Faraday cup procedure to align ion beam current

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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.

faraday cup sample mount

 

 

 

 

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.

faraday cup

 

 

 

 

 

 

 

 

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

 

  1. 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.
  2. Turn the ion beam ON (make sure the electron beam is off).
  3. 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.
  4. 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.
  5. 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.

Ion sputter rate table

 

 

 

 

Here is a link to a technical report in the Journal of Surface Analysis, which provides additional information:

http://www.sasj.jp/JSA/CONTENTS/vol.14_2/Vol.14%20No.2/Vol.14%20No.2%20124-130.pdf

And here is a link to a video that shows an ion source being aligned using a Faraday cup – http://www.youtube.com/watch?v=uKg9GLkXT3s

LaB6 filament Rejuvenation

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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.

Unstable Auger data

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:

  1. Set the beam voltage to 1kV and the emission voltage to 100% (or the maximum for that beam voltage).
  2. 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.
  3. Bleed O2 into the system to about 5 X 10 -7 Torr.
  4. 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

Ion Beam Induced Low Energy Electrons

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For the purpose of checking the performance of a surface analysis spectrometer such as a cylindrical mirror analyzer (CMA) or spherical capacitive analyzer (SCA), looking at an ion induced low energy electron peak can be extremely helpful. The peak typically occurs at about 20 to 50 eV and the size if the peak is directly related to both the alignment of the ion beam to the analyzer as well as the amount of ion current.

Checking XPS Performance

Set up an alignment for a range of zero to 100 eV kinetic. The eV range in binding energy depends on which anode energy you have selected in the software. See the table below.  Most systems use an Al anode, so the energy would be 1480 to 1380 eV (which is about 0 to 100 eV kinetic).

  1. Using a blank sample mount, position a sample to the focal point of the analyzer.
  2. Look under the Hardware Properties Menu for XPS and note the X-ray Anode type.
  3. Set up an alignment with the following parameters:
  • Upper Limit 1480 eV if Al is the anode, 1250 eV if Mg is the anode.
  • Lower limit 1380 eV if Al is the anode, 1150 eV if Mg is the anode
  • EV per step .5 (or the closest selection .5)
  • Time per step 20 ms
  1. Start the alignment and turn on the ion gun (no raster). You should have a low energy peak at around 20 to 50 eV kinetic.
  2. If necessary, reduce the ion gun beam current to prevent the detector from saturating. (You can increase the ion gun condenser lens setting or reduce the emission current in order to reduce the ion beam  current).

If you do not get the peak, then you have a problem with the analyzer or analyzer electronics. If you do, then the analyzer and  electronics are probably OK.

This is a very useful technique for isolating a low signal XPS problem between the analyzer and the X-ray source. You can also use the low energy peak to rough in the alignment of the ion gun to the XPS analyzer focal point.

Low energy peak of Mg anode

 

 

 

Checking AES Noise Level

Analyzer noise (noisy data) can be caused by these things:

  • Poor contact between the inner and outer cylinder terminating ceramics
  • Analyzer control
  • Electron Multiplier supply
  • Electron gun control or Electron gun high voltage supply

This technique will help isolate analyzer noise by determining if it is related to the electron gun, which in tern would be caused by the electron gun control or electron gun high voltage supply.

Overview

This procedure uses both the electron gun and ion gun as a source to generate low energy electrons. By comparing the relative noise levels, you can determine if the problem is related to the electron beam only, or both beams.  If it is related only to the electron beam, then the problem is in the electron gun control or electron gun high voltage supply.

If both the electron and ion beams are noisy, then the problem is either the analyzer control, multiplier supply or poor contact in the analyzer.  The analyzer control and electron multiplier supplies can be tested for noise using the appropriate calibration procedure.

 Procedure

This procedure was written specifically for a Physical Electronics 600 scanning auger system, but the principles can be applied to other systems as well.

Set up an alignment with these parameters:

Lower Limit 0, Upper Limit 100, EV per step 1, Time per step 20 ms

In AugerScan, go to the Multiplier Properties dialog box and uncheck the Auto EMS box. This will keep the computer from trying to automatically set up the electron multiplier voltage.

  1. In AugerScan, go to the Hardware Properties dialog box and make sure the input is VF1.
  2. With the electron beam on and set up for a normal elastic peak, start the acquisition and manually adjust the 32-100 CMA electron multiplier until you have a maximum count rate of approximately 100Kcps.  You will see a low energy peak around 20 to 50 eV depending on your sample.
  3. Use the yellow cycle stop button to end the alignment and then save the file.
  4. Blank the electron beam and turn on the ion gun. Do not use any raster.
  5. Start the acquisition and manually adjust the 32-100 CMA electron multiplier until you have a maximum count rate of approximately 100Kcps.
  6. Use the yellow cycle stop button to end the alignment and then save the file.

Compare the two files to determine whether or not they have similar amounts of noise.  In the examples shown below, then electron gun as a source exhibits more noise than the ion gun as a source.  In this instance the problem was isolated to a noisy emission supply in the 20-610 High Voltage supply on a 600 system.

Electron gun noise

 

 

 

 

 

ion gun noise

 

 

 

 

 

Ion Gun Alignment

On systems that do not have scanning electronic guns for TV imaging, you can use the low energy peak to center the ion beam with respect to the analyzer focal point. If you have scanning then you can simply look at the ion beam in real time on a SiO2 sample.

 AES Ion Gun Alignment Procedure (for non-scanning AES):

Using a blank sample mount, position a sample to the focal point of the analyzer (Elastic peak).

  1. Set up an alignment with the following parameters:
  • Lower limit 0 eV
  • Upper Limit 100 eV
  • Time per step 20 ms
  1. In the Multiplier Properties dialog box, un-check the Auto EMS Box.
  2. In the Hardware Properties dialog box, make sure the input is V/F1.
  3. On the 32-100, set the CMA multiplier switch to Analog and make sure the potentiometer is fully CCW.
  4. Start the alignment and turn on the ion gun (no raster).
  5. Slowly turn up the 32-100 CMA multiplier supply (or the 20-075 multiplier supply if you have an older system) until you have about a 100K cps low energy electron peak at 20 to 50eV.  This should occur at no more than 2000 volts on the multiplier (5.0 on the 32-100 potentiometer).
  6. Finally, adjust the X and Y position of the ion gun for maximum signal. The ion gun is now aligned to the focal point of the analyzer.

XPS Ion Gun Alignment Procedure:

Using a blank sample mount, position a sample to the focal point of the analyzer.

  1. Look under the Hardware Properties Menu for XPS and note the X-ray Anode type.
  2. Set up an alignment with the following parameters:
  • Upper Limit 1480 eV if Al is the anode, 1250 eV if Mg is the anode.
  • Lower limit 1380 eV if Al is the anode, 1150 eV if Mg is the anode
  • EV per step .5 (or the closest selection to .5)
  • Time per step 20 ms
  • Pass Energy 100 (or the closest selection to 100)
  1. Start the alignment and turn on the ion gun (no raster). You should have a low energy electron peak at around 20 to 50 eV kinetic.
  2. If necessary, reduce the ion gun beam current to prevent the detector from saturating. (You can increase the condenser lens setting or reduce the emission current in order to reduce the ion beam  current).
  3. Finally, adjust the X and Y position of the ion gun for maximum signal. The ion gun is now aligned to the focal point of the analyzer.  Once roughed in you can use a piece of TaO5 to check the alignment of the ion gun with respect to the system microscope because when you burn through the oxide layer you will see a blue ring on the TaO5 sample. RBD Instruments provides TaO5 samples for this purpose.