High current power supply for vacuum sputtering deposition - project and questions. / High Voltage / Forums

Forums

4hv.org :: Forums :: High Voltage

« Previous topic | Next topic »

High current power supply for vacuum sputtering deposition - project and questions.

1 2 next

Move Thread

LAN_403

stamasd

Wed May 23 2018, 03:46PM

Registered Member #61903 Joined: Sat Nov 11 2017, 12:04AM
Location:
Posts: 8

Hi there.

Let me start with a preamble regarding my project. I've been planning to build eventually a Farnsworth fusor, and as part of that larger project I have studied high voltage/high power PS designs; I have an industrial Spellman 50kV PS that I have modified for higher current etc. However I have run out of funds (temporarily) for parts needed for the fusor, so in the meantime I am concentrating on other projects that don't require that much money. In particular I am doing some electrochemistry experiments for which I will need platinum electrodes. The commercially available platinum-coated titanium electrodes to the amateur are not quite up to snuff (platinum coating too thin) so I thought about making my own, in particular by sputtering deposition in vacuum of platinum onto a titanium plate. I have already started building the vacuum assembly and now I have come to the question of the power supply. (sorry for the long background rant but this is how my mind functions).

I won't need very high voltages, I figure at most 1500-2000V for ignition of the plasma and probably 500-800V for maintenance of the discharge. I will need high currents though, probably at least 400mA or up to 1A for my purpose. I also want to get this done as cheaply as possible for the reasons above.

I have some parts which I think would be appropriate for building such a beast. Maybe not with the parameters above, but at least it will be a start. I have 3 MOTs as well as HV diodes (MOD and other kinds) and HV capacitors. I plan on using 2 MOTs with primaries in antiparallel, grounded cores and the live wires in secondaries connected in a full-wave rectifier configuration and a filtering capacitor bank. Essentially this (ignore values in the image, this is not my picture , just linked here for clarity) but with the primaries in parallel of course, not series:
A7b00f1e22ac2d07837f64dee42f07e1

The input would come from a variac as I will need to control the output and likely underdrive the MOTs. I have a 30A variac which should be enough. Of course, full metering will be used (AC voltmeter on input, mA meter and HV meter on output) because I will need to know what the running parameters are in order to control them. I will probably use a 3rd MOT as ballast if the variac isn't enough to give a smooth operation. MOTs will be cooled with at least one big fan, or possibly submerged in mineral oil. Full HV precautions in use, and the setup will be completely enclosed.

Now, with this setup I don't expect to get more than 200mA, possibly 300mA at best in a stable manner for longer runs (10-20min or more). I have browsed all 101 pages of threads on this forum, and have found very little in terms of discussions about increasing the current output of similar setups. Again, it's not higher voltage that I need but higher current, and preferably in a sustainable manner for a significant amount of time. The MOTs I have are all different, and I can get more if needed but again they will all probably be dissimilar - thus simply parallelling the outputs would not work trouble-free.

Please comment on my plan, and I welcome any suggestions.

(edit) The Spellman PS I have mentioned above is inadequate for this purpose, even modded I only get about 9mA from it).

Edit2: here's a quick drawing on what I mean, better than the picture above (dropbox link):

2Spoons

Wed May 23 2018, 11:41PM

Registered Member #2939 Joined: Fri Jun 25 2010, 04:25AM
Location:
Posts: 615

Would it not be far simpler to electroplate a thicker layer of platinum onto a commercial platinized titanum electrode? Or even get a commercial plater to do it?

There's also DC magnetron sputtering, which doesn't need high power high voltage to work. An NST is sufficient.

Are you not concerned about coating the entire inside of your vacuum chamber with expensive platinum? Do you have a recovery process planned?

I applaud your effort to build your own sputtering system, but it seems to me to be a hard way to make electrodes.

stamasd

Thu May 24 2018, 12:17AM

Registered Member #61903 Joined: Sat Nov 11 2017, 12:04AM
Location:
Posts: 8

Thanks for the reply.

Yes, it seems a convoluted way to obtain what I want, but the pleasure is also in the journey not only the destination. :) Also this will be a test of my building skills for high vacuum and high voltage systems.

While I could start with commercial electrodes, those are not deposited on sheet titanium but on mesh. The mesh has big holes and that would lead to a lot of the sputtered material being lost and, as you mention, be deposited on the inside of the chamber. That will be minimized by using sheet titanium. Also for independent reasons I prefer to have my electrodes in sheet form rather than mesh.

I do have a recovery method, of course. Besides in the amounts I'll be using, platinum is not that expensive (I paid $40 for a 1g bar of pure platinum; one bar would be enough in theory to deposit onto about 1 square meter of titanium, much more than I intend to do).

DC magnetron sputtering is what I intend to use, with a home-made magnetron. But the 20-30mA that a NST provides will definitely not be enough to generate efficient amounts of plating. I have run some rough calculations, and at 30mA given the sputtering efficiency of platinum (about 1.2, rounded to 1 for simplicity) and the size of my titanium plates (10x10cm) it would take about 2h or more to coat one side of the plate with the minimum thickness I want; and that assumes 100% run time, which will not be the case (repositioning of the plates for optimal coverage, which involves repressurizing and pumping down the chamber each time etc). Therefore my opting for the higher current.

2Spoons

Thu May 24 2018, 01:36AM

Registered Member #2939 Joined: Fri Jun 25 2010, 04:25AM
Location:
Posts: 615

stamasd wrote ...

Yes, it seems a convoluted way to obtain what I want, but the pleasure is also in the journey not only the destination. :)

Fair enough. I built my own kayak for pretty much the same reason.

Have you considered a motorised platform to hold your substrate, to avoid repeated repositioning/ pump down cycles? Not trivial to do, but then this whole project is non-trivial, and you'd probably get a more even coating.
You might consider having two anodes: one high voltage/ low current one to initiate the plasma with the second to provide the higher current once the plasma is lit. There will be a limit to how much current you can have before the plasma collapses into an arc, there should be data somewhere for the pressure you intended to use.
The other things to look at are TIG welder supplies, which have a high voltage igniter for a low voltage high current arc - could be some good inspiration there. Maybe you could hack the supply from a used one.

stamasd

Thu May 24 2018, 08:12AM

Registered Member #61903 Joined: Sat Nov 11 2017, 12:04AM
Location:
Posts: 8

I'm hoping to get away without needing multiple anodes, probably by creatively manipulating the variac dial. :) This is also where a long run time would become tedious. I am worried about the plasma stability, but that will depend on how good a vacuum I can achieve with this setup. My goal for that is 100-150 microns. Hopefully my brand new dual stage mechanical pump can achieve that. I'd rather not complicate things by having to include my diffusion pump in the system - but if it comes to that it would be doable. I am using a lot of information from a PhD thesis I found online which while not directly discussing platinum sputtering, still gave me lots of information on magnetron geometry, current profiles, deposition rates, layer microstructure etc. Another source of inspiration was a series of videos made a few years ago by Ben Krasnow (Applied Science on youtube). My setup would be similar to his, only rougher as I don't have access to precision machining tools.

And another reason why I want a sputtering system is its potential for use in other cool projects.

Oh, and as for motorized platform I have considered moving my 3D printer skeleton inside the chamber, but that would require a chamber about 10x bigger than what I intend to use. Not to mention that it's not made from vacuum-friendly or plasma-safe materials.

About the TIG generator: it's an interesting idea, and I do eventually want to get one for its intended purpose. However right now it's not a viable option:
1. They all require a 220V connection and I don't have one; that would require modifications to my home electrical network
2. Even used they"re out of my current budget.

stamasd

Thu May 24 2018, 02:26PM

Registered Member #61903 Joined: Sat Nov 11 2017, 12:04AM
Location:
Posts: 8

Here is my magnetron design. I use Sm/Co magnets as they are more tolerant of higher temperatures than the more usual Nd ones. Also it's unbalanced on purpose with a higher magnetic field on the outside than in the center. For that a small disc magnet is used in the center, with polarity opposed to the outer ring. This helps focus the plasma better and reduce ion losses. Will need to experiment with exactly how strong that magnet needs to be for best results (in the paper I mentioned above they used an electromagnet but that's a complication I'd like to avoid).

In the picture, blue is copper for cooling. It will be water cooled. Will need an outside dielectric shroud to minimize losses by corona and other parasitic discharges. I'm thinking Pyrex or quartz glass should do (haven't sourced that yet). "Target" is sputtering parlance for the material to be sputtered (Pt in this instance). It is the "target" of the ion bombardment.

Dropbox link:

2Spoons

Fri May 25 2018, 05:10AM

Registered Member #2939 Joined: Fri Jun 25 2010, 04:25AM
Location:
Posts: 615

Interesting design. Would you mind posting a link to the thesis? I'd love to read it - building a sputtering system is on my (rather long) to-do list.

Another supply idea that might work well is something I saw many years ago for running a HeNe laser. Basic plan is to put a voltage multiplier on the end of your high current (AC) supply, but with lower value caps. While there is no plasma the multiplier will charge until ignition occurs, after which the load impedance is much lower than the multiplier impedance so the current flows straight through the diode string without any voltage boost.

So if you were to use a couple of MOTs for the main supply, with a variac, and probably some kind of ballast too, then add a weak multiplier (with fat diodes!) on the output of that, I think that would do what you want.

Thinking about it a bit more, what you really want is a fixed supply voltage with a variable ballast, since you are really trying to control the plasma current, rather than the voltage. Variac as a variable ballast? I reckon that could work...

stamasd

Fri May 25 2018, 12:21PM

Registered Member #61903 Joined: Sat Nov 11 2017, 12:04AM
Location:
Posts: 8

Here's the thesis (I had to search for it again because I downloaded a copy but hadn't bookmarked the site). It's actually a ME thesis, not PhD, my bad:

So the variac used as a variable ballast would have the output connected in series with the MOT primaries, and the input unconnected correct? Interesting.

I had thought of a doubler, but I'm concerned of high ripple (especially with small capacitors) which would make the plasma unstable.

2Spoons

Fri May 25 2018, 08:52PM

Registered Member #2939 Joined: Fri Jun 25 2010, 04:25AM
Location:
Posts: 615

I think you are going to get high ripple regardless - since the plasma has a negative resistance slope its going to dump the charge out of your smoothing caps pretty easily - and if you make your caps bigger I think you will simply encourage arc formation. The only way I see to stop this is to use a large ballast resistor on the DC side, but thats going to waste a lot of power.

Is ripple actually bad? Would it not help prevent arc formation?

stamasd

Fri May 25 2018, 09:50PM

Registered Member #61903 Joined: Sat Nov 11 2017, 12:04AM
Location:
Posts: 8

I don't know. I will find out. :)

How about a large inductor in the DC path? I'm not afraid to roll my own if I have to. In fact meet my little friend :D

Hvt

1 2 next

Moderator(s): Chris Russell, Noelle, Alex, Tesladownunder, Dave Marshall, Dave Billington, Bjørn, Steve Conner, Wolfram, Kizmo, Mads Barnkob