Critical overlay control plays an important role in producing high yield. The drawback in current overlay control is that each layer is controlled independently. Thus, wafer yield is limited by residual overlays from each layer. However, interactions between different layers (or combinations of different layers) determine final yield instead of a single layer. This invention proposes to ^-optimize multiple layer overlay control to achieve better yield. One example is to utilize residual overlay information from previous layers and feed forward the residual overlay information to control of subsequent layers.
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