U.S. patent application number 12/034478 was filed with the patent office on 2008-09-25 for articles comprising nanoscale patterns with reduced edge roughness and methods of making same.
This patent application is currently assigned to PRINCETON UNIVERSITY. Invention is credited to Stephen Y. Chou, Wei Wu, Zhaoning Yu.
Application Number | 20080230947 12/034478 |
Document ID | / |
Family ID | 46300500 |
Filed Date | 2008-09-25 |
United States Patent
Application |
20080230947 |
Kind Code |
A1 |
Chou; Stephen Y. ; et
al. |
September 25, 2008 |
Articles Comprising Nanoscale Patterns With Reduced Edge Roughness
and Methods of Making Same
Abstract
In accordance with the invention, an article comprising a
nanoscale surface pattern, such as a grating, is provided with a
nanoscale patterns of reduced edge and/or sidewall roughness.
Smooth featured articles, can be fabricated by nanoimprint
lithography using a mold having sloped profile molding features.
Another approach uses a mold especially fabricated to provide
smooth sidewalls of reduced roughness, and a third approach adds a
post-imprint smoothing step. These approaches can be utilized
individually or in various combinations to make the novel
articles.
Inventors: |
Chou; Stephen Y.;
(Princeton, NJ) ; Yu; Zhaoning; (Levittown,
PA) ; Wu; Wei; (Mountain View, CA) |
Correspondence
Address: |
POLSTER, LIEDER, WOODRUFF & LUCCHESI
12412 POWERSCOURT DRIVE SUITE 200
ST. LOUIS
MO
63131-3615
US
|
Assignee: |
PRINCETON UNIVERSITY
Princeton
NJ
|
Family ID: |
46300500 |
Appl. No.: |
12/034478 |
Filed: |
February 20, 2008 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
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10732038 |
Dec 10, 2003 |
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12034478 |
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10046594 |
Oct 29, 2001 |
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10732038 |
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09107006 |
Jun 30, 1998 |
6309580 |
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10046594 |
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08558809 |
Nov 15, 1995 |
5772905 |
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09107006 |
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60432213 |
Dec 10, 2002 |
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60432216 |
Dec 10, 2002 |
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Current U.S.
Class: |
264/225 |
Current CPC
Class: |
B29C 2043/023 20130101;
B29C 2043/025 20130101; B29C 43/003 20130101; G03F 7/0002 20130101;
B29C 33/62 20130101; B29C 59/022 20130101; B29C 43/222 20130101;
G03F 9/7053 20130101; B82Y 40/00 20130101; B29C 33/60 20130101;
B29C 2059/023 20130101; B29C 59/026 20130101; B82Y 10/00 20130101;
B29C 43/021 20130101 |
Class at
Publication: |
264/225 |
International
Class: |
B29C 59/02 20060101
B29C059/02 |
Goverment Interests
STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH
[0003] The invention was sponsored in part by the Government under
grant 170-6394 and the Government has certain rights to the
invention.
Claims
1. A method of making an article comprising a nanoscale surface
pattern having reduced edge and/or sidewall roughness comprising
the steps of: providing a mold with a molding surface having one or
more nanoscale protruding regions having smooth sidewalls with a
surface roughness less than about 5 nanometers, the protruding
regions arranged complementary to the pattern; providing a work
piece with a moldable surface; pressing together the molding
surface and moldable surface to reduce the thickness of the
moldable surface under the protruding features; and separating the
mold from the moldable surface.
2. The method of claim 1 wherein providing the mold comprises the
steps of: providing a mold substrate of (110) crystalline material
with a nanoscale pattern of etch resistant material;
anisotropically etching the masked mold substrate with a wet
etchant having an etching rate in the (111) crystal plane slower
than the etching rate in the (100) and (110) planes to etch smooth
sidewalls.
3. The method of claim 2 wherein the article comprises a mold for
imprinting a nanoscale surface pattern.
4. A method of masking an article comprising a nanoscale surface
pattern having reduced edge and/or sidewall roughness comprising
the steps of: providing a mold with a molding surface having one or
more protruding regions arranged complementary to the pattern;
providing a work piece with a moldable surface of material having a
glass transition temperature; pressing together the molding surface
and the moldable surface to reduce the thickness of the moldable
surface under the protruding features, thereby imprinting the
pattern on the moldable surface; separating the mold from the
moldable surface; and heating the moldable surface above the glass
transition temperature to smooth the edges and sidewalls of the
imprinted pattern.
5. The method of claim 4 wherein the article comprises a mold for
imprinting a nanoscale surface pattern.
Description
CROSS-REFERENCE TO RELATED APPLICATIONS
[0001] The present application claims priority from, and is a
division of, U.S. patent application Ser. No. 10/732,038 filed on
Dec. 10, 2003, which in turn, is a continuation-in-part of U.S.
patent application Ser. No. 10/046,594 filed on Oct. 29, 2001,
which claims priority to U.S. patent application Ser. No.
09/107,006 filed on Jun. 30, 1998 (now U.S. Pat. No. 6,309,580
issued Oct. 30, 2001) and which, in turn, claims priority to U.S.
patent application Ser. No. 08/558,809 filed on Nov. 15, 1995 (now
U.S. Pat. No. 5,772,905 issued Jun. 30, 1998). The foregoing '038,
'594, '006, and '809 applications are each incorporated herein by
reference.
[0002] The '038 Application further claims the benefit of U.S.
Provisional Application Ser. No. 60/432,213 filed on Dec. 10, 2002
and also claims the benefit of U.S. Provisional Application Ser.
No. 60/432,216 filed on Dec. 10, 2002. The foregoing '213 and '216
provisional applications are each incorporated herein by
reference.
BACKGROUND OF THE INVENTION
[0004] Nanoscale patterned articles, such as nanoscale gratings and
wires, have many important applications in optics, electronics,
biotechnology, and micro-fluidics. They can be used to filter and
direct light, to facilitate fabrication of nanoscale mechanical and
electronic devices, and to analyze biological molecules.
[0005] A typical nanoscale patterned article comprises a substrate
with a microscopically patterned surface. A nanoscale grating, for
example, can comprise a substrate, such as silicon or resist coated
silicon, having a surface array of protruding parallel lines
separated by intervening recessed lines. The lines can have a
minimum dimension of under 100 nanometers and the spacing between
successive lines can be on the order of 200 nm or less. Other
articles use different surface patterns of comparably small feature
size.
[0006] The roughness of line edges and sidewalls in such nanoscale
surface patterns has an important bearing on device performance.
Studies have shown that roughness causes scattering loss in optical
devices, impedes electron transport through nanoscale wires and
degrades performance in bio-analytic and micro-fluidic systems.
[0007] A variety of approaches have been proposed for the
fabrication of smooth nanoscale surface patterned devices, but most
are unsuitable for large-scale production. Previous fabrication
methods include electron-beam lithography and interference
lithography. Electron beam lithography, however, is a serial
processing technique of inherently low throughput. Interference
lithography is affected by random factors such as disturbances and
instabilities in the exposure system which contribute to
roughness.
[0008] Other approaches to reducing roughness include anisotropic
wet etching and thermal oxidation of pattern sidewalls with
etch-back. Anisotropic wet etching, however, can only be used on a
limited class of crystalline materials. And thermal oxidation
requires high temperature processing incompatible with many
desirable materials.
[0009] Nanoimprint lithography (NIL) is a promising approach to
patterning smooth nanoscale features. In NIL, a nanofeatured
molding surface is typically imprinted into a surface, such as a
polymer-coated substrate. The imprinted pattern can then be coated,
as with metal, or the imprinted material can be selectively removed
to expose the substrate surface for further processing. Further
details concerning nanoimprint lithography are set forth in
applicant's U.S. Pat. No. 5,772,905 issued Jun. 30, 1998 and
entitled "Nanoimprint Lithography" and U.S. Pat. No. 6,482,742
issued Nov. 19, 2002 and entitled "Fluid Pressure Imprint
Lithography." The '905 and '742 patents are incorporated herein by
reference.
[0010] The present invention provides articles comprising nanoscale
patterns with reduced edge and sidewall roughness through
adaptations in NIL processing.
BRIEF SUMMARY OF THE INVENTION
[0011] Briefly stated, the present disclosure provides an article
comprising a nanoscale surface pattern, such as a grating, is
provided with a nanoscale patterns of reduced edge and/or sidewall
roughness. Smooth featured articles, can be fabricated by
nanoimprint lithography using a mold having tapered profile molding
features. Another approach especially fabricates the mold to
provide smooth sidewalls of reduced roughness, and yet a third
approach provides the article with a post-imprint smoothing step.
These approaches can be utilized individually or in combination to
make the novel smooth featured articles.
[0012] The foregoing features, and advantages set forth in the
present disclosure as well as presently preferred embodiments will
become more apparent from the reading of the following description
in connection with the accompanying drawings.
BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWINGS
[0013] In the accompanying drawings which form part of the
specification:
[0014] FIG. 1 is a cross-sectional view showing the steps in
fabricating a triangle profile mold;
[0015] FIG. 2 graphically illustrates the silicon etching rate in
(111) direction as a function of temperature in a KOH:H2O:isopropyl
alcohol mixture;
[0016] FIG. 3 is a scanning electron micrograph of a triangle
profile mold etched in a (100) silicon substrate with a 200 nm
grating period. FIG. 3A shows the grating before the removal of the
original mask. FIG. 3B shows the grating with the mask removed.
[0017] FIG. 4 shows steps in creating a triangle-profile relief
pattern in resist, by first pressing a triangle mold into the
resist, and then removing the mold from the resist.
[0018] FIG. 5A and FIG. 5B are scanning electron micrographs (SEMs)
of triangle profile resist gratings created by nanoimprinting with
triangle profile molds, with periods of 1 .mu.m and 200 nm,
respectively.
[0019] FIG. 6 shows steps in the creation deposited masks on the
distal tips of the resist triangles using shadow evaporation and
then removing the uncovered portions of the resist to expose the
underlying substrate.
[0020] FIG. 7 is a scanning electron micrograph of the top view of
a 200 nm period grating with square profile and smooth sidewalls
transferred into the silicon dioxide layer on top of a silicon
substrate.
[0021] FIG. 8 is a scanning electron micrograph of a 200 nm period
grating with square profile in silicon dioxide.
[0022] FIG. 9A is a schematic a cross-sectional view of a triangle
resist grating during shadow evaporation. The line width of the
evaporated mask wires is determined by the apex angle of the
triangles in the resist and the angle of incidence.
[0023] FIG. 9B graphs the calculated and measured grating duty
cycle of a grating as a function of the angle of incidence for
shadow evaporation, when a wet-etched triangle profile mold in
(100) silicon substrate was used, and the resist was shadowed twice
from opposite directions.
[0024] FIG. 10 illustrates that a triangle mold for imprinting, can
achieve better line-width uniformity despite variations in the
original mask.
[0025] FIG. 11 is a cross-sectional view showing mold separation
for triangle and square profile molds.
[0026] FIG. 12 compares experimentally measured peak separation
forces for different mold profiles;
[0027] FIG. 13 illustrates fabrication of a anisotropic-etching
nano-grating mold. 13A shows a substrate of (110) Si with a
Si0.sub.2 grating mask aligned in the <110> direction and
patterned using interference lithography. 13B shows a grating
pattern transferred into Si using KOH wet etch. 13C illustrates a
Si0.sub.2 mask removed using buffered HF etch.
[0028] FIG. 14 shows a grating in (110) Si after the KOH wet-etch,
with the Si0.sub.2 mask still in place. Although the SiO.sub.2 mask
shows rough edges, the relief grating structure in Si has extremely
smooth sidewalls.
[0029] FIG. 15 is a SEM of a 200 nm period grating mold etched into
(110) Si after removal of the Si0.sub.2 mask. Smooth sidewalls are
obtained.
[0030] FIG. 16 is a schematically illustrates pattern duplication
by NIL. In 16A the mold and substrate are pressed together. On 16B
the mold and substrate are pressed together when the temperature is
elevated to make the resist viscous. In 16C the mold is separated
from the substrate after cooling. In 16D, an O.sub.2 RIE is used to
remove remaining resist in the recessed regions.
[0031] FIGS. 17A and 17B illustrate a 200 nm period resist grating
patterned by NIL using a (110) Si mold.
[0032] FIG. 18 illustrates reducing edge roughness by NIL and
thermal annealing. 13A shows grating with rough sidewalls is
patterned in the resist by NIL. 18B illustrates RIE carried out to
isolate neighboring lines. 18C shows the resist becomes viscous
after being heated up, resulting in a grating pattern with reduced
roughness.
[0033] FIG. 19 is a SEM of a 200 nm period resist grating showing
the effect of thermal annealing on roughness reduction. 19A depicts
a resist grating with rough edges before the thermal treatment. 19B
shows the edge roughness of the resist grating is significantly
reduced after a 100.degree. C., 10 min thermal treatment.
[0034] FIG. 20 is a schematic perspective view illustrating
features of an article comprising a nanoscale patterned
surface.
[0035] FIG. 21 is a schematic perspective view illustrating
features of an article comprising a nanoscale pyramidal patterned
surface.
[0036] Corresponding reference numerals indicate corresponding
parts throughout the several figures of the drawings. It is to be
understood that the drawings are for illustrating the concepts set
forth in the present disclosure and are not to scale. Before any
embodiments of the invention are explained in detail, it is to be
understood that the invention is not limited in its application to
the details of construction and the arrangement of components set
forth in the following description or illustrated in the
drawings.
DETAILED DESCRIPTION
[0037] The following detailed description illustrates the invention
by way of example and not by way of limitation. The description
enables one skilled in the art to make and use the present
disclosure, and describes several embodiments, adaptations,
variations, alternatives, and uses of the present disclosure,
including what is presently believed to be the best mode of
carrying out the present disclosure.
[0038] The description is divided into four parts. Part I describes
an exemplary article having a nanoscale pattern with smooth edges
and/or sidewalls, and Parts II, III and IV describe approaches for
making such articles.
[0039] I. Exemplary Article
[0040] Referring to the drawings, FIG. 20 is a schematic top view
of an exemplary article 100 comprising a nanoscale patterned
surface 101. Typical useful nanoscale surface patterns 101 are
comprised of a plurality of protruding features 102 and one or more
recessed features 103 having at least one protruding feature with a
minimum lateral dimension I of less than 100 nm. State of the art
is less than 25 nm and as small as 10 nm or less. The depth d
between a protruding feature and a recessed feature is typically
less than 250 nm and can be as small as 5 nm. Molds for making the
patterns have complementary patterns with correspondingly small
dimensions. In accordance with the invention, the surface patterns
are provided with smooth pattern edges 104 and/or smooth pattern
sidewalls 105 having reduced roughness as compared with
conventional nanoscale surface patterns. The article 100 differs
from conventional nanoscale gratings in that the edges 102 have a
low roughness of less than about 10 nanometers (i.e. no protrusions
from the edge larger than 10 nanometers will appear on an SEM) and
preferably less than about 5 nanometers and/or the sidewalls 103
have a low roughness of less than about 5 nanometers.
[0041] A particular embodiment of such an article can be a large
area sub-200 nanometer period grating. In a typical grating, the
protruding features form an array of parallel lines separated by
intervening recessed regions. Such gratings are cornerstone
structures for many applications. They can be used in UV optical
filters, polarizers, sub-wavelength optical devices and ultrahigh
density patterned magnetic media.
[0042] II. NIL Using Tapered Molds to Reduce Pattern Edge
Roughness
[0043] Unlike previous imprint-based patterning techniques, the
present approach uses grating molds with tapered and preferably
triangle-shaped profiles. The complement of the triangle profile
relief pattern on the mold is then transferred into a resist thin
film carried on a substrate by pressing the mold into the resist
and removing the mold.
[0044] In essence, this first approach to making a nanoscale
surface patterned article with smooth pattern edges and sidewalls
comprises the steps of 1) providing a mold with a molding surface
having a plurality of tapered regions; 2) providing a work piece
with a moldable surface; 3) pressing the molding surface and the
moldable surface together to reduce the thickness of the moldable
surface under the protruding features. This step produces reduced
thickness regions; and 4) separating the mold and the moldable
layer. The work piece can then be further processed in the
patterned regions to complete a nanoscale surface patterned
article.
[0045] In exemplary subsequent steps, metal (or other suitable
material) can be coated selectively onto the tips of the resist
triangles through oblique angle deposition (e.g. shadow
evaporation). After removing the portions of the resist that are
not covered by the evaporated material to expose the underlying
substrate, the grating pattern in the resist can be replicated in a
material that is added onto the substrate or can be replicated
directly into the substrate.
[0046] This approach offers many advantages over the prior art.
First, the mold can be patterned to achieve extremely smooth (on
the atomic level) sidewalls. The smoothness can be preserved and
replicated in the resist (and the underlying substrate) because of
the high-resolution (<10 nm) of nano-imprint lithography. Thus
the approach produces patterns with smoothness unattainable by the
prior art.
[0047] Second, this approach offers an advantageous way for
controlling the grating duty cycle simply by changing the angle of
incidence for shadow evaporation. Different line-widths can be
obtained even when using the same mold. Our experiments show the
line-width is linearly dependent on the angle of incidence, and it
provides a line-width tunable range from 25% to 75% of the grating
period. This is believed unachievable with the prior art.
[0048] Thirdly, the approach utilizes the natural crystalline
orientations of the material that makes up the mold body. The
resulting triangle grating lines will have the same apex angle in
spite of the possible variations of line-width in the original
pattern. Thus, the approach standardizes the mold topology and, at
the same time, improves line-width uniformity across a wafer and
from wafer to wafer.
[0049] We now describe how to provide a mold with tapered
projecting features in FIG. 1 and how to use the mold to make a
nanoscale patterned article in FIG. 4 et seq.
[0050] FIG. 1A shows an exemplary mold body 12 which can comprise a
crystalline material. Crystals of silicon are preferred but other
crystalline materials can be used. The surface of the mold body
(which will be patterned with the desired features) carries a mold
mask layer 10. This surface of body 12 can be prepared in a way so
that it is parallel to one of the (100) crystalline planes of the
mold body material. The mask layer 10 can be grown or deposited
through any appropriate technique such as thermal oxidation or
chemical vapor deposition (CVD).
[0051] The mask layer 10 is then patterned into mask portions 14
(here a grating) to expose portions 16 of the mold surface (FIG.
1B) using suitable patterning techniques such as interference
lithography, imprint lithography, electron beam lithography, or
pattern transfer techniques such as lift off or reactive ion
etching (RIE).
[0052] FIG. 1C shows the grating pattern etched into the mold
surface using a suitable anisotropic etching process such as wet
chemical etching. Generally this etching process should be highly
selective in its etching rates between the directions normal to
(100) and (111) crystal planes of the material comprising the mold
body, with the etching rate normal to (100) plane higher than the
rate normal to (111) plane. This anisotropy is chosen to produce a
tapered triangle-shaped etching profile, exposing the highly smooth
(111) crystal planes 18. FIG. 1D shows the cross-sectional view of
the finished mold after the removal of the remaining mask material
14.
[0053] In one experiment, the mold body 12 was made in a (100)
silicon substrate, with the mask material 10 being a layer of
thermally grown silicon dioxide. The thickness of the oxide
typically ranges from 30 nm to 300 nm. The mask layer 10 was then
patterned into portions 14 of a 200 nm period grating using
interference lithography and reactive ion etching.
[0054] The substrate was briefly dipped into a diluted hydrofluoric
acid (HF) to remove oxide that may have remained in the regions 16
between the mask lines 14. A mixture of 500 g potassium hydroxide
(KOH), 1600 ml deionized (DI) water, and 400 ml isopropyl alcohol
(IPA) was used for the wet chemical etching step indicated in FIG.
1C. The etching selectivity between the direction normal to (100)
plane and the direction normal to (111) plane is greater than
20.
[0055] FIG. 2 shows the measured etching rate normal to (111) plane
(R.sub.111) as a function of the temperature for this wet etching
recipe. Data in FIG. 2 indicates the etching rate is less sensitive
to temperature variations at 65.degree. C., which was chosen for
the etching of the mold to achieve better uniformity across a
4-inch wafer. The optimum etching time depends on the grating
period, line-width of the mask 14, and the material of the mold
body 12. For this embodiment using (100) silicon substrates and a
200 nm grating period, the etching time usually lies between 30 and
90 seconds. A triangle profile mold was obtained after the
remaining oxide mask 14 was finally removed using hydrofluoric
acid.
[0056] It should be understood that methods for creating a triangle
profiled grating mold are not limited to those described here. For
example, instead of patterning a deposited film 10, material can be
added onto the substrate to create the grating mask 14 through
suitable means such as evaporation and lift-off.
[0057] FIG. 3B is a scanning electron micrograph of a perspective
view of a triangle-profile grating mold created in (100) silicon
using the steps illustrated in FIG. 1. The surfaces of these
features are extremely smooth due to the wet chemical etching. FIG.
3A is the grating with the original rough-edged mask lines, which
clearly shows the effectiveness of wet-etching in reducing edge
roughness.
[0058] FIGS. 4A-4C show steps using the triangle-profile grating
mold 20 for patterning resist using nanoimprint lithography. A work
piece comprising an imprint resist thin film 24 supported on a
substrate 26 is brought in contact with the mold 20. The mold is
pressed into the resist 24, typically after heating to allow
sufficient softening of the resist 24. Mold 20 is removed from the
resist 24 after cooling to leave an imprinted relief pattern 28 in
the resist 24, which compliments the shape of the features 46 in
the mold, e.g. recessed regions 46 in the mold shape protruding
features 28 in the resist.
[0059] FIG. 5A and FIG. 5B are scanning electron micrographs of
cross-sections of resist gratings patterned by nanoimprint
lithography using triangle-profile molds in accordance with the
steps described in FIG. 4. The resist patterns conform well to the
shapes of the features on the molds because of the high-resolution
(<10 nm) of nanoimprint lithography.
[0060] FIGS. 6A-6D show how to create a resist grating with smooth,
almost vertical sidewalls starting from a triangle-profile relief
pattern 28 in the patterned resist. FIG. 6A shows the resist
profile after imprinting. FIG. 6B shows a layer of suitable mask
material 30 (which can be metal, dielectric, or semiconductor,
ceramic or a combination thereof) deposited onto one side of resist
triangles 28 through an oblique angle coating process, such as
shadow evaporation. FIG. 6C shows another optional deposition step
consisting of oblique angle coating the tips of the resist
triangles 28 from another side of the grating. FIG. 6D shows the
step of removing the resist in the regions unprotected by the mask
material 30 by an anisotropic etching process (reactive ion
etching, chemical etching, etc.) to expose the portions 32 of the
underlying substrate 26 between the mask lines 30.
[0061] After this removal step, the grating pattern can be
replicated in a material that is added on substrate 26 or can be
replicated directly into substrate 26 by etching.
[0062] FIG. 7 is a scanning electron micrograph of a top view of a
200 nm period grating formed in a layer of silicon dioxide on a
silicon substrate using steps depicted in FIG. 1, FIG. 4, and FIG.
6. FIG. 8 is a scanning electron micrograph of a cross-sectional
view of such a grating. The sidewalls of the gratings are extremely
smooth due to the wet-chemical etching mold preparation. This
degree of smoothness remains an elusive and most often,
unattainable goal for conventional production techniques.
[0063] In addition to the extreme smoothness of the sidewalls, the
present approach provides a convenient way to control grating
line-width. FIG. 9A shows an embodiment in which the mask material
30 is coated by shadow evaporation on both sides of the resist
triangle 28 in two consecutive deposition steps. The line-widths in
the final replicated pattern are determined by the width 40 of the
portions of the resist protected by the oblique angle coated mask
30. This width can be easily changed by adjusting the angle of
incidence 34 for the oblique coating step. The change depends on
the geometry of the triangle profile of the grating, described by
angle 32. In the case of using a wet-etched (100) silicon mold, the
angles 32 and 36 in the resist are 70.52.degree. and 54.74.degree.,
respectively. Preferably the angle 32 is in the range 25.degree. to
75.degree..
[0064] FIG. 9B shows calculated and experimentally measured grating
duty cycles (the ratio of line-width 40 over period 38) as a
function of the angle of incidence for the shadow evaporation. The
data in FIG. 9B shows that the duty cycle can be easily changed
over a range from 25% to 75%, which is about 400% wider than the
reported range of from 50% to 60% when using interference
lithography.
[0065] Also, since the duty cycle/angle of incidence dependence is
almost linear, the change of duty cycle can be achieved simply by
using different angles of incidence, without the need of any
modification of the mold or the imprinted resist profile. Thus the
duty cycle can be readily varied or controlled in a production
line.
[0066] In contrast, in conventional interference lithography, the
change of grating duty cycle is typically achieved by using
different doses for the exposure of photo-resist, the duty
cycle/dose dependence generally is not linear, the tuning range is
small, the process is hard to control and not easily repeatable
because line-widths are also affected by factors such as random
disturbances and instability of the exposure system. The present
approach also offers the added benefit for improving grating
line-width uniformity, as depicted in FIG. 10.
[0067] FIG. 10A.1 and FIG. 10A.2 show two wet-etched molds (or two
different portions on the same mold) masked by gratings 14 with
different line-widths. FIG. 10B.1 and FIG. 10B.2 show the resist
profiles (together with the shadow-evaporation coated mask 30)
imprinted using these two molds, correspondingly. Although the
resist triangles have different heights, they all have the same
apex angle, since the apex angle is solely determined by the
orientations of the crystal planes of the mold body material. FIG.
10C.1 and FIG. 10C.2 show that after removing resist in the regions
unprotected by mask 30 using an anisotropic etching process, the
gratings in both cases will have the same line-width, because the
line-width is affected only by the covered upper part of the resist
triangles. Thus even with line-width variations from sample to
sample, the present invention can improve the line-width uniformity
by compensating the differences through the process depicted in
FIG. 10.
[0068] Good mold release properties are important in fabricating
nanoscale features by nanoimprint lithography. Using a triangle
shaped grating profile instead of a square greatly facilitates mold
release.
[0069] There are two contributing factors to this improvement:
First, for two molds with the same grating period 38 and feature
depth 42 but with different profiles (triangle and square) as shown
in FIG. 11A and FIG. 11B, the contact area with the resist for a
triangle profile mold 20 is only about 65% of the contact area for
a square mold 44. This means a reduction in the total surface
energy that needs to be overcome for mold separation. Second, the
mold separation processes are different in the two cases. For a
square mold 44, the sidewalls of the protruding features on the
mold remain in contact with the resist until the tops of the mold
features are moved out of the openings of the recesses in the
patterned resist (FIG. 11B), which increases resistance to the mold
release. FIG. 11A shows the removal of a triangle mold 20 from the
resist 24, once the mold is raised relative to the substrate, there
is no further contact between the mold features and the printed
resist. The absence of "stickiness" in this case facilitates
separation by reducing the resistance.
[0070] FIG. 12 shows the measured peak separation forces/unit mold
area for molds of different profiles. The period of the gratings is
200 nm, both molds have the same feature height of 150 nm and they
were treated together with the same surfactant, and the same resist
(NP-60) was used in the experiment. The separation force for a flat
mold (with no pattern) is also plotted for comparison.
[0071] The measurement clearly indicates that the triangle profile
greatly reduced the total force needed to separate the mold from
the resist, compared with a mold with square profile. Easier
separation implies more flexibility in designing and choosing the
imprint resists, which is very important and valuable for the
implementation of large-scale production of nano-gratings by
nano-imprint lithography.
[0072] III. NIL Using Smooth Walled Molds to Reduce Pattern Edge
Roughness
[0073] Because line edge roughness in the master mold will also be
duplicated in the resist, it is desirable to reduce sidewall
roughness of the master mold. This approach to making a nanoscale
surface patterned article with smooth pattern edges and sidewalls
comprises the steps of 1) providing a mold with a molding surface
having a plurality of protruding regions with smooth walls; 2)
providing a work piece with a moldable surface; 3) pressing the
molding surface and the moldable surface together to reduce the
thickness of the moldable surface under the protruding features to
produce reduced thickness regions; and 4) separating the mold from
the moldable surface. The work piece can then be further processed
in the patterned regions in accordance with methods well known in
the art.
[0074] The wet etching technique used in the mold fabrication step
illustrated in FIG. 1 provides desirable smooth side walls.
However, as we will show in connection with FIG. 13, that wet
etching can even improve molds having vertical rather than tapered
sidewalls.
[0075] FIG. 13 is a schematic of an exemplary process for the
fabrication of smooth-walled grating molds. Instead of a (100) Si
substrate, a (110) Si substrate 12A is used as the mold substrate.
60 nm thick oxide 10 was then thermally grown on the substrate.
Gratings were carefully aligned parallel the (111) crystal plane
during interference lithography and were later transferred into the
oxide layer using a CHF.sub.3 reactive ion etching (RIE) process to
form masking elements 14A. (FIG. 13A). A KOH:deionized
water:isopropyl alcohol anisotropic wet-etch was used to further
transfer the grating into the underlying (110) Si substrate, with
the oxide serving as an etching mask. (FIG. 13B) Because the
etching rate in the (111) crystal plane direction is much slower
than the etching rates in the directions of (100) and (110) plane,
this process creates a Si grating mold 20A with extremely smooth
substantially vertical sidewalls 18A. (FIG. 13C). FIG. 14 shows the
effect of this anisotropic etching process, although the original
grating in the oxide shows a high degree of edge roughness, this
raggedness is not reproduced in the underlying Si grating
sidewalls. Finally, as shown in FIG. 15 the oxide mask was removed
using a buffered HF wet-etch.
[0076] Grating patterns are duplicated onto a surface patterned
article using those Si surface relief gratings as master molds by
NIL, the schematic of which is shown in FIG. 16. FIG. 16A shows the
provision of a mold 20B having a smooth molding surface, and a work
piece comprising a substrate 26B having a moldable surface 24B such
as a resist. The molding surface has a plurality of protruding
regions with extremely smooth substantially vertical sidewalls 18B
as described above.
[0077] FIG. 16B illustrates pressing the molding surface and the
moldable surface together to reduce the thickness of the moldable
surface under the protruding features. This pressing produces
reduced thickness regions.
[0078] In FIG. 16C, the mold 20B is separated from the moldable
surface 24B leaving an imprinted pattern.
[0079] FIG. 16D shows the work piece further processed, as by
etching away the moldable material in the reduced thickness
regions, to produce an article with a nanoscale patterned surface.
Because NIL is a high resolution (sub-10 nm) lithography, line-edge
smoothness of the master molds is retained during the duplication
process.
[0080] FIG. 17A shows a top-view of the grating in NIL resist after
imprinting using the smooth Si mold, and FIG. 17B shows the resist
profile. The resist grating has vertical and smooth sidewalls, and
pattern contrast is high. These are desirable characteristics not
easily achievable using conventional interference lithography.
[0081] IV. NIL Using Post-Imprinting Treatment to Reduce
Roughness
[0082] A third approach uses a moldable surface that becomes
viscous when heated. The approach comprises the steps of providing
a work piece with a moldable surface of such material, patterning
the moldable surface into a nanoscale surface pattern and then,
after the patterning, heating the moldable surface material to its
viscous state. In a preferred embodiment, the method comprises the
steps of 1) providing a mold with a molding surface having a
plurality of protruding regions; 2) providing a work piece with a
moldable surface having a glass transition temperature; 3) pressing
together the molding surface and the moldable surface to reduce the
thickness of the moldable surface under the protruding features to
produce reduced thickness regions; 4) separating the mold from the
moldable surface; and 5) heating the moldable surface above the
glass transition temperature.
[0083] This approach can be used to even further smooth moldable
surfaces imprinted by the tapered mold process of FIG. 4 or the
smooth side-wall molding process of FIG. 16. It can also be used to
smooth surfaces imprinted by even rough sidewall conventional
molds.
[0084] FIG. 18 illustrates an example of this approach to
smoothing. A grating mold is patterned as by interference
lithography. The mold can have rough sidewalls. The mold is then
imprinted into a substrate 26C having a molding surface 24C that
has a glass transition temperature. After imprinting, a grating
with rough edges was reproduced in the imprint resist. (FIG. 18A)
Then an O.sub.2 RIE process was carried out to remove the remaining
resist in the recessed region and to isolate the neighboring lines
from each other (FIG. 18B). After RIE, the sample was heated so the
resist becomes viscous. Because a smooth surface is energetically
favorable, this thermal treatment will result in a pattern of
resist elements 24D with rounded profile and significantly reduced
surface roughness (FIG. 18C).
[0085] FIG. 19 shows the effect of thermal annealing on the edge
roughness. In this experiment we used a polystyrene-based resist
which has a flow temperature of 100.degree. C. FIG. 19A is the top
view of the resist nano-grating after imprinting using a rough-edge
grating mold. The sample was then baked at 100.degree. C. on a
hot-plate for 10 minutes. FIG. 19B shows a top view of the resist
pattern with smooth sidewalls after the thermal treatment.
[0086] It should be pointed out that although we used a specific
imprint resist NP-60 in this experiment, this technique is not
limited to this single resist. It can be applied to other
thermoplastic polymers such as PMMA with similar thermal
characteristics as well.
[0087] Using these NIL-based line edge roughness reduction
techniques, we have successfully fabricated nano-scale gratings
over large area (4-inch wafer) on various substrates. The smallest
grating pitch achievable is around 190 nm, which is determined by
the wavelength of the laser (351.1 nm) used in interference
lithography.
[0088] These nano-scale gratings with smooth sidewalls have many
important applications, including sub wavelength optics,
micro/nano-fluidic devices and bio-analysis for the manipulation of
single biological molecules.
[0089] In sub wavelength optical applications, because transmission
loss increases as the second exponential of roughness' as light
propagates inside the gratings, smoothing technologies can be used
for the fabrication of highly-efficient sub wavelength devices.
[0090] Recently we have fabricated and demonstrated a fluidic
device consisting of sealed nano-channels so that double-stranded
DNA molecules can be stretched and moved along these channels.
Fabrication of super smooth gratings is a critical step in this
application because of the small dimensions (<100 nm) of the
channels, resistance encountered by the molecules as they move in
the channels increases drastically as the sidewall roughness
increases.
[0091] In conclusion, we have developed sidewall smoothing
technologies based on nanoimprint lithography. Using these
technologies, we have fabricated nano-gratings with extremely
smooth sidewalls over large area. Compared with other sidewall
smoothing technologies, ours are low-cost, effective and can be
applied to a variety of materials and substrates. These smooth
gratings have a variety of applications in optics,
micro/nano-fluidic devices and biotechnology.
[0092] It should be understood that the invention is not limited to
the specific techniques and materials described herein, and may be
implemented in any appropriate forms. For example other types of
materials can be used for the mold body. Besides Si0.sub.2, other
materials (e.g. Si.sub.3N.sub.4, metal, and polymer) and processes
(e.g. lift-off and electron-beam lithography) can be used to
pattern the initial wet-etch mask. Other etching processes and
conditions can be used to etch the desired features into the mold.
In some embodiments, a dry plasma etching using appropriate gases
can be utilized in mold preparation to achieve a similar etching
selectivity between different crystal planes, so that a comparable
effect as wet chemical etching can be obtained.
[0093] It should also be pointed out that although the fabrication
of one-dimensional grating structure is used here for the purpose
of demonstration. The same principle can be applied to the
fabrication of two-dimensional structures as well.
[0094] To fabricate a two-dimensional structure, the mask 14
depicted in FIG. 1 will comprise a two-dimensional array of dots or
holes. Using the steps described in FIG. 1, a mold 200 with
pyramidal surface features 201 (or the reverse of the pyramids,
depending on whether the original two-dimensional mask pattern is a
dot array or an array of holes) can be formed as shown in FIG.
21.
[0095] Pattern transfer using the two-dimensional mold can be
carried out in the same spirit as the steps described in FIG. 4 and
FIG. 5. The fabricated features can be arrays of dots or holes,
depending on the mold and the particular pattern transfer process
used. However, the same principle of feature dimension control by
using different incident angle 34 as depicted in FIG. 9A also
applies in this case.
[0096] It can now be seen that one aspect of the invention is a
nanoscale patterned article having on at least one surface a
pattern comprising a plurality of protruding features and recessed
features, at least one protruding feature having smooth edges,
smooth sidewalls and a minimum lateral dimension between the
sidewalls, the edges and sidewalls having a maximum surface
roughness less than 5 nanometers and the minimum lateral dimension
less than 100 nanometers. Typically a plurality of the protruding
features protrude from the surface by a distance in the range 5 to
250 nanometers. In a typical embodiment, a plurality of protruding
features comprise an array of parallel lines separated by recessed
features to form a grating.
[0097] Another aspect of the invention is a method of making an
article comprising a nanoscale surface pattern having reduced edge
and/or sidewall roughness. The method comprises the steps of: 1)
providing a mold with a molding surface having one or more
nanoscale protruding regions tapering toward the end distal the
surface, the protruding regions arranged complementary to the
pattern; 2) providing a work piece with a moldable surface; 3)
pressing together the molding surface and the moldable surface to
reduce thickness of the moldable surface under the protruding
features; and 4) separating the mold from the moldable surface.
[0098] A third aspect of the invention involves the use of very
smooth mold sidewalls. Specifically, a method of making an article
comprising a nanoscale surface pattern having reduced edge and/or
sidewall roughness comprising the steps of: 1) providing a mold
with a molding surface having one or more nanoscale protruding
regions having smooth sidewalls with a surface roughness of less
than about 5 nanometers, the protruding regions arranged
complementary to the pattern; 2) providing a work piece with a
moldable surface and the moldable surface; 3) pressing together the
molding surface and the moldable surface to reduce the thickness of
the moldable surface under the protruding features; and 4)
separating the mold from the moldable surface.
[0099] Yet another aspect of the invention involves the use of a
moldable surface that becomes viscous upon heating. In a preferred
embodiment, it is a method of making an article comprising a
nanoscale surface pattern having reduced edge and/or sidewall
roughness comprising the steps of: 1) providing a mold with a
molding surface having one or more protruding regions arranged
complementary to the pattern; 2) providing a work piece with a
moldable surface of material having a glass transition temperature;
3) pressing together the molding surface and the moldable surface
to reduce the thickness of the moldable surface under the
protruding features, thereby imprinting the pattern on the moldable
surface; 4) separating the mold from the moldable surface; and 5)
heating the moldable surface above the glass transition temperature
to smooth the edges and sidewalls of the imprinted pattern.
[0100] As various changes could be made in the above constructions
without departing from the scope of the disclosure, it is intended
that all matter contained in the above description or shown in the
accompanying drawings shall be interpreted as illustrative and not
in a limiting sense.
* * * * *