# Patent

## Title
Phase change current density control structure

## Abstract

A phase change memory element and method of forming the same. The memory element includes first and second electrodes. A first layer of phase change material is between the first and second electrodes. A second layer including a metal-chalcogenide material is also between the first and second electrodes and is one of a phase change material and a conductive material. An insulating layer is between the first and second layers. There is at least one opening in the insulating layer providing contact between the first and second layers.

## Background
CROSS-REFERENCE TO RELATED APPLICATION
This application is a divisional application of U.S. patent application Ser. No. 11/304,593, filed Dec. 16, 2005, now U.S. Pat. No. 7,943,921 the subject matter of which is incorporated in its entirety by reference herein.


FIELD OF THE INVENTION
The present invention relates to semiconductor devices, and in particular phase-change memory devices and methods of forming the same.
BACKGROUND OF THE INVENTION
Non-volatile memories are important elements of integrated circuits due to their ability to maintain data absent a power supply. Phase change materials have been investigated for use in non-volatile memory cells. Phase change memory cells include phase change materials, such as chalcogenide alloys, which are capable of stably transitioning between amorphous and crystalline phases. Each phase exhibits a particular resistance state and the resistance states distinguish the logic values of the memory cell. Specifically, an amorphous state exhibits a relatively high resistance, and a crystalline state exhibits a relatively low resistance.
A typical phase change cell has a layer of phase change material between first and second electrodes. As an example, the phase change material is a chalcogenide alloy, such as Ge2Sb2Te5 or SbTeAg. See, e.g., Lankhorst et al., Low-cost and nanoscale non-volatile memory concept for future silicon chips, NATURE MATERIALS, vol. 4 pp. 347-352 (April 2005).
A portion of the phase change material is set to a particular resistance state according to the amount of current applied via the electrodes. To obtain an amorphous state, a relatively high write current pulse (a reset pulse) is applied through the phase change cell to melt a portion of the material for a short period of time. The current is removed and the cell cools rapidly to a temperature below the glass transition temperature, which results in the portion of the material having an amorphous phase. To obtain a crystalline state, a lower current write pulse (a set pulse) is applied to the phase change cell for a longer period of time to heat the material to a temperature below its melting point. This causes the amorphous portion of the material to re-crystallize to a crystalline phase that is maintained once the current is removed and the cell is rapidly cooled.
A sought after characteristic of non-volatile memory is low power consumption. Often, however, phase change memory cells require large operating currents. It is therefore desirable to provide a phase change memory cell with reduced current requirements. For phase change memory cells, it is necessary to have a current density that will heat the phase change material past its melting point and quench it in an amorphous state. One way to increase current density is to decrease the size of a bottom electrode; another way is to deposit small conductive crystals on the bottom electrode. These methods maximize the current density at the bottom electrode interface to the phase change material.
It would be desirable, however, to maximize the current density at a location above the bottom electrode in certain applications.
BRIEF SUMMARY OF THE INVENTION
Embodiments of the invention provide phase change memory elements and methods of forming the same. An exemplary memory element includes first and second electrodes. A first layer of phase change material is located between the first and second electrodes. A second layer comprising a metal chalcogenide material is also between the first and second electrodes and is one of a phase change material and a conductive material. An insulating layer is between the first and second layers. There is at least one opening in the insulating layer providing contact between the first and second layers.



BRIEF DESCRIPTION OF THE DRAWINGS
The foregoing and other advantages and features of the invention will become more apparent from the detailed description of exemplary embodiments provided below with reference to the accompanying drawings in which:
FIG. 1 depicts a phase change memory element according to an exemplary embodiment of the invention;
FIGS. 2A-2I depict the formation of the memory element of FIG. 1 at different stages of processing; and
FIG. 3 is a block diagram of a system including a memory element according to an exemplary embodiment of the invention.



DETAILED DESCRIPTION OF THE INVENTION
In the following detailed description, reference is made to various specific embodiments of the invention. These embodiments are described with sufficient detail to enable those skilled in the art to practice the invention. It is to be understood that other embodiments may be employed, and that various structural, logical and electrical changes may be made without departing from the spirit or scope of the invention.
The term “substrate” used in the following description may include any supporting structure including, but not limited to, a semiconductor substrate that has an exposed substrate surface. A semiconductor substrate should be understood to include silicon, silicon-on-insulator (SOI), silicon-on-sapphire (SOS), doped and undoped semiconductors, epitaxial layers of silicon supported by a base semiconductor foundation, and other semiconductor structures. When reference is made to a semiconductor substrate or wafer in the following description, previous process steps may have been utilized to form regions or junctions in or over the base semiconductor or foundation. The substrate need not be semiconductor-based, but may be any support structure suitable for supporting an integrated circuit, including, but not limited to, metals, alloys, glasses, polymers, ceramics, and any other supportive materials as is known in the art.
The invention is now explained with reference to the figures, which illustrate exemplary embodiments and throughout which like reference numbers indicate like features. FIG. 1 depicts an exemplary embodiment of a phase change memory element 100 constructed in accordance with the invention. The element 100 shown in FIG. 1 is supported over a substrate 101. Over the substrate 101 is a first insulating layer 110.
A first electrode 102 and second insulating layer 103 are over the first insulating layer 110 and substrate 100. The first electrode 102 can be any suitable conductive material, such as platinum or tungsten, among others. The second insulating layer 103 can be a nitride, such as silicon nitride (Si3N4); a low dielectric constant material; an insulating glass; or an insulating polymer; among other materials.
As shown in FIG. 1, a first layer 104 of phase change material, preferably a chalcogenide material, is over the first electrode 102 and the second insulating layer 103. The first layer 104 is electrically coupled to the first electrode 102 through an opening in the second insulating layer 103. In the illustrated embodiment, the first layer 104 is a germanium-telluride layer. Other exemplary chalcogenide compositions for the first layer 104 include concentrations of Te below about 70%. The germanium concentration is preferably above about 10%. The first layer 104 can include additional elements, for example antimony. The percentages given are atomic percentages which total 100% of the atoms of the constituent elements. In the illustrated embodiment, the first layer 104 is about 300 Å to about 500 Å thick and is in electrical contact with the underlying first electrode 102.
A third insulating layer 105 is over the first layer 104. In the illustrated embodiment, the second insulating layer 103 is amorphous carbon, but other insulating materials can be used. An opening 106 extends through the third insulating layer 105 over the phase change material of the first layer 104. The opening 106 is at a location such that it is at least partially directly above the first electrode 102. The opening 106 has an area smaller than the surface area of the first electrode 102. Optionally, there can be more than one opening 106 within the insulating layer 105.
Over the third insulating layer 105 and within the opening 106 is a second layer 107 of metal-chalcogenide material. Alternatively, the second layer could be a phase change material, and further can be the same material as the first layer 104. In the illustrated embodiment, the second layer 107 is, for example, a tin-telluride layer having about 50% tin and about 50% tellurium and is about 500 Å to about 700 Å thick.
Although second layer 107 is shown over the chalcogenide material of the first layer 104, it should be understood that the orientation of the layers can be altered. For example, the first layer 104 may be over the second layer 107 and within the opening 106.
Over the second layer 107 is a second electrode 108. The second electrode 108 can be any suitable conductive material, for example, platinum, among others. Tungsten in the illustrated embodiment.
For operation, a pulse generator 35 is used to apply a reset pulse to the element 100. The pulse generator 35 can be, for example, memory access circuitry, such as DRAM memory cell access circuitry, among others. The reset pulse melts at least a portion of one or more of the germanium-telluride first layer 104, the layer 107 or the interface between layers 104 and 107. This leaves the melted portion(s) in a high resistance, amorphous state. A set pulse crystallizes at least a portion of the germanium-telluride first layer 104, leaving the first layer 104 in a low resistance state. During operation, current 120 is channeled through the opening 106 to achieve an increased current density at a distance H from the first electrode 102 and at the interface of the first and second layers 104, 107. In the illustrated embodiment, the distance H is the thickness of the first layer 104. It should be understood that the element 100 can be configured such that H is a different distance from the first electrode 102. The height H is selected to achieve the increased current density at a desired height and will depend on the particular material(s) used in the element 100.
FIGS. 2A-2H are cross sectional views of a wafer fragment depicting the formation of the memory element 100 according to an exemplary embodiment of the invention. No particular order is required for any of the actions described herein, except for those logically requiring the results of prior actions. Accordingly, while the actions below are described as being performed in a specific order, the order is exemplary only and can be altered if desired. Although the formation of a single memory element 100 is shown, it should be appreciated that the memory element 100 can be one memory element in an array of memory elements, which can be formed concurrently.
As shown by FIG. 2A, a substrate 101 is initially provided. As indicated above, the substrate 101 can be semiconductor-based or another material useful as a supporting structure as is known in the art. A first insulating layer 110 is formed over the substrate 101. A first electrode 102 and second insulating layer 103 are formed over the first insulating layer 110. The first electrode 102 is formed within an opening in the second insulating layer 103 such that the surface of the first electrode 102 is exposed. The first insulating layer 110 can be, for example, silicon dioxide. The second insulating layer 103 can be silicon nitride, a low dielectric constant material, or other suitable insulators known in the art, and may be formed by any method known in the art.
As shown in FIG. 2B, a first layer 104 of phase change material, e.g., germanium-telluride, is formed over the first electrode 102 and second insulating layer 103. Formation of the first layer 104 may be accomplished by any suitable method. In the depicted embodiment, the first layer 104 is formed having a thickness of about 300 Å about 500 Å.
FIG. 2C depicts the formation of the third insulating layer 105. In the illustrated embodiment, the third insulating layer 105 is an amorphous carbon layer, but other materials may be used. The insulating layer 105 is formed at a distance H over the first electrode 102 and can be formed by any suitable technique. In the illustrated embodiment, H is the thickness of the first layer 104. H, however, could be a different distance. Additionally, the third insulating layer 105 could instead be formed over more than one layer.
Referring to FIG. 2D, a layer 201 of photoresist is formed over the third insulating layer 105. The photoresist layer 201 is patterned to have an opening 206 to expose the insulating layer 105. The opening 206 is formed at a location such that the opening 206 is formed at least partially directly above the first electrode 102. The size and location of the opening 206 determines the size and location of the opening 106 (FIG. 1) to be formed in the insulating layer 105.
Alternatively, more than one opening 206 can be patterned in the photoresist layer 201 above the first electrode 102, as shown in FIG. 2E. Additionally, an optional process can be used to further reduce the size of the opening 106 (FIG. 1) that will be formed. For this optional process, a second layer of material 202 is formed over the photoresist layer 201 and lining the sidewalls of openings 206. Openings 207 are formed in the pattern created by the photoresist layer 201 and the material 202 together. Thereby the openings 207 have a reduced size as compared to the openings 206. Once the openings 207 are formed the material 202 is removed.
As depicted in FIG. 2F, the portions of the third insulating layer 105 exposed by the opening 206 are removed to form opening(s) 106. In the case of the process described in connection with FIG. 2E, the portions of the third insulating layer 105 exposed by the openings 207 are removed to form a plurality of openings 106. The photoresist layer 201 (and layer 202 shown in FIG. 2E) are also removed by conventional methods. Hereinafter, the invention is illustrated has having one opening 106 for exemplary purposes only, but, as shown in FIG. 2I, there may be a plurality of openings 106.
A second layer 107 of a metal-chalcogenide material, e.g., tin-telluride, is formed over the third insulating layer 105 and within the opening 106, as shown in FIG. 2G. The second layer 107 can be formed by any suitable method, e.g., physical vapor deposition, chemical vapor deposition, co-evaporation, sputtering, among other techniques. In the illustrated embodiment, the second layer 107 is formed having a thickness of about 500 Å to about 700 Å.
As illustrated in FIG. 2H, a conductive material is deposited over the second layer 107 to form a second electrode 108. Similar to the first electrode 102, the conductive material for the second electrode 108 may be any material suitable for a conductive electrode, for example, platinum or tungsten, among others. In the illustrated embodiment, the layers 104, 105, 107, 108 are formed as blanket layers.
Although the memory element 100 is shown having first and second layers 104, 107, the element 100 can include additional layers of the same or different materials described above between the first and second electrodes 102, 108. Further, the insulating layer 105 can be between any of the additional layers to achieve an increased current density at a desired distance H from the first electrode 102.
Additional processing steps can be performed, for example, to form connections to other circuitry of the integrated circuit (e.g., logic circuitry, sense amplifiers, etc.) of which the memory element 100 is a part, as is known in the art.
FIG. 3 illustrates a processor system 300 which includes a memory circuit 348, e.g., a memory device, which employs memory array 301, which includes at least one memory element 100 constructed according to the invention. The processor system 300, which can be, for example, a computer system, generally comprises a central processing unit (CPU) 344, such as a microprocessor, a digital signal processor, or other programmable digital logic devices, which communicates with an input/output (I/O) device 346 over a bus 352. The memory circuit 348 communicates with the CPU 344 over bus 352 typically through a memory controller.
In the case of a computer system, the processor system 300 may include peripheral devices such as a floppy disk drive 354 and a compact disc (CD) ROM drive 356, which also communicate with CPU 344 over the bus 352. Memory circuit 348 is preferably constructed as an integrated circuit, which includes a memory array 301 having at least one memory element 100 according to the invention. If desired, the memory circuit 348 may be combined with the processor, for example CPU 344, in a single integrated circuit.
The above description and drawings are only to be considered illustrative of exemplary embodiments, which achieve the features and advantages of the present invention. Modification and substitutions to specific process conditions and structures can be made without departing from the spirit and scope of the present invention. Accordingly, the invention is not to be considered as being limited by the foregoing description and drawings, but is only limited by the scope of the appended claims.

## Claims

1. A method of forming a memory element, the method comprising the acts of:
forming a first electrode;
forming a first region of phase change material over the first electrode;
forming an insulating material over the first region;
forming a second region comprising one of a phase change material and a conductive material over the insulating layer;
forming at least two openings in the insulating layer to provide contact between the first and second regions to define a single memory element; and
forming a second electrode over the second region, wherein at least a portion of each of the at least two openings is located directly between the first electrode and second electrodes.

2. The method of claim 1, wherein the first region is formed comprising a chalcogenide material.

3. The method of claim 2, wherein the first region is formed comprising germanium-telluride.

4. The method of claim 1, wherein the first region is formed having a thickness of about 300 Å to about 500 Å.

5. The method of claim 1, wherein the second region is formed comprising a metal-chalcogenide material.

6. The method of claim 5, wherein the second region is formed comprising tin-telluride.

7. The method of claim 1, wherein the second region is formed having a thickness of about 500 Å to about 700 Å.

8. The method of claim 1, wherein the insulating material is formed comprising amorphous carbon.

9. The method of claim 1, wherein at least one of the first and second electrodes are formed comprising tungsten.

10. The method of claim 1, wherein the at least one opening is formed at least partially directly above the second electrode.

11. The method of claim 1, wherein the act of forming the at least one opening in the insulating material comprises forming a layer of photoresist in contact with the insulating material and patterned to have at least one opening having a first size.

12. The method of claim 11, wherein the at least one opening having the first size defines the at least one opening in the insulating material.

13. The method of claim 11, wherein the act of forming the at least one opening in the insulating material further comprises the act of reducing the first size to a second size by forming a third layer on surfaces of the photoresist layer, wherein the second size defines the at least one opening in the insulating material.

14. The method of claim 13, further comprising the acts of removing the photoresist layer and the third layer.

15. The method of claim 1, wherein the act of forming the at least one opening comprises forming the at least one opening having a surface area less than a surface area of the first electrode.

16. A method of forming a memory element comprising:
forming a first electrode;
forming a first region of phase change material adjacent the first electrode;
forming an insulating material adjacent the first region;
forming at least two openings in the insulating material;
forming a second region spaced from the first region, such that the insulating material is between the first region from the second region and the openings provide contact between the first region and second region to define a single memory element, the second region comprising one of a phase change material and a conductive material; and
forming a second electrode spaced from the first electrode, such that the first region, second region and insulating material are located between the first and second electrodes and at least a portion of each of the at least two openings is located directly between the first and second electrodes.

17. The method of claim 16, wherein the first region comprises a chalcogenide material.

18. The method of claim 17, wherein the first region comprises germanium-telluride.

19. The method of claim 16, wherein the first region has a thickness of about 300 Å to about 500 Å.

20. The method of claim 19, wherein the second region comprises tin-telluride.

21. The method of claim 16, wherein the second region has a thickness of about 500 Å to about 700 Å.

22. The method of claim 16, wherein the insulating material comprises amorphous carbon.

23. The method of claim 16, wherein at least one of the first and second electrodes comprise tungsten.

24. The method of claim 16, wherein the at least two openings are at least partially directly above the second electrode.

25. The method of claim 16, wherein the at least two openings are configured to increase current density during operation of the element.

26. The method of claim 16, wherein the at least two openings have a surface area less than a surface area of one of the first and second electrodes.