U.S. patent application number 10/614175 was filed with the patent office on 2004-01-15 for optical device, method for producing the same, and method for producing polyimide film.
This patent application is currently assigned to Sharp Kabushiki Kaisha. Invention is credited to Fujita, Hideaki, Ishii, Yorishige, Kurata, Yukio, Tamura, Toshihiro.
Application Number | 20040008963 10/614175 |
Document ID | / |
Family ID | 26432008 |
Filed Date | 2004-01-15 |
United States Patent
Application |
20040008963 |
Kind Code |
A1 |
Fujita, Hideaki ; et
al. |
January 15, 2004 |
Optical device, method for producing the same, and method for
producing polyimide film
Abstract
A method for producing an optical device having a polyimide film
through which a light beam is transmitted, which comprises applying
a solution containing a polyamic film-forming starting material on
a substrate and then baking the resultant under vacuum to form the
polyimide film.
Inventors: |
Fujita, Hideaki; (Shiki-gun,
JP) ; Ishii, Yorishige; (Yamatotakada-shi, JP)
; Tamura, Toshihiro; (Shiki-gun, JP) ; Kurata,
Yukio; (Tenri-shi, JP) |
Correspondence
Address: |
BIRCH STEWART KOLASCH & BIRCH
PO BOX 747
FALLS CHURCH
VA
22040-0747
US
|
Assignee: |
Sharp Kabushiki Kaisha
|
Family ID: |
26432008 |
Appl. No.: |
10/614175 |
Filed: |
July 8, 2003 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
|
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10614175 |
Jul 8, 2003 |
|
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|
09539385 |
Mar 31, 2000 |
|
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6618543 |
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Current U.S.
Class: |
385/129 |
Current CPC
Class: |
G02B 6/138 20130101;
G02B 1/046 20130101; C08L 79/08 20130101; G02B 1/046 20130101 |
Class at
Publication: |
385/129 |
International
Class: |
G02B 006/10 |
Foreign Application Data
Date |
Code |
Application Number |
Jul 9, 1999 |
JP |
HEI 11-195388 |
Mar 31, 1999 |
JP |
HEI 11-090528 |
Claims
What is claimed is:
1. A method for producing an optical device having an organic
polymer film through which a light beam is transmitted, which
comprises applying a solution containing an organic polymer
film-forming starting material on a substrate to form the applied
film, and then baking the applied film under vacuum of 1 Torr or
lower to form the organic polymer film, wherein the organic polymer
film has a high transmittance when the light beam has a wavelength
of 1.5 .mu.m or shorter.
2. A method for producing an optical device having an organic
polymer film through which a light beam is transmitted, which
comprises applying a solution containing an organic polymer
film-forming starting material on a substrate to form the applied
film, and then baking the applied film under gaseous nitrogen to
form the organic polymer film, wherein the organic polymer film has
a high transmittance when the light beam has a wavelength of 1.5
.mu.m or shorter.
3. A method according to claim 1, wherein the organic polymer film
has an absorptivity coefficient of light of not more than 1.6
mm.sup.-1 in the wavelength of 650 nm.
4. A method according to claims 1 or 2, wherein the light beam has
a wavelength of 500 nm to 800 nm.
5. A method according to claims 1 or 2, wherein the organic polymer
film is a polyimide resin film.
6. A method according to claim 1, wherein the polyimide resin film
is a photosensitive polyimide resin film.
7. A method according to claim 2, wherein the polyimide resin film
is a photosensitive polyimide resin film.
8. A method according to claim 1, wherein the organic polymer film
has a thickness not less than 5 .mu.m and not more than 200
.mu.m.
9. A method according to claim 1, wherein the applied film is
preparatorily heated under atmospheric pressure, before the baking
under vacuum.
10. A method according to claim 1, wherein the baking is performed
under a vacuum of 1.times.10.sup.-2 Torr.
11. A method according to claim 6, wherein the photosensitive
polyimide type resin film is an acetophenone type resin film.
12. A method according to claim 6, wherein the photosensitive
polyimide type resin film includes a tertiary amine.
13. A method according to claim 1, wherein the organic polymer
film-forming starting material is a precursor of a polyimide type
resin.
Description
CROSS-REFERENCES TO RELATED APPLICATIONS
[0001] This application is related to Japanese patent applications
No. Hei 11(1999)-195388 filed on Jul. 9, 1999 and No. Hei
11(1999)-090528 filed on Mar. 31, 1999, whose priorities are
claimed under 35 USC .sctn.119, the disclosure of which is
incorporated herein by reference in its entirety.
TECHNICAL FIELD OF THE INVENTION
[0002] The present invention relates to an optical device capable
of making it compact and a method for producing the same, and to a
method for producing a polyimide film.
BACKGROUND OF THE INVENTION
[0003] With the practice in the variety of mobile electronic
appliances and network equipments, further development for a more
compact and lightweight optical device having a higher performance
is required. Main types of conventionally known optical devices
include, firstly, an optical waveguide formed on a substrate as an
optical device for communication or the like, and secondly, a
bulk-type optical device comprising a combination of various types
of optical components.
[0004] The optical waveguide referred above requires that the light
loss is small, that it is easily produced, that the difference in
refraction index between the core layer and the clad layer is
controllable, that it has excellent heat resistance, etc. However,
since a quartz based optical waveguide require a process at a
temperature as high as 1,000.degree. C. or even higher, there are
such problems that the substrate on which the optical waveguide is
formed is limited, and that the waveguide cannot be formed after
the optical components are mounted on the substrate.
[0005] In the case of optical communication systems using a plastic
optical fiber (POF) as the transmission medium, which is recently
attracting attention, a thick film optical waveguide capable of
being efficiently connected with a large POF having a diameter of
about 1 mm is necessary. However, it is difficult to obtain thick
films of a quartz based optical waveguide due to the problems
encountered in their production.
[0006] On the other hand, many are interested in the recently
proposed plastic optical waveguide, because it enables waveguides
at lower temperatures, and thick films thereof are relatively
easily achieved. A polyimide optical waveguide using polyimide,
which boasts the highest heat resistance among the plastics, can be
generally produced by a method as follows:
[0007] (1) producing a thin film of polyimide by coating a
substrate with a solution of polyamic acid or polyimide by means
of, for instance, spin coating, and then heating it remove the
solvent by volatilization; and
[0008] (2) obtaining a polyimide optical waveguide by patterning
the resulting thin film into a desired shape by means of, for
example, dry etching using gaseous oxygen.
[0009] In the case of an optical device using such as POF as the
transmission medium, the loss increases in the wavelength region of
from about 600 nm to800 nm. Thus, in a practical application, it is
necessary to minimize the loss. Accordingly, in JP-A-Hei4-9807 (the
term "JP-A-" as referred herein signifies "an unexamined published
patent application") is proposed a polyimide optical waveguide
having a lower loss. The proposed method provides a polyimide
having reduced loss by fluorinating the diamine which constitutes
the polyimide.
[0010] However, the production process for the plastic optical
waveguide above had disadvantages that the resulting polyimide
optical waveguide exhibits a tanned color characteristic of
polyimide; an optical waveguide having low loss was thereby
unfeasible.
[0011] A fluorinated polyimide disclosed in JP-A-Hei4-9807 was so
expensive that an optical device using such a material also
suffered a disadvantage of increased cost. Furthermore, in case of
processing the polyimide by means of dry etching using gaseous
oxygen, the polyimide was found unsuitable for use as a mask,
because the selective ratio between the polyimide and a positive
resist using novolak based resin was so small, and this inevitably
required the use of, for example, silicon oxide, aluminum, etc.
This also led to a problematic increase in production cost due to
the need of a complicated production process. Although there is
known a method of simplifying the production process comprising
using a polyimide having a photosensitive group (a photosensitive
polyimide), the application of such photosensitive polyimides to
optical devices is not studied heretofore because such polyimides
undergo coloring upon baking.
[0012] As described above, in a plastic optical waveguide
heretofore using a polyimide or a like organic polymer, which is
relatively easy to obtain thick films therefrom, it has been found
difficult to achieve both high performance and low cost at the same
time due to the absorption of the core material or to the large
optical loss caused by the scattering at the interface between the
core and the cladding. Furthermore, it has been found difficult to
obtain the final products at a low cost due to the complicated
production process.
SUMMARY OF THE INVENTION
[0013] In the light of the aforementioned circumstances, the
present invention has been accomplished in order to solve the
problems above. Accordingly, an object of the present invention is
to provide a low cost optical device which can be easily made
compact and lightweight, and to provide a method for producing the
same.
[0014] The present inventors have found that a polyimide film
having high transmittance can be achieved by forming a coating of a
polyimide or photosensitive polyimide on a substrate, and by then
baking it in vacuum. This allows a low cost optical device yet
having high performance by etching the film into a desired
shape.
[0015] Thus, in accordance with an aspect the present invention,
there is provided a method for producing an optical device having
an organic polymer film through which a light beam is transmitted,
which comprises applying a solution containing an organic polymer
film-forming starting material on a substrate to form the applied
film and then baking the applied film under vacuum to form the
organic polymer film.
BRIEF DESCRIPTION OF THE DRAWINGS
[0016] FIGS. 1(a) to 1(g) are schematic sectional views showing a
method of producing an optical waveguide according to an embodiment
of the present invention;
[0017] FIG. 2 is a graphical representation showing the wavelength
dependence of the transmittance of polyimide samples baked under
the conditions A to C;
[0018] FIGS. 3(a) to 3(c) are schematic sectional views showing a
method of producing a polyimide optical waveguide using a
photosensitive polyimide according to a second embodiment of the
present invention;
[0019] FIG. 4 is a graphical representation showing the wavelength
dependence of the transmittance of photosensitive polyimide samples
baked under the same conditions A to C;
[0020] FIG. 5 is a graphical representation showing the wavelength
dependence of the transmittance of photosensitive polyimide samples
baked under conditions C, D and E;
[0021] FIG. 6 is a diagram showing the infrared absorption spectra
in the wavenumber for photosensitive polyimide samples fabricated
under conditions A to C and F;
[0022] FIG. 7 is an enlarged diagram showing spectra in the
wavenumber ranging from 1,200 to 500 cm.sup.-1; and
[0023] FIG. 8 is a perspective view of an optical communication
device using a polyimide optical waveguide according to a third
embodiment of the present invention.
PREFERRED EMBODIMENTS OF THE INVENTION
[0024] In the present invention, a polyimide, for example, may be
mentioned as a polyimide type resin of an organic polymer usable
for forming the organic polymer film. The polyimide may have a
photosensitive group and may further contain another organic
polymer which does not prevent the passage of the light beam.
[0025] In the case where the organic polymer is a polyimide, a
precursor of the polyimide such as polyamic acid may be mentioned
as the above-mentioned organic polymer film-forming starting
material. The precursor changes to the polyimide by ring closure
with dehydration at baking.
[0026] Furthermore, the method of the present invention is
characterized by that the baking is performed under a vacuum degree
of 1 Torr or lower.
[0027] In accordance with a further other aspect of the present
invention, there is provided an optical device having on a
substrate an organic polymer film through which a light beam is
transmitted, wherein the organic polymer film is formed by applying
a solution containing an organic polymer film-forming starting
material on the substrate and then baking the resultant under
vacuum. Here, the organic polymer film has preferably a thickness
not less than 5 .mu.m and not more than 200 .mu.m.
[0028] In case that the organic polymer film is a photosensitive
polyimide film, it is preferable to be able to transmit a light
beam having a wavelength of 1.5 .mu.m or shorter. Furthermore, in
case that the organic polymer film is a polyimide film, it is
preferable to be able to transmit a light beam having a wavelength
of 500 to 800 nm.
[0029] According to still other aspect of the present invention,
there is provided an optical device having an optical waveguide for
sending light to or receiving light from a plastic optical fiber,
wherein the optical waveguide includes a core made of an organic
polymer film, the organic polymer film being formed by applying a
solution containing an organic polymer film-forming starting
material on a substrate and then baking the resultant under
vacuum.
[0030] According to a yet other aspect of the present invention,
there is provided a method for producing a polyimide film, which
comprises applying a solution containing a polyimide film-forming
starting material on a substrate and then baking the resultant
under vacuum to form the polyimide film.
[0031] The process for forming an optical waveguide according to
the present invention is described in further detail by making
reference to preferred embodiments below.
[0032] [First Embodiment]
[0033] The first embodiment refers to a case in which a polyimide
film and a method for producing the same according to the present
invention is applied to an optical waveguide. In the explanation
below, the polyimide refers to a case in which an acid dianhydride
and diamine as the starting materials are not fluorinated.
[0034] FIGS. 1(a) and 1(g) are schematic sectional views showing
the method of producing the optical waveguide according to the
first embodiment of the present invention. The method for producing
the polyimide optical waveguide is described below by making
reference to FIGS. 1(a) to 1(g).
[0035] (1) First, a substrate 1 made of Si or the like, whose
surface is covered with a silicon oxide or organic polymer film
provided as a buffer layer 2, is coated with a core layer 3 by
applying an organic solvent solution of a polyimide (i.e., a
polyamic acid that is the precursor thereof having a refractive
index higher than the buffer layer 2 by spin coating, and a part of
the organic solvent is volatilized and removed therefrom by
prebaking the coating in air. Thus, the core layer 3 made from a
polyimide film is finally obtained by baking the resulting coat
under vacuum to volatilize the organic solvent and carry out
imidization (FIG. 1(a)).
[0036] (2) An etching mask 4 for the core layer 3 is provided by
depositing a film of copper, aluminum, silicon oxide, etc., by
means of, for example, sputtering (FIG. 1(b)).
[0037] (3) After coating a photoresist 5, patterning thereof is
performed by means of a photolithographic process (FIG. 1(c)).
[0038] (4) A mask pattern is transferred to the etching mask 4 by
means of dry etching or wet etching (FIG. 1(d)).
[0039] (5) The core layer 3 is etched by means of reactive ion
etching (RIE) using gaseous oxygen (FIG. 1(e)).
[0040] (6) The etching mask 4 is removed by wet etching and the
like (FIG. 1(f)).
[0041] (7) An over cladding 6 is formed by coating an organic
polymer having a refractive index lower than that of the core layer
3 by means of spin coating, followed by baking (FIG. 1(g)).
[0042] In the optical waveguide above, the core layer 3 having a
high refractive index and made of a polyimide film is interposed
between the buffer layer 2 and the over cladding layer 6 both lower
in refractive index, and hence, a guided light wave is transmitted
inside the waveguide, being confined in the core layer 3.
[0043] Accordingly, the polyimide film constituting the core layer
3 preferably has a higher transmittance for a light having the
desired wavelength (the wavelength of the light waveguided through
the waveguide). Since a polyimide obtained by using fluorinated
acid dianhydride or diamine generally yields a higher transmittance
for the light in the wavelength region ranging from visible to near
infrared regions, it is attracting much attention as a device
material suitable for optical communication, but is expensive.
However, even in case of using a low cost polyimide (the polyimide
according to the present invention), it is possible to increase the
transmittance thereof by carrying out the baking (imidization)
under vacuum.
[0044] The relation between the baking conditions of polyimide and
the transmittance thereof is explained below. A commercially
available polyimide, PIX-3400 (trade name) manufactured by Hitachi
Chemical Co, Ltd., was applied to a glass substrate 0.55 mm in
thickness by spin coating to obtain a pre-baked film 25 .mu.m in
thickness. The film was obtained by baking under conditions as
follows:
[0045] (Condition A) Baking at 350.degree. C. in air for 1 hour
[0046] (Condition B) Baking at 350.degree. C. under gaseous
nitrogen for 1 hour
[0047] (Condition C) Baking at 350.degree. C. under vacuum of
1.times.10.sup.-2 Torr for 1 hour
[0048] In the baking above, prebaking was performed at 130.degree.
C. by using a hot plate in air for 5 minutes after spin coating.
Baking was carried out from 100.degree. C. at a heating rate of
3.degree. C./min.
[0049] The wavelength dependence of transmittance was measured on
each of the samples prepared under the conditions above by using
UV365A manufactured by Shimadzu Corporation. The results are shown
in FIG. 2. The same glass substrate was used as the reference
material.
[0050] It can be understood from the results that the transmittance
for a light 1 .mu.m in wavelength is improved for the samples in
the order of the conditions A, B, and C. In particular, the
transmittance of a sample prepared under the condition C is
distinguished for a light in the wavelength region of 500 to 800
nm, from the conditions A and B. For instance, for a light 650 nm
in wavelength generally used in optical communication using POF as
the transmission medium or in optical disk devices such as a DVD, a
transmittance of 89%, 91%, and 96% were obtained for the samples
prepared under the conditions of A, B, and C, respectively.
[0051] As described above, polyimide films having the same
composition but differing in transmittance can be obtained
depending on the baking condition, and a polyimide film having high
transmittance can be obtained by baking under vacuum. In case of a
light from 500 to 800 nm in wavelength, in particular, the
polyimide films formed under the conditions A and B above were
found practically unfeasible for optical devices which transmit
light due to their low transmittance for the light in the
wavelength band range above. However, a polyimide film sufficiently
high in transmittance for practical application can be obtained by
preparing the polyimide film by baking under the condition C in
vacuum.
[0052] As described in an embodiment according to the present
invention, the transmittance of the film can be improved by baking
the pre-baked film in vacuum, and hence, optical waveguides and
optical devices having low loss can be obtained at a low cost.
[0053] The production process described in the embodiment according
to the present invention is provided only as an example, and
modifications of the process are acceptable so long as the baking
of polyimide is carried out in vacuum. Thus, for instance, the
polyimide film may be patterned by using any process other than
RIE, or a partially modified process can be used. Furthermore,
other optical elements such as prisms and lenses can be fabricated
by applying a similar process.
[0054] [Second Embodiment]
[0055] FIGS. 3(a) to 3(c) are schematic sectional views showing a
method of producing a polyimide optical waveguide using a
photosensitive polyimide according to a second embodiment of the
present invention. Referring to FIG. 3, the process for fabricating
a polyimide optical waveguide is described below. In FIGS. 3(a) to
3(c), the same constituents as those shown in FIGS. 1(a) to 1(c)
are referred by the same numerals.
[0056] (1) A substrate 1 made of Si or the like, whose surface is
covered with a silicon oxide or organic polymer film provided as a
buffer layer 2, is coated with a core layer 3 by applying a
solution in an organic solvent of photosensitive polyimide (i.e.,
polyamic acid that is the precursor thereof) having a refractive
index higher than the buffer layer 2 by spin coating, and a part of
the organic solvent is volatilized and removed therefrom by
prebaking the coating in air (FIG. 3(a)).
[0057] (2) The core layer is patterned by means of a
photolithographic process using a desired mask pattern (FIG.
3(b)).
[0058] (3) By performing baking under vacuum, the organic solvent
is volatilized at the same time with imidization.
[0059] (4) An organic polymer having a refractive index lower than
that of the core layer 3 is applied and baked to obtain an
overcladding 6 (FIG. 3(c)).
[0060] In the optical waveguide above, the core layer 3 having a
high refractive index and made of a polyimide film is interposed
between the buffer layer 2 and the overcladding layer 6 both having
a lower refractive index. In this manner, the wave guided light is
confined inside the core layer 3 and guided through the
waveguide.
[0061] The fabrication process described in the embodiments
according to the present invention is characterized by that the
baking of photosensitive polymide is carried out under vacuum, and
the process above is shown only as an example. Accordingly, a part
of the process may be employed; for instance, the step of coating
the overcladding 6 may be omitted to use air as the overcladding.
Furthermore, other optical elements such as prisms and lenses can
be fabricated in the same manner by applying a similar fabrication
process; likewise, the devices above may be fabricated integrally
on the same substrate.
[0062] By using a photosensitive polyimide as described in the
embodiment above, a polyimide optical waveguide and optical devices
can be fabricated by a further simplified process than that shown
in the first embodiment of the present invention. In general, a
photosensitive polyimide material heretofore yielded a lower light
transmittance as compared with an ordinary polyimide due to
coloring during baking, and was therefore unfeasible for an optical
material. However, as shown in the first embodiment according to
the present invention, the transmittance thereof can be greatly
improved by carrying out the baking under vacuum.
[0063] In FIG. 4 is shown the measured results on the wavelength
dependence of the transmittance of a photosensitive polyimide
(trade name HD-6100, manufactured by Hitachi Chemical--Dupont
Microsystems Co. Ltd.) baked under the same conditions A to C as
used in the first embodiment. It can be seen that the transmittance
differs depending on the baking condition of the photosensitive
polyimide, and that the transmittance for the light 1.5 .mu.m or
less in the wavelength region (particularly for the light 400 nm to
1.2 .mu.m in wavelength) is far higher for the polyimide baked
under the condition C as compared with those obtained under the
condition A or B. For instance, for a light 650 nm in wavelength
generally used in optical communication using POF as the
transmission medium or in optical disk devices such as a DVD, a
transmittance of 10%, 20%, and 94% were obtained for the samples
prepared under conditions of A, B, and C, respectively.
[0064] As described above, the transmittance of a light in the
visible region 1.5 .mu.m or less in wavelength can be greatly
improved by performing the baking of photosensitive polyimide in
vacuum. In the case the light includes radiations in the wavelength
region of from 400 nm to 1.2 .mu.m, the polyimide films prepared
under the conditions A and B were not feasible for practical
application because the transmittance were low, but by performing
baking in vacuum under the condition C, it was possible to obtain a
polyimide film having a transmittance in a sufficiently useful
level.
[0065] Furthermore, transmittance was measured on a sample
fabricated under the condition C and then heated in air at
350.degree. C. to find the sample to be stable, yielding the same
value as that obtained under the condition C.
[0066] As shown above by the embodiment according to the present
invention, an optical device was found to be easily fabricated by
the use of a photosensitive polyimide, and by performing the baking
under vacuum, the transmittance can be improved. Thus, the present
invention is applicable to optical waveguides and to various types
of optical devices.
[0067] Then, the upper limit of the vacuum degree for improving the
transmittance of the polyimide film formed by baking a
photosensitive polyimide as above is described below.
[0068] By using the photosensitive polyimide (trade name HD-6100,
manufactured by Hitachi Chemical--Dupont Microsystems Co. Ltd.)
shown in the embodiment above, samples were prepared under the
conditions D and E below to measure the transmittance thereof.
[0069] (Condition D) Baking at 350.degree. C. under vacuum of 1
Torr for 1 hour
[0070] (Condition E) Baking at 350.degree. C. under vacuum of 10
Torr for 1 hour
[0071] FIG. 5 shows the measured results on the wavelength
dependence of the transmittance of a photosensitive polyimide baked
under the condition C (reference can be made to the first
embodiment) and the conditions D and E above. It can be seen that
the transmittance differs depending on the vacuum degree on baking,
and for a light 650 nm wavelength, transmittance of 94%, 93%, and
84% were obtained for the samples prepared under conditions of C,
D, and E, respectively.
[0072] At a vacuum degree of 1 Torr or lower, little difference
occurs on the transmittance. The polyimide obtained under a vacuum
degree of 10 Torr yields a sufficiently high transmittance, and
although it is practically usable, particularly preferred is to
control the vacuum degree of 1 Torr or lower from the viewpoint of,
for instance, the difficulty in controlling the vacuum apparatus at
a high pressure.
[0073] To further investigate the reason for the change in
transmittance with differing baking conditions, a sample was
prepared under condition F below.
[0074] (Condition F) No Baking
[0075] The non-baked sample fabricated under the condition F was
subjected to infrared absorption spectroscopy by using an infrared
spectrometer, Model FTS6000 manufactured by Nippon Bio-Rad
Laboratories KK.
[0076] In FIG. 6 are shown the infrared absorption spectra
(observed wavenumber range:2,200 to 400 cm.sup.-1; resolution: 4
cm.sup.-1) for the photosensitive polyimides fabricated under the
conditions A to C (reference can be made to the first embodiment)
and F. By taking the transmittance of an infrared radiation into
consideration, a high resistivity silicon was used for the
substrate.
[0077] On comparing the spectra for the samples fabricated under
the conditions of A to C with that fabricated without baking under
condition F, it can be seen that imide rings are formed and that
sufficient imidization occurred on the former samples by the
dehydration cyclization reaction of polyamic acid independent to
the atmospheric conditions of baking.
[0078] Furthermore, the reason why the photosensitive polyimide
obtained by baking in air under the condition A undergo coloring,
i.e., show decrease in transmittance, while that baked in vacuum
under condition C does not. The photosensitive group used in a
photosensitive polyimide is an acetophenone based tertiary amine.
By taking this fact into consideration, the difference in infrared
absorption spectra for the tertiary amine was investigated in
further detail.
[0079] FIG. 7 shows an enlarged spectra in the wavenumber range of
from 1,200 to 500 cm.sup.-1. The absorption peak observed at ca.
1,040 cm.sup.-1 is assigned to the C-N stretching vibration of
tertiary amine (i.e., the photosensitive group), and the intensity
thereof for the samples decreases in the order of the conditions F,
A, B, and C. From the absorption characteristics in the visible
region for the non-baked sample (i.e., the sample obtained under
condition F), it is known that the photosensitive group does not
contribute to the decrease in transmittance in the visible region.
However, the results shown in FIG. 7 suggest that the
photosensitive groups, which should be normally released and
volatilized in the subsequent step of baking, undergo modification
(change in bonding state) by baking and reside in the film. Such
residual groups may influence the transmitting characteristics.
[0080] In the method according to the present invention, the baking
step is carried out in vacuum. Presumably, the release of the
photosensitive groups is accelerated by some effect to realize a
state free of photosensitive groups. In this manner, the
deterioration in transmittance in the visible wavelength region due
to the modification of the photosensitive groups as described above
is prevented from occurring.
[0081] [Third Embodiment]
[0082] FIG. 8 is a schematically drawn optical communication device
using the polyimide optical waveguide according to a third
embodiment of the present invention.
[0083] The optical communication device comprises, as the main
components, a principal waveguide 3a which transfers the received
light incident from an optical fiber 7 to a light receptor device 9
such as a photodiode, and a sub waveguide 3b which transfers the
sending light emitted from a light emitting device 8 such as a
semiconductor laser to the principal waveguide 3a. These waveguides
are formed together with a control unit 11 which controls the light
receptor device 9 and the light emitting device 8, and with a
monitoring photodiode 10 which monitors the output from the light
emitting device 8 on a substrate 1 made from silicon and the like.
The sub waveguide 3b is coupled with the principal waveguide 3a by
the side of the principal waveguide 3a. The principal waveguide 3a
and the sub waveguide 3b are provided by using a polyimide optical
waveguide fabricated by the production method shown by the first or
the second embodiment according to the present invention.
[0084] The light emitting device 8 is controlled by the control
unit 11 to emit a sending light. The sending light emitted from the
light emitting device 8 is coupled with the sub waveguide 3b, and
coupled with the principal waveguide 3a after being transmitted
through the sub waveguide 3b. Finally, the light is transmitted
through the principal waveguide 3a and coupled with the optical
fiber 7. A part of the light emitted from the light emitting device
8 is received by the monitoring photodiode 10, and is controlled as
such that the emission intensity at the light emitting device 8 is
maintained constant. On the other hand, the received light incident
from the optical fiber 7 is coupled with the principal waveguide
3a, and is coupled with the light receptor device 9 by a tapered
coupler 3c comprising a principal waveguide 3a whose thickness is
gradually reduced. Finally, the received light is converted into an
electric signal by using the control unit 11.
[0085] As described in the first embodiment of the present
invention, the principal waveguide 3a and the sub waveguide 3b
having low loss and yet reduced in production cost by using a
polyimide optical waveguide obtained by baking under vacuum. Thus,
an optical communication device having high efficiency can be
realized at low cost. Furthermore, as shown in the second
embodiment according to the present invention, the production
process can be simplified by using a photosensitive polyimide as
the optical waveguide material. This enables an optical
communication device further reduced in cost.
[0086] As the optical fiber 7, usable are the multimode optical
fibers such as POF. A POF comprises a core made of a plastics such
as PMMA, polycarbonate, etc., and the clad is constructed by a
plastics having a refractive index lower than that of the core. The
core diameter ranges from about 200 .mu.m to 1 mm, and has a larger
aperture as compared with a quartz optical fiber.
[0087] Thus, in case POF is coupled with a quartz based optical
waveguide having a thickness of several micrometers, the coupling
loss becomes too large. A polyimide optical waveguide can be formed
relatively easily into a thick film several tens to several
hundreds of micrometers in thickness, and can be coupled with a POF
having a large diameter at high efficiency.
[0088] As the principal waveguide 3a and the sub waveguide 3b, the
thickness thereof is preferably in a range of from 5 to 200 .mu.m.
If the waveguides are provided thinner than the thickness range
above, the coupling efficiency with the POF becomes inferior, or an
optical communication device having lower loss becomes available by
using a quartz based optical waveguide. In the case the waveguides
are provided thicker, it becomes difficult to form uniform films on
the substrate 1. The polyimide film is formed by generally applying
a polyimide solution to the substrate 1 by means of spin coating,
and, although depending on the viscosity and the concentration of
the solution, difficulties are found in obtaining a uniform film
with a thickness of about 40 .mu.m or more. Thus, to obtain a film
thicker than the limit above, it is necessary to superpose a
plurality of films, or to employ coating methods other than spin
coating. However, even if a plurality of films are superposed, the
number of films is limited to several layers. As a coating method
other than spin coating, there is known a method of flow coating
the substrate 1 with a polyimide solution using a doctor blade and
the like. In this method again, it is difficult to obtain a uniform
coating 200 .mu.m or more in thickness due to the self weight of
the polyimide solution. It can be seen from above that the film
thickness which enables an optical device having a higher
performance and yet capable of providing a uniform coating on the
substrate 1 is limited to a range of from 5 to 200 .mu.m.
[0089] As described above, a high performance optical communication
device yet capable of production at a low cost can be obtained by
using an optical waveguide described in the first and the second
embodiments of the present invention. Furthermore, since the
optical waveguides above can be fabricated by a semiconductor
process, they can be aligned with other optical devices at high
precision.
[0090] The embodiments of above are provided only as examples, and
are applicable to other optical communication devices using a
polyimide film formed by baking a polyimide or a photosensitive
polyimide under vacuum, optical elements (e.g., optical elements
such as prisms, mirrors, lenses, etc., which are assembled into an
optical disk device using light flux comprising radiations ranging
in the wavelength of form 400 nm to 1.2 .mu.m), orientation
controlling film of liquid crystal display devices, color filters
and the protective films thereof, overcoating materials represented
by the protective films for semiconductor lasers and photodiodes,
and optical devices having transparent films and the like of a heat
resistant laminated materials, etc. Furthermore, because polyimide
(photosensitive polyimide) excels in resistances against heat and
chemicals as well as in electric insulation properties, it can be
applied to the surface of an electronic component to provide a
protective coating having electric insulating properties.
[0091] (III) As described above, the method for producing an
optical device according to the present invention enables an
optical device having excellent light transmitting properties.
[0092] Furthermore, the method for producing a polyimide film
according to the present invention provides polyimide films having
favorable light transmittance, yet reduced in optical loss and
imparted with heat resistance. Moreover, by baking a photosensitive
polyimide, an optical device can be easily produced without
employing a complicated production process. More specifically, by
forming an organic polymer film on a substrate and patterning it
thereafter, a compact and lightweight optical device can be
obtained with high precision.
[0093] While the invention has been described in detail by making
reference to specific examples, it should be understood that
various changes and modifications can be made without departing
from the scope and the spirit of the present invention.
* * * * *