1. Field of the Invention
The present invention relates to a magnetic tunnel junction (hereinafter, referred to as an “MTJ”) device and a method for fabricating the same. More particularly, the present invention relates to an MTJ device with a reduced junction resistance and a method for fabricating the same.
2. Description of the Related Art
An MTJ device is a junction structure including a sandwich of two ferromagnetic layers separated by a thin insulating layer, in which the amount of tunneling current is varied according to a relative magnetic direction of each of the ferromagnetic layers. The MTJ device has been used in a nonvolatile magnetic memory device, a read head of a highly integrated magnetic storing medium, and the like, and has recently attracted scientific attention due to a phenomenon depending not on electric charges but on electric spins.
To embody a high-performance highly integrated magnetic memory device, an MTJ device with a high magnetoresistance (MR) and a low junction resistance is required. In particular, the resistance-area (RA) value, obtained by multiplying the resistance value of an MTJ by the area of the MTJ, is an important variable that determines the signal to noise (S/N) ratio and the resistance capacitor (RC) time constant.
Referring to
Even though methods for reducing the RA value to several tens of Ωμm2 have been extensively investigated, an MTJ device having a low RA value still cannot maintain an optimized high MR ratio. Accordingly, an MTJ device including a tunnel barrier having a predetermined thickness to obtain good uniformity, and having a high MR ratio and a low RA value is needed.
The present invention provides an MTJ device with a reduced magnetic tunnel junction resistance and good uniformity and a method for fabricating the same.
In accordance with a feature of an embodiment of the present invention, there is provided an MTJ device including a substrate and a fixed layer, a tunnel barrier, and a free layer sequentially stacked on the substrate, wherein a magnetoresistance buffer layer formed of a metallic nitride is interposed between the fixed layer and the tunnel barrier, and the entire magnetic tunnel junction device is thermally treated to reduce the magnetic junction resistance thereof. Preferably, nitrogen is combined with elements of the tunnel barrier during the thermal treatment to form a nitrogen rich layer at the tunnel barrier.
The fixed layer preferably includes a seed layer, a pinning layer, and a pinned layer, which are sequentially deposited. The seed layer is preferably a ferromagnetic layer formed of one selected from the group consisting of NiFe, Ru, and Ir. The pinning layer is preferably a semi-ferromagnetic layer formed of one selected from the group consisting of FeMn and IrMn. The pinned layer is preferably a ferromagnetic layer formed of one selected from the group consisting of NiFe and CoFe. The magnetoresistance buffer layer is preferably a metallic nitride layer formed of FeN. The tunnel barrier is preferably an insulating layer formed of AlOx. The thermal treatment preferably includes heating the magnetic tunnel junction device at a temperature of 150 to 300° C. and slowly cooling the magnetic tunnel junction device.
In accordance with another feature of an embodiment of the present invention, there is provided a method for fabricating an MTJ device, which includes (a) depositing a fixed layer on a substrate and processing the surface of the fixed layer using nitrogen plasma, (b) sequentially stacking a tunnel barrier, a free layer, and a capping layer on the fixed layer and thermally treating the tunnel barrier, the free layer, and the capping layer to thereby fabricate the MTJ device with a reduced magnetoresistance.
The fixed layer, the tunnel barrier, the free layer, and the capping layer are preferably deposited by sputtering.
In (a), the nitrogen plasma processing preferably includes applying a direct power to a nitrogen atmosphere under a predetermined pressure to generate nitrogen plasma and bringing the nitrogen plasma into contact with the fixed layer.
In (b), the thermal treatment preferably includes heating and then slowly cooling the tunnel barrier, the free layer, and the capping layer one or more times, wherein each heating is performed at a temperature between 150° C. and 300° C. Also in (b), a magnetic field is preferably applied to the MTJ device during the thermal treatment. The thermal treatment preferably leads nitrogen to combine with elements of the tunnel barrier.
The fixed layer preferably includes a seed layer, a pinning layer, and a pinned layer, which are sequentially stacked on the substrate.
Here, the seed layer is preferably a ferromagnetic layer formed of one of NiFe, Ru, and Ir, the pinning layer is preferably a semi-ferromagnetic layer formed of one selected from the group consisting of FeMn and IrMn, and the pinned layer is preferably a ferromagnetic layer formed of one selected from the group consisting of NiFe and CoFe.
The magnetoresistance buffer layer is preferably a metallic nitride layer formed of FeN, and the tunnel barrier is preferably an insulating layer formed of AlOx.
According to the present invention, the top surface of a fixed layer is processed with nitrogen plasma, and after a tunnel barrier is deposited on the fixed layer, an MTJ device is thermally treated. Thus, the MTJ device with a reduced magnetic junction resistance can be fabricated.
The above and other features and advantages of the present invention will become more apparent by describing in detail preferred embodiments thereof with reference to the attached drawings in which:
Korean Patent Application No. 2002-71046, filed on Nov. 15, 2002, and entitled: “Magnetic Tunnel Junction Device And Method For Fabricating The Same,” is incorporated by reference herein in its entirety.
An MTJ device and a method for fabricating the same according to an embodiment of the present invention will be described more fully hereinafter with reference to the accompanying drawings, in which preferred embodiments of the invention are shown. The invention may, however, be embodied in different forms and should not be construed as limited to the embodiments set forth herein. Rather, these embodiments are provided so that this disclosure will be thorough and complete, and will fully convey the scope of the invention to those skilled in the art. In the drawings, the thickness of layers and regions are exaggerated for clarity. It will also be understood that when a layer is referred to as being “on” another layer or substrate, it can be directly on the other layer or substrate, or intervening layers may also be present. Further, it will be understood that when a layer is referred to as being “under” another layer, it can be directly under, and one or more intervening layers may also be present. In addition, it will also be understood that when a layer is referred to as being “between” two layers, it can be the only layer between the two layers, or one or more intervening layers may also be present. Like numbers refer to like elements throughout.
Referring to
The seed layer 12 is formed of one of NiFe, Ru, and Ir, the pinning layer 13 is formed of a semi-ferromagnetic material such as FeMn and IrMn, and the pinned layer 15 is formed of a fixed ferromagnetic layer such as NiFe and CoFe.
Unlike conventional MTJ devices, the MTJ device according to the present invention further comprises a magnetoresistance buffer layer 17 between the pinned layer 15 and the tunnel barrier 19. The magnetoresistance buffer layer 17 is formed of a nitride such as FeN, which is obtained by processing the top surface of the pinned layer 15 using nitrogen plasma.
The tunnel barrier 19 is formed using AlOx or AlNxOx on the magnetoresistance buffer layer 17. The free layer 21 is formed using a ferromagnetic material such as NiFe on the tunnel barrier 19. Also, the capping layer 23 is formed using Ru on the free layer 21.
As shown in
Next, the surface of the pinned layer 15 is processed with nitrogen plasma, thereby forming the magnetoresistance buffer layer 17, as shown in
Referring to
A seed layer formed of Ta, a pinning layer formed of NiFe, a pinned layer formed of FeMn, and a buffer layer NiFe are sequentially deposited on an Si/SiO2 substrate using a dc or rf magnetron sputtering system having a degree of vacuum of 8×10−8 Torr or less. Here, the seed layer, the pinning layer, the pinned layer, and the buffer layer are formed to a thickness of 10 nm, 14 nm, 10 nm, and 6 nm, respectively. Then, immediately after depositing the seed layer, the pinning layer, the pinned layer, and the buffer layer, the nitrogen plasma processing is conducted using a direct power of 3.5 W under a pressure of 100 mTorr in an atmosphere of N2 .
Next, an Al layer, a NiFe layer, and an Au layer are deposited by sputtering to thicknesses of 1.58 nm, 20 nm, and 20 nm, respectively, and then are thermally treated. The thermal treatment is carried out in a vacuum state having a pressure of 5×10−6 Torr. During the thermal treatment, a magnetic field having 150 Oe is applied in parallel with the magnetic axis of the resultant structure. The thermal treatment includes heating the resultant structure, where the Al layer, the NiFe layer, and the Au layer are formed, three times, for a duration of 30 minutes each time, at temperatures of 180° C., 230° C. and 270° C., respectively, and slowly cooling the resultant structure after each heating.
The resultant MTJ device includes Ta/NiFe/FeMn/NiFe/Al2O3 /NiFe/Au. All properties of the MTJ device are measured using a direct four-electrode method at a normal temperature, i.e., room temperature.
The MR ratio is defined in Equation 1 as:
In Equation 1, Rap is the resistance of the MTJ in a case where a magnetic direction of the pinned layer is not parallel with that of the free layer, while Rp is the resistance of the MTJ in a case where a magnetic direction of the pinned layer is parallel with that of the free layer. As the MR ratio increases, it is easier to determine a direction of spin in each of the pinned layer and the free layer. Thus, data recorded in bits of the MTJ device can be read out at a high speed.
As shown in
As the temperature of the thermal treatment is increased up to 230° C., the MR ratio of the nitrogen-unprocessed junction f1 increases from 14% to 17.5%. However, as the temperature of the thermal treatment becomes higher than 230° C., the MR ratio begins to decrease. A similar temperature change in the thermal treatment of the nitrogen-processed junction g1 results in the MR ratio of the nitrogen-processed junction g1 being varied within a much wider range than that of the nitrogen-unprocessed junction f1. When the thermal treatment is performed at 230° C., the MR ratio of the nitrogen-processed junction g1 is 18.7%, which is higher than that of the nitrogen-unprocessed junction f1 under the same conditions. The sharp increase in the MR ratio of the nitrogen-processed junction g1 in response to the thermal treatment indicates that the Al2O3 layer is uniformly formed by redistribution of oxygen, and that interracial characteristics between the tunnel barrier and the pinned layer are improved due to a change in distribution of nitrogen affecting the pinned layer.
Referring to the inner graph, before the thermal treatment, the RA value of the nitrogen-unprocessed junction f3 is 390 kΩμm2. As the temperature of the thermal treatment increases, the RA value also increases up to 418 kΩμm2 at a temperature of 230° C. where the MR ratio has a maximum. As the temperature of the thermal treatment becomes higher than 230° C., the RA value decreases again. This change in the graph is similar to that of the nitrogen-unprocessed junction f1 shown in
Before the thermal treatment, the RA value of the nitrogen-processed junction g3 is 100 kΩμm2, which is less than that (390 kΩμm2 ) of the nitrogen-unprocessed junction f3. The RA value of the nitrogen-processed junction g3 increases slightly until the temperature reaches 180° C., and then the RA value of the nitrogen-processed junction g3 decreases to 78 kΩμm2, which is less than the RA value (100 kΩμm2) obtained before the thermal treatment.
The large decrease in the RA values is because nitrogen, which was mostly distributed between the NiFe layer and the Al2O3 layer by the nitrogen plasma processing, is redistributed by the thermal treatment. As shown in the graph, since the RA value of the nitrogen-processed junction g3 is lower before the thermal treatment, when the Al layer is deposited, the nitrogen, which contacts the surface of the NiFe layer by the nitrogen plasma processing, is assumed to be partially used to form the AlN. Also, it is inferred that when the thermal treatment is performed at 230° C., more nitrogen flows into the Al2O3 layer to increase the MR ratio and decrease the RA value, thus enabling an optimum distribution of nitrogen. The above inferences are valid considering a thermodynamic result that the enthalpy (−76 Kcal/mol) required for forming AlN is lower than that required for forming a transitional metallic nitride, such as FeN4 (−2.5 Kcal/mol) or Ni3N (0.2 Kcal/mol), which may be formed on the surface of the NiFe.
Referring to
Referring to
Referring to
Referring to
In particular, when the exposure time tex is 60 seconds and the temperature of the thermal treatment increases from 0 to 180° C., the MR ratio is highly increased from about 0 to about 10%. Then, when the temperature increases up to 230° C., the MR ratio is increased up to more than 14%. Accordingly, it can be seen that the nitrogen plasma processing degrades the characteristics of the MR ratio while the thermal treatment improves them. This change is similar to that of the case where the exposure time tex is 10 seconds or 30 seconds.
Referring to
In a case where an exposing time tex is 30 seconds, the binding energy has a peak at 395˜398 eV where no peak is shown when the exposing time tex is 0 second. That is, after the MTJ device is exposed to the nitrogen plasma, AlN and/or FeN are generated. Therefore, the MTJ device including Ta/NiFe/FeMn/NiFe/Fe is processed with the nitrogen plasma, thus forming an MTJ device including Ta/NiFe/FeMn/NiFe/FeN/AlOx(1.32 nm)/NiFe/Au or Ta/NiFe/FeMn/NiFe/FeN/AlN/AlOx(1.32 nm)/NiFe/Au.
As explained so far, a method for fabricating an MTJ device includes depositing a pinned layer, forming a magnetoresistance buffer layer on the pinned layer by nitrogen plasma processing, depositing a tunnel barrier, a free layer, and a capping layer, and thermally treating the resultant structure. According to this method, a high-performance MTJ device with a high MR ratio and a low RA value can be fabricated. Also, because the magnetoresistance buffer layer is formed to be coupled with the tunnel barrier, the MTJ device with improved uniformity can be fabricated. As a result, sensing errors can be reduced during recording/reading of data.
While the present invention has been particularly shown and described with reference to preferred embodiments thereof, it should be appreciated that the scope of the invention is not limited to the detailed description of the invention hereinabove, which is intended merely to be illustrative, but rather comprehends the subject matter defined by the following claims. For example, those of ordinary skill in the art can fabricate a magnetoresistance buffer layer by forming another metal layer between a pinned layer and a tunnel barrier, processing the metal layer with nitrogen plasma, and thermally treating the metal layer.
Number | Date | Country | Kind |
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2002-71046 | Nov 2002 | KR | national |
This is a divisional application based on pending application Ser. No. 10/713,215, filed Nov. 17, 2003, the entire contents of which is hereby incorporated by reference.
Number | Date | Country | |
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Parent | 10713215 | Nov 2003 | US |
Child | 11707949 | Feb 2007 | US |