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
If you try to start an ion pump when the vacuum in the chamber is in the mid 10-4 range, the gas load will be high enough to produce a visible ion plasma. Normally you don’t start the ion pumps until the vacuum is pulled down to the low 10-5 range by the turbo pump. But, sometimes you want to deliberately generate an ion plasma to help clean the ion pump elements.
Just start the ion pumps when the vacuum reaches the mid to low 10-4 Torr range. You may see that the pressure in the chamber rises to the 10-3 Torr range when the ion pump high voltage is turned on. That is OK; keep the ion pumps on while pumping the chamber with the turbo pump. You can leave them on for 5 minutes or so, then shut off the ion pump supply and let them cool down for 5 minutes. Then repeat the process. After a number of cycles, vacuum will be in the low 10-5 range and the ion pumps will start. You know when the ion pumps start because the vacuum goes into the 10-6 range and keeps improving slowly. By forcing the ion pumps to start in the high 10-4 range the resultant ion plasma helps to clean the ion pump elements.
If the pumps are loaded with argon or contaminated with hydrocarbons, you want to use oxygen to produce the ion plasma because oxygen will react with the contaminants. Assuming the ion pumps are started, back fill oxygen into the vacuum chamber to 5 X 10-5 Torr. Turn off the ion gauge and monitor the current on the ion pump control. Increase the oxygen until you get about 50mA of current on the ion pump control. Adjust the oxygen leak valve as needed to maintain 50mA or so of current. Maintain this condition for about 30 minutes, and then turn the oxygen off. As the pumps cool down the vacuum will recover and typically by the next day the ion pumps are happy once again.
For more info on ion pumps type Ion Pump Element rebuild procedure in the RBD TechSpot search box
Using Ta2O5 or SiO2 works well for aligning an ion beam to the focal point of an X-ray photoelectron or scanning Auger electron spectrometer. But, in order to optimize the ion beam focus at larger beam sizes, a Faraday cup is required.
The Faraday cup used on many Physical Electronics/PHI surface analysis systems comprises a specially configured sample mount with a molybdenum aperture that has a diameter of 250um.
Because the current measured into the Faraday cup is in the low nA range, a picoammeter (such as the RBD Instruments Inc. 9103 USB picoammeter) and bias box are required. When the bias box is set to the ion input, the target is grounded and the output of the bias box is routed from the ion lead (Faraday cup) on the specimen stage to the input of the picoammeter. When an ion beam is larger than the 250um Faraday cup aperture, only the portion of the beam that is 250um or smaller is measured. By adjusting the ion beam focus and position for maximum current into the Faraday cup, the ion beam can be aligned and the current density can be optimized for any ion beam condition. In general, larger beam sizes result in more total current and faster sputter rates.
Another benefit of using a Faraday cup is that you can also determine the electric current density using a multiplication factor. Ion current density is rated in mA/cm2.Dividing the area of the 250um Faraday cup hole into one square centimeter gives us a factor of 2037.18. So, to calculate the ion beam current density using a 250um Faraday cup, measure the ion current that enters the Faraday cup and multiply it by 2037.18 to get the current density in mA/cm2. For example, the PHI 04-303 5kV differential ion source has a maximum current density specification of 600 mA/cm2 at 5kV ion beam voltage and 25 MPa of argon gas pressure. That works out to just under 300nA of ion current passing into the Faraday cup. Typically, though, the 04-303 ion source is operated at 3-to-4kV with 15MPa of argon pressure. Therefore, the maximum ion current passing into a Faraday cup under those conditions is more in the range of 150 to 200nA.
Procedure to Maximize the Ion Current Passing into a Faraday Cup
First you need to align the Faraday cup to the focal point of the analyzer. For Auger electron spectrometers, acquire an elastic peak just to the side of the Faraday cup hole and then move the Faraday cup hole to the center of the TV image. For X-ray photoelectron spectroscopy systems, move the Faraday cup hole to the center of the system microscope’s image at the highest possible magnification setting.
Turn the ion beam ON (make sure the electron beam is off).
Set the bias box to Ion and the bias to ON. This will ground the target and apply +90V to the ion lead on the specimen stage (which in turn makes the electrical contact to the Faraday cup). Note that there are different versions of the bias box used on PHI systems. Some systems do not have bias boxes. In those cases, short out the target and and connect the picoammeter to the ion lead on the specimen stage.
While observing the picoammeter, adjust the focus (objective) and condenser on the ion gun control and the mechanical offsets (thumbscrews) on the ion gun for maximum current into the Faraday cup. This will take several iterations to optimize. Once the mechanical offsets on the ion gun have been adjusted to where no further increase in current is noted, lock them down securely and also make sure that the ion gun housing is tight. Do not adjust the mechanical offsets for subsequent focus adjustments at different condenser (COND) or beam voltage settings. Instead, you can optimize the position of the ion beam into the Faraday cup by using the offset adjustments on the ion gun control if necessary.
Note the measured current and ion gun settings in a form such as the table shown below. By optimizing a few ranges of current and using those parameters to acquire depth profiles on a standard such as SiO2 or TaO5 (both available from RBD) you can create a matrix of reproducible sputter rates.
Here is a link to a technical report in the Journal of Surface Analysis, which provides additional information:
Lanthanum hexaboride (LaB6 ) filaments provide a very stable emission of electron current in the hot cathode electron sources used in many scanning Auger electron spectrometers. However, this type of filament is susceptible to deactivation from vacuum contaminants such as fluorine.
If your LaB6 filament becomes contaminated it may exhibit symptoms such as unstable emission current or no emission current at all. The Auger data below shows instability in the background that was caused by unstable emission current from the cathode.
Usually it is possible to rejuvenate a LaB6 filament by backfilling the chamber with Oxygen while monitoring the emission current as outlined in the procedure below.
LaB6 filament: rejuvenation procedure:
Set the beam voltage to 1kV and the emission voltage to 100% (or the maximum for that beam voltage).
Increase the filament current up to the normal operating value. 1.3 to 1.5 amps is typical for a PHI 600 or 660 scanning auger spectrometer.
Bleed O2 into the system to about 5 X 10 -7 Torr.
Slowly reduce the emission voltage until you get about 50uA of emission current. Keep an eye on it, as the O2 cleans the filament the emission will rise and you will need to increase the emission voltage in order to keep the emission current from going up too much. The maximum recommended emission current is 100uA.
Once the emission current is stable then you can turn off the O2. This process typically takes 5 to 20 minutes. In some cases the vacuum chamber may have some low level contamination where the emission current of the filament will drop once the O2 is turned off. In those cases, you may want to leave the O2 on for an extended period of time at a higher vacuum such as 2 X10-8 Torr.
If rejuvenating the filament does not work then the filament may need to be replaced. RBD Instruments Inc. provides LaB6 filaments for the Physical Electronics PHI 590 through 660 series scanning auger spectrometers. Visit us at rbdinstruments dot com
