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Terahertz real-time imaging uncooled array based on antenna- and cavity-coupled bolometers rsta.royalsocietypublishing.org
Research Cite this article: Simoens F, Meilhan J. 2014 Terahertz real-time imaging uncooled array based on antenna- and cavity-coupled bolometers. Phil. Trans. R. Soc. A 372: 20130111. http://dx.doi.org/10.1098/rsta.2013.0111
One contribution of 11 to a Discussion Meeting Issue ‘Beyond Moore’s law’.
Subject Areas: microsystems, nanotechnology, electromagnetism, solid state physics Keywords: antenna-coupled bolometer terahertz array, terahertz real-time imaging, terahertz uncooled two-dimensional camera Author for correspondence: François Simoens e-mail: [email protected]
François Simoens and Jérôme Meilhan CEA-Leti MINATEC, 17 rue des Martyrs, Grenoble Cedex 9 38054, France The development of terahertz (THz) applications is slowed down by the availability of affordable, easyto-use and highly sensitive detectors. CEA-Leti took up this challenge by tailoring the mature infrared (IR) bolometer technology for optimized THz sensing. The key feature of these detectors relies on the separation between electromagnetic absorption and the thermometer. For each pixel, specific structures of antennas and a resonant quarter-wavelength cavity couple efficiently the THz radiation on a broadband range, while a central silicon microbridge bolometer resistance is read out by a complementary metal oxide semiconductor circuit. 320 × 240 pixel arrays have been designed and manufactured: a better than 30 pW power direct detection threshold per pixel has been demonstrated in the 2–4 THz range. Such performance is expected on the whole THz range by proper tailoring of the antennas while keeping the technological stack largely unchanged. This paper gives an overview of the developed bolometer-based technology. First, it describes the technology and reports the latest performance characterizations. Then imaging demonstrations are presented, such as realtime reflectance imaging of a large surface of hidden objects and THz time-domain spectroscopy beam twodimensional profiling. Finally, perspectives of camera integration for scientific and industrial applications are discussed.
1. Introduction Terahertz (THz) radiation, loosely defined to be between 0.3 and 10 THz, combines many unique properties: non-ionizing penetration through non-metallic and nonpolar materials, high sensitivity to water content, spatial resolution 10 times shorter than for millimetre wave radiation, and specific spectral fingerprints for many 2014 The Author(s) Published by the Royal Society. All rights reserved.
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Without modifying the three-dimensional pixel architecture of a standard IR bolometer, THz absorption can be optimized by the adjustment of the equivalent impedance of the standard
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substances. These features open the way to promising applications in many interdisciplinary fields [1–4], many of which require two-dimensional imaging. The first demonstrations of THz two-dimensional imaging [5] were based on optoelectronic THz time-domain spectroscopy (THz-TDS). Since then, various different imaging modalities have been developed and demonstrated. However, most of them use a single detector and therefore samples need to be scanned in two dimensions relative to the fixed terahertz beam, or vice versa. Mechanical scans limit inexorably the data acquisition rate and point-to-point measurements are preferably applied to stationary objects. Additionally, such systems tend to be bulky and complicated. The spread of applications using THz imaging would be greatly increased by the availability of affordable, compact, easy-to-use and highly sensitive detectors. In particular, large-format (manypixel) focal plane arrays (FPAs) integrated in compact and hand-held cameras would enable fast two-dimensional image acquisition; the camera would operate in video mode, recording twodimensional data in real time without the need for raster scanning. Such features are possessed by thermal infrared (IR) array sensors based on bolometers [6]. These monolithic FPAs are collectively processed above complementary metal oxide semiconductor (CMOS) read-out integrated circuits (ROICs) with standard silicon technology. The optically sensitive surface may gather a large number of pixels [7]. These sensors operate at room temperature. And finally the CMOS ROIC provides advanced functions of signal processing before delivering video output. Since the 1990s CEA-Leti [8,9] has been developing such detectors with the choice of amorphous silicon (a-Si) as the thermometer material, whereas most of the other players have chosen vanadium oxide. In 2005, the Massachusetts Institute of Technology group [10] demonstrated real-time THz imaging using a commercial uncooled bolometer IR camera in association with quantum cascade lasers (QCLs). Several other institutes and companies [11,12] tested this imaging setup configuration, including the authors’ group [13]. In a common way, these tests showed that significant improvements in sensitivity could be advantageously achieved by designing bolometer FPAs specifically for THz frequencies. CEA-Leti took up this challenge by tailoring its proprietary bolometer technology for optimized THz sensing. Such a development was initiated a few years ago with two constraints as guidelines. First, the pixel structure had to be as close as possible to the existing a-Si IR bolometer stack in order to benefit from the maturity of this technology that has demonstrated high performance and high fabrication yield [14]. The second constraint involved the design of a technological flow chart fully compatible with standard CMOS microelectronic equipment. Indeed, similarly to other running developments of THz arrays based on CMOS field effect transistors [15,16] or Schottky diodes [17], bolometers can reach very-large-scale integration as a result of silicon process technologies. The combination of these two requirements meets the criteria of low cost and low SWAP (size, weight and power) that any commercial camera has to target. This trend is followed by the IR thermal bolometer arrays: the prices have decreased steadily and now meet the demands of large-volume applications, such as automotive [18] or home appliances. This paper summarizes the results of the Leti bolometer-based technology development. First, it describes the pixel array structure and the chips prototyped with two different FPA formats. Then it reports the modelling and the characterizations of the main figures of merit. The following section shows imaging tests carried out with a prototype of a compact camera integrating the FPA. Then a specific section focuses on the demonstration of terahertz imaging capabilities of a complete real-time reflection imaging system. Perspectives of camera integration for scientific and industrial applications conclude this paper.
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microbridge structure. It can be done in a quite straightforward way by tuning the sheet resistance of a metallic film deposited on the existing suspended membrane. This method has been applied for the first time by NEC [19], and then followed by INO [20]. Such cameras are now commercialized but still the lack of a quarter-wavelength cavity necessarily hampers the optical coupling efficiency. In addition, when THz frequencies smaller than 3 THz are being targeted, the mismatch between membrane size and wavelength will quickly degrade the published noise equivalent power (NEP) (100 pW in the 3–4 THz range). The CEA-Leti THz pixel three-dimensional [21] stack differs substantially from standard IR bolometers. The key feature of this architecture relies on the separation between the two main functions of such sensors, i.e. the electromagnetic absorption and the thermometer. Complementary to an optimized quarter-wavelength cavity, antennas are coupled to the bolometer to perform optical radiation absorption. Crossed quasi-double-bowtie resistively loaded antennas are designed to collect the THz flux of any polarization (figure 1a). The longer bowtie antenna located on the suspended membrane, named ‘DC’ for direct coupling, is directly excited by the incident THz wave whose polarization is aligned along the axis of this antenna. In order to couple the other impinging polarization, a large antenna, capacitive coupling (CC), is located below the microbridge. The surface currents induced in this antenna are coupled via a capacitive mechanism to metallic planes deposited on the suspended membrane. This two-storied antenna architecture permits the tuning of the optical coupling of both polarizations independently and with no mutual interferences, as shown later. The residual radiation that is not absorbed by the antennas is partially recovered owing to a specific resonant quarter-wavelength cavity; as a result, the optical absorption is typically enhanced from 60–70% to 80–90% at resonance. Surface currents created on the metallic antenna planes are coupled onto the central silicon microbridge that integrates matched antenna resistive loads. The Joule power dissipated in the resistive loads induces a bolometer resistance change that is read out at the pixel level by a front-end CMOS circuit. The process above the integrated circuit (IC) of the complete THz bolometer pixel stack is fully CMOS foundry compatible. THz arrays are collectively manufactured above the IC wafer using only the classical steps of microelectronic photolithography, deposition process and etching. The main technological key [22] is the etching and metallization of through-oxide-vias (TOVs) through the thick dielectric cavity. TOVs electrically connect the bolometer resistance with the CMOS upper metallic contacts. This process ensures the fabrication of a monolithic two-dimensional
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Two formats of THz FPAs have been designed, processed, tested and used for imaging demonstrations. First, a large 320 × 240 pixel chip has been developed and integrated in a compact video camera housing [25]. A second chip integrates THz bolometers with visible and IR pixels to act as a tri-spectral monolithic detector [26]. The 320 × 240 array (figure 1b) is composed of 50 µm pitch pixels designed for an optimized sensitivity in the 2–4 THz range where most of the explosive fingerprints are located and QCLs are efficient [27,28]. Extensive three-dimensional finite-element method (FEM) simulations [29] were carried out with the commercial High Frequency Structure Simulator (HFSS) and Comsol codes to design two kinds of pixels. They are optimized at 1.7 and 2.4 THz for CC polarization while maintaining maximum absorption at 1.35 THz for the DC direction. A dedicated CMOS application-specific integrated circuit (ASIC) has been designed by CEALeti teams to ensure low-noise measurement of the resistances of the 320 × 240 bolometers and to format the resulting signals into a single video data stream. Thanks to state-of-the-art Si microelectronic facilities and robust bolometer technology knowhow, a very high yield has been achieved: more than 99.5% of the 320 × 240 pixels are functional for 56 out of the 63 chips available per 200 mm wafer. The monolithic tri-spectral array detector integrates three types of sensors, all of them operating at room temperature. For the visible channel, photodiodes are implemented within the ASIC while the IR and THz detectors consist, respectively, of a-Si 25 µm pitch and 50 µm pitch microbolometers. The central region of the FPA is occupied by 32 × 32 THz pixels while the surroundings combine IR and visible pixels. One major issue [30] was to work out a technological stack compatible with the simultaneous process of IR and THz bolometric pixels above the CMOS wafer. Another important challenge was the design of an ASIC that efficiently reads and outputs the signal generated by the three channels [31]. In spite of this very challenging tri-spectral integration gathering very diverse pixels, a satisfactory fabrication yield has been achieved: several chips have been integrated in vacuum packaging with diamond windows and tested for real-time imaging, as presented in §4. As common features, the THz bolometric pixels of these arrays present thermal time constants between 20 and 40 ms and thermal resistances of 50 MK W−1 .
4. Modelled and measured figures of merit Extensive work on modelling of the figures of merit has been carried out, of both the collection and thermometer functions. Following this design phase, specific experimental developments and tests of the prototyped chips were undertaken in order to assess the numerical results.
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sensor with a large number of pixels. Moreover, the resulting vertical architecture means that pixels are separated from the CMOS ROIC by the metallic reflector plane; such a feature prevents electromagnetic perturbation of the optical coupling by ROIC metallic lines as encountered in [23]. The antenna sizes and shapes can be chosen independently from the bolometric device to match the illumination characteristics, in particular the frequency range and the polarization. This arrangement is very versatile with respect to frequency [24]. When designing the pixel, the antennas and the resonant cavity can be sized to match the wavelength, whereas the central bolometer structure can be kept small; as a result, the response time and the heat to current conversion efficiency are preserved. Crossed polarized antenna structures have been implemented to make the bolometer sensitive to both transverse electric (TE) and transverse magnetic (TM) polarizations, but it is possible to integrate only one of these two polarizations in the pixel structure.
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(a) Spectral absorption One major figure of merit of any FPA is the spectral absorption. Few publications show spectral absorption characterized continuously in a large THz range. However, this feature represents the optical collection efficiency and is then directly related to the FPA sensitivity. Actual instruments and procedures are lacking. Hence, special efforts have been dedicated to developing experimental methods to extract spectral features of our two-dimensional array sensor with the help of Bruker Optics Vertex 80 V Fourier transform spectrometers (FTSs). First, the reflectivity of the sensor surface was measured. A preliminary reference interferogram is acquired with a metallic mirror, and a second spectrum is recorded in which the FPA replaces the mirror. Assuming that the reference mirror is perfectly reflective, the Fourier transform of the ratio of sample to reference interferograms represents the spectral reflectivity of the array. This measurement was first applied to a 160 × 120 pixel prototype array processed above a raw Si wafer instead of an ASIC. Without an ASIC to ensure the multiplexing, specific metallic meander lines at the reflector backplane level address electrically 16 pixels. Hence, this configuration makes possible the reading of the signal sensed by individual pixels selected within the array. The pixels of this array are designed for optimized coupling at 1.35 and 1.7 THz, respectively, for the ‘DC’ and ‘CC’ antennas. Figure 2a,b shows the spectral reflectivity Ropt (f ) measured, respectively, for the DC and CC. Polarization is controlled with a wire grid polarizer inserted in the set-up. This experimental result is compared with the |S11 |2 parameter (dotted line of figure 2) extracted from threedimensional FEM simulations in which each pixel of the array is considered as a Floquet cell [29]. A very good match between simulations and measurements is observed over the whole frequency range: a distinct absorption peak arises (at 1.35 and 1.7 THz) as a dip in reflectivity and the shapes follow the same trends. However, this measurement is not sufficient to assess the useful pixel absorption efficiency that is the ratio between the thermal heating of the antenna loads and the incident wave power. Moreover, this method overestimates the spectral absorption deduced from αopt (f ) = 1 − Ropt (f ). Indeed, some power losses in the stack—such as Joule dissipation within the dielectric cavity—do not contribute to the bolometric signal.
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In order to ascertain the bolometric conversion efficiency, direct measurement of the spectral response of the sensor has to be performed. The collimated beam delivered by the interferometer is deflected outside of the FTS and is focused on the detector array enclosed inside a vacuum laboratory test vessel. Resulting interferograms include spectral absorption of the devices convoluted to the spectral features of the source and of the optical chain. This measurement is referenced to an interferogram obtained in a similar operation scheme with the FTS internal pyroelectric detector to retrieve the relative spectral response of the bolometer. The relative spectral response is normalized to the maximum peak. As shown by figure 2c,d, good agreement is noted between the empirical and theoretical curves. One can observe comparable performance and a large absorption peak in the vicinity of 1.35 and 1.7 THz, respectively, for DC and CC antennas. The observed weak oscillations and slight shift of the experimental resonance comes mainly from the atmospheric signatures that alter the signal during its transmission in air between the FTS and the sensor. The absorption spectra extracted from modelling and measurements (figure 3) are in agreement with the ones obtained in the reflectivity configuration. These characterizations give us an upper bound for the useful absorption of our device and locations of resonance peaks. The developed procedures make possible the evaluation of the ‘useful’ spectral absorption of the THz array sensor that is an essential figure of merit when one seeks to estimate and to compare devices’ performance.
(b) Noise equivalent power Another key performance metric for the detector is the NEP. Mathematically, it is the ratio between the output noise voltage spectral density vn and the detector voltage responsivity Rv . This quantity can be reported at various circuit levels, from the direct signal generated at the pixel output, i.e. the current/voltage change in the resistance for a resistive bolometer, to the output of the overall FPA imaging chain operated in video mode. The latter NEP requires only an a priori knowledge of the total impinging source power. This method is applied to the prototyped 320 × 240 pixel array with the 2.4 THz design, and is operated at a video rate of 25 Hz. A rowwise reading scheme of the ROIC allows the current of the bolometer to be integrated by the 5 pF capacitance of its associated trans-impedance amplifier during 125 µs per pixel. The sensitivity measurement set-up that combines a QCL [32] emitting at 2.5 THz and a Thomas Keating Instruments power meter is detailed in a previous paper [33]. In a general way, no filter is used during these experiments.
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Real-time active imaging capabilities of the prototyped two-dimensional FPAs have been tested in several set-ups, in both reflection and transmission configurations. First, simple optical arrangements were tested in which samples intercept the collimated beam of a THz QCL. As shown in figure 4a,b, the 320 × 240 THz array is able to deliver real-time images of objects, respectively a scalpel blade and Leti characters written with metallic ink on paper, both concealed in a postal envelope [28]. Speckle patterns arising from the QCL source coherence degrade the image quality. THz antenna- and cavity-coupled bolometers also present ultra-wideband detection capabilities. Even if the prototyped 320 × 240 FPAs are optimized for centre resonance frequencies in the 2 THz range, significant absorption remains below 1 THz. Therefore, the array can deliver real-time frames of the THz beam produced by longer wavelength sources. Two essay campaigns demonstrated sensitive two-dimensional real-time profiling of the beam delivered by classical TDS set-ups [34]. During these experiments, the beam generated by the photoconductive emitter propagates in the ambient atmosphere and is focused on the FPA by off-axis parabolic mirrors. In figure 4c,d [35], the beam delivered by the TDS is imaged with a 29 dB signal to noise ratio (SNR) at peak; detected integrated THz power of the source emitting as low as 25 nW has been resolved with sufficient SNR to observe tiny details of the beam shape. The characterization of the second type of prototyped array described in §3 (figure 5b) showed state-of-the-art performance for each channel [26]. The figure 5a presents the visible and THz channel outputs when a paper optical test pattern is standing in the way of a QCL beam optical path. The 32 × 32 THz pixels of the array centre profile the QCL beam while visible photodiodes image the object at the periphery. In figure 5c, thermal IR imaging by the IR pixels of the two-dimensional sensor is demonstrated separately using f /1 germanium optics.
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First, the responsivity at 2.5 THz corresponding to the CC polarization is measured. As a total power of 238 µW illuminates the device, the sum of the pixel voltages over the array is 3000 V. The FPA responsivity is calculated as the ratio of the integrated signal to the total impinging −1 power and is found to be RCC vtotal = 12.6 MV W . The same measurement performed for the cross-polarization state, i.e. the DC antenna, leads to a lower total responsivity RDC vtotal = 4.3 MV W−1 . Imaging RMS noise vn including pixel noise, ROIC and digitizing chain contributions is measured to be 400 µV RMS. The smallest variation of impinging power that can be detected is calculated as the ratio of RMS noise to the device responsivity. This detection power threshold reaches 32 pW for the CC polarization at 2.5 THz and is better than 30 pW at the resonance frequency. During the design of a detector sensitive to unpolarized radiation, one of the main challenges is to obtain cross-polarized antennas featuring both high absorption efficiency and no mutual interferences. Cross antennas built at the same technological level suffer from mutual coupling that averages and lowers the absorption efficiency independently from the polarization state. As described in §2, the two-storied crossed antenna architecture was chosen to remove this unwanted mutual coupling. This property has been experimentally tested by rotating the FPA angle θ from 0◦ to 90◦ with respect to the linearly polarized incident QCL beam—controlled with wire grid polarizers. For θ = 0◦ , the TM polarization of the QCL corresponds to the DC antenna orientation. As illustrated in figure 3b, the resulting output integrated signal of DC cos(θ)2 + V CC sin(θ)2 . Moreover, the ratio of the device follows a mathematical function Vout out integrated output signals is in correspondence to the optical absorption efficiencies of DC and CC antennas (figure 3a). These results demonstrate the decoupling property of this crossed antenna three-dimensional structure. However, one has to point out that, if needed, it is possible to design a square law detector in which the CC and DC antennas spectral absorptions are similar.
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Figure 4. Examples of frames extracted from video sequences. (a) Images of a scalpel blade and of metallic ink Leti characters written on paper (b), both concealed in an envelope. Profiles of the beam emitted by a TDS emitter before (c) and after (d) optical alignment. (Online version in colour.) (a)
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5. Imaging tests in a reflection system Imaging demonstrations shown in the previous section have motivated the development of a complete reflection active THz imaging demonstrator combining QCLs with the 320 × 240 THz FPA (figure 6a).
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The illumination beam is delivered by multiple QCL ridges in order to optimize the available optical power. The number of QCL ridges operating in continuous wave mode was chosen to fit the cooling power of the cryocooler. An innovative pulsed tube (PT) cryocooler developed by CEA-INAC has enabled such integration. This PT provides a 20 W cooling power on the cold finger, ending up with the choice of a 30 K temperature operation and four combined QCL ridges. The source is capable of delivering continuously tens of mW with dedicated fast driving electronics and switches to gate the laser. The optical beam is then guided by a specific optical setup that points the beam towards the scene. A metallic horn ensures beam mixing to provide better illumination uniformity. A specifically designed Fresnel lens is interposed in the beam optical path to magnify the output of the rectangular horn. The imaging plane is located at 1 m from the demonstrator housing, whose inside atmosphere is dried to minimize water vapour absorption. The illuminated area size is 40 × 60 mm2 . A large plane mirror is used to illuminate the scene and to collect the part of the beam that is reflected and backscattered by the imaged objects. Then a folded Newton telescope composed of an 80 cm paraboloid mirror focuses the beam onto the camera FPA with an equivalent f /0.8 relative aperture. The optical path from the QCL-based source to the camera is 4 m long, of which 2 m are in air. The camera comprises a dedicated vacuum-sealed packaging topped by a Zeonex window. The operation of the array under the vacuum atmosphere—typically 10−3 mbar—is necessary for high sensitivity, as for any bolometer sensor. The camera also includes the front-end electronics made of two different cards. The first card delivers low-noise analogue voltages and digitizes the chip output voltage. The second card, a field-programmable gate array (FPGA) board, drives the sensor and ensures synchronization between the camera and external devices. Evaluation of optical quality and resolution has been performed with the help of absorbing optical test patterns placed in front of a metallic plane mirror. The image of the Siemens star is flawed by specular interferences induced by the coherence of the source (figure 6b). Then the slow time response of the bolometer is turned into an advantage to blur the speckle patterns: by oscillating axially the Fresnel lens, a large improvement of image quality is obtained as illustrated in figure 6c. A second imaging test has been carried out with a Leti writing test pattern of various sizes (figure 6d). Letter details of dimensions close to 2 mm are being resolved and represent the outmost resolution that can be achieved by this reflection imaging system demonstrator. In spite
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An innovative monolithic two-dimensional array sensor technology has been developed with the aim of fulfilling the need for affordable, easy-to-use and highly sensitive detectors for realtime THz imaging. Derived from the mature uncooled thermal IR bolometers, an innovative architecture of monolithic array based on the use of antennas and of a resonant quarterwavelength cavity has been designed. Thanks to the maturity of bolometer technology and to full compatibility with silicon CMOS processes, technological barriers have been lifted. Two formats of THz arrays optimized for sensing in the 1–5 THz range have been processed above CMOS ASICs: a 320 × 240 THz sensor and a tri-spectral chip. A high manufacturing yield has been observed with, for example, 56 out of 63 chips available in a 200 mm wafer. A NEP better than 30 pW per pixel has been characterized at 2.5 THz for the 320 × 240 FPA integrated in a camera and operated in video mode, making this uncooled sensor the state of the art of such devices. Three-dimensional modelling studies of this sensor have shown good agreement with the measured features, such as the broadband spectral absorption and non-mutual coupling between the two crossed polarizations that this pixel structure can collect. Real-time imaging tests have highlighted the ease of use and the high sensitivity and resolution achieved by this broadband array detector. Further technological developments will target performance improvement of the prototyped sensor fed by ongoing progress in IR bolometer technology. Moreover, the confirmed good mastery of the electromagnetic design of our structure motivates the development of an array optimized in the 0.6–1 THz range where atmospheric and material transmission are more favourable. The short-term aim of developing a commercial camera for scientific use has proved to be very valuable in helping to align any THz set-up in real time. Meanwhile, industrial applications requiring penetrating imaging with high resolution are being investigated through tests of this array in a transmission or reflection configuration. A compact active imaging system would fulfil the demands of various fields of applications, such as homeland security, biology and medical sciences, non-destructive controls for industry (e.g. pharmaceutical, aerospace and microelectronics), quality control of food and agricultural products, global environmental monitoring, and the study of historical and archaeological work. If appropriate, the imaging functionality can be considered in combination with spectral analysis for chemical identification, provided by the sensor itself associated with multicolour illumination or possibly with a separate spectrometer. Acknowledgements. The support from CEA as well as from the EU FP7 project MUTIVIS is gratefully acknowledged. The authors also acknowledge the Leti team and academic institutions—in France, University of Savoie, University of Paris 7, University of Rennes, Ecole Normale Supérieure de Paris, and in Italy, the research centre ‘Fondazione di Bruno Kessler’—for their fruitful collaboration.
References 1. Jansen C et al. 2010 Terahertz imaging: applications and perspectives. Appl. Opt. 49, E48–E57. (doi:10.1364/AO.49.000E48) 2. Mittleman DM, Jacobsen RH, Nuss MC. 1996 T-ray imaging. IEEE J. Sel. Top. Quantum Electron. 2, 679–692. (doi:10.1109/2944.571768) 3. Withayachumnankul W et al. 2007 T-ray sensing and imaging. Proc. IEEE 95, 1528–1558. (doi:10.1109/JPROC.2007.900325) 4. Tonouchi M. 2007 Cutting-edge THz technology. Nat. Photonics 1, 97–105. (doi:10.1038/ nphoton.2007.3)
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of imperfectly homogeneous illumination, the SNR is sufficient to perform real-time imaging of hidden objects because this ‘Leti’ test pattern has been successfully imaged (figure 6d) once hidden below a nylon clean room coat [33].
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5. Hu BB, Nuss MC. 1995 Imaging with terahertz waves. Opt. Lett. 20, 1716–1718. (doi:10.1364/ OL.20.001716) 6. Simoens F. 2014 THz bolometer detectors. In Physics and Applications of Terahertz Radiation (eds M Perenzoni, DJ Paul), pp. 35–75. Springer Series in Optical Sciences, vol. 173. Dordrecht, The Netherlands: Springer Science+Business Media. 7. Tissot JL, Tinnes S, Durand A, Minassian C, Robert P, Vilain M. 2010 High performance uncooled amorphous silicon VGA and XGA IRFPA with 17µm pixel-pitch. Proc. SPIE 7834, 78340K-1-8. (doi:10.1117/12.864867) 8. Yon JJ, Astier A, Bisotto S, Chamings G, Durand A, Martin JL, Mottin E, OuvrierBuffet JL, Tissot JL. 2005 First demonstration of 25 µm pitch uncooled amorphous silicon microbolometer IRFPA at LETI-LIR. Proc. SPIE 5783, 432–440. (doi:10.1117/12.606487) 9. Yon JJ, Mottin E, Tissot JL. 2008 Latest amorphous silicon microbolometer developments at LETI-LIR. Proc. SPIE 6940, 6940–6961. (doi:10.1117/12.780538) 10. Lee AWM, Hu Q. 2005 Real-time, continuous-wave terahertz imaging by use of a microbolometer focal-plane array. Opt. Lett. 30, 2563–2565. (doi:10.1364/OL.30.002563) 11. Behnken BN, Karunasiri G, Chamberlin DR, Robrish PR, Faist J. 2008 Real-time imaging using a 2.8 THz quantum cascade laser and uncooled infrared microbolometer camera. Opt. Lett. 33, 440–442. (doi:10.1364/OL.33.000440) 12. Oda O. 2008 Detection of terahertz radiation from quantum cascade laser, using vanadium oxide microbolometer focal plane arrays. Proc. SPIE 6940, 69402Y. (doi:10.1117/12.781630) 13. Simoens F, Durand T, Meilhan J, Gellie P, Maineult W, Sirtori C, Barbieri S, Beere H, Ritchie D. 2009 Terahertz imaging with a quantum cascade laser and amorphous-silicon microbolometer array. In Proc. SPIE Millimetre Wave and Terahertz Sensors and Technology, Berlin, Germany, 1–3 September 2009 (eds KA Krapels, NA Salmon), p. 74850M. Proceedings of SPIE, vol. 7485. Bellingham, WA: SPIE. (doi:10.1117/12.830350) 14. Crastes A, Tissot JL, Vilain M, Legras O, Tinnes S, Minassian C, Robert P, Fieque B. 2008 Uncooled amorphous silicon 1/4 VGA IRFPA with 25 µm pixel-pitch or high end applications. In Proc. Infrared Technology and Applications XXXIV, Orland, FL, 16 March 2008 (eds BF Andresen, GF Fulop, PR Norton), p. 69401X. Proceedings of SPIE, vol. 6940. Bellingham, WA: SPIE. (doi:10.1117/12.779488) 15. Al Hadi R, Sherry H, Grzyb J, Zhao Y, Förster W, Keller HM, Cathelin A, Kaiser A, Pfeiffer UR. 2012 A 1 k-pixel video camera for 0.7–1.1 terahertz imaging applications in 65-nm CMOS. IEEE J. Solid-State Circuits 47, 1–14. (doi:10.1109/JSSC.2012.2217851) 16. Boppel S, Lisauskas A, Max A, Krozer V, Roskos HG. 2012 CMOS detector arrays in a virtual 10-kilopixel camera for coherent terahertz real-time imaging. Opt. Lett. 37, 536–538. (doi:10.1364/OL.37.000536) 17. Han R, Zhang Y, Coquillat D, Videlier H, Knap W, Brown E, O KK. 2011 A 280-GHz Schottky diode detector in 130-nm digital CMOS. IEEE J. Solid-State Circuits 46, 2602–2612. (doi:10.1109/JSSC.2011.2165234) 18. Bercier E, Robert P, Pochic D, Tissot JL, Arnaud A, Yon JJ. 2012 Far infrared imaging sensor for mass production of night vision and pedestrian detection systems. In Advanced Microsystems for Automotive Applications 2012: Smart Systems for Safe, Sustainable and Networked Vehicles (ed. G Meyer), pp. 301–312. Berlin, Germany: Springer. 19. Oda N et al. 2012 Development of terahertz focal plane arrays and handy camera. Proc. SPIE 8012, 80121B. (doi:10.1117/12.888992) 20. Bolduc M et al. 2011 Noise-equivalent power characterization of an uncooled microbolometerbased THz imaging camera. Proc. SPIE 8023, 80230C. (doi:10.1117/12.883507) 21. Peytavit E, Agnèse P, Ouvrier Buffet J-L, Beguin A, Simoens F. 2005 Room temperature terahertz microbolometers. Proc. IRMMW-THz 1, 257–258. 22. Pocas S et al. 2013 Technological customization of uncooled amorphous silicon microbolometer for THz real time imaging. Proc. SPIE 8624, 862419. (doi:10.1117/12.2001259) 23. Luukanen A, Miller AJ, Grossman EN. 2004 Active millimeter-wave video rate imaging with a staring 120-element microbolometer array. Proc. SPIE 5410, 195–201. (doi:10.1117/12.543488) 24. Meilhan J, Simoens F, Lalanne-Dera J, Gidon S, Lasfargues G, Pocas S, Nguyen DT, OuvrierBuffet J-L. 2012 Terahertz frequency agility of uncooled antenna-coupled micro-bolometer arrays. In Proc. 37th Int. Conf. IEEE Infrared, Millimeter, and Terahertz Waves (IRMMW-THz), Wollongong, NSW, 23–28 September 2012. Piscataway, NJ: IEEE. 25. Simoens F, Meilhan J, Pocas S, Ouvrier-Buffet JL, Maillou T, Gellie P, Barbieri S. 2010 THz uncooled microbolometer array development for active imaging and
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spectroscopy applications. In Proc. 35th Int. Conf. IEEE Infrared Millimeter and Terahertz Waves (IRMMW-THz), Houston, TX, 5–10 September 2010, pp. 1–2. Piscataway, NJ: IEEE. (doi:10.1109/ICIMW.2010.5612553) Perenzoni M, Massari N, Stoppa D, Pocas S, Delplanque B, Meilhan J, Simoens F, Rabaud W. 2012 A 160 × 160-pixel image sensor for multispectral visible, infrared and terahertz detection. In Proc. 38th European Solid-State Circuits Conference (ESSCIRC 2012), Bordeaux, France, 17–21 September 2012, pp. 93–96. Piscataway, NJ: IEEE. (doi:10.1109/ESSCIRC.2012.6341264) Simoens F et al. 2010 Development of uncooled antenna-coupled microbolometer arrays for explosive detection and identification. Proc. SPIE 7837, 78370B. (doi:10.1117/12.865189) Meilhan J et al. 2011 Active THz imaging and explosive detection with uncooled antennacoupled microbolometer arrays. Proc. SPIE 8023, 80230E. (doi:10.1117/12.883898) Nguyen DT, Simoens F, Ouvrier-Buffet J-L, Meilhan J, Coutaz J-L. 2012 Simulations and measurements of the electromagnetic response of broadband THz uncooled antennacoupled microbolometer array. IEEE Trans. Terahertz Sci. Technol. 2, 299–305. (doi:10.1109/ TTHZ.2012.2188395) Pocas S et al. 2012 Innovative monolithic detector for tri-spectral (THz, IR, Vis) imaging. In Proc. Millimetre Wave and Terahertz Sensors and Technology V, Edinburgh, UK, 24 September 2012 (eds NA Salmon, EL Jacobs), p. 85440F. Proceedings of SPIE, vol. 8544. Bellingham, WA: SPIE. (doi:10.1117/12.974970) Perenzoni M, Massari N, Pocas S, Meilhan J, Simoens F. 2010 A monolithic visible, infrared and terahertz 2D detector. In Proc. 35th Int. Conf. IEEE Infrared Millimeter and Terahertz Waves (IRMMW-THz), Houston, TX, 5–10 September 2010. Piscataway, NJ: IEEE. Barbieri S, Alton J, Beere HE, Fowler J, Linfield EH, Ritchie DA. 2004 2.9THz quantum cascade lasers operating up to 70 K in continuous wave. Appl. Phys. Lett. 85, 1674. (doi:10.1063/ 1.1784874) Simoens F et al. 2012 Complete THz system for reflection real-time imaging with uncooled antenna-coupled bolometer arrays. In Proc. 37th Int. Conf. IEEE Infrared, Millimeter, and Terahertz Waves (IRMMW-THz), Wollongong, NSW, 23–28 September 2012. Piscataway, NJ: IEEE. Oden J, Meilhan J, Lalanne-Dera J, Roux J-F, Garet F, Coutaz J-L, Simoens F. 2013 Imaging of broadband terahertz beams using an array of antenna-coupled microbolometers operating at room temperature. Opt. Expr. 21, 4817–4825. (doi:10.1364/OE.21.004817) Meilhan J, Oden J, Cavalié P, Dillhon S, Roux J-F, Coutaz J-L, Tignon J, Simoens F. 2013 Roomtemperature video-rate imaging of terahertz pulses generated by femtosecond lasers using an array of micro-bolometers. In Proc. Int. Workshop on Optical Terahertz Science and Technology 2013, Kyoto Terrsa, Japan, 1–5 April 2013. Kyoto, Japan: Kyoto University.
Terahertz real-time imaging uncooled array based on antenna- and cavity-coupled bolometers rsta.royalsocietypublishing.org
Research Cite this article: Simoens F, Meilhan J. 2014 Terahertz real-time imaging uncooled array based on antenna- and cavity-coupled bolometers. Phil. Trans. R. Soc. A 372: 20130111. http://dx.doi.org/10.1098/rsta.2013.0111
One contribution of 11 to a Discussion Meeting Issue ‘Beyond Moore’s law’.
Subject Areas: microsystems, nanotechnology, electromagnetism, solid state physics Keywords: antenna-coupled bolometer terahertz array, terahertz real-time imaging, terahertz uncooled two-dimensional camera Author for correspondence: François Simoens e-mail: [email protected]
François Simoens and Jérôme Meilhan CEA-Leti MINATEC, 17 rue des Martyrs, Grenoble Cedex 9 38054, France The development of terahertz (THz) applications is slowed down by the availability of affordable, easyto-use and highly sensitive detectors. CEA-Leti took up this challenge by tailoring the mature infrared (IR) bolometer technology for optimized THz sensing. The key feature of these detectors relies on the separation between electromagnetic absorption and the thermometer. For each pixel, specific structures of antennas and a resonant quarter-wavelength cavity couple efficiently the THz radiation on a broadband range, while a central silicon microbridge bolometer resistance is read out by a complementary metal oxide semiconductor circuit. 320 × 240 pixel arrays have been designed and manufactured: a better than 30 pW power direct detection threshold per pixel has been demonstrated in the 2–4 THz range. Such performance is expected on the whole THz range by proper tailoring of the antennas while keeping the technological stack largely unchanged. This paper gives an overview of the developed bolometer-based technology. First, it describes the technology and reports the latest performance characterizations. Then imaging demonstrations are presented, such as realtime reflectance imaging of a large surface of hidden objects and THz time-domain spectroscopy beam twodimensional profiling. Finally, perspectives of camera integration for scientific and industrial applications are discussed.
1. Introduction Terahertz (THz) radiation, loosely defined to be between 0.3 and 10 THz, combines many unique properties: non-ionizing penetration through non-metallic and nonpolar materials, high sensitivity to water content, spatial resolution 10 times shorter than for millimetre wave radiation, and specific spectral fingerprints for many 2014 The Author(s) Published by the Royal Society. All rights reserved.
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Without modifying the three-dimensional pixel architecture of a standard IR bolometer, THz absorption can be optimized by the adjustment of the equivalent impedance of the standard
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substances. These features open the way to promising applications in many interdisciplinary fields [1–4], many of which require two-dimensional imaging. The first demonstrations of THz two-dimensional imaging [5] were based on optoelectronic THz time-domain spectroscopy (THz-TDS). Since then, various different imaging modalities have been developed and demonstrated. However, most of them use a single detector and therefore samples need to be scanned in two dimensions relative to the fixed terahertz beam, or vice versa. Mechanical scans limit inexorably the data acquisition rate and point-to-point measurements are preferably applied to stationary objects. Additionally, such systems tend to be bulky and complicated. The spread of applications using THz imaging would be greatly increased by the availability of affordable, compact, easy-to-use and highly sensitive detectors. In particular, large-format (manypixel) focal plane arrays (FPAs) integrated in compact and hand-held cameras would enable fast two-dimensional image acquisition; the camera would operate in video mode, recording twodimensional data in real time without the need for raster scanning. Such features are possessed by thermal infrared (IR) array sensors based on bolometers [6]. These monolithic FPAs are collectively processed above complementary metal oxide semiconductor (CMOS) read-out integrated circuits (ROICs) with standard silicon technology. The optically sensitive surface may gather a large number of pixels [7]. These sensors operate at room temperature. And finally the CMOS ROIC provides advanced functions of signal processing before delivering video output. Since the 1990s CEA-Leti [8,9] has been developing such detectors with the choice of amorphous silicon (a-Si) as the thermometer material, whereas most of the other players have chosen vanadium oxide. In 2005, the Massachusetts Institute of Technology group [10] demonstrated real-time THz imaging using a commercial uncooled bolometer IR camera in association with quantum cascade lasers (QCLs). Several other institutes and companies [11,12] tested this imaging setup configuration, including the authors’ group [13]. In a common way, these tests showed that significant improvements in sensitivity could be advantageously achieved by designing bolometer FPAs specifically for THz frequencies. CEA-Leti took up this challenge by tailoring its proprietary bolometer technology for optimized THz sensing. Such a development was initiated a few years ago with two constraints as guidelines. First, the pixel structure had to be as close as possible to the existing a-Si IR bolometer stack in order to benefit from the maturity of this technology that has demonstrated high performance and high fabrication yield [14]. The second constraint involved the design of a technological flow chart fully compatible with standard CMOS microelectronic equipment. Indeed, similarly to other running developments of THz arrays based on CMOS field effect transistors [15,16] or Schottky diodes [17], bolometers can reach very-large-scale integration as a result of silicon process technologies. The combination of these two requirements meets the criteria of low cost and low SWAP (size, weight and power) that any commercial camera has to target. This trend is followed by the IR thermal bolometer arrays: the prices have decreased steadily and now meet the demands of large-volume applications, such as automotive [18] or home appliances. This paper summarizes the results of the Leti bolometer-based technology development. First, it describes the pixel array structure and the chips prototyped with two different FPA formats. Then it reports the modelling and the characterizations of the main figures of merit. The following section shows imaging tests carried out with a prototype of a compact camera integrating the FPA. Then a specific section focuses on the demonstration of terahertz imaging capabilities of a complete real-time reflection imaging system. Perspectives of camera integration for scientific and industrial applications conclude this paper.
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microbridge structure. It can be done in a quite straightforward way by tuning the sheet resistance of a metallic film deposited on the existing suspended membrane. This method has been applied for the first time by NEC [19], and then followed by INO [20]. Such cameras are now commercialized but still the lack of a quarter-wavelength cavity necessarily hampers the optical coupling efficiency. In addition, when THz frequencies smaller than 3 THz are being targeted, the mismatch between membrane size and wavelength will quickly degrade the published noise equivalent power (NEP) (100 pW in the 3–4 THz range). The CEA-Leti THz pixel three-dimensional [21] stack differs substantially from standard IR bolometers. The key feature of this architecture relies on the separation between the two main functions of such sensors, i.e. the electromagnetic absorption and the thermometer. Complementary to an optimized quarter-wavelength cavity, antennas are coupled to the bolometer to perform optical radiation absorption. Crossed quasi-double-bowtie resistively loaded antennas are designed to collect the THz flux of any polarization (figure 1a). The longer bowtie antenna located on the suspended membrane, named ‘DC’ for direct coupling, is directly excited by the incident THz wave whose polarization is aligned along the axis of this antenna. In order to couple the other impinging polarization, a large antenna, capacitive coupling (CC), is located below the microbridge. The surface currents induced in this antenna are coupled via a capacitive mechanism to metallic planes deposited on the suspended membrane. This two-storied antenna architecture permits the tuning of the optical coupling of both polarizations independently and with no mutual interferences, as shown later. The residual radiation that is not absorbed by the antennas is partially recovered owing to a specific resonant quarter-wavelength cavity; as a result, the optical absorption is typically enhanced from 60–70% to 80–90% at resonance. Surface currents created on the metallic antenna planes are coupled onto the central silicon microbridge that integrates matched antenna resistive loads. The Joule power dissipated in the resistive loads induces a bolometer resistance change that is read out at the pixel level by a front-end CMOS circuit. The process above the integrated circuit (IC) of the complete THz bolometer pixel stack is fully CMOS foundry compatible. THz arrays are collectively manufactured above the IC wafer using only the classical steps of microelectronic photolithography, deposition process and etching. The main technological key [22] is the etching and metallization of through-oxide-vias (TOVs) through the thick dielectric cavity. TOVs electrically connect the bolometer resistance with the CMOS upper metallic contacts. This process ensures the fabrication of a monolithic two-dimensional
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Two formats of THz FPAs have been designed, processed, tested and used for imaging demonstrations. First, a large 320 × 240 pixel chip has been developed and integrated in a compact video camera housing [25]. A second chip integrates THz bolometers with visible and IR pixels to act as a tri-spectral monolithic detector [26]. The 320 × 240 array (figure 1b) is composed of 50 µm pitch pixels designed for an optimized sensitivity in the 2–4 THz range where most of the explosive fingerprints are located and QCLs are efficient [27,28]. Extensive three-dimensional finite-element method (FEM) simulations [29] were carried out with the commercial High Frequency Structure Simulator (HFSS) and Comsol codes to design two kinds of pixels. They are optimized at 1.7 and 2.4 THz for CC polarization while maintaining maximum absorption at 1.35 THz for the DC direction. A dedicated CMOS application-specific integrated circuit (ASIC) has been designed by CEALeti teams to ensure low-noise measurement of the resistances of the 320 × 240 bolometers and to format the resulting signals into a single video data stream. Thanks to state-of-the-art Si microelectronic facilities and robust bolometer technology knowhow, a very high yield has been achieved: more than 99.5% of the 320 × 240 pixels are functional for 56 out of the 63 chips available per 200 mm wafer. The monolithic tri-spectral array detector integrates three types of sensors, all of them operating at room temperature. For the visible channel, photodiodes are implemented within the ASIC while the IR and THz detectors consist, respectively, of a-Si 25 µm pitch and 50 µm pitch microbolometers. The central region of the FPA is occupied by 32 × 32 THz pixels while the surroundings combine IR and visible pixels. One major issue [30] was to work out a technological stack compatible with the simultaneous process of IR and THz bolometric pixels above the CMOS wafer. Another important challenge was the design of an ASIC that efficiently reads and outputs the signal generated by the three channels [31]. In spite of this very challenging tri-spectral integration gathering very diverse pixels, a satisfactory fabrication yield has been achieved: several chips have been integrated in vacuum packaging with diamond windows and tested for real-time imaging, as presented in §4. As common features, the THz bolometric pixels of these arrays present thermal time constants between 20 and 40 ms and thermal resistances of 50 MK W−1 .
4. Modelled and measured figures of merit Extensive work on modelling of the figures of merit has been carried out, of both the collection and thermometer functions. Following this design phase, specific experimental developments and tests of the prototyped chips were undertaken in order to assess the numerical results.
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sensor with a large number of pixels. Moreover, the resulting vertical architecture means that pixels are separated from the CMOS ROIC by the metallic reflector plane; such a feature prevents electromagnetic perturbation of the optical coupling by ROIC metallic lines as encountered in [23]. The antenna sizes and shapes can be chosen independently from the bolometric device to match the illumination characteristics, in particular the frequency range and the polarization. This arrangement is very versatile with respect to frequency [24]. When designing the pixel, the antennas and the resonant cavity can be sized to match the wavelength, whereas the central bolometer structure can be kept small; as a result, the response time and the heat to current conversion efficiency are preserved. Crossed polarized antenna structures have been implemented to make the bolometer sensitive to both transverse electric (TE) and transverse magnetic (TM) polarizations, but it is possible to integrate only one of these two polarizations in the pixel structure.
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(a) Spectral absorption One major figure of merit of any FPA is the spectral absorption. Few publications show spectral absorption characterized continuously in a large THz range. However, this feature represents the optical collection efficiency and is then directly related to the FPA sensitivity. Actual instruments and procedures are lacking. Hence, special efforts have been dedicated to developing experimental methods to extract spectral features of our two-dimensional array sensor with the help of Bruker Optics Vertex 80 V Fourier transform spectrometers (FTSs). First, the reflectivity of the sensor surface was measured. A preliminary reference interferogram is acquired with a metallic mirror, and a second spectrum is recorded in which the FPA replaces the mirror. Assuming that the reference mirror is perfectly reflective, the Fourier transform of the ratio of sample to reference interferograms represents the spectral reflectivity of the array. This measurement was first applied to a 160 × 120 pixel prototype array processed above a raw Si wafer instead of an ASIC. Without an ASIC to ensure the multiplexing, specific metallic meander lines at the reflector backplane level address electrically 16 pixels. Hence, this configuration makes possible the reading of the signal sensed by individual pixels selected within the array. The pixels of this array are designed for optimized coupling at 1.35 and 1.7 THz, respectively, for the ‘DC’ and ‘CC’ antennas. Figure 2a,b shows the spectral reflectivity Ropt (f ) measured, respectively, for the DC and CC. Polarization is controlled with a wire grid polarizer inserted in the set-up. This experimental result is compared with the |S11 |2 parameter (dotted line of figure 2) extracted from threedimensional FEM simulations in which each pixel of the array is considered as a Floquet cell [29]. A very good match between simulations and measurements is observed over the whole frequency range: a distinct absorption peak arises (at 1.35 and 1.7 THz) as a dip in reflectivity and the shapes follow the same trends. However, this measurement is not sufficient to assess the useful pixel absorption efficiency that is the ratio between the thermal heating of the antenna loads and the incident wave power. Moreover, this method overestimates the spectral absorption deduced from αopt (f ) = 1 − Ropt (f ). Indeed, some power losses in the stack—such as Joule dissipation within the dielectric cavity—do not contribute to the bolometric signal.
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In order to ascertain the bolometric conversion efficiency, direct measurement of the spectral response of the sensor has to be performed. The collimated beam delivered by the interferometer is deflected outside of the FTS and is focused on the detector array enclosed inside a vacuum laboratory test vessel. Resulting interferograms include spectral absorption of the devices convoluted to the spectral features of the source and of the optical chain. This measurement is referenced to an interferogram obtained in a similar operation scheme with the FTS internal pyroelectric detector to retrieve the relative spectral response of the bolometer. The relative spectral response is normalized to the maximum peak. As shown by figure 2c,d, good agreement is noted between the empirical and theoretical curves. One can observe comparable performance and a large absorption peak in the vicinity of 1.35 and 1.7 THz, respectively, for DC and CC antennas. The observed weak oscillations and slight shift of the experimental resonance comes mainly from the atmospheric signatures that alter the signal during its transmission in air between the FTS and the sensor. The absorption spectra extracted from modelling and measurements (figure 3) are in agreement with the ones obtained in the reflectivity configuration. These characterizations give us an upper bound for the useful absorption of our device and locations of resonance peaks. The developed procedures make possible the evaluation of the ‘useful’ spectral absorption of the THz array sensor that is an essential figure of merit when one seeks to estimate and to compare devices’ performance.
(b) Noise equivalent power Another key performance metric for the detector is the NEP. Mathematically, it is the ratio between the output noise voltage spectral density vn and the detector voltage responsivity Rv . This quantity can be reported at various circuit levels, from the direct signal generated at the pixel output, i.e. the current/voltage change in the resistance for a resistive bolometer, to the output of the overall FPA imaging chain operated in video mode. The latter NEP requires only an a priori knowledge of the total impinging source power. This method is applied to the prototyped 320 × 240 pixel array with the 2.4 THz design, and is operated at a video rate of 25 Hz. A rowwise reading scheme of the ROIC allows the current of the bolometer to be integrated by the 5 pF capacitance of its associated trans-impedance amplifier during 125 µs per pixel. The sensitivity measurement set-up that combines a QCL [32] emitting at 2.5 THz and a Thomas Keating Instruments power meter is detailed in a previous paper [33]. In a general way, no filter is used during these experiments.
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Real-time active imaging capabilities of the prototyped two-dimensional FPAs have been tested in several set-ups, in both reflection and transmission configurations. First, simple optical arrangements were tested in which samples intercept the collimated beam of a THz QCL. As shown in figure 4a,b, the 320 × 240 THz array is able to deliver real-time images of objects, respectively a scalpel blade and Leti characters written with metallic ink on paper, both concealed in a postal envelope [28]. Speckle patterns arising from the QCL source coherence degrade the image quality. THz antenna- and cavity-coupled bolometers also present ultra-wideband detection capabilities. Even if the prototyped 320 × 240 FPAs are optimized for centre resonance frequencies in the 2 THz range, significant absorption remains below 1 THz. Therefore, the array can deliver real-time frames of the THz beam produced by longer wavelength sources. Two essay campaigns demonstrated sensitive two-dimensional real-time profiling of the beam delivered by classical TDS set-ups [34]. During these experiments, the beam generated by the photoconductive emitter propagates in the ambient atmosphere and is focused on the FPA by off-axis parabolic mirrors. In figure 4c,d [35], the beam delivered by the TDS is imaged with a 29 dB signal to noise ratio (SNR) at peak; detected integrated THz power of the source emitting as low as 25 nW has been resolved with sufficient SNR to observe tiny details of the beam shape. The characterization of the second type of prototyped array described in §3 (figure 5b) showed state-of-the-art performance for each channel [26]. The figure 5a presents the visible and THz channel outputs when a paper optical test pattern is standing in the way of a QCL beam optical path. The 32 × 32 THz pixels of the array centre profile the QCL beam while visible photodiodes image the object at the periphery. In figure 5c, thermal IR imaging by the IR pixels of the two-dimensional sensor is demonstrated separately using f /1 germanium optics.
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First, the responsivity at 2.5 THz corresponding to the CC polarization is measured. As a total power of 238 µW illuminates the device, the sum of the pixel voltages over the array is 3000 V. The FPA responsivity is calculated as the ratio of the integrated signal to the total impinging −1 power and is found to be RCC vtotal = 12.6 MV W . The same measurement performed for the cross-polarization state, i.e. the DC antenna, leads to a lower total responsivity RDC vtotal = 4.3 MV W−1 . Imaging RMS noise vn including pixel noise, ROIC and digitizing chain contributions is measured to be 400 µV RMS. The smallest variation of impinging power that can be detected is calculated as the ratio of RMS noise to the device responsivity. This detection power threshold reaches 32 pW for the CC polarization at 2.5 THz and is better than 30 pW at the resonance frequency. During the design of a detector sensitive to unpolarized radiation, one of the main challenges is to obtain cross-polarized antennas featuring both high absorption efficiency and no mutual interferences. Cross antennas built at the same technological level suffer from mutual coupling that averages and lowers the absorption efficiency independently from the polarization state. As described in §2, the two-storied crossed antenna architecture was chosen to remove this unwanted mutual coupling. This property has been experimentally tested by rotating the FPA angle θ from 0◦ to 90◦ with respect to the linearly polarized incident QCL beam—controlled with wire grid polarizers. For θ = 0◦ , the TM polarization of the QCL corresponds to the DC antenna orientation. As illustrated in figure 3b, the resulting output integrated signal of DC cos(θ)2 + V CC sin(θ)2 . Moreover, the ratio of the device follows a mathematical function Vout out integrated output signals is in correspondence to the optical absorption efficiencies of DC and CC antennas (figure 3a). These results demonstrate the decoupling property of this crossed antenna three-dimensional structure. However, one has to point out that, if needed, it is possible to design a square law detector in which the CC and DC antennas spectral absorptions are similar.
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Figure 4. Examples of frames extracted from video sequences. (a) Images of a scalpel blade and of metallic ink Leti characters written on paper (b), both concealed in an envelope. Profiles of the beam emitted by a TDS emitter before (c) and after (d) optical alignment. (Online version in colour.) (a)
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Figure 5. Frames extracted from real-time visible and THz (a) and thermal IR video output (c) of the MUTIVIS tri-spectral monolithic array sensor (b). (Online version in colour.)
5. Imaging tests in a reflection system Imaging demonstrations shown in the previous section have motivated the development of a complete reflection active THz imaging demonstrator combining QCLs with the 320 × 240 THz FPA (figure 6a).
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The illumination beam is delivered by multiple QCL ridges in order to optimize the available optical power. The number of QCL ridges operating in continuous wave mode was chosen to fit the cooling power of the cryocooler. An innovative pulsed tube (PT) cryocooler developed by CEA-INAC has enabled such integration. This PT provides a 20 W cooling power on the cold finger, ending up with the choice of a 30 K temperature operation and four combined QCL ridges. The source is capable of delivering continuously tens of mW with dedicated fast driving electronics and switches to gate the laser. The optical beam is then guided by a specific optical setup that points the beam towards the scene. A metallic horn ensures beam mixing to provide better illumination uniformity. A specifically designed Fresnel lens is interposed in the beam optical path to magnify the output of the rectangular horn. The imaging plane is located at 1 m from the demonstrator housing, whose inside atmosphere is dried to minimize water vapour absorption. The illuminated area size is 40 × 60 mm2 . A large plane mirror is used to illuminate the scene and to collect the part of the beam that is reflected and backscattered by the imaged objects. Then a folded Newton telescope composed of an 80 cm paraboloid mirror focuses the beam onto the camera FPA with an equivalent f /0.8 relative aperture. The optical path from the QCL-based source to the camera is 4 m long, of which 2 m are in air. The camera comprises a dedicated vacuum-sealed packaging topped by a Zeonex window. The operation of the array under the vacuum atmosphere—typically 10−3 mbar—is necessary for high sensitivity, as for any bolometer sensor. The camera also includes the front-end electronics made of two different cards. The first card delivers low-noise analogue voltages and digitizes the chip output voltage. The second card, a field-programmable gate array (FPGA) board, drives the sensor and ensures synchronization between the camera and external devices. Evaluation of optical quality and resolution has been performed with the help of absorbing optical test patterns placed in front of a metallic plane mirror. The image of the Siemens star is flawed by specular interferences induced by the coherence of the source (figure 6b). Then the slow time response of the bolometer is turned into an advantage to blur the speckle patterns: by oscillating axially the Fresnel lens, a large improvement of image quality is obtained as illustrated in figure 6c. A second imaging test has been carried out with a Leti writing test pattern of various sizes (figure 6d). Letter details of dimensions close to 2 mm are being resolved and represent the outmost resolution that can be achieved by this reflection imaging system demonstrator. In spite
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An innovative monolithic two-dimensional array sensor technology has been developed with the aim of fulfilling the need for affordable, easy-to-use and highly sensitive detectors for realtime THz imaging. Derived from the mature uncooled thermal IR bolometers, an innovative architecture of monolithic array based on the use of antennas and of a resonant quarterwavelength cavity has been designed. Thanks to the maturity of bolometer technology and to full compatibility with silicon CMOS processes, technological barriers have been lifted. Two formats of THz arrays optimized for sensing in the 1–5 THz range have been processed above CMOS ASICs: a 320 × 240 THz sensor and a tri-spectral chip. A high manufacturing yield has been observed with, for example, 56 out of 63 chips available in a 200 mm wafer. A NEP better than 30 pW per pixel has been characterized at 2.5 THz for the 320 × 240 FPA integrated in a camera and operated in video mode, making this uncooled sensor the state of the art of such devices. Three-dimensional modelling studies of this sensor have shown good agreement with the measured features, such as the broadband spectral absorption and non-mutual coupling between the two crossed polarizations that this pixel structure can collect. Real-time imaging tests have highlighted the ease of use and the high sensitivity and resolution achieved by this broadband array detector. Further technological developments will target performance improvement of the prototyped sensor fed by ongoing progress in IR bolometer technology. Moreover, the confirmed good mastery of the electromagnetic design of our structure motivates the development of an array optimized in the 0.6–1 THz range where atmospheric and material transmission are more favourable. The short-term aim of developing a commercial camera for scientific use has proved to be very valuable in helping to align any THz set-up in real time. Meanwhile, industrial applications requiring penetrating imaging with high resolution are being investigated through tests of this array in a transmission or reflection configuration. A compact active imaging system would fulfil the demands of various fields of applications, such as homeland security, biology and medical sciences, non-destructive controls for industry (e.g. pharmaceutical, aerospace and microelectronics), quality control of food and agricultural products, global environmental monitoring, and the study of historical and archaeological work. If appropriate, the imaging functionality can be considered in combination with spectral analysis for chemical identification, provided by the sensor itself associated with multicolour illumination or possibly with a separate spectrometer. Acknowledgements. The support from CEA as well as from the EU FP7 project MUTIVIS is gratefully acknowledged. The authors also acknowledge the Leti team and academic institutions—in France, University of Savoie, University of Paris 7, University of Rennes, Ecole Normale Supérieure de Paris, and in Italy, the research centre ‘Fondazione di Bruno Kessler’—for their fruitful collaboration.
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of imperfectly homogeneous illumination, the SNR is sufficient to perform real-time imaging of hidden objects because this ‘Leti’ test pattern has been successfully imaged (figure 6d) once hidden below a nylon clean room coat [33].
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