Protect the CF flange knife edge

Posted on by
4
Print Friendly, PDF & Email

The CF flange (also commonly referred to as the Varian trade-marked Conflat) uses a knife edge that cuts into a soft metal gasket (usually copper) to provide a leak-tight metal-to-metal seal. “Protect the knife edge at all costs” is a common catchphrase at RBD Instruments that we like to drill into anyone who works on a UHV vacuum chamber. Simply put, it means to use care when removing used copper gaskets from Conflat (CF) flanges, and to protect the flange edge with aluminum foil when it is being stored on a shelf. Even a small scratch on the knife edge can result in a leaky seal on the flange.

Specialized tools are available for removing copper gaskets, but most labs do not have them. So, what is the best way to remove a copper gasket from a CF flange if you do not have a special tool? IMO you should use a clean regular channel lock or vice grip type of pliers whenever possible to remove used copper gaskets instead of a flat blade screwdriver. You need to use pliers that have teeth, not the smooth ones. In some cases you will not be able to use pliers as there is not enough room to get the pliers onto the gasket without risking damage to a ceramic or some other optic part. In those cases, you will need to use a straight blade screwdriver.

To use pliers, you simply position the jaws of the pliers on the inner and outer diameters of the gasket and tilt the pliers back. The applied leverage will pop the gasket out of the groove. Be sure to get a good grip on the edge of the gasket as quite often the gasket is pressed in very tightly and will be somewhat difficult to remove. Other times the gaskets will practically fall out by themselves. It just depends on the fit of the flanges and whether or not the system has been baked out (the copper expands during bake-out).

When using a screwdriver, extreme care must be taken to ensure that the screwdriver does not slip off the side of the gasket and damage the knife edge once the gasket pops out of the flange recess. You can reduce the chances of damage by using a screwdriver that has a tip that is flat with sharp edges and slightly thinner than the height of the exposed gasket edge. Apply downward pressure on the tip of the screwdriver as you try to pry the gasket up. Sometimes the gaskets can be wedged in very tightly after compression and be difficult to remove. Use the tip of the screwdriver to leverage the gasket out; do not push the screwdriver. This is when one is most likely to damage a knife edge: if the screwdriver slips as the gasket comes loose, it may scrape across the knife edge and cause a large scratch or gouge. Once that happens, the flange may no longer seal properly.

If you do gouge a knife edge you might be able to still get it to seal by using a small piece of a soft vacuum-compatible metal such as gold or platinum to fill in the void on the gouge. Small scratches can be polished out with some ultra-fine emery cloth. If the gouge is too big then the flange may need to have the knife edge touched-up with a metal lathe. That is not always practical or even possible. So, the best thing to do is to… protect the knife edge at all costs.

When flanges are being stored for long periods of time or are being stacked up on a shelf, the best way to protect them is to wrap them in UHV aluminum foil. You can purchase UHV compatible aluminum foil from All-Foils Inc. http://www.allfoils.com/single-product/uhv-foil/

Below are some pictures of the correct and incorrect techniques to remove copper gaskets from CF flanges. If you need to purchase some copper gaskets for your UHV vacuum chamber and also want to pay the lowest possible price, RBD Instruments Inc. now provides copper gaskets for CF flanges in small or large quantities at low prices.  Click on this link for more information // Copper gaskets for CF flanges // or visit us at our website by going to RBD Instruments dot com.

Ion Plasma to clean ion pumps

Posted on by
1
Print Friendly, PDF & Email

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.

http://youtu.be/vqTTybwSDl0

There are two ways to do this.

  1. 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.
  2. 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

Faraday cup procedure to align ion beam current

Posted on by
Reply
Print Friendly, PDF & Email

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