Monte Carlo simulations of image analysis for flexible and high-resolution registration metrology

Abstract

The continuous progress of PROVE, the new photomask registration and overlay measurement tool currently under development at Carl Zeiss has been reported at mask related conferences since it's first publication at EMLC 2008. The project has moved in the past year from a final design on paper to functional hardware in the lab. Major tool components such as the climate control unit, the automated mask handling system and the metrology stage have been assembled and successfully tested. The scope of this paper is to report on the current status of PROVE and furthermore present results from simulations utilizing the image analysis routines of the tool. Monte-Carlo simulations were used to analyze the impact of several realistic tool limitations (camera noise, stage and focus noise and imaging telecentricity) on the image analysis process. The evaluation itself was based on a conventional threshold approach to perform both registration and CD measurement simultaneously. The results show, that the routines can deal with the tool imperfections and limit the contribution to the reproducibility error for standard registration markers to a negligible part. Even single contact holes suffer only from small errors, when camera noise is low and image averaging is increased. Employing a generally used test pattern the CD test results also confirm a sufficiently small error contribution to the CD non-uniformity reproducibility . MOTIVATIONPhotomask registration measurement has gained importance with the introduction of the 45 nm technology node and the discussion of double patterning schemes for several device applications. Measuring and analyzing pattern placement is not just a qualification task of high-end photomasks for the most advanced nodes; it also enables the continuous improvement of writing tools and the optimization of the overlay performance for the complete lithography process. Last year at EMLC 2008, the authors informed the mask making community about PROVE, the registration and overlay metrology tool which is currently under development at Carl Zeiss. PROVE addresses in-die measurement capability on production features by means of a high-resolution 193 nm optics, as well as optimized illumination for best contrast and pellicle compatibility. Within the last 12 months the project has made steady progress towards functional hardware as described in the following section of this paper. Meanwhile, the image analysis software has also been programmed, and was utilized for a Monte-Carlo simulation to test its functionality and assess its contribution to the overall error budget. In the main part of the paper, we present the effect of unavoidable machine inaccuracies, such as lateral positioning errors, focus errors, camera noise and telecentricity in the imaging path on the image analysis for registration and CD metrology. PROGRESS IN PROVEThe basic working principles of PROVE, focusing on the optical beam path including the new auto focus concept and illumination conditions have already been presented at photomask rel...

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