The development of semiconductor devices, which opens technological innovation in mobile devices, automobiles, industrial equipment, etc. is accelerating year by year to achieve miniaturization with a higher integration. In this situation, a three-dimensional design, or a vertical integration, has emerged as a trend. For example, in Fin-Field-Effect Transistors (Fin-FETs), the aspect ratios (ARs) of the fin structure have reached as high as 10. Another example is a three-dimensional (3D) NAND flash memory of which ARs have exceeded 40 [1,2]. In the near future, we expect that the device structure will become much more complex and the ARs of 3D structures further increase. In order to fabricate those devices with a high yield, extremely strict cleanliness is required on the complexed 3D structures. In contrast to a large number of reports on cleaning and its evaluation on a flat surface, those on 3D structures are limited.
Based on this background, we aim at developing a novel non-destructive method to evaluate the cleaning performance of contaminants and oxides at the bottoms of 3D nanostructures, which are probably the most difficult areas to make clean.
We apply Angle-Resolved X-ray Photoelectron Spectroscopy (AR-XPS) for 3D structures with high ARs such as deep trenches and pores. In order to collect photoelectrons from their bottoms, we need to align the detector with the bottoms by adjusting the take-off angle of photoelectrons with a high accuracy. At this specific angle, we collect photoelectrons emitted from both the surface and the bottom. On the other hand, at a very shallow take-off angle, XPS spectra represent the condition of solely the surface. By subtracting the latter signal from the former one, we expect to extract information only from the bottoms. In this scheme, a key is to guarantee that we can detect signals from the bottoms, not from sidewalls, on the nanostructures. In order to achieve this, we propose to embed a heterogeneous “landmark” element at some bottoms.
To test the feasibility of the proposed method, we took XPS spectra of Au embedded at the bottoms of trenches with different ARs (1–6) on Si. Our concept is schematically depicted in Fig. 1(a). The sample was fabricated by metal-assisted chemical etching in solutions [3]. Figures 1(b) and 1(c) show the XPS spectra of Si2p and Au4f orbitals, respectively, at different take-off angles of photoelectrons from a Si surface consisting of trenches with an aspect ratio of approximately 6. Because Au resides only at the bottoms of the trenches, the Au intensity attenuates more significantly than that of Si as the take-off angle decreased. And it is barely detectable at angles below 60°. Figure 1(d) summarizes the area ratios of Au to Si signals as a function of take-off angles for samples with different ARs. It shows that the signal intensity of Au is strong only at an angle of 90°, and attenuates steeply at lower take-off angles. This trend is clearer for samples with higher ARs. These results demonstrate that the signal intensity from the “landmark” element, or Au in this experiment, serves as a guarantee to detect photoelectrons from the bottoms of 3D nanostructures. This will be more important when we analyze spectra on a complex structure with a higher AR. We expect that the combined method of AR-XPS measurements with our unique sample structure contributes to unveiling the cleaning performance at the bottoms of 3D structures.
References
[1] Yue-Gie Liaw et al, Solid-State Electronics 126(2016)46-50.
[2] Chong-Jhe Sun et al, IEEE journal of the Electron Devices Society 8(2020)1016-1020.
[3] Tomoki Higashi et al., Annual Meeting of JVSS 2023 [to be presented].
