Ion beam etch for the patterning of advanced absorber materials for EUV masks

The EUV mask absorber structure is currently ~ 80nm thick TaN-based film, subtractively patterned through resist mask using dry reactive etch. A thin (2-3nm) Ru layer, below the absorber, protects the reflective Mo/Si multilayer, and acts as an etch stop. For future nodes, shadowing from an 80nm thick feature is considered prohibitive. Accordingly, more highly absorbing materials are being investigated, to allow a thinner absorber and reduced 3-D effects. For example, Ni, Pt and Pd are practical materials with EUV extinction coefficient more than double that of TaN, allowing for 25-40 nm absorber thickness. A challenge for Ni, Pt, or Pd--based absorbers is that reactive etch processes are unavailable. We discuss patterning of these materials by physical ion beam etch (IBE), the etch method of choice for these materials in magnetic or electrode applications. We demonstrate IBE etch rates up to ~80-200 nm/minute, implying process times of less than 20s. Via material and ion species optimization, we demonstrate etch selectivity to Ru up to ~ 1.8:1. Using SRIM simulations, we investigate ion damage through the underlying Ru layer, versus ion species and ion energy. Simulation predicts that any damage can be confined within a protective 2.5nm Ru layer, for ion energies of 200-400V. Depth of damage is reduced from ~8 nm to ~ 1.8 nm by reduction of the beam energy from 1200V to 200V. Based on the angular dependence of the IBE rates, we simulate IBE patterning of absorber structures, and demonstrate effective patterning down to 48nm mask half-pitch (~ 24nm wafer pitch for a 5nm node). Etched feature sidewall angles of 81-86o are demonstrated.