Patent Application
20070278547
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 of the invention 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 of the invention or the following detailed description of the invention.
The exemplary embodiment described herein is a SAF structure exhibiting a well-defined high Hflop (flop field) using a combination of high Hk (uniaxial anisotropy), high Hsat (saturation field), and ideal soft magnetic properties exhibiting well-defined easy and hard axes. The exemplary embodiment comprises an amorphous Cobalt-Iron-Boron-based (CoFeB) SAF grown on Tantalum (Ta), with a Cobalt-Iron (CoFe) insertion layer at the bottom interface of the spacer to boost Hsat. This structure has been shown to have the combination of sufficiently high Hsat and well-defined high Hk with the desired soft magnetic behavior. This exemplary embodiment eliminates the need for a pinning layer in MRAM devices.
CoFe-based alloys are generally known to exhibit high saturation fields; however, other alloy SAFs exhibiting desirable properties as described herein can be used as well. A well-defined and high flop field is an essential requirement in addition to a high saturation field. The first step is to grow soft/well-behaved (well-defined easy and hard axis, low coercivity and high Hk) single layer films of CoFe/CoFe-based alloys. Films with these above mentioned desirable properties are achieved when grown on a Ta seed layer. Using the above mentioned seed, soft CoFe-based films (CoFe, CoFeB with different Boron compositions) with coercivities<5.0 Oe and Hk>30.0 Oe are achievable.
SAFs grown with these alloys exhibit high Hsat. CoFe provides the highest Hsat; however, at or near balance these SAFs did not exhibit a well-defined Hflop. The flop field shows up in a magnetization-versus-field (M-H) loop as a sharp slope change from a flat region at low field to a steep slope when the SAF moves approximately 90 degrees to the applied field. With the CoFe SAFs, the flat-region disappears, indicating changes in the magnetization direction, something that broadens the free layer transition. Amorphous CoFeB SAFs, on the other hand, retain the flat, well-defined flop at balance. When the boron composition in the CoFeB is decreased too much, the alloy becomes crystalline and the flat region is no longer well defined, i.e., crystalline CoFeB behaves similar to CoFe and is therefore not suitable. These SAFs, which have no well-defined flop, are not rigid and will move when a field is applied, though the saturation fields are high. Amorphous CoFeB (B>9%) with a thin CoFe at the bottom interface to boost Hsat exhibits desirable properties. Hsat preferably is greater than 1500 Oe. These SAFs exhibit similar MR as the ones with pinned SAF. Having CoFe at one interface is desirable for increasing Hsat, but introducing CoFe on both the interfaces caused the disappearance of the flop region similar to that observed in an all CoFe or crystalline CoFeB SAFs.
MRAM bit 10 is sandwiched between a bit line 20 and a digit line 30. Bit line 20 and digit line 30 include conductive material such that a current can be passed there through. In this illustration, bit line 20 is positioned on top of MRAM bit 10 and digit line 30 is positioned on the bottom of MRAM bit 10 and is directed at a 90° angle to bit line 20 as shown in
MRAM bit 10 includes a free magnetic region 15, a tunneling barrier 16, and a fixed magnetic region 17, wherein tunneling barrier 16 is sandwiched between free magnetic region 15 and fixed magnetic region 17. In this exemplary embodiment, free magnetic region 15 includes a tri-layer structure 18, which has an anti-ferromagnetic coupling spacer layer 65 sandwiched between two ferromagnetic layers 45 and 55. Anti-ferromagnetic coupling spacer layer 65 has a thickness 86 and ferromagnetic layers 45 and 55 have thicknesses 41 and 51, respectively. Further, fixed magnetic region 17 has a tri-layer structure 19, which has an anti-ferromagnetic coupling spacer layer 66 sandwiched between two ferromagnetic layers 46 and 58. The fixed magnetic region 17 is formed on a suitable seed layer 54 such as Tantalum. The magnetic bit 10 contacts the bit line 30 and digit line 20 through bottom and top electrodes (not shown). Anti-ferromagnetic coupling spacer layer 66 has a thickness 87 and ferromagnetic layers 46 and 58 have thicknesses 42 and 52, respectively.
Generally, anti-ferromagnetic coupling spacer layers 65 and 66 include at least one of the elements Ruthenium, Rhodium, Chromium, Vanadium, Molybdenum, for example, or combinations thereof and alloys of these such as Ruthenium-Tantalum. Further, ferromagnetic layers 45 and 55 include at least one of elements Nickel, Iron, Cobalt, or combinations thereof. In accordance with the exemplary embodiment, ferromagnetic layers 46 and 58 comprise Cobalt-Iron-Boron. Also, it will be understood that magnetic regions 15 and 17 can include material structures other than tri-layer structures and the use of tri-layer structures in this embodiment is for illustrative purposes only. For example, one such material structure could include a five-layer stack of a ferromagnetic layer/anti-ferromagnetic coupling spacer layer/ferromagnetic layer/anti-ferromagnetic coupling spacer layer/ferromagnetic layer structure. Another exemplary material structure for layer 15 would include a single ferromagnetic layer.
Ferromagnetic layers 45 and 55 each have a magnetic moment vector 57 and 53, respectively, that are usually held anti-parallel by coupling of the anti-ferromagnetic coupling spacer layer 65. Also, in some embodiments, the fixed magnetic region 17 has a resultant magnetic moment vector 50. The resultant magnetic moment vector 50 is oriented along an anisotropy easy-axis in a direction that is at an angle, for example between 30° to 60°, but preferably 45°, from bit line 20 and digit line 30 (see
The MRAM bit 10 does not have a pinning layer positioned contiguous to and for influencing the vector 50 within the fixed magnetic region 17. This is accomplished by fabricating the ferromagnetic layers 46 and 58 so that they exhibit a well-defined and sufficiently-high Hflop; using a combination of high Hk (uniaxial anisotropy), high Hsat (saturation field), and ideal soft magnetic properties exhibiting well-defined easy and hard axes.
The fixed SAF with no pinning layer provides several advantages over the conventional pinned SAFs. Some of these advantages are: (a) Simplified MTJ stack, (b) reduced cost (PtMn, pinning material typically used is the most expensive part of the MTJ stack), (c) an anneal step that is optimized for the MTJ stack rather than the pinning material (which is usually a higher temperature and for a longer time) and (d) reduced Mn diffusion related problems.
The exemplary embodiment comprises an amorphous Cobalt-Iron-Boron-based fixed SAF with different Boron compositions, grown on Tantalum, with a CoFe insertion layer 49 at the bottom interface of the spacer to boost Hsat. The CoFe-based films preferably comprise coercivities of less than 5.0 Oe and Hk greater than 10 Oe and preferably greater than 15 Oe. This structure has been shown to have the combination of sufficiently high Hsat and well-defined high Hk with the desired soft magnetic behavior.
While anti-ferromagnetic coupling layers 65 and 66 are illustrated between the two ferromagnetic layers in each tri-layer structure 18 and 19, it will be understood that the ferromagnetic layers 45, 55, 46, 58 could be anti-ferromagnetically coupled through other means, such as magnetostatic fields or other features. For example, when the aspect ratio of a cell is reduced to five or less, the ferromagnetic layers are anti-parallel coupled from magnetostatic flux closure.
MRAM bit 10 has tri-layer structures 18 that have a length/width ratio in a range of 1 to 5 for a non-circular plan. It will be understood that MRAM bit 10 can have various shapes, such as square, elliptical, rectangular, or diamond, but is illustrated in
Further, during fabrication of MRAM array 3, each succeeding layer (i.e. 30, 54, 58, 66, 46, 16, 55, 65, etc.) is deposited or otherwise formed in sequence and each MRAM bit 10 may be defined by selective deposition, photolithography processing, etching, etc. in any of the techniques known in the semiconductor industry. During deposition of at least the ferromagnetic layers 45 and 55, a magnetic field may be provided to set a preferred easy magnetic axis for this pair (induced anisotropy). Similarly, a strong magnetic field applied during the post-deposition anneal step may induce a preferred easy axis. In addition, the bits are typically patterned to be longer in one direction, resulting in a shape anisotropy that favors the long axis of the bit.
It is desirable to have the magnetic vector within the ferromagnetic layer 46 pointing in a particular direction. However, in case of a balanced SAF, this is difficult because the symmetric nature of the structure means it is equally likely for the structure to end up in either of the two stable states after the high-field anneal process step. The measured resistance vs. field (R-H) loops for perfectly balanced SAFs illustrate that, since some curves have increasing resistance as the external field H is turned up and others have decreasing resistance with increasing H, some of the bottom SAFs set in one direction and some in the other.
This is controllable by introducing symmetry breakers in the reference SAF. A slightly magnetically imbalanced structure makes the fixed SAFs fall into a preferred state when removed from the high-field used for the high-field anneal step. An imbalance may be accomplished by making one layer slightly thicker than the other, generating a net magnetic moment 50 for the trilayer structure 10. A small amount of imbalance is sufficient to make the fixed layer in all the bits to go to the same direction. Too much imbalance in the SAF is not preferred due to various reasons. If the imbalance is too high, then we have a magnetic field acting on the bits from the imbalanced layer which influences device characteristics, which is undesirable. Also, too much of an imbalance will change flop behavior of the SAF. For best operation, the imbalance should be between 0.5% and 10%.
Another structure involving symmetry breaking without imbalancing the moment comprises the fixed magnetic region 17 (SAF) where the intrinsic Hk is different for the two ferromagnetic layers 46 and 58 (see
Yet another symmetry breaking structure for the fixed magnetic region 17 comprises a triple SAF shown in
While at least one exemplary embodiment has been presented in the foregoing detailed description of the invention, 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.
