X-ray microanalysis of optical materials for 157nm photolithography

Next generation microelectronic circuits will have minimum dimensions below 100 nm. It is envisioned that 157 nm laser lithography will be the next step of optical lithography, A.C. Cefalas, E. Sarantopoulou, Microelectronic Engineering, V53, (2000) 465, followed by lithographies at shorter wavelengths e.g. 13 nm. At 157 nn vacuum ultraviolet (VUV) illumination of the mask target lithographic features with dimensions less than 100 nm on the photoresist could be achieved. However, there are problems related with the design of the optical projection system. This is mainly because most of the optical materials in one hand have high absorption coefficient and their optical properties degrade constantly with time under VUV irradiation. Taking into consideration the imaging requirements for this type of application, the refractive index variation over the illuminated volume of the optical material should be better than 10^-6, and hence optical elements should be prepared from ultra high purity materials. Crystals have been examined with the Jeol 2010 F microscope equipped by the energy dispersive X-ray spectroscopy (EDXS), and it has been proved to be an efficient quality control technique for identifying defects and impurities in crystal samples. A non-uniform distribution of concentration of various elements in wide band gap dielectric crystals in confined space regions from 2 to 50 nm was found, and this result sets the limitations in the optical quality of the crystals.