Thanks to their unique combination of properties: non-volatility, speed, density and write endurance, spintronic memory called spin transfer-torque Magnetic Random Access Memory (STT-MRAM) is expected to play a major role in the future development of the Internet of Things (IoT) and more generally in information and communication technologies. This type of spintronic device is usually made of materials, some of which can be classified as critical. Recent studies have evaluated critical materials contained in magnetic random access memory [1,2]. However, in those cases the type of memory analyzed belongs to the first generation of MRAMs developed in the early 2000s. Nowadays, the memory devices are magnetized perpendicularly to the plane of the layers and contain a synthetic antiferromagnet (SAF) that provides a high coercivity to the STT-MRAM reference layer with reduced stray field. This SAF is typically made of cobalt (Co) and platinum (Pt) multilayers antiferromagnetically coupled across a thin ruthenium (Ru) layer. Due to the high-embodied energy of platinum group metals (PGMs), a common concern when evaluating these materials is the environmental risk associated with their production. An evaluation of the environmental and economic risks of using such multilayers is first reported here, followed by a discussion of its supply risk. Substitution of Co/Pt multilayers by Co/Ni multilayers can lead to a reduction by 3-4 orders of magnitude in terms of energy requirements or global warming potential (GWP) associated with the use of these multilayers. An alternative concept based on perpendicular shape anisotropy (PSA) can also yield a reduction by 1-2 orders of magnitude in these quantities. However, for the case of STT-MRAM, tiny quantities of PGM layers are used in comparison with the mass of the silicon wafer on which these type of devices are grown. Therefore, the environmental and economic impact of the silicon wafer fabrication is found to be much higher than that of the PGM materials incorporated in the STT-MRAM stacks. Nonetheless, the high supply risk associated with PGMs remains a reason for awareness. One explored possibility is a SAF structure based on Co/Ni multilayers which can have similar performance. A second more challenging alternative is also proposed based on the aforementioned PSA concept. Finally, we address the case of several other metals identified by the European Commission as critical and used in MRAM such as W or Ta, both recently included in the EU's Conflict Minerals Regulation released in January 2021 [3].
The concept of Perpendicular Shape Anisotropy (PSA) spin transfer torque (STT) MRAM has been recently proposed as a solution to achieve downsize scalability of MRAM below sub-10 nm technology nodes, down to 3-4 nm cell size lateral dimensions. In conventional p-STT-MRAM, at sub-20 nm diameters, the perpendicular anisotropy arising from the MgO/CoFeB interface becomes too weak to ensure sufficient stability of the storage layer magnetization. In addition, this interfacial anisotropy decreases rapidly with increasing temperature, resulting in a significant drawback for applications having to operate on a wide temperature range. Here, we combine both coercivity and electron holography measurements as function of temperature to show that in a PSA storage layer, the source of anisotropy is much more robust versus temperature compared to the interfacial anisotropy of conventional STT-MRAM stacks. This allows to considerably reduce the temperature dependence of coercivity. This property is quite beneficial for applications having to operate on an extended temperature range, such as automotive (-40°C to 150°C), or to fulfill solder reflow compliance requiring 1 minute retention at 260°C.
The fabrication of muti-gigabit magnetic random access memory (MRAM) chips requires the patterning of magnetic tunnel junctions at very small dimensions (sub-30 nm) and very dense pitch. This remains a...
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