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Using evaporators

Differentially pumped ports

On the analysis chamber two CF38 gate valves with roughing ports are installed. On the preparation chamber - bottom one such valve can be installed. These gate valves, accompanied by a z-retractor and pumping valve connection, permit to exchange the inserted evaporator without venting the whole chamber. The pumping is done by an auxiliary turbopumping station.

The inserted evaporator can be up to 35 cm long, which allows the minimum sample-to-evaporator working distance about 12 cm.

Evaporator exchange:

  1. Disconnect the electrical and water (if any) connection from the old evaporator. Blow out the residual water from inside using compressed air.
  2. Carefully retract the old evaporator using the corresponding z-retractor. It is good if the device is shaking a bit which allows to pass safer through the copper gaskets. Stop and shake more if you feel sudden change of resistance during the retraction.
  3. Fully close the gate valve.
  4. Check that the pumping valve is closed.
  5. Slowly loosen the nuts on the evaporator flange. The area will be vented through this flange. Observe the pressure in the chamber all the time. If it starts increasing considerably tighten the gate valve more in order to minimize the leak.
  6. Remove the old evaporator and prepare the new one. Use a new copper gasket.
  7. Install the new evaporator into the retractor. Be careful with its rotaton. Some parts too out of axis might interfere later with the retractor mechanism during the insertion. Moreover, gravity might influence the alignment of the internal part of the evaporator; the parts too out of axis (at the chosen rotation) might not pass well through the gate valve.
  8. Tighten the nuts on the evaporator flange.
  9. Perform an electric check of the evaporator feedthroughs (that filament or thermocouple resistances are correct with no short contacts).
  10. Connect the big turbopumping staton to the roughing valve of the evaporator.
  11. Open the roughing valve.
  12. Start the turbopump. The pressure should decrease fast in the 10-5 mbar range or below.
  13. Repeat the electric check of the evaporator.
  14. Perform a bakeout of the evaporator area. A 165 W/1.1 m-long heating tape and several layers of aluminum foil are enough to reach 130 °C. Reasonable bake-out time is several hours or overnight.
  15. When the evaporator cools down a bit remove the aluminum foil and the heating tape.
  16. Once again repeat the electric check of the evaporator.
  17. Slowly open the gate valve towards the chamber. Observe the pressure in the chamber. It should not increase above 10-8 mbar.
  18. Carefully insert the evaporator in the chamber using the z-retractor. Again, it is good if the device is shaking a bit which allows to pass safer through the copper gaskets. Stop, go back and shake more if you feel sudden change of resistance during the insertion.
  19. When the evaporator is fully inserted close the roughing valve.
  20. Connect electric cables and water cooling pipes (if any) to the evaporator.
  21. Perform a degas procedure of the evaporator.


Knudsen-cell evaporators

We have two simple home-made Knudsen-cell evaporators. They have no shutters and no water cooling. They can be used only in the preparation chamber - bottom.



The cell consists of a crucible made of alumina ceramic tube with a tantalum heating spiral wound around it. The backside of the ceramic tube is closed with a piece of aluminum foil into which the K-type thermocouple for temperature reading is inserted. Temperatures up to 400 °C can be reached.


For the operation we use the dedicated controller containing pdfDelta Elektronika ES015-10 power supply and Eurotherm pdfEPC3016 temperature regulator. The regulation loop is active when the switch is in the KEEP TEMPERATURE position. For the most typical temperatures up to 200 °C (set by up/down arrows)  the CURRENT LIMIT should be set to 2.0-2.5 A.
 

Alternatively, PS2342-06B DC power supply and thermocouple voltage reading on multimeter can be used (recalculate voltage to temperature using  a table).

The deposition is timed by rotating and/or translating the sample holder attached to the transfer rod. The evaporator is to be preheated sufficiently in advance, in order to provide constant deposition rate. The rate can be checked by quartz crystal microbalance and adjusted by the heating current while observing the crucible temperature.

After use the crucibles should be properly cleaned (in supersonic bath, solvents, furnace...) and degassed by annealing in vacuum in order to avoid cross-contamination of the following material to be evaporated.



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 an 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 emitted 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 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 e-beam evaporators have shutters for well-defined deposition timing and are water-cooled.


Tectra evaporators (single)

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

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

There is a thermocouple attached to the Cu shield that allows to check its temperature 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 for the wire/rod shortening due to its evaporation. We mount there wires or rods. High voltage is connected to the HV connector on the retractor, in the evaporator axis.
  • Pockets 2, 3 and 4 are not retractable. We mount there crucibles. High voltage of these three pockets is connected to another HV connector, places at the top of the evaporator and inclined.
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 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
    (horizontal)
    pocket 1
    Oxford evaporator
    (horizontal)
    pocket 2
    Oxford evaporator
    (horizontal)
    pocket 3
    Oxford evaporator
    (horizontal)
    pocket 4
      usually Pd rod Ce in Mo crucible usually Co rod Pt wire Sn in Mo crucible Au in C crucible In in Mo crucible
    x 25 13 5 5 5 5 5
    y 25 5 15 22 30 30 22
    z 245 100 93 97 97 87 87
    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 analysis 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. Warming up huge crucibles is significantly 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.


Quartz crystal microbalance

The sensor is installed on the z-retractor (15 cm travel) in the preparation chamber - bottom. It can be used for the determination of the thickness of the deposited layers.

The sensor is to be connected to the water cooling pipes and to the corresponding control unit. It shows the deposited thickness in kiloångströms (kÅs).

The manual of the sensor can be downloaded here. The manual of the control unit is available here.


Estimated deposition rates

For vapour pressure data of materials and for the selection of suitable crucibles (if applicable) you can consult the following evaporation guides provided by:

At the Materials Science Beamline we obtained the following approximative parameters using e-beam or other evaporators:

Element Form High voltage Emission Rate Date Note
Ag in Mo crucible 1000 V 2 mA 0.6 Å/min Aug 2010  
Al in Al2O3 crucible 0.7 Å/min Sep 2008 Knudsen-cell
Au In C crucible 1000 V 20 mA 0.2 Å/min May 2011  
B     Feb 2015 user's source
Ce in Mo crucible 1000 V 20-26 mA 0.3 Å/min May 2016  
Cl     Jun 2012 user's source
Co rod Ø 2 mm 1000 V 8 mA 0.1 Å/min Sep 2015  
Cu in C crucible 1000 V 30 mA 0.1 Å/min Aug 2011  
Fe wire Ø 2 mm 1000 V 7 mA 0.3 Å/min Aug 2011  
Ga in quartz crucible 0.1 Å/min Aug 2009 Knudsen-cell
In in Mo crucible 400 V 5 mA 0.6 Å/min Jul 2012  
Ir wire Ø 1 mm 1000 V 18 mA 0.02 Å/min Nov 2015  
K alkali metal dispenser 0.7 Å/min Dec 2015 direct current
Mg in crucible 4 Å/min Oct 2009 Knudsen-cell
Mo wire Ø 1 mm 1200 V 30 mA 0.05 Å/min Feb 2007  
Nb rod Ø 2 mm 1000 V 55 mA 0.7 Å/min Mar 2007  
Ni wire Ø 1 mm 1000 V 4 mA 0.2 Å/min Oct 2014  
Pb in C crucible 400 V 5 mA 0.6 Å/min Aug 2011  
Pd wire Ø 1 mm 1000 V 3 mA 0.1 Å/min Nov 2014  
Pt wire Ø 0.5 mm 1000 V 5 mA 0.08 Å/min May 2015  
Rh wire Ø 0.5 mm 1000 V 11 mA 1 Å/min Mar 2014  
Si 1×2 mm2 wafer strip 1000 V 7 mA 0.5 Å/min Jul 2011 or direct current
Sn in Mo or C crucible 1000 V 6 mA 0.4 Å/min May 2015  
Ta wire Ø 0.5 mm 1000 V 55 mA 0.1 Å/min Mar 2009  
Ti wire Ø 1 mm 1200 V 9 mA   Nov 2008 or direct current
V wire Ø 0.5 mm 1400 V 6 mA 0.5 Å/min Sep 2011  
W wire Ø 1 mm 2000 V 30 mA 0.3 Å/min Jan 2010  
Zn     Oct 2009 user's source
Last Updated on Tuesday, 24 November 2020 15:10