Abstract
A metamaterial pixel providing an electromagnetic emitter and/or en electromagnetic detector operating within a limited bandwidth. The metamaterial pixel is comprised of plasmonic elements arranged within a periodic photonic crystal array providing an electromagnetic emitter and/or an electromagnetic detector adapted in embodiments for operation at selected bandwidths within the wavelength range of visible out to a millimeter. Performance of the pixel in applications is enhanced with nanowires structured to enhance phononic scattering providing a reduction in thermal conductivity. In embodiments multiple pixels are adapted to provide a spectrometer for analyzing thermal radiation or electromagnetic reflection from a remote media. In other embodiments emitter and detector pixels are adapted to provide an absorptive spectrophotometer. In other embodiments one or more of metamaterial pixels are adapted as the transmitter and/or receiver within a communication system. In a preferred embodiment the pixel is fabricated using a silicon SOI starting wafer.
Claims
1. An apparatus comprising a metamaterial thermal pixel, wherein the pixel comprises: a thermal micro-platform, the thermal micro-platform having a support layer that is suspended by nanowires at a perimeter thereof, and an active layer disposed on a portion of the support layer; an off-platform region, the off-platform region surrounding the micro-platform; a plurality of the nanowires comprised of a first layer having phononic scattering and/or phononic resonant structures physically adapted to reduce thermal conductivity, and wherein one or more of the thermal micro-platform is comprised of an arrayed metamaterial structure providing one or more of an emitter and/or detector for electromagnetic radiation.
2. The apparatus of claim 1 wherein the thermal micro-platform is comprised of a temperature control element further comprised of one or more of a resistive heater or a Peltier thermoelectric cooler.
3. The apparatus of claim 1 wherein the thermal micro-platform is comprised of a temperature sensing element further comprised of one or more of Seebeck thermoelectric devices, a thermistor, and a subthreshold MOST, PTAT bandgap diode.
4. The apparatus of claim 1 wherein one or more of the thermal micro-platform is comprised of a periodic array of metallic, dielectric or semiconductor elements shaped variously as, without limitation, squares, crossbars, circles, dipole antennas, and split ring resonant (SRR) structures.
5. The apparatus of claim 1, wherein the one or more of the thermal micro-platform comprises a reflecting metallic film providing an increased reflective plasmon confinement at the wavelength band or bands of interest.
6. The apparatus of claim 1 wherein the one or more of thermal micro-platform is comprised of one or more composite levels of plasmonic resonant structures providing operation within one or more wavelength bands of interest.
7. The apparatus of claim 1 comprising a plurality of thermal micro-platforms, each platform having one or more of the emitter and/or the detector.
8. The apparatus of claim 1 wherein the first layer of the plurality of nanowires has phonon mean-free-paths greater than the distance between atomic- or nano-scaled boundaries, providing a means for reduction in thermal conductivity.
9. The apparatus of claim 1 wherein the first layer of the plurality of nanowires is a semiconductor active layer.
10. The apparatus of claim 1 wherein the thermal micro-platform and the nanowires are comprised of the active layer of a silicon SOI starting wafer.
11. The apparatus of claim 1 wherein the plurality of nanowires is comprised of a first and second layer, the second layer comprising a metal selected from the group, without limitation, tungsten, palladium, platinum, molybdenum, and aluminum providing an electrical connection of increased electrical conductivity.
12. The apparatus of claim 1 wherein the plurality of nanowires is comprised of a third layer further comprised of a dielectric selected from the group comprising, without limitation, silicon nitride, silicon oxynitride, aluminum oxide, and silicon dioxide, and further wherein the dielectric provides a reduction of stress across the micro-platform.
13. The apparatus of claim 1 wherein the active layer is a semiconductor comprised of, without limitation, silicon, germanium, silicon-germanium, gallium arsenide, gallium nitride, indium phosphide, silicon carbide and alloys thereof.
14. The apparatus of claim 1 wherein the one or more thermal micro-platform is covered with random matrices of carbon nanotubes or graphene disposed to provide a further enhancement of emissivity or absorptivity.
15. The apparatus of claim 1 wherein the pixel is maintained under vacuum and is comprised of a resistive heater having a gettering material providing a means of degassing within the vacuum volume.
16. The apparatus of claim 1 wherein the one or more of the thermal micro-platform is adapted to provide a standoff spectral reflectance analyzer for a remote media including agricultural soils and food products.
17. The apparatus of claim 1 wherein the thermal micro-platform is adapted to provide a standoff temperature sensor for monitoring the temperature of a remote media.
18. The apparatus of claim 1 wherein the one or more thermal platform is adapted to provide a spectrophotometer for spectral analysis wherein an electromagnetic beam is sourced by the emitter, transmitted through or reflected from an analyte comprised of a gas, vapor, particulate or surface, and detected by the detector.
19. The apparatus of claim 1 wherein the emitter and detector provide one or more of a transmitter and/or a receiver within an infrared communication system.
20. The apparatus of claim 1 wherein the emitter and/or detector operate within one or more wavelength bands of limited bandwidth, the wavelength bands comprised of visible light, infrared and millimeter wavelengths.
Description
BRIEF DESCRIPTION OF THE FIGURES
[0050] FIG. 1 is prior art plan view depicting a thermal micro-platform.
[0051] FIG. 2 is a prior art illustrative view depicting a nanowire with phononic structures providing a reduction in thermal conductivity.
[0052] FIG. 3 is a prior art cross-sectional view depicting a micro-platform released from a surrounding support platform using a backside etch.
[0053] FIG. 4 is a prior art cross-sectional view depicting a micro-platform released from a surrounding support platform using a topside etch.
[0054] FIG. 5A is a prior art cross-sectional view depicting a section of a nanowire comprised of a first layer providing a reduced thermal conductivity.
[0055] FIG. 5B is a prior art cross-sectional view depicting a section of a nanowire comprised of a first and second layer in accordance with embodiments of the invention.
[0056] FIG. 5C is a prior art cross-sectional view depicting a section of a nanowire comprised of a first, second and third layer in accordance with embodiments of the invention.
[0057] FIG. 6A is a plan view depicting arrays of the metamaterial plasmonic elements disposed on the micro-platform in accordance with embodiments of the invention.
[0058] FIG. 6B is a plan view depicting additional arrays of the metamaterial plasmonic elements disposed on the micro-platform in accordance with embodiments of the invention.
[0059] FIG. 7 is a plan view depicting the pixel configured to provide a metamaterial emitter in accordance with embodiments of the invention.
[0060] FIG. 8 is a plan view depicting the pixel configured to provide a metamaterial detector in accordance with embodiments of the invention.
[0061] FIG. 9 depicts a standoff infrared analyzer for monitoring the temperature of a remote surface incorporating the pixel in accordance with embodiments of the invention.
[0062] FIG. 10 depicts a reflective spectrometer providing reflectance spectra from a remote media incorporating the pixel in accordance with embodiments of the invention.
[0063] FIG. 11 depicts an absorptive spectrophotometer incorporating the pixel in accordance with embodiments of the invention.
[0064] FIG. 12 depicts a transmitter and receiver within a communication system incorporating the pixel in accordance with embodiments of the invention.
DETAILED DESCRIPTION
[0065] Definitions: The following terms are explicitly defined for use in this disclosure and the appended claims:
[0066] micro-platform means a platform having a maximum dimension of about 100 nanometers on a side up to about 1 centimeter. micro-platform comprised of refers to both the underlying platform structure such as a the patterned active region of a silicon SOI starting wafer in addition to thermal elements and metamaterial structures physically disposed on the platform.
[0067] metamaterial structure means a structural component of a pixel providing characteristics not generally found in nature with application as an emitter and/or detector based on structural configurations which affect the movement of photons, electrons, phonons and energy couplings thereof. The metamaterial structure may be non-plasmonic or plasmonic.
[0068] phononic nanowire means a suspended nanowire comprised of a phononic structure providing a reduction in thermal conductivity.
[0069] metamaterial pixel in the present invention means a pixel structurally configured with one or more of phononic crystal, photonic crystal, scattering, superlattice, quantum mechanical tunneling, resonant, and plasmonic structures.
[0070] phononic crystal (PnC) means a metamaterial structure comprised of periodic subwavelength phononic nanostructure that affects the thermal energy transport of phonons.
[0071] photonic crystal (PhC) in this invention means a metamaterial structure comprised of periodic subwavelength optical nanostructure that affects the transport of photons.
[0072] surface plasmonic polariton (SPP) means a surface electromagnetic wave guided along a metametarial patterned surface or ALD film wherein the surface or film has sufficient electrical conductivity to support associated charge motion. In this invention, SPPs within the metamaterial can be excited from an integral photon or electron source such as an internal black body structure, internally-sourced tunneling electrons or from an external photon beam source. A SPP is a type of bosonic quasiparticle.
[0073] nanowire means a suspended structure providing support for a micro-platform having some structural dimensions of less than 1000 nm.
[0074] emitter means the metamaterial structure sourcing electromagnetic radiation in the spectral range including ultraviolet, visible light, infrared and into millimeter wavelengths.
[0075] detector means the metamaterial structure sensitive to incident electromagnetic radiation in the spectral range including ultraviolet light, visible light, infrared, and into millimeter wavelengths.
[0076] thermoelectric device means any device for conversion of thermal energy into electrical energy or visa versa. This term refers to both temperature control elements and temperature sensing elements.
[0077] temperature control element means a device for heating such as a heated resistor or a device for cooling such as a Peltier cooler.
[0078] temperature sensing element means a device for temperature sensing such as a Seebeck thermocouple sensor, thermister, IPTAT, VPTAT, MOST, bipolar transistor or bolometer sensor.
[0079] Cross-sectional views depicting metamaterial plasmonic elements as disposed on the micro-platform are presented in FIG. 6A and FIG. 6B. These elements are disposed in a periodic matrix over an underlying dielectric film. In some embodiments, the metamaterial structure includes a metallic film underlying overall. In embodiments, the metamaterial structure is a type of photonic crystal. This metamaterial structuring provides an enhanced emissivity and absorptivity for emitter and detector functions, respectively. Each element supports a local resonant electromagnetic field which couples with fields originating from nearby elements within the larger metamaterial matrix. The entire metamaterial structure, when heated, provides an efficient emitter of radiation, and, when not heated, provides an efficient absorber of incident radiation.
[0080] Each panel of FIG. 6A (501-508) shows portions of a larger array of patterned metamaterial filter structure disposed on the micro-platform 610. In this case the micro-platform 110 supports a field confinement adjacent to the surface elements 620. The individual elements 501-508 are of subwavelength dimension (as referred to free space wavelength). The principal wavelength of filter 501 is generally lower than that of filter 508. Panel 502 shows a 1-D Brag grating structure which is polarization sensitive. Panel 503 shows split-ring resonant SRR elements which are typically used in filters with center frequencies in the very long wave infrared region. Panel 504 presents polarization-sensitive simple dipoles and a folded dipole resonant antenna forming a cell within a larger periodic plasmonic array. All panels except 504 and 507 each provide a single, primary bandwidth filter band while panels 504 and 507 are characterized by multiple primary operational wavelength bands. Secondary bands are generally observed which derive from minor resonances associated with specific dimensions and couplings beyond nearest neighbor elements. Surface plasmonic coupling generally involves polariton waves associated with individual resonant elements.
[0081] FIG. 6B is a plan view depicting additional arrays of metamaterial plasmonic elements. Panels 509 through 516 present portions of periodic arrays comprised of split-ring resonators. Panel 514 depicts an array portion providing filter with at least two primary wavelengths.
[0082] FIG. 6C is a cross-sectional view depicting metamaterial plasmonic elements 620 disposed on portions of a micro-platform 610 in FIG. 6A and FIG. 6B. The metal film is patterned to provide raised, pillar-like structures above the micro-platform 110.
[0083] Metal films are chosen as the surface element 620 in most embodiments for operation in the visible, near infrared, and long-wave infrared wavelength region because metals provide a high plasma frequency and an increased density of electrons compared to a semiconducting structural element. In embodiments, semiconductor surface plasmonic structures such as are depicted in FIGS. 6A and 6B can provide operation at mid-infrared and longer wavelength regions.
[0084] FIG. 6D is a cross-sectional view depicting plasmonic elements 620 disposed on the micro-platform 610 with an intermediate dielectric film 630. This is typically a film selected for low loss at the wavelength of interest and in some embodiments a film selected to reduce overall stress across the micro-platform. The dielectric film 630 is generally of thickness ranging from 30 nm to 1 micrometer.
[0085] FIG. 6E is a cross-sectional view depicting surface plasmonic elements 620 disposed with three films disposed on the micro-platform 610. This tri-level film sandwich is comprised of an intermediate dielectric film 630 and a metallic film 640 of typically of thickness of 1 micrometer or less. In embodiments, the metallic film provides increased confinement of the electromagnetic field associated with the surface plasmonic structures with an accompanying increase in overall emissivity or absorptivity.
[0086] The metallic surface and reflecting structures in many embodiments are comprised of metals to reduce losses at shorter infrared wavelengths. A preferred metal for performance over a wide range of wavelengths is Ag, W, Pd, Pt, Ni, Al, and Ti. In some non-CMOS embodiments, the surface metal is Au. The patterned metallic metamaterial elements are typically of thickness in the range of 1 nm to 1000 nm.
[0087] In other embodiments, nonmetallic arrayed surface elements 610 depicted in FIG. 6A and FIG. 6B are suitable for pixels adapted for operation in the very longwave infrared regions out to about 1 mm. At these wavelengths a semiconductor can provide lower loss compared with metallic structures. In these embodiments, the surface film element 620 is selected to provide a dielectric constant compared with an underlying film 620 of lower dielectric constant depicted in FIGS. 6A-6E. 6. In embodiments, these structures may be raised areas, pillars, cavities, holes or structures embedded within a dielectric film.
[0088] Tri-level metamaterial filters of FIG. 6E based on the panels of FIG. 6A and FIG. 6B in many cases provide an electromagnetic bandwidth quality factor Q of 10 or higher. In embodiments, the multi-layering concept of FIG. 6E is extended to provide more than three layers. Appropriate stacked structuring with vertical plasmonic coupling between metallic elements at different stack levels provides a 3-D metamaterial structure. These 3-D stacked metamaterial structures can be optimized to provide a further narrowing of the bandwidth of the metamaterial filter with an accompanying increase in the quality factor Q.
[0089] FIG. 7 is a plan view depicting a semiconductor chip adapted to provide a metamaterial emitter. Thermoelectric control elements disposed on the micro-platform 110 are resistive heaters (710 and 711). This embodiment a micro-platform 110 provides a uniform stress into the platform and generally permits fabrication of larger micro-platforms over the underlying cavity 125. The micro-platform 110 is supported by nanowires 730 comprised of a level 1 film and nanowires 740 comprised of a level 1 film covered with a level 2 metal film. The nanowires are tethered onto surrounding support platform 102 which provides a thermal heat sink. The micro-platform 110 is heated by a first heating element 710 disposed between bonding pads 740 and 750. In this embodiment a second heating element structured similar to the first heating element, is contacted through bonding pads 760 and 770 disposed on the surrounding support platform 102.
[0090] In embodiments comprising the emitter of FIG. 7, a first level of the metamaterial structure may be selected, without limitation, from among the structural options of FIG. 6. In the embodiment of FIG. 7, the micro-platform formed of the active layer from a silicon SOI starting wafer is heated to a temperature of 400 C. and higher when a high temperature metal such as tungsten is used on the nanowires and micro-platform. If a lower temperature metallization such as aluminum is used, the maximum operational temperature is limited to around 400 C. In other embodiments, a pixel similar to that of FIG. 7 comprising a semiconductor active layer such as silicon carbide or gallium nitride and with dielectric passivation films of silicon nitride permits heating of a micro-platform to temperatures over 1000 C.
[0091] Tungsten and aluminum films are deposited using a DC magnetron tool. Any dielectric film chosen is generally deposited by RF sputtering. Patterning of these thin films is accomplished using a resist such as patterned PMMA with a lift-off process. Other patterning techniques are used with thicker films. Backside etch to form the cavity 125 is accomplished with DRIE or with patterned TMAH or KOH at an elevated temperature. Topside formation of the cavity 126 is accomplished using a hot vapor HF etch and with a patterned passivation layer of material such as Si.sub.3N.sub.4 protecting certain topside areas as desired.
[0092] FIG. 8 is a plan view depicting a semiconductor chip adapted to provide a metamaterial detector. In this illustrative embodiment, the detector is comprised of a Seebeck sensing element and a Peltier controlled-cooling element. The detector pixel is comprised of a micro-platform 110 disposed over cavity 125. Nanowires of types 820 and 840 support the micro-platform 110 and are tethered onto the surrounding micro-platform 102. Nanowires of type 820 are comprised of heavily doped p+ and n-couplings 840 connected in series to provide a thermocouple array with couplings disposed on the micro-platform 110 and the off-platform heat sink area 102. The thermocouple may be operated in either a Seebeck sensor or Peltier cooling mode. The thermocouple is comprised of a metallic on-platform ohmic connection and an off-platform interconnecting trace 850. The two thermocouples are electrically connected between bonding pads 810 and 820 disposed on the surrounding support platform 102. The thermocouples sense the minute differential temperature difference between the micro-platform 110 and the surrounding heat sink 102 resulting from absorbed incident radiation into the metamaterial structure 780.
[0093] It will be noted the Seebeck sensor array is depicted in FIG. 8 with only two thermocouples. In embodiments, the micro-platform is populated with over 2000 series-connected thermocouples, providing an increase in overall pixel detectivity D* and responsivity (Volts/Watt) for pixel operation as a detector. In embodiments comprising the detector of FIG. 8, a metamaterial structure may include, without limitation, the first layer structure options of FIGS. 6A-6D.
[0094] In many embodiments, including the embodiment of FIG. 8, the micro-platform 110 is formed of the high resistivity active layer of a starting silicon wafer having a resistivity of over 1000 Ohm-cm. The heavily doped thermocouple regions are diffused directly into the high resistivity micro-platform 110. Sensed signal loss due to the shunt effect of parasitic resistance in the high resistivity areas is designed to be minimal. The heavily doped thermocouple regions of p+ type 820 and n type 830 semiconductor are typically formed by diffusion from a patterned spin-on glass formed with boron or phosphorus in the illustrative silicon process embodiment. For detector pixels operated at temperatures of less than 400 C., DC sputtered aluminum is used for metallization. Selected ALD dielectric films are generally deposited by RF plasma sputtering or physical evaporation. Patterning is generally accomplished using a PMMA or similar resist with micro-dimensioning obtained with e-beam lithography or optical lithography as appropriate.
[0095] In the illustrative embodiments of FIG. 7 and FIG. 8, the plasmonic absorber 780 can be identical for both the emitter of FIG. 7 and the detector of FIG. 8. The use of a certain commonality in the cleanroom process for both the emitter pixel and the detector pixel embodiments permits a lower cost production processing.
[0096] In embodiments, the emissivity/absorptivity of the metamaterial structure can be enhanced by growing or depositing carbon nanotubes (CNT), especially vertical multiwall carbon nanotubes (VWCNT) or graphene. Carbon nanotubes are grown typically using an acetylene precursor in a CVD reactor. In embodiments, graphene is generally deposited as a random mesh over the metamaterial.
[0097] In embodiments, the pixel is mounted in a package backfilled with a gas of low thermal conductivity such as Xe, Kr or Ar. This reduces the parasitic loss due to thermal conductivity of atmosphere between the micro-platform and the surrounding heat sink. In embodiments, the pixel is disposed within a vacuum package for the purpose of reducing heating or cooling of the micro-platform due to undesirable convective and conductive heat dissipation.
[0098] In some package embodiments, the pixel is sealed in an oxygen environment. An additional resistive heater is disposed on the micro-platform in thermal contact with a gettering material. When the additional resistive heater is powered the gettering material is activated and an outgassing of the pixel environment is achieved providing a vacuum.
Example 1
Multi-Wavelength Pyrometer
[0099] FIG. 9 depicts an apparatus comprised of multiple detector pixels adapted as a standoff infrared analyzer monitoring the temperature of a standoff media 920. Multiple detectors 940 are sensitive to separate wavelength bands of thermal radiation 910 emitted from standoff media 920. Optics 930 focus the radiation 910 from the remote media 910 onto the detectors 940. In this embodiment, signal conditioning circuitry 950 with an interface to a digital bus permits a determination of the temperature of a standoff media based on differential spectral analysis of the emitted thermal radiation and an estimate or calibration of thermal emissivity of the standoff media 920. In embodiments, this adaptation is implemented with multiple detectors providing a multi-wavelength pyrometer.
Example 2
Reflective Spectrometer
[0100] FIG. 10 depicts the pixel configured to provide a reflective spectrometer for spectral analysis of reflectance from standoff media. The spectrometer is comprised of both an emitter 1010 which illuminates a standoff media through focusing optics 1040 and detectors 1050 and 1060 monitoring the return beam. The emitter and detector pixels are comprised of metamaterial plasmonic devices. The reflectance 1030 from the standoff media 1020 is determined by the surface and near surface permittivity at various depths from the surface of the standoff media 1020. The detectors 1050 and 1060 are structured to provide sensitivity over selected wavelength bands within the emitted spectrum of the emitter 1010. The emitter and detectors are disposed on at least two different micro-platforms within one or more pixels. The spectrometer is comprised of circuits 1070 for powering the emitter and providing signal conditioning for the detectors. In application the spectrometer may provide monitoring of spectral reflectivity of processed food, agricultural products and epidermal human skin or tissue.
Example 3
Absorptive Spectrometer
[0101] FIG. 11 depicts the pixel adapted to provide an absorptive spectrometer in this illustrative embodiment comprised of a broadband emitter 1120 and detector pixels 1150-1154 with an analyzing beam transmitted through a media of interest 1140. Optics 1130 is used to collimate the broadband emitted beam through the media 1140. Controller 1110 powers the temperature dynamics of the micro-platform of emitter 1120. Multiple detectors 1150-1154 detect the beam modulated by its traverse through the media of interest 1140. In embodiments, the emitters and detectors are comprised of plasmonic or nonplasmonic metamaterial devices. The detectors 1150-1154 are disposed on separate micro-platforms within one or more pixels. Control circuit 1110 implements a synchronized sampling link 1170 providing double-switched sampling of each detector 1150-1154. This synchronized sampling reduces noise originating from sources external to the emitter 1120, media 1140 and detectors 1150-1154.
Example 4
Infrared Communication System
[0102] FIG. 12 depicts a full duplex communication system comprised of a forward path emitter1 1220, detector1 1230 return path emitter2 1250 and detector2 1270. Transmit control1 and transmit control2 modulate the intensity of respective emitters 1220 and 1250. Receiver control1 and receiver control2 provide signal conditioning for respective detectors 1230 and 1270. The emitters and detectors are comprised of metamaterial plasmonic devices. In other embodiments, the system adapted with additional metamaterial thermal structures provide communication over multiple wavelength bands and with communication protocols such as FSK, FHSS and DSSS protocols.
[0103] It is to be understood that although the disclosure teaches many examples of embodiments in accordance with the present teachings, many additional variations of the invention can easily be devised by those skilled in the art after reading this disclosure. As a consequence, the scope of the present invention is to be determined by the following claims.
