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DESIGN METHOD FOR COMPLEX LENSES BY DIVIDING THEM INTO PARTS

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The paper deals with a method of ultraviolet objective design for optical lithography. The approach to such complex lens design is to split the complex objective into the two simpler ones. The front objective has the object at the finite distance and the image placed at the infinity. The second (receiver) part of lithographic lens acts as an ordinary photographic objective. The separate parts of complex objective have been calculated independently and combined into a single complex lens with the subsequent cross-cutting optimization of parameters. The two independent parts of lithographic lens are combined in an aperture stop plane. The both lenses are designed according to the scheme  with an external entrance pupil. In order to generate starting points of optical systems, we have used the elements of artificial intelligence in lens design software, SYNOPSYS, OSD. The proposed method describes the steps of obtaining feasible starting points and solves the typical optimization challenges within the systems with high aperture. The calculations of characteristics are explained on an example of bi-telecentric lithographic objective. The objective is optimized as a diffraction-limited system for the spectral range from 362 to 368 mm where the principal color is 365 nm. The Strehl ratio at the principal color of 365 nm at the edge of the field is equal to 0.989. The objective has a total track of 630 mm, consists of the 18 lenses with 4 aspherical surfaces. The total magnification is 0.2, with the distortion less than 1 %. The image size is 22×22 mm2. We have used S-FPL53, S-FPL51Y, BAL15Y glass material for the positive lenses and PBM2Y, PBL25Y for the negative lenses. The simulated internal transmission after the adding of antireflection coating is equal to 43 %.

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