Alignment and overlay metrology using a spectroscopic diffraction method

As lithographic technology drives the integrated-circuit feature size towards 0.1 micron and below, overlay and alignment tolerances are becoming increasingly severe. State-of-the-art aerial imaging overlay metrology systems are limited in accuracy due to inherent limitation on tool imaging resolution and aberrations inherent to the optical system. These in effect result in measurement inaccuracy exceeding industry requirements for next generation overlay tolerances. In this paper, a spectroscopic, diffraction based, technique is proposed as an alternative solution for overlay metrology and alignment measurement in sub 0.1 micron node. With one diffraction grating on the surface overlaying a second diffraction grating on a reference layer, the spectroscopic reflection is modulated by the relative position of the two gratings. Thus, the alignment error can be extracted from broadband diffraction efficiency of measurement pads constituent to the alignment target. This novel diffraction-based overlay metrology has inherent advantages over the traditional image-based overlay metrology: the targets are less sensitive to process variations, and spectroscopic diffraction measurements are less affected by the inherent aberration of the optical system. For these reasons, a diffraction-based spectroscopic metrology has higher potential to generate overlay data that is directly related to registration errors due to alignment and not due to the process or tool aberrations. As a consequence, this technology has a higher potential to generate accurate data to correct registration errors during the lithographic process. Feasibility and accuracy of the technique is studied through a set of experiments and of rigorous simulations on grating targets with various line-width and line-to-space ratios. Correlation to aerial imaging overlay measurements is demonstrated. The results suggest the technique is capable of achieving or exceeding overlay and alignment control accuracy requirements for sub 0.1 micron design rules.