Hole Pattern Fabrication using Halftone Phase-Shifting Masks in KrF Lithography

A new concept for enhancing the depth of focus of fine hole patterns is proposed. The concept is based on modulating the complex amplitude of the hole pattern image. It is shown that shifting the origin of the complex amplitude plane enhances the depth of focus. Simulations show that the resolution limit improves to 0.5 λ/NA with the depth of focus of ±1.0 λ/N A 2. It is also shown that halftone phase-shifting masks can be used to shift the origin. The optimum intensity transmittance of the halftone region for maximizing depth of focus is 17.3% for a 0.6 λ/NA hole pattern. Using the optimized halftone phase-shifting mask, a quarter-micron hole pattern can be fabricated with the large depth of focus of 1.4 µm in KrF lithography. Proximity effects for holes arrayed in a row are also discussed.