The present invention generally relates to magnetic random access memory devices and more particularly to the magnetic tunnel junction stack of spin-transfer based MRAM devices.
Magnetic random access memory (MRAM) is a nonvolatile memory technology that uses magnetization to represent stored data, in contrast to older RAM technologies that use electronic charges to store data. One primary benefit of MRAM is that it retains the stored data in the absence of electricity, i.e., it is a nonvolatile memory. Generally, MRAM includes a large number of magnetic cells formed on a semiconductor substrate, where each cell represents one data bit. A bit is written to a cell by changing the magnetization direction of a magnetic element within the cell, and a bit is read by measuring the resistance of the cell (low resistance typically represents a “0” bit and high resistance typically represents a “1” bit).
Magnetoresistive random access memories (MRAMs) combine magnetic components to achieve non-volatility, high-speed operation, and excellent read/write endurance. In a standard MRAM device 100, such as that illustrated in
The traditional MRAM switching technique depicted in
In spin-transfer MRAM (ST-MRAM) devices, such as that shown in
ST-MRAM virtually eliminates the problem of bit disturbs, results in improved data retention, and enables higher density and lower power operation for future MRAM. Since the current passes directly through the MTJ stack, the main requirements of the magnetic tunneling barrier for ST-MRAM include: low resistance-area product (RA), high magnetoresistance (MR), and high breakdown voltage. Moderate to low magnetization and low magnetic damping of the free layer is required for the devices to have low switching current density. MTJ material with MgO tunnel barriers and CoFeB (CFB) free layers are used in ST-MRAM as these result in very high MR at low RA. However, to obtain very high MR with MgO, the devices typically have to be annealed at temperatures as high as 350° C. or above. However, as the annealing temperature of the tunneling barrier increases, the breakdown voltage decreases. At temperatures above 325° C., the breakdown voltage of the tunneling barrier degrades significantly. This results in an unfavorable ratio of the required critical voltage (Vc) for switching to the breakdown voltage of the tunnel barrier layer (Vbd), which results in a high proportion of devices not able to switch before breaking down. Also, the damping of the free layer, which determines the rate of energy loss from a precessing magnetic moment to the lattice, increases as the anneal temperature of the material increases. This may be a result of metal diffusion at high temperatures, typically from the top electrode covering the free layer. The increased damping results in high switching currents. Therefore, although very high MR can be obtained by annealing the MTJ stack at higher temperatures, the overall performance of the device decreases.
Accordingly, it is desirable to provide a structure and method for obtaining high MR while maintaining low damping and a favorable low Vc/Vbd ratio. Furthermore, other desirable features and characteristics of the present invention will become apparent from the subsequent detailed description and the appended claims, taken in conjunction with the accompanying drawings and this background.
Embodiments of the present invention will hereinafter be described in conjunction with the following drawing figures, wherein like numerals denote like elements, and
The following detailed description is merely exemplary in nature and is not intended to limit the invention or the application and uses of the invention. Furthermore, there is no intention to be bound by any theory presented in the preceding background or the following detailed description.
A magnetic tunnel junction (MTJ) structure includes an MgO tunnel barrier and a CoFeB free layer that require a low temperature anneal, for example, less than 300° C., but results in high magnetoresistance (MR), low damping and an improved ratio Vc/Vbd of critical switching voltage to tunnel barrier breakdown voltage for improved spin torque yield and reliability. By adding very thin layers of pure Fe at the interface between the MgO tunneling barrier and the CoFeB free layer and using annealing temperatures of less than 300° C., higher MR values than the conventional stack and process (no Fe and 350° C. anneal) are obtained. This improved structure also has a very low resistance-area product (RA) MgON diffusion barrier between the CoFeB free layer and a top Ta electrode or cap to prevent the diffusion of Ta into CoFeB, which assists in keeping the damping, and therefore also the switching voltage low. With the annealing temperature below 300° C., the breakdown voltage is high, thus resulting in a favorable ratio of Vc/Vbd and in a high proportion of devices switching before breakdown, therefore improving the yield and reliability of the devices.
Though the exemplary embodiment of the MTJ is described with reference to spin-transfer MRAM (ST-MRAM), it may also be used in toggle-MRAM and magnetic sensors.
First and second conductor 302, 314 are formed from any suitable material capable of conducting electricity. For example, conductors 302, 314 may be formed from at least one of the elements Al, Cu, Au, Ag, Ta or their combinations.
In the illustrated embodiment, fixed magnet element 304 is located between insulator 306 and electrode 302. Fixed magnet element 304 has a fixed magnetization that is either parallel or anti-parallel to the magnetization of free magnetic element 310. In the practical embodiment, fixed magnet element 304 includes a template or seed layer 316 formed on the electrode 302 for facilitating the formation of a pinning layer 318, for example IrMn, PtMn, FeMn, thereon. The template/seed layer 316 is preferably a non magnetic material, for example Ta, Al, Ru, but can also be a magnetic material, for example NiFe, CoFe. The pinning layer 318 determines the orientation of a magnetic moment of the pinned layer 320 formed thereon. The fixed layer 324 is formed on the spacer layer 322. The pinned magnetic layer 320 and fixed magnetic layer 324 have anti-parallel magnetizations, and may be formed from any suitable magnetic material, such as at least one of the elements Ni, Fe, Co, B, or their alloys as well as so-called half-metallic ferromagnets such as NiMnSb, PtMnSb, Fe3O4, or CrO2. Spacer layer 322 is formed from any suitable nonmagnetic material, including at least one of the elements Ru, Os, Re, Cr, Rh, Cu, or their combinations. Synthetic antiferromagnet structures are known to those skilled in the art and, therefore, their operation will not be described in detail herein.
An insulator layer 306 is formed on the fixed magnetic element 304, and more specifically, on the fixed magnetic element 324. The insulator layer 306 comprises insulator materials such as AlOx, MgOx, RuOx, HfOx, ZrOx, TiOx, or the nitrides and oxidinitrides of these elements like MgON thereon.
In this exemplary embodiment, insulator 306 is located between free magnetic element 310 and fixed magnet element 304. More specifically, insulator 306 is located between free magnetic element 310 and fixed magnetic layer 324. Insulator 306 is formed from any suitable material that can function as an electrical insulator. For example, insulator 306 may be formed preferably from MgO, or from a material such as oxides or nitrides of at least one of Al, Si, Hf, Sr, Zr, Ru or Ti. For purposes of MRAM cell 300, insulator 306 serves as a magnetic tunnel barrier element and the combination of free magnetic element 310, insulator 306, and fixed magnet element 304 form a magnetic tunnel junction.
In the illustrated embodiment, free magnetic element 310 is located between the insulator material 306 and the electrode 314. Free magnetic element 310 is formed from a magnetic material having a variable magnetization. For example, free magnetic element 310 may be formed from at least one of the elements Ni, Fe, Co, B or their alloys as well as so-called half-metallic ferromagnets such as NiMnSb, PtMnSb, Fe3O4, or CrO2. As with conventional MRAM devices, the direction of the variable magnetization of free magnetic element 310 determines whether MRAM cell 300 represents a “1” bit or a “0” bit. In practice, the direction of the magnetization of free magnetic element 310 is either parallel or anti-parallel to the direction of the magnetization of fixed magnet element 324.
Free magnetic element 310 has a magnetic easy axis that defines a natural or “default” orientation of its magnetization. When MRAM cell 300 is in a steady state condition with no current 328 applied (when transistor 329 is not activated), the magnetization of free magnetic element 310 will naturally point along its easy axis. As described in more detail below, MRAM cell 300 is suitably configured to establish a particular easy axis direction for free magnetic element 310. From the perspective of
Electrode 314 serves as the data read conductor for MRAM cell 300. In this regard, data in MRAM cell 300 can be read in accordance with conventional techniques: a small current flows through MRAM cell 300 and electrode 314, and that current is measured to determine whether the resistance of MRAM cell 300 is relatively high or relatively low. The read current is much smaller than the current required to switch the free layer by spin-transfer in order to avoid disturbs caused by reading the cell.
In practice, MRAM cell 300 may employ alternative and/or additional elements, and one or more of the elements depicted in
The spin-transfer effect is known to those skilled in the art. Briefly, a current becomes spin-polarized after the electrons pass through the first magnetic layer in a magnet/non-magnet/magnet trilayer structure, where the first magnetic layer is substantially thicker or has a substantially higher magnetization than the second magnetic layer. The spin-polarized electrons cross the nonmagnetic spacer and then, through conservation of angular momentum, place a torque on the second magnetic layer, which switches the magnetic orientation of the second layer to be parallel to the magnetic orientation of the first layer. If a current of the opposite polarity is applied, the electrons instead pass first through the second magnetic layer. After crossing the nonmagnetic spacer, a torque is applied to the first magnetic layer. However, due to its larger thickness or magnetization, the first magnetic layer does not switch. Simultaneously, a fraction of the electrons will then reflect off the first magnetic layer and travel back across the nonmagnetic spacer before interacting with the second magnetic layer. In this case, the spin-transfer torque acts so as to switch the magnetic orientation of the second layer to be anti-parallel to the magnetic orientation of the first layer.
In accordance with the exemplary embodiment, a thin layer 308 of iron (Fe) is formed between the insulator 306 and the free magnetic element 310. The thickness of the layer 308 may be in the range of 1-5 Å, but preferably is in the range of 2.5 Å-5 Å (see U.S. Pat. No. 7,098,495 assigned to the assignee of the present application regarding high polarization insertion layers). By adding very thin layers of pure Fe at the interface between insulator 306 and the free magnetic element 310 and using annealing temperatures less than 350° C., and preferably less than 300° C., and more preferably at about 265° C., one can obtain MR values higher than the conventional stack and process (no Fe and 350° C. anneal). With the annealing temperatures below 300° C., the breakdown voltages are high, thus resulting in a favorable ratio of Vc/Vbd and in a high proportion of devices switching before breakdown, hence improving the yield and reliability of the devices. Further in accordance with the exemplary embodiment, a diffusion barrier 312 is formed between the free layer 310 and the electrode 314 (see U.S. Pat. No. 6,544,801 assigned to the assignee of the present application regarding diffusion barriers). This diffusion barrier 312 preferably is formed of low RA magnesium oxinitride (MgON) and has a thickness in the range of 8-20 Å, but preferably is in the range of 12 Å-16 Å. The diffusion barrier 312 prevents diffusion of tantalum into the free layer 310, thereby keeping the damping low and reducing critical currents.
While at least one exemplary embodiment has been presented in the foregoing detailed description, it should be appreciated that a vast number of variations exist. It should also be appreciated that the exemplary embodiment or exemplary embodiments are only examples, and are not intended to limit the scope, applicability, or configuration of the invention in any way. Rather, the foregoing detailed description will provide those skilled in the art with a convenient road map for implementing an exemplary embodiment of the invention, it being understood that various changes may be made in the function and arrangement of elements described in an exemplary embodiment without departing from the scope of the invention as set forth in the appended claims.