Evaluating an alternative Pt-W alloy as a low-n EUV mask absorber: Film properties and imaging performance

The mask three-dimensional (M3D) effect poses a significant challenge to the advancement of extreme ultraviolet (EUV) lithography, particularly in high-numerical-aperture systems. This effect critically degrades key imaging metrics—such as the normalized image log-slope (NILS), telecentricity error (TCE), and best-focus variation (BFV)—limiting the achievable resolution. The adoption of absorber materials with a low refractive index (low-n) offers a promising pathway to mitigate M3D effects. In this study, we investigate an alternative Pt-W alloy as a next-generation low-n EUV mask absorber. The surface morphology, roughness, and intrinsic stress of Pt-W films with varying compositions were systematically characterized. The films exhibited excellent stability, both thermally (at operational temperatures <200°C) and chemically (against standard mask-cleaning solutions), which is essential for prolonging mask lifetime. The incorporation of tungsten was found to enhance the dry-etching performance. The imaging performance, simulated using the optically characterized film properties, was compared against a reference TaBN absorber. Simulations reveal that the Pt-W alloy enables superior imaging performance at a reduced absorber thickness, effectively alleviating M3D effects compared to the TaBN benchmark.