Effects of stress on the stability of x‐ray masks

Accurate reproduction and overlay of submicron features with x‐ray lithography puts very stringent requirements on the mask. The stability, flatness, and durability of the mask depend on, among other factors, the stress of the blank membrane and the total stress of the overlaying films. Stress is monitored using three different techniques. Metal absorber stress σm=0.1×109 dyn/cm2 or less with membrane stress σBN∼10 σm is needed for masks with distortion less than 5 ppm. Long‐term stability of a boron nitride (BN) mask has been measured to be better than ±0.03 μm (1σ). Mask blanks with flatness <0.2 μm are routinely fabricated with high stress BN and flat glass. High durability membranes (E/1−ν=2.4×1012 dyn/cm2) with low defect counts (<1/cm2) make feasible direct metal deposition on BN and eliminate the use of polyimide.