U.S. patent application number 12/073057 was filed with the patent office on 2008-09-04 for silver selenide film stoichiometry and morphology control in sputter deposition.
Invention is credited to Keith Hampton, Jiutao Li, Allen McTeer.
Application Number | 20080210921 12/073057 |
Document ID | / |
Family ID | 31976442 |
Filed Date | 2008-09-04 |
United States Patent
Application |
20080210921 |
Kind Code |
A1 |
Li; Jiutao ; et al. |
September 4, 2008 |
Silver selenide film stoichiometry and morphology control in
sputter deposition
Abstract
A method of sputter depositing silver selenide and controlling
the stoichiometry and nodular defect formations of a sputter
deposited silver-selenide film. The method includes depositing
silver-selenide using a sputter deposition process at a pressure of
about 0.3 mTorr to about 10 mTorr. In accordance with one aspect of
the invention, an RF sputter deposition process may be used
preferably at pressures of about 2 mTorr to about 3 mTorr. In
accordance with another aspect of the invention, a pulse DC sputter
deposition process may be used preferably at pressures of about 4
mTorr to about 5 mTorr.
Inventors: |
Li; Jiutao; (Boise, ID)
; Hampton; Keith; (Boise, ID) ; McTeer; Allen;
(Meridian, ID) |
Correspondence
Address: |
DICKSTEIN SHAPIRO LLP
1825 EYE STREET NW
Washington
DC
20006-5403
US
|
Family ID: |
31976442 |
Appl. No.: |
12/073057 |
Filed: |
February 28, 2008 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
|
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10230279 |
Aug 29, 2002 |
7364644 |
|
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12073057 |
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Current U.S.
Class: |
257/2 ;
257/E47.001 |
Current CPC
Class: |
H01L 21/02631 20130101;
Y10S 428/938 20130101; C23C 14/0623 20130101; C23C 14/548 20130101;
Y10T 428/12896 20150115; C23C 14/3407 20130101; C23C 14/0021
20130101; C23C 14/3492 20130101; H01L 21/02266 20130101 |
Class at
Publication: |
257/2 ;
257/E47.001 |
International
Class: |
H01L 47/00 20060101
H01L047/00 |
Claims
1-75. (canceled)
76. A resistance variable memory device comprising: a
silver-selenide film deposited onto a chalcogenide glass layer, the
silver-selenide film deposited using a sputter deposition pressure
varying in a range from about 0.3 mTorr to about 10 mTorr, having a
formula of Ag.sub.xSe, where x is about 2, and having substantially
no nodular defects.
77. The device of claim 76 wherein said silver-selenide film has a
silver concentration of less than about 67.5 atomic weight
percent.
78. The device of claim 77 wherein said silver-selenide film has a
silver concentration of about 67 atomic weight percent.
79. The device of claim 77 wherein said silver-selenide film has a
silver concentration of about 66.7 atomic weight percent.
80. A resistance variable memory device comprising: a
silver-selenide film comprising: a silver-selenide target formed
onto a chalcogenide glass layer, wherein the silver-selenide target
has a silver concentration of less than about 67.5 atomic weight
percent, has the formula of Ag.sub.xSe, where x is about 2, and has
substantially no nodular defects.
81. The device of claim 80 wherein said silver-selenide film has a
silver concentration of about 67 atomic weight percent.
82. The device of claim 81 wherein said silver-selenide film has a
silver concentration of about 66.7 atomic weight percent.
83. A resistance variable memory device comprising: a
silver-selenide target pulse DC sputter deposited onto a
chalcogenide glass layer deposited at a pressure range between
about 0.3 mTorr and about 10 mTorr, said silver-selenide target
having a silver concentration of about 66.7 atomic weight percent,
and a silver-selenide film having substantially no nodular defects
from said sputtered target, the silver-selenide film having the
formula of Ag.sub.xSe, where x is about 2.
84. The device of claim 83 wherein said silver-selenide film has a
silver concentration of less than about 67.5 atomic weight
percent.
85. The device of claim 84 wherein said silver-selenide film has a
silver concentration of about 67 atomic weight percent.
86. The device of claim 84 wherein said silver-selenide film has a
silver concentration of about 66.7 atomic weight percent.
87. The device of claim 83 wherein said pressure range is between
about 4 mTorr and about 5 mTorr.
88. A resistance variable memory device comprising: a
silver-selenide target having a silver concentration of about 66.7
atomic weight percent; and a silver-selenide film formed from said
sputtered target, said silver-selenide film having the formula of
Ag.sub.xSe, where x is about 2 and having substantially no nodular
defects.
89. A resistance variable memory device comprising: a
silver-selenide film deposited onto a chalcogenide glass layer, the
silver-selenide film having a formula of Ag.sub.xSe, where x is
about 2, and having substantially no nodular defects.
90. The device of claim 89 wherein said silver-selenide film has a
silver concentration of less than about 67.5 atomic weight
percent.
91. The device of claim 90 wherein said silver-selenide film has a
silver concentration of about 67 atomic weight percent.
92. The device of claim 90 wherein said silver-selenide film has a
silver concentration of about 66.7 atomic weight percent.
Description
FIELD OF THE INVENTION
[0001] The invention relates to the field of resistance variable
memory devices formed using a chalcogenide glass and, in
particular, to an improved method of depositing a silver-selenide
film on a chalcogenide glass.
BACKGROUND OF THE INVENTION
[0002] Chalcogenide materials are presently of great interest for
use in resistance variable memory devices compared to memory
technologies currently in use, due to potential advantages in
switching characteristics, non-volatility, memory speed,
reliability, thermal characteristics, and durability. Research in
this area is reported in the articles "High Speed Memory Behavior
and Reliability of an Amorphous As.sub.2S.sub.3 Film doped with Ag"
by Hirose et al., Phys. Stat. Sol. (1980), pgs. K187-K190;
"Polarity-dependent memory switching and behavior of Ag dendrite in
Ag-photodoped amorphous As.sub.2S.sub.3 films" by Hirose et al.,
Journal of applied Physics, Vol. 47, No. 6 (1976), pgs. 2767-2772;
and "Dual Chemical Role of Ag as an Additive in Chalcogenide
Glasses" by Mitkova et al., Physical Review Letters, Vol. 83, No.
19 (1999), pgs. 3848-3851, the disclosures of which are
incorporated herein by reference.
[0003] In many memory cell designs employing chalcogenide
materials, a film of silver-selenide (Ag.sub.2Se) is incorporated
with a chalcogenide material layer. The silver-selenide film is
important for electrical performance. Accordingly, silver-selenide
deposition is an important aspect of fabricating the resistance
variable memory device. Most available research in silver-selenide
deposition is limited and evaporation deposition is normally chosen
for silver-selenide film formation.
[0004] Silver-selenide deposition by evaporation has an attendant
problem because the dissociative properties of silver-selenide make
it impossible to achieve precision stoichiometries of
silver-selenide. It is believed that in evaporation techniques, as
the silver starts to diffuse to a lower concentration, it begins to
agglomerate. As the silver is tied up in clusters or agglomerates,
selenium is more readily available for evaporation in the beginning
of the evaporation process. Thus, during evaporation techniques,
selenium is evaporated more quickly, causing the deposition target
to become silver-rich. Near the end of the evaporation process
little to no selenium is left for deposition onto the substrate,
leaving mostly silver available for deposition. Thus substantial
amounts of selenium are deposited on the substrate followed by
deposition of primarily silver. Accordingly, the evaporation
technique therefore does not uniformly deposit the silver-selenide
and controlling the stoichiometry and surface morphology of
evaporated silver-selenide is difficult.
[0005] Furthermore, evaporation deposition is not conducive to
industrial application. Sputter deposition is more readily
available for industrial processes and sputter deposition has many
advantages compared to evaporation deposition techniques. For
example, sputter deposition provides better film thickness and
quality control.
[0006] Generally, sputter deposition, or sputtering, is performed
by placing a substrate in a deposition chamber which is evacuated
or pressurized to a desired pressure. A particle stream of the film
material usually generated from a target is then generated within
the chamber and the deposition occurs by condensation of the
particles onto the substrate. In another sputtering technique,
often referred to as ion beam bombardment sputtering, a high-energy
source beam of ions is directed toward the target. The force of the
bombarding ions imparts sufficient energy to the atoms of the
target to cause the energized atoms to leave the target and form a
particle stream. The resulting deposition upon the substrate forms
a thin film.
[0007] Due to the high diffusion property of silver, low melting
point of selenium, and the memory properties of silver-selenide,
controlling the stoichiometry and morphology of the silver-selenide
film during sputter deposition is difficult. For instance,
silver-selenide bulk material is conductive, but its conductivity
(about thousands ohm.sup.-1 cm.sup.-1) is relatively lower than
that of most metals. Also, silver concentration is critical for
electrical performance of the device, thus it is necessary to
maintain the silver concentration close to about 66.7 atomic weight
percent (herein after represented "%"). With silver concentrations
higher than about 67.5%, many nodular defects are formed in and/or
on the silver-selenide film. The size of these defects can be about
a tenth of a micrometer, which could have severe negative impact on
sub micron device fabrication. Although the exact mechanism by
which these defects are formed are unknown, it is believed that
these defects are caused by excess silver, beyond the desired
stoichiometric silver concentration requirements of the
silver-selenide film.
[0008] It would be desirable to have an improved method of
depositing a silver-selenide film. It would also be desirable to
have a method of controlling the stoichiometry and morphology of
silver-selenide for sputter deposition.
BRIEF SUMMARY OF THE INVENTION
[0009] An exemplary embodiment of the present invention includes a
method of depositing a silver-selenide film on a substrate. The
method includes using a low pressure sputter deposition process.
Preferred sputter deposition processes include RF sputtering or
pulse DC sputtering. Preferably, the sputter deposition will occur
in pressures ranging from about 0.3 mTorr to about 10 mTorr. The
invention is particularly useful for depositing a silver-selenide
film with better stoichiometric precision. The invention is also
particularly useful for sputter depositing a silver-selenide film
while avoiding nodular defect formation throughout and on the
surface of the silver-selenide film.
[0010] These and other features and advantages of the invention
will be better understood from the following detailed description,
which is provided in connection with the accompanying drawings.
BRIEF DESCRIPTION OF THE DRAWINGS
[0011] FIG. 1(a) is an SEM image of a pulse DC sputter deposited
silver-selenide film deposited using a pressure of 20 mTorr.
[0012] FIG. 1(b) is an SEM image of a pulse DC sputter deposited
silver-selenide film deposited using a pressure of 10 mTorr.
[0013] FIG. 1(c) is an SEM image of a pulse DC sputter deposited
silver-selenide film deposited using a pressure of 3 mTorr.
DETAILED DESCRIPTION OF THE INVENTION
[0014] In the following detailed description, reference is made to
various specific structural and process embodiments of the
invention. These embodiments are described with sufficient detail
to enable those skilled in the art to practice the invention. It is
to be understood that other embodiments may be employed, and that
various structural, logical and electrical changes may be made
without departing from the spirit or scope of the invention.
[0015] The term "silver-selenide" is intended to include various
species of silver-selenide, including some species which have a
slight excess or deficit of silver, for instance, Ag.sub.2Se,
Ag.sub.2+xSe, and Ag.sub.2-xSe.
[0016] The term "chalcogenide glass" is intended to include various
composition structures based on elements from Group VIA (S, Se, Te,
Po, O) alone or in combination with elements from group IV (Si, Ge)
and/or group V (P, As, Sb, Bi).
[0017] The present invention relates to a process for depositing
silver-selenide. In accordance with the invention, low pressures,
of for example, 0.3 mTorr to about 10 mTorr, are used to sputter
deposit silver-selenide. Also in accordance with the invention,
silver-selenide is preferably deposited using an RF sputtering
process or pulse DC sputtering process.
[0018] Silver-selenide itself has electrical memory properties,
i.e. conductivity, and sputter deposition processes normally
involve strong current, voltage and ion bombardment. Therefore,
both electrical and thermal effects from the sputter deposition
process can influence the silver-selenide sputter target and
deposited silver-selenide film. For the above reason, sputter
deposition requires consideration on how to apply electrical power
to silver-selenide targets.
[0019] Since the conductivity of silver-selenide is relatively
lower than that of most metals, D.C. sputtering has not worked.
Regular DC magnetron sputtering attempts have not been effective,
primarily because the plasma is not easily ignited.
[0020] Depending on the target age, sputter deposition at higher
pressures, e.g., about 20 mTorr or greater, result in films with
either lower or higher silver concentrations than the desired
stoichiometric silver concentration of about 66.7%. It has been
observed that high pressure deposition, e.g., about 20 mTorr or
greater, of relatively new targets using RF or pulse DC magnetron
sputter deposition result in silver-selenide films having silver
concentrations of only about 60%, which is much lower than the
desired stoichiometric silver concentration of 66.7%. It has also
been observed that high pressure deposition, e.g., about 20 mTorr
or greater, of relatively old targets using RF or pulse DC
magnetron sputter deposition result in silver-selenide films having
silver concentrations higher than about 67.5%.
[0021] The inventors have discovered that RF or pulse DC magnetron
sputter deposition processes at low pressures ranging from about
0.3 mTorr to about 10 mTorr may be used to deposit more precise
stoichiometric silver-selenide films while avoiding nodular defects
formation in the film. It has also been discovered that the
silver-selenide target composition changes over the lifetime of the
target, and that the use of a low pressure sputter deposition
process allows for precise stoichiometric deposits from both old
and new silver-selenide targets.
[0022] FIG. 1 shows SEM images of substrates formed of production
grade silicon wafers with silicon nitride films having a pulse DC
sputter deposited silver-selenide film of about 500 Angstroms
thick. The silver-selenide films shown in FIG. 1 were pulse DC
sputter deposited using a Denton Vacuum Discovery.RTM. 24 at 200
kHz with a 1056 ns pulse width, and a constant power supply of 150
W. A silver-selenide target having a stoichiometric silver
concentration of about 66.7% was used to deposit the
silver-selenide film. Comparing the SEM images of the pulse DC
sputter deposited silver-selenide films at various pressures
indicate that low pressure sputter deposition ranging from about
0.3 mTorr to about 10 mTorr reduces and eliminates nodular defect
formations. It was observed that a silver-selenide film deposited
using high pressure, i.e., about 20 mTorr, has a silver
concentration higher than about 67.5% and has nodular defect
formations on the surface and through out the film as shown in FIG.
1(a); as shown in FIG. 1(b) a deposited silver-selenide film formed
using a low pressure of 10 mTorr has relatively few nodular defect
formations; and as shown in FIG. 1(c) a deposited film using an
even lower pressure of 3 mTorr has a smooth surface with no nodular
defects.
[0023] In accordance with a first embodiment of the invention, a
silver-selenide target is sputter deposited using an RF sputter
deposition process at a low pressure ranging from about 0.3 mTorr
to about 10 mTorr, and more preferably about 2 mTorr to about 3
mTorr, to provide a silver-selenide film having little to no
nodular defects and a silver concentration of about equivalent to
the silver concentration of a silver-selenide target used to
sputter deposit the silver-selenide film. For example, where a
silver-selenide target having a silver concentration of about 66.7%
is used in the RF sputter deposition process, the deposited
silver-selenide film will have a silver concentration of less than
about 67.5% and preferably about 67% and more preferably about
66.7%. A process in accordance with the first embodiment of the
invention may be used for silver-selenide targets of any age, while
still providing a sputter deposited silver-selenide film having a
silver concentration about equivalent to that of the
silver-selenide target used to deposit the silver-selenide
film.
[0024] In a sputtering process in accordance with the first
embodiment of the invention, the sputtering deposition generally
takes place in a chamber. An initial base vacuum pressure is
established first. The initial base vacuum pressure may be any
suitable pressure, including pressures higher than about 10 mTorr,
which may help ignite the plasma. During the sputtering process,
process gas should be maintained at a pressure ranging from about
0.3 mTorr to about 10 mTorr, and preferably ranging from about 2
mTorr to about 3 mTorr. The process gas may be any suitable
sputtering process gas, for example, krypton, xenon, helium, neon,
argon or combinations thereof. The preferred process gas is argon.
Although not wishing to be limited by to any particular amounts of
power, power applied during the sputtering process preferably may
range, for example, between about 100 watts to about 500 watts and
is most preferably about 150 watts. Power density and power
requirements may vary and depend on the chosen system or size of
the target. For example, targets four inches or larger may require
more power. The preferred RF frequency is between about 100 kHz and
about 20 MHz and is preferably 13.5 MHz. An exemplary sputter
deposition system is the Denton Vacuum Discovery.RTM. 24.
[0025] In accordance with a second embodiment of the invention, a
silver-selenide target is sputter deposited using a pulse DC
sputter deposition process at low pressures ranging from about 0.3
mTorr to about 10 mTorr to provide a silver-selenide film having a
silver concentration of about equivalent to the silver
concentration of a silver-selenide target used to sputter deposit
the silver-selenide film. For example, where a silver-selenide
target having a silver concentration of about 66.7% is used in the
pulse DC sputter deposition process the deposited silver-selenide
film will have a silver concentration of less than about 67.5% and
preferably about 67% and more preferably about 66.7%. A low
pressure of from about 4 to about 5 mTorr is preferred. There is a
difference between RF sputter deposition and pulse DC sputter
deposition in that for pulse DC sputtering a deposition pressure of
from about 4 to about 5 mTorr, produces a deposited silver-selenide
film having a silver concentration of substantially equivalent to
the silver concentration of the silver-selenide target, for example
66.7%. However, generally low pressure deposition provides smoother
sputter deposited silver-selenide films having a more precise
silver-selenide stoichiometry. The preferred low pressure used may
vary depending on the condition of the target, for example, age of
the target.
[0026] Similar to the process described above in accordance, with
the first embodiment of the invention, the sputtering deposition in
accordance with the second embodiment also takes place in a
chamber, for example, in a Denton Vacuum Discovery.RTM. 24, where a
suitable initial base vacuum pressure is established first and a
suitable process gas is employed. However, in accordance with the
second embodiment, during the sputtering process, the process gas
should be maintained at a pressure ranging from about 0.3 mTorr to
about 10 mTorr, and preferably ranging from about 4 mTorr to about
5 mTorr. Although not wishing to be limited by to any particular
amounts of power, the power applied during the sputtering process
preferably may range, for example, between about 100 watts to about
500 watts and is most preferably 150 watts and the preferred pulse
DC frequency may range, for example, between about 100 kHz and
about 250 kHz and is preferably about 200 kHz. However, power
density and power requirements may vary and t will depend on the
chosen system and/or size of the target. For example, targets four
inches or larger may require more power. The pulse width should
range from about 1000 ns to about 1200 ns and is preferably about
1056 ns.
[0027] Although the exact mechanism to explain the origin of
experimental observations is unknown there is a connection between
sputter pressure, ion kinetic energy, scattering induced energy
reduction, and/or RF and pulse DC plasma electrical properties. For
practical application, the inventors propose to use an RF sputter
deposition process or pulse DC sputter deposition process at lower
pressure to deposit better precision stoichiometric silver-selenide
films and avoid nodular defect formations on the film. Accordingly,
pressure may be varied within the low pressure range of from about
0.3 mTorr to about 10 mTorr to fine tune the silver concentration
of the silver-selenide film. The power sources may be varied as
well. This is of great importance in device fabrication in that
many devices require elemental concentrations slightly deviated
(i.e., .+-.2% at. concentration) from the preferred value of about
66.7%. Accordingly, since low pressure sputter deposition can also
be used on relatively old targets while still providing more
precise stoichiometric concentrations of silver, the invention
expands the target lifetime thus reducing process costs.
[0028] While exemplary embodiments of the invention have been
described and illustrated, various changes and modifications may be
made without departing from the spirit or scope of the invention.
Accordingly, the invention is not limited by the foregoing
description, but is only limited by the scope of the appended
claims.
* * * * *