I never found that we would need any cryo-stuff on our SXFiveFE (turbo+scroll) or SX100 (diffusion+oil pumps). What kind of materials are those in picture? I am aware that too long a beam exposure would take coated carbon from sample (so there is thinning of the coating until it completely thins-out at hit spot) - that leaves mark in the coating. Carbon thinning is main factor limiting beam exposure on sample. At least I was so thinking, hitherto.
I am not doing metals often.
Your picture is suggesting that there is e-beam induced deposition of organics. I am aware that such deposition can be witnessed in its all gory details on apertures (for some apertures used for too many years it looks like shield volcano ) or on sample stage beam parking position (our FEG instrument). I started to wonder why in one case we get coating disintegration and removal, and in other organic (hydrocarbon) deposition? But maybe I am completely wrong with a first, maybe there is no thinning of carbon coating at all, and those marks are deposited organic too? But why carbon coating quality then matters? considering same coating thickness of 20nm, with rod-based shitty coating the coating will go off in few minutes with 10nA; with carbon-thread single pulse coating coating stays for ten minutes with up to 100nA; with multiple-carbon thread composite layer coating 700nA for 10 minutes can be achieved. If there would be only the e-beam deposition of hydrocarbons, and no carbon removal by e-beam, there should be no such differences between coatings, beam dose for when absorption current abruptly drops, and charging starts should be similar for all case - but it is not.
So why in some cases do we observe contamination, while in other we get removal (cleaning) with same e-beam on same machine?
Good questions. Yes, these are all uncoated metal specimens because the client was interested in trace carbon.
I really cannot speak about electron beam damage to carbon coats but I have observed a change in the coating coloration on coated metal specimens.
On uncoated samples Ben Buse and I (and others I am sure) have measured a decrease in the carbon signal in the first 5 to 10 seconds (or longer) of beam exposure which I attribute to volatilization of the native hydrocarbon layer.
This by the way, is the rationale for the so called "decontamination time" option in the Acquisition Options dialog in Probe for EPMA. The idea being to delay the start of counting after the faraday cup is removed, until this native hydrocarbon layer is removed by beam exposure.
This same parameter can also be utilized for beam sensitive samples for the so called "incubation time", in this case waiting for the sample to heat up (and ion migration to commence), in order to ensure a linear (log) decay of the alkali intensities.
My observations on uncoated samples are that as (native) carbon is removed directly underneath the beam spot, elemental carbon is deposited from hydrocarbons in the vacuum system becoming "cracked", and then deposited adjacent to the heated beam spot where it is slightly cooler. This is the genesis (I suspect) of the "carbon ring" we often observe after acquisition.
Of course these effects are extremely dynamic and depend entirely on the cleanliness of the vacuum system and sample, the beam size and current (electron dose areal density), and the thermal conductivity of the specimen itself.