KEYWORDS: Scanning electron microscopy, 3D image processing, 3D metrology, Design rules, Critical dimension metrology, Monte Carlo methods, Metrology, Manufacturing
As the manufacturing process is more integrated, in the field of metrology, there is an increasing demand for method to monitor local dispersion of measurement value that can trace randomly generated week points in the chip. Since it is difficult to obtain local dispersion with an optical method where the relatively large spot compared to the size of the target structure, many attempts have been made to use other method such as scanning electron microscopy (SEM). It is clear that SEM is suitable for obtaining local dispersion thanks to its high resolution, but it is difficult to obtain thickness information because only contrast data is included in the image. From these demands, a method using gray level (GL) index of the SEM image to estimate the depth of the target pattern has been proposed. However, because it is an index that simply correlates the GL value to the depth without considering pattern geometry, it follows different trends depending on the design rule and dispersion of the critical dimension (CD) and depth. In order to overcome this inhomogeneity of the GL index, in this study, we propose 3D GL index considering the field of view (FOV) of secondary electron (SE) emission according to the 3D geometry of the pattern. We apply effective FOV derived from SE emission function estimated by Lambertian distribution and CD and depth of the pattern to conventional GL index. As a result of applying it to the polysilicon hole pattern and comparing it with the vertical-SEM depth measurement, unlike the existing index, 3D GL index shows a clear linear trend with high correlation R2 of 0.781 regardless of design rule and dimension variation. In that it can more accurately and robustly respond to process variation, the proposed 3D GL index can increase the utilization of the depth monitoring method using SEM image.
In semiconductor industry, depth measurement is usually performed using the optical critical dimension (OCD) metrology, the atomic force microscopy (AFM) and transmission electron microscope (TEM). However, there are some limits in the depth measurements using OCD metrology or AFM. Therefore, this paper presents a new monitoring method for local depth using scanning electron microscopy (SEM) and a feasible verification method using AFM. This paper considers monitoring of local hole depth for the direct contact (DC). First, this paper proposes a depth monitoring index based on gray level (GL) of SEM image. The index includes not only the GL of the hole but also the GL of the background to reduce the effect of the GL inconsistency. Second, this paper verifies the effectiveness of the proposed depth monitoring index using in-FAB AFM. The reliability of the in-FAB AFM measurements as the reference for local depth is verified by TEM.
The evolution of 3D NAND memory devices is increasing the depth of HAR (High Aspect Ratio) hole structure. Consequently the technology to measure the shape of the structure is also becoming more difficult. In general, optical measurement method such as OCD (Optical Critical Dimension) is mainly used for measurement of the HAR structure, but optic technology has limitation in measurement of hole structure independently. To overcome this, SEM (Scanning Electron Microscopy) with high acceleration voltage of electron beam can be used for the measurement of bottom CD (Critical Dimension, diameter of a hole) of the hole structure. However this technology also has challenge in that the measured CD does not always represent the exact bottom CD of the structure. In order to solve this problem, we propose a method of inferring the actual depth where the measured CD is located by examining the change of the acceleration voltage and the angle of incident electron beam. The CDs of real product hole pattern were measured according to the change of landing energy of electron beam and the measured depth was calculated using proposed method. After inferring the CD measured from the actual hole structure, the method is verified in a sample having known structure figures. The proposed method can be used for 3D microstructure measurements using SEM technology in the future.
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