Hi Everyone,
We are evaluating a proposal for SEM. The instrument is offered with a standard Tungsten filament or LaB6 filaments. I know LaB6 is better in terms of image quality, spot size, and filament life as compared to Tungsten filament (of course price is high). What are the other points we should consider such as high vacuum chamber, maintenance cost, operational cost, etc? Any help from an experienced user will be much appreciated.
Dear Dr. Muhammad Zahid ,
I suggest to have a look at the following, interesting document:
- Choosing the Right Microscope – Part 1
Choosing the Right Microscope: a Step-by-Step Guide – #1 Tungsten, LaB6, or Field Emission By Tss Microscopy (2019)
Available at: https://tssmicroscopy.com/resources/news-and-press/choosing-the-right-scope-a-step-by-step-guide-1-tungsten-lab6-or-field-emission/
My best regards, Pierluigi Traverso.
Hi,
We have an LaB6 source on our conventional Zeiss EVO50EP SEM and chose one over a tungsten filament, mainly because of the pros for us :
- Multi-user SEM : not all the users are familiar with or even willing to change the source ; LaB6 can be changed once or twice a year depending on beam time by experienced users only.
- High beam time : we prefer to block the SEM for two days once/twice a year to change the LaB6 source, rather than every two weeks for a W filament (tried that but you MUST have two Wehnelts to minimize downtime : one in use and the other stored ready in the chamber for replacement).
- Our SEM has VP and environmental capability : don't even consider tungsten for this ; LaB6 brightness is mandatory (in my opinion)
- LaB6 brightness is also a HUGE advantage if you want to run SEM-EDS on light-elements samples : much more counts for the same beam current ; key advantage on sensitive organic samples AND when doing EDS in VP/EP mode.
Some cons however have to be remembered :
- Prepare to train yourself to change the LaB6 source : it is slightly more difficult than changing a W filament and you should ask for training at the installation.
- You will need a higher gun vacuum (ion pump) : will need changing in some years, more expensive maintenance. The column and chamber vacuum will mostly be the same as with W SEMs. The gun vac gauge probe will also need changing (yearly or every two years).
- Electron source cost will be higher (but not so much vs. its advatanges) : LaB6 Kimball source = ca. 1700 €
Having worked for 15 years with an LaB6, I personally will never go back to a tungsten source. (But would gladly go for a FEG, when we can afford it!).
Some other LaB6 tips :
- keep a replacement W equipped Wehnelt in the chamber in case your SEM cannot afford to have sudden downtime in case of the LaB6 flashing ; you can swap the LaB6 for a W source in 15 minutes to finish your job and schedule LaB6 change later.
- always keep a spare LaB6 AND a Wehnelt aperture : if the LaB6 flashes suddenly, you'll not only need the replacement source but also the aperture (sometimes, they are too polluted to be re-used after cleaning)
- if you go for a maintenance contract, schedule source replacement while they do the maintenance : saves time and allows column apertures replacement while you air the colum and the gun !
- Try and learn to work both the first and second saturation points with a LaB6. The first one is exceptionally resoluted and bright (for VP and quick EDS) but very small and thus, instable ; the second is very stable (for image analysis or long EDS) but will lower your source life.
Not many of us LaB6 users, but I'm a believer. ;o)
Dear Pierre Caulet,
Many thanks for your time and reply. I really appreciate you spared some time and wrote a detailed review. This is the exact information I was looking for.
Best regards,
In addition to the mentioned advantages, LaB6 filaments last considerably longer. We have been getting about one year between filament changes.
Pierre Caulet's reply is really spot on, but I would like to add a word of caution when considering "brightness" of W vs. LaB6. Yes, it's true a LaB6 is brighter than a W filament in the same microscope- but that does NOT mean that all LaB6 scopes will outperform all W filament scopes. You must also consider the column optics, especially at lower kV's the beam coherence is crucial. Across different manufacturers and different models you will see a significant variation, which is compounded at lower kV's and therefore more noticeable.
I also wanted to point out that Hitachi W filaments on the new Hitachi SU3800/SU3900's can last for over 300 run hours with patented Intelligent Filament Technology. Of course, LaB6 filaments are still longer, but another con to consider (beyond price) is long warm up time and if not properly followed the filament can crack.
Bottom line is always, always... try before you buy! With your own sample and doing your own adjustments in person, if possible. Good luck!
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