U.S. patent application number 12/603242 was filed with the patent office on 2013-07-18 for sub-wavelength metallic apertures as light enhancement devices.
The applicant listed for this patent is Sachin Attavar, Steven M. Blair, Alexander Chagovetz, Mark Alan Davis, Mohit Diwekar, John Dredge. Invention is credited to Sachin Attavar, Steven M. Blair, Alexander Chagovetz, Mark Alan Davis, Mohit Diwekar, John Dredge.
Application Number | 20130182315 12/603242 |
Document ID | / |
Family ID | 42164967 |
Filed Date | 2013-07-18 |
United States Patent
Application |
20130182315 |
Kind Code |
A9 |
Blair; Steven M. ; et
al. |
July 18, 2013 |
SUB-WAVELENGTH METALLIC APERTURES AS LIGHT ENHANCEMENT DEVICES
Abstract
Light enhancement devices, applications for the light
enhancement devices, and methods for making the light enhancement
devices are provided. The light enhancement devices include a
substrate and a film of metal disposed over the substrate, the film
of metal including at least one cavity. The cavity may be of
various shapes depending on the desired application.
Inventors: |
Blair; Steven M.; (Salt Lake
City, UT) ; Diwekar; Mohit; (Salt Lake City, UT)
; Attavar; Sachin; (Salt Lake City, UT) ;
Chagovetz; Alexander; (Salt Lake City, UT) ; Davis;
Mark Alan; (Springville, UT) ; Dredge; John;
(Cedar Hills, UT) |
|
Applicant: |
Name |
City |
State |
Country |
Type |
Blair; Steven M.
Diwekar; Mohit
Attavar; Sachin
Chagovetz; Alexander
Davis; Mark Alan
Dredge; John |
Salt Lake City
Salt Lake City
Salt Lake City
Salt Lake City
Springville
Cedar Hills |
UT
UT
UT
UT
UT
UT |
US
US
US
US
US
US |
|
|
Prior
Publication: |
|
Document Identifier |
Publication Date |
|
US 20100118390 A1 |
May 13, 2010 |
|
|
Family ID: |
42164967 |
Appl. No.: |
12/603242 |
Filed: |
October 21, 2009 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
|
|
11497581 |
Aug 2, 2006 |
|
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12603242 |
|
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|
|
61114322 |
Nov 13, 2008 |
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61169113 |
Apr 14, 2009 |
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61177891 |
May 13, 2009 |
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60705216 |
Aug 2, 2005 |
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60795110 |
Apr 26, 2006 |
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Current U.S.
Class: |
359/346 |
Current CPC
Class: |
G02B 5/008 20130101;
G02B 6/1226 20130101; G02B 1/118 20130101; B82Y 20/00 20130101 |
Class at
Publication: |
359/346 |
International
Class: |
H01S 3/08 20060101
H01S003/08 |
Claims
1. A light enhancement device comprising a) a substrate; and b) a
film of metal disposed over the substrate, the film of metal
comprising at least one cavity exposing the substrate, the at least
one cavity comprising a tapered sidewall characterized by an angle,
wherein the angle of the tapered sidewall with respect to a surface
parallel to the substrate is sufficiently different than 90.degree.
to provide an enhancement of the transmission of light through the
cavity, an enhancement of the intensity of light within the cavity,
or both, that is greater than the enhancement if the angle was
90.degree..
2. The device of claim 1, wherein the at least one cavity has the
shape of a truncated cone.
3. The device of claim 1, wherein a surface of the film of metal
opposite the substrate is substantially flat.
4. The device of claim 1, wherein the film of metal comprises two
or more layers of metal.
5. The device of claim 1, further comprising at least one
biomolecule disposed in the at least one cavity.
6. The device of claim 1, further comprising a passivation layer
disposed over the film of metal, wherein the passivation layer is
capable of preventing adsorption of a molecule of interest to the
film of metal.
7. The device of claim 6, wherein the passivation layer comprises
an alkyl phosphonic acid.
8. The device of claim 6, wherein the passivation layer comprises a
self-assembled monolayer of an alkyl phosphonic acid.
9. The device of claim 6, wherein the film of metal comprises
aluminum and the passivation layer comprises butyl phosphonic acid,
decyl phosphonic acid, or combinations thereof.
10. A light enhancement device comprising a) a substrate; b) a film
of metal disposed over the substrate, the film of metal comprising
at least one cavity exposing the substrate, the at least one cavity
comprising a tapered sidewall characterized by an angle, wherein
the angle of the tapered sidewall with respect to a surface
parallel to the substrate is sufficiently different than 90.degree.
to provide an enhancement of the transmission of light through the
cavity, an enhancement of the intensity of light within the cavity,
or both, that is greater than the enhancement if the angle was
90.degree.; c) at least one biomolecule disposed in the at least
one cavity; and d) a passivation layer disposed over the film of
metal, wherein the passivation layer is capable of preventing
adsorption of a molecule of interest to the film of metal.
11. A light enhancement device comprising: a) a substrate; b) a
film of metal disposed over the substrate; c) at least one cavity
in the metal film exposing the substrate and having a sidewall; and
d) at least one change in the sidewall within the cavity including
a change in angle, a change in material, a change in width, or
combinations thereof sufficient to provide an enhancement of the
transmission of light through the cavity, an enhancement of the
intensity of light within the cavity, or both, that is greater than
the enhancement without the change in the sidewall.
12. The device of claim 11 wherein the sidewall includes arcuate
sections.
13. The device of claim 11, wherein the film of metal comprises two
or more layers of metal.
14. The device of claim 11 wherein the at least one cavity is an
elongated trench.
15. The device of claim 11, further comprising at least one
biomolecule disposed in the at least one cavity.
16. The device of claim 11, further comprising a passivation layer
disposed over the film of metal, wherein the passivation layer is
capable of preventing adsorption of a molecule of interest to the
film of metal.
17. The device of claim 16, wherein the passivation layer comprises
an alkyl phosphonic acid.
18. The device of claim 16, wherein the passivation layer comprises
a self-assembled monolayer of an alkyl phosphonic acid.
19. The device of claim 16, wherein the film of metal comprises
aluminum and the passivation layer comprises butyl phosphonic acid,
decyl phosphonic acid, or combinations thereof.
20. A light enhancement device comprising: a) a substrate; b) a
film of metal disposed over the substrate; c) at least one cavity
in the metal film exposing the substrate and having a sidewall; d)
a change in the sidewall within the at least one cavity, the at
least one change including a change in angle, a change in material,
a change in width, or a combination thereof; e) at least one
biomolecule disposed in the at least one cavity; and f) a
passivation layer disposed over the film of metal, wherein the
passivation layer is capable of preventing adsorption of a molecule
of interest to the film of metal.
21. A light enhancement device comprising: a) a substrate; b) a
film of metal disposed over the substrate having a thickness less
than 500 nm; c) at least one nanoaperture in the metal film
exposing the substrate and having a width less than 500 nm; and d)
at least a portion of a sidewall of the at least one nanoaperture
being non-parallel with respect to a surface normal to the
substrate.
22. The device of claim 21 wherein the at least a portion of the
sidewall includes a taper, a change in angle, a change in material,
a change in width, or a combination thereof.
Description
CLAIM OF PRIORITY
[0001] Priority of U.S. Provisional Patent Application Ser. No.
61/114,322, filed on Nov. 13, 2008 is claimed; and is herein
incorporated by reference.
[0002] Priority of U.S. Provisional Patent Application Ser. No.
61/169,113, filed on Apr. 14, 2009 is claimed; and is herein
incorporated by reference.
[0003] Priority of U.S. Provisional Patent Application Ser. No.
61/177,891, filed on May 13, 2009, is claimed; and is herein
incorporated by reference.
BACKGROUND
[0004] It has been demonstrated that when illuminated with light,
metallic cavity arrays support extraordinary transmission with
resonances at specific frequencies, which are strongly related to
the cavity array periodicity. See T. W. Ebbesen, H. J. Lezec, H. F.
Gaemi, T. Thio, and P. A. Wolff, "Extraordinary optical
transmission through sub-wavelength cavity arrays," Nature (London)
391, 667 (1998). Several models have been suggested to describe
this phenomenon. See L. Martin-Moreno, F. J. Garcia-Vidal, H. J.
Lezec, K. M. Pellerin, T. Thio, J. B. Pendry, and T. W. Ebbesen,
"Theory of Extraordinary Optical Transmission through Subwavelength
Cavity Arrays," Phys. Rev. Lett. 86, 1114 (2001); C. Genet, M. P.
van Exter, J. P. Woerdman, "Fano-type interpretation of red shifts
and red tails in cavity array transmission spectra," Opt. Commun.
225, 331 (2003); and H. J. Lezec, T. Thio, "Diffracted evanescent
wave model for enhanced and suppressed optical transmission through
subwavelength cavity arrays," Opt. Exp. 12, 3629 (2004). Most of
these invoke the role of surface plasmon polaritons (SPPs). SPPs
are surface electromagnetic waves formed by collective oscillation
of electrons at a metal-dielectric interface. See H. Raether,
Surface Plasmons on Smooth and Rough Surfaces and on Gratings,
(Springer-Verlag, Berlin, 1988). These models indicate that the
extraordinary transmission occurs when the incident excitation
matches the surface plasmon resonances. The light is strongly
localized on subwavelength scales as plasmonic excitations and a
resonance effect is accompanied by field enhancement.
[0005] One of the main possible areas of use for such metallic
cavity arrays is in the microarray diagnostic technologies. The
substrates generally used in a microarray platform consist of an
array of microscopic spots of immobilized DNA oligonucleotides,
peptides, or proteins. The complementary or desired sequence of
another molecule, such as ssDNA attached or tagged with a
fluorescent molecule (often with absorption maxima at 488 nm, 532
nm and 635 nm) hybridizes to complementary probes on the substrate.
After the hybridization reaction these substrates are excited by
laser sources corresponding to the fluorescent molecules used, and
fluorescence intensity is read or scanned with a microarray
scanner. The concentrations of DNA oligomers immobilized on such
substrates are typically in the nanomolar to picomolar ranges. The
metallic cavity arrays under illumination redistribute light inside
the cavities through the excitation of surface plasmons thereby
increasing the local intensity. By immobilizing the DNA
oligonucleotides inside the cavities and using them as tiny
reaction chambers for hybridization, it is possible to take
advantage of the local intensity enhancements for improving the
emitted fluorescence intensity. See M. J. Heller, "DNA microarray
technologies: Devices, systems and applications," Annu. Rev.
Biomed. Eng., 4, 129 (2002); Y. Liu, F Mandavi, and S. Blair
"Enhanced Fluorescence Transduction Properties of Metallic cavity
Arrays," IEEE J. Selected Topics in Quantum Electronic 11, 778
(2005); and S. Fore, Y, Yuen, L. Hesselink, T. Huser,
"Pulsed-interleaved excitation FRET measurements on single duplex
DNA molecules inside C-shaped cavities" Nano. Lett. 7 1749
(2007).
[0006] However, many conventional metallic cavity arrays are
limited in the ability to control or tune the enhancement in light
transmission through the cavities and/or light intensity within the
cavities. As a result, the sensitivity, accuracy, and specificity
of assays using such cavity arrays is limited.
SUMMARY
[0007] Provided herein are light enhancement devices, applications
for the light enhancement devices, and methods for making the light
enhancement devices. The disclosed light enhancement devices
include a substrate and a film of metal disposed over the
substrate, the film of metal including at least one cavity. The
present invention is based, in part, upon the inventors' discovery
that by adjusting the angle of the sidewall of the cavity with
respect to a surface normal to the substrate, it is possible to
achieve an enhancement of the transmission of light through the
cavity, an enhancement of the intensity of light within the cavity,
or both, than the enhancement if the sidewall of the cavity was
straight. Large enhancement factors, including enhancement factors
of 15 or more, may be achieved for specific ranges of sidewall
angles. As a result, light enhancement devices including the
disclosed cavities are capable of providing significantly more
sensitive, accurate, and specific bioassays as compared to
conventional light enhancement devices.
[0008] In addition to an angled sidewall, further enhancement of
the transmission of light through the cavity can be obtained by
including one or more changes in the sidewall within the cavity,
including a change in angle, a change in material, a change in
width, or a combination thereof. In addition, further enhancement
of the transmission of light through the cavity may be obtained
through creation of additional nodes. In this application, a "node"
means a location in the film of metal where the angle of the
sidewall of the cavity with respect to a surface of the substrate
is substantially changed. For example, the cavity of FIG. 1 has two
nodes at locations 14 and 16. As another example, the cavity of
FIG. 2 has three nodes at locations 22, 24 and 26. The spacing
between the nodes and the width of the cavity at each node may also
be adjusted to allow tuning the cavity to multiple or different
wavelengths of incoming light.
[0009] The shape and dimensions of the cavities may vary. In some
embodiments, the cavity is in the shape of a truncated cone,
although other shapes are possible. The dimensions of the cavities
may be on the nanometer scale.
[0010] Light enhancement devices including a plurality of any of
the disclosed cavities in the metallic film are also provided. In
some embodiments, the plurality of cavities may be arranged in a
periodic array. The shape, dimensions, and the magnitude of the
angle of a tapered sidewall of the cavities within such arrays may
be the same or different from one another. The light enhancement
devices may be used with a variety of wavelengths of light.
[0011] In another aspect, applications involving any of the
disclosed light enhancement devices are provided. In some
embodiments, the light enhancement devices may comprise at least
one biomolecule disposed in the cavity and may be used as
biosensors. This non-limiting application is further described
below. Light enhancement devices including a passivation layer
disposed over the metallic film in order to prevent the adsorption
of molecules of interest to the metallic film are also described,
as are methods for forming such light enhancement devices.
[0012] In yet another aspect, methods for making any of the
disclosed light enhancement devices are provided. The methods
involve forming a film of metal over a substrate and forming at
least one cavity in the film of metal. Techniques for forming the
film of metal and forming the cavities are provided herein.
BRIEF DESCRIPTION OF THE DRAWINGS
[0013] FIG. 1 is a cross-sectional view of a light-enhancement
device.
[0014] FIG. 2 is a cross-sectional view of a light-enhancement
device.
[0015] FIG. 3 is a cross-sectional view of a light-enhancement
device.
[0016] FIG. 4 is a cross-sectional view of a light-enhancement
device.
[0017] FIG. 5 is a cross-sectional view of a light-enhancement
device.
[0018] FIG. 6 is a cross-sectional view of a light-enhancement
device.
[0019] FIG. 7 is a cross-sectional view of a light-enhancement
device.
[0020] FIG. 8 is a cross-sectional view of a light-enhancement
device.
[0021] FIG. 9 is a cross-sectional view of a light-enhancement
device.
[0022] FIG. 10 is a cross-sectional view of a light-enhancement
device.
[0023] FIG. 11 is a cross-sectional view of a light-enhancement
device.
[0024] FIG. 12 is a cross-sectional view of a light-enhancement
device.
[0025] FIG. 13 is a cross-sectional view of a light-enhancement
device.
[0026] FIG. 14 is a cross-sectional view of a light-enhancement
device.
[0027] FIG. 15 is a cross-sectional view of a light-enhancement
device.
[0028] FIG. 16 is a cross-sectional view of a light-enhancement
device.
[0029] FIG. 17 is a cross-sectional view of a light-enhancement
device.
[0030] FIGS. 18a-h are top-views of various shapes of a
light-enhancement devices.
[0031] FIG. 19 is a cross-sectional view of a light-enhancement
device.
[0032] FIG. 20 is a cross-sectional view of a light-enhancement
device.
[0033] FIG. 21 is a cross-sectional view showing a method of making
a light-enhancement device.
[0034] FIG. 22 is a cross-sectional view showing a method of making
a light-enhancement device.
[0035] FIG. 23 is a cross-sectional view showing a method of making
a light-enhancement device.
[0036] FIG. 24 is a cross-sectional view showing a method of making
a light-enhancement device.
[0037] FIG. 25 is a cross-sectional view showing a method of making
a light-enhancement device.
[0038] FIG. 26 is a cross-sectional view showing a method of making
a light-enhancement device.
[0039] FIG. 27 is a cross-sectional view showing a method of making
a light-enhancement device.
[0040] FIG. 28 is a cross-sectional view showing a method of making
a light-enhancement device.
[0041] FIG. 29 is a cross-sectional view showing a method of making
a light-enhancement device.
[0042] FIG. 30 shows an image of an array of conical cavities in a
metallic film (aluminum).
[0043] FIG. 31 shows the calculated transmittance through a cavity
(with d=200 nm) in aluminum film with a thickness of 100 nm on a
glass substrate with different taper angles. The cavity under
consideration was illuminated from the air side (A) and the glass
side (B).
[0044] FIG. 32 shows the normalized optical power flow through a
single cavity as evaluated at the bottom of the conical cavity as
seen in an XY cross-section. The cavity dimensions were d=200 nm
and t=100 nm and the cavity was excited from the air side.
[0045] FIG. 33 shows the normalized optical power flow through a
single cavity as evaluated at the bottom of the conical cavity as
seen in a YZ cross-section. The cavity dimensions were d=200 nm and
t=100 nm and the cavity was excited from the air side.
[0046] FIG. 34 shows the calculated intensity enhancement at the
bottom of a single conical cavity in a 100 nm thick aluminum film.
The cavity had a diameter of d=200 nm and was illuminated from the
air side (A) and the glass side (B).
[0047] FIG. 35 shows the calculated intensity enhancement at the
bottom of a single conical cavity in a 100 nm thick aluminum film.
The cavity had a diameter of d=200 nm and was illuminated from the
air side (A) and the glass side (B).
[0048] FIG. 36 shows the calculated transmittance through a cavity
array in a 100 nm aluminum film on glass with different taper
angles. The array parameters were a.sub.0=500 nm and d=200 nm. The
illumination was from the air side (A) and the glass side (B).
[0049] FIG. 37 shows the measured transmittance through an array of
cavities in a 100 nm aluminum film on glass, for both sides of
illumination. The array parameters in were a.sub.0=500 nm and d=200
nm.
[0050] FIG. 38 shows the normalized optical power flow through the
cavity array as evaluated at the bottom of a conical cavity. The
cavity array parameters were a.sub.0=500 nm, d=200 nm, and t=100
nm. The array was excited from the air side.
[0051] FIG. 39 shows the normalized optical power flow through the
cavity array as evaluated at the bottom of a conical cavity as seen
in a YZ cross-section. The cavity array parameters were a.sub.0=500
nm, d=200 nm, and t=100 nm. The array was excited from the air
side.
[0052] FIG. 40 shows the calculated intensity enhancement through a
cavity array in a 100 nm aluminum film on glass with different
taper angles. The array parameters were a.sub.0=500 nm and d=200
nm. The illumination was from the air side (A) and the glass side
(B).
[0053] FIG. 41 shows the calculated intensity enhancement through a
cavity array in a 100 nm aluminum film on glass with different
taper angles. The array parameters were a.sub.0=500 nm and d=200
nm. The illumination was from the air side (A) and the glass side
(B).
[0054] FIG. 42 shows the calculated intensity enhancement through a
cavity array in a 100 nm aluminum film on glass with different
taper angles. The array parameters were a.sub.0=550 nm and d=200
nm. The illumination was from the air side (A) and the glass side
(B).
[0055] FIG. 43 shows the calculated intensity enhancement through a
cavity array in a 100 nm gold film on glass with different taper
angles. The array parameters were a.sub.0=450 nm and d=200 nm. The
illumination was from the air side (A) and the glass side (B).
[0056] FIG. 44 shows the calculated intensity enhancement through a
cavity array in a 100 nm gold film on glass with different taper
angles. The array parameters were a.sub.0=550 nm and d=200 nm. The
illumination was from the air side (A) and the glass side (B).
DETAILED DESCRIPTION
[0057] Provided herein are light enhancement devices, applications
for the light enhancement devices, how to make light enhancement
devices, and working examples.
Light Enhancement Devices
[0058] The light enhancement devices include a substrate and a film
of metal disposed over the substrate, the film of metal including
at least one cavity or nanoaperture. By "cavity," it is meant an
opening having dimensions on the nanometer scale that extends
through the metallic film, exposing the underlying substrate. A
non-limiting exemplary light enhancement device 10 is shown in FIG.
1. The light enhancement device 10 includes a substrate 11 and a
film of metal 12 disposed over the substrate, the film of metal
including a cavity 13. The metallic layer 12 can be any conductive
pure metal, metallic alloy, or metallic compound. In all
embodiments, layer 12 can be a single material or multiple layers
of different materials. The substrate 11 can be any dielectric
material and can be a single material or multiple layers of
different materials.
[0059] As shown in FIG. 1, the cavity 13 includes a tapered
sidewall 18 characterized by an angle A of approximately 30.degree.
with respect to a surface normal 11n to the substrate 11 (or an
angle B of approximately 120.degree. with respect to the surface
11s of the substrate). Thus, the cavity 13 has a sidewall 18 that
is non-parallel with respect to the surface normal 11n to the
substrate, or that is non-orthogonal to the surface 11s of the
substrate. In some of the disclosed cavities, the angle with
respect to a surface normal 11n to the substrate 11 is sufficiently
different from 0.degree. to provide an enhancement of the
transmission of light through the cavity, the intensity of light
within the cavity, or both, that is greater if the angle was
0.degree.. The cavity 13 has a shape of a truncated cone or
frustum.
[0060] The cavity 13 of FIG. 1 has a continuous change in width
along a depth of the cavity. In addition, the cavity 13 of FIG. 1
has two nodes 14 and 16. A light enhancement or resonance of
electromagnetic waves, such as visible light, can be created in the
cavity between and/or at the nodes 14 and 16, and between the
sidewall 18. The spacing between nodes will typically be less than
the wavelength of the electromagnetic waves used. For example, if
visible light is used, then the distance between nodes may be less
than about 400 nm. For infrared, the distance between nodes may be
less than about 1 mm. In this patent application, for simplicity,
the term "light" will be used for electromagnetic waves of any
wavelength, including visible light.
[0061] The resonance created results in an amplification of the
light transmitted through the cavity. One practical use of the
surface plasmon resonance effect is in identification or
quantification of a chemical sample, such as at least one
biomolecule. A chemical in the cavity can absorb the light and
fluoresce the light at a different wavelength than the incoming
light. A comparison of the amplitude and/or wavelength of light
transmitted without the chemical in the cavity to the amplitude
and/or wavelength of light transmitted with the chemical in the
cavity can allow determination of whether a specific chemical is
present and in what quantity.
[0062] Multiple factors may be changed to affect the output light
signal, such as the thickness of the metal, the type of metal, the
shape of the hole or cavity, the shape of the nodes or corners
(blunt corner or rounded edge), and the wavelength of light. Any or
all of these factors may be adjusted to obtain the optimal signal
amplification for ease in identification or quantification of the
specific chemical placed in the cavity.
[0063] The dimensions of the cavities may vary. The cavities may be
defined by a top width, a bottom width, widths at each node within
the cavity, and a depth. Top width means the width of the cavity at
the top surface of the metallic film. Bottom width means the width
of the cavity at the bottom surface of the metallic film (which is
also the interface of the metallic film and the substrate). The
depth of the cavity may be determined by the thickness (labeled "t"
in FIG. 1) of the metallic film. Each of these dimensions may vary.
In some embodiments, such as the embodiment of FIG. 1, the top
width is greater than the bottom width. In other embodiments, the
top width may be less than the bottom width (as in FIG. 8). The
thickness of the film may also vary. Experiments have been
performed with thicknesses varying between 50 nm to about 500 nm.
If a film is made up of multiple layers, each layer may have a
thickness of around 500 nm or less. In some applications,
thicknesses of greater than 500 nm may be desirable.
[0064] The surface characteristics of the metallic film may vary.
In some embodiments, the top surface of the metallic film, e.g.
element 12t in FIG. 1, is substantially flat. By "substantially
flat" it is meant that the top surface of the metallic film is
flat, but not necessarily perfectly flat. In such embodiments, the
top surface of the metallic film does not comprise any surface
features, such as raised or depressed regions in the form of
protrusions, dimples, grooves, ribs, corrugations, etc., on the
surface of the metallic film. However, in other embodiments, the
top surface of the metallic film may comprise such surface features
or similar such features.
[0065] A variety of metals or metal alloys may be used to form the
metallic films. The film of metal may include two or more layers of
metal. The film of metal may also include layers of dielectric
material. Because the enhancement of the transmission of light
through the cavity and the intensity of light within the cavity may
vary depending upon the choice of metal, and whether layers of
dielectric are included, multi-layer films provide the ability to
tune these enhancement effects as desired.
[0066] Similarly, the composition of the substrate may vary. In
some embodiments, the substrate comprises a transparent material. A
variety of transparent materials may be used, including but not
limited to glass, quartz, silicon, fused silica, or optical
plastics. Silicon is particularly suitable for infrared wavelengths
of light.
[0067] In some embodiments, the light enhancement devices include a
plurality of cavities disposed in the metallic film. The plurality
of cavities may be randomly distributed throughout the metallic
film or may be arranged in a periodic array. For periodic arrays,
the periodicity (the distance between the centers of adjacent
cavities) may vary. In some embodiments, the periodicity is about
200 nm. In other embodiments, the periodicity is about 300 nm,
about 400 nm, about 500 nm, or about 600 nm. However, other
periodicities are possible. The patterns formed by the periodic
array may vary. Non-limiting examples of patterns include square
arrays, rectangular arrays, triangular arrays, and hexagonal
arrays. In the arrays, the shape, dimensions, and the magnitude of
the angle of a tapered sidewall of one cavity may be the same or
different as the shape, dimensions, and the magnitude of the angle
of a tapered sidewall of another cavity in the array. In some
embodiments, the shape, dimensions, and the magnitude of the angle
of a tapered sidewall of one cavity are substantially the same as
each of these characteristics of another cavity in the array.
[0068] The disclosed light enhancement devices having cavities with
tapered sidewalls and/or non-linear sidewalls are capable of
enhancing the transmission of light through the cavities; enhancing
the intensity of light within the cavities; or both; as compared to
light enhancement devices having cavities with straight sidewalls
orthogonal to the surface of the substrate. The enhancement factors
may vary depending upon the angle of the tapered sidewall. By
"enhancement factor" it is meant the factor by which the light
transmission through, or the light intensity within, the cavity
having tapered sidewalls is increased over the transmission
through, or intensity within, a cavity having straight sidewalls.
Other variables may affect the enhancement factor, including, but
not limited to the dimensions of the cavity, the periodicity of the
cavities within an array of cavities; the type of metal(s) used in
the metallic film, the wavelength of light illuminating the cavity,
and whether the illumination is above the metallic film through the
air or below the metallic film through the substrate. However,
enhancement factors as high as 15 have been observed in some of the
disclosed light enhancement devices.
[0069] As shown in FIG. 2, a cavity 13a or sidewall of another
light enhancement device 10a can have a change 24 in width, a
change in angle, or both, so that at least a portion of the
sidewall of the cavity is non-linear and non-orthogonal to the
surface of the substrate. The change in the sidewall can be
distinct and can divide the cavity into one or more different
portions with different characteristics, such as different shapes,
different widths, different depths, different materials, different
volumes, different angular sidewalls, or combinations thereof. The
light can react differently, or different wavelengths of light can
react differently, in the different portions due to the different
characteristics. In addition, the cavity 13a can have three nodes
22, 24 and 26. The cavity 13a can include an enlarged portion, or
greater diameter, at the upper surface of the metallic layer 12,
and a reduced portion, or lesser diameter, at the substrate 11. In
addition, the enlarged portion of the cavity 13a can include curved
or arcuate walls. There can be resonance at or between the nodes
22, 24 and 26. The distance between the nodes and the width of the
cavity or portions can be adjusted to match a specific wavelength
of light. Note that the widths of the two portions are different.
If this difference in widths is sufficient, the resonance between
the portions can occur at a different wavelengths. This resonance
at multiple wavelengths can be helpful in identifying a specific
chemical compound. For example, the width of the upper portion can
be designed for resonance with one specific chemical subgroup and
the width of the lower portion can be designed for resonance with a
different specific chemical subgroup of the chemical one is seeking
to identify.
[0070] The width of the lower portion can be constant, or
cylindrical as in FIG. 2. With constant width, the resonance will
be amplified. Increased amplification at a specific wavelength can
result in a larger light signal that is easier to quantify. The
distance between nodes 24 and 26, or depth of the lower portion,
can be the same as or different than the width of the upper portion
or the width of the lower portion. With more nodes or widths having
equal distances, the signal has more amplification. With more nodes
or widths of different distances, resonance will occur at more
wavelengths. This may be useful in identifying a specific chemical
in the cavity, especially if that chemical is very complex, with
multiple different chemical functional groups. Cavity design is
selected to optimize the desired use of the cavity.
[0071] Cavity 13b of another light enhancement device 10b of FIG. 3
has another node 32, than cavity 13a of FIG. 2, and has a
difference in width and two changes in angle. Depending on the
width at node 32 and adjacent nodes, these nodes can aid in signal
amplification or in allowing resonance to occur at different
wavelengths. The cavity 13b can have a stepped cross-section with
an upper enlarged or greater diameter portion and a lower reduced
or lesser diameter portion.
[0072] Cavity 13c of another light enhancement device 10c of FIG. 4
has the same node structure as cavity 13b of FIG. 3, but is
manufactured of multiple metallic materials, 41 and 42. Similar to
metallic materials 12 discussed previously, metallic materials 41
and 42 can be pure metals, metallic alloys, or metallic compounds
and each layer 41 or 42 can be a single material or layers of
different materials. In addition, cavity 13c has a change in width,
a change in material or layers, and two changes in angle.
[0073] An alternative structure, for providing more differences
between nodes and widths, and thus more wavelengths at which
resonance can occur, is shown in cavity 13d of another light
enhancement device 10d of FIG. 5. Angle A in cavity 13d is greater
than ninety degrees, as compared with an angle approximately equal
to ninety degrees at a similar location in cavity 13a of FIG. 2.
This results in a different width of the lower portion. Thus, the
walls of the cavity 13d can be angled or inclined, and can be
curved or arcuate in cross-section. A lower portion of the cavity
13d can have an inverted frusto-conical shape. In addition, cavity
13d has a change in width, and two changes in angle.
[0074] The difference between cavity 13d of another light
enhancement device 10d of FIG. 5 and cavity 13e of another light
enhancement device 10e of FIG. 6 is similar to the difference
between cavity 13a of FIG. 2 and cavity 13b of FIG. 3. Cavity 13d
has a change 24 in width, and a change 24 in angle. The change 24
divides the cavity into an upper portion with curved sidewall and a
lower portion with angled sidewall. Cavity 13e has two changes 24
and 62 in width and angle dividing the cavity into an upper
truncated cone and a lower truncated cone, both with angled
sidewalls. Cavity 13d has three nodes 22, 24 and 56; while cavity
13e has three nodes 22, 62, 24 and 56. In the cavity 13e, the
curved shape of the sidewall, between nodes 22 and 24 has been
replaced by a corner, resulting in additional node 62 in cavity
13e. Depending on the spacing between these nodes and other nodes
in cavity 13e, additional node 62 may result in increased
wavelength amplification or they may result in a new wavelength at
which resonance can occur. Differences between cavity 13b in FIG. 3
and cavity 13e in FIG. 6 are angles A and B, which are greater than
zero with respect to the surface normal or greater than ninety
degrees with respect to the surface. Thus, the cavity 13e can have
an upper inverted frusto-conical shape larger than a lower inverted
frusto-conical shape. Cavity 13f of another light enhancement
device 10f of FIG. 7 is similar to cavity 13e with the difference
that metallic layer 12 has been divided into at least two different
metallic layers 41 and 42. Thus, cavity 13f also has a change of
material.
[0075] In cavity 13g of another light enhancement device 10g of
FIG. 8, the width of the cavity at the top or opening is different
from the width of the cavity at the bottom, due to angled sidewall
of the cavity which has an angle C greater than zero with respect
to the surface normal or an angle C greater than ninety degrees
with respect to the surface of the substrate. As discussed
previously, this may allow resonance at an additional wavelength. A
diameter of the cavity 13g at the substrate can be larger than a
diameter of the cavity at the upper surface of the metallic layer,
resulting in a truncated cone shape. In addition, the cavity has
two nodes 82 and 84.
[0076] There are three nodes 92, 94 and 96 in the hourglass-shaped
cavity 13h of another light enhancement device 10h of FIG. 9. The
cavity 13h has a change 94 in width and a change 94 in angle
dividing the cavity into two truncated cones of opposite
orientation. The width of a midpoint of the cavity is less than the
width of the cavity at the top and the bottom, resulting in the
hourglass shape. The width of the top and bottom can be the same or
different. The metallic layer 12 of FIG. 9 can be divided into
multiple metallic layers 101 and 102 as shown in the cavity 13i of
another light enhancement device 10i of FIG. 10. Thus, cavity 13i
also has a change in material.
[0077] Cavity 13j of another light enhancement device 10j of FIG.
11 has four nodes 14, 112, 114, and 16, allowing more resonance. In
addition, the cavity has two changes 112 and 114 in angle and width
dividing the cavity into upper and lower cylindrical portions with
an annular groove section therebetween. The cavity 13j includes an
annular groove formed in the cylindrical wall of the cavity
intermediate the top and bottom of the cavity. Thus, the sidewall
of the cavity is non-linear. The sidewall of the groove can be
curved, as shown. In FIG. 12, cavity 13k of another light
enhancement device 10k has two more nodes 122 and 124 than cavity
13j. Due to the curved cavity walls of the groove in the cavity 13j
of FIG. 11, between nodes 112 and 114, cavity 13j of FIG. 11 does
not have nodes 122 and 124, which are present in cavity 13k of FIG.
12. The structure in FIG. 12 has a dielectric layer 125 between
metallic layers 12a and 12b. Metallic layers 12a and 12b may be the
same material or they may be different materials, and can be pure
metals, metallic alloys, or metallic compounds. In addition to more
nodes for resonance in cavity 13k, the dielectric layer 125 can act
as a wave guide. The thicknesses of the metallic layers 12a and 12b
and the dielectric layer 125 can be modified to adjust spacing
between nodes in order to match the desired wavelength(s) for
optimal resonance. The cavity in the middle or dielectric layer 125
can be larger, or have a greater diameter, than the cavity in the
metallic layers 12a and 12b. In addition, a portion of the cavity
can be formed in the substrate 11. Thus, the cavity 13k has four
changes 114, 124, 122 and 112 in angle, two or three changes in
material, and two changes in width dividing the cavity into three
different cylindrical shapes, and with a non-linear sidewall. In
addition, the cavity 13k has six nodes 14, 112, 122, 124, 114 and
16.
[0078] In cavity 13L of another light enhancement device 10L of
FIG. 13, there are at least three metallic layers 12 with a
dielectric layer 125 between each metallic layer 12. All of the
metallic layers 12 may be made of the same material or the
different layers may be made of different materials. Similarly, all
of the dielectric layers 125 may be made of the same material or
the different layers may be made of different materials. More
layers results in more nodes and increased resonance. Note that in
FIG. 13, the metal layers 12 in the cavity 13L, align with the
dashed line 131. Line 131 is perpendicular to the surface plane of
the substrate 11.
[0079] Cavity 13m of another light enhancement device 10m of FIG.
14 is similar to cavity 13L with the exception that the metal
layers are not in alignment with line 131. The cross-sectional
shape of cavity 13m is circular or elliptical. Note that there are
many different distances between nodes in this structure, allowing
resonance at many different wavelengths. Many other shapes may also
be made, such as the "V" shaped cavity 13n of another light
enhancement device 10n of FIG. 15. Note the alignment of metal
layers with "V" shaped line 151.
[0080] Cavity 13o of another light enhancement device 10o in FIG.
16 shows a similar structure to 13b in FIG. 3. The lower portion
162 of the cavity 13o does not have to be centered or aligned under
the upper portion 161 of the cavity. The cavity 13o has two changes
24 and 32 in angle and one change in width dividing the cavity into
upper and lower portions that are not collinear, and with a
sidewall that is non-linear. With lower cavity 162 off-center from
upper cavity 161, there can be resonance at one wavelength between
the nodes and resonance at a different wavelength between the
nodes.
[0081] The structure of another light enhancement device 10p of
FIG. 17, like that of FIG. 16, is also non-symmetrical. This
principle of non-symmetry can also be applied to other embodiments
discussed previously. As in cavity 13o, the lower portion 172 of
the cavity 10p does not have to be centered or aligned under the
upper portion 171 of the cavity. The nodes in cavity 13p are
situated differently than the nodes in cavity 13o. Note that in
cavity 13p, node 173 hangs over or partially encloses the lower
portion 172 of the cavity.
[0082] As shown in FIG. 18a-h, the shape of a cavity, as seen
looking down on the metal with the substrate beneath, can be
various shapes, including square 181, rectangular 182, circular
183, elliptical 184, channel 185, a non-symmetrical shape 187, or a
figure eight 188. Cavity, as used in this application, includes not
only a pit or hole but also a channel. A channel 185 does not need
to continue in a straight line, but can bend or curve, so that the
channel shape, as seen from the surface of the metal, may form an L
shape, an S shape, or any other desired shape or pattern, such as
186. For further clarification, a three dimensional view of a
champagne glass-shaped cavity 187 as a channel of another light
enhancement device 10q is shown in FIG. 19 and as a circular hole
13a in FIG. 20. All structures in this patent application may use
any of the cavity shapes of FIG. 18. The pattern or mask designed
or selected determines the cavity shape as seen looking down on the
metal with the substrate beneath.
Applications
[0083] The disclosed light enhancement devices find use in a
variety of applications. These applications are not particularly
limited, but rather may be driven by the need for an increase in
light transmission or light intensity. By way of example only, the
light enhancement devices may find use in biological applications
as biosensors. The basic principle underlying use of sub-wavelength
metallic apertures as biosensors involves the detection of
fluorescently labeled biomolecules within the cavities. Because the
transmission of light through and the intensity of light within the
disclosed cavities is greatly enhanced, the sensitivity, accuracy,
and specificity of biosensors including the disclosed cavities is
greatly improved over conventional biosensors. In addition, the
ability to tune the enhancement of light transmission and light
intensity by adjusting the angle of the tapered sidewall, the
cavity shape, and other factors, for a particular wavelength of
light is extremely useful as it allows a single biosensor to be
used with a broader range of biomolecules and fluorescent
molecules. Thus, in some embodiments, the disclosed light
enhancement devices may further comprise at least one biomolecule
disposed in the cavity. A variety of biomolecules may be used,
including, but is not limited to DNA, RNA, or proteins. The light
enhancement devices may further include any of the accessories
necessary for biosensing such as delivery systems for supplying the
biomolecules, light sources, and detectors. Exemplary biosensors,
accessories, and methods for using the biosensors are disclosed in
U.S. Ser. No. 11/497,581 and in International Publication Number WO
2007/094817, both of which are hereby incorporated by reference in
their entirety.
[0084] For sensor applications especially, the disclosed light
enhancement devices may be treated to facilitate the adsorption and
immobilization of molecules of interest to specific regions on the
light enhancement devices, e.g., the substrate surface at the
bottom of the cavity. Molecules of interest include, but are not
limited to biomolecules, such as those described above. Similarly,
the disclosed light enhancement devices may be treated to prevent
the adsorption of such molecules to specific regions on the light
enhancement devices, e.g., the surface of the metallic film. As
used herein, the phrase "passivating" and "passivation" are used to
refer to the prevention of the adsorption of molecules of interest
to specific regions on the light enhancement devices.
[0085] Functionalized silane molecules are often used to facilitate
the adsorption and immobilization of biomolecules to glass
surfaces. Silane molecules form stable bonds with glass surfaces
via Si--O--Si bond formation. Silanes may be functionalized with a
variety of molecular groups for coupling to biomolecules, thereby
immobilizing the biomolecules on silane-treated glass. Such
functionalized silanes (e.g., biotin-PEG-silane) are known, as are
methods of treating glass surfaces with such molecules. Other
molecules for facilitating the adsorption of biomolecules to glass
surfaces are known, including, but not limited to those disclosed
in U.S. Ser. No. 11/497,581, International Publication Number WO
2007/094817, and U.S. Pat. Pub. No. 2008/0032301, which are hereby
incorporated by reference in their entirety.
[0086] To ensure that molecules of interest adsorb to specific
regions on the light enhancement devices, the other regions of the
light enhancement devices may be passivated. The types of molecules
used for passivation depend upon the region to be passivated (i.e.,
the metallic film or the substrate material). For passivation of
metallic films, the types of molecules used for passivation also
depend upon the identity of the metal.
[0087] By way of example only, gold surfaces may be passivated with
a variety of alkylthiols, including, but not limited to PEG-thiol.
Alkylthiols are known, as are methods of treating gold surfaces
with such molecules. See, e.g., International Publication Number WO
2007/094817 and K. L. Prime and G. M. Whitesides, "Adsorption of
proteins onto surfaces containing end-attached oligo(ethylene
oxide): a model system using self-assembled monolayers," Journal of
the American Chemical Society, 115, 10714-10721 (1993).
[0088] For other metals, including aluminum, another approach is
possible. Aluminum may be classified as a very active metal due to
its ability to oxidize very quickly. The aluminum oxide layer is
chemically bound to the surface and it seals the core aluminum from
any further reaction. Since silane molecules attach to both
aluminum (Al--O--Si) and glass (Si--O--Si), passivation of aluminum
is important for restricting the adsorption of silane molecules
(and subsequently coupled biomolecules) to the substrate surface at
the bottom of the cavity. Alkyl phosphonic acids may be used to
passivate a variety of metal oxides, such as titanium oxide and
aluminum oxide, while not binding to silica surfaces in an aqueous
medium. See Korlach, J. et al., Selective aluminum passivation for
targeted immobilization of single DNA polymerase molecules in
zero-mode waveguide nanostructures. Proceedings of the National
Academy of Sciences 2008, 105, (4), 1176-1181; Mutin, P. H., et
al., Selective Surface Modification of SiO2−TiO2 Supports with
Phosphonic Acids. Chemistry of Materials 2004, 16, (26), 5670-5675;
Michel, R. et al., Selective Molecular Assembly Patterning: A
New Approach to Micro- and Nanochemical Patterning of Surfaces for
Biological Applications. Langmuir 2002, 18, (8), 3281-3287. In
addition, alkyl phosphonic acids form self-assembled monolayers
(SAMs) on a number of oxide surfaces, such as tantalum oxides (See
Brovelli, D., et al., Langmuir 1999, 15, 4324), aluminum oxides,
(See Hauffman, T., et al., Study of the Self-Assembling of
n-Octylphosphonic Acid Layers on Aluminum Oxide. Langmuir 2008, 24,
(23), 13450-13456; Hoque, E., et al., J. Chem. Phys. 2006, 124,
174710) and titanium oxide (See Adden, N., et al., Phosphonic Acid
Monolayers for Binding of Bioactive Molecules to Titanium Surfaces.
Langmuir 2006, 22, (19), 8197-8204; Mutin, P. H., et al., Selective
Surface Modification of SiO2−TiO2 Supports with Phosphonic Acids.
Chemistry of Materials 2004, 16, (26), 5670-5675). One of the main
reasons for using phosphonic acids rather than carboxylic acids is
their stronger binding with the oxide. As noted above, aluminum
forms a native oxide when exposed to an oxygen-containing
environment. Phosphonic acids are generally deposited from an
organic or water-diluted (10.sup.-3 mol/L) solution. Phosphonic
acids interact with the aluminum hydroxyl groups, where an increase
in the amount of surface hydroxyls enhances the phosphonic acid
deposition. See Hoque, E., et al., J. Chem. Phys. 2006, 124,
174710. Phosphonic acid specifically reacts to hydrated aluminum
oxide, through Al--O--P bond. The Si--O--P bond formed on glass
substrates are easily hydrolysable. The phosphonic acid prevents
subsequent chemical treatments, such as exposure to silane
containing molecules, from reacting with the aluminum. Then,
molecules of interest can be attached to non-aluminum surfaces via
reaction with a specific functional group of the silane molecule,
as described above.
[0089] Any of the disclosed light enhancement devices may further
include a passivation layer disposed over the film of metal,
wherein the passivation layer is capable of preventing the
adsorption of a molecule of interest to the film of metal. The
passivation layer may be disposed over the film of metal, including
the metallic sidewalls of the cavity, but not over the exposed
substrate surface at the bottom of the cavity. In some embodiments,
the passivation layer comprises an alkylthiol or an alkyl
phosphonic acid. In some embodiments, the passivation layer
comprises a self-assembled monolayer of an alkylthiol or an alkyl
phosphonic acid. Any of the alkylthiol molecules disclosed above,
as well as any of the methods for passivating surfaces with such
molecules, may be used. Similarly, a variety of alkyl phosphonic
acids or combinations of alkyl phosphonic acids may be used,
including, but not limited to those in which the alkyl chain is a
substituted or unsubstituted, straight chain or branched alkyl
having 1 to 25 carbons, e.g., from 4 to 20 carbons, or 8 to 15
carbons, etc. Any of the alkyl phosphonic acids disclosed in U.S.
Pat. Pub. No. 2008/0032301, which is hereby incorporated by
reference in its entirety, may also be used. Similarly, any of the
methods for passivating surfaces with such molecules disclosed in
this reference may be used. In some embodiments, the alkyl
phosphonic acid is butyl phosphonic acid or decyl phosphonic acid,
or combinations thereof. The examples below further describe
methods of passivating aluminum surfaces with butyl phosphonic acid
and decyl phosphonic acid.
[0090] Finally, other techniques for passivating the disclosed
light enhancement devices are possible, including, but not limited
to those disclosed in U.S. Ser. No. 11/497,581, International
Publication Number WO 2007/094817, and U.S. Pat. Pub. No.
2008/0032301, which are hereby incorporated by reference in their
entirety.
How to Make
[0091] Also provided are methods for making the disclosed light
enhancement devices. The methods involve forming a film of metal
over a substrate and forming at least one cavity in the film of
metal. The formed cavities are characterized as described above.
Techniques for forming films of metal over substrates are known. By
way of example only, physical vapor deposition (PVD) techniques or
chemical vapor deposition (CVD) techniques may be used to deposit
thin metal films on substrates.
[0092] The composition and characteristics of the metallic film and
the substrate may vary. In some embodiments, the film of metal may
comprise gold, aluminum, silver, copper, platinum, or combinations
thereof. However, other metals, metal alloys, or metallic compounds
are possible. The film of metal may be a single layer of metal, but
in other embodiments, the film of metal may comprise two, three,
four, or more layers of metal. Other layers may be incorporated
into the metallic film, including, but not limited to layers of
dielectric materials such as metal oxides or perovskites.
Non-limiting examples of dielectric materials include
Al.sub.2O.sub.3, TiO.sub.2, Ta.sub.2O.sub.5, TiWO.sub.3, and the
like. The surface characteristics of the metallic film may vary. In
some embodiments, the top surface of the metallic film is
substantially flat, but in other embodiments the top surface may
include a variety of surface features as described herein. Specific
examples of substrates are also provided herein.
[0093] The methods for forming any of the disclosed cavities are
based on known semiconductor processing methods. Such methods
include, but are not limited to, focused ion beam lithography;
electron beam lithography and reactive ion etching; and optical
interference lithography. See U.S. Ser. No. 11/497,581;
International Publication Number WO 2007/094817; "S. C. Lee and S.
R. Brueck, "Nanoscale two-dimensional patterning on Si(001) by
large-area interferometric lithography and anisotropic wet
etching," Journal of Vacuum Science & Technology B 22,
1949-1952 (2004), and S. Y. Chou, P. R. Krauss, and P. J. Renstrom
"Nanoimprint lithography," Journal of Vacuum Science &
Technology B 14, 4129-4133 (2004). These references describe each
of these techniques in detail and are hereby incorporated by
reference in their entirety.
[0094] Regarding focused ion beam lithography, this technique can
be used to directly mill cavities in metallic films. The diameter
of the cavity may be adjusted by adjusting the diameter of the ion
beam. Cavities having tapered sidewalls may be formed by adjusting
the diameter of the ion beam during the milling process, e.g.,
decreasing the diameter of the ion beam as the beam mills through
the metallic film.
[0095] Regarding electron beam lithography and reactive ion
etching, this technique involves exposing a substrate covered with
a resist to an electron beam to pattern the resist; selectively
removing either the exposed or non-exposed regions of the resist;
and transferring the pattern to the substrate by etching. Cavities
having tapered sidewalls may be formed by adjusting certain
parameters during the electron beam lithography step, the reactive
ion etching step, or both. During the electron beam lithography
step, the intensity of the electron beam affects the degree to
which chemical bonds in the resist material are broken (i.e., the
degree to which the resist becomes "exposed"), and thus, the
diameter of the pattern formed in the resist. Adjusting the
intensity of the electron beam during exposure of the resist can
provide a diameter that either increases or decreases along the
depth of the resist, thereby forming a tapered sidewall. During the
reactive ion etching step, the gas composition, the gas flow, the
gas pressure, and the RF power may be adjusted in order to achieve
a desired sidewall angle.
[0096] Following are more detailed descriptions of how to make the
various embodiments shown. To make the conical cavity of FIG. 1,
apply a thin metal layer through sputtering, CVD, or other desired
method on a substrate. Apply a resist to the metal layer. Pattern
the resist per standard lithography techniques. Perform a heavy
mechanical isotropic etch resulting in resist erosion and causing
formation of the conical shaped cavity.
[0097] To make the champagne glass shaped structure of FIG. 2, a
thin film 12 is adhered to substrate 11 through sputtering, CVD, or
other desired method. As shown in FIG. 21, the mask or pattern 210
will match the pattern of node 24. Initially, an isotropic wet etch
is used to create the cup portion of the champagne glass shape. The
type and time of isotropic wet etch will determine the width and
depth of the upper portion. Following the isotropic etch, an
anisotropic etch is used to create the lower portion of the cavity.
The mask 210 is then removed.
[0098] A double mask can be used to make the structures shown in
FIG. 3 and FIG. 4. The first mask 221 is made to align with node 26
as shown in FIG. 22. An anisotropic etch is used to etch all the
way through to the substrate. The first mask 221 is removed and a
second mask 231 is made to align with node 32 as shown in FIG. 23.
An anisotropic etch is then used to etch part way through metallic
layer 12 of FIG. 3 or all the way through metallic layer 41 of FIG.
4. The second mask 231 is then removed.
[0099] To make cavity 13d, shown in FIG. 5, begin with an isotropic
wet etch 242 of mask 241 as shown in FIG. 24. Remove the first mask
241 and apply a second mask 251 as shown in FIG. 25. Do an
isotropic dry etch 252. Cavity 13e, shown in FIG. 6, can be made
the same as cavity 13d with the exception that the first etch is a
resist erosion isotropic dry etch instead of an isotropic wet etch.
Cavity 13f, shown in FIG. 7, can be etched the same as cavity 13e.
The difference in making cavity 13f is that at least two layers 41
and 42 are applied prior to patterning and etching.
[0100] To make the cavity 13g of FIG. 8, an initial anisotropic
etch (indicated by the shorter dashed lines) is followed by an
isotropic dry etch (indicated by the longer dashed lines), as shown
in FIG. 26. Note that the mask 261 is aligned with node 82. The
isotropic dry etch widens the cavity to the width of node 84.
[0101] To make the cavity 13h of FIG. 9, an initial isotropic dry
etch 272 is followed by an anisotropic etch 273 then a second
isotropic dry etch 274 as shown in FIG. 27. Note that the
anisotropic etch is indicated by the shorter dashed lines and the
isotropic etches are indicated by the longer dashed lines. Mask 271
is aligned with node 94. The isotropic dry etch widens the cavity
to the width of node 92 at the top of the cavity and to the width
of node 96 at the bottom of the cavity. Cavity 13i of FIG. 10 is
made in a similar manner. The difference is that multiple metallic
layers 41 and 42 are applied to the substrate.
[0102] To make cavity 13j of FIG. 11, an anisotropic etch is used
down to the desired depth of nodes 111 and 112. This is followed by
an isotropic wet etch to the desired depth of nodes 113 and 114.
The final etch is anisotropic down to the substrate 11.
[0103] To make cavity 13k of FIG. 12, a metallic layer 12b is
sputtered on top of a substrate 11, followed by a dielectric layer
125 and another metallic layer 12a. See FIG. 28. An anisotropic
etch etches through both metallic layers 12a and 12b and through
the dielectric layer 125. Note that the mask 281 is aligned with
node 16. A selective isotropic etch then etches laterally into
dielectric layer 125 and also into the substrate, the etch rate
being selective between the dielectric and the metallic layers.
Cavity 13L of FIG. 13 is made by the same method. A difference
between making cavity 13L and cavity 13k is that in cavity 13L more
alternating layers of metal and dielectric are applied prior to
etch.
[0104] To make the elliptical or circular cross-sectional shape of
cavity 13m in FIG. 14, the initial etch is anisotropic. The etch is
changed to more and more isotropic while progressing towards the
center of the cavity. This isotropic etch cuts the layers back away
from line 131 towards the center of the cavity. After passing the
center, the etch is made more and more anisotropic until it is
primarily anisotropic when the substrate is reached. Following this
etch, a selective isotropic etch is used to etch back the
dielectric layers as was done with cavity 13L.
[0105] To make the "V" shaped cavity of 13n in FIG. 15, the initial
etch is isotropic. The etch is changed to more and more anisotropic
while progressing towards the bottom of the cavity. The mask would
be aligned with the cavity width at the bottom of the cavity.
Following this etch, a selective isotropic etch is used to etch
back the dielectric layers as was done with cavity 13L.
[0106] The method for making the structure in FIG. 16 is similar to
the method as for making the structure of FIG. 3. To make cavity
13o, following the first etch, align the second mask off-center
from the first cavity. To make cavity 13p of FIG. 17, layer 42 is
patterned and etched to form cavity 172. Another layer 41 is
applied on top of layer 42. A resist 291 is applied on top of layer
41. The resist and layer 41 are etched, as indicated by the dashed
lines, to form cavity 171. The resist 291 is then removed.
Working Examples
[0107] Single cavity and sub-wavelength cavity arrays in optically
opaque Al films (with thickness 100 nm) on glass were studied. An
example is shown in FIG. 30. Array parameters (periodicity: a.sub.0
and cavity diameter: d) were optimized for higher enhancements for
the excitation wavelengths 532 nm and 635 nm. Higher enhancements
for the periodicities of 500 nm, 550 nm with corresponding cavity
diameters of 200 nm, 250 nm each were observed. Below, the study on
a cavity and cavity array in Al film with a.sub.0=500 nm and d=200
nm and also with a.sub.0=550 nm and d=250 nm with a range of taper
angles from 0.degree. to 60.degree. is discussed. A similar cavity
array in a 100 nm thick Au film was also considered for enhancement
effects with a.sub.0=450 nm and d=200 nm along with a.sub.0=550 nm
and d=200 nm for a range of taper angles. Other array geometries
showed very similar behavior.
[0108] Electromagnetic calculations were performed using COMSOL
multiphysics v3.5a. A glass substrate was assumed on top of which
an aluminum film was placed; the upper region was air. The
dielectric properties of metal were incorporated via the complex
dielectric constant as measured by spectroscopic ellipsometry from
300-1600 nm. The size of the computational space used was set by
the cavity array periodicity in the x-y direction with periodic
boundary conditions applied on the faces. The cavity arrays were
excited by light polarized along the y-direction and the
enhancements were calculated for both directions of incidence (air
side (above), substrate side (below)). The enhancement was
calculated by integrating total field intensity within a volume of
a 10 nm slice at the bottom of a cavity and dividing by the total
integrated intensity within the same volume but in the absence of
the metallic film. In the calculations for a single cavity as a
comparison with the cavity array all of the above conditions were
similar except radiation boundary conditions were applied on the
faces. See F. Mandavi, Y. Liu, and S. Blair, "Modeling Fluorescence
Enhancement from Metallic Nanocavities," Plasmonics 2, 129
(2007).
Example 1
Single Conical Aperture
[0109] The calculated normalized transmission spectra for a single
cavity is shown in FIG. 31. A rapid increase of transmittance with
increasing taper angle and a systematic spectral peak shift toward
longer wavelengths was observed when the illumination was from the
air side (FIG. 31A). The transmittance peaks also became wider with
increasing taper angle. The transmittance did not have such a
drastic effect when illuminated from glass side (FIG. 31B).
[0110] The normalized power flow through such a cavity as evaluated
at the bottom of the cavity for a sidewall taper angle of
45.degree. was about 3 times higher at 532 nm and about 5 times
higher at 635 nm when compared to the one with straight sidewalls.
The cavity showed considerable intensity enhancement with
increasing taper angle (taper angle as measured from a plane
perpendicular to the surface of the substrate) for the cavity
side-walls for both directions of incidence (air side, substrate
side) as seen in FIGS. 31 through 33. The calculated enhancement at
the bottom of such a cavity varied over a wide range of wavelengths
for both directions of illumination as seen in FIG. 34. The
enhancement at particular wavelengths of interest (488 nm, 532 nm,
635 nm) could further be tuned by varying the cavity sidewall taper
angle for both directions of illumination (FIG. 35). However the
air side illumination had a larger improvement in enhancement as a
function of taper angle as compared to substrate side illumination.
With increasing taper angles ( >50.degree.) the cavity becomes
"shallower" and the light confinement effect became weaker which
was clearly evident from a drop in enhancement (FIG. 35).
Example 2
Regular Arrays of Conical Apertures
[0111] In a metal film with a periodic array of cavities, the
periodicity allows for grating coupling of the SPPs to light that
results in resonantly enhanced transmission bands, known as
"extraordinary of enhanced optical transmission" (EOT). The main
effect of arranging the cavities in this particular manner is to
enhance the collection efficiency of the incident light. The
transmittance of a single cavity gets modulated because of this
coupling between holes through propagating SPPs (surface waves).
The transmission spectrum of a cavity array depends both on the
periodicity as well as the cavity size in a similar manner.
[0112] The transmission spectra of cavity arrays with different
cavity sidewall taper angles is shown in FIG. 36. The transmission
spectra red-shifted, broadened and the transmission increased with
increasing taper angles when the array was illuminated from air
side. The observed increase in the width of the transmitted peaks
with increasing cavity size indicates that the radiative damping of
the SPPs increases with cavity size. The transmittance minima
however does not have a significant shift with increasing taper
angle.
[0113] As seen in FIG. 37, the transmittance spectra through such
an array of cavities was measured over a broad range of wavelengths
(of about 50-100 nm) using a fiber coupled halogen white light
source and the Avantes multichannel spectrometer for both
directions of illumination at normal incidence. It was seen that
the transmission through these conical shaped apertures does not
depend very strongly on whether light enters from the air side or
the substrate side. (The intensity enhancement, however, does
depend on which way light enters, as properly represented in other
figures.)
[0114] The net incident optical power flow through the metallic
cavity array when illuminated from the air side was also studied.
The electric field remained finite at the metallic boundaries, and
a considerable flux moved downward along the tapered cavity side
walls. The sidewalls thus represent sinks of radiation. The
"funnel" effect in these tapered cavities is particularly evident
in FIGS. 38 and 39. The normalized power flow at the bottom of a
cavity in case of a side wall taper angle of 45.degree. was about 5
times higher at 532 nm and about 10 times higher at 635 nm as
compared to a cavity with straight side-walls.
[0115] The enhancement at the bottom of a conical cavity in this
metallic array varied over a wide range of wavelengths as shown in
FIG. 40. The average enhancement factor was as high as 15 over a
broad wavelength range of about 250 nm for air side illumination;
and also about 8 over a wavelength range of 250 nm for glass side
illumination for =40.degree.. As shown in FIG. 41, for an array of
such cavities with conical side walls the enhancement can increase
twenty-fold as compared to such an array with same size cavity but
with straight side-walls. As seen in the case of a single cavity,
the enhancement has a much weaker dependence on the cavity taper
angle when the array is excited from glass side at a particular
excitation wavelength (532 nm or 635 nm) as the cavity diameter is
always fixed at 200 nm in this case. The effect of having a wider
cavity on the exit side (air) is not so much of importance to field
localization at the cavity bottom, hence to the enhancement, as the
SPP coupling is initiated from the first film interface
(metal-glass) which propagates along it and down to the second
interface (metal-air). This offers a possible way of getting
selective-enhancement at both the wavelengths of interest (532 nm
and 635 nm) by carefully designing the metal cavity array profile.
For example, it is possible to select a at which enhancement both
at 532 nm and 635 nm is achieved. At the taper angles of up to
about 45.degree., enhancement can be gained at both of these
wavelengths, but if the taper angle is greater than about
45.degree. the enhancement at 635 nm is greatly improved.
[0116] The dependence of enhancement on the taper angle in case of
a single cavity was shown in FIG. 35 in the previous section. For
the cavity arrays, a near linear increase with increasing taper
angle for wavelengths 532 nm and 635 nm when excited from the air
side was observed. However, not a significant increase in
enhancement with was seen at 488 nm, the average enhancement being
by a factor of about 2. It is noted that the cavity array under
consideration has a periodicity of 500 nm. The transmission
spectrum as well as the enhancement spectrum has a minima around
a.sub.0 which corresponds to the coupling of top SPP interface
(metal-air) which can be accurately described as a Fano-type
anti-resonance. This coupling corresponds to the interference
between two contributions which arise from resonant and
non-resonant elements: in this case light directly transmitted
through the metal film which interferes with the one reradiated
after exciting the localized cavity modes. This minima has a nearly
no shift with increasing . The SPPs on top interface are still
excited in this case, but the effective coupling strength between
these and the bottom SPPs (metal-substrate interface) is much
weaker. There is not a large variation in metal dielectric constant
(real and imaginary parts) at these wavelengths. In the case of a
single cavity, a totally different behavior was observed. This
effect became more apparent in the case of a similar array with a
different periodicity of 550 nm as seen by a near-linear increase
in enhancement with taper angle for a wavelength of 488 nm (see
FIG. 42). In order to further generalize this effect, the effect of
enhancement and light gathering capacity on taper angle was also
studied from an array of cavities in 100 nm Au film shown in FIGS.
43 and 44 for different periodicities. Thus, the role of
periodicity is clearly apparent.
Example 3
Passivation of Aluminum Surfaces
[0117] Substrate Preparation: Plain glass substrates and glass
substrates coated with a 100 nm thick aluminum film were provided
by Moxtek Inc., Provo (SEM shown in FIG. The substrates were
cleaned using solvent wash. The wash included acetone, isopropyl
alcohol and methanol. After the solvent wash, the substrates were
rinsed with ddH.sub.2O and dried using nitrogen, followed by argon
plasma cleaning using Harrick plasma cleaner. The plasma cleaner
was operated at medium power setting (200 W). At this point, the
substrates were exposed to oxygen stream to create an oxide layer
on top of the aluminum. The oxidized substrates were dipped in
boiling water for 5 minutes.
[0118] Phosphonic Acid Self Assembled Monolayers: Two phosphonic
acids were used, n-Decylphosphonic acid (DPA) and n-Butylphosphonic
acid (BPA) from Alfa Aesar (purity 98%), as received. Phosphonic
acid solutions of 1 mM were made in methanol, a concentration at
which the molecules behave as free species in the solution. A
passivation layer was self-assembled onto the substrates by leaving
it in phosphonic acid solution for 16 hours. After passivation, the
passivated substrates were annealed for 4 hrs at 90.degree. C.
Physi-sorbed phosphonic acid was removed using triple methanol
washes.
[0119] Silanization: After cleaning, the substrates were placed in
a Fisher Scientific Company oven at 115.degree. C. with a small
vial containing 1.5 ml of 3-glycidoxypropyltrimethoxysilane (GPS)
(Sigma-Aldrich). The oven was sealed, pumped down, and purged 3
times with ultrapure nitrogen. After 8 hours, the oven was purged
with nitrogen and the substrates were removed.
[0120] Surface Characterization: Surface wettability was
investigated by measuring the advancing contact angles in a sessile
water drop experiment. A water drop of 1 .mu.L volume was used in
each measurement. Three independent readings were taking for each
substrate.
[0121] XPS analyses were performed on an Axis Ultra spectrometer
from Kratos (Manchester, U.K.) equipped with a concentric
hemispherical analyzer and using a mono-chromatized aluminum anode
X-ray source maintained at 15 KeV. The substrates were investigated
under ultrahigh vacuum conditions: 10.sup.-8-10.sup.-7 Pa.
Substrates were analyzed with a pass energy of 160.0 eV for survey
scans and 20.0 eV for high energy resolution elemental scans.
[0122] Static ToF-SIMS (Cameca/ION-TOF TOF-SIMS IV) was performed
with a monoisotopic 25 keV .sup.69Ga.sup.+ primary ion source. The
primary ion (target) current was typically 2 pA, and the raster
area of the beam was 500.times.500 .mu.m.sup.2.
[0123] Radio-Labeling: The probe oligonucleotides were
3'-end-labeled with [.alpha.-32P] dATP using Terminal Transferase
(New England Biolabs) labeling kit. The reaction mixture consisted
of 5 pmols of 5' end amine terminated oligonucleotide, 5 .mu.L of
10.times. NE buffer 4, 5 .mu.L of 2.5 mM CoCl.sub.2, 0.5 .mu.L
Terminal Transferase (20 units/.mu.l), 2.5 .mu.L of a [.alpha.-32P]
dATP 6000 ci/mmol (Perkin Elmer) and ddH.sub.20 to a final volume
of 50 .mu.L. The mixture was incubated at 37.degree. C. for 30
minutes. 10 .mu.L of 0.2M EDTA (pH8.0) was added to terminate the
reaction. The products were purified using spehadex g25 columns.
The purified product was spiked with 245 pmols of unlabeled
oligonucleotide. The solution was dried using speed vac. Dried
oligonucleotide was re-suspended in 150 mM phosphate buffer
(pH=8.5). Silanized substrates were spotted with 1 .mu.L of 500,
50, 5 and 0.5 .mu.M solutions of oligonucleotide. After spotting,
the substrates were kept at room temperature in a humid chamber for
at least 12 hours. The substrates were then rinsed with ddH.sub.2O
and blown dry with N.sub.2. These substrates were scanned using
phosphor-screen.
[0124] The contact angle on cleaned aluminum and glass substrates
were almost zero, indicating that the droplets were completely
wetting these surfaces. After surface treatment with the alkyl
phosphonic acids, the aluminum surfaces became hydrophobic. The
contact angle observed for BPA coated aluminum was 82.5 and 103.2
for DPA coated aluminum. This corresponds well with the length of
the alkyl chain of these molecules. The contact angle on glass was
about 15, probably due to accumulation of adventitious carbon. The
contact angle on silanized glass substrates was 61.
[0125] To further analyze the films, XPS characterization was done
to understand the chemical identity of the surface. First, the
formation of an oxide layer on aluminum was confirmed by XPS
characterization. XPS spectra of Al 2p were taken on oxidized,
unmodified Al substrates. An Al 2p spectrum was resolved into a
metallic and oxide component by fitting in the 70-80 eV binding
energy regions. The fitted spectrum illustrated the presence of an
oxide peak at the binding energy of 75.2 eV as well as an Al metal
peak at 72.3 eV. This agrees well with binding energy separation of
2.8 eV reported in XPS-spectra handbook. The chemical surface
composition of clean and unmodified Al was determined by XPS to be
37, 49, and 14 at. % for Al, O, and C, respectively. As stated
earlier, the presence of oxide on the Al surface is required for
the phosphonate reaction.
[0126] Next, XPS characterization was used to confirm the
adsorption of the alkyl phosphonic acids to aluminum. XPS spectra
of modified aluminum substrates showed phosphorus peaks 2s and 2p
peaks, indicating modification of the Al surface. These peaks were
absent for glass substrates after treating with a similar process
of alkyl phosphonic acid modification as with aluminum.
High-resolution spectra were collected for O 1s, C 1s, P 2p and Al
2p peaks and the atomic % of these peaks are shown in Table 1,
below.
TABLE-US-00001 TABLE 1 Atomic % calculated from high resolution XPS
scan O C P Al Al + BPA 30.17 19.35 5.11 46.37 Al + DPA 28.15 29.86
2.43 39.56
[0127] As determined from Table 1, The C/P ratio for DPA was 12.3.
This is close to the theoretical value of 10. This shift is
probably due to either adventitious carbon or due to protruding
long alkyl chains, resulting in a higher C atomic % as compared to
P due to the higher exit thickness for the energetically lower P
photoelectrons. Similar trends have been reported by other papers
when they observed complete coverage. See Hogue, E., et al.,
Alkylphosphonate Modified Aluminum Oxide Surfaces. The Journal of
Physical Chemistry B 2006, 110, (22), 10855-10861. The C/P value
for BPA was lower than theoretical value, which may mean that BPA
does not form a complete monolayer. This trend has been reported in
other studies with smaller alkyl chain SAMs, because they tend to
form irregular structured films. Alkyl chain length has a strong
influence on the molecular packing during self-assembly; the longer
the chain length, the better the orientation of the molecules on
the surfaces. The longer chains are better able to self-assemble
due to an increase in van der Waals (vdW) attractive forces with
increasing chain length, because the strength of the vdW
interactions per adsorbate is proportional to the number of
methylene units in the adsorbate. See Ulman, A., Formation and
Structure of Self-Assembled Monolayers. Chemical Reviews 1996, 96,
(4), 1533-1554; Spori, D. M.; Venkataraman, N. V.; Tosatti, S. G.
P., et al., Influence of Alkyl Chain Length on Phosphate
Self-Assembled Monolayers. Langmuir 2007, 23, (15), 8053-8060.
[0128] Surface imaging mode was used to investigate the coverage of
the phosphonic acid layer. Positive secondary ion spectra did not
show any characteristic peak for the modified aluminum surface.
However characteristic peaks for phosphonic acids were observed in
the negative ion spectra. Two fragmentation peaks of the phosphonic
acid group, PO.sub.2.sup.-, and PO.sub.3.sup.-, confirmed the
presence of the acids on the surface of aluminum coated substrates.
The phosphate ion fragment peak signals were almost down to the
background level in case of glass substrates, which confirmed the
selective formation of phosphonic layer on aluminum coated
substrates.
[0129] Radio-labeling experiments showed the effectiveness of the
passivation layer at preventing silanization and oligo/DNA
immobilization. As noted above, silanized glass and silanized
aluminum substrates were each spotted with four serial dilution
spots (at increasing concentration) of oligonucleotide. A
phosphor-screen was used to observe the immobilized
oligonucleotides. On each of these substrates, the phosphor-screen
showed four visible spots, indicating attachment of both silane
molecules and the oligonucleotides.
[0130] However, the aluminum substrates which were subjected to the
alkyl phosphonic acid passivation treatment showed either no
immobilized oligonucleotides or much less immobilized
oligonucleotide than the unpassivated aluminum substrates. In the
case of the DPA modified aluminum substrate, no oligonucleotide
spots were visible, which indicated that a passivation of 1/1000
was possible with these substrates. In the case of BPA modified
aluminum substrate, the highest concentration spot of
oligonucleotide was slightly visible, but the other three spots at
lower concentrations were absent. Thus, the BPA modified aluminum
substrates also exhibited the ability to passivate against
silanization and oligonucleotide attachment, but to a slightly
lesser degree than DPA.
[0131] As will be understood by one skilled in the art, for any and
all purposes, particularly in terms of providing a written
description, all ranges disclosed herein also encompass any and all
possible subranges and combinations of subranges thereof. Any
listed range can be easily recognized as sufficiently describing
and enabling the same range being broken down into at least equal
halves, thirds, quarters, fifths, tenths, etc. As a non-limiting
example, each range discussed herein can be readily broken down
into a lower third, middle third and upper third, etc. As will also
be understood by one skilled in the art all language such as "up
to," "at least," "greater than," "less than," and the like include
the number recited and refer to ranges which can be subsequently
broken down into subranges as discussed above.
[0132] All publications, patent applications, issued patents, and
other documents referred to in this specification are herein
incorporated by reference as if each individual publication, patent
application, issued patent, or other document were specifically and
individually indicated to be incorporated by reference in its
entirety. Definitions that are contained in text incorporated by
reference are excluded to the extent that they contradict
definitions in this disclosure.
[0133] For the purposes of this disclosure and unless otherwise
specified, "a" or "an" means "one or more."
* * * * *