Intensity spatial filtering applied to defect detection in integrated circuit photomasks

An intensity spatial filter has been devised and used for the semiautomatic photoelectric detection of 2.5-µ defects in 5-cm
²
photolithography masks for silicon integrated circuits. The filter is based on a simple geometric approximation to the form factor or envelope function for the intensity in the Fourier-transform plane, and permits small-area diffraction-limited illumination. This approach complements the Watkins' method.