Excimerlaser

Excimer laser are gas lasers which can generate electromagnetic radiation in the ultraviolet wavelength range. Examples are the surgical correction of myopia, the photolithography for the production of highly integrated semiconductor devices or the micro - material processing (eg, the "Drilling" extremely fine nozzles for inkjet printers ).

The word excimer is from the contraction of the English excited ( stimulated dt ) and the term dimer formed and describes the laser active medium. A dimer consists basically of two identical atoms or molecules. However, today rare-gas halides are used as the active laser medium priority. Thus, the correct term is actually Exciplexlaser ( from excited and complex), but this name is rarely used in practice.

The first excimer laser was founded in 1970 by Nikolai Basov, VA Danilychew and Yu. M. Popov constructed in Moscow at the PN Lebedev Physics Institute. They used the xenon dimer Xe2 and an electron beam for excitation. The first commercial excimer laser was built by Lambda Physik 1977.

Operation

The data used as excimers noble gas halides can only exist as excited molecules and are not stable in the ground state. They can be formed by the chemical reaction between the noble gas and the halogen is steered by an electric discharge or a strong electron beam in the desired direction of the excited Edelgashalogenides. Since the excited molecules are metastable, the noble gas halide is first enriched and there is a population inversion, ie there are more molecules in the excited state than in the ground state ( see figure). The excited molecules can release the stored energy in the form of ultraviolet radiation, they go into the unstable ground state and immediately break down into its components. This transition can be triggered simultaneously by incident ultraviolet light of the same wavelength at all excited molecules, whereby a laser beam is produced.

Excimer laser can only be operated pulsed. The pulse duration is typically between 4 and 40 ns. Repetition current excimer laser are in the range of up to a few kilohertz. In the industrial sector excimer laser are used to achieve the pulse energies of up to 1200 milli- joules.

The wavelength of an excimer laser is determined by the resulting in the excitation molecule. The corresponding starting materials ( gases) are provided, for example, in gas cylinders. The gas mixture, which typically consists of a few percent of the active gas components and a buffer gas (helium or neon ) in the laser cavity from which the laser-active excimers or exciplexes are produced, must be replaced regularly, since as both a longer service life and by the operation to change the properties of the gas mixture such that the pulse energy falls below an acceptable level.

Applications

KrF and ArF excimer lasers have been used since the mid-1990s in photolithography to expose photosensitive resists. The low wavelength allows the manufacture of structures of 28 nm in width (2011 ) and thus form the basis for the production of all modern microelectronic devices. Excimer lasers are also used for direct editing of virtually all materials ( ceramics, metals, plastics, etc. ) for the production of structures with lateral dimensions in the sub- micrometer range. Examples are the production of fiber Bragg gratings ( FBGs ) or micromachining of surfaces.

Also in the medical excimer numerous applications. Example, they are used to cut human tissue. For pulsed laser radiation ( frequencies from 100 to 200 Hz) is used in the rule, which results in that the surrounding tissue is not heated and allows for a wound healing process without great pain. With each pulse can be worn up to 2 microns tissue. That and the very small focus diameter make excimer lasers attractive for applications in ophthalmology, such as LASIK, and are increasingly replacing " hot cutting methods " using argon, Nd: YAG and CO2 lasers, which have higher penetration depth in human tissue. In dermatology XeCl excimer lasers for the treatment of UVB - sensitive dermatoses such as psoriasis (psoriasis) or atopic eczema ( neurodermatitis), etc. employed.