High-power lasers pumped by a gas discharge are extensively used in industrial applications. Of particular importance are lasers pumped by an electric discharge in excimer gas mixtures because this allows the generation of powerful ultraviolet radiation (UV), with wavelengths below 350 nm. Due to the short wavelength, these lasers offer the unique possibility to structure materials on the sub-micron scale, when focused or with mask techniques. Also, the absorption of UV radiation is strong in many materials, which allows an improved control of the ablation depth. Also when larger structures are to be fabricated, excimer lasers can be of advantage, e.g. for hole drilling in materials which are too brittle for mechanic processing, such as ceramics. The excimer lasers used predominantly so far are XeCl lasers and KrF lasers but these show important limitations. Although XeCl lasers can be operated with a high spatial beam quality, close to the fundamental limit of diffraction, the comparatively long wavelength (308 nm) often leads to an undesirably deep penetration of the radiation, which leads to problems with the quality of machining. KrF lasers emit at a shorter wavelength (248 nm), which reduces the absorption depth. However, the spatial beam quality of KrF lasers is rather poor. This limitation arises because, with decreasing wavelength, discharges in excimer are known to become spatially inhomogeneous through instabilities leading to filamentation, within typically only ten or a few tens of nanoseconds after the ignition of the discharge. This excludes the buildup of an unperturbed, high-quality beam in sufficiently many roundtrips through the laser resonator. High-precision processing may still be done with low quality beams, by spatial filtering with masks or diaphragms. But this strongly reduces the efficiency because only a small fraction of the radiation reaches the work piece. What is actually required is an excimer laser, which emits a diffraction-limited, high quality beam but at a much shorter wavelength than that of XeCl lasers, possibly even below the wavelength of KrF lasers.
|Award date||25 May 2007|
|Place of Publication||Enschede|
|Publication status||Published - 25 May 2007|