MRAM (Magnetic Random Access Memory)
Among the emerging technologies that have been developed to overcome the scaling problem of current Si-based electronics, MRAM, made of magnetic thin films, seems particularly promising as it allows the development of non-volatile memory device with high operation speed and high density. For that reason, MRAM is widely investigated by major Semiconductor companies for next generation memory device.
MRAM is not only suitable as conventional memory device, but it can also be used in alternative computing system such as 'Neuromorphic computing', 'Probabilistic computing' and 'Processing in memory'. These new technologies are at the center of interest in current Semiconductor industry and research.
Magnetic tunnel junction (MTJ)
MTJ is a unit device that is the core of spintronics. An MTJ is a simple FM/Oxide/FM structure (FM=Ferromagnet, Oxide=AlOx or MgO). The figure on the left shows a simple schematic of an MTJ. Relative alignment of the magnetization of Ferromagnets (Green and Blue) determine the resistance state of an MTJ. P-state represent '0 and AP-state represent '1'. An MTJ allows the conversion of spin-information to electronic information (Resistance, Voltage, Current), which gives compatibility to current electronic system.
TMR (tunnel magnetoresistance), which is the ratio between AP and P-state resistance, is typically around 100~200%.
Parallel state (P): Low resistance (RP)
Anti-Parallel state (AP): High resistance (RAP)
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Manipulation of MRAM
MRAM can be manipulated by 3-different methods
1. Magnetic field (Toggle MRAM)
1st generation of MRAM was manipulated by inducing magnetic field with electric current flow. The figure on the right shows the schematic of a Toggle MRAM (by Avalanche). Current flowing through Write line 1& 2 induce orthogonal magnetic field that can manipulate the magnetization direction of an MTJ. However, as device size scales, it becomes more difficult to switch the magnetization and more current is required. Thus, due to weak scalability of the architecture, Toggle MRAM is only used with small capacity (~kB) and used for special applications such as space and other very harsh conditions.
2. Spin-transfer torque (STT-MRAM)
Spin-transfer torque is a method that transfers the torque (polarized current) from one FM to the other. Advantage of an STT-MRAM is that it gives good scalability. As the device size scales, smaller current is required to switch the magnetization. Moreover, compact size (2-terminal) gives high density chip layout. Thus, these features have allowed the development of STT-MRAM in major foundry companies. STT-MRAM provides non-volatility, high speed and good data retention for memory applications. Furthermore, good MTJ tunability has broaden the application of STT-MRAM in SoC.
3. Spin-orbit torque (SOT-MRAM)
Spin-orbit torque is a new physics that was discovered in 2011. This method creates spin current through spin-orbit coupling in HM/FM/oxide structure. Main advantage of SOT-MRAM is that SOT provides higher speed, better endurance and more design margin than STT-MRAM. Critical disadvantage is that it requires symmetry breaking (such as application of external magnetic field) and has lower density than STT-MRAM. Currently, SOT-MRAM is widely studied in spintronics community. We will also focus on developing and studying SOT-MRAM to make it more efficient.
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Voltage controlled magnetic anisotropy
Voltage controlled magnetic anistropy (VCMA) allows the manipulation of magnetic anisotropy with external magnetic field. This allows the control of energy barrier that separates digital 0 and 1 in MRAM, thus allowing us to either selectr a device or utilize low energy, high speed MRAM.
Spin Logic Devices
Spintronic devices are non-volatile and multi-functional, which is not possible in current Si-based devices. Thus, we work to produce logic devices, logic-in-memory devices and more complex computing units using spintronics devices.
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SOT-based logic & logic-in-memory
Spin-orbit torque devices have unique device geometry which helps in making logic devices. Using the three-terminal geometry and non-volatile property of MTJs, we can produce simple logic devices using SOT device. Current is injected into the SOT channel and the torque generated acts on MTJs and switches the magnetization. A single write line can operate multiple MTJs and the amplitude of the input current determines which MTJs will switch.
[1] S.-h. C. Baek et al., 'Novel Operation of a Multi-Bit SOT Memory Cell Addressed With a Single Write Line', IEEE Trans. Magn. 53(11), 3401405 (2017)
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SOT + VCMA MRAM for logic-in-memory application
E-field controlled magnetic anisotropy (or voltage controlled magnetic anisotropy, VCMA) is a useful technique to control the magnetic anisotropy of a magnetic device. E-field (Voltage) is applied in order to lower the magnetic anisotropy energy of the magnetic device, which lowers the critical switching current of the device. Thus, this allows for not only low-power switching, but it can be also used for cell selection when working with SOT-logic. Moreover, the VCMA behavior can be tuned by controlling the oxidation condition of the gate oxide where the E-field is applied.
[2] S.-h. C. Baek et al., 'Complementary logic operation based on electric-field controlled spin-orbit torques', Nature Electronics 1, 398-103 (2018)
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Reseach topics for MRAM
1. MTJ based logic computing
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Device architecture: Not only SOT-MRAM but STT-MRAM can be studied in order to find good application for MTJs
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Multi-MTJ based neuromorphic device
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PUFs
2. Spin-orbit torque MRAM (SOT-MRAM)
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Device architecture: Find applications for SOT-MRAM (eg. logic-in-memory, Neuromorphic etc.)
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Electric field assisted switching: Combination of Ferroelectric material for low power switching / multi-level state
3. Process Engineering
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High TMR materials
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Etch optimization