I believe we’re over the vacuum difficulties. She’s buzzing along at a nice deep vacuum in the SPEC, INT, and GUN chambers. Last night the computer finally booted up without the dreaded CHECK VACUUM alert.
Next up will be an extended 30 hour bake session to try to revive the emitter. I don’t trust her to run the heater unattended, so I’ll be staying over at the shop to keep an eye on things.
Keep your fingers crossed. With any luck I might have some images to post on Sunday evening!
At this magnification, the tip of the thread looks a bit like lumpy clay.
Can we get closer? Of course.
Steel is a decent electrical conductor, so it’s relatively easy to image. Insulators (like most organic matter) are a lot tougher to shoot without special processing (such as sputter coating).
Here is a seashell:
Here is the same shell at low magnification:
See the jets of “water” shooting off of the spikes to the left? That’s not photoshop, that’s physics.
As the electron beam scans the surface, the shell (being a poor conductor) accumulates charge. Over time the shell becomes more negatively charged. Like charges repel, so the electron beam is deflected.
The field in the shell will be strongest in the places with the strongest curvature (the tips of those spines). Since we’re using the beam to make the image itself, that’s exactly where the image will be the most distorted.
At higher magnification the problem only becomes worse.
I’m glad Meryl is finally taking decent (if not quite breathtaking) photos. If you could look at whatever you wanted under an SEM, what would it be?
Meryl was a little disheveled when she arrived at the shop. She had been in storage for a few years, and while mostly complete, she looked a little ragged around the edges.
Fortunately she arrived with a full set of schematics and manuals. Less fortunately, they look like 5th generation faxes and are written in a charming Japanese-English patois.
The first order of business was getting the column connected to the main console. I was extremely fortunate that the cable ends were meticulously labeled, so this was mostly a matter of plugging the proper connector into the proper board.
Dozens of twisted pair, ground strap, and coaxial connections later, I finally ran out of things to plug in. The umbilical cord connecting the two consoles was getting to be pretty impressive.
With everything connected and tidy, I was finally ready to power her up for the first time.
I carefully adjusted the variac to 100V and threw the main breaker. The lights dimmed, and a satisfying hum came from the machine. She lived!
And 20 minutes later, she died. The Vent and EVAC lights were flashing, meaning that a rough vacuum could not be drawn down to the point that the diffusion pump would take over.
Time to debug.
A poor vacuum could be due to a bad seal. But there were dozens of seals to check, and disturbing a perfectly working vacuum seal is just asking for trouble. At this point, I wasn’t even sure if the roughing pump was working as well as it should. I decided to start there.
Vacuum pumps really suck. But how much?
Meryl’s roughing pump is an oil-sealed rotary vane pump. It’s a pretty simple device, essentially a motor connected to rotor in an oil-filled chamber. The rotor has a series of vanes that grab a little bit of air at a time and compress it towards a discharge port. When the pressure is high enough, a valve opens and releases it to the atmosphere.
This pump was already leaking a little oil out of the bottom, but it seemed to run well enough. But was that well enough? I didn’t have a proper vacuum gauge handy, so I had to get creative to see if the pump was the source of the problem.
Experiment #1: vapor pressure of water
One simple test of the vacuum would be to see if I could boil water at room temperature. The vapor pressure of water at room temperature is around 17 Torr (regular atmosphere is around 760 Torr). If the pump could evacuate a chamber with a little water in the bottom and cause it to boil, then the vacuum must be at least that strong.
So the pump could make it down to < 17 Torr. But how much less? And would it be enough for the diffusion pump?
Experiment #2: use your brain
After some reflection, it occurred to me that I did in fact have a vacuum gauge handy– the Pirani gauge on Meryl herself!
By a stroke of luck, the gauge fit neatly inside the vacuum hose on the pump.
By connecting the gauge directly to the pump, I could bypass the entire microscope (with all of its potentially leaky seals) and see if the pump could pull hard enough to make the system happy.
The ever-polite and inscrutable manual informed me that TP#3 would show the voltage reading of the Pirani gauge… But what was the expected range? No word on that.
Digging through the schematics, I miraculously found a sticky note with some voltages scrawled in pencil indicating that the diffusion pump wouldn’t be happy until the Pirani read 2.5 volts or higher.
The pump could only pull it up to 1.8V.
Time for an oil change
Reasoning that the pump had sat for quite a while, it was probably long overdue for an oil change. But what kind of oil?
It had come with a mostly empty jar of TKO-19 Ultra Vacuum Pump Oil. Sounds expensive, no?
A quick Google around showed that it was not only expensive, it was discontinued. A couple of forum posts suggested replacing it with a more recent, equally expensive formulation.
I recently became the proud owner of a couple of discarded JEOL scanning electron microscopes. The big one (“Milly“) is a JSM-6320F from the early 1990s. The little one (“Meryl“) is a JSM-5600 from the early 2000s.
They sat in storage for about five years, and the previous owner had a lot of trouble getting them running again. That trouble has now passed on to me, and I’m in the process of restoring them to their former glory.
Where to begin
Of the two, Milly is a lot more technically interesting. She uses an FEG emitter (requiring an ultra-high vacuum to operate). She has an SEI imager and an X-ray backscatter detector, and was once capable of producing extremely impressive images at about 1nm resolution.
On the downside, she’s a little complicated. Her vacuum system uses two roughing pumps (not supplied), two diffusion pumps, and three ion pumps. Getting the vacuum down below 10-9 Torr requires perfect seals and a finicky bake-out process. Her power requirements are a little fancy. And being late 1990s technology, her “computer” looks like something used on one of the Apollo missions.
In addition to these challenges, I’ve never actually used (let alone worked on) an SEM. While Milly might eventually take stunning images, I have a feeling that the road to getting there may be a long one.
Lucky for me, Milly has a little sister.
And then there’s Meryl
Meryl is a much simpler SEM. She uses a thermionic emitter (a simple tungsten filament) rather than an FEG. She doesn’t require UHV, so her seals are simply rubber gaskets. There is only one roughing pump (provided!) and a single diffusion pump. She only needs 100V AC, which is easily converted from standard 110 with a supplied variac. She only takes up about half the space of her big sister. Best of all, she even came with a few spare parts, which considering my inexperience, I fully expect to install.
Watch this space for updates as the great microscope adventure unfolds.