Abstract:
The present invention discloses a magnetoresistive random access memory (MRAM) and a manufacturing method thereof. The whole cells are connected to each other by using a substrate as a ground terminal and a vertical structure field effect transistor (FET) for connecting the cells to the bit line. Thus, the MRAM is easily manufactured without requiring a special process for isolation of each cell. The MRAM uses the vertical structure FET to simplify the whole manufacturing process. An MTJ cell mask process which is essential in a general horizontal structure FET is omitted, to improve a speed of the MRAM and attain high integration of the MRAM.

Description:
BACKGROUND OF THE INVENTION  
         [0001]    1. Field of the Invention  
           [0002]    The present invention relates to a magnetoresistive random access memory (MRAM) and a manufacturing method thereof, and more particularly to an MRAM and a manufacturing method thereof which can simplify the whole process and increase a memory speed by improving an MRAM cell structure.  
           [0003]    2. Description of the Background Art  
           [0004]    Most of the semiconductor memory manufacturing companies have developed an MRAM using a ferromagnetic material as one of the next generation memory devices.  
           [0005]    The MRAM is a memory device for reading and writing data by forming multi-layer ferromagnetic thin films, and sensing current variations according to magnetization directions of the respective thin films. The MRAM has a high speed, low power consumption and high integration density because of the special properties of the magnetic thin films, and performs a nonvolatile memory operation like a flash memory.  
           [0006]    The MRAM research is still in its early stage, and mostly concentrated on the formation of multi-layer magnetic thin films, less on a unit cell structure and a peripheral sensing circuit, etc.  
           [0007]    [0007]FIG. 1 is a cross-sectional diagram illustrating a magnetic tunnel junction (MTJ) cell having a multi-layer magnetic thin film structure for storing data in the conventional MRAM.  
           [0008]    In general, the MTJ cell  5  includes a anti-ferroelectric thin film  1 , a pinned ferromagnetic thin film  2 , a thin insulation layer  3  which tunneling current flows through, and a free ferromagnetic thin film  4 .  
           [0009]    Here, the magnetization direction of the pinned ferromagnetic thin film  2  is pinned to one direction, and the anti-ferroelectric thin film  1  fixes the magnetization direction of the pinned ferromagnetic thin film  2 . Conversely, the magnetization direction of the free ferromagnetic thin film  4  is changed by an external magnetic field.  
           [0010]    When current flows through the MTJ cell  5  in the vertical direction, tunneling current is generated through the insulation layer  3 . When the pinned ferromagnetic thin film  2  and the free ferromagnetic thin film  4  have the same magnetization direction, the tunneling current increases, and when the pinned ferromagnetic thin film  2  and the free ferromagnetic thin film  4  have opposite magnetization directions, the tunneling current decreases.  
           [0011]    This phenomenon is called a tunneling magnetoresistance (TMR) effect. The magnetization direction of the free ferromagnetic thin film  4  can be sensed by sensing the amount of the tunneling current, and thus data stored in the cell can be read in the read operation.  
           [0012]    In the write operation, the magnetization direction of the free ferromagnetic thin film  4  is decided according to polarity of current by a level of voltage transmitted to a write word line, and thus the MTJ cell  5  stores a data of ‘0’ or ‘1’.  
           [0013]    [0013]FIG. 2 a  is an exemplary diagram illustrating an MRAM cell using a horizontal structure field effect transistor (FET).  
           [0014]    The unit cell of the MRAM includes one horizontal structure FET  9 , an MTJ cell  5 , a read word line  6 , a write word line  8  and a bit line  7 .  
           [0015]    The read word line  6  is used to read data to control the FET  9 . The write word line  8  receives current, forms an external magnetic field, and changes the magnetization direction of the free ferromagnetic thin film  4  of the MTJ cell  5  by using the external magnetic field, to store data. The bit line  7  transmits current to the MTJ cell  5  in the vertical direction, to sense the magnetization direction of the free ferromagnetic thin film  4 .  
           [0016]    In the read operation, the conventional MRAM transmits voltage to the read word line  6  to operate the FET  9 , transmits current to the bit line  7 , and senses the amount of current flowing through the MTJ cell  5 .  
           [0017]    In the write operation, the MRAM maintains an off state of the FET  9 , transmits current to the write word line  8  and the bit line  7 , and thus changes the magnetization direction of the free ferromagnetic thin film  4  of the MTJ cell  5  by the external magnetic field.  
           [0018]    Here, the current is transmitted to the bit line  7  and the write word line  8  at the same time because the magnetic field is most actively generated in the vertical crossing point of the two metal lines. Accordingly, one cell can be selected from a few cell arrangements,  
           [0019]    [0019]FIG. 2 b  is a cross-sectional diagram illustrating an MRAM corresponding to the conventional MRAM cell of FIG. 2 a.    
           [0020]    A ground line  12  is formed on a source  10  of the horizontal structure FET  9 , the read word line  6  is formed on a gate thereof, and a conductive layer  13 , a contact plug  14 , a conductive layer  15  and a contact plug  16  are sequentially formed on a drain  11  thereof. A connection layer  17  is formed on the write word line  8 , and the MTJ cell  5  and the bit line  7  are stacked on the connection layer  17 .  
           [0021]    One of the biggest problems in the manufacturing process of the MRAM is to control surface roughness of the lower structure in which one MTJ cell  5  is formed below a few nm. In the MRAM of FIG. 2b, the write word line  8  and the connection layer  17  are formed in the lower portion of the MTJ cell  5 , and thus surface roughness is hardly controlled below a few nm.  
           [0022]    In addition, the conventional MRAM includes two word lines and one bit line per unit cell as well as a ground line for grounding, namely four metal lines, which increases surface resistances. It is very important to control the resistances.  
           [0023]    When the transistor is small, it is much more difficult to control the external resistance of the transistor. Such resistances in combination with the resistances of the MTJ cell have detrimental effects on the cell operation. A short channel effect and resistance variation of the horizontal structure FET deteriorate a few hundreds Giga-level integration of the MRAM.  
         SUMMARY OF THE INVENTION  
         [0024]    The present invention is achieved to solve the above problems. Accordingly, it is a primary object of the present invention to provide a high integration magnetoresistive random access memory which can minimize a short channel effect and resistance variation of a transistor by using a vertical structure transistor.  
           [0025]    Another object of the present invention is to provide a high integration magnetoresistive random access memory which can simplify the whole process by forming a magnetic tunnel junction cell according to a self-alignment method, without using a special mask process.  
           [0026]    In order to achieve the above-described objects of the invention, there is provided a magnetoresistive random access memory including a vertical structure field effect transistor, wherein the vertical structure field effect transistor has its channel column and drain sequentially formed on an insulation substrate, its source area formed on the insulation substrate on which the channel columns have not been formed, and its read word lines formed around the channel columns to serve as gates, a contact line, a magnetic tunnel junction cell, a bit line and a write word line are sequentially formed on the drain area, and patterns of the magnetic tunnel junction cell are formed according to the self-alignment method, without requiring a special mask process for forming the patterns.  
           [0027]    According to one aspect of the invention, a manufacturing method of a magnetoresistive random access memory includes: a first process for forming a vertical structure field effect transistor by sequentially forming a channel column and a drain on an insulation substrate, a source area on the insulation substrate on which the channel columns have not been formed, and read word lines around the channel columns serving as gates; a second process for forming a magnetic tunnel junction cell and a bit line on the vertical structure field effect transistor, and patterning the magnetic tunnel junction cell according to the self-alignment method aligning the magnetic tunnel junction cell in a matrix shape, by using the bit line and read word line masks; and a third process for patterning and etching a write word line on the bit line in the right angle direction to the bit line. 
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0028]    The present invention will become better understood with reference to the accompanying drawings which are given only by way of illustration and thus are not limitative of the present invention, wherein:  
         [0029]    [0029]FIG. 1 is a cross-sectional diagram illustrating a conventional MTJ cell;  
         [0030]    [0030]FIGS. 2 a  and  2   b  are an exemplary diagram and a cross-sectional diagram illustrating a conventional MRAM using a horizontal structure FET;  
         [0031]    [0031]FIGS. 3 a  and  3   b  are an exemplary diagram and a cross-sectional diagram illustrating an MRAM using a vertical structure FET in accordance with the present invention; and  
         [0032]    [0032]FIGS. 4 a  to  4   n  are flowcharts showing sequential steps of a manufacturing method of an MRAM using a vertical structure FET in accordance with the present invention. 
     
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS  
       [0033]    An MRAM and a manufacturing method thereof in accordance with a preferred embodiment of the present invention will now be described in detail with reference to the accompanying drawings.  
         [0034]    [0034]FIG. 3 a  is an exemplary diagram illustrating an MRAM using a vertical structure FET in accordance with the present invention.  
         [0035]    The MRAM includes a vertical structure FET  29 , an MTJ cell  30 , a read word line  31 , a write word line  32  and a bit line  33 .  
         [0036]    The read word line  31  is used to read data to control the vertical structure FET  29 . The write word line  32  receives current, forms an external magnetic field, and stores data according to variations of the magnetization direction of a free ferromagnetic thin film of the MTJ cell  30 . The bit line  33  transmits current to the MTJ cell  30  in the vertical direction, to sense the magnetization direction of the free ferromagnetic thin film.  
         [0037]    [0037]FIG. 3 b  is a cross-sectional diagram illustrating the MRAM using the vertical structure FET corresponding to FIG. 3 a  in accordance with the present invention.  
         [0038]    A source  35 , a channel  50  and a drain  36  are stacked on an insulation substrate  34 , and the read word lines  31  separated by an oxide film  38  are formed around the channel  50 .  
         [0039]    A drain contact line  37  is formed on the drain  36 , and the MTJ cell  30  is stacked on the contact line  37 . In addition, the bit line  33  is formed on the MTJ cell  30 , and the write word line  32  is formed on the bit line  33 .  
         [0040]    The MRAM using the vertical structure FET  29  simplifies the lower structure of the MTJ cell  30 , and thus easily controls surface toughness thereof. Moreover, the MRAM attains high speed and high density by simplifying the whole process.  
         [0041]    On the other hand, FIGS. 4 a  to  4   n  are flowcharts showing sequential steps of a manufacturing method of the MRAM in accordance with the present invention.  
         [0042]    Referring to FIG. 4 a , a cylindrical Si column  50  is masked on the insulation substrate  34 , to form a vertical structure channel.  
         [0043]    As shown in FIG. 4 b , the source  35  and the drain  36  are formed according to N+ ion implantation.  
         [0044]    As depicted in FIG. 4 c , the gate oxide film  38  is deposited on the whole surfaces of the source  35  and the drain  36  formed in the process of FIG. 4 b  and on the sidewalls of the cylindrical Si column  50 , to form the vertical structure FET  29 .  
         [0045]    As illustrated in FIG. 4 d , the read word line  31  comprised of polysilicon is deposited on the whole surface of the vertical structure FET  29  formed in the process of FIG. 4 c , and a planarization process is performed thereon.  
         [0046]    As shown in FIG. 4 e , an interlayer insulation film  39  is deposited on the whole surface of the read word line  31 .  
         [0047]    Referring to FIG. 4 f , a contact hole  51  is formed on the interlayer insulation film  39  for electrical connection of the MTJ cell  30  formed later and the drain  36  of the vertical structure FET  29 .  
         [0048]    As illustrated in FIG. 4 g , a polysilicon contact material is deposited on the whole surface of the interlayer insulation film  39 , to form the contact line  37 . Accordingly, the upper MTJ cell  30  and the drain  36  of the vertical structure FET  29  are connected through the contact hole  51 .  
         [0049]    As shown in FIG. 4 h , the MTJ cell  30  comprised of four thin films is sequentially deposited on the contact line  37  formed in the process of FIG. 4 g.    
         [0050]    As depicted in FIG. 4 i , photoresist  40  is patterned by using the read word line  31  mask.  
         [0051]    As shown in FIG. 4 j , the resulting structure are etched to expose the side of the read wordline by using the photoresist  40  patterned in the process of FIG. 4i, thereby forming the gate. The MTJ cell  30  thin films are patterned in the same shape as the gate.  
         [0052]    As illustrated in FIG. 4 k , an interlayer insulation film  41  is deposited on the whole surface of the MTJ cell  30  patterned in the process of FIG. 4 j , and a planarization process is performed thereon.  
         [0053]    Referring to FIG. 41, a metal line composing the bit line  33  is deposited on the whole surface of the interlayer insulation film  41  formed in the process of FIG. 4k. Here, the interlayer insulation film  41  prevents the bit line  33  from being connected to the vertical structure FET  29 , when the bit line  33  is deposited. Thereafter, photoresist  42  is patterned in the right angle direction to the read word line  31  by using the bit line  33  mask.  
         [0054]    As illustrated in FIG. 4 m , the resulting structure are etched to expose the side contact line  37  by using the photoresist  42  patterned in the process of FIG. 41, so that the MTJ cell  30  and the contact line  37  can be formed in the matrix shape. Here, the line-shaped thin films of the MTJ cell  30  are automatically transformed into the matrix thin films by the right angle direction bit line  33  mask. That is, the MTJ cell  30  can be formed according to the above-mentioned method, namely the self-alignment method, without using a special MTJ cell mask.  
         [0055]    As shown in FIG. 4 n , an interlayer insulation film  43  is deposited on the whole surface of the bit line  33  patterned in the process of FIG. 4 m , a planarization process is performed thereon, and the write word line  32  is deposited on the whole surface of the interlayer insulation film  43 . Then, the write word line  32  is patterned in the right angle direction to the bit line  33 , and etched. Here, the interlayer insulation film  43  prevents the bit line  33  from being connected to the write word line  32 , when the write word line  32  is deposited.  
         [0056]    In accordance with the present invention, the MTJ cell  30  is formed according to the self-alignment method without requiring a special mask process. The mask process is performed merely to form the read word line and bit line patterns, thereby simplifying the whole process and improving integration of the MRAM.  
         [0057]    As discussed earlier, in accordance with the present invention, surface roughness of the lower portion of the MTJ cell is easily controlled by using the vertical structure FET. In addition, the whole cells are grounded on the substrate and thus connected to each other. As a result, the MRAM is easily manufactured without requiring a special process for isolation of each cell. Furthermore, process expenses and time are reduced and integration of the MRAM is improved due to simplification of the process.  
         [0058]    As the present invention may be embodied in several forms without departing from the spirit or essential characteristics thereof, it should also be understood that the above-described embodiment is not limited by any of the details of the foregoing description, unless otherwise specified, but rather should be construed broadly within its spirit and scope as defined in the appended claims, and therefore all changes and modifications that fall within the metes and bounds of the claims, or equivalences of such metes and bounds are therefore intended to be embraced by the appended claims.