A novel robust diffraction-based metrology concept for measurement and monitoring of critical layers in memory devices

Current image based overlay metrology accuracy will not be suitable for the critical layers of near future memory production. At current nodes, measurement reproducibility of 0.6nm or better is required. The number of sampling points is also expected to increase due to the need for higher order process corrections on the exposure tool. To maintain or improve total measurement cost, these requirements should be met without negatively impacting throughput. In this paper we will study a novel, diffraction-based system especially designed to meet these challenging requirements for next generation memory devices. In addition to overlay metrology, the system is capable of measuring CD and side wall angle (SWA) within the same measurement cycle. The system can also be used to monitor exposure tool overlay and focus stability. In this paper we intend to examine the metrics used to evaluate the overlay metrology performance critical for a DRAM production environment. We also intend to spend much of the paper taking a deeper look at how we can combine the overlay and CD metrology functionalities to examine the asymmetric profile of target gratings. One of the critical applications for diffraction based overlay metrology is in understanding the asymmetric properties of target gratings across a wafer. Reconstructing asymmetric profiles quickly, effectively and with a suitable degree of sensitivity, will allow measurement accuracy to be further enhanced and will open the door to numerous applications within the memory fab environment including process monitoring and improvement. In this paper, we intend to investigate techniques for detecting asymmetric structures and also for the more complex issue of reconstructing the shape of these structures.