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 mainly determined by the Al metal-thickness uniformity. Modeling based on Simmons’ theory supports our experimental finding. This illustrates the importance in improving Al metal-film uniformity for producing MTJ with ultra-uniform resistance.
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.
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