Implementation of autofocus in alignment system for layered imprint fabrication

Abstract

Abstract：Autofocus method based on the analysis of image content information is investigated to reduce the alignment error resulting from mark positioning uncertainty due to defocus in microstructure layered fabrication process based on multilevel imprint lithography. The applicability of several autofocus functions to the alignment mark images is evaluated concerning their uniformity, sharpness near peak, reliability and measure computation efficiency and the most suitable one based on power spectrum in frequency domain (PSFD) is adopted. To solve the problem of too much computation amount needed in PSFD algorithm, the strategy of interested region detection and effective image reconstruction is proposed and the algorithm efficiency is improved. The test results show that the computation time is reduced from 0.316 s to 0.023 s under the same conditions while the other merits of the function are preserved, which indicates that the modified algorithm can meet the mark image autofocusing requirements in response time, accuracy and robustness. The alignment error due to defocus which is about 0.5 μm indicated by experimental results can be reduced or eliminated by the autofocusing implementation. Keywords：autofocus algorithm; alignment; imprint lithography; layered fabrication Three-dimensional microstructure fabrication techniques are important for a variety of applications since fabrication in the third dimension facilitates higher speed, higher packing density, less energy loss, more efficient reaction, etc [1,2] . The final goal of our research is to develop a set of standard 3D microstructure fabrication process based on multilevel imprint lithography which is an efficient, low cost and high resolution method for the parallel replica of microstructures [3,4] . One of the critical issues of the multilevel imprint is the precision alignment between layers. So an alignment system with a resolution of 0.2 μm and an overlay accuracy of 1.5 μm based on video image is developed.Since the depth of the field of microlens is very short in the alignment system with an alignment accuracy of micros or sub-micros, it is necessary to focus the alignment mark carefully to obtain a clear image. When focusing manually, defocus error will be generated due to individual evaluation criteria for well-focused image. Moreover, to attenuate the influence of temperature variation and cycling airflow on accuracy, the alignment setup should be placed in a closed enclosure, which makes it difficult to focus manually. So it is necessary to construct or choose an autofocus function suitable for the specific system. In this paper, several autofocus algorithms are compared concerning their applicability to the mark images and the optimal one is adopted and modified. The improved focusing algorithm is applied to the alignment system.

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