Stitching Technology for Nanometer Periodic Interference Gratings
Date Issued
2009
Date
2009
Author(s)
Chen, Cheng-Hung
Abstract
A step-and-align interference lithography (SAIL) system for manufacturing continuous periodic nano-structures on 4-inches wafers is developed. The system establishes a large square interference image by interfering two expanded Gaussian laser beams onto a proximity approach mask. The servo system then moves the wafer around to stitch the images into a large grating. The resulted patterning shows high successful rate and potential to achieve two-dimensional structures.The key technologies in the SAIL are roughly classified as optical and mechanical. The optical technique focuses on yielding an available interfered image on the substrate and is the fusion of many different topics. The mechanical engineering subsystem includes positioning system, wafer holding system, and measurement system. The positioning accuracy and long-distance movement of the wafer chuck is archived by a dual-actuated stage, and positioning signals are feedback by high-stability and high resolution laser interferometers.To stitch the small exposure area onto the full wafer, it is essential that the stage steps at integer multiple of the fringe period. The period measurement system by geometric method is able to calculate nanometer structure period before the development of the wafer. The measurements resulted in 250 nm periodical structures with good error bound under 0.5 nm. Since the circulation of cooling water in the laser tube causes vibration on the main optical table, it is necessary to place the argon-ion laser on a separate optical table. This generates serious relative motion between the two tables and has to be eliminated by a beam stabilization/steering system. The experiments show that the variance of the position shift and angle change is under 3μm and 10 μrad, 3σ, respectively.For stitching the single small interfered area over the 4" wafer, we propose a stitching method using a complex servo system composed of dual actuator positioning stage with laser interferometer. The system operation is then based on the high resolution laser interferometer coordinate. The dual actuator system provided a wide travel range using the linear positioning stage and fed the positioning error back to the piezo-actuated stage to achieve nanometer accuracy. Hybrid control architecture was introduced due to the different communication interface between the two sets of positioning systems. The stability of positioning system in X-axis, Y-axis, and Theta_Z are 11.34 nm, 9.25 nm, and 240.6 nrad, 1σ, respectively. The positioning system can exactly lock on to the target vibration with about 10 nm error. This is less than one tenth of the fringe period and is sufficient to satisfy the accuracy requirement.The micrographs of SEM in 600 nm and 250 nm one-dimensional structures of full wafer and 250 nm two-dimensional structures of single exposure have been shown and calculated the period of nanometer structures by SEM images. The continuity of the interference fringes in the wafer has been examined using SEM. There were no discontinuous of the fringes over a long distance in the crossover region.
Subjects
Step-and-align interference lithography (SAIL)
large area
periodic nanometer structures
hybrid control architecture
dual-actuator stage
Type
thesis
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