Thermal Evaporation and the Kurt J. Lesker PRO Line PVD 75

Thermal Evaporation

Thermal evaporation is a physical vapour deposition (PVD) technique in which a solid source material is heated inside a high-vacuum chamber until it melts and evaporates. The vapour travels in a near-linear trajectory towards the substrate, where it condenses to form a thin film. Because no plasma or reactive gas is required, the process can reach base pressures of \(10^{-6}\)–\(10^{-7}\ \mathrm{mbar}\), minimising scattering and contamination.

The mean free path \(\lambda\) of evaporated atoms depends on temperature, gas pressure \(p\), and the diameter \(d\) of the gas molecules:

\[\lambda = \frac{k_B T}{\sqrt{2}\,\pi d^2\, p}\]

At \(10^{-6}\ \mathrm{mbar}\), nitrogen has \(\lambda \approx 70\ \mathrm{m}\) so evaporation is effectively collision-free and directional. Water vapour is particularly harmful: at the same pressure it reduces \(\lambda\) to roughly \(10\ \mathrm{cm}\), sufficient to cause significant scattering and film non-uniformity. For this reason the chamber is always vented with dry nitrogen rather than air.

Heating is achieved by passing a high DC current through a tungsten boat that holds the source material, or through tungsten/molybdenum/tantalum wires wound around an insulating crucible of boron nitride or quartz. The source material must have a lower evaporation temperature than the boat material to avoid co-evaporation of the crucible. A quartz crystal microbalance (QCM) above the source monitors the deposition rate in real time; a source shutter prevents the substrate from seeing the source until a stable rate is established.

Kurt J. Lesker PVD 75 — Step-by-Step Protocol

Preliminary machine check

  • If the last use was more than two hours ago, verify that the chamber is under vacuum: the VARIAN gauge reader (second row) should read \(< 10^{-1}\ \mathrm{mbar}\), in the VAC menu, all pumps should be off, and the icons TURBO, TURBO BK, ROUGH, and VENT should appear red;
  • gently pull the chamber door to confirm it is locked by differential pressure.

Opening the vacuum chamber and mounting samples

  • Press the VENT icon (turns green) to admit air, monitor the Pirani gauge as pressure rises towards \(10^{+3}\ \mathrm{mbar}\),
  • while venting, press ROUGH to start the scroll pump warm-up,
  • when the chamber reaches atmospheric pressure open the door wearing nitrile gloves and an FFP3 dust mask,
  • go to the DEP menu and open the sample shutter pressing SUB SHTR icon (turns green) before opening the door,
  • open the door, lift the sample holder off its pegs, and pull it towards you,
  • turn the holder upside down, place it on the table, and mount the sample,
  • invert the holder again to check substrate stability, then reinsert it into the chamber,
  • manually rotate the holder slowly to align a sample with the vertical axis of the metal source.

Crucible setup and metal loading

  • Select the source according to the material and the lab usage policy:
  • Sources 1 & 2 — gold or aluminium (verify sufficient material is present).
  • Source 4 — magnetically active metals (chromium-coated tungsten rod, or the dedicated nickel crucible stored in the grey cabinet to the left of the evaporator).
  • Source 3 (or 4 for magnetic materials) — any other metal; consult an experienced user and have them assist during the first evaporation.
  • Check crucible integrity and confirm the source material is properly seated.

Vacuum pump-down

  • Close the door, in the VAC menu press TURBO BK, the door will lock and the pressure reat with the Pirani will begin to fall,
  • when pressure reaches \(5 \times 10^{-1}\ \mathrm{mbar}\) (within ~15 min) press TURBO to start the turbo pump,
  • wait for the turbo to reach 100% speed,
  • after 5 minutes of turbo operation the Penning gauge (first line of the Varian CC2C reader, active below \(5 \times 10^{-3}\ \mathrm{mbar}\)) should read \(< 5 \times 10^{-4}\ \mathrm{mbar}\).
  • typical pump-down times after a major chamber cleaning:
Pumping time (h) Vacuum (mbar)
1 \(5 \times 10^{-6}\)
4 \(2 \times 10^{-6}\)
16 \(8 \times 10^{-7}\)
≥ 24 \(4 \times 10^{-7}\)

Deposition

  • On the source selector knob, select the source to be used,
  • on the crystal controller, go to Quick Edit → Material and select the metal to evaporate, return to the main menu,
  • turn on the POWER supply, then enable Output to send current to the source,
  • record the base pressure (first line of the Varian reader) in the logbook,
  • confirm the sample shutter is closed (SUB SHTR icon red in the DEP menu),
  • slowly increase the Manual Setpoint knob to 0.8. Some red LEDs on the current indicator should illuminate and the chamber pressure should rise by roughly half a decade (e.g. \(6 \times 10^{-7} \to 2 \times 10^{-6}\ \mathrm{mbar}\)),
  • wait for the pressure to drop again, then continue increasing current in 0.3-point steps. Visually monitor the crucible for melting, once the metal is liquid a positive rate will appear on the thickness monitor,
  • adjust power gradually to reach the target rate (typically 5 Å/s for gold or aluminium).
  • once the rate is stable, zero the thickness monitor and simultaneously open the sample shutter. Note the process pressure, a good evaporation should stay below \(5 \times 10^{-5}\ \mathrm{mbar}\),
  • adjust current continuously to keep the rate as stable as possible,
  • when the target thickness is reached, close the sample shutter, turn the current knob back to zero, set Output to OFF, and turn off the power supply.

Reopening the chamber and retrieving samples

  • In the VAC menu, press TURBO to turn off the turbo pump,
  • close the valve between the primary pump and the turbo (TURBO BK icon),
  • wait until the turbo slows to 70% speed,
  • quickly press and release the VENT icon twice in rapid succession: the pressure will rise to a fraction of an atmosphere and the turbo will decelerate rapidly. When it reaches 20%, fully open the VENT valve,
  • once atmospheric pressure is reached, open the door, remove the sample holder, retrieve your samples, and replace the empty holder in the chamber.

Leaving the machine

  • Close the chamber door,
  • pump down using the primary pump only pressing TURBO BK (valve open) and ensure ROUGH is running,
  • wait until the pressure drops below \(10^{-1}\ \mathrm{mbar}\),
  • close the TURBO BK valve and turn off the ROUGH pump.
  • the machine is now left under vacuum with all pumps off,
  • complete the logbook entry.