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before subjecting it to the “hard” working conditions might be (at least in my experience really ‘is’) of benefi t regarding beam stability as well as maintaining your fi lament’s lifetime. “Slow heating” means [aſt er change of fi lament] at least running the new fi lament 1–2 hours “under- under”-saturated aſt er achievement of an evacuation optimum of the chamber (not knowing whether you could see anyhow the fi lament’s image in such a condition like in a TEM on the screen). You easily would / could see (if you e.g. have permanent vacuum-measurement monitoring ready) that on starting to “heat” (increase) the fi lament (current), the vacuum decreases signifi cantly (at least slightly) because of “evaporation” of metal & impurities / contaminants. T en increase fi lament current a bit more.... and let “equilibrate” vacuum conditions. You might even fi nd such decreasing vacuum aſt er a third increasing of the fi lament heating near the “saturation” point. Another helpful requisite also would be to use - without using or even heating the fi lament fi rst, an ACD (anti-contamination device - if available) with at least one if not two cycles of cooling fi rst and pumping the chamber aſt erwards to get rid of most organic molecules you brought into the fi lament chamber by your cleaning and mounting procedures - let warm up the ACD and pump (RV and HV if possible) again. According to the specifi cations of the Hitachi S3400N VPSEM (I found on: https://www. ntnu.edu/documents/140082/1269041159/S-3400NSpecifi cations.pdf/6 b94c26f-a9d4-4f36-82b3-7e9eb3603922 ) the possibility of doing so might be impossible due to the automatic settings of diverse functions but you should know about those possibilities from practical experience. Unfortunately, a modern VPSEM is not comparable to such an old piece of equipment like the TEM ZEISS 109 (vintage 1976) I worked with where maintaining and handling vacuum matters relatively easy... And, BTW, life time of a tungsten fi lament always depends on the “technical quality” and the handling - when mounting into the chamber - too.... Wolfgang Muss wij.muss@aon.at First, what kind of fi lament life are you currently getting? If you are close to 100 hours, I would be satisfi ed. If you were only getting 30, then I would look for a problem. We got better life when we ran at a consistent voltage and did not have users doing the saturating. We went from average lifetimes of 25 hours to 80 hours. We also operated just under saturation conditions. We simply turned off the high voltage at the end of the session. We needed to check saturation and reduce current over time. As the fi lament thinned, it needed less current to reach the same temperature. Finally, we ran the fi lament a little further back from the front of the Wehnelt. T at lessened brightness but increased life. Warren Straszheim wesaia@iastate.edu Fri Feb 9 Do you have a high vacuum gauge on the scope? With both our new Hitachi (SU3500) and our old JEOL (5600), the “ready” state comes on when the vacuum is in the high 10 -4 Torr range (or even higher if there was a vacuum leak, outgassing sample, or other problem). Running a fi lament at such a poor vacuum will drastically reduce fi lament life and generally contaminate the column faster. I insist on having a gauge on any scope I run, and don’t turn the beam on until the vacuum is at least mid-10 -5 Torr. With vacuum gauges under $1000, they will quickly pay for themselves in terms of fi laments and time lost changing them. I have ranted to both JEOL and Hitachi that they should make them standard on all scopes. By their own admission, half of their service calls are vacuum related, and with a gauge, the end user can oſt en fi nd a leak and save a service visit. In our case with the closest service in Ontario, it’s a $2000 fl ight to get here. Should be a simple budgetary calculation, but Hitachi says I am the only customer in Canada who ever requested a vacuum gauge. I also let the fi lament cool 5 to 10 minutes whenever possible before venting. I do not have any hard evidence, but it just seems logical to me that exposing a hot tungsten fi lament to atmosphere is going to induce more oxidation (or whatever) compared to a cool one. In my


60


experience, these precautions contribute more to fi lament life than slight undersaturation. Of course, you want to make sure you are not oversaturated, but I get 3-400 hours of the Hitachi fi laments running in the “medium” gun setting. Jim Ehrman jehrman@mta.ca Sat Feb 10 T anks to all who responded to my S3400N VPSEM tungsten


fi lament questions. We do try to keep users from venting for 5 minutes aſt er turning off the HV. We probably average 50 hours per fi lament lately, which is what aggravated me a bit, because the last box was more like 80 hours. One challenge is that some users change samples multiple times in a session. T en the next user is in variable pressure changing mag and kV, vacuum, etc... then someone will run an EBSD map overnight at 30 kV, 100 probe current. We ask a lot from the SEM, in general it is a workhorse. I did have several suggestions that seem promising: T e Hitachi person called and suggested new fi laments should not be turned on until Vacuum has worked for a couple of hours. He said they try to wait more like 15 minutes between HV off and venting. Several responders further emphasized “gently” saturating the fi lament over a few hours, keeping it slightly under saturated for general use, and using some gun bias to concentrate the emission area. One respondent made a great point about using a vacuum gauge on the gun and waiting until Vacuum is in the 10 to the minus 5 range. I honestly do not know what the vacuum is when the Hitachi lets you turn on the HV. Adding more spacers to push the grid cap further away from the fi lament tip was another suggestion. I was under the impression that however many spacers that each individual fi lament came with was the correct distance as measured at the factory to keep emission current appropriate? Otherwise I handle everything with gloves and care when changing fi laments and occasionally use metal polish on the grid cap and threaded holder as well as the anode. More oſt en, I will sonicate the grid cap and holder in methanol when the gun is open to change a fi lament. T anks again for all the suggestions, I will repost this in hopes this collection of information might be useful to others with tungsten fi lament questions. Bil Schneider wfschneider@wisc.edu Sat Feb 10 In my experience, there are 3 very simple rules to extend the


tungsten fi lament lifetime: 1. Vent the microscope by nitrogen and always exchange the sample and pump the chamber quickly. T is will minimize the contamination of the vacuum chamber by water and speed up the pumping. 2. Aſt er getting HV ready signal (when the gun/chamber is pumped), wait several tens of seconds or longer before heating the fi lament - the fi lament lifetime depends on the pressure exponentially. Reaching lower 10 -3 Pa in the gun is perfect to extend the fi lament lifetime. 3. Turn off fi lament heating manually and wait several minutes before venting the chamber. Like that, the fi lament will cool down properly. Venting the fi lament when it is hot decreases its lifetime signifi cantly. By utilizing long waiting time, the fi lament lifetime around 1000 hours can be obtained easily if there is no vacuum leak in the gun. So sometimes it is a question of whether to wait longer and reach a very long lifetime or wait a shorter time and exchange fi laments more frequently (changing tungsten fi lament is cheap and easy). For someone and on some machines, exchanging the fi lament might take several minutes only so it does not make sense to wait several minutes during each sample exchange procedure. For other users/machines, exchanging the fi lament might be a nightmare so it would be better to wait longer and extend the fi lament lifetime. Tomas Hrncir tomas.hrncir@tescan.com Mon Feb 12 I have just picked up this off ering and with 50 years EM experience I fully agree with Tomas’ posting. T is procedure should be in your standard operating procedure for running a scanning electron microscope, any other way is just not good enough! Steve Chapman protrain@emcourses.com Tue Feb 13


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