Abstract:
A drain loaded 1T1R resistive memory device and 1T1R resistive memory array are provided. The resistive memory array comprises an array of drain loaded 1T1R resistive memory device structures. Word lines are connected across transistor gates, while a resistive elements are connected between transistor gates and bit lines. The resistive element comprises a material with a resistance that is changed electrically, for example using a sequence of electric pulses. The resistive element may comprise PCMO.

Description:
CROSS-REFERENCE  
       [0001]    This application is a continuation-in-part of U.S. patent application Ser. No. 10/256,362, filed on Sep. 26, 2002, entitled Method of Fabrication 1T1R Resistive Memory Array. The above-identified application is incorporated herein by reference. 
     
    
     
       BACKGROUND OF THE INVENTION  
         [0002]    The present invention relates to information storage devices, and more specifically to a memory cell array incorporating a resistive memory element.  
           [0003]    New materials, referred to herein as resistive memory materials, are now making it possible to produce non-volatile memory cells based on a change in resistance. Materials having a perovskite structure, among them colossal magnetoresistance (CMR) materials, are materials that have electrical resistance characteristics that can be changed by external influences.  
           [0004]    For instance, the properties of materials having perovskite structures, especially CMR materials, can be modified by applying one or more short electrical pulses to a thin film or bulk material. The electric field strength or electric current density from the pulse, or pulses, is sufficient to switch the physical state of the materials so as to modify the properties of the material. The pulse is of low enough energy so as not to destroy, or significantly damage, the material. Multiple pulses may be applied to the material to produce incremental changes in properties of the material. One of the properties that can be changed is the resistance of the material. The change may be at least partially reversible using pulses of opposite polarity, or the same polarity but with different pulse widths, from those used to induce the initial change.  
         SUMMARY OF THE INVENTION  
         [0005]    Accordingly, a 1T1R resistive memory structure is provided. The resistive memory device structure comprises a substrate with a transistor formed thereon. The transistor comprises a gate, a source and a drain. A bottom electrode overlies the drain and is electrically connected to it. A resistive memory material overlies the bottom electrode and is electrically connected to the bottom electrode. A top electrode overlies the resistive memory material. The top electrode may be used to read out the value of the resistive memory material. The gate may be used to select the bit to read.  
           [0006]    Individual 1T1R resistive memory structures may be combined to form a memory array. The top electrodes may act as bit lines. Word lines are electrically connected to the gates to select, or assist in selecting, one or more bits.  
           [0007]    A resistive memory array is also provided. The array may comprise a word line connected to at least two transistor gates. Each transistor drain is connected to a bit line through a resistive memory element. Adjacent transistor sources may be connected together to a common source line. The source lines are connected to a source voltage, which may be ground. Multiple 1T1R resistive memory device structures may be arranged as an array, whereby multiple drains can share a bit line. Word lines connected to gates may then be used to select multiple bits within a single word, such that one or more bits may be read or written. 
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0008]    [0008]FIG. 1 is a schematic view of a resistive memory array.  
         [0009]    [0009]FIG. 2 is a planar view of a resistive memory array.  
         [0010]    [0010]FIG. 3 is a cross-sectional view of the resistive memory array taken through a bit line.  
         [0011]    [0011]FIG. 4 is a cross-sectional view of the resistive memory array taken from between adjacent bit lines.  
         [0012]    [0012]FIG. 5 is a cross-sectional view of the resistive memory array taken through a bit line.  
         [0013]    [0013]FIG. 6 is a cross-sectional view of the resistive memory array taken from between adjacent bit lines.  
         [0014]    [0014]FIG. 7 is a cross-sectional view of the resistive memory array taken through a bit line.  
         [0015]    [0015]FIG. 8 is a cross-sectional view of the resistive memory array taken from between adjacent bit lines.  
         [0016]    [0016]FIG. 9 is a cross-sectional view of the resistive memory array taken through a bit line.  
         [0017]    [0017]FIG. 10 is a cross-sectional view of the resistive memory array taken from between adjacent bit lines.  
         [0018]    [0018]FIG. 11 is a cross-sectional view of the resistive memory array taken through a bit line.  
         [0019]    [0019]FIG. 12 is a cross-sectional view of the resistive memory array taken from between adjacent bit lines.  
     
    
     DETAILED DESCRIPTION OF THE INVENTION  
       [0020]    [0020]FIG. 1 illustrates a schematic view of an embodiment of a 1T1R memory array  10 . A 24 bit 1T1R memory array is shown. As shown, there are four word lines  12 , labeled W 1 -W 4 , and six bit lines  14 , labeled B 1 -B 6 . Each bit  16  (indicated by dashed lines) is formed by a transistor  18  and a resistive element  20 , accordingly this memory element may be referred to as a 1-transistor, 1-resistor memory bit, or a 1T1R memory bit. Each transistor  18  has a gate  22 , which is connected to one of the word lines  12 . The resistive element  20  is connected between a drain  24  of a transistor  18  and a bit line  14 . The transistor  18  has a source  26  connected to a common source  28  (designated Vs). As shown in this embodiment, the sources  26  of adjacent transistors  18  are connected together, which may reduce array area.  
         [0021]    [0021]FIG. 2 illustrates a planar view of a 1T1R memory array  10  during processing. As shown in this embodiment, there are four word lines  12 , labeled W 1 -W 4 , and three bit lines  14 , labeled B 1 -B 3 , which form a 12 bit memory array.  
         [0022]    [0022]FIG. 3 shows a cross-section of the memory array of FIG. 2 taken through one of the bit lines  14 , and corresponds to the cross-section identified as “A-A” in FIG. 2. FIG. 4 shows a cross-section that corresponds to “B-B” in FIG. 2, which is a cross-section taken between two adjacent bit lines  14 . A standard process, which is well known to those of ordinary skill in the art, may be used to form any desired wells and shallow trench isolation (STI)  48  on a substrate  50 . A gate oxide  52  is grown over the substrate  50 . A layer of polycide  54  is deposited, followed by a layer of oxide  56 , and a layer of nitride  58 . The term oxide as used herein refers to silicon oxide, including silicon dioxide. The term nitride refers generally to silicon nitride. For example, the polycide  54  may be between approximately 100 nm and 200 nm thick; the oxide  56  may be between approximately 100 nm and 200 nm thick, and the nitride between approximately 50 nm and 100 nm thick. Photoresist is deposited and patterned. The layers of polycide  54 , oxide  56  and nitride  58  are then etched to form gate stacks  60 , as shown in FIG. 3 and FIG. 4. Phosphorous or arsenic N+ source/drain ion implantation is then performed to produce source regions  62  and drain regions  64 . The N+ ion implantation may include lightly doped drain (LDD). The N+ ion implantation may include Halo ion implantation. Both of these implantation processes may be used in connection with support circuitry, if any, so that these processes performed in connection with the memory array need not add process steps to the total process.  
         [0023]    A layer of nitride is deposited, preferably to a thickness of between approximately 50 nm and 150 nm, and etched to form nitride sidewalls  66 , as shown in FIGS. 5 and 6. FIG. 5 corresponds to FIG. 3 following formation of the nitride sidewalls  66 . FIG. 6 corresponds to FIG. 4 following formation of the nitride sidewalls  66 . A salicide process is then performed to salicide the N+ areas, which correspond to the source/drain regions  62  and  64 , and the P+ areas, which within the memory array correspond to the p-well tie (not shown). The salicide process may be used to form common source lines, for example between adjacent transistors. If the memory array is being formed simultaneously with support circuitry the P+ areas may also correspond to source/drain regions of some of the support circuitry (not shown).  
         [0024]    Silicon oxide  70  is deposited by a CVD process to a thickness suitable for planarization using a CMP process. For example, the silicon oxide may be deposited to a thickness of about 1.5 times the height of the gate stack  60 . The silicon oxide  70  is then planarized using a CMP process. In one embodiment the planarization will be stopped at the nitride  58 , resulting in the structure shown in FIG. 7, which corresponds to FIG. 5 following deposition and planarization of silicon oxide  70 , and FIG. 8, which likewise corresponds to FIG. 6 following additional processing.  
         [0025]    Photoresist is applied and patterned for bit contact etch. A selective etch of the oxide is used to open bit contacts. Because of the high selectivity of oxide to nitride etch, overlap of the mask pattern over the nitride  58  is tolerable. Due to the selectivity of the etch process the silicon oxide is etched without etching the nitride on top of the gate stack, this provides at least some self alignment of the bit contacts. A barrier metal, such as, TiN, TaN, TaAlN x  is deposited to form a thin barrier layer (not shown). A bottom electrode material is then deposited. For example the bottom electrode material may be platinum or iridium. The bottom electrode material is planarized, for example using CMP, to the level of the nitride  58  to produce bottom electrodes  74 . The resulting structure is shown in FIG. 9, which corresponds to the cross-section at the bit line, and FIG. 10, which corresponds to the cross-section between adjacent bit lines.  
         [0026]    In one embodiment, a resistive memory material  76  is deposited over the bottom electrodes  74  across the memory array  10 . Alternatively, the resistive memory material  76  is deposited over an entire wafer and removed from areas outside the memory array  10 . The resistive memory material  76  is composed of any material that is capable of changing resistance in response to electrical pulses, for example a CMR and HTSC materials, such as PCMO. A top electrode  78  is then formed by depositing a top electrode material, such as platinum or iridium, patterning and etching the top electrode material to form one, or more, top electrodes  78 , which correspond to the bit lines  14 . The resulting memory array structure is illustrated by the cross-sectional view shown in FIG. 11, which corresponds to the cross section at a bit line, and FIG. 12, which corresponds to the cross section between adjacent bit lines.  
         [0027]    In a second embodiment, a layer of resistive memory material  76  is deposited overlying the memory array and etched to form resistive memory studs (not shown) overlying the bottom electrodes  74 . A thin layer of between approximately 10 and 50 nm of a barrier insulator, such as Si 3 N 4 , Al 3 O 5  or TiO 2  is deposited, followed by a layer of oxide. The layer of oxide has a thickness suitable for CMP planarization, for example 1.5 times the height of the resistive memory studs. The layer of oxide is then planarized level with the resistive memory studs, possibly using CMP. The planarization process removes the barrier insulator from the tops of the resistive memory studs, prior to formation of the top electrodes  78 .  
         [0028]    In a third embodiment, resistive memory studs are formed using a single Damascene process. A layer of oxide is deposited to a thickness of between approximately 100 nm and 300 nm. Trenches are etched through the oxide to the bottom electrodes  74 . A thin layer of barrier insulator, such as Si 3 N 4 , Al 3 O 5  or TiO 2 , between approximately 10 nm and 50 nm thick is deposited along the trenches, including on trench sidewalls. The barrier insulator is plasma etched to remove barrier insulator from planar surfaces, including the bottom electrodes  74 , leaving barrier insulator on the trench sidewalls. The resistive memory material  76  is deposited and planarized to form resistive memory studs (not shown). Top electrodes  78  are then formed overlying the resistive memory studs.  
         [0029]    Although the above embodiment, utilized an N+ ion implant for the formation of the source and drain regions, a P+ ion implant could have been used instead.  
         [0030]    One process of forming transistors has been described in connection with the formation of the 1T1R resistive memory array. This process may be used to form support electronics as well as the memory array. For example, the support electronics and the memory array transistors may be formed using at least some of the process steps described above. An alternative process for forming transistors may be used, including for example a process that incorporates a high-k dielectric material. Once the transistors are formed, contact is made to the drain and a resistive memory material is deposited, as described above to form a 1T1R resistive memory array.  
         [0031]    A 1T1R resistive memory device structure along with a memory array comprising multiple 1T1R bits has been provided, and described. The present invention is not limited to any particular array size or configuration. Other variations and embodiments of the invention may occur to those of ordinary skill in the art. The scope of the invention shall be defined by the claims, without being limited by any preferred embodiment.