Documenting the inception and build of the Heatsync Labs Scanning Electron Microscope
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 ;)
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…
HDPE is not the best material for a high vacuum, but it does OK under low vacuum…
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.
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.