Semiconductor memory device

A semiconductor memory device according to an embodiment includes: a plurality of magnetic tunnel junction elements arranged on a semiconductor substrate; and a plurality of selection transistors electrically connected to first ends of the plurality of magnetic tunnel junction elements. A plurality of first bit lines are respectively connected to the first ends of the magnetic tunnel junction elements via one or more of the selection transistors. A plurality of upper electrodes are respectively connected to second ends of the plurality of magnetic tunnel junction elements. A plurality of second bit lines are respectively connected to the second ends of the magnetic tunnel junction elements via the upper electrodes. The upper electrodes extend along the second bit lines, and one of the upper electrodes is commonly connected to the second ends of the plurality of magnetic tunnel junction elements arranged in an extending direction of the second bit lines.

CROSS REFERENCE TO RELATED APPLICATIONS

This application is based upon and claims the benefit of priority from the prior Japanese Patent Application No. 2010-281947, filed on Dec. 17, 2010, the entire contents of which are incorporated herein by reference.

FIELD

The embodiments of the present application relate to a semiconductor memory device.

BACKGROUND

As resistance change type elements for storing data using a change in resistance of elements, magnetic random access memories (MRAMs) have been developed. A current is caused to flow through magnetic tunnel junction (MTJ) elements to thereby write data into the MTJ elements.

One end of each MTJ element is connected to a corresponding first bit line via a corresponding selection transistor, and another end of each MTJ element is connected to a corresponding second bit line via a corresponding upper electrode. Each upper electrode is typically connected to a corresponding second bit line via a single via contact.

To cause a sufficient write current to flow through MTJ elements, a plurality of selection transistors may be provided to respectively correspond to the MTJ elements. In this case, a large current flows through via contacts between the upper electrodes and the second bit lines during data write operation. Each upper electrode is commonly connected to two MTJ elements neighboring in the word line direction in some cases. In such cases, the current flowing through the via contacts further increases.

If the current is concentrated on the via contacts, metal materials constituting the via contacts may deteriorate due to electromigration. The deterioration of the via contacts causes lowering of the reliability of a semiconductor memory device.

DETAILED DESCRIPTION

A semiconductor memory device according to an embodiment of the present invention includes: a plurality of magnetic tunnel junction elements arranged on a semiconductor substrate; and a plurality of selection transistors electrically connected to first ends of the plurality of magnetic tunnel junction elements. A plurality of first bit lines are respectively connected to the first ends of the magnetic tunnel junction elements via one or more of the selection transistors. A plurality of upper electrodes are respectively connected to second ends of the plurality of magnetic tunnel junction elements. A plurality of second bit lines are respectively connected to the second ends of the magnetic tunnel junction elements via the upper electrodes. The upper electrodes extend along the second bit lines, and one of the upper electrodes is commonly connected to the second ends of the plurality of magnetic tunnel junction elements arranged in an extending direction of the second bit lines.

First Embodiment

FIG. 1is a partial plan view of a memory cell array of an MRAM according to a first embodiment.FIG. 2is a sectional view taken along the line2-2ofFIG. 1.FIG. 3is a sectional view taken along the line3-3ofFIG. 1.FIG. 4is a sectional view taken along the line4-4ofFIG. 1.

The MRAM according to the first embodiment is a spin injection write type MRAM. Typically, methods for writing data into MRAMs are divided into a magnetic field write method and a spin injection write method. In the magnetic field write method, when the size of each magnetic tunnel junction element (MTJ element) is reduced, a coercive force increases. As a result, a write current tends to increase. On the other hand, the spin injection write method uses a spin transfer torque (STT) write method. Accordingly, as the size of a magnetic material decreases, a spin injection current necessary for flux reversal decreases. Therefore, MTJ elements employing the spin injection write method are advantageous in terms of higher integration, lower power consumption, and enhanced performance. Meanwhile, in the magnetic field write method, erroneous writing into a non-selected memory cell may be caused due to spreading of a magnetic field. In the spin injection write method, however, such erroneous writing into a non-selected memory cell does not occur.

As shown inFIG. 1, a plurality of word lines WL extend in a row direction. First bit lines BL1and second bit lines BL2extend in a column direction. The first bit lines BL1and the second bit lines BL2extend in parallel with each other and are orthogonal to the word lines WL. Each of the second bit lines BL2is provided to correspond to two first bit lines BL1and is disposed between every two first bit lines BL1. The first bit lines BL1are provided above MTJ elements and are provided on contacts CBa/V1a. The second bit lines BL2are provided on via contacts V1b. The first bit lines BL1are not in direct contact with the MTJ elements, and are connected to selection transistors ST via the contacts CBa/V1a. The second bit lines BL2are connected to upper electrodes UE via the via contacts V1b. That is, in the first embodiment, the upper electrodes UE also function as local interconnections. In other words, the upper electrodes UE are also called local interconnections. In the plan view ofFIG. 1, the positions of contact plugs CBa and via contacts V1aare represented by “CBa/V1a”.

The MTJ elements, which are magnetic tunnel junction elements, are two-dimensionally arranged in a matrix form on a semiconductor substrate SUB. As shown inFIG. 2, two selection transistors ST neighboring in the column direction are provided to correspond to one MTJ element. These two selection transistors ST simultaneously operate to allow a write current from the first bit lines BL1to flow through the corresponding MTJ element. That is, in the first embodiment, one memory cell is composed of one MTJ element and two selection transistors ST. During data write operation, two word lines WL corresponding to a selected memory cell are driven to render two selection transistors ST conductive. This allows the two selection transistors ST to supply the MTJ element with a sufficient current for writing data.

As shown inFIG. 1, in the first embodiment, one memory cell (unit cell UC) has a layout area of 12F2(4F×3F). F represents a feature size which is a minimum processing dimension in a semiconductor production process. If one selection transistor ST can supply the corresponding MTJ element with a sufficient current for writing data, each memory cell may be composed of one selection transistor ST and one MTJ element. This allows a reduction in the area of the unit cell UC.

As shown inFIG. 2, each selection transistor ST is formed on the semiconductor substrate SUB. A first diffusion layer D1of each selection transistor ST is connected to the corresponding first bit line BL1via the corresponding contact plug CBa and the corresponding via contact V1a. A second diffusion layer D2of each selection transistor ST is connected to a lower end (lower electrode) of the corresponding MTJ element via the corresponding contact plug CBb. That is, each first bit line BL1is electrically connected to the lower end of the corresponding MTJ element via two selection transistors ST neighboring in the column direction. In the plan view ofFIG. 1, each selection transistors ST is provided between each of the contacts CBa/V1aand each of the MTJ elements.

The upper electrodes UE are provided on the MTJ elements. As shown inFIGS. 1 and 3, the upper electrodes UE are connected to upper ends of two MTJ elements neighboring in the row direction. In addition, as shown inFIGS. 1 and 4, the upper electrodes UE extend in the column direction along the second bit lines BL2below the second bit lines BL2. Further, the upper electrodes UE are commonly connected to the upper ends of a plurality of MTJ elements arranged in an extending direction (column direction) of the second bit lines BL2. The second bit lines BL2are electrically connected to the upper electrodes UE via the via contacts V1b. As shown inFIGS. 1 and 3, each via contact V1bis shared by two MTJ elements between the two adjacent MTJ elements neighboring in the row direction. In addition, as shown inFIGS. 1 and 4, each via contact V1bis arranged in the column direction below the second bit lines BL2, and is provided between adjacent two word lines WL. That is, in the first embodiment, the upper electrodes UE are formed in a comb shape. Further, each of the upper electrodes UE is shared by all the MTJ elements which are arranged below two first bit lines BL1corresponding to the second bit line BL2connected to the corresponding upper electrode UE.

The word lines WL function not only as word lines but also as gate electrodes CG of the selection transistors ST. Alternatively, the gate electrodes CG of the selection transistors ST may be separately provided, and the word lines WL may be formed so as to be electrically connected to the gate electrodes CG in a layer different from that of the gate electrodes CG.

Each first bit line BL1is electrically connected to a lower end of the corresponding MTJ element through the corresponding via contact V1a, the corresponding selection transistor ST, and the corresponding contact plug CBb. Each second bit line BL2is electrically connected to an upper end of the corresponding MTJ element through the corresponding via contact V1band the corresponding upper electrode UE. That is, each MTJ element and each selection transistors ST are connected in series between each first bit line BL1and each second bit line BL2.

The via contacts V1bbetween the second bit lines BL2and the upper electrodes UE shown inFIG. 4are formed in the same process as that of the via contacts V1abetween the first bit lines BL1and the contact plugs CBa shown inFIG. 2. However, the via contacts V1aare formed at positions deeper than the via contacts V1b. Thus, during formation of the via contacts V1aand V1b, the upper electrodes UE function as etching stoppers.

As shown inFIG. 3, the first bit lines BL1and the MTJ elements are formed above active areas AA which extend in the column direction. The selection transistors ST are also formed on the active area AA. On the other hand, the second bit lines BL are formed above an element isolation region STI (Shallow Trench Isolation) between the adjacent active areas AA. Accordingly, the first bit lines BL1and the second bit lines BL2can be formed in the same metal wiring layer (M1).

During data write operation, a pair of two word lines WL provided on both sides of a selected MTJ element are driven to render two selection transistors ST corresponding to the two word lines WL conductive. This allows a write current to flow through the selected MTJ element between the two selection transistors ST, thereby selectively writing data into current elements of the MTJ element.

Each pair of word lines WL provided on both sides of each non-selected MTJ element remain in a non-active state. Accordingly, the write current does not flow through the non-selected MTJ elements, so that no data is written into the non-selected MTJ elements.

The write current is fed to the selection transistors ST via the contacts CBa/V1afrom the first bit lines BL1, and is further supplied to the MTJ elements via the selection transistors ST. The write current passing through the MTJ elements flows through the second bit lines BL2via the upper electrodes UE and the via contacts V1b. This allows the write current to flow through the MTJ elements.

In the MRAM according to the first embodiment, as shown inFIG. 1, each of the upper electrodes UE is commonly connected to two MTJ elements neighboring in the row direction. In addition, like the second bit lines BL2, each of the upper electrodes UE extends in the column direction and is commonly connected to the plurality of MTJ elements neighboring in the column direction. As a result, the upper electrodes UE and the second bit lines BL2are connected via the plurality of via contacts V1bwhich are arranged in the column direction. Accordingly, the resistance between each upper electrode UE and each second bit line BL2decreases, and the write current flows from the upper electrodes UE to the second bit lines BL2via the plurality of via contacts V1b. That is, the write current flows through the plurality of via contact V1bin a dispersed manner, thereby preventing the write current from being concentrated on the local via contacts V1b. Consequently, it is possible to provide an MRAM capable of suppressing electromigration in the via contacts V1aand having high reliability.

As a comparative example, a case is assumed in which each of the upper electrodes UE is commonly connected to two MTJ elements neighboring in the row direction and does not extend in the column direction. In this comparative example, the entire write current passing through the two MTJ elements neighboring in the row direction flows through one via contact V1bprovided between the two MTJ elements in a concentrated manner. This increases the possibility of occurrence of electromigration in the via contact V1b.

On the other hand, in the first embodiment, the write current flows through the plurality of via contacts V1bin a dispersed manner as described above, thereby suppressing electromigration in the via contact V1a.

Further, the MRAM according to the first embodiment can be produced only by changing the mask layout of the upper electrodes UE of the comparative example. Accordingly, the MRAM according to the first embodiment can be easily produced at low cost with a small change of the production process.

In the first embodiment, each of the upper electrodes UE is commonly connected to all MTJ elements sharing the corresponding second bit line BL2so as to reduce the resistance of each via contact V1b. That is, each of the upper electrodes UE is connected to all the via contacts V1bwhich are provided below the corresponding second bit line BL2. However, the upper electrodes UE do not necessarily have to extend the same length as the second bit lines BL2. That is, as long as the upper electrodes UE are connected to two or more via contacts V1bneighboring in the column direction, the upper electrodes UE do not necessarily have to be connected to all the via contacts V1bbelow the corresponding second bit line BL2. For example, even when the upper electrodes UE are configured to be connected to two via contacts V1bneighboring in the column direction, the effects of the first embodiment cannot be impaired.

Second Embodiment

FIG. 5is a partial plan view of a memory cell array of an MRAM according to a second embodiment.FIG. 6is a sectional view taken along the line6-6ofFIG. 5. The sectional view taken along the line2-2ofFIG. 5and the sectional view taken along the line3-3ofFIG. 5correspond toFIG. 2andFIG. 3, respectively.

In the second embodiment, each of the via contacts V1bis provided not only between two adjacent MTJ elements neighboring in the row direction, but also between two contacts CBa/V1aneighboring in the row direction. That is, each of the via contacts V1ais provided between every pair of adjacent word lines WL in the plan view of the semiconductor substrate SUB viewed from the top. The other configuration of the second embodiment may be similar to the corresponding configuration of the first embodiment.

In this configuration, the density of the via contacts V1aincreases, and the resistance between each upper electrode UE and each second bit line BL2further decreases. The density of the via contacts V1ain the second embodiment is twice as large as that of the first embodiment, and the density of the current flowing through the via contacts V1ain the second embodiment is half that of the first embodiment. As a result, the second embodiment can more effectively suppress electromigration in the via contacts V1a. Further, the second embodiment can provide the same effects as those of the first embodiment.

Third Embodiment

FIG. 7is a partial plan view of a memory cell array of an MRAM according to a third embodiment.FIG. 8is a sectional view taken along the line8-8ofFIG. 5.FIG. 9is a sectional view taken along the line9-9ofFIG. 5. The sectional view taken along the line2-2ofFIG. 5corresponds toFIG. 2.

In the third embodiment, the second bit lines BL2are buried by a damascene process so as to reach the upper electrodes UE. Accordingly, as shown inFIGS. 8 and 9, each second bit line BL2is in direct contact with the entire upper surface of the corresponding upper electrode UE. The second bit lines BL2and the upper electrodes UE are each formed as a two-layer interconnection. This eliminates the need for the via contacts V1a, and thus the via contacts V1aare not provided. The upper electrodes UE are integrally formed with the second bit lines BL2. In other words, the upper electrodes UE also function as the second bit lines BL2. The other configuration of the third embodiment may be similar to the corresponding configuration of the first embodiment.

In the third embodiment, each of the second bit lines BL2is in direct contact with the entire upper surface of the corresponding upper electrode UE, and the second bit lines BL2and the upper electrodes UE are each formed as a two-layer interconnection. Accordingly, the resistance between each second bit line BL2and each upper electrode UE further decreases, so that the electromigration in the via contacts V1acan be completely eliminated. Further, the third embodiment can provide the same effects as those of the first embodiment.

Fourth Embodiment

FIG. 10is a partial plan view of a memory cell array of an MRAM according to a fourth embodiment.FIG. 11is a sectional view taken along the line11-11ofFIG. 10. The sectional view taken along the line2-2ofFIG. 10and the sectional view taken along the line3-3ofFIG. 10correspond toFIG. 2andFIG. 3, respectively.

In the fourth embodiment, the second bit lines BL2are provided to respectively correspond to the first bit lines BL1. That is, the first bit lines BL1and the second bit lines BL2are alternately arranged in the extending direction of the word lines WL. Each of the upper electrodes UE is commonly connected to the plurality of MTJ elements arranged in the extending direction (column direction) of the corresponding first bit line BL1and the corresponding second bit line BL2. The upper electrodes UE are formed in a comb shape in the plan view.

The upper electrodes UE are electrically connected to the second bit lines BL2via the via contacts V1b, as with the first embodiment. Accordingly, each of the via contacts V1bis commonly connected to the plurality of MTJ elements arranged in the column direction. In the fourth embodiment, however, each of the upper electrodes UE is not commonly connected to the MTJ elements neighboring in the row direction. In the fourth embodiment, the unit cell UC has a layout area of 16F2(4F×4F).

In the MRAM according to the fourth embodiment, like the second bit lines BL2, each of the upper electrodes UE extends in the column direction and is commonly connected to the plurality of MTJ elements neighboring in the column direction. In addition, the upper electrodes UE are connected to the second bit lines BL2via the plurality of via contacts V1bwhich are arranged in the column direction. Therefore, although each of the upper electrodes UE is not commonly connected to the MTJ elements neighboring in the row direction, the fourth embodiment can provide the same effects as those of the first embodiment.

In the fourth embodiment, the number of the via contacts V1amay be increased as in the second embodiment. Further, each of the second bit lines BL2may be in direct contact with the entire surface of the corresponding upper electrode UE, as with the third embodiment. In short, the fourth embodiment may be combined with the second or third embodiment. Hence, the fourth embodiment can also provide the same effects as those of the second or third embodiment.