Serrated Fourier filters and inspection systems

A Fourier filter configured to block a portion of light from a wafer, comprising one or more blocking elements configured to block the portion of the light, wherein the one or more blocking elements have a transition region across which transmission varies, wherein the transition region is not physically smooth due to periodic serrations formed on edges of the one or more blocking elements, and wherein the transition region is substantially optically smooth.

An inspection system configured to detect defects on a wafer, comprising:

• a Fourier filter comprising periodic serrations formed on edges of one or more blocking elements, wherein the one or more blocking elements are configured to block a portion of light returned from the wafer, wherein the periodic serrations define a transition region of the one or more blocking elements, and wherein the periodic serrations are configured to vary transmission across the transition region such that variations in the transmission across the transition region are substantially smooth;
• and
• a detector configured to detect light transmitted by the Fourier filter, wherein signals generated by the detector can be used to detect the defects on the wafer.