Residual aberrations in optical lithography systems used to produce integrated circuits can significantly affect the image quality produced at the wafer. Thus, the development of a simple and reliable technique for quantifying aberrations is of great importance. A theoretical foundation has been given for the ability of programmed probe based aberration targets to measure individual Zernike aberration terms. The optimum targets are inverse Fourier transforms of the Zernike polynomials and this allows the main features of the family of targets to be predicted in advance. Simulation of discretized versions shows an impressive 27 to 36% increase, per 0.01 waves of rms aberration, in the intensity at the center of the target relative to the clear field intensity. The cross contamination by other targets is about 1/6th as large and it is thus possible to measure spherical aberration independent of focus. The theoretical foundation of this work, as well as initial simulation results are presented in . An invention disclosure was filed on this work and the university filed for a provisional patent application in September 2001.
Once the aberration fingerprint of a lens or a family of lenses is determined, this data can be input into the powerful PatternMatch software created by Frank Gennari. Device designers can use this software to discover where their layouts will be affected by the aberrations present and design accordingly. This offers a significant link between designers and lithographers, which will only become increasingly important as low k1 lithography solutions are implemented.
Figure 1: An example of the digitized spherical aberration target and the response of the central probe image intensity on the wafer to no aberration, 0.05 waves (peak) of defocus, and 0.05 waves of spherical aberration.