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.

Adaptive Weighted Error-Correction Method Based on the Error Distribution Characteristics of Multi-Channel Alignment

“…In order to obtain better alignment accuracy and the positioning flexibility of the alignment mark, the current grating pitch of the alignment mark is getting smaller from 16 µm to 1 µm∼5 µm [16,38,39]. If the period of the grating is in the range of 2∼5 times the wavelength, the diffraction results calculated using scalar theory and vector theory have certain differences [40].…”

“…To realistically mimic the lithography process, the simulated grating has a pitch of 2 µm, a phase depth of 160 nm, and a duty ratio of 0.5. Compared with 16 µm, the grating pitch of 2 µm reduces the effect of mark asymmetry, improves the alignment accuracy, and reduces the occupied area on the wafer [16,39]. The alignment mark material is set to Si, while the upper medium material is set to SiO 2 with a thickness of 800 nm.…”