Stability testing of surface analysis optics

There are two easy ways to check the stability of the electron or photon source on an X-ray photoelectron spectrometer, Auger electron spectrometer or Scanning Electron microscope:

  1. Measure the target current and plot the results vs. time using a data logging picoammeter such as RBD’s 9103.
  2. Acquire a depth profile region over a wide energy range but do not turn on the sputter ion gun.

Method 1 – Plot the target current vs. time.

As shown in the pictures below, plotting the target current versus time shows the stability of the electron beam as well as trends in the current.  In this case the current being measured is an electron beam in the range of approximately 300nA.

Electron Current vs time
current measurement display

By changing the scale of the plot, you can see finer details of the current stability and any trend. In this measurement the current drifted up by about 30 nA over a 2 hour period and started to stabilize after the first hour. Room temperature changes can effect the stability of electron optics as can thermal mass of the electron source.

close up of current measurement

Measuring target current vs. time works well for electron beams on Auger spectrometers and SEMs, as well as secondary electrons generated by X-ray sources on XPS systems.  The secondary electron current generated by X-ray sources is directly proportional to the X-ray flux.

Method 2 – Wide energy range depth profile.

For this method you want to set up a region for a depth profile that is at least 1000 eV wide.  In the example below we acquired from 1000 to 0 eV on a silver sample, 2 sweeps per cycle. Normally the ion gun is turned on for a depth profile but for this test the ion gun is not turned on.

In the picture below you can see the Profile vs. time display where the highest count in each cycle is displayed.   

Depth profile vs time

This picture of all 95 Ag cycles super imposed shows that the stability is pretty good.   A depth profile test like this tests not only the X-ray source stability, but also the analyzer voltages, electron multiplier and detector electronics. You can do the same test with an AES system which would test the electron gun as well as the analyzer,electron multiplier and detector electronics.

This picture below shows the first cycle and the next picture shows the last cycle.  If you look very closely you will see a small increase in the carbon peak that coincides with the over all slight drop in the intensity of the Ag profile vs. time display.  Carbon will typically increase over time in UHV systems due to adsorption and desorption effects.

If you have some extra time you may want to run one of these test methods on your XPS, AES or SEM.   The results can be interesting and if nothing else will let you know that your system is stable.