Hiroaki Narisawa

A Novel Resistance Memory with High Scalability and Nanosecond Switching

We report a novel nonvolatile dual-layered electrolytic resistance memory composed of a conductive Cu ion activated layer and a thin insulator for the first time. An ON/OFF mechanism of this new type memory is postulated as follows: Cu ions pierce through the insulator layer by applied electric field, the ions form a Cu conductive bridge in the insulator layer, and this bridge dissolves back to the ion activated layer when the field is reversed. The 4 kbit memory array with 1T-1R cell structure was fabricated based on 180 nm CMOS process. Set/reset pulses were 5 ns, 110 muA and 1 ns, 125 muA, respectively. Those conditions provide large set/reset resistance ratio of over 2 orders of magnitude and satisfactory retention. Essential characteristics for high capacity memories including superb scalability down to 20 nmphi, sufficient endurance up to 107 cycles and preliminary data for 4-level memory are also presented. These characteristics promise the memory being the next generation high capacity nonvolatile memory even before the scaling limitation of flash memories is encountered.

Thermal activation effect on spin transfer switching in magnetic tunnel junctions

A calculation of the intrinsic spin transfer switching current Ic0 is realized in the magnetic tunnel junctions (MTJs). Ic0 is determined by the distribution of Ic in repeated measurements, assuming the linear decrease of the thermal stability by increasing the spin-polarized current. The spin transfer torque is found to be proportional to η(I)I with the applied current I and I-dependent spin polarization η(I). Even in case that I is limited less than Ic due to the tunnel barrier breakdown, Ic0 is calculated with an assisting external magnetic field and making better use of the magnetic phase diagram. In our MTJs, the average intrinsic switching current density was 6.4MA∕cm2.

Magnetoresistive random access memory operation error by thermally activated reversal (invited)

The reliability in magnetoresistive random access memory (MRAM) write operation was investigated for both toggle and asteroid memory chips developed with 0.18μm CMOS process. Thermally activated magnetization reversal, being the dominant origin of the intrinsic write error, was studied theoretically and experimentally. For asteroid MRAM, the bit line or word line disturbing error on half selected bits was proved to have significant effect on the write operation margin, even in the ideal case free from bit-to-bit switching field distribution and the hysteresis loop shift. For toggle MRAM, on the other hand, the dominant origin of the error occurs for selected bits, although its impact is much smaller than in the case of asteroid MRAM. As was expected from the estimation based on the single domain model, more than 10mA operation margin with the error rate of <10−9, which is sufficiently small for semiconductor IC memory, was achieved for the toggle MRAM.

Performance characteristics of a 4-terminal vertical magnetoresistive/inductive head

A 4-terminal vertical magnetoresistive (MR)/inductive head using the current bias technique has been developed. In the new head, a bias current conductor is connected to the rear lead of the MR element in series. With this design, this type of MR/inductive head can have a second harmonic distortion (SHD) of less than -25 dB with a 6 to 16 mA sense current. The linear recording density at D/sub 50/ is about 3.15 kfr/mm. The effective track width of the MR sensor is almost the same as its optical track width (2.5 /spl mu/m), so high track density (TPI) is possible. The stability of an MR element with a 2 /spl mu/m track width was also investigated. >

Advanced vertical MR head

A newly designed vertical magnetoresistive (MR)/inductive head has been developed. In this new head, the read element has the tri-layer structure, in which the insulator layer is sandwiched between the two permalloy layers, and both ends of the elements are filled with the insulator. The sense current lead is connected only to the upper NiFe layer. As a result, not only the sense current density is enlarged but also the film Hk is reduced. The sensor sensitivity is estimated to be 2.5 times as large as that of the conventional vertical MR sensor. The output level of about 700 /spl mu/Vpp is obtained at the read track width of 1.5 /spl mu/m. The D/sub 50/ is at about 5.8 kfc/mm.