Patent Publication Number: US-11380841-B2

Title: Magnetoresistive effect element and method of manufacturing the same

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application is a Divisional Application of U.S. application Ser. No. 15/916,964, filed on Mar. 9, 2018, which is based upon and claims the benefit of priority from Japanese Patent Application No. 2017-180050, filed Sep. 20, 2017, the entire contents of all of which are incorporated herein by reference. 
    
    
     FIELD 
     Embodiments relate generally to a magnetoresistive-effect element and a method of manufacturing the same. 
     BACKGROUND 
     Magnetoresistive-effect elements, which exhibit the magnetoresistive effect, are known. The magnetoresistive effect is a phenomenon in which a magnetoresistive-effect element exhibits different resistances when respective magnetization directions of two ferromagnets are parallel and anti-parallel. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  illustrates a cross-sectional structure of a part of a magnetoresistive memory device of one embodiment; 
         FIG. 2  illustrates a magnified view of a cross-sectional structure of a part of the magnetoresistive memory device of the embodiment; 
         FIG. 3  illustrates one state in a manufacturing process of the magnetoresistive memory device of the embodiment; 
         FIG. 4  illustrates absolute values of standard enthalpy of formation of materials; 
         FIG. 5  illustrates absolute values of standard enthalpy of formation of other materials; 
         FIG. 6  illustrates the state subsequent to  FIG. 3 ; 
         FIG. 7  illustrates the state subsequent to  FIG. 6 ; 
         FIG. 8  illustrates a cross-sectional structure of a part of a magnetoresistive memory device of a modification of the embodiment; 
         FIG. 9  illustrates one state in a manufacturing process of the magnetoresistive memory device of the modification of the embodiment; 
         FIG. 10  illustrates the state subsequent to  FIG. 9 ; and 
         FIG. 11  illustrates the state subsequent to  FIG. 10 . 
     
    
    
     DETAILED DESCRIPTION 
     In general, according to one embodiment, a method of manufacturing a magnetoresistive-effect element includes: forming a second layer on a stack of layers, the stack of layers including a ferromagnetic layer and a first layer, the first layer comprising magnesium oxide, the second layer and magnesium oxide having a selected ratio larger than 1 to first etching by ion beams; and etching the stack of layers through the first etching with the second layer used as a mask. 
     Embodiments will now be described with reference to the figures. In the following description, components with substantially the same functionalities and configurations will be referred to with the same reference numerals, and repeated descriptions may be omitted. The figures are schematic, and the relations between the thickness and the area of a plane of a layer and ratios of thicknesses of layers may differ from actual ones. Moreover, the figures may include components which differ in relations and/or ratios of dimensions in different figures. Each embodiment illustrates the device and method for materializing the technical idea of that embodiment, and the technical idea of an embodiment does not specify the quality of the material, shape, structure, arrangement of components, etc. to the following. 
       FIG. 1  illustrates a cross-sectional structure of a part of a magnetoresistive memory device of one embodiment. As illustrated in  FIG. 1 , a magnetic tunnel junction (MTJ) element  2  is disposed on the conductor  1 . The conductor  1  may be an electrode above a silicon substrate (not shown), or may be a portion of a silicon substrate with impurities introduced therein. 
     The MTJ element  2  has a shape of a substantial circle along the xy-plane, for example. The MTJ element  2  includes at least a ferromagnet  11 , a nonmagnet  12 , and a ferromagnet  13 . The nonmagnet  12  is located between the ferromagnets  11  and  13 , and, for example, is in contact with the ferromagnets  11  and  13  between the ferromagnets  11  and  13 . Any of the ferromagnets  11  and  13  may be located lower. The ferromagnet  11 , the nonmagnet  12 , and the ferromagnet  13  are stacked along the z-axis. 
     The ferromagnet  11  includes or is made of one or more conductive ferromagnetic elements. Specifically, the ferromagnet  11  includes one or more of iron (Fe), boron (B), and cobalt (Co), and includes, for example, cobalt iron boron (CoFeB) or boride iron (FeB). Alternatively, the ferromagnet  11  includes or is made of an alloy of two or more elements of Co, Fe, and B. The ferromagnet  11  is magnetized in the direction along a particular axis, and has a magnetization easy axis along an axis which penetrates the boundaries of the ferromagnet  11 , the nonmagnet  12 , and the ferromagnet  13 , such as the-axis. The direction of the magnetization of the ferromagnet  11  can be switched by a current (write current) which flows through the ferromagnet  11 , the nonmagnet  12 , and the ferromagnet  13 . The ferromagnet  11  may be generally referred to as a storage layer, etc. 
     The nonmagnet  12  includes or is made of one or more nonmagnetic insulative elements, and serves as a tunnel barrier. For example, the nonmagnet  12  includes or is made of magnesium oxide (MgO). 
     The ferromagnet  13  includes or is made of one or more conductive ferromagnetic elements. For example, the ferromagnet  13  includes or is made of cobalt platinum (CoPt), cobalt nickel (CoNi), or cobalt palladium (CoPd). The ferromagnet  13  may also include a stack of two or more layers that respectively includes different ones of cobalt (Co), platinum (Pt), palladium (Pd), and nickel (Ni), or the alloy of two or more of these elements. The ferromagnet  13  has the magnetization easy axis along an axis which penetrates the boundaries of the ferromagnet  11 , the nonmagnet  12 , and the ferromagnet  13 . The ferromagnet  13  has a magnetization with a fixed or invariable direction, and has a larger coercivity than that of the ferromagnet  11 , for example. The magnetization direction of the ferromagnet  13  being “fixed” or “invariable” refers to the magnetization direction of the ferromagnet  13  not being switched by a write current of a magnitude that switches the magnetization direction of the ferromagnet  11 . The ferromagnet  13  may be generally referred to as a reference layer, etc. 
     The set of the ferromagnet  11 , the nonmagnet  12 , and the ferromagnet  13  exhibits the magnetoresistive effect. Specifically, the MTJ element  2  exhibits the minimum and maximum resistances when the magnetization direction of the ferromagnet  11  is parallel and antiparallel with the magnetization direction of the ferromagnet  13 , respectively. 
     The MTJ element  2  may include an additional layer. The details of layers included in the MTJ element  2  do not limit the embodiments. By way of example only, the MTJ element  2  includes a conductor  14  and a ferromagnet  15 , and the figures and the following description are based on such an example. The conductor  14  is located on the ferromagnet  13 , and the ferromagnet  15  is located on the conductor  14 . The conductor  14  has a function to antiferromagnetically couple the ferromagnets  13  and  15 , and includes or is made of ruthenium (Ru), for example. The ferromagnet  15  has a function to suppress or offset a magnetic field that is generated by the ferromagnet  13  and applied to the ferromagnet  11 , or a stray magnetic field. 
     A tantalum layer  21  is disposed on the top of the MTJ element  2 . The tantalum layer  21  includes tantalum, and, for example, is substantially made of tantalum. The term “substantially” is herein intended to represent that the tantalum layer  21  is made of tantalum, but it contains unintentionally introduced impurities. 
     As illustrated in  FIG. 2 , a part  31 B of a hard mask  31 A may partially remain on the tantalum layer  21  unintentionally. The hard mask  31 A will be described in full detail below. 
     An insulator  24  is disposed around the MTJ element  2 . An electrode  26  is disposed on the tantalum layer  21 . 
     Referring to  FIGS. 3 to 7 , a manufacturing method of the structure of  FIG. 1  will be described. As illustrated in  FIG. 3 , a stack of to-be-processed layers  2 A is deposited on the conductor  1 . The stack of layers  2 A is a stack of to-be-processed layers that will be partially removed through etching to be processed into layers included in the MTJ element  2 . Specifically, in an example of the MTJ element  2  made of the ferromagnet  11 , the nonmagnet  12 , the ferromagnet  13 , the conductor  14 , and the ferromagnet  15 , the stack of layers  2 A includes a ferromagnet  11 A, a nonmagnet  12 A, a ferromagnet  13 A, a conductor  14 A, and a ferromagnet  15 A. The ferromagnet  11 A, the nonmagnet  12 A, the ferromagnet  13 A, the conductor  14 A, and the ferromagnet  15 A are stacked in this order along the z-axis, and are layers to be etched into the ferromagnet  11 , the nonmagnet  12 , the ferromagnet  13 , the conductor  14 , and the ferromagnet  15 , respectively. For a case of the MTJ element  2  including one or more additional layers, the stack of layers includes one or more additional to-be-processed layers that will be respectively processed into the additional layers. 
     A tantalum layer  21 A is formed on the stack of layers  2 A. The tantalum layer  21 A is a layer part of which will be removed through etching to be processed into the tantalum layer  21 . 
     A hard mask  31 A is formed on the stack of layers  2 A. The hard mask  31 A has one or more of the features described in the following. 
     The hard mask  31 A is a conductor. The hard mask  31 A is removed after being used in a subsequent process, and it is desirable that the hard mask  31 A is removed completely. The hard mask  31 A may, however, not be completely removed depending on the characteristics of the material of the hard mask  31 A, and the details of etching. It is desirable that the hard mask  31 A can send a current through the hard mask  31 A even in such a case. 
     The hard mask  31 A has a high hardness. The hard mask  31 A is used as a mask during subsequent physical etching of the stack of layers  2 A and the tantalum layer  21 A into the MTJ element  2  and the tantalum layer  21 , respectively. To this end, the hard mask  31 A is formed into a hard mask pattern  31  with a plane shape corresponding to the shape along the xy-plane (plane shape) of the MTJ element  2 , and the stack of layers  2 A and the tantalum layer  21 A are etched through the hard mask pattern  31 . In order to form the MTJ element  2  with a small plane shape for improving the density of components in the magnetoresistive memory device, the hard mask pattern  31  also has a small plane shape. On the other hand, the physical etching lowers the top of the hard mask pattern  31 , and therefore the hard mask pattern  31  needs to have a certain thickness to avoid the hard mask pattern  31  from being etched off from its top and the tantalum layer  21 A from being exposed. Because of such requests, the hard mask pattern  31  has a high aspect ratio. The hard mask pattern  31  with a very high aspect ratio can fall down due to its own weight or other factors during the etching. In order to reduce such a possibility, the hard mask  31 A has a high hardness. Moreover, the harder the hard mask  31 , the higher the resistance of the hard mask  31  against the physical etching. From this perspective, the hard mask  31  has a high hardness. 
     A material with a high hardness is, for example, a material harder than MgO. The hardest target film in the physical etching with the hard mask pattern  31  is MgO used for the nonmagnet  12 . For this reason, the hard mask  31  needs to have a resistance to the etching for forming the stack of layers  2 A, in particular MgO. Therefore, the material for the hard mask  31  needs to be harder than MgO, and is a material to allow the etching for forming MgO to have an etch selection ratio larger than 1 with MgO. As a hard material for the hard mask  31 , a compound can be used, for example. 
     Moreover, it is known that a material with a high absolute value of standard enthalpy of formation (|[Δ]fH0|) or a high bonding energy has a high hardness.  FIGS. 4 and 5  illustrate the absolute values of the standard enthalpy of formation of various materials. In general, materials that have been actually used as or studied for the possibility as a material of the hard mask for the physical etching of MTJ elements include aluminum nitride (AlN), boron carbide (B 4 C), boron nitride (BN), carbon (C), gallium nitride (GaN), indium nitride (InN), silicon carbide (SiCβ), tantalum carbide (TaC), titanium boride (TiB 2 ), titanium carbide (TiC), titanium nitride (TiN), etc. Those materials have only absolute values of standard enthalpies of formation lower than the absolute value of the standard enthalpy of formation of MgO. 
       FIGS. 4 and 5  also illustrate the absolute values of standard enthalpies of formation other than those materials, and illustrate the absolute values of standard enthalpies of formation of silver sulfate (I) (Ag 2 SO 4 ), aluminum chloride (AlCl 3 ), aluminum fluoride (AlF 3 ), mullite (3Al 2 O 3 .2SiO 2 ), aluminum sulfate (A 2 (SO 4 ) 3 ), arsenic pentoxide (As 2 O 5 ), arsenic trioxide (As 4 O 6 ), aluminum oxide (Al 2 O 3 ), witherite (poison weight) (BaCO 3 ), barium fluoride (BaF 2 ), barium nitrate (Ba(NO 3 ) 2 ), alexandrite (BeAl 2 O 4 ), beryllium oxide (BeO), calcium hydroxide (Ca(OH) 2 ), a calcium chloride (CaCl 2 )), calcite (CaCO 3 ), fluorite (CaF 2 ), dolomite (CaMg(CO 3 ) 2 ), calcium oxide (CaO), perovskite (CaTiO 3 ), cesium chloride (CsCl), malachite (Cu 2 (CO) 3 (OH) 2 ), fayalite (Fe 2 SiO 4 ), pyrite (FeS 2 ), hexagonal boron nitride (h-BN), cinnabar (HgS), lanthanum oxide (La 2 O 3 ), magnesium hydroxide (Mg(OH) 2 ), forsterite (Mg 2 SiO 4 ), spinel (MgAl 2 O 4 ), magnesite (MgCO 3 ), magnesium oxide (MgO), manganese oxide (II) (MnO), manganese dioxide, manganese oxide (IV) (MnO 2 ), cryolite (Na 3 AlF 6 ), sodium chloride (NaCl), nickel oxide (NiO), millerite (NiS), palladium oxide (PdO), silicon nitride (SiN 4 ), β2-tridiymite (SiO 2 ), samarium oxide (Sm 2 O 3 ), strontium oxide (SrO), rutile (TiO 2 ), wurtzite boron nitride (w-BN), tungsten carbide (W 2 C, WC), zinc oxide (ZnO), sphalerite (ZnS), corundum (α-Al 2 O 3 ), lead oxide (litharge) (α-PbO), crystal (α-quartz) (α-SiO 2 ), and β-quartz (β-SiO 2 ). As can be seen from  FIGS. 4 and 5 , among those materials, silver sulfate (I), aluminum chloride, aluminum fluoride, mullite, aluminum sulfate, arsenic pentoxide, arsenic trioxide, aluminum oxide, witherite, barium fluoride, barium nitrate, alexandrite, calcium hydroxide, calcium chloride, calcite, fluorite, dolomite, calcium oxide, perovskite, malachite, fayalite, lanthanum oxide, magnesium hydroxide, forsterite, spinel, magnesite, cryolite, silicon nitride, β2-tridiymite, samarium oxide, and rutile have absolute values of standard enthalpies of formation higher than that of MgO (illustrated by the alternate long and short dash line). Therefore, the material for the hard mask  31  can include silver sulfate (I), an aluminum chloride, aluminum fluoride, mullite, aluminum sulfate, arsenic pentoxide, arsenic trioxide, aluminum oxide, witherite, barium fluoride, barium nitrate, alexandrite, calcium hydroxide, calcium chloride, calcite, fluorite, dolomite, calcium oxide, perovskite, malachite, fayalite, lanthanum oxide, magnesium hydroxide, forsterite, spinel, magnesite, cryolite, silicon nitride, β2-tridiymite, samarium oxide, and rutile. 
     The hard mask  31  may have crystallinity. In general, it is known that many hard materials are crystalline and the hard mask  31  of a high hardness has crystallinity. For example, the material for the hard mask  31  is a material that can grow epitaxially on the tantalum layer  21 A, and a material that can be formed by the lattice strain of 4% or less, for example. 
     The material for the hard mask  31  may be a material with a high affinity with tantalum, in other words a material that easily crystallizes on tantalum. This is because materials that easily crystallize tend to have a high hardness, and the hard mask  31  is formed on the tantalum layer  21 A, as described above. The material for the hard mask  31  has a high hardness if it is lattice-matched; however it is not necessarily lattice-matched, but it only needs to have a high hardness. 
     Referring back  FIG. 3 , a photoresist  33  is formed on the hard mask  31 A after the formation of the hard mask  31 A. The photoresist  33  remains above an area in which the MTJ element  2  will be formed and has, in the remaining portions, openings  34  that reach the bottom of the photoresist  34 . 
     As illustrated in  FIG. 6 , the photoresist  33  is used as a mask and the hard mask  31  is etched. As a result, a hard mask pattern  31  is formed from the hard mask  31 . The hard mask pattern  31  is located above an area in which the MTJ element  2  will be formed, has a plane shape corresponding to or substantially the same plane shape of the to-be-formed MTJ element  2 , and has an independent and/or isolated plane shape. The hard mask pattern  31  has a high aspect ratio due to the MTJ element  2  having a small plane shape and the hard mask pattern  31  having a thickness to prevent the following physical etching from etching off the hard mask pattern from its top. The photoresist  33  is then removed. 
     As illustrated in  FIG. 7 , the hard mask pattern  31  is used as a mask, and the stack of layers  2 A and the tantalum layer  21 A is physically etched through ion beam etching (IBE). The IBE is generally performed in an atmosphere of rare gas, and, for example, in gas containing one or more of argon (Ar), neon (Ne), krypton (Kr), and xenon (Xe). The stack of layers  2 A and the tantalum layer  21 A are patterned by the physical etching, thereby the MTJ element  2  and the tantalum layer  21  are formed. The etching lowers the top of the hard mask pattern  31  somewhat. The hard mask pattern  31  is then removed by wet or dry etching, for example. 
     An insulator  24  is then formed in the surroundings of the MTJ element  2  and the electrode  26  is formed on the insulator  24  and the tantalum layer  21  afterwards, thereby the structure of  FIG. 1  is obtained. 
     According to the embodiment, the MTJ element  2  with a small diameter can be formed. The details are as follows. 
     As described above, a diameter of an MTJ element is desirably smaller for improved integration of magnetoresistive memory devices. The smaller the diameter of an MTJ element, the smaller the diameter of the pattern of a hard mask used to etch a stack of layers into the MTJ element. On the other hand, with physical etching, such as the IBE, used to etch the stack of layers, the hard mask pattern needs to be thick to prevent the physical etching from etching off the hard mask pattern. In particular, with conventional materials used for the hard mask, the hard mask pattern needs to be considerably thick due to their low hardness. Thus, an increasingly high aspect ratio is required for the hard mask pattern in order to have a small diameter and a large thickness. 
     The higher the aspect ratio of the hard mask pattern, the lower the instability thereof, which makes it fall down easily. For improved stability of the hard mask pattern to prevent it from falling down, the aspect ratio of the hard mask pattern needs to be reduced. Thus, the aspect ratio of the hard mask pattern needs to be high to keep it from being etched off and to make a small MTJ element, whereas it needs to be low to suppress the collapse of the hard mask pattern. Therefore, the advance of reduction of the diameter of the MTJ elements is limited by the maximum aspect ratio for the hard mask patterns. In fact, with the materials that realize the hard mask patterns of diameters so far without falling down, a hard mask pattern of a smaller diameter cannot be formed without falling down. 
     The description so far relates to a structure where the ferromagnet  11  with switchable magnetization direction is located under the ferromagnet  13 , or a structure where the ferromagnet  11 , the nonmagnet  12 , the ferromagnet  13 , the conductor  14 , and the ferromagnet  15  are stacked in this order on the conductor  1 . The embodiment, however, is applicable to a structure where the ferromagnet  15 , the conductor  14 , the ferromagnet  13 , the nonmagnet  12 , and the ferromagnet  11  are stacked in this order on the conductor  1 , as illustrated in  FIG. 8 . 
     The structure of  FIG. 8  can be formed through the process of  FIGS. 9 to 11 .  FIGS. 9 to 11  respectively correspond to  FIG. 3 ,  FIG. 6 , and  FIG. 7 , respectively differ from  FIG. 3 ,  FIG. 6 , and  FIG. 7  in the order of layers in the stack of layers  2 A and the order of the layers in the MTJ element  2 , and are applied with descriptions for  FIG. 3 ,  FIG. 6 , and  FIG. 7  for the remaining features. 
     According to the embodiment, for the hard mask pattern  31  used in physical etching for forming the MTJ element  2 , a compound with an aspect ratio with MgO larger than one to the physical etching is used. Therefore, the hard mask pattern  31  has a higher resistivity against the physical etching than materials conventionally used and MgO. This allows the hard mask pattern  31  to be thinner, for a particular diameter, than that of conventional materials, which is necessary to keep it from falling down. Therefore, the hard mask pattern  31  can have a significantly small diameter that would result in a high aspect ratio with which the hard mask pattern  31  would otherwise easily fall down. This enables formation of the MTJ element  2  of a small diameter that is unrealizable with the conventional materials. 
     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 methods and systems described herein may be embodied in a variety of other forms; furthermore, various omissions, substitutions and changes in the form of the methods and systems 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.