Format Focal Plane Arrays Research Articles

Comparative Study on Planar Type-II Strained-Layer Superlattice Infrared Photodetectors Fabricated by Ion-Implantation

Recent progress in Type-II strained layer superlattice (SLS) material systems has offered viable alternatives towards achieving large format, small-pitch, and low-cost focal plane arrays for different military and commercial applications. For focal plane array fabrication, in order to address difficulties associated with mesa-isolation etching or the complex surface treatment/ passivation process, planar structures have been considered. In this work, a comparative study on the recent progress on the planar SLS photodetector using ion-implantation for device isolation is presented. The devices presented here are nBn and pBn heterostructure InAs/InAsSb SLS photodetectors, where Zn and Si were chosen as the ion implants, respectively. The electrical and optical performance of the planar devices were compared to each other and with associated mesa-etched fabricated devices, to give a deeper view of the device performance.

A Time-Efficient Self-Test Method for Evaluating Thermal Parameters of Uncooled Infrared Detectors

Uncooled infrared (IR) detectors are widely used in industry and civil imaging fields. Moreover, thermal parameters have a strong impact on detectors’ performance, especially under the trend towards higher spatial resolution. Therefore, evaluating them is important for detectors improvement. In this work, inspired by the traditional optical frequency-domain concept, a self-test method for evaluating thermal parameters of an uncooled IR focal plane array (FPA) is proposed. Utilizing a frequency-adjustable pulsed Joule heating power stimulus, the relationship between the Joule heating power, pixels’ thermal parameters, and pixels’ response voltage was firstly analyzed and established. This method was validated experimentally using a 3 × 3 format diode-based detector. For comparison, an experiment applying the steady-state method was also conducted. The results of all experiments were in good mutual agreement, demonstrating the reliability of the proposed method. For a 1280 × 1024 format FPA, the steady-state method needs approximately 90 h to finish measurements, while the proposed method requires only 5 min, illustrating that the proposed self-test method could considerably reduce the thermal parameter testing time for large-scale uncooled IR FPA.

Fabrication of 12 µm pixel-pitch 1280 × 1024 extended short wavelength infrared focal plane array using heterojunction type-II superlattice-based photodetectors

We present an intial demonstration of a 1280 × 1024 extended short-wavelength infrared focal plane array (FPA) imager with 12 μm pixel-pitch based on type–II InAs/AlSb/GaSb superlattice heterojunction photodetectors, with a novel bandstructure-engineered photo-generated carrier extractor as the window layer in the hetero structure to efficiently extract the photo-generated carriers. This heterostructure with a larger bandgap top window/contact layer leads to the device having lower dark current density compared to conventional pn junction devices. The large format FPA was fabricated with 12 μm pixel-pitch using a developed fabrication process. Test pixels fabricated separately exhibit 100% cut–off wavelengths of ∼2.22, ∼2.34 μm, and ∼2.45 μm at 150, 200 K, and 300 K. The test devices achieve saturated quantum efficiency values under zero bias of 54.3% and 68.4% at 150 and 300 K, under back-side illumination and without any anti-reflection coating. At 150 K, these photodetectors exhibit dark current density of 1.63 × 10−7 A cm−2 under −20 mV applied bias providing a specific detectivity of 1.01 × 1011 cm · Hz1/2/W at 1.9 μm.

A Dual-Band Focal Plane Array of Kinetic Inductance Bolometers Based on Frequency-Selective Absorbers

Passive imaging cameras at submillimeter wavelengths with large format focal plane arrays (FPAs) are being developed as the next generation of security screening systems. In this contribution, a dual-band FPA for security imagers at submillimeter wave frequencies is presented. The detectors are based on bolometric superconducting kinetic inductance resonators, which allow the development of large FPAs at medium-cooled temperatures. Two frequency-selective absorber (FSA) sets coupled to superconductive resonator lines are designed to implement a dual-color security imager. The performance of the dual-band imager is evaluated using spectral analysis approach that combines Fourier optics with a Floquet-mode field representation. The geometry of the unit cells is based on a Jerusalem cross configuration and the designed FSAs show a stable angular response and a rejection 1 to 3 of the undesired bandwidth. The detectors in the dual-band FPA are distributed over a hexagonal grid to maximize their physical size and then improve their sensitivity. The effective point spread function of the imager coupled to a black-body point source over a wide frequency band (1:6) was demonstrated experimentally with excellent agreement to the one estimated by using the proposed spectral technique.

A back side configured pointed dipole plasmonic optical antenna array enhanced quantum dot infrared photodetector

In this paper, we report a back side configured pointed dipole optical antenna array enhanced quantum dot infrared photodetector (QDIP). In this configuration, the light illumination is from the substrate side (i.e. back side illumination), goes through the QDIP and then reaches the optical antenna array. The near-field electric fields (E-fields) are simulated for both top and back side illuminations. The back side configured pointed dipole antenna array gives stronger E-fields than the top side configuration. The physical mechanism for the difference is analyzed. A longwave infrared QDIP with the back side configured pointed dipole optical antenna array was fabricated and tested. The measurements agree well with the simulation. Compared with the top side configured optical antenna array, the back side configured optical antenna array allows easier fabrication of large format (e.g. 640 × 512 or 1024 × 1024) focal plane arrays and provides higher plasmonic enhancement.

Terahertz Metamaterial Absorbers Implemented in CMOS Technology for Imaging Applications: Scaling to Large Format Focal Plane Arrays

We present the design and fabrication of terahertz (THz) metamaterial (MM) absorbers and their monolithic integration into a commercial CMOS technology along with its respective readout electronics to produce a low-cost, uncooled, and high resolution THz camera. We first describe the work done on single band and broadband MM absorbers on custom substrates, then progress with a description of the integration of such resonators into a six metal layer 180 nm CMOS process and its coupling with two types of microbolometer sensors: Vanadium oxide (VOx) and silicon (Si) pn diode. Additionally, we demonstrate the integration of the THz sensors with readout electronics to form a monolithic THz focal plane array (FPA). Reflection images of a metallic object hidden in a manila envelope are recorded using both the VOx and Si pn diode detectors, demonstrating the suitability of the technology for stand-off detection of concealed objects. Finally, we present the current work toward scaling this technology into a 64 × 64 FPA.

Dynamic power reduction in digital pixel design for large format focal plane arrays

This paper presents a design and analytical approach to significantly reduce the dynamic power consumption of front-end pixel design for digital readout integrated circuits (DROICs) in digital pixel sensor (DPS) arrays. DPS architecture relies on coarse quantization with pulse frequency modulation (PFM) and a novel approach of extended integration incorporated to achieve lower noise. The design is fabricated in 90nm CMOS process with pixel pitch of 30µm. Proposed architecture can attain eminently high charge handling capacity of 2.2Ge- with a quantization noise of 1072e- and extremely low power dissipation of 14.28mW. The proposed dynamic power reduction paradigm enables to alleviate the overall power consumption to 35% as compared to state-of- art PFM based 256×256 DPS array with the lowest Figure of Merit (FoM) of 297fJ/LSB reported earlier. The power reduction escalates further for higher detector currents and large format Focal Plane Arrays (FPA). The proposed design is tested and compared to our previous DROIC measurement results and other works in terms of power and quantization noise.

MCT-Based LWIR and VLWIR 2D Focal Plane Detector Arrays for Low Dark Current Applications at AIM

We present our latest results on n-on-p as well as on p-on-n low dark current planar mercury cadmium telluride (MCT) photodiode technology long wavelength infrared (LWIR) and very long wavelength infrared (VLWIR) two-dimensional focal plane arrays (FPAs) with quantum efficiency (QE) cut-off wavelength >11 μm at 80 K and a 512 × 640 pixel format FPA at 20 μm pitch stitched from two 512 × 320 pixel photodiode arrays. Significantly reduced dark currents as compared with Tennant’s “Rule 07” are demonstrated in both polarities while retaining good detection efficiency ≥60% for operating temperatures between 30 K and 100 K. This allows for the same dark current performance at 20 K higher operating temperature than with previous AIM INFRAROT-MODULE GmbH (AIM) technology. For p-on-n LWIR MCT FPAs, broadband photoresponse nonuniformity of only about 1.2% is achieved at 55 K with low defective pixel numbers. For an n-on-p VLWIR MCT FPA with 13.6 μm cut-off at 55 K, excellent photoresponse nonuniformity of about 3.1% is achieved at moderate defective pixel numbers. This advancement in detector technology paves the way for outstanding signal-to-noise ratio performance infrared detection, enabling cutting-edge next-generation LWIR/VLWIR detectors for space instruments and devices with higher operating temperature and low size, weight, and power for field applications.

Latest Developments in Long-Wavelength and Very-Long-Wavelength Infrared Detection with p-on-n HgCdTe

We report recent developments at Commissariat a l’Energie Atomique-Laboratoire d’Electronique des Technologies de l’Information Infrared Laboratory on the processing and characterization of p-on-n HgCdTe (MCT) planar infrared focal plane arrays (FPAs) operating in the long-wavelength infrared (LWIR) and very-long-wavelength infrared (VLWIR) spectral bands. The active layers in these FPAs were grown by liquid phase epitaxy (LPE) on a lattice-matched CdZnTe substrate. This technological process results in lower dark current and lower serial resistance than for n-on-p vacancy-doped architecture and thus is better adapted for lower flux detection or higher operating temperature. This architecture was evaluated for space applications in the LWIR and VLWIR spectral bands with cutoff wavelengths from 10 to 17 μm at 78 K. Innovations have been introduced to the technological process to form a heterojunction by use of an LPE growth technique. The initial objective was to reduce the dark current at low temperatures, by reducing the transition temperature from diffusion-limited to depletion-limited dark current. Another advantage is that the wider bandgap obtained in the vicinity of the junction ensures less sensitivity to the defects present at the interface between MCT and passivation layers. Electro-optical characterization of p-on-n photodiodes was performed on quarter video graphics array format FPAs with 25 and 30 μm pixel pitches. The results revealed excellent operabilities in current and responsivity, with low dispersion, and noise limited by current shot noise. Studies performed on the dark current show that dark current densities at 78 K are consistent with the heuristic prediction law “Rule 07”. Below this temperature, dark current varies as a pure diffusion current for a variety of devices from different manufacturers, introducing a temperature range limitation in the description of the “Rule 07” law.

<inline-formula> <tex-math notation="TeX">\(640\times 512\) </tex-math></inline-formula> Extended Short Wavelength Infrared In<sub>0.83</sub>Ga<sub>0.17</sub>As Focal Plane Array

We present the detailed characteristics of solid source molecular beam epitaxy (MBE) grown large format ( \(640\times 512\) ) extended short wavelength infrared In0.83Ga0.17As sensor with desirable performance at both pixel and focal plane array (FPA) levels. The FPA pixels in the mesa structure grown on a graded AlInAs buffer layer with 2.65- \(\mu \) m, 300-K cutoff wavelength exhibited 300- and 200-K peak detectivities as high as \(\sim \!\! 2.5\times 10^{10 }\) and \(\sim \!\!1\times 10^{12}\) cmHz \(^{1/2}\) /W, which are both equivalent to the theoretical limits set by the Johnson noise of the detector. Dark current analysis of the pixels displayed no considerable tunneling component with the dark current being dominated by generation-recombination and shunt leakage mechanisms >200 K up to a reverse bias voltage of 3 V. Moreover, the noise measurements displayed no 1/ \(f\) noise in the FPA pixels. In spite of the large lattice mismatch, the FPA yielded very good response linearity, as well as impressively good responsivity nonuniformity and pixel operability of 5.5% and 99.8%, respectively. The results showing the feasibility of both solid source MBE and graded AlInAs buffer for the growth of extended InGaAs photodetectors are very encouraging for the large format FPA implementation of these detectors with desirable imaging performance and an extended cutoff wavelength as high as \(\sim 2.7\) \(\mu \) m.

Quantum Well Infrared Photodetector Technology and Applications

Quantum well infrared photodetectors (QWIPs) are known for their stability, high pixel-to-pixel uniformity, and high-pixel operability, which are essential for large area imaging arrays. In this paper, we discuss the initial demonstration of QWIP devices, and the many years of progress that propelled this technology toward the demonstration of large format focal plane arrays. In addition, we present some potential applications of this technology in science and medicine.

A monolithic resonant terahertz sensor element comprising a metamaterial absorber and micro‐bolometer

AbstractIn this article a monolithic resonant terahertz sensor element with a noise equivalent power superior to that of typical commercial room temperature single pixel terahertz detectors and capable of close to real time read‐out rates is presented. The detector is constructed via the integration of a metamaterial absorber and a micro‐bolometer sensor. An absorption magnitude of 57% at 2.5 THz, a minimum NEP of and a thermal time constant of 68 ms for the sensor are measured. As a demonstration of detector capability, it is employed in a practical Nipkow terahertz imaging system. The monolithic resonant terahertz detector is readily scaled to focal plane array formats by adding standard read‐out and addressing circuitry enabling compact, low‐cost terahertz imaging.

Recent progress in InSb based quantum detectors in Israel

InSb is a III–V binary semiconductor material with a bandgap wavelength of 5.4μm at 77K, well matched to the 3–5μm MWIR atmospheric transmission window. When configured as a Focal Plane Array (FPA) detector, InSb photodiodes offer a large quantum efficiency, combined with excellent uniformity and high pixel operability. As such, InSb arrays exhibit good scalability and are an excellent choice for large format FPAs at a reasonable cost. The dark current is caused by Generation–Recombination (G–R) centres in the diode depletion region, and this leads to a typical operating temperature of ∼80K in detectors with a planar implanted p–n junction. Over the last 15years SCD has developed and manufactured a number of different 2-dimensional planar FPA formats, with pitches in the range of 15–30μm.In recent years a new epi-InSb technology has been developed at SCD, in which the G–R contribution to the dark current is reduced. This enables InSb detector operation at 95–100K, with equivalent performance to standard InSb at 80K. In addition, using a new patented XBnn device architecture in which the G–R current is totally suppressed, epitaxial InAsSb detectors have been developed with a bandgap wavelength of 4.2μm, which can operate in the 150–170K range.In this short review of the past two decades, a number of key achievements in SCD’s InSb based detector development program are described. These include High Operating Temperature (HOT) epi-InSb FPAs, large format megapixel FPAs with high functionality using a digital Read Out Integrated Circuit (ROIC), and ultra low Size, Weight and Power (SWaP) FPAs based on the HOT XBnn architecture.

Compressed sensing for practical optical imaging systems: a tutorial

The emerging field of compressed sensing has potentially powerful implications for the design of optical imaging devices. In particular, compressed sensing theory suggests that one can recover a scene at a higher resolution than is dictated by the pitch of the focal plane array. This rather remarkable result comes with some important caveats however, especially when practical issues associated with physical implementation are taken into account. This tutorial discusses compressed sensing in the context of optical imaging devices, emphasizing the practical hurdles related to building such devices, and offering suggestions for overcoming these hurdles. Examples and analysis specifically related to infrared imaging highlight the challenges associated with large format focal plane arrays and how these challenges can be mitigated using compressed sensing ideas.

InAs/GaSb superlattice based long-wavelength infrared detectors: Growth, processing, and characterization

We report growth, processing, and characterization of antimonide superlattice long-wavelength infrared photodetectors based on the complementary barrier infrared detector (CBIRD) design. We used photoluminescence measurements for evaluating detector material and studied the influence of the material quality on the intensity of the photoluminescence. We performed direct noise measurements of the superlattice detectors and demonstrated that while intrinsic 1/ f noise is absent in superlattice heterodiode, side-wall leakage current can become a source of strong frequency-dependent noise. We developed an effective dry etching process for these complex antimonide-based superlattices that enabled us to fabricate single pixel devices as well as large format focal plane arrays.

Corrugated Quantum-Well Infrared Photodetector Focal Plane Arrays

Corrugated quantum-well infrared photodetectors (C-QWIPs) have been proposed for long-wavelength infrared detection. In this work, we optimize the detector structure and produce a number of large format focal plane arrays (FPAs). Specifically, we adopt one-corrugation-per-pixel geometry to increase the active detector volume and incorporate a composite cover layer to preserve the large sidewall reflectivity, which results in a large detector quantum efficiency. We also optimize the detector material structure such as the final state energy, the doping density, and the number of quantum well periods to improve the FPA operation under the existing readout electronics. As a result, high FPA sensitivity has been achieved, and their characteristics are in agreement with the detector model. Based on this model, we perform a systematic analysis on the FPA performance with a wide range of detector and system parameters. We find that C-QWIP FPAs are capable of high-speed imaging especially for those with longer cutoff wavelengths.

QWIP focal plane arrays on InP substrates for single and dual band thermal imagers

Alternative material systems on InP substrate provide certain advantages for mid-wavelength infrared (MWIR), long-wavelength infrared (LWIR) and dual band MWIR/LWIR quantum well infrared photodetector (QWIP) focal plane arrays (FPAs). While InP/InGaAs and InP/InGaAsP LWIR QWIPs provide much higher responsivity when compared to the AlGaAs/GaAs QWIPs, AlInAs/InGaAs system facilitates completely lattice matched single band MWIR and dual band MWIR/LWIR FPAs. We present an extensive review of the studies on InP based single and dual band QWIPs. While reviewing the characteristics of InP/InGaAs and InP/InGaAsP LWIR QWIPs at large format FPA level, we experimentally demonstrate that the cut-off wavelength of AlInAs/InGaAs QWIPs can be tuned in a sufficiently large range in the MWIR atmospheric window by only changing the quantum well (QW) width at the lattice matched composition. The cut-off wavelength can be shifted up to ∼5.0 μm with a QW width of 22 Å in which case very broad spectral response (Δ λ/ λ p = ∼30%) and a reasonably high peak detectivity are achievable leading to a noise equivalent temperature difference as low as 14 mK ( f/2) with 25 μm pitch in a 640 × 512 FPA. We also present the characteristics of InP based two-stack QWIPs with wavelengths properly tuned in the MWIR and LWIR bands for dual color detection. The results clearly demonstrate that InP based material systems display high potential for dual band MWIR/LWIR QWIP FPAs needed by third generation thermal imagers.

Lattice-matched AlInAs–InGaAs mid-wavelength infrared QWIPs: characteristics and focal plane array performance

The AlInAs/InGaAs material system is promising for mid-wavelength infrared (MWIR) and multi-band quantum well infrared photodetectors (QWIPs) as a lattice-matched alternative to the strained AlGaAs/InGaAs system. In this paper, we report a large format (640 × 512) AlInAs/InGaAs QWIP focal plane array (FPA) with 4.9 µm cut-off wavelength and assess the performance of this material system for MWIR QWIP applications at both pixel and large format FPA level. We also experimentally demonstrate that the cut-off wavelength of AlInAs/InGaAs QWIPs can be tuned in a sufficiently large range in the MWIR atmospheric window by changing only the quantum well (QW) width at the lattice-matched composition. The cut-off wavelength is shifted from 4.15 to 4.9 µm when the QW width is decreased from 30 to 23 Å, in which case a very broad spectral response (Δλ/λp = 32%) with a reasonably high peak detectivity (5.5 × 1010 cm Hz1/2 W−1, f/1.5) is achievable. The AlInAs/InGaAs QWIP FPA yielded excellent sensitivity with a noise equivalent temperature difference as low as 22 mK and a background limited performance (BLIP) temperature as high as 115 K with f/1.5 aperture and 300 K background. The results clearly demonstrate the potential of this material system for completely lattice-matched dual- or multi-band QWIP FPAs for third-generation thermal imagers.

High-quality large-area MBE HgCdTe/Si

HgCdTe offers significant advantages over other similar semiconductors, which has made it the most widely utilized variable-gap material in infrared (IR) focal plane array (FPA) technology. HgCdTe hybrid FPAs consisting of two-dimensional detector arrays that are hybridized to Si readout circuits (ROIC) are the dominant technology for second-generation infrared systems. However, one of the main limitations of the HgCdTe materials system has been the size of lattice-matched bulk CdZnTe substrates, used for epitaxially grown HgCdTe, which have been limited to 30 cm2 in production. This size limitation does not adequately support the increasing demand for larger FPA formats which now require sizes up to 2048×2048, and only a single die can be printed per wafer. Heteroepitaxial Si-based substrates offer a cost-effective technology that can be scaled to large wafer sizes and further offer a thermal-expansion-matched hybrid structure that is suitable for large format FPAs. This paper presents data on molecular-beam epitaxy (MBE)-grown HgCdTe/Si wafers with much improved materials characteristics than previously reported. We will present data on 4- and 6-in diameter HgCdTe both with extremely uniform composition and extremely low defects. Large-diameter HgCdTe/Si with nearly perfect compositional uniformity and ultra low defect density is essential for meeting the demanding specifications of large format FPAs.

Laboratory and field performance of megapixel QWIP focal plane arrays

We present the results of laboratory and field testing of a very large format (1024 × 1024 pixels) focal plane array (FPA) made using quantum well infrared photodetector (QWIP) technology. The FPA was produced by QWIP Technologies, Inc. and tested at the Army Research Laboratory (ARL). Tests performed included spectral response, dark current, 3d-noise, minimum resolvable temperature (MRT) and optical crosstalk. The spectral response was found to peak at 8.55 μm. The device was found to have conversion efficiency close to 10% and the level of dark current was low enough to allow background limited infrared performance (BLIP) at 76 K. The pixel operability was in excess of 97% and there were no large cluster defects. The FPA was thinned to less than 20 μm to allow thermal cycling and sharp focus. We will show indoor and outdoor imagery acquired with the FPA. The FPA was installed in a camera system that was taken to a major field test. We will show initial results of the analysis of the image data collected at that test. Vehicles of military interest were imaged at ranges between 500 m and 5 km.