exert almost no pressure on the inside wall.
Removing more doesn’t change much. If
a container can withstand 10–1 torr, it can
probably safely hold 10–11.
a hole in the plate to fit the diffusion pump
and machined a water-cooled baffle to go
between the pump and plate (Figure C).
The baffle prevents diffusion pump oil from
migrating into the bell jar. Boiling oil gets
messy, and getting even small amounts of oil
into the sensitive parts of a SEM would cause
many problems. Air molecules can pass
through the baffle’s tortuous pathway, but hot
oil molecules condense on its water-cooled
surfaces and drip back down.
I cut another hole in the aluminum base
plate and added an additional vacuum monitor called a Penning gauge, also bought on
eBay for about $250. This device measures
vacuum from 0.001 down to 10–8 torr, and will
indicate when the diffusion pump has taken
the chamber pressure down to the range
necessary for SEM operation.
The first time I pumped down the jar,
I started the rotary pump, then exited the
garage and shut the door behind me. If the
jar imploded, I would be far enough away to
escape the wreckage. But below a pressure
of 0.01 torr, variations in pressure don’t much
affect the strength needed for a vacuum
chamber. This is a key point that often tricks
people. Once you remove 99% of the air
The Electron Gun
There are many ways of generating electrons
for an electron microscope, but the easiest
is to simply heat up a piece of wire. This goes
by the exciting name of thermionic emission,
and these filaments are used in vacuum tubes
and cathode ray tubes; they make the orange
glow inside the back of old TVs and radios.
From eBay I bought a set of tungsten filaments
with ceramic insulator holders that were originally made for use in commercial SEMs.
I connected the filament to a low-voltage
power supply that I built from a Variac variable
transformer, isolation transformer, bridge
rectifier, and smoothing capacitors. I originally
fed low-voltage AC to the filament, but that
resulted in image quality issues, so I designed
an unregulated but smoothed DC power supply.
Once the filament is glowing, it emits lots
of electrons in all directions. To motivate
them into a single direction, you need to apply
high voltages across pieces of metal strategically arranged around the filament. The
whole assembly is termed an electron gun,
and when the applied voltage is 10kV, my gun
C
Spark Plug Power
Ordinary automotive spark plugs are
designed to supply insulated high voltages
through metal walls and across pressure
differentials, so I used them to bring power
for the electron gun into the SEM chamber.
I drilled and tapped a series of holes in
the base plate to hold the spark plugs, and
added O-ring glands. I also made some low-voltage pass-through connections for other
circuitry using wide-head screws sealed to
the plate with Buna-N (nitrile) washers. And
to let users move a small stage to locate the
specimen under the electron beam, I added
spring-loaded teflon shaft seals that transfer
rotary motion through the base plate while
the chamber is under vacuum.
58 Make: makezine.com/31
