Spincoating emulsion

keithwms · Jan 10, 2012

At "wildbillbugman" Bill's suggestion, I am hereby mentioning that I have access to a spincoater that can handle plates up to 6" diam. I spincoat quartz and silicon wafers with photo- and e-beam resist as a routine part of my research work. I believe that the technique may work well for film emulsions and should result in very high uniformity except at the very edges. I am happy to try it out and will do so as soon as I find the time.

Has anybody else here tried spincoating?

Hexavalent · Jan 10, 2012

I have tried this, albeit not with any sort of professional equipment.
What I discovered is that managing temperature (and viscosity) of the gelatin is critical and well beyond my capabilities.

Looking forward to hearing about your experiments!

keithwms · Jan 10, 2012

Cool. More info on the type of spincoater I've been using may be found here:

http://www.laurell.com/index.php

It is possible to control pretty much everything with these, the chucks can even be heated or warmed air can be flowed into the coating area. I currently have the simplest version, though.

wildbillbugman · Jan 11, 2012

keithwms,
I too have tried spin coating. But my" equipment" was just a tad less sophisticated. An old record turntable at 33 and 70 RPM. My bigest problem was uneven thickness. T he deposit thinned the further out from the center it got. No need to say that I had no temperature control. Back then, I was working with gelatin emulsions. Now I work with modifiede PVA based emulsions, and temperature is not quite as critical. The PVA emulsions do not "set up" at low temperatures like gelatin.
Two questions: Would the cost of this equipment be prohibitive to most people? The research I have done indicat 10s of thousands of bucks for these spin coaters-used.
Can rectangular plates be spun? All the panels for microchips I have seen are circuler.
That having been asked, I am all for a try at it.
Bill

keithwms · Jan 11, 2012

You get a more even spin if you use circular disks, but it'd be no problem to make a disk with an indentation to accept a rectangular plate, that should take care of most of that issue. But anyway, when I spin square substrates, I only see a bit of nonuniformity right at the corners.

The speeds are controlled to several thousand rpm and can be programmed, for multilayer coating etc.

The coater I have ran $2k or so, but it was brand new. These things come on the market for decent prices whenever semiconductor fab shops retool or whatever.

I typically spin films of a couple hundred nanometers, but it depends on viscosity and spin speed and time. It is also no big deal to drop fluid as the thing is spinning.

I don't have a heating chuck for curing, but the thermal conductivity of glass is very low, so you'd probably just want to blow in some warm air anyway, if you do want to warm the emulsion.

polyglot · Jan 11, 2012

Photoresist - that's basically photo emulsion. You're already there, though I guess the things you're currently coating with (and the very thin layers you're making) mean you're using much less viscous emulsions than the typical photo emulsion.

Why does the flow rate not increase proportional to the radius (alpha = radius * omega^2) ? I would have expected the resulting coating to come out with a 1/r thickness profile unless there are thin-film effects that cause flow to cease at a certain minimum thickness. Can't say I know nearly enough fluid dynamics actually...

keithwms · Jan 11, 2012

I can tune the viscosity of the resists that I spin, just by adding solvent. I guess the same thing could be done with photo emulsions... but my thought was simply to use temperature to tune the viscosity.

The flow rate for thin films isn't quite as you describe; in the thin-film limit, the interaction between substrate and wetting fluid really starts to matter. I don;t know offhand what the profile is, but the thickness profile is rather flat except near the edges. You can judge the film uniformity, directly, by watching color changes in the resist... it goes through a rainbow of colors as the film spins thinner, kind of like a soap-bubble effect.

What one does is work out some "spin curves".... film thickness as a function of speed, time, and solution viscosity. I have no idea what the ideal thickness of a photo emulsion is... a few microns? That'd be step 1, to try to get that right. Might require a few passes. If the solution is very viscous and there is a radial profile change, who knows, it might actually be interesting to see.

Hexavalent · Jan 11, 2012

Are those photoresists suspensions, or solutions?

Photographic (silver halide) 'emulsion' is really a suspension of particles in a colliod. I'm wondering if there might not be unwanted "centrifugal" separation effects.

I coat paper and film between 5-10 mils when wet.

Keith, could you delay your regular work and get on this right away?

keithwms · Jan 11, 2012

Hmm, these are not colloids that I am spinning, so yes, there could be some radial intrigue. On the other hand, if the solution is sufficiently viscous then the Stokes force should keep things more or less together.

I could also mount the glass off center so that the material sweeps across it more like a rainbow curve.

It'll just have to be tried.

Suggestions as to which emulsion to use?

Hexavalent · Jan 11, 2012

keithwms said:
Hmm, these are not colloids that I am spinning, so yes, there could be some radial intrigue. On the other hand, if the solution is sufficiently viscous then the Stokes force should keep things more or less together.

I could also mount the glass off center so that the material sweeps across it more like a rainbow curve.

It'll just have to be tried.

Suggestions as to which emulsion to use?

Perhaps some lampblack wc paint in 10% gelatin could be used for rough tests - it would give a visual indication of coating uniformity, and be a tad cheaper than using silver.

Photo Engineer · Jan 11, 2012

I think it should be tried. I'm not aware of this method ever having been used due to the one off nature of the product.

I think it is an interesting idea. Maybe I could whip up some emulsion and send you a jar.

PE

keithwms · Jan 11, 2012

Photo Engineer said:
I think it should be tried. I'm not aware of this method ever having been used due to the one off nature of the product.

I think it is an interesting idea. Maybe I could whip up some emulsion and send you a jar.

PE

Please do! I will happily test it immediately on receipt.

pgrasso · Jan 11, 2012

I am an application engineer for Laurell Technologies. We manufacture spin coaters as mentioned in keithwms 12:38am post. Spin coating is used with all types of monomers and polymers, aqueous solutions, solutions with suspended nanoparticles, epoxies, optical lens coatings, paint, colloids, etc... Round, square, rectangular and irregular shapes can be used within a spin coater. Coating uniformity, when using a square or rectangular substrate, can be maximized out to the edges, when using a properly designed substrate chuck. Spin speeds can be up to 12,000rpm. If anyone has questions about spin coating, feel free to contact me at pgrasso[at]laurell.com

wildbillbugman · Jan 11, 2012

Hi,
If you are willing to test samples, it might be wothwhile to test one of my emulsions. I think that everyone on this forum makes gelatin based emulsions. I am an an Old Hippy and make emulsions based on modified PVA. The rheology is very different. I will send you some. Just tell me where to send it.
I assume you are equiped for protecting your liquids and spun plates drom light? To keep it simple, I can send you a sample sensitive to only blue light.
How large a sample do you need ?
Bill

dwross · Jan 11, 2012

Intriguing! If it works, it might be an option for a cottage industry dry plate company. I can very much see (or at least hope) that that kind of new-to-us small company will ascend again. Some of the new technologies, like 3D printing, might be about to change a lot of our concepts about manufacturing. Back to the future, anyone?

In the meantime...
don't overlook that a couple of bucks for some glass, a silicone baking sheet and a catheter syringe will get you perfect plates.

http://www.amazon.com/Catheter-Tip-60-CC-Syringe/dp/B0002YFRBG
http://www.thelightfarm.com/cgi-bin/htmlgen.py?content=03Nov2011

d

keithwms · Jan 11, 2012

pgrasso said:
I am an application engineer for Laurell Technologies. We manufacture spin coaters as mentioned in keithwms 12:38am post. Spin coating is used with all types of monomers and polymers, aqueous solutions, solutions with suspended nanoparticles, epoxies, optical lens coatings, paint, colloids, etc... Round, square, rectangular and irregular shapes can be used within a spin coater. Coating uniformity, when using a square or rectangular substrate, can be maximized out to the edges, when using a properly designed substrate chuck. Spin speeds can be up to 12,000rpm. If anyone has questions about spin coating, feel free to contact me at pgrasso[at]laurell.com

Et voila. Thanks for chiming in. Perhaps you'd be willing to run some tests

Laurell makes some very fine equipmenmt at reasonable price.

keithwms · Jan 11, 2012

wildbillbugman said:
I assume you are equiped for protecting your liquids and spun plates drom light?

Most of us who do spinning work only with amber or red lights anyway, as the resists are of course light sensitive. So that's no big deal. I have a whole orange room that I built out of orange plexiglas. Making a red box would be no big deal.

The Laurell unit I have also has a handy closing shield/top, it keeps everything tidy and you can purge with inert gas or warm air whatever you please. Easy peasy. The unit's inner surfaces are also seem to be made mostly of PTFE- very easy to clean.

Photo Engineer · Jan 11, 2012

The only thing I have right now Keith is some Azo type. I have used up all others. I have a very foggy high speed emulsion that could be used for test. It is about 10% iodide.

Contact me.

PE

keithwms · Jan 11, 2012

Okay, anything with realistic viscosity is fine.

wildbillbugman · Jan 11, 2012

Keithwms,
Now all you need do is provide an adress to send samples to. Also please let us know what a reasonable sample size would be, bearing in mind that Ag is expensive these days.
Bill

keithwms · Jan 11, 2012

My shipping address is Keith Williams, UVA Physics, 382 McCormick Road, Charlottesville VA 22904. Sample size... well I can do very small samples if desired. I was planning to use ordinary microscope coverslips cut into ~1x1" squares. 1mL would easily coat that, many times over.

Question is whether you want me to test exposure, or you... I could do simple tests.

Wish I had some blank plates for my rb, it has plate holders for that, let me see what I can come up with....

The big advantage of spin coating is that it is extremely economical in material. Whatever your emulsion costs, I can tell you that some of the resist I use are far more costly. And unused material can actually be recovered... partially at least.

I suppose now would be a good time to mention that I am also looking into adapting an inkjet for paper coating...

Photo Engineer · Jan 11, 2012

Keith;

I would like to remind you that the dry layer should be about 10 - 100 microns depending on emulsion which means that the emulsion itself must be put down at about 5 - 7 mils thickness. Can you do that? Resists are put down much thinner.

PE

keithwms · Jan 11, 2012

I can't promise Ron, but I can certainly try, and in the worst case, multilayer spinning is not difficult.

Regarding typical thicknesses, I wonder if that's really a necessity or if that's just what the bulk coaters can do best....

wildbillbugman · Jan 11, 2012

Keif,
My newwest batch is 0.25 M, based on the MW of AgNO3. Would 100 grams be enough to do all the tesing you would like to do?.
I will supply it in a light-proof container. Refridgeration is not required.. It is a blue sensitive emulsion in its current form. My "standard" developer is undiluted Kodak D19 for 8.5 minutes..
I do not have any glass plates that would fit your coating machine, as my usual plate size is 5"X7". Maybe someone who is sending you a sample could send you some 4"X5" plates. I think that your amber safe lights should suffice for this emulsion.
I am quite excited about this emulsion because, even with extended soaks in developer, it shows no signe of fogging.
I will get this off to you tomorow, Thurday.
Many thanks for your intrest and desire to help,
Bill

keithwms · Jan 11, 2012

Cool Bill, I look forward to seeing whether it works. Yes, 100 gr is probably, what, 25-50mL? Should be plenty. I can get some plates, no worries.

Spincoating emulsion

Privacy & Transparency

Privacy & Transparency