Patent Publication Number: US-2015069544-A1

Title: Magneto-resistive element

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application claims the benefit of U.S. Provisional Application No. 61/874,612, filed Sep. 6, 2013, the entire contents of which are incorporated herein by reference. 
    
    
     FIELD 
     Embodiments described herein relate generally to a magneto-resistive element comprising a cap layer. 
     BACKGROUND 
     Recently, large-capacity magneto-resistive random access memories (MRAMs) have been attracting attention, with expectations. An MRAM employs a magnetic tunnel junction (MTJ) element which exploits the tunnel magneto-resistive (TMR) effect. Each MTJ element in an MRAM comprises two ferromagnetic layers (CoFeB) between which a tunnel barrier layer (MgO) is interposed, one of the two ferromagnetic layers being a magnetization fixed layer (reference layer) in which the direction of magnetization is fixed and so does not change, and the other being a magnetization free layer (memory layer) the direction of magnetization of which is capable of being easily changed. The states in which the directions of magnetization of the reference layer and memory layer are mutually parallel and anti-parallel are respectively defined as binary 0 and binary 1 on the basis of which data can be stored. 
     More specifically, when the directions of magnetization of the reference and memory layers are parallel, the resistance of the tunnel barrier layer (that is, the barrier resistance) is low, and the tunnel current is greater than that when the directions of magnetization are antiparallel. The MR ratio is defined as: resistance in antiparallel state-resistance in parallel state/resistance in parallel state. Because stored data is read by detecting differences in resistance due to the TMR effect, it is preferable when reading data that the ratio of resistive difference (MR ratio) by the TMR effect should be high. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIGS. 1A to 10  are schematic diagrams illustrating the operation of embodiments; 
         FIG. 2  is a view of an example of a surface tension in each layer of a magneto-resistive element; 
         FIG. 3  is a cross sectional view showing a basic structure of an MTJ element; 
         FIG. 4  is a diagram showing a relationship between the surface tension and standard electrode potential in each of various metal materials; 
         FIG. 5  is a cross sectional view showing a brief structure of the magneto-resistive elements of the embodiments; and 
         FIGS. 6A to 6H  are cross sectional views of production steps of the magneto-resistive element shown in  FIG. 4 . 
     
    
    
     DETAILED DESCRIPTION 
     In general, according to one embodiment, there is provided a magneto-resistance element comprising: a first ferromagnetic layer formed on an underlying substrate; a tunnel barrier layer formed on the first ferromagnetic layer; a second ferromagnetic layer formed on the tunnel barrier layer; a cap layer formed on the second ferromagnetic layer, wherein a surface tension of the cap layer is equal to or less than that of the second ferromagnetic layer. 
     According to the conventional method of manufacturing an MTJ element, Ta is formed as a cap layer immediately above a CoFeB ferromagnetic layer. It should be noted here that, this method, however, entails the following drawbacks. That is, since the surface tension of Ta is higher than that of CoFeB, Ta easily grow in an island shape on CoFeB, and a portion of the Ta layer grown to have an island shape sinks into CoFeB. Then, when a layer formed of CoFeB—MgO—CoeB is annealed to promote (001)-orientation, Ta easily diffuses into CoFeB. This causes the degradation of magnetic properties of CoFeB. As a result, a high MR ratio cannot be achieved. This embodiment has been proposed to solve the above-mentioned drawback, as a technique for obtaining a high MR ratio. 
     (Basic Principle of Embodiments) 
       FIGS. 1A to 1C  are schematic diagrams illustrating a state in which a liquid-like deposit layer  202  is formed on an underlying layer  201 , to explain the operation of this embodiment. 
       FIG. 1A  shows a case where surface tension of the deposit layer  202  is lower than that of the underlying layer  201 . The figure illustrates a surface tension γ SV  of the underlying layer  201 , a surface tension γ LV  of the deposit layer  202 , an interface tension γ SL , a resistance R and a contact angle θ. In this case, the contact angle θ is sufficiently small, and the deposit layer  202  is formed to be conformal. Even in the case where the surface tension of the deposit layer  202  is equal to that of the underlying layer  201 , the deposit layer  202  is formed to be conformal. 
       FIG. 1B  shows a case where the surface tension of the deposit layer  202  is higher than that of the underlying layer  201 . In this case, the contact angle θ is large, and the deposit layer  202  grows in an island fashion. Further, as shown in  FIG. 1C , the deposit layer  202  sink in the underlying layer  201 , and with this structure, componential materials of the deposit layer can easily diffuse in the underlying layer  201 . 
     In order to form the deposit layer  202 , as a cap layer, to be conformal on the underlying layer  201 , as a ferromagnetic layer, it suffices if the surface tension of the cap layer is set to be equal to or less than that of the underlying ferromagnetic layer. 
       FIG. 2  illustrates the surface tension of each of various kinds of metal materials which constitute the MTJ element. The properties illustrated here are of an example of the MTJ element, in which a CoFeB layer (first ferromagnetic layer)  103 , an MgO layer (tunnel barrier layer)  104 , a CoFeB layer (second ferromagnetic layer)  105 , a Ta layer (cap layer)  106  and a Cu layer (upper layer)  107  are stacked on the underlying layer as shown in  FIG. 3 . 
     A surface tension  203  of the Ta cap layer  106 , which is located immediately above the CoFeB layer  105 , is higher than that of CoFeB, and therefore it can easily grow in an island shape. In order to suppress the island-like growth, it suffices if the surface tension of the cap layer  106 , which is located immediately above the CoFeB layer  105 , is set within or lower than that indicated by reference numeral  204  in  FIG. 2 . 
     In general, the surface tension of an alloy of two kinds of metals falls in a range between that of an alloy having a higher surface tension and that of the other having a lower surface tension, and is determined by the composition of these metals. In the case of an alloy of three or more types of metals, the surface tension falls in a range between that of an alloy having the highest surface tension and that of the one having the lowest surface tension. 
     Therefore, in order to equalize the surface tension of the cap layer  106  with that of the underlying ferromagnetic layer, it suffices if the surface tension of the cap layer  106  is set higher than that of the element having the lowest surface tension among those constituting the underlying ferromagnetic layer, but lower than that of the element having the highest surface tension among those constituting the second ferromagnetic layer. Further, in order to reliably set the surface tension of the cap layer  106  lower than that of the underlying ferromagnetic layer, it suffices if the surface tension of the cap layer  106  is set lower than that of the element having the lowest surface tension among those constituting the underlying ferromagnetic layer. 
     For the prevention of the above-mentioned island-like growth shown in  FIG. 1B , the upper limit of the surface tension of the cap layer  106  must be equal to or less than that of the element having the highest surface tension among those constituting its underlying ferromagnetic layer  105 . On the other hand, the upper layer is formed further above the cap layer  106 , and therefore if the surface tension of the cap layer  106  is excessively low, the island-like growth of the upper layer is enhanced. Therefore, in order to suppress the island-like growth of the upper layer, the surface tension of the cap layer  106  should not be set greatly lower than that of its underlying ferromagnetic layer  105 , but should be equal to or slightly lower than the surface tension of the ferromagnetic layer  105 . In order for this, the surface tension of the cap layer  106  should preferably set equal to or higher than that of the element having the lowest surface tension among those constituting the underlying ferromagnetic layer. 
       FIG. 4  is a diagram showing the relationship between the surface tension and standard electrode potential in each of various metal materials. For the suppression of the island-growth of the cap layer, a combination desirable for the materials of the cap layer can be selected from the vertical axis of  FIG. 4 . 
     When the cap layer  106  has a standard electrode potential lower than those of Fe and Co and also an appropriately adjusted surface tension, the cap layer  106  can supply electrons to the CoFeB layer  105  to charge it negative, thereby preventing oxidization of the interface between the CoFeB layer  105  and the tunnel barrier  104 . Further, diffusion of the materials from the cap layer  106  to the CoFeB layer  105  can be suppressed, thereby preventing the degradation of the magnetic properties. Thus, a high MR ratio can be achieved. 
     On the other hand, when the cap layer  106  has a standard electrode potential higher than those of Fe and Co but has an appropriately adjusted surface tension, it is possible that the cap layer  106  captures electrons from the CoFeB layer  105  to charge it positive, thus oxidizing the interface between the CoFeB layer  105  and the tunnel barrier  104 . However, the diffusion of the materials from the cap layer  106  to the CoFeB layer  105  can be suppressed, and therefore still, an MR ratio higher than those of the conventional techniques, can be achieved. 
     In addition, with regard to the standard electrode potentials of Fe and Co, a combination of a material having a higher potential than those and another material having a lower potential than those, can be employed as well. In general, a material having a standard electrode potential lower than those of Fe and Co, exhibits good magnetic properties but the thermal resistance thereof is low. On the other hand, a material having a standard electrode potential higher than those of Fe and Co, exhibits a good thermal resistance but the magnetic properties thereof are low. However, with an alloy of a material having a standard electrode potential lower than those of Fe and Co and another material having a standard electrode potential higher than those of Fe and Co, the thermal resistance can be improved while maintaining good magnetic properties. Such phenomena have been confirmed in tests carried out by the inventors of the embodiments. 
     Therefore, by selecting a combination desirable for the cap layer material not only from the vertical axis but also the horizontal axis of  FIG. 4 , further more excellent properties can be obtained. 
     From  FIGS. 2 and 4 , it can be understood that when CoFeB is used as the ferromagnetic layer, selection of preferable materials for setting the surface tension of the cap layer should be one of the followings: 
     (1) a single elemental metal having a surface tension lower than that of B or an alloy of such metals; 
     (2) a single elemental metal having a surface tension lower than those of Fe and Co but higher than that of B or an alloy of such metals; 
     (3) an alloy of a metal having a surface tension lower than those of Fe and Co but higher than that of B and another metal having a surface tension lower than that of B; and 
     (4) an alloy of a metal having a surface tension higher than those of Fe and Co and another metal having a surface tension lower than that of B. 
     More specifically, the materials of category (1) are elemental metals of Al, Mn, Zn, Mg, Ag, Sn and Pb and an alloy of a combination of any of these. More preferably, the material should be an alloy of one or more of Al, Mn, Zn and Mg and one or more of Ag, Sn and Pb. 
     The materials of category (2) are elemental metals of Ti, Hf, Cr, Zr, Pt, Pd, Cu and Au and an alloy of a combination of any of these. More preferably, the material should be an alloy of one or more of Ti, Hf, Cr and Zr and one or more of Pt, Pd, Cu and Au. 
     The materials of category (3) are alloys of one or more of Ti, Hf, Cr, Zr, Pt, Pd, Cu and Au and one or more of Al, Mn, Zn, Mg, Ag, Sn and Pb. More preferably, the material should be an alloy of one or more of Ti, Hf, Cr and Zr and one or more of Ag, Sn and Pb, or an alloy of one or more of Pt, Pd, Cu and Au and one or more of Al, Mn, Zn and Mg. 
     The materials of category (4) are alloys of one or more of Ta, V, Nb, W, Mo, Ru, Ir and Rh and one or more of Al, Mn, Zn, Mg, Ag, Sn and Pb. More preferably, the material should be an alloy of one or more of Ta, V and Nb and one or more of Ag, Sn and Pb, or an alloy of one or more of W, Mo, Ru, Ir and Rh and one or more of Al, Mn, Zn and Mg. 
     In the present embodiments, the above-listed materials are selected as the cap layer formed on the CoFeB ferromagnetic layer, and thus the cap layer can be formed conformally. Thus, the embodiments can contribute to the realization of an MTJ element having a high MR ratio. 
     The magneto-resistive element according to the embodiment and the manufacturing method thereof will now be explained in more detail. 
     Embodiment 
       FIG. 5  is a cross section of a brief structure of a magneto-resistive element of this embodiment. The magneto-resistive element of this embodiment is an MTJ element used in an MRAM. 
     A lower wiring layer  101  of Ta or the like is formed on a substrate (not shown), and an underlying layer  102  of Ru or the like, a first ferromagnetic layer  103  comprising CoFeB, a tunnel barrier layer  104  comprising MgO, a second ferromagnetic layer  105  comprising CoFeB, a cap layer  106  and an upper layer  107  of Al, Cu or the like are stacked on the lower wiring layer. These stacked layer structural components are processed in an island shape. 
     Here, the cap layer  106  should only be selected from the materials explained above, and it is formed of, for example, an Al—Ni alloy. 
     An insulation layer  108  of SiN or the like is formed on side surfaces of the MTJ portion processed into the island shape and also on the underlying wiring layer  101  in order to protect the MTJ portion. 
     Further, an insulation layer  109  of SiO 2  or the like is formed on the side surfaces of the MTJ portion such as to interpose the insulation layer  108  between each side surface and itself, as it is embedded therein. 
     An insulation layer  110  of SiO 2  or the like is formed on the insulation layer  109  and the MTJ portion, and a contact hole  111  is formed in the insulation layer  110  to open a section above the MTJ portion. Then, an upper wiring layer  112  of Al, Cu or the like is formed on the insulation layer  110  to fill in the contact hole  11 , and the upper wiring layer  112  is processed into a wiring pattern. 
     It should be noted here that although it is not shown in the figure, the magneto-resistive element of this embodiment has a configuration in which the element is disposed at each intersection of bit lines BL and word lines WL arranged to intersect with each other, and each element is configured to function as a memory cell of MRAM. 
     Next, a method of manufacturing a magneto-resistive element of the present embodiment will now be described with reference to  FIGS. 6A to 6H . 
     First, as shown in  FIG. 6A , on a lower wiring layer  101  of Ta or the like having a thickness of 5 nm, formed are an underlying layer  102  of Ru or the like having a thickness of 2 nm, a CoFeB layer (first ferromagnetic layer)  103  having a thickness of 1.5 nm, an MgO (tunnel barrier layer)  104  having a thickness of 1 nm, and a CoFeB (second ferromagnetic layer)  105  having a thickness of 1.5 nm. The underlying layer  102  may also function as a reference layer. The first ferromagnetic layer  103  may be used as a reference layer or memory layer. 
     The method of forming the tunnel barrier  104  may be any of the followings: direct sputtering of the target of oxidation by RF; post-oxidation of a metal layer by oxygen gas, oxygen plasma, oxygen radical or ozone, a molecular beam epitaxy (MBE) method, an atomic layer deposition (ALD) method, an molecular beam epitaxy (MBE) and a chemical vapor deposition (CVD), etc. Further, the method of forming the ferromagnetic layers  103  and  105  may be any of the sputtering, MBE and ALD methods. 
     Next, as shown in  FIG. 6B , the alloy cap layer  106  to which the embodiment is applied is formed. More specifically, the cap layer  106  is formed of an Al—Ni alloy on the CoFeB ferromagnetic layer  105  by the sputtering method. As can be seen from  FIG. 4 , the surface tension of the cap layer  106  of the Al—Ni alloy is equal to or lower than that of the underlying CoFeB, and therefore the cap layer  106  is formed conformally on the ferromagnetic layer  105 . 
     Next, as shown in  FIG. 6C , the upper layer  107  of Al, Cu or the like is formed on the cap layer  106 . The upper layer  107  may be used as an etching mask, a reference layer, a surface protection layer or an upper wiring connection layer. It should be noted that the surface tension of Al or Cu is equal to or lower than that of the Al—Ni alloy, and therefore the upper layer  107  is formed conformally on the cap layer  106 . 
     Next, as shown in  FIG. 6D , the upper layer  107 , the cap layer  106 , the second ferromagnetic layer  105 , the tunnel barrier layer  104 , the first ferromagnetic layer  103  and the underlying layer  102  are etched selectively in this order by, for example, the ion milling method, and thus the stacked structure portion comprising the underlying layer  102  to the upper layer  107  is processed into an island shape. 
     Subsequently, as shown in  FIG. 6E , the insulation layer  108  configured to protect the MTJ portion in the next step is formed by, for example, the sputtering method, CVD method or ALD method. The insulation layer  108  is formed of, for example, SiN, SiOx, MgO and AlOx, on an upper surface and side surfaces of the MTJ portion and an exposed upper surface of the lower wiring layer  101 . 
     Next, the lower wiring layer  101  is selectively etched by, for example, the reactive ion etching (RIE) method. Note that the processed section of the lower wiring layer  101  is located on, for example, the front side and further side of the page on  FIG. 6E , and not shown. During the etching, the MTJ portion is protected by the insulation layer  108  shown in  FIG. 6E . 
     Next, as shown in  FIG. 6F , the insulation layer  109  is formed on the insulation layer  108  such as to bury the MTJ portion by, for example, the sputtering method or CVD method. The insulation layer  109  is formed of, for example, SiOx. 
     Next, as shown in  FIG. 6G , the insulation layer is subjected to etchback by, for example, the chemical mechanical polishing (CMP) method or gas phase etching method, and thus an upper surface of the upper layer  107  of the MTJ portion is exposed. 
     Next, as shown in  FIG. 6H , the insulation layer  110  is formed on the MTJ portion and the insulation layer  109 , and thereafter, the contact hole  111  is formed in the upper section of the MTJ portion by, for example, the RIE method. The insulation layer  110  is formed of, for example, SiOx. 
     From this stage on, the upper wiring layer  112  made of Al, Al, Cu or the like, is formed and then selectively etched into a wiring pattern by, for example, the RIE method, and thus a magneto-resistive element having the structure shown in  FIG. 5  is completed. 
     As described above, according to this embodiment, the cap layer  106  which has a surface tension equal to or less than that of the second ferromagnetic layer  105 , is formed on the ferromagnetic layer  105  in the magneto-resistive element. With this structure, the island-like growth of the cap layer  106  can be prevented, and therefore the cap layer  106  is formed conformally on the ferromagnetic layer  105 . Thus, the diffusion of the materials from the cap layer  106  to the ferromagnetic layer  105  can be suppressed, thereby preventing the degradation of the magnetic properties of the ferromagnetic layer  106 . Consequently, a high MR ratio can be achieved. 
     Further, with selection of an alloy of a combination of an element having a standard electrode potential higher that those of Co and Fe which constitute the ferromagnetic layer  105  and an element having a lower potential, the embodiment exhibits an advantageous effect of being capable of improving the thermal resistance while maintaining the good magnetic properties. Further, Al—Ni is used as the cap layer  106 , the surface tension thereof is not excessively lowered. Thus, the embodiment exhibits another advantage of being capable of preventing the island-like growth of the upper wiring layer  107  even in the case where Al, Cu or the like is used as the upper wiring layer  107 . 
     Therefore, a magneto-resistive element with excellent properties can be realized as a memory device of an MRAM, and the availability thereof is very high. 
     MODIFIED EXAMPLE 
     Note that the embodiments are not limited to the one explained above. 
     The material of the cap layer is not limited to the Al—Ni alloy, but may be replaced as needed by any of those elected from  FIG. 4  described above, according to the material of the underlying ferromagnetic layer. 
     More specifically, it suffices if the material is of the type which has a surface tension equal to or less than that of the second ferromagnetic layer, and it can be categorized into the followings. 
     (1) The surface tension of the cap layer is lower than that of the element having the lowest surface tension among those constituting the second ferromagnetic layer. That is, the cap layer is made of a single elemental metal having a surface tension lower than that of the element having the lowest surface tension among those constituting the second ferromagnetic layer, or an alloy of such metals. 
     (2) The surface tension of the cap layer is higher than that of the element having the lowest surface tension among those constituting the second ferromagnetic layer, but lower than that of the element having the highest surface tension among those constituting the second ferromagnetic layer. That is, the cap layer is made of a single elemental metal having a surface tension higher than that of the element having the lowest surface tension among those constituting the second ferromagnetic layer, but lower than that of the element having the highest surface tension among those constituting the second ferromagnetic layer, or an alloy of such metals. 
     (3) The cap layer is made of an alloy of a metal having a surface tension higher than that of the element having the lowest surface tension among those constituting the second ferromagnetic layer, but lower than that of the element having the highest surface tension among those constituting the second ferromagnetic layer, and a metal having a surface tension lower than that of the element having the lowest surface tension among those constituting the second ferromagnetic layer. 
     (4) The cap layer is made of an alloy of a metal having a surface tension lower than that of the element having the lowest surface tension among those constituting the second ferromagnetic layer, and a metal having a surface tension higher than that of the element having the highest surface tension among those constituting the second ferromagnetic layer. 
     In addition, the ferromagnetic layers are not limited to CoFeB, but various types of ferromagnetic materials can be employed. When selecting the ferromagnetic material, it suffices only if the cap layer falls within the range which satisfies the conditions (1) to (4) indicated above. Further, the tunnel barrier layer is not limited to MgO, but AlN, AlON, Al 2 O 3 , etc. may be used. 
     While certain embodiments have been described, these embodiments have been presented by way of example only, and are not intended to limit the scope of the inventions. Indeed, the novel embodiments described herein may be embodied in a variety of other forms; furthermore, various omissions, substitutions and changes in the form of the embodiments described herein may be made without departing from the spirit of the inventions. The accompanying claims and their equivalents are intended to cover such forms or modifications as would fall within the scope and spirit of the inventions.