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The Life and Death of a Tungsten Hairpin Filament | |
Diagram 1 Diagram 2
Diagram 3
Diagram 4
Diagram 5
Table 1
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Many
words have been written about filament saturation and filament life, not
all with years of experience behind them!
Sometimes when visiting laboratories one has the feeling that the
life of the filament is far more important than the quality of the
results, so let us try to demystify filament life? The
life of a tungsten hairpin filament very much depends upon its use, the
applications in a particular laboratory.
Medium to low magnification applications in the transmission
electron microscope (TEM) may only require low emission currents (10 to
15 micro amps) with a long filament life resulting.
High resolution studies require higher operating currents (20 to
45 micro amps) and when demanding more emission the life of the filament
will suffer. Whilst a long
filament life is good for the laboratory it is not worthy of boasting,
for those who have a short filament life may well be using the
instrument near to the limit of its performance, a far more relevant
boast! Similar
situations in filament life relate to the scanning electron microscope
(SEM). However where a
hundred plus hours may be possible in a TEM used at low magnification
levels (<30,000X), one would expect 60 to 70 hours in a SEM used at
low levels of magnification (<5,000X) or for analytical work.
At high resolution or at low accelerating voltages, constantly
pushing the instrument to its limit, SEM filament life may well be only
15 to 20 hours. So
why do we have these differences? It's
all about filament position and emission.
The more emission you demand from the filament the hotter it
needs to be to reach saturation, the result a higher the level of
oxidation and evaporation and a shorter filament life.
However whilst you cannot have something for nothing it is
possible to obtain even more from your filament if you do run it hard.
Diagram 1 demonstrates the change in performance of a typical SEM
when the filament to cathode cap position is varied.
As resolution is improved filament life is lost, but through
optimisation you are able to make a spectacular change to most SEM's
performance. Make similar
changes to a TEM gun, through shorter filament to cathode distances, and
you have the opportunity for smaller apertures in the condenser system
and smaller spot sizes enabling higher coherent images with a visible
improvement in image quality. So
how do you saturate the filament? I
believe we would all understand this procedure a good deal better if we
called it "saturating the cathode", because that is what we
are doing. Most TEM
operators heat the filament whilst watching the image of the virtual
source which we see at condenser crossover (Diagram 2).
Many believe that one needs to oversaturate, overheat the
filament, to run on the flat part of the graph (Diagram 2).
The original reason for this was to compensate for filament
supply instabilities, I believe this is not necessary today.
Take a look at the virtual source of your TEM at 50,000X
undersaturated, watch it for some time, if the instrument is working
correctly it will not change at all; who needs to oversaturate?
As an ex demonstrator but still a TEM operator I do not saturate
completely, I leave just a few dark spots in the source for a deliberate
reason. If you are able to
see the edge of the beam as you bring it to condenser crossover it is
easy to see that condition. But,
higher up the magnification range it is not possible to see the edge of
an aligned beam, but the dark patches that undersaturating places on the
image are an excellent indication that you are at condenser cross over.
Why is this so important? High
coherence is almost always a TEM operator's objective and the best way
to do this is to run the condenser system in an overfocus condition,
turn clockwise from condenser crossover! In
the SEM to obtain any level of performance from a hairpin filament you
need to saturate fully. It
is no good running at first peak as the source is too large and you
sacrifice too much probe current using the condenser lenses to obtain a
suitable sized probe for high performance.
Once again there is no substitute for what I believe are the
correct emission current figures set out here in Table 1.
The "correct emission" is attained either through bias
adjustment, or in an ideal situation, through moving the filament
forward with the bias in its central position. No matter how you use your microscope the filament will eventually break giving you the opportunity to analise the filament and its ceramic. In
the TEM a normal break occurs to one side of the tip, the break should
be between two tapered ends (Diagram 3 (A)).
In the SEM we drive the filament much harder so that a normal
break may well have blobs of metal at the point of failure (Diagram 3
(B)). Never
panic with small blobs at the filament break in a SEM, if an operator is
unlucky this can happen during saturation to the most experienced of us.
In the
TEM an overheated filament will break as above, except the ends may not
taper, they will either be in the form of blobs of metal, or if the
blobs have fallen off, two blunt ends (Diagram 3 (C)).
A similar situation may occur with SEM filaments.
A filament blowing through high voltage discharge will have its
end blown away, fortunately only seen in an unhappy TEM! (Diagram 3 (D)) A
filament effected by a very poor vacuum will break due to severe
oxidation. The filament will
seem to break with the normal taper, but its life will have been very
short. You will notice how
the filament is very thin, far thinner than for a normal break (Diagram
3 (E)). Even if the gauges
indicate the vacuum is good, this is not often a true representation of
the actual gun vacuum, the ceramic will tell us even more. If
the filament has been carefully saturated in a TEM, not overheated, the
base will be a light blue in colour (Diagram 4 (a)).
The colour comes from a light coating of evaporated tungsten.
If the filament has been run very hard (typical of a SEM), either
due to overheating in error, or due to the filament being placed very
close to the cathode in order to obtain improved emission levels, the
ceramic will be a dark blue in colour (Diagram 4 (b)).
An orange to brown coloration is due to contamination, the
filament had been operating in a poor vacuum environment (Diagram 4
(c)). This coloration would
be expected if the filament had failed due to excess oxidation. There
are other reasons for a short filament life; namely that caused through
poor quality wire being used to make the filament. From time to time the wire used to manufacture filaments is not up to standard, it contains a level of garbage; no criticism of the manufacturer of the filaments, more a criticism of the wire maker! In these circumstances the filaments thin normally but a good deal quicker and with far more "evaporated materials". Diagram 5 demonstrates the build up of contaminants within the cathode due to poor filament quality. As many of you will know I have worked with several electron microscope manufacturers and every few years or so this type of problem crops up. My advice to EM operators is NEVER use a complete box of filaments that have served you well; always save two. Then in the situation that we now describe you could pop in one of a good box to prove if you have a problem, or that the new box of filaments is of a poor quality. So how should you proceed – 1. You need to run another filament from the new/problem box with great care, keep an eye on the operators to make sure you do not have an over saturation problem with one of them 2. If the problem is repeated replace with a filament from a good box. 3. If the problem continues you probably have a leak in the gun area, forget what the gauges say unless they are actually in the gun area. 4. If the problem went away you have proven the new filaments to be at error, return them to the supplier with an explanation of your problem 5. Other areas of investigation if this does not solve your problem are- a) The colour of the ceramic - yellow/brown suggests a vacuum leak b) A smelly gun chamber - smells of oil/ozone - suggests a vacuum leak c) Normal filament break but THE WHOLE OF THE FILAMENT HAS THINNED, not just the tip - gas attack - suggests a vacuum leak So
there we are, in terms of filament life you get what you pay for!
To work with low levels of intensity in the SEM and TEM may be
fine for some. However if
you want to obtain more from your microscope this will almost certainly
be at a cost - that cost will be filament life! DiagramsNo.1
Filament Life, Position and Performance No.2
Filament Saturation No.3
Filament Breaks No.4
Filament Base Colours No.5
Contaminated Cathode Apertures TableNo.
1
Optimal Emission Currents |
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