Choosing the Right Microscope: a Step-by-Step Guide – #1 Tungsten, LaB6, or Field Emission

“Would you like that with tungsten, LaB6, or field emission?”

Ordering a coffee used to be easy, but these days you rehearse how you will ask for your drink while you stand in a line because of the complexity. Ordering a new microscope, though, hasn’t always been easy. Like the coffee, there are many things to consider as you make your decision: flavor, size, etc. If you get the coffee order wrong, it is annoying for a short amount of time. If you get the SEM order wrong, it can be a very costly mistake with long term consequences.

Here at TSS, we like to make sure that our customers are confident that they are choosing the right ‘scope for their requirements. We appreciate that not everyone who contacts us has decades of experience in microscopy, so we decided to create some brief tutorials in our new article series “Choosing the Right ‘Scope: a Step by Step Guide.”

A great place to start the series is at the source…literally. Will you need a basic tungsten filament, a LaB6 emitter, or the ultimate electron source: field emission?  This is a critical choice that will affect not only the performance, image quality and resolution of your new electron microscope, but also the price.

Tungsten and LaB6 sources use thermionic emission to generate electrons. Basically, heat overcomes the work function and electrons start to emit from the material. Tungsten and LaB6 have a relatively low brightness factor and can drift over time as the filament heats up and settles in (not a big deal in practice really).  A Field Emission source (FE) produces electrons by placing the filament in an electrical potential gradient. The FE source is usually a wire that is coated with zirconium oxide – typically that famous Schottky Emitter that we know and love.

So how do you choose?  Ultimately it depends on one key thing: the features that you need to see:
1.    Do you typically look at features that are 20 microns or larger? Then the tungsten microscope will meet your imaging needs. Tungsten ‘scopes are easy to use and easy to maintain. Granted, you will have to change the filament every 20 to 100 hours, depending on how you use it, but an experienced operator can go from a dead filament to imaging again in 20 minutes. At TSS we train our customers how to change filaments, and also how to operate the emitter to get the most lifetime from it.
2.    Do you typically look at features in the 10–20 nm feature size range? If yes, then a LaB6 scope is a great choice. It can give you the benefits of slightly better imaging than tungsten, but with reduced source maintenance time because you do not have to change the LaB6 emitter as often.  The benefits of LaB6 over tungsten are incremental, so our advice is to not get too hung up on this one.
3.    If you are looking at features that are sub-5 or 10 nm, you may need to consider the top of the line source: FE. FE sources are low maintenance for users because they can often run for 1–5 year, again, depending on how it is used. And, FE sources provide the ultimate in resolution. There are several nuances to changing the source, however, so this task is really best left to service engineers, like our experienced crew at TSS Microscopy. This is typically done during a Preventive Maintenance visit if you have one of our Service Contracts.

Pictured above is a high voltage supply cable for an FEG SEM system

There are some other factors related to the above that should be considered – like what your samples are made of, do they charge up under the electron beam, do they outgas a lot, are they particle based or thin film, etc.  These factors are a bit more complex and subtle but can make a difference in your SEM selection.

Your samples may not fit neatly into these three metaphorical buckets, which is why we recommend you give us a call to talk to you about your imaging needs so that you can decide on the best electron source for your work. And we promise that this is an easier conversation than if you were to ask “what do you recommend” at the local coffee bar.  Stay caffeinated and see you at the next installment…

Stay tuned for Article 2 of our Choosing the Right ‘Scopeseries coming soon –
Chamber and Stage Size – Why it is Important and How to Choose it.