U.S. patent application number 10/254652 was filed with the patent office on 2003-03-20 for photo-electric combined substrate.
This patent application is currently assigned to NEC CORPORATION. Invention is credited to Fujiwara, Masahiko, Itoh, Masataka, Kaneyama, Yoshinobu, Kitajo, Sakae, Oda, Mikio, Shimada, Yuzo.
Application Number | 20030053765 10/254652 |
Document ID | / |
Family ID | 27322822 |
Filed Date | 2003-03-20 |
United States Patent
Application |
20030053765 |
Kind Code |
A1 |
Oda, Mikio ; et al. |
March 20, 2003 |
Photo-electric combined substrate
Abstract
The present invention provides a photo-electric combined
substrate comprising an electric interconnection part having an
electric interconnection layer and an electric insulating layer as
well as an optical waveguide part consisting of a core and a clad,
where the electric insulating layer in the electric interconnection
part and the optical waveguide part are made of the same material;
a ceramic substrate comprising an optical device and an electric
device where a ceramic substrate has a concave where the concave is
filled with a resin, and where at least an optical device is
mounted on the ceramic substrate while an electric device on the
resin in the substrate concave; and an optical waveguide comprising
a core and a clad having a refractive index lower than that of the
core, where the core is made of a fluorene-unit-containing epoxy
acrylate resin.
Inventors: |
Oda, Mikio; (Tokyo, JP)
; Kitajo, Sakae; (Tokyo, JP) ; Shimada, Yuzo;
(Tokyo, JP) ; Itoh, Masataka; (Tokyo, JP) ;
Kaneyama, Yoshinobu; (Tokyo, JP) ; Fujiwara,
Masahiko; (Tokyo, JP) |
Correspondence
Address: |
FOLEY AND LARDNER
SUITE 500
3000 K STREET NW
WASHINGTON
DC
20007
US
|
Assignee: |
NEC CORPORATION
|
Family ID: |
27322822 |
Appl. No.: |
10/254652 |
Filed: |
September 26, 2002 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
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10254652 |
Sep 26, 2002 |
|
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09593128 |
Jun 13, 2000 |
|
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|
6477284 |
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Current U.S.
Class: |
385/88 ;
385/14 |
Current CPC
Class: |
G02B 6/10 20130101; G02B
6/43 20130101; H05K 1/0306 20130101; H05K 3/4629 20130101; H05K
3/4605 20130101; H01L 2224/16225 20130101; H05K 1/0274 20130101;
H01L 2924/15151 20130101; G02B 6/138 20130101; H05K 1/183
20130101 |
Class at
Publication: |
385/88 ;
385/14 |
International
Class: |
G02B 006/42; G02B
006/12 |
Foreign Application Data
Date |
Code |
Application Number |
Jun 14, 1999 |
JP |
11-167222 |
Jun 17, 1999 |
JP |
11-171096 |
Jun 24, 1999 |
JP |
11-178766 |
Claims
What is claimed is:
1. A ceramic substrate comprising an optical device and an electric
device where a ceramic substrate has a concave where the concave is
filled with a resin, and where at least an optical device is
mounted on the ceramic substrate while an electric device on the
resin in the substrate concave.
2. The ceramic substrate comprising an optical device and electric
device as claimed in claim 1 where the ceramic substrate is a
multilayer interconnection substrate.
3. The ceramic substrate comprising an optical device and an
electric device as claimed in claim 1 where the surfaces of the
resin filled in the concave and of the ceramic substrate are at the
same level.
4. The ceramic substrate comprising an optical device and an
electric device as claimed in claim 1 where a fine electric
interconnection layer is formed on the concave using the resin
filled in the concave as an electric insulating layer.
5. The ceramic substrate comprising an optical device and an
electric device as claimed in claim 1 where the resin filled in the
concave is cured in the concave.
6. The ceramic substrate comprising an optical device and an
electric device as claimed in claim 1 where the resin filled in the
concave is cured into the shape of the concave and then fitted into
the concave.
7. The ceramic substrate comprising an optical device and an
electric device as claimed in claim 1 where a material whose
coefficient of thermal expansion is substantially equal to that of
the material for the optical device is used as a main ceramic
material for the ceramic substrate.
Description
CROSS-REFERENCE TO RELATED PATENT APPLICATIONS
[0001] This application is a division of application Ser. No.
09/593,128, filed Jun. 13, 2000, now pending, and based on Japanese
Patent Application No. 11-167222, filed Jun. 14, 1999, by Mikio
Oda, Sakae Kitajo, Yuzo Shimada, Masataka Itoh, Yoshinobu Kaneyama
and Masahiko Fujiwara This application claims only subject matter
disclosed in the parent application and therefore presents no new
matter.
BACKGROUND OF THE INVENTION
[0002] 1. Field of the Invention
[0003] This invention relates to a photo-electric combined
substrate having a photo-electric transducing function used in,
e.g., optical communication, in which an optical waveguide is
combined with an electric interconnection, and a manufacturing
process therefor.
[0004] This invention also relates to an optical waveguide, in
particular an optical waveguide with a higher heat resistance and a
higher economic efficiency which can be manufactured by a
relatively convenient process, and to a manufacturing process
therefor.
[0005] 2. Description of the Prior Art
[0006] Apparatuses such as an optical exchanger and a
photo-interconnection device have been intensely investigated and
developed for achieving large-capacity and high-speed
communication. These apparatuses comprise an electric signal
processing portion, an optical signal processing portion, and a
transducing portion of an electric signal to an optical signal or
vice versa. The transducing portion comprises a photo-electric
transducing device (optical device) such as a laser diode (LD) and
a photodiode (PD), and an electric element for operating the
optical device or amplifying the signal.
[0007] In a conventional photo-interconnection, a silicon substrate
is, in the light of its properties, used as a substrate on which an
optical waveguide is formed and an optical device is mounted, while
a ceramic substrate or printed board is frequently used as a
substrate on which an electric interconnection is formed and an
electric device is mounted. These substrates are mutually connected
via a bonding wire in a manner that the substrate for the optical
device is placed over the electric substrate.
[0008] In the conventional technique where the substrate on which
an optical waveguide is formed and an optical device is mounted, is
connected, via a bonding wire, with the substrate on which an
electric interconnection is formed and an electric device is
mounted, however, the wire is relatively longer. Therefore, when
increasing an operating frequency for increasing a transmission
capacity, a noise is overlaid on a signal. Thus, a higher frequency
cannot be achieved.
[0009] In an attempt for solving the problem, several techniques
have been proposed, in which both optical and electric devices are
mounted on a single substrate; for example, JP-A 9-236731 has
disclosed a ceramic substrate on which both optical and electric
devices are mounted.
[0010] When both electric and optical devices are mounted on a
ceramic substrate and these devices are closely mounted for
high-speed operation of the optical device, the ceramic is
responsible for heat insulation. However, a ceramic does not have a
sufficiently low heat conductivity to prevent thermal interference
between the electric and the optical devices.
[0011] On the other hand, when an optical device is mounted on a
resin heat-insulating material on a ceramic substrate, the resin is
so soft that an optical axis tends to be not in the right
position.
[0012] For high-speed operation of an optical device, it is
necessary to reduce the length of the electric interconnection
between the electric and the optical devices. When the electric
device and the optical device are mounted by separate methods,
there is a restriction in reducing the length of the electric
interconnection between the electric and the optical devices. There
are also limitations in densification of, e.g., the electric and
the optical devices.
[0013] A combined substrate described above has a configuration
where an optical waveguide consisting of a siloxane polymer is
formed on a ceramic multilayer interconnection substrate. Thus,
when the interconnection substrate and the optical waveguide are
made of different materials, it is necessary to form the electric
insulating layer of the interconnection substrate and the optical
waveguide with different materials by separate processes. It has
been, therefore, difficult in the combined substrate to realize a
complete three-dimensional combination or an adequately reduced
cost for the optical waveguide and the electric interconnection.
Furthermore, for a siloxane polymer used as a resin for an optical
waveguide in the combined substrate, it is difficult to form a fine
interconnection or a via-hole by photolithography process. The
polymer cannot be, therefore, as a material for an electric
insulating film.
[0014] In JP-A 3-245586, a semiconductor laser device is mounted on
a resin such as a fluororesin (Teflon; trade mark) as an insulating
material for preventing heat from being transferred from an
electric device to the semiconductor laser device as an optical
device.
[0015] Among others, an optical waveguide made of a resin has been
intensely investigated and developed because it can be formed by a
low-temperature and low-cost process into various types of
substrates, leading to reduction in an overall cost for an optical
module.
[0016] For example, F. Shimokawa et al., In Pr43rd ECTC (1993), p.
705-710 and T. Matsuura et al., MES'97 (the Seventh
Microelectronics Symposium), p. 77-80 have disclosed an example
where a fluorinated polyimide is used as a material for forming an
optical waveguide.
[0017] According to these publications, the fluorinated polyimide
is applied on a substrate and then heated to 300 to 400.degree. C.
to form a film. Then, the optical waveguide core is processed into
a desired shape by reactive ion etching. A copolymerization ratio
between two polyimides can be varied to adjust a refractive index.
The glass-transition temperature of the fluorinated polyimide is
about 300.degree. C.
[0018] Furthermore, K. Enbutsu et al., MOC/GRIN'97 Tech Digest, P3,
p. 394 and 1998 Electronic Information Communication Association
Electronics Society Meeting Proceeding C-3-69 have disclosed an
example where an ultraviolet (UV) curable resin (photosensitive
epoxy resin) is used as a material for forming an optical
waveguide.
[0019] As indicated in these publications, an ultraviolet curable
resin has an advantage that only the core of the optical waveguide
can be irradiated with UV to be cured into a desired shape. A main
component in the UV curable resin can be varied to adjust a
refractive index. The glass-transition temperature of the UV
curable resin is about 250.degree. C.
[0020] In JP-A 10-170738, an optical waveguide is made of an
asymmetric-spiro-ring containing epoxy acrylate resin.
[0021] However, when a fluorinated polyimide is used as a material
for an optical waveguide, reactive ion etching is employed in
forming the optical waveguide into a desired shape, leading to a
longer etching duration, and thus it cannot be conveniently formed.
In addition, available substrate materials are limited due to
process factors such as a higher deposition temperature.
Furthermore, a fluorinated polyimide has a disadvantage of a higher
material cost.
[0022] On the other hand, an UV curable resin can be used to
conveniently form a core shape because only UV irradiation is
required. It is, however, used for a multi-mode optical waveguide
whose core cross section has a width and a height of several ten
micrometers because of an insufficient resolution for forming a
fine pattern. Thus, it is not be applied to a single-mode optical
waveguide whose core cross section has a width and a height of
about several micrometers.
[0023] Furthermore, the UV curable resins as described in the prior
art publications have a glass-transition temperature of about
250.degree. C., so that it cannot endure an optical device mounting
step (about 300.degree. C.) using a gold/tin solder (melting point:
280.degree. C.) having a self-alignment effect, i.e., an effect
that an optical device is drawn to a desired position by surface
tension of a solder ball.
SUMMARY OF THE INVENTION
[0024] An object of this invention is to provide a photo-electric
combined substrate in which an optical waveguide and an electric
interconnection can be three-dimensionally combined with a reduced
cost, and a manufacturing process therefor.
[0025] Another object of this invention is to provide a ceramic
substrate comprising an optical device and an electric device which
can operate with a high speed.
[0026] Another object of this invention is to provide a material
for an optical waveguide which has good optical transparency at
communication frequencies of 1.3 and 1.55 .mu.m and adequate heat
resistance and can be conveniently shaped.
[0027] In the first aspect, it provides a photo-electric combined
substrate comprising an electric interconnection part having an
electric interconnection layer and an electric insulating layer as
well as an optical waveguide part consisting of a core and a clad,
where the electric insulating layer in the electric interconnection
part and the optical waveguide part are made of the same
material.
[0028] The photo-electric combined substrate of this aspect
described above allows the electric interconnection part and the
optical waveguide part to be formed by the same process, so that
the optical waveguide part and the electric interconnection part
can be three-dimensionally mounted and the photo-electric combined
substrate can be prepared with a reduced cost.
[0029] The electric interconnection part and the optical waveguide
part may be formed as separate structures.
[0030] Alternatively, the optical waveguide part may be placed on
the electric interconnection part, or the optical waveguide part
may be formed in the electric insulating layer of the electric
interconnection part, which allows the photo-electric combined
substrate to be further densified.
[0031] The material for the above-mentioned electric insulating
layer and the optical waveguide may be a photosensitive resin whose
refractive index depends on an exposure dose, and the core of the
optical waveguide may be formed by scanning an exposure light while
focusing on a desired position in the photosensitive resin such
that the refractive index of the part to be the core of the
photosensitive resin is higher than that of the part to be the clad
of the photosensitive resin.
[0032] Moreover, the electric interconnection part and the optical
waveguide part may be formed on the same substrate.
[0033] The above substrate may be a ceramic substrate, a
single-layer interconnection substrate or a multilayer
interconnection substrate.
[0034] This aspect also provides a process for manufacturing a
photo-electric combined substrate comprising an electric
interconnection part having an electric interconnection layer and
an electric insulating layer as well as an optical waveguide part
consisting of a core and a clad, which comprises steps of forming
the electric interconnection part and forming the optical waveguide
part, where the electric insulating layer in the electric
interconnection part and the optical waveguide part are made of the
same material.
[0035] It allows the electric interconnection part and the optical
waveguide part to be formed by the same process. Furthermore, the
optical waveguide part can be three-dimensionally mounted with the
electric interconnection part, and the photo-electric combined
substrate can be prepared with a reduced cost.
[0036] The steps of forming the electric interconnection part and
forming the optical waveguide part may comprise the step of forming
the electric interconnection part and the optical waveguide part as
separate structures.
[0037] The step of forming the electric interconnection part and
the optical waveguide part as separate structures may comprise the
steps of depositing the material while separately forming the
electric interconnection part and the optical waveguide part in
given areas of the deposited material, and removing the deposited
material where the electric interconnection part or the optical
waveguide part is not to be formed.
[0038] The step of forming the optical waveguide part may comprise
the step of forming the optical waveguide part on the electric
interconnection part. Alternatively, in terms of the steps of
forming the electric interconnection part and forming the optical
waveguide part, the optical waveguide part may be formed in the
electric insulating layer of the electric interconnection part
during forming the electric interconnection part. It provides a
further-densified photo-electric combined substrate.
[0039] The process of this aspect may comprise the step of forming
the core of the optical waveguide part by scanning an exposure
light while focusing on a desired position in the photosensitive
resin such that the refractive index of the part to be the core of
the photosensitive resin is higher than that of the part to be the
clad of the photosensitive resin, using a photosensitive resin
whose refractive index depends on an exposure dose.
[0040] The process of this aspect may comprise the step of forming
the electric interconnection part and the optical waveguide part on
the substrate and the substrate may be a ceramic substrate, a
single-layer interconnection substrate or a multilayer
interconnection substrate.
[0041] In the second aspect of the present invention, it provides a
ceramic substrate having a concave where the concave is filled with
a resin, and where at least an optical device is mounted on the
ceramic substrate while an electric device on the resin in the
substrate concave. Particularly, on the resin-filled concave, a
fine electric interconnection layer using the resin as an electric
insulating layer is formed.
[0042] This aspect also provides a process for manufacturing the
above ceramic substrate comprising an optical device and an
electric device, comprising the steps of forming a concave on a
ceramic substrate; filling a resin in the concave; and mounting an
optical device on the ceramic substrate while mounting an electric
device on the resin filled in the concave.
[0043] An optical device, particularly a light emitting diode such
as an LD significantly susceptible to heat may be mounted on a
ceramic substrate while an electric device on a resin filled in a
concave on the ceramic substrate. It can prevent thermal
interference between the electric and the optical devices and thus
it hardly brings about misalignment of an optical axis.
Furthermore, the surfaces of the ceramic substrate and of the resin
filled in the concave may be at the same level to reduce the wiring
length between the electric and the optical devices and to further
promote densification. In particular, a fine electric
interconnection layer may be formed in the concave using the resin
filled in the concave as an electric insulating layer and then an
electric device may be mounted on the interconnection layer to meet
the need of an electric device with a finer interconnection.
[0044] In the third aspect of the present invention, it provides an
optical waveguide comprising a core and a clad having a refractive
index lower than that of the core, where the core is made of a
fluorene-unit-containing epoxy acrylate resin.
[0045] The clad is preferably made of a fluorene-unit-containing
epoxy acrylate resin whose refractive index is lower than the
material for the core.
[0046] The core or both of the core and the clad formed using the
fluorene-unit-containing epoxy acrylate resin preferably have a
glass-transition temperature of 260.degree. C. or higher, a light
propagation loss of 0.5 dB/cm or less at a wavelength of 1.3 .mu.m,
and a light propagation loss of 0.5 dB/cm or less at a wavelength
of 1.55 .mu.m.
[0047] A particularly preferable fluorene-unit-containing epoxy
acrylate resin described above is, for example, the following
compound represented by formula (1): 1
[0048] wherein X is the chemical structure represented by formula
(2), Y is hydrogen or methyl, and n is an integer of 0 or more:
2
[0049] wherein *s in the benzene rings indicate bonding positions
to the chemical structure X in formula (1), and the positions * may
be independently selected from ortho-, meta- and para-positions to
the bonding position of the fluorene unit with the benzene rings;
R1 to R16 are independently selected from a hydrogen atom, an alkyl
group, an alkoxy group, an alkoxycarbonyl group, an aryl group and
an aralkyl group.
[0050] This aspect of the present invention also provides a process
for manufacturing an optical waveguide, comprising the steps
of:
[0051] forming a lower clad layer on a substrate (Step 1);
[0052] forming a fluorene-unit-containing epoxy acrylate resin
layer on the lower clad layer (Step 2);
[0053] exposing and etching the fluorene-unit-containing epoxy
acrylate resin layer to form a core (Step 3); and
[0054] post-baking the substrate comprising the core (Step 4).
[0055] For forming a fluorene-unit-containing epoxy acrylate resin
layer as the lower clad layer in the above Step 1, the
fluorene-unit-containing epoxy acrylate resin layer is formed; then
the resin layer is exposed with a dose more than that during the
step of forming the core to control the refractive index of the
lower clad layer to be lower than the refractive index of the core,
and then the substrate comprising the lower clad layer is
post-baked.
[0056] After the above Step 4, for forming a
fluorene-unit-containing epoxy acrylate resin layer as an upper
clad layer covering the core layer, the fluorene-unit-containing
epoxy acrylate resin layer is formed; then the resin layer is
exposed with a dose more than that during the step of forming the
core to control the refractive index of the upper clad layer to be
lower than the refractive index of the core, and then the substrate
comprising the upper clad layer is post-baked.
[0057] The post baking may be conducted at a temperature of 160 to
250.degree. C.
[0058] The material for the optical waveguide according to the
third aspect has the following features; first, it has a good
optical transparency at communication wavelengths of 1.3 and 1.55
.mu.m; second, it has a glass-transition temperature of 260.degree.
C. or higher, i.e., exhibits higher heat resistance; third, it is
curable by UV irradiation owing to its epoxy acrylate groups, so
that the core may have a cross section having a height and a width
of several to several ten micrometers by UV exposure and
development and thus the core may be readily formed; fourth, the
refractive index of the fluorene-unit-containing epoxy acrylate
resin may be controlled within an appropriate range by controlling
an exposure dose, so that using the same material, a layer can be
formed and then the clad and the core can be formed only by
adjusting an exposure dose; and fifth, in contrast to a fluorinated
polyimide, the film-formation of the resin requires a relatively
lower temperature, resulting in a low-temperature process.
[0059] As described above, a fluorene-unit-containing epoxy
acrylate resin can be used as a material for an optical waveguide
to conveniently form a heat-resistant optical waveguide.
BRIEF DESCRIPTION OF THE DRAWINGS
[0060] FIG. 1 is a cross section illustrating the first embodiment
of a photo-electric combined substrate according to the first
aspect of this invention.
[0061] FIGS. 2(a) to (d) are cross sections illustrating a
manufacturing process for the combined substrate of FIG. 1.
[0062] FIG. 3 is a side view of the optical waveguide in the
combined substrate of FIG. 1.
[0063] FIG. 4 is a cross section illustrating the second embodiment
of the combined substrate according to the first aspect of this
invention.
[0064] FIGS. 5(a) to (d) are cross sections illustrating a
manufacturing process for the combined substrate of FIG. 4.
[0065] FIG. 6 is a cross section illustrating the third embodiment
of the combined substrate according to the first aspect of this
invention.
[0066] FIGS. 7(a) to (d) are cross sections illustrating a
manufacturing process for the combined substrate of FIG. 6.
[0067] FIG. 8 shows the first embodiment of the second aspect
according to this invention, where there is formed, on a concave in
a ceramic multilayer interconnection substrate, a fine
interconnection using a resin as an electric insulating layer, and
an electric device is mounted on the interconnection while an
optical device on the ceramic substrate.
[0068] FIG. 9 shows the second embodiment of the second aspect
according to this invention, where a resin is filled in a concave
in a ceramic multilayer interconnection substrate, and an electric
device is mounted on the resin while an optical device on the
ceramic substrate.
[0069] FIG. 10 illustrates a process for manufacturing the
substrate of FIG. 8.
[0070] FIG. 11 illustrates a process for manufacturing the
substrate of FIG. 9.
[0071] FIG. 12 is a plan view of a ceramic multilayer
interconnection substrate for illustrating Step (b) in Example 2 in
detail.
[0072] FIG. 13 is a plan view of a ceramic multilayer
interconnection substrate for illustrating Step (b) in Examples 3
or 4 in detail.
[0073] FIG. 14 shows a structure in Example 5.
[0074] FIG. 15 is a schematic cross section of a built-in optical
waveguide as an embodiment of the third aspect according to this
invention.
[0075] FIG. 16 is a schematic cross section of a built-in optical
waveguide as another embodiment of the third aspect according to
this invention.
[0076] FIG. 17 is a cross section of a ridge type of optical
waveguide of the third aspect according to this invention.
DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS
[0077] Several embodiments of the first aspect according to this
invention will be described with reference to the drawings.
[0078] (Embodiment 1)
[0079] FIG. 1 is a cross section illustrating the first embodiment
of a photo-electric combined substrate according to the first
aspect of this invention.
[0080] The photo-electric combined substrate (hereinafter, referred
to as a "combined substrate") of this embodiment comprises a
ceramic multilayer interconnection substrate 1 where an electric
interconnection is formed on both surfaces and its inside via a
copper interconnection 2 and an interlayer via hole 3; a fine
interconnection part on the ceramic multilayer interconnection
substrate 1 which consists of an electric insulating layer 4 and a
fine copper interconnection 5; and an optical waveguide also on the
ceramic multilayer interconnection substrate 1 which consists of an
optical waveguide clad layer 6 and an optical waveguide core layer
7. On the ceramic multilayer interconnection substrate 1 are
mounted an LD (laser diode) 8 via a high-melting solder bump 9 and
a driver silicon LSI 10 via a solder bump 11. A control silicon LSI
12 is mounted on the fine electric interconnection.
[0081] In the combined substrate configured as described above, the
control silicon LSI 12 controls the driver silicon LSI 10, which
then drives the LD 8. The LD 8 is optically connected to the
optical waveguide core 7.
[0082] The manufacturing process for the combined substrate
illustrated in FIG. 1 will be described with reference to FIG. 2,
which consists of cross sections illustrating a process for
manufacturing the combined substrate of FIG. 1.
[0083] In the manufacturing process for the combined substrate, a
ceramic multilayer interconnection substrate 1 is first formed as
shown in FIG. 2(a).
[0084] Alumina powder, a flux, an organic binder, a solvent and a
plasticizer are well mixed in a ball mill. The mixture is applied
and extended on a carrier tape by a blade and dried to form a green
sheet. Then, the green sheet is punched with a die, the hole is
filled with a conductor paste made of a metal powder, and a given
conductor pattern is printed on the green sheet. A plurality of
green sheets prepared as described above are then piled and baked.
A ceramic multilayer interconnection substrate 1 is thus
prepared.
[0085] Then, as shown in FIG. 2(b), a fine electric interconnection
part and an optical waveguide part are formed on the ceramic
multilayer interconnection substrate 1.
[0086] On the ceramic multilayer interconnection substrate 1 is
spin-coated a photosensitive resin which is to be a lower clad
layer 6a in an optical waveguide and an electric insulating layer 4
in a fine electric interconnection. It is then exposed and
developed to form a via pattern in an area to be used as the fine
electric interconnection, to leave an area to be used as the lower
clad layer 6a as an even resin film and to remove the
photosensitive resin in an area where the surface of the ceramic
multilayer interconnection substrate 1 is to be exposed. Herein,
the photosensitive resin is a resin whose refractive index depends
on an exposure dose, and in the exposure-development process the
dose is adjusted such that the refractive index of the lower clad
layer 6a is slightly lower than that of the optical waveguide core
7 described later. The photosensitive resin is then cured at a
unique temperature for the resin. A plating resist is then applied
by spin coating. It is then exposed and developed, and patterned
for forming a fine copper interconnection. Copper plating is then
applied to form a fine copper interconnection 5 and the plating
resist is removed.
[0087] Subsequently, a photosensitive resin is spin-coated, which
is to be an optical waveguide core layer 7 and an electric
insulating layer 4 in a fine electric interconnection. It is
exposed and developed to form a via pattern in an area to be used
as a fine electric interconnection, to form a core pattern in an
area to be used as an optical waveguide core 7, and to remove the
photosensitive resin in an area where the surface of the ceramic
multilayer interconnection substrate 1 is to be exposed. Herein,
the photosensitive resin is a resin whose refractive index depends
on an exposure dose, and in the exposure-development process, while
patterning the optical waveguide core layer 7, the dose is adjusted
such that the refractive index of the core layer 7 is slightly
higher than that of the lower clad layer 6a. The photosensitive
resin is then cured at a unique temperature for the resin. A
plating resist is then applied by spin coating. It is then exposed
and developed, and patterned for forming a fine copper
interconnection. Copper plating is then applied to form a fine
copper interconnection 5 and the plating resist is removed.
[0088] Subsequently, a photosensitive resin is spin-coated, which
is to be an upper clad layer 6b in the optical waveguide clad layer
6 and an electric insulating layer 4 in a fine electric
interconnection. It is exposed and developed to form a via pattern
in an area to be used as a fine electric interconnection, to form a
core pattern in an area to be used as an upper clad layer 6b, and
to remove the photosensitive resin in an area where the surface of
the ceramic multilayer interconnection substrate 1 is to be
exposed. Herein, the photosensitive resin is a resin whose
refractive index depends on an exposure dose, and in the
exposure-development process, the dose is adjusted such that the
refractive index of the upper clad layer 6b is slightly lower than
that of the optical waveguide core layer 7. The photosensitive
resin is then cured at a unique temperature for the resin. A
plating resist is then applied by spin coating. It is then exposed
and developed, and patterned for forming a fine copper
interconnection. Copper plating is then applied to form a fine
copper interconnection 5 and the plating resist is removed.
[0089] Then, as shown in FIG. 2(c), an LD 8 is mounted on the
ceramic multilayer interconnection substrate 1 using a high-melting
solder 9.
[0090] Finally, as shown in FIG. 2(d), a driver silicon LSI 10 is
mounted on the ceramic multilayer interconnection substrate 1 via a
solder bump 11 and a control silicon LSI 12 is mounted on the fine
electric interconnection.
[0091] The above process can provide a combined substrate
comprising a three-layered fine electric interconnection part
consisting of an electric insulating layer 4 and a fine copper
interconnection 5 and an optical waveguide part, in which an LD 8
and LSIs 10, 12 are mounted as an optical device and electric
devices, respectively. According to the manufacturing process
described above, the fine electric interconnection part and the
optical waveguide part can be formed in separate areas, using the
same material for the electric insulating layer 4 and the optical
waveguide by the same process. The number of the interconnection
layers in the fine electric interconnection part can be selected as
appropriate. The optical waveguide is not limited to the above
three-dimensional configuration.
[0092] FIG. 3 is a side view of the optical waveguide in the
combined substrate of FIG. 1.
[0093] As seen in FIG. 3, an optical waveguide is formed in a
configuration where an optical waveguide clad layer 6 surrounds the
optical waveguide core layer 7. Although FIG. 3 shows an example
comprising three optical waveguide core layers 7, the number of the
core layers 7 may be varied, depending on, e.g., a desired
application for an optical circuit.
[0094] (Embodiment 2)
[0095] FIG. 4 is a cross section illustrating the second embodiment
of a combined substrate according to the first aspect of this
invention.
[0096] The combined substrate of this embodiment comprises a
ceramic multilayer interconnection substrate 21 where an electric
interconnection is formed on both surfaces and its inside via a
copper interconnection 22 and an interlayer via hole 23. A fine
electric interconnection layer consisting of an electric insulating
layer 24 and a fine copper interconnection 25 is formed on the
whole upper face of the interconnection substrate 21. Moreover, an
optical waveguide part consisting of an optical waveguide core
layer 27 and an optical waveguide clad 26 is formed in a given area
on the fine electric interconnection layer. The electric insulating
layer 24 and the optical waveguide part are made of the same
material. On the fine electric interconnection layer are mounted an
LD 28 via a high-melting solder bump 29 as well as a driver silicon
LSI 30 and a control silicon LSI 32 via a solder bump 31.
[0097] In the combined substrate configured as described above, the
control silicon LSI 32 controls the driver silicon LSI 30, which
then drives the LD 28. The laser diode 28 is optically connected to
the optical waveguide core 27.
[0098] The manufacturing process for the combined substrate
illustrated in FIG. 4 will be described with reference to FIG. 5,
which consists of cross sections illustrating a process for
manufacturing the combined substrate of FIG. 4.
[0099] In the manufacturing process for the combined substrate, a
ceramic multilayer interconnection substrate 21 is first formed as
shown in FIG. 5(a).
[0100] Alumina powder, a flux, an organic binder, a solvent and a
plasticizer are well mixed in a ball mill. The mixture is applied
and extended on a carrier tape by a blade and dried to form a green
sheet. Then, the green sheet is punched with a die, the hole is
filled with a conductor paste made of a metal powder, and a given
conductor pattern is printed on the green sheet. A plurality of
green sheets prepared as described above are then piled and baked.
A ceramic multilayer interconnection substrate 21 is thus
prepared.
[0101] Then, as shown in FIG. 5(b), a fine electric interconnection
part is formed on the whole upper surface of the ceramic multilayer
interconnection substrate 21.
[0102] On the ceramic multilayer interconnection substrate 21 is
spin-coated a photosensitive resin which is to be an electric
insulating layer 24. It is then exposed and developed to form a
pattern for an electric insulating layer, and heated at a unique
temperature for the resin to be cured. A plating resist is applied
by spin coating, and exposed and developed. After patterning for an
electric conductor layer, copper plating is applied to form a fine
copper interconnection 25.
[0103] The above steps of forming the electric insulating layer and
of forming the fine copper interconnection are repeated by adequate
times to form a desired fine electric interconnection layer on the
ceramic multilayer interconnection substrate 21.
[0104] Subsequently, as shown in FIG. 5(c), an optical waveguide
part is formed on the fine electric interconnection part consisting
of the electric insulating layer 24 and the fine copper
interconnection 25 on the ceramic multilayer interconnection
substrate 21.
[0105] On the fine electric interconnection is first spin-coated
the same resin as that for the electric insulating layer 24 to be a
lower clad layer 26a in the optical waveguide. It is exposed and
developed to pattern the optical waveguide lower clad layer 26a
into a given shape. The resin is cured at a unique temperature for
the resin. Then, the same resin as that for the electric insulating
layer 24 to be an optical waveguide core layer 27 is spin-coated.
It is then exposed and developed to pattern a given area into an
optical waveguide core layer 27. The resin is cured at a unique
temperature for the resin.
[0106] Subsequently, the same resin as that for the electric
insulating layer 24 to be an optical waveguide upper clad layer 26b
is spin-coated. It is then exposed and developed to pattern a given
the upper clad layer 26b into a given shape. The resin is cured at
a unique temperature for the resin. The clad layers 26a, 26b and
the core layer 27 are made of a resin whose refractive index
depends on an exposure dose. In the process of exposure and
development, while pattering, the dose is adjusted such that the
refractive index of the optical waveguide core layer 27 is slightly
lower than that of the clad layer 26.
[0107] Finally, as shown in FIG. 5(d), an LD 28 is mounted on the
fine electric interconnection part via a high-melting solder 29
while a driver silicon LSI 30 and a control silicon LSI 32 are
mounted on the fine electric interconnection part via a solder bump
31.
[0108] The above process can provide the combined substrate of this
embodiment. According to the manufacturing process described above,
the optical waveguide part can be formed on the fine electric
interconnection part of the ceramic multilayer interconnection
substrate 21, using the same material for the electric insulating
layer 24 and the optical waveguide by the same process. The number
of the interconnection layers in the fine electric interconnection
part can be selected as appropriate. The optical waveguide is not
limited to the above three-dimensional configuration.
[0109] (Embodiment 3)
[0110] FIG. 6 is a cross section illustrating the third embodiment
of a combined substrate according to the first aspect of this
invention.
[0111] The combined substrate of this embodiment comprises a
ceramic multilayer interconnection substrate 41 where an electric
interconnection is formed on both surfaces and its inside via a
copper interconnection 42 and an interlayer via hole 43. A fine
electric interconnection layer consisting of an electric insulating
layer 44 and a fine copper interconnection 45 is formed on the
whole upper face of the interconnection substrate 41. Moreover, an
optical waveguide consisting of an optical-transmitting core layer
53 and an optical-receiving core layer 55 is formed in the fine
electric interconnection layer. The electric insulating layer 44
and the optical waveguide are made of the same material. On the
fine electric interconnection layer are mounted an LD 48 via a
high-melting solder bump 49 as well as a driver silicon LSI 50 and
a control silicon LSI 52 via a solder bump 51.
[0112] In the combined substrate configured as described above, the
control silicon LSI 52 controls the driver silicon LSI 50, which
then drives the LD 48. The LD 48, the optical-transmitting core 53,
the PD 54 and the optical-receiving core 55 are optically connected
with each other.
[0113] The manufacturing process for the combined substrate
illustrated in FIG. 6 will be described with reference to FIG. 7,
which consists of cross sections illustrating a process for
manufacturing the combined substrate of FIG. 6.
[0114] Alumina powder, a flux, an organic binder, a solvent and a
plasticizer are well mixed in a ball mill. The mixture is applied
and extended on a carrier tape by a blade and dried to form a green
sheet. Then, the green sheet is punched with a die, the hole is
filled with a conductor paste made of a metal powder, and a given
conductor pattern is printed on the green sheet. A plurality of
green sheets prepared as described above are then piled and baked.
A ceramic multilayer interconnection substrate 41 is thus prepared
as shown in FIG. 7(a).
[0115] Then, as shown in FIG. 7(b), a fine electric interconnection
part is formed on the whole upper surface of the ceramic multilayer
interconnection substrate 41.
[0116] On the ceramic multilayer interconnection substrate 41 is
spin-coated a photosensitive resin which is to be an electric
insulating layer 44. It is then exposed and developed to form a
pattern for an electric insulating layer, and heated at a unique
temperature for the resin to be cured. A plating resist is applied
by spin coating, and exposed and developed. After patterning for an
electric conductor layer, copper plating is applied to form a fine
copper interconnection 45.
[0117] The above steps of forming the electric insulating layer and
of forming the fine copper interconnection are repeated by adequate
times to form a desired fine electric interconnection layer on the
ceramic multilayer interconnection substrate 41.
[0118] Subsequently, as shown in FIG. 7(c), an optical waveguide is
formed in the electric insulating layer 44 of the fine electric
interconnection on the ceramic multilayer interconnection substrate
41. The photosensitive resin is a resin whose refractive index
depends on a dose of laser beam as an exposing light. The laser
beam is scanned while focusing on areas where photosensitive-resin
core layers 53, 55 are to be formed such that the refractive index
of the optical waveguide core layers 53, 55 is slightly higher than
that of the surrounding electric insulating layer 44, to
three-dimensionally draw the optical waveguide core layers 53,
55.
[0119] Finally, as shown in FIG. 7(d), an LD 48 is mounted on the
fine electric interconnection part via a high-melting solder 49
while a driver silicon LSI 50 and a control silicon LSI 52 are
mounted on the fine electric interconnection part via a solder bump
51.
[0120] The above process can provide the combined substrate of this
embodiment. According to the manufacturing process described above,
the optical waveguide can be formed on the fine electric
interconnection of the ceramic multilayer interconnection substrate
41, using the same material for the electric insulating layer 44
and the optical waveguide by the same process.
[0121] The second aspect of this invention will be described.
[0122] Mounting an optical device, particularly a light emitting
diode such as an LD, on a ceramic substrate while mounting an
electric device on a resin filled in a concave on the ceramic
substrate can more effectively prevent thermal interference between
the electric and the optical devices than the case where an optical
and an electric devices are mounted using only a ceramic substrate,
because a resin has a lower thermal conductivity than a ceramic. In
particular, when a fine electric interconnection is formed using a
resin as an electric insulating layer, an electric device with a
finer interconnection which cannot be achieved in a ceramic
electric interconnection, can be mounted. Furthermore, the optical
device is mounted on the ceramic substrate which is harder than a
resin, and thus misalignment of an optical axis can be effectively
prevented. Moreover, an electric and an optical devices can be
mounted by an equivalent process such as bare-chip mounting. These
devices can be, therefore, closely mounted to reduce the length of
the electric interconnection between the devices, leading to
further densification. Herein, it is desirable that the surfaces of
the ceramic substrate on which an optical device is mounted and of
the resin filled in the concave where an electric device is mounted
are at the substantially same level to avoid difficulty in forming
an electric interconnection.
[0123] The ceramic substrate may be a multilayer ceramic
interconnection substrate prepared by molding into a desired shape
ceramic powder such as alumina, aluminum nitride, silicon carbide
and beryllium oxide, which is optionally combined with, e.g., a
binder; forming an interconnection pattern on its surface to form a
green sheet; and then piling a plurality of green sheets which are
then baked. In addition, materials such as mullite
(3Al.sub.2O.sub.3.2SiO.sub.2), glass ceramic, aluminum nitride,
silicon carbide and beryllium oxide may be used alone.
[0124] The concave on the ceramic substrate may be formed by an
appropriate process such as, but not limited to, piling a given
number of green sheets which have been preliminary punched out the
area to form the concave, depending on the depth of the concave; or
applying a photoresist, exposing an area to be processed and
sand-blasting the area.
[0125] A resin material filled in the concave on the ceramic
substrate may be any resin which is commonly used for a usual
electric device, preferably an epoxy or polyimide resin.
[0126] The resin may be filled by either applying the resin on the
whole surface of the substrate including the concave by an
application technique such as bar coating, knife coating and spin
coating, or applying the resin only on the concave using a means
such as a dispenser. The surfaces of the resin and of the ceramic
can be at the same level by applying a suitable curing technique to
the resin used and then grinding the surface for leveling.
Alternatively, the resin may be separately cured into a shape
corresponding to the concave on the ceramic substrate and it may be
then adhered to the concave on the ceramic substrate. As used
herein, the term "at the same level" should not be necessarily
construed to indicate that these surfaces must provide a strictly
even plane with no tiny steps. A small step may be acceptable as
long as an interconnection between an optical and an electric
device can be formed without disconnection.
[0127] (Embodiment 1)
[0128] FIG. 8 shows the first embodiment of the second aspect. A
ceramic multilayer interconnection substrate 81 has a concave 82
filled with a resin 83. The surfaces of the ceramic multilayer
interconnection substrate 81 and the resin 83 filled in the concave
82 are at the same level. An optical device 85 is mounted on the
ceramic multilayer interconnection substrate 81 while an electric
device 86 with a cooling fin 87 on the resin 83 filled in the
concave 82 of the ceramic multilayer interconnection substrate 81.
The optical and the electric devices are electrically connected via
an electric interconnection 84 with the shortest distance.
[0129] (Embodiment 2)
[0130] FIG. 9 shows the second embodiment of the second aspect. A
ceramic multilayer interconnection substrate 91 has a concave 92 in
which a fine electric interconnection layer 98 consisting of an
electric insulating resin layer 981 and an electric conductor layer
982 is formed. The surfaces of the ceramic multilayer
interconnection substrate 91 and the fine electric interconnection
layer 98 in the concave 92 are at the same level. An optical device
95 is mounted on the ceramic multilayer interconnection substrate
91 while an electric device 96 with a cooling fin 97 on the fine
electric interconnection layer 98 in the concave 92 of the ceramic
multilayer interconnection substrate 91. The optical and the
electric devices 95, 96 are electrically connected via an electric
interconnection 94 with the shortest distance.
[0131] The third aspect of this invention will be described.
[0132] An optical waveguide according to the third aspect of this
invention essentially consists of a core and a clad. There are no
restrictions for its structure; it may be a built-in type or ridge
type of optical waveguide.
[0133] A fluorene-unit-containing epoxy acrylate resin used in this
invention comprises the fluorene unit represented by formula (3)
and an epoxy acrylate (including an epoxy methacrylate) unit which
is a UV-curable functional group. 3
[0134] Such a resin is particularly used as a material for an
optical waveguide because 1) it comprises both fluorene and epoxy
acrylate units which allow the material to be deposited at a lower
temperature while being heat resistant adequately to endure a
photo-irradiation mounting process; 2) it exhibits a small light
propagation loss at communication wavelengths of 1.3 and 1.55 .mu.m
so that it is very effective as a material for an optical
waveguide; and 3) it allows the refractive indices of the core and
the clad layers to be conveniently controlled only by changing an
exposure dose, which is required during forming an optical
waveguide.
[0135] A particularly preferable epoxy acrylate resin comprising a
fluorene unit is a compound represented by formula (1): 4
[0136] wherein X is the chemical structure represented by formula
(2), Y is hydrogen or methyl, and n is an integer of 0 or more:
5
[0137] wherein *s in the benzene rings indicate bonding positions
to the chemical structure X in formula (1), and the positions * may
be independently selected from ortho-, meta- and para-positions to
the bonding position of the fluorene unit with the benzene rings;
R1 to R16 are independently selected from a hydrogen atom, an alkyl
group, an alkoxy group, an alkoxycarbonyl group, an aryl group and
an aralkyl group.
[0138] The substituents R1 to R16 are selected from the group
consisting of C.sub.1-8 alkyl group such as methyl, ethyl, propyl
and butyl; C.sub.1-8 alkoxy group such as methoxy, ethoxy, propoxy
and butoxy; C.sub.2-9 alkoxycarbonyl group such as methoxycarbonyl,
ethoxycarbonyl, propoxycarbonyl and butoxycarbonyl; aryl group such
as phenyl, tolyl, xylyl, mesylyl, cumenyl, naphthyl and
anthracenyl; and aralkyl group such as benzyl, phenetyl,
1-phenylethyl, 1-methyl-1-phenylethyl and methylbenzyl. For the
above substituents including a branched structure such as C.sub.4
or more alkyl, their isomers are also encompassed within this
invention.
[0139] A radical initiator may be added to the resin to promote
photopolymerization. A radical initiator may be selected from, but
not limited to, known peroxide initiators and azobis-type
initiators. Peroxide initiators include methyl ethyl ketone
peroxide, isobutyl peroxide and cumene hydroperoxide. Azobis-type
initiators include 1,1'-azobiscyclohexane-1-carbonitrile and
4,4'-azobis(4-cyanovaleric acid).
[0140] A fluorene-unit-containing epoxy acrylate resin including
these compositions is dissolved into a solvent, the resulting
solution is applied to a given substrate to form a layer, and it is
then subject to pre-heating for evaporating the solvent to provide
a dried film.
[0141] The solvent can preferably homogeneously dissolve or
otherwise homogeneously disperse the fluorene-unit-containing epoxy
acrylate resin, and can be appropriately selected, taking into its
boiling point consideration. For example, the resin may be
dissolved or dispersed in an organic solvent such as cellosolves or
a photopolymerizable compound which is liquid at an ambient
temperature such as a lower molecular weight epoxy (meth)acrylate,
as long as the properties of the optical waveguide such as heat
resistance, a refractive index and an optical-propagation loss are
not deteriorated.
[0142] The concentration of the fluorene-unit-containing epoxy
acrylate resin in the solution or homogeneous dispersion may be
appropriately selected, depending on, e.g., a designed film
thickness.
[0143] A substrate to which the resin is applied may be, for
example, a silicon, ceramic or printed substrate.
[0144] When the film is used as a core, a given core-shaped area is
subject to UV curing by partial exposure using a mask, where the
refractive index of the core depends on a UV dose. Furthermore, it
is soaked in an amine-based alkaline developing solution for
developing and etching the unexposed areas to form a core.
[0145] When the film is used as a clad layer, UV is irradiated with
a dose more than a predetermined dose to a core area to adjust the
refractive index of the clad to be lower than that of the core.
[0146] An exemplary relationship between a dose and a refractive
index is shown in Table 1.
1 TABLE 1 Refractive index Dose (measuring wavelength: 1.3
(mJ/cm.sup.2) .mu.m) 0 1.57078 100 1.56782 800 1.56671
[0147] The data in Table 1 were determined as described below.
[0148] On a substrate was spin-coated a solution of a
fluorene-unit-containing epoxy acrylate resin represented by
formula (4) under the conditions of 800 rpm for 10 sec. and 4000
rpm for 30 sec. to give a film. After pre-drying at 75.degree. C.
for about 10 min., it was exposed and heated (post-baked) at
230.degree. C. for about 30 min. under an atmosphere of nitrogen to
give an even film with a thickness of 4.7 .mu.m. Under three
different conditions (dose: 0, 100 and 800 mJ/cm.sup.2), three
samples were prepared and then determined for their refractive
indices by a prism coupler technique. 6
[0149] wherein the moiety X.sub.1 is represented by formula (5) in
which both attachment positions *s are at para positions to the
fluorene unit. 7
[0150] It is most preferable to adjust an UV dose for controlling a
refractive index. Alternatively, the refractive index may be
controlled by using different types of fluorene-unit-containing
epoxy acrylate resin in the clad and the core layers or adjusting a
mix ratio for a composition comprising the fluorene-unit-containing
epoxy acrylate resin.
[0151] Thus, a core or both core and clad can be formed using a
fluorene-unit-containing epoxy acrylate resin.
[0152] The resin may be of a higher crosslink density and higher
heat resistance by post-baking (a post-heating process) at a
temperature of 160 to 250.degree. C., preferably 230 to 250.degree.
C. for about 30 to 90 min.
[0153] There are, for example, the following relationship between a
glass-transition temperature of the compound represented by formula
(4) and a post-baking temperature; when a post-baking temperature
is 200, 210 or 230-250.degree. C., a glass-transition temperature
is generally 250, 260 or 300.degree. C., respectively.
[0154] After the above post-baking process, the refractive index of
a fluorene-unit-containing epoxy acrylate resin is fixed at a
certain value. Therefore, when forming each layer of an optical
waveguide using a fluorene-unit-containing epoxy acrylate resin,
the layer is preferably post-baked before forming a subsequent
layer. Thus, post-baking after forming each layer can prevent
fluctuation in a refractive index during subsequent UV irradiation
or heating.
[0155] This invention will be specifically described with reference
to non-limiting examples.
EXAMPLE 1
[0156] A material with which fine processing can be performed with
a high optical transparency and a low cost, is required for
preparing a combined substrate according to the first embodiment of
this invention. For example, a fluorene-unit-containing epoxy
acrylate resin (V259PA, NIPPON STEEL CHEMICAL CO., LTD.) meets
these conditions, i.e., its optical loss as a single-mode optical
waveguide is about 0.3 dB/cm, which is comparable to other polymer
materials developed for an optical waveguide. The above epoxy
acrylate resin is UV sensitive and has an adequate resolution for
miniaturization, so that a several-micron level of electric
interconnection can be formed with a low cost.
[0157] There will be described a process for manufacturing an
optical waveguide using a fluorene-unit-containing epoxy acrylate
resin.
[0158] On a substrate in which an optical waveguide is to be formed
is applied a coating solution of a fluorene-unit-containing epoxy
acrylate resin by a common technique such as spin coating and dip
coating to give an applied film. It is then baked for evaporating
the solvent and then exposed to adjust the refractive index of a
lower clad to an appropriate value. Then, it is heated (post-baked)
at 160 to 250.degree. C. for about 30 to 90 min. to solidify the
exposed area to form a lower clad. On the lower clad is applied a
coating solution of a fluorene-unit-containing epoxy acrylate resin
by a common technique such as spin coating and dip coating to give
an applied film. It is baked for evaporating the solvent and then
exposed through a patterned glass mask to adjust the refractive
index of a core while curing a desired area. The refractive index
of the core must be lower by up to 1% than that of the surrounding
clad.
[0159] The above substrate is soaked in a potassium hydroxide
solution or an amine-based alkaline developing solution for
dissolving the unexposed area and developing. It is then heated
(post-baked) at 160 to 250.degree. C. for about 30 to 90 min. to
solidify the exposed area to shape an optical waveguide core.
Subsequently, on the core and the lower clad is deposited a
fluorene-unit-containing epoxy acrylate resin as an upper clad as
described for the lower clad. Thus, the above process can
conveniently provide a heat-resistant single-mode optical waveguide
having a core-section height and width of about several micrometers
or a multi-mode optical waveguide having a core-section height and
width of about several ten micrometers.
[0160] In another example, e.g., in a combined substrate
illustrated in FIG. 1, a material having a high thermal
conductivity such as aluminum nitride, silicon carbide and
beryllium oxide may be used as a ceramic material for a ceramic
multilayer interconnection substrate 1 for effectively releasing
heat from an LD 8.
[0161] In another example, e.g., in a combined substrate
illustrated in FIG. 1, a material whose coefficient of thermal
expansion is substantially equal to that of the LD 8 material may
be used as a ceramic material for the ceramic multilayer
interconnection substrate 1 for minimizing distortion in the LD 8
due to a difference in a thermal expansion between the LD 8 and the
ceramic multilayer interconnection substrate 1 caused by
temperature variation during die bonding of the LD 8.
[0162] In the above embodiments and examples, there is illustrated
a case where an electric interconnection part and an optical
waveguide part are mounted on a ceramic multilayer interconnection
substrate, but it is not always necessary in a photo-electric
combined substrate according to the first aspect of this invention
that an electric interconnection part and an optical waveguide part
are mounted on the substrate. Furthermore, in a configuration where
an electric interconnection part and an optical waveguide part are
mounted on a substrate, the substrate may be, besides a ceramic
multilayer interconnection substrate, an appropriately-configured
substrate such as a simple ceramic substrate and a single-layer
interconnection substrate on which an electric interconnection part
and an optical waveguide part are mounted.
[0163] This invention should not be limited to the above
embodiments or examples. Various changes or modifications can be
allowed without departing from the scope of this invention.
[0164] An example of the second aspect of this invention will be
described.
EXAMPLE 2
[0165] The structure illustrated in FIG. 8 is prepared by a process
shown in FIG. 10. In step (a), a ceramic multilayer interconnection
substrate 81 is prepared. Alumina powder, a flux, an organic
binder, a solvent and a plasticizer are well mixed in a ball mill.
The mixture is applied and extended on a carrier tape by a blade
and dried to form a green sheet. Then, the green sheet is punched
with a die, the hole is filled with a conductor paste made of a
metal powder, and a given conductor pattern is printed on the green
sheet. A plurality of green sheets thus prepared are then piled and
baked to provide a ceramic multilayer interconnection substrate 81.
A concave is formed by punching a required number of green sheets
with a die.
[0166] In step (b), a resin 83 is filled in the concave 82 of the
ceramic multilayer interconnection substrate 81. After spin-coating
the resin 83, the resin 83 is heated at a unique temperature for
the resin to be cured. The resin applied the areas other than the
concave is ground to make the surfaces of the resin 83 of the
concave and of the ceramic multilayer interconnection substrate 81
at the same level. Herein, the resin used was a polyimide. A
solution of a precursor polyamic acid in N-methylpyrrolidone was
applied the surface of the substrate 81 including the concave 82 by
spin coating, it was heated at 350.degree. C. to be cured into a
polyimide, and it was ground with a grindstone until the ceramic
substrate was completely exposed to build the resin in the concave.
Then, a resist is applied on the whole surface of the ceramic
substrate including the resin 83 filled in the concave 82, exposed
and developed, patterned for forming an electric interconnection 84
between an electric device 86 and an optical device 85 and an
electric interconnection for the electric device 86. Copper is
plated. Then, on the ceramic multilayer interconnection substrate
81 and the resin surface in the concave 82 are formed electric
interconnections 84 between an electric device 86 mounted on the
resin and an optical device 85 mounted on the ceramic substrate 81
as well as between the ceramic multilayer interconnection substrate
81 and the electric device 86.
[0167] FIG. 12 is a plan view of the ceramic multilayer
interconnection substrate illustrating forming the electric
interconnection 84 in step (b). In FIG. 12(a), there are formed an
electrode 89 for mounting the optical device 85 and an electrode 90
for the electric device around the resin 83 in the concave 82 on
the ceramic multilayer interconnection substrate 81. The structure
of FIG. 12(a) is subject to the process of step (b) in FIG. 10 to
form the electric interconnection 84 from the electrode on the
ceramic multilayer interconnection substrate 81 to the surface of
the resin 83 in the concave 82 (FIG. 12(b)).
[0168] In step (c), the optical device (a light emitting diode) 85
is mounted on the ceramic substrate 81 using, e.g., a gold-tin
solder ball. In step (d), the electric device 86 is mounted on the
electric interconnection formed on the surface of the resin 83
filled in the concave 82 in the ceramic substrate 81 using, e.g., a
gold-tin solder ball, and a cooling fin 87 is attached. Thus, the
structure illustrated in FIG. 8 can be prepared.
EXAMPLE 3
[0169] The structure illustrated in FIG. 9 is prepared by a process
shown in FIG. 11. In step (a), a ceramic multilayer interconnection
substrate 91 is prepared. Alumina powder, a flux, an organic
binder, a solvent and a plasticizer are well mixed in a ball mill.
The mixture is applied and extended on a carrier tape by a blade
and dried to form a green sheet. Then, the green sheet is punched
with a die, the hole is filled with a conductor paste made of a
metal powder, and a given conductor pattern is printed on the green
sheet. A plurality of green sheets thus prepared are then piled and
baked to provide a ceramic multilayer interconnection substrate 91.
A concave is formed by punching a required number of green sheets
with a die.
[0170] In step (b), a fine electric interconnection layer 98 is
formed on the concave 92 in the ceramic multilayer interconnection
substrate 91. A solution of photosensitive polyamic acid in
N-methylpyrrolidone for electric insulation was applied by spin
coating to the whole surface of the ceramic multilayer
interconnection substrate 91 including the concave 92. The
photosensitive polyamic acid solution applied to the concave as an
electric insulating layer 981 is applied to the area lower than the
ceramic surface. The applied solution is, therefore, exposed to a
dose of about 500 mJ/cm.sup.2 at a wavelength of 365 nm using,
e.g., a stepper. It is developed to remove the resin applied on the
areas other than the concave 92 while forming a pattern for an
electric insulating layer on the resin layer formed in the concave
92. Then, it is heated at 350.degree. C. under an atmosphere of
nitrogen to cure the photosensitive polyamic acid into a polyimide.
While forming the polyimide film in the concave 92, a polyimide
film 981 is deposited on the side of the concave 92. In the light
of the fact, the polyimide film other than the area around the side
of the concave whose film thickness is constant is used as an
insulating film for an electric interconnection. Since around the
side in the concave 92 the polyimide is deposited to a level higher
than the ceramic surface, the polyimide is ground with a grindstone
to level the ceramic substrate and the surface. Subsequently, a
plating resist is applied using an appropriate means such as a spin
coater and a spray type of applicator, exposed and developed,
patterned the electric conductor layer. Copper is plated to form an
electric conductor layer 982. The above steps of forming the
electric insulating layer and of forming the electric conductor
layer are repeated by required times to form a desired fine
electric interconnection layer 98 in the concave 92 of the ceramic
multilayer interconnection substrate 91. A fine electric
interconnection layer 98 is formed on the concave to the level of
the ceramic substrate. Then, a resist is applied to the whole
surface of the ceramic substrate including the fine electric
interconnection layer 98 in the concave 92, exposed and developed,
patterned for forming an electric interconnection between an
electric and an optical devices. Copper is plated to form an
electric interconnection between an electric device mounted on the
fine electric interconnection 98 and an optical device 95 mounted
on the ceramic substrate on the substrate surface.
[0171] FIG. 13 is a plan view of the ceramic multilayer
interconnection substrate 91 illustrating forming the electric
interconnection 94 in step (b). In FIG. 13(a), there are formed an
electrode 99 for mounting the optical device 95 in the vicinity of
the fine electric interconnection layer 98 in the concave 92 on the
ceramic multilayer interconnection substrate 91. The structure of
FIG. 13(a) is subject to the process of step (b) in FIG. 11 to form
the electric interconnection 94 from the electrode 99 on the
ceramic multilayer interconnection substrate 91 to the electric
conductor layer 982 in the concave 92 (FIG. 13(b)).
[0172] In step (c), the optical device (a light emitting diode) 95
is mounted on the ceramic substrate using, e.g., a gold-tin solder
ball. In step (d), the electric device 96 is mounted on the
electric interconnection formed on the fine electric
interconnection layer formed in the concave of the ceramic
substrate, and a cooling fin 97 is attached. Thus, the structure
illustrated in FIG. 9 can be prepared.
EXAMPLE 4
[0173] The structure illustrated in FIG. 9 is prepared by a process
shown in FIG. 11. In step (a), a ceramic multilayer interconnection
substrate 91 is prepared. Alumina powder, a flux, an organic
binder, a solvent and a plasticizer are well mixed in a ball mill.
The mixture is applied and extended on a carrier tape by a blade
and dried to form a green sheet. Then, the green sheet is punched
with a die, the hole is filled with a conductor paste made of a
metal powder, and a given conductor pattern is printed on the green
sheet. A plurality of green sheets thus prepared are then piled and
baked to provide a ceramic multilayer interconnection substrate 91.
A concave is formed by punching a required number of green sheets
with a die.
[0174] In step (b), a fine electric interconnection layer 98 is
formed on the concave 92 in the ceramic multilayer interconnection
substrate 91. A solution of polyamic acid in N-methylpyrrolidone
for electric insulation was applied by spin coating to the whole
surface of the ceramic multilayer interconnection substrate 91
including the concave 92. Then, it is heated at 350.degree. C.
under an atmosphere of nitrogen to cure the polyamic acid into a
polyimide. While forming the polyimide film in the concave 92, a
polyimide film 981 is deposited on the side of the concave 92. In
the light of the fact, the polyimide film 981 other than the area
around the side of the concave whose film thickness is constant is
used as an insulating film for an electric interconnection. The
polyimide film 981 is irradiated with a laser to form a via hole to
form an insulating pattern. Subsequently, a plating resist is
applied using an appropriate means such as a spin coater and a
spray type of applicator, exposed and developed, patterned the
electric conductor layer. Copper plating is applied to form an
electric conductor layer 982. The above steps of forming the
electric insulating layer and of forming the electric conductor
layer are repeated by required times to form a desired fine
electric interconnection layer 98 in the concave 92 of the ceramic
multilayer interconnection substrate 91. The resin deposited on the
areas other than the concave is removed by grinding with a
grindstone. A fine electric interconnection layer 98 is formed on
the concave to the level of the ceramic substrate. Then, a resist
is applied to the whole surface of the ceramic substrate including
the fine electric interconnection layer in the concave, exposed and
developed, patterned for forming an electric interconnection
between an electric and an optical devices. Copper plating is
applied to form an electric interconnection between an electric
device mounted on the fine electric interconnection 98 and an
optical device 95 mounted on the ceramic substrate on the substrate
surface.
[0175] Subsequently, as described in Example 3, the structure of
FIG. 13(a) is subject to the process of step (b) in FIG. 11 to form
the electric interconnection 94 from the electrode 99 on the
ceramic multilayer interconnection substrate 91 to the electric
conductor layer 982 in the concave 92 as illustrated in FIG.
13(b).
[0176] In step (c), the optical device (a light emitting diode) 95
is mounted on the ceramic substrate using, e.g., a gold-tin solder
ball. In step (d), the electric device 96 is mounted on the
electric interconnection formed on the fine electric
interconnection layer formed in the concave of the ceramic
substrate, and a cooling fin 97 is attached. Thus, the structure
illustrated in FIG. 9 can be prepared.
EXAMPLE 5
[0177] In this example, a substrate is prepared as described in
Example 2 except modification only in step (b). Specifically, a
resin 83 to be filled in a concave is formed not directly on the
ceramic multilayer interconnection substrate 81, but on a separate
substrate by spin coating and heating. Then, the resin 83 to be
filled is cut into pieces with the size of the concave 82 in the
ceramic multilayer interconnection substrate 81 and released from
the separate substrate. Alternatively, a photosensitive resin may
be exposed and developed to provide pieces having the size of the
concave 82, and then released from the separate substrate. The
resin 83 thus obtained is adhered to the concave 82 in the ceramic
multilayer interconnection substrate 81. Adhesives which may be
used include epoxy, polyimide and silicone adhesions.
EXAMPLE 6
[0178] In this example, a substrate is prepared as described in
Examples 3 and 4 except modification only in step (b).
Specifically, a fine electric interconnection layer 98 is formed
not directly on the ceramic multilayer interconnection substrate 91
having a concave 92, but on a separate substrate as described in
the procedure for forming the fine electric interconnection layer
98 in Examples 3 and 4. Then, the fine electric interconnection
layer 98 is cut into the pieces with the size of the concave 92 in
the ceramic multilayer interconnection substrate 91 and released
from the separate substrate. As illustrated in FIG. 14, the ceramic
multilayer interconnection substrate is electrically connected to
the fine electric interconnection layer 98 via a solder 100 in the
concave 92 of the ceramic multilayer interconnection substrate
91.
EXAMPLE 7
[0179] In the structure illustrated in FIG. 8 or 9, a material
having a high thermal conductivity such as aluminum nitride,
silicon carbide and beryllium oxide is used as a ceramic material
for a ceramic multilayer interconnection substrate 81 (91) for
effectively releasing heat from an optical device 85 (95).
EXAMPLE 8
[0180] In the structure illustrated in FIG. 8 or 9, a material
whose coefficient of thermal expansion is substantially equal to
that of the optical device 85 (95) material is used as a ceramic
material for the ceramic multilayer interconnection substrate 81
(91) for minimizing distortion in the optical device due to a
difference in a thermal expansion between the optical device 85
(95) and the ceramic multilayer interconnection substrate 81 (91)
caused by temperature variation during die bonding of the optical
device 85 (95). Herein, the ceramic material was aluminum nitride
(coefficient of thermal expansion: 4.6.times.10.sup.-6 (1/K)) while
the optical device was a GaAs laser diode (coefficient of thermal
expansion: 4.7.times.10.sup.-6 (1/K)).
[0181] The number of the concaves in the ceramic multilayer
interconnection substrate may be selected as necessary. Any
alternative may be selected for the ceramic material, a
manufacturing process for the ceramic multilayer interconnection
substrate, a manufacturing process for the high-density multiple
interconnection layer in the concave, a procedure for filling the
resin into the concave or a method for mounting the electric or
optical device. This invention should not be construed to be
limited to the above examples, and various alterations and
modifications can be allowed without any deviation from the scope
of this invention.
[0182] Examples of the third aspect of this invention will be
described.
EXAMPLE 9
[0183] FIG. 15 is a schematic cross section of a built-in type of
optical waveguide as an embodiment of this invention.
[0184] On a substrate 101 were sequentially deposited a lower clad
layer 102 and a core 103. On the lower clad 102 was an upper clad
covering the core 103. It is necessary that the upper and the lower
clads have herein a lower refractive index than the core.
[0185] The lower and the upper clads 102, 104 were, in this
example, made of a fluorinated polyimide with a refractive index of
1.560.
[0186] The optical waveguide illustrated in this figure was
prepared as shown in FIG. 16.
[0187] In step (a), a fluorinated polyimide was deposited as a
lower clad 122 on a substrate 121.
[0188] In step (b), on the lower clad 122 was spin-coated or
dip-coated a coating solution of a fluorene-unit-containing epoxy
acrylate resin to form an applied film. The film was subject to
pre-heating at 75.degree. C. for about 10 min. Then, the solvent
was evaporated to form a core layer (a layer of the
fluorene-unit-containing epoxy acrylate resin) 123.
[0189] The fluorene-unit-containing epoxy acrylate resin was a
resin represented by formula (4), wherein the moiety X.sub.1 is
represented by formula (5) in which both attachment positions *s
are at para positions to the fluorene unit. 8
[0190] In step (c), the product was exposed through a glass mask
having a given pattern at an exposure wavelength of 365 nm and an
exposure dose of 300 mJ/cm.sup.2.
[0191] Then, it was soaked in a potassium hydroxide solution or an
amine-based alkaline developing solution for dissolving the
unexposed area and developing. It is then heated (post-baked) at
230.degree. C. for about 30 min to solidify the exposed area to
shape an optical waveguide core 123a. The above process can provide
a single-mode optical waveguide having a core-section height and
width of about several micrometers or a multi-mode optical
waveguide having a core-section height and width of about several
ten micrometers.
[0192] After the post-curing, the core had a glass-transition
temperature of 300.degree. C. and a refractive index of 1.567. A
glass-transition temperature of 300.degree. C. or higher may
provide adequate heat resistance to a mounting process for an
optical device. As described above, heating at an elevated
temperature of 230.degree. C. or higher can provide a
glass-transition temperature of 300.degree. C. or higher.
[0193] In step (d), a fluorinated polyimide as an upper clad 124
was deposited on the lower clad 122, covering the core 123a, to
give an optical waveguide.
[0194] The optical waveguide was determined for its optical
propagation loss at a wavelength of 1.3 and 1.55 .mu.m, exhibiting
a good result of 0.3 dB/cm.
[0195] An optical propagation loss was determined as follows. A
laser beam was introduced from one side of the optical waveguide
using an optical fiber and propagated in the optical waveguide. The
beams at the inlet and from the outlet were introduced into a
photodetector using an optical fiber to determine light quantities.
For a cut optical waveguide, similar measurement was conducted to
determine an optical propagation loss from the relationship between
the length of the optical waveguide and a light quantity from a
photodetector.
[0196] In this example, a fluorinated polyimide as a core material
was not used. A reactive ion etching can be, therefore, eliminated
for forming a core, allowing an optical waveguide with high heat
resistance to be prepared by a convenient process.
EXAMPLE 10
[0197] In this example, there will be described with reference to
FIG. 16, an optical waveguide having the same structure as that
illustrated in FIG. 15 for Example 9, where a
fluorene-unit-containing epoxy acrylate resin is used for all of a
core, a lower clad and an upper clad layers.
[0198] The fluorene-unit-containing epoxy acrylate resin was the
resin represented by formula (4) in Example 9.
[0199] In step (a), on a substrate 121 was spin-coated or
dip-coated a coating solution of a fluorene-unit-containing epoxy
acrylate resin to form an applied film. The film was subject to
pre-heating at 75.degree. C. for about 10 min. Then, the solvent
was evaporated to form a lower clad layer (a layer of the
fluorene-unit-containing epoxy acrylate resin) 122.
[0200] Then, the product was exposed with a dose of 800 mJ/cm.sup.2
to adjust the refractive index of the lower clad 122 to 1.5667.
[0201] The exposed area was cured and the refractive index was
fixed by heating (post-baking) under an atmosphere of nitrogen at
230.degree. C. for about 30 min.
[0202] In step (b), a core layer (a layer of the
fluorene-unit-containing epoxy acrylate resin) 123 was formed on
the lower clad 122 as described in Example 9 except changing a dose
to 100 mJ/cm.sup.2. The refractive index was 1.5678.
[0203] In step (c), an optical waveguide core 123a was formed as
described in Example 9.
[0204] In step (d), on the lower clad 122 was formed a
fluorene-unit-containing epoxy acrylate resin layer covering the
core 123a, and it was exposed with a dose of 800 mJ/cm.sup.2 to
adjust the refractive index of the upper clad 124 to 1.5667. It was
then post-baked under an atmosphere of nitrogen at 230.degree. C.
for 30 min to provide an optical waveguide.
[0205] The glass-transition temperature of the core and the clad
was 300.degree. C.
[0206] The optical waveguide was determined for its optical
propagation loss at a wavelength of 1.3 and 1.55 .mu.m, exhibiting
a good result of 0.3 dB/cm.
[0207] In this example, both of the core and the clad were formed
using the fluorene-unit-containing epoxy acrylate resin. This
example is characterized in that the clad and the core were formed
using the same material only by altering a UV dose. Such a process
allows an optical waveguide to be prepared more conveniently than
Example 9.
[0208] An optical waveguide may have any structure including a
built-in structure as illustrated in FIG. 15 and a ridge structure
as illustrated in FIG. 17 where a substrate 131 and a lower clad
132 are sequentially formed and a ridge-shaped core 133 is formed
on the lower clad 132. The optical waveguide may be prepared by a
dry process such as reactive ion etching, and may be etched by a
known process.
* * * * *