In recent years, laser applications have been gaining in importance in the field of micro and macro-processing. An increasing number of laser systems are used, and more and more requirements are being placed on these systems. This also holds for optics, in particular for excimer lasers of shorter wavelengths (193 nm or 157 nm). The optical thin-film technology is a chosen method for providing the high efficiency and high resolution which is necessary, e.g. in laser medicine, or semiconductor lithography.

    The number of suitable coating materials is, however, highly restricted in this spectral range. Apart from the oxides SiO₂ and Al₂O₃, only fluoride materials (LaF₃, NdF₃, DyF₃, GdF₃, CaF_2, LiF, MgF₂, AIF_3, and Cryolite) apply. Substrates may well be coated conventionally, but using pure fluorine, or fluorine compounds as reactive gas components leads to a higher quality of the optics. This, however, is associated with industrial safety and environmental problems. Therefore, LTS (Chemical) Inc. has conducted investigations on alternative coating methods in thin-film technology in the UV range. Especially reactive gas processing, being highly dangerous for the environment, must be replaced.

    a) One solution is our novel production method allows us to introduce excess fluorine (LaF₃) in a controlled manner.
    b) Second solution to the problem is to mix the existing fluoride compounds, which makes aggressive reactive gas unnecessary. Fluoride compounds can be combined in multiple ways, also with regard to different quantity ratios, which provides a large variety of suitable coating materials. The coating properties can be optimally adapted, and adhesion problems on the substrate can be overcome e.g. by adding other suitable fluoride compounds. One way to introduce fluorine is to maintain levels of fluoride in the starter material through the introduction of a 2nd Halide as a doping component, which is a constituent of liquid phase formulation. As a result of such research, we offer LaF₃:HfF₄ for DUV coating.

    Varied mixing ratios also allow further improvements of the coatings’ optical properties. The stability of the coatings can be improved considerably through an optimized adaptation of the structure, and pollutant deposits in the coating can be reduced. This leads to an improved lifetime of the optical components. The project was supported by the International SEMATECH initiative.
 

* LTS (Chemical) Inc. is the producer of DFM^® fluoride materials