edfas.org 21 ELECTRONIC DEVICE FAILURE ANALYSIS | VOLUME 26 NO. 4 specimens. Specimen thinning to electron transparency using the Xe pFIB is not trivial because of the high energy beam surface damage and the large Xe beam diameters and the prominent beam tails of the Xe beam, making fine-scale milling complex.[5] This article presents concentrated Ar ion beam milling using low energy (< 1 kV) as a post-pFIB final thinning step of plan view TEM specimens from the PCM device. Consequently, precise control of specimen thinning is achieved, which results in high-quality specimens with pristine surfaces and a large field of view for TEM characterization. MATERIALS AND METHODS PLASMA FIB SPECIMEN PREPARATION GeTe-based mushroom cells were fabricated, electrically tested, and left in the partial SET state by applying a rectangular RESET electrical pulse (8 V and 200 ns), followed by a partial SET pulse (1.8 V and 50 ns). Crosssection and plan view TEM specimens from the PCM were prepared in a Helios dual beam pFIB from Thermo Fisher Scientific. In this system, the electron source and Xe ion source traverse at 52°. Cross-section and plan view TEM specimens were successfully prepared using the pFIB milling adjustments suggested by Vitale and Sugar.[5] The specimen preparation is based on a modified plan view Ga FIB preparation.[11] The plan view pFIB preparation was discussed in more detail; however, the final polishing steps are the same for both specimen types. Details of pFIB preparation are in Bonifacio et al.[12] For this work, the total milling time for plan view TEM preparation was reduced from ~3 hours using the Ga FIB to ~1.5 hours with Xe pFIB due to higher milling rates in the pFIB. Because GeTe is susceptible to ion beam damage, depositing a protective Pt layer on the device (labeled area in Fig. 1) in preparation for the plan view lift-out is imperative. A 1 µm thick Pt layer was deposited using the electron beam (2 kV and 1.4 nA) followed by the Xe ion beam (8 kV and 3.7 nA). After welding the specimen to the grid, the back of the bulk plan view TEM specimen with the bottom device structure and Si substrate is wedge-shaped (Fig. 2a). Specimen bulk thinning using the Xe pFIB differed significantly from the Ga FIB. The placement of the milling box was crucial in successfully milling the PCM specimen using the pFIB. The milling box is placed away from the edge of the specimen (rectangular box in Fig. 2a) during pFIB milling. After Xe pFIB polishing steps, the front side of the plan view TEM specimen (Fig. 2b) was the top device surface, specifically some Pt capping layer covering the TiN layer. The back side of the specimen (Fig. 2c) included the bottom device structure of the W pillar contact and GeTe layer; most of the Si substrate and the continuous W layer were removed. Based on the image contrast, the specimen thickness varies; the bottom part of the plan view TEM specimen is the thinnest (Fig. 2b). Also, curtaining artifacts on the specimen’s back side are visible and some of the W layer remains (Fig. 2c). Fig. 1 Scanning electron microscopy top view image of the GeTe-based phase change memory (PCM) device with the position of the GeTe layer identified. The schematic on the right is a cross-section view across the GeTe layer labeled as PCM. Fig. 2 Scanning electron microscopy image of the GeTe-based PCM device after bulk milling and lift-out to the grid (a) and after thinning steps at 30 kV and 5 kV of the front (b) and back (c) sides of the plan view specimen. The green rectangular box represents the position of the milling box during pFIB milling, which is away from the specimen edge. (a) (b) (c)
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