U.S. patent application number 11/149225 was filed with the patent office on 2005-12-29 for resistance variable memory elements based on polarized silver-selenide network growth.
Invention is credited to Campbell, Kristy A..
Application Number | 20050286294 11/149225 |
Document ID | / |
Family ID | 34795469 |
Filed Date | 2005-12-29 |
United States Patent
Application |
20050286294 |
Kind Code |
A1 |
Campbell, Kristy A. |
December 29, 2005 |
Resistance variable memory elements based on polarized
silver-selenide network growth
Abstract
The invention relates to a resistance variable memory element
including polarizable metal-chalcogen regions within a doped
chalcogenide glass. A method for physically aligning the
polarizable metal-chalcogen regions to form a conducting channel is
provided. The invention also relates to a resistance variable
memory element including metal-chalcogen regions within a
chalcogenide glass backbone. The metal-chalcogen regions and glass
regions bond to form a conducting channel. In addition, a method of
operating such memory elements is provided in which metal ions move
in and out of the conducting channels in response to applied
voltages, thereby affecting the resistance of the memory
elements.
Inventors: |
Campbell, Kristy A.; (Boise,
ID) |
Correspondence
Address: |
DICKSTEIN SHAPIRO MORIN & OSHINSKY LLP
2101 L Street, NW
Washington
DC
20037
US
|
Family ID: |
34795469 |
Appl. No.: |
11/149225 |
Filed: |
June 10, 2005 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
|
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11149225 |
Jun 10, 2005 |
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10765393 |
Jan 28, 2004 |
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Current U.S.
Class: |
365/163 ;
257/E45.002 |
Current CPC
Class: |
G11C 2013/009 20130101;
G11C 2213/11 20130101; G11C 13/0069 20130101; H01L 45/1658
20130101; G11C 13/02 20130101; H01L 45/1616 20130101; H01L 45/1233
20130101; H01L 45/1266 20130101; H01L 45/085 20130101; H01L 45/1608
20130101; H01L 45/1625 20130101; G11C 2213/51 20130101; H01L 45/143
20130101 |
Class at
Publication: |
365/163 |
International
Class: |
G11C 011/00 |
Claims
1. A memory element comprising: at least one resistance variable
material layer; at least one metal-containing layer; and at least
one conducting channel formed within said resistance variable
material layer, said conducting channel configured to receive and
expel metal ions in response to write, erase, and read voltages
applied to said memory element.
2. The memory element of claim 1, wherein said resistance variable
material layer is a chalcogenide glass layer.
3. The memory element of claim 2, wherein said chalcogenide glass
layer has a stoichiometry of Ge.sub.xSe.sub.100-x.
4. The memory element of claim 3, wherein said chalcogenide glass
layer has a stoichiometry from about Ge.sub.18Se.sub.82 to
Ge.sub.25Se.sub.75.
5. The memory element of claim 4, wherein said chalcogenide glass
layer is a doped chalcogenide glass layer.
6. The memory element of claim 5, wherein said doped chalcogenide
glass layer is doped with metal ions.
7. The memory element of claim 6, wherein said metal ions are
silver ions.
8. The memory element of claim 7, wherein said doped chalcogenide
glass layer is from about 150 .ANG. to about 600 .ANG. thick.
9. The memory element of claim 8, wherein said doped chalcogenide
glass layer has metal-chalcogen regions which are aligned to form
said conducting channel.
10. The memory element of claim 9, wherein said metal-chalcogen
regions are Ag.sub.2Se regions within a GeSe glass backbone.
11. The memory element of claim 10, wherein said Ag.sub.2Se regions
become aligned upon application of a conditioning voltage to the
memory element.
12. The memory element of claim 11, wherein said conditioning
voltage is greater than subsequent write, read, and erase
voltages.
13. The memory element of claim 12, wherein the Ag.sub.2Se regions
form at least one conducting channel within the doped chalcogenide
glass layer.
14. The memory element of claim 11, wherein prior to application of
said conditioning voltage, said memory element has a first
resistance state and after application of said conditioning voltage
to said memory element, said memory element has a second resistance
state lower than said first resistance state.
15. The memory element of claim 14, wherein said write, erase, and
read voltages have an absolute magnitude lower than that of said
conditioning voltage.
16. The memory element of claim 15, wherein said write voltage
produces a third resistance state lower than the second resistance
state.
17. The memory element of claim 16, wherein a second write voltage
produces a fourth resistance state lower than said third resistance
state.
18. The memory element of claim 3, wherein said chalcogenide glass
layer has a stoichiometry from about Ge.sub.20Se.sub.80 to
Ge.sub.43Se.sub.57.
19. The memory element of claim 18, wherein said chalcogenide glass
layer has a stoichiometry of Ge.sub.40Se.sub.60.
20. The memory element of claim 18, wherein said chalcogenide glass
layer is from about 150 .ANG. to about 500 .ANG. thick.
21. The memory element of claim 20, wherein the at least one
metal-containing layer is formed over said chalcogenide glass
layer.
22. The memory element of claim 21, wherein said at least one
metal-containing layer is from about 300 .ANG. to about 1200 .ANG.
thick.
23. The memory element of claim 1, wherein said at least one
metal-containing layer is an Ag.sub.2Se layer.
24. The memory element of claim 1, wherein said resistance variable
material has a germanium-selenide glass backbone.
25. The memory element of claim 24, wherein when a conditioning
pulse is applied to the memory element, Ag.sub.2Se is driven into
said germanium-selenide glass backbone.
26. The memory element of claim 25, wherein said conditioning pulse
has a pulse duration of from about 10 to about 500 ns and greater
than about 700 mV.
27. The memory element of claim 26, wherein the Ag.sub.2Se is
bonded to the germanium-selenide glass backbone forming at least
one conducting channel within the chalcogenide glass layer.
28. The memory element of claim 27, further comprising a second
metal-containing layer formed over the first metal-containing
layer.
29. The memory element of claim 28, wherein said second
metal-containing layer comprises silver ions.
30. The memory element of claim 29, wherein the silver ions are
driven in and out of the at least one conducting channel by
applying different voltages.
31. A memory element comprising: at least one doped chalcogenide
glass layer, said doped chalcogenide glass layer comprising
polarized metal-chalcogen regions within a glass backbone, wherein
said polarized metal-chalcogen regions form at least one conducting
channel for receiving and expelling metal ions within said doped
chalcogenide glass layer in response to write, erase, and read
voltages applied to said memory element, wherein a conditioning
voltage is applied that changes said memory element from a first
resistance state to a second resistance state, said second
resistance state being lower than said first resistance state; and
first and second electrodes electrically coupled to said doped
chalcogenide glass layer.
32-39. (canceled)
40. A memory element comprising: at least one chalcogenide glass
layer, said chalcogenide glass layer further comprising bonded
regions of metal and glass, wherein said bonded regions of metal
and glass form at least one conducting channel within said
chalcogenide glass layer; at least one metal-containing layer
formed over said chalcogenide glass layer; and first and second
electrodes electrically coupled to said chalcogenide glass
layer.
41-119. (canceled)
120. A method of operating a memory element comprising a conducting
at least one conducting channel formed within a chalcogenide glass
material, said method comprising: applying a conditioning voltage
to physically align metal-chalcogen regions which form said at
least one conducting channel within a chalcogenide glass material,
said first voltage moving the memory element from a first to a
second resistance state, said first resistance state exhibiting a
higher resistance than said second resistance state; and applying a
first write voltage to move metal ions into said conducting channel
and placing the memory element in a third resistance state, said
third resistance state being lower than said second resistance
state.
121. The method of claim 120, further comprising applying a second
write voltage to move the memory element into a fourth resistance
state, said fourth resistance state being equal to or lower than
said third resistance state.
122. The method of claim 121, further comprising applying an erase
voltage to move the memory element into a fifth resistance state,
said fifth resistance state being higher than said second and third
resistance state.
123-126. (canceled)
127. The method of claim 120, wherein the write voltage is less
than the conditioning voltage in absolute amplitude.
128. The method of claim 121, wherein the second write voltage is
less than or equal to the first write voltage in absolute
amplitude.
129. The method of claim 122, wherein the erase voltage is applied
with inverse polarity compared to the write voltage.
130. A method of operating a memory element comprising a
chalcogenide glass material with at least one conducting channel
formed from bonded metal and glass regions, said method comprising:
applying a conditioning voltage to condition the memory element,
said first voltage moving the memory element from a first
resistance state into a second resistance state, wherein said
second resistance state is less than said first resistance state;
and applying a write voltage to move the memory element into a
third resistance state, said third resistance state being less than
said second resistance state.
131-143. (canceled)
Description
FIELD OF THE INVENTION
[0001] The invention relates to the field of random access memory
(RAM) devices formed using a chalcogenide-based resistance variable
memory element.
BACKGROUND OF THE INVENTION
[0002] A well-known semiconductor memory component is a random
access memory (RAM). RAM permits repeated read and write operations
on memory elements. Typically, RAM memory elements are volatile, in
that stored data is lost once the power source is disconnected or
removed. Non-limiting examples of RAM devices which contain such
memory elements include dynamic random access memory (DRAM),
synchronized dynamic random access memory (SDRAM) and static random
access memory (SRAM). DRAM's and SDRAM's typically store data in
capacitors which require periodic refreshing to maintain the stored
data.
[0003] Recently, resistance variable memory elements have been
investigated for suitability as semi-volatile and non-volatile
random access memory elements. A class of such devices include an
insulating material formed of a chalcogenide glass disposed between
two electrodes. A conductive material is incorporated into the
material. The resistance of the material can be changed between
high and low resistance states by application of suitable voltages
across the memory element. D. D. Thornburg has discussed
polarization of arsenic triselenide in an electric field. For
instance, the polarization of arsenic triselenide allows the memory
device to switch between different memory states. See Thornburg, D.
D., Memory Switching in Amorphous Arsenic Triselenide, J.
NON-CRYST. SOLIDS 11 (1972), at 113-120; Thornburg, D. D. and
White, R. M., Electric Field Enhanced Phase Separation and Memory
Switching in Amorphous Arsenic Triselenide, J. APPL. PHYS. (1972),
at 4609-4612.
[0004] When set in a particular resistance state, the particular
resistance state of the memory element will remain intact for
minutes, hours, or longer even after the voltage potentials are
removed. Such a device can function, for example, as a semi or
non-volatile resistance variable memory element having two
resistance states, which in turn can define two logic states.
BRIEF SUMMARY OF THE INVENTION
[0005] In one aspect, exemplary embodiments of the invention
provide a resistance variable memory element and a method of
forming the same in which a doped chalcogenide glass contains
regions of polarizable metal-chalcogen material forming a
conducting channel present within a chalcogenide glass backbone.
The conducting channel can receive and expel metal ions in and out
of it to set a particular resistance state for the memory element
in response to write and erase voltages.
[0006] In another aspect, exemplary embodiments of the invention
provide a resistance variable memory element and a method of
forming the same in which the resistance variable memory element
comprises at least one chalcogenide glass layer and at least one
metal-containing layer formed between two electrodes. The
chalcogenide glass layer further comprises a conducting channel
formed from at least partially bonded regions of metal-chalcogen
and glass. The conducting channel can receive and expel metal ions
in and out of it to set a particular resistance state for the
memory element in response to write and erase voltages.
[0007] In another aspect, embodiments of the invention provide a
method for changing the resistance state of a resistance variable
memory element. A conditioning voltage is applied to produce a
conducting channel within a glass network. The conducting channel
can receive and expel metal ions to set a particular resistance
state for the memory element through subsequent programming
voltages, such as write and erase voltages.
[0008] These and other features and advantages of exemplary
embodiments 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
[0009] FIGS. 1A-1F illustrates a cross-sectional view of a
resistance variable memory device fabricated in accordance with a
first embodiment of the invention.
[0010] FIGS. 2A-2F illustrates a cross-sectional view of a
resistance variable memory device fabricated in accordance with a
second embodiment of the invention.
[0011] FIGS. 3A-3F illustrates a cross-sectional view of a
resistance variable memory device fabricated in accordance with a
third embodiment of the invention.
[0012] FIGS. 4A-4F illustrates a cross-sectional view of a
resistance variable memory device fabricated in accordance with a
fourth embodiment of the invention.
[0013] FIG. 5 illustrates a processor-based system having one or
more memory devices that contains resistance variable memory
elements according to the various embodiments of the invention.
DETAILED DESCRIPTION OF THE INVENTION
[0014] 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.
[0015] The term "substrate" used in the following description may
include any supporting structure including but not limited to a
glass, plastic, or 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 which may not be silicon-based. When
reference is made to a semiconductor substrate in the following
description, previous process steps may have been utilized to form
regions or junctions in and/or over the base semiconductor or
foundation.
[0016] Applicant has discovered that metal-chalcogen regions, such
as silver-selenide, formed in a chalcogenide glass layer, for
example, germanium-selenide, can be polarized upon application of a
conditioning voltage and align to form a conducting channel within
the chalcogenide glass layer. The conducting channel alters the
resistance state of the glass from a very high resistance state,
e.g. 1 G.OMEGA., to a medium resistance state, e.g., 10 M.OMEGA.. A
subsequently applied write voltage, having an energy lower than
that of the conditioning voltage, can then program the glass to a
lower resistance state, e.g. 10 k.OMEGA., by causing available
metal ions to move into the conducting channels where they remain
after the write voltage is removed. The metal ions within the
conducting channel can be removed by application of a negative
polarity erase voltage. Because the conducting channel is
previously formed by the conditioning voltage before a write or
erase operation occurs, higher speed switching of the glass between
resistance states can be achieved compared to trying to form and
completely decompose conductive pathways each time the glass is
written or erased to a resistance state. The glass layer can be
used to construct memory elements.
[0017] For purposes of a simplified description, memory elements
are described below in which a metal-chalcogen is described as
silver-selenide, and the chalcogenide glass as germanium-selenide.
However, these specific materials are not considered as limiting
the invention.
[0018] The invention will now be explained with reference to FIGS.
1A-F; 2A-2F; 3A-3F; and 4A-4-F, which respectively illustrate
exemplary embodiments of a resistance variable memory element 100,
101, 102 and 103, and their methods of formation and operation, in
accordance with the invention.
[0019] FIG. 1A depicts a first exemplary embodiment of a resistance
variable memory element 100 and its method of formation in
accordance with the invention. A first electrode 2 is formed over a
substrate 1. The first electrode 2 may comprise a conductive
material, for example, various metals such as, one or more of
tungsten, tantalum, aluminum, platinum, silver, or titanium
nitride, among others. In addition, the first electrode 2 can
comprise a conductively-doped semiconductor material. The first
electrode 2 should preferably not produce or expel metal ions, as
discussed below.
[0020] Although FIG. 1A illustrates a first electrode 2 provided on
substrate 1, it should be appreciated that additional layers may be
provided between electrode 2 and the substrate 1. For instance, a
barrier layer may be used to prevent migration of metal ions from
layer 2. In addition, a semiconductor substrate 1 containing
circuit layers covered with an insulating layer can be provided
below first electrode 2 if desired.
[0021] Next, a glass layer 4 is formed over the first conductive
electrode 2. The glass layer 4 is electrically coupled to electrode
2. The glass layer 4 is preferably a chalcogenide glass layer 4
that has been doped, e.g. photodoped, with a metal ion such as
silver (Ag), and is more preferably a germanium-selenide glass
layer 4 having a Ge.sub.xSe.sub.100-x stoichiometry doped with Ag
ions. The stoichiometric range for glass layer 4 is preferably from
about Ge.sub.18Se.sub.82 to about Ge.sub.25Se.sub.75, and is more
preferably about Ge.sub.25Se.sub.75 when metal ions, such as Ag
ions, are provided in the glass layer 4 by a doping process. Glass
layer 4 is formed to a thickness of from about 150 .ANG. to about
600 .ANG. thick, and is preferably about 500 .ANG. thick.
[0022] Although glass layer 4 is described as a chalcogenide glass
layer, other suitable glass layers may be employed as well. For
instance, suitable glass material compositions for the glass layer
4 can include, but are not limited to, AsSe (arsenic-selenide, such
as As.sub.3Se.sub.2), GeS (germanium-sulfide), and combinations of
Ge, Ag, and Se, among others. Any one of the suitable glass
materials for glass layer 4 and may further comprise small
concentrations of dopants such as nitrogen nitrides, metals, and
other group 13-17 elements from the periodic table.
[0023] The formation of the chalcogenide glass layer 4 having a
stoichiometric composition such as Ge.sub.25Se.sub.75 in accordance
with one exemplary embodiment of the invention, can be accomplished
by any suitable method. For instance, by evaporation, co-sputtering
germanium and selenium in the appropriate ratios, sputtering using
a germanium-selenide target having the desired stoichiometry, or
chemical vapor deposition with stoichiometric amounts of GeH.sub.4
and SeH.sub.2 gases (or various compositions of these gases), which
result in a germanium-selenide film of the desired stoichiometry,
are some non-limiting examples of methods which can be used to form
the glass layer 4.
[0024] After the chalcogenide glass layer 4 is formed, metal ions
are doped into the glass layer 4 by a photodoping process. For
instance, the chalcogenide glass layer 4, such as
Ge.sub.25Se.sub.75, can be photodoped with a metal such as Ag.
Metal ions can be driven into the glass layer 4 by applying a metal
(Ag) layer on top of the glass layer 4 and exposing the glass layer
4 and metal layer to visible radiation. The metal layer can be
formed over the glass layer 4, for example, by sputtering, physical
vapor deposition, or other well-known techniques in the art.
[0025] The metal doping of glass layer 4 causes the glass layer 4
to contain polarizable metal-chalcogen regions 30 and glass
backbone regions 50, as shown in FIG. 1B. In this context,
polarizable regions are regions which can physically align in the
presence of a suitable voltage applied across a memory element.
[0026] It should be appreciated that the polarizable
metal-chalcogen regions 30 are distinct regions within the glass
layer 4. When glass layer 4 is doped with a metal ion, the glass
layer 4 will phase separate into polarizable metal-chalcogen
regions 30 and glass backbone regions 50. See Maria Mitkova, et
al., Dual Chemical Role of Ag as an Additive in Chalcogenide
Glasses, PHYSICAL REVIEW LETTERS, Nov. 8, 1999, at 3848-3851. If Ag
is used as the dopant metal ion and germanium selenide, e.g.
Ge.sub.25Se.sub.75, is used for glass layer 4, the polarizable
metal-chalcogen regions 30 are Ag.sub.2Se regions within a
germanium selenide backbone. In essence, the glass backbone regions
50 are non-metal containing glass regions with a stoichiometry
determined by the loss of selenium (Se) from the germanium selenide
glass to the formation of Ag.sub.2Se.
[0027] Referring back to FIG. 1A, a second electrode 10 is then
formed over the glass layer 4 and any residual metal ions, e.g. Ag,
remain in glass layer 4 to complete the formation of the resistance
variable memory element 100. The second electrode 10 may comprise
any conductive material, for example, various metals, such as, one
or more of tungsten, tantalum, aluminum, platinum, silver, or
titanium nitride among others. In addition, the second electrode 10
can comprise a conductively-doped semi-conductive material, e.g.,
doped polysilicon.
[0028] Although electrode 10 may be directly applied to glass layer
4, in a preferred embodiment, the second electrode 10 is in contact
with an intermediate metal-containing layer 5, which is provided
over glass layer 4. This intermediate metal-containing layer 5 is
preferably a layer comprising Ag.
[0029] Although the doping of metal, e.g., Ag, into the glass layer
4 will produce metal ions used, e.g, Ag ions within glass layer 4,
the presence of the metal-containing layer 5 will serve as an
additional source and receptacle for metal ions during write and
erase operations. For example, for a germanium selenide glass layer
4 backbone containing polarizable Ag.sub.2Se metal-chalcogen
regions 30, during a `write` process, the metal-containing layer 5
can be Ag and is a source of metal ions, e.g., Ag, which enter
glass layer 4. During an `erase` process, the metal-containing
layer 5 is the receptacle of the metal ions, e.g., Ag that move out
of the glass layer 4.
[0030] In another exemplary embodiment, the metal containing layer
5 is omitted and the top electrode 10 is formed of a material
capable of donating and receiving metal ions. For example, top
electrode 10 can be made of silver which would be in contact with
glass layer 4, and would then become a source and receptacle of
metal ions during a write and erase operation. It is also possible
to use an electrode 10, which donates or receives metal ions with
metal-containing layer 5, which also donates or receives metal
ions, in combination.
[0031] It should be appreciated that the first electrode 2 and the
second electrode 10 can comprise the same or different materials.
However, for example, if the first electrode 2 and the second
electrode 10 comprise the same material such as tungsten or any
other non-metal ion comprising metal, one side of the memory
element 100, preferably the side with the second electrode 10, must
have an excess of metal ions, e.g., Ag which, in the preferred
embodiment, is metal-containing layer 5.
[0032] Because the metal ions, e.g., Ag ions which enter and leave
glass 4 comes from layer 5, if provided, or layer 10, if layer 5 is
not provided, it is preferable that electrode 2 not donate any
metal ions. As a result, although intervening barrier layers are
not illustrated in the FIG. 1A embodiment, intervening barrier
layers can be present to prevent metal ion migration from electrode
2 into glass layer 4, or from electrode 10 into glass layer 4. The
barrier layers, if provided, should not contain mobile metal
ions.
[0033] A method of operating and manipulating the resistance state
of the memory element depicted in FIG. 1A will now be described in
reference to FIGS. 1B-1F.
[0034] Reference is now made to FIG. 1B which is a cross-sectional
view of the memory element 100 of FIG. 1A prior to application of a
conditioning voltage. As FIG. 1B illustrates, the chalcogenide
glass regions 50 and polarizable metal-chalcogen regions 30 have no
long range order, i.e., they are randomly distributed. For a
germanium selenide glass layer 4 and Ag ion configuration, the
doped chalcogenide glass layer 4, contains regions of
germanium-selenide 50 and regions of polarizable silver-selenide
30. Free Ag ions may also be present in glass layer 4. The
germanium-selenide regions 50 serve as the glass backbone for
memory element 100.
[0035] Referring now to FIG. 1C, when a conditioning voltage
(V.sub.1) of suitable energy, for example, is applied from a
voltage source (DC) 20, one or more regions of silver-selenide 30
will polarize, that is, align to form a conducting channel 60. The
aligned silver-selenide regions 40 form conducting channel 60 which
spans the entire thickness of glass layer 4. The application of
V.sub.1 induces the alignment of the polarizable silver-selenide
regions. It has been found that a conditioning voltage (V.sub.1)
about 200 mV under DC switching conditions and about 1.7V, 500 ns
under AC conditions, is sufficient to physically align the
Ag.sub.2Se regions 30 and form the conducting channel 60.
[0036] After the conditioning voltage V.sub.1 is applied, memory
element 100 is in a `medium` state of resistance. Prior to
application of the conditioning voltage illustrated in FIG. 1B, the
memory element 100 is in a `high` state of resistance. As a
non-limiting example, a `high` state of resistance for memory
element 100 can be greater than 1 G.OMEGA.. A `medium` state of
resistance, produced by the alignment of the Ag.sub.2Se regions 30
can be around 1 M.OMEGA.. FIG. 1C illustrates the memory element
100 in the `medium` state of resistance after applying a
conditioning voltage V.sub.1.
[0037] The conditioning voltage V.sub.1 is at a higher potential
than subsequent potentials used to write, read, or erase the memory
element 100. This is due to the initial disorder of the polarizable
silver-selenide regions 30, as illustrated in FIG. 1B. Prior to
application of the conditioning voltage V.sub.1, the structure of
the memory element 100 is in the most disordered state. Thus, to
bring the memory element 100 into a more structured state
illustrated in FIG. 1C, a conditioning voltage V.sub.1 which is
larger than subsequently applied write, erase or read voltages is
required.
[0038] Any suitable number, including all of the polarizable
regions of silver-selenide 30, can be polarized to form conducting
channel 60 which extends throughout the thickness of glass layer 4.
The amplitude of V.sub.1, necessary to induce formation of
conducting channel 60, will depend upon the pulse width, glass
layer 4 composition, and thickness. With an exemplary embodiment of
Ag.sub.2Se formed within a glass with an initially
Ge.sub.2Se.sub.75 stoichiometry before addition of Ag, having a
thickness of 500 .ANG., and a metal-containing layer 5 of Ag having
a thickness of 200 .ANG., a 1.7V pulse having a duration of 500 ns
was found to be sufficient.
[0039] The preestablished conducting channel 60 allows additional
metal ions within the glass layer 4, or from the metal-containing
layer 5, and/or electrode 10, to move into and out of the
conducting channel 60 upon application of a potential across the
two electrodes. Thus, the resistance state of the memory element
100 can be changed quickly. In other words, the movement of the Ag
ions in the conducting channel 60 upon application of a write
voltage V.sub.2, which has a potential less than the conditioning
voltage V.sub.1, can lower the resistance state of the memory
element 100 from the medium resistance state of e.g., 1 M.OMEGA.,
to a lower resistance state of e.g., 10 k.OMEGA.. When an erase
voltage of inverse polarity to the write voltage is applied, the
silver ions are driven out of the conducting channel 60 and back
into the glass layer 4 and metal-containing layer 5, and/or
electrode 10, increasing the resistance state of the memory element
100 back to the medium resistance state. The erase voltage need
only be sufficient to drive Ag ions out of the conducting channel
60. The conducting channel 60 is not dispersed and is still
maintained.
[0040] Because the conducting channel 60 always remains intact, the
switching speeds and response of the memory element 100 are
enhanced because the Ag ions can move into and out of the
conducting channel 60 without the need to reform the channel 60
every time the memory element 100 is switched.
[0041] It should be appreciated that the presence of the glass
regions 50, here, germanium-selenide regions 50, serve to isolate
the polarizable metal-chalcogen regions 30 from each other, here,
polarizable silver-selenide regions 30. The germanium-selenide
regions 50 also restrict the mobility and provide isolation between
the polarized silver-selenide regions 40. As a result, once memory
element 100 is written to a low resistance state, enhanced data
retention arises due to the polarized silver-selenide regions 40
being held more rigidly in the glass backbone 50.
[0042] It should also be appreciated that although only one
conducting channel 60 is illustrated in FIGS. 1C-1F, one or more
conducting channels 60 may be formed in glass layer 4. Moreover,
conditioning of the memory element 100 is conducted only once, and
after the memory element 100 is conditioned, the conditioned
structure 100 will operate through normal write and erase
operations.
[0043] FIG. 1D illustrates the memory element 100 after a write
operation is performed. A write voltage V.sub.2 is applied from a
voltage source 20, to `write` information into the resistance
variable memory element 100. The write voltage V.sub.2, occurs at a
lower potential than the conditioning voltage V.sub.1. For example,
if the conditioning pulse is 200 mV, memory element 100 should be
written with a `write` voltage V.sub.2 of less than 200 mV. As
shown in FIG. 1D, the additional Ag ions enter the preestablished
conductive channel 60 to further complete the conductive path and
lower the resistance of the memory element 100. With this exemplary
embodiment of Ag.sub.2Se formed within a glass with an initially
Ge.sub.25Se.sub.75 stoichiometry before addition of Ag, having a
thickness of 500 .ANG., and a metal-containing layer 5 of Ag of
thickness 200 .ANG., a write voltage of about 700 mV with a pulse
width of about 100 ns was found to be sufficient.
[0044] Applying the write voltage V.sub.2 causes Ag ions to move
into the conducting channel 60 from the side with the positive
potential, here electrode 10. The Ag ions are supplied from any
free Ag ions within glass layer 4, and the metal-containing layer
5, and/or electrode 10. FIG. 1D illustrates memory element 100 in
the `low` state of resistance, e.g., around 10 k.OMEGA..
[0045] It should be appreciated that the preestablished conductive
channels 60 may be affected by high temperatures as the metal ions
and polarizable metal-chalcogen regions 30 become more mobile.
Accordingly, when this occurs, it may be desirable to periodically
refresh the memory elements by periodically reapplying the
conditioning pulse V.sub.1 to reset the conductive channel 60 and
thereby resetting the memory element 100 to its medium resistance
state.
[0046] Referring now to FIG. 1E, a `read` operation is illustrated
in which a read potential V.sub.3, which is less than write
potential V.sub.2, is applied to the memory element 100. Current
flow through the memory element 100 is sensed by a current sensing
amplifier 32, which provides an output representing the resistance
state of the memory element 100.
[0047] A read voltage V.sub.3, which is below the threshold for
writing the memory element 100, e.g., V.sub.2, is sufficient. Where
the write voltage V.sub.2 is about 700 mV with a pulse duration of
100 ns, the read voltage V.sub.3 can then be a potential less than
about 200 mV with a pulse width less than about 500 ns. The read
voltage V.sub.3 does not disturb other memory elements in a memory
element array, which are in the pre-conditioned medium resistance
`OFF` state, since the read voltage V.sub.3 is lower than the write
voltage V.sub.2. The read voltage V.sub.3 may be applied in various
manners, such as a sweep voltage, pulse voltage, or step voltage,
among other methods.
[0048] FIG. 1F illustrates the memory element 100 when an erase
voltage is applied across the electrodes 2 and 10 from voltage
source 20. An erase voltage V.sub.4 having an inverse polarity from
the write voltage V.sub.2 is applied from electrode 10 to electrode
2 to erase the memory element 100. The erase voltage V.sub.4 may
also be of a smaller absolute magnitude than the write voltage
V.sub.2.
[0049] The application of an erase voltage V.sub.4 moves Ag ions
out of the conducting channel 60 toward the electrode with the
negative potential. This is electrode 10 in FIG. 1F. The Ag ions
will move out of the conducting channel 60 and into the
metal-containing layer 5, the glass layer 4, and/or electrode 10 if
the electrode 10 contains Ag. Stated in another way, the erase is
essentially a function of removing Ag ions from conducting channel
60. As a result, the erase is complete when the resistance of the
memory element 100 returns to the `medium` state of resistance,
i.e., around 1 M.OMEGA..
[0050] It is important to note that after an erase operation, the
conducting channel 60 of FIG. 1C remains intact; thus, allowing for
faster write and erase switching times. In other words, the erase
voltage V.sub.4 returns the memory element 100 to the state of
resistance illustrated in FIG. 1C. Thus, the erase voltage V.sub.4
should be low enough to cause a resistance shift in the memory
element 100, but not of a magnitude which would destroy the
conducting channel 60.
[0051] It should be appreciated that the conditioning, write, and
erase pulse widths are dependent on the electric field amplitude,
i.e., the applied voltage. Thus, shorter pulses will require higher
voltages for V.sub.1, V.sub.2, and V.sub.4 and vice versa. This is
expected since the movement of Ag ions in the conducting channel 60
has an energy requirement for movement into and out of the
conducting channel 60, which in turn is dependent upon the
concentration of Ag ions which enters or leaves the conducting
channel 60.
[0052] Reference is now made to FIG. 2A which shows another
exemplary embodiment of the invention and its method of
formation.
[0053] The FIG. 2A embodiment has a first electrode 2 formed over a
substrate 1. The first electrode 2 may comprise any of the
conductive materials listed above for the same electrode as in the
FIG. 1A embodiment. Similar to the FIG. 1A embodiment, additional
barrier layers may be provided between electrode 2 and substrate 1,
if required to prevent metal ion migration. The barrier layers, if
provided, should not contain mobile metal ions.
[0054] Next, a glass layer 4 such as a chalcogenide glass layer 4
is formed over the first electrode 2. The glass layer 4 is
electrically coupled to electrode 2. The glass layer 4 is
preferably a chalcogenide glass layer 4, and more preferably, a
germanium-selenide glass layer 4 having a Ge.sub.xSe.sub.100-x
stoichiometry. The stoichiometric range for chalcogenide glass
layer 4 as depicted in the FIG. 2A embodiment is preferably from
about Ge.sub.20Se.sub.80 to about Ge.sub.43Se.sub.57, and is more
preferably about Ge.sub.40Se.sub.60. Glass layer 4 is formed to a
thickness of from about 150 .ANG. to about 500 .ANG. thick, and
preferably is about 150 .ANG. thick.
[0055] For purposes of a simplified description, the glass layer 4
of structure 101 is described further below as a chalcogenide glass
layer 4 and more specifically, a Ge.sub.40Se.sub.60 layer. However,
other suitable glass or polymer layers may be employed without
affecting the utility of the invention. For instance, suitable
glass material compositions for the glass layer 4 can include but
are not limited to, SiSe (silicon-selenide), AsSe
(arsenic-selenide, such as As.sub.3Se.sub.2), GeS
(germanium-sulfide), and combinations of Ge, Ag, and Se, among
others. Any one of the suitable glass materials may further
comprise small concentrations of dopants such as nitrogen nitrides,
metals, and group 1, 2, and 13-17 elements from the periodic
table.
[0056] The formation of the glass layer 4 having a stoichiometric
composition such as Ge.sub.40Se.sub.60 in accordance with one
exemplary embodiment of the invention, can be accomplished by any
suitable method. For instance, evaporation, co-sputtering germanium
and selenium in the appropriate ratios, sputtering using a
germanium-selenide target having the desired stoichiometry, or
chemical vapor deposition with stoichiometric amounts of GeH.sub.4
and SeH.sub.2 gases (or various compositions of these gases), which
result in a germanium-selenide film of the desired stoichiometry,
are some non-limiting examples of methods which can be used to form
the glass layer 4. It should be appreciated that the glass layer 4
may comprise one or more layers of a glass material.
[0057] Still referring to FIG. 2A, a metal-containing layer 6,
preferably silver-selenide, is deposited over the chalcogenide
glass layer 4. However, any suitable metal-containing layer 6 may
be used so long as it interacts with the glass backbone such that
it allows the transfer of metal ions into glass layer 4 upon
application of a sufficient voltage across a memory element of
which layers 4 and 6 are a part. For instance, besides
silver-selenide, the metal-containing layer 6 may comprise silver,
copper, or other transition metals. Other suitable metal-containing
layers 6 which may be used include glass layers doped with a
metal.
[0058] Preferably, the metal-containing layer 6 will comprise the
same type of chalcogen component as is present in glass layer 4.
For example, if glass layer 4 is Ge.sub.xSe.sub.100-x,
metal-containing layer 6 may be Ag.sub.2Se. The metal-containing
layer 6 is formed to a thickness of from about 300 .ANG. to about
1200 .ANG. thick, and preferably is about 470 .ANG. thick.
[0059] It should be appreciated that excess metal ions need to be
provided either by metal-containing layer 6 itself, or through some
other means for donation to glass layer 4. For instance,
metal-containing layer 6 can be formed to contain excess metal
ions. That is, Ag.sub.2+xSe, where x represents excess Ag ions.
Alternatively, if the excess metal ions are not part of the
metal-containing layer 6, a separate second metal-containing layer
7 with a sufficient thickness that gives the desired excess amount
of metal ions, e.g., Ag ions to the metal-containing layer 6 may be
provided over or beneath the first metal-containing layer 6.
[0060] An electrode 10 is provided over the conducting
metal-containing layer 6, if metal-containing layer 7 is omitted,
or is provided over metal-containing layer 7 if the latter is
provided. Electrode 10 need not donate any metal ions if one or
both of layers 6 and 7 provide sufficient metal ions for memory
element operation.
[0061] In an exemplary embodiment, the second metal-containing
layer 7 is provided and serves as a source and receptacle for metal
ions doing write and erase operations. In another exemplary
embodiment, the top electrode 10 may be the source and receptacle
of additional metal ions when the second metal-containing layer 7
is omitted, and excess metal ions are not available in layer 6. In
this case, the top electrode 10 may comprise silver, which donates
and receives silver ions to and from glass layer 4.
[0062] Some non-limiting examples of forming the metal-containing
layer 6 are physical vapor deposition techniques such as
evaporative deposition, sputtering, chemical vapor deposition,
co-evaporation, or depositing a layer of selenium above a layer of
silver to form silver-selenide (Ag.sub.2Se) can also be used. It
should be appreciated that the metal-containing layer 6 may
comprise one or more layers of a metal-containing material. For
purposes of a simplified description, FIGS. 2A-2F refer to the
first metal-containing layer 6 as a silver-selenide layer 6 and the
second metal-containing layer 7 as a silver layer 7. In this case
electrode 10 does not contribute metal ions to, or receive metal
ions from glass layer 4.
[0063] The second electrode 10 may comprise any of the materials
described above for electrode 10 of the FIG. 1A embodiment. FIG. 2A
illustrates that the second electrode 10 is in contact with an
upper surface of the second metal-containing layer 7; however,
intervening layers may be provided between layers 7 and 10, if
desired. As described above, the second metal-containing layer 7
can provide additional Ag ions. In addition, the second electrode
10 can comprise Ag which can also provide additional Ag ions if the
second metal-containing layer 7 is omitted from memory element
101.
[0064] In another exemplary embodiment, the metal containing layer
7 is omitted and the top electrode 10, which is in contact with
metal-containing layer 6, can provide and receive metal ions, e.g.,
Ag ions, and can be the source and receptacle of metal ions during
a write and erase operation. It is also possible to use an
electrode 10, which donates or receives metal ions with
metal-containing layer 7, which also donates or receives metal ions
in combination. It should be appreciated that the first electrode 2
and the second electrode 10 can comprise the same or different
materials.
[0065] As described above, the first and second metal-containing
layers 6 and 7 can provide Ag ions. In addition, the second
electrode 10 can comprise Ag, if used, which can also provide Ag
ions if the second metal-containing layer 7 is omitted from memory
element 101, and layer 6 does not have excess Ag ions. Because the
metal ions, e.g., Ag ions which enter and leave glass 4 is coming
from the second metal-containing layer 7, if provided, or electrode
10, if layer 7 is not provided and layer 6 does not have excess Ag
ions., it is preferable that electrode 2 not donate any metal
ions
[0066] As a result, although intervening barrier layers are not
illustrated in the FIG. 2A embodiment, intervening barrier layers
can be present to prevent metal ion migration from electrode 2 into
glass layer 4, or from electrode 10 into glass layer 4. Barrier
layers, if provided, should not contain mobile metal ions.
[0067] A method of operating and manipulating the resistance state
of the memory element 101 depicted in FIG. 2A will now be described
in reference to FIGS. 2B-2F. For exemplary purposes, the methods of
operation described in FIGS. 2B-2F is for a memory element 101
comprising a Ge.sub.40Se.sub.60 glass layer 4 that is 150 .ANG.
thick, an Ag.sub.2Se layer 470 .ANG. thick, and a silver layer 200
.ANG. thick.
[0068] Reference is now made to FIG. 2B which is a cross-sectional
view of the glass layer 4 in memory element 101 of FIG. 2A prior to
application of a conditioning voltage across memory element 101. As
FIG. 2B illustrates, the germanium-selenide glass layer 4 has no
long range order, i.e., it has non-uniform distribution of Ge and
Se within the glass layer 4. FIG. 2B further illustrates the
presence of germanium-germanium (Ge--Ge) bonds 17 throughout the
glass layer 4. The presence of another species which can provide a
more thermodynamically favorable energy than the Ge--Ge bond energy
will ultimately break the Ge--Ge bonds 17 and bond with the
previously bonded Ge.
[0069] Referring now to FIG. 2C, a conditioning pulse having a
potential V.sub.1, is applied across memory element 101. The
conditioning pulse causes metal chalcogenide, e.g., Ag.sub.2Se from
the layer 6 to enter into glass layer 4, thereby breaking Ge--Ge
bonds 17 in the glass layer 4 and to form conductive channel 11
within the glass 4 backbone. The conditioning pulse's parameters
are dependent upon the composition and thickness of the various
layers comprising memory element 101.
[0070] For a Ge.sub.40Se.sub.60 glass layer 4, a first Ag.sub.2Se
layer 6, and a second Ag layer 7 having the construction described,
a conditioning pulse having a pulse duration from about 10 to about
500 ns and greater than about 700 mV has been found sufficient to
form conductive channel 11. The amplitude of the conditioning pulse
will depend on the pulse width. The conducting channel 11 will form
in the weakest part of the chalcogenide glass material 4, i.e., in
the areas that require the least amount of energy to form the
conductive channel 11. The conditioning pulse causes the conducting
channel 11 to form by re-orientation of the GeSe and Ag.sub.2Se
regions, as shown in FIG. 2C.
[0071] The Ag.sub.2Se provided from the first metal-containing
layer 6, and driven into the glass layer's 4 backbone, assists in
forming the conducting channel 11 as it bonds with the
germanium-selenide, i.e., as the Ag.sub.2Se bonds to the glass. It
should be appreciated that a plurality of conducting channels 11
can be formed in the chalcogenide glass layer 4. For purposes of a
simplified description, only one conducting channel 11 is
illustrated in FIGS. 2C-2F.
[0072] After application of the conditioning pulse, memory element
101 is in a `medium` state of resistance. Prior to application of a
conditioning pulse, and as illustrated in FIG. 2B, memory element
101 is in a `high` state of resistance where the germanium-selenide
regions are not oriented. For instance, a `high` state of
resistance which arises after the conditioning pulse is applied to
memory element 101 can be about 1 G.OMEGA.. A `medium` state of
resistance can be around 1 M.OMEGA..
[0073] In the medium resistance state, the memory element 101 is
still considered `OFF` and remains in this conditioned state, with
the Ag.sub.2Se regions, polarized and aligned in the direction of
current flow, until the conducting channel 11 receives excess metal
ions from the first metal-containing layer 6, if excess metal ions
are present, and from the second metal containing layer 7, and/or
electrode 10 during a `write` operation.
[0074] Therefore, applying a conditioning pulse across the memory
element 101 breaks the weak Ge--Ge bonds 17 within the
Ge.sub.40Se.sub.60 glass layer 4 and allows Ag.sub.2Se and ions
thereof to bond to germanium-selenide sites. In part, the
conditioning pulse V.sub.1, reorients the non-uniform state of the
chalcogenide glass layer 4 (FIG. 2B) into a more organized and
structured state having aligned Ag.sub.2Se areas as illustrated in
FIG. 2C. However, the memory element 101 of FIG. 2C is still in a
medium or `OFF` state of resistance.
[0075] Referring now to FIG. 2D, during a `write` operation, excess
Ag ions from metal-containing layer 6, Ag ions from
metal-containing layer 7, and/or electrode 10 enter the glass 4 and
will cluster in the conducting channel 11, and more specifically,
cluster to the germanium-selenide and Ag.sub.2Se bonded regions;
thus, forming a low resistance conductive path as clustering
structures 12 in FIG. 2D illustrate. These clustering structures
12, i.e., regions of Ag/Ag+, are formed throughout the conducting
channel 11. The presence of the clustering structures 12 provides a
low resistance state for memory element 101. A `write` mode exists
when a voltage V.sub.2 less than the conditioning voltage V.sub.1
is applied across memory element 101, thereby generating an `ON`
(low resistance) state for memory element 101. Low resistance is
about 10 k.OMEGA..
[0076] It should be appreciated that the portion of the glass
backbone around the conducting channel 11 does not contain much Ag
ions. In fact, the majority of the chalcogenide glass layer 4 does
not contain Ag ions. The Ag ions from a `write` operation proceed
into the conducting channel 11 from the first metal-containing
layer 6 if excess metal ions are present, the second
metal-containing layer 7, and/or electrode 10. One exemplary write
potential V.sub.2 is preferably a pulse from about 8 to about a 150
ns that is less than the potential of V.sub.1, e.g., less than 700
mV. A write potential V.sub.2 of about 400 mV has been found to be
adequate with the memory element 101. The amplitude of the write
potential will vary depending on the pulse width.
[0077] As a result, during a write operation the Ag ions take the
path of least resistance into glass layer 4. In this case, the path
of least resistance is provided by the conducting channel 11. The
Ag ions will migrate toward the negative potential, here, electrode
2, when applied across the memory element 101. Accordingly, the
movement of the Ag ions into the conducting channel 11 renders
channel 11 more conductive.
[0078] When an erase potential V.sub.4, having an inverse polarity
to that of the write potential V.sub.2 is applied to the memory
element 101, the Ag ions will leave conductive channel 11 and move
back into the first metal-containing layer 6. The memory element
101 reverts back to the `medium` state resistance, as illustrated
in FIG. 2E. An exemplary erase potential V.sub.4 is a pulse from
about 8 to about 150 ns that is from about negative 400 mV (-400
mV) to about negative 700 mV (-700 mV) in amplitude. As with the
write potential V.sub.2, the amplitude of the erase potential
V.sub.4 will depend on the pulse width.
[0079] It should be appreciated that application of an erase
potential V.sub.4 from voltage source 20 across the structure 101,
serves only to drive the free Ag ions (unbound Ag ions) out of the
conducting channel 11 back to their original source The conducting
channel 11 remains in place even after an erase potential V.sub.4
is applied across the memory element 101, so long as the erase
potential V.sub.4 does not greatly exceed the magnitude of the
conditioning potential V.sub.1 in reverse polarity.
[0080] It has been further discovered that applying another
positive potential V.sub.3, to an already `ON` memory element 101
structure, i.e, one in a low resistance state, illustrated in FIG.
2D, results in an even lower resistance `ON` state caused by the
presence of additional Ag/Ag+ ion clusters 12, as illustrated in
FIG. 2F. In essence, applying multiple `write` pulses to memory
element 101 can reduce the resistance state of memory element 101
to a much lower resistance. In some instances, the resistance of
memory element 101 can be well below 10 k.OMEGA..
[0081] In an exemplary embodiment of memory element 101, a 10 ns,
1.7V conditioning pulse V, was applied to cause the memory element
101, initially at 1 G.OMEGA., to move into a `medium` state of
resistance of approximately 1 M.OMEGA.. A 10 ns, 700 mV write pulse
V.sub.2 was applied to the memory element 101 to move it to a low
resistance state of approximately 10 k.OMEGA.. A 10 ns, negative
550 mV erase pulse V.sub.4 was applied to return the memory element
101 to a medium resistance state. It was also observed that
applying a 10 ns, 700 mV write pulse V.sub.3 repeatedly to the
memory element 101 yielded lower and lower resistance states below
10 k.OMEGA.. Thus, the memory element 101 could be used to set
different detectable logic states in accordance with the number of
applied write pulses V.sub.3.
[0082] For instance, a `read` operation in which a read potential
V.sub.5, which is less than write potential V.sub.2, can applied to
the memory element 101. Current flow through the memory element 101
can be sensed by a current sensing amplifier, which can provide an
output representing the resistance state of the memory element 101
(not pictured).
[0083] A read voltage V.sub.5, which is below the threshold for
writing the memory element 101, e.g., V.sub.1, is sufficient. Where
a 10 ns, 700 mV write pulse V.sub.2 is used, the read voltage
V.sub.5 can then be in the range from any pulse less than about 500
ns and less than or equal to about 200 mV. The read voltage V.sub.5
does not disturb other memory elements in a memory element array,
which are in the pre-conditioned medium resistance `OFF` state,
since the read voltage V.sub.5 is lower than the write voltage
V.sub.2. The read voltage Vs may be applied in various manners,
such as a sweep voltage, pulse voltage, or step voltage, among
other methods.
[0084] FIG. 3A-3F depicts a third exemplary embodiment of a
resistance variable memory device 102 constructed in accordance
with the invention.
[0085] A first electrode 2 is formed over a substrate 1. The first
electrode 2 may comprise any of the conductive materials listed
above as in the FIGS. 1A and 2A embodiments.
[0086] Next, a first glass layer 4 is formed over the first
electrode 2. The first glass layer 4 is electrically coupled to
electrode 2. The first glass layer 4 can comprise the same
materials as in prior embodiments and have the same stoichiometric
ranges as the glass layer 4 in FIG. 2A. For purposes of a
simplified description, the first glass layer 4 is described
further below as a Ge.sub.40Se.sub.60 chalcogenide glass layer 4.
Glass layer 4 is formed to a thickness of from about 150 .ANG. to
about 500 .ANG. thick, and preferably is about 150 .ANG. thick.
[0087] The formation of the chalcogenide glass layer 4, having a
stoichiometric composition, such as Ge.sub.40Se.sub.60, can be
accomplished by any of the methods described above for forming the
glass layer 4 of FIG. 2A. The first glass layer 4 may comprise one
or more layers of a glass material.
[0088] Still referring to FIG. 3A, a first metal-containing layer
6, preferably silver-selenide, is formed over the first
chalcogenide glass layer 4. The formation of the first
metal-containing layer 6, such as silver-selenide, can be
accomplished by any of the methods described above for forming the
metal-containing layer 6 of FIG. 2A. The first metal-containing
layer 6 may comprise one or more layers of a metal-containing
material. The first metal-containing layer 6 is formed to a
thickness of from about 300 .ANG. to about 1200 .ANG. thick, and
preferably is about 470 .ANG. thick.
[0089] Next, a second glass layer 8 is formed over the first
metal-containing layer 6. The second glass layer 8 allows
deposition of silver above a silver-selenide layer 6, for instance.
The second glass layer 8 can be utilized as a diffusion control
layer to prevent metal ions from migrating from electrode 10 into
the memory element 102. The second glass layer 8 is formed to a
thickness of from about 100 .ANG. to about 300 .ANG. thick, and
preferably is about 150 .ANG. thick.
[0090] The formation and composition of the second glass layer 8 is
the same as described above for the formation and composition of
the glass layer 4 of FIG. 3A. For purposes of a simplified
description, the second glass layer 8 is described as a
chalcogenide glass layer having a stoichiometry similar to the
first glass layer 4 i.e., Ge.sub.40Se.sub.60. Further, one or more
layers of glass material can be provided if desired for glass layer
8. The second glass layer 8 may be formed to a thickness of from
about 100 .ANG. to about 300 .ANG. thick, and preferably is about
150 .ANG. thick.
[0091] Although the first glass layer 4 and the second glass layer
8 are described above as having a stoichiometry and material
composition similar to each other i.e., Ge.sub.40Se.sub.60, it
should be appreciated that the first glass layer 4 and the second
glass layer 8 can possess different stoichiometries from each
other, different thicknesses, and they can even be formed of
different glasses.
[0092] As in the second embodiment, excess metal ions need to be
provided in this embodiment as well, either by excess metal ions in
the first metal-containing layer 6, by an optional second
metal-containing layer 7 provided above glass layer 8, or by upper
electrode 10. For instance, the first metal-containing layer 6 can
be formed containing excess metal ions. Alternatively, if the
excess metal ions are not part of the first metal-containing layer
6, i.e., added specifically or deposited with an excess metal, the
metal ions need to be added as a separate second metal-containing
layer 7 and/or an upper electrode 10. The second metal-containing
layer 7 should have a sufficient thickness that gives the desired
excess amount of metal ions to the glass layer 4.
[0093] The second metal-containing layer 7 can comprise any metal
ions so long as it provides metal ions to enable formation of a
conducting channel in the glass layer 4 after application of a
conditioning pulse. For instance, the second metal-containing layer
7 may comprise silver or copper. In an exemplary embodiment, the
second metal-containing layer 7 is present and serves as a source
and receptacle for additional metal ions.
[0094] A second electrode 10 is formed over the second glass layer
8 or over the second metal containing layer 7, if provided, as
shown in FIG. 3A, to complete the formation of the memory element
102. The second electrode 10 may comprise any of the conductive
materials listed above for the electrode 10 described in reference
to FIGS. 1A and 2A. FIG. 3A illustrates that the second electrode
10 is in contact with an upper surface of the second
metal-containing layer 7; however, intervening layers may be
provided between layers 7 and 10, if desired. As described above,
the second metal-containing layer 7 can provide additional Ag ions.
In addition, the second electrode 10 can comprise Ag which can also
provide additional Ag ions if the second metal-containing layer 7
is omitted from structure 102.
[0095] It should be appreciated that the first electrode 2 and the
second electrode 10 can comprise the same or different materials.
However, for example, if the first electrode 2 and the second
electrode 10 comprise the same material, such as tungsten or any
other non-metal ion comprising metal, one side of the memory
element 102, preferably the side with the second electrode 10, must
have an excess of metal ions, Ag in the preferred embodiment,
either in layer 6 or preferably as the second metal-containing
layer 7.
[0096] As a result, although intervening barrier layers are not
illustrated in the FIG. 3A embodiment, intervening barrier layers
can be present to prevent metal ion migration from electrode 2 into
glass layer 4, or from electrode 10 into glass layer 4, when the
excess metal ions are provided by layers 6 and/or 7. Barrier
layers, if provided, should not contain mobile metal ions.
[0097] A method of operating and manipulating the resistance state
of the memory element 102 depicted in FIG. 3A is described below in
reference to FIGS. 3B-3F. For exemplary purposes, the methods of
operation described in FIGS. 3B-3F is for a memory element 102
comprising a first Ge.sub.40Se.sub.60 glass layer 4 that is 150
.ANG. thick, an Ag.sub.2Se layer 470 .ANG. thick, a second
Ge.sub.40Se.sub.60 glass layer 8 that is 150 .ANG. thick and a
silver layer 200 .ANG. thick.
[0098] Reference is now made to FIG. 3B which is a cross-sectional
view of the glass layer 4 of memory element 102 of FIG. 3A prior to
application of a conditioning pulse V.sub.1. As FIG. 3B
illustrates, the glass layer 4, formed of Ge.sub.40Se.sub.60, has
no long range order. FIG. 3B further illustrates the presence of
germanium-germanium (Ge--Ge) bonds 17 throughout the glass layer 4.
The presence of another species which can provide a more
thermodynamically favorable energy will break the Ge--Ge bonds 17
and bond with the previously bonded Ge. Accordingly, the Ge--Ge
bond 17 is not strong and can easily be broken.
[0099] Referring to FIG. 3C, when a conditioning pulse V.sub.1 is
applied across memory element 102, excess Ag ions from the first
and/or second metal-containing layers 6, 7 and/or from electrode 10
(if ions are available), enter into glass layer 4 and break some of
the Ge--Ge bonds 17. This forms conducting channel 11 via
incorporation of Ag.sub.2Se from the first metal-containing layer 6
and is illustrated in FIG. 3C. The conditioning pulse's V.sub.1
parameters are dependent upon composition and thickness of the
layers comprising memory element 102. Moreover, the methods of
operating memory element 101 depicted in FIGS. 2C-2F, is similar to
the methods of operating memory element 102 for write, read and
erase operations. Thus, as described above with reference to FIGS.
2C-2F, the methods of operating memory element 102 can proceed in a
similar manner as illustrated in FIGS. 3C-3F.
[0100] FIG. 4A depicts a fourth exemplary embodiment of a
resistance variable memory device 103 constructed in accordance
with the invention.
[0101] A first electrode 2 is formed over a substrate 1. The first
electrode 2 may comprise any of the conductive materials listed
above for the electrode 2 described in the FIGS. 1A, 2A and 3A
embodiments. Next, a first glass layer 4 is formed over the first
electrode 2. The first glass layer 4 is electrically coupled to
electrode 2. The first glass layer 4 can comprise the same material
as provided for the glass layer 4 in FIGS. 2A and 3A.
[0102] For purposes of a simplified description, the first glass
layer 4 is described further below as a Ge.sub.40Se.sub.60
chalcogenide glass layer 4. The formation of the first chalcogenide
glass layer 4 can be accomplished by any of the methods described
above for forming the glass layer 4 of FIGS. 2A and 3A. The first
glass layer 4 may comprise one or more layers of a glass material.
The first glass layer 4 is formed to a thickness of from about 150
.ANG. to about 500 .ANG. thick, and preferably is about 150 .ANG.
thick.
[0103] Still referring to FIG. 4A, a first metal-containing layer
6, preferably silver-selenide, is formed over the first
chalcogenide glass layer 4. The formation of the first
metal-containing layer 6 can be accomplished by any of the methods
described above for forming the metal-containing layer 6 of FIGS.
2A and 3A. The first metal-containing layer 6 may comprise one or
more layers of a metal-containing material. The first
metal-containing layer 6 is formed to a thickness of from about 300
.ANG. to about 1200 .ANG. thick, and preferably is about 470 .ANG.
thick.
[0104] Next, a second glass layer 8 is formed over the first
metal-containing layer 6. The second glass layer 8 may be used as a
diffusion control layer to control the migration of metal ions into
the glass layer 4. The formation and composition of the second
glass layer 8 is the same as described above for the formation and
composition of the glass layer 4 of FIGS. 2A and 3A. For purposes
of a simplified description, the second glass layer 8 is described
as a chalcogenide glass layer having a stoichiometry similar to the
first glass layer 4 e.g., Ge.sub.40Se.sub.60. Further, one or more
layers of glass material can be provided if desired.
[0105] Although the first glass layer 4 and the second glass layer
8 are described above as having a stoichiometry and material
composition similar to each other, e.g., Ge.sub.40Se.sub.60, it
should be appreciated that the first glass layer 4 and the second
glass layer 8 can possess different stoichiometries from each
other, be different thicknesses, and they can even be different
glasses. The second glass layer 8 may be formed to a thickness of
from about 100 .ANG. to about 300 .ANG. thick and preferably is
about 150 .ANG. thick.
[0106] Next, a second metal-containing layer 9, preferably silver,
is formed over the second glass layer 8. The formation of the
second metal-containing layer 9 can be accomplished by any of the
methods described above for forming the metal-containing layer 6 of
FIGS. 2A and 3A. The second metal-containing layer 9 may comprise
one or more layers of a metal-containing material. The second
metal-containing layer 9 is formed to a thickness of from about 100
.ANG. to about 500 .ANG. thick, and preferably is about 200 .ANG.
thick.
[0107] It should be appreciated that excess metal ions need to be
provided either by the first metal-containing layer 6 or second
metal-containing layer 9, and/or second electrode 10. For instance,
the first metal-containing layer 6 can be formed containing excess
metal ions. Alternatively, if the excess metal ions are not part of
the first metal-containing layer 6 i.e., added specifically or
deposited with an excess metal, the additional metal ions can be
provided from the second metal-containing layer 9 and/or second
electrode 10. The second metal-containing layer 9 should have a
sufficient thickness that gives the desired excess amount of metal
ions to the first metal-containing layer 6. In an alternate
embodiment, a third metal-containing layer 7 can be provided, if
desired. Preferably, the third metal-containing layer comprises
silver.
[0108] The second metal-containing layer 9 can comprise any metal
ions so long as it provides metal ions to the conducting channel 11
formed in the chalcogenide glass layer 4 after application of a
conditioning voltage across the electrodes 2 and 10.
[0109] In an exemplary embodiment, the presence of the third
metal-containing layer 7 serves as the source and receptacle for
metal ions. For example, during a `write` process, the third
metal-containing layer 7 is the source of the metal ions that move
into the conducting channel 11. During an `erase` process, the
third metal-containing layer 7 is the receptacle of the metal ions
that move out of the conducting channel 11.
[0110] A second electrode 10 is next formed over the third
metal-containing layer 7, as illustrated in FIG. 4A, to complete
the formation of the memory device 103. The second electrode 10 may
comprise any of the conductive materials listed above for the
electrode 10 as described above in the FIGS. 2A and 3A embodiments.
FIG. 4A illustrates that the second electrode 10 is in contact with
an upper surface of the third metal-containing layer 7; however,
intervening layers may be provided between layers 7 and 10, if
desired. As described above, the third metal-containing layer 7 can
provide additional Ag ions. In addition, the second electrode 10
can comprise Ag which can also provide additional Ag ions if the
third metal-containing layer 7 is omitted from memory element
103.
[0111] In another exemplary embodiment, the third metal containing
layer 7 is omitted and the second metal-containing layer 9, can
provide and receive metal ions, e.g., Ag ions, and can be the
source and receptacle of metal ions during a write and erase
operation. It is also possible to use an electrode 10, which
donates or receives metal ions with the third metal-containing
layer 7 or second metal-containing layer 9, which also donates or
receives metal ions in combination.
[0112] It should be appreciated that the first electrode 2 and the
second electrode 10 can comprise the same or different materials.
However, for example, if the first electrode 2 and the second
electrode 10 comprise the same material such as tungsten or any
other non-metal ion comprising metal, one side of the memory
element 103, preferably the side with the second electrode 10, must
have an excess of metal ions, e.g., Ag.
[0113] As described above, the third metal-containing layer 7 can
provide Ag ions or the second metal-containing layer 9 can. In
addition, the second electrode 10 can comprise Ag, if used, which
can also provide Ag ions if the third metal-containing layer 7 is
omitted from memory element 103, if desirable. Because the metal
ions, e.g., Ag ions which enter and leave glass 4 is coming from
the third metal-containing layer 7, if provided, or layer 10, or
layer 9, if layer 7 is not provided, it is preferable that
electrode 2 not donate any metal ions
[0114] As a result, although intervening barrier layers are not
illustrated in the FIG. 4A embodiment, intervening barrier layers
can be present to prevent metal ion migration from electrode 2 into
glass layer 4, or from electrode 10 into glass layer 4. Barrier
layers, if provided, should not contain mobile metal ions.
[0115] A method of operating and manipulating the resistance state
of the memory element depicted in FIG. 4A is described below in
reference to FIGS. 4B-4F. For exemplary purposes, the methods of
operation described in FIGS. 4B-4F is for a memory element 103
comprising a first Ge.sub.40Se.sub.60 glass layer 4 that is 150
.ANG. thick, a first Ag.sub.2Se layer 470 .ANG. thick, a second
Ge.sub.40Se.sub.60 glass layer 8 that is 150 .ANG., a second
Ag.sub.2Se layer 200 .ANG. thick, and a silver layer 300 .ANG.
thick.
[0116] Reference is now made to FIG. 4B which is a cross-sectional
view of the memory element 103 of FIG. 4A prior to application of a
conditioning pulse. As FIG. 4B illustrates, the Ge.sub.40Se.sub.60
glass layer 4 has no long range order. FIG. 4B further illustrates
the presence of germanium-germanium (Ge--Ge) bonds 17 throughout
the glass layer 4. The presence of another species which can
provide a more thermodynamically favorable energy than the Ge--Ge
bond energy will break the Ge--Ge bonds 17 and bond with the
previously bonded Ge. Accordingly, the Ge--Ge bonds 17 are not
strong and can easily be broken.
[0117] Referring now to FIG. 4C, a conditioning pulse V, is applied
to memory element 103. Memory element 103 is conditioned in a
similar manner as described above with regard to memory elements
101 and 102. Conditioning the memory element 103, is done only once
and after the memory element 103 is conditioned, the conditioned
memory element 103 will operate through normal write and erase
operations.
[0118] Accordingly, applying a conditioning pulse from the voltage
source 20 forms a conducting channel 11 via incorporation of
Ag.sub.2Se from the metal-containing layer 6 into the glass
backbone which is illustrated in FIG. 4C. The Ag.sub.2Se becomes
polarized and aligned within the glass 4 backbone to form
conductive channel 11. The conditioning pulse's parameters are
dependent upon the thickness of the layers comprising memory
element 103. Moreover, similar to the methods of operating memory
elements 101 and 102 depicted in FIGS. 2C-2F and 3C-3F, the method
of operating memory element 103 depicted in FIGS. 4C-4F, proceeds
in an analogous manner as described above with reference to FIGS.
2C-2F and 3C-3F.
[0119] It should be further appreciated that with regard to memory
elements 101, 102 and 103, the presence of an additional
metal-containing layer 7, which provides the Ag ions, can enhance
the switching characteristics of the memory elements 101, 102 and
103. For instance, since there are more available Ag ions to move
in and out of the preformed conducting channels 11, the memory
elements 101, 102 and 103 can operate with greater speed.
[0120] Although the embodiments described above in FIGS. 1A-4F,
refer to the formation of only one resistance variable memory
element 100, 101, 102 and 103, it must be understood that the
invention contemplates the formation of any number of such memory
elements. A plurality of resistance variable memory elements can be
fabricated in a memory array and operated with memory access
circuits. Thus, the resistance variable memory elements 100, 101,
102 and 103 can be utilized in many electronic devices.
Specifically, the methods and operation of the memory elements
disclosed herein, can be used in any device whenever it is desired
to have a resistance variable memory element with faster switching
times.
[0121] The resistance variable memory elements 100, 101, 102 and
103 of the invention may be used in memory applications as well as
in creating various CMOS type circuits.
[0122] The invention is not limited to the details of the
illustrated embodiments. Accordingly, the above description and
drawings are only to be considered illustrative of exemplary
embodiments which achieve the features and advantages of the
invention. Modifications and substitutions to specific methods,
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.
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