Formation of monolithic masks for 0.25 μm x-ray lithography

We have constructed monolithic masks for proximity x-ray lithography by forming 1 μm thick polycrystalline Si membranes directly on glass support frames. Finished masks are 78 mm in diameter and ∼5 mm thick, with the Si membrane spanning 27 mm. The monolithic design provides simple processing and unprecedented flatness of 30 nm across the membrane and <500 nm across the entire disk. Mask blanks were metallized with 500 nm of W that was sputter deposited under conditions that hold the film stress <50 MPa. Tungsten was patterned by reactive ion etching to form features as small as 0.25 μm.