The Effect Of Semiconductor Processing Upon The Focusing Properties Of Fresnel Zone Plates Used As Alignment Targets

J M. Lavine; M T. Mason; D. R Beaulieu

doi:10.1117/12.941901

29 June 1984 The Effect Of Semiconductor Processing Upon The Focusing Properties Of Fresnel Zone Plates Used As Alignment Targets

J M. Lavine, M T. Mason, D. R Beaulieu

Author Affiliations +

Proceedings Volume 0470, Optical Microlithography III: Technology for the Next Decade; (1984) https://doi.org/10.1117/12.941901
Event: 1984 Microlithography Conferences, 1984, Santa Clara, United States

Abstract

The use of Fresnel Zone Plates as alignment targets for both x-rayl and optical2 lithography has generated a need for understanding their focusing properties after they have been exposed to semiconductor processing procedures. Semiconductor processing can result in variation of radial dimensions, variation of contrast, and in variation of topography, all of which can impact upon the focusing properties. In this paper we report our measurements and calculations of the axial and radial intensity distribution of monochromatic light on zone plates used for optical alignment purposes. Measurements of the axial and radial intensity distribution were made with a conventional metallurgical microscope with the white-light source replaced with a HeNe laser. Calculations were an approximate solution of the Fresnel-Kirchoff Integral and an exact solution of Maxwell's Equations. 2 For Fresnel Zone Plates exhibiting r2 symmetry where the radii are given by rm = mAfl, m = 1,2,3.....2N, where N= the number of periods and fl = the primary focus, theory indicates that equal intensities be observed at ± fl, Ã‚Â± f3, ± f5,.....where f3, f5, are the odd submultiple foci. Even submultiples are zero. The linewidth decreases as the submultiplier increases. For a Fresnel Zone Plate with f1 = 200 microns, N=4 and X = 632.8nm, we observe equal intensities at ± f1 and ± f3 and reduced intensity at ± f5. The linewidth at fl is about a factor of ten larger than at f3. With alteration of the r2 symmetry, even-valued submultiple foci appear first, and with continued variation a total redistribution of the light along the z-axis is observed. Variation of the reflectivity of the zones, variation of the thin-film thicknesses and variation of the optical path difference bewteen zones can result in large intensity variations at the primary focus. The superposition of photoresist with neither conformal nor planarizing topography can result in loss of intensity. However, if the photoresist thickness is properly chosen this loss is not significant. Several processing examples are given.

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J M. Lavine, M T. Mason, and D. R Beaulieu "The Effect Of Semiconductor Processing Upon The Focusing Properties Of Fresnel Zone Plates Used As Alignment Targets", Proc. SPIE 0470, Optical Microlithography III: Technology for the Next Decade, (29 June 1984); https://doi.org/10.1117/12.941901

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