Development and application of a new tool for lithographic mask evaluation, the stepper equivalent Aerial Image Measurement System, AIMS : Optical lithography

This paper describes the development of a new tool for evaluating lithographic masks, its software, and its application to the development of advanced mask designs, including phase-shifted features. This mask-imaging system, known as the stepper equivalent Aerial Image Measurement System (AIMS), provides a means for rapid evaluation of masks. The key feature of AIMS is that the mask is imaged under conditions that emulate the image produced by a given lithographic exposure tool onto a resist layer. In the AIMS microscope, the image obtained is enlarged so as to permit quantitative measurement with a low-noise CCD camera. A quantitative record of selected features of the mask is useful in predicting the printability window for given mask and stepper combinations. Details of the optical system and extensive software capability are given, and examples are presented of feature printability of phase-shifted features, optical proximity, and other effects. Applications include the prediction of key critical mask dimensions as a function of exposure and depth of focus and the rapid checking of the effectiveness of repair actions prior to validation by resist runs. The AIMS microscope system is available as the Carl Zeiss MSM100 Microlithography Simulation Microscope and is now in use in a number of companies as a new tool for mask fabrication and development.