U.S. patent application number 12/150063 was filed with the patent office on 2008-12-04 for method of forming catalyst nanoparticles for nanowire growth and other applications.
Invention is credited to Yong Chen, Theodore I. Kamins, Philip J. Kuekes.
Application Number | 20080296785 12/150063 |
Document ID | / |
Family ID | 32107210 |
Filed Date | 2008-12-04 |
United States Patent
Application |
20080296785 |
Kind Code |
A1 |
Kamins; Theodore I. ; et
al. |
December 4, 2008 |
Method of forming catalyst nanoparticles for nanowire growth and
other applications
Abstract
Methods for forming a predetermined pattern of catalytic regions
having nanoscale dimensions are provided for use in the growth of
nanowires. The methods include one or more nanoimprinting steps to
produce arrays of catalytic nanoislands or nanoscale regions of
catalytic material circumscribed by noncatalytic material.
Inventors: |
Kamins; Theodore I.; (Palo
Alto, CA) ; Kuekes; Philip J.; (Menlo Park, CA)
; Chen; Yong; (Redwood City, CA) |
Correspondence
Address: |
HEWLETT PACKARD COMPANY
P O BOX 272400, 3404 E. HARMONY ROAD, INTELLECTUAL PROPERTY ADMINISTRATION
FORT COLLINS
CO
80527-2400
US
|
Family ID: |
32107210 |
Appl. No.: |
12/150063 |
Filed: |
April 23, 2008 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
|
|
10281678 |
Oct 28, 2002 |
7378347 |
|
|
12150063 |
|
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Current U.S.
Class: |
257/786 ;
257/E23.01 |
Current CPC
Class: |
H01L 2924/0002 20130101;
C30B 29/605 20130101; C30B 11/12 20130101; B82Y 10/00 20130101;
C30B 11/00 20130101; H01L 2924/0002 20130101; B82Y 30/00 20130101;
H01L 2924/00 20130101 |
Class at
Publication: |
257/786 ;
257/E23.01 |
International
Class: |
H01L 23/48 20060101
H01L023/48 |
Claims
1-23. (canceled)
24. A device comprising: i) a mold with nanoscale protrusions
forming a desired pattern; and ii) catalytic material coating the
protrusions
25. A device comprising: i) a layer of masking material; and ii) an
underlying layer said masking material having a desired pattern of
nanoscale depressions exposing the underlying layer.
26. The device of claim 25, wherein the underlying layer contains
catalytic material.
27. The device of claim 25, further comprising catalytic material
deposited within the nanoscale depressions.
28. A device comprising: i) a substrate: ii) a catalytic layer on
the substrate; iii) a non-catalytic layer over the catalytic layer;
and iv) a layer of masking material overlying the noncatalytic
layer, said masking material having a desired pattern of nanoscale
depressions exposing the non-catalytic layer.
29. The device of claim 28, wherein the nanoscale depressions also
expose nanoscale regions of the catalytic layer.
30. A device comprising: i) a substrate; and ii) a regular array of
catalytic nanoislands on the substrate.
31. A device comprising: i) a substrate; ii) a catalytic layer on
the substrate; and iii) a non-catalytic layer over the catalytic
layer, said non-catalytic layer having a regular pattern of
nanoscale openings exposing the catalytic layer.
Description
CROSS-REFERENCE TO RELATED APPLICATION
[0001] This application is a divisional of prior application Ser.
No. 10/281,678, filed on Oct. 28, 2002.
BACKGROUND
[0002] Growth of silicon nanowires offers the possibility of
forming arrays with a large surface-to-volume ratio. These arrays
can be used for chemical or environmental sensing, for electrical
transduction, or for electron emission.
[0003] Bulk synthesis of semiconductor nanowires has been
traditionally achieved using several variations of transition metal
catalyzed techniques such as vapor-liquid-solid (VLS) synthesis.
See, e.g., Kamins et al., J. Appl. Phys. 89:1008-1018 (2001) and
U.S. Pat. No. 6,248,674. In standard vapor-liquid-solid (VLS)
synthesis techniques used for producing silicon nanowires, each
wire grows from a single particle of gold, cobalt, nickel or other
metal. A vapor-phase silicon-containing species transported to the
catalyst inside a high-temperature furnace condenses on the surface
of the molten catalyst, where it crystallizes to form silicon
nanowires.
[0004] Silicon nanowires produced by the standard VLS process are
composed of a single crystal. In the standard process, the size of
the catalytic particle controls the diameter of the nanowire grown
from it. Thus, in order to obtain a uniform nanowire diameter
distribution, monodispersed catalyst particles need to be created
on a solid substrate. However, creation of nanometer size catalyst
droplets is a non-trivial task. The nanoparticles can be formed by
deposition techniques, such as chemical vapor deposition or
physical vapor deposition. Although they can be registered to
previously formed patterns, creating these pattern requires
additional processing, usually involving costly lithography. In
addition, conventional lithography processes cannot readily form
nanoparticles of the desired small dimensions Thus, there is a need
for improved methods of forming evenly spaced catalytic particles
having dimensions in the nanometer range.
SUMMARY OF THE INVENTION
[0005] The present invention is directed to nanoimprinting or soft
lithography methods for creating arrays of catalyst nanoparticles
useful for forming nanoscale wires for device applications. The
methods of the present invention are capable of forming smaller
catalyst islands, move rapidly and less expensively than is
possible with conventional lithography or even with electron-beam
lithography.
[0006] In one embodiment, the method of forming an array of
catalytic nanoparticles includes the steps of (1) providing a mold
with nanoscale protrusions forming a desired pattern; (2) coating
the protrusions with catalytic material; and (3) transferring the
desired pattern of catalytic material to a substrate by contacting
the substrate with the catalytic material.
[0007] In another embodiment, nanoscale regions of catalyst are
localized within depressions of a non-catalytic layer by a method,
which includes the steps of (1) depositing a layer of masking
material on an underlying layer; (2) providing a mold with
nanoscale protrusions forming a desired pattern; (3) pressing the
protrusions of the mold into the masking material so that
depressions are formed in the masking layer in the desired pattern;
(4) exposing the underlying layer in the depressions; and (5)
localizing catalytic material in the depressions. In a preferred
embodiment, the underlying substrate itself is the source of the
catalytic material localized in the depressions. Alternatively, the
catalytic material is selectively deposited in the depressions.
[0008] Another embodiment is a method of forming nanoscale regions
of exposed catalyst, comprising the steps of: (1) obtaining a
substrate; (2) providing a catalytic layer on the substrate; (3)
forming a non-catalytic layer over the catalytic layer; (4)
depositing a layer of masking material on the non-catalytic layer;
(5) providing a mold with nanoscale protrusions forming a desired
pattern; (6) pressing the protrusions of the mold into the masking
material so that depressions are formed in the masking layer in the
desired pattern; (7) exposing regions of the non-catalytic layer in
the depressions; (8) etching the exposed non-catalytic regions to
expose regions of the catalytic layer; and (9) removing the masking
material.
[0009] In a preferred embodiment a mold formed from parallel layers
of nanoscale thickness, are used for the imprinting process. The
mold can be made by: (1) providing a plurality of alternating
layers of a first material and a second material forming a stack of
parallel layers, wherein the first material is dissimilar from the
second material, each layer having a nanoscale thickness; (2)
cleaving and/or polishing the stack normal to its parallel layers,
thereby creating an edge wherein each layer of the first and second
materials is exposed; and (3) creating a mold having a pattern of
alternating recessed and protruding lines by etching the edge of
the stack in an etchant that attacks the first material at a
different rate than the second material, thereby creating said
pattern on the edge of the stack. The mold can then be used to
create linear patterns of catalyst in further steps, which include:
(1) providing a catalytic layer overlying a substrate and coating
the catalytic layer with a masking material layer; and (2) forming
a first set of nanoimprinted lines in the masking material layer,
by pressing the protruding lines of the mold into the masking
material layer exposing strips of the catalytic Iayer; and (3)
etching the exposed strips of the catalytic layer to form lines of
catalyst having a nanoscale width. Preferably, the method further
comprises the steps of: (1) rotating the mold; and (2) applying the
rotated mold to the masking material, thereby creating a second set
of lines in the masking material, which intersect the first set of
lines; and (3) etching the catalytic material that is not protected
by the masking material. If the two sets of lines created by
sequential application of the mold are orthogonal, a rectangular
array of squares is created. Alternatively, the two sets of lines
can be oriented at a non-perpendicular angle, thereby creating a
skewed array of parallelograms.
[0010] Yet another embodiment of the present invention is a method
for exposing nanoscale regions of catalytic material surrounded by
a noncatalytic layer. Starting materials include a multilayered
composite comprised of a substrate, a layer of catalytic material
covering the substrate, and a masking layer formed over the
catalytic layer. A first set of lines is imprinted in the masking
layer using a mold having a patterned edge of alternating recessed
and protruding nanoscale strips. The mold is then rotated and
reapplied to form polygons of masking material in a regular array.
The catalytic material that is not protected by the masking
material is then covered with a non-catalytic material and the
masking material is removed to expose nanoscale regions of the
catalytic material circumscribed by non-catalytic material. The
exposed regions of catalytic material can then be used as catalysts
for nanowire growth.
BRIEF DESCRIPTION OF THE DRAWINGS
[0011] These and other features, aspects, and advantages of the
present invention will become better understood with respect to the
following description, appended claims, and accompanying drawings
where:
[0012] FIGS. 1A-1C shows steps for transferring a catalyst to a
substrate using a mold in accordance with an embodiment of the
present invention;
[0013] FIGS. 2A-2D show steps for imprinting a layer of masking
material and exposing an underlying layer in accordance with an
embodiment of the present invention;
[0014] FIGS. 3A-3D show steps for exposing a catalytic layer below
a noncatalytic layer after imprinting in accordance with an
embodiment of the present invention;
[0015] FIGS. 4A-4B show steps for selectively adding catalyst to an
exposed surface in accordance with an embodiment of the present
invention;
[0016] FIG. 5A-5D show steps to make and use a superlattice mold
for nanoimprinting one or more sets of lines in accordance with an
embodiment of the present invention; and
[0017] FIGS. 6A-6C show steps for exposing catalyst regions
surrounded by a noncatalytic layer in accordance with an embodiment
of the present invention.
DETAILED DESCRIPTION
[0018] In accordance with the present invention, methods are
provided for creating arrays of catalytic material useful for
forming nanowires for device applications. For controlled
application of the nanowires, they should be regularly spaced or
positioned on the substrate in a predetermined pattern. Growth of
each nanowire generally proceeds from a catalyst nanoparticle on
the substrate surface. As shown here, when the catalyzing
nanoparticles do not have to be registered to underlying structure,
they can be formed by "soft lithography," or nanoimprinting, which
involves forming impressions of a mold, having nanoscale features,
onto a layer of underlying material.
[0019] FIGS. 1A-1C shows steps for transferring a catalyst to a
substrate using a mold in accordance with one version of the
present invention. The mold typically contains an array of
protruding and/or recessed regions having nanoscale dimensions. In
FIG. 1A, a mold 10 is provided with nanoscale protrusions 20
forming all the individual elements of the desired pattern. A
suitable mold for use in nanoimprint lithography is disclosed in
U.S. Pat. Nos. 5,772,905 and 6,309,580 (incorporated herein by
reference).
[0020] The mold 10 can be patterned with protruding features 20,
such pillars, stripes, rectangular solids, or other
three-dimensional designs. Protruding features having a minimum
lateral size of 25 nm can be obtained using electron beam
lithography, reactive ion etching (RIE) and other appropriate
technology. Preferably, protruding features of the mold will have a
lateral dimension of 5 nm to 20 nm. A mold 10 having the desired
pattern of protruding nanoscale features at resolution levels much
less than that of the-state-of-the-art e-beam lithography can be
made according to methods described in further detail below, or as
disclosed in the related application of Chen et. al. [HP Docket No,
100110197-1] (incorporated herein by reference). The typical depth
of a protruding feature is from 5 nm to 500 nm, depending on the
desired lateral dimension and the depth of the desired impression
to be made.
[0021] In general, the mold 10 should be made of a relatively hard
material capable of retaining its shape and integrity under the
pressure applied during a nanoimprinting process. Accordingly, the
mold can be made of materials such as metals, dielectrics,
semiconductors, ceramics, or their combination.
[0022] As shown in FIGS. 1B and 1C, the protrusions 20 of mold 10
are coated with a material containing the desired catalyst, and the
catalytic material 30 is then transferred to a substrate 40 having
a non-catalytic surface 50 by physical contact, or possibly by an
energetic or chemical attraction between the catalytic material and
the surface of the substrate when they are brought close
together.
[0023] In general, the catalytic material 30 includes a catalyst
capable of catalyzing the growth of nanowires. Accordingly, the
catalytic material can include metals used to generate silicon
nanowires, such as titanium, gold, zinc, silver, copper, platinum,
palladium, nickel, and manganese. Alternatively, the catalytic
material can include a catalyst capable of catalyzing the growth of
carbon nanotubes or metal nanowires.
[0024] In this version of the present invention, the substrate 40
can be any material having a noncatalytic surface 50 capable of
accepting the catalytic nanoparticles transferred from the mold,
e.g., a silicon, silicon dioxide, silicon nitride or alumina
substrate.
[0025] FIGS. 2A-2D show steps for imprinting a layer of masking
material and exposing an underlying layer in accordance with
another version of the present invention. As shown in FIG. 2A,
before imprinting, a thin masking layer 100 is deposited on top of
an underlying layer 110. The masking layer 100 should be a
relatively soft material capable of retaining an impression from
the mold. For example, the masking layer may comprise a
thermoplastic polymer deposited by an appropriate technique, such
as spin casting. The underlying layer can be substrate, a
non-catalytic layer, or a catalytic layer.
[0026] As shown in FIGS. 2B and 2C, the mold creates patterned
masking regions 130 punctuated by depressions 120 in the thin
masking layer 100 overlying the underlying layer 110. In FIG. 2B,
the depressions 120 formed by the nanoimprinting technique uncover
exposed regions 140 of the underlying layer 110. Alternatively, as
shown in FIGS. 2C and 2D, compressed regions, which do not contact
the underlying layer, are formed in the masking material that
generally conform to the pattern of the mold. The underlying layer
is exposed subsequently by directional etching through the
remaining thickness of the imprinted masking material by an etching
process such as reactive-ion etching (RIE).
[0027] In one version of the invention, the exposed regions 140 of
underlying material 110 in the depression 120 can be the catalyst.
In an alternative version, the exposed underlying material 110 can
be a non-catalytic material on which the catalyst can be deposited
selectively without any deposition on the surrounding region. Such
selective deposition can be accomplished by, for example, chemical
vapor deposition or liquid-phase deposition.
[0028] FIGS. 3A-3D show steps for exposing a catalytic layer below
a noncatalytic layer after imprinting in accordance with another
version of the present invention. As shown in FIG. 3A, the
following multi-layered structure is prepared before imprinting:
(1) a catalytic layer 220 is provided overlying a substrate 230;
(2) a non-catalytic layer 210 is provided overlying the catalytic
layer 220; and (3) a thin masking layer 200 is deposited on top of
the underlying non-catalytic layer 210. Generally, the thin masking
layer is composed of material resistant to etching procedures, such
as a polymer material, whereas the material comprising the
non-catalytic layer is susceptible to etching in suitably selected
etchants. For example, a silicon oxide layer can be selectively
etched in a wet etchant that contains hydrogen fluoride (HF) or by
dry etching in a fluorcarbon gas.
[0029] As shown in FIG. 3B, the mold creates a patterned masking
layer 240 punctuated by depressions uncovering exposed regions 250
of non-catalytic layer 210. As shown in FIG. 3C, the exposed
regions of the non-catalytic layer are then subjected to an etching
step to form an etched non-catalytic layer 260 punctuated by
exposed catalytic regions 270. Preferably, the etching step is
conducted using a directional etching process, such as reactive-ion
etching (RIE), to avert undercutting portions of the non-catalytic
layer protected by the patterned masking layer 240. As shown in
FIG. 3D, the masking material can be removed by, for example,
selectively dissolving the masking material in a solvent, thereby
uncovering the top surface of the etched non-catalytic layer.
[0030] FIGS. 4A-4B shows steps for selectively adding catalyst to
an exposed surface in accordance with some versions of the present
invention. More particularly, some versions of the preceding
procedures produce a patterned masking layer 310 punctuated by
regions exposing a surface of an underlying layer 300. As shown in
FIG. 4A, when the underlaying layer does not contain a catalyst, an
array of regularly spaced catalyst nanoislands 320 can be generated
by selectively depositing catalytic material on the exposed
surfaces of the underlaying layer 300 interposed throughout the
patterned masking layer. As shown in FIG. 4B, the patterned masking
layer can then be removed to uncover the underlying layer, dotted
with slightly elevated catalyst nanoislands.
[0031] In any case, an array of discrete nanoscale regions of
catalytic material are formed at the locations determined by the
pattern on the mold, and nanowires are then grown by catalytic
decomposition of a silicon-containing gas such as silane
(SiH.sub.4) or dichlorosilane (SiH.sub.2Cl.sub.2).
[0032] FIGS. 5A-5D show steps to make a regular array of uniformly
spaced nanoparticles using a nanoimprinting device in accordance
with another version of the present invention. A nanoimprinting
device can be provided using previously described techniques [U.S.
Pat Nos. 6,294,450, 6,365,059 and 6,407,443; and U.S. Patent
Application Number 2001/0044300, incorporated herein by reference].
As shown in FIG. 5A, a mold 400 is constructed by growing or
depositing a plurality of alternating layers of two dissimilar
materials, comprising a layer of first material 410 and a layer of
second material 420, on a mold substrate 430. The alternating
layers form a stack, with the thickness of each layer determined by
the required nanoparticle dimension and spacing. Typical dimensions
and spaces are 5 nm to 100 nm. The stack will have a major surface
parallel to that of the substrate 430. The stack is cleaved and/or
polished normal to its major surface to expose the plurality of
alternating layers. The exposed layers are then etched to a chosen
depth using an etchant that etches the first material 410 at a
different rate than the second material 420, thereby creating a
pattern of recessed strips on the edge of the stacked structure
400.
[0033] As shown in FIG. 5B, the edge structure can then be used to
form nanoimprinted lines 450 in a layer of masking material
overlying a layer of catalyst 440. The material not protected by
the pattern can be etched at this time, creating nanoimprinted
lines, as described previously (supra).
[0034] FIG. 5C shows the results of further processing in
accordance with one version of the present invention. After the
first set of lines 450 is formed, as in FIG. 5B, the mold 400 is
rotated and then applied to the structure again, creating a second
set of lines in the masking material. Both patterns can be defined
in the masking material before etching the underlying device
material, or the device material can be etched after each
nanoimprinting step. FIG. 5C also shows the device material and the
masking material established on a substrate 460.
[0035] The two sets of intersecting lines can be orthogonal,
creating a rectangular array of square masking elements 470, as in
FIG. 5C or they can be purposely oriented at a non-perpendicular
angle, creating a skewed array of parallelograms. Moreover, the
mold can be rotated and applied again to create polygonal shaped
masking elements.
[0036] As shown in FIG. 5D, catalyst nanoislands 480 can be created
by etching exposed regions of the catalyst and removing the
protective masking elements. Alternatively, if non-catalytic
materials are initially used, the nanoislands can be treated to
create catalyst islands. For example, the catalyst can be added
chemically by selective addition to the islands formed in FIGS. 5C
and 5D, or physically using the different elevations of the islands
and the background regions. These catalyst nanoislands can then be
used as catalysts for nanowire growth.
[0037] FIGS. 6A-6D. show another version of a method for exposing
catalyst regions surrounded by a noncatalytic layer. As shown in
FIGS. 6A and 6B, a layer of the catalytically active material 500
is formed on a substrate 510, and a layer that is not catalytically
active 530 is formed over the catalytically active layer 500. FIG.
6A shows that before forming the non-catalytic layer 530, the
method described above, i.e., of imprinting a first set of lines,
then rotating and reapplying the mold to create a regular array of
polygons in the masking material, is used to form a mask 520 over
the desired catalytically active regions. Another alternative (not
shown) would include a harder masking material underlying a top
masking layer, which is made of material suitably soft for
nanoimprinting. The pattern would be transferred to the underlying
layer of hard masking material by etching. FIG. 6B shows the
surrounding areas being covered by the noncatalytically active
material 530, which is not deposited on (or removed by, e.g.,
chemical-mechanical polishing from) the protected regions of the
catalyst. As shown in FIG. 6C, the masking material is then
removed, leaving nanometer size exposed regions of the
catalytically active material 540 surrounded by material that is
not catalytically active.
[0038] The previously described versions of the present invention
have many advantages. In particular, the methods of the present
invention are capable of forming smaller catalyst islands than are
possible with conventional lithography or even with electron-beam
lithography. The present methods can also form catalyst islands
more rapidly and less expensively than electron-beam lithography
because nanoimprinting is a parallel process (forming many patterns
at the same time), rather than a serial process (forming patterns
sequentially) like electron-beam lithography.
[0039] Although the present invention has been described in
considerable detail with reference to certain preferred versions
thereof, other versions are possible. For example, the technique
described above, of rotating a nanoscale mold having a pattern of
recessed strips, can also be used to form an intermediate mold
having a regular array of polygonal protrusions. The entire pattern
of small nanoislands can then be formed in or on an underlying
material in one impression. Alternatively, pattern definition and
lift-off techniques can be used, in which a nanoscale pattern is
formed in an underlying material and later removed along with any
material deposited on top of the pattern. Therefore, the spirit and
scope of the appended claims should not be limited to the
description of the preferred versions contained herein.
* * * * *