Enhanced annealing stability and perpendicular magnetic anisotropy in perpendicular magnetic tunnel junctions using W layer

Abstract

The magnetic properties of the perpendicular storage electrode (buffer/MgO/FeCoB/Cap) were studied as a function of annealing temperature by replacing Ta with W and W/Ta cap layers with variable thicknesses. W in the cap boosts up the annealing stability and increases the effective perpendicular anisotropy by 30% compared to the Ta cap. Correspondingly, an increase in the FeCoB critical thickness characterizing the transition from perpendicular to in-plane anisotropy was observed. Thicker W layer in the W(t)/Ta 1 nm cap layer makes the storage electrode highly robust against annealing up to 570 °C. The stiffening of the overall stack resulting from the W insertion due to its very high melting temperature seems to be the key mechanism behind the extremely high thermal robustness. The Gilbert damping constant of FeCoB with the W/Ta cap was found to be lower when compared with the Ta cap and stable with annealing. The evolution of the magnetic properties of bottom pinned perpendicular magnetic tunnel junctions (p-MTJ) stack with the W2/Ta1 nm cap layer shows back-end-of-line compatibility with increasing tunnel magnetoresistance up to the annealing temperature of 425 °C. The pMTJ thermal budget is limited by the synthetic antiferromagnetic hard layer which is stable up to 425 °C annealing temperature while the storage layer is stable up to 455 °C.

Physicochemical origin of improvement of magnetic and transport properties of STT-MRAM cells using tungsten on FeCoB storage layer

“…[12][13][14] However, a cap layer used on top of storage layer, can also indirectly influence the magnetic and transport properties of the pMTJ by modifying the interfacial and bulk physicochemical and electronic properties. [15][16][17][18] The main physicochemical modifications are Boron absorption from the storage layer 17 or interdiffusion during sputtering or post deposition anneals. 15 In an earlier paper, we reported significant improvement of annealing tolerance using thick W/Ta cap as compared to Ta because of structural stiffening of the stack and much weaker interdiffusion with W than with Ta.…”

“…13,14 As a consequence, the effective magnetic critical thicknesses (tc), listed in antiferromagnetically coupled by 0.9 nm Ru layer. 16,21 Co/Pt MLs based SAF are widely used in the pMTJ stacks 22,23 because of their large PMA [24][25][26] , which makes them good candidates for realizing magnetically hard layers.…”

“…4). In our previous work 16 , the improvement of thermal tolerance was explained in terms of overall improvement of the mechanical stiffness due to the incorporation of W as its melting temperature is very high, 3422°C. The observation that W is nanocrystalline or even amorphous further supports this explanation ruling out the possibility of crystallinity of W being responsible for high thermal tolerance as stated before by G. An et al 30 The thermal stability factors of patterned memory cells were calculated from statistical measurements of resistance versus magnetic field (R(H)) loops using the switching field distribution model.…”

Modulation of Heavy Metal/Ferromagnetic Metal Interface for High‐Performance Spintronic Devices

“…Experimental results demonstrated that after annealing at 420 °C for 10 min, the p‐MTJ showed a high TMR ratio of 138% and the resistance area product (RA) of 0.5 MΩ µm 2 . Subsequently, Chatterjee et al reported that after annealing at 425 °C, the p‐MTJ with W(2 nm)/Ta(1 nm) capping layer exhibited a TMR ratio of 117% and the low RA of 8 Ω µm 2 , which can be attributed to the thin MgO layer …”

A Recent Progress of Spintronics Devices for Integrated Circuit Applications

“…In the double CoFeB-MgO interface structure, a thin metal layer such as Ta, W, or Mo was inserted to absorb boron from the CoFeB layers (MgO/CoFeB/insertion layer/CoFeB/MgO) for high tunnel magnetoresistance (TMR) ratio and perpendicular anisotropy [15,16,29,[33][34][35][36][37][38][39][40][41][42][43][44]. First, Ta was used as insertion material because Ta has a bcc crystal structure and is a good boron absorber, as mentioned above.…”