Ding-Yeong Wang

Reduction in critical current density for spin torque transfer switching with composite free layer

A composite free layer (CFL) consisting of a soft layer and a hard layer exchange coupled in parallel is proposed. The experimental results showed that the critical current density (Jc) can be reduced from 7.05×106A∕cm2 of strong coupled CoFeB(12.5A)∕Ru(4A)∕NiFe(17.5A) CFL to 2.65×106A∕cm2 of weak coupled CoFeB(12.5A)∕Ru(15A)∕NiFe(17.5A) CFL. The macrospin simulations showed that the soft layer of CFL can assist the hard layer of CFL to switch at a lower Jc. These results suggest that by using CFL, it is possible to reduce the Jc of spin torque transfer switching without compromising the thermal stability.

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.

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

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

A novel 1T2UMTJ toggle MRAM with high read/write bandwidth interface

A novel one-transistor-and-two-uneven-magnetic tunneling junctions (1T2UMTJ) cell with toggle switching mode is proposed to increase the packing density and to secure the writing selectivity. Besides, a four-state sense amplifier is proposed to reduce the read access time to less than 25 ns for two accessing data in one cycle. With adequate I/O stages, the 1T2UMTJ toggle MRAM architecture has a double word length or alternatively has a high read/write bandwidth. The two-bit data can be read from or written into a selected memory cell of 1T2UMTJ toggle MRAM within one clock cycle, therefore, demonstrating the promising property of high bandwidth processing capability.

Improvement switching characteristics of toggle magnetic random access memory with dual polarity write pulse scheme

The writing probability of toggle magnetic random access memory (MRAM) at built-in bias field is studied by micromagnetic simulation and a dual polarity write pulse scheme has been proposed to enhance the toggle probability at low writing field. The critical writing field can be reduced to 19Oe at strong built-in bias field from CoFe(1.0nm)∕CoFeB(2.0nm)∕Ru∕CoFe(5.0nm) pinned layer structure. From the simulation and experimental results, it is proven that the toggle MRAM can be operated at lower writing field by dual polarity write pulse scheme.

Hardware implementation of physically unclonable function (puf) in perpendicular STT MRAM

The pSTT MRAM array with stepped structure MTJ is fully integrated with standard 0.18 µm CMOS process. Using a low power etch process greatly improves the uniformity of MR and RP. The bit pattern is randomized with a voltage method and a cost-effective field method; the latter can be incorporated into wafer processing. We showed the bit pattern is unpredictable under the same sub-critical stimulation, thus, practically unclonable. The Hamming Weight and inter-chip Hamming Distance are both ∼50%, an evidence of sufficient uniqueness. A single embedded STT-MRAM PUF can cover many needs of security electronics.

On the Hardware Implementation of MRAM Physically Unclonable Function

Intrinsic properties of magnetic tunnel junctions (MTJs) are exploited for low-power security electronics. The combination of: 1) fast and nonvolatile storage; 2) stochastic nature of subcritical switching; and 3) random variation in the magnetic anisotropy of MTJ make MRAM array ideal for implementing physically unclonable function (PUF). We develop two bit-pattern randomization procedures, one of which can be exercised at wafer level in process line, while the other can be completed in the in-line tester. The procedures generate unpredictable and unclonable bit patterns in a spin-transfer torque (STT)-MRAM array and store them securely in the array. We show the resistance of MTJ is stable through rounds of thermal baking, the readback from the PUF shows no ambiguity from 25 °C to 75 °C. A single embedded STT-MRAM PUF can cover many needs of security electronics.