US 20050260772 A1
An MTJ (magnetic tunneling junction) device particularly suitable for use as an MRAM (magnetic random access memory) or a tunneling magnetoresistive (TMR) read sensor, is formed on a seed layer which allows the tunneling barrier layer to be ultra-thin, smooth, and to have a high breakdown voltage. The seed layer is a layer of NiCr which is formed on a sputter-etched layer of Ta. The tunneling barrier layer for the MRAM is formed from a thin layer of Al which is radically oxidized (ROX), in-situ, to form the layer with characteristics described above. The tunneling barrier layer for the read sensor formed from a thin layer of Al or a HfAl bilayer which is naturally oxidized (NOX), in-situ, to form the barrier layer. The resulting device has generally improved performance characteristics in terms of GMR ratio and junction resistance.
11. A method of forming a magnetic tunneling junction (MTJ) MRAM device with an ultra-thin tunneling barrier layer of high smoothness and breakdown voltage comprising:
providing a substrate having a substantially planar upper surface;
forming a first NiCr seed layer on said substrate;
forming a non-magnetic metal layer on said seed layer;
forming a Ta overlayer on said metal layer and sputter-etching said overlayer;
forming a second NiCr seed layer on said sputter-etched Ta overlayer;
forming an AFM pinning layer on said seed layer;
forming a pinned layer on said pinning layer;
forming a layer of Al on said pinned layer; and
oxidizing said Al layer in a plasma oxidation chamber, by a process of radical oxidation, to form a tunneling barrier layer on said pinned layer, said tunneling barrier layer being ultra-thin, smooth and having a high breakdown voltage as a result of the NiCr seed layer formed on the sputter-etched Ta overlayer; and
forming a free layer on said tunneling barrier layer;
forming an upper capping layer on said free layer.
12. The method of
13. The method of
14. The method of
15. The method of
16. The method of
forming a first ferromagnetic layer, which is a layer of CoFe(10%), to a thickness between approximately 15 and 25 angstroms;
forming a coupling layer of Ru, to a thickness between approximately 7 and 8 angstroms on said first layer;
forming a second ferromagnetic layer, which is a layer of CoFe(25%) or CoFe(50%), to a thickness between approximately 10 and 20 angstroms;
magnetically coupling the two CoFe layers with antiparallel magnetizations.
17. The method of
18. The method of
placing the Al layer into a plasma oxidation chamber that includes an upper electrode, a lower electrode and a grid positioned between said electrodes;
placing said Al layer in contact with the lower electrode;
feeding the upper electrode within the chamber with 0.5 liters of oxygen gas while providing power to the upper electrode at a rate of between 500 and 800 watts to produce a shower of oxygen radicals through said grid which impinge on said Al layer.
19. The method of
20. The method of
30. A method of forming a tunneling magnetoresistive (TMR) read head with an ultra-thin tunneling barrier layer of high smoothness and breakdown voltage comprising:
providing a substrate, which is an NiFe lower shield and conducting lead layer having a substantially planar upper surface;
forming a Ta overlayer on said substrate;
sputter-etching said Ta overlayer, reducing it in thickness and rendering its upper surface smooth and amorphous;
forming an NiCr seed layer on said sputter-etched Ta overlayer;
forming an AFM pinning layer on said seed layer;
forming a synthetic pinned layer on said pinning layer;
forming a naturally oxidized tunneling barrier layer on said pinned layer, said tunneling barrier layer being smooth and homogeneous as a result of being formed on said Ta sputter-etched overlayer and said NiCr seed layer;
forming a free layer on said tunneling barrier layer;
forming an upper capping layer on said free layer; and
forming an NiFe upper shield and conducting lead layer on said capping layer.
31. The method of
32. The method of
33. The method of
34. The method of
35. The method of
forming a first ferromagnetic layer of CoFe(10%) formed to a thickness between approximately 20 and 25 angstroms;
forming a coupling layer of Ru on said first ferromagnetic layer, to a thickness between approximately 7 and 8 angstroms forming a second ferromagnetic layer of CoFe(50%) on said Ru layer, to a thickness between approximately 25 and 30 angstroms.
36. The method of
forming a layer of Al or a bilayer of HfAl on said pinned layer;
placing said formation into an oxidation chamber;
purging said chamber with oxygen gas at approximately 75 millitorr pressure;
leaving the fabrication in the chamber for approximately 15 minutes.
37. The method of
38. The method of
This application is related to Docket No. HT 02-019, Ser. No. 10/371,841, filing date Feb. 20, 2003, Docket No. HT 03-016, Ser. No. 10/820,391, filing date Apr. 8, 2004, and Docket No. HT 02-032, Ser. No. 10/768,917, filing date Jan. 30, 2004, assigned to the same assignee as the current invention.
1. Field of the Invention
This invention relates generally to magnetic tunneling junction (MTJ) devices such as MRAMs and read-heads and more particularly to the use of a novel seed layer that allows the formation of a junction layer of superior physical properties.
2. Description of the Related Art
The magnetic tunneling junction device (MTJ device) is essentially a variable resistor in which the relative orientation of magnetic fields in an upper and lower very thin dielectric layer (the tunneling barrier layer) formed between those electrodes. As electrons pass through the upper electrode they are spin polarized by its magnetization direction. The probability of an electron tunneling through the intervening tunneling barrier layer then depends on the magnetization direction of the lower electrode. Because the tunneling probability is spin dependent, the current depends upon the relative orientation of the magnetizations of magnetic layers above and below the barrier layer. Most advantageously, one of the two magnetic layers (the pinned layer) in the MTJ has its magnetization fixed in direction, while the other layer (the free layer) has its magnetization free to move in response to an external stimulus. If the magnetization of the free layer is allowed to move continuously, as when it is acted on by a continuously varying external magnetic field, the device acts as a variable resistor and it can be used as a read-head. If the magnetization of the free layer is restricted to only two orientations relative to the fixed layer (parallel and anti-parallel), the first of which produces a low resistance (high tunneling probability) and the second of which produces a high resistance (low tunneling probability), then the device behaves as a switch, and it can be used for data storage and retrieval (a MRAM).
Magnetic tunneling junction devices are now being utilized as information storage elements in magnetic random access memories (MRAMs). Typically, when used as an information storage or memory device, magnetic fields produced by orthogonally intersecting current carrying lines (digit and bit lines) orient the magnetization of the free layer so that it is either parallel or anti-parallel to the pinned layer; at a later time a sensing current passed through the MTJ indicates if it is in a high (antiparallel) or low (parallel) resistance state.
When used as a read head, (called a TMR read head, or “tunneling magnetoresistive” read head) the free layer magnetization is moved by the influence of the external magnetic fields of a recorded medium, such as is produced by a moving hard disk or tape. As the free layer magnetization varies in direction, a sense current passing between the upper and lower electrodes and tunneling through the barrier layer feels a varying resistance and a varying voltage appears across the electrodes. This voltage, in turn, is interpreted by external circuitry and converted into a representation of the information stored in the medium.
Whether it is used as an MRAM or as a TMR read head, fabrication of a high quality MTJ device presents considerable difficulties due to the necessity of forming layers of extreme thinness. Sun et al. (U.S. Pat. No. 6,574,079) provides a particularly well written statement of some of these difficulties. First, to obtain effective spin polarization of the conduction electrons, the magnetization of the electrode layers must be strong. This is itself a problem, since the layers are exceptionally thin. Second, the resistance of the tunneling barrier layer is typically high, which results in a poor ratio of signal-to-noise (S/N) in read head applications. If the resistance of the barrier layer is lowered by excessively thinning that layer, then fabrication processes such as lapping the air-bearing surface of the read head can create shorts through the barrier layer. Sun et al. teach the formation of a thin barrier layer within a general configuration of the following form:
Applicants have discovered other recent prior art (commercially produced) TMR read head configurations analogous to that taught by Sun et al., including:
Applicants have found that a tunneling junction with improved performance over those of the prior art, particularly those cited above, can be made using a novel seed layer and method of forming it. The tunneling junction so formed, which can also be used in either a TMR configuration or an MRAM configuration, is capable of producing a junction resistance of RA≅1 Ωμm2 and a GMR ratio, DR/R>10% and a dielectric breakdown voltage, Vb>0.5 volts.
A first object of this invention is to provide a method of forming a smooth, ultra thin tunneling barrier layer suitable for incorporation in a TMR read head or an MTJ MRAM.
A second object of this invention is to provide such a tunneling barrier layer with low junction resistance, high GMR ratio and high breakdown voltage which can be used in a read head suitable for reading recordings of high density (>100 Gb/in2).
A third object of this invention is to provide a method of forming an MTJ MRAM element with a very smooth and flat bottom electrode (pinned layer), on which can be formed a tunnel barrier layer having the properties listed above.
A fourth object of the present invention is to provide an MTJ MRAM element having well controlled free layer magnetization, well controlled magnetization of the pinned layer as evidenced by a large exchange field and thermal stability and a tunneling barrier layer with physical integrity.
The objects of the present invention will be achieved in both an TMR read head or an MTJ MRAM configuration by a method that improves the topography (reduces roughness) of the bottom electrode in the MRAM or the pinned layer of the TMR read head and allows the formation of an ultra-thin tunneling barrier layer that meets the requirements set forth above.
An MRAM bottom electrode configuration normally has the form:
To meet the all the objects of this invention, it was decided to use NiCr as a buffer (seed) layer rather than the conventional NiFe and to form it on the Ta layer after a sputter-etch process of the Ta layer. The Ta layer is itself formed on a NiCr/Ru conducting lead layer. It has already been demonstrated in commonly assigned Application HT 02-019, which is fully incorporated herein by reference, that a Ru lead layer formed on a NiCr seed layer is advantageous for the further deposition of smooth layers, so the formation of a sputtered Ta layer on the Ru layer produces a smooth surface also on the Ta. The Ta is then sputter-etched and a second NiCr seed layer is formed upon it. It is the formation of this second NiCr seed layer on the sputter-etched Ta underlayer that allows the objects of this invention to be achieved as follows.
Two differently configured sensor stack configurations for a TMR sensor are used in this invention:
An antiferromagnetic pinning layer, symbolized AFM, is deposited on the NiCr. The AFM can be MnPt 150 angstroms thick (or IrMn 90 angstroms thick). A synthetic antiferromagnetic pinned layer (SyAP) is formed on the AFM, in this case the SyAP is CoFe(10%)/Ru7.5/CoFe(50%), or CoFe(10%)/Ru7.5/CoFe(25%), where the numbers in percents (%) refer to atom percentages of Fe in the CoFe alloy. A layer of aluminum (Al), 5.75 angstroms thick is then deposited on the SyAP and is oxidized by a process of in-situ natural oxidation (NOX) to form a tunnel barrier layer. It is to be noted that oxidation of such a thin Al layer is most advantageously done by NOX, whereas oxidation of thicker layer, such as 7-12 angstroms of Al, is more advantageously done by radical oxidation (ROX), which is fully explained in related Application HT 03-016 which is fully incorporated herein by reference. As will be further discussed, the 5.75 angstrom Al layer is a double atomic layer of Al. This NOX process is taught in commonly assigned Application HT 02-032, fully incorporated herein by reference. Note that the difference between configurations (A) and (B) is that in (B) the NOX barrier layer is a bilayer of Al 4.5-Hf 1.5. A free layer of CoFe(10%)-NiFe(18%) is then formed over the naturally oxidized aluminum (or aluminum-hafnium) and a second Ta capping layer is formed over the free layer.
The importance of the sputter-etched Ta layer on which is grown the NiCr seed layer in achieving the objects of this invention (high GMR ratio, low junction resistance, high breakdown voltage) is shown by the results of experiments we performed. In these experiments we compared prior art methods of forming magnetic tunnel junction (MTJ) MRAM devices with the method of the present invention. The prior art methods had already been shown to provide very high quality performance, but the method of the present invention provided improvements in all areas. In this experiment we formed four different bottom electrodes for an MRAM device and formed identical ROX aluminum barrier layers of initial 10 angstrom thickness. The four configurations are shown in the following table and are compared in terms of their GMR ratio (DR/R), junction resistance, RA, and breakdown voltage, Vb.
Structures #1,2 and 3 are all formed on a sputter-etched (SE) Ta layer and are superior in all parameters over structure #4 in which the Ta layer is not sputter-etched. Structures #2 and 3 include the sputter-etched Ta layer but use the prior art NiFe seed layer. Structures #2 and 3 differ only in the material of the AFM pinning layer, ie. MnPt vs. IrMn. The measured differences between #2 and #3 are small. Structure #1 is the present invention, and embodies the sputter-etched Ta on which is formed the NiCr seed layer. Structure #1 is superior in all respects to structures #2, #3 and #4. In particular, the high junction resistance indicates a proper degree of Al oxidation, whereas the lower junction resistance of structures #2,3 and 4 indicate under-oxidation has occurred. As is disclosed in HT 03-016, the ROX treatment of all structures is carried out in a plasma oxidation chamber furnished with a grid between the electrode that ionizes the oxygen and the surface being oxidized. The ionized oxygen, in passing through the grid, creates a shower of oxygen radicals, including atomic, molecular and ionized oxygen, which impinge on the surface with less energy than in plasma oxidation, where the oxygen is not reduced in energy by the grid. In addition to the measured values noted in the table above, high-resolution TEM images strongly support the effectiveness of the present invention and show a very smooth and conformal oxidized Al layer when the method of the present invention is used.
The bottom electrode, #1 in the table, would be incorporated in the following MRAM element that would achieve the objects of the present invention:
The bottom electrode, #1, above would serve as a pinned layer for the formation of a TMR read sensor of either of the following configurations:
The present invention, in a first preferred embodiment, is a method of forming an MTJ MRAM by the use of a novel NiCr seed layer formed on a sputter-etched Ta layer so that the subsequently formed tunneling junction layer is ultra-thin and smooth and has a high breakdown voltage. In a second preferred embodiment the present invention is a method of forming a TMR read head having a high GMR ratio, low junction resistance and high tunneling layer breakdown voltage, using the novel NiCr seed layer formed on a sputter-etched Ta layer.
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There is shown first a substrate (10), which in this embodiment is a silicon substrate on which will be formed the MRAM element. It is understood that the single MRAM element to be described can be one of an array of such elements and that such element or array may be further connected to associated circuitry used in changing, storing and retrieving information. On the substrate is deposited a first seed layer (20), which in this embodiment is a layer of NiCr(35%-45%) formed to a thickness between approximately 50 and 100 angstroms, with approximately 50 angstroms being preferred. On the first seed layer is then formed a layer of non-magnetic metal (30), which in this embodiment is a smooth layer of Ru, formed to a thickness between approximately 250 and 1000 angstroms, with approximately 400 angstroms being preferred. On the Ru layer there is then formed a capping overlayer (40), which in this embodiment is a layer of Ta formed to a thickness between approximately 60 and 80 angstroms, with approximately 60 angstroms being preferred.
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This ROX tunneling barrier layer is formed to exceptional smoothness and uniformity and has a high breakdown voltage, all being a result of its formation over the sputter-etched Ta overlayer (40) and NiCr layer (85).
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As is understood by a person skilled in the art, the preferred embodiments of the present invention are illustrative of the present invention rather than limiting of the present invention. Revisions and modifications may be made to methods, materials, structures and dimensions employed in forming and providing an MTJ device in either a TMR read head configuration or in an MRAM configuration, said devices having a smooth, uniform and ultra-thin tunneling barrier layer of high breakdown voltage, while still forming and providing such a device and its method of formation in accord with the spirit and scope of the present invention as defined by the appended claims.