Frits Zernike and microlithography

Zernike polynomial surface and wavefront descriptions have been used in the manufacture of projection optics for microlithography since the 1970’s. This is because the optical tolerances are so small that one cannot rely on trial-anderror to achieve diffraction-limited wavefront correction. No manufactured optical surface can be considered to be spherical or even rotationally symmetrical; they have to be measured and systematically compensated. Over the last few decades of Moore’s Law there have been continuing decreases in wavefront tolerances and a consequent increase in sophistication of deterministic optical polishing and compensation strategies for residual surface and alignment errors. Optical designs have evolved from all-spherical to the inclusion of rotationally symmetric aspheric surfaces, more recently in the form of Forbes Q-type polynomials, to Zernike polynomials that include bilaterally symmetric terms. These historical trends and their application to EUV projection optics are reviewed and illustrated with two recent optical designs.