U.S. patent application number 10/320869 was filed with the patent office on 2004-01-22 for fabrication of finely featured devices by liquid embossing.
This patent application is currently assigned to Massachusetts Institute of Technology. Invention is credited to Bulthaup, Colin A., Hubert, Brian N., Jacobson, Joseph M., Wilhelm, Eric J..
Application Number | 20040013982 10/320869 |
Document ID | / |
Family ID | 27387490 |
Filed Date | 2004-01-22 |
United States Patent
Application |
20040013982 |
Kind Code |
A1 |
Jacobson, Joseph M. ; et
al. |
January 22, 2004 |
Fabrication of finely featured devices by liquid embossing
Abstract
Elastomeric stamps facilitate direct patterning of electrical,
biological, chemical, and mechanical materials. A thin film of
material is deposited on a substrate. The deposited material,
either originally present as a liquid or subsequently liquefied, is
patterned by embossing at low pressure using an elastomeric stamp
having a raised pattern. The patterned liquid is then cured to form
a functional layer. The deposition, embossing, and curing steps may
be repeated numerous times with the same or different liquids, and
in two or three dimensions. The various deposited layers may, for
example, have varying electrical characteristics, interacting so as
to produce an integrated electronic component.
Inventors: |
Jacobson, Joseph M.;
(Newton, MA) ; Bulthaup, Colin A.; (Boston,
MA) ; Wilhelm, Eric J.; (Somerville, MA) ;
Hubert, Brian N.; (Menlo Park, CA) |
Correspondence
Address: |
Testa, Hurwitz & Thibeault, LLP
Patent Administrator
High Street Tower
125 High Street
Boston
MA
02110
US
|
Assignee: |
Massachusetts Institute of
Technology
Cambridge
MA
|
Family ID: |
27387490 |
Appl. No.: |
10/320869 |
Filed: |
December 17, 2002 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
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10320869 |
Dec 17, 2002 |
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09525734 |
Mar 14, 2000 |
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6517995 |
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60153776 |
Sep 14, 1999 |
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60167847 |
Nov 29, 1999 |
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Current U.S.
Class: |
430/320 ;
257/E21.024; 257/E21.174; 257/E21.577; 264/1.1; 264/1.24; 264/1.36;
264/1.38; 264/2.3; 430/322; 430/330; 438/455; 438/754 |
Current CPC
Class: |
B01J 2219/0063 20130101;
B01J 2219/00382 20130101; H01L 51/0014 20130101; B01J 2219/00621
20130101; B01J 2219/00635 20130101; B01J 2219/00659 20130101; B01J
2219/00608 20130101; B01J 2219/00585 20130101; H01L 21/0271
20130101; C40B 60/14 20130101; H01L 21/76817 20130101; H01L 51/0004
20130101; G03F 7/0017 20130101; B01J 2219/00725 20130101; C40B
40/06 20130101; H01L 27/28 20130101; B01J 19/0046 20130101; B01J
2219/00605 20130101; H01L 21/288 20130101; H01L 21/76802 20130101;
B01J 2219/00722 20130101; C40B 40/12 20130101; H01L 51/0017
20130101; B01J 2219/0061 20130101; B82Y 10/00 20130101; H01L
2251/105 20130101; G03F 7/0002 20130101; B82Y 40/00 20130101; B01J
2219/00596 20130101; C40B 40/10 20130101; B01J 2219/00731
20130101 |
Class at
Publication: |
430/320 ;
430/322; 430/330; 438/455; 438/754; 264/1.36; 264/1.1; 264/1.24;
264/1.38; 264/2.3 |
International
Class: |
B31F 001/07 |
Claims
What is claimed is:
1. A method of fabricating a functional component, the method
comprising the steps of: a. providing a thin film of a curable
material in liquid form; b. patterning the curable material by
embossing the material at low pressure with an elastomeric stamp
having a raised pattern thereon; c. curing the patterned material;
and d. repeating steps (a) to (c) a plurality of times with
materials which, when cured, have varying functional
characteristics, the cured layers interacting so as to produce a
functional component.
2. The method of claim 1 wherein the stamp comprises an elastomeric
polymeric matrix with a rigidity-conferring material entrained
therein.
3. The method of claim 1 further comprising the step of forming the
elastomeric stamp by: a. creating a negative impression of the
pattern in a substrate; b. enclosing the pattern; c. pouring a
liquid elastomeric precursor into the enclosure, the precursor
flowing into the negative impression of the pattern d. curing the
elastomeric precursor into an elastomer; and e. removing the
elastomer from the substrate.
4. The method of claim 1 further comprising the step of forming the
elastomeric stamp by: a. providing a photosensitive elastomer; b.
exposing the elastomer to actinic radiation so as to render the
pattern; and c. photochemically developing the exposed elastomer to
produce the pattern.
5. The method of claim 1 further comprising the step of cleaning
the stamp by applying a liquid polyimide thereto, curing the
polyimide, and removing the cured polyimide from the stamp.
6. The method of claim 1 wherein the curable material is applied as
a liquid.
7. The method of claim 6 wherein the liquid is applied onto a
smooth, flat support as a bead and drawn into a uniform film.
8. The method of claim 1 wherein the curable material is applied as
a non-liquid and subsequently liquefied.
9. The method of claim 8 wherein the material is applied to a
support and liquefied by heating the support.
10. The method of claim 8 wherein the material is liquefied by
heating the stamp.
11. The method of claim 1 wherein the raised pattern comprises
convex surfaces.
12. The method of claim 1 wherein the stamp is applied to the
patterned material and removed therefrom with a rocking motion.
13. The method of claim 1 wherein the material is present on a
support and is at least partially cured with the stamp held against
the support.
14. The method of claim 1 wherein the stamp is removed from the
material prior to curing the material, the material retaining the
pattern despite removal of the stamp.
15. The method of claim 1 wherein the material is present on an
uneven surface, the stamp patterning the material without
substantial lateral deflection.
16. The method of claim 1 wherein the material is present on an
uneven surface, the stamp having unraised portions which, with the
raised features in contact with the surface, planarize the material
in contact with the unraised portions.
17. The method of claim 1 wherein a plurality of contiguous layers
is patterned with elastomeric stamps at least some of which have
different patterns, at least some of the stamps having raised
features in common locations to create vias between non-adjacent
layers.
18. The method of claim 17 wherein at least some of the vias extend
through a plurality of layers.
19. The method of claim 17 wherein the vias are filled by deposited
material forming one of the layers, said material being planarized
as said layer is patterned.
20. The method of claim 1 wherein the material of at least one of
the layers is a suspension of nanoparticles in a carrier
liquid.
21. The method of claim 20 wherein the material is cured by
evaporating the carrier liquid, the nanoparticles coalescing into a
substantially continuous patterned film.
22. The method of claim 21 wherein the nanoparticles are metal, the
film being conductive.
23. The method of claim 21 wherein the nanoparticles are
semiconductive, the film being semiconductive.
24. The method of claim 1 wherein at least one of the layers is
soluble in a solvent, and further comprising the step of removing
the at least one layer by exposure of the component to the
solvent.
25. The method of claim 1 wherein, for each layer, the stamp is
applied at a plurality of locations to produce a two-dimensional
repetitive pattern.
26. The method of claim 1 wherein steps (a) to (d) are repeated a
plurality of times so that the cured layers are formed
repetitively.
27. The method of claim 1 wherein application of the stamp to the
thin film results in adherence of material to the raised stamp
pattern, the embossing step comprising transferring the adhered
material to a substrate for curing.
28. The method of claim 27 wherein transfer is accomplished by
application of low pressure to the stamp.
29. The method of claim 1 wherein the thin film is formed by
deposition of the curable material in droplet form followed by
application of the stamp thereto so as to form a thin film having a
pattern complementary to the stamp pattern.
30. The method of claim 1 wherein the functional component is a
micromechanical structure.
31. The method of claim 1 wherein the functional component is an
integrated circuit, the cured layers comprising conducting,
dielectric, and semiconducting layers.
32. The method of claim 1 wherein the functional component is a
biochip.
33. The method of claim 1 wherein the functional component is a
field-emission display.
34. The method of claim 33 wherein the curable material is a
suspension of metal nanoparticles and carbon nanotubes and the
pattern comprises first and second sets of interdigitated lines
having nanotubes protruding therefrom, the repeating step
comprising applying a suspension of metal nanoparticles so as to
cover the first set of interdigitated lines and curing the
metal-nanoparticle suspension thereover.
35. The method of claim 1 wherein the functional component is an
optical waveguide.
36. An integrated circuit fabricated in accordance with claim
31.
37. A biochip fabricated in accordance with claim 32.
38. A field-emission display fabricated in accordance with claim
38.
39. An optical waveguide fabricated in accordance with claim
35.
40. A method of fabricating a functional component, the method
comprising the steps of: a. providing a thin film of a liquid on a
support; b. patterning the liquid by embossing it at low pressure
with an elastomeric stamp having a first raised pattern thereon,
the raised pattern displacing the liquid when brought into contact
with the support; and c. bringing into contact with the support a
substrate having thereon a second raised pattern, the liquid, where
present on the support, adhering to the second raised pattern.
41. The method of claim 40 wherein the liquid comprises a
biological material.
42. The method of claim 40 wherein the liquid comprises a
biological resist, and further comprising the step of exposing the
substrate to a biological material, the biological material not
adhering to raised portions of the substrate that have received the
resist.
43. A method of fabricating a functional component, the method
comprising the steps of: a. providing a thin film of a liquid on a
support; b. patterning the liquid by embossing it at low pressure
with an elastomeric stamp having a raised pattern thereon, the
raised pattern having at least some features with submicron
dimensions and displacing the liquid when brought into contact with
the support; and c. curing the patterned material.
44. Apparatus for fabricating a functional component, the apparatus
comprising: a. means for applying a thin film of a curable material
in liquid form; b. an elastomeric stamp having a raised pattern
thereon, the raised pattern having at least some features with
submicron dimensions; c. means for applying the elastomeric stamp
to the curable material so that the raised pattern displaces the
material; and d. means for curing the patterned material.
Description
PRIOR APPLICATION
[0001] This application stems from U.S. Provisional Application
Serial No. 60/153,776, filed on Sep. 14, 1999, and No. 60/167,847,
filed on Nov. 29, 1999.
FIELD OF THE INVENTION
[0002] The present invention relates to fabrication of finely
featured electronic, chemical, and mechanical devices.
BACKGROUND OF THE INVENTION
[0003] Electronic and electromechanical components are presently
fabricated in large, immobile manufacturing facilities that are
tremendously expensive to build and operate. For example,
semiconductor device fabrication generally requires specialized
microlithography and chemical etching equipment, as well as
extensive measures to avoid process contamination. The total amount
of time required for processing of a single chip may be measured in
days, and typically requires repeated transfer of the chip into and
out of vacuum conditions.
[0004] In addition to their expense, the fabrication processes
ordinarily employed to create electronic and electromechanical
components involve harsh conditions such as high temperatures
and/or caustic chemicals, limiting the ability to integrate their
manufacture with that of functionally related but environmentally
sensitive elements. For example, the high temperatures used in
silicon processing may prevent three-dimensional fabrication and
large-area fabrication; these temperatures are also incompatible
with heat-sensitive materials such as organic and biological
molecules. High temperatures also preclude fabrication on
substrates such as conventional flexible plastics, which offer
widespread availability and low cost.
[0005] Despite intensive effort to develop alternatives to these
processes, no truly feasible techniques have yet emerged. U.S. Pat.
No. 5,817,550, for example, describes a low-temperature
roll-to-roll process for creating thin-film transistors on plastic
substrates. This approach faces numerous technical hurdles, and
does not substantially reduce the large cost and complexity
associated with conventional photolithography and etching
processes.
[0006] U.S. Pat. No. 5,772,905 describes a process called
"nanoimprint lithography" that utilizes a silicon mold, which is
pressed under high pressure and temperature into a thin film of
material. Following cooling with the mold in place, the material
accurately retains the features of the mold. The thin film may then
be treated to remove the small amount of material remaining in the
embossed areas. Thus patterned, the film may be used as a mask for
selectively etching underlying layers of functional materials. This
process is capable of producing patterns with very fine resolutions
at costs significantly below those associated with conventional
processes. But it is quite complicated, requiring numerous
time-consuming steps to create a single layer of patterned
functional material. The technique requires high application
pressures and temperatures at very low ambient pressures, thereby
imposing significant complexity with attendant restriction on the
types of materials that can be patterned. Perhaps most importantly,
this technique is limited to producing single-layer features,
thereby significantly limiting its applicability to device
fabrication.
[0007] U.S. Pat. No. 5,900,160 describes the use of an elastomeric
stamp to mold functional materials. In particular, the stamp is
deformed so as to print a self-assembled molecular monolayer on a
surface. This process, also called MIMIC (Micromolding Against
Elastomeric Masters), is significantly simpler than nanoimprint
lithography, and can be performed at ambient temperatures and
pressures. But the technique is generally limited to low-resolution
features (in excess of 10 .mu.m), and more importantly, the types
of geometries amenable to molding by this technique are
limited.
DESCRIPTION OF THE INVENTION
OBJECTS OF THE INVENTION
[0008] It is, accordingly, an object of the present invention to
provide an easily practiced, low-cost process for directly
patterning functional materials without the need for multistage
etching procedures.
[0009] Another object of the invention is to increase the speed
with which layers of functional materials can be patterned.
[0010] Still another object of the invention is to provide a
fabrication process that requires no unusual temperature, pressure,
or ambient conditions, thereby increasing the range of materials
amenable to patterning.
[0011] A further object of the invention is to facilitate
convenient nanoscale patterning of multiple adjacent layers.
[0012] Yet another object of the invention is to planarize
deposited materials as part of the application process, eliminating
the need for additional planarizing processes (such as chemical
mechanical polishing), thereby facilitating fabrication of complex
three-dimensional devices employing many (e.g., in excess of 100)
layers.
BRIEF SUMMARY OF THE INVENTION
[0013] To achieve the foregoing and other objects, the present
invention utilizes an elastomeric stamp to facilitate direct
patterning of electrical, biological, chemical, and mechanical
materials. In accordance with the invention, a thin film of
material is deposited on a substrate. The deposited material,
either originally present as a liquid or subsequently liquefied, is
patterned by embossing at low pressure using an elastomeric stamp
having a raised pattern. The patterned liquid is then cured to form
a functional layer. The deposition, embossing, and curing steps may
be repeated numerous times with the same or different liquids, and
in two or three dimensions. The various deposited layers may, for
example, have varying electrical characteristics, interacting so as
to produce an integrated electronic component.
BRIEF DESCRIPTION OF THE DRAWINGS
[0014] The foregoing discussion will be understood more readily
from the following detailed description of the invention, when
taken in conjunction with the accompanying drawings, in which:
[0015] FIGS. 1A-1D are greatly enlarged sectional views
illustrating fabrication of an elastomeric stamp in accordance with
the present invention;
[0016] FIGS. 2A and 2B are side elevations illustrating application
of a thin, uniform film of liquid onto a substrate;
[0017] FIGS. 3A-3C and 3D-3F are sectional views illustrating,
respectively, the embossing process of the present invention as
applied to planar surfaces and non-planar surfaces;
[0018] FIGS. 3G-3I are sectional views illustrating planarization
and the creation of vias using the present invention;
[0019] FIGS. 4A-4F are sectional views illustrating fabrication of
an electronic inverter in accordance with the present
invention;
[0020] FIGS. 5A-5F are plan views of the structures shown
sectionally in FIGS. 4A-4F;
[0021] FIGS. 6A-6G are sectional views illustrating fabrication of
a microelectromechanical device in accordance with the present
invention;
[0022] FIGS. 7A-7G are plan views of the structures shown
sectionally in FIGS. 6A-6G;
[0023] FIGS. 8A-8F are sectional views illustrating fabrication of
a biochip in accordance with the present invention;
[0024] FIGS. 9A-9C schematically illustrate, respectively, a single
SRAM circuit, a two-dimensional array of such circuits, and a
three-dimensional array of such circuits;
[0025] FIGS. 10A and 10B are sectional views illustrating
fabrication of a field-emission display device in accordance with
the present invention;
[0026] FIG. 11 is a block diagram of a preferred nano-embossing
system implementing the present invention; and
[0027] FIGS. 12A and 12B schematically illustrate alternative
configurations for synthesizing nanoparticles.
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS
[0028] FIGS. 1A-1D illustrate an exemplary approach to fabricating
an elastomeric stamp useful in the practice of the present
invention. As shown in FIG. 1A, a substrate 100 is patterned with a
series of recessed features 105 and projecting features 110. These
features correspond in size and arrangement (but not in depth) to
the pattern ultimately desired for a component layer. Accordingly,
the features 105, 110 may be inscribed using conventional
techniques such as photolithography, e-beam, focused ion-beam,
micromachining, or other lithographic approaches. Feature sizes as
small as 150 nm have been accurately obtained and utilized,
although even smaller features are of course possible. Substrate
100 may, for example, be any surface of sufficient smoothness that
may be conveniently patterned, and which will not bond to the
material from which the stamp is to be formed. Suitable materials
include, for example, silicon, metal wafers, and exposed
photoresist.
[0029] As shown in FIG. 1B, a raised enclosure 115 is applied to
substrate 100 so as to surround the pattern of features 105, 110.
Enclosure 115 may be, for example, a metal or plastic wall, the
outer contour of which is designed to fit within the device that
will apply the stamp as hereinbelow described. An uncured elastomer
120 in liquid form is poured into the well 125 formed by enclosure
115 and features 105, 110. Preferably, elastomer 120 is a curable
rubber or silicone material such as polydimethylsiloxane (PDMS),
e.g., the SYLGARD-184 material supplied by Dow Corning Co. To
prevent seepage, enclosure 115 is desirably held against the
surface of substrate 100 with a modest pressure or set within a
conforming groove in substrate 100.
[0030] A sufficient amount of elastomer 120 is poured into well 125
to completely fill features 105 and to provide a stable body mass
for stamping operations. The elastomer 120 is then cured into a
solid plug 130. For example, the PDMS material mentioned above may
be cured by heating in an oven at 80.degree. C. for 2 h. Other
silicone elastomers may be cured by exposure to moisture, e-beam or
actinic (e.g., ultraviolet) radiation, or by addition of a
cross-linking agent.
[0031] The solid plug 130 is separated, with or without enclosure
115, from substrate 100 as shown in FIG. 1D to form a finished
stamp 132. The underside of plug 130 has a series of projecting and
recessed features 135, 140 complementary to the features 105, 110
of substrate 100, which are left undamaged by the foregoing process
steps; moreover, little if any elastomer is desirably left on the
substrate 100 when plug 130 is removed.
[0032] Enclosure 115 may be removed along with plug 130 as shown in
FIG. 1D, or it may instead be left in place on substrate 100. If it
is removed and its association with plug 130 retained, it may serve
several purposes: facilitating mechanical attachment to the
stamping device, assisting with alignment of the stamp (for
example, enclosure 115 may have an alignment tab that mates with a
complementary recess in the stamping device), and limiting lateral
deformation of plug 130. To further limit lateral deformation, plug
130 may be made relatively thin (by pouring the liquid elastomer
120 to a level not substantially above the surface of substrate
100) and capped by a solid support structure. A fenestrated film or
other rigidity-conferring filler material may be added to liquid
elastomer 120 prior to curing, thereby integrating within the
resulting polymer matrix to further enhance the rigidity of plug
130.
[0033] Other techniques may also be used to fabricate the stamp
132. For example, a stamp may be patterned by selectively curing a
thin film of the elastomer by exposure to actinic radiation through
a mask followed by photochemical development (to remove the exposed
or the unexposed areas), or by selective thermal curing with an
atomic force microscope (AFM) thermal tip. The stamp 132 may also
be fabricated from non-elastomeric stiff materials for better
control of deformation. For example, the procedures described above
can be carried out with a polyacrylate rather than an elastomer.
Suitable polyacrylates include polyfunctional acrylates or mixtures
of monofunctional and polyfunctional acrylate that may be cured by
e-beam or ultraviolet (UV) radiation.
[0034] If the stamp 132 becomes soiled, it may be cleaned by
coating the patterned surface with a liquid polyimide such as
Japanese Synthetic Rubber, curing the polyimide in place, and then
peeling it off the stamp. This process will remove dust and debris
without harming the patterned stamp surface.
[0035] The stamp is applied to a liquid which, when cured, provides
a desired electrical, chemical, biological, and/or mechanical
functionality. For example, the liquid may be a colloidal
dispersion of nanoparticles or carbon nanotubes; an uncured
polyimide; a solution of biological material; or a solution of a
suitable sacrificial or release layer which may later be dry- or
wet-etched (e.g., PMMA). In general, the liquid is present on a
substrate (or on a previously deposited and cured layer) as a thin,
uniform film. A deposited liquid can be drawn into such a film by
various techniques, one of which is illustrated in FIGS. 2A and 2B.
A substrate 200--which may be a glass slide, a silicon wafer, a
sheet of plastic, or other smooth material--receives a bead 210 of
liquid. A smooth rod 220 (which may be glass or a flexible
material) is dragged across substrate 200 in the direction of the
arrow, drawing the bead 210 into a uniform film 230. In general,
the pressure between rod 220 and substrate 200 can vary without
affecting the resultant thickness of film 230; indeed, rod 220 can
even be held slightly above substrate 200 (so that no contact is
actually made). The speed with which rod 220 is drawn across
substrate 200 does affect the thickness of film 230, however, with
faster travel resulting in a thinner film. Accordingly, for a film
of uniform thickness, rod 220 should be drawn at a constant speed,
and should not be allowed to rotate as it is drawn. The film
thickness is also affected by the size (diameter) of rod 220.
[0036] After rod 220 has been fully drawn across substrate 200, the
film 230 will typically still be in a liquid state. Depending on
the liquid, substantial loss of volume may occur by evaporation;
indeed, a loss of 90% of the initial height of the film is not
unusual. Thus, a thin film of liquid initially 100.+-.10 nm in
height may dry down to a film 10.+-.1 nm in height. We have
routinely obtained dry films with heights less than 40 nm using
this technique.
[0037] For some materials, the use of a rod to produce a thin film
is not an option. For example, the material may not wet to the
surface of substrate 200, or the solvent may evaporate almost
instantly. An alternative application technique useful in such
cases utilizes a stamp having a patterned surface as described
above. A small line of the liquid material to be drawn into a film
is deposited onto substrate 200. One edge of the stamp is brought
into contact with substrate 200 immediately next to the line of
liquid. The stamp is then lowered into contact with the substrate,
displacing the liquid in front of it and producing a thin,
patterned layer of material under the stamp.
[0038] Another alternative involves application of the material to
be patterned as a droplet, either to the surface of the receiver
substrate or to the raised-pattern surface of the stamp. The stamp
is then brought into contact with the substrate surface, thereby
molding the applied material in the pattern of the stamp. The
material may be cured (e.g., thermally) with the stamp in contact
with the substrate. For example, this approach has been applied to
liquid-phase polyimide, vinyl, and nanoparticle metal inks, which
are cured by activating a hotplate underlying the substrate
following patterning. It is found, however, that this approach is
most useful for insulators (such as polymers) because the resulting
patterned film is contiguous. The process also works best with
viscous materials that exhibit limited outgassing during cure
(although PDMS stamps are to some degree porous to may outgassing
components).
[0039] FIGS. 3A-3C illustrate the embossing technique of the
present invention as applied to a planar surface. A substrate 300
is coated with a thin, uniform film 305 of liquid as described
above. An elastomeric stamp 310 having a pattern of projecting and
recessed features 315, 320 is lowered until the projecting features
315 make contact with substrate 300, thereby displacing liquid 305
at the regions of contact. The height (or heights) of the recessed
features 320 exceeds that of the liquid that will be displaced
therein. The area dimensions of projecting features 315 are
constrained by the need for these features to push aside the liquid
305 and either make contact with substrate 300 or at least displace
enough liquid to facilitate its convenient subsequent removal. The
maximum areas of features 315 depend greatly on the viscosity of
the liquid, the thickness of the film 305, and the nature of the
stamp elastomer. For metallic nanoparticles in suspension (15%)
with a wet film thickness of 500 nm, it has been found that an
elastomeric stamp formed from PDMS can completely transport the
nanoparticle-containing liquid over a distance greater than 5
.mu.m. In order to enhance the transport capability of features
315, these may have convex, rather than flat, surfaces; for
example, the features may be domed, peaked, or otherwise shaped to
make contact with substrate 300 at a small region, progressively
moving more liquid as stamp 310 is pressed against substrate 300
and the features 315 flatten.
[0040] Stamp 310 is preferably lowered onto substrate 300 using a
slight rocking motion. Since the stamp is elastomeric, it may be
slightly flexed so that one edge first makes contact with the
substrate before the rest of the stamp rolls into place. This
approach prevents or reduces the formation of air bubbles. No
unusual pressure, temperature, or ambient conditions are necessary
for the embossing process. Very light or no pressure is applied to
the stamp 310 so the projecting features 315 penetrate the liquid
film 305. Any attractive force between projecting features 315 and
substrate 300 will assist with the transport of liquid into
recesses 320, and may also allow pressure to be removed--so that
features 315 merely rest against substrate 300--without sacrificing
contact.
[0041] With the stamp 310 against substrate 300 as shown in FIG.
3B, the film 305 may be partially or completely cured. The curing
mode is dictated by the nature of the liquid, and may include one
or more process steps such as heating, evaporating a solvent (to
which the elastomer of stamp 310 is permeable), UV exposure, laser
annealing, etc. Stamp 310 is removed from substrate 300 as shown in
FIG. 3C, leaving a pattern of fully or partially cured film traces
325 that correspond to the pattern of recesses 320. Preferably,
stamp 310 is removed using a rocking motion. Smooth, uniform motion
improves the quality of the pattern 325 and prevents damage thereto
from minuscule bursts of air.
[0042] It is found that even if the liquid 305 is not cured while
stamp 310 is in contact with substrate 300, it will tend
nonetheless to retain the pattern 325 when stamp 310 is removed so
long as the thickness of the liquid is sufficiently small. That is,
there will be no detectable flow of liquid back into areas
displaced by the projecting regions of stamp 310, probably due both
to the absolute height of liquid 305 and the small contact angle
between the liquid and substrate 300. Moreover, so long as the
surface energies of the substrate 300 and the stamp 310 are
sufficiently mismatched, there will be no withdrawal of substrate
material by stamp 310. As a result, stamp 310 may be immediately
reused without cleaning.
[0043] If uncured or partially cured, the patterned liquid 325 may
at this point be cured into full solidity. In addition to the
curing techniques discussed above, the absence of the stamp 310
facilitates such additional mechanisms as vacuum evaporation and
chemical modification (e.g., by addition of a cross-linker). In
this regard, it should be noted that the film patterned by the
stamp 310 may begin as a solid rather than a liquid. For example,
the film may be heated to decrease viscosity before stamp 310 is
brought into contact therewith. Alternatively, stamp 310 may itself
be heated to a temperature sufficient to melt the solid film upon
contact.
[0044] The film patterned by stamp 310 need not be planar; indeed,
in constructions involving multiple deposited and patterned layers,
coplanarity among layers may frequently be disrupted to achieve
desired three-dimensional configurations. FIG. 3D shows a substrate
300 having a previously patterned layer of a first material 330. A
thin film 335 of liquid is drawn over material 330 and, where
exposed, substrate 300; the liquid 335 is generally conformal,
resulting in an uneven liquid surface. Maintaining precise
alignment among patterned layers is obviously vital to proper
functioning of the finished device.
[0045] An elastomeric stamp 340 is well-suited to patterning such
an uneven surface while maintaining precise rendition of the stamp
pattern. As shown in FIG. 3E, stamp 340 is lowered as discussed
previously. Because of its elastic character, stamp 340 deforms to
allow different projecting features 345 to reach solid surfaces of
different heights without substantial lateral deflection. As a
result, the pattern 350 of material 335 that remains upon removal
of stamp 340 is substantially complementary to the pattern of
projecting features 345, notwithstanding the different heights of
the embossed regions. Naturally, the degree of fidelity to the
stamp pattern depends on the degree of elasticity inherent in the
stamp and the differences in height that must be accommodated.
[0046] Following removal of stamp 340, the embossed pattern of
material 350 is cured. Of course, the curing mode chosen must not
damage the previously cured layer 330.
[0047] As explained above, a thin film of deposited may be
conformal, resulting in a surface of varying heights (rather than
filling recesses to create a planar surface). The embossing
technique of the present invention can be used not only to
planarize such deposited layers, but also to create "vias" that
interconnect layers not directly in contact with each other. With
reference to FIG. 3G, a substrate 300 is patterned with a
previously deposited and embossed layer of a first material 360. A
thin film 365 of liquid is drawn over material 360 and, where
exposed, substrate 300; once again the liquid 365 is generally
conformal, resulting in an uneven liquid surface. In many
applications, it is desirable for the component layer formed from
liquid 365 to be planar rather than conformal. For example,
planarization is essential for microelectromechanical (MEM)
structures and many-layer three-dimensional circuits. The present
invention can accomplish both planarization and the creation of
vias among non-adjacent stratified layers.
[0048] As shown in FIG. 3G, the projecting features of a stamp 370
(representatively indicated at 375) have elevations chosen such
that, with the surfaces 377 of the projections in contact with
substrate 373, the recessed portions of stamp 370 (representatively
indicated at 380) make contact with the surface of liquid 365. As
shown in FIG. 3H, the result is planarization of the liquid layer
365 where it is in contact with stamp surfaces 380. When stamp 370
is removed (FIG. 3I), that layer is substantially planar with the
exception of edge ridges shown at 385. Moreover, a via 390 is
established between the surface of substrate 300 and the top
surface of layer 365. A layer subsequently deposited on layer 365,
therefore, can make contact with substrate 300, and this
subsequently deposited layer can also be planarized in the manner
just described. Alternatively, the via 390 can be made to persist
through multiple layers by embossing with a similar projecting
feature as each such layer is applied. In this way, contact between
distant layers may be effectuated.
[0049] If the elevation of projecting features 377 is insufficient,
there will be no contact with substrate 300 and via 390 will not
form. If the elevation of projecting features 377 is excessive,
then liquid 365 will not fully planarize; via 390 will effectively
be stepped, with an intervening ridge or shoulder. Nonetheless, the
latter sizing error is preferable, since the via 390 will be
functional and, moreover, the configuration shown in FIG. 3I can
still be achieved by compression of features 375 (if substrate 300
can tolerate some applied pressure).
[0050] Liquid 365 may or may not be cured (totally or partially)
before stamp 370 is withdrawn in the manner hereinabove described.
Following curing, the liquid 365 may decrease in height,
jeopardizing planarity. This problem can be overcome by applying
additional layers of the same material and embossing with the same
pattern of features 377, 380. The ability to planarize and pattern
in the same step represents a significant fabrication capability
and improvement over the prior art.
[0051] The foregoing approach, in which a stamp is made from a
master and then used repeatedly, may not be suitable for all
applications. An alternative arrangement utilizes a device which,
under computer control, is capable of changing its surface topology
in accordance with a desired pattern and then acting as a stamp.
Such a device may be built, for example, using an array of MEM
elements that are actuated electrostatically, thermally,
magnetically, piezoelectrically or by other computer-controllable
means, actuation of an element causing it to alter the degree or
manner in which it projects from the surface of the array. One such
device useful in the present application is a micro-mirror array in
which an array of elements is caused to tilt either out of plane or
lie flat depending on an electrical signal (see Kim et al., Society
for Information Display 99 Digest, p. 982 (1999)).
[0052] The approach of the present invention can be used to create
arbitrary functional devices. The technique is negative-working, in
the sense that the pattern of projecting features corresponds to
the material that will be removed rather than deposited. This
design methodology is apparent from FIGS. 4A-4F and 5A-5F, which
illustrate fabrication of a two-transistor electronic inverter.
Each of FIGS. 4A-4F is a section taken from the corresponding one
of FIGS. 5A-5F along the line labeled with the figure number.
Functional layers are built up on a substrate 400 (FIGS. 4A, 5A),
which may be, for example, a glass slide, a plastic sheet, a
silicon wafer, or any other material having a sufficiently smooth
surface 400s. Each added layer is patterned by a different
stamp.
[0053] As shown in FIGS. 4B, 5B, a patterned conductive metal layer
410 is established on surface 400s of substrate 400. This is
accomplished by first applying a thin film of a metal-containing
liquid, such as a suspension of gold or silver nanoparticles in a
suitable carrier liquid (see, e.g., U.S. Pat. No. 5,756,197, the
entire disclosure of which is hereby incorporated by reference).
The applied liquid is patterned with a stamp as described above so
as to create a series of channels that reveal the surface 400s of
substrate 400. The liquid is then cured (e.g., in the case of a
metal nanoparticle suspension, the carrier is evaporated so that
the metal particles coalesce into a substantially continuous,
conductive patterned film). The pattern formed includes a pair of
transistor gaps 412, a ground rail 414, and a V.sub.cc rail
416.
[0054] A semiconductive layer 420 is deposited onto the conductive
layer 410. Layer 420 completely fills and is planarized over the
channels 412, so that in these locations, layer 420 is in contact
with substrate 400. Otherwise, the pattern of layer 420
substantially matches that of layer 410 so that the semiconductor
420 does not bridge between metal lines. In some areas 422, layer
420 is removed by the embossing process to reveal the underlying
layer 410, while in other areas 424 overlying channels previously
defined through layer 410, substrate 400 is revealed.
Semiconductive layer 420 may be applied as a liquid suspension of
semiconductor (e.g., silicon, germanium, CdSe, etc.) nanoparticles
as described, for example, in U.S. Pat. No. 5,534,056 (the entire
disclosure of which is hereby incorporated by reference). Again,
following patterning, the layer may be cured by evaporating the
carrier so as to coalesce the particles into a continuous patterned
film.
[0055] An insulating layer 430 is applied over semiconductive layer
420 as shown in FIGS. 4D, 5D. Layer 430 completely fills the vias
424, and is planarized thereover. A via 432, slightly smaller in
diameter than the via 422 (see FIG. 4C) created earlier, is formed
through that via 422 to reveal layer 410. The insulating layer may
be applied as an uncross-linked liquid polymer precursor, such as a
radiation-cure coating (polyacrylates and polymethacrylates, for
example, are suitable for this purpose). Following patterning and
removal of the stamp, the polymer precursor may be cured (i.e.,
cross-linked) into solidity by exposure to UV or e-beam
radiation.
[0056] With reference to FIGS. 4E, 5E, a second metal layer 435 is
applied to insulating layer 430 and patterned by stamping. A plug
of the metal layer 435 completely fills the via 432 created
previously and connects to metal layer 410; because via 432 has a
smaller diameter than via 424, a layer of insulating material
separates the plug of metal from semiconductor layer 420 within the
via 432. The second metal layer 430 forms the gates 440 of the two
transistors.
[0057] An encapsulant 450, such as a UV-cured polymer, epoxy or
spin-on glass is applied as a coating over layer 435 to protect all
underlying functional layers from contamination or physical damage.
The encapsulant, which is applied at a sufficient thickness to fill
all exposed channels, also adds structural rigidity to the finished
device.
[0058] FIGS. 6A-6G and 7A-7G illustrate fabrication of a freely
rotating MEM wheel. Each of FIGS. 6A-6G is a section taken from the
corresponding one of FIGS. 7A-7G along the line labeled with the
figure number. The structure includes a first sacrificial or
release layer, a second sacrificial or release layer, a first metal
layer, a third sacrificial or release layer, and a second metal
layer. After all layers are applied, a final release step etches
away the release layers to liberate a purely metallic structure.
Each layer is patterned using an elastomeric stamp as described
above.
[0059] A substrate 600 (FIGS. 6A, 7A), which may be a glass slide,
a plastic sheet, a silcion wafer, or any other appropriately smooth
surface (for MEM applications a relatively stiff substrate may be
desirable), receives a first release layer 610 as shown in FIGS.
6B, 6C. Release layer 610 may be, for example, a polymer (such as
PMMA) soluble or wet-etchable in a solvent (such as acetone), or
etchable by dry-etch techniques (such as plasma etching); or may be
a spin-on glass etchable in hydrofluoric acid. Release layer 610
completely covers substrate 600 with the exception of a hole 612
patterned in the release layer by means of the elastomeric stamp.
This hole 612 will receive material for the axle of the wheel.
[0060] The second release layer 620 is patterned as shown in FIGS.
6C, 7C. The pattern includes a series of depressions 622. These
will be filled with metal to create dimples on the rotating wheel.
The hole 612 is patterned in the center of layer 620.
[0061] With reference to FIGS. 6D, 7D, the first metal layer 630
fills the holes 612, 622 (see FIG. 6C) patterned in the first two
release layers 610, 620. Layer 630 is planarized over these holes.
Stamping eliminates metal from a pair of concentric circular
regions 632, 635. Region 632 defines the edge of the wheel, and
region 635 faciliatates separation of the wheel from the axle. The
bottom of the wheel fills the depressions 622 (FIGS. 6C, 7C),
forming dimples that will reduce stiction between the wheel and
substrate 600. Not shown are small holes patterned in the wheel to
allow etchant to reach the underlying release layers 610, 620.
[0062] The third release layer 640 is added and patterned as shown
in FIGS. 6E, 7E. This layer uses a stamp identical to that employed
to pattern the first release layer 610, forming a hole 645 in the
center for the axle of the wheel.
[0063] The second metal layer 650 (FIGS. 6F, 7F) is patterned to
create a cap 652 on the axle of the wheel. This cap prevents the
wheel from leaving the axle after all release layers are etched
away. Metal layer 650 is also crosshatched to create small islands
655 of metal. These islands represent excess material and will be
removed when the release layers are etched away, but are included
to facilitate separation of the release layers. During the release
step it may be necessary to use a supercritical CO.sub.2 release to
avoid suckdown problems between the wheel and the substrate.
[0064] After the release layers 610, 620, 640 are etched away by
exposure to a suitable solvent, the device assumes the
configuration shown in FIGS. 6G, 7G. The finished device is a wheel
660 with dimples 662 on its bottom surface, an axle 665 about which
the wheel 660 is free to rotate, and a cap 650 that holds the wheel
660 in place on the axle 665.
[0065] Other MEM structures amenable to production using the
present invention include, for example, so-called heatuators,
linear comb drives, and combustion engines.
[0066] FIGS. 8A-8F illustrate use of the present invention to
create a so-called "biochip," i.e., an electronically active or
readable substrate having a dense array of different biological
materials (e.g., DNA probes, protein probes, carbohydrates). Such a
chip can be used, for example, to identify samples of interest or
to test for the presence of various molecular sequences. See, e.g.,
U.S. Pat. Nos. 5,605,662, 5,874,219, 5,744,305 and 5,837,832. If a
sufficiently large array of different oligonucleotides can be
deposited onto a surface, then one may in principle obtain full
genome sequence information via the method of sequencing by
hybridization (Skiena et al., Proc. 36th Ann. Symp. on Foundations
of Comp. Sci., pp.613-20 (1995)). As shown in FIG. 8A, an
elastomeric stamp 810 has a series of projecting features 815. A
substrate 820 has deposited thereon a thin film of biological
material 822.
[0067] Stamp 810 is lowered until projecting features 815 penetrate
and displace the liquid film 812 to make contact with the
underlying substrate 820 (FIG. 8B). The stamp 810 is then removed
from contact with the substrate 810, leaving a pattern 825 of
biological material and a complementary pattern of regions 827 from
which biological material has been removed (FIG. 8C).
[0068] FIG. 8D shows a second substrate 830 having an array of
projecting features 832 each with a biological receptor 835 bonded
thereto. This biological receptor uniquely bonds to constituents of
the biological material 822; for example, biological material 822
may be a protein solution, and the receptor 835 an antibody
specific for the protein. The second substrate 830 is aligned above
the original substrate 820.
[0069] The second substrate 830 is brought into contact with
substrate 820 (FIG. 8E); some of the projecting features 832
overlie biological material 825, while others overlie voids 827.
Biological material binds to receptors attached to projecting
features that penetrate the liquid, while projecting features
brought into contact with (or proximity to) void areas 827 remain
unmodified. FIG. 8F shows the second substrate 830 removed from
contact with substrate 820. Biological material on the original
substrate was selectively transferred to certain projecting
features 832 of the second substrate 830 and not to others; the
second substrate 830, thus selectively patterned (with features 832
on the order of 10 nm-100 .mu.m) and chemically reacted, may serve
as a biochip. The liquid material remaining on the original
substrate 820 may be used to produce additional biochips.
[0070] If desired, the biochip may be brought into contact with a
third substrate having a different biological material, and which
has been patterned with the original stamp 810 or with a different
stamp. In this way, a second layer of biological material can be
selectively added to various of of the projecting features 832.
[0071] In an alternative approach, a biological resist layer is
patterned by an elastomeric stamp in accordance with the invention,
and is then brought into contact with a substrate having projecting
features. The resist material binds selected projecting features
based on the respective patterns of the features and the resist.
The entire structure is then immersed in a functional biological
material, which binds only to projecting features that have not
received resist. Finally, the structure is immersed in an etch bath
that removes the resist material (and any biological material that
may have bound to it), but leaving undisturbed biological material
bound to features that did not receive resist.
[0072] In a second alternative, biological material may be directly
transferred from the projecting features of the elastomeric stamp
onto selected sites (e.g., raised features) on the substrate. Areas
of the stamp corresponding to recessed features do not transfer
material. In this fashion the substrate may be patterned without
the need for an intermediate transfer step. Spreading of the
transferred material is avoided by maintaining only a very thin
film of material in the plate from which the stamp is "inked." It
is important, of course, that the receiver surface exhibit a higher
affinity for the biological material than the stamp. PDMS has a
very low surface energy, making it ideal for transferring a wide
range of materials.
[0073] It should be stressed that this "direct pattern transfer"
approach to patterning can be employed in connection with materials
other than biological liquids. For example, a metal nanoparticle
dispersion may be applied as a thin film to a flat surface such as
glass or plastic. A patterned elastomeric stamp is brought into
contact with the film of material and withdrawn, and the material
adhering to the stamp transferred to a second surface. Using this
technique, we have obtained conducting structures with edge
resolutions on the order of 300 nm.
[0074] Existing methods for making DNA chips, such as described in
U.S. Pat. No. 5,744,305, are limited in resolution and in requiring
DNA arrays to be constrained to planar and non-porous surfaces.
Using the stamping methods of the present invention and standard
nucleotide chemistry (such as that used in gene-assembly machines),
a DNA biochip may be fabricated in which nucleotides are added one
base unit at a time to build up an array of spatially separated
oligonucleotides that differ in their sequences as a function of
location. For example, chemical synthesis of DNA can be
accomplished by sequential addition of reactive nucleotide
derivatives. Each new nucleotide in the sequence is first blocked
by reaction with 4',4'dimethoxytrityl (DMT) and then combined with
a highly reactive methylated diisopropyl phosphoramidite group,
which links the nucleotide with the one previously added. The
blocking group is removed by detritylation, which renders the newly
linked nucleotide available for linkage to a further nucleotide.
When synthesis is complete, all methyl groups are removed by
exposure to alkaline pH.
[0075] Similarly, by employing the standard chemistries used in
protein-assembly machines (e.g., repeated sequences of chemically
blocking an amino acid, activation, linkage to the most recently
added amino acid, followed by unblocking), carbohydrate-assembly
machines, protein or carbohydrate biochips may be fabricated. In
such biochips it may be desirable to have good separation between
biological domains (such as between oligonucleotides of different
sequence). This may be accomplished by stamping such sequences onto
a non-planar or porous surface. In this context, the term "porous"
refers to non-planar features that physically separate unique
nucleotide sequences (or other chemically distinct biomolecules).
For example, each sequence may be patterned on the top surface of a
raised pillar, each of which is physically separated from its
neighbors. This design allows for convenient removal unwanted
chemistries, since these can be continuously withdrawn as they
accumulate at the bases of the pillars. Alternatively, each
nucleotide sequence may be deposited into a separate recessed
well.
[0076] The stamping process of the present invention can be
efficiently deployed to produce repetitive circuit patterns in two
or three dimensions using a single set of stamps. FIG. 9A
schematically illustrates a single SRAM 900 circuit with a power
rail V.sub.cc 910 and a ground rail 915. The SRAM 900 is addressed
using a horizontal control line 920 and a vertical control line 925
which, when both high, activate the split-gate transistor structure
930 and connect the read/write line 935 to the memory cell. The
volatile memory is stored in a pair of cross-coupled inverters 940.
This circuit can be fabricated using the embossing technique with
five different elastomeric stamps: two metal layers, a
semiconducting layer, a thin insulating layer, and a planarizing
layer with vias.
[0077] FIG. 9B shows the manner in which the basic circuit 900 can
be utilized as a "tile" in a two-dimensional array of such
circuits. In the figure, the circuit 900 is replicated 16 times in
a contiguous, 4.times.4 two-dimensional array 950. This memory
array 950 has power and ground rails, the horizontal control lines
running along the left and right edges 955, and the vertical
control lines and read/write lines running along the top and bottom
edges. The array 950 is produced by applying, in the pattern of the
array, the same five stamps over each applied layer. The stamped
regions interact to form the continuous circuit 950.
[0078] As shown in FIG. 9C, the array can be extended into three
dimensions by replicating the two-dimensional array 950 in a
vertical stack 970. A memory address is divided so that the first
bits of the address decode into a set of horizontal control lines
that all lie in the same two-dimensional position but are stacked
vertically, and the last bits of the address decode the vertical
control lines in the same way. In this fashion a word of memory is
stored in the same two-dimensional position of different arrays in
the vertical stack (so that the number of bits in a word of data
corresponds to the number of vertically stacked memory arrays). The
decoding circuitry on the edges of the memory may also be produced
using the same five masks repeated for each layer with vias
interconnecting the layers.
[0079] This approach is well-suited to construction of so-called
"cellular automata," which are interconnected processing cells that
interact with neighbors to compute in parallel. Cellular automata
are often used to simulate three-dimensional environments, but
conventional approaches are inherently two-dimensional and
therefore limited in processing capacity. By creating circuits in
three dimensions with many layers, it is possible to overcome this
scaling limitation. A cellular-automata device would include many
two-dimensional arrays of cells stacked vertically to create an
interconnected three-dimensional array.
[0080] Another example of three-dimensional devices amenable to
fabrication in accordance with the present invention is a neuronal
structure consisting of many individual electronic "neurons" (each
represented by a processor) arranged in three-dimensions with many
"dendritic" interconnects between neighboring devices. Each neuron
is affected by all of its surrounding neurons and in turns affects
the neurons to which it is connected. Neural networks created in
three-dimensions avoid many of the scaling problems that plague
today's two-dimensional circuits.
[0081] Another application of the stamping process of the present
invention involves creation of electron-emission structures for use
in field-emission displays (FEDs). Today, these devices are
typically fabricated in silicon and are quite expensive and
complicated to produce; the most common structure used is a
Spindt-tip. Recently, research has shown that by using materials
with a lower work function (e.g., single-wall carbon nanotubes),
much simpler structures can be fabricated with equal or better
efficiency than typical silicon emitters (Choi et.al., Society for
Information Display 99 Digest, p. 1134 (1999)). Unfortunately, the
growth temperatures for producing nanotubes are well above the
melting point for glass or plastic substractes (exceeding
800.degree. C.) and have thus not been integrated with processes
employing such substrates.
[0082] In accordance with the present application, a slurry of
metallic (preferably gold) nanoparticles and chopped up nanotubes
(nanopipes) is dissolved in a solvent. As shown in FIG. 10A, this
slurry is then patterned, by stamping, onto a substrate 1010 (e.g.,
a glass sheet) as sets 1020, 1025 of interdigitated lines; some
carbon nanotubes 1030 will protrude from the surfaces of the lines
1020, 1025. Through any of various available techniques (e.g.,
application of an electric field, or exploiting the flow of the
liquid as the stamp is released), these nanotubes may be positioned
to all point in the same directions. Lines 1020, 1025 are then
cured at temperatures below 300.degree. C.
[0083] With reference to FIG. 10B, another layer 1040 of the
nanoparticle slurry is applied so as to completely cover one set of
lines 1020, thereby fully enclosing the carbon nanotubes. This set
of lines 1020 represents the gate of the FED, whereas the set of
lines 1025 represents the cathode. In operation, a phosphored anode
1050 is disposed proximately and in opposition to lines 1020, 1025,
and a high vacuum established between anode 1050 and substrate
1010. Two parameters govern the operation of the FED: the voltage
between the anode 1050 and the cathode lines 1025 (V.sub.ac), and
the voltage between the gate lines 1020 and the cathode lines 1025
(V.sub.gc). The FED is either on or off. To set the FED to the "on"
state, V.sub.ac is set to about 20V and V.sub.gc is set to 0V;
electrons will stream from the cathode lines 1025 to the anode 1050
due to the low work function of the carbon nanotubes, but electrons
will not stream from the gate lines 1020 to the anode 1050. To set
the FED to the "off" state, V.sub.ac remains at about 20V but
V.sub.gc is set to 5V; the electrons from cathode lines 1025 will
then stream to the gate lines 1020 and no electrons will stream to
the anode 1050. A visual display is caused by selective,
line-by-line activation of the cathode lines 1025 to cause electron
streaming therefrom.
[0084] In another application, the stamping process of the present
invention may be combined with existing chip-fabrication processes.
For example, the current high-end microprocessor production process
can be divided into two major steps: the "front-end" processing,
which consists of all steps necessary to produce a working
transistor (e.g., silicon growth, gate oxide, doping, transistor
fabrication); and the "back-end" processing of the wafer that
creates the metal interconnects and vias which establish
connections among the transistors. For high-end chips there may be
a total of 30 mask sets, 18 for front-end processing and 12 for
back-end processing; the complexity and cost of a chip is generally
determined by the number of mask sets employed in its
fabrication.
[0085] In accordance with the present invention, stamping is used
to produce the metal back end for an otherwise typically fabricated
silicon-wafer front end. A wafer is produced using standard silicon
front-end processes up until the point when metal would first be
deposited. Then, instead of evaporating aluminum and applying it
using plasma etching, CPVD, CMP, Damascene planarization, and/or
the other traditional processes (which tend to be expensive,
lengthy, difficult, and wasteful), layers of metallic nanoparticles
are patterned by nanoscale embossing to form the interconnect
layers; in particular, a thin film of a metal nanoparticle solution
is applied (e.g., by a spin-on technique) onto the wafer, and the
film is patterned by embossing as described above to form metal
interconnects and to fill the vias to underlying layers. The
conducting traces thus formed are cured, and a layer of a
dielectric nanoparticle material is deposited thereon. This layer
is then embossed to pattern vias between metal layers, and then
cured. The steps of depositing, patterning, and curing conductive
and insulating layers are repeated until the desired number of
layers is attained.
[0086] This approach offers advantages in terms of cost, time,
waste, and difficulty of production; but, in addition, it also has
the advantage of being self-planarizing. As a result, each layer of
dialectric can be planarized through the stamping process, so that
it is possible to create many more layers than can be obtained
using current, conventional processes. In addition, since the
stamping process is conformal to underlying layers, the quality of
the planarization is not critical (as is the case, for example, in
photolithography, where each layer must be planar to within a few
hunder nanometers).
[0087] Still another application the stamping process of the
present invention is fabrication of organic light emitters, organic
logic, and organic transistors. Organic light emitters and logic
materials such as PPV (poly(p-phenylene vinylene) and thiophene are
difficult to pattern using standard lithographic processes because
the etch process can degrade the organic material. One alternative
approach is to use ink jet (Shimoda et al., Society for Information
Display 99 Digest, p. 376 (1999)), but the resolution of this
process is limited to above 10 .mu.m. The stamping process
described herein facilitates patterning of significantly finer
features.
[0088] Yet another application of the stamping process of the
present invention is patterning of optical waveguides. An optical
waveguide is a structure in which a first region possesses a first
index of refraction and a second region possesses a second index of
refraction. A very simple optical waveguide may be made by simply
embossing a rectangular ridge in an optically transparent material
(such as spin-on glass or UV optical polymer) surrounded by air.
Light directed into one end of the ridge will emerge at the other
end. By combining such printed optical waveguides with printed
light emitters such as organic electroluminescent materials,
inorganic electroluminescent materials or hybrid electroluminescent
materials and with printed detectors (such as phototransistors or
photodiodes) and switches (such as electro-optical switches), it is
possible to construct an "all-printed" or partially printed
switching fabric for control of incoming optical signals and
transmission of output optical signals for various
optical-telecommunications applications.
[0089] FIG. 11 shows a block diagram of a preferred nano-embossing
system, indicated generally at 1100. The system operates on a
substrate 1110, which is secured to a Z-translation stage 1115. The
Z-translation stage is secured to a 360.degree. theta stage 1120,
which rotates in the XY plane. Theta stage 1120 is itself secured
to a carrier 1125 on a gantry system 1130 adapted for
two-dimensional movement in the XY plane. These components can
transport substrate 1110 to any spatial position within the limit
of movement, and with arbitrary XY rotation. A series of functional
modules are suspended above substrate 1120, each module performing
a different step in the embossing process: depositing thin films of
material on the substrate, patterning the thin film, and curing the
film following embossing.
[0090] In particular, thin films of liquid are produced on
substrate 1110 by a metal rod 1135.sub.1 and an ejection device
1140.sub.1 (e.g., an ink jet head or pipet) that deposits a small
amount of liquid as described above in connection with FIGS. 2A,
2B. Additional sets of metal rods and ejection devices
(representatively indicated at 1135.sub.2, 1140.sub.2) are
available for deposition of different liquids. The deposited liquid
films are patterned by an elastomeric stamp, which may be selected
from a plurality of available stamps representatively shown at
1150.sub.1, 1150.sub.2. The stamps are each retained within a
suitable stamping press (not shown), the outer contours of the
stamps fitting within complementary recess within the stamping
equipment.
[0091] The patterned films are cured by a device 1160 (e.g., a
thermal lamp, a UV lamp, a laser, etc.) as appropriate to the film.
The substrate 1110 travels back and forth between these different
modules and an aribtrary number of layers may be patterned thereon.
Alignment of these different modules with respect to substrate 1110
can be accomplished, for example, using optical fiduciary marks as
commonly used for silicon mask alignment. In addition, fine-grained
alignment of the stamps 1150 may be performed using physical
self-alignment of the stamp. For example, each stamp 1150 may
contain deeply recessed triangular features that merge with raised
alignment features on the substrate 1110. The stamp itself is
preferably capable of translation and rotation during
alignment.
[0092] Alternatively, a nano-embossing system in accordance with
the present invention may comprise a "roll-to-roll" process
facilitating continuous production of functional devices. A
roll-to-roll process resembles conventional letterpress printing
processes, with the stamps of the present invention configured as
elastomeric letterpress plates. A plate is rotated on a drum,
making gentle contact with a moving substrate onto which the
curable liquid has been deposited.
[0093] Nanoparticles in solution for use with the present invention
may be fabricated using a process similar to chemical vapor
deposition (CVD), alternative configurations for which are
illustrated in FIGS. 12A and 12B. With reference to FIG. 12A,
controlled flows of a CVD precursor gas and an inert carrier gas
are introduced into a heated vacuum chamber 1200 through respective
mass-flow controllers 1210, 1215. The chamber 1200 is generally
tubular in shape and is heated by a surrounding resistive coil. The
wall of chamber 1200 is substantially transparent to radiation from
a pair of orthogonally oriented lasers 1225.sub.1, 1225.sub.2. The
organic capping material is introduced in vapor form into chamber
1200, downstream of lasers 1225.sub.1, 1225.sub.2, by means of a
flow controller 1230. A collecting table 1232 is disposed within
chamber 1200 still further downstream, and is chilled by
recirculation of a cooling fluid through a pair of valves
1235.sub.1, 1235.sub.2. Gaseous material is drawn through chamber
1200 in the direction of the arrow by a vacuum source (not
shown).
[0094] As the CVD precursor travels through chamber 1200, it is
dissociated by a combination of the elevated temperature in the
chamber and energy imparted by lasers 1225.sub.1, 1225.sub.2. The
respective concentrations of CVD precursor and inert carrier are
chosen such that mean free path of the chemically pure, dissociated
elements or molecules permits, on a probabilistic basis, only
hundreds of collisions with other like species before the organic
vapor introduced through flow controller 1230 is encountered. With
each collision, more and more of the dissociated species come
together, thereby forming larger particles. Capping this growing
particle with an organic shell prevents it from further increasing
in size. The inert gas carries the growing particles from the
dissociation region to the capping region at a known rate, and once
capped, the particles are collected on chilled collecting table
1232. The carrier gas and unreacted precursor exit the chamber
1200. The resulting nanoparticles 1240, in the form of a paste on
the plate 1232, are then removed from the vacuum chamber and put
into solution. The solution is subjected to gravity or
centrifuging, and the nanoparticles of the smallest size are
skimmed off the top.
[0095] Suitable CVD precursors include silane, TIBA
(tri-isobutyl-Al), WF.sub.6, and Cu(hfac).sub.2 (i.e., copper
hexafluoroacetylacetonate) with helium and argon as inert carrier
gasses. Suitable organic capping groups include straight-chain
alkyl groups that chemically bond to the particle, or groups that
interact with the particle surface through a heteroatom such as
sulfur, oxygen, nitrogen, or silicon. Other suitable organics, as
disclosed in U.S. Pat. No. 5,750,194, include alpha-terpineol,
methyl oleate, butyl acetate, glyceride linoleate, glyceride
linolenate, glyceride oleate, citronellol, geraniol, phenethyl
alcohol, and nerol.
[0096] As shown in FIG. 12B, the use of more reactive species
justifies a simpler configuration that may include a vacuum chamber
1250, which is evacuated by a vacuum pump 1260 operating through a
valve 1260. A CVD precursor gas and an organic capping group in
vapor form are introduced into vacuum chamber 1250 through
respective mass-flow controllers 1260, 1265. The CVD precursor
quickly agglomerates into particles, and is capped by the organic
vapor. The particles 1270 collect on a chilled table 1275, and are
collected as described above.
[0097] Although the present invention has been described with
reference to specific details, it is not intended that such details
should be regarded as limitations upon the scope of the invention,
except as and to the extent that they are included in the
accompanying claims.
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