U.S. patent application number 12/842400 was filed with the patent office on 2010-11-11 for film-forming composition, insulating film with low dielectric constant, formation method thereof, and semiconductor device.
Invention is credited to Takeshi Asano, Yoshitaka Hamada, Hideo Nakagawa, Masaru Sasago, Fujio Yagihashi.
Application Number | 20100283133 12/842400 |
Document ID | / |
Family ID | 39779669 |
Filed Date | 2010-11-11 |
United States Patent
Application |
20100283133 |
Kind Code |
A1 |
Hamada; Yoshitaka ; et
al. |
November 11, 2010 |
FILM-FORMING COMPOSITION, INSULATING FILM WITH LOW DIELECTRIC
CONSTANT, FORMATION METHOD THEREOF, AND SEMICONDUCTOR DEVICE
Abstract
In the invention, a silica sol prepared by hydrolyzing and
condensing a silane compound represented by the following formula:
Si(OR.sup.1).sub.4 or R.sup.2.sub.nSi(OR.sup.3).sub.4-n wherein
R.sup.1s, R.sup.2(s) and R.sup.3(s) may be the same or different
when a plurality of them are contained in the molecule and each
independently represents a linear or branched C.sub.1-4 alkyl group
in the presence of a hydrophilic basic catalyst and a hydrophobic
basic catalyst is used for a conventional porous-film forming
composition.
Inventors: |
Hamada; Yoshitaka;
(Joetsu-shi, JP) ; Yagihashi; Fujio; (Joetsu-shi,
JP) ; Asano; Takeshi; (Joetsu-shi, JP) ;
Nakagawa; Hideo; (Shiga, JP) ; Sasago; Masaru;
(Osaka, JP) |
Correspondence
Address: |
MYERS BIGEL SIBLEY & SAJOVEC
PO BOX 37428
RALEIGH
NC
27627
US
|
Family ID: |
39779669 |
Appl. No.: |
12/842400 |
Filed: |
July 23, 2010 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
|
|
12029569 |
Feb 12, 2008 |
7786022 |
|
|
12842400 |
|
|
|
|
Current U.S.
Class: |
257/632 ;
106/482; 257/E21.24; 257/E29.001; 438/780 |
Current CPC
Class: |
C08G 77/08 20130101;
Y10T 428/31663 20150401; C08L 83/04 20130101; H01L 21/02216
20130101; H01L 21/02282 20130101; H01L 21/31695 20130101; H01L
21/02126 20130101; C09D 183/04 20130101; H01B 3/46 20130101; H01L
21/3122 20130101; H01L 21/02203 20130101 |
Class at
Publication: |
257/632 ;
438/780; 106/482; 257/E21.24; 257/E29.001 |
International
Class: |
H01L 29/00 20060101
H01L029/00; H01L 21/31 20060101 H01L021/31; C09D 5/25 20060101
C09D005/25 |
Foreign Application Data
Date |
Code |
Application Number |
Feb 16, 2007 |
JP |
2007-036345 |
Claims
1. A silica sol prepared by hydrolyzing and condensing a
hydrolyzable silane compound in the presence of at least one
hydrophilic basic catalyst selected from alkali metal hydroxides
and quaternary ammonium hydroxides represented by the following
formula (1): (R.sup.1).sub.4N.sup.+OH.sup.- (1) wherein, R.sup.1s
may be the same or different and each independently represents a
hydrocarbon group which may contain an oxygen atom and the cationic
portion [(R.sup.1).sub.4N.sup.+] satisfies the following
relationship (2): (N+O)/(N+O+C).gtoreq.1/5 (2) in which, N, O and C
are the numbers of nitrogen, oxygen and carbon atoms contained in
the cationic portion, respectively, and at least one hydrophobic
basic catalyst selected from quaternary ammonium hydroxides which
do not satisfy the above-described relationship (2).
2. A composition for forming a porous film comprising the silica
sol of claim 1.
3. A porous film formed using the porous-film forming composition
of claim 2.
4. A method for forming a porous film, which comprises applying the
porous-film forming composition of claim 2 to form a thin film and
sintering the thin film.
5. A method for manufacturing of a semiconductor device, which
comprises forming an interlayer insulating film by using the method
for forming a porous film of claim 4.
6. A semiconductor device comprising the porous film of claim 3 as
an interlayer insulating film.
Description
CROSS-RELATED APPLICATIONS
[0001] This application is a divisional of U.S. patent application
Ser. No. 12/029,569 filed Feb. 12, 2008, currently pending, which
claims priority from Japanese Patent Application No. 2007-036345;
filed Feb. 16, 2007, the disclosures of which are incorporated
herein by reference in their entireties.
BACKGROUND OF THE INVENTION
[0002] 1. Field of the Invention
[0003] The present invention relates to a silica sol capable of
providing a porous film excellent in dielectric properties,
adhesion, uniformity of thin film, and mechanical strength, a film
forming composition, a method for forming a porous film, a porous
film formed thereby, and a semiconductor device having the porous
film therein.
[0004] 2. Description of the Related Art
[0005] In the fabrication of semiconductor integrated circuits, as
their integration degree becomes higher, an increase in
interconnect delay time due to an increase in interconnect
capacitance, which is a parasitic capacitance between metal
interconnects, prevents their performance enhancement. The
interconnect delay time is called an RC delay which is in
proportion to the product of electric resistance of metal
interconnects and the static capacitance between interconnects.
Reduction in the resistance of metal interconnects or reduction in
the capacitance between interconnects is necessary for reducing
this interconnect delay time. The reduction in the resistance of an
interconnect metal or the interconnect capacitance can prevent even
a highly integrated semiconductor device from causing an
interconnect delay, which enables miniaturization and high speed
operation of the semiconductor device and moreover, minimization of
the power consumption.
[0006] In order to reduce the resistance of metal interconnects,
copper interconnects have recently replaced conventional aluminum
interconnects.
[0007] Use of copper interconnects alone, however, has limits in
accomplishing performance enhancement so that reduction in the
interconnect capacitance is an urgent necessity for further
performance enhancement of semiconductor devices.
[0008] One method for reducing interconnect capacitance may be to
reduce the dielectric constant of an interlayer insulating film
disposed between metal interconnects. It is the common practice to
prepare a material having a dielectric constant of 2.5 or less by
introducing pores therein to make it porous.
[0009] When an interlayer insulating film is made porous, however,
reduction in mechanical strength and adsorption of moisture tend to
deteriorate the film so that reduction in dielectric constant (k)
by introduction of pores in the film and maintenance of sufficient
mechanical strength and hydrophobicity are big challenges to be
solved.
[0010] For satisfying both introduction of pores and sufficient
mechanical strength, proposed is a method of introducing zeolite or
zeolite-like structure, as ultimately hard particles, into a film
to raise its strength or forming crystals to reduce remaining
silanol groups, thereby maintaining sufficient hydrophobicity. For
example, California University/USA has proposed a method for
forming a zeolite film (silica zeolite film having an MFI crystal
structure) on a semiconductor substrate by using a suspension
obtained by separating and removing particles of a relatively large
particle size from zeolite fine particles obtained by hydrolysis,
in the presence of tetrapropylammonium hydroxide (TPAOH), of
tetraethylorthosilicate (TEOS) dissolved in ethyl alcohol (refer
to, for example, US Patent Application Publication No. 2002/0060364
A1, Advanced Material, 13, No. 19, 1453-1466(2001)). Although the
zeolite film obtained by the above-described method has a Young's
modulus of from 16 to 18 GPa, it cannot be suited for practical use
because due to high hygroscopicity of the film, it absorbs
atmospheric moisture and drastically raises its dielectric constant
(for example, it increases from 2.3 to 3.9). There is therefore
proposed a method of keeping a dielectric constant of the film to
from 2.1 to 2.3 by silane treatment for making the film surface
hydrophobic.
[0011] There is also proposed a method for heightening the strength
by using zeolite particles/zeolite-like particles and an
alkoxysilane hydrolysate in combination (refer to, for example,
Japanese Patent Provisional Publication No. 2004-153147). In this
method, zeolite particles or zeolite-like particles are formed
first and they are mixed with the alkoxysilane hydrolysate,
optionally followed by a ripening reaction. The method for forming
crystalline zeolite thus requires such a complex operation.
[0012] A synthesis method of zeolite having a low impurity content
and suitable for use in semiconductor devices as described above is
very cumbersome. There are many attempts to obtain a
low-dielectric-constant film by using a silicon oxide-based polymer
which is advantageous to an industrial process application compared
to zeolite. For example, in Japanese Patent Provisional Publication
No. 2004-149714, recommended is a method for improving a pore
density of a film by using a large amount of tetrapropylammonium
hydroxide acting as a structure directing agent upon synthesis of
zeolite to partially form a zeolite-like structure, thereby forming
zeolite-like micropores in the film during film formation.
[0013] The film strength itself not only depends on the physical
properties of a material used for a film forming composition but
also depends on the behaviors of the material during film
formation. According to the report (for example, Japanese Patent
Provisional Publication No. 2005-216895) by the present inventors,
a high strength film can be formed by the steps of: modifying a
surface of a silica sol or zeolite particles with a crosslinking
group having a high crosslinkability between particles or between a
particle and a silicon-oxide-based resin to be added
simultaneously; temporarily losing the crosslinkability with a
protective means for preventing the crosslink formation or
deactivation of the crosslinking groups during stable storage; and
sintering after application for removing the protective means and
developing the high crosslinkability again.
SUMMARY OF THE INVENTION
[0014] A silica sol can be prepared far easier than zeolite so that
it is a preferable material for industrial uses. The conventional
silica sol particles cannot have enough pore density and it is not
suitable for a material for maintaining pores therein. The film
made of the conventional silica sol particles may have much
inferior mechanical strength to that of zeolite particles. If a
silica sol having high strength can be prepared, not only such a
silica sol is industrially advantageous but also the particles may
hold pore spaces therearound during the sintering process.
Preparation of a low-dielectric-constant film having a high
porosity in spite of having high strength can therefore be
expected.
[0015] An object of the invention is therefore to provide a silica
sol suitable for industrial use and capable of providing a porous
film excellent in mechanical strength, a film forming composition
containing the silica sol, a method for forming a porous film, and
a porous film formed thereby.
[0016] Another object of the present invention is to provide a
high-performance and high-reliability semiconductor device having a
porous film prepared using the above-described advantageous
material.
[0017] The present inventors made a working hypothesis in order to
improve the performance of a porous-film-forming coating solution
containing a silica.
[0018] Described specifically, when pores are introduced into a
film of a silica in order to reduce its dielectric constant, pore
does not contribute to mechanical strength of the resulting film.
Additionally, the pore-introduced film has much lower mechanical
strength than a pore-free material having the same composition due
to vulnerability of the surface of the pores. An improvement was
made as described in Japanese Patent Provisional Publication No.
2005-216895 based on a concept that silica particles are added as a
structure as in the case of zeolite particles. If the mechanical
strength of silica particles can be improved further, a film having
high strength as formed using zeolite may be available.
[0019] Based on the above-described working hypothesis, the present
inventors have carried out an extensive investigation. As a result,
they succeed in increasing the mechanical strength of a porous film
by using a silica sol prepared under the following specified
conditions for a conventionally-used composition for forming a
porous film containing silicon as a main component, thereby
imparting, to the porous film, mechanical strength resulting from
an increase in the strength of the silica sol. Moreover, they have
accomplished a preparation method of an additive capable of
improving the physical properties of the film even to a level
applicable to a semiconductor fabrication process, and completed
the invention.
[0020] Silica sol is a generic name of noncrystalline silicic acid
polymers. Silica sols in various forms or having various properties
are known and they are different each other depending on the
reaction degree of a hydrolyzable silane compound and water. A
silica sol having a lower condensation degree has higher
hydrophilicity and lower strength, while a silica sol having a
higher condensation degree has higher hydrophobicity and higher
strength. As a result of an extensive investigation on a method for
maximizing the condensation degree of a silica sol, the present
inventors have succeeded in obtaining a porous film having a high
mechanical strength, which strength may result from an increase in
the strength of a silica sol, by adding a silica sol prepared under
the following specified conditions to a porous-film forming
composition. It has also been found that the dielectric constant of
the porous film thus obtained is sufficient for application to
semiconductor fabrication, leading to the completion of the
invention.
[0021] In one aspect of the present invention, there is thus
provided a method for preparing a silica sol, which comprises
hydrolyzing and condensing a hydrolyzable silane compound in the
presence of at least one hydrophilic basic catalyst selected from
alkali metal hydroxides and quaternary ammonium hydroxides
represented by the following formula (1):
(R.sup.1).sub.4N.sup.+OH.sup.- (1)
(wherein, R.sup.1s may be the same or different and independently
represent a hydrocarbon group which may contain an oxygen atom and
the cationic portion [(R.sup.1).sub.4N.sup.+] satisfies the
following relationship (2):
(N+O)/(N+O+C).gtoreq.1/5 (2)
in which, N, O and C are the numbers of nitrogen, oxygen and carbon
atoms contained in the cationic portion, respectively) and at least
one hydrophobic basic catalyst selected from quaternary ammonium
hydroxides which do not satisfy the above-described relationship
(2). By using both a basic catalyst having high hydrophilicity and
a hydrophobic basic catalyst during preparation of a silica sol, a
silica sol capable of imparting high strength to a film can be
prepared.
[0022] The hydrophilic basic catalyst is selected preferably from
lithium hydroxide, sodium hydroxide, potassium hydroxide, cesium
hydroxide, tetramethylammonium hydroxide and choline.
[0023] The hydrophobic basic catalyst is selected preferably from
quaternary organic ammonium hydroxides represented by the following
formula (3):
(R.sup.2).sub.4N.sup.+OH.sup.- (3)
(wherein, R.sup.2s may be the same or different and each
independently represents a linear or branched C.sub.1-8 alkyl group
with the proviso that all the R.sup.2s do not represent a methyl
group simultaneously).
[0024] The hydrolyzable silane compound contains preferably at
least one silane compound selected from those represented by the
following formulas (4) and (5):
Si(OR.sup.3).sub.4 (4)
R.sup.4.sub.nSi(OR.sup.5).sub.4-n (5)
(wherein, R.sup.3s may be the same or different and each
independently represents a linear or branched C.sub.1-4 alkyl
group. R.sup.4(s) may be the same or different when there are
plural R.sup.4s and each independently represents a linear or
branched C.sub.1-4 alkyl group which may have a substituent,
R.sup.5(s) may be the same or different when there are plural
R.sup.5s and each independently represents a linear or branched
C.sub.1-4 alkyl group, and n is an integer from 1 to 3). Use of
these compounds as a silicon source facilitates obtaining a
material less contaminated with a metal or halogen.
[0025] The method for preparing a silica sol according to the
invention may further have, after the hydrolysis and condensation
reactions, a step of temporarily losing the crosslinkability on the
surface of the silica sol. Examples of the losing step include a
method of adding a carboxylic acid having at least two carboxyl
groups in one molecule thereof.
[0026] The silica sol prepared by the above-described method is
also one aspect of the invention. A porous film having high
strength as described later can be obtained by adding the silica
sol of the invention for a porous-film forming composition. This
effect on strength is particularly marked when the preparation
method has the above-described post-step further.
[0027] In a further aspect of the invention, there is also provided
a composition for forming a porous film containing a silica sol
prepared by either one of the above-described preparation method. A
porous film with high strength is available by using the
composition containing the silica sol according to the
invention.
[0028] In a further aspect of the present invention, there is also
provided a porous film formed using the composition. The porous
film of the invention can have higher strength than that of a
porous film formed using a silica sol prepared in a conventional
manner having the same dielectric constant attained by the
conventional silica sol.
[0029] The porous film of the invention is available by the
formation process having a step of applying the porous-film forming
composition to form a thin film and a step of sintering the thin
film. This formation process can be applied to the formation of an
interlayer insulating film for semiconductor fabrication. In other
words, an important user of the porous film of the invention is a
semiconductor device having the porous film as an interlayer
insulating film.
[0030] Although the silica sol of the invention is available
without cumbersome operations as used for preparing zeolite fine
particles, it has mechanical strength equal to that of zeolite and
in addition, can provide a low-dielectric-constant insulating film
having high performance.
BRIEF DESCRIPTION OF THE DRAWINGS
[0031] FIG. 1 is a schematic cross-sectional view illustrating one
example of a semiconductor device according to the present
invention;
[0032] FIG. 2 is a graph showing mechanical strength as a function
of dielectric constant; and
[0033] FIG. 3 is an X-ray diffraction chart of a film obtained
using a film forming composition of Comparative Example 3.
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT(S)
[0034] The present invention now will be described more fully
hereinafter in which embodiments of the invention are provided with
reference to the accompanying drawings. This invention may,
however, be embodied in many different forms and should not be
construed as limited to the embodiments set forth herein; rather,
these embodiments are provided so that this disclosure will be
thorough and complete, and will fully convey the scope of the
invention to those skilled in the art.
[0035] The terminology used in the description of the invention
herein is for the purpose of describing particular embodiments only
and is not intended to be limiting of the invention. As used in the
description of the invention and the appended claims, the singular
forms "a", "an" and "the" are intended to include the plural forms
as well, unless the context clearly indicates otherwise.
[0036] Unless otherwise defined, all technical and scientific terms
used herein have the same meaning as commonly understood by one of
ordinary skill in the art to which this invention belongs.
[0037] Hereinafter, preferred embodiments of the present invention
will be described. However, it is to be understood that the present
invention is not limited thereto.
[0038] The synthesis method of a silica sol having high strength,
which method is discovered by the present inventors, comprises
hydrolyzing and condensing a hydrolyzable silane compound in the
presence of at least one hydrophilic basic catalyst selected from
alkali metal hydroxides and quaternary ammonium hydroxides
represented by the following formula (1):
(R.sup.1).sub.4N.sup.+OH.sup.- (1)
(wherein R.sup.1s may be the same or different and each
independently represents a hydrocarbon group which may contain an
oxygen atom and the cationic portion [(R.sup.1).sub.4N.sup.+]
satisfies the following relationship (2):
(N+O)/(N+O+C).gtoreq.1/5 (2)
in which N, O and C are the numbers of nitrogen, oxygen and carbon
atoms contained in the cationic portion, respectively) and at least
one hydrophobic basic catalyst selected from quaternary ammonium
hydroxides which do not satisfy the above-described relationship
(2).
[0039] R.sup.1 is an organic group composed of carbon, hydrogen and
oxygen and examples of such a group include C.sub.1-20 alkyl groups
which may have a hydroxyl group or may have a --O--, --(C.dbd.O),
or --(C.dbd.O)O-- structure in the alkyl group.
[0040] The hydrophilic basic catalyst is an alkali metal hydroxide
or organic ammonium hydroxide having a low carbon ratio. Such a
basic catalyst does not function well for forming an association
state with silane having a silanol group but is active as a
condensation reaction catalyst. Preferred examples of the
hydrophilic basic catalyst include alkali metal hydroxides such as
lithium hydroxide, sodium hydroxide, potassium hydroxide and cesium
hydroxide, and quaternary ammonium salts such as
tetramethylammonium hydroxide and choline. Of these,
tetramethylammonium hydroxide and choline are especially preferred
because use of a catalyst having a small metal impurity content
enables elimination of demetallization treatment or reduction of a
burden due to such demetallization treatment after preparation.
[0041] Of the organic quaternary ammonium hydroxides as the
hydrophobic basic catalyst, those having a higher carbon ratio and
a higher capacity of forming an association state with silane
having a silanol group than the hydrophilic ones are preferred, of
which those represented by the following formula (3):
(R.sup.2).sub.4N.sup.+OH.sup.- (3)
(wherein, R.sup.2s may be the same or different and each
independently represents a linear or branched C.sub.1-8 alkyl group
with the proviso that all the R.sup.2s do not represent a methyl
group simultaneously) are selected as more preferred ones. It is
very important to select a proper hydrophobic base catalyst.
Catalysts showing good dispersibility in an aqueous solution and
having low micelle forming properties though being hydrophobic are
preferred, of which those having no micelle forming properties are
more preferred. Specific examples include the following ammonium
salts such as:
[0042] ethyltrimethylammonium hydroxide, propyltrimethylammonium
hydroxide, butyltrimethylammonium hydroxide,
pentyltrimethylammonium hydroxide, hexyltrimethylammonium
hydroxide, heptyltrimethylammonium hydroxide,
octyltrimethylammonium hydroxide, diethyldimethylammonium
hydroxide, dipropyldimethylammonium hydroxide,
dibutyldimethylammonium hydroxide, dipentyldimethylammonium
hydroxide, dihexyldimethylammonium hydroxide,
diheptyldimethylammonium hydroxide, dioctyldimethylammonium
hydroxide, triethylmethylammonium hydroxide,
tripropylmethylammonium hydroxide, tributylmethylammonium
hydroxide, tripentylmethylammonium hydroxide,
trihexylmethylammonium hydroxide, triheptylmethylammonium
hydroxide, trioctylmethylammonium hydroxide, tetraethylammonium
hydroxide, propyltriethylammonium hydroxide, butyltriethylammonium
hydroxide, pentyltriethylammonium hydroxide, hexyltriethylammonium
hydroxide, heptyltriethylammonium hydroxide, octyltriethylammonium
hydroxide, dipropyldiethylammonium hydroxide,
dibutyldiethylammonium hydroxide, dipentyldiethylammonium
hydroxide, dihexyldiethylammonium hydroxide,
diheptyldiethylammonium hydroxide, dioctyldiethylammonium
hydroxide, tripropylethylammonium hydroxide, tributylethylammonium
hydroxide, tripentylethylammonium hydroxide, trihexylethylammonium
hydroxide, triheptylethylammonium hydroxide, trioctylethylammonium
hydroxide, tetrapropylammonium hydroxide, butyltripropylammonium
hydroxide, pentyltripropylammonium hydroxide,
hexyltripropylammonium hydroxide, heptyltripropylammonium
hydroxide, octyltripropylammonium hydroxide,
dibutyldipropylammonium hydroxide, dipentyldipropylammonium
hydroxide, dihexyldipropylammonium hydroxide,
diheptyldipropylammonium hydroxide, dioctyldipropylammonium
hydroxide, tributylpropylammonium hydroxide,
tripentylpropylammonium hydroxide, trihexylpropylammonium
hydroxide, triheptylpropylammonium hydroxide,
trioctylpropylammonium hydroxide, tetrabutylammonium hydroxide,
pentyltributylammonium hydroxide, hexyltributylammonium hydroxide,
heptyltributylammonium hydroxide, octyltributylammonium hydroxide,
dipentyldibutylammonium hydroxide, dihexyldibutylammonium
hydroxide, diheptyldibutylammonium hydroxide,
dioctyldibutylammonium hydroxide, tripentylbutylammonium hydroxide,
trihexylbutylammonium hydroxide, triheptylbutylammonium hydroxide,
trioctylbutylammonium hydroxide, tetrapentylammonium hydroxide,
hexyltripentylammonium hydroxide, heptyltripentylammonium
hydroxide, octyltripentylammonium hydroxide,
dihexyldipentylammonium hydroxide, diheptyldipentylammonium
hydroxide, dioctyldipentylammonium hydroxide,
trihexylpentylammonium hydroxide, triheptylpentylammonium
hydroxide, trioctylpentylammonium hydroxide, tetrahexylammonium
hydroxide, heptyltrihexylammonium hydroxide, octyltrihexylammonium
hydroxide, diheptyldihexylammonium hydroxide,
dioctyldihexylammonium hydroxide, triheptylhexylammonium hydroxide,
trioctylhexylammonium hydroxide, tetraheptylammonium hydroxide,
octyltriheptylammonium hydroxide, dioctyldiheptylammonium
hydroxide, trioctylheptylammonium hydroxide and tetraoctylammonium
hydroxide.
[0043] The amount of the basic catalyst, as the total amount of the
hydrophobic basic catalyst and the hydrophilic basic catalyst, is
from 1 to 50 mole %, preferably from 5 to 30 mole %, more
preferably from 10 to 20 mole % per mole of the total amount of the
hydrolyzable silane compound which will be described later. Amounts
of the catalyst exceeding the above-described range may hinder the
sufficient growth of silica sol particles, making it difficult to
prepare a low-k film. Amounts of the catalyst below the
above-described range, on the other hand, do not cause condensation
of siloxane sufficiently, making it impossible to obtain a film
having intended strength. With regard to a mixing ratio of the
hydrophobic catalyst and the hydrophilic catalyst, it is desired to
add from 0.2 to 2.0 moles of the hydrophilic basic catalyst to 1
mole of the hydrophobic basic catalyst.
[0044] As described later, a porous film obtained from a film
forming composition containing the silica sol prepared in the
presence of such a combination of the basic catalysts has higher
strength than a film prepared by a conventional synthesis method
not depending on the above-described combination.
[0045] The present inventors presume that this high mechanical
strength of the film is attributable to the strength of the silica
sol itself. The reason for the high strength of the silica sol of
the invention obtained by the preparation method of the invention
may be as follows, which does not limit the technical scope of the
invention.
[0046] When the hydrophobic basic catalyst to be used in the
invention is used alone as in the conventional manner, an
association state may be formed with an alkoxysilane, which is also
publicly known as a structure directing agent for determining the
crystal type of zeolite, or silanol due to high affinity therewith
(so-called hydrophobic interaction). However, the catalyst may have
poor affinity with water and low reactivity with water molecules
due to its hydrophobicity so that a hydrolysis reaction or a
dehydration condensation reaction may occur only slowly and a
sufficient condensation reaction may not proceed.
[0047] The hydrophilic basic catalyst may be highly effective for
promoting a hydrolysis reaction or dehydration condensation
reaction, but may not have a sufficient capacity of forming an
association state with a silane source so that the condensation
occurs speedily but at random and a portion having an internal
strain appears. As a result, sufficient bonds may not be formed in
some portions.
[0048] When the hydrophobic basic catalyst and the hydrophilic
basic catalyst are used in combination, on the other hand, an
association state may be formed between the hydrophobic basic
catalyst and alkoxysilane by the hydrophobic interaction and such
association state may be maintained by the static interaction
between a silanol (silicate) and ammonium cation even after partial
progress of hydrolysis of the alkoxysilane into silanol. The
hydrophilic basic catalyst may then act on the association to
promote the condensation reaction of the silanol. As the reaction
may proceed sufficiently, a siloxane backbone having a high spatial
crosslinking proportion may be formed at the associated site. The
hydrophobic basic catalyst may act to form another association
state between the alkoxysilane and silica surface. The hydrophilic
basic catalyst then may promote the condensation. The growth of a
silica sol may proceed by the repetition of such reactions. In a
film obtained from the silica sol prepared by this method, no or
almost no micropores may be observed, suggesting that the silica
sol may not be of a type partially having a zeolite-like crystal
structure. Due to the combined use with the hydrophilic basic
catalyst, the growth of the silica sol may proceed according to the
above-described mechanism in which a large amount of the
hydrophobic basic catalyst does not remain in the silica sol. Such
mechanism may enable to form amorphous silica with less internal
strains and a high crosslinking ratio instead of forming a crystal
such as a zeolite structure. Moreover, the silica gel whose
condensation reaction may proceed while sufficiently relaxing the
internal strain during condensation does not have therein many
remaining silanol groups and therefore may be rigid and may have
high hydrophobicity. When a low dielectric constant film is formed
as described later, the film may therefore have both high strength
and a stable low dielectric constant.
[0049] As the silicon source to be used for the preparation of the
silica sol of the invention, silicon sources similar to those used
conventionally for the preparation of a silica sol are principally
usable. The preferably hydrolyzable silane compounds are
represented by the following formula (4) or (5):
Si(OR.sup.3).sub.4 (4)
R.sup.4.sub.nSi(OR.sup.5).sub.4-n (5)
(wherein, R.sup.3s may be the same or different and each
independently represents a linear or branched C.sub.1-4 alkyl
group, R.sup.4(s) may be the same or different when there are
plural R.sup.4s and each independently represents a linear or
branched C.sub.1-4 alkyl group which may have a substituent,
R.sup.5(s) may be the same or different when there are plural
R.sup.5s and each independently represents a linear or branched
C.sub.1-4 alkyl group, and n is an integer from 1 to 3).
[0050] Examples of the preferably-used silane compounds represented
by the formula (4) include, but not limited to, tetramethoxysilane,
tetraethoxysilane, tetrapropoxysilane, tetrabutoxysilane,
tetraisopropoxysilane, tetraisobutoxysilane,
triethoxymethoxysilane, tripropoxymethoxysilane,
tributoxymethoxysilane, trimethoxyethoxysilane,
trimethoxypropoxysilane and trimethoxybutoxysilane.
[0051] Examples of the silane compounds represented by the formula
(5) include methyltrimethoxysilane, methyltriethoxysilane,
methyltri-n-propoxysilane, methyltri-i-propoxysilane,
methyltri-n-butoxysilane, methyltri-s-butoxysilane,
methyltri-i-butoxysilane, methyltri-t-butoxysilane,
ethyltrimethoxysilane, ethyltriethoxysilane,
ethyltri-n-propoxysilane, ethyltri-i-propoxysilane,
ethyltri-n-butoxysilane, ethyltri-s-butoxysilane,
ethyltri-i-butoxysilane, ethyltri-t-butoxysilane,
n-propyltrimethoxysilane, n-propyltriethoxysilane,
n-propyltri-n-propoxysilane, n-propyltri-i-propoxysilane,
n-propyltri-n-butoxysilane, n-propyltri-s-butoxysilane,
n-propyltri-i-butoxysilane, n-propyltri-t-butoxysilane,
i-propyltrimethoxysilane, i-propyltriethoxysilane,
i-propyltri-n-propoxysilane, i-propyltri-i-propoxysilane,
i-propyltri-n-butoxysilane, i-propyltri-s-butoxysilane,
i-propyltri-i-butoxysilane, i-propyltri-t-butoxysilane,
n-butyltrimethoxysilane, n-butyltriethoxysilane,
n-butyltri-n-propoxysilane, n-butyltri-i-propoxysilane,
n-butyltri-n-butoxysilane, n-butyltri-s-butoxysilane,
n-butyltri-i-butoxysilane, n-butyltri-t-butoxysilane,
i-butyltrimethoxysilane, i-butyltriethoxysilane,
i-butyltri-n-propoxysilane, i-butyltri-i-propoxysilane,
i-butyltri-n-butoxysilane, i-butyltri-s-butoxysilane,
i-butyltri-i-butoxysilane, i-butyltri-t-butoxysilane,
s-butyltrimethoxysilane, s-butyltriethoxysilane,
s-butyltri-n-propoxysilane, s-butyltri-i-propoxysilane,
s-butyltri-n-butoxysilane, s-butyltri-s-butoxysilane,
s-butyltri-i-butoxysilane, s-butyltri-t-butoxysilane,
t-butyltrimethoxysilane, t-butyltriethoxysilane,
t-butyltri-n-propoxysilane, t-butyltri-i-propoxysilane,
t-butyltri-n-butoxysilane, t-butyltri-s-butoxysilane,
t-butyltri-i-butoxysilane, t-butyltri-t-butoxysilane,
dimethyldimethoxysilane, dimethyldiethoxysilane,
dimethyldi-n-propoxylsilane, dimethyldi-i-propoxysilane,
dimethyldi-n-butoxysilane, dimethyldi-s-butoxysilane,
dimethyldi-i-butoxysilane, dimethyldi-t-butoxysilane,
diethyldimethoxysilane, diethyldiethoxysilane,
diethyldi-n-propoxylsilane, diethyldi-i-propoxysilane,
diethyldi-n-butoxysilane, diethyldi-s-butoxysilane,
diethyldi-i-butoxysilane, diethyldi-t-butoxysilane,
di-n-propyldimethoxysilane, di-n-propyldiethoxysilane,
di-n-propyldi-n-propoxylsilane, di-n-propyldi-i-propoxysilane,
di-n-propyldi-n-butoxysilane, di-n-propyldi-s-butoxysilane,
di-n-propyldi-i-butoxysilane, di-n-propyldi-t-butoxysilane,
di-i-propyldimethoxysilane, di-i-propyldiethoxysilane,
di-i-propyldi-n-propoxylsilane, di-i-propyldi-i-propoxysilane,
di-i-propyldi-n-butoxysilane, di-i-propyldi-s-butoxysilane,
di-i-propyldi-i-butoxysilane, di-i-propyldi-t-butoxysilane,
di-n-butyldimethoxysilane, di-n-butyldiethoxysilane,
di-n-butyldi-n-propoxylsilane, di-n-butyldi-i-propoxysilane,
di-n-butyldi-n-butoxysilane, di-n-butyldi-s-butoxysilane,
di-n-butyldi-i-butoxysilane, di-n-butyldi-t-butoxysilane,
di-i-butyldimethoxysilane, di-i-butyldiethoxysilane,
di-i-butyldi-n-propoxylsilane, di-i-butyldi-i-propoxysilane,
di-i-butyldi-n-butoxysilane, di-i-butyldi-s-butoxysilane,
di-i-butyldi-i-butoxysilane, di-i-butyldi-t-butoxysilane,
di-s-butyldimethoxysilane, di-s-butyldiethoxysilane,
di-s-butyldi-n-propoxylsilane, di-s-butyldi-i-propoxysilane,
di-s-butyldi-n-butoxysilane, di-s-butyldi-s-butoxysilane,
di-s-butyldi-i-butoxysilane, di-s-butyldi-t-butoxysilane,
di-t-butyldimethoxysilane, di-t-butyldiethoxysilane,
di-t-butyldi-n-propoxylsilane, di-t-butyldi-i-propoxysilane,
di-t-butyldi-n-butoxysilane, di-t-butyldi-s-butoxysilane,
di-t-butyldi-i-butoxysilane, di-t-butyldi-t-butoxysilane,
trimethylmethoxysilane, trimethylethoxysilane,
trimethyl-n-propoxysilane, trimethyl-i-propoxysilane,
trimethyl-n-butoxysilane, trimethyl-s-butoxysilane,
trimethyl-i-butoxysilane, trimethyl-t-butoxysilane,
triethylmethoxysilane, triethylethoxysilane,
triethyl-n-propoxylsilane, triethyl-i-propoxysilane,
triethyl-n-butoxysilane, triethyl-s-butoxysilane,
triethyl-i-butoxysilane, triethyl-t-butoxysilane,
tri-n-propylmethoxysilane, tri-n-propylethoxysilane,
tri-n-propyl-n-propoxysilane, tri-n-propyl-i-propoxysilane,
tri-n-propyl-n-butoxysilane, tri-n-propyl-s-butoxysilane,
tri-n-propyl-i-butoxysilane, tri-n-propyl-t-butoxysilane,
tri-i-propylmethoxysilane, tri-i-propylethoxysilane,
tri-i-propyl-n-propoxylsilane, tri-i-propyl-i-propoxysilane,
tri-i-propyl-n-butoxysilane, tri-i-propyl-s-butoxysilane,
tri-i-propyl-i-butoxysilane, tri-i-propyl-t-butoxysilane,
tri-n-butylmethoxysilane, tri-n-butylethoxysilane,
tri-n-butyl-n-propoxylsilane, tri-n-butyl-i-propoxysilane,
tri-n-butyl-n-butoxysilane, tri-n-butyl-s-butoxysilane,
tri-n-butyl-i-butoxysilane, tri-n-butyl-t-butoxysilane,
tri-i-butylmethoxysilane, tri-i-butylethoxysilane,
tri-i-butyl-n-propoxylsilane, tri-i-butyl-i-propoxysilane,
tri-i-butyl-n-butoxysilane, tri-i-butyl-s-butoxysilane,
tri-i-butyl-i-butoxysilane, tri-i-butyl-t-butoxysilane,
tri-s-butylmethoxysilane, tri-s-butylethoxysilane,
tri-s-butyl-n-propoxylsilane, tris-butyl-i-propoxysilane,
tri-s-butyl-n-butoxysilane, tri-s-butyl-s-butoxysilane,
tri-s-butyl-i-butoxysilane, tri-s-butyl-t-butoxysilane,
tri-t-butylmethoxysilane, tri-t-butylethoxysilane,
tri-t-butyl-n-propoxylsilane, tri-t-butyl-i-propoxysilane,
tri-t-butyl-n-butoxysilane, tri-t-butyl-s-butoxysilane,
tri-t-butyl-i-butoxysilane, and tri-t-butyl-t-butoxysilane.
[0052] According to the method of the invention, the silane
compounds may be used either singly or in combination. Use of at
least one of the compounds represented by the formula (4) and at
least one of the compounds represented by the formula (5) in
combination is especially preferred. A ratio of the compound of the
formula (4) to the compound of the formula (5) is preferably from
5:95 to 95:5, more preferably from 25:75 to 75:25. A hydrolyzable
silane compound other than the above-described silane compound may
be added.
[0053] In the reaction, a silane compound other than the
above-described trivalent or tetravalent hydrolyzable silane
compounds having a single silicon atom may be added. Examples of
such a silane compound include, but not limited to, divalent
hydrolyzable silane compounds such as dimethyldimethoxysilane and
dimethyldiethoxysilane and hydrolyzable silane compounds having
plural silicon atoms such as hexamethoxydisiloxane,
methylenebistrimethoxysilane, methylenebistriethoxysilane,
1,3-propylenebistrimethoxysilane, 1,4-(butylene)bistrimethoxysilane
and 1,4-phenylenebistrimethoxysilane. It is to be noted that the
amount of such a silane compound is preferably 30 mole % or less in
terms of silicon.
[0054] Water for hydrolysis to be added to the reaction system is
from 0.5 to 100 times the mole, more preferably from 1 to 10 times
the mole of the moles necessary for completely hydrolyzing the
silane compound.
[0055] When a polymer solution is prepared by hydrolyzing and
condensing the hydrolyzable silane compound, it may contain, in
addition to water, a solvent such as alcohol corresponding to the
alkoxy group of the silane compound. Examples include methanol,
ethanol, isopropyl alcohol, butanol, propylene glycol monomethyl
ether, propylene glycol monopropyl ether, propylene glycol
monopropyl ether acetate, ethyl lactate and cyclohexanone.
[0056] The amount of the solvent other than water is preferably
from 0.1 to 500 times the mass, more preferably from 1 to 100 times
the mass of the silane compound.
[0057] The hydrolysis and condensation reactions of the silane
compound are performed under ordinarily employed conditions. The
reaction temperature typically ranges from 0.degree. C. to the
boiling point of an alcohol generated by the hydrolysis and
condensation reactions, preferably from room temperature to
80.degree. C.
[0058] A more convenient reaction process for the formation and
growth of a silica sol is dropwise addition of the hydrolyzable
silane compound, directly or after dissolving it in the
above-described solvent, to an aqueous solution of the hydrophobic
basic catalyst and hydrophilic basic catalyst adjusted to the
reaction temperature or sometimes in a reaction solution obtained
by adding the above-described organic solvent to the aqueous
solution. The dropwise addition is performed typically from 10
minutes to 24 hours, more preferably for from 30 minutes to about 8
hours. The time varies, depending on a reaction apparatus and scale
of the reaction.
[0059] Since the reactions proceed speedily after completion of the
dropwise addition, it is not necessary to have a long ripening time
after completion of the dropwise addition. A long ripening time
however does not cause any marked deterioration. A film obtained by
ripening for 4 hours or longer after completion of the dropwise
addition and then temporarily losing the neutralization reaction
however tends to have decreased strength. The film tends to have
higher strength when neutralization reaction is terminated within 1
hour after completion of the dropwise addition.
[0060] By the above-described reactions of the invention, a silica
sol strong enough to provide a porous film having high strength and
a low dielectric constant as will be described later can be
obtained. The silica sol can be made more desirable by adding a
step of temporarily losing the crosslinkability of a silanol after
the hydrolysis and condensation reactions as disclosed in Japanese
Patent Provisional Publication No. 2004-149714.
[0061] Described specifically, aggregation of silica particles in
the reaction mixture is suppressed by the presence of the
hydrophobic basic catalyst so that gelation hardly occurs in spite
of a high-temperature reaction for long hours. Inactivation of the
catalyst in the post-treatment step however facilitates
association/condensation of silica particles, leading to
considerable deterioration of storage stability. When the material
having deteriorated stability is used for a film forming
composition solution as is, it seems to lose the crosslinkability
before film formation, though the reason for it is not clear, and
the film thus obtained cannot have sufficient strength. It is
therefore preferred to introduce a step of protecting a surface
active silanol as soon as the completion of the condensation
reaction by the basic catalyst. Described specifically, the active
silanol is protected by adding a divalent or polyvalent carboxylic
acid compound after the neutralization reaction of the basic
catalyst but before the disappearance of the crosslinkability, more
preferably immediately after the neutralization reaction or by
carrying out the neutralization reaction itself with a divalent or
polyvalent carboxylic acid, thereby carrying out neutralization and
silanol protection simultaneously, whereby the crosslinkability can
be lost until the decomposition of the carboxylic acid compound at
the time of film formation.
[0062] Preferred examples of the carboxylic acid having, in the
molecule thereof, at least two carboxyl groups include oxalic acid,
malonic acid, malonic anhydride, maleic acid, maleic anhydride,
fumaric acid, glutaric acid, glutaric anhydride, citraconic acid,
citraconic anhydride, itaconic acid, itaconic anhydride and adipic
acid. Such a carboxylic acid acts effectively when added in an
amount ranging from 0.05 mole % to 10 mole %, preferably from 0.5
mole % to 5 mole % based on the silicon unit.
[0063] Preparation of a film forming composition using the silica
sol of the invention is performed in accordance with a method for
preparing a film forming composition containing a conventional
silica sol.
[0064] When the film forming composition is used as a material for
a semiconductor insulating film which will be described later and
an alkali metal hydroxide is used as the hydrophilic basic
catalyst, demetallization treatment must be performed at any stage
during from above-described termination of the reaction to
preparation of a coating composition solution. Many examples of
demetallization treatment have already been proposed, but metals
are typically removed by a method using an ion exchange resin or
washing of the organic solvent solution with water. When a silica
sol is prepared in the presence of a combination of only ammonium
catalysts not containing metal impurities during reaction, such
demetallization treatment is not necessary, but is usually added
similarly.
[0065] A solvent such as water used for preparing the
silica-sol-containing composition is typically exchanged with a
coating solvent which will be described later. There are many known
examples of it, but even if the silica sol of the invention is
subjected to the above-described stabilizing treatment, an
operation for isolating it by completely removing the solvent is
not preferred.
[0066] A number of solvents to be used for preparing a solution of
a film forming coating composition are known. Similar solvents are
usable for the film forming composition of the invention. Specific
examples include aliphatic hydrocarbon solvents such as n-pentane,
isopentane, n-hexane, isohexane, n-heptane, 2,2,2-trimethylpentane,
n-octane, isooctane, cyclohexane and methylcyclohexane; aromatic
hydrocarbon solvents such as benzene, toluene, xylene,
ethylbenzene, trimethylbenzene, methylethylbenzene,
n-propylbenzene, isopropylbenzene, diethylbenzene, isobutylbenzene,
triethylbenzene, diisopropylbenzene and n-amylnaphthalene; ketone
solvents such as acetone, methyl ethyl ketone, methyl n-propyl
ketone, methyl n-butyl ketone, methyl isobutyl ketone,
cyclohexanone, 2-hexanone, methylcyclohexanone, 2,4-pentanedione,
acetonylacetone, diacetone alcohol, acetophenone, and fenthion;
ether solvents such as ethyl ether, isopropyl ether, n-butyl ether,
n-hexyl ether, 2-ethylhexyl ether, dioxolane, 4-methyldioxolane,
dioxane, dimethyldioxane, ethylene glycol mono-n-butyl ether,
ethylene glycol mono-n-hexyl ether, ethylene glycol monophenyl
ether, ethylene glycol mono-2-ethylbutyl ether, ethylene glycol
dibutyl ether, diethylene glycol monomethyl ether, diethylene
glycol dimethyl ether, diethylene glycol monoethyl ether,
diethylene glycol diethyl ether, diethylene glycol monopropyl
ether, diethylene glycol dipropyl ether, diethylene glycol
monobutyl ether, diethylene glycol dibutyl ether, tetrahydrofuran,
2-methyltetrahydrofuran, propylene glycol monomethyl ether,
propylene glycol dimethyl ether, propylene glycol monoethyl ether,
propylene glycol diethyl ether, propylene glycol monopropyl ether,
propylene glycol dipropyl ether, propylene glycol monobutyl ether,
dipropylene glycol dimethyl ether, dipropylene glycol diethyl
ether, dipropylene glycol dipropyl ether and dipropylene glycol
dibutyl ether, ester solvents such as diethyl carbonate, ethyl
acetate, .gamma.-butyrolactone, .gamma.-valerolactone, n-propyl
acetate, isopropyl acetate, n-butyl acetate, isobutyl acetate,
sec-butyl acetate, n-pentyl acetate, 3-methoxybutyl acetate,
methylpentyl acetate, 2-ethylbutyl acetate, 2-ethylhexyl acetate,
benzyl acetate, cyclohexyl acetate, methylcyclohexyl acetate,
n-nonyl acetate, methyl acetoacetate, ethyl acetoacetate, ethylene
glycol monomethyl ether acetate, ethylene glycol monoethyl ether
acetate, diethylene glycol monomethyl ether acetate, diethylene
glycol monoethyl ether acetate, diethylene glycol mono-n-butyl
ether acetate, propylene glycol monomethyl ether acetate, propylene
glycol monoethyl ether acetate, dipropylene glycol monomethyl ether
acetate, dipropylene glycol monoethyl ether acetate, dipropylene
glycol mono-n-butyl ether acetate, glycol diacetate,
methoxytriglycol acetate, ethyl propionate, n-butyl propionate,
isoamyl propionate, diethyl oxalate, di-n-butyl oxalate, methyl
lactate, ethyl lactate, n-butyl lactate, n-amyl lactate, diethyl
malonate, dimethyl phthalate and diethyl phthalate;
nitrogen-containing solvents such as N-methylformamide,
N,N-dimethylformamide, acetamide, N-methylacetamide,
N,N-dimethylacetamide, N-methylpropionamide, and
N-methylpyrrolidone, and sulfur-containing solvents such as
dimethyl sulfide, diethyl sulfide, thiophene, tetrahydrothiophene,
dimethyl sulfoxide, sulfolane, and 1,3-propanesultone.
[0067] These solvents may be used either singly or in
combination.
[0068] By dilution, the composition is provided as a final product
for obtaining a target film. The degree of dilution differs,
depending on the viscosity or intended film thickness, but the
composition is diluted while adjusting the amount of the solvent
typically from 50 to 99 mass %, more preferably from 75 to 98 mass
%.
[0069] As another material to be added to the film forming
composition, a number of film forming aids including surfactants
are known and basically, any of them can be added to the film
forming composition of the invention. As the film forming aid,
surfactants, silane coupling agents and radical generators
described in, for example, Japanese Patent Provisional Publication
No. 2001-354904 can be used.
[0070] A proportion of the film forming aid, if it is added, in the
total solid content of the film forming composition of the
invention is from 0.001 to 10 mass % in terms of a solid
content.
[0071] As a silicon-based polymer component, a polysiloxane
prepared by a method other than that described herein can be
incorporated in the film forming composition of the invention, but
a proportion of such a polysiloxane in the total solid content must
be adjusted to 59 mass % or less, preferably 20 mass % or less in
order to fulfill the advantage of the invention.
[0072] As the polysiloxane prepared by a method other than that
described herein and miscible in the silica-sol-containing film
forming composition of the invention, following ones are preferred
additives because they are not only useful as a binder or film
forming aid but also can improve the binding force between silica
sols, thereby improving the mechanical strength of the film without
impairing the dielectric constant which the film is expected to
have.
[0073] Polysiloxane compounds having the above-described function
and preferred as an additive contain a high concentration of
silanol groups and are synthesized in the following manner.
[0074] A starting material is a mixture of a hydrolyzable silane
compound containing at least one tetrafunctional alkoxysilane
compound represented by the following formula (6):
Si(OR.sup.6).sub.4 (6)
(wherein, R.sup.6s may be the same or different and each
independently represents a linear or branched C.sub.1-4 alkyl
group) and/or at least one alkoxysilane compound represented by the
following formula (7):
R.sup.7.sub.nSi(OR.sup.8).sub.4-n (7)
(wherein, R.sup.8(s) may be the same or different when there are
plural R.sup.8s and each independently represents a linear or
branched C.sub.1-4 alkyl group, R.sup.7(s) may be the same or
different when there are plural R.sup.7s and each independently
represents a linear or branched C.sub.1-4 alkyl group which may
have a substituent, and n is an integer from 1 to 3).
[0075] A proportion of the compound of the formula (6) is, in terms
of silicon atoms, preferably 25 mole % or greater but not greater
than 100 mole % based on the total moles of the entire hydrolyzable
silane compounds, that is, the compounds (6) and (7).
[0076] Preferred examples of R.sup.7 of the silane compound (7)
include alkyl groups such as methyl, ethyl, n-propyl, iso-propyl,
n-butyl, iso-butyl, sec-butyl, t-butyl, n-pentyl, 2-ethylbutyl,
3-ethylbutyl, 2,2-diethylpropyl, cyclopentyl, n-hexyl and
cyclohexyl, alkenyl groups such as vinyl and allyl, alkynyl groups
such as ethynyl, aryl groups such as phenyl and tolyl, aralkyl
groups such as benzyl and phenethyl, and other unsubstituted
monovalent hydrocarbon groups. They may each have a substituent
such as fluorine. Of these, methyl, ethyl, n-propyl, iso-propyl,
vinyl and phenyl groups are especially preferred.
[0077] As R.sup.6 and R.sup.8, those providing an alcohol, which
appears as a by-product after hydrolysis, having a boiling point
lower than that of water are preferred. Examples include methyl,
ethyl, n-propyl and iso-propyl.
[0078] The polysiloxane compound can be obtained by hydrolyzing and
condensing such a silane compound in the presence of an acid
catalyst. In order to obtain a polysiloxane compound capable of
heightening a binding force between silica sols, however, it is
preferred to carry out hydrolysis and condensation reactions in the
presence of an acid catalyst not in a conventional manner but under
such conditions as to hydrate silanol generated during hydrolysis
and thereby prevent gelation.
[0079] A method of obtaining a siloxane compound by hydrolysis and
condensation reactions of a hydrolyzable silane compound in the
presence of an acid catalyst is performed under reaction control.
The reaction control is necessary because in the hydrolysis and
condensation reactions of a hydrolyzable silane compound in the
presence of an acid catalyst, a hydrolysis speed is higher than a
condensation speed so that when a trivalent or tetravalent
hydrolyzable silane compound is used as a raw material, the
concentration of active silanol groups in the reaction mixture
becomes too high without any reaction control and a large amount of
an intermediate having many active reaction active sites is formed,
which may cause gelation. For the reaction control to prevent
gelation, either a method of controlling generation of silanol
groups or a method of directly controlling gelation reaction of
silanol groups generated by hydrolysis is used. These two
controlling methods differ in an addition manner of the
hydrolyzable silane compound and an amount of water added for
hydrolysis.
[0080] Of these two methods, the method of controlling generation
of silanol groups is more typical. In condensation in the presence
of an acid catalyst under ordinary conditions, water is added
dropwise to the reaction mixture containing a hydrolyzable silane
compound. This makes it possible to provide a sufficient time for
silanol groups generated by hydrolysis to be consumed for
condensation, control a rise in the concentration of the silanol
groups and thereby prevent gelation. In addition, gelation is
prevented by using a larger amount of an organic solvent having a
relatively low polarity while decreasing the total amount of water,
thereby avoiding contact between water and the hydrolyzable silane
compound as much as possible and condensing the silanol groups
while storing the alkoxy groups without causing an abrupt increase
in the concentration of the silanol groups. In the particular case
where no organic solvent is used, an amount of water must be
adjusted to 1 mole or less per mole of the hydrolyzable group in
the hydrolyzable silane compound. Even in the typical case where an
organic solvent is used, an amount of water is often adjusted
similarly to 1 mole or less per mole of the hydrolyzable group in
the hydrolyzable silane compound. Apart from actual use, an upper
limit of the amount of water is at most three times or five times
larger than the amount necessary for hydrolysis in a patent
literature which has a large margin. When the amount of water
exceeded 1 mole per 1 mole of a hydrolyzable group in the actual
use as described above, there is a risk of gelation. When water is
added in an amount of two times the amount necessary for hydrolysis
of all hydrolyzable groups, a polysiloxane compound cannot be taken
out from the reaction mixture due to gelation thereof. In addition,
the polysiloxane compound is synthesized while suppressing an
increase in the concentration of silanol groups so that its content
is low. For example, preparation of a polysiloxane compound in an
amount of 5 mole % or greater, in terms of entire silicon atoms,
usually leads to gelation.
[0081] The method of directly controlling a gelation reaction is on
the other hand characterized by the use of a large excess of water.
Active silanol groups are hydrated with a large excess of water,
whereby the gelation reaction is controlled. More preferably,
hydrolysis is performed using a large excess of water instead of
using a large amount of an organic solvent which disturbs
hydration. In the ordinary reaction operation, the hydrolyzable
silane compound is charged in a reaction mixture of hydrolysis
which constantly contains water in an amount exceeding the molar
equivalent of the hydrolyzable groups already charged. It is more
common to charge a large excess of water and an acid catalyst in a
reaction tank in advance and add the hydrolyzable silane compound
dropwise thereto. Such a design enables prompt hydration of silanol
groups generated by the hydrolysis. Although a large amount of
silanol groups is generated in the reaction mixture, sufficient
hydration always occurs due to existence of a large amount of water
and as a result of control of the activity of the silanol groups by
hydration, gelation is prevented. Moreover, a polysiloxane compound
available by this method is known to have, in the molecule thereof,
a high content of silanol groups.
[0082] In the above-described method, an amount of water used for
hydrolysis of the monomer must be, at the same time, sufficient for
hydrating the silanol groups generated in the reaction system. It
is preferred to add the water in an amount of 3 moles or greater,
preferably 5 moles or greater, per mole of the hydrolyzable group
contained in the monomer. Gelation can usually be prevented by the
addition of water in an amount greater than 5 moles. Described
specifically, assuming that the lower limit of the preferred amount
of water is 5 moles as described above and the upper limit is 100
moles as described later, each per mole of the hydrolyzable group
contained in the monomer, when a polysiloxane compound is prepared
from the tetravalent hydrolyzable silane compound of the formula
(6) and the trivalent compound, among the compounds represented by
the formula (7), the following relationship holds:
100.times.(4.times.Q+3.times.T).gtoreq.X.gtoreq.5.times.(4.times.Q+3.tim-
es.T)
(wherein Q represents the mole of the compound of the formula (6),
T represents the mole of the compound of the formula (7), and X
represents the mole of water). By carrying out hydrolysis and
condensation reactions in the presence of an acid catalyst while
using such a large amount of water, a polysiloxane compound having
a high silanol content is available without causing gelation.
Addition of water in an amount exceeding 100 moles may be
uneconomical because it only enlarges an apparatus used for
reactions, though depending on the amount, and raises a cost for
drainage treatment.
[0083] As the acid catalyst, any known ones are basically usable by
properly adjusting the reaction conditions. Use of a catalyst
selected from organic sulfonic acids which are said to be strongly
acidic among organic acids, and inorganic acids which are said to
be more strongly acidic is preferred to allow hydrolysis and
condensation reactions to proceed completely. Examples of the
inorganic acids include hydrochloric acid, sulfuric acid, nitric
acid, and perchloric acid, while those of the organic sulfonic
acids include methanesulfonic acid, tosic acid and
trifluoromethanesulfonic acid. The amount of the strong acid used
as the catalyst is from 10.sup.-6 moles to 1 mole, preferably
10.sup.-5 to 0.5 mole, more preferably 10.sup.-4 to 0.3 mole per
mole of the silicon-containing monomer.
[0084] A divalent organic acid may be added further in order to
heighten the stability of the polysiloxane derivative during the
reaction. Examples of such an organic acid include oxalic acid,
malonic acid, methylmalonic acid, ethylmalonic acid, propylmalonic
acid, butylmalonic acid, dimethylmalonic acid, diethylmalonic acid,
succinic acid, methylsuccinic acid, glutaric acid, adipic acid,
itaconic acid, maleic acid, fumaric acid, and citraconic acid. Of
these, oxalic acid an maleic acid are especially preferred. An
amount of the organic acid other than the organic sulfonic acid is
from 10.sup.-6 moles to 10 moles, preferably 10.sup.-5 to 5 moles,
more preferably 10.sup.-4 to 1 mole per mole of the
silicon-containing monomer.
[0085] The hydrolysis and condensation reactions are started by
dissolving the catalyst in water and then adding the monomer to the
resulting solution. At this time, an organic solvent may be added
to the aqueous solution of the catalyst or the monomer may be
diluted in advance with the organic solvent. The reaction
temperature is from 0 to 100.degree. C., preferably from 10 to
80.degree. C. It is also preferred to keep the temperature in the
range from 10 to 50.degree. C. during dropwise addition of the
monomer and then ripen the reaction mixture in the range from 20 to
80.degree. C.
[0086] Preferred examples of the organic solvent include methanol,
ethanol, 1-propanol, 2-propanol, 1-butanol, 2-butanol,
2-methyl-1-propanol, acetone, acetonitrile, tetrahydrofuran,
toluene, hexane, ethyl acetate, cyclohexanone,
methyl-2-n-amylketone, propylene glycol monomethyl ether, ethylene
glycol monomethyl ether, propylene glycol monoethyl ether, ethylene
glycol monoethyl ether, propylene glycol dimethyl ether, diethylene
glycol dimethyl ether, propylene glycol monomethyl ether acetate,
propylene glycol monoethyl ether acetate, ethyl pyruvate, butyl
acetate, methyl 3-methoxypropionate, ethyl 3-ethoxypropionate,
tert-butyl acetate, tent-butyl propionate, propylene glycol
mono-tert-butyl ether acetate, and .gamma.-butyrolactone, and
mixtures thereof.
[0087] Of these solvents, water soluble ones are preferred.
Examples include alcohols such as methanol, ethanol, 1-propanol and
2-propanol, polyols such as ethylene glycol and propylene glycol,
polyol condensate derivatives such as propylene glycol monomethyl
ether, ethylene glycol monomethyl ether, propylene glycol monoethyl
ether, ethylene glycol monoethyl ether, propylene glycol monopropyl
ether, and ethylene glycol monopropyl ether, acetone, acetonitrile
and tetrahydrofuran.
[0088] The organic solvent added in an amount of 50 mass % or
greater hinders progress of hydrolysis and condensation reactions
so that the amount must be less than 50 mass %. Per mole of the
monomer, preferably from 0 to 1,000 ml of the organic solvent is
added. Use of a large amount of the organic solvent is uneconomical
because it requires an unnecessarily large reactor. The amount of
the organic solvent is preferably 10 mass % or less based on water.
It is most preferred to perform the reactions without the organic
solvent. The hydrolysis and condensation reactions are, if
necessary, followed by the neutralization reaction of the catalyst.
In order to smoothly conduct the following extraction operation
further, the alcohol generated during the hydrolysis and
condensation reactions is preferably removed under reduced pressure
to obtain an aqueous solution of the reaction mixture. The amount
of an alkaline substance necessary for the neutralization is
preferably from 1 to 2 equivalents of the inorganic acid or organic
sulfonic acid. As the alkaline substance, any substance is usable
insofar as it is alkaline in water. Heating temperature of the
reaction mixture varies, depending on the kind of the alcohol to be
removed, but preferably from 0 to 100.degree. C., more preferably
from 10 to 90.degree. C., still more preferably from 15 to
80.degree. C. The degree of vacuum varies, depending on the kind of
the alcohol to be removed, exhaust apparatus, condensing apparatus
or heating temperature, but is preferably not greater than
atmospheric pressure, more preferably an absolute pressure of 80
kPa or less, still more preferably an absolute pressure of 50 kPa
or less. It is difficult to know the precise amount of the alcohol
to be removed, but about at least 80 mass % of the alcohol
generated during the reactions is preferably removed.
[0089] In order to remove the catalyst used for the hydrolysis and
condensation reactions from the aqueous solution, the polysiloxane
derivative is extracted with an organic solvent. As the organic
solvent, those capable of dissolving therein the polysiloxane
derivative and separating a mixture with water into two layers are
preferred. Examples include methanol, ethanol, 1-propanol,
2-propanol, 1-butanol, 2-butanol, 2-methyl-1-propanol, acetone,
tetrahydrofuran, toluene, hexane, ethyl acetate, cyclohexanone,
methyl-2-n-amylketone, propylene glycol monomethyl ether, ethylene
glycol monomethyl ether, propylene glycol monoethyl ether, ethylene
glycol monoethyl ether, propylene glycol monopropyl ether, ethylene
glycol monopropyl ether, propylene glycol dimethyl ether,
diethylene glycol dimethyl ether, propylene glycol monomethyl ether
acetate, propylene glycol monoethyl ether acetate, ethyl pyruvate,
butyl acetate, methyl 3-methoxypropionate, ethyl
3-ethoxypropionate, tert-butyl acetate, tert-butyl propionate,
propylene glycol mono-tert-butyl ether acetate,
.gamma.-butyrolactone, methyl isobutyl ketone and cyclopentyl
methyl ether, and mixtures thereof.
[0090] Mixtures of a water soluble organic solvent and a sparingly
water-soluble organic solvent are especially preferred. Preferred
examples of the combination include, but not limited to,
methanol+ethyl acetate, ethanol+ethyl acetate, 1-propanol+ethyl
acetate, 2-propanol+ethyl acetate, propylene glycol monomethyl
ether+ethyl acetate, ethylene glycol monomethyl ether+ethyl
acetate, propylene glycol monoethyl ether+ethyl acetate, ethylene
glycol monoethyl ether+ethyl acetate, propylene glycol monopropyl
ether+ethyl acetate, ethylene glycol monopropyl ether+ethyl
acetate, methanol+methyl isobutyl ketone, ethanol+methyl isobutyl
ketone, 1-propanol+methyl isobutyl ketone, 2-propanol+methyl
isobutyl ketone, propylene glycol monomethyl ether+methyl isobutyl
ketone, ethylene glycol monomethyl ether+methyl isobutyl ketone,
propylene glycol monoethyl ether+methyl isobutyl ketone, ethylene
glycol monoethyl ether+methyl isobutyl ketone, propylene glycol
monopropyl ether+methyl isobutyl ketone, ethylene glycol monopropyl
ether+methyl isobutyl ketone, methanol+cyclopentyl methyl ether,
ethanol+cyclopentyl methyl ether, 1-propanol+cyclopentyl methyl
ether, 2-propanol+cyclopentyl methyl ether, propylene glycol
monomethyl ether+cyclopentyl methyl ether, ethylene glycol
monomethyl ether+cyclopentyl methyl ether, propylene glycol
monoethyl ether+cyclopentyl methyl ether, ethylene glycol monoethyl
ether+cyclopentyl methyl ether, propylene glycol monopropyl
ether+cyclopentyl methyl ether, ethylene glycol monopropyl
ether+cyclopentyl methyl ether, methanol+propylene glycol methyl
ether acetate, ethanol+propylene glycol methyl ether acetate,
1-propanol+propylene glycol methyl ether acetate,
2-propanol+propylene glycol methyl ether acetate, propylene glycol
monomethyl ether+propylene glycol methyl ether acetate, ethylene
glycol monomethyl ether+propylene glycol methyl ether acetate,
propylene glycol monoethyl ether+propylene glycol methyl ether
acetate, ethylene glycol monoethyl ether+propylene glycol methyl
ether acetate, propylene glycol monopropyl ether+propylene glycol
methyl ether acetate, and ethylene glycol monopropyl
ether+propylene glycol methyl ether acetate.
[0091] The mixing ratio of the water soluble organic solvent and
the sparingly water-soluble organic solvent is determined as
needed, but the water soluble organic solvent is added in an amount
of from 0.1 to 1000 parts by mass, preferably from 1 to 500 parts
by mass, more preferably from 2 to 100 parts by mass, based on 100
parts by mass of the sparingly water-soluble organic solvent.
[0092] The organic layer obtained after the removal of the catalyst
used for the hydrolysis and condensation reactions is mixed in a
porous-film forming composition after partial distillation of the
solvent under reduced pressure and solvent substitution by
re-dilution.
[0093] An undesirable impurity which is thought to be a microgel is
sometimes mixed in the reaction mixture due to fluctuations in the
conditions during hydrolysis reaction or concentration. The
microgel can be removed by washing with water prior to mixing of
the polysiloxane compound. When washing with water is not so
effective for the removal of the microgel, this problem may be
overcome by washing the polysiloxane compound with acidic water and
then with water.
[0094] The acidic water usable for the above purpose contains
preferably a divalent organic acid, more specifically, oxalic acid
or maleic acid. The concentration of the acid contained in the
acidic water is from 100 ppm to 25 mass %, preferably from 200 ppm
to 15 mass %, more preferably from 500 ppm to 5 mass %. The amount
of the acidic water is from 0.01 to 100 L, preferably from 0.05 to
50 L, more preferably from 0.1 to 5 L per L of the polysiloxane
compound solution obtained in the above-described step. The organic
layer may be washed in a conventional manner. Both of them are
charged in the same container, stirred, and left to stand to
separate a water layer from the mixture. The washing may be
performed at least once. Washing ten times or more does not bring
about reasonable effects so that the washing is performed
preferably from once to about five times.
[0095] The acid used for washing is then removed by washing with
neutral water. It is only necessary to use, for this washing, water
called deionized water or ultrapure water. The neutral water is
used in an amount of from 0.01 to 100 L, preferably from 0.05 to 50
L, more preferably from 0.1 to 5 L per L of the polysiloxane
compound solution washed with the acidic water. The washing is
performed in the above-described manner, more specifically, by
charging them in the same container, stirring the resulting mixture
and leaving it to stand to separate a water layer from the mixture.
The washing may be performed at least once. Washing ten times or
more does not bring about reasonable effects so that the washing is
performed preferably from once to about five times.
[0096] To the polysiloxane compound solution which has finished
washing, a solvent for preparing a coating composition, which will
be described later, is added. By performing a solvent exchange
under reduced pressure, a mother solution to be added to the
porous-film forming composition can be obtained. This solvent
exchange may be carried out after addition of silicon oxide fine
particles which will be described later. The solvent exchange is
conducted at a temperature which varies, depending on the kind of
the extraction solvent to be removed, but is preferably from 0 to
100.degree. C., more preferably from 10 to 90.degree. C., still
more preferably from 15 to 80.degree. C. The degree of vacuum
varies depending on the kind of the extraction solvent to be
removed, exhaust gas apparatus, condensing apparatus or heating
temperature, but is preferably not greater than the atmospheric
pressure, more preferably an absolute pressure of 80 kPa or less,
still more preferably an absolute pressure of 50 kPa or less.
[0097] When the solvent is exchanged, nanogel may be generated due
to loss of stability of the polysiloxane compound. The generation
of the nanogel depends on the affinity between the final solvent
and polysiloxane compound. An organic acid may be added to prevent
the generation of it. As the organic acid, divalent ones such as
oxalic acid and maleic acid, and monovalent carboxylic acids such
as formic acid, acetic acid and propionic acid are preferred. The
amount of the organic acid is from 0 to 25 mass %, preferably from
0 to 15 mass %, more preferably from 0 to 5 mass % based on the
polymer in the solution before the solvent exchange. When the
organic acid is added, its amount is preferably 0.5 mass % or
greater. If necessary, the acid may be added to the solution before
the solvent exchange and then, solvent extracting operation may be
performed.
[0098] As described above, the polysiloxane compound obtained in
the above-described method can have, in the molecule thereof, a
greater amount of silanol groups compared with that obtained by the
conventional method using hydrolysis and condensation reactions.
Described specifically, when the polysiloxane compound is composed
of units represented by the following formulas:
##STR00001##
(wherein, Q means a unit derived from a tetravalent hydrolyzable
silane, T means a unit derived from a trivalent hydrolyzable
silane, and R in T1 to T3 indicates that a bond represented by
Si--R is a bond between silicon and a carbon substituent),
component ratios (molar ratios) (q1 to q4, t1 to t3) of the units
(Q1 to Q4, T1 to T3) in the polysiloxane compound as measured by
.sup.29Si--NMR satisfies the following relationships:
(q1+q2+t1)/(q1+q2+q3+q4+t1+t2+t3).ltoreq.0.2 and
(q3+t2)/(q1+q2+q3+q4+t1+t2+t3).gtoreq.0.4.
The polysiloxane compound which can satisfy the above-described
relationships may improve the binding force between the
above-described silica sols.
[0099] If the condensation rate of the polysiloxane compound is
calculated on the basis of a remaining amount of silanol groups or
alkoxy groups (both groups are generically referred to as
"hydrolyzable group"), a polysiloxane compound having a silanol
content of 5 mole % or greater in terms of silicon atoms is
obtained by the above-described method. Use of such a polysiloxane
compound can improve the binding force between silica sols.
[0100] If the polysiloxane compound obtained in the above-described
method is added, a film-forming composition is obtained by mixing a
solution of this polysiloxane compound in a coating solvent with
the above-described solution containing the silica sol of the
invention while adjusting the viscosity and the like as described
above.
[0101] After preparation of a porous-film forming composition in
the above-described manner, the composition is spin coated onto a
target substrate at an adequate rotation speed while controlling
the solute concentration of the composition, whereby a thin film
having a desired thickness can be formed.
[0102] A thin film having a thickness of about 0.1 to 1.0 .mu.m is
typically formed in practice, but the film thickness is not limited
thereto. A thin film with a greater thickness can be formed by
carrying out coating of the composition plural times.
[0103] Not only spin coating but also another application method
such as scan coating can be employed.
[0104] A thin film thus formed can be converted into a porous film
in a known manner. Described specifically, the porous film is
available as a final product by removing the solvent from the thin
film by using an oven in a drying step (typically called pre-baking
step in a semiconductor fabrication process) to heat it to
preferably from 50 to 150.degree. C. for several minutes and then
sintering it in the range from 350 to 450.degree. C. for from about
5 minutes to 2 hours. A curing step with ultraviolet radiation or
electron beam may be added further.
[0105] The porous film thus obtained has excellent mechanical
strength because the film as a whole is composed mainly of silica
gel particles having high mechanical strength. It has hardness, as
measured by nanoindentation, of 0.5 to 2 GPa and a modulus of
elasticity from about 4 to 15 GPa. This is remarkably high
mechanical strength considering that a conventional porous
material, which is obtained by adding a thermally decomposable
polymer to a silicone resin and then removing the polymer by
heating to form pores, has only hardness from 0.05 to 1 GPa and
modulus of elasticity from 1.0 to 4.0 GPa. The porous film of the
invention has higher strength even if their dielectric constants
are of the same level than a porous film obtained in the
conventional manner, for example, by using a silica sol prepared
from tetrapropylammonium hydroxide alone.
[0106] The mechanism of the porous film of the invention having
both high strength and low dielectric constant is considered as
below. A space between particles in the film becomes a pore as the
solvent evaporates during application, film formation and sintering
steps, whereby a film having a low dielectric constant can be
obtained. When the silica particles have low strength, they undergo
deformation or shrinkage during the formation of pores, which
reduces the size of the pores. If the silica particles have high
strength as in the invention, on the other hand, the pores do not
shrink in size and the porous film of the invention can have higher
strength when compared with a film equal in dielectric constant. As
shown later in Examples, neither a zeolite-like repeating structure
in the atomic arrangement nor micropores are observed in the porous
film of the invention. The film of the invention may be therefore
utterly different in concept from the conventional film having
improved strength. The above-described observation may support the
presumption of the inventors.
[0107] A low-dielectric-constant porous film to be used for
semiconductor devices has conventionally a problem of deterioration
in the mechanical strength of the film because introduction of
pores into the film to reduce its dielectric constant and make the
film porous decreases the density of the material constituting the
film. The deterioration in the mechanical strength not only has an
influence on the strength of semiconductor devices themselves but
also causes peeling due to lack of sufficient strength against
chemical mechanical polishing typically employed for the
semiconductor fabrication process,
[0108] The porous film obtained using the composition for forming a
porous film composed mainly of a silica gel and prepared by the
method of the invention can have both a low dielectric constant and
high mechanical strength simultaneously. In particular, when the
porous film is used as an interlayer insulating film of
semiconductor devices, it does not cause such peeling and enables
fabrication of highly-reliable, high-speed and small-sized
semiconductor devices because it has high mechanical strength in
spite of a porous film and also has low dielectric constant,
[0109] A semiconductor device having, as an interlayer insulating
film thereof, the porous film is also one of the inventions. The
term "interlayer insulating film" as used herein may mean a film
for electrically insulating conductive sites present in a layer or
conductive sites present in different layers. Examples of the
conductive sites include metal interconnects.
[0110] One embodiment of the semiconductor device of the invention
will next be described based on FIG. 1.
[0111] As substrate 1, Si semiconductor substrates such as Si
substrate and SOI (Si On Insulator) substrate can be employed.
Alternatively, it may be a compound semiconductor substrate such as
SiGe or GaAs.
[0112] Interlayer insulating films illustrated in FIG. 1 are
interlayer insulating film 2 of a contact layer, interlayer
insulating films 3, 5, 7, 9, 11, 13, 15, and 17 of interconnect
layers, and interlayer insulating films 4, 6, 8, 10, 12, 14, and 16
of a via layer.
[0113] The interconnect layers from the interlayer insulating film
3 of the bottom interconnect layer to the interlayer insulating
film 17 of the uppermost interconnect layer are referred to as M1,
M2, M3, M4, M5, M6, M7 and M8, respectively in the order from the
bottom to the top. The layers from the interlayer insulating film 4
of the lowermost via layer to the interlayer insulating film 16 of
the uppermost via layer are referred to as V1, V2, V3, V4, V5, V6
and V7, respectively in the order from the bottom to the top.
[0114] Some metal interconnects are indicated by numerals 18 and 21
to 24, respectively, but even if such a numeral is omitted,
portions with the same pattern as that of these metal interconnects
illustrate metal interconnects.
[0115] A via plug 19 is made of a metal and it is typically copper
in the case of a copper interconnect. Even if a numeral is omitted,
portions with the same pattern as that of these via plugs
illustrate via plugs.
[0116] A contact plug 20 is connected to a gate of a transistor
(not illustrated) formed on the uppermost surface of the substrate
1 or to the substrate.
[0117] As illustrated, the interconnect layers and the via layers
are stacked alternately. The term "multilevel interconnects"
typically means M1 and layers thereabove. The interconnect layers
M1 to M3 are typically called local interconnects; the interconnect
layers M4 to M5 are typically called intermediate or semi-global
interconnects; and the interconnect layers M6 to M8 are typically
called global interconnects.
[0118] In the semiconductor device illustrated in FIG. 1, the
porous film of the invention is used as at least one of the
interlayer insulating films 3, 5, 7, 9, 11, 13, 15, and 17 of the
interconnect layers and the interlayer insulating films 4, 6, 8,
10, 12, 14 and 16 of the via layers.
[0119] For example, when the porous film of the invention is used
as the interlayer insulating film 3 of the interconnect layer (M1),
a capacitance between the metal interconnect 21 and metal
interconnect 22 can be reduced greatly. When the porous film of the
invention is used as the interlayer insulating film 4 of the via
layer (V1), a capacitance between the metal interconnect 23 and
metal interconnect 24 can be reduced greatly. Thus, use of the
porous film of the invention having a low dielectric constant for
the interconnect layer enables drastic reduction of the capacitance
between metal connects in the same layer. In addition, use of the
porous film of the invention having a low dielectric constant for
the via layer enables drastic reduction in the capacitance between
the metal interconnects above and below the via layer. Accordingly,
use of the porous film of the invention for all the interconnect
layers and via layers enables great reduction in the parasitic
capacitance of interconnects.
[0120] In addition, use of the porous film of the invention as an
insulating film for interconnection is free from a conventional
problem, that is, an increase in a dielectric constant caused by
moisture absorption of porous films during formation of multilevel
interconnects by stacking them one after another. As a result, the
semiconductor device featuring high speed operation and low power
consumption can be obtained.
[0121] In addition, due to high strength of the porous film of the
invention, the semiconductor device thus obtained has improved
mechanical strength. As a result, the semiconductor device thus
obtained has greatly improved production yield and reliability.
[0122] The present invention will hereinafter be described in
detail by Examples. It should be noted that the scope of the
invention is not limited to or by these Examples.
Example 1
[0123] A mixture of 21.7 g of methyltrimethoxysilane and 24.3 g of
tetramethoxysilane was slowly added dropwise under stirring to a
solution of 11.5 g of a 20% aqueous solution of tetrapropylammonium
hydroxide, 6.2 g of a 25% aqueous solution of tetramethylammonium
hydroxide, 81 g of ultrapure water and 180 g of ethanol which
solution had been heated to 70.degree. C. in advance. Immediately
after completion of the dropwise addition, the reaction mixture was
neutralized with 23 g of a 20% aqueous solution of oxalic acid.
Propylene glycol monomethyl ether (200 ml) was added and the
mixture was concentrated under reduced pressure to remove ethanol.
Ethyl acetate (300 ml) was then added and washing with 200 ml of
ultrapure water was repeated until the mixture became pH 7.
Propylene glycol monomethyl ether (200 ml) was added again and the
mixture was concentrated under reduced pressure until its
nonvolatile residue became 7% or less. Propylene glycol monomethyl
ether was added again to adjust the nonvolatile residue content to
about 7 mass %, whereby a porous-film forming composition was
obtained. Three porous-film forming compositions (Examples 1-(1) to
(3)) were prepared in a similar manner except that only the
reaction time was changed ((1) one hour, (2) four hours, and (3)
eight hours, respectively).
Example 2
[0124] A mixture of 21.7 g of methyltrimethoxysilane and 24.3 g of
tetramethoxysilane was slowly added dropwise under stirring to a
solution of 21.5 g of a 20% aqueous solution of tetrapropylammonium
hydroxide, 8.2 g of a 25% aqueous solution of tetramethylammonium
hydroxide, 81 g of ultrapure water and 180 g of ethanol which
solution had been heated to 80.degree. C. in advance. Immediately
after completion of the dropwise addition, the reaction mixture was
neutralized with 23 g of a 20% aqueous solution of oxalic acid.
Propylene glycol monomethyl ether (200 ml) was added and the
mixture was concentrated under reduced pressure to remove ethanol.
Ethyl acetate (300 ml) was then added and washing with 200 ml of
ultrapure water was repeated until the mixture became pH 7.
Propylene glycol monomethyl ether (200 ml) was added again and the
mixture was concentrated under reduced pressure until its
nonvolatile residue became 7% or less. Propylene glycol monomethyl
ether was added again to adjust the nonvolatile residue to about 7
mass %, whereby a porous-film forming composition was obtained.
Three porous-film forming compositions (Examples 2-(1) to (3)) were
prepared in a similar manner except that only the reaction time was
changed (to (1) one hour, (2) four hours, and (3) eight hours,
respectively).
Example 3
[0125] A mixture of 21.7 g of methyltrimethoxysilane and 24.3 g of
tetramethoxysilane was slowly added dropwise under stirring to a
solution of 11.5 g of a 25% aqueous solution of tetrabutylammonium
hydroxide, 6.2 g of a 25% aqueous solution of tetramethylammonium
hydroxide, 81 g of ultrapure water and 180 g of ethanol which
solution had been heated to 70.degree. C. in advance. Immediately
after completion of the dropwise addition, the reaction mixture was
neutralized with 23 g of a 20% aqueous solution of oxalic acid.
Propylene glycol monomethyl ether (200 ml) was added and the
mixture was concentrated under reduced pressure to remove ethanol.
Ethyl acetate (300 ml) was then added and washing with 200 ml of
ultrapure water was repeated until the mixture became pH 7.
Propylene glycol monomethyl ether (200 ml) was added again and the
mixture was concentrated under reduced pressure until its
nonvolatile residue became 7% or less. Propylene glycol monomethyl
ether was added again to adjust the nonvolatile residue to about 7
mass %, whereby a porous-film forming composition was obtained.
Three porous-film forming compositions (Examples 3-(1) to (3)) were
prepared in a similar manner except that only the reaction time was
changed ((1) one hour, (2) four hours, and (3) eight hours,
respectively).
Comparative Examples 1-(1) to (3)
[0126] A mixture of 21.7 g of methyltrimethoxysilane and 24.3 g of
tetramethoxysilane was slowly added dropwise under stirring to a
solution of 8.2 g of a 25% aqueous solution of tetramethylammonium
hydroxide, 81 g of ultrapure water and 180 g of ethanol, which
solution had been heated to 60.degree. C. in advance. Immediately
after completion of the dropwise addition, the reaction mixture was
neutralized with 23 g of a 20% aqueous solution of oxalic acid.
Propylene glycol monopropyl ether (200 ml) was added and the
mixture was concentrated under reduced pressure to remove ethanol.
Ethyl acetate (300 ml) was then added and washing with 200 ml of
ultrapure water was repeated until the mixture became pH 7.
Propylene glycol monopropyl ether (200 ml) was added again and the
mixture was concentrated under reduced pressure until its
nonvolatile residue became 7% or less. Propylene glycol monopropyl
ether was added again to adjust the nonvolatile residue to about 7
mass %, whereby a comparative composition was obtained. Three
comparative compositions were prepared in a similar manner except
that only the reaction time was changed ((1) one hour, (2) four
hours, and (3) eight hours, respectively).
Comparative Examples 2-(1) and (2)
[0127] A mixture of 21.7 g of methyltrimethoxysilane and 24.3 g of
tetramethoxysilane was slowly added dropwise under stirring to a
solution of 34.5 g of a 20% aqueous solution of tetrapropylammonium
hydroxide, 81 g of ultrapure water and 180 g of ethanol, which
solution had been heated to 80.degree. C. in advance. Immediately
after completion of the dropwise addition, the reaction mixture was
neutralized with 23 g of a 20% aqueous solution of oxalic acid.
Propylene glycol monopropyl ether (200 ml) was added and the
mixture was concentrated under reduced pressure to remove ethanol.
Ethyl acetate (300 ml) was then added and washing with 200 ml of
ultrapure water was repeated until the mixture became pH 7.
Propylene glycol monopropyl ether (200 ml) was added again and the
mixture was concentrated under reduced pressure until its
nonvolatile residue became 7% or less. Propylene glycol monopropyl
ether was added again to adjust the nonvolatile residue to about 7
mass %, whereby a comparative composition was obtained. Two
comparative compositions were prepared in a similar manner except
that only the reaction time was changed ((1) four hours and (2)
eight hours, respectively).
Comparative Example 3
[0128] After 23.0 g of a 15% aqueous solution of
tetrapropylammonium hydroxide was mixed with 27.0 g of
tetraethoxysilane and the mixture was stirred at room temperature
for 3 days, the reaction mixture was stirred under heating at
80.degree. C. for 35 hours to yield a zeolite-crystal-containing
solution having a particle size peak at 550 nm. A mixture of 6.2 g
of tetraethoxysilane and 28.4 g of methyltriethoxysilane was slowly
added dropwise under stirring to a solution which had been obtained
by adding 4.1 g of a 25% aqueous solution of tetramethylammonium
hydroxide, 62.6 g of ultrapure water and 180 g of ethanol to the
resulting zeolite-crystal-containing solution and had been heated
to 80.degree. C. in advance. Immediately after completion of the
dropwise addition, the reaction mixture was neutralized with 23 g
of a 20% aqueous solution of oxalic acid. Propylene glycol
monopropyl ether (200 ml) was added and the mixture was
concentrated under reduced pressure to remove ethanol. Ethyl
acetate (300 ml) was then added and washing with 200 ml of
ultrapure water was repeated until the mixture became pH 7.
Propylene glycol monopropyl ether (200 ml) was added again and the
mixture was concentrated under reduced pressure until the
nonvolatile residue content became 7% or less. Propylene glycol
monopropyl ether was added again to adjust the nonvolatile residue
to about 7 mass %, whereby a comparative composition was
obtained.
Example 4
[0129] Porous films were formed using the porous-film forming
compositions obtained in Examples 1 to 3 (nine compositions in
total) and the comparative compositions obtained in Comparative
Examples 1 to 3 (six compositions in total) in accordance with the
following process and their physical properties were evaluated
(Examples 4-(1) to (3), Examples 5-(1) to (3), Examples 6-(1) to
(3), Comparative Examples 4-(1) to (3), Comparative Examples 5-(1)
and (2), Comparative Example 6).
[0130] The physical properties of each of the porous films were
measured by the following methods.
[0131] 1. Dielectric constant (k) was measured using "495-CV
System" (product of SSM Japan) in accordance with C-V measurements
with an automatic mercury probe.
[0132] 2. Mechanical strength (modulus of elasticity) was measured
using a nano indenter (product of Nano Instruments).
[0133] Each of the porous-film forming compositions was spin-coated
onto an 8-inch silicon wafer at 4,000 rpm for 1 minute by a spin
coater. A thin film thus obtained was heated at 120.degree. C. for
2 minutes by using a hot plate. After heating further for 3 minutes
at 250.degree. C., it was heated at 450 .degree. C. for one hour in
a clean oven in a nitrogen atmosphere, whereby a porous film was
obtained. The films thus obtained using the above-described
compositions each has a thickness of about 3,000 .ANG..
[0134] The dielectric constant and modulus of elasticity of the
porous films thus formed are shown in Table 1. The relationship
between the dielectric constant and mechanical strength of these
films is shown in the graph of FIG. 2.
[0135] It is to be noted that the approximate line in FIG. 2 is
determined by the least-squares method.
TABLE-US-00001 TABLE 1 Dielectric Mechanical Coating composition
constant k strength (GPa) Example 4-(1) Composition of Ex. 1-(1)
2.68 9.24 Example 4-(2) Composition of Ex. 1-(2) 2.49 7.82 Example
4-(3) Composition of Ex. 1-(3) 2.38 6.82 Example 5-(1) Composition
of Ex. 2-(1) 2.51 7.84 Example 5-(2) Composition of Ex. 2-(2) 2.32
6.17 Example 5-(3) Composition of Ex. 2-(3) 2.18 5.99 Example 6-(1)
Composition of Ex. 3-(1) 2.54 8.08 Example 6-(2) Composition of Ex.
3-(2) 2.36 6.4 Example 6-(3) Composition of Ex. 3-(3) 2.26 5.65
Comp. Ex. 4-(1) Composition of Comp. Ex. 1-(1) 2.5 7.23 Comp. Ex.
4-(2) Composition of Comp. Ex. 1-(2) 2.32 5.73 Comp. Ex. 4-(3)
Composition of Comp. Ex. 1-(3) 2.13 3.81 Comp. Ex. 5-(1)
Composition of Comp. Ex. 2-(1) 2.56 5.56 Comp. Ex. 5-(2)
Composition of Comp. Ex. 2-(2) 2.4 5.33 Comp. Ex. 6 Composition of
Comp. Ex. 3 2.54 7.71
(Reference Test 1)
[0136] The compositions of Example 2-(3) and Comparative Example 3
were each applied onto a silicon wafer and then baked to form a
porous film having a thickness of 3000 .ANG.. As a result of X-ray
diffraction of the film, a signal based on the presence of zeolite
was observed (FIG. 3) from the porous film obtained using the
composition of Comparative Example 3, while no signal was observed
from the porous film obtained using the composition of Example 2.
The position of a standard peak of zeolite is shown by an arrow in
FIG. 3 and this suggests that the film in this chart contains
zeolite. The signal with the symbol * in FIG. 3 has remained
because signals derived from a silicon wafer of a substrate cannot
be cancelled from the background. Nothing but noise was observed
from the film obtained from the composition of Example 2-(3).
(Reference Test 2)
[0137] The micropores of the films obtained in Reference Test 1
were measured. Nitrogen adsorption technique using Quantachrome's
Autosorb 1 was employed for the measurement. As a result,
distribution of pores was observed in a micropore region of 1 nm or
less only in Comparative Example 6, but the distribution of pores
was observed in a mesopore region of 2 nm or greater in the other
sample.
[0138] When a low-dielectric-constant insulating film is designed,
it is necessary, as a method of reducing only its dielectric
constant, to heighten the porosity by controlling the size of
particles contained in a film forming composition to raise a void
ratio or using a pore-forming agent such as porogen. The film
however has no mechanical strength at the porous portion thereof.
When films are made of the same material, there is a trade-off
relationship between a porosity and mechanical strength. As actual
examples in FIG. 2 show, there is typically a linear relationship,
within a narrow range, that is, a range of a dielectric constant
from 2.1 to 2.7, between a low dielectric constant and mechanical
strength of films available from materials synthesized using the
same material and catalyst. In order to verify whether a
low-dielectric-constant insulating film with high mechanical
strength is formed or not, the mechanical strength relative to the
dielectric constant must be compared between these films.
[0139] As is apparent from FIG. 2, the low-dielectric-constant
insulating films of the invention obtained in Examples each has
high mechanical strength at each dielectric constant compared with
the mechanical strength/dielectric constant of a film formed in a
conventional manner, for example, the low-dielectric-constant
insulating film which is obtained in Comparative Example 4 using a
silica sol synthesized by the conventional method and showing
relatively high mechanical strength at each dielectric constant.
This tendency is marked at a dielectric constant of 2.5 or
less.
[0140] Moreover, from Reference Tests 1 and 2, it has been
elucidated that the film obtained using the silica sol of the
invention has neither a zeolite-like crystal structure nor
zeolite-like micropores in the film. As the above-described results
of mechanical strength/dielectric constant have revealed, when the
films obtained in Examples are compared with the film obtained
using the composition of Comparative Example 3 containing zeolite
fine particles derived from crystals and having considerably high
strength, the low-dielectric-constant insulating films obtained in
Examples have mechanical strength comparable to that of the
low-dielectric-constant insulating film having zeolite particles
incorporated therein. In short, it has been found that although the
silica sol of the invention is available without a cumbersome
operation necessary for preparation of zeolite fine particles, it
provides equal mechanical strength.
[0141] The method for preparing a porous-film forming composition
according to the invention is effective for preparing a material
for forming a low-dielectric-constant insulating film with high
mechanical strength.
[0142] The porous-film forming composition according to the
invention is effective as a material for forming a
low-dielectric-constant insulating film with high mechanical
strength.
[0143] The method for forming a porous film according to the
invention is effective for preparing a material for forming a
low-dielectric-constant insulating film with high mechanical
strength.
[0144] The porous film according to the invention is effective as a
material for forming a low-dielectric-constant insulating film with
high mechanical strength.
[0145] The semiconductor device according to the invention is
effective as a high-performance semiconductor device capable of
achieving high speed and low power consumption operation.
[0146] It is to be understood that the present invention is not
limited to the embodiments given above. The embodiments given above
are merely illustrative, and those having substantially the same
configuration as the technical concept defined by the appended
claims of the present invention and having similar functions and
effects are considered to fall within the technical scope of the
present invention.
* * * * *