4 - Techniques and Tools for Photo Metrology

Abstract

This chapter discusses the critical dimension (CD) scanning electron microscope (CD-SEM) and the optical overlay tools which provide the most important final measurements for lithographic control. Electrical CD measurement is also discussed, because of its growing importance as a CD control method in cutting edge microprocessor fabrication. This chapter also covers optical metrology tools for film thickness control. A guide to the statistical interpretation of metrology data is also given. The purpose of the CD-SEM is still to gain from a top-down scan or image sufficient information about a complex cross-sectional shape to predict electrical device performance. This has become a difficult task with multiple requirements. The scan or image have high resolution to represent details of the top surface, the side wall, the foot, and any residual material in between features. The CD-SEM consists basically of an ultrahigh vacuum electron column which produces a tiny electron probe on the wafer surface; an accurate stage for locating the features to be measured on the wafer a detector which collects the electrons that arise from the impact of the probe upon the wafer surface (called “secondary” electrons or “SE”); and a sophisticated computer for interpreting the images and controlling the entire system.