edfas.org ELECTRONIC DEVICE FAILURE ANALYSIS | VOLUME 26 NO. 4 24 lowed by Ar ion milling were determined by analyzing the cross-section TEM specimen (Fig. 4). EDS acquired from the GeTe layer post-pFIB Ar milling (Fig. 4b) revealed the absence of Xe signal compared to post Xe pFIB milling. This indicates the removal of pFIB- induced artifacts during the Ar ion milling step. Si and Mo EDS signals are from the SiN layer and TEM grid, respectively. Furthermore, the partial SET specimen in Fig. 4c prepared by pFIB milling followed by post-pFIB Ar ion beam milling (Fig. 4c) was of high quality and suitable for high resolution TEM imaging; the crystalline and amorphous areas are easily differentiated. At partial SET state, the initial polycrystalline GeTe layer has a 160 nm wide affected area (marked with a dashed line, Fig. 4c) off-center from the W electrode. This affected region is composed of a crystalline area over a 107 nm wide amorphous dome. Using PED, area I (I in Fig. 4d) was found to be a mixture of crystalline and amorphous areas based on the distinct diffraction intensities and diffused ring near the center. Adjacent to area I was area II with a diffused ring of diffraction intensity (II in Fig. 4d) corresponding to amor- phous GeTe. EXTENT OF AFFECTED AREA BY PARTIAL SET PROCESS The plan view TEM specimens after Ar ion beam milling were electron-transparent and suitable for highresolution imaging. The affected region with the amorphous and crystalline areas ini- tially observed in the cross-section speci- men was identified on the plan view specimen. Analysis of the high-resolution TEM images and its FFTs distinguished the crystalline and amorphous areas on the GeTe layer from the plan view specimen. Details of the analyses and results are in Bonifacio et al[12]. From these results, the amorphous and partially crystalline GeTe areas were identified (Fig. 5a). The affected regions on the plan view specimen were analyzed—the inner area near the W bottom electrode as amorphous GeTe and the outer areas as partially crystallized GeTe (Fig. 5a). This region was measured as 237 nm in diameter; within this area is a 130 nm diameter amorphous region. The amorphous GeTe area was off-center from the W bottom electrode, which was similar to the cross-section specimen results. With the same applied partial SET electrical-pulse programming, the extent of the affected region was established to be larger than what was observed from the cross-section TEM specimen: 237 nm in diameter (Fig. 5a) compared to 160 nm width (Fig. 4c). Such results demonstrate the advantage of preparing a plan view specimen over cross-section specimens; it provides a large field of view that captures variation in the device because of electrical programming. The dark field (DF) image (Fig. 5a) revealed areas of dark intensity contrast in the DF-STEM image. For high-angle Fig. 4 Dark-field STEM image (a) at 300 kV of the cross-section specimen from GeTe-based PCM device prepared using the Xe pFIB, followed by concentrated Ar ion beam milling. Normalized EDS data (b) shows the absence of the Xe signal, which confirms the removal of Xe pFIB damage from the specimen. The area marked by a yellow rectangle in (a) indicates the EDS acquisition area. After partial SET operation, a high-resolution TEM image (c) displays the change in the initial polycrystalline GeTe layer forming an area (marked by arrow) with a mixture of crystalline and amorphous areas (a-GeTe) just above the W bottom electrode. A precession electron diffraction (PED) image (d) confirmed areas with mixed crystalline and amorphous region (I in d) and fully amorphous region (II in d) corresponding to the a-GeTe observed in (c). (a) (b) (c) (d) PRECISE FINAL SPECIMEN THINNING BY CONCENTRATED ARGON ION BEAM MILLING (continued from page 22)
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