Metamaterial-based devices and methods for fabricating the same

Various embodiments of the present invention are directed to metamaterial-based devices and to methods of fabricating metamaterial-based devices. In one embodiment, a metamaterial-based device comprises a channel layer, a top metallic layer, and a bottom metallic layer. The channel layer has a top and a bottom surfaces, and at least one channel configured to transmit at least one material. The top metallic layer has a top surface and a bottom surface attached to the top surface of the channel layer and has a first lattice of openings extending between the top and bottom surfaces of the top metallic layer. The bottom metallic layer has a top surface and a bottom surface, wherein the top surface of the bottom metallic layer is attached to the bottom surface of the channel layer.

TECHNICAL FIELD

Embodiments of the present invention relate to metamaterial-based devices, and, in particular, to metamaterial-based devices that can be used in chemical sensors that can be included in lab-on-a-chip devices.

BACKGROUND

Analyte detection technology is currently employed in a wide range of disciplines ranging from electrochemical analysis through measurements to detect the presence and amount of biological compounds to pollution monitoring and industrial control. For example, chemical sensors have been developed to determine carbon dioxide levels in underground parking structures and in industrial manufacturing plants and to detect the presence of certain toxic chemicals in homes and in mines. Federal, state, and local governments have become increasingly aware of the dangers posed by many airborne pollutants and have begun to regularly monitor the levels of pollutants using chemical sensors. In addition, the threat of terrorist attacks employing toxic chemical weapons has created public concern and a demand for chemical sensors that can detect particular chemical weapons so that government authorities can respond accordingly. In the medical fields sensors have also been developed to detect quantities of certain biological compounds.

Although advancements in engineering and scientific disciplines have made it possible to fabricate chemical sensors to detect a variety of different analytes, a typical chemical sensor is often limited to detection of a single analyte or a small number of different kinds of analytes. In addition, a number of steps may be needed to prepare an analyte for detection. For example, certain chemical sensors employ a fluorescent material immobilized on an optical-fiber core. An analyte is detected by observing a color change that results from the fluorescent material reacting with the analyte. However, in order to detect a different analyte, the fluorescent material needs to be changed to one that fluoresces when reacted with the different analyte. Certain types of biosensors may employ active biological or biologically derived components which form chemical bonds with an analyte and hold the analyte in position for detection by a chemical sensor. An indirect approach is to use an enzyme that catalyzes a chemical reaction when an analyte is present to produce a product that can be detected by a chemical sensor. The presence of the product is assumed to confirm the existence of the analyte.

In recent years, Raman spectroscopic methods have also been developed to detect analytes. A typically analyte molecule has a unique Raman spectra that can be used to identify the analyte. For example, Raman spectra obtained from gas and liquid phase Raman scattering have been used to identify certain unknown analyte molecules. However, the intensities associated with these Raman spectra are often weak. In more recent years, surface-enhanced Raman spectroscopy (“SERS”) has been developed as an analyte-detection tool. Raman scattering from an analyte adsorbed on or even within a few Angstroms of a metal surface can be 103-106times greater than Raman scattering observed for the same analyte in gas or liquid phases. SERS enhances the Raman spectra of an analyte via two mechanisms. The first mechanism is an enhanced electromagnetic field called a surface plasmon produced at the surface of a metal. The surface plasmon can be created when the wavelength associated with incident electromagnetic radiation is close to the plasma wavelength of the metal surface. Molecules adsorbed or in close proximity to the surface experience a larger electromagnetic field than the field used to produce the Raman scattering in the liquid and gas phase. The second mechanism of enhancement results from the formation of a charge-transfer complex between the metal surface and the analyte. However, even SERS is not able to provide the enhancement often needed to identify a wide variety of analytes.

SUMMARY

Various embodiments of the present invention are directed to metamaterial-based devices and to methods of fabricating metamaterial-based devices. In one embodiment of the present invention, a metamaterial-based device comprises a channel layer, a top metallic layer, and a bottom metallic layer. The channel layer has a top surface, a bottom surface, and at least one channel configured to transmit at least one material. The top metallic layer has a top surface and a bottom surface attached to the top surface of the channel layer and has a first lattice of openings extending from the top surface to the bottom surface of the top metallic layer. The bottom metallic layer has a top surface and a bottom surface, wherein the top surface of the bottom metallic layer is attached to the bottom surface of the channel layer.

DETAILED DESCRIPTION

Various embodiments of the present invention are directed to metamaterial-based devices that can be used in chemical sensors and to methods for fabricating these devices. The term “metamaterial” refers to devices comprised of composite materials that are configured to have electromagnetic responses that are different from the materials comprising the devices. In other words, metamaterial-based devices of the present invention can be configured to exhibit responses to particular wavelength ranges of electromagnetic radiation that are different from responses exhibit by the materials comprising the devices. Metamaterial-based devices of the present invention can be used in chemical sensors to identify certain gaseous or liquid materials using Raman spectroscopic methods.

FIG. 1Ashows an isometric view of a first metamaterial-based device100in accordance with embodiments of the present invention. Metamaterial-based device100comprises a channel layer102sandwiched between a top-metallic layer104and a bottom-metallic layer106. As shown inFIG. 1A, channel layer102includes two channels108and110, which can be used to transmit material in either a gas or a liquid state. Top-metallic layer104and bottom-metallic layer106are “mesh-like” metallic structures. In other words, top-metallic layer104includes a lattice of tapered openings, such as tapered opening112, that progressively narrow from the top surface to the bottom surface of top-metallic layer104. Bottom-metallic layer106also includes a lattice of openings (shown in subsequent figures), such as opening114, that progressively narrow from the top surface to the bottom surface of bottom-metallic layer106. Metamaterial-based device100is supported by a dielectric substrate116.

FIG. 1Bshows an exploded isometric view of metamaterial-based device100in accordance with embodiments of the present invention. As shown inFIG. 1B, channels108and110span the width of channel layer102so that a material can be transmitted through channels108and110.FIG. 1Balso reveals the lattice of openings in bottom-metallic layer106. The openings in top-metallic layer104are larger than the openings in bottom-metallic layer106, and each opening in top-metallic layer104is substantially aligned with a corresponding opening in bottom-metallic layer106. For example, double-headed arrow118indicates that opening112in top-metallic layer104is substantially aligned with opening114in bottom-metallic layer106. Although, the tapered openings in metallic layers104and106are rectangular, in other embodiments of the present invention, the tapered openings can be square, circular, elliptical, irregularly shaped, or have any other suitable shape. In various embodiments of the present invention, metallic layers104and106can be comprised of silver, gold, aluminum, titanium, copper, or any other suitable metal or metal alloy that allows for the formation of plasmons, as described below with reference toFIGS. 5-6.

FIG. 1Cshows a cross-sectional view of metamaterial-based device100along a line1C-1C, shown inFIG. 1A, in accordance with embodiments of the present invention. The openings in top-metallic layer104are located above the openings in bottom metallic layer106. For example, an opening120in top-metallic layer104is located above an opening122in bottom-metallic layer106.FIG. 1Creveals that the openings in metallic layers104and106taper toward substrate116. Regions of top-metallic layer104between openings and regions between openings of bottom-metallic layer106are tapered away from substrate116and are located above one another. For example, a tapered region124in top-metallic layer104is located above a tapered region126in bottom-metallic layer106. Channels108and110in channel layer102are configured and positioned between metallic layers104and106so that edges of the tapered regions can be exposed to materials transmitted in channels108and110. For example, edges130-133of tapered regions124and126are exposed in channel108.

FIG. 1Dshows a cross-sectional view of a second metamaterial-based device200in accordance with embodiments of the present invention. Metamaterial-based device200is identical to metamaterial-based device100except, as shown inFIG. 1D, the openings in the lattice of openings of bottom-metallic layer106are filled with the same material comprising substrate116. For example, opening122is filled with the material comprising substrate116.

FIG. 2Ashows an isometric view of a third metamaterial-based device200in accordance with embodiments of the present invention. Metamaterial-based device200is substantially identical to metamaterial-based device100except bottom-metallic layer106of metamaterial-based device100has been replaced by a bottom-metallic layer202that does not include a lattice of openings.

FIG. 2Bshows an exploded isometric view of metamaterial-based device200in accordance with embodiments of the present invention.FIG. 2Breveals that unlike bottom-metallic layer106of metamaterial-base device100, bottom-metallic layer202does not include a lattice of openings. Bottom metallic layer202can be comprised of silver, gold, titanium, copper, or any other suitable metal or metal alloy that allows for the formation of surface plasmons, as described below with reference toFIGS. 5-6.

FIG. 2Cshows a cross-sectional view of metamaterial-based device200along a line2C-2C, shown inFIG. 2A, in accordance with embodiments of the present invention. Channels108and110in channel layer102are configured and positioned between metallic layers104and202so that edges of the tapered regions can be exposed to materials transmitted in channels108and110. For example, edges130-131of tapered region124are exposed in channel108.

In other embodiments of the present invention, the sidewalls forming the regions between openings in metallic layers104and106can be substantially vertical. In other embodiments of the present invention, openings can taper away from substrate116, and regions of metallic layers104and106between openings can taper toward substrate116. In still other embodiments of the present invention, openings in metallic layer104can also be filled with a suitable dielectric material that seals metallic layer104.

Channel layer102can be comprised of a group IV, a group III-V, a group II-VI semiconductor, SiO2, Si3N4, a transparent polymer material, or a metal oxide. Channel layer102can also be configured with numerous channels, each of which can be used to transmit a different gaseous or liquid material.FIG. 3Ashows a top view of channel layer102having five separate channels301-305that can each be used to separately transmit a different material in accordance with embodiments of the present invention. Channel layer102can also be configured with a number of mixing chambers for combining reacts to form products.FIG. 3Bshows a top view of a channel layer102having 3 separate mixing chambers306-308for combining different reactants in accordance with embodiments of the present invention. Gaseous or liquid reacts can be introduced via separate channels310and312to form products in mixing chamber306. The products formed in mixing chamber306exit channel layer102via channel314. Gaseous or liquid reacts can be introduced one at a time via channel316to form products in mixing chambers307and308. The products formed in mixing chambers307and308exit channel layer102via channel318.

The thickness of the layers102,104, and106, the size and aspect ratio of the openings, and the lattice constants associated with the lattice of openings can all be varied depending on the range of radiation wavelengths needed to produce a photonic interaction with metamaterial-based devices of the present invention.FIGS. 4A-4Bshow parameters associated with metamaterial-based device embodiments that can be varied in accordance with embodiments of the present invention.FIG. 4Ashows parameters associated with a top view of four tapered openings. Outer rectangles, such as rectangle402, represent the widest portion of each tapered opening, and inner rectangles, such as rectangle404, represent the narrowest portion of each tapered opening. Parameter A represents the spacing between adjacent openings or the lattice constant, and parameters W1, W2, and W3represent widths between openings.FIG. 4Bshows a cross-sectional view of a first tapered region406in top-metallic layer104and a second tapered region408in bottom-metallic layer106. Parameters T1, T2, and T3represent thicknesses associated with top-metallic layer104, channel layer102, and bottom-metallic layer106, respectively.

The parameters described above with reference toFIG. 4can be varied so that devices of the present invention interact with certain wavelength ranges of incident electromagnetic radiation to exhibit negative effective permittivity ∈eff, negative effective permeability μeff, and a negative refractive index n. In other words, an electromagnetic wave propagating at a speed v=c/n, where c is the speed of electromagnetic radiation in vacuum, in metamaterial-based devices of the present invention propagates along a direction opposite the incident direction of the electromagnetic wave. The metallic regions of the metallic layers104and106produce a negative permeability and negative permittivity via a plasmonic response. The magnetic component of the electromagnetic radiation threading through openings induces current loops linking metallic regions of the metallic layers104and106, as schematically represented by directional arrows410and412inFIG. 2B, resulting in μeff<0.

A refractive index n and an effective permeability μeffobtained from simulating an electromagnetic wave propagating through a hypothetical metamaterial-based device in accordance with embodiments of the present invention are provided inFIGS. 5A-5B, respectively. Horizontal axes502and503represent a range of wavelengths associated with electromagnetic waves propagating through the hypothetical metamaterial-based device, and vertical axes504and505represent values of real and imaginary components of the refractive index n and the effective permeability μeff. The parameters used for the simulation are displayed in Table 1:

TABLE 1LengthParameter(nm)A320W1110W2124W3218T125T280T325
The simulation was performed using the well-known Finite Difference in Time Domain method (“FDTD”) (for a detailed description of FDTD, see A. Taflove and S. C. Hagness,Computational Electrodynamics,2nd Edition (Artech House, Boston, 2000).

FIG. 5Ashows a plot of real and imaginary components of a refractive index n versus a range of wavelengths for an electromagnetic wave propagating through the hypothetical metamaterial-based device. Curve508represents the real component of n, Re[n], and curve510represents the imaginary component of n, Im[n]. Curve508is negative for electromagnetic waves with wavelengths greater than about 1.4 μam, and has the approximate value −1.7 at an approximate resonant wavelength 1.72 μm. Electromagnetic waves propagating through the hypothetical metamaterial-based device with wavelengths greater than about 1.4 μm experience a negative refractive index, and, as a result, have a negative angle of refraction.

A property of metamaterial-based device embodiments of the present invention is that incident electromagnetic waves having an appropriate wavelength create resonant electromagnetic surface waves, called “surface plasmons,” which are localized along the surface and edges of metallic layers104and106. Surface plasmons are formed along the surface of metallic objects when the real part of the effective permittivity and permeability is negative. Surface plasmons formed on metallic layers104and106can propagate on the surfaces and can be localized to openings in metallic layers104and106.FIG. 5Bshows a plot of real and imaginary components of the effective permeability μeffversus a range of wavelengths for the same hypothetical metamaterial-based device. A curve512represents the real component of μeff, Re[μeff], and a curve514represents the imaginary component of μeff, Im[μeff]. Curve512is negative for electromagnetic waves with wavelengths between about 1.65 μm to about 1.8 μm. This signifies the presence of an electromagnetic resonance whereby surface plasmons are formed at the surface of metallic layers104and106in this range of wavelengths of incident radiation, with strongly enhanced local electric field facilitating an enhanced Raman scattering.

Metamaterial-based devices of the present invention can be included in chemical sensors to intensify Raman scattering associated with Raman spectra that are used to identify unknown gaseous or liquid analytes transmitted via the channels of the channel layer. Raman spectra can also be used to identify products formed in mixing chambers of the channel layer.FIG. 6shows a schematic representation of a chemical sensor600that includes metamaterial-based device100in accordance with embodiments of the present invention. Chemical sensor600also includes an electromagnetic radiation source602and a detector604. Source602can be positioned so that emitted electromagnetic radiation is incident upon the metamaterial-based device100and can be configured to emit electromagnetic radiation with wavelengths that induce the formation of surface plasmons on metallic layers104and106. The electric field of plasmons formed along the edges of the openings in the metallic layers104and106is considerably enhanced over those formed along surfaces. For example, referring toFIG. 1C, plasmon enhancement is strongest along edges130-133of metallic regions124and126. Analytes606and608are injected into channels108and110, respectively. The analyte molecules are attached to or are in close proximity to exposed surfaces and edges of metallic layers104and106along channels108and110. The plasmon electric field component excites vibrational modes of the analyte molecules. Analyte molecules experience stronger plasmon electric field components along the edges of the openings than the plasmon electric field components formed along the surface of metallic layers104and106. As a result, Raman scattering is enhanced for analytes molecules located close to the edges than for analyte molecules located on or close to surfaces of metallic layers104and106. Detector604can be positioned and configured to detect electromagnetic radiation from analytes606and608. In other embodiments of the present invention, metamaterial-based device100of chemical sensor600can be configured with mixing chambers, as described above with reference toFIG. 3, so that chemical sensor600can be used to detect formation of products. Note that, in other embodiments of the present invention, a chemical sensor substantially identical to chemical sensor600can include metamaterial-based device200rather than metamaterial-based device100.

Chemical sensor embodiments of the present invention can be included in lab-on-a-chip devices. Lab-on-a-chip devices integrate microfluidic systems on a microchip in order to automate many standard laboratory practices. Lab-on-a-chip devices may include a network of channels, mixers, reservoirs, and diffusion chambers that are etched into glass or a polymer chip and may also include integrated electrodes, pumps, valves, and other suitable miniature devices.FIG. 7shows a schematic representation of a lab-on-a-chip device700that includes a chemical sensor in accordance with embodiments of the present invention. An unknown analyte702is injected into a sample preparation device704, which may include an injector, dispenser, and preconcentrator. The prepared analyte is then transmitted to a multiplexer706that separates the analyte into components that are transmitted separately to a chemical sensor708. Chemical sensor708can then be used to produce a Raman spectrum associated with each analyte.

FIGS. 8A-8Jshow isometric and cross-sectional views that correspond to steps of a method for fabricating metamaterial-based device100, shown inFIG. 1, in accordance with embodiments of the present invention. First, as shown inFIGS. 8A-8B, a bottom-metallic layer802is deposited on a substrate804using evaporation, sputtering, atom layer deposition (“ALD”), or wafer bonding. Bottom-metallic layer802can be comprised of silver, gold, aluminum, titanium, copper, or another suitable material, and substrate804can be comprised of SiO2, Si3N4, or another suitable dielectric material.

Next, as shown inFIG. 8C, electron beam lithography (“EBL”), x-ray lithography, photolithography, focused ion beam lithography, extreme UV lithography (“EUVL”), or nanoimprint lithography (“NIL”) can be used to form a lattice of openings, such as opening806, in bottom-metallic layer802. In other method embodiments of the present invention, a lift-off process can be used to form metallic layer802with the lattice openings. First, a resist is deposited on substrate804and patterned with crossing channels that correspond to the mesh-like structure shown inFIGS. 1-2leaving pillars that correspond to openings in bottom-metallic layer802using any one of various lithography techniques. Second, silver, gold, aluminum, titanium, copper, or another suitable material is deposited over the resist and in the channels. Third, the resist is dissolved leaving bottom-metallic layer802with a lattice of openings. The openings in the lattice of openings can be rectangular, square, circular, elliptical, irregularly shaped, or another suitable shape, and the side walls of each opening can be tapered toward substrate804, or substantially vertical. In an optional step of fabricating metamaterial-based device100, as shown in a cross-sectional view ofFIG. 8D, the lattice of openings in bottom-metallic layer802can be back filled with the same material comprising substrate804and the top surface of bottom-metallic layer802planarized. For example, region808is an opening in bottom-metallic layer802that has been back filled with substrate804material.

Note that the steps described with reference toFIGS. 8C and 8Dare optional. For example, in other method embodiments of the present invention, forming the lattice of openings in bottom metallic layer802can be omitted in order to fabricate a metamaterial-based device with no lattice of openings in bottom metallic layer802, such as bottom metallic layer202of metamaterial-base device200shown inFIG. 2.

Next, in a cross-sectional view shown inFIG. 8E, a channel layer810is formed on the top surface of bottom-metallic layer802using CVD, MBE, ALD, evaporation, sputtering, or wafer bonding. The channel layer can be comprised of a group IV semiconductor, a group III-V semiconductor, a group II-VI semiconductor, polymers, or another suitable material. Next, in a cross-sectional view as shown inFIG. 8F, an opening812can be formed in channel layer810using electron beam lithography, nanoimprint lithography, reactive ion etching, chemically assisted ion etching, or focused ion beam milling, or in other embodiments, the lift-off process described above can be used. The opening812may represent a channel or a mixing chamber formed in channel layer810. Next, in a cross-section view shown inFIG. 8G, opening812is filled with a polymer814using CVD spin-coating, or spray-coating and the surface can be planarized.

Next, in a cross-sectional view shown inFIG. 8H, a top-metallic layer816is deposited on channel layer810using evaporation, sputtering, ALD, or wafer bonding. EBL, x-ray lithography, photolithography, focused ion beam lithography, EUVL, or NIL can be used to form a lattice of openings in top-metallic layer816. Top-metallic layer816can be comprised of silver, gold, aluminum, copper, or another suitable material. Next, in a cross-sectional view shown inFIG. 8I, polymer814is removed from opening812by applying an appropriate solvent that dissolves polymer814.FIG. 8Jshows an isometric view of a metamaterial-based device800fabricated in accordance with the steps described above with reference toFIGS. 8A-8I. In other method embodiments of the present invention, before the polymer814is removed from opening812, a dielectric material can be deposited in the openings of the lattice of openings in metallic layer816. In a subsequent, an appropriate solvent can then be used to dissolve polymer814. As a result, openings in the lattice of openings in top metallic layer816are filled with dielectric material.