# Patent

## Title
Phase change memory

## Abstract

A PCRAM cell has a high resistivity bottom electrode cap to provide partial heating near the interface between the cell and the bottom electrode, preventing separation of the amorphous GST region from the bottom electrode, and reducing the programming current requirements.

## Background
FIELD
The present disclosure relates generally to phase change memories and in particular the present disclosure relates to phase change memory electrodes.
BACKGROUND
Phase change random access memory (PCRAM) is a non-volatile form of memory that uses the reversible process of changing the state of an alloy containing one or more elements from Group V or VI of the periodic table between amorphous and crystalline states upon application of an electric current, and wherein the two states have substantially different electrical resistance. Typical current phase change memories use a chalcogenide alloy, such as a Germanium-Antimony-Tellurium (GeSbTe, or GST, most commonly Ge2Sb2Te5) alloy. The amorphous (a-GST) and crystalline (c-GST) states of the material have largely different resistivity, on the order of three orders of magnitude, so that a determination of the state is easily done. The crystalline state has typical resistance on the order of kiloOhms (kΩ), whereas the amorphous state has typical resistance on the order of megaOhms (MΩ). The states are stable under normal conditions, so the PCRAM cell is a non-volatile cell with a long data retention. When the GST is in its amorphous state, it is said to be RESET. When the GST is in its crystalline state, it is said to be SET. A PCRAM cell is read by measuring its resistance.
The structure of a typical vertical PCRAM cell in a SET state 100 as shown in FIG. 1 includes a bottom metal contact 102, a bottom electrode 104 surrounded by dielectric material 106, a chalcogenide (GST) 108 having a crystalline portion (c-GST) 112, a top electrode 114, a metal top contact 116, and a cell select line 118. The GST 108 being all c-GST means that the GST has a high conductivity, and low resistance, typically on the order of kΩ. The bottom electrode 104 is sometimes referred to as a heater.
A RESET structure of the PCRAM cell 100 is shown in FIG. 2. The bottom electrode 104 is typically a high conductivity, low resistivity metal or alloy (less than 1 milliOhms.cm (mΩ.cm)). To change the cell 100 from a SET state to a RESET state, a current is passed through the bottom metal contact 102 and bottom electrode 104. This current heats a programmable volume region of the GST 108 near the top of the bottom electrode 104 to a temperature sufficient to melt the GST in that region. Typical melting points for many GST materials are in the range of 600 degrees C., although the melting point differs for other chalcogenides. When the current is removed, a section of the programmable volume of GST 108 that has been heated to its melting point rapidly cools due to heat dissipation into the surrounding materials. This rapid cooling does not allow the melted programmable volume region to cool in a crystalline state. Instead, a region of amorphous GST (a-GST 110) remains at or near the top of the heater 104.
The desired a-GST region is a hemispherical region covering the top of the bottom electrode 104 and extending slightly into the field of c-GST. This allows for a high resistance of the GST 108, as the resistances of the c-GST 112 and a-GST 110 portions behave electrically as series a connected resistance. This is shown in FIG. 3.
The majority of the heat generated by the current passing through the bottom electrode 104 does not contribute to heating of the GST 108, since the heat is dissipated by the surrounding dielectric material 106. Therefore, most of the heating of the programmable volume region of GST 108 is due to resistive heating near the top of the heater 106.
In typical PCRAM cells, the cell (the GST layer) and the top electrode are patterned together with the current flowing from the top electrode contact to the bottom electrode. In this arrangement, current density is mostly symmetric. In an ideal RESET state, a hemispheric region of GST covering the entire area of the bottom electrode contact is converted to the amorphous state (a-GST 110), to prevent a parallel leakage path.
The hottest region in the GST programmable volume is typically about 20 nanometers above the interface between the bottom electrode 104 and the GST 108 due to heat loss through bottom electrode 104. The inefficient heating of low resistance bottom electrodes 104 combined with the hottest region being above the interface between the bottom electrode 104 and the GST 108 can create an amorphous GST region that is separated from the bottom electrode as shown in FIG. 4. This leads to a parallel resistance connection for the a-GST and c-GST regions, and the current flows though the low resistance path of the parallel circuit, the result being that the cell is stuck at a low resistance state and the GST cannot be converted back to a high resistance state.
Still further, a RESET current pulse that is too large will form an ideal hemispherical amorphous region covering the bottom electrode 104, but will create a region of the GST that is too hot, often in excess of 900 degrees C. This hot spot can cause bubbling, sublimation, or composition change.
To switch the cell 100 from a RESET state to a SET state, a SET current is passed through the metal contact 102 and bottom electrode 104 to heat the a-GST section 110 near the top of the bottom electrode 104 to a temperature below the melting point, but sufficiently high (on the order of 350 degrees C. for typical GST materials, but different for other chalcogenides) at which the mobility of atoms in the region near the top of the bottom electrode 104 allows them to rearrange from an amorphous state to a crystalline state. The resulting configuration has a GST 108 that is all crystalline, as is shown in FIG. 1.
The currents used to SET and RESET the cell are typically as follows. A SET state is achieved by applying a voltage or current pulse sufficient to raise the GST temperature in the programmable volume to below the melting point but above its crystallization temperature, and is held for a sufficient time to allow the rearranging of the atoms to a crystalline state. A RESET state is achieved by applying a voltage or current pulse sufficient to raise the GST temperature in the programmable volume to the melting point, and is held typically for a shorter time than the SET pulse. The SET pulse is typically longer in duration but of lower amplitude than the RESET pulse. The RESET pulse is typically shorter in duration but of higher amplitude than the SET pulse. The actual amplitudes and durations of the pulses depend upon the size of the cells and the particular phase change materials used in the cell. RESET currents for many GST cells are currently in the 400 to 600 microAmpere (μA) range, and have durations in the 10-50 nanosecond range, whereas SET currents are currently in the 100 to 200 μA range and have durations in the 50-100 nanosecond range. Read currents are lower than either SET or RESET currents. As cell size continues to decrease, the currents involved and the durations thereof also continue to decrease.
For the reasons stated above, and for other reasons stated below which will become apparent to those skilled in the art upon reading and understanding the present specification, there is a need in the art for improved PCRAM structures and methods for phase change memory switching.



BRIEF DESCRIPTION OF DRAWINGS
FIG. 1 is a cross-sectional view of a typical phase change memory cell in a SET state;
FIG. 2 is a cross-sectional view of a typical phase change memory cell in a RESET state;
FIG. 3 is a partial cross-sectional view of a desired RESET structure in a phase change memory cell;
FIG. 4 is a partial cross-sectional view of a failure state RESET structure in a phase change memory cell;
FIG. 5 is a cross-sectional view of a phase change memory cell according to one embodiment;
FIGS. 6A to 6E are in-process cross-sectional views of formation of an electrode cap according to another embodiment;
FIGS. 7A to 7D are in-process cross-sectional views of formation of an electrode cap according to another embodiment;
FIGS. 8A to 8C are in-process cross-sectional views of formation of an electrode cap according to another embodiment;
FIG. 9 is a simplified circuit diagram of a portion of a memory array according to another embodiment;
FIG. 10 is a simplified circuit diagram of a portion of a memory array according to another embodiment; and
FIG. 11 is a simplified circuit diagram of a portion of a memory array according to another embodiment.



DETAILED DESCRIPTION
In the following detailed description of the embodiments, reference is made to the accompanying drawings that form a part hereof. In the drawings, like numerals describe substantially similar components throughout the several views. These embodiments are described in sufficient detail to enable those skilled in the art to practice the embodiments. Other embodiments may be utilized and structural, logical, and electrical changes may be made without departing from the scope of the application.
The following detailed description is, therefore, not to be taken in a limiting sense, and the scope of the present disclosure is defined only by the appended claims, along with the full scope of equivalents to which such claims are entitled.
Embodiments disclosed herein use a high resistivity cap on a bottom electrode of a PCRAM cell to increase the contribution of the bottom electrode to heating of a programmable region of a phase change material of the cell. While GST is used in the description herein, it should be understood that other phase change materials including other chalcogenides, are amenable to use with the various embodiments. For example only, phase change materials include but are not limited to GeTe, In—Se, Sb2Te3, GaSb, InSb, As—Te, Al—Te, Ge—Sb—Te, Te—Ge—As, In—Sb—Te, Te—Sn—Se, Ge—Se—Ga, Bi—Se—Sb, Ga—Se—Te, Sn—Sb—Te, In—Sb—Ge, Te—Ge—Sb—S, Te—Ge—Sn—O, Te—Ge—Sn—Au, Pd—Te—Ge—Sn, In—Se—Ti—Co, Ge—Sb—Te—Pd, Ge—Sb—Te—Co, Sb—Te—Bi—Se, Ag—In—Sb—Te, Ge—Sb—Se—Te, Ge—Sn—Sb—Te, Ge—Te—Sn—Ni, Ge—Te—Sn—Pd, Ge—Te—Sn—Pt, and the like. For purposes of this application, resistivity refers to electrical resistivity.
FIG. 5 shows a PCRAM cell 500 in cross section. Cell 500 includes a mostly typical set of components similar to those shown in FIGS. 1 and 2, and operates under the same general principles. A lower metal contact 502 has thereon a bottom electrode 504 surrounded by dielectric material 506. A phase change material 508, such as a chalcogenide or GST material, is above the bottom electrode 504, and is topped with a top electrode 514, a top metal contact 516, and a cell select line 517. The phase change material 508 is shown in FIG. 5 having an amorphous region 510 and a crystalline region 512. The bottom electrode 504 has two parts, a main, lower portion, element 518 and a top, upper portion, electrode cap 520.
In one embodiment, for a total bottom electrode height of approximately 100 nm, the top electrode cap 520 thickness is in a range of 2 nm to 20 nm (currently approximately 2 to 20% of the total height of the bottom electrode), and it is formed of a high resistivity material (from 1 mΩ.cm to 1000 mΩ.cm). The main element 518 is formed of a low resistivity material. The high resistivity material close to the GST programmable volume creates a partial heating of the GST programmable volume by the resistive heating in the electrode cap 520. This heating serves to move the hottest region of the GST closer to the interface between the bottom electrode 504 and the GST 508, and to prevent the formation of an amorphous region of GST separated from the top of the bottom electrode 504.
One problem with simply making the entire heater a high resistivity material is that partial heating of the cell GST will occur, but a majority of the heat generated by the current passing through the high resistivity heater will be dissipated into the surrounding dielectric without contributing to the heating of the GST material. Further, power consumption will increase due to the high amounts of voltage required to get current to the GST region through the high resistivity heater element.
The embodiments herein concentrate heating due to the bottom electrode 504 at its top where the high resistivity cap 520 is, that is, near the interface between the bottom electrode 504 and the GST 508. The heat produced by the high resistivity cap 520 is close to the cell (GST 508) interface, and provides efficient heating of the programmable volume, and prevents the formation of a crystalline GST region between the bottom electrode 504 and the amorphous GST region formed at the top of the bottom electrode 504. Further, since high electrical resistivity material has a lower thermal conductivity than low electrical resistivity material, the traditional heat sink effect of a low electrical resistivity heater element is reduced at or near the interface between the heater element and the GST. In combination, the programming current requirements can also be reduced.
As an example, a typical PCRAM cell such as that shown in FIGS. 1 and 2 with a bottom electrode diameter of 50 nanometers (nm) and a height of 100 nm, having an undoped GST thickness of 100 nm, has a SET resistance of approximately 2 kΩ (with a typical GST resistivity of 4 mΩ.cm). With a bottom electrode material of TiAlN (resistivity 4 mΩ.cm), the series resistance from the bottom electrode is approximately 2 kΩ, which consumes almost the same energy as a RESET operation. The majority of the energy is lost without contribution to switching of the GST state. Therefore, commonly used bottom electrodes are lower resistivity materials. For example, TiN, having a resistivity of 0.17 mΩ.cm equates to a series resistance of 87 Ω.
In the embodiment shown in FIG. 5, on the other hand, assuming the same component sizes as in the previous example, that is, a bottom electrode diameter of 50 nm and a height of 100 nm, 20 nm of which is TiAlN instead of TiN, and having an undoped GST thickness of 100 nm, still has a SET resistance of 2 kΩ (with GST resistivity of 4 mΩ.cm). However, with the cap electrode of TiAlN, which may have a resistivity on the order of 20 mΩ.cm, the resistance of the cap 520 is approximately 2 kΩ, which consumes the same energy as the GST cell RESET operation, but provides the majority of the energy to heat up the GST near the interface between the bottom electrode and the GST. This in turn promotes the formation of a proper hemispheric a-GST region covering and in contact with the bottom electrode.
The electrode cap 520 described above can be formed in a number of ways. For example, FIGS. 6A-6E show the formation of an electrode cap such as cap 520 in a series of in-process cross-sectional views. During formation of the bottom electrode, usually TiN or Tungsten (W), an overlayer of electrode material is deposited as is shown in FIG. 6A. FIG. 6B shows the results of an appropriate chemical mechanical planarization (CMP) process and overpolish, resulting in a 20 nm or less recess at the top of the bottom electrode. A thin layer of high resistivity material is deposited either with plasma vapor deposition or chemical vapor deposition, the results of which are shown in FIG. 6C. Following another CMP buffering, a 20 nm or less high resistivity cap remains on the bottom electrode as shown in FIG. 6D. The crystalline GST layer is then deposited as is shown in FIG. 6E. Alternatively, the recess can be formed by a wet etch process, a plasma dry etch process, or the like.
Alternatively, an electrode cap such as cap 520 can be formed as shown in FIGS. 7A-7D, a series of in-process cross-sectional views. During formation of the bottom electrode, usually TiN or Tungsten (W), an overlayer of electrode material is deposited as is shown in FIG. 7A. FIG. 7B shows the results of an appropriate chemical mechanical planarization (CMP) process and overpolish, resulting in a 20 nm or less recess at the top of the bottom electrode. A thin layer of high resistivity material is deposited either with plasma vapor deposition or chemical vapor deposition in the recess and over the dielectric as shown in FIG. 7C, and is patterned together with the GST layer as shown in FIG. 7D. The resistivity of the tin layer (from 2 nm to 20 nm) is high enough so that it does not change the electric current distribution in the bottom electrode much, but still forms a local heater above the low resistivity bottom electrode to promote localized GST heating at the interface between the cap and the GST.
Another method for forming the bottom electrode with a high resistivity cap is shown in FIGS. 8A-8C, and uses low energy plasma source implantation to dope the top of a bottom electrode plug with an appropriate dopant to form a thin layer of high resistivity material as is shown in FIGS. 8A and 8B. In one embodiment, the bottom electrode plug is TiN and the dopant is Al, or the bottom electrode plug is TiAl and the dopant is N, or the plug is TaN and the dopant is N to form a top cap layer of Nitrogen-rich high resistivity TaN. It should be understood that various dopants and bottom electrode plug materials could be used to create a top cap on the bottom electrode, the top cap having a much higher resistivity than the plug.
It should be noted that the resistivity of materials such as TiAlN and TaN can be increased by several orders of magnitude by increasing Nitrogen concentrations in the compounds. This increase in Nitrogen concentration can be accomplished, for example, by adjusting Nitrogen-containing gas ratio during chemical vapor deposition or physical vapor deposition of the bottom electrode material or low energy Nitrogen plasma source implantation.
The low resistivity material typically used for bottom electrodes or plugs is as low a resistance conductor as can be formed, for example W, TiN, TiW, Pt, TiAlN (with low Nitrogen concentrations) and the like. The various electrode caps and high resistivity layers of the various embodiments can be formed of any number of high resistivity materials, including by way of example only, TiAlN, AlPdRe, HfTe5, TiNiSn, PbTe, Bi2Te3, TiN, ZrN, HfN, VN, NbN, TaN, TiC, ZrC, HfC, VC, NbC, TaC, TiB2, ZrB2, HfB2, VB2, NbB2, TaB2, Cr3C2, Mo2C, WC, CrB2, Mo2B5, W2B5, TiAlN, TaSiN, TiCN, SiC, B4C, WSi(x), MoSi2, or the like.
PCRAM memory arrays can take several different forms, each of which are amenable to use with the bottom electrode cap configuration PCRAM cells described above. Examples of PCRAM memory arrays include an array of PCRAM cells each comprising an access transistor (metal oxide semiconductor field effect transistor (MOSFET) or bipolar transistor) and one PCRAM cell, in other words a 1T1C configuration. The resistance of the PCRAM cell can be switched between high and low states by resetting the GST of the cell to an amorphous state (high resistance) or setting the cell to a crystalline state (low resistance). Both set and reset currents are provided through the access transistor. An example of a portion of a PCRAM array of this type is shown in FIG. 9. A cell is selected by selecting its corresponding word line and cell select line. Bitlines may be tied to a common voltage source or individually selected. To RESET a cell, a large short pulse is applied to the corresponding cell select line while its word line is turned on. The RESET current flows through the selected memory element and resets the cell. To SET a cell, a smaller but longer pulse is applied to the cell select line to heat the memory element above its crystallization temperature but below its melting point. To read a cell, a voltage smaller than the threshold switching voltage of amorphous phase change material is applied to the cell select line.
Another PCRAM memory array uses a large block of phase change material and a top electrode, and is shown in general in FIG. 10. A common voltage is applied to the top electrode to bias all memory bits. A memory element is selected by selecting its word line and bitline.
Yet another PCRAM memory array is shown in FIG. 11. Diode-accessed cross-point PCRAM arrays select a memory element by biasing its word line high and non-selected word lines low, while biasing its selected bitline low and non-selected bitlines high. Only the diode connected to the selected cell is forward biased. All other diodes are reverse biased or do not have sufficient bias to overcome their threshold voltage, and no current flows except in the selected cell.
PCRAM arrays can be used in various memory devices, and may be coupled to a processor or memory controller, and may form part of an electronic system, including but not limited to memory modules for computers, cameras, portable storage devices, digital recording and playback devices, PDAs, and the like.
CONCLUSION
PCRAM cells and methods of forming them have been described that include a high resistivity cap on the bottom electrode to provide localized heating of a GST layer of the cell, preventing separation of an amorphous GST region from the top of the bottom electrode, and reducing its programming current requirements.
Although specific embodiments have been illustrated and described herein, it will be appreciated by those of ordinary skill in the art that any arrangement, which is calculated to achieve the same purpose, may be substituted for the specific embodiments shown. This application is intended to cover any adaptations or variations of the embodiments. Therefore, it is manifestly intended that this application be limited only by the claims and the equivalents thereof.

## Claims

1. A phase change memory cell, comprising:
a bottom electrode having a lower portion of a first material and an electrode cap of a second material, the first material having a resistivity substantially lower than the second material;
a phase change material on top of the bottom electrode; and
a top electrode over the phase change material.

2. The memory cell of claim 1, wherein the depth of the cap is approximately 2-20% of the total depth of the bottom electrode.

3. The memory cell of claim 1, wherein the first material has a resistivity less than 1 milliOhm.cm.

4. The memory cell of claim 1, wherein the second material has a resistivity of greater than 1 milliOhm.cm.

5. The memory cell of claim 1, wherein the phase change material is a chalcogenide.

6. The memory cell of claim 1, wherein the phase change material is a Germanium-Antimony-Tellurium alloy.

7. A memory device, comprising:
an array of phase change memory cells arranged in rows and columns;
control circuitry to read, write and erase the memory cells; and
address circuitry to latch address signals provided on address input connections;
wherein each memory cell comprises:
a bottom electrode with a lower portion of a first material and an electrode cap of a second material, the cap material having a resistivity substantially higher than the resistivity of the lower portion;
a phase change material on top of the bottom electrode; and
a top electrode over the phase change material.

8. The memory device of claim 7, wherein the cap has a thickness of approximately 2 to 20% of a total height of the bottom electrode.

9. The memory device of claim 7, wherein the cap has a resistivity of at least 1 milliOhm.cm.

10. The memory device of claim 7, wherein the phase change material is a chalcogenide.

11. The memory device of claim 7, wherein the phase change material is a Germanium-Antimony-Tellurium alloy.

12. The memory device of claim 7, wherein the second material has a resistivity of greater than 1 milliOhm.cm.

13. A processing system, comprising:
a processor; and
a memory device coupled to the processor to store data provided by the processor and to provide data to the processor, the memory comprising:
an array of phase change memory cells arranged in rows and columns and accessed by bitlines and word lines;
control circuitry to read, write and erase the memory cells; and
address circuitry to latch address signals provided on address input connections;

wherein each memory cell comprises:
a phase change memory cell having a bottom electrode with a main lower portion and an electrode cap, the cap having a resistivity substantially higher than the main lower portion.

14. The processing system of claim 13, wherein the cap comprises a material having a resistivity of at least 1 milliOhm.cm.