Smaller, smarter, faster, and more accurate: The new overlay metrology

Abstract

With the introduction of double patterning, overlay capability below 5nm is required for optical lithography density scaling to the 22nm node and beyond. Commensurate overlay metrology must enable dense sampling of all patterned area to control single-nanometer systematic sources of error among an increasing number of device layers. This translates to the need for sub-second measurement of microscopic targets representing multiple layers within a metrology tool field of view, all while improving accuracy. Blossom (BLO) is the overlay metrology of record for the IBM 32nm technology. As we will describe here, the densely packed array of layers represented in a single BLO target has enabled us to conduct within-field in-line sampling on our most critical layers. We will also report the significant improvements to metrology performance that have resulted from our migration of BLO technology to a new measurement platform. In addition, as 22nm development proceeds, we are shrinking our overlay targets further. A target suitable for within-chip insertion, a 10μm square micro-Blossom (μBLO) target, can accommodate up to 8 layers. Correlation of μBLO to BLO measurements on a layer pair shows excellent agreement, and despite an approximately 10X area shrink relative to BLO, the μBLO measurement uncertainty remains comfortably below 0.5nm. Our paper presents details of our target layout, measurement, and analysis approach. In addition, we detail data representative of overlay variation in state-of-the-art lithographic processes, along with our outlook for overlay metrology implementation for future technologies. © 2010 Copyright SPIE - The International Society for Optical Engineering.