Title: Method of molecular-scale pattern imprinting at surfaces
Abstract: A method for mask-free molecular or atomic patterning of surfaces of reactive solids is disclosed. Molecules adsorb at surfaces in patterns, governed by the structure of the surface, the chemical nature of the adsorbate, and the adsorbate coverage at the surface. The surface is patterned and then imprinted with the pattern by inducing localized chemical reaction between adsorbate molecules and the surface of the solid, resulting in an imprint being formed in the vicinity of the adsorbate molecules. When the imprinted molecular patterns are conjugated chains containing .pi. bonds along which electrical charge can flow the molecular patterns constitute molecular wires or the imprinted molecules constitute a molecular-scale device. The surface of the substrate can be doped by including n- or p-type dopants in the adsorbate molecules. These molecular wires are anchored to the substrate by using conjugated chains which can be chemically bound at intervals along the chains to the substrates.
Patent Number: 6,878,417 Issued on 04/12/2005 to Polanyi, et al.
| Inventors:
|
Polanyi; John C. (86 Willcocks Street, Toronto Ontario, CA M5S 1C8);
Rogers; Duncan (3801 W. Spring Creek Pkwy., Plano, TX 75023)
|
| Assignee:
|
Polanyi; John C. (Tornoto, CA);
Rogers; Duncan (Ibaraki, JP)
|
| Appl. No.:
|
276828 |
| Filed:
|
April 8, 2003 |
| PCT Filed:
|
May 18, 2001
|
| PCT NO:
|
PCT/CA01/00695
|
| 371 Date:
|
April 8, 2003
|
| 102(e) Date:
|
April 8, 2003
|
| PCT PUB.NO.:
|
WO01/88960 |
| PCT PUB. Date:
|
November 22, 2001 |
| Current U.S. Class: |
427/533; 427/552; 427/510; 427/497; 438/515; 438/694; 438/766; 438/795 |
| Intern'l Class: |
B05D 003//06; H01L 021//26.3; H01L 021//32.15 |
| Field of Search: |
427/510,511,515,497,498,503,504,526,527,533,576,581,596,597,595
216/2.12,65,66,94
438/514-536,694,710,712,766,765,767,769,795,796,798
|
References Cited [Referenced By]
U.S. Patent Documents
| 4340617 | Jul., 1982 | Deutsch et al. | 427/581.
|
| 4566937 | Jan., 1986 | Pitts | 216/13.
|
| 4608117 | Aug., 1986 | Ehrlich et al.
| |
| 4615904 | Oct., 1986 | Ehrlich et al.
| |
| 4701347 | Oct., 1987 | Higashi.
| |
| 5035782 | Jul., 1991 | Tamura et al. | 427/504.
|
| 5112434 | May., 1992 | Goldberg | 427/581.
|
| 5273788 | Dec., 1993 | Yu | 427/581.
|
| 5279867 | Jan., 1994 | Friedt et al. | 427/581.
|
| 5322988 | Jun., 1994 | Russell et al.
| |
| 5393699 | Feb., 1995 | Mikoshiba et al. | 427/576.
|
| 5405481 | Apr., 1995 | Licoppe et al.
| |
| 5492734 | Feb., 1996 | Matsumoto et al. | 427/576.
|
| 5512328 | Apr., 1996 | Yoshimura et al. | 427/503.
|
| 5645897 | Jul., 1997 | Andra.
| |
| 5935454 | Aug., 1999 | Tada et al. | 427/552.
|
| 6007969 | Dec., 1999 | Hatakeyama et al. | 427/552.
|
| 6432317 | Aug., 2002 | Douglas et al. | 216/62.
|
| 6517401 | Feb., 2003 | Ogawa et al. | 427/553.
|
| 6630404 | Oct., 2003 | Babcock | 438/694.
|
| 6737286 | May., 2004 | Tao et al. | 438/694.
|
| 2004/0023519 | Feb., 2004 | Clark et al. | 438/795.
|
| Foreign Patent Documents |
| 3725169 | Feb., 1989 | DE.
| |
Other References
Placement of conjugated oligomers in an alkanethiol matrix by scanned probe
microscope lithography; Chen et al.; Applied Physics Letters, vol. 75, No.
5; Aug. 2, 1999 pps 624-626.
Selective nanoscale growth of titanium on the Si(001) surface using an
atomic hydrogen resist; Mitsui et al.; Journal of Applied Physics; vol.
86, No. 3; Aug. 1, 1999, pps 1676-1679.
Organo-germanium adsorption on a silicon surface by excimer-light
irradiation; Ohshima; Applied Surface Science 107 (1996) 85-89, no month.
The adsorption of C.sub.6 H.sub.5 Cl on Si(111)7.times.7 studies by STM;
Chen et al.; Surface Science 340 (1995) p. 224-230, no month.
"Laser-induced microscopic etching of GaAs and InP," D. J. Ehrlich, R. M.
Osgood, Jr., and T.F. Deutsch, 1980 American Institute of Physics; Appl.
Phys. Lett 36(8), Apr. 15, 1980 pps.698-700.
"Conversion of SiCl Pair and Island Sites to SiCl Single Site upon
Annealing of Ci/Si(111)-7.times.7 Surfaces", Chun Yan, John A. Jensen and
Andrew C. Kummel, 1995 American Chemical Society, J. Phys. Chem. vol. 99,
No. 16, 1995, pps. 6084-6090, no month.
Anonymous, "Fabrication of Gold Nanostructures by Lithography with
Self-Assembled Monolayers", IBM Technical Disclosure Bulletin, vol. 39,
No. 12, Dec. 1996, pp. 235-238, NY.
M. Balooch and W.J. Siekhaus, "Spontaneous and STM-Induced Reaction of XeF2
with Si(111)-7.times.7 at Low Coverage", Nanotechnology 7, 1996, no month
pp. 356-359, IOP Publishing Ltd., UK.
Chen X. H. et al., "Photoetching of Si(111)-(7.times.7) Studied by STM",
Surface Science, Apr. 10, 1997, Elsevier, Netherlands, vol. 376, No. 1-3,
pp. 77-86.
Chen X. H. et al., "Adsorption of C.sub.6 H.sub.5 Cl on Si(111) 7.times.7
Studied by STM", Surface Science, Oct. 20, 1995, Elsevier Netherlands,
vol. 340, No. 3, pp. 224-230.
|
Primary Examiner: Padgett; Marianne
Attorney, Agent or Firm: Schumacher; Lynn C.
Hill & Schumacher
Parent Case Text
CROSS REFERENCE TO RELATED PATENT APPLICATION
This patent application is a continuation-in-part patent application of
U.S. patent application Ser. No. 09/573,683 filed on May 19, 2000 entitled
METHOD OF MOLECULAR-SCALE PATTERN IMPRINTING AT SURFACES, which and which
is incorporated herein by reference in its entirety.
Claims
What is claimed is:
1. A method of patterning a surface of a solid on a molecular scale,
comprising:
providing a reactive solid having a surface;
forming a pattern of adsorbed molecules at sites on said surface of said
reactive solid by providing a plurality of preselected molecules and
exposing said surface to said preselected molecules; and
forming an imprinted pattern covering less than every surface atom on said
surface by irradiating the surface and the pattern of the adsorbed
molecules with effective excitation energy to form a chemical bond between
at least one constituent of the adsorbed molecules and the reactive solid
locally to the sites where the adsorbed molecules are adsorbed on said
surface, wherein said pattern of adsorbed molecules include elongated
chains along which electrical charge can flow, including selected
functional groups chemically bound to selected constituents in said
elongated chains at intervals therealong, and wherein when said pattern of
adsorbed molecules is irradiated said chemical bond between said selected
functional groups and said constituent is broken, and wherein said
selected constituent forms a chemical bond with said surface of said
substrate at said intervals to anchor the imprinted elongated chains to
said surface.
2. The method according to claim 1 wherein said elongated chains include
conjugated diene or aromatic chains, and wherein said selected
constituents are side chains spaced along said elongated chains.
3. The method according to claim 1 wherein said selected functional group
is chlorine.
4. The method according to claim 1 wherein the imprinted elongated chains
along which electrical charge can flow are nanoscale wires.
5. The method according to claim 1 wherein said preselected molecules
include a functional group having at least one constituent capable of
interaction with a constituent in a functional group in a neighboring
preselected molecule adsorbed on said surface wherein said adsorbed
molecules form a two dimensional network on said surface in which said
dopant atoms are substantially regularly spaced.
6. The method according to claim 5 wherein said interaction is hydrogen
bonding.
7. A method of patterning a surface of a solid on a molecular scale,
comprising:
providing a reactive solid having a surface;
forming a pattern of adsorbed molecules at sites on said surface of said
reactive solid by providing a plurality of preselected molecules and
exposing said surface to said preselected molecules;
exposing said surface to an effective blocking agent subsequent to exposing
said surface to said preselected molecules to militate against surface
diffusion of said adsorbed molecules; and
forming an imprinted pattern covering less than every surface atom on said
surface by irradiating the surface and the pattern of the adsorbed
molecules with effective excitation energy to form a chemical bond between
at least one constituent of the adsorbed molecules and the reactive solid
locally to the sites where the adsorbed molecules are adsorbed on said
surface, wherein said blocking agent does not imprint on said surface
under irradiation,
wherein said pattern of adsorbed molecules include elongated conjugated
chains along which electrical current can flow, including selected
functional groups chemically bound to a selected constituent in said
conjugated chains at intervals therealong, and wherein when said pattern
of adsorbed molecules is irradiated said chemical bond between said
functional group and said constituent is broken, and wherein a dangling
bond from said selected constituent chemically binds to an atom in said
surface of said substrate to anchor said elongated conjugated chains to
said surface at said intervals along said conjugated chains.
8. The method according to claim 7 wherein said elongated conjugated chains
include conjugated diene or aromatic chains.
9. The method according to claim 7 wherein said selected constituents are
side chains spaced along said elongated conjugated chains.
10. A method of patterning a surface of a solid on a molecular scale,
comprising:
providing a reactive solid having a surface;
forming a pattern of adsorbed molecules at sites on said surface of said
reactive solid by providing a plurality of preselected molecules and
exposing said surface to said preselected molecules; and
forming an imprinted pattern covering less than every surface atom on said
surface by irradiating the surface and the pattern of the adsorbed
molecules with effective excitation energy to form a chemical bond between
at least one constituent of the adsorbed molecules and the reactive solid
locally to the sites where the adsorbed molecules are adsorbed on said
surface, wherein said preselected molecules include at least one dopant
atom which is either a p-type or an n-type dopant atom, and wherein said
at least one constituent of the adsorbed molecule is said dopant atom, and
wherein said imprinted pattern is a nanoscale wire.
11. The method according to claim 10 wherein said preselected molecules
include a functional group having at least one constituent capable of
interaction with a constituent in a functional group in a neighboring
preselected molecule adsorbed on said surface wherein said adsorbed
molecules form a two dimensional network on said surface in which said
dopant atoms are substantially regularly spaced.
12. The method according to claim 10 wherein said substrate is a
semiconductor and said dopant atoms are aluminum atoms.
13. The method according to claim 10 wherein said substrate is a
semiconductor and said dopant atoms are arsenic atoms.
14. The method according to claim 11 wherein said interaction is hydrogen
bonding.
15. A method of patterning a surface of a solid on a molecular scale,
comprising:
providing a reactive solid having a surface;
forming a pattern of adsorbed molecules at sites on said surface of said
reactive solid by providing a plurality of preselected molecules and
exposing said surface to said preselected molecules;
exposing said surface to an effective blocking agent subsequent to exposing
said surface to said preselected molecules to militate against surface
diffusion of said adsorbed molecules; and
forming an imprinted pattern covering less than every surface atom on said
surface by irradiating the surface and the pattern of the adsorbed
molecules with effective excitation energy to form a chemical bond between
at least one constituent of the adsorbed molecules and the reactive solid
locally to the sites where the adsorbed molecules are adsorbed on said
surface, wherein said blocking agent does not imprint on said surface
under irradiation,
wherein said preselected molecules include at least one dopant atom which
is either a p-type or an n-type dopant atom, and wherein said at least one
constituent of the adsorbed molecule is said dopant atom.
16. A method of patterning a surface of a solid on a molecular scale,
comprising:
providing a reactive solid having a surface with spaced electrically
conducting electrodes formed on said surface;
forming a pattern of adsorbed molecules at sites on said surface of said
reactive solid by providing a plurality of preselected molecules and
exposing said surface to said preselected molecules; and
forming an imprinted pattern covering less than every surface atom on said
surface by irradiating the surface and the pattern of the adsorbed
molecules with effective excitation energy to form a chemical bond between
at least one constituent of the adsorbed molecules and the reactive solid
locally to the sites where the adsorbed molecules are adsorbed on said
surface, wherein said pattern of adsorbed molecules comprise elongated
conjugated chains along which electrical charge can flow, wherein each
elongated chain includes a selected functional group chemically bound to a
selected constituent in said elongated chains, and wherein when said
pattern of adsorbed molecules is irradiated said chemical bond between
said selected functional group and said selected constituent is broken
whereafter said selected constituent forms a chemical bond with an
electrically conducting electrode on said surface, and wherein the
electrical contact between the electrically conducting electrode and the
elongated chain chemically bound thereto is an ohmic electrical contact.
17. A method of patterning a surface of a solid on a molecular scale,
comprising:
providing a reactive solid having a surface;
forming a pattern of adsorbed molecules at sites on said surface of said
reactive solid by providing a plurality of preselected molecules and
exposing said surface to said preselected molecules;
exposing said surface to an effective blocking agent subsequent to exposing
said surface to said preselected molecules to militate against surface
diffusion of said adsorbed molecules; and
forming an imprinted pattern covering less than every surface atom on said
surface by irradiating the surface and the pattern of the adsorbed
molecules with effective excitation energy to form a chemical bond between
at least one constituent of the adsorbed molecules and the reactive solid
locally to the sites where the adsorbed molecules are adsorbed on said
surface, wherein said blocking agent does not imprint on said surface
under irradiation,
and including spaced electrically conducting electrodes formed on said
surface, wherein said pattern of adsorbed molecules comprise elongated
conjugated diene or aromatic chains along which electrical charge can
flow, wherein each elongated chain includes a selected functional group
chemically bound to a selected constituent in said elongated chains, and
wherein when said pattern of adsorbed molecules is irradiated said
chemical bond between said selected functional group and said selected
constituent is broken whereafter said selected constituent forms a
chemical bond with an electrically conducting electrode on said surface,
and wherein the electrical contact between the electrically conducting
electrode and the elongated chain chemically bound thereto is an ohmic
electrical contact.
18. The method according to claim 17 wherein said electrically conducting
electrodes are metallic or semiconductor.
Description
FIELD OF THE INVENTION
The present invention relates to a method for pattern imprinting, on an
atomic or molecular-scale, of the surface of a solid by inducing localized
chemical reaction between adsorbate molecules and the surface of the
solid.
BACKGROUND OF THE INVENTION
Progress in the hundred billion dollar semi-conductor industry depends, in
part, on the ability to mark (i.e. write, dope or etch) a surface with
small features at controlled separations. The current limit is the making
of marks separated by a few tenths of a nanometer (commonly 0.3 microns,
i.e. 3,000 A, which is roughly one thousand atoms separation). Patterns of
these dimensions constitute the lower limit of what can be achieved by the
conventional method of marking, which involves the use of a patterned mask
to shield portions of the surface from the agent (electrons, light or
chemicals) used in order to mark the surface. It has not proved possible
to make patterned masks having features smaller than tenths of a micron.
Moreover, masks with such small features already suffer from
irreproducibility.
U.S. Pat. No. 5,645,897 issued to Andra discloses a method for surface
modification by ion bombardment of the surface or the region in front of
the surface portion being etched or coated. The ion source is chosen to
produce ions which are highly charged and possessing kinetic energies
sufficiently high to permit the ions to approach the surface but low
enough to prevent penetration of the surface. A stated advantage of the
process of this patent is that the highly charged state of the ions and
their low kinetic energies results in very localized energy deposition
thereby giving rise to improved spatial resolution in the imprinting of
patterned masks for etching or coating the surface. This patent also
discloses combining the feature of localized energy deposition using the
ion beams with conventional lithographic masking techniques for producing
precise etching patterns.
U.S. Pat. No. 5,405,481 issued to Licoppe et al. is directed to a gas
photonanograph device for production of nanometer scale surface patterns.
The device includes a head comprising a fiber optic cable terminating in a
tip and microcapillary channels also terminating at the tip that feed
reactive gas from a gas reservoir. The tip is spaced from the area of the
substrate surface being light activated. Nanopatterns can be produced by
scanning this device, as one might write with a pen, the tip of the pen
here being a focused light source.
U.S. Pat. No. 4,701,347 issued to Higashi specifically mentions the
photolysis of molecules adsorbed on a surface as a method for growing
patterned metal layers on semiconductor. However, in common with earlier
patents cited therein, going back to U.S. Pat. No. 3,271,180 issued on
Sep. 6, 1966, the pattern of photolytic and thermal reaction induced by
illumination of the adsorbate derives from the presence of a mask between
the light source and the adsorbed layer.
U.S. Pat. No. 5,322,988, in common with U.S. Pat. No. 4,701,347 referred to
above, uses laser irradiation to induce photochemical and thermal reaction
between an adsorbate layer and the underlying substrate, but the reaction
etches rather than writes (the etching is termed "texturing"). Reaction,
it is stated, only occurs where the laser is impinging with sufficient
fluence, i.e. patterned illumination (as beneath a "mask") is the source
of patterned etching.
D. J. Ehrlich et al. in Appl. Phys. Lett. 36, 698 (1980) describe a method
of mask-free etching of semiconductors based on the ultraviolet photolysis
of gaseous methyl halides. The place of the patterned mask is taken by an
interference pattern, i.e. it derives, once more, from patterned
irradiation of the surface.
U.S. Pat. Nos. 4,608,117 and 4,615,904 issued to Ehrlich et al., disclose
maskless growth of patterned films. This method describes a two-step
process. In step one a pattern is written on the surface using a focused
light-beam or electron-beam as a pen, and photodissociation as the agent
for writing. Once a 1-2 monolayer pattern of metal or semiconductor has
been written in this fashion, step two involves uniform irradiation of the
gaseous reagent and the surface which results in the accumulation of
material on the "prenucleated sites", i.e. in the close vicinity of the
pattern of deposition formed in step one. Consequently this second
growth-phase is mask-free. In the mask-free film-growth phase "atoms are
provided dominantly by direct photodissociation of the gas-phase
organometallic molecules." (U.S. Pat. No. 4,608,117, column 2, lines 12
and 13). Film growth, it is stated, occurs selectively in the prenucleated
regions where impinging atoms originating in the gas phase have a higher
sticking coefficient at the surface.
M. Balooch and W. J. Siekhaus, Nanotechnology, 7, (1996) 365-359, report on
the adsorption of XeF.sub.2 on a Si surface. They teach how to produce a
silicon vacancy by bringing the tip of the STM down to the surface and
then applying a voltage pulse between the STM tip and the surface. An
etching reaction occurs at the point where the STM tip produces a highly
localized and strong electric field. Balooch teaches producing an
individual mark comprising ejection of a silicon atom. Such a method of
marking a surface is not amenable to producing large scale patterns across
the surface as required in many applications, due to the length of time
needed to re-position the STM each time to produce an atomic scale mark
and the .about.10.sup.10 or more atoms in a macroscopic device.
U.S. Pat. No. 5,129,991 issued to Gilton describes an alternative scheme
for mask-free etching. An adsorbed etch-gas (a chloride or fluoride) is
present on a substrate which has macroscopic regions fabricated from
different materials having different photoemission threshold-values for
the release of electrons. This substrate is illuminated with a wavelength
of light selected to give electron emission from some regions but not from
others. The emitted electrons cause etching to occur only on those regions
of the substrate which are composed of materials with a low enough
photoemission threshold to emit electrons; i.e., reaction is localised,
but localised to macroscopic areas.
C. Yan et al., J. Phys. Chem., 99 6084 (1995), have reported that molecular
chlorine impinging as an energetic (0.11 eV) beam of molecules on a
Si(111) 7.times.7 substrate reacts directly from the gas to halogenate the
substrate preferentially at surface silicon-atom sites which are adjacent
to one another (70% adjacent, 30% non-adjacent). Though these chlorinated
pairs of sites recur randomly across the surface, they constitute
short-range order, i.e., a simple form of molecular-scale patterning.
It is well known that one can produce a pattern on a surface by adsorbing a
weakly-bound layer of molecules which, in their most stable configuration,
form a pattern. The existence of such adsorbate patterns has been shown
incontrovertibly by Scanning Tunneling Microscopy (STM) which reveals the
locations and separations of the adsorbate molecules (refs. 1-4). The
origin of these spontaneously-formed molecular patterns has been the
subject of theoretical analysis (e.g., refs. 5-7). The patterns are
governed by the effect on the adsorbate of the regular arrangement of the
atoms in the underlying crystal (termed the "substrate"), by the size and
shape of the adsorbed molecules themselves which can interact with one
another to form an adsorbate pattern due to that cause alone, and by the
coverage which determines the inter-adsorbate separation and hence affects
the favored pattern of adsorption. Through the choice of these variables
(substrate, adsorbate and coverage) one obtains differing patterns. These
adsorbate patterns repeat at intervals as small as a few atomic diameters.
Typically, what are termed `adsorbate layers` have heats of adsorption of
0-1 eV. At the low end of this range they are said to be `physisorbed`,
and at the high end `chemisorbed`. They are adsorbate layers by virtue of
the fact that they are subject to desorption, with a heat of desorption
corresponding to the prior heat of adsorption, by warming the surface.
It would be advantageous to exploit the existence of these molecular-scale
adsorbate patterns as a means to mark the underlying surface. This would
provide an avenue to mask-free marking and, more importantly, to marking
at separations at least a hundred times less than the lowest limit
achievable through the use of the present procedures employing masks.
SUMMARY OF THE INVENTION
It is an object of the present invention to provide a process for marking
solid surfaces with patterns of molecular dimensions. In the present
invention a molecular-scale pattern, that derives from the presence of a
pre-existent molecular-scale pattern in an adsorbate layer, is imprinted
upon the surface. If an adsorbate is to be found attached to preferred
sites at a surface, that recur at intervals across the surface, we refer
to the adsorbate as `patterned`.
The present invention provides a method of patterning a surface of a solid
on a molecular scale, comprising:
providing a reactive solid having a surface;
forming a pattern of adsorbed molecules at sites on said surface of said
reactive solid by providing a plurality of preselected molecules and
exposing said surface to said preselected molecules;
exposing said surface to an effective blocking agent subsequent to exposing
said surface to said preselected molecules to militate against surface
diffusion of said adsorbed molecules; and
forming an imprinted pattern covering less than every surface atom on said
surface by irradiating the surface and the pattern of the adsorbed
molecules with effective excitation energy to form a chemical bond between
at least one constituent of the adsorbed molecules and the reactive solid
locally to the sites where the adsorbed molecules are adsorbed on said
surface, wherein said blocking agent does not imprint on said surface
under irradiation.
The present invention provides a method of patterning a surface of a solid
on a molecular scale, comprising:
providing a reactive solid having a surface;
exposing said surface to an effective blocking agent;
forming a pattern of adsorbed molecules at sites on said surface of said
reactive solid by providing a plurality of preselected molecules and
exposing said surface to said preselected molecules, wherein said blocking
agent blocks preselected surface sites from said preselected molecules;
and
forming an imprinted pattern covering less than every surface atom on said
surface by irradiating the surface and the pattern of the adsorbed
molecules with effective excitation energy to form a chemical bond between
at least one constituent of the adsorbed molecules and the reactive solid
locally to the sites where the adsorbed molecules are adsorbed on said
surface, wherein said blocking agent does not imprint on said surface
under irradiation.
In another aspect of this invention there is provided a method of
patterning a surface of a solid on a molecular scale, comprising:
providing a reactive solid having a surface;
forming a pattern of adsorbed molecules at sites on said surface of said
reactive solid by providing a plurality of preselected molecules and
exposing said surface to said preselected molecules; and
forming an imprinted pattern covering less than every surface atom on said
surface by irradiating the surface and the pattern of the adsorbed
molecules with effective excitation energy to form a chemical bond between
at least one constituent of the adsorbed molecules and the reactive solid
locally to the sites where the adsorbed molecules are adsorbed on said
surface, wherein said imprinted pattern includes a first imprinted pattern
formed from first preselected electropositive atoms or molecules, said
imprinted pattern including a second imprinted pattern formed from second
preselected molecules wherein said first preselected electropositive atoms
or molecules are selected to promote aggregation of said second
preselected molecules into nanoscale sized clusters locally to binding
sites where the first electropositive atoms or molecules are chemically
bound to said surface.
The present invention provides a method of patterning a surface of a solid
on a molecular scale, comprising:
providing a reactive solid having a surface;
forming a pattern of adsorbed molecules at sites on said surface of said
reactive solid by providing a plurality of preselected molecules and
exposing said surface to said preselected molecules; and
forming an imprinted pattern covering less than every surface atom on said
surface by irradiating the surface and the pattern of the adsorbed
molecules with effective excitation energy to form a chemical bond between
at least one constituent of the adsorbed molecules and the reactive solid
locally to the sites where the adsorbed molecules are adsorbed on said
surface, wherein said imprinted pattern includes a first imprinted pattern
formed from first preselected electronegative atoms or molecules, said
imprinted pattern including a second imprinted pattern formed from second
preselected molecules wherein said first preselected electronegative atoms
or molecules are selected to inhibit aggregation of said second
preselected molecules locally to binding sites where the first
electronegative atoms or molecules are chemically bound to said surface.
The present invention provides a method of patterning a surface of a solid
on a molecular scale, comprising:
providing a reactive solid having a surface, said reactive solid being
selected from the group consisting of metals and semiconductors;
forming a pattern of adsorbed molecules at sites on said surface of said
reactive solid by providing a plurality of preselected molecules and
exposing said surface to said preselected molecules; and
forming an imprinted pattern covering less than every surface atom on said
surface by irradiating the surface and the pattern of the adsorbed
molecules with effective excitation energy to form a chemical bond between
at least one constituent of the adsorbed molecules and the reactive solid
locally to the sites where the adsorbed molecules are adsorbed on said
surface.
In another aspect of the invention there is provided a method of patterning
a surface of a solid on a molecular scale, comprising:
providing a reactive solid having a surface;
forming a pattern of adsorbed molecules at sites on said surface of said
reactive solid by providing a plurality of preselected molecules and
exposing said surface to said preselected molecules; and
forming an imprinted pattern covering less than every surface atom on said
surface by irradiating the surface and the pattern of the adsorbed
molecules with effective excitation energy to form a chemical bond between
at least one constituent of the adsorbed molecules and the reactive solid
locally to the sites where the adsorbed molecules are adsorbed on said
surface.
In this aspect of the invention the pattern of adsorbed molecules may
include elongated conjugated chains along which electrical current can
flow, including selected functional groups chemically bound to a selected
constituent in said conjugated chains at intervals therealong, and wherein
when said pattern of adsorbed molecules is irradiated said chemical bond
between said functional group and said constituent is broken, and wherein
a dangling bond from said selected constituent chemically binds to an atom
in said surface of said substrate to anchor said elongated conjugated
chains to said surface at said intervals along said conjugated chains.
In this aspect of the invention the preselected molecules may include at
least one dopant atom which is either a p-type or an n-type dopant atom,
and wherein said at least one constituent of the adsorbed molecule is said
dopant atom.
In this aspect of the invention the substrate may include spaced
electrically conducting electrodes formed on said surface, wherein said
pattern of adsorbed molecules comprise elongated conjugated diene or
aromatic chains along which electrical charge can flow, wherein each
elongated chain includes a selected functional group chemically bound to a
selected constituent in said elongated chains, and wherein when said
pattern of adsorbed molecules is irradiated said chemical bond between
said selected functional group and said selected constituent is broken
whereafter said selected constituent forms a chemical bond with an
electrically conducting electrode on said surface, and wherein the
electrical contact between the electrically conducting electrode and the
elongated chain chemically bound thereto is an ohmic electrical contact.
BRIEF DESCRIPTION OF THE DRAWINGS
The method of marking or patterning a surface on a molecular scale forming
the subject of this invention will now be described, reference being made
to the accompanying Figures, in which:
FIG. 1(a) to FIG. 1(b) are illustrative of the Scanning Tunneling
Microscopy (STM) images from which the distributions of the individual
ClBz (chlorobenzene) molecules and Cl (chlorine bound to Si adatoms) over
the F (faulted) and U (unfaulted) halves of the Si(111)7.times.7 unit
cells were obtained. FIG. 1(a) is an STM image of clean Si(111)7.times.7.
The unit cell indicated is 0.00269 microns (26.9 A) along one side. FIG.
1(b) is a similar surface of FIG. 1(a) dosed with 1 L of ClBz. The dark
shadows are the ClBz molecules; it is evident that they adsorb
preferentially on the F sites. FIG. 1(b) shows two scans of a partially
chlorinated region of an Si(111)7.times.7 surface (previously exposed to
Cl.sub.2, for this illustration). The dark shadows at -1 V show themselves
to be Cl bound to Si since they "light up" (i.e., current flows) when the
STM tip-bias is changed to -3V. Chlorobenzene (ClBz) does not "light up"
in the range -1 to -3V.
FIG. 2(a) is a bar graph obtained by counting individual ClBz molecules at
an average coverage of 48% prior to irradiation, on F and U, and then
counting Cl atoms, on F and U, which have been formed by irradiating the
ClBz. The radiation was 193 nm ultraviolet from an excimer laser. Within
the experimental (counting) error the F/U population ratio is the same for
the parent ClBz (labeled `P`, for parent) as for the daughter Cl (labeled
`D`), indicative of "local" reaction. Both the ClBz and the Cl (P and D,
respectively) are 27% more likely to be on the F sites than on the U sites
at this ClBz coverage.
FIG. 2(b) shows a bar graph obtained by counting individual ClBz molecules
at an average coverage of 38% prior to irradiation, on F and U, and
counting Cl atoms bound to Si adatoms on F and U following irradiation by
electrons.
FIG. 3(a) shows a bar graph showing the distribution of parent (ClBz) and
daughter (Cl) species over adjacent atomic sites termed "middle atoms"
designated (M) and corner atoms labeled (C) following irradiation by
photons.
FIG. 3(b) shows a bar graph showing the distribution of parent (ClBz) and
daughter (Cl) species over adjacent atomic sites termed "middle atoms"
designated (M) and corner atoms labeled (C) following irradiation by
electrons.
FIG. 4(a) shows a pattern of Cl atoms produced by irradiation using
electrons from a scanning tunneling microscope (STM) tip of ClBz adsorbed
on Si(111)7.times.7. The interrupted line of white spots was recorded with
the tip at -3V, at which voltage Cl atoms `light up` to form white spots.
The -4V pulses at the STM tip were spaced at equal intervals of
approximately 59 .ANG..
FIG. 4(b) shows a pattern of Cl atoms produced by irradiation using
electrons from a scanning tunneling microscope (STM) tip of ClBz adsorbed
on Si(111)7.times.7. the continuous line of white spots was recorded with
the tip at -3V. For this experiment the -4V pulses were applied in rapid
sequence (approx. every 29 .ANG.) to the STM tip as it moved down the
figure from top to bottom and the irradiation-induced reaction gave rise
to a continuous line of Cl atoms. The line of white spots was recorded
with the tip at -3V, at which voltage Cl atoms `light up` to form white
spots.
DETAILED DESCRIPTION OF THE INVENTION
The present invention provides a process for the formation of regular
patterns upon solid surfaces, repeated at intervals of less than or of the
order of 0.3 microns (3,000 A). The material to be marked is exposed to a
gas or liquid that adsorbs to form, spontaneously, an ordered monolayer.
This ordering will in general be most complete if the adsorbate is
"annealed", i.e., warmed sufficiently to render it mobile. In some cases
(e.g., that exemplified here) the adsorbate is sufficiently mobile that
annealing is unnecessary.
In one aspect of the invention there is provided a method of patterning a
surface of a solid on a molecular scale. The method comprises providing a
reactive solid having a surface;
forming a pattern of adsorbed molecules at sites on said surface of said
reactive solid by providing a plurality of preselected molecules and
exposing said surface to said preselected molecules; and
forming an imprinted; pattern covering less than every surface atom on said
surface by irradiating the surface and the pattern of the adsorbed
molecules with effective excitation energy to form a chemical bond between
at least one constituent of the adsorbed molecules and the reactive solid
locally to the sites where the adsorbed molecules are adsorbed on said
surface. In this aspect of the invention the reactive solid may be a
crystalline solid or an amorphous solid.
In another aspect of this invention there is provided a method of
patterning a surface of a solid on a molecular scale comprising exposing a
surface of a reactive crystalline solid to an effective imprinting agent
to form a pattern of adsorbed molecules on at least a portion of the
surface; and imprinting a pattern on the surface by applying effective
excitation energy to induce the adsorbed molecules to chemically react
locally to their site of adsorption, with the surface.
Some factors governing the pattern of order in an adsorbate layer
(substrate crystal structure, adsorbate molecular-shape, and coverage)
were given in the previous section. We note here that control over
adsorbate patterning can also be achieved using different crystal faces or
by the deliberate introduction of patterned defects. Patterned defects are
commonly obtained by cutting a substrate crystal at a known angle to
create a predictable pattern of steps and terraces on the atomic scale
(see ref. 8). A further means of obtaining patterned defects, now well
established, is through the application of voltage or current pulses to
the tip of an STM, which removes or adds atoms where the tip is applied
(see ref. 9). Whichever of these means are used to introduce a pattern of
defects at the substrate surface, a corresponding patterning will be
obtained in the adsorbate layer that is subsequently deposited.
The ordered adsorbate layer on to the surface, may be excited by
irradiation using light (photons) or charged particle bombardment
(electrons or ions) so that a reaction between the patterned adsorbate and
the substrate occurs. This has the consequence that the adsorbate pattern
is imprinted upon the surface. Since the adsorbate pattern repeats at
molecular intervals, so does the imprinted pattern.
The method of imprinting the adsorbate pattern disclosed herein relies upon
exciting the adsorbate using generalized irradiation wherein the plurality
of molecules making up the adsorbate pattern are simultaneously induced to
chemically react with the surface. This permits a rapid imprinting of the
pre-existent adsorbate pattern on a time scale which is determined by the
irradiation time. For example, a pulse from a laser having a pulse width
from nanoseconds to picoseconds may easily be used to imprint the entire
pattern of .about.10.sup.10 atoms required for a macroscopic device--an
improvement of some twenty orders-of-magnitude in time required as
compared with atom-by-atom construction.
In one aspect of the invention the surface is imprinted by the intact
adsorbate molecule which has been excited by irradiation of the adsorbate
and surface.
In another aspect of the invention the surface is imprinted by a chemically
reactive fragment formed from the adsorbate molecule as a result of the
joint irradiation of the adsorbate and substrate.
In one aspect of the invention the agent which excites the adsorbate is the
direct absorption of ultraviolet radiation by the adsorbate molecules.
In another aspect of the invention the agent which excites the adsorbate is
the impact of substrate electrons at the sites of adsorbed molecules,
these substrate electrons having themselves been excited by impacting
photons, electrons or ions which energize a portion of the substrate
electrons.
In a variant on substrate-mediated adsorbate excitation, electronic
excitation of the substrate leads to energy-transfer to the adsorbate
without transferring an electron, by the so-called
`electronic-to-electronic energy-transfer` mechanism.
The above aspects contribute to the example of chlorobenzene adsorbed on a
silicon wafer irradiated by ultraviolet radiation, or electrons disclosed
herein. The novelty, and unexpectedly surprising results forming the
present invention, consists not in the presence of patterned adsorbates,
nor in the direct photoexcitation and photodissociation of adsorbates, nor
in the electron-impact excitation and dissociation of adsorbate by
substrate electrons, nor in the use of an external electron beam to induce
adsorbate excitation and reaction, nor in the use of other
charged-particle beams namely ions to induce adsorbate excitation, but in
the localised nature of the ensuing reaction, which is responsible for the
transference of the adsorbate pattern to the substrate.
A variant of the invention disclosed herein is that electrons coming from
an external source can take the place of light as the primary agent that
excites the patterned adsorbate and causes it to imprint its pattern upon
the underlying surface. Electron irradiation and photoirradiation are
known to have comparable effects. It follows that other charged particles,
namely charged molecules termed ions, can also be used for irradiation.
The reaction of the irradiated adsorbate with the substrate may
equally-well be that of an excited intact adsorbate molecule, or of a
molecular fragment of that molecule, or of its atomic fragment. Following
the reaction the product may be chemically bound at the surface (localised
"writing") or imbedded in the surface (localised "doping") or may evidence
itself as a localised removal of surface atoms as a consequence of the
localised reaction (localised "etching") or evidence itself by the
preferential atomic-scale pitting of the surface at the site of attachment
of a chemically-bound species in the course of subsequent irradiation. The
last scenario makes it possible to translate the patterned attachment of
chemical groups ("writing") described here, into a similar pattern of
"pits" ("etched" hollows, one or more atoms in diameter) thus further
extending the range of useful application.
A further aspect of this invention is its "reinforcing" nature. Successive
adsorption-irradiation cycles will cause the annealed adsorbate layer
(prior to irradiation) to seek out, or in some cases, avoid, the sites at
which the first pattern was imprinted. Thus, by way of example, adsorbates
will collect preferentially in the region of a "pit" in the surface to
form a pattern. When, therefore, a second application of adsorbate is
followed by irradiation, reaction will occur once more preferentially at
the sites of the prior imprinting (this being in general the site of
secondary adsorption). Thus secondary and subsequent patterned-adsorption
plus irradiation-imprinting can be used to enlarge, or chemically change,
a primary imprint. The primary imprint, it should be understood, may have
been made by the present method or alternatively by the tip of an atomic
writing/etching instrument such as the Scanning Tunneling Microscope (STM)
(see ref. 9). This "reinforcing" application will be important in
permitting the uses of this method of adsorption-plus-irradiation to
increase the size of primary "pits" in secondary and subsequent "etches",
and to write or dope with selected chemical agents in the vicinity of
prior marks.
Since each reactive or etching event is triggered by the arrival at the
adsorbate-plus-surface of a photon, electron or ion, and since the number
of such photons or electrons is readily counted and controlled, both the
primary and subsequent reaction, dope, or etch, is controllable as to the
number of atoms involved. What is being described is, therefore, a means
for patterned writing, etching and doping that is subject to digital
control.
The method for marking a surface on an atomic or molecular scale disclosed
herein will be described and illustrated hereinafter using a non-limiting,
illustrative example in which a crystalline silicon wafer is marked, using
as the adsorbate molecule, chlorobenzene. However, it is to be understood
by those skilled in the art that the invention is in no way limited to
this system but rather the chlorobenzene-silicon system serves only to
illustrate the principles of the present invention. As used herein, the
phrase "imprinting agent" refers to species (liquid or vapor) which, when
exposed to the surface of a reactive solid, forms an adsorbate pattern on
the exposed surface.
At the outset of the illustrative studies used to demonstrate the principle
of the method of marking forming this invention, a clean Si(111)7.times.7
wafer at room temperature was shown to be free of contamination at the
atomic level by means of Scanning Tunneling Microscopy (STM). The wafer
was then exposed to approximately 1 Langmuir (1L; 10.sup.-7 torr for 10
seconds) of chlorobenzene vapour in an ultrahigh vacuum vessel. The vessel
was then re-evacuated to ultrahigh vacuum (UHV). All experiments were
performed under UHV. Re-examination of the surface by STM showed partial
coverage of the surface by chlorobenzene (ClBz) molecules which evidenced
themselves as dark spots of roughly molecular dimensions, i.e., as locally
reduced current from the negatively charged STM tip (-1V) to the crystal.
As expected the chlorobenzene molecule was stable in the adsorbed state on
silicon.
FIGS. 1(a), 1(b) and 1(c) illustrate the type of scanning tunneling
microscopy (STM) data from which the essential findings disclosed in FIG.
2 discussed hereinafter were obtained. The STM images show clean silicon
(Si(111)7.times.7), ClBz covered silicon, and Cl covered silicon. Atomic
chlorine can be distinguished from the chlorobenzene by increasing the
negative tip-bias to -2V or -3V, whereupon the chlorobenzene dark spots
are unaffected, but the dark spots corresponding to Cl become bright. It
should be noted that the spots correspond to individual ClBz molecules or
Cl atoms.
The locations of the dark spots for the chlorobenzene molecules constituted
a pattern that repeated itself across the Si surface. The spots were
predominantly located on the faulted (F) half of each unit cell rather
than on the unfaulted (U) half (FIG. 1). This selectivity decreased with
increasing coverage. This suggests that the marked preference for F over U
is due to stronger adsorbate-to-substrate (ClBz to Si) bonding for F than
for U rather than to adsorbate-adsorbate (ClBz-ClBz) interaction.
Ordering of the adsorbate need not in all cases be due to
adsorbate-substrate forces, but can be due to adsorbate-adsorbate
interactions as in so-called "SAMs" (Self-Assembled Monolayers, made up,
for example, of long-chain molecules) which favour patterned geometries
even on amorphous solids. In this case, however, the extent of adsorbate
ordering will increase with increasing coverage. An example of SAM-type
behavior is likely to be that of brominated long-chain hydrocarbons which
have been shown to form highly-ordered adlayers on graphite at -1 ML
coverage (see ref. 4).
The chlorobenzene on silicon, used here, distributes itself in a recurring
pattern over the F and U sites. The triangular F and U sites alternate
across the entire silicon-wafer surface. The separation between the site
centers, F and U, is 0.00155 microns (15.5 A). The triangles (a pair of
which, F+U, make up each unit cell) are equilateral with sides of 0.0027
microns. Patterned adsorption over F and U is achieved at room temperature
without annealing. By analogy with benzene (see ref. 2), it is thought
that the monochlorinated benzene employed here at first adsorbs weakly in
a mobile "precursor" state and then chemisorbs (with a heat of adsorption
of .about.1 eV) at the preferred site. The binding in the
chemically-adsorbed condition is believed to involve a benzene ring (or in
the present case a monochlorinated benzene ring) lying approximately flat
upon the surface with its delocalized .pi.-bonds overlapping the surface
dangling-bonds located on the Si adatoms.
Those skilled in the art will understand that the experimental findings
disclosed herein do not depend upon the correctness of the foregoing
interpretation of the nature of the binding. Moreover, the theory being
qualitative in nature does not purport to explain the preference for
(benzene and) chlorobenzene adsorption at F in preference to U sites.
Preferential binding of adsorbates to particular sites at surfaces (i.e.,
to particular atoms, or arrangements of atoms as in the case of the
faulted half of the unit cell termed F) is a well known phenomenon. Since
the preferred sites recur at regular intervals across the surface, the
adsorbate molecules also recur, forming a pattern.
The experiments reported herein illustrate not only adsorbate patterning
(required for the method disclosed herein) but also that effective
excitation, such as by irradiation of the surface of a reactive solid,
produces a pattern imprinted on the surface (a second requirement of the
present method). The example of photopatterning that is disclosed herein
is one in which a pattern of atoms is deposited on the surface in a
chemically-bound state as a result of irradiation by ultraviolet or
irradiation by electrons. Central to the present method of photopatterning
is that the adsorbate pattern or a closely-related one is imprinted on the
surface by radiation-induced reaction with the surface, which reaction is
localised to regions adjacent to the adsorbate molecules. Localisation may
be so complete as to restrict reaction to the site of adsorption, though
in the example of ClBz (as we show) the preference is for reaction at the
neighboring atom (see FIG. 3). It is only if reaction is "localised" that
it will transfer the adsorbate pattern to the surface.
It will also be understood by those skilled in the art that statistically,
not every molecule forming part of the adsorbate pattern will necessarily
react and/or be imprinted but a majority of the adsorbate molecules
reacting locally to their site of adsorption will be sufficient to imprint
an effective adsorbate pattern.
The existence of molecularly "localised scattering" of photofragments from
substrates has been known, see refs. 17, 18. The surprising new finding
disclosed herein, namely "localised reaction", takes the place of
"localised scattering" when (due to such factors as the impact energy at
the scattering site, the angle of impact and the nature of the impacted
atom(s)) the localised scattering event is pre-empted by a localised
perturbation of the surface leading to bond-formation at the site of
impact. Hence the molecular or atomic species that would have been
"scattered", instead "reacts" to bind chemically with the substrate
surface to form, under the prevailing experimental conditions, involatile
or (in the case of etching) volatile product. Reaction, rather than
scattering, was obtained by replacing the inert halide substrate used in
the scattering studies by the silicon employed here, well known to be
reactive. It is recognised however, that even an "inert" substrate can
exhibit reaction if the impacting species are themselves sufficiently
reactive and energetic.
In view of the mobility of many species at surfaces, particularly energetic
species formed as a result of radiation, the existence of localised
reaction was not evident prior to its being demonstrated in the
experiments disclosed herein. We return to this point later in this
disclosure.
As we have noted, it is not the adsorbate patterning, described above, but
the method that exploits such patterns through imprinting that constitutes
the present invention. In the case that the adsorbate is chlorobenzene,
the silicon surface has been imprinted on by irradiation of the adsorbate,
as a pattern of chlorine atoms attached to Si dangling bonds not already
occupied by the chlorobenzene adlayer. On the basis of extensive prior
work (for example see ref 19) it is known that irradiation by ultraviolet
(UV), or (in the case of low work-function materials) visible light, or
irradiation by an external electron beam (see for example refs. 13, 14)
causes reactions to occur. Such reactions may be initiated by
photodissociation of the adlayer, or by interaction of the intact
electronically-excited adlayer molecules with the substrate. Both
categories of reaction can be assisted by the heating of the surface by
the irr
