Exploratory Test Structures for Image Evaluation in Optical Projection Printing

Abstract

Geometrical patterns with high sensitivity to optical imaging system parameters have been incorporated into electrical test masks and evaluated. The exploratory structures consist of targets such as fine-lines and small-dots, checkerboards with modified sizing, subimageable defects which interact with features, and lines along the inside and outside corners of large opaque regions. The patterns are sensitive to effects such as dose, focus error, lens and illumination aberrations, and flare. The patterns were projection printed at λ = 436 nm, NA 0.28 and σ = 0.7 on doped and annealed polysilycon, thin polysilicon and titanium coated wafers. The patterns after etching were automatically probed, analyzed and plotted on a flexible probing system. Significant flare and moderate linewidth variations across a die were observed. The exploratory focus targets gave greater sensitivity than the standard linewidth structures but were also strongly affected by exposure and bias both in mask making and in pattern transfer. A test pattern with programmed defects and an associated model for quantitative interpretation gave promising results for electrically monitoring the printability of defects.