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BACKGROUND OF THE INVENTION
1. Field of the Invention
This invention relates to the formation of structures on a nanometer size
scale.
2. Description of the Prior Art
In recent years interest has increased in the fabrication of electronic,
optical and/or biomolecular devices of nanometer size (nanometer
structures). An attractive feature, of such molecular size devices is the
vast number that may be packed into a small area, i.e., the high density
that is possible. Computers which incorporate such devices would have
significantly increased memory and speed. The incorporation of biological
molecules into such devices is also desirable and would be facilitated by
the ability to fabricate structures on a nanometer scale. The fabrication
of devices on the nanometer level also is desirable since new physical
effects not obtainable for larger size devices beome important. An example
is the patterning of surfaces for utilization of surface enhanced Raman
scattering phenomena.
Current methods of producing such nanometer devices include writing on the
surface of a substrate or on a substrate covered with an electron
sensitive resist material with a focused electron beam. This serial method
of device fabrication is, however, limited by the long times required to
produce the vast number of nanometer devices possible in small areas
(i.e., over 10,000 devices in a micron square area) Additionally,
conventional methods do not presently offer convenient means of
incorporating active biological molecules into nanometer size devices.
A particularly advantageous method of producing microdevices and
micropatterns consists of lithographic reproduction of an existing pattern
from a suitable mask onto the device substrate. Such a preexisting pattern
is transferred by placing the mask in proximity to the underlying
substrate. Then the pattern is etched into or applied to the substrate
using a variety of methods including exposure to a beam consisting of
reactive ions, electromagnetic radiation or reactive molecules The mask is
normally produced by ,a serial writing method such as focusing an electron
beam on a suitably sensitive material such as a resist material Current
lithographic methods are limited by the size of the individual features
which can be embedded in a mask, the size of the mask as well as the time
necessary to produce these masks In particular, currently employed
lithographic masks will typically have features which are micron in
dimension whereas nanometer microdevice fabrication requires masks
containing nanometer scale details. While serial writing of such a pattern
on a nanometer scale is possible, using known methods such as focused
electron beams, the necessary time for production of such a pattern is a
serious limitation for the practical fabrication of nanometer devices See,
for example, U.S. Pat. Nos. 4,103,064 and 4,103,073, Craighead et al,
"Ultra-Small Metal Particle Arrays Produced by High Resolution
Electron-Beam Lithography", J. Appl. Phys. 53 (11) (Nov. 1982), pp.
7186-7188, Mochel et al "Electron Beam Writing on a 20-A Scale in Metal
8-Aluminas", Appl. Phys. Lett. 42 (4) (Feb. 1983), pp. 392-394, Isaacson
"Electrons, Ions and Photons in Submicron Research", Submicron Research,
Cornell University (1984), pp. 28-32.
Attempts have also been made to formulate techniques to fabricate nanometer
scale devices "in parallel" wherein a number of devices are made at the
same time in a relatively few number of steps. For example, it has been
suggested that large biological molecules may be used as a mask to apply a
nanometer scale pattern of material on a substrate. Suggestions for the
particular large molecule to be employed have included DNA and denatured
proteins., See Carter, "Molecular Level Fabrication Techniques and
Molecular Electron Devices", J. Vac. Sci. Technol. Bl (4) (Oct.-Dec.
1983), pp. 959-968, Carter, "Biotechnical Synergism in Molecular
Electronics", Nonlinear Electrodynamics in Biological Systems, Plenum
Press, pp. 260-273 and Tucker, "Biochips: Can Molecules Compute?", High
Technology, Vol. 4, No. 2 (Feb. 1984), pp. 36-47 and 79, see particularly
p. 46.
SUMMARY OF THE INVENTION
The present invention advances techniques for creating nanostructures
beyond those techniques known to date as described above. In accordance
with the present invention, arrays consisting of numerous identical
nanometer scale structures are created in a few steps. Such nanostructures
are based on nanometer patterns created by self-assembled two-dimensional
molecular arrays.
In the present invention, a substrate surface, which can either serve as a
passive support such as a carbon film or have intrinsic solid state
properties used in the device, such as silicon, supports a self-assembled
two-dimensional molecular array exhibiting density, thikness and/or
chemical reactivity variations. The array may typically consist of
two-dimensionally crystallized nondenatured proteins which retain their
native properties of a very regular nature. Alternatively, the array may
consist of other non-biological material.
The two-dimensional self-assembled molecular array may be used to transfer
a pattern contained in the molecula,r array onto either an underlying
substrate or an overlying coating. In a simple embodiment of the present
invention, a self-assembled molecular array includes a two-dimensionally
ordered array of holes formed by nondenatured protein molecules bound to a
substrate base. A functional material to be used in the nanostructure is
deposited onto the substrate surface through the holes. The molecular
array is then removed to leave a substrate surface covered by a periodic
array of islands composed of the material. A particular advantage of
employing nondenatured protein molecules is their extremely regular
nature.
In another embodiment, the two-dimensional self-assembled molecular array
which is bound to a base support substrate is overcoated with a thin film
of material to be used in the nanostructure. The film may be of metal,
insulator or semiconductor. The film is then irradiated with an ion beam,
for example, to replicate the pattern of holes in the underlying molecular
array. In fact, the present invention may be employed to create improved
masks and templates to be used in lithographic production of nanometer
structures.
Alternatively, holes through the selfassembled molecular array may be
employed as a pattern for etching the substrate.
Composite devices, biomolecular-solid state heterostructures consisting of
biologically active molecules and other functionable materials may be
produced according to the present invention.
BRIEF DESCRIPTION OF THE DRAWINGS
These and other objects and advantages of this invention will become more
apparent and more readily appreciated from the following detailed
description of the presently preferred exemplary embodiments of the
invention taken in conjunction with the accompanying drawings, of which:
FIG. 1 is a schematic diagram of a nanostructure including a self-assembled
two-dimensional molecular array used as a template for an overlayer;
FIG. 2 is a schematic diagram of a nanostructure consisting of a
self-assembled two-dimensional molecular array used as a mask for the
deposition of additional material on a substrate;
FIG. 3 is a schematic diagram of a nanostructure consisting of a
self-assembled two-dimensional molecular array used as a mask for removing
material from a substrate;
FIG. 4 is a schematic diagram of the electron density of the self-assembled
two-dimensional molecular array from Sulfolobus acidocaldarius
superimposed with the results of metal deposition;
FIG. 5 is a schematic diagram of one possible finished nanostructure
according to the present invention;
FIGS. 6A-6H schematically illustrate the process for forming the structure
of FIG. 5;
FIG. 6I represents a nanostructure according to another embodiment of the
present invention;
FIGS. 7A-7D schematically illustrate the method of forming a nanostructure
in accordance with yet another embodiment of the present invention; ,
FIGS. 8A-8D schematically illustrate the steps of forming a structure
according to the present invention with a multilayered substrate having
material deposited therein and having an overlying pattern of material
commensurate with the deposits in the substrate; and
FIG. 8E is a schematic illustration of a nanostructure according to the
present invention with a multilayered substrate having deposits therein
and an overlying pattern of material incommensurate with the deposits in
the substrate.
DETAILED DESCRIPTION Of THE PRESENTLY PREFERRED EXEMPLARY EMBODIMENTS
A fundamental structural unit that forms the basis of all embodiments of
this application and is illustrated in FIGS. 1-5 includes a substrate 10
supporting a self-assembled two-dimensionally ordered molecular array
containing thickness, density or chemical reactivity variations in a
spatial pattern having a characteristic dimension of 1-50 nanometers. Of
course, a pattern of variations in thickness may very well be a pattern of
holes extending through array 12.
Substrate 10 may be made of a passive supporting material such as a
molecularly smooth amorphous carbon, crystalline mica or graphite or a
functional material to be used in the ultimate nanodevice such as silicon.
The substrate base may also consist of a multilayer composite such as
layers 10a illustrated in FIGS. 8A-8E. Multilayered substrate 10a may
include silicon coated by a thin insulating layer of silicon dioxide or
other material, for example.
In some cases, the substrate base may be a well defined area which is part
of a larger base support. For example, the base may contain a micropattern
consisting of a thin layer of conductive material such as aluminum which
is deposited by conventional lithographic methods. In this case, array 12
could be bound selectively to specific areas on base 10 delineated by the
micropattern. This can be accomplished by using a variety of conventional
methods which serve to activate the specific areas delineated by the
micropattern so that they bind array 12. For example, array 12 could be
made of a material that normally carries an electrical charge and which is
attracted to the patterned conducting layer through application of a
voltage differential between this layer and a second electrode placed
above substrate 10. Alternatively, a pattern which is lithographically
transferred to a substrate base using conventional methods can be
selectively activated by using glow discharge. In this case, surrounding
areas which do not contain the micropattern would be prevented from
developing an attractive charge because they would be coated with an
insulating layer.
Self-assembled array 12 can be conventionally produced using a variety.of
protein molecules including hemocyanin,,cytochrome oxidase, porin from the
E. coli outer membrane, acetylcholine from nicotinic end-plate receptors,
rhodopsin from photoreceptor membranes and in some cases are found to
occur, in nature such as in the S-layer from Sulfolobus acidocaldarius or
the purple membrane from Halobacterium halobium. Approximately 1000
distinct two-dimensional crystallizable proteins are known. A general
method for production of such a self-assembled protein array is described
by Keegstra and van Bruggen, "Electron Microscopy at Molecular Dimensions"
Springer-Verlag, New York (1980), pp. 318-327.
A second example of array 12 involving the membrane protein rhodopsin is
given by B.L. Scott et al, "Two-Dimensional Crystals in
Detergent-Extracted Disk Membranes From Frog Rod Outer Segments (ROS)"
Bio. Phys. J. Vol. 33, (1981) p. 293a. Self-assembled membrane arrays 12
are typically 5-20 nanometers in thickness and normally have an overall
diameter between 0.1 and 1 micron. It is possible, however, to form larger
membranes using established procedures such as membrane fusion which
consists of adding a detergent such as cetyltriammonium bromide (CTAB) to
a suspension of membranes and then incubating at room temperature for
several days.
Examples of array 12 which contain an ordered pattern of through holes
include the two-dimensional array formed by hemocyanin, a respiratory
protein from the spiny lobster Panulirus interruptus, according to the
method of Keegstra and van Bruggen, supra, and the S-layer which is
naturally found in the facultatively sulphur oxidizing microorganism,
Sulfolobus acidocaldarius and isolated according to the method of Michel
et al, Electron Microscopy at Molecular Dimension, Ed. by W. Baumesiter
and W. Vogell, Springer-Verlag (1980), p. 27. Numerous other examples of
array 12 which form naturally or can be formed using known methods include
those formed from the proteins cytochrome oxidase, porin and connexon.
Bacteriorhodopsin in the purple membrane of Halobacterium halobium and
rhodopsin isolated from frog or bovine retinas form arrays 12 which have a
pattern of two-dimensional density and thickness variations. The
two-dimensional crystals formed from ribosomes, well known in the
literature, form a biomolecular assembly containing both protein and
nucleic acid which may be used as array 12.
Array 12 may also have chemical reactivity variations spatially distributed
thereover Examples of materials which may be used for array 12 exhibiting
chemical reactivity variations include protein molecules which exhibit
catalytic activity or act as a coenzyme for catalytic activity. Array 12
of purple membrane formed from bacteriorhodopsin molecules exhibit such a
spatially varying chemical reactivity since it acts as a light-driven
proton pump. Hence, a local pH gradient will be created at the site of
each bacteriorhodopsin molecule during illumination of the entire array
with light of 570 nm wavelength. This local pH gradient could be used to
activate pH sensitive reactions in the molecules of an overlayer thereby
producing a structural pattern within the overlayer on a nanometer scale
dimension. A large number of reactions are known which are pH sensitive
including reactions involving molecules contained in many commercially
available pH sensing papers.
Array 12 can also consist of the surface layer of a three-dimensional
crystal. For example, most three-dimensional crystals formed from protein
molecules consist of large void spaces filled with water which comprises
up to 80% of the total volume of the crystal. The top surface layer
exhibits a large variation in contour which is ideal for acting as a
template in the same manner as a monolayer two-dimensional crystal as
described herein. Examples include crystals formed from the protein
molecules lysozyme and hemoglobin.
Additionally, many examples of two- and three-dimensional array 12 of
non-biological origin exist which are useful in the present invention. For
example, zeolites from a class of crystalline materials characterized by
periodic arrays of nanometer dimension holes may be employed in the
present invention, as illustrated by Wright et al, "Localizing Active
Sites in Zeolitic Catalysts", Nature, Vol. 318, p. 611 (1985). These
non-biological arrays could be used as single crystal surfaces or in the
form of thin films, grown epitaxially as unit cell monolayers on a
crystalline substrate. Microscopic colloidal particles of uniform size are
another non-biological example useful as array 12. Such an array is formed
by slowly altering the chemical properties of a colloidal particle
suspension in contact with substrate 10 to be coated in such a way as to
cause particles to come out of suspension and deposit on the surface of
substrate 10. In an aqueous suspension of polymer spheres, for example,
the salt concentration can be increased. When this, process is done
slowly, ordered arrays of particles of diameters from 5 nm to about 1000
nm can be deposited on the surface.
Various techniques may be employed to bind array 12 to substrate 10 or
array 12 may be formed on substrate 10 in situ. Binding can be
accomplished electrostatically which is typically achieved by treating
substrate 10 chemically or by placing substrate 10 in a glow discharge
chamber. Array 12 is then applied as an aqueous suspension of membrane
fragments on substrate 10 which is subsequently dried by evaporation,
blotting with an adsorbant medium or by mechanical shaking. Alternatively,
array 12 can be bound using a binding layer between substrate 10 and array
12 which can consist of an application of polylysine or another molecule
which binds preferentially to substrate 10 and array 12.
To chemically bind array 12 to substrate 10, substrate 10 may be chemically
treated or coated in such a way as to promote the binding of array 12. For
example, a substrate of carbon may be coated with an adsorbed molecular
layer of the cationic polyamino acid poly-l-lysine in order to bind array
12 with an anionic surface. Alternatively, for example, the anionic
polyamino acid poly-l-glutamate will bind cationic array 12 to substrate
10. For example, substrate 10 may be placed in a vacuum and exposed to a
mild plasma etch (glow discharge) prior to the application of the
polyamino acid. A 10% solution of the polyamino acid is applied to
substrate 10 and left to incubate for several minutes. Substrate 10 is
then washed with distilled water and air dried. Array 12 is then applied
to the coated substrate 10.
Array 12 may be bound to substrate 10 by electrical means when substrate 10
is selected to be electrically conducting. Conducting substrate 10 may be
placed in contact with an aqueous suspension of two-dimensional ordered
membrane sheets which carry a surface charge. Applying an electric field
between substrate 10 and a second electrode in contact with: the
suspension can both orient the sheets with one side preferentially facing
substrate 10 and move the membrane sheets onto substrate 10. For example,
purple membrane has been shown to exhibit ordering in suspension in
applied electric fields. See, Keszthelyi, "Orientation of Purple Membrane
by Electric Fields" Methods in Enzymology, Vol. 88, L. Packer (Editor),
pp. 287-297.
Related orientational effects can be achieved by magnetic fields.
Electro-chemical means may be employed to bind array 12 to substrate 10. As
an example, substrate 10 may have a patterned Ta/W film on one surface
with an ordered array of holes in the Ta/W film exposing a carbon base. An
oxide film is rapidly produced on the Ta/W film by applying 0.1 N NaOH to
substrate 10 while applying a voltage between the metal film and the NaOH.
This is accomplished by connecting the grid on which substrate 10 sits to
the positive terminal of a battery and connecting the negative terminal to
the NaOH solution via a small electrode. A resistor in series with the
grid controls the current draw. In this fashion, an oxide layer is
produced on the metal, while no oxide is formed on the exposed carbon
holes. After deposition of the oxide layer, the grid is washed, dried and
then glow discharged in order to make the carbon hydrophilic, for the
subsequent adsorption of a different molecule, such as the protein
ferritin. The ferritin is preferentially adsorbed on the exposed carbon
holes and not on the metal oxide layer surrounding them.
It is also possible to crystallize array 12 directly on the surface of
substrate 10 using the properties of the substrate to promote
crystallization. This is accomplished using methods of two-dimensional
protein crystallization well-known in the literature. An example given by
Keegstra and van Bruggen, supra, involves the assembly of two-dimensional
arrays of Panulirus interruptus hemocyanin. A small drop of hemocyanin
protein in a 10 millimolar (mM) sodium acetate buffer is placed on an
electron microscope grid having a parlodion-carbon supporting substrate:
The grid, with the protein solution on top, is floated on a 50 mM sodium
acetate solution for a time from 30 minutes to several hours at 4.degree.
C. with the specimen supporting substrate 10 acting as a membrane for
dialysis. Then, the grid is washed and dried.
From the basic structure of array 12 on base 10, a number of alternative
embodiments can be created. For example, as illustrated in FIG. 1,
overlayer 14 may be applied on top of array 12. Overlayer 14 may consist
of a thin layer typically 1-2.5 nanometers thick of a material selected
for its ultimate function in subsequent steps in the nanometer structure
fabrication. For example, when pattern 16 in array 12 includes holes,
pattern 16 may be transferred into overlayer 14 by ion milling to form
holes 18. As will be explained below, overlayer 14 may then be removed and
used as a nanometer mask for further steps in the nanodevice fabrication.
In this case, the material for overlayer 14 may be chosen for its
flexibility and structural integrity. Alternatively, overlayer 14 may
comprise a functional material in the ultimate nanodevice and be chosen
for its optical/electronic properties.
A specific example of the embodiment illustrated in FIG. 1 will now be
provided. In this embodiment, array 12 is the crystalline proteinaceous
cell wall surface layer (S-layer) from the thermophilic bacterium
Sulfolobus acidocaldarius a sulfur-oxidizing microorganism whose natural
habitat is hot acidic springs. The S-layer consists of 10 nm thick
monolayer periodic array of a single glycoprotein having a molecular
weight in the range of 140-170 kdaltons. Specimens of purified S-layer
show an array of protein dimers arranged as shown in FIG. 4, a basis of
three dimers on a two-dimensional triangular lattice having a 22 nm
lattice parameter. The three-dimensional structure is porous, the protein
occupying 30% of the volume within the 10 nm thick sheet containing the
S-layer.
Substrate 10 in this embodiment of the nanostructure device consists of a
30 nm thick amorphous carbon film which provides a molecularly smooth
surface. The carbon film is deposited on an electron microscope grid 3
millimeters in diameter using conventional methods. The S-layers are
employed in the form of aqueous suspensions (12 mg/ml) of primarily
monolayer crystalline fragments of varying size up to 0.5 micron diameter.
Adsorption of the S-layer to the carbon substrate is accomplished by
exposing the substrate to a mild plasma etch (glow discharge) in order to
remove organic contaminants and render carbon substrate base 10
hydrophilic. A drop of S-layer suspension is then placed in contact with
substrate 10 and rinsed with double distilled water. Excess water is drawn
off with filter paper and the preparation air dried.
Overlayer 14 is then deposited by evaporation. In this embodiment,
overlayer 14 is a thin layer Ta/W which is chosen because of its fine
grain. Application of overlayer 14 is accomplished by e-beam evaporation
of 80% tantalum and 20% tungsten (by weight) at 2.times.10.sup.-6 torr at
room temperature at an incident angle of 40 degrees from the normal to
substrate 10. This produced an average overlayer thickness of 1.2 nm as
determined by a quartz crystal monitor and grain sizes of 2-3 nm as
determined by electron microscopy.
To pattern overlayer 14 to assume pattern 16 of thickness or density
variations in array 12, overlayer 14 is exposed to an argon ion mill for a
short period of time in order to punch holes corresponding to variations
16 in array 12.
Substrate 10 in this example may consist of a large variety of materials.
For example, for the purpose of electrical contact, substrate 10 may be
fabricated from a conductor such as aluminum. Alternatively, substrate 10
may consist of a layered structure such as a conductor coated by a thin
insulator such as aluminum and aluminum oxide or a semiconductor material
such as a P-doped silicon. To provide a guide for binding array 12, or as
a suitable surface for crystallization, substrate 10 may consist of a
patterned surface produced by conventional scanning beam methods or by
other methods described herein.
Alternatively, a liquid surface may be utilized since it is well known that
many arrays such as purple membrane can be layered on the surface of a
liquid such as water. All of the formation techniques described herein may
be carried out with an array supported by such a liquid surface. One
advantage of such a substrate 10 is the ease by which the patterned
nanostructure is removed from liquid substrate 10.
Thus, some proteins, in particular biomembrane based systems, are easier to
grow as large single crystals with the proteins and a lipid matrix at an
air-water interface. Once formed, a two-dimensional protein crystal at an
air-liquid interface could be used as a template for nanopatterning an
evaporated metal overlayer 14 as has been described above. Patterned metal
layer 14 could then be transferred to a solid substrate by the usual
techniques employed to deposit surfactant (Langmuir-Blodgett) films.
Alternatively, overlayer 14 once patterned, could be removed from
two-dimensional array 12 by immersing substrate 10 in a fluid and floating
it on the air-fluid interface as is routinely done for carbon-platinum
freeze-fracture replicas.
Overlayer 14 may be part of a larger pattern formed using conventional
methods of integrated circuit ,fabrication. In this case, a suitable mask
formed from a resist is used to allow deposition to occur only in the
desired areas including the area overlying array 12.
Overlayer 14 may also consist of a thin layer of material which has been
patterned using the methods described herein. For example, a thin layer
metal which has been patterned into a metal screen containing nanometer
sized holes can be produced as described herein. This layer can be
selectively bound directly over a two-dimensional self-assembled array 12
to serve as overlayer 14.
Alternatively, a nonpatterned overlayer 14 may be deposited on an array,
and then coated with a second array 12. The second array is used to
pattern the layer using masking techniques to be described and then
removed. In a second fabrication step, the underlying array serves as a
template for further patterning as described above. The two arrays could
be commensurate, producing a periodic structure, or incommensurate,
producing a modulated structure, as will be described below with respect
to FIGS. 8A-8E.
A variety of overlayers 14 may be patterned using arrays 12 as templates,
for example thin metal films as has been described above. As other
examples, overlayer 14 itself may be a layered structure produced by
multistage evaporation or by chemical treatment, e.g., oxidation, of an
initially homogeneous film. A first layer of Ta/W may be followed by Pt,
for example.
Instead of using array 12 as a template for producing a patterned overlayer
14, array 12 may be used as a mask for applying material on or in
substrate 10 or.removing material from substrate 10. In FIG. 2, array 12
is employed as a mask for forming deposits 20 on substrate 10. In FIG. 3,
mask 12 is employed to control the removal of material from substrate 10
forming pits 22. In the embodiment of FIG. 2, array 12 advantageously has
a pattern of through holes allowing penetration of atoms or molecules
through array 12 to substrate 10. In the embodiment of FIG. 3, density or
thickness variations in array 12 can be transferred directly into
substrate 10 utilizing penetrating X-ray or electron beams which then
react with substrate resist material. To form pits 22, an agent may be
applied to the assembly which reacts directly with substrate 10 or is
activated by the additional exposure of a penetrating beam such as visible
light. Alternatively, the removal or alteration of the composition of base
material 22 might occur through electrical conduction of ions through the
channels formed in array 12.
The embodiment illustrated in FIG. 5 is a more detailed version of that
illustrated in FIG. 1 in that it consists of array 12 on substrate 10 with
overlayer 14 having holes 18 therein. In this embodiment, holes 18 have a
diameter of approximately 15 nm arranged on a triangular lattice with 20
nm periodicity. Holes 18 are formed in overlayer 14 which is best
characterized as a lattice or screen consisting of two intersecting
periodic sets of metal strips. Film 14, which is approximately 1 square
micron in area, is part of larger micropattern which has been fabricated
using conventional methods. Conducting electrod,e material 24 is disposed
adjacent to array 12 and overlayer 14 to complete the assembly.
The method of forming the embodiment of FIG. 5 is illustrated in FIGS.
6A-6H. First, as illustrated in FIG. 6A, the 1 square micron area of
exposed carbon layer 10 to which the S-layer array 12 is bound is
fabricated by depositing a photoresist material 26 onto a larger
carbonffilm and then exposing and developing this resist using
conventional prior art microdevice fabrication methods in the area where
the nanostructure is to be formed. A hydrophobic resist material is chosen
to minimize binding of the S-layer to the surface of resist 26. Then,
array 12, consisting of the S-layer is disposed in the space on substrate
10 layed open after removal of the exposed photoresist. Then, as
illustrated in FIG. 6B, a one nanometer thick layer of Ta/W is deposited
as described above with respect to FIG. 1. That is, overlayer 14 is
deposited using evaporative beam techniques applied at a 40.degree. angle
to the normal. It should be noted that the undeveloped layers of
photoresist 26, i.e., those areas where the carbon layer of substrate 10
is not exposed, will also be coated with the Ta/W.
In FIG. 6C, holes 18 are formed in overlayer 14 using argon ion mlling at
normal incidence. In this example, a 2 KV beam of 0.2 ma/cm.sup.2 is
employed for 25 seconds This has the effect of transferring spatially
ordered pattern 16 in array 12 to overlayer 14., Note that the size of the
screen formed depends on the time of exposure to the argon ion milling For
example, thinner sized screen strips and larger holes can be produced by
increasing the exposure time to ion milling from 25 to 30 seconds.
Conditions, of course, will vary depending upon the thickness of metal
film 14 and exact specifications of the ion mill.
In addition to ion milling, holes 18 can be formed by X-ray irradiation,
electromagnetic irradiation, electron beam irradiation, particle beam
irradiation, etching, chemical removal, solvent removal, plasma treatment
or any combination of these various techniques.
In FIG. 6D, the areas of substrate 10 which had been covered by photoresist
26 and film 14 are removed by exposing and developing the photoresist
using conventional methods.
In FIGS. 6E-6H, conductors 24 of micron dimension are fabricated on
substrate 10 leading to and from the nanostructure device using
conventional methods In FIG. 6E, photoresist layer 28 is placed over the
entire area of substrate 10 including areas covered by the 1 nanometer
thick Ta/W film 14 which contains the nanometer screen pattern. A wire
pattern is exposed on photoresist 28 and upon developing, the photoresist
is removed from areas where the wire pattern occurs in FIG. 6F. In FIG.
6G, conducting metal layer 30, such as aluminum, is deposited. In those
areas where the wire pattern has been developed, metal 30 will directly
coat carbon base 10. In step 6H, photoresist 28 and overlying metal 30 are
removed, leaving only conducting metal layer 24 to form,the wires.
Although the nanostructure described was produced on a carbon layer, it is
to be understood that a similar method can be used to produce nanometer
patterned devices on other useful solid state surfaces such as silicon. In
addition, while the fabrication of only a single nanometer patterned
microdevice was described, a similar multistep method could be used to
fabricate in parallel many such nanodevices which are arranged in a
complex pattern comprising an electronic or optical integrated circuit.
Although conventional electrodes have been utilized in the example of FIGS.
6A-6H which directly connect to the nanostructure array, alternative
methods of interacting directly with such a patterned array are also
possible. For example, each individual nanostructure can be addressed
using a scanning tunnelling microscope probe. In the case of optically
sensitive material, entire arrays can be addressed with a focused laser
beam.
The device as illustrated in FIGS. 1 and 5 can be considered a
microsubstrate comprising a base substrate, a lithographic mask or
template material containing an ordered nanometer scale pattern and
overlying material to which the patterned nanometer scale detail has been
transferred. The method described in FIGS. 6A-6H demonstrates that such a
microsubstrate can be used to produce a microdevice based on the nanometer
patterned array. In contrast to conventional methods for production of
micropatterns, the methods thus far described and the articles produced
therefrom.allow significantly smaller patterns to be produced. For
example, conventional photo-lithographic methods for production of
integrated circuits result in microdevices typically one micron square
area. The embodiment described in FIGS. 6A-6H results in the patterning of
several thousand nanostructures in a one micron square area.
In addition, as described with respect to FIG. 6A-6H, conventional
lithography can be used in conjunction with the microsubstrate to produce
a nanometer pattern within the normally unpatterned area used to produce
conventional micro devices. The present invention also offers significant
advantages over more conventional methods of writing nanometer size
patterns on a substrate surface with an electron beam. The present
invention allows for parallel production of large numbers of nanometer
structures thus offering a significant decrease in the time it would take
to produce the same microdevices with the serial write method, for example
with a scanning electron beam.
FIGS. 7A-7D illustrate a manner in which the structure of FIG. 2 and other
useful structures can be formed. As illustrated in FIG. 7A, array 12 is
formed on substrate 10. Then, as illustrated in FIG. 7B, a three nanometer
thick layer 33 on array 12 and pro3ections 32 in holes 16 through array 12
is formed using an evaporative beam at normal incidence. This structure
may be created in the same manner as illustrated in FIGS. 6A-6B except
that the metal is applied at normal incidence. Layer 33 and projections 32
may be a metal, such as Ta/W or any other material.
In FIG. 7C, array 12 is lifted off with layer 33 to leave projections 32
deposited on substrate 10. Projections 32 obviously have the same pattern
as holes, 16 in array 12. Array 12 can be lifted off using a number of
techniques which break up the organic biomolecular array and do not effect
projections 32. For example, array 12 may be exposed to a detergent
solution such as 5% SDS (sodium dodecylsulfate), proteolyt enzymes which
specifically degrade protein, or acid such as HCl, or a c | | |
