Questions and thought experiment about metal vacuum deposition

[keegreil]

Hi all, I'm new here. My professional background is in computers and airplanes, but about a year ago I started getting really curious about how telescope mirrors were made. I know that typically they are coated with aluminum in a very good vacuum. What follows is a dreamed up alternative process that is almost certainly less effective (or totally useless?), but since this is far out of my profession I need some help coming up with some reasonable hypotheses for what might happen if this were tried (not just what I wish would happen). Right now I live in a hotel room for work, but perhaps later this year I will have a garage again and may try it myself.

 

Most hobbyists, when finishing a telescope mirror, send the glass out to get professionally coated. Most hobbyists (not all!) cannot build a good enough vacuum chamber to successfully vapor deposit aluminum themselves. My understanding is that the reasons a high vacuum is required are twofold. 1) In a low vacuum, even if successfully vaporized, the aluminum will bump into other air molecules and cool off to a solid along its transit, and so would deposit as a (black?) powder. 2) In the presence of oxygen, aluminum quickly forms aluminum oxide, which deposits as a white powder. A vacuum solves both of these problems. Are these assumptions correct and are there other reasons a vacuum is required?

 

Here is my hypothetical experiment.

1. Place a small piece of aluminum inside an empty jar, preferably made of Pyrex

2. Add a small amount of water to the jar (enough to cover the aluminum?)

3. Cover the jar with a flat piece of glass to be coated

4. Place a second jar full of water on top of the flat glass

5. Put the whole stack on a heat source, VERY slowly bringing it up enough to melt the aluminum

 

Then wait (hours? days?). The water in the first jar will have all boiled, hopefully forcing all of the air (especially O2) out the top and filling the space with superheated steam, at the melting point of aluminum (660 C I think). The water in the second, top jar, will be boiling and, by evaporative cooling, self regulating its own temperature to 100 C. The glass plate on which it rests should be slightly above 100 C.

 

In my ideal imaginary world, the aluminum would (albeit slowly) vaporize and be carried by the superheated steam to the cool plate, where it would instantly deposit itself in a nice shiny coating. By filling the chamber with 700 C steam, in theory we have displaced all of the oxygen, and prevented the aluminum from cooling to a solid prematurely. This would allow simple hobbyists like myself to perform aluminum vapor deposition without the need for a high vacuum system.

 

So here are my questions. While I feel like this shouldn't work (it would be too easy), why exactly wouldn't it?

1. Would the aluminum still cool off because of a boundary layer of cooler steam on the plate? Is it ok if the aluminum atoms become supercooled before freezing onto the glass plate?

2. Is the vapor pressure of liquid aluminum just too low to get any useful deposition rates?

 

Perhaps trying this with tin first would be a better idea, less extreme temperatures required Thanks in advance for any input or other wild ideas, cheers!

 

-- Keegan

 

PS: Attached is an embarrassingly crude drawing of the idea, if that helps picture it.

[Enthalpy]

Gaseous aluminium is extremely reactive. It reacts immediately if encountering a molecule of oxygen, or nitrogen, or water vapour. Even dry argon would contain too many impurities that would catch the aluminium atoms.

[keegreil]

Wow, I see. I never realized aluminum would react with water. Guess that answers my question then, you really do need a vacuum. Just out of curiosity, would doing this with tin work? Or is tin to reactive as well? Thanks!

[Enthalpy]

Any metal in atomic form is very reactive. I have no doubt with tin, and would expect even atomic gold to react with vapour.

 

Some reactions in liquids can deposit silver, which makes mirrors but not quite good enough for astronomy. Maybe a wet reaction exists to deposit gold of good quality? (Aluminium I guess not, as reactions in water would release hydrogen instead)

 

Now we're typically in a question of chemistry, so someone else should jump in.

[John Cuthber]

I'm not sure that it's chemical reactivity that would stop this working with tin (or other fairly un-reactive metals.)

The metal would "bury" water molecules in the deposited layer and that would spoil it.

 

Silver mirrors deposited from solution were the medium of choice for a long time. They are clearly good enough for astronomy since they were used for it.

There are reactions which deposit gold mirrors (and also copper and nickel ones)

[Enthalpy]

Silver is not very durable, a reason why astronomers prefer aluminium or gold.

 

Is there a solution that deposits gold without using cyanides? Few gold salts are soluble.

[keegreil]

Thanks for the ideas everyone, I may look into actually trying to make a vacuum. Might be a fun project, if difficult. Does anyone have experience with creating vacuums? Specifically with evap getters?

 

My latest idea is this. Get a $100 Harbor Freight pump to take the chamber down to about 10^-3 Torr.

Place the mirror in the top of the chamber facing down, covered by a remote controlled shutter.

Place the aluminum in a small crucible, and gradually evaporate it.

 

At first, the Al atoms will react with everything in the chamber, hopefully turning them into solid products and condensing in ugly black powder all over the walls. Basically, we are using it as a "getter" to take the vacuum down to about 10^-6 Torr (ish?). After a while, the Al will have "gotten" all of the gas in the chamber and will start depositing as a clean shiny layer. When that happens, remotely open the shutter, uncovering the mirror and coating it perfectly (in theory ).

 

My theory is that by making an Al evap getter of sorts, we could get around having to use other more exotic pumps like oil diffusion or turbo-molecular pumps.

 

Another advantage is that we don't need to figure out a way to measure when the vacuum is good enough to deposit Al (though that admittedly should be easier than making a pump). When the Al starts making shiny layers, then the vacuum is good enough. No fancy vacuum gauges required?

 

I know this idea is probably ridiculous too, but thanks for entertaining my curiosity!

[Enthalpy]

When we deposited aluminium on silicon wafers we needed a secondary vacuum seriously better than 1e-6 torr. Was it 1-e9? I've forgotten.

 

Aluminium will get oxygen and hydrogen from the chamber, but not necessarily hydrogen, argon, carbon monoxide (once aluminium has converted the dioxide).

 

Whatever your means to create the deep vacuum, it will fight against the leaks from the primary pump, and I doubt metal evaporation has the necessary throughput. Titanium evaporation is used for that purpose, but only after a secondary pump has already done its mission.

 

An oil diffusion pump isn't that exotic... It's a very simple pump, reliable, easy to operate, efficient.

 

My suggestions would be:

- Find a university with a microelectronics lab, make some deal to get your mirrors coated. Like: invite them to observation nights.

- Buy a used aluminium evaporation machine. It would be third-hand nowadays, probably dirt-cheap.

- Candy wrapping still uses aluminium evaporation on polyester films, in huge machines.

- Do not try to assemble the machine by yourself. Evaporation is usually done by an electron beam so the crucible remains luke-warm an doesn't pollute the aluminium - nothing simple to my eyes.

- Check the price of aluminium, I've doubts on that one. A <100nm layer that resists corrosion must be pure.

- Try wet methods to deposit gold.

========================

 

Seen evaporation targets of aluminium on eBay, item #390317323315 there, they want 330 usd for 1 pound, quality is not specified.

 

For mechanical parts, you get alloy 99,8% pure, but I doubt it suffices for a mirror's ultra-thin layer. Purer exists to make electrolytic capacitors, but where to get it? For microelectronics, we had <<1ppm impurities.

[John Cuthber]

Thanks for the ideas everyone, I may look into actually trying to make a vacuum. Might be a fun project, if difficult. Does anyone have experience with creating vacuums? Specifically with evap getters?

 

My latest idea is this. Get a $100 Harbor Freight pump to take the chamber down to about 10^-3 Torr.

Place the mirror in the top of the chamber facing down, covered by a remote controlled shutter.

Place the aluminum in a small crucible, and gradually evaporate it.

 

At first, the Al atoms will react with everything in the chamber, hopefully turning them into solid products and condensing in ugly black powder all over the walls. Basically, we are using it as a "getter" to take the vacuum down to about 10^-6 Torr (ish?). After a while, the Al will have "gotten" all of the gas in the chamber and will start depositing as a clean shiny layer. When that happens, remotely open the shutter, uncovering the mirror and coating it perfectly (in theory ).

 

My theory is that by making an Al evap getter of sorts, we could get around having to use other more exotic pumps like oil diffusion or turbo-molecular pumps.

 

Another advantage is that we don't need to figure out a way to measure when the vacuum is good enough to deposit Al (though that admittedly should be easier than making a pump). When the Al starts making shiny layers, then the vacuum is good enough. No fancy vacuum gauges required?

 

I know this idea is probably ridiculous too, but thanks for entertaining my curiosity!

You would do better with a 2 stage process. A magnesium getter because it removes nitrogen as well as oxygen.

Then the aluminium.

If you can get a sealed system and fire a Mg getter then only the argon (and a few trace gases) will be left behind. Since only about 1% of the air is argon the pressure drops a hundredfold.

As you say, no need to measure it at that point. If you start with a good vacuum, it suddenly gets 100 times better

Better yet, flush the system with clean nitrogen first, then drop the pressure then getter the remaining N2 with Mg.

You can calculate the mass of Mg needed- it's not a lot if you start with a fair vacuum.

 

In any event, the easy way is, of course, to get someone with the right kit to do it for you.

[Enthalpy]

To go from 10^-3 torr to 10^-10 torr, the kind of vacuum needed for aluminium deposition, argon must be removed as well, and worse, hydrogen. Flushing with nitrogen is done, but traditionally it doesn't suffice to get the amount of hydrogen low enough.

 

I see two "oil diffusion pump" (search word) at eBay.com for 500 usd.

[John Cuthber]

You can aluminise a mirror at 5X 10^-4 mm Hg (0.5 microns)

http://link.springer.com/content/pdf/10.1007%2FBF00674994.pdf#page-1

so 10^-10 is overkill.

 

A decent mechanical pump can get close

http://www.yellowjacket.com/node/890

will get to 15 microns

 

Mg gettering should just about get to 0.15 microns by stripping everything but argon. (Of course this assumes a very good vacuum system with no real or virtual leaks. Ypu probably won't make one at home at the first attempt)

But, if you vac the chamber down with the pump, then back fill with nitrogen twice you should remove practically all the argon.

The air starts at about 1 metre pressure and is dropped about a million fold by the pump so that should take out all but a millionth of the argon.

Fill it with pure N2 and there's only a part per million or so of argon present. Repeat the process and, in principle you get to just 1 part argon in 10^12- obviously, the nitrogen will have more than that much argon as an impurity.

The residual pressure will depend on how pure the nitrogen was.

 

Other metals like barium would be a better getter, but are less easy to get hold of.

 

Hydrogen is often derived from moisture so a baked out system will help.

 

It's probably easiest to get someone else to silver the mirror.

[Enthalpy]

I don't know (or remember) in detail why we needed such a pressure for integrated circuits. I do remember that the substrates had to be warm to obtain uniform, smooth, well-connected aluminium. Possibly the layer would oxidize in real time if it grew in a moderate vacuum. 5*10^-4 torr is definitely easier to achieve.

 

Titanium was considered a good getter back then, as it also adsorbs hydrogen, in addition to its reaction with oxygen and nitrogen. A flame at low pressure would be difficult, but a filament or foil of getter metal can be heated by electric current.

 

As well, evaporation of the coating metal looks easiest in the shape of a filament heated by a current. This enables more refractory metals: gold.

 

Liquid nitrogen tends to be pure because many compounds are solid at that temperature, so it's more a question of circuit cleanliness after the nitrogen evaporates.

 

Is there a known easy process (wet chemical, like for silver) to deposit gold? Gold is the best mirror silver, if I dare to say.

[John Cuthber]

Semiconductor manufacture is a whole different ball-game. You do need UHV for that because you have to ensure that not too many "foreign" atoms end up in the working parts.

 

Titanium is a great getter, but it's melting + boiling points are much higher than Mg (though, to be fair, you don't need to melt it to get it to work as a getter.)

 

Argon has roughly the same boiling and freezing points as oxygen so it's quite a common impurity liquid oxygen, but there's usually more than a part in a million million in nitrogen.

 

There are wet process methods for gold, but, like silver, they only work well for back silvered mirrors and those don't get used much in telescopes.

[Enthalpy]

Maybe I had a wrong figure in memory with 10^-9 torr: websites cite rather 10^-6 torr for aluminium deposition. I suspect this isn't linked with purity, since silicon isn't sensitive to hydrogen, oxygen, nitrogen, argon, carbon (it's more gold, sodium and a few more which are unwanted) but with the spontaneous oxidation of aluminium during the deposition.

 

Getter: if it's a filament I don't worry about the needed temperature, which a current achieves easily.

 

What about household aluminium foil as a getter? At least it's inexpensive, large (wide access is necessary at low pressure), and some mechanism can move it between two rolls. A flat coil could be supplied with power at a reasonable impedance and induce the heating current in the foil.

[keegreil]

Hi all, thanks for the ideas! Sorry for the late reply, I've been in the middle of moving...

 

Enthalpy, I do not doubt at all that you needed a seriously good vacuum for semiconductor work. For a real scientific grade telescope mirror a super clean vacuum like 10^-9 torr may be desired. I was going for something much less impressive, I'm really just interested in testing the boundaries of what is possible for an amateur. Likewise, electron beam evaporation of the target Al is definitely the ideal method to eliminate contamination, but for an amateur attempt I think a simple tungsten filament or other crucible is easier. Interesting point about the substrate being heated, I haven't heard of that being done for telescope mirrors but I may look into it.

 

I agree oil diffusion pumps are simple in principle, I had read however (can't find my source now) that the oil for them is pretty specialized and can be rather expensive (i.e., not motor oil or olive oil, haha). That's why I was hoping to avoid those pumps. Lastly, gold is very reflective, but not as much as Al in the visible range. I understand it is the ideal material for Infrared mirrors (such as those used in CO₂ lasers).

 

Using a Mg getter is a great idea, thanks! I have a question that kind of goes back to my first post though. If the aluminum will not react with Nitrogen and remove it from the chamber, why exactly does the Nitrogen need to be removed at all? Is it that the aluminum would cool off when bumping into N₂ before it reached the glass?

 

I also really like the idea of a roll of Aluminum foil as a getter, I'm all for using household materials! Can you help me flesh out how exactly that process would work? I'm picturing a roll of foil lying next to a inductive heater coil, being stretched across the heater and rolled up by a motor on the other side. Is it alright to just be heating a small portion of the foil at a time? Or to work properly does the whole thing need to be heated at once? My guess is if done right, the foil would be rolled up and protected before cooling off, preventing the recently released molecules from reattaching right away. After the whole roll has been heated and re-rolled and the mechanical pump did it's work, slowly unroll it again while cold and wait for the vacuum to drop.

[Klaynos]

For the amature I'd have thought that sputtering deposition is more assessable. The evaporator systems I've used have used traditional vacuum pumps as a backing pump with either turbo pumps or differential pimps using liquid nitrogen to get to a decent pressure.

[Enthalpy]

I had in mind that big telescopes for visible wavelengths were coated with gold, but apparently it's aluminium or silver, protected by Si₃N₄. My bad.

Reflectance versus wavelength, from Wiki:

even metals looking very white, like platinum, reflect less.

[John Cuthber]

Thank you for the graph which confirms that gold is gold, while silver is silver.

[AbeMichelson]

For home hobbyist I would say there are two acceptable methods of making silvered mirrors:

1) electrodeposition

2) Tollen's reagent (ingredients can be bought through Amazon and shouldn't put you on a watch list). It's a fun experiment and worth doing either way. In undergrad we made Christmas ornaments using this and small round bottom flasks.

 

After you make your mirror, be sure to coat the backside (silver side) with some sort of coating (paint, acrylic...), otherwise the silver will tarnish.

Edited January 8, 2014 by AbeMichelson

[Enthalpy]

Err, expect no acrylic on a telescope mirror - their metal is on the front side. Recent ceramics for astronomy mirrors aren't even transparent.

[Sensei]

Thanks for the ideas everyone, I may look into actually trying to make a vacuum. Might be a fun project, if difficult. Does anyone have experience with creating vacuums?

 

One way is to fill hermetic container (with little hole on bottom) with distill water. You have to place it in even bigger container with water, so water will be able to escape on demand.

Then put pipes from setup where is done f.e. electrolysis of water, where is produced oxygen (on + electrode) to container.

Oxygen will push away water from container.

After that container is closed from bottom and contain very pure oxygen.

We now need to get ride of oxygen by burning some metal inside of this container.

Pass large current through wires, and metal will react with oxygen and burn.

Oxide of metal will remain inside hermetic container as dust.