Yung-Hung Wang

Interfacial and annealing effects on magnetic properties of CoFeB thin films

The interfacial and annealing effects on magnetic properties of CoFeB thin films with Ta and Ru interfaces were investigated. It is found that the thickness of magnetically dead layer of as-deposited CoFeB film strongly depends on the interfaces. After annealing above 150°C boron atoms seem to diffuse out of CoFeB film and the effective saturation magnetization of CoFeB film increases with annealing temperature and annealing time. In addition, CoFeB film with Ta interfaces remains amorphous while that with Ru interfaces starts to crystallize during the annealing above 250°C. Furthermore, the annealing-induced anisotropy field of CoFeB film can be modified by using different seed and capping materials.

Effect of Adjacent Layers on Crystallization and Magnetoresistance in CoFeB/MgO/CoFeB Magnetic Tunnel Junction

Crystallization must occur in order to obtain lattice matching between CoFeB and MgO in amorphous CoFeB/MgO/amorphous CoFeB magnetic tunnel junction (MTJ). However, the interface layer effect on the crystallization behavior is not yet clear. An aim of our experiments was to investigate the effects of various layers including Ta, MgO, Ru, and NiFe on the crystallization and transport properties in CoFeB/MgO/CoFeB MTJs deposited on SiO2/Si and glass. It was found that the onset of crystallization of CoFeB and its specific orientation were dependent on the layer adjacent to CoFeB. The effects of crystallization on transport properties are also discussed

Wide operation margin of toggle mode switching for magnetic random access memory with preceding negative pulse writing scheme

In this work, a writing scheme with preceding negative pulse wave form for toggle magnetic random access memory (MRAM) is proposed to enhance the switching yield and enable a low current switching. The failure mechanism of toggle switching is studied by micromagnetic analysis. As a result of broadened operation window and reduced switching current, the scalability of MRAM is feasible with the robust toggle operation.

Impact of stray field on the switching properties of perpendicular MTJ for scaled MRAM

For the first time, we show the stray field (Hs) from the pinned layer of perpendicular magnetic tunnel junctions (pMTJs) plays multiple important roles: its out-of-plane component degrades pinned layer stability of scaled MTJ, while its in-plane component assists spin-transfer torque switching. Through stray field engineering, one can retain its advantage, and minimized its detrimental effect. We also show that etching process plays a vital role in the stray-field engineering. Our pMTJ exhibits robust pinned layer performance, symmetrical R-H loop and balanced spin-torque switching current. The switching current of our pMTJ is ~60uA@20us for a 80nm diameter pMTJ.

High density and low power design of MRAM

MRAM structures based on 1T2UMTJ cell and PWWL architecture are proposed to shrink the bit size with a potential down to 6 F/sup 2/ by a so-called ExtVia process and reduce the writing current by a factor of two, combined with the nature of non-volatility and high speed, making the MRAM suitable for universal memory applications.

Improvement of Transport Properties in Magnetic Tunneling Junctions by Capping Materials

The capping effects on the properties of magnetic tunneling junctions (MTJs) have been investigated to improve the magnetoresistive (MR) ratio [Nagamine , Applied Physics, vol. 99, p. 08K703, 2006] and thermal stability [Fukumoto , Japanese Journal of Applied Physics, vol. 45, p. 3829, 2006. In this paper, MgO/Mg and MgO capping layers for MTJs were used to replace our previously used Ta and Ru capping layer [Yen , IEEE Transactions on Magnetics, vol. 42, no. 10, p. 862, 2004]. The MTJs with NiFe and CoFeB free layers were patterned to ellipses of 0.36 mum times 0.72 mum. The MR ratios of the MTJs with an NiFe free layer were enhanced from 26% to about 33% by capping MgO/Mg bilayers. A great improvement was also found on the MTJs with the CoFeB free layer, where the MR ratio was 50.4% with the MgO cap layer enhanced from 44.7% of Ru-capped MTJs. In addition, the influences of those capping layers included Ta, Ru, MgO, and MgO/Mg on thermal stability were also discussed

A 6-F/sup 2/ bit cell design based on one transistor and two uneven magnetic tunnel junctions structure and low power design for MRAM

Novel cell structures based on one transistor and two uneven magnetic tunnel junction cell and pillar write word line architecture are proposed to shrink the bit size with a potential down to 6 F2 by a so-called extended via process, and to reduce the writing current by a factor of 2, combined with the nature of nonvolatility and high speed, making the magnetoresistive random access memory suitable for universal memory applications

Scaling Properties of Step-Etch Perpendicular Magnetic Tunnel Junction With Dual-CoFeB/MgO Interfaces

We built and studied the size scaling effect of perpendicular magnetic tunnel junctions (p-MTJs) with stepetch structure and dual-MgO/CoFeB interfaces. The step-etch structure yields symmetrical R-H loop, while dual-MgO/CoFeB interfaces raises cell anisotropy, thus the data retention time. The p-MTJ of 45-nm diameter shows spin-transfer torque switching voltage Vsw with tight temporal sigma (σ(Vsw) <;3.7%). The thermal stability factor is 60. Although the critical switching current (Ic0) reduces with MTJ area, its density (Jc0) increases. One plausible explanation of this observation is that the magnetization reversal of small MTJ follows the single-domain macrospin model, while that of the larger MTJ may be affected by the nucleation of domain during the magnetization reversal, and the wall motion leads to Jc0 lowering; the other may be due to process-induced film damage. Fortunately, the switching efficiency (Eb/|Ic0|) is higher for smaller p-MTJ.

Evidences of Reactive-Ion-Etching-Induced Damages to the Ferromagnet of Perpendicular Magnetic Tunnel Junctions

To minimize the R-H loop shift of a perpendicular magnetic tunnel junction (p-MTJ), a stop-on-tunnel-barrier etch is required to pattern the reference layer such that it is wider than the free layer. Our experiments show that reactive ion etching can induce penetrating damages to the ferromagnetic layers beyond the etched surface. Sufficient sacrificial layer atop the tunnel barrier must be kept to prevent both free and reference layers from damage, which forms a magnetic “step” in the reference layer and results in the R-H loop shift. The optimized p-MTJs revealed an average offset field of 1.4 Oe and also exhibited steady switching behaviors with one-sigma switching voltage distributions less than 6% for both resistance states. A low switching current density of 1.63 × 106 A/cm2 of an 80-nm p-MTJ was measured with a 10-μs pulse at zero external offset field.

Adjacent-Reference and Self-Reference Sensing Scheme with Novel Orthogonal Wiggle MRAM Cell

A novel orthogonal wiggle cell for either adjacent-reference architecture or self-reference architecture is proposed to enhance the read/write operation of MRAM. With reliability and non-disturbance of the device being verified, the mass production of MRAM is feasible because of the stabilized functionality and improved performance