In recent years, development of nonvolatile semiconductor storage devices that use resistance elements as storage elements such as an MRAM (Magnetoresistive Random Access Memory), a PCRAM (Phase Change Random Access Memory), and an RRAM (Resistive Random Access Memory) is underway.
As for the MRAM among these storage devices, magnetic field writing and spin-transfer torque writing are normally known as data writing techniques and these techniques are advantageous in high speed processing because of the use of the magnetization reversal of magnetic bodies. The spin-transfer torque writing has a characteristic such that a spin injection current becomes smaller in an amount necessary for magnetization reversal as magnetic bodies is smaller in size. The amount of a current necessary for spin-transfer magnetization reversal (a reversal threshold current) is defined by the density of a current flowing to a magnetoresistive element (an MTJ element). For this reason, the reversal threshold current decreases as the area of the MTJ element is reduced. That is, when the area of the MTJ element is reduced, the reversal threshold current is downscaled. Therefore, the MRAM based on the spin-transfer torque writing is advantageous in high integration, low power consumption, and high performance, and is expected as a nonvolatile semiconductor storage device that can replace a DRAM.
However, in conventional memory cell layouts, MTJ elements are not arranged uniformly on a semiconductor substrate and some adjacent MTJ elements are arranged at a smaller interval. Therefore, the MRAM has problems such that it is more difficult to perform exposure and processing in lithographic and etching steps with the progress of downscaling.
Meanwhile, there has been known a method of apparently uniformly arranging MTJ elements on a semiconductor substrate by arranging dummy MTJ elements in the spaces between adjacent MTJ elements. In this method, a stable shape is ensured for the MTJ elements because of the uniform arrangement; however, this method disadvantageously creates other problems in processing steps such as the need to selectively and electrically isolate only the dummy MTJ elements from other electrodes, wirings, and the like. In addition, the memory size cannot be reduced for the area by which the dummy MTJ elements are arranged. Under these circumstances, conventional MRAMs cannot simultaneously meet the demands of downscaling and mass storage capacity.
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