Keiji Hosotani

Effect of Self-Heating on Time-Dependent Dielectric Breakdown in Ultrathin MgO Magnetic Tunnel Junctions for Spin Torque Transfer Switching Magnetic Random Access Memory

Spin torque transfer switching magnetic random access memory (Spin-MRAM) using MgO-magnetic tunnel junction (MTJ) is considered to be the most promising candidate for high-density, high-speed, and non-volatile RAM. Notwithstanding its excellent potential, the breakdown mechanism of MgO-MTJ has not been well understood although a thorough understanding is essential for commercialization of Spin-MRAM. In this paper, we demonstrate for the first time the modeling of dielectric breakdown phenomena of MgO-MTJ by time-dependent dielectric breakdown (TDDB) measurement concerning the effect of self-heating using simulation and conclude that E-model with the effect of self-heating at MgO-MTJ during current stress (power) removed gives the best fitting as a degradation model of MgO-MTJ ultrathin dielectrics.

Resistance drift of MgO magnetic tunnel junctions by trapping and degradation of coherent tunneling

Magnetoresistive Random Access Memory (MRAM) is a promising device for high-density (over Gbits scale), high-speed (equal to DRAM or better) non-volatile RAM, and much research has been done over several years with a view to overcoming the problems regarding practical use. Spin Torque Transfer switching MRAM (STT-MRAM) is considered to be the most promising candidate and there already are some papers on this new device. MgO is expected to be the best material for magnetic tunnel junction (MTJ) of STT-MRAM, because MgO-MTJ is known to show large Magnetoresistance (MR) and enhance spin polarization by the coherent tunneling effect, resulting in decrease of writing current of MTJ. MgO-MTJ has been shown to be an excellent barrier with little resistance drift compared with MTJ using alumina. Notwithstanding its excellent potential, the degradation mechanism of MgO-MTJ has not been well understood. In this paper, we will demonstrate for the first time the degradation of coherent tunneling and trapping phenomena of MgO-MTJ and discuss its mechanism.

A 16Mb MRAM with FORK Wiring Scheme and Burst Modes

A 16Mb MRAM based on 0.13mum CMOS and 0.24mum MRAM process achieves a 34ns asynchronous access and 100MHz synchronous operation, compatible with pseudo-SRAM for mobile applications. By implementation of FORK wiring scheme, the cell efficiency is raised to 39.9% and the disturb robustness of half-selection state is improved

Switching Current Fluctuation and Repeatability for MRAM With Propeller-Shape MTJ

The writing region and the repeatability of the function test and switching current fluctuation of magnetoresistive random access memory (MRAM) with a propeller shape magnetic tunnel junction (MTJ) array is evaluated. The effect of the write sequence is also investigated. The writing region is larger when the easy-axis field pulse is turned on prior to the hard-axis field than that in the case of the opposite sequence. However, the total margin in the latter sequence is larger after the repeated tests because of the smaller switching field fluctuation. The average 1-sigma value of the switching field fluctuation is 1.7%, which is mainly caused by the thermal fluctuation. The probability of the write error is estimated to be less than 10-16 by the bit line writing region and the thermal stability

1.8 V Power Supply 16 Mb-MRAMs With 42.3% Array Efficiency

Technologies for realizing high density MRAM were developed. First, new circuitry to lower the resistance of programming wires was developed. Second, both MTJ plane shape and cross-sectional structure were optimized to lower the programming current. Based on these two technologies, 16 Mb MRAM was designed, fabricated with 130 nm CMOS process and 240 nm back end MTJ process. As a result, a 1.8 V power supply MRAM with 42.3% array efficiency was successfully demonstrated

Effect of Interface Buffer Layer on the Reliability of Ultra-Thin MgO Magnetic Tunnel Junctions for Spin Transfer Switching MRAM

Study of the reliability of ultra-thin MgO tunneling barriers for spin transfer switching magnetoresistive random access memory (MRAM) revealed MgO to be an excellent film with little resistance drift. Precise control of CoFeB/MgO/CoFeB interface was found to be important for making highly reliable tunneling barriers.

Temperature dependence of tunnel resistance for CoFeB∕MgO∕CoFeB magnetoresistive tunneling junctions: The role of magnon

Tunnel resistance for CoFeB∕MgO∕CoFeB junctions with a wide variety of resistance has been investigated as a function of bias voltage Vb and temperature to elucidate bias voltage dependence of the magnetoresistance ratio. Comparison with a conventional NiFe∕AlOx∕CoFe junction is also discussed. In the case of a parallel alignment of the magnetic moments with a MgO barrier, the Vb dependence was much smaller than that of the antiparallel (AP) alignment with a MgO barrier and of both alignment with an AlOx barrier. This probably originates from the unique tunnel mechanism with a MgO barrier: coherent tunneling of Δ1 electron states. In the case of AP alignment with a MgO barrier, distinctive features were observed: temperature coefficient of tunnel resistance steeply decreased with increasing absolute value of Vb at −0.2V<Vb<0.2V. This suggests that inelastic tunneling with excitation of magnon modes plays a crucial role.

MRAM write error categorization with QCKBD

A new test pattern, quadruplet checker board (QCKBD), is proposed which enables to evaluate magnetic crosstalk from the neighbor write lines. At first, some conventional test patterns changing the write points were applied to categorize magnetic random access memory (MRAM) write errors. But magnetic crosstalk from the neighbor write lines could not be isolated by these conventional tests since magnetic crosstalk error was caused when the neighbor cell is written. Whereas the QCKBD results from 4Kb test vehicles show that magnetic crosstalk restricts the write margin. By changing the cell structure in order to suppress magnetic crosstalk, the write margin is improved from 3.3 to 7.3

Electric field dependent switching and degradation of Resistance Random Access Memory

We have developed a novel method and model to describe the switching and degradation phenomena of uni-polar type ReRAM using conventional electric field dependent dielectric breakdown model. By this method, we can clearly describe the relationship between “forming”, “set”, “reset”, and degradation process of TiO2 based ReRAM. In this paper, we will demonstrate our method and discuss the switching and degradation model of uni-polar type ReRAM regarding its future potential for commercialization.

Self-aligned MTJ etching technique using side walls for high-density 8F2 MRAMs

M. Yoshikawa, M. Amano, T. Ueda, E. Kiatagawa, S. Takahashi, T. Kai, T. Kishi, N. Shimomura, H. Aikawa, T. Kajiyama, K. Hosotani, Y. Asao, K. Suemitsu, H. Hada, S. Tahara, and H. Yoda Corporate Research & Development Center, Toshiba Corporation 1, Komukai Toshiba-cho, Saiwai-ku, Kawasaki 212-8582, Japan Phone: +81-44-549-2115, Fax: +81-44-520-1275, E-mail: masa.yoshikawa@toshiba.co.jp SoC Research & Development Center, Toshiba Corporation, Yokohama, Kanagawa 235-8522, Japan System Devices Research Laboratories, NEC Corporation, Sagamihara, Kanagawa 229-1198, Japan