Servo System Design and Analysis for Fabricating Large Area Sub-Micron-Period Interference Gratings
Date Issued
2008
Date
2008
Author(s)
Chen, Lien-Sheng
Abstract
In this thesis, we have designed and implemented a complex servo system for stitching interference gratings over large area which mainly included two sub-systems. One of them is the so-called beam steering system, which eliminates low frequency position and angle drifting of the incoming beam. Experimental results have shown less than 1.3 μrad beam angle instability and 0.16 μm position instability during sixty seconds time duration (1σ). And combined with analysis given in existed literature, 4.22 nm of fringe period variation (1σ) during this period is guaranteed.he other sub-system which has even more crucial influence on stitching results is the wafer positioning system. We have implemented a dual-stage actuating system, in which one is responsible for long stroke coarse positioning over a two dimensional plane (the motor stages) and the other one (the PZT stage) provides short-range high-bandwidth fine precision positioning. Performance is found to be limited by two factors: positioning accuracy and steady state residual instability. A brief study of how external loads of the PZT stage influence its resonant frequency and therefore have impacts on stage instability is initiated. Special servo architecture used in this system is introduced, in which a high-bandwidth controller is running in the front end and a servo system based on global high-accuracy interferometer measurement is running in back end. The high-update-rate signal is processed by a digitally-implemented low-pass filter to avoid over-reacting of the PZT stage. The design process of this filter and the fine servo rate of the PZT stage are described. Experimental challenges are initiated to verify system performance. Averaged positioning errors are found to be less than 5 nm in two linear axes and 100 nrad in rotational axis, and steady-state instabilities are less than 20 nm in X- and Y-directions and 310 nrad in θ-direction respectively, 1σ. Also, a special angular error due to error-alignment of optical and positioning coordinate axes is explained, and a measurement and correction method is proposed. The average measurement result of this angle is 3478.50 μrad with 24.92 μrad error bound.xposure and fringe-stitching experiments with both 1025 nm and 551 nm fringe period are executed and experimental results are observed by employing micrographic tools. Positioning errors can be observed directly in micrographs near stitching edges and greatly improved with the wafer positioning system. Overlapping area of exposures has to be minimized since contrast loss is severe in these areas. Optimal “zero-overlapping” step sizes in X- and Y-directions are found experimentally.inally, full four inch wafer fringe-stitching with 1025 nm grating period is initiated successfully and results are present.
Subjects
interference lithography
interference fringe stitching
beam steering
wafer positioning
servo system
large area periodic microstructures
Type
thesis
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