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Dear All,
I have several questions for LaB6 microscopes users:
At Burnham we have been having problems with the scope ( FEI T12) when changing the filaments. We have worked with Kimball, Denka and Applied Physics.
In some cases we see dark current. Other times we get a very low emission for the beginning and sometimes the filament last only few days.
We go over 4 or 5 filaments before we find one that works fine.

-Have anybody had similar problems?
-is anybody following a particular procedure to avoid this?
-Could somebody give the values they get for a new filament: Emission, Saturation point, and life time ( hours)

-Is anybody lowering the heat of the filament when inserting holders? We have been told that may increase the life time of the filament.

-Lat week our beam was flickering and the illumination was fluctuating without changing anything in the scope. Anybody experienced anything like that?

Thanks to all.


Wendy

Ok firstly I apologise if this is too basic but I have no idea of your user level. All these operations should be done with powder free gloves, tweezers and generally vacuum operation.

I think it is near impossible that such a large number of LaB6 filament would be faulty so it is definitely the system. For a LaB6 filament to fail in a number of days would require an incredibly poor vacuum. What do the filaments look like under a light microscope when they fail?

Typically when the vacuum is < 5 x 10^-7 mBar (Log value of 27) the filament will last 1500+ hours if properly saturated. The rough saturation level of a LaB6 is around 25 on a T12 but this should easily be able to be found.

Flickering of the image if completely random and from what you are seeing with filament performance is High Tension discharging in the gun chamber. Can you hear discharging, see IGP vacuum bursts or sharp rises in the emission current?

I recommend the following:

Vent the gun chamber and take the wehnelt out. Inspect the gun chamber for any signs of arching (black streeks) or dust especially on the white insulator. If you see black marks on the insultor then call FEI for a service engineer to clean it.

Take the wehnelt apart and remove the 500um wehnelt aperture from the top piece and discard. Replace this with a spare 500um wehnelt aperture after cleaning. While this may seem clean LaB6 filaments leave a clear insulating layer that causes problems.

Now inspect the top part of the wehnelt for scratches and if there are any that seem deep you will need to polish this with "Pol" or "Wehnol". Wash with Acetone after polishing to remove the polish.

If the top has been polish or not clean the top thoroughly with Jif (Cif in some countries) under water. Rinse with water then place in 100% ethanol and sonicate for 10 minutes. Then sonicate in fresh ethanol for another 10 minutes.

After sonicating bake in a 80 degree Celcius oven for 30 minutes or dry with a hair drier then wrap in Aluminium foil.

Take the base of the wehnelt and sonicate in ethanol the same as the top. Dont use Jif as it can get trapped in the moving parts.

Place on a new 500um wehnelt aperture and assemble the wehnelt. Set the filament distance to 0.25mm (five minor divisions)

Before putting the wehnelt back in take a clean piece of small tubing and attach to the end of a vacuum cleaner. With the clean end of the tube vacuum the white insulator in the gun chamber by keeping the hose above the surface and not touching it.

Pump down the gun chamber. After pumping slowly condition the High Tension up to 120kV and leave it on at this setting overnight. I would actually recommend leaving the High Tension on at all times and turn the filament current off when not in use.

This should hopefully fix the problem if not then you need to place a service call with FEI as it could be a more serious problem.

It is a good idea to turn down the filament when exchanging samples but if you have LN2 on the column (cold trap) the IGP gun vacuum should not spike more than say 40 and drop back down quickly.

I hope this was helpful.