Topic:
Glossary: L
Here you are going to find explications of important terms in plasma technology:
- layer properties
- Substantial criteria for the evaluation of a coating are the chemical resistance, including corrosion protection, optical properties, abrasion behaviour, electrical properties; for the evaluation of the adhesion of further layers or bondings, the surface energy is the important variable.
- layer thickness
- Distance between surface and another interface.
- lead frame
- Adaptor base for chip carriers; looks like a spider.
- LIGA process (lithography, electroforming, moulding)
- The LIGA method combines the processing steps lithography,
electroplating, and (if necessary) moulding (LIGA, or LiGA (including
moulding), LiGa (without moulding)), in order to produce plastic or
metal structures with lateral dimensions, respectively lateral
dimension tolerances, in the micrometer scale. LIGA is applied in the
field of microelectromechanical systems, nevertheless microoptics, in
particular when structures with high aspect ratios are to be produced.
Depending on the type of lithography, there is a differentiation
between roentgen-LIGA, where x-rays (commonly from a synchrotron) are
applied, and UV-LIGA, where ultraviolet light is used such as in common
semiconductor technology. UV-LIGA was rendered possible during the last
decade of the 20th century through the development of new photoresists,
SU-8 in particular, which was designed for high aspect ratios. Another
option is to generate a master from a silicon wafer via
photolithography and deep silicon etching (e.g. Advanced Silicon
Etching® ASE, the so-called “Bosch process”), followed by
electroplating, and moulding (if necessary). Based on the traditional
UV-LIGA processes, this technique – that was, like the UV-LIGA,
introduced during the last decade of the 20th century – is as well
called silicon-LIGA.
Process steps:
I. The feedstock is an even substrate, e.g. a silicon wafer or a polished disk of beryllium, copper, titan or other.
II. The substrate, if not yet electroconductive, is provided with a metallic “starting layer”, mostly by sputtering or vaporisation.
III. Upon the starting layer, a thick resist (sensitive to light or x-rays) is applied.
IV. The resist is exposed and processed. After the processing, a negative matrix remains, which is to be generated in the electroplating process.
V. In a galvanic process, a metal is deposited on the areas from which the resist was removed (thus the starting layer uncovered) during the processing.
After the removal of the resist, it is the substrate, the starting layer and the galvanically deposited metal which remain. Now there are different options for the following process:
A. By etching the starting layer (that is now acting as victim layer) and possibly the substrate away, (little) metal components can be manufactured directly.
Through further electroplating (“overgrowing”), and adjacent removal of substrate and starting layer, a mould insert can be generated from the lithographically created microstructure, which is to be installed in a casting tool through which the finally wanted plastic part is formed, e.g. through injection moulding or hot stamping.
B. Alternatively, the mould insert can be cut out from the lithographically created microstructure (e.g. via spark erosion) and then be installed in a casting tool.
If the steps exposure, processing, and electroplating are repeated several times, then more complex structures can be designed that must reduce in width towards the substrate (the wafer), though, otherwise the part would not come off the mould. This is a limit to the complexity of the structure to be generated.
If the mould created through LIGA is used for injection moulding, one distinctiveness in contrast to the production of macroscopic parts can be observed: it is not necessary to make arrangements for small boreholes for the elusion of the air present in the mould, since the unevennesses in the contact area between mould and counterpart are sufficient for the elusion of the air. Only the boreholes for the material supply need to be generated. Examples of use for the LIGA technology are toothed wheels for miniature gears and microfine jets for filters. - lightning discharge
- Electrical discharge at high gas pressure that takes place between thundercloud and earth. The strength of current in a lightning channel reaches peak values of 10,000 A. The plasma and the neutral gas in the lightning channel are heated up strongly because of the current flow. The hereby caused sudden enlargement of the lightning channel generates the thunder. Because of recent findings it is known that lightning discharges take place between cloud and ionosphere as well.
- low-pressure plasma
- All plasma applications that are neither executed under atmospheric
(normal-pressure plasma) nor under high pressure, but under low
pressures instead, are called low-pressure plasma. Low-pressure plasmas
are non-thermal plasmas which means that the electrons’ temperature is
much higher than the ion temperature.
Typical technical low-pressure plasmas are operated in a pressure range of a few mbar with process gases like e.g. inert gases (helium, argon), oxygen, hydrogen, nitrogen, or tetrafluormethane. Characteristic of low-pressure plasmas is a mean free path (of the electrons) that is larger than the Debye-length. Low-pressure plasmas are found in the universe in glowing gas nebulae, and on earth as polar lights. Of big technical importance are low-pressure plasmas for the production of microelectronic components (plasma etching).
Furthermore, low-pressure plasmas are applied for a multitude of coating tasks; examples are optics (glasses, lenses...), tools (hardened surfaces). In low-pressure plasmas, electron temperatures of some eV (several 10000°K) can be reached via selective heating of the electrons, while the neutral gas’ temperature lies just above room temperature.
Thus, even thermally sensitive materials, like plastics, can be treated by low-pressure plasmas. A disadvantage of low-pressure plasmas is the poor integratability into production and processing lines. - low-pressure plasma system
- see „low-pressure plasma“
- low-pressure plasma technology
- see „low-pressure plasma“
- low-temperature plasma
- see „low-pressure plasma“
