U.S. patent application number 08/985904 was filed with the patent office on 2002-01-03 for low refractive index sio2 film and process for producing the same.
This patent application is currently assigned to DAI NIPPON PRINTING CO., LTD. Invention is credited to ICHIMURA, KOJI.
Application Number | 20020001725 08/985904 |
Document ID | / |
Family ID | 26568491 |
Filed Date | 2002-01-03 |
United States Patent
Application |
20020001725 |
Kind Code |
A1 |
ICHIMURA, KOJI |
January 3, 2002 |
LOW REFRACTIVE INDEX SIO2 FILM AND PROCESS FOR PRODUCING THE
SAME
Abstract
A low refractive index SiO.sub.2 film is provided which uses a
starting material for forming an SiO.sub.2 film and has a lower
refractive index than the conventional SiO.sub.2 film. A starting
material gas comprising a gas containing a fluorine atom, a gas
containing a silicon atom and an alkyl group having 1 to 4 carbon
atoms or an alkyl group having 1 to 4 carbon atoms with a part or
the whole of hydrogen atoms substituted by a fluorine atom, and a
gas containing an oxygen atom is subjected to plasma CVD in a
vacuum chamber 1 to form an SiO.sub.2 film on a web 2 in a plasma
zone 5. The SiO.sub.2 film thus formed has, introduced thereinto,
at least one low refractive index element selected from a fluorine
atom, an alkyl group having 1 to 4 carbon atoms, and an alkyl group
having 1 to 4 carbon atoms with a part or the whole of hydrogen
atoms substituted by a fluorine atom, and the SiO.sub.2 film with
the low refractive index element introduced thereinto has a lower
refractive index than an SiO.sub.2 film with the low refractive
index element not introduced thereinto.
Inventors: |
ICHIMURA, KOJI; (TOKYO-TO,
JP) |
Correspondence
Address: |
ROGER W PARKHURST
PARKHURST & WENDEL
1421 PRINCE STREET
SUITE 210
ALEXANDRIA
VA
223142805
|
Assignee: |
DAI NIPPON PRINTING CO.,
LTD
|
Family ID: |
26568491 |
Appl. No.: |
08/985904 |
Filed: |
December 5, 1997 |
Current U.S.
Class: |
428/446 ;
427/497; 427/503; 427/568; 427/574; 428/447; 428/451 |
Current CPC
Class: |
C23C 16/401 20130101;
Y10T 428/31663 20150401; Y10T 428/31667 20150401; G02B 1/111
20130101 |
Class at
Publication: |
428/446 ;
428/447; 428/451; 427/497; 427/503; 427/568; 427/574 |
International
Class: |
G02B 001/10 |
Foreign Application Data
Date |
Code |
Application Number |
Dec 18, 1996 |
JP |
354141/1996 |
Oct 31, 1997 |
JP |
315992/1997 |
Claims
1. A low refractive index SiO.sub.2 film comprising an SiO.sub.2
film with a low refractive index element introduced thereinto, the
low refractive index element being at least one member selected
from the group consisting of a fluorine atom, an alkyl group having
1 to 4 carbon atoms, and an alkyl group having 1 to 4 carbon atoms
with a part or the whole of hydrogen atoms substituted by a
fluorine atom.
2. The low refractive index SiO.sub.2 film according to claim 1,
which is adapted for use as an antireflection film.
3. The low refractive index SiO.sub.2 film according to claim 1,
which has been prepared by CVD using at least one starting material
selected from the group consisting of a gas containing a fluorine
atoms, a starting material containing a silicon atom and an alkyl
group having 1 to 4 carbon atoms, and a starting material
containing a silicon atom and an alkyl group having 1 to 4 carbon
atoms with a part or the whole of hydrogen atoms substituted by a
fluorine atom.
4. The low refractive index SiO.sub.2 film/according to claim 3,
wherein the gas containing a fluorine atom is selected from the
group consisting of CF.sub.4, C.sub.2F.sub.6, C.sub.3F.sub.6, and
SF.sub.6.
5. The low refractive index SiO.sub.2 film according to claim 3,
wherein the starting material containing a silicon atom and an
alkyl group having 1 to 4 carbon atoms comprises at least one
compound selected from the group consisting of HMDSO, TMDSO,
octamethylcyclotetrasiloxane, TEOS, MTMOS, methylsilane,
dimethylsilane, trimethylsilane, diethylsilane, propylsilane, and
phenylsilane.
6. The low refractive index SiO.sub.2 film according to claim 3,
wherein the starting material containing a silicon atom and an
alkyl group having 1 to 4 carbon atoms with a part or the whole of
hydrogen atoms substituted by a fluorine atom comprises at least
one compound selected from the group consisting of HMDSO, TMDSO,
octamethylcyclotetrasiloxane, TEOS, MTMOS, methylsilane,
dimethylsilane, trimethylsilane, diethylsilane, propylsilane, and
phenylsilane, a part or the whole of hydrogen atoms in each of said
compounds being substituted by a fluorine atom.
7. A process for producing a low refractive index SiO.sub.2 film,
comprising forming a thin film on a substrate by CVD using a
starting material gas comprising a gas containing a fluorine atom,
a gas containing a silicon atom and an alkyl group having 1 to 4
carbon atoms or an alkyl group having 1 to 4 carbon atoms with a
part or the whole of hydrogen atoms substituted by a fluorine atom,
and a gas containing an oxygen atom.
8. A process for producing a low refractive index SiO.sub.2 film,
comprising the steps of: (1) introducing a mixed gas, comprising a
gas containing a fluorine atom, a volatile gas of an organic
compound containing a silicon atom and an alkyl group having 1 to 4
carbon atoms or an alkyl group having 1 to 4 carbon atoms with a
part or the whole of hydrogen atoms substituted by a fluorine atom,
and a gas containing an oxygen atom, into a vacuum chamber
maintained in a vacuum of 10.sup.-3 to 1 mmHg (Torr) and bringing
the mixed gas to a plasma stream by glow discharge; and (2)
bringing the plasma stream into contact with the surface of a
substrate disposed within the vacuum chamber to form a thin film on
the substrate.
Description
BACKGROUND OF THE INVENTION
[0001] The present invention relates to a low refractive index
SiO.sub.2 film which is applicable to a light antireflection film
on various surfaces of curve mirrors, back mirrors, goggles, window
glasses, displays of personal computers and word processors, and
other various commercial displays.
[0002] In recent years, antireflection films have been applied to
surfaces of various displays of word processors, computers, and
televisions, surfaces of various optical lenses and optical
articles, and surfaces of window glasses of automobiles and
electric railcars to prevent the reflection of light from the above
surfaces.
[0003] An about 0.1 .mu.m-thick thin film of MgF.sub.2 formed on
glass will be described as an example of the antireflection film.
It is already known that, when incident light perpendicularly
enters a thin film, in order for the antireflection film to prevent
the reflection of light by 100% and to pass light by 100%
therethrough, relationships represented by the equations (1) and
(2) should be met (see "Science Library" Physics=9 "Optics,"
pp.70-72, 1980, Science Sha Ltd., Japan).
n.sub.0={square root}{square root over (n.sub.g)} (1)
n.sub.0h=.lambda..sub.0/4 (2)
[0004] wherein .lambda..sub.0 represents a specific wavelength,
n.sub.0 represents the refractive index of the antireflection film
at this wavelength, h represents the thickness of the
antireflection film, and n.sub.g represents the refractive index of
the substrate.
[0005] It is already known that the refractive index n.sub.g of
glass is about 1.5, the refractive index no of MgF.sub.2 film is
1.38, and the wavelength .lambda..sub.0 of incident light is 5500
.ANG. (reference). When these values are substituted in the
equation (2), the results of calculation show that the thickness h
of the antireflection film is about 0.1 .mu.m which is the
thickness of an optical thin film. For this reason, an optical thin
film having such thickness has been used as the antireflection
film. Vacuum process, such as vacuum deposition, sputtering, ion
plating, and plasma CVD, have been known to be suitable for the
formation of such an optical thin film.
[0006] From the equation (1), it is apparent that prevention of the
reflection of light by 100% can be attained by the selection of
such a material that the refractive index of the upper coating is
approximately equal to a value of square root of the refractive
index of the lower coating. The antireflection of light by
utilizing the above principle to make the refractive index of the
upper coating lower than the refractive index of the lower coating,
that is, by providing a thin film of a high refractive index layer
and a thin film of a low refractive index layer in that order on a
substrate, has hitherto been performed in the art.
[0007] An SiO.sub.2 film is generally known as a low refractive
index film and hence has been extensively used as antireflection
films and the like. Antireflection films having various layer
constructions, however, have been desired to have lower refractive
index.
DISCLOSURE OF THE INVENTION
[0008] Accordingly, an object of the present invention is to
provide a low refractive index SiO.sub.2 film which uses a starting
material for forming an SiO.sub.2 film and has a lower refractive
index than the conventional SiO.sub.2 film.
[0009] According to one aspect of the present invention, there is
provided a low refractive index SiO.sub.2 film comprising an
SiO.sub.2 film with a low refractive index element introduced
thereinto, the low refractive index element being at least one
member selected from the group consisting of a fluorine atom, an
alkyl group having 1 to 4 carbon atoms, and an alkyl group having 1
to 4 carbon atoms with a part or the whole of hydrogen atoms
substituted by a fluorine atom.
[0010] Formation of a film in such a state that at least one low
refractive index element selected from the group consisting of a
fluorine atom, an alkyl group having 1 to 4 carbon atoms, and an
alkyl group having 1 to 4 carbon atoms with a part or the whole of
hydrogen atoms substituted by a fluorine atom is introduced into an
SiO.sub.2-forming material, can provide a film having a lower
refractive index than the SiO.sub.2 film per se. The SiO.sub.2 film
with an low refractive index element introduced thereinto according
to the present invention has lower refractive index than the
SiO.sub.2 film per se and hence is useful as a film as the upper
layer in an antireflection film.
[0011] An SiO.sub.2 film with a fluorine atom introduced thereinto
is usually in such a state that a silicon atom is bonded to a
fluorine atom. Upon exposure of the SiO.sub.2 film having this bond
to an atmosphere having an excessive water vapor content, the bond
is likely to be hydrolyzed by moisture in air, causing the low
refractive index effect to be lost. Therefore, according to a
preferred embodiment of the present invention, an alkyl group with
a part or the whole of hydrogen atoms substituted by a fluorine
atom is used as the starting material. In this case, the fluorine
atom introduced into the SiO.sub.2 film is bonded to the carbon
atom, causing no hydrolysis, which permits the low refractive index
effect to be maintained for a long period of time.
[0012] According to another aspect of the present invention, there
is provided a process for producing a low refractive index
SiO.sub.2 film, comprising forming a thin film on a substrate by
CVD using a starting material gas comprising a gas containing a
fluorine atom, a gas containing a silicon atom and an alkyl group
having 1 to 4 carbon atoms or an alkyl group having 1 to 4 carbon
atoms with a part or the whole of hydrogen atoms substituted by a
fluorine atom, and a gas containing an oxygen atom.
[0013] According to a further aspect of the present invention,
there is provided a process for producing a low refractive index
SiO.sub.2 film, comprising the steps of:
[0014] (1) introducing a mixed gas, comprising a gas containing a
fluorine atom, a volatile gas of an organic compound containing a
silicon atom and an alkyl group having 1 to 4 carbon atoms or an
alkyl group having 1 to 4 carbon atoms with a part or the whole of
hydrogen atoms substituted by a fluorine atom, and a gas containing
an oxygen atom, into a vacuum chamber maintained in a vacuum of
10.sup.-3 to 1 mmHg (Torr) and bringing the mixed gas to a plasma
stream by glow discharge; and
[0015] (2) bringing the plasma stream into contact with the surface
of a substrate disposed within the vacuum chamber to form a thin
film on the substrate.
[0016] According to the present invention, since a mixed gas,
comprising a gas containing a fluorine atom, a volatile gas of an
organic compound containing a silicon atom and an alkyl group
having 1 to 4 carbon atoms or an alkyl group having 1 to 4 carbon
atoms with a part or the whole of hydrogen atoms substituted by a
fluorine atom, and a gas containing an oxygen atom, is used in the
formation of a film by CVD, the resultant CVD film comprises a
substantially inorganic SiO.sub.2 film formed by oxidation of Si
atom and, introduced into the SiO.sub.2 film, a fluorine atom
and/or an alkyl group having 1 to 4 carbon atoms or an alkyl group
having 1 to 4 carbon atoms with a part or the whole of hydrogen
atoms substituted by a fluorine atom, that is, a low refractive
index element. The SiO.sub.2 film with a low refractive index
element introduced thereinto has a lower refractive index than an
SiO.sub.2 film not using a gas containing a low refractive index
element, that is, an SiO.sub.2 film per se. Further, the low
refractive index film has high hardness and bond strength.
BRIEF DESCRIPTION OF THE DRAWING
[0017] FIG. 1 is a schematic diagram showing an embodiment of the
construction of a plasma CVD apparatus for producing the low
refractive index SiO.sub.2 film according to the present
invention.
BEST MODE FOR CARRYING OUT THE INVENTION
[0018] A schematic diagram showing an embodiment of the
construction of a plasma CVD apparatus for producing the low
refractive index SiO.sub.2 film according to the present invention
is shown in FIG. 1. In FIG. 1, numeral 1 designates a vacuum
chamber the interior of which can be regulated to a desired degree
of vacuum. A winding/unwinding mechanism containing a pair of rolls
3, 4, which can unwind or rewind a web 2 and function to travel the
web 2 in a positive direction or in a reverse direction, is
disposed within the vacuum chamber 1. A plasma zone 5 is disposed
so as to face the surface of the web 2 travelled between the roll 3
and the roll 4. The plasma zone 5 for plasma CVD treatment
comprises a section for projecting a mixed starting material gas, a
flat electrode 6, and a film forming drum 7 which can stably travel
the web 2 and is grounded. The flat electrode 6 may be a gas
projecting electrode which projects an inert gas to create
plasma.
[0019] A gas containing a fluorine atom, a gas containing a silicon
atom and an alkyl group having 1 to 4 carbon atoms or an alkyl
group having 1 to 4 carbon atoms with a part or the whole of
hydrogen atoms substituted by a fluorine atom, and a gas containing
an oxygen atom are used as the starting material for forming a low
refractive index SiO.sub.2 film by CVD according to the present
invention.
[0020] The gas containing a fluorine atom is added in order to make
the refractive index of the resultant film lower than that of the
SiO.sub.2 film per se, and examples thereof include CF.sub.4,
C.sub.2F.sub.6, C.sub.3F.sub.6, and SF.sub.6.
[0021] The material containing a silicon atom and an alkyl group
having 1 to 4 carbon atoms is a main starting material for the low
refractive index film. The alkyl group having 1 to 4 carbon atoms,
particularly a methyl or ethyl group, is introduced in order to
make the refractive index of the resultant film lower than that of
the SiO.sub.2 film per se. The starting material containing a
silicon atom and an alkyl group having 1 to 4 carbon atoms is
gasified upon introduction into the vacuum chamber. In the present
invention, the reason why the number of carbon atoms of the alkyl
group in the material to be gasified is limited to 1 to 4 is that a
starting material containing an alkyl group having 5 or more carbon
atoms is less likely to be vaporized in CVD, making it difficult to
form a low refractive index film.
[0022] Gasifiable starting materials containing a silicon atom and
an alkyl group having 1 to 4 carbon atoms include
silicon-containing organic compounds, such as hexamethyldisiloxane
(abbreviation: HMDSO), tetramethyldisiloxane (abbreviation: TMDSO),
octamethylcyclotetrasiloxane- , tetraethoxysilane (abbreviation:
TEOS), methyltrimethoxysilane (abbreviation: MTMOS), methylsilane,
dimethylsilane, trimethylsilane, diethylsilane, propylsilane, and
phenylsilane. Preferred are HMDSO, TMDSO, and TEOS. HMDSO is
particularly preferred. For each of these preferred and
particularly preferred compounds, the molecule contains an alkyl
group having 1 to 4 carbon atoms, such as a methyl or ethyl group,
a silicon atom, and an oxygen atom.
[0023] According to a further preferred embodiment of the present
invention, the material containing a silicon atom and an alkyl
group having 1 to 4 carbon atoms as the main starting material is
such that a part or the whole of hydrogen atoms in the alkyl group
having 1 to 4 carbon atoms has been substituted by a fluorine atom.
In this case, the fluorine atom is stably bonded to the carbon atom
to offer the effect of stably maintaining the low refractive index
effect.
[0024] Gasifiable starting materials, containing a silicon atom and
an alkyl group having 1 to 4 carbon atoms with a part or the whole
of hydrogen atoms substituted by a fluorine atom, usable herein
include HMDSO, TMDSO, octamethylcyclotetrasiloxane, TEOS, MTMOS,
methylsilane, dimethylsilane, trimethylsilane, diethylsilane,
propylsilane, and phenylsilane, a part or the whole of hydrogen
atoms in each of the above compounds being substituted by a
fluorine atom.
[0025] The gas containing an oxygen atom is used in order to
oxidize the Si material to give SiO.sub.2.
[0026] Further, other gaseous materials may be added to the mixed
gas composed of the above gases from the viewpoint of imparting
desired properties.
[0027] In the introduction of the mixed gas comprising the Si
material gas and the fluorine gas into the vacuum chamber, the flow
(SLM) ratio is preferably 1:0.1 to 20.
[0028] In plasma CVD, the vacuum chamber is preferably kept at a
pressure of 10.sup.-3 to 1 mmHg (Torr).
[0029] An antireflection film will be described as a preferred
example of products with the low refractive index SiO.sub.2 film,
according to the present invention, formed thereon. A polyethylene
terephthalate (PET) substrate (a product of Toray Industries, Inc.,
tradename: Lumirror T-60, thickness: 100 .mu.m) is provided as a
substrate. A 125 .ANG.-thick ITO layer, a 250 .ANG.-thick SiO.sub.2
layer, a 1000 .ANG.-thick ITO layer, and a 800 .ANG.-thick
SiO.sub.2 layer are laminated in that order on the PET substrate.
The ITO layer may be formed by any means. In general, however,
sputtering is applicable. Instead of the ITO layer, a TiO.sub.2 or
other layer formed of a high refractive index material may be used.
In this case, the higher the degree to which the refractive index
of the layer underlying the SiO.sub.2 layer is above the refractive
index of the SiO.sub.2 layer, the better the antireflection effect
and consequently the better the performance of the antireflection
film. Further, when the ITO layer is used, electrical conductivity
can be imparted to the film, permitting the film to exhibit an
electromagnetic shielding property and an antistatic property. A
fluororesin layer having a thickness of not more than 100 .ANG. may
be wet coated thereon from the viewpoint of forming an antismudge
coating which functions to enhance the water repellency of the
surface, thereby preventing the adhesion of a stain such as a
fingerprint.
[0030] In the above antireflection film, the ITO layer and the
SiO.sub.2 layer, which are first formed on the hard coat layer on
the substrate, may be replaced with a wet coat of an organic
material. Formation of the ITO layer by sputtering takes a lot of
time which is causative of increased production cost. Such a change
in film forming means is advantageous from the viewpoint of the
cost. The adhesion of the ITO layer formed on the wet coat layer of
the organic material is not very good. In this case, a layer having
a small thickness (usually not more than 100 .ANG.) of other
material corresponding to an adhesive may be formed between the
organic layer and the ITO layer.
[0031] If the antireflection property required is not very high
(for example, when the formation of a low reflective film is
contemplated), an SiO.sub.2 layer alone may be formed on the hard
coat. This layer construction is extensively adopted in the
antireflection film for LCD from the viewpoint of the cost. In this
case, TAC (a product of Fuji Photo Film Co., Ltd., tradename: Fuji
Tac, thickness: 100 .mu.m) possessing optical properties, that is,
not having birefringence, may be used as the substrate.
[0032] The process for producing the low refractive index SiO.sub.2
film of the present invention by plasma CVD will be described with
reference to the following examples.
EXAMPLE 1
[0033] TMDSO, oxygen, and CF.sub.4 were used as starting material
gases, and argon was used as a gas for producing plasma. These
gases were introduced, as a mixed gas composed of TMDSO (0.1 SLM),
oxygen (20 SLM), CF.sub.4 (0.22 SLM), and argon (0.35 SLM), into a
vacuum chamber of a plasma CVD apparatus with a pressure regulated
to 40 mTorr (4.times.10.sup.-2 mmHg), and an electric power of 2.1
kW and 40 kHz was applied between the film forming drum and the gas
projecting electrode to produce plasma. On the other hand, a PET
film (a product of Toray Industries, Inc., Japan; tradename:
Lumirror T-60, thickness: 100 .mu.m) as a substrate was fed into
and passed through a plasma producing zone at a feed rate of 1.5
m/min to form an SiO.sub.2 film on the substrate.
[0034] The antireflection effect of the SiO.sub.2 film was
confirmed as follows. A black vinyl tape was applied to the back
surface of the substrate to eliminate the reflection of the back
surface. In this state, the reflectance of the front surface in the
visible light region was measured at an incident angle of
5.degree., and the refractive index of the SiO.sub.2 film was
evaluated based on the minimum reflectance. As a result, it was
found that the minimum reflectance was 1.76% and the refractive
index was 1.47.
COMPARATIVE EXAMPLE 1
[0035] For comparison, a film of Comparative Example 1 was formed
in the same manner as in Example 1, except that CF.sub.4 alone was
omitted.
[0036] As a result, the SiO.sub.2 film formed without use of
CF.sub.4 had a minimum reflectance of 1.95% and a refractive index
of 1.483. Thus, the SiO.sub.2 film formed using CF.sub.4 in Example
1 had lower reflectance and lower refractive index.
EXAMPLE 2
[0037] Oxygen was used as a part of the starting material, and
argon was used as a carrier gas for producing plasma. These gases
were introduced, as a mixed gas composed of oxygen (30 sccm) and
argon (30 sccm), into a vacuum chamber of a plasma CVD apparatus
with a pressure regulated to 40 mTorr. HMDSO (hexamethyldisiloxane)
as a monomer material was bubbled by this carrier gas to feed the
monomer material into the vacuum chamber of the CVD apparatus with
a pressure regulated to 40 mTorr. The bubbling temperature was room
temperature. Plasma was produced by applying an electric power of
100 W and 13.56 MHz across the upper electrode and the earth
electrode.
[0038] A PET film [a product of Toray Industries, Inc., tradename:
Lumirror T-60, thickness: 100 .mu.m] was used as a substrate, and
the film formation time was 10 min. The refractive index of the
film thus formed was measured by ellipsometry and found to be 1.42.
The film was analyzed by IR spectrometry for the presence of a
CH.sub.3 group. As a result, a peak derived from stretching of an
Si--CH.sub.3 bond appeared around 1277 cm.sup.-1, indicating the
presence of the CH.sub.3 group.
COMPARATIVE EXAMPLE 2
[0039] For comparison, the procedure of Example 2 was repeated,
except that the flow rate of oxygen was 100 sccm. Thus, an
SiO.sub.2 film of Comparative Example 2 was formed. The SiO.sub.2
film was analyzed by IR spectroscopy. As a result, the presence of
a CH.sub.3 group was hardly observed. The refractive index of the
SiO.sub.2 film formed in Comparative Example 2 was measured by
ellipsometry and found to be 1.44, which was higher than the
refractive index of the SiO.sub.2 film, with the CH.sub.3 group
introduced thereinto, prepared in Example 2.
EXAMPLE 3
[0040] A PET film (a product of Toray Industries, Inc., tradename:
Lumirror T-60, thickness: 100 .mu.m) was provided as a substrate.
Oxygen, argon, and CF.sub.4 were used as the starting material
gases. Further, HMDSO (hexamethyldisiloxane) was used as the
monomer material. The gas flow rate was 30 sccm for oxygen, 30 sccm
for argon, and 30 sccm for CF.sub.4. The monomer material was
bubbled using the argon gas as a carrier gas and fed into a vacuum
chamber of a plasma CVD apparatus with the pressure being regulated
to 40 mTorr. The bubbling temperature was room temperature.
[0041] An electric power of 100 W and 13.56 MHz was applied across
the upper electrode and the earth electrode to produce plasma. The
film formation time was 10 min. The refractive index of the
SiO.sub.2 film with fluorine atom introduced thereinto was measured
by ellipsometry and found to be 1.40.
COMPARATIVE EXAMPLE 3
[0042] For comparison, in the formation of an SiO.sub.2 film,
CF.sub.4 was not used to form an SiO.sub.2 film with a fluorine
atom not introduced thereinto, and the refractive index of the
SiO.sub.2 film with no fluorine atom introduced thereinto was
measured by ellipsometry and found to be 1.42, which was higher
than the refractive index (1.40) of the SiO.sub.2 film, with the
fluorine atom introduced thereinto, prepared in Example 3.
EXAMPLE 4
[0043] A PET film (a product of Toray Industries, Inc., tradename:
Lumirror T-60, thickness: 100 .mu.m) was provided as a substrate.
Oxygen and argon were used as the starting material gases. Further,
HMDSO (hexamethyldisiloxane) with H in the CH.sub.3 thereof being
substituted by F was used as the monomer material. The gas flow
rate was 30 sccm for oxygen and 30 sccm for argon. The monomer
material was bubbled using the argon gas as a carrier gas and fed
into a vacuum chamber of a plasma CVD apparatus with the pressure
being regulated to 40 mTorr. The bubbling temperature was room
temperature.
[0044] An electric power of 100 W and 13.56 MHz was applied across
the upper electrode and the earth electrode to produce plasma. The
film formation time was 10 min. The refractive index of the
SiO.sub.2 film was measured by ellipsometry and found to be
1.40.
[0045] For both the SiO.sub.2 film formed in this example (Example
4) and the SiO.sub.2 film formed in Example 3, the initial
refractive index was 1.40. These SiO.sub.2 films were allowed to
stand for three days under high temperature and high humidity
conditions (80.degree. C., 95% RH). As a result, the SiO.sub.2 film
formed in Example 4 underwent no change in refractive index,
whereas the refractive index of the SiO.sub.2 film prepared in
Example 3 was increased to 1.45. This demonstrates that, in the
SiO.sub.2 film with a fluorine atom introduced thereinto, use of
HMDSO, with H in the CH.sub.3 group substituted by F, rather than
use of CF.sub.4 resulted in better high temperature and high
moisture resistance.
[0046] The refractive index SiO.sub.2 film with at least one low
refractive index element selected from a fluorine atom, an alkyl
group having 1 to 4 carbon atoms, and an alkyl group having 1 to 4
carbon atoms with a part or the whole of hydrogen atoms thereof
being substituted by a fluorine atom has lower refractive index
than the SiO.sub.2 film with no low refractive index element
introduced thereinto.
* * * * *