[labnetwork] LED Lights for Lithography

[martin at algoshift.com]

Based on a couple of responses it looks like I did a confusing job of 
explaining what I am working on.

I am focusing on lights to illuminate the lab, the room, not to cure 
PMMA.  So, yes, this is about lights on the ceiling.

Also, I said "above 500 nm" when I was thinking frequency.  I should 
have said "below 500 nm".  The point is, as I understand it today, the 
goal is to not have much energy in the blue and UV range of emissions.

Sorry for the confusion.

Thanks,

-Martin

---


On 2020-02-21 23:55, martin at algoshift.com wrote:
> I am currently working on the development of LED-based lights for
> lithography applications.  I came across this list and was kindly
> allowed to join.
> 
> I have a background in high-performance, high accuracy LED-based
> applications going back some twenty years.  In addition to that I
> worked in aerospace engineering, robotics and other work I can't talk
> about (most recently, SpaceX).
> 
> At this stage in my new mission I am trying to confirm what I have
> learned in order to start developing a few prototypes for testing.
> This is what I know and don't know so far:
> 
> - Energy above 500 nm should be below 0.001%
> - Operator metamerism doesn't seem to be much of a concern in these
> environments (?)
> - Outgassing is not desirable (I don't have any kind of a
> specification for this)
> - No specification on acceptable flicker
> - No specification on required efficiency (Lumen/Watt)
> - No specification on the amount of light required, either:
>     - Illuminance (intensity of light on a surface, lux) or,
>     - Luminance (light energy emitted, lumens)
> 
> Frankly, there really isn't very much data out there.  It also seems
> that semiconductor companies keep their lithography illumination
> requirements somewhat close to the vest.  At least this is what I've
> come across.  I wonder if this is because these kinds of
> specifications might reveal process details?  Don't know.
> 
> The three main trades I have in front of me at the moment for this 
> design are:
> 
> - White LEDs with carefully selected film or coating-based filter to
> cut blue + UV
> - Green and Red LEDs only, no blue; filtration is still needed
> - A combination of carefully selected white LEDs with low spectral
> power above 500 nm along with, perhaps, green and red to enhance;
> filtration still needed
> 
> To clarify, the Green+Red LED option still requires filtration because
> green LEDs produce some energy above 500 nm.  If I am to take the
> 0.001% specification to be true, an optical filter would still be
> required.
> 
> White LED's, which, of course, are nothing more than blue LEDs with a
> phosphor coating are the most readily available high efficiency units
> in the market.  Frankly, if high light output at the lowest possible
> cost is a requirement it is hard to beat them with a combination of
> red and green LEDs.  That said, depending on how they are selected, a
> significant portion of the spectral power they emit will have to be
> converted into heat at a filter or bad things will happen in the lab.
> 
> The third option involves selecting very warm white LEDs that have
> almost no blue spike.  This means less heating of the filter element
> and, likely, longer life.  This could be an interesting solution.
> 
> Plastic film based filters degrade over time, particularly if there's
> a lot of heating due to having too much energy in the undesirable
> portion of the spectrum.  This is where thin film deposition
> (sputtering?) could exhibit far more favorable band-pass
> characteristics as long a longevity.  Cost, of course, could be an
> issue.
> 
> I am very familiar with material out-gassing issues in the context of
> aerospace applications.  Not so for lab usage.  Understanding where
> these limits might lie would be very useful.  The perfectionist in me
> wants to design a T5-class 4 ft LED light fully encased and
> appropriately sealed in a durable glass tube that is both internally
> and externally coated to not pass light above 500 nm.  At the same
> time, I do understand that a real solution has to fit a budget as well
> as technical specifications.  Not sure where that intersection lies
> but I am aware of it.
> 
> I introduced a term above that might not be familiar to everyone here;
> observer metamerism.  This is a by-product of the spectral power
> distributions of light, reflection and the human vision system
> interacting in such a way that two colors that are different might
> appear the same (or, in general, you have trouble discerning colors
> that are easy to see under different conditions).  If you've ever
> tried to determine if a steak is well done under a typical white LED
> light and could not, that's observer metamerism.  Mitigation requires
> "filling in" the emitted spectra in areas relevant to the task at
> hand.
> 
> This is why I asked myself this question in the red+green LED case.
> Both of these have narrow emission spectra.  Our brains can function
> with this kind of light and, yes, we will see it as yellow.  However,
> any colors in the portion of the visible spectrum lacking energy will
> become challenging to deal with.  It's like being color blind.  Given
> that lithography labs are already built to work with yellow light, I
> find myself wondering how much of a problem, if any, might be posed by
> observer metamerism in the case of the proposed red+green LED
> solution.
> 
> I think that's the basics at this point.  I would appreciate any and
> all feedback, questions and even a good shove in the right direction.
> 
> Thank you,
> 
> Martin Euredjian
> AlgoShift, LLC
> Los Angeles, CA
> 661-305-9320
> 
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