Abstract:
A magnetoresistive element has a crystalline structural quality and magnetic anisotropy enhancement bilayer (CSMAE bilayer) with a). enhanced the crystalline structural quality, hence fabrication yield, of the resulting magnetoresistive element; and b). enhanced the magnetic anisotropy of the recording layer whereby achieving a high MR ratio for the magnetoresistive element with a simultaneous reduction of an undesirable spin pumping effect. As the magnetoresistive film is thermally annealed, a crystallization process occurs to form bcc CoFe grains having epitaxial growth with (100) plane parallel to the surface of the tunnel barrier layer as Boron elements migrate into the impurity absorbing layer. Removing the top portion of the impurity absorbing layer by means of sputtering etch or RIE etch processes followed by optional oxidization process, a thin but thermally stable portion of impurity absorbing layer is formed on top of the magnetoresistive element with a low damping constant. Accordingly, a reduced write current can be achieved for spin-transfer torque MRAM application.

Description:
RELATED APPLICATIONS 
       [0001]    This application is a continued application of prior application Ser. No. 14/695,286 filed on Apr. 24, 2015, which is a divisional application of prior application Ser. No. 14/073,844 filed on Nov. 6, 2013. This application seeks priority to application Ser. No. 14/695,286 with its priority date as that of U.S. Utility patent application Ser. No. 14/073,844 filed on Nov. 6, 2013 and U.S. Provisional Patent Application No. 61,745,757, filed on Dec. 24, 2012; the entire contents of each of which are incorporated herein by reference. 
     
    
     BACKGROUND OF THE INVENTION 
       [0002]    1. Field of the Invention 
         [0003]    This invention relates to the field of magnetoresistive elements. More specifically, the invention comprises spin-transfer-torque magnetic-random-access memory (MRAM) using magnetoresistive elements as basic memory cells which potentially replace the conventional semiconductor memory used in electronic chips, especially mobile chips for power saving and non-volatility. 
         [0004]    2. Description of the Related Art 
         [0005]    In recent years, magnetic random access memories (hereinafter referred to as MRAMs) using the magnetoresistive effect of ferromagnetic tunnel junctions (also called MTJs) have been drawing increasing attention as the next-generation solid-state nonvolatile memories that can cope with high-speed reading and writing, large capacities, and low-power-consumption operations. A ferromagnetic tunnel junction has a three-layer stack structure formed by stacking a recording layer having a changeable magnetization direction, an insulating spacing layer, and a fixed reference layer that is located on the opposite side from the recording layer and maintains a predetermined magnetization direction. 
         [0006]    As a write method to be used in such magnetoresistive elements, there has been suggested a write method (spin torque transfer switching technique) using spin momentum transfers. According to this method, the magnetization direction of a recording layer is reversed by applying a spin-polarized current to the magnetoresistive element. Furthermore, as the volume of the magnetic layer forming the recording layer is smaller, the injected spin-polarized current to write or switch can be also smaller. Accordingly, this method is expected to be a write method that can achieve both device miniaturization and lower currents. 
         [0007]    Further, as in a so-called perpendicular MTJ element (equivalently referred to as the “magnetoresistive element”), both two magnetization films have easy axis of magnetization in a direction perpendicular to the film plane due to their strong magnetic crystalline anisotropy (shape anisotropies are not used), and accordingly, the device shape can be made smaller than that of an in-plane magnetization type. Also, variance in the easy axis of magnetization can be made smaller. Accordingly, by using a material having a large perpendicular magnetic crystalline anisotropy, both miniaturization and lower currents can be expected to be achieved while a thermal disturbance resistance is maintained. 
         [0008]    There has been a known technique for achieving a high MR ratio in a perpendicular MTJ element by forming an underneath MgO tunnel barrier layer and a bcc or hcp-phase cap layer that sandwich a thin recoding layer having an amorphous CoFeB ferromagnetic film and accelerate crystallization of the amorphous ferromagnetic film to match interfacial grain structure to MgO layer through a thermal annealing process. The recording layer crystallization starts from the tunnel barrier layer side to the cap layer and forms a CoFe grain structure having a perpendicular magnetic anisotropy, as Boron elements migrate into the cap layer. Accordingly, a coherent perpendicular magnetic tunneling junction structure is formed. By using this technique, a high MR ratio can be achieved. 
         [0009]    However, where a cap layer is used for achieving a high MR ratio in an MTJ element, the cap layer may increase the damping constant of the recording layer, due to the so-called spin-pumping effect. Further, the damping constant of the recording layer may also increase from the additional cap layer material diffusion during the heat treatment in the device manufacturing process. 
         [0010]    In a spin-injection MRAM using either a perpendicular or planar magnetization film, a write current is proportional to the damping constant and inversely proportional to a spin polarization. Therefore, reduction of the damping constant and increase of the spin polarization are mandatory technologies to reduce the write current. 
       BRIEF SUMMARY OF THE PRESENT INVENTION 
       [0011]    Present invention illustrates a method of making one or more perpendicular magnetoresistive elements for perpendicular spin-transfer-torque MRAM with improved quality and yield. 
         [0012]    The method presented in present invention makes a perpendicular magnetoresistive element to be sandwiched between an upper electrode and a lower electrode of each MRAM memory cell, together with a write circuit which bi-directionally supplies a spin polarized current to the magnetoresistive element and a select transistor electrically connected between the magnetoresistive element and the write circuit. 
         [0013]    The perpendicular magnetoresistive element comprises: a reference layer having magnetic anisotropy in a direction perpendicular to a film surface of the reference layer and having an invariable magnetization direction; a tunnel barrier layer on the reference layer; a recording layer having magnetic anisotropy in a direction perpendicular to a film surface of the reference layer and having a variable magnetization direction on the tunnel barrier layer; and a “crystalline structural quality and magnetic anisotropy enhancement bilayer” (CSMAE bilayer) comprising a recording layer and a novel cap multilayer (equivalently referred to as the “cap layer” or the “impurity absorbing layer”) provided on a surface of the recording layer, wherein at least the top portion of the cap multilayer (a sacrificial layer) is later removed after conducting a thermal annealing process on the magnetoresistive film and leaves a thermally stable bottom cap layer interfacing to the recording layer. 
         [0014]    As an amorphous ferromagnetic material, like CoFeB, in the recording layer is thermally annealed, a crystallization process occurs to form bcc CoFe grains having epitaxial growth with (100) plane parallel to the surface of the tunnel barrier layer to form a perpendicular anisotropy as Boron elements migrate into the Boron-absorbing cap layer. Removing the top portion of the cap layer by means of sputtering etching including ion bean etching (IBE) or reactive ion etching (RIE) process(es) followed by an optional oxidization process, a thin thermally stable portion of cap layer is preserved on top of the recording layer, accordingly, a recording layer having a low damping constant and a reduced write current is achieved. 
         [0015]    The present invention also comprises planar magnetoresistive elements and methods of manufacturing such planar magnetoresistive elements for planar, also known as parallel (parallel to a film surface of the reference layer), spin-transfer-torque MRAM. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0016]      FIG. 1A  is a cross-sectional view showing a configuration of an MTJ element  10  as deposited, according to the method illustrated in first embodiment. 
           [0017]      FIG. 1B  is a cross-sectional view showing a configuration of an MTJ element  10  after IBE etching process to remove the protective layer and top portion of the impurity absorbing layer, according to the method illustrated in first embodiment. 
           [0018]      FIG. 2  is a cross-sectional view showing a configuration of an MTJ element  10  after an optional top surface oxidization of the remaining cap layer, followed by deposition of a upper-contact layer film comprising a buffer layer and a photoresist layer. 
           [0019]      FIG. 3A  is a cross-sectional view showing a configuration of an MTJ element  10  as deposited, according to the method illustrated in second embodiment. 
           [0020]      FIG. 3B  is a cross-sectional view showing a configuration of an MTJ element  10  after the 1 st  RIE or IBE etching process to remove the protective layer, according to the method illustrated in second embodiment. 
           [0021]      FIG. 3C  is a cross-sectional view showing a configuration of an MTJ element  10  after the 2 nd  RIE etching process to remove the second cap sub-layer (equivalently referred to as the “second impurity absorbing sub-layer”), according to the second embodiment; 
           [0022]      FIG. 4A  is a cross-sectional view showing a configuration of an MTJ element  10  as deposited, according to the method illustrated in third embodiment. 
           [0023]      FIG. 4B  is a cross-sectional view showing a configuration of an MTJ element  10  after the 1 st  RIE or IBE etching process to remove the protective layer and the third cap sub-layer (equivalently referred to as the “third impurity absorbing sub-layer”), according to the method illustrated in third embodiment. 
           [0024]      FIG. 4C  is a cross-sectional view showing a configuration of an MTJ element  10  after the 2 nd  RIE etching process to remove the second impurity absorbing sub-layer, according to the method illustrated in third embodiment. 
           [0025]      FIG. 5  is a cross-sectional view showing a configuration of an MTJ element  10  as deposited, according to the method illustrated in fourth embodiment. 
           [0026]      FIG. 6  is a cross-sectional view showing a configuration of an MTJ element  10  after an optional top surface oxidization of the remaining first cap sub-layer (equivalently referred to as the “first impurity absorbing sub-layer”), followed by deposition of a upper-contact layer film comprising a buffer layer and a photoresist layer, according to the fourth embodiment. 
       
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
       [0027]    In general, according to one embodiment in present invention, making a magnetoresistive element comprising: 
         [0028]    forming a reference layer having magnetic anisotropy and having an invariable magnetization direction; 
         [0029]    forming a tunnel barrier layer atop the reference layer; 
         [0030]    forming a recording layer atop the tunnel barrier layer and having a variable magnetization direction; 
         [0031]    forming an impurity absorbing layer, with one or more impurity sublayer(s), i.e., a cap layer or cap multilayer, atop the recording layer; 
         [0032]    forming a protective layer atop the cap multilayer, wherein the protective layer and at least the top portion of the cap layer or cap multilayer are later removed or trimmed after conducting a thermal annealing process on the magnetoresistive film but keeping the remaining portion of the cap layer, interfacing the recording layer, with a reasonable thickness that has an increased thermal stability due to annealing; 
         [0033]    employing an oxidization process to oxidize the top surface of the remaining cap layer when necessary; and 
         [0034]    forming an upper-contact layer provided on the remaining cap layer, comprising a buffer layer and a photoresist layer for further photo-lithographic processes of making a magnetoresistive element. After all above processes, the remained portion of the impurity absorbing layer is called “magic layer”, that has a significantly reduced thickness. 
       First Embodiment 
       [0035]      FIG. 1A  and  FIG. 1B  are cross-sectional views showing a configuration of an MTJ element  10  as deposited according to the method described in first embodiment. The MTJ element  10  is configured by stacking a bottom electrode  11 , a base layer (equivalently referred to as the “seed layer”)  12 , a reference layer  13 , a tunnel barrier layer  14 , a recording layer  15 , a cap layer  16 , and a protective layer  17  in this order from the bottom. 
         [0036]    The reference layer  13  and the recording layer  15  are made of a ferromagnetic material, and have uni-axial magnetic anisotropy in a direction perpendicular to the film surfaces. Further, directions of easy magnetization of the reference layer  13  and the recording layer  15  are also perpendicular to the film surfaces. In other words, the MTJ element  10  is a perpendicular MTJ element in which magnetization direction of the reference layer  13  and the recording layer  15  faces in a direction perpendicular to the film surfaces. A direction of easy magnetization is a direction in which the internal magnetic energy is at its minimum where no external magnetic field exists. Meanwhile, a direction of hard magnetization is a direction in which the internal energy is at its maximum where no external magnetic field exists. 
         [0037]    The recording layer  15  has a variable (reversible) magnetization direction. The reference layer  13  has an invariable (fixing) magnetization direction. The reference layer  13  is made of a ferromagnetic material having a perpendicular magnetic anisotropic energy which is sufficiently greater than the recording layer  15 . This strong perpendicular magnetic anisotropy can be achieved by selecting a material, configuration and a film thickness. In this manner, a spin polarized current may only reverse the magnetization direction of the recording layer  15  while the magnetization direction of the reference layer  13  remains unchanged. An MTJ element  10  which comprises a recording layer  15  having a variable magnetization direction and a reference layer  13  having an invariable magnetization direction for a predetermined write current can be achieved. 
         [0038]    The tunnel barrier layer  14  is made of a non-magnetic insulating metal oxide or nitride. 
         [0039]    The cap layer  16  is made of a low electro-negativity and low diffusivity metal layer or a metal alloy layer having a bcc or hcp-phase and having at least a thickness of  50  angstroms. The cap layer serves to introduce and to enhance crystalline structural quality and perpendicular magnetic anisotropy of the recording layer  15 . As an amorphous ferromagnetic material, like CoFeB, in the recording layer is thermally annealed, a crystallization process occurs to form bcc CoFe grains having epitaxial growth with (100) plane parallel to surface of the tunnel barrier layer and a perpendicular anisotropy is induced in the recording layer, as Boron elements migrate into the cap layer having a low electro-negativity. A damping constant of the recording layer  15  sometimes increases (deteriorates) depending on a material in contact with the recording layer  15 , which is known as a spin pumping effect. The cap layer  16  may also have a function to prevent increase of the damping constant of the recording layer  15  by reducing the spin pumping. Further, the thickness of the cap layer  16  is selected to be big enough that it serves as a good absorber for the Boron elements from the recording layer to achieve better epitaxial CoFe crystal grains in the recoding layer having an ultra-low damping constant. 
         [0040]    An example configuration of the MTJ element  10  will be described below. The reference layer  13  is made of CoFeB (around 2 nm)/TbCoFe (around 20 nm). The tunnel barrier layer  14  is made of MgO (around 1 nm). The recording layer  15  is made of CoFeB (around 1.2 nm). The cap layer  16  is made of Ti (around 10 nm). The protective layer  17  is made of Ru (around 10 nm). The base layer  12  is made of Ta (around 20 nm)/Cu (around 20 nm)/Ta (around 20 nm). Each element written in the left side of “/” is stacked above an element written in the right side thereof. 
         [0041]    The CoFeB (with Boron content no less than 10% and no more than 30%) layer comprised in the recording layer  15  is in an amorphous state as deposited. The MgO layer comprised in the tunnel barrier layer  14  is formed into rocksalt crystal grains with the (100) plane parallel to the substrate plane. During a thermal annealing with a temperature higher than 250-degree C., the Boron elements of the CoFeB migrate into its above Ti cap layer to form TiB2 since Ti has much lower electro-negativity than Co and Fe, and the amorphous CoFeB is crystallized to form bcc CoFe grains having epitaxial growth with (100) plane parallel to the surface of the MgO crystal tunnel barrier layer. A thick Ti cap layer is essential to absorb as many as Boron atoms as possible and achieve better epitaxial bcc CoFe crystal grains. A relatively pure CoFe film has a lower damping constant than an amorphous CoFeB film. A typical damping constant for a pure CoFe is around 0.003, while CoFeB has a damping constant of 0.01. Accordingly, a perpendicular magnetization having a low damping constant is induced in the recording layer. 
         [0042]    After the thermal annealing process, an IBE etching process is adopted to etch away the Ru protective layer and the top portion of the Ti cap layer, leaving a much thinner remaining Ti cap layer for an easy integration with very small dimension lithographic patterning process, as shown in  FIG. 1B . An upper contact layer  19  (not shown) comprising a buffer layer and a photoresist layer is then deposited on the top of MTJ film after the etching. 
         [0043]    Then, if necessary, a surface oxidization process may also be added before the upper contact layer deposition. A surface oxidization process, i.e. by using of a mixed gas containing natural oxygen (O 2 ), or radical oxygen and Argon (Ar), may also be adopted before the deposition of the upper contact layer. Doing so, a thin oxide layer  16 B is formed between the remaining cap layer  16 A and the upper contact layer  20  for better interfacial thermal stability and less diffusion. The final configuration of the MTJ element  10 , which is ready for MTJ element photolithographic patterning process, is shown in  FIG. 2 . 
       Second Embodiment 
       [0044]      FIG. 3A  is a cross-sectional view showing an example configuration of the MTJ element  10  as deposited according to the method described in second embodiment. As shown in  FIG. 3A , the reference layer  13  is made of CoFeB (around 2 nm)/TbCoFe (around 20 nm). The tunnel barrier layer  14  is made of MgO (around 1 nm). The recording layer  15  is made of CoFeB (around 1.2 nm). The first cap sub-layer (equivalently referred to as the “first impurity absorbing sub-layer”)  16  is made of Ti (around 2 nm). The second cap sub-layer (equivalently referred to as the “second impurity absorbing sub-layer”)  17  is made of Ta (around 10 nm). The protective layer  18  is made of Ru (around 10 nm). The base layer is made of Ta (around 20 nm)/Cu (around 20 nm)/Ta (around 20 nm). Each element written in the left side of “/” is stacked above an element written in the right side thereof. 
         [0045]    Similar to the first embodiment, the CoFeB (with Boron content no less than 10% and no more than 30%) layer comprised in the recording layer  15  is formed into an amorphous state as deposited. During a thermal annealing with a temperature higher than 250-degree, the Boron elements of the CoFeB migrate first into its above thin Ti cap sub-layer  16  and further across Ti cap sub-layer into the Ta cap sub-layer  17 , since Ta atom has a even lower electro-negativity and a stronger Boron-bonding than Ti atom. The ionicity of metal-Boron bonds decreases in the following order: Mg, Al, Mn, Y, Cr, Zr, Hf, Nb, Ta, V and Ti. Accordingly, the amorphous CoFeB in the recording layer is crystallized to form bcc CoFe grains having epitaxial growth with (100) plane parallel to surface of the MgO crystal tunnel barrier layer, and a perpendicular magnetization having a low damping constant is induced in the recording layer. 
         [0046]    After the thermal annealing process, a RIE etching process utilizing CH3OH gas, or NH3+CO mixed gas chemistry can be employed to etch away the protective Ru layer  18  and the second cap Ta sub-layer serves as its etch-stop layer, as shown in  FIG. 3B . Alternatively, an IBE etching process is utilized to etch away the protective Ru layer  18  and the upper portion of the second cap Ta sub-layer  17 . The remaining Ta cap sub-layer is readily removed by the 2 nd  RIE etching process utilizing CF4 gas chemistry, leaving a thin Ti first cap sub-layer, as shown in  FIG. 3C , for an easy integration with very small dimension lithographic patterning process. Following an optional surface oxidization process if necessary, also similar to the first embodiment, an upper contact layer  20  comprising a buffer layer and a photoresist layer is then deposited on the top of MTJ film, and the final configuration of the MTJ element  10 , which is ready for MTJ element photolithographic patterning process, is shown in  FIG. 2 . 
       Third Embodiment 
       [0047]      FIG. 4A  is a cross-sectional view showing an example configuration of the MTJ element  10  according to the third embodiment. As shown in  FIG. 4A , the reference layer  13  is made of CoFeB (around 2 nm)/TbCoFe (around 20 nm). The tunnel barrier layer  14  is made of MgO (around 1 nm). The recording layer  15  is made of CoFeB (around 1.2 nm). The first cap sub-layer  16  is made of Ti (around 2 nm). The second cap sub-layer  17  is made of Ta (around 3 nm). The third cap sub-layer layer (equivalently referred to as the “third impurity absorbing sub-layer”)  18  is made of Hf (around 10 nm). The protective layer is made of Ru (around 10 nm). The base layer is made of Ta (around 20 nm)/Cu (around 20 nm)/Ta (around 20 nm). Each element written in the left side of “/” is stacked above an element written in the right side thereof. 
         [0048]    Similar to the first and second embodiments, the CoFeB (with Boron content no less than 10% and no more than 30%) layer comprised in the recording layer  15  is formed into an amorphous state as deposited. During a thermal annealing with a temperature higher than 250-degree, the Boron elements of the CoFeB migrate first into its above thin Ti cap sub-layer  16  and thin Ta cap sub-layer  17 , and further across them into the Hf cap sub-layer  18 , since Hf atom has a even lower electro-negativity and a stronger Boron-bond than both of Ti atom and Ta atom. From the first cap sub-layer to the third cap sub-layer, the bond iconicity with Boron atom gradually increases and more effectively attracts or absorbs Boron elements from the CoFeB recording layer. Accordingly, the amorphous CoFeB in the recording layer is crystallized to form better bcc CoFe grains having epitaxial growth with (100) plane parallel to surface of the MgO crystal tunnel barrier layer, and a perpendicular magnetization having a low damping constant is induced in the recording layer. 
         [0049]    After the thermal annealing process, the  1 st RIE etching process using CH3OH gas, or NH3+Co mixed gas chemistry is employed to etch away the protective Ru layer  19  and the third cap Hf sub-layer  18 , stopping at Ta cap sub-layer  17 , as shown in  FIG. 4B . The remaining Ta cap sub-layer is then readily removed by the 2 nd  RIE etching process using CF4 gas chemistry, leaving a thin Ti first cap sub-layer, as shown in  FIG. 4C , for an easy integration with very small dimension lithographic patterning process. Following an optional surface oxidization process, also similar to the first embodiment, an upper contact layer  20  comprising a buffer layer and a photoresist layer is then deposited on the top of MTJ film, and the final configuration of the MTJ element  10 , which is ready for MTJ element photolithographic patterning process, is shown in  FIG. 2 . 
       Fourth Embodiment 
       [0050]    Similar to above embodiments, the same cap multilayer can be adopted in a planar magnetoresistive element.  FIG. 5  is a cross-sectional view showing an example configuration of the MTJ element  10  according to the method described in fourth embodiment. All layered configuration can be identical to those of the third embodiment, except that the reference layer has an invariable magnetization in a direction parallel to a film surface and the recording layer has a variable magnetization in a direction parallel to a film surface. After the same thermal annealing and etching processes are utilized, the final configuration of the MTJ element  10 , which is ready for MTJ element photolithographic patterning process, is shown in  FIG. 6 . 
         [0051]    While certain embodiments have been described above, 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.