Soft-X-Ray Projection Imaging Using a 1:1 Ring-Field Optic

The first demonstration of diffraction-limited imaging at 14 nm in the soft-x-ray region, which resulted in the printing of 0.05 μm wide lines and spaces in a 60 nm thick film of PMMA resist, was produced using a multilayer-coated 20:1 reduction Schwarzschild optic.1 Unfortunately, a Schwarzschild optic possesses a central obscuration and a small image field and, hence, is not a very practical camera. A slightly more complicated optical system, but one that has already been used in a practical camera at visible wavelengths, is the 1:1 Offner ring- field optic.2 In theory a 0.0835 NA ring-field optic should be able to image 0.1 μm lines and spaces in a 100 micron wide 50 mm radius ring-shaped field at high contrast when illuminated with radiation at wavelengths shorter than 15 nm.3 In fact, an iridium-coated Offner 1:1 ring field camera was recently used to carry out projection imaging using 42 nm radiation from an undulator in the vacuum ultraviolet storage ring at Brookhaven National Laboratory.4