I was testing to see if I could wipe away the hazing.

Hi Garen,

Assuming you use similar thicknesses on both samples, this is probably due to the different morphology you get from the two deposition methods. Have you had a look at that with a SEM? It is know from literature that in general evaporated samples are more porous in nature than the sputtered ones (you can find more information on this in textbooks on deposition techniques, e.g. Thornton, Thin Film Processes. During sputtering you get a peening effect (expecially at low pressures) which creates more dense layers. I take it that when you have more porous samples the hazing will be more severe since there is more access via the pores of the structure (more active surface). I don't know the thickness of your layers but if they are 50-100 nm you might see the difference in porosity by the amount of light transmitting through the Ag film. With thicker samples this is usually not possible anymore because they then become optically closed.

Best regards,

Peter

Actually I am using 150nm silver films. I would like to thank you for directing me to the peening effect, it was most helpful in understanding the difference between the sputter and evaporation deposited films.

To be honest, I am, rather reluctantly, moving away from a chemical source for hazing towards this mechanical source. I will have a look at the hazed samples under SEM (the thermally hazed samples & not my chemically hazed ones) and see if I can catch anything significant at the the hazed silver/clear silver boundary.

Garen,

You could consider doing a surface chamical analyses like XPS so you can see what kind of contamination you might have. AgO, AgS or any other other component of chemical bond will show up in XPS.

best regards,

Peter

If it is really hazy/opaque I would suspect that there are structures larger than the wavelenght of visible light, maybe cracking/partial peeling off of your silver film. A uniform oxidation/sulfurisation would still leave an optically flat and therefore reflecting or transparant film. Of course it could be that the chemical change is not so uniform, the fact that you can induce the hazing locally by touching the substate with your finger is suspect. I am curious what you will find by SEM, and if you can do XPS or EDX (or maybe XRD?) that would also be interesting, to detect chemical changes.

I concur with PIeter that the hazing could be related to adhesion issues. Especially the fact that hazing occurs when touching the surface. Ag as a noble metal in itself does not act as a adhesion materials (like Ti, Mo, Nb, V, Cr). It is my own experiece that Au layer after deposition adhese for some time on Si and glass but after a while will come loose. To check for this "adhesion" problem you could try to sputter a Ti or Mo adhesion layer t and then the Ag layer. This is a combination which will stick to glass.

Alternatively a simple tape test on the Ag, directly after sputtering and after a few hours, should also give you indication if the layer is adhesing.

Hello Peter,

I am having a similar problem encountering hazing along the surface after sputtering Ag to glass. It is also random, they are some samples made by other people where the surface stays shinny and some other where the surfaces changes into a foggy and opaque silver.

We are all using gloves of course during the sputtering process.

My question is, did you find any patterns as far as when the surface gets hazy? Could it be that the handling is different and that is why the results change from one person to another?

Your posted question was really helpful already, thank you.

Hi Sonia,

As suggested in all of the comments in this discussion the reason could be:

1. Adhesion of Ag to glass is not very good. Most noble metals do not stick well to substrates like glass, SiO2. You might have initial binding through Van der Waals forces but the layer might come loose after a while. To check for this problem you could try using a adhesion layer of Ti, Mo, Cr to make sure you do not have a adhesion problem. You can also use scotch tape to check if the adhesion of the layer is ok. If you have good adhesion with scotch tape you should not be able to peel of the Ag. Most industrial coating (like mirrors) use an adhesion layer of Cr on glass before Ag is applied.

2. You can improve adhesion of Ag by using a pre-sputter etch of the substrate but I doubt this will be good enough for longtime adhesion.

3. Check for O2 and H20 levels in your sputter chamber. Start the deposition when base pressure of < 10-6 Torr is reached. The P_O2 level is then < 10-8 Torr.

4. Since your silver layers become foggy and opaque, I think you have a adhesion problem. If it was an environmental contamination (e.g. sulfur for which Ag is highly sensitive) the layers will turn brown (AgS).

I will look into it then Peter. Thank you so much for your answers!

Hi Peter, its been a while. I was meaning to report on progress a little earlier, but I got busy and distracted.

I have been running some experiments and the following is a list of what I have tried/come to understand:

i. The surface features are due to surface migration

ii. The surface features are due to delamination of Ag from SiO

My reservation comes because

i. In one of my papers, I performed an order of magnitude assessment of surface migration of Ag atoms on a nanowire and found the rate of surface migration of the Ag atoms to be too slow in a nitrogen environment to cause these effects.

ii. On the other hand, I applied some adhesive materials between the oxide and and Ag layer, but do not have definitive evidence that this adhesion layer is helping very much

I'm still interested in the topic, but looks like I will have less time on my hands nowadays. Meanwhile, hopefully by leaving this information here, someone else who is looking into the issue can build on this understanding, come to identify the source of the problem and together we can build a solution to this rather interesting and unexpected phenomena.

Thanks!

Can you provide your SEM images? Maybe that will help to find out more about the phenomenon.

Indeed aluminum and nickel should normally stick reasonably well to SiO2 and silver I think, even though they are not as good as Cr, Ti and Ta. Still, it would be interesting to see if they would further decrease the hazing effect.

Hi Garen,

As Pieter mentioned a SEM image would be nice. Do you know what the thickness was of the Ni and Al adhesion layers you used? And what is the hotplate temperature and time at which you see hazing? It might be that with thin adhesion layers there still might be some oxygen migration/diffusion toward the Ag layer @ hotplate temperatures

Hi guys,

So I used 3nm of Ni for adhesion and I have included the SEM images here. One is that of the hazy portion and the other of the as-deposited film. The surface features are distinct in the hazy sample. I don't know how to label these images, but I suppose they are pretty self-evident.

I did not take note of when the haze occurred as I put it on the hot-plate as a form of accelerated degradation, but in some cases, leaving the samples in air over-night was sufficient for hazing to appear.

Hi Garen,

From this it looks you have a very nice fine-grained or T-zone deposition but after the hotplate treatment you see a complete recrystalisation of the surface and void formation. I don't know the fase diagrams of Ni-Ag but i would not be surprised if the metal layers are intermixing and you form some low temperature eutectic alloy using the hotplate. This surely does not look like oxidation. The rough structure will affect the specular reflectance of your samples and might be the explanation for the hazy appearance. So far we discussed oxidation or reaction with sulfur of the Ag layer. You mentioned the hazing also happens after a while at room temperature. The hotplate experiment now raises the question whether this recrystallisation also takes place at room temperature? If so, you probably have found the cause of your problem: alloying of thin film layers. If a room temperature this re-crystalisation does not take place your hotplate experiment probably is a different kind of hazing and you need to look further what happens at room temperature.

Did the hazy parts also look like this in the SEM before you used the adhesion layer? At least these experiments/pictures for me me vindicate the 'delamination' theory. The hazing indeed seems to be caused by a chemical reaction. I would do EDX (or even better: XPS) to get an idea of what substance you have on your surface.

How thick is the SiOx? Have you determined the Si to O ratio? Unoxidized Si will migrate, preferentially, to the surface of the Ag and oxidize. This happens rapidly at room temperature.

Ionic radius of silver is so big than lacuna radius in SiO2 matrix

Hi Peter, the NiAg phase diagram shows that no alloying is possible, hence the choice of material. But yes, based on everything shown so far, it does look like some form of surface reorganization that occurs at both RT and elevated temperatures.

Hi Pieter, the adhesion between SiO/Ni/Ag is actually really good. It is unlikely any delamination occured and the EDX results I had showed no evidence of oxygen.

Hi Joseph, based on what I have done so far, it is a silicon poor oxide, perhaps SiO1.9. Would it still be possible for Si to migrate, especially with the Ni layer?

Thanks all for the ideas!

Hi Garen,

Looking at the phase diagram I think your right it's not just some eutectic recrystallisation of Ni and Ag. Since you do see this hazing at room temperature, do you see this recrystalllisation also at room temperature? If not, it's a different issue and might me caused by SiO2 interacting with NiAg, maybe the formation of a silicate. I would try a different substrate layer (e.g. LPCVD Si3N4) to check this.

I suggested to do the EDX on the hazy silver to see if you have any other elements present there, that may help you to identify what reaction has taken place.

Garen,

Yes. The Si will migrate through the Ni. Try removing the haze with HF. It should be easily removed. If it is SiO2 on the surface it will come back.

There is also the possibility that Ni is migrating.

I think its quite clear to me know (having gone through all the postulates and data again) that the haze that I am observing can be attributed to silver relaxation. I have been able to reproduce the problem under various conditions (room temperature, elevated temperature, different substrates, different blocking layers, so on...) and can break the problem into two parts.

While this does not exclude the problems of halide formation and other forms of silver tarnishing, these are not problems that I face on my films.

Thanks again everyone for looking at this with me and I hope that this has helped you as much as it has helped me.

Hi Garen,

Very nice to hear you tackled the problem. As I mentioned in one of my first responses, noble metals like Au, Ag and Pt initially adhere to oxide surfaces through vd Waals forces but after a while they will come loose. The only way to solve this really is by using an adhesion layer like Cr, Ti, Ni, Al. These materials react with the oxide to form intermediate layers (e.g.  Si-O-Ti) which have strong chemical bonds. These are the ideal adhesion layers for any other metal. Noble metals will not form these intermediate layers since they do not readily oxidize and do not form these strong bonds.

Garen Kwan I have issue with silver delamination with thermal evaporated silver on silicon surface. Any idea why silver delamination happen from silicon surface?