Precise measurement of thin-film thickness in 3D-NAND device with CD-SEM

A method for the inline measurement of the tunnel oxide–nitride-blocking oxide (ONO) film thickness in 3D-NAND devices was studied. The ONO film, whose thickness is critical to the device properties, cannot be measured with conventional methods because it is deposited on the sidewall of a memory hole. Thus, a method to measure the thickness of this vertical film is required. We propose a critical dimension-scanning electron microscope (CD-SEM) measurement. The film thickness can be obtained by measuring the hole diameter before and after the film deposition. Namely, the decrease in the hole diameter should be twice of the thickness in principle. However, its applicability to the actual 10-nm-thick ONO film has not been verified. In this study, the measurement precision and the validity of the method were examined with actual ONO film in the 3D-NAND test wafers. The results showed excellent precision (0.08 nm) and good consistency with planar transmission electron microscope (TEM) and ellipsometry results. In addition, the method revealed the subnanometer thickness difference depending on the nominal hole diameter and the hole density. It suggests the impact of inhomogeneity in the source gas supply during the film deposition. These results ensure that the method is sufficiently precise for the inline local thickness measurement of the ONO film. With this method, yield and reliability managements in the 3D-NAND device manufacturing would be improved.