Free-standing subwavelength grid infrared rejection filter of 90 MM diameter for LPP EUV light source

A subwavelength grid infrared (IR) filter as large as 90 mm in diameter was fabricated and tested on a 6 inch Si wafer for a laser-produced plasma (LPP) extreme ultraviolet (EUV) light source used in the next generation lithography tools. The IR filter is a grid type, which has higher thermal stability compared with a conventional multilayer metal membrane filter. The grid is a free-standing Mo-coated Si honeycomb structure with a thickness of 5 μm, a wire width of only 0.35 μm and a pitch of 4.5 μm. Such a large-size free-standing microstructure was successfully fabricated by carefully balancing film stress at each process step. The fabricated IR filter demonstrated 99.7 % rejection for 10.6 μm IR light.