Faraday cup procedure to align ion beam current

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

Sputter rate standard

 Ta2O5Sputter-Rate standard Recipe

You can make your own 1,000 Å anodized tantalum sputter rate standard samples using the procedure below. Note that storing, using and disposing of any acid can be dangerous. Do not attempt this procedure unless you have the proper safety equipment and a system for handling the waste.

  1. “Polish” two .125 mm thick foils of tantalum by dipping them for up to 1 or 2 minutes in an acid solution (59.0% H2S O4; 17.0% HF; 24% HN03).
  2. Pass the samples through two rinses of deionized H20.
  3. Blow them dry with filtered dry Nitrogen gas (N2).
  4. . Using one “polished” foil as an anode and another as a cathode, apply 66.6 V DC between them while they are suspended in an electrolyte (94.3% deionized H20; 5.7% HN03) One of these plates is sufficiently anodized when the current drops to zero.
  5. Rinse the “gold” anodized specimen in acetone. The “gold” color indicates 1,000 Å of Ta2O5.
  6. Blow it dry with filtered dry Nitrogen gas (N2).

If you don’t want to deal with the chemicals that are involved to make your own 1,000 Å thick Ta2O5 sputter rate standard, you can purchase one from RBD Instruments at this link: RBD Sputter Standards 

SiO2 sputter rate standard

RBD Instruments also provides a 1000 SiO2 sputter rate standard.  The figure below shows the orientation of the wafer which makes it easy to break the wafer into smaller pieces which you can mount into your system.

how to cut SiO2 wafer

 

 

 

 

 

 

 

Why do you need sputter rate standards anyway? Each ion source will produce a different sputter rate depending on the conditions that the ion source is operated at, as well as other factors such as the angle of the ion source to the sample. Changing the beam voltage, condenser and focus (beam size), pressure (amount of argon or other gas) and raster area all affect the sputter rate. By using a sputter rate standard you can characterize your ion source for a particular set of operating conditions for a known thickness of standard material (Ta2O5 or SiO2).

To further complicate things, the sputter rate of different materials varies greatly and that makes it very difficult to accurately know the true sputter rate for compounds.

Follow the link to this article for some very helpful insights into sputter rates:

Sputter Rate Information

RBD Instruments can provide sputter rates for some common materials upon request.  To contact us for that information, please go to the Technical Support page of our website at rbdinstruments dot com and go to Support – Technical Support

 

 

How to align the 04-303 ion gun

This post explains how to align the Physical Electronics 04-303 ion gun typically found on PHI Auger electron spectroscopy and X-ray photoelectron spectroscopy systems. The alignment principles explained here will apply to just about any surface analysis ion source.

First, here is a link to a video that explains all of the alignment methods: 04-303 Ion gun Alignment Video

Next, here is a link to a tech tip that explains the theory and operation of the 04-303 ion gun: 04-303 Ion Source Theory and Alignment

Finally, here is the basic operation and alignment taken from the tech tip:

04-303 Ion Gun Operation

Basic Operation:

1. On the 11-065, set the Emission/Pressure meter switch to Emission. Make sure that the scale switch is in the X1 (times one) position.

2. Press the Diff Pump Ion Gun button on the AVC remote, or manually pump the ion gun.

3. Slowly turn up the Emission knob until you have 25mA of emission current (X1 position).

4. Switch the Emission/Pressure meter switch to Pressure.

5. Slowly open the argon leak valve on the 04-303 ion gun until you have 15 mPa of pressure on the meter. This corresponds to approximately 2 x 10-8 torr when differentially pumped, and 2 x 10-7 torr when not differentially pumped.

You are now ready to sputter. When you turn the ion beam voltage on, the ion gun will be sputtering.

Alignment: Visual Method

This works in both ABS and SED image modes. SED mode is sometimes easier to work with.

1. Insert a SiO2 sample and position it to the focal point of the analyzer. Use 30o to 60o of tilt.

2. Get a low magnification image of the SiO2. Use a low electron beam voltage, such as 1.5kV in order to get the largest possible image size (the lowest possible magnification).

3. Set up the ion gun as discussed above. Set the condenser to 5.00 (the smallest spot size) and the objective to 3.40.

4. Turn on the ion gun beam voltage. If the electron beam current and the ion beam current are approximately the same value, the ion beam spot should be visible on the TV monitor.

5. Mechanically adjust the position of the ion gun (turn the thumb screws) to center the ion beam spot on the TV monitor. Adjust the OBJ for the smallest spot size.

For more information or to order a replacement ionizer for your 04-303 ion gun, visit our website at www dot rbdinstruments dot com