1. Field of the Invention
The present invention relates to a semiconductor device and a method of manufacturing the same, particularly, to a magnetic memory apparatus (MRAM: Magnetic Random Access Memory) using a tunneling magneto resistive (TMR) element as a memory element and a method of manufacturing the same.
2. Description of the Related Art
In recent years, a magnetic random access memory (MRAM) utilizing the tunneling magneto resistive effect has been proposed as an information memory element.
FIG. 57 is an oblique view schematically showing a conventional semiconductor memory device. The construction of the MRAM will now be described briefly with reference to FIG. 57.
As shown in FIG. 57, a plurality of bit lines 23 and a plurality of write word lines 13 are arranged to cross each other at right angles so as to form a matrix configuration, and a TMR element 24 is arranged at each intersection between the bit line 23 and the write word line 13. The TMR element 24 is connected to the bit line 23 through an upper electrode (not shown) and is also connected to a switching element (MOSFET) 5 through a lower electrode 17. The gate electrode of the MOSFET acts as a read word line 3.
The TMR element 24 comprises a magnetically fixed layer 18 connected to the lower electrode 17, a magnetic recording layer 20 connected to the bit line 23 through upper electrode, and a tunnel barrier layer (tunnel junction film) 19 interposed between the magnetically fixed layer 18 and the magnetic recording layer 20.
The magnetically fixed layer 18 has a direction of magnetization fixed to an easy axis direction (EA direction). On the other hand, the magnetic recording layer 20 has two magnetization directions determined by the mutual function between the magnetic recording layer 20 and the magnetically fixed layer 18, said two magnetization directions corresponding to the information memory states of “1” and “0”. The resistance of the tunnel junction is rendered lowest when the direction of magnetization of the magnetic recording layer 20 is made equal to the magnetization direction of the magnetically fixed layer 18, and is rendered highest when the direction of magnetization of the magnetic recording layer 20 is made opposite to that of the magnetically fixed layer 18. The particular change in the resistance is read by allowing an electric current to flow through the TMR element 24 so as to make it possible to judge the information memory states of “1” and “0”.
The MRAM memory cell of the particular construction is designed such that the magnetization direction of only the magnetic recording layer 20 is reversed by the magnetic field synthesized by the current magnetic field generated from the current flowing through both the selected bit line 23 and the selected write word line 13, though the magnetization direction of the magnetically fixed layer 18 remains unchanged. Therefore, where data is written in an optional cell, the magnetization direction of the magnetic recording layer 20 is reversed as described above so as to write information in the selected cell. On the other hand, where data is read from an optional cell, the bit line 23 and the read word line 13 are selected and a current value flowing from the bit line 23 through the TMR element 24, the lower electrode 17, and the switching MOSFET 5 is compared with, for example, a reference cell so as to judge the information memory states of “1” and “0” denoting the resistance state of the cell.
FIG. 58 shows by arrows the state of magnetization of the magnetic recording layer included in the conventional semiconductor memory device. As shown in FIG. 58, domains 100 in which the magnetization vectors in the longitudinal direction are turned are actually formed in both edge portions of the magnetic recording layer 20, though it is ideal for all the magnetization directions 28 to be aligned in the easy axis direction (EA direction) in the magnetic recording layer 20. A so-called “diamagnetic field” is generated by the presence of these domains 100. As a result, in the region in which the diamagnetic field has been generated, it is impossible to maintain uniform the original tunnel resistance corresponding to the information memory states of “1” and “0”. This gives rise to the problem that the S/N ratio of the signals of “1” and “0” that can be output is degraded, resulting in failure to ensure a sufficient operating margin and to read out data.
In order to overcome the above-noted problem, the length in the longitudinal direction of the cell is increased in the prior art so as to achieve a vertical-longitudinal ratio of, for example, at least 3, thereby ensuring an area required for the reading of data even if a diamagnetic field is generated in each of the both edges of the cell. However, the measure pointed out above leads to enlargement of the cell area so as to provide a large obstacle to the miniaturization of the MRAM cells in the future.
As described above, the prior art is defective in that the domains 100 generated in the cell degrade the operating margin in reading the data and make it difficult to miniaturize the cell.