U.S. patent application number 10/995608 was filed with the patent office on 2005-07-28 for custom electrodes for molecular memory and logic devices.
Invention is credited to Beck, Patricia A., Li, Zhiyong, Ohlberg, Douglas A., Stewart, Duncan.
Application Number | 20050164412 10/995608 |
Document ID | / |
Family ID | 32850612 |
Filed Date | 2005-07-28 |
United States Patent
Application |
20050164412 |
Kind Code |
A1 |
Beck, Patricia A. ; et
al. |
July 28, 2005 |
Custom electrodes for molecular memory and logic devices
Abstract
A method for tailoring at least portions of an exposed
non-planar layered surface of a conductive layer formed on a
substrate having a first surface roughness to provide the exposed
surface with a second surface roughness. The method includes:
forming the conductive layer on the substrate; and tailoring at
least portions of the exposed surface of the conductive layer in a
plasma to at least smooth the exposed surface of the conductive
layer, whereby the second surface roughness is essentially the same
as the first surface roughness.
Inventors: |
Beck, Patricia A.; (Palo
Alto, CA) ; Ohlberg, Douglas A.; (Mountain View,
CA) ; Stewart, Duncan; (Menlo Park, CA) ; Li,
Zhiyong; (Mountain View, 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: |
32850612 |
Appl. No.: |
10/995608 |
Filed: |
November 22, 2004 |
Related U.S. Patent Documents
|
|
|
|
|
|
Application
Number |
Filing Date |
Patent Number |
|
|
10995608 |
Nov 22, 2004 |
|
|
|
10405294 |
Apr 2, 2003 |
|
|
|
6855647 |
|
|
|
|
Current U.S.
Class: |
438/1 |
Current CPC
Class: |
G11C 13/02 20130101;
H01L 2924/0002 20130101; H01L 2924/0002 20130101; H01L 27/28
20130101; H01L 2924/00 20130101 |
Class at
Publication: |
438/001 |
International
Class: |
H01L 021/00 |
Claims
What is claimed is:
1. A method for tailoring at least portions of an exposed
non-planar layered surface of a layer of conductive material formed
on a substrate having a first surface roughness to provide said
exposed surface with a second surface roughness, said method
including: forming said conductive layer on said substrate; and
tailoring at least portions of said exposed surface of said
conductive layer in a plasma to at least smooth said exposed
surface of said conductive layer, whereby said second surface
roughness is essentially the same as said first surface
roughness.
2. The method of claim 1 wherein said tailoring is performed in a
plasma to additionally accomplish at least one of the following:
(a) rearrange said conductive layer, (b) alter the hydrophilicity
of said exposed layer, and (c) provide a barrier layer due to the
presence of an oxide film on said exposed surface.
3. The method of claim 2 wherein said plasma is selected from the
group consisting of oxygen alone to provide a hydrophilic surface,
oxygen and subsequent argon to provide a less hydrophilic, more
hydrophobic surface, argon alone to provide a hydrophobic surface,
or a sequence of oxygen and hydrogen to provide a smooth surface
with reduced oxygen, which is passivated.
4. The method of claim 3 wherein said plasma contains oxygen,
leaving an oxide film on said conductive layer, and wherein said
oxide film is subsequently removed, leaving said smooth exposed
surface of said conductive layer.
5. The method of claim 1 wherein said tailoring includes at least
one of the following steps: cleaning and oxidizing to a
predetermined level.
6. The method of claim 1 wherein said tailoring includes at least
one of the following steps: actively smoothing, actively oxidizing,
actively removing said oxide without re-roughening, and actively
passivating.
7. The method of claim 1 wherein said conductive material comprises
a material selected from Rows 1 B-7B and 8 of the Periodic
Table.
8. The method of claim 7 wherein said conductive material is
selected from the group consisting of platinum, tungsten, silver,
aluminum, palladium, copper, nickel, chromium, molybdenum,
titanium, and tantalum.
9. The method of claim 8 wherein said conductive material consists
essentially of platinum.
10. The method of claim 1 wherein said second surface roughness is
less than 8 .ANG. RMS.
11. The method of claim 10 wherein said conductive layer has a
thickness and wherein said second surface roughness is less than
0.8% of said thickness of said conductive layer.
12. The method of claim 10 wherein said second surface roughness is
approximately 4 A .ANG. RMS.
13. The method of claim 12 wherein said conductive layer has a
thickness and wherein said second surface roughness is
approximately 0.4% of said thickness of said conductive layer.
14. A method of reliably fabricating a molecular electronic device
comprising at least a first electrode and a molecular switch film
thereon, said method comprising: providing a substrate; forming
said first electrode on said substrate, said first electrode
comprising a non-planar surface of tailored conductive material,
said non-planar surface comprising either a layer or a nanowire;
and forming said molecular film on at least said first electrode,
wherein said first electrode is formed by a process including:
cleaning portions of said substrate where said first electrode is
to be deposited; pre-sputtering said portions where said non-planar
surface comprises said layer; and forming said conductive layer
having said non-planar surface on at least said portions.
15. The method of claim 14 wherein said non-planar surface
comprises said layer.
16. The method of claim 15 wherein said substrate is provided with
a coating on which said first electrode is deposited.
17. The method of claim 16 wherein said coating is subjected to
said cleaning step and said pre-sputtering step before depositing
said conductive layer.
18. The method of claim 15 wherein cleaning is performed with an
oxygen plasma to remove organic contaminants.
19. The method of claim 15 wherein said pre-sputtering is performed
under conditions to further clean said surface and remove
environmental contaminants.
20. The method of claim 15 wherein said conductive layer is formed
to a thickness of 50 to 5,000 .ANG..
21. The method of claim 15 wherein a resist is formed on a coating
on said substrate and patterned, said pattern comprising an array
of said first electrodes, wherein said patterning is done by
removing resist from those areas where said conductive layer is to
be deposited to form said first electrodes.
22. The method of claim 21 wherein said first electrode is formed
by imprinting or molding.
23. The method of claim 21 wherein: said exposed areas are cleaned
with an oxygen plasma to remove organic contaminants; said exposed
areas are pre-sputtered to further clean said surface and remove
environmental contaminants; said conductive layer is
blanket-deposited everywhere, to deposit a layer about 50 to 5,000
.ANG. thick; and said conductive layer is patterned to form said
first electrodes.
24. The method of claim 14 wherein said non-planar surface
comprises said nanowire.
25. The method of claim 24 wherein said substrate is provided with
a coating on which said first electrode is formed.
26. The method of claim 24 wherein cleaning is performed with an
oxygen plasma to remove organic contaminants.
27. The method of claim 24 wherein said conductive layer is formed
to a diameter of 50 to 5,000 .ANG..
28. The method of claim 24 wherein a resist is formed on a coating
on said substrate and patterned, said pattern comprising growth
initiation sites for an array of said first electrodes, wherein
said patterning is done by removing resist from those areas where
said conductive layer is to be deposited to form said first
electrodes.
29. The method of claim 28 wherein said first electrode is formed
by imprinting or molding.
30. The method of claim 14 further including tailoring properties
of the exposed surface of said conductive layer following its
deposition.
31. The method of claim 30 wherein said tailoring is performed in a
plasma to accomplish at least one of the following: (a) rearrange
said conductive layer, (b) smooth said exposed surface of said
conductive layer, (c) alter the hydrophilicity of said exposed
layer, and (d) provide a barrier layer due to the presence of an
oxide film on said exposed surface.
32. The method of claim 31 wherein said plasma is selected from the
group consisting of oxygen alone to provide a hydrophilic surface,
oxygen and subsequent argon to provide a less hydrophilic, more
hydrophobic surface, argon alone to provide a hydrophobic surface,
or a sequence of oxygen and hydrogen to provide a smooth surface
with reduced oxygen, which is passivated.
33. The method of claim 32 wherein said plasma contains oxygen,
leaving an oxide film on said conductive layer, and wherein said
oxide film is subsequently removed, leaving said smooth exposed
surface of said conductive layer.
34. The method of claim 30 wherein said tailoring includes at least
one of the following steps: cleaning and oxidizing to a
predetermined level.
35. The method of claim 30 wherein said tailoring includes at least
one of the following steps: actively smoothing, actively oxidizing,
actively removing said oxide without re-roughening, and actively
passivating.
36. The method of claim 14 wherein said molecular device comprises
an electrical element formed with two or more electrodes.
37. The method of claim 36 wherein said molecular device is
selected from the group consisting of switches, diodes, resistors,
transducers, and transistors.
38. The method of claim 37 further including forming a second
contact on said molecule film and over said first layer to form a
switch.
39. The method of claim 38 wherein said second contact is selected
from the group consisting of second electrodes, circular
electrodes, tip addressing, and a nanopore over said molecular film
covered with an electrode.
40. The method of claim 14 wherein said conductive material
comprises a material selected from Rows 1B-7B and 8 of the Periodic
Table.
41. The method of claim 40 wherein said conductive material is
selected from the group consisting of platinum, tungsten, silver,
aluminum, palladium, copper, nickel, chromium, molybdenum,
titanium, and tantalum.
42. The method of claim 41 wherein said conductive material
consists essentially of platinum.
43. A method of forming a nano-imprinted or molded layer of
conductive material on a substrate having a first surface
roughness, said conductive layer having a second surface roughness,
where said second surface roughness is approximately the same as
said first surface roughness, said method comprising: cleaning
portions of said substrate where said first electrode is to be
deposited; pre-sputtering said portions; and depositing said
conductive layer on at least said portions.
44. The method of claim 43, wherein said conductive layer is
deposited on at least said portions without formation of any
sticking layer prior to depositing said conductive layer.
45. The method of claim 43 wherein cleaning is performed with an
oxygen plasma to remove organic contaminants.
46. The method of claim 43 wherein said pre-sputtering is performed
under conditions to further clean said surface and remove
environmental contaminants.
47. The method of claim 43 wherein said depositing of said
conductive layer is performed to a thickness of 50 to 5,000
.ANG..
48. The method of claim 43 further including tailoring properties
of the exposed surface of said conductive layer following its
deposition.
49. The method of claim 48 wherein said tailoring is performed in a
plasma to accomplish at least one of the following: (a) rearrange
said conductive layer, (b) smooth said exposed surface of said
conductive layer, (c) alter the hydrophilicity of said exposed
layer, and (d) provide a barrier layer due to the presence of an
oxide film on said exposed surface.
50. The method of claim 49 wherein said plasma is selected from the
group consisting of oxygen alone to provide a hydrophilic surface,
oxygen and subsequent argon to provide a less hydrophilic, more
hydrophobic surface, argon alone to provide a hydrophobic surface,
or a sequence of oxygen and hydrogen to provide a smooth surface
with reduced oxygen, which is passivated.
51. The method of claim 50 wherein said plasma contains oxygen,
leaving an oxide film on said conductive layer, and wherein said
oxide film is subsequently removed, leaving said smooth exposed
surface of said conductive layer.
52. The method of claim 48 wherein said tailoring includes at least
one of the following steps: cleaning and oxidizing to a
predetermined level.
53. The method of claim 48 wherein said tailoring includes at least
one of the following steps: actively smoothing, actively oxidizing,
actively removing said oxide without re-roughening, and actively
passivating.
54. The method of claim 43 wherein said conductive material
comprises a material selected from Rows 1B-7B and 8 of the Periodic
Table.
55. The method of claim 54 wherein said conductive material is
selected from the group consisting of platinum, tungsten, silver,
aluminum, palladium, copper, nickel, chromium, molybdenum,
titanium, and tantalum.
56. The method of claim 55 wherein said conductive material
consists essentially of platinum.
57. The method of claim 43 wherein said conductive layer has a
thickness and wherein said second surface roughness is less than
0.8% of said thickness of said conductive layer.
58. The method of claim 57 wherein said conductive layer has a
thickness and wherein said second surface roughness is
approximately 0.4% of said thickness of said conductive layer.
59. A method of tailoring the surface of a nanowire conductive
layer on a substrate having a first surface roughness, said
conductive layer having a second surface roughness, where said
second surface roughness is approximately the same as said first
surface roughness, said method comprising: cleaning portions of
said substrate where said nanowire is to be formed; forming said
nanowire on said substrate; and performing said tailoring in a
plasma to accomplish at least one of the following: (a) rearrange
said conductive layer, (b) smooth said exposed surface of said
conductive layer, (c) alter the hydrophilicity of said exposed
layer, and (d) provide a barrier layer due to the presence of an
oxide film on said exposed surface.
60. The method of claim 59 wherein said plasma is selected from the
group consisting of oxygen alone to provide a hydrophilic surface,
oxygen and subsequent argon to provide a less hydrophilic, more
hydrophobic surface, argon alone to provide a hydrophobic surface,
or a sequence of oxygen and hydrogen to provide a smooth surface
with reduced oxygen, which is passivated.
61. The method of claim 60 wherein said plasma contains oxygen,
leaving an oxide film on said conductive layer, and wherein said
oxide film is subsequently removed, leaving said smooth exposed
surface of said conductive layer.
62. The method of claim 59 wherein said tailoring includes at least
one of the following steps: cleaning and oxidizing to a
predetermined level.
63. The method of claim 59 wherein said tailoring includes at least
one of the following steps: actively smoothing, actively oxidizing,
actively removing said oxide without re-roughening, and actively
passivating.
64. The method of claim 59 wherein said conductive material is
selected from the group consisting of platinum, tungsten, silver,
aluminum, palladium, copper, nickel, chromium, molybdenum,
titanium, and tantalum.
65. The method of claim 64 wherein said conductive material
consists essentially of platinum.
Description
CROSS-REFERENCE TO RELATED APPLICATIONS
[0001] The present application is a continuation-in-part of
application Ser. No. 10/405,294, filed Apr. 2, 2003.
[0002] The present application is related to U.S. Pat. No.
6,459,095, issued Oct. 1, 2002, entitled "Chemically Synthesized
and Assembled Electronic Devices", which is directed to the
formation of nanowires used for nano-scale computing and memory
circuits. The present application is also related to U.S. Pat. No.
6,314,019, issued Nov. 6, 2001, entitled "Molecular Wire Crossbar
Interconnect (MWCI) for Signal Routing and Communications", and to
U.S. Pat. No. 6,128,214, entitled "Molecular Wire Crossbar Memory",
issued on Oct. 3, 2000, as well as to applications Ser. No.
09/280,045, entitled "Molecular Wire Crossbar Logic (MWCL)", and
Ser. No. 09/280,188, entitled "Molecular Wire Transistor (MWT)",
both filed on Mar. 29, 1999, which are all directed to various
aspects of memory and logic circuits utilized in nanocomputing. The
present application is also related to application Ser. No.
09/823,195, filed Mar. 29, 2001, entitled "Bistable Molecular
Mechanical Devices with a Band Gap Change Activated by an Electric
Field for Electronic Switching, Gating, and Memory Applications",
and to U.S. Pat. No. 6,458,621, entitled "Batch Fabricated
Molecular Electronic Devices with Cost-Effective Lithographic
Electrodes", issued on Oct. 1, 2002. The foregoing items are all
incorporated herein by reference.
TECHNICAL FIELD
[0003] The present application is generally directed to microscale
and nanoscale computing and memory circuits, and, more
particularly, to the formation of wires and contacts for device
applications, specifically, to the fabrication of electrodes
employed in such devices. The term "microscale" reflects that
either the horizontal or vertical dimensions or the electrical
pathway between electrodes is measured in micrometers. The term
"nanoscale" reflects that either the horizontal or vertical
dimensions or the electrical pathway between electrodes is measured
in nanometers.
BACKGROUND ART
[0004] As feature sizes of integrated-circuit devices continue to
decrease, it becomes increasingly difficult to design well-behaved
devices. The fabrication is also becoming increasingly difficult
and expensive. In addition, the number of electrons either accessed
or utilized within a device is decreasing, which produces increased
statistical fluctuations in the electrical properties. In the
limit, device operation depends on a single electron, and
traditional device concepts must change.
[0005] Molecular electronics has the potential to augment or even
replace conventional devices with electronic elements, can be
altered by externally applied voltages, and has the potential to
scale from micron-size dimensions to nanometer-scale dimensions
with little change in the device concept. The molecular switching
elements can be formed by solution techniques, as is well known.
The self-assembled switching elements may be integrated on top of a
semiconductor integrated circuit so that they can be driven by
conventional semiconductor electronics in the underlying substrate.
To address the switching elements, interconnections or wires are
used.
[0006] For nanoscale electronic circuits, it is necessary to invent
new materials with the functions envisioned for them and new
processes to fabricate them. Nanoscale molecules with special
functions can be used as basic elements for nanoscale computing and
memory applications.
[0007] While self-assembled techniques may be employed and while
redox reaction-based molecules may be used, such as rotaxanes,
pseudorotaxanes, and catenanes, other techniques for assembling the
devices and other molecular systems may alternatively be employed.
An example of such other techniques comprises lithographic
techniques adapted to feature sizes in the micrometer-size range,
as well as feature sizes in the nanometer-size range. An example of
other molecular systems involves electric-field-induced band gap
changes, such as disclosed and claimed in patent application Ser.
No. 09/823,195, filed Mar. 29, 2001, which is incorporated herein
by reference. While prior references have employed the term "band
gap", this term more precisely is used for semiconductors. The
corresponding term with regard to molecules is "HOMO-LUMO gap"
(highest occupied molecular orbital - lowest unoccupied molecular
orbital), and that is the term that will be used throughout.
[0008] Examples of molecules used in the electric-field-induced
HOMO-LUMO gap change approach include molecules that evidence:
[0009] (1) molecular conformation change or an isomerization;
[0010] (2) change of extended conjugation via chemical bonding
change to change the HOMO-LUMO gap; or
[0011] (3) molecular folding or stretching.
[0012] Changing of extended conjugation via chemical bonding change
to change the HOMO-LUMO gap may be accomplished in one of the
following ways:
[0013] (a) charge separation or recombination accompanied by
increasing or decreasing HOMO-LUMO localization; or
[0014] (b) change of extended conjugation via charge separation or
recombination and .pi.-bond breaking or formation.
[0015] Molecular electronic devices hold promise for future
electronic and computational devices. Examples of such molecular
electronic devices include, but are not limited to, crossed wires,
nanoporous surfaces, and tip addressable circuitry which forms
switches, diodes, resistors, transducers, transistors, and other
active components. For instance, a crossed wire switch may comprise
two wires, or two electrodes, for example, with a molecular
switching species between the two electrodes. Thin single or
multiple molecular layers can be formed, for example, by
Langmuir-Blodgett (LB) techniques or self-assembled monolayer (SAM)
on a specific site. Well-controlled properties, such as roughness
and hydrophilicity of the underlying surface are needed to allow
optimal LB film formation.
[0016] Prior work in the field of molecular electronics has
utilized electrodes of gold, aluminum, and polysilicon.
[0017] Gold has a low melting point, low bulk modulus, and high
diffusivity, making it less stable with respect to external stress
and incompatible with a standard CMOS process, although it has the
advantages of no oxide and the chemical stability of a noble metal.
Aluminum forms a poorly controlled native oxide that acts as a
natural barrier to electronic transport. Polysilicon is a
semi-conductor with associated semiconductor properties, giving it
lower conductivity than a metal and an oxide barrier to transport.
Polysilicon electrode molecular devices have been fabricated and
shown to display switching.
[0018] Platinum is difficult to maintain in a stable form. During
the interval following Pt deposition and preceding the next
processing step, an "environmental" film (carbon, etc.) will form
on the surface. This is a particular issue when the active
molecular layer may be on the order of 20 .ANG. thick, which, for
reference, is the same magnitude as a native silicon oxide. Working
with a just-deposited-film (perhaps the "cleanest" way) is
difficult and impractical. Even a "just-deposited" blanket film
will require time to move to the next process, which will not be in
ultrahigh vacuum (UHV). Until alternate means of forming patterned
contacts are readily realizable, lithography is presently the most
likely technology to use. Shadow masks avoid lithographic process,
but are dimensionally limited (to large micron-sized dimensions,
sparsely placed). Even nanoimprinting exposes surfaces to organic
chemicals that are potentially incompatible with the use of organic
active layers. Therefore, the most practical way to fabricate
electrodes incorporating molecules is to pattern the electrode with
a flexible geometry in a cost-efficient, time efficient, flexible
geometry way and then clean the organics from the surface before
subsequent processing. This electrode patterning may include a
textured or untextured surface (distinguished from "roughness" by
the prearrangement of pattern and length scale. The electrode layer
may be deposited and formed into a pattern by etching the gross
feature (e.g., ion etch), or formed through a pattern (e.g., shadow
mask or by lift-off) or by a process of stamping, also called
imprinting or molding. The electrodes need not be planar, but could
be formed by the growth and arrangement of nanowires or on
pretextured substrates such as those commonly used in the
micromachining and nanomachining communities (e.g., MEMS and
NEMS).
[0019] Thus, a method for preparing platinum, and other conductive
electrodes, that avoid most, if not all, of the foregoing problems
is required for use with molecular films for forming molecular
electronic devices. In addition, it would be an advantage to tailor
the surface (of the Pt) to desired device specifications for use
even if lithographic steps are not employed.
DISCLOSURE OF INVENTION
[0020] In accordance with an embodiment disclosed herein, a method
is provided for tailoring at least portions of an exposed
non-planar surface of a layer of conductive material to provide a
smooth surface that can be as smooth as the non-planar surface of
the underlying substrate supporting the conductive layer. By
"tailoring" is meant a process involving the preparation of the
surface. By "conductive layer" is meant a layer comprising a
material having a resistivity of less than 1375 micro-ohm-cm,
wherein the material is capable of forming a solid-state oxide that
is stable under ambient conditions. The method includes
[0021] forming the conductive layer on the substrate; and
[0022] tailoring at least portions of the exposed surface of the
conductive layer in a plasma to at least smooth the exposed surface
of the conductive layer, whereby the surface roughness is
essentially the same as that of the substrate.
[0023] In accordance with another embodiment, a method of reliably
fabricating a molecular electronic device comprising at least a
first electrode and a molecular switch film thereon is provided.
The method comprises:
[0024] providing a substrate;
[0025] forming the first electrode on the substrate, the first
electrode comprising a non-planar surface of tailored conductive
material, the non-planar surface comprising either a layer or a
nanowire; and
[0026] forming the molecular film on at least the first
electrode,
[0027] wherein the first electrode is formed by a process
including:
[0028] cleaning portions of the substrate where the first electrode
is to be deposited;
[0029] pre-sputtering the portions where the non-planar surface
comprises the layer; and
[0030] forming the conductive layer having the non-planar surface
on at least the portions.
[0031] In accordance with a further embodiment, a method is
provided for forming a nano-imprinted or molded layer of conductive
material on a substrate having a first surface roughness, with the
conductive layer having a second surface roughness, where the
second roughness is approximately the same as the first surface
roughness. The method comprises the steps of cleaning portions of
the substrate where the first electrode is to be deposited;
pre-sputtering the cleaned portions; and depositing the conductive
layer on at least the presputtered and cleaned portions.
[0032] In accordance with yet another embodiment, a method of
tailoring the surface of a nanowire conductive layer on a substrate
having a first surface roughness is provided, where the conductive
layer has a second surface roughness and where the second surface
roughness is approximately the same as the first surface roughness.
The method comprises:
[0033] cleaning portions of the substrate where the nanowire is to
be formed;
[0034] forming the nanowire on the substrate; and
[0035] performing the tailoring in a plasma to accomplish at least
one of the following: (a) rearrange the conductive layer, (b)
smooth the exposed surface of the conductive layer, (c) alter the
hydrophilicity of the exposed layer, and (d) provide a barrier
layer due to the presence of an oxide film on the exposed
surface.
BRIEF DESCRIPTION OF THE DRAWINGS
[0036] FIGS. 1a-1d are top plan views of one embodiment of a
process for fabricating molecular devices (the embodiment depicted
is of a crossed wire device, but the embodiments herein are not so
limited);
[0037] FIG. 2 is a cross-sectional view (side elevation) taken
through the line 2-2 of FIG. 1d;
[0038] FIG. 3 is a flow chart depicting the process;
[0039] FIG. 4 is a top plan view similar to FIG. 1d, but depicting
an embodiment in which the second, or top, electrode is
circular;
[0040] FIG. 5 is a cross-sectional view, similar to FIG. 2, taken
through the line 5-5 of FIG. 4;
[0041] FIG. 6 is a top plan view similar to FIG. 1d, but depicting
an embodiment in which the second, or top, electrode is a probe;
and
[0042] FIG. 7 is a cross-sectional view, similar to FIG. 2, taken
along the line 7-7 of FIG. 6.
BEST MODES FOR CARRYING OUT THE INVENTION
DEFINITIONS
[0043] As used herein, the term "self-aligned" as applied to
"junction" means that the junction that forms the switch and/or
other electrical connection between two electrodes is created
wherever portions of the two electrodes, either of which may be
coated or functionalized, overlap.
[0044] The term "device" means a switch, diode, resistor,
transducer, transistor, or other electrical element formed with two
or more electrodes.
[0045] The term "self-assembled" as used herein refers to a system
that naturally adopts some regular pattern because of the identity
of the components of the system; the system achieves at least a
local minimum in its energy by adopting this configuration.
[0046] The term "singly configurable" means that a device can
change its state only once via an irreversible process such as an
oxidation or reduction reaction; such a device can be the basis of
a programmable read-only memory (PROM), for example.
[0047] The term "reconfigurable" means that a device can change its
state multiple times via a reversible process such as an oxidation
or reduction; in other words, the device can be opened and closed
multiple times, such as the memory bits in a random access memory
(RAM).
[0048] The term "bi-stable" as applied to a molecule means a
molecule having two relatively low energy states. The molecule may
be either irreversibly switched from one state to the other (singly
configurable) or reversibly switched from one state to the other
(reconfigurable).
[0049] "Micron-scale dimensions" refers to dimensions that range
from 1 micrometer to a few micrometers in size.
[0050] "Sub-micron scale dimensions" refers to dimensions that
range from 1 micrometer down to 0.05 micrometers.
[0051] "Nanometer scale dimensions" refers to dimensions that range
from 0.1 nanometers to 50 nanometers (0.05 micrometers).
[0052] "Micron-scale wires" refers to rod or ribbon-shaped
conductors or semiconductors with widths or diameters having the
dimensions of 1 to 10 micrometers or larger, heights that can range
from a few tens of nanometers to a few micrometers, and lengths of
up to several micrometers or more.
[0053] "Nanometer-scale wires" refers to rod or ribbon-shaped
conductors or semiconductors with widths or diameters having the
dimension of 1 to 50 nanometers, heights that can range from 0.3 to
100 nm, and lengths of up to several micrometers or more.
[0054] Molecular Devices.
[0055] FIGS. 1a-1d depict one embodiment for the fabrication of
molecular devices 10. As shown in FIG. 1a, a substrate 12 is
provided. Next, a first electrode 14 is formed on a portion of the
surface of the substrate 12, as shown in FIG. 1b. A molecular
switch film 16 is formed on the surface of the substrate 12,
covering the first electrode 14. Finally, a second electrode 18,
generally at right angles to the first electrode 14, is applied on
the molecular film 16. The completed molecular device 10 is shown
in FIG. 2. Although a two-terminal device is shown and described
herein, it will be readily apparent to those skilled in this art
that the present teachings are not limited to two-terminal devices,
but is also applicable to three-terminal, and higher, devices.
[0056] Further details of the formation of a molecular device 10,
such as shown in FIG. 2, are available in above-mentioned U.S. Pat.
No. 6,458,621. Briefly, the substrate 12 comprises a material
selected from the group consisting of semiconductors, insulating
plastics, polymers, crystalline ceramics, and amorphous ceramics.
Preferably, the substrate 12 includes a coating 12a formed thereon,
such as an insulating layer formed on a semiconductor wafer, such
as SiO.sub.2 on Si.
[0057] The first electrode 14 comprises a material selected from
the group consisting of platinum, tungsten, aluminum,
polycrystalline silicon, single crystal silicon, amorphous silicon,
and conductive polymers.
[0058] The molecular film 16 typically comprises a material capable
of switching/changing in the presence of an applied electric field.
It can be a film that allows the growth of filaments.
[0059] Another example of the molecule film 16 includes molecular
materials that evidence an electric field induced HOMO-LUMO
(highest occupied molecular orbital-lowest unoccupied molecular
orbital) gap change and are selected from the group consisting of:
(1) molecular conformation change or an isomerization; (2) change
of extended conjugation via chemical bonding change to change the
HOMO-LUMO gap; and (3) molecular folding or stretching, wherein the
change of extended conjugation via chemical bonding change to
change the HOMO-LUMO gap is selected from the group consisting of:
(2a) charge separation or recombination accompanied by increasing
or decreasing electron localization; and (2b) change of extended
conjugation via charge separation or recombination and .pi.-bond
breaking or formation.
[0060] As noted above, such switch films 16, which are primarily
discussed in terms of switches, may also be used in a variety of
devices, including, but not limited to, diodes, resistors,
transducers, transistors, etc.
[0061] The second electrode 18 is selected from the same list of
materials as the first electrode 14, and may be the same or
different, with the caveat that there is usually, but not always, a
sticking layer (e.g., Ti). Such a sticking layer may account for
some of the switching activity, i.e., it may be the difference
between the Pt and Ti that is involved in the switching and so the
choice of electrode may well tailor the effect. Also, the second
electrode may not even be part of the stack, but rather part of a
moveable-tip addressable scheme.
[0062] Specific examples of second contacts 18 further include
circular electrodes and nanopores over the molecular film 16
covered with an electrode. The nanopore serves to limit the extent
of the second contact.
PRESENT EMBODIMENTS
[0063] The embodiments herein are directed to the improved
fabrication of conductive electrodes, e.g., platinum (Pt),
electrodes for use in molecular electronic devices 10, particularly
first electrodes 14. This material has been fabricated as the first
electrode 14 in a device stack 10 as shown in FIG. 2. The platinum
electrodes 14 have been tested with a 2-station [2] rotaxane
molecular film and eicosanoic acid film 16. These molecular devices
10 have displayed both diode behavior and switch behavior. However,
while the following description is specifically directed to
platinum electrodes, the electrode may comprise any conductive
material that forms a solid oxide film that is stable under ambient
conditions (e.g., standard temperature and pressure--STP).
Advantageously, the conductive electrode properties include: low or
controlled oxide formation (or possibly passivated), high melting
point, high bulk modulus, and low diffusion. Further, the
conductive material forming the first electrode 14 has a
resistivity less than 1375 micro-ohm-cm, and may comprise any of
the elements in rows 1B-7B and 8 of the Periodic Table. Examples
include platinum, tungsten, silver, aluminum, copper, nickel,
chromium, molybdenum, titanium, and tantalum. Of these, platinum is
preferred because it is compatible with CMOS-type back-end
processing and packaging, i.e., oxide/nitride films and high
temperature steps.
[0064] The deposition of platinum first electrodes 14 employing
prior art procedures results in metal layers having a smoothness of
8 to 10 .ANG. (the smoothness of the coating 12a is typically about
4 .ANG.). It is noted that prior deposition techniques that use a
typical sticking layer increase the roughness. Unless the adhesion
is carefully controlled, Pt deposited in any useful thickness
simply lifts from the surface, especially under liquid conditions
such as SAM or LB deposition. Also prior depositions make no
mention of tailoring the surface; the Pt is just deposited. Herein,
the formation of the electrode may or may not include deposition of
a conductive layer; for example, the formation of the electrode may
comprise nanowire growth. In any event, the surface is tailored for
smoothness, hydrophilicity, and barrier layer. The terms "tailored"
or "tailoring" refer to a process involving the preparation of the
surface, and further includes any of the following: (a) actively
smoothing, (b) actively oxidizing, which produces a very
hydrophilic surface good for Langmuir-Blodgett films, (c) actively
removing the oxide without re-roughening, and (d) actively
passivating. By "actively" is meant that an operation is performed
or a sequence of predetermined steps is set in motion to accomplish
a specific desired result.
[0065] With regard to nanowires, even though the deposition
techniques described herein do not apply, nevertheless, the
described tailoring techniques do hold for nanowires. Indeed, the
tailoring techniques described herein are applicable to electrodes
having a non-planar surface. The term "non-planar" includes
three-dimensional surfaces, such as textured surfaces (either
deliberately formed or due to conformal deposition on a textured
substrate, such as a micro-machined substrate) and nanowires, but
does not include surface roughness that arises without willful
intent.
[0066] The following description of the formation of the first
electrode 14 on a coated substrate 12, 12a is intended to be
exemplary only. FIG. 3 illustrates the flow chart for the process
disclosed herein.
[0067] The substrate 12 is provided (step 30). In the prior art
approach, the first electrode 14 is formed on the substrate (step
32). Next, the molecular film 16 is formed on the first electrode
(step 34). In accordance with the embodiments disclosed herein, a
pattern (if any) is formed for deposition (step 36a), exposed
portions of the substrate 12 are cleaned, if necessary (step 36b),
those portions are pre-sputtered (step 36c), the Pt first electrode
14 is formed on those portions (step 36d), the pattern is finished,
if necessary (step 36e), residual material, if any, is removed
(step 36f), and the properties of the exposed surface of the Pt
electrode 14 are cleaned (step 36g) and tailored (step 36h).
Following tailoring of the exposed surface properties, the
molecular film 16 is deposited on the Pt electrode 14. The details
of the process are now described. The following description with
regard to the formation of the first electrode 14 is provided in
terms of forming a planar surface. Descriptions of forming a
non-planar surface follow.
[0068] The substrate 12 comprises <100> SEMI-grade prime
silicon wafer (alternatively, an extra smooth substrate, such as
cleaved mica, may be used). If a silicon wafer is used, it is
cleaned as is conventional in the semiconductor art for a
pre-diffusion clean such as an RCA-clean.
[0069] Next, a layer of tight knit, or dense, thermal oxide 12a is
grown on the silicon wafer 12 (or deposited on a non-silicon
wafer). If non-thermal oxide is deposited, it will most likely
require densification. If a non-silicon substrate, such as mica, is
used, then the oxide may not be needed, as the substrate may not be
electrically conducting. As is well-known, tight-knit thermal oxide
is grown to be close-packed, thereby avoiding a separate
densification step that would increase the process time.
[0070] An oxide, or other suitable material as is known in the art,
is needed on silicon to provide an insulating substrate 12a, and
thereby electrically isolate the subsequent platinum layer from
silicon 12. Otherwise, a metal on semiconductor would result, and
device properties would be more coupled to the substrate, which is
less desirable than metal on insulator. Direct contact may also
produce metal-silicon intermixing. If an insulating non-silicon
crystal 12, such as mica, is used, then the insulating layer 12a is
superfluous and can be eliminated, as noted above.
[0071] The thermal oxide 12a is grown to a preferable thickness of
about 2,000 .ANG.. The layer could be thicker than 2,000 .ANG., but
must not be so thick that undue stress on the wafer 12 or in the
film develops. On the other hand, the thickness of the thermal
oxide 12a should be greater than 1,000 .ANG. for electrical
isolation.
[0072] A silicon nitride, Si.sub.xN.sub.y, where x=1-3 and y=1-4
(stoichiometric Si.sub.xN.sub.y is Si.sub.3N.sub.4), could be grown
in place of silica, but is less preferable, due to the lack of
stoichiometric control that is obtainable with SiO.sub.2.
[0073] If desired, a resist is formed and patterned for
conventional lift-off (step 36a). Any of the resist materials
commonly employed in this art may be used. The pattern is the array
of one or more first electrodes 14. The resist is removed from
those areas where the platinum is to be deposited to form the first
electrodes. Removal of the resist is also conventional. A dry etch
of the metal would produced a somewhat sharper profile, which is
not necessarily desirable where molecular coverage on the order of
30 .ANG. is attempted. Indeed, etching (wet/dry/milling, etc.)
techniques may be done, although they may involve multiple steps
for fabricating desired profiles. Another method of producing a
pattern to be filled with platinum would be the well known
shadow-masking process.
[0074] Another method of forming the electrode 14 would be to grow
a nanowire in the desired area or to form or imprint (or mold) the
pattern. Also, the substrate surface could be patterned in a
three-dimensional fashion on several levels (such as with wells)
for micromechanical, sensor, or off-plane applications. It will be
appreciated that nanowires may sit up off the surface (i.e., in a
vertical or near-vertical direction), but not always. A nanowire
may or may not need an insulating area under it. Examples of the
growth of nanowires are given in U.S. Pat. No. 6,773,616, issued to
Yong Chen et al on Aug. 10, 2004; and U.S. Pat. No. 6,656,573,
issued to Yong Chen et al on Dec. 2, 2003, while examples of the
formation of nanowires by imprinting (or molding) are given in U.S.
Pat. No. 6,579,742, issued to Yong Chen on Jun. 17, 2003; U.S. Pat.
No. 6,432,740, issued to Yong Chen on Aug. 13, 2002; and U.S. Pat.
No. 6,407,443, issued to Yong Chen et al on Jun. 18, 2002. The
teachings of the foregoing references are incorporated herein by
reference.
[0075] Once the areas for Pt deposition have been exposed, these
open areas are cleaned (descummed), such as with an oxygen plasma
(step 36b). The specific parameters for de-scumming depend on the
particular plasma system used; for an RIE System 1700, the
conditions were 100 mTorr, 100 Watts, for 2 minutes, using forward
power control. The time may range from 1 to 5 minutes, but no
further significant improvement is seen after 5 minutes. More
sputtering, which is undesirable, results from higher power.
Pressures in the range of 50 to 200 mTorr and powers up to 100
Watts have been used. It will be noted that this cleaning step is
not applicable to nanowire growth.
[0076] Next, a pre-sputter of the exposed areas is performed (step
36c). A 5 min. argon (Ar) pre-sputter was performed in an SFI DC
Magnetron sputter system at 6.5 sccm Ar, 0.9 mTorr. This
pre-sputter further cleans the surface (the above O.sub.2 plasma
removes organics) and removes environmental contaminants. Without
this pre-sputter step, the subsequent Pt layer 14 lifts off under
duress, while too much sputtering increases the surface roughness
of the substrate coating 12a. It will be noted that this
pre-sputtering step is not applicable to nanowire growth.
[0077] The advantage of the pre-sputter step is that no "sticking"
layer, or adhesive layer, is required, as is conventional practice
in the art, in order to deposit the platinum layer 14 and maintain
it on the surface of the substrate 12 or coating 12a. This avoids
the extra steps required and potential increased surface roughness
resulting from the deposition of these layers(s) otherwise
required, e.g., Ti, Cr, Ta, conventionally used to adhere a
platinum layer to a surface.
[0078] However, experiments were performed to provide adequate
sticking without sacrificing smoothness. Further, for films
immersed in liquid, it is not always apparent that the layer is
going to peel when dry. For LB coating and SAM deposition, the Pt
film must be well adhered. Some deposited Pt films, which seem to
be adequately adhered without the process disclosed herein, simply
roll up like a window shade when the substrate is immersed in
fluid.
[0079] In a preferred embodiment, the platinum layer 14 is
blanket-deposited everywhere, using, for example, a DC magnetron
sputtering system (step 36d). As an example of operating
parameters, present sample values for cleaned and reconfigured
system are: cathode: 6.7 A, 6.7 V; beam: 15 mA, 348 V; accelerator
1.3 mA, 150.5 V; neutralizer: 5.61 A; emission: 16.8 mA to deposit
a layer of Pt about 1,000 .ANG. thick. The Pt layer 14 can be
thinner or thicker than 1,000 .ANG., but must be thick enough to
provide good conduction, but not so thick as to provide a large
step for the molecular switch film 16 to cover. By "good"
conduction is meant that the platinum layer 14 can pass a desired
current through a probe. The thickness of the Pt layer 14 is in the
range of 50 to 5,000 .ANG., No lumps/asperities of platinum were
observed on the surface from this system for a thickness of 1,000
.ANG.. A desired profile without sharp edges is achieved through
lift-off techniques. Fine line liftoff is achieved with thinner
depositions, without undue experimentation. While liftoff is
preferred, shadow-masking and etching may alternatively be
performed.
[0080] In some embodiments, the formation of the Pt layer 14 is
completed by performing the lift-off, to remove resist(s) (and the
metal covering that resist) from unwanted regions (step 36e). A
conventional solvent, such as N-methyl-pyrrolidone, followed by a
water rinse, may be used. Again, combinations of techniques well
known in the semiconductor art, though not as preferred, may be
used. If no pre-patterning was done, then at this step, the blanket
platinum would be masked and etched, again, using techniques well
known in the art.
[0081] Platinum may alternatively be deposited by evaporation, such
as e-beam evaporation, also blanketly deposited, or formed through
wire growth, as described above.
[0082] The remaining Pt bottom conductor areas 14 are cleaned,
which again is system-dependent (step 36f). If there is resist
remaining from a previous step, this step serves to remove any
residual material. The removal of such residual material could be
as restrained as the cleaning/tailoring step described immediately
below. Alternatively, depending on the quality and quantity of
residual material, the removal step could be much more aggressive,
using various combinations of plasma etching, wet or dry etching,
etc.
[0083] In some embodiments, step 36f is omitted, and an O.sub.2
plasma is used to clean, as well as rearrange and smooth the
surface of the remaining Pt layer 14 (step 36g). An example of such
O.sub.2 cleaning/tailoring is performed in an RIE System 1700; the
conditions were 80 sccm O.sub.2, 100 mTorr, 100 Watts, for 5
minutes, operating under forward power control with a HIVAC base
pressure of 2.0.times.10 .sup.-5 Torr. It appears that the surface
is physically distinct, based on Atomic Force Microscopy images. It
appears that the oxygen plasma is sufficient to cause some physical
bombardment of the surface. At lower powers with higher pressures,
no rearrangement of the surface is observed.
[0084] Essentially, at relatively low pressure and high power (not
too much gas in the chamber, physical bombardment), there is a
sputtering component that increases with the mass of the species.
On the other hand, at relatively high pressure, low power (lots of
gas; less acceleration), then mostly a chemical reaction occurs.
Under the conditions of moderate pressure and power is where the
desired rearrangement is obtained. As with the foregoing processes,
this step is machine-dependent, and the operating parameters will
vary from one machine to another. However, the determination of
such operating parameters for a specific machine is not considered
to be undue, based on the teachings herein.
[0085] The tailoring step is performed in an oxygen plasma to
rearrange the platinum layer and to smooth the exposed surface of
the platinum layer. This step alters the hydrophilicity of the Pt
layer to render it more hydrophilic and also provides a barrier
layer (due to the presence of the PtO.sub.2 on the surface). This
is important, since the Pt surface is very hydrophilic when the
oxide is present and seems to be the key to obtaining a desirable
uniform Langmuir-Blodgett film. This is true no matter how the Pt
was deposited or grown. Specifically, this tailoring step works not
only for planar surfaces but also for nanowire surfaces and
surfaces that are not totally planar.
[0086] An oxygen plasma, as described in the previous paragraph,
provides a hydrophilic Pt surface. Use of an oxygen plasma and a
subsequent argon plasma may alternatively be used; this combination
provides a less hydrophilic, more hydrophobic Pt surface. Yet
alternatively, an argon plasma alone may be used, which also
provides a hydrophobic surface. Finally, a sequence of oxygen, then
hydrogen plasmas may be used, to provide a smooth surface with
reduced oxygen, which is passivated.
[0087] The foregoing Pt deposition procedure yields a surface
roughness that is less than 8 .ANG. RMS, and can be as small as 4
.ANG. RMS which is about as good as the substrate coating 12a. It
also yields at this point an oxygenated surface and a hydrophilic
surface. It will be noted that the foregoing surface roughness is
produced on films of 800 to 1000 .ANG.. Thus, the surface roughness
may be considered to be in the range of about 0.8 to 1% to as small
as 0.4 to 0.5% of the thickness of the conductive layer 14.
[0088] Without subscribing to any particular theory, it appears
that the reason why a smooth platinum surface is obtained is based
on the following: (1) prior to the platinum deposition, the process
starts with smooth surface, with smooth oxide thereon (or cleaved
insulator, such as mica); (2) no sticking layer is used for
adhesion of the Pt layer (sticking layers, such as Ti, Cr, Ta,
increase the surface roughness); and (3) subsequent to Pt
deposition, the O.sub.2 plasma removes any remaining polymer,
rearranges and smoothes the surface, without pitting it, thereby
tailoring the Pt exposed surface. It will be appreciated that the
O.sub.2 plasma also rearranges and smoothes even when no polymer
(the resist) contact is initiated.
[0089] The oxygenated layer may be removed in an argon plasma in
the same RIE machine, either immediately following or at a later
time. The conditions of 40 mTorr, Ar (80 sccm), and 15 W forward
power remove the oxygenated layer, maintain the smoothness of the
rearranged surface, and produce a surface which wets identically to
"as-deposited" platinum, with only trace amounts of oxide
present.
[0090] Summarizing the foregoing, in some embodiments, the first
electrode 14 may be applied and tailored over a textured
surface.
[0091] In an embodiment, the first electrode 14 may be applied and
tailored over a pre-formed surface with varying heights (such a
pre-formed surface is to be distinguished from a rough surface in
that the height is a process variable, such as the formation of
micromachined pits).
[0092] In an embodiment, the first electrode 14 is formed from a
film that is then patterned by imprinting (or molding), as
described above.
[0093] In an embodiment, the second electrode 18 is circular or
semi-circular. FIG. 4 depicts this configuration, which is
analogous to FIG. 1d, but for the configuration of the second
electrode. FIG. 5 is a cross-sectional view of FIG. 4.
[0094] In an embodiment, the first electrode 14 is not deposited in
a layer, but grown, as in a nanowire, as described above. FIGS. 6
and 7 depict such a configuration, wherein the first electrode 14
is a nanowire and wherein the second electrode 18' is a probe tip.
FIG. 7 is a cross-sectional view of FIG. 6. A functionalized layer
16' may be formed on the surface of the nanowire 14. Functionalized
layers are well known and are described elsewhere; see. e.g., U.S.
Pat. No. 6,459,095, issued Oct. 1, 2002. In particular, some
functionalization or tailoring may be added to the surface of the
nanowire prior to SAM or other molecular deposition. The
functionalized layer 16' may then comprise molecular film 16 with
additional binders below (or above) it. The nanowire may be
oxidized and certain groups would preferentially stick to that. If
the nanowire is Pt, it would be treated no differently than the
sheet Pt for molecular deposition.
[0095] In an embodiment, after the conductive layer is deposited,
then the properties of the exposed surface of the conductive layer
are tailored.
[0096] In an embodiment, a conductive layer having a smooth surface
is provided, wherein the conductive layer essentially replicates
the smooth surface of the underlying substrate.
[0097] In an embodiment, a contact or second electrode is formed
over the first electrode, which may be oriented at a non-zero angle
with respect thereto, such as with a crossbar device, e.g., a
switch. For pores, dots, tip addressing, etc., there may be an
electrode or alternatively brief contact may be made, such as with
a dot.
[0098] Following the last step (depositing the conductive layer,
forming the nanowire, or the tailoring step), the molecule or
molecular film is formed on the surface.
[0099] Advantageously, conductive electrode properties include: a
controlled oxide formation (under certain circumstances), a high
melting point, high bulk modulus, low diffusion, some degree of
stability (which depends on surface preparation). Smooth deposited
film surfaces are compatible with Langmuir-Blodgett molecular film
deposition. The metallic nature gives high conductivity connection
to molecules. Barrier layers may be added to the device stack,
i.e., Al.sub.2O.sub.3 over the conductive layer. The surface
tailoring is especially advantageous for self-assembled monolayer
(SAM) work.
[0100] The embodiments disclosed and claimed herein, while
including the deposition/growth of the conductive layer, are not to
be construed as limiting to just the deposition, but optionally
includes the tailoring of the conductive surface through plasma
exposure. Such tailoring of the conductive surface is apparently
unknown heretofore. Essentially, the physical structure is combined
with chemical features to produce films uniquely suited for the
application of molecular films through a wide variety of formats,
including, but not limited to, Langmuir-Blodgett (LB),
self-assembled monolayer (SAM), spin-coat, etc.
[0101] The surface may be further tailored to include oxide or no
oxide while maintaining the low surface roughness (and maintaining
any texturing on a scale longer than roughness), which also changes
the wetting properties, etc.
EXAMPLES
[0102] Experimental Procedure
[0103] Both the blanket and photolithographically-modified Pt films
were sputter deposited on Si wafers with a 100 nm silicon dioxide
layer. The typical Pt thickness was 100 nm. The plasma treatment
was performed in a RIE.RTM. model 1700 system. Freshly deposited Pt
films and films exposed to various plasma treatments were analyzed
with contact angle and ellipsometry measurements within 10 minutes
of preparation and by XPS and Auger with controls.
[0104] For contact angle measurements a droplet of 2 .mu.L 18
M.OMEGA..multidot.cm water was injected onto the sample surface
from a syringe. An image of the static water droplet was recorded
with a digital camera and analyzed to yield a sessile contact
angle, averaging at least three readings.
[0105] Ellipsometric measurements were performed using a laser with
a wavelength of 532 nm and an incident angle of 58 degrees. A
simple model was used to derive the optical constants, n and k. The
platinum was approximated by an infinite thickness. The reported
values represent an average of three readings from different
locations.
[0106] The surface morphology of the Pt films was monitored with a
commercial atomic force microscope operated under ambient
conditions in tapping mode. The surface roughness is calculated
over a 1 .mu.m.sup.2 area.
[0107] XPS spectra were acquired on either a Surface Science
Instruments spectrometer or a PHI Quantum 2000 spectrometer with
monochromated Al K.alpha. 1486.6 eV X-ray source. Take-off angles
in the two instruments were set at 35.degree. and 45.degree.,
respectively. All the photoemission peak positions were corrected
to opportunistic C1s at 284.8 eV binding energy.
[0108] Auger analysis was performed on a PHI 670 Scanning Auger
Microprobe with a CMA analyzer, 20 KeV, 10 nA beam energy and 45
degree tilt.
[0109] Results and Discussions
[0110] A. Optical Constants
[0111] Previous ellipsometric study has shown that the optical
constants of Pt thin films were strongly dependent on the film
deposition conditions. In this study, the optical constants,
refractive index (n) and extinction coefficient (k), of films with
different plasma treatments were derived from single-wavelength
ellipsometry with a single-layer model. The films with different
plasma treatments fell into two classes based on their optical
constants measured at 532 nm: a larger value class with n
.about.2.5 and k .about.4.2 and a smaller value class with n
.about.1.8 and k .about.3.4. The films treated with argon plasma
and those treated with argon after oxygen behaved similarly to the
as-deposited film. They all exhibited larger optical constants. In
contrast, measurements of the platinum films exposed only to oxygen
plasma resulted in optical parameters belonging to the smaller
values class. Films intentionally introduced to photochemicals
before plasma treatment showed no variation from the above.
[0112] Although there was only a slight decrease of the n and k
values over several hours, contact angle measurements exhibited a
larger change. Ellipsometry appears not to be sensitive to the
changes that do occur.
[0113] B. Contact Angle Measurement
[0114] Water contact angle is a direct measure of surface
hydrophilicity. Sessile water contact angles of the Pt thin films
were recorded in parallel with the optical constants. Under ambient
conditions, contact angles increased markedly within in the first
three hours, changing slowly thereafter. As a catalytic material, a
variety of chemical species can adsorb onto platinum surfaces. As
the surface adsorbs CO, hydrocarbons, and other organic compounds,
the surface free energy decreases and a higher water contact angle
is observed. Contact angle studies by other investigators also have
documented a hydrophilic nature migrating toward hydrophobic within
minutes of exposure to the laboratory atmosphere. Hydrophobic is
defined as a contact angle greater than 30 degrees.
[0115] The platinum films could also be divided into two classes,
based upon the time dependence of the water contact angle. The
samples in the higher contact angle group consisted of: the fresh
as-deposited film and films treated with an argon plasma. The
samples exhibiting values in the lower contact angle group were the
films treated with an oxygen plasma (and no subsequent argon
plasma). This is consistent with the ellipsometric
measurements.
[0116] Both measurements reveal that an oxygen plasma treatment
changes some platinum thin film properties, while an argon plasma
treatment can restore some properties of freshly deposited Pt
films. The oxygen plasma treated surfaces are initially more
hydrophilic than the freshly deposited or argon plasma treated
surfaces, but the rate of increase of the contact angle is similar
for both classes. In order to understand why and how the oxygen
plasma treatment can change surface properties so dramatically,
x-ray photoelectron spectroscopy was utilized to examine the
surface chemical composition of the platinum thin films.
[0117] C. X-Ray Photoelectron Spectroscopy (XPS) and Auger Electron
Spectroscopy (Auger)
[0118] The survey and Pt 4f region spectra of four platinum thin
films were scanned. The four films were (1) a fresh as-deposited
thin film, (2) a film treated with argon plasma (5 min. at 100 W
and 100 mTorr; "AR1") alone, (3) a film treated with only oxygen
plasma (5 min. at 100 W and 100 mTorr; "OX1"), and (4) a film
treated with oxygen plasma (5 min. at 100 W and 100 mTorr) followed
by argon (5 min. at 100 W and 100 mTorr) plasma. Only Pt, C, and O
were observed on all samples. The presence of carbon and oxygen was
unavoidable because of surface adsorption of hydrocarbons and
species with C--O functionalities. The peak position and intensity
of C, O, and Pt were almost identical on the fresh as-deposited
thin film, the film treated with argon plasma, and the film treated
with oxygen plasma plus argon plasma. However, a significant
increase of the O 1s peak intensity at 532 eV was observed in the
film treated with oxygen plasma alone. In addition, a new set of Pt
4f peaks appeared on this sample at higher binding energy. The new
peaks, Pt 4f.sub.7/2 at 74.7 eV and Pt 4f.sub.5/2 at 78.0 eV, are
conclusive evidence of platinum oxide formation. This result is
also consistent with the XPS result for a previously reported
PtO.sub.2 thin film prepared by reactive sputtering in the presence
of oxygen gas.
[0119] Combining all the pieces of information derived from optical
constant measurements, contact angle measurement, XPS, and Auger
studies, it is clear that the oxygen plasma treatment forms an
oxide layer on the Pt thin film surface and changes the surface
properties dramatically. In order to understand the relationship
between oxide generation and the oxygen plasma condition,
high-resolution spectra of platinum thin films treated with a
somewhat aggressive oxygen plasma treatment (5 min. at 100 W and
100 mTorr), OX1, and with a less aggressive plasma (2 min at 50 W
and 50 mTorr), OX2, were studied. The relative atomic
concentrations of all the fitted components are listed in Table 1,
after the absolute peak areas were corrected with the sensitivity
factor of each element.
1TABLE 1 The relative atomic concentration (%) of fitted peaks at
different chemical states. Pt 4f peaks O 1s peaks 2 + 2' 1 1 + 1'
(PtO or 3 + 3' (metal 2 C 1s Samples* (Pt.sup.0) Pt(OH).sub.2)
(PtO.sub.2) oxide) (C--O related) peaks OX1 6.4 4.5 17.1 32.5 19.1
20.3 OX2 7.9 5.1 16.7 30.7 20.2 19.9 OX1 + 53.9 1.0 0.1 2.7 3.6
38.6 AR2 OX2 + 55.1 0.8 0.0 3.1 2.2 38.8 AR2 OX1 = O.sub.2 plasma:
5 min. 100 W 100 mTorr; OX2 = O.sub.2 plasma: 2 min. 50 W 50 mTorr;
AR2 = Ar plasma: 1 min. 15 W 40 mTorr.
[0120] The majority of the Pt, 56% to 61%, within the XPS sampling
depth (usually less than 50 .ANG.) of films treated with oxygen
plasma was in the PtO.sub.2 chemical state as denoted 3 and 3'. The
O to Pt atomic ratio is nearly 2:1, provided that the Pt.sup.0
(denoted as 1 and 1') was excluded in these samples. A small
portion of Pt, 16% to 17%, was assigned tentatively as PtO or
Pt(OH).sub.2 chemical state as denoted 2 and 2'. The more
aggressive oxygen plasma produces only slightly more oxide than the
less aggressive oxygen plasma, based on the ratio of Pt in oxide
chemical states vs. Pt in the metallic state.
[0121] Estimation of thickness of platinum oxide from
high-resolution XPS spectra was performed using the simple
substrate-overlayer model and the thickness of oxide in the Pt film
treated with the aggressive and less aggressive oxygen plasmas was
calculated to be 2.4 nm and 2.7 nm, respectively. Auger data, which
follows, differs with respect to this thickness.
[0122] XPS shows about 98% of Pt exists in the metallic chemical
state (Pt.sup.0) after a further treatment with the AR2 argon
plasma. The stated argon plasma condition is the minimal possible
power and flow to generate a stable plasma in the RIE instrument.
Any platinum oxides were present in quantities below the XPS
detection limit. The oxygen atomic concentration dropped to less
than 6% among the elements detected on these samples and could be
mainly attributed to the surface adsorbed species with C--O
functional groups. A high percentage of C was also detected in
these metallic platinum film surfaces from various adsorbed
species.
[0123] The Auger Electron Spectroscopy results showed similar
elements but differed with respect to oxide thickness. The elements
detected on the surface of each of the samples were primarily
platinum plus carbon and oxygen. By elemental analysis of the etch
products, seeking the point at which oxygen from the sample became
undetectable during etching, it was concluded that the oxide (PtO,
PtO.sub.2, Pt(OH).sub.2) was less than 5 .ANG. in thickness (for a
sample treated with OX1), actual depth, full width, half maximum
(FWHM). The oxygen content of as-deposited and OX1+AR2 treated
samples was minimal and their oxide thicknesses were less than 2
.ANG..
[0124] The ion-gun etch rate was experimentally determined to be
5.2 .ANG./min (actual depth in Pt(O) by AFM measurement) The
calculated conversion factor between the Pt(oxide) etch rate and
SiO.sub.2 calibration material was consistent with that for other
heavy metals. Survey scans of the samples were presented as plots
of the first derivative of the number of electrons detected as a
function of energy. Depth profiles were obtained by alternating an
acquisition cycle with a sputter cycle. During the acquisition
cycle selected elemental peak intensities were collected. The
sputter cycle removed material from the surface of the sample using
a 2 keV Ar.sup.+ source rastered over a 5 mm.times.5 mm area. In
order to eliminate crater wall effects, the data was acquired from
a much smaller region in the center of the sputtered area.
[0125] For a sample subjected to OX1, slight shifts in the platinum
peak position due to chemical state allowed the Pt (oxide) and Pt
(metal) components of the metal to be separated using a linear
least squares (LLS) curve fitting routine. No correction to the
relative sensitivity factor was made for the Pt (oxide) trace for
stoichiometry and therefore error may be present in the atomic
compositions reported.
[0126] The PtO.sub.2 peaks dominate the OX1 spectrum where
.about.61% of the Pt is present as PtO.sub.2. The remaining Pt is
present in two or three different states and in the initial XPS
data these states were separated into Pt.sup.0 (metal) and
PtO/Pt(OH).sub.2. Due to the strong peaks of PtO.sub.2 and PtO, the
PtO and Pt(OH).sub.2 chemical states could not be accurately
separated.
[0127] Using the OX1+AR2 treated sample as a reference for spectral
subtraction and assuming that this sample is representative of the
surface after cleaning and after exposure to air, the reference
spectrum of the sample with treatment OX1+AR2 is seen as primarily
Pt.sup.0 with trace amounts of PtO/Pt(OH).sub.2. Scaling and
subtracting the spectrum of the sample treated with OX1+AR2 from
that treated with OX1 alone produces the chemical difference
between the two samples, i.e., the effect of the oxygen plasma. In
this subtracted spectrum, the primary peaks are associated with the
presence of PtO.sub.2 but minor states are also present.
Curve-fitting the spectrum reveals PtO.sub.2 and two additional
chemical states that correlate to PtO and Pt(OH).sub.2. The data
shows an approximately 2 eV difference between these two chemical
states, which is corroborated by available literature. The
narrowness of the fitted peaks cause some ambiguity as to the
precise ratios of these two chemical states, but both are present
in the sample treated with OX1.
[0128] The ratios of PtO2:PtO:Pt(OH).sub.2 were found to be:
[0129] PtO.sub.2: 87.4%
[0130] PtO: .about.5.1%
[0131] Pt(OH).sub.2: .about.7.5%
[0132] In conclusion, the spectral subtraction shows more clearly
the difference between samples treated with OX1 alone and OX1+AR2.
These differences include the presence of three additional chemical
states for platinum: PtO.sub.2 (predominantly) and lesser amounts
of both PtO and Pt(OH).sub.2.
[0133] D. Atomic Force Microscopy (AFM)
[0134] Plasma treatment of the platinum thin films also altered the
morphology. Investigation was carried out to achieve surfaces with
as smooth as possible morphology. The surface roughness was
monitored by AFM, and the data is listed in Table 2, along with
other surface properties. The sputtering deposition condition used
in this laboratory produces platinum thin films with RMS roughness
of 5.4 .ANG. over an area of 1 .mu.m.sup.2.
2TABLE 2 The surface properties of platinum thin film treated with
different plasma conditions. Water RMS Contact roughness angle in
Process condition* (degrees) n k 1 .mu.m.sup.2 (.ANG.) Fresh
as-deposited Pt 32 2.53 4.26 5.4 5 min O.sub.2 plasma (OX1) alone w
1.85 3.35 3.4 5 min Ar plasma alone 30 2.47 4.18 8.1 OX1 + 5 min Ar
plasma 30 2.50 4.21 5.7 OX1 + 3 min Ar plasma 25 2.51 4.23 6.0 OX1
+ 1 min Ar plasma 25 2.45 4.15 5.6 OX1 + 1 min Ar plasma 31 2.48
4.18 4.8 (50 W, 50 mTorr) OX1 + 1 min Ar plasma 32 2.47 4.18 4.4
(25 W, 50 mTorr) OX1 + 1 min Ar plasma 27 2.40 4.07 3.8 (15 W, 40
mTorr) (AR2) OX1 + 1 min Ar plasma w 1.90 3.41 3.1 (20 W, 25 mTorr,
no plasma is generated) O.sub.2 or Ar plasma: 100 W 100 mTorr,
unless otherwise specified. w: water readily wetted the surface
producing a contact angle of generally less than 10 degrees, so it
was difficult to obtain an accurate reading.
[0135] Argon plasma exposure, particularly, "high" power plasma,
will roughen the platinum surface. An 8.1 .ANG. RMS roughness was
observed for the surface treated with argon plasma for 5 min. at
100 W and 100 mTorr. Heavy argon atoms under a high power plasma
condition can bombard the Pt thin film and roughen the surface.
Oxygen plasma exposure did not roughen the surface, but rather
smoothed it, as suggested by a 3.4 .ANG. roughness over an area of
1 .mu.m.sup.2 recorded for the surface treated oxygen plasma for 5
min. at 100 W and 100 mTorr. These numbers are for films of nominal
thickness 1000 .ANG..
[0136] A series of lower power/shorter duration argon plasmas was
evaluated for its ability to minimize the effect of roughening. By
using a minimal argon plasma, 1 min. 15 W at 40 mTorr, little
roughening (3.8 .ANG. RMS roughness in 1 .mu.m.sup.2) of the
platinum thin film surface occurred, yet the oxide was removed and
surface properties dramatically changed. These events also occurred
on film of 1000 .ANG. thickness.
[0137] While the foregoing examples involved planar first
electrodes, it is expected that essentially the same results will
be obtained for non-planar electrodes.
[0138] Conclusion
[0139] The properties of platinum thin films are strongly affected
by the plasma treatment conditions. Argon-treated Pt thin films
behaved similarly to as-deposited untreated films with respect to
water contact angle and ellipsometrically measured optical
properties. Oxygen plasma treatment resulted in marked change of
the surface chemical properties. XPS and Auger studies confirmed
the formation of platinum oxides, PtO.sub.2, PtO and Pt(OH) after
the film was treated with oxygen, even under modest plasma
conditions. The change in the surface properties was attributed to
the formation of such an oxide layer on the film surface. Further
treatment with argon plasma diminished the oxide layer; however,
aggressive argon plasmas roughened the surface. In order to
minimize the surface roughness, a minimal argon plasma recipe
subsequent to oxygen plasma treatment was developed to produce
clean, metallic Pt thin films with a roughness of less than 4 .ANG.
within a 1 .mu.m.sup.2 area.
[0140] Initial experiments indicate that hydrogen plasma will also
remove the oxide and may offer some passivation advantages.
INDUSTRIAL APPLICABILITY
[0141] The method of fabricating a platinum layer having a
relatively smooth surface and tailored mechanical, physical and
chemical properties in a molecular electronic device is expected to
find use in nanoscale computing and memory circuits.
* * * * *