Transcript No Slide Title

UofO- Geology 619
Electron Beam MicroAnalysisTheory and Application
Electron Probe MicroAnalysis (EPMA)
Electrons:
Instrumentation and Theory of
Electron Solid Interactions
Electron Microprobe Instrumentation
What Makes a Microprobe?
•High Stability Electron Source
•Focussing WDS X-ray Optics
•High Precision Stage
•Reflected Light Optics
•Beam Current (Faraday Cup)
Electron Microprobe Instrumentation - cont’d
Vacuum: What’s the point?
•Column and chamber vacuum
to maximize electron-sample
interactions (not electron-gas
molecules).
•Gun vacuum to prolong the life
of the electron source and avoid
arcing.
•Automatic microprocessor
control and integration of
vacuum reading, venting, and
gun and column settings to
avoid catastrophes.
Electron Microprobe Instrumentation - cont’d
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)
Electron Microprobe Instrumentation - cont’d
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.
“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.
Gas molecules in the pressure
range here (from atm down to
10-2 - 10 -3 torr) move via
Oil-sealed rotary-vane rough vacuum pump
viscous flow.
Rough vacuum pumps serve several functions: to “rough” out
chambers vented to atmosphere, and also to “back” higher
Bigelow, Fig 4.1, p. 135
vacuum pumps (e.g. diffusion pumps).
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)
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.
Backstreaming
•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.
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.
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.
Ion pump-1
magnets
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.
element
housing
old
cathode
new
anodes
new
cathode
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
Measuring Vacuum
No single gauge can
measure pressure
from atmosphere to
UHV. Different
gauges are used to
measure vacuum over
different pressure
segments.
1)
Mechanical use
diaphragms that
change position due
to force of the gas
molecules.
2) Gas property
gauges measure a
bulk property such as
as thermal conductivity or viscosity.
3) Ionization gauges operate by measuring the current
flowing across ionized gas molecules in the gauge
Thermocouple 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.
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).
Electron-Solid Interactions
Beam Penetration
•Beam penetration decreases with Z
•Beam penetration increases with energy
•Electron range ~ inelastic processes
•Electron scattering (aspect) ~ elastic processes
Elastic and inelastic scattering
(a) Elastic Interactions
•Backscattering of electrons (BSE)
(b) Inelastic Interactions
•Plasmon excitation (in metals, loosely bound
outer-shell electrons are excited)
•Phonon excitation (lattice oscillations, i.e.,
heating)
•Secondary electron excitation (SE)
E0 = accelerating voltage
(of electrons emitted from
gun); usually 5-25 keV
•Inner-shell ionization (Auger electrons, Xrays)
•Bremsstrahlung (continuum) x-ray
generation
•Cathodoluminescence radiation (non-metal
valence shell phenomenon)
Scattering lexicon
•Cross section: a measure of the probability that an event of a
certain kind will occur, e.g. K-shell cross section.
•Q = N/nint, where N=events of certain type/vol (sites/cm3),
ni=number incident particles/unit area (particles/cm2), and
nt=number target sites/vol (sites/cm3).
•Q has units of cm2 and is thought of as an effective ‘size’ which
the atom presents as a target to incident particle. The Q for elastic
scattering is ~10-17 cm2 and for K-shell ionization is ~10-20 cm2.
•Mean free path: average distance an electron travels within a
specimen between events of a specific type.
•MFP=A/(NArQ) where A is atomic wt (g/mol), NA is Avogadro’s
number, r is density (g/cm3).
Backscatter Electron Production
Elastic process
Backscatter Electron Detection
In-Lens and Energy Selective BSE
A solid-state (semi-conductor)
backscattered electron detector (a) is
energized by incident high energy
electrons (~90% E0), wherein
electron-hole pairs are generated and
swept to opposite poles by an applied
bias voltage. This charge is collected
and input into an amplifier. (b) It is
positioned directly above the
specimen, surrounding the opening
through the polepiece.
Secondary Electron Production
High keV beam electron
Virtually identical keV
Few eV secondary electron
SE imaging: the signal is from the
top 5 nm in metals, and the top 50
nm in insulators. Thus, fine scale
surface features are imaged. The
detector is located to one side, so
there is a shadow effect – one side
is brighter than the opposite.
Pollen
Secondary Electrons
Inelastic scattering of HV beam electron can promote loosely
bound electrons from valence to conduction band in
semiconductor or insulator with enough energy to move thru
the solid (in metals, promotion from conduction-band
directly). Backscattered electrons can also produce secondary
electrons.
By definition,
these secondary
electrons are
<50 eV, with
most <10 eV.
(Goldstein et al, 1992, p. 107)
a) Complete energy distribution of electrons emitted from target.
Region I and II are BSE, Region III secondary. b) Secondary
electron energy distribution, measured (points) and modeled (lines)
Characteristics of Secondary and BSE Electrons
Energy distribution of all electrons
emitted from specimen under keV
electron bombardment:
SEs (eV)
SE: Topographic
BSE: Compositional
BSEs (keV)
N(E/E0)
E/E0
SEs are VERY low energy electrons!
Cat flea
Cathodo-luminescence:
Zircon CL image from a ultra high pressure rock (China)
Auger electron spectra of silver with an
incident beam energy of 1 keV.
Derivative and integral spectra are
compared (after Goldstein et al. 1981)
CL image of showing growth zones in a zircon
X-ray Generation
Fluorescent Yield:
Electron Range:
Mean Free Path (electrons)
At very low energies, the
electrons do not have a very
efficient way of losing energy
to the crystal lattice, so the
mean free path is long.
At very high energies, the
electrons are moving so fast
that they literally "zip by" the
other atoms so fast that the
electrons of the stationary
atoms do not have time to
respond (or get excited).
At intermediate energies (on
the order of 50 eV), the
incident electrons can very
easily lose energy by creating
electronic excitations or
ionization events in the solid,
and the mean free path is very
short.
It all ends up as heat eventually!
Material
obsidian glass
zircon
quartz
calcite
mica
iron metal
epoxy
k
0.014
0.042
0.10
0.05
0.006
0.80
0.002
T oK (15 keV, 0.02 uA, 2 um)
51
17
7.2
14
120
0.9
360
T oK (20 keV, 0.05 uA, 2 um)
171
57
24
48
400
3
1200