Currently the most demanding application of extreme-ultraviolet (EUV) optics is in lithography. A commercial extreme-ultraviolet stepper will likely have six or more normal-incidence reflective optics, and the largest of these will be tens of cm in diameter. Each of the Mo/Si multilayer stepper mirrors must have the highest attainable reflectivity at 13 nm, but more importantly the central wavelength of the reflectivity must be controlled to better than 0.01 nm and the peak reflectivity of the mask reticle to better than 0.1%. In order to meet these demands, our group at the National Institute of Standards and Technology (NIST) has implemented several improvements to our reflectometer. Additionally, the recent upgrade of the Synchrotron Ultraviolet Radiation Facility (SURF III) allows reliable operation at a variety of stored beam energies. Utilizing this capability, coupled with different filtering schemes, we analyzed the out-of-band radiation in the system. Under optimum conditions, the out-of-band contribution of the beam incident on the sample is held to 0.4%, and is primarily due to correctible effects of near-band scatter. We estimate that our total uncertainty, due to both systematic effects (accuracy of our out-of-band corrections) and random effects (noise, optics heating, etc.) is better than 0.3%. We will also discuss improvements that have been designed and are yet to be implemented.
Issue: No. 1
Pub Type: Journals
extreme ultraviolet, lithography, metrology, reflectometry