Optical systems for extreme ultraviolet (EUV) lithography require optical elements with wavefront aberrations limited to a fraction of an EUV wavelength to achieve diffraction-limited performance. Achieving wavefront and surface figure metrology at this level of accuracy is one of the key challenges in the development of EUV lithography. We have successfully built and operated a prototype EUV point diffraction interferometer which is capable of performing wavefront measurement of EUV optical elements at their operational wavelength. Initial experiments to characterize the interferometer, and to measure the optical wavefront diffracted from a Fresnel zone plate lens are discussed.

We have demonstrated the operation of a high average power, all solid state laser and target system for EUV lithography. The laser operates at 1.06 µm with a pulse repetition rate of 200 Hz. Each pulse contains up to 400 mJ of energy and is less than 10 ns in duration. The EUV conversion efficiency measured with the laser is independent of the laser repetition rate. Operating at 200 Hz, the laser has been used for lithography using a 3 bounce Kohler illuminator.

Point diffraction interferometry is a common-path technique that is suitable for at-wavelength testing of EUV lithography optics. We have constructed and successfully tested such a device at the Advanced Light Source at Lawrence Berkeley Laboratory. Preliminary results on the characterization of the wavefront produced by a Fresnel zone plate lens are presented. Reference wavefront quality and thermal effects are also discussed.

To achieve the image performance necessary for soft x-ray projection lithography, interferometric testing at the design wavelength is required to accurately characterize the wavefront of the imaging system. The wavefront depends not only on the surface figure of the individual optics and on their relative alignment, but also on aperture dependent phase shifts induced by the resonant multilayer coatings on the optical surfaces. This paper describes the design and lithographic fabrication of an array of point diffraction interferometers on a Si 3 N 4 membrane that has been over-coated with a spatially graded absorbing film to provide fringe contrast control. Experimental results using a visible light analogue (larger pinholes and different absorption gradient) will be presented.