U.S. patent application number 17/394223 was filed with the patent office on 2022-06-30 for methods for creating large-area complex nanopatterns for nanoimprint molds.
The applicant listed for this patent is The Trustees of Princeton University. Invention is credited to Stephen Y. CHOU, Fei DING.
Application Number | 20220205920 17/394223 |
Document ID | / |
Family ID | |
Filed Date | 2022-06-30 |
United States Patent
Application |
20220205920 |
Kind Code |
A1 |
CHOU; Stephen Y. ; et
al. |
June 30, 2022 |
METHODS FOR CREATING LARGE-AREA COMPLEX NANOPATTERNS FOR
NANOIMPRINT MOLDS
Abstract
Some embodiments of the invention provide methods that can
create large area complex patterns for nanoimprint molds without or
with very litter of the use of the charged beam or photon beam
direct-writing of nanostructures. Some embodiments of the invention
use (i) Fourier nanoimprint patterning (FNP), (ii) edge-guided
nanopatterning (EGN), and (iii) nanostructure self-perfection, and
their combinations.
Inventors: |
CHOU; Stephen Y.;
(Princeton, NJ) ; DING; Fei; (Princeton,
NJ) |
|
Applicant: |
Name |
City |
State |
Country |
Type |
The Trustees of Princeton University |
Princeton |
NJ |
US |
|
|
Appl. No.: |
17/394223 |
Filed: |
August 4, 2021 |
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16038963 |
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International
Class: |
G01N 21/65 20060101
G01N021/65; B29C 37/00 20060101 B29C037/00; B29C 59/02 20060101
B29C059/02; G03F 7/00 20060101 G03F007/00; B29C 33/56 20060101
B29C033/56; G01N 21/64 20060101 G01N021/64 |
Claims
1. A method for creating a complex periodic nanopattern,
comprising: (a) obtaining a fig nanoimprint mold having a pattern
that is a first Fourier components of the complex nanopattern; (b)
patterning a surface of a daughter mold substrate by (i) imprinting
using the first mold in a resist, (ii) transferring the imprinted
resist pattern onto the daughter mold to create the first Fourier
component on the daughter mold, and (iii) removing the resist; (c)
obtaining a second nanoimprint mold having a pattern that is a
second Fourier component of the complex nanopattern; (d) performing
(b) using the second nanoimprint mold; and (e) optionally,
repeating steps (a) to (d), until the final pattern on the daughter
mold is substantially similar to the complex nanopattern.
2. The method of claim 1, wherein the first nanoimprint mold or the
second nanoimprint mold has a period of 200 nm.
3. The method of claim 1, wherein the first nanoimprint mold or the
second nanoimprint mold has periodic patterns that are linear
periodic grating.
4. The method of claim 1, wherein the first nanoimprint mold or the
second nanoimprint mold has periodic patterns that are periodic
arrays of pillars.
5. The method of claim 1, wherein material of the first nanoimprint
mold or the second nanoimprint mold is plastic.
6. The method of claim 1, wherein material of the first nanoimprint
mold or the second nanoimprint mold is silicon dioxide.
7. A method for creating a mold of a periodic nanopattern that is
superposition of periodic patterns, comprising the steps in
sequence: (a) obtaining a master nanoimprint mold having a period
pattern of a period P; (b) having a daughter mold substrate for
creating a daughter mold; (c) creating, on a surface of the
daughter mold substrate, a first etching masking layer using
nanoimprint lithography with the master nanoimprint mold followed
by a liftoff; (d) creating, on the surface of the daughter mold
substrate, a second etching masking layer using nanoimprint
lithograph with the master nanoimprint mold and a liftoff, wherein
during the nanoimprint lithography the periodic pattern on the
master nanoimprint mold is, relative to the periodic pattern
created in (c), rotated by an angle or shifted by a distance, and
wherein the second etching masking layer is superpostioned on the
first etching masking mask to form a superposition mask; and (e)
transferring the pattern of the superposition mask onto the
daughter mold substrate, by etching the daughter mold substrate,
using the superposition mask; wherein in the nanoimprint
lithography of (c) and (d), the mold imprinting into a nanoimprint
resist; wherein in the liftoff of (c) and (d), the masking material
is deposited onto the imprinted resist profile and the daughter
mold surface that is not covered by the resist, and the imprint
resist is, after the evaporation, is removed, removed, which
removes the masking material deposited on the resist and keeps the
masking the material deposited on the daughter mold surface.
8. The method of claim 7, wherein the patterns of the
superstructure mask is a periodic structure that has a period that
is equal to the period of the master mold.
9. The method of claim 7, wherein the patterns of the
superstructure mask is a periodic structure that has a period that
is larger than the period of the master mold.
10. The method of claim 7, wherein the method further comprises the
steps that repeat the steps of (a) to (e) in claim 7, wherein the
master mold is the daughter mold formed in claim 1.
11. The method of claim 7, wherein the method further comprises the
steps of: (i) modifying, after step (e), the structures on the
daughter mold; (ii) repeating, after (i) the step (a) to (e),
wherein the master mold is the daughter mold formed in (i).
12. The method of claim 8, wherein the period of the master mold
has a period of 200 nm.
13. The method of claim 7, wherein the periodic patterns of the
master mold is linear periodic grating.
14. The method of claim 7, wherein the periodic patterns of the
master mold is a periodic array of pillars.
15. The method of claim 7, wherein the masking material is Cr
(chromine).
16. The method of claim 7, wherein the mold material is
plastic.
17. The method of claim 7, wherein the mold material is silicon
dioxide.
18. The method of claim 7, wherein the method further comprises a
step of self-perfecting.
19. The method of claim 7, wherein the method further comprise a
step of edge patterning.
Description
CROSS-REFERENCING
[0001] This application claims the benefit of: provisional
application Ser. No. 61/801,424, filed Mar. 15, 2013 (NSNR-004PRV),
provisional application Ser. No. 61/801,096, filed Mar. 15, 2013
(NSNR-005PRV), provisional application Ser. No. 61/800,915, filed
Mar. 15, 2013 (NSNR-006PRV), provisional application Ser. No.
61/793,092, filed Mar. 15, 2013 (NSNR-008PRV), provisional
Application Ser. No. 61/801,933, filed Mar. 15, 2013 (NSNR-009PRV),
provisional Application Ser. No. 61/794,317, filed Mar. 15, 2013
(NSNR-010PRV), provisional application Ser. No. 61/802,020, filed
Mar. 15, 2013 (NSNR-011PRV) and provisional application Ser. No.
61/802,223, filed Mar. 15, 2013 (NSNR-012PRV), all of which
applications are incorporated by reference herein for all
purposes.
BACKGROUND
[0002] One of the most critical obstacles in developing large-area
nanosystems is lack of the ability to nanomanufacture over large
areas. Today we have the tools and technologies to make
nanostructures in small area (e.g. microchips), or microstructures
in large area (e.g. large screen TVs); but we do not have the tools
and technologies to make complex nanostructures over large areas.
For examples, the direct writing of nanostructures by charged
(electron and ion) or photonic beams is too slow for large area.
This has severely impeded the development and commercialization of
many large area nanosystems. Clearly a viable solution to the
problem can revolutionize the field. The current invention is
related to the methods that can over this problem. The methods
invented can create large area complex patterns for nanoimprint
molds without or with very litter of the use of the charged beam or
photon beam direct-writing of nanostructures
SUMMARY
[0003] The following brief summary is not intended to include all
features and aspects of the present invention, nor does it imply
that the invention must include all features and aspects discussed
in this summary.
[0004] The invention is related to creating large area complex
patterns for nanoimprint molds.
BRIEF DESCRIPTION OF THE DRAWINGS
[0005] The skilled artisan will understand that the drawings,
described below, are for illustration purposes only. The drawings
are not intended to limit the scope of the present teachings in any
way. Some of the drawings are not in scale.
[0006] FIG. 1. Multi-set nanopatterning (MSN)based on multiple
processing by different nanoimprint molds to form the final mold.
Example-1 is a large area 100 nm nanodot array formed by
fabrication using a large area nano-grating mold. Example-2 shows
that MSN can use a large-area nano-grating mold to create
large-area nano-dots, which then to the nano-rings, which then the
split nano-rings.
[0007] FIG. 2. One embodiment of Multi-set nanopatterning (MSN) is
to "convert" microstructures on a large area mold to
"nanostructures" and to add new microstructures.
[0008] FIG. 3. Multi-set nanopatterning (MSN), a new innovative
path-changing approach, offers a viable solution to a central
challenge in nanomanufacturing (including nanoimprint): generation
of complex nanostructures over large-area (>1 meters) without
using electron beam lithography or the like.
[0009] FIG. 4. Shows some of the principles of interference
lithography.
[0010] FIG. 5. Schematic of generation of nanopatterns with varying
shape, spacing and density by FNP. (a) Double imprint cycles of a
linear grating with an angle (e.g. 858) to create a pillar array
mold. (b) Double cycles of imprinting of the pillar array mold to
the final patterns of pillars with varying shape, spacing and
density. (chou group
[0011] FIG. 6. Nanodots array with varying shape, spacing and
density by FNP. The rotation angle is 85 degree. Note the gaps as
small as 2 nm are produced.
[0012] FIG. 7. Nanodots array with varying shape, spacing and
density by FNP. The rotation angle is 30 degree, which produces a
period short than 85 degree. Again 2 nm gaps are produced.
[0013] FIG. 8. SEM of ring array with 200 nm pitch, 40 nm and 140
inner and outer diameter, respectively. Fabricated by FNP and
EGN.
[0014] FIG. 9. Fabrication process of nanoimprint mold for
split-ring devices: (a) SiO2 pillars fabricated by interference
lithography and nanoimprint; (b) conformal SiNx growth over the
pillars fabricated in (a); (c) etching down SiNx by reactive ion
etching to expose the SiO2 pillar; (d) oblique evaporation of Cr
using the SiO2 pillar as shadow mask; (e) etching into shadowed
SiNx to make a cut on the SiNx ring; (f) removal of SiO2 pillar by
HF.
[0015] FIG. 10. SEM pictures of the split-ring fabrication. (a)
Conformal SiNx deposition; (b) Selectively etching down SiNx to
expose central SiO2 pillar; (c) Single cut on the surrounding SiNx
ring. (Scale bar: 200 nm).
[0016] FIG. 11. SEM of single split ring mold by FNP.
[0017] 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.
[0018] 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 OF EXEMPLARY EMBODIMENTS
[0019] The following detailed description illustrates some
embodiments of the invention by way of example and not by way of
limitation.
[0020] The following patent applications are incorporated by
reference in their entirety: 61/802,020, filed Mar. 15, 2013 and
provisional application Ser. No. 61/802,223, filed Mar. 15,
2013.
[0021] The methods invented, that can create large area complex
patterns for nanoimprint molds without or with very litter of the
use of the charged beam or photon beam direct-writing of
nanostructures, comprise several basic methods and different of
combination of basic methods, that lead to the final desire complex
over large area. The technology is termed Multi-set nanopatterning
(MSN) and also termed "compositional imprint lithography" (CIL).
The two terms are interchangeable in this disclosure.
[0022] The basic method A, that creates complex structures on a
substrate surface comprises multiple steps of fabrication of
daughter molds and multiple superposition of nanoimprint of
periodic patterns.
[0023] For examples, as shown in FIG. 1, we start with a linear
grating and after created a daughter linear grating of the same
period, we will do the second imprint with the same linear grating
mater with rotating 90 degree angle to create a pillar daughter
mold. Now the pillar daughter can become master and being used
twice imprint and etching to create aperiodic lattices (see later
section of the disclosures). Such lattice are very useful for light
extraction in light emitting diodes (LEDs). There many different
ways to create complex patterns by using different cycles of the
same mater mold, and then use different daughter molds to create
granddaughter mold and grand grand daughters, until the final
desired complex patterns are achieved. This can be very large area,
since nanoscale grating can be produced today meter size. Such
periodic pattern superposition is in fact a Fourier transform, with
each periodic pattern representing a Fourier component. Detailed
analysis and examples are given in the paper attached here as a
part of the application.
[0024] The basic method B is to have pattern of either micron or
nanosize, by shadow evaporation patterns can formed at the edges of
these patterns, and its lateral dimension is determined by the film
thickness, in this way micro patterns become nanoscale patterns.
One also can oxidize or conformal depositions to form nanometer
edges patterns.
[0025] By doing multiple steps of A and B to create daughter and
granddaughter and grand grand daughter molds, complex patterns over
large area can be created without using the writing of a focused
beam.
[0026] In other embodiments, the MSN is based on the two
principles: (a) the large-area nanoimprint mold with the desired
nanostructures can be composed (fabricated) by using several
primary molds with each of them having only a part of the
structures on the final mold, and (b) the nanostructures on a
primary mold can be made by "converting" a microstructure to a
nanostructure, or "converting" large-area simple nanostructures
(e.g. by interference lithography) into large area complicated
structures using conventional micro fabrication methods. Each of
two processes can be used intermixed and multiple times to build
very complicated nanostructures over large area. Once a large area
master mold is made, it can be duplicated to multiple copies for
mass production.
[0027] FIG. 2 illustrates that a nanogate/nanowire can be
fabricated by using edge patterning (film deposition or shadow
evaporation) of microstructure. The Example-1 in FIG. 1 shows that
a large area 100 nm dots can be formed by two cycles of nanoimprint
and etching using a large area grating mold (the second imprint
rotates the mold by 90 degree). The Example-2 shows the SEMs
(demonstration) that MSN can use a large-area nano-grating mold to
form large-area nano-dots, which then to form the nano-rings, which
then the split nano-rings, and which then split-nano-ring. The ring
is formed by oxidizing a pillar or by conformal thin film
deposition of pillar (and etch away the unwanted parts). The
(single or double) split of the ring is formed by shadow
evaporation. All these fabrication steps (except nanoimprint) use
only the conventional microfabrication processes.
[0028] One central issue in nanomanufacturing is to generate
complex nanopatterns over large area with high-throughput and low
cost, particularly for the feature size less than 100 nm and
pattern pitch less than 200 nm (i.e. below 100 nm node in
semiconductor ICs) in an area larger than 100 cm2 (or even to wall
pager size). Advance in this area will have significant impact to a
wide range of industrial applications, well beyond semiconductor
ICs, such as new materials, solar cells, solid state lighting, fuel
cells, data storage, optical communication, displays,
biotechnology, to name a just a few.
[0029] For fabricating sub-100 nm patterns of sub-200 nm pitch,
scanning laser does not have sufficient resolution nor the needed
throughput, scanning electron lithography is too slow, and deep-UV
lithography (main workhorse for semiconductor ICs) is too expensive
for the most products outside of IC and is unapplicable to the
flexible or thin film substrates and/or the area larger than 300 mm
diameter.
[0030] Nanoimprint is regarded as one of emerging technologies that
will change the world and one of the most important manufacturing
technologies in the 21.sup.st century.
[0031] However, nothing can be imprinted without a mold (template);
one must have a mold first in nanoimprint. Thus, nanoimprint faces
the same challenge as other nanomanufacturing: generation complex
nanostructures over large area--although in nanoimprint, such
generation is needed only once: the master mold. Hence, making the
master mold with large area complex nanostructures is the most
serious obstacle or the Achilles in nanoimprint.
[0032] Clearly, conventional approaches are impossible to meet
needs for nanoimprint master mold fabrication and
nanomanufacturing. Hence, a solution to large-area complex
nanostructure generation will have transformative impact to
nanomanufacturing, and nanotechnology.
[0033] The invention is related to the methods to generate complex
nanostructures over a large area without using electron beam
lithography (EBL) or the like.
[0034] The proposed research is to explore a new innovative
path-changing approach, termed "multi-set nanopatterning" (MSN),
that offers a viable solution to a central challenge in
nanomanufacturing (including nanoimprint):
[0035] The MSN comprises three innovative nonconventional
technologies and their creative superposition(s) (i.e. multiple
uses). The three technologies are (i) Fourier nanoimprint
patterning (FNP), (ii) edge-guided nanopatterning (EGN), and (iii)
nanostructure self-perfection. Just each individual technology
alone, it already can create new complex nanostructures over a
large area that could not be generated before; but when combined
together, they can generate far more complex nanopatterns over a
large area. Furthermore, the proposed research will advance new
approaches in nanoimprint mold duplication and will use large area
nanoplasmonics and roll-to-roll nanoimprint as test bed for the
technologies to be developed. The research outcomes are expected to
have transformative impacts to nanomanufacturing and multiple
multi-billion-dollar industrial fields.
[0036] The invention is related to a number of different new
approaches for large-area complex nanostructure generation for many
years. Based on the innovation developments made by our group and
others, plus new concepts we generated recently, the proposed
research aims at a new innovative approach to generate large area
complex nanostructures without using EBL, termed
"multi-set-nanopatterning" (MSN). MSN includes three types of
paradigm-shift nanofabrication technologies as well as creative
superposition(s) (i.e. multiple uses and different combinations) of
them (FIG. 2). The three paradigm-shift technologies are (i)
Fourier nanoimprint patterning (FPN) (FIG. 5), (ii) edge-guided
nanopatterning (EGN) (FIG. 2.2), and (iii) nanostructure
self-perfection by liquefaction (SPEL). These technologies will be
developed with the test bed of nanoplasmonics for solar cells,
solid state lighting and other optical applications, and of
roll-to-roll nanoimprint master molds.
[0037] Fourier Nanoimprint Patterning (FNP) creates a desired final
complex nanopattern by superpositioning its Fourier components.
Specifically, the final nanopattern is a result of several
nanoimprints of "simple patterns", each of them is one or several
Fourier components of the final nanopattern (e.g. gratings or grids
with different periods) The superposition is done by sequentially
adding a new simple pattern onto the patterns that are already
imprinted and fabricated on a substrate. FNP can generate
large-area complex nanostructures, because (a) currently large-area
linear nanogratings become commercially available, and (b)
nanoimprint allows making daughter molds for intermediate and final
nanostructures to be made, which greatly simplifies the fabrication
process and cost of FNP.
[0038] Edge-guided nanopatterning (EGN) uses an edge(s) of an
existing micro/nanoscale pattern to create smaller and/or complex
patterns. For example, EGN "converts" a micro-rectangle into a
nanowire, and a nano-cylinder into nanoring; and for a lesser known
example, a micro-rectangle into nanosplit and a symmetric
micropattern into an asymmetric nanopattern (FIG. 10, 11).
[0039] Nanostructure self-perfection is a class of methods that
changes an imperfect structure into perfect one. The main focus
here will be "nanostructure self-perfection by liquefaction"
(SPEL), although other self-perfection methods will also be
used.
1. Fourier Nanoimprint Patterning (FNP)
[0040] Fourier nanoimprint patterning (FNP) creates a final complex
nanopattern by superpositioning several nanoimprints of "simple
patterns", where each of them is one or several Fourier components
of the final nanopattern. The superposition is done by adding the
simple pattern sequentially onto the patterns that are already
imprinted (and fabricated) on a substrate (FIG. 5).
[0041] FNP can create complex nanopatterns over an area, as large
as several meter squares, because of four facts: (1) every pattern
is a superposition of Fourier components which in turn are either
simple linear gratings themselves or a superposition of linear
gratings; (2) large-area (square-meters area) nanogratings are now
commercially available; (3) multiple methods can be used to
manipulate some Fourier components, and (4) nanoimprint can be used
to make daughter molds for FPN's intermediate nanostructures, which
greatly simplifies the fabrication process and cost of FNP.
[0042] The FNP concept was originated by the PI in 1998 for forming
large-area periodic nanopillar/hole array]. Recently, the PI
proposed new approaches to further advance FNP concept for
fabrication of much more complex nanostructures over a large area,
which will be a key part of the proposed research. Below, we use
some of our work as examples to illustrate FNP and discuss the
proposed research in FNP. However, it should be pointed out: FNP
can be used many different ways to generate different
nanostructures, hence the illustrations here are just a very small
set of broad possibilities.
Example 1.1 Making Large-Area Nanopillar or Nanogrid Array by
FNP
[0043] A 2D nanopillar/nanogrid array can be generated by
superpositioning two linear gratings (each represents one Fourier
component) with one in x-direction and another in y-direction.
Specifically, first, we fabricated a master mold of 200 nm period
linear grating over entire 4'' wafer by interference lithography
(FIG. 4)]. In FNP, the linear grating master mold is used to
fabricate a daughter mold in two cycles of nanoimprinting and Cr
lift-off. The first cycle creates a Cr linear grating on the
daughter mold substrate. The second cycle, which puts the master
mold perpendicular to the direction of the first Cr grating,
generates a final Cr pattern of nanogrid on the daughter mold
substrate. Finally, Cr grid is used as an etching mask in etching
the substrate and is removed after the etching, leaving a 2D
nanogrid daughter mold (FIG. 3).
[0044] Many variations can be used here. For examples, (a) if the
rotation angle of the master mold in the second imprint and
lift-off cycle is different from 90 degree, the parallelogram shape
array will be generated; (b) the use of two different period of
linear grating creates rectangle grid, rather than square grid; (c)
with a nanogrid mold, nanopillar mold can be created by imprint and
etching; and (d) the square pillars can be changed into circular
pillars with a smooth vertical sidewall by SPEL (See Task-3) and
other techniques.
Example-1.1 Making Nanopillar Array with Varying Period,
Separations by FNP
[0045] In many applications, they need nanostructures that have a
broad band of resonant frequencies rather than a single frequency
for manipulating light. Such applications include solar cells and
LEDs which are broad band optical devices. A broad band frequency
can be only achieved by using nanostructures that are equivalent of
a superposition of multiple different-frequency Fourier components
(i.e. multiple different period linear gratings). To have
polarization independence, it requires the nanopatterns symmetrical
in both x and y-directions. Such broad brand structures are very
difficult to generate by conventional approaches.
[0046] Recently, we invented in a new innovative method, termed
"double Imprint of pillar-array" (DIP), to generate such broad-band
patterns using FNP. The principle of DIP is to create a final
complex broad-band structure by double cycles of nanoimprint and Cr
deposition using a pillar-array mold. The pillar mold was made, in
turn, by double cycles of imprinting/fabrication of a linear
grating mold, and the two gratings are not in 90 degrees
(orthogonal direction) but having h a rotation angle offset of
.theta..sub.1 (FIG. 5). And in the double imprinting of the pillar
mold, the two imprints are further offset by another angle offset
of .theta..sub.2. After using Cr as an etching mask to etch the
substrate, the 3D Cr become a complex 2D patterns in the daughter
mold, that have nanopillar array with linearly varying pillar
shape, spacing (pillar period) and density. For using 200 nm period
grating, we have demonstrated a 1 nm/200 nm pitch linear increment
in pillar's shape and spacing (FIG. 6A, B).
[0047] Mathematically, we have:
L = p sin .function. ( .theta. 2 ) ##EQU00001## .DELTA. = 2 .times.
p sin .function. ( .theta. 1 ) .times. sin .function. ( .theta. 2 2
) ##EQU00001.2##
[0048] where L is the repeat unit length of the superpositioned
pattern (i.e. the (rhombus) edge length in FIG. 6B); p is the
original linear grating period; and .DELTA. is defined as the
linear increment of the center-to-center distance of the adjacent
dots along the long range pattern unit edges.
[0049] When .theta..sub.1.apprxeq.90.degree. and
.theta..sub.2.apprxeq.0.degree., it can be further simplified to be
.DELTA.=p.theta..sub.2=p.sup.2/L.
For example, if grating pitch is 200 nm and misalignment angle is
0.5.degree., the long range rhombus unit edge length will be L=22.9
.mu.m, with the linearly increment as .DELTA.=1.74 nm/200 nm pitch.
Sub-nm increment could be achieved with this method, if the second
alignment mismatch angle can be further reduced (e.g. smaller than
0.28.degree. for 200 nm pitch). Moreover, the density of the dots
will be doubled after the center-to-center distance goes larger
than the dot diameter, or, in other words, after the dots separate.
Detailed derivation of the center positions of pillars is shown in
supplement material.
[0050] Clearly, such broad pillar arranges have enormous
application solar cells, solid state lighting, chemical and
bio-sensing and other areas, where there are enormous developments
in devices (including the PI's group).
Example 1.3 Other FPN Patterns Methods
[0051] Clearly, there are unlimited ways to do FPN to create
desired final structure (the only limitation might be our
imagination). If m is the number of times of nanoimprint of linear
grating masters/etchings, the final superpositioned pattern will be
mathematically given by [Chou, et al, patent appl.]
M = i = 1 m .times. .times. a i .times. sin .function. ( k .fwdarw.
r .fwdarw. + b i ) ##EQU00002##
[0052] Where for the i.sup.th grating {right arrow over (k.sub.i)}
is the grating wavevector (|{right arrow over
(k.sub.i)}|=2.pi./p.sub.i, p.sub.i=period of i.sup.th grating, and
the direction is normal to the grating direction), {right arrow
over (r)} is the position vector in x-y plane, a.sub.i is the
i.sup.th amplitude, and b.sub.i is the phase difference (duo to the
linear shift).
[0053] To explore the new desired nanopatterns over a large area by
FNP, we will, in the proposed research: (1) study and improve the
FNP's feature size control and fidelity; (2) further explore
various nanostructures made by FNPs that are important for solar
cells, solid state lighting, and bio/chemical sensing, particularly
those demonstrated in Sec. 12; and (3) use, improve, and develop
the FNP methods that can reduce the final nano-pattern's period and
feature sizes. (Note: Currently, we produce 200 nm period grating
on 4'' wafers and get 140 nm period commercially, which is very
expensive for the proposal budget).
2. Edge-Guided Nanopatterning (EGN)
[0054] Edge-guided nanopatterning (EGN) uses an edge(s) of an
existing micro/nanoscale pattern to guide the creation of smaller
and/or complex nanopatterns. EGN provides three powerful and unique
capabilities in nanofabrication. (1) EGN creates a nanostructure
from a microstructure and does not need nanostructure mask/mold. A
well-known example is a shadowing or deposition on the sidewall of
a microstructure. Therefore large area microstructures (e.g.
display size) can be created first and then are turned into
large-area nanostructures (FIG. 7). The PI used such an approach to
fabricate 60 nm MOSFETs about .about.30 years ago; and EGN is also
used in modern IC fabrication, being called "double patterning".
(2) EGN changes nanostructure into different often hard-to-make
shapes. And (3) EGN creates aperiodic structure from a periodic
structures. Let us give examples in (2) and (3).
Example 2.1 Nano-Rings from Nano-Pillars by EGN
[0055] Once a large-area nano-pillar array is fabricated (e.g.
using FPN discussed before), it can be turned into a large-area
nano-ring array by EGN. One way to do it is to conformably deposit
a thin layer on the pillars and etch away the materials deposited
on the pillar's top and foot, but not on the sidewall, and then
selectively etching away the pillars, leaving the materials
deposited on the sidewall on the substrate, forming the ring array
(e.g. SiO2 pillars with SiNx as the deposited material). In case of
Si pillars, the conformal material deposition also can be replaced
by oxidization of Si (e.g. the nano-rings we fabricated FIG. 8)
Example 2.2 Nanoscale Single-Split-Rings from Nano-Pillars by
EGN
[0056] Large-area nanoscale single-split ring array also can be
fabricated from nanopillar array using EGN. The first few steps of
the fabrication are similar to that for the nanoring array. But
during the anisotropic (vertically) etching of the deposited
material on the top and bottom of the pillar, the etching time is
made to be sufficient longer, so that after the etching the height
of the material deposited on pillar sidewall is much lower than
pillar height, making a part of the pillar stick out (e.g. SiO2
pillar SiNx conformal deposition). Then a shadow evaporation of Cr
from an angle will deposit Cr everywhere on the sample surface,
except behind the stick-out pillar (similar to the shadow of a
telephone pole under the 10 am sun). The Cr will be used as an
etching mask, and the etching will etch only the area that is not
covered by the Cr (i.e. the shadow), which cuts through each ring.
After removing the Cr and the pillars, a single-split ring mold is
generated (FIG. 9. 10, 11).
Example 2.3 Nanoscale Double-Split-Ring Array from Nano-Pillar
Array by EGN
[0057] EGN can be used to generate large area double-split nanoring
arrays. The starting nanopattern is a square pillar array. An EGN
is used to create a square-ring mold. The mold then imprints square
ring trenches in a resist on a substrate. Then in a second EGN,
where three shadow evaporations of Cr from different angles guided
by the edges of each square ring, creates Cr square rings, each has
a double split. The Cr is used as the etching mask in etching the
substrate, which becomes the final daughter mold of double-split
ring array.
Example 2.4 New Innovations and Research in EGN
[0058] Clearly, again, the above are just a few examples that EGN
can create; and there are a plenty of new ways yet to be explored
(again our imagination is the limit). We will focus on the
researches in two areas.
[0059] (i) New EGN approaches to generate other complex patterns.
In EGN, the final complex pattern is determined by the starting
material shape, the edges used as the guiding, deposition (or
growth) of the materials, the deposition angles, the number of
depositions, etc. as well as the combination (and or repeat) of
different individual parameters. Clearly, the possibilities of EGN
are unlimited, as said before. We will explore each of these
parameters and their combinations. We will develop new EGN for
fabricating the popular complex nanopatterns that currently must
use EBL. Such patterns include triangle array, bowtie array,
various complex patterns needed for our nanoplasmonic solar cell
and LED test-bed (See Task 6).
[0060] (ii) Scaling the size of complex patterns created by EGN. We
will explore the key factors that affect the feature size and size
control in EGN, such as resist thickness, etching depth, and
deposition thickness. To measure the material layer thickness and
other vertical critical dimensions by various techniques such as
ellipsometry, spectrometry, and scanning probe-based methods will
be used. For measuring lateral dimensions and directly observing 3D
topology, high-resolution SEM, TEM, and atomic force microscope
(AFM) will be used. The measurements will be feedback to
fabrication controls, and fabrication equipment will be improved to
increase accuracy.
3. Nanostructures Self-Perfection by Liquefaction (SPEL)
[0061] Nanopatterning defects, the deviations of nanostructure
shapes from the ideal design, are unavoidable in any
nanofabrication methods today, and become worse as a
nanofabrication method is near its intrinsic limit. The defects
include edge roughness, slopped sidewall, rounded top, small
aspect-ratio, and non-circularness of circles/disks, etc.
[0062] Clearly, we have not found a new nanofabrication method that
can avoid the defects. Therefore, an alternative approach to remove
the fabrication defects is to remove the defects after the
fabrication, rather than change the fabrication method. To remove
the defects over a large area in a short time, the removal method
must be "self-perfecting", which means "one simple action" removes
the defects everywhere on the entire large-area sample.
[0063] Recently, we have been developing a new self-perfection
method, termed "self-perfection by liquefaction" (SPEL). SPEL turns
an imperfect structure into perfect one by fast liquefying the
structure while subjecting the structure to a set of boundary
conditions. Under the set of boundary conditions, the perfect shape
of the nanostructure in liquid is the energy minimum, while the
imperfect one is not; so that in reaching the thermal equilibrium,
the imperfect shape "automatically" changes to the perfect one.
[0064] Clearly, in SPEL, the key is to find the "correct set" of
boundary conditions for the desired perfect structure. Again there
are no limits in designing suitable boundary conditions. Below, we
will discuss innovations that we have developed, and discuss the
proposed research.
[0065] It is a well-known and well-practiced method to thermal flow
a polymer structure (e.g. resist structure) to remove LER or form
circular patterns, since, for the given condition, the smooth edges
and circular shape are the energy minimum of liquid in the thermal
process. However, such a process has two problems: (a) the shapes
are not perfect shapes (as desired), since after the heating, the
shape has slopped sidewall, wider foot print, rounded top and
smaller aspect ratio, and (b) the process can be applied only to
the polymer with low flow temperature, not to the hard materials
such as metals and inorganic semiconductors (e.g. Si).
[0066] To solve these problems to turn a defected shape into a real
perfect one, we made two innovations (1) we invented several sets
of new boundary conditions, where the perfect shapes are the energy
minimum, and (2) we used a pulse (20 ns) excimer laser to
selectively melt a hard material (e.g. Si or Cr) without
significantly heating the substrate (the excimer layer melts only
.about.300 nm thick surface materials that absorb the laser,
similar to LASIK eye surgery). The innovation allows
self-perfection in hard materials. SPEL was used to smooth Si
lines.
[0067] Capped-SPEL (C-SPEL) For example, one new set of boundary
condition that we developed is the "capped-SPEL" (C-SPEL), which
puts a single plate on top of the structures to be perfected (an
individual plate per structure). Under the new boundary condition,
the top surface of the structure is no longer free, but has to be
flat and in contact with the top plate during the flow ("liquid")
state, leading to a flat top and vertical smooth sidewall after the
flow. (Note: The vertical sidewall is also required by the energy
minimum, which first observed by us and later proved theoretically
using a simple parallel capacitor model).
[0068] Gapped-SPEL. Another new set of boundary conditions that we
developed is "Gapped-SPEL" (G-SPEL), which puts a single plate on
top of the structures to be perfected but with a gap between the
two (i.e. no contact). Under the G-SPEL boundary condition, during
the flow, the structure to be perfected raises up to touch the
surface of the top plate, making the structure narrower, taller,
and with smooth, vertical sidewall. G-SPEL has been demonstrated in
both for Si and Cr.
[0069] In SPEL, we used micro-spacers to control the final gap
between the guiding plate and the substrate, and Air Cushion Press
to apply uniform pressure between them.
[0070] Other self-perfection technologies will also be used in MNF,
although it is not the major research focus here. Such technologies
include (i) crystalline anisotropic etching to remove line edge
roughness (LER) (e.g. etching with the grating lines aligned to the
(111) planes of a (110) orientation Si wafer), and (ii)
guided-self-assembly of diblock copolymers, where we have extended
researches previously.
[0071] Our research will focus on (1) the exploration of new
boundary conditions to different self-perfection requirements,
which include the change of surface energies, multi-layer and
multi-material systems, (2) the methods of scaling up to very large
area (e.g. Air Cushion Press), (3) fundamental science of SPEL, and
(4) different applications of SPEL in MSN.
4. Exploration of Multi-Set-Nanopatterning (MSN) for Nanoplasmonics
and Roll-to-Roll Nanoimprint
[0072] Multi-set-nanopatterning (MSN) is a paradigm-shift approach
to creating complex nanostructures over a large area, which uses
three types of nonconventional nanofabrication technologies and
creative superposition(s) (or multiple uses) of these technologies.
As discussed before, the three paradigm-shift technologies are (i)
Fourier nanoimprint patterning, (ii) edge-guided nanopatterning,
and (iii) nanostructure self-perfection by liquefaction.
[0073] Clearly, there are unlimited ways for MSN. Our research here
will be in three areas: (1) explore some creative ways to combine
(superposition) the three fundamental technologies for making the
nanostructures without EBL that is drastically different from what
have made today; (2) explore the specific MSN that will make
nanostructures needed for solar energy, solid-state lighting, and
other metallic metamaterials (e.g. negative index materials); and
(3) explore the MSN approaches that are important to the
roll-to-roll master mold fabrication.
[0074] An example of the test bed is a new nanoplasmonic solar cell
that we proposed and demonstrated. The new solar cell, termed
plasmonic cavity with subwavelength hole-array" (PlaCSH) solar
cells with a 85 nm thick photovoltaic layer (poly
(3-hexylthiophene)/[6,6]-phenyl-C61-butyric acid methyl ester
(P3HT/PCBM) bulk hetero-junction) have: (a) A light
coupling-efficiency/absorptance as high as 96%, average 90%,
broad-band, and Omni acceptance; and (b) a power conversion
efficiency under standard solar irradiation that is 52% higher than
the same structure except the cavity, and nearly 180% when in the
cloudy day, due to the light acceptance is nearly independent of
the incident angle.
[0075] As the first team to demonstrate roller nanoimprint (as well
as planar nanoimprint), PI's group has several roller nanoimprint
tools. The roller nanoimprint tools will be used as demonstrate for
the large-area complex nanostructures.
5. High-Fidelity Replication of Nanoimprint Molds
[0076] High fidelity and fast duplication of daughter molds are
critical to the proposed research and future nonmanufacturing.
Recently, we have developed an approach in large-area mold
duplication that offers a solution to this issue. We will use and
improve this approach in the proposed research.
[0077] Our approach has several novelties: (1) the mold duplication
process is simple and fast by depositing polymer layers and bonding
a backplane, (2) the front size layer is customer high-Young's
modulus polymer rather than soft PDMS, giving high imprint
resolution and fidelity, (3) the mold has an easy demolding
surface, and (4) the mold is flexible.
[0078] Specifically, our mold, termed "High-fidelity flexible mold"
(HiF2M) consists of a 3-layer structure. The very top layer is
high-fidelity fluorinated polymer, as carrier layer of
nanopatterns; the top layer is bonded by the middle layer on to the
flexible substrate. The high-fidelity fluorinated polymer features
a fluorine-rich backbone structure with optimal molecular weight,
so it exhibits high stiffness (>90 MPa), low surface energy, and
high chemical stability. Those features allow sub-30 nm
high-resolution patterns, easy mold release, and chemical mold
cleaning, respectively.
[0079] We achieved sub-30 nm feature size in the 3-layer HiF2M
mold. We even observed a 2 nm-gap between nanopillars on HiF2M
mold. We also fabricated large-area HiF2M mold (50 cm by 20 cm) on
a PET sheet. By comparison of the master mold, daughter mold, and
imprints by the daughter mold, we found that the duplication method
exhibits high fidelity. The pattern pitch on HiF2M is only 0.8%
deviated from the original master mold with feature size of only
0.7% deviation. Both are smaller than measurement error (1%),
indicating that there is no substantial distortion associated with
the mold duplication process. The high fidelity and high uniformity
in duplication promises the feasibility of future mold
manufacture.
[0080] Our research will use the above technologies in MSN
developments, and will further advance such mold technologies,
including the uses of the front surface materials with even higher
mold fidelity and durability, the refinement of each layer (their
materials' properties and thickness) and the material formations
(by ourselves).
[0081] Although the foregoing invention has been described in some
detail by way of illustration and example for purposes of clarity
of understanding, it is readily apparent to those of ordinary skill
in the art in light of the teachings of this invention that certain
changes and modifications may be made thereto without departing
from the spirit or scope of the appended claims.
[0082] Accordingly, the preceding merely illustrates the principles
of the invention. It will be appreciated that those skilled in the
art will be able to devise various arrangements which, although not
explicitly described or shown herein, embody the principles of the
invention and are included within its spirit and scope.
Furthermore, all examples and conditional language recited herein
are principally intended to aid the reader in understanding the
principles of the invention and the concepts contributed by the
inventors to furthering the art, and are to be construed as being
without limitation to such specifically recited examples and
conditions. Moreover, all statements herein reciting principles,
aspects, and embodiments of the invention as well as specific
examples thereof, are intended to encompass both structural and
functional equivalents thereof. Additionally, it Is intended that
such equivalents include both currently known equivalents and
equivalents developed in the future, i.e., any elements developed
that perform the same function, regardless of structure. The scope
of the present invention, therefore, is not intended to be limited
to the exemplary embodiments shown and described herein. Rather,
the scope and spirit of present invention is embodied by the
appended claims.
* * * * *