Phase Shifting Diffraction Interferometry for Measuring Extreme Ultraviolet Optics

Extreme ultraviolet projection lithography operating at a wavelength of 13nm requires surface figure accuracy on each mirror to be better than 0.25nm rms. A new type of interferometry, based on the fundamental process of diffraction, is described that can intrinsically achieve the required accuracy. Applying this principle, two independent spherical wavefronts are generated - one serves as the measurement wavefront and is incident on the optic or optical system under test and the other serves as the reference wavefront. Since they are generated independently their relative amplitude and phase can be controlled, providing contrast adjustment and phase shifting capability. Using diffraction from a single mode optical fiber, different interferometers can be configured to measure individual mirrors or entire imaging systems. Measurement of an EUV projection system is described.