In the past, optimum usage of SEM meant baking out one’s column and being cautious about wearing gloves when dealing with samples but as feature sizes decrease this may not be enough.
Hydrocarbon buildup in SEM vacuum chambers can result in less than ideal resolution. The electron beam reacts with contamination and then deposits carbon on the sample which can result in charging and degrade the image contrast. Traditionally, oxygen plasma has been employed for cleaning surfaces in a vacuum. O2 is split into oxygen ions and radicals which effectively removes organic contaminants.
Low Energy Plasma Cleaners
Low energy plasma cleaners that pull oxygen ions and radicals out of “thin air” have been produced by manufacturers. Although this may seem like “snake oil”, it actually does work. Although the approach is very slow, it really works, has very little setup, and is easy to use. A “dirty” system will involve days or even weeks of the initial cleaning.
The Evactron® De-Contaminator was modified to use welding-grade oxygen. The investigation performed denotes that there are a day and night comparison for speed and effectiveness. When compared to using ambient air for weeks, one or two four-hour sessions with oxygen were shown to be more effective.
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Figure 1. Oxygen tank attached to Evactron Decontaminator low energy plasma cleaner. Red tubing connects the bottle/regulator to the intake port of the Evactron.
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Figure 2. Before (left) and after (right) 4 hours of chamber cleaning with oxygen plasma.
Working Principle
Plasma works by transforming gas atoms into radicals and ions. Gas atoms are oscillated into a frenzy by high-frequency magnetic fields, breaking bonds and emitting a luminescent glow. The radicals and ions then perform all the work. Oxygen ions and radicals are very effective as cleaning agents. During the cleaning process, the oxygen gas radicals and ions react with the hydrocarbons within the SEM. These reactions produce CO2, CO, and H2O that are then eliminated from the system by the vacuum pumps.
When air is used as the process gas, nitrogen ions are also produced in addition to oxygen ions and radicals. Nitrogen ions contribute to the cleaning process and also recombine with oxygen ions and radicals. However, the probability of long-lived oxygen reactive species is less, as air is 70% nitrogen. When their lifetime is reduced, their mean free path is also reduced, making cleaning farther away from the plasma less likely. When using oxygen as the process gas, there is no predatory nitrogen to get in the way. As oxygen ions and radicals have fewer restrictions, they can move farther and clean better.
Conclusion
It is interesting to know how a low-energy plasma system can be transformed to work with oxygen. A trip to a welding supply store produced a regulator and a bottle of O2. XEI Scientific was all set to start with a bit of tubing and some fittings. With the regulator in place, the bottle is opened and the regulator is hardly opened to generate a constant wisp of oxygen flow. Once the tubing is attached to the leak port of the plasma system, it is ready to go.
References and Further Reading
- Neal Sullivan, et al., Microscopy and Microanalysis 2002
- T.C. Isabell AND P.E. Fischione, Plasma Science, 1998.
- http://en.wikipedia.org/wiki/Plasma_cleaning
This information has been sourced, reviewed and adapted from materials provided by XEI Scientific.
For more information on this source, please visit XEI Scientific.