WO2006132813A1

WO2006132813A1 - Memory device with switching glass layer

Info

Publication number: WO2006132813A1
Application number: PCT/US2006/020242
Authority: WO
Inventors: Kristy A. Campbell
Original assignee: Micron Technology, Inc.
Priority date: 2004-07-19
Filing date: 2006-05-26
Publication date: 2006-12-14

Abstract

A memory device (100) , such as a PCRAM, including a chalcogenide glass backbone material with germanium telluride glass (18) in contact with a metal-chalcogenide (20) such as tin selenide and methods of forming such a memory deviceare disclosed.

Description

MEMORY DEVICE WITH SWITCHING GLASS LAYER

[0001] This application is a continuation-in-part of U.S. Patent Application Ser.

No. 10/916,421, filed August 12, 2004, entitled PCRAM Device With Switching Glass

Layer, and is also a continuation-in-part of U.S. Patent Application Ser. No. 10/893,299,

filed July 19, 2004, entitled Resistance Variable Memory Device and Method of

Fabrication. The entirety of each of these applications is hereby incorporated by

reference herein.

FIELD OF THE INVENTION

[0002] The invention relates to the field of random access memory (RAM)

devices formed using a resistance variable material.

BACKGROUND

[0003] Resistance variable memory elements, which include Programmable

Conductive Random Access Memory (PCRAM) elements, have been investigated for

suitability as semi- volatile and non- volatile random access memory devices. In a

typical PCRAM device, the resistance of a chalcogenide glass backbone can be

programmed to stable lower conductivity (i.e., higher resistance) and higher

conductivity (i.e., lower resistance) states. An unprogrammed PCRAM device is

normally in a lower conductivity, higher resistance state. [0004] A conditioning operation forms a conducting channel of a metal-

chalcogenide in the PCRAM device, which supports a conductive pathway for altering

the conductivity/resistivity state of the device. The conducting channel remains in the

glass backbone even after the device is erased. After the conditioning operation, a write

operation will program the PCRAM device to a higher conductivity state, in which

metal ions accumulate along the conducting channel(s). The PCRAM device may be

read by applying a voltage of a lesser magnitude than required to program it; the

current or resistance across the memory device is sensed as higher or lower to define

the logic "one" and "zero" states. The PCRAM may be erased by applying a reverse'

voltage (opposite bias) relative to the write voltage, which disrupts the conductive

pathway, but typically leaves the conducting channel intact. In this way, such a device

can function as a variable resistance memory having at least two conductivity states,

which can define two respective logic states, i.e., at least a bit of data.

[0005] One exemplary PCRAM device uses a germanium selenide (i.e., GexSeioo-x)

chalcogenide glass as a backbone. The germanium selenide glass has, in the prior art,

incorporated silver (Ag) by (photo or thermal) doping or co-deposition. Other

exemplary PCRAM devices have done away with such doping or co-deposition by

incorporating a metal-chalcogenide material as a layer of silver selenide (e.g., Ag2Se),

silver sulfide (AgS), or tin selenide (SnSe) in combination with a metal layer, proximate a chalcogenide glass layer, which during conditioning of the PCRAM provides material

to form a conducting channel and a conductive pathway in the glass backbone.

[0006] Extensive research has been conducted to determine suitable materials

and stoichiometries thereof for the glass backbone in PCRAM devices. Germanium

selenide having a stoichiomety of about

(i.e_v Ge2Se3), as opposed to Ge23Se77 or

Ge3oSe7o, for example, has been found to function well for this purpose. A glass

backbone of with an accompanying metal-chalcogenide (e.g., typically silver

selenide) layer, enables a conducting channel to be formed in the glass backbone during

conditioning, which can thereafter be programmed to form a conductive pathway. The

metal-chalcogenide is incorporated into chalcogenide glass layer at the conditioning

step. Specifically, the conditioning step comprises applying a potential (about 0.20 V)

across the memory element structure of the device such that metal-chalcogenide

material is incorporated into the chalcogenide glass layer, thereby forming a

conducting channel within the chalcogenide glass layer. It is theorized that Ag2Se is

incorporated onto the glass backbone at Ge-Ge sites via new Ge-Se bonds, which allows

silver (Ag) migration into and out of the conducting channel during programming.

Movement of metal (e.g., typically silver) ions into or out of the conducting channel

during subsequent programming and erasing forms or dissolves a conductive pathway

along the conducting channel, which causes a detectible conductivity (or resistance)

change across the memory device. [0007] It has been determined that Ge4oSeδo works well as the glass backbone in a

PCRAM device because this stoichiometry makes for a glass that is rigid and

incorporates thermodynamically unstable germanium-germanium (Ge-Ge) bonds. The

presence of another species, such as silver selenide provided from an accompanying

layer, can, in the presence of an applied potential, break the Ge-Ge bonds and bond

with the previously homopolar bonded Ge to form conducting channels. These

characteristics make this "40/60" stoichiometry optimal when using a germanium

selenide chalcogenide glass with respect to the formation of a conducting channel and

conductive pathway.

[0008] While germanium-chalcogenide (e.g., GeωSeβo) glass layers are highly

desirable for PCRAM devices, other glasses may be desirable to improve switching

properties or thermal limitations of the devices.

SUMMARY

[0009] The invention provides embodiments of a method of determining suitable

glass backbone material, which may be used in place of GeωSeβo glass in a resistance

variable memory device, such as a PCRAM, with other materials, a method of forming

memory devices with such materials, and devices constructed in accordance with these

methods. [0010] The chalcogenide glass material may be represented by AxBioo-x, where A

is a non-chalcogenide material selected from Groups 3-15 of the periodic table and B is

a chalcogenide material from Group 16. The method of selecting a glass material

includes: (1) selection of a non-chalcogenide component A from Groups 3-15 that will

exhibit homopolar bonds; (2) selection of a chalcogenide component B from Group 16

for which component A will have a bonding affinity, relative to the A-A homopolar

bonds; (3) selection of a stoichiometry (i.e., x of AχBioo-x) that will allow the homopolar

A-A bonds to form; and (4) confirmation that the glass AχBioo-_X/ at the selected

stoichiometry (i.e., x), will allow a conducting channel and a conductive pathway to

form therein upon application of a conditioning voltage (when a metal-chalcogenide

layer and metal ions are proximate the glass).

[0011] An exemplary memory device constructed in accordance with an

embodiment of the invention uses a germanium telluride glass backbone having a

GexTeioo-χ stoichiometry and a metal-chalcogenide layer proximate thereto for a memory

cell. In a specific exemplary embodiment, x is between about 44 and about 53. Also,

the metal-chalcogenide layer can be a tin selenide with a stoichiometry of about SnSe,

Other layers may also be associated with this glass backbone and metal-chalcogenide

layer. [0012] The above and other features and advantages of the invention will be

better understood from the following detailed description, which is provided in

connection with the accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

[0013] FIGs. 1-3 show graphs of Raman shift analysis of germanium selenide

glass, which may be used in selecting glass backbone materials in accordance with the

invention;

[0014] FIG. 4 shows an exemplary embodiment of a memory device in

accordance with the invention;

[0015] FIG. 5 shows an exemplary embodiment of a memory device in

accordance with the invention;

[0016] FIGs. 6-11 show a cross-section of a wafer at various stages during the

fabrication of a device in accordance with an embodiment of the invention;

[0017] FIG. 12 shows a resistance-voltage curve of a first (conditioning) write and

second (programming) write for a 0.13 μm device in accordance with the invention;

[0018] FIG. 13 shows an exemplary processor-based system incorporating

memory devices in accordance with the invention; [0019] FIGs. 14a-14h are graphs showing experimental results of thermal testing

conducted with devices fabricated in accordance with exemplary embodiments of the

invention; and

[0020] FIG. 15 shows a graph of Raman shift analysis of germanium telluride

glass.

DETAILED DESCRIPTION

[0021] 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.

[0022] 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.

[0023] The term "chalcogenide" is intended to include various alloys,

compounds, and mixtures of chalcogens (elements from Group 16 of the periodic table,

e.g., sulfur (S), selenium (Se), tellurium (Te), polonium (Po), and oxygen (O)).

[0024] Embodiments of the invention provide a method of selecting a glass

backbone material for use in a resistance variable memory device, such as a PCRAM.

The backbone material (i.e., backbone glass layer 18 of FIGs. 4 and 5) may be

represented by the formula AxBioo-x, where A is a non-chalcogenide material selected

from Groups 3-15, and preferably 13, 14, and 15, of the periodic table and B is a

chalcogenide material. The glass backbone may also be represented by the formula

(AxBioo-x)Cy, where C represents one or more additional, optional components, which

may be present in some glass formulations, but which may be omitted; therefore, the

description hereafter will focus on two components (A and B) for simplicity sake. The

ultimate choice for the material AxBioo-x depends, in part, on the make up of an adjacent

metal-chalcogenide layer (e.g., layer 20 of FIGs. 4 and 5) with which it operationally

engages. The component A should have an affinity for the chalcogenide component B,

and preferably for the chalcogenide material (which is preferably also component B) of the metal-chalcogenide layer. Since Ge4oSeeo glass has been experimentally observed to

have good glass backbone properties in PCRAM devices, a backbone material

represented by the formula AxBioo-x should have properties similar to Ge4oSeeo glass,

such as homopolar bond properties and affinity of the non-chalcogenide component,

e.g., Ge, for the chalcogenide component in the metal-chalcogenide layer.

[0025] Taking these characteristics into consideration, a primary consideration in

selecting components A and B and the stoichiometry for a glass backbone material is

that the resulting material contain thermodynamically unstable homopolar bonds of

component A, meaning that the non-chalcogenide component A may form a bond with

another component A in the glass, as initially formed, only if there is an insufficient

amount of component B to satisfy the coordination number requirement for component

A, which allows for the formation homopolar A-A bonds. The A-A homopolar bonds

in such a glass material are thermodynamically unstable and will themselves break

when the device is programmed and a conducting channel is formed in the glass

backbone by the metal-chalcogenide layer when the chalcogenide component for the

metal-chalcogenide layer bonds to the component A participating in the homopolar

bonds. This property is dependent on the stoichiometry of the material AxBioo-x in that

excess chalcogenide component B will inhibit the formation of homopolar A-A bonds.

[0026] What "excess" chalcogenide component B means in relation to the

material AxBioo-x and its stoichiometry may be determined by whether the material exhibits the homopolar bonds or not. Raman spectroscopy can be a useful analytical

tool for determining the presence of homopolar bonds when selecting a material AxBioo-x

for the glass backbone in PCRAM devices. Raman Spectroscopy is based on the Raman

effect, which is the inelastic scattering of photons by molecules. A plot of Raman

intensity (counts) vs. Raman shift (cm'¹) is a Raman spectrum, which is the basis for

FIGs. 1-3.

[0027] Referring now to FIG. 1, which is a Raman spectrum for bulk Gei3Se77

glass, it can be observed by the Raman Shift peaks at about 200 cm."¹ and at about 260

cm"¹ that the glass incorporates Ge-Se bonds and Se-Se (i.e., chalcogenide) bonds. This

is an undesirable stoichiometry for the glass backbone since it lacks the homopolar Ge-

Ge (i.e., non-chalcogenide) bonds desirable for switching in a device comprising a

germanium selenide glass. Compare FIG. 1 to FIG. 2, the latter of which is a Raman

spectrum for bulk GeωSeβo glass, which shows a Raman Shift peak at about 175 cm"¹ that

corresponds to Ge-Ge homopolar bonds and a peak at about 200 cnr¹ that corresponds

to Ge-Se bonds. The prevalence of non-chalcogenide (i.e., Ge-Ge) homopolar bonds

found in Ge-ωSeβo is a characteristic sought in materials for the glass backbone in

PCRAM. This characteristic may also be seen in a thin film of GeωSeβo using Raman

spectra, as shown in FIG. 3. Similar comparisons of Raman spectra may be made for

other materials AχBioo-x of varying stoichiometrics to find spectra showing peaks demonstrating homopolar bonding of the non-chalcogenide component (i.e., A-A) in

the material, which indicates that it has suitable properties for a glass backbone.

[0028] Taking the aforementioned desired characteristics into consideration, the

method of detecting a suitable glass backbone may be performed by the following

steps: (1) selection of a non-chalcogenide component A from Groups 3-15 that will

bonds; (3) selection of a stoichiometry (i.e., x of AxBioo-x) that will provide for a

thermodynamically unstable glass and will allow the homopolar A-A bonds to form;

and (4) confirmation that the glass AxBioo-x, at the selected stoichiometry (i.e., x), will

allow a conducting channel and a conductive pathway to form therein upon application

of a conditioning voltage (when a metal-chalcogenide, e.g., M_yBioo-_y, and metal ions are

proximate the glass).

[0029] Using the above-discussed methodology for selecting glass backbone

materials AxBioo-x, at least four have been found preferable for use in PCRAM devices.

These materials include arsenic selenide, represented by formula AssoSeso, tin selenide,

represented by formula SnsoSeso, antimony selenide, represented by formula SbxSeioo-χ,

and germanium telluride, represented by formula GexTeioo-_*. As shown by FIG. 15, the

Raman shift peaks for germanium telluride glass are at about 140 counts/cm-¹ (for Ge-

Ge bonds) and about 180 counts/cm-i (for Te-Te bonds), demonstrating at least one favorable characteristic of the glass for PCRAM. Although each of these exemplary

materials include selenium or tellurium for component B, other chalcogenides may be

used as well.

[0030] The invention is now explained with reference to the other figures, which

illustrate exemplary embodiments and throughout which like reference numbers

indicate like features. FIG. 4 shows an exemplary embodiment of a memory device 100

constructed in accordance with the invention. The device 100 shown in FIG. 4 is

supported by a substrate 10. Over the substrate 10, though not necessarily directly so,

is a conductive address line 12, which serves as an interconnect for the device 100

shown and for a plurality of other similar devices of a portion of a memory array of

which the shown device 100 is a part. It is possible to incorporate an optional

insulating layer (not shown) between the substrate 10 and address line 12, and this may

be preferred if the substrate 10 is semiconductor-based. The conductive address line 12

can be any material known in the art as being useful for providing an interconnect line,

such as doped polysilicon, silver (Ag), gold (Au), copper (Cu), tungsten (W), nickel

(Ni), aluminum (Al), platinum (Pt), titanium (Ti), and other materials,

[0031] Over the address line 12 is a first electrode 16, which can be defined

within an insulating layer 14 (or may be a common blanket electrode layer; not shown),

which is also over the address line 12. This electrode 16 can be any conductive material

that will not migrate into chalcogenide glass, but is preferably tungsten (W). The insulating layer 14 should not allow the migration of metal ions and can be an

insulating nitride, such as silicon nitride (Si3N4), a low dielectric constant material, an

insulating glass, or an insulating polymer, but is not limited to such materials.

[0032] A memory element, i.e., the portion of the memory device 100 which

stores information, is formed over the first electrode 16. In the embodiment shown in

FIG. 4, a chalcogenide glass layer 18 is provided over the first electrode 16. The

chalcogenide glass layer 18 has the stoichiometric formula AxBioo-x, with A being a non-

chalcogenide component and B being a chalcogenide component as discussed above.

The material AxBioo-x may be many materials with the appropriate characteristics (e.g.,

homopolar bonds, homopolar bond strength, thermodynamic instability, etc.) and an

appropriate stoichiometry, as discussed above, but is preferably be selected from

SnsoSeso, SbxSeioo-x, AssoSeso, and GeχTeioo-x, which have been found to be suitable by

following the methodology discussed above. Germanium telluride with a formula

GexTeioo-x, where x is between about 44 and 53, is the preferred material for layer 18.

More preferably, x is between 46 and 51 and most preferably, x is about 47. This may

be written as Ge«Tes4 to GesiTe49, Germanium telluride is a particularly good selection

for the chalcogenide glass layer 18 because, as shown by the Raman data in FIG. 15, it

exhibits the Ge-Ge homopolar bonds desirable for such glass. Also, when used with a

metal-chalcogenide layer 20 of tin selenide (SnSe), the tin selenide allows for the formation of a Ge-Se bond and the development of a channel for metal (e.g., Ag) ion

migration during operation of the memory device.

[0033] The layer of chalcogenide glass 18 is preferably between about 100 A and

about 1000 A thick, most preferably about 300 A thick. Layer 18 need not be a single

layer of glass, but may also be comprised of multiple sub-layers of chalcogenide glass

having the same or different stoichiometries. This layer of chalcogenide glass 18 is in

electrical contact with the underlying electrode 16.

[0034] Over the chalcogenide glass layer 18 is a layer of metal-chalcogenide 20,

which may be any combination of metal component M, which may be selected from

any metals, and chalcogenide component B, which is preferably the same chalcogenide

as in the glass backbone layer 18, and may be represented by the formula M_yBioo-_y. As

with the glass backbone layer 18 material, other components may be added, but the

metal-chalcogenide will be discussed as only two components M and B for simplicity

sake. The metal-chalcogenide may be, for example, silver selenide (AgySe, y being

about 20) or, preferably, tin selenide (Smo+/-_ySe, where y is between about 10 and 0). The

metal-chalcogenide layer 20 is preferably about 500 A thick; however, its thickness

depends, in part, on the thickness of the underlying chalcogenide glass layer 18. The

ratio of the thickness of the metal-chalcogenide layer 20 to that of the underlying

chalcogenide glass layer 18 should be between about 5:1 and about 1:1, more preferably

about 2.5:1. [0035] Still referring to FIG. 4, a metal layer 22 is provided over the metal-

chalcogenide layer 20, with the metal of layer 22 preferably incorporating some silver, if

not being exclusively silver. The metal layer 22 should be about 500 A thick. The metal

layer 22 assists the switching operation of the memory device 100. Over the metal layer

22 is a second electrode 24. The second electrode 24 can be made of the same material

as the first electrode 16, but is not required to be so. In the exemplary embodiment

shown in FIG. 4, the second electrode 24 is preferably tungsten (W). The device(s) 100

may be isolated by an insulating layer 26.

[0036] FIG. 5 shows another exemplary embodiment of a memory device 101

constructed in accordance with the invention. Memory device 101 has many

similarities to memory device 100 of FIG. 4 and layers designated with like reference

numbers are preferably the same materials and have the same thicknesses as those

described in relation to the embodiment shown in FIG. 4. For example, the first

electrode 16 is preferably tungsten. The chalcogenide glass layer 18 material AxBioo-x is

selected according to the methodology detailed above and can be germanium telluride;

it is preferably about 150 A thick. As with device 100 of FIG- 4, the metal-chalcogenide

layer 20 may be any combination M_yBioo-_y, but can be tin selenide; it is preferably about

470 A thick. The metal layer 22 preferably contains some silver, but can be mostly or

wholly silver; it is preferably about 200 A thick. The primary difference between device 100 and device 101 is the addition to device 101 of additional second and third

chalcogenide layers 18a and 18b.

[0037] The second chalcogenide glass layer 18a is formed over the metal-

chalcogenide layer 20 and is preferably about 150 A thick. Over this second

chalcogenide glass layer 18a is metal layer 22. Over the metal layer 22 is a third

chalcogenide glass layer 18b, which is preferably about 100 A thick. The third

chalcogenide glass layer 18b provides an adhesion layer for subsequent electrode

formation. As with layer 18 of FIG. 4, layers 18a and 18b are not necessarily a single

layer, but may be comprised of multiple sub-layers. Additionally, the second and third

chalcogenide layers 18a and 18b may be a different glass material from the first

chalcogenide glass layer 18 or from each other. Glass material preferred for layers 18a

and 18b is germanium selenide (Ge_xSeioo-x), more preferably Ge2Se3, but other materials

may be useful as well, including germanium telluride (GexTeioo-x), arsenic selenide

(AsxSeioo-x), tin selenide (SnxSeioo-x), antimony selenide (SbxSeioo-x), , germanium sulfide

(GexSioo-χ), and combinations of germanium (Ge), silver (Ag), and selenium (Se). The

second electrode 24 is preferably tungsten (W), but may be other metals also.

[0038] As shown by FIGs. 14a-14h, PCRAM devices in accordance with the

above-discussed embodiment (FIG. 5) were experimentally tested for memory

operation under varied thermal conditions. Each chart (FIGs. 14a-14h) represents a set

of thermal tests on a respective PCRAM device. Similar to the device shown in FIG. 5, each tested device had a tungsten (W) first electrode (e.g., layer 16), a 3OθA germanium

telluride (GexTeioo-x, x s 44 to 53) layer (e.g., layer 18) thereover, a 900A tin selenide

(SnSe) layer (e.g., layer 20) thereover, a 150A germanium selenide (Ge2Se3) layer (e.g.,

layer 18a) thereover, a 500A silver (Ag) layer (e.g., layer 22) thereover, a IOOA

germanium selenide (Ge2Se3) layer (e.g., layer 18b) thereover, and a tungsten (W)

second electrode (e.g., layer 24).

[0039] The testing was conducted by positioning wafers, which supported the

PCRAM devices, on a temperature-controllable chuck and DC programming ten (10)

devices at the temperatures shown on the charts in FIGs. 14a-14h. The DC probing

procedure was conducted on each device as follows: (1) sweep potential from 0 to 800

mV and read resistance of cell at 10 mV to determine the initial resistance (Ri); (2)

sweep the potential from 0 to 10 mV and record the resistance at 10 mV to obtain the

write resistance (RwI); (3) sweep the device from 0 to -1 V and record the potential at

which the device erased and the current at that erase potential to determine the erase

voltage and erase current; (4) sweep the device from 0 to 800 mV and read the

resistance at 10 mV (this is the Rerase) and record the potential at which the device

switched (i.e., was written; the Vw2); and (5) sweep the potential from 0 to 10 mV and

record the resistance at 10 mV (this is the Rw2). The charts of FIGs. 14a-14h show these

measured parameters (i.e., VwI, Vw2, Ri, erase current, erase voltage, RwI, Rw2,

Rerase) for 10 experimental devices constructed in accordance with the invention at the temperatures shown along the x-axis of the charts. The results show that germanium

tellurium based PCRAM cells have thermal tolerances suitable for use in memory

devices.

[0040] The above-discussed embodiments are exemplary embodiments of the

invention; however, other exemplary embodiments may be used which combine the

first electrode layer 16 and address line layer 12. Another exemplary embodiment may

use blanket layers (e.g., layers 16, 18, 20, and 22 of FIG. 4) of the memory cell body,

where the memory cell is defined locally by the position of the second electrode 24 over

the substrate 10. Another exemplary embodiment may form the memory device within

a via. Additional layers, such as barrier layers or alloy-controlling layers, not

specifically disclosed in the embodiments shown and discussed above, may be added

to the devices in accordance with the invention without departing from the scope

thereof.

[0041] FIGs. 6-11 illustrate a cross-sectional view of a wafer during the

fabrication of a memory device 100 as shown by FIG. 1. Although the processing steps

shown in FIGs. 6-11 most specifically refer to memory device 100 of FIG. 1, the methods

and techniques discussed may also be used to fabricate memory devices of other

embodiments (e.g., device 101 of FIG. 5) as would be understood by a person of

ordinary skill in the art. [0042] As shown by FIG. 6, a substrate 10 is provided. As indicated above, the

substrate 10 can be semiconductor-based or another material useful as a supporting

structure as is known in the art. If desired, an optional insulating layer (not shown)

may be formed over the substrate 10; the optional insulating layer may be silicon

nitride or other insulating materials used in the art. Over the substrate 10 (or optional

insulating layer, if desired), a conductive address line 12 is formed by depositing a

conductive material, such as doped polysilicon, aluminum, platinum, silver, gold,

nickel, but preferably tungsten, patterning one or more conductive lines, for instance

with photolithographic techniques, and etching to define the address line 12. The

conductive material may be deposited by any technique known in the art, such as

sputtering, chemical vapor deposition, plasma enhanced chemical vapor deposition,

evaporation, or plating.

[0043] Still referring to FIG. 6, over the address line 12 is formed an insulating

layer 14. This layer 14 can be silicon nitride, a low dielectric constant material, or many

other insulators known in the art that do not allow metal (e.g., silver, copper, or other

metal) ion migration, and may be deposited by any method known in the art, An

opening 14a in the insulating layer is made, for example, by photolithographic and

etching techniques, thereby exposing a portion of the underlying address line 12. Over

the insulating layer 14, within the opening 14a, and over the address line 12 is formed a

conductive material, preferably tungsten (W). A chemical mechanical polishing step may then be utilized to remove the conductive material from over the insulating layer

14, to leave it as a first electrode 16 over the address line 12, and planarize the wafer.

[0044] FIG. 7 shows the cross-section of the wafer of FIG. 6 at a subsequent stage

of processing. A series of layers making up the memory device 100 (FIG. 4) are blanket-

deposited over the wafer. A chalcogenide glass layer 18 is formed to a preferred

thickness of about 300A over the first electrode 16 and insulating layer 14. The

chalcogenide glass layer 18 is GexTeioo-x, where x is between about 44 to 53, but may also

be selected from other materials such as AssoSeso, SnsoSeso, and SbxSeioo-x, and as

described above, may be selected from many materials AxBioo-x with appropriate

characteristics and of an appropriate stoichiometry for memory function.

[0045] The steps in selecting a chalcogenide glass layer 18 material are: (1)

selection of a non-chalcogenide component A from Groups 3-15 that will exhibit

homopolar bonds; (2) selection of a chalcogenide component B from Group 16 for

which component A will have a bonding affinity, relative to the A-A homopolar bonds;

(3) selection of a stoichiometry (i.e., x of AχBioo-x) that will provide for

thermodynamically unstable homopolar A-A bonds; and (4) confirmation that the glass

AxBioo-x, at the selected stoichiometry (i.e., x), will allow a conducting channel and a

conductive pathway to form therein upon application of a conditioning voltage when a

metal-chalcogenide layer 20 is proximate the glass layer 18. Once the materials are

selected, deposition of the chalcogenide glass layer 18 may be accomplished by any suitable method, such as evaporative techniques or chemical vapor deposition;

however, the preferred technique utilizes either sputtering or co-sputtering.

[0046] Still referring to FIG. 7, a metal-chalcogenide layer 20, e.g., M_yBioo-_y, is

formed over the chalcogenide glass layer 18. The metal-chalcogenide layer 20 is

preferably tin selenide (SnSe), particularly when germanium telluride is used as the

chalcogenide glass layer 18. Physical vapor deposition, chemical vapor deposition, co-

evaporation, sputtering, or other techniques known in the art may be used to deposit

layer 20 to a preferred thickness of about 500 A. Again, the thickness of layer 20 is

selected based, in part, on the thickness of layer 18 and the ratio of the thickness of the

metal-chalcogenide layer 20 to that of the underlying chalcogenide glass layer 18 is

preferably from about 5:1 to about 1:1, more preferably about 2.5:1. It should be noted

that, as the processing steps outlined in relation to FIGs. 6-11 may be adapted for the

formation of other devices in accordance the invention, e.g., the layers may remain in

blanket-deposited form, a barrier or alloy-control layer may be formed adjacent to the

metal-chalcogenide layer 20, on either side thereof, or the layers may be formed within

a via.

[0047] Still referring to FIG. 7, a metal layer 22 is formed over the metal-

chalcogenide layer 20. The metal layer 22 preferably incorporates at least some silver

(Ag), if not exclusively being silver (Ag), but may be other metals as well, such as copper (Cu) or a transition metal, and is formed to a preferred thickness of about 300 A.

The metal layer 22 may be deposited by any technique known in the art.

[0048] Still referring to FIG. 7, over the metal layer 22, a conductive material is

deposited for a second electrode 24. Again, this conductive material may be any

material suitable for a conductive electrode, but is preferably tungsten; however, other

materials may be used such as titanium nitride or tantalum, for example.

[0049] Now referring to FIG. 8, a layer of photoresist 28 is deposited over the top

electrode 24 layer, masked and patterned to define the stacks for the memory device

100, which is but one of a plurality of like memory devices of a memory array. An

etching step is used to remove portions of layers 18, 20, 22, and 24, with the insulating

layer 14 used as an etch stop, leaving stacks as shown in FIG. 9. The photoresist 30 is

removed, leaving a substantially complete memory device 100, as shown by FIG. 9. An

insulating layer 26 may be formed over the device 100 to achieve a structure as shown

by FIGs. 4, 10, and 11. This isolation step can be followed by the forming of connections

(not shown) to other circuitry of the integrated circuit (e.g., logic circuitry, sense

amplifiers, etc.) of which the memory device 100 is a part, as is known in the art,

[0050] As shown in FIG. 10, a conditioning step is performed by applying a

voltage pulse of about 0.20 V to incorporate material from the metal-chalcogenide layer

20 into the chalcogenide glass layer 18 to form a conducting channel 30 in the

chalcogenide glass layer 18. The conducting channel 30 will support a conductive pathway 32, as shown in FIG. 11, upon application of a programming pulse of about

0.17 V during operation of the memory device 100.

[0051] The embodiments described above refer to the formation of only a few

possible resistance variable memory device structures (e.g., PCRAM) in accordance

with the invention, which may be part of a memory array. It must be understood,

however, that the invention contemplates the formation of other memory structures

within the spirit of the invention, which can be fabricated as a memory array and

operated with memory element access circuits.

[0052] FIG. 12 shows a resistance-voltage curve of a first write, which

corresponds to a conditioning voltage, and a second write, which corresponds to a

programming voltage, for a 0.13 μm device such as device 100 or 101 shown in FIGs. 4

and 5, respectively. The device represented by the curve of FIG. 12 has an AssoSeso

chalcogenide glass layer 18 (See FIGs. 4 and 5). FIG. 12 shows that the first write is at a

slightly higher potential than the second write (i.e., about 0.2 V. compared to about 0.17

V, respectively). This is because the first write conditions the device in accordance with

the processing shown at FIG. 10 by forming a conducting channel 30, which remains

intact after this first write. The second write requires less voltage because the stable

conducting channel 30 is already formed by the conditioning write and the conductive

pathway 32 is formed more easily. Application of these write voltages programs the

device to a non-volatile higher conductivity, lower resistivity, memory state. These observed programming parameters utilizing a chalcogenide glass layer 18 selected in

accordance with the invention show that the tested devices work as well as when

Ge4oSeβo is used as the glass backbone.

[0053] The erase potential for a device having an AssoSeso chalcogenide glass

layer (e.g., layer 18) is also similar to a device having GeωSeβo glass. This erase voltage

curve is not shown in FIG. 12; however, the erase potential is about -0.06 V, which

returns the device to a non-volatile higher resistance, lower conductivity memory state.

[0054] FIG. 13 illustrates a typical processor system 400 which includes a

memory circuit 448, e.g., a PCRAM device, which employs resistance variable memory

devices (e.g., devices 100 and 101) fabricated in accordance with an embodiment the

invention. A processor system, such as 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.

[0055] In the case of a computer system, the processor system 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 one or more resistance variable memory devices, e.g., device 100. If desired, the memory circuit 448 may be

combined with the processor, for example CPU 444, in a single integrated circuit.

[0056] The above description and drawings should only be considered

illustrative of exemplary embodiments that achieve the features and advantages of the

invention. Modification and substitutions to specific process conditions and structures

can be made without departing from the spirit and scope of the 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.

[0057] What is claimed as new and desired to be protected by Letters Patent of

the United States is:

Claims

1. A method of forming a memory device, comprising:

providing a first electrode and a second electrode;

providing a metal-chalcogenide layer between said first and second electrodes

providing a germanium telluride glass between said first and second electrodes and in contact with said metal-chalcogenide layer.

2. The method of claim 1, wherein said metal-chalcogenide layer is between said chalcogenide glass layer and said second electrode.

3. The method of claim 1, wherein said metal-chalcogenide layer comprises silver selenide.

4. The method of claim 1, wherein said metal-chalcogenide layer comprises tin selenide.

5. The method of claim 4, further comprising providing a metal layer between said metal-chalcogenide layer and said second electrode.

6. The method of claim 5, wherein said metal layer comprises silver.

7. The method of claim 5, further comprising providing a first chalcogenide glass layer between said metal-chalcogenide layer and said metal layer.

8. The method of claim 9, further comprising providing a second chalcogenide glass layer between said metal layer and said second electrode.

9. The method of claim 1, wherein said germanium telluride layer has a stoichiometric formula GexTeioo-x, where x is about 44 to about 53.

10. The method of claim 1, wherein said germanium telluride layer has a stoichiometric formula GexTeioo-x, where x is about 46 to about 51.

11. The method of claim I₁ wherein said germanium telluride layer has a stoichiometric formula GexTeioo-x, where x is about 47.

12. A method of forming a memory cell, comprising:

forming a first electrode over a substrate;

forming a memory body over said first electrode, said memory body comprising a germanium telluride glass layer and a tin selenide layer;

forming a chalcogenide glass layer over said memory body;

forming a silver-containing layer over said chalcogenide glass layer;

forming an adhesion layer over said silver-containing layer; and

forming a second electrode over said adhesion layer.

13. The method of claim 12, wherein said germanium telluride glass layer has a stoichiometric formula GexTeioo-x, where x is about 44 to about

53.

14. The method of claim 12, wherein said germanium telluride glass layer has a stoichiometric formula GexTeioo-χ, where x is about 46 to about 51.

15. The method of claim 12, wherein said germanium telluride glass layer has a stoichiometric formula GexTeioo-χ, where x is about 47.

16. The method of claim 12, wherein said germanium telluride glass layer is in contact with said first electrode.

17. The method of claim 12, wherein the various layers of said memory cell are arranged vertically.

18. The method of claim 12, wherein said chalcogenide glass layer is

19. The method of claim 12, wherein said adhesion layer is Ge2Se3.

20. A method of forming a memory cell, comprising

providing a substrate;

forming a first tungsten electrode over said substrate;

forming a germanium telluride layer over said first tungsten electrode, wherein said germanium telluride layer has a stoichiometric formula GexTeioo-x, x being between about 44 and about 53; forming a tin selenide layer over said germanium telluride layer;

forming a first germanium selenide layer over said tin selenide layer;

forming a silver-containing layer over said first germanium selenide layer;

forming a second germanium selenide layer over said silver- containing layer;

forming a second tungsten electrode over said second germanium selenide layer; and

forming a conducting channel in said germanium telluride layer.

21. A memory device, comprising:

a first electrode;

a second electrode;

a memory element between said first and said second electrodes, said memory element comprising a germanium telluride layer and a metal-chalcogenide layer between said germanium telluride layer and said second electrode.

22. The memory device of claim 21, wherein said germanium telluride layer has a stoichiometric formula GeχTeioo-x, where x is about 44 to about 53.

23. The memory device of claim 21, wherein said germanium telluride layer has a stoichiometric formula GexTeioo-x, where x is about 46 to about 51.

24. The memory device of claim 21, wherein said germanium telluride layer has a stoichiometric formula GexTeioo-χ, where x is about 47.

25. The memory device of claim 21, further comprising a conducting channel within said germanium telluride layer, said conducting channel comprising said metal-chalcogenide material.

26. The memory device of claim 25, further comprising a conductive pathway associated with said conducting channel, said conductive pathway being provided when said memory device is programmed to a first memory state.

27. The memory device of claim 21, wherein said metal-chalcogenide material comprises tin selenide.

28. The memory device of claim 21, further comprising a metal layer between said metal-chalcogenide layer and said second electrode.

29. The memory device of claim 28, further comprising a chalcogenide glass layer between said metal-chalcogenide layer and said metal layer.

30. The memory device of claim 29, further comprising a second chalcogenide glass layer between said metal layer and said second electrode.

31. The memory device of claim 21, wherein said memory device is part of a processor system.

32. A memory cell, comprising:

a first electrode over a substrate;

a germanium telluride layer over said first electrode;

a tin selenide layer over said germanium telluride layer;

a first germanium selenide layer over said tin selenide layer;

a silver-containing layer over said first germanium selenide layer;

a second germanium selenide layer over said silver-containing layer; and

a second electrode over said second germanium selenide layer.

33. The memory cell of claim 32, wherein said germanium telluride layer has a stoichiometric formula GexTeioo-x, where x is about 44 to about 53.

34. The memory cell of claim 32, wherein said germanium telluride layer has a stoichiometric formula GeχTeioo-x, where x is about 46 to about 51.

35. The memory cell of claim 32, wherein said germanium telluride layer has a stoichiometric formula GexTeioo-x, where x is about 47.

36. The memory cell of claim 32, further comprising a conducting channel within said germanium telluride layer, said conducting channel comprising said metal-chalcogenide material.

37. The memory cell of claim 36, further comprising a conductive pathway associated with said conducting channel, said conductive pathway being provided when said memory device is programmed to a first memory state.

38. The memory cell of claim 32, wherein said first and second germanium selenide layers comprise Ge2Se3.

39. The memory cell of claim 32, wherein said layers making up said cell are stacked vertically.

40. The memory cell of claim 32, wherein said layers making up said cell are in contact with one another.

41. A PCRAM memory cell, comprising:

a substrate;

a first tungsten electrode over said substrate;

a germanium telluride layer over said first tungsten electrode, wherein said germanium telluride layer has a stoichiometric formula GexTeioo-x, x being between about 44 and about 53;

a tin selenide layer over said germanium telluride layer;

a first germanium selenide layer over said tin selenide layer; a silver-containing layer over said first germanium selenide layer;

a second germanium selenide layer over said silver-containing layer;

a second tungsten electrode over said second germanium selenide layer; and

a conducting channel in said germanium telluride layer.

42. A processor system, comprising:

a processor and a memory device; wherein said memory device comprises:

a first electrode;

a second electrode;

43. The processor system of claim 42, wherein said germanium telluride layer has a stoichiometric formula GexTeioo-x, where x is about 44 to about 53.

44. The processor system of claim 42, wherein said germanium telluride layer of said memory device has a stoichiometric formula Ge_xTeioo-x, where x is about 46 to about 51.

45. The processor system of claim 42, wherein said germanium telluride layer of said memory device has a stoichiometric formula GexTeioo-x, where x is about 47.

46. The processor system of claim 42, further comprising a conducting channel within said germanium telluride layer of said memory device, said conducting channel comprising said metal- chalcogenide material.

47. The processor system of claim 42, further comprising a conductive pathway associated with said conducting channel, said conductive pathway being provided when said memory device is programmed to a first memory state.

48. The processor system of claim 42, wherein said metal-chalcogenide material of said memory device comprises tin selenide.

49. The processor system of claim 42, further comprising a metal layer between said metal-chalcogenide layer and said second electrode of said memory device.

50. The processor system of claim 49, further comprising a chalcogenide glass layer between said metal-chalcogenide layer and said metal layer of said memory device.

51. The processor system of claim 50, further comprising a second chalcogenide glass layer between said metal layer and said second electrode of said memory device.