A blog on the repair, operation and calibration of surface analysis systems and components including electron spectrometers, sputter ion guns and vacuum related hardware. Click on the Index tab below to see a list of all posts. Visit our website at http://www.rbdinstruments.com
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:
Measure the target current and plot the results vs. time using a data logging picoammeter such as RBD’s 9103.
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.
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.
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.
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.
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.
Vent the chamber and remove the specimen stage.
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.
Remove the end screw and washer and slide out the first long ceramic.
Install a new long ceramic and replace the washer and screw.
Repeat for the other long ceramic.
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.
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.
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
1-30-20 I updated this blog post to include a video on how to isolate a thermally unstable electronic component using a heat gun and cool spray.
When a problem develops with your XPS or AES spectrometer and you contact RBD Instruments for assistance, we frequently hear the same question – “What could be the source of the problem?”. The troubleshooting electronics tips below may help to find out.
Most problems on XPS and AES spectrometers can be broken into two broad categories – Optics problems and electronics problems. Optics (electron optics) are parts of the system that are installed on the vacuum chamber and include things like spherical capacitive analyzers, cylindrical mirror analyzers, sputter ion guns, x-ray sources, neutralizers and so on. Electronics include units such as power supplies and controllers.
Optics can generally (but not always) easily be tested by simply measuring the resistances on the electrical feedthroughs as specified in the optic’s manual. For example, a 04-303 ion gun should have contact between pins 1 and 2 (the filament) only. All other pins should be open to each other and to ground. By open, we mean that the electrical resistance is infinite. If pin 3 were shorted to ground, we would know that the objective lens inside the ion gun has a flake that is shorting it out, and the ion gun needs to be taken apart and rebuilt. But an optic component can have correct resistances and still have a problem (such as an open contact). Sometimes a short in an optic can also cause a failure in an electronic unit.
Electronic units are the source of problems with XPS and AES spectrometers more often than optics. Most of the students who use XPS and AES spectrometers in a university environment do not have enough training or experience with electronics to really dig into the electronic circuitry, but there are still a number of simple steps that can be taken that may result in a successful repair.
First of all, make sure that all power is off to any electronic unit that you work with. Measurements of voltages should always be performed by personal who have been training to work with electronic components and (often) high voltage. Potentially lethal voltages are present in these types of systems. If you are not properly trained, do not attempt to measure voltages in electronic units.
But, as long as the power is off and the unit is unplugged, you can safely visually inspect the unit and the boards inside of the unit for obvious signs of damage. For the most part, these are indicated by discoloration of the circuit boards, melted traces, melted transistors, dried up capacitors, and burnt resistors. If you find some components that are visibly damaged, then replacing them may solve the problem. Or, there may also be additional problems and the damaged components are just a symptom.
Some things to look for include:
Heat damage to a circuit board: Discoloration of the board which indicates that a component has been running very hot.
Melted components: Transistors (the plastic type) can actually melt when overheated. This is easy to spot.
Cracked capacitors: When capacitors dry out (after say 20 years or more) they can fail. See the links below for detailed instructions on how to test a capacitor. Be careful with larger value capacitors as they can hold a charge for a long time. Just to be safe, discharge capacitors before you measure them by clipping a resistor (a few hundred to a few thousand ohms is typical) across the contacts. This is not necessary for small value low voltage capacitors.
Burnt resistors: Resistors that burn up can turn to carbon and be difficult to identify without a schematic and board layout. Measure the resistance of suspect resistors with an ohmmeter. Sometimes you need to lift one end of the resistor (desolder one end of the resistor from the board) when measuring resistors in circuit as other components that may be in parallel with the resistor can affect the reading.
Shorted diodes and transistors: You can use a DVM in the diode check mode to test diodes and transistors for shorts. See the links below for detailed instructions on how to do that.
Dullness on transistor housing: If a metal housing (cover) transistor overheats often the finish on the transistor housing may be dull as a result of overheating. You can compare the finish to other similar transistors and then use a DVM in the diode check mode to test the suspected transistor. Plastic housing transistors that have melted are readily apparent.
Below I have listed some links to troubleshooting electronics resources. If you can’t fix the problem on your older Physical electronics (PHI) electronic unit yourself after following the steps in this post, we can provide you with technical support, repair services, loaners or exchange units. Contact us for more info.
