Design and fabrication of diffractive phase element for minimizing the focusing spot size beyond diffraction limit

This paper proposes a fabrication apparatus of high numerical aperture (NA) diffractive lens (Concentric Chirped Grating, CCG). The fabrication scheme is based on photolithography incorporating with Double Patterning (DP) technique and Litho-Etch-Litho-Etch (LELE) process. The CCG element having NA up to 1.4 in a glass substrate (n=1.5) at 940 nm wavelength and feature size down to 320 nm is successfully fabricated. The fabricated element can be very useful in integrated Surface Plasmon sensors and beam shaping applications. When controlling the shift between the odd and even rings during the second exposure, the novel phase element that can minimize the optical beam spot size beyond the diffraction limit is generated.