Srinivas V. Pietambaram

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

MgO-Based Tunnel Junction Material for High-Speed Toggle Magnetic Random Access Memory

We report the first demonstration of a magnetoresistive random access memory (MRAM) circuit incorporating MgO-based magnetic tunnel junction (MTJ) material for higher performance. We compare our results to those of AlOx-based devices, and we discuss the MTJ process optimization and material changes that made the demonstration possible.We present data on key MTJ material attributes for different oxidation processes and free-layer alloys, including resistance distributions, bias dependence, free-layer magnetic properties, interlayer coupling, breakdown voltage, and thermal endurance. A tunneling magnetoresistance (TMR) greater than 230% was achieved with CoFeB free layers and greater than 85% with NiFe free layers. Although the TMR with NiFe is at the low end of our MgO comparison, even this MTJ material enables faster access times, since its TMR is almost double that of a similar structure with an AlOx barrier. Bit-to-bit resistance distributions are somewhat wider for MgO barriers, with sigma about 1.5% compared to about 0.9% for AlOx. The read access time of our 4 Mb toggle MRAM circuit was reduced from 21 ns with AlOx to a circuit-limited 17 ns with MgO.

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.

Low-power switching in magnetoresistive random access memory bits using enhanced permeability dielectric films

We reduced the switching field (Hsw) in arrays of single-layer magnetoresistive random access memory elements using enhanced permeability dielectric (EPD) films. This reduction is due to an increased magnetic flux density produced at the bit by increasing the permeability μ of the surrounding dielectric. The authors produced EPD films by embedding superparamagnetic nanoparticles of various sizes in oxides of Al, Mg, or Si. For bits surrounded by EPD, Hsw decreased linearly as μ increased. Using this approach, we reduced Hsw by ≈40% for μ=3.5, without changing the energy barrier to magnetization reversal.

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.

Toggle MRAM with CoFeB-Based Synthetic Antiferromagnet Free Layers

For toggle MRAM, the free layer is a synthetic antiferromagnet (SAF) that must have specific magnetic properties such as: low magnetostriction, a repeatable saturation field (Hsat) that can be adjusted within a specific range, and a well-defined and reproducible intrinsic anisotropy axis. The results from fully-functional Mb-scale MRAM circuits using CoFeB SAF free layers optimized for toggle switching is reported. A 40% improvement in useable MR in 4Mb MRAM circuits using optimized CoFeB SAF free layers is demonstrated, with switching comparable to standard NiFe material. The critical properties optimized for switching, electrical properties of the devices, and thermal endurance are described.