U.S. patent application number 11/201790 was filed with the patent office on 2005-12-08 for process for manufacturing integrated resistor and phase-change memory element including this resistor.
This patent application is currently assigned to STMicroelectronics S.r.l.. Invention is credited to De Santi, Giorgio, Marangon, Maria Santina, Zonca, Romina.
Application Number | 20050269667 11/201790 |
Document ID | / |
Family ID | 8185827 |
Filed Date | 2005-12-08 |
United States Patent
Application |
20050269667 |
Kind Code |
A1 |
Zonca, Romina ; et
al. |
December 8, 2005 |
Process for manufacturing integrated resistor and phase-change
memory element including this resistor
Abstract
A vertical-current-flow resistive element includes a monolithic
region having a first portion and a second portion arranged on top
of one another and formed from a single material. The first portion
has a first resistivity, and the second portion has a second
resistivity, lower than the first resistivity. To this aim, a
monolithic region with a uniform resistivity and a height greater
than at least one of the other dimensions is first formed; then the
resistivity of the first portion is increased by introducing, from
the top, species that form a prevalently covalent bond with the
conductive material of the monolithic region, so that the
concentration of said species becomes higher in the first portion
than in the second portion. Preferably, the conductive material is
a binary or ternary alloy, chosen from among TiAl, TiSi,
TiSi.sub.2, Ta, WSi, and the increase in resistivity is obtained by
nitridation.
Inventors: |
Zonca, Romina; (Paullo,
IT) ; Marangon, Maria Santina; (Merate, IT) ;
De Santi, Giorgio; (Milano, IT) |
Correspondence
Address: |
SEED INTELLECTUAL PROPERTY LAW GROUP PLLC
701 FIFTH AVE
SUITE 6300
SEATTLE
WA
98104-7092
US
|
Assignee: |
STMicroelectronics S.r.l.
Agrate Brianza
ID
Ovonyx Inc.
Boise
|
Family ID: |
8185827 |
Appl. No.: |
11/201790 |
Filed: |
August 11, 2005 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
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11201790 |
Aug 11, 2005 |
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10345129 |
Jan 14, 2003 |
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6946673 |
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Current U.S.
Class: |
257/530 ;
257/E45.002 |
Current CPC
Class: |
H01L 45/16 20130101;
H01L 45/1233 20130101; G11C 13/0069 20130101; G11C 13/0004
20130101; H01L 45/06 20130101; G11C 2213/52 20130101; G11C 2013/008
20130101; H01L 45/126 20130101; H01L 45/144 20130101 |
Class at
Publication: |
257/530 |
International
Class: |
H01L 029/00 |
Foreign Application Data
Date |
Code |
Application Number |
Jan 17, 2002 |
EP |
02425013.6 |
Claims
1. A method of making a vertical-current-flow resistive element,
comprising: forming a monolithic region of a single material having
a height greater than at least one other dimension, the forming
including: forming first and second portions of the monolithic
region arranged on top of one another, by forming said first
portion with a first resistivity and said second portion with a
second resistivity lower than said first resistivity.
2. The method according to claim 1, wherein forming said first
portion includes adding to the monolithic region a first
concentration of species forming a prevalently covalent bond with
said single material, and forming said second portion includes
adding to the monolithic region a second concentration of said
species, lower than said first concentration, wherein said single
material is conductive.
3. The method according to claim 2, wherein said species have a
decreasing concentration starting from said first portion towards
said second portion.
4. The method according to claim 2, wherein said single material is
a binary or ternary alloy.
5. The method according to claim 4, wherein said conductive
material is chosen from among TiAl, TiSi, TiSi.sub.2, Ta, and
WSi.
6. The method according to claim 2, wherein said species comprise
nitrogen.
7. The method according to claim 1, wherein said first portion has
an approximately constant resistivity.
8. The method according to claim 1, wherein said second portion has
a gradually decreasing resistivity starting from said first
portion.
9. The method according to claim 1, further comprising: forming a
top electrode of conductive material in electrical contact with
said first portion; and forming a bottom electrode of conductive
material in direct electrical contact with said second portion.
10. The method according to claim 1, wherein said monolithic region
has a shape chosen from among a column shape, having a height, a
width and a depth, wherein the height is greater than the width and
the depth; a wall shape, having a height, a width and a depth,
wherein the depth and the height are greater than the width: and a
closed shape having a height, a width and a perimeter, wherein the
height and the perimeter are greater than the width.
11. A process for manufacturing a vertical-current-flow resistive
element, comprising: forming a monolithic region of conductive
material having a uniform resistivity and having a first portion
and a second portion arranged on top of one another; and increasing
the resistivity of said first portion so that said first portion
has a greater resistivity than said second portion.
12. The process according to claim 11, wherein said step of
increasing the resistivity comprises enriching said first portion
with species that form a prevalently covalent bond with said
conductive material.
13. The process according to claim 12, wherein said step of
enriching comprises implanting or introducing said species from
plasma.
14. The process according to claim 12, wherein said species
comprise nitrogen.
15. The process according to claim 11, wherein said conductive
material is a binary or ternary alloy.
16. The process according to claim 11, wherein said conductive
material is chosen from among TiAl, TiSi, TiSi.sub.2, Ta, and
WSi.
17. The process according to claim 11, further comprising the steps
of forming a bottom electrode of conductive material in direct
electrical contact with said second portion and forming a top
electrode of conductive material in electrical contact with said
first portion.
18. A method of making a phase-change memory element, comprising:
forming a programmable element of chalcogenic material; and forming
a resistive element having a first end in direct electrical contact
with said programmable element, the resistive element including a
monolithic region having a first portion and a second portion
arranged on top of one another, the first portion having a first
resistivity and said second portion having a second resistivity
lower than said first resistivity.
19. The method of claim 18, wherein forming the resistive element
includes forming the monolithic region of a conductive material;
forming the first portion by adding to the monolithic region a
first concentration of a species covalently bonded with the
conductive material; and forming the second portion by adding to
the monolithic region a second concentration of the species
covalently bonded with the conductive material, the first
concentration being greater than the second concentration.
20. The method of claim 19, wherein the conductive material is
chosen from among TiAl, TiSi, TiSi.sub.2, Ta, and WSi and the
species include nitrogen.
21. The method of claim 18 wherein the step of forming the
resistive element includes: forming the monolithic region of
conductive material having a uniform resistivity; and increasing
the resistivity of the first portion so that the first portion has
a greater resistivity than the second portion.
22. The method of claim 18, further comprising: forming a
conductive electrode; forming an insulating layer on the electrode;
and forming an opening that extends through the insulating layer
and exposes a portion of the electrode, wherein forming the
resistive element includes forming the monolithic region in the
opening and in contact with the electrode.
23. The method of claim 22 wherein forming the resistive element
includes depositing a conductive material in the opening, removing
an excess portion of the conductive material from a surface of the
insulating layer, and nitridating the conductive material in the
opening without using a mask.
Description
BACKGROUND OF THE INVENTION
[0001] 1. Field of the Invention
[0002] The present invention relates to an integrated resistor, a
phase-change memory element including this resistor, and a process
for the fabrication thereof.
[0003] 2. Description of the Related Art
[0004] As is known, phase-change memory elements, or PCM elements,
exploit the characteristics of a class of materials able to change
between two phases having distinct electrical characteristics. For
example, these materials may change from an amorphous, disorderly
phase to a crystalline or polycrystalline, orderly phase, and the
two phases are associated to considerably different values of
resistivity.
[0005] At present, alloys of elements of group VI of the periodic
table, such as Te or Se, referred to as chalcogenides or
chalcogenic materials, can advantageously be used in phase-change
cells. The chalcogenide that currently offers the most promise is
formed by a Ge, Sb and Te alloy (Ge.sub.2Sb.sub.2Te.sub.5) and is
widely used for storing information in overwritable disks.
[0006] In chalcogenides, the resistivity varies by two or more
orders of magnitude when the material passes from the amorphous
phase (more resistive) to the crystalline phase (more conductive)
and vice versa. The characteristics of the chalcogenides in the two
phases are shown in FIG. 1. As may be noted, at a given read
voltage, here designated by Vr, there is a resistance variation of
more than 10.
[0007] Phase change may be obtained by locally increasing the
temperature, as shown in FIG. 2. Below 150.degree. C. both phases
are stable. Above 200.degree. C. (nucleation start temperature,
designated by T.sub.x), fast nucleation of the crystallites takes
place, and, if the material is kept at the crystallization
temperature for a sufficient length of time (time t.sub.2), it
changes its phase and becomes polycrystalline. To bring the
chalcogenide back into the amorphous state, it is necessary to
raise the temperature above the melting temperature T.sub.m
(approximately 600.degree. C.) and then to cool the chalcogenide
off rapidly (time t.sub.1).
[0008] From the electrical standpoint, it is possible to reach both
the critical temperatures, namely crystallization and melting
temperature, by causing a current to flow through a resistive
element which heats the chalcogenic material by Joule effect.
[0009] The basic structure of a phase-change memory element 1 which
operates according to the principles described above is shown in
FIG. 3 and comprises a resistive element 2 (heater) and a
programmable element 3. The programmable element 3 is made of a
chalcogenide and is normally in the polycrystalline state in order
to enable a good flow of current. One part of the programmable
element 3 is in direct contact with the resistive element 2 and
forms the area involved in the phase change, hereinafter referred
to as phase-change portion 4.
[0010] If an electric current having an appropriate value is made
to pass through the resistive element 2, it is possible to heat the
phase-change portion 4 selectively up to the crystallization
temperature or to the melting temperature and to cause phase
change. In particular, if a current I is made to pass through a
resistive element 2 having resistance R, the heat generated is
equal to I.sup.2R.
[0011] At present, the resistive element 2 is obtained by
deposition--using PVD (Physical Vapor Deposition), Reactive PVD and
CVD (Chemical Vapor Deposition)--of materials having a resistivity
of between a few hundred .mu..OMEGA.cm and a few ten m.OMEGA.cm.
The material thus obtained has a substantially homogeneous
resistance in all directions.
[0012] The memory element described above is disadvantageous since
it has a high dissipation on account of the high resistance of the
resistive element, even if the portion useful for generating the
phase change heat for the memory element 1 is only one part of its
volume. A high level of dissipation may, in fact, be harmful for
the materials and components integrated in the chip. The problems
associated with dissipation of the entire resistive element
moreover impose design constraints on the values of resistivity
that can be used for the resistive element, as well as on the
programming currents and voltages, giving rise to high levels of
consumption.
BRIEF SUMMARY OF THE INVENTION
[0013] An embodiment of the present invention provides a resistive
element that overcomes the described disadvantages.
[0014] The resistive element is a vertical-current-flow resistive
element that includes a monolithic region having a first portion
and a second portion arranged on top of one another. The monolithic
region is formed by a single material and has a height greater than
at least one other dimension. The first portion has a first
resistivity and the second portion has a second resistivity lower
than the first resistivity.
BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWING(S)
[0015] For a better understanding of the present invention, a
preferred embodiment thereof is now described, purely by way of
non-limiting example, with reference to the attached drawings,
wherein:
[0016] FIG. 1 shows the current-versus-voltage characteristic of a
phase-change material;
[0017] FIG. 2 shows the temperature-versus-current plot of a
phase-change material;
[0018] FIG. 3 shows the basic structure of a PCM element;
[0019] FIG. 4 is a cross-section of a resistive element according
to the invention;
[0020] FIG. 5 presents a concentration plot of ions designed to
increase the resistivity of the resistive element of FIG. 4;
and
[0021] FIGS. 6, 7 and 8 are top plan views of the resistive element
of FIG. 4, according to three different embodiments.
DETAILED DESCRIPTION OF THE INVENTION
[0022] FIG. 4 shows a PCM memory element 10 comprising a bottom
electrode 11, of electrically conductive material; an insulating
layer 16, arranged on top of the bottom electrode 11; a resistive
element 12, which extends vertically inside the insulating layer 16
and is in contact with the bottom electrode 11; a polycrystalline
layer 13, which extends on top of the insulating layer 16 and has a
portion (hereinafter referred to as phase-change portion 14) in
contact with the resistive element 12; and a top electrode 15, of
conductive material, which extends on top of the polycrystalline
layer 13.
[0023] The resistive element 12, of overall height H, is of the
vertical-current-flow type and has a height or thickness H in the Z
direction much greater than at least one of the other dimensions
(width in the X direction and depth in the Y direction). In
particular, the resistive element 12 may be column-shaped, with a
depth (in the Y direction) comparable to the width in the X
direction, as shown in the schematic top plan view of FIG. 6, or
else wall-shaped, having a depth much greater than the width in the
X direction, as shown in the schematic plan view of FIG. 7, or yet
again may have a closed shape, such as the annular shown in the
schematic top plan view of FIG. 8.
[0024] The resistive element 12 is formed by a monolithic region
made of a material selected among TiAlN, TiSiN, TiSi.sub.2N, TaN,
WSiN and has a first portion 12a, having high resistivity, and a
second portion 12b, of lower resistivity, arranged on top of one
another. In the example illustrated, the first portion 12a is
arranged at the top, and the second portion 12b is arranged at the
bottom. The resistivity of the resistive element 12 may vary
gradually, or else sharply, between the first portion 12a and the
second portion 12b.
[0025] The resistive element 12 of FIG. 4 is obtained starting from
a material having an intrinsically medium-to-low resistivity, such
as TiAl, TiSi, TiSi.sub.2, Ta, WSi, or another binary or ternary
alloy with similar characteristics, and is subsequently treated so
as to increase the resistivity of the first portion 12a with
respect to the second portion 12b.
[0026] Preferably, the starting material of the resistive element
12 is enriched with nitrogen ions or nitrogen radicals, so as to
increase local resistivity. For example, the enrichment may be
achieved by plasma implantation or nitridation. Possibly,
afterwards the resistive element 12 may be subjected to a thermal
process whereby the introduced nitrogen forms amorphous,
temperature-stable clusters.
[0027] As is known, nitrogen contributes to forming covalent bonds,
rather than metallic bonds, and consequently determines a decrease
in the electrons present in the conduction band, and thus increases
the value of resistivity of the material into which it has been
introduced.
[0028] FIG. 5 shows the distribution of the nitrogen-ion content in
the vertical direction (Z axis) inside the resistive element 12,
after nitridation. In the example illustrated, in the first portion
12A (the top one) there is a concentration of nitrogen ions which
to a first approximation is constant and has a higher value,
corresponding to a high resistivity, whereas in the second portion
12b (the bottom one) the concentration of nitrogen ions is smaller
and decreases almost down to zero in proximity of the interface
with the bottom electrode 11. For example, the first portion 12a
has a resistivity of approximately 10 m.OMEGA.cm, whilst in the
second portion 12b the resistivity is reduced to approximately 1
m.OMEGA.cm. The profile of the nitrogen concentration, and hence of
the resistivity, can in any case be engineered according to the
particular requirements.
[0029] As indicated in FIG. 4 by the arrows 20, the current flows
in a vertical direction (Z direction), i.e., parallel to the height
of the resistive element 12, in contrast to barrier regions, made
of similar alloys, wherein normally the thickness of the layer is
much smaller than its width, and the current flows in a direction
transverse to the larger dimension (width). In barrier regions,
moreover, the resistivity is approximately uniform in the direction
of the flow of current.
[0030] The resistive element 12 of FIG. 4 is obtained as described
hereinafter. After depositing and patterning the bottom electrode
11, on top of the substrate (not shown) the insulating layer 16 is
deposited and planarized, so as to have, at the end, a height H.
The insulating layer 16 is then etched to form an opening where the
resistive element 12 is to be made.
[0031] Next, the starting material of the resistive element 12, for
example TiAl, TiSi, TiSi.sub.2, Ta, or WSi, is deposited, and the
excess material is removed from the surface of the insulating layer
16, for example by etch-back or CMP (Chemical Mechanical
Polishing).
[0032] The resistive element 12 is then nitridated, for instance by
an N implantation or a nitrogen-plasma implantation ("Remote Plasma
Nitridation" or "Decoupled Plasma Nitridation"), or, in general,
using any process that generates reactive nitrogen species
(nitrogen ions or nitrogen radicals). The processes enable
engineering of the nitrogen profile in the Z direction (as shown,
for example, in FIG. 5), thus enabling modulation of the
resistivity of the resistive element 12. Preferably, the
nitridation step is carried out without the use of masks.
[0033] Next, the polycrystalline layer 13 and the layer intended to
form the top electrode 15 are deposited and are then defined so as
to form a strip that extends perpendicular (at least locally) to
the resistive element 12. In practice, the width direction of the
resistive element 12 is parallel to the direction of extension of
the strip in the area of mutual contact.
[0034] The advantages of the resistive element described are
illustrated hereinafter. First, modulation of the resistivity in
the vertical direction enables minimization of the heat dissipation
and of the voltage drop in the portion distant from the
phase-change region 14 (second portion 12b in contact with the
bottom electrode 11) and maximization of the same quantities in the
first portion (i.e., the one in contact with the phase-change
region 14), where it is important to have a good generation of heat
in order to control phase change of the phase-change region 14.
Thus a high local dissipation of heat is obtained in contact with
the phase-change region 14 and a low dissipation elsewhere, with a
consequent reduction in the risks of damage to the materials and
components integrated in the chip.
[0035] The optimization of the resistivity profile moreover enables
the use of programming voltages and currents lower than those
required for a uniform resistive element. Consequently, it is
possible to achieve better performance of the device, reduce energy
consumption, and simplify the design of the components intended to
generate and transport said currents and voltages.
[0036] Finally, it is clear that numerous modifications and
variations may be made to the resistive element described and
illustrated herein, all falling within the scope of the invention,
as defined in the attached claims. For example, using a heavy
implantation, it is possible to nitride preferentially the deep
portion of the resistive element 12, obtaining a nitrogen and
resistivity profile opposite to the one of FIG. 5. In addition, by
engineering the nitridation technique, it is possible to modify the
concentration profile so as to obtain, instead of a gradual
reduction of the resistivity in the second portion, a sharp
reduction of the resistivity, or else so as to obtain a portion of
reduced thickness with a high resistivity, or yet again a profile
with gradual variation of the resistivity throughout the height of
the resistive element. In addition, one could use a material other
than nitrogen to adjust the resistivity profile of the resistive
element 12.
[0037] All of the above U.S. patents, U.S. patent application
publications, U.S. patent applications, foreign patents, foreign
patent applications and non-patent publications referred to in this
specification and/or listed in the Application Data Sheet, are
incorporated herein by reference, in their entirety.
* * * * *