DISTORTION MEASUREMENT OF EMBEDDED ABSORBER (SILICON MEMBRANE) AND CONVENTIONAL (DIAMOND MEMBRANE) X-RAY MASKS

Distortion of x-ray masks remains an important issue in the development of x-ray lithography. In x-ray masks employing the embedded absorber structure, the absorber material is deposited selectively in the trenches at an elevated temperature. Thermal stresses develop upon cooling the mask due to the mismatch in the coefficients of linear expansion between the absorber and the membrane resulting in distortions. Such x-ray masks were fabricated with 0.45-mu-m thick tungsten absorber and 2.5-mu-m thick silicon membrane; the tungsten was deposited by chemical vapor deposition (CVD) at 300-degrees-C. The resulting in-plane-distortions (IPD) were experimentally measured by using a special purpose mask consisting of equal lines and spaces and a large array of fine crosses; the IPD was found by measuring the locations of these crosses at the beginning and the completion of the mask processing. It was found that the maximum IPD is 0.27-mu-m over a 7.5 X 7.5 mm2 square die for a typical value of fractional absorber coverage (FAC) of 50%. The results are in agreement with the calculations obtained from a previously developed analytical model suggesting that the distortions are entirely due to thermal stresses, predictable and correctable. Large IPD can be greatly reduced by using a membrane whose Young's modulus is much larger than that of the absorber. A conventional mask was fabricated by patterning 0.2-mu-m thick evaporated tungsten on a 1.5-mu-m thick diamond membrane; it was found that the distortions were reduced to less than 0.07-mu-m over a 7.5 X 7.5 mm2 square die in spite of the large intrinsic stress (500 MPa) in the evaporated absorber.