Reliability Of Magnetic Tunnel Junctions Research Articles

High-Density SOT-MRAM Based on Shared Bitline Structure

This brief proposes a new design technique for spin-orbit torque magnetic random access memory (SOT-MRAM), suitable for high-density and low-power on-chip cache applications. A bitline of the proposed memory bit cell is shared with that of an adjacent bit cell leading to a reduction in the number of metals along the column direction. This results in higher integration density due to improved metal pitch limited region. The proposed memory can optimize the bit-cell area by aggressively reducing the size of write access transistor, since SOT-based switching operation of SOT devices translates to smaller size of write access transistor. Furthermore, the proposed SOT-MRAM still retains the advantages of SOT-MRAM such as low write energy dissipation, high read-disturb margin, and improved reliability of magnetic tunnel junction. In comparison with the conventional SOT-MRAM bit cells, our proposed MRAM bit cell can have 20% less bit-cell area. Even compared with the standard STT-MRAM, our proposed bit cell still achieves higher integration density. Moreover, the shared bitline SOT-MRAM achieves >6× lower write power and higher read-disturb margin than does the STT-MRAM.

Interview

Professor Yun Heub Song, from Hanyang University in Korea, talks about the work reported in the paper ‘Reliability of magnetic tunnel junctions with a spinel MgAl2O4 film’, page 119. The field we are researching is the reliability of magnetic tunnel junction (MTJ) devices, which is the most important field within magnetoresistive random-access memory (MRAM). Research on reliability of SiO2 has been performed previously, but on reliability of similar oxide material with MTJ was nearly not studied. Magnetic Tunnel Junctions (MTJs) are the most important parts of MRAMs and consist of two ferromagnetic layers with a tunnel barrier between them. Of the two ferromagnetic layers, one is a pinned layer in which electron spins are pinned, and the other is a free layer in which electron spins are not pinned. The resistance of MTJ differs when the electron spin of the free layer is parallel or anti-parallel to the electron spin of the pinned layer, and MTJ stores the data using the resistance difference between the two. Currently, we are researching breakdown time prediction modelling according to MTJ conditions and mechanisms of breakdown and technology for screening failure MTJs with poor reliability. We compared the reliability between MTJ with MgAl2O4 tunnel barrier, which is a new material, as well as conventional MTJ with an MgO tunnel barrier. As result, MTJ with MgAl2O4 tunnel barrier showed almost similar Time-Dependent Dielectric-Breakdown (TDDB) with different bias polarities. This is because the lattice mismatch of ferromagnetic layer and tunnel barrier is remarkably low in new MTJ, and this characteristic is applied for both top and bottom interface. To improve the reliability of MTJ, the roughness of interface between ferromagnetic layer (FM)/Tunnel barrier/FM should be uniform. If the roughness is poor, trap sites are generated at the interface creating current path, and this eventually leads to the breakdown of MTJ. As such, it is important to ensure that the interface is of excellent quality MTJ shows different reliability characteristics according to fabrication conditions, such as conditions of roughness, tunnel barrier materials and MTJ size. It would take a very long time to test the reliability of every MTJ with various conditions. The material for better characteristics is found to secure lower resistance-area (RA), higher tunnel magnetoresistance (TMR) and better thermal stability, and MTJ size is becoming smaller to satisfy the Spin-Transfer-Torque (STT). As the material changes and MTJ size becomes smaller, the need for research on reliability will become greater. In the future, we will be looking to secure more MTJs under various conditions to make it possible to predict their reliability, as well as the failure cell screening technology for the commercialization of MRAM. We are analysing the reliability with switching the state of MTJ by securing MTJ samples that are capable of STT. Also, getting over conventional DC conditions, we are analysing the reliability under AC conditions, which MTJs are actually operating.

Reliability of magnetic tunnel junctions with a spinel MgAl 2 O 4 film

We investigated the stress-induced resistance drift and breakdown characteristics in MgAl2O4 (MAO)-based magnetic tunnel junctions (MTJs) with the MAO thickness of ∼0.9 nm. The occurrence of trap sites around the tunnel barrier–electrode interfaces was characterised via interval voltage stress (IVS) and constant voltage stress (CVS) experiments, in which the occurrence of trap sites affects the resistance drift in IVS and time-dependent dielectric breakdown in CVS. The observed resistance drift ratios were almost the same for positive and negative applied voltages, indicating that the difference in the interface structure was negligibly small between the top and bottom MAO/CoFe interfaces in the MAO-based MTJs. More interestingly, the resistance drift ratios were very low compared with the results of a previous study on MgO-based MTJs. MAO-based MTJs is likely to be an effective alternative that can be used to improve MTJ device reliability.

Area-Efficient SOT-MRAM With a Schottky Diode

This letter presents a spin-orbit torque magnetic random access memory (SOT-MRAM) for high-density, reliable, and energy-efficient on-chip memory application. Unlike the conventional SOT-MRAM requiring two access transistors, the proposed MRAM uses only one access transistor along with a Schottky diode in order to achieve high integration density while maintaining the advantages of SOT-MRAM, such as low write energy and enhanced reliability of magnetic tunnel junction. The Schottky diode is forward-biased during read, whereas it is reverse-biased during write to prevent sneak current paths. Our simulation results show that the proposed MRAM can achieve 30% and 50% reduction in bit-cell area in comparison to conventional spin-transfer torque MRAM (STT-MRAM) and SOT-MRAM, respectively, and $\sim 2.5\times $ improvement in write power compared with the STT-MRAM.

Study on initial current leakage spots in CoFeB-capped MgO tunnel barrier by conductive atomic force microscopy

Although a microscopic study on a MgO tunnel barrier by atomic force microscopy has been required to study the reliability of magnetic tunnel junctions, the deterioration of bare MgO due to the adsorption of H2O and CO2 has been a problem. For an accurate evaluation of the initial current leakage spots distributed in a MgO tunnel barrier, a CoFeB-capped MgO tunnel barrier structure is proposed for evaluation by means of conductive atomic force microscopy. The CoFeB capping layer thickness was optimized to be 2.0 nm to prevent H2O and CO2 adsorption on the MgO and to minimize the series resistance due to the capping layer. The initial current leakage spot density of the MgO tunnel barrier with the optimized CoFeB capping layer exponentially increased as the thickness of the MgO tunnel barrier decreased from 1.6 to 0.8 nm, and was 157 spots/µm2 at the MgO thickness of 1.2 nm and the bias voltage of 0.5 V.

Switching and reliability issues of magnetic tunnel junctions for high-density memory device

Abstract Magnetoresistive random access memory (MRAM) is one of the promising universal memories, which combines the high speed of static RAM, high-density of dynamic RAM, and non-volatility of FLASH memory. However, MRAM faces several challenges prior to the appearance at the commercial market on a large scale. The most important challenge among them will be an issue, related to the magnetic switching. The conventional MRAM structures and writing methods cause several issues to be overcome for high packing density without cross-talk. Furthermore, small ferromagnetic elements will require high magnetic field, which will increase the power consumption of the devices. This article will review new writing schemes, such as thermal-assisted switching and spin-transfer switching methods. In addition, reliability characteristics and thermal stability of magnetic tunnel junctions will be presented, because ensuring the reliability of magnetic tunnel junctions emerges as an another challenging problem for the successful application of the new writing schemes to the high-density MRAM devices in the next generation.

Effect of nitrogen incorporation to oxidation process on the reliability of magnetic tunnel junctions

Magnetic tunnel junctions (MTJs) were fabricated using nitrogen-mixed oxygen plasma (O/sub 2/:N/sub 2/=10:1). From the measurements of time-dependent dielectric breakdown (TDDB) under a constant voltage stress, the reliability of MTJs with an oxy-nitride barrier was investigated and compared with that of MTJs fabricated using pure oxygen plasma. The reliability of MTJs with an oxy-nitride barrier is much improved with the incorporation of nitrogen to oxidation process. In addition, the reliability of the oxy-nitride barrier is gradually enhanced with increasing oxy-nitridation time even after the time exceeds the optimal value. It is believed that the enhancement is due to the bonding of nitrogen to electron traps both in the oxide barriers and at the bottom interface. The characteristics of the bonding were examined by XPS measurements, which reveal a nitrogen 1s peak in Al-N bond. The lifetime of the two barriers was also estimated for comparison.

Enhanced reliability of magnetic tunnel junctions with thermal annealing

Time-dependent dielectric breakdown (TDDB) of magnetic tunnel junctions (MTJs) under a constant voltage stress, tunneling magnetoresistance (TMR), and barrier properties, e.g. effective barrier height and thickness, were investigated as a function of thermal annealing temperatures. A Weibull failure distribution function was plotted in terms of time to breakdown (T BD ) of MTJs. The T BD when 63% fraction of as-fabricated MTJs cumulatively failed increased significantly after thermal annealing at 210 °C while the TMR also increased from 8.85% to 14.22% before and after thermal annealing at 210 °C, respectively. We believe that the enhanced reliability of the MTJs is due to healing effect of bulk defects in the barrier during the annealing process, likely leading to the reduction of defect trap density. The reduction of bulk defects in the barrier was also confirmed by the lowered 1/f noise power spectral density from voltage fluctuation measurements.