Excimer laser
Excimer laser are gas lasers , the electromagnetic radiation in the ultraviolet wavelength range can produce. Application examples are the surgical correction of myopia , photolithography for the production of highly integrated semiconductor components or micro-material processing (e.g. the "drilling" of extremely fine nozzles for inkjet printers ).
The word excimer is formed from the contraction of the English excited (dt. Excited ) and the term dimer and describes the laser-active medium. A dimer basically consists of two identical atoms or molecules. However today are primarily inert gas - halides used as the lasing medium. So the correct name is actually exciplex laser (from excited and complex ), but this name is rarely used in practice.
The first excimer laser was made in 1970 by Nikolai Bassow , WA Danilitschew and Ju. M. Popow at the PN Lebedev Physics Institute in Moscow . They used the xenon dimer Xe 2 and an electron beam for excitation. The first commercial excimer laser was built by Lambda Physik in 1977 .
functionality
The noble gas halides used as excimers can only exist as excited molecules and are not stable in the ground state. They can be formed by directing the chemical reaction between the noble gas and the halogen by an electrical discharge or a strong electron beam in the desired direction of the excited noble gas halide. Since the excited molecules are metastable, the noble gas halide is first enriched and a population inversion occurs, which means that 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, whereby they pass into the unstable ground state and immediately break down into their components. This transition can be triggered by incident ultraviolet light of the same wavelength for all excited molecules at the same time, creating a laser beam.
Most excimer lasers can only be operated in pulsed mode. The pulse duration is between 300 fs and 40 ns. Repetition rates of today's excimer lasers are a maximum of a few kilohertz . In the industrial sector, excimer lasers with pulse energies of up to 1.2 joules are used.
The wavelength of an excimer laser is determined by the molecule produced during the excitation . The corresponding starting materials (gases) are z. B. provided in gas cylinders . The gas mixture, which 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 generated, must be replaced regularly, as longer downtimes and change the properties of the gas mixture during operation in such a way that the pulse energy falls below an acceptable value.
Applications
| molecule | wavelength |
|---|---|
| Ar 2 | 126 nm |
| Kr 2 | 146 nm |
| F 2 | 157 nm |
| Xe 2 | 172 nm |
| ArF | 193.3 nm |
| Kr Cl | 222 nm |
| KrF | 248.35 nm |
| Xe Br | 282 nm |
| XeCl | 308 nm |
| XeF | 351 nm |
KrF and ArF excimer lasers have been used in photolithography for exposure of photosensitive photoresists since the mid-1990s . The short wavelength enables the production of structures with a width of 28 nm (as of 2011) and thus forms the basis for the production of all modern microelectronic components . Excimer lasers are also used for the direct processing of practically all materials (ceramics, metals, plastics, etc.) for the production of structures with lateral dimensions in the sub-micrometer range. Examples of this are the production of fiber Bragg gratings (FBG) or the micromachining of surfaces.
Excimer lasers also find numerous applications in medicine. They are used, for example, to cut human tissue. For this purpose, pulsating laser radiation (frequencies between 100 and 200 Hz) is usually used, which means that the surrounding tissue is not heated and enables a wound healing process without major pain. With each pulse, up to 2 µm of tissue is removed. This and the very small focus diameter make excimer lasers attractive for applications in ophthalmology , for example LASIK , and are increasingly replacing “hot-cut methods” using argon , Nd: YAG and CO 2 lasers , which penetrate deeper into human tissue. In dermatology , XeCl excimer lasers are used to treat UVB-sensitive dermatoses such as psoriasis vulgaris ( psoriasis ) or atopic eczema ( neurodermatitis ) and many more.
literature
- D. Basting, K. Pippert, U. Stamm: History and future prospects of excimer laser technology . In: 2nd International Symposium on Laser Precision Microfabrication . 2001, p. 14–22 ( PDF [accessed July 26, 2010]).
- PR Herman, KR Beckley, BC Jackson, D. Moore, J. Yang, K. Kurosawa, T. Yamanishi: Processing applications with the 157-nm fluorine excimer laser . In: Proc. SPIE 2992, Excimer Lasers, Optics and Applications . tape 84 , 1997, ISSN 0277-786X , p. 86-95 , doi : 10.1117 / 12.270086 .
Individual evidence
- ↑ NG Basov, VA Danilychev, Y. Popov, DD Khodkevich: Laser for vacuum of the region spectrum with excitation of liquid xenon by on electron beam . In: Zh. Eksp. Fiz. i Tekh. Pis'ma. Red . No. 12 , 1970, pp. 473-474 .
- ↑ NG Basov, VA Danilychev, Y. Popov, DD Khodkevich: Laser Operating in the Vacuum Region of the Spectrum by Excitation of Liquid Xenon with on Electron Beam . In: Journal of Experimental and Theoretical Physics Letters . No. 12 , 1970, pp. 329 .
- ↑ Appl. Phys. B 44, 199-204 (1987)
- ↑ Jürgen Eichler, Hans-Joachim Eichler: Lasers: designs, beam guidance, applications . Springer, 2010, ISBN 978-3-642-10461-9 , pp. 128 .
- ↑ a b H. Frowein, P. Wallenta: Compact excimer lasers for industrial use. In: Photonics. 34, 2002, pp. 46-49. PDF ( Memento from October 19, 2011 in the Internet Archive )