U.S. patent application number 09/837885 was filed with the patent office on 2002-03-21 for mesoporous silica film from a solution containing a surfactant and methods of making same.
Invention is credited to Baskaran, Suresh, Birnbaum, Jerome C., Coyle, Christopher A., Domansky, Karel, Fryxell, Glen E., Kohler, Nathan J., Li, Xiaohong, Liu, Jun, Thevuthasan, Suntharampillai.
Application Number | 20020034626 09/837885 |
Document ID | / |
Family ID | 27499216 |
Filed Date | 2002-03-21 |
United States Patent
Application |
20020034626 |
Kind Code |
A1 |
Liu, Jun ; et al. |
March 21, 2002 |
Mesoporous silica film from a solution containing a surfactant and
methods of making same
Abstract
The present invention is a mesoporous silica film having a low
dielectric constant and method of making having the steps of
combining a surfactant in a silica precursor solution, spin-coating
a film from this solution mixture, forming a partially hydroxylated
mesoporous film, and dehydroxylating the hydroxylated film to
obtain the mesoporous film. It is advantageous that the small
polyoxyethylene ether surfactants used in spin-coated films as
described in the present invention will result in fine pores
smaller on average than about 20 nm. The resulting mesoporous film
has a dielectric constant less than 3, which is stable in moist air
with a specific humidity. The present invention provides a method
for superior control of film thickness and thickness uniformity
over a coated wafer, and films with low dielectric constant.
Inventors: |
Liu, Jun; (West Richland,
WA) ; Domansky, Karel; (Cambridge, MA) ; Li,
Xiaohong; (Richland, WA) ; Fryxell, Glen E.;
(Kennewick, WA) ; Baskaran, Suresh; (Kennewick,
WA) ; Kohler, Nathan J.; (Richland, WA) ;
Thevuthasan, Suntharampillai; (Kennewick, WA) ;
Coyle, Christopher A.; (Richland, WA) ; Birnbaum,
Jerome C.; (Richland, WA) |
Correspondence
Address: |
MARGER JOHNSON & McCOLLOM, P.C.
1030 S.W. Morrison Street
Portland
OR
97205
US
|
Family ID: |
27499216 |
Appl. No.: |
09/837885 |
Filed: |
April 18, 2001 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
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09837885 |
Apr 18, 2001 |
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09413062 |
Oct 4, 1999 |
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09837885 |
Apr 18, 2001 |
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09361499 |
Jul 23, 1999 |
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09361499 |
Jul 23, 1999 |
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09335210 |
Jun 17, 1999 |
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09335210 |
Jun 17, 1999 |
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09220882 |
Dec 23, 1998 |
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Current U.S.
Class: |
428/312.6 ;
257/E21.273; 427/240; 427/385.5; 428/313.5 |
Current CPC
Class: |
H01L 21/31695 20130101;
H01L 21/02282 20130101; Y10T 428/249972 20150401; H01L 21/02222
20130101; H01L 21/02164 20130101; C01B 37/02 20130101; H01L
21/02216 20130101; Y10T 428/249969 20150401; H01L 21/02203
20130101; H01L 21/02337 20130101 |
Class at
Publication: |
428/312.6 ;
427/240; 427/385.5; 428/313.5 |
International
Class: |
B32B 003/26 |
Goverment Interests
[0002] This invention was made with Government support under
Contract DE-AC0676RL01830 awarded by the U.S. Department of Energy.
The Government has certain rights in the invention.
Claims
We claim:
1. A mesoporous silica film prepared from a surfactant containing
solution, having a dielectric constant less than 3 that has both a
relative stability and an absolute stability in a humid atmosphere,
a film thickness from about 0.1 .mu.m to about 1.5 .mu.m, and an
average pore diameter less than or equal to about 20 nm.
2. The mesoporous silica film as recited in claim 1, wherein said
average pore diameter is less than or equal to about 10 nm.
3. The mesoporous silica film as recited in claim 1, wherein said
thickness has a standard deviation less than +/-5%.
4. The mesoporous silica film as recited in claim 1, wherein a
porosity of said mesoporous silica film is disordered.
5. A mesoporous silica film having a thickness from about 0.1 .mu.m
to about 1.5 .mu.m and a standard deviation about said thickness,
wherein said standard deviation is less than +/-5%.
6. The mesoporous silica film as recited in claim 5, wherein a
dielectric constant of said mesoporous silica film is less than
3.
7. The mesoporous silica film as recited in claim 5, having a
dielectric constant with a relative stability and an absolute
stability.
8. The mesoporous silica film as recited in claim 5, having an
average pore size less than or equal to about 20 nm.
9. The mesoporous silica film as recited in claim 5, having a
porosity that is disordered.
10. A mesoporous silica film prepared from a surfactant containing
solution, comprising a porosity that is disordered, said porosity
having an average pore diameter of less than or equal to about 20
nm, and a film thickness from about 0.1 .mu.m to about 1.5
.mu.m.
11. The mesoporous silica film as recited in claim 10, having a
dielectric constant less than 3, said dielectric constant having
both a relative stability and an absolute stability.
12. A method of making a mesoporous film comprising the steps of:
(a) combining a silica precursor with an aqueous solvent, a
catalyst and a surfactant into a precursor solution; (b) spin
coating said precursor solution into a templated film; (c) removing
said aqueous solvent, said catalyst and said surfactant from said
templated film and forming a hydroxylated film with disordered
porosity; and (d) dehydroxylating said hydroxylated film and
obtaining said mesoporous film.
13. The method as recited in claim 12, wherein said surfactant is a
polyoxyethylene ether surfactant.
14. The method as recited in claim 13, wherein said polyoxyethylene
ether surfactant is C.sub.12H.sub.25 (CH.sub.2CH.sub.2O).sub.10OH
also known as C.sub.12EO.sub.10 or 10 lauryl ether;
C.sub.16H.sub.33(CH.sub.2CH.sub.2O)- .sub.10OH also known as
C.sub.16EO.sub.10 or 10 cetyl ether; C.sub.18H.sub.37
(CH.sub.2CH.sub.2O).sub.10OH also known as C.sub.18EO.sub.10 or 10
stearyl ether; C.sub.12H.sub.25(CH.sub.2CH.sub.2O- ).sub.4OH also
known as C.sub.12EO.sub.4 or 4 lauryl ether;
C.sub.16H.sub.33(CH.sub.2CH.sub.2O).sub.2OH also known as
C.sub.16EO.sub.2 or 2 cetyl ether or combinations thereof.
15. The method as recited in claim 12, wherein said surfactant is
in combination with a chemical agent selected from the group of a
second surfactant, smaller hydrophilic molecular compounds, and
with organic co-solvents.
16. The method as recited in claim 15, wherein said second
surfactant is selected from the group consisting of non-ionic
surfactant, cationic surfactant, anionic surfactant, amphoteric
surfactant and combinations thereof.
17. The method as recited in claim 16, wherein said cationic
surfactant is an ammonium-based surfactant.
18. The method as recited in claim 15, wherein said smaller
hydrophilic molecular compounds are selected from the group
consisting of glycerol, propylene glycol, and ethylene glycol.
19. The method as recited in claim 15, wherein said organic
co-solvents are selected from the group consisting of mesitylene,
octane and combinations thereof.
20. The method as recited in claim 12, wherein said silica
precursor is selected from the group consisting of tetraethyl
orthosilicate (TEOS), tetramethyl orthosilicate (TMOS), methyl
triethoxysilane, phenyl triethoxy silane, dimethyl dimethoxy silane
ethyl triethyoxysilane, and combinations thereof.
21. The method as recited in claim 12, wherein said aqueous solvent
comprises ethanol and water.
22. The method as recited in claim 12, wherein said acid is
selected from the group consisting of inorganic acid, organic acid
and combinations thereof.
23. The method as recited in claim 12, wherein said precursor
solution includes at least one other surfactant.
24. The method as recited in claim 12, wherein said precursor
solution includes at least one smaller hydrophilic molecular
compound.
25. The method as recited in claim 12 wherein said precursor
solution includes at least one organic co-solvent.
26. The method as recited in claim 23, wherein said at least one
other surfactant is selected from the group consisting of non-ionic
surfactant, cationic surfactant, anionic surfactant, amphoteric
surfactant and combinations thereof.
27. The method as recited in claim 24, wherein said at least one
smaller hydrophilic molecular compound is selected from the group
consisting of glycerol, propylene glycol, ethylene glycol and
combinations thereof.
28. The method as recited in claim 25, wherein said at least one
organic co-solvent is selected from the group consisting of
mesitylene, octane and combinations thereof.
29. The method as recited in claim 12, wherein dehydroxylating
occurs in the presence of a silicon-based organic compound in the
vapor phase.
30. The method as recited in claim 29, wherein the silicon-based
organic compound is a silane.
31. The method as recited in claim 30, wherein the silane is
selected from the group consisting of trimethyl iodosilane,
trimethyl chlorosilane, dimethyl dimethoxy silane, demethyl
dichloro silane, hexaphenyl disilazane, diphenyl tetramethyl
silazane and hexamethyl disilazane .
32. A method of making a mesoporous film with a surfactant
containing solution, the method comprising the steps of: (a)
combining a silica precursor with an aqueous solvent, a catalyst
and a surfactant that is a polyoxethylene ether surfactant into a
precursor solution; (b) spin coating said precursor solution into a
templated film; (c) removing said aqueous solvent, said catalyst
and said surfactant forming a hydroxylated film having porosity;
and (d) dehydroxylating said hydroxylated film and obtaining said
mesoporous film.
33. The method as recited in claim 32, wherein said polyoxyethylene
ether surfactant is C.sub.12H.sub.25 (CH.sub.2CH.sub.2O).sub.10OH
also known as C.sub.12EO.sub.10 or 10 lauryl ether;
C.sub.16H.sub.33(CH.sub.2CH.sub.2))- .sub.10OH also known as
C.sub.16EO.sub.10 or 10 cetyl ether; C.sub.18H.sub.37
(CH.sub.2CH.sub.2O).sub.10OH also known as C.sub.18EO.sub.10 or 10
stearyl ether; C.sub.12H.sub.25(CH.sub.2CH.sub.2O- ).sub.4OH also
known as C.sub.12EO.sub.4 or 4 lauryl ether;
C.sub.16H.sub.33(CH.sub.2CH.sub.2O).sub.2OH also known as
C.sub.16EO.sub.2 or 2 cetyl ether or combinations thereof.
34. The method as recited in claim 32, wherein said porosity is
disordered as indicated by an absence of an x-ray diffraction peak
in the range of 2 to 6 degrees 2-theta.
35. The method as recited in claim 32, wherein said porosity is
disordered, lacking a regular geometric arrangement of pores, and
the pore structure is characterized by an x-ray diffraction peak
between about 0.75 and about 2 degrees 2-theta.
36. The method as recited in claim 32, wherein said precursor
solution includes at least one other surfactant.
37. The method as recited in claim 32, wherein said precursor
solution includes at least one smaller hydrophilic molecular
compound.
38. The method as recited in claim 32, wherein said precursor
solution includes at least one organic co-solvent.
39. The method as recited in claim 32, wherein said precursor
solution includes an agent selected from the group consisting of a
second surfactant, a smaller hydrophilic molecular compound, an
organic co-solvent and combinations thereof.
40. The method as recited in claim 36, wherein said at least one
other surfactant is selected from the group consisting of non-ionic
surfactant, cationic surfactant, anionic surfactant, amphoteric
surfactant and combinations thereof.
41. The method as recited in claim 37, wherein said at least one
smaller hydrophilic molecular compound is selected from the group
consisting of glycerol, propylene glycol, ethylene glycol and
combinations thereof.
42. The method as recited in claim 38, wherein said at least one
organic co-solvent is selected from the group consisting of
mesitylene, octane and combinations thereof.
43. The method as recited in claim 39, wherein said second
surfactant is selected from the group consisting of non-ionic
surfactant, cationic surfactant, anionic surfactant, amphoteric
surfactant and combinations thereof.
44. The method as recited in claim 39, wherein said smaller
hydrophilic molecular compound is selected from the group
consisting of glycerol, propylene glycol, ethylene glycol and
combinations thereof.
45. The method as recited in claim 39, wherein said organic
co-solvent is selected from the group consisting of mesitylene,
octane and combinations thereof.
46. The method as recited in claim 32 wherein said silica precursor
is selected from the group consisting of tetraethyl orthosilicate
(TEOS), tetramethyl orthosilicate, methyl triethoxysilane, phenyl
triethoxy silane, dimethyl dimethoxy silane and combinations
thereof.
47. The method as recited in claim 32 wherein said aqueous solvent
includes ethanol.
48. The method as recited in claim 32, wherein said catalyst is
selected from the group consisting of inorganic acid, organic acid
and combinations thereof.
49. The method as recited in claim 48, wherein said organic acid is
carboxylic acid selected from the group consisting of methanoic
acid (formic acid), ethanoic acid (acetic acid), ethandioic acid
(oxalic acid), butanoic acid (butyric acid), and combinations
thereof.
50. The method as recited in claim 32, wherein dehydroxylating
occurs in the presence of a silicon-based organic compound in the
vapor phase.
51. The method as recited in claim 50, wherein the silicon-based
organic compound is a silane.
52. The method as recited in claim 51, wherein the silane is
selected from the group consisting of trimethyl iodosilane,
trimethyl chlorosilane, dimethyl dichloro silane, hexaphenyl
disilazane, diphenyl tetramethyl silazane and hexamethyl
disilazane.
53. A mesoporous silica film made by the method of claim 32,
comprising: a disordered porosity, lacking a regular geometric
arrangement of pores, and characterized by an x-ray diffraction
peak between about 0.75 and about 2 degrees 2-theta; a dielectric
constant less than 3.0 that is stable; a film thickness from about
0.1 .mu.m to about 1.5 .mu.m; and an average pore diameter less
than or equal to about 20 nm.
54. A mesoporous silica film made by the method of claim 32,
comprising: a disordered porosity as indicated by an absence of an
XRD peak in the range from 2 to 6 degrees 2-theta; a dielectric
constant less than 3.0 that is stable; a film thickness from about
0.1 .mu.m to about 1.5 .mu.m; and an average pore diameter less
than or equal to about 20 nm.
55. A mesoporous film made by the method of claim 12, comprising: a
dielectric constant less than 3.0 that is stable; a film thickness
from about 0.1 .mu.m to about 1.5 .mu.m; and an average pore
diameter less than or equal to about 20 nm.
56. A method of making a mesoporous film comprising the steps of:
(a) combining a silica precursor with an aqueous solvent, an acid
and a polyoxethylene ether surfactant into a precursor solution;
(b) spin-coating said precursor solution into a templated film; (c)
removing said aqueous solvent, said acid and said surfactant
forming a hydroxylated film; and (d) dehydroxylating said
hydroxylated film and obtaining said mesoporous film.
57. The method as recited in claim 56, wherein said polyoxyethylene
ether surfactant is C.sub.12H.sub.25 (CH.sub.2CH.sub.2O).sub.10OH
also known as C.sub.12EO.sub.10 or 10 lauryl ether;
C.sub.16H.sub.33(CH.sub.2CH.sub.2O)- .sub.10OH also known as
C.sub.16EO.sub.10 or 10 cetyl ether; C.sub.18H.sub.37
(CH.sub.2CH.sub.2O).sub.10OH also known as C.sub.18EO.sub.10 or 10
stearyl ether; C.sub.12H.sub.25(CH.sub.2CH.sub.2O- ).sub.4OH also
known as C.sub.12EO.sub.4 or 4 lauryl ether;
C.sub.16H.sub.33(CH.sub.2CH.sub.2O).sub.2OH also known as
C.sub.16EO.sub.2 or 2 cetyl ether or combinations thereof.
58. The method as recited in claim 57, wherein said polyoxyethylene
ether surfactant is in combination with other small surfactants,
with smaller hydrophilic molecules, and with organic
co-solvents.
59. The method as recited in claim 58, wherein said small
surfactants are ammonium-based surfactants.
60. The method as recited in claim 59, wherein said ammonium-based
surfactants are cetyl trimethyl ammonium chloride.
61. The method as recited in claim 58, wherein said smaller
hydrophilic molecules are selected from the group consisting of
glycerol, propylene glycol, and ethylene glycol.
62. The method as recited in claim 58, wherein said organic
co-solvents are selected from the group consisting of mesitylene
and octane.
63. The method as recited in claim 56, wherein said silica
precursor is tetraethyl orthosilicate (TEOS).
64. The method as recited in claim 56, wherein said aqueous solvent
comprises ethanol and water.
65. The method as recited in claim 56, wherein said acid is
hydrochloric acid.
66. A mesoporous film having a dielectric constant less than 2.5, a
film thickness from about 0.2 .mu.m to about 1.5 .mu.m, and an
average pore diameter less than or equal to about 5 nm.
67. A mesoporous film having a thickness from about 0.2 .mu.m to
about 1.5 .mu.m and a standard deviation about said thickness that
is less than +/-5%.
68. A mesoporous silica film prepared from a surfactant containing
solution, having a dielectric constant less than 3 that has both a
relative stability and an absolute stability in a humid atmosphere,
a film thickness from about 0.1 .mu.m to about 1.5 .mu.m, an
average pore diameter less than or equal to about 20 nm, and a
porosity that is disordered.
69. The mesoporous silica film as recited in claim 68, wherein
disordered is indicated by the absence of an X-ray diffraction peak
in the range of about 2 to about 6 degrees 2-theta.
70. The mesoporous silica film as recited in claim 68, wherein
disordered porosity is characterized by an X-ray diffraction peak
between about 0.75 and about 2 degrees 2-theta.
71. A method of making a mesoporous film comprising the steps of:
(a) combining a silica precursor with an aqueous solvent, a
catalyst and a surfactant into a precursor solution; (b) spin
coating said precursor solution into a templated film; (c) removing
said aqueous solvent, said catalyst and said surfactant from said
templated film and forming a hydroxylated film; and (d)
dehydroxylating said hydroxylated film with a gaseous silicon-based
organic compound and obtaining said mesoporous film.
72. The method of claim 71, wherein the silicon-based organic
compound is a silane.
73. The method of claim 71, wherein said dehydroxylation of said
film occurs in alternating exposures of said film to a vacuum and
to the gaseous silane.
74. The method of claim 71, wherein said silane is selected from
the group consisting of trimethyl iodosilane, trimethyl
chlorosilane, dimethyl dimethoxy silane, dimethyl dichloro silane,
hexaphenyl disilazane, diphenyl tetramethyl silazane and hexamethyl
disilazane.
Description
CROSS REFERENCE TO RELATED INVENTION
[0001] This application is a Continuation-In-Part of application
Ser. No. 09/361,499, filed Jul. 23, 1999, now abandoned, which is a
Continuation-in-Part of application Ser. No. 09/335,210, filed Jun.
17, 1999, now abandoned, which is a Continuation-In-Part of
application Ser. No. 09/220,882 filed Dec. 23, 1998, now
abandoned.
FIELD OF THE INVENTION
[0003] The present invention relates generally to porous silica
film with nanometer-scale porosity produced from solution
precursors. More specifically, the present invention relates to
mesoporous silica film from a solution containing a surfactant
(surfactant templated) and the use of specific surfactants to
template porosity with the characteristic pore size being defined
by the surfactant micelle size. The present invention also relates
to the use of dehydroxylation in combination with surfactant
templated mesoporous silica films to obtain a dielectric constant
less than 3 under ambient humid conditions.
[0004] As used herein, the term "silica" means a compound having
silicon (Si) and oxygen (O) and possibly additional elements.
[0005] Further, as used herein, "mesoporous" refers to a size range
which is greater than 1 nm, but significantly less than a
micrometer. In general, this refers most often to a size range from
just over 1.0 nm (10 angstroms) to a few tens of nanometers.
[0006] The term "stable" can mean an absolute stability, a relative
stability or a combination thereof. Relative stability means that a
dielectric constant increases no more than about 20% when a
surfactant templated mesoporous film is taken from an equilibrated
condition of 0.0% relative humidity or vacuum to an equilibrated
condition of 50% relative humidity. Absolute stability means that
the dielectric constant remains less than 3 under any conditions
including humid conditions of at least 40% relative humidity.
[0007] The term "hydroxylated" encompasses partially and fully
hydroxylated. The term "dehydroxylating" encompasses partial or
total removal of hydroxyl groups from surface(s) of the surfactant
templated mesoporous silica film.
BACKGROUND OF THE INVENTION
[0008] Porous silica films are potentially useful as low dielectric
constant intermetal materials in semiconductor devices, as low
dielectric constant coatings on fibers and other structures, and in
catalytic supports. Most of the U.S. semiconductor industry is
presently (1998) in the process of implementing interlevel
dielectric films that are silica films, or derivatives of silica
and silicates, or polymeric films, with less than 25% or no
porosity with dielectric constant (k') in the range of 3.0 to 4.0.
Further reductions in dielectric constant are desired to improve
the operating speed of semiconductor devices, reduce power
consumption in semiconductor devices and reduce overall cost of
semiconductor devices by decreasing the number of metallization
levels that are required.
[0009] Since air has a dielectric constant of 1.0, the introduction
of porosity is an effective way of lowering the dielectric constant
of a film. In addition, because silica dielectrics have been a
standard in microelectronic devices, silica films with porosity are
very attractive to the semiconductor industry for advanced devices
that require low dielectric constant materials. The feature size or
design rule in the semiconductor interconnect will be sub-150 nm in
ultralarge scale integration; and pore sizes to achieve lower
dielectric constant (k<3) must be significantly smaller than the
intermetal spacing.
[0010] Dielectric constant of porous films is dependent on the
material and pore structure. For porous silica films for use in
microelectronic devices, material and pore structure must result in
uniform dielectric constants across the wafers and in different
directions on the wafer. In general, isotropic material and pore
structures are expected to provide the desired uniformity in film
dielectric constant compared to anisotropic material and pore
structures.
[0011] Also, low dielectric constant mesoporous films used
commercially need to be prepared in a manner compatible with a
semiconductor device manufacturing process line, for example spin
coating. For large-area circular wafers, other coating techniques
such as dip coating are not as convenient because dip coating
requires masking of the backside to prevent contamination.
[0012] Surface topography is also very critical to fabrication of a
multilevel interconnect structure. In the "damascene" process for
copper interconnects intended for ultralarge scale integration on
semiconductor chips, each dielectric layer is etched, following
which copper is deposited, and the structure planarized by
chemical-mechanical polishing (CMP). The initial planarity and the
absence of surface texture in the low k dielectric film is very
critical in maintaining planarity at each level of the
interconnect.
[0013] Another important concern with porous dielectric films is
mechanical integrity. Because of their fragility, it appears
unlikely that porous films will be directly polished using
conventional chemical-mechanical-polishing (CMP) equipment, but a
dense "cap" layer of silica or another material on the porous low K
film will be planarized. However, even with a cap layer, the porous
low K material must have adequate stiffness, compressive and shear
strengths, to withstand the stresses associated with the CMP
process.
[0014] Silica films with nanometer-scale (or mesoporous) porosity
may be produced from solution precursors and classified into two
types (1) "aerogel or xerogel" films (aerogel/xerogel) in which a
random or disordered porosity is introduced by controlled removal
of an alcohol-type solvent, and (2) "mesoporous"
surfactant-templated silica films in which the pores are formed
with ordered porosity by removal of a surfactant. Heretofore, the
most successful demonstration of low dielectric constant silica
films with dielectric constant of 3.0 or less has been with
aerogel/xerogel-type porous silica films. However, disadvantages of
aerogel/xerogel films include (1) deposition of aerogel/xerogel
films requires careful control of alcohol removal (e.g. maintaining
a controlled atmosphere containing solvent or gelling agent during
preparation) for formation of the pore structure (2) the smallest
pore size typically possible in aerogel/xerogel films falls in the
size range of 10- 100 nm, and (3) limited mechanical strength
compared to dense selica films. These disadvantages have hindered
implementation of these aerogel/xerogel porous silica films in
semiconductor devices.
[0015] In order to obtain a porous film with a low dielectric
constant of any material made by any process, it is necessary to
minimize the number of hydroxyl groups in the structure, especially
at pore surfaces. The dielectric films must be made hydrophobic in
order for the electrical properties to be stable in humid air.
Hydroxylated surfaces in porous silica films result in a dielectric
constant exceeding that of dense silica (i.e. approximately 4.0).
Physisorption of water molecules by hydroxylated surfaces can
further increase the dielectric constant and effective capacitance
of a mesoporous silica film. Physisorption of water molecules can
be avoided by handling films in non-humid atmospheres or vacuum, or
by minimizing exposure of films to humid conditions. Hydroxyl
groups and physisorbed water molecules may be removed from silica
surfaces at very high temperatures. C. J. Brinker and G. W.
Scherer, in Sol-Gel Science, Academic Press, New York, N.Y. (1990)
(Brinker et al. 1990) discuss thermal dehydroxylation of silica by
exposure to very high temperatures of over 800.degree. C. However,
semiconductor devices with dielectric films and metal lines cannot
usually be processed over about 500.degree. C. Thus, other methods
of dehydroxylation are needed for porous silica films on
semiconductors.
[0016] E. F. Vansant, P. Van der Voort and K. C. Vrancken, in
Characterization and Chemical Modification of the Silica Surface,
Vol. 93 of Studies in Surface Science and Catalysis, Elsevier, New
York, N.Y. (1995), and Brinker et al., 1990, cite procedures for
hydroxylation of silica surfaces by fluorination or by treatment
with silane solutions. Aerogel/Xerogel-type films have been
dehydroxylated by both (a) fluorination treatment, and (b) a
two-step dehydroxylation method of (1) initial silane solution
treatment (e.g. trimethylchlorosilane or hexamethyldisilazane
(HMDS) in a solvent), and then (2) following this solution
treatment with a treatment in hydrogen-containing gases (e.g. 10%
hydrogen in nitrogen) at moderately high temperatures of
300-450.degree. C. The silane/forming gas(H.sub.2 in N.sub.2)
treatment is discussed in U.S. Pat. No. 5,504,042 and some of the
other related patents by Smith and colleagues that are referenced
therein.
[0017] In the surfactant-templated films, the pores form ordered
(e.g. hexagonal) arrays, with the characteristic pore size being
defined by the surfactant micelle size. The surfactant templated
route allows control of the porosity, pore size and pore shape
using the properties of the surfactants and their interactions with
the silica species. For a given level of porosity, this control in
pore size and architecture and structure of the pore walls can also
result in good mechanical properties. More specifically, smaller
and uniform pores can impart better mechanical properties than
larger and non-uniform pores. Although easier to produce (no need
for controlled atmosphere to form the porosity), mesoporous
surfactant templated silica films have not been demonstrated with
low dielectric constant.
[0018] U.S. patent application 08/921,754 filed Aug. 26, 1997 by
Bruinsma et al, now U.S. Pat. No. 5,922,299, describes the
preparation of mesoporous surfactant templated silica films with
ordered porosity by spin coating. The surfactant used was a
cationic ammonium-based surfactant. A goal of this work was
low-dielectric constant interlayers in microelectronic devices.
[0019] U.S. Pat. No. 5,858,457 by Brinker et al also reports a dip
coating procedure for making a surfactant-templated mesoporous
silica film with ordered porosity, where the surfactant used was
also a ammonium-based surfactant. Brinker et al measured the
dielectric constant using a mercury dot electrode on the film,
reporting a value for the dielectric constant of 2.37.
[0020] However, surfactant templated mesoporous silica films
prepared with ammonium surfactants and tested after pyrolysis
(thermal removal) of the surfactant have been found to adsorb
moisture under ambient humid conditions, and therefore do not have
a low dielectric constant under the ambient humid conditions of
normal manufacturing and operating conditions for semiconductor
devices. No dehydroxylation steps are reported in either Bruinsma
et al. or Brinker et. al.
[0021] The paper Continuous Mesoporous Silica Films With Highly
Ordered Large Pore Structures, D. Zhao, P. Yang, N. Melosh, J.
Feng, B F Chmelka, and G D Stucky, Advanced Materials, vol. 10 No.
16, 1998, pp 1380-1385, discusses the formation of directional or
ordered large pore structures in films by dip coating silica based
solutions containing non-ionic poly(alkalene oxide) triblock
copolymers and low molecular weight alkyl(ethylene oxide)
surfactants. Low dielectric constants (1.45-2.1) were reported for
these films as measured after calcination of the films. However, a
disadvantage of ordered porosity, for example hexagonal porosity,
is the uncertainty in uniformity of dielectric constant in
different directions on large wafers. Furthermore, no
dehydroxylation procedures, that are useful for maintaining low
values of dielectric constant, are reported in the paper by Zhao et
al.
[0022] Thus, there is a need for a surfactant templated mesoporous
silica films and method of making them that provides a dielectric
constant less than 3, and that meets engineering requirements
including but not limited to control of film thickness and
thickness uniformity, minimum surface texture, and mechanical
integrity. The dielectric constant must be relatively stable under
normal operating conditions which include humid air at room
temperature, and must be uniform across large wafers .
SUMMARY OF THE INVENTION
[0023] It is therefore an object of the present invention to
provide a surfactant templated mesoporous silica film which has
properties including but not limited to dielectric constant less
than 3, film thickness from about 0.1 .mu.m to about 1.5 .mu.m,
standard deviation of film thickness less than or equal to +/-5%
standard deviation, average pore sizes smaller than about 20 nm,
low dielectric constant and combinations thereof.
[0024] The present invention includes a method of making a
surfactant templated mesoporous film having the same general steps
as described in co-pending U.S. patent application 08/921,754.
Thus, the present invention is a method of making a mesoporous
silica film having the steps of combining a surfactant in a silica
precursor solution, spin-coating a film, heating the film to remove
the surfactant to form a mesoporous film that is at least partially
hydroxylated, and dehydroxylating the partially hydroxylated film
to obtain the mesoporous film. According to the present invention,
selection of surfactant, selection of concentrations of silica
precursor solution constituents and combinations thereof provide a
film having one or more of the features set forth above.
[0025] The advantage of low dielectric constant (k<3) that is
stable at ambient humid conditions is achieved in accordance with
the present invention in combination with dehydroxylation which
involves partial or complete removal of hydroxyl groups at
temperatures within electronic component processing temperatures.
During dehydroxylation, hydroxyl groups may be replaced with
hydrophobic groups such as organic alkyl groups, siloxane
(--Si--O--Si--) bonds or combinations thereof on internal pore
surfaces as well as external surfaces of the surfactant templated
mesoporous film.
[0026] It is advantageous that the surfactants used in spin-coated
surfactant templated mesoporous films as described in the present
invention will result in fine pores smaller than about 20 nm. Most
often the average pore size can be tailored with surfactants in the
size range of about 1 to about 20 nm. This pore size range is
desirable in interlevel dielectric films that separate
metallization lines in semiconductor devices to minimize diffusion
of metal species during repeated heat treatments. Further
advantages of the present invention include a method which provides
for superior control of film thickness and thickness uniformity
across a coated wafer, films with low dielectric constant that is
stable; as well as disordered porosity which increases confidence
in uniformity of dielectric constant in different directions on
large wafers.
[0027] The subject matter of the present invention is particularly
pointed out and distinctly claimed in the concluding portion of
this specification. However, both the organization and method of
operation, together with further advantages and objects thereof,
may best be understood by reference to the following description
taken in connection with accompanying drawings wherein like
reference characters refer to like
elements. BRIEF DESCRIPTION OF THE DRAWINGS
[0028] FIG. 1 shows film porosity as a function of surfactant/TEOS
mole ratio in a spin-coating solution containing polyoxyethylene
ether surfactants as determined by nuclear reaction analysis (NRA)
for the C.sub.XEO.sub.10 polyoxyethylene ether surfactant
series.
[0029] FIG. 2 shows the dielectric constant (measured at room
temperature under ambient conditions in humid air) of a surfactant
templated mesoporous film prepared with C.sub.12EO.sub.10
polyoxyethylene ether surfactant as a function of dehydroxylation
procedures.
[0030] FIG. 3 shows the dielectric constant (measured at room
temperature under ambient conditions in humid air) of a surfactant
templated mesoporous film prepared with C.sub.16EO.sub.10
polyoxyethylene ether surfactant as a function of dehydroxylation
procedures.
[0031] FIG. 4a shows the low angle x-ray diffraction spectrum for
mesoporous silica film prepared with a C.sub.12EO.sub.10
polyoxyethylene ether surfactant. The x-ray beam was along the
radial direction of the circular wafer.
[0032] FIG. 4b shows the low angle x-ray diffraction spectrum for
mesoporous silica film prepared with a C.sub.12EO.sub.10
polyoxyethylene ether surfactant. The x-ray beam was along the
tangential direction of the circular wafer.
[0033] FIG. 5 is a transmission electron micrograph showing
microstructure of the mesoporous silica film prepared with a
C.sub.12EO.sub.10 polyoxyethylene ether surfactant. FIG. 6a is a
surface contour map of a mesoporous silica film prepared with a
C.sub.12EO.sub.10 polyoxyethylene ether surfactant.
[0034] FIG. 6b is a surface profile of a mesoporous silica film
prepared with a C.sub.12EO.sub.10 polyoxyethylene ether
surfactant.
[0035] FIG. 7 is a graph of elastic modulus of a mesoporous silica
film measured by picoindentation, as a function of the indentation
load.
[0036] FIG. 8a shows the low angle x-ray diffraction spectrum for
mesoporous silica film prepared with a C.sub.12EO.sub.10
polyoxyethylene ether surfactant. Surfactant/TEOS mole ratio was 0.
19. The x-ray beam was along the radial direction of the circular
wafer.
[0037] FIG. 8b shows the low angle x-ray diffraction spectrum for
mesoporous silica film prepared with a C.sub.12EO.sub.10
polyoxyethylene ether surfactant. Surfactant/TEOS mole ratio was 0.
19. The x-ray beam was along the tangential direction of the
circular wafer.
[0038] FIG. 9a shows the low angle x-ray diffraction spectrum for
mesoporous silica film prepared with a C.sub.12EO.sub.10
polyoxyethylene ether surfactant. Surfactant/TEOS mole ratio was
0.30. The x-ray beam was along the radial direction of the circular
wafer.
[0039] FIG. 9b shows the low angle x-ray diffraction spectrum for
mesoporous silica film prepared with a C.sub.12EO.sub.10
polyoxyethylene ether surfactant. Surfactant/TEOS mole ratio was
0.30. The x-ray beam was along the tangential direction of the
circular wafer.
[0040] FIG. 10a shows the low angle x-ray diffraction spectrum for
mesoporous silica film prepared with a mixture of C.sub.12EO.sub.10
and C.sub.12EO.sub.4 polyoxyethylene ether surfactant. The total
surfactant to TEOS mole ratio was 0.20. The x-ray beam was along
the radial direction of the circular wafer.
[0041] FIG. 10b shows the low angle x-ray diffraction spectrum for
mesoporous silica film prepared with a mixture of C.sub.12EO.sub.10
and C.sub.12EO.sub.4 polyoxyethylene ether surfactant. The total
surfactant to TEOS mole ratio was 0.20. The x-ray beam was along
the tangential direction of the circular wafer.
[0042] FIG. 11a shows the low angle x-ray diffraction spectrum for
mesoporous silica film prepared with a C.sub.16EO.sub.10
polyoxyethylene ether surfactant. Surfactant/TEOS mole ratio was
0.20. The x-ray beam was along the radial direction of the circular
wafer.
[0043] FIG. 11b shows the low angle x-ray diffraction spectrum for
mesoporous silica film prepared with a C.sub.16EO.sub.10
polyoxyethylene ether surfactant. Surfactant/TEOS mole ratio was
0.20. The x-ray beam was along the tangential direction of the
circular wafer.
[0044] FIG. 12a shows the low angle x-ray diffraction spectrum for
mesoporous silica film prepared with a C.sub.18EO.sub.10
polyoxyethylene ether surfactant. Surfactant/TEOS mole ratio was
0.20. The x-ray beam was along the radial direction of the circular
wafer.
[0045] FIG. 12b shows the low angle x-ray diffraction spectrum for
mesoporous silica film prepared with a C.sub.18EO.sub.10
polyoxyethylene ether surfactant. Surfactant/TEOS mole ratio was
0.20. The x-ray beam was along the tangential direction of the
circular wafer.
[0046] FIG. 12c shows the low angle x-ray diffraction spectrum for
mesoporous silica film prepared with a C.sub.18EO.sub.10
polyoxyethylene ether surfactant. Surfactant/TEOS mole ratio was
0.20. The x-ray beam was along the radial direction of the circular
wafer. The area scanned was located about 90 degrees (rotation)
away from the area scanned in FIG. 12a and b.
[0047] FIG. 12d shows the low angle x-ray diffraction spectrum for
mesoporous silica film prepared with a C.sub.18EO.sub.10
polyoxyethylene ether surfactant. Surfactant/TEOS mole ratio was
0,20. The x-ray beam was along the tangential direction of the
circular wafer. The area scanned was located about 90 degrees
(rotation) away from the area scanned in FIG. 12a and b.
DESCRIPTION OF THE PREFERRED EMBODIMENT
[0048] The present invention is a mesoporous silica film made from
a surfactant containing solution (surfactant templated mesoporous
silica film) which has properties including but not limited to a
dielectric constant less than 3, film thickness from about 0.1
.mu.m to about 1.5 .mu.m, also from about 0.2 .mu.m to about 1.5
.mu.m standard deviation of film thickness less than or equal to
+/-5% standard deviation, average pore sizes smaller than about 20
nm, more preferably less than about 10 nm and most preferably less
than about 5 nm, ordered or disordered porosity, and combinations
thereof. According to the present invention, porosity is greater
than 30%, preferably greater than 40% and more preferably greater
than 50%.
[0049] The present invention includes a method of making a
mesoporous silica film by templating and spin-coating silica
precursor solutions containing a surfactant to form a hydroxylated
film which are the same general steps as described in co-pending
U.S. patent application 08/921,754, and that application is thus
incorporated herein by reference, and then chemically
dehydroxylating the hydroxylated film to form the mesoporous silica
film. Therefore, the present invention is a method of making a
mesoporous silica film having the steps of combining a surfactant
in a silica precursor solution, forming a film by spin-coating,
heat treating the film to remove the surfactant and forming a
mesoporous film that is hydroxylated, and chemically
dehydroxylating the hydroxylated film to obtain the mesoporous
silica film with a low dielectric constant.
[0050] The silica precursor solution includes a silica precursor,
an aqueous solvent, a catalyst and a surfactant. A film is made by
spin-coating a mixture of the silica precursor solution and
surfactant, after which the aqueous solvent, the catalyst, and the
surfactant are removed by heating to form mesoporous silica film
that is hydroxylated. Chemically dehydroxylating the hydroxylated
film results in a mesoporous silica film with a low dielectric
constant. The chemical dehydroxylating is preferably achieved by
exposing the hydroxylated film separately to a silicon-based
organic compound such as a silane, either as the pure liquid or
pure vapor or as a solution, or as a vapor in a carrier gas or gas
mixture, and a dehydroxylating gas. The resulting mesoporous film
has a dielectric constant less than 3 that remains less than 3 in a
humid environment. According to a further preferred embodiment of
the present invention, low dielectric constant (k<3) mesoporous
surfactant-templated films may be obtained by using one or more
dehydroxylation step(s) that includes removing hydroxyl groups from
surfaces of the mesoporous material. In this embodiment, the
surfactant may be any surfactant including but not limited to
non-ionic surfactant, cationic surfactant, anionic surfactant,
amphoteric surfactant, and combinations thereof.
[0051] The precursor solution may include a chemical agent
including but not limited to a second surfactant, a smaller
hydrophilic molecular compound, an organic co-solvent and
combinations thereof. A second surfactant includes but is not
limited to non-ionic surfactant, cationic surfactant, anionic
surfactant, amphoteric surfactant and combinations thereof. Smaller
hydrophilic molecular compound includes but is not limited to
glycerol, propylene glycol, ethylene glycol and combinations
thereof. Organic co-solvent includes but is not limited to
mesitylene, octane and combinations thereof.
[0052] The silica precursor includes but is not limited to
tetraethyl orthosilicate (TEOS), tetramethyl orthosilicate (TMOS)
methyl triethoxysilane, phenyl triethoxy silane, dimethyl dimethoxy
silane, ethyl triethoxysilane and combinations thereof.
[0053] In a preferred embodiment, the aqueous solvent includes
ethanol.
[0054] The catalyst includes but is not limited to inorganic acid
including but not limited to hydrochloric acid, nitric acid,
sulfuric acid; organic acid including but not limited to carboxylic
acid, amino acid and combinations thereof. Carboxylic acid includes
but is not limited to methanoic acid (formic acid), ethanoic acid
(acetic acid), ethandioic acid (oxalic acid), butanoic acid
(butyric acid), and combinations thereof. Amino acid includes but
is not limited to glycine, nitromethane and combinations
thereof.
[0055] A preferred non-ionic surfactant is a polyoxyethylene ether
surfactant. The term "non-ionic" refers to a surfactant chemistry
where cationic (e.g. ammonium or sodium ions) or anionic (e.g.
sulfonate, sulfate or halide) species are not present. The
non-ionic polyoxyethylene ether surfactants described in this
application are small molecules containing carbon, hydrogen and
oxygen, with only a hydroxyl (--OH) group at the hydrophilic end of
the polymer. With the use of these surfactants, in combination with
the dehydroxylation procedure, low dielectric constants (i.e. low
capacitance in films) are obtained using simple synthesis and
processing conditions. Additionally, greater film thickness
uniformity, minimum surface texture, and stability of dielectric
constant are obtained through the use of these surfactants.
[0056] Surfactants in this polyoxyethylene ether family include but
are not limited to C.sub.12H.sub.25 (CH.sub.2CH.sub.2O).sub.10OH
also known as C.sub.12EO.sub.10 or 10 lauryl ether;
C.sub.16H.sub.33(CH.sub.2CH.sub.- 2O).sub.10OH also known as
C.sub.16EO.sub.10 or 10 cetyl ether; C.sub.18H.sub.37
(CH.sub.2CH.sub.20).sub.10OH also known as C.sub.18EO.sub.10 or 10
stearyl ether; C.sub.12H.sub.25(CH.sub.2CH.sub.2O- ).sub.4OH also
known as C.sub.12EO.sub.4 or 4 lauryl ether;
C.sub.16H.sub.33(CH.sub.2CH.sub.2O).sub.2OH also known as
C.sub.16EO.sub.2 or 2 cetyl ether, and combinations thereof.
[0057] Additionally, polyoxyethylene ether surfactant may be used
in conjunction with a chemical agent including but not limited to
other surfactants, smaller hydrophilic molecular compounds
compatible with the ethanol and water present in the aqueous
solvent, organic co-solvents compatible with the surfactant(s) and
combinations thereof. The surfactants include but are not limited
to ammonium-based cationic surfactants such as cetyl trimethyl
ammonium chloride. The organic co-solvents include but are not
limited to mesitylene, octane and combinations thereof. The smaller
hydrophilic molecular compounds include but are not limited to
glycerol, propylene glycol, ethylene glycol, and combinations
thereof. The smaller hydrophilic molecular compounds have much
higher boiling points compared to water and ethanol as well as low
vapor pressures. These smaller hydrophilic molecular compounds are
likely to reside as inclusions in the silica-rich walls that have
formed around the surfactant micelles upon spin-coating and drying,
and upon calcination, these inclusions can leave behind finer scale
porosity in the silica walls.
[0058] The silica precursor solution is made up of four solution
compounds of (1) a silica precursor, preferably tetraethyl
orthosilicate (TEOS); (2) an aqueous solvent, for example, ethanol,
water and combinations thereof; (3) a catalyst for hydrolysis of
the silica precursor, preferably an acid, for example nitric acid
or hydrochloric acid, and (4) a surfactant. Because TEOS is not
soluble in water alone, a co-solvent, preferably ethanol, is added.
Although a preferable solution mixture contains mole ratios of:
TEOS 1.0; water 5; ethanol 5; HC1 0.05; and surfactant 0.17, the
surfactant/TEOS mole ratio can be varied to control the pore-volume
fraction in the final film and to vary the pore structure. Also, it
will be recognized by those skilled in the art that a much wider
range of surfactant sizes and amounts in this family of small
polyoxyethylene ethers may be possible with different solvent
amounts. It is important to avoid precipitation of the silica
precursor in the solution prior to spin coating. Precipitation of
the silica precursor may be avoided by the use of alcohol as a
co-solvent, preferably as a primary solvent, in combination with
acidic pH. Alternatively, precipitation may be avoided by
controlling the water to TEOS mole ratio alone or in combination
with control of pH, addition of alcohol, or both.
[0059] A templated film is made by spin-coating the silica
precursor solution. The solution is dispensed onto the surface of a
substrate and spun using a spin-coater, for example at 2000 rpm for
30 seconds. The substrate is preferably a silicon wafer or an
aluminum-coated silicon wafer, but it is not limited to these
substrates.
[0060] The spin-coating technique used in the present invention
requires no atmosphere control when used with these
surfactant-containing solutions, and the method should be readily
applicable to microelectronics manufacturing. The technique
produces films with good thickness uniformity across wafers ranging
from small to large surface area. Films produced by the method of
the present invention have film thickness from about 0.2 .mu.m to
about 1.5 .mu.m with a thickness variation having a standard
deviation of less than +/-5%. For example, one film with a
thickness of about 0.8 .mu.m had a thickness variation with a
standard deviation of less than 25 nanometers (0.3%) across a
4-inch wafer. The film thickness can be controlled by adjusting the
relative ratios of the solution compounds, and also by varying the
spinning rate during deposition.
[0061] After spin-coating, the surfactant-templated film is formed
into a hydroxylated mesoporous film by removal of the aqueous
solvent, the acid, and the surfactant. Aqueous solvent removal is
typically achieved by heating the spin-coated film. For example,
exposing the spin-coated film to a temperature of 115.degree. C.
for 1 hour completes drying and increases condensation of the
silica. Further heat treatment (calcination) of the film, for
example at a temperature of 475.degree. C. for 5 hours, or at
400.degree. C. on a hot plate for 5 minutes in N2 gas, removes the
surfactant and forms a mesoporous film that is partially
hydroxylated.
[0062] The partially hydroxylated film is chemically dehydroxylated
into a mesoporous silica film by exposing the partially
hydroxylated film to a silicon-based organic compound such as a
silane, either as the pure liquid or pure vapor or as a solution,
or as a vapor in a carrier gas or gas mixture. The silane can be
chosen from the following and not limited to trimethyl iodosilane,
trimethyl chlorosilane, dimethyl dimethoxy silane, hexamethyl
disilazan dimethyl dichlorosilane hexaphenyl disilazane, and
diphenyl tetramethyl silazane. Additionally, the silane exposed
film may be further exposed to, a dehydroxylating gas or to a heat
treatment. The silane treatment may be proceeded and followed by a
vacuum treatment or a treatment in an inert gas or forming gas, or
both. The partially hydroxylated film is preferably dehydroxylated
in a two-step process which includes a soak treatment in a solution
of hexamethyl disilazane in an organic solvent and exposure to an
H.sub.2 in N.sub.2 gas at an elevated temperature. The partially
hydroxylated film is more preferably dehydroxylated in a multiple
step high temperature process, which includes an initial vacuum
treatment, followed by a vapor phase silane treatment, followed by
a second vacuum treatment. The silane/vacuum treatment step is
preferably repeated using the same silane or a different silane and
is followed by a high temperature inert gas or forming gas
treatment.
[0063] For example, soaking the hydroxylated film for 24 hours in a
10% solution of hexamethyl disilazane in toluene and then exposing
it to 2% H.sub.2 in N.sub.2 gas at 400.degree. C. for 2 hours
results in effective dehydroxylation of the mesoporous film, which
then exhibits stable dielectric properties in moist air. This
sequence of dehydroxylation process steps is preferably repeated
once. The resulting mesoporous film has a dielectric constant
typically less than 2.5 under ambient humid conditions, and the
dielectric constant of the film is stable in moist or humid
atmosphere over long periods of time.
EXAMPLE 1
[0064] An experiment was conducted to demonstrate the efficacy of a
preferred embodiment of the present invention. Three different
surfactants in the polyoxyethylene ether family were investigated:
(1) C.sub.12H.sub.25 (CH.sub.2CH.sub.2O).sub.10OH, also known as
C.sub.12EO.sub.10 or 10 lauryl ether; (2)
C.sub.16H.sub.33(CH.sub.2CH.sub- .2O).sub.10OH, also known as
C.sub.16EO.sub.10 or 10 cetyl ether; and (3) C.sub.18H.sub.37
(CH.sub.2CH.sub.2O).sub.10OH, also known as C.sub.18EO.sub.10 or 10
stearyl ether. All the films with these surfactants were prepared
using a solution with the following molar ratios:
TEOS:H20:ethanol:hydrochloric acid=1: 5 : 5 : 0.05.
[0065] The surfactant/TEOS mole ratio was varied from about 0.10 to
about 0.50. All the components except for the TEOS were mixed until
a homogeneous solution was obtained. When the surfactant/TEOS mole
ratio is greater than about 0.2 and ratios of TEOS:H20:ethanol are
about 1: 5 : 5, homogenaity is more readily achieved by heating the
solution from about 40.degree. C. to about 50.degree. C.,
especially for polyoxethylene ether surfactants. Heating may not be
needed for more dilute solutions.
[0066] TEOS was then added and the solution was stirred. Following
addition of TEOS, the solution was aged for 20 hours at room
temperature. No precipitate was formed under these solution
conditions.
[0067] The aged solution was dispensed onto the surface of polished
4-inch Si wafers by spin-coating at 2000 rpm for 30 seconds using a
spin-coater.
[0068] The resulting surfactant-templated films were converted to a
mesoporous film by removing the aqueous solvent, the acid, and the
surfactant. This removal was achieved by subjecting the templated
films to a temperature of 115.degree. C. for 1 hour. Complete
removal of the surfactant from the films was achieved by
calcination (heat treatment) at 475.degree. C. for 5 hours.
[0069] Prior to making electrical/capacitance measurements, the
calcined films were characterized by nuclear reaction analysis
(NRA) to determine porosity, and by profilometry to measure
thickness. The NRA porosity data was not used as an exact measure
of porosity, but rather was used for guidance to help determine
which films to select for further electrical/capacitance
measurements.
[0070] FIG. 1 shows the porosity determined by NRA for the
C.sub.XEO.sub.10 polyoxyethylene ether surfactant series. The graph
shows only porosity values using the different surfactants for only
specific surfactant/TEOS values. For several higher surfactant/TEOS
ratios the film quality was not acceptable for evaluation of
electrical properties, and films formed with such ratios were
therefore not investigated further. For consideration as dielectric
films in semiconductor devices, the film thickness should be in the
range of about 0.5 to about 1.2 .mu.m. In addition, the films
should be uniform in thickness, crack-free, and without major
blemishes or surface defects. Films with non-wetted islands,
cracks, ring-like structures, serrated patterns or cloudy
inclusions were not considered for electrical evaluation. Defects
such as comets (e.g. due to dust particles on the wafer) on
otherwise uniform films were considered acceptable, as these could
not be attributed to inherent solution properties. The table E1-1
lists the observations in terms of film quality with these
surfactants at different concentrations.
1TABLE E1-11 Film Quality for C.sub.xEO.sub.10 based Films
Surfactant>>>> Surfactant/TEOS Film Quality Film
Quality Film Quality mole ratio (below) C.sub.12EO.sub.10
C.sub.16EO.sub.10 C.sub.18EO.sub.10 0.10 Good Acceptable Acceptable
0.17 Good Poor Poor 0.24 Poor Acceptable Poor 0.30 Poor Acceptable
Poor 0.40 Poor Poor Poor 0.50 Poor Poor Poor
[0071] Based on the NRA porosity data shown in FIG. 1 and the
observations concerning film quality, two films were selected for
electrical measurements. These two films as shown in Table 1 were
those prepared with solutions containing (1) C.sub.12EO.sub.10,
surfactant/TEOS mole ratio of 0.17; and (2) C.sub.16EO.sub.10,
surfactant/TEOS mole ratio of 0.30.
[0072] Initial electrical testing of these calcined films for
capacitance using a precision LCR meter yielded dielectric
constants (i.e. capacitance) much higher than expected for porous
films, because the film still contained a significant amount of
hydroxyl (--OH) groups.
[0073] Each of these two partially hydroxylated films was therefore
dehydroxylated by exposing the hydroxylated film separately to a
silane and a dehydroxylating gas. The films were dehydroxylated by
treatments of soaking for 24 hours in a 10% solution of hexamethyl
disilazane in toluene and exposure for 2 hours to 2% H.sub.2 in
N.sub.2 gas at 400.degree. C. This sequence of dehydroxylation
process steps was repeated once on each film, and the dielectric
constant was measured after each of these steps.
[0074] The capacitance measurements were performed as follows. The
backside of the wafer was scratched/etched to expose bare silicon
surface and a layer of gold was then sputter-deposited. On the top
film side, an array of gold dots approximately 2.8 mm in diameter
was formed by sputtering using a shadow mask. Capacitance was
measured at room temperature at ambient conditions for four dots on
each sample, and the dielectric constant was calculated using the
film thickness and dot diameter. The dielectric constant data
obtained in this way is shown in FIG. 2 and FIG. 3 for the two
different films.
[0075] The data in FIG. 2 shows that a dielectric constant of 1.80
can be obtained for the film synthesized with the C.sub.12EO.sub.10
surfactant. The data in FIG. 3 shows that a dielectric constant of
1.85 can be obtained for the fihn synthesized with the
C.sub.16EO.sub.10 surfactant. Such low dielectric constants
indicate tremendous promise for application of such mesoporous
silica films prepared with small polyoxyethylene ether surfactants
in semiconductor devices. The low dielectric constants obtained
with these films are also relatively stable, increasing by less
than 5% over a period of one day in ambient laboratory conditions
with temperatures at 20-22.degree. C. and a relative humidity of
40-65%. The dielectric constants did not increase in value
thereafter.
[0076] The pore structure of the surfactant templated mesoporous
film was probed by low angle x-ray diffraction (XRD) and
transmission electron microscopy (TEM). The XRD spectra of the film
on the wafer for two different directions, radial and tangential,
are shown in FIGS. 4A, 4B. The spectra did not contain any peak in
the range of 2 to 6 degrees 2-theta, thereby indicating that pores
are not ordered. A TEM micrograph of a section of the film in FIG.
5 indicates that the pores are disordered with an isotropic
nanoporous structure. Pore sizes were estimated to be less than 3
nm from TEM micrographs, and from nitrogen adsorption/desorption
analysis of powders prepared from the solution by a rapid spray
drying process.
[0077] Film planarity and surface topography were measured by
optical profilometry. Minimal striation-type surface texture was
observed in these films. A surface contour map of the film is shown
in FIG. 6a. Roughness is generally less than .+-.50 angstroms over
length scales of tens of microns (FIG. 6b).
[0078] The elastic modulus of the mesoporous silica film was
measured with a Hysitron Picoindenter.TM. using a Berkowich diamond
tip. The instrument and tip were calibrated on a dense silica
standard with a modulus of 70 GPa. A range of indentation loads
(50-300 .mu.N) and residence time (50-900 s) at maximum load were
studied. For the measurement parameters used, indentation depths
were less than 10% of the film thickness, and therefore substrate
effects were not expected to affect the measured values. The effect
of indentation load on measured modulus values for a residence time
of 300 s is shown in FIG. 7 for the mesoporous silica film. The
modulus is in the range of 14-17 GPa for a highly porous film
prepared with the C.sub.12EO.sub.10 poyloxyethylene ether
surfactant with a porosity of =55%. The relative modulus of the
porous silica film with respect to dense silica is in reasonable
agreement with calculations for porous solids based on either
closed or open porosity. The relatively high modulus for the porous
film indicates promise for withstanding CMP in interconnect
fabrication.
EXAMPLE 2
[0079] An experiment was conducted to demonstrate use of various
polyoxethelene ether surfactants alone and in combination in
preparation of a mesoporous silica film with low dielectric
constants. The components of the spin coating solutions including
the surfactant type and amount used in each solution, as well as
the dielectric constant of selected films are shown in Table E2-1.
All the components except for the TEOS were mixed until a
homogeneous solution was obtained. In this experiment, the
components were added in the following order: surfactant, ethanol,
water and acid. When the surfactant was a solid at room
temperature, the surfactant was heated to about 30 to 40.degree. C.
to melt the surfactant, before adding other solution components.
Heating the surfactant is not always necessary, but a homogeneous
solution could be more readily obtained by this procedure. TEOS was
then added and the solution was stirred. Following addition of
TEOS, the solution was aged for 20 hours at room temperature and
dispensed onto the surface of polished 4-inch Si wafers by
spin-coating at 2000 rpm for 30 seconds using a spin-coater.
[0080] The resulting surfactant-templated films were converted to
mesoporous film by heating on a series of three hot plates. The
highest hot plate temperature was about 400.degree. C. Selected
films from this set were subjected to a dehydroxylation procedure
including treatment in hexamethyl disilazane solution followed by
treatment in 2% H.sub.2/N.sub.2 as described previously, and the
film dielectric constants measured. The table shows that film
dielectric constants of less than 2.25 can be obtained using more
than one surfactant.
2 TABLE E2-1 Composition: Mole Ratio Sample # Teos H.sub.2O ETOH
HNO3 Surfactant 1 Surfactant 2 K' CC24 1 5 10 0.05 0.1
C.sub.12EO.sub.10 0.1 C.sub.16EO.sub.10 CC25 1 5 10 0.05 0.13
C.sub.12EO.sub.10 0.13 C.sub.16EO.sub.10 CC26 1 5 10 0.05 0.15
C.sub.12EO.sub.10 0.15 C.sub.16EO.sub.10 2.16 CC27 1 5 5 0.05 0.06
C.sub.12EO.sub.10 0.06 C.sub.18EO.sub.20 CC28 1 5 5 0.05 0.1
C.sub.12EO.sub.10 0.1 C.sub.18EO.sub.20 2.11 CC29 1 5 5 0.05 0.1
C.sub.12EO.sub.10 0.1 C.sub.12EO.sub.4 CC30 1 5 5 0.05 0.13
C.sub.12EO.sub.10 0.13 C.sub.12EO.sub.4 CC31 1 5 5 0.05 0.15
C.sub.12EO.sub.10 0.15 C.sub.12EO.sub.4 2.23
EXAMPLE 3
[0081] The disordered pore structures of films prepared with
polyoxethelene ether surfactants were probed more extensively by
low angle x-ray diffraction to determine any characteristic
features in the x-ray spectra of these films. The components of the
spin coating solutions including the surfactant type and amount
used in each solution are shown in Table E3-1. All the components
except for the TEOS were mixed until a homogeneous solution was
obtained. In this experiment, the components were added in the
following order: surfactant, ethanol, water and acid. When the
surfactant was a solid at room temperature, the surfactant was
heated to about 30 to 40.degree. C. to melt the surfactant, before
adding other solution components. Heating the surfactant is not
always necessary, but a homogeneous solution could be more readily
obtained by this procedure. TEOS was then added and the solution
was stirred. Following addition of TEOS, the solution was aged for
20 hours at room temperature and dispensed onto the surface of
polished 4-inch Si wafers by spin-coating at 2000 rpm for 30
seconds using a spin-coater.
3TABLE E3-1 Composition: Mole Ratios Sample # TEOS H.sub.2O EtOH
HCl HNO3 Surfactant 144-3-I-D 1 5 5 0.05 0.19 C.sub.12EO.sub.10
CC22C 1 5 10 0.05 0.3 C.sub.12EO.sub.10 CC29A 1 5 5 0.05 0.1
C.sub.12EO.sub.10 0.1 C.sub.12EO.sub.4 CC81-1B 1 5 20 0.05 0.2
C.sub.16EO.sub.10 CC83-1B 1 5 20 0.05 0.2 C.sub.18EO.sub.10
[0082] The resulting surfactant-templated films were converted to
mesoporous films by heating on a series of three hot plates. The
highest hot plate temperature was about 400.degree. C. Two films
from this set, 143-3-I-D and CC22C were subjected to a
dehydroxylation procedure including a treatment in hexamethyl
disilazane solution followed by treatment in 2% H.sub.2/N.sub.2 as
described previously.
[0083] The films were probed by x-ray diffraction, using the
experimental parameters below. X-ray spectra were obtained on a
scanned area about 1 cm.times.1 cm in extent, with the centroid of
the scanned area being located about 3.5 cm from the center of the
wafer. Spectra were obtained in both the radial and tangential
directions of the x-ray beam with respect to the circular
wafer.
[0084] Scan Range: 1.00-6.00 deg (2 Theta)
[0085] Scan Rate: 0.05 deg / 10 sec
[0086] Scan Type: Continuous (i.e., not Step-Scan)
[0087] Diffractometer: Philips X'Pert MPD (Model PW3040/00)
[0088] X-ray Source: Sealed Ceramic Tube, Long-Fine Focus (LFF) Cu
Anode (Cu K alpha radiation)
[0089] X-ray Power: 40 kV, 50 mA (2000 W)
[0090] Gonoimeter Radius: 250 mm.
[0091] Incident Beam Optics:
[0092] 0.04 rad Soller Slit
[0093] Programmable, Automatic Divergence Slit (10 mm spot
length)
[0094] 10 mm Beam Mask (10 mm spot width)
[0095] Receiving Optics:
[0096] 0.04 rad Soller Slit
[0097] Programmable, Automatic Anti-Scatter Slit (10 mm spot
length)
[0098] Programmable Receiving Slit (0.2 mm)
[0099] Curved Graphite Monochromator
[0100] Detector: Xe Proportional Counter
[0101] 144-3-I-D: X-ray spectra corresponding to the radial and
tangential directions are shown in FIG. 8a, FIG. 8b, respectively.
The intensity of the diffracted or reflected beam steadily
increases as lower angles are approached, because a greater
percentage of the direct beam reaches the detector, in spite of
careful alignment of the system components and control of the
sample height relative to the path of the incident and reflected
beam. In spite of this increasing intensity, there is evidence of a
peak near 1.1 degrees 2-theta in both spectra. Transmission
electron microscopy of a thin section of this film showed no
evidence of ordered porosity. The areas of the film that were
studied did not show any regular geometric arrangement of pores,
especially long-range geometric arrangement.
[0102] CC22C: X-ray spectra corresponding to the radial and
tangential directions are shown in FIG. 9a, FIG. 9b respectively.
In the tangential directions, there is evidence of a peak at about
1.1 degrees, but in the radial direction, a clear peak is not
evident. Only increasing intensity with lower angles is
observed.
[0103] CC29C: X-ray spectra corresponding to the radial and
tangential directions are shown in FIG. 10a, FIG. 10b respectively.
In both the radial and the tangential directions, there is evidence
of a peak at about 1.1 to 1.2 degrees 2-theta.
[0104] CC81-1B: X-ray spectra corresponding to the radial and
tangential directions are shown in FIG. 11a, FIG. 11b respectively.
In both the radial and the tangential directions, there is evidence
of a peak at about 1.1 to 1.2 degrees 2-theta.
[0105] CC83-1B: Two sets of x-ray spectra obtained on this sample
are shown in FIGS. 12a-12d. One set (radial and tangential
direction) was obtained about a quarter-wafer away (about 90
degrees rotation of the wafer) from the other. In FIGS. 12a, 12b,
the spectra obtained in the radial and tangential direction in one
area do not show clear evidence of a peak at low angles. However,
each of the spectra in FIGS. 12c, 12d from the other area on the
sample contains a single peak at around 1.1-1.2 degrees 2-theta.
Transmission electron microscopy of a thin section of this film
showed no evidence of ordered porosity. The areas of the fihn that
were studied did not show any regular geometric arrangement of
pores, especially long-range geometric arrangement.
[0106] The observations concerning x-ray reflections at low angle
and transmission electron microscopy in this example, in
combination with the observations concerning x-ray reflections and
the microstructure by TEM in Example 1 are consistent with a pore
structure that does not have any ordered geometric "crystalline"
arrangement, especially long range. This disordered porosity was
characterized by an X-ray diffraction peak at very low angles
(about 0.75 to about 2 degrees 2- theta). It is to be noted that
this peak is not observed 100% of the time for disordered
porosity.
EXAMPLE 4
[0107] Dehydroxylation of mesoporous silica films utilizing a
silane in the vapor form at room temperature can produce dielectric
constants less than 2.5. A mesoporous film on a silicon wafer was
placed in a stainless steel reaction vessel having an internal
volume of .about.0.081 cm.sup.3. The reaction vessel (equipped with
inlet and outlet high temperature valves) was connected to a high
vacuum line via vacuum tubing. The reactor was placed in a sand
bath and temperatures were monitored employing thermocouples
deployed uniformly about the reactor. The initial heating up step
(0 min. to 2 hrs) was conducted with the chamber placed under high
vacuum (.about.10.sup.-5 torr). After the reaction chamber had
achieved the desired temperature it was opened to the silane in
vapor phase. The pressure of the silane vapor was dependent on the
silane's vapor pressure at or near its boiling point. After the
desired time had elapsed the chamber was placed under vacuum. The
treatments of the mesoporous films essentially consisted of a
vacuum treatment followed by one or more silane treatments in vapor
phase followed by one or more vacuum treatments at temperatures
ranging from 298 to 723 K (25 C to 450 C). Vacuum treatments and
silane treatments were varied in duration from 5 minutes to 2
hours. This procedure was repeated for a number of cycles for the
samples illustrated in Table E4-1. Upon cooling, the wafer was
removed from the reaction vessel and, following deposition of gold
electrodes on the surfaces, was placed in a tube furnace and
treated with forming gas (2%H.sub.2/N.sub.2) at 673 K (400 C) for
two hours. The capacitance of the film was measured under ambient
conditions. The films were also placed in a flow of dry nitrogen
gas and film capacitance measured. Finally the wafer was placed in
a sealed glass container containing a beaker of water to simulate
100% relative humidity for time periods ranging from 1 to 3 days,
and the sample then was removed and the capacitance measured again
in room air. Several different silanes were investigated, including
trimethyl iodosilane, trimethyl chlorosilane, dimethyl dimethoxy
silane, and hexamethyl disilazane. The results of experiments with
trimethyl iodosilane and hexamethyl disilazane are set forth in
Table E4-1. These results illustrate that, depending on the silane
and the treatment conditions employed, low dielectric constants
(<2.5) on mesoporous silica films can be achieved with a
procedure that includes an exposure of the mesoporous silica film
to silane. This exposure may occur in high humidity conditions.
These results indicate that dehydroxylation by silanes may be most
effective with a procedure that includes removal of gas-phase or
physisorbed species in the porous film before and/or after the
silane treatment step. This removal of gas-phase or physisorbed
species was carried out by treatment in vacuum or by treatment in
flowing forming gas, but may also be accomplished by treatment in
other flowing inert gases such as high purity nitrogen or
argon.
4TABLE E4-1 Number K' Silane Total of cycles K' (in air, (pressure
Mmole Temp Time* (silane K' (in flowing after 100% Sample # in
torr) silane .degree. C. (min) treatment) (in air) nitrogen)
humidity)** JB-3 (CH.sub.3) 0.16 275 60 1 1.66 1.57 1.91 .sub.3SiI
36 torr JB-6 CH.sub.3) 0.17 400 10 3 1.73 1.65 1.72 .sub.3SiI 30
torr JB-8 HMDS*** 0.17 350 10 5 1.77 1.67 1.86 19 torr *Silane
treatment time, in minutes. Generally, vacuum time is the same.
**Exposure to 100% humidity for .about.15 hours, capacitance
immediately measured thereafter in ambient air. ***Hexamethyl
disilazane
[0108] It is believed that such treatment may be best accomplished
in a chamber wherein the film temperature can be controlled, and
where the required gases can be fed into the chamber in the proper
sequence, and the chamber pumped down in vacuum before and/or after
silane exposure. We designed an experimental reaction chamber to be
used for 20 dehydroxylation of mesoporous silica films supported on
silicon wafers which could be an independent chamber or part of an
integrated spin-track tool. The stainless steel chamber is
constructed to hold 4, 6, 8 and 12 inch wafers. Under high vacuum
the chamber will support an outer pressure of one atmosphere, and
the cooled seal on the front-opening door will maintain a vacuum of
10.sup.-5 torr. The internal self-heating shelves will heat to 500
C, and internal circulation is assured with a fan. After an initial
vacuum treatment, a gaseous silane is pumped into the chamber at
the desired pressure, and thereafter a vacuum is again applied. The
cycle may be repeated as many times as necessary to achieve the
desired degree of dehydroxylation. After the last vacuum treatment,
forming or an inert gas is pumped into the chamber.
EXAMPLE 5
[0109] Mono- and di-alkyl substituted alkoxysilanes can be used as
additional silica precursors in the surfactant-containing spin
coating solution used to prepare low dielectric constant mesoporous
silica films with dielectric constants of <2.5. A series of
solutions were prepared as described in example 1 except that
methyl triethoxysilane and dimethyl dimethoxysilane were added to
the one mole ratio of tetraethoxysilane. Molar ratios of 0.95 :
0.05 to 0.25: 0.75 of TEOS to the alkyl-ethoxysilane respectively
were prepared. The surfactant used was 10 lauryl ether. The
surfactant to silica precursor mole ratio was 0.17. Silicon wafers
were spin coated with these solutions and heat-treated as described
in example 2. Selected coated wafers were subjected to the
dehydroxylation treatment as follows. The coated silicon wafer was
placed in a stainless steel reaction vessel having an internal
volume of .about.0.081 cm.sup.3. The reaction vessel (equipped with
inlet and outlet high temperature valves) was connected to a high
vacuum line via vacuum tubing. The reactor was placed in a sand
bath and temperatures were monitored employing thermocouples
deployed uniformly about the reactor. The initial heating up step
(0 min. to 2 hrs) was conducted with the chamber placed under high
vacuum (.about.10.sup.-5 torr). After the reaction chamber had
achieved the desired temperature it was opened to the silane in
vapor phase. The pressure of the silane vapor was dependent on the
silane's vapor pressure at or near its boiling point. After the
desired time had elapsed the chamber was placed under vacuum. The
treatments of the mesoporous films essentially consisted of a
vacuum treatment followed by one or more silane treatments in vapor
phase followed by one or more vacuum treatments at temperatures
ranging from 298 to 723 K (25 C to 450 C). Vacuum treatments and
silane treatments were varied in duration from 5 minutes to 2
hours. Upon cooling, the wafer was removed from the reaction vessel
and gold electrodes deposited on the surfaces. The capacitance of
the film was measured under ambient conditions. The films were also
placed in a flow of dry nitrogen gas and film capacitance measured.
The results of one wafer are given in Table E5-1.
5TABLE E5-1 Dehydroxy Siloxane lation Total molar silane mmol Time
Temp No. of K' K' Sample # ratio (pressure) silane (min) (.degree.
C.) cycles Air nitrogen JB-21 0.85 (CH3)3SiI 0.23 10 390 5 2.24
2.23 TEOS 17 torr 0.15 Methyl triethoxide
CLOSURE
[0110] While a preferred embodiment of the present invention has
been shown and described, it will be apparent to those skilled in
the art that many changes and modifications may be made without
departing from the invention in its broader aspects. The appended
claims are therefore intended to cover all such changes and
modifications as fall within the true spirit and scope of the
invention.
* * * * *