Figure: Perpendicular magnetic anisotropy (Keff) of FeCoB storage electrode of MTJ stacks with W lamination of various thicknesses inserted between the FeCoB electrode and the Ta cap layer as a function of annealing temperature.
Magnetic tunnel junctions are the basic elements of a new class of magnetic memory called MRAM (Magnetic Random Access Memory). These are about to enter in volume production at major microelectronics foundries (Samsung, TSMC, Global Foundries…). To move towards high density memory (several Gbit), short access time (sub 3ns), and elevated temperature of operation (150°C required for automotive applications), their magnetic and electrical properties must still be improved. In particular, the magnetic anisotropy of the magnetic electrodes which determines the memory retention must be increased as well as the tunnel magnetoresistance amplitude (TMR). The heart of a magnetic tunnel junction is a stack consisting of Buffer/FeCoB/MgO/FeCoB/Cap. The magnetic electrodes of FeCoB 1nm to 2nm thick are initially amorphous while the MgO tunnel barrier is polycrystalline. The buffer and protecting layer (Cap) are most often in tantalum (Ta). These stacks must be annealed after deposition in order to improve the crystallinity of the MgO barrier and provoke the crystallization of the FeCoB electrodes. This crystallization is required to obtain a large tunnel magnetoresistance (TMR). The higher the annealing temperature, the better the crystallization. However, the annealing temperature is usually limited to about 300°C by interdiffusion phenomena taking place in the metallic layers of the stack, in particular of Ta in FeCoB. It has discovered at SPINTEC that by introducing lamination of a refractory metal in the stack, in particular of tungsten whose melting temperature is 3422°C, the annealing temperature can be increased up to 570°C. The refractory metal mechanically stiffens the whole stack. The resulting tunnel magnetoresistance is increased by about 30%.
Enhanced annealing stability and perpendicular magnetic anisotropy in perpendicular magnetic tunnel junctions using W layer , J. Chatterjee, R. C. Sousa, N. Perrissin, S. Auffret, C. Ducruet, and B. Dieny, Appl. Phys. Lett. 110, 202401 (2017)
Last update : 01/24 2018 (1280)