U.S. patent application number 12/795415 was filed with the patent office on 2010-09-23 for phase change memory device using carbon nanotube.
This patent application is currently assigned to Korea Advanced Institute of Science & Technology. Invention is credited to YANG-KYU CHOI, KUK-HWAN KIM.
Application Number | 20100237318 12/795415 |
Document ID | / |
Family ID | 38014828 |
Filed Date | 2010-09-23 |
United States Patent
Application |
20100237318 |
Kind Code |
A1 |
CHOI; YANG-KYU ; et
al. |
September 23, 2010 |
PHASE CHANGE MEMORY DEVICE USING CARBON NANOTUBE
Abstract
Provided are a phase change memory device that can operate at
low power and improve the scale of integration by reducing a
contact area between a phase change material and a bottom
electrode, and a method for fabricating the same. The phase change
memory comprises a current source electrode, a phase change
material layer, a plurality of carbon nanotube electrodes, and an
insulation layer. The current source electrode supplies external
current to a target. The phase change material layer is disposed to
face the current source electrode in side direction. The carbon
nanotube electrodes are disposed between the current source
electrode and the phase change material layer. The insulation layer
is formed outside the carbon nanotube electrodes and functions to
reduce the loss of heat generated at the carbon nanotube
electrodes.
Inventors: |
CHOI; YANG-KYU; (Daejeon,
KR) ; KIM; KUK-HWAN; (Daejeon, KR) |
Correspondence
Address: |
FOLEY AND LARDNER LLP;SUITE 500
3000 K STREET NW
WASHINGTON
DC
20007
US
|
Assignee: |
Korea Advanced Institute of Science
& Technology
|
Family ID: |
38014828 |
Appl. No.: |
12/795415 |
Filed: |
June 7, 2010 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
|
|
11610341 |
Dec 13, 2006 |
7749801 |
|
|
12795415 |
|
|
|
|
Current U.S.
Class: |
257/4 ;
257/E45.001; 977/750 |
Current CPC
Class: |
Y10S 977/70 20130101;
H01L 45/126 20130101; H01L 45/16 20130101; H01L 45/1233 20130101;
G11C 13/025 20130101; Y10S 977/752 20130101; B82Y 10/00 20130101;
H01L 45/06 20130101; G11C 13/0004 20130101; Y10S 977/75 20130101;
Y10S 977/742 20130101 |
Class at
Publication: |
257/4 ; 977/750;
257/E45.001 |
International
Class: |
H01L 45/00 20060101
H01L045/00 |
Foreign Application Data
Date |
Code |
Application Number |
Jan 5, 2006 |
KR |
10-2006-001336 |
Claims
1. A phase change memory device using carbon nanotubes comprising:
a current source electrode supplying external current to a target;
a phase change material layer disposed to face the current source
electrode in side direction; a plurality of carbon nanotube
electrodes disposed between the current source electrode and the
phase change material layer; and an insulation layer formed outside
the carbon nanotube electrodes.
2. The phase change memory device of claim 1, wherein each of the
carbon nanotube electrodes has a diameter ranging from
approximately 1 nm to 100 nm.
3. The phase change memory device of claim 1, wherein the carbon
nanotube electrodes are formed in a single wall type.
4. A phase change memory device using carbon nanotubes comprising:
a current source electrode supplying external current to a target;
a phase change material layer disposed to face the current source
electrode in side direction; a plurality of carbon nanotube
electrodes disposed between the current source electrode and the
phase change material layer, one portion of the carbon nanotube
electrodes extending to overlap with the phase change material
layer; and an insulation layer formed outside the carbon nanotube
electrodes.
5. The phase change memory device of claim 4, wherein each of the
carbon nanotube electrodes has a diameter ranging from
approximately 1 nm to 100 nm.
6. The phase change memory device of claim 4, wherein the carbon
nanotube electrodes have an overlapping length with the phase
change material layer in a range of approximately 1/10 to 8/10 of
the total length of the carbon nanotube electrodes.
7. The phase change memory device of claim 4, wherein the carbon
nanotube electrodes are formed in a single wall type.
8. A phase change memory device using carbon nanotubes comprising:
a current source electrode supplying external current to a target;
a phase change material layer disposed to face the current source
electrode in side direction; a plurality of carbon nanotube
electrodes disposed between the current source electrode and the
phase change material layer; an insulation layer formed outside the
carbon nanotube electrodes; and a heat generating resistance layer
disposed between the carbon nanotube electrodes and the phase
change material layer in contact with the carbon nanotube
electrodes.
9. The phase change memory device of claim 8, wherein each of the
carbon nanotube electrodes has a diameter ranging from
approximately 1 nm to 100 nm.
10. The phase change memory device of claim 8, wherein the carbon
nanotube electrodes are formed in a single wall type.
Description
CROSS-REFERENCE TO RELATED PATENT APPLICATIONS
[0001] This application is a division of application Ser. No.
11/610,341, filed Dec. 13, 2006, now pending, and based on South
Korean Patent Application No. 10-2006-001336, filed Jan. 5, 2006,
by Yang-Kyu Choi and Kuk-Hwan Kim, which are incorporated herein by
reference in their entirety. This application claims only subject
matter disclosed in the parent application and therefore presents
no new matter.
BACKGROUND OF THE INVENTION
[0002] 1. Field
[0003] The present invention relates to a phase change memory
device, and more particularly, to a phase change memory device
using a carbon nanotube to allow operation at low power and large
scale of integration, and a method for fabricating the same.
[0004] 2. Description of the Background Art
[0005] A phase change memory device is a type of memory that stores
information using an electrical conductivity difference between a
crystalline phase and an amorphous phase of a specific
material.
[0006] Such a phase change memory device has received a great
attention as a next generation non-volatile memory due to its
unique characteristics such as large threshold voltage margin,
rapid operation speed, excellent durability, and long data
retention time. Furthermore, many researchers have recently
reported about successful mass production of phase change memory
devices that can be scaled equivalent to commercial flash memory
devices.
[0007] The size of memory cells and operation of memory cells need
to be maintained uniform in order for phase change memory devices
to become a major next generation memory. The magnitude of
operation current affects the size of the memory cells, and a
contact area between a phase change material and a bottom electrode
affects the magnitude of the necessary operation current.
Therefore, the contact area between the phase change material and
the bottom electrode is aimed to be reduced so as to have high
density of the operation current with a small amount of operation
current.
[0008] However, when etching is performed to form a bottom
electrode in a phase change memory device, it is usually difficult
to form a contact between the bottom electrode and a phase change
material with a uniform diameter. Also, the size of the contact
needs to be small to have high current density; however, in
addition to the difficulty in obtaining the uniform contact size,
downsizing the contact is another limitation in fabricating highly
integrated phase change memory devices. As a result, improving
reliability of phase change memory devices and the scale of
integration may be limited.
[0009] Two approaches are suggested to achieve the large scale of
integration in phase change memory devices based on the reduction
in the size of the memory cells.
[0010] First, the size of the memory cells of the phase change
memory devices can be reduced by reducing the contact area between
the phase change material and the bottom electrode.
[0011] Second, resistance of the bottom electrode, which acts as a
heating material, is increased to generate a large amount of heat
under the same current density. As a result of this heat
generation, the size of the memory cells of the phase change memory
devices can be reduced. According to the known Joule definition,
heat transferred to the phase change material is proportional to
the resistance of a heat generating material and to the square of
an amount of current flowing through the heat generating
material.
[0012] On the basis of the above facts, a structure of a typical
phase change memory device will be described hereinafter.
[0013] Small openings are formed in a bottom portion of a phase
change material, and a bottom electrode, which is a heat generating
material, fills the openings. As a result, the contact area between
the bottom electrode and the phase change material is
two-dimensional. In the typical phase change memory device, since
operation current supplied from outside flows widely as much as the
contact area, it is often difficult to obtain an amount of heat
sufficient to cause a phase transition.
[0014] Hence, a method of forming a ring-type contact is introduced
to overcome the above difficulty. In a phase change memory device
using this ring-type contact, a heat generating material, i.e., the
bottom electrode, fills only the surface of small openings, and an
insulation material fills the rest of the small openings.
[0015] FIG. 1a is a perspective view of a typical phase change
memory device structure. FIG. 1b is a sectional view of the typical
phase change memory device structure cut in a 1b-1b' direction
illustrated in FIG. 1a. FIG. 1c is a top view of a bottom electrode
of the typical phase change memory device.
[0016] Referring to FIG. 1a, in the typical ring-type phase change
memory device, an external current source electrode 101 that
supplies external current to a target and a phase change material
layer 105 that shows the characteristics of the phase change memory
device face to each other in side direction. A bottom electrode
102, which is a heat generating material, is formed in a ring shape
between the external current source electrode 101 and the phase
change material layer 105. An insulation material 103 fills the
inside of the bottom electrode 102 in the form of a circle to
prevent loss of heat outside. A dielectric material 104 encompasses
the bottom electrode 102 and the insulation material 103.
[0017] In the typical ring-type phase change memory device, the
phase change material layer 105 and the bottom electrode 102
contact with each other one-dimensionally in circumference. Thus,
as compared with the typical phase change memory device showing the
two-dimensional surface contact between the phase change material
and the bottom electrode, the ring-type phase change memory device
can have high density of operation current even with a small amount
of the operation current. As mentioned above, since the insulation
material 103 encompasses the bottom electrode 102, the loss of heat
generated at the bottom electrode 102 can be blocked.
[0018] However, the bottom electrode 102 in the ring-type phase
change memory device needs to fill inside of the small openings,
and thus, a material for the bottom electrode 102 is selected with
limitation. Also, despite of the ring formation method, the scale
of integration in the ring-type phase change memory device is
usually 50% of that of a currently fabricated flash memory. Hence,
even with this ring-type phase change memory device structure,
achieving the same or greater scale of integration is limited.
[0019] Accordingly, another phase change memory device structure
that requires a small amount of operation current needs to be
developed.
SUMMARY OF THE INVENTION
[0020] Accordingly, the present invention is directed to solve at
least the limitations and disadvantages of the background art.
[0021] One embodiment of the present invention is directed to
provide a phase change memory device using a carbon nanotube that
can operate at low power and improve the scale of integration by
reducing a contact area between a phase change material and a
bottom electrode.
[0022] Another embodiment of the present invention is directed to
provide a method for fabricating a phase change memory device using
a carbon nanotube in which a contact area between a phase change
material and a bottom electrode is reduced.
[0023] According to one embodiment of the present invention, there
is provided a phase change memory using carbon nanotubes comprising
a current source electrode supplying external current to a target,
a phase change material layer disposed to face the current source
electrode in side direction, a plurality of carbon nanotube
electrodes disposed between the current source electrode and the
phase change material layer, and an insulation layer formed outside
the carbon nanotube electrodes.
[0024] Consistent with the embodiment of the present invention,
each of the carbon nanotube electrodes may have a diameter ranging
from approximately 1 nm to 100 nm.
[0025] Consistent with the embodiment of the present invention, the
carbon nanotube electrodes may be formed in a single wall type.
[0026] According to another embodiment of the present invention,
there is provided a method for fabricating a phase change memory
device using carbon nanotubes, the method comprising disposing a
catalyst for forming a plurality of carbon nanotubes over
predetermined regions of a current source electrode supplying
external current to a target, growing the carbon nanotubes in
vertical direction using the catalyst as a seed to form carbon
nanotube electrodes, depositing an insulation layer over the
current source electrode in a manner to cover the carbon nanotube
electrodes, polishing the insulation layer until flush with the
carbon nanotube electrodes, and forming a phase change material
layer over the planarized insulation layer in contact with the
carbon nanotube electrodes.
[0027] Consistent with the other embodiment of the present
invention, disposing the catalyst over the predetermined regions of
the current source electrode may comprise forming the catalyst
using one selected from a group consisting of Fe.sub.2O.sub.3, Pt,
Co, Ni, Ti, Mo, and a combination thereof.
[0028] Consistent with the other embodiment of the present
invention, growing the carbon nanotubes in vertical direction using
the catalyst as the seed may comprise forming the carbon nanotube
electrodes in a single wall type.
[0029] Consistent with the other embodiment of the present
invention, growing the carbon nanotubes in vertical direction using
the catalyst as the seed may comprise forming the carbon nanotube
electrodes to have a diameter ranging from approximately 1 nm to
100 nm.
[0030] According to still another embodiment of the present
invention, there is provided a phase change memory using carbon
nanotubes comprising a current source electrode supplying external
current to a target, a phase change material layer disposed to face
the current source electrode in side direction, a plurality of
carbon nanotube electrodes disposed between the current source
electrode and the phase change material layer, one portion of the
carbon nanotube electrodes extending to overlap with the phase
change material layer, and an insulation layer formed outside the
carbon nanotube electrodes.
[0031] Consistent with the still other embodiment of the present
invention, each of the carbon nanotube electrodes may have a
diameter ranging from approximately 1 nm to 100 nm.
[0032] Consistent with the still other embodiment of the present
invention, the carbon nanotube electrodes may have an overlapping
length with the phase change material layer in a range of
approximately 1/10 to 8/10 of the total length of the carbon
nanotube electrodes.
[0033] Consistent with the still other embodiment of the present
invention, the carbon nanotube electrodes may be formed in a single
wall type.
[0034] According to further another embodiment of the present
invention, there is provided a method for fabricating a phase
change memory device using carbon nanotubes, the method comprising
disposing a catalyst for forming a plurality of carbon nanotubes
over predetermined regions of a current source electrode supplying
external current to a target, growing the carbon nanotubes in
vertical direction using the catalyst as a seed to form carbon
nanotube electrodes, depositing an insulation layer over the
current source electrode in a manner to cover the carbon nanotube
electrodes, polishing the insulation layer until flush with the
carbon nanotube electrodes, selectively etching the planarized
insulation layer to make the carbon nanotube electrodes exposed
substantially at the same level of the planarized insulation layer
protrude, and forming a phase change material layer over the etched
insulation layer such that the carbon nanotube electrodes overlap
with the phase change material layer.
[0035] Consistent with the further other embodiment of the present
invention, disposing the catalyst over the predetermined regions of
the current source electrode may comprise forming the catalyst
using one selected from a group consisting of Fe.sub.2O.sub.3, Pt,
Co, Ni, Ti, Mo, and a combination thereof.
[0036] Consistent with the further other embodiment of the present
invention, growing the carbon nanotubes in vertical direction using
the catalyst as the seed may comprise forming the carbon nanotube
electrodes in a single wall type.
[0037] Consistent with the further other embodiment of the present
invention, growing the carbon nanotubes in vertical direction using
the catalyst as the seed may comprise forming the carbon nanotube
electrodes to have a diameter ranging from approximately 1 nm to
100 nm.
[0038] Consistent with the further other embodiment of the present
invention, selectively etching the planarized insulation layer to
make the exposed carbon nanotube electrodes protrude may comprise
etching the planarized insulation layer such that a protruding
length of the carbon nanotube electrodes ranges from approximately
1/10 to 8/10 of the total length of the carbon nanotube
electrodes.
[0039] According to still further another embodiment of the present
invention, there is provided a phase change memory using carbon
nanotubes comprising a current source electrode supplying external
current to a target, a phase change material layer disposed to face
the current source electrode in side direction, a plurality of
carbon nanotube electrodes disposed between the current source
electrode and the phase change material layer, an insulation layer
formed outside the carbon nanotube electrodes, and a heat
generating resistance layer disposed between the carbon nanotube
electrodes and the phase change material layer in contact with the
carbon nanotube electrodes.
[0040] Consistent with the still further other embodiment of the
present invention, each of the carbon nanotube electrodes may have
a diameter ranging from approximately 1 nm to 100 nm.
[0041] Consistent with the still further other embodiment of the
present invention, the carbon nanotube electrodes may be formed in
a single wall type.
[0042] According to even further another embodiment of the present
invention, there is provided a method for fabricating a phase
change memory device using carbon nanotubes, the method comprising
disposing a catalyst for forming a plurality of carbon nanotubes
over predetermined regions of a current source electrode supplying
external current to a target, growing the carbon nanotubes in
vertical direction using the catalyst as a seed to form carbon
nanotube electrodes, depositing an insulation layer over the
current source electrode in a manner to cover the carbon nanotube
electrodes, polishing the insulation layer until flush with the
carbon nanotube electrodes, depositing a heat generating resistance
layer over the planarized insulation layer to contact the carbon
nanotube electrodes exposed substantially at the same level of the
planarized insulation layer, and forming a phase change layer over
the heat generating resistance layer.
[0043] Consistent with the even further other embodiment of the
present invention, disposing the catalyst over the predetermined
regions of the current source electrode may comprise forming the
catalyst using one selected from a group consisting of
Fe.sub.2O.sub.3, Pt, Co, Ni, Ti, Mo, and a combination thereof.
[0044] Consistent with the even further other embodiment of the
present invention, growing the carbon nanotubes in vertical
direction using the catalyst as the seed may comprise forming the
carbon nanotube electrodes in a single wall type.
[0045] Consistent with the even further embodiment of the present
invention, growing the carbon nanotubes in vertical direction using
the catalyst as the seed may comprise forming the carbon nanotube
electrodes to have a diameter ranging from approximately 1 nm to
100 nm.
[0046] The present invention will be described more fully with
reference to the accompanying drawings, in which embodiments of the
invention are shown. This invention may, however, be embodied in
many different forms and should not be construed as being limited
to the embodiments set forth herein; rather, these embodiments are
provided so that this disclosure will be thorough and complete, and
will fully convey the concept of the invention to those skilled in
the art. In the drawings, the same reference numerals in different
drawings represent the same element.
BRIEF DESCRIPTION OF THE DRAWINGS
[0047] The accompanying drawings, which are comprised to provide a
further understanding of the invention and are incorporated in and
constitute a part of this specification, illustrate embodiments of
the invention and together with the description serve to explain
the principles of the invention. In the drawings:
[0048] FIG. 1a is a perspective view of a typical phase change
memory device structure;
[0049] FIG. 1b is a sectional view of the typical phase change
memory device structure cut in a 1b-1b' direction illustrated in
FIG. 1a;
[0050] FIG. 1c is a top view of a bottom electrode of the typical
phase change memory device;
[0051] FIG. 2a is a perspective view of a phase change memory
device structure according to an embodiment of the present
invention;
[0052] FIG. 2b is a sectional view of the phase change memory
device structure cut in a 2b-2b' direction illustrated in FIG.
2a;
[0053] FIG. 2c is a top view of a bottom electrode of the phase
change memory device according to the embodiment of the present
invention;
[0054] FIGS. 3a to 3e are sectional views to illustrate a method
for fabricating a phase change memory device according to an
embodiment of the present invention;
[0055] FIG. 4 is a sectional view of a phase change memory device
structure according to another embodiment of the present invention;
and
[0056] FIG. 5 is a sectional view of a phase change memory device
structure according to further another embodiment of the present
invention.
DETAILED DESCRIPTION OF EMBODIMENTS
[0057] Various embodiments of a phase change memory device using a
carbon nanotube and a method for fabricating the same will be
described in a more detailed manner with reference to the attached
drawings.
[0058] FIG. 2a is a perspective view of a phase change memory
device structure according to an embodiment of the present
invention. FIG. 2b is a sectional view of the phase change memory
device structure cut in a 2b-2b' direction illustrated in FIG. 2a.
FIG. 2c is a top view of a bottom electrode of the phase change
memory device according to the embodiment of the present
invention.
[0059] Referring to FIG. 2a, the phase change memory device
comprises a current source electrode 201, a phase change material
layer 207, a plurality of carbon nanotube electrodes 203, and an
insulation layer 205. The current source electrode 201 supplies
external current to a target. The phase change material layer 207
is disposed to face the current source electrode 201 in side
direction. The carbon nanotube electrodes 203 are arranged between
the current source electrode 201 and the phase change material
layer 207, and the insulation layer 205 is formed outside the
carbon nanotube electrodes 203.
[0060] The current source electrode 201 supplies external current
to a target to achieve an intended level of current density
necessary for inducing a phase change in the phase change material
layer 207.
[0061] The carbon nanotube electrodes 203 are arranged between the
current source electrode 201 and the phase change material layer
207 and make contact with the phase change material layer 207. The
carbon nanotube electrodes 203 transfer the external current, which
is necessary for inducing the phase change in the phase change
material layer 207, from the current source electrode 201 to the
phase change material layer 207. In addition to this role, the
carbon nanotube electrodes 203 function as a heat generating
material, functionally corresponding to the typical bottom
electrode 102 (see FIG. 1a).
[0062] As illustrated in FIG. 2c, the carbon nanotube electrodes
203 may be formed in a predetermined pattern having certain
regularity in size and arrangement. However, the carbon nanotube
electrodes 203 may still be formed without having the predetermined
pattern.
[0063] A diameter `d` of each of the carbon nanotube electrodes 203
ranges from approximately 1 nm to 100 nm, and this range means that
the carbon nanotube electrodes 203 have a small area being close to
the form of a dot. Particularly, the carbon nanotube electrodes 203
may be formed in a single wall type, which usually has high
resistance. The reason for forming the single wall type carbon
nanotube electrodes 203 is to generate a large amount of heat
substantially with the same current density through increasing the
resistance of the carbon nanotube electrodes 203, which serve as a
heat generating material. Detailed description of the heat
generation will be described later.
[0064] The insulation layer 205 encompasses the outside of the
carbon nanotube electrodes 203 disposed between the current source
electrode 201 and the phase change material layer 207. This
structural characteristic of the insulation layer 205 disallows
heat generated at the carbon nanotube electrodes 203 to be
transferred outside. The insulation layer 205 comprises one
selected from a group consisting of SiO.sub.2, Si.sub.4N.sub.4,
HfO.sub.2, ZrO.sub.2, Ta.sub.2O.sub.5, Al.sub.2O.sub.3,
La.sub.2O.sub.3, and CeO.sub.2.
[0065] The phase change material layer 207 is a memory layer that
stores information using an electrical conductivity difference in a
phase change material. The phase change material usually has two
different phases such as amorphous phase and crystalline phase.
Since amorphous phase has high specific resistance than crystalline
phase, amorphous and crystalline phases of the phase change
material can be distinguished from each other. Therefore, as the
phase change material layer 207 is electrically heated by which
current flows from the carbon nanotube electrodes 203, the
amorphous and crystalline phases of the phase change material are
changed reversibly from each other, allowing storing
information.
[0066] In the phase change memory device according to the
embodiment of the present invention, the carbon nanotube electrodes
203, which are heat generating materials, contact the phase change
material layer 207. At this time, the contact area is small,
substantially corresponding to the diameter of the carbon nanotube
electrodes 203. Thus, as compared with the ring-type phase change
memory device in which the phase change material layer and the heat
generating material make one dimensional surface contact in
circumference, the contact area of the phase change memory device
according to the present invention decreases to a great extent.
[0067] As a result of this effect, the phase change memory device
according to the present invention shows several characteristics
that allow more enhanced scale of integration as compared with the
typical phase change memory device.
[0068] In detail, first, the carbon nanotubes are used to form
bottom electrodes of the phase change memory device. As mentioned
above, the bottom electrodes are the current passage between the
current source electrode 201 and the phase change material layer
207. As a result, an amount of operation current necessary for the
phase change can be reduced.
[0069] That is, each of the carbon nanotube electrodes 203 arranged
between the current source electrode 201 and the phase change
material layer 207 have a size of approximately 1 nm to 100 nm,
thereby forming the small contact area with the phase change
material layer 207. Hence, high current density can be obtained
even with a small amount of current. Consequently, a large amount
of operation current, which often put a burden on improving the
scale of integration, is not a limiting factor for improving the
scale of integration.
[0070] Second, using the carbon nanotubes as the bottom electrodes
functioning as the current passage between the current source
electrode 201 and the phase change material layer 207 allows
utilizing high thermal conductivity of the carbon nanotubes.
[0071] In general, the thermal conductivity of a carbon nanotube is
two times higher than diamond. For instance, a carbon nanotube is
usually known to have a thermal conductivity of approximately 6,000
W/mk. The higher the thermal conductivity, the easier the transfer
of heat generated at the carbon nanotube to the outside.
[0072] Third, in the phase change memory device according to the
embodiment of the present invention, the carbon nanotubes (i.e.,
the bottom electrodes) are arranged uniformly around the phase
change material layer 207, a region responsible for the phase
change can be widened.
[0073] This widened region allows increasing a threshold voltage
margin of the phase change memory device, and thus, multi-level
cell (MLC) technology that enables storage of several bits on a
single device can be implemented. As a result, the scale of
integration of the phase change memory device can be improved.
[0074] Fourth, the phase change memory device has different heat
generation efficiency depending on electrical properties of the
carbon nanotubes.
[0075] More specifically, various electrical characteristics appear
in the phase change memory device depending on the diameter of the
carbon nanotubes and chirality.
[0076] Although multi-wall nanotubes (MWNTs) show approximately 99%
of similarity in electrical characteristics to metal, MWNTs are not
often suitable to be used as bottom electrodes due to low
resistance of MWNTs.
[0077] Depending on the chirality, single wall nanotube (SWNTs) can
be classified into a group of the nanotubes that exhibit
semiconductor characteristics and a group of the nanotubes that
exhibit metallic characteristics. By using the nanotubes that
exhibit the semiconductor characteristics, the resistance of the
carbon nanotubes can be increased, and the heat generation
efficiency can be improved.
[0078] Accordingly, as compared with the typical phase change
memory device, the phase change memory device according to the
present invention can have an amount of operation current reduced
to a great extent, and can be integrated in large scale.
[0079] Hereinafter, a method for fabricating the above-described
type of phase change memory device will be described.
[0080] FIGS. 3a to 3e are sectional views to illustrate a method
for fabricating a phase change memory device according to an
embodiment of the present invention.
[0081] According to the embodiment of the present invention, the
method comprises placing a catalyst over predetermined regions of a
current source electrode; vertically growing a seed (i.e., the
catalyst) to form carbon nanotubes; depositing an insulation layer
over the current source electrode in a manner to cover the carbon
nanotubes; polishing the surface of the insulation layer; and
forming a phase change material layer.
[0082] Referring to FIG. 3A, a catalyst 302 is placed over
predetermined regions of a current source electrode 301 supplying
external current that is necessary for inducing a phase change to a
target. The catalyst 302 is to form carbon nanotubes.
[0083] The catalyst 302 comprises one selected from a group
consisting of Fe.sub.2O.sub.3, Pt, Co, Ni, Ti, Mo, and a
combination thereof. As illustrated in FIG. 3a, the catalyst 302
may be formed in a predetermined pattern having regularity in size
and arrangement. However, the catalyst 302 may still be formed
without having the predetermined pattern.
[0084] Referring to FIG. 3b, using the catalyst 302, which serves
as a seed for carbon nanotubes, the carbon nanotubes are grown
vertically in a pillar shape to thereby form carbon nanotube
electrodes 303.
[0085] The carbon nanotube electrodes 303 are formed in a single
wall type with high resistance to generate a large amount of heat
due to high resistance of the bottom electrodes (i.e., the carbon
nanotube electrodes 303). A diameter of each of the carbon nanotube
electrodes 303 ranges from approximately 1 nm to 100 nm. This
magnitude of the diameter is small, and thus, the carbon nanotube
electrodes 303 have a small sectional area that is very close to a
dot.
[0086] Referring to FIG. 3c, an insulation layer 305 is deposited
over the current source electrode but widely enough to cover the
pillar-type carbon nanotube electrodes 303.
[0087] Referring to FIG. 3d, the surface of the insulation layer
305 is planarized using chemical mechanical polishing (CMP) until
the insulation layer 305 is flushed with the carbon nanotube
electrodes 303 (i.e., until the carbon nanotube electrodes 303 are
exposed substantially at the same level of the planarized
insulation layer 305).
[0088] Referring to FIG. 3e, a phase change material layer 307 is
deposited over the planarized insulation layer 305 such that the
phase change material layer 307 and the carbon nanotube electrodes
303 make contact with each other. At this point, the contact area
between the phase change material layer 307 and the carbon nanotube
electrodes 303 is a sectional area corresponding to the diameter of
each of the carbon nanotube electrodes 303. That is, the contact
area between the phase change material layer 307 and the carbon
nanotube electrodes 303 is small, being close to a dot-like
form.
[0089] On the basis of the above described sequential processes,
the phase change memory device with the carbon nanotube-based
bottom electrodes can be fabricated.
[0090] FIG. 4 is a sectional view of a phase change memory device
structure accruing to another embodiment of the present
invention.
[0091] The phase change memory device according to the other
embodiment of the present invention comprises a current source
electrode 401, a phase change material layer 407, a plurality of
carbon nanotube electrodes 403, and an insulation layer 405. The
current source electrode 401 supplies external current, which is
necessary for inducing a phase change, to a target. The phase
change material layer 407 is disposed to face the current source
electrode 401 in side direction. The carbon nanotube electrodes 403
are arranged between the current source electrode 401 and the phase
change material layer 407, and one portion of the carbon nanotube
electrodes 403 extends to the phase change material layer 407,
thereby overlapping with the phase change material layer 407. The
insulation layer 405 is formed to encompass the outside of the
carbon nanotube electrodes 403 formed between the current source
electrode 401 and the phase change material layer 407. The
insulation layer 405 particularly prevents the loss of heat
generated at the carbon nanotube electrodes 403 to the outside.
[0092] An overlapping length of the carbon nanotube electrodes 403
with the phase change material layer 407 may be approximately 1/10
to 8/10 of the entire length of the carbon nanotube electrodes
403.
[0093] Different from the phase change memory device according to
the embodiment of the present invention, wherein the carbon
nanotube electrodes are formed to contact the phase change material
layer, the carbon nanotube electrodes 403 in the phase change
memory device according to the other embodiment of the present
invention extend to the phase change material layer 407 to overlap
with the phase change material layer 407. As mentioned above, the
carbon nanotube electrodes 403 are used as the current passage
between the current source electrode 401 and the phase change
material layer 407. Due to this overlapping structure, heat
generated at the carbon nanotube electrodes transfers to the phase
change material layer 407 that causes a phase change, enlarging the
area of the phase change material layer 407 to a great extent.
[0094] Accordingly, the phase change memory device according to the
other embodiment of the present invention can maintain high current
density even with a small amount of operation current. Also, a
threshold voltage margin of the phase change memory device is
improved, and this effect allows the implementation of the MLC
technology that enables storage of information in several bits on a
single device.
[0095] A method for fabricating the phase change memory device
according to the other embodiment of the present invention further
comprises selectively etching the insulation layer 405 after
polishing the insulation layer described in the fabrication method
according to the embodiment of the present invention.
[0096] That is, the method for fabricating the phase change memory
device according to the other embodiment of the present invention
comprises disposing a catalyst 402 for forming a plurality of
carbon nanotubes over predetermined regions of the current source
electrode 401 that supplies external current necessary for inducing
a phase change to a target; vertically growing carbon nanotubes
using the catalyst 402 as a seed; depositing the insulation layer
405 over the current source electrode 401 in a manner to cover the
carbon nanotube electrodes 403; polishing the insulation layer 405
until flush with the carbon nanotube electrodes 403; selectively
etching the insulation layer 405 to make the exposed carbon
nanotube electrodes 403 exposed substantially at the same level of
the planarized insulation layer 405 protrude; and forming the phase
change material layer 407 over the insulation layer 405 such that
the protruding carbon nanotube electrodes 403 overlap with the
phase change material layer 407.
[0097] When selectively etching the insulation layer 405 to make
the exposed carbon nanotube electrodes 403 protrude, since the
protruding portions of the carbon nanotube electrodes 403 overlap
with the phase change material layer 407, the insulation layer 405
is etched until the length of the protruding portions of the carbon
nanotube electrodes 403 is in a range of approximately 1/10 to 8/10
of the entire length of the carbon nanotube electrodes 403.
[0098] Herein, those elements and fabrication processes of the
second exemplary phase change memory device that are similar to or
same as those described in the above embodiment of the present
invention will not be described in detail.
[0099] FIG. 5 is a sectional view of a phase change memory device
structure according to further another embodiment of the present
invention.
[0100] The phase change memory device according to the further
embodiment of the present invention further comprises a heat
generating resistance layer 509 that has high resistance to
overcome low heat efficiency caused by the low resistance of the
carbon nanotube electrodes in the phase change memory device
according to the embodiment of the present invention.
[0101] In more detail, the phase change memory device according to
the further embodiment of the present invention comprises a current
source electrode 501, a phase change material layer 507, a
plurality of carbon nanotube electrodes 503, an insulation layer
505, and a heat generating resistance layer 509. The current source
electrode 501 supplies external current necessary for inducing a
phase change to a target. The phase change material layer 507 is
disposed to face the current source electrode 501 in side
direction. The carbon nanotube electrodes 503 are disposed between
the current source electrode 501 and the phase change material
layer 507. The insulation layer 505 is formed to encompass the
outside of the carbon nanotube electrodes 503, and functions to
prevent the loss of heat generated at the carbon nanotube
electrodes 503 to the outside. The heat generating resistance layer
509 is disposed between the carbon nanotube electrodes 503 and the
phase change layer 507 in contact with the carbon nanotube
electrodes 503.
[0102] When the carbon nanotube electrodes 503 are used as the
current passage, electrons are less likely to scatter in the
direction of growing the carbon nanotubes. Thus, the carbon
nanotube electrodes 503 allow a flow of a large amount of current,
e.g., approximately 10.sup.10 Acm.sup.-2. Although the carbon
nanotubes are advantageous of obtaining high current density or
conducting current, when the carbon nanotubes are used as a heat
generating material, a sufficient amount of heat that causes a
phase change in the phase change material may not be generated due
to the low resistance of the carbon nanotubes.
[0103] Therefore, the heat generating resistance layer 509 is
deposited thinly between the carbon nanotube electrodes 503 and the
phase change material layer 507 in contact with the phase change
material layer 507. As a result, the high current density of the
carbon nanotube electrodes 503 is provided to the heat generating
resistance layer 509. Consequently, the heat generation efficiency
can be improved using the high resistance of the heat generating
resistance layer 509.
[0104] The heat generating resistance layer 509 comprises one
selected from a group consisting of W, Mo, Ta, Ni, Cr, and
nichrome.
[0105] A method for fabricating the phase change memory device
according to the further embodiment of the present invention
comprises disposing a catalyst 502 for forming a plurality of
carbon nanotubes over predetermined regions of the current source
electrode 502 supplying external current necessary for inducing a
phase change; vertically growing carbon nanotubes using the
catalyst 502 as a seed to thereby form the carbon nanotube
electrodes 503; depositing the insulation layer 505 over the
current source electrode 501 in a manner to cover the carbon
nanotube electrodes 503; polishing the insulation layer 505 until
the insulation layer is flushed with the carbon nanotube electrodes
503; depositing the heat generating resistance layer 509 in contact
with the carbon nanotube electrodes exposed substantially at the
same level of the planarized insulation layer 505; and forming the
phase change material layer 507 over the heat generating resistance
layer 509.
[0106] Herein, those elements and fabrication processes of the
third exemplary phase change memory device that are similar to or
same as those described in the above embodiment of the present
invention will not be described in detail.
[0107] Instead of carbon nanotubes, dot-shaped electric hot-wires
can be formed using silicon nanowires, SiGe nanowires, or ZnO
nanowires as a heat generating material, and can function as the
current passage between the current source electrode and the phase
change material layer.
[0108] According to various embodiments of the present invention,
the phase change memory device uses carbon nanotubes to form the
typical bottom electrodes. Thus, the phase change memory device can
maintain the high current density even with a small amount of
operation current as compared with the typical phase change memory
device. Also, a threshold voltage margin of the phase change memory
device according to the embodiments of the present invention can be
increased, and as a result, the MLC technology can be implemented
to the phase change memory device.
[0109] In addition, since the phase change memory device according
to the embodiments of the present invention can be scaled down, a
large amount of operation current, which is the typically observed
limitation in achieving the large scale of integration, can be
overcome. Furthermore, the phase change memory device according to
the embodiments of the present invention can be highly integrated
and operate at low power with high efficiency.
[0110] While the present invention has been described with respect
to specific embodiments, it will be apparent to those skilled in
the art that various changes and modifications may be made without
departing from the spirit and scope of the invention as defined in
the following claims.
[0111] Since the specific embodiments of the present invention are
provided to show the technical scope and spirit of the present
invention, these embodiments should not be construed as limitive
but merely illustrative, and are intended to be included within the
scope of the following claims.
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