Micro-focusing Ultraviolet light/laser with microscope objective?

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Retired Lab Rat
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Micro-focusing Ultraviolet light/laser with microscope objective?

#1 Post by Retired Lab Rat » Thu Mar 21, 2019 1:58 am

Hello! New to this forum. I'm wondering if anyone has experience with focusing ultraviolet light through a microscope objective. I'm an inveterate tinkerer and it seems that very nearly every project that I have ever dreamed up has called for a custom made electronic circuit board. I have, for a very long time, wanted to build a circuitboard "printer". The general idea would be to build a small X-Y table that moves a tightly focused, modulated light source, (probably ultraviolet), over a photo-sensitized circuitboard blank. The light either sets or un-sets the photo-resist emultion, (positive or negative photo mask), and then the board is "developed" to wash away the etch resist in areas to etched. I figure that the best spectrum will be shortwave UV, (~250nm ~350nm). An ordinary UV LED would probably require calumniation to focus to a clean spot, but a UV laser diode might require little or no pre-focusing optics. I would use precision pinhole discs to define the spot and a microscope objective to shrink the spot size and concentrate the light. I'm not entirely sure but I don't expect that I would need all that intense of a light source. That issue would be driven by the photo-speed of the emulsion and the travel speed, (dwell time), of the imaged spot. The first question that I have is: how do the optics of your average microscope objective handle UV light. I have seen optics that had filter coatings that were specifically designed to block UV and/or IR light. How likely am I to run into that obstacle? Any and all thoughts and ideas are welcome here. My biggest obstacle is that, being a Retired Lab Rat, my tinkering budget contains more lint than glint.

Retired Lab Rat,
Austin, TX

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75RR
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Re: Micro-focusing Ultraviolet light/laser with microscope objective?

#2 Post by 75RR » Thu Mar 21, 2019 7:01 am

Hi,

just posting to suggest that you might find the photomacrography.net forum quite useful, particularly the macro setups.

Many of them have stepper motors installed on the x and y stage axes.

Here is a sample of what these DIYers get up to.

https://www.photomacrography.net/forum/ ... lectronics

http://www.photomacrography.net
Zeiss Standard WL (somewhat fashion challenged) & Wild M8
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Re: Micro-focusing Ultraviolet light/laser with microscope objective?

#3 Post by MichaelG. » Thu Mar 21, 2019 8:32 am

Retired Lab Rat wrote:... The general idea would be to build a small X-Y table that moves a tightly focused, modulated light source, (probably ultraviolet), over a photo-sensitized circuitboard blank. The light either sets or un-sets the photo-resist emultion, (positive or negative photo mask), and then the board is "developed" to wash away the etch resist in areas to etched. I figure that the best spectrum will be shortwave UV, (~250nm ~350nm). An ordinary UV LED would probably require calumniation to focus to a clean spot, but a UV laser diode might require little or no pre-focusing optics. I would use precision pinhole discs to define the spot and a microscope objective to shrink the spot size and concentrate the light.
A couple of preliminary thoughts:
1. You will probably find that 405nm is the most cost-effective wavelength
2. Your description of the optics is basically that of a 'Spatial Filter'

https://www.thorlabs.com/newgrouppage9. ... p_id=10768

MichaelG.
Too many 'projects'

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Re: Micro-focusing Ultraviolet light/laser with microscope objective?

#4 Post by apochronaut » Thu Mar 21, 2019 11:24 am

Microscope optics have been used for decades for laser delivery. In this capacity they were used as mini condensers , reversed. However designers quickly learned that although the theory was obvious, they had to design purpose built versions that were entirely air spaced because microscope objectives use lens assemblies that are cemented together. Lasers cooked the cement.

Low power lasers, probably aren't as hard on the cement but you are talking UV here, so I would imagine air spacing is absolutely necessary, so any old microscope objective won't really do.

There were a couple of Bausch & Lomb ones on ebay a while ago and from time to time they show up. AO also made them as I imagine did others. One of the major laser optics supplier's stuff until recently was cookie cutter Bausch & Lomb and was probably made there and even now one or two of their designs might be. I think someone in the Rochester area is making those now.

The problem is in identifying them. The B & L ones were marked . I think they had a temperature threshold on them too. Perhaps if you search for some old catalogues or brochures; you might come up with some details.

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Re: Micro-focusing Ultraviolet light/laser with microscope objective?

#5 Post by Hobbyst46 » Thu Mar 21, 2019 12:28 pm

Retired Lab Rat wrote:...I figure that the best spectrum will be shortwave UV, (~250nm ~350nm). An ordinary UV LED would probably require calumniation to focus to a clean spot, but a UV laser diode might require little or no pre-focusing optics. I would use precision pinhole discs to define the spot and a microscope objective to shrink the spot size and concentrate the light. I'm not entirely sure but I don't expect that I would need all that intense of a light source. That issue would be driven by the photo-speed of the emulsion and the travel speed, (dwell time), of the imaged spot. The first question that I have is: how do the optics of your average microscope objective handle UV light. I have seen optics that had filter coatings that were specifically designed to block UV and/or IR light...
Firstly, and irrespective of the coating, most glasses do not pass UV below about 300nm. Quarz does, but that will not be ordinary microscope objectives. Even above 300nm and up to about 350nm, light transmission through the objective is considerably less than 100%. There are special objectives, mainly optimized for fluorescence microscopy, that transmit UV better, but I believe that they are still limited to 350nm and above, since for biology, 365nm is useful (also 330-340nm, just recalled). Very short wavelength light is liable to cause photodamage of cells and tissues.
Secondly, lasers (not laser diodes!) are routinely used on confocal microscopes. At all wavelengths from about 360nm (and possibly shorter - I am not sure) and up to the visible and near IR, with ordinary objectives (or the UV-optimized objectives), without problems. Since they are epi-illumination systems, the full power of the excitation beam - say, at 365nm - passes through the objective. I can't say, though, if the transmitted instensity will be sufficient to work on your photoresist masks.
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Re: Micro-focusing Ultraviolet light/laser with microscope objective?

#6 Post by wporter » Thu Mar 21, 2019 6:40 pm

If you're still at the conceptual stage, and wonder if indeed 405nm would work, you may want to get one of these, and do some experimentation. The price is right, and you would also not have to worry about collimation, power supply, hardware, ad nauseum, while you get some data. I would also wear red safety laser goggles, but I'm sure you, as an experienced tech, would do so.

I have one of these, and though I haven't done much with it yet, I did accidentally blind myself for a month after looking down the barrel when there was a battery in the chamber (just kidding!):

https://www.ebay.com/itm/Military-20-Mi ... 2761637766

The good thing about this brand is that it has a key for a key switch built into the end of the thing.

EDIT: url corrected as per MichaelG below.
Last edited by wporter on Fri Mar 22, 2019 3:28 am, edited 1 time in total.

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Re: Micro-focusing Ultraviolet light/laser with microscope objective?

#7 Post by MichaelG. » Thu Mar 21, 2019 11:30 pm

wporter wrote:wow, that's some url...
Indeed it is ... and most of it is superfluous [everything from the question mark onwards is just ebay tracking the route you took to get there]

Try this: https://www.ebay.com/itm/Military-20-Mi ... 2761637766

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Re: Micro-focusing Ultraviolet light/laser with microscope objective?

#8 Post by wporter » Fri Mar 22, 2019 12:39 am

Thank you, MichaelG. I was heading out the door (being yanked, more like) and didn't take the time to examine it. Good to know ebay is tracking my movements.

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Re: Micro-focusing Ultraviolet light/laser with microscope objective?

#9 Post by Retired Lab Rat » Fri Mar 22, 2019 2:59 am

Wow! Lots of good ideas. Yeah, if I had taken a look at the transmission spectrum of BS glass I would have known that I couldn't put shortwave UV through it. Having looked up that information, it would seem that for shortwave UV my choices are Quartz (crystal), Fused Silica, Calcium Fluorite and Magnesium Fluorite optics. I checked some of the usual suspects for fused silica objectives and found that they run $2K -$4K each. Not happening, and I should expect that fused silica will be the "cheap" type. So it may be that I will have to walk away from deep UV. The problem is that since most photoresist films function by crosslink polymerization, the rate of polymerization is, roughly, proportional to the photon energy of the light. That is, the "photo-speed" goes down as the wavelength gets longer. I would have to increase the intensity of the light and/or exposure time, and at some point, I would destroy the polymer from simple thermal degradation. About the blue lasers... I would love to be able to use one of those. The ones sold on EBAY for wood engraving are about 75% to where I need to go. They have mountable housings and current drivers that have modulation inputs, but I would still have to mount my own optics. I should imagine that they use gallium nitride LEDs, so they will have an output around 450 nm. I just don't know if that is deep enough into the blue/UV spectrum to be effctive for exposing the photoresist. It's nearly impossible to get enough specification detail on the photoresists to predict their spectral sensitivity. I think that I may try to order sample quantities of several different photoresists and just experiment with various wavelengths to see what will work.

Retired Lab Rat,
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Re: Micro-focusing Ultraviolet light/laser with microscope objective?

#10 Post by MichaelG. » Fri Mar 22, 2019 6:39 am

wporter wrote:Thank you, MichaelG. I was heading out the door (being yanked, more like) and didn't take the time to examine it. Good to know ebay is tracking my movements.
You're welcome

I'm generally in favour of 'disclosure' but ebay does seem to be working-it-to-death.
One small mercy: They have at least kept it easy to find the part that's relevant to us.

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Re: Micro-focusing Ultraviolet light/laser with microscope objective?

#11 Post by Hobbyst46 » Fri Mar 22, 2019 9:24 am

Retired Lab Rat wrote:Wow! Lots of good ideas. Yeah, if I had taken a look at the transmission spectrum of BS glass I would have known that I couldn't put shortwave UV through it. Having looked up that information, it would seem that for shortwave UV my choices are Quartz (crystal), Fused Silica, Calcium Fluorite and Magnesium Fluorite optics. I checked some of the usual suspects for fused silica objectives and found that they run $2K -$4K each. Not happening, and I should expect that fused silica will be the "cheap" type. So it may be that I will have to walk away from deep UV. The problem is that since most photoresist films function by crosslink polymerization, the rate of polymerization is, roughly, proportional to the photon energy of the light. That is, the "photo-speed" goes down as the wavelength gets longer. I would have to increase the intensity of the light and/or exposure time, and at some point, I would destroy the polymer from simple thermal degradation. About the blue lasers... I would love to be able to use one of those. The ones sold on EBAY for wood engraving are about 75% to where I need to go. They have mountable housings and current drivers that have modulation inputs, but I would still have to mount my own optics. I should imagine that they use gallium nitride LEDs, so they will have an output around 450 nm. I just don't know if that is deep enough into the blue/UV spectrum to be effctive for exposing the photoresist. It's nearly impossible to get enough specification detail on the photoresists to predict their spectral sensitivity. I think that I may try to order sample quantities of several different photoresists and just experiment with various wavelengths to see what will work.

Retired Lab Rat,
Austin TX
Surfing has brought me this communication "[PDF]Exposure of Photoresists - MicroChemicals" a freely available web document. They state that there are photoresists that work at 400-435nm, although the general trend in this area is the reverse, towards extremely short UV irradiation.

BTW, photo-lithography is heavily used in the R&D of microfluidics, for the preparation of PDMS channeled blocks. You might find some information searching in that direction, although the amount of articles on microfluidics is huge.
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Re: Micro-focusing Ultraviolet light/laser with microscope objective?

#12 Post by MichaelG. » Fri Mar 22, 2019 11:45 am

Hobbyst46 wrote:Surfing has brought me this communication "[PDF]Exposure of Photoresists - MicroChemicals" a freely available web document. ..
That's an interesting document ... Thanks
https://www.microchemicals.com/technica ... resist.pdf

Incidentally: I presume the trend towards shorter wavelengths is to do with microcircuitry fabrication, rather than amateur P.C.B. manufacturing.
405nm was developed for BluRay DVDs, and I think that's fine enough work for most of us.

MichaelG.
Too many 'projects'

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