U.S. patent application number 11/620671 was filed with the patent office on 2008-07-10 for uniform critical dimension size pore for pcram application.
Invention is credited to Matthew J. Breitwisch, Roger W. Cheek, Eirc A. Joseph, Chung H. Lam, Hsiang-Lan Lung, Alejandro G. Schrott.
Application Number | 20080164453 11/620671 |
Document ID | / |
Family ID | 39593485 |
Filed Date | 2008-07-10 |
United States Patent
Application |
20080164453 |
Kind Code |
A1 |
Breitwisch; Matthew J. ; et
al. |
July 10, 2008 |
UNIFORM CRITICAL DIMENSION SIZE PORE FOR PCRAM APPLICATION
Abstract
A memory cell and a method of making the same, that includes
insulating material deposited on a substrate, a bottom electrode
formed within the insulating material, a plurality of insulating
layers deposited above the bottom electrode and at least one of
which acts as an intermediate insulating layer. A via is defined in
the insulating layers above the intermediate insulating layer. A
channel is created for etch with a sacrificial spacer. A pore is
defined in the intermediate insulating layer. All insulating layers
above the intermediate insulating layer are removed, and the
entirety of the remaining pore is filled with phase change
material. An upper electrode is formed above the phase change
material.
Inventors: |
Breitwisch; Matthew J.;
(Yorktown Heights, NY) ; Cheek; Roger W.; (Somers,
NY) ; Lam; Chung H.; (Peekskill, NY) ; Lung;
Hsiang-Lan; (Elmsford, NY) ; Joseph; Eirc A.;
(White Plains, NY) ; Schrott; Alejandro G.; (New
York, NY) |
Correspondence
Address: |
LAW OFFICE OF IDO TUCHMAN (YOR)
ECM #72212, PO Box 4668
New York
NY
10163-4668
US
|
Family ID: |
39593485 |
Appl. No.: |
11/620671 |
Filed: |
January 7, 2007 |
Current U.S.
Class: |
257/3 ;
257/E21.158; 257/E47.001; 438/652 |
Current CPC
Class: |
H01L 45/144 20130101;
H01L 45/1233 20130101; H01L 45/1683 20130101; H01L 45/148 20130101;
H01L 45/06 20130101 |
Class at
Publication: |
257/3 ; 438/652;
257/E47.001; 257/E21.158 |
International
Class: |
H01L 47/00 20060101
H01L047/00; H01L 21/28 20060101 H01L021/28 |
Claims
1. A method of forming a memory cell, the method comprising:
forming a plurality of insulating layers over a substrate; forming
a bottom electrode within at least one of the insulating layers;
defining a via through at least one of the insulating layers above
the bottom electrode, the via and bottom electrode being separated
by at least one intermediate insulating layer; forming a
sacrificial spacer in the via above the intermediate insulating
layer, the sacrificial spacer including a channel having a smaller
diameter than a diameter of the via; defining a pore through the
intermediate insulating layer below the sacrificial spacer and
above the bottom electrode such that the channel continues through
the intermediate insulating layer to the bottom electrode; removing
the sacrificial spacer; depositing phase change material in the
pore, the phase change material filling the entire pore; and
forming an upper electrode above the phase change material.
2. The method of claim 1, further comprising forming an undercut in
at least one of the insulating layers, the undercut defining an
overhang above the via.
3. The method of claim 1, wherein forming the sacrificial spacer
includes: depositing a sacrificial spacer layer within the via, the
conformality of deposition of the sacrificial spacer layer being
such that a cavity is formed by the sacrificial spacer layer; and
etching the sacrificial spacer layer such that the area below the
cavity forms a ridge within the sacrificial spacer.
4. The method of claim 1, wherein the pore is tubular.
5. The method of claim 1, wherein the surface of the pore is
substantially planar.
6. The method of claim 1, wherein the side walls of the
intermediate insulating layer defining the pore are substantially
perpendicular to an upper surface of the intermediate insulating
layer.
7. The method of claim 1, wherein the diameter of the side walls of
the intermediate insulating layer defining the pore is
substantially less than the diameter of the via.
8. The method of claim 1, further comprising: removing all
sacrificial layers above the intermediate insulating layer prior to
forming the phase change material.
9. A memory cell comprising: a substrate; an insulating layer
formed over the substrate; a bottom electrode formed within the
insulating layer; a pore in the insulating layer above the bottom
electrode; phase change material formed within the pore, the phase
change material filling the entire pore; and an upper electrode
formed above the phase change material.
10. The memory cell of claim 9, wherein the pore is tubular.
11. The memory cell of claim 10, wherein the surface of the pore is
substantially planar.
12. The memory cell of claim 9, wherein the side walls of the
intermediate insulating layer defining the pore are substantially
perpendicular to an upper surface of the intermediate insulating
layer.
13. The memory cell of claim 9, wherein the phase change material
and upper electrode are patterned for bit line connections.
14. The memory cell of claim 9, wherein the diameter of the side
walls of the intermediate insulating layer defining the pore is
substantially less than the diameter of the via.
15. An integrated circuit comprising one or more memory cells, at
least one of the memory cells comprising: a substrate; an
insulating layer formed over the substrate; a bottom electrode
formed within the insulating layer; a pore in the insulating layer
above the bottom electrode; phase change material formed within the
pore, the phase change material filling the entire pore; and an
upper electrode formed above the phase change material.
16. The integrated circuit of claim 15, wherein the pore is
tubular.
17. The integrated circuit of claim 15, wherein the pore is
substantially planar.
18. The integrated circuit of claim 15, wherein the phase change
material and upper electrode are patterned for bit line connection.
Description
FIELD OF THE INVENTION
[0001] The present invention is directed toward computer memory,
and more particularly to a non-volatile phase change memory
device.
BACKGROUND OF THE INVENTION
[0002] There are two major groups in computer memory: non-volatile
memory and volatile memory. Constant input of energy in order to
retain information is not necessary in non-volatile memory but is
required in the volatile memory. Examples of non-volatile memory
devices are Read Only Memory, Flash Electrical Erasable Read Only
Memory, Ferroelectric Random Access Memory, Magnetic Random Access
Memory, and Phase Change Memory. Examples of volatile memory
devices include Dynamic Random Access Memory (DRAM) and Static
Random Access Memory (SRAM). The present invention is directed to
phase change memory. In phase change memory, information is stored
in materials that can be manipulated into different phases. Each of
these phases exhibit different electrical properties which can be
used for storing information. The amorphous and crystalline phases
are typically two phases used for bit storage (1's and 0's) since
they have detectable differences in electrical resistance.
Specifically, the amorphous phase has a higher resistance than the
crystalline phase.
[0003] Glass chalcogenides are a group of materials commonly
utilized as phase change material. This group of materials contain
a chalcogen (Periodic Table Group 16/VIA) and a more
electropositive element. Selenium (Se) and tellurium (Te) are the
two most common semiconductors in the group used to produce a glass
chalcogenide when creating a phase change memory cell. An example
of this would be Ge.sub.2Sb.sub.2Te.sub.5 (GST), SbTe, and
In.sub.2Se.sub.3. However, some phase change materials do not
utilize chalcogen, such as GeSb. Thus, a variety of materials can
be used in a phase change material cell as long as they can retain
separate amorphous and crystalline states.
[0004] The amorphous and crystalline phases in phase change
material are reversible. As shown in FIG. 1, this is achieved by
forming a via 104 lined with insulating material 106. A lower
electrode 102 (also referred to as the source) is formed below the
phase change material 107 and an upper electrode 101 (also referred
to as the drain) is formed above the phase change material 107.
This allows an electrical pulse to travel through the phase change
material when electricity is applied from the source 102 to the
drain 101. Due to ohmic heating, the phase change material 107
changes its phase. A relatively high intensity, short duration
current pulse with a quick transition at the trailing edge results
in the phase change material 107 melting and cooling quickly. The
phase change material 107 does not have the time to form organized
crystals, thereby creating an amorphous solid phase. A relatively
low intensity, long duration pulse allows the phase change material
107 to heat and slowly cool, thus crystallizing into the
crystalline phase. It is possible to adjust the intensity and
duration of the pulses to produce a varying degree of resistance
for multi-bit storage in a memory cell.
[0005] A phase change cell is read by applying a pulse of
insufficient strength to program, i.e. to alter the phase of, the
material 107. The resistance of this pulse can then be read as a
"1" or "0". The amorphous phase which carries a greater resistance
is generally used to represent a binary 0. The crystalline phase
which carries a lower resistance can be used to represent a binary
1. In cells where there are varying degrees of resistance, the
phases can be used to represent, for example, "00", "01", "10", and
"11".
SUMMARY OF THE INVENTION
[0006] An exemplary aspect of the invention is a method of forming
a memory cell. The method for forming the memory cell begins with
standard front end of line (FEOL) wafers generally forming with a
plurality of insulating layers over a substrate. A bottom electrode
is formed within at least one of the insulating layers. A via is
defined by etching through at least one of the insulating layers
above the bottom electrode. The via and bottom electrode are
separated by at least one intermediate insulating layer. A
sacrificial spacer is formed in the via above the intermediate
insulating layer. A channel with a smaller diameter than the
diameter of the via is defined within the sacrificial spacer walls.
A pore is created in the intermediate insulating layer below the
sacrificial spacer and above the bottom electrode such that the
channel continues through the intermediate insulating layer to the
bottom electrode. The sacrificial spacer is then removed and phase
change material is deposited into the pore, filling the entire
pore. Finally, an upper electrode is deposited above the phase
change material.
[0007] Another exemplary aspect of the present invention is a
memory cell. The memory cell includes a substrate, an insulating
layer formed over the substrate, a bottom electrode formed within
the insulating layer, a pore in the insulating layer above the
bottom electrode, phase change material formed within the pore,
with the phase change material filling the entire pore, and an
upper electrode formed above the phase change material.
[0008] Another exemplary aspect of the present invention is an
integrated circuit comprising one or more memory cells with at
least one of the memory cells comprising a substrate, an insulating
layer formed over the substrate, a bottom electrode formed within
the insulating layer, a pore in the insulating layer above the
bottom electrode, phase change material formed within the pore,
with the phase change material filling the entire pore, and an
upper electrode formed above the phase change material.
Additionally, the upper electrode may be patterned for bit line
connections.
BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWINGS
[0009] FIG. 1 is a cross sectional view of a memory cell of the
present invention.
[0010] FIG. 2 is a cross sectional view of a FEOL wafer with
insulating layers.
[0011] FIG. 3 is a cross sectional view of the creation of a via
and undercut in the insulating layers.
[0012] FIG. 4 is a cross sectional view illustrating the deposition
of insulating material into the via.
[0013] FIG. 5 is a cross sectional view of the creation of a
sacrificial spacer.
[0014] FIG. 6 is a cross sectional view of the creation of a
pore.
[0015] FIGS. 7A and 7B are cross sectional views illustrating the
removal of the insulating layer(s).
[0016] FIGS. 8A and 8B are cross sectional views illustrating the
deposition of phase change material and an upper electrode.
DETAILED DESCRIPTION OF THE INVENTION
[0017] The present invention is described herein with reference to
embodiments of the invention. Throughout the description of the
invention reference is made to FIGS. 1-8. When referring to the
figures, like structures and elements shown throughout are
indicated with like reference numerals.
[0018] FIG. 1 illustrates the cross sectional view of an exemplary
memory cell 102 contemplated by the present invention. The
exemplary memory cell 102 is comprised of an insulating layer 104,
a bottom electrode 106, an intermediate insulating layer 108, a
pore 114 within the intermediate insulating layer that contains
phase change material 110, and an upper electrode 112. The memory
cell 102 is typically formed on a substrate with
metal-oxide-semiconductor field-effect transistors (MOSFETs) (not
shown). Other switching devices known to those skilled in the art,
such as junction FETs and bipolar junction transistors, may be used
with the present invention.
[0019] In FIG. 2 an exemplary embodiment of a starting front end of
line (FEOL) wafer with insulating layer depositions is shown. The
exemplary FEOL wafer is comprised of the insulating layer 104. The
insulating layer 104 may be composed of, but not limited to,
silicon dioxide (SiO.sub.2). The bottom electrode 106 may be, but
is not limited to, titanium nitride (TiN), tungsten (W), silver
(Ag), gold (Au), or aluminum (Al).
[0020] In a particular embodiment of the invention, the thickness
of the insulating layer 104 and the bottom electrode 106 is greater
than 50 nm. The dimension of the bottom electrode is such that its
diameter is larger than the diameter of the pore 114 (see FIG. 1)
plus tolerance for overlay so that adequate electrical contact is
made. In a particular embodiment the diameter of the bottom
electrode 106 is at least 80 nm.
[0021] Insulating layers disposed above the starting FEOL wafers
are the intermediate insulating layer 108, a silicon dioxide layer
202, and an upper insulating layer 204. The intermediate insulating
layer 108 may be comprised of, but not limited to, silicon nitride
(SiN.sub.x). The silicon dioxide layer 202 may also be comprised
of, but not limited to, amorphous silicon/polysilicon (Si), or any
material which can be removed selectively to the intermediate
insulating layer 108.) The upper insulating layer 204 may also be
comprised of silicon nitride. The insulating materials, SiO.sub.2
and SiN.sub.x, can be formed in one plasma enhanced chemical vapor
deposition (PECVD) chamber sequentially or formed separately. In a
particular embodiment of the invention, the intermediate insulating
layer 108 is approximately 30 nm thick, the silicon dioxide layer
202 approximately 250 nm thick, and the upper insulating layer 204
is approximately 30 nm. It is contemplated that substitute
insulating materials may be used for the insulating layer 104 with
the present invention, such silicon oxycarbide (SiOC). The
intermediate insulating layer 108 and upper insulating layer 204
may also be comprised of alternate insulating materials. An example
of alternate insulating materials would be the aforementioned
SiO.sub.2 and SiN.sub.x, aluminum oxide (Al.sub.2O.sub.3), tantalum
pentoxide (Ta.sub.2O.sub.5), etc. Additionally, the SiO2 layer 202
may be comprised of polysilicon/amorphous silicon
[0022] In an alternate embodiment of the starting FEOL wafer with
insulating layer deposition, the wafer is comprised of a silicon
dioxide insulating layer 104, a bottom electrode 106, an
intermediate insulating layer 108, a silicon dioxide layer 202, and
an upper insulating layer 204. The bottom electrode 106 may be, but
is not limited to, titanium nitride or tungsten. The intermediate
insulating layer 108 may be comprised of, but not limited to,
SiN.sub.x. The silicon dioxide layer 202 may be comprised of, but
not limited to, silicon dioxide and may containany material which
can be removed selectively to the intermediate insulating layer.
The upper insulating layer 204 may be comprised of, but not limited
to, silicon nitride.
[0023] Starting with FIG. 2 and turning to FIG. 3, a via 302 is
etched into the silicon dioxide layer 202 and upper insulating
layer 204. The via 302 stops at the intermediate insulating layer
108. Defining the via 302 can be performed by first forming a
lithography mask with photo resist (not shown) above the upper
insulating layer 204 and the silicon layer 202. The photo resist is
pattern so that the area above the bottom electrode 106 is exposed
to the proceeding etch. The etch can then be performed using an
anisotropic reactive-ion etch (RIE) process. The photo resist is
then stripped from the surface of the upper insulating layer 204.
The undercut 304 can be formed by performing a dilute HF wet etch
where the HF attacks the silicon dioxide more rapidly than the
silicon nitride or amorphous silicon. In a particular embodiment of
the invention, the via 302 is approximately 200 nm in diameter and
250 nm in height. The undercut amount 304 is approximately 15 nm
per side.
[0024] FIG. 4 illustrates the deposition of a conformal insulating
layer 402 and a cavity 404 formed therein. In one embodiment of the
invention, amorphous silicon is used as the conformal insulating
layer 402. The conformal insulating layer 402 can be deposited by
chemical vapor deposition (CVD). The thickness of the conformal
insulating layer 402 should be greater than the radius of the via
302 in order to create the cavity 404 therein. The size of the
undercut 304 in the silicon dioxide layer 202 correlates to the
size of the cavity 404 formed within the conformal insulating layer
402. The diameter of the cavity 404 is approximately twice the size
of the undercut 304 of the silicon dioxide layer 202. For example,
a 30nm undercut creates a 60nm diameter cavity 404. Furthermore,
the diameter of the cavity 404 will be independent of the diameter
of the via 302, providing that the silicon dioxide layer thickness
202 is greater than or equal to a minimum value H.sub.min.
Mathematically, this value can be represented by equation 1 and
describes the point at which the cavity dimension is below the
triangular pinch-off.
H.sub.min=r+ {square root over ((2r-.DELTA.).DELTA.)} Eq. 1
Here H.sub.min is the silicon dioxide layer thickness 202, .DELTA.
the size of the undercut 304 (half the cavity diameter) and r the
radius of the via 302.
[0025] In another embodiment however, the diameter of the cavity
404 can be modulated by the profile of the via 302. Specifically,
if a controlled taper angle is present in the via, the cavity
diameter will decrease according to equation 2, where .delta. is
the effective size of the reduction.
.theta. = tan - 1 [ ( .DELTA. - .delta. ) ( 2 r - ( .DELTA. -
.delta. ) ) .delta. ] Eq . 2 ##EQU00001##
[0026] In FIG. 5, a sacrificial spacer 502 is defined by
anisotropic selective reactive-ion etch. The etch removes all of
the conformal insulating material above and below the cavity 404
(see FIG. 4) and stops on intermediate insulating layer 108.
Additionally, the etch removes the upper insulating layer 204 (see
FIG. 4). A channel 504 is created within the sacrificial spacer 502
during this process. The channel allows further etching to be
concentrated onto a small region of the intermediate insulating
layer 108 above the bottom electrode 106.
[0027] FIG. 6 shows the process step for defining the pore 114.
Defining the pore 114 in the intermediate insulating layer 108 may
be performed by a selective and anisotropic reactive ion etch
process (to maintain the sacrificial spacer critical dimension) or
by aphosphoric acid wet etch (if dimension is not critical). The
phosphoric acid etches the channel 504 within the sacrificial
spacer 502 into the intermediate insulating layer 108, stopping at
the bottom electrode 106. Consequently, if a phosphoric acid wet
etch is used, the upper insulating layer 204 is also removed. The
resulting radius of the pore 114 is that of the channel 504 and
substantially smaller than that of the via 302 (see FIG. 4).
Furthermore, the pore radius is substantially uniform throughout.
The height of the pore 114 created is that of the thickness of the
intermediate insulating layer 108. Additionally, the surface of the
pore 114 is substantially planar and perpendicular to the side
surfaces of the intermediate insulating layer 108. In a particular
embodiment of the invention, the pore 114 is approximately 30 nm in
diameter and 30 nm in height.
[0028] Illustrated in FIG. 7A is the removal of the sacrificial
spacer 502 (see FIG. 6) and the silicon dioxide layer 202. In this
exemplary embodiment dilute HF is used to etch the silicon dioxide
layer 202. The sacrificial spacer 502 is etched with dilute
potassium hydroxide (KOH). In an alternate embodiment, KOH is used
to etch the amorphous silicon from the sacrificial spacer 502.
Dilute HF is used to etch the SiO.sub.2 from the silicon dioxide
layer 202. The remaining surface is that of the intermediate
insulating layer and that of the top surface of the bottom
electrode 106 at the bottom of the pore 114. To ensure that the
surface is planar a chemical mechanical polish (CMP) can be
performed. Additionally the CMP will remove and excess insulating
material above the intermediate insulating layer 108.
[0029] In another alternate embodiment, illustrated in FIG. 7B, the
silicon dioxide layer 202 is retained. KOH is used to remove the
sacrificial spacer 502 and the dilute HF step is omitted. A channel
202H is created within the silicon dioxide layer 202.
[0030] In FIG. 8A, the phase change material 110 is deposited above
the intermediate insulating layer 108 and filling the entirety of
the pore 114. The phase change material 110 can be comprised of a
chalcogenide. Chalcogenides are comprised of a chalcogen (Periodic
Table Group 16/Group VIA) and a more electropositive element. An
example of phase change materials would be GeSb and SbTe. An upper
electrode 112 is then formed above the phase change material 110.
The upper electrode 112 may be comprised of, but not limited to,
silver (Ag), gold (Au), tungsten (W), or aluminum (Al).
[0031] In this exemplary embodiment, a phase change region 116 is a
region of the phase change material 110 that changes phases. The
remaining phase change material 110 above the intermediate
insulating layer 108 acts as a conductive passage for an electrical
current. This current runs from the bottom electrode 106, to the
phase change region 116, through the phase change material 110 and
up to the upper electrode 112. It is contemplated that the phase
change material 110 and the upper electrode 112 above the
intermediate insulating layer 108 and away of the pore 114 may be
removed with CMP.
[0032] In FIG. 8B, the phase change material 110A is deposited into
the channel 202H, within the silicon dioxide layer 202, and into
the pore 114. The phase change material 110A fills the entirety of
the channel 202H and pore 114. The phase change material within the
pore is the phase change region 116. In this alternate embodiment,
the phase change material 110A does not require additional etching
as explained below.
[0033] Returning to FIG. 1, the phase change material 110 of the
completed memory cell 102 above the intermediate insulating layer
108 and the upper electrode 112 are patterned for bit line
connections. This may be accomplished by forming a lithography mask
with photo resist, performing a reactive-ion etch on the regions
exposed with the mask, and then stripping the photo resist from the
memory cell 102. A Reactive Ion Etching or Ion Milling process can
be used to etch the upper electrode 112 and phase-change material
110.
[0034] To program the memory cell 102, an electrical pulse is
applied beginning at the bottom electrode 106, to phase change
region 116, into the phase change material 110 above the
intermediate insulating layer 108, and finally up to the upper
electrode 112. Ohmic heating created by the resistance heats the
phase change material 110 in the phase change region 116 and
changes its resistive properties. A short, strong electrical pulse
causes the phase change region 116 to heat and cool quickly
resulting in an amorphous phase. A long, weaker electrical pulse
causes the phase change region 116 to heat and cool slowly, thereby
allowing the phase change region 116 to crystallize. The amorphous
and crystalline phases exhibit, respectively, higher and lower
resistive properties. The stored data can be retrieved by reading
the resistance of a particular cell with an electrical pulse that
is either too weak or too short to alter the phase in the phase
change region 116.
[0035] The manufacture of an integrate circuit of cells is achieved
by producing the cells in an array so that rows and columns are
formed. These cells are then linked together at the FET gates in
the MOSFET creating a "word" line. The wiring, used also as the
upper electrode 112, is linked together perpendicular to the FET
gate linkage creating a "bit" line. This allows each cell to be
read or programmed individually by mapping its "word" and "bit"
line coordinates.
[0036] The description of the present invention has been presented
for purposes of illustration and description, but is not intended
to be exhaustive or limited to the invention in the form disclosed.
Many modifications and variations will be apparent to those of
ordinary skill in the art without departing from the scope and
spirit of the invention. The embodiment was chosen and described in
order to best explain the principles of the invention and the
practical application, and to enable others of ordinary skill in
the art to understand the invention for various embodiments with
various modifications as are suited to the particular use
contemplated. Having thus described the invention of the present
application in detail and by reference to embodiments thereof, it
will be apparent that modifications and variations are possible
without departing from the scope of the invention defined in the
appended claims.
[0037] The terminology used herein is for the purpose of describing
particular embodiments only and is not intended to be limiting of
the invention. As used herein, the singular forms "a", "an" and
"the" are intended to include the plural forms as well, unless the
context clearly indicates otherwise. It will be further understood
that the terms "comprises" and/or "comprising," when used in this
specification, specify the presence of stated features, integers,
steps, operations, elements, and/or components, but do not preclude
the presence or addition of one or more other features, integers,
steps, operations, elements, components, and/or groups thereof.
[0038] The corresponding structures, materials, acts, and
equivalents of all means or step plus function elements in the
claims below are intended to include any structure, material, or
act for performing the function in combination with other claimed
elements as specifically claimed. The description of the present
invention has been presented for purposes of illustration and
description, but is not intended to be exhaustive or limited to the
invention in the form disclosed. Many modifications and variations
will be apparent to those of ordinary skill in the art without
departing from the scope and spirit of the invention. The
embodiment was chosen and described in order to best explain the
principles of the invention and the practical application, and to
enable others of ordinary skill in the art to understand the
invention for various embodiments with various modifications as are
suited to the particular use contemplated.
[0039] Having thus described the invention of the present
application in detail and by reference to embodiments thereof, it
will be apparent that modifications and variations are possible
without departing from the scope of the invention defined in the
appended claims.
* * * * *