SPI Tungsten Hairpin
Brand Filaments and Electron Sources
Some easy to understand comments about tungsten hairpin vs. LaB6 sources for electron microscopes
This is not intended to be a detailed description of how the electron
sources in a microscope operate, but to outline the salient details.
Detailed descriptions abound in the literature and various EM reference
books (see for example. Reimer - Scanning Electron Microscopy Springer-
Verlag 1985)
The first thing to make clear is that the accelerating voltage of the
microscope (HV) does not have a great deal to do with electron emission per se.
There is an interaction of the filament, it's power supplies and the HV power supplies.
This is due to the fact the the filament and it's power supply must reside within the HV
tank and operate while electrically floating at the high end of the HV terminal,
also it is relevant to design the electronics to simplify operation and
hence there is some interconnections between the HV and the bias voltages.
The important point to keep in mind here is that the primary function of the
HV is to accelerate the electrons into the electron optical column. The HV
itself is not the source of the electron beam. It is the function of the
filament to be the source of these electrons.
There are 3 common types of electron sources
in use today: Thermionic emission, Partial Field Emitters, and Pure Field
Emitters.
In the case of a thermal emitter, electrons are emitted from the hot
material in accord with the
Richardson equation for thermionic (heated) emission. The function of the
filament (heater) current is to give the electrons in the source enough
thermal energy to over come the energy barrier which is
prohibiting their escape. All metals will give off electrons when heated
and the higher the temperature the more electrons are emitted, however
most
materials do not survive very long at the temperatures required for this
process to yield a significant amount of electrons.
Tungsten (W) has a sufficiently high melting temperature (3650K)
that it survives for a longer time at elevated temperatures ( ~ 2600- 3000K)
before it fails an it is the material of choice for a typical thermionic
emitter.
Another way to get emission is to decrease the "work function (energy barrier)"
which is limiting emission. Materials with lower
work functions emit at lower temperatures (LaB6 for example). These
materials are sometime referred to as partial field emitters since they
exploit the fact that their reduced work function enables their operation at
lower temperatures than W (~1400-2000K). The use of this "partial field
emitter" nomenclature is not rigorously true, since the primary excitation
mechanism is still thermal activation.
Finally field emission sources operate due to a decrease of their energy
barrier width by an applied field permitting quantum tunneling of
electrons through the narrowed barrier into "vacuum". This is a different
process all together than thermionic emission and is governed by the Fowler-
Nordheim relations. There are two types of Field emitters - Thermally
assisted (sometimes called Schottky) emitters and Cold Field Emitters. As
the names imply, the thermally assisted Field emitters are heated, while the
Cold Field emitters are not, both require the application of an external
field to "extract" the electron beam from a very small tip.
The electrons which are emitted from the filament regardless of the type
of emitter are "replaced" by virtue
of the fact that there is a connection to ground and the electron reservoir
in metals is a continuum, hence the filament remains overall neutral. If
this ground connection was not present, the material would become in effect,
locally charged, and the electron emission would start to decline, and
eventually stop (the local energy barrier would continually increase).
For the case of the Thermionic & LaB6 Guns, the filament, wehnelt cap and
anode from a triode system. By judicious design of the power supply the wehnelt is
made slightly negative with the filament and the resulting the electrostatic
field which acts as a small lens and produces a focussing of the
electrons which are emitted only from a limited region of the
hot tip of the W source. This voltage difference is usually called the
"bias" voltage
of the gun. In the Field emitters a multiple anode system is employed
instead of the triode configuration. Here one applies an extraction voltage
instead of the bias voltage. Bias voltages are usually a few hundred volts,
while the extraction voltage on a FEG is usually in the range of 3-5 kV.
The filament and wehnelt (or last anode for an FEG) are also connected to
the negative terminal of the HV supply. Thus, the beam of electrons which
leaves the wehnelt is accelerated by the
HV toward the nearest grounded anode, which is judiciously placed at the
base of the gun. This acceleration gives the electrons their Z (axial) velocity.
Some of these electrons make it through a small aperture in the grounded
anode which is the entrance to the electron column.
Once past the final gun electrode (ie. the one at ground potential ) the Z
acceleration of the electrons is essentially zero and they are traveling at
constant velocity (determined by the HV) until they hit the sample.
The function of the remaining lenses and deflectors in the body of the
microscope are to further deflect/focus this beam onto the specimen in
the manner chosen by the analyst.
Lastly you should not that the electrons are emitted from a thermionic
filament much the same way as they are in a light bulb. As in the case of
the light bulb, failure of the filament is usually due to mass
transfer/melting. This does involve movement of metal atoms away from the
point of failure (even though you are below the melting point). Over heating
the filament causes this to happen faster and thus decreased the filament
life.
The above information was taken from an E-mail reply drafted by
Dr. Nestor
Zaluzec to answer someone's question and we re-publish his E-mail comments here with
minor editing for greater clarity.
We wish to thank Dr. Zaluzec, Argonne National Laboratories for permitting
us to use this short description of the differences in the workings of the
different electron sources.
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Friday July 04, 2008
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