Interview

Abstract

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.