With the shortening printing wavelength and increasing numerical aperture of lithographic tool, the depth of
focus(DOF) sees a rapidly drop down trend, reach a scale of several hundred nanometers while the repeatable accuracy
of focusing and leveling must be one-tenth of DOF, approximately several dozen nanometers. For this feature, this article
first introduces several focusing technology, Obtained the advantages and disadvantages of various methods by
comparing. Then get the accuracy of dual-grating focusing method through theoretical calculation. And the dual-grating
focusing method based on photoelastic modulation is divided into coarse focusing and precise focusing method to
analyze, establishing image processing model of coarse focusing and photoelastic modulation model of accurate
focusing. Finally, focusing algorithm is simulated with MATLAB. In conclusion dual-grating focusing method shows
high precision, high efficiency and non-contact measurement of the focal plane, meeting the demands of focusing in
193nm projection lithography.
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