<title>Image Quality Enhancements For Raster Scan Lithography</title>

Abstract

Raster scan lithography systems, such as scanned-laser and most E-beam mask writers, produce images through a mosaic of discrete picture elements (pixels). Image qualities of the printed mask (or wafer) are governed by the interplay of several printing variables, including size and shape of the writing spot, pitch and orientation of the pixel grid, relative intensities of pixels, and exposure characteristics of the resist. We will review the theoretical foundations of raster imaging and show how these variables affect several key measures of lithographic image quality, including minimum feature size, edge placement resolution and accuracy, dimensional uniformity, and edge roughness. We will present three quality enhancement techniques and compare their performances to that provided by a basic raster printing scheme where pixels of binary (on or off) values are printed on a cartesian grid. The first technique involves rotating the printing grid 45 degrees to the main axis of the data coordinate system. We will demonstrate that, for lithographic images where most edges are parallel to the data axes, this grid provides 41% more addressable edge positions than a non-rotated grid with the same pixel density. The second technique, adapted from computer-graphics "antialiasing" applications, involves modulating the intensity of pixels along the edges of features to finely control the shape of the aerial image. This provides a vernier mechanism for the placement of exposed edges between grid locations and results in finer effective addressability and smoother edges. Third, we will review how multiple pass printing (a.k.a. vote-taking) reduces random errors, and show how it also reduces systematic errors when certain printing parameters are alternated between passes. Finally, we will present a single printing strategy in which all three techniques are combined to yield high accuracy, high-resolution images with economic use of printing pixels.