U.S. patent application number 10/263745 was filed with the patent office on 2003-02-06 for microstructure array, and methods of fabricating a microstructure array, a mold for forming a microstructure array, and a microlens array.
Invention is credited to Teshima, Takayuki, Yagi, Takayuki.
Application Number | 20030024820 10/263745 |
Document ID | / |
Family ID | 14110214 |
Filed Date | 2003-02-06 |
United States Patent
Application |
20030024820 |
Kind Code |
A1 |
Teshima, Takayuki ; et
al. |
February 6, 2003 |
Microstructure array, and methods of fabricating a microstructure
array, a mold for forming a microstructure array, and a microlens
array
Abstract
In a method for fabricating an array of microstructures, a
substrate with an electrically-conductive portion is provided, an
insulating mask layer is formed on the electrically-conductive
portion of the substrate, a plurality of openings are formed in the
insulating mask layer to expose the electrically-conductive
portion, and a first plated or electrodeposited layer is deposited
in the openings and on the insulating mask layer by electro- or
electroless-plating, or electrodeposition. At least a surface of
the first plated or electrodeposited layer is made electrically
conductive. After that, the insulating mask layer is removed, and a
second plated layer is formed on the first plated or
electrodeposited layer and on the electrically-conductive portion
by electroplating to firmly fix the first plated or
electrodeposited layer to the substrate.
Inventors: |
Teshima, Takayuki;
(Kanagawa-ken, JP) ; Yagi, Takayuki;
(Kanagawa-ken, JP) |
Correspondence
Address: |
FITZPATRICK CELLA HARPER & SCINTO
30 ROCKEFELLER PLAZA
NEW YORK
NY
10112
US
|
Family ID: |
14110214 |
Appl. No.: |
10/263745 |
Filed: |
October 4, 2002 |
Related U.S. Patent Documents
|
|
|
|
|
|
Application
Number |
Filing Date |
Patent Number |
|
|
10263745 |
Oct 4, 2002 |
|
|
|
09536128 |
Mar 28, 2000 |
|
|
|
Current U.S.
Class: |
205/118 |
Current CPC
Class: |
G02B 3/0031 20130101;
G02B 3/0056 20130101; G02B 3/0012 20130101 |
Class at
Publication: |
205/118 |
International
Class: |
C25D 005/02 |
Foreign Application Data
Date |
Code |
Application Number |
Apr 1, 1999 |
JP |
1999/94436 |
Claims
What is claimed is:
1. A method for fabricating an array of microstructures comprising
the steps of: preparing a substrate with an electrically-conductive
portion; forming an insulating mask layer on the
electrically-conductive portion; forming a plurality of openings in
the insulating mask layer to expose the electrically-conductive
portion; forming a first plated or electrodeposited layer in the
openings and on the insulating mask layer by electro- or
electroless-plating, or electrodeposition, with at least a surface
of the first plated or electrodeposited layer being electrically
conductive; removing the insulating mask layer; and forming a
second plated layer on the first plated or electrodeposited layer
and on the electrically-conductive portion by electroplating.
2. A fabrication method according to claim 1, wherein the step of
forming the first plated or electrodeposited layer is stopped after
a thickness of the first plated or electrodeposited layer reaches
half a designed final height or more of the second plated layer
above a central portion of the opening.
3. A fabrication method according to claim 1, wherein the first
plated or electrodeposited layers are formed in the step of forming
the first plated or electrodeposited layer such that the insulating
mask layer between the adjacent first plated or electrodeposited
layers is not completely covered.
4. A fabrication method according to claim 1, wherein the step of
forming the second plated layer is stopped after a thickness of the
second plated layer reaches a thickness of the insulating mask
layer or more.
5. A fabrication method according to claim 1, wherein the opening
has a circular shape and the microstructure is a semispherical
microstructure.
6. A fabrication method according to claim 1, wherein the opening
has an elongated stripe shape and the microstructure is a
semicylindrical microstructure.
7. A fabrication method according to claim 1, further comprising a
step of forming a mold on the substrate with the first plated or
electrodeposited layer and the second plated layer; and a step of
separating the mold from the substrate.
8. A fabrication method according to claim 7, wherein the mold is
formed using electroplating.
9. A fabrication method according to claim 7, wherein the mold is a
mold for fabricating a microlens array.
10. A fabrication method according to claim 9, further comprising a
step of coating a light-transmitting material on the mold; a step
of hardening the light-transmitting material; and a step of
separating the material from the mold to obtain the microlens
array.
11. A microstructure array comprising: a substrate having an
electrically-conductive portion; a first plated or electrodeposited
layer formed on the electrically-conductive portion by electro- or
electroless-plating or electrodeposition using a plurality of
openings formed in an insulating mask layer formed on the
electrically-conductive portion of the substrate to expose the
electrically-conductive portion, at least a surface of the first
plated or electrodeposited layer being electrically conductive, and
the insulating mask layer being finally removed; and a second
plated layer formed on the first plated or electrodeposited layer
and on the electrically-conductive portion by electroplating.
12. A microstructure array according to claim 11, wherein a height
of the second plated layer formed on the first plated or
electrodeposited layer above a central portion of the opening is in
a range from 1 .mu.m to 100 .mu.m.
13. A microstructure array according to claim 11, wherein a
thickness of the second plated layer formed on the first plated or
electrodeposited layer above a central portion of the opening is
equal to or smaller than a thickness of the first plated or
electrodeposited layer above the central portion of the
opening.
14. A microstructure array according to claim 11, wherein the first
plated or electrodeposited layers are formed so as not to
completely cover the electrically-conductive portion.
15. A microstructure array according to claim 11, wherein a
thickness of the second plated layer is equal to or larger than a
spacing between the first plated layer and the
electrically-conductive portion.
16. A microstructure array according to claim 11, wherein the
opening has a circular shape and the microstructure is a
semispherical microstructure.
17. A microstructure array according to claim 11, wherein the
opening has an elongated stripe shape and the microstructure is a
semicylindrical microstructure.
Description
BACKGROUND OF THE INVENTION
[0001] 1. Field of the Invention
[0002] The present invention relates to a method for fabricating a
microstructure array, a method for fabricating a mold or a master
of a mold (in the specification the term "mold" is chiefly used in
a broad sense including both a mold and a master of a mold) for
forming a microstructure array, a method for fabricating a
microstructure array using the mold, and a microstructure array.
This invention particularly relates to a mold for forming a
microlens array, a method for fabricating the mold, and a method
for fabricating the microlens array using the mold.
[0003] 2. Description of the Related Background Art
[0004] A microlens array typically has a structure of arrayed
minute lenses each having a diameter from about 2 or 3 microns to
about 200 or 300 microns and an approximately semispherical
profile. The microlens array is usable in a variety of
applications, such as liquid-crystal display devices, optical
receivers and inter-fiber connections in optical communcation
systems.
[0005] Meanwhile, earnest developments have been made with respect
to a surface emitting laser and the like which can be readily
arranged in an array form at narrow pitches between the devices.
Accordingly, there exists a significant need for a microlens array
with narrow lens intervals and a large numerical aperture (NA).
[0006] Likewise, a light receiving device, such as a charge coupled
device (CCD), has been repeatedly decreased in size as
semiconductor processing techniques have been developed and
advanced. Therefore, also in this field, the need for a microlens
array with narrow lens intervals and a large NA is increasing.
[0007] In the field of such a microlens, a desirable structure is a
microlens with a large light-condensing efficiency which can highly
efficiently utilize light incident on its lens surface.
[0008] Further, similar desires exist in the fields of optical
information processing, such as optical parallel
processing-operations, and optical interconnections. Furthermore,
active or self-radiating type display devices, such as
electroluminescence (EL) panels, have been enthusiastically studied
and developed, and a highly-definite and highly-luminous display
has been proposed. In such a display, there is a heightened desire
for a microlens array which can be produced at a relatively low
cost and with a large area, as well as with a small lens size and a
large NA.
[0009] There are presently a number of prior art methods for
fabricating microlenses.
[0010] In a prior art microlens-array fabrication method using an
ion exchange method (see M. Oikawa, et al., Jpn. J. Appl. Phys.
20(1) L51-54, 1981), a refractive index is raised at plural places
in a substrate of multi-component glass. A plurality of lenses are
thus formed at the places with a high-refractive index. In this
method, however, the lens diameter cannot be large, compared with
the intervals between lenses. Hence, it is difficult to design a
lens with a large NA. Further, the fabrication of a large-area
microlens array is not easy since a large scale manufacturing
apparatus, such as an ion diffusion apparatus, is required to
produce such a microlens array. Moreover, an ion exchange process
is needed for each glass, in contrast with a molding method using a
mold. Therefore, variations of lens quality, such as a focal
length, are likely to increase between lots unless the management
of fabrication conditions in the manufacturing apparatus is
carefully conducted. In addition to the above, the cost of this
method is relatively high, as compared with the method using a
mold.
[0011] Further, in the ion exchange method, alkaline ions for
ion-exchange are indispensable in a glass substrate, and therefore,
the material of the substrate is limited to alkaline glass. The
alkaline glass is, however, unfit for a semiconductor-based device
which needs to be free of alkaline ions. Furthermore, since a
thermal expansion coefficient of the glass substrate greatly
differs from that of a substrate of a light radiating or receiving
device, misalignment between the microlens array and the devices is
likely to occur due to a misfit between their thermal expansion
coefficients as an integration density of the devices
increases.
[0012] Moreover, a compressive strain inherently remains on the
glass surface which is processed by the ion exchange method.
Accordingly, the glass tends to warp, and hence, a difficulty in
joining or bonding between the glass and the light radiating or
receiving device increases as the size of the microlens array
increases.
[0013] In another prior art microlens-array fabrication method
using a resist reflow (or melting) method (see D. Daly, et al.,
Proc. Microlens Arrays Teddington., p23-34, 1991), resin formed on
a substrate is cylindrically patterned using a photolithography
process and a microlens array is fabricated by heating and
reflowing the resin. Lenses having various shapes can be fabricated
at a low cost by this resist reflow method. Further, this method
has no problems of thermal expansion coefficient, warp and so
forth, in contrast with the ion exchange method.
[0014] In the resist reflow method, however, the profile of the
microlens is strongly dependent on the thickness of resin, wetting
conditions between the substrate and resin, and the heating
temperature. Therefore, variations between lots are likely to occur
while fabrication reproducibility per a single substrate surface is
high.
[0015] Further, when adjacent lenses are brought into contact with
each other due to the reflow, a desired lens profile cannot be
secured due to the surface tension. Accordingly, it is difficult to
achieve a high light-condensing efficiency by bringing the adjacent
lenses into contact and decreasing an unused area between the
lenses. Furthermore, when a lens diameter from about 20 or 30
microns to about 200 or 300 microns is desired, the thickness of
deposited resin must be large enough to obtain a spherical surface
by the reflow. It is, however, difficult to uniformly and thickly
deposit the resin material having desired optical characteristics
(such as refractive index and optical transmissivity). Thus, it is
difficult to produce a microlens with a large curvature and a
relatively large diameter.
[0016] In another prior art method, an original plate of a
microlens is fabricated, lens material is deposited on the original
plate and the deposited lens material is then separated. The
original plate or mold is fabricated by an electron-beam
lithography method (see Japanese Patent Application Laid-Open No. 1
(1989)-261601), or a wet etching method (see Japanese Patent
Application Laid-Open No. 5 (1993)-303009). In these methods, the
microlens can be reproduced by molding, variations between lots are
unlikely to occur, and the microlens can be fabricated at a low
cost. Further, the problems of alignment error and warp due to the
difference in the thermal expansion coefficient can be solved, in
contrast to the ion exchange method.
[0017] In the electron-beam lithography method, however, an
electron-beam lithographic apparatus is expensive and a large
investment in equipment is needed. Further, it is difficult to
fabricate a mold having a large area more than 100 cm.sup.2 (100
cm-square) because the electron beam impact area is limited.
[0018] Further, in the wet etching method, since an isotropic
etching using a chemical action is principally employed, an etching
of the metal plate into a desired profile cannot be achieved if the
composition and crystalline structure of the metal plate vary even
slightly. In addition, etching will continue unless the plate is
washed immediately after a desired shape is obtained. When a minute
microlens is to be formed, a deviation of the shape from the
desired one is possible due to etching lasting during a period from
the time a desired profile is reached to the time the microlens is
reached.
[0019] Further, there also exists a mold fabrication method using
an electroplating technique (see Japanese Patent Application
Laid-Open No. 6 (1994)-27302). In this method, as illustrated in
FIGS. 1A and 1B, an insulating film 103 having a conductive layer
101 formed on one surface thereof and an opening 104 is used, the
electroplating is performed with the conductive layer 101 acting as
a cathode, and a protruding portion or plated layer 105 acting as a
mother mold for a lens is formed on a surface of the insulating
film 103. A resist layer 110 is formed on the other surface of the
conductive layer 101 to prevent the formation of a plated layer on
this surface. The process of fabricating the mold by this method is
simple, and cost is reduced.
[0020] In the method of FIGS. 1A and 1B, the diameter of the
opening 104 needs to be less than 20 or 30 microns when a minute
microlens of about 20 or 30 microns in diameter is required to be
fabricated. In such a case, since a contact area between the plated
layer 105 and the conductive layer 101 is small, there is a great
fear that the protruding portion 105 falls due to a shearing stress
occurring between those layers 101 and 105 when a lens or mold is
formed by this structure. An anchor portion is provided in a bottom
portion of the plated layer 105 to prevent that falling, but this
is not enough to solve the problem.
[0021] There exists another mold fabrication method using the
electroplating technique (see Japanese Patent Publication No. 64
(1989)-10169). In this method, as illustrated in FIGS. 2A to 2D,
after a convex plated layer 205 is formed, a photoresist insulating
layer 203 is removed, except its portion between the plated layer
205 and an electrode layer 201, and another plated layer 206 is
thickly formed on the plated layer 205 and the electrode layer 201
to form a mold. There is, however, a fear that the mold deforms or
cracks occur in the mold when heating of transparent resin to be
molded and pressure molding are repeatedly conducted using the
thus-fabricated mold. Those phenomena are due to the fact that
thermal and mechanical strains tend to be accumulated since
mechanical characteristics, such as Young's modulus and yielding
strength, of the photoresist 203 left between the plated layer 205
and the electrode layer 201 are far smaller than those of the other
elements, and the fact that the molecular weight of high polymer
resin, such as the photoresist, tends to be lowered and hence the
resin is gasified.
SUMMARY OF THE INVENTION
[0022] An object of the present invention is to provide a
fabrication method for fabricating a microstructure array
(typically a microlens array such as a semispherical microlens
array, a flyeye lens and a lenticular lens) with a high resistivity
flexibly, readily and stably, a fabrication method of a mold for
forming a microstructure array, a fabrication method of a
microstructure array using the mold, and so forth.
[0023] The present invention is generally directed to a fabrication
method for fabricating an array of microstructures which includes
the following steps:
[0024] preparing a substrate with an electrically-conductive
portion;
[0025] forming an insulating mask layer on the
electrically-conductive portion;
[0026] forming a plurality of openings in the insulating mask layer
to expose the electrically-conductive portion;
[0027] forming a first plated or electrodeposited layer in the
opening and on the insulating mask layer by electro- or
electroless-plating or electrodeposition, with at least a surface
of the first plated or electrodeposited layer being electrically
conductive;
[0028] removing the insulating mask layer; and
[0029] forming a second plated layer on the first plated or
electrodeposited layer and on the electrically-conductive portion
by electroplating.
[0030] More specifically, the following constructions are possible
based on the above fundamental construction.
[0031] The step of forming the first plated or electrodeposited
layer may be stopped after a thickness or height of the first
plated or electrodeposited layer (i.e., the distance between the
exposed electrically-conductive portion and the top of the first
plated layer) reaches half a designed final height or more of the
second plated layer above a central portion of the opening.
[0032] When the electroplating is performed after the mask layer of
insulating material such as resist is removed, the cathode is
composed of the electrically-conductive portion and the first
plated or electrodeposited layer. In such a case, a current tends
to be concentrated on a top portion of the first layer, and hence,
the semispherical or semicylindrical profile of the plated layer is
likely to be deformed. Further, the height of the plated layer is
likely to differ between a peripheral portion of its array and a
central portion of its array. In contrast, when the electroplating
is performed with the mask layer present, the cathode is composed
of the plated or electrodeposited layer only. In such a state, an
approximately uniform current density can be obtained over the
semispherical or semicylindrical plated or electrodeposited layer.
Therefore, it is preferable to form the plated or electrodeposited
layer with a desired profile under the condition that the mask
layer is present, as far as possible. For this purpose, the
first-layer forming step is continued after its thickness reaches
or exceeds half a designed final height of the second plated layer
above the central portion of the opening (i.e., the distance
between the exposed electrically-conductive portion and the top of
the second plated layer). In this case, the height of the plated or
electrodeposited layer is proportional with a curvature radius
thereof.
[0033] The first plated or electrodeposited layers may be formed in
the step of forming the first plated or electrodeposited layer such
that the insulating mask layer between the adjacent first plated or
electrodeposited layers is not completely covered.
[0034] The step of forming the second plated layer may be stopped
after a thickness of the second plated layer (i.e., the distance
between the top of the first plated layer and the top of the second
plated layer) reaches a thickness of the insulating mask layer or
more. This is needed to completely fill the space created by the
removal of the mask layer with the second plated layer and to
firmly fix the first layer to the substrate.
[0035] The opening may have a circular shape and the microstructure
may be a semispherical microstructure, or an elongated, striped
shape, and the microstructure may be a semicylindrical
microstructure.
[0036] The fabrication method may further include a step of forming
a mold on the the substrate with the first plated or
electrodeposited layer and the second plated layer, and a step of
separating the mold from the substrate. In this case, the mold may
be formed using electroplating, and the mold may be a mold for
fabricating a microlens array.
[0037] In this method, the mold can be directly formed by
electroplating or the like. Therefore, no expensive equipment is
needed, costs can be reduced, and the size of the mold can be
enlarged readily. Furthermore, the size of the plated layer can be
controlled in situ, and the lens diameter and the like can be
readily and precisely controlled by controlling electroplating time
and temperature.
[0038] The fabrication method may further include a step of coating
a light-transmitting material on the mold, a step of hardening the
light-transmitting material, and a step of separating the material
from the mold to obtain the microlens array.
[0039] The present invention is also directed to a microstructure
array including:
[0040] a substrate having an electrically-conductive portion;
[0041] a first plated or electrodeposited layer formed on the
electrically-conductive portion by electro- or electroless-plating
or electrodeposition using a plurality of openings formed in an
insulating mask layer formed on the electrically-conductive portion
of the substrate to expose the electrically-conductive portion, at
least a surface of the first plated or electrodeposited layer being
electrically conductive, and the insulating mask layer being
finally removed; and
[0042] a second plated layer formed on the first plated or
electrodeposited layer and on the electrically-conductive portion
by electroplating.
[0043] More specifically, the following structures are possible
based on the above fundamental structure.
[0044] A height of the second plated layer formed on the first
plated or electrodeposited layer above a central portion of the
opening (i.e., the distance between the exposed
electrically-conductive portion and the top of the second plated
layer) may be in a range from 1 .mu.m to 100 .mu.m.
[0045] A thickness of the second plated layer formed on the first
plated or electrodeposited layer above a central portion of the
opening (i.e., the distance between the top of the first plated
layer and the top of the second plated layer) may be equal to or
smaller than a thickness or height of the first plated or
electrodeposited layer above the central portion of the opening
(i.e., the distance between the exposed electrically-conductive
portion and the top of the first plated layer).
[0046] The first plated or electrodeposited layers may be formed so
as not to completely cover the electrically-conductive portion.
[0047] A thickness of the second plated layer may be equal to or
larger than a spacing between the first plated layer and the
electrically-conductive portion.
[0048] These advantages and others will be more readily understood
in connection with the following detailed description of the more
preferred embodiments in conjunction with the drawings.
BRIEF DESCRIPTION OF THE DRAWINGS
[0049] FIGS. 1A and 1B are cross-sectional views illustrating a
conventional method of fabricating a microstructure array,
respectively.
[0050] FIGS. 2A to 2D are cross-sectional views illustrating
another conventional method of fabricating a microstructure array,
respectively.
[0051] FIGS. 3A to 3F are cross-sectional views illustrating steps
of a method for fabricating a mold for a microstructure array, such
as a microlens array, and of embodiments according to the present
invention, respectively.
[0052] FIG. 4 is a plan view illustrating a state in which a radius
in a horizontal direction of a plated layer grown from an opening
reaches above a half of an interval between adjacent plated
layers.
[0053] FIGS. 5A to 5E are cross-sectional views illustrating steps
of a method for fabricating a microlens array mold according to the
present invention, respectively.
[0054] FIGS. 6A to 6D are cross-sectional views illustrating steps
of a method for fabricating a microlens array mold according to the
present invention, respectively.
[0055] FIGS. 7A to 7C are cross-sectional views illustrating steps
of a method for fabricating a microlens array mold according to the
present invention, respectively.
[0056] FIG. 8 is a view illustrating an electroplating apparatus
used in the fabrication method of the present invention.
[0057] FIG. 9 is a view illustrating a principle of forming a
semispherical microstructure by electro- or electroless-plating or
electrodeposition.
DESCRIPTION OF THE PREFERRED EMBODIMENTS
[0058] (First Embodiment)
[0059] A first embodiment of a fabrication method of a
semispherical microstructure array will be described by reference
to FIGS. 3A to 3F.
[0060] Initially, a silicon wafer of 1 inch in diameter is
thermally oxidized using an oxidizing gas, and layers of silicon
dioxide with a thickness of 1 .mu.m are formed on opposite surfaces
of the wafer. This wafer is used as a substrate 1 illustrated in
FIGS. 3A to 3F. Cr and Au are continuously layered with thicknesses
of 10 nm and 200 nm on the above wafer, respectively, using an
electron beam vacuum-evaporation method which is one suitable
thin-film forming method. An electrode layer 2 is thus formed.
Aromatic polyamide acid solution is then spin-coated and this
coating is thermally treated to form a mask layer 3 of polyimide
with a thickness of 1 .mu.m as illustrated in FIG. 3A.
[0061] Any material, such as metal, semiconductor (a silicon wafer
or the like) and insulating substance (such as glass, quartz and
polymer film), can be used as the substrate material. When the
metal material is used as the substrate 1, there is no need to form
the electrode layer 2. Further, when the semiconductor is used, the
electrode layer is not necessarily needed if the semiconductor has
enough conductivity to enable electroplating. However, where metal
or semiconductor is used as the substrate, a plated layer will also
be formed on a portion other than the microstructure forming
portion, since the entire substrate is immersed in electroplating
liquid. Therefore, when the plated layer is desired to be formed on
a predetermined portion only, the insulating substance can be
preferably used as the substrate. Alternatively, a metal or
semiconductor, whose surface is partially insulated, may also be
used.
[0062] Materials of the electrode layer and the substrate are
selected from materials which are not corrosive to the electro- or
electroless-plating or electrodepositing solution used since the
electrode layer is exposed to such a solution. The mask layer 3 may
be formed of any inorganic or organic insulating material that is
also anticorrosive to the electro- or electroless-plating or
electrodepositing solution.
[0063] Further, coating, exposure and development of photoresist
are conventionally performed using photolithography to form an
opening in the resist. The mask layer 3 at the resist opening is
etched by a reactive ion etching using oxygen. Thus, the electrode
layer 2 is exposed and an opening 4 is formed as illustrated in
FIG. 3B. The photoresist is removed thereafter. When the mask layer
3 is formed of photoresist, the above etching process can be
omitted.
[0064] As formed, the opening 4 has a circular shape and a diameter
of 10 .mu.m. A plurality of the openings 4 are formed in a
two-dimensional matrix array of 500.times.500. Intervals between
the adjacent openings 4 are 50 .mu.m.
[0065] Ni (nickel) electroplating is then performed at a bath
temperature of 50.degree. C. and a cathodic current density of 5
A/dm.sup.2 as illustrated in FIG. 8. The above substrate 1 for
electroplating is used as a base 9, and the electrode layer 2 is
used as the cathode. Ni electroplating bath 10 containing nickel
(II) sulfate, nickel (II) chloride, boric acid and brightener is
used. An external electric power source 12 is connected between the
electrode layer 2 and an anodic plate 11.
[0066] Ni plated layer 5 is initially deposited in the opening 4
and grown therein. The plated layer 5 expands onto the mask
insulating layer 3, and the semispherical microstructure 5 is thus
formed as illustrated in FIGS. 3D. The plated layer 5 is deposited
until its radius reaches about 25 .mu.m above a center of the
opening 4. FIG. 4 shows a plan view of FIG. 3D.
[0067] Where the electroplating is effected at the opening 4 in the
electroplating solution 10 containing metal ions, metal ions in the
electroplating solution 10 move toward the plated layer 5, and
hence, deposition of the electroplating proceeds with its growth
direction being isotropic. Thus, a semispherical or semicylindrical
plated layer can be formed. When the size of the opening 4 is
sufficiently smaller than the size of the anodic plate 11 and metal
ions are uniformly dissolved in the electroplating solution 10, the
growth direction of the plated layer is isotropic. Typically, a
microlens array has a structure of arrayed minute lenses each
having a diameter from about 2 or 3 microns to about 200 or 300
microns, and the size of the opening is made smaller than the
desired diameter of the microlens. In order to better achieve an
isotropic growth of the plated or electrodeposited layer, the size
of the opening is less than the diameter of the semispherical
structure.
[0068] The plated layer is formed by the deposition of metal ions
in the electroplating bath caused by the electrochemical reaction.
The thickness of the plated layer can be readily controlled by
controlling the electroplating time and temperature. The following
materials can be used as electroplating metal. For example, as a
single metal, Ni, Au, Pt, Cr, Cu, Ag, Zn and the like can be
employed. As an alloy, Cu--Zn, Sn--Co, Ni--Fe, Zn--Ni and the like
can be used. Any material can be used so long as electroplating is
possible.
[0069] The first plated layer 5 can include a portion formed by
electrodeposition or electroless plating. The surface of the first
plated layer 5 only needs to be electrically conductive such that a
second layer can be formed on this surface using electroplating or
electrodeposition.
[0070] Turning now to processes of FIGS. 3A to 3F, the mask layer 3
is then removed by plasma ashing as illustrated in FIG. 3E. As the
removal method, organic cleansing, inorganic cleansing,
ultraviolet-ray ozone ashing, plasma ashing and so forth can be
employed if only none of the substrate 1, electrode layer 2 and
plated layer 5 are corroded thereby.
[0071] When the adjacent plated layers 5 are joined to each other,
the mask layer 3 can be removed if the mask layer 3 is not
completely covered with the first plated layers 5 (see FIG. 4).
Thus, the mask layer 3 can be removed without residue.
[0072] After that, another Ni electroplating is performed at a bath
temperature of 50.degree. C. and a cathodic current density of 5
A/dm.sup.2. The same Ni electroplating bath containing nickel (II)
sulfate, nickel (II) chloride, boric acid and brightener is used. A
second plated layer 6 is thus grown until its radius reaches about
30 .mu.m above the center of the opening 4 as illustrated in FIG.
3F. Thereby, a contact area between the semispherical plated layers
5 and 6 and the electrode layer 2 increases, and hence, the
semispherical first plated layer 5 is firmly fixed to the electrode
layer 2. In such a mold for forming a microlens array, there occurs
neither mold deformation nor cracks due to the molding pressure
when used, and excellent mechanical characteristics, such as a high
shearing stress, can be obtained. Therefore, when a lens material
layer or a mold material layer formed on the above structure is
separated from this structure, the collapse of the first plated
layer 5 can be prevented. Further, the first plated layer 5 does
not drop even if a strong stirring plating is conducted to deposit
the mold material layer on the above structure. Leveling is also
achieved by the second plated layer 6, so that a plated surface
with a large glossiness can be obtained. Furthermore, since a
portion of the mask layer 3 between the first plated layer 6 and
the electrode layer 2 is removed, the problem of deformation of the
plated layers 5 and 6 due to thermal stress of the mask layer can
be eliminated and a mold for forming a microstructure array with a
high thermal resistivity can be obtained.
[0073] When the second plated layer 6 is formed, the forming method
is preferably limited to electroplating since it can take metal
ions even into narrow areas, since the second plated layer 6 needs
to be formed in such a narrow area between the first plated or
electrodeposited layer 5 and the electrode layer 2.
[0074] A fabrication process for forming a microlens array by using
the above structure will be described with reference to FIGS. 6A to
6D. A resin 14 of ultraviolet-ray hardening photopolymer is
deposited on the plated layers 5 and 6 as illustrated in FIG. 6A.
After a support substrate 15 of glass is placed on the resin 14,
the resin 14 is hardened by exposing the resin 14 to ultraviolet
rays as illustrated in FIG. 6B. The resin 14 of a microlens array
can be separated from the substrate 1 and the plated layers 5 and 6
by lifting the glass substrate 15. Thus, the concave resin 14 can
be formed as illustrated in FIG. 6C. Here, a thousand photopolymer
structures 14 for the microlens array could be formed by the same
microlens array mold 1, 2 and 6, using the same method. During such
a process, there was no collapse of the semispherical plated layers
5 and 6.
[0075] In the fabrication method for a mold for a microlens array
according to this embodiment, the resistivity of the mold is
increased, and a large number of the concave resins 14 with the
same profile can be fabricated using the same mold.
[0076] Another resin 16 with a larger refractive index than that of
the resin 14 is further dropped on the concave resin 14, and the
resin 16 is hardened. Thus, a plane microlens array as illustrated
in FIG. 6D can be obtained.
[0077] In the above method, the alkaline glass is not indispensable
for forming a microlens, so that materials which can be used for
the microlens and the substrate are less restricted than in the ion
exchange method.
[0078] The above microlens array may be fabricated by other
methods, such as a method in which a conventional thermoplastic
resin is used and a heated mold is stamped on this resin, a method
in which a thermosetting resin is laid over a mold and then heated
to be hardened, and a method in which an electron-beam hardening
resin is coated on a mold and the resin is hardened by electron
beam irradiation.
[0079] (Second Embodiment)
[0080] A second embodiment of a fabrication method of a
semispherical microstructure array will be described by reference
to FIGS. 3A to 3F, 5A to 5E and 7A to 7C.
[0081] Also in the second embodiment, substrate 1, electrode layer
2, insulating mask layer 3, openings 4 are formed as in the first
embodiment.
[0082] A Cu (copper) electroplating is then performed at a bath
temperature of 55.degree. C. and a cathodic current density of 4
A/dm.sup.2 as illustrated in FIG. 8. The above substrate 1 for
electroplating is used as a base 9, and the electrode layer 2 is
used as the cathode. A Cu electroplating bath containing copper
sulfate, sulfuric acid, chloric acid and brightener is used. A Cu
plated layer 5 is initially deposited in the opening 4 and grown
therein. The plated layer 5 expands onto the mask insulating layer
3, and the semispherical microstructure 5 is thus formed as
illustrated in FIG. 3D. The plated layer 5 is deposited until its
radius reaches about 20 .mu.m above a center of the opening 4.
[0083] The mask layer 3 is then removed by plasma ashing as
illustrated in FIG. 3E. After that, Ni electroplating is performed
at a bath temperature of 50.degree. C. and a cathodic current
density of 5 A/dm.sup.2. Ni electroplating bath containing nickel
(II) sulfate, nickel (II) chloride, boric acid and brightener is
used. A second plated layer 6 is thus grown until its radius
reaches about 25 .mu.m above the center of the opening 4 as
illustrated in FIG. 3F. Thereby, a contact area between the
semispherical plated layers 5 and 6 and the electrode layer 2
increases, and hence, the semispherical first plated layer 5 is
firmly fixed to the electrode layer 2. Also in such a mold for
forming a microlens array, there occur neither mold deformation nor
cracks due to the molding pressure when used, and excellent
mechanical characteristics can be obtained.
[0084] A process for fabricating a mold for forming a microlens
array will be described by reference to FIGS. 5A to 5E. The above
structure is used as a master. PSG (phospho-silicate glass) of 1
.mu.m in thickess is deposited at 350.degree. C. by an
atmospheric-pressure chemical vapor deposition (CVD) method to form
a sacrificial layer 7 as illustrated in FIG. 5A. Ti and Au are
continuously layered with thicknesses of 10 nm and 200 nm on the
above wafer, respectively, using the electron-beam evaporation
method. An electrode layer 8 for electroplating the mold is thus
formed as illustrated in FIG. 5B.
[0085] Ni electroplating is then performed at a bath temperature of
50.degree. C. and a cathodic current density of 5 A/dm.sup.2 as
illustrated in FIG. 5C. The above master is used as a base, and the
above electrode layer 8 is used as the cathode. Ni electroplating
bath containing nickel (II) sulfate, nickel (II) chloride, boric
acid and brightener is used. Thus, a concave mold 13 is formed as
illustrated in FIG. 5C.
[0086] The wafer of FIG. 5C is then immersed in a mixture solution
of hydrofluoric acid and ammonium fluoride to etch and remove the
sacrificial layer 7 of PSG. The substrate 1 and the mold 13 can be
separated from each other as illustrated in FIG. 5D. The Ti of the
electrode layer 8 for electroplating the mold can be removed
simultaneously. After that, the Au of the electrode layer 8 is
etched by a mixture solution of iodine and potassium iodide. The
mold 13 for a convex microlens array can be thus produced as
illustrated in FIG. 5E.
[0087] The separated substrate has a semispherical microstructure.
Here, a thousand molds 13 for a microlens array could be formed by
repeating the same steps as illustrated in FIGS. 5A to 5E. During
such a process, there was no collapse of the semispherical plated
layers 5 and 6.
[0088] In the fabrication method of the mold for a microlens array
according to this embodiment, a plurality of molds 13 with the same
profile can be readily fabricated since the mold 13 is formed by
molding. In this embodiment, the resistivity of the mold master is
increased, and a plurality of molds for a microlens array with the
same profile can be fabricated using the same mold master.
[0089] After ultraviolet-ray hardening resin of photopolymer 24 is
laid over the mold 13 for a convex microlens array fabricated by
the above method, as illustrated in FIG. 7A. A glass substrate 25
as a supporting substrate is then placed on the resin 24 as
illustrated in FIG. 7B. The resin 24 is exposed to ultraviolet rays
through the glass 25 to be hardened. After that, the glass 25 with
the resin 24 is separated from the mold 13. Thus, a convex
microlens array with a large glossiness is obtained as illustrated
in FIG. 7C. A thousand microlens arrays of photopolymer could be
formed by repeating the same steps as illustrated in FIGS. 7A to
7C, using the same mold 13 for a microlens array.
[0090] The above microlens array may also be fabricated by other
methods, such as the above-described methods using thermoplastic
resins, thermosetting resins and electron-beam hardening
resins.
[0091] (Third Embodiment)
[0092] A third embodiment of a method of fabricating a
semispherical microstructure array will be described by reference
to FIGS. 3A to 3F, 5A to 5E and 7A to 7C.
[0093] Initially, a silicon wafer of 1 inch in diameter is
thermally oxidized using an oxidizing gas, and layers of silicon
dioxide with a thickness of 1 .mu.m are formed on opposite surfaces
of the wafer. This wafer is used as a substrate 1 illustrated in
FIGS. 3A to 3F. Cr is layered with a thickness of 200 nm on the
above wafer, using the electron beam vacuum-evaporation method. An
electrode layer 2 of Cr is thus formed. Aromatic polyamide acid
solution is then spin-coated and this coating is thermally treated
to form a mask layer 3 of polyimide with a thickness of 1 .mu.m as
illustrated in FIG. 3A.
[0094] Further, coating, exposure and development of photoresist
are conventionally performed using photolithography to form an
opening in the resist on the mask layer 3. The mask layer 3 at the
resist opening is etched by the reactive ion etching using oxygen.
Thus, the electrode layer 2 is partly exposed through the mask
layer 3 and an opening 4 is formed as illustrated in FIG. 3B. The
photoresist formed on the mask layer 3 is removed thereafter.
[0095] As formed, the opening 4 has a circular shape and a diameter
of 5 .mu.m. A plurality of the openings 4 are formed in a
two-dimensional matrix array of 500.times.500. Intervals between
the adjacent openings 4 are 50 .mu.m.
[0096] A Cr (chromium) electroplating is then performed at a bath
temperature of 45.degree. C. and a cathodic current density of 20
A/dm.sup.2 as illustrated in FIG. 8. The above substrate 1 for
electroplating is used as a base 9, and the electrode layer 2 is
used as the cathode. A Cr electroplating bath containing chromic
acid and sulfuric acid is used. A Cr plated layer 5 is initially
deposited in the opening 4 and grew therein. The plated layer 5
expands onto the mask layer 3, and the semispherical microstructure
5 is thus formed as illustrated in FIG. 3D. The plated layer 5 is
deposited until its radius reaches about 12 .mu.m above a center of
the opening 4.
[0097] The mask layer 3 is then removed by plasma ashing as
illustrated in FIG. 3E. After that, another Cr electroplating is
performed at a bath temperature of 45.degree. C. and a cathodic
current density of 20 A/dm.sup.2. A Cr electroplating bath
containing sulfuric acid and brightener is used. A second plated
layer 6 is thus grown until its radius reaches about 20 .mu.m above
the center of the opening 4 as illustrated in FIG. 3F. Thereby, the
semispherical first plated layer 5 is firmly fixed to the electrode
layer 2. In such a mold for forming a microlens array, there occur
neither mold deformation nor cracks due to the molding pressure
when used, and a mold master for a microlens array with excellent
mechanical characteristics can be obtained.
[0098] A fabrication process for a mold will be described by
reference to FIGS. 5A to 5E. The above structure is used as a
master. PSG of 1 .mu.m in thickess is deposited at 350.degree. C.
by the atmospheric-pressure CVD method to form a sacrificial layer
7 as illustrated in FIG. 5A.
[0099] Ni electroless plating is then performed at a bath
temperature of 90.degree. C. Ni electroless plating bath containing
nickel (II) sulfate, ethylene diamine, sodium hypophosphite and
additive is used. Thus, a mold 13 is formed as illustrated in FIG.
5C.
[0100] The wafer of FIG. 5C is then immersed in a mixture solution
of hydrofluoric acid and ammonium fluoride to etch and remove the
sacrificial layer 7 of PSG. The substrate 1 and the mold 13 can be
separated as illustrated in FIG. 5D. Since the electroless plating
is used in this embodiment, the process for forming the electrode
layer 8 on the sacrificial layer 7 is unnecessary, in contrast to
the second embodiment.
[0101] The separated substrate has a semispherical microstructure.
Here, a thousand molds 13 for forming a microlens array could be
formed by repeating the same steps as illustrated in FIGS. 5A to
5E. During such a process, there was no collapse of the
semispherical plated layers 5 and 6.
[0102] In the fabrication method of the mold for forming a
microlens array according to this embodiment, a plurality of molds
13 with the same profile can be readily fabricated since the mold
13 is formed by molding. In this embodiment, the resistivity of the
mold master is increased, and a plurality of molds for a microlens
array with the same profile can be fabricated using the same mold
master.
[0103] After ultraviolet-ray hardening resin of photopolymer 24 is
laid over the mold 13 for a convex microlens array fabricated by
the above method as illustrated in FIG. 7A. A glass substrate 25 as
a supporting substrate is then placed on the resin 24 as
illustrated in FIG. 7B. The resin 24 is exposed to ultraviolet rays
through the glass 25 to be hardened. After that, the glass 25 with
the resin 24 is separated from the mold 13. Thus, a convex
microlens array with a high glossiness is obtained as illustrated
in FIG. 7C. A thousand microlens arrays of photopolymer could be
formed by repeating the same steps as illustrated in FIGS. 7A to
7C, using the same mold 13 for a microlens array.
[0104] While the present invention has been described with respect
to what are presently considered to be the preferred embodiments,
it is to be understood that the invention is not limited to the
disclosed embodiments. The present invention is intended to cover
various modifications and equivalent arrangements included within
the spirit and scope of the appended claims.
* * * * *