Experimental investigation of pupil-fill factors as process window indicators for dry optical lithography
Kanjirakkattu Joseph Jaison.
Date of Issue2008
School of Mechanical and Aerospace Engineering
The underlying fundamental working principle of a OPL system is an inverse of the microscope. Instead of projecting a magnified image from a small object, a step-scan machine, as an OPL system is alternatively known, produces either a demagnified aerial image in air or a demagnified latent image in a photosensitive polymeric film from anobject, a photomask in this instant. Hence, classical theory formulated for microscopy, such as those of Abbe and Kohler, has been adapted to model a step-scan machine. This work reports an investigation of the accuracy of a recently proposed pupil-till factor as a process window indicator, for the purpose of rapid optimization of resolution enhancement techniques that employ attenuated phase shift mask and off-axis illumination. The concept of pupil-fill factor (PFF) formulation is based on and adapted from the Abbe’s summation of coherent source point theory for partial coherent imaging as well as classical waves interference theory. In a nutshell, the pupil fill factor correlates the weightage of the zeroth diffraction order and first diffraction order of the illumination at the fourier plane of the step-scan system to final image. The investigation focuses on the lithographic printing of periodic line-space features and process parameters that influence the optical wavefront such as transmission values of the phase-shifting features on attenuated phase shift mask, partial coherence of off-axis illumination source and pitch size of the periodic line features. The process parameters are varied and the corresponding variations of the PFF values are compared with that of process window results obtained experimentally and numerically. According to the proposed hypothesis, the PIT values would vary inversely with respect to that of the process window results. The results suggest that the PFF ‘values provides a rather good indication of the experimental process window results in some data set but lacks the consistency in this context. The lack of consistency may be attributed to the negligence of considerations such as the photochemistry interaction between the photoresist and optical irradiation, numerical aperture and feature sizc. The PFF is intended to be developed as a fast approximation tool to assist in computation time reduction of numerical optimization of resolution enhancement techniques employed in step-scan machines. Time efficient simulation plays an important role in semiconductor manufacturing, an industry characterized by its high cost and high volume of production. High resolution optical microscopy computation maybe another possible field in which PFF may be applied. Keywords: pupil-fill factor, process window, resolution enhancement techniques, optical projection lithography.