U.S. patent application number 09/941544 was filed with the patent office on 2003-03-13 for stoichiometry for chalcogenide glasses useful for memory devices and method of formation.
Invention is credited to Campbell, Kristy A..
Application Number | 20030047765 09/941544 |
Document ID | / |
Family ID | 25476667 |
Filed Date | 2003-03-13 |
United States Patent
Application |
20030047765 |
Kind Code |
A1 |
Campbell, Kristy A. |
March 13, 2003 |
Stoichiometry for chalcogenide glasses useful for memory devices
and method of formation
Abstract
A method of forming resistance changing elements with improved
operational characteristics for use in memory devices and the
resulting structures are disclosed. A chalcogenide glass having the
formula (Ge.sub.x1Se.sub.1-x1).sub.1-y1Ag.sub.y1, wherein
18.ltoreq.x.sub.1.ltore- q.28, or the formula
(Ge.sub.x2SE.sub.1-x2).sub.1-y2Ag.sub.y2, wherein
39.ltoreq.x.sub.2.ltoreq.42, and wherein in both the silver is in a
concentration which maintains the germanium selenide glass in the
glass forming region is used in a memory cell. The glass may also
have a glass transition temperature (Tg) near or higher than
typical temperatures used for fabricating and packaging memory
devices containing the memory cell.
Inventors: |
Campbell, Kristy A.; (Boise,
ID) |
Correspondence
Address: |
DICKSTEIN SHAPIRO MORIN & OSHINSKY LLP
2101 L STREET NW
WASHINGTON
DC
20037-1526
US
|
Family ID: |
25476667 |
Appl. No.: |
09/941544 |
Filed: |
August 30, 2001 |
Current U.S.
Class: |
257/298 ;
257/296; 257/300; 257/E45.002 |
Current CPC
Class: |
G11C 13/0004 20130101;
H01L 45/1233 20130101; H01L 45/085 20130101; H01L 45/1625 20130101;
H01L 45/1658 20130101; H01L 45/143 20130101; H01L 45/1616
20130101 |
Class at
Publication: |
257/298 ;
257/296; 257/300 |
International
Class: |
H01L 029/76; H01L
027/108; H01L 029/94; H01L 031/119 |
Claims
What is claimed as new and desired to be protected by Letters
Patent of the United States is:
1. A memory cell comprising: a chalcogenide glass doped with a
metal, said chalcogenide glass having a stoichiometry and a maxium
allowable amount of metal dopant which causes said chalcogenide
glass to remain in a glass forming region; a first electrode and a
second electrode in contact with said chalcogenide glass; and a
dendrite formed between said first and second electrodes when
voltage is applied to said first and second electrodes.
2. The memory cell of claim 1, wherein said chalcogenide glass has
a glass transition temperature which is about or higher than the
highest temperature used in the formation and packaging of a memory
device containing said memory cell.
3. The memory cell of claim 1, wherein said chalcogenide glass
comprises a material selected from the group consisting of oxygen,
sulfur, tellurium and selenium.
4. The memory cell of claim 3, wherein said chalcogenide glass
comprises oxygen.
5. The memory cell of claim 3, wherein said chalcogenide glass
comprises sulfur.
6. The memory cell of claim 3, wherein said chalcogenide glass
comprises tellurium.
7. The memory cell of claim 3, wherein said chalcogenide glass
comprises selenium.
8. The memory cell of claim 1, wherein said metal is selected from
the group consisting of silver, copper, platinum, gold, cadmium,
ruthenium, cobalt, zinc, chromium, maganese and nickel.
9. The memory cell of claim 1, wherein said chalcogenide glass is a
germanium selenide glass.
10. The memory cell of claim 9, wherein said chalcogenide glass is
a germanium selenide glass doped with silver.
11. A non-volatile memory cell comprising: a germanium selenide
glass doped with silver, said silver doping being in a
concentration which maintains said germanium selenide glass in the
glass forming region; a first electrode and a second electrode in
contact with said doped germanium selenide glass; and a dendrite
formed between said first and second electrodes when voltage is
applied to said first and second electrodes.
12. The non-volatile memory cell of claim 11, wherein said
germanium selenide glass has a glass transition temperature which
is about or higher than the highest temperature used in the
fabrication and packaging of a memory device containing said
non-volatile memory cell.
13. The non-volatile memory cell of claim 11, wherein said
germanium selenide glass comprises a material having the formula
(Ge.sub.x1Se.sub.1-x1).sub.1-y1Ag.sub.y1, wherein
18.ltoreq.x.sub.1.ltore- q.28.
14. The non-volatile memory cell of claim 13, wherein y.sub.1
represents a silver atomic percentage which is less than or equal
to that which approximately satisfies equation y.sub.1=19+15 *
sin[0.217*x.sub.1+3.23].
15. The non-volatile memory cell of claim 11, wherein said
germanium selenide glass comprises a material having the formula
(Ge.sub.x2Se.sub.1-x2).sub.1-y2Ag.sub.y2, wherein
39.ltoreq.x.sub.2.ltore- q.42.
16. The non-volatile memory cell of claim 15, wherein y.sub.2
represents a silver atomic percentage which is less than or equal
to that which approximately satisfies equation y.sub.2=21-11.5
*exp[-(ln(x.sub.2/44.4)/- (0.84).sup.2)].
17. A chalcogenide glass material having the formula
(Ge.sub.x1Se.sub.1-x1).sub.1-y1Ag.sub.y1, wherein
18.ltoreq.x.sub.1.ltore- q.28 and wherein said silver is in a
concentration which maintains said germanium selenide glass in the
glass forming region.
18. The chalcogenide glass material of claim 17, wherein
x.sub.1=23.
19. The chalcogenide glass material of claim 17, wherein
x.sub.1=25.
20. The chalcogenide glass material of claim 17, wherein
x.sub.1=20.
21. The chalcogenide glass material of claim 17, wherein y.sub.1
represents a silver atomic percentage which is less than or equal
to that which approximately satisfies equation y.sub.1=19+15 *
sin[0.217*x.sub.1+3.23].
22. A chalcogenide glass material having the formula
(Ge.sub.x2Se.sub.1-x2).sub.1-y2Ag.sub.y2, wherein
39.ltoreq.x.sub.2.ltore- q.42 and wherein said silver is in a
concentration which maintains said germanium selenide glass in the
glass forming region.
23. The chalcogenide glass material of claim 22, wherein y.sub.2
represents a silver atomic percentage which is less than or equal
to that which approximately satisfies equation y.sub.2=21-11.5
*exp[-(ln(x.sub.2/44.4)/(0.84).sup.2)].
24. A memory cell comprising: a germanium selenide glass having the
formula (Ge.sub.x1Se.sub.1-x1).sub.1-y1Ag.sub.y1,wherein
18.ltoreq.x.sub.1.ltoreq.28 and wherein said silver is in a
concentration which maintains said germanium selenide glass in the
glass forming region; and, at least two electrodes in contact with
said germanium selenide glass, said germanium selenide glass
forming a dendrite between at least two electrodes in response to a
voltage applied across said at least two electrodes.
25. A memory cell comprising: a germanium selenide glass having the
formula (Ge.sub.x2Se.sub.1-x2).sub.1-y2Ag.sub.y2, wherein
39.ltoreq.x.sub.2.ltoreq.42 and wherein said silver is in a
concentration which maintains said germanium selenide glass in the
glass forming region; and at least two electrodes in contact with
said germanium selenide glass, said germanium selenide glass
forming a dendrite between at least two electrodes in response to a
voltage applied across said at least two electrodes.
26. A method of forming a memory cell comprising the steps of:
providing a chalcogenide glass over a substrate; doping said
chalcogenide glass with a metal to form a doped chalcogenide glass,
said doped chalcogenide glass having a stoichiometry which causes
said doped chalcogenide glass to be in a glass forming region, said
doped chalcogenide glass having a glass transition temperature
which is about or higher than the highest temperature used in the
formation and packaging of a memory device containing said memory
cell; and, forming a plurality of electrodes in contact with said
doped chalcogenide glass.
27. A method of forming a memory cell comprising the steps of:
providing a germanium selenide glass having the formula
(Ge.sub.x1Se.sub.1-x1).sub.1-- y1Ag.sub.y1, wherein
18.ltoreq.x.sub.1.ltoreq.28 over a substrate, and wherein said
silver is in a concentration which maintains said germanium
selenide glass in the glass forming region; and, forming at least
two electrodes in contact with said germanium selenide glass at
locations which permit said glass to transition between high and
low resistance states in response to signals applied to said
electrodes.
28. The method of claim 27, wherein x.sub.1=23.
29. The method of claim 27, wherein x.sub.1=25.
30. The method of claim 27, wherein x.sub.1=20.
31. A method of forming a memory cell comprising the steps of:
providing a germanium selenide glass having the formula
(Ge.sub.x2Se.sub.1-x2).sub.1-- y2Ag.sub.y2, wherein
39.ltoreq.x.sub.2.ltoreq.42 and wherein said silver is in a
concentration which maintains said germanium selenide glass in the
glass forming region; and, forming at least two electrodes in
contact with said germanium selenide glass at locations which
permit said glass to transition between high and low resistance
states in response to signals applied to said electrodes.
32. A method of operating a memory cell comprising the steps of:
applying a voltage across a germanium selenide glass having the
formula (Ge.sub.x1Se.sub.1-x1).sub.1-y1Ag.sub.y1, wherein
18.ltoreq.x.sub.1.ltore- q.28 and wherein said silver is in a
concentration which maintains said germanium selenide glass in the
glass forming region, to change the resistance state of said
glass.
33. The method of claim 32, wherein x.sub.1=23.
34. The method of claim 32, wherein x.sub.1=25.
35. The method of claim 32, wherein x.sub.1=20.
36. A method of operating a memory cell comprising the steps of:
applying a voltage across a germanium selenide glass having the
formula (Ge.sub.x2Se.sub.1-x2).sub.1-y2Ag.sub.y2, wherein
39.ltoreq.x.sub.2.ltore- q.42 and wherein said silver is in a
concentration which maintains said germanium selenide glass in the
glass forming region, to change the resistance state of said
glass.
37. A processor system comprising: a processor; and an integrated
circuit coupled to said processor, at least one of said processor
and integrated circuit including a memory cell, said memory cell
comprising: a germanium selenide glass having the
formula(Ge.sub.x1Se.sub.1-x1).sub.1-y1Ag.sub.y1- , wherein
18.ltoreq.x.sub.1.ltoreq.28 and wherein said silver is in a
concentration which maintains said germanium selenide glass in the
glass forming region; and at least two electrodes in contact with
said doped germanium selenide glass, said germanium selenide glass
changing a resistance state in response to application of a voltage
across said at least two electrodes.
38. The processor system of claim 37, wherein said processor and
said integrated circuit are integrated on same chip.
39. The processor system of claim 37, wherein x.sub.1=23.
40. The processor system of claim 37, wherein x.sub.1=25.
41. The processor system of claim 37, wherein x.sub.1=20.
42. A processor system comprising: a processor; and an integrated
circuit coupled to said processor, at least one of said processor
and integrated circuit including a memory cell, said memory cell
comprising: a germanium selenide glass having the formula
(Ge.sub.x2Se.sub.1-x2).sub.1-y2Ag.sub.y- 2, wherein
39.ltoreq.x.sub.2.ltoreq.42 and wherein said silver is in a
concentration which maintains said germanium selenide glass in the
glass forming region; and at least two electrodes in contact with
said doped germanium selenide glass, said germanium selenide glass
changing a resistance state in response to application of a voltage
across said at least two electrodes.
43. The processor-based system of claim 42, wherein said processor
and said integrated circuit are integrated on same chip.
Description
FIELD OF THE INVENTION
[0001] The present invention relates to the field of semiconductor
devices formed using chalcogenide glasses.
BACKGROUND OF THE INVENTION
[0002] One type of integrated circuitry currently used in the
semiconductor industry comprises memory circuitry where information
is stored in the form of binary data. The circuitry can be either
volatile or non-volatile. Volatile storing memory devices result in
loss of data when power is interrupted. In contrast, non-volatile
memory circuitry retains the stored data even when power is
interrupted.
[0003] The operation of memory circuitry, and particularly that of
programmable metallization cells, has been disclosed in the Kozicki
et al. U.S. Pat. Nos. 5,761,115; 5,896,312; 5,914,893; and
6,084,796, the disclosures of which are incorporated by reference
herein. Such a cell includes an insulating dielectric material
disposed between opposing electrodes. A conductive material is
doped into the dielectric material. The resistance of such material
can be changed between highly insulative and highly conductive
states. In its normal high resistive state and to perform a write
operation, a voltage potential is applied across the opposing
electrodes. The electrode having the positive voltage applied
thereto functions as an anode, while the electrode held at a lower
potential functions as a cathode. The conductively-doped dielectric
material has the capability of undergoing a structural change at a
certain applied voltage. With such voltage applied, a conductive
dendrite or filament extends between the electrodes, effectively
interconnecting the top and bottom electrodes.
[0004] The dendrite remains when the voltage potentials are
removed. This way, the resistance of the conductively-doped
dielectric material between electrodes could drop by several orders
of magnitude. Such material can be returned to its highly resistive
state by reversing the voltage potential between the anode and
cathode, effectively disrupting the dendrite connection between the
top and bottom electrodes. Again, the highly resistive state is
maintained once the voltage potential is removed. This way, such a
device can function, for example, as a programmable memory
cell.
[0005] The preferred resistance-variable material received between
the electrodes typically comprises a chalcogenide material having
metal ions diffused therein. A specific example is germanium
selenide (Ge.sub.xSe.sub.1-x) diffused with silver (Ag) ions. One
method of diffusing the silver ions into the germanium selenide
material is to initially evaporate the germanium selenide glass and
then deposit a thin layer of silver upon the glass, for example by
sputtering, physical vapor deposition, or other known technique in
the art. The layer of silver is irradiated, preferably with
electromagnetic energy at a wavelength less than 600 nanometers, so
that the energy passes through the silver and to the silver/glass
interface, to break a chalcogenide bond of the chalcogenide
material. As a result, the glass is doped with silver. If, however,
too much silver is doped into the chalcogenide material, the
chalcogenide material changes from an amorphous state to a
crystalline one and, consequently, the operation of the
programmable memory cell is adversely affected.
[0006] When a chalcogenide glass is used in a memory device to
insure that its properties do not change during various processing
steps associated with fabrication of the memory device, the
chalcogenide glass must have a glass transition temperature (Tg)
which is about or higher than the fabrication and processing
temperatures used in the subsequent steps of memory device
fabrication. If the processing and/or packaging temperatures are
higher than the glass transition temperature, the amorphous state
of the chalcogenide material may change to a crystalline state or
the glass stoichiometry may change or the mean coordination number
of the glass may change and the operation of the memory cell
affected. As such, the glass stoichiometry of the chalcogenide
glass must be chosen so that the glass backbone (before and after
metal doping) and/or metal-doped glass has a glass transition
temperature which is about or higher than the processing
temperatures subsequent to the glass deposition or subsequent to
metal doping of the glass.
[0007] Accordingly, there is a need for a chalcogenide glass
material that will remain in a glass forming region when doped with
a metal such as silver and which allows maximization of subsequent
possible processing temperatures, as well as a method of forming
such a non-volatile memory element.
BRIEF SUMMARY OF THE INVENTION
[0008] The present invention provides a method of forming
non-volatile or semi-volatile memory elements using a metal doped
chalcogenide glass which has a stoichometry which keeps the glass
in the glass forming region. The glass also has a glass transition
temperature (Tg) which is about or higher than typical processing
and/or packaging temperatures used for memory device formation.
[0009] According to an exemplary embodiment of the present
invention, germanium selenide glasses for use as memory elements
are selected from a range of germanium selenide glasses having
stoichiometries that fall within a first stoichiometric range
R.sub.1 including Ge.sub.18Se.sub.82 (with a maximum atomic
percentage of Ag when doped of about 30% or less) continuously to
Ge.sub.28Se.sub.72 (with a maximum atomic percentage of Ag when
doped of about 20% or less) and which have the general formula
(Ge.sub.x1Se.sub.1-x1).sub.1-y1Ag.sub.y1, wherein
18.ltoreq.x.sub.1.ltore- q.28 and wherein y.sub.1 represents the
fit silver (Ag) atomic percentage which is the maximum amount which
will keep the glass in the glass forming region. Typically, y.sub.1
is less than or equal to that which approximately satisfies
equation (1):
y.sub.1=19+15*sin[0.217*x.sub.1+3.23] (1)
[0010] According to another embodiment of the present invention,
germanium selenide glasses for memory elements are selected from a
range of germanium-selenide glasses having stoichiometries that
fall within a second stoichiometric range R.sub.2 of doped
chalcogenide glasses including Ge.sub.39Se.sub.61 (with a maximum
atomic percentage of Ag when doped of about 20% or less)
continuously to Ge.sub.42Se.sub.58 (with a maximum atomic
percentage of Ag when doped of about 15% or less) and which have
the general formula (Ge.sub.x2Se.sub.1-x2).sub.1-y2Ag.sub.y2,
wherein 39.ltoreq.x.sub.2.ltoreq.42 and wherein y.sub.2 represents
the fit silver (Ag) atomic percentage which is the maximum amount
which will keep the glass in the glass forming region. Typically,
y.sub.1 is less than or equal to that which approximately satisfies
equation (2):
y.sub.2=21-11.5*exp[-(ln(x.sub.2/44.4)/(0.84).sup.2)]tm (2)
[0011] If the Ag-doped germanium selenide material has a
stoichiometry that falls within the first or second stoichiometric
range R1, R2, the doped germanium selenide glass will remain
amorphous enabling its use in a memory device. If, however, the
Ag-doped germanium selenide material has a stoichiometry that does
not fall within the first or second stoichiometric range R.sub.1,
R.sub.2, the doped germanium selenide glass becomes crystalline
precluding its use in a non-phase change-type memory device.
[0012] According to another embodiment of the present invention,
and to produce an optimum non-volatile memory cell, the doped
germanium selenide glass is selected to fall within the first or
second stoichiometric range R.sub.1, R.sub.2 and to have a glass
transition temperature (Tg) which is about or higher than the
highest processing and/or packaging temperatures used for memory
device formation.
BRIEF DESCRIPTION OF THE DRAWINGS
[0013] FIG. 1 illustrates a ternary phase diagram showing glass
forming regions for (GE.sub.xSe.sub.1-x).sub.1-yAg.sub.y
compounds.
[0014] FIG. 2 illustrates a cross-sectional view of the early
stages of fabrication of a a memory device in accordance with an
embodiment of the present invention.
[0015] FIG. 3 illustrates a cross-sectional view of the memory
device of FIG. 2 at a stage of processing subsequent to that shown
in FIG. 2.
[0016] FIG. 4 illustrates a cross-sectional view of the memory
device of FIG. 2 at a stage of processing subsequent to that shown
in FIG. 3.
[0017] FIG. 5 illustrates a cross-sectional view of the memory
device of FIG. 2 at a stage of processing subsequent to that shown
in FIG. 4.
[0018] FIG. 6 illustrates a cross-sectional view of the memory
device of FIG. 2 at a stage of processing subsequent to that shown
in FIG. 5.
[0019] FIG. 7 illustrates a cross-sectional view of the memory
device of FIG. 2 at a stage of processing subsequent to that shown
in FIG. 6.
[0020] FIG. 8 illustrates a cross-sectional view of the memory
device of FIG. 2 at a stage of processing subsequent to that shown
in FIG. 7.
[0021] FIG. 9 illustrates a cross-sectional view of the memory
device of FIG. 2 at a stage of processing subsequent to that shown
in FIG. 8.
[0022] FIG. 10 illustrates a cross-sectional view of the memory
device of FIG. 2 at a stage of processing subsequent to that shown
in FIG. 9.
[0023] FIG. 11 illustrates a computer system having a memory cell
formed according to the present invention.
DETAILED DESCRIPTION OF THE INVENTION
[0024] In the following detailed description, reference is made to
various specific embodiments in which the invention may be
practiced. These embodiments are described with sufficient detail
to enable those skilled in the art to practice the invention, and
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.
[0025] The term "silver" is intended to include not only elemental
silver, but silver with other trace metals or in various alloyed
combinations with other metals as known in the semiconductor
industry, as long as such silver alloy is conductive, and as long
as the physical and electrical properties of the silver remain
unchanged. Similarly, the terms "germanium" and "selenium" are
intended to include not only elemental germanium and selenium, but
germanium and selenium with other trace metals or in various
alloyed combinations with other metals as known in the
semiconductor industry, as long as the physical and electrical
properties of the germanium or selenium remain unchanged.
[0026] The term "non-volatile memory device" is intended to include
not only non-volatile memory device but also semi-volatile memory
devices and any memory device which is capable of maintaining its
memory state after power is removed from the device.
[0027] The present invention relates to a method of forming
non-volatile memory elements under varying glass stoichiometries.
The formation of a non-volatile memory device 100 (FIG. 10)
including a memory element which comprises a doped chalcogenide
glass having a selected stoichiometry will be explained below with
reference to FIGS. 2-10. For a better understanding of the
invention, however, the selection of a chalcogenide glass having a
stoichiometry selected in accordance with the present invention and
which is employed in the non-volatile memory device 100 (FIG. 10)
is first explained below with reference to FIG. 1.
[0028] Referring to the drawings, where like elements are
designated by like reference numerals, FIG. 1 illustrates a ternary
phase diagram 200 showing glass forming regions for
(Ge.sub.xSe.sub.1-x).sub.1-yAg.sub.y compounds, as studied by
Mitkova et al. in Dual Chemical Role of Ag as an Additive in
Chalcogenide Glasses, Phys. Rev. Letters, Vol. 83, No 19 (Nov.
1999), the disclosure of which is incorporated by reference herein.
According to Mitkova et al. and as shown in FIG. 1, ternary
(Ge.sub.xSe.sub.1-x).sub.1-yAg.sub.y glasses which comprise
germanium selenide glasses with silver (Ag) as an additive form in
two distinct compositional regions: a selenium-rich region labeled
region I (FIG. 1) and a germanium-rich region labeled region II
(FIG. 1). As also shown in FIG. 1, a corridor 88 separates the
selenium-rich region I from the germanium-rich region II. Mitkova
et al. mentions that no bulk glass formation occurs along the
corridor 88 until the silver (Ag) concentration exceeds
y.gtoreq.0.2 and the two selenium-rich and germanium-rich regions I
and II coalesce.
[0029] The study conducted by Mitkova et al. concluded that silver
(Ag) acts as a "network modifier" for the glass compositions of the
selenium-rich region I (FIG. 1). In this region, silver (Ag) phase
separates into an Ag.sub.2Se-rich phase and a Ge.sub.tSe.sub.1-t
phase which has less Se than the starting Ge.sub.xSe.sub.1-x
material. In contrast, in the germanium-rich region II (FIG. 1),
silver (Ag) acts as a "network former" for glass compositions,
forms part of the backbone and reduces the connectivity of the
glass.
[0030] Referring back to FIG. 1 and further analyzing the ternary
phase diagram with glass forming regions for
(Ge.sub.xSe.sub.1-x).sub.1-yAg.sub- .y compounds, Applicant has
discovered that the careful selection of the stoichiometry of a
chalcogenide glass is directly correlated to the ability of the
doped chalcogenide glass to maintain an amorphous state over a wide
and continuous range of dopant metal concentrations and, therefore,
to successfully function as a programmable memory cell.
[0031] Accordingly, Applicant has discovered that, contrary to
current belief in the semiconductor art, not all doped germanium
selenide glass stoichiometries could be successfully used as
non-volatile or semi-volatile memory devices. Applicant has
discovered that optimization of a doped germanium selenide glass
for switching operations in memory devices requires the doped
germanium selenide glass all fully within a glass forming region,
such as the glass forming regions I and II of FIG. 1 of Mitkova et
al. In addition, for use in a memory device, the germanium selenide
glass must have a glass transition temperature (Tg) high enough to
allow the doped germanium selenide glass to withstand temperatures
of subsequent wafer processing and/or chip packaging processes, for
example wire bonding or encapsulation.
[0032] According to the present invention, germanium selenide glass
compositions capable of creating functional non-volatile memory
devices require glass stoichiometries to fall in one of the
following two ranges:
[0033] a first stoichiometric range R.sub.1 including
Ge.sub.18Se.sub.82 (with a maximum atomic percentage of Ag when
doped of about 30% or less) continuously to Ge.sub.28Se.sub.72
(with a maximum atomic percentage of Ag when doped of about 20% or
less) and which have the general formula
(Ge.sub.x1Se.sub.1-x1).sub.1-y1Ag.sub.y1, wherein
18.ltoreq.x.sub.1.ltore- q.28 and wherein y.sub.1 represents the
fit silver (Ag) atomic percentage which is the maximum amount which
will keep the glass in the glass forming region. Typically, y.sub.1
is less than or equal to that which approximately satisfies
equation (1):
y.sub.1=19+15*sin[0.217*x.sub.1+3.23] (2)
[0034] or
[0035] a second stoichiometric range R.sub.2 of doped chalcogenide
glasses including Ge.sub.39Se.sub.61 (with a maximum atomic
percentage of Ag when doped of about 20% or less) continuously to
Ge.sub.42Se.sub.58 (with a maximum atomic percentage of Ag when
doped of about 15% or less) and which have the general formula
(Ge.sub.x2Se.sub.1-x2).sub.1-y2Ag.sub.y2, wherein
39.ltoreq.x.sub.2.ltoreq.42 and wherein y.sub.2 represents the fit
silver (Ag) atomic percentage which is the maximum amount which
will keep the glass in the glass forming region. Typically, y.sub.1
is less than or equal to that which approximately satisfies
equation (2):
y.sub.2=21-11.5*exp[-(ln(x.sub.2/44.4)/(0.84).sup.2)] (2)
[0036] For example, germanium selenide glasses having a selenium
(Se) composition of about 62% to about 71% will not be able to form
functional memory devices as the doped glass falls within the
coridor 88 (FIG. 1) and out of the first and second stoichiometric
ranges R.sub.1, R.sub.2, described above. For example, a memory
device using a doped germanium selenide glass having a selenium
(Se) composition of about 63.5% and a silver (Ag) doping between
about 7% to about 22% fails after one write/erase data retention
cycle.
[0037] Similarly, germanium selenide glasses having a selenium (Se)
composition greater than about 82% will also not be able to form
functional memory devices, as they fall out of the first and second
stoichiometric ranges R.sub.1, R.sub.2 described above, when the
amount of silver (Ag) dopant is sufficient for the switching
operation. Doped chalcogenide germanium selenide glasses having a
selenium (Se) composition less than about 58% will also be
incapable of forming functional memory devices since the maximum
amount of silver (Ag) dopant allowable to remain in glass forming
region R.sub.2 is insufficient for the switching operation (the
maximum silver atomic percentage is lower than about 7%).
[0038] The following Table 1 is a compilation of data on
silver-doped germanium selenide glasses used as non-volatile memory
cells obtained by the Applicant. Carefully choosing the
stoichiometry of the silver-doped germanium selenide glass to fall
either within the first or second stoichiometric range R.sub.1,
R.sub.2 described above allows the silver-doped germanium selenide
glass to function as a non-volatile memory cell. This is because,
above 82% Se, the maximum allowable Ag falls rapidly to less than
10% Ag allowed to remain in the glass forming region. This amount
of Ag is insufficient to obtain good electrical switching.
1 TABLE 1 Edge Glass forming region Ag Ag-doped glass in Lot #, Wfr
# at. % Se at. % Ge Functional? ternary phase diagram glass forming
region? 0641274, wfr 5 63.5 36.5 Initially Yes; Fails >.about.7
and <.about.22 at. % Ag Only initially. after a write/erase data
retention cycle. 0440263, wfr 14 68.5 31.5 No. Poor write erase
>.about.18 and <.about.30 at. % Ag No. characteristics and
limited data retention. 0540868, wfr 5 83.9 16.1 No. Devices would
<.about.10 at. % Ag No. not switch, remaining in low resistance
state characteristic of too much Ag in the glass. 0440263, wfr 12
85.2 14.8 No. Devices that <.about.7 at. % Ag No. would write
were threshold switches. 0440263, wfr 8 80 20 Yes. Good write up to
34 at. % Ag Yes. characteristics and data retention. Good
subsequent erases. 1344272, wfr 1 77 23 Yes. Good write and up to
.about.33 at. % Ag Yes. erase characteristics. 2349273, wfr 7 75 25
Yes. Good write and up to .about.33 at. % Ag Yes. erase
characteristics
[0039] The Ge.sub.20Se.sub.80 glass doped with Ag up to 34% (Table
1) falls entirely within the first stoichiometric range R.sub.1 of
doped germanium selenide glasses and, therefore, falls within the
glass forming regions of the present invention. Memory cells
employing such Ge.sub.20Se.sub.80 glass doped with Ag up to 34%
exhibit good write/erase characteristics and are fully functional.
Additionally, memory cells with Ge.sub.23Se.sub.77 and
Ge.sub.25Se.sub.75 doped with up to 33% Ag exhibit good write/erase
characteristics. In contrast, the first four silver-doped germanium
selenide compositions of Table 1 fall out of the glass forming
regions when doped with an adequate amount of Ag for good
electrical switching. Accordingly, the memory cells which use such
silver-doped germanium selenide compositions are all non-functional
because the devices do not switch and/or have poor write/erase
characteristics. Out of the four non-functional silver-doped
germanium selenide compositions of Table 1, only the
Ge.sub.36.5Se.sub.63.5 doped with silver (Ag) with an atomic
percentage greater that about 7% but smaller than about 22%
exhibits initially good write/erase characteristics, but fails
after one cycle.
[0040] The data of Table 1 supports Applicant's observation that
functional non-volatile memory devices based on a doped germanium
selenide glass composition require such glass composition to have a
particular stoichiometry that falls within one of the first or
second stoichiometric range R.sub.1, R.sub.2 described above.
However, as noted above, optimization of functional memory devices
based on doped germanium selenide glasses requires glass transition
temperatures (Tg) that allow the doped germanium selenide glasses
to withstand temperatures for conventional fabrication and/or
packaging processes, for example, wire bonding or encapsulation.
Thus, in accordance with an embodiment of the present invention,
germanium selenide glasses for non-volatile or semi-volatile memory
devices have stoichiometries that fall within the two
stoichiometric ranges R.sub.1, R.sub.2 described above, and have
also a glass transition temperature (Tg) which is about or higher
than the processing and/or packaging temperatures.
[0041] Table 2 lists glass transition temperatures (Tg) measured
for nine germanium selenide chalcogenide glasses:
2TABLE 2 Tg at. % Ge at. % Se 107.39 12 88 165.54 18 82 183.91 20
80 209.37 23 77 228.57 24 76 249.1 25 75 334.83 30 70 415.76 33 67
346.67 40 60
[0042] Typical temperatures for packaging of non-volatile memory
devices are of about 170.degree. C. to about 190.degree. C. (e.g.,
for encapsulation) and can be as high as 230.degree. C. (e.g., for
wire bonding). Typical processing steps during the fabrication of
such non-volatile memory devices, for example photoresist and/or
nitride deposition processes, can also take place at temperatures
of about 200.degree. C. Accordingly, to obtain a viable
chalcogenide glass composition for a memory cell of a memory
device, the stoichiometry must fall within the first or second
stoichiometric ranges R.sub.1, R.sub.2 discussed above and must
have a glass transition temperature (Tg) which is about or higher
than the highest packaging and/or processing temperatures used
during the formation of the memory device or of the packaging of
the memory device itself. This way, the selection of a germanium
selenide glass for a functional memory cell accounts for both a
stoichiometry that falls within glass forming regions and an
adequate glass transition temperature (Tg). For example, a
Ge.sub.25Se.sub.75 glass is a good candidate for a non-volatile
memory device because the Ge.sub.25Se.sub.75 glass falls within the
first stoichiometric range R.sub.1 described above and it also has
a glass transition temperature (Tg) of about 250.degree. C. Another
good candidate is a Ge.sub.40Se.sub.60 glass because it also falls
within the second stoichiometric range R.sub.2 described above and
has a glass transition temperature (Tg) of about 347.degree. C.
[0043] Reference is now made to FIGS. 2-10 which illustrate an
exemplary embodiment of a non-volatile memory device 100 (FIG. 10)
using a doped germanium selenide glass selected in accordance with
the present invention. FIG. 2 depicts a portion of an insulating
layer 12 formed over a semiconductor substrate 10. The insulating
layer 12 may be formed by any known deposition methods, such as
sputtering by chemical vapor deposition (CVD), plasma enhanced CVD
(PECVD) or physical vapor deposition (PVD), among others. The
insulating layer 12 may be formed of a conventional insulating
oxide, such as silicon oxide (SiO.sub.2), a silicon nitride
(Si.sub.3N.sub.4), or a low dielectric constant material, among
many others.
[0044] A first electrode 14 is next formed over the insulating
layer 12, as also illustrated in FIG. 2. The first electrode 14
comprises any conductive material, for example, tungsten, tantalum,
titanium, platinum, or silver, among many others. A dielectric
layer 15 (FIG. 2) is next formed over the first electrode 14. The
dielectric layer 15 may comprise similar materials to those
described above with reference to the insulating layer 12.
[0045] Referring now to FIG. 3, an opening 13 is formed in the
dielectric layer 15 and extending to the first electrode 14. The
opening 13 may be formed by known methods of the art, for example,
by a conventional patterning and etching process. A chalcogenide
glass 17 is next formed over the dielectric layer 15, to fill in
the opening 13, as shown in FIG. 4.
[0046] According to an embodiment of the present invention, the
chalcogenide glass 17 is a germanium selenide glass having a
Ge.sub.23Se.sub.77 stoichiometry that falls within the first
stoichiometric range R.sub.1 and within a glass forming region of
the present invention. The formation of the germanium selenide
glass 17 with Ge.sub.23Se.sub.77 stoichiometry in accordance with
one exemplary embodiment may be accomplished by evaporating a
germanium selenide glass which has been synthesized with the exact
stoichiometries, i.e. 23% germanium and 77% selenium. In accordance
with another exemplary embodiment, the germanium selenide glass 17
with Ge.sub.23Se.sub.77 stoichiometry is formed by co-sputtering
germanium and selenium in the appropriate ratios, or by sputtering
using a Ge.sub.23Se.sub.77 target. In yet another embodiment of the
invention, the germanium selenide glass 17 with Ge.sub.23Se.sub.77
stoichiometry is formed by 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 Ge.sub.23Se.sub.77
film.
[0047] Once the germanium selenide glass 17 with the desired
stoichiometry has been formed, the doping concentration of the
silver dopant is selected with a maximum concentration in
accordance with the ternary phase diagram of FIG. 1 and the
equations (1) and (2) outlined above. Accordingly, for the
germanium selenide glass 17 with a Ge.sub.23Se.sub.77
stoichiometry, the maximum silver doping is about 33%.
[0048] Referring now to FIG. 5, incorporation of silver into the
Ge.sub.23Se.sub.77 glass 17 may be accomplished by photodoping,
that is depositing a thin layer 18 comprising silver, preferably
predominantly elemental silver, over the Ge.sub.23Se.sub.77 glass
17 and then "driving" the silver atoms within the
Ge.sub.23Se.sub.77 glass by using light (FIG. 6), or by
co-sputtering with Ag, Ge and Se, or Ag and a Ge.sub.23Se.sub.77
target, or Ag.sub.2Se and Ge.sub.xSe.sub.1-x. The thickness of the
layer 18 comprising silver is selected so that, when the silver is
subsequently diffused into the germanium selenide glass layer 17,
the atomic percentage of Ag in resulting silver-doped chalcogenide
glass 20 (FIG. 7) will allow such glass to fall within a glass
forming region R.sub.1 or R.sub.2.
[0049] Depending upon the glass stoichiometry, the silver atoms
will either incorporate themselves into the glass backbone (the
Ge--Se structure) or react with Se to form Ag.sub.2Se, leaving
behind a silver-doped germanium selenide glass 20 (FIG. 7) with a
new Ge--Se stoichiometry. Thus, when about 33% of silver is
incorporated into the Ge.sub.23Se.sub.77 glass, the system phase
separates into an Ag.sub.2Se phase and a Ge.sub.30Se.sub.70
backbone glass.
[0050] As mentioned above, the proper selection of the germanium
selenide glass for the memory element 100 (FIG. 10) requires the
doped germanium selenide glass to fall within the glass forming
region and to have a glass transition temperature (Tg) which is
about or higher than the highest fabrication and/or packaging
processing temperatures. Thus, for the exemplary embodiment
described above, the silver-doped Ge.sub.23Se.sub.77 glass 20 of
the memory device 100 (FIG. 10) can withstand processing
temperatures at least as high as about 210.degree. C.
[0051] Referring now to FIG. 8, a second conductive electrode
material 16 is formed over the doped germanium selenide glass 20.
The second conductive electrode material 16 may comprise any
electrical conductive material, for example, tungsten, tantalum,
titanium, or silver, among many others, as long as it is a
different material than the first electrode 14.
[0052] After the formation of the second conductive electrode
material 16 (FIG. 8), further steps to create a functional memory
cell may be carried out. Patterning by photolithography, for
example, may be employed to produce memory element 20a and second
electrode 16a, illustrated in FIG. 9. Referring now to FIG. 10, one
or more dielectric layers 30 are formed over the second electrode
16a and the dielectric layer 15 to complete the formation of the
non-volatile memory device 100 (FIG. 10). Conventional processing
steps can be further carried out to electrically couple the second
electrode 16a to various circuits of memory arrays. Alternatively,
additional multilevel interconnect layers and associated dielectric
layers could be formed from the memory cell 100 to appropriate
regions of the substrate 10, as desired.
[0053] Although only two electrodes 14, 16a are shown in FIGS.
2-10, it must be readily apparent to those skilled in the art that
in fact any number of such electrodes may be formed. In addition,
although the embodiments described above refer to the formation of
only one non-volatile memory cell 100, it must be understood that
the present invention contemplates the formation of any number of
such non-volatile memory cells.
[0054] Although an exemplary memory cell fabrication has been
described above using a Ge.sub.23Se.sub.77 composition, other Ge/Se
stoichiometries for the glass composition within the R1, R2 ranges
described above, besides Ge.sub.23Se.sub.77, can be used. For
example, Ge.sub.25Se.sub.75 and Ge.sub.20Se.sub.80 compositions
have been found to be particularly good compositions for memory
cell fabrication.
[0055] Although the present invention has been explained with
reference to the formation of a doped germanium selenide glass with
a stoichiometry selected according to the present invention, the
invention is not limited to this embodiment and has applicability
to other chalcogenide glasses. Accordingly, the stoichiometry of
any chalcogenide glass comprising any one of oxygen (O), sulfur
(S), selenium (Se) and tellurium (Te) and doped with a metal dopant
may be selected so that the doped chalcogenide glass maintains an
amorphous state over a wide and continuous range of dopant metal
concentrations. Thus, the present invention contemplates any doped
chalcogenide glass that falls fully within a glass-forming region
(corresponding to a respective ternary phase diagram for a
particular chalcogenide glass) and has a glass transition
temperature (Tg) which is about or higher than the highest
processing temperature for memory device fabrication.
[0056] Further, although the invention has been explained with
reference to the formation of a germanium selenide glass doped with
silver, other dopants may be used also, depending on the device
characteristics and as desired. Thus, the invention also
contemplates chalcogenide glasses doped with copper, platinum,
gold, silver, cadmium, iridium, ruthenium, cobalt, chromium,
maganese or nickel, among many others.
[0057] A typical processor-based system 400 which includes a memory
circuit 448, for example a PCRAM, one or both of which contain
non-volatile or semi-volatile memory cells, such as the
non-volatile memory cell 100 according to the present invention is
illustrated in FIG. 11. 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 448
communicates with the system over bus 452.
[0058] In the case of a computer system, the processor system may
include peripheral devices such as a floppy disk drive 454 and a
compact disk (CD) ROM drive 456 which also communicate with CPU 444
over the bus 452. Memory 448 is preferably constructed as an
integrated circuit, which includes one or more non-volatile memory
cells 100. If desired, the memory 448 may be combined with the
processor, for example CPU 444, in a single integrated circuit.
[0059] The above description and drawings are only to be considered
illustrative of exemplary embodiments which achieve the features
and advantages of the present invention. Modification and
substitutions to specific process conditions and structures can be
made without departing from the spirit and scope of the present
invention. Accordingly, the invention is not to be considered as
being limited by the foregoing description and drawings, but is
only limited by the scope of the appended claims.
* * * * *