Physical vapour deposition (PVD) is a variety of vacuum deposition techniques in which the material goes from a condensed phase to a vapour phase and then back to a thin film condensed phase. Sputtering and evaporation are the two most common PVD methods used in PV manufacturing.
Sputtering involves a target or source material being bombarded by high energy particles, ejecting atoms of this material which are subsequently deposited onto a substrate to form thin film layers. A schematic description of a sputtering system is shown in Figure 1. The sputtering system consists of a high-vacuum chamber, a gas inlet (typically for an inert gas such as argon), a pump connection, a sputter target which is negatively charged (the cathode), and the sample which is positively charged (the anode). By applying a direct-current (DC) or alternative-current [AC or radiofrequency (RF)] excitation, a plasma is generated. When the plasma energy is sufficiently high, the kinetic energy of the bombarding particles is much higher than conventional thermal energies resulting in material removal at the sputter target. DC discharges are usually preferred for electrically conductive materials, while an RF plasma is suitable for all materials including dielectric target materials. There are two cycles in a RF sputtering process. Firstly, the target material is negatively charged which causes the polarisation of atoms; the ionized Ar+ ions are attracted to the target surface. Secondly, the target is positively charged, which causes the ejection of gas ions and source atoms due to reverse polarisation. Subsequently, the source atoms are accelerated towards the substrate resulting in thin film deposition. On the other hand, during DC sputtering the sputtering target is negatively charged throughout the process. This explains why this can only be used for conductive sputter targets because the material has to be sufficiently conductive to avoid accumulation of gaseous ions at the target. To increase the sputter yield often a so-called magnetron sputtering process is used which uses magnetic fields to confine the plasma to the sputter yield.In addition to the sputtering of pure materials such as aluminium or silver there are also two types of sputtering processes which can be used to make binary, tertiary, or even more complex materials. The easiest way to grow more complex materials is to sputter various elements at the same time and/or using multi-element sputtering targets. In reactive sputtering in addition to inert gasses such as argon, reactive gasses such as oxygen are used in the sputtering process. These reactive gasses can react with the target material to form compound materials on the sample. For example, molybdenum oxide can be sputtered using a pure molybdenum target in combination with O2 plasma. As reactive sputtering involves chemical reactions, it is not classified as physical vapour deposition technique.
Like most of the other thin film deposition techniques, evaporation also takes place a high-vacuum atmosphere (10-5~10-9 Torr). The working principle is quite straightforward: the target material is heated up to a temperature with a significant vapour pressure and the high vacuum allows the vaporised material to flow directly to the substrate where it condensates into a thin film. There are two major types of evaporation, thermal evaporation (or resistance heating evaporation) and electron beam (e-beam) evaporation. The schematic in Figure 2 shows the difference between these two types. During thermal evaporation, the material is heated by passing an electrical current passing through a material holder (normally a crucible or boat), the deposition rate can be controlled by regulating the current. E-beam evaporation involves a high energy electron beam bombarding the material directly to generate heat and vaporise the material. E-beam evaporation has the advantage that as the electron beam can be focussed; i.e. localised heating of the material is possible. It is also more suitable for high melting point materials, and the growth rate can be better controlled compared to the thermal evaporation process. However, thermal evaporation is a “milder process” resulting in a lower damage on the substrate.