Method For Finely Adjusting The Spatial Distribution Of Etch Depths Of The Beam Sampling Grating

The research work consists of two parts. The first part focuses on the theoretical analysis and experimental verification of the method for finely adjusting the spatial dis-tribution of etch depths. In the second part, we introduce the variable IBF and conduct simulation analysis of the IBF process with variable-spot ion beams.The important findings and innovations in this thesis include:1. We proposed a new solution that can improve the uniformity of diffraction effi-ciency and ensure higher productivity of BSGs. There have been two approaches:the researchers from Lawrence Livermore National Laboratory invented a technique, wet etch figuring, that is similar to the conventional IBF; the research group in National Syn-chrotron Radiation Laboratory, University of Science and Technology of China applied the Chemical Mechanical Polishing (CMP) into the post-processing of the ion-etched BSG. Though both approaches have approved effective in improving the uniformity of diffraction efficiency, their productivities are very low. Our solution is control-lably adjusting the spatial distribution of etch depths via a dynamic leaf, which par-tially shields the broad ion beam in the course of large-area scanning ion beam etching (IBE). The large-area scanning IBE contributes to higher productivity, thus our solution has a higher productivity compared to the previous two. In addition, the key point or innovation is how to realize the fine adjustment of etch depths during IBE.2. We introduced a practical method for quantitatively analyzing the process tol-erances during the fabrication of diffraction gratings. For the BSG, we conduct the tolerance analysis on two key structural parameters, i.e., groove depth and duty cycle, accounting for the uniformity of diffraction efficiency. The multi-variable error bars are used to describe the tolerance limits of different parameters and the fabrication process of diffraction gratings is modeled as a nonlinear programming problem. The tolerance analysis on BSGs indicates that the uniformity of diffraction efficiency of 5% RMS can be realized when the measurement accuracy of duty cycle is not less than ±0.01 and the control precision of etch depth is not less than ±0.2 nm.3. We put forward a practical innovation for finely adjusting the spatial distribu-tion of etch depths. On the one hand, we built a fine-adjustment system to conduct the experimental verification. On the other, we developed a fine-adjustment algorithm to simulate the fine-adjustment process and to perform optimization on the motion trajec-tory of the fine-adjustment system. Moreover, some new concepts are introduced, such as the shielding time and shielding rate, the spatio-temporal analysis of the etch process, and S-curved transformation.4. We invented the variable-spot IBF and developed a solving method based on the integral etching time by applying the fine-adjustment algorithm into the ultra-precise optical polishing. It has many advantages over the conventional IBF. For example, it is adaptive to the surface errors of different spatial frequencies; the variable-spot IBF is more stable and reliable; the solution of etching time is more accurate. Other fields of advanced optical fabrication could also benefit from the idea of adopting variable-spot tool.