Sub-nanometer interferometry for aspheric mirror fabrication

摘要

Aspheric mirrors for extreme ultraviolet lithography (EUVL) at a wavelength of211u001e13nm require surface figure accuracy approaching 0.10 nm rms. A new type of 211u001einterferometry, based on the fundamental process of diffraction, is described 211u001ethat has the intrinsically ability to achieve this accuracy on aspherical 211u001esurfaces. However, care must be taken in the design and implementation of the 211u001eoptical system that images the aspheric mirror onto the CCD camera. Non-common 211u001epaths of the measurement and reference wavefronts within the optical system, as 211u001ewell as distortion of the image of aspheric mirror on the CCD, must be addressed 211u001ein order to realize sub-nanometer accuracy. The phase shifting diffraction 211u001einterferometer and the mitigation of potential imaging errors are described for 211u001emeasuring the surface figure on aspheric mirrors.