Patent Abstract:
A memory element for a magnetic RAM, having a first magnetic portion in a first recess of a first insulating layer; and a non-magnetic portion and a second magnetic portion in a second recess of a second insulating layer covering the first insulating layer, the second recess exposing the first magnetic portion and a portion of the first insulating layer around the first magnetic portion, the non-magnetic portion being interposed between the first and second magnetic portions.

Full Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to a magnetic random access memory or MRAM and a method for manufacturing such a memory. 
     2. Discussion of the Related Art 
       FIG. 1  illustrates the operation of a magnetic RAM. Such a memory comprises an array of memory elements arranged in rows and in columns, a single memory element  10  being shown in  FIG. 1 . Each memory element  10  is formed of the stacking of three layers: a first layer  12  formed of a magnetic material, for example, cobalt, having a fixed magnetic orientation, a second layer  14  formed of an insulator, and a third layer  16  formed of a ferromagnetic material, for example, cobalt and iron alloy or a nickel and iron alloy, the magnetic orientation of which can vary. Insulating layer  12  behaves as a barrier to prevent the alloying between magnetic layer  12  and ferromagnetic layer  16  and enable the passing of electrons, the spin of which must be kept. Generally, each layer of the memory element may itself be formed of several layers. All the memory elements  10  of a same array column are connected to a conductive track  18 , behaving as a bit line. A conductive track  20  is arranged above the memory elements  10  of a same array row but is not in electric contact with the memory elements of the row. 
     For each memory element  10  in the array, first layer  12  is connected via a portion  22  of connection to the drain (or to the source) of an N- or P-channel MOS transistor  24  having its source (or its drain) connected to a reference voltage, for example, ground GND. The gate of MOS transistor  24  is controlled by a gate control signal S G . The MOS transistor associated with each memory element may be replaced with a diode circuit. MOS transistor  24  has the function of selecting in read mode the memory element  10  to be addressed. 
     As an example, magnetic layer  12  of memory element  10  has a magnetic moment vector with a fixed orientation, whatever the amplitude of the magnetic field in which the memory element is bathed. Ferromagnetic layer  16  then has a magnetic moment vector with an orientation that can be modified by applying a magnetic field. As an example, binary data may be stored in the memory element by orienting the magnetic moment vector of ferromagnetic layer  16  in parallel or in antiparallel with respect to the magnetic moment vector of magnetic layer  12 . 
     A data write operation into memory element  10  is performed by flowing a current through bit line  18  and bit line  20  associated with the memory element. The flowing of a current in bit line  18  causes the forming of a magnetic field having the general orientation of the field lines represented by arrow  26 . Similarly, the flowing of a current in word line  20  causes the forming of a magnetic field having the general orientation of its field lines represented by arrow  28 . According to the flow direction of the current in bit line  18  and word line  20 , the magnetic moment vector of ferromagnetic layer  16  is oriented in parallel or in antiparallel with respect to the magnetic moment vector of magnetic layer  12 . In a write operation, MOS transistor  24  is on. 
     An operation of reading of the binary data stored in memory element  10  is performed by turning off transistor  24  associated with memory element  10  and by flowing a current therein via bit line  18 . The determination of the data stored in the memory element is based on the difference of the resistance of memory element  10  according to the orientation difference of the magnetic moment vectors of ferromagnetic layer  16  and of magnetic layer  12 . 
       FIGS. 2A to 2G  show successive steps of a conventional example of a method for manufacturing such a memory element  10  in integrated form. Such a method is especially described in U.S. Pat. No. 6,673,675, which is incorporated herein by reference. 
     As shown in  FIG. 2A , the magnetic memory is formed on a substrate  30 , for example, polysilicon, comprising insulation trenches  32  insulating the memory elements from one another. Two N-type doped regions  34 ,  36  form the source and drain regions of MOS transistor  24 . The gate of MOS transistor  24  is formed of the stacking of a gate oxide layer  38 , for example, silicon oxide, and of a gate layer  40 , for example, polysilicon. Substrate  30  and the gate of MOS transistor  24  are covered with an insulating layer  42 . A connection portion  44 , for example, metallic, is buried at the surface of insulating layer  42  and is connected to doped region  36  via a contact  46 . A connection portion  48 , for example, metal, is buried at the surface of insulating layer  42  and is connected to doped region  34  via a via  50 . Connection portion  48  is intended to be grounded. A conductive track  52 , for example, metal, is buried at the surface of insulating layer  42  and forms word line  20 . 
       FIG. 2B  shows the structure obtained after having covered insulating layer  42  with an insulating layer  54 , and having formed, in insulating layer  54 , a connection portion  56 , for example, metal, in contact with connection portion  44 . 
       FIG. 2C  shows the structure obtained after having covered insulating layer  54  with an insulating layer  58  and formed, in insulating layer  58 , a connection portion  60 , for example, metal, in contact with connection portion  56  and which extends substantially above word line  52 . 
       FIG. 2D  shows the structure obtained after having covered insulating layer  54  with an insulating layer  62  and etched a recess  64  with substantially straight sides in insulating layer  54 , exposing a portion of connection portion  60 . 
       FIG. 2E  shows the structure obtained after having deposited, for example, by vapor phase deposition or cathode sputtering, on insulating layer  62 , a magnetic layer  66 , an insulating layer  68 , a ferromagnetic layer  70 , and a conductive layer  72 , for example, metal. The deposited layers penetrate into recess  64  so that magnetic layer  66  is in contact with connection portion  60 . Generally, magnetic layer  66  has a thickness of approximately some ten nanometers, insulating layer  68  has a thickness of a few nanometers, and ferromagnetic layer  70  has a thickness of from some ten nanometers to a few tens of nanometers. 
       FIG. 2F  shows the structure obtained after a chem./mech polishing (CMP) of layers  66 ,  68 ,  70 ,  72  down to insulating layer  62 . A memory element  73  formed of the stacking of magnetic, insulating, and ferromagnetic portions  74 ,  75 , and  76  is thus insulated. Portions  74 ,  75 ,  76  thus defined comprise corner areas  77 ,  78 ,  79 . In other words, the resulting structure of memory element  73  after the planarization step has a “U”-shaped cross-section. Such corner areas  77 ,  78 ,  79  are undesirable since it is difficult to control the thickness of insulating portion  75  at the level of corner area  77 . In particular, there is a risk for the thickness of insulating portion  75  to be locally decreased at the level of corner area  77 . This may cause the occurrence of leakage currents between magnetic portion  74  and ferromagnetic portion  76 , altering the operation of memory element  73 . It is thus desirable to eliminate corner areas  77 ,  78 ,  79 . 
       FIG. 2G  shows the structure obtained after etching of corner areas  77 ,  78 ,  79  of memory element  73 . A memory element  73  in which magnetic, insulating, and ferromagnetic portions  74 ,  75 , and  76  are substantially planar is then obtained. 
     A disadvantage is that the materials generally used to form the memory elements are little reactive with the chemical etches conventionally used in integrated circuit manufacturing processes, since there is no forming of volatile compounds. It is thus necessary to use RIE-type etches (reactive ion etching) to eliminate corner areas  77 ,  78 ,  79  from memory element  73 . A disadvantage of such etchings is that the materials etched by an RIE-type etch tend to deposit back on the walls of the etch chamber and/or on other portions of the integrated circuit. This may result in a soiling of the etch chamber, and/or, which is much more disturbing, the occurrence of defects at the integrated circuit level. 
     SUMMARY OF THE INVENTION 
     The present invention aims at obtaining a memory element for a magnetic RAM exhibiting no “corner area” and capable of being formed by a process comprising no RIE-type etch steps. 
     Another object of the present invention is to provide a method for manufacturing such a memory element which is compatible with manufacturing processes currently used for integrated circuits. 
     Another object of the present invention is to provide a method for manufacturing such a memory element which only slightly modifies the steps of the general RAM manufacturing process. 
     For this purpose, the present invention provides a memory element for a magnetic RAM, comprising a first magnetic portion in a first recess of a first insulating layer; and a non-magnetic portion and a second magnetic portion in a second recess of a second insulating layer covering the first insulating layer, the second recess exposing the first magnetic portion and a portion of the first insulating layer around the first magnetic portion, the non-magnetic portion being interposed between the first and second magnetic portions. 
     According to an embodiment of the present invention, the first magnetic portion is connected to a source or drain region of a field-effect transistor. 
     The present invention also provides a magnetic RAM comprising an array of memory elements, such as described previously, distributed in rows and columns, and comprising, for each row, a conductive track extending along the row and intended for the writing of data into the memory elements of the row, the memory elements of the row being arranged above the conductive track with an interposed insulating layer. 
     The present invention also provides a magnetic RAM comprising an array of memory elements, such as previously described, arranged in rows and columns, and comprising, for each row, two conductive tracks extending along the row and intended for the writing of data into the memory elements of the row, the memory elements of the row being arranged at the level of the plane equidistant from the two conductive tracks. 
     The present invention also provides a method for manufacturing a magnetic memory element comprising digging a first recess into a first insulating layer; forming a first magnetic layer in the first recess and on the first insulating layer; etching, by chem/mech polishing, the first magnetic layer and a portion of the first insulating layer to delimit a first magnetic portion in the first recess; forming a second insulating layer; digging a second recess into the second insulating layer exposing the first magnetic portion and a portion of the first insulating layer around the first magnetic portion; forming a non-magnetic layer and a second magnetic layer in the second recess and on the second insulating layer; and etching, by chem/mech polishing, the second magnetic layer, the non-magnetic layer, and a portion of the second insulating layer to delimit a non-magnetic portion and a second magnetic portion in the second recess. 
     According to an embodiment of the present invention, the method comprises the previous steps of providing a silicon substrate at the level of which is formed a doped region; forming an insulating layer; forming a connection portion connected to the doped region and a conductive track adjacent to the connection portion, the conductive track being intended for the writing of data into the memory element; forming an insulating layer; forming a connection portion in contact with the intermediary connection portion and overhanging the conductive track; and forming said memory element above the conductive track, the first magnetic portion being connected to the connection portion. 
     According to an embodiment of the present invention, the method comprises the previous steps of providing a silicon substrate at the level of which is formed a doped region; forming an insulating layer; forming a connection portion connected to the doped region and two conductive tracks on either side of the connection portion, the two conductive tracks being intended for the writing of data into the memory element; and forming said memory element at the level of the plane equidistant from the two conductive tracks, the first magnetic portion being connected to the connection track. 
     The foregoing and other objects, features, and advantages of the present invention will be discussed in detail in the following non-limiting description of specific embodiments in connection with the accompanying drawings. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1 , previously described, illustrates the operation of a magnetic RAM; 
         FIGS. 2A to 2G , previously described, illustrate successive steps of an example of a conventional method for manufacturing a magnetic RAM memory element; 
         FIGS. 3A to 3E  illustrate successive steps of a first example of a manufacturing process according to the present invention of a magnetic RAM memory element; and 
         FIGS. 4A to 4D  illustrate steps of a second example of a magnetic RAM memory element manufacturing process according to the present invention. 
     
    
    
     DETAILED DESCRIPTION 
     For clarity, the same elements have been designated with the same reference numerals in the different drawings and, further, as usual in the representation of integrated components, the various drawings are not to scale. 
     A first example of a process according to the present invention for manufacturing a magnetic RAM memory element will now be described in relation with  FIGS. 3A to 3E . The initial steps of the first method example correspond to the steps previously described in relation with  FIGS. 2A to 2D . 
       FIG. 3A  shows the structure obtained after having deposited, for example, by vapor phase deposition, a layer  80  of a magnetic material, for example, cobalt, on insulating layer  62  and in recess  64 . Magnetic layer  80  penetrates into recess  64  to be in contact with connection portion  60 . According to a variation of the present invention, insulating layers  58  and  62  correspond to a single insulating layer which is deposited after forming of connection portion  60 . 
       FIG. 3B  shows the structure obtained after a step of chem/mech polishing of magnetic portion  80  and of a portion of insulating layer  62  to delimit a magnetic portion  82  at the level of recess  64 . 
       FIG. 3C  shows the structure obtained after a step of deposition of an insulating layer  84  on the structure of  FIG. 3B , of etching of a recess  86  in insulating layer  84  to expose the entire magnetic portion  82  and a portion of insulating layer  64  surrounding magnetic portion  82 , and of successive depositions, for example, by vapor phase deposition, of an oxide layer  88 , and of a layer of a ferromagnetic material  90 , for example, a cobalt and iron alloy or a nickel and iron alloy, on insulating layer  84  and in recess  86 . 
       FIG. 3D  shows the structure obtained after a step of etching by chem/mech polishing of ferromagnetic and oxide layers  90  and  89  and of a portion of insulating layer  84  to delimit at the level of recess  86  an oxide portion  92  and a ferromagnetic portion  94 . A memory element  96  comprising a “corner” area  95  at the level of the periphery of oxide portion  92  is thus obtained. 
       FIG. 3E  shows the structure obtained after a step in which insulating layer  84  has been covered with an insulating layer  97 , a via  98  has been formed, in insulating layer  97 , coming to contact ferromagnetic portion  94 , and a conductive track  100  has been formed on insulating layer  97  in contact with via  98 . Conductive track  100  corresponds to the bit line associated with the column of the magnetic RAM to which memory element  96  belongs. 
     According to a variation of the present invention, a metal layer is deposited above ferromagnetic layer  90 . After the etch step, previously described in relation with  FIG. 3D , a metal portion is then delimited at the level of ferromagnetic portion  94 . Via  98  is then formed at the contact of the metal portion. 
     The “active” region of memory element  96  corresponds to the region of oxide portion  92  for which magnetic portion  82  and ferromagnetic portion  94  are opposite. Corner area  95  of oxide portion  92  is not disturbing since it is not located at the level of the active region of memory element  96 . A local decrease in the thickness of oxide portion  92  at the level of corner area  95  thus does not disturb the operation of memory element  96 . Further, the present manufacturing method comprises no RIE-type etch steps since memory element  96  is only delimited by chem/mech polishing steps. Thereby, the risk of uncontrolled deposition of the materials forming the memory element in the etch chamber or on the integrated circuit, characteristic of an RIE-type etch, is avoided. 
     A second example of a method for manufacturing according to the present invention a magnetic memory will now be described in relation with  FIGS. 4A to 4D . 
       FIG. 4A  shows a structure similar to  FIG. 2A . However, conversely to the structure shown in  FIG. 2A , two conductive tracks  110 ,  112  corresponding to two word lines are provided for each row of the magnetic RAM. For each memory element of a same row, conductive tracks  110 ,  112  extend on either side of connection portion  44 . 
       FIG. 4B  shows the structure obtained after deposition of an insulating layer  114  on insulating layer  42 , the etching of a recess  116  in insulating layer  42  which exposes connection portion  44 , and the deposition of a layer of a magnetic material  118 , for example, cobalt-based, on insulating layer  114 . Magnetic layer  118  penetrates into recess  116  to contact connection portion  44 . 
       FIG. 4C  shows the structure obtained after a chem/mech polishing of magnetic layer  118  and of a portion of insulating layer  114  to delimit a magnetic portion  120  in recess  116 . 
       FIG. 4D  shows the structure obtained after implementation of steps similar to those illustrated in relation with  FIGS. 3C and 3D  of the first example of embodiment. 
     The structure of memory element  96  obtained by the second example of a manufacturing process according to the present invention is identical to that obtained by the first example of a manufacturing process according to the present invention. In particular, corner area  95  of oxide portion  92  is insulated from the active region of memory element  96  and does not disturb its operation. 
     An operation of data writing into memory element  96  is performed by running a current in the bit line and currents of opposite directions in word lines  110 ,  112 . A magnetic field having its maximum amplitude substantially at the level of a plane equidistant from word lines  110 ,  112 , that is, substantially at the level of magnetic memory element  96  is then obtained. 
     In the first example of embodiment in which a single word line  52  is associated with each row of the magnetic RAM, it is necessary for memory element  96  to be arranged above word line  52  to benefit from a magnetic field of maximum amplitude in a write generation. In the second method example, the magnetic field has a maximum amplitude at the level of the plane equidistant from the two word lines  110 ,  112 . This enables leaving memory element  96  above connection portion  44 . It is then no longer necessary to provide the steps of deposition of insulating layers  54  and  58  and the steps of forming of connection portions  56  and  60  of the first method example. The second method example thus enables reducing the number of masks to be provided for the memory element manufacturing. 
     According to a variation of the previously-described examples of embodiment, the MOS transistor associated with each memory element and used for the reading of the data stored at the level of the memory element may be replaced with a diode circuit. 
     According to another variation of the previously-described examples of embodiment, a single word line is associated with each row of the magnetic RAM and is connected to all the memory elements in the row. Each memory element is then caught between the bit line and the word line associated with the memory element. An operation of reading of the data stored at the level of a memory element is then performed by running a current through the memory element via the bit line and the word line associated with the memory element. Such an alternative embodiment enables suppressing the MOS transistor associated with each memory element. 
     The present invention has many advantages. 
     First, it enables obtaining a magnetic RAM for which, at the level of the active region of each memory element, the thickness of the oxide portion is relatively uniform. 
     Second, the steps of the manufacturing process of each memory element according to the present invention relative to the etching of the materials forming the memory element only implement chem/mech polishing steps instead of RIE-type etchings. The disadvantages of RIE-type etchings are thus avoided. 
     Third, the manufacturing process according to the present invention only implements layer deposition steps and chem/mech polishing etch steps, which are compatible with usual integrated circuit manufacturing processes. 
     Fourth, the manufacturing process according to the present invention comprises but a small number of additional steps and thus only slightly modifies usual magnetic RAM manufacturing steps. 
     Of course, the present invention is likely to have various alterations, modifications, and improvements which will occur to those skilled in the art. In particular, the magnetic layer, the oxide layer, and the ferromagnetic layer based on which the memory element is formed may each be formed of the stacking of several layers. 
     Such alterations, modifications, and improvements are intended to be part of this disclosure, and are intended to be within the spirit and the scope of the present invention. Accordingly, the foregoing description is by way of example only and is not intended to be limiting. The present invention is limited only as defined in the following claims and the equivalents thereto.

Technology Classification (CPC): 7