Magnetic memory

A magnetic memory according to an embodiment includes: a first wiring; a second wiring; a first switching element disposed between the first wiring and the second wiring; a first magnetic member extending in a first direction and disposed between the first switching element and the second wiring; a third wiring disposed between the first magnetic member and the second wiring; a first magnetoresistive element disposed between the third wiring and the second wiring; and a second switching element disposed between the first magnetoresistive element and the second wiring.

CROSS REFERENCE TO RELATED APPLICATIONS

This application is based upon and claims the benefit of priority from the prior Japanese Patent Application No. 2019-168715, filed on Sep. 17, 2019, the entire contents of which are incorporated herein by reference.

FIELD

Embodiments described herein relate generally to magnetic memories.

BACKGROUND

A magnetic memory is known, in which domain walls of a magnetic member are moved (shifted) due to a current caused to flow through the magnetic member. In such a magnetic memory, a first wiring is electrically connected to one end of the magnetic member, and a second wiring is electrically connected to the other end via a magnetoresistive element (for example, magnetic tunnel junction (MTJ) element) and a switching element, and the domain walls of a magnetic member are moved when a shift current for shifting the domain walls is caused to flow between the first wiring and the second wiring. The magnetic memory including such a configuration has a problem in that it is difficult to keep a sufficient operational margin.

DETAILED DESCRIPTION

A magnetic memory according to an embodiment includes: a first wiring; a second wiring; a first switching element disposed between the first wiring and the second wiring; a first magnetic member extending in a first direction and disposed between the first switching element and the second wiring; a third wiring disposed between the first magnetic member and the second wiring; a first magnetoresistive element disposed between the third wiring and the second wiring; and a second switching element disposed between the first magnetoresistive element and the second wiring.

First Embodiment

FIG. 1shows a magnetic memory according to a first embodiment. The magnetic memory according to the first embodiment includes a plurality of (four inFIG. 1) memory cells1011,1012,1021, and1022arranged in an array form, bit lines (first wirings) BL1and BL2, data lines (second wirings) DL1and DL2, source lines (third wirings) SL1and SL2, and control circuits101,102, and103. The memory cell10ij(i, j=1, 2) includes first to third terminals, in which a first terminal11aijis electrically connected to the data line DLi, a second terminal11bijis electrically connected to the bit line BLi, and a third terminal11cij, is electrically connected to the source line SLj. The data lines DL1and DL2are electrically connected to the control circuit101, the source lines SL1and SL2are electrically connected to the control circuit102, and the bit lines BL1and BL2are electrically connected to the control circuit103. Although three control circuits controls the data lines, the source lines, and the bit lines inFIG. 1, a single control circuit may control all lines.

The description “A and B are electrically connected” herein means that A and B may be directly connected or indirectly connected via a conducting member. Although the magnetic memory shown inFIG. 1includes four memory cells, an m×n array of memory cells where each of m and n is a natural number may be provided. In such a case, the magnetic memory includes m data lines DL1to DLm, m bit lines BL1to BLm, and n source lines SL1to SLn.

Each memory cell10ij(i, j=1, 2) includes a magnetic member12ij, a magnetoresistive element14ij, a switching element16ij, including two terminals, and a switching element18ij, including two terminals.

The magnetic member12ij(i, j=1, 2) is formed of a vertically magnetized material and extends in a z direction (the vertical direction inFIG. 1). One end of the magnetic member12ij(i, j=1, 2) is electrically connected to the source line SLj, via the third terminal11cij, and the other end is electrically connected to the bit line BLivia one of the terminals of the switching element18ij. The one end of the magnetic member12ij(i, j=1, 2) is preferably in contact with the source line SLj; at the third terminal11cij. The data lines DL1and DL2and the bit lines BL1and BL2extend in an x direction (the horizontal direction inFIG. 1), and the source lines SL1and SL2extend in a y direction that is perpendicular to the z direction and the x direction.

The magnetoresistive element14ij(i, j=1, 2) reads information (magnetization direction) written to the magnetic member12ij. For example, a magnetic tunnel junction (MTJ) element is used as the magnetoresistive element14ij. In the following descriptions, the magnetoresistive element14ij(i, j=1, 2) is an MTJ element. In the MTJ element14ij(i, j=1, 2), a first terminal is electrically connected to the source line SLj, and a second terminal is electrically connected to one of the terminals of the switching element16ij.

The other terminal of the switching element16ij(i, j=1, 2) is electrically connected to the data line DLivia the first terminal11aij, and the other terminal of the switching element18ij(i, j=1, 2) is electrically connected to the bit line BLivia the second terminal11bij.

The configuration of the memory cell10ij(i, j=1, 2) will then be described in detail with reference toFIG. 2.FIG. 2is a cross-sectional view of the magnetic memory taken along the data line DL1.

The magnetic member12ij(i, j=1, 2) is, for example, in a cylindrical shape extending in the z direction, and has a first end portion12aijand a second end portion12bij. The cross section of the magnetic member12ij(i, j=1, 2) taken along the x-y plane has, for example but not limited to, a ring shape. The peripheral shape of the cross section may be in a circular, an oval, or a polygonal shape.

The magnetic member12ij(i, j=1, 2) is formed of a multi-layer film including layers of cobalt or nickel for example. In addition to cobalt or nickel, an alloy containing an element selected from iron, cobalt, platinum, palladium, magnesium, and a rare earth element may also be used to form the magnetic member12ij, (i, j=1, 2).

The magnetic member12ij(i, j=1, 2) includes a plurality of regions12cij, arranged along the z direction. The regions12cijmay be separated from one another by narrow portions12dijdisposed on an outer surface of the magnetic member12ij. Each region12cij(i, j=1, 2) has at least one magnetic domain. When a drive current (shift current) is supplied between the first end portion12aijand the second end portion12bijof the magnetic member12ij(i, j=1, 2), the domain walls of the magnetic member12ijmove in the z direction. When no drive current is supplied, the domain walls stay at the narrow portions12dij. The first end portion12aijof the magnetic member12ij(i, j=1, 2) is electrically connected to the source line SLij, and preferably in contact with the source line SLij.

The MTJ element14ij(i, j=1, 2) includes a free layer14aijin which the magnetization direction is changeable, a fixed layer14bijin which the magnetization direction is fixed, and a non-magnetic insulating layer (tunnel barrier layer)14cijdisposed between the free layer14aijand the fixed layer14bij. In the MTJ element14ij(i, j=1, 2), the free layer14aijis electrically connected to the first end portion12aijof the magnetic member12ij, and the fixed layer14bijis electrically connected to the one of the terminals of the switching element16ij. The free layer14aijof the MTJ element14ij(i, j=1, 2) is preferably in contact with the source line SLj. In other words, the free layer14aijof the MTJ element14ij(i, j=1, 2) is preferably in contact with a surface of the source line SLj(the lower surface inFIG. 2). The other terminal of the switching element16ij(i, j=1, 2) is electrically connected to the data line DLi. A magnetoresistive element obtained by replacing the non-magnetic insulating layer of the MTJ element with a non-magnetic metal layer may be used instead of the MTJ element.

The second end portion12bijof the magnetic member12ij(i, j=1, 2) is electrically connected to one end of a magnetic member19ijvia a non-magnetic conducting member17ijdisposed to be in contact with the inner surface of the second end portion12bij. The one end of the magnetic member19ij(i, j=1, 2) is disposed to be inserted into the second end portion12bijof the magnetic member12ij. The other end of the magnetic member19ijis electrically connected to the one of the terminals of the switching element18ij. The magnetic member19ij(i, j=1, 2) is formed of, for example, a soft magnetic material.

The other of the terminals of the switching element18ij(i, j=1, 2) is electrically connected to the bit line BLi. A field line (“FL”)201is disposed on a side in the x direction (on the left side inFIG. 2) of the magnetic member1911, and a field line202is disposed on the other side (on the right side). The field line202is also on a side in the x direction (on the left side inFIG. 2) of the magnetic member1912, and a field line203is disposed on the other side (on the right side). The field line202is therefore disposed between the magnetic member1911and the magnetic member1912. The field lines201,202, and203extend in the y direction.

As will be described later in the descriptions of the write method, data (magnetization direction) is written to each of the magnetic members1911-1922by a magnetic field generated by a write current flowing through the field line, and the written data is moved to the first end portion12aijof the magnetic member12ijwhen a drive current (shift current) is supplied between the first end portion12aijand the second end portion12bijof the corresponding magnetic member12ij(i=1, j=1, 2). The data is then read by detecting a strayed magnetic field from the first end portion12aij(i=1, j=1, 2) at the free layer14aijof the MTJ element14ij.

The switching element16ij(i, j=1, 2) and the switching element18ij(i, j=1, 2) may be, for example, two-terminal switching elements. When the value of a voltage applied across the two terminals is equal to or less than a threshold value, the switching elements16ijand18ij(i, j=1, 2) are in a “high-resistance” state, which is an electrically nonconductive state, for example. When the value of the voltage applied across the two terminals is more than the threshold value, the switching elements16ijand18ij(i, j=1, 2) are in a “low-resistance” state, which is an electrically conductive state, for example. When in the ON state, the switching elements16ijand18ij(i, j=1, 2) keep the ON state as long as a current having a value equal to or more than a holding current value flows. The switching elements16ijand18ij(i, j=1, 2) may have this function regardless of the polarity of the voltage. The switching elements16ijand18ij(i, j=1, 2) are formed of at least one chalcogen element selected from a group of Te, Se, and S. A chalcogenide, which is a compound of one or more of the above elements, may also be used. The switching elements may also be formed of at least one of the elements selected from a group of B, Al, Ga, In, C, Si, Ge, Sn, As, P, and Sb.

Operations of the magnetic memory according to the first embodiment will now be described with reference toFIGS. 2 to 6.

First, a write operation will be described below. An example in which data is written to the memory cell1011will be described. Currents are caused to flow in the opposite directions through the field lines201and202to generate a magnetic field for controlling the magnetization of the magnetic member1911. The magnetization of the magnetic member1911controls, via the conducting member1711, the magnetization in the region12c11that is the closest to the end portion12b11of the magnetic member1211to write data (magnetization direction). The control operation for causing the currents to flow through the field lines201to203may be performed by any of the control circuits101,102, and103shown inFIG. 1, or any other control circuit that is not shown.

The switching element1811is then brought into the ON state by the control circuits102and103, and a shift current is caused to flow between the bit line BL1and the source line SL1to move the data written to the region12c11toward the end portion12a11side of the magnetic member1211.

FIG. 3shows the voltage conditions in a case where a ground voltage Vss is applied to the source line SL1during the shift operation, andFIG. 4shows the voltage conditions in a case where a negative voltage Vnn is applied to the source line SL1. In the cases shown inFIGS. 3 and 4, the domain walls move in a direction along which the current flows. In the case shown inFIG. 3, the control circuit103applies a shift voltage Vshift to the bit line BL1, and the control circuit102applies the ground voltage Vss to the source line SL1. At this time, an intermediate voltage Vmid between the shift voltage Vshift and the ground voltage Vss is applied to the wirings that are not connected to the memory cell1011, for example the bit line BL2, the source line SL2, and the data lines DL1and DL2. In the case shown inFIG. 4, the control circuit103applies the shift voltage Vshift to the bit line BL1, and the control circuit102applies the negative voltage Vnn to the source line SL1. The control circuits101,102, and103apply the ground voltage Vss to the wirings that are not connected to the memory cell1011, for example, the bit line BL2, the source line SL2, and the data lines DL1and DL2. In both cases, the shift current flows from the bit line BL1to the source line SL1via the switching element1811, the magnetic member1911, the conducting member1711, and the magnetic member1211, but does not flow through the magnetic member1212, the magnetic member1221, the magnetic member1222, and the switching elements1812,1821, and1822. The shift current does not flow through the MTJ elements1411,1412,1421, and1422, and the switching elements1611,1612,1621, and1622, either.

A read operation will be described below with respect to a case where data is read from the memory cell1011. First, the control circuit101and the control circuit102turn on the switching element1611.FIG. 5shows the voltage conditions in a case where the control circuit101applies a ground voltage Vss to the source line SL1, andFIG. 6shows the voltage conditions in a case where the control circuit101applies a negative voltage Vnn to the source line SL1. In the case shown inFIG. 5, the control circuit101applies a read voltage Vread to the data line DL1, and the control circuit102applies the ground voltage Vss to the source line SL1. At this time, the control circuits101,102, and103apply an intermediate voltage Vmid between the read voltage Vread and the ground voltage Vss to the wirings that are not connected to the MTJ element1411, for example the bit lines BL1and BL2, the source line SL2, and the data line DL2. In the case shown inFIG. 6, the control circuit101applies the read voltage Vread to the data line DL1, and the control circuit102applies the negative voltage Vnn to the source line SL1. The control circuits101,102, and103apply the ground voltage Vss to the wirings that are not connected to the memory cell1011, for example the bit lines BL1and BL2, the source line SL2, and the data line DL2. In both cases, the read current flows from the data line DL1to the source line SL1via the switching element1611and the MTJ element1411, but does not flow through the MTJ elements1412,1421, and1422and the switching elements1612,1621, and1622. The read current does not flow through the magnetic members1211,1212,1221, and1222and the switching elements1811,1812,1821, and1822, either. The free layer of the MTJ element1411has a magnetization direction that responds to the strayed magnetic field from the region12c11of the magnetic member1211, which is the closest to the MTJ element1411. Therefore, the read data corresponds to the data stored in the region12c11.

Although the domain walls move in the direction along which the current flows in this embodiment, the domain walls may move in a direction that is opposite to the direction along which the current flows. The moving direction of the domain walls may be controlled by such factors as the material of the magnetic member, the material and the location of the conducting member stacked on the magnetic member, and the manufacturing conditions. The material of the conducting member stacked on the magnetic member may be Pt, W, or Ta, for example, but not limited to those materials. The movement of the domain walls may be controlled by using the spin orbit torque (SOT) effect that is dependent on the material of the conducting member.

As described above, in the magnetic memory according to this embodiment, the current path of the shift current for moving the domain wall is separated from the current path of the read current. Therefore, erroneous shifting caused by the read current, read disturb, may be prevented. As a result, the operational margin determined by the current variation distribution may be broadened. The broadening of the operational margin will be described below with reference to a comparative example.

COMPARATIVE EXAMPLE

FIG. 7Ais a schematic diagram showing the configuration of a magnetic memory according to a comparative example, and

FIG. 7Bshows a current distribution of the magnetic memory according to the comparative example. As shown inFIG. 7A, the magnetic memory according to the comparative example has a memory cell10including, between two terminals11aand11b, a magnetic member12, an MTJ element14, and a switching element16. The MTJ element14is disposed between the magnetic member12and the switching element16. The terminal11ais electrically connected to the switching element16, and the terminal11bis electrically connected to the magnetic member12.

In the magnetic memory according to the comparative example, the read operation and the shift operation for shifting the domain walls are performed by causing a read current Iread or a shift current Ishift between the terminal11aand the terminal11b.

The magnetic memory including the above-described configuration has the current distribution shown inFIG. 7B. A holding current Ihold, which flows through the switching element16to maintain the ON state of the switching element16, is smaller than the read current Iread of the magnetic memory. A threshold current Ic for shifting the domain walls of the magnetic member12is greater than the read current Iread. The shift current Ishift for shifting the domain walls is greater than the threshold current Ic and smaller than a breakdown current Ibd with which the tunnel barrier of the MTJ element14is broken. Thus, in the magnetic memory according to the comparative example, a current distribution needs to be set in such a manner that the read current Iread and the shift current Ishift are between the holding current Ihold of the switching element16and the breakdown current Ibd of the MTJ element.

In contrast, the magnetic memory according to this embodiment has three terminals as shown inFIG. 8A. Specifically, the magnetic memory according to this embodiment includes the memory cell10, in which the switching element16, the MTJ element14, the magnetic member12, and the switching element18are disposed between the terminal11aand the terminal11b, and the terminal11cis provided between the MTJ element14and the magnetic member12. In the magnetic memory shown inFIG. 8A, the read operation is performed by causing the read current Iread to flow between the terminal11aand the terminal11c, and the shift operation is performed by causing the shift current Ishift between the terminal11band the terminal11c.

Since the shift current path and the read current path are separated from each other in the magnetic memory shown inFIG. 8A, the shift current Ishift may have a current distribution shown inFIG. 8B, and the read current Iread may have a current distribution shown inFIG. 8C. Specifically, the shift current Ishift is greater than the holding current Ihold of the switching element18and the threshold current Ic for shifting the domain walls, and the read current Iread is greater than the holding current Ihold of the switching element16and smaller than the breakdown current Ibd of the MTJ element14. Thus, the operational margin of the magnetic memory according to this embodiment is broader than that of the magnetic memory according to the comparative example. As described above, in the magnetic memory according to this embodiment, the operational margin may be broadened since the shift current path and the read current path are separated from each other. Furthermore, erroneous shifting (read disturb) caused by the read current may be prevented. Since the shift current does not flow through the MTJ elements, the change in magnetoresistance (MR) of the MTJ element does not affect the pulse shape of the shift current. Furthermore, since the shift current does not flow through the MTJ element, no voltage stress is applied to the MTJ element, improving the durability of the MTJ element. Since the MTJ element and the magnetic member are not directly connected to each other, the effective MR of the MTJ element may be increased. Since the operational margin may be broadened, the read current may be increased, thereby facilitating the maintaining of the ON state of the switching element.

Second Embodiment

A second embodiment will now be described. The second embodiment is a method of manufacturing the magnetic memory according to the first embodiment.FIGS. 9 to 17show the respective steps of the method.

First, a metal layer300of an aluminum oxide is formed on a silicon substrate200, or a substrate300of aluminum is bonded to the silicon substrate200(FIG. 9). Subsequently, anode oxidation is performed on the metal layer300. During the anode oxidation, the metal layer300or the silicon substrate200is set as an anode, and a current is caused to flow in an electrolytic solution (for example, any of or any combination of sulfuric acid, oxalic acid, and phosphoric acid). The metal layer (aluminum) is then oxidized, forming metal ions and dissolving. The metal ions are combined with oxygen in the electrolytic solution to make a metal oxide (aluminum oxide), which grows on the surface of the metal layer300. As the dissolving and the growth advance at the same time, minute holes302surrounded by the aluminum oxide are formed on the surface of the aluminum of the metal layer300. While the holes302are formed, a second voltage is periodically added, the second voltage being different from the first voltage applied to form the holes. While the second voltage is applied, portions302ahaving smaller dimensions (diameter) in the x direction and the y direction shown inFIG. 2are formed. The aluminum in the regions around the hoes302is changed to aluminum oxide300A (FIG. 10).

Subsequently, as shown inFIG. 11, a magnetic layer304covering the inner surface of each hole302is formed. The magnetic layer304corresponds to the magnetic members1211and1212shown inFIG. 2. Thereafter, a non-magnetic insulating film (for example, a silicon oxide film)306is formed to fill into each hole302, except for the upper portion of the hole302, as shown inFIG. 12.

A non-magnetic conductor layer308covering the side surface of the upper portion of each hole302is then formed, as shown inFIG. 13. The conductor layer308corresponds to the conductor layers1711and1712shown inFIG. 2. Thereafter, a non-magnetic insulating film (for example, a silicon oxide film)310is formed to fill the hole302and to cover the top surface of the aluminum oxide300A. Openings are then formed in the insulating film310using the photolithographic technique. The top surface of the insulating film306and the inner side surface of the conductor layer308are exposed in each opening. A magnetic member (for example, a soft magnetic member)319is then filled into each opening. The magnetic member319corresponds to a part of the magnetic members1911and1912shown inFIG. 2. Wirings3201,3202, and3023are then formed on the insulating film310. The wirings3201,3202, and3023correspond to the field lines201,202, and203shown inFIG. 2, respectively (FIG. 14).

A non-magnetic insulating film (for example, a silicon oxide film)322covering the wirings3201,3202, and3023is then formed (FIG. 15). Openings are formed through the insulating film322by using the photolithographic technique to expose the top surface of the magnetic member319. A magnetic member (for examples, a soft magnetic member)324is filled into each opening. The magnetic members324correspond to the rest of the magnetic members1911and1912shown inFIG. 2. A non-magnetic insulating film (for example, a silicon oxide film)332covering the magnetic members324is then formed on the insulating film322. Openings are formed through the insulating film332using the photolithographic technique to expose the top surface of the magnetic member324. Switching elements3301and3302are formed to fill the openings. The switching elements3301and3302correspond to the switching elements1811and1812shown inFIG. 2. A wiring340electrically connected to the switching elements3301and3302are formed on the insulating film332(FIG. 15). The wiring340corresponds to the bit line BL1shown inFIG. 2. Subsequently, a non-magnetic insulating film (for example, a silicon oxide film), which is not shown, is formed to cover the wiring340. The non-magnetic insulating film is smoothed by chemical mechanical polishing (CMP) to expose the surface of the wiring340.

A CMOS circuit including the control circuits101,102, and103shown inFIG. 1is formed on another substrate400.

The substrate400with the CMOS circuit is reversed and joined to the substrate on which the magnetic layer304, the magnetic member324, the switching elements3301and3302, and the wiring340are formed as shown inFIG. 16. The substrates are joined so that the surface of the substrate400on which the CMOS circuit is formed faces the wiring340. The wirings340,3201,3202, and3203shown inFIG. 16are electrically connected to the CMOS circuit.

The silicon substrate200is then polished from the back side by CMP for example, until the surface of the aluminum oxide300A is exposed. The end portion of the magnetic layer304is also exposed at this time. Subsequently, wirings5001and5002each electrically connected to the magnetic layer304are formed on the exposed surface of the aluminum oxide. The wirings5001and5002correspond to the source lines SL1and SL2shown inFIG. 2. Thereafter, a non-magnetic insulating film (for example, a silicon oxide film)502is formed to cover the wirings5001and5002. The insulating film502is smoothed by using CMP for example to expose the surfaces of the wirings5001and5002. MTJ elements5161and5162that are electrically connected to the exposed surfaces of the wirings5001and5002respectively are then formed. The MTJ element516i(i=1, 2) includes a fixed layer514in which the magnetization direction is fixed, a free layer510, which is disposed between the fixed layer514and the wiring500i, and in which the magnetization direction may be changed, and a non-magnetic insulating layer (tunnel barrier layer)512disposed between the fixed layer514and the free layer510.

Subsequently, a non-magnetic insulating film (for example, a silicon oxide film)520is formed to cover the MTJ elements5161and5162, as shown inFIG. 17. Openings are then formed through the insulating film520by using the lithographic technique until the top surface of the fixed layer514included in each of the MTJ elements5161and5162is exposed, and switching elements5241and5242that are each electrically connected to the fixed layer514are formed. The switching elements5241and5242correspond to the switching element1611,1612shown inFIG. 2. Thereafter, a wiring530electrically connected to the switching elements5241and5242is formed. The wiring530corresponds to the data line DL1shown inFIG. 2. A non-magnetic insulating film (for example, a silicon oxide film) that is not shown is formed to cover the wiring530, and smoothed by CMP. The MTJ elements5161and5162and the wirings530,5001, and5002are electrically connected to the CMOS circuit formed on the substrate400through vias embedded in minute holes (for examples the holes302shown inFIG. 10) formed in the aluminum oxide300A.

The magnetic layer304shown inFIG. 11is not formed in the holes in which the vias are embedded. However, a dummy magnetic layer may be embedded in such holes.

The magnetic memory according to the first embodiment is manufactured in the above-described manner.

In the magnetic memory manufactured according to the second embodiment, the path of the shift current for moving the domain walls in a memory cell is different from a current path for reading data from the memory cell, as is explained in the descriptions of the first embodiment. As a result, the erroneous shifting of the domain walls caused by the read current, the read disturb, can be avoided. This enables the broadening of the operational margin caused by variations of current distribution.