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Using evaporators

E-beam evaporators


These evaporators can be used for deposition of materials that exhibit sufficiently high vapour pressures only at high temperatures. The material can be:
  • in a form of a wire/rod of diameter between 0.5 and 2 mm, if evaporating below its melting point, or
  • placed in the electrically conductive crucible (usually C, Mo, Ta), if mechanically unstable or evaporating above its melting point.
The material or the crucible is heated by electrons emited from a hot W filament and accelerated by high voltage. The filament is covered by a water-cooled Cu shield in order to prevent outgasing due to warming up during operation.

The most difficult materials evaporated so far at the Materials Science Beamline were Nb, Ta and W.

The evaporation can be non-reactive (in vacuum) or reactive (e.g. Ce in O2 for deposition of CeO2). Maximum allowed pressure is 1×10-6 mbar.

All our evaporators have shutters for well-defined deposition timing and have water cooling.


Tectra evaporators (single)

This evaporator can be fed by wires, rods or crucibles. The position is retractable by 25 mm which can compensate the wire/rod shortening due to its evaporation.

The filament is floating but one of its sides is grounded in the connected power supply.

There is a thermocouple attached to the Cu shield that allows to check its temperature (used mainly during degassing).



Oxford evaporator (quadruple)

This evaporator has 4 different pockets but they cannot be operated at the same time:
  • Pocket 1 is retractable by 25 mm which can compensate the wire/rod shortening due to its evaporation. We mount there wires or rods. The high voltage is connected to the HV connector on the retractor.
  • Pockets 2, 3 and 4 are not retractable. We mount there crucibles. The high voltage of these three pockets is connected to another HV connector in the top part of the evaporator.
One side of every filament is grounded to the evaporator body,, the other side is connected to the filament vacuum feedthrough as shown in the following scheme:

Therefore only the active wire (marked red) of the cable coming from the power supply is to be connected to the pin corresponding to the desired pocket. The current returns to the power supply through the grounding wire connected on the CF63 viewport flange.

There is no thermocouple on the Cu shield so we never degas this evaporator without water cooling.

Power supplies

We have two power supplies. They both have a feedback loop keeping the emission current stable via automatic regulation of the filament current.




Usage

  1. Check water and electrical connections. Especially in the case of the quadruple Oxford evaporator be careful to connect the HV and filament connectors to the contacts corresponding to the desired pocket of the four possible ones.
  2. Open water cooling circuit, first the left (inlet) valve fully and then the right (outlet) valve just a bit while observing the pressure on the manometer. It should be around 1.5 bar. Check that there are no water leaks.
  3. Check that the electropneumatic valve 10 towards the beamline is closed and that ion gun, LEED, electron analyzer, X-ray source etc. are switched off.
  4. Set the sample position for evaporation. Approximate values are:

      Top retractable
    evaporator
    pointing downwards)
    Short Tectra
    evaporator
    (horizontal)
    Bottom retractable
    evaporator
    (horizontal)
    Oxford evaporator
    pocket 1
    (horizontal)
    Oxford evaporator
    pocket 2
    (horizontal)
    Oxford evaporator
    pocket 3
    (horizontal)
    Oxford evaporator
    pocket 4
    (horizontal)
    x 25 13 5 5 5 5 5
    y 25 5 15 23 23 ?? ??
    z 260 120 113 118 108 108 118
    Θ 180° 30° 60° 90° 90° 90° 90°

    Risky positions (marked in red) should be always set while observing the situation inside the chamber. There is a high risk of crash!
  5. Check through the viewport that the shutter is correctly closed, especially on the Oxford evaporator with four possible pockets.
  6. Check that all 3 knobs (high voltage, emission and filament) on the corresponding power supply are fully countherclockwise. Then switch on the power supply.
  7. Set high voltage to the desired value (usually 1 kV) on the kV-meter..
  8. Set the filament knob to the desired filament current limit but never more than half of the range. It corresponds to 8 A limit. Higher currents might damage the filament.
  9. Set the emission knob to 0.1 (on the knob scale). Wait and observe the filament current increase on the A-meter. It should slowly rise up to 2-4 A and then stop. It means that the emission-filament feedback loop is synchronized.
  10. Now, using the emission knob, slowly increase the emission (read on the mA-meter switched to the correct range below) to the desired value while observing the pressure in the chamber.

    Note: The red LED labelled under emission is normally off and the filament currents are 5 A (Tectra) or 6 A (Oxford). The LED turns on during the operation if the desired emission cannot be reached with the filament limit. It can mean that:
    • the evaporator is broken or incorrectly connected (if the A-meter shows 0), or
    • the filament knob is set too low (if the A-meter shows <5 A), or
    • the evaporated wire/rod needs to be inserted more inside the experimental chamber (if the A-meter shows 5-8 A).
  11. Wait at least 5 minutes to preheat the evaporated material in order to reach stable deposition rate. Especially, warming up huge crucibles is much slower thin rods or wires.
  12. Open the shutter and start the timer. Check through the viewport that the shutter is correctly open.
  13. During the deposition observe the pressure in the chamber and the stability of the parameters on the power supply. Adjust if necessary. Write down all parameters into the labbook.
  14. When the timer beeps close the shutter. Check through the viewport that the shutter is correctly closed, especially on the Oxford evaporator with four possible pockets.
  15. Slowly decrease emission to zero, then filament to zero, and then high voltage to zero.
  16. Set the sample position for the next experimental procedure. Again, observe the situation inside the chamber all the time and be careful not to crash anything!
  17. After several minutes (or at the end of your shift) close water cooling circuit, first the right (outlet)  valve and then the left (inlet) valve.
  18. Switch off the power supply.

Last Updated on Friday, 22 February 2019 13:39