For the high volume manufacturing at the 45nm node and beyond it is crucial to match the OPC behaviour of all scanners used at a given process step. For this task the ASML LithoTuner PatternMatcher software was used. LithoTuner PatternMatcher is a tool to improve the proximity differences between a reference scanner and one or more so called 'to be matched' scanners. The optimization uses the concept of sensitivities of CDs of critical features towards adjustable scanner parameters in combination with the delta CD's of those critical features. To perform the scanner matching it is very important to have accurate and repeatable CD data. Therefore we investigated the use of scatterometry as a replacement for the traditional CDSEM measurement. Scatterometry significantly enhances the measurement precision while simultaneously reduces the measurement time. In a first step we determined the sensitivities of the structures by measuring the CD response to small perturbations of the individual scanner parameter settings. CD through pitch and repeating 2 dimensional line end structures were measured using the ASML YieldStar tool and a Hitachi CDSEM. The scatterometry- and CDSEM based sensitivities of the scanner parameter settings are compared. Finally a scanner matching based on both sets of sensitivities has been performed. In this article we will show that both methods are suited to perform the scanner matching. We will also present the differences between the two sets of sensitivities obtained with scatterometry and CDSEM. At the end we will present the results of the tool matching and show the results of a cross check. In the cross check sensitivities obtained with the use of scatterometry were used for the scanner matching next to SEM metrology used for verification.
As optical lithography pushes towards the 32nm node and as the k1 factor moves toward 0.25, scanner performance and operational stability are the key enablers to meet device scaling requirements. Achieving these requirements in production requires stable lithography tools and processes. Stable performance is tracked with respect to pattern to pattern overlay, nominal focus and critical dimension uniformity (CDU). Within our paper we will characterize the intrinsic lithographic performance of the scanner and will discuss a new method of machine control to improve the stability and thus the overall performance of the lithographic solution. This is achieved by measuring specific monitor wafers, modeling the results by a new software algorithm and constantly feeding back corrective terms to the scanner. Diffraction-based optical dimensional scatterometry was selected because of its precision, its ability to measure overlay and focus with a single metrology recipe and its capability to generate greater amounts of measurement data in a shorter time period than other metrology techniques and platforms. While monitor wafer performance can be indicative, we will discuss the impact of the new control loop on product. We will take a closer look at possible interactions with the existing process control loops and work through the configuration of both internal and fab control loops. We will show improvements in the focus performance on product wafers by using scatterometry as well. Most importantly we will demonstrate that the newly implemented control loop resulted in a significant improvement of the CD and overlay performance of critical product layers. This had a very positive impact on overall process variation and the rework rate at lithography.
Differences in imaging behaviour between lithographic systems of the same wavelength result in variations of optical proximity effects (OPE). A way to compensate these irregularities is through scanner tuning. In scanner tuning, scanner specific adjustments are obtained through the determination of scanner knob sensitivities of relevant structures followed by an optimization to adjust the structure CD values to be close to the desired values. Traditionally, scanner tuning methods have relied heavily on wafer-based CD metrology to characterize both the initial mismatch as well as the sensitivities of CDs to the scanner tuning knobs. These methods have proven very successful in reducing the mismatch, but their deployment in manufacturing has been hampered by the metrology effort. In this paper, we explore the possibility of using ASML's LithoTuner PatternMatcher FullChip (PMFC) computational lithography tool to reduce the dependence on wafer CD metrology. One tuning application using flexray illumination instead of traditional scanner knobs is presented in this work; in this application individual critical features in wafer printing are improved without affecting other sites. The limited impact of tuning on other structures is verified through full-chip LMC runs. Potential uses of this technology are for process transfers from one fab to another where the OPC signature in the receiving fab is similar but not identical to the signature of the originating fab. The tuning application is investigated with respect to its applicability in a production environment, including further metrology effort reduction by using scatterometry tools.
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