FABRICATION OF ASPHERIC HIGH NUMERICAL APERTURE REFLECTIVE DIFFRACTIVE OPTIC ELEMENTS USING ELECTRON-BEAM LITHOGRAPHY

Electron beam lithography and reactive ion etching have been used to fabricate four level aspheric reflecting focusing diffractive optical elements. A fast (f/2), 1 mm diam, four-phase-level, reflecting, off-axis imaging diffractive optical element, without spherical aberration, coma, and astigmatism, has been fabricated on silicon. Alignment and stitching errors have been held to less than 40 nm and the smallest pattern feature is 0.6 mum. The silicon grating is coated with gold to enhance reflectivity with an efficiency of 73%. The quality of diffractive optical elements is limited by the fidelity of the fabrication steps, and methods are demonstrated for proximity effect correction, alignment, high selectivity mask creation, and reactive ion etching that can be used for high quality diffractive optic element fabrication of essentially arbitrary type.