Self-compensation for trefoil aberration of symmetric dioptric microlithographic lens

摘要

The i-line microlithographic lens with unity magnification can be applied for the 3D integrated circuit steppers. The configuration of the microlithographic lens can be divided into three types: the dioptric type, the catoptric type, and the mixed catoptric and dioptric type. The dioptric type with unity magnification is typically designed as symmetry about the aperture stop on both image and object sides to counterbalance aberrations effectively. The lens mounting is substantially critical for the diffraction-limit microlithographic lens, because mounting stresses and gravity degrade image quality severely. The surface deformation of the kinematic mounting is ultimately low, but the disadvantage is high cost and complicated structures. The three-point mounting belongs to the semi-kinematic mounting without over constrain to decrease the surface deformation significantly instead of the ring mounting; however, the disadvantage is the trefoil aberration caused from large-aperture lenses due to gravity. Clocking lenses is a practical method of compensating the surface figure error for optimum wavefront aberration during pre-assembly phase, and then the time and cost spent on the post-assembly for fine alignment reduce much. The self-compensation by two pairs of symmetric lenses on both sides with 60-degree angle difference is beneficial to compensate the trefoil aberration effectively, and it is a cost-effective method to achieve the wavefront error close to the design value. In this study, the self-compensation method for the trefoil deformation of large-aperture lenses employed in the symmetric dioptric microlithographic lens is successfully verified in simulation.