| Popis: |
In our study we have investigated the mask 3D impact on best focus (BF) shifts, which may occur on more complex 2D patterns, by looking at simplified line/space test patterns at various pitches. Several test masks were created; with different absorber thicknesses or different quartz etch depths, all containing ASML L/S test patterns. These test patterns consist of 40 and 45nm horizontal lines at multiple pitches (80-315 nm) and 90nm vertical lines. Wafers were exposed on an NXT:1950i, and the critical dimensions (CDs) were measured through focus to get the best focus (Bossung top) for the different features. In this paper we demonstrate that optimizing the mask absorber thickness for 6% att.PSM will reduce feature-to-feature best focus offsets (~25nm smaller BF range measured on L/S test features) and thus improve the Overlapping Depth of Focus. The change in absorber thickness has limited impact on exposure latitude, but will impact the print CDs. Next to the impact of the absorber thickness on best focus shifts we also derived a 1.33 nm/nm sensitivity of the best focus range to etch depth variations for the ASML L/S test features, and show that the over-etch needs to be carefully controlled, as it also impacts the best focus range. |
