U.S. patent application number 09/860037 was filed with the patent office on 2002-04-11 for microscale patterning and articles formed thereby.
Invention is credited to Chou, Stephen Y., Zhuang, Lei.
Application Number | 20020042027 09/860037 |
Document ID | / |
Family ID | 26800858 |
Filed Date | 2002-04-11 |
United States Patent
Application |
20020042027 |
Kind Code |
A1 |
Chou, Stephen Y. ; et
al. |
April 11, 2002 |
Microscale patterning and articles formed thereby
Abstract
The present invention is directed to a lithographic method and
apparatus for creating micrometer, more particularly sub-micrometer
patterns in a thin film coated on a substrate. The present
invention utilizes the self-formation of periodic, supramolecular
(micrometer scale) pillar arrays in a thin melt to form the
patterns. The self-formation was induced by placing a second plate
or mask a distance above the polymer film. The pillars bridge the
plate and the mask, having a height equal to the plate-mask
separation (preferably 2-7 times that of the film's initial
thickness). If the surface of the mask has a protruding pattern
(e.g., a triangle or rectangle), the pillar array is formed with
the edge of the pillar array aligned to the boundary of the mask
pattern.
Inventors: |
Chou, Stephen Y.;
(Princeton, NJ) ; Zhuang, Lei; (Princeton,
NJ) |
Correspondence
Address: |
Raymond A. Miller
Benesch, Friedlander, Coplan & Aronoff LLP
2300 BP Tower
200 Public Square
Cleveland
OH
44114
US
|
Family ID: |
26800858 |
Appl. No.: |
09/860037 |
Filed: |
September 24, 2001 |
Related U.S. Patent Documents
|
|
|
|
|
|
Application
Number |
Filing Date |
Patent Number |
|
|
09860037 |
Sep 24, 2001 |
|
|
|
09807266 |
Jun 11, 2001 |
|
|
|
09807266 |
Jun 11, 2001 |
|
|
|
PCT/US99/23717 |
Oct 8, 1999 |
|
|
|
60103790 |
Oct 9, 1998 |
|
|
|
Current U.S.
Class: |
430/322 ;
430/330 |
Current CPC
Class: |
Y10T 428/268 20150115;
G03F 7/0002 20130101; Y10T 428/1157 20150115; B82Y 40/00 20130101;
Y10T 156/1002 20150115; B82Y 30/00 20130101; B82Y 10/00
20130101 |
Class at
Publication: |
430/322 ;
430/330 |
International
Class: |
G03F 007/00 |
Goverment Interests
[0002] This invention was made with Government support under
Contract No. 341-6086 awarded by DARPA. The Government has certain
rights in this invention.
Claims
What is claimed is:
1. A method of forming. a pattern on a surface comprising: placing
a plate above a surface layer of a material; maintaining said plate
above said surface of said material; allowing pattern formation to
occur via interaction between said plate and said surface
layer.
2. The method of claim 1, wherein the step of allowing pattern
formation to occur includes rendering said surface deformable.
3. The method of claim 2, wherein said material is a polymer.
4. The method of claim 3, wherein said polymer is rendered
deformable by heating the polymer to the polymer's glass transition
temperature.
5. The method of claim 1, wherein said material is a thin film
deposited on a substrate.
6. The method of claim 5, wherein the substrate is selected from
the group consisting of semiconductors, dielectrics, metals,
polymers and combination thereof.
7. The method of claim 1, wherein said material is selected from
the group consisting of a homoploymer, a copolymer, a polymer
blend, a liquid, a liquid polymer, liquid crystals, a
semiconductor, a metal, and a dielectric material.
8. The method of claim 1, wherein said pattern is comprised of a
plurality of pillars.
9. The method of claim 8, wherein said plurality of pillars is in a
periodic array.
10. A method for forming a pattern on a surface, comprising the
steps of: obtaining a substrate; depositing a polymer film on the
substrate; placing a mask above the film, said mask having a
protruding feature; and heating the polymer film to thereby form a
contact between said film and said protruding feature.
11. A method of nanolithography comprising the steps of: depositing
a material on a substrate; placing a mask a distance above said
material, maintaining said mask above said material, said material
and substrate interacting to form a pattern in said material on
said substrate.
12. The method of claim 11, wherein the material comprises a
thermoplastic polymer.
13. The method of claim 11, further including heating the material
to said material's glass transition temperature.
14. The method of claim 11, wherein the substrate is selected from
the group consisting of semiconductors, dielectrics, metals,
polymers and combination thereof.
15. The method of claim 11, further including the step of removing
said mask after said pattern is formed.
16. The method of claim 11, wherein said pattern is comprised of a
plurality of pillars.
17. The method of claim 16, wherein said plurality of pillars is
formed as a periodic array.
18. A method of forming a relief pattern on a surface of a material
composed of: positioning a mask a predetermined distance above the
surface of the material; and altering the surface of the material
to a deformable surface, said mask and said deformable surface
interacting to form said relief pattern.
19. The method of claim 18, wherein said relief pattern has a
height of about 10 nm to about 1,000 nm.
20. The method of claim 18, wherein said relief pattern has a
height of about 50 nm to about 750 nm.
21. The method of claim 18, wherein said relief pattern has a
height of about 100 nm to about 700 nm.
22. The method of claim 18, wherein said surface is altered by
heating to a glass transition temperature of said material.
23. The method of claim 18, wherein said mask has a pattern formed
thereon.
24. The method of claim 18, wherein said relief pattern is
patterned after said pattern on said mask.
25. The method of claim 18, wherein said relief pattern is
comprised of a plurality of pillars.
26. The method of claim 18, wherein said relief pattern has a
height of less than about 1 .mu.m.
27. The method of claim 18, further including the step of cooling
said material after said relief pattern is formed.
28. The method of claim 18, wherein said predetermined distance is
about 2 to about 7 times a thickness of said deformable surface of
said material.
29. The method of claim 28, wherein said deformable surface
thickness is in a range of about 1 nm to about 2,000 nm.
30. The method of claim 29, wherein said deformable thickness is in
a range of about 5 nm to about 1,000 nm.
31. The method of claim 30, wherein said deformable thickness is in
a range of about 50 nm to about 500 nm.
32. The method of claim 31, wherein said deformable thickness is in
a range of about 75 nm to about 250 nm.
33. The method of claim 32, wherein said deformable thickness is
about 100 nm.
34. The method of claim 18, wherein said mask is dielectric.
35. The method of claim 18, wherein said material is a viscous
liquid.
36. The method of claim 18, wherein said material is a polymer.
37. The method of claim 18, wherein said polymer is a
homopolymer.
38. A microscale pattern forming assembly comprised of: a
substrate; a material deposited on said substrate; and a mask
positioned a predetermined distance above said material.
39. The microscale pattern forming assembly of claim 38, further
including a spacer interposed between said material and said mask
to maintain said mask at said predetermined distance.
40. The microscale pattern forming assembly of claim 39, wherein
said mask has a protruding pattern formed thereon.
41. The microscale pattern forming assembly of claim 38, wherein
said substrate has a higher glass transition temperature than said
material.
42. The microscale pattern forming assembly of claim 38, wherein
said mask is dielectric.
43. The microscale pattern forming assembly of claim 38, wherein
said material is a viscous liquid.
44. The microscale pattern forming assembly of claim 38, wherein
said material is a polymer.
45. The microscale pattern forming assembly of claim 38, wherein
said mask has a pillar formed from said material in contact
therewith.
46. The microscale pattern forming assembly of claim 38, wherein
said mask and said material have a plurality of pillars formed
there between.
47. A method of nanolithography comprising: depositing a material
on a substrate; placing a mask a distance above said material, said
mask having protrusion patterns formed thereon; and forming a
pattern in the material corresponding to said protrusion patterns,
said pattern being a result of an interaction between said
protrusion patterns and said material.
48. The method of claim 47, wherein said protrusion patterns is
comprised of a first protrusion pattern and a second protrusion
pattern, said first and second protrusion pattern being of
different length.
49. The method of claim 47, wherein said method is coated with a
surface coating.
50. An article having nanoscale patterning, said article being
comprised of a plurality of pillars, said plurality of pillars
having a height ranging from above 1 nm to below 1 .mu.m.
51. The article of claim 50, wherein said height is in the range of
about 100 nm to about 700 nm.
52. The article of claim 50, wherein said height is in the range of
about 250 nm to about 550 nm.
53. The article of claim 50, wherein said pillar has a diameter,
said pillar height to pillar diameter ratio being in a range of
about 0.1 to about 0.5.
54. The article of claim 50, wherein said plurality of pillars are
in a periodic array.
55. The article of claim 50, wherein said plurality of pillars has
a period of about 1 .mu.m to about 10 .mu.m.
56. The article of claim 50, which said plurality of pillars has a
boundary defined by a pattern on a mask used to form said plurality
of patterns.
57. The article of claim 50, wherein said plurality of pillars are
connected to form a lithographically-induced self-construction.
58. The article of claim 50, wherein said nanoscale patterning is
substantially identical in lateral size as a mask used to form said
nanoscale patterning.
Description
CROSS REFERENCE TO RELATED APPLICATION
[0001] This is a continuation of application U.S. Ser. No.
09/807,266, filed on April 9, 2001, which is a national phase
filing based on International Application No. PCT/US99/23717, filed
on Oct. 8, 1999, which claimed the benefit of priority from U.S.
Provisional Patent Application Ser. No. 60/103,790, filed on Oct.
9, 1998.
FIELD OF THE INVENTION
[0003] The present invention relates generally to forming patterns
on or in a surface material, assemblies used therefor, and articles
formed thereby. More specifically, the present invention relates to
microscale patterning and/or lithography. Microscale patterning and
microscale lithography have a broad spectrum of applications, e.g.
in the production of integrated circuits, microdevices, and the
like. The patterns formed can be utilized to perform an array of
functions, including electrical, magnetic, optical, chemical and/or
biological functions.
BACKGROUND
[0004] One of the key processing methods in fabrication of
semiconductors, integrated electrical circuits, integrated optical,
magnetic, and mechanical circuits and microdevices is forming very
small patterns.
[0005] Lithography is often used to create a pattern in a thin film
carried on a substrate so that, in subsequent process steps, the
pattern will be replicated in the substrate or in another material
which is added onto the substrate. One purpose the thin film
satisfies is protecting a part of the substrate so that in
subsequent replication steps, the unprotected portion can be
selectively etched or patterned. Thus, the thin film is often
referred to as a resist.
[0006] A typical lithography process for the integrated circuits
fabrication involves exposing a resist with a beam of energetic
particles which are electrons, or photons, or ions, by either
passing a flood beam through a mask or scanning a focused beam. The
particle beam changes the chemical structure of the exposed area of
the film, so that when immersed in a developer, either the exposed
area or the unexposed area of the resist will be removed to
recreate the pattern or obverse of the pattern, of the mask. A
limitation on this type of lithography is that the resolution of
the image being formed is limited by the wavelength of the
particles, the particle scattering in the resist, the substrate,
and the properties of the resist. Although pattern sizes greater
than 200 nm can be achieved by photolithography, and pattern sizes
in the range of 30 nm to 200 nm can be achieved utilizing electron
beam lithography, these methods are resource intensity and suffer
from low resolution.
[0007] U.S. Pat. No. 5,772,905 describes a method and apparatus for
performing ultra-fine line lithography wherein a layer of thin film
is deposited upon a surface of a substrate and a mold having at
least one protruding feature and a recess is pressed into the thin
film.
[0008] An alternative strategy to those described above is to use a
"naturally occurring" or "self-assembly" structure as a template
for subsequent parallel fabrication. For example, U.S. Pat. No.
4,407,695 and U.S. Pat. No. 4,801,476 describe a spin coating
technique to prepare close-packed monolayers or colloidal
polystyrene spheres with diameters of typically 0.1-10 microns on
solid substrates. The pattern is then replicated by a variety of
techniques, including evaporation through the interstices, ion
milling of the spheres and/or the substrates, and related
techniques. Highly ordered biologically membranes ("S-layers") have
also been suggested as starting points for fabrication. Close
packed bundles of cylindrical glass fibers, which could be
repeatedly drawn and repacked to reduce the diameters and lattice
constant have also been used. Block copolymer films have been
suggested for use as lithography masks wherein micelles of the
copolymer which form on the surface of a water bath are
subsequently picked up on a substrate.
[0009] To date, the focus of "self-assembly" has been primarily on
either phase separation of a polymer blend, of di-block copolymers,
or of local modification of surface chemistry (i.e., chemical
lithography). In self-assembly by phase separation, the periodic
structures are multidomain, and their orientation and locations are
uncontrollable and random. A long-sought after goal in
self-assembly is precise control of the orientation and location of
a self-assembled polymer structure.
[0010] There is an ongoing need to produce progressively smaller
pattern size. There also exist a need to develop low-cost
technologies for mass-producing microscale and sub-micron (e.g.
nanometer) structures. Microscale, indeed nanoscale and smaller,
pattern technology will have an enormous impact in many areas of
engineering and science. Both the future of semiconductor
integrated circuits and the commercialization of many innovative
electrical, optical, magnetic, and mechanical microdevices that are
far superior to current devices will depend on such technology.
SUMMARY OF THE INVENTION
[0011] Technologically, self-assembly promises not only low-cost
and high-throughput, but also other advantages in patterning
microstructures, which may be unavailable in conventional
lithography.
[0012] The present invention is generally directed to the formation
of patterns in a material through deformation induced by a mask
placed above a material, as well as assemblies used therefor, and
products formed thereby. An important aspect of the present
invention is novel method, referred to herein as
"lithographically-induced self-assembly" (LISA). In this process a
mask is used to induce and control self-assembly of a deformable
surface, preferably a thin film into a pre-determined pattern. One
advantage of the present invention is relatively accurate control
of the lateral location and orientation of a self-assembled
structure. Preferably, a substantially uniform, film is cast on a
substrate. A mask, preferably with protruding patterns representing
the pattern to be formed in or on the film, is placed above the
film, but physically separated from the film by a gap. The mask,
the film, and the substrate are manipulated, if necessary, to
render the film deformable. For example in the case of a polymer,
the polymer film may be heated to a temperature above the polymer's
glass transition temperature and then cooled down to room
temperature. During the heat-cool cycle, the initially flat film
assembles into discrete periodic pillar arrays. The pillars, formed
by rising against the gravitational force and surface tension,
bridge the two plates to form periodic pillar arrays. The pillars
generally have a height equal to the plate-mask separation.
Moreover, if the surface of the mask has a protruding pattern, the
pillar array is generally formed only under the protruding pattern
with the edge of the array generally aligned to the boundary of the
mask pattern. After the pillar formation, due to a constant polymer
volume, there is little polymer left in the area between pillars.
The shape and size of the mask pattern can be used to determine the
pillar array's lattice structure. The location of each pillar can
be controlled by the patterns on the mask. This process can be used
repeatedly to demagnify the self-assembled pattern size. This
demagnification permits a self-assembled structure to have a size
smaller than that of the mask pattern(s). If the demagnification is
used repeatedly, a size much smaller than that by a single
self-assembly process can be achieved. This would allow for
progressively smaller pattern-mask-patterns to be formed. The basic
LISA process can also be modified to form a non-pillared pattern
that is substantially identical to the features of the mask.
[0013] One embodiment of the present invention is a patterning
method or method of patterning which comprises depositing a
material on a substrate. The material and substrate may be already
formed, and the material and substrate may be the same or
different. In this case the step of depositing a material would not
be necessary, but rather a surface layer(s) would be selectively
manipulated so that a predetermined thickness of surface material
is deformable. This thickness must be small enough that the mask
can interact with the material through the separation distance to
form a contact therebetween. As is described more fully herein, the
thin film or surface layer(s) preferably has a thickness in the
range of about 1 nm to about 2,000 nm, more preferably about 10 nm
to about 1,000 nm, more preferably about 100 nm to about 500 nm and
even more preferably about 50 nm to about 250 nm. If the deposited
material is deformable at room temperature (e.g., a liquid polymer
or polymer dispersion, the material may not need to be deformed).
If a liquid polymer is used, it may be cured (e.g., photo curing)
after either pillar formation, usually before removal of the mask.
For a solid material, it may be necessary to render the material
deformable, e.g. by heating to a temperature where the material may
flow. Deforming by heat is a preferred route, but the material or
surface layer(s) may also be deformed by, other routes (e.g.,
chemical reactions). Heating may occur by any conventional means
(e.g., laser, light sources, heat radiating or microwave
induction), and the heat may be pulsed or continuous.
[0014] It is important that the mask be maintained above the
material or film. A spacer (which may be integrally or
non-integrally formed with the mask) is convenient to this end.
However, an assembly may be used wherein the mask is maintained
above said material without resorting to a spacer.
[0015] The substrate can be any number of compositions which are
capable of supporting the film, but the present invention has
particular applicability to substrates which are, themselves
intended to be processed to have patterns formed thereon or
therein. The substrate can have pre-existing relief patterns or be
flat.
[0016] The mask can be of any suitable material as described
herein. In many cases, the mask, will often be very similar in
composition to the underlying substrate. Indeed, it is envisioned
to use a suitably patterned substrate from a previous LISA process
in a second or more LISA process or LISC process. The mask can have
any suitable surface coating and the protrusion may be formed from
a surfactant or other suitable protruding material (e.g.,
monolayers or self-assembled monolayers) with a different surface
energy. The protruding pattern may be of varying heights on the
same pattern resulting in like pillars. Of course, any combination
of protrusion pattern protrusion coating or monolayer material
pattern may be used to form the relief structure.
[0017] Another embodiment of the present invention is the relief
structure formed by either or both the LISA and LISC process.
BRIEF DESCRIPTION OF THE DRAWINGS
[0018] The features, aspects, and advantages of the present
invention will become better understood with regard to the
following description, appended claims, and accompanying drawings
wherein:
[0019] FIG. 1 schematically illustrates lithographically-induced
self-assembly (LISA): (a) a flat substrate, (b) a thin layer of
deformable material deposited thereon, (c) a mask with a protruding
pattern a distance above the deformable material; and (d)
self-assembly into a periodic supramolecular pillar array after a
heat-and-cool cycle;
[0020] FIG. 2 is an (a) optical and (b) AFM images of periodic
pillars formed using a mask of a plain flat surface. The pillars
have a closely-packed hexagonal lattice and are multi-domain,
covering the entire wafer with a single-domain size of about 50
.mu.m;
[0021] FIG. 3 is an optical micrograph of (a) a protruding triangle
pattern on the mask and (b) pillar array formed under the triangle
pattern using LISA, and (c) AFM of the pillar array;
[0022] FIG. 4 is an optical and AFM images of the LISA pillar
arrays formed under protruding square patterns of a side of (a) 10
.mu.m, (b) 14 .mu.m, and (c) 14 .mu.m. The separation between the
mask and the substrate (a) 430 nm, (b) 280 nm, and (c) 360 nm,
respectively;
[0023] FIG. 5 is (a) optical micrograph of a protruding line
pattern spelling "PRINCETON" on the mask and (b) AFM image of
pillars formed under the mask pattern;
[0024] FIG. 6 is an AFM image of a pillar array formed under a
grid-line mask pattern with each pillar aligned under an
intersection of the grid;
[0025] FIG. 7 (a) illustrates schematically
lithographically-induced self-assembly using a surfactant as the
pattern: (i) A thin layer of homopolymer cast on a flat silicon
wafer. (ii) A mask of surfactant patterns placed a distance above
the PMMA film, but separated by a spacer. (iii) During a
heat-and-cool cycle, the polymer film self-assembled into a
periodic supramolecular pillar array. (b) Schematic of the
experimental setup;
[0026] FIG. 8 shows the observed dynamic behavior of the growth of
the first pillar under a square mask pattern at 120.degree. C. (a)
The polymer was featureless before heating the system. The
beginning of 120.degree. C. was set as time zero. (b)-(f) At
120.degree. C., the polymer under the corners of the mask pattern
is being pulled up; (g) the first pillar just touched the mask;
(h)-(i) the pillar expanded to its final size;
[0027] FIG. 9 shows the observed dynamic behavior of the growth of
an array formed under the square mask pattern at 120.degree. C.
from the first pillar to the last pillar. The pillars were formed
in an orderly manner, one by one, first under the corners of the
mask pattern, then along the edges, later new corners and new
edges, until the area under the mask pattern was filled with
pillars. The completion of the first pillar was set as time
zero;
[0028] FIG. 10 is the atomic force image of the same LISA pillar
array (5.times.5 pillars) as in FIG. 8 and 9. The pillars with a
flat top have a height, diameter, and period of 310 nm, 5 .mu.m,
and 9 .mu.m, respectively The array has a simple cubic lattice;
[0029] FIG. 11 schematically illustrates a proposed model for LISA:
(a) surface roughening, (b) roughening enhancement due to a long
range attractive force, (c) pillar formation, and (d)
self-organization;
[0030] FIG. 12 schematically illustrates lithographically induced
self-construction ("LISC") schematic of LISC: (a) a thin polymer
film cast on a flat substrate (e.g. silicon), (b) a mask with
protruding patterns placed a distance above the polymer film, (c)
during a heat-and-cool cycle, the polymer film self-constructs into
a mesa under a mask protrusion. The mesa has a lateral dimension
identical to that of the mask protrusion, a height equal to the
distance between the mask and the substrate, and a steep side
wall;
[0031] FIG. 13 shows (a) optical image of protruded pattern of
"PRINCETON" on the mask, and (b) AFM image of the LISC patterns
formed underneath the mask pattern The LISC pattern duplicates the
lateral dimension of the mask pattern.
[0032] FIG. 14 illustrates Self-Assembly (SALSA) of a Random-Access
Electronic Device.
DETAILED DESCRIPTION OF THE PRESENT INVENTION
[0033] It is to be understood that the invention is not limited to
a specific article of manufacture or technique described herein,
and may be implemented in any appropriate assembly or process.
Additionally, it should be understood that the invention is not
limited to any particular material or substrate. As is described
herein, a variety of types of materials and/or substrate may be
used.
[0034] As described herein, placing a mask a distance above a
surface, preferably a thin deformable surface results in the
formation of a self-assembled periodic supramolecular array of
pillars during a heat-and-cool cycle. The pillars are formed in the
area under the protruding mask pattern and are normal to the
substrate. The pillars generally extend all the way to the mask (or
to protrusion patterns on the mask). The location of each pillar as
well as the size, shape, and lattice structure of the array can be
controlled by the patterns on the mask with a great deal of
precision. The period and diameter of the pillars also can be
controlled, depending, for example, on the molecular weight of the
polymer Both the period and diameter generally become smaller as
the lower molecular weight of the polymer is lowered. Although not
intending to be bound by theory, it appears the LISA process
involves a delicate interplay of surface-roughening, long-range Van
der Waals forces between the surface and the mask, surface melt
flow, wetting properties of both mask and substrate, trapping of
triple-phase lines, and balance of attractive and repulsive forces.
LISA is extendable to other materials such as semiconductors,
metals, and biological materials. The application of LISA and
related technologies described herein opens up numerous
applications in microelectronics, information storage, new
materials, biology, and chemistry.
[0035] Turning with specificity to the figures, FIG. 1
schematically illustrates the lithographically induced
self-assembly of the present invention. Onto the substrate 31 is
layered a material 33, which, in the preferred embodiment, is a
thin layer of a homopolymer, preferably polymethyl-methacrylate
(PMMA). The PMMA was first spun on substrate 31, in this case, a
silicon wafer having a substantially plain flat surface, followed
by baking at 80.degree. C. to drive out the solvent. Next a mask
35, typically made of silicon dioxide, with a protruding pattern 37
on its surface that faces a deformable material 33 is placed above
the PMMA film 33. As is shown in FIG. 1, the mask 35 is separated,
using a spacer 39, from the PMMA by several hundred nanometers. The
distance between the protrusion 37 and film 33 is preferably in the
range of about 10 nm to about 1000 nm, more preferable 50 nm to 800
nm, and never more preferably that about 100 nm to about 700 nm.
The spacer 39 may be either integrally formed with the mask or a
separate element (see e.g. FIG. 12).
[0036] Pattern forming assembly 41 was heated from room temperature
to a temperature above the glass transition temperature of the PMMA
film 33, and then cooled back to room temperature. During the
heat-cool cycle, a press or chuck 51 (shown in FIG. 8b) was used to
hold the substrate 31, the spacer 39, and the mask 35 (i.e. the
pattern forming assembly) in their respective positions, thereby
preventing substantial relative movement and maintaining the
mask-substrate separation constant. The open space between the
initial PMMA film 33 and the mask 35 gives the PMMA film 33 freedom
to deform three-dimensionally. Preferably, the substrate 31 is wet
to the deformable material 33. In the case of a silicon substrate,
the silicon substrate surface preferably has a thin layer of native
oxide, making it a high energy surface and wet to a PMMA melt. In a
preferred embodiment of the present invention, the mask surface 47
has a monolayer surfactant, making it a low energy surface and
non-wet to a PMMA melt. The heat-cool cycling was performed either
in atmosphere or a vacuum of 0.3 torr, which has little effects on
the final results. The height of the protruding patterns 37 on the
mask 35 is typically micro scale and generally in the range of
about 50 nm to about 500 nm (in this example 150 nm).
[0037] It was observed that without a mask placed on the top, after
a heat-cool cycle, the PMMA film 33 remains flat and featureless.
But, with a mask 35 placed a certain distance above the surface of
the PMMA film 33, after the same heat-cool cycle, the initially
flat PMMA film 33 became self-assembled into periodic
supramolecular pillars 49 shown in FIG. 1. The pillars 49 were
formed only under the protruding patterns 37 of the mask 35, but
not under the recessed areas of the mask 35. The pillars 49 are
normal to the substrate 31 and extend all the way from the
substrate 31 to the mask 35, making their height generally equal to
the initial separation 43 between the substrate 31 and the surface
of the mask 35. The location of each pillar 49 as well as the size,
shape and lattice structure of the array is determined by the
pattern 37 on the mask 35.
[0038] A variety of masks having protruding patterns were made.
Masks were formed in a variety of shapes, such as triangles,
rectangles, squares, lines, and grids. As can be seen in FIG. 2,
for a mask without any pattern (i.e., a plain flat surface) placed
165 nm above the surface of the PMMA film, the original flat PMMA
film became, after a heat-cool cycle, periodic PMMA 15 pillar
arrays with a close-packed hexagonal structure of 3.4 .mu.m period,
2.7 .mu.m pillar diameter, and 260 nm height. The optical images
showed that the array of pillars 49 are multidomain and everywhere
over the entire sample. The average size of a single domain is
about 50 .mu.m (i.e.--15 periods). The atomic force microscopy
(AFM) showed that the top of each pillar 49 is flat (due to contact
with the mask) and the sidewall is quite vertical (due to the tip
size effect, the AFM tip is not good for profiling the sidewall).
Two-dimensional Fourier transform of the AFM images showed six
sharp points arranged in a hexagonal shape in the k-space, further
confirming the hexagonal lattice structure of the pillars. The
initial PMMA thickness was about 95 nm. The substrate and the mask
were heated to 130.degree. C. and were held together for 20 min by
a pressure of 300 psi. Then they were cooled down for 10 min to
room temperature before being separated.
[0039] As can be seen in FIG. 3, for a protruding triangle pattern
67 on the mask, a single-domain PMMA pillar 79 array of a
close-packed hexagonal lattice is formed under the mask pattern 67.
Both optical and AFM images show that the shape and size of the
pillar 79 array are identical to that of the mask pattern, with the
pillars 79 on the edges of the array aligned along the edges of the
triangle mask pattern 67. The polymer initially under the recessed
area of the mask 35 is depleted after the LISA process and no
pillars 79 are formed under the area. In this example initial
thickness of the PMMA film 33 was 95 nm. The initial separation
between the substrate and the mask pattern 67 was 530 nm. The LISA
pillar array is a close-packed hexagonal structure that has a
periodicity of 3 .mu.m, an average pillar diameter of 1.6 .mu.m.
The triangle mask pattern 67 has a side of 53 .mu.m; a larger size
will make the pillar array multi-domain.
[0040] As can be seen in FIG. 4, when the protruding patterns on
the mask are rectangles and squares, the pillar arrays 99a formed
in LISA also have a shape and size identical to the mask patterns
with the pillars 99a at the edges aligned to the edges of the mask
patterns, just as in the case of the triangle mask pattern.
However, the lattice structures of the pillar arrays are not
hexagonal. It appears that the pillars 99a on the edges are formed
and aligned to the mask pattern edges first. Then the other pillars
will be formed later according to the position of the edge pillars.
The shape of the mask and the pillar height determine the final
lattice structure of a pillar array. FIG. 4(b) and (c) show that a
same mask pattern geometry but different mask-substrate separations
lead to two different lattice structures (pillars 99b and 99c).
Moreover, the pillars at the edges appear to have a diameter
slightly larger than other pillars.
[0041] The size and shape of a number mask used in our experiment
as well as the diameter, period and height of the pillars formed in
LISA are summarized in Table 1 below.
[0042] Table 1 below provides a summary of the geometry of the mask
patterns and the LISA Pillars.
1 TABLE 1 Side Pillar Pillar Pillar Mask Length Height Period
Diameter Geometry (um) (nm) (um) (um) Plain N/A 260 3.4 2.7
Triangle 53 530 3.0 1.6 Rectangle 24 .times. 12 440 3.3 2.0 Square
10 430 3.5 2.0 14 280 3.5 2.1 14 360 3.5 2.0 Line N/A 530 4.8
2.1
[0043] The pillar diameter seems to decrease, with increasing the
pillar height (i.e., the separation between the substrate and the
mask), that can be understood from the fact that the total polymer
volume is constant. The pillar period was found to vary slightly
with different mask pattern geometry. Further experiments showed
that the pillar period and size depend on the polymer molecular
weight. When PMMA of 15K molecular weight (made by a vendor
different from 2K PMMA) was used, the pillar period and diameter
became about 8 .mu.m and about 5 .mu.m, respectively. It was also
found that as the heating time could impact pillar formation. An
example of high ratio of pillar height to the pillar diameter we
achieved is 0.5 (800 nm height and 1.6 .mu.m diameter).
[0044] As can be seen in FIG. 5, if the mask pattern is a
protruding line of a width less than 5 um, a single pillar line
will be formed and aligned under the line pattern.
[0045] As can be seen in FIG. 6, if the mask pattern is a grid, a
pillar 109 is formed and aligned under each intersection of the
grid mask pattern. In this case, the pillar 109 period is fixed by
the period of the grid on the mask.
[0046] In LISA, the plate or mask, when placed a distance above a
thin melt film, preferably a single-homopolymer melt film, causes
the polymer film, initially flat on another plate, to self-assemble
into periodic pillar arrays. The pillars form by raising against
the gravitational force and the surface tension, bridge the two
plates, having a height equal to the plate-mask separation. If the
mask surface has either a protruding pattern, a surfactant pattern
(e.g. with a shape of a triangle or rectangle, etc.), or a
combination of the two, a pillar array is formed under the pattern
with its boundary aligned to the boundary of the mask pattern and
with its lattice structure determined by the mask pattern
geometry.
[0047] To monitor the development of the pattern forming process of
LISA in a polymer film, a monitoring assembly 110 was used. The
monitoring assembly 110 is shown in FIG. 7b. In this case, the mask
was made of glass, thus allowing for observation through the mask
using an optical microscope 112. A CCD camera 114 and video
recorder 116 with a time resolution of 30 millisecond recorded the
pattern forming behavior. The sample consists of 2,000 molecular
weight (2K) PMMA polymer 103 cast on a silicon substrate 101 that
has a surface that wets the PMMA polymer 103. The glass transition
temperature of the PMMA was found to be 96.degree. C. The mask 105
has various patterns of a monolayer of self-assembled surfactant
117. The surfactant 117 was applied to the mask 105 via
photolithography and a lift-off. The substrate 101 and mask 105
were separated by a 220 nm spacer 109 and were held together by a
metal holder comprised of a chuck 51 and a screw 53 with the
pattern forming assembly interposed therebetween. The entire
pattern forming assembly 111 was heated by a hot plate 55. The
temperature was monitored by a thermocouple mounted on the
holder.
[0048] The dynamic behavior of a LISA pillar array formation under
a square mask pattern is summarized in FIGS. 8 and 9. The PMMA was
135 nm thick. FIG. 8 shows the growth of the first pillar in the
array. The sample holder was heated from room temperature to
120.degree. C. and maintained that temperature for the remainder of
the experiment. As can be seen in FIG. 8a, before heating the
system, the PMMA was featureless. The temperature was increased at
a rate of about 10.degree. C./min up to 100.degree. C. and then at
1.degree. C./min after that. Once the temperature exceeded 11 0C, a
very faint image showing the outline of a pattern could be
observed. As can be seen in FIG. 6 this latent image was clearly
visible once the system reached 120.degree. C. The beginning of
120.degree. C. was chosen as the zero time reference in FIG. 8. The
latent pattern indicates the onset of pattern formation and is
visible because the polymer in that region is several tens of
nanometers higher than the surrounding film. For a square mask
pattern a latent image formed first under the corners of the mask
pattern and then the edges. As can be seen in FIG. 8c-e, gradually,
the latent image at the corners becomes much more pronounced,
indicating the growth of polymer pillars at the corners (FIG.
8c-e). It was observed that one pillar always grew faster than the
rest. In this particular example, it took 58 minutes for the first
pillar to reach the mask. When a pillar just touched the mask, its
image became a black point in the center of a bright circle (see
FIG. 8g), and then expanded into a bright dot of 5 .mu.m diameter
in 6 seconds (see FIG. 8i). This suggests that pillars first grow
as a cone-shaped spike in the polymer film and then, after touching
the mask, reshape into a pillar with a flat top. The mask surface
should be a low energy surface to limit the distance that a pillar
can spread.
[0049] FIG. 9 shows the growth behavior from the first pillar to
the last pillar of a LISA array formed under the square mask
pattern. The time zero in this figure is set at the completion of
the first pillar. The formation of the second pillar was completed
9 seconds after the first pillar, in a corner of the mask pattern
adjacent to the first pillar (See FIG. 9b). The third pillar was
completed 58 seconds later and was in a corner adjacent to the
second pillar (See FIG. 9c). And the fourth pillar was formed at 2
min 59 seconds (See FIG. 9d). After the pillars at the corners were
completed, new pillars started to form at the edges of the mask
pattern (See FIG. 9e-g). A new edge pillar was observed to always
form adjacent to an existing pillar. After the first ring of
pillars was completed, which took about 50 minutes beyond the first
pillar formation, the second latent pattern images formed just
inside the ring.
[0050] In a fashion similar to the formation of pillars in the
first ring, a new pillar was formed at a new corner, then the
adjacent corners, later the new edges. As the process continued,
pillar formation propagated from the corners to the edges and from
outside to inside (See FIG. 9h-l). Similar dynamic behavior has
been observed in square patterns with different sizes as well as
with mask patterns with different shapes (e.g., triangles and
rectangles). The atomic force microscope image of the LISA pillar
array shown in FIG. 10 shows that the diameter of each pillar is
uniform and that the top of each pillar is substantially flat. The
pillar height, diameter, and period is 310 nm, 5 .mu.m, and 9
.mu.m, respectively. The measured height suggests that the actual
mask-substrate spacing was 310 nm and that the spacer was pressed
45 nm into the PMMA.
[0051] While not wishing to be bound by theory, FIG. 11 illustrates
a proposed model for the formation of periodic supramolecular
pillar arrays in a film utilizing the LISA process. LISA appears to
occur in four stages. The first stage is the surface roughening
shown in FIG. 11a. When a polymer 133 is heated above its glass
transition temperature, it becomes a deformable and/or viscous
liquid than can flow. Since there is an open space between the
polymer 133 and the mask 135, the polymer 133 will flow and deform
three-dimensionally to relieve the polymer film's internal stress
and surface tension, roughening the surface of the polymer film
surface.
[0052] The second stage is the enhancement of the polymer surface
roughening shown in FIG. 11b. Placing a mask 135, preferably a
dielectric mask polymer 133, on top of the PMMA can create a Van
der Waals attractive force, which is long-range and inversely
proportional to a power of the distance between the film 133 and
the mask 135. The closer to the mask 135, the larger the attractive
force on the polymer 133, making the film roughness grow until some
polymer touches the surface 147 of the mask.
[0053] The third stage is the pillar 149 formation shown in FIG. 1
ic. In order to minimize the total free energy, the low energy
surface of the mask forces the polymer melt 133 to ball up on the
mask surface 147, forming round pillars 149. On the other hand, the
high energy surface of the substrate 131 always keeps its surface
148 covered with a thin layer of polymer 133. The thin film layer
connects all polymer pillars 149, allowing a polymer mass flow
between the pillars 149. The thin film layer also acts as a polymer
reservoir, supplying polymer to the pillars 149. The connectivity
and the polymer mass-transfer equalize the pressure inside all
pillars, and hence the pillar diameter. The final diameter of a
pillar 149 also depends on the other forces applied to the pillar
149, as discussed in the next paragraph.
[0054] The fourth and final stage is the self-organization shown in
FIG. 11d. Initially, the polymer pillars 149 have random locations
and various diameters, and can move around inside the area defined
by a mask pattern 137. But, once a pillar 149 moves to an edge of
the mask pattern 137, part of its triple-phase line (i.e. the line
that intersects the liquid, solid and vapor phase) is trapped to
the edge, limiting the pillar's movement to only along the edge.
When a pillar 149 reaches a corner of the mask pattern 137, another
part of its triple-line is trapped by another edge. Then that
pillar cannot move any more, trapped at the corner, because a
pillar 149 cannot move in two different directions at the same
time; breaking away from one of the edges requires extra energy and
is unlikely. Once pillars occupy the corners, other pillars start
to self-organize on the edges. When the self-organization on the
edges finishes, the self-organization of pillars propagates
gradually into the center of the mask pattern 137. During the
cooling process, the polymer pillars solidify and maintain the
self-assembled patterns.
[0055] It appears the self-organization of pillars is due to the
balance between long-range attractive force and the short-range
repulsive force. The attractive force brings the pillars close
together, while the repulsive force keeps the pillars apart. The
cross-over of the two forces fixes the distance between the
pillars. This is similar to the self-organization in colloids. We
believe that the surface of the PMMA in this case has like-charges.
Therefore, the pillars appear to be attractive when they are a
certain distance away, but repulsive when the pillars are very
close. The attractive force between like-charges could be induced
by the substrate and mask, similar to the situation of two
microspheres between two glass walls. In the self-organization
stage (the fourth stage), the pillar diameter continues to adjust
to balance the surface tension, the repulsive force and the
attractive force. Since the pillars at corners have less repulsive
force than those in the center, the diameter of the corner pillars
is slightly bigger, as observed in our experiments.
[0056] From the above observations and others, it appears that the
LISA process is related to (i) a long-range attractive force
between the polymer melt film and the mask, (ii) the hydrodynamic
forces in the polymer melt, (iii) the surface tension, and (iv) the
interplay of all the forces. The long range force could be
electrostatic, dipole, or Van der Waals forces, or a combination of
all. It appears that electrostatic force is the driving force. The
patterns are formed as a result of interplay and instability of
charges in a polymer melt, image charges in a mask, and
hydrodynamic force in the polymer melt. We observed that because
the polymer melt thickness is ultra-thin, LISA is not due to the
instabilities from the thermal convection (Rayleigh-Benard
instability) or the surface tension driven Benard convention, which
also could lead to the pattern formation.
[0057] If there is no mask placed on top of the PMMA melt, the
charges in the PMMA film should be uniformly distributed due to a
flat surface and symmetry. However, if there is a mask with a
finite conductivity placed near the PMMA melt, an image charge will
be induced in the mask. The interplay of the charges and the image
charges can cause instability and pattern formations. Again, not
wishing to be bound by theory, we consider the case that the mask
has a protruding square. Since the charge tends to accumulate at
corners, there will be more image charge in the corners than other
places on the protruding square, causing a nonuniform charge
distribution in the mask. The nonuniform distribution of the image
charge will cause redistribution of the charges in PMMA film. The
process continues in a positive feedback fashion. Eventually,
enough charges and image charges will build up at the corners of
the square mask pattern and in the PMMA areas under the corners, so
that the electrostatic force between the corners of the mask
patterns and the PMMA under the corners exceeds the gravitational
force. The PMMA melt in those areas, which initially were flat,
starts to be pulled up into smaller cones. The charge will move
into the sharp point of the cones, hence inducing more image
charges at the corners of the mask. If the mask is not too far away
from the PMMA, the charges and the image charges will build up a
local electric field, that can overcome the surface tension. In
this case, the small PMMA cones will grow. The growth will reduce
the distance between the charges and the image charges, hence
increasing the strength of the electrostatic force and speeding up
the growth. Finally the PMMA pillars reach the mask, forming a full
pillar. Once the full PMMA pillars are formed at the corners, the
charges and image charges must redistribute. The pillars formed
become a boundary for the capillary waves in the PMMA melt surface.
The capillary wave, a linear wave of amplitude of about
one-hundredth of the film thickness (less than 1 nm in our case)
will form standing waves set by the boundary. If the standing wave
peak next to a boundary pillar has an amplitude slightly larger
than the rest of the peaks, more charges will be accumulated in
that peak and more image charges in the mask area above the peak,
making the peak grow into a full pillar. Once the pillars reach the
mask, the process will repeat, until all small amplitude capillary
peaks under the mask protruding patterns develop into full
pillars.
[0058] Therefore, the formation of the PMMA pillars starts at the
corners, then the edges, and later propagates into the center of
the mask protruding pattern. On the other hand, the polymer under
the recess areas of the mask is too far away to have an
electrostatic force to overcome surface tension to develop into
full pillars.
[0059] The protruding patterns on the mask guide the boundary of
the pillar array. The pillar array has a size, shape and period,
that are riot only different, but smaller than the features on the
mask. Such demagnification is technologically significant and could
be used repeatedly to achieve even smaller patterns. With a
suitable set of polymers of desired properties (e.g., viscosity,
surface tension, etc) and a repeated usage of LISA, the diameter of
the pillars can be "demagnified." Furthermore, LISA showed for the
first time the role of trapping the three-phase lines by a mask
pattern in self-organization of a polymer melt.
[0060] The LISA process would appear to be applicable to other
polymers and materials, especially single-phase materials, such as
semiconductors, metals, and biological materials. The periodic
arrays formed by LISA have many applications, such as memory
devices, photonic materials, new biological materials, just to name
a few. With a proper design, a single crystal lattice of pillar
array with predetermined diameter, period, location, and
orientation could be achieved over an entire wafer.
[0061] Utilizing the principles elucidated in LISA, we have been
able to control the surface energy and form patterns with a size
identical to the patterns on the mask can be formed. We refer to
this as lithographically-induced self-construction (LISC). It
differs from LISA in that the relief pattern is substantially
identical in lateral dimensions to the patterns on the mask as
opposed to the pillar arrays found in LISA. LISC offers a unique
way to pattern polymer electronic and optoelectronic devices
directly without using the detrimental photolithography
process.
[0062] As can be seen in FIG. 12, in LISC, a mask 235 with a
protruded pattern 237 is placed a certain distance above an
initially flat polymer 233 that is cast on a substrate 231. During
a heating process that raises the temperature above the polymer's
glass transition temperature and during cooling back to the room
temperature, the polymer was attracted, against gravitational force
and surface tension, to mask protrusions 237, but not to the recess
areas of the mask, forming the polymer mesas 249 on their own. The
mesas have a lateral dimension substantially identical to the
protruded patterns on the mask 235, a height equal to the distance
between the mask 235 and the substrate, and a relatively steep
sidewall.
[0063] In the LISC experiments, both the mask and the substrate are
made of silicon. The protrusions with a variety of shapes have a
height of -300 nm. The polymer is polymethal methalcrylate (PMMA)
which was spin-cast on the substrate and was baked at 80.degree. C.
to drive out the solvent. The molecular weight and thickness is
typically 2000 and 100 nm, respectively. The gap between the
initially flat polymer film and the mask protrusions ranged from
100 to 400 nm, and was controlled by a spacer. The temperature was
cycled from room temperature to 170.degree. C. The heat was from
the top and bottom of the sample, making the thermal gradient on
the sample very small. A press was used to supply the heat and to
hold the gap constant. A surfactant with a low surface energy was
coated on the mask to facilitate the mask-sample separation after
LISC. We found that the materials (for the mask and substrate) and
the parameters (e.g., the protrusion height, polymer thickness,
polymers molecular weight, gap, etc.) are not very critical to
LISC. LISC can be formed over a wide range of these parameters. The
typical diameter of the masks and substrate is larger than 2 cm.
The masks are made by photolighography and etching. The temperature
was cycled to 175.degree. C. At present, the minimum size of the
polymer microstructures formed by LISC is limited by the patterns
on the mask. However, the demagnification effect observed in LISA
could be used to form a resist wherein the substrate is etched in
the recessed areas of the pattern to thereby form smaller and
smaller patterns on masks.
[0064] To further test the ability of LISC in forming an arbitrary
pattern, we again created the protrusions of the word "PRINCETON"
on a LISC mask. This is shown in FIG. 13, comparison of PMMA LISC
patterns with the mask showed that the polymer mesas formed in LISC
duplicate the patterns on the mask very well. The linewidth and the
height of the pattern are 3 .mu.m and 230 nm, respectively. The
initial PMMA film thickness is only 100 nm.
[0065] In LISA, an array of periodic polymer pillars was formed
under a single mask protrusion, instead of a single polymer mesa
with the same lateral dimension as the mask protrusion is formed as
in LISC. The key to have a LISC rather than LISA appears to reduce
the difference of the surface tensions of the polymer and the mask.
When the difference is small enough, each polymer pillar formed in
the initial phase of LISC will spread and merge with other pillars
to form a single mesa under each mask protrusion. Either using a
different surfactant on the mask or increasing the processing
temperature (which would reduce the polymer surface tension) can
reduce the surface tension difference.
[0066] Another embodiment of the present invention is self-aligned
self-assembly (SALSA) of random access electronic device arrays.
The conventional approach in fabricating such an array is to
fabricate each individual device first, then connect them with word
lines and bit lines. As the devices become smaller, the precision
alignment between the wires and devices becomes more difficult to
fabricate, substantially increasing the fabrication cost. Using the
LISA principle, we can first fabricate a word-line array and a
bit-line array in two different substrates, and then let the device
self-assemble between the word-line and bit-line. FIG. 14
illustrates the applicability of SALSA to Random Access Electronic
Devices. A word line assembly is fabricated utilizing a silicon
wafer as is known in the art (e.g., by acid etching). Similarly, a
bit line is fabricated with a silicon wafer as is known in the art.
A thin film or polymer 73 (e.g., PMMA) is deposited on the word
line assembly 77. The bit line 75 is placed a pre-determined
distance above the word line 77 or vice versa, e.g. a distance of
less than 1 micron and preferably in the range of about 100 to 400
nm. The temperature is cycled from room temperature to the glass
transition temperature of the polymer 73 and then cooled back down,
to thereby form a pillar structure at the juncture of each word/bit
line.
[0067] Although the present invention has been described with
reference to preferred embodiments, one skilled in the art will
recognize that changes may be made in form and detail without
departing from the spirit and scope of the invention.
* * * * *