Integrated interferometry for in-process monitoring of critical dimension in vertical NAND flash memory dry etch

Integrated interferometry installed on top of dry etch equipment was developed for the in-process measurement of critical dimension (CD) in vertical NAND flash memory dry etch forming word-line steps, also known as word-line processes. The etching process sequentially consists of mask trimming and mold etching (multilayered structure of Si3N4 and SiO2). In-process real-time CD measurements can be made in a trimming step that etches the photoresist (PR) to form a vertical mask shape. The measurement principle is based on the analysis of the interference fringe signal of light reflection to a single point in the center of the wafer. To check the measurement accuracy of the interferometer, the CD measured with interferometry was compared with an in-line SEM (scanning electron microscope). The correlation coefficient between the interferometric measurements measured during the mask trimming step and the in-line SEM measured after the completed process was 86%. On the other hand, when the mask trimming and the mold etching steps including Si3N4 over etching were added, the total measurement results showed that the correlation coefficient was 95%. This is because the CD value was derived based on the phenomenon that the PR thickness change measured by interferometry in the trimming step was a linear relationship with the change in the lateral PR amount, so the lateral PR amount was weakly etched in the mold etching step after the trimming step, which eventually affected the final formed CD. This metrology method can be simply applied to dry etching equipment in real production lines.