Patent Publication Number: US-2023136082-A1

Title: Micro-electromechanical system (mems) interferometer for ft-mir spectroscopy

Description:
TECHNICAL FIELD 
     The present disclosure is directed to an interferometer for a Fourier transform infrared spectrometer. More specifically, the interferometer is based on a micro-electromechanical system (MEMS) used in conjunction with an uncooled near-zero index metasurface detector. 
     BACKGROUND 
     Infrared spectrometers have been deployed in a wide range of applications that benefit from non-invasive chemical analysis. For the oil &amp; gas industry, the potential to migrate this technology into downhole logging application holds important benefits in the identification and analysis of in situ hydrocarbons particularly with respect to the mid-infrared regime which could allow real-time chemical analysis and quantification of saturate, aromatic, resin, and asphaltenic (SARA) components. However, the constraints on size and thermal control with existing miniaturization applications are relatively benign in comparison to those for downhole, which have been a significant obstacle to migration of the technology into oilfield sensing applications. For instance, the environmental conditions in downhole logging while drilling operations can exceed 175° C. and 200 MPa with sensor packages confined to less than a few centimeters in diameter. While production logging conditions are more benign, generally less than 125° C. and 100 MPa, still sensor packages less than a couple of centimeters in diameter are required. None of these type downhole applications are amenable to integration of the cryogenic cooling systems typical with laboratory grade detectors. 
     SUMMARY 
     An embodiment described in examples herein provides a microelectromechanical (MEMS) interferometer. The MEMS interferometer includes a pair of movable mirrors that are positioned along perpendicular axes, wherein each of the pair of movable mirrors is coupled to a mechanism. The mechanism includes an electrostatic actuator driving a displacement amplification mechanism, and the displacement amplification mechanism driving each of the pair of the movable mirrors. The MEMS interferometer includes a beam splitter that is positioned at an intersection of the perpendicular axes extending through each movable mirror and the beam splitter. The MEMS interferometer also includes a metasurface microbolometer placed in line with the beam splitter to measure an intensity of a recombined beam from the pair of movable mirrors. 
    
    
     
       BRIEF DESCRIPTION OF DRAWINGS 
         FIG.  1    is a schematic diagram of a Fourier transform infrared (FTIR) spectrometer. 
         FIGS.  2 A and  2 B  are schematic diagrams of a micro-electromechanical system (MEMS) interferometer for an FTIR. 
         FIGS.  3 A and  3 B  are cross-sections of the mechanism used for moving each mirror in the MEMS interferometer. 
         FIGS.  4 A,  4 B,  4 C, and  4 D  are drawings showing the metasurface geometry of the detector. 
         FIG.  5    is a plot showing the absorption spectrum for three crude oils, each from different global reservoirs with the absorption spectrum for the metasurface superimposed. 
         FIGS.  6 A and  6 B  are drawings of an uncooled microbolometer using a metasurface geometry. 
         FIGS.  7 A and  7 B  are plots showing a comparison of the thermometric properties for doped vanadium oxide (VO2) films and an undoped vanadium oxide (VO2) film. 
         FIG.  8    is a plot showing a comparison of spectral detectivity for the metasurface detector, in comparison to various commercially available IR detectors operated at different temperatures. 
     
    
    
     DETAILED DESCRIPTION 
     With the lack of infrared spectroscopic devices adaptable to the constraints of downhole application, realization of the concept for “FT-IR on a chip” in the form of an uncooled miniaturized laboratory grade infrared spectrometer would have disruptive effects on in situ downhole chemical analysis in the oilfield. Development of an uncooled MEMS based interferometer is a critical initial step. 
     Embodiments described herein provide a monolithic silicon MEMS interferometer utilizing an uncooled metasurface absorber detector to enable a substantial size reduction in an FTIR amenable for downhole application. The techniques may be used to provide chemical spectroscopy downhole in a wellbore in real-time. The metasurface absorber detector has unusual electromagnetic absorption properties that enable laboratory quality detectivity at higher temperatures than other detectors. 
     The metasurface elemental structure is derived from a geometric inversion of the canonical Rhodonea, or more commonly four-leaf roses, conformal mapping contours and was found to exhibit a near zero index metamaterial (NZIM) behavior. The near zero index properties of the metasurface lead to an absorption phenomenon characterized by surface plasmon resonances that confine the absorption mechanism within the ultrathin (λ/375) metasurface plane and make the absorption properties of the microbolometer design relatively insensitive to moderate changes of the material properties of the remaining laminae. 
     Accordingly, this unusual feature allows the metasurface to be integrated on a single VO 2  material thermometric layer which can then be operated at downhole elevated temperatures within the VO 2  metal-insulator-transition region. Within this region the VO 2  layer exhibits more than an order of magnitude enhancement in its ambient thermometric properties. 
     This provides detector performance levels that, using other types of detectors, would be achievable only with cryogenic cooling. As these technologies are limited to laboratory environments, the metasurface microbolometer represents a significant advancement, enabling uncooled IR spectroscopy using miniaturized sensor devices. These devices may be used in any number of field type applications, such as oilfield exploration and production applications 
       FIG.  1    is a schematic diagram of a representative Fourier transform infrared (FTIR) spectrometer  100 . Fourier transform infrared spectroscopy generally uses an interferometer, such as a Michelson interferometer based on MEMS technology, termed a MEMS interferometer  102  herein, to collect data on a sample  104  based on path length differences. After collection, the data is processed using a Fourier transform resulting in an IR spectrum. 
     The operation of the MEMS interferometer  102  is based upon separating an incident or input beam  106  of radiation into two beams  108  and  110  by means of a beamsplitter  112 , whereupon a path length difference between the separated beams is introduced by antisymmetric movement  114  of both of two reflecting elements, for example, a pair of mirrors  116 . The path length difference creates constructive and destructive interference in the recombined beam  118  at the beamsplitter  112 . 
     Thus, radiation originating from a source  120  passes through input optics  122 , forming an approximately collimated input beam  124 . The approximately collimated input beam  124  passes through the sample  104  and into the MEMS interferometer  102 . The constructive and destructive interference of the recombined beam  118  results in a change in the intensity of the output beam  126  as a function of the relative path length difference, termed an interferogram. The output beam  126  is passed through output optics  128  to be focused on a detector  130 , such as the metasurface microbolometer described herein. 
     The intensity of the interferogram can be monitored as a function of path difference, for example, the relative displacement of the reflecting elements over time, using the detector  130 . Fourier transformation techniques are applied to the raw interferogram data to convert the spectra from the relative displacement domain to the wavelength domain, resulting in an absorption spectrum. The absorption spectra can be analyzed to determine the chemical composition of the sample. 
     As described herein, decreasing the size of the FTIR is a prerequisite to their widespread deployment in environments with limited space, such as downhole. Miniaturization of infrared spectrometers has been difficult due to the need for cryogenically cooled detectors, limitations of the space available for downhole logging devices, and the development of instruments having sufficient spectral resolution to discriminate between the varieties of chemicals that may be encountered in a wellbore. 
     A significant impediment for achieving high quality mid-IR (MIR) spectroscopy, as determined by spectral range, is the need for cooling of most detector technologies. Generally, uncooled detectors have limited responsiveness in longer wavelength regimes, such as in the mid-IR range, for example, 250 wavenumbers (cm −1 ) to 2000 cm −1 . The design of the metasurface may be adjusted or tuned to cover other spectral ranges, for example, by changing the size of the features in the pattern. Thus, as described herein, a detector formed using a metamaterial can provide the needed detectivity without cooling. 
     Generally, metamaterials may be used to obtain effective properties at any specific frequency, by manipulating the design of subwavelength resonator elements, or shapes, of the metamaterial. Accordingly, obtaining the desired properties in the metamaterial is a matter of development of the appropriate geometric elements for the frequency range and electromagnetic response of interest. As described herein, a meta-material for infrared sensing is based upon thermal detection using arrays of very small thermal mass detector elements that interact with one or more electromagnetic modes. These are used to make broadband devices for spectroscopic chemical detection which rely upon the tailored broadband characteristics of the metamaterial design. In embodiments described herein, the metamaterials are used in uncooled MIR microbolometer technologies in performance regimes currently occupied by cryogenically cooled detector systems. Further, in various embodiments, the microbolometer detector  130  is incorporated into a miniaturized Michelson interferometer based on a micro-electromechanical system (MEMS), termed a MEMS interferometer  102 , herein. 
       FIGS.  2 A and  2 B  are schematic diagrams of a micro-electromechanical system (MEMS) interferometer  102  for an FTIR. Like numbered items are as described with respect to  FIG.  1   .  FIG.  2 A  is a top view of the MEMS interferometer  102 , while  FIG.  2 B  is a perspective view of the MEMS interferometer  102 . In various embodiments, the MEMS interferometer  102  is formed into a single monolithic chip or block of substrate, with dimensions  202  of about 12 mm×12 mm. 
     As described herein, in various embodiments the FTIR MIR spectrometer is based on a metasurface microbolometer, used as the detector  130 , which exhibits good absorption in the biological “fingerprint” region of the electromagnetic spectrum, e.g., from about 500 cm −1  to about 2000 cm −1 . This region is useful for identifying and analyzing many hydrocarbons and wellbore fluids. The MEMS interferometer  102  utilizes a pair of mechanisms that drive movable micromirrors, or mirrors  116 . Each mechanism includes an electrostatic actuator  204  on the chip that drives a mirror  116  through a displacement amplification mechanism  206 . Each mirror  116  is placed along a perpendicular axis extending through a beamsplitter  112 . The input beam  106  is divided by the beamsplitter  112  and sent to each movable mirror  116 , then recombined to create constructive and destructive interference in a beam from the beamsplitter  112  to the detector  130 . As described herein, in various embodiments the detector  130  is a metasurface detector that does not require cryogenic cooling. 
     The displacement amplification mechanism  206  increases the spectral resolution of the MEMS interferometer  102  by increasing the amplitude of the motion of each mirror  116 . The displacement amplification mechanism  206  increases the motion of each mirror  116  by a ratio of about 8.8:1 over the input motion from the electrostatic actuator  204 . This is described in further detail for a single mechanism with respect to  FIGS.  3 A and  3 B . 
       FIGS.  3 A and  3 B  are cross-sections of the mechanism used for moving each mirror  116  in the MEMS interferometer  102 . Like numbers are as described with respect to  FIGS.  1  and  2   . Each mirror  116  is controlled by a coupled electrostatic actuator  204  that uses a comb drive mechanism  302 . 
     The motion of the electrostatic actuator  204  drives the displacement amplification mechanism  206 . The comb drive mechanism  302  includes a sway stabilizer  304  that is attached to the substrate at two attachment points  306 . As described herein, the sway stabilizer  304  assists in keeping the motion of the comb drive mechanism  302  linear at high drive voltages. The comb drive mechanism  302  includes a central actuator  308  that is attached to movable combs  310  that has grounded tines that are positioned between tines extending from positive combs  312  (positive tines) and negative combs  314  (negative tines), which are fixed in place. Each tine from the movable comb  310  is positioned about 70 μm from a tine on one of the other combs  312  or  314 . Applying a voltage potential between the positive combs  312  and the negative combs  314  will cause the movable combs  310  to oscillate between the positive combs  312  and the negative combs  314 , moving the central actuator  308 . The amplitude 315 of the motion is proportional to the potential difference between the positive combs  312  and the negative combs  314 . 
     As described herein, the displacement amplification mechanism  206  amplifies the motion of the comb drive mechanism  302  to increase the total displacement of the mirror  116 . The displacement amplification is created through the combination of a symmetric fulcrum about the axis of the central actuator  308  and three pairs of serpentine moment release flexures  316 ,  318 , and  320 . The comb drive mechanism  302  imparts motion on the central actuator  308 , which activates the fulcrum lever about the constraint, or attachment, points  322  resulting in an amplification of the motion  324  at the mirror  116 . The three pairs of serpentine moment release flexures  316 ,  318 , and  320  are designed to function as quasi-perfect hinge joints at each location. The degree of departure from the perfect hinge moment release degrades the mirror and actuator amplification ratio of the motion  324  of the mirror  116 . For the idealized case in which the three pairs of serpentine release flexures  316 ,  318 , and  320  could be replaced by perfect ball-joints, the amplification ratio would be approximately 10:1 whereas in the practical design case involving the serpentine release flexures  316 ,  318 , and  320  as built, the amplification ratio of the motion  316  of the central actuator  308  is about 8.8:1, due to the incomplete release of the moment constraints. 
     The sway stabilizer  304  allows an increase in the vibrational loading at which the comb drive mechanism  302  experiences lateral instability. The sway stabilization  304  mechanism is integrated at the extreme location of the central actuator  308  from the displacement amplification mechanism  206 . 
       FIG.  3 B  is a schematic diagram of the motion of the mirror  116 . As illustrated  FIG.  3 B , a displacement of about 18.6 micrometers (μm) is increased to about 164.9 μm by the action of the amplification mechanism. Thus, with the amplification of the motion of the two moveable mirrors the total motion is about 329.8 μm, which provides an interferometric spectral resolution of about 15 cm −1  over the mid-IR spectral range of 2000-500 cm −1 . 
       FIGS.  4 A,  4 B,  4 C, and  4 D  are drawings showing the metasurface geometry of the microbolometer or detector  130 .  FIG.  4 A  is a drawing of the geometry for the rhodonea conformal mapping contours. 
     As described herein, the metasurface is derived from a geometric inversion of the rhodonea conformal mapping contours, shown in  FIG.  4 A , or more commonly called four-leaf roses, conformal mapping contours.  FIG.  4 B  shows the pattern after geometrical inversion of the base conformal contours. The metasurface detector is based upon an electrically conductive geometric pattern imprinted onto the surface of a dielectric substrate (Si 3 N 4 ) then both formed on a single layer of thermometric material (VO 2 ) using the pattern of  FIG.  4 B .  FIG.  4 C  is a drawing of the final metasurface geometry formed along the inverted contours, wherein the dimensions are in microns.  FIG.  4 D  is a magnified view of a portion of the metasurface, showing the patterns used for the present wavenumber range. The metasurface develops more than 90% infrared absorption in the biological “fingerprint” region, for example, in the wavenumber range of about 1500-600 cm −1 . 
     The metasurface was found to exhibit a near zero index metamaterial behavior. The near zero index properties of the metasurface lead to an absorption phenomenon created by surface plasmon resonances. This phenomenon confines the absorption to the ultrathin metasurface, which makes the absorption properties of the detector practically independent of the material properties of the remaining materials that may comprise the microbolometer. This allows integration of the metasurface absorber with a common thermometric material layer, undoped VO 2 , which exhibits a metal-insulator-transition (MIT) region. In the region where the thermometric material is transitioning from an insulator to an electrically conductive metal, the thermometric properties improve by more than an order of magnitude. This allows for the performance of an uncooled detector technology to reach levels previously requiring active cooling. 
     The detector is based on the integration of the metasurface absorber in a microbolometer construction with a single VO 2  material thermometric layer that is temperature controlled to operate at 60° C., for example, within the metal-insulator-transition region. Within this transition region the VO 2  layer has effectively transitioned from a dielectric to a metallic electrical conductor and acquires more than a 50 fold enhancement in the thermometric properties compared to the room temperature dielectric state. Thus, by controlling the detector cavity temperature at 60° C., the detectivity performance matches or exceeds conventional detector technologies cooled at cryogenic conditions, for example, at less than about −200° C. As a result, in some embodiments the detector technology described herein enables a downhole spectroscopic instrument with performance matching lab instruments. 
       FIG.  5    is a plot showing the absorption spectrum for three crude oils, each from different global reservoirs. The SARA fractions (saturate, aromatic, resin, and asphaltene) for the crude oil samples are summarized in Table 1. As can be seen in the plot, the detectivity  502  of the metasurface detector, using a detector with a 150 μm diameter formed from a layer of VO 2  on Si 3 N 4  would be sufficiently high to characterize all three crude oil samples. The uncooled microbolometer as described with respect to  FIG.  6   , exhibits a predicted maximum absorption of 99.5% at 870 cm −1  and an absorption bandwidth of 150% full-width at half-maximum (FWHM) on 1070 cm −1  center wavenumber, coincident with important chemical spectra of downhole hydrocarbons as shown in the spectra overlay plot of  FIG.  5   . The spectra in  FIG.  5    show the differences between crudes of different SARA fractions, and emphasize the high distinctions that exist in the lower wavenumber range below 1000 cm −1  in which the metasurface absorptivity remains generally above 95%. 
     
       
         
           
               
             
               
                 TABLE 1 
               
             
            
               
                   
               
               
                 SARA fractions of three crude oil samples 
               
            
           
           
               
               
               
               
               
               
               
            
               
                   
                   
                   
                   
                 Asphal- 
                   
                 Refer- 
               
               
                   
                 Saturates 
                 Aromatics 
                 Resins 
                 tenes 
                 Density 
                 ence 
               
               
                 Origin 
                 (wt. %) 
                 (wt. %) 
                 (wt. %) 
                 (wt. %) 
                 (g/cc) 
                 Number 
               
               
                   
               
            
           
           
               
               
               
               
               
               
               
            
               
                 North Sea 
                 82.7 
                 13.4 
                 3.9 
                 0 
                 0.839 
                 504 
               
               
                 West 
                 42.4 
                 36.1 
                 20.5 
                 1 
                 0.921 
                 506 
               
               
                 Africa 
               
               
                 France 
                 24.2 
                 43.4 
                 19.9 
                 12.4 
                 0.939 
                 508 
               
               
                   
               
            
           
         
       
     
       FIGS.  6 A and  6 B  are drawings of an uncooled microbolometer  602  using a metasurface geometry. For in situ downhole chemical analysis applications, a sample rate on the order of once every second or so is minimally acceptable. For the 15 cm −1  resolution achievable with the described MEMS FT-MIR interferometer and a minimum modulation frequency of 500 Hz, a 1 Hz sample rate to analyze the chemical spectral range of about 2000-500 cm −1  constrains the system to using only four interferogram sweeps per sample. This limited number of interferogram sweeps may not provide sufficiently low noise levels in the analysis spectra and the eventual applications may constrain the operation to lower frequency sample rates below 1 Hz. 
     Thus, in some embodiments, a much higher modulation frequency is used. For a 1000 Hz modulation frequency, the corresponding number of interferogram sweeps increases to eight, which should provide a lower noise level in the analysis spectra. This option though does degrade the detectivity of the sensor by about 35% so a trade-off in analysis would be required. 
     The change in resistance of the thermometric layer due to a temperature change caused by the absorption of radiation by the metasurface  604  is the response (or intensity) measured by the readout integrated circuit (ROIC)  606  in the form of a change in voltage drop across the contacts of the bridge under constant bias current. The bridge structure, including the metasurface  604 , dielectric substrate  608  and the thermometric layer (located underneath the dielectric substrate  608 ), is suspended over the ROIC  606  with an air gap using a set of thermal isolators  610  in order to minimize the thermal conduction path to heat generated in the absorbing layer, allowing the ROIC  606  to compound the effects of incident radiation and enhance the electrical signal created in response to changing field thermography. 
     A reduced thermal conduction path, though, must be balanced against increasing the thermal time constant and reducing the responsiveness to changing incident radiation. The mass of the bridge structure can be reduced in order to improve the response time, but may decrease electromagnetic absorptivity and increase voltage noise level on the detector. More rapid frame rates limit responsivity and detectivity while increased temperatures contribute to noise levels. Thus, high performance applications involving near-background radiation limited performance at rapid frame rates have been limited in practice to systems with active cooling. The predicted detector performance characteristics from integration of the inverted rhodonea geometry metasurface into a conventional uncooled microbolometer architecture can be determined, providing a figure of merit. 
     Detector Figures of Merit 
     In order to make a normalized comparison of the performance of different detectors, three parameters are generally used as figures of merit. These are voltage responsivity (R v ), signal to noise detectivity (D*), and total voltage noise level, usually given in terms of a noise equivalent difference temperature (NEDT). The voltage responsivity, R v , is a function of the output voltage signal and the temperature responsivity with changes in incident electromagnetic flux on the detector, and is given by the relation shown in Equation 1. 
     
       
         
           
             
               
                 
                   
                     R 
                     v 
                   
                   = 
                   
                     
                       
                         I 
                         b 
                       
                       ⁢ 
                       R 
                       ⁢ 
                       β 
                       ⁢ 
                       
                         R 
                         T 
                       
                     
                     = 
                     
                       
                         I 
                         b 
                       
                       ⁢ 
                       R 
                       ⁢ 
                       β 
                       ⁢ 
                       
                         
                           Δ 
                           ⁢ 
                           
                             T 
                             _ 
                           
                         
                         
                           Δ 
                           ⁢ 
                           
                             
                               ϕ 
                               _ 
                             
                             0 
                           
                         
                       
                     
                   
                 
               
               
                 
                   ( 
                   1 
                   ) 
                 
               
             
           
         
       
     
     In Equation 1, I b  is the bias current (amps, A), R is the bolometer electrical resistance (ohms, Ω), β is the thermometric layer temperature coefficient of resistance (TCR, 1/K), R T  is the temperature responsivity of the detector (K/W), ΔT is the complex variation in temperature of the detector (K), and Δφ 0  is the complex variation in incident radiation (W). 
     The detector signal to noise detectivity D* is defined as shown in Equation 2. 
     
       
         
           
             
               
                 
                   
                     D 
                     ⋆ 
                   
                   = 
                   
                     
                       R 
                       v 
                     
                     ⁢ 
                     
                       
                         
                           A 
                           d 
                         
                         
                           
                             4 
                             ⁢ 
                             kTR 
                           
                           + 
                           
                             
                               α 
                               H 
                             
                             [ 
                             
                               
                                 
                                   I 
                                   b 
                                   2 
                                 
                                 ⁢ 
                                 
                                   R 
                                   2 
                                 
                               
                               Nf 
                             
                             ] 
                           
                         
                       
                     
                   
                 
               
               
                 
                   ( 
                   2 
                   ) 
                 
               
             
           
         
       
     
     In equation 2, A d  is the detector area confronting the incident radiation, k=1.38×10 − 23 n−m/K (Boltzmann&#39;s constant), T is the absolute temperature (K) of the bridge structure, α H =0.002 (Hooge coefficient for homogenous semiconductor films), f is the modulation frequency, and N is the number of free carriers (electrons) in the thermometric material. 
     The noise equivalent difference temperature (NEDT) denotes the temperature change of a detector due to incident radiation that corresponds to an output signal equal to the RMS total noise level (a signal-to-noise ratio of 1). This is a fundamental parameter of the detector performance and represents the minimum temperature difference that can be discerned above the background noise. The NEDT is defined as shown in Equation 3. 
     
       
         
           
             
               
                 
                   NEDT 
                   = 
                   
                     
                       Δ 
                       ⁢ 
                       
                         V 
                         n 
                       
                       ⁢ 
                       
                         
                           Δ 
                           ⁢ 
                           T 
                         
                         
                           Δ 
                           ⁢ 
                           
                             V 
                             s 
                           
                         
                       
                     
                     = 
                     
                       Δ 
                       ⁢ 
                       
                         V 
                         n 
                       
                       ⁢ 
                       
                         
                           R 
                           T 
                         
                         
                           R 
                           v 
                         
                       
                     
                   
                 
               
               
                 
                   ( 
                   3 
                   ) 
                 
               
             
           
         
       
     
     In Equation 3, ΔV s  is the voltage change for a temperature change of ΔT on the detector, and ΔV n  is the root mean square (RMS) total noise voltage level as calculated by the relation shown in Equation 4. 
     
       
         
           
             
               
                 
                   
                     
                       Δ 
                       ⁢ 
                       
                         V 
                         n 
                         2 
                       
                     
                     
                       Δ 
                       ⁢ 
                       f 
                     
                   
                   = 
                   
                     
                       4 
                       ⁢ 
                       kTR 
                     
                     + 
                     
                       
                         α 
                         H 
                       
                       [ 
                       
                         
                           
                             I 
                             b 
                             2 
                           
                           ⁢ 
                           
                             R 
                             2 
                           
                         
                         Nf 
                       
                       ] 
                     
                   
                 
               
               
                 
                   ( 
                   4 
                   ) 
                 
               
             
           
         
       
     
     In equation 4, f is the modulation bandwidth. 
     In one embodiments, the metasurface detector design is based upon integration with a dielectric layer  608  formed of a single layer of Si 3 N 4  of about 200 nm in thickness  612 . The thermometric layer of VO 2 , located under the dielectric substrate  608 , is about 500 nm in thickness  614 . This is enabled by the low mass loading of the metasurface  604 , which is a gold layer of about 120 nm in thickness  616 . Specifically, the metasurface geometry has a 35% fill factor within a 150 μm diameter. As used herein, fill factor represents the amount of active material in the pattern of the metasurface  604 , for example, the gold forming a metasurface. 
     In another embodiment, the detector  602  has the dimensions shown in Table 1. In this embodiment, the thermometric layer of VO 2  is about 35 nm in thickness  614 , the dielectric substrate of Si 3 N 4  is about 100 nm in thickness  612 , and the metasurface  604  is about 27 nm in thickness  616 . As a result, the mass loading develops a maximum bending stress in the substrate of 6.3 kPa/g. The tensile strength of the VO 2  substrate is α ult =172 MPa giving an ultimate shock acceleration capability of greater than about 27000 g&#39;s, which is greater than required to sustain the expected worst case shock loads that could be experienced downhole in a production logging environment, for example, less than about 100 g&#39;s. As a result, confinement to single thermometric and dielectric layers is acceptable with this ultrathin metasurface for the expected downhole vibration and shock environments. 
     
       
         
           
               
             
               
                 TABLE 1 
               
               
                   
               
               
                 Summary of metamaterial microbolometer design properties 
               
               
                 (f = 500 Hz, Δf = 10 Hz). 
               
               
                   
               
             
            
               
                   
               
            
           
           
               
               
            
               
                 Si 3 N 4  dielectric substrate dimensions 
                 152 × 152 × 0.100 μm 3   
               
               
                 VO 2  thermometric substrate dimensions 
                 152 × 152 × 0.035 μm 3   
               
               
                 Metasurface envelope 
                 ø150 × 0.027 μm 3   
               
               
                 Maximum Absorption 
                 99.5% 
               
               
                 Ti electrode dimensions (4) 
                 1 × 0.5 × 150 μm 3   
               
            
           
           
               
               
               
            
               
                 Resistance, R 
                 9910 
                 Ω 
               
               
                 Bias Current, I b   
                 75 
                 μA 
               
               
                 Resistive Temperature Rise, ΔT   
                 2 
                 K 
               
            
           
           
               
               
            
               
                 TCR, β 
                 0.859 1/K    60° C. 
               
               
                 Thermal Conductance, G th   
                 3.0 × 10 −7  W/K 
               
               
                 Thermal Capacitance, C th   
                 7.8 × 10 −9  J/K 
               
            
           
           
               
               
               
            
               
                 Thermal Time Constant,    th   
                 26 
                 ms 
               
               
                 NEDT 
                 1 
                 mK 
               
               
                 Maximum Responsivity, R   
                 26 
                 kV/W 
               
            
           
           
               
               
            
               
                 Maximum Detectivity, D* 
                 1.06 × 10 10  cm{square root over (Hz)}/W    500 Hz 
               
               
                   
               
               
                     indicates data missing or illegible when filed 
               
            
           
         
       
     
       FIGS.  7 A and  7 B  are plots showing a comparison of the thermal properties for doped vanadium oxide (VO2) films  702  and  704  and an undoped vanadium oxide (VO2) film  706 .  FIG.  7 A  shows plots of the temperature dependence of electrical resistivity.  FIG.  7 B  shows plots of the temperature dependence of the thermal coefficient of resistance (TCR). 
     The normalized detectivity (D*) as given by Equation 2 is dependent upon the electrical resistivity and thermal coefficient of resistance of the thermometric VO 2  layer, while the noise equivalent difference temperature (NEDT) as given by Equation 3 is dependent upon the specific carrier density. The plots in  FIG.  7    clearly illustrates the metal-insulator-transition (MIT). For undoped VO 2  film  706  the data in  FIG.  7 A  indicates a resistivity at 60° C. (333 K) of about 3.4×10 − 2Ω−cm. The theoretical electron density of VO 2  has been calculated to be about 4×10 18 /cm 3 . Using these material properties for the VO 2  thermometric layer, along with Equation 1 for responsivity R v  Equation 2 for normalized detectivity (D*), and Equation 3 for noise equivalent difference temperature (NEDT) and simulation results for the metasurface absorptivity (using the MultiPhysics simulation software available from Comsol® of Stockholm, Sweden) predictions can be made for the metasurface detector performance figures of merit. Using the detector figures of merit as a set of discriminators, a series of analytical trade-off studies was conducted to optimize detector performance for a controlled detector cavity temperature of 60° C. and 500 Hz modulation frequency. The optimized detector figures of merit are detectivity (D*) of 1×10 10  cm*sqrt(Hz)/W at 333 K, and an NEDT of 1 mK. The results are based upon 75 μA bias current, which creates a latent resistive temperature rise of 2.0 K in the microbolometer. 
       FIG.  8    is a plot showing a comparison of spectral response for the metasurface detector  802 , in comparison to various commercially available IR detectors operated at different temperatures. The modulation frequency for all detectors is 1000 Hz, except for the state of the art uncooled thermistor bolometers  804  at 10 Hz and the metasurface detector  802  at 500 Hz. 
     The direct comparison in  FIG.  8    includes the detectivity spectrum of the metasurface detector at 500 Hz modulation frequency superimposed onto the spectra for various commercially available infrared and THz detector technologies operated at the noted temperatures and over the wavenumber range from 10000-250 cm −1 . The superimposed metasurface microbolometer spectrum indicates a maximum detectivity D* of 1.0×10 10  cm*sqrt(Hz)/W, which is comparable to the performance for the state of the art cryogenically cooled detectors. 
     In this work we present a mid-IR perfect metasurface absorber (PMA) design formed from a geometric inversion of the rhodonea conformal mapping contours. The PMA behaves as a near zero index metamaterial (NZIM) having intrinsic multiple coupled absorption resonances that combine to form broadband infrared absorption characteristics of more than 90% in the wavenumber range 1500-600 cm −1 . An uncooled microbolometer design is described that uses the metasurface geometry imprinted on a single Si 3 N 4  dielectric substrate with a single VO 2  thermometric substrate leading to a mid-IR detector with predicted maximum absorption of 99.5% at 870 cm −1  and an absorption bandwidth of 156% full-width half-maximum (FWHM) on 1070 cm −1  center wavenumber, coincident with important chemical spectra of downhole hydrocarbon fluids and emulsions. Figures of merit analyses for the uncooled microbolometer result in predicted maximum detectivity D*=1×10 10  cm*sqrt(Hz)/W and noise equivalent difference temperature NEDT of 1 mK at a modulation frequency of 500 Hz and a microbolometer temperature of 60 C. These uncooled microbolometer parameters indicate mid-IR interferometers can be miniaturized for downhole applications of in situ FT-THz spectroscopy. 
     Embodiments 
     An embodiment described in examples herein provides a microelectromechanical (MEMS) interferometer. The MEMS interferometer includes a pair of movable mirrors that are positioned along perpendicular axes, wherein each of the pair of movable mirrors is coupled to a mechanism. The mechanism includes an electrostatic actuator driving a displacement amplification mechanism, and the displacement amplification mechanism driving each of the pair of the movable mirrors. The MEMS interferometer includes a beam splitter that is positioned at an intersection of the perpendicular axes extending through each movable mirror and the beam splitter. The MEMS interferometer also includes a metasurface microbolometer placed in line with the beam splitter to measure an intensity of a recombined beam from the pair of movable mirrors. 
     In an aspect, the MEMS interferometer includes a single chip. In an aspect, the single chip is 12 mm×12 mm. 
     In an aspect, the electrostatic actuator includes a central actuator attached to a movable comb, wherein the movable comb include grounded tines. The electrostatic actuator also includes a positive comb including positive tines, wherein the positive tines are interspersed with the grounded tines on a first side of the movable comb. The electrostatic actuator also includes a negative comb including negative tines, wherein the negative tines are interspersed with the grounded tines on a second side of the movable comb. A sway stabilizer is attached to the central actuator at one end, and a coupling from the central actuator is attached to the displacement amplification mechanism at an opposite end from the sway stabilizer. 
     In an aspect, the displacement amplification mechanism includes a symmetric fulcrum coupled to a central actuator of the electrostatic actuator, and three serpentine release flexures, wherein the serpentine release flexures amplify the displacement from the central actuator to increase a motion of the movable mirror. In an aspect, the displacement amplification mechanism increases the motion of the movable mirror by a factor of eight over the motion of the central actuator. 
     In an aspect, the metasurface microbolometer includes a metasurface tuned to absorb a radiation in a range of frequencies in the mid infrared, a thermometric layer in contact a dielectric layer in contact with the metasurface, wherein the thermometric layer changes in resistivity with temperature changes. The metasurface microbolometer also includes a dielectric substrate supporting the thermometric layer and the metasurface, and a readout integrated circuit to measure a response from the thermometric layer including a voltage drop across the contacts of the bridge with a constant bias current. 
     In an aspect, the metasurface is an electrically conductive geometric pattern based on a geometrical inversion of rhodonea conformal mapping contours. In an aspect, the metasurface includes gold. In an aspect, the metasurface absorbs radiation through surface plasmon resonances. In an aspect, the metasurface is about 120 nm in thickness. In an aspect, the metasurface is less than 30 nm in thickness. In an aspect, the metasurface has a diameter of about 150 μm. in an aspect, the metasurface has a 35% fill factor. In an aspect, the metasurface has a detectivity (D*) of about 1×10 10  cm*sqrt(Hz)/W at 333 K at a bias current of 75 μA. In an aspect, the metasurface has a noise equivalent difference temperature (NEDT) of about 1 mK at a bias current of 75 μA. 
     In an aspect, the thermometric layer includes undoped vanadium oxide (VO 2 ). In an aspect, the thermometric layer is about 500 nm in thickness. In an aspect, the thermometric layer is less than 40 nm in thickness. 
     In an aspect, the dielectric substrate includes silicon nitride (Si 3 N 4 ). In an aspect, the dielectric substrate is about 200 nm in thickness. In an aspect, the dielectric substrate is about 100 nm in thickness. In an aspect, the dielectric substrate is separated from the readout integrated circuit by an airgap. 
     In an aspect, the metasurface microbolometer has a broadband absorption of more than 90% in a wavenumber range of about 1500 to about 600 cm −1 . In an aspect, the metasurface microbolometer has an absorption bandwidth of 156% of full width half maximum (FWHM) centered on a wavenumber of 1070 cm −1 . 
     Other implementations are also within the scope of the following claims.