Thin layers
Among thin layers , the thin film or film ( English thin films , and thin layer ) is understood to mean layers of solid materials in the micro- or nanometer range . These thin layers often show a physical behavior (strength, electrical conductivity, etc.) that differs from the solid body made of the same material. In this way, properties can also be achieved that are otherwise not available. Thin layers are used in surface finishing and microelectronics .
Thin layers are z. B. by sputtering or molecular beam epitaxy (MBE) produced or vapor-deposited. The methods of applying thin layers or the relevant specialist knowledge is referred to as thin-layer technology , but the term stands in particular for passive electronic components manufactured using this technology.
Thin layers can often only be produced up to a maximum layer thickness - they often have such high internal stresses that they would flake off at higher layer thicknesses.
History and thin layers in everyday life
Originally, this was the sole meaning of the term film (e.g. thin films ), but it changed after the invention of photography and especially cinema . Today, however, the terms thin layers and thin-layer technology are only used for layer thicknesses of up to approx. 1 µm. The use of plural (thin layers) characterizes the special properties of particularly thin layers across a wide range of applications (optical filters, mirror coatings, material technology, diffusion protection, hard material layers, light protection, thin-film solar cells, etc.).
Well-known examples from everyday life, such as the 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 on individual or several such layers. Thin layers are not only used in science and technology - they find a variety of useful applications in our everyday environment. Examples are aluminum-coated foils for packaging (coffee) and rescue blankets.
Economical meaning
The economic importance of thin layers results from the special properties associated with the small thickness (interference, sensors , etc.), from the material economy and from the continuously improved processes for large-scale mass production ( coating processes , mask technology). With the help of thin-film technology, various processes can be used to manufacture micro-technical components or other functional layers. Typical layer thicknesses are in the micrometer and nanometer range up to monomolecular layers. This also makes the use of expensive materials economical if the desired effect can be achieved despite small amounts (example: platinum film resistors instead of wire resistors for temperature measurement).
Wear and tear can result in high costs. Hard material layers on cheaper, softer materials can reduce damage and improve service life (tools) and quality (e.g. plastic glasses).
Corrosion protection layers can reduce damage caused by corrosion.
The greatest economic importance is attached to thin layers in microelectronics. Most microelectronic components such as B. processors, memory modules, monitors, but also storage media such as CDs / DVDs and hard drives are manufactured using thin-film technology.
application
optics
In the optical thin films play a major role, they are used to control the reflection and transmission behavior of surfaces and optical component for UV - VIS and - IR radiation to change. The reflection behavior of a surface can be changed significantly by thin layers. Typical applications are the manufacture of reflective elements, such as mirrors , or the anti-reflective coating of lenses. There are essentially two groups of materials used: metals (high absorption and reflection) and dielectric materials (high transparency).
The most important property for such layers is the (complex) refractive index , which essentially determines the reflection and transmission behavior (cf. Fresnel equations ). Depending on the area of application, thin layers must meet additional technical requirements, for example so that an anti-reflective lens is suitable for everyday use. This includes:
- mechanical resilience ( adhesive strength , hardness , abrasion resistance, scratch resistance, etc.)
- thermal resistance to cold and heat as well as similar thermal expansion coefficients of the layer and the substrate to prevent damage - usually the layers already have a much higher internal stress during production than are caused by temperature differences
- chemical resistance to solvents , cleaning agents , UV radiation , moisture , etc.
The combination of all requirements means that only a few substances are even considered as layer materials. Because of the small selection of layer materials, not every arbitrary refractive index is available.
Metallic thin layers
Smooth metal bodies, i.e. metal bodies with a surface roughness that is significantly smaller than the wavelength of the light in question, generally have a degree of reflection between 92% and 98% in the VIS and IR range . This property of metals is used to improve the reflectivity of any body by coating it with a thin layer of metal. Layer thicknesses of a few hundred nanometers are typical here. In this area, the metal layers, usually made of aluminum , silver and gold , show the properties of thick layers. In this way, large and comparatively light mirrors can also be produced inexpensively, for example as a reflector for solar systems.
Metallic mirrors are 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. Dielectric layers are used to further improve the reflective properties of surfaces. With multiple layers , wavelength-selective mirrors ( dichroic mirrors ) can be produced, which have a significantly higher degree of reflection than metal layers at their specified wavelength.
On the other hand, thinner metal layers of up to around 50 nanometers are partially transparent. Such metal vapor coatings can, for example, be applied to multi-pane insulating glass and serve as so-called heat protection glazing (“thermal windows”). The thickness of the metal layer is chosen so that it is sufficiently transparent for visible light, but reflects comparatively strongly long-wave infrared radiation (“thermal radiation”). The metal vapor deposition also creates a mirror effect and is therefore also used for architectural design. A side effect of this coating is that even longer-wave radio waves are shielded (see Faraday cage ).
Thin metal layers can also be used for the production of simple polarizers . A metal film is deposited in a fine strip structure on a substrate. The wire grid polarizers produced in this way only allow electromagnetic waves that are linearly polarized transversely to the strip structure through the filter.
Dielectric thin layers
| Molecular formula | Surname | Refractive index |
|---|---|---|
| MgF 2 | Magnesium fluoride | 1.38 |
| SiO 2 | Silicon dioxide | 1.46 |
| Al 2 O 3 | Alumina | 1.7 |
| ZrO 2 | Zirconia | 2.05 |
| PrTiO 3 | Praseodymium titanium oxide | 2.1 |
| TiO 2 | Titanium oxide | 2.3 |
| ZnS | Zinc sulfide | 2.3 |
Dielectric thin layers enable significantly more and more specialized applications than metallic thin layers or the dichroic mirror mentioned in the previous section with different reflectivities for visible and infrared light, which are also based on dielectric layers. With them it is possible to control the degree of reflection between 0 and 100% even in very narrow spectral ranges or to influence the polarization of the transmitted or reflected light. A large number of transparent materials are available for optical applications, for example magnesium fluoride or titanium dioxide .
The properties of dielectric thin layers for optical applications are essentially based on the interference of light in these thin layers. The decisive factor here is the occurrence of multiple reflections at one of the interfaces between the front and back of the layer and the superposition of the partial beams or partial waves. These interfere with each other, i. that is, they cancel or reinforce each other. Therefore, it depends on the distance covered in the thin layer, the refractive index of the layer and the wavelength of the light whether an incident light beam (in comparison without a layer ) is reflected more intensely (in the case of constructive interference ) or less (in the case of destructive interference ) . Both cases and their combination are used in technology.
Even single coatings enable an anti-reflective coating for optical elements made of glass, for example a thin layer of magnesium fluoride on glass can reduce the degree of reflection from 4.25% to around 1.25%. By cleverly combining these materials in sometimes very complex layer systems, surfaces with a defined refractive index can be produced in a more or less large spectral range. In this way, the rather unsatisfactory reduction in reflection (strong dependence on wavelength or angle of incidence) can be significantly further improved by a single layer. In practice, a triple layer of materials with different refractive indices and thicknesses that work over the entire visible area is usually sufficient. The aforementioned dichroic mirrors, for example so-called warm and cold light mirrors, can also be produced by means of multiple layers . Warm light mirrors ( English hot mirrors ) are characterized by a high degree of transmission for visible light and a high degree of reflection for infrared heat radiation. Cold-light mirror (engl. Cold mirrors ) on the other hand act exactly opposite, they reflect visible light well, let pass infrared radiation but, z. B. with cold light mirror lamps . Furthermore, multiple coatings make it possible to manufacture interference filters as well as splitter and one-way mirrors . Polarization filters are also possible through the use of optically anisotropic or active materials .
The precision in the production of these layers has to be very high and decides whether the desired interference effects can occur. It should also be noted that the degree of transmission and reflection of such a coated system depends to a large extent on the angle of incidence and the wavelength used. The selection of the respective coating therefore depends heavily on the desired area of application.
Surface finishing
Thin layers are also used to refine and functionalize surfaces. This is understood to mean the improvement of surfaces in terms of their functional (e.g. 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 dirt-repellent surfaces on glass and ceramics ( lotus effect ).
electronics
Schematic cross-section through a CMOS chip with a complex stack of structured, thin layers.
Flexible thin-film solar cells
In electronics and especially in semiconductor electronics , thin layers play a decisive role; they form the basis for the manufacture and functioning of transistors or diodes and thus for all microelectronic products . Other important areas of application are electronic displays, such as LC displays or OLED displays , as well as photovoltaics , for example in the form of thin-film solar cells or the top electrode of conventional solar cells .
Almost all material groups are used, ranging from metals for conductor tracks and electrical contacts, such as copper , silver , aluminum or gold , to semiconductors such as silicon , germanium or gallium arsenide , to non-conductors ( dielectric ) such as silicon dioxide or titanium dioxide .
Thin layers are also used in conventional electronics, for example as a thin insulation layer applied by spray coating to coil wires.
Other uses
Further areas of application for thin layers are:
- Dental treatment ( amine fluoride to seal and harden the tooth surface)
- Medicine (coating of prostheses )
- Food packaging (coated PA films as a gas and aroma barrier for packaging meat, sausage and cheese)
Manufacturing
Thin layers differ widely in terms of thickness (a few nanometers up to several micrometers) and the material used (metals, dielectrics, organic materials, etc.) or the material combination (alloys, layer stacks). Production takes place using the coating methods of thin-film technology. First and foremost, these are chemical (CVD) and physical (PVD) vapor deposition processes, in which the material is produced either through the reaction of volatile starting materials or through condensation from the vapor phase on an existing layer or substrate surface. Furthermore, there are numerous other processes based on liquid raw materials, such as electrostatic spraying, painting , dipping processes or rotation coating , as well as electroplating .
The electrostatic spray process is used particularly often . Both the substrate and the paint are electrically charged so that the paint is automatically drawn onto the substrate. With this process, painting across corners and angles is even possible. A wafer-thin wet layer thickness of 1 to 2 μm is applied here. Since the paint is mainly based on organic solvents , which evaporate after the painting process, an extremely thin dry layer thickness of 0.01 to 0.02 μm is created.
The process conditions under which this deposition takes place have a significant influence on the layer properties. These include the substrate temperature, contamination on the substrates, growth rate and process pressure. The most important layer properties include layer thickness, surface roughness , crystal morphology , density , contamination and doping as well as the resulting properties such as refractive index , layer adhesion , hardness , etc.
The optical properties of thin layers can be determined in the course of determining the coating parameters using spectrophotometric or ellipsometric measuring methods. Spectrophotometry supplies transmission spectra to which theoretical transmission curves can be fitted or which can be evaluated using the envelope method. The optical properties determined in this way are used to correct or optimize the process conditions.
See also
Literature and Sources
- Werner Bausch, Frank L. Pedrotti, Leno S. Pedrotti: Optics for engineers. Basics. Springer, Berlin 2005, ISBN 3-540-22813-6
- Aicha Elshabini-Riad, Fred D. Barlow III: Thin Film Technology Handbook. McGraw-Hill, New York 1998, ISBN 0-07-019025-9
Web links
- Thin Films and You . 2011 (Animated short film about thin layers in everyday life; English)