Abstract:
A vertical magnetic memory device includes a pinned layer including a plurality of first ferromagnetic layers that are alternately stacked with at least one first spacer, wherein the pinned layer is configured to have a vertical magnetization, a free layer including a plurality of second ferromagnetic layers that are alternately stacked with at least one second spacer, and a tunnel barrier coupled between the pinned layer and the free layer.

Description:
CROSS-REFERENCES TO RELATED APPLICATION 
       [0001]    The present application claims priority under 35 U.S.C. §119(a) to Korean application number 10-2011-0078270, filed on Aug. 5, 2011, in the Korean Intellectual Property Office, which is incorporated herein by reference in its entirety as set forth in full. 
       to BACKGROUND 
       [0002]    1. Technical Field 
         [0003]    The present invention relates to a semiconductor memory device, and more particularly, to a magnetic memory device and a fabrication method thereof. 
         [0004]    2. Related Art 
         [0005]    A magnetic memory device stores information using a magnetic field and provides low power consumption, durability and fast operation speeds. Moreover, since a magnetic memory device has a nonvolatile characteristic where data can be maintained even in a power-off state, its use as a portable memory is being considered. 
         [0006]    As an example of a magnetic memory device, an MRAM (magnetoresistive random access memory) with a gigabit storage is being developed using a tunnel magnetoresistance (TMR) device. 
         [0007]    Here, a tunnel magnetoresistance effect is obtained by a pair of ferromagnetic layers and a tunnel insulation layer interposed therebetween. With respect to the tunnel magnetoresistance effect, since magnetic coupling does not substantially occur between the ferromagnetic layers, a large magnetic resistance can be obtained even in a weak magnetic field condition. Compared to a giant magnetoresistance (GMR) device, a TMR device may have a higher magnetoresistance and lower switching current for programming data. 
         [0008]    In being manufactured, a magnetic memory device has developed from a device in which ferromagnetic layers are horizontally magnetized to a device in which ferromagnetic layers are vertically magnetized. While CoFeB has been used as a ferromagnetic substance for causing horizontal magnetization, CoFeB may also be used as a vertical magnetization substance. 
         [0009]      FIG. 1  is a diagram illustrating the structure of a typical vertical magnetic memory device. 
         [0010]    Referring to  FIG. 1 , the vertical magnetic memory device has a structure in which a seed layer, a pinned layer, a tunnel barrier, a free layer and a capping layer are stacked. As a material of the pinned layer and the free layer, CoFeB may be used. 
         [0011]    Here, in fabricating a vertical magnetic memory device using CoFeB, the thickness of each of the pinned layer and the free layer may be limited to 2.2 nm or less because at a larger thickness, vertical magnetization characteristics start to disappear and horizontal magnetization characteristics start to gain strengths. Thus, when using CoFeB for a vertical magnetic memory device, the thickness of each of the pinned layer and the free layer is to be maintained at 2.2 nm or less. However, if the thickness of the pinned layer or the free layer decreases to 2.2 nm or less, thermal stability starts to deteriorate. 
         [0012]    During experiments, in the case of a magnetic memory device using CoFeB, thermal stability was detected to be about 43 at a device manufactured using  40  nm process. However, a magnetic memory device is desired to have a thermal stability target of about 60. Thus, in a vertical magnetic memory device using CoFeB, adequate thermal stability is difficult to obtain. 
         [0013]    Since, as discussed above, in using CoFeB in a magnetic memory device, while vertical magnetization characteristics may be obtained while thermal stability deteriorates when CoFeB layer is 2.2 nm or less in thickness and opposite characteristics are obtained when CoFeB layer is larger than 2.2 nm in thickness, it is difficult to use CoFeB in a vertical magnetic memory device. 
       SUMMARY 
       [0014]    In one embodiment of the present invention, a vertical magnetic memory device includes: a pinned layer including a plurality of first ferromagnetic layers that are alternately stacked with at least one first spacer, wherein the pinned layer is configured to have a vertical magnetization; a free layer including a plurality of second ferromagnetic layers that are alternately stacked with at least one second spacer; and a tunnel barrier coupled between the pinned layer and the free layer. 
         [0015]    In another embodiment of the present invention, a vertical magnetic memory device includes: capping layer and formed by alternately and repeatedly stacking a plurality of ferromagnetic layers with a plurality of spacers, wherein two of the ferromagnetic layers contact the seed layer and the capping layer, respectively. 
         [0016]    In another embodiment of the present invention, a method for fabricating a vertical magnetic memory device including a pinned layer, a free layer, and a tunnel barrier formed between the pinned layer and the free layer includes: forming the pinned layer by stacking a plurality of first ferromagnetic layers alternately with at least one first spacer, wherein the pinned layer is configured to have a vertical magnetization; and forming the free layer by stacking a plurality of second ferromagnetic layers with at least one second spacer. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0017]    Features, aspects, and embodiments are described in conjunction with the attached drawings, in which: 
           [0018]      FIG. 1  is a diagram illustrating the structure of a conventional vertical magnetic memory device; 
           [0019]      FIG. 2  is a configuration diagram of a magnetic memory device in accordance with an embodiment of the present invention; 
           [0020]      FIG. 3  is a configuration diagram of a magnetic memory device in accordance with another embodiment of the present invention; and 
           [0021]      FIG. 4  is a graph illustrating coupling characteristics between a ferromagnetic layer and a spacer in the magnetic memory device according to the present invention. 
       
    
    
     DETAILED DESCRIPTION 
       [0022]    Hereinafter, a magnetic semiconductor device and a fabrication method thereof according to the present invention will be described below with reference to the accompanying drawings through exemplary embodiments. 
         [0023]      FIG. 2  is a configuration diagram of a magnetic memory device in accordance with an embodiment of the present invention. 
         [0024]    Referring to  FIG. 2 , a vertical magnetic memory device  10  in accordance with an embodiment of the present invention has a structure in which a seed layer  110 , a pinned layer  120 , a tunnel barrier  130 , a free layer  140  and a capping layer  150  are stacked. 
         [0025]    In the pinned layer  120  and the free layer  140 , ferromagnetic layers  1210  and  1410  and spacers  1220  and  1420  may be alternately stacked in a repeated manner. In this regard, the pinned layer  120  is formed to have the overall height greater than that of the free layer  140  so that proper functions of the pinned layer  120  are maintained. 
         [0026]    In forming the pinned layer  120  to be higher than the free layer  140 , the number of stacked layers or the height of each stacked layer may be controlled. 
         [0027]    The vertical magnetic memory device  10  shown in FIG.  2  represents the case in which the number of stacked layers in the pinned layer  120  is controlled to be greater than the number of stacked layers of the free layer  140 . This is described in detail as follows. 
         [0028]    In  FIG. 2 , the pinned layer  120  may be formed by repeatedly stacking m (m is a natural number greater than or equal to or 2) layers of a compound material including CoFe as a constituent and the spacer  1220  in total, where the top layer is the ferromagnetic layer  1210 . 
         [0029]    The free layer  140  may be formed by repeatedly stacking n (n is a natural number smaller than m) layers of a compound material including CoFe as a constituent and the spacer  1420  in total, where the top layer is the ferromagnetic layer  1410 . 
         [0030]    The ferromagnetic layers  1210  and  1410  forming the pinned layer  120  and the free layer  140 , respectively, may each be formed of a compound material including CoFe as a constituent such as CoFeB, CoFe, CoFeBTa, and CoFeBSl. The thickness of each of the ferromagnetic layers  1210  and  1410  may be set to 0.1˜2.2 nm. Each of the spacers  1220  and  1420  forming the pinned layer  120  and the free layer  140  may have a thickness of 0.2˜2 nm and may be formed as an oxide spacer such as a MgO layer, a metal oxide spacer such as Al 2 O 3 , TiO 2 , HfO 2 , ZrO 2  or Ta 2 O 3  layer or a metal spacer such as Ru, Ta, W, Al or Ti layer. Here, the spacers  1210  and  1410  each cause an appropriate magnetic coupling so that the pinned layer  120  and the free layer  140  that are each made up of multiple layers can operate as if it was made with a single, unitary layer while maintaining thermal stability by having a sufficient overall thickness and avoiding a loss of vertical magnetization by having each individual layer with a thickness less than or equal to 2.2 nm despite having the overall thickness of the pinned layer  120  and the free layer  140  being greater than 2.2 nm. 
         [0031]    As the tunnel barrier  130 , an MgO layer may be used. In this regard, when MgO is grown on a crystal face (for example,  110 ), TMR may be increased by a factor or about 10 at a room temperature. 
         [0032]      FIG. 3  is a configuration diagram of a magnetic memory device in accordance with another embodiment of the present invention. 
         [0033]    Referring to  FIG. 3 , a vertical magnetic memory device  20  in accordance with another embodiment of the present invention includes a seed layer  210 , a pinned layer  220 , a tunnel barrier  230 , a free layer  240  and a capping layer  250 . 
         [0034]    In the exemplary embodiment, the pinned layer  220  and the free layer  240  have structures in which ferromagnetic layers  2210  and  2410  and spacers  2220  and  2420  are alternately stacked a number of times such that the ferromagnetic layers  2210  and  2410  are the top layers in each of the pinned layer  220  and the free layer  240 . Here, in order to form the pinned layer  220  to have an overall height greater than that of the free layer  240 , the height of each of the ferromagnetic layers  2210  constituting the pinned layer  220  is controlled to be higher than the height of each of the ferromagnetic layers  2410  constituting the free layer  240 . Here, the heights of the ferromagnetic layers  2210  may be the same or different, and the heights of the ferromagnetic layers  2410  may be the same or different. 
         [0035]    The pinned layer  220  may be formed by repeatedly stacking x (x is a natural number equal to or greater than 2) number of ferromagnetic layers  2210  of a compound material including CoFe as a constituent and the spacer  2220  in total so that the top layer is the ferromagnetic layer  2210 . 
         [0036]    The free layer  240  may be formed by repeatedly stacking the x number of the ferromagnetic layer  2410  that are each made of a compound material including CoFe as a constituent and the spacer  2420 , where the top layer is the ferromagnetic layer  2410 . 
         [0037]    The ferromagnetic layers  2210  and  2410  of the pinned layer  220  and the free layer  240 , respectively, may be formed of a compound material substance including CoFe as a constituent such as CoFeB, CoFe, CoFeBTa and CoFeBSi. According to an example, the thickness of the ferromagnetic layer  2210  of the pinned layer  220  is set to 0.1˜2.2 nm, and the thickness of the ferromagnetic layer  2410  constituting the free layer  240  is set to be smaller than the thickness of the ferromagnetic layer  2210  in the pinned layer  220 . 
         [0038]    Each of the spacers  2220  and  2420  of the pinned layer  220  and the free layer  240  may be formed to have a thickness of 0.2˜2 nm and may be formed of an oxide spacer such as an MgO spacer, a metal oxide spacer such as an Al 2 O 3 , TiO 2 , HfO 2 , ZrO 2  and Ta 2 O 3  spacer or a metal spacer such as a Ru, Ta, W, Al and Ti spacer. 
         [0039]    As the tunnel barrier  230 , a MgO layer may be used. Here, when MgO is grown on a crystal face (for example,  210 ), TMR may be increased by a factor of about 10 at a room temperature. 
         [0040]    In reference to the vertical magnetic memory devices shown in  FIGS. 2 and 3 , formation of an oxide spacer between ferromagnetic layers constituting a pinned layer and a free layer by using MgO are as follows. 
         [0041]    When the spacers  1220 ,  1420 ,  2220  and  2420  are formed using MgO, the thickness of each of the ferromagnetic layers  1210 ,  1410 ,  2210  and  2410  may be decreased without sacrificing the overall functions. Also, the adjacent ones of constituent magnetic layers of each of ferromagnetic layers  1210 ,  1410 ,  2210  and  2410  are ferromagnetically and antiferromagnetically coupled with each other, as appropriate, by the MgO spacers  1220 ,  1420 ,  2220 , and  2420 . Here, a sufficient overall volume/thickness for each of the pinned layers  120  and  220  and the free layers  140  and  240  are obtained to avoid a loss of vertical magnetization while decreasing the thickness of each of the ferromagnetic layers  1210 ,  1410 ,  2210  and  2410  to avoid of a vertical magnetization. Here, when a compound material including CoFe as a constituent is used as the material of the pinned layers  120  and  220  and the free layers  140  and  240 , the ferromagnetic layers  1210 ,  1410 ,  2210  and  2410  may be formed to have a thickness equal to or less than 2.2 nm so that vertical magnetization characteristics are not lost and a sufficient overall volume/thickness for each of the pinned layers  120  and  220  and the free layers  140  and  240  are obtained so that adequate thermal stability is obtained. 
         [0042]      FIG. 4  is a graph illustrating coupling characteristics between a ferromagnetic layer and a spacer in the magnetic memory device according to an exemplary embodiment of the present invention. 
         [0043]      FIG. 4  shows coupling characteristics between contact surfaces when a MgO layer is placed as a spacer between two ferromagnetic layers. 
         [0044]    In terms of ferromagnetic coupling characteristics, it is shown that a coupling energy J (erg/cm 2 ) reaches a maximum A when the thickness of a MgO layer serving as a spacer is 0.9 nm. In the case of antiferromagnetic coupling characteristics, it is shown that a coupling energy J (erg/cm 2 ) reaches a maximum B when the thickness of a MgO layer serving as a spacer is 0.6˜0.7 nm. 
         [0045]    Here, when the MgO spacer is interposed between ferromagnetic layers, any of the ferromagnetic coupling characteristics and the antiferromagnetic coupling characteristics may be obtained by adjusting the thickness of the MgO spacer, and thus, the two ferromagnetic layers may be coupled antiferromagnetically or ferromagnetically. By using appropriate magnetic couplings in a pinned layer or a free layer, the thickness of each of the ferromagnetic layers may be minimized while a sufficient overall volume/thickness of the pinned layer or the free layer is obtained. 
         [0046]    The magnetic memory devices shown in  FIGS. 2 and 3  include ferromagnetic layers having, for example, CoFeB magnetic layers and MgO spacers that are alternately stacked between the seed layers  110  and  210  and the capping layers  150  and  250 , respectively. Here, by using a MgO spacer as a tunnel barrier, a magnetic element formed on one side of the tunnel barrier may serve as a pinned layer, and a magnetic element formed on the other side of the tunnel barrier may serve as a free layer. 
         [0047]    In having the pinned layer to operate independently of the magnetization direction of the free layer, the tunnel barrier is formed (for example, by selecting one of MgO spacers that are alternatively stacked with CoFeB magnetic layers to form the free layer and the pinned layer) so that the height of the pinned layer is higher than the height of the free layer, where, according to an example, such a height determines the independence of the operation. 
         [0048]    In the exemplary embodiments of the present invention, when fabricating a vertical magnetic memory device, a pinned layer and a free layer are formed using a compound material including CoFe as a constituent. According to an example, by alternately stacking ferromagnetic layers made of a compound material including CoFe as a constituent and having a height of 2.2 nm or less, the vertical magnetization characteristics of a ferromagnetic layer may be maintained and the overall volume/thickness of each of the pinned layer and the free layer may be sufficient to obtain adequate thermal stability. 
         [0049]    Here, when under 40 nm processes are used for manufacturing semiconductor devices such as a 2× nm level, vertical magnetization characteristics are maintained while obtaining sufficient volumes/thicknesses of a pinned layer and a free layer to obtain thermal stability. Thus, a vertical magnetic memory device having smaller dimensions may be obtained. 
         [0050]    While specific embodiments have been described above, they are exemplary only. Accordingly, a magnetic semiconductor device and the fabrication method thereof as described herein should not be limited to the specific embodiments but should be broadly construed to include any other reasonably suitable devices/methods consistent with the above-described features of the exemplary embodiments.