Eugene Youjun Chen

Progress and outlook for MRAM technology

We summarize the features of existing semiconductor memories and compare them to Magnetoresistive Random Access Memory (MRAM),a semiconductor memory with magnetic bits for nonvolatile storage. MRAM architectures based on Giant Magnetoresistance (GMR) and Magnetic Tunnel Junction (MTJ) cells are described. This paper will discuss our progress on improving the material structures, memory bits, thermal stability of the bits, and competitive architectures for GMR and MTJ based MRAM memories as well as the potential of these memories in the commercial memory market.

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.

High density submicron magnetoresistive random access memory (invited)

Various giant magnetoresistance material structures were patterned and studied for their potential as memory elements. The preferred memory element, based on pseudo-spin valve structures, was designed with two magnetic stacks (NiFeCo/CoFe) of different thickness with Cu as an interlayer. The difference in thickness results in dissimilar switching fields due to the shape anisotropy at deep submicron dimensions. It was found that a lower switching current can be achieved when the bits have a word line that wraps around the bit 1.5 times. Submicron memory elements integrated with complementary metal–oxide–semiconductor (CMOS) transistors maintained their characteristics and no degradation to the CMOS devices was observed. Selectivity between memory elements in high-density arrays was demonstrated.

End domain states and magnetization reversal in submicron magnetic structures

Patterned submicron magnetic thin films of various geometries have been systematically studied. We have observed end domain states in rectangular elements, which is in excellent agreement with micromagnetic simulation results. Significant deviation from single domain behavior has been found in low aspect ratio elements. We will show that this deviation is attributed to behavior of the end domains.

Submicron spin valve magnetoresistive random access memory cell

Spin valve magnetoresistive random access memory cells with widths varying from 1.5 to 0.25 μm and an aspect ratio of length/width more than 10 were fabricated and tested. In general, the switching field of the free magnetic layer was found to be inversely proportional to the width of the cell. Adequate pinning was shown for cell width down to 0.75 μm. For 0.5 and 0.25 μm wide cells, the switching field of the free magnetic layer is comparable to the pinning field of the other magnetic layer. So the pinned magnetic layer rotates with the free magnetic layer. The giant magnetoresistive ratio of the cell drops dramatically. Potentially, this may be a fundamental problem for this memory mode. Solutions are proposed.

Comparison of oxidation methods for magnetic tunnel junction material

Advances in reducing the resistance and enhancing the magnetoresistance (MR%) of the magnetic tunnel junction (MTJ) material has made it useful for magnetoresistive random access memory as well as magnetic field sensing applications. One of the most important aspects for producing the MTJ material is the method used for forming the tunnel barrier, and its impact on the properties of MTJ such as resistance and area product (RA), MR%, and RA uniformity across a large area. We have explored forming the aluminum oxide tunnel barrier with air; reactive sputtering; plasma oxidation with plasma source; plasma oxidation with power introduced from the target side; and plasma oxidation with power introduced from the substrate side. Our results show that all techniques can be made to work. Plasma oxidation is favored due to its simplicity and manufacturing compatibility. It was also discovered that different oxidation methods used in this study caused little difference in MTJ resistance uniformity. The latter is mai...

High density nonvolatile magnetoresistive RAM

Non-volatile memory cells based on ferromagnetically coupled giant magneto-resistive (GMR) material were patterned into submicron feature sizes. Switching characteristics of such cells allow for bipolar signal reading which is twice the intrinsic magnetoresistance change of the material. Excellent thermal stability is reported for deep submicron memory cells.

High-Speed Characterization of Submicrometer Giant Magnetoresistive Devices

A microwave test structure has been designed to measure the high-speed response of giant magnetoresistive (GMR) devices. The test structure uses microwave transmission lines for both writing and sensing the devices. Pseudo-spin-valve devices, with line widths between 0.4 and 0.8 μm, were successfully switched with pulses whose full width at half-maximum was 0.5 ns. For small pulse widths τpw the switching fields are observed to increase linearly with 1/τpw. The increase in switching fields at short pulse widths is characterized by a slope which, for the current devices, varies between 4 and 16 μA s/m (50–200 Oe ns). The magnetoresistive response during rotation and switching was observed. For small rotations (∼45° between layer magnetizations) the GMR response pulses had widths of 0.46 ns, which is at the bandwidth limit of our electronics. For larger rotations (∼90°) the response pulses broadened considerably as the magnetic layers were rotated near the unstable equilibrium point perpendicular to the dev...

Experimental and analytical properties of 0.2 micron wide, multi-layer, GMR, memory elements

Memory elements 0.2 microns wide made from multilayer GMR material have been studied experimentally and analytically. When etched into narrow elements, the 12 to 15% material exhibits a memory mode with large outputs of /spl plusmn/8%. Analysis shows that the mode is suitable for multi-megabit die with high performance if yield can be achieved.

Switching Characteristics Of Submicron Dimension Permalloy Sandwich Films

Submicron magnetoresistive sandwich cells of various sizes are fabricated and their magnetic properties are studied. A novel coupling mechanism of magnetostatic origin has been found to be mainly responsible for unique switching characteristics in cells with small length to width ratio.