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How can I etch dichroic glass?

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GreySoul

Tue Mar 20 2007, 02:58AM

Registered Member #546 Joined: Fri Feb 23 2007, 11:43PM
Location: Albuquerque, NM
Posts: 239

Hi all - I can't seem to google up any info on this, figured a chemical wiz might have an idea.

I have a sheet of dichroic borosilicate float glass. the coating process involves vapor deposition of metals on the glass surface to make optical bandpass filters... anyways

I need to very selectively etch small diameter holes in the dichroic layer and not affect the optical qualities of the glass below it. 2" sheet, need a 0.1" holes sorta evenly spaced...so....

I've done plenty of silkscreen PCBs and I figure I would just make a resist stencil on a screen and knock these out by the dozen (if not hundreds)

But I can't find anything that will eat the dichro off. So far I have tried 40% nitric acid, Datek pcb etching grade ferric chloride, and "Sparex" pickel which I think is weak sulphuric acid - no results.

Any suggestions?

Thanks all,

-Doug

Chris

Tue Mar 20 2007, 04:07AM

Registered Member #8 Joined: Thu Feb 02 2006, 04:34AM
Location: Harlowton, MT, United States
Posts: 214

What kind of metals or compounds are deposited on the surface? Aqua regia can dissolve most metals (especially when thin or finely divided) except tantalum, iridium, and maybe a few others. You really need some good ventilation for it, of course, since the vapors contain free chlorine and nitrosyl chloride. Elemental bromine should attack any metal to some degree, but it is nasty to work with and much more difficult to find a resist material for than acids (you could use teflon, glass, or maybe some fluoroelastomer such as fluorosilicone or viton). You can extract usefully concentrated bromine electrolytically from bromide salt with a separatory funnel since it is dense and relatively insoluble, though.

If the layer is conductive, an electrolytic etch will be even more precise, faster, and use much less harsh or dangerous chemicals (at least before you power the cell up). A simple aqueous sodium chloride electrolytic etch works well on most metals but will produce chlorine gas (no problem with good ventilation though). That method is often used for etching relatively thick steel such as for pulsejet valves. More vigorous but also more toxic, would be a similar fluoride bath (though it could attack the glass too).

For the most powerful electrolytic etching, an anhydrous bath of fused (molten) chloride salt could be used. A eutetic 60/40 mix of lithium and potassium chloride melts very easily and below the strain point of even soda-lime glass. Also 60/40 calcium/sodium chloride melts at less than 600C. It can attack all metals at the anode, even iridium, at some rate or other. Similarly a fluoride bath would be even more vigorous, but far too toxic to be worthwhile, and chloride is sufficient for any metal anyway. Remember though that hot glass is conductive in such salt baths, but with a very brief exposure (maybe even a single low voltage capacitor discharge) it could work without damaging the glass to any appreciable degree (depending I guess on how high purity you need this). Also you would have minimal production of free halogen in the brief electrolysis.

Bored Chemist

Tue Mar 20 2007, 06:58AM

Registered Member #193 Joined: Fri Feb 17 2006, 07:04AM
Location: sheffield
Posts: 1022

I think they will have used dielectric coatings rather than metals. Without kinowing what they are it's difficult to know how to dissolve them- particularly without damaging the glass.

Steve Conner

Tue Mar 20 2007, 11:52AM

Registered Member #30 Joined: Fri Feb 03 2006, 10:52AM
Location: Glasgow, Scotland
Posts: 6706

Our optics whiz says those coatings are extremely tough and have to be got off mechanically.

GreySoul

Tue Mar 20 2007, 03:15PM

Registered Member #546 Joined: Fri Feb 23 2007, 11:43PM
Location: Albuquerque, NM
Posts: 239

so some digging,...the coatings are titanium and quartz.

The quartz is probably why nothing has worked yet.\

And steve - I've seen this done before. most comanies use lasers, but before lasers were commonly available people used acids.

Also - anyone have experiance with imagon film? I have a roll, and if it would work better than silk screened resist I could use it.... just thinking aloud on that one.

-Doug

Steve Conner

Tue Mar 20 2007, 03:28PM

Registered Member #30 Joined: Fri Feb 03 2006, 10:52AM
Location: Glasgow, Scotland
Posts: 6706

Hmm, well if they used acid, it would probably have been HF.

Bored Chemist

Tue Mar 20 2007, 06:50PM

Registered Member #193 Joined: Fri Feb 17 2006, 07:04AM
Location: sheffield
Posts: 1022

Titanium dioxide isn't a lot easier to get through than quartz and would have ignored the "etchants" you have tried so far. HF would work but it will mess up the sufrace of the substrate. What are you trying to do?

GreySoul

Tue Mar 20 2007, 09:52PM

Registered Member #546 Joined: Fri Feb 23 2007, 11:43PM
Location: Albuquerque, NM
Posts: 239

I'm trying to etch patterns onto dichro for an art project. the evenly spaced dots are just one facet of this - once i get a process down I';m going to expand into other patterns and such.

I'm well aware of companies that do this work, but the whole point of the project is I do it myself.

Would a weak hf solution slowly etch the material to where I could watch the process and remove the glass before it etches the udnerlying glass? Obviously the bottom and edges would have to be masked.... but could that work?

Also - my local chem supplier closed down I guess (if anyone knows anything about Albuchem, let me know) so... any reasonable source of HF? I'm pretty certain it's one of the harder acids to come by, right?

...anyways...thanks so far, and I'll keep updating progress if I make any.

-Doug

Chris

Tue Mar 20 2007, 11:13PM

Registered Member #8 Joined: Thu Feb 02 2006, 04:34AM
Location: Harlowton, MT, United States
Posts: 214

so some digging,...the coatings are titanium and quartz.

The quartz is probably why nothing has worked yet.\

Actually the etchants you have tried would have a hard time with titanium too. Is it just titanium or titanium dioxide?

Hazmatt_(The Underdog)

Wed Mar 21 2007, 03:40AM

Registered Member #135 Joined: Sat Feb 11 2006, 12:06AM
Location: Anywhere is fine
Posts: 1735

If you're trying to etch Ti or any of the same group metals, you use a mixture of Nitric and Hydroflouric Acids. It's a standard De-ox that my boss uses seldom because of the potency of the stuff. You can also use a Nitric BiFlouride mixture, it will be very corrosive to Ti, SiO2, and just about anything you can think of.
When using it will produce Nitrogen Oxides immediately as brown clouds of nasty toxic gas, so you have to do it out in the open, and hopefully with protection.
If you have any left over it will need to be stored in a poly jar of some sort because itll eat through most things.

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