Soft errors in 10-nm-scale magnetic tunnel junctions exposed to high-energy heavy-ion radiation

Abstract

The influences of various types of high-energy heavy-ion radiation on 10-nm-scale CoFeB–MgO magnetic tunnel junctions with a perpendicular easy axis have been investigated. In addition to possible latent damage, which has already been pointed out in previous studies, high-energy heavy-ion bombardments demonstrated that the magnetic tunnel junctions may exhibit clear flips between their high- and low-resistance states designed for a digital bit 1 or 0. It was also demonstrated that flipped magnetic tunnel junctions still may provide proper memory functions such as read, write, and hold capabilities. These two findings proved that high-energy heavy ions can produce recoverable bit flips in magnetic tunnel junctions, i.e., soft errors. Data analyses suggested that the resistance flips stem from magnetization reversals of the ferromagnetic layers and that each of them is caused by a single strike of heavy ions. It was concurrently found that an ion strike does not always result in a flip, suggesting a stochastic process behind the flip. Experimental data also showed that the flip phenomenon is dependent on the device and heavy-ion characteristics. Among them, the diameter of the device and the linear energy transfer of the heavy ions were revealed as the key parameters. From their dependences, the physical mechanism behind the flip was discussed. It is likely that a 10-nm-scale ferromagnetic disk loses its magnetization due to a local temperature increase induced by a single strike of heavy ions; this demagnetization is followed by a cooling period associated with a possible stochastic recovery process. On the basis of this hypothesis, a simple analytical model was developed, and it was found that the model accounts for the results reasonably well. This model also predicted that magnetic tunnel junctions provide sufficiently high soft-error reliability for use in space, highlighting their advantage over their counterpart conventional semiconductor memories.

Radiation Effects in Advanced and Emerging Nonvolatile Memories

“…When radiation-induced errors do occur, it most often a result of interaction with the select device or supporting CMOS peripheral circuitry. However, there are some reports of device-level SBUs in highly scaled STT-MRAM bits (MTJs) [92] and PCRAM bits [139] which merit further investigation.…”

“…Damage and bit upsets induced by heavy ions were studied by Kobayashi et al [91], specifically investigating the effect of 15-MeV silicon atoms on the CoFeB/MgO/CoFeB pMTJ devices with a diameter of 70 nm and did not find resistance change. Later, the same group found that singleevent bit flips can occur in an MTJ cell resulting from heavyion exposure, depending on both the LET and properties of the bit [92]. In this second study, MgO/CoFeB pMTJ with diameters between 40 and 80 nm were subject to heavy-ion bombardment.…”

“…(c) Summary of LET versus feature size/bit diameter for the experiment described in the text. Copyright (2017) The Japan Society of Applied Physics.Reprinted with permission, from[92].…”

Ar+ ion irradiation of magnetic tunnel junction multilayers: impact on the magnetic and electrical properties

“…That hardness makes MRAM promising for applications in extreme environments 1 . Still, magnetic tunnel junctions (MTJs) are not tolerant to all sorts of radiation [2][3][4] : indeed, ion-irradiation-induced modifications of MTJs have been observed, extending from soft errors (undesired but recoverable magnetization switching, provoked by localized heating 4 ) to permanent changes in magnetic and electrical properties produced by structural modifications. The degree of such modifications is governed by the theelastic an inelastic energy losses in the materials, which are dependent on the ion mass and charge state, kinetic energy, fluence, target composition; also the chemical bonding in the target and the temperature of the latter are crucial.…”

Magnetic Tunnel Junction Applications

“…Even if MTJ has high tolerance to radiation [45], the MOS access transistors in MTJ-based memory structures may be impacted by radiation. Among other factors, tolerance to radiation is one important and studied field [46][47][48][49] when it is necessary to take into account performance and risks of MRAM in the integration process of MOS technology.…”