VP-WDS, heating stage

[Jason in Singapore]

Hi all,

This is a general request for thoughts or advice. We are in the exploring the idea of purchasing a low vacuum or enviro SEM equipped with a high-temperature (800-1200C) heating stage. Is anybody here currently doing high-T experiments in an SEM or EPMA?

Since EDS won’t work once the sample is glowing hot, we are considering a WDS system as an attachment. From our asking around, it seems that the Bruker XSense is the only WDS system on the market that can operate in low pressure and enviro modes (is this true?). We were told that the MagnaRay (Thermo) cannot operate in low vacuum because “coronal discharge will occur on the x-ray counter which will damage the preamp”…

Does anybody have any positive or negative feedback on parallel beam WDS systems in general? I can see that they have some advantages for light elements, which we are not particularly interested in for this instrument. It also seems like there should be significant compromise of spectral resolution, which is unfortunate. For this instrument, the idea is to use the WDS for x-ray imaging rather than quantitative analysis. 

Any ideas, advice, or experience that anyone is willing to share is welcomed.

Cheers,
Jason Herrin
NTU Singapore
jsherrin@ntu.edu.sg

[Karsten Goemann]

Hi Jason,

Not sure if you're aware, but one big issue with x-ray microanalysis at variable pressure is reduced spatial resolution due to primary beam scattering. The primary electron "skirt" can reach hundreds of microns depending on gas type/pressure, mean free path of the electrons in the gas etc. Does your instrument have means to minimise that path length such like PLA "cones"?

We've tried EDS at low vacuum (at room temperature) for different projects in the past and found the spatial resolution insufficient even after pushing those parameters (gas pressure, free path length...) as much as we could.

Cheers,
Karsten

[Jason in Singapore]

Hi Karsten,

Thanks for the reply. Primary electron scattering will certainly be an issue. We seem to do okay with EDS imaging at 50-100 Pa on our FE-SEM, but I can imagine this will become more of a problem at 500-2000 Pa. Quantitative analysis aside, what would you say is the practical upper pressure limit for x-ray imaging?

We only recently learned about cone systems to shield the beam (FEI). It seems like it should help, although it must compromise the field of view. I suppose that working at 30 kV must also help a bit. 

-Jason

[Karsten Goemann]

Hi Jason,

Yes, the cones make a significant difference but as you say they restrict the field of view. The ones we have are 500um.

Using 30kV definitely helps.

The upper pressure limit for x-ray imaging really depends on what sort of spatial resolution you need, diameter of the pressure limiting aperture, what the free path length of the electrons is in the gas (e.g. if you don't have cones, what's your analytical working distance and is that flexible to some extent), and what type of gas. If you can use He for example that should work better in terms of scattering than N2 or air, or water vapour.

Cheers,
Karsten

[Probeman]

Since EDS won’t work once the sample is glowing hot, we are considering a WDS system as an attachment. From our asking around, it seems that the Bruker XSense is the only WDS system on the market that can operate in low pressure and enviro modes (is this true?). We were told that the MagnaRay (Thermo) cannot operate in low vacuum because “coronal discharge will occur on the x-ray counter which will damage the preamp”…

Hi Jason,
It occurs to me that because the Cameca EPMA utilizes column separation windows between the column and the spectrometers, one might be able to operate a Cameca instrument in a variable pressure mode.

Of course one would have to modify (differentially pump) the gun (and turn off the secondary detector), but it might be possible to run at elevated pressures in the column.  But crazy.
john

[Jason in Singapore]

Hi John,

Thanks for the reply. I wonder if anybody has tried operating their EPMA in variable pressure mode yet? If not, then I guess it’s only a matter of time. We have spoken with some folks from Shimadzu about developing one, but so far they haven’t taken the bait.

Just an update, it turns out that the Bruker XSense is limited to less than about 3.6 keV, which is a bit restrictive if we are working with a 30 kV beam. We have learned from EDAX that their TEXS WDS system is also capable of operating in VP/enviro mode. Its range extends up to 15 keV with the proper crystal configuration. Unfortunately, it is also a parallel beam WDS system, so I can image there should be be significant loss of signal and spectral resolution for higher energy x-rays. Also, the x-ray optics need to be near to the sample, so that’s a limitation for heating stage experiments. It seems Oxford is the only manufacturer still using Rowland circle geometry, but their system is not currently compatible with VP/enviro. 

Another idea is to forget WDS and simply add a thick window to an EDS system in order to block out IR and visible coming off the glowing hot sample (sort of like sunglasses). Bruker had previously developed a removable polymer thick window for an EDS fitted to a beamline heating stage system. EDAX, meanwhile, can manufacture a permanent SiN thick window (1 micron thickness), to achieve the same result.

Cheers,
Jason