Dr. Jens Rafaelsen – Applications Engineer, EDAX
One of the things I learned during the 2015 Microscopy and Microanalysis meeting was just how efficient plasma cleaners really are and this is a short story about how it saved the day for us. We had shipped our older Hitachi S3400N microscope from Mahwah to Portland for the show and had tested everything before it went on the truck. The meeting opened Monday August 3 at noon so Sunday was set aside for getting everything set up and calibrated. While our service group had done most of the work, I had a bit of data I wanted to collect for the days to follow. So I sat down at the microscope, turned on the beam, and stared at the current meter showing next to nothing. I checked the usual microscope settings and fidgeted with the apertures but still couldn’t get a decent current down through the column. Since we were a little short on time and the Hitachi booth was close by, we went over and looked sufficiently desperate for the Hitachi service guys to take pity on us and come to help.
I noticed the Hitachi guys going through the same steps I had done and end up with the same problem, so at least it wasn’t just down to my short comings regarding microscope service. As the last step they pulled out the aperture strip and the black gunk covering all three apertures gave us a pretty good indication of the problem: the beam was being severely attenuated simply because the apertures were clogged up with carbon contamination. Of course the Hitachi guys’ immediate question was “Did you bring a new aperture strip?” and my answer was a meek “No…”. But then I remembered that I did bring a plasma cleaner. I didn’t really believe that it would be able to do much with the level of contamination that we had on the apertures but it was still worth a shot. So I put the aperture strip in the cleaner chamber and ran it at a pressure of 2*10-2 mbar with a power of 50 W.
I have to say that I was extremely surprised when the aperture strip looked as good as new after only 10 minutes of plasma exposure. Both the EDAX and Hitachi service guys were equally impressed and after mounting the strip back in the column we were up and running again. So 10 minutes of plasma cleaning saved us from having to either try to have an aperture strip shipped in overnight or run the microscope with no aperture and ensuing risk of sample damage and reduced imaging capability. Unfortunately I didn’t take any pictures before and after cleaning, as I honestly was not expecting it to work, but the picture below shows us busy running demos on the Hitachi during the show.
At this point you might wonder why I had brought a plasma cleaner in the first place. Well, one of the things that we were highlighting with the new Octane Elite detector we launched at the show was the silicon nitride window and its durability. I had run a test on my office desk with a live detector mounted directly on an asher chamber (shown in Figure 1) that I borrowed from Vince Carlino of ibss Group, Inc. When the asher chamber is running, it looks like something out of a science fiction movie so we wanted to do something similar at the M&M meeting as a visual prop.
Since a full detector takes up space we simply put a single detector module directly in the asher chamber and started the cleaning process on Monday when the exhibition began. I took pictures of the controller for the system and the module at the start and end of each day as can be seen in the picture sequence below.
After almost 76 hours of continuous plasma exposure, the silicon nitride window shows no signs of degradation and knowing what plasma cleaning did to the aperture strip, I am pretty certain that was absolutely no carbon contamination on the window. Of course this is more of a show-and-tell kind of experiment and the testing I did before this involved detailed monitoring of the module performance and temperature to detect any pin-holes that would not be visible by eye. That report will be available shortly.
The next step will be to try the same with a polymer window but I am still thinking about exactly how to design the experiment. Of course I could just clean it for an extended period of time and see if the window is still intact but it would be nice to have a metric of how fast the damage occurs (or not). One idea would be to use a bare window and correlate the ratio of the carbon and aluminum signal of the window to the silicon peak from the support grid in order to monitor any changes in thickness, but if anyone has other suggestions, I would be happy to hear them.
Even though the tabletop plasma cleaner has been around for a number of years, its complete usefulness is sometimes is overlooked because it is a small piece of auxillary equipment. Sometimes, however, the smallest of equipment can provide the largest benefit!