Wavefront metrology for EUV projection optics by soft x-ray interferometry in the NewSUBARU

Precise measurement of the wavefront errors of projection optics with 0.1 nm RMS accuracy is necessary to develop extreme ultraviolet (EUV) lithography. To accomplish this, an experimental EUV interferometer was developed and installed at the NewSUBARU SR facility, with which various types of interferometry experiments can be carried out by replacing optical parts easily. The wavefront error of a Schwarzschild-type test optics was measured by several methods. Finally, reproducibility below 0.045 nm RMS was achieved with the point diffraction interferometer (PDI) method, and the residual systematic error was reduced to 0.066 mn RMS excluding axial symmetric aberration.