Transcript Electron probe microanalysis EPMA - UW

UW- Madison Geoscience 777
Electron Probe Microanalysis
EPMA
Vacuum Systems
1/16/13
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What’s the point?
We need a high vacuum in the column and
chamber to maximize electron-sample
interactions (not electron-gas molecules). We
need a high vacuum in the gun to prolong the life
of the electron source and avoid arcing. We need
automatic microprocessor control and integration
of vacuum reading, venting, and gun and column
settings to avoid catastrophes.
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Key points
• Description of numbers of air molecules
with different pumps
• Rough vacuum pumps
• Diffusion and turbo pumps
• Ion pumps
• How vacuum is measured
Units of Vacuum
The two main units used to measure
pressure (vacuum) are torr and Pascal.
Atmospheric pressure (STD) = 760 torr or
1.01x105 Pascal.
One torr = 133.32 Pascal
One Pascal = 0.0075 torr
An excellent vacuum in the electron
microprobe chamber is 4x10-5 Pa (which
is 3x10 -7 torr)
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Why the fuss?
Anthony Buonaquisti wrote an excellent article “If
you hate vacuum systems, read on” published in
Microscopy Today. No one got involved in electron
microscopy in order to learn about vacuum science. But the
equipment we use responds to poor vacuum practice with
poor vacuum quality -- which translates to equipment that
doesn’t work well, or doesn’t work at all.
In the following table, he demonstrates the
magnitude of the presence of gas molecules, which species
dominate at different pressure ranges, the vacuum we
achieve with different pumps, and the average distance
between molecules colliding with each other (MFP).
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Vacuum Regimes: What’s what
What
# of gas
molecules
present
Atmospheric
Pressure
10 gas
Mechanical
Pumping
1014 gas
Diffusion
Pumping
10 gas
Ultra High
Vacuum
10 gas
Dominant gas
species
19
molecules/cm
molecules/cm
H2O>N,O
MFP = 1 cm
10-3 torr
H2 O
MFP=10 cm
H, He
MFP=10 m
760 torr
3
3
9
molecules/cm
Pressure
N>O>H2O
Mean Free
Path (between
collisions)
MFP=0.1 um
5
10 torr
-6
6
10
3
6
molecules/cm3
-10
torr
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Pumps
Electron microprobes (and SEMs and
TEMs) all have similiar pumping systems, being
combinations of at least 2 different pump types.
To go from atmospheric to moderate vacuum,
rough vacuum pumps are utilized. Once the
chamber is pumped to a level of ~10-3 torr, high
vacuum is acheived via either a diffusion or
turbo pump. Some instruments (e.g. Cameca)
include additional (“differential”) pumping for
the gun, via an ion pump.
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“Rough vacuum” pump
Gas molecules from the
volume being pumped diffuse
into the space between the
rotor and chamber case, and
are compressed by the rotating
rotor until they have a pressure
high enough to force upon the
exhaust valve. They then exit,
through the oil, to the outlet
port.
Oil-sealed rotary-vane rough vacuum pump
Bigelow, Fig 4.1, p. 135
Gas molecules in the pressure
range here (from atm down to
10-2 - 10 -3 torr) move via
viscous flow.
Rough vacuum pumps serve several functions: to “rough”
out chambers vented to atmosphere, and also to “back”
higher vacuum pumps (e.g. diffusion pumps).
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Oil Diffusion Pump
Bigelow, Fig 5.1& 2, p. 173
Bigelow suggests this well-known pump might
be better called a “vapor jet pump”. High
molecular mass oil is heated and moves
vertically at 300-400 m/s, compressing against
the jets any air molecules that have diffused into
the vicinity. The oil molecules and now attached
air molecules fall downward, cooling to a liquid
against the water-cooled outer jacket. There is
thus a build up of air molecules in the lower
region, adjacent to the port that is attached to a
second pump (e.g. rotary-vane rough vacuum
pump), which then remove these air
molecules.The pressures (to 10-7 torr) this pump
operates at are appropriate for the gas molecules
to move by molecular flow (not viscous flow) -leading to backstreaming of oil vapor
(explained later)
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Molecular flow vs viscous flow
Initial pumping of volumes exposed
to the atmosphere proceed through the
viscous flow regime, where there are so
many gas molecules that their mean
free path (MFP) is so short that they
collide more readily with each other
than with the walls of the tube. They
move as a mass in the general direction
of low pressure.
When gas pressure drops enough
that the MFP is greater than the
internal tube diameter, individual gas
molecules do not encounter other gas
molecules necessarily moving in one
direction (to low pressure). Rather, in
this molecular flow regime, the flow of
gas in independent of pressure
gradient, and depends mainly on
tube dimensions and molecule
speed (~temperature). In this
case, backstreaming of molecules
into the high vacuum chamber is
possible.
Bigelow, Fig 2,1, p. 31
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Backstreaming
Backstreaming refers to the
movement of gases (including
pump oil vapor) from pumps into
the vacuum chamber. It can be an
important issue with diffusion
pumps.
Design of diffusion pumps
can make some difference.
Placement of a continuous
operation cold plate over the
diffusion would be the best
solution, but it is rarely included
in microprobe design.
Oil diffusion pumps have a
long history and are
considered by many to be less
costly and easier to use in a
multiple user facility.
However, the alternative is the
turbo pump.
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Turbo Pumps
Turbomolecular pumps use
no oil (though they may have
greased bearings) and operate
like jet engines. Momentum is
imparted to gas molecules by
disks rotating at very high
speeds. Gas molecules
randomly entering the entrance
collide with the spinning rotor
blade, and are propelled toward
the pump’s exhaust vent.
Turbo pumps can reach 10-7 to
10 -10 torr.
Turbo pumps are nearly
free of oil backstreaming (if
certain operating procedures are
carefully followed), as the high
molecular mass oil vapor is
compressed to a ratio > 1040 ,
versus values of 1010 for
nitrogen.
Bigelow, Fig 6.1, p. 229
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Scroll Pumps
However, to eliminate
ANY possibility of oil
backstreaming, oil
rough pumps need to
be replaced by oil-free
pumps. Scroll pumps
are one such pump.
A scroll compressor
uses 2 interleaving
scrolls to pump
gases. One of the
scrolls is often
fixed, while the
other orbits
eccentrically
From http://www.anest-iwata.co.jp
without rotating,
thereby trapping and
pumping or compressing pockets of gas between the scrolls. (wikipedia)
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SEM vacuum setup
This is a typical vacuum setup, with one
high vacuum pump (diffusion or turbo) and
one rough pump, and a series of valves.
1) Initial pump down: V1 and V2 closed, V3
open, and chamber, manifold and gun
pumped out.
2) When chamber pressure low enough, V3
closes, V2 opens and roughs out the
diffusion/turbo pump.
3) When the pressure in the diffusion/turbo
pump is low enough, V1 opens and pumps
out the chamber and gun to the high
operating vacuum.
Not shown is an airlock chamber that would
have its own vacuum tubing and valves.
Bigelow, Fig 2,6, p. 42
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Ion pump-1
Ion pumps are used for ultra high
vacuums; they might also be called
‘Getter pumps.’ They consist of short
stainless steel cylinders (anodes) between
two metal (Ti, or Ti and Ta) plates
(cathodes), all sitting within a strong
magnetic field parallel to the cylinder
axes.
A high voltage is applied between
the anodes and cathodes, with the
resulting electrons from the cathodes
moving in long helical trajectories
through the anode tubes, increasing the
probability of collision with gas
molecules.
magnets
element
housing
old
cathode
new
anodes
new
cathode
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Ion pump-2
Pumping of gas molecules in an
ion pump occurs by 4 mechanisms:
1) Surface burial: Ionized gas
molecules then accelerate into the
cathode, sputtering Ti everywhere. Gas
molecules on surfaces are buried by Ti
atoms. This is the main pumping
mechanism.
2) Chemisorption: Reactive gases (O,
N, CO and H) react with fresh Ti to
form stable oxides, carbides, nitrides
and hydrides.
3) Ion implantation occurs when the
electric potential gives some positive
gas ions sufficient energy to penetrate
the cathodes.
4) Neutral atom implantation: some
gas ions strike the cathode and gain
electrons, becoming neutral atoms.
If they rebound with enough
kinetic energy, they will become
buried in the surface of the anode
or opposite cathode.
One problem with some ion
pumps is “Argon instability”,
particularly if there is a leak of P10
detector gas (90% Ar) into the
chamber.
Bigelow, Fig 7,1, p. 277
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Measuring vacuum
torr
No one gauge can measure pressure
from atmosphere to UHV. Different
gauges are used to measure vacuum
over different pressure segments.
There are 3 basic mechanisms:
1) Mechanical use diaphragms that
change position due to force of the
gas molecules.
2) Gas property gauges measure a bulk
property such as thermal conductivity or
viscosity.
3) Ionization gauges operate by
measuring the current flowing across
ionized gas molecules in the gauge.
Left image from Bigelow; Right image from Physics Today advertisement (MKS = company)
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Thermacouple Gauges
These gas property gauges
find much usage in our
instruments. A thermocouple
measures the temperature of a
heated wire inside a tube. As
the number of gas molecules
hitting the wire (and thus
conducting heat away from the
wire) decreases as the pressure
decreases, the temperature of
the wire increases. As
temperature rises, the voltage
generated by the thermocouple
increases. This is calibrated
and gives a precise reading.
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Ionization Gauges
Cold cathode ionization (aka
Penning) gauges do not have
filaments, and rely on an external
event (cosmic ray, radioactive event)
to start the action. Once started, the
magnetic field constrains the
electron to a long helical path with a
high probability of of ionizing gas
molecules. The current that flows
across the gap between anode and
cathode is measured with a sensitive
microammeter calibrated in pressure
units (as less molecules ionized, the
current is lower).
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RGA
Residual gas analyzers are specialized mass
spectrometers, used to detect and quantify the gas partial
pressures in a vacuum chamber. They may be of the
magnetic sector design, or quadrupole design (above). Gas
molecules are ionized, and then accelerated into an ion
detector, separated by their mass-to-charge ratio (m/z)
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A snapshot of the vacuum
in our SX51 chamber
Residual Gas Analyzers
(RGAs) are rarely utilized
in electron microprobes.
They are valuable for
diagnosing vacuum
problems, as well as giving
us an appreciation that a
‘vacuum’ is full of gas
molecules. The fact that
the N (28) peak is 10x the
water (18) peak indicates
that there is a minor but
significant leak of room air
into the chamber. Normally
immediately after pumpdown,
N drops quickly with H2O
N=28,14
H2O=18
O=32
H=2
Ar=40
Oil=43
RGA spectrum from April 2000; x-axis is M/Z
being the dominant gas, and then H2O slowly drops too.
From prior records of ‘good vacuum’, we can deduce that
the atm gases are at least one order of magnitude too high.
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References
Many vacuum product
distributors publish
catalogs which include
excellent descriptions of
vacuum system
operations.
One I find particularly
useful is Kurt. J. Lesker
Company, and their
catalog is also online at
www.lesker.com