Thin film

In thin films, thin film or movie (english thin films, and thin layer ) refers to layers of solid materials in the micro - or nanometer range. These thin layers are often exhibit physical behavior (hardness, electrical conductivity, etc.) which is different from the solid body of the same material. As also properties can be achieved which would otherwise not exist. Thin films are used in surface finishing and microelectronics.

Thin films are produced and deposited, for example, by sputtering or molecular beam epitaxy ( MBE). The methods of applying thin layers or the related knowledge is referred to as thin-film technology, the term is, however, particularly for made ​​in this technology, passive electronic components.

Thin layers can often produce only up to a maximum layer thickness - they often possess such high residual stresses that they would flake off at higher film thicknesses.

History and thin layers in everyday life

Originally, this was the sole meaning of the term film ( eg thin films), but after the invention of photography and especially of the movie underwent a change. The terms thin films and thin film technology are however common only for film thicknesses up to about 1 micron today. The use of the plural (thin layers ) characterizes the specific properties of very thin layers of the broad range of applications ( optical filters, mirror coatings, materials, diffusion, protection, hard coatings, light, thin-film solar cells, etc.).

Well-known examples from everyday life, such as rainbow colors of thin oil films on water and soap bubbles as well as the brilliant colors of peacock feathers or butterfly wings are caused by light interference at single or multiple such layers. Thin layers are applied not only in science and technology - you can find a variety nutzvoller applications in our daily environment. Examples are aluminum coated films for packaging ( coffee) and emergency blankets.

Economic Importance

The economic importance of thin films arises from the risks associated with the small thickness particular characteristics (interference, sensors, etc. ), from the material economy and from the ever- improved methods for large-scale mass production ( coating method, mask technology). With the help of thin film technology can be customized with various methods of micro technical components or other functional layers can be produced. Typical layer thicknesses are in the micrometer and nanometer range, to monomolecular layers. This also makes the use of expensive materials economically, if in spite of small amounts, the desired effect can be achieved (for example, platinum layer resistors instead of wire resistors for temperature measurement ).

Due to wear caused great damage. Through hard coatings on cheaper, softer materials, damage can be reduced and the service life (tools ) and quality ( eg plastic lenses ) improve.

Corrosion protection layers can reduce damage caused by corrosion.

The highest economic significance of thin layers in microelectronics. Most microelectronic components such as processors, memory chips, monitors, as well as storage media such as CDs / DVDs and hard drives are manufactured using thin-film technology.

Application

Optics

In optics, thin films play an important role, they are used to change the reflection and transmission behavior of surfaces and optical component for UV, VIS and IR radiation. The reflectance behavior of a surface can be substantially altered by thin layers. Typical applications include the production of reflecting elements such as mirrors, lenses, or antireflection coatings. Here, two main groups of materials are used: metals ( high absorption and reflection) and dielectric materials (high transparency).

The most important feature of such layers is the (complex) refractive index that substantially the reflection and transmission behavior determined (see Fresnel equations). Depending on the application, thin layers must meet additional technical requirements, for example, so that an anti-reflective lens is suitable for everyday use. These include:

• Mechanical strength ( adhesion, hardness, abrasion resistance, scratch resistance, etc. )
• Thermal resistance to heat and cold, and similar coefficients of thermal expansion of the layer and the substrate to prevent damage - most of the layers have been by manufacturing a much higher residual stress than by temperature differences arise
• Chemical resistance to solvents, detergents, UV light, humidity, etc.

The combination of all requirements means that only a few substances come at all as a layer materials in question. Thus is due to the small selection of layer materials not any refractive index available.

Metallic thin films

Smooth metal body, that is, metal body having a surface roughness much less than the wavelength of light in question generally have in the VIS - and IR - region has a degree of reflection between 92% and 98%. This characteristic of metals is utilized in order to improve the reflectivity of any bodies by coating them with a thin metal layer. Typically all the layer thicknesses of a few hundred nanometers. In this area indicate the metal layers, commonly made ​​of aluminum, silver and gold, the properties of thick layers. Thus, even large and relatively light mirror produced at low cost, for example, as a reflector for solar systems.

Metallic mirror sufficient as a reflective layer for everyday objects and many technical applications. However, there are applications in which such reflection losses of 2 to 10% can no longer be tolerated, for example laser systems. In order to improve the reflective properties of surface again, dielectric layers may be used. With multiple layers can be wavelength-selective mirror ( dichroic mirror ) establish that a much higher reflectance have at their specified wavelength than metal layers.

Thinner metal layers, to about 50 nanometers, however, are partially transparent. Such metal vapor coats for instance, could be applied to multi-pane insulated glass, and serve as so-called " heat protection glazing " ( " thermal window "). The thickness of the metal layer is selected such that it is sufficiently transparent to visible light, but a long -wave infrared radiation ( " heat radiation" ) relatively highly reflective. The metal vapor also causes a mirror effect and is therefore also used for architectural design. A side effect of this coating is that even long-wave radio waves to be shielded (cf. Faraday cage).

Thin metal layers can also be used for the production of simple polarizers. In this case, a metal film is deposited in a fine stripe pattern on a substrate. The wire grid polarizers prepared in this way can only electromagnetic waves which are polarized transversely to the strip a linear structure, through the filter.

Dielectric thin films

Dielectric thin layers allow significantly more and more specialized applications as metallic thin layers or mentioned in the previous section dichroic mirrors with different reflectivity for visible and infrared light, which are also based on dielectric layers. With them it is possible to control the reflectivity between 0 and 100 %, even in very narrow spectral ranges or to influence the polarization of the transmitted or reflected light. For optical applications, there is a plurality of transparent materials are available, for example magnesium fluoride or titanium oxide.

The characteristics of dielectric films for optical applications are essentially based on the interference of light in these thin layers. The decisive factor here is the presence of multiple reflection at the interface of the front and rear sides of the layer and the superimposition of the partial beams or partial waves. These interfere with each other, that is, they cancel each other out or reinforce. Therefore, it is dependent on the distance in the film path, the refractive index of the layer and the wavelength of the light, whether a incident light beam (in comparison with non-layered ) amplified is reflected (in the case of constructive interference ) or decreased (in the case of destructive interference ). Both cases and their combination are used in the art.

Simple coatings provide an anti-reflection coatings for optical elements of glass, for example, a thin layer of magnesium fluoride on glass to reduce the reflectance of 4.25% to about 1.25 %. By cleverly combining these materials in often very complex layer systems, surfaces with defined refractive index in a more or less wide spectral range can be produced. In this way the rather unsatisfactory reflection reduction (strong wavelength or incident angle dependence ) can be significantly improved by a single layer. In practice, is usually sufficient, a triple lamination of materials having different refractive indices and thickness to function over the entire visible range. With multiple layers, the aforementioned dichroic mirror can be produced, for example, so-called hot and cold mirror. Hot mirror (english hot mirrors ) are distinguished by a high transmittance for visible light and high reflectivity for infrared radiation. Cold Mirrors ( engl. cold mirrors ), however, have opposite effects exactly, they reflect visible light well, let pass infrared radiation, however, eg dichroic lamps. Furthermore, it is possible by multiple coating to produce interference filters and splitter and one-way mirror. By the use of optically anisotropic or active materials, the polarization filter is possible.

The precision in the preparation of these layers must be very high, and decides whether it can cause the desired interference effects. Furthermore, it is noted that the transmittance and reflectance of such a coated system is strongly dependent on the incident angle and the wavelength used for the part. The selection of the particular coating is highly dependent on the desired application.

Surface finishing

Thin films are also used for finishing and surface functionalization. This refers to the improvement of surfaces in terms of their functional (eg corrosion protection, wear protection, etc. ), decorative (degree of gloss, color, etc. ) properties, or a combination of both. Examples include improving the scratch resistance of plastic or metal parts, such as DVDs or tools or the production of stain-resistant surfaces on glass and ceramic (lotus effect).

Electronics

Flexible thin-film solar cells

In electronics and especially the semiconductor electronics thin layers play a crucial role, they form the basis for the manufacture and operation of transistors or diodes, and thus for all microelectronic products. Other important applications include electronic displays, such as LCD displays or OLED devices, as well as photovoltaic, for example in the form of thin film solar cells, or the top electrode of conventional solar cells.

Here, almost all material groups are used, ranging from metals for interconnects and electrical contacts, such as copper, silver, aluminum or gold, semiconductors such as silicon, germanium or gallium arsenide, to non-conductors ( dielectric) such as silica or titanium dioxide.

Also found in conventional electronics thin layers application, for example, as applied by spray coating thin insulating layer at coil wires.

Other applications

Other applications for thin layers are:

• Dental treatment (amine fluoride for sealing and hardening the tooth surface )
• Medicine ( coating of prostheses)
• Food packaging (coated PA films as gas and aroma barrier for packaging meat, sausage and cheese)

Production

Thin films exhibit large differences in the thickness ( a few nanometers up to several micrometers ) and the material used ( metals, dielectrics, organic materials, etc. ) or the combination of materials (alloys, layer stack ). The production is done using the coating methods of thin film technology. Are primarily the chemical (CVD) and physical ( PVD) vapor deposition method in which the material. Either by reaction of the volatile starting materials or by condensation from the vapor phase onto an existing layer or substrate surface In addition, there are numerous other methods based on liquid starting materials, such as paints, dip-coating or spin coating, and electroplating.

The process conditions under which the deposition is going on, while having significant influence on the film properties. These include the substrate temperature, pollution on the substrates, growth rate and process pressure. Among the most important film properties include the layer thickness, surface roughness, crystal morphology, density, contamination and doping and the consequent devoted properties such as refractive index, adhesion, hardness, etc.