Chapter 6 - Alignment Techniques in Optical and X-Ray Lithography

Abstract

Publisher Summary This chapter describes a wide range of optical alignment techniques. It discusses the basic principles important for optical alignment and states their advantages and disadvantages. Many systems are based on several of these principles. Alignment is the process of positioning an alignment mark on one circuit level with a corresponding mark on another level. Perfect alignment does not guarantee perfect overlay accuracy, or registration, which are terms that apply to relative position throughout the field. Pattern placement errors including magnification errors and mask distortion affect overlay accuracy, even in the presence of perfect alignment. Often, magnification differences between mask and wafer dominate the distortion. These may arise from differences in temperature between when the mask or wafer was patterned and when it is used or from wafer stresses arising from processing steps. Nonlinear in-plane distortions can be minimized by careful wafer processing and by ensuring that both mask and wafer are held flat in their chucks.