Variable line spacing diffraction grating fabricated by direct write lithography for synchrotron beamline applications

A Variable Line Spacing (VLS) diffraction grating has been fabricated using an optical direct write technique. This grating is now in use at the Advanced Light Source, in beamline 12.0.1, delivering light for EUV lithography. Direct Write Lithography (DWL) with focused light at λ = 442 nm was used for the first time to record a VLS grating pattern on a substrate coated with a photoresist. The pattern was transferred to the Si substrate surface using reactive plasma etch. Precision of groove placement was verified by wavefront measurements of a witness grating recorded simultaneously with the VLS pattern. Atomic force microscope measurements confirmed near ideal groove shape and high smoothness of the grating grooves. The grating coated with a Ru coating demonstrated diffraction efficiency of 39.5% in the negative first diffraction order which corresponds to theoretical efficiency at the wavelength of 13.5 nm. This work validates the DWL approach as a promising technique for advanced grating fabrication.