Thin Co/Ni-based bottom pinned spin-transfer torque magnetic random access memory stacks with high annealing tolerance

Abstract

Spin-transfer torque magnetic random access memory (STT-MRAM) is considered as a replacement for next generation embedded and stand-alone memory applications. One of the main challenges in the STT-MRAM stack development is the compatibility of the stack with CMOS process flows in which thermal budgets up to 400 °C are applied. In this letter, we report on a perpendicularly magnetized MgO-based tunnel junction (p-MTJ) on a thin Co/Ni perpendicular synthetic antiferromagnetic layer with high annealing tolerance. Tunnel magneto resistance (TMR) loss after annealing occurs when the reference layer loses its perpendicular magnetic anisotropy due to reduction of the CoFeB/MgO interfacial anisotropy. A stable Co/Ni based p-MTJ stack with TMR values of 130% at resistance-area products of 9 Ω μm2 after 400 °C anneal is achieved via moment control of the Co/Ta/CoFeB reference layer. Thinning of the CoFeB polarizing layer down to 0.8 nm is the key enabler to achieve 400 °C compatibility with limited TMR loss. Thinning the Co below 0.6 nm leads to a loss of the antiferromagnetic interlayer exchange coupling strength through Ru. Insight into the thickness and moment engineering of the reference layer is displayed to obtain the best magnetic properties and high thermal stability for thin Co/Ni SAF-based STT-MRAM stacks.

Seed layer impact on structural and magnetic properties of [Co/Ni] multilayers with perpendicular magnetic anisotropy

“…Note that for the [Co/Ni] on NiCr, the M s loss is large and the hysteresis loop becomes bow-tie like with coercivity (µ 0 H c ) increase (Fig.4(a)). The large µ 0 H c enables [Co/Ni] on NiCr seed to function as hard layer in MTJ stacks [12][13][14] . Fig.5 Commonly, highly fcc(111) textured [Co/Ni] films result in large PMA.…”

“…Besides, [Co/Ni] has been researched as alternative PMA material and has been employed in p-MTJ because of its high spin polarization and low Gilbert damping constant [8][9][10][11] . Also, [Co/Ni] has been incorporated in an ultrathin SAF [12][13][14] . Recently, the use of [Co/Ni] in the free layer material was proposed to enable free layers with high thermal stability needed at CD below 20nm 15 .…”

Exchange-driven Magnetic Logic

“…Figure 9b shows how the exchange-driven magnetic logic concept can be isolated, using a thin layer of MgO (1 nm), to enable voltage pulse application that will excite the required magnetic field pulses ( H R ). Additionally, this scheme is CMOS integration compatible, as thin MgO layers for isolation have been used in conjunction with Co/Ni multilayers in spin-transfer torque magnetic random access memory (STT-MRAM) stacks 26 .…”

Novel multifunctional RKKY coupling layer for ultrathin perpendicular synthetic antiferromagnet

“…Thinner pSAF structures composed of buffer layer /[ Co/X ] m/Co / Ru / Co / texture breaking layer / FeCoB / tunnel barrier were proposed to contribute reducing the generation of non-volatile etch product during etching 12 . However, these thin pSAF stacks do not exhibit sharp magnetic reversal with high squareness after annealing at 400 °C temperature 13 . This is most likely due to inter-diffusion of the texture breaking material (for instance Ta) into the FeCoB layers.…”