Understanding optical end of line metrology

Optical end of line metrology (OELM) is a new method to measure relative li ne shortening effects using conventional optical overlay instruments. In th is technique, a frame that has two adjacent sides that are constructed of l ines and spaces is imaged onto a wafer. Since gratings below 0.5 mu m canno t be resolved using conventional optics, the alignment tool sees the sides composed of lines and spaces as solid edges. The purpose of this paper is t o characterize the errors implicit with this approach. OELM results are com pared with scanning electron microscopy (SEM) line shortening measurements, which showed that optical measurements exaggerated the physical effect. A simple aerial image analogy predicts that optical line shortening measureme nts are pitch and duty cycle dependent. OELM measurements require calibrati on as a function of pitch and line width. (C) 2000 Society of Photo-Optical Instrumentation Engineers. [S0091-3286(00)02707-0].