Multi-layer overlay metrology

A novel approach to overlay metrology, called Blossom, maximizes the number of layers measurable within a single optical field of view (FOV). As chip processing proceeds, each layer contributes a set of at least four marks, arranged symmetrically on concentric circles, to create a 90 degrees rotationally invariant array of marks that "blossoms" to fill the FOV. Radial symmetry about the target center is maintained at each layer to minimize susceptibility to metrology lens aberrations. Overlay combinations among detectable marks within the target can be measured simultaneously. In the described embodiment, 28 distinct layers are represented within a 50 mu m square FOV. Thus, all the layers of a functional chip can be represented in a single target. Blossom achieves several benefits relative to overlay methods currently in practice: center dot Compression (> 30X) of area required for overlay targets. center dot Nullification of within-target proximity effects. center dot Suppression of optical mark fidelity (OMF) errors. center dot Reduction of sensitivity to across-target detection noise. center dot Elimination of overlay error random walk among layers. center dot Reference mark redundancy for detection flexibility and robustness. center dot Integration of multi-layer and within-layer overlay control schema. center dot Simplification of overlay recipe creation and management. center dot Capture and visualization of overlay performance through the entire chip fabrication. Blossom results from 65 nm products in manufacturing are described.