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 f abricate four level aspheric reflecting focusing diffractive optical e lements. A fast (f/2), 1 mm diam, four-phase-level, reflecting, off-ax is imaging diffractive optical element, without spherical aberration, coma, and astigmatism, has been fabricated on silicon. Alignment and s titching errors have been held to less than 40 nm and the smallest pat tern feature is 0.6 mum. The silicon grating is coated with gold to en hance reflectivity with an efficiency of 73%. The quality of diffracti ve optical elements is limited by the fidelity of the fabrication step s, and methods are demonstrated for proximity effect correction, align ment, high selectivity mask creation, and reactive ion etching that ca n be used for high quality diffractive optic element fabrication of es sentially arbitrary type.