X-ray mask image-placement studies at the Microlithography Mask Development Center

A continuing trend in X-ray lithography is the requirement for high accuracy masks. Image placement, or the ability to pattern images in the correct locations, is one of the most critical requirements. It is driven by a number of parameters, including the electron-beam lithography system and precision of the metrology system. Also, because the X-ray mask substrate consists of a thin membrane, it is very susceptible to the stresses of the resist film, absorber material, and plating base. An extensive analysis of the contributors to image placement was performed to determine the relative contribution of each. This analysis highlighted those contributors which caused the largest distortions and which, therefore, presented the most opportunities for improvement. Several changes were then implemented which resulted in a 50 percent overall improvement to placement of the X-ray mask images. The experimental design and detailed results are discussed.