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Claims  |
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What is claimed:
1. A method of forming a fluorinated silicon oxide layer on the surface of
a substrate using a plasma chemical vapor deposition apparatus which
includes a process chamber and a plasma chamber, comprising the steps of:
positioning the substrate within said process chamber;
introducing into said plasma chamber an inert gas and oxygen gas, said
inert gas being selected from at least one of the inert gases of the group
consisting of argon, neon, krypton or xenon;
introducing r.f. power into said plasma chamber such that a plasma is
created in the plasma chamber, and wherein the stability of the plasma is
enhanced by introduction of said inert gas;
introducing a silicon-containing gas into said process chamber adjacent the
surface of said substrate; and
introducing into one of said chambers a fluorine-containing gas whereby
said fluorine gas is available at the surface of said substrate, and said
silicon-containing gas and said fluorine-containing gas are excited by
said plasma and interact proximate to the surface of said substrate to
form a fluorinated silicon oxide layer on the surface of said substrate.
2. The method of claim 1 further comprising the step of simultaneously
applying an r.f. bias to said substrate during the formation of said
fluorinated silicon oxide layer such that the surface of said substrate is
sputtered by ions of said inert gas and etched by ions of said
fluorine-containing gas to enhance the filling of gaps on the surface of
said substrate.
3. The method of claim 1 wherein the flow rate of said oxygen gas is
approximately twice the flow rate of each of said silicon and
fluorine-containing gases.
4. The method of claim 1 wherein said plasma comprises an ion density of
greater than 10.sup.11 ions/cm.sup.3.
5. The method of claim 1 wherein said fluorine-containing gas is a
fluorocarbon represented by the general formula C.sub.n F.sub.2+2n,
wherein n is an integer of 1 to 3.
6. The method of claim 5 wherein said fluorocarbon is tetrafluoromethane
(CF.sub.4).
7. The method of claim 1 wherein said silicon-containing gas is silane.
8. The method of claim 1 wherein said fluorine-containing gas is silicon
tetrafluorine (SiF.sub.4).
9. The method of claim 1 wherein said substrate is maintained at a
temperature of not more than 450.degree. C.; and said process chamber is
maintained at a pressure of not more than 20 mTorr.
10. A method of forming a fluorinated silicon oxide layer on the surface of
a substrate using a plasma chemical vapor deposition apparatus which
includes a process chamber and a plasma chamber, comprising the steps of:
positioning the substrate within said process chamber;
introducing into said plasma chamber an inert gas and oxygen gas, said
inert gas being selected from at least one of the inert gases of the group
consisting of argon, neon, krypton and xenon;
introducing r.f. power into said plasma chamber such that a plasma is
created in the plasma chamber, said plasma containing an ion density of
greater than 10.sup.11 ions/cm.sup.3 and wherein the stability of the
plasma is enhanced by introduction of said inert gas;
maintaining said substrate at a temperature of not more than 450.degree.
C.;
maintaining said process and plasma chambers at a pressure of not more than
20 mTorr;
introducing a silane gas into said process chamber adjacent the surface of
said substrate; and
introducing into one of said chambers a fluorine-containing gas, whereby
said fluorocarbon gas is available at the surface of said substrate, and
said silane gas and said fluorine-containing gas are excited by said
plasma and interact proximate to the surface of said substrate to form a
fluorinated silicon oxide layer on the surface of said substrate.
11. The method of claim 10 wherein the flow rate of said oxygen gas is
approximately twice the flow rate of each of said silicon and
fluorine-containing gases.
12. The method of claim 10 further comprising the step of simultaneously
applying an r.f. bias to said substrate during the formation of said
fluorinated silicon oxide layer such that the surface of said substrate is
sputtered by ions of said inert gas etched by and ions of said
fluorine-containing gas to enhance the filling of gaps on the surface of
said substrate.
13. The method of claim 10 wherein said fluorine-containing gas is
tetrafluoromethane (CF.sub.4).
14. The method of claim 10 wherein said fluorine-containing gas is a
fluorocarbon represented by the general formula C.sub.n F.sub.2+2n,
wherein n is an integer of 1 to 3.
15. The method of claim 10 wherein said fluorine-containing gas is silicon
tetrafluorine (SiF.sub.4).
16. A method of forming a fluorinated silicon oxide layer on the surface of
a substrate using a plasma chemical vapor deposition apparatus which
includes a process chamber and a plasma chamber, comprising the steps of:
positioning the substrate within said process chamber;
introducing into said plasma chamber an inert gas and oxygen gas;
introducing r.f. power into said plasma chamber such that a plasma is
created in the plasma chamber, said plasma containing an ion density of
greater than 10.sup.11 ions/cm.sup.3 and wherein the stability of the
plasma is enhanced by introduction of said inert gas;
maintaining said substrate at a temperature of not more than 450.degree.
C.; maintaining said process and plasma chambers at a pressure of not more
than 20 Mtorr;
introducing a silane gas into said process chamber adjacent the surface of
said substrate; and
introducing silicon tetrafluorine gas into one of said chambers, whereby
said silicon tetrafluorine gas is available at the surface of said
substrate and said silane gas and said silicon tetrafluorine gas are
excited by said plasma and interact proximate to the surface of said
substrate to form a fluorinated silicon oxide layer on the surface of said
substrate.
17. The method of claim 16 further comprising the step of simultaneously
applying an r.f. bias to said substrate during the formation of said
fluorinated silicon oxide layer such that the surface of said substrate is
sputtered by ions of said inert gas and ions of said fluorine-containing
gas to enhance the filling of gaps on the surface of said substrate.
18. A method of forming a fluorinated silicon oxide dielectric layer on the
surface of a substrate using a plasma chemical vapor deposition apparatus
which includes a process chamber and a plasma chamber, comprising the
steps of:
positioning the substrate within said process chamber;
introducing into said plasma chamber argon and oxygen gas, said inert gas
being selected from at least one of the inert gases of the group
consisting of argon, neon, krypton and xenon;
introducing r.f. power into said plasma chamber such that a plasma is
created in the plasma chamber, said plasma containing an ion density of
greater than 10.sup.11 ions/cm.sup.3 and wherein the stability of the
plasma is enhanced by introduction of said inert gas;
maintaining said substrate at a temperature of not more than 450.degree.
C.;
maintaining said process and plasma chambers at a pressure of not more than
20 Mtorr;
introducing a silane gas into said process chamber adjacent the surface of
said substrate; and
introducing tetrafluoromethane gas into one of said chambers, whereby said
tetrafluoromethane gas is available at the surface of said substrate, and
said silane gas and said tetrafluoromethane gas are excited by said plasma
and interact proximate to the surface of said substrate to form a
fluorinated silicon oxide layer on the surface of said substrate.
19. The method of claim 18 further comprising the step of simultaneously
applying an r.f. bias to said substrate during the formation of said
fluorinated silicon oxide layer such that the surface of said substrate is
sputtered by ions of said argon gas and etched by ions of said
fluorine-containing gas to enhance the filling of gaps on the surface of
said substrate. |
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Claims |
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Description |
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BRIEF DESCRIPTION OF THE INVENTION
This invention relates generally to the formation of a dielectric layer on
the surface of a substrate or wafer, and more particularly to a method for
depositing a fluorinated silicon oxide dielectric layer on the surface of
a substrate or wafer.
BACKGROUND OF THE INVENTION
A dielectric layer is an important component in the manufacture of
integrated circuits. A dielectric is used generally to electrically
isolate conductive layers and enable useful interconnects between such
layers. As device densities increase, multiple dielectric layers may be
used to isolate stacked device features. When forming such multilayer
dielectric it is desirable to provide a dielectric film with good gap
fill, isolation, stress and step coverage properties on patterned material
layers. These properties become critical as device dimensions shrink.
Dielectric layers are often formed by Chemical Vapor Deposition (CVD). The
CVD process deposits a material on a surface by transport and reaction of
certain gaseous precursors on such surface. Plasma may be used to assist
decomposition of certain gaseous chemicals. CVD apparatus come in many
forms. Low pressure CVD systems and atmospheric pressure CVD systems
operate on thermal CVD principles. Plasma CVD systems operate by
disassociation and ionization of gaseous chemicals and are able to operate
at lower temperatures than conventional thermal CVD systems. Such lower
temperature methods are desirable and will minimize diffusion of shallow
junctions and inter-diffusion of metals.
In addition to good gap fill and step overage properties it is desirable to
provide a dielectric layer with a low dielectric constant. As the
intra-layer metal width and spacing of the interconnections decrease, the
wiring (or sometimes referred to as line-to-line) capacitance increases
and becomes the major factor contributing to the total capacitance.
Another factor contributing to the total capacitance, but to a lesser
extent, is the inter-layer capacitance. The total capacitance, limits the
operating speed of such devices. A layer with a low dielectric constant
provides an immediate performance improvement due to a reduction in
capacitance.
It is desirable to develop a suitable method of forming a dielectric which
provides low dielectric constants, having all of the necessary film
properties such as stability, density, gap fill, low film stress and step
coverage.
OBJECTS AND SUMMARY OF THE INVENTION
It is an object of this invention to provide an improved method for
formation of dielectric layers.
More particularly, it is an object of this invention to provide an improved
method for formation of a fluorinated silicon oxide dielectric layer.
A further object of this invention is to provide a method for formation of
a fluorinated silicon oxide layer with an improved dielectric constant.
A still further object of the invention is to provide a method for
formation of a fluorinated silicon oxide dielectric layer which has
desirable gap fill on patterned materials, low film stress and step
coverage properties.
These and other objects are achieved by the method herein disclosed
comprising the steps of forming a fluorinated silicon oxide dielectric
layer on a substrate using a plasma chemical vapor deposition apparatus
which includes a process chamber and a plasma chamber, wherein a substrate
is positioned within the process chamber. An inert gas such as argon, and
oxygen gas are introduced into the plasma chamber. Radio frequency (r.f.)
power is introduced into the plasma chamber such that a plasma is created
in the plasma chamber. A silicon-containing gas is introduced into the
process chamber adjacent to the surface of the substrate. A
fluorine-containing gas is introduced into either the plasma chamber or
the process chamber such that the fluorine gas is available at the surface
of the substrate. The silicon-containing gas and the fluorine-containing
gas are excited by the plasma and interact to form a layer of fluorinated
silicon oxide on the surface of the substrate.
BRIEF DESCRIPTION OF THE DRAWINGS
Other objects and advantages of the invention become apparent upon reading
of the detailed description of the invention provided below and upon
reference to the drawings in which:
FIG. 1 is a cross-sectional view of an apparatus used for formation of a
fluorinated silicon oxide layer (SiOF) according to one embodiment of the
invention.
FIG. 2 is a cross-sectional view of an apparatus used for formation of a
fluorinated silicon oxide layer according to an alternative embodiment of
the invention.
FIG. 3 is a graph depicting a Fourier transform infrared absorption (FTIR)
spectrum of a SiOF layer formed according to the invention.
FIG. 4 is a graph comparing the compressive stress of the SiOF film
deposited according to the method of this invention and the flow rate of
the fluorine-containing gas.
FIGS. 5a and 5b are photographs made by Scanning Electron Microscope (SEM)
of a cross-section of a fluorinated silicon oxide layer showing the gap
fill and step coverage achieved according to the method of the invention.
DESCRIPTION OF THE PREFERRED EMBODIMENT
Turning to the drawings, wherein like components are designated by like
reference numerals, FIGS. 1 and 2 represent apparatus that can be used to
form a fluorinated silicon oxide dielectric layer in accordance with the
method of this invention. The apparatus includes a plasma chamber 10 and a
process chamber 11. The chambers 11 and 10 are evacuated through port 32
to a vacuum in the range of 0 to 20 mTorr. Coils 12 are arranged around
the plasma chamber 10 to excite gases into a plasma state. Various
arrangements of coils known in the art may be used to generate the plasma.
A substrate 16 is placed on a support 17 (sometimes referred to as a
chuck) positioned within process chamber 11, such that a surface of the
substrate is facing upwards. The support 17 may be biased by application
of r.f. bias power through r.f. generator 19 and transmission line 18. A
mechanical support or an electrostatic support, known in the art may be
used.
In FIG. 1, the preferred embodiment of the invention, a first gas stream is
introduced into the plasma chamber 10 proximate to coils 12 through gas
inlet line 22. Preferably, the first gas stream includes a mixture of
oxygen and argon gases which are introduced to inlet line 22. The desired
flow rates of the oxygen and argon are controlled by mass flow controllers
25 and 26, respectively. The invention may also be carried out with inert
gases other than argon. The inert gas provides a sputter etch component
for simultaneous etching of the fluorinated silicon oxide layer during its
deposition when the support 17 is biased. Other inert gases that may be
used are, any one of: argon, neon, xenon and krypton, or any mixture
thereof. Helium may also be used in combination with any one of argon,
neon, xenon and krypton, or in combination with any mixture of argon,
neon, xenon and krypton.
To generate a plasma, r.f. energy 13 is directed into plasma chamber 10
through coils 12 arranged around plasma chamber 10. It is preferred that
the frequency of the r.f. energy be 13.56 MHz, which is a commercial
standard frequency. In such a configuration, a plasma is generated in the
chamber 10 whereby a large percentage of the gaseous molecules introduced
by said first gas stream are dissociated to form reactive species,
including ionized atoms. An ion density of greater than 10.sup.11
ions/cm.sup.3 is achieved, and is referred to as a high density plasma.
The plasma contains electrons with very high energy compared to the other
species present. The high electron energy increases the density of
disassociated reactant species available for deposition.
A second gas stream which includes a silicon-containing gas and a
fluorine-containing gas is introduced into the process chamber 11 through
gas inlet line 23. The silicon-containing gas and the fluorine-containing
gas are introduced at a desired flow rate by way of mass flow controllers
28 and 27. Preferably the gases are SiH.sub.4 and CF.sub.4, respectively.
The gases mix in gas inlet line 23 as they enter the process chamber 11. A
gas distribution ring 24 is placed inside the process chamber 11 adjacent
to substrate 16 to receive and disperse the second gas stream. The gas
distribution ring 24 contains a plurality of distribution holes 29 which
are placed equally around said ring 24. The second gas is distributed
substantially uniformly adjacent the surface of the substrate 16 through
the plurality of distribution holes 29. As the silicon- and
fluorine-containing gases exit the gas ring 24, they are disassociated and
activated by the plasma which has entered the process chamber 11 from the
plasma chamber 10. In this disassociated and activated state, the silicon
and fluorine gaseous chemicals react to form a layer of fluorinated
silicon oxide on the surface of the substrate 16. The plasma has excited
the silicon and fluorine gases, and this allows the CVD reaction to occur
at lower temperatures than conventional thermal CVD processes. In the
method of the invention, the temperature of the substrate is in the range
of substantially 100.degree. C. to 400.degree. C. This low temperature
range is desirable because it is well below the melting point of any
metallic interconnects or components, and is below deformation modes of
the materials used and thus prevents defects such as stress induced voids,
expansion mismatch and hillock formation.
The silicon source and the fluorine source gases are introduced into the
chambers 11 and 12 at approximately the same flow rate. The flow rate of
the inert gas may vary from approximately the same as the silicon and
fluorine gases, up to at least approximately twice that of the silicon and
fluorine gases. To produce good quality fluorinated silicon oxide, oxygen
gas is introduced at a flow rate of least approximately twice that of the
silicon and fluorine gases. The actual flow rates of the gases are
dependent upon the vacuum system, the gas ring design and other equipment
configurations, however the flow rate ratio will continue to apply.
In an alternative embodiment of the invention, depicted in FIG. 2, the
fluorine-containing gas is introduced into plasma chamber 10 as a
constituent of the first gas stream via gas inlet line 22. Thus, in this
embodiment, the first gas stream contains a mixture of oxygen, argon and a
fluorine-containing gas. The desired flow rates of the oxygen, argon and
fluorine gas are controlled by mass flow controllers 25, 26 and 27,
respectively. A second gas stream comprises a silicon-containing gas which
is conveyed into the process chamber 11 via gas inlet line 23. The
silicon-containing gas is conveyed to gas distribution ring 24 and is
distributed substantially uniformly adjacent the surface of said substrate
16 through a plurality of distribution holes 29. In this embodiment, the
silicon and fluorine source gases do not mix inside the gas inlet line 23
or the distribution ring 24. Instead, the fluorine source is contained in
the plasma, with a portion of the fluorine source in a disassociated and
ionized state, and enters the process chamber 11 from the plasma chamber
10 whereby it interacts with the silicon gas as the silicon gas exits the
distribution holes 29 proximate the substrate 16 to form a layer of
fluorinated silicon oxide on the surface of the substrate 16.
In the preferred embodiment, the fluorine-containing gas will be comprised
of a fluorocarbon represented by the general formula C.sub.n F.sub.2+2n,
where n is an integer of 1 to 3, and in particular the fluorine source
will be tetrafluoromethane (chemical formula: CF.sub.4). Preferably, the
silicon-containing gas will be silane (chemical formula: SiH.sub.4), and
the inert gas will be argon (chemical formula Ar). In this embodiment of
the invention, the chemical reaction is represented by:
##STR1##
The invention can also be carried out with an alternate chemistry where the
fluorine-containing gas is comprised of silicon tetrafluorine (chemical
formula: SiF.sub.4); represented by the chemical reaction:
##STR2##
As discussed above, it is desirable to reduce the dielectric constant of
the layer deposited on the substrate 16. It has been found that a low
dielectric constant will be a function of the fluorine concentration in
the layer. The fluorine concentration of the layers formed according to
the inventive method were determined by Rutherford Back-scattering
Spectroscopy (RBS). Two different layers were tested. Both layers were
deposited under the following process conditions: SiH.sub.4, CF.sub.4 and
Ar were each introduced at a flow rate of 40 sccm and O.sub.2 was
introduced at 80 sccm. The pressure in the chambers 11 and 12 was in the
range of 4-5 mTorr. An r.f. power of 5 kW was applied to the coils 12 and
the support 17 was not biased. One layer had a deposited SiOF thickness of
7600 angstroms with the resulting chemical concentration: 11.9 atomic
percent fluorine, 38.8 atomic percent silicon and 49.3 atomic percent
oxygen. The other layer had a SiOF deposited thickness of 950 angstroms
with a chemical concentration of: 10.2 atomic percent fluorine, 41.4
atomic percent silicon and 48.4 atomic percent oxygen. While the thickness
of the two SiOF layers differ significantly, the fluorine concentration in
the layers is fairly consistent at above 10 atomic percent fluorine. The
RBS analysis showed no detectable carbon contamination. Carbon
contamination in the two layers was also tested by X-ray Photoelectron
Spectroscopy (XPS) and by Secondary Ion Mass Spectroscopy (SIM) analysis.
In both tests the carbon contamination was less than 0.02 percent. Another
important quality in a dielectric layer is resistance to moisture
absorption. FIG. 3 illustrates the fourier transform infrared absorption
(FTIR) spectrum for a SiOF layer exposed to air for two days after it was
formed in accordance with the method of the invention. The layer was
deposited with SiH.sub.4, CF.sub.4 and Ar each at a flow rate of 40 sccm,
and O.sub.2 at a flow rate of 80 sccm. The chambers 10 and 11 were
evacuated to a pressure of 5 mTorr, and the support 17 was biased with an
r.f. power of 600 watts. The absorption peaks depicted in FIG. 3
correspond to Si-O and Si-F, at wave numbers of approximately 1077/cm and
930/cm, respectively. The absorption peak intensity corresponding to the
water or the hydroxyl radical which would occur between 3300-3600/cm is
not detected, indicating that the water content is near zero.
It is important for a film to exhibit low film stress. FIG. 4 is a graph
illustrating the compressive stress of the fluorinated silicon oxide film
deposited at various fluorine gas flow rates in accordance with the
inventive method. The CF.sub.4 gas flow rate was tested in the range of 30
to 50 sccm. The SiH.sub.4 , Ar and O.sub.2 flow rates were held constant
at 70 sccm, 100 sccm and 140 sccm, respectively. As shown in FIG. 4, the
compressive film stress results were in the range of approximately 100 to
50 MPa, with the film stress decreasing as the CF.sub.4 flow rate
increased.
It is desirable for a layer to provide good gap fill and step coverage on
patterned substrates. To further increase such desirable film properties
etching of the substrate during deposition may be employed. Referring
again to FIG. 1, the method of the invention provides for etching the
substrate whereby an r.f. bias is applied to the support 17 through r.f.
generator 19 and transmission line 18. The r.f. bias is applied creating a
negative dc bias voltage on the support 17. The negative dc bias will
accelerate ions towards the surface of substrate 16.
In the embodiment of the invention when the support 17 is biased, there is
a combination of two etchants applied to the surface of the substrate 16.
Free active fluorine ions will produce reactive ion etching at the surface
of the substrate 16, while the argon ions (or other inert gas ions) will
sputter etch the surface of the substrate 16. According to the method of
this invention, the etching occurs simultaneously with the deposition of
the fluorinated silicon oxide layer. FIGS. 5a and 5b illustrate excellent
gap fill and step coverage of a layer produced with biasing of the chuck
17 in accordance with the present inventive method. Such excellent
properties are achieved without resort to separate iterative deposition
and etch steps. In addition, sputtering with argon ions is found to
enhance the density of the deposited film.
EXAMPLE: In an example, a fluorinated silicon oxide layer was deposited in
the apparatus of FIG. 1 operated pursuant to the Table set forth below.
The temperature of the substrate was maintained below 400.degree. C.
TABLE
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Flow Rates: SiH.sub.4 40 sccm
CF.sub.4 40 sccm
O.sub.2 80 sccm
Ar 40 sccm
r.f. Frequency 13.56 MHz
r.f. Bias Power 700 Watts
Pressure 4-10 MTorr
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The method was performed in a single step whereby deposition and etching
occur simultaneously. A deposition rate of 1300 angstroms/min was
observed.
With application of the r.f. bias, the substrate temperature increases. It
is important to keep the substrate temperature below the melting point of
aluminum. To control the temperature a cooling medium is circulated
through the support 17. It has been found that keeping the wafer chuck
cool is important to stabilize the fluorine concentration in the deposited
film. If the chuck is too hot, substantially above 400.degree. C., the
resulting film contains low fluorine concentration.
Film characteristics were tested for the SiOF layer deposited in the
Example. The dielectric constant was 3.5, significantly below that of a
conventional silicon dioxide film, which is reported in the range of 4.0
to 4.3 This low constant indicates that tightly bound Si-F networks with
less residual OH radicals are present in the film. Water optical
absorption was below the IR detection limit. After the wafer was exposed
to air for ten days the moisture content was 0.575% according to an MEA
(moisture evolution analysis) measurement. The refractive index is 1.43 to
1.44. No significant change of the refractive index was observed after
annealing of the sample at 900 C. for one half hour in a nitrogen
environment, indicating film stability. Impurity levels were below XPS
detection limit, and compressive film stress was below 100 MPa. As the
results just stated show, the inventive method disclosed herein has
produced a film which possesses desirable film properties along with a
dielectric constant significantly below a conventional silicon oxide film.
The excellent step coverage and gap fill achieved can be appreciated by
reference to FIGS. 5a and 5b, a SEM photograph of portion the cross
section of an eight inch wafer with a fluorinated silicon oxide layer 33.
In FIG. 5a the wafer contains aluminum lines 31 and 32 formed on the
surface of substrate 30. The lines 31 and 32 were spaced apart at 0.35
microns. The aspect ratio of the gap between lines 31 and 32 was 1.5 to
1.0. An SiOF layer 33 was deposited atop the lines 31 and 32 and the
substrate 30. The layer 33 was deposited under the following flow rate
conditions: SiH.sub.4 at 26 sccm, CH.sub.4 at 20 sccm, O.sub.2 at 100 sccm
and Ar at 100 sccm. The support (not shown) was biased with an r.f. bias
power of 700 watts, and chamber pressure was at about 5 mTorr. The layer
was deposited in a single step. As shown be FIG. 5a, the SiOF layer has
uniformly filled the 0.35 micron gap without any voids, hillocks or other
defects.
FIG. 5b illustrates a SEM photograph of the cross section of a different
area of the same wafer and SiOF layer 33 depicted in FIG. 5a. This portion
of the wafer contains aluminum lines 35, 36, 37 and 38 formed on the
surface of substrate 30. The lines 35, 36, 37 and 38 were spaced at 0.6
microns, and the aspect ratio of such lines was 1 to 1. Again, excellent
step coverage and gap fill is achieved without the formation of voids,
hillocks and other defects. The chemistry and method disclosed in this
invention can successfully fill 0.35 micron gaps, and has achieved such
excellent gap fill at both the center and on the edge of an eight inch
substrate.
While the invention has been described in connection with specific
embodiments it is evident that many variations, substitutions,
alternatives and modifications will be apparent to those skilled in the
art in light of the foregoing description. Accordingly, this description
is intended to encompass all such variations, substitutions, alternatives
and modifications as fall within the spirit of the appended claims.
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