edfas.org 25 ELECTRONIC DEVICE FAILURE ANALYSIS | VOLUME 26 NO. 4 annular dark-field (HAADF)-STEM imaging, these areas of lower Z-contrast are attributed to lower local density or voids. These voids (Fig. 5a) were located mostly near the W electrode and were verified to be inherent to the device because of the electrical programming, i.e., they were not caused by the specimen preparation. Previous work from Padilla et al.[15] has shown similar voids on a GeSbTe-based PCM device, which were attributed to element segregation and density change by electrical programming. Figure 5b is a combined EDS map for Ge and Te revealing the distribution of Ge and Te in the PCM device after RESET biasing followed by partial SET biasing. The nonuniform Ge and Te distributions near the electrode match with the affected region identified in the STEM image (Fig. 5a). A line profile (Fig. 5c) acquired across the affected region from the W electrode showed the elemental segregation in the affected region. From Fig. 5c, the amorphous GeTe area was found to have high distribution of Ge, while the partially crystallized GeTe was Te-rich. The void with low dark field STEM intensity contrast (Fig. 5a) on the amorphous GeTe, positioned between 57.5 nm to 85 nm, was found to be Te-rich (Fig. 5c). It can be concluded that during partial SET process, Ge is driven toward the W bottom electrode and leaves behind a Te-rich layer. During the partial SET bias, the Ge-rich area was amorphous, which enabled the segregation of elements. The subsequent voids were created due to elemental redistribution, and were frozen during the quench stage.[15] CONCLUSION Plan view TEM specimen preparation of the PCM device using the Xe pFIB with post-pFIB Ar ion beam milling was established. The phase transformation and elemental distribution within the GeTe layer in the partial SET state of the device were identified by TEM techniques. The area affected by the partial SET state was identified as Te-rich, partially crystallized GeTe, and a Ge-rich, amorphous GeTe with voids found close to the W bottom electrode. Most importantly, post-pFIB Ar ion beam milling was an important preparation step to achieve electron-transparent specimens by precise control of thinning of both cross-section and plan view TEM specimens. Specimens prepared by Xe pFIB can be thicker, and then further polished using Ar ion beam milling. The result is high-quality specimens with large fields of view and pristine surfaces for TEM characterization. REFERENCES 1. J. Zhu, et al.: “Tri-Directional TEM Failure Analysis on Sample Prepared by In-Situ Lift-Out FIB and Flipstage,” Proc. Int. Symp.Test. Fail. Anal. (ISTFA), 2014, doi.org/10.31399/asm.cp.istfa2014p0480. 2. T. Meyer, et al.: “Site-specific Plan-view TEM Lamella Preparation of Pristine Surfaces with a Large Field of View,” Ultramicroscopy, 2021, 228, p. 113320, doi.org/10.1016/j.ultramic.2021.113320. 3. F.A. Stevie, et al.: “Plan View TEM Sample Preparation using the Focused Ion Beam Lift-out Technique,” AIP Conf. Proc., 1998, 449, p. 868–872, doi.org/10.1063/1.56881. 4. X. Zhong, et al.: “Comparing Xe+ pFIB and Ga+ FIB for TEM Sample Preparation of Al Alloys: Minimising FIB‐induced Artefacts,” J. Microsc., 2021, 282(2), p. 101-112, doi.org/10.1111/jmi.12983. 5. S. Vitale and J.D. Sugar: “Using Xe Plasma FIB for High-Quality TEM Sample Preparation,” Microscopy and Microanalysis, 2022, 28(3), p. 646–658, doi.org/10.1017/s1431927622000344. 6. C.S. Bonifacio, et al.: “Removal of Ga Implantation on FIB-prepared Atom Probe Specimens using Small Beam and Low Energy Ar+ Milling,” Microscopy and Microanalysis, 2018, 24(S1), p. 1118–1119, doi.org/10.1017/S1431927618006074. 7. P. Nowakowski, et al.: “Accurate Removal of Implanted Gallium and Amorphous Damage from TEM Specimens after Focused Ion Beam (FIB) Preparation,” Microscopy and Microanalysis, 2018, 23(S1), p. 300–301, doi.org/10.1017/S1431927617002185. 8. C. Bonifacio, et al.: “Post-FIB Cleaning of TEM Specimens from 14 nm and Other FinFETs by Concentrated Argon Ion Milling,” Electronic Device Failure Analysis, 2019, 21(4), p. 4–12, doi.org/10.31399/asm. edfa.2019-4.p004. 9. G.W. Burr, et al.: “Recent Progress in Phase-Change Memory Technology,” IEEE Journal on Emerging and Selected Topics in Fig. 5 STEM dark-field image of the plan view specimen from the GeTe-based PCM device at partial SET state (a) labeled with the amorphous and partially crystalline areas. The corresponding elemental distribution is shown in the combined Ge and Te energy dispersive x-ray spectroscopy (EDS) map (b). Line profile from the EDS map (b) and the associated DF-STEM intensity (a) were acquired and plotted in (c). (a) (b) (c)
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