Multilayer Dielectric Gratings: In-situ Monitoring Of Duty Cycle Of Photoresist Mask And Ion-Beam-Etched Groove Depth

Due to the lack of effective real-time monitoring and control techniques in the fabrication of gratings, typically a large number of experiments have to be completed to ascertain a set of appropriate fabrication parameters. Therefore, the fabrication of high-quality gratings entails strict requirements on the expertise of the technicians. To minimize the empirism and randomness and increase the quantitativeness in fabricating high-quality multilayer dielectric gratings, the research presented in this thesis aims to study how to fabricate high-efficiency multilayer dielectric gratings, via the approach of comparison. Since the diffraction efficiency of the multilayer dielectric gratings is most influenced by its duty cycle (ratio of ridge width to period) and groove depth, which are essentially determined at the fabrication procedures of development and ion-beam-etching, respectively, the focus in this thesis is on how to realize effective control on the duty cycle of the photoresist gratings and the groove depth of the ion-beam-etched gratings.A new technique for controlling the duty cycle of photoresist gratings made on top of a multilayer dielectric stack is studied. It is used in conjunction with the in-situ monitoring technique during photoresist development, and based on the principle that the effective refractive index of the leaky mode that has a strong evanescent tail in the cladding varies with the duty cycle of the grating situated at the interface between the top dielectric layer and the cladding. By applying the relationship between the coupling angle and duty cycle, a desired duty cycle can be obtained by terminating development when the leaky mode is excited under a preset incident angle. Our experimental results showed that the duty cycle of the resulting grating can be adjusted by changing the incident angle. The work preliminarily establishes the feasibility of the technique. Experimental setup and procedure are presented detailedly, and the error sources and their influences are discussed.