MRAM is a memory type that uses the orientation of magnetization as the elementary unit of information, the bit. In the latest generation of MRAMs, magnetization is oriented perpendicular to the plane of the material layers, deposited in sequence to form a material stack and create a magnetic tunnel junction (MTJ). The properties of these layers are improved by annealing after deposition. However, the maximum annealing temperature of such a junction is limited to 300°C, while some manufacturing processes in the microelectronics industry require annealing temperatures of 400°C. Is it possible to remove this limitation and extend the application field of MRAMs?
At the heart of MRAM is a perpendicular magnetic tunnel junction based on a stack of CoFeB/MgO/CoFeB (Figure) for which researchers at IRIG’s Spintronics and Component Technology laboratory (Spintec) are at the forefront. In order to improve its magnetic and electrical properties, a so-called annealing step after deposition is necessary. During this annealing, the boron atoms of the CoFeB layers migrate and allow crystallization of the junction. To limit the diffusion of boron into other parts of the MTJ, tantalum (Ta) with higher affinity for boron is used as a protective capping layer. During this annealing, Ta captures part of the iron atoms of the MTJ, which degrades the MRAM. This undesirable capture appears above 300°C.
Material stack and thickness in nm of the different layers of materials used in magnetic tunnel junctions after annealing at 425°C.
IRIG researchers had the idea of replacing Ta with tungsten with better refractory properties. They then observed that after annealing at 400°C tungsten had a lower tendency to capture iron. The layers then remain more homogeneous, which improves the properties observed at their interfaces. It becomes even possible to increase the annealing temperature up to 450°C while significantly improving the magnetic performance of the active part of the junction. Integrating MRAMs into standard microelectronics processes is becoming a reality.
During this annealing, tantalum tends to absorb boron from the tunnel barrier. It also absorbs some of the iron from the magnetic electrode and changes its chemical composition. The MRAM's magnetoresistance and information retention time are then degraded. Tungsten, on the other hand, captures less iron during annealing, leading to improved properties after annealing.