MASKS FOR LITHOGRAPHIC PATTERNING USING OFF-AXIS ILLUMINATION

摘要

In a lithographical tool utilizing off-axis illumination, masks to provide increased depth of focus and minimize CD differences between certain features are disclosed. A first mask for reducing proximity effects between isolated and densely packed features and increasing depth of focus (DOF) of isolated features is disclosed. The first mask comprises additional lines (214) referred to as scattering bars, disposed next to isolated edges. The bars are spaced a distance from isolated edges such that isolated and densely packed edge gradients substantially match so that proximity effects become negligible. The width of the bars is set so that a maximum DOF range for the isolated feature is achieved. A second mask, that is effective with quadrupole illumination only, is also disclosed. This mask "boosts" intensity levels and consequently DOF ranges for smaller square contacts so that they approximate intensity levels and DOF ranges of larger elongated contacts. Increasing the intensity levels in smaller contacts reduces critical dimension differences between variably sized contact patterns when transferred to a resist layer. The second mask comprises additional openings, referred to as anti-scattering bars, disposed about the square contact openings. The amount of separation between the edge of the smaller contact and the anti-scattering bars determines the amount of increased intensity. The width of the anti-scattering bars determines the amount of increase in DOF range. Both scattering bar and anti-scattering bars are designed to have widths significantly less than the resolution of the exposure tool so that they do not produce a pattern during exposure of photoresist.