Method for determination of the radiation stability of crystals

#The present invention relates to methods of determination of radiation stability of crystals, especially of crystals for optical elements, such as lenses, and to the use of crystals of a given radiation stability for manufacture of optical components and electronic units.

#It has been shown that color centers arise in crystals at crystal defects cause by the presence of foreign atoms and at crystal lattice imperfections due to irradiation of the crystals. This means that the more light or electromagnetic radiation that is propagated in a crystal, the greater the number of color centers formed in the crystal. Because of this the light absorption of the crystal increases or the light transmittance decreases. The formation of color centers and the decrease in radiation transmission caused by that increase has proven troublesome or problematic, especially in optical components, through which energetic light, such as laser light, is conducted, as in steppers, with which structures of integrated circuits are optically projected on a photo-lacquer-coated wafer.

#This formation of color centers plays an important role in optical structuring by lasers. It has already been attempted to make crystals of sufficient perfection and sufficient purity so that they are nearly free of foreign atoms and filter defects. However since impurities of less than 1 ppm in optical components, especially those used for DUV (Deep Ultra Violet, λ<250 nm), already cause notable difficulties, each individual crystal must be tested for radiation stability prior to use in an optical component. The procedure up to now has been to cut a sample with a length of about 1 to 10 cm and a cross-section of about 2.5 cm×2.5 cm of the crystal to be tested. Subsequently the front face of the sample was finely polished and irradiated by means of a laser, usually with a working wavelength of 157 nm generated by an F2 excimer laser and/or with a current working wavelength for a stepper of 193 nm generated by an ArF excimer laser. The usual energy density amounts to from 1 to 100 mJ/cm2 with a pulse frequency of 50 to 500 Hz and a pulse rate of 104 to 107. The absorption of the sample before and after laser irradiation at the respective working wavelengths was measured with a spectrophotometer. The laser-induced transmission decrease was calculated at both values. The conversion to the absorption coefficient was then performed by calculation according to the method described by K. R. Mann and E. Eva in “Characterizing the absorption and aging behavior of DUV Optical Material by High-resolution Excimer Laser Calorimetry”, SPIE, Vol. 3334, p. 1055.

#The objects, features and advantages of the invention will now be illustrated in more detail with the aid of the following description of the preferred embodiments, with reference to the accompanying sole FIGURE which is a graphical illustration of a correlation of laser-induced damage caused by an ArF excimer laser and Röntgen damage.

#The crystals tested were made according to the Bridgeman-Stockbarger Method. For example synthetic CaF2 powder was filled in a vessel. The powder was melted at a temperature of from 1400 to 1460° C. Crystals were drawn into a temperature field by lowering of the vessel. The crystal growth occurred under a vacuum of 10−4 to 10−5 Torr.

#Other known crystal growing processes, such as the Czochralski and the Nacken-Kyropoulus Process, which are generally for the commercial growth of CaF2 crystals, have no or only a subordinate importance here (Lit.: K. -Th. Wilke and J. Bohm, “Crystal Growth”, Verlag Harri Deutsch, Thun, Frankfurt/Main 1998).

#The crystalline CaF2 is cleaved along the {1,1,1}-direction of the crystal, in contrast to amorphous quartz glass, which is also used for optical components in DUV photolithography.