IOE made great progress in 193nm Lithography system.

Currently, 193nm Lithography system is the key equipment of semiconductor industry. To achieve high numerical aperture (NA), some optical components of the lithography projective lenses are designed to have strongly curved concave or convex shapes. The large steepness results in wide range of angle of incidence (AOI) that can vary from 0° for near-axis rays to over 60° for off-axis rays. Now, the common thermal evaporation and planetary deposition could lead to the film thickness uniformity over the spherical surface. Meantime, the refractive index of the high and low refractive index sublayers such as lanthanum fluoride (LaF3) and magnesium fluoride (MgF2) also changed at different positions on the spherical surface. The spectrum of the (antireflective) AR coating over a wide range of incident angles is clearly inconsistent.  

After overcome the problem of film thickness uniformity on the large-diameter spherical lenses in 2012, Cunding Liu, the member of Youth Innovation Promotion Association CAS from IOE, has researched the optical properties of the films at different locations of the spherical substrate. The micro-structures of the fluoride films at the different locations were analyzed by scanning electron microscope (SEM), the column slanting angles of the fluoride films increased gradually from the center to the brim of the spherical substrate. By the computer simulation, the vector growth model was agreement with the coating process of the lithography system. The results show that the increase of the column slanting angels led to the increase gradually of the refractive index inhomogeneity from the center to the brim of the spherical substrate. The research results were published in Thin solid films [612, 296-302 (2016)]. 

Reflectance spectra and angle-resolved reflectance (inset) of 193 nm AR coatings in coating/ substrate/ coating stacks. (a) The black (dashed) line is from theoretical design with traditional methods. The red dots are reflectance from the measurement and the solid (green) lines depict the reversely engineered spectra taking the interface roughness and varying porosity of the sublayers into account. (b) The solid (green) lines and red dots are reflectance of the stack designed with the optimized model and from experimental measurements, respectively. 

Based on the structure and optical properties of the single-layer film on the spherical substrate, researchers obtained the method to improve spectral uniformity. The studies revealed that the spectral performance of the fluoride multilayers depend on the thickness control monitor factors, interface roughness and refractive index inhomogeneity. The thickness control monitor factors led to the actual thickness of every layer film differ from the theoretical design, and the influence was obtained by measuring the corresponding actual thicknesses of optical spectra of one group single-layer films with different monitored thicknesses. The interface roughness and refractive index inhomogeneity were the primary reason to lead to spectrum non-uniformity of AR coatings on steep spherical substrate. 

By considering the interface roughness and refractive index non-uniformity, researchers proposed more accurate model to describe the multilayer fluoride films. The interface roughness is obtained from atomic force microscopy (AFM) to measure one group samples with gradually increasing number of layers. The interface roughness and refractive index non-uniformity not only affect the film thickness, but also cause the refractive index change between different layers of the same material. The results show that the interfacial roughness and refractive index non-uniformity of the multilayer fluoride film could be obtained by reversely engineered spectral characteristics of the AR coatings. The method obviously improved the accuracy and reliability of the theoretical design of the AR coatings on the wide angle of incidence of the spherical lens (as shown in the Figure), and accurately improved the spectral uniformity of the AR coating on the large-diameter spherical lens. The research results have important guiding values for the high numerical aperture 193nm lithography system, such as immersion lithography system. The research results were published in the Optics Express (26, 19524 (2018)). 

The work was funded by Youth Innovation Promotion Association CAS.