JEOL FE-EPMA Beam Stability survey (JXA-iHP200F, JXA-8530F, JXA-8500F, etc.)

[Peng Jiang]

Aloha EPMA folks,

Yesterday I discussed the beam stability of our newly installed JXA-iHP200F at the University of Hawaiʻi at Mānoa with John Donovan. He was surprised by the long waiting time (8–12 hours) for the beam to stabilize and suggested that I make a post here.

I also think that a ~12-hour stabilization period is quite long, even though this is a new emitter and a new instrument. I'm curious how other newly installed JXA-iHP200F probes or other JEOL probes have behaved in terms of beam stability and overall stabilization time. I greatly appreciate your experience and thoughts

Below is a quick summary of how our new probe beam behaves:
1. When switching from a high accelerating voltage to a lower one (e.g., 20 kV to 10 or 5 kV), the beam current increases rapidly during the first 8–12 hours, followed by a much slower increase thereafter as it gradually stabilizes.
2. When switching from a lower accelerating voltage to a higher one (e.g., 5 to 10 kV, or 10-15 to 20 kV), the beam current first increases rapidly and is then followed by a gradual decrease, stabilizing around 12 hours.

After stabilized, the beam variation was < 1% within 12 hours.

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- Above was a test with the voltage switched from high (15-20kV) to low (5kV).

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- Above was a test with the voltage switched from low (5kV) to high (10kV).

JEOL engineers suspected that room temperature might be affecting low voltage beam stability. So we added a temperature logger and showed that temperature was not the factor (image below).

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- Above was a test under 10kV, 50nA, for 20h. No significant temperature change but beam dropped and it took about 12 hours to stabilize.

I also noticed that, even under the same voltage (stabilized for a few days), changing beam current also appeared to "trigger" the stabilizing period.
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- Above was a test under 10kV 50nA for 36h. Beam current was changed from 100nA (used for X-ray mapping) to 50nA (for beam stability test). It took about 4 hours for beam to stabilize.

[Probeman]

I am surprised that Peng's FEG takes so long to stabilize.

I know that Cameca had a lot of trouble with their early FEG but I'd never heard of any stability issues with JEOL's.

In addition to switching beam currents for samples with both major and trace elements, there's the multi-voltage method for thin film analysis:

https://smf.probesoftware.com/index.php?topic=111.0

Having a stable beam after switching beam current and/or beam voltage is pretty darn essential.

With our Cameca W gun at UofO, there are zero stability issues with changes in beam current or voltage. Though early on we had relatively short filament life, but once the column has run for six months or so, the filament life improved significantly.

If you do have a FEG instrument, please vote in Peng's poll at the beginning of this topic.

[sem-geologist]

We have both SX100 and SXFiveFE and so I can see fundamental differences between using W-hairpin and Shottky's emitter. They require absolutely different analytical strategies, on one hand "Field Emission" opens new horizons, on other hand it brings pretty nasty not advertised disadvantages. Because of these instabilities we were running our SXFiveFE at constant 15kV, and EHT never OFF. Rarely switching to 25kV for sulphide analytical week batch session. (stability within 0.3% of set beam current value in all cases). As for beam current drifts we had experienced such, but not so severe (rather stabilizes in 30 min instead of 4h, also it works worse with beam regulator, than without beam regulator). That requires different analytical approach polling similar condition requiring analytical works into longer sessions.

I actually am not surprised about long stabilization at all (I voted "I am using Cameca FE-EPMA", and as I couldn't check two positions in the poll, for stabilization time it would be "<1h" - but that is such a slippery slope where it is easy to go also into >12h (or one more position could be added named as "never"). What I am surprised actually that this is brought to wider attention so much late. This aspect of Thermally Enhanced Field emission (known as Shottky's emitters, further abbreviated by me here as TFE) of electron beam is completely left out in most of SEM handbooks. For thin-film analysis simple W-hairpin emitter is the only sane option.

As for Cameca FE machines, so far from my 12 year experience with such instrument - for 3 first months after new emitter installed it would be able to stabilize in less than 20 min after switching conditions. However, if being run by Cameca instructions and protocols such tip would start deteriorate and could reach the state where stabilization falls into "never" (I don't know if any of these had changed, my knowledge about improper protocols are from 2019; they had a chance to improve from then). Pushing Cameca service aside (starting from 2019), and running with improved my own developed protocols preventing tip deterioration, I could keep switching condition stability <30min, even with 2 year old tip. Then there are other variables which influence beam drift after condition switching - contaminated splash aperture under the C2, which in case of all Cameca SX instruments are not easily accessible, and thus In my case was not replaced for 5 years for life-time of TFE.

Why it takes so long to stabilize the beam current after changing the HV?
Because TFE is not thermionic emitter, but field emitter directly depending from electric field which changes with any change of EHT (we have two overlapping fields: between emitter and Extractor, and between emitter and Annode - that is surprisingly somehow neglected from all practical handbooks and general TFE knowledge) - The Extractor's voltage set value stays the same when switching between the EHT conditions (that is applied voltage between emitter and extractor), but the field affecting the tip actually changes as it is a sum of mentioned fields! That is a bit of oversimplification, and also could highly depend from topology of HV supply (i.e. it could show in the software that extractor voltage is the same, but it could use  a voltage divider which would swing extractor voltage with change of EHT).

That was the easiest part to grasp. Then lets go to more complicated part: 4 horses of apocalypse, or 4 variables which all depends from each other – emission, temperature, field, geometry.

Geometry of tip
Jeol and Cameca EPMA uses TFE designed for SEM work - they are designed to achieve sub 10nm beams, and thus the diameter of tip surface often is about 300-200nm (the small beam diameter is achieved with utilization of virtual source, which is kind like small point within the tip of emitter, where described few hundred nm tip surface acts like kind of projecting "lens"). This geometry reacts to field changes and is able to erode atom by atom from emitting facet and recrystallize those at its sides increasing diameter, or changing its shape (from round into square/Rhomboid). Emission is not uniform from this tip surface - normally it is weakest at middle, and highest at rims, and for SEM/EPMA work we normally use the middle emission part discarding outer parts with help of numerous apertures and condenser lenses... unless column is badly aligned or optical path is physically bent making it impossible to align the emission across all conditions. After changing the field (i.e. changing EHT) the previous geometry becomes unstable and it starts to change for least energy in the newly set field. The pace of that crystallization depends proportionally to the temperature of the tip and geometry itself. I.e. if tip end diameter grows above 1µm (or starts like that) it becomes less affected by field and temperature (That is why Shimadzu EPMA-FE use such specifically tailored TFE for EPMA, not SEM).

Temperature of the tip.
Default old-knowledge based SEM TFE datasheets defines the optimal heat current to give about 1800K (that is for new tip) - the temperature which allows to produce less noisy emission, while still preventing the TFE from complete disintegration - temperature optimal for SEM/TEM high resolution, low noise applications. Setting temperature above would disintegrate tip, where setting it below would start introducing white noise. The problem starts with aging TFE, as it loose mass (mostly through loss of H2O and OH- from ZrOx reservoir, and Zr itself) and the heat current I_new from datasheet (for 1800K) would make higher tip temperature exceeding that (optimal) 1800K (less mass to heat up with same current). Many SEM OEMs ignore that (or are unaware) and that basically kills TFE tips in 1-3 years. Correct heat current decreasing protocol depending from tip aging is hard to implement as there are no way to measure TFE temperature directly (which is different from production setup, where datasheets for I_heat vs T_emitter are defined from direct temperature measurements). And then there is environmental factor - Datasheet I_heat/T_emitter != I_heat/T_emitter on SEM/EPMA. On SEM/EPMA when setting initial 1800K I_heat current values from datasheets tends to actually set about 10 degrees more, as SEM and EPMA have other heat sources heating the casing of EPMA/SEM itself (especially Cameca SX line with its 50 °C stage always-power-on motors). Also Air conditioning can swing these temperatures (AC failure could lead to catastrophic failure of TFE when run at 1800K. Also there is huge thermal capacity of metal case of SEM/EPMA, which adds few more degrees up when TFE is working for at least a week.

Emission of the tip
Emission is proportional to geometry, temperature and the field. It makes it absolutely counterintuitive when we observe changes in emission made by change of the field. In most of case if we see direct response of increase of emission when changing the field, we can be confident that it will drift to lower in long term (few hours, days). And otherwise - direct decrease of the emission after changing the field could be observed to drift to even higher emission than initial after some hours or days. There are few loop-feedbacks of these cause and effects - emission also influence of tip temperature by Nothingham's effect - higher emission means more cooling, lower emission means less cooling by that effect. Take this as lax example: less cooling -> higher temperature -> larger elongation of tip -> emission surface brought closer to extractor -> field increase -> emission increase -> more cooling -> emission decrease -> less cooling...
So on one hand Emission depends from geometry, temperature and field, but also it influence these variables.

Actually there is a fifth "horse of apocalypse" - the ZrOx reservoir, which is depleting in long term and influencing the 4 above presented parameters, while depletion ratio is depending from temperature.

So what could be done to decrease these instabilities?:
A. decrease the I_heat to bring it 30 - 40K below 1800K - in case of new tip it is easy as TFE datasheet can be used for that. It rather wont stop geometry changes, but it would slow it down significantly making it very slow drift up/down or down/up where any beam current regulators should cope with such slow drifts. It also slows down ZrOx depletion. For aged tip it can be rather tricky as most likely tip is already running at 1840K and further compensation of I_current would be needed. (I use this method on our SXFiveFE)
B. decrease the I_heat to bring it 100K lower (i.e. 1700K) - that will produce more noisy images, albeit introduced random white noise wont affect the analytical work, but should stop recrystallization of tip completely. For EPMA work and SEM TFE that could be a bit tricky to achieve unless very high beam currents are not used at all (i.e. 600-1000nA).
C. Ditch SEM tailored TFE and install EPMA tailored TFE with at least 1000nm tip diameter, set it to around 1790K and forget.
D. Do nothing, let the Tip deteriorate (shorten up the tip and widen up diameter) - the observed instabilities between EHT conditions will disappear, emission will drop significantly, the noise and other source of drifts will dominate the stability issues, so that EHT cross condition instability will become insignificant issue completely burried to other instabilities (20% and more of set value).

(experimental)
E. Increase extractor to its max (6kV? 7.2kV?) so that tip would get one and only one shape of highest  possible field (extractor field would dominate, and EHT field would be insignificant) (that albeit increases the risk of catastrophic failure of Arching in case of Vacuum problems). This would allow to use SEM TFE which would keep the same shape across changing EHT and also should eliminate the setting time completely. This however tend to add new source of noise at least on Cameca SXFiveFE, which I still can't simply find the explanation. It could be HV power supply depending issue.

Chose your poison wisely  .

[Probeman]

This is a wonderful explanation of FEG operation from SEM Geologist!

It also would be great to hear from JEOL EPMA FEG users what they have found on JEOL instruments...

[Ben Buse]

Ok I'm just running a chart recorder for 30minutes, switching from 20kV 80nA to 15kV 20nA and will post the results here. I haven't noticed any long term stablisation drift

[Ben Buse]

One obvious point is how much does it matter when your measuring beam current before and after? And what is the specification?

[John Donovan]

One obvious point is how much does it matter when your measuring beam current before and after? And what is the specification?

Probe for EPMA averages the before and after beam current measurements with every analysis, so as long as the drift is relatively linear, the beam drift correction should be a pretty good interpolation.

[Ben Buse]

Ok I'm just running a chart recorder for 30minutes, switching from 20kV 80nA to 15kV 20nA and will post the results here. I haven't noticed any long term stablisation drift

Here's data attached, but realised you might want a longer time frame...
We are still on our original FEG tip installed 2012

[sem-geologist]

Ok I'm just running a chart recorder for 30minutes, switching from 20kV 80nA to 15kV 20nA and will post the results here. I haven't noticed any long term stablisation drift

Here's data attached, but realised you might want a longer time frame...
We are still on our original FEG tip installed 2012

Despite short time frame I find it extremely interesting.
Tip from 2012 in 2026 would make it 14 year old TFE ! 
That is old. I doubt there is any ZrOx reserve left if it was running for so long, even if temperature of tip would had been reduced to 1700K from the initial install time, which I doubt from the date and historical track of Schottky's TFE stability research. In case it was set at 1800K initially, it had to go through numerous "Ring-collapse" events during those 14 years and thus would had achieved its final stable form (with diameter closer to 600-1000 nm). In that case I don't think You will see any such drifts compared to Peng Jiang's shown examples. Such drifts as he shown is an issue of a healthy freshly produced FEG SEM tip, where diameter of tip is still 200-300nm and ZrOx reservoir is still full, for such new tip changing a field will always impact the shape and do some re-crystallization of the emitting facet (which takes few hours). Old tip, after dozens of ring-collapse events (unless the temperature of tip was reduced down from default 1800K) and after practically completed depletion of ZrOx reservoir, would decrease angular current significantly (and thus also would decrease maximum achievable beam current) – and would not react to the change of emission field.

What maximum achievable beam current can you get?
Maybe You could read the extractor value somewhere? The plot suggests me that it was not pushed too high, but I could be wrong.

[Peng Jiang]

So what could be done to decrease these instabilities?:
A. decrease the I_heat to bring it 30 - 40K below 1800K - in case of new tip it is easy as TFE datasheet can be used for that. It rather wont stop geometry changes but would slow it down significantly making it very slow drift up/down or down/up where any beam current regulators should cope with such slow drifts. For aged tip it can be rather tricky as most likely tip is already running at 1840K and further compensation of I_current would be needed.
B. decrease the I_heat to bring it 100K lower (i.e. 1700K) - that will produce more noisy images, albeit introduced random white noise wont affect the analytical work, but should stop recrystalization of tip completely.
C. Ditch SEM tailored TFE and install EPMA tailored TFE with at least 1000nm tip diameter, set it to around 1790K and forget.
D. Do nothing, let the Tip deteriorate (shorten up the tip and widen up diameter) - the observed instabilities between EHT conditions will disappear, emission will drop significantly, the noise and other source of drifts will dominate the stability issues, so that EHT cross condition instability will become insignificant issue completely burried to other instabilities (20% and more of set value)

Thanks SEM-Geologist! This is a very thorough review of possible causes of beam instabilities and approaches to make beam stable. This all look like a JEOL engineer's job to complete, and maybe a replacement of a new emitter.

The new probe here started its emitter in middle June. So it's already been two months, which is supposed to be more stable than it should be.

[Peng Jiang]

One obvious point is how much does it matter when your measuring beam current before and after? And what is the specification?

JEOL has beam stability specification of ±0.3% per hour and ±1.0% per 12 hours for a test under 10 kV, 50 nA. The beam current variation becomes an issue for long-time counting or X-ray mapping, especially during the stabilizing period, where the variation does not seem to be linear.

[Ben Buse]

So here's the plot for the next 16 hours (continuing from above the switching from 20kV 80nA to 15kV 20nA), which might in part be natural drift rather than stablisation.  ca 1.5% rel  in 12 hours.

Good point about x-ray mapping

Also shown is a long plot for the 2 hrs after 1st 30 mins.

[Ben Buse]

What maximum achievable beam current can you get?
Maybe You could read the extractor value somewhere? The plot suggests me that it was not pushed too high, but I could be wrong.

Over a uA beam current achievable at around 20-25kV.

Extract current try to keep around 80uA.

[Peng Jiang]

So here's the plot for the next 16 hours (continuing from above the switching from 20kV 80nA to 15kV 20nA), which might in part be natural drift rather than stablisation.  ca 1.5% rel  in 12 hours.

Good point about x-ray mapping

Also shown is a long plot for the 2 hrs after 1st 30 mins.

Thanks Ben for doing this test. This is very informative. It appears that your Cameca probe got instant beam current stabilization for the first 30 mins ~ 2 hours, then it started to drift up before reaching another stabilized beam current, or drift down (hard to predict). The time period was also about 12 hours. This is very interesting. Our JXA-iHP200F probe (which has its emitter 'started' two months ago) could stay at a stabilized beam current for a few days, but sometimes also drift up or down (gradually, not like constant fluctuation).

These are all very interesting. I would be interested to see how other new / old JEOL and Cameca FE probes behave as regards to beam stability when changing accelerating voltages.

[Ben Buse]

Thanks Ben for doing this test. This is very informative. It appears that your Cameca probe got instant beam current stabilization for the first 30 mins ~ 2 hours, then it started to drift up before reaching another stabilized beam current, or drift down (hard to predict).

Sorry I didn't say this is a JEOL8530F