Control of fixturing‐induced distortion in x‐ray masks

Mask distortion can be one of the major contributions to overlay error in x‐ray lithography. Although some overlay errors due to mask patterning errors (limited by the accuracy of the electron‐beam writing tool) will always be present, a significant amount of additional overlay error can be attributed to mask distortion caused in part by the way the mask is mechanically held in place. This fixturing‐induced distortion can be present not only during the x‐ray exposure step, but also during the mask writing itself. It is clearly important to reduce such distortion to a minimum if x‐ray lithography is to fulfill its potential. We have developed a kinematic mask holder to effectively eliminate the distortion due to fixturing that has been observed with other mask holders. A description of the fixture is presented, along with experimental results showing the resulting distortion when used in an electron‐beam writing tool, a Nikon 2I measuring tool, and an x‐ray stepper. The stepper results are also compared to...