SEM Buildblog

Documenting the inception and build of the Heatsync Labs Scanning Electron Microscope var _gaq = _gaq || []; _gaq.push(['_setAccount', 'UA-23025630-1']); _gaq.push(['_trackPageview']); (function() { var ga = document.createElement('script'); ga.type = 'text/javascript'; ga.async = true; ga.src = ('https:' == document.location.protocol ? 'https://ssl' : 'http://www') + '.google-analytics.com/ga.js'; var s = document.getElementsByTagName('script')[0]; s.parentNode.insertBefore(ga, s); })();

[hsl-sem]

Sad and excited to say that the HSL SEM project is on the back burner (but not on hold! Big difference!) while I send some things to the ISS.

BRB. (Go Antares!)

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My talk at EHSM 2012 Berlin! 

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TIG Welding for Fun and Profit

So I recently championed the acquisition of a nice TIG welder at the Heatsync Labs hackerspace (special thanks to Karl at K-Zell Metals for getting us such a good deal). I want us to always be able to make anything, even space ships, so I want us to have the tools we need, be it laser cutters or TIG welders. However, my real reason for wanting a good welder was to weld vacuum chambers, fittings, tee's, etc. 

I've since discovered that I'm not very good at welding. But I'm trying. Which illustrates exactly my passion for this project! The SEM project truly encompasses every conceivable form of fabrication that I can think of. Digital electronics, power electronics, high voltage, RF, analog front ends, welding, machining, vacuum physics, mechanical assembly, materials science, particle physics, electromagnetic  software design, human interface systems... the list goes on and on and on!!

Literally everyone who walks into the hackerspace has something, or more realistically, multiple things to offer this project. I have learned so many skills by passionately chasing this dream around for the past two years and for that I am so thankful! I am thankful that a creative community such as Heatsync Labs, and the entire maker movement in general, exists. This is something that didn't exist when I was little, and to be a part of it is awe inspiring. 

Whew! Yeah ok I'm done, for now ;)

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EHSM! on Flickr.

http://ehsm.eu

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it happens on Flickr.

Sh-- er Crap. Electron gun Mark II bit the dust because someone forgot to tell me that glass is brittle and not at all flexible. Who knew? But we just utter some colorful words and smile and make it again, amirite?

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Electron Gun Mark II

I kicked around a few ideas about how to build a free standing "developmental" version of the gun, with an improved rigid support structure. Also shows is an all-metal version which should prove to be better as a final version, though it sadly hides the entire discharge tube from sight and therefore is not a good candidate for a "development" item. Call it future idea #7633...

A quick search on http://outgassing.nasa.gov/ shows that HDPE is actually not bad at all, total mass loss (TML) is 0.22% per day and condensate (CVCM) was 0.04% of that. Low outgassing is defined as TML < 1% and CVCM < 0.1% and HDPE certainly qualifies for this, though its thermal properties leave much to be desired. Bake-out is not really possible with HDPE, but I am OK with this for now. 

A quick lathing operation and a cleanse with some brake degreaser and the gun is ready to go. Except I don't have a pic of the finished gun. Why don't I have a pic of that.... Hang on...

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Electron Gun Mark I on Flickr.

yes, I realize the tungsten electrode is off center. You try precisely lathe boring natural rubber! It kinda moves all over!

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A Better Cold Cathode Electron Gun

Time to make things better. The first prototype cold cathode electron gun used a piece of aluminum foil as a beam aperture. Talk about a not at all precise piece of optics. The new Mark I gun uses a lathe turned rubber stopper to support a piece of lathe turned tungsten and a small peiece of #32 AWG wire to bias a lathe turned aluminum anode with a 0.46mm aperture. 

Interestingly enough the electron spot is much larger even though the length of the exit aperture is longer (more tunnel, less hole in foil) but it IS larger in diameter. Not to worry because a larger divergence (high optical numerical aperture or N.A.) actually means a smaller spot size once we start using the lenses to focus things down. But it makes me wonder if there is some scattering off of the inside walls of the aperture bore. Hmm... 

But there are still problems. The rubber stopper is very difficult to center and likes to wobble. Its impossible to keep the electron gun on center and the spot wanders because of this. More than once the glass tube supporting the gas discharge fell off inside the beamline and had to be fished out after damaging the phosphor coated detector. Also, the aperture is jam fit to the glass and has caused the glass to crack more than once. Its a bad idea, the whole assembly really needs the be rigidly attached. Mark II is on its way already... But its still progress, and its still really pretty to see!

Oh, and please ignore my crappy attempt at a beam current detector. It really didn't work at all. Which makes sense considering how low the beam current is. 

[hsl-sem]

An Awful Horrible Deliciously Hacked Up Electron Lens

Its not pretty. (Is... is she using a... harbor freight clamp to hold the lens at the focal point? yikes...) I'm sorry, because, it is in fact indeed awful and horrible and hideous. Its arguably uglier than my current divorce... (hilarious! ... *crickets* ...*clears throat*) But you know what it is? 

Its a gap-solenoid electron lens! Its a baby step, and I might even go so far as to say that it is even more than just a baby step because it is an electron spot, and an electron spot is exactly what an SEM needs to be... well... an SEM! But its also a low-vacuum electron spot and totally affordable and hackable electron spot. Its.. its progress!

The lens consists of a piece of lathe turned 1018 steel. Its a spool shaped central piece which is cut in half to allow for a small 1mm thick brass washer to be inserted. The 500 turns of #20 AWG magnet wire is wrapped around either side of the gap washer and a sleeve of lathe turned 1018 steel is slid over and clamped onto the spool shape. The end result is a bit like a hollow donut with 0.070" thick walls and a rectangular cross section instead of the familiar circular one we all know and love and dip in coffee. Except its steel.

The resulting focal length is about 6 inches with 800mA of lens current at the 10kV acceleration voltage. The actual beam energy has yet to be measured and is surely less than this due to the voltage drop across the plasma used to create the beam. This is probably somewhat excessive and the number of turns should be doubled to reduce lens heating. But it works for now so I don't really want to touch it.

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Happy Accidents

What happens when water gets into the beamline during a leak check? Amazing accidental plasma art!!

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Cold Cathode Electron Gun Part 1

Please bare with me, this narration is coming from the past. I try to always take pics and document everything but life happens and the blarg doesn't get updated and then I have to do this :)

Using what I saw on Mr Styner's amazing and truly humbling website, I constructed a simple experimental electron gun using a copper wire in a capillary tube, a rubber stopper, and a turned stainless steel needle. Its really not even worth describing what I did because it looked horrible and didn't even work. It did make a nice discharge tube tough. Mmmm delicious nitrogen plasma... Several things were wrong, the polarity of the chamber vs the gun caused the plasma to extend all the way down the beamline. Also, it was an unfocused electron spray, not really a directional spot.

  With try number 2 I decided to actually put a little work into it. Ugh! You know how lazy I am... ;) I attempted to create an inverted design, so instead of shielding the accelerating anode wire I shielded the discharge area, with the tube within a tube design shown above. One of the huge problems with the stainless steel needle was the rate of sputter coat. The glass was almost black with metalization after only a short period of time, tens of minutes if that. I used a 1/16th inch TIG welding tungsten electrode for the cathode, and a piece of aluminum foil with a pinhole poked into it for the anode. The pinhole wasn't very round and I really didn't measure it... But oh well.

The supporting structure is a lathe turned rubber stopper. Yes, to my surprise you really can lathe natural rubber stoppers quite well! Go figure! I lathed a nipple for the small glass tube and a shoulder for the beamline tube. A 5C collet was used to hold the rubber stopper (mostly centered.. it is squishy rubber) and a hole was drilled for the main centered tungsten cathode. a small 30 gauge wire was pushed through the stopper for the anode. No epoxy was used, the rubber made its own seal which didn't seem to leak horribly.

 I applied about 10kV using a flyback transformer driving from a 12V 1.5A laptop power supply through a 10 megaohm 5 watt resistor to the cathode and I grounded the anode. A target was constructed using a stainless steel rod coated with powder from the inside of a CFL and placed into the beamline tube. 

I was nervous as all hell during pumpdown using only the roughing pump.. I could see the Faraday space of the discharge tube getting longer and longer and longer until it touched the anode... then I looked down and saw a faint orange (orange? what the hull? everyone knows electrons are green...) spot appear on the target!! It was... really off center, but who cares. As I watched it grew brighter and brighter and brighter until it winked out all together as the discharge went out. Oh no did I blow a power supply?!?! Nope, the vacuum just became too deep. No worries, shut off the pump, wait 5 minutes, hey its back again!

ELECTRON BEAM SUCCESS 

(Shh, I know its not focused yet, we'll get to that later!)

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Inspiration From an Old Hacker Hero

More about the cold cathode low pressure electron gun I spoke of in the last post.

Onward!!!

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Completed Vacuum System on Flickr.

Hey look, its a fully functional vacuum system!

What does that mean? That means that the turbo-pump and the backing pump have their own lines into the chamber. The chamber can be roughed, then switched over to turbopump control if needed.

So what's the plan from here? The plan is to develop two beamlines. The first beamline will use a cold cathode emission system which operates at low pressures and uses the residual air in the chamber and a HV DC source to form a plasma from which electrons are harvested and injected down into the optical system.

The second beamline will be a standard thermionic emitter using a tungsten hairpin filament and a high current DC source. The downside of this is that the system needs to be running at high vacuum which means turbopump. Turbopumps are nice, but they are not hackable. We can't go get one for less than the price of the entire completed system, and that's just not the point! 

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DV-6M with turbopump calibration and leak test on Flickr.

That's the power of pinesol turbomolucular pumps!

This plot shows the leak rate of the chamber over a 100 minute period after a quick pumpdown using the turbopump. It leaves much to be desired, but its not bad for never having been baked out or electropolished. 

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Steel machined Vacuum Valve Knob by hslphotosync on Flickr.

The Art of Scrounging and Hacking or Time vs Money.

It just so happened that these cheap high vacuum pneumatic valves were on ebay for $60 apiece. What a steal. Let's convert them to manual by adding a lathe turned knurled steel knob!

And of course, should you ever want to do this yourself here's what the steel knobbie looks like drawing on purple engineering paper. 

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As presented to Heatsync Labs, the home of the HSL SEM!

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Turbomolecular porn by hslphotosync on Flickr.

The turbomolecular pump has been attached! The largest flange on the chamber is a KF-50 for various reasons including cost of fitting hardware. Above this size, ISO fittings must be used and these are much more expensive than the KF style.

The turbopump that was donated to the project uses a 4.5" CFF conflat style metal knife gasket mount. 4.5"CFF to KF-50 adapters are several hundred dollars so it was decided instead to machine an adapter out of aluminum and use a square lathe-cut viton gasket instead of the soft metal gasket. The gasket was purchased from ANCorp for $14.50. The 3x5" aluminum stock was purchased from Industrial Metal Supply for $22. But of course nothing is free, the 10 hours or so it took to machine the piece is worth something right?! Luckily time IS free!

The profile of the piece was turned first, with the KF-50 end on one side and a square shoulder on the other. 

Though I suppose its not completely necessary when using viton rubber instead of metal as a gasket material, I attempted to machine the knife edge into the face of the CFF half. The circular sawtooth would normally bite into a soft copper gasket but of course that's not possible when using aluminum for the flange material as the aluminum is too easily deformed. Stainless is too expensive and I'm not sure I have the skill required to pull it off just yet... The knife edge came out OK, though slightly over sized. I must've been tired or something. 

Finally the hole pattern was added using a mill and a (horribly undersized) rotary table. A very long day at the lathe, and a somewhat tired yet nonetheless glorious victory!

Edit: If I made this piece again, I would probably have attempted to make it tapered from the CFF bore to the KF-50 bore. If I ever come across enough free time and another hunk of metal I might just try to machine another one and compare the pumping speed between the two.