edfas.org 27 ELECTRONIC DEV ICE FA I LURE ANALYSIS | VOLUME 24 NO . 4 from the NVM and the technical parameters from the modality equipment. After a relationship is identified, a mathematical expression can be formulated to calculate a numerical metric. The metric should be generic for a given modality. Finally, once a quantifiable metric is established, the security thresholds can be defined. The range of values corresponding to a more secure design should be proposed. Device-specific security analysis: The emerging devices carry huge complexity and diversity in terms of their physical functionality. This introduces newvulnerabilities to attacks that can be launched by constantly evolving, advanced FA tools. Since the device-level physics varies between the emerging devices, the range of imaging modalities that canbeused toattack themalso varies from one NVM to another. Therefore, it is necessary to identify which imaging modalities pose a threat to a given NVM technology, and which of the many modalities a given NVM technology is immune to. This section provides a thorough analysis for the NVM technologies, identifying weaknesses to different modalities. There is also a brief discussion for a couple of emerging NVM types. STT-RAM cells typically have a large number of magnetic and non-magnetic layers, making themagnetic free layer nearly in themiddle of the entire device stack. As the magnetic free layer is positioned deep inside the whole STT-RAM device stack, it is difficult to probe directly in a nondestructive manner with MOCI. The widely used scanning electron microscopy (SEM) technique canbe applied to anSTT-RAMcell to identify the electrical resistance contrast. The generationof secondary electrons from the top-most electrode are responsible for SEM imaging. Therefore, getting distinguishable images from the active data storage (magnetic free) layer from the large layer stack of STT-RAM cells is not possible, evenwith high beamenergy and highmagnifications (see Fig. 4). Indistinguishable images make it less vulnerable to SEM imaging.[13] However, if adversaries can take a cross-sectional image or remove stack layers until the data storage layer is exposed, they can observe information about stored bits by SEM imaging. FeRAM uses zirconate titanate or hafnium oxide in a capacitor as a ferroelectric material with polarization properties, but no ferromagnetic properties. As the MOCI technique depends on the magnetic properties of the materials, FeRAMmaynot bevulnerable toMOCI. However, since scanningmicrowave impedancemicroscopy (sMIM) distinguishes between changes in capacitance and resistance, and SCM detects capacitance variation, FeRAM may potentially be vulnerable to sMIM and SCM. Electron beam induced resistance change (EBIRCH) can also be consideredanapplicable attackmodality since the change in resistance can be measured through physical failure inspection tools like electron beam induced current, electron beam absorbed current, and EBIRCH. The scanning transmissionelectronmicroscopy (STEM) imaging technique is suitable for investigating the microstructure of phase change materials. STEM can also be used to enhance the contrast between amorphous and crystalline material in a PCM cell. Since current for the amorphous and crystalline states of the materials will be different, conductive atomic force microscopy (CAFM) can detect the current state of thematerial and determine whether it is in the “0” or “1” state. Therefore, PCM may potentially be vulnerable to CAFM. In this scenario, an attacker would have to remove layers until exposing the (a) (b) Fig. 4 SEM imaging at 30 kV of two different STT-RAM cells comparison (a) HRS state (b) LRS state. No obvious distinctions are remarked between cells in HRS and LRS.[13]
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