Author Topic: Cameca Gun Ion Pump Glitches  (Read 2943 times)

Probeman

  • Emeritus
  • *****
  • Posts: 2051
  • Never sleeps...
    • John Donovan
Re: Cameca Gun Ion Pump Glitches
« Reply #15 on: May 15, 2020, 09:13:15 am »
Somehow I missed that, but I can confirm that our ion pump is working great since we changed out the bad column separation window.

Here's a question: how does JEOL prevent "poisoning" of their ion pumps from P-10 argon if they don't have column separation windows? They must have very "tight" detector windows!
The only stupid question is the one not asked!

JonF

  • Professor
  • ****
  • Posts: 55
Re: Cameca Gun Ion Pump Glitches
« Reply #16 on: July 17, 2020, 01:32:41 am »
Update on ours: I found a bad detector window and having checked all the column separation windows, ended up replacing one thick film window that looked a bit blown (couldn't really see any holes though). Surprisingly, the thin film windows all looked pristine, so I'm not convinced any of the column windows were actually bad.

I stuck the detector window under a microscope and took a few pictures.





For scale, the window is approx 1 cm across the short dimension, the hole up to maybe 1 mm in length.

Typically, this window was from SP5 and was a pig to get out as we have the liquid nitrogen anti-contamination dewar fitted, severely reducing access.

Top tip: fill all the gaps in the table underneath with tin foil or blue roll! When the circlip holding the detector pops off, or when you drop it on refitting, it will fall in to the tin foil and not down in to the bottom of the instrument amongst the pipes and cabling. Saved my bacon, that did! 

I'm waiting a while for the vacuum to recover before I restart the ion pump, but fingers crossed this has sorted out the ion pump tripping out!

sem-geologist

  • Professor
  • ****
  • Posts: 31
Re: Cameca Gun Ion Pump Glitches
« Reply #17 on: July 17, 2020, 10:02:16 am »
I think this is good place to share my 2 cents.

This is going to be about SXFive FE, but don't discard this fast, particularly if you are planning to run LaB6 or CeB6 on your SX100 as that I think applies to that too (unless your ion pump is noble diode or StarCell type). So I see you guys had described here some serious leaks, and after fixing Ar leak from spectrometers the issues were fixed for your machines. If you are running with tungsten tip that is good approach and probably you won't need to look any further.

But for tips operating continuously a year and further you should be aware that there are also not-obvious Ar contamination sources. First of all very tiny part of Ar diffuses through separation window (despite tight seal). Secondly, there is always residue air on sample surface, pores, and air contains 1% of argon. This can look insignificant in short term, but is of key importance in long-term. These argon bits accumulate in the diode ion getter pump(s), but differently than other getterable gases, argon is temporary buried on cathode by sputtering process. The problem is that then pump gets critical mass of argon in the pump it is no more capable to manage to hold it all, and even worse - it looses ability to hold already collected argon... and so pump releases most of the argon in an "avalanche" fashion (thus You would see in the logged data huge spike).

I am describing here argon cycles, there can be other issues on SXFive FE like leaky metal gaskets (these can be checked or ruled out by changing room temperature as that is sensitive to that), and can produce similar patterns, which is hard to distinguish (devil in the details). It is good to understand how argon capture works, and how spiking can be overcome.

SXFive FE is equipped with two ion getter pumps (both simple Diode type). The big primary pump is situated close to the Schottky's emitter, and small (20 l) is situated below the primary. This small one buffers column vacuum in between chamber and gun containment section, and as discussed latter is first in the line to catch and suffer from argon accumulation. The small one starts to be Argon saturated after more-less 1 year (that is with excellent separation windows and also careful procedures limiting leakage of samples in the chamber; if spectrometer would leak argon, then pump would start to suffer much more sudden).

So whats going with that released argon from the pump? In closed system (sealed vacuum containment and single ion pump) that would go into "perpetual" well defined periodic spiking, as after argon release the same pump tries to catch that argon back.  But after crossing some stability threshold it releases all captured argon again and repeats the process many times. On the microprobe it is a partially open system and part of released argon travels further in vacuum system where it is taken care. And so the affected pump with every cycle needs to deal with lesser amount of argon and duration in between spikes is increasing until finally it stops spiking. A small part of released argon escapes through column down to the chamber (if column-chamber valve is not closed) where it is taken out with turbo pumps, another small part is captured by other, in our case primary, pump. The affected pump keeps spiking despite lessened amount of argon as, differently to initially gradually accumulated argon, it needs to pump huge amount of argon at once. The newly formed argon burial structures are very unstable and so pump releases argon again very shortly after previous spike (minutes to hours to days). If part of argon can escape during spiking, then cycles will increase periodicity (longer time between spikes) as after some days/weeks/months there will be manageable amount of argon left and ion pump will cease spiking, but not forever, just for some longer time (months, half year...).

Looks good? Problem is self-solved? Not so fast. During smaller (secondary) pump release of argon, the primary ion pump is significantly being contaminated with argon with every spike of secondary pump, which is not good as primary pump being of simple Diode type is also susceptible for argon release. It is a larger pump and thus it can capture (temporary) much more argon... and so release much more argon. The safety vacuum threshold for Schottky emitter is set lax so that it would not get triggered with spikes originated in the secondary ion pump, but when primary pump gets argon saturated and releases all its argon - vacuum goes few orders over safety limit and trips the emitter.

If nothing is done with argon accumulation, then we experience that Primary pump releases argon in about 1.5 years from last baking. It actually did for us last week, that is why I am writing it here. I still have fresh mind after doing research on ion pump workings and workarounds. This is not first time it happen to us. We had similar "1.5 Year old" ion pump incident while running previous emitter. It then went worse than now as amount of argon released by primary pump was so large that secondary pump had gave up (that was 1.5 year ago, during Christmas-New Years eve brake, I was quite shocked to find such a mess getting back from holiday). This time secondary ion pump had survived, but I guess it is as I run it with manually set 5000V voltage, so it has much better response to pressure increase. But it was still enough to trip the field emission gun.

So I thought up some very simple procedure to get rid of Argon without baking the pumps, and I found it worked for our SXFive FE. It is based on 2 actually 3 assumptions:

1) If I want that no argon would be caught by any of the ion pumps, then both ion pumps should be off, as there is no possibility to isolate one from other.
2) Excess argon needs some open way to get out.
3) ion getter (Diode) pump needs energy to keep the Ar from escaping.

So with these in mind the procedure is clear:
(1) shut both ion pumps off
(2) while keeping column-chamber valve (EP 6) open.
(and obviously - the FEG needs to be shutdown before)

I had monitored the vacuum by separate vacuum gauge (IMG2(?)), which is installed near primary ion pump. The vacuum instantly worsened up.... but then as expected by my assumption, started to improve. As it had plateaued in a half hour, I had restarted ion pumps - vacuum got improved - no more spikes. Actually funnily, the primary pump did not wanted to restart, and controller showed "too low pressure" (too good vacuum) - seriously. But if waiting for a bit it gets correctly. It got back to ultimate vacuum in an hour.

BTW, emitter is today back on and running for SXFiveFE record, I have plans for it to be able to work stable (200 nA/1h within specification) up to 18000 hours... 5000h more to go. When FEG is run correctly it runs stable and also the vacuum baseline is really good. After argon riddance, IP1 (primary pump) shows 1.7E-7 Pa  - that is with Shottky tip operational (!), before spikes (two weeks ago) it increased from 1.8E-7 to 2.0E-7 Pa. From previous experience, when FEG is run in nonsense default mode - the primary vacuum would raise to 2.8E-7 Pa in a few months from 1.7E-7 Pa achieved during tip installation. This demonstrates that actually any baking of ion pumps is not needed on FEG instrument (after installment) at all when everything is run using brains. (just for comparison - our ZEISS FEG SEM manuals highly recommend baking ion pumps once 1-3 months)
« Last Edit: July 17, 2020, 03:14:47 pm by sem-geologist »