Abstract:
A magnetic memory device includes a substrate, a magnetic tunneling junction (MTJ) structure disposed on the substrate, and a capping layer disposed on the MTJ structure. By adding a capping layer on the MTJ structure, the property of the magnetic memory device is improved, the magnetoresistance (MR) ratio is raised, and the time cost by the magnetic memory device to process data is effectively reduced.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
       [0001]    This non-provisional application claims priority under 35 U.S.C. § 119(a) on Patent Application No(s). 096104417 filed in Taiwan, R.O.C. on Feb. 7, 2007, the entire contents of which are hereby incorporated by reference. 
       BACKGROUND OF THE INVENTION 
       [0002]    1. Field of Invention 
         [0003]    The present invention relates to a magnetic memory device, and more particularly to a technology of using a capping layer to improve the property of the magnetic memory device. 
         [0004]    2. Related Art 
         [0005]    Magnetic random access memory (MRAM) is a new memory device having the advantages of being non-volatile, having high density, high read/write speed, and anti-radiation property. When writing data, a common method employs two current lines, a bit line and a word line, to sense magnetic memory cells selected by the intersection of magnetic fields, so as to change the magnetoresistance (MR) value by changing the magnetization vector direction of a free layer (or referred as a sensing layer). When reading data from the memory, the selected magnetic memory cells are connected to the read current source, so that the digital value of the memory data can be determined from the read resistance value. 
         [0006]    Such a magnetic memory cell has a multi-layered stacking structure of magnetic materials, which is formed by stacking an anti-ferromagnetic material layer, a fixing layer formed by ferromagnetic/non-magnetic metal/ferromagnetic materials, a tunneling barrier insulating layer, and a free layer made of ferromagnetic materials. The status of the memory “0” or “1” is determined by a parallel or anti-parallel structure of the magnetization vector of two layers of ferromagnetic materials. However, when the magnetic memory is designed towards the trend of having a high density, the size of the magnetic memory cell must be reduced, resulting in a greater switching field of the free layer, which not only requires a greater current, but also causes a great challenge for the circuit design. Therefore, in designing a magnetic memory device, besides reducing the current, another important task is to achieve the consistency in switching the free layer of all the magnetic memory cells. 
         [0007]    Moreover, how to shorten the response time required by the magnetic memory device for processing data and raise the MR ratio of the magnetic device is also an important issue in developing magnetic memory devices. Thus, currently, the MR ratio can be raised in two aspects. One aspect lies in researching and developing the tunneling layer (for example, MgO) of the magnetic device, through which it is found that the MR ratio can be effectively raised to above 200%. Meanwhile, in order to take the magnetic switching property into consideration, the material of the free layer must be changed from CoFeB with a high MR ratio into NiFe magnetic materials with a low magnetostriction coefficient, so as to solve the consistency problem in switching the free layer. However, the MR ratio would be greatly decreased, which is great problem in the design of the magnetic device. The other aspect lies in development of AIO x  tunneling layer device, in which different capping layers are employed to improve the thermal stability and the MR ratio. 
       SUMMARY OF THE INVENTION 
       [0008]    Accordingly, it is an important task in the technology of developing magnetic memory devices on how to improve the property of the magnetic memory device, raise the MR ratio, and effectively reduce the access time by the magnetic memory device in processing data. 
         [0009]    The present invention discloses a magnetic memory device, which includes a substrate, a first ferromagnetic layer disposed on the substrate, an insulating layer disposed on the first ferromagnetic layer, a second ferromagnetic layer disposed on the insulating layer, and a capping layer stacked on the second ferromagnetic layer. Herein, stacking the capping layer on the second ferromagnetic layer aims at preventing metal ions in the etching mask from diffusing to the second ferromagnetic layer, so as to prevent the switching property of the magnetic memory device from getting worse. Furthermore, the capping layer can be utilized to effectively raise the MR ratio and maintain a preferred magnetic switching property of the memory device. 
         [0010]    Further scope of applicability of the present invention will become apparent from the detailed description given hereinafter. However, it should be understood that the detailed description and specific examples, while indicating preferred embodiments of the invention, are given by way of illustration only, since various changes and modifications within the spirit and scope of the invention will become apparent to those skilled in the art from this detailed description. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0011]    The present invention will become more fully understood from the detailed description given herein below for illustration only, which thus is not limitative of the present invention, and wherein: 
           [0012]      FIG. 1  is a side view of a magnetic memory device according to one embodiment of the present invention; and 
           [0013]      FIG. 2  is a side view of a magnetic memory device according to another embodiment of the present invention. 
       
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
       [0014]    A magnetic device is provided through the embodiments of the present invention. In the magnetic device, a capping layer is added to raise the MR ratio of the whole magnetic device, and meanwhile, maintain a preferred magnetic switching motion of the magnetic device. 
         [0015]    Referring to  FIG. 1 , a magnetic memory unit is constituted by a multi-layered magnetic structure, which is generally formed by sequentially stacking three layers: a first ferromagnetic layer  11 , an insulating layer  12  or composite ferromagnetic layers, and a second ferromagnetic layer  13 . The first ferromagnetic layer  11  is a pinned layer, the insulating layer  12  is a tunneling barrier layer, and the second ferromagnetic layer  13  is a free layer. Memory data is determined by the magnetic moment arrangement status of the first, second ferromagnetic layers  11 ,  13  respectively the insulating layer  12  is stacked between these ferromagnetic layers. During the etching process of the magnetic memory unit, an adopted etching mask  18  is a metal mask, which is generally made of Ta or TaN. However, during the fabrication process, the Ta ions are easily diffused and result in damaging the second ferromagnetic layer  13 . A capping layer  16  is added on the second ferromagnetic layer  13  to prevent the inter-diffusion between the etching mask  18  and the second ferromagnetic layer  13 . Furthermore, the capping layer  16  can also be used to adjust the magnetic property of the second ferromagnetic layer (the free layer)  13 . Furthermore, in this embodiment, more importantly, it is found that, the capping layer  16  can significantly raise the MR ratio of the whole magnetic memory device, accelerate the data-reading speed, and improve the thermal stability of the device. 
         [0016]    Next, referring to  FIG. 2 , it shows a magnetic memory device according to another preferred embodiment of the present invention. The magnetic memory device includes a substrate  100 , a first ferromagnetic layer  102 , an insulating layer  104 , a second ferromagnetic layer  106 , a capping layer  108 , and an etching mask  110 . The first ferromagnetic layer  102  is disposed on the substrate  100 , the insulating layer  104  is disposed between the first ferromagnetic layer  102  and the second ferromagnetic layer  106 , the capping layer  108  is disposed on the second ferromagnetic layer  106 , and the etching mask  110  is disposed on the capping layer  108 . 
         [0017]    Herein, the first ferromagnetic layer  102  is a pinned layer, the second ferromagnetic layer  106  is a free layer, and the insulating layer  104  is a tunneling barrier layer. Therefore, the first ferromagnetic layer  102 , the insulating layer  104 , and the second ferromagnetic layer  106  can be sequentially stacked to form a magnetic tunneling junction (MTJ) structure. In this embodiment, the material of the first ferromagnetic layer  102  can be various magnetic materials such as CoFe or CoFeB. The material of the insulating layer  104  can be, but not limited to, AIO x , MgO, or Fe 3 O 4 . The material of the second ferromagnetic layer  106  can be various magnetic materials such as NiFe or CoFeB. 
         [0018]    In this embodiment, in order to protect the MTJ structure in the magnetic memory device, a single oxide layer, a single metal layer, or a oxide/metal composite layer serves as the capping layer  108 , to prevent the cross-diffusion from occurring between the etching mask  110  on the capping layer  108  and the free layer (second ferromagnetic layer)  106 . Herein, the material of the single oxide layer can be MgO, the material of the single metal layer can be Ta or Ru, and the material of the oxide/metal composite layer can be MgO/Mg or Ru/MgO. Herein, the oxide/metal composite layer serving as the capping layer  108  can achieve the most preferred effect. For example, if the material of the insulating layer  104  is AIO x  and the material of the second ferromagnetic layer  106  is NiFe, the MgO or MgO/Mg composite layer serves as the capping layer  108  to improve the magnetic switching property, and to raise the MR ratio of the magnetic memory device from the original 20% to about 30% to 33%. 
         [0019]    Furthermore, another advantage of using the MgO/Mg composite layer as the capping layer  108  lies in effectively preventing Ta contained in the etching mask  110  from diffusing under a high temperature, so as to maintain the property of the second ferromagnetic layer (the free layer)  106 . 
         [0020]    The invention being thus described, it will be obvious that the same may be varied in many ways. Such variations are not to be regarded as a departure from the spirit and scope of the invention, and all such modifications as would be obvious to one skilled in the art are intended to be included within the scope of the following claims.