[time-nuts] Maser info (vacuum levels)

Mark J. Blair nf6x at nf6x.net
Fri Sep 3 07:25:44 UTC 2010

On Sep 2, 2010, at 9:34 PM, jimlux wrote:
> maybe 1E-6 micron (1E-9 torr)..

Maybe that's closer to what it was, and I just misremembered the value. It's been a long time, and that was my only experience with high vacuum equipment.

> Sounds about right.. the mechanical pump will pull it down to a few microns in a minute or so (I assume it's like a bell jar with maybe 50 liters total volume?)

As I recall, there was a machined stainless steel lid with a glass viewing port over the evaporation chamber, then a large bell jar lowered over that. The heating filament was pretty substantial, and we'd run around a hundred amps through it as I recall, controlled by dialing up a large variac while watching a current meter. The filament was near the bottom of the chamber, with a perforated stainless steel plate over it. We'd place the wafers to be plated face-down over holes in that plate, and also set a glass slide in a designated spot on that plate. The viewing port allowed the filament to be viewed through the glass slide, and we'd control the amount of metal to be deposited by turning off the heat once the glowing filament was no longer visible through the metal deposited onto the glass slide.

As a side effect, the used glass slides made nice gold- or aluminum-plated first-surface mirrors. Rumor was that they'd recycle the glass and discard the gold, which was too thin to be worth salvaging! That may be a tall tale, though.

> Lots of interlocks to keep you from doing dumb stuff (e.g. venting to atmosphere with the diff pump hot and connected), actually not all that dirty.. you probably weren't sticking complex mechanical stuff in there.. basically a wafer that you'd put next to the evaporator source.  So no issues with virtual leaks, etc.

Yes, just a few wafers and a glass slide on the perforated plate, and a small bit of metal wire on the filament. We weren't burdened with maintaining the machine (aside from topping up the LN trap), but I'd guess that the teaching assistant would have to clean the chamber occasionally, i.e. to remove goopy photoresist residue from the perforated plate. I don't know whether the evaporator was made on-site or purchased, but I would guess that it was made on-site.

> At work, we've got tons (well, tens) of these little evaporation workstation things.. A rolling cart about a meter by half a meter, and a meter high, with a bell jar on top.  A mechanical two stage pump and a 3" diff pump under the plate.  A couple of feedthroughs for current to heat the evaporation source.  A couple toggle switches, a ion and a thermocouple gage..  We don't use the for evaporating metal (at least I and the folks in my section don't)... we use them to test electronics under vacuum..

Cool. It'd be funny if the cleaning crew surreptitiously plated their hubcaps and stuff late at night...

Mark J. Blair, NF6X <nf6x at nf6x.net>
Web page: http://www.nf6x.net/
GnuPG public key available from my web page.

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