Abstract
A thermal pixel configured as an electromagnetic emitter and/or an electromagnetic detector operating within a limited bandwidth. The thermal pixel comprises a micro-platform thermally isolated from a surrounding off-platform region by phononic nanowires. In embodiments, the micro-platform is comprised of metamaterial and/or photonic crystal filters providing operation over a limited bandwidth. In other embodiments, the micro-platform is comprised of nanotube structure providing a broadband emission/absorption spectral response. Structural configurations for the pixel take advantage of the Kirchhoff law of thermal radiation which states that a good thermal emitter is also a good absorber. In a preferred embodiment the pixel is fabricated using a silicon SOI starting wafer.
Claims
1. A pixel comprising a photonic source and/or photonic detector, wherein the pixel further comprises: a substrate having a substantially planar surface; a cavity formed from the substrate; a micro-platform disposed within the cavity, wherein the thermal micro-platform is suspended by nanowires, each of the nanowires suspended from the substrate, and the micro-platform comprised of one or more thermal elements and photonic structure, wherein: one or more of the nanowires is comprised of a first layer, the first layer comprised of phononic scattering structure and/or phononic resonant structure, wherein the phononic scattering structure and/or the phononic resonant structure reduces the thermal conductivity; the first layer is comprised of a semiconductor, wherein the semiconductor provides electrical conductivity; the one or more thermal elements selected from the group further comprising a temperature sensor device, a resistive heater and a cooling structure, and the photonic structure comprises one or more of photonic resonant structure and/or photonic nonresonant structure, wherein said structure enhances emissivity and/or absorptivity of electromagnetic radiation, the electromagnetic radiation comprising one or more bands within the range of visible, NIR, MWIR, LWIR and millimeter wavelengths or broadband radiation.
2. The pixel of claim 1 wherein the pixel is disposed within an array comprising a plurality of the pixel.
3. The pixel of claim 1 wherein the phononic scattering structure comprises scattering sites separated by distances less than the mean-free-path of heat-conducting phonons, wherein the phononic scattering structure decreases thermal conductivity in the first layer.
4. The pixel of claim 3 wherein the phononic scattering structure comprises porous silicon, wherein the porous silicon is obtained using a metal-assisted chemical etch.
5. The pixel of claim 1 wherein the phononic resonant structure comprises a phononic crystal (PnC) having a phononic bandgap, wherein the phononic bandgap decreases thermal conductivity in the first layer.
6. The pixel of claim 1 wherein the phononic scattering structure and/or phononic resonant structure comprises random or periodically disposed holes, vias, surface pillars, surface dots, plugs, cavities, ion-implanted elemental species and embedded particulates.
7. The pixel of claim 1 wherein the semiconductor of the first layer is selected from a group including Si, Ge, SiGe, GaAs, GaN, InP, SiC, TIN, Bi.sub.2Te.sub.3, Bi.sub.2Se.sub.3, CoSb.sub.3, Sb.sub.2Te.sub.3, La.sub.3Te.sub.4, SnSe, ZnS, CdS and alloys/superlattice combinations thereof.
8. The pixel of claim 1 wherein the first layer is further comprised of an ALD metal layer or metallic interstitial layer selected from the group aluminum, tungsten, palladium, platinum, molybdenum, gold and silver, the metallic layer or metallic interstitial providing an increase in nanowire electrical conductivity.
9. The pixel of claim 1 wherein the first layer is further comprised of a dielectric layer selected from the group comprising one or more of silicon dioxide, silicon nitride, hafnium oxide, silicon oxynitride, and aluminum oxide, the dielectric layer providing insulation between electrically conducting layers and/or a control of mechanical interlayer stress.
10. The pixel of claim 1 wherein the photonic resonant structure is comprised of metamaterial resonators operational with surface plasmons, split ring resonators (SRR) and photonic crystal (PhC) having deep subwavelength dimensions wherein the photonic resonant structure provides an increase in emissivity and/or absorptivity within a limited wavelength range.
11. The pixel of claim 1 wherein the photonic resonant structure comprises one or more of wavelength antennas, fractile antenna adaptations and LC resonant circuits, and further wherein the photonic resonant structure provides an increase in photonic emissivity and/or absorptivity within a limited wavelength range.
12. The pixel of claim 1 wherein the photonic nonresonant structure is comprised of one or more of carbon nanotubes, graphene mesh, gold black, carbon black and silicon grass, the photonic nonresonant structure providing an increase of the photonic emissivity and/or absorptivity of electromagnetic radiation over a broadband spectral range.
13. The pixel of claim 1 disposed within a plurality of the pixels wherein the cavities of each pixel share a common cavity space maintained in a vacuum condition, wherein the common cavity space comprises at least one unique pixel, the unique pixel further comprising a thermal element having a resistive heater, the resistive heater comprising a gettering material, and wherein the heater, when heated, creates an increased vacuum level within the common space.
14. The pixel of claim 1 comprising a photonic detector wherein the thermal element comprises a temperature sensor device selected from the group comprising a Seebeck thermoelectric device, thermistor, subthreshold MOST or bolometer.
15. The pixel of claim 1 comprising a photonic detector wherein a reverse-biased pn junction diode disposed either on the micro-platform or on the substrate and further wherein the reverse-biased pn junction provides sensitivity to visible and NIR radiation.
16. The pixel of claim 1 comprising a photonic detector further wherein the thermal element comprises a cooling structure, further comprising a Peltier thermoelectric device, and the Peltier thermoelectric device is operated to cool the thermal micro-platform.
17. The pixel of claim 1 comprising a photonic detector, wherein the cavity is hermetically sealed and maintained in a vacuum condition, and further wherein the nanowires provide thermal isolation from the substrate sufficient for the micro-platform to cool spontaneously via emitted thermal radiation.
18. The pixel of claim 1 comprising a photonic source, wherein the thermal element comprises a resistive heater, the resistive heater powered from an external power source, wherein the pixel provides a light emitting micro-platform.
19. The pixel of claim 1 configured for operation within one or more of an absorptive spectrophotometer, a reflective spectrometer and a multi-wavelength pyrometer.
20. The pixel of claim 1 comprising a photonic source and/or photonic detector, adapted to provide a transmitter and/or detector within a photonic communication system.
Description
BRIEF DESCRIPTION OF THE FIGURES
[0062] FIG. 1 is prior art plan view depicting a thermal micro-platform.
[0063] FIG. 2 is a prior art illustrative view depicting a nanowire with phononic structures providing a reduction in thermal conductivity.
[0064] FIG. 3 is a prior art cross-sectional view depicting a micro-platform released from a surrounding support platform using a backside etch.
[0065] FIG. 4 is a prior art cross-sectional view depicting a micro-platform released from a surrounding support platform using a topside etch.
[0066] 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.
[0067] 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.
[0068] FIG. 5C is a prior art cross-sectional view depicting a section of a nanowire comprised of a first, second and third layer.
[0069] FIG. 6A is a plan view depicting arrays of one and two-dimensional photonic structures comprised of a patterned ALD metal layer disposed on a micro-platform in accordance with embodiments of the invention.
[0070] FIG. 6B is a plan view depicting metamaterial photonic structures disposed on a micro-platform in accordance with embodiments of the invention.
[0071] FIGS. 6C, 6D and 6E depict cross-sectional views of metamaterial or photonic crystal structures comprising 2-layers, 3-layers and 4-layers, respectively, in accordance with embodiments of the invention.
[0072] FIG. 7 is a plan view depicting the pixel configured to provide a metamaterial infrared source in accordance with embodiments of the invention.
[0073] FIG. 8 is a plan view depicting the pixel configured to provide a metamaterial infrared detector in accordance with embodiments of the invention.
[0074] FIG. 9 depicts an apparatus comprising the pixel as a detector physically configured to provide a reflective spectrometer or pyrometer for monitoring the temperature of a remote surface in accordance with embodiments of the invention.
[0075] FIG. 10 depicts a reflective spectrophotometer incorporating pixels comprising an infrared beam source and an absorptive detector for analyzing spectra from a remote media in accordance with embodiments of the invention.
[0076] FIG. 11 depicts an absorptive spectrophotometer incorporating pixels in accordance with embodiments of the invention.
[0077] FIG. 12 depicts a transmitter and receiver within a communication system incorporating the pixel in accordance with embodiments of the invention.
[0078] FIG. 13A depicts a cross-sectional view of the pixel configured for operation as a passive blackbody with a self-cooled micro-platform in accordance with embodiments of the invention.
[0079] FIG. 13B depicts a cross-sectional view of the micro-platform of FIG. 13A physically configured as a self-cooled thermal detector in accordance with embodiments of the invention.
[0080] FIG. 14 depicts a cross-sectional view of the pixel configured for operation as a passive blackbody infrared emitter with a cooled micro-platform, partially exposed to incident LWIR radiation in accordance with embodiments of the invention.
[0081] FIG. 15 depicts an imaging detector pixel comprising the thermal pixel disposed within an imaging system for visible and LWIR.
DETAILED DESCRIPTION
[0082] Definitions: The following terms are explicitly defined for use in this disclosure and the appended claims:
[0083] micro-pIatform means a platform having a maximum dimension of about 100 nanometers on a side up to about 1 centimeter.
[0084] metamaterial (MM) structure as defined uniquely for this specification means a photonic structure within the micro-platform having deep subwavelength dimensions providing an enhancement of electromagnetic emissivity and/or absorptivity or a reduction in thermal conductivity. The metamaterial structure may be plasmonic or non-plasmonic.
[0085] metamaterial pixel or MM pixel in the present invention means a pixel structurally configured as a metamaterial (MM) structure.
[0086] photonic crystal (PhC) structure means a photonic structure comprising areas of varying permittivity within the micro-platform and having dimensions on order of a wavelength providing an enhancement of electromagnetic emissivity and/or absorptivity.
[0087] surface plasmonic polariton (SPP) means a surface electromagnetic wave guided along a metamaterial-patterned surface having sufficient electrical conductivity to support associated electron motion.
[0088] phononic crystal (PnC) means a metamaterial structure comprised of periodic nanostructure that affects the thermal energy transport of phonons.
[0089] nanowire means a suspended structure providing support for a micro-platform.
[0090] phononic nanowire means a suspended nanowire comprising phononic scattering and/or phononic resonant structure providing a reduction in thermal conductivity.
[0091] infrared as defined uniquely for this specification means electromagnetic radiation including NIR, SWIR, MWIR, LWIR, and millimeter wavelengths.
[0092] emitter or IR source means a source of electromagnetic radiation within the spectrum including visible light, SWIR, MWIR, LWIR millimeter wavelengths.
[0093] absorber means an absorber of electromagnetic radiation within the spectrum including visible, SWIR, MWIR, LWIR and millimeter wavelengths.
[0094] thermoelectric device means a semiconductor junction device for conversion of thermal energy into electrical energy as a Seebeck device or vice versa as a Peltier device.
[0095] bolometer means a very sensitive temperature sensing device wherein its resistance varies with temperature.
[0096] temperature control element means a thermal device such as an electrically-heated resistor for heating, a Peltier thermoelectric device for cooling, and a passive blackbody structure configured for cooling by thermal radiation.
[0097] temperature sensing element means a device for sensing temperature such as a Seebeck thermoelectric sensor, thermister, subthreshold MOS transistor (MOST), bandgap diode and a bolometer.
[0098] m, mm, um and pm respectively means a unit of length as meter, millimeter, micrometer, nanometer and picometer, respectively.
[0099] Cross-sectional views depicting the photonic surface elements of micro-platforms configured for infrared emission and absorption are depicted in FIG. 6A and FIG. 6B. These elements are disposed in a periodic matrix over an underlying dielectric film. In FIG. 6A all surface elements may comprise MMs and are scaled with deep subwavelength dimensions with the exception of element 504. Element 504 is an array of quarterwave antennas with primary application for absorptive heating with incident mm-wave irradiation. Element 502 of FIG. 6A in embodiments is also not necessarily an MM and may be configured as a conventional Bragg resonator, typically with spacings of near half-wavelength.
[0100] Elements 509-516 of FIG. 6B depict a micro-platform having a patterned metal surface of MM emitters and/or absorbers providing a surface plasmonic polariton (SPP) resonance with sensitivity over a limited bandwidth. Individual cells comprise single- and double-split ring resonators (SRRs). Element 514 comprises an array comprising two distinctively different single-SRR cells providing an emitter and/or absorber with two different or overlapping wavelengths.
[0101] The micro-platform can be configured with a photonic surface having a plurality of adjacent wavelength bands to provide an increased bandwidth. In embodiments, a micro-platform configured with two or more emitting and/or absorbing structures of different periodicity or shape is used to advantageously provide operation within the extended wavelength band 8-14 urn corresponding to a room temperature blackbody radiation primary spectrum.
[0102] MM structure provides an enhanced emissivity and absorptivity for emitter and detector functions. Each MM element supports a local resonant electromagnetic field which couples with fields originating from nearby elements within the larger MM matrix. When heated, the MM structure provides an efficient emitter of radiation, and, when not heated, it provides an efficient absorber of incident radiation. In some embodiments, the MM structure provides a thermal self-cooling when sufficient thermal isolation is provided to the micro-platform from local heat sources.
[0103] In embodiments, the photonic structure is a photonic crystal (PhC) having defect structure tuned for emission and/or radiation within an infrared bandwidth.
[0104] Each panel of FIG. 6A (501-508) shows portions of a larger array of patterned MM filter structures disposed on the micro-platform 610. The arrays of top-level structures depicted in FIG. 6A in embodiments comprise a patterned metal film over an underlying dielectric and metallic reflector. The individual elements 501-508 are of subwavelength dimension (as referred to free space wavelength). As an example, the principal wavelength of filter 501 is lower than that of filter 508 when the elements are disposed within a specific area. Panel 502 shows a 1-D Bragg grating structure which is polarization sensitive and configured as a MM element. 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 antenna array. All panels except 504 and 507 each provide a single, primary bandwidth filter band. Panels 504 and 507 are characterized by multiple primary operational wavelength bands. Secondary bands within each format are generally observed which derive from minor resonances associated with specific dimensions and couplings beyond nearest neighbor elements. Bandwidth is increased wherein elements are spaced more closely.
[0105] FIG. 6B is a plan view depicting additional arrays of MM plasmonic metamaterial elements. Panels 509 through 516 present portions of periodic arrays comprised of split-ring resonators. Panel 514 depicts an array portion operational with two primary wavelengths.
[0106] FIG. 6C is a cross-sectional view depicting MM plasmonic or PhC surface elements 620, such as those of FIG. 6A and FIG. 6B, disposed on a micro-platform substrate 610. The micro-platform 110 supports a field confinement adjacent to the surface elements 620. The patterned metal film 620 comprises raised, pillar-like structures on an underlying dielectric or semiconductor 110.
[0107] Metal films are chosen as the surface element 620 in many embodiments because metals provide a high plasma frequency and an increased density of electrons, especially for the shorter wavelengths, compared to a semiconducting or dielectric structural element. Advantageous maximum conductivity for these structured films is obtained with Au, Ag, and Cu metallization. Other preferred metals for performance over a wide range of wavelengths include W, Pd, Pt, Ni, Al, and Ti. The patterned, metallic MM elements are typically of thickness in the range of 10 nm to 200 nm.
[0108] FIG. 6D is a cross-sectional view depicting plasmonic elements 620 disposed on a micro-platform substrate 610 with an intermediate dielectric film 630. The dielectric selected for low loss at the wavelength of interest and in some embodiments it is 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 um.
[0109] 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 metallic films 620, 640. In embodiments, the metallic films increase confinement of the electromagnetic field associated with the surface plasmonic structures 620 and increase overall emissivity and/or absorptivity.
[0110] In other embodiments, a semiconductor or dielectric is structured as a PhC to provide emission and/or absorption over a limited wavelength range. In embodiments, these PhC structures may be raised areas, pillars, cavities, and holes wherein radiation wavelengths are determined by the defect structure within the PhC.
[0111] Tri-level MM photonic structures 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 some embodiments, the multi-layering concept of FIG. 6E is extended to provide more than three layers. In some embodiments, stacked layers provide an emitter and/or absorber with bandwidth determined by both surface MM structures and thickness of multiple underlying dielectric layers (as in E. Raphaeli et al, 2013). 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 or widening of the emission and/or absorption bandwidth.
[0112] In embodiments, the bandwidth for operation can be significantly reduced by designing structures for Fano resonance which is typically of high-Q. This resonance can be obtained with plasmonic nanostructures of several types including plasmonic holey or particle arrays and even Bragg diffraction gratings.
[0113] FIG. 7 is a plan view depicting a semiconductor chip configured with a MM surface 780 to provide an infrared emitter. The surface 780 acts as a filter for blackbody-type thermal radiation spectrum derived from heating of the micro-platform 110. Thermal control elements disposed on the micro-platform 110 are resistive heaters (710 and 711). The resistive heaters receive power through contact pads 770, 750, 760, and 770. In some embodiments, one resistor 711 is operated as a resistive heater and resistor 710 is operated as a thermistor temperature sensor. Temperature control elements 710, 711 are typically an ALD metal film electrically insulated from the underlying micro-platform 110. The circular micro-platform 110 can have reduced physical stress within itself and onto support nanowires 720,730 permitting fabrication of larger micro-platforms 110 over the underlying cavity 125. 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.
[0114] In embodiments comprising the emitter of FIG. 7, the radiative structure 780 may comprise a selection including, without limitation, the structural options of FIGS. 6A and 6B. In the illustrative embodiment of FIG. 7, micro-platform 110 and nanowires formed of the active layer from a silicon SOI starting wafer can provide a controlled heating to temperatures as high as around 550 C. In other embodiments based in FIG. 7, the micro-platform and nanowires are comprised of silicon carbide or gallium nitride semiconductor films providing structural rigidity at temperatures of over 1000 C. Dielectric passivation films typically comprise one or more of silicon nitride, aluminum oxide, silicon dioxide and hafnium oxide.
[0115] Metal films are typically deposited prior to lithographic patterning with a DC magnetron tool. Dielectric films are generally deposited by RF sputtering. Patterning of these thin films is accomplished using a resist such as patterned PMMA with a lift-off process.
[0116] Backside etch to form cavity 125 is accomplished with DRIE or with patterned TMAH or KOH at an elevated temperature. Alternatively, topside formation of the cavity 125 is accomplished using a hot vapor HF etch or XeF.sub.2 with a patterned passivation layer of material such as Si.sub.3N.sub.4 protecting certain topside areas as desired.
[0117] FIG. 8 is a plan view depicting a pixel adapted with surface structure 780 to comprising any of the illustrative filters of FIGS. 6A and 6B to provide an infrared detector. In this illustrative embodiment, the detector is comprised of a Seebeck sensing element and a Peltier controlled-cooling element. Micro-platform 110, supported by nanowires (depicted by 820,830), is disposed over cavity 125 with surrounding support platform 102.
[0118] Nanowires are comprised of heavily doped p+ and n couplings (depicted as 820 and 840) connected in series to provide a first thermocouple series-connected between bonding pads 810,820. The first thermocouple is operated as a Seebeck sensing element to monitor incremental changes in temperature of the micro-platform in response to absorbed incident infrared radiation. A second series connection of thermocouples connected between bonding pads 860,880 is operated as a Peltier cooler. This Peltier cooler reduces micro-platform 110 temperature thereby advantageously reducing the thermal noise from the first thermoelectric sensing element.
[0119] In this illustrative depiction, the Seebeck sensor array of FIG. 8 comprises only two thermocouples. In embodiments, the micro-platform is populated with up to 2000 series-connected thermocouples, providing an increase in overall pixel responsivity (Volts/Watt) for pixel operation as a detector. In embodiments comprising the detector of FIG. 8, a MM infrared absorbing structure may include, without limitation, the first layer structure options of FIGS. 6A-6D providing sensitivity over a limited wavelength range. In some embodiments, the infrared absorbing structure 780 is physically configured alternatively as a broadband absorber comprising one or more of carbon nanotubes, a graphene mesh, carbon black, gold black and silicon grass.
[0120] In 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 in Seebeck and thermistor structures due to the shunt effect of parasitic resistance in the high resistivity areas is thereby advantageously designed to be minimal. For pixels formed from a starting silicon SOI wafer, 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 having boron or phosphorus. DC sputtered aluminum interconnections are typically provided for metallization on micro-platforms 110 operated at temperatures up to 550 C. In application, standard process integrated circuits formed in the surrounding support platform 102 are maintained at room temperature. 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.
[0121] Pixels depicted in illustrative examples FIG. 7 and FIG. 8 permit fabrication of both infrared sources and detectors based on similar processes with lower cost production. The pixels of FIG. 7 and FIG. 8 are, in some embodiments, fully compatible with industry CMOS foundry production cleanroom processes.
[0122] 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 pixel environment. In other embodiments, the pixel is disposed within a vacuum package wherein a further thermal isolation of the micro-platform 110 is provided.
[0123] In some pixel encapsulation embodiments, an additional resistive heater is disposed off-platform in thermal contact with a gettering material such as Bi or Ti. This heater for gettering may be disposed on a dedicated thermal micro-platform. When the resistive heater is powered, the gettering material is activated and the target gas component within the pixel enclosure is reduced.
Example 1
Multi-Wavelength Pyrometer
[0124] FIG. 9 depicts an apparatus comprised of multiple detector micro-platforms similar to the depiction of FIG. 8, physically configured to provide a standoff infrared analyzer for monitoring the temperature of thermal radiation 910 from 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 920 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. The pyrometer is calibrated using standoff media 920 of known temperature or known emissivity.
Example 2
Reflective Photospectrometer
[0125] FIG. 10 depicts the pixel configured to provide a reflective photospectrometer comprising both an infrared source and an infrared detector for spectral analysis of reflectance from a standoff media. This configuration permits the use of a pulsed, infrared emitter 1010 and synchronous detection with detectors 1050, 1060 thereby increasing the signal-to-noise ratio of detected infrared reflection from media 1020. The spectrometer is comprised of both an emitter 1010 which illuminates a standoff media 1020 through focusing optics 1040. Detectors depicted as 1050 and 1060 monitor different wavelengths of reflection from media 1020. Platforms comprising the emitter 1010 and detector 1050,1060 are typically comprised of MM plasmonic absorbers. 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 applications, the spectrometer may provide monitoring of spectral reflectivity for building structures, processed food, agricultural fields, agricultural products and surface/internal imaging of objects.
Example 3
Absorptive Photospectrometer
[0126] FIG. 11 depicts the pixel adapted to provide an absorptive photospectrometer wherein synchronization for double-switched noise is provided with control and detection circuits. This type of synchronized switching is also known as correlated double-sampling (CDS) and is well known to those skilled in the art as a means for reducing noise originating between the two sampling switches. This illustrative embodiment is comprised of an infrared emitter 1120 which transmits an infrared broadband beam with collimating optics 1130 transmitting through a semi-transparent media of interest 1140. The infrared beam attenuated by media 1140 terminates into infrared detectors 1150-1154. These detectors are each sensitive to a preselected wavelength band or multiple wavelength bands. The plurality of detectors tuned to various infrared wavelengths are disposed on separate micro-platforms. Controller 1110 temperature-cycles the micro-platform in synchronization with the detector MUX, LNA and digitizing signal conditioning circuits. In a typical embodiment, the photospectrometer components are disposed within a single enclosure having a cavity or other structure for the media of interest 1140. In embodiments, the emitter and detectors are comprised of plasmonic or nonplasmonic metamaterial devices. In some embodiments, the emitter and/or detectors are comprised of PhC filters.
Example 4
Emitter and Receiver for Infrared Communication
[0127] In embodiments, the thermal micro-platform is physically configured as an antenna within an infrared communication apparatus. Infrared emission from the thermal micro-platform is modulated by pulsed heating of the micro-platform thereby providing a transmitting antenna for the communication system. The same micro-platform or a different micro-platform is configured as a detector providing a receiving antenna for the communication system.
[0128] Photonic structure in the micro-platform may comprise a MM filter to provide communication over a limited infrared wavelength range. In some embodiments, the micro-platform may be configured for broadband communication with a non-resonant filter comprised of one or more of vertically-aligned nanotubes (especially carbon nanotubes), mesh of graphene, gold black, silicon grass or carbon black.
[0129] FIG. 12 depicts pixels disposed within a full duplex communication system comprised of a forward path emitter1 1220 as transmitting antenna providing an infrared beam controlled by transmit controller 1230. The forward beam is received into detector1 1230 comprising a thermal micro-platform receiving antenna and processed further through receiver control1 1240. Similarly, the return signal transmitted as an infrared beam from emitter2 1460 as a transmitting antenna and received into detector2 1270 with further processing through receiver control2 1280. The emitters and detectors are comprised of M and/or PhC devices within separate pixels to provide communication over a limited bandwidth. Detection is obtained with Seebeck thermal sensing elements disposed in detectors 1230 and 1270. In embodiments, the apparatus is physically configured with additional photonic thermal emitters and detectors to provide communication over multiple wavelength bands and with communication protocols such as FSK, FHSS and DSSS protocols.
Example 5
Passive Micro-Refrigerator
[0130] FIGS. 13A and 13B depict the pixel wherein the micro-platform is physically configured as a passive blackbody cooler based on its infrared source of radiation 1311. In this embodiment, blackbody radiation from the micro-platform 1301 provides a net thermal cooling of the micro-platform. Cooling of the micro-platform is obtained by reducing external heating to extreme levels. In this embodiment, the micro-platform is cooled without enabling Peltier thermoelectric cooling. The pixel is physically configured with phononic nanowires 214 of sufficiently low thermal conductivity and disposed within an enclosure wherein the internal surface of the enclosure does not radiate. Micro-platform 1301 is maintained under vacuum or a gas of low thermal conductivity within cavity125.
[0131] The thermally-isolated micro-platform 1301 has application as a passive cooler for electronic components dissipating extremely low power, typically providing a reduction in thermal noise for the electronic components.
[0132] FIG. 13A depicts a cross-sectional view of a passive, self-cooled pixel wherein micro-platform 1301 is configured to provide a maximum blackbody emissivity over the spectral range 8 to 14 um wavelength of room temperature radiation and thus maximize the cooling effect. In this wavelength range the spectral radiance of a blackbody is in the range of 55 W/m.sup.2/sr at room temperature. In this embodiment, micro-platform 1301 comprises active silicon layer 1307 covered by a 3-layer metamaterial sandwich further comprised of a topside array of patterned MM structure 1309 over dielectric film 1308 and reflector layer 1310. The 3-layer sandwich comprises a MM emitter, typically designed for surface plasmonic polaritons tuned for electromagnetic emission 1311 in the NIR, MWIR and LWIR wavelength range. The electromagnetic emission 1311 originates as spontaneous blackbody radiation from the surface of micro-platform 1301. It propagates through vacuum cavity 125 and is collected into the field of non-reflecting, cone-shaped silicon absorbers 1304. Absorber 1304 is a micro-scale analog of the cone-structures of RF anechoic chamber walls. A shiny metal film 1306 is disposed on the upper surface of cap structure 1302 to reflect unwanted heat from external sources such as solar. The topside structure is formed from a silicon wafer and bonded to the underside structure foundation 342. The wall 1303 is defined with a through-silicon-via (TSV) etch process step following wafer bonding.
[0133] In another embodiment, the micro-platform 1301 of FIG. 13A is configured only with a field of nanotubes, typically vertical wall carbon nanotubes (VWCNT) thermally-connected with a support micro-platform and without MM structuring. The nanotube textured surface also provides an increased spectral range for broadband blackbody emission.
[0134] Blackbody electromagnetic radiation within the increased wavelength range 2 to 20 um provided by nanotube absorbers is in the range of 108 W/m.sup.2/sr. In embodiments, the micro-platform infrared-absorbing area ranges from 10.sup.10 to 10.sup.4 m.sup.2. In embodiments of the present invention, wherein the thermal conductivity of supporting nanowires and other thermal conductors is reduced into the nanoWatt/ C. range, self-cooling can occur.
[0135] Other salient features in FIG. 13 are the supporting nanowires 214, silicon oxide layer 344 of the starting SOI wafer, eutectic or conducting epoxy bond layer 354 between the silicon foundation 342 and header 352. Bonding pads 350 depict electrical connections through nanowires to passive and nanopower devices and circuits disposed on micro-platform 1301.
[0136] A very, very large-scale example of passive radiative surface cooling by blackbody-type radiative emission is provided by the ground surface in Antarctica where thermal ground radiation exits through the atmosphere due to extremely low humidity and limited solar heating. Ground surface temperatures in Antarctica are cooled below the temperature of surrounding atmosphere. This occurs because ground surface temperature is reduced to provide thermodynamic equilibrium with thermally radiated heat exiting through the atmosphere. This is in contrast with ground surfaces elsewhere in the world wherein thermal radiation from atmospheric water vapor and increased solar energy absorbed into the ground is greater than the surface radiative cooling.
Example 6
Passively-Cooled Infrared Detector
[0137] FIG. 14 depicts the pixel comprising micro-platform 1301 in another embodiment wherein the micro-platform is cooled by radiative blackbody radiation 1311 into space 125. In this embodiment, radiation 1312 originating externally is limited and the net radiation 1312, 1311 cools the micro-platform 1301. This embodiment differs from that depicted in FIG. 13 wherein blackbody radiation from photonic structure passes through a pixel window that is transparent for LWIR radiation 1312, 1311 entering from both directions. In a typical embodiment, the enclosing header 352 is extended to comprise an infrared-transparent cover, and micro-platform 1311 is maintained under vacuum or a gas of lowest thermal conductivity.
[0138] In this embodiment, thermal radiation 1312 originating from a remote object or scene is focused onto the self-cooled micro-platform 1301 to provide an incremental heating, typically at the nanoWatt level. In this embodiment, an apparatus is physically configured with appropriate optics and an external IR source is focused onto the micro-platform 1301. This incident radiation 1312 heats micro-platform 1301 and is sensed by Seebeck sensing elements disposed on the micro-platform. In other embodiments, the micro-platform is configured with a field of nanotubes providing a broadband blackbody emissivity for cooling. We note that the intensity of external radiation 1312 is restricted by design and therefore the sensitivity of the detector to incident radiation is reduced.
Example 8
Pixel within a Focal Plane Array Imager
[0139] FIG. 15 depicts the thermal pixel disposed within the imaging detector pixel of a focal plane array (FPA) imager 1500 comprised of both thermoelectric Seebeck elements 1631,1604 for sensing incident infrared radiation and a pn junction photodiode element 1620 for sensing visible and NIR radiation. FIG. 15 also depicts the peripheral CMOS circuitry within the pixel including row address drivers 1504, column drivers 1506 and column signal conditioning circuits 1508. The pixel signal output is provided as differential levels through column wires 1625 and 1630. The imaging pixel is replicated within the FPA in a two-dimensional matrix. Each pixel is powered through lines V.sub.CS 1606 and V.sub.GND 1638. Each pixel is addressed through lines V.sub.CS, V.sub.IR/VIS and V.sub.REF.
[0140] The thermoelectric sensor elements provide a passive signal source and are directly addressed through lines V.sub.IR/VIS and V.sub.REF. The imaging pixel is switched between infrared and visible wavelength bands through line V.sub.IR/VIS. The thermoelectric signal is amplified through separate inverters T.sub.1 1624, T.sub.2 1622 and T.sub.4 1626, T.sub.5 1634. The thermoelectric sensor elements provide a differential signal with respect to ground line V.sub.GND 1638.
[0141] Detection of visible light is obtained with pn junction sensor element D 1620, reverse biased by gate transistors T.sub.7 1621 and T.sub.8 1632 to charge capacitor C.sub.1 1633 in response to incident visible radiation. Sensor element D is disposed outside the micro-platform 1636. The signal on capacitor C.sub.1 1633 is further amplified through the inverter comprising T.sub.1 1624, T.sub.2 1622 providing signal to lines S.sub.1 1625 and V.sub.GND 1638.
[0142] Signals received during successive time intervals from enabled imaging pixels are processed by the column signal conditioning circuit and formatted appropriately for image display or other purposes.
[0143] 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.
