This blog will describe the symptoms and solution for lens elements that charge up due to an oxidized graphite coating.
Overview – The Physical Electronics SCA (Spherical Capaitive Analyzer) has a series of 2 or 4 lenses that focus electrons into the energy analyzer section of the SCA. The first two lenses determine the analysis area and the second two lenses focus the electrons into the SCA for optimal counts and energy resolution. The voltages applied to the lenses change as a function of the kinetic energy of the electrons being detected. The sketch below shows the general concept on an SCA that uses a Position Sensitive Detector. Most SCAs today have a MCD multi-channel detector but the lenses work the same way.
The 5600 XPS system lenses are constructed out of stainless steel. In order to reduce the secondary electron yield (a lower secondary electron yeild improves energy resolution), Aquadag is sprayed on the inside of the lens surfaces. Aquadag is basically pure carbon, and carbon has a low secondary electron yield.
Problem – unstable data at higher pass energies, larger analysis areas, or higher x-ray source power.
Normally charging presents itself as unstable data with very high spikes in the counts followed by rapid discharges. In this case, the problem presented itself more like a digital step problem with very repeatable steps in the data at particular eVs. Another interesting effect was the ratio of peak heights would also change as a function of the pass energy, analysis area or x-ray source power.
Initially the symptoms pointed to the MCD multi channel detector or the chevron plates. The MCD was pulled and inspected an no problems were seen. The chevron (channel) plates were replaced and that did not change the symptoms.
Another clue was that if the lens cables were removed from the SCA and the lens elements shorted to ground, the data looked correct. In addition, one lens could be grounded and the other lens could have voltage applied to it and the data would also look correct. However, if the area of one of the the lenses were changed (by selecting large area mode) the problem would return, even with the other lens still grounded.
The conclusion was that the surfaces of the lenses must be charging, but only at large areas where more electrons would fill the lenses.
The SCA lens was removed and the resistivity of the lens elements were measured. The resistance of the lens coating would vary from tens of ohms to thousands of ohms depending on where the measurements were made. These resistance measurements matched the symptoms as a high resistance surface would not conduct the electrons that hit the inside of the lens cylinders.
Aquadag works well to reduce secondary electrons. But if exposed to air for extended periods of time it (evidently) can form an oxide layer which increases the resistance of the coating substantially.
A clean stainless steel scouring pad with a very light touch was used to break the oxide layer without removing too much of the Aquadag coating.
The technique used was to lightly rotate the scouring pad inside the lens elements and then check the resistance of the lens coating. The resistance would gradually drop with each rotation of the scouring pad. When the resistance dropped close to a few ohms, no further scouring was done.
When this process was completed, the inside of the lens elements were conductive but still black, so most of the Aquadag coating was still intact.
After reinstalling the lens, pumping down and baking the vacuum chamber, the SCA performed correctly.
If you are experiencing this problem please contact RBD Instruments for more details.