Characteristic study of image-based alignment for increasing accuracy in lithography application

Abstract

Image-based overlay and alignment is an effective technology for finding wafer positions precisely. Methods to increase the alignment accuracy and decrease the alignment tolerance for the lithography application are important and challenging, which need more study to consider the problems in a broader context. In view of this, the inner characteristics of image-based alignment are investigated in this paper by studying the case with phase difference only as well as the case with both power and phase differences. Based on the study, the authors propose some recommendable conclusions. For the case with phase difference only, the mark period should be about four times of optical spot size, the light spot size needs to be minimized for higher resolution, and the subwavelength alignment mark is not recommended to use due to its larger cross-correlation curve width. By contrast, for the case that both power and phase differences exist in application, subwavelength property has great advantages in choosing the best alignment reference waveform and higher robustness to handle the edge profile and mark thickness variation. In reality, the measured waveforms can be changed easily due to the thickness variation of the films above the alignment mark, which may cause center position finding error. The authors suggest that it is necessary to establish a reference waveform library with all kinds of shapes, through comparing all maximal cross-correlation values with the real waveform to find the best reference one and the center position. The authors believe image-based alignment method will play an important role in advanced lithography node for its robustness for most kinds of film stacks.