Shan-Yi Yang

Preparation of gradient-index (GRIN) polymer fibers for imaging applications

A closed extrusion process combined with a core-shell separation die design is developed in this study for preparing gradient index (GRIN) polymer fibers with a quadratic distribution of the refractive index. The material system used in this investigation is methyl methacrylate (MMA, n = 1.49) and benzyl methacrylate (BzMA, n = 1.568). The refractive index differences between the center and the periphery (Δn) of the prepared polymer fibers increase from 0.0115 to 0.020 when BzMA in the reactant mixtures increases from 17 to 28%. This finding would suggest that increasing the high refractive index monomer in the reactant mixture increased Δn. The Δn values decrease from 0.018 to 0.0135 when the diffusion zone temperature increases from 70 to 90°C. The diffusion rates of the monomers increase with the temperature, thereby causing BzMA and MMA to distribute more uniformly at a higher temperature than at a lower one. This uniform distribution leads to the decrease of Δn with an increasing temperature. The prepared GRIN polymer fibers have potential applications as imaging lenses for scanners, fax machines, and copiers.

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.

A novel method for preparing gradient index (GI) plastic rods

SummaryThe initiator diffusion technique was used in this study to prepare the gradient index(GI) plastic rods, in which two kinds of initiators were used, i.e., thermal initiator and redox initiator. Experimental results showed that a GI plastic rod with a parabolic profile of refractive index was obtained by the initiator diffusion technique. The difference of the refractive index from the center to the periphery (Δn) of the plastic rods was in the range of 0.004–0.014. Δn increased with increasing temperature due to the increase of the gel effect and diffusion rates of monomers at a higher temperature. Furthermore, the GI plastic rod had substantially less defects, e.g., bubbles and opaqueness, than that prepared from interfacial-gel copolymerization.

IC process compatible anodic electrode structures for unipolar HfOx-based RRAM

HfOx RRAM is a most promising candidate for next generation nonvolatile memory [1,2] with highest endurance, speed till now but bipolar switching affects the selection of steering device, performance and applications. Bipolar and unipolar RRAMs have their advocators [1,3]. Cell area of 4F2 and/or 3D stacking for high density applications is the determining factor of preferring unipolar device. High operation power, low endurance and slow speed are most crucial issues. Recently, polarity of RRAM reported to be a strong function of top (anodic) electrode (TE) material [4,5]. In this work, HfOx-based RRAM is studied by selection of TE stacking structures to find unipolar HfOx-based RRAM device structures with better performance.

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.

Stabilization of resistive switching with controllable self-compliant Ta 2 O 5 -based RRAM

Ta/Ta₂O₅ RRAMs show self-compliant characteristics in some Ta or Ta₂O₅ thickness range but Ti/TaO_x RRAMs always need current compliance due to totally consumption of SC conduction layer.

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.

Back-hopping phenomenon in perpendicular mangetic tunnel junctions

This study shows a clear correlation between back hopping, write error rate, bias-dependent coercivity (Hc) and the switching phase diagram (SPD) of p-MTJs. Data support that the bias-induced fieldlike torque, in addition to the Slonczewski anti-damping torque, at higher bias voltages in dual-MgO p-MTJ, and a thinner MgO capping layer may suppress the occurrence of back hopping.