edfas.org 23 ELECTRONIC DEV ICE FA I LURE ANALYSIS | VOLUME 24 NO . 4 • Describe the attacking capabilities of physical modalities, threat models, and assumptions. • Present a comprehensive taxonomy of NVMs, showing which physical modality each type is vulnerable to. • Develop a framework of metrics for quantitatively assessing the vulnerability of NVM cells to different physical attacks. • Discuss potential countermeasures against physical attacks, at design, device, and package levels. BACKGROUND EMERGING NVM DEVICES Technical limitations of memory devices have motivated researchers to continue innovating. Generally, when compared to traditional nonvolatile memory, the new emerging devices have significantly less energy consumption, higher density, and faster read andwrite times. [3] Figure 1 portrays a taxonomy of nonvolatile memory devices, including the emerging technologies. Nonvolatile semiconductor memories are presented, based on their functional properties and themechanismfor storing data. Flash memory is a nonvolatile memory, based on the MOSFET device, that can be erased and reprogrammed electrically. The information is stored in a floating gate (FG) memory, with programming and erasing operations carried out with a tunneling current.[4] The oxide layers surrounding the floating gate keep electrons trapped, even after power is removed. The thin tunnel oxide layer allows more carriers into the floating gate to enhance the functionality. The state of the floating gate is determined by injecting and discharging electrons. The presence of electrons in the floating gate hampers the current flow in the transistor, which is how the “0” bit state is determined. On the other hand, the bit state “1” is determined when electrons are removed from the floating gate, allowing current flow. NOR and NAND are twomajor flashmemory types that are distinguished by the structure of the memory cells. When the cells are connected in parallel to the bit lines, it is designated as NOR flash. NOR flash allows the cells to read data in a faster manner and be programmed individually. Meanwhile, series connectionof the cells is designated as NAND flash, which ismore dense, cost-effective, andpower efficient thanNOR flash. Adownside of flash memory is that the extra thin tunnel oxide layer can have reliability issues, resulting in lost charge. Also, from a broad perspective, flash memory can often violate hypothetical design requirements, like low power consumption, low latency, and number of write cycles. A popular emerging device technology is magnetoresistive randomaccessmemory (MRAM), whichuses amagnetic tunneling junction (MTJ) device that aids in storing information. MTJ is made up of memory cells with two magnetic storage layers; one has a fixedmagnetic polarity (pinned layer) while the other features a switchable magnetic polarity (free layer).[5] These two layers are separated by a thin tunneling oxide layer. More generally, MRAM is a type of memory that stores data in a magnetic material that is integrated with silicon circuitry. The cell state is determined by sensing the electrical resistance when the current passes through. If themagnetic orientationof both Fig. 1 Taxonomy of nonvolatile memory devices, showing the physical attack modalities that can be used to attack them.
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