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  1. #1
    keithwms's Avatar
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    Spincoating emulsion

    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?
    "Only dead fish follow the stream"

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    Hexavalent's Avatar
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    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!
    - Ian

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    keithwms's Avatar
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    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.
    "Only dead fish follow the stream"

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  4. #4

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    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

  5. #5
    keithwms's Avatar
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    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.
    "Only dead fish follow the stream"

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    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...

  7. #7
    keithwms's Avatar
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    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.
    "Only dead fish follow the stream"

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    Hexavalent's Avatar
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    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?
    - Ian

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    keithwms's Avatar
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    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?
    "Only dead fish follow the stream"

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  10. #10
    Hexavalent's Avatar
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    Quote Originally Posted by keithwms View Post
    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.
    - Ian

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