Aerial plane inspection for advanced photomask defect detection

A new methodology - Aerial Plane Inspection (API) - has been developed to inspect advanced photomasks used for the 45 nm node and beyond. Utilizing images from a high resolution mask inspection system, a mask image is recovered by combining the transmitted and reflected images. A software transformation is then performed to replicate the aerial image planes produced in a photolithography exposure system. These aerial images are used to compare adjacent die in a Die-Die inspection mode in order to find critical defects on the photomask. The mask recovery process and modeling of the aerial plane image allows flexibility to simulate a wide range of lithographic exposure systems, including immersion lithography. Any source shape, Sigma, and numerical aperture (NA) can be used at all common lithographic wavelengths. Sensitivity of the inspection can be fully adjusted to match photomask specifications for CD control, lineend shortening, OPC features, and for small and large defective areas. An additional adaptive sensitivity option can be utilized to automatically adjust sensitivity as a function of MEEF. Using the Aerial Plane Inspection to compare pattern images has the benefit of filtering out non-printing defects, while detecting very small printing defects. In addition, defects that are not printing at ideal exposure condition, but may be reducing the lithographic process window, can also be detected. Performing defect detection at the aerial image plane is more tolerant to small Optical Proximity Correction (OPC) sub-resolution assist features (SRAFs) that are difficult to inspect at the reticle image plane.