Abstract
We have investigated the use of annular illumination on a KrF excimer laser stepper ( λ=248 nm). The stepper has an NA=0.44 and a partial coherence (σ) of 0.45. The central obscuration used to produce annular illumination was 75%. This illumination technique, combined with a negative tone surface-imaging resist process, has demonstrated a depth of focus (DOF) of 1.6μm for 0.25μm dense lines and spaces and 1.3μm DOF for 0.3μm dense contacts.
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