<title>Coherent Illumination Improves Step-And-Repeat Printing On Wafers</title>

Abstract

VLSI circuit fabrication will require new optical projection systems with micron resolution. If conventional incoherent illumination is used, large numerical aperture optics will be required, giving rise to smaller fields and reduced depth of focus. An attractive alternate approach is the use of nearly spatially coherent illumination which, for the same numerical aperture, gives higher resolution and better size control without sacrificing field size and depth of focus. Partially coherent illumination is defined and different designs for arc and laser sources are given. The effects on photoresist profile formation are analysed by using a resist behaviour model. It is shown that with partially coherent illumination, linewidths are less sensitive to variations of the energy absorbed in the resist and, consequently, are better controlled across oxide steps. A set of curves is given to determine the useful resolution and the linewidth variation amplitudes versus the coherence degree and the numerical aperture of the lenses. SEM micrographs confirm these results.© (1979) COPYRIGHT SPIE--The International Society for Optical Engineering. Downloading of the abstract is permitted for personal use only.