U.S. patent application number 09/527468 was filed with the patent office on 2001-11-29 for electrode for optical waveguide element.
This patent application is currently assigned to FUJITSU PHOTO FILM CO., LTD.. Invention is credited to Tsuruma, Isao.
Application Number | 20010046722 09/527468 |
Document ID | / |
Family ID | 17683186 |
Filed Date | 2001-11-29 |
United States Patent
Application |
20010046722 |
Kind Code |
A1 |
Tsuruma, Isao |
November 29, 2001 |
ELECTRODE FOR OPTICAL WAVEGUIDE ELEMENT
Abstract
When forming electrodes for an optical waveguide element, a
metal film is formed on a surface of a substrate, and openings of
predetermined shapes are formed in the metal film. Then proton
exchange is carried out on the surface of the substrate with the
metal film used as a mask, and optical channel waveguides are thus
formed. At least a part of edge portions of the metal film defining
the openings is left on the substrate and the metal film is plated
with plating metal. The metal film plated with the plating metal is
processed into electrodes of predetermined shapes for applying an
electric voltage to the optical channel waveguides.
Inventors: |
Tsuruma, Isao;
(Kanagawa-ken, JP) |
Correspondence
Address: |
Sughrue Mion Zinn MacPeak and Seas PLLC
2100 Pennsylvania Avenue NW
Washington
DC
20037-3213
US
|
Assignee: |
FUJITSU PHOTO FILM CO.,
LTD.
|
Family ID: |
17683186 |
Appl. No.: |
09/527468 |
Filed: |
March 17, 2000 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
|
|
09527468 |
Mar 17, 2000 |
|
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|
08958581 |
Oct 28, 1997 |
|
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|
6060334 |
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Current U.S.
Class: |
438/31 |
Current CPC
Class: |
G02F 1/035 20130101 |
Class at
Publication: |
438/31 |
International
Class: |
H01L 021/00 |
Foreign Application Data
Date |
Code |
Application Number |
Oct 28, 1996 |
JP |
284800/1996 |
Claims
What is claimed is;
1. A method of forming electrodes for an optical waveguide element
comprising the steps of forming a metal film on a surface of a
substrate, forming openings of predetermined shapes in the metal
film, carrying out proton exchange on the surface of the substrate
with the metal film used as a mask, thereby forming optical channel
waveguides, leaving at least a part of edge portions of the metal
film defining the openings, plating the metal film with plating
metal, and processing the metal film plated with the plating metal
into electrodes of predetermined shapes for applying an electric
voltage to the optical channel waveguides.
2. A method of forming electrodes for an optical waveguide element
as defined in claim 1 in which said substrate is of a material
selected from the group consisting of a LiNb.sub.xTa.sub.1-xO.sub.3
(0.ltoreq.x.ltoreq.1) crystal, a Mg-doped
LiNb.sub.xTa.sub.1-xO.sub.3 (0.ltoreq.x.ltoreq.1) crystal and a
Zn-doped LiNb.sub.xTa.sub.1-xO.sub.3 (0.ltoreq.x.ltoreq.1)
crystal.
3. A method of forming electrodes for an optical waveguide element
as defined in claim 1 further comprising the steps of applying
negative photo-resist to the substrate after said proton exchange
and before said plating, exposing the photo-resist to light from
the backside of the substrate using the metal film as a photo-mask,
removing the photo-resist with the part of the photo-resist which
is on the optical channel waveguides and accordingly exposed to
light left there, and effecting the plating using as a mask the
part of the photo-resist left on the substrate.
4. A method of forming electrodes for an optical waveguide element
as defined in claim 3 in which said substrate is of a material
selected from the group consisting of a LiNb.sub.xTa.sub.1-xO.sub.3
(0.ltoreq.x.ltoreq.1) crystal, a Mg-doped
LiNb.sub.xTa.sub.1-xO.sub.3 (0.ltoreq.x.ltoreq.1) crystal and a
Zn-doped LiNb.sub.xTa.sub.1-xO.sub.3 (0.ltoreq.x.ltoreq.1)
crystal.
5. A method of forming electrodes for an optical waveguide element
comprising the steps of forming a metal film on a surface of a
substrate, forming openings of predetermined shapes in the metal
film, carrying out proton exchange on the surface of the substrate
with the metal film used as a mask, thereby forming optical channel
waveguides, processing the metal film into metal film fractions of
predetermined shapes corresponding to the shapes of electrodes to
be formed, each metal film fraction including at least a part of an
edge portion defining one of the openings, plating the metal film
fractions with plating metal, and using the metal film fractions
plated with the plating metal as the electrodes for applying an
electric voltage to the optical channel waveguides.
6. A method of forming electrodes for an optical waveguide element
as defined in claim 5 in which said substrate is of a material
selected from the group consisting of a LiNb.sub.xTa.sub.1-xO.sub.3
(0.ltoreq.x.ltoreq.1) crystal, a Mg-doped
LiNb.sub.xTa.sub.1-xO.sub.3(0.l- toreq.x.ltoreq.1) crystal and a
Zn-doped LiNb.sub.xTa.sub.1-xO.sub.3 (0.ltoreq.x.ltoreq.1)
crystal.
7. A method of forming electrodes for an optical waveguide element
as defined in claim 5 further comprising the steps of applying
negative photo-resist to the substrate after said proton exchange
and before said plating, exposing the photo-resist to light from
the backside of the substrate using the metal film as a photo-mask,
removing the photo-resist with the part of the photo-resist which
is on the optical channel waveguides and accordingly exposed to
light left there, and effecting the plating using as a mask the
part of the photo-resist left on the substrate.
8. A method of forming electrodes for an optical waveguide element
as defined in claim 7 in which said substrate is of a material
selected from the group consisting of a LiNb.sub.xTa.sub.1-xO.sub.3
(0.ltoreq.x.ltoreq.1) crystal, a Mg-doped
LiNb.sub.xTa.sub.1-xO.sub.3 (0.ltoreq.x.ltoreq.1) crystal and a
Zn-doped LiNb.sub.xTa.sub.1-xO.sub.3 (0.ltoreq.x.ltoreq.1)
crystal.
9. An electrode for an optical waveguide element which is formed on
a substrate, on which an optical channel waveguide is formed by
proton exchange, with its one edge aligned with one edge of the
optical channel waveguide and is for applying an electric voltage
to the optical channel waveguide, wherein the improvement comprises
that the electrode comprises a metal film fraction which is a part
of metal film used as a mask when the optical channel waveguide is
formed by the proton exchange and a plating metal layer formed on
the metal film fraction by plating.
10. An electrode for an optical waveguide element as defined in
claim 9 in which a buffer layer is formed between the substrate and
the metal film.
11. An electrode for an optical waveguide element as defined in
claim 9 in which said substrate is of a material selected from the
group consisting of a LiNb.sub.xTa.sub.1-xO.sub.3
(0.ltoreq.x.ltoreq.1) crystal, a Mg-doped
LiNb.sub.xTa.sub.1-xO.sub.3 (0.ltoreq.x.ltoreq.1) crystal and a
Zn-doped LiNb.sub.xTa.sub.1-xO.sub.3 (0.ltoreq.x.ltoreq.1) crystal.
Description
BACKGROUND OF THE INVENTION
[0001] 1. Field of the Invention
[0002] This invention relates to an electrode for applying an
electric voltage to an optical channel waveguide of an optical
waveguide element in which the optical channel waveguide is formed
by proton exchange, and a method of forming the electrode.
[0003] 2. Description of the Related Art
[0004] There have been provided various optical waveguide elements
having an optical channel waveguide formed on a substrate. As a
method of forming the optical channel waveguide, there has been
known a proton exchange process.
[0005] In the proton exchange process, metal film is first formed
on a surface of a substrate, an opening is formed in the metal film
by etching and proton exchange is carried out on the surface of the
substrate using the metal film as a mask.
[0006] Generally an electric voltage is applied to the optical
channel waveguide through electrodes disposed near or just above
the optical channel waveguide.
[0007] A conventional method of forming the electrodes for applying
an electric voltage to the optical channel waveguide will be
described with reference to FIGS. 9A to 9H, hereinbelow.
[0008] Metal film 2 such as of Cr is first formed on a substrate 1
as shown in FIG. 9A.
[0009] A resist layer 3 is formed on the metal film 2 in a
predetermined pattern by photolithography as shown in FIG. 9B.
[0010] Then the metal film 2 is etched to form openings 4 in a
predetermined pattern in the metal film 2 using the resist layer 3
as a mask, and the resist layer 3 is removed as shown in FIG.
9C.
[0011] Thereafter proton exchange is carried out using the metal
film 2 with the openings 4 as a mask, thereby forming optical
channel waveguides 5 on the surface of the substrate 1 as show in
FIG. 9D.
[0012] The metal film 2 is then removed by etching as shown in FIG.
9E and the substrate 1 is annealed as required.
[0013] Thereafter a conductive film 7 such as of aluminum is formed
over the surface of the substrate 1 as shown in FIG. 9F.
[0014] A resist layer 8 is formed over the conductive film 7 with
portions opposed to the optical channel waveguides 5 exposed by
photolithography as shown in FIG. 9G.
[0015] Then the conductive film 7 is removed at the portions
opposed to the optical channel waveguides 5 by etching using the
resist 8 as a mask as shown in FIG. 9H.
[0016] When the resist 8 is thereafter removed, the conductive
films 7 are left on opposite sides of each optical channel
waveguide 5. The conductive films 7 on opposite sides of each
optical channel waveguide 5 can be used as electrodes for applying
an electric voltage to the optical channel waveguide 5.
[0017] However this method is disadvantageous in the following
point. That is, when the resist mask 8 is formed over the
conductive film 7 with the portions opposed to the optical channel
waveguides 5 exposed, the edge of the resist mask 8 circumscribing
the optical channel waveguide 5 cannot be precisely aligned with
the edge of the optical channel waveguide 5 due to fluctuation in
skill of the operator and/or in precision of the exposure device.
Accordingly, the edges of the electrodes (conductive film) hang
over the optical channel waveguide or are positioned away from the
edge of the optical channel waveguide 5 as shown in FIG. 10 in an
enlarged scale, which results in fluctuation in performance of the
optical waveguide element or deterioration in yield. In FIG. 10, L
denotes the alignment error.
[0018] In Japanese Unexamined Patent Publication No.
7(1995)-146457, there is disclosed a method of forming the
electrodes for a optical waveguide element which can overcome such
a problem. In the method, the metal film which is used as a mask
for setting the pattern of the optical waveguide upon proton
exchange is left there and used as the electrodes. That is, metal
film is formed on a surface of a substrate, openings of
predetermined shapes are formed in the metal film, proton exchange
is carried out on the surface of the substrate with the metal film
used as a mask, thereby forming optical channel waveguides, and the
metal film is removed with at least a part of the edges of the
openings left there. The metal film fractions are used as the
electrodes.
[0019] In the method the openings of predetermined shapes are
formed in the metal film generally by etching though liftoff may be
used.
[0020] When metal film is processed by etching or liftoff, the
thickness of the metal film should be several hundred namometers
(nm) at most. When such thin metal film is used as an electrode,
the resistance of the electrodes becomes high and accordingly
optical waveguide elements provided with such electrodes are hard
to operate at high speed (e.g., high speed modulation at several
hundred MHz or higher).
SUMMARY OF THE INVENTION
[0021] In view of the foregoing observations and description, the
primary object of the present invention is to provide an electrode
for an optical waveguide element which is formed with its one edge
precisely aligned with one edge of the optical channel waveguide
and at the same time makes it feasible to operate the optical
waveguide element at high speed.
[0022] Another object of the present invention is to provide a
method of forming such electrodes for an optical waveguide
element.
[0023] In the method of the present invention, a part of the metal
film which is used as a mask for setting the pattern of the optical
waveguide upon proton exchange is left there as in the above
identified Japanese patent publication (Japanese Unexamined Patent
Publication No. 7(1995)-146457), then the metal film is plated and
used as the electrodes.
[0024] That is, in accordance with a first aspect of the present
invention, there is provided a method of forming electrodes for an
optical waveguide element comprising the steps of
[0025] forming a metal film on a surface of a substrate,
[0026] forming openings of predetermined shapes in the metal
film,
[0027] carrying out proton exchange on the surface of the substrate
with the metal film used as a mask, thereby forming optical channel
waveguides,
[0028] leaving at least a part of edge portions of the metal film
defining the openings,
[0029] plating the metal film with plating metal, and
[0030] processing the metal film plated with the plating metal into
electrodes of predetermined shapes for applying an electric voltage
to the optical channel waveguides.
[0031] In the method of forming electrodes for an optical waveguide
element in accordance with a second aspect of the present
invention, the metal film used as a mask in the proton exchange is
first processed into metal film fractions of predetermined shapes
corresponding to the shapes of electrodes to be formed, each metal
film fraction including at least a part of an edge portion defining
one of the openings, and then the metal film fractions are plated
with plating metal and used as the electrodes for applying an
electric voltage to the optical channel waveguides.
[0032] In the method of forming electrodes for an optical waveguide
element in accordance with a third aspect of the present invention,
in the method of the first or second aspect of the present
invention, negative photo-resist is applied to the substrate after
said proton exchange and before said plating, then the photo-resist
is exposed to light from the back side of the substrate using the
metal film as a photo-mask, the photo-resist is subsequently
removed with the part of the photo-resist which is on the optical
channel waveguides and accordingly exposed to light left there, and
then the plating is effected using as a mask the part of the
photo-resist left on the substrate.
[0033] In accordance with a fourth aspect of the present invention,
there is provided an electrode for an optical waveguide element
which is formed on a substrate, on which an optical channel
waveguide is formed by proton exchange, with its one edge aligned
with one edge of the optical channel waveguide and is for applying
an electric voltage to the optical channel waveguide, wherein the
improvement comprises that the electrode comprises a metal film
fraction which is a part of metal film used as a mask when the
optical channel waveguide is formed by the proton exchange and a
plating metal layer formed on the metal film fraction by
plating.
[0034] Preferably a buffer layer is formed between the substrate
and the metal film.
[0035] The metal film used as a mask for setting the pattern of the
optical channel waveguide upon proton exchange naturally has an
edge aligned with an edge of the optical channel waveguide.
Accordingly when an electrode is formed by plating the metal film
including at least a part of an edge portion defining one of the
openings, the edge of the electrode can be precisely aligned with
the edge of the optical channel waveguide.
[0036] When the metal film plated with metal is used as an
electrode, the thickness of the electrode increases and the
resistance of the electrode lowers as compared with when the metal
film is used as an electrode as it is. Accordingly the optical
waveguide element in which an electric voltage is applied to the
optical channel waveguide through the electrode can be operated at
high speed.
[0037] When the plating metal layer comes to hang out over the
optical channel waveguide from the edge of the metal film, the
finished electrode cannot have an edge aligned with the edge of the
optical channel waveguide even if the metal film on which the
plating metal is plated has an edge aligned with the edge of the
optical channel waveguide, which results in the same problem as
that described above in conjunction with FIG. 10.
[0038] The method in accordance with the third aspect of the
present invention can overcome this problem. That is, when the
negative photo-resist applied to the substrate after the proton
exchange is exposed to light from the back side of the substrate
using the metal film as a mask, the exposed part of the
photo-resist has an edge precisely aligned with the edge of the
optical channel waveguide. Accordingly, by removing the negative
photo-resist with the exposed part left there and effecting the
plating using the exposed part of the photo-resist as a mask, the
plating metal cannot hang out over the optical channel waveguide
and the edge of the electrode plated with the plating metal can be
precisely aligned with the edge of the optical channel
waveguide.
[0039] When a buffer layer is formed between the substrate and the
metal film, light propagation loss due to the electrode can be
reduced.
BRIEF DESCRIPTION OF THE DRAWINGS
[0040] FIGS. 1A to 1H are views for illustrating the procedure of
forming the electrodes of an optical waveguide element by a method
in accordance with a first embodiment of the present invention,
[0041] FIGS. 2A to 2F are plan views showing the state of the
substrate at different steps shown in FIGS. 1A to 1H,
[0042] FIGS. 3A to 3H are views for illustrating the procedure of
forming the electrodes of an optical waveguide element by a method
in accordance with a second embodiment of the present
invention,
[0043] FIGS. 4A to 4H are plan views showing the state of the
substrate at different steps shown in FIGS. 3A to 3H,
[0044] FIGS. 5A to 5E are views for illustrating the procedure of
forming the electrodes of an optical waveguide element by a method
in accordance with a third embodiment of the present invention,
[0045] FIGS. 6A to 6D are plan views showing the state of the
substrate at different steps shown in FIGS. 5A to 5E,
[0046] FIGS. 7A to 7E are views for illustrating the procedure of
forming the electrodes of an optical waveguide element by a method
in accordance with a fourth embodiment of the present
invention,
[0047] FIGS. 8A to 8C are plan views showing the state of the
substrate at different steps shown in FIGS. 7A to 7E,
[0048] FIGS. 9A to 9H are views for illustrating the procedure of
forming the electrodes of an optical waveguide element by a
conventional method, and
[0049] FIG. 10 is a view for illustrating alignment error of the
electrodes formed by the conventional method.
DESCRIPTION OF THE PREFERRED EMBODIMENTS
[0050] A finished optical waveguide element will be first described
with reference to FIG. 2F for the purpose of simplicity of
understanding. As shown in FIG. 2F, the finished optical waveguide
element comprises a substrate 10 which may comprise an x-plate of,
for instance, a LiNbO.sub.3 crystal. A pair of optical channel
waveguides 11, which form a directional photocoupler, are formed on
a surface of the substrate 10 to extend in Y-direction, and three
electrodes 12, 13 and 14 are formed on the surface of the substrate
10 on opposite sides of the portions of the optical channel
waveguides 11 where they extend in parallel to each other close to
each other and between the portions. The electrodes 12 and 14 are
connected to a drive circuit (not shown) and the electrode 13 is
connected to the drive circuit by way pad electrodes 16. A
predetermined electric voltage is applied to each of the optical
channel waveguides 11 through the electrodes 12, 13 and 14.
[0051] Each of the edges of the electrodes 12, 13 and 14 adjacent
to the optical channel waveguides 11 should be precisely aligned
with the corresponding edge of the optical channel waveguides 11.
Otherwise it becomes difficult to precisely apply the predetermined
voltage to each optical channel waveguide 11.
[0052] A method in accordance with a first embodiment of the
present invention which makes it feasible to form the electrodes
12, 13 and 14 in the desired manner will be described with
reference to FIGS. 1A to 1H, hereinbelow. FIGS. 1A to 1H are
cross-sectional views taken along line A-A in FIG. 2A.
[0053] Resist layers 20 are first formed by known lithography on
the surface of a substrate 10 in the shape of optical channel
waveguides 11 to be formed as shown in FIGS. 1A and 2A.
[0054] Then a Ta layer 21, an Au layer 22 and a Ta layer 23 are
formed by sputtering in this order on the surface of the substrate
10 over the resist layers 20 as shown in FIG. 1B. The layers 21, 22
and 23 are, for instance, 15 nm, 10 nm and 15 nm respectively in
thickness.
[0055] The substrate 10 carrying thereon the resist layers 20 and
the metal layers 21, 22 and 23 is then dipped in acetone and
subjected to ultrasonic cleaning, and the resist layers 20 and the
Ta layer 21, the Au layer 22 and the Ta layer 23 on the resist
layers 20 are removed from the substrate 10 by liftoff as shown in
FIGS. 1C and 2B.
[0056] The substrate 10 is dipped in pyrophosphoric acid heated to
150.degree. C. to 200.degree. C. for a predetermined time, whereby
the exposed part of the substrate 10 is subjected to proton
exchange and optical channel waveguides 11 are formed on the
surface of the substrate 10 as shown in FIG. 1D. Since the
fractions of the metal film comprising the Ta layer 21, the Au
layer 22 and the Ta layer 23 left on the substrate 10 function as a
mask upon proton exchange, the optical channel waveguides 11 formed
are in the shape of the resist layers 20.
[0057] The resulting substrate 10 carrying thereon the optical
channel waveguides 11 and the fractions of the metal layer are
cleaned and subjected to heat treatment at 340.degree. C. to
400.degree. C. for a predetermined time. Then the upper Ta layer 23
is removed by etching with fluoronitric acid (1:2) as shown in FIG.
1E.
[0058] In order to form wirings for connecting the central
electrode 13 and the pad electrodes 16 (FIG. 2F), a resist pattern
having openings in portions where the wirings are to be formed is
formed by lithography. Then Au/Cr is deposited on the substrate 10
by low resistance heating vacuum deposition and the resist pattern
is removed by liftoff, leaving wirings 25 of Au/Cr as shown in FIG.
2C.
[0059] Thereafter negative photo-resist 26 is applied to the
substrate 10, and light is projected onto the surface of the
substrate with the photo-resist 26 covered with a photo-mask having
openings respectively corresponding to the electrodes 12, 13 and 14
and the pad electrodes 16, thereby exposing the portion of the
photo-resist 26 not covered with the photo-mask. Then light is
projected onto the backside of the substrate 10 so that the
portions of the photo-resist 26 just above the optical channel
waveguides 11 are exposed with the Ta layer 21 and the Au layer 22
functioning as a mask. (The portions corresponding to the wiring 25
are not exposed.) When the photo-resist is developed, only the
exposed portions of the photo-resist 26 are left on the substrate
10 as shown in FIGS. 1F and 2D. The order of the exposure from the
front side of the substrate and the exposure from the backside of
the substrate may be reversed.
[0060] Then Au layer 27 of, for instance, 1 to 4 .mu.m is formed on
the substrate 10 by electrolytic plating with the pattern of the
photo-resist 26 used as a mask as shown in FIG. 1G.
[0061] The resist layer 26 is removed by a plasma asher or resist
release solution as shown in FIG. 2E, and the Au layer 22 and the
Ta layer 21 are removed by etching using the Au layer 27 formed by
the plating as a mask as shown in FIG. 1H. Thus the electrodes 12,
13 and 14 and the pad electrodes 16 consisting of the Ta layer 21,
the Au layer 22 and the plated Au layer 27 are formed as shown in
FIG. 1H and 2F. Also the wirings 25 are plated with Au 27 and have
an increased thickness.
[0062] Since the electrodes 12, 13 and 14 are formed by leaving the
edges of the Ta layer 21 and the Au layer 22 which define the
optical channel waveguides 11 and at the same time the Au layer 27
on the Ta layer 21 and the Au layer 22 is formed using the
photo-resist pattern 26, which precisely conforms to the shape of
the optical channel waveguides 11, as a mask so that the Au layer
27 cannot hang out over the optical channel waveguides 11, the
edges of the electrodes 12, 13 and 14 facing the optical channel
waveguides 11 are precisely in alignment with the edges of the
optical channel waveguides 11.
[0063] Since the electrodes 12, 13 and 14 and the pad electrodes 16
consist of the Ta layer 21 and the Au layer 22 plated with a thick
Au layer 27, they are low in electric resistance and can apply an
electric voltage at a high frequency not lower than several hundred
MHz, whereby high speed drive of the optical waveguide element can
be realized.
[0064] Further when the resist layers 20 and the Ta layer 21, the
Au layer 22 and the Ta layer 23 on the resist layers 20 are removed
from the substrate 10 by liftoff in the step shown in FIG. 1C, the
Au layer 22 is not exposed. Accordingly, the problem of short
circuit due to adhesion of particles of Au to the surface of Au
layer and/or deterioration in bonding due to stain of the surface
can be avoided.
[0065] By forming the openings for defining the shape of the
optical channel waveguides 11 by liftoff of the resist layers 20
and the Ta layer 21, the Au layer 22 and the Ta layer 23 on the
resist layers 20, the dimensional accuracy of the openings can be
higher than by forming the same by etching, which results in a
higher dimensional accuracy of the optical channel waveguides
11.
[0066] When the Ta layer 23, which is apt to be stained, is removed
after the proton exchange, a clean surface of the Au layer 22 is
exposed, whereby the contact resistance between the Au layers 22
and the wirings 25 can be reduced and at the same time, a uniform
Au layer 27 can be obtained since the Au layer 22 wets well with
plating solution.
[0067] The width of the wirings 25 which extend across the optical
channel waveguides 11 as measured in the direction in which an
optical wave is guided is preferably not larger than 50 .mu.m,
whereby light propagation loss due to the wirings 25 can be reduced
to about 5%.
[0068] Further even when fine dust on the surface of the substrate
is removed by a mechanical process, for instance, by brush scrape
before the proton exchange, the Ta layer 23, which is relatively
hard, cannot be damaged and accordingly the pattern of the optical
channel waveguides 11 cannot be adversely affected.
[0069] Though, in the first embodiment, the Ta layer 21 and the Au
layer 22 are processed to a predetermined shape after plated with
Au 27, the Ta layer 21 and the Au layer 22 are processed to a
predetermined shape before plated with Au 27 in a second embodiment
described hereinbelow with reference to FIGS. 3A to 3H and 4A to
4H. FIGS. 4A to 4H are cross-sectional views taken along line B-B
in FIG. 3A.
[0070] Resist layers 20 are first formed by known lithography on
the surface of a substrate 10 in the shape of optical channel
waveguides 11 to be formed as shown in FIGS. 3A and 4A.
[0071] Then a Ta layer 21, an Au layer 22 and a Ta layer 23 are
formed by sputtering in this order on the surface of the substrate
10 over the resist layers 20 as shown in FIG. 3B. The layers 21, 22
and 23 are, for instance, 15 nm, 100 nm and 15 nm respectively in
thickness.
[0072] The substrate 10 carrying thereon the resist layers 20 and
the metal layers 21, 22 and 23 is then dipped in acetone and
subjected to ultrasonic cleaning, and the resist layers 20 and the
Ta layer 21, the Au layer 22 and the Ta layer 23 on the resist
layers 20 are removed from the substrate 10 by liftoff as shown in
FIGS. 3C and 4B.
[0073] The substrate 10 is dipped in pyrophosphoric acid heated to
150.degree. C. to 200.degree. C. for a predetermined time, whereby
the exposed part of the substrate 10 is subjected to proton
exchange and optical channel waveguides 11 are formed on the
surface of the substrate 10 as shown in FIG. 3D. Since the
fractions of the metal film comprising the Ta layer 21, the Au
layer 22 and the Ta layer 23 left on the substrate 10 function as a
mask upon proton exchange, the optical channel waveguides 11 formed
are in the shape of the resist layers 20.
[0074] The resulting substrate 10 carrying thereon the optical
channel waveguides 11 and the fractions of the metal layer are
cleaned and subjected to heat treatment at 340.degree. C. to
400.degree. C. for a predetermined time. Then the upper Ta layer 23
is removed by etching with fluoronitric acid (1:2) as shown in FIG.
3E.
[0075] Thereafter, a resist pattern 30 for defining the shapes of
the electrodes is formed on the substrate 10 as shown in FIG. 4C
and the Ta layer 21 and the Au layer 22 are etched as shown in FIG.
4D using the resist pattern as a mask.
[0076] In order to form wirings for connecting the central
electrode 13 and the pad electrodes 16 (FIG. 3F), a resist pattern
having openings in portions where the wirings are to be formed is
formed by lithography. Then Au/Cr is deposited on the substrate 10
by low resistance heating vacuum deposition and the resist pattern
is removed by liftoff, leaving wirings 25 of Au/Cr as shown in FIG.
4E.
[0077] Thereafter negative photo-resist 26 is applied to the
substrate 10, and light is projected onto the backside of the
substrate 10 so that the portions of the photo-resist 26 just above
the optical channel waveguides 11 are exposed with the Ta layer 21
and the Au layer 22 functioning as a mask. (The portions
corresponding to the wirings 25 are not exposed.) When the
photo-resist is developed, only the exposed portions of the
photo-resist 26 are left on the substrate 10 as shown in FIGS. 3F
and 4F.
[0078] Then Au layer 27 of, for instance, 1 to 4 .mu.m is formed on
the substrate 10 by electrolytic plating with the pattern of the
photo-resist 26 used as a mask as shown in FIG. 3G.
[0079] The resist layer 26 is removed by a plasma asher or resist
release solution as shown in FIG. 4G. Then the substrate 10 is cut
along the chained lines shown in FIG. 4G. Thus the electrodes 12,
13 and 14 and the pad electrodes 16 consisting of the Ta layer 21,
the Au layer 22 and the plated Au layer 27 are formed as shown in
FIGS. 3H and 4H.
[0080] The advantage of the second embodiment is basically the same
as that of the first embodiment.
[0081] A third embodiment of the present invention will be
described hereinbelow with reference to FIGS. 5A to 5E and 6A to
6E. FIGS. 5A to 5E are cross-sectional views taken along line C-C
in FIG. 6A.
[0082] A substrate 10 carrying thereon the optical channel
waveguides 11 and the fractions of the metal layers 21 and 22 as
shown in FIGS. 5A and 6A is obtained in the same manner as in the
first embodiment.
[0083] Thereafter, positive photo-resist 40 is applied to the
substrate and a photo-mask 41 shown in FIG. 6C is disposed on the
backside of the substrate, and light is projected onto the backside
of the substrate through the photo-mask 41, thereby exposing the
photo-resist 40 to light through the mask 41. When the photo-resist
40 is subsequently developed, a pattern of the photo-resist 40
having one opening just above each of the optical channel
waveguides 11 is formed as shown in FIGS. 5B and 6B.
[0084] Then buffer layers 42 of SiO.sub.2 are formed in a thickness
of 100 nm to 500 nm, for instance, by sputtering using the resist
pattern 40 as a mask as shown in FIG. 5C.
[0085] Thereafter the photo-resist 40 is removed by liftoff. In
this manner a buffer layer 42 of SiO.sub.2 is formed on each of the
optical channel waveguides 11 as shown in FIGS. 5D and 6D.
[0086] Wirings 25 of Cr/Au are formed on the buffer layers 42 as
shown in FIGS. 5E and 6E.
[0087] Thereafter electrodes for applying an electric voltage to
the optical channel waveguides 11 are formed, for instance, in the
manner described above in conjunction with the first
embodiment.
[0088] By thus forming buffer layers 42 of SiO.sub.2 between the
optical channel waveguides 11 and the wirings 25 extending across
the optical channel waveguides 11, light propagation loss due to
the wirings 25 can be reduced.
[0089] A fourth embodiment of the present invention will be
described hereinbelow with reference to FIGS. 7A to 7E and 8A to
8D. FIGS. 7A to 7E are cross-sectional views taken along line D-D
in FIG. 8A.
[0090] A substrate 10 carrying thereon the optical channel
waveguides 11 and the fractions of the metal layers 21 and 22 as
shown in FIGS. 7A and 8A is obtained in the same manner as in the
first embodiment.
[0091] Thereafter, positive photo-resist 40 is applied to the
substrate and a photo-mask 41 shown in FIG. 8C is disposed on the
backside of the substrate, and light is projected onto the backside
of the substrate through the photo-mask 41, thereby exposing the
photo-resist 40 to light through the mask 41. When the photo-resist
40 is subsequently developed, a pattern of the photo-resist 40
having one opening just above each of the optical channel
waveguides 11 is formed as shown in FIGS. 7B and 8B.
[0092] Then buffer layers 42 of SiO.sub.2 and Cr/Au layers 45 are
formed in this order, for instance, by sputtering using the resist
pattern 40 as a mask as shown in FIG. 7C.
[0093] Thereafter the photo-resist 40 is removed by liftoff. In
this manner, a buffer layer 42 of SiO.sub.2 and a wiring of Cr/Au
layer 45 are formed on each of the optical channel waveguides 11 as
shown in FIGS. 7D and 8D.
[0094] Then the substrate is plated with Au 27 except the portions
just above the optical channel waveguides 11. Thus a thick Au layer
27 is formed on the Ta layer 21 and the Au layer 22 which form the
electrodes for applying an electric voltage to the optical channel
waveguides 11 and on the Cr/Au layer 45 forming the wirings as
shown in FIG. 7E.
[0095] Thereafter the composite metal layer comprising the Au layer
27, the Au layer 22 and the Ta layer 21 is processed to
predetermined shapes of the electrodes, for instance, by etching.
Thus electrodes and wirings having increased thicknesses can be
formed.
* * * * *