The method of monitoring superposition is used in a manufacturing process in which continuous layers are deposited on top of another to form a stack. Each layer may include a periodic structure, such as a diffraction grating, that will be aligned with the periodic structure within another layer. The stacked periodic structures can be irradiated to form + and -1st order diffraction patterns from the periodic structures. Images of stacked periodic structures can be captured containing + and-diffraction patterns. The + and-diffraction patterns can be compared to calculate the superposition between successive layers.
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