From emsuite wiki

The Life and Death of a Tungsten Hairpin Filament

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 prov
e 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!
Optimal Emission Currents

Optimal Emission Currents

Tungsten Hairpin Currents
ABS			100 microamps+kV
ISI			100 microamps+kV
Topcon			100 microamps+kV
Comscan			120 microamps
Hitachi			100 microamps
JEOL			100 microamps+kV
LEO			400 microamps
Cambridge			400 microamps (engineer adjusted)
Phillips			50 microamps
LaB6
LaB6			settings tend to be 25-50% of these values
FEG
FEG			between 8 and 20 microamps