U.S. patent number 6,482,742 [Application Number 09/618,174] was granted by the patent office on 2002-11-19 for fluid pressure imprint lithography.
Invention is credited to Stephen Y. Chou.
United States Patent |
6,482,742 |
Chou |
November 19, 2002 |
Fluid pressure imprint lithography
Abstract
An improved method of imprint lithography involves using direct
fluid pressure to press the mold into a substrate-supported film.
Advantageously the mold and/or substrate are sufficiently flexible
to provide wide area contact under the fluid pressure. Fluid
pressing can be accomplished by sealing the mold against the film
and disposing the resulting assembly in a pressurized chamber. It
can also be accomplished by subjecting the mold to jets of
pressurized fluid. The result of this fluid pressing is enhanced
resolution and high uniformity over an enlarged area.
Inventors: |
Chou; Stephen Y. (Princeton,
NJ) |
Family
ID: |
24476613 |
Appl.
No.: |
09/618,174 |
Filed: |
July 18, 2000 |
Current U.S.
Class: |
438/690; 264/293;
425/385; 438/945 |
Current CPC
Class: |
B29C
43/003 (20130101); B29C 43/021 (20130101); B82Y
10/00 (20130101); B82Y 40/00 (20130101); G03F
7/0002 (20130101); B29C 59/022 (20130101); B29C
2043/025 (20130101); B29C 2043/3233 (20130101); B29C
2043/3238 (20130101); B29C 2043/566 (20130101); B29C
2059/023 (20130101); H01L 51/0004 (20130101); Y10S
438/945 (20130101) |
Current International
Class: |
G03F
7/00 (20060101); H01L 51/40 (20060101); H01L
51/05 (20060101); H01L 021/302 (); H01L 021/461 ();
A01J 021/00 (); B28B 011/08 (); B29C 059/00 () |
Field of
Search: |
;438/20,945,690,700
;425/385 ;264/156,293 ;156/320,643,65.1 |
References Cited
[Referenced By]
U.S. Patent Documents
Primary Examiner: Niebling; John F.
Assistant Examiner: Simkovic; Viktor
Attorney, Agent or Firm: Lowenstein Sandler PC
Claims
What is claimed:
1. A method for processing a surface of a substrate comprising the
steps of: applying a moldable layer on the surface of the
substrate; providing a mold with a molding surface having a
plurality of protruding features; pressing the molding surface and
the moldable layer together by direct fluid pressure to reduce the
thickness of the moldable layer under the protruding features to
produce reduced thickness regions; and withdrawing the mold from
the moldable layer.
2. The method of claim 1 further comprising the steps of: removing
the material of the moldable layer from the reduced thickness
regions to selectively expose regions of the substrate; and further
processing the substrate selectively in the exposed regions.
3. The method of claim 2 wherein the further processing comprises
doping the substrate with impurities, removing material from the
substrate, or adding material on the substrate.
4. The method of claim 1 further comprising the step of hardening
the moldable layer after pressing.
5. The method of claim 1 wherein the pressing comprises sealing a
region between the mold and the moldable layer and subjecting the
mold and the substrate to pressurized fluid.
6. The method of claim 5 wherein the sealing comprises sealing a
region between the mold and the substrate from the pressurized
fluid.
7. The method of claim 1 wherein the substrate or the mold or both
are sufficiently flexible to conform together under the fluid
pressure.
8. The method of claim 1 wherein the pressing comprises pressing
the mold and moldable layer together by streaming pressurized
fluid.
9. The method of claim 1 wherein at least two of the protruding
features of the molding surface are laterally spaced apart by less
than 200 nm.
10. The method of claim 1 where the thickness of the moldable layer
is in the range 0.1 nm to 10 .mu.m.
11. A process for patterning a mask layer on a semiconductor
substrate comprising: applying the mask layer to the semiconductor
substrate; disposing a mold having a patterned surface adjacent the
mask layer; filling a chamber with pressurized fluid; and
subjecting the mold or the substrate to pressurized fluid from the
chamber to press together the mold and the mask layer.
12. The process of claim 11 wherein the material of the mask layer
comprises a polymer and further comprising the step of curing the
polymer after performing the pressing.
13. The process of claim 12 wherein the curing includes
illuminating the layer with radiation.
14. The process of claim 12 wherein the cured mask layer conserves
an imprinted pattern from the mold.
15. The process of claim 11 further comprising cooling the mask
layer to a temperature at which the material of the mask layer
hardens.
16. The process of claim 11 wherein the material of the mask layer
comprises resist.
17. The process of claim 16 wherein the material of the mask layer
comprises a liquid polymer.
18. The process of claim 11 further comprising heating the mask
layer prior to pressing to a temperature at which the material of
the mask layer is pliable.
19. The process of claim 11 wherein the pressing comprises applying
the fluid pressure to a surface of the mold to push the patterned
face of the mold towards the substrate.
20. The process of claim 11 the pressing comprises applying the
fluid pressure to a surface of the substrate to push the substrate
towards the patterned face of the mold.
21. The process of claim 11 further comprising: removing the mold
from the mask layer leaving molded recesses in the mask layer; and
cleaning the mask material from the molded recesses to expose
regions of the substrate.
22. The process of claim 21 further comprising one or more of the
following steps: a selective etch of the exposed substrate, a
selective diffusion of impurities into the exposed substrate, and a
selective deposition of material on the exposed substrate.
23. The process of claim 11 further comprising positioning a
sealing material to isolate a region between the mold and the mask
layer from the fluid pressure in the chamber.
24. The process of claim 23 wherein the positioning includes
placing a ring of material around a region between the mold and the
mask layer.
25. The process of claim 23 wherein the positioning comprises
placing at least one flexible membrane between the pressure chamber
and at least one of the mold and the substrate.
26. A process of treating a semiconductor substrate, comprising the
steps of: disposing a layer of mask material on the substrate;
positioning a mold with a patterned surface adjacent the layer of
mask material; positioning a sealing device to isolate the layer of
mask material from a pressure chamber; disposing the masked
substrate and the mold in a pressure chamber; and increasing a
pressure of pressurized fluid in the pressure chamber to force
together the patterned face of a mold and the layer of mask
material.
27. The process of claim 26, wherein the positioning of the sealing
device hermetically isolates a region between the layer of mask
material and the mold from pressurized fluid in the pressure
chamber.
28. The process of claim 26 further comprising heating the mask
layer prior to the increasing of pressure.
29. The process of claim 26 further comprising curing the mask
layer after the pressing to harden deformations caused by the
mold.
30. The process of claim 29 further comprising removing the mold
from contact with the mask layer after the curing.
31. The process of claim 30 further comprising the step of removing
contaminants from the mask layer after removing the mold.
32. The process of claim 29 further comprising cleaning the mask
material from the deformations.
33. The process of claim 32 including further processing the
substrate by one or more of the following steps: selectively
etching from the substrate, selectively doping impurities in the
substrate, and selectively adding material on the substrate.
34. The process of claim 26, wherein increasing the pressure
comprises applying pressure to a fluid in the chamber.
35. The process of claim 26 where the fluid comprises gas.
36. The process of claim 26 where the fluid is liquid.
37. The method of claim 1 wherein the substrate and the mold are
made of the same material to minimize differential thermal
expansion or contraction.
38. The method of claim 1 wherein the moldable layer includes a
previously formed pattern and the mold is aligned to the previously
formed pattern before pressing the molding surface and the moldable
layer together.
39. The method of claim 1 wherein the pressing by direct fluid
pressure comprises filling a chamber with pressurized fluid and
subjecting the mold or the substrate to pressurized fluid from the
chamber.
40. The method of claim 1 wherein the pressing by direct fluid
pressure comprises: positioning a sealing device to isolate the
layer of mask material from a pressure chamber; disposing the
substrate and the mold in a pressure chamber; and increasing the
pressure of a pressurized fluid in the pressure chamber to force
together the patterned surface of the mold and the moldable layer.
Description
FIELD OF THE INVENTION
This invention relates to imprint lithography and, in particular,
to imprint lithography wherein direct fluid pressure is used to
press a mold into a thin film. The process is particularly useful
to provide nanoimprint lithography of enhanced resolution and
uniformity over an increased area.
BACKGROUND OF THE INVENTION
Lithography is a key process in the fabrication of semiconductor
integrated circuits and many optical, magnetic and micromechanical
devices. Lithography creates a pattern on a thin film carried on a
substrate so that, in subsequent process steps, the pattern can be
replicated in the substrate or in another material which is added
onto the substrate.
Conventional lithography typically involves applying a thin film of
resist to a substrate, exposing the resist to a desired pattern of
radiation, and developing the exposed film to produce a physical
pattern. In this approach, resolution is limited by the wavelength
of the radiation, and the equipment becomes increasingly expensive
as the feature size becomes smaller.
Nanoimprint lithography, based on a fundamentally different
principle offers high resolution, high throughput, low cost and the
potential of large area coverage. In nanoimprint lithography, a
mold with nanoscale features is pressed into a thin film, deforming
the shape of the film according to the features of the mold and
forming a relief pattern in the film. After the mold is removed,
the thin film can be processed to remove the reduced thickness
portions. This removal exposes the underlying substrate for further
processing. Details of nanoimprint lithography are described in
applicant's U.S. Pat. No. 5,772,905 issued Jun. 30, 1998 and
entitled "Nanoimprint Lithography". The '905 patent is incorporated
herein by reference.
The usual method of pressing the mold into the thin film involves
positioning the mold and the substrate on respective rigid plates
of a high precision mechanical press. With such apparatus, the
process can generate sub-25 nm features with a high degree of
uniformity over areas on the order of 12 in.sup.2. Larger areas of
uniformity would be highly advantageous to increase throughput and
for many applications such as displays.
SUMMARY OF THE INVENTION
An improved method of imprint lithography involves using direct
fluid pressure to press a mold into a substrate-supported film.
Advantageously the mold and/or substrate are sufficiently flexible
to provide wide area contact under the fluid pressure. Fluid
pressing can be accomplished by sealing the mold against the film
and disposing the resulting assembly in a pressurized chamber. It
can also be accomplished by subjecting the mold to jets of
pressurized fluid. The result of this fluid pressing is enhanced
resolution and high uniformity over an enlarged area.
BRIEF DESCRIPTION OF THE DRAWINGS
The advantages, nature and various additional features of the
invention will appear more fully upon consideration of the
illustrative embodiments now to be described in detail in
connection with the accompanying drawings. In the drawings:
FIG. 1 is a schematic flow diagram of the steps in an improved
method of imprint lithography;
FIG. 2 illustrates a typical mold and a substrate bearing a
moldable film for use in the improved method of FIG. 1;
FIG. 3 illustrates apparatus for practicing the improved method of
FIG. 1;
FIGS. 4A, 4B and 4C show the moldable layer and substrate at
various stages of the process of FIG. 1;
FIGS. 5A, 5B and 5C illustrate various further processing steps
that can be performed on the substrate;
FIGS. 6A-6E illustrate alternative sealing arrangements useful in
the method of FIG. 1; and
FIG. 7 shows alternative apparatus for practicing the method of
FIG. 1.
It is to be understood that these drawing are for purposes of
illustrating the concepts of the invention and are not to
scale.
DETAILED DESCRIPTION
The use of a high precision mechanical press to press a mold into a
thin film presents tolerance problems in replicating small patterns
over large areas. Presses move on guide shafts through apertures,
and the spacings between the shafts and their respective apertures
can be large compared to the features to be replicated. Such
spacings permit undesirable relative translational and rotational
shifts between the substrate and the mold. Moreover, despite the
most careful construction, the molds and the substrates used in
lithography are not perfectly planar. When these molds and
substrates are disposed on the rigid plates of a press, the
deviations from planarity over large areas can result in variations
in the molding pressure and depth of imprint. Accordingly, it is
desirable to provide a method of imprint lithography which avoids
the limitations of mechanical presses.
In accordance with the invention, the problem of unwanted lateral
movements of mechanical presses is ameliorated by using direct
fluid pressure to press together the mold and the moldable surface.
The inventive methods apply fluid pressure over a surface of the
mold, the substrate supporting the moldable surface or both.
Because the fluid pressure is isostatic, no significant unbalanced
lateral forces are applied. Direct fluid pressure also includes
fluid pressure transmitted to the mold or substrate via a flexible
membrane, as it does not interfere with the transmission of
isostatic pressure from the fluid. And streaming pressurized fluid
from numerous openings in a pressure vessel can also apply nearly
isostatic direct fluid pressure on the mold or substrate.
It is contemplated that the invention will have important
applications in the molding of a pattern on a previously patterned
substrate. The mold can be aligned with the previous pattern using
conventional alignment techniques, and imprinting by direct fluid
pressure minimizes any relative lateral shifts with consequent
improvement in the alignment of the two patterns.
Referring to the drawings, FIG. 1 is a schematic flow diagram of an
improved process for imprint lithography using direct fluid
pressure. An initial step shown in Block A, is to provide a mold
having a plurality of protruding features and a substrate-supported
thin film of moldable material. The protruding features are
preferably micrometer scale features and, more advantageously,
nanoscale features. The method is highly advantageously where the
mold surface has at least two protruding features spaced apart by
at least one lateral dimension less than 200 nm. A moldable
material is one which retains or can be hardened to retain the
imprint of protruding features from a mold surface.
FIG. 2 illustrates a typical mold 10 with protruding features and a
substrate 20 bearing a moldable thin film 21 for use in the process
of FIG. 1. The mold comprises a body 11 and a molding layer 12
including a plurality of protruding features 13 having a desired
shape. The mold body 11 and the molding layer 12 are typically
fused quartz, glass or ceramic. The molding layer 12 can be
patterned into features 13 of nanoscale dimensions using electron
beam lithography and etching techniques well known in the art. The
thickness of layer 21 is typically in the range 0.1 nm-10 .mu.m,
and the extent of protruding features 13 is typically in the range
0.1 nm-10 .mu.m.
The substrate typically comprises a semiconductor wafer such as a
substantially planar wafer of monocrystalline silicon. The
substrate could also be plastic, glass or ceramic. The moldable
thin film 21 can be any polymer that can be made pliable to
pressure and can retain a pressure-imprinted deformation or
pattern. It can be a thermoplastic polymer, such as polycarbonate
or polymethyl methacrylate (PMMA), which temporarily softens in
response to heat. Alternatively it can be a liquid, such as a
UV-curable silicone, which hardens in response to radiation or a
liquid which cures with heat. It can also be a composite layer of
polymer and hardenable liquid. The thin film is typically applied
to the substrate by spraying or spinning. Advantageously the film
polymer does not adhere to the mold surface. If necessary, the mold
surface can be coated with a release agent to prevent such
adherence.
In high resolution applications, the mold and the substrate are
advantageously made of the same material in order to minimize
misalignment due to differential thermal expansion or
contraction.
Preferably the mold body 11, the substrate 20 (or both) is flexible
so that, under the force of fluid pressure, the mold and the
substrate will conform despite deviations from planarity. Silicon
substrates of thickness less than 2 mm exhibit such flexibility for
typical imprint pressures.
The next step, shown in Block B, is to place the mold and the
thin-film together and to seal the interface of the mold with the
thin film, forming a mold/film assembly. If the thin film already
includes a previously formed pattern, then the pattern of the mold
should be carefully aligned with the previous pattern on the film
in accordance with techniques well known in the art. The objective
of the sealing is to permit external fluid pressure to press the
mold into the film. The sealing can be effected in a variety of
ways such as by providing a ring of material such as an elastomeric
gasket around the area to be molded and peripherally clamping the
assembly.
The third step (Block C) is to press the mold into the film by
direct fluid pressure. One method for doing this is to dispose the
assembly in a pressure vessel and introduce pressurized fluid into
the vessel. The advantage of fluid pressure is that it is
isostatic. The resulting force uniformly pushes the mold into the
thin film. Shear or rotational components are de minimus. Moreover
since the mold and/or substrate are flexible rather than rigid,
conformation between the mold and the film is achieved regardless
of unavoidable deviations from planarity. The result is an enhanced
level of molding resolution, alignment and uniformity over an
increased area of the film.
The pressurized fluid can be gas or liquid. Pressurized air is
convenient and typical pressures are in the range 1-1000 psi. The
fluid can be heated, if desired, to assist in heating the moldable
thin film. Cooled fluid can be used to cool the film.
FIG. 3 illustrates a sealed mold/film assembly 30 disposed within a
pressure vessel 31. The assembly 30 is sealed by a peripheral
elastomeric gasket 32, extending around the area to be molded. The
periphery of the assembly can be lightly clamped by a clamp (not
shown) to effectuate the seal. The vessel 31 preferably includes a
valve-controlled inlet 34 for the introduction of pressurized fluid
and a valve controlled outlet 35 for the exit of such fluid. The
vessel 31 may optionally include a heater 36 for heating a
thermoplastic or heat curable thin film and/or a transparent window
37 for introducing radiation to cure or cross link the film. A
sealable door 38 can provide access to the interior of the
vessel.
The next step shown in Block D, is to harden the moldable thin
film, if necessary, so that it retains the imprint of the mold and
to remove the mold. The process for hardening depends on the
material of the thin film. Some materials will maintain the imprint
with no hardening. Thermoplastic materials can be hardened by
preliminarily heating them prior to molding and permitting them to
cool after imprint. PMMA, for example, can be suitably softened by
heating to 200.degree. C. prior to molding and hardened by cooling
after imprint. Heat curable materials can be hardened by applying
heat during imprint. The above-described eater 36 and/or the use of
a heated pressurized fluid can effectuate such hardening. Radiation
curable materials can be hardened by the application of UV
radiation during imprint. Such radiation can be supplied through
the window 37 of the pressure vessel. The mold can be made of
transparent material to permit the radiation to reach the film.
Alternatively, the substrate can be transparent and the window
positioned to illuminate the film through the substrate.
The fifth step shown in Block E is optional in some applications.
It is to remove contaminants (if any) and excess material from the
recesses of the molded thin film. The molded film will have raised
features and recesses. In many lithographic operations it is
desirable to eliminate the material from the recesses so that the
underlying substrate is exposed for further processing. This can be
conveniently accomplished using reactive ion etching.
FIGS. 4A, 4B and 4C show the moldable layer and substrate at
various stages of the process. FIG. 4A illustrates the layer 21
during imprinting by mold 10 pressed by fluid pressure in the
direction of arrow 40. The protruding features 13 of the mold press
into layer 21, producing thinned regions 41. The recessed regions
42 of the mold between successive protruding features leave layer
21 with regions 43 of greater thickness.
FIG. 4B shows the layer 21 after hardening and removal of the mold.
The layer 21 retains the thinned regions 41 and thick regions 43 in
accordance with the pattern imprinted by the mold.
FIG. 4C illustrates the layer and substrate after removal of the
excess layer material in the recesses, exposing nanoscale regions
44 of the substrate 20.
In important applications the resulting structure is a
resist-covered semiconductor substrate with a pattern of recesses
extending to the substrate as shown in FIG. 4C. Such a structure
can be further processed in a variety of ways well-known in the
art. For example, the molded film can be used as a mask for the
removal of surface layers in exposed regions of the substrate, for
doping exposed regions or for growing or depositing materials on
the exposed regions.
FIGS. 5A, 5B and 5C illustrate such further processing. In FIG. 5A,
the substrate can include a surface dielectric layer 50 (such as
SiO.sub.2 on Si) and the mask layer can permit removal of the
dielectric at exposed regions. In FIG. 5B impurity regions 51 can
be diffused or implanted into the semiconductor selectively in
those regions which are exposed, altering the local electrical or
optical properties of the doped regions. Alternatively, as shown in
FIG. 5C new material layers 52 such as conductors or epitaxial
layers can be deposited or grown on the exposed substrate within
the recesses. After processing, the remaining material of the
molded layer can be removed, if desired, using conventional
techniques. PMMA, for example, can be cleaned away by washing with
acetone. A substrate can be subjected to additional lithographic
steps to complete a complex device such as an integrated
circuit.
As mentioned above, there are a variety of ways of sealing the
mold/film assembly 30 so that pressurized fluid will press the mold
into the film. FIGS. 6A-6D illustrate several of these ways.
FIG. 6A schematically illustrates an arrangement for sealing a
mold/film assembly by disposing the assembly within a sealed
covering of flexible, fluid-impermable membrane 40 (e.g. a plastic
bag). In this arrangement the region between the mold and the
moldable layer is sealed in relation to an external pressure
vessel. Preferably the air is removed from the bag before
molding.
FIG. 6B shows an alternate sealing arrangement wherein the assembly
30 is sealed by a peripheral sealing clamp 61 which can be in the
form of a hollow elastic torroid. Sealing can be assisted by
providing the mold with a protruding region 62 extending around the
region to be molded. In use, the clamp and pressurized fluid will
press the protruding region 62 into the moldable film, sealing the
molding region. FIG. 6C illustrates a sealing arrangement in which
the assembly 30 is sealed by applying a peripheral tube or weight
63 which lightly presses the mold onto the moldable film. A
peripheral protruding region 62 can assist sealing.
FIG. 6D shows an alternative sealing arrangement wherein the
assembly 30 is sealed by a sealing o-ring 64 between the mold and
the substrate. Preferably the o-ring seats within peripheral
recesses 65, 66 in the mold and the substrate, respectively. Light
pressure from a peripheral tube or weight 63 can assist
sealing.
FIG. 6E shows yet another sealing arrangement in which the assembly
30 is disposed between flexible membranes 40A and 40B is enclosed
within a pair of mating cylinders 67A, 67B. Application of fluid
pressure to the interior of the cylinders would press the mold and
moldable surface together.
Alternatively, two the cylinders could lightly seal against the
mold and the substrate, respectively, before pressurization. Yet
further in the alternative, the substrate could rest upon a support
and a single cylinder lightly seal against the mold or a
membrane.
FIG. 7 illustrates alternative molding apparatus 70 where the
assembly is disposed adjacent openings 71 in a hollow pressure cap
72 and the mold 10 is pressed into the moldable layer 21 by jets of
pressurized fluid escaping through the openings 71. The cap 72
(analogous to vessel 31) has an internal chamber 73 for pressurized
fluid. The region between the mold and the moldable film is
effectively sealed from the pressure vessel by the upper surface of
the mold.
In operation, the substrate and mold are placed on a substrate
holder 79. The cap 72 can be held in fixed position above the mold
10, as by bars 74, 75. High pressure fluid, preferably gas, is
pumped into chamber 73 through an inlet 76. The high pressure fluid
within the chamber produces a fluid jet from each opening 71. These
jets uniformly press the mold 10 against the moldable layer to
imprint the mold features.
Advantageously, the cap 72 can include a groove 77 along a
perimeter of the face adjacent the mold 10. The groove 77 can hold
an o-ring 78 between the cap 72 and the mold 20. The o-ring
decreases fluid outflow between the cap 72 and the mold 10,
increasing the molding pressure and making it more uniform.
Alternatively, the substrate holder 79 can have the same structure
as cap 72 so that the substrate is also pressed by jets of
pressurized fluid.
EXAMPLES
The invention may now be better understood by consideration of the
following specific examples.
Example 1
A silicon wafer of 4" diameter is coated with a layer of PMMA 150
nm thick. A mold is made of a 4" diameter silicon wafer and has
plural silicon dioxide protruding patterns 100 nm thick on one side
of its surface. The mold is placed on the PMMA layer with the
protruding patterns facing the PMMA. The mold and the substrate are
sealed in a plastic bag within a chamber, and the chamber is
evacuated. Then nitrogen gas at 500 psi is introduced in the
chamber. A heater in the chamber heats the PMMA to 170.degree. C.,
which is above the glass transition temperature of the PMMA,
softening the PMMA. Under the gas pressure, the mold is pressed
into the PMMA. After turning off the heater and introducing a cold
nitrogen gas, the PMMA temperature drops below its glass transition
temperature, and the PMMA hardens. Then the nitrogen gas is vented
to the atmosphere pressure. The mold and substrate assembly is
removed from the chamber. The bag is cut off, and the mold is
separated from the substrate.
Example 2
A silicon wafer of 4" diameter is coated with a layer of PMMA 150
nm thick and is placed on a chuck. The chuck has a plurality of
small holes on its surface. The holes can be connected either to
vacuum or to pressurized gas. When the holes are connected to
vacuum, the chuck holds the wafer on the chuck's surface. A mold
made of a 4" diameter silicon wafer has a plurality of silicon
dioxide protruding patterns (100 nm thick) on one of its surfaces.
The mold is held by a second chuck, which has the same design as
the substrate chuck. The mold is placed on top of the PMMA layer
with the protruding patterns facing the PMMA. The mold and the
substrate are placed in a chamber. The PMMA can be heated from the
chuck.
During the imprint process, the PMMA is first heated above its
glass transition temperature. A ring pattern on the mold seals off
the mold pattern inside the ring from the pressure outside. Then
the holes of both chucks are changed from vacuum to a gas pressure
of 500 psi. The pressurized gas presses the mold protruding
patterns into PMMA. Importantly, the pressurized gas presses the
mold protruding pattern into the PMMA uniformly in submicron scale,
despite the roughness of the backsides of the mold and the
substrate as well as the roughness of the chuck surfaces.
It is to be understood that the above described embodiments are
illustrative of only a few of the many embodiments which can
represent applications of the invention. Numerous and varied other
arrangements can be made by those skilled in the art without
departing from the spirit and scope of the invention.
* * * * *