Multi layer overlay measurement recent developments

One of the main challenges related to the growing number of Litho layers and most specifically to Multi Patterning, is the ability to align to many layers at once. In the past things were simple, the alignment tree was set so that every layer aligns to one layer and at the most is measured versus two layers, such as contact to poly and Isolation. Today, even at the 20 nm node there are double and triple patterning for critical layers such as Isolation, poly, contact and Metal 1. This forces a much more complex alignment tree and Overlay (OVL) measurement. Layers are sometimes aligned to an average of previous layers, to different layers at different orientations and disposition is done based on several measurements. This growing challenge increases the number of Overlay measurements significantly, increases the target area and present the need to make many measurement from different layers consistent. Another challenge is the increased number of recipes and the need for flexible alignment tree scheme during development. These challenges are addressed by Multi layer targets such as Triple AIM, Multilayer AIMid and the Blossom and micro-Blossom targets where alignment marks from multiple patterning steps and layers were densely populated. A single OVL reading is calculated by the metrology tool on a selected pair or multiple pair average¹. Here we propose the Multi-Layer measurement that provides an additional degree of metrology and solution to these challenges: in one measurement several overlay results are achieved, the results are always self-consistent. It allows at the same measurement grab to look back and disposition previous layers after their processing was completed. It allows a flexible alignment tree without the need to add or change targets, even during ramp and production. It reduces the number of recipes that need to be created and managed. And it also reduces significantly the area needed for the targets. In this paper we will show recent results from IMEC, on Back-End (BE) stack of four layers including one double patterning layer. We compared several target sizes, showing that such a target can fit within the Indie requirements of 10x10 μm. Results show that there is not a lot of need to compromise on performance in order to get good Multi-Layer measurements. Eventually we will describe process compatible targets which are needed more in the Front End (FE) layers. Looking forward at the increased complexity needed for future nodes and multiple pitch splitting lithography, it is encouraging to see that for Overlay we can simplify metrology instead of making it follow the complexity trend.