U.S. patent application number 11/226188 was filed with the patent office on 2007-10-11 for method for manufacturing nanostructure.
This patent application is currently assigned to Canon Kabushiki Kaisha. Invention is credited to Yoshihiro Ishibe, Kazumi Kimura, Yasuhiro Matsuo, Kentarou Nomura.
Application Number | 20070235342 11/226188 |
Document ID | / |
Family ID | 35636882 |
Filed Date | 2007-10-11 |
United States Patent
Application |
20070235342 |
Kind Code |
A1 |
Matsuo; Yasuhiro ; et
al. |
October 11, 2007 |
Method for manufacturing nanostructure
Abstract
A method for manufacturing a nanostructure is provided, in which
pores (3) having diameters on the order of nanometers (nm), the
diameter decreasing as in a tapered shape, can be manufactured. The
method includes repeated cycles of the alternate step of anodizing
Al or an Al alloy (1) to form an anodic oxide film (2) having pores
(3) in an anodizing step and increasing diameter of the pore (3) in
a pore-widening step, so that the diameter of the pore (3) is
varied in the depth direction.
Inventors: |
Matsuo; Yasuhiro;
(Kawasaki-shi, JP) ; Ishibe; Yoshihiro;
(Utsunomiya-shi, JP) ; Kimura; Kazumi; (Toda-shi,
JP) ; Nomura; Kentarou; (Tsuchiura-shi, JP) |
Correspondence
Address: |
CANON U.S.A. INC. INTELLECTUAL PROPERTY DIVISION
15975 ALTON PARKWAY
IRVINE
CA
92618-3731
US
|
Assignee: |
Canon Kabushiki Kaisha
Ohta-ku
JP
|
Family ID: |
35636882 |
Appl. No.: |
11/226188 |
Filed: |
September 13, 2005 |
Current U.S.
Class: |
205/175 ;
257/E21.291; 257/E21.309 |
Current CPC
Class: |
B82Y 30/00 20130101;
H01L 21/32134 20130101; B82Y 20/00 20130101; G02B 26/125 20130101;
C25D 11/12 20130101; G02B 2207/101 20130101; H01L 21/31687
20130101; H01L 21/02258 20130101; H01L 21/02203 20130101 |
Class at
Publication: |
205/175 |
International
Class: |
C25D 11/12 20060101
C25D011/12 |
Foreign Application Data
Date |
Code |
Application Number |
Oct 1, 2004 |
JP |
2004-290051(PAT.) |
Claims
1. A method for manufacturing a nanostructure, the method
comprising the steps of: a) an anodizing step of anodizing Al or an
Al alloy to form an anodic oxide film having first pores, b) an
increasing step of increasing diameters of the first pores in the
anodic oxide film, and c) an anodizing step of anodizing the anodic
oxide film to form a second pore at the bottom of each first pore,
wherein the second pores have different diameters to the first
pores.
2. The method for manufacturing a nanostructure according to claim
1, further comprising the step of repeating the steps a), b) and c)
at least once.
3. The method for manufacturing a nanostructure according to claim
1, wherein the diameters of the pores decrease between the first
and second pores in the depth direction from the nanostructure
surface.
4. The method for manufacturing a nanostructure according to claim
3, wherein the diameter decrease is stepwise.
5. The method for manufacturing a nanostructure according to claim
3, wherein the diameter decrease is tapered.
6. The method for manufacturing a nanostructure according to any
one of claim 1, wherein intervals between the pores are controlled
by adjusting the composition, the temperature, and the applied
voltage of an acidic electrolytic solution used in the anodizing
step.
7. The method for manufacturing a nanostructure according to any
one of claim 1, wherein the diameters of the pores are controlled
by adjusting the composition, the temperature, and the treatment
time of an etching solution used in the increasing step.
8. The method for manufacturing a nanostructure according to any
one of claim 1, wherein the acidic electrolytic solution and the
etching solution are phosphoric acid.
9. A nanostructure manufactured by the manufacturing method
according to any one of claim 1.
10. A method for manufacturing a mold for molding a scanning lens,
the method comprising the step of producing a nanostructure on a
mold surface in the shape of a curved surface by the manufacturing
method according to any one of claim 1.
11. A fine structure grating produced using a nanostructure
manufactured by the manufacturing method according to claim 9.
12. A scanning lens produced using a mold manufactured by the
manufacturing method according to claim 10 and comprising the fine
structure grating on a lens surface.
13. An optical scanning apparatus comprising a scanning lens
according to claim 12 and an imaging optical system to direct a
light beam reflected from an optical deflector to a surface to be
scanned.
14. An image forming apparatus comprising an optical scanning
apparatus according to claim 13 and a photosensitive drum disposed
on the surface to be scanned.
Description
BACKGROUND OF THE INVENTION
[0001] 1. Field of the Invention
[0002] The present invention relates to a method for manufacturing
a microstructure (nanostructure) having a periodic structure of a
few tens of nanometers to a few hundred nanometers, and is suitable
for manufacturing elements related to electronics, biology, and
optics.
[0003] The present invention also relates to a fine structure
grating produced by using the nanostructure.
[0004] Furthermore, the fine structure grating of the present
invention may be disposed on a surface of a scanning lens used in
an optical scanning apparatus, and has an antireflective
function.
[0005] 2. Description of the Related Art
[0006] Thin films, thin lines, dots, and the like of metals and
semiconductors having sizes smaller than a specific length may
exhibit unique electric, optical, and chemical properties.
[0007] From this point of view, there has been a growing interest
in a structure (microstructure) on the order of nanometers as a
functional material, the microstructure having a structure finer
than 1 micrometer (.mu.m).
[0008] A typical method for manufacturing the microstructure on the
order of nanometers is, for example, a semiconductor manufacturing
process by using photolithography. However, there are many problems
in compatibility between an increase in area and a reduction in
manufacturing cost.
[0009] A technique of so-called self-organization based on a
naturally formed regular structure has attracted attention as a
technology for ensuring the compatibility between an increase in
area and a reduction in manufacturing cost, and considerable
research has been conducted. Examples of techniques of
self-organization include anodization of Al (refer to R. C.
Furneaux, W. R. Rigby & A. P. Davidson, NATURE, Vol. 337, P147,
(1989)).
[0010] FIG. 5 is a diagram showing an anodic oxide film having
cylindrical pores (nanoholes). As shown in the drawing, when an Al
plate 21 is anodized in an acidic electrolyte, an anodic oxide film
22 having pores 23 is formed.
[0011] These pores 23 are characterized by being very fine
cylindrical pores of a few tens of nanometers to a few hundred
nanometers in diameter and having a specific geometric structure in
which the pores are arranged in parallel at intervals of a few tens
of nanometers to a few hundred nanometers.
[0012] These cylindrical pores 23 have large aspect ratios and the
diameter of the cross-section is highly uniform. The diameters and
the intervals of these pores 23 can be controlled to some extent by
adjusting the current and the voltage during anodization.
[0013] Various proposals have been attempted in consideration of
this specific geometric structure of the Al anodic oxide film
(refer to Masuda, KOTAI BUTSURI (Solid State Physics), Vol. 31,
493, (1996)). Examples of applications will be described below,
while detailed explanations have been made in the above-described
document by Masuda.
[0014] Examples thereof include an application to films taking
advantage of the abrasion resistance and the insulating property of
the anodic oxide film.
[0015] Various applications to coloring, magnetic recording media,
EL light emitting elements, electrochromic elements, optical
elements, solar cells, gas sensors, and others are attempted by the
use of a technology for filling a metal, semiconductors, and the
like in nanoholes and a technology for replicating nanoholes.
[0016] Furthermore, applications to many areas, for example,
quantum effect devices, e.g., quantum wires and MIM elements, and
molecular sensors through the use of nanoholes as chemical reaction
fields, are expected.
[0017] On the other hand, the technique of self-organization, in
particular the technique of Al anodization, has an advantage that
the microstructure on the order of nanometers can easily be formed
with good controllability.
[0018] According to this technique, in general, a nanostructure
having a large area can be formed. Various proposals for the use of
this technique have been made previously (refer to Japanese Patent
Laid-Open No. 63-187415 and Japanese Patent Laid-Open No.
11-200090).
[0019] Japanese Patent Laid-Open No. 63-187415 discloses a magnetic
recording medium in which an electrochemically stable substrate
layer having electrical conductivity is laminated on a base
material, an anodic oxide film of Al or an Al alloy is laminated on
the substrate layer, and magnetic materials are filled in
micropores disposed in the anodic oxide film.
[0020] Here, a material, e.g., Rh, Nb, Ta, Au, Ir, Pt, Ti, Cr, Pd,
Ru, Os, Ga, Zr, Ag, Sn, Cu, Hf, or Be, is used as the substrate
layer.
[0021] It is described that the use of the above-described 18 types
of material exhibits an effect of making the depths of micropores
formed in the anodic oxide film during anodization of Al or the Al
alloy uniform.
[0022] Japanese Patent Laid-Open No. 11-200090 describes the
formation of electrically conductive paths on the bottoms of pores
and, thereby, the pores are formed uniformly while the bottoms of
pores have excellent electrical conductivity.
[0023] The pore on the order of nanometers produced by Al
anodization has a pore diameter (minor axis) exhibiting excellent
uniformity in the depth direction (major axis).
[0024] Various applications are able to take advantage of the
above-described advantageous features. However, in some cases
related to electronics, biology, and optics, it is desirable that
the pores have a nonuniform diameter.
[0025] For example, in the case where pores are used as mother dies
and transferred members are used as optical elements, periodic
structures on the order of nanometers may perform an antireflective
function, and particularly excellent antireflective properties may
be exhibited in the case that the transferred member is in the
shape of a circular cone rather than a cylinder.
SUMMARY OF THE INVENTION
[0026] The present invention provides a method for manufacturing a
microstructure, in which pores having pore diameters on the order
of nanometers and decreasing in a tapered shape can be
manufactured.
[0027] A method for manufacturing a nanostructure according to an
aspect of the present invention includes a) an anodizing step of
anodizing Al or an Al alloy to form an anodic oxide film having
first pores, b) an increasing step of increasing diameters of the
first pores in the anodic oxide film, and c) an anodizing step of
anodizing the anodic oxide film to form a second pore at the bottom
of each first pore, wherein the second pores have different
diameters to the first pores.
[0028] Further features of the present invention will become
apparent from the following description of exemplary embodiments
with reference to the attached drawings.
BRIEF DESCRIPTION OF THE DRAWINGS
[0029] FIGS. 1A to 1D are diagrams showing steps according to a
first embodiment of the present invention.
[0030] FIGS. 2A to 2C are diagrams schematically showing
cross-sections of pores according to the first embodiment of the
present invention.
[0031] FIG. 3 is a diagram showing an outline of an anodization
apparatus according to the first embodiment of the present
invention.
[0032] FIG. 4 is a diagram showing an outline of a pore-widening
apparatus according to the first embodiment of the present
invention.
[0033] FIG. 5 is a diagram showing an anodic oxide film having
pores known from the prior art.
[0034] FIG. 6 is a sectional view showing a main scanning
cross-section of an optical scanning apparatus according to a
second embodiment of the present invention.
DESCRIPTION OF THE EMBODIMENTS
[0035] The definition of a fine structure grating in the present
invention will be described below.
[0036] For a fine structure grating, a lattice pitch P that
satisfies a condition for a so-called zero-order lattice is
selected. The fine structure grating is referred to as an SWS
(subwave structure).
[0037] The lattice pitch is less than or equal to the order of the
wavelength of a light source to be used (on the order of
nanometers), and the fine structure grating is designed to be used
for zeroth-order light having no diffraction effect (refer to US
Patent Application No. 20020179827 A1).
[0038] A zero-order lattice is a lattice that produces no
diffracted light other than zeroth-order light in a periodic fine
structure grating.
[0039] In a periodic fine structure grating, diffraction light is
generally produced at a diffraction angle that satisfies the
following conditional expression for diffraction: P(Nssin
.theta.m-Nisin .theta.i)=m.lamda. (a) where P is a lattice pitch,
Ni is a refractive index (of a medium of the fine structure
grating) on an incident side, .theta.i is an incident angle,74 m is
an m-order diffraction angle, Ns is a refractive index (of the
medium of the fine structure grating) on an exit side, m is a
diffraction order, and .lamda. is an operating wavelength. As is
obvious from the equation (a), with respect to the diffraction
angle, .theta.m.gtoreq..theta.1 (m=1) holds.
[0040] In the case of normal incidence, the condition under which
no +1st-order diffracted light is produced is defined as
.theta..sub.+1.gtoreq.90.degree. (b) and, therefore, it is clear
that P<.lamda./(Ns+Nisin .theta.i) (c) is the condition for a
zero-order lattice.
[0041] .theta..sub.+1 becomes 90 degrees or more at the most
off-axis position, and hence the lattice pitch P becomes a smaller
Pa. When the incident angle is other than 0 degrees, the lattice
pitch P must be further reduced.
[0042] The embodiments of the present invention will be described
below with reference to the drawings.
First Embodiment
[0043] FIGS. 1A to 1D are explanatory diagrams showing an outline
of the steps of a process for manufacturing a microstructure
(nanostructure) according to the first embodiment of the present
invention.
[0044] In the drawings, reference numeral 1 denotes aluminum (Al)
or an Al alloy, reference numeral 2 denotes an anodic oxide film,
and reference numeral 3 denotes a pore.
[0045] In the present embodiment, when Al or an Al alloy 1 is
anodized in the anodizing step (the first step), an anodic oxide
film 2 including pores 3 having a relatively small pore diameter D
is formed in accordance with the conditions (composition,
temperature, and applied voltage of an acidic electrolytic
solution) for anodization (FIG. 1B).
[0046] The pore diameter (thickness) D of the pore 3 is increased
to a pore diameter Da in the pore-widening step (the second step)
in accordance with the conditions (composition, temperature, and
treatment time of an etching solution) for pore-widening (FIG.
1C).
[0047] In another anodizing step (the third step), another pore
having a pore diameter of Db is further formed at the bottom of the
pore having the pore diameter of Da.
[0048] Consequently, a pore 3 is formed which consists of a pore
having a small diameter Db stepwise disposed at the bottom of the
pore having a large diameter Da (FIG. 1D).
[0049] The anodizing step of the present embodiment is a step in
which Al or an Al alloy is immersed as an anode in an acidic
electrolytic solution, a cathode is similarly immersed, and a
direct-current power supply is connected between the anode and the
cathode to supply power, so that Al or the Al alloy is oxidized and
pores are self-organized by one operation to form an anodic oxide
film.
[0050] The pore diameters of the above-described pores are on the
order of nanometers. The above-described acidic electrolytic
solution is a solution containing at least one of sulfuric acid,
oxalic acid, and phosphoric acid.
[0051] At this time, intervals between the pores can be controlled
to some extent at a few tens of nanometers to a few hundred
nanometers by appropriately selecting the composition, the
temperature, and the applied voltage of the acidic electrolytic
solution, and the depths of the pores can be controlled to some
extent by the application time.
[0052] The pore-widening step is a step in which the anodic oxide
film provided with pores is immersed in an etching solution that is
an acidic or alkaline solution and, thereby, side walls of the
pores are dissolved, so as to increase the pore diameter.
[0053] At this time, the pore diameters can be controlled to some
extent by adjusting the composition, the temperature, and the
treatment time of the etching solution.
[0054] In the present embodiment, the anodic oxide film formed
through the anodizing step and the pore-widening step is anodized
again, so that another pore is further formed at the bottom of the
pore having the pore diameter increased by the pore-widening.
[0055] That is, the anodizing step and the pore-widening step are
performed alternately and, thereby, pores having different
dissolution histories are formed while being stacked in a direction
of the depth of the pore. As a result, a pore having a diameter
varying, specifically decreasing, in the depth direction is
formed.
[0056] The diameter of the pore formed through the above-described
procedure is decreased stepwise or as in a tapered shape by
appropriately selecting the anodizing condition and the
pore-widening condition.
[0057] Since the pore has a wide mouth, when the pores are used as
molds, a different type of material can be filled therein with
relative ease, and can also be released therefrom with relative
ease.
[0058] Each of FIGS. 2A, 2B, and 2C is a sectional view
schematically showing a key portion of a microstructure according
to the first embodiment of the present invention.
[0059] FIG. 2A shows cross-sections of pores 3 formed in the anodic
oxide film 2 when the anodizing step and the pore-widening step are
repeated alternately twice. The pore 3 has a two-step configuration
and the diameter is decreased stepwise in the depth direction.
[0060] FIG. 2B shows cross-sections of pores 3 formed when the
anodizing step and the pore-widening step are repeated alternately
five times. The pore 3 has a five-step configuration and the
diameter is decreased stepwise in the depth direction.
[0061] FIG. 2C shows cross-sections of pores 3 formed when the
anodizing step and the pore-widening step are repeated alternately
a plurality of times (the operation time is fragmented
adequately).
[0062] A pore having a diameter varying smoothly in the depth
direction is formed by performing anodization with fragmentary
operation times and repeating short-time pore-widening. The pore is
in a tapered shape having a diameter decreasing in the depth
direction.
[0063] Anodizing Step
[0064] In the anodizing step of the present embodiment, Al or the
Al alloy is immersed in the acidic electrolytic solution, and a
direct-current voltage is applied between Al or the Al alloy and a
cathode.
[0065] FIG. 3 is a schematic diagram of an anodization apparatus
used in the present step. In the drawing, reference numeral 11
denotes a sample made of Al or an Al alloy, reference numeral 12
denotes a cathode, reference numeral 13 denotes an acidic
electrolytic solution, reference numeral 14 denotes a
direct-current power supply, reference numeral 15 denotes a switch,
reference numeral 16 denotes a reaction vessel, and reference
numeral 17 denotes a constant-temperature water bath.
[0066] In addition, a timer to control the anodization time, an
ammeter to monitor the current, circuits to detect the anodization
time and the current value and to operate the switch, and the like
are incorporated, although not shown in FIG. 3.
[0067] In FIG. 3, the sample 11 and the cathode 12 are immersed in
the acidic electrolytic solution 13 in the reaction vessel 16,
which is kept at a constant temperature by the constant-temperature
water bath 17, and the switch 15 is closed for a predetermined time
to apply a constant voltage from the direct-current power supply
14, so that anodization is effected.
[0068] Examples of acidic electrolytic solutions used for the
anodization include phosphoric acid, oxalic acid, and sulfuric
acid. The conditions of anodization, that is, the composition, the
temperature, the applied voltage, and the application time of the
acidic electrolytic solution, are appropriately set in accordance
with the intervals and depths of the pores to be produced.
[0069] Pore-Widening Step
[0070] In the pore-widening step used in the present embodiment,
the pore diameter is increased by dissolving the side wall of the
pore formed in the anodic oxide film in the anodizing step.
[0071] FIG. 4 is a schematic diagram of a pore-widening apparatus
used in the present step. In the drawing, reference numeral 18
denotes a sample having been subjected to the anodizing step,
reference numeral 19 denotes an etching solution, and reference
numeral 20 denotes a constant-temperature oven.
[0072] In addition, a thermometer to detect the surface temperature
of the sample, a timer to control the pore-widening time, a
handling mechanism to detect the temperature and the pore-widening
time and to take out the sample, and the like are incorporated,
although not shown in the drawing.
[0073] The etching solution used for the pore-widening step is
either an acidic solution or an alkaline solution. The conditions
of pore-widening, that is, the composition, the treatment time, and
the temperature of the etching solution, are appropriately set in
accordance with the diameter of the pore to be produced.
[0074] In the present invention, since the anodizing step and the
pore-widening step must be performed alternately, it is desirable
that the acidic electrolytic solution used in the anodizing step
and the etching solution used in the pore-widening step are the
same type of solution.
[0075] For example, it is particularly desirable to use a
phosphoric acid solution for both the anodizing step and the
pore-widening step.
SPECIFIC EXAMPLES AND COMPARATIVE EXAMPLES
[0076] The specific examples and comparative examples of the method
for manufacturing a microstructure according to the present
embodiment will be described below.
Specific Example 1
[0077] A 3-inch Si wafer having a resistance value of 0.01
.OMEGA.cm or less was prepared, and an Al film of 500 nm was formed
by sputtering. Thereafter, an electrode was fixed to the back, and
the electrode, all over the back, and the wafer side surface were
covered with an insulating waterproof coating, so that a sample was
produced.
[0078] The sample was anodized using an anodization apparatus as
shown in FIG. 3. The sample was immersed together with a cathode in
a 5 percent by weight phosphoric acid aqueous solution controlled
at a temperature of 10.degree. C. in a constant-temperature water
bath. A direct-current power supply was connected, and 120 V was
applied for a predetermined time, so that the Al film was anodized
by one-half the film thickness.
[0079] After the sample was taken out, the surface of the anodic
oxide film was viewed with a scanning electron microscope. It was
observed that pores were formed at intervals of about 300 nm, and
the pore diameter was about 20 nm.
[0080] The sample was subjected to pore-widening by using the
pore-widening apparatus shown in FIG. 4. The sample was immersed in
a 5 percent by weight phosphoric acid aqueous solution controlled
at a temperature of 25.degree. C. in a constant-temperature oven.
After the sample was taken out, the surface of the anodic oxide
film was viewed with a scanning electron microscope. It was
observed that the pore diameter was about 100 nm.
[0081] Anodization was performed again on the same condition by
using the anodization apparatus shown in FIG. 3. The sample was
immersed together with a cathode in a 5 percent by weight
phosphoric acid aqueous solution controlled at a temperature of
10.degree. C. in the constant-temperature water bath. A
direct-current power supply was connected, and 120 V was applied,
so that the Al film was anodized throughout the film thickness.
[0082] After the sample was taken out, the surface of the anodic
oxide film was viewed with a scanning electron microscope. It was
observed that a pore having a pore diameter of about 20 nm was
formed at the bottom of the pore having the pore diameter of about
100 nm.
[0083] The anodic oxide film was cut perpendicularly with a focused
ion beam, and the cross-section was viewed with a scanning electron
microscope. It was observed that the cross-sectional shape of the
pore was narrowed in two steps in the depth direction as shown in
FIG. 2A.
[0084] The depth of the pore was 500 nm.
Comparative Example 1
[0085] A comparative example relative to the above-described
Specific example 1 will be described below.
[0086] Each of the anodizing step and the pore-widening step was
performed once, and the cross-sectional shape of the pore was
observed.
[0087] A sample similar to that in the above-described Specific
example 1 was prepared, and anodization was performed by using the
anodization apparatus shown in FIG. 3. The sample was immersed
together with a cathode in a 5 percent by weight phosphoric acid
aqueous solution controlled at a temperature of 10.degree. C. in a
constant-temperature water bath. A direct-current power supply was
connected, and 120 V was applied, so that the Al film was anodized
throughout the film thickness.
[0088] After the sample was taken out, the surface of the anodic
oxide film was viewed with a scanning electron microscope. It was
observed that the pore diameter was about 20 nm.
[0089] Subsequently, the sample was subjected to pore-widening by
using the pore-widening apparatus shown in FIG. 4. The sample was
immersed for 30 minutes in a 5 percent by weight phosphoric acid
aqueous solution controlled at a temperature of 25.degree. C. in a
constant-temperature oven.
[0090] After the sample was taken out, the surface of the anodic
oxide film was viewed with a scanning electron microscope. It was
observed that the pore diameter was about 100 nm.
[0091] The anodic oxide film was cut perpendicularly with a focused
ion beam, and the cross-sectional shape of the pore was viewed with
a scanning electron microscope. It was observed that the diameter
of the pore was substantially uniform in the depth direction.
[0092] As described above, in Specific example 1, the anodizing
step and the pore-widening step are repeated alternately and,
thereby, the pore diameter of the pore can be varied in the depth
direction, in contrast to Comparative example 1. In this manner, a
microstructure applicable to a higher-performance device can be
provided.
Specific Example 2
[0093] Specific example 2 of the method for manufacturing a
microstructure according to the present invention will be described
below.
[0094] The anodizing step and the pore-widening step were repeated
10 times with fragmentary operation times, and the cross-sectional
shape was observed.
[0095] A sample similar to that in the above-described Specific
example 1 was prepared, and the conditions other than the treatment
time were set similarly to the conditions in Specific example 1.
The anodization time and the pore-widening time were set to be
one-fifth the predetermined time shown in Specific example 1.
[0096] After the anodizing step and the pore-widening step were
repeated 10 times alternately, the surface of the anodic oxide film
was viewed with a scanning electron microscope. It was observed
that the intervals of the pores were about 300 nm.
[0097] The anodic oxide film was cut perpendicularly with a focused
ion beam, and the cross-sectional shape of the pore was viewed with
a scanning electron microscope. It was observed that the pore
diameter was decreased smoothly in the depth direction, as in the
above-described FIG. 2C, the pore diameter was about 200 nm at the
mouth and about 20 nm at the bottom and, therefore, the diameter at
the mouth was large as in the tapered shape shown in FIG. 2C.
[0098] In the present invention, the ratio of the pore diameter
r.sub.1 at the mouth to the pore diameter r.sub.2 at the bottom can
satisfy r.sub.1/r.sub.2.gtoreq.5. In the present embodiment, since
r.sub.1=200 nm and r.sub.2=20 nm, r.sub.1/r.sub.2=10 holds and,
therefore, r.sub.1/r.sub.2>5 is satisfied.
[0099] This is so that, in the case where the fine structure
grating of Specific example 2 is produced by using the
nanostructure shown in FIG. 2C as a mold, the fine structure
grating can be released from the mold with relative ease.
Second Embodiment
[0100] An embodiment is described, in which the fine structure
grating produced by using the nanostructure of the first embodiment
(Specific example 1 and Specific example 2) is applied to the
surface of a scanning lens (imaging optical element) of an optical
scanning apparatus used for an image forming apparatus, e.g., a
laser beam printer or a digital copier.
[0101] Examples of techniques for transferring a fine structure
grating from the nanostructure include for example the methods of
injection molding, press molding, replication, electrolytic
deposition, and imprinting. However, injection molding, press
molding, and replication are particularly desirable since the fine
structure grating can be produced together with a base
material.
[0102] In the present embodiment, an example in which the
application is performed by injection molding as a manufacturing
means for a scanning lens (imaging optical element) is
described.
[0103] A mold having a free-form surface (lens surface) to mold a
scanning lens (imaging optical element) was prepared. Uniform films
of a primer layer and then an aluminum layer were formed on the
free-form surface by sputtering, so that a free-form surface
covered with aluminum was produced.
[0104] A positive electrode was attached to a part of surfaces
other than the free-form surface, and the entire mold was covered
with a masking tape to expose the free-form surface only, so that
those other than the free-form surface were brought into an
insulating waterproof state.
[0105] Subsequently, the aluminum film was anodized by one-half the
film thickness, and was dissolved to increase the pore diameter of
the pore. The remaining aluminum film was anodized again, and the
pore diameter was increased again, so that pores having the pore
diameter decreased stepwise were formed on the mold surface.
[0106] In the anodization, the film was immersed together with a
negative electrode in a 5 percent by weight phosphoric acid aqueous
solution controlled at a temperature of 10.degree. C., and the
direct-current voltage of 120 V was applied for a predetermined
time, so that pores were formed up to about one-half the thickness
of the aluminum film.
[0107] The pore diameter was increased to about 100 nm by immersion
in a 5 percent by weight phosphoric acid aqueous solution to effect
etching.
[0108] The film was immersed again together with the negative
electrode in the 5 percent by weight phosphoric acid aqueous
solution controlled at a temperature of 10.degree. C.
[0109] The direct-current voltage of 120 V was applied and
energization continued until the passage of current became
sufficiently feeble, so that the second stage pores were
formed.
[0110] Thereafter, immersion in a 5 percent by weight phosphoric
acid aqueous solution at room temperature was performed to effect
etching. Consequently, pores were produced on the mold surface in
such a way that a pore having a diameter of about 100 nm was formed
stepwise at the bottom of each pore having a pore diameter of about
200 nm.
[0111] The mold produced by the above-described procedure was
placed on each of the incident surface side and the exit surface
side, and a cycloolefin polymer (produced by ZEON Corporation) was
injection-molded with an injection molding machine (SS180: produced
by Sumitomo Heavy Industries, Ltd.), so that a scanning lens was
produced.
[0112] At this time, the temperature of a molten resin was set at
270.degree. C., and the pressure was kept at 700 kg/cm during
injection of the resin. When the resin was released from the mold,
the releasing was performed so that the resin (fine structure
grating) filled in the pores was able to elastically deform.
[0113] The incident surface of the thus produced scanning lens was
viewed with a scanning electron microscope. It was observed that
fine structure gratings were randomly disposed all over the
free-form surface and the diameters were decreased in two
steps.
[0114] It was ascertained that an individual lattice (projection)
stood in the direction of the normal of the surface at an interval
of about 300 nm.
[0115] P-polarized laser beam with a wavelength of 780 nm was
incident perpendicularly on the scanning lens and the reflectance
was measured. It was observed that the reflectance was reduced to
one-quarter the reflectance of a mirror-finished surface with no
fine structure grating.
[0116] In the second embodiment, fine structure gratings 108 are
disposed on the surfaces of the scanning lens (imaging optical
element) in order to prevent reflection of the light beam incident
on the scanning lens (imaging optical element).
[0117] In recent years, scanning lenses (imaging optical elements)
of scanning optical systems are generally produced from plastic
since aspherical shapes are easily configured and are manufactured
easily.
[0118] However, with plastic lenses it is difficult to apply an
antireflective coating to the lens surface for reasons of
technology and cost. Therefore, Fresnel reflection occurs at each
lens surface.
[0119] Consequently, in the second embodiment, the fine structure
gratings 108 are disposed on the incident surface and the exit
surface of the scanning lens (imaging optical element).
[0120] In the present specification, a direction in which a light
beam is reflected and deflected (deflected and scanned) by a
deflection device is defined as a main scanning direction, and a
direction orthogonal to the optical axis of an imaging optical
system and the main scanning direction is defined as a subscanning
direction.
[0121] In FIG. 6, reference numeral 101 denotes a light source
device composed of, for example, a semiconductor laser. Reference
numeral 102 denotes a condenser lens (collimator lens) for
converting a divergent light beam emitted from the light source
device 101 into a substantially collimated light beam or a
convergent light beam.
[0122] Reference numeral 103 denotes an aperture stop for shaping a
beam shape by regulating the passage of the light beam.
[0123] Reference numeral 104 denotes a cylindrical lens that has a
predetermined power in only the subscanning direction and forms an
image of the light beam passed through the aperture stop 103 on a
deflection surface (reflection surface) of an optical deflector
105, described below, as a substantially linear image in a
subscanning cross-section.
[0124] Reference numeral 105 denotes the optical deflector serving
as a deflection device. The optical deflector 105 is composed of,
for example, a polygon mirror having a tetrahedral arrangement
(rotating polyhedral mirror), and is rotated by a driving device,
e.g., a motor (not shown in the drawing) at a constant rate in a
direction indicated by an arrow A in the drawing.
[0125] Reference numeral 106 denotes an image formation lens system
serving as an imaging optical system having a beam-focusing
function and f.theta. characteristics, and including two scanning
lenses (imaging optical elements), that is, a first scanning lens
106a and a second scanning lens 106b, made of a plastic material
(transparent resin material).
[0126] The light beam that is based on image information and
reflected and deflected by the optical deflector is made to form an
image on the surface to be scanned 107. In addition, a function for
compensating for tilt is provided by establishing a conjugated
relationship between the deflection surface 105a of the optical
deflector 105 and the surface to be scanned 107 in the subscanning
cross-section.
[0127] In general, a photosensitive drum is disposed on the surface
to be scanned.
[0128] The light beam from the light source device 100 may be
incident directly on the optical deflector 105 without using the
above-described optical elements 102, 103, and 104.
[0129] Each of the lens surfaces of the two scanning lenses, that
is, the first and second scanning lenses 106a and 106b, is in the
shape of a spherical or aspherical curved surface in the main
scanning cross-section shown in FIG. 6.
[0130] Each of the lens surfaces of the two scanning lenses, that
is, the first and second scanning lenses 106a and 106b, is
basically in the shape of a known special aspherical surface having
a curvature varying as the position changes from an on-axis
position (center of scanning) in an off-axis position (periphery of
scanning) in the subscanning cross-section.
[0131] In the second embodiment, fine structure gratings 108 are
disposed all over the incident surface (the surface nearest to the
optical deflector) 106a1 and the exit surface 106a2 of the first
scanning lens and the incident surface 106b1 and the exit surface
(the surface nearest to the surface to be scanned 107) 106b2 of the
second scanning lens.
[0132] These fine structure gratings 108 are configured to perform
an antireflective function in accordance with the incident angle of
the light beam incident on the scanning lens, so that the reflected
beam from the lens surface of the imaging optical system 106 is
restricted to become incident on the surface to be scanned 107.
[0133] In the present invention, as described in the second
embodiment, the fine structure grating including periodically
arranged projections of nanometers (nm) produced by using the
nanostructure of Specific example 1 and Specific example 2 (shown
in FIGS. 2A to 2C) is applied to the scanning lens. Therefore, an
excellent antireflective property is achieved.
[0134] The shape of the fine structure grating is a shape in which
the diameter is decreased in the height direction.
[0135] While the present invention has been described with
reference to exemplary embodiments, it is to be understood that the
invention is not limited to the disclosed exemplary embodiments.
The scope of the following claims is to be accorded the broadest
interpretation so as to encompass all modifications, equivalent
structures and functions.
* * * * *