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BACKGROUND OF THE INVENTION
1. Field of the invention
The present invention relates to a birefringent plate. The birefringent
plate is utilized as an optically functional device, such as a
1/4-wavelength phase-difference plate and a 1/2-wavelength
phase-difference plate, in a pick-up for a CD-ROM player or a DVD player.
2. Description of the Related Art
A birefringent plate has been used conventionally. In the birefringent
plate, an obliquely deposited film is formed on a transparent glass
substrate, and is made from a dielectric material, such as oxide, sulfide
and fluoride, which is transparent in the visible light to the
near-infrared region. The "obliquely deposition" herein means a method to
form a film on a substrate surface which is inclined with respect to the
flying direction of a depositing material. The structure of the obliquely
deposited film is observed as a columnar structure which has an assembly
of fine columns. The fine columns have circular cross-sections, and are
inclined at a definite predetermined angle with respect to the surface of
the substrate. The density of the columns exhibits anisotropy in a plane,
and the refractive index exhibits anisotropy in a plane. As a result, the
obliquely deposited film effects the birefringence. By utilizing the
birefringence of the obliquely deposited film, the birefringent plate is
applied to optical devices, such as a 1/4-wavelength phase-difference
plate and a 1/2-wavelength phase-difference plate.
One of the problems associated with the obliquely-deposited birefringent
plate is that the phase difference varies because of a change of the
refractive index in accordance with an amount of the water which is
occluded in the obliquely deposited film. The birefringent refractive
index of the obliquely deposited film depends on the packed rate of the
columnar structure and the refractive index of the columns and the
substance occupying the spaces, etc. It is known that the birefringent
refractive index of the obliquely deposited film nearly monotonously
decreases when the refractive index of the substance occupying the spaces
enlarges. For example, when all of the substance occupying the spaces is
water whose refractive index n is 1.33 (i.e., n=1.33), the birefringent
refractive index is reduced by half with respect to the case where all of
the substance occupying the spaces is air whose refractive index n is 1.0
(i.e., n=1.0).
Tantalum pentoxide (Ta.sub.2 O.sub.5) is one of the film materials which
are often used for the obliquely deposited birefringent plate. Immediately
after the preparation, a Ta.sub.2 O.sub.5 obliquely deposited film is
reduced to exhibit liver brown, but is turned into transparent by carrying
out a bleaching (oxidation) treatment. The bleaching treatment is carried
out in a dry-air atmosphere whose temperature is 90.degree. C. or less, or
in a highly-humid atmosphere whose relative humidity is 85% or less.
Thereafter, the Ta.sub.2 O.sub.5 obliquely deposited film is left in a
constant temperature-and-humidity atmosphere, for example, at 25.degree.
C. and in a relative humidity of 60%.
As earlier mentioned, the obliquely deposited film is composed of the
columnar structure of a low density (a packed rate of from 70% to 80%),
and has spaces in an amount of from 20% to 30% by volume. In the bleaching
step and under the constant temperature-and-humidity atmosphere, a large
amount of water is adsorbed and occluded in the spaces of the obliquely
deposited film. This phenomenon was confirmed by an infrared spectroscopy
analysis. The amount of the occluded water is saturated when water is
adsorbed in all of the spaces of the columnar structure. The amount of the
occluded water in the obliquely deposited film varies in accordance with
the temperature. When the obliquely deposited film is exposed to an
atmosphere of 100.degree. C. or more, the occluded water evaporates.
Accordingly, air is the main component occupying the spaces. When the
temperature is decreased to room temperature, the obliquely deposited film
retrieves the water vapor in air. Then, the amount of the occluded water
is recovered to the original amount.
Thus, the occluded water comes in and out of the obliquely deposited film
in accordance with the temperature. Consequently, when the amount of the
occluded water varies in the obliquely deposited film in accordance with
the temperature, the refractive index of the substance occupying the
spaces changes. As a result, the birefringent refractive index of the
obliquely deposited film varies so that the phase difference fluctuates.
Japanese Unexamined Patent Publication (KOKAI) No. 1-312,507 discloses a
countermeasure to the problem. For instance, a transparent resin, such as
an epoxy resin, is injected into the spaces in the columnar structure,
thereby improving the temperature and moisture characteristics of the
obliquely deposited film. In this method, however, since the injected
resin exhibits a high refractive index, the birefringent refractive index
of the resulting birefringent plate decreases greatly. As a result, it is
necessary to thicken the obliquely deposited film in order to obtain
desired characteristics.
Another one of the problems associated with the obliquely-deposited
birefringent plate is that the phase difference fluctuates depending on
the measurement positions within the identical substrate when the phase
difference was examined with a coherent light source, for example, a laser
beam despite the fact that the characteristics of the obliquely deposited
film, such as the film thickness and the birefringent refractive index,
are constant. This phenomenon does not occur when an incoherent light is
spectroscopically separated and is used as a light source. This is an
adverse characteristic when a monochromatic light, such as a laser beam,
is used as a light source.
Usually, the distribution of the phase difference is periodic. For
instance, in a glass substrate of 5 cm.times.5 cm in size, the phase
difference fluctuates by 10 deg. or more in a certain case. When the
obliquely-deposited birefringent plate is used as a 1/4-wavelength
phase-difference plate or a 1/2-wavelength phase-difference plate, the
fluctuation of the phase difference results in a decreased yield of
product and a remarkably increased cost because it is necessary to inspect
every single device of 3 mm.times.3 mm in size.
Since the distribution of the phase difference occurs periodically, one of
the causes of the fluctuation of the phase difference is considered to be
the uneven thickness-wise inclination of the glass substrate. Normally, as
for a substrate used in the obliquely deposited birefringent plate, a flat
glass substrate having a surface roughness of 20 .ANG. or less is used.
When the irregularity is large on the surfaces, the resulting obliquely
deposited film whitens because the diameters of the columns enlarge.
Regarding the flat glass substrate, a float glass substrate and a polished
glass substrate have been known. However, in these glass substrates as
well, there arise uneven thickness-wise inclinations which are equivalent
to the wavelength of a laser beam.
Generally, it is possible to reduce the flatness of the surfaces of a glass
substrate sufficiently less than the wavelength of a light. However, it is
difficult to keep the parallelism between the front and opposite surfaces
which is the thickness of a glass substrate of a large area. Whilst, it is
relatively easy to make the thickness of the obliquely deposited film even
by optimizing the geometric arrangement of the deposition. Accordingly, as
illustrated in FIG. 8, the cross-section of an actual obliquely deposited
film is made in such a manner that an obliquely deposited film of an even
thickness is formed on a glass substrate whose size is a couple of
centimeters and which has a thickness inclination nearly equal to the
wavelength of a light.
When a highly coherent light, such as a laser beam, enters the thus
constructed obliquely deposited birefringent film, the light interference
occurs not only in the obliquely deposited film but also between the
opposite surface of the glass substrate and the surface of the obliquely
deposited film. The phase-difference distribution is caused in a plane of
the birefringent plate by the light interference between the opposite
surface of the glass substrate and the surface of the obliquely deposited
film.
Namely, as illustrated in FIG. 8, the actually observed light is the
superimposition of a beam 1 and a beam 2 which pass in the obliquely
deposited film exhibiting the birefringence. For example, the beam 1
influences the beam 2 differently in the case where the effective optical
thickness, the summed thickness of the obliquely deposited film and the
glass substrate, is (2 m+1).lambda./2 from in the case where it is (2
m+1).lambda./4.
Accordingly, when the glass substrate has a definite predetermined
thickness inclination as illustrated in FIG. 8, the phase difference
varies periodically as the phase difference is measured along the
direction of the inclination. Likewise, when the glass substrate has
irregular thicknesses, the phase-difference distribution is observed in a
plane.
The phase difference varies as aforementioned when the following 4
conditions are overlapped: 1) when the obliquely deposited film exhibits a
high refractive index; 2) when the glass substrate has the periodic
thickness inclination; 3) when the incident light reflects at the opposite
surface of the glass substrate; and 4) when the measuring light source is
a highly coherent laser beam. If a birefringent plate is constituted by an
obliquely deposited film which is made by using SiO.sub.2 or MgF.sub.2
exhibiting a low refractive index, the problem can be solved to a certain
extent. However, SiO.sub.2 and MgF.sub.2 cannot be used in optically
functional component parts, such as a 1/4-wavelength phase-difference
plate, because they exhibit a small birefringent refractive index and
whiten. On the other hand, a Ta.sub.2 O.sub.5 obliquely deposited film
does not whiten, but exhibits a large birefringent refractive index.
However, Ta.sub.2 O.sub.5 exhibits a refractive index of 1.86, and
produces an interference reflection of 10.7%. Moreover, it is difficult to
eliminate the periodic thickness inclination of the glass substrate by
ordinary methods for forming the glass substrate.
SUMMARY OF INVENTION
The present invention has been developed in order to solve the
aforementioned problems of the birefringent plate made by forming the
obliquely deposited film on the glass substrate. It is therefore an object
of the present invention to provide an obliquely deposited birefringent
plate whose phase difference is less dependent on the temperature, and
whose phase-difference distribution is less likely to fluctuate in a plane
even when a highly coherent light source, such as a laser beam, is used.
The inventors of the present invention discovered that the aforementioned
disadvantages can be solved by reducing the reflection at the opposite
surface of the glass substrate and by forming a protective film which is
formed on an obliquely deposited film to hold the occluded water of the
obliquely deposited film within the obliquely deposited film, or
alternatively a protective film which reduces the light interference
between the opposite surface of the glass substrate and the surface of the
obliquely deposited film in addition to the occluded-water holding
function. Thus, the inventors completed the present invention.
A birefringent plate according to a first aspect of the present invention
comprises:
a transparent glass substrate;
an obliquely deposited film formed on one of the surfaces of the glass
substrate by obliquely deposition of a dielectric material with respect to
the normal of the glass substrate; and
a protective film formed on the obliquely deposited film for holding the
occluded water of the obliquely deposited film within the obliquely
deposited film.
A birefringent plate according to a second aspect of the present invention
comprises:
a transparent glass substrate;
an obliquely deposited film formed on one of the surfaces of the glass
substrate by obliquely deposition of a dielectric material with respect to
the normal of the glass substrate; and
a protective film formed on the obliquely deposited film for holding the
occluded water of the obliquely deposited film within the obliquely
deposited film and reducing the light interference between the opposite
surface of the glass substrate and the surface of the obliquely deposited
film.
A birefringent plate according to a third aspect of the present invention
comprises:
a transparent glass substrate;
an antireflection coating formed on one of the surfaces of the glass
substrate;
an obliquely deposited film formed on the other one of the surfaces of the
glass substrate by obliquely deposition of a dielectric material with
respect to the normal of the glass substrate; and
a protective film formed on the obliquely deposited film for holding the
occluded water of the obliquely deposited film within the obliquely
deposited film.
A birefringent plate according to a fourth aspect of the present invention
comprises:
a transparent glass substrate;
an antireflection coating formed on one of the surfaces of the glass
substrate;
an obliquely deposited film formed on the other one of the surfaces of the
glass substrate by obliquely deposition of a dielectric material with
respect to the normal of the glass substrate; and
a protective film formed on the obliquely deposited film for holding the
occluded water of the obliquely deposited film within the obliquely
deposited film and reducing the light interference between the opposite
surface of the glass substrate and the surface of the obliquely deposited
film.
A birefringent plate according to a fifth aspect of the present invention
comprises:
a transparent glass substrate;
an antireflection coating formed on one of the surfaces of the glass
substrate; and
an obliquely deposited film formed on the other one of the surfaces of the
glass substrate by obliquely deposition of a dielectric material with
respect to the normal of the glass substrate.
In the birefringent plates according to the first, second, third and fourth
aspects of the present invention, the protective film is applied on the
obliquely deposited film. As a result, it is possible to manufacture an
obliquely deposited birefringent plate whose phase difference is less
dependent on the temperature. In particular, in the birefringent plate
according to the second or fourth aspect of the present invention, the
protective film reduces the light interference between the opposite
surface of the glass substrate and the surface of the obliquely deposited
film so that the phase difference is effectively inhibited from
fluctuating at the measurement positions.
Moreover, in the birefringent plates according to the third, fourth and
fifth aspects of the present invention, the antireflection coating is
formed on one of the surfaces (e.g., the opposite surface) of the glass
substrate. In particular, in the birefringent plate according to the
fourth aspect of the present invention, in addition to the antireflection
coating, the protective film is applied on the obliquely deposited film to
reduce the light interference between the opposite surface of the glass
substrate and the surface of the obliquely deposited film. As a result,
even when the phase difference of the birefringent plates is measured with
a light source, such as a laser beam, the phase difference is furthermore
effectively inhibited from fluctuating at the measurement positions.
BRIEF DESCRIPTION OF THE DRAWINGS
A more complete appreciation of the present invention and many of its
advantages will be readily obtained as the same becomes better understood
by reference to the following detailed description when considered in
connection with the accompanying drawings and detailed specification, all
of which forms a part of the disclosure:
FIG. 1 is a cross-sectional view of Example No. 1 of a birefringent plate
according to the present invention in which a three-layered antireflection
coating is formed on the opposite surface of a glass substrate;
FIG. 2 is a cross-sectional view of Example No. 2 of a birefringent plate
according to the present invention in which a protective film is disposed
on an obliquely deposited film;
FIG. 3 is a cross-sectional view of Example No. 3 of a birefringent plate
according to the present invention in which a three-layered antireflection
coating is formed on the opposite surface of a glass substrate and a
protective film is disposed on an obliquely deposited film;
FIG. 4 is a cross-sectional view of Example No. 4 of a conventional
birefringent plate in which only an obliquely deposited film is disposed
on a glass substrate;
FIG. 5 is a diagram for illustrating the temperature dependencies of the
birefringent refractive indices (.DELTA.n) exhibited by Example Nos. 1
through 3 of the present birefringent plate and Example No. 4 of the
conventional birefringent plate;
FIG. 6 is a diagram for illustrating the phase-difference distributions in
a plane exhibited by Example Nos. 1 through 3 of the present birefringent
plate and Example No. 4 of the conventional birefringent plate;
FIG. 7 is a diagram for illustrating the relationships between the
reflectances and the phase-difference-fluctuation widths exhibited by
Example Nos. 1 through 3 of the present birefringent plate and Example No.
4 of the conventional birefringent plate; and
FIG. 8 is a cross-sectional view for illustrating the light interference
between the surface of an obliquely deposited film and the opposite
surface of a glass substrate.
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS
Having generally described the present invention, a further understanding
can be obtained by reference to the specific preferred embodiments which
are provided herein for the purpose of illustration only and not intended
to limit the scope of the appended claims.
In the first aspect of the present invention, the protective film is formed
on the obliquely deposited film for holding the occluded water of the
obliquely deposited film within the obliquely deposited film. Accordingly,
even when the birefringent plate is heated to 100.degree. C. or more, the
occluded water contained in the obliquely deposited film does not
evaporate. Hence, the birefringent refractive index can be kept constant,
and the phase difference little fluctuates in accordance with the
temperature.
In the second aspect of the present invention, the protective film holds
the occluded water contained in the obliquely deposited film within the
obliquely deposited film. Accordingly, the phase difference does not
fluctuate in accordance with the temperature. Simultaneously, the
protective film functions to reduce the light interference between the
opposite surface of the glass substrate and the surface of the obliquely
deposited film. Consequently, the evenness can be remarkably improved in
the phase difference when the phase difference is measured by using a
light source, such as a laser beam. In order to obtain this advantage, the
protective film can employ not only a single-layered construction but also
a more than one multi-layered construction.
In the case of the single-layered construction, a low refractive-index
material which exhibits an optical film thickness (2 m-1).lambda./4 in
which m=1-5 is formed on the surface of the obliquely deposited film. For
example, the low refractive index n herein falls in the range of from 1.25
to 1.45 (i.e., n=1.25-1.45). The low refractive-index material can be
fluorocarbon, such as polytetrafluoroethylene (hereinafter abbreviated to
as "PTFE"), furan, NaF, MgF.sub.2, CaF.sub.2 and SiO.sub.2. Here, .lambda.
is the wavelength of a used laser beam. Note that it is preferable to
select a material of high mechanical strength as the low refractive-index
material to be formed on the obliquely deposited film.
In the case of the two-layered construction, on the surface of the
obliquely deposited film, there are formed a high refractive-index
material, which exhibits an optical film thickness .lambda./2 and whose
refractive index falls in the range of from 2.0 to 2.4 (i.e., n=2.0-2.4),
and a low refractive-index material, which exhibits an optical film
thickness (2 m-1).lambda./4 in which m=1-5 and whose refractive index
falls in the range of from 1.25 to 1.45 (i.e., n=1.25-1.45). The low
refractive-index material can be PTFE, furan, NaF, MgF.sub.2, CaF.sub.2
and SiO.sub.2. The high refractive-index material can be Ta.sub.2 O.sub.5,
ZrO.sub.2, CeO.sub.2 and TiO.sub.2. In the case of the two-layered
construction, the characteristics can be furthermore enhanced when the
optical film thickness of the obliquely deposited film is set at
m.lambda./4. Here, .lambda. is the wavelength of a used laser beam. Note
that the low refractive-index materials whose refractive indices are
n=1.25-1.45 can be combined as a multi-layered including the two-layered
construction to form the protective film. If such is the case, the optical
film thicknesses of the multi-layered low refractive-index materials are
set in total at (2 m-1).lambda./4 in which m=1-5. Here, .lambda. is the
wavelength of a used laser beam.
In the third aspect of the present invention, the antireflection coating
reduces the reflection at the opposite surface of the glass substrate.
Accordingly, the evenness can be remarkably improved in the phase
difference when the phase difference is measured by using a light source,
such as a laser beam. Moreover, the protective film is applied on the
obliquely deposited film, and inhibits the occluded water of the obliquely
deposited film from evaporating. Hence, even when the temperature of the
birefringent plate is increased, the birefringent refractive index can be
kept at a constant value, and the phase difference little fluctuates in
accordance with the temperature. The protective film can employ not only a
single layered construction but also a more than one multi-layered
construction.
In the fourth aspect of the present invention, the antireflection coating
reduces the reflection at the opposite surface of the glass substrate, and
the protective film functions to reduce the light interference between the
opposite surface of the glass substrate and the surface of the obliquely
deposited film. Accordingly, the evenness can be remarkably improved in
the phase difference when the phase difference is measured by using a
light source, such as a laser beam. Moreover, similarly to the third
aspect of the present invention, the protective film holds the occluded
water contained in the obliquely deposited film within the obliquely
deposited film. Consequently, the phase difference does not fluctuate in
accordance with the temperature. In order to obtain these advantages, the
protective film can employ not only a single-layered construction but also
a more than one multi-layered construction.
In the fifth aspect of the present invention, the antireflection coating is
applied on one of the surfaces of the transparent glass substrate, and can
be either a single-layered or a multi-layered construction. In this
structure, the reflection is reduced sharply at the opposite surface of
the glass substrate. Accordingly, the evenness can be remarkably improved
in the phase difference when the phase difference is measured by using a
light source, such as a laser beam. However, when the temperature is
increased to 100.degree. C. or more, the birefringent refractive index
increases because the occluded water contained in the obliquely deposited
film evaporates.
The antireflection coating, which is formed on one of the surfaces of the
transparent glass substrate, can be formed by the methods, which have been
known conventionally. For example, when the antireflection coating employs
a three-layered construction, Al.sub.2 O.sub.3, ZrO.sub.2 and MgF.sub.2
are laminated in this order starting from one of the surfaces of the
substrate, and are formed so as to have optical thicknesses of .lambda./4,
.lambda./2 and .lambda./4, respectively.
The dielectric material, which is obliquely deposited on the other one of
the surfaces of the glass substrate, is not limited in particular as far
as it is transparent in the visible light to the near-infrared region and
exhibits birefringence. For example, as the dielectric material, it is
possible to employ tantalum pentoxide (Ta.sub.2 O.sub.5), bismuth trioxide
(Bi.sub.2 O.sub.3), cerium dioxide (CeO.sub.2) and titanium dioxide
(TiO.sub.2). Known methods are employed to form the obliquely deposited
film. For instance, it is possible to employ an electron-beam deposition
method and a sputtering method.
The obliquely deposited film is formed on the surface of the transparent
glass substrate by obliquely depositing of the dielectric material, for
example, in the oblique direction of from 60 deg. to 80 deg., with respect
to the normal of the substrate. The deposition angle and the thickness of
the obliquely deposited film can be determined appropriately to grow the
columnar structure so that a sufficient birefringent refractive index can
be obtained. Note that, when the oblique deposition is carried out in one
direction, the distribution arises in the film thickness so that the phase
difference fluctuates. Accordingly, in order to make the distribution of
the film thickness even, a repetitive deposition is employed in which the
obliquely deposited film is formed in the two directions by turning the
substrate by 180 deg. as disclosed in Japanese Unexamined Patent
Publication (KOKAI) No. 63-132,203.
The protective film, which is formed on the surface of the obliquely
deposited film, functions not only to keep the occluded water in the
obliquely-deposited film from evaporating but also simultaneously to
reduce the light interference between the opposite surface of the glass
substrate and the surface of the obliquely deposited film. The following
are the conditions of the protective film satisfied with both of the
functions:
1) the protective film is made from a material having a blocking effect
against the water molecules and the water vapor;
2) the protective film is made from a material which fills up the fine
irregularity in the surface of the obliquely deposited film, and which
exhibits favorable adhesion to the obliquely deposited film;
3) the protective film is made from a low refractive-index material (e.g.,
n=1.25-1.45); and
4) the protective film is made from a material which exhibits a small
internal stress and which hardly gives damages to the obliquely deposited
film, and is made by a process which hardly gives damages to the obliquely
deposited film.
The film-forming materials, which satisfy these conditions relatively well,
can be PTFE, furan, NaF, CaF.sub.2, MgF.sub.2 and SiO.sub.2. The film
structure can be formed not only as a single-layered construction but also
as a more than one multi-layered construction. By forming the
multi-layered protective film, the functions can be enhanced.
Specifically, a first film, which is made from a material exhibiting a
favorable adhesion to the obliquely deposited film, is formed on the
obliquely deposited film, and thereafter a second film is formed on the
first film with a substance, which exhibits a blocking effect against the
water molecules and the water vapor. When the protective film is formed
free from pin holes, and even when the protective film has a thickness of
100 nm only, the protective film fully inhibits the occluded water
contained in the obliquely deposited film from evaporating. Note that,
when the thickness of the protective film is adjusted to (2 m-1).lambda./4
in which m=1-5, the protective film not only inhibits the occluded water
contained in the obliquely-deposited film from evaporating but also
simultaneously reduces the light interference between the opposite surface
of the glass substrate and the surface of the obliquely deposited film.
Here, .lambda. is the wavelength of a used laser beam. Regarding the
method for preparing the protective film, it is possible to employ
sputtering, vacuum deposition, ion plating, plasma polymerization or CVD
(chemical vapor deposition).
Here, MgF.sub.2 is widely used as a material for forming an antireflection
coating on a glass substrate. However, since MgF.sub.2 exhibits a high
mechanical strength, it is necessary to form its film on a substrate,
which is heated to 250.degree. C. or more, in order to prepare a film
exhibiting a good spectroscopic characteristic. The thus prepared
MgF.sub.2 film exhibits a large internal stress. Namely, when the
MgF.sub.2 film is formed on a deposition film which exhibits a low
mechanical strength, there arises a problem in that the MgF.sub.2 film
peels off or cracks in a certain occasion.
SiO.sub.2 exhibits a satisfactory adhesion to the obliquely deposited film,
and its film exhibits a small internal stress. However, since SiO.sub.2
has a slightly large refractive index, it is slightly inferior in the
function of reducing the light interference between the opposite surface
of the glass substrate and the surface of the obliquely deposited film.
The polymer film, such as a PTFE film formed by a vacuum deposition method
and a plasma-polymerized film made from furan, exhibits a low mechanical
strength, and is inferior in the adhesion to the obliquely deposited film.
However, since the polymer film exhibits a small internal stress and
produces the blocking effect against the water molecules, it is provided
with the favorable properties as the protective film.
As having described so far, in the present birefringent plate, the
reflection is suppressed at the opposite surface of the glass substrate,
and the protective film is applied on the obliquely deposited film. The
fluctuation of the phase-difference distribution is suppressed in a plane
even when the phase difference is measured by using a light source, such
as a laser beam. Moreover, the phase difference little fluctuates even at
elevated temperatures of 100.degree. C. or more.
The preferred embodiments of the present birefringent plate will be
hereinafter described with reference to the drawings.
First Preferred Embodiment
As illustrated in FIG. 1, a glass substrate 2, borosilicate crown glass of
5 cm.times.5 cm, was prepared. The glass substrate 2 was first washed with
acetone, and was dried fully. Whilst, a four-source electron-beam
deposition apparatus was prepared. The four-source electron-beam
deposition apparatus working as a film-forming apparatus included an
obliquely deposited film-forming jig in addition to an ordinary rotary
jig. The glass substrate 2 was attached to the ordinary rotary jig. Then,
the film-forming apparatus was evacuated to a vacuum of 1.times.10.sup.-6
Torr. Thereafter, the glass substrate 2 was heated to a temperature of
300.degree. C. or less. Then, Al.sub.2 O.sub.3, ZrO.sub.2 and MgF.sub.2
were laminated in this order on a surface of the glass substrate 2, and
were formed so as to have optical thicknesses of .lambda./4, .lambda./2
and .lambda./4, respectively to form a three-layered antireflection
coating 3. Note that .lambda. was herein set at 780 nm.
Subsequently, the glass substrate 2 was removed from the film-forming
apparatus. Then, the glass substrate 2 was washed again with acetone, and
was dried fully. Thereafter, the glass substrate 2 was attached to the
obliquely deposited film-forming jig in the film-forming apparatus. Then,
the film-forming apparatus was evacuated to a vacuum of 1.times.10.sup.-6
Torr. Thereafter, Ta.sub.2 O.sub.5 was deposited in a thickness of 2,400
nm at room temperature on the other one of the surfaces of the glass
substrate 2 at an obliquely deposition angle of 70 deg. with respect to
the normal of the glass substrate 2, thereby forming an obliquely
deposited film 4a. Then, the obliquely deposition angle was changed to -70
deg. with respect to the normal of the glass substrate 2, and Ta.sub.2
O.sub.5 was further deposited in a thickness of 2,400 nm at room
temperature on the surface of the obliquely deposited film 4a, thereby
forming an obliquely deposited film 4b. Thus, a birefringent film was
obtained which had a two-layered construction. This preferred embodiment
is a birefringent plate which corresponds to the fifth aspect of the
present invention and will be hereinafter referred to as Example No. 1.
Second Preferred Embodiment
As illustrated in FIG. 2, a glass substrate 2 which was identical with that
of the First Preferred Embodiment was prepared, and was washed with
acetone. Then, the glass substrate 2 was attached to the obliquely
deposited film-forming jig in the film-forming apparatus. Thereafter, the
film-forming apparatus was evacuated to a vacuum of 1.times.10.sup.-6
Torr. Then, Ta.sub.2 O.sub.5 was deposited in a thickness of 2,400 nm at
room temperature on one of the surfaces of the glass substrate 2 at an
obliquely deposition angle of 70 deg. with respect to the normal of the
glass substrate 2, thereby forming an obliquely deposited film 4a.
Thereafter, the obliquely deposition angle was changed to -70 deg. with
respect to the normal of the glass substrate 2, and Ta.sub.2 O.sub.5 was
further deposited in a thickness of 2,400 nm at room temperature on the
surface of the obliquely deposited film 4a, thereby forming an obliquely
deposited film 4b. Thus, a birefringent film was obtained which had a
two-layered construction.
Subsequently, the glass substrate 2 was re-attached to the ordinary rotary
jig in the film-forming apparatus. Then, the film-forming apparatus was
evacuated. Thereafter, a protective film 5 was formed on the surface of
the obliquely deposited film 4b. The protective film 5 was formed as a
two-layered construction which was composed of a Ta.sub.2 O.sub.5 film in
a thickness of 195 nm and a PTFE film in a thickness of 150 nm. The
protective film 5 was formed in order to hold the occluded water of the
obliquely deposited film 4 within the obliquely deposited film 4 and to
reduce the light interference between the opposite surface of the glass
substrate 2 and the surface of the obliquely deposited film 4. This
preferred embodiment is a birefringent plate which corresponds to the
first or second aspect of the present invention and will be hereinafter
referred to as Example No. 2. Note that it is unnecessary to limit the
thickness of the protective film 5 when the protective film 5 functions as
a simple protective film.
Third Preferred Embodiment
As illustrated in FIG. 3, a glass substrate 2 which was identical with that
of the First Preferred Embodiment was prepared. The glass substrate 2 was
washed with acetone, and was dried fully. Thereafter, the glass substrate
2 was attached to the ordinary rotary jig in the film-forming apparatus.
Then, the film-forming apparatus was evacuated to a vacuum of
1.times.10.sup.-6 Torr. Thereafter, the glass substrate 2 was heated to a
temperature of 300.degree. C. or less. Then, Al.sub.2 O.sub.3, ZrO.sub.2
and MgF.sub.2 were laminated in this order on a surface of the glass
substrate 2, and were formed so as to have optical thicknesses of
.lambda./4, .lambda./2 and .lambda./4, respectively. Thus, a three-layered
antireflection coating 3 was formed. Note that .lambda. was herein set at
780 nm.
Subsequently, the glass substrate 2 was removed from the film-forming
apparatus. Then, the glass substrate 2 was washed again with acetone, and
was dried fully. Thereafter, the glass substrate 2 was attached to the
obliquely deposited film-forming jig in the film-forming apparatus. Then,
the film-forming apparatus was evacuated to a vacuum of 1.times.10.sup.-6
Torr. Thereafter, Ta.sub.2 O.sub.5 was deposited in a thickness of 2,400
nm at room temperature on the other one of the surfaces of the glass
substrate 2 at an obliquely deposition angle of 70 deg. with respect to
the normal of the glass substrate 2, thereby forming an obliquely
deposited film 4a. Then, the obliquely deposition angle was changed to -70
deg. with respect to the normal of the glass substrate 2, and Ta.sub.2
O.sub.5 was further deposited in a thickness of 2,400 nm at room
temperature on the surface of the obliquely deposited film 4a, thereby
forming an obliquely deposited film 4b. Thus, a birefringent film was
obtained which had a two-layered construction.
Still subsequently, the glass substrate 2 was re-attached to the ordinary
rotary jig in the film-forming apparatus. Then, the film-forming apparatus
was evacuated. Thereafter, a protective film 5, a PTFE film, was formed in
a thickness of 150 nm on the surface of the obliquely deposited film 4b.
The protective film 5 was formed in order to hold the occluded water of
the obliquely deposited film 4 within the obliquely deposited film 4 and
to reduce the light interference between the opposite surface of the glass
substrate 2 and the surface of the obliquely deposited film 4. This
preferred embodiment is a birefringent plate which corresponds to the
third or fourth aspect of the present invention and will be hereinafter
referred to as Example No. 3.
COMPARATIVE EXAMPLE
As illustrated in FIG. 4, a glass substrate 2 which was identical with that
of the First Preferred Embodiment was prepared. The glass substrate 2 was
washed with acetone, and was dried fully. Then, the glass substrate 2 was
attached to the obliquely deposited film-forming jig of the film-forming
apparatus. Thereafter, the film-forming apparatus was evacuated to a
vacuum of 1.times.10.sup.-6 Torr. Then, Ta.sub.2 O.sub.5 was deposited in
a thickness of 2,400 nm at room temperature on one of the surfaces of the
glass substrate 2 at an obliquely deposition angle of 70 deg. with respect
to the normal of the glass substrate 2, thereby forming an obliquely
deposited film 4a. Thereafter, the obliquely deposition angle was changed
to -70 deg. with respect to the normal of the glass substrate 2, and
Ta.sub.2 O.sub.5 was further deposited in a thickness of 2,400 nm at room
temperature on the surface of the obliquely deposited film 4a, thereby
forming an obliquely deposited film 4b. Thus, a birefringent film was
obtained which had a two-layered construction. This birefringent plate is
a comparative example, which was provided with the obliquely deposited
film 4 only, and will be hereinafter referred to as Example No. 4.
(Performance Comparison Test)
In order to examine the temperature dependencies of the thus prepared four
birefringent plates, the birefringent plates were measured for the phase
differences (deg.) by using an ellipsometer. As for the light source
employed herein, a continuous light was separated spectroscopically. The
measurements were carried out at 2 levels, namely, at room temperature and
at 100.degree. C. The birefringent refractive indices were calculated from
the resulting phase differences (deg.) by using the following equation:
Birefringent Refractive Index (.DELTA.n)=.lambda./d in which .lambda. is
the phase difference (nm), d is the film thickness (nm), and the phase
difference (nm) equals the phase difference (deg.)/360.times.780.
The four birefringent plates were also examined for the distributions of
the phase differences (deg.) in a plane by using an ellipsometer. The
measurements were carried out in the X-axis direction of the birefringent
plates (i.e., in the direction perpendicular to the deposition direction)
at intervals of 2.5 mm (or 5 mm). In addition, in order to investigate the
relationships between the distributions of the phase differences (deg.) in
a plane and the reflectances which were exhibited by the four birefringent
plates, the four birefringent plates were measured for the spectral
reflectance characteristics by using a visible light. The reflectances
were average values which were obtained by measuring with the length of
780.+-.20 nm at intervals of 5 nm. Table 2 and FIG. 7 show the
relationships between the fluctuation widths of the phase differences
(deg.) in the X-axis direction and the reflectances exhibited by the four
birefringent plates. Here, the fluctuation widths of the phase differences
(deg.) were obtained by subtracting the minimum phase-difference values
from the maximum phase-difference values.
TABLE 1
Identi- Phase Difference Phase Difference
fication at Room Temp. (deg.) at 100.degree. C. (deg.)
Example No. 1 99.0 104.8
Example No. 2 98.6 98.8
Example No. 3 97.9 98.0
Example No. 4 100.5 105.6
TABLE 2
Identi- Phase-Difference- Reflectance
fication Fluctuation Width (deg.) (%)
Example No. 1 4.2 6.7
Example No. 2 2.0 4.9
Example No. 3 1.5 1.0
Example No. 4 10.3 10.7
(Reduction in Temperature Dependencies in Birefringent Plates)
Following are apparent from Table 1 and FIG. 5: the birefringent plates had
no protective film applied on the obliquely deposited film 4 (e.g.,
Example Nos. 1 and 4) exhibited the phase-difference increment of about 5
deg., respectively, and the birefringent refractive indices increased by a
couple of % with the temperature change of from room temperature to
100.degree. C.; whereas the birefringent plates which were provided with
the protective film 5 applied on the obliquely deposited film 4 (e.g.,
Example Nos. 2 and 3) hardly exhibited the phase-difference increment even
when the temperature was elevated to 100.degree. C., and the birefringent
refractive indices were held at constant substantially. These results
imply that the protective film 5 inhibited the occluded water contained in
the obliquely deposited film 4 from evaporating even at 100.degree. C. and
the refractive indices were kept constant at the spaces in the obliquely
deposited film 4. Thus, it is now possible to prepare a birefringent
plate, which is less dependent on the temperature, by applying the
protective film on the obliquely deposited film.
(Reduction in Phase-Difference Distribution in Birefringent Plates)
It is understood from FIG. 6 that the phase difference of the conventional
birefringent plate (Example No. 4) was fluctuated greatly at different
measurement positions. As earlier mentioned, in order to suppress the
fluctuation of the | | |
