Abstract:
A MRAM cell structure includes a bottom electrode; a magnetic tunnel junction unit disposed on the bottom electrode; a top electrode disposed on the magnetic tunnel junction unit; and a blocking layer disposed on the top electrode, wherein the blocking layer is wider than the magnetic tunnel junction unit for preventing against formation of a short circuit between a contact and the magnetic tunnel junction unit.

Description:
BACKGROUND 
       [0001]    The present invention relates generally to magnetoresistive random access memory (MRAM), and more particularly to a MRAM cell structure with a blocking layer for avoiding short circuits. 
         [0002]    MRAM is a type of memory device containing an array of MRAM cells that store data using their resistance states instead of electronic charges. Each MRAM cell includes a magnetic tunnel junction (MTJ) unit whose resistance can be adjusted to represent a logic state “0” or “1.” Conventionally, the MTJ unit is comprised of a fixed magnetic layer, a free magnetic layer, and a dielectric tunnel layer disposed there between. The resistance of the MTJ unit can be adjusted by changing the direction of the magnetic moment of the free magnetic layer. When the magnetic moment of the free magnetic layer is parallel to that of the fixed magnetic layer, the resistance of the MTJ unit is low, whereas when the magnetic moment of the free magnetic layer is anti-parallel to that of the fixed magnetic layer, the resistance of the MTJ unit is high. The MTJ unit is coupled between top and bottom electrodes, and an electric current flowing through it can be detected for determining its resistance, and therefore the logic state of the MRAM cell. 
         [0003]    One drawback of the conventional MRAM cell structure is that an undesired short circuit often occurs when forming a contact on the top electrode. For example,  FIG. 1  illustrates a cross-sectional view of a conventional MRAM cell structure  100  where an undesired short circuit is formed between a via contact  102  and a MTJ unit  104 . During the formation of the via contact  102 , an etching process is performed to create a via on the top electrode  106  in the inter-metal dielectric layer  108 , and the via is then filled with conductive materials to provide the via contact  102 . As integrated circuits continue to scale down, the MRAM cells are susceptible to misalignment of the via contact  102 , which increases the difficulty in controlling the end point of the via etching process, and therefore increases the possibility of a short circuit between the contact and the MTJ unit. 
         [0004]      FIG. 2  illustrates a cross-sectional view of another conventional MRAM cell structure  200  where an undesired short circuit is formed between a direct contact  202  and a MTJ unit  204 . Again, due to the difficulty in controlling the end point of an etching process, the front end of the direct contact  202  can extend beyond the top electrode  206  and reach the MTJ unit  204 . 
         [0005]    Such short circuits can cause the MRAM cell structures  100  and  200  to fail. Thus, what is needed is a MRAM cell structure that can avoid those undesired short circuits between the contacts and the MTJ units. 
       SUMMARY 
       [0006]    The present invention discloses a MRAM cell structure with a blocking layer for avoiding short circuits. In one embodiment of the invention, the MRAM cell structure includes a bottom electrode; a magnetic tunnel junction unit disposed on the bottom electrode; a top electrode disposed on the magnetic tunnel junction unit; and a blocking layer disposed on the top electrode, wherein the blocking layer is wider than the magnetic tunnel junction unit for preventing against formation of a short circuit between a contact and the magnetic tunnel junction unit. 
         [0007]    The construction and method of operation of the invention, however, together with additional objectives and advantages thereof will be best understood from the following description of specific embodiments when read in connection with the accompanying drawings. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0008]      FIG. 1  illustrates a cross-sectional view of a conventional MRAM cell structure where a via contact is shorted with a MTJ unit. 
           [0009]      FIG. 2  illustrates a cross-sectional view of another conventional MRAM cell structure where a direct contact is shorted with a MTJ unit. 
           [0010]      FIGS. 3-14  illustrate a series of cross-sectional views showing a process flow of fabricating a MRAM cell structure in accordance with one embodiment of the present invention. 
       
    
    
     DESCRIPTION 
       [0011]    This invention describes a MRAM cell structure with a blocking layer for avoiding undesired short circuits. The following merely illustrates various embodiments of the present invention for purposes of explaining the principles thereof. It is understood that those skilled in the art will be able to devise various equivalents that, although not explicitly described herein, embody the principles of this invention. 
         [0012]      FIGS. 3-14  illustrate a series of cross-sectional views showing a process flow of fabricating a MRAM cell structure in accordance with one embodiment of the present invention.  FIG. 3  illustrates a cross-sectional view  300  where a contact  302  is constructed in a dielectric layer  304 , on which another dielectric layer  306  is disposed. The dielectric layer  304  can be an inter-metal dielectric layer under which semiconductor devices, such as transistors, resistors, and diodes, are constructed on a semiconductor substrate (not shown in the figure). Those devices can be coupled to the contact  302  via embedded wiring that is not shown in the figure. 
         [0013]    Referring to  FIG. 4 , an opening  308  is formed through the dielectric layer  306  to partially expose the contact  302  by a series of processing steps, such as photolithography and etching. The sidewalls of the opening  308  are tapered to a certain degree in order to facilitate depositing materials thereon in a later stage. Referring to  FIG. 5 , a first conductive layer  310  with a thickness ranging approximately between 100 Å and 600 Å is deposited over the dielectric layer  306  and the exposed portion of the contact  302  by methods such as chemical vapor deposition (CVD) or plasma enhanced chemical vapor deposition (PECVD). The first conductive layer  310  can be made of materials, such as copper, aluminum, and platinum. A composite MTJ layer  312  comprised of a free magnetic layer, a dielectric tunnel layer, and a fixed magnetic layer is subsequently deposited on the first conductive layer  310  by methods such as CVD or PECVD. The free and fixed magnetic layers are made of ferromagnetic materials where one is a permanent magnet set to a particular polarity, and the other&#39;s magnetic field changes to match that of an external magnetic field. The composite MTJ layer has a total thickness ranging approximately between 200 Å and 600 Å. A second conductive layer  313  with a thickness approximately ranging from 100 Å to 600 Å is deposited on the composite MTJ layer  312  by methods such as CVD and PECVD. Due to the opening  308  shown in  FIG. 4 , the first conductive layer  310  is electrically coupled to the contact  302 , and a recess is formed on the surface of the second conductive layer  313  above the contact  302 . 
         [0014]    Referring to  FIG. 6 , a layer of bottom anti-reflecting coating (BARC)  314  is disposed on the second conductive layer  313  by methods such as spin-on coating, CVD or PECVD in order to enhance control of critical dimensions during a photolithographic process. A photoresistor layer  316  with a pattern defined by a photolithographic process is formed on the BARC  314 . Parts of the BARC  314  that are not covered by the photoresistor layer  316  are etched away such that the remaining BARC  314  would have a pattern substantially identical to that of the photoresistor layer  316 . Thereafter, the photoresistor  316  is stripped and the remaining BARC  314  is used as a hard mask to protect its underlying layers from being removed in a subsequent etching step that partially removes the second conductive layer  313  and the composite MTJ layer  312  until the first conductive layer  312  is exposed. Then, the reaming BARC  314  is stripped to render a semiconductor structure with a profile as the cross-sectional view shown in  FIG. 7 . 
         [0015]    It is noted that in  FIG. 7 , an optional cap layer  318  is deposited over the first conductive layer, the remaining composite MTJ layer  312  and the remaining second conductive layer  313 . This cap layer  318  is designed, among other things, to protect the composite MTJ layer  312  from being shorted with a contact to be constructed in a later stage. As such, it is desired that the cap layer  318  be made of dielectric materials, such as silicon oxide, silicon nitride, silicon oxynitride, and silicon carbide. 
         [0016]    Referring to  FIG. 8 , an inter-metal dielectric layer  320  is deposited over the cap layer  318 . The inter-metal dielectric layer  320  has a relatively uneven surface as it reflects the contour of the cap layer  318 . The inter-metal dielectric layer  320  is leveled by methods such as chemical mechanical polishing (CMP) or an etching back process until the second conductive layer  313  is exposed to render a semiconductor structure with a profile as the cross-sectional view shown as  FIG. 9 . In  FIG. 9 , the optional cap layer  318  remains on the sidewalls of the second conductive layer  313  to protect the top electrode  313  from being shorted with a contact to be constructed in a later stage. 
         [0017]    Referring to  FIG. 10 , a blocking layer  322  with a thickness ranging approximately between 100 Å and 600 Å is deposited over the inter-metal dielectric layer  320 , the second conductive layer  313 , and the cap layer  318  on the sidewalls of the second conductive layer  313 . The blocking layer  318  is made of conductive materials, such as copper, aluminum, platinum, which can be the same materials making up the second conductive layer  313 , to from an electrical connection with the second conductive layer  313 . An optional etch stop layer  318  can be deposited on the blocking layer  322  for controlling the end point of a subsequent etch process. Referring to  FIG. 11 , a photoresistor layer  326  is disposed on the etch stop layer to define a MRAM cell structure. An etching process is performed to remove parts of the etch stop layer  324 , the blocking layer  322 , the inter-metal dielectric layer  320 , the cap layer  318  and the first conductive layer  310  that are not covered by the photoresistor layer  326 . Thereafter, the photoresistor layer  326  is stripped to render a semiconductor structure with a profile as the cross-sectional view shown in  FIG. 12 . 
         [0018]    In  FIG. 12 , the remaining first conductive layer  316 , composite MTJ layer  312  and second conductive layer  313  function as the bottom electrode, the MTJ unit and the top conductive layer  313  of a MRAM cell structure, respectively. The blocking layer  322  and the etch stop layer  324  have a width that is substantially the same as that of the bottom electrode  310  measured horizontally. An inter-metal dielectric layer  328  is deposited over the etch stop layer  324  and the exposed dielectric layer  306  to render a semiconductor structure with a profile as the cross-sectional view shown in  FIG. 13 . 
         [0019]    Referring to  FIG. 14 , a contact  330  is constructed in the inter-metal dielectric layer  328  and the etch stop layer  324  to form an electrical connection with the blocking layer  322 . The contact  330  is made by forming a contact opening and then filling the opening with conductive materials by methods such CVD, PECVD, and electric plating. During operation, a voltage is applied to the top electrode  313  through the contact  330  and the blocking layer  322 , resulting in a current flowing through the MTJ unit  312  that can be detected at via the contact  302  coupled to the bottom electrode  310 . As discussed above, the resistance of the MTJ unit  312  can be adjusted by changing the magnetic moment of the free magnetic layer to represent a logic state, which can be read by detecting the current flowing from the contact  330  to the contact  302 . 
         [0020]    In this embodiment, the contact  330  is shown as a direct contact without any via constructed between the contact itself and the blocking layer  322 . Alternatively, a via contact similar to the one shown in  FIG. 1  can be constructed on the blocking layer  322  in accordance with another embodiment of the present invention. 
         [0021]    The blocking layer  322  and the cap layer  318  on the sidewalls of the top electrode  313  protect the MTJ unit  312  from being shorted with the contact  330  caused by over etching during the formation thereof. Even if a misalignment of the contact  330  occurs, the blocking layer  322  and the cap layer  318  are able to prevent the misaligned contact from penetrating into the MTJ unit  312 . As such, the reliability of the MRAM cell structure as shown in  FIG. 14  can be improved. 
         [0022]    Moreover, another advantage of the present invention is that the proposed process does not require an additional mask and is compatible with the standard backend process. Thus, the proposed MRAM cell structure would not incur a significant increase in costs. 
         [0023]    The above illustration provides many different embodiments or embodiments for implementing different features of the invention. Specific embodiments of components and processes are described to help clarify the invention. These are, of course, merely embodiments and are not intended to limit the invention from that described in the claims. 
         [0024]    Although the invention is illustrated and described herein as embodied in one or more specific examples, it is nevertheless not intended to be limited to the details shown, since various modifications and structural changes may be made therein without departing from the spirit of the invention and within the scope and range of equivalents of the claims. Accordingly, it is appropriate that the appended claims be construed broadly and in a manner consistent with the scope of the invention, as set forth in the following claims.