Monday, July 30, 2012

AuPd Coating Procedure Part 3: Configuring Plasma

For sputtering, a partial pressure of Ar is required in the sputtering chamber. As such, it is counter productive to rapidly pump away the Ar introduced as a sputter agent. To increase the Ar base pressure in the sputtering chamber we first set the pump to "half speed" by pressing the "L" button on the turbo pump controller.  Look here if you can't find the half speed button. The turbo will gradually slow down, but will do so more quickly as the gas load is increased when argon is introduced. As the turbo slows several of the green LED's on the pump controller will dim. Look here to locate the half-speed button and the turbo speed indicating LED's.

First one opens the Ar cylinder using the valve V1. Make sure the regulated pressure is just a few PSI as indicated by the red arrow. One is leaking a very small amount of Ar into the sputter chamber, so a large pressure head is not required as there is very little gas flow. Open V2 to open the gas cylinder to the sputter coater.

Ar can now be leaked into the sputter coater using the leak valve shown to the left. This valve is easily damaged, so do not over tighten. Only finger tighten to close. Again, without a deposition rate monitor, the film thickness can only be determined by recipe, and for the current deposition calibration an Ar partial pressure of 5x10-2 mbar was used. As Ar is leaked into the chamber the turbo pump, now set to half-speed, will start to slow-- which will reduce the pumping efficiency. As a result the Ar partial pressure will be unstable until the turbo pump fully reaches half speed. Once the pressure is stable one can move on to the next step which is setting the high voltage for the desired plasma current.

Make sure the sputter coater is configured for sputtering by setting switch "B" to HTS which stands for "high tension supply". Now knob "C" controls the high voltage supply. On the high tension supply unit set switch "N" to SPUT for sputtering.

At this point we have a predetermined sample-target distance of 25 mm and a Ar partial pressure of 5x10-2 mbar. We now need to set a fixed deposition time. As determined in the previous AuPd coater calibration, for the parameters above a 55 s deposition time yields a 25 nm coating at 30 mA emission current.

The deposition time can be set using the set of push buttons indicated by "Q".

Now with the shutter closed, press the green high voltage START button "O" and turn the knob "C" up to somewhere between 5-6 to read a plasma current of 30 mA on meter "R". When the START button "O" is pushed, the timer will start counting down as indicated by the LCD display "P". If the timer times out then the voltage will turn off and the plasma current will naturally drop to 0 mA. Press "O" again and continue to adjust knob "C" to set the desired plasma emission current. It's important to perform this step with the shutter closed or material will be deposited on the samples while one adjusts the coater parameters.

Since the plasma current is a function of the Ar partial pressure, the plasma current reading at meter "R" will vary over time if the turbo pump is not fully at half-speed and the Ar partial pressure isn't stable.

In summary:
  • Set coater to HTS mode.
  • Set high tension supply to SPUT.
  • Make sure target shutter is closed.
  • Set timer to the desired deposition time.
  • Press high tension supply START button-- timer starts.
  • Adjust knob "C" to desired emission current on meter "R".
  • Proceed to AuPd Coating Procedure Part 4: Coating and Venting

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