# Patent

## Title
Systems and methods for fabricating self-aligned memory cell

## 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 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.

## Background
CROSS-REFERENCES TO RELATED APPLICATIONS
This application is a continuation of U.S. patent application Ser. No. 13/727,620 filed Dec. 27, 2012, which is a continuation of U.S. patent application Ser. No. 13/092,830 filed Apr. 22, 2011, which in turn, 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 incorporated by reference herein in their entirety for all purposes.


FIELD OF INVENTION
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
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.
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.
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.
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
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.
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 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.
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.
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.
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(LaNi03). As chromium and LNO are “sticky” materials, an adhesion layer is not needed.
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.
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
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:
FIGS. 1A-1B show an exemplary process to fabricate a self-aligned memory cell.
FIG. 1C shows an exemplary memory structure with an adhesion layer.
FIG. 1D shows an exemplary memory structure without an adhesion layer.
FIG. 1E shows an exemplary memory structure with platinum plating.
FIG. 2 shows a first exemplary RRAM memory array with two-terminal memory cells.
FIG. 3 depicts a second exemplary RRAM array with a complementary output (three terminal memory cell).
FIG. 4 shows an exemplary RRAM array.
FIG. 5 shows a cross sectional TEM view of an exemplary PCMO RRAM Memory Cell.
FIG. 6 shows an exemplary multi-level RRAM.



DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS
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.
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.
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).
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).
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.
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.
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.
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.
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.
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.
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 Si02 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, Si02 or other insulators.
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.
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.
The-one-step programming process will be discussed first. The selected cell is W2B2 and a first related bit line, B22, will be programmed to high voltage bit and a second related bit line, B22, will be programmed to low voltage bit. The word line, W2 is floated. All other word lines are biased with half-programming pulse voltage. Bit line B22 is grounded. Programming pulse, VP is applied to B22. All other bit lines are biased to the ground potential. As a result the resistor R22A is at the low resistance-state and the resistance B22B is at the high resistance-state. All memory resistor in the bit 2 other than that connected to W2 word lines are biased with half of the programming voltage. Therefore, there is no resistance change. Similarly each bit lines along the W2 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.
The two-step programming is more conventional. Again, cell W2B2 is selected in this example. A first selected memory resistor, R22A and memory resistor R22B are programmed to a low resistance state and to a high resistance-state, respectively. The selected word line, W2, 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 B2 and !B2, 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, R22A and R22B, 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.
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.
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.
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 (Si02), titanium nitride (TiN), platinum (Pt), and PCMO material, respectively.
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.
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.

## Claims

1. A stacked resistive random access memory (RRAM), comprising:
a first metal electrode layer;
a first insulator above the first metal electrode layer wherein the first insulator is patterned to expose one or more first metal portions of the first metal electrode layer;
a first Pr1-x CaxMnO3 (PCMO) layer above the first insulator and the first metal portions, wherein x is between approximately 0.3 and approximately 0.5;
a second metal electrode layer;
a second insulator located above the second metal electrode layer, wherein the second insulator is patterned to expose one or more second metal portions of the second metal electrode layer;
a second Pr1-x CaxMnO3 (PCMO) layer above the second insulator and the second metal portions; and
a last metal electrode layer, thus comprising a two-layer, resistive random access memory.

2. The stacked RRAM of claim 1, wherein the first metal electrode layer is above a wafer.

3. The stacked RRAM of claim 2, further comprising an insulator layer between the wafer and the first metal electrode layer.

4. The stacked RRAM of claim 3, further comprising vias connecting the first metal electrode layer and circuits on the wafer.

5. The stacked RRAM of claim 3, further comprising an adhesion layer next to the first metal electrode layer.

6. The stacked RRAM of claim 1, wherein any of the first, second or last metal electrode layer is a chemical mechanical polished layer.

7. The stacked RRAM of claim 1, wherein the first, second or last metal electrode layer comprises one of: platinum, chromium, iridium, ruthenium, nickel, silicides, nickel, and titanium.

8. The stacked RRAM of claim 1, further comprising a passivation film above the last metal electrode layer.

9. A stacked resistive random access memory (RRAM), comprising:
a first metal electrode layer;
a first insulator located above the first metal electrode layer wherein the first insulator is patterned to expose one or more first metal portions of the first metal electrode layer;
a first Pr1-x CaxMnO3 (PCMO) layer above the first insulator and the first metal portions, wherein x is between approximately 0.3 and approximately 0.5;
a second metal electrode layer;
a second insulator located above the second metal electrode layer, wherein the second insulator is patterned to expose one or more second metal portions of the second metal electrode layer;
a second Pr1-x CaxMnO3 (PCMO) layer above the second insulator and the second metal portions;
a third metal electrode;
a third insulator located above the third metal electrode layer, wherein the third insulator is patterned to expose one or more third metal portions of the third metal electrode layer; and
a third Pr1-x CaxMnO3 (PCMO) layer above the third insulator and the third metal portions; and
a last metal electrode layer, thus comprising a three-layer, stacked resistive random access memory.

10. The stacked RRAM of claim 9, wherein the first, second, third or last metal electrode layer comprises one of: platinum, chromium, iridium, ruthenium, nickel, silicides, nickel, and titanium.

11. A stacked resistive random access memory (RRAM), comprising:
a first metal electrode layer, wherein the first metal layer forms a set of first metal lines oriented in an X-direction;
a first insulator formed above the first metal electrode layer wherein the first insulator is patterned to expose one or more first metal portions of the first metal electrode layer;
a first Pr1-x CaxMnO3 (PCMO) layer above the first insulator and the first metal portions, wherein the first PCMO layer forms a substantially amorphous material above the first insulator and the first PCMO layer forms a substantially polycrystalline material above the first metal portion;
a second metal electrode layer, wherein the second metal layer forms a set of second metal lines oriented in a Y-direction;
a second insulator formed above the second metal electrode layer wherein the second insulator is patterned to expose one or more second metal portions of the second metal electrode layer;
a second Pr1-x CaxMnO3 (PCMO) layer above the second insulator and the second metal portions, wherein the second PCMO layer forms a substantially amorphous material above the second insulator and the second PCMO layer forms a substantially polycrystalline material above the second metal portion; and
a last metal electrode layer, thus forming a stacked resistive random access memory.

12. The stacked RRAM of claim 11, wherein the Y-direction is substantially perpendicular to the X-direction.

13. The stacked RRAM of claim 11, further comprising:
a third metal electrode layer, third insulator, and a third PCMO layer above the second PCMO layer; and
a fourth metal electrode layer, fourth insulator, and a fourth PCMO layer above the third PCMO layer, thus forming a four-layer stacked resistive random access memory.

14. The stacked RRAM of claim 11, further comprising vias connecting the first metal electrode layer to circuits on the wafer.

15. The stacked RRAM of claim 11, further comprising an adhesion layer with the first metal electrode layer.

16. The stacked RRAM of claim 11, wherein any of the first, second or last metal electrode layer are chemical mechanical polished metal layers.

17. The stacked RRAM of claim 11, wherein the first, second or last metal electrode layer comprises one of: platinum, chromium, iridium, ruthenium, nickel, silicides, nickel, and titanium.

18. The stacked RRAM of claim 11, further comprising a passivation film above the last metal electrode layer.