U.S. patent application number 10/215531 was filed with the patent office on 2003-04-24 for simplified method to produce nanoporous silicon-based films.
Invention is credited to Brungardt, Lisa, Drage, James S., Ramos, Teresa A., Smith, Douglas M., Wu, Hui-Jung.
Application Number | 20030077918 10/215531 |
Document ID | / |
Family ID | 24262260 |
Filed Date | 2003-04-24 |
United States Patent
Application |
20030077918 |
Kind Code |
A1 |
Wu, Hui-Jung ; et
al. |
April 24, 2003 |
Simplified method to produce nanoporous silicon-based films
Abstract
An improved nanoporous dielectric film useful for the production
of semiconductor devices, integrated circuits and the like, is
provided, together with novel processes for producing these
improved films. The improved films are produced by a process that
includes (a) preparing a silicon-based, precursor composition
including a porogen, (b) coating a substrate with the silicon-based
precursor to form a film, (c) aging or condensing the film in the
presence of water, (d) heating the gelled film at a temperature and
for a duration effective to remove substantially all of said
porogen, and wherein the applied precursor composition is
substantially aged or condensed in the presence of water in liquid
or vapor form, without the application of external heat or exposure
to external catalyst.
Inventors: |
Wu, Hui-Jung; (Fremont,
CA) ; Drage, James S.; (Fremont, CA) ;
Brungardt, Lisa; (Albuquerque, NM) ; Ramos, Teresa
A.; (US) ; Smith, Douglas M.; (Albuquerque,
NM) |
Correspondence
Address: |
ROBERTS & MERCANTI, LLP
P.O. BOX 484
PRINCETON
NJ
08542-0484
US
|
Family ID: |
24262260 |
Appl. No.: |
10/215531 |
Filed: |
August 9, 2002 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
|
|
10215531 |
Aug 9, 2002 |
|
|
|
09566287 |
May 5, 2000 |
|
|
|
Current U.S.
Class: |
438/781 ;
257/E21.273 |
Current CPC
Class: |
H01L 21/02282 20130101;
Y10S 438/96 20130101; H01L 21/02126 20130101; H01L 21/02203
20130101; H01L 21/31695 20130101; H01L 21/02216 20130101 |
Class at
Publication: |
438/781 |
International
Class: |
H01L 021/31 |
Claims
What is claimed is:
1. A method of producing a nanoporous silica dielectric film by a
process comprising (a) preparing a silicon-based, precursor
composition comprising a porogen, (b) coating a substrate with the
silicon-based precursor to form a film, (c) aging or condensing the
film in the presence of water, (d) heating the gelled film at a
temperature and for a duration effective to remove substantially
all of said porogen, and wherein said precursor composition is
substantially aged or condensed in the presence of water in liquid
or vapor form, without the application of heat or exposure to
external catalyst.
2. The method of claim 1 wherein the silicon-based precursor
composition comprises water in a molar ratio of water to Si ranging
from about 2:1 to about 0:1.
3. The method of claim 1 wherein the silicon-based precursor
composition comprises a monomer or prepolymer of Formula I:
Rx--Si--Ly (Formula I) wherein x is an integer ranging from 0 to
about 2, and y is an integer ranging from about 2 to about 4; R is
independently selected from the group consisting of alkyl, aryl,
hydrogen and combinations thereof; L is an electronegative moiety,
independently selected from the group consisting of alkoxy,
carboxy, amino, amido, halide, isocyanato and combinations
thereof.
4. The method of claim 3 wherein the silicon-based precursor
composition further comprises a polymer formed by condensing a
monomer or prepolymer according to Formula I, wherein the number
average molecular weight of said polymer ranges from about 150 to
about 10,000 amu.
5. The method of claim 3 wherein the silicon-based precursor
composition comprises a monomer or precursor that is selected from
the group consisting of an acetoxysilane, an ethoxysilane, a
methoxysilane, and combinations thereof.
6. The method of claim 5 wherein the silicon-based precursor
composition comprises a monomer or precursor that is selected from
the group consisting of tetraacetoxysilane, a C.sub.1 to about
C.sub.6 alkyl or aryl-triacetoxysilane, and combinations
thereof.
7. The method of claim 6 wherein said triacetoxysilane is
methyltriacetoxysilane.
8. The method of claim 3 wherein the silicon-based precursor
composition comprises a monomer or precursor that is selected from
the group consisting of tetrakis(2,2,2-trifluoroethoxy)silane,
tetrakis(trifluoroacetoxy)silane, tetraisocyanatosilane,
tris(2,2,2-trifluoroethoxy)methylsilane,
tris(trifluoroacetoxy)methylsila- ne, methyltriisocyanatosilane and
combinations thereof.
9. The method of claim 1 wherein at least a portion of the water of
step (c) is absorbed from atmospheric water vapor.
10. The method of claim 1 wherein all of the water of step (c) is
absorbed from atmospheric water vapor.
11. The method of claim 9 wherein the atmospheric partial pressure
of water vapor ranges from about 5 mm Hg to about 20 mm Hg,
12. The method of claim 9 wherein the film is exposed to
atmospheric water vapor for a time period effective for aging the
applied film.
13. The method of claim 12 wherein the film is exposed to
atmospheric water vapor for a time period ranging from about 20
seconds to about 5 minutes.
14. The process of claim 1 further comprising a curing step
conducted at a temperature and for a duration sufficient to render
the thickness and density of the produced film stable for use in a
semiconductor device.
15. The process of claim 1 wherein the porogen has a boiling point,
sublimation point or decomposition temperature ranging from about
175.degree. C. to about 450.degree. C.
16. The process of claim 1 wherein heating step (d) comprises
heating the film at a temperature ranging from about 175.degree. C.
to about 300.degree. C., for a time period ranging from about 30
seconds to about 5 minutes, to remove substantially all
porogen.
17. The process of claim 1 wherein the porogen is selected to
covalently bond to a silicon component of the precursor
composition, and remains covalently bonded thereto, until the
heating of step (d).
18. The process of claim 1 wherein the porogen has a molecular
weight ranging from about 100 to about 10,000 amu,
19. The process of claim 18 wherein the porogen has a molecular
weight ranging from about 100 to about 3,000 amu,
20. The process of claim 1 wherein the porogen comprises a reagent
comprising at least one reactive hydroxyl or amino functional
group, and said reagent is selected from the group consisting of an
organic compound, an organic polymer, an inorganic polymer and
combinations thereof.
21. The process of claim 1 wherein the porogen is a compound
selected from the group consisting of 1-adamantanol, 2-adamantanol,
1-adamantanamine, 4-(1-adamantyl)phenol,
4,4-(1,3-adamantanediyl)diphenol, a-D-cellobiose octaacetate, and
cholesterol.
22. The process of claim 1 wherein the porogen is selected from the
group consisting of a polyalkylene oxide, a monoether of a
polyalkylene oxide, an aliphatic polyester, an acrylic polymer, an
acetal polymer, a poly(caprolatactone), a poly(valeractone), a
poly(methyl methacrylate), a poly (vinylbutyral) and combinations
thereof.
23. The process of claim 22 wherein the polyalkylene oxide
monoether comprises a C.sub.1 to about C.sub.6 alkyl chain between
oxygen atoms and a C1 to about C6 alkyl ether moiety, and wherein
the alkyl chain is substituted or unsubstituted.
24. The process of claim 23 wherein the polyalkylene oxide
monoether is a polyethylene glycol monomethyl ether or
polypropylene glycol monobutyl ether.
25. The process of claim 1 wherein the porogen is present in the
composition in a ratio ranging from about 2 to about 20 weight
percent.
26. The process of claim 1 wherein the silicon-based, precursor
composition further comprises a solvent.
27. The process of claim 26 wherein the silicon-based, precursor
composition comprises solvent in an amount ranging from about 10%
to about 90% by weight.
28. The process of claim 26 wherein the solvent has a boiling point
ranging from about 50 to about 175.degree. C.
29. The process of claim 26 wherein the solvent is selected from
the group consisting of hydrocarbons, esters, ethers, ketones,
alcohols, amides and combinations thereof.
30. The process of claim 26 wherein the solvent is not an alcohol
when the silicon based monomer or precursor comprises an
acetoxy-functional group.
31. The process of claim 26 wherein the solvent does not comprise
hydroxyl or amino groups.
32. The process of claim 26 wherein the solvent is selected from
the group consisting of di-n-butyl ether, anisole, acetone,
3-pentanone, 2-heptanone, ethyl acetate, n-propyl acetate, n-butyl
acetate, 2-propanol, dimethyl acetamide, propylene glycol methyl
ether acetate, and/or combinations thereof.
33. A nanoporous dielectric film produced on a substrate by the
process of claim 1.
34. A semiconductor device comprising a nanoporous dielectric film
of claim 33.
35. The semiconductor device of claim 34 that is an integrated
circuit.
Description
FIELD OF THE INVENTION
[0001] The present invention relates to novel nanoporous silica
dielectric films having improved mechanical strength, and to
semiconductor devices comprising these improved films. The present
invention also provides improved processes for producing the same
on substrates suitable for use in the production of semiconductor
devices, such as integrated circuits. The nanoporous films of the
invention are prepared using silicon-based starting materials and
polymers, copolymers, oligomers, and/or compounds, and are prepared
by a simplified process that, in one embodiment, allows for aging
or gelation without heating.
BACKGROUND OF THE INVENTION
[0002] As feature sizes in integrated circuits approach 0.25 .mu.m
and below, problems with interconnect RC delay, power consumption
and signal cross-talk have become increasingly difficult to
resolve. It is believed that the integration of low dielectric
constant materials for interlevel dielectric (ILD) and intermetal
dielectric (IMD) applications will help to solve these problems.
While there have been previous efforts to apply low dielectric
constant materials to integrated circuits, there remains a
longstanding need in the art for further improvements in processing
methods and in the optimization of both the dielectric and
mechanical properties of such materials used in the manufacture of
integrated circuits.
[0003] Nanoporous Films
[0004] One material with a low dielectric constant are nanoporous
films prepared from silica, i.e., silicon-based materials. Air has
a dielectric constant of 1, and when air is introduced into a
suitable silica material having a nanometer-scale pore structure,
such films can be prepared with relatively low dielectric constants
("k"). Nanoporous silica materials are attractive because similar
precursors, including organic-substituted silanes, e.g.,
tetraethoxysilane ("TEOS"), are used for the currently employed
spin-on-glasses ("S.O.G.") and chemical vapor deposition ("CVD") of
silica SiO.sub.2. Nanoporous silica materials are also attractive
because it is possible to control the pore size, and hence the
density, mechanical strength and dielectric constant of the
resulting film material. In addition to a low k, nanoporous films
offer other advantages including: 1) thermal stability to
900.degree. C., 2) substantially small pore size, i.e., at least an
order of magnitude smaller in scale than the microelectronic
features of the integrated circuit, 3) as noted above, preparation
from materials such as silica and TEOS that are widely used in
semiconductors, 4) the ability to "tune" the dielectric constant of
nanoporous silica over a wide range, and 5) deposition of a
nanoporous film can be achieved using tools similar to those
employed for conventional S.O.G. processing.
[0005] Thus, high porosity in silica materials leads to a lower
dielectric constant than would otherwise be available from the same
materials in nonporous form. An additional advantage, is that
additional compositions and processes may be employed to produce
nanoporous films while varying the relative density of the
material. Other materials requirements include the need to have all
pores substantially smaller than circuit feature sizes, the need to
manage the strength decrease associated with porosity, and the role
of surface chemistry on dielectric constant and environmental
stability.
[0006] Density (or the inverse, porosity) is the key parameter of
nanoporous films that controls the dielectric constant of the
material, and this property is readily varied over a continuous
spectrum from the extremes of an air gap at a porosity of 100% to a
dense silica with a porosity of 0%. As density increases,
dielectric constant and mechanical strength increase but the degree
of porosity decreases, and vice versa. This suggests that the
density range of nanoporous films must be optimally balanced
between the desired range of low dielectric constant and the
mechanical properties acceptable for the desired application.
[0007] Nanoporous silica films have previously been fabricated by a
number of methods. For example, nanoporous films have been prepared
using a mixture of a solvent and a silica precursor, which is
deposited on a substrate suitable for the purpose. Broadly, a
precursor in the form of, e.g., a spin-on-glass composition is
applied to a substrate, and then polymerized in such a way as to
form a dielectric film comprising nanometer-scale voids.
[0008] When forming such nanoporous films, e.g., by spin-coating,
the film coating is typically catalyzed with an acid or base
catalyst and water to cause polymerization/gelation ("aging")
during an initial heating step.
[0009] More recently, U.S. Pat. No. 5,895,263 describes forming a
nanoporous silica dielectric film on a substrate, e.g., a wafer, by
applying a composition comprising decomposable polymer and organic
polysilica i.e., including condensed or polymerized silicon
polymer, heating the composition to further condense the
polysilica, and decomposing the decomposable polymer to form a
porous dielectric layer. This process, like many of the previously
employed methods of forming nanoporous films on semiconductors, has
the disadvantage of requiring heating for both the aging or
condensing process, and for the removal of a polymer to form the
nanoporous film. Furthermore, there is a disadvantage that organic
polysilica, contained in a precursor solution, tends to increase in
molecular weight after the solution is prepared; consequently, the
viscosity of such precursor solutions increases during storage, and
the thickness of films made from stored solutions will increase as
the age of the solution increases. The instability of organic
polysilica thus requires short shelf life, cold storage, and fine
tuning of the coating parameters to achieve consistent film
properties in a microelectronics/integrated circuit manufacturing
process.
[0010] As mentioned supra, there is a continuing need in the
microelectronics industry to provide improved materials allowing
for semiconductor devices, such as integrated circuits, with
increased circuit density, and increased processing speed and
power. This is coupled with a continuing desire to reduce the cost
in time, money and manufacturing equipment of producing such
semiconductor devices. Thus, there remains this ongoing need for
further improvements in both the desirable properties of nanoporous
dielectric films, as well as an ongoing need for further
improvements in methods for producing such nanoporous dielectric
films.
SUMMARY OF THE INVENTION
[0011] In order to solve the above mentioned problems and to
provide other improvements, the invention provides novel nanoporous
silica dielectric films with a low dielectric constant ("k"), e.g.,
typically ranging from about 1.5 to about 3.8, as well as novel new
methods of producing these dielectrics films.
[0012] Broadly, the dielectric nanoporous films of the invention
are prepared by a method that includes the following process
steps:
[0013] (a) preparing a silicon-based, precursor composition
including a porogen,
[0014] (b) coating a substrate with the silicon-based precursor to
form a film,
[0015] (c) aging or condensing the film in the presence of
water,
[0016] (d) heating the gelled film at a temperature and for a
duration effective to remove substantially all of the porogen.
[0017] Advantageously, in the above-described process steps, the
precursor composition is substantially aged or condensed in the
presence of water in liquid or vapor form, without the application
of external heat or exposure to external catalyst.
[0018] The artisan will appreciate that the molar ratio of water to
Si can be readily determined by the desired rate of condensation
and the successful production of nanoporous silica dielectric
films. In particular embodiments, the molar ratio of water to Si
ranges, e.g., from about 2:1 to about 0:1.
[0019] Broadly, the silicon-based precursor composition includes a
monomer or prepolymer according to Formula I:
Rx--Si--Ly (Formula I)
[0020] wherein x is an integer ranging from 0 to about 2, and y is
an integer ranging from about 2 to about 4;
[0021] R is independently selected from the group consisting of
alkyl, aryl, hydrogen and combinations thereof;
[0022] L is an electronegative moiety, such as, e.g., alkoxy,
carboxy, amino, amido, halide, isocyanato and combinations
thereof.
[0023] The silicon-based precursor composition optionally includes
one or more monomers or prepolymers of Formula I, as well as a
polymer formed from the condensation of one or more different
monomers or prepolymers according to Formula I. The polymer formed
from Formula I has a molecular weight, for example, that ranges
from about 150 to about 10,000 amu.
[0024] Useful monomers or prepolymers include, e.g., acetoxysilane,
an ethoxysilane, a methoxysilane, and combinations thereof.
Particular monomers or prepolymers useful according to the
invention also include, e.g., tetraacetoxysilane, a C.sub.1 to
about C.sub.6 alkyl or aryl-triacetoxysilane, and combinations
thereof. The triacetoxysilane is, for example, a
methyltriacetoxysilane. Further monomers or prepolymers useful
according to the invention also include, e.g.,
tetrakis(2,2,2-trifluoroethoxy)silane,
tetrakis(trifluoroacetoxy)silane, tetraisocyanatosilane,
tris(2,2,2-trifluoroethoxy)methylsilane,
tris(trifluoroacetoxy)methylsilane, methyltriisocyanatosilane and
combinations thereof.
[0025] Optionally, the water employed for the processes of the
invention is added to the silicon-based, precursor composition
prior to the application of the precursor composition to the
substrate. Variations on this process include the addition of
amounts of water to the silicon-based, precursor composition
insufficient to fully condense or age the applied film, and
completing the aging process by exposing the applied film to
environmental water vapor. In one particularly convenient
embodiment of the invention, no water is added to the
silicon-based, precursor composition prior to application to the
substrate. Instead, all of the water for aging the film is provided
by environmental, e.g., atmospheric water vapor present in the
controlled atmosphere of the processing facility. The atmospheric
partial pressure of water vapor in the processing facility can be
adjusted to range, for example, from about 5 mm Hg to about 20 mm
Hg, The time required for achieving water-mediated aging depends on
the materials selected, on the source of the water, e.g., mixed
into the precursor or from environmental water vapor, and the
desired degree of aging. The time period ranges, for example, from
about 20 seconds to about 5 minutes, or more.
[0026] A useful porogen according to the invention is added to the
precursor in an amount ranging from about 2 to about 20 weight
percent. The porogen also has a boiling point, sublimation point or
decomposition temperature ranging, e.g., from about 175.degree. C.
to about 450.degree. C. The porogen also has a molecular weight
ranging, e.g., from about 100 to about 10,000 amu, or more
particularly, from about 100 to about 3,000 amu. In addition, the
porogen is selected to be readily removed from the applied and aged
film, e.g., by heating at a temperature ranging from about
175.degree. C. to about 300.degree. C., for a time period ranging
from about 30 seconds to about 5 minutes to remove substantially
all of the porogen.
[0027] A solvent is also optionally provided to reduce precursor
viscosity and aid film spreading, as required. When a solvent is
present, the silicon-based, precursor composition includes a
solvent or mixture of solvents in an amount ranging, for example,
from about 10% to about 90% by weight. The solvent has a boiling
point ranging, for instance, from about 50 to about 175.degree. C.
and is selected, for example, from hydrocarbons, esters, ethers,
ketones, alcohols, amides and combinations thereof. However, to
avoid undesirable interactions, the solvent is not an alcohol when
the silicon based monomer or precursor comprises an
acetoxy-functional group. To avoid cross-linkage of the solvent to
the precursor, it should be noted that the solvent optionally does
not include hydroxyl or amino groups. Nanoporous dielectric films
prepared by the methods of the invention, as well as semiconductor
devices and/or integrated circuits manufactured with such films,
are also provided.
BRIEF DESCRIPTION OF THE FIGURE
[0028] FIG. 1 Illustrates the pore volume distribution (Y-axis)
platted against the log of the pore size (X-axis).
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT
[0029] Accordingly, nanoporous silica dielectric films having a
dielectric constant, or k value, ranging from about 1.5 to about
3.8, can be produced by the methods of the invention. Typically,
silicon-based dielectric films, including nanoporous silica
dielectric films, are prepared from a suitable silicon-based
dielectric precursor composition, e.g., a spin-on-glass ("S.O.G.")
material blended with one or more optional solvents and/or other
components. The dielectric precursor composition is applied to a
substrate suitable, e.g., for production of a semiconductor device,
such as an integrated circuit ("IC"), by any art-known method.
[0030] The films produced by the processes of the invention have a
number of advantages over those previously known to the art,
including improved mechanical strength, that enables the produced
film to withstand the further processing steps required to prepare
a semiconductor device on the treated substrate, and a low and
stable dielectric constant. The property of a stable dielectric
constant is advantageously achieved without the need for further
surface modification steps to render the film surface hydrophobic,
as was formerly required by a number of processes for forming
nanoporous silica dielectric films. Instead, the silica dielectric
films as produced by the processes of the invention are
sufficiently hydrophobic as initially formed.
[0031] Further, in one embodiment, the processes of the invention
advantageously require no heating step or steps for the initial
polymerization (i.e., gelling or aging) of an applied precursor
composition onto a substrate. Instead, the precursor composition is
selected to be water-polymerizable, and water is either blended
with the precursor prior to application to the desired substrate,
and/or after application to a substrate, atmospheric moisture
facilitates the aging process, in situ. Further still, the
processes of the invention provided for a nanometer scale (10 nm or
less) diameter pore size, which is also uniform in size
distribution.
[0032] In order to better appreciate the scope of the invention, it
should be understood that unless the "SiO.sub.2" functional group
is specifically mentioned when the term "silica" is employed, the
term "silica" as used herein, for example, with reference to
nanoporous dielectric films, is intended to refer to dielectric
films prepared by the inventive methods from an organic or
inorganic glass base material, e.g., any suitable silicon-based
material. It should also be understood that the use of singular
terms herein is not intended to be so limited, but, where
appropriate, also encompasses the plural, e.g., exemplary processes
of the invention may be described as applying to and producing a
"film" but it is intended that multiple films can be produced by
the described, exemplified and claimed processes, as desired.
[0033] Additionally, the term "aging" refers to gelling,
condensing, or polymerization, of the combined silica-based
precursor composition on the substrate after deposition, induced,
e.g., by exposure to water and/or an acid or base catalyst. The
term "curing" refers to the removal of residual silanol (Si--OH)
groups, removal of residual water, and the process of making the
film more stable during subsequent processes of the microelectronic
manufacturing process. The curing process is performed after
gelling, typically by the application of heat, although any other
art-known form of curing may be employed, e.g., by the application
of energy in the form of an electron beam, ultraviolet radiation,
and the like. The terms, "agent" or "agents" herein should be
considered to be synonymous with the terms, "reagent" or
"reagents," unless otherwise indicated.
[0034] Dielectric films, e.g., interlevel dielectric coatings, are
prepared from suitable precursors applied to a substrate. Art-known
methods for applying the dielectric precursor composition, include,
but are not limited to, spin-coating, dip coating, brushing,
rolling, spraying, and/or by chemical vapor deposition. Prior to
application of the base materials to form the dielectric film, the
substrate surface is optionally prepared for coating by standard,
art-known cleaning methods. The coating is then processed to
achieve the desired type and consistency of dielectric coating,
wherein the processing steps are selected to be appropriate for the
selected precursor and the desired final product. Further details
of the inventive methods and compositions are provided below.
[0035] Substrates
[0036] Broadly speaking, a "substrate" as described herein includes
any suitable composition formed before a nanoporous silica film of
the invention is applied to and/or formed on that composition. For
example, a substrate is typically a silicon wafer suitable for
producing an integrated circuit, and the base material from which
the nanoporous silica film is formed is applied onto the substrate
by conventional methods, e.g., including, but not limited to, the
art-known methods of spin-coating, dip coating, brushing, rolling,
and/or spraying. Prior to application of the base materials to form
the nanoporous silica film, the substrate surface is optionally
prepared for coating by standard, art-known cleaning methods.
[0037] Suitable substrates for the present invention
non-exclusively include semiconductor materials such as gallium
arsenide ("GaAs"), silicon and compositions containing silicon such
as crystalline silicon, polysilicon, amorphous silicon, epitaxial
silicon, and silicon dioxide ("SiO.sub.2") and mixtures thereof. On
the surface of the substrate is an optional pattern of raised
lines, such as metal, oxide, nitride or oxynitride lines which are
formed by well known lithographic techniques. Suitable materials
for the lines include silica, silicon nitride, titanium nitride,
tantalum nitride, aluminum, aluminum alloys, copper, copper alloys,
tantalum, tungsten and silicon oxynitride. These lines form the
conductors or insulators of an integrated circuit. Such are
typically closely separated from one another at distances of about
20 micrometers or less, preferably 1 micrometer or less, and more
preferably from about 0.05 to about 1 micrometer. Other optional
features of the surface of a suitable substrate include an oxide
layer, such as an oxide layer formed by heating a silicon wafer in
air, or more preferably, an SiO.sub.2 oxide layer formed by
chemical vapor deposition of such art-recognized materials as,
e.g., plasma enhanced tetraethoxysilane oxide ("PETEOS"), plasma
enhanced silane oxide ("PE silane") and combinations thereof, as
well as one or more previously formed nanoporous silica dielectric
films.
[0038] The nanoporous silica film of the invention can be applied
so as to cover and/or lie between such optional electronic surface
features, e.g., circuit elements and/or conduction pathways that
may have been previously formed features of the substrate.
[0039] Such optional substrate features can also be applied above
the nanoporous silica film of the invention in at least one
additional layer, so that the low dielectric film serves to
insulate one or more, or a plurality of electrically and/or
electronically functional layers of the resulting integrated
circuit. Thus, a substrate according to the invention optionally
includes a silicon material that is formed over or adjacent to a
nanoporous silica film of the invention, during the manufacture of
a multilayer and/or multicomponent integrated circuit.
[0040] In a further option, a substrate bearing a nanoporous silica
film or films according to the invention can be further covered
with any art known non-porous insulation layer, e.g., a glass cap
layer.
[0041] Water Condensable Precursor Compositions
[0042] Broadly, the precursor composition employed for forming
silica-dielectric films according to the invention includes one or
more silicon-based monomers and/or polymers that are readily
condensed in the presence of water. The water can be optionally
supplied during preparation of the liquid precursor composition,
absorbed from environmental water vapor, and/or combinations of
these.
[0043] For a silicon based monomer or pre-polymer to be reactive
with water, the monomer or prepolymer must have at least two
reactive groups that can be hydrolyzed. Such reactive groups
include, e.g., alkoxy (RO), acetoxy (AcO), etc. Without meaning to
be bound by any theory or hypothesis as to how the methods and
compositions of the invention are achieved, it is believed that
water hydrolyzes the reactive groups on the silicon monomers to
form Si--OH groups (silanols). The latter will undergo condensation
reactions with other silanols or with other reactive groups, as
illustrated by the following formulas:
[0044] Si--OH+HO--Si.fwdarw.Si--O--Si+H.sub.2O
[0045] Si--OH+RO--Si.fwdarw.Si--O--Si+ROH
[0046] Si--OH+AcO--Si.fwdarw.Si--O--Si+AcOH
[0047] R=alkyl or aryl
[0048] Ac=acyl (CH.sub.3CO)
[0049] These condensation reactions lead to formation of silicon
based polymers. Generally speaking, an acid or base is used to
catalyze both the hydrolysis and the condensation reactions. It
should be mentioned that when adding volatile acid or base
catalysts to the precursor composition there is the potential that
some or all of the catalyst will evaporate during deposition of the
precursor onto the substrate. While such evaporative losses can be
controlled or compensated for, certain embodiments of the present
invention advantageously avoid this difficulty by forming an acid
catalyst in situ by reaction with water. In particular, the
reaction of water with acetoxy groups produces acetic acid; the
latter compound is a catalyst for hydrolysis and condensation
reactions. Therefore, when using acetoxy and other precursors of
this type, it is not necessary to add catalyst to the precursor
composition;
[0050] Thus, in one embodiment of the invention, the dielectric
precursor composition includes a compound, or any combination of
compounds, denoted by Formula I:
Rx--Si--Ly (Formula I)
[0051] wherein x is an integer ranging from 0 to about 2 and y is
an integer ranging from about 2 to about 4),
[0052] R is independently alkyl, aryl, hydrogen and/or combinations
of these,
[0053] L is independently selected and is an electronegative group,
e.g., alkoxy, carboxy, halide, isocyanato and/or combinations of
these.
[0054] Particularly useful monomers or precursors are those
provided by Formula I when x ranges from about 0 to about 2, y
ranges from about 2 to about 4, R is alkyl or aryl or H, and L is
an electronegative group, and wherein the rate of hydrolysis of the
Si--L bond is greater than the rate of hydrolysis of the
Si--OCH.sub.2CH.sub.3 bond. Thus, for the following reactions
designated as (a) and (b):
[0055] a) Si--L+H.sub.2O.fwdarw.Si--OH+HL
[0056] b)
Si--OCH.sub.2CH.sub.3+H.sub.2O.fwdarw.Si--OH+HOCH.sub.2CH.sub.3
[0057] The rate of (a) is greater than rate of (b).
[0058] Examples of suitable compounds according to Formula I
include, but are not limited to:
1 Si(OCH2CF.sub.3).sub.4 tetrakis(2,2,2-trifluoroethoxy)- silane,
Si(OCOCF.sub.3).sub.4 tetrakis(trifluoroacetoxy)silane*,
Si(OCN).sub.4 tetraisocyanatosilane, CH.sub.3Si(OCH2CF.sub.3).sub.3
tris(2,2,2-trifluoroethoxy)methylsilane,
CH.sub.3Si(OCOCF.sub.3).sub.3 tris(trifluoroacetoxy)methylsilane*,
CH.sub.3Si(OCN).sub.3 methyltriisocyanatosilane, [*These generate
acid catalyst upon exposure to water] and or combinations of any of
the above.
[0059] In another embodiment of the invention, the dielectric
precursor composition includes a polymer synthesized from compounds
denoted by Formula I by way of hydrolysis and condensation
reactions, wherein the number average molecular weight ranges from
about 150 to about 10,000 amu.
[0060] In a further embodiment of the invention, silicon-based
dielectric precursors useful according to the invention include
organosilanes, including, for example, alkoxysilanes according to
Formula II, as taught, e.g., by co-owned U.S. Ser. No. 09/054,262,
filed on Apr. 3, 1998, the disclosure of which is incorporated by
reference herein in its entirety. 1
[0061] Optionally, Formula II is an alkoxysilane wherein at least 2
of the R groups are independently C.sub.1 to C.sub.4 alkoxy groups,
and the balance, if any, are independently selected from the group
consisting of hydrogen, alkyl, phenyl, halogen, substituted phenyl.
For purposes of this invention, the term alkoxy includes any other
organic group which can be readily cleaved from silicon at
temperatures near room temperature by hydrolysis. R groups can be
ethylene glycoxy or propylene glycoxy or the like, but preferably
all four R groups are methoxy, ethoxy, propoxy or butoxy. The most
preferred alkoxysilanes nonexclusively include tetraethoxysilane
(TEOS) and tetramethoxysilane.
[0062] In a further option, for instance, especially when the
precursor is applied to the substrate by chemical vapor deposition,
e.g., as taught by co-owned patent application Ser. No. 09/111,083,
filed on Jul. 7, 1998, and incorporated by reference herein in its
entirety, the precursor can also be an alkylalkoxysilane as
described by Formula II, but instead, at least 2 of the R groups
are independently C.sub.1 to C.sub.4 alkylalkoxy groups wherein the
alkyl moiety is C.sub.1 to C.sub.4 alkyl and the alkoxy moiety is
C.sub.1 to C.sub.6 alkoxy, or ether-alkoxy groups; and the balance,
if any, are independently selected from the group consisting of
hydrogen, alkyl, phenyl, halogen, substituted phenyl. In one
preferred embodiment each R is methoxy, ethoxy or propoxy. In
another preferred embodiment at least two R groups are alkylalkoxy
groups wherein the alkyl moiety is C.sub.1 to C.sub.4 alkyl and the
alkoxy moiety is C.sub.1 to C.sub.6 alkoxy. In yet another
preferred embodiment for a vapor phase precursor, at least two R
groups are ether-alkoxy groups of the formula (C.sub.1 to C.sub.6
alkoxy).sub.n wherein n is 2 to 6.
[0063] Application Ser. No. 09/111,083, mentioned above, also
teaches that preferred silica precursors for chemical vapor
deposition include, for example, any or a combination of
alkoxysilanes such as tetraethoxysilane, tetrapropoxysilane,
tetraisopropoxysilane, tetra(methoxyethoxy)silane,
tetra(methoxyethoxyethoxy)silane which have four groups which may
be hydrolyzed and than condensed to produce silica,
alkylalkoxysilanes such as methyltriethoxysilane silane,
arylalkoxysilanes such as phenyltriethoxysilane and precursors such
as triethoxysilane which yield SiH functionality to the film.
Tetrakis(methoxyethoxyethoxy)silane, tetrakis(ethoxyethoxy)silane,
tetrakis(butoxyethoxyethoxy)silane, tetrakis(2-ethylthoxy)silane,
tetrakis(methoxyethoxy)silane, and tetrakis(methoxypropoxy)silane
are particularly useful for the invention.
[0064] In a still further embodiment of the invention, the
alkoxysilane compounds described above may be replaced, in whole or
in part, by compounds with acetoxy and/or halogen-based leaving
groups. For example, the precursor compound may be an acetoxy
(CH.sub.3--CO--O--) such as an acetoxy-silane compound and/or a
halogenated compound, e.g., a halogenated silane compound and/or
combinations thereof. For the halogenated precursors the halogen
is, e.g., Cl, Br, I and in certain aspects, will optionally include
F. Preferred acetoxy-derived monomers or precursors include, e.g.,
tetraacetoxysilane, methyltriacetoxysilane and/or combinations
thereof.
[0065] In one particular embodiment of the invention, the
silicon-based precursor composition includes a monomer or polymer
precursor such, for example, acetoxysilane, an ethoxysilane,
methoxysilane and/or combinations thereof.
[0066] In a more particular embodiment of the invention, the
silicon-based precursor composition includes a tetraacetoxysilane,
a C.sub.1 to about C.sub.6 alkyl or aryl-triacetoxysilane and
combinations thereof. In particular, as exemplified below, the
triacetoxysilane is a methyltriacetoxysilane.
[0067] Processes for Forming Water Condensed Nanoporous Dielectric
Films
[0068] Broadly, in one embodiment of the invention the method of
the invention is conducted by preparing a silicon-based precursor
composition that includes a porogen that is selected to age or
condense in the presence of water, without the application of an
external source of heat. A desired substrate is coated with the
silicon-based precursor by any standard, art-known method, to form
a film. The applied film is then aged or condensed in the presence
of water, without the application of any external heat. The aged
film is then heated at a temperature and for a duration effective
to remove substantially all of the porogen, to provide a silica
dielectric film having a nanometer scale pore structure, with the
desired low range of dielectric constant.
[0069] In the absence of water these reactions will not occur. The
precursor compositions of this invention will be very stable over
time, until water is added, because polymer growth will not occur.
While certain embodiments of the invention use a precursor
comprising water, in many production situations it is preferred
that the precursor be prepared without water to avoid the aging and
thickening of the precursor between the time of mixing and the time
of application to a substrate. A thicker, more viscous precursor
results in thicker applied films. To avoid this potential problem,
one option is to add the water to the precursor formulation just
prior to application to the substrate. Avoiding the potential
problems of precursor viscosity increasing over time is a very
important benefit of having the precursor aged or condensed by
environmental water vapor after coating, e.g., during the
manufacture of semiconductor devices.
[0070] In overview, the precursor is deposited on the substrate at
room temperature (15-25.degree. C.), usually by spin coating. Once
the precursor composition is deposited onto the substrate, the
resulting firm will absorb water from the environment (typically
chip manufacturing clean rooms have relative humidity >30%,
typically about 40%). This water vapor is sufficient to age the
film in situ. Optionally, the film may be heated at 25-200.degree.
C. to hasten the solidification of the film.
[0071] Once the film has aged, i.e., once it is sufficiently
condensed to be solid or substantially solid, the porogen can be
removed. The latter should be sufficiently non-volatile so that it
does not evaporate from the film before the film solidifies.
Generally, the porogen is removed by heating the film at or above
200.degree. C.
[0072] The precursor composition preferably contains an acid
catalyst, either volatile or nonvolatile, depending on processing
requirements. Acid catalysts include, e.g., organic acids, such as
glacial acetic acid; and inorganic acids such as nitric acid and
hydrogen chloride; this catalyst will accelerate the hydrolysis and
condensation reactions. A base catalyst (such as amine compounds)
may also be added to the precursor composition for this purpose. Of
particular value is the in-situ generation of catalyst by reaction
of water with the monomer or polymer contained in the precursor
composition. For example, reaction of water with tetraacetoxysilane
produces acetic acid.
[0073] A film is judged to be porous if its refractive index (n) is
less than 1.44. A non-porous film made from the liquid precursor of
this invention will have a refractive index of 1.44. Air has a
refractive index of 1.0. The porosity of a film is the % of its
volume that is air. The porosity of a film is inversely
proportional to n-1. For example, a film with n=1.27 will have
porosity=100*[(1.44-1)-(1.27-1)]/(1.44-1)=39%.
[0074] Selection of a Porogen or Porogens
[0075] A porogen may be a compound or oligomer or polymer and is
selected so that, when it is removed, e.g., by the application of
heat, a silica dielectric film is produced that has a nanometer
scale porous structure. The scale of the pores produced by porogen
removal is proportional to the effective steric diameters of the
selected porogen component (as noted supra, the use of a singular
term encompasses the plural as well). The need for any particular
pore size range (i.e., diameter) is defined by the scale of the
semiconductor device in which the film is employed. Thus, for
microelectronic applications in which the minimum feature size is
less than 100 nm, a pore size of 10 nm or less is required.
Furthermore, the porogen should not be so small as to result in the
collapse of the produced pores, e.g., by capillary action within
such a small diameter structure, resulting in the formation of a
non-porous (dense) film. Further still, there should be minimal
variation in diameters of all pores in the pore population of a
given film.
[0076] Given the above, it is preferred that porogen is a compound
that has a substantially homogeneous molecular weight and molecular
dimension, and not a statistical distribution or range of molecular
weights, and/or molecular dimensions, in a given sample. The
avoidance of any significant variance in the molecular weight
distribution allows for a substantially uniform distribution of
pore diameters in the film treated by the invetive processes. If
the produced film has a wide distribution of pore sizes, the
likelihood is increased of forming one or more large pores, i.e.,
bubbles, on a dimensional scale of 10 nm or greater, that could
interfere with the production of reliable semiconductor devices
having a minimum feature size of less than 100 nm,
[0077] Preferrably, the porogen has reactive groups, such as
hydroxyl or amino. The reactive groups will react with the silicon
precursor to form Si--O--R or Si--NHR bonds (R represents an
independently selected organic moiety, e.g., an aryl or alkyl
group, substituted or unsubstituted). These bonds will minimize the
chance of phase separation between silicon monomer and porogen
during film deposition (spin coating); minimal phase separation
will lead to the best possible film appearance and thickness
uniformity, and also minimize the pore size and distribution in the
final film. Optionaly the porogen may have more than one reactive
group of the same or different function (e.g., one or more of --OH
and/or (--NH.sub.2, etc.). Simply by way of example, such groups
are believed to form a covalent linkage between a porogen and the
Si component of the resulting film in the form of Si--O--C or
Si--N--C linkages.
[0078] Furthermore, the porogen should have a molecular weight and
structure such that it is readily and selectively removed from the
film without interfering with film formation. This is based on the
nature of semiconductor devices, which typically have an upper
limit to processing temperatures. Broadly, a porogen should be
removable from the newly formed film at temperatures below, e.g.,
about 450.degree. C. In particular embodiments, depending on the
desired post film formation fabrication process and materials, the
porogen is selected to be readily removed at temperatures ranging
from about 300.degree. C. to about 400.degree. C. during a time
period ranging, e.g., from about 30 seconds to about 5 minutes. For
instance, the very highest limit for multi-level interconnect
processing in IC fabrication is 450.degree. C., and many IC
manufacturers require the highest limit be 400.degree. C. Further,
in certain particular embodiments, the porogen is selected to be
removed at a temperature of less than 300.degree. C. The removal of
the porogen may be induced by heating the film at atmospheric
pressure or under a vacuum, or by exposing the film to radiation,
or both.
[0079] Porogens which meet the above characteristics include those
compounds and polymers which have a boiling point, sublimation
temperature, and/or decomposition temperature (at atmospheric
pressure) range, for example, from about 175.degree. C. to about
450.degree. C., or in this range, to less than about 450.degree. C.
In particular embodiments, the boiling point, sublimation
temperature, and/or decomposition temperature(s) of the porogen (at
atmospheric pressure) are less than about 400.degree. C., and in
even more particular embodiments are less than about 300.degree. C.
In addition, poregens suitable for use according to the invention
include those having a molecular weight ranging, for example, from
about 100 to about 10,000 amu, and more preferably in the range of
100-3,000 amu.
[0080] Broadly, porogens suitable for use in the processes and
compositions of the invention include polymers, preferably those
which contain one or more reactive groups, such as hydroxyl or
amino. Advantageously, the molecular weights of selected polymers
useful as porogens ranges, for example, from about 100 to about
10,000 amu. In particular embodiments, the molecular weight of the
previously mentioned polymers range, from about 100 to about 3,000
amu. Within these general parameters, a suitable polymer porogen
for use in the compositions and methods of the invention is, e.g.,
a polyalkylene oxide, a monoether of a polyalkylene oxide, an
aliphatic polyester, an acrylic polymer, an acetal polymer, a
poly(caprolatactone), a poly(valeractone), a poly(methyl
methacrylate), a poly(vinylbutyral) and/or combinations thereof.
When the porogen is a polyalkylene oxide monoether, one particular
embodiment is a C.sub.1 to about C.sub.6 alkyl, e.g., polyethylene
glycol monomethyl ether, or polypropylene glycol monomethyl
ether.
[0081] Additional porogens suitable for use in the processes and
compositions of the invention include organic compounds. Specific
compounds useful as porogens include, for example: 1-adamantanol
(CAS # 768-95-6), 2-adamantanol (CAS # 700-57-2), 1-adamantanamine
(CAS # 768-94-5), 4-(1-adamantyl)phenol (CAS # 29799-07-03),
4,4'-(1,3-adamantanediyl)diphenol (CAS # 37677-93-3),
a-D-cellobiose octaacetate (CAS # 5346-90-7), and cholesterol (CAS
# 57-88-5).
[0082] Without meaning to be bound by any theory or hypothesis as
to how the invention might operate, it is believed that porogens
that are, "readily removed from the film" undergo one or a
combination of the following events: (1) physical evaporation of
the porogen during the heating step, (2)degradation of the porogen
into more volatile molecular fragments, (3) breaking of the bond(s)
between the porogen and the Si containing component, and subsequent
evaporation of the porogen from the film, or any combination of
modes 1-3. The porogen is heated until a substantial proportion of
the porogen is removed, e.g., at least about 50% by weight, or
more, of the porogen is removed. More particularly, in certain
embodiments, depending upon the selected porogen and film
materials, at least about 75% by weight, or more, of the porogen is
removed. Thus, by "substantially" is meant, simply by way of
example, removing from about 50% to about 75%, or more, of the
original porogen from the applied film.
[0083] A porogen or porogens are present in the liquid precursor
composition, for example, in a percentage ranging from about 1 to
about 40 weight percent, or more. More particularly, a porogent or
porogens are present in the liquid precursor composition, e.g., in
a percentage ranging from about 2 to about 20 weight percent.
[0084] Solvents for Precursor Composition
[0085] The precursor composition optionally includes a solvent
system. Reference herein to a "solvent" should be understood to
encompass a single solvent, polor or nonpolar and/or a combination
of compatible solvents forming a solvent system selected to
solubilize the precursor monomer or pre-polymer components,
together with the other required components of the precursor
composition. A solvent is optionally included in the precursor
composition to lower its viscosity and promote uniform coating onto
a substrate by art-standard methods (e.g., spin coating, spray
coating, dip coating, roller coating, and the like).
[0086] In order to facilitate solvent removal, the solvent is one
which has a relatively low boiling point relative to the boiling
point of any selected porogen and the other precursor components.
For example, solvents that are useful for the processes of the
invention have a boiling point ranging from about 50 to about
175.degree. C. For instance, when the solvent boiling point is
175.degree. C., the boiling points and/or sublimation temperature
of the porogen and silicon based monomer is greater than
175.degree. C., simply to allow the solvent to evaporate from the
applied film and leave the active portion of the precursor
composition in place. In order to meet various safety and
environmental requirements, the solvent preferably has a high flash
point (generally greater than 40.degree. C.) and relatively low
levels of toxicity. A suitable solvent includes, for example,
hydrocarbons, as well as solvents having the functional groups
C--O--C (ethers), --CO--O (esters), --CO-- (ketones), --OH
(alcohols), and --CO--N--(amides), and solvents which contain a
plurality of these functional groups.
[0087] Simply by way of example, and without limitation, solvents
for the precursor composition include di-n-butyl ether, anisole,
acetone, 3-pentanone, 2-heptanone, ethyl acetate, n-propyl acetate,
n-butyl acetate, ethyl lactate, ethanol, 2-propanol, dimethyl
acetamide, propylene glycol methyl ether acetate, and/or
combinations thereof. It is preferred that the solvent not react
with the Si-based monomer or polymer component of the precursor
composition. Instead it is preferred that only the porogen can
react with the Si-based component. Therefore, the solvent should
preferrably not contain hydroxyl or amino groups.
[0088] The solvent component is preferably present in an amount of
from about 10% to about 90% by weight of the overall blend. A more
preferred range is from about 20% to about 75% and most preferably
from about 20% to about 60%. The greater the percentage of solvent
employed, the thinner is the resulting film. The greater the
percentage of porogen employed, the greater is the resulting
porosity
[0089] Water as an Aging or Gelling Agent
[0090] As noted supra, in one embodiment of the invention the
composition of the silicon-based precursor composition is selected
to undergo aging or gelling in the presence of water, either liquid
or water vapor. Preferably, the precursor composition is applied to
a desired substrate and then exposed to an ambient atmosphere that
includes water vapor, e.g., at standard temperatures and standard
atmospheric pressure. The higher the relative humidity of the
surrounding atmosphere, the faster the aging process will occur.
Preferably, the approximately 40 percent relative humidity of the
processing clean room is employed, so that no special additional
processing equipment or chambers are required. Of course, the
relative humidity can be varied as required for particular film
forming conditions, to range, e.g., from a relative humidity of
about 5% to about 99%. Measured another way, for example, the
desired exposure to water vapor is provided when the atmospheric
partial pressure of water vapor ranges from, e.g., about 5 mm Hg to
about 20 mm Hg,
[0091] As will be readily appreciated, the time period for exposure
of the applied film to environmental water vapor is a time that is
sufficient for the film to attain sufficient condensation so that
removal of the porgen is successfully accomplished. Simply by way
of example, the time for exposure to environmental water vapor
ranges from a minimum of about 5 to 10 seconds, up to 30 minutes or
longer. Simply by way of example, the applied precursor film is
exposed to environmental water vapor for a time period ranging,
e.g., from about 20 seconds to about 5 mintues, or more
particularly, for a time period ranging, e.g., from about 5 seconds
to about 60 seconds.
[0092] Optionally, the precursor composition is prepared prior to
application to a substrate to include water in a proportion
suitable for initiating aging of the precursor composition, without
being present in a proportion that results in the precursor
composition aging or gelling before it can be applied to a desired
substrate. The water in the precursor is optionally present in an
amount sufficient to fully age the film without any need for
further exposure to environmental water vapor, although any
convenient combination of these sources of water to support the
aging process is conveniently employed for particular desired film
forming situations.
[0093] Simply by way of example, the water is mixed into the
precursor composition in a proportion ranging from
[0094] The mole ratio of water to the Si atoms in the monomer or
polymer component is preferably from about 0 to about 50. A more
preferred range is from about 0.1 to about 10 and most preferably
from about 0.5 to about 1.5. The acid may be present in a catalytic
amount which can be readily determined by those skilled in the art.
Preferably the molar ratio of acid to silane ranges from about 0 to
about 10, more preferably from about 0.001 to about 1.0, and most
preferably from about 0.005 to about 0.02.
[0095] Condensation Catalysts
[0096] The precursor composition preferrably contains an acid
catalyst, such as glacial acetic acid; this catalyst will
accelerate the hydrolysis and condensation reactions. Optionally, a
base catalyst (such as an alkali, ammonia or amine compounds with a
pK.sub.b ranging from less than 0 to about 9 can be added to the
precursor composition for this purpose. Amines, such as primary,
secondary and tertiary alkyl amines, aryl amines, alcohol amines
and mixtures thereof which have a boiling point of about
200.degree. C. or less, preferably 100.degree. C. or less and more
preferably 25.degree. C. or less. Other amines, include, e.g.,
monoethanol amine, tetraethylenepentamine,
2-(aminoethylamino)ethanol, 3-aminopropyltriethoxy silane,
3-amino-1,2-propanediol, 3-(diethylamino)-1,2-propanediol,
n-(2-aminoethyl)-3-aminopropyl-trimetho- xy silane,
3-aminopropyl-trimethoxy silane, methylamine, dimethylamine,
trimethylamine, n-butylamine, n-propylamine, tetramethyl ammonium
hydroxide, piperidine and 2-methoxyethylamine.
[0097] It should also be mentioned, simply by way of example, that
when the precursor comprises acetoxy silicon monomers, the
water-induced hydrolysis of the acetoxy compounds liberates acetic
acid. The acetic acid catalyzes the further hydrolysis, and also
accelerates condensation of the silicon precursor.
[0098] Optional Curing Steps
[0099] The artisan will appreciate that specific temperature ranges
for curing substrates comprising nanoporous dielectric films
according to the invention will depend upon the selected materials,
substrate and desired nanoscale pore structure, as is readily
determined by routine manipulation of these parameters. Generally,
the curing step comprises heating the previously prepared film at a
temperature of at least 400.degree. C. and for a time period
ranging from about 10 to about 60 minutes, and is desirably
conducted in the absence of oxygen, e.g., under an inert gas such
as N.sub.2, or in a vacuum. As exemplified herein, the films are
cured at about 425.degree. C. for about 30 minutes under
vacuum.
[0100] The artisan will also appreciate that any number of
additional art-known curing methods are optionally employed,
including the application of sufficient energy to cure the film by
exposure of the film to electron beam energy, ultraviolet energy,
microwave energy, and the like, according to art-known methods.
EXAMPLES
[0101] The following non-limiting examples serve to illustrate the
invention.
Example 1
Nanoporous Silica Dielectric Film with 550 MW PEGMME with Heat
Aging
[0102] A precursor was prepared by combining, in a 100 ml
roundbottom flask (containing a magnetic stirring bar), 10 g
tetraacetoxysilane (United Chemical), 10 g methyltriacetoxysilane
(United Chemical), and 30 g acetone (Pacific Pac). These
ingredients were combined within an N.sub.2-environment (N.sub.2
glove bag).
[0103] The flask was also connected to an N.sub.2 environment to
prevent environmental moisture from entering the solution (standard
temperature and pressure).
[0104] After 20 minutes of stirring using the magnetic stirring
bar, 1.5 g of water was added to the flask. After 3 hours of
continued stirring of the water-containing solution, 6.81 g of
polyethylene glycol monomethylether ("PEGMME" Aldrich; MW550 amu)
was added as a porogen, and stirring continued for another 2 hrs.
Thereafter, the resulting solution was filtered through a 0.2
micron filter to provide the precursor solution for the next
step.
[0105] The precursor solution was then deposited onto a series of 4
inch silicon wafers, each on a spin chuck and spun at 2500 rpm for
30 seconds. The presence of water in the precursor resulted in the
film coating being substantially condensed by the time that the
wafer was inserted into the first oven. Insertion into the first
oven, as discussed below, took place within the 10 seconds of the
completion of spinning.
[0106] Each coated wafer was then transferred into a sequential
series of ovens preset at specific temperatures, for one minute
each. In this example, there were three ovens, and the preset oven
temperatures were 80.degree. C., 175.degree. C., and 300.degree.
C., respectively. The PEGMME was driven off by these sequential
heating steps as each wafer was moved through each of the three
respective ovens.
[0107] Each wafer was cooled after receiving the three-oven stepped
heat treatment, and the produced dielectric film was measured using
ellipsometry to determine its thickness and refractive index. Each
film-coated wafer was then further cured at 425.degree. C. for 30
minutes under vacuum.
[0108] Results
[0109] A film is judged to be porous if its refractive index (n) is
less than 1.44. A non-porous film made from the liquid precursor of
this invention will have a refractive index of 1.44. In comparison,
air has a refractive index of 1.0. The porosity of a nanoporous
film of the invention, is therefore proportional to the percentage
of its volume that is air. The porosity of a film is inversely
proportional to n-1. For example, a film with n=1.27 will have
porosity of: 100*[(1.44-1)-(1.27-1)]/(1.44-1)=39%.
[0110] The measurements obtained from a representative nanoporous
silica dielectric film produced by the methods of this example on a
wafer, are shown in Table 1, below.
2TABLE 1 Bake Thickness (.ANG.) Bake RI# Cure Thickness (.ANG.)
Cure RI 6622 1.255 5902 1.250 #Refractive Index
[0111] Application of the above-equation indicates that the cured
film produced by the above-described method has a porosity of about
43%.
Example 2
Nanoporous Silica Dielectric Film with 550 MW PEGMME with H.sub.2O
Aging
[0112] A precursor was prepared as described for Example 1, supra.
The prepared precursor mixture was deposited onto a series of 4
inch silicon wafers, which were each mounted on a spin chuck. Each
coated wafer was spun at 2500 rpm for 30 seconds, and the presence
of water in the precursor resulted in the film coating being
substantially condensed by the time that the wafer was inserted
into the oven. Insertion into the oven, as discussed below, took
place within the 10 seconds of the completion of spinning.
[0113] The coated wafers were each then heated for 1 or 3 minutes
in a single oven, at one of the following alternative temperatures:
200.degree. C., 225.degree. C., 250.degree. C., 270.degree. C., or
300.degree. C., to determine the optimum temperature range for
porogen removal.
[0114] Each heated wafer was then cooled, and the resulting film on
each wafer was measured using ellipsometry to determine its
thickness and refractive index (which can be correlated to the
film's porosity.) Each film was then cured at 425.degree. C. for 30
minutes under vacuum.
[0115] Results
[0116] Measurements of film thickness and refractive index for
nanoporous silica dielectric films produced and heated as described
above are summarized in Table 2, below.
3TABLE 2 Bake Bake Bake Time Thickness Cure Thickness Temp. (min)
(.ANG.) Bake RI# (.ANG.) Cure RI# 200.degree. C. 1 9392 1.443 6619
1.251 3 8047 1.375 N/A* cracked 225.degree. C. 1 9127 1.440 6537
1.258 3 7442 1.348 N/A* cracked 250.degree. C. 1 7801 1.291 N/A*
cracked 3 7613 1.286 N/A* cracked 270.degree. C. 1 7704 1.254 6730
1.247 3 7588 1.252 6380 1.261 300.degree. C. 1 7697 1.240 6405
1.258 3 7550 1.238 6499 1.258 #Refractive Index *Cure thickness and
Cure RI is not available when cured film too extensively
cracked.
[0117] As can be appreciated from an inspection of the above
results, the best results were obtained with a brief 1 or 3 minute
oven treatment at 270.degree. C. or greater, which both drove off
the porogen and substantially cured the film. Heating at
300.degree. C. certainly drove off most or all of the porogen. The
films after a final cure step show a slightly decreased film
thickness and a slightly greater RI, confirming that curing does
further increased the film density. However, the changes are
relatively minor, and it is expected that the cure step will be
rendered unnecessary by a modest increase in the duration and
temperature of the porogen removal step.
[0118] In addition, the film made using bake temperature of
270.degree. C. and bake time of 1 minute (See Table 2, supra) was
confirmed by measurements to have an average pore size of about 20
.ANG. (2 nanometers). In contrast, films obtained by previously
employed methods have average pore diameters of, e.g., 60 .ANG. (6
nanometers). Further, the film produced by the instant example had
virtually no pores larger than 100 .ANG. (10 nanometers). The pore
size distribution was obtained by the art-known method of
isothermal nitrogen adsorption which is based upon the Brunauer
Emmett Teller (BET) and Kelvin theories (see, e.g., Ralph K. Iler,
1979, Chemistry of Silica, John Wiley and Sons, PP467 and 488-502,
the disclosure of which is incorporated by reference herein). This
measurement data confirms that films having nanometer scale pore
structure can be produced by the method of this invention.
Example 3
Nanoporous Silica Dielectric Film with 550 MW PPGMBE with Heat
Aging
[0119] A precursor was prepared as described for Example 1,supra,
but a different porogen, polypropylene glycol monobutyl ether
(PPGMBE; mw 340 amu), was employed. The prepared precursor was then
deposited onto 4 inch silicon wafers, each on a spin chuck. Each
wafer was spun at 2500 rpm for 30 seconds. The presence of water in
the precursor resulted in the film coating being substantially
condensed by the time that the wafer was inserted into the oven.
Insertion into first oven, as discussed below, took place within
the 10 seconds of the completion of spinning.
[0120] As for Example 1, supra, each coated wafer was then
transferred into a sequential series of ovens preset at specific
temperatures, for one minute each as preset temperatures of
80.degree. C., 175.degree. C., and 300.degree. C., respectively.
Each wafer was cooled after receiving the three-oven stepped heat
treatment, and the produced dielectric film measured using
ellipsometry to determine its thickness and refractive index. Each
film-coated wafer was then further cured at 425.degree. C. for 30
minutes under vacuum.
4 Bake Thickness (.ANG.) Bake RI Cure Thickness (.ANG.) Cure RI
8168 1.277 7064 1.251
[0121] The above results confirm that films can be made with a
second porogen.
Example 4
Nanoporous Silica Dielectric Film with 340 MW PPGMBE with H.sub.2O
Aging
[0122] Precursor was made as described above for Example 3,supra.
The resulting films were measured and the data summarized in the
following table.
5 Bake Bake Cure Bake Time Thickness Thickness Temp. (min) (.ANG.)
Bake RI (.ANG.) Cure RI 200.degree. C. 1 6312 1.374 4881 1.269 3
6187 1.374 4676 1.272 250.degree. C. 1 6593 1.322 4911 1.279 3 5844
1.306 4847 1.273 270.degree. C. 1 6253 1.279 4982 1.283 3 6042
1.285 5099 1.285 300.degree. C. 1 6557 1.270 5076 1.278 3 6199
1.269 5228 1.279
Example 5
Control Experiment--Water Added
[0123] Precursor was made as described for Example 1, supra. The
mixture was deposited onto a 4 inch silicon wafer on a spin chuck.
It was spun at 2500 rpm for 30 seconds. The film was heated for 1
minute in ovens at 80.degree. C., 175.degree. C., and 300.degree.
C. The wafer was then cooled and the film measured using
ellipsometry to determine its thickness and refractive index (which
can be correlated to the film's porosity.) The film was then cured
at 425.degree. C. for 30 minutes under nitrogen.
[0124] The cohesive strength was measured by the "stud pull method"
which is described, for example, in co-owned U.S. Ser. No.
09/111,084, filed Jul. 7, 1998, incorporated by reference herein in
its entirety.
6 Cure Cohesive Strength Thickness (.ANG.) Cure RI k.sup.1 (kpsi)
4572 1.2468 2.3 7.5 .sup.1Dielectric constant.
Example 6
No Water Added
[0125] Precursor was made by mixing 10 g tetraactoxysilane (United
Chemical), 10 g methyltriacetoxysilane (United Chemical), 30 g
acetone (Pacific Pac), and 1.5 ml of dried glacial acetic acid
(Aldrich) in a dry bag. After 3 hours of mixing, 6.81 g of 550 MW
polyethyleneglycol monomethylether (Aldrich) was added as a
porogen. This was then mixed for 2 hrs and then filtered with 0.2
micron filter.
[0126] The processing facility is one in which the relative
humidity of the air is maintained at about 40% relative
humidity.
[0127] The mixture was deposited onto a 4 inch silicon wafer on a
spin chuck. It was spun at 2500 rpm for 30 seconds. Sufficient
moisture was absorbed into the applied precursor during processing
to substantially age or condense the applied film. The time between
the end of spinning and oven insertion was, as described
previously, about 10 seconds.
[0128] The film was heated for 1 minute in ovens @ 80.degree. C.,
175.degree. C., and 300.degree. C. The wafer was then cooled and
the film was measured using ellipsometry to determine its thickness
and refractive index (which can be correlated to the films
porosity.) The film was then cured at 425.degree. C. for 30 minutes
with nitrogen.
7 Cure Cohesive Strength Thickness (.ANG.) Cure RI k (kpsi) 5240
1.215 1.99 5.5
[0129] The above results confirm that water induced aging or
condensation was achieved simply by absorption of water vapor from
environmental air, in an environmentally controlled processing
facility.
DISCUSSION
[0130] Comparative examples 5 and 6, described supra, confirm that
the water-aged film has substantial cohesive strength relative to
film aged without water. The cohesive strength measurement obtained
above, e.g., Example 5, confirm that the nanoporous dielectric
silica films produced by the methods of the present invention are
substantially stronger than those obtained by application of
previous methods, while retaining similar dielectric, constant
values.
* * * * *