Author Topic: LaB6 on SX100  (Read 1442 times)

Philipp Poeml

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LaB6 on SX100
« on: February 12, 2016, 10:00:42 am »
Dear all,

Is anyone of you using a LaB6 cathode on a SX100? Or does anyone know someone and get me in touch? I would like to try this adventure. I just mounted it and I am playing around with it. But I am lacking experience in what emission current to use and what lifetime to expect. And I would prefer to run it slightly undersaturated.

There is a way from Cameca to calculate the heat value for 1800 K cathode temperature. They also say, to drop the temperature by 100 K one would need to lower the heat by about 20.

So, as I have no experience with LaB6 whatsoever, I would be grateful for any comment.

Cheers
Philipp

Mike Jercinovic

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Re: LaB6 on SX100
« Reply #1 on: February 12, 2016, 01:33:07 pm »
Hi Philipp
The quick suggestions...We generally have been using CeB6 on the SX-Ultrachron, but have used LaB6 also.  We think we get better lifetime out of the CeB6 (three years or so) compared to LaB6, but our statistics are not so good having had only a few of them in there.  Generally we bring up a new one at 85% of the optimum heat value, and bring it up to maximum over about six months.  Of course, there is no need to ever bring it up to the optimum heat value unless you need that resolution.  Always leave the heat on at your operating value unless you have to vent the gun for some reason, so just HV off for sample exchange.  For emission current, there are some variables (hardware, firmware, Wehnelt-tip distance).  Start at biasing to give 100uA, then align and adjust manually to see if you can find a maximum output, then leave it there for awhile.  Good center brightness in gun scan high and gun scan low images is a good target, but at "undersaturated" conditions, you may get some off-axis emission.  There is probably a lot of latitude here if you are not so interested in maximum brightness and spatial resolution.  If it is stable and image quality is good = success.

Now, here is maybe more information than you want...Our situation may be somewhat different than yours as this machine has a power supply developed to specifically optimize brightness for REE hexaborides, and controls the condensers using the so-called geochron firmware that operates the three condensers independently, which also results in greatly improved current density at the sample.  We order cathodes from Applied Physics Technologies, and specify a 30 micron flat tip area (rather than the typical 16um flat).  We tune the bias setting mostly based on the gun scan images, getting deep red center figures with both high and low gun scan images.  The behavior of the bias is certainly affected by the exact setting of tip height relative to the Wehnelt, as well as the emission area of the tip.  Note of course that the Wehnelt assembly itself is different for CeB6 (or LaB6), compared to W.
I am guessing that you have the old type HV power supply and old-style ion pump (four quadrant elements with magnetic shield).  We have the new style, off axis ion pump that eliminates the magnetic shield.  This difference can be a factor in performance of the gun (vacuum dependent), and we would recommend you switch off the ion pump after any gun maintenance until the chamber vacuum is pretty good, then turn it back on manually.  The firmware tends to turn on the ion pump a bit early in the sequence which may actually cause some plasma formation and may not be so good for the cathode so it is good to be patient with the vacuum system overall.  We do find that after awhile we need to go back in and bring the tip back very slightly nearer to the Wehnelt as there is some recession with time (there is an effect noted in the bias behavior which you may not see with the old style power supply).  We use CeB6 on our old SX50 also, and have been very happy with how it works, with much improved brightness relative to W,  and very long lifetimes.  We find it requires much less regular alignment, and users need not worry about figuring out saturation heat values.

Ben Vos

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Re: LaB6 on SX100
« Reply #2 on: March 09, 2018, 07:11:42 am »
Hi Philipp

We converted  from W to LaB6 about one year ago and I’m very happy with the result now.

I confirm the advantages of LaB6 as explained by Mike such as very long lifetime that you can expect (> 1 year continuously heating on).  We replaced LaB6 after one year of use since emission was somewhat lower then at the begin of life.  Perhaps only a readjustment of tip in Wehnelt could restore original emission again.  The replaced LaB6 tip has in each case no sign of damage when observed with OM.  It would be very interesting to share information how to adjust LaB6 tip in Wehnelt.  Step by Step procedure? Tools needed.  Perhaps you or Mike can organize a class for that?

We order standard LaB6 from CAMECA that have 16µm flat tip.  With C1C2 = 1200 (= CAMECA service magic number) we can already obtain Ibmax = 600 nA.  With a 30 micron flat tip area this Ib max value must be very huge and much much higher then you ever can get with W-filament.   

One fancy trick with LaB6 is that you also can use Ie (gun emission current) as a proxy for Ib (beam current) on sample.  The image stays perfectly focussed and shift of beam < 0.5 µm in the range Ib 30 nA – 400 nA.  This is clearly shown on pictures in attached file.  Note that the SE image is a bit more blurred at high Ib current but that is an effect of a bit larger spotsize on sample.

So if one is a bit lazy and doesn’t want to readjust all the time Focus, Demagnetize lens, Beam- alignment, Astigmatism …  just do it once in the morning and you can take advantage of that trick for the rest of the day.

Did you already convert to LaB6?  I’m very interested in the results that you get on your shielded SX100 anyway.  It would be very appreciated if you could share this information with me.
Thanks,
Benedict
« Last Edit: March 09, 2018, 11:11:22 am by John Donovan »

JonF

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Re: LaB6 on SX100
« Reply #3 on: April 30, 2018, 04:55:19 am »
Note of course that the Wehnelt assembly itself is different for CeB6 (or LaB6), compared to W.

Does anyone have any pictures showing the difference between LaB6 and W Wehnelt assemblies? I've got two here, but they look the same to me! I'm interested in switching our SX100 to LaB6.

Thanks!

Mike Jercinovic

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Re: LaB6 on SX100
« Reply #4 on: May 01, 2018, 08:47:57 am »
Here are a couple of pictures.  The one on the left is the older version of the LaB6 Wehnelt, originally for the SX50, the one on the right is the newer version available for the later SX100s.

Mike Jercinovic

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Re: LaB6 on SX100
« Reply #5 on: May 01, 2018, 08:51:16 am »
more of a side view here...

JonF

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Re: LaB6 on SX100
« Reply #6 on: May 01, 2018, 09:44:56 am »
They're not like either of my two, guess I have two W filament wehnelts then.

Thanks for taking the pictures!