Abstract:
This invention describes a novel tunnel magnetoresistive (TMR) deposition process that can enhance the signal-to-noise ratio (SNR) of a TMR reader. A method of manufacturing a tunnel magnetoresistive sensor includes providing a substrate; forming a first portion of a magnetic tunnel junction (MTJ) structure on the substrate; forming a second portion of the MTJ structure on the substrate; forming a tunnel barrier layer of the MTJ structure between the first portion and the second portion; heating the first portion of the MTJ structure before forming the tunnel barrier layer or after forming at least a portion of the tunnel barrier layer; and cooling the tunnel barrier layer.

Description:
FIELD 
       [0001]    Aspects of the present invention relate to magnetoresistive structures, and more particularly methods for manufacturing a magnetoresistive head. 
       BACKGROUND OF RELATED ART 
       [0002]    A tunneling magnetoresistance or magnetoresistive (TMR) reader is a type of magnetic sensor that utilizes a magnetic tunnel junction (MTJ). In general, a TMR reader includes two ferromagnetic layers that are separated by a barrier layer (e.g., MgO). In a current-perpendicular-to-plane (CPP) geometry type TMR structure, the resistance of the structure for a current flowing perpendicular to the barrier layer depends on the relative angle between the magnetization of both ferromagnetic layers. The quality of such a TMR device is characterized by its magnetoresistance ratio (dR/R) that indicates the magnitude change in resistance of a material due to an external magnetic field. The TMR device exhibits magnetoresistance at the magnetic tunnel junction. Generally, the magnetoresistance ratio decreases with both increasing temperature and increasing bias voltage. 
         [0003]    The signal-to-noise ratio (SNR) of the TMR reader depends on both the signal (which is proportional to amplitude and thus dR/R of the thin film stack of the TMR reader) and noise. The noise can be partitioned into magnetic noise and electrical noise. While the magnetic noise is correlated to the magnitude of the signal, the electrical noise is proportional to the resistance-area (RA) product of the device. Therefore, the SNR of the TMR reader may be increased by increasing the signal (e.g., increasing the magnetoresistance ratio dR/R of the film stack) and/or reducing the noise (e.g., reducing the RA to decrease electrical noise). In a generally known TMR structure, however, dR/R will typically decrease when RA is reduced (e.g., due to wave function overlap because of the reduced barrier thickness as well as increase in interlayer coupling). 
         [0004]    In the related art, in situ-heating has been used after the deposition of the tunnel barrier (e.g., a MgO tunnel barrier layer) to improve the dR/R for a given RA. This is done to improve the crystallinity of the MgO tunnel barrier layer before deposition of the next layer. However, it is desirable to further improve the SNR performance of a TMR device (e.g., a reader). 
       SUMMARY 
       [0005]    Embodiments of the present invention are directed toward tunnel magnetoresistive (TMR) structures and methods for manufacturing the same. 
         [0006]    According to an embodiment of the present invention, a method for manufacturing a tunnel magnetoresistive (TMR) structure is provided. The method includes forming a reference layer on a substrate, forming a tunnel barrier layer on the substrate, forming a free layer on the substrate, and heating and cooling the TMR structure after forming the reference layer. 
         [0007]    According to another embodiment of the present invention, a method of manufacturing a tunnel magnetoresistive sensor is provided. The method includes providing a substrate, forming a first portion of a magnetic tunnel junction (MTJ) structure on the substrate, forming a second portion of the MTJ structure on the substrate, forming a tunnel barrier layer of the MTJ structure between the first portion and the second portion, heating the first portion of the MTJ structure before forming the tunnel barrier layer or after forming at least a portion of the tunnel barrier layer, and cooling the tunnel barrier layer. 
         [0008]    According to another embodiment of the present invention, a method of manufacturing a magnetic tunnel junction (MTJ) head for a magnetic storage device is provided. The method includes providing a substrate, forming a first portion of the MTJ head on the substrate, forming a second portion of the MTJ head on the substrate, forming a tunnel barrier layer of the MTJ head between the first portion and the second portion, heating the first portion of the MTJ head before forming the tunnel barrier layer or after forming at least a portion of the tunnel barrier layer, and cooling the tunnel barrier layer. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0009]    The above and other features and aspects of the present invention will become more apparent by describing in detail embodiments thereof with reference to the attached drawings in which: 
           [0010]      FIG. 1  is a conceptual cross-sectional view of a tunneling magnetoresistance (TMR) structure according to an embodiment of the present invention. 
           [0011]      FIG. 2  is a flowchart illustrating a method for manufacturing a TMR structure according to an embodiment of the present invention. 
           [0012]      FIG. 3  is a conceptual cross-sectional view of a tunneling magnetoresistance (TMR) structure according to an embodiment of the present invention. 
           [0013]      FIG. 4  is a flowchart illustrating another method for manufacturing a TMR structure according to an embodiment of the present invention. 
           [0014]      FIG. 5  is a graph illustrating a comparison of magnetoresistance between a TMR structure of a comparative example and a TMR structure according to an embodiment of the present invention. 
       
    
    
     DETAILED DESCRIPTION 
       [0015]    Aspects of embodiments according to the present invention are directed toward methods for manufacturing a magnetoresistive structure such as a head for a magnetic storage device (e.g., a hard drive). The methods utilize a combination of in-situ heating and in-situ cooling below room temperature (e.g., cryogenic cooling) during the manufacturing process. In the following description of the present invention, processes or constructions that are well-known to one of ordinary skill in the art may be omitted. 
         [0016]      FIG. 1  is a conceptual cross-sectional view of a tunneling magnetoresistance (TMR) structure  100  (e.g., TMR reader) according to an embodiment of the present invention. The TMR structure  100  includes a shield layer  12  on a substrate  10 , a seed layer  14  on the shield layer  12 , an anti-ferromagnetic (AFM) layer  16  (e.g., Ir—Mn) on the seed layer  14 , a P1 layer  18  (e.g., a ferromagnetic layer made of CoFe or CoFeB) on the AFM layer  16 , a spacer  20  (e.g., Ru) on the P1 layer  18 , a P2 layer  22  (e.g., a ferromagnetic layer made of CoFe or CoFeB) on the spacer  20 , a tunnel barrier layer  24  (e.g., MgO) on the P2 layer  22 , a free layer  26  (e.g., CoFe/NiFe) on the tunnel barrier layer  24 , and a cap layer  28  (e.g., Ta/Ru) on the free layer  26 . 
         [0017]    In the embodiment of  FIG. 1 , one or more of the seed layer  14  and the AFM layer  16  may be referred to as a pinning layer, and one or more of the P1 layer  18 , the spacer  20 , and the P2 layer may be referred to as a pinned layer. One or more of the pinning layer and the pinned layer may be referred to as a reference layer on the substrate  10 . For example, the reference layer may denote a combination of the P1 layer  18 , the spacer  20 , and the P2 layer  22 . In another example, the reference layer may denote only the P2 layer  22 . In still another example, the reference layer may denote the seed layer  14 , the AFM layer  16 , the P1 layer  18 , the spacer  20 , and the P2 layer  22  and other various layers below the tunnel barrier layer  24 . However, the present invention is not limited to the above described structure and materials. In some embodiments, some of the various layers may be omitted or substituted by other suitable layers in various order. In some embodiments, the order of the various layers in  FIG. 1  is partially or completely reversed. In addition, the thicknesses of the various layers illustrated in the drawings may be exaggerated for clarity and may not be representative of the actual thicknesses. 
         [0018]    Processes for fabricating the above described TMR structure  100  of  FIG. 1  will be described more fully below according to an embodiment of the present invention. The processes can significantly enhance the dR/R of the TMR structure  100  (e.g., thin film stack) and also help to maintain a reasonable dR/R at low RA. The increase of the dR/R of the TMR structure  100  may be attributed to, but not limited to, an improved lattice matching between the tunnel barrier layer  24  and the free layer  26 . In several embodiments, the free layer  26  will remain substantially disordered when deposited on the cold substrate  10  and then only crystallize during the subsequent annealing step to be described in more detail below. 
         [0019]    Referring to  FIG. 1 , the substrate  10  that is made of a suitable material (e.g., AlTiC) is first provided, and the shield layer  12  is deposited on the substrate  10 . The surface of the shield layer  12  on the substrate  10  may be etched using conventional plasma treatment to remove any native oxide and contamination. The above described layers of the TMR structure  100  shown in  FIG. 1  are sequentially deposited until the tunnel barrier layer  24  is deposited. In several embodiments, the tunnel barrier layer  24  may be a MgO layer or a MgO/Mg layer. In other embodiments, other suitable materials can be used. Then, the partially formed TMR structure  100  is heated to between about 50° C. and about 400° C., inclusive. In several embodiments, the partially formed TMR structure  100  can be heated within a smaller range between 100° C. and about 300° C., inclusive. The heating process may be performed in a suitable heating chamber, and the heating time can be between about one minute and about fifty minutes, inclusive. In several embodiments, the heating time can be in a smaller range that is between about five minutes and about fifteen minutes, inclusive. 
         [0020]    After heating, the partially formed TMR structure  100  is sent to a cooling chamber or cooling station for cooling. During cooling, the TMR structure  100  is cooled to a temperature between about −223° C. (50K) to about 20° C. (293K), inclusive. In several embodiments, the cooling temperature is between about −173° C. (100K) and about 0° C. (273K). The cooling time can be between about five minutes and about sixty minutes, inclusive. In several embodiments, the cooling time can be in a smaller range of between about ten minutes and about thirty minutes, inclusive. It should be understood that the above recited temperatures and time durations used in the heating and cooling processes are illustrative embodiments, and the present invention is not limited thereto. Therefore, in other embodiments, other suitable temperatures and heating/cooling time may be used. 
         [0021]    In some embodiments, the tunnel barrier layer  24  includes a tunnel barrier layer  24   a  (e.g., a MgO layer) and an interlayer  24   b  (e.g., a layer of Mg, Zn, Al, AlSi, Ca, Si, or other suitable materials) deposited on the tunnel barrier layer  24   a . The heating and cooling of the TMR structure  100  may be performed in different orders. In several embodiments, the heating step may be performed before or after at least a part of the tunnel barrier layer  24   a  is deposited, and before or after the interlayer  24   b  is formed. After the heating step is performed, the cooling step may be performed before or after the interlayer  24   b  is formed, and before or after at least a part of the free layer  26  is formed. In several embodiments, the cooling step is performed after a portion of the free layer  26  is formed. After cooling, the cap layer  28  is deposited on the cooled partially formed TMR structure  100  to completely form the TMR structure  100 . Subsequently, optimized high vacuum field annealing is performed on the completed TMR structure  100 . 
         [0022]      FIG. 2  is a flowchart illustrating a process for manufacturing a TMR structure according to an embodiment of the present invention. In particular embodiments, the process can be used to form the TMR structure of  FIG. 1 . According to the process of  FIG. 2 , a reference layer is formed on a suitable substrate (S 10 ), a tunnel barrier layer is formed on the substrate (S 12 ), and a free layer is formed on the substrate (S 14 ). In addition, heating and cooling of the TMR structure is performed after forming the reference tunnel barrier layer (S 16 ). Although the steps S 10 , S 12 , S 14 , and S 16  are illustrated in  FIG. 2  in a particular order. The present invention is not limited thereto. To the contrary, the order of these steps may be performed in various suitable orders. In one embodiment, the process can perform the sequence of actions in a different order. In another embodiment, the process can skip one or more of the actions. In other embodiments, one or more of the actions are performed simultaneously. In some embodiments, additional actions can be performed. Also, it is noted that the above disclosed process may not include all the various steps of manufacturing a TMR structure because some steps or processes that are generally known in the art and not necessary for the understanding of the present invention may be omitted for clarity. 
         [0023]      FIG. 3  is a conceptual cross-sectional view of a tunneling magnetoresistance (TMR) structure  200  according to an embodiment of the present invention. In  FIG. 3 , the order in which the various layers of the TMR structure  200  are formed has a reversed order as compared to the TMR structure  100  of  FIG. 1 . During fabrication, the heating and cooling of the TMR structure  200  may be performed in various suitable orders. After forming the free layer  36 , the heating step may be performed before or after at least a part of the tunnel barrier layer  34  is deposited, and before or after the interlayer  32  is formed. After the heating step is performed, the cooling step may be performed before or after at least a part of the interlayer  32  is formed, and before or after at least a part of the reference layer  30  is formed. A description of some of the processes used during the fabrication of the TMR structure  200  that are substantially similar to those of the TMR structure  100  is omitted for clarity. For example, the above described heating and cooling temperatures and time durations in reference to the embodiment of  FIG. 1  are applicable in the fabrication of the TMR structure  200  of  FIG. 3 . 
         [0024]      FIG. 4  is a flowchart illustrating another process for manufacturing a TMR structure according to an embodiment of the present invention. In particular embodiments, the process can be used to form the TMR structure of  FIG. 3 . According to the process of  FIG. 4 , a suitable substrate is provided (S 20 ), a first portion of a magnetic tunnel junction (MTJ) structure is formed on the substrate (S 22 ), a second portion of the MTJ structure is formed on the substrate (S 24 ), and a tunnel barrier layer of the MTJ structure is formed between the first portion and the second portion (S 26 ). In addition, the first portion of the MTJ structure is heated before forming the tunnel barrier layer or after forming at least a portion of the tunnel barrier layer (S 28 ). After heating, cooling is performed on the tunnel barrier layer (S 30 ). Although steps S 20  through S 30  are shown in a particular order in  FIG. 4 , the present invention is not limited thereto. To the contrary, steps S 20  through S 30  may be performed in various suitable orders. In one embodiment, the process can perform the sequence of actions in a different order. In another embodiment, the process can skip one or more of the actions. In other embodiments, one or more of the actions are performed simultaneously. In some embodiments, additional actions can be performed. Also, it is noted that the above disclosed process may not include all the various steps of manufacturing a TMR structure because some steps or processes that are generally known in the art and not necessary for the understanding of the present invention may be omitted for clarity. 
         [0025]      FIG. 5  is a graph illustrating a comparison of magnetoresistance between a TMR structure of a comparative example and a TMR structure according to an embodiment of the present invention. The vertical axis and horizontal axis of  FIG. 5  represent magnetoresistance (MR) in arbitrary units and RA, respectively. In  FIG. 5 , the lower curve C 1  represents the magnetoresistance—of the TMR structure of the comparative example in which no cooling process was performed, and the upper curve C 2  represents the magnetoresistance—of the TMR structure according to the embodiment of the present invention in which cooling process was performed after heating. It can be seen in  FIG. 5  that the magnetoresistance of the embodiment is substantially increased when cooling is performed during fabrication. That is, for the same RA, higher magnetoresistance (i.e., curve C 2 ) is observed for the TMR structure fabricated according to the embodiment. Therefore, the SNR of the TMR structure fabricated according to the embodiment of the present invention may be improved. 
         [0026]    In the above described embodiments, the cooling process may be performed in cryogenic temperature range (e.g., about 100K), and if the cooling process is performed at higher temperatures (e.g., room temperature), the resulting TMR increase may be smaller. However, even if the TMR structure  100  or  200  is only cooled back to room temperature, one can still observe a TMR increase. Cooling to room temperature (i.e., not to cryogenic temperature) can also be carried out by simply letting the TMR structure to cool down for a period of time after the heating process. 
         [0027]    In several embodiments, the heating and cooling steps may be performed in various different orders as illustrated in the table below. 
         [0000]    
       
         
               
             
           
               
                   
               
             
             
               
                 Reference layer/Tunnel barrier/Heating/Cooling/Interlayer/Free layer 
               
               
                 Reference layer/Tunnel barrier/Heating/Interlayer/Cooling/Free layer 
               
               
                 Reference layer/Tunnel barrier/Interlayer/Heating/Cooling/Free layer 
               
               
                 Reference layer/Tunnel barrier/Interlayer/Heating/Free layer part1/Cooling/Free layer part2 
               
               
                 Reference layer/Tunnel barrier/Heating/Interlayer/Free layer part1/Cooling/Free layer part2 
               
               
                 Reference layer/Tunnel barrier part1/Heating/Tunnel barrier part2/Cooling/Interlayer/Free layer 
               
               
                 Reference layer/Tunnel barrier part1/Heating/Tunnel barrier part2/Interlayer/Cooling/Free layer 
               
               
                 Reference layer/Tunnel barrier part 1/Heating/Tunnel barrier part2/Interlayer/Free layer part1/Cooling/ 
               
               
                 Free layer part2 
               
               
                 Reference layer/Heating/Tunnel barrier/Cooling/Interlayer/Free layer 
               
               
                 Reference layer/Heating/Tunnel barrier/Interlayer/Cooling/Free layer 
               
               
                 Reference layer/Heating/Tunnel barrier/Interlayer/Free layer part1/Cooling/Free layer part2 
               
               
                 Free Layer/Tunnel barrier/Heating/Cooling/Interlayer/Reference Layer 
               
               
                 Free Layer/Tunnel barrier/Heating/Interlayer/Cooling/Reference Layer 
               
               
                 Free Layer/Tunnel barrier/Interlayer/Heating/Cooling/Reference Layer 
               
               
                 Free Layer/Tunnel barrier/Interlayer/Heating/Reference Layer part1/Cooling/Reference Layer part2 
               
               
                 Free Layer/Tunnel barrier/Heating/Interlayer/Reference Layer part1/Cooling/Reference Layer part2 
               
               
                 Free Layer/Tunnel barrier part1/Heating/Tunnel barrier part2/Cooling/Interlayer/Reference Layer 
               
               
                 Free Layer/Tunnel barrier part1/Heating/Tunnel barrier part2/Interlayer/Cooling/Reference Layer 
               
               
                 Free Layer/Tunnel barrier part 1/Heating/Tunnel barrier part2/Interlayer/Reference Layer part1/Cooling/ 
               
               
                 Reference Layer part2 
               
               
                 Free Layer/Heating/Tunnel barrier/Cooling/Interlayer/Reference Layer 
               
               
                 Free Layer/Heating/Tunnel barrier/Interlayer/Cooling/Reference Layer 
               
               
                 Free Layer/Heating/Tunnel barrier/Interlayer/Reference Layer part1/Cooling/Reference Layer part2 
               
               
                   
               
             
          
         
       
     
         [0028]    It is noted that the above table is not an exhaustive list of all the possible combinations according to the present invention. To the contrary, other suitable orders of performing the various steps are possible according to the present invention. In each of the above combinations, the process starts from the leftmost step to the rightmost step. In some combinations, a particular layer may be formed in two steps (e.g., part 1 and part 2) with an intervening heating or cooling step. 
         [0029]    In several embodiments, the above described methods may be used to fabricate a head (e.g., magnetic tunnel junction head) of a magnetic storage device (e.g., a hard drive). 
         [0030]    While the present invention has been particularly shown and described with reference to exemplary embodiments thereof, it will be understood by those of ordinary skill in the art that various changes in form and details may be made therein without departing from the spirit and scope of the present invention as defined by the following claims and their equivalents.