Excimer Lasers – The powerful light source in the UV and VUV

Abstract

The powerful light source in the UV and VUV Excimer lasers are pulsed gas lasers operating with a special mixture of noble gases and halogens. They emit laser radiation in the UV and VUV spectra, at discrete wavelengths between 351 nm and 157 nm. Continuous improvements over recent years have resulted in excimer lasers becoming the tool of choice for many applications.Starting as "exotic" lasers in the laboratories of universities and research centres, they now operate within the industry in key technologies such as, for example, optical lithography for the manufacture of highly integrated semiconductor devices. The main medical application is the use of excimer lasers at 193 nm in ophthalmology for vision-correction systems. In both applications a total of about 10,000 lasers are currently operating worldwide. The theoretical basis for the laser was established in 1917, when Einstein postulated the theoretical concept of stimulated emission [1]. The first excimer laser was built in 1971 by Basow, 54 years later [2]. This was 11 years after the first ruby laser had been built by Mainman. The term "excimer" is a shortened form of "excited dimer", which describes a molecule formed from two identical noble gas atoms. These atoms will only bond with each other in an excited state. The term "excimer" was later expanded to include all combinations of rare gases and halides. Therefore, in laser physics, all rare-gas halides and similar molecules are excimers, which are not really dimers. Nowadays excimer lasers are the dominant UV source in many applications. Using an excimer laser is the easiest way to generate photons in the deep ultraviolet. Although it is possible to generate radiation below 350 nm with solid-state lasers using non-linear optical effects, tremendous effort is involved in this process. A frequency-tripled Nd-YAG laser, for example, emits at 355 nm, but to ANDREAS GÖRTLERAndreas Görtler is senior scientist in the R&D department at TuiLaser AG. He studied physics in Erlangen, Germany and for his Ph.D. he developed low-pressure gas discharge switches for pulsed power applications. He has more than 10 years of experience in the development of excimer lasers, especially highly repetitive systems for industrial applications. Since 2003 he has been chair of the section Short Timescale Physics of the German Physical Society (DPG). THE AUTHORS Basics of Excimer lasersThe laser is a source of an intensive coherent light field. It has a close analogy with an electric amplifier, which has a positive feedback loop. It is well known that this arrangement starts to oscillate if the feedback is positive and the amplification is higher than the losses.Oscillation condition: Amplification > lossesThe same thing occurs in a laser. The light is amplified by a (light) amplifying medium. In the case of an excimer laser, this medium is a gas mixture. Normally the lower energy level in the molecules is more populated than the upper level. But under certain conditions, this population state can be inverted, the upper level...

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