Multi-state magnetoresistance random access cell with improved memory storage density

A multi-state magnetoresistive random access memory device having a pinned ferromagnetic region with a magnetic moment vector fixed in a preferred direction in the absence of an applied magnetic field, an non-ferromagnetic spacer layer positioned on the pinned ferromagnetic region, and a free ferromagnetic region with an anisotropy designed to provide a free magnetic moment vector within the free ferromagnetic region with N stable positions, wherein N is a whole number greater than two, positioned on the non-ferromagnetic spacer layer. The number N of stable positions can he induced by a shape anisotropy of the free ferromagnetic region wherein each N stable position has a unique resistance value.

FIELD OF THE INVENTION

This invention relates to semiconductor memory devices and, more particularly, the present invention relates to semiconductor random access memory devices that utilize a magnetic field.

BACKGROUND OF THE INVENTION

Traditional semiconductor memory devices store a memory state by storing an electronic charge. However, magnetoresistive random access memory (hereinafter referred to as “MRAM”) devices store a memory state by utilizing the direction of the magnetic moment vector created in a magnetic material or structure. Thus, a memory state in a MRAM device is not maintained by power, but rather by the direction of the magnetic moment vector. To be commercially viable, however, MRAM must 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.

In previous MRAM technology, 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. Thus, a single MRAM device typically stores one bit of information and to increase the memory density, the MRAM device must be scaled laterally to smaller dimensions.

As the bit dimension shrinks, however, three problems occur. First, the switching field increases for a given shape and film thickness, requiring more current to switch. Second, the total switching volume is reduced so that the energy barrier for reversal, which is proportional to volume and switching field, drops. 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 thermal fluctuations if the barrier is too small. Finally, because the switching field is produced by shape, the switching field becomes more sensitive to shape variations as the bit shrinks in size. With photolithography scaling becoming more difficult at smaller dimensions, MRAM devices will have difficulty maintaining tight switching distributions.

Accordingly, it is an object of the present invention to provide a new and improved magnetoresistive random access memory device which can store multiple states.

SUMMARY OF THE INVENTION

To achieve the objects and advantages specified above and others, a multi-state MRAM cell is disclosed. In the preferred embodiment, the multi-state MRAM cell has a resistance and includes a multi-state MRAM device sandwiched therebetween a first conductive line and a base electrode. Further, a second conductive line is positioned proximate to the base electrode. In the preferred embodiment, the multi-state MRAM device includes a pinned synthetic anti-ferromagnetic region positioned adjacent to the base electrode. The pinned synthetic anti-ferromagnetic region includes an anti-ferromagnetic pinning layer and a pinned ferromagnetic layer which has a pinned magnetic moment vector oriented in a preferred direction at a first nonzero angle relative to the first conductive line. Further, a non-ferromagnetic spacer layer is positioned on the pinned synthetic anti-ferromagnetic region.

A free ferromagnetic region is positioned on the non-ferromagnetic spacer layer and adjacent to the second conductive line. The free ferromagnetic region has a free magnetic moment vector that is free to rotate in the presence of an applied magnetic field and, in the preferred embodiment, has a shape designed to allow more than two, e.g. four, stable states, as will be discussed presently.

In the preferred embodiment, the free magnetoresistive region includes a tri-layer structure that includes an anti-ferromagnetic coupling spacer layer sandwiched therebetween two ferromagnetic layers. Further, the purpose of the first conductive line is to act as a bit line and the purpose of the second conductive line is to act as a switch line. These conductive lines supply current pulses to the MRAM device to induce a magnetic field for aligning the free magnetic moment vector in a desired state.

The multiple states of the MRAM device are created by an induced anisotropy within the free ferromagnetic region. In the preferred embodiment, the shape of the free ferromagnetic region is chosen to create multiple magnetic states wherein a first hard axis is oriented parallel with the first conductive region and a second hard axis is oriented parallel with the second conductive region. Consequently, the free magnetic moment vector will not be stable in either of these two directions.

Also, in the preferred embodiment, the shape of the free ferromagnetic region is chosen so that a first easy axis and a second easy axis are both oriented at nonzero angles to the first hard axis, the second hard axis, and the pinned magnetic moment vector. The first easy axis and the second easy axis are also chosen to be oriented at a 90° angle relative to each other. The first easy axis creates a first stable position and a third stable position wherein the first stable position and the third stable position are oriented anti-parallel along the first easy axis. The second easy axis creates a second stable position and a fourth stable position wherein the second stable position and the fourth stable position are oriented anti-parallel along the second easy axis.

Thus, in the preferred embodiment, four stable positions have been created by the shape induced anisotropy of the free ferromagnetic region. However, it will be understood that other methods can be used to create more than two stable positions in the free ferromagnetic region. Further, the resistance of the MRAM device depends on which stable position the free magnetic moment vector is aligned with because each stable position is oriented at a unique angle relative to the pinned magnetic moment vector. Hence, the four stable positions can be measured by measuring the resistance of the MRAM device.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

Turn now toFIG. 1, which illustrates a simplified sectional view of a multi-state MRAM cell5in accordance with the present invention. Multi-state MRAM cell5includes a multi-state MRAM device7sandwiched therebetween a base electrode14and a conductive line36wherein multi-state MRAM device7has a resistance, R. Further, a conductive line12is positioned proximate to base electrode14and an isolation transistor10is electrically connected to base electrode14and an electrical ground13as illustrated.

Multi-state MRAM device7includes a pinned synthetic anti-ferromagnetic region19positioned adjacent to base electrode14. Pinned synthetic anti-ferromagnetic region19includes an anti-ferromagnetic pinning layer16positioned on base electrode14, a pinned ferromagnetic layer17positioned on layer16, an anti-ferromagnetic coupling layer18positioned on layer17, and a fixed ferromagnetic layer22positioned on layer18. Further, fixed ferromagnetic layer22has a fixed magnetic moment vector20oriented in a fixed preferred direction (seeFIG. 3) at a first nonzero angle relative to conductive line36. It will be understood that region19can be substituted by many configurations, including using a single fixed layer, and the use of four layers in this embodiment is for illustrative purposes only.

A non-ferromagnetic spacer layer24with a thickness is positioned on pinned synthetic anti-ferromagnetic region19. It will be understood that non-ferromagnetic spacer layer24can include multiple layers, but is shown as one layer for illustrative purposes. Also, it will be understood that non-ferromagnetic spacer layer24can include a dielectric material, such as aluminum oxide (AlO), wherein multi-state MRAM device7behaves as a tunneling junction device. Layer24is typically thin enough to allow a spin polarized tunneling current to flow between fixed ferromagnetic layer22and ferromagnetic layer28. In other embodiments, non-ferromagnetic spacer layer24can include a conductive material, such as copper (Cu), wherein multi-state MRAM device7behaves as a giant magnetoresistive device. In general, however, non-ferromagnetic spacer layer24can include any suitable non-ferromagnetic material which results in a device with a substantial magnetoresistive ratio.

A free ferromagnetic region26is positioned on non-ferromagnetic spacer layer24and adjacent to conductive line36. Ferromagnetic layer28has a free magnetic moment vector30that is free to rotate in the presence of an applied magnetic field. In the preferred embodiment, free ferromagnetic region26is designed to provide free magnetic moment vector30with more than two stable positions, as will be discussed presently. In the preferred embodiment, free magnetoresistive region26includes a tri-layer structure that includes an anti-ferromagnetic coupling spacer layer32sandwiched therebetween a ferromagnetic layer28and a ferromagnetic layer34.

It will be understood that free magnetoresistive region26can include more than three layers and that the use of three layers in this embodiment is for illustrative purposes only. For example, a five-layer stack of a ferromagnetic layer/anti-ferromagnetic coupling spacer layer/ferromagnetic layer/anti-ferromagnetic coupling spacer layer/ferromagnetic layer could also be used. Further, it will be understood that region26could include less than three layers such as a single free ferromagnetic layer.

It will also be understood that the free ferromagnetic layer26and the pinned synthetic anti-ferromagnetic layer19can have their positions exchanged in multi-state MRAM device7(SeeFIG. 5). The free ferromagnetic layer26would be positioned on the base electrode14, the non-ferromagnetic spacer layer24would be positioned on the free ferromagnetic layer26, and the pinned synthetic anti-ferromagnetic layer19would be positioned between the non-ferromagnetic spacer layer24and the conductive line36.

In the preferred embodiment, pinned ferromagnetic layer22and ferromagnetic layer28have a split band structure with respect to electron spin that causes polarization of the conduction band electrons. In general, any material with a split band structure resulting in spin polarization of the conduction band electrons can be included in layers22and28. The spin polarization of these materials results in a device structure that depends on the relative orientation of magnetic moment vectors20and30. Further, in some embodiments, at least one of fixed ferromagnetic layer22and free ferromagnetic layer28can include half-metallic materials. It is well known to those skilled in the art that half-metallic materials ideally have 100% spin polarization, but in practice typically have at least 80% spin polarization. The use of half-metallic materials greatly increases a signal-to-noise ratio of multi-state MRAM cell5.

Turn now toFIG. 2which illustrates a simplified plan view of multi-state MRAM cell5. To simplify the description of the operation of multi-state MRAM device7, all directions will be referenced to an x- and y-coordinate system40as shown. In coordinate system40, an angle θ is defined to be 0° along the positive x-axis, 90° along the positive y-axis, 180° along the negative x-axis, and 270° along the negative y-axis.

Further, in the preferred embodiment, a bit current, IB, is defined as being positive if flowing in the positive x-direction and a switch current, IS, is defined as being positive if flowing in the positive y-direction. The purpose of conductive line12and conductive line36is to create a magnetic field that acts upon multi-state MRAM device7. Positive bit current, IB, will induce a circumferential bit magnetic field, HB, and positive switch current, IS, will induce a circumferential switch magnetic field, HS. Since conductive line36is above multi-state MRAM device7, in the plane of the element, HBwill be applied to multi-state MRAM device7in the positive y-direction for positive bit current, IB. Similarly, since conductive line12is positioned below multi-state MRAM device7, in the plane of the element, HSwill be applied to multi-state MRAM device7in the positive x-direction for positive switch current, IS.

It will be understood that the definitions for positive and negative current flow are arbitrary and are defined here for illustrative purposes and convenience. The effect of reversing the current flow is to change the direction of the magnetic field induced within multi-state MRAM device7. The behavior of a current induced magnetic field is well known to those skilled in the art and will not be elaborated upon further here.

As discussed previously, free ferromagnetic region26is designed to provide free magnetic moment vector30with more than two stable positions. One method to create more than two stable positions is to manipulate the shape of free ferromagnetic region26. For example, in one embodiment, free ferromagnetic region26can have a shape that is defined by a polar equation given as r(θ)=1+|A·cos(N·θ−α)|, wherein N is approximately half the number of stable positions and is a whole number greater than one, θ is an angle relative to conductive lines12and36, r is a distance in polar coordinates that is a function of the angle θ in degrees, α is an angle in degrees that determines the angle of the lobes with respect to conductive line12, and A is a constant. The angle θ has continuous values in the range between 0° and 360° and, in this embodiment, the constant A has a value between 0.1 and 2.0.

In the preferred embodiment, the equation for polar function r(θ) is chosen so that at least one lobe is oriented parallel with conductive line12(i.e. θ=90°). It will be understood that polar function r(θ) traces out the outer edge of free ferromagnetic region26to illustrate the basic shape of the region and not the dimensions. Also, it will be understood that other equations are possible to describe the basic shape and that other shapes are possible to induce a shape anisotropy that creates more than two stable states.

In general, however, other methods of inducing an anisotropy could be used alone or in combination with shape anisotropy. For example, an intrinsic anisotropy of a magnetic material included in free ferromagnetic region26, generally thought to arise from atomic-level pair ordering, can be used. Also, the direction of the intrinsic anisotropy can be set by applying a magnetic field during deposition of free ferromagnetic region26or during a post deposition anneal. A magnetocrystalline anisotropy of the magnetic material included in free ferromagnetic region26could also be used by growing a magnetic material with a preferred crystalline orientation. Further, an anisotropy induced by certain anisotropic film growth methods could also be used to induce an anisotropy wherein the induced anisotropy is thought to originate from a shape asymmetry of the growing clusters or crystallites.

However, it will be understood that in the preferred embodiment, the anisotropy of free ferromagnetic region26is created by a shape anisotropy. A shape anisotropy is used in the preferred embodiment for illustrative purposes only and it will be understood that other methods are available to create an anisotropy, and, consequently, more than two stable states in free ferromagnetic region26.

In the preferred embodiment, it is assumed that multi-state MRAM cell5illustrated inFIG. 2has four stable states wherein N is equal to four, A is equal to 0.5, and α is equal to 180°, and that the four stable states are created by the shape anisotropy of free ferromagnetic region26. Further, it is assumed that the shape of free magnetoresistive region26induces an easy axis44and an easy axis42which are oriented at a nonzero angle relative to one another, wherein the nonzero angle is 90° in the preferred embodiment. Further, easy axis44and easy axis42are oriented at a nonzero angle relative to pinned magnetic moment vector20(not shown), conductive line36, and conductive line12.

Also in the preferred embodiment, it is assumed that the shape of free magnetoresistive region26induces a hard axis46and a hard axis48wherein hard axis46is oriented parallel to conductive line36and hard axis48is oriented parallel to conductive line12. It will be understood that the magnetization directions of the stable states may be complex, with the magnetization bending or curling to minimize its energy, but it is assumed for illustrative purposes that the magnetization directions in this embodiment are oriented 90° apart along easy axes42and44, as discussed above.

Further, it will be understood that the magnetization is not generally uniform in the same direction over the area of the bit, but is assumed to be uniform in this embodiment for simplicity. Thus, for simplicity, the easy axis is defined as being an axis which is oriented with a center of MRAM cell5when the magnetic moment vector is in a stable rest state.

In this embodiment, easy axis44creates a stable position50and a stable position56and easy axis42creates a stable position52and a stable position54. Hence, stable position54is oriented 90° relative to stable position56, stable position50is oriented 180° relative to stable position56, and stable position52is oriented 270° relative to stable position56. Thus, in this embodiment, four stable positions have been created in free magnetoresistive region26for free magnetic moment vector30to align with. It will be understood that the angles between adjacent stable positions are chosen to be 90° for illustrative purposes and other angles could be chosen.

The relationships between free magnetic moment vector30and the resistance are illustrated in graph81inFIG. 3where free magnetic moment vector30is shown oriented in stable positions50,52,54, and56along a R-axis. The R-axis is defined as a resistance axis directed anti-parallel with the direction of pinned magnetic moment vector20.

Resistance, R, of multi-state MRAM cell5depends on the position of free magnetic moment vector30relative to pinned magnetic moment vector20. It is well known by those skilled in the art that the resistance of a magnetoresistive device, such as a magnetic tunnel junction or a spin valve device, varies between a minimum value, Rmin, and a maximum value, Rmax, by approximately as the cosine of an angle, φ, between magnetic moment vectors20and30according to the relationship given as

R≈12⁢(Rmin+Rmax)-12⁢(Rmax-Rmin)⁢cos⁡(ϕ).
R is at its maximum value, Rmax, when vectors20and30are anti-parallel (i.e. φ=180°) so that cos(φ))=−1. R is at its minimum value, Rmin, when vectors20and30are parallel (i.e. φ=0°) so that cos(φ))=1. It is further understood by those skilled in the art that if one of layers22or28has an opposite polarization of the conduction electrons then the principles of the device are unchanged, but the sign of the cosine dependence is opposite. This mathematical cosine relationship is shown graphically as the projection of the magnetic moment vector20on the R-axis in graph81.

In the preferred embodiment, R has a value R11when free magnetic moment vector30is in stable position56, a value R01when free magnetic moment vector30is held in stable position54, a value R00when free magnetic moment vector is held in stable position50, and a value R10when free magnetic moment vector is held in stable position52. Further, it will be understood that in this illustration R00<R01<R10<R11.

For example, if free magnetic moment vector30is oriented in stable position50, then multi-state MRAM device7has a resistance value of R00, which is the projection of free magnetic moment vector30in stable position50onto the R-axis. Similarly, the projections of magnetic moment vector30in stable positions52,54, and56have corresponding resistance values of R10, R01, and R11, respectively. Thus, the state of multi-state MRAM device7can be read by measuring its resistance.

The method of writing to multi-state MRAM cell5involves supplying currents in conductive line12and conductive line36such that free magnetic moment vector30can be oriented in one of four stable positions in the preferred embodiment. To fully elucidate the writing method, specific examples describing the time evolution of circumferential bit magnetic field, HB, and circumferential switch magnetic field, HS, are now given.

Turn now toFIG. 4which illustrates a magnetic pulse sequence100. Illustrated are the magnetic pulse sequences used to write multi-state MRAM cell5to various states. The writing method involves applying current pulses in conductive line12and conductive line36to rotate free magnetic moment vector30in a direction parallel to one of the four stable positions in the preferred embodiment. The current pulses are then turned off so that free magnetic moment vector30is aligned in one of the four stable positions.

It will be understood that for a memory array one can arrange a plurality of MRAM cells on a grid with lines12and36electrically connected to an array of cells arranged in rows and columns to form a cross point array. With multi-state MRAM cells, as with conventional two state MRAM cells, the bits ideally must not switch when exposed to a field generated by a single conductive line. It is desired to have only the bit which is at the cross-point of two active conductive lines be switched.

In the preferred embodiment, MRAM cell5requires a much higher magnetic field to switch 180° compared to 90°. The currents used for writing are designed to generate magnetic fields that are above the threshold for 90° switching but below the threshold required for 180° switching. In this way a sequence of current pulses can be applied to the lines that will switch only the MRAM cell at the cross-point and will move its magnetic moment vector in a sequence of 0° and/or 90° switches until it reaches the desired final state regardless of the initial state.

InFIG. 4, for example, the longer pulses of HBdetermine if the final state will be in the negative or positive y-direction while the bipolar pulse of HSdetermines if the final state will be in the positive or negative x-direction. Due to the symmetry of MRAM cell5, an equivalent writing scheme can be constructed by reversing the roles of HSand HBso that HSpulses are long and HBpulses are short and bipolar. If δ is the duration of a short pulse, δ=ti+1−t1, then the total duration of the write cycle shown inFIG. 4is 4·δ. Since the active part of the cycle for any given bit is only the part of the cycle with the bipolar pulses on HS, the same result could be obtained with slightly different write circuitry that only executes the active part of the cycle needed to write the desired state. Thus the total cycle time can be reduced to 2·δ.

It is understood by those skilled in the art that the current pulses in a circuit may have variations in shape and duration, such as finite rise time, overshoot, and finite separation between pulses and therefore may not appear exactly as illustrated inFIG. 4.

One principle of operation in writing to MRAM cell5is having a single long pulse on one conductive line coincident with a bipolar pulse on another conductive line to move the magnetic moment of the MRAM bit to the desired state. In particular, a finite separation between the current pulses may be desirable so that the magnetic state of the free layer moves to an equilibrium state before the beginning of the next current pulse.

For example, by using a magnetic pulse sequence102for HSand a magnetic pulse sequence112for HB, multi-state MRAM cell is written into a ‘11’ state. In particular, at a time t0, HBis pulsed to a negative value while HSis zero. Since only one write line is on, the magnetic moment of the MRAM bit will not change states.

At a time t2, HBis pulsed to a positive value while HSis pulsed to a negative value. If the initial state was in direction56or50, then this will cause a 90 rotation of the magnetic moment direction54. If the initial state is54or52, then there will be no rotation. At a time t3when the negative HSpulse is complete, the bit can only be oriented with its magnetic moment substantially along axis42inFIG. 2. At time t3, HBis kept at its positive value while HSis pulsed to a positive value. This has the effect of orienting free magnetic moment vector30toward direction56. At a time t4, HBand HSare both made zero so a magnetic field force is not acting upon free magnetic moment vector30and the moment settles into equilibrium position56. It will be understood that in this illustration t0<t1<t2<t3<t4.

Consequently, free magnetic moment vector30will become oriented in the nearest stable position to minimize the anisotropy energy, which in this case is stable position56. As discussed previously and illustrated graphically inFIG. 3, stable position56is defined as the ‘11’ state. Hence, multi-state MRAM cell5has been programmed to store a ‘11’ by using magnetic pulse sequences102and112. Similarly, multi-state MRAM cell5can be programmed to store a ‘10’ by using magnetic pulse sequences104and112, to store a ‘01’ by using magnetic pulse sequences106and112, and to store a ‘00’ by using magnetic pulse sequences108and112. It will be understood that the states used in this illustration are arbitrary and could be otherwise defined.

Thus, multi-state MRAM cell5can be programmed to store multiple states without decreasing the dimensions of multi-state MRAM cell5. Consequently, in the preferred embodiment, the memory storage density is increased by a factor of two. Also, a writing method has been demonstrated in the preferred embodiment so that one of four possible states can be stored in the MRAM device regardless of the initial stored state.