Yiming Huai

Perpendicular magnetic tunnel junction with thin CoFeB/Ta/Co/Pd/Co reference layer

Integration of high density spin transfer torque magnetoresistance random access memory requires a thin stack (less than 15 nm) of perpendicular magnetic tunnel junction (p-MTJ). We propose an innovative approach to solve this challenging problem by reducing the thickness and/or moment of the reference layer. A thin reference layer structure of CoFeB/Ta/Co/Pd/Co has 60% magnetic moment of the conventional thick structure including [Co/Pd] multilayers. We demonstrate that the perpendicular magnetization of the CoFeB/Ta/Co/Pd/Co structure can be realized by anti-ferromagnetically coupling to a pinned layer with strong perpendicular anisotropy via Ruderman-Kittel-Kasuya-Yosida exchange interaction. The pMTJ with thin CoFeB/Ta/Co/Pd/Co reference layer has a comparable TMR ratio (near 80%) as that with thick reference layer after annealing at 280 °C. The pMTJ with thin reference layer has a total thickness less than 15 nm, thereby significantly increasing the etching margin required for integration of high density pMTJ array on wafers with form factor of 300 mm and beyond.

Bit error rate investigation of spin-transfer-switched magnetic tunnel junctions

A method is developed to enable a fast bit error rate (BER) characterization of spin-transfer-torque magnetic random access memory magnetic tunnel junction (MTJ) cells without integrating with complementary metal-oxide semiconductor circuit. By utilizing the reflected signal from the devices under test, the measurement setup allows a fast measurement of bit error rates at >106, writing events per second. It is further shown that this method provides a time domain capability to examine the MTJ resistance states during a switching event, which can assist write error analysis in great detail. BER of a set of spin-transfer-torque MTJ cells has been evaluated by using this method, and bit error free operation (down to 10−8) for optimized in-plane MTJ cells has been demonstrated.

Progress and outlook for STT-MRAM

New product applications have an increasing demand for a non-volatile memory (NVM) exhibiting higher speeds, extended endurance and lower power consumption as existing solutions are not fully capable to deliver on all of these attributes. Of the group of new NVMs, Phase-change RAM (PRAM), Resistive RAM (RRAM) and Spin-Transfer Torque based MRAM (STT-MRAM) [1–8], STT-MRAM has the most attractive combination of fast read and write speed (<30 ns) and high endurance (>1015) along with non-volatility. We will review the recent progress on STT-MRAM at both the MTJ device and CMOS integrated chip levels. In the technical area, the key focus is switching current (or voltage) reduction at short pulse width (<10 ns) while maintaining high magnetic bit thermal stability. Results from novel MTJs achieving low switching current while maintaining high thermal stability will be presented. First generation STT-MRAM products are being targeted using in-plane based MTJs, and future products below 45 nm will likely require perpendicular MTJ (pMTJ) designs. The later is the key focus of recent research in this area. We will review the current state of pMTJ development, along with micromagnetic modeling results for key understanding of STT switching dynamics and latest experimental data of pMTJs. Finally, we discuss the market positioning of the STT-MRAM. While it is currently being targeted for stand-alone commodity memory (replacing DRAM) [4], and embedded memory (replacing embedded Flash, SRAM and DRAM) [6], new market applications such as storage-class solid state drives and embedded solutions in advanced System-on-Chip (SoC) are also being explored. We will compare key value adds from STT-MRAM in these emerging market segments.

Different dielectric breakdown mechanisms for RF-MgO and naturally oxidized MgO

We investigate the voltage breakdown of the spin transfer torque magnetic random access memory (STT-MRAM) with perpendicular magnetic tunnel junctions (pMTJs). Different breakdown behaviors are observed for RF-MgO pMTJs and naturally oxidized MgO pMTJs. While the time-tofailure body distribution of the naturally oxidized MgO follows the Weibull distribution, that of RF-MgO follows the lognormal distribution. This result suggests distinctly different dielectric breakdown mechanisms for naturally oxidized MgO and RF-MgO. For low failure probability, the progressive voltage breakdown of RF-MgO (associated with the lognormal distribution) results in an order-of-magnitude reliability improvement over the abrupt breakdown of the naturally oxidized MgO. We show that RF-MgO is suitable for perpendicular STT-MRAM applications.

Dramatic reduction of read disturb through pulse width control in spin torque random access memory

Magnetizations dynamic effect in low current read disturb region is studied both experimentally and theoretically. Dramatic read error rate reduction through read pulse width control is theoretically predicted and experimentally observed. The strong dependence of read error rate upon pulse width contrasts conventional energy barrier approach and can only be obtained considering detailed magnetization dynamics at long time thermal magnetization reversal region. Our study provides a design possibility for ultra-fast low current spin torque random access memory.

Enhanced ferromagnetic resonance linewidth of the free layer in perpendicular magnetic tunnel junctions

We report the frequency dependence of the ferromagnetic resonance linewidth of the free layer in magnetic tunnel junctions with all perpendicular-to-the-plane magnetized layers. While the magnetic-field-swept linewidth nominally shows a linear growth with frequency in agreement with Gilbert damping, an additional frequency-dependent linewidth broadening occurs that shows a strong asymmetry between the absorption spectra for increasing- and decreasing external magnetic field. Inhomogeneous magnetic fields produced during reversal of the reference and pinned layer complex is demonstrated to be at the origin of the symmetry breaking and the linewidth enhancement. Consequentially, this linewidth enhancement provides indirect information on the magnetic coercivity of the reference and pinned layers. These results have important implications for the characterization of perpendicular magnetized magnetic random access memory bit cells.

Zero-field spin transfer oscillators based on magnetic tunnel junction having perpendicular polarizer and planar free layer

We experimentally studied spin-transfer-torque induced magnetization oscillations in an asymmetric MgO-based magnetic tunnel junction device consisting of an in-plane magnetized free layer and an out-of-plane magnetized polarizer. A steady auto-oscillation was achieved at zero magnetic field and room temperature, with an oscillation frequency that was strongly dependent on bias currents, with a large frequency tunability of 1.39 GHz/mA. Our results suggest that this new structure has a high potential for new microwave device designs.

Perpendicular magnetic tunneling junction switching dynamic modes, extreme events, and performance scaling

The performance of the state-of-the-art perpendicular magnetic tunneling junction (pMTJ) device is fundamentally determined by the physics of material “extreme events.” A dynamic mode approach is used to study “extreme events” of stochastic nonlinear magnetization switching, including magnetic interactions and non-uniform magnetization dynamics. Our theory and experiment show that the magnetization switching “extreme events” are well characterized by the dynamic modes of interacting magnetic systems. The dynamic modes provide a clear understanding of the physical processes of the magnetization switching “extreme events.” We predict markedly different pMTJ scaling behaviors for spin transfer torque, spin-orbit-interaction torque, and thermal fluctuations at different operation speeds and bit error rate conditions. Understanding these scaling behaviors is critical for existing and emerging pMTJ device applications.