Systems and methods for reading and writing a magnetic memory device

An MRAM cell comprises a magnetic metal layer and a magnetic sensing device in close proximity to the magnetic metal layer. One end of the magnetic metal layer is coupled with a word line transistor, while the other end of the magnetic metal layer is coupled to a first bit line. The magnetic sensing device can be coupled with a second bit line. The magnetic metal layer can be used to both program and read the cell, eliminating the need for a second current lien in the cell.

RELATED APPLICATION INFORMATION

This application is related to co-pending U.S. patent application Ser. No. 11/255,606, entitled, “A Magnetic Memory Device and Methods for Making a Magnetic Memory Device,” filed Oct. 21, 2005, which is incorporated herein in its entirety as if set forth in full.

BACKGROUND

1. Field of the Invention

The invention relates in general to memory devices for use as computer main storage, and in particular to memory arrays that use magnetic memory elements as the individual memory cells.

2. Background of the Invention

The desired characteristics of a memory cell for computer main memory are high speed, low power, nonvolatility, and low cost. Low cost is accomplished by a simple fabrication process and a small surface area. Dynamic random access memory (DRAM) cells are fast and expend little power, but have to be refreshed many times each second and require complex structures to incorporate a capacitor in each cell. Flash type EEPROM cells are nonvolatile, have low sensing power, and can be constructed as a single device, but take microseconds to write and milliseconds to erase, which makes them too slow for many applications, especially for use in computer main memory. Conventional semiconductor memory cells such as DRAM, ROM, and EEPROM have current flow in the plane of the cell, i.e., “horizontal”, and therefore occupy a total surface area that is the sum of the essential memory cell area plus the area for the electrical contact regions, and therefore do not achieve the theoretical minimum cell area.

Unlike DRAM, magnetic memory cells that store information as the orientation of magnetization of a ferromagnetic region can hold stored information for long periods of time, and are thus nonvolatile. Certain types of magnetic memory cells that use the magnetic state to alter the electrical resistance of the materials near the ferromagnetic region are collectively known as magnetoresistive (MR) memory cells. An array of magnetic memory cells is often called magnetic RAM or MRAM.

To be commercially practical MRAM should have comparable memory density to current memory technologies, be scalable for future generations, operate at low voltages, have low power consumption, and have competitive read/write speeds.

For an MRAM device, the stability of the nonvolatile memory state, the repeatability of the read/write cycles, and the memory element-to-element switching field uniformity are three of the most important aspects of its design characteristics. A memory state in MRAM is not maintained by power, but rather by the direction of the magnetic moment vector. Storing data is accomplished by applying magnetic fields and causing a magnetic material in a MRAM device to be magnetized into either of two possible memory states. Recalling data is accomplished by sensing the resistive differences in the MRAM device between the two states. The magnetic fields for writing are created by passing currents through strip lines external to the magnetic structure or through the magnetic structures themselves.

As the lateral dimension of an MRAM device decreases, three problems occur. First, the switching field increases for a given shape and film thickness, requiring a larger magnetic field to switch. Second, the total switching volume is reduced so that the energy barrier for reversal decreases. The energy barrier refers to the amount of energy needed to switch the magnetic moment vector from one state to the other. The energy barrier determines the data retention and error rate of the MRAM device and unintended reversals can occur due to thermofluctuations (superparamagnetism) if the barrier is too small. A major problem with having a small energy barrier is that it becomes extremely difficult to selectively switch one MRAM device in an array. Selectablility allows switching without inadvertently switching other MRAM devices. Finally, because the switching field is produced by shape, the switching field becomes more sensitive to shape variations as the MRAM device decreases in size. With photolithography scaling becoming more difficult at smaller dimensions, MRAM devices will have difficulty maintaining tight switching distributions.

These problems often associated with conventional MRAM devices result in other problems. For example, it takes high currents in order to change the state of the magnetic sensing device in order to program a conventional MRAM device. These high currents create several problems including high power consumption which makes MRAM devices unsuitable for many portable applications. Moreover, the magnetic field resulting from the currents is often difficult to control which leads to cross talk problems especially in MRAM devices with decreased lateral dimensions as described above.

Another problem with conventional MRAM devices is that two current lines are typically required for generating the currents and associated magnetic field needed to program the magnetic sensing device included in the MRAM device. The inclusion of two current lines limits the ability to shrink the MRAM device and achieve the greatest possible densities in size reductions.

SUMMARY

An MRAM cell comprises a magnetic metal layer and a magnetic sensing device in close proximity to the magnetic metal layer. One end of the magnetic metal layer is coupled with a word line transistor, while the other end of the magnetic metal layer is coupled to a first bit line. The magnetic sensing device can be coupled with a second bit line.

In one aspect, the MRAM cell can be read by turning on the word line transistor, letting the first bit line float, and coupling the second bit line with a current sense amplifier. This configuration allows current to flow through the word line transistor, the magnetic metal layer, the magnetic sensing device, and into the current sense amplifier. A current sense amplifier can be configured to sense the current flowing from the magnetic sensing device in order to determine the program state of the magnetic sensing device.

In another aspect, the MRAM cell can be programmed to a first state by turning on the word line transistor and applying a voltage difference via the first bit line to the other side of the magnetic metal layer. This will cause current to flow from the first bit line through the magnetic metal layer and to ground through the word line transistor.

In another aspect, the MRAM cell can be programmed to another state by again turning on the word line transistor and applying a voltage difference to the other side of the magnetic metal layer via the first bit line. The direction of the current flowing through the magnetic metal layer would determine the programming state of the MRAM cell.

These and other features, aspects, and embodiments of the invention are described below in the section entitled “Detailed Description.”

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

FIG. 1Ais a diagram illustrating an example embodiment of an MRAM cell100that can be included in an MRAM device configured in accordance with one embodiment of the systems and methods described herein. It will be apparent that not all the layers, structures, and/or circuits included in MRAM cell100, or the MRAM device in which MRAM cell100is included, are illustrated inFIG. 1A. Only certain elements, layers, and/or aspects associated with MRAM cell100are shown inFIG. 1Afor the sake of convenience. Methods for fabricating an MRAM device that includes an MRAM cell100are described in detail in co-pending patent application Ser. No. 11/255,606. The description of the Ser. No. 11/255,606 application describes in detail the layers comprising an MRAM device that includes MRAM cell100and methods for fabricating the layers and other circuits. Thus, while not all of the layers, elements, and circuits associated with MRAM cell100are illustrated inFIG. 1A, this should not be seen as limiting MRAM cell100to any particular construction or as excluding any of these various layers, elements, and/or circuits. Further, while the layers illustrated inFIG. 1Aare shown in two dimensions, it will be clear that the layers are actually three-dimensional.

As can be seen, MRAM cell100includes a magnetic metal layer102and a magnetic sensing device104in close proximity to magnetic metal layer102. Magnetic sensing device104is separated from magnetic metal layer102, in certain embodiments, via a proximity conductor layer106. Magnetic metal layer102can have an associated length, height, and width that can range anywhere from 10 nm to 10 μm. Similarly, magnetic sensing device104can have an associated length, height, and width that can range anywhere from 5 nm to 10 μm depending on the embodiment.

Magnetic metal layer102can have a permeability in the range from about 10 to 108. Magnetic metal layer102is conductive and has a resistivity (ρ) that ranges from about 4 μΩ-cm to 108μΩ-cm. Magnetic metal layer102can also have a saturation magnetization (MS) that ranges from about 10 Guass to 2.5 Tesla. The material used to construct magnetic metal layer102can include at least one element with a crystallization phase. For example, magnetic metal layer102can include Ni, Fe, Co, B, Mo, Zn, Pb, Si, C, O, and/or any other material that can provide the ρ and MSdescribed above.

Proximity conductor106can be configured to connect magnetic sensing device104and magnetic metal layer102. The resistivity (ρ) of proximity conductor106can be in the range of about 1 to 1010μΩ-cm. Proximity conductor106can be metal, a conductive compound, semi-conductor material, or any other material that has a resistivity within the range described above. These materials can include, for example, Cu, TiN, TaN, Si, W, Ag, Ru, Ir, Pt, etc.

Magnetic sensing device104can include a single or multi-layer layer ferro/anti-ferro magnetic device. Such magnetic devices can include, for example, a Magnetic Tunnel Junction (MTJ) device, a Giant Magneto Resistance (GMR) device, a Colossal Magneto Resistance (CMR) device, or an Anisotropic Magneto Resistance (AMR) device, Magneto Optical (MO) element, or magnetic disc. For example, magnetic sensing device106can include an MTJ device comprising of a ferro magnetic layer, an insulator, another ferro magnetic layer, and an anti-ferromagnetic layer. Alternatively, magnetic sensing device104can include an MTJ device that includes a ferromagnetic layer, an insulator layer, and another ferromagnetic layer, or an MTJ device that includes an anti-ferro magnetic layer, a ferromagnetic layer, an insulator, and another ferromagnetic layer.

In other embodiments, magnetic sensing device104can include a GMR device that includes a ferromagnetic layer, a thin conductive layer, another ferromagnetic layer, and an anti-ferromagnetic layer. Alternative GMR devices that can be used in conjunction with the systems and methods described herein can include a ferromagnetic layer, a thin conductive layer, and another ferromagnetic layer, or an anti-ferromagnetic layer, a ferromagnetic layer, a thin conductive layer, and another ferromagnetic layer.

Alternatively, a CMR device comprising a Mn-based compound with at least two elements, such as LaSrMnO, PrCaMnO, LaCaMnO, etc., can be used for magnetic sensing device104. In still other embodiments, an AMR device, MO elements, or a magnetic disc comprising 3d transition ferromagnetic elements or alloys with other elements can be used for magnetic sensing device106.

The ferromagnetic layers referred to above can, depending on the embodiment, include 3 d transition ferromagnetic elements or alloys with other elements such as CoFe, NiFe, CoFeB, Fe, Co, etc. The anti-ferromagnetic layers described above can include transition anti-ferromagnetic elements or alloys with other elements, such as FeMn, IrMn, NiO, PtMn, NiMn, CoO, etc. Other anti-ferromagnetic layers referred to above can include ferromagnetic anti-layers with or without anti-ferromagnetic material, such as CoFe/Ru/CoFe, CoFe/Ru/CoFe/IrMn, etc. Insulator layers referred to above can include elements such as Al2O3, MgO, etc., and the thin conductive layers described above, can include materials such as Cu, Ag, Cr, Ru, Ir, etc.

It will be understood that the devices, layers, and materials described above are by way of example only and should not be seen as limiting the systems and methods described herein to any particular device structure and/or materials.

As will be described in more detail below, magnetic metal layer102can be used to conduct currents that create magnetic fields that can be used to program magnetic sensing device104to one of two states. Further, magnetic metal layer102can be used to conduct currents that enable the state of magnetic sensing device104to be determined. By using magnetic layer102to conduct read and write currents in this manner, lower current levels can be used in the read and write operations as compared to conventional devices. Further, the low currents in magnetic layer102reduces and can even eliminate any cross talking problems. Moreover, the number of metal bit lines can be reduced relative to conventional MRAM devices, which allows for greater reduction in size and increase densities.

FIG. 2is a diagram illustrating the current lines202and204and magnetic sensing device206for an exemplary MRAM cell200. As explained above, magnetic sensing device206comprises magnetic material that can be configured to store information as the orientation of the magnetization of a ferromagnetic region within magnetic sensing device206. The orientation of the magnetization can be effected by magnetic fields By and Bx that result from current flowing through current lines202and204.

Current lines202and204are typically constructed from non-magnetic materials, such as Cu. Magnetic fields By and Bx generated by currents Ix and Iy flowing through current lines202and204are generated in accordance with Ampere's law. If the sum of the magnetic fields (Bx+By) is greater than the coercive field of magnetic sensing device206, then magnetic sensing device206can be programmed to one of two programming states.

For example, when currents Ix and Iy are flowing in the directions shown, i.e., from right to left and into the page, and the currents are sufficient magnitude such that Bx+By is greater than the coercive field of magnetic sensing device206, then the magnetic moment vector for magnetic sensing device206can be switched to one of the two programming states. Reversal of the directions for currents Ix and Iy will then switch the magnetic moment vector in the other direction programming magnetic sensing device206to the other of the two programming states.

Unfortunately, in part because current lines202and204are constructed from non-magnetic materials, it takes large amounts of current, i.e., several mA to several tens of mA, to generate sufficient magnetic fields Bx and By to overcome the energy barrier from magnetic sensing device206. Further, the distribution of magnetic fields By and Bx cannot be controlled sufficiently to avoid cross talk between cells.

In the example ofFIG. 3, which illustrates the current lines302and310and magnetic sensing device306for another exemplary MRAM cell, current lines302and310are surrounded by magnetic material304and308. For current line302this is illustrated by the side view on the right ofFIG. 3. MRAM cell300operates in the same fashion as MRAM cell200; however, due to the resistivity (ρ) difference between current lines302and310and the magnetic material304and308surrounding current lines302and310respectively, most of the current Ix and Iy flows through the non-magnetic material of current lines302and310. In addition, due to the confining operation of magnetic material304and308, which can be referred to as magnetic clamps, the magnetic fields Bx and By generated by currents Ix and Iy respectively, are confined and the distribution is better controlled. Essentially, magnetic clamps304and308act as U-shaped magnets surrounding current lines302and310. Thus, most of magnetic fields Bx and By are focused within clamps304and308.

For the cell ofFIG. 3, currents Ix and Iy, required to program magnetic sensing device306and MRAM cell300, are less than those required for MRAM cell200; however, several mA are still required. Problems with cross talking are also improved, but the device ofFIG. 3is more complicated to manufacture than that ofFIG. 2.

FIG. 4is a diagram illustrating the current lines402and410and magnetic sensing device406for still another exemplary MRAM cell400. As with MRAM cell300, current lines402and410are clamped by magnetic material404and408. Here however magnetic sensing device406uses Synthetic Antiferromagnetic Coupling (SAF). In order to use SAF coupling, magnetic sensing device406is constructed from a plurality of layers. These layers include a first ferromagnetic layer, a very thin conductive layer, e.g., approximately 0.7 nm, and a second ferromagnetic layer. Further, currents Ix and Iy are pulsed on different time sequences. The differential pulsing switches the magnetization of the first and second ferromagnetic layers at different times. If the magnetization of both the first and the second ferromagnetic layers are switched then magnetic sensing device406is programmed to one of the two programming states. If the magnetization is not switched, then magnetic sensing device406remains programmed to its current state.

Use of SAF technology is effective for eliminating cross talk; however, very large currents are typically required to program magnetic sensing device406. For example, several tens of mA are required in order to change the state of magnetic sensing device406. In addition, the very thin conductive layer required for magnetic sensing device406can be very difficult to manufacture and control. If the thickness of the thin conductive layer varies too much, then the cell will not operate correctly.

Moreover, each of the cells illustrated inFIGS. 2-4require two current lines to program the magnetic sensing device included therein. Conversely, in cell100, only magnetic metal layer102is needed to program magnetic sensing device104. Thus, one current line can be eliminated by implementing the structure illustrated inFIG. 1.

FIG. 5is a diagram illustrating a side view of an MRAM device500comprising two exemplary MRAM cells100.FIG. 5illustrates some key dimensions related to MRAM cells100including the width (Wμ) of magnetic metal layers102, the width (Wm) of magnetic sensing device104, the distance (d) between magnetic sensing device104and magnetic metal layer102, and the spacing (S) between magnetic metal layers102. Example ranges for these various dimensions were provided above.

As illustrated inFIG. 6, a writing current602can flow in either direction along magnetic metal layer102. As illustrated inFIG. 7, current602will create a magnetic field704within magnetic metal layer102and will also cause a magnetic field702to flow through magnetic sensing device104. InFIG. 7, the current is shown flowing into the page creating clockwise magnetic field702and704.

Magnetic field704created within magnetic metal layer102is proportional to the permeability of magnetic metal layer102in accordance with Ampere's law. The higher the permeability (μ), the greater the magnetic field that is generated. Magnetic field704will leak from magnetic metal layers102at the boundary creating an external magnetic field702within magnetic sensing device104as illustrated. If the “leaked” magnetic field702is greater than the coercive field of magnetic sensing device104, then programming can occur.

InFIG. 8, current602is reversed such that it is coming out of the page, which creates counterclockwise magnetic field704within magnetic metal layer102and “leaked” magnetic field702within magnetic sensing device104. Again, if the “leaked” field702is larger than the coercive field of magnetic sensing device104then programming can occur, this time to the opposite state that would result from magnetic field702illustrated inFIG. 7.

Using magnetic metal layer102to program magnetic sensing device104in this manner can significantly reduce, or even eliminate, cross talk between cells100. Further, significantly lower currents are needed to create a sufficient magnetic field to overcome the coercive field of magnetic sensing device104. For example a current of as little as 440 μA can generate a magnetic field702of 2,800 G; however, the magnetic field outside of a target cell100drops to almost zero with a log decay. As a result, using magnetic metal layer102in the manner described results in a low current, cross talk free MRAM cell100.

In addition, only a single current line is needed to program magnetic sensing device104as opposed to two current lines as with conventional MRAM designs.

Not only does cell100include fewer current lines and conventional MRAM cells, MRAM cell100also does not require separate writing and reading current paths as with conventional devices. The reading operation of a conventional cell1200is illustrated inFIG. 12. In order to read the state of magnetic sensing device1210, a word line transistor1206and sense amplifier1212are required. When word line transistor1206is turned on, a current1208flows up through the cell and through magnetic sensing device1210and then down to current sense amplifier1212, which can be configured to determine the programming state of magnetic sensing device1210based on the value of current

As can be seen, read current1208flows through a different path than writing currents Ix and Iy flowing in current lines1202and1204. Further, several layers1216,1218,1220, and1222are needed within cell1200in order to provide a current path for current1208to flow through transistor1206and into sensing amplifier1212.

FIG. 9is a diagram illustrating how the state of magnetic sensing device104can be read in an MRAM cell100in accordance with one embodiment in the systems and methods described herein. Here, a turn on voltage can be applied to the gate of word line transistor108, such as a 1.6 volt turn on voltage. A voltage difference can then be applied to magnetic metal layer102and magnetic sensing device104. This will cause a current902to flow through word line transistor108into magnetic metal layer102and up into the magnetic sensing device104as illustrated. The current can then flow through BL2and into sense amplifier110, which can be configured to sense the state of magnetic sensing device104. Sense amplifier110is configured to compare the current on BL2to a reference current112. By sensing the difference between the current on BL2relative to reference current112, sense amplifier110can be configured to distinguish the logic state of magnetic sensing device104. BL1can be left floating during this read operation.

FIG. 10is a diagram illustrating a write operation in which magnetic sensing device104is programmed to one of two programming states in accordance with one embodiment of the systems and methods described herein. Here, a turn on voltage, e.g., of 1.6 volts, can be applied to word line transistor108turning it on. A voltage difference is then applied to the other side of magnetic metal layer102via BL1. This causes a writing current1002to flow from BL1through magnetic metal layer102to word line transistor108. As explained above, current1002will create a magnetic field of sufficient strength to overcome the energy barrier of magnetic sensing device104and thus switch the magnetic moment vector for magnetic sensing device104. For example, in one embodiment a 50 mA current1002is generated in magnetic metal layer102. BL2can be left floating through this operation.

FIG. 11is a diagram illustrating a writing operation in which magnetic sensing device104is programmed to the other state. Here the operation is similar to the operation depicted and described in relation toFIG. 10; however, the voltage difference applied to the other side of magnetic metal layer102via BL1can be the opposite of the voltage difference applied in the process described in relation toFIG. 10. This will cause a writing current1102to flow in the opposite direction in magnetic metal layer102switching the magnetic moment vector of magnetic sensing device104and programming magnetic sensing device to the other state. BL2can be left floating through this operation. Again, in one example embodiment a 50 mA current1102can be generated in order to program magnetic sensing device104to the other state.

As can be seen, not only does MRAM cell100include a single current line102, it also eliminates the need for, and complexity associated with, having separate read and write paths through the cell.

While certain embodiments of the inventions have been described above, it will be understood that the embodiments described are by way of example only. Accordingly, the inventions should not be limited based on the described embodiments. Rather, the scope of the inventions described herein should only be limited in light of the claims that follow when taken in conjunction with the above description and accompanying drawings.