U.S. patent application number 15/898571 was filed with the patent office on 2019-08-22 for air gap three-dimensional cross rail memory device and method of making thereof.
The applicant listed for this patent is SANDISK TECHNOLOGIES LLC. Invention is credited to Satoru MAYUZUMI, Vincent SHIH, Yuji TAKAHASHI.
Application Number | 20190259772 15/898571 |
Document ID | / |
Family ID | 67543710 |
Filed Date | 2019-08-22 |
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United States Patent
Application |
20190259772 |
Kind Code |
A1 |
TAKAHASHI; Yuji ; et
al. |
August 22, 2019 |
AIR GAP THREE-DIMENSIONAL CROSS RAIL MEMORY DEVICE AND METHOD OF
MAKING THEREOF
Abstract
A memory device includes first conductive rails laterally
extending along a first horizontal direction over a substrate, a
rectangular array of first memory pillar structures, each
containing a memory element, overlying top surfaces of the first
conductive rails, second conductive rails laterally extending along
a second horizontal direction and overlying top surfaces of the
rectangular array of first memory pillar structures, and a
one-dimensional array of first cavities free of solid material
portions therein, laterally extending along the second horizontal
direction and located between neighboring pairs of the second
conductive rails.
Inventors: |
TAKAHASHI; Yuji; (Yokkaichi,
JP) ; MAYUZUMI; Satoru; (Yokkaichi, JP) ;
SHIH; Vincent; (San Jose, CA) |
|
Applicant: |
Name |
City |
State |
Country |
Type |
SANDISK TECHNOLOGIES LLC |
Plano |
TX |
US |
|
|
Family ID: |
67543710 |
Appl. No.: |
15/898571 |
Filed: |
February 17, 2018 |
Current U.S.
Class: |
1/1 |
Current CPC
Class: |
H01L 45/1675 20130101;
H01L 27/11556 20130101; H01L 27/2409 20130101; H01L 45/1233
20130101; H01L 21/8221 20130101; H01L 45/143 20130101; H01L 27/0688
20130101; H01L 27/1021 20130101; H01L 27/2481 20130101; G11C
16/0441 20130101; H01L 27/2427 20130101; H01L 45/144 20130101; H01L
45/06 20130101 |
International
Class: |
H01L 27/11556 20060101
H01L027/11556; H01L 27/102 20060101 H01L027/102; H01L 27/06
20060101 H01L027/06; H01L 21/822 20060101 H01L021/822; G11C 16/04
20060101 G11C016/04; H01L 45/00 20060101 H01L045/00 |
Claims
1. A memory device, comprising: first conductive rails laterally
extending along a first horizontal direction over a substrate; a
rectangular array of first memory pillar structures overlying top
surfaces of the first conductive rails, wherein each first memory
pillar structure comprises a respective memory element; second
conductive rails laterally extending along a second horizontal
direction and overlying top surfaces of the rectangular array of
first memory pillar structures; and a one-dimensional array of
first cavities free of solid material portions therein laterally
extending along the second horizontal direction and located between
neighboring pairs of the second conductive rails; wherein each
first memory pillar structure comprises a series connection of a
respective first selector element and a respective first memory
element; and wherein each first selector element comprises: an
ovonic threshold switch material portion; an upper amorphous carbon
portion that contacts a top surface of the ovonic threshold switch
material portion; and a lower amorphous carbon portion that
contacts a bottom surface of the ovonic threshold switch material
portion.
2. The memory device of claim 1, wherein each of the first cavities
vertically extends from a horizontal plane including bottom
surfaces of the second conductive rails to a horizontal plane
including top surfaces of the second conductive rails.
3. (canceled)
4. The memory device of claim 1, wherein: each of the second
conductive rails comprises a vertical stack of a lower second
conductive rail portion and an upper second conductive rail
portion; and each of the first cavities vertically extends at least
from a horizontal plane including interfaces between the first
selector elements and first memory elements to a horizontal plane
including interfaces of the lower second conductive rail portions
and the upper second conductive rail portions.
5. The memory device of claim 1, wherein each of the first cavities
includes a respective portion that protrudes above a horizontal
plane including topmost surfaces of the second conductive
rails.
6. The memory device of claim 1, wherein: each of the upper second
conductive rail portions has tapered sidewalls and a greater width
at a respective bottom surface than at a respective top surface
along the first horizontal direction; and each of the first
cavities has a lesser width at a level of lower portions of the
upper second conductive rail portions than at a level of upper
portions of the lower second conductive rail portions.
7. The memory device of claim 1, wherein: each of the first
cavities has a vertical cross-sectional shape within a vertical
plane that is perpendicular to the first horizontal direction; and
a top portion of the vertical cross-sectional shape includes two
vertically protruding regions laterally spaced by a vertically
recessed region.
8. The memory device of claim 7, wherein the vertical
cross-sectional shape is entirely enclosed by an inner surface of
at least one dielectric liner located between a respective
neighboring pair of second conductive rails and between a
respective pair of neighboring rows of the first memory pillar
structures that are spaced apart along the first horizontal
direction.
9. The memory device of claim 8, wherein the at least one
dielectric liner comprises: an outer dielectric liner laterally
contacting the respective neighboring pair of second conductive
rails and between the respective pair of neighboring rows of the
first memory pillar structures; and an inner dielectric liner
defining a boundary of a respective first cavity and located
between the respective first cavity and the outer dielectric
liner.
10. The memory device of claim 8, further comprising: a rectangular
array of second memory pillar structures overlying top surfaces of
the second conductive rails, wherein each second memory pillar
structure comprises a series connection of a respective second
selector element and a respective second memory element; and a
rectangular array of dielectric isolation pillars located between
each neighboring pair of second memory pillar structures spaced
along the first horizontal direction, wherein each first cavity
underlies a respective one of the dielectric isolation pillars.
11. The memory device of claim 10, wherein: each of the dielectric
isolation pillars includes a pair of concave bottom surfaces that
laterally extend along the second horizontal direction and a
downward-protruding portion located between the pair of concave
bottom surfaces; and each pair of concave bottom surfaces of the
dielectric isolation pillars contacts a respective one of the at
least one dielectric liners.
12. (canceled)
13. The memory device of claim 1, wherein each first memory element
comprises a phase change memory element including, from bottom to
top: a first lower conductive liner plate comprising a first
metallic material; a first phase change memory material pillar
comprising a first phase change memory material; and a first upper
conductive liner plate comprising a second metallic material.
14. The memory device of claim 13, wherein the first phase change
memory material is selected from germanium antimony telluride
compounds, indium germanium telluride compounds, aluminum selenium
telluride compounds, indium selenium telluride compounds, and
aluminum indium selenium telluride compounds.
15. The memory device of claim 14, further comprising a
two-dimensional array of barrier elements disposed between the
first phase change memory elements and a proximal set among a set
of the first conductive rails and a set of the second conductive
rails.
16-23. (canceled)
24. A memory device, comprising: first conductive rails laterally
extending along a first horizontal direction over a substrate; a
rectangular array of first memory pillar structures overlying top
surfaces of the first conductive rails, wherein each first memory
pillar structure comprises a respective memory element; second
conductive rails laterally extending along a second horizontal
direction and overlying top surfaces of the rectangular array of
first memory pillar structures; a one-dimensional array of first
cavities free of solid material portions therein laterally
extending along the second horizontal direction and located between
neighboring pairs of the second conductive rails; a rectangular
array of second memory pillar structures overlying top surfaces of
the second conductive rails, wherein each second memory pillar
structure comprises a series connection of a respective second
selector element and a respective second memory element; and a
rectangular array of dielectric isolation pillars located between
each neighboring pair of second memory pillar structures spaced
along the first horizontal direction, wherein: each first memory
pillar structure comprises a series connection of a respective
first selector element and a respective first memory element; each
of the first cavities has a vertical cross-sectional shape within a
vertical plane that is perpendicular to the first horizontal
direction; a top portion of the vertical cross-sectional shape
includes two vertically protruding regions laterally spaced by a
vertically recessed region; the vertical cross-sectional shape is
entirely enclosed by an inner surface of at least one dielectric
liner located between a respective neighboring pair of second
conductive rails and between a respective pair of neighboring rows
of the first memory pillar structures that are spaced apart along
the first horizontal direction; each first cavity underlies a
respective one of the dielectric isolation pillars; each of the
dielectric isolation pillars includes a pair of concave bottom
surfaces that laterally extend along the second horizontal
direction and a downward-protruding portion located between the
pair of concave bottom surfaces; and each pair of concave bottom
surfaces of the dielectric isolation pillars contacts a respective
one of the at least one dielectric liners.
Description
FIELD
[0001] The present disclosure relates generally to the field of
semiconductor devices and specifically to three-dimensional cross
rail memory structures including air gaps and methods of forming
the same.
BACKGROUND
[0002] A phase change material (PCM) memory device is a type of
non-volatile memory device that stores information as a resistive
state of a material that can be in different resistive states
corresponding to different phases of the material. The different
phases can include an amorphous state having high resistivity and a
crystalline state having low resistivity (i.e., a lower resistivity
than in the amorphous state). The transition between the amorphous
state and the crystalline state can be induced by controlling the
rate of cooling after application of an electrical pulse that
renders the phase change memory material in a first part of a
programming process. The second part of the programming process
includes control of the cooling rate of the phase change memory
material. If rapid quenching occurs, the phase change memory
material can cool into an amorphous high resistivity state. If slow
cooling occurs, the phase change memory material can cool into a
crystalline low resistivity state.
SUMMARY
[0003] According to an aspect of the present disclosure, a memory
device is provided, which comprises: first conductive rails
laterally extending along a first horizontal direction over a
substrate; a rectangular array of first memory pillar structures
overlying top surfaces of the first conductive rails, wherein each
first memory pillar structure comprises a respective memory
element; second conductive rails laterally extending along a second
horizontal direction and overlying top surfaces of the rectangular
array of first memory pillar structures; and a one-dimensional
array of first cavities free of solid material portions therein
laterally extending along the second horizontal direction and
located between neighboring pairs of the second conductive
rails.
[0004] According to another aspect of the present disclosure, a
method of forming a memory device is provided, which comprises the
steps of: forming first conductive rails laterally extending along
a first horizontal direction, a rectangular array of first memory
pillar structures, and second conductive rails laterally extending
along a second horizontal direction over a substrate, wherein the
first conductive rails and rows of the first memory pillar
structures are laterally spaced by first trenches, and lower
portions of the second conductive rails and columns of the first
memory pillar structures are laterally spaced by second trenches,
and upper portions of the second conductive rails are laterally
spaced by third trenches; forming sacrificial semiconductor
structures by selectively growing a semiconductor material from
sidewalls of the second conductive rails; forming dielectric
isolation structures in upper portions of the third trenches by
depositing at least one dielectric material therein; forming fourth
trenches laterally extending along the first horizontal direction
through the vertical stacks and the dielectric isolation
structures, wherein a rectangular array of second memory pillar
structures are formed by remaining portions of the vertical stacks;
and removing the sacrificial semiconductor structures from
underneath the fourth trenches to form a one-dimensional array of
first cavities free of solid material portions therein, laterally
extending along the second horizontal direction, wherein the second
conductive rails are laterally spaced among one another by the
one-dimensional array of first cavities.
BRIEF DESCRIPTION OF THE DRAWINGS
[0005] FIG. 1A is a top-down view of an exemplary structure for
forming a three-dimensional phase change memory device after
formation of a first layer stack including a first bottom
conductive layer, a first selector layer, a first phase change
memory layer, and an optional first barrier layer according to an
embodiment of the present disclosure.
[0006] FIG. 1B is a vertical cross-sectional view of the exemplary
structure along the vertical plane B-B' of FIG. 1A.
[0007] FIG. 2A is a top-down view of the exemplary structure after
formation of first stacked rail structures laterally spaced by
first trenches through the first layer stack according to an
embodiment of the present disclosure.
[0008] FIG. 2B is a vertical cross-sectional view of the exemplary
structure along the vertical plane B-B' of FIG. 2A.
[0009] FIG. 2C is a vertical cross-sectional view of the exemplary
structure along the vertical plane C-C' of FIG. 2A.
[0010] FIG. 3A is a top-down view of the exemplary structure after
formation of first dielectric isolation structures according to an
embodiment of the present disclosure.
[0011] FIG. 3B is a vertical cross-sectional view of the exemplary
structure along the vertical plane B-B' of FIG. 3A.
[0012] FIG. 3C is a vertical cross-sectional view of the exemplary
structure along the vertical plane C-C' of FIG. 3A.
[0013] FIG. 4A is a top-down view of the exemplary structure after
formation of a first top conductive layer and a patterned second
photoresist layer over the first stacked rail structures and the
first dielectric isolation structures according to an embodiment of
the present disclosure.
[0014] FIG. 4B is a vertical cross-sectional view of the exemplary
structure along the vertical plane B-B' of FIG. 4A.
[0015] FIG. 4C is a vertical cross-sectional view of the exemplary
structure along the vertical plane C-C' of FIG. 4A.
[0016] FIG. 5A is a vertical cross-sectional view of the exemplary
structure after formation of second trenches through the first top
conductive layer, the first stacked rail structures, and the first
dielectric isolation structures and formation of second lower
conductive rails according to an embodiment of the present
disclosure.
[0017] FIG. 5B is a vertical cross-sectional view of the exemplary
structure along the vertical plane B-B' of FIG. 5A.
[0018] FIG. 5C is a vertical cross-sectional view of the exemplary
structure along the vertical plane C-C' of FIG. 5A.
[0019] FIG. 6A is a top-down view of the exemplary structure after
formation of first sacrificial trench fill structures according to
an embodiment of the present disclosure.
[0020] FIG. 6B is a vertical cross-sectional view of the exemplary
structure along the vertical plane B-B' of FIG. 6A.
[0021] FIG. 6C is a vertical cross-sectional view of the exemplary
structure along the vertical plane C-C' of FIG. 6A.
[0022] FIG. 7A is a top-down view of the exemplary structure after
formation of a second layer stack including a second bottom
conductive layer, a second selector layer, a second phase change
memory layer stack, and an optional second barrier layer according
to an embodiment of the present disclosure.
[0023] FIG. 7B is a vertical cross-sectional view of the exemplary
structure along the vertical plane B-B' of FIG. 7A.
[0024] FIG. 7C is a vertical cross-sectional view of the exemplary
structure along the vertical plane C-C' of FIG. 7A.
[0025] FIG. 8A is a top-down view of the exemplary structure after
formation of second upper conductive rails and second stacked rail
structures laterally spaced by third trenches through the second
layer stack according to an embodiment of the present
disclosure.
[0026] FIG. 8B is a vertical cross-sectional view of the exemplary
structure along the vertical plane B-B' of FIG. 8A.
[0027] FIG. 8C is a vertical cross-sectional view of the exemplary
structure along the vertical plane C-C' of FIG. 8A.
[0028] FIG. 9A is a top-down view of the exemplary structure after
removal of first sacrificial trench fill structures and extension
of the third trenches according to an embodiment of the present
disclosure.
[0029] FIG. 9B is a vertical cross-sectional view of the exemplary
structure along the vertical plane B-B' of FIG. 9A.
[0030] FIG. 9C is a vertical cross-sectional view of the exemplary
structure along the vertical plane C-C' of FIG. 9A.
[0031] FIG. 10A is a top-down view of the exemplary structure after
formation of sacrificial semiconductor structures by a selective
deposition process according to an embodiment of the present
disclosure.
[0032] FIG. 10B is a vertical cross-sectional view of the exemplary
structure along the vertical plane B-B' of FIG. 10A.
[0033] FIG. 10C is a vertical cross-sectional view of the exemplary
structure along the vertical plane C-C' of FIG. 10A.
[0034] FIG. 11A is a top-down view of the exemplary structure after
formation of dielectric liners in upper portions of the third
trenches according to an embodiment of the present disclosure.
[0035] FIG. 11B is a vertical cross-sectional view of the exemplary
structure along the vertical plane B-B' of FIG. 11A.
[0036] FIG. 11C is a vertical cross-sectional view of the exemplary
structure along the vertical plane C-C' of FIG. 11A.
[0037] FIG. 12A is a top-down view of the exemplary structure after
formation of second dielectric isolation structures according to an
embodiment of the present disclosure.
[0038] FIG. 12B is a vertical cross-sectional view of the exemplary
structure along the vertical plane B-B' of FIG. 12A.
[0039] FIG. 12C is a vertical cross-sectional view of the exemplary
structure along the vertical plane C-C' of FIG. 12A.
[0040] FIG. 13A is a top-down view of the exemplary structure after
formation of a second top conductive layer and a patterned fourth
photoresist layer over the second stacked rail structures and the
second dielectric isolation structures according to an embodiment
of the present disclosure.
[0041] FIG. 13B is a vertical cross-sectional view of the exemplary
structure along the vertical plane B-B' of FIG. 13A.
[0042] FIG. 13C is a vertical cross-sectional view of the exemplary
structure along the vertical plane C-C' of FIG. 13A.
[0043] FIG. 13D is a vertical cross-sectional view of the exemplary
structure along the vertical plane D-D' of FIG. 13A.
[0044] FIG. 13E is a vertical cross-sectional view of the exemplary
structure along the vertical plane E-E' of FIG. 13A.
[0045] FIG. 14A is a vertical cross-sectional view of the exemplary
structure after formation of fourth trenches through the second top
conductive layer, the second stacked rail structures, and the
second dielectric isolation structures and formation of third lower
conductive rails according to an embodiment of the present
disclosure.
[0046] FIG. 14B is a vertical cross-sectional view of the exemplary
structure along the vertical plane B-B' of FIG. 14A.
[0047] FIG. 14C is a vertical cross-sectional view of the exemplary
structure along the vertical plane C-C' of FIG. 14A.
[0048] FIG. 14D is a vertical cross-sectional view of the exemplary
structure along the vertical plane D-D' of FIG. 14A.
[0049] FIG. 14E is a vertical cross-sectional view of the exemplary
structure along the vertical plane E-E' of FIG. 14A.
[0050] FIG. 15A is a vertical cross-sectional view of the exemplary
structure after removal of the sacrificial semiconductor structures
to form first cavities according to an embodiment of the present
disclosure.
[0051] FIG. 15B is a vertical cross-sectional view of the exemplary
structure along the vertical plane B-B' of FIG. 15A.
[0052] FIG. 15C is a vertical cross-sectional view of the exemplary
structure along the vertical plane C-C' of FIG. 15A.
[0053] FIG. 15D is a vertical cross-sectional view of the exemplary
structure along the vertical plane D-D' of FIG. 15A.
[0054] FIG. 15E is a vertical cross-sectional view of the exemplary
structure along the vertical plane E-E' of FIG. 15A.
[0055] FIG. 16A is a vertical cross-sectional view of the exemplary
structure after conformal deposition of an outer dielectric liner
according to an embodiment of the present disclosure.
[0056] FIG. 16B is a vertical cross-sectional view of the exemplary
structure along the vertical plane B-B' of FIG. 16A.
[0057] FIG. 16C is a vertical cross-sectional view of the exemplary
structure along the vertical plane C-C' of FIG. 16A.
[0058] FIG. 16D is a vertical cross-sectional view of the exemplary
structure along the vertical plane D-D' of FIG. 16A.
[0059] FIG. 16E is a vertical cross-sectional view of the exemplary
structure along the vertical plane E-E' of FIG. 16A.
[0060] FIG. 17A is a vertical cross-sectional view of the exemplary
structure after non-conformal deposition of a dielectric fill
material to form an inner dielectric liner around each first cavity
according to an embodiment of the present disclosure.
[0061] FIG. 17B is a vertical cross-sectional view of the exemplary
structure along the vertical plane B-B' of FIG. 17A.
[0062] FIG. 17C is a vertical cross-sectional view of the exemplary
structure along the vertical plane C-C' of FIG. 17A.
[0063] FIG. 17D is a vertical cross-sectional view of the exemplary
structure along the vertical plane D-D' of FIG. 17A.
[0064] FIG. 17E is a vertical cross-sectional view of the exemplary
structure along the vertical plane E-E' of FIG. 17A.
[0065] FIG. 18A is a vertical cross-sectional view of the exemplary
structure after formation of dielectric fill material portions by a
planarization process according to an embodiment of the present
disclosure.
[0066] FIG. 18B is a vertical cross-sectional view of the exemplary
structure along the vertical plane B-B' of FIG. 18A.
[0067] FIG. 18C is a vertical cross-sectional view of the exemplary
structure along the vertical plane C-C' of FIG. 18A.
[0068] FIG. 18D is a vertical cross-sectional view of the exemplary
structure along the vertical plane D-D' of FIG. 18A.
[0069] FIG. 18E is a vertical cross-sectional view of the exemplary
structure along the vertical plane E-E' of FIG. 18A.
[0070] FIG. 19A is a vertical cross-sectional view of the exemplary
structure after formation of a rectangular array of third memory
pillar structures, additional conductive rails, second cavities,
and additional dielectric fill material portions according to an
embodiment of the present disclosure.
[0071] FIG. 19B is a vertical cross-sectional view of the exemplary
structure along the vertical plane B-B' of FIG. 19A.
[0072] FIG. 19C is a vertical cross-sectional view of the exemplary
structure along the vertical plane C-C' of FIG. 19A.
[0073] FIG. 19D is a vertical cross-sectional view of the exemplary
structure along the vertical plane D-D' of FIG. 19A.
[0074] FIG. 19E is a vertical cross-sectional view of the exemplary
structure along the vertical plane E-E' of FIG. 19A.
DETAILED DESCRIPTION
[0075] As discussed above, the present disclosure is directed to
three-dimensional cross rail phase change memory structures
including air gaps and methods of forming the same, the various
aspects of which are described below. The air gaps reduce
capacitive coupling of conductive lines in a three-dimensional
phase change memory device, which improves the device performance
by reducing the RC delay. The drawings are not drawn to scale.
Multiple instances of an element may be duplicated where a single
instance of the element is illustrated, unless absence of
duplication of elements is expressly described or clearly indicated
otherwise. Unless otherwise indicated, a "contact" between elements
refers to a direct contact between elements that provides an edge
or a surface shared by the elements. Ordinals such as "first,"
"second," and "third" are employed merely to identify similar
elements, and different ordinals may be employed across the
specification and the claims of the instant disclosure. A same
reference numeral refers to a same element or a similar element.
Unless otherwise noted, elements with a same reference numeral are
presumed to have a same material composition.
[0076] As used herein, a "layer" refers to a material portion
including a region having a thickness. A layer may extend over the
entirety of an underlying or overlying structure, or may have an
extent less than the extent of an underlying or overlying
structure. Further, a layer may be a region of a homogeneous or
inhomogeneous continuous structure that has a thickness less than
the thickness of the continuous structure. For example, a layer may
be located between any pair of horizontal planes between, or at, a
top surface and a bottom surface of the continuous structure. A
layer may extend horizontally, vertically, and/or along a tapered
surface. A substrate may be a layer, may include one or more layers
therein, and/or may have one or more layer thereupon, thereabove,
and/or therebelow.
[0077] As used herein, a "layer stack" refers to a stack of layers.
As used herein, a "line" or a "line structure" refers to a layer
that has a predominant direction of extension, i.e., having a
direction along which the layer extends the most.
[0078] As used herein, a "semiconducting material" refers to a
material having electrical conductivity in the range from
1.0.times.10.sup.-6 S/cm to 1.0.times.10.sup.5 S/cm. As used
herein, a "semiconductor material" refers to a material having
electrical conductivity in the range from 1.0.times.10.sup.-6 S/cm
to 1.0.times.10.sup.5 S/cm in the absence of electrical dopants
therein, and is capable of producing a doped material having
electrical conductivity in a range from 1.0 S/cm to
1.0.times.10.sup.5 S/cm upon suitable doping with an electrical
dopant. As used herein, an "electrical dopant" refers to a p-type
dopant that adds a hole to a valence band within a band structure,
or an n-type dopant that adds an electron to a conduction band
within a band structure. As used herein, a "conductive material"
refers to a material having electrical conductivity greater than
1.0.times.10.sup.5 S/cm. As used herein, an "insulator material" or
a "dielectric material" refers to a material having electrical
conductivity less than 1.0.times.10.sup.-6 S/cm. As used herein, a
"heavily doped semiconductor material" refers to a semiconductor
material that is doped with electrical dopant at a sufficiently
high atomic concentration to become a conductive material, i.e., to
have electrical conductivity greater than 1.0.times.10.sup.5 S/cm.
A "doped semiconductor material" may be a heavily doped
semiconductor material, or may be a semiconductor material that
includes electrical dopants (i.e., p-type dopants and/or n-type
dopants) at a concentration that provides electrical conductivity
in the range from 1.0.times.10.sup.-6 S/cm to 1.0.times.10.sup.5
S/cm. An "intrinsic semiconductor material" refers to a
semiconductor material that is not doped with electrical dopants.
Thus, a semiconductor material may be semiconducting or conductive,
and may be an intrinsic semiconductor material or a doped
semiconductor material. A doped semiconductor material can be
semiconducting or conductive depending on the atomic concentration
of electrical dopants therein. As used herein, a "metallic
material" refers to a conductive material including at least one
metallic element therein. All measurements for electrical
conductivities are made at the standard condition.
[0079] Referring to FIGS. 1A and 1B, an exemplary structure for
forming a three-dimensional phase change memory device is
illustrated, which includes a substrate 8. The substrate 8 includes
an insulating material layer in an upper portion, and may
optionally include additional layers (not illustrated) underneath,
which can include, for example, a semiconductor material layer and
interconnect level dielectric layers embedding metal interconnect
structures therein. In one embodiment, semiconductor devices such
as field effect transistors may be provided on the semiconductor
material layer, and the metal interconnect structures can provide
electrically conductive paths among the semiconductor devices. The
exemplary structure includes a memory array region, which is
illustrated herein, and a peripheral region (not illustrated)
including interconnect structures and/or peripheral devices. Memory
cells are subsequently formed in the memory array region.
[0080] A first vertical stack (12L, 14L, 16L, 17L, 173L), which is
also referred to as a first layer stack, is formed over the
substrate 8. The first vertical stack (12L, 14L, 16L, 17L, 173L)
can include a first bottom conductive layer 12L, a first selector
layer 14L, a first phase change memory layer 16L, an optional first
barrier layer 17L, and an optional first hard mask layer 173L. Each
layer in the first vertical stack (12L, 14L, 16L, 17L, 173L) can be
formed as a blanket material layer, i.e., an unpatterned material
layer that laterally extend along a first horizontal direction hd1
and a second horizontal direction hd2 that is perpendicular to the
first horizontal direction hd1.
[0081] The first bottom conductive layer 12L includes at least one
conductive material layer, which can be at least one metallic
material layer. For example, the first bottom conductive layer 12L
can include a layer stack, from bottom to top, of a first metal
layer 122L (such as a tungsten layer) and a first metal nitride
layer 124L (such as a tungsten nitride layer or a titanium nitride
layer). The thickness of the first metal layer 122L can be in a
range from 20 nm to 100 nm, such as from 30 nm to 70 nm, although
lesser and greater thicknesses can also be employed. The thickness
of the first metal nitride layer 124L can be in a range from 1 nm
to 10 nm, such as from 1.5 nm to 5 nm, although lesser and greater
thicknesses can also be employed.
[0082] The first selector layer 14L includes a non-Ohmic material
that provides electrical connection of electrical isolation
depending on the magnitude and/or the polarity of an externally
applied voltage bias thereacross. In one embodiment, the first
selector layer 14L includes at least one threshold switch material
layer. The at least one threshold switch material layer includes
any suitable threshold switch material which exhibits non-linear
electrical behavior, such as an ovonic threshold switch material or
a diode threshold switch material (e.g., materials for p-n
semiconductor diode, p-i-n semiconductor diode, Schottky diode or
metal-insulator-metal diode). As used herein, an ovonic threshold
switch (OTS) is a device that does not crystallize in a low
resistance state under a voltage above the threshold voltage, and
reverts back to a high resistance state when not subjected to a
voltage above the threshold voltage across the OTS material layer.
As used herein, an "ovonic threshold switch material" refers to a
material that displays a non-linear resistivity curve under an
applied external bias voltage such that the resistivity of the
material decreases with the magnitude of the applied external bias
voltage. In other words, an ovonic threshold switch material is
non-Ohmic, and becomes more conductive under a higher external bias
voltage than under a lower external bias voltage.
[0083] An ovonic threshold switch material (OTS material) can be
non-crystalline (for example, amorphous) in a high resistance
state, and can remain non-crystalline (for example, remain
amorphous) in a low resistance state during application of a
voltage above its threshold voltage across the OTS material. The
OTS material can revert back to the high resistance state when the
high voltage above its threshold voltage is removed. Throughout the
resistive state changes, the ovonic threshold switch material can
remain non-crystalline (e.g., amorphous). In one embodiment, the
ovonic threshold switch material can comprise layer a chalcogenide
material which exhibits hysteresis in both the write and read
states. The chalcogenide material may be a GeTe compound or a
Ge--Se compound doped with a dopant selected from As, N, and C,
such as a Ge--Se--As compound semiconductor material. The ovonic
threshold switch material layer can include a first ovonic
threshold switch material layer 144L which contains any ovonic
threshold switch material. In one embodiment, the first ovonic
threshold switch material layer 144L can include, and/or can
consist essentially of, a GeSeAs alloy, a GeSe alloy, a SeAs alloy,
a GeTe alloy, or a SiTe alloy.
[0084] In one embodiment, the material of the first ovonic
threshold switch material layer 144L can be selected such that the
resistivity of the first ovonic threshold switch material layer
144L decreases at least by two orders of magnitude (i.e., by more
than a factor of 100) upon application of an external bias voltage
that exceeds a critical bias voltage magnitude (also referred to as
threshold voltage). In one embodiment, the composition and the
thickness of the first ovonic threshold switch material layer 144L
can be selected such that the critical bias voltage magnitude can
be in a range from 1 V to 4 V, although lesser and greater voltages
can also be employed for the critical bias voltage magnitude. The
thickness of the first ovonic threshold switch material layer 144L
can be, for example, in a range from 5 nm to 50 nm, such as from 10
nm to 30 nm, although lesser and greater thicknesses can also be
employed.
[0085] The first selector layer 14L can include an optional first
upper barrier material layer 146L overlying the first ovonic
threshold switch material layer 144L and an optional first lower
barrier material layer 142L underlying the first ovonic threshold
switch material layer 144L. The optional first upper and/or lower
barrier material layers (146L, 142L) include a material that
suppresses diffusion of the material of the first ovonic threshold
switch material layer 144L. For example, the first upper and/or
lower barrier material layers (146L, 142L) can include amorphous
carbon or diamond-like carbon (DLC). In one embodiment, the first
upper barrier material layers 146L can include an upper amorphous
carbon layer that contacts a top surface of the first ovonic
threshold switch material layer 144L, and the first lower barrier
material layer 142L can include a lower amorphous carbon layer that
contacts a bottom surface of the first ovonic threshold switch
material layer 144L. The thickness of the first upper barrier
material layers 146L can be in a range from 4 nm to 40 nm, such as
from 8 nm to 20 nm, although lesser and greater thicknesses can
also be employed. The thickness of the first lower barrier material
layer 142L can be in a range from 4 nm to 40 nm, such as from 8 nm
to 20 nm, although lesser and greater thicknesses can also be
employed.
[0086] The first phase change memory layer 16L includes a first
phase change memory material layer 164L. The first phase change
memory material layer 164L include a phase change memory material.
As used herein, a "phase change memory material" refers to a
material having at least two different phases providing different
resistivity. The at least two different phases can be provided, for
example, by controlling the rate of cooling from a heated state to
provide an amorphous state having a higher resistivity and a
polycrystalline state having a lower resistivity. In this case, the
higher resistivity state of the phase change memory material can be
achieved by faster quenching of the phase change memory material
after heating to an amorphous state, and the lower resistivity
state of the phase change memory material can be achieved by slower
cooling of the phase change memory material after heating to the
amorphous state
[0087] Exemplary phase change memory materials include, but are not
limited to, germanium antimony telluride compounds such as
Ge.sub.2Sb.sub.2Te.sub.5 (GST), germanium antimony compounds,
indium germanium telluride compounds, aluminum selenium telluride
compounds, indium selenium telluride compounds, and aluminum indium
selenium telluride compounds. These compounds (e.g., compound
semiconductor material) may be doped (e.g., nitrogen doped GST) or
undoped. Thus, the phase change memory material layer can include,
and/or can consist essentially of, a material selected from a
germanium antimony telluride compound, a germanium antimony
compound, an indium germanium telluride compound, an aluminum
selenium telluride compound, an indium selenium telluride compound,
or an aluminum indium selenium telluride compound. The thickness of
the first phase change memory material layer 164L can be in a range
from 1 nm to 60 nm, such as from 3 nm to 40 nm and/or from 10 nm to
25 nm, although lesser and greater thicknesses can also be
employed.
[0088] The first phase change memory layer 16L can optionally
include a first lower conductive liner layer 162L underlying the
first phase change memory material layer 164L, and can optionally
include a first upper conductive liner layer 166L overlying the
first phase change memory material layer 164L. The optional first
lower conductive liner layer 162L and/or the optional first upper
conductive liner layer 166L, if present, include a conductive
metallic material. In one embodiment, the first lower conductive
liner layer 162L and/or the first upper conductive liner layer 166L
can include a conductive metallic nitride such as titanium nitride,
tantalum nitride, or tungsten nitride. The thickness of each of the
first lower conductive liner layer 162L and the first upper
conductive liner layer 166L can be in a range from 1 nm to 10 nm,
such as from 1.5 nm to 5 nm, although lesser and greater
thicknesses can also be employed.
[0089] The optional first barrier layer 17L, if present, includes a
barrier material, and can be formed on top of the first phase
change memory layer 16L. The barrier material is a material that
can prevent diffusion of, and provide effective encapsulation of,
the first phase change memory material. In one embodiment, the
barrier material can include, and/or can consist essentially of,
amorphous carbon. The thickness of the first barrier layer 17L can
be in a range from 12 nm to 75 nm, such as from 20 nm to 60 nm,
although lesser and grater thicknesses can also be employed.
[0090] The optional first hard mask layer 173L includes a hard mask
material that can be employed as a planarization stopping structure
in a subsequent planarization process. The first hard mask layer
173L can include a material selected from a metal, a dielectric
material, or a semiconductor material. For example, the first hard
mask layer 173L can include silicon nitride, a dielectric metal
oxide, or a metal. In one embodiment, the first hard mask layer
173L can include silicon nitride. The thickness of the first hard
mask layer 173L can be in a range from 3 nm to 30 nm, such as from
6 nm to 15 nm, although lesser and greater thicknesses can also be
employed.
[0091] Referring to FIGS. 2A-2C, a first photoresist layer 197 can
be applied over the first vertical stack (12L, 14L, 16L, 17L,
173L), and can be lithographically patterned to form a line and
space pattern. For example, the first photoresist layer 197 can be
patterned to form line trenches that laterally extend along the
first horizontal direction hd1 and laterally spaced apart along the
second horizontal direction hd2. The line trenches can have a
uniform width that is invariant with translation along the first
horizontal direction hd1. The pattern in the first photoresist
layer 197 can be a periodic pattern that is repeated along the
second horizontal direction hd2 with a pitch that is equal to the
sum of the width of a line trench in the first photoresist layer
197 and the width of a patterned portion of the first photoresist
layer 197. The pitch can be in a range from 32 nm to 600 nm,
although lesser and greater pitches can also be employed.
[0092] An anisotropic etch process is performed employing the
patterned portions of the first photoresist layer 197 as an etch
mask. The anisotropic etch process etches through portions of the
first vertical stack (12L, 14L, 16L, 17L, 173L) that are not masked
by the first photoresist layer 197. The chemistry of the
anisotropic etch process can be sequentially modified to etch
through the various material layers of the first vertical stack
(12L, 14L, 16L, 17L, 173L). The anisotropic etch process can stop
at, or below, the top surface of the substrate 8. First trenches 11
laterally extending along the first horizontal direction hd1 are
formed by the anisotropic etch. The first trenches 11 can be line
trenches having a uniform vertical cross-sectional shape within
vertical planes that are perpendicular to the first horizontal
direction hd1. The uniform vertical cross-sectional shape of each
first trench 11 can be invariant with translation along the first
horizontal direction hd1. Each first trench 11 can vertically
extend from the horizontal plane including the bottom surface of
the first photoresist layer 197 to the horizontal plane including
the top surface of the substrate 8. Each first trench 11 can have a
vertical cross-sectional shape of an inverted trapezoid.
[0093] Remaining portions of the first vertical stack (12L, 14L,
16L, 17L, 173L) include first stacked rail structures (12, 14',
16', 17', 173'), each of which laterally extends along the first
horizontal direction hd1. The first stacked rail structures (12,
14', 16', 17', 173') are laterally spaced apart by the first
trenches 11 along the second horizontal direction hd2. As used
herein, a "rail" or a "rail structure" refers to a structure that
extends along a lengthwise direction, and optionally with a uniform
cross-sectional shape within planes that are perpendicular to the
lengthwise direction that is invariant under translation along the
lengthwise direction. As used herein, a "stacked rail" or a
"stacked rail structure" refers to a contiguous stack of at least
two rails that laterally extend along a same lengthwise
direction.
[0094] Each first stacked rail structure (12, 14', 16', 17', 173')
includes, from bottom to top, a first conductive rail 12 that is a
patterned portion of the first bottom conductive layer 12L, a first
selector rail 14' that is a patterned portion of the first selector
layer 14L, a first phase change memory rail 16' that is a patterned
portion of the first phase change memory layer 16L, a first barrier
rail 17' that is a patterned portion of the first barrier layer
17L, and an optional first hard mask strip 173' that is a patterned
portion of the first hard mask layer 173L. As used herein, a
"strip" refers to a rail having a thickness that is less than the
width. The first stacked rail structures (12, 14', 16', 17', 173')
laterally extend along the first horizontal direction hd1, are
laterally spaced among one another by the first trenches 11, and
are located over the substrate 8.
[0095] In one embodiment, each of the first conductive rails 12 can
include a vertical stack of a first metal rail 122 that is a
patterned portion of the first metal layer 122L and a first metal
nitride strip 124 that is a patterned portion of the first metal
nitride layer 124L. In one embodiment, each first selector rail 14'
can include a vertical stack of a first lower barrier material
strip 142', a first ovonic threshold switch material rail 144', and
a first upper barrier material strip 146'. The first lower barrier
material strip 142' is a patterned portion of an optional first
lower barrier material layer 142L. The first ovonic threshold
switch material rail 144' is a patterned portion of the first
ovonic threshold switch material layer 144L. The first upper
barrier material strip 146' is a patterned portion of the optional
first upper barrier material layer 146L. In one embodiment, each
first phase change memory rail 16' can include, from bottom to top,
a first optional lower conductive liner strip 162', a first phase
change memory material rail 164', and a first optional upper
conductive liner strip 166'. Each first lower conductive liner
strip 162' is a patterned portion of the first lower conductive
liner layer 162L, each first phase change memory material rail 164'
is a patterned portion of the first phase change memory material
layer 164L, and each first upper conductive liner strip 166' is a
patterned portion of the first upper conductive liner layer 166L.
The first photoresist layer 197 can be subsequently removed, for
example, by ashing. The first stacked rail structures (12, 14',
16', 17', 173') laterally extend along the first horizontal
direction hd1, and are laterally spaced among one another by the
first trenches 11 over the substrate 8.
[0096] Referring to FIGS. 3A and 3B, a first continuous dielectric
liner layer can be optionally deposited on sidewalls and bottom
surfaces of the first trenches 11 and over the first hard mask
strips 173'. The first continuous dielectric liner layer includes a
dielectric material such as silicon nitride, a dielectric metal
oxide (such as aluminum oxide), or silicon oxide. The first
continuous dielectric liner layer can be deposited by a conformal
deposition method such as chemical vapor deposition (CVD) or atomic
layer deposition (ALD). The thickness of the first continuous
dielectric liner layer can be in a range from 1 nm to 10 nm, such
as from 2 nm to 6 nm, although lesser and greater thicknesses can
also be employed. The first continuous dielectric liner layer
includes a material different from the material of the first
barrier rails 17'.
[0097] A first dielectric fill material layer can be deposited on
the first continuous dielectric liner layer. The first dielectric
fill material layer includes a planarizable dielectric material
such as undoped silicate glass (e.g., silicon oxide), doped
silicate glass, or a spin-on glass (SOG). The first dielectric fill
material layer can be deposited by a conformal deposition method or
by a self-planarizing deposition method (such as spin coating). In
one embodiment, the first continuous dielectric liner layer can
include, and/or can consist essentially of, a material selected
from silicon nitride and a dielectric metal oxide, and the first
dielectric fill material layer can include, and/or can consist
essentially of, a material selected from doped silicate glass and
undoped silicate glass.
[0098] Portions of the first continuous dielectric liner layer and
the first dielectric fill material layer can be removed from above
the horizontal plane including top surfaces of the first hard mask
strips 173' by a planarization process. The planarization process
can include chemical mechanical planarization (CMP) and/or a recess
etch process. Each remaining portion of the first continuous
dielectric liner layer constitutes a first dielectric liner 152,
and each remaining portion of the first dielectric fill material
layer constitutes a first dielectric fill material portion 154.
Each contiguous set of a first dielectric liner 152 and a first
dielectric fill material portion 154 constitutes a first dielectric
isolation structure (152, 154), which is a dielectric isolation
structure having a shape of a rail. A first dielectric isolation
structure (152, 154) is formed in each of the first trenches 11 as
a rail structure. The first dielectric isolation structures (152,
154) laterally extend along the first horizontal direction hd1, and
are laterally spaced among one another along the second horizontal
direction hd2. The top surfaces of the first dielectric isolation
structures (152, 154) can be coplanar with the top surfaces of the
first hard mask strips 173'.
[0099] If the first hard mask strips 173' are present, top surfaces
of the first dielectric isolation structures (152, 154) can be
recessed by an etch process, which can be an isotropic etch process
or an anisotropic etch process. The duration of the etch process
can be selected such that the recessed top surfaces of the first
dielectric isolation structures (152, 154) is at about the level of
the top surfaces of the first barrier rails 17'. The first hard
mask strips 173' can be subsequently removed. For example, if the
first hard mask strips 173' include silicon nitride, the first hard
mask strips 173' can be removed by a wet etch employing hot
phosphoric acid.
[0100] Referring to FIGS. 4A-4C, a first top conductive layer 18L
can be formed over the one-dimensional array of first stacked rail
structures (12, 14', 16', 17') spaced by the first dielectric
isolation structures (152, 154). The first top conductive layer 18L
includes at least one conductive material layer, which can be at
least one metallic material layer. For example, the first top
conductive layer 18L can include a metal layer (such as a tungsten
layer). The thickness of the first top conductive layer 18L can be
in a range from 20 nm to 100 nm, such as from 30 nm to 70 nm,
although lesser and greater thicknesses can also be employed. The
thickness of the first top conductive layer 18L can be in a range
from 1 nm to 10 nm, such as from 1.5 nm to 5 nm, although lesser
and greater thicknesses can also be employed.
[0101] A second photoresist layer 199 having a line and space
pattern can be formed over the first top conductive layer 18L.
Portions of the second photoresist layer 199 laterally extend along
the second horizontal direction hd2 with a respective uniform
width. Thus, line trenches are present among the portions of the
second photoresist layer 199. The line trenches laterally extend
along the second horizontal direction hd2, and are laterally spaced
apart along the first horizontal direction hd1. The line trenches
can have a uniform width that is invariant with translation along
the second horizontal direction hd2. The pattern in the second
photoresist layer 199 can be a periodic pattern that is repeated
along the first horizontal direction hd1 with a pitch that is equal
to the sum of the width of a line trench in the second photoresist
layer 199 and the width of a patterned line-shaped portion of the
second photoresist layer 199. The pitch can be in a range from 32
nm to 600 nm, although lesser and greater pitches can also be
employed.
[0102] Referring to FIGS. 5A-5C, second trenches 15 are formed
through the first top conductive layer 18L, the first stacked rail
structures (12, 14', 16', 17'), and the first dielectric isolation
structures (152, 154). An anisotropic etch process can be performed
to etch portions of the first top conductive layer 18L, the first
barrier rails 17', the first phase change memory rails 16', the
first selector rails 14', and upper regions of the first dielectric
isolation structures (152, 154) that are not masked by the second
photoresist layer 199. The materials of the first top conductive
layer 18L, the first barrier rails 17', the first phase change
memory rails 16', the first selector rails 14', and the first
dielectric isolation structures (152, 154) can be etched selective
to the material in the top portions of the first conductive rails
12 by the anisotropic etch process. Volumes from which the
materials of the first top conductive layer 18L, the first barrier
rails 17', the first phase change memory rails 16', the first
selector rails 14', and upper regions of the first dielectric
isolation structures (152, 154) are removed constitute the second
trenches 15. The second trenches 15 laterally extend along the
second horizontal direction hd2.
[0103] The chemistry of the anisotropic etch process can be
sequentially modified to etch through the various materials of the
first top conductive layer 18L, the first barrier rails 17', the
first phase change memory rails 16', the first selector rails 14',
and the first dielectric isolation structures (152, 154). The
anisotropic etch process can stop at the top surface of, or within,
the first conductive rails 12. For example, the anisotropic etch
process can stop on the first metal nitride strips 124. The second
trenches 15 can be line trenches having a uniform vertical
cross-sectional shape within vertical planes that are perpendicular
to the second horizontal direction hd2. The uniform vertical
cross-sectional shape of each second trench 15 can be invariant
with translation along the second horizontal direction hd2. Each
second trench 15 can vertically extend from the horizontal plane
including the bottom surface of the second photoresist layer 199 to
the horizontal plane including top surfaces of the first conductive
rails 12 (which can be word lines for example). Each second trench
15 can have a vertical cross-sectional shape of an inverted
trapezoid such that each second trench 15 has a pair of tapered
lengthwise sidewalls and a lesser width at a bottom portion than at
a top portion.
[0104] The first top conductive layer 18L is divided into a
one-dimensional array of conductive rails, which are herein
referred to as lower second conductive rail portions 18 (which can
be portions of bit lines for example). Each lower second conductive
rail portion 18 is subsequently incorporated into a respective
second conductive rail that includes a respective upper second
conductive rail portion to be subsequently formed thereabove.
[0105] Each vertical stack of a first barrier rail 17', a first
phase change memory rails 16', and a first selector rails 14' is
divided into a row of first memory pillar structures (14, 16, 17)
that are laterally spaced apart long the first horizontal direction
hd1. The combination of the first barrier rails 17', the first
phase change memory rails 16', and the first selector rails 14' is
divided into a two-dimensional array of first memory pillar
structures (14, 16, 17), which may be a periodic two-dimensional
array. The two-dimensional array of first memory pillar structures
(14, 16, 17) can form a rectangular periodic two-dimensional array
having a first pitch along the first horizontal direction hd1 and a
second pitch along the second horizontal direction hd2.
[0106] As used herein, a "pillar" or a "pillar structure" refers to
a structure that extends along a vertical direction such that each
sidewall of the structure is vertical or is substantially vertical.
As used herein, a sidewall is "substantially vertical" if the
sidewall is tapered, i.e., tilted, with respect to a vertical
direction by a tilt angle that is less than 10 degrees.
[0107] Each first memory pillar structure (14, 16, 17) includes,
from bottom to top, a first selector pillar 14 that is a patterned
portion of a first selector rail 14', a first phase change memory
pillar 16 that is a patterned portion of a phase change memory rail
16', and an optional first barrier plate 17 that is a patterned
portion of a first barrier strip 17'. As used herein, a "plate"
refers to a structure with a vertical thickness (such as a uniform
vertical thickness) that is less than the maximum lateral dimension
of the structure in any horizontal direction.
[0108] Each first selector pillar 14 is a selector element, i.e.,
an element that provides non-linear voltage-current characteristics
such that the element functions as a conductor under first voltage
bias conditions and as an insulator under second voltage bias
conditions. In one embodiment, each first selector pillar 14 can
include a vertical stack of a first lower barrier material portion
142 that is a patterned portion of a first lower barrier material
strip 142', a first ovonic threshold switch material portion 144
that is a patterned portion of a first ovonic threshold switch
material rail 144', and a first upper barrier material portion 146
that is a patterned portion of a first upper barrier material strip
146'.
[0109] In one embodiment, each first selector pillar 14 includes a
first ovonic threshold switch material portion 144, an upper
amorphous carbon portion that contacts a top surface of the first
ovonic threshold switch material portion 144 as a first upper
barrier material portion 146, and a lower amorphous carbon portion
that contacts a bottom surface of the first ovonic threshold switch
material portion 144 as a first lower barrier material portion
142.
[0110] Each first phase change memory pillar 16 is a phase change
memory element, i.e., a structure that changes the resistance
depending on the phase of a material therein. In one embodiment,
each first phase change memory pillar 16 can include a vertical
stack of an optional first lower conductive liner plate 162 that is
a patterned portion of a first optional lower conductive liner
strip 162', a first phase change memory material pillar 164 that is
a patterned portion of a first phase change memory material rail
164', and a first optional upper conductive liner plate 166 that is
a patterned portion of a first upper conductive liner strip
166'.
[0111] Each first dielectric isolation structure (152, 154) has a
laterally undulating height after the anisotropic etch process.
Specifically, the height of each first dielectric isolation
structure (152, 154) can be the same as the total thickness of a
first stacked rail structure (12, 14', 16', 17') as provided at the
processing steps of FIGS. 4A and 4B underneath each portion of the
second photoresist layer 199, and can be about the same as the
height (i.e., the thickness) of a first conductive rail 12
underneath each second trench 15. Each first dielectric isolation
structure (152, 154) can continuously extend underneath a plurality
of second trenches 15.
[0112] The first barrier plates 17 are barrier elements, and can be
arranged in a two-dimensional periodic array. While FIGS. 5A-5C
illustrate an embodiment in which a first phase change memory
pillar 16 overlies a first selector pillar within each first memory
pillar structure (14, 16, 17), embodiments are expressly
contemplated herein in which a first phase change memory pillar 16
underlies a first selector pillar within each first memory pillar
structure (14, 16, 17). In each case, if a first barrier plate 17'
is present within a first memory pillar structure (14, 16, 17), a
two-dimensional array of barrier elements (i.e., first barrier
plates 17) may be disposed between the first phase change memory
elements (i.e., the first phase change memory pillars 16) and a
proximal set among a set of the first conductive rails 12 and a set
of the lower second conductive rail portions 18.
[0113] Referring to FIGS. 6A-6C, the second photoresist layer 199
can be subsequently removed, for example, by ashing. A first
sacrificial dielectric liner layer can be optionally deposited on
sidewalls and bottom surfaces of the second trenches 15 and over
the lower second conductive rail portions 18. The first sacrificial
dielectric liner layer includes a dielectric material such as
silicon nitride, a dielectric metal oxide (such as aluminum oxide),
or silicon oxide. The first sacrificial dielectric liner layer can
be deposited by a conformal deposition method such as chemical
vapor deposition (CVD) or atomic layer deposition (ALD). The
thickness of the first sacrificial dielectric liner layer can be in
a range from 1 nm to 10 nm, such as from 2 nm to 6 nm, although
lesser and greater thicknesses can also be employed.
[0114] A first sacrificial fill material layer can be deposited on
the first sacrificial dielectric liner layer. The first sacrificial
fill material layer includes a planarizable dielectric material
such as undoped silicate glass, doped silicate glass, or a spin-on
glass (SOG). The first sacrificial fill material layer can be
deposited by a conformal deposition method or by a self-planarizing
deposition method (such as spin coating). In one embodiment, the
first sacrificial dielectric liner layer can include, and/or can
consist essentially of, a material selected from silicon nitride
and a dielectric metal oxide, and the first sacrificial fill
material layer can include, and/or can consist essentially of, a
material selected from doped silicate glass and undoped silicate
glass.
[0115] Portions of the first sacrificial dielectric liner layer and
the first sacrificial fill material layer can be removed from above
the horizontal plane including top surfaces of the lower second
conductive rail portions 18 by a planarization process. The
planarization process can include chemical mechanical planarization
(CMP) and/or a recess etch process. Each remaining portion of the
first sacrificial dielectric liner layer constitutes a first
sacrificial liner 155, and each remaining portion of the first
sacrificial fill material layer constitute a first sacrificial fill
material portion 157. Each contiguous set of a first sacrificial
liner 155 and a first sacrificial fill material portion 157
constitutes a first sacrificial trench fill structure (155, 157),
which has a shape of a rail. A first sacrificial trench fill
structure (155, 157) is formed in each of the second trenches 15 as
a rail structure. The first sacrificial trench fill structures
(155, 157) laterally extend along the second horizontal direction
hd2, and are laterally spaced among one another along the first
horizontal direction hd1. The top surfaces of the first sacrificial
trench fill structures (155, 157) can be coplanar with the top
surfaces of the lower second conductive rail portions 18.
[0116] Referring to FIGS. 7A-7C, a second vertical stack (22L, 24L,
26L, 27L, 273L), which is also referred to as a second layer stack,
is formed over the lower second conductive rail portions 18 and the
first sacrificial trench fill structures (155, 157). The second
vertical stack (22L, 24L, 26L, 27L, 273L) can include a second
bottom conductive layer 22L, a second selector layer 24L, a second
phase change memory layer 26L, an optional second barrier layer
27L, and an optional second hard mask layer 273L. Each layer in the
second vertical stack (22L, 24L, 26L, 27L, 273L) can be formed as a
blanket material layer.
[0117] The second bottom conductive layer 22L includes at least one
conductive material layer, which can be at least one metallic
material layer. For example, the second bottom conductive layer 22L
can include a layer stack, from bottom to top, of a second metal
layer 222L (such as a tungsten layer) and a second metal nitride
layer 224L (such as a tungsten nitride layer or a titanium nitride
layer). The thickness of the second metal layer 222L can be in a
range from 20 nm to 200 nm, such as from 30 nm to 70 nm, although
lesser and greater thicknesses can also be employed. The thickness
of the second metal nitride layer 224L can be in a range from 2 nm
to 20 nm, such as from 2.5 nm to 5 nm, although lesser and greater
thicknesses can also be employed.
[0118] The second selector layer 24L includes a layer of a
non-Ohmic material. The non-Ohmic material of the second selector
layer 24L can be any of the non-Ohmic materials that may be
employed for the first selector layer 14L. In one embodiment, the
second selector layer 24L includes a second ovonic threshold switch
material layer 244L. In one embodiment, the material of the second
ovonic threshold switch material layer 244L can be selected such
that the resistivity of the second ovonic threshold switch material
layer 244L decreases at least by two orders of magnitude (i.e., by
more than a factor of 100) upon application of an external bias
voltage that exceeds a critical bias voltage magnitude. In one
embodiment, the composition and the thickness of the second ovonic
threshold switch material layer 244L can be selected such that the
critical bias voltage magnitude can be in a range from 2 V to 4 V,
although lesser and greater voltages can also be employed for the
critical bias voltage magnitude. The thickness of the second ovonic
threshold switch material layer 244L can be, for example, in a
range from 5 nm to 50 nm, such as from 20 nm to 30 nm, although
lesser and greater thicknesses can also be employed.
[0119] The second selector layer 24L can include an optional second
upper barrier material layer 246L overlying the second ovonic
threshold switch material layer 244L and an optional second lower
barrier material layer 242L underlying the second ovonic threshold
switch material layer 244L. The optional second upper and/or lower
barrier material layers (246L, 242L) include a material that
suppresses diffusion of the material of the second ovonic threshold
switch material layer 244L. For example, the second upper and/or
lower barrier material layers (246L, 242L) can include amorphous
carbon or diamond-like carbon (DLC). In one embodiment, the second
upper barrier material layers 246L can include an upper amorphous
carbon layer that contacts a top surface of the second ovonic
threshold switch material layer 244L, and the second lower barrier
material layer 242L can include a lower amorphous carbon layer that
contacts a bottom surface of the second ovonic threshold switch
material layer 244L. The thickness of the second upper barrier
material layers 246L can be in a range from 4 nm to 40 nm, such as
from 8 nm to 20 nm, although lesser and greater thicknesses can
also be employed. The thickness of the second lower barrier
material layer 242L can be in a range from 4 nm to 40 nm, such as
from 8 nm to 20 nm, although lesser and greater thicknesses can
also be employed.
[0120] Alternatively or additionally, the second selector layer 24L
may include an alternative non-Ohmic material layer such as a p-n
or p-i-n junction diode layer. In this case, the second selector
layer 24L becomes conductive only under electrical bias condition
of one polarity, and becomes electrically non-conductive under
electrical bias condition of the opposite polarity.
[0121] The second phase change memory layer 26L includes a second
phase change memory material layer 264L. The second phase change
memory material layer 264L include a phase change memory material.
The phase change memory material of the second phase change memory
material layer 264L can be any phase change memory material that
may be employed for the first phase change memory material layer
164L. The thickness of the second phase change memory material
layer 264L can be in a range from 2 nm to 60 nm, such as from 3 nm
to 40 nm and/or from 10 nm to 25 nm, although lesser and greater
thicknesses can also be employed.
[0122] The second phase change memory layer 26L can optionally
include a second lower conductive liner layer 262L underlying the
second phase change memory material layer 264L, and can optionally
include a second upper conductive liner layer 266L overlying the
second phase change memory material layer 264L. The optional second
lower conductive liner layer 262L and/or the optional second upper
conductive liner layer 266L, if present, include a conductive
metallic material. In one embodiment, the second lower conductive
liner layer 262L and/or the second upper conductive liner layer
266L can include a conductive metallic nitride such as titanium
nitride, tantalum nitride, or tungsten nitride. The thickness of
each of the second lower conductive liner layer 262L and the second
upper conductive liner layer 266L can be in a range from 2 nm to 20
nm, such as from 2.5 nm to 5 nm, although lesser and greater
thicknesses can also be employed.
[0123] The optional second barrier layer 27L, if present, includes
a barrier material, and can be formed on top of the second phase
change memory layer 26L. The barrier material is a material that
can prevent diffusion of, and provide effective encapsulation of,
the second phase change memory material. In one embodiment, the
barrier material can include, and/or can consist essentially of,
amorphous carbon. The thickness of each second barrier layer 27L
can be in a range from 22 nm to 75 nm, such as from 20 nm to 60 nm,
although lesser and grater thicknesses can also be employed.
[0124] The optional second hard mask layer 273L includes a hard
mask material that can be employed as a planarization stopping
structure in a subsequent planarization process. The second hard
mask layer 273L can include a material selected from a metal, a
dielectric material, or a semiconductor material. For example, the
second hard mask layer 273L can include silicon nitride, a
dielectric metal oxide, or a metal. In one embodiment, the second
hard mask layer 273L can include silicon nitride. The thickness of
the second hard mask layer 273L can be in a range from 3 nm to 30
nm, such as from 6 nm to 25 nm, although lesser and greater
thicknesses can also be employed.
[0125] A third photoresist layer 297 can be applied over the second
vertical stack (22L, 24L, 26L, 27L, 273L), and can be
lithographically patterned to form a line and space pattern. For
example, the third photoresist layer 297 can be patterned to form
line trenches that laterally extend along the second horizontal
direction hd2 and laterally spaced apart along the first horizontal
direction hd1. The line trenches can have a uniform width that is
invariant with translation along the second horizontal direction
hd2. The pattern in the third photoresist layer 297 can be a
periodic pattern that is repeated along the first horizontal
direction hd1 with a pitch that is equal to the sum of the width of
a line trench in the third photoresist layer 297 and the width of a
patterned portion of the third photoresist layer 297. The pitch can
be in a range from 32 nm to 600 nm, although lesser and greater
pitches can also be employed. In one embodiment, the pattern of the
third photoresist layer 297 can be the same as the pattern in the
second photoresist layer 199.
[0126] Referring to FIGS. 8A-8C, an anisotropic etch process is
performed employing the patterned portions of the third photoresist
layer 297 as an etch mask. The anisotropic etch process etches
through portions of the second vertical stack (22L, 24L, 26L, 27L,
273L) that are not masked by the third photoresist layer 297. The
chemistry of the anisotropic etch process can be sequentially
modified to etch through the various material layers of the second
vertical stack (22L, 24L, 26L, 27L, 273L). The anisotropic etch
process can stop at, or below, the top surface of the substrate 8.
Third trenches 21 laterally extending along the second horizontal
direction hd2 are formed by the anisotropic etch. The third
trenches 21 can be line trenches having a uniform vertical
cross-sectional shape within vertical planes that are perpendicular
to the second horizontal direction hd2. The uniform vertical
cross-sectional shape of each third trench 21 can be invariant with
translation along the second horizontal direction hd2. Each third
trench 21 can vertically extend from the horizontal plane including
the bottom surface of the third photoresist layer 297 to the
horizontal plane including the top surfaces of the first
sacrificial trench fill structures (155, 157). Each third trench 21
can have a vertical cross-sectional shape of an inverted
trapezoid.
[0127] Remaining portions of the second vertical stack (22L, 24L,
26L, 27L, 273L) include second stacked rail structures (22, 24',
26', 27', 273'), each of which laterally extends along the second
horizontal direction hd2. The second stacked rail structures (22,
24', 26', 27', 273') are laterally spaced apart by the third
trenches 21 along the first horizontal direction hd1.
[0128] Each second stacked rail structure (22, 24', 26', 27', 273')
includes, from bottom to top, an upper second conductive rail
portion 22 that is a patterned portion of the second bottom
conductive layer 22L, a second selector rail 24' that is a
patterned portion of the second selector layer 24L, a second phase
change memory rail 26' that is a patterned portion of the second
phase change memory layer 26L, a second barrier rail 27' that is a
patterned portion of the second barrier layer 27L, and an optional
second hard mask strip 273' that is a patterned portion of the
second hard mask layer 273L. The second stacked rail structures
(22, 24', 26', 27', 273') laterally extend along the second
horizontal direction hd2, are laterally spaced among one another by
the third trenches 21 along the first horizontal direction, and are
located over a one-dimensional array of lower second conductive
rail portions 18.
[0129] In one embodiment, each of the upper second conductive rail
portions 22 can include a vertical stack of a second metal rail 222
that is a patterned portion of the second metal layer 212L and a
second metal nitride strip 224 that is a patterned portion of the
second metal nitride layer 224L. In one embodiment, each second
selector rail 24' can include a vertical stack of a second lower
barrier material strip 242', a second ovonic threshold switch
material rail 244', and a second upper barrier material strip 246'.
The second lower barrier material strip 242' is a patterned portion
of an optional second lower barrier material layer 242L. The second
ovonic threshold switch material rail 244' is a patterned portion
of the second ovonic threshold switch material layer 244L. The
second upper barrier material strip 246' is a patterned portion of
the optional second upper barrier material layer 246L. In one
embodiment, each second phase change memory rail 26' can include,
from bottom to top, a second optional lower conductive liner strip
262', a second phase change memory material rail 264', and a second
optional upper conductive liner strip 266'. Each second lower
conductive liner strip 262' is a patterned portion of the second
lower conductive liner layer 262L, each second phase change memory
material rail 264' is a patterned portion of the second phase
change memory material layer 264L, and each second upper conductive
liner strip 266' is a patterned portion of the second upper
conductive liner layer 266L. The second stacked rail structures
(22, 24', 26', 27', 273') laterally extend along the second
horizontal direction hd2, and are laterally spaced among one
another by the third trenches 21 over the substrate 8.
[0130] Each vertical stack of a lower second conductive rail
portion 18 and an upper second conductive rail portion 22
constitutes a second conductive rail (18, 22) that laterally
extends along the second horizontal direction. The second
conductive rails (18, 22) can be formed as a periodic
one-dimensional array. In one embodiment, the width of the top
surface of a lower second conductive rail portion 18 can be
narrower than the width of the bottom surface of an upper second
conductive rail portion 22 within each second conductive rail (18,
22). In this case, the sidewalls of each second conductive rail
(18, 22) can have a laterally undulating vertical cross-sectional
profile.
[0131] In the exemplary structure, the first conductive rails 12
(e.g., word lines) laterally extend along the first horizontal
direction hd1. A rectangular array of first memory pillar
structures (14, 16, 17) is provided on top surfaces of the first
conductive rails 12. Second conductive rails (18, 22) (e.g., bit
lines) laterally extend along the second horizontal direction hd2.
The first conductive rails 12 and rows of the first memory pillar
structures (14, 16, 17) are laterally spaced by first trenches 11,
which are filled with the first dielectric isolation structures
(152, 154). Lower portions of the second conductive rails (18, 22),
i.e., lower second conductive rail portions 18, and columns of the
first memory pillar structures (14, 16, 17) are laterally spaced by
third trenches 21, which are filled with the first sacrificial
trench fill structures (155, 157). Each first memory pillar
structure (14, 16, 17) comprises a series connection of a
respective first selector element 14 and a respective first phase
change memory element 16. Vertical stacks of a second selector rail
24' and a second phase change memory rail 26' are formed over the
second conductive rails (18, 22). The vertical stacks (24', 26')
and upper portions of the second conductive rails (18, 22), i.e.,
the upper second conductive rail portions 22, are laterally spaced
by third trenches 21 that laterally extend along the second
horizontal direction hd2.
[0132] Referring to FIGS. 9A-9C, the first sacrificial trench fill
structures (155, 157) can be removed to vertically extend the third
trenches 21. In one embodiment, an anisotropic etch process that
employs the third photoresist layer 297 can be performed to etch
the material of the first sacrificial fill material portions 157.
For example, if the first sacrificial fill material portions 157
include silicon oxide, a reactive ion etch process employing
CF.sub.4 and O.sub.2 can be performed to remove the first
sacrificial fill material portions 157. Alternatively, an isotropic
etch process may be employed to remove the material of the first
sacrificial fill material portions 157. For example, if the first
sacrificial fill material portions 157 include silicon oxide, a wet
etch employing hydrofluoric acid can be performed to remove the
first sacrificial fill material portions 157. Subsequently, the
first sacrificial liners 155 can be removed by another etch
process, which may be an anisotropic etch process or an isotropic
etch process. For example, if the first sacrificial liners 155
include silicon nitride, a wet etch employing hot phosphoric acid
can be employed to remove the first sacrificial liners 155. The
third photoresist layer 297 can be subsequently removed, for
example, by ashing. Top surfaces of the first conductive rails 12
are physically exposed at the bottom of each third trench 21.
Further, sidewalls and recessed top surfaces of the first
dielectric isolation structures (152, 154) are physically exposed
at a bottom portion of each third trench 21.
[0133] In an alternative embodiment, only the top portions of the
first sacrificial trench fill structures (155, 157) can be removed
(i.e., to only partially remove the first sacrificial trench fill
structures (155, 157)) to vertically extend the third trenches 21.
In this alternative embodiment, the top surface of the remaining
first sacrificial trench fill structures (155, 157) is located
below the optional barrier plate 17 such that the sidewalls of the
second conductive rails (18, 22) are exposed in the third trenches
21.
[0134] Referring to FIGS. 10A-10C, sacrificial semiconductor
structures 212 can be grown from the physically exposed surfaces of
the second conductive rails (18, 22) within each third trench 21.
Specifically, the sacrificial semiconductor structures 212 can be
formed by selectively growing a semiconductor material from
sidewalls of the second conductive rails (18, 22) while preventing
growth of the semiconductor material from physically exposed
sidewalls of the rectangular array of first memory pillar
structures (14, 16, 17) and the vertical stacks including a
respective second selector rail 24' and a respective second phase
change memory rail 26'. In one embodiment, the lower second
conductive rail portions 18 and the second metal rails 222 can
consist essentially of an elemental metal or an intermetallic alloy
that is conducive to select growth of a semiconductor material
during a selective semiconductor deposition process that inhibits
growth of the semiconductor material from insulating surfaces, the
chalcogenide surfaces, the carbon surfaces or metallic nitride
surfaces.
[0135] In an illustrative example, the lower second conductive rail
portions 18 and the second metal rails 222 can consist essentially
of tungsten, which is conducive to selective growth of a
semiconductor material such as amorphous silicon, polysilicon, or a
silicon-germanium alloy therefrom while the semiconductor material
does not grow from the surfaces of the surfaces of the first metal
nitride strips 124, the second metal nitride strips 224, the first
dielectric isolation structures (152, 154), the first selector
pillars 14, the first phase change memory pillars 16, the first
barrier plates 17, the second selector rails 24', the second phase
change memory rails 26', the second barrier rails 27', and the
optional second hard mask strips 273'.
[0136] The selective semiconductor deposition process that can be
employed to form the sacrificial semiconductor structures 212 can
be a low pressure chemical vapor deposition process in which a
reactant and an etchant are simultaneously or alternately flowed
into a process chamber including the exemplary structure. The
reactant can include one or more of silane, disilane,
dichlorosilane, trichlorosilane, germane, and digermane, and the
etchant can include gas phase hydrogen chloride. The reaction
temperature can be in a range from 450 degrees Celsius to 900
degrees Celsius, such as from 500 degrees Celsius to 700 degrees
Celsius. The partial pressure of the reactant can be in a range
from 1 mTorr to 1 Torr, and the partial pressure of the etchant can
be in a range from 1 mTorr to 1 Torr. A carrier gas such as
hydrogen or nitrogen may be employed to increase the total pressure
during processing to a range from 200 mTorr to 200 Torr. The
semiconductor material has a greater growth rate on metal surfaces
(such as tungsten surfaces) of the lower second conductive rail
portions 18 and the second metal rails 222 than on surfaces of
metal nitride materials, the surfaces of chalcogenide materials,
the surfaces of the carbon material, surfaces of metallic nitride
material or surfaces of dielectric materials. The flow rate of the
etchant is selected such that the etch rate provided by the etchant
is greater than the deposition rate on surfaces of metal nitride
materials, the surfaces of chalcogenide materials, the surfaces of
the carbon material, surfaces of metallic nitride material or
surfaces of dielectric materials than on the metal surfaces of the
lower second conductive rail portions 18 and the second metal rails
222. Thus, deposition of the semiconductor material proceeds only
from the surfaces of the lower second conductive rail portions 18
and the second metal rails 222.
[0137] The duration of the selective semiconductor deposition
process is selected such that two sacrificial semiconductor
structures 212 that grow from opposite sides within each third
trench 21 merge to form a seam along a center vertical plane of the
third trench 21 that extend along the second horizontal direction
hd2. A sealed cavity 211 extending along the second horizontal
direction hd2 is formed underneath each adjoined pair of
sacrificial semiconductor structures 212, and a recess cavity 213
extending along the second horizontal direction hd2 is formed above
each adjoined pair of sacrificial semiconductor structures 212. As
used herein, a cavity refers to a continuous volume that is free of
any solid material portion therein. A cavity may be filled with a
gas phase material only, or may be under vacuum. As used herein, a
"sealed cavity" or an "encapsulated cavity" refers to a cavity
bounded by a set of solid surfaces that do not include any opening
therein. A sealed cavity or an encapsulated cavity is defined
entirely by surfaces of a sealed volume. A recess cavity refers to
a cavity that extends downward from en empty volume located above.
Each sacrificial semiconductor structure 212 can have an upper
convex surface that laterally extends along the second horizontal
direction hd2, and a lower convex surface that laterally extends
along the second horizontal direction hd2. Generally, a pair of
sacrificial semiconductor structures 212 can be formed within each
third trench 21 by selective growth of the semiconductor material.
Convex surfaces (such as the upper convex surfaces) of the pair of
sacrificial semiconductor structures 212 can be adjoined to provide
an edge that extend along the second horizontal direction hd2
within each third trench 21.
[0138] Referring to FIGS. 11A-11C, a conformal dielectric material
layer is deposited in the recess trenches 213, which are upper
portions of the third trenches 21 that overlie the sacrificial
semiconductor structures 212. The conformal dielectric material
layer includes a dielectric material such as silicon nitride. The
thickness of the conformal dielectric material layer can be in a
range from 1 nm to 12 nm, such as from 1.5 nm to 6 nm, although
lesser and greater thicknesses can also be employed. The conformal
dielectric material layer can be deposited, for example, by low
pressure chemical vapor deposition process.
[0139] An anisotropic etch process can be performed to remove
horizontal portions of the conformal dielectric material layer.
Remaining portions of the conformal dielectric material layer after
the anisotropic etch process constitute dielectric liners 241'. The
dielectric liners 241' are formed on the sidewalls of the recess
trenches 213, which are upper portions of the third trenches 21
that overlie the sacrificial semiconductor structures 212.
[0140] Referring to FIGS. 12A-12C, a dielectric fill material can
be deposited on the dielectric liners 241' to fill the recess
trenches 213. The dielectric fill material can be a planarizable
dielectric material such as undoped silicate glass, doped silicate
glass, or a spin-on glass (SOG). The dielectric fill material can
be deposited by a conformal deposition method or by a
self-planarizing deposition method (such as spin coating). In one
embodiment, the dielectric fill material can include, and/or can
consist essentially of, a material selected from doped silicate
glass and undoped silicate glass.
[0141] Portions of the dielectric fill material can be removed from
above the horizontal plane including top surfaces of the second
hard mask strips 273' by a planarization process. The planarization
process can include chemical mechanical planarization (CMP) and/or
a recess etch process. Each remaining portion of the dielectric
fill material constitutes a dielectric fill material portion 243'.
Each contiguous set of a dielectric liner 241' and a dielectric
fill material portion 243' constitutes a second dielectric
isolation structure (241', 243'), which is a dielectric isolation
structure having a shape of a rail. A second dielectric isolation
structure (241', 243') is formed in an upper portion of each third
trench 21 above a respective pair of sacrificial semiconductor
structures 212 as a rail structure. The second dielectric isolation
structures (241', 243') laterally extend along the second
horizontal direction hd2, and are laterally spaced among one
another along the first horizontal direction hd1. The top surfaces
of the second dielectric isolation structures (241', 243') can be
coplanar with the top surfaces of the second hard mask strips
273'.
[0142] If the second hard mask strips 273' are present, top
surfaces of the second dielectric isolation structures (241', 243')
can be recessed by an etch process, which can be an isotropic etch
process or an anisotropic etch process. The duration of the etch
process can be selected such that the recessed top surfaces of the
second dielectric isolation structures (241', 243') is at about the
level of the top surfaces of the second barrier rails 27'. The
second hard mask strips 273' can be subsequently removed. For
example, if the second hard mask strips 273' include silicon
nitride, the second hard mask strips 273' can be removed by a wet
etch employing hot phosphoric acid.
[0143] The second dielectric isolation structures (241', 243') are
formed in the third trenches 21 as dielectric isolation rail
structures that laterally extend along the second horizontal
direction hd2. The second dielectric isolation structures (241',
243') are formed on the convex surfaces of the sacrificial
semiconductor structures 212.
[0144] Referring to FIGS. 13A-13E, a second top conductive layer
28L can be formed over the one-dimensional array of second stacked
rail structures (22, 24', 26', 27') spaced by the second dielectric
isolation structures (241', 243'). The second top conductive layer
28L includes at least one conductive material layer, which can be
at least one metallic material layer. For example, the second top
conductive layer 28L can include a metal layer (such as a tungsten
layer). The thickness of the second top conductive layer 28L can be
in a range from 20 nm to 200 nm, such as from 30 nm to 70 nm,
although lesser and greater thicknesses can also be employed.
[0145] A fourth photoresist layer 299 having a line and space
pattern can be formed over the second top conductive layer 28L.
Portions of the fourth photoresist layer 299 laterally extend along
the first horizontal direction hd1 with a respective uniform width.
Thus, line trenches are present among the portions of the fourth
photoresist layer 299. The line trenches laterally extend along the
first horizontal direction hd1, and are laterally spaced apart
along the second horizontal direction hd2. The line trenches can
have a uniform width that is invariant with translation along the
first horizontal direction hd1. The pattern in the fourth
photoresist layer 299 can be a periodic pattern that is repeated
along the second horizontal direction hd2 with a pitch that is
equal to the sum of the width of a line trench in the fourth
photoresist layer 299 and the width of a patterned line-shaped
portion of the fourth photoresist layer 299. The pitch can be in a
range from 32 nm to 600 nm, although lesser and greater pitches can
also be employed. In one embodiment, the pattern of the fourth
photoresist layer 299 can be the same as the pattern of the first
conductive rails 12.
[0146] Referring to FIGS. 14A-14E, fourth trenches 25 are formed
through the second top conductive layer 28L, vertical stacks (24',
26', 27') of a second selector rail 24', a second phase change
memory rail 26', and an optional second barrier rail 27', and the
second dielectric isolation structures (241', 243'). An anisotropic
etch process can be performed to etch portions of the second top
conductive layer 28L, the second barrier rails 27', the second
phase change memory rails 26', the second selector rails 24', and
the second dielectric isolation structures (241', 243') that are
not masked by the fourth photoresist layer 299. The materials of
the second top conductive layer 28L, the second barrier rails 27',
the second phase change memory rails 26', the second selector rails
24', and the second dielectric isolation structures (241', 243')
can be etched selective to the material in the top portions of the
second conductive rails (18, 22) (for example, selective to the
second metallic nitride strips 224) by the anisotropic etch
process. Volumes from which the materials of the second top
conductive layer 28L, the second barrier rails 27', the second
phase change memory rails 26', the second selector rails 24', and
the second dielectric isolation structures (241', 243') are removed
constitute the fourth trenches 25. The fourth trenches 25 laterally
extend along the first horizontal direction hd1. Surfaces of the
sacrificial semiconductor portions 212 are physically exposed
within each of the fourth trenches 25.
[0147] The chemistry of the anisotropic etch process can be
sequentially modified to etch through the various materials of the
second top conductive layer 28L, the second barrier rails 27', the
second phase change memory rails 26', the second selector rails
24', and the second dielectric isolation structures (241', 243').
The anisotropic etch process can stop at the top surface of, or
within, the second conductive rails (18, 22). For example, the
anisotropic etch process can stop on the second metal nitride
strips 224. The sacrificial semiconductor structures 212 may be
partially etched by the anisotropic etch process. Each fourth
trench 25 can vertically extend from the horizontal plane including
the bottom surface of the fourth photoresist layer 299 to the
horizontal plane including top surfaces of the second conductive
rails (18, 22) or to the top surfaces of the sacrificial
semiconductor structures 212.
[0148] The second top conductive layer 28L is divided into a
one-dimensional array of conductive rails, which are herein
referred to as lower third conductive rail portions 28. Each lower
third conductive rail portion 28 can be subsequently incorporated
into a respective third conductive rail that includes a respective
upper third conductive rail portion to be subsequently formed
thereabove. Alternatively, if the level of the second top
conductive layer 28L is the level of the topmost conductive rail
structures, the one-dimensional array of conductive rails can
constitute third conductive rails (e.g., word lines).
[0149] Each vertical stack of a second barrier rail 27', a second
phase change memory rails 26', and a second selector rails 24' is
divided into a row of second memory pillar structures (24, 26, 27)
that are laterally spaced apart long the second horizontal
direction hd2. The combination of the second barrier rails 27', the
second phase change memory rails 26', and the second selector rails
24' is divided into a two-dimensional array of second memory pillar
structures (24, 26, 27), which may be a periodic two-dimensional
array. The two-dimensional array of second memory pillar structures
(24, 26, 27) can form a rectangular periodic two-dimensional array
having the first pitch along the first horizontal direction hd1 and
the second pitch along the second horizontal direction hd2.
[0150] Each second memory pillar structure (24, 26, 27) includes,
from bottom to top, a second selector pillar 24 that is a patterned
portion of a second selector rail 24', a second phase change memory
pillar 26 that is a patterned portion of a phase change memory rail
26', and an optional second barrier plate 27 that is a patterned
portion of a second barrier strip 27'. The second memory pillar
structures (24, 26, 27) can be arranged as a rectangular
two-dimensional array.
[0151] Each second selector pillar 24 is a selector element. In one
embodiment, each second selector pillar 24 can include a vertical
stack of a second lower barrier material portion 242 that is a
patterned portion of a second lower barrier material strip 242', a
second ovonic threshold switch material portion 244 that is a
patterned portion of a second ovonic threshold switch material rail
244', and a second upper barrier material portion 246 that is a
patterned portion of a second upper barrier material strip
246'.
[0152] In one embodiment, each second selector pillar 24 includes a
second ovonic threshold switch material portion 244, an upper
amorphous carbon portion that contacts a top surface of the second
ovonic threshold switch material portion 244 as a second upper
barrier material portion 246, and a lower amorphous carbon portion
that contacts a bottom surface of the second ovonic threshold
switch material portion 244 as a second lower barrier material
portion 242.
[0153] Each second phase change memory pillar 26 is a phase change
memory element. In one embodiment, each second phase change memory
pillar 26 can include a vertical stack of an optional second lower
conductive liner plate 262 that is a patterned portion of a second
optional lower conductive liner strip 262', a second phase change
memory material pillar 264 that is a patterned portion of a second
phase change memory material rail 264', and a second optional upper
conductive liner plate 266 that is a patterned portion of a second
upper conductive liner strip 266'.
[0154] The fourth trenches 25 divide the second dielectric
isolation structures (241', 243') into a two-dimensional array of
dielectric isolation pillar structures (241, 243). Each second
dielectric isolation structure (241', 243') is divided into a
one-dimensional array of dielectric isolation pillar structures
(241, 243). A two-dimensional array of dielectric isolation pillar
structures (241, 243) can be formed. Each dielectric isolation
pillar structure (241, 243) can include a pair of dielectric liner
segments 241 that are patterned portions of dielectric liners 241',
and a dielectric pillar 243 that is a patterned portion of a
dielectric fill material portion 243'.
[0155] The second barrier plates 27 are barrier elements, and can
be arranged in a two-dimensional periodic array. While FIGS.
14A-14E illustrate an embodiment in which a second phase change
memory pillar 26 overlies a second selector pillar within each
second memory pillar structure (24, 26, 27), embodiments are
expressly contemplated herein in which a second phase change memory
pillar 26 underlies a second selector pillar within each second
memory pillar structure (24, 26, 27). In each case, if a second
barrier plate 27' is present within a second memory pillar
structure (24, 26, 27), a two-dimensional array of barrier elements
(i.e., second barrier plates 27) may be disposed between the second
phase change memory elements (i.e., the second phase change memory
pillars 26) and a proximal set among a set of the second conductive
rails (18, 22) and a set of the lower third conductive rail
portions 28 (or third conductive rails).
[0156] Referring to FIGS. 15A-15E, the sacrificial semiconductor
structures 212 are removed from underneath the fourth trenches 25
selective to the remainder of the exemplary structure by an
isotropic etch process. The sacrificial semiconductor structures
212 are removed from underneath the two-dimensional array of
dielectric isolation pillar structures (241, 243) selective to the
materials of the dielectric isolation pillar structures (241, 243)
to form a one-dimensional array of first cavities 215. Each first
cavity 215 laterally extends along the second horizontal direction
hd2 underneath a row of dielectric isolation pillar structures
(241, 243) that are arranged along the second horizontal direction
hd2.
[0157] The selective isotropic etch process etches the material of
the sacrificial semiconductor structures 212 selective to the
remainder of the exemplary structure. As used herein, a "selective"
etch process is an etch process that etches a first material with a
first etch rate without etching a second material or while etching
the second material at a second etch rate such that the ratio of
the first etch rate to the second etch rate is at least 3. If the
second etch rate is finite, the ratio of the first etch rate to the
second etch rate is referred to as the "selectivity" of the etch
process. In this case, the selective etch process etches the first
material "selective to" the second material. The selectivity of the
selective etch process is at least 3, and can be in a range from 3
to 100,000 such as from 10 to 1,000. A wet etch employing a KOH
solution or a trimethyl(2-hydroxyethyl)ammonium hydroxide (TMY)
solution can be used to selectively etch polysilicon or amorphous
silicon sacrificial semiconductor structures 212.
[0158] An upper portion of each first cavity 215 is joined to lower
portions of a plurality of fourth cavities 25. A lower portion of
each fourth cavity 25 is jointed can be joined to an upper portion
of each first cavity 215 within the one-dimensional array of first
cavities 215. Each first cavity 215 can vertically extend at least
from a first horizontal plane including interfaces between the
first selector elements 14 and the first phase change memory
elements 16 to a second horizontal plane including bottom surfaces
of the second conductive rails (18, 22). In one embodiment, each
first cavity 215 can vertically extend from top surfaces of the
first conductive rails 12 to a horizontal plane including top
surfaces of the second conductive rails (18, 22). In one
embodiment, each first cavity 215 can vertically extend from top
surfaces of the first metal nitride strips 122 to the horizontal
plane including top surfaces of the second conductive rails (18,
22). In one embodiment, the second conductive rails (18, 22) can be
laterally spaced among one another by the one-dimensional array of
first cavities 215.
[0159] Each dielectric isolation pillar structure (241, 243) can
include a pair of concave bottom surfaces that are adjoined by a
ridge extending along the second horizontal direction hd2. Each
concave bottom surface of the dielectric isolation pillar
structures (241, 243) can be physically exposed to a respective one
of the first cavities 215.
[0160] Referring to FIGS. 16A-16E, an outer dielectric liner 252
can be deposited on the physically exposed surfaces of the
exemplary structure. The physically exposed surfaces on which the
outer dielectric liner 252 is deposited include sidewalls of the
first memory pillar structures (14, 16, 17) that are parallel to
the second horizontal direction hd2, sidewalls of the second memory
pillar structures (24, 26, 27) that are parallel to the first
horizontal direction hd1, sidewalls of the second conductive rails
(18, 22) that extend along the second horizontal direction hd2, and
sidewalls of the lower third conductive rail portions 18 (or of
third conductive rails) that extend along the first horizontal
direction hd1. The outer dielectric liner 252 is deposited on
bottom surfaces of the dielectric isolation pillar structures (241,
243), which become physically exposed after removal of the
sacrificial semiconductor structures 212. Thus, the outer
dielectric liner 253 is deposited on bottom surfaces of the
dielectric liner segments 241, and on each pair of concave bottom
surfaces of the dielectric pillars 243. The outer dielectric liner
252 can be formed by a conformal deposition process such as low
pressure chemical vapor deposition (LPCVD). The outer dielectric
liner 252 includes a dielectric material that can protect and/or
passivate the materials of the first memory pillar structures (14,
16, 17) and the second memory pillar structures (24, 26, 27). For
example, the outer dielectric liner 252 can include silicon
nitride. The thickness of the outer dielectric liner 252 can be in
a range from 1 nm to 10 nm, such as from 2 nm to 5 nm, although
lesser and greater thicknesses can also be employed.
[0161] Referring to FIGS. 17A-17E, a dielectric fill material can
be non-conformally deposited to form a dielectric fill material
layer 254L. The dielectric fill material can include, for example,
undoped silicate glass or doped silicate glass. The non-conformal
deposition process employed to deposit the dielectric fill material
can be, for example, physical vapor deposition, plasma enhanced
chemical vapor deposition (PECVD), vacuum evaporation, or any
directional dielectric material deposition process. The dielectric
fill material can be deposited in portions of the first cavities
215 that underlie the fourth trenches 25, i.e., within portions of
the first cavities 215 that overlap in area with overlying fourth
trenches 25. The regions of the dielectric fill material layer 254L
that is deposited on the sidewalls of the outer dielectric liner
252 around remaining volumes of the first cavities 215 are herein
referred to as inner dielectric liners 2541. Depending on the
degree of anisotropy in the deposition of the dielectric fill
material, the inner dielectric liners 2541 may continuously extend
along the second horizontal direction hd2 to continuously cover
sidewalls of the outer dielectric liner 252 within each first
cavity 215, or may be formed as discrete portions that are
discontinuous underneath the dielectric isolation pillar structures
(241, 243). The inner sidewalls of the first cavities 215 may, or
may not, be physically exposed to the remaining portions of the
first cavities 215 depending on the anisotropy of the deposition
process that deposits the dielectric fill material.
[0162] The deposited dielectric fill material seals each upper
opening of the fourth trench 25. Each first cavity 215 becomes a
first encapsulated cavity 255 that extends along the second
horizontal direction hd2. Each first encapsulated cavity 255 may be
filled with a gas phase material only, or may be under vacuum. The
maximum width of each first encapsulated cavity 255 can be in a
range from 30% to 99%, such as from 50% to 90%, of the maximum
lateral separation distance between a neighboring pair of second
conductive rails (18, 22). The bottom surfaces of the portions of
the dielectric fill material layer 254L that define the top
surfaces of the first encapsulated cavities 255 may have a
vertically undulating profile as illustrated in FIG. 17D. Further,
the top surfaces of the portions of the portions of the dielectric
fill material layer 254L that define the bottom surfaces of the
first encapsulated cavities 255 may have a vertically undulating
profile as illustrated in FIG. 17D. In some cases, a second
encapsulated cavity 257 may be formed in the dielectric fill
material layer 254L located in each fourth trench 25 depending on
the anisotropy of the deposition process employed to deposit the
dielectric fill material.
[0163] In the alternative embodiment in which only the top portions
of the first sacrificial trench fill structures (155, 157) are
removed at the step of FIGS. 9A-9C, the top surface of the
remaining first sacrificial trench fill structures (155, 157) is
located below the optional barrier plate 17 such that the first
encapsulated cavities 255 are located above the remaining first
sacrificial trench fill structures (155, 157) (e.g., the first
encapsulated cavities 255 having a bottom surface at a level below
the second conductive rails (18, 22) but at or above the level of
the top surface of the first phase change elements 16). Thus, in
various embodiment, each of the first encapsulated cavities 255
vertically extends at least from a horizontal plane including
bottom surfaces of the second conductive rails (18, 22) to at least
a horizontal plane including top surfaces of the second conductive
rails (18, 22).
[0164] Referring to FIGS. 18A-18E, portions of the dielectric fill
material layer 254L that overlie the top surfaces of the lower
third conductive rail portions 28 (or third conductive rails) can
be removed by a planarization process (such as chemical mechanical
planarization or a recess etch). Remaining portions of the
dielectric fill material layer 254L constitute dielectric fill
material portions 254. The dielectric fill material portions 254
include the inner dielectric liners 2541 contacting inner sidewalls
of the outer dielectric liners 252 at the level of the first
encapsulated cavities 255. In one embodiment, regions of the
dielectric fill material portions 254 that fill the fourth trenches
25 can be interconnected among one another via regions of the
dielectric fill material portions 254 that line the first
encapsulated cavities 255 underneath the two-dimensional array of
dielectric isolation pillar structures (241, 243). In this case, a
single continuous dielectric material structure can include each
region of the dielectric fill material portions 254 via
interweaving connections among the various regions of the
dielectric fill material portions 254 located in the fourth
cavities 25 and around the first encapsulated cavities 255.
[0165] Generally, the dielectric fill material portions 254 can be
formed by anisotropically depositing a dielectric material in the
fourth trenches 25 and outer regions of volumes of the
one-dimensional array of first cavities 215. The first cavities 215
shrink in volume by deposited dielectric material by less than 50%
of the volumes of the one-dimensional array of first cavities 215
as formed by removal of the sacrificial semiconductor structures
212 from underneath the fourth trenches 25 at the processing steps
of FIGS. 15A-15E.
[0166] The set of processing steps of FIGS. 7A-7C, 8A-8C, 9A-9C,
11A-11C, 12A-12C, 13A-13E, 14A-14E, 15A-15E, 16A-16E, 17A-17E, and
18A-18E can be repeated mutatis mutandis as many times as needed to
form additional levels of a rectangular array of pillar structures
and additional level of conductive rails. The adjustments in
subsequent sets of processing steps can include rotation of
patterns by 90 degrees at each level so that a cross-bar array of
conductive rails can be provided. Volumes temporarily filled with
dielectric fill material portions during formation of an underlying
level can be converted into regions including a respective
encapsulated cavity during a subsequent set of processing steps for
formation of an immediately overlying level.
[0167] For example, upon a single repetition of the processing
steps of FIGS. 7A-7C, 8A-8C, 9A-9C, 11A-11C, 12A-12C, 13A-13E,
14A-14E, 15A-15E, 16A-16E, 17A-17E, and 18A-18E, the exemplary
structure illustrated in FIGS. 19A-19E can be obtained. The
exemplary structure of FIGS. 19A-19E include upper third conductive
rail portions 32. Each combination of a lower third conductive rail
portion 28 and an upper third conductive rail portion 32
constitutes a third conductive rail (28, 32).
[0168] A two-dimensional array of third pillar structure (34, 36,
37) can be provided. Each third pillar structure (34, 36, 37) can
include, from bottom to top, a third selector pillar 34, a third
phase change memory pillar 36, and an optional third barrier plate
37.
[0169] In one embodiment, each third selector pillar 34 includes a
third ovonic threshold switch material portion 344, an upper
amorphous carbon portion that contacts a top surface of the third
ovonic threshold switch material portion 344 as a third upper
barrier material portion 346, and a lower amorphous carbon portion
that contacts a bottom surface of the third ovonic threshold switch
material portion 344 as a third lower barrier material portion
342.
[0170] Each third selector pillar 34 is a selector element. In one
embodiment, each third selector pillar 34 can include a vertical
stack of a third lower barrier material portion 342, a third ovonic
threshold switch material portion 344, and a third upper barrier
material portion 346.
[0171] Each third phase change memory pillar 36 is a phase change
memory element. In one embodiment, each third phase change memory
pillar 36 can include a vertical stack of an optional third lower
conductive liner plate 362, a third phase change memory material
pillar 364, and a third optional upper conductive liner plate
366.
[0172] A two-dimensional array of dielectric isolation pillar
structure (341, 343) can be provided. Each dielectric isolation
pillar structure (341, 343) can include a pair of dielectric liner
segments 341 and a dielectric pillar 343.
[0173] Additional dielectric fill material portions 354 fill
additional trenches. Second cavities 355 can laterally extend along
the first horizontal direction. Outer dielectric liners 352 can
line the second cavities 355. The additional dielectric material
portions 354 can include inner dielectric liners 3541 that line the
second cavities 355. Lower fourth conductive rail portions 38 can
laterally extend along the second horizontal direction hd2.
[0174] As discussed above, additional levels can be formed by
repetition of a set of processing steps needed to form one level
with suitable changes in the processing conditions. At each level,
encapsulated cavities (255, 355) can be formed between each
neighboring pair of conductive rails {(18, 22), (28, 32)}.
[0175] While a PCM memory device was described as an exemplary
memory device above, it should be understood that any other type of
memory device, such as a magnetic random access memory (MRAM) or a
metal oxide resistive random access memory (ReRAM) may be formed
instead with the above described cavities (e.g., air gaps). Thus,
in alternative embodiments, the series connection of a phase change
memory element (such as a first phase change memory element 16) and
a selector element (such as a first selector element 14) and the
optional barrier plate 17 may be replaced with any other type of
memory element such as a magnetic memory element or a metal oxide
(e.g., titanium oxide or nickel oxide) resistive memory element
with or without a selector element (e.g., a diode steering
element). As such, each rectangular array of memory pillar
structures formed at each memory level can include any type of
memory pillar structures known in the art. All such variations are
expressly contemplated herein.
[0176] Referring to all drawings and according to various
embodiments of the present disclosure, a memory device is provided,
which comprises: first conductive rails 12 laterally extending
along a first horizontal direction hd1 over a substrate 8; a
rectangular array of first memory pillar structures overlying top
surfaces of the first conductive rails 12, wherein each first
memory pillar structure comprises a respective memory element;
second conductive rails (18, 22) laterally extending along a second
horizontal direction hd2 and overlying top surfaces of the
rectangular array of first memory pillar structures; and a
one-dimensional array of first cavities 255 free of solid material
portions and therein laterally extending along the second
horizontal direction hd2 and located between neighboring pairs of
the second conductive rails (18, 22).
[0177] In one embodiment, a phase change memory device is provided,
which comprises: first conductive rails 12 laterally extending
along a first horizontal direction hd1 over a substrate 8; a
rectangular array of first memory pillar structures (14, 16, 17)
overlying top surfaces of the first conductive rails 12, wherein
each first memory pillar structure (14, 16, 17) comprises a series
connection of a respective first selector element 14 and a
respective first phase change memory element 16; second conductive
rails (18, 22) laterally extending along a second horizontal
direction hd2 and overlying top surfaces of the rectangular array
of first memory pillar structures (14, 16, 17); and a
one-dimensional array of first cavities 255 free of solid material
portions therein (e.g., air gaps) laterally extending along the
second horizontal direction hd2.
[0178] In one embodiment, the one-dimensional array of first
cavities 255 vertically extends at least from a first horizontal
plane including interfaces between the first selector elements 14
and first phase change memory elements 16 to a second horizontal
plane including bottom surfaces of the second conductive rails (18,
32).
[0179] In one embodiment, each of the second conductive rails (18,
22) comprises a vertical stack of a lower second conductive rail
portion 18 and an upper second conductive rail portion 22, and each
of the first cavities 255 vertically extends above a third
horizontal plane including interfaces of the lower second
conductive rail portions 18 and the upper second conductive rail
portions 22.
[0180] In one embodiment, each of the first cavities 255 includes a
respective portion that protrudes above a fourth horizontal plane
including topmost surfaces of the second conductive rails (18, 22).
In one embodiment, each of the upper second conductive rail
portions 22 has tapered sidewalls and a lesser width at a
respective bottom surface than at a respective top surface along
the first horizontal direction hd1, and each of the first cavities
255 has a lesser width at a level of lower portions of the upper
second conductive rail portions 22 than at a level of upper
portions of the lower second conductive rail portions 18.
[0181] In one embodiment, each of the first cavities 255 has a
vertical cross-sectional shape within a vertical plane that is
perpendicular to the first horizontal direction hd1, and a top
portion of the vertical cross-sectional shape includes two
vertically protruding regions (corresponding to a pair of convex
surfaces of an overlying dielectric isolation pillar structure
(242, 243)) laterally spaced by a vertically recessed region.
[0182] In one embodiment, the vertical cross-sectional shape is
entirely enclosed by an inner surface of at least one dielectric
liner (which includes an outer dielectric liner 252 and an inner
dielectric liner 2541, if present, having the same material as the
dielectric fill material portions 254 as provided at the processing
steps of FIGS. 18A-18E)) located between a respective neighboring
pair of second conductive rails (18, 22) and between a respective
pair of neighboring rows of the first memory pillar structures (14,
16, 17) that are spaced apart along the first horizontal direction
hd1.
[0183] In one embodiment, the at least one dielectric liner
comprises an outer dielectric liner 252 laterally contacting the
respective neighboring pair of second conductive rails (18, 22) and
between the respective pair of neighboring rows of the first memory
pillar structures (14, 16, 17), and an inner dielectric liner 2541
defining a boundary of a respective first cavity 255 and located
between the respective first cavity 255 and the outer dielectric
liner 252.
[0184] In one embodiment, the phase change memory device can
further comprise: a rectangular array of second memory pillar
structures (24, 26, 27) overlying top surfaces of the second
conductive rails (18, 22), wherein each second memory pillar
structure (24, 26, 27) comprises a series connection of a
respective second selector element 24 and a respective second phase
change memory element 26; and a rectangular array of dielectric
isolation pillars (241, 243) located between each neighboring pair
of second memory pillar structures (24, 26, 27) spaced along the
first horizontal direction hd1, wherein each first cavity 255
underlies a respective one of the dielectric isolation pillars
(241, 243).
[0185] In one embodiment, each of the dielectric isolation pillars
(241, 243) includes a pair of concave bottom surfaces that
laterally extend along the second horizontal direction hd2 and a
downward-protruding portion located between the pair of concave
bottom surfaces. In one embodiment, each pair of concave bottom
surfaces of the dielectric isolation pillars (241, 243) contacts a
respective one of the at least one dielectric liners (252,
254).
[0186] The various encapsulated cavities (255, 355) of the
embodiments of the present disclosure can be employed to reduce
capacitive coupling between neighboring pair of conductive rails
{(18, 22), (28, 32)}, which can be employed as word lines and bit
lines for the rectangular arrays of memory pillar structures {(14,
16, 17), (24, 26, 27)} including respective phase change memory
elements (16, 26). In other words, the various encapsulated
cavities (255, 355) of the present disclosure provide air gap
structures that reduce capacitive coupling among conductive line
structures. By reducing the capacitive coupling among the word
lines and/or among the bit lines, the RC delay in the word lines
and/or the bit lines can be reduced in the three-dimensional phase
change memory array of the present disclosure, thereby providing
enhanced device performance. The sacrificial semiconductor
structures 212 provide a desired controlled height for the
encapsulated cavities (e.g., air gaps) without requiring a
difficult deposition of an insulating layer in only the upper part
of the respective trenches, which may form air gaps with greater
than or less then a desirable height.
[0187] Although the foregoing refers to particular preferred
embodiments, it will be understood that the disclosure is not so
limited. It will occur to those of ordinary skill in the art that
various modifications may be made to the disclosed embodiments and
that such modifications are intended to be within the scope of the
disclosure. Where an embodiment employing a particular structure
and/or configuration is illustrated in the present disclosure, it
is understood that the present disclosure may be practiced with any
other compatible structures and/or configurations that are
functionally equivalent provided that such substitutions are not
explicitly forbidden or otherwise known to be impossible to one of
ordinary skill in the art. All of the publications, patent
applications and patents cited herein are incorporated herein by
reference in their entirety.
* * * * *