Jason Allen Janesky

A 4-Mb toggle MRAM based on a novel bit and switching method

A 4-Mb magnetoresistive random access memory (MRAM) with a novel magnetic bit cell and toggle switching mode is presented. The circuit was designed in a five level metal, 0.18-mum complementary metal-oxide-semiconductor process with a bit cell size of 1.55 mum2. The new bit cell uses a balanced synthetic antiferromagnetic free layer and a phased write pulse sequence to provide robust switching performance with immunity from half-select disturbs. This switching mode greatly improves the operational performance of the MRAM as compared to conventional MRAM. A detailed description of this 4-Mb toggle MRAM is presented

Recent developments in magnetic tunnel junction MRAM

We summarize our progress on Magnetoresistive Random Access Memory (MRAM) based on Magnetic Tunnel Junctions (MTJ). We have demonstrated MTJ material in the 1-1000 k/spl Omega/-/spl mu/m/sup 2/ range with MR values above 40%. The switching characteristics are mainly governed by the magnetic shape anisotropy that arises from the element boundaries. The switching repeatability, as well as hard axis selectability, are shown to be dependent on both shape and aspect ratio. MTJ memory elements were successfully integrated with 0.6 /spl mu/m CMOS technology, achieving read and program address access times of 14 ns in a 256/spl times/2 MRAM.

A Fully Functional 64 Mb DDR3 ST-MRAM Built on 90 nm CMOS Technology

A spin torque magnetoresistive random access memory (ST-MRAM) holds great promise to be a fast, high density, nonvolatile memory that can enhance the performance of a variety of applications, particularly when used as a non-volatile buffer in data storage devices and systems. Towards that end, we have developed a fully functional 64 Mb DDR3 ST-MRAM built on 90 nm CMOS technology. The memory is organized in an 8-bank configuration that can sustain 1.6 GigaTransfers/s (DDR3-1600). We have run standard memory tests, such as a March6N pattern, on the full 64 Mb at 800 MHz with 0 fails for greater than 10 5 cycles. Full functionality was also verified from 0°C to 70°C with no significant change in performance. The bits are magnetic tunnel junctions (MTJs) having an MgO tunnel barrier and a magnetic free layer made of a CoFeB-based alloy with an in-plane magnetization, but with an out-of-plane anisotropy reduced by more than 50% due to an enhanced perpendicular surface anisotropy. To enable the 64 Mb performance, we developed an MTJ stack that has low switching voltage (Vsw), high breakdown voltage (Vbd), and excellent switching reliability with tight distributions. The ST switching distribution has σ ≈ 10%, and we found excellent agreement with a single Gaussian distribution down to an error rate . For our optimized material, the Vsw/Vbd ≈ 0.3, and the separation between Vsw and Vbd is ≈ 25σ. The energy barrier to magnetization reversal (Eb) was characterized using both time-dependent coercivity and higher temperature to accelerate reversal. We found the average Eb ≈ 70kbT.

Demonstrated reliability of 4-mb MRAM

The successful commercialization of MRAM will rely on providing customers with a robust and reliable memory product. The intrinsic reliability of magnetoresistive tunnel junction (MTJ) memory bits and the metal interconnect system of MRAM are two areas of great interest due to the new materials involved in this emerging technology. Time dependent dielectric breakdown (TDDB) and resistance drift were the two main failure mechanisms identified for intrinsic memory bit reliability. Results indicated that a lifetime over 10 years is achievable under the operating condition. For metal interconnect system, the initial results of Cu with magnetic cladding have met the reliability performance of typical nonclad Cu backend process in electromigration (EM) and iso-thermal annealing (ITA). Finally data retention is demonstrated over times orders of magnitude longer than 10 years.

Exchange coupling control and thermal endurance of synthetic antiferromagnet structures for MRAM

Synthetic antiferromagnet (SAF) structures are a key element of TMR and GMR read heads and MRAM devices. Control of the SAF coupling strength and thermal endurance are key issues for these technologies. We find that the coupling strength increases with stronger crystalline texture in polycrystalline NiFe SAFs, and, surprisingly, we observe a strong dependence on seed layer in amorphous CoFeB SAFs. We also have developed an analysis method for evaluating thermal endurance of SAFs and show that failure of the SAF can be modeled as a thermally activated diffusion process. The analysis is used to predict the time to failure at any temperature, thus allowing accelerated failure analysis for SAF-based devices. The stability improves dramatically with increasing Ru spacer thickness. The time to failure for typical NiFe SAFs was found to be >10 years at 120/spl deg/C.

The switching properties of patterned synthetic ferrimagnetic structures

We investigated the switching properties of patterned submicrometer synthetic ferromagnetic (SF) Ni65Fe15Co20(t1nm)∕Ru0.8nm∕Ni65Fe15Co20(t2nm) tri-layers. By changing t1 and t2, the shape anisotropy field, Hksh∝∣t1−t2∣, was changed from 36 to 18Oe, and the effective material anisotropy field, Hkeff,∝α=(t1+t2)∕∣t1−t2∣, was changed from 28 to over 60Oe. We found that a hard axis field, Hhd, is less effective at reducing the easy axis switching field, Hsw, as α is increased, with α=3.7 requiring twice the relative magnitude of Hhd for the same relative reduction in Hsw as a single magnetic layer. In addition, by repeating the basic SF tri-layer structure in circular elements, we demonstrated improved stability against thermal activation by a factor of 2 with no associated increase in Hsw.

Reliability of 4 Mbit MRAM

Prior to our recent publication (J. Akerman et al, IEEE Trans. Dev. Mat. Rel. 4, p.428-435, 2004), little data on magnetoresistive random access memory (MRAM) reliability were available in the literature. In this paper, we present additional reliability data taken in full 4 Mb MRAM arrays, significantly extending the scope of our original study. In particular, we present good reliability over 10 years against dielectric breakdown, resistance drift, drift of programming currents, electromigration, and also demonstrate solid data retention. This study further solidifies our expectations of MRAM becoming the memory technology of choice for high-performance non-volatile memory applications where endurance and reliability are crucial requirements.

Chapter 5 – Magnetic tunnel junction based magnetoresistive random access memory

Publisher Summary This chapter presents the salient features of state-of-the-art magnetic tunnel junctions (MTJ)-based magnetoresistive random access memory (MRAM). The chapter provides a description of 0.18 um MRAM technology and its implementation in a 1 Mb memory array. MRAM bit size scaling and challenges associated with continued miniaturization are discussed. A novel switching approach with significantly improved scaling properties is also presented. The ability to scale the MRAM bit cell to smaller dimensions is essential for MRAM to be a competitive memory technology. As the bit size is reduced, write performance could be affected by several parameters—switching field, write line field generation, bit-to-bit variation of the switching field within an array, hard-axis field response, susceptibility to thermal fluctuations, and magnetostatic interactions between neighboring bits. Magnetostatic interactions between neighboring bits must also be considered as the size of the bit cell is reduced and the density of the array is increased. A given bit experiences different values and configurations of stray fields depending on the details of the magnetization directions of surrounding bits. Savtchenko switching relies on the unique behavior of a synthetic antiferromagnet (SAF) free layer that is formed from two ferromagnetic layers separated by a nonmagnetic coupling spacer layer. The moments of the balanced SAF free-layer are antiparallel in zero-field and the coupled system therefore responds to an applied magnetic field in a manner that is different from the single ferromagnetic layer of conventional MRAM.

Toggle MRAM: A highly-reliable Non-Volatile Memory

Magnetoresistive Random Access Memory (MRAM) is based on magnetic tunnel junction devices integrated with standard CMOS, resulting in high-speed read and write, unlimited endurance, and the highest reliability of any non-volatile memory. The first commercially available MRAM product, Freescales 4Mb MR2A16A Toggle MRAM, was released for production in 2006 and is now in volume production. In this paper we provide an overview of Freescales MRAM technology and describe the performance and reliability attributes of the MR2A16A.

Switching Energy Barrier Study of Toggle MRAM Using a Novel Pulse Technique

We present a complete study of the influence of thermal activation on the DW mode, the toggle mode, and on the separate first and second pulses of the toggle sequence. To obtain these results, we developed a novel technique that employs a train of three-pulse packets. We found good agreement with a single energy barrier (Eb) thermal activation model for both the DW and toggle modes, indicating excellent bit switching quality. We also measured the Eb vs. bit size with no changes in the material stack, and found that Eb >70 kbT to below 0.1 um, which is sufficient stability for scaling toggle MRAM to beyond the 65 nm CMOS node.