2. Experimental techniques

## 2.1. Metallization

There are principally three different methods of depositing metal on a substrate: plating, metal evaporation and sputtering. Metal plating is generally used to deposit thick layers. Only metal evaporation and sputtering were used in the work reported on in this chapter.

### 2.1.1. Metal evaporation

Evaporation techniques are based on heating up a source to a temperature where the material starts vaporizing. The vaporized material is then deposited on the sample and cools down forming a thin film. Thermal evaporation can either be achieved by heating the source with a resistive element or by using an electron beam. Resistive heating takes place by passing a current through a heating element, often made out of tungsten, which heats up a crucible containing the source material. Resistive evaporation has the disadvantage of potential contamination from the crucible if the melting temperature of the crucible is close to the melting temperature of the source material, resulting in a poor film quality. Electron beam evaporation uses an electron beam generated from a cathode to heat up the source material locally. The crucibles are water cooled to minimize contamination. The electron beam is generated by a thermionic emission filament and is accelerated towards the crucible using a high accelerating voltage. The beam is then focused into a spot on the surface of the source material and the interaction between the accelerated electrons and the source material will cause the material to start heating up and vaporize. The combination of local heating and water cooled sources prevents crucible metal contamination, resulting in a high purity film deposited on the substrate. The evaporation processes take place under high vacuum (10�<sup>3</sup> –10�<sup>4</sup> mTorr) in order to create a mean free path of the evaporating flux, which is greater than the distance between the source and the sample.

### 2.1.2. Sputtering

While evaporation requires a source to be heated to produce a flux of gas, sputtering targets make use of a physical plasma process rather than heat. The plasma is formed using an inert gas (normally Argon) and is excited by either a direct current (DC) or radio frequency (RF) source. The target source is negatively biased and the plasma sputters neutral atoms of source material away from the target towards an anode, where the neutral atoms are deposited on the sample. Since a plasma is required, the working pressures of sputtering systems are relatively high (≈10�<sup>1</sup> mTorr). The sputtering method used for most of the work reported on in this chapter is RF magnetron sputtering. Radio frequency magnetron sputtering is an enhanced sputter method which enables a higher deposition rate at low operating pressure together with the possibility of obtaining high quality films at low as well as high substrate temperatures. A schematic diagram of the experimental setup for this method is shown in Figure 16.

In the chamber filled with the Ar gas, a high voltage is applied at high frequency between the target and the sample. The surface atoms of the target material are removed and deposited onto the substrate by bombarding the target with the ionized Ar atoms. The magnet, located behind the target, enhances ionization and effectively directs the sputtered atoms towards the substrate.
