Patent Publication Number: US-6905937-B2

Title: Methods of fabricating a cross-point resistor memory array

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application is a continuation-in-part of application Ser. No. 10/345,547, filed Jan. 15, 2003, entitled “Electrically Programmable Resistance Cross Point Memory Structure”, invented by Sheng Teng Hsu and Wei-Wei Zhuang, which is a divisional of application Ser. No. 09/894,922, filed Jun. 28, 2001, entitled “Electrically Programmable Resistance Cross Point Memory,” invented by Sheng Teng Hsu, and Wei-Wei Zhuang, now U.S. Pat. No. 6,531,371, issued Mar. 11, 2003. 
     Application Ser. No. 10/345,547, filed Jan. 15, 2003, entitled “Electrically Programmable Resistance Cross Point Memory Structure”, invented by Sheng Teng Hsu and Wei-Wei Zhuang is incorporated herein by reference. 
    
    
     BACKGROUND OF THE INVENTION 
     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. 
     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 wider width, from those used to induce the initial change. 
     SUMMARY OF THE INVENTION 
     Accordingly, a memory structure is provided, which comprises a substrate with a plurality of doped lines, for example n-type bit lines, with regions of the opposite dopant, for example p-type regions, formed into the n-type bit lines to form diodes. Bottom electrodes overly the diodes. A layer of resistive memory material overlies the bottom electrodes. Top electrodes overly the resistive memory material. In a preferred embodiment, the top electrodes form a cross-point array with the doped lines, and the diodes are formed at each cross-point. 
     A method of manufacturing the memory structure is also provided. A substrate is provided and a plurality of doped lines, such as n-type bit lines, are formed on the substrate. Diodes are formed at what will become each cross-point of the cross-point array. The diodes are formed by doping a region of the doped lines to the opposite polarity, for example by implanting ions. Bottom electrodes are then formed over the diodes. A layer of resistive memory material is deposited over the bottom electrodes. Top electrodes are then deposited overlying the resistive memory material above the diodes such that a cross-point array is defined by the doped lines and the top electrodes, with a diode located at each cross-point. It may be possible, or even preferred, to achieve the method of manufacture in such a way the doped line, the diode formation, and the bottom electrode formation are all self aligned. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a top view on a resistive memory array. 
         FIGS. 2A and 2B  are a cross-section corresponding to A-A′ and B-B′ respectively in  FIG. 1  during initial processing. 
         FIGS. 3A and 3B  are a cross-section corresponding to A-A′ and B-B′ respectively in  FIG. 1  during processing. 
         FIGS. 4A and 4B  are a cross-section corresponding to A-A′ and B-B′ respectively in  FIG. 1  during processing. 
         FIGS. 5A and 5B  are a cross-section corresponding to A-A′ and B-B′ respectively in  FIG. 1  during processing. 
         FIGS. 6A and 6B  are a cross-section corresponding to A-A′ and B-B′ respectively in  FIG. 1  during processing. 
         FIGS. 7A and 7B  are a cross-section corresponding to A-A′ and B-B′ respectively in  FIG. 1  during processing. 
         FIGS. 8A and 8B  are a cross-section corresponding to A-A′ and B-B′ respectively in  FIG. 1  during processing. 
         FIGS. 9A and 9B  are a cross-section corresponding to A-A′ and B-B′ respectively in  FIG. 1  as shown. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
       FIG. 1  shows a cross-point memory array area  10 . The memory array area  10  comprises a substrate with a plurality lines  14  formed thereon. The lines  14  may be doped lines. Diodes  15  may comprise a doped portion of the lines  14  with the opposite polarity dopants. An active layer  16  of resistive memory material overlies the plurality of lines  14 . A plurality of top electrodes  18  overly the active layer  16 , such that the active layer  16  is interposed between the diodes  15  and the top electrodes  18 . 
     The top electrodes  18  and the lines  14  are each preferably substantially parallel rows. The top electrodes  18  and the lines  14  are arranged in a cross-point arrangement such that they cross each other in a regular pattern. A cross-point refers to each position where a top electrode  18  crosses a line  14 . As shown, the top electrodes and the lines are arranged at substantially 90 degrees with respect to each other. The top electrodes and the lines can each function as either word lines or bit lines as part of a cross-point memory array. As shown, the lines  14  are bit lines that have been doped as n-type lines, which are also referred to as N+ bit lines when they are heavily doped n-type lines. 
       FIG. 1  shows just the memory array area. It should be clear that in an actual device, the substrate, the lines  14  and the top electrodes  18  may extend well beyond the memory array area, which is defined by the active layer  16 . In one embodiment the active layer is substantially continuous, such that the active layer extends across more than one cross-point. The lines  14  and the top electrodes  18  may connect to other support circuitry, which is not shown, on the same substrate. 
       FIGS. 2-9  illustrate the process for forming a resistive memory array. Those figures denoted with an A correspond to a cross-section taken along A-A′ in FIG.  1 . Likewise, those figures denoted with a B correspond to a cross-section taken along B-B′ in FIG.  1 . 
     Follow any state of the art process to form the supporting electronics. The resistive memory array will preferably be fabricated in a p-well or using a p-type substrate. Support electronics are defined here as any non-memory devices, which may be connected to the resistive memory array, such as coding, decoding, data processing or computing circuitry. 
     Referring now to  FIGS. 2A and 2B , in one embodiment a layer of oxide  20  is deposited overlying the substrate  12 . The substrate is any suitable substrate material, for example silicon. The layer of oxide  20  is preferably in the range of between approximately 100 nm and 500 nm. Photoresist, which is not shown, is then deposited and patterned to produce a pattern of preferably parallel lines over the memory array area  10 . The layer of oxide  20  is then etched to form a series of preferably parallel lines exposing the underlying substrate  12 . 
     In an alternative embodiment, a layer of polysilicon, not shown, may be deposited over the layer of oxide  20  prior to depositing the photoresist. The layer of polysilicon is preferably between approximately 50 nm and 100 nm. The layer of polysilicon is also patterned along with the layer of oxide  20 . This optional layer of polysilicon may be used as a polishing stop for a subsequent CMP polishing step. 
     An n-type dopant, such as phosphorous, or arsenic, is implanted into exposed substrate  12  to form n-type bit lines  14  as shown in  FIGS. 3A and 3B . 
     A silicon nitride layer  22  is deposited overlying the layer of oxide  20 , and the n-type bit lines. The silicon nitride layer  22  is deposited to a thickness of preferably between approximately 100 nm and 500 nm. The silicon nitride layer  22  is patterned. Preferably, the silicon nitride layer  22  will be formed as parallel lines which are perpendicular to the bit lines  14 , as shown in  FIGS. 4A and 4B . Preferably, the memory cells will be formed in the area where the silicon nitride layer lines cover the n-type bit lines following subsequent processing. 
     In an alternative embodiment, a silicidation process may be performed to form a silicide where the n-type bit lines  14  are exposed. This silicidation process may reduce the bit line resistance. 
     Oxide  24  is then deposited to a thickness of between approximately 200 nm and 700 nm, as shown in  FIGS. 5A and 5B . 
     The oxide  24  and the silicon nitride layer  22  are then polished, preferably using CMP. The oxide  24  and the silicon nitride layer  22  are preferably polished to stop at the layer of oxide  20 . Alternatively, if a layer of polysilicon was deposited over the layer of oxide  20  prior to depositing the silicon nitride layer  22 , the layer of polysilicon may be used as a polishing stop. If a layer of polysilicon is used as the polishing stop, the remaining polysilicon is removed following the polishing. Regardless whether a polysilicon polish stop is used and removed, or not used at all, the resulting structure is substantially as shown in  FIGS. 6A and 6B . 
     After polishing the oxide  24  and the silicon nitride layer  22 , the silicon nitride layer  22  is removed, for example using a wet etch. A CVD oxide is then deposited overlying the substrate, including the remaining portions of the oxide  24 . The CVD oxide is preferably deposited to a thickness of between approximately 10 nm and 50 nm. A plasma etch is used to etch the CVD oxide stopping at the substrate  12 . The CVD oxide deposition and plasma etch forms oxide spacers  26  as shown in  FIGS. 7A and 7B , while exposing a region within the n-type bit lines  14 . 
     Referring now to  FIGS. 8A and 8B , P+ dots  30  are formed within the exposed regions of the n-type bit lines  14 . The P+ dots  30  may be formed by ion implantation forming a shallow P+ junction. In one embodiment boron ions are implanted using energies in the range of between approximately 5 keV and 15 keV at a dose of between approximately 1×10 15 /cm 2  and 5×10 15 /cm 2 . In an alternative embodiment, BF 2  ions are implanted at energies between approximately 40 keV and 80 keV at a dose of between approximately 1×10 15 /cm 2  and 5×10 15 /cm 2 . 
     A bottom electrode material, such as platinum, iridium, ruthenium, or other suitable material, is deposited to a thickness of between approximately 20 nm and 500 nm over the substrate  12 , including the P+ dots  30 . The bottom electrode material is then planarized, for example using CMP, to form the bottom electrodes  32 . 
     In a preferred embodiment, a layer of barrier material, not shown, is deposited to a thickness of between approximately 5 nm and 20 nm prior to depositing the bottom electrode material. The barrier material is preferably TiN, TaN, WN, TiTaN or other suitable barrier material. The barrier material will also be planarized along with the bottom electrode material. The presence of the barrier material reduces, or eliminates, the formation of silicide at the interface between the bottom electrodes  32  and the P+ dots  30 . 
     The n-type bit lines  14 , the P+ dots  30  and the bottom electrodes  32  are preferably self-aligned using the process described. This self-alignment will preferably minimize the cell size of each memory cell within the memory array. 
     Referring now to  FIGS. 9A and 9B , a layer of resistive memory material  40  is deposited over the bottom electrodes within the memory array area. The resistive memory material  40  is preferably a perovskite material, such as a colossal magnetoresistive (CMR) material or a high temperature superconducting (HTSC) material, for example Pr 0.7 Ca 0.3 MnO 3  (PCMO). Another example of a suitable material is Gd 0.7 Ca 0.3 BaCo 2 O 5+5 . The resistive memory material  40  is preferably between about 5 nm and 500 nm thick. The resistive memory material  40  can be deposited using any suitable deposition technique including pulsed laser deposition, rf-sputtering, e-beam evaporation, thermal evaporation, metal organic deposition, sol gel deposition, and metal organic chemical vapor deposition. The resistive memory material  40  is removed from outside the memory array area by ion milling or other suitable process thereby forming the active layer  16 . It is also possible to form a large recessed area to deposit perovskite material over and then use chemical mechanical polishing (CMP) to form the active layer  16 . 
     Top electrodes  18  are formed over the resistive memory material  40  forming the active layer  16  by depositing and patterning a layer of platinum, iridium, copper, silver, gold, or other suitable material. The top electrodes are preferably parallel to each other and preferably perpendicular to the n-type bit lines  14 . The structures shown in  FIGS. 9A and 9B  correspond cross-sections of the top view shown in FIG.  1 . 
     In one embodiment, the memory array structure is passivated and interconnected to supporting circuitry or other devices formed on the same substrate. It may also be possible to combine some of the steps discussed above, with those used to form the support circuitry. 
     The examples provided above all utilized n-type doped lines on a p-type substrate or p-well, with P+ dots to form the diodes. In this configuration the doped lines may act as the bit lines. However, the n-type lines may alternatively act as word lines by changing the polarity of the electrical signal used in connection with the memory array. It is also possible to construct a resistive memory array with the opposite polarity. The doped lines would be p-type lines, formed in an n-type substrate or n-well, with N+ dots to form the diodes. The p-type lines would either act as word lines or bit lines depending on the electrical polarity used in connection with the resistive memory array. 
     Although various exemplary embodiments have been described above, it should be understood that additional variations may be made within the scope of the invention, which is defined by the claims and their equivalents.