1. Field of the Invention
This invention relates generally to a current perpendicular to plane random access memory (CPP-MRAM) cell formed as a magnetic tunneling junction (MTJ) and using a spin transfer effect with enhanced spin torque.
2. Description of the Related Art
The conventional magnetic tunneling junction (MTJ) device is a form of ultra-high magnetoresistive device in which the relative orientation of the magnetic moments of parallel, vertically separated, upper and lower magnetized layers controls the flow of spin-polarized electrons tunneling through a very thin dielectric layer (the tunneling barrier layer) formed between those layers. When injected electrons pass through the upper layer they are spin polarized by interaction with the magnetic moment of that layer. The majority of the electrons emerge polarized in the direction of the magnetic moment of the upper layer, the minority being polarized opposite to that direction. The probability of such a polarized electron then tunneling through the intervening tunneling barrier layer into the lower layer then depends on the availability of states within the lower layer that the tunneling electron can occupy. This number, in turn, depends on the magnetization direction of the lower electrode. The tunneling probability is thereby spin dependent and the magnitude of the current (tunneling probability times number of electrons impinging on the barrier layer) depends upon the relative orientation of the magnetizations of magnetic layers above and below the barrier layer. The MTJ device can therefore be viewed as a kind of multi-state resistor, since different relative orientations (e.g. parallel and antiparallel) of the magnetic moments will change the magnitude of a current passing through the device. In a common type of device configuration (spin filter), one of the magnetic layers has its magnetic moment fixed in direction (pinned) by exchange coupling to an antiferromagnetic layer, while the other magnetic layer has its magnetic moment free to move (the free layer). The magnetic moment of the free layer is then made to switch its direction from being parallel to that of the pinned layer, whereupon the tunneling current is large, to being antiparallel to the pinned layer, whereupon the tunneling current is small. Thus, the device is effectively a two-state resistor. The switching of the free layer moment direction (writing) is accomplished by external magnetic fields that are the result of currents passing through conducting lines adjacent to the cell.
FIG. 1 is a highly schematic drawing showing an overhead view of a conventional MRAM cell between orthogonal word (200) and bit (100) lines. The cell (1000) is drawn with a slightly elliptical horizontal cross-section because such a shape produces a magnetic anisotropy within the free layer that assists its magnetic moment in retaining a thermally stable fixed position after switching fields have been turned off. The fields produced by currents in each of the two lines are between about 30 to 60 Oersteds in magnitude. According to the diagram, the word line field will be along the hard axis of the cell, the bit line field will be along the easy axis (the longer axis of the ellipse).
The use of magnetic fields externally generated by current carrying lines (as in FIG. 1) to switch the magnetic moment directions becomes problematic as the size of the MRAM cells decreases and, along with their decrease, so does the width of the current carrying lines. The smaller width lines require greater current densities to produce the necessary switching fields on the MTJ elements, greatly increasing power consumption.
For this reason, a new type of magnetic device, called a spin transfer device and described by Slonczewski, (U.S. Pat. No. 5,695,164) and by Redon et al. (U.S. Pat. No. 6,532,164) has been developed and seems to eliminate some of the problems associated with the excessive power consumption necessitated by external switching fields. The spin transfer device shares some of the operational features of the conventional MTJ cell described above, except that the switching of the free layer magnetic moment is produced by the spin polarized current itself. In this device, unpolarized conduction electrons passing through a first magnetic layer having its magnetic moment oriented in a given direction (such as the pinned layer) are preferentially polarized by their passage through that layer by a quantum mechanical exchange interaction with the polarized bound electrons in the layer. Such a polarization can occur to conduction electrons that reflect from the surface of the magnetized layer as well as to those that pass through it. When such a stream of polarized conduction electrons subsequently pass through a second magnetic layer whose polarization direction is not fixed in space (such as the free layer), the polarized conduction electrons exert a torque on the bound electrons in the magnetic layers which, if sufficient, can reverse the polarization of the bound electrons and, thereby, reverse the magnetic moment of the magnetic layer. The use of a spin-polarized current internal to the cell to cause the magnetic moment reversal requires much smaller currents than those required to produce an external magnetic field from adjacent current carrying lines to produce the moment switching. Recent experimental data (W. H. Rippard et al., Phys. Rev. Lett., 92, (2004)) confirm magnetic moment transfer as a source of magnetic excitation and, subsequently, magnetic moment switching. These experiments confirm earlier theoretical predictions (J. C. Slonczewski, J. Magn. Mater. 159 (1996) LI, and J. Z. Sun, Phys. Rev. B., Vol. 62 (2000) 570). These latter papers show that the net torque, Γ, on the magnetization of a free magnetic layer produced by spin-transfer. from a spin-polarized DC current is proportional to:Γ=s nm x (nsx nm),  (1)Where s is the spin-angular momentum deposition rate, ns is a unit vector whose direction is that of the initial spin direction of the current and nm is a unit vector whose direction is that of the free layer magnetization and x symbolizes a vector cross product. According equation (1), the torque is maximum when ns is orthogonal to nm.
Referring to FIG. 2, there is shown a schematic illustration of an exemplary prior art MTJ element being contacted from above by a bit line (100) and from below by a bottom electrode (200). Moving vertically upward, there is shown a seed layer (1), an antiferromagnetic pinning layer (2), a synthetic antiferromagnetic (SyAF) pinned reference layer (345), consisting of a first ferromagnetic layer (3), a non-magnetic spacer layer (4) and a second ferromagnetic layer (5), a non-conducting tunneling barrier layer (6), a ferromagnetic free layer (7) and a non-magnetic capping layer (8). Arrows, (20) and (30), indicate the antiparallel magnetization directions of the two ferromagnetic layers (3) and (5) of the SyAF pinned layer (345). The double-headed arrow (40) in layer 7 indicates that this layer is free to have its magnetic moment directed in either of two directions.
Referring again to FIG. 2 it is noted that when a critical current (arrow (50) is directed from bottom to top (layer (1) to layer (8)), the free layer magnetization (40) would be switched to be opposite to the direction of the reference layer's magnetization (30) by the spin-transfer torque. This puts the MTJ cell into its high resistance state.
Conversely, if the current is directed from top to bottom, the free layer magnetization (40) would be switched, by torque transfer of angular momentum, to the same direction as that of the pinned reference layer (30), since the conduction electrons have passed through that layer before entering the free layer. The MTJ element is then in its low resistance state.
Referring again to FIG. 2, there is shown some additional circuitry, specifically a transistor (500) to inject current into the cell element whenever the cell element is selected to be written upon. The transistor is electrically connected to the cell through a conducting via (80) which allows a current to pass vertically between the bottom electrode (300) and the bit line (100). The word line (200), which can contact the transistor gate activates the transistor so as to inject the writing current. In this way one can create a single spin-RAM memory cell that utilizes the spin transfer effect (denoted hereinafter as an STT-RAM) for switching an MTJ type element. In this paper, we will use the term “element” to describe the basic MTJ structure comprising a tunneling barrier layer sandwiched between ferromagnetic fixed and free layers. We shall use the term “memory cell” to denote the combination of the MTJ element incorporated within circuitry that permits the element to be written on and read from. The word line provides the bit selection (i.e., selects the particular cell which will be switched by means of a current passing through it between the bit line and the source line) and the transistor provides the current necessary for switching the MTJ free layer of the selected cell. Although it is not shown in this simplified figure, the cell is read by applying a bias voltage between the bit line and source line, thereby measuring its resistance and comparing that resistance with a standard cell in the circuit. It is to be noted that large cell arrays are subject to difficulties that arise from statistical variations in the magnetic and electrical properties of each cell. For example, to decide whether a cell is in its high or low resistance state, its resistance must be compared to that of a reference cell that is in a known resistance state. However, statistical variations in the high and low resistance values of the array cells and the reference cells often make it possible to incorrectly interpret the resistance value of a cell. For this reason, the maximum variations of the resistance values of cells must fall within a “read margin” (of error) so that a correct interpretation of a resistance value is made in all cases. In fact, it is the goal of the present invention to provide a method of improving this read margin without sacrificing any of the features of the memory cell.
The critical current for spin transfer switching, Ic, is generally a few milliamperes for a 180 nm sub-micron MTJ cell (of cross-sectional area A approximately A=200 nm×400 nm). The corresponding critical current density, Jc, which is Ic/A, is on the order of several 107 Amperes/cm2. This high current density, which is required to induce the spin transfer effect, could destroy the insulating tunneling barrier in the MTJ cell, such as a layer of AlOx, MgO, etc.
During the reading of data, a small current flows across the MTJ cell and its resistance is compared with a pre-written MTJ cell (not shown) called a reference cell, to determine whether the cell being read is in a high or low resistance state. Typically, the reading margin is determined by the ratio between the magneto-resistive ratio, dR/R (the difference between the maximum and minimum resistance of the cell divided by its maximum resistance) and the coefficient of resistance variance, σ/μ, (the ratio between resistance standard deviation σ and resistance mean value μ).
Normally, the write current density required to switch the direction of the free layer magnetization is mainly determined by the free layer magnetic moment, damping ratio and spin-angular momentum deposition rate, which depend on the MTJ film materials and their quality. As the MTJ device is microminiaturized to nanometer scale dimensions, the write current density is unchanged, giving a much smaller write current which is scalable to the shrinking MTJ cell dimensions. Hence, power consumption in the device is reduced.
However, as the MTJ cell dimensions become smaller and smaller, the MTJ resistance variation rapidly increases. For example. using the same MTJ film materials and deposition processes, the coefficient of MTJ resistance variance is found to be inversely proportional to the square root of each MTJ junction area. This makes the reading process very difficult, even impossible, without a great increase in the magneto-resistive ratio dR/R. To address this problem, a spin transfer MRAM structure with a special arrangement of MTJ cells is proposed to reduce the resistance variance.
Various combinations of MTJ cells can be found in the prior art. Huai et al. (U.S. Pat. No. 7,009,877) shows an MTJ element and a spin transfer element arranged vertically. In this invention they use an MTJ and a CPP-GMR immediately connected together to achieve a low write switching current. They also include the combination of two different MTJ cells, one with a smaller dR/R than the other, to achieve a low write switching current.
Hosotani (US Patent Application 2006/0221680) and Ju et al. (US Patent Application 2006/0202244) and Nickel et at (US Patent Application 2005/0195649) all disclose two MTJ elements connected in series and are used to write two bits per cell. In these applications two different MTJ cells are connected together with different anisotropy directions in their layers, obtained either by different shape orientations or different magnetic materials.
Nguyen et al (U.S. Pat. No. 6,992,359) disclose a method for reducing write current density for spin transfer by using a free layer having a high perpendicular magnetic anisotropy. The purpose is to achieve a low write switching current for a spin-transfer MRAM.
None of the above prior art discuss a method for reducing resistance variation, which will help the reading process for a spin-transfer MRAM. To address this problem, we propose a spin-transfer MRAM cell structure with a special arrangement of MTJ elements designed to reduce the resistance covariance. In this arrangement, electric current flows across two or more identical MTJ elements (denoted “sub-cells”) substantially identical in structure to the MTJ element shown in FIG. 2. The configuration will be described in greater detail with reference to FIGS. 3a and 3b below.
During the writing process, the required critical current for switching the magnetization direction of an MTJ free layer is the same for all MTJ sub-cells connected in series. Thus, the same size current-supplying local transistor is required as would be needed for writing a single MTJ element. During the reading process, if each individual sub-cell has a mean resistance value Rp and if the resistance values are distributed with a standard deviation σ, the mean value of the total resistance in one MTJ MRAM cell unit containing N sub-cells is the product (N)(Rp), while the standard deviation of total resistance of many such MRAM units is the product σ(N)1/2. Therefore, given that the coefficient of resistance of one MTJ sub-cell is σ/Rp, the coefficient of resistance among the MTJ MRAM units is the product (N)−1/2 (σ/Rp). In other words, the coefficient of resistance is reduced by a factor of (N)1/2, yielding a greatly increased reading margin.