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Two Layer Mask Misalignment
Mask Registration Error
Many circuit features such as transistors, contacts and vias, require two (or more) masks to form the circuit feature. During fabrication there is an error distribution for X shift and Y shift of the mask misalignment called the Overlay Error. between the two consecutive masks that must be overlaid.
The Overlay Error is usually within the range of 5nm to 10nm but this error is in addition to the focus and exposure conditions for the two masks which may result in a true worst case condition where the mask error, focus, exposure and mask misalignment conditions are all at different corners producing an extreme condition in the process.
During fabrication there is an error distribution for the Mask Misalignment, also called Mask Registration Error, between two consecutive masks that are overlaid to produce a circuit feature.
The misalignment errors are named the H Shift and V Shift (also called the X Shift and Y Shift) which denote the relative mask misalignments to each other. While the shapes of the error distributions of the X shift and Y shift errors are usually similar, the error distribution parameters (e.g. the mean and stadard deviation for a Gaussain distribution) for the X shifts and the error distribution parameters for the Y shifts are not identical.