Control of fixturing-induced distortion in x-ray masks

Abstract

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 results using a nonkinematic fixture. In all cases, the kinematic fixture results in distortion of less than 0.02 μm (1σ), consistent with the noise of the measurements in all three tools.