Structure and method for amplifying target overlay errors using the synthesized beat signal between interleaved arrays of differing periodicity

摘要

The amplification of target overlay errors of interleaved arrays in semiconductor fabrication is achieved by calculating the synthesized beat signal on a set of targets that are imaged using conventional microscopy and measured using a geometrical image processing algorithm. The interleaved arrays have differing periodicities resulting in a phase shift. The difference in periodicity distinguishes the arrays and amplifies the sensitivity to the overlay error. The phase-shift ensures that the elements of the arrays are interleaved and not overlapped. The beat signal has a zero crossing location that is proportional to the overlay error between the interleaved arrays, with a proportionality constant much greater than one. The overlay error is amplified by this proportionality constant.In an alternative embodiment, the geometrical image processing algorithm is first digitally filtered prior to obtaining the overlay error. This spatial filtering allows for noise suppression. A bandpass filter algorithm is employed to eliminate all frequencies other than the fundamental spatial frequencies of the arrays from the remainder of the overlay error calculation.