Thermal evaporation
Thermal evaporation (including vapor deposition or sputtering , engl. Thermal evaporation ) is a the PVD process associated high vacuum-based coating technique . This is a process in which the entire starting material is heated by an electrical heater (resistive or inductive) to temperatures close to the boiling point, a material vapor moves to a substrate and condenses there to form a layer . It is one of the simplest evaporation processes in coating technology.
In a broader sense, thermal evaporation is understood as a group of PVD processes in which the starting material is heated in various ways. This group includes, for example, evaporation methods using lasers , electron beams or an electric arc . The molecular belongs to this group. On the other hand, processes in which the material vapor is subsequently modified by a plasma, as in ion plating , are not included in the group of evaporation processes.
functionality
In thermal evaporation, the starting material is heated to temperatures close to the boiling point. Individual atoms , “ atom clusters ” or molecules dissolve , that is, they evaporate, and migrate through the vacuum chamber. Due to the arrangement between the evaporation source and the substrate, the material vapor hits the opposite, cooler substrate and is deposited there ( condensation ). A thin layer of the evaporated material forms on the substrate . The disadvantage of this method is that the material vapor in the vacuum chamber spreads in all directions and therefore part of the material is inevitably also deposited on the vessel wall of the recipient .
Like most other PVD processes, thermal evaporation is a high vacuum process. Typical process pressures are 10 −6 mbar . There are various reasons for this, on the one hand, the low pressure minimizes collisions with gas particles still present in the vacuum (in this pressure range the mean free path is much greater than the distance between the evaporator source and the substrate), and on the other hand, the process pressure must be below the gas pressure of the material to be evaporated.
Collisions with other atoms or molecules should be avoided because the material can react chemically with them. For example, part of a metal vapor can oxidize so that the deposited layers are contaminated. In extreme cases, this could lead to the deposition of metal oxide layers. This is generally undesirable, but can also be used in a targeted manner in the case of reactive evaporation by letting ionized oxygen into the vacuum chamber. In this way, for example, the deposition of indium tin oxide layers (ITO layers) can be improved or the deposition of black nickel (NiO) can be achieved; both materials are used in photovoltaics.
In the deposition of alloys , the different vapor pressures of the individual components and thus the different deposition rates are problematic. In this case, individual components are usually evaporated from separate sources at different temperatures. If the residual pressure of the vacuum is too high, less dense layers with different material properties can arise.
Evaporator Sources
As mentioned in the previous section, thermal evaporation is divided into the following subgroups. The classification is based on the evaporator used:
- Process in which the material is completely melted
- Resistance evaporator: With thermal evaporation from a boat, the material container is heated by a current flowing through it until the evaporation material evaporates. The boat is often made of molybdenum , tungsten or tantalum . Alternatively, a tungsten filament with Al 2 O 3 or a boron nitride container with the vapor deposition material is used. A disadvantage of this method is the risk of contamination with the container material.
- Induction heater : Here the conductive material in an insert (liner) is heated directly by inductive heating (eddy current).
- Process in which only part of the material is melted
- Electron beam evaporator: When using an electron beam evaporator, the evaporation material is heated by an electron beam. The kinetic energy of the electrons is transferred to the material to be evaporated by inelastic collisions. It is located in a water-cooled copper crucible or in a liner made of molybdenum, tantalum, boron nitride or graphite in this copper crucible. With this method, contamination with crucible material is almost impossible.
- Arc : see arc evaporation
- pulsed laser : see laser beam evaporation
Areas of application
Typical materials for this process are metals (e.g. copper , silver , gold ), but other materials such as silicon dioxide , indium tin oxide or organic semiconductors (e.g. pentacene ) can also be deposited in this way. Due to this diversity, the process temperature is very different, metals are vaporized at 1000–3400 ° C. Other materials, however, require significantly lower temperatures (e.g. pentacene at approx. 290 ° C or indium tin oxide at approx. 600 ° C).
Temperature control is an important factor here, because even small changes in temperature can result in large differences in the evaporation rate. The regulation is not possible via a constant energy supply to the evaporator, because the heat balance u. a. depends on the level. The separation control and thus the energy supply to the heater is carried out via layer thickness measurements using a quartz oscillator . The parameters must be determined beforehand with a test.
literature
- KS SreeHarsha: Principles of physical vapor deposition of thin films . Elsevier, 2006, ISBN 978-0-08-044699-8 .
Individual evidence
- ↑ KS SreeHarsha: Principles of physical vapor deposition of thin films . Elsevier, 2006, ISBN 978-0-08-044699-8 , pp. 367-452 (Section 5. Thermal Evaporation Sources).
- ^ X. Zeng et al .: Morphological characterization of pentacene single crystals grown by physical vapor transport . In: Applied Surface Science . tape 253 , 2007, p. 3581-3585 , doi : 10.1016 / j.apsusc.2006.07.068 .