DOI: 10.1007/s00339-002-1446-0 Appl. Phys. A 75, 637–640 (2002) Rapid communication Materials Science & Processing Applied Physics A j. heber 1, ✉ c. m ¨ uhlig 2 w. triebel 2 n. danz 1 r. thielsch 3 n. kaiser 1 Deep UV laser induced luminescence in oxide thin films 1 Fraunhofer Institut für Angewandte Optik und Feinmechanik, Schillerstraße 1, 07745 Jena, Germany 2 Institut für Physikalische Hochtechnologie e.V. Jena, Winzerlaer Straße 10, 07745 Jena, Germany 3 Southwall Europe GmbH, Southwallstraße 1, 01900 Großröhrsdorf, Germany Received: 20 February 2002/Accepted: 11 April 2002 Published online: 5 July 2002 • © Springer-Verlag 2002 ABSTRACT Time-resolved luminescence experiments have been set up in order to study the interaction of 193-nm laser radiation with dielectric thin films. At room tem- perature, Al 2 O 3 coatings show photoluminescence upon ArF excimer laser irradiation, with significant intensity contributions besides the known substrate emission. Time- and energy-resolved measurements indicate the presence of oxygen-defect centers in Al 2 O 3 coatings, which suggests a strong single-photon interaction at 193 nm by F + and F center absorption. Measurements on highly reflective thin-film stacks, consist- ing of quarter-wave Al 2 O 3 and SiO 2 layers, indicate similar UV excitations, mainly from color centers of Al 2 O 3 . PACS 78.55.-m; 78.20.-e; 77.55.+f 1 Introduction The nature of defect forma- tion upon intense UV laser irradiation of wide-band-gap materials has been of continuing interest for some time. Extensive work has been performed on bulk lens materials (SiO 2 , CaF 2 ) at excimer laser wavelengths of 248 nm and 193 nm. Several radiation interac- tions have been identified as multipho- ton absorption, color-center formation, and compaction [1–5]. Early studies of UV coatings have indicated that inter- action phenomena such as two-photon absorption and compaction are signifi- cant in thin films [6] or even more pronounced [7] than in the bulk ma- terial. With regard to color centers in deep UV coatings, however, no ex- perimental investigation has been pub- lished in the literature to the best of our knowledge. Bearing in mind the high defect density in oxide UV coatings, which can be clearly inferred by re- cent absorption studies of Al 2 O 3 thin films [8–10], photoluminescence (PL) ✉ Fax: +49-3641/807-601, E-mail: heber@iof.fraunhofer.de analysis of thin-film materials is poten- tially of great interest. Even the small excitation volume of coating samples, which decreases by about five orders of magnitude compared to bulk sam- ples, can be considered to be an ac- cessible range for experimental inves- tigation, due to the high defect-center concentration. In this paper, we present first results of luminescence properties of Al 2 O 3 coatings for deep UV applications. Characteristic fluorescence spectra pro- vide evidence of the potential role of oxygen vacancies for 193-nm laser in- teractions. In agreement with previous work [11], oxygen vacancies in thin- film samples are not necessarily cre- ated by the UV irradiation itself. Their primary source is the preceding elec- tron beam deposition process. Lumines- cence properties of the thin-film mate- rial will be discussed with the aid of time- and energy-resolved spectra. Fur- thermore, the fluorescence spectra of highly reflective mirror systems suggest that Al 2 O 3 provides the main source of radiation interactions in multilayer stacks. 2 Experimental procedure The coatings investigated con- sist of SiO 2 and Al 2 O 3 single layers of 100-nm thickness and highly reflective multilayer stacks of SiO 2 /Al 2 O 3 . Sin- gle layers were deposited by reactive electron-beam deposition of 99.99% pu- rity oxides onto 370 K heated substrates (0.3 nm/s deposition rate, 10 -6 mbar starting pressure, 5 × 10 -4 mbar partial oxygen pressure during evaporation). The dielectric mirrors of Al 2 O 3 /SiO 2 were deposited in a high-vacuum, plasma ion assisted electron-beam de- position process [12] onto fused-silica substrates at room temperature. A quar- ter-wave-thickness design [(HL)20 H] has been chosen at 193-nm center wave- length for 15 ◦ angle of incidence, corres- ponding to the measurement configuration. Photoluminescence measurements of UV coatings have been performed similarly to a previously described ex- perimental system [13]. Excitation was carried out with an unpolarized excimer laser at 193 nm, a nearly top-hat-shaped beam profile of 5 mm × 5 mm, a repeti- tion rate of 10 Hz, and an integral-square pulse width [14] of about 25 ns. Laser radiation was directed at 15 ◦ angle of incidence on the sample surface. The lu- minescence response was detected close to the reflected laser beam (‘front-face’ arrangement). The luminescence sig- nal was guided by an optical fiber onto the spectrometer. An intensified, gated optical multichannel analyzer (OMA) system served for detection. The laser scattering was shielded from the detec-