Three-dimensional magnetic memory array with a minimal number of access conductors therein

[0002] This invention relates to magnetic thin film semiconductor devices and, more particularly, to a thin film magnetoresistive memory device.

[0003] Memory devices are used in electronic systems and computers to store information in the form of binary data. These memory devices may be characterized as either volatile memory, where the stored data is lost if the power source is disconnected or removed or non-volatile, where the stored data is retained even during power interruption. An example of a non-volatile memory device is the magnetic random access memory (MRAM).

[0004] An MRAM can be formed, to considerable advantage, based on the storage of digital bits as alternative states of magnetization of magnetic materials in each memory cell, typically thin-film materials. These films may be thin magnetic films having information stored therein based on the direction of the magnetization occurring in those films. The information is typically obtained either by inductive sensing to determine the magnetization state, or by magnetoresistive sensing of each state.

[0005] An arrangement for sensing states of magnetization in thin film magnetic material portions used in memory cells for storing bits is based on choosing a thin film magnetic material which also exhibits a sufficient magnetoresistance property. Changes in electrical resistance of such a material with the application, removal or change in magnitude of a magnetic field do not depend on the dimensions of the film portion. Thus the film portion to store a bit can be made very small to improve the packing density of cells in a magnetic memory.

[0006] Such magnetic thin-film memories may be conveniently provided on the surface of a monolithic integrated circuit to provide easy electrical interconnection between the memory cells and the memory operating circuitry on the monolithic integrated circuit. When so provided, it is desirable to reduce the size and increase the packing density of the magnetic thin-film memory cells to achieve a significant density of stored digital bits.

[0007] Typically, a thin-film magnetic memory includes a number of bit lines intersected by a number of word lines. At each intersection, a thin film of magnetically coercive material is interposed between the corresponding word line and bit line. The magnetic material at each intersection forms a magnetic memory cell in which a bit of information is stored.

[0008] The word lines are often provided on a first metal interconnect layer and the bit lines are provided on another. In each case, the metal interconnect layers must typically be connected to supporting circuitry or other underlayer structures on the monolithic integrated circuit for the memory to function. In addition, portions of the first metal interconnect layer are often connected to portions of the second metal interconnect layer to complete selected circuit elements.

[0009] The number of metal interconnect layers, typically copper interconnect layers, in the proceeding arrangement requires two metal interconnects for each plane (or level) of magnetic memory cells. As density becomes an issue, the amount of substrate space required must be taken into account. The structural arrangement of a magnetoresistive memory device is a significant focus of the present invention.

[0010] Exemplary embodiments of the present invention include a structure and method for forming a magnetic memory having a number N of levels of magnetic memory cells by forming at least one magnetic memory core structure having first and second surfaces, forming a first access conductor connecting to the first surface, forming a second access conductor connecting to the second surface, wherein N+1 access conductors are employed per number N of levels of the magnetic memory cells. The structure comprises a magnetic memory having a number N of levels of magnetic memory cells, each including at least one magnetic memory core structure having first and second surfaces, the first and second surfaces each connecting to an individual access conductor, wherein N+1 access conductors are required per number N of levels of the magnetic memory cells.

[0011] FIG. 1 is a cross-sectional view depicting a semiconductor substrate covered with tetraethylorthosilicate (TEOS) dielectric film and a first copper strip, which forms the first conductor of a first level magnetic memory cell.

[0012] FIG. 2 is a subsequent cross-sectional view taken from FIG. 1 after patterning and etching of a sense layer and a pinned layer to a first level magnetic memory cell.

[0013] FIG. 3 is a subsequent cross-sectional view taken from FIG. 2 following the formation of a second copper strip running substantially parallel to the first copper strip, which forms the second conductor of a first level magnetic memory cell and which also forms a first conductor of a second level magnetic memory cell.

[0014] FIG. 4 is a subsequent cross-sectional view taken from FIG. 3 after patterning and etching of a sense layer and a pinned layer to a second level magnetic memory cell.

[0015] FIG. 5 is a subsequent cross-sectional view taken from FIG. 4 following the formation of a third copper strip running substantially parallel to the first and second copper strips, which forms the second conductor of a second level magnetic memory cell and which may also be used as a first conductor of an additional level magnetic memory cell.

[0016] FIG. 6 is a subsequent cross-sectional view taken from FIG. 5 depicting a repeating pattern for forming multiple levels of magnetic memory cells, which illustrates that the number of conductive lines required for a desired number of levels of magnetic memory cells is N+1.

[0017] FIGS. 7a-7care cross-sectional views depicting a semiconductor substrate fabricated with barrier layer variations using a tetraethylorthosilicate (TEOS) dielectric film and a first copper strip, which forms the first conductor of a first level magnetic memory cell.

[0018] FIG. 8 is a subsequent cross-sectional view taken from FIG. 7 after patterning and etching of a sense layer and a pinned layer to a first level magnetic memory cell.

[0019] FIG. 9 is a subsequent cross-sectional view taken from FIG. 8 following the formation of a second copper strip running substantially perpendicular to the first copper strip, which forms the second conductor of a first level magnetic memory cell and which also forms a first conductor of a second level magnetic memory cell.

[0020] FIG. 10 is a subsequent cross-sectional view taken from FIG. 9 after patterning and etching of a sense layer and a pinned layer to a second level magnetic memory cell.

[0021] FIG. 11 is a subsequent cross-sectional view taken from FIG. 10 following the formation of a third copper strip running substantially perpendicular to the second copper strip and substantially parallel to the first copper strip, which forms the second conductor of a second level magnetic memory cell and which may also be used as a first conductor of an additional level magnetic memory cell.

[0022] FIG. 12 is a subsequent cross-sectional view taken from FIG. 11 depicting a repeating pattern for forming multiple levels of magnetic memory cells, which illustrates that the number of conductive lines required for a desired number of levels of magnetic memory cells is N+1.

[0023] Exemplary implementations of the present invention, as depicted respectively in FIGS. 1-6 and FIGS. 7-12, are directed to a magnetic memory array structure for a magnetic thin film semiconductor device, such as a thin film magnetoresistive memory device.

[0024] The following exemplary implementation is in reference to a thin film magnetoresistive memory device (MRAM). While the concepts of the present invention are conducive to MRAMs, the concepts taught herein may be applied to other semiconductor devices that would likewise benefit from the use of the structure disclosed herein. Therefore, the depiction of the present invention in reference to the manufacture of a MRAM (the preferred embodiment), is not meant to so limit the extent to which one skilled in the art might apply the concepts taught hereinafter.

[0025] As described above, a magnetic thin film semiconductor device may be implemented in various different technologies. One such application is in MRAM devices, an exemplary implementation of which is depicted in FIGS. 1-6. Referring now to FIG. 1, a semiconductive substrate 10, such as a silicon wafer, is prepared for the processing steps of the present invention. Insulating material 11, such as tetraethylorthosilicate (TEOS), is formed over substrate 10. Next, a conductive strip 12, such as copper, is formed over insulating material 11. Conductive strip 12 will function as a first conductor (or first bitline) to a first level of magnetic memory cells of the MRAM device.

[0026] Referring now to FIG. 2, a dielectric material 20 is patterned and etched to allow the formation of a pinned layer 21, typically comprising a seed layer, such as NiFe, an antiferomagnetic layer, such as IrMn, and a ferromagnetic layer, such as NiFe, and the formation of a tunnel dielectric layer 22, such as Al₂O₃, and an overlying sense layer 23, typically NiFeCo, which combine to form the makeup of the magnetic memory core of the magnetic memory cell. The pinned layer is deposited so that it maintains a certain magnetic pole orientation. For example, a conductive material is deposited and then subjected to a large magnetic field in order to create a desired pole orientation. The combination of layers 21, 22 and 23 may comprise several materials that will respond as required for utilization in a magnetic memory core. The present invention does not limit what type of materials nor the combinations of materials used to construct the magnetic core of the memory cell, as the structural design of the present invention is adaptable to any suitable materials used. However, in the exemplary implementations of the present invention all materials are created in an elemental composition that classifies them as either antiferomagnetic or ferromagnetic materials.

[0027] Referring now to FIG. 3, a second conductive strip 30, such as copper, which runs substantially parallel to the first conductor, is formed to create a second conductor (or wordline) for the first level of magnetic memory cells. Because of the unique structural arrangement of the present invention, this second conductor for the first level of magnetic memory cells can also serve as the first conductor of a second level of magnetic memory cells.

[0028] Referring now to FIG. 4, the steps of FIG. 2 are repeated as a dielectric material 40 is patterned and etched to allow the formation of a pinned layer 41, comprising a seed layer, such as NiFe, an antiferomagnetic layer, such as IrMn, and a ferromagnetic layer, such as NiFe, and the formation of a tunnel dielectric layer 42, such as Al₂O₃, and an overlying sense layer 43, typically NiFeCo, which combine to form the makeup of the magnetic memory core of the magnetic memory cell. As discussed in the text with FIG. 2, the pinned layer is deposited so that it maintains a certain magnetic pole orientation and the combination of layers 41, 42 and 43 may comprise several materials that will respond as required for utilization in a magnetic memory core.

[0029] Referring now to FIG. 5, a third conductive strip 50, such as copper, which runs substantially parallel to the second conductor, is formed to create a second conductor (or second bitline) for the second level of magnetic memory cells. Because of the unique structural arrangement of the present invention, this second conductor for the second level of magnetic memory cells can also serve as the first conductor of a third level of magnetic memory cells. The MRAM is then completed in accordance with fabrication steps used by those skilled in the art.

[0030] A final MRAM array structure, depicted in FIG. 6, further illustrates the concept of the present invention. Referring now to FIG. 6, multiple levels of magnetic memory cells are shown. The formation of multiple levels of magnetic memory cells is basically the repeat of the fabrication process depicted in FIG. 2 through FIG. 5, with each level of magnetic memory cells building one on top another. FIG. 6 demonstrates a concept of the present invention in that each cell comprises a first conductive strip and a second conductive strip to function as a first and second conductor to any given level of magnetic memory cells of the CMAM device. Sandwiched between the first and second conductors is the magnetic core, which is used in conjunction with the first and second conductors to store the state of the memory cell.

[0031] As shown in FIG. 6 a first level of magnetic memory comprises first conductor 12 and second conductor 30, with magnetic core materials 21 and 23 and tunnel dielectric layer 22 sandwiched therebetween. The presence of dielectric 20 keeps conductors 12 and 30 physically separated from one another. The second or higher level N of magnetic memory cells includes a first conductor comprising conductor 30 of the next lower level magnetic memory cells and a second conductor 64, with magnetic core material 61 and 63 and tunnel dielectric layer 62 sandwiched therebetween.

[0032] A second exemplary implementation of the present invention for an MRAM device is depicted in FIGS. 7a-12. Referring now to FIG. 7a, a semiconductive substrate 70, such as a silicon wafer, is prepared for the processing steps of the present invention. Insulating material 71, such as tetraethylorthosilicate (TEOS), is formed over substrate 70. Next, a first thin barrier layer 72.1, such as tantalum (Ta) and a first conductive strip 72.2, such as copper, is formed over insulating material 71.

[0033] Alternative variations of a barrier layer are depicted in FIGS. 7band 7c. As shown in FIG. 7b, the barrier layer 72.1 is formed and patterned directly on the first conductive strip 72.2 and thus will ultimately separate the first conductive strip and the subsequent second conductive strip.

[0034] As shown in FIG. 7c, the barrier layer is made up of two portions. A first barrier layer 72.1 is formed, followed by the formation of the first conductive strip 72.2 as described in FIG. 7a. After the first barrier layer 72.1 and first conductive strip 72.2 are planarized a second barrier layer 72.3 is formed and patterned directly on first conductive strip 72.2 to make a barrier layer that completely surrounds first conductive strip 72.2. These alternatives would carry though the rest of the fabrication process.

[0035] Referring now to FIG. 8, first thin conductive layer 72.1 and conductive strip 72.2 are planarized and will function as a first conductor 72 (or first bitline) to a first level of magnetic memory cells of the MRAM device. Next, a dielectric material 80 is patterned and etched to allow the formation of a pinned layer 81, typically comprising a seed layer, such as NiFe, an antiferomagnetic layer, such as IrMn, and a ferromagnetic layer, such as NiFe, and the formation of a tunnel dielectric layer 82, such as Al₂O₃, and an overlying sense layer 83, typically NiFeCo, which combine to form the makeup of the magnetic memory core to the magnetic memory cell. Pinned layer 81 is either deposited so that it maintains a certain magnetic pole orientation or is annealed later in a magnetic field to set the desired orientation.

[0036] For example, a magnetic material is deposited and then subjected to a large magnetic field in order to create the desired pole orientation. The combination of layers 81, 82 and 83 may comprise several materials that will respond as required for utilization in a magnetic memory core. The present invention does not limit what type of materials nor the combinations of materials used to construct the magnetic core of the memory cell, as the structural design of the present invention is adaptable to any suitable materials used.

[0037] Referring now to FIG. 9, a second conductive strip 90, such as copper, which runs substantially perpendicular to the first conductor, is formed to create a second conductor (or wordline) for the first level of magnetic memory cells. Due to the unique structural arrangement of the present invention, this second conductor for the first level of magnetic memory cells can also serve as the first conductor of a second level of magnetic memory cells.

[0038] Referring now to FIG. 10, the steps of FIG. 8 are repeated as a dielectric material 100 is patterned and etched to allow the formation of pinned layer 101, comprising a seed layer, such as NiFe, an antiferomagnetic layer, such as IrMn, and a ferromagnetic layer, such as NiFe, and the formation of a tunnel dielectric layer 102, such as Al₂O₃, and an overlying sense layer 103, typically NiFeCo, which combine to form the makeup of the magnetic memory core of the magnetic memory cell. As discussed in the text with FIG. 8, the pinned layer is deposited so that it maintains a certain magnetic pole orientation and the combination of layers 101, 102 and 103 may comprise several materials that will respond as required for utilization in a magnetic memory core.

[0039] Referring now to FIG. 11, dielectric layer 110 is formed and patterned in preparation for the subsequent formation of a second conductor. Next, a second thin conductive layer 111.1 and a third conductive strip 111.2, such as copper, which runs substantially perpendicular to the second conductor, are formed and planarized to create a second conductor (or second bitline) for the second level of magnetic memory cells. Because of the unique structural arrangement of the present invention, this second conductor for the second level of magnetic memory cells can also serve as the first conductor of a third level of magnetic memory cells. If so desired, the first and third conductors can be fabricated to run at an angle of 1-89° to the second conductor so that the first and second bitlines intersect the wordline at an angle ranging from 1 to 89°. The MRAM is then completed in accordance with fabrication steps used by those skilled in the art.

[0040] A final MRAM array structure, depicted in FIG. 12, further illustrates the concept of the present invention. Referring now to FIG. 12, multiple levels of magnetic memory cells are shown. The formation of multiple levels of magnetic memory cells is basically the repeat of the fabrication process depicted in FIG. 8 through FIG. 11, with each level of magnetic memory cells building one on top another. FIG. 12 demonstrates a concept of the present invention in that each cell comprises a first conductive strip and a second conductive strip to function as a first and second conductor to any given level of magnetic memory cells of the MRAM device. Sandwiched between the first and second conductors of each cell is the magnetic core, which is used in conjunction with the first and second conductors to store the state of the memory cell.

[0041] As shown in FIG. 12 a first level of magnetic memory comprises first conductor 72 and second conductor 90, with magnetic core materials 81 and 83 and tunnel dielectric layer 82 sandwiched there between. In a second or higher level N of magnetic memory cells, the first conductor comprises a conductor 90 of the next lower level magnetic memory cells and a second conductor 125 with magnetic core material 121 and 123 and tunnel dielectric layer 122 sandwiched therebetween.

[0042] FIGS. 6 and 12 each demonstrate that each adjacent level of magnetic memory cells will utilize a common conductor. Such a pattern allows for the fabrication of a magnetic memory having N levels of magnetic memory cells only requiring N+1 conductors. It is conceivable that only fabrication limitations and desired device operational characteristics would limited the number of levels of magnetic memory cells that those skilled in the art could build. Even with these possible constraints, the advantage of employing the structural design of the present invention would provide motivation to use the design as the number of conductors needed is only N+1 conductors versus conventional designs which use 2N conductors per N levels of magnetic memory cells.

[0043] It is to be understood that, although the present invention has been described with reference to a preferred embodiment, various modifications, known to those skilled in the art, may be made to the process disclosed herein without departing from the invention as recited in the several claims appended hereto.