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The reduced critical dimensions of semiconductor devices place more stringent requirements on the uncertainty (including precision and accuracy) of overlay metrology tools used to monitor stepper feature placement. The use of mix and match technologies of phase shift with 1x and 5x stepper techniques and step and scan exposure tools further stresses the need for accuracy in overlay metrology tools. In addition to these pressures it is widely recognized that overlay measurement accuracy can be compromised by misalignments and measurement errors in the optical measurement tool, referred to as a tool-induced-shift. Further errors in the measurement process result from asymmetries of the specimen creating an erroneous overlay shift, referred to as wafer-induced-shift. Our optical overlay metrology program at NIST is currently addressing these measurement concerns with the development of a high-accuracy overlay measurement system. This system will be used for overlay measurement research and the calibration of standard reference artifacts designed to improve standard optical alignment procedures as well as artifacts specifically designed for overlay instrument calibration. We first discuss the design features of our new instrument: the optical measurement system, sample-stage dynamics, the metrology reference frame, and position-measurement systems. We then discuss the fundamental differences between measurement tool errors and wafer- induced errors and the phase-dependent nature of the interactions of the electric fields in the image which result from these errors. Next, difficulties encountered in trying to separate these effects by a simple rotation of the sample followed by analysis of the sum and difference components of the intensity image are investigated. We then briefly describe our design and development approach of two artifacts, one for alignment and scale calibration and the other a conventional box-in-box overlay-specific test structure. This approach is taken to best separate and reduce the error contribution from tool-induced-shift.
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