To measure patterns and shapes on the mask, photomask makers use traditional critical-dimension scanning electron microscopes (CD-SEMs). In general, the CD-SEM is the workhorse metrology tool in the mask shop. It uses electron beams to form images of tiny structures at high magnifications.
The CD-SEM is sufficient to measure traditional mask shapes. But the CD-SEM could struggle, or may not work, amid the shift to more complex patterns and shapes on the mask. Most complex mask patterns do not have uniform CDs after optical proximity correction (OPC) or inverse lithography technology (ILT).
Indeed, there are several challenges in mask metrology, particularly with the CD-SEM. For one thing, mask makers must take more measurements than ever before with the CD-SEM. In addition, the number of mask defect issues flagged during mask inspection increases, but not all of these will actually result in yield problems on the wafer.