U.S. patent application number 12/841035 was filed with the patent office on 2011-01-20 for silver selenide sputtered films and method and apparatus for controlling defect formation in silver selenide sputtered films.
Invention is credited to Jiutao Li, Allen McTeer.
Application Number | 20110014115 12/841035 |
Document ID | / |
Family ID | 34573480 |
Filed Date | 2011-01-20 |
United States Patent
Application |
20110014115 |
Kind Code |
A1 |
Li; Jiutao ; et al. |
January 20, 2011 |
SILVER SELENIDE SPUTTERED FILMS AND METHOD AND APPARATUS FOR
CONTROLLING DEFECT FORMATION IN SILVER SELENIDE SPUTTERED FILMS
Abstract
Method and apparatus for sputter depositing silver selenide and
controlling defect formation in and on a sputter deposited silver
selenide film are provided. A method of forming deposited silver
selenide comprising both alpha and beta phases is further provided.
The methods include depositing silver selenide using sputter powers
of less than about 200 W, using sputter power densities of less
than about 1 W/cm.sup.2, using sputter pressures of less than about
40 mTorr and preferably less than about 10 mTorr, using sputter
gasses with molecular weight greater than that of neon, using
cooling apparatus having a coolant flow rate at least greater than
2.5 gallons per minute and a coolant temperature less than about
25.degree. C., using a magnetron sputtering system having a
magnetron placed a sufficient distance from a silver selenide
sputter target so as to maintain a sputter target temperature of
less than about 350.degree. C. and preferably below about
250.degree. C. during sputter deposition, and heating the sputter
deposition substrate to greater than about 30.degree. C.
Inventors: |
Li; Jiutao; (Boise, ID)
; McTeer; Allen; (Eagle, ID) |
Correspondence
Address: |
Thomas J. D'Amico;DICKSTEIN SHAPIRO MORIN & OSHINSKY LLP
2101 L. Street NW
Washington
DC
20037-1526
US
|
Family ID: |
34573480 |
Appl. No.: |
12/841035 |
Filed: |
July 21, 2010 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
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10712106 |
Nov 14, 2003 |
7799180 |
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12841035 |
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Current U.S.
Class: |
423/509 ;
204/192.15 |
Current CPC
Class: |
C23C 14/3492 20130101;
C23C 14/0623 20130101 |
Class at
Publication: |
423/509 |
International
Class: |
C01B 19/04 20060101
C01B019/04 |
Claims
1-84. (canceled)
85. A silver selenide film, comprising: deposited silver selenide,
wherein said silver selenide comprises both alpha silver selenide
and beta silver selenide.
86. The silver selenide film of claim 85, wherein said deposited
silver selenide primarily comprises alpha silver selenide.
Description
FIELD OF THE INVENTION
[0001] This invention relates generally to a method and apparatus
for sputtering silver selenide and more particularly to a method
and apparatus for controlling defect formation and structural
phases in sputtered silver selenide layers.
BACKGROUND OF THE INVENTION
[0002] Noble metal doped chalcogenide glasses, such as silver
selenide, are presently of great interest for use in non-volatile
memory devices. Non-volatile memory devices employing thin films of
doped chalcogenide glasses offer several advantages over
conventional devices, especially with regard to non-volatile memory
speed, thermal characteristics, durability and reliability.
[0003] Thin film doped deposition is an important aspect of
fabricating memory devices containing metal doped chalcogenide
glasses. Because these films play an important role in the
electrical performance of such memory devices, it is desirable that
these deposited layers be defect free. This is especially true when
such memory devices are used in ultra large scale integration
(ULSI) devices, whose sub-micron structure greatly increases the
need for thin film defect control. For example, thin film defects
may be 10 to 100 times larger than typical ULSI device structures
and thus can easily cause shorting and other electrical performance
problems.
[0004] Silver selenide is a desirable chalcogenide glass material
for use in fabricating non-volatile memory devices. To date,
however, there has been only limited research into the fundamental
properties of silver selenide thin films. Furthermore, most of this
research involves forming silver selenide films via an evaporation
deposition technique. Evaporation deposition, however, is not well
suited for industrial application and has an attendant problem in
that the dissociative properties of silver selenide make it
difficult to achieve precision stoichiometries in the deposited
film.
[0005] Physical vapor deposition, also known as "sputtering," is
more easily adaptable to industrial applications and also provides
better coating thickness and quality control than evaporation
deposition techniques. Silver selenide, however, exhibits defect
formation during conventional sputter deposition due to localized
high temperatures, which occur during the sputtering operation.
[0006] Sputtering devices have long been used by the semiconductor
processing industry to coat substrates (e.g., silicon wafers) with
various materials (e.g., aluminum, titanium, gold, etc.) during the
manufacture of integrated circuits. Generally, in a sputtering
device, the material to be deposited or sputtered onto the
substrate is contained in a target. The substrate is placed on a
substrate support table in a sputtering chamber. Air in the
sputtering chamber is evacuated and replaced with an inert gas such
as argon, usually at a low pressure. An electric field is then
established between an anode, such as the walls that line the
sputtering chamber, and the target, which acts as an cathode
(electron source). The resulting potential gradient causes
electrons to be emitted from the target surface. As these electrons
are drawn toward the anode by the electric field, they strike and
ionize some of the inert gas molecules. These positively charged
inert gas ions are then drawn toward and collide with the
negatively charged target. The ions impact the target with
sufficient energy to dislodge, or sputter, particles of target
material into the sputtering chamber. The substrate to be coated,
which is usually positioned in the chamber with its surface facing
the target, receives some of the sputtered target particles, which
adhere to and coat the substrate. The cloud of free electrons,
inert gas atoms, inert gas ions, and sputtered target particles
that exists near the target sputtering surface is termed a "plasma
discharge."
[0007] The location of the plasma discharge may be controlled using
magnetron sputtering devices, which introduce a magnetic field
adjacent to the sputtering surface of the target in the sputtering
chamber. The magnetic field is generated by a rotating magnetic
circuit located on the side of the target opposite the sputtering
surface. The magnetic field acts to trap electrons in a desired
region, thus producing a region of high-density plasma. This region
of high-density plasma rotates with the magnetic circuit about an
axis that is perpendicular to the target sputtering surface. Thus,
deep erosion of the target sputtering surface occurs in the region
where the high density plasma is produced, while other portions of
the target are hardly sputtered at all. This preferentially
sputtered region of the target is often referred to as the
racetrack portion, due to the characteristic ovoid-shaped area of
the target that is eroded by the high-density plasma discharge.
[0008] It is generally known that the sputtering process generates
a substantial amount of energy, which results in heating of the
sputtering target. This heating is caused by the high electrical
potential and current applied to the target material and by the
energy delivered to the target by the bombarding ions. The heat
generated during sputtering needs to be dissipated; otherwise it
may damage the target and other components of the magnetron
sputtering device and, in the case of silver selenide targets, may
cause defect formation on the target material. In one previously
known approach for cooling a sputtering target, a water-tight
cooling chamber is formed on the side of the target opposite the
target sputtering surface. The non-sputtering surface of the target
forms one wall of the cooling chamber. The cooling chamber is
filled with coolant (e.g., water), which floods the non-sputtering
surface of the target and dissipates the heat generated during
sputtering.
[0009] During the conventional sputter deposition of silver
selenide using a magnetron sputtering device, millimeter sized
defects rapidly form on the surface of the target, especially in
the target racetrack area. Silver selenide target surface defect
formation causes increased defect formation in and on the deposited
film. This defect formation results from localized heating of the
target during sputtering coupled with both the relatively low phase
transition point of selenide and the low melting point of silver.
It is thought that the associated heating of the Ag.sub.2Se target
during sputtering promotes the diffusion of silver into the
Ag.sub.2Se target, which causes natural protrusions to grow on the
target. These defects are then transferred to the deposited film
during sputtering, causing undesired localized stoichiometric
variations in the deposited film.
[0010] Thus, in view of the foregoing, there is a desire and need
for an improved method of sputtering a silver selenide target with
a reduced target temperature, which in turn reduces target and
deposited film defect formation.
BRIEF SUMMARY OF THE INVENTION
[0011] Embodiments of the invention provide a method and apparatus
for sputter depositing a silver selenide film on a substrate, while
maintaining a sputter target temperature at least below about
350.degree. C. and preferably below about 250.degree. C. Desirable
sputtering processes include magnetron RF, DC, and pulse DC
sputtering.
[0012] In one embodiment, silver selenide films are sputter
deposited by employing a silver selenide target having a
temperature maintained below about 350.degree. C., more preferably
below about 250.degree. C., wherein said target temperature is
maintained by conducting the sputtering process under a sputter
power below about 200 W, more preferably below about 100 W.
[0013] In another embodiment, silver selenide films are sputter
deposited by employing a silver selenide target having a
temperature maintained below about 350.degree. C., more preferably
below about 250.degree. C., wherein said target temperature is
maintained by conducting the sputtering process with a sputter gas
maintained at a pressure of less than about 40 mTorr, more
preferably less than about 10 mTorr.
[0014] In another embodiment, silver selenide films are sputter
deposited by employing a silver selenide target having a
temperature maintained below about 350.degree. C., more preferably
below about 250.degree. C., wherein said target temperature is
maintained by conducting the sputtering process employing a sputter
gas with a molecular weight greater than that of neon.
[0015] In yet another embodiment, silver selenide films are sputter
deposited by employing a silver selenide target having a
temperature maintained below about 350.degree. C., more preferably
below about 250.degree. C., wherein said target temperature is
maintained by conducting the sputtering process using a target
cooling apparatus with coolant flow rates at least above 2.5
gal/min and coolant temperature at least below 25.degree. C.
[0016] In another embodiment, silver selenide films are sputter
deposited by employing a silver selenide target having a
temperature maintained below about 350.degree. C., more preferably
below about 250.degree. C., wherein said target temperature is
maintained by conducting a sputter process under a sputter power
density of less than about 1 W/cm.sup.2.
[0017] In another embodiment, silver selenide films are sputter
deposited by positioning a magnetron of the sputtering system such
that the silver selenide target temperature is maintained below
about 350.degree. C., more preferably below about 250.degree.
C.
[0018] Desirable sputter apparatuses of the present invention
include sputtering systems with cooling apparatuses capable of
maintaining a silver selenide target at a temperature at least
below about 350.degree. C. and preferably below about 250.degree.
C. during sputter deposition. Other desired sputtering apparatuses
include magnetron sputtering systems using a magnetron positioned
so as to maintain a target temperature of less than about
350.degree. C. and preferably less than about 250.degree. C. during
sputter deposition.
[0019] The methods and apparatuses of the present invention are
particularly useful for controlling defect formation during sputter
deposition of silver selenide thin films.
BRIEF DESCRIPTION OF THE DRAWINGS
[0020] The features mentioned above and other features and
advantages of the present invention will be better understood from
the following detailed description, which is provided in connection
with the accompanying drawings in which:
[0021] FIG. 1 is a graph showing total particle count vs. target
lifetime for a silver selenide sputtering target;
[0022] FIGS. 2(a) and 2(b) illustrate scanning electron microscope
images showing typical defects deposited on and in a sputter
deposited silver selenide film;
[0023] FIGS. 3(a) and 3(b) show images of typical black porous
defects formed on an Ag.sub.2Se sputter target after 3 kwh of
target consumption during sputter deposition;
[0024] FIG. 4(a) shows a deposited silver selenide film formed
according to one embodiment of the prior art;
[0025] FIG. 4(b) shows a silver selenide film structure change when
the silver selenide film of FIG. 4(a) is annealed at 130.degree.
C.;
[0026] FIG. 4(c) shows further silver selenide film structure
changes when the silver selenide film of FIG. 4(a) is annealed at
250.degree. C.;
[0027] FIG. 4(d) shows cone defect formation when the silver
selenide film of FIG. 4(a) is annealed at 350.degree. C.;
[0028] FIG. 4(e) shows whisker-like defect formation when the
silver selenide film of FIG. 4(a) is annealed at 380.degree.
C.;
[0029] FIG. 4(f) shows millimeter-sized whisker-like defect
formation when the silver selenide film of FIG. 4(a) is annealed at
410.degree. C.;
[0030] FIG. 5 is a graph illustrating the relationship between
target temperature and sputter power during silver selenide
deposition;
[0031] FIG. 6 is a graph illustrating the relationship between
target temperature and sputter pressure during silver selenide
sputter deposition;
[0032] FIG. 7 is a graph illustrating the relationship between
target temperature and Ne/Ar sputter gas flow ratio at varying
pressures during silver selenide sputter deposition;
[0033] FIG. 8 is a graph illustrating the relationship between
target temperature and Xe/Ar flow ratio at varying pressures during
silver selenide sputter deposition;
[0034] FIG. 9 is a graph illustrating the relationship between
target temperature and sputter gas;
[0035] FIG. 10 is a graph illustrating the relationship between
target temperature and effective sputter power density;
[0036] FIG. 11 shows a simplified example of a magnetron sputtering
device to be used to carry out the embodiments of the
invention;
[0037] FIG. 12(a) shows a cross sectional view of a non-uniform
target sputter profile;
[0038] FIG. 12(b) shows a top view of a non-uniform target sputter
profile, which forms a characteristic racetrack shape; and
[0039] FIG. 12(c) shows a cross sectional view of a uniform target
sputter profile.
[0040] FIG. 13 shows two x-ray diffraction scans of deposited
silver selenide films sputtered under different sputter pressures
and powers.
DETAILED DESCRIPTION OF THE INVENTION
[0041] 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.
[0042] 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.
[0043] The term "defect free silver selenide film" as used in this
application is defined as silver selenide films having less than
about 0.16 defect counts/cm.sup.2.
[0044] The present invention relates to a method of controlling
defect formation in deposited silver selenide and to a method of
depositing a silver selenide film having a mixture of alpha and
beta phases. In accordance with the invention, silver selenide is
deposited using a sputtering process that maintains the silver
selenide target temperature at less than about 350.degree. C. and
preferably less than about 250.degree. C.
[0045] During the conventional sputter deposition of silver
selenide, target defect count has been observed to increase from
less than fifty to over thousands per wafer after only very short
target consumption (2-3 kwh), as illustrated in FIG. 1. Although
these defects can be reduced to under fifty counts by gently wiping
the target, after another several kwh sputter deposition, the
surface of the target will again exhibit severe defect formation.
These target defects in turn cause increasing defect formation in
and on the deposited film. FIGS. 2(a) and 2(b) represent typical
scanning electron microscope (SEM) images of defects formed on
deposited silver selenide films.
[0046] Based on the film deposition rate for silver selenide,
target consumption of about 2-3 kwh represents deposition of silver
selenide on only several hundred wafers. This wafer deposition rate
is not practical for commercial production. Furthermore, in order
to obtain the proper film composition and resistivity, the silver
selenide target requires at least 0.5 kwh of conditioning each time
the target chamber is vented for target cleaning. Thus, the usable
target life between cleanings is even further reduced.
[0047] The defect shapes shown in FIGS. 2(a) and (b) lead the
present inventors to believe that the deposited silver selenide
defects could be due to a micro arching on the silver selenide
target, which subsequently forms a splatter defect in and on the
silver-selenide film. This belief is supported by FIGS. 3(a) and
(b), showing two images of an Ag.sub.2Se target after 3 kwh of
target consumption. The images clearly show many black porous
chips, which are approximately millimeter sized. FIGS. 3(a) and (b)
also show that the defect density reaches its highest on the
racetrack portion of the target.
[0048] Silver selenide (e.g. Ag.sub.2Se) is well known for its low
temperature phase transition point of 406 K (about 130.degree. C.).
At temperatures below 406 K, Ag.sub.2Se forms an orthorhombic
structure, known as the "beta phase." At temperatures above 406 K
(about 133.degree. C.), Ag.sub.2Se undergoes a structural change in
which the Se forms a body-centered cubic sublattice, while the Ag
undergoes a melting transition. In this so-called "alpha phase" or
"superionic phase," the Ag ions exhibit liquid-like diffusion. At
around 1170 K (897.degree. C.), the Se sublattice will also undergo
a melting transition.
[0049] During magnetron sputter deposition, the high electrical
potential and current applied to the target as well as the energy
imparted to the target via bombardment by the inert gas ions can
cause localized heating of the sputter target in the regions where
the high-density plasma discharge is concentrated. This heating
effect is sufficient to raise the temperature of localized regions
of the Ag.sub.2Se target at least enough to accelerate the
diffusion of Ag through the Ag.sub.2Se target. This diffusion can
cause natural protrusion growth on the target, which in turn
explains the origin of the porous chip defects formed on the
Ag.sub.2Se target.
[0050] The origin of the black porous chip defects is further
explained with reference to FIGS. 4(a), (b), (c), (d), (e), and
(f), which show a series of SEM images taken on silicon oxide
wafers on which Ag.sub.2Se films were sputter deposited and then in
situ annealed at varying temperatures. In this experiment, wafer
stages were brought up to various temperatures at least thirty
minutes prior to loading a wafer onto the stage. Wafers were then
loaded onto the wafer stages and annealed for 5 minutes. Each wafer
was then loaded into a cooling chamber and cooled back to room
temperature. All deposition, annealing, and cooling processes were
in situ processes, which reduces the risk of contamination
problems.
[0051] FIG. 4(a) shows a silicon wafer with a deposited silver
selenide film at room temperature. The as-deposited film appears
smooth. FIG. 4(b) shows a silicon wafer with a deposited silver
selenide film after annealing at 130.degree. C. As illustrated in
FIG. 4(b), the silver selenide film clearly shows a structure
change. FIG. 4(c) shows a silicon wafer with a deposited silver
selenide film after annealing at 250.degree. C., while FIG. 4(d)
shows a silicon wafer with a deposited silver selenide film after
annealing at 350.degree. C. The silver-selenide film of FIG. 4(d)
clearly shows the development of a cone-like defect. FIGS. 4(e) and
4(f) show silicon wafers with deposited silver selenide films after
annealing at 380.degree. C. and 410.degree. C. respectively. The
silver selenide films of FIGS. 4(e) and 4(f) show a progression in
defect size from the cone defect of FIG. 4(d) to larger
whisker-like defects. These whisker-like defects are on the
millimeter scale in size and are clearly visible on the wafer with
the unaided eye. Accordingly, comparison of the defect formation on
the annealed silver selenide films with the defect formation
observed on the Ag.sub.2Se target during sputtering lead the
present inventors to understand that controlling the target
temperature during Ag.sub.2Se deposition is desirable for
controlling defect formation in and on the target and the deposited
film.
[0052] FIG. 11 shows a simplified example of a magnetron sputtering
device 100, similar to that used in the exemplary embodiments
described below. Generally, the device 100 comprises a substrate
support table 102, target 104, target backing plate 105, and a
magnetron 108. During sputtering, a substrate 101 is placed on the
substrate support table 102 and raised to a position near the
target 104 inside the chamber 116. Using a pump 122, air in the
sputtering chamber is evacuated and replaced with an inert sputter
gas, usually at a low pressure. The sputter gas may be any suitable
sputtering gas such as argon, neon, xenon and combinations thereof.
A motor assembly 112 provides rotational motion to the magnetron
108 through a shaft 114 to rotate the magnetron 108 at about 100
rpm. The plasma discharge occurs in the space between the substrate
and the target 104, coating the substrate with sputtered target
particles. As described above, this process may cause heating of
the target, which is disadvantageous to silver selenide
deposition.
[0053] An exemplary system of the magnetron sputtering device 100
described above is the Applied Materials Endura magnetron
sputtering system using an ENI pulsed DC power supply. Accordingly,
except where indicated, silver selenide deposition according to the
exemplary embodiments of the present invention discussed below was
performed using an Applied Materials Endura magnetron sputtering
system using an ENI pulsed DC power supply with a 350 W, 200 KHz,
1056 ns pulse width. The sputtering target used in each exemplary
embodiment was a silver selenide target approximately 13 inches in
diameter and approximately 1/8 inch thick. The silver selenide
target purity was at least 99.95%. All embodiments of the present
invention are designed to maintain a silver selenide target
temperature at less than about 350.degree. C. and preferably less
than about 250.degree. C. during sputtering.
[0054] As discussed above, silver selenide undergoes a structural
change from the "beta phase" to the "alpha phase," in which the Se
forms a body-centered cubic sublattice, at temperatures above 406 K
(about 133.degree. C.). In accordance with the present invention,
it is desirable for the deposited silver selenide film to be formed
from a mixture of the alpha and beta silver selenide phases. Such
mixture is critical for the optimal performance of certain devices
formed from silver selenide films. Conventional silver selenide
sputter techniques produce deposited silver selenide films
predominantly consisting of beta silver selenide. According to the
present invention, silver selenide films containing both alpha and
beta phases may be formed via a sputtering process under conditions
of lower power/higher pressure. Furthermore, formation of alpha
phase silver selenide is enhanced by heating of the substrate.
[0055] FIG. 13 shows two x-ray diffraction (XRD) scans 300, 320 of
sputter deposited silver selenide films. Scan 1 shows an XRD scan
300 of a silver selenide film sputter deposited at a sputter power
of 350 W and an argon sputter gas flow rate of 50 sccm, which
corresponds to a sputter gas pressure of about 7 mTorr. Scan 2
shows an XRD scan 320 of a silver selenide film sputter deposited
at a sputter power of 350 W and an argon sputter gas flow rate of
75 sccm, which corresponds to a sputter gas pressure of about 10
mTorr. The scans 300, 320 show that, under the latter conditions,
alpha and beta peaks appear around 23 and 37 degrees in a glancing
mode 2.theta. XRD scan.
[0056] While not being limited by theory, based on the XRD scans
300, 320, it appears that a lower energy impact of silver selenide
with the substrate during sputtering yields more alpha phase silver
selenide in the deposited film. Lower sputter power and higher
sputter pressure decrease the kinetic energy imparted to the
sputtered silver selenide molecules, thus resulting in a lower
energy impact of silver selenide with the substrate. Furthermore,
it also appears that heating the substrate to an a temperature
above room temperature (about 25 C) also enhances the alpha phase
in the deposited film.
[0057] While the crystalline structure of silver selenide (i.e.
mixture of alpha and beta phases) is important for the optimal
performance of devices employing such films, the film texture (e.g.
smoothness), and defects in or on the film also affect device
performance. Thus, to the extent decreases in sputter power or
increases in sputter pressure result in films exhibiting a rough or
defect-laden surface, a balance in the sputter power and pressure
parameters must be struck in order to produce smooth, defect-free
silver selenide films, whose structure exhibits both alpha and beta
phases. However, it should be emphasized that, independent of the
process method used to produce the film, it is desirable to have
the alpha phase present in deposited silver selenide films.
[0058] In a sputtering process in accordance with a first exemplary
embodiment of the invention, a silver selenide film is sputter
deposited using the magnetron sputtering system described above,
but maintaining a sputter power such that the temperature of the
silver selenide target 104 is maintained at less than about
350.degree. C. and preferably less than about 250.degree. C. during
sputtering.
[0059] Generally, reducing the sputter power reduces the kinetic
energy imparted to the target during sputtering and, thus, lowers
target temperature. As shown in FIG. 5, target temperature is
directly related to sputter power such that increasing sputter
power increases target temperature. For the sputtering system 100
described above and as illustrated in FIG. 5, target temperature is
maintained at less than 350.degree. C. at sputter powers below
about 200 W. As also shown in FIG. 5, target temperature can be
further reduced to below about 250.degree. C. at sputter powers
less than about 100 W.
[0060] In accordance with a second exemplary embodiment of the
invention, a silver selenide film is deposited using the Applied
Materials Endura magnetron sputtering system described above, but
maintaining a sputter gas pressure less than about 40 mTorr and
preferably less than about 10 mTorr so as to maintain a temperature
of the silver selenide target of less than about 350.degree. C. and
preferably less than about 250.degree. C. during sputtering.
[0061] As indicated in FIG. 6, increasing the sputter gas pressure
lowers the target temperature during sputtering. This reduction in
temperature as sputter gas pressure increases is due to a reduction
in the kinetic energy of bombardment imparted to the target by the
impact of the inert gas ions during sputtering. FIG. 6 illustrates
the correlation between target temperature and sputter pressure for
the sputter process described above, using argon as the sputter
gas. According to FIG. 6, silver selenide target temperature is
reduced to less than about 350.degree. C. at a sputter pressures
greater than about 5 mTorr. Furthermore, target temperature is
reduced to less than about 250.degree. C. at sputter pressures
greater than about 15 mTorr.
[0062] Referring now to FIGS. 7-9 and in accordance with a third
exemplary embodiment of the invention, a silver selenide film is
sputter deposited using the Applied Materials Endura magnetron
sputtering system described above, but using a sputter gas with a
molecular weight at least greater than that of neon such that the
silver selenide target temperature is maintained at about
350.degree. C. and preferably less than about 250.degree. C. during
sputtering. While the preferred sputtering gas is xenon, the
present invention is not so limited. The inventors have discovered
that at least xenon, argon, and mixtures of these induce the
desired reduction in silver selenide target temperature as shown in
FIGS. 8 and 9.
[0063] Sputter gasses with higher molecular weight than that of
neon, such as argon and xenon, reduce the kinetic energy imparted
to the target during sputtering. FIG. 7 shows the correlation
between the sputter target temperature and Ne/Ar sputter gas flow
ratio at various sputter gas pressures. According to FIG. 7, at
least at the lower sputter pressures indicated, increasing the
proportion of neon in the sputter gas mix induces higher sputter
target temperatures. Furthermore, when the Ne gas in the Ne/Ar
sputter gas flow ratio of FIG. 7 is replaced with xenon gas, as
shown in FIG. 8, there is a clear decrease in target temperature at
the sputter gas pressures indicated. FIG. 9 also shows the
relationship between target temperature and sputter gas at various
sputter gas pressures. As shown in FIG. 9, using a sputter gas
containing a mixture of argon and neon results in higher target
temperatures than use of either argon or xenon alone.
[0064] FIG. 8 also shows that, as the molecular weight of the
sputter gas increases (i.e. increasing Xe/Ar ratio) target
temperature is quickly reduced to at least less than about
350.degree. C. at a sputter pressure of 2 mTorr. At the relatively
higher pressures shown in FIG. 8 (10 mTorr and 20 mTorr), the
target was cooled to less than about 250.degree. C. Sputter gas
pressures of less than about 10 mTorr are desirable because they
have been shown to prevent the formation of nodular defects in the
deposited silver selenide films with high silver concentrations
(e.g. higher than about 67.5% Ag). Furthermore, lower sputter gas
pressures allow for the deposition of silver selenide films with
greater stoichiometric precision.
[0065] In accordance with a fourth exemplary embodiment of the
invention, silver selenide target cooling is achieved by improving
the target cooling apparatus of the sputter deposition system.
Referring to FIG. 11, a typical magnetron sputtering device 100,
described above, comprises a magnetron 108, which is disposed
within a cooling chamber 116. The cooling chamber 116 is defined by
a top 117, two sides 119 and the target backing plate 105. A
cooling fluid, such as water, enters the cooling chamber 116 at an
inlet 118, circulates around the magnetron 108, and exits through
an outlet 120. The arrows A-F show the generalized water flow paths
around the magnetron 108.
[0066] The present inventors have discovered that silver selenide
target cooling to at least below 350.degree. C. and preferably less
than about 250.degree. C. can be achieved by increasing the coolant
flow between the magnetron 108 and the target backing plate 105 (as
shown by arrows C and D), decreasing coolant temperature, and
improving the thermal conductivity between the coolant and target
backing plate 105. For example, coolant flow rates are at least
above 2.5 gal/min and coolant temperature is at least below
25.degree. C. to allow selenide target cooling to at least below
350.degree. C. and preferably less than about 250.degree. C.
Furthermore, the backing plate should be constructed from materials
designed to optimize backing plate/coolant contact and the target
backing plate should be colored black in order to improve heat
radiation.
[0067] In accordance with a fifth exemplary embodiment of the
invention, silver selenide target temperature is maintained below
about 350.degree. C. and preferably less than about 250.degree. C.
during sputtering by maintaining an effective sputter power density
on the target of less than about 1 W/cm.sup.2.
[0068] The target sputter area of a sputtering apparatus will vary
according to the particular apparatus used. As described below, the
area of the target actually used during sputtering can range from
only a portion of the entire target, as shown in FIGS. 12(a) and
12(b), to the entire target, as shown in FIG. 12(c). According to
the present invention, sputter parameters, such as the sputter
power, for a particular sputter apparatus are varied such that the
effective sputter power density of the sputter apparatus is less
than about 1 W/cm.sup.2.
[0069] As described above, FIG. 5 shows a graph of Target
Temperature v. Sputter Power for the Applied Materials Endura
Sputter system described above, using argon as the sputter gas, and
clearly indicates an increase in target temperature as sputter
power is increased. However, the sputter area of the target, also
impacts target temperature. Thus, controlling effective sputter
power density (sputter power per unit of sputter area) provides
more accurate control of silver selenide target temperature. Silver
selenide target temperature as a function of effective sputter
power density is plotted in FIG. 10 and clearly shows that silver
selenide target temperature is maintained below about 350.degree.
C. and preferably less than about 250.degree. C. during sputtering
by maintaining an effective sputter power density on the target of
less than about 1 W/cm.sup.2.
[0070] FIG. 12(a) shows a cross sectional view of a non-uniform
target sputter profile, which represents the sputter area of the
target. During sputtering, areas of target erosion, S.sub.1 and
S.sub.2, are caused by the high-density plasma as it moves across
the target within the magnetic filed generated by the magnetron. As
described above, this preferentially sputtered region of the target
is often referred to as the racetrack portion, due to the
characteristic ovoid-shaped area of the target that is eroded by
the high-density plasma discharge. FIG. 12(b) illustrates a typical
racetrack area 200 formed on a target 104 during sputtering. As
shown, the total sputter area S.sub.total of the target is
determined by the formula:
S.sub.total=.pi.(R.sub.2.sup.2-R.sub.1.sup.2)
Effective sputter power density on the target can then be
determined by dividing the sputter power of the sputtering system
by S.sub.total.
[0071] FIG. 12(c) shows a cross sectional view of a uniform target
sputter profile. In this case, the sputter power is spread across
the entire target area S.sub.total. Similar to the non-uniform
target sputter profile, sputter power density on the target can be
determined by dividing the total sputter power by S.sub.total.
[0072] In accordance with a sixth exemplary embodiment of the
present invention, the magnetron in a magnetron sputtering system
is positioned such that the target temperature is maintained below
about 350.degree. C. and preferably less than about 250.degree. C.
during sputtering.
[0073] As described above, FIG. 11 shows a typical magnetron
sputtering device 100 comprising a target 104, target backing plate
105, and a magnetron 108. As the distance between the magnetron and
the sputter target 107 is increased, the distribution of the
magnetic field on the target also increases, causing the sputter
area of the target to increase. Increasing sputter area has the
effect of decreasing sputter power density, which, as described
above, lowers target temperature during sputtering.
[0074] In accordance with a seventh exemplary embodiment of the
present invention, a silver selenide film is sputter deposited
under sputter conditions such that the structure of said silver
selenide film comprises both the alpha phase and beta phases.
Desirable sputter deposited silver selenide films consist primarily
of alpha phase silver selenide. For example, using the applied
Materials Endura magnetron sputtering system described above,
silver selenide is sputter deposited using a sputter power of less
than about 250 W and a sputter pressure of at least about 10 mTorr.
Furthermore, under sputter conditions wherein the sputter power is
less than about 200 W and the sputter pressure is about 10 mTorr or
less, a defect-free silver selenide film comprising both the alpha
and beta structural phases may be formed on a substrate.
[0075] Regardless of the sputtering apparatus used, heating the
substrate to greater than about 30.degree. C. results in greater
formation of alpha phase silver selenide in the deposited film.
[0076] The present invention uses novel methods and apparatuses for
silver selenide sputter deposition that decrease defects in
deposited silver selenide films. Such methods and apparatuses of
silver selenide defect control are useful in the production of
certain non-volatile memory devices that require virtually
defect-free silver selenide film layers. Furthermore, the present
invention provides novel methods and apparatuses for silver
selenide sputter deposition that are suitable to industrial
application.
[0077] While separate exemplary embodiments of the invention have
been described and illustrated, practice of the present invention
is not limited to use of only one of these exemplary embodiments.
One or more of the embodiments of the invention can be used
separately or together to maintain a sputter target at a
temperature of less than about 350.degree. C. and preferably less
than about 250.degree. C.
[0078] Furthermore, 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.
* * * * *