Overlay Quality Metric Guy Cohen, Eran Amit, Dana Klein, Daniel Kandel, Vladimir B. Levinski KLA-Tencor Corporation, 1 Halavyan St. Migdal Haemek, 23100, Israel ABSTRACT As overlay budget continues to shrink, an improved analysis of the different contributors to this budget is needed. A major contributor that has never been quantified is the accuracy of the measurements. KLA-Tencor developed a quality metric, that calculates and attaches an accuracy value to each OVL target. This operation is performed on the fly during measurement and can be applied without affecting MAM time or throughput. Using a linearity array we demonstrate that the quality metric identifies targets deviating from the intended OVL value, with no false alarms. Keywords: Overlay, Imaging, Accuracy, TMU
1. INTRODUCTION Traditionally the quantity characterizing the quality of an overlay (OVL) measurement and therefore the quality of a measuring technology is the total measurement uncertainty (TMU). This term incorporates the estimated error of a measurement due to precision and tool induced shift (TIS) of each measurement and additionally the tool to tool matching error. These errors are assumed to be of statistical nature and a proper sampling plan is used to assure OVL control in production. It is also well known that the metrology target OVL values are shifted with respect to the device OVL values as deduced from yield data or measurement by a reference metrology. In practice proper experiments are performed to characterize this shift and the results are utilized by fab control systems. A large part of the device to metrology target OVL deviation is resulting from the device characteristics and their interactions with the process and the scanner. These effects are discussed elsewhere. A portion of device to metrology OVL deviation can be attributed to the accuracy of the OVL measurement. As will be clear from the examples below, target inaccuracy is not a global issue, and is not stable along time. It is specific to the local target under measurement and is affected by measurement technology, process, layer, and local surroundings. Furthermore, the magnitude of target inaccuracy may exceed significantly that of the TMU. As the OVL budget shrinks, it is particularly critical to characterize the accuracy of the targets and appropriately handle the results to assure proper process control. KLA-Tencor developed a quality metric that delivers a quantitative evaluation of the accuracy of an OVL result for a given target. The quality metric is applicable to all imaging targets – BiB, AIM, AIMid, Blossom and multilayer AIMid. Each OVL measurement is accompanied by a quality metric value related to the accuracy of this specific OVL measurement value. In part 2 below, we introduce the quality metric concept and supporting simulations. In Part 3 we show typical experimental results. In part 4 we demonstrate the ability of the quality metric to detect outliers using a linearity array as a reference.
2. THE ORIGIN OF THE QUALITY METRIC AND SIMULATION EXAMPLE A first step in improving the accuracy is to define what we mean by this term. A more complete theoretical explanation of the accuracy appears in Daniel Kandel et al., Overlay Accuracy Fundamentals, Proceedings of SPIE (2012), paper 8324-41 (current proceedings). In this article we will only demonstrate by simulation a simple example of inaccuracy. It is well known that when a target is symmetric the measured OVL value is not sensitive to measurement details. This is demonstrated in the simulation below ( Figure 1). In this simulation the OVL value of a symmetric target having an intended OVL value of 0nm is calculated. These calculations result in a single OVL value that is equal to the intended OVL value, independent of the measurement wavelength. The situation is more complicated when asymmetry is introduced to the target as is observed in
Metrology, Inspection, and Process Control for Microlithography XXVI, edited by Alexander Starikov, Proc. of SPIE Vol. 8324, 832424 · © 2012 SPIE · CCC code: 0277-786X/12/$18 · doi: 10.1117/12.916379
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