Memory elements having patterned electrodes and method of forming the same

Abstract

A memory element having a resistance variable material and methods for forming the same are provided. The method includes forming a plurality of first electrodes over a substrate and forming a blanket material stack over the first electrodes. The stack includes a plurality of layers, at least one layer of the stack includes a resistance variable material. The method also includes forming a first conductive layer on the stack and etching the conductive layer and at least one of the layers of the stack to form a first pattern of material stacks. The etched first conductive layer forming a plurality of second electrodes with a portion of the resistance variable material located between each of the first and second electrodes.

Description

FIELD OF THE INVENTION

The invention relates to the field of random access memory (RAM) devices formed using a resistance variable material.

BACKGROUND OF THE INVENTION

Resistance variable memory elements, which include Programmable Conductive Random Access Memory (PCRAM) elements using chalcogenides, have been investigated for suitability as semi-volatile and non-volatile random access memory devices. A typical chalcogenide resistance variable memory element is disclosed in U.S. Pat. No. 6,348,365 to Moore and Gilton.

In a typical chalcogenide resistance variable memory element, a conductive material, for example, silver, tin and copper, is incorporated into a chalcogenide glass. The resistance of the chalcogenide glass can be programmed to stable higher resistance and lower resistance states. An unprogrammed chalcogenide variable resistance element is normally in a higher resistance state. A write operation programs the element to a lower resistance state by applying a voltage potential across the chalcogenide glass and forming a conductive pathway. The element may then be read by applying a voltage pulse of a lesser magnitude than required to program it; the resistance across the memory device is then sensed as higher or lower to define two logic states.

The programmed lower resistance state of a chalcogenide variable resistance element can remain intact for an indefinite period, typically ranging from hours to weeks, after the voltage potentials are removed; however, some refreshing may be useful. The element can be returned to its higher resistance state by applying a reverse voltage potential of about the same order of magnitude as used to write the device to the lower resistance state. Again, the higher resistance state is maintained in a semi- or non-volatile manner once the voltage potential is removed. In this way, such an element can function as a variable resistance memory having at least two resistance states, which can define two respective logic states, i.e., at least a bit of data.

One exemplary chalcogenide resistance variable device uses a germanium selenide (i.e., Ge_xSe_100−x) chalcogenide glass as a backbone. The germanium selenide glass has, in the prior art, incorporated silver (Ag) and silver selenide (Ag_2+/−xSe) layers in the memory element. FIG. 1 depicts an example of a conventional chalcogenide variable resistance element 1. A semiconductive substrate 10, such as a silicon wafer, supports the memory element 1. Over the substrate 10 is an insulating material 11, such as silicon dioxide. A conductive material 12, such as tungsten, is formed over insulating material 11. Conductive material 12 functions as a first electrode for the element 1. An insulating material, 13 such as silicon nitride, is formed over conductive material 12. A glass material 51, such as Ge₃Se₇, is formed within via 22.

A metal material 41, such as silver, is formed over glass material 51. An irradiation process and/or thermal process are used to cause diffusion of metal ions into the glass material 51. A second conductive electrode 61 is formed over dielectric material 13 and residual metal material 41.

The element 1 is programmed by applying a sufficient voltage across electrodes 12 and 61 to cause the formation of a conductive path between the two electrodes 12 and 61, by virtue of a conductor (i.e., such as silver) that is present in metal ion laced glass layer 51. In the illustrated example, with the programming voltage applied across electrodes 12 and 61, the conductive pathway forms from electrode 12 towards electrode 61.

It is desirable to have additional methods of forming memory elements. In particular, it is desirable to have techniques for forming memory elements in a high density.

BRIEF SUMMARY OF THE INVENTION

Exemplary embodiments of the invention provide memory elements having a resistance variable material and methods for forming the same. The method includes forming a plurality of first electrodes over a substrate and forming a blanket material stack over the first electrodes. The stack includes a plurality of layers, at least one layer of the stack includes a resistance variable material. The method also includes forming a first conductive layer on the stack and etching the conductive layer and at least one of the layers of the stack to form a first pattern of material stacks. The etched first conductive layer forming a plurality of second electrodes with a portion of the resistance variable material located between each of the first and second electrodes.

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 illustrates a cross sectional view of a conventional resistance variable memory element;

FIG. 2A depicts a portion of a memory array including memory elements according to an exemplary embodiment of the invention;

FIG. 2B is a top down view of a portion of the memory array of FIG. 2A;

FIG. 2C. illustrates a cross sectional view of a portion of the memory array shown in FIG. 2B along line 2C-2C′;

FIGS. 3A-3F depict the formation of the memory elements of FIG. 2A at different stages of processing; and

FIG. 4 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-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. Additionally, for purposes of this specification, a substrate can include layers and structures over a semiconductor substrate, wafer, or other material, such as conductive lines and/or insulating layers.

The invention is now explained with reference to the figures, which illustrate exemplary embodiments and throughout which like reference numbers indicate like features. FIG. 2A depicts a cross section of a portion of a memory array 200 according to an exemplary embodiment of the invention. The memory array 200 includes a plurality of individual resistance variable memory elements 201.

The array 200 is supported by a substrate 210. Over the substrate 210, though not necessarily directly so, is a first (e.g., a bottom) electrode 212 for each memory element 101a, 101b. This electrode 212 is preferably tungsten (W). An insulating layer 214 is between the first electrodes 212 and can be, for example, silicon nitride (Si₃N₄), a low dielectric constant material, an insulating glass, or an insulating polymer, but is not limited to such materials.

Memory elements 201 are formed over each first electrode 212. The memory elements are generally represented by the reference numeral 201 and generally correspond to each portion of a memory stack 240 (described below) addressable by a first and second electrode 212, 251, respectively. The memory elements 201 can be formed directly over each first electrode 212 such that one memory element 201 corresponds to each first electrode 212; or can be offset from the first electrodes 212, as shown in FIG. 2A, such that a single first electrode 212 is shared by more than one (e.g., two) memory elements 201. A memory stack 240 is formed over the first electrodes 212. The memory stack 240 includes one or more layers 241 of resistance variable material and may include layers 242, 243 of additional materials. As shown in FIG. 2A, at least a portion of the memory stack 240 is further formed into element stacks 202.

In the exemplary embodiment shown in FIG. 2A, for example, the memory stack 240 includes a chalcogenide material layer 241, e.g., germanium selenide (Ge_xSe_100−x). The germanium selenide may be within a stoichiometric range of about Ge₃₃Se₆₇ to about Ge₆₀Se₄₀. The chalcogenide material layer 241 may be between about 100 Å and about 1000 Å thick, e.g., about 300 Å thick. Layer 241 need not be a single layer, but may also be comprised of multiple chalcogenide sub-layers having the same or different stoichiometries. The chalcogenide material layer 241 is in electrical contact with the underlying electrodes 212. The chalcogenide material layer 241 is a blanket layer and can be shared by all memory elements 201 of the array 200. Other resistance variable materials can be used for layer 241, including, as non-limiting examples, other chalcogenide glasses; chalcogenide glasses comprising a metal, such as silver, tin, copper, among others; a polymer, such as polymethylphenylacetylene, copperphtalocyanine, polyparaphenylene, polyphenylenevinylene, polyaniline, polythiophene and polypyrrole; and amorphous carbon.

Optionally, over the chalcogenide material layer 241 is a layer of metal-chalcogenide 242, such as tin-chalcogenide (e.g., tin selenide) or a silver chalcogenide (e.g., silver selenide). For purposes of the illustrated embodiment, the optional metal-chalcogenide layer 242 is a tin-chalcogenide layer, for example tin selenide. It is also possible that other chalcogenide materials may be substituted for selenium, such as sulfur, oxygen, or tellurium. The metal-chalcogenide layer 242 may be about 100 Å to about 400 Å thick; however, its thickness depends, in part, on the thickness of the underlying chalcogenide material layer 241. The ratio of the thickness of the metal-chalcogenide layer 242 to that of the underlying chalcogenide material layer 241 should be between about 5:1 and about 1:3.

An optional metal layer 243 is provided over the metal-chalcogenide layer 242, with silver (Ag) being the exemplary metal. This metal layer 243, if employed, is desirably between about 300 Å and about 500 Å thick. In the illustrated embodiment, the metal-chalcogenide layer 242 and the metal layer 243 form a portion of the element stacks 202.

The memory stack 240 can include additional layers (not shown). For example, stack 240 can include a second chalcogenide material layer over the metal layer 243. Such a second chalcogenide layer can be a same material as chalcogenide material layer 241, or it can be a different material.

Over the metal layer 243 is a second electrode 251, which is included in the element stack 202. The second electrode 251 can be made of the same material as the first electrode 212, but is not required to be so. In the exemplary embodiment shown in FIGS. 2A and 2B, the second electrode 251 is preferably tungsten (W).

In the illustrated embodiment, the electrodes 251 are electrode lines extending across at least a portion of the array 200, as shown in the FIG. 2B. FIG. 2B is a top down view of an end portion of the array 200. When second electrodes 251 are electrode lines, the second electrode lines 251 can be electrically coupled to circuitry peripheral to the array 200 through conductive via 270 at an end of the array 200. FIG. 2C is a cross sectional view of an end portion of the array 200 and along line 2C-2C′, and depicting the connection of a second electrode 251 to a conductive via 270. Referring to FIG. 2C, the memory stack 240 ends and the electrode 251 extends along a lateral side of the stack 240 and along, e.g., insulating layer 214 to contact via 270. An optional insulating layer 260 is between the lateral side of the stack 240 and the second electrode 251.

Between the element stacks 202 and over the chalcogenide material layer 241, is an etch stop layer. The etch stop layer 231 facilitates the formation of the element stacks 202 as described in more detail below in connection with FIGS. 3A-3F.

While the invention is not to be bound by any specific theory, it is believed that upon application of a conditioning voltage, metal ions from the metal-chalcogenide layer 242 form one or more conducting channels within the chalcogenide material layer 241. Specifically, the conditioning step comprises applying a potential across the memory elements 201 such that material from the metal-chalcogenide layer 242 is incorporated into the chalcogenide material layer 241, thereby forming conducting channels corresponding to each element 201 through the layer 241. Movement of ions from the layer 242 into or out of a respective conducting channel during subsequent programming of a particular element 201 forms a conductive pathway, which causes a detectible resistance change across that memory element 201.

In the illustrated example, the layers 242, 243 are conductive. Therefore, to avoid second electrodes 251 from being shorted together, layers 242, 243 are patterned in a similar manner to the second electrodes 251 to form element stacks 202. Accordingly, in the illustrated embodiment, the layers 242, 243, 251 are formed as lines. Layers 242, 243, 251 could instead be formed in an alternative pattern as desired. Where stack 240 includes different layers and/or different material, the conductive layers of the stack 240 can be pattered in a similar manner to the second electrodes 251. Other layers of the stack 240, e.g., layers of resistance variable material, such as a layer of chalcogenide glass that supports the formation of conductive pathways, can be blanket layers shared by more than one element 201 of the array 200, or can be patterned in a similar manner to the second electrodes 251 if desired.

FIGS. 3A-3F are cross sectional views of a wafer in various stages of fabrication depicting the formation of the memory array 200 according to an exemplary method 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 general order, the order is exemplary only and can be altered if desired. Although the formation of only a portion of an array 200 is shown, it should be appreciated that the memory array 200 can include additional memory elements 201, which can be formed concurrently.

As shown by FIG. 3A, a substrate 210 is initially provided. As indicated above, the substrate 210 can be semiconductor-based or another material useful as a supporting structure. An insulating layer 214 is formed over the substrate 210. The insulating layer 214 can be silicon nitride, a low dielectric constant material, or other insulators known in the art, and may be formed by any known method. Preferably, the insulating layer 214 (e.g., silicon nitride) does not allow metal ion migration from the optional metal-chalcogenide layer 242. An opening 214a in the insulating layer 214 is made, for instance by photolithographic and etching techniques, exposing a portion of the substrate 210. A first electrode 212 is formed within the opening 214a, by forming a layer of conductive material over the insulating layer 214 and in the opening 214a. A chemical mechanical polishing (CMP) step is performed to remove the conductive material from over the insulating layer 214. Desirably, the first electrode 212 is formed of tungsten.

At least one layer of a memory stack 240 is formed over the insulating layer 214 and first electrodes 212, as depicted in FIG. 3B. In the illustrated embodiment, a chalcogenide material layer 241 is formed over the first electrodes 212 and insulating layer 214. Formation of the chalcogenide material layer 241 may be accomplished by any suitable method, for example, by sputtering.

As shown in FIG. 3C, an etch stop layer 231 is formed over the chalcogenide material layer 241. The etch stop layer 231 is chosen to have a high selectivity to the etch chemistry used to etch certain layers of the memory stack 240 (FIG. 2A). Accordingly, the particular etch stop layer may depend on the composition of the memory stack 240 (FIG. 2A). In the illustrated embodiment, an exemplary etch stop layer is transparent carbon, although other materials can be used.

The etch stop layer 231 is patterned in a first pattern to provide openings 231a over the layer 241. The openings can be made directly over the first electrodes 212 or, as shown in FIG. 3C, the openings 231a can be made offset from the first electrodes 212. Where the openings 231a are formed offset from the first electrodes 212, the openings 231a preferably overlap at least a portion 212a of the first electrodes 212, with an opening overlying two adjacent electrodes 212.

As shown in FIG. 3D, additional layers of the memory stack 240 (FIG. 2A) are formed over the etch stop layer and in opening 231a. In the illustrated embodiment, the optional metal-chalcogenide layer 242 (e.g., tin chalcogenide) is formed over the etch stop layer and in opening 231a and in contact with the chalcogenide material layer 241. The metal-chalcogenide layer 242 can be formed by any suitable method, e.g., physical vapor deposition, chemical vapor deposition, co-evaporation, sputtering, among other techniques. Optionally, a metal layer 243 is formed over the metal-chalcogenide layer 242. The metal layer 243 is preferably silver (Ag), or at least contains silver, and is formed to a preferred thickness of about 300 Å to about 500 Å. The metal layer 243 may be deposited by any technique known in the art.

Referring to FIG. 3E, a conductive material is deposited over the metal layer 243 to form a second electrode layer 251. Where second electrodes 251 are to be connected to peripheral circuitry as shown in FIGS. 2B and 2C, the second electrode layer 251 is formed to extend beyond an end of the layers 241, 242, 243 and formed in contact with vias 270 (FIGS. 2B-2C). Similar to the first electrode 212, the conductive material for the second electrode 251 may be any material suitable for a conductive electrode. In one exemplary embodiment the second electrode 251 is tungsten.

As illustrated in FIG. 3F, a photoresist layer 232 (or other mask layer) is deposited over the second electrode layer 251, and patterned in a second pattern to define locations of element stacks 202. Each element stack 202 forms a portion of a memory element 201. In one exemplary embodiment, the mask layer 232 is formed to define locations of stacks 202 such that the stacks 202 have a width 282, which is larger than the width 281 of the opening 231a. This provides for an alignment margin between the mask layers used to define openings 231a and the photoresist layer 232. In the illustrated embodiment, the second pattern of the photoresist layer (and therefore stacks 202) is approximately a negative image of the first pattern for openings 231a (FIG. 3C).

An etching step is used to remove portions of layers 251, 243 and 242. The etching stops at the etch stop layer 231, leaving stacks 202 as shown in FIG. 2A. The photoresist layer 232 is removed, leaving the structure shown in FIGS. 2A-2C.

Additional steps may be performed to complete the memory array 200. For example, an insulating layer (not shown) may be formed over and between the stacks 202. Also, depending on the composition and/or properties of the etch stop layer 231, it may be desirable to ultimately remove the etch stop layer 231. Also, other processing steps can be conducted to electrically couple the array 200 to circuitry for accessing electrodes 212, 251 (e.g., access circuitry described in U.S. Patent Application, assigned to Micron Technology, Inc.) and peripheral circuitry (not shown), e.g., forming conductive via 270 (FIG. 2B), and to include the array 200 in an integrated circuit or processor system, e.g., processor system 400 described below in connection with FIG. 4.

FIG. 4 illustrates a processor system 400 which includes a memory circuit 448, e.g., a memory device, which employs a memory array 200 according to the invention. The processor system 400, which can be, for example, a computer system, generally comprises a central processing unit (CPU) 444, such as a microprocessor, a digital signal processor, or other programmable digital logic devices, which communicates with an input/output (I/O) device 446 over a bus 452. The memory circuit 448 communicates with the CPU 444 over bus 452 typically through a memory controller.

In the case of a computer system, the processor system 400 may include peripheral devices such as a floppy disk drive 454 and a compact disc (CD) ROM drive 456, which also communicate with CPU 444 over the bus 452. Memory circuit 448 is preferably constructed as an integrated circuit, which includes a memory array 200 (FIGS. 2A and 2B). If desired, the memory circuit 448 may be combined with the processor, for example CPU 444, 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 memory array comprising: a plurality of first electrodes, each having first and second lateral edges over a substrate; a resistance variable material over the plurality of first electrodes; and a plurality of material stacks over the resistance variable material, each material stack located between adjacent first electrodes and comprising at least a conductive material and a second electrode, wherein a first material stack is directly over a first lateral edge of a first electrode to form a first memory element, the first material stack not being directly over a second lateral edge of the first electrode, and a second material stack is directly over the second lateral edge of the first electrode to form a second memory element, the second material stack not being directly over the first lateral edge of the first electrode, the first and second memory elements being electrically connected to the first electrode.

2. The memory array of claim 1, wherein each material stack corresponds to at least one memory element, and wherein the resistance variable material is common to each memory element.

3. The memory array of claim 1, wherein the resistance variable material comprises a chalcogenide material.

4. The memory array of claim 3, wherein the chalcogenide material comprises chalcogenide glass.

5. The memory array of claim 4, wherein the conductive material comprises a metal-chalcogenide material.

6. The memory array of claim 4, wherein the conductive material further comprises a metal.

7. The memory array of claim 5, wherein the metal-chalcogenide material comprises tin selenide.

8. The memory array of claim 5, wherein the metal-chalcogenide material comprises tin telluride.

9. The memory array of claim 6, wherein the metal comprises silver.

10. The memory array of claim 1, wherein the second electrode comprises tungsten.

11. The memory array of claim 1, wherein the resistance variable material is in contact with the first electrode.

12. The memory array of claim 1, wherein each material stack is in contact with the resistance variable material.

13. The memory array of claim 1, wherein each material stack is partially offset from the first electrode.

14. The memory array of claim 1, wherein each material stack corresponds to a single memory element.

15. A processor system comprising: a processor; and a memory device coupled to the processor, the memory device comprising: a plurality of first electrodes, each having first and second lateral edges over a substrate; a resistance variable material˜over the plurality of first electrodes; and a plurality of material stacks over the resistance variable material, each material stack located between adjacent first electrodes and comprising at least a conductive material and a second electrode, wherein a first material stack is directly over a first lateral edge of a first electrode to form a first memory element, the first material stack not being directly over a second lateral edge of the first electrode, and a second material stack is directly over the second lateral edge of the first electrode to form a second memory element, the second material stack not being directly over the first lateral edge of the first electrode, the first and second memory elements being electrically connected to the first electrode.

16. The system of claim 15, wherein each material stack corresponds to at least one memory element, and wherein the resistance variable material is common to each memory element.

17. The system of claim 15, wherein the resistance variable material comprises a chalcogenide material.

18. The system of claim 15, wherein the conductive material comprises a metal-chalcogenide material.

19. The system of claim 18, wherein the conductive material further comprises a metal.

20. The system of claim 15, wherein the resistance variable material is in contact with the first electrode.

21. The system of claim 20, wherein each material stack is in contact with the resistance variable material.

22. The system of claim 15, wherein each material stack is partially offset from the at least one first electrode.

23. The system of claim 15, wherein each material stack corresponds to a single memory element.