Patent Publication Number: US-2015068887-A1

Title: Manufacturing method of magnetoresistive element and manufacturing apparatus of the same

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application claims the benefit of U.S. Provisional Application No. 61/875,488, filed Sep. 9, 2013, the entire contents of which are incorporated herein by reference. 
    
    
     FIELD 
     Embodiments described herein relate generally to a manufacturing method of magnetoresistive element used for a magnetoresistive random access memory, and a manufacturing apparatus of the same. 
     BACKGROUND 
     Nowadays, large-capacity magnetoresistive random access memories (MRAMs) using a magnetic tunnel junction (MTJ) element exploiting the tunnel magnetoresistive (TMR) effect have gained attention and raised expectations. In an MTJ element used for an MRAM, one of two ferromagnetic layers (CoFeB) holding a tunnel barrier layer (MgO) therebetween is used as a magnetization fixed layer (reference layer) in which the direction of magnetization is fixed and prevented from changing, and the other is used as a magnetization free layer (storage layer) in which the direction of magnetization is easily reversed. The state where the directions of magnetization of the reference layer and the storage layer are parallel and the state where the directions of magnetization are antiparallel are correlated with binary “0” and “1”, respectively, and thereby information can be stored. 
     When the directions of magnetization of the reference layer and the storage layer are parallel with each other, the resistance (barrier resistance) of the tunnel barrier layer is lower than that in the case where the directions of magnetization are antiparallel, and has a greater tunnel current. The equation “MR ratio=(resistance in the antiparallel state-resistance in the parallel state)/resistance in the parallel state” holds. Stored information is read out by detecting a change in resistance caused by the TMR effect. Thus, it is preferable that a resistance change rate (MR ratio) caused by the TMR effect is large in reading. 
     In a method of manufacturing an MTJ element, a sputtering apparatus is generally used for forming MgO as a tunnel barrier layer. However, this method cannot produce MgO with high quality and promoted (001) orientation, and it is difficult to express a high MR ratio through the whole substrate. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWING 
         FIG. 1  is a schematic diagram illustrating positional relationship between a target and a substrate to be treated. 
         FIG. 2  is a schematic diagram illustrating results of experiments in dependence of an MR ratio on the position on the substrate. 
         FIG. 3  is a plan view illustrating an intermittent irradiation region on a substrate to be treated. 
         FIG. 4  is a cross-sectional view illustrating a schematic structure of a magnetoresistive element manufacturing apparatus according to a first embodiment. 
         FIG. 5  is a plan view illustrating positional relationship between a substrate to be treated and a substrate shutter. 
         FIGS. 6A to 6I  are cross-sectional views illustrating a process for manufacturing a magnetoresistive element using the apparatus of  FIG. 4 . 
         FIG. 7  is a cross-sectional view illustrating a schematic structure of a magnetoresistive element manufacturing apparatus according to a second embodiment. 
         FIG. 8  is a plan view illustrating arrangement relationship between first and second rotary stages in  FIG. 7 . 
         FIG. 9  is a cross-sectional view illustrating a schematic structure of a magnetoresistive element manufacturing apparatus according to a third embodiment. 
     
    
    
     DETAILED DESCRIPTION 
     In general, according to one embodiment, a method of manufacturing a magnetoresistive element comprises: intermittently exposing a surface of a base substrate to sputter particles from a sputter target, and thereby forming a thin film on the base substrate. 
     Basic Principle of Embodiments 
     A basic principle of embodiments will be explained hereinafter, before explanations of the embodiments. 
       FIG. 1  is a schematic diagram illustrating positional relationship between substrates  111  to be treated and a sputter target  121  formed of MgO, and illustrating distances (T/S) between the target and the substrate. In  FIG. 1 , the distance (T/S-2) is greater than the distance (T/S-1). The arrow  123  in  FIG. 1  indicates a direction of applying sputtering particles and O −  ions from the target  121 . O −  ions are applied together with the sputtering particles to the surface of the substrates  111  to be treated. 
       FIG. 2  illustrates results of experiments in dependence of the MR ratio on the position on the substrate, in the case where an MgO film is formed by the apparatus of  FIG. 1  with the target  121  to produce an MTJ element (magnetoresistive element). In the results, it is inferred that change in the MR ratio depends on a difference in irradiation with O −  ions. 
     As illustrated in  FIG. 2 , a edge portion of the substrate  111  has a higher MR ratio than that of a central portion of the substrate  111 . This tendency holds even when the distance (T/S) is changed. Thus, the MR ratios illustrated in  FIG. 2  are mainly caused by a difference in the in-plane position on the substrate  111  to be treated, rather than change in influence of O −  ion irradiation according to distance (T/S). 
     As illustrated in  FIG. 3 , when the substrate  111  to be treated is rotating in a direction of arrow  115  around the center thereof, an edge portion of the substrate  111  with higher MR ratio corresponds to an intermittent irradiation region of sputtering particles, that is, an intermittent irradiation region  140  of O −  ions. Although the central portion of the substrate  111  to be treated is included in part of a region in which O −  ions spread, the central portion of the substrate  111  to be treated is continuously irradiated with O −  ions, in the structure of an ordinary film formation apparatus. Thus, the MR ratio in the edge portion of the substrate  111  is increased as illustrated in  FIG. 2 , because MgO is repeatedly damaged and relieved by intermittent irradiation of O −  ions, and thereby MgO is formed with higher quality and promoted (001) orientation. 
     Specifically, the results in  FIG. 2  show that expression of high MR ratio requires intermittent irradiation with O −  ions emitted from the MgO target. In the present embodiment, an MTJ element having high MR ratio is produced by performing intermittent irradiation. 
     The following is explanation of method of manufacturing a magnetoresistive element and a manufacturing apparatus of the same according to the present embodiment. 
     First Embodiment 
       FIG. 4  is a cross-sectional view illustrating a schematic structure of a magnetoresistive element manufacturing apparatus according to the first embodiment, and illustrating an example of a sputtering apparatus.  FIG. 5  is a plan view illustrating positional relationship between a substrate to be treated and a substrate shutter in  FIG. 4 . 
     A first rotary stage  110 , on which a substrate  111  to be treated is to be placed, is installed in a film formation chamber  100  used for sputtering. The stage  110  is provided to be rotatable by a motor (not shown), and to rotate the substrate  111  in a direction of the arrow  115  around the center of the substrate  111 . The substrate  111  to be treated is used for forming an MTJ element. For example, in the substrate  111 , a first ferromagnetic layer, such as CoFeB, is formed on a base substrate. 
     A sputter target  121  is placed in a position opposing the stage  110  in the chamber  100 . Although a normal vector of the center of the target  121  is directed to a central portion of the substrate, it may be shifted from the center of the substrate, since the sputter particles and O −  ions going from the target  121  in a direction of arrow  123  spread on the surface of the substrate  111 . The target  121  is sputtered by RF electric power applied to a space between the target  121  and the chamber  100  or the stage  110 . The target  121  functions as a tunnel barrier layer of the MTJ element, and is formed of, for example, MgO. 
     A substrate shutter  130  to isolate the substrate  111  and the target  121  from each other is placed in a position between the stage  110  and the target  121  and near the stage  110 . The substrate shutter  130  has a length about several times as long as the diameter of the substrate  111 , and a width approximately equal to the diameter of the substrate  111 . The substrate shutter  130  has an axis  133  in a position distant from the center of the stage  110 , and is rotatable in a direction of arrow  135 . Rotation of the substrate shutter  130  intermittently isolates the substrate  111  from the target  121 . 
     A target shutter  122  is disposed in a position between the target  121  and the stage  110  and near the target  121 . The target shutter  122  prevents damage in the chamber  100  caused by unstable spread of plasma, and contamination of the substrate surface by particles caused by massive discharge of films from the surface of the target, when electric discharge is generated on the surface of the target. The target shutter  122  is controlled independently of the substrate shutter  130 . 
     In the above apparatus, the whole surface of the substrate  111  is intermittently irradiated with sputter particles and O −  ions in periods of a second or less, by rotation of the stage  110 , on which the substrate  111  is placed, and the substrate shutter  130 . Specifically, intermittent O −  ion treatment is performed. Although the substrate shutter  130  may be driven to perform straight-line motion or arc-like reciprocal motion, it is preferable to adopt rotary motion to perform high-speed driving with less load on the driving motor and less malfunction frequency. 
     To start film formation in the above apparatus, the target  121  is sputtered by RF discharge in a state where the target shutter  122  is closed. By the sputtering, sputter particles are discharged from the target  121 , and O −  ions are also discharged. Then, after sputtering becomes stable, the target shutter  122  is opened, and sputtering film formation is started. 
     When sputtering film formation is started, the stage  110  is rotated, and the substrate shutter  130  is rotated in advance at a speed of about 100 rpm. Since the substrate  111  is rotated by rotation of the stage  110 , the whole surface of the substrate  111  is uniformly irradiated with the sputter particles and O −  ions. In addition, since the substrate shutter  130  is rotating, the whole surface of the substrate  111  is intermittently irradiated with the sputter particles and O −  ions from the target  121 . Since O −  ions are intermittently applied, MgO formed on the substrate  111  is damaged and relieved repeatedly, and MgO with higher quality and promoted (001) orientation is formed. 
     Although the rotational speed of the substrate shutter  130  is not specifically limited, too low a speed reduces the effect obtained by repeated damage and relief, and thus certain high speed should be adopted. Experiments performed by the inventors of the present invention proved that sufficient rotational speed of the substrate shutter  130  was speed with periods of a second or more. 
     The sputtering apparatus is not always limited to RF sputtering, but DC sputtering may be adopted. In either of RF sputtering and DC sputtering, the surface of the substrate is exposed to damage caused by sputter plasma. By subjecting the whole surface of the substrate to intermittent treatment as in the present embodiment, the film on the surface of the substrate is repeatedly damaged and restored, arrangement of atoms forming the film is optimized, and the property of the film is improved on the whole surface of the substrate. In the above treatment, the damage source is, for example, O −  or recoil sputter gas atoms in RF sputtering, and electrons or recoil sputter gas atoms in DC sputtering. 
     Next, a method of manufacturing magnetoresistive element using the sputtering apparatus of  FIG. 4  will be explained hereinafter, with reference to step cross-sectional views of  FIGS. 6A to 6I . 
     First, as illustrated in  FIG. 6A , an underlayer  12  formed of Ru and having a thickness of 2 nm, and a CoFeB layer (first ferromagnetic layer)  13  having a thickness of 2 nm are formed on a lower interconnect layer  11  formed of Ta and having a thickness of 5 nm. The method for forming the underlayer  12  and the first ferromagnetic layer  13  may be any of sputtering, molecular beam epitaxy (MBE), atomic layer deposition (ALD), and chemical vapor deposition (CVD), or another method. The underlayer  12  may also serve as a lower electrode layer or a reference layer. The ferromagnetic layer  13  may be used as a reference layer or a storage layer. 
     Next, as illustrated in  FIG. 6B , an MgO tunnel barrier layer  14  is formed. The MgO tunnel barrier layer  14  has been subjected to intermittent irradiation with O −  ions over the whole substrate by the manufacturing apparatus, to which the present embodiment is applied. 
     Specifically, the structure illustrated in  FIG. 6A  is used as a substrate to be treated, and placed on the stage  110  of the apparatus illustrated in  FIG. 4 . Then, the substrate shutter  130  is rotated together with rotation of the stage  110  to subject the MgO target  121  to RF sputtering, and thereby an MgO layer (tunnel barrier layer)  14  having a thickness of 1 nm is formed on the ferromagnetic layer  13 . By intermittent O −  ion irradiation using the substrate shutter  130 , the MgO layer  14  is repeatedly damaged and relieved, and thereby has high quality and promoted (001) orientation. 
     Next, as illustrated in  FIG. 6C , a CoFeB layer (second ferromagnetic layer)  15  having a thickness of 2 nm is formed on the tunnel barrier layer  14 , and an upper layer  16  formed of Ta is formed thereon. The ferromagnetic layer  105  may be used as a storage layer or a reference layer. The upper layer  106  may be used as a etching mask, a reference layer, a surface protective layer, or an upper interconnect connection layer. 
     Next, as illustrated in  FIG. 6D , the upper layer  16 , the second ferromagnetic layer  15 , the tunnel barrier layer  14 , the first ferromagnetic layer  13 , and the underlayer  12  are successively and selectively etched by ion milling or the like, to form a laminate structure part formed of the underlayer  12  to the upper layer  16  with an island shape. 
     Then, as illustrated in  FIG. 6E , an insulation layer  17  to protect an MTJ part is formed in the next step, by sputtering, CVD, or ALD. The insulation layer  17  is, for example, SiN, SiOx, MgO, or AlOx, and formed on an upper surface and side surfaces of the MTJ part and an exposed upper surface of the lower interconnect layer  11 . 
     Next, the lower interconnect layer  11  is selectively etched by, for example, reactive ion etching (RIE). The processed portions of the lower interconnect layer  11  are located in the front part and the rear part of  FIG. 6E , and not shown. In the etching, the MTJ part is protected by the insulation layer  17  illustrated in  FIG. 6E . 
     Then, as illustrated in  FIG. 6F , an insulation layer  18  is formed on the insulation layer  17  by sputtering or CVD or the like, to bury the MTJ part. The insulation layer  18  is, for example, SiOx. 
     Next, as illustrated in  FIG. 6G , the insulation layer  18  is subjected to etchback by chemical mechanical polishing (CMP) or gas phase etching, to expose an upper surface of the upper layer  16  of the MTJ part. 
     Then, as illustrated in  FIG. 6H , an insulation layer  19  is formed on the MTJ part and the insulation layer  18 , and then a contact hole  20  is opened on the MTJ part. The insulation layer  19  is, for example, SiOx. 
     Next, as illustrated in  FIG. 61 , an upper interconnect layer  21  formed of Al or Al—Cu is formed, and subjected to selective etching to have an interconnect pattern by RIE or the like. Thereby, a magnetoresistive element is finished. 
     As described above, according to the present embodiment, the whole substrate is intermittently exposed to sputter particles and O −  ions in a region of a normal vector direction of the center of the MgO target, when the MgO tunnel barrier layer  14  of the magnetoresistive element is formed. Thereby, MgO of the whole substrate is repeatedly subjected to damage and relief caused by O −  ions, and obtains improved quality. Then, the (001) orientation of MgO is promoted, and thereby high MR ratio is expressed over the whole substrate. 
     Thus, the present embodiment enables production of magnetoresistive elements having excellent property as memory elements of MRAMs, which is extremely effective. The sputtering apparatus thereof is obtained by only providing a conventional apparatus with the rotatable substrate shutter  130 , and can be achieved without large change in a conventional apparatus. 
     Second Embodiment 
       FIG. 7  is a cross-sectional view illustrating a schematic structure of a magnetoresistive element manufacturing apparatus according to a second embodiment, and illustrating an example of a sputtering apparatus.  FIG. 8  is a plan view illustrating positional relationship between a substrate to be treated and a substrate shutter in  FIG. 7 . 
     In the present embodiment, a second rotary stage  210  is installed in a chamber  100 , instead of the substrate shutter  130  illustrated in  FIG. 4 . In addition, a first rotary stage  110  is placed in a region on the stage  210 , which is shifted from the center of the stage  210 . Specifically, the second rotary stage  210  has a diameter at least twice as large as a diameter of the first rotary stage  110 , and rotates in a direction of arrow  215  on an axis  213  that is distant from the center of the first rotary stage  110 . Thereby, the substrate  111  to be treated rotates on its own axis by rotation of the stage  110 , and revolves (around the axis  213 ) by rotation of the stage  210 . 
     When the substrate  111  to be treated revolves (around the axis  213 ), the surface of the substrate  111  to be treated is exposed to sputter particles and O −  ions from the target  121  in a position where the substrate  111  is opposed to the target  121 , but not exposed to sputter particles or O −  ions in other positions. Specifically, the whole surface of the substrate  111  to be treated is intermittently irradiated with O −  ions from the target  121 , like the case where the substrate shutter  130  is rotated. Although the second rotary stage  210  may be driven to reciprocally move in a straight-line direction, it is preferable to adopt rotary motion to perform high-speed driving with less load on the driving motor and less malfunction frequency. Although the rotational speed of the second rotary stage  210  is not specifically limited, the rotational speed is preferably a speed at which the stage  210  performs one rotation in a second or less, like the rotational speed of the substrate shutter  130 . 
     The specific process of manufacturing the MTJ element using the present apparatus is similar to the first embodiment, as illustrated in  FIGS. 6A to 6I . 
     As described above, according to the present embodiment, the second rotary stage  210  is rotated together with the first rotary stage  110 , and thereby the whole surface of the substrate  111  can be intermittently exposed to a region of a normal vector region of the center of the MgO target, like the first embodiment. Thus, high MR ratio can be obtained through the whole substrate, and it is possible to manufacture magnetoresistive elements having excellent property for MRAMs, in the same manner as the first embodiment. 
     Third Embodiment 
       FIG. 9  is a cross-sectional view illustrating a magnetoresistive element manufacturing apparatus used for a third embodiment. 
     The apparatus includes a ferro-magnetic layer film-formation device, in addition to a film-formation device for forming a tunnel barrier layer of an MTJ element. 
     A film-formation chamber  300  of a DC sputtering apparatus for a ferromagnetic layer is installed adjacent to the film-formation chamber  100  of the RF sputtering apparatus described in the first or second embodiment. 
     A rotary stage  310  to place a substrate  111  to be treated on is installed in the chamber  300 , and a sputter target  321  is placed in a position opposed to the stage  310 . The chambers  100  and  300  are connected to each other by a gate valve  351 . In addition, the chamber  300  is provided with a gate valve  352  to take out and put in the substrate  111  to and from the outside (atmosphere). 
     The ferromagnetic layer film-formation device is not limited to a DC sputtering apparatus, but may be an MBE device, an ALD device, or a CVD device. 
     In the present embodiment, the substrate  111  to be treated is conveyed into the chamber  300  and placed on the stage  310 , in a state where the gate valve  352  is opened. Then, after the gate valve  352  is closed, the first ferromagnetic layer  13  illustrated in  FIG. 6A  is formed by sputtering. The substrate  111  has a structure in which the underlayer  12  is formed on the lower interconnect layer  11 . The underlayer  12  may be formed in the chamber  300  before the ferromagnetic layer  13  is formed, by preparing a target for the underlayer  12  in the chamber  300 . 
     Next, the gate valve  351  is opened, and then the substrate  111  is conveyed into the chamber  100  and placed on the stage  110 . Then, after the gate valve  351  is closed, the tunnel barrier layer  14  illustrated in  FIG. 6B  is formed by sputtering, while the stage  110  and the substrate shutter  130  are rotated at high speed. 
     After the tunnel barrier layer  14  is formed, the gate valve  351  is opened, and then the substrate  111  is returned into the chamber  300  and placed on the stage  310 . Then, after the gate valve  351  is closed, the second ferromagnetic layer  15  illustrated in  FIG. 6C  is formed by sputtering. The steps after this are similar to the steps illustrated in  FIGS. 6D to 6I . 
     As described above, according to the present embodiment, it is possible to successively form an MTJ film, in which the tunnel barrier layer  14  with high quality and promoted (001) orientation is held between the first and second ferromagnetic layers  13  and  15 . It is thus possible to produce MTJ elements with high MR ratio. 
     Modification 
     The present invention is not limited to the above embodiments. 
     Although the substrate shutter and the second rotary stage are rotated at rotational speed of 100 rpm in the embodiments, their rotational speeds are not limited. However, too low rotational speed reduces the effect obtained by repeatedly damaging and relieving MgO, and they preferably performs one rotation per second or less. 
     In addition, the size of the substrate shutter in the first embodiment is not necessarily several times as large as the diameter of the substrate, but it suffices that the size of the substrate shutter is larger than the diameter of the substrate. The method for driving the substrate shutter is not limited to rotation, but may be any method that enables the shutter to be put into or taken out of the space between the target and the substrate at high speed. 
     Although single-wafer processing apparatuses are explained in the embodiments, the present invention is not limited to them, but may be applied to a batch apparatus that simultaneously performs treatment for a plurality of substrates to be treated. For example, in the apparatus illustrated in  FIG. 7 , a plurality of substrates  111  to be treated are placed on the second rotary stage  210 , and thereby tunnel barrier layers can be simultaneously formed on the respective substrates  111 . 
     In addition, the target material is not limited to MgO, but may be any metal that can function as a tunnel barrier layer. For example, Al 2 O 3  may be used as the target material. The target material is not limited to a simple substance thereof, but may be a material including one of them as a main component. 
     The present invention is not limited to formation of a tunnel barrier layer, but is applicable to formation of a ferromagnetic layer. Application of the present invention to formation of a ferromagnetic layer such as CoFeB reduces plasma damage. As a result, the layer has improved flatness, and increase in pressure resistance of MgO and reduction in variations are achieved when MgO or the like is formed on CoFeB. In addition, the MR ratio is improved. 
     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.