Development of extreme ultraviolet interferometry for laser plasma source operation

When characterizing an extreme ultraviolet (EUV) lithographic optical system, visible light interferometry is limited to measuring wavefront aberration caused by surface figure error while failing to measure wavefront errors induced by the multilayer coatings. This fact has generated interest in developing interferometry at an EUV camera's operational wavelength (at-wavelength testing), which is typically around 13 nm. While a laser plasma source (LPS) is being developed as a lithography production source, it has generally been considered that only an undulator located at a synchrotron facility can provide the necessary laser-like point source for EUV interferometry. Although an undulator-based approach has been successfully demonstrated, it would be advantageous to test a camera in its operational configuration. We are developing the latter approach by utilizing extended source size schemes to provide usable flux throughput. A slit or a grating mounted at the source plane can provide the necessary spatial coherence for lateral shearing interferometry. The usable source size is limited only by the well-corrected field of view of the camera under test. The development of this interferometer will be presented.