Device design and process integration of high density nonvolatile memory devices




Ferdousi, Fahmida

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This research focuses on device design and process integration of high density nonvolatile memory devices. Research was carried out to improve scaling of floating gate memories by increasing charge density as well as spin-based memories by reducing critical switching current. This work demonstrates fabrication of CMOS-compatible nonvolatile hybrid memory device using fullerene molecules as a floating gate. Molecules have dimensions of several Angstroms resulting in an electron density of ~10¹³ cm⁻² or higher. In hybrid MOSCAPs, fullerenes were encapsulated between inorganic oxides, i.e. SiO₂ as a tunnel oxide and HfO₂ as a control oxide. Introduction of a high-k material as a control oxide improves capacitive coupling between control gate and floating gate as well as the program/erase efficiency. The MOS capacitors demonstrate nonvolatile memory operation at room temperature. The device data infers that program/erase mechanism in fullerene devices is Fowler-Nordheim tunneling; however, retention is determined by trap-assisted tunneling. The next part of the work focused on spin-transfer-torque (STT) based magnetic memory. Spin-based memory has the unique potential to be the universal memory because of its high density, fast switching, and nonvolatility. This work presents STT switching of perpendicular magnetic anisotropy (PMA) spin-valves with tilted magnetization using point contact measurement. The PMA materials have high coercivity resulting in good retention and tilted magnetization induces precessional switching resulting in a lower switching current density. First, micromagnetic simulations were performed for spin-valves with tilted magnetization and precessional switching was observed to reduce the switching current. Then, spin-valve structures were fabricated by e-beam evaporation. The structure consisted of Co/Pt and Co/Ni layers, where the thickness of the layers was optimized to obtain different amount of tilt in magnetization. Point contact measurements of tilted spin-valves show STT switching, where the switching field of the free layer varies with the magnitude and sign of the applied current. The observed STT effect is stronger in a 45° tilted spin-valve compared to a 12° tilted device presumably due to the tilted spin polarization. However, tilting introduces nonuniform effective field and canting of the domains which affect the STT.



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