Evaporation (deposition)

Thermal evaporation (even vapor deposition or vapor deposition, Eng. Thermal evaporation ) is associated with the PVD process high-vacuum based coating technology. This is a method in which the starting material is heated by an electric heater ( resisitiv or inductively ) to temperatures near the boiling point, steam moves a material to a substrate, where it condenses to form a layer. It thus represents one of the simplest evaporation method in coating technology dar.

In a broader sense, the thermal evaporation is to be understood as a group of the PVD process, in which the starting material is heated in various ways. This group includes, for example, evaporation methods by laser, electron beam or an electric arc. The molecular beam epitaxy is one of the group. In contrast, methods in which the material vapor is subsequently modified by a plasma, such as ion plating, not included in the group of the evaporation process.

Operation

In thermal evaporation, the source material is heated to temperatures near the boiling point. Here, single atoms, " atom clusters " or molecules dissolve, that is, they evaporate and travel through the vacuum chamber. Because of the arrangement between the evaporation source and substrate of the Matrialdampf strikes the opposite, cooler substrate and beats down there ( condensation). It is formed on the substrate, a thin layer of the vaporized material. The disadvantage of this method is that the material vapor in the vacuum chamber spreads in all directions and therefore inevitably reflected some of the material also on the vessel wall of the recipient.

Like most other PVD - method is also the thermal evaporating a high- vacuum process. Typical process pressures are 10-6 mbar. There are several reasons, Firstly, due to the low pressure collision with remaining in the vacuum gas particles is minimized (in this pressure region the mean free path is very much greater than the distance between the evaporation source to the substrate), on the other hand has to process pressure by the gas pressure of the material are to be vapor.

Collisions with other atoms or molecules are to be avoided, as the material can react chemically with them. For example, to oxidize a portion of a metal vapor so that the deposited layers are contaminated. In extreme cases, it could result from the deposition of metal oxide layers. This is usually undesirable, but can be deliberately exploited also in the case of reactive evaporation by ionized oxygen is introduced into the vacuum chamber. In this way, for example, the deposition of the indium tin oxide layers improves (ITO ) layers or the deposition of black nickel oxide (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, most individual components from separate sources evaporated at different temperatures. With too high a residual pressure of vacuum less dense layers with different material properties can occur.

Evaporation sources

As mentioned in the previous section, the thermal evaporation is divided into the following subgroups. The classification is done on the basis of the evaporator used:

• Resistance Evaporators: In thermal evaporation from a boat of the material container is heated by current flow until the vapor deposition material evaporated. The boat is often made of molybdenum, tungsten or tantalum. Alternatively, a tungsten filament with Al2O3 or boron nitride container is used with the vapor deposition. A disadvantage of this method is the risk of contamination the container with material.
• Induction Heater: Here, the conductive material in an insert (liner ) by inductive heating ( eddy current ) is directly heated.

• Electron beam evaporator: When using electron beam evaporators from the vapor deposition material is heated by an electron beam. Thereby the kinetic energy of the electrons transmitted through inelastic collisions of the material to be vaporized. It is located on that in a water-cooled copper crucible or in an insert (English liner) made ​​of molybdenum, tantalum, boron nitride or graphite in this copper crucible. In this method, the contamination is virtually eliminated with crucible material.
• Arc: see arc evaporation
• Pulsed laser: see Laserstrahlverdampfen

Applications

Typical materials for this process include metals (eg, copper, silver, gold ), but also other materials, such as silica, indium tin oxide, or organic semiconductors (e.g., pentacene ), can be deposited. The process temperature is very different because of this diversity, as metals are vaporized at 1000-3400 ° C. Other materials, however, require much lower temperatures (e.g., pentacene at about 290 ° C, or indium tin oxide at about 600 ° C).

Temperature control is an important factor, because even small temperature changes can result in large differences in the evaporation rate. The control is not possible on a constant supply of energy to the evaporator, since the heat balance is inter alia dependent on the level. The Abscheideregelung and thus the power supply to the heater is effected on the layer thickness measurements by means of a quartz oscillator. The parameters must be determined beforehand for a test.