Inspection of integrated circuit photomasks with intensity spatial filters

Spatial filtering is shown to be an effective method for integrated circuit photomask inspection. Results show that errors of 0.2 mils can be detected using f/9.5 lenses and that the use of f/2.0 lenses detects errors of below 0.1 mil. Contamination, missing and added features, and step and repeat errors are all displayed. The method allows the observation of nonperiodic errors and can be used immediately for contamination inspection with very little increase in sensitivity. The procedure only needs alignment of the mask in rotation, and means that the mask can be totally inspected by scanning it through a laser beam.