Abstract:
Systems and methods are disclosed to form a resistive random access memory (RRAM) by forming a first metal electrode layer; depositing an insulator above the metal electrode layer and etching the insulator to expose one or more metal portions; depositing a Pr 1-x Ca x MnO 3  (PCMO) layer above the insulator and the metal portions, wherein X is between approximately 0.3 and approximately 0.5, to form one or more self-aligned RRAM cells above the first metal electrode; and depositing a second metal electrode layer above the PCMO layer.

Description:
CROSS-REFERENCES TO RELATED APPLICATIONS 
       [0001]    This application is a continuation of U.S. patent application Ser. No. 13/092,830 filed Apr. 22, 2011, which is a divisional application of U.S. patent application Ser. No. 11/486,472, filed Jul. 14, 2006, entitled “SYSTEMS AND METHODS FOR FABRICATING SELF-ALIGNED MEMORY CELL”, and is related to U.S. patent application Ser. No. 11/388,528, filed Mar. 25, 2006, entitled “SYSTEMS AND METHODS FOR FABRICATING SELF-ALIGNED MEMORY CELL.” The disclosures of all of the above mentioned applications are all incorporated by reference herein in their entirety for all purposes. 
     
    
     FIELD OF INVENTION 
       [0002]    The present invention relates generally to a self-aligned storage node for a memory device such as a nonvolatile memory device. 
       BACKGROUND OF THE INVENTION 
       [0003]    A memory device such as random access memory (RAM) is a semiconductor device for storing digital information. Data, as digital information, can be written to and read from a RAM. RAMs are fabricated using integrated circuit technology. For example, a DRAM is made of many storage nodes or memory cells and each memory cell has a memory cell transistor and a capacitor. The capacitor is an important element of the memory cell because it stores the digital information. Trench capacitors and stack capacitors are the two major types of DRAM cell capacitors. 
         [0004]    DRAMs are volatile and thus require power to maintain the data from being lost. Non-volatile alternatives include resistive random access memory (RRAM) devices, each being a cross point type memory array of a planar matrix of spaced memory cells sandwiched between two meshes of conductors running in orthogonal directions above and below the cells. The row conductors running in one direction are referred to as the word lines, and the column conductors extending in a second direction usually perpendicular to the first direction are referred to as the bit lines. The memory cells are usually arranged in a square or rectangular array so that each memory cell unit is connected with one word line and an intersecting bit line. 
         [0005]    In the RRAM array, the resistance of each memory cell has more than one state, and the data in the memory cell is a function of the resistive state of the cell. The resistive memory cells may include one or more magnetic layers, a fuse or anti-fuse, or any element that stores or generates information by affecting the magnitude of the nominal resistance of the element. Other types of resistive elements used in a resistive RAM array include poly-silicon resistors as part of a read-only memory, and floating gate transistors as part of optical memory, imaging devices or floating gate memory devices. 
         [0006]    One type of resistive random access memory is a magnetic random access memory (MRAM), in which each memory cell is formed of a plurality of magnetic layers separated by insulating layers. One magnetic layer is called a pinned layer, in which the magnetic orientation is fixed so as not to rotate in the presence of an applied magnetic field in the range of interest. Another magnetic layer is referred to as a sense layer, in which the magnetic orientation is variable between a state aligned with the state of the pinned layer and a state in misalignment with the state of the pinned layer. An insulating tunnel barrier layer sandwiches between the magnetic pinned layer and the magnetic sense layer. This insulating tunnel barrier layer allows quantum mechanical tunneling to occur between the sense layer and the pinned layer. The tunneling is electron spin dependent, causing the resistance of the memory cell, a function of the relative orientations of the magnetizations of the sense layer and the pinned layer. The variations in the junction resistance for the two states of the sense layer determine the data stored in the memory cell. U.S. Pat. No. 6,169,686, granted to Brug et al. on Jan. 2, 2001 discloses such a magnetic memory cell memory. U.S. Pat. No. 6,385,079 discloses a method for designing a resistive random access memory array in which elements are selected with values of resistances that are correlated to maintain a signal-to-noise ratio of 20 decibels or more for the array. A plurality of memory cells are selected and spaced from each other in a matrix of rows and columns, each memory cell being selected to have a junction resistance value of between 0.25 megaohms and 3.60 megaohms. A plurality of conductive row lines are selected and connected between a number N of memory cells in each row, each row and column line being selected to have a row or column unit line resistance value below 0.38 ohms, so that the values of junction resistance are correlated with the values of the row and column unit line resistance to provide a signal-to-noise ratio of 20 decibels or more for the memory array. The values of the row and column unit line resistance are selected so that the total row line resistance for each row is approximately equal to the total column line resistance for each column. The ratio of the junction resistance to the unit line resistance is approximately five million to one, in order to maintain a signal-to-noise ratio of at least 20 decibels in the resistive memory array. For an equal number N of row and column elements, the total row or column line resistance must be greater than approximately five million to N. If N is equal to approximately 1000, the ratio of junction resistance to total row or column line resistance must be approximately 5,000 or greater. 
       SUMMARY OF THE INVENTION 
       [0007]    In one aspect, systems and methods are disclosed to form a resistive random access memory (RRAM) by forming a first metal electrode layer; depositing an insulator above the metal electrode layer and etching the insulator to expose one or more metal portions; depositing a Pr1-X CaXMnO3 (PCMO) layer above the insulator and the metal portions, wherein X is between approximately 0.3 and approximately 0.5, to form one or more self-aligned RRAM cells above the first metal electrode; and depositing a second metal electrode layer above the PCMO layer. 
         [0008]    In another aspect, a resistive random access memory (RRAM) includes a first metal electrode layer; an insulator material deposited and patterned above the metal electrode layer; a Pr1-X CaXMnO3 (PCMO) layer forming one or more self-aligned RRAM cells above the first metal electrode layer, where X is between approximately 0.3 and approximately 0.5; and a second metal electrode layer deposited above the first metal electrode layer. 
         [0009]    In yet another aspect, a resistive random access memory (RRAM) includes an insulator material deposited and patterned; and a PCMO layer deposited above the patterned insulator material forming one or more self-aligned RRAM cells without any etching of the PCMO layer. 
         [0010]    In another aspect, a resistive random access memory (RRAM) includes an amorphous PCMO or silicon material portion and a polycrystalline PCMO portion adjacent the amorphous material to form one or more self-aligned RRAM cells without any etching thereto. 
         [0011]    Implementations of the above aspects may include one or more of the following. The PCMO layer is simply deposited over the insulator to form amorphous PCMO. Where the insulator has been patterned to expose the metal, polycrystalline PCMO forms the RRAM without requiring etching of the PCMO layer. This simplifies the manufacturing process significantly and allows smaller geometry to be achieved. The metal layer can be a transition metal such as chromium, platinum, nickel, ruthenium, copper, and others, a silicide such as nickel silicide, and a Mott insulator such as LNO(LaNiO3). As chromium and LNO are “sticky” materials, an adhesion layer is not needed. 
         [0012]    In other implementations, an insulator layer can be formed between a wafer and the first metal electrode layer. The first metal electrode layer can be formed above a wafer. Via holes connect the first metal electrode layer to circuit on the wafer. An adhesion layer can be formed above the first metal electrode layer. A metal can be formed above the via holes, and the metal is polished using chemical mechanical polishing. The first or second metal electrode layer can be platinum, chromium, iridium, ruthenium, nickel, silicide, nickel, titanium or LNO. A passivation film can be formed above the second electrode metal layer. Each memory cell can be one of: a two terminal memory device, a three terminal memory device. The PCMO layer can be deposited above the insulator material to form an amorphous PCMO. Alternatively, the PCMO layer can be deposited above the metal openings to form polycrystalline PCMO. A plurality of metal layers and vias can connect the RRAM cells to a wafer circuit. The PCMO layers can be stacked to form a 3-dimensional RAM structure. 
         [0013]    Advantages of the invention may include one or more of the following. The system provides a low-cost high density solid state memory device. The memory units can be interconnected within a wafer, and the wafers are interconnected in a stacked wafer construction of a memory system. The resulting semiconductor device offers high density storage at a very low cost. Moreover, the process can be done at low temperature. The substrate temperature in forming a thin film approximately at the range of 300-400 deg. C. and the process requires a short time. Since the thin film is formed at a very low temperature during substantially the whole process, the process can be applied to a highly integrated device to deposit an additional memory layer with a plurality of elements without damaging other elements previously deposited using conventional deposition. 
     
    
     
       BRIEF DESCRIPTION OF DRAWINGS 
         [0014]    In order that the manner in which the above-recited and other advantages and features of the invention are obtained, a more particular description of the invention briefly described above will be rendered by reference to specific embodiments thereof, which are illustrated, in the appended drawings. Understanding that these drawings depict only typical embodiments of the invention and are not therefore to be considered to be limiting of its scope, the invention will be described and explained with additional specificity and detail through the use of the accompanying drawings in which: 
           [0015]      FIGS. 1A-1B  show an exemplary process to fabricate a self-aligned memory cell. 
           [0016]      FIG. 1C  shows an exemplary memory structure with an adhesion layer, while 
           [0017]      FIG. 1D  shows an exemplary memory structure without an adhesion layer. 
           [0018]      FIG. 1E  shows an exemplary memory structure with platinum plating. 
           [0019]      FIG. 2  shows a first exemplary RRAM memory array with two-terminal memory cells. 
           [0020]      FIG. 3  depicts a second exemplary RRAM array with a complementary output (three terminal memory cell). 
           [0021]      FIG. 4  shows an exemplary RRAM array. 
           [0022]      FIG. 5  shows a cross sectional TEM view of an exemplary PCMO RRAM Memory Cell. 
           [0023]      FIG. 6  shows an exemplary multi-level RRAM. 
       
    
    
     DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS 
       [0024]    Referring now to the drawings in greater detail, there is illustrated therein structure diagrams for a semiconductor processing system and logic flow diagrams for processes a system will utilize to deposit a memory device, as will be more readily understood from a study of the diagrams. 
         [0025]      FIGS. 1A-1B  show an exemplary process to fabricate a self-aligned memory cell. The process uses CMOS wafers and conventional CMOS circuits or structures may be formed on the wafer prior to the memory cell fabrication. One or more electronic devices can be formed on the wafer. The process forms non-volatile memory such as RRAM. Unlike conventional DRAM, which uses electrical cells (e.g., capacitors) to store data, RRAM uses resistance cells. Because resistance memory cells maintain their state even when power is removed, RRAM possesses a distinct advantage over electrical cells. 
         [0026]    Turning now to  FIG. 1A , the process deposits an insulator such as silicon dioxide, silicon nitride, silicon oxynitride, or any other suitable low-K materials ( 1 ). Next, via holes are formed to provide connections to the CMOS circuits ( 2 ). The process then optionally deposits an adhesion layer such as titanium nitride, chromium, or tantalum nitride, among others ( 3 ). Next, the process deposits via metal and performs Chemical Mechanical Polishing (CMP) as well as Chemical Vapor Deposition (CVD), among others ( 4 ). Next, metal is deposited and patterned to form bottom electrodes using metals such as platinum, chromium, iridium, ruthenium, nickel, or silicides such as nickel or titanium, among others ( 5 ). 
         [0027]    In  FIG. 1B , the process deposits an insulator material such as silicon dioxide, silicon nitride, silicon oxynitride or any suitable low K materials ( 6 ). Next, the insulator material is patterned ( 7 ). PCMO is then deposited ( 8 ). The polycrystalline PCMO is deposited on the metal exposed area and the amorphous PCMO is deposited on the insulator material. A via hole is then formed to connect the PCMO to a top electrode that in turn is connected to a CMOS circuit. Next, metal is deposited and patterned to form top electrodes using metals such as platinum, chromium, iridium, ruthenium, nickel, or silicides such as nickel or titanium, among others ( 8 ). The metal is patterned to form a top electrode ( 9 ). The process then deposits a passivation film layer such as silicon nitride, silicon oxynitride, or silicon dioxide, among others, and then perform CMP as necessary ( 10 ). 
         [0028]    The process of  FIGS. 1A-1B  forms a variable resistor which can be programmed to have high resistance or low resistance (in two-state memory circuits), or any intermediate resistance value (in multi-state memory circuits). The different resistance values of the RRAM cell represent the information stored in the RRAM circuit. 
         [0029]    The resulting RRAM is simple and small in size with advantages from the nonvolatile characteristic of the resistor memory cell and the stability of the memory state. Since resistor is a passive component and cannot actively influence nearby electrical components, a basic RRAM cell can be just a variable resistor, arranged in a cross point resistor network to form a cross point memory array. To prevent cross talk or parasitic current path, a RRAM cell can further include a diode, and this combination is sometimes called a 1R1D (or 1D1R) cross point memory cell. To provide better access, a RRAM can include an access transistor, as in DRAM or FRAM cell, and this combination is sometimes called a 1R1T (or 1T1R) cross point memory cell. 
         [0030]    The resistance state of a RRAM cell is referred to the storing (writing) or sensing (reading) methodology of the RRAM circuit. The term resistance state is related to the resistance value of the memory resistor (the resistance state can then be said to be the resistance of the memory resistor), but sensing the resistance value of the memory resistor often means sensing the voltage across the memory resistor (the resistance state can then be said to be the voltage across the memory resistor), or sensing the current through the memory resistor (the resistance state then can be said to be the current through the memory resistor). The resistance states of the RRAM can be represented by different techniques such as structural state, polarization, or magnetization. 
         [0031]    The PCMO layer is simply deposited over the insulator to form amorphous PCMO which is a typical insulator. Where the insulator has been patterned away to expose the metal layer, the PCMO layer is polycrystalline PCMO which exhibits RRAM capability. The RRAM is formed and does not need etching of PCMO, simplifying the manufacturing process significantly and allowing smaller geometry to be achieved. The metal layer can be a transition metal such as chromium, nickel, ruthenium, copper, or nickel silicide, among others. As chromium is a “sticky” material, an adhesion layer is not needed. If the metal layer is platinum, an adhesive layer may be used to allow other materials to adhere to the platinum layer. 
         [0032]      FIG. 1C  shows an exemplary memory structure where the metal material such as Pt, Ru, Ir, among others, requires an adhesion layer with an insulator interface. In  FIG. 1C , an insulator is formed above a wafer such as a CMOS wafer. The insulator is etched and an adhesion layer such as TiN, TaN or Cr is deposited, resulting in a via with a metal such as CVD W and CMP. Next, a bottom electrode and metal wire is formed. An insulator is deposited and suitably patterned. A PCMO is then deposited. A second adhesion layer of TiN or Cr is then deposited, and a top electrode and metal line is formed. A passivation insulator is then formed to protect the circuitry.  FIGS. 1F-1G  show exemplary electrical characteristics of the memory device of  FIG. 1C . 
         [0033]      FIG. 1D  shows an exemplary memory structure where the metal material such as Cr, among others, does not need an adhesion layer with an insulator interface. Similar to  FIG. 1C , an insulator is formed above a wafer such as a CMOS wafer. The insulator is etched and an adhesion layer such as TiN or Cr is deposited in a via with a metal such as CVD W and CMP. Next, a bottom electrode and metal wire is formed. An insulator is deposited and patterned. Then a PCMO layer is deposited. Where the PCMO is deposited above the insulator, an amorphous PCMO is formed. Where the PCMO is deposited to exposed metal surface, a polycrystalline PCMO is formed. A top electrode and metal line is formed, and a passivation insulator is then formed to protect the circuitry. 
         [0034]      FIG. 1E  shows an exemplary cross sectional view of an exemplary PCMO memory. In  FIG. 1E , the selective-plating with platinum (Pt) forms the bottom electrode on the metal wire. The crystalline PCMO grows on a platinum surface and no crystalline PCMO was grown on insulators such as SiO2 and SiN, among others. The metal layer can be aluminum, tungsten, chromium, nickel or copper, or a suitable metal. The metal can also be a noble metal such as Pt or In, but these materials are difficult to work with. The insulator can be SiN, SiON, SiO2 or other insulators. 
         [0035]    Referring to  FIG. 2 , a first exemplary RRAM memory array  30  with two-terminal memory cells is shown. A sense amplifier  32  is connected to the bit line  34  of a selected memory cell  36 . Each cell  36  has logic states “1” and “0” that correspond to a low resistance state and a high resistance state, respectively. The cell  36  can also have multiple states, enabling multiple states such as 256 states to be stored in one cell, resulting in high density storage cells. The resistance of a selected memory cell  36  may be changed by applying currents to a word line  12  and a bit line  14  crossing the selected memory cell. A voltage Vr is applied to the word line  38  of the selected memory cell  36 , and sense amplifier  32  applies a voltage to the bit line  34  of cell  36 . The sense amplifier  32  provides an amplified output  39  reflecting the state of the memory cell  36 . The same bit line voltage is applied to all of the bit line  34 , effectively biasing all the cells on unselected rows to zero potential. This action isolates the bit line currents from one another, effectively blocking most of the leakage current that might otherwise flow through secondary paths where such leakage may possibly cause errors in the sensing function of the selected memory cell. 
         [0036]      FIG. 3  depicts a second exemplary RRAM array with a complementary output (three terminal memory cell). Each memory cell consists of two memory resistors formed in accordance with  FIGS. 1A-1B . Each of the complementary bit lines is connected to the respect input of a differential amplifier, which was not shown in this figure. The output of the differential amplifier may be connected to an inverter for a digital output at either “0” or “1” state. The RRAM thin film can have a uniform material property, in which case the RRAM can be programmed in one step. Alternatively, for high density or for processes that do not provide uniform thin film deposition, a two-step programming may be applied to any non-uniform property RRAM thin film memory cell. 
         [0037]    The-one-step programming process will be discussed first. The selected cell is W 2 B 2  and a first related bit line, B 22 , will be programmed to high voltage bit and a second related bit line, B 22 , will be programmed to low voltage bit. The word line, W 2  is floated. All other word lines are biased with half-programming pulse voltage. Bit line B 22  is grounded. Programming pulse, VP is applied to B 22 . All other bit lines are biased to the ground potential. As a result the resistor R 22 A is at the low resistance-state and the resistance B 22 B is at the high resistance-state. All memory resistor in the bit  2  other than that connected to W 2  word lines are biased with half of the programming voltage. Therefore, there is no resistance change. Similarly each bit lines along the W 2  may be properly biased to program one word at a time. After programming, the two memory cell resistors in any given bit are at the high resistance-state and low resistance-state, respectively. 
         [0038]    The two-step programming is more conventional. Again, cell W 2 B 2  is selected in this example. A first selected memory resistor, R 22 A and memory resistor R 22 B are programmed to a low resistance state and to a high resistance-state, respectively. The selected word line, W 2 , is set to ground potential, and all other word lines are biased to 0.5 VP. A negative programming pulse, having pulse amplitude of −VP, and a positive programming pulse, having amplitude of +VP, with appropriate pulse widths, are applied to bit lines B 2  and !B 2 , respectively. The positive and the negative programming pulses do not have to be applied simultaneously, and the A resistor and the B resistor may be programmed separately. Memory resistors, R 22 A and R 22 B, are, therefore, programmed to a low resistance state, RL and a high resistance state, RH, respectively. All other memory resistors are either pulsed with half of the programming pulse voltage, or not pulsed. Therefore, the resistance of the non-selected memory resistors will not change during this programming operation. 
         [0039]    The memory array may be read by applying a read voltage to the word line and detecting (reading) the voltage difference between the bit lines which share the memory cell. 
         [0040]    In one exemplary memory cell array shown in  FIG. 4 , word lines for selecting rows and bit lines for selecting columns are arranged to intersect at right angles. Memory cells are formed at intersections, and a peripheral driver circuit for selectively allowing information to be written into or read from the memory cells and an amplifier circuit which for reading the information are also formed. The peripheral circuit section includes a word line driver circuit and bit line driver circuit and a signal detecting circuit such as a sense amplifier, for example. 
         [0041]      FIG. 5  shows a cross sectional TEM view of an exemplary PCMO RRAM memory cell. IN this RRAM embodiment, the memory device is formed of silicon (Si), silicon oxide (SiO2), titanium nitride (TiN), platinum (Pt), and PCMO material, respectively. 
         [0042]      FIG. 6  shows an exemplary multi-level RRAM. In  FIG. 6 , vias  402  enable connections to be made from a wafer  401  to a plurality of metal lines  410 - 416  along the X and Y axis to select and access a plurality of RRAM cells  404  formed above the wafer  401 . 
         [0043]    It is to be understood that various terms employed in the description herein are interchangeable. Accordingly, the above description of the invention is illustrative and not limiting. Further modifications will be apparent to one of ordinary skill in the art in light of this disclosure.