9103 USB Picoammeter Filter Settings – Part 1

Print Friendly, PDF & Email

The 9103 Picoammeter uses a continuously sampling A/D when measuring current. These samples are then averaged using an low-pass infinite impulse response (IIR) filter.

Filter Settings

When using the 9103 to sample current, you have control over the filter response and the degree of smoothing (both in Actuel in when programming the unit). The filter setting will make little difference for most constant signals, but for dynamic and periodic signals, the filter can be set to attenuate noise, or to provide detail and catch peaks.

A filter coefficient that is user-programmable determines the amount of smoothing the filter will apply. The higher the value, the more smoothing of the signal.

The filter can be set to 0, 2, 4, 8, 16, 32, and 64. A value of 0 is essentially the same as bypassing the filter. A value of 64 is the greatest amount of filtering. For most cases, values of 4, 8, and 16 will work best. Higher values may produce more accurate results for stable signals, but it will take longer for measurements to stabilize.

Example

In the examples below, a 1 Hz sine wave is sampled at a 25 mS rate, yielding 40 discrete data points per cycle. Each data point is comprised of multiple filtered A/D readings.

The filter settings used in the examples are 2, 4, 8, 16, and 32.

filter setting 2
Filter Setting 2
filter setting 4
Filter Setting 4
filter setting 8
Filter Setting 8
filter setting 16
Filter Setting 16
filter setting 32
Filter Setting 32

There’s quite a bit of noise present when using low filter values of 2 and 4, while values of 16 and 32 reduce the noise but also attenuate the signal somewhat. For this application, a value of 8 produces the most accurate result.

Conclusion

In general, any low-pass filter will of course mask high-frequency data. While the 9103 is not typically used to measure periodic signals, the filter’s effect on your application may be significant. When in doubt, start with the filter set to 8 for some noise reduction without significant smoothing or signal attenuation.

In Part 2 we’ll discuss use of the additional first-level filter implemented in the high-speed model of the 9103.

Stability testing of surface analysis optics

Print Friendly, PDF & Email

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.

Model 290 Hot/Cold module leakage current

Print Friendly, PDF & Email

The 10-325 specimen stage, which is commonly used on PHI Physical Electronics 5600 through 5800 XPS X-ray photo-electron spectrometer surface analysis systems, is often equipped with the model 290 Hot/Cold module.

The design of the Model 290 Hot/Cold module incorporates some zirconium oxide ceramics to electrically and thermally isolate the specimen holder from the specimen stage. Zirconium oxide has a very low thermal conductivity that is about 10% of the thermal conductivity of the more common alumina (aluminum oxide) ceramic material used on many UHV optics units. Zirconium oxide is a good choice for hot or cold specimen stages.

Over time, though, the evaporation of materials from specimens, as well as the evaporation of the filament on the Model 290, can coat the zirconium oxide ceramics. This coating can cause the specimen sample holder to become resistive to ground, which in turn affects the accuracy of target current measurements.  The leakage current of a coated ceramic might be several hundred nano amps.   This means that only target currents above that several hundred nano amps value would be measurable.   ISS or static SIMS (both options on XPS systems) need the ion current to be set in the range of 1 to 2 nA.  And aligning the monochromator requires measuring currents of 3 nA or less. If the specimen stage leakage current is high, then it would be impossible to accurately measure or set those currents.

To test your target leakage current, simply measure the target current using the +90V bias box and picoammeter without any electron, ion, or X-ray source turned on.   Typically, the leakage current will be just a few 10s of picoamps.  If the leakage current is much higher than low picoamps, the zirconium ceramics shown in the image below may be coated and should be replaced the next time the system is up to air for maintenance.

The replacement procedure is simple.

  1. Vent the chamber and remove the specimen stage.
  2. Place the specimen stage on a workbench with some aluminum foil (UHV foil preferred) so that you can catch parts that will come off the specimen stage while you are working on it.
  3. Remove the end screw and washer and slide out the first long ceramic.
  4. Install a new long ceramic and replace the washer and screw.
  5. Repeat for the other long ceramic.
  6. For the back short ceramic, first remove the bearing by removing the C clamp on the shaft and sliding the shaft out. Note that two thin washers that will come off the bearing shaft as you pull it out.
  7. Next loosen the two pan head slotted screws and slide out the old ceramic and replace it with the new one.  Tighten the 2 flat head slotted screws and then replace the bearing.
  8. For the front short ceramic, just loosen the two pan head slotted screws and remove the old ceramic and replace it with the new ceramic. Then tighten the two pan head slotted screws.

After replacing all of the zirconium oxide ceramics, the leakage current should be minimal. 

Reinstall the 10-325 specimen stage then pump down and bake out the chamber.

The RBD part number for the set of 4 ceramics is 290CK-4RE.    Contact us for more information

Coated Ceramics
Coated Ceramics
clean-and-dirty-ceramic
clean-and-dirty-ceramic

.

Insert new ceramic
Insert new ceramic
New ceramics installed
New ceramics installed