Effect of gravity deformation on the wavefront of 90 nm deep ultraviolet lithography projection optics

The gravity deformation of the lenses is controlled by force mounting in their under surface margin. Finite Element Analysis (FEA) method is used to analyze the relationship between the lens surface's non-spherical deformation and the compensating force. The deformation data are fitted to Zernike coefficients. Compared with the wavefront of projection optics before and after compensation, the influence of gravity deformation can be evaluated. The results show that the max non-spherical deformation RMS is decreased from 50.877 nm to 26.675 nm because of compensating force. However, the max wavefront error-root mean square value of projection optics is increased from 0.041λ to 0.055λ. Although the gravity deformation can be controlled by the compensating force efficiently, the wavefront aberration of projection optics is increased. The gravity deformation cooperation of the lenses should be considered so as to achieve better wavefront.