Optical Inspection of Photolithographic Masks
Original Publication Date: 1986-Jul-01
Included in the Prior Art Database: 2005-Mar-09
The effects that particles of a given size in a photolithographic mask have on the operation of the finished product depend on their local environment. To discriminate between harmful and harmless particles, a dark-field laser scanning microscope with a threshold analyzer in its output is operated using an optical resolution (aperture) matched to the size and neighborhood of the harmful particles. As an example, Fig. 1 shows the scanner output signal of two closely spaced particles 1 and 2 viewed at high resolution; as scanner threshold TH is not reached, the particles remain undetected, although they could lead to defects if the mask were to be printed by a low-resolution optical system. If the resolution of the scanner is reduced, a signal as shown in Fig. 2 is obtained, which will be detected in the inspection process.