PATENT ABSTRACT
An “integrated” Fabry-Perot interferometer, such as for use in a spectrophotometer, is fabricated by attaching two micro-machined semiconductor-on-insulator wafers to one another. One mirror is formed on each micro-machined wafer. One mirror is supported by a thermally insulated, suspended micro-platform. In some embodiments, interferometer cavity length is adjustable. Detectors are disposed at least partially within the micro-platform. In some embodiments, the interferometer, a light source, and other circuitry and components, such as wireless communications components, are contained in a sealed package that includes a sampling region, thereby providing an integrated spectrophotometer. The integrated spectrophotometer can be implanted, for example, in animal tissue environments, such as for analyzing various compounds in the blood.

PATENT DESCRIPTION
FIELD OF THE INVENTION 
     The present invention pertains to spectrophotometers. 
     BACKGROUND OF THE INVENTION 
     Spectrophotometry is the quantitative measurement of the reflection or transmission properties of a material as a function of wavelength. Spectrophotometry is commonly used to measure the transmittance or reflectance of solutions, transparent or opaque solids, or gases. The device that performs this measurement is known as a “spectrophotometer”. 
       FIG. 1  depicts a block diagram of a typical prior-art spectrophotometer  108  in use performing a spectral assay of media  104 . Spectrophotometer  108  includes Fabry-Perot interferometer  112 , detector(s)  116 , and processor  120 . 
     Interrogating light  102  emitted from broadband light source  100  is directed towards media  104 . The light is dispersed, via reflection, absorption, etc., as it passes through media  104 . The dispersion alters the spectral content of the interrogating light. The specifics of the alteration depend on and can be characteristic of media  104 . As a consequence, analysis of spectrally altered light  106  can provide information about media  104 . This information is “extracted” using interferometer  112 , detector(s)  116 , and processor  120 , as discussed further below. 
     Spectrally altered light  106  enters Fabry-Perot interferometer  112 . Wavelengths of spectrally altered light  106  that resonate within interferometer  112  form filtered exit light  110 . In this fashion, interferometer  112  selectively filters spectrally altered light  106 . 
     Filtered light  110  exits interferometer  112  and is directed to detector(s)  116 . In some prior-art spectrophotometers, detector(s)  116  are sensitive to certain wavelengths of electromagnetic (EM) radiation and generate electrical signals  118  (i.e., a photocurrent) when such wavelengths are detected. The amplitude of signals  118  is indicative of the light intensity at the particular wavelength. Signal(s)  118  from detector  116  are conditioned (analog-to-digital conversion, etc.) and transmitted to processor  120 . In the processor, signal(s)  118  are processed via a Fourier transform or related algorithms to provide assay  124  of the spectral content of filtered exit light  110 . 
     As previously noted, filtered exit light  110  will contain wavelengths corresponding to the resonances of the interferometer cavity. Analysis of those particular wavelengths will rarely provide a complete spectral analysis of spectrally altered light  106 . Consequently, as part of the spectrophotometry process, the resonant wavelengths of interferometer  112  are altered. In some spectrophotometers, this alteration is implemented by changing the cavity length of the interferometer, such as via cavity-length controller  114 . Each such alteration will change the spectral content of exit light  110 . In this fashion, a wavelength sweep is performed, wherein for each change in cavity length (and, hence, spectral content of the exit light  110 ), the detection operation is repeated. This ultimately provides a complete spectral analysis of media  104  (assuming that the frequency sweep is large enough). The spectral analysis, which provides light intensity as a function of wavelength, can be used as a fingerprint (for identification purposes) and/or as a means to quantify the amount of media that is present. Identification and/or quantification involves a comparison of the spectral analysis to a database that provides compound identification as a function of spectrum or concentration (of a particular media) as a function of spectrum. 
     An embodiment of conventional Fabry-Perot interferometer  112  is depicted in  FIG. 2 . Interferometer  112  consists of two spaced-apart mirrors  226  and  228 . The mirrors are typically “highly” reflective, such that most of the light impinging on them is reflected. The change in the “thickness” of the lines that are representative of light “beam” is intended to be (qualitatively) indicative of the attenuation of the transmitted intensity resulting from reflections at mirror surfaces. 
     The portion of light  106  entering interferometer  112 A makes multiple (partial) reflections between mirrors  226  and  228 . Although depicted as a single coherent beam (like a laser beam), spectrally altered light  106  is in the form of a broad plane wave comprising multiple wave fronts. Constructive interference (resonance) occurs if the transmitted light is in phase, and this corresponds to a high-transmission peak of the interferometer. If the transmitted light is out-of-phase, destructive interference occurs and this corresponds to a transmission minimum. 
     The resonant wavelengths of a Fabry-Perot interferometer are a function of the angle that light travels through the interferometer, the size of gap between the mirrors (i.e., cavity length) and the refractive index of the medium between mirrors. For fixed values of those parameters, the wavelength of the reflected light determines whether that light is “in phase” or “out-of-phase”. 
     The resonant wavelengths of a Fabry-Perot interferometer can be altered by changing its cavity length. Cavity length can be changed via cavity-length controller  114  (see  FIG. 1 ), which in interferometer  112  depicted in  FIG. 2  comprises electrostatic actuator  230 . 
     Electrostatic actuator  230  includes controlled voltage source  232 . Mirrors  226  and  228  are electrically conductive, so that when a voltage is applied across them, an electrostatic force of attraction results. Mirror  226  is suspended (e.g., from a stationary substrate, etc.) via tethers  234  that enable mirror  226  to move. Consequently, when a voltage is applied across mirrors  226  and  228  creating an electrostatic force of attraction, tethered mirror  226  moves toward mirror  228 . This movement reduces the size of gap G compared to the quiescent state in which no voltage is applied. Within the range of movement of mirror  226 , the size of gap G is a function of voltage. Since, as already indicated, a change in cavity length alters the resonances of the interferometer, the transmission spectrum as a function of wavelength for interferometer  112  can be altered via electrostatic actuator  230 . 
     Most prior-art spectrophotometers are fabricated with minimal integration of elements. This affects cost and also limits the type of applications in which such spectrophotometers can be used. 
     SUMMARY OF THE INVENTION 
     Embodiments of the present invention provide a highly integrated Fabry-Perot interferometer and a highly integrated spectrophotometer. 
     In some embodiments, the invention provides an “integrated” Fabry-Perot interferometer with an adjustable cavity length. The integrated interferometer includes (in addition to the interferometer itself), one or more actuation structures for controlling cavity length and one or more detectors. 
     In the illustrative embodiment, the integrated interferometer is fabricated by attaching two micro-machined semiconductor-on-insulator wafers to one another. One mirror is formed on each such wafer. In the illustrative embodiment, one of the wafers is machined to provide a thermally insulated, suspended “micro-platform” comprising at least a layer of single crystal silicon. The micro-platform supports one of the two interferometer mirrors. Detectors are formed at least partially within the micro-platform. In the illustrative embodiment, the detectors are thermal detectors. Very small electrical conductors, referred to herein as “nanowires,” which can form part of the detectors, provide electrical connection between the micro-platform and “off-platform” electrical contacts, located elsewhere in the interferometer, for extracting the detector signals for processing. The nanowires are also used for applying a voltage across the mirrors for electrostatic control of interferometer cavity length. 
     In some alternative embodiments, rather than having an adjustable cavity length, an interferometer in accordance with the present teachings includes multiple cavities, each having a different fixed cavity length. 
     In some embodiments, the integrated Fabry-Perot interferometer forms part of a spectrophotometer. In addition to the interferometer, the spectrophotometer includes hardware/software for processing the signals generated by the detectors of the interferometer. 
     In some embodiments, the spectrophotometer also includes one or more of: a light source, a region for receiving an analyte of interest, a means of calibration, and a power supply (optionally energy-harvesting). In embodiments that include a light source and power supply, the spectrophotometer can be contained within a sealed package and configured for remote wireless (e.g., RFID, etc.) operation. Remote operation enables, for example, implanting the spectrophotometer in animal tissue environments, such as for analyzing various compounds in the blood. 
     Embodiments of the spectrophotometer can be used, for example, to determine the identity of a compound (e.g., glucose, oxygen, markers, etc.) and its concentration in a media (e.g., blood or other fluid, etc.). This is accomplished, for example, by comparing measurements obtained by the spectrophotometer to a reference file for the compound. The reference file includes information such as the wavelength spectrum of the compound, intensity-versus-wavelength values for the compound at varying concentrations, and the like. 
     As previously noted, in some embodiments, an integrated spectrophotometer is provided. The integrated spectrophotometer is physically adapted to be implanted within animal tissue, including a human body (earlobe, finger, or skin flap, etc.) to provide an assay of one or more of glucose, oxygen, and other analytes in body fluid. Physical adaptations for such applications include, without limitation, an ability to transmit data and/or receive power wirelessly. 
     In some other embodiments, the spectrophotometer is used in non-biological applications, such to analyze feed and effluent streams for laboratory chemical reactors or analytical instruments, and can even be used within such reactors and instruments. The spectrophotometer can be configured for placement in chemical production facilities to detect leaks and products of chemical reactions. The spectrophotometer can be configured for placement down-hole with petroleum exploratory drilling rigs for the purpose of analyzing a liquid or gas. This enables in-situ analysis without having to extracting the liquid/gas to the surface. 
     The spectrometer can be used with sources of electromagnetic energy such as sunlight, emissions from an explosion or combustion event, blackbody emissions from a remote scene, or modulated signal beams. Consequently, embodiments of the spectrophotometer can configured to detect and analyze spectral components of light created during explosions, including detection of toxic gases. The spectrophotometer can be configured to monitor absorption spectra of sunlight as filtered by a media, such as smokestack effluents, thereby monitoring, for example, coal ash and the like. In some embodiments, the spectrophotometer can be configured for analyzing multiple infrared wavelengths so as to monitor the“blackbody” emission spectrum from remote scenes and objects to determine the surface temperature thereof. 
     The spectrophotometer can be configured for hand-held use thereby providing a highly mobile unit enabling movement and placement not previously practical with many prior-art spectrophotometers. 
    
    
     
       BRIEF DESCRIPTION OF THE FIGURES 
         FIG. 1  depicts a prior-art spectrophotometer including a conventional Fabry-Perot interferometer configured with discrete components. 
         FIG. 2  depicts a conventional Fabry-Perot interferometer. 
         FIG. 3  depicts a spectrophotometer including an integrated Fabry-Perot interferometer in accordance with an illustrative embodiment of the present invention. 
         FIG. 4  depicts an integrated spectrophotometer in accordance with an illustrative embodiment of the present invention. 
         FIG. 5  depicts an integrated Fabry-Perot interferometer in accordance with a first illustrative embodiment of the present invention. 
         FIG. 6A  depicts starting semiconductor-on-insulator wafers for fabricating the integrated Fabry-Perot interferometer in accordance with an illustrative embodiment of the present invention. 
         FIG. 6B  depicts the starting wafers of  FIG. 6A  partially patterned. 
         FIG. 7  depicts further detail of an embodiment of the integrated Fabry-Perot interferometer of  FIG. 5  utilizing the wafers of  FIGS. 6A and 6B . 
         FIG. 8A  depicts a cross-sectional view of the integrated Fabry-Perot interferometer shown in  FIG. 6  through the line A-A in the direction shown. 
         FIG. 8B  depicts a perspective view of a portion of the integrated Fabry-Perot interferometer shown in  FIG. 6 . 
         FIG. 9  depicts a cross-section view of an integrated Fabry-Perot interferometer in accordance with a second embodiment of the present invention. 
         FIG. 10A  depicts a cross-sectional view of an integrated Fabry-Perot interferometer in accordance with a third embodiment of the present invention. 
         FIG. 10B  depicts a cross-sectional view of the integrated Fabry-Perot interferometer shown in  FIG. 10A  through the line B-B in the direction shown. 
     
    
    
     DETAILED DESCRIPTION 
     Definitions 
     The following terms are explicitly defined for use in this disclosure and the appended claims:
         “infrared” refers to the broad range of photon wavelengths in the range from visible light at 700 nm to 100 microns, including the NIR, Mid-IR, LWIR, and ULWIR wavelength bands.   “micro-platform” means a patterned layer having dimensions of about 100 nanometers on a side up to about 1 centimeter on a side.   “nano-dimensioned” or “nano-sized” or “nanometer sized” means a structure whose controlled dimension is less than 1 micron (1000 nanometers).   “nanowires” are very small (nano-dimensioned) electrically conductive elements. Although nanowires can include metallization (they could alternatively be appropriately doped to provide electrical conductivity), the structure thereof is based on a non-metallic material, such as a semiconductor or electrically insulating material.   “quiescent state” means a non-actuated or non-energized state.   “RFID” refers to a two-way wireless communications protocol.   “Semiconductor-on-insulator” refers to a wafer typically having a three layers including an “active” layer, a “buried oxide layer” (“box”) layer, and a “handle” layer. The box layer is sandwiched by the active and handle layers. The most common semiconductor-on-insulator wafer has traditionally included a silicon device layer, a silicon dioxide box layer, and a silicon handle. This wafer is usually referred to as an “SOI” wafer. More recently, semiconductor-on-insulator wafers including: silicon-germanium alloy/silicon oxide/silicon handle, germanium/silicon oxide/silicon handle, and other combinations including various semiconductors and dielectric films are now available.   “supported by” means that, for example, one layer is supported by, but not necessarily disposed on, another layer. For example, if a third layer is disposed on a second layer that is, in turn, disposed on a first layer, the third layer is “supported by” (but not “disposed on”) the first layer.       

       FIG. 3  depicts the salient features of spectrophotometer  308  in accordance with the present teachings. The spectrophotometer includes integrated Fabry-Perot interferometer  312  and electronic digital and analog circuitry and software  120 . 
     In operation, Interrogating light  102  is emitted from light source  100 . Light source  100  can be a broad-band or narrow-band source of light for emitting wavelengths of interest, including visible and infrared light, as a function of the analyte being interrogated. Light source  100  can be, without limitation, an LED, a quantum cascade laser, or a heated blackbody including environmental sunlit scenes. 
     Interrogating light  102  is passed through analyte  104 , which is at least partially transparent. More precisely, the analyte is typically not transparent. However, in such cases, it is usually dispersed within a transparent or partially transparent media, such as blood, water, other liquids, gases, etc. The spectral content of interrogating light  102  is altered by virtue of passing through the analyte, resulting in spectrally altered light  106 . This spectral alteration is due to the absorption and/or dispersion of certain wavelengths of the interrogating light. Spectrally altered light  106  is directed into integrated Fabry-Perot interferometer  312 . 
     With continued reference to  FIG. 3  and now referring to  FIG. 5 , integrated Fabry-Perot interferometer  312  includes two partially and highly reflective mirrors  504  and  506 . Cavity  502  is defined between the mirrors; the length of cavity  502  is the size of the gap between the two mirrors. Mirror  504  is disposed on a platform that resides on a suspended support layer. As a consequence of this arrangement, mirror  504  is movable. On application of a voltage across mirrors  504  and  506 , which results in an electrostatic force-of-attraction, the platform and mirror  504  move towards mirror  506 . This alters the gap between the mirrors (i.e., alters the length of cavity  502  of interferometer  312 ). In this context, the mirrors must be electrically conductive. More precisely, either the mirrors, or a layer associated therewith, must be electrically conductive. Thus, as used herein, the term “electrically conductive,” when used to describe a mirror or a highly-reflective surface, means that the either the mirror/surface or something attached to it is electrically conductive. 
     The light is spectrally filtered in Fabry-Perot interferometer  312  in conventional fashion. As mentioned in the Background section, the filtering is a function of the resonant wavelengths of the interferometer and those resonances are a function of interferometer cavity length, among other parameters. Although the spectral filtering is dependent on other parameters as previously mentioned, it is cavity length that is varied in the illustrative embodiments. 
     The spectrally filtered light exiting interferometer cavity  502  through mirrors  504  and  506  passes into adjacent layers of material. Light passing through mirror  504  enters the platform, which contains detectors. In the illustrative embodiment, the detectors are thermocouples that are series connected to form a thermopile. The filtered light raises the temperature of the platform above that of the surrounding layer. As a consequence, a (Seebeck) voltage is generated, in known fashion, from the thermocouple array. The thermal detectors operate in analog fashion; that is, the amplitude of the voltage generated is proportional to the power absorbed by the platform from the light. Thus, the amplitude of the voltage is a function of the light intensity at a particular wavelength. The first mirror on the micro-platform is electrically connected by nanowires to an aluminum interconnect patterned on the first silicon substrate. 
     Cavity length is periodically changed to alter the resonant frequencies of interferometer  312 . As previously noted, cavity length is changed electrostatically by applying a voltage across the mirrors. The voltage is applied via a controlled voltage source. For each such periodic change, signal voltages are generated by the detectors. 
     The relationship between the applied voltage and the wavelengths of the light exiting the interferometer can be determined in known fashion. Using that relationship, in conjunction with the amplitude of the voltage generated by the detectors during each period, information concerning light intensity as a function of wavelength can be obtained. 
     The detector signals  118  are transmitted off-platform to electrical contacts situated elsewhere in the interferometer. From these contacts, signals  118  are transmitted to electronic circuitry  120  (external to the interferometer). Electronic circuitry  120  includes, without limitation, signal conditioning (reduce noise), an analog-to-digital converter, a suitably programmed processor, processor-accessible memory, and wires for conducting electrical signals to and from various components/structures of integrated Fabry-Perot interferometer  312 . The processor includes, without limitation, algorithms for processing the detector signals, such as via a Fourier transform or variations thereof, algorithms for controlling and varying the cavity length, and, optionally, algorithms for comparing the processed information with reference information about the analyte that is stored in the processor-accessible memory. In this fashion, a complete spectral assay of the light that resulted from interrogation of the analyte is obtained and can be used to determine qualitative and quantitative information about the analyte. 
     Further detail of an embodiment of integrated Fabry-Perot interferometer  312  is provided later in this specification in conjunction with  FIGS. 6A, 6B, 7, 8A, and 8B . 
       FIG. 5  depicts light  106  entering interferometer  312  via “upper” mirror  506 . Those skilled in the art will appreciate that light  106  could also (or alternatively) enter interferometer  312  through “lower” mirror  504  and be processed in essentially the same fashion. 
     In spectrophotometer  308 , neither light source  100  nor electronic circuitry  120  is co-located in a housing with integrated Fabry-Perot interferometer  312 . 
     In accordance with some embodiments of the invention, a fully integrated spectrophotometer is provided. An embodiment of fully integrated spectrophotometer  408  in accordance with the present invention is depicted in  FIG. 4 . 
     Fully integrated spectrophotometer  408  includes housing  438  that hermetically seals its contents, including light source  100 , integrated Fabry-Perot interferometer  312 , and various electronic devices and circuitry (e.g., processor  444 , low-noise signal conditioning  446 , analog-to-digital conversion  448 , light source drivers  450 , power supply  452 , RF antenna  454 , and RFID transponder  456 ). 
     Housing  438  is configured to provide sampling region  440 . The sampling region is defined in a region that is external to housing  438  and is thus exposed to the ambient environment. In the illustrative embodiment, sampling region  440  is formed by creating an “inlet” wherein the walls of the housing extend inwardly for a distance. This inlet has a “u” shape, wherein the two “legs” of the “u” are windows  442 A and  442 B. The windows are leak proof and transparent to the interrogating light emitted from light source  100 . As depicted in  FIG. 4 , when spectrophotometer  408  is placed in a fluid, the fluid readily enters sampling region  440 . In operation, light from light source  100  is directed through window  442 A to sampling region  440 , which contains an analyte of interest. 
     The interrogating light passes through the media containing the analyte in sampling region  440  and is spectrally altered as previously discussed. The spectrally altered light then re-enters housing  438  through second window  442 B. In the illustrative embodiment, integrated Fabry-Perot interferometer  312  is situated behind window  442 B, so that the spectrally altered beam passes through that window and into the interferometer. 
     The spectrally altered beam is spectrally filtered in interferometer  312  in the manner previously discussed. The output from the detectors is extracted from interferometer  312  and is transmitted to low-noise signal conditioning circuitry  446  and then to analog-to-digital convertor circuitry  448 . The resulting digital signal is then sent to processor  444 . 
     In some embodiments, the processor includes processor-accessible memory containing software for controlling and varying the cavity length, software for controlling light-source drivers  450 , and software for controlling communications and power functions. In such embodiments, the minimally processed data is transmitted from integrated spectrophotometer  408  to an external processor. The external processor generates the spectral assay, etc., via Fourier-transform processing or variations thereof. The external processor also compares the spectral assay to reference information, such as for qualitative (analyte identification) or quantitative (analyte concentration) determinations. 
     In some other embodiments, processor  444  can generate the spectral assay and, optionally, the qualitative and quantitative determinations. 
     In the illustrative embodiment, spectrophotometer  408  receives power and communication control through integral antenna  454  that is sensitive to electromagnetic or magnetic fields sourced from an external RFID interrogator. In the illustrative embodiment, spectrophotometer  408  includes passive RFID transponder  456  that communicates with the external interrogator by wireless means through antenna  454 . The implementation of passive RFID transponder  456  is within the capabilities of those skilled in the art. 
     In some embodiments, spectrophotometer  408  can be powered with energy harvested from remote electromagnetic or magnetic field sources at RF wavelength bands including low frequency, high frequency, or ultra-high frequency and communicated using a wireless telemetry link. In some such embodiments, antenna  454  is operated as a “rectenna,” which is a portmanteau word meaning “rectifying antenna”. A rectanna is an antenna that is used to convert incident electromagnetic or magnetic energy into direct current. In its simplest form, the rectanna is implemented by connecting an RF diode connected across the dipole elements of antenna  454 . The diode rectifies the AC voltage induced in the antenna to produce DC power. In such an embodiment, “power supply  452 ” is an appropriately connected RF diode and a capacitor for energy storage. 
       FIGS. 6A, 6B, 7, and 8A-8B  depict further detail of an embodiment of integrated Fabry-Perot interferometer  312 . 
     The inventor recognized that it is particularly advantageous to fabricate some embodiments of integrated Fabry-Perot interferometer  312  (as well as embodiments of other versions of the interferometer disclosed later in this specification) using semiconductor-on-insulator wafers. In particular, the alternating layer structure, the thickness of the layers, as well as the material characteristics thereof in such wafers are well suited for fabricating at least some embodiments of an integrated Fabry-Perot interferometer in accordance with the present teachings. 
     As will be appreciated by comparing  FIGS. 6A, 6B, and 7 , in the illustrative embodiment, integrated Fabry-Perot interferometer  312  is formed from two semiconductor-on-insulator wafers  602  and  610  as well as non-electrically conductive substrate  600 .  FIG. 6A  depicts starting wafers  602  and  610  prior to any patterning steps. During fabrication, various additional layers of material are formed on one or both of the wafers.  FIG. 6B  depicts wafers  602  and  610  after some patterning has been completed. 
     The illustrative embodiments disclose the use of silicon-on-insulator wafers, which have a device layer of single crystal silicon, a box layer of silicon dioxide, and a handle layer of silicon. Suitable semiconductor-on-insulator wafers for use in conjunction with the present invention are not limited to such silicon-on-insulator wafers. In some other embodiments, the device layer is an alloy film of silicon-germanium. Silicon-germanium offers advantages for the device layer; in particular, it has lower thermal conductivity than silicon. This is particularly useful for embodiments in which the detectors operate as thermal detectors (bolometers), since the region in which detectors reside should be thermally insulated from sources of heat other than what is delivered from the electromagnetic radiation exiting the interferometer cavity. In further embodiments those skilled in the art can easily recognize that other semiconductor-on-insulator starting wafer combinations also offer potential advantages. For instance, a wafer with a device layer of bismuth telluride or derivatives thereof can offer an increased Seebeck sensitivity compared with silicon or silicon-germanium. 
     Semiconductor-on-insulator wafers  602  and  610  includes three layers: a “device” layer of silicon, a buried oxide (“box”) layer of silicon dioxide, and a “substrate” or “handle” layer of silicon. In wafer  602 , those layers are: layer  608  (device layer), layer  606  (box layer), and layer  604  (handle layer). In wafer  610 , those layers are: layer  616  (device layer), layer  614  (box layer), and layer  612  (handle layer). 
     Typically, the device layer is single crystal silicon (about 10-2000 nanometers in thickness), the box layer is SiO 2  (about 0.5 to 4 microns in thickness) and the handle layer is single crystal silicon (&gt;250 microns in thickness). As discussed later in further detail, in some embodiments, layer  608  of wafer  602  comprises high resistivity silicon that is doped appropriately during processing. 
     As depicted in  FIG. 6B , a portion of device layer  608  of wafer  602  is patterned to form micro-platform  626 . The portion of layer  606  immediately surrounding micro-platform  626  is patterned to create structures  630 , referred to herein as “nanowires,” which will ultimately function as electrically conductive wires for conducting electrical signals to and from micro-platform  626 . They are referred to as “nanowires” because at least the controlled dimension thereof is less than 1 micron, such as the width of nanowire  630 . The portion of handle layer  604  below micro-platform  626  and nanowires  630  is removed, thereby creating region  622 . This “releases” the portion of box layer  606  below micro-platform  626  and nanowires  630  such that the released portion is not supported by any underlying material. The unsupported portion of layer  606  is designated “support layer  624 ”. 
       FIG. 6B  depicts an additional layer  618  of electrically insulating material formed on active layer  616  of wafer  610 . In the illustrative embodiment, layer  618  is a layer of silicon dioxide. A portion of layer  618  and a portion of layer  616  are removed, forming cavity  628 . Wafers  602  and  610  are aligned so that “device” layers  608  and  616  are facing one another and micro-platform  626  is approximately centered with respect to cavity  628  of wafer  610 . The two wafers are bonded together at layers  608  and  618  via solder or epoxy preforms  620  or direct wafer-to-wafer bonding. Wafer  602  and substrate  600  are bonded together via solder or epoxy preforms. Substrate  600  can be ceramic, quartz, or other suitable, non-electrically conductive material. Additional layers (metallization and/or insulator) may be grown/deposited on the various exposed layers of wafers  602  and  610 . Such details and further description of the fabrication process is provided later in this specification. 
       FIG. 7  depicts further detail of an embodiment of interferometer  312 . It will be apparent that the basic structure of interferometer  312  results from joining wafers  602  and  610  to one another (at interface  752 ) and from joining wafer  602  to substrate  600  at interface  754 . The structure of integrated interferometer  312  provides optical filtering, detection, and electrical connectivity, as previously discussed and as discussed further below. 
     Optical Filtering. 
     Interferometer  312  includes highly (but partially) reflective surfaces  732  and  734 . These reflective surfaces are implementations of mirrors  504  and  506  ( FIG. 5 ). In the illustrative embodiment, reflective surfaces  732  and  734  are aluminum having a thickness in the range of about 10 to 100 nanometers. In other embodiments, materials such as gold, silver, copper, etc., and combinations thereof can be used. In yet further embodiments, the reflective surfaces can be multi-layer dielectric sandwiches of appropriate thickness. In still further embodiments, the reflective surfaces can be combinations of metals and dielectrics. The fabrication of mirrors is within the capabilities of those skilled in the art. The space between highly reflective surfaces  732  and  734  defines optical cavity  736 . The length of optical cavity  736  is equal to gap G. 
     Detection. 
     Micro-platform  626  is an effectively isothermal region comprising materials suitable for (1) absorbing radiation in the visible and/or IR band and (2) for detecting such radiation. Micro-platform is effectively isothermal because the layer from which it is formed (layer  608 ) has high thermal conductivity and for the most part, micro-platform  626  is isolated from other layers. To detect radiation, micro-platform  626  includes detectors. In the illustrative embodiment, the detectors are embodied as thermal detectors; in particular, thermocouples. The portion of the thermocouple positioned within micro-platform  626  becomes the heated end; the other end of each thermocouple is located in the “field” region of layer  608 , which is not heated and therefore provides a reference temperature. 
     Operating in Seebeck thermovoltaic mode, the thermocouples generate a voltage proportional to the temperature difference between micro-platform  626  and surrounding field region of layer  608 . Thus, the voltage generated is proportional to the power absorbed from the light exiting the reflective surface  732 . 
     In some other embodiments, the detectors are embodied as thermistors, and in some further embodiments, the detectors are embodied as band gap detectors. The detectors are described in further detail in conjunction with  FIGS. 8A and 8B . 
     In some embodiments, interferometer  312  includes infrared (IR) absorber  756  for enhanced absorption of infrared radiation. IR absorber  756  is disposed on the “underside” of support layer  624 . In the illustrative embodiment, IR absorber  756  is a dense grouping of individual structures having a relatively high length to width (or diameter) ratio. Such an absorber is particularly effective for enhancing the absorption of mid- to long-wave IR. 
     In some embodiments, IR absorber  756  is implemented as silicon structures (e.g., pedestals, etc.) referred to herein as “silicon grass”. The spacing between adjacent “blades” of silicon grass is the range of nanometers. The silicon grass is not necessarily uniform in structure. The presence of the silicon grass greatly increases the absorption efficiency of IR, as opposed to an un-patterned layer of the same material. In some embodiments, the “height” of the silicon grass is at least one-quarter wavelength of the incident IR. Since the shortest wavelength IR is about 700 nanometers, this equates to a minimum height for the grass of about 175 nanometers. Typical width or diameter of the silicon grass is in the range of about 1-10 nanometers, giving a minimum L/D greater than 15 and a typical L/D in excess of 100. Silicon grass can be formed, for example, using DRIE (deep reactive ion etching). 
     In some further embodiments, IR absorber  756  is implemented as vertical multiwall carbon nanotubes. This can be accomplished, for example, by a first atomic layer deposition, which serves as a catalyst for growth. This deposition is followed by chemical vapor deposition (“CVD”) process with an acetylene precursor to grow the VMWCNTs. The L/D for the VMWCNTs can be tens of thousands. 
     Electrical Connectivity. 
     Interferometer  312  is able to: (1) apply a voltage across highly reflective surfaces  732  and  734  for electrostatic control of cavity length and (2) conduct electrical signals from micro-platform  626  to electrical contacts located elsewhere in the interferometer and, finally, to processing electronics located external to the interferometer. 
     Arrangement for Applying a Voltage to Highly Reflective Surfaces. 
     The length of cavity  736  (gap G) can be altered by applying a voltage across reflective surfaces  732  and  734 . In this context, the reflective surfaces function as electrodes of an electrostatic actuator. Since, in the illustrative embodiment, the reflective surfaces comprise metal, electrical connection to surfaces  732  and  734  is trivial. 
     Voltage is applied to reflective surface  734  (i.e., the “upper fixed mirror”) using contacts  748 B and  738 . Contact  748 B is an ohmic contact to layer  612  and contact  738  is an ohmic contact between layer  612  and reflective surface  734 . Electrical interconnect  758  couples contact  748 B to contact  750 B. Contact  750 B is coupled to a controlled voltage source (not depicted). 
     Voltage can be applied to reflective surface  732  (i.e., the “lower movable mirror) using contacts  748 A or  748 C. Interferometer  312  is typically arranged, however, to provide only one electrical path to reflective surface  732  for the application of a voltage. In the illustrative embodiment, that path is through contact  748 A. 
     Through-wafer vias  740 A and  740 B are used to access electrical contact layer  608 , which is the layer on which electrical traces reside. Vias  740 A and  740 B extend all the way through “upper” wafer  610  to “lower” wafer  602 . More particularly, these through-wafer vias extend through layers  612 ,  614 ,  616 ,  618 , “exposing” layer  608 . Insulating layer  742  (e.g., silicon dioxide, etc.) is disposed on the sidewalls of vias  740 A and  740 B and layer  744  of an electrical conductor, such as aluminum, etc., is disposed on insulating layer  742 . Electrical contacts  746 A and  746 B are formed at the base of vias  740 A and  740 B, respectively. 
     As described in further detail in conjunction with  FIG. 8A , in the illustrative embodiment, electrically conductive trace  874  disposed on the “upper” surface of electrical contact layer  608  electrically couples contact  746 A to one nanowire  630 . Electrically conductive trace  876 , which is disposed on micro-platform  626 , couples the one nanowire to reflective surface  732  to complete the electrical path from contact  748 A. A layer of an electrically insulating material, such as silicon dioxide, is disposed between electrical contact layer  608  and the metallization. In embodiments in which metal is used for electrical conduction, a layer of insulator is disposed between the metal and the “supporting” layer to the extent needed to provide electrical insulation from underlying silicon. 
     In some alternative embodiments, rather than creating electrical paths via metallic traces, layer  608  is doped to provide electrically conductive paths. In such embodiments, to maintain electrical isolation between such conductive paths, layer  608  must comprise a high resistivity material, such as high resistivity silicon. As discussed further below, nanowires  630  are not metallized; rather, electrical conductivity is provided by doping the nanowires. 
     Arrangement for Conducting Electrical Signals from the Micro-Platform to Off-Platform Contacts and External Circuitry. 
     As previously mentioned, in the illustrative embodiment, detectors (partially) within micro-platform  626  are implemented as thermal detectors. Such detectors will generate a voltage when they detect heat. The voltage signals generated by the detectors are ultimately processed as part of the spectrophotometry process. To do so, such signals must be transmitted to external circuitry (e.g., for analog to digital conversion, for Fourier algorithmic processing, etc.). 
     The detector signals are electrically conducted off of micro-platform  626  via nanowires  630 , which are described in further detail in conjunction with  FIGS. 8A and 8B . Electrically conductive traces disposed on layer  608  electrically couple the signals from nanowires  630  to electrical contacts  746 A and  746 B located at the “base” of through-wafer vias  740 A and  740 B. These electrical contacts are electrically coupled to respective contacts  748 A and  748 C disposed “on top” of interferometer  312  in conjunction with metallization layer  744  disposed on the “right-hand” sidewall of the respective through-wafer vias. Contacts  748 A and  748 C are electrically coupled to contacts  750  on substrate  600 , at which point the signals can be transmitted to external circuitry. Because of the preponderance of electrical traces on the “field” region of layer  608 , that region is referred to herein as the “electrical contact layer”. 
       FIG. 8A  depicts a cross sectional view of interferometer  312  along the line A-A in  FIG. 7  and in the direction shown.  FIG. 8A  is effectively a plan view of interferometer  312  with all layers above layer  608  removed. 
     Contacts  746 A and  746 B are the electrical contacts that are disposed at the base of through-wafer vias  740 A and  740 B (see  FIG. 7 ), as previously discussed. Electrical traces  874  (on field region  608 ) and  876  (on micro-platform  626 ), in conjunction with a nanowire  630 , place contact  746 A and reflective surface  732  in electrical contact for the application of a voltage, such as for electrostatically adjusting interferometer cavity length. 
     A plurality of detectors  862  are formed in/on micro-platform  626 . In the illustrative embodiment, the detectors are thermal detectors—in particular thermocouples—that are series-connected to form a thermopile. One end of the thermopile is electrically coupled, via metallization trace  870 , to contact  746 A. The other end of the thermopile is electrically coupled, via metallization trace  872 , to contact  746 B. 
     In the illustrative embodiment, each detector  862  comprises a Seebeck junction and two arms. The two arms are implemented via two nanowires  630 , one of which is n-doped and the other of which is p-doped. Junction  864  is disposed in/on micro-platform  626  and is formed by appropriately doping (with p-material and n-material) the region of micro-platform  626  between the ends of two nanowires. Micro-platform  626  is also pattern-doped in the region between the end of each nanowire  630  and its respective junction  864  to create electrically conductive path  866  that places the nanowire and p-n junction in electrical contact with one another. Path  866  is doped with the same material as the associated nanowire  630 . Dopant materials include, for example, phosphorus, arsenic, and boron. Electrical traces  868  disposed on electrical contact layer  608  (with an intervening layer of insulator) electrically connect detectors  862  to one another in the off platform of layer  608  to provide the series connection. 
     In some other embodiments, thermal detectors  862  are thermistors. The thermistors are formed by pattern-doping the active layer with one or more of phosphorus, arsenic, and boron. In yet some further embodiments, the detector is a small band-gap semiconductor junction or a high-Z thermoelectric junction. In such embodiments, the junction is formed of InAs, GaAs, InAs, HgCdTe or other appropriate semiconductor materials obtained variously through CVD deposition, sol-gel deposition, and patterned-doping processes. 
       FIG. 8B  depicts a perspective view of micro-platform  626  and two detectors  862 . For clarity, other detectors and nanowires are not depicted in  FIG. 8B , it being understood that additional detectors having nanowires  630  extending from all four sides of micro-platform  626  are present, as depicted in  FIG. 8A . 
     As shown in  FIG. 8B , nanowires  630  are patterned from layer  608  and have a thickness equal to that of layer  608 , but have an exceedingly small width (10 to 2000 nanometers). It is notable that in  FIG. 7 , nanowires  630  are illustrated with a “sawtooth” profile, similar to the manner in which a “resistor” is normally depicted. Nanowires  630  are not resistors; they are drawn in this fashion to be readily distinguishable, for example in  FIGS. 5, 6B , and  7 , from the unpatterned material of layer  608  and micro-platform  626 . 
     With continued reference to  FIGS. 8A and 8B , nanowires must be electrically conductive yet, at the same time, they should exhibit low thermal conductivity to keep the amount of heat that they conduct on or off micro-platform  626  to a practical minimum (for embodiments in which detectors  862  are implemented as thermal detectors). For this reason, in the illustrative embodiment, the upper surface of nanowires  630  is not metallized. That is, although such metallization would readily provide electrically conductive paths for conducting a voltage on to, or electrical signals off of, micro-platform  626 , metal is an excellent conductor of heat. 
     The thermal conductivity of nanowire  630  is a function of the thermal energy conducted through charge carriers and lattice-energy transfer mechanisms. For silicon semiconductor nanowires, the thermal conductivity is primarily determined by phonon scattering, which is, in turn, a function of nanowire cross-section and the presence of internal scattering structures. The greater the scattering, the lower the thermal conductivity. 
     In accordance with some embodiments, nanowires  630  include a physical adaptation that reduces their ability to conduct heat. In some embodiments, the physical adaptation is a plurality of “scattering holes” (not depicted) to scatter phonons, thereby reducing thermal conductivity along the length of each nanowire  630 . The spacing between the scattering holes on each nanowire is approximately the phonon scattering length and greater than the scattering length for electrical charge carriers (i.e., electrons or holes). In particular, the phonon scattering length in silicon (about 50 to 500 nanometers) is typically about 10× greater than the scattering length for electrical charge carriers (about 5 to 50 nanometers). The presence of these scattering holes results in an increase in the ratio of electrical conductivity to thermal conductivity of each nanowire  630 . For additional disclosure concerning nanowires and other aspects of micro-platform  626 , see, U.S. patent application Ser. No. 14/245,598, which is incorporated by reference herein in its entirety. 
     To further increase the thermal isolation of micro-platform  626 , in some embodiments, a portion of support layer  624  below nanowires  630  is removed. 
     Fabrication. 
     Processing of the “Lower” Wafer  602 . 
     Referring generally to  FIGS. 6A, 6B, 7, 8A, and 8B , device layer  608  of SEMICONDUCTOR-ON-INSULATOR wafer  602  is appropriately patterned to create micro-platform  626  and the nanowires  630 . The micro-platform is lithographically patterned, for example, via reactive ion etching (RIE). In the illustrative embodiment, layer  606 , which is silicon dioxide, is used as an etch stop. 
     Micro-platform  626  is doped to form detectors  862 . As previously discussed, in the illustrative embodiment, the detectors are thermal detectors, such as thermocouples. The thermocouples are formed by pattern-doping micro-platform  626  to form a Seebeck junction and nanowires  630 , in alternating fashion, with n-type material and p-type material. The dopants can be one or more of phosphorus, arsenic, or boron. In some embodiments, the thermal detector is a thermistor. The thermistors are formed by doping the appropriate regions with a high-resistivity active silicon layer with one or more of phosphorus, arsenic, or boron. 
     Micro-platform  626  is covered by a thin (submicron) layer of a dielectric, such as silicon dioxide. Silicon dioxide can be deposited, for example, from a TEOS precursor via a low pressure chemical vapor deposition (“LPCVD”) tool. In some other embodiments, the thin dielectric film is deposited from a silane/ammonia precursor in a similar CVD tool. The thin dielectric film is appropriately lithographically patterned. 
     In the illustrative embodiment, highly reflective surface  732  is formed by evaporating or sputtering a metal, such as aluminum, onto the topside of the thin dielectric film and appropriately patterning the metal. In some other embodiments, gold, silver, copper, dielectric sandwiches, or combinations of these materials (including aluminum) can suitably be used to form surface  732 . 
     Highly reflective surface  732  is partially reflecting. In the illustrative embodiment in which surface  732  is formed from aluminum, the thickness thereof is in the range of about 10 to about 100 nanometers. Another film of aluminum that provides electrical contacts and interconnects with the detector is also deposited and patterned. 
     The portion of layer  606  underlying micro-platform  626  (i.e., layer  624 ) serves as a support therefor. Support layer  624  is “released” by etching into layer  604  (i.e., the handle of semiconductor-on-insulator wafer  602 ), creating cavity  622 . Layer  606 / 624  serves as an etch-stop for the etch process. The etchants used are preferably anisotropic, such as, without limitation, TMAH or KOH. Alternatively, deep reactive ion etching (“DRIE”) can be used can be used to create cavity  622 . 
     In some embodiments, IR absorber  756  is formed on the “under side” of support layer  624 . In embodiments in which IR absorber  756  are carbon nanotubes, they are grown, in known fashion, in a reactor using a catalyst film of iron oxide a few nanometers in thickness followed by CNT growth from a H 2 C 2  precursor. 
     Processing of the “Upper” Wafer  610 . 
     Layer  618  of an electrically insulating material, such as silicon dioxide, is formed on device layer  616  of semiconductor-on-insulator wafer  610 . Layer  618  has a thickness in the range of about 50 to about 500 nanometers. Layer  618  can be formed via oxidation in a furnace. Cavity  628  is formed in layers  618  and  616  via reactive ion etching. Layer  614  is used as an etch stop. Device layer  616  and insulator layer  618  thus serves as a spacer to define the nominal “gap” (i.e., cavity length) for interferometer  312 . For operation at mid- and long-wavelength infrared, the thickness of layer  616  is in the range of about 1 to about 20 microns. 
     A layer of aluminum, which will serve as highly reflective surface  734  (i.e., the “upper” mirror of interferometer  312 ), is evaporated or sputtered onto layer  614  of semiconductor-on-insulator wafer  610 . In some other embodiments, films of gold, copper, multi-layer dielectrics, or combinations of these materials (including aluminum) can suitably be used to form the reflective layer. This is followed by rapid thermal annealing (“RTA”) to form the ohmic contact  738  between layer  612  and surface  734 . This enables a voltage to be applied to the surface  734 , as required when the surface functions as an electrode for electrostatic actuation. The aluminum film covering other portions of layer  616  (i.e., outside of cavity  628 ) is removed by chemical/mechanical polishing (“CMP”). 
     Through-wafer vias  740 A and  740 B are formed using, for example, DRIE, and are then coated with a film of a dielectric material, such as silicon dioxide, etc. A film of metal, such as aluminum, is deposited on the dielectric material in the vias and then patterned. This additional film is used to form the electrical connection with the detectors and highly reflective surface  732  on micro-platform  626 . 
     Bonding the First and Second Wafers Together. 
     Wafers  602  and  610 , after processing as described above, are aligned and bonded together at interface  752  using one or more of anodic, direct semiconductor-to-semiconductor, cement, or eutectic alloy bonding processes. The bonded wafers are then sawed into individual die, which are bonded at interface  754 , to substrate  600 . In some embodiments, this bonding is implemented with an electrically conductive epoxy perform. In the illustrative embodiment, substrate  600  is a ceramic header with appropriately patterned electrical pins and interconnects. In some other embodiments, substrate  600  is another suitable material, such as epoxy, very high-resistivity silicon, etc. In the illustrative embodiment, the integrated structure (within a packaging header) is wired to bonding pads  750  via an ultrasonic wire bonder. 
       FIG. 9  depicts integrated Fabry-Perot interferometer  912 , which is a variation of integrated Fabry-Perot interferometer  312 . In this embodiment, the placement of wafer  602  and wafer  610  ( FIG. 6A ) is reversed such that the movable mirror is situated “above” the fixed mirror. That is, micro-platform  626  and highly reflective surface  732  are disposed “above” highly reflective surface  734 . Interferometer  912  has the same basic structure as interferometer  312 , being based on two semiconductor-on-insulator wafers and a ceramic, etc., substrate. 
     Integrated Fabry-Perot interferometer  912  includes three vias  940 A,  940 B, and  940 C, which all provide electrical access to electrical connections (that ultimately connect to micro-platform  626 ) on layer  608 . Contact  750  on hermetic seal  976  and ohmic contact  738  provide electrical connection to highly reflective layer  734 . An aluminum film at interface  752  is patterned to provide electrical connection between appropriate nanowires  630  and respective vias  940 A,  940 B, and  940 C. 
     In this embodiment, light enters the interferometer through region  977 , thereby ensuring that the light reaches interferometer cavity  732  before it encounters IR absorber  756 . In some other less preferred embodiments, light enters via cavity  622 , thereby encountering IR absorber  756  before reaching interferometer cavity  732 . If light enters via cavity  622 , the spectral finesse of interferometer  912  is likely to be degraded. 
     The same techniques that were used to fabricate interferometer  312  are used to fabricate interferometer  912 . However, for interferometer  912 , IR absorber  756  is formed after through-wafer vias  940 A,  940 B, and  940 C. 
       FIGS. 10A and 10B  depict integrated Fabry-Perot interferometer  1012 , which is another variation of integrated Fabry-Perot interferometer  312 . In this embodiment, cavity length is fixed. Interferometer  1012  includes multiple cavities, each tuned to filter a different selected wavelength based on cavity length. 
     As depicted in  FIG. 10B , interferometer  1012  includes six cavities, three of which:  1036   1-1 ,  1036   2-1 ,  1036   3-1 , are visible in  FIG. 10A . Cavity  1036   1-1  has a cavity length of G 1 , cavity  1036   2-1  has a cavity length of G 2 , and cavity  1036   3-1  has a cavity length of G 3 . Each cavity functions as a discrete interferometer in the manner previously discussed, wherein detectors  862  ( FIG. 10B ) within each micro-platform generate a voltage in response to heating and electrically conducting nanowires  630  conduct the detector signals off-platform. As in other embodiments depicted, all structures external to each micro-platform  626  are effectively an isothermal reference mass. The thermocouple arrays in this embodiment provide sensitivity for thermal sensing with incident radiation wavelengths longer than typically 2 microns. IR absorber  756  enhances absorption of IR radiation in micro-platform  626 . 
     Referring to  FIG. 10B , the six arrays of detectors  862  associated with the six cavities are electrically connected to a common first interconnect  1084 . The first interconnect is electrically coupled to electrical contact  1086 . This arrangement simplifies the interconnection for detector readout. Electrical contact  1086  couples to electrical contact  1078 A disposed on layer  1081 . Electrical contact  1078 A is coupled to a first electrical contact  750  on substrate  600 . 
     Each array of detectors  862  has its own unique second electrical contact for readout:  1082   i,j , wherein i=1, 3 and j=1, 3. Each second electrical contact  1082   i,j  is electrically connected to electrical contact  1078 B disposed on layer  1081 . Electrical contact  1078 B is coupled to a second electrical contact  750  on substrate  600 . Light is pulsed sequentially into the various cavities. This configuration permits a parallel simultaneous readout of signals from all detectors  862  in the array via electrical contacts  1082   i,j . 
     Referring to  FIG. 10A , insulator layer  1081  is disposed on layer  608 . That is, an additional insulator layer (e.g., silicon dioxide, etc.) is added to the device layer of the lower starting semiconductor-on-insulator wafer. Layer  1081  provides electrical isolation between the interconnections that are patterned onto electrical contact layer  608 . This layer also improves the finesse of interferometer  1012  by reducing the penetration of the evanescent wave to the detector. Anti-reflection layer  1080  is disposed on layer  612  to reduce reflection of the incident radiation. In some embodiments, layer  1080  is a quarter-wave thickness of a single dielectric film or a sandwich of multiple dielectrics. 
     The same techniques that were used to fabricate interferometer  312  are used to fabricate interferometer  1012 . 
     The “pedestals” defined by portions of layer  608 , layer  1081 , and layer  616  serve as both an isothermal reference for thermal detectors in the micro-platforms  626  and as a support for electrical interconnects. The differences in cavity length between the cavities result from etching a different distance into layer  612 . The pedestals also effectively extend the two “inner pedestals” of layer  604  that support layer  606 . This helps to reduce or eliminate variations in the gaps in each cavity due to bowing of the layers  604  and  612 . 
     Although the embodiment of integrated Fabry-Perot interferometer  1012  depicted in  FIGS. 10A and 10B  has six interferometric cavities, in some other embodiments, interferometer  1012  can include fewer or more than six such cavities. 
     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.