An automated mask inspection system—AMIS

Photolithographic masks used in the fabrication of silicon integrated circuits must be inspected for defects. If the density of defects on a given mask is excessive, the mask is discarded. This paper describes an automated mask inspection system (AMIS) used for inspection of step-and-repeat masks. The limitations of human inspectors are outlined, and the considerations that led to the development of the present system are indicated. Detailed discussions are given of the optical, mechanical, and electronic subsystems, along with a description of the software, including control systems and data processing. AMIS is routinely used to inspect both masters and copies for defects as small as 2 µm, and is capable of measuring individual feature linewidths to less than M 0.1 µm. The long-term stability of AMIS and the absence of human fatigue assure consistently accurate measurement of 1) mask defect density, and 2) the fraction of chip sites which are free of defects of size greater than 2 µm.