Abstract:
A method of fabricating a magnetoresistive random access memory device comprising the steps of providing a substrate, forming a conductive layer positioned on the substrate, forming a magnetoresistive random access memory device positioned on conductive layer, forming a metal cap on the magnetoresistive random access memory device, and electroless plating a bump metal layer on the metal cap. The bump metal layer acts as a self-aligned via for a bit line subsequently formed thereon.

Description:
FIELD OF THE INVENTION  
         [0001]    This invention relates to semiconductor memory devices.  
           [0002]    More particularly, the present invention relates to improved methods of fabricating semiconductor random access memory devices that utilize a magnetic field.  
         BACKGROUND OF THE INVENTION  
         [0003]    A magnetoresistive random access memory (hereinafter referred to as “MRAM”) device has a structure which includes ferromagnetic layers separated by a non-ferromagnetic layer. Information is stored as directions of magnetization vectors in magnetic layers. Magnetic vectors in one magnetic layer, for instance, are magnetically fixed or pinned, while the magnetization direction of the other magnetic layer is free to switch between the same and opposite directions which are called “Parallel” and “Anti-parallel” states, respectively. In response to Parallel and Anti-parallel states, the magnetic memory element represents two different resistances. The resistance indicates minimum and maximum values when the magnetization vectors of two magnetic layers point in substantially the same and opposite directions, respectively. Accordingly, a detection of changes in resistance allows an MRAM device to provide information stored in the magnetic memory element.  
           [0004]    A MRAM device integrates magnetic memory elements and other circuits, for example, a control circuit for magnetic memory elements, comparators, for detecting states in a magnetic memory element, input/output circuits, etc. These circuits are fabricated in the process of complimentary metal oxide semiconductor (hereinafter referred to as “CMOS”) technology in order to lower the power consumption of the MRAM device. The CMOS process requires high temperature steps which exceeds 300° C. for depositing dielectric and metal layers and annealing implants, for example.  
           [0005]    Magnetic layers employ ferromagnetic materials such as nickel-iron, cobalt-iron, and nickel-iron-cobalt which require processing below 300° C. in order to prevent intermixing of magnetic materials caused by high temperatures. Accordingly, magnetic memory elements need to be fabricated at a different stage after CMOS processing.  
           [0006]    Further, magnetic memory elements contain components that are easily oxidized and also sensitive to corrosion. To protect magnetic memory elements from degradation and keep the performance and reliability of the MRAM device, a passivation layer is formed over magnetic memory elements.  
           [0007]    In addition, a magnetic memory element includes very thin layers, some of which are tens of angstroms thick. The performance of the magnetic memory element is sensitive to the surface conditions on which magnetic layers are deposited. Accordingly, it is necessary to make a flat surface to prevent the characteristics of a MRAM device from degrading. Also, magnetic memory elements are typically very small which makes it extremely difficult to connect the magnetic memory element to circuitry by using photolithography processes where the alignment tolerances are tight. Further, the materials comprising the ferromagnetic layers are difficult to etch because they are typically non-volatile in conventional low temperature plasmas and are very thin which makes them sensitive to corrosion from conventional chlorine based chemistries.  
           [0008]    It would be highly advantageous, therefore, to remedy the foregoing and other deficiencies inherent in the prior art.  
         SUMMARY OF THE INVENTION  
         [0009]    To achieve the objects and advantages specified above and others, an improved method of fabricating a MRAM device with a self-aligned via is disclosed. The method involves forming magnetic memory elements on circuitry for controlling operations of magnetic memory elements. First, the circuitry is formed on a substrate under the CMOS process which requires a heat treatment of 300° C. or more. While fabricating the circuitry, conductive lines are also formed, which are used to create magnetic fields for writing and/or reading states in the magnetic memory element. The metal lines may or may not be enclosed by high permeability material such as a permalloy layer which facilitates magnetic fields to concentrate on the magnetic memory element. After completion of the circuitry, a surface of a layer including the circuitry is polished by the chemical mechanical polishing (hereinafter referred to as “CMP”) process which produces a flat surface on the layer including the circuitry, then the magnetic memory element has a metal cap formed thereon. The flat surface prevents the characteristics of the magnetic memory element from degrading. Fabrication of the magnetic memory element after the CMOS process improves the performance and reliability of the magnetic memory element and avoids thermal degradation of the magnetic memory element.  
           [0010]    Further, the metal cap on the MRAM device is used as a seed layer to electrochemically deposit a bump metal layer. By using the metal cap as the seed layer, difficult and expensive photolithography processing steps typically used to form an electrically conductive via are avoided. A dielectric layer is deposited on the exposed surface and CMP or a similar process is used to expose the bump metal layer and form a flat surface wherein a bit line is formed adjacent to the bump metal layer.  
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0011]    The foregoing and further and more specific objects and advantages of the instant invention will become readily apparent to those skilled in the art from the following detailed description of a preferred embodiment thereof taken in conjunction with the following drawings:  
         [0012]    [0012]FIGS. 1 through 4 are simplified cross-sectional views illustrating several steps in a method of fabricating a MRAM device with a self-aligned via in accordance with the present invention. 
     
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT  
       [0013]    Turn now to FIG. 1, which illustrates a simplified cross-sectional view of a partially fabricated MRAM cell  5  with a self-aligned via contact in accordance with the present invention. MRAM cell  5  includes a substrate  10  onto which a circuit (not shown) for controlling the operation of a MRAM device  25  is positioned. The circuit includes, for instance, NMOS switching transistors which are fabricated by the well known CMOS process. Other circuit elements, for example, input/output circuits, data/address decoders, and comparators, may be contained in the MRAM device, however they are omitted from the drawings for simplicity. Further, MRAM cell  5  includes a plug conductor  12  positioned on substrate  10 .  
         [0014]    A dielectric layer  11  is positioned on substrate  10  and plug conductor  12  and patterned and etched to form a metal layer  13  positioned on plug conductor  12 . Dielectric layer  11  typically includes silicon oxide or another suitable dielectric material. Plug conductor  12  and metal layer  13  allow electrical contact from the circuit to subsequent layers grown thereon as will be discussed presently.  
         [0015]    Further, dielectric layer  11  is also patterned and etched to form a digit line  16  which is positioned on substrate  10  and proximate to the circuit. Digit line  16  carries a current to generate a magnetic field which causes magnetic memory elements to store states. A thin field focusing layer  14  partially surrounds digit line  16  except for a top surface  17  directed toward MRAM device  25 . Thin field focusing layer  14  has a high permeability and includes a material such as nickel-iron. In order to improve adhesion of thin field focusing layer  14  and to provide a barrier for nickel or iron diffusion into dielectric layer  11 , a thin film of Ta or TaN or other such materials could be added between thin field focusing layer  14  and dielectric layer  11 . Focusing layer  14 , for example, facilitates the magnetic field to focus on a magnetic memory element placed proximate to digit line  16  through top surface  17  not covered by layer  14 .  
         [0016]    A dielectric layer  18  is positioned on the circuit and digit line  16  wherein a trench is formed in dielectric layer  18  adjacent to metal layer  13 . Dielectric layer  18  behaves as an etch stop layer and typically includes aluminum nitride, aluminum oxide, silicon nitride, or another suitable dielectric material. Instead of etch stop layer  18 , other techniques such as endpoint etches may be used. Dielectric layer  18  also serves to electrically isolate digit line  16  from MRAM device  25 .  
         [0017]    A conductive layer  20  is positioned within the trench and electrically connected to the circuit through metal layer  13  and plug conductor  12 . Further, a portion of conductive layer  20  is positioned on dielectric layer  18  wherein dielectric layer  18  separates digit line  16  and conductive layer  20 . In order to form magnetic memory elements on conductive layer  20 , a top surface of conductor layer  20  needs to be smooth and flat because magnetic memory elements have very thin films, thereby a good condition for a magnetic memory element is attained. Conductive layer  20  has a flat surface formed by using chemical mechanical polishing or another similar technique. Conductive layer  20  typically includes a conductor metal, such as aluminum, aluminum alloys, copper, copper alloys, tantalum, Tantalum nitride, titanium, titanium nitride, titanium tungsten, and the like.  
         [0018]    MRAM device  25  has a surface and is positioned on the flat surface of conductive layer  20  wherein MRAM device  25  is magnetically coupled to digit line  16 . MRAM device  25  can be formed using techniques well known to those skilled in the art. One typical method is to deposit blanket layers of magnetic material then use a hard mask to define specific MRAM cells and surround the MRAM cells with a dielectric layer  22 . A second method is by using a dielectric layer  22  positioned on conductive layer  20  and dielectric layer  18  to form a trench wherein MRAM device  25  is deposited. Dielectric layer  22  typically includes silicon dioxide or another suitable dielectric material. Also, it will be understood that dielectric layer  22  can include multiple layers, but is illustrated as a single layer in this embodiment for illustrative purposes only.  
         [0019]    In the preferred embodiment, MRAM device  25  includes a pinned synthetic anti-ferromagnetic region  24  positioned on the flat surface of conductive layer  20  whereon a non-ferromagnetic spacer layer  26  is positioned. Further, a free ferromagnetic region  28  is positioned on non-ferromagnetic spacer layer  26 . Non-ferromagnetic spacer layer  26  forms a quantum mechanical tunneling barrier between pinned synthetic anti-ferromagnetic region  24  and free ferromagnetic region  28  and typically includes aluminum oxide. It will be understood that the positions of pinned synthetic anti-ferromagnetic region  24  and free ferromagnetic region  28  could be reversed, but in this embodiment pinned synthetic anti-ferromagnetic region  24  is positioned adjacent to conductor layer  20  for illustrative purposes only. Pinned synthetic anti-ferromagnetic region  24 , non-ferromagnetic spacer layer  26 , and free ferromagnetic region  28  are illustrated in this embodiment as including a single layer for simplicity, but it will be understood that multiple layers could be used.  
         [0020]    Further, in this specific embodiment, MRAM device  25  is a standard MRAM bit formed according to well-known practices in the art. While a standard MRAM bit is illustrated herein for convenience, it will be understood by those skilled in the art that many other types of semiconductor devices could be provided. Also, while a single MRAM bit is illustrated for convenience it should be understood that, for example, a complete array of devices or control/driver circuits around the periphery of an array of magnetic memory bits may be formed.  
         [0021]    A metal cap  30  is positioned on the surface of MRAM device  25  in either of the above-mentioned processes. Metal cap  30  can include titanium-tungsten, tungsten, aluminum, or another suitable material. A portion of second dielectric layer  22  is removed to expose a flat surface  31  of metal cap  30 . The portion of second dielectric layer  22  can be removed by using CMP or the like. Metal cap  30  is used as a seed layer as will be discussed presently.  
         [0022]    Turning now to FIG. 2, an optional process of making electrical contact with metal cap  30  is disclosed. MRAM cell  5  is immersed in an electrochemical deposition bath to form a bump metal layer  32  positioned on flat surface  31  of metal cap  30 . Electrochemical deposition can include using electroless plating or immersion. Further, bump metal layer  32  includes a material more noble than the material included in metal cap  30  wherein bump metal layer  32  is substantially deposited only on metal cap  30  and forms a self-aligned via contact, as will be discussed presently. A dielectric layer  34  is then positioned on dielectric layer  22  and bump metal layer  32 .  
         [0023]    Turning now to FIG. 3, a portion of dielectric layer  34  is removed by using CMP or a similar technique to form a flat surface  35  on bump metal layer  32 . Turn now to FIG. 4 which illustrates a partial cross-sectional view through bit line  16 , MRAM device  25 , and bump metal layer  32  as indicated by arrows  7 - 7 . In FIG. 4, a bit line  36  is positioned on flat surface  35  of the bump metal layer  32 , wherein bit line  36  is oriented perpendicular to digit line  16 . Bit line  36  is partially surrounded by a high permeability layer  38  except a surface toward MRAM device  25 . A dielectric layer  40  is then deposited on dielectric layer  34  and high permeability layer  38 .  
         [0024]    Thus, an improved and novel fabrication method for a magnetic memory element is disclosed. Circuitry for controlling magnetic memory element is fabricated first under the process that requires a high temperature processing, for example the CMOS process, and then the magnetic memory elements are formed on the circuitry. Accordingly, MRAM devices are integrated with circuit elements fabricated by the CMOS process and are prevented from degradation of metal composition caused by high temperatures. Further, electrochemical deposition is used to form a self-aligned via by using the hard mask layer as a seed layer. Because the MRAM devices are formed with a self-aligned conductive via contact, expensive and difficult photolithography processing steps are avoided.  
         [0025]    Various changes and modifications to the embodiments herein chosen for purposes of illustration will readily occur to those skilled in the art. To the extent that such modifications and variations do not depart from the spirit of the invention, they are intended to be included within the scope thereof which is assessed only by a fair interpretation of the following claims.