Design and fabrication of low noise high diffraction efficiency diffractive optical elements

We demonstrate a method to fabricate high quality and environmentally rugged monolithic Diffractive Optical Elements (DOEs). Analog direct-write e-beam lithography was used to produce analog resist profiles that were transferred into their substrates using Chemically Assisted Ion Beam Etching (CAIBE) in one single etching step. The Point Spread Function of the e-beam during exposure caused by forward electron scattering in the resist and back scattering from the substrate was determined by measuring the exposure of a step function. An iterative method that makes use of the point spread function was developed to adjust the electron exposure file and compensate for the proximity effect caused by electron scattering. Slope dependent etch rates that occur during the microstructure transfer process were characterized and also compensated for by exposure file adjustment. Finally, the DOE was divided into regions with different periodicity ranges. For each periodicity range the range of clock speed for the exposure is set to achieve even and accurate feature depth in the final element. Many DOEs have been fabricated by this technique including a Fresnel lens of 32 phase levels. DOEs fabricated using this technique, can be used as high quality masters for a following replication process based on molding, casting etc. Moreover comparing with conventional binary optics fabrication methods, which require multiple exposure and processing steps for master generation, our approach requires only a single lithography and etching step. Therefore the fabrication method presented in this paper will not only yield high quality masters, but will also result in a general cost reduction and reduce the turnaround time between design and replication