The authors estimate signal-to-noise ratios (SNRs) and contrasts for both InGaAs SWIR camera (cut-off wavelength λco~1.7 μm) and type II superlattice (T2SL) SWIR camera (λco~2.3 μm), under such situations as human skin as an object and vegetation as surroundings which are illuminated only by OH night airglow. In estimating the number of signal electrons, the measured spectral properties of quantum efficiencies for both InGaAs and T2SL detectors are used along with reflectance spectra of human skin and materials, while atmospheric transmission spectra are calculated with MODTRAN. As to noise electrons, shot noise resulting from dark current of InGaAs or T2SL detector is added to photon noise and ROIC (Read-Out Integrated Circuit) noise. The SNR values for the T2SL camera are found larger than those for the InGaAs camera. The contrasts of human skin vs surroundings are positive for the T2SL camera, while those for the InGaAs camera are negative.
Milliwatt average power terahertz quantum cascade lasers (THz-QCLs) combined with microbolometer focal plane array cameras allow for acquisition rates on the order of 1×106 pixels/s. This system enables real-time imaging in transmission and reflection modes with signal to noise ratios of >25 dB per pixel. While these system allow rapid imaging for fairly transparent samples, signal to noise ratios of > 90 dB can be achieved with single element detectors where the samples are more opaque or require higher SNR. Systems using LongWave's terahertz QCLs and single/multi-element detectors will be presented.
The author summarizes development of uncooled microbolometer terahertz (THz) focal plane arrays (FPAs) and real-time cameras for sub-THz and THz wave detection. The array formats are 320x240 and 640x480, and the cameras have several functions, such as lock-in imaging, external-trigger imaging, image processing (pixel binning and frame integration), beam profiling and so on. The FPAs themselves are sensitive to sub-THz, THz and infrared radiations. Active imaging systems based on the imagers are described. One of them is a real-time transmission-type THz microscope which contains a THz camera and a quantum cascade laser (QCL). The other one is an active sub-THz imaging system, where a transmission imaging mode and a reflection imaging mode can be switched with one-touch operation. Strong THz emitters, such as far-infrared gas lasers and QCLs, are strongly coherent and often produce interference fringes in an image. A method of reducing the interference fringes (beam homogenizing) is described. Microbolometer FPAs developed by other groups, antenna-coupled CMOS FPA, array detectors based on GaAs high-mobility heterostructure and so on are also summarized, which operate in real-time and at room temperature. A fair method of evaluating performance of detectors with different sizes and at different wavelengths is explained and the performances of the detectors are compared.
The authors developed terahertz (THz) imager which incorporates 320x240 focal plane array (FPA) with enhanced sensitivity in sub-THz region (ca. 0.5 THz). The imager includes functions such as external-trigger imaging, lock-in imaging, beam profiling and so on. The function of the external-trigger imaging is mainly described in this paper, which was verified in combination of the THz imager with the pulsed THz free electron laser (THz-FEL) developed by Osaka University.
The THz-FEL emits THz radiation in a wavelength range of 25 - 150 μm at repetition rates of 2.5, 3.3, 5.0 and 10 pulses per second. The external trigger pulse for the THz imager was generated with a pulse generator, using brightening pulse for THz-FEL. A series of pulses emitted by the THz-FEL at 86 μm were introduced to the THz imager and Joule meter via beam splitter, so that the output signal of THz imager was normalized with the output of the Joule meter and the stability of the THz radiation from FEL was also monitored. The normalized output signals of THz imager (digits/μJ) obtained at the repetition rates mentioned above were found consistent with one another. The timing-relation of the external trigger pulse to the brightening pulse was varied and the influence of the timing-relation on beam pattern is presented. These experimental results verify that the external trigger imaging function operates correctly.
Uncooled microbolometer-type 640x480 and 320x240 Terahertz (THz) focal plane arrays (FPAs) with enhanced sensitivity in sub-THz region are developed, and incorporated into 640x480 and 320x240 cameras, respectively. The pixel in the THz-FPA has such a structure that an area sensitive to electromagnetic wave is suspended above read-out integrated circuit (ROIC). A thin metallic layer is formed on the top of the sensitive area, while a thick metallic layer is formed on the surface of ROIC. The structure composed of the thin metallic layer and the thick metallic layer behaves as an optical cavity. The THz-FPAs reported in this paper have a modified pixel structure which has several times longer optical-cavity length than NEC’s previous pixel does, by forming a thick SiN layer on the ROIC. The extended optical-cavity structure is favorable for detecting electromagnetic wave with lower frequency. Consequently, the Minimum Detectable Power per pixel (MDP) is improved ten times in sub-THz region, especially 0.5-0.6 THz. This paper presents spectral frequency dependences of MDP values for THz-FPA with the modified pixel structure and THz-FPA with the previous pixel structure, using THz free electron laser (FEL) developed by Osaka University. The modification of pixel structure extends high sensitivity region to lower frequency region, such as sub-THz region, and the wider spectral coverage of THz camera surely expands its applicability
Image reconstruction method for non-synchronous THz signals was developed for a combination of THz Free Electron
Laser (THz-FEL) developed by Osaka University with THz imager. The method employs a slight time-difference
between repetition period of THz macro-pulse from THz-FEL and a plurality of frames for THz imager, so that image
can be reconstructed out of a predetermined number of time-sequential frames. This method was applied to THz-FEL
and other pulsed THz source, and found very effective. Thermal time constants of pixels in 320x240 microbolometer
array were also evaluated with this method, using quantum cascade laser as a THz source.
This paper describes features of uncooled palm-size and real-time Terahertz (THz) imager. The THz imager and
powerful THz quantum cascade laser were assembled into THz microscope with which THz images of narrow string
were obtained at 4.3 and 2.0 THz. The analyses on these images show that spatial resolutions evaluated at two
frequencies are consistent with Fraunhofer diffraction limit. THz imager has been applied to investigate beam patterns
for a variety of THz sources. The experimental results on beam patterns show that THz imager plays an important role in
developing THz sources. A method for reducing non-uniformity due to strong coherency of THz sources is finally
presented.
This paper describes a real-time transmission-type Terahertz (THz) microscope, with palm-size THz camera and compact quantum cascade laser (QCL). The THz camera contains 320x240 microbolometer focal plane array which operates at 30 Hz frame rate and has lock-in imaging function as well as integration functions such as frame integration and spatial filter. These functions are found very powerful in improving signal-to-noise ratio. QCL is installed in compact Stirling cycle cooler. A variety of QCLs covers frequency range from 1.5 to 5 THz and provides time-average power of 0.5~2 mW. The illumination area for sample is changed by adjusting one lens in the illumination optics. Performances of the THz microscope, such as signal-to-noise ratio and so on, were measured and are found consistent with the calculations. THz images taken with the THz microscope are finally presented.
This paper describes a real-time terahertz (THz) imaging system, using the combination of a palm-size THz camera
with a compact quantum cascade laser (QCL). The THz camera contains a 320x240 microbolometer focal plane array
which has nearly flat spectral response over a frequency range of ca. 1.5 to 100 THz, and operates at 30 Hz frame rate.
The QCL is installed in compact cryogen-free cooler. A variety of QCLs are prepared which can cover frequency range
from ca. 1.5 to 5 THz. THz images of biochemical samples will be presented, using the combined imaging system.
Performance of the imaging system, such as signal-to-noise ratio of transmission-type THz microscope, is predicted.
Terahertz imaging has attracted much attention in recent years, because the technique can be applied to many
application fields such as nondestructive analysis and imaging method through opaque materials. A terahertz real-time
imaging technique (Terahertz Camera) considered increasingly important in the future has been developed. The terahertz
camera consists of a light source (Terahertz quantum cascade laser) and an un-cooled micro-bolometer array, which can
easily get real-time terahertz-image. As an application of the terahertz camera, a stand-off imaging system that could be
useful in a fire disaster relief and a label-free bio-materials detection system have developed and demonstrated.
Uncooled Terahertz (THz) focal plane array (FPA), 320x240 format-23.5 μm pitch, and THz imager were
developed. There are two types of THz-FPAs, i.e., broad-band type and narrow-band type. Since broad-band
type THz-FPA was developed, a couple of modifications have been made to improve Noise Equivalent Power.
The narrow-band type THz-FPA has such a new structure that Si cover is put above thermal isolation structure
of broad-band type THz-FPA at a distance of half of wavelength of interest. Measurements on responsivities
of narrow-band type FPAs show enhancement by a factor of ca. 3. Lock-in imaging technique has been
developed, which increases signal-to-noise ratio as a function of square root of the number of frames of
integration. Both passive and active THz imaging experiments were finally described.
The authors summarize the past 40-years history on the development of HgCdTe infrared detectors in Japan. At the
early stage of development of material growth, high-quality HgCdTe layers were obtained by liquid phase epitaxy
technique, owing to lattice-matched CdZnTe substrates. Hetero-epitaxial growth techniques of HgCdTe were also
successfully developed to obtain epilayers on much larger and cheaper substrates such as GaAs and Si, using methods of
metal-organic chemical vapor deposition and molecular beam epitaxy, where key issues were controlling surface
orientation, surface polarity and so forth. Fabrication process of p-on-n junction photodiodes was developed with various
improvements on ion implantation and surface passivation. On the basis of technologies mentioned above, large-scale
infrared focal plane arrays were realized with superior thermal images.
The authors tried real-time imaging of THz radiation from Quantum Cascade Laser (QCL), using
vanadium oxide (VOx) microbolometer focal plane arrays (FPAs) of 320x240 with pitches of 37 μm and 23.5
μm as well as 640x480 with 23.5μm pitch. The QCL has such parameters as 3.1 THz emission frequency
(97μm in wavelength), 300-400 nsec pulse width, 1.07 msec repetition period, 30 mW peak intensity, 15K
operation temperature. The THz radiation from QCL is collimated by off-axis parabola (OAP) and focused on
FPA by another OAP. The 10 μm range infrared radiation from scene is blocked by sapphire disk or metal
mesh filter. Noise Equivalent Power (NEP) at 3.1 THz is estimated to be 200~400 pW.
The authors have successfully developed versatile uncooled infrared sensor modules, which have the dual dynamic
range drive (DDRD) feature enabling both a wide object temperature range and a high temperature resolution and are
applicable to a variety of uses. These modules allow users to get not only thermal images of room temperature objects at
a high temperature resolution but also those of high temperature objects. The sensor driven by the modules is of the
bolometer type having the pixel pitch of 23.5um, which may be the smallest pitch among commercial uncooled sensors
in the world. The article describes the characteristics and performance of the modules and introduces the DDRD feature.
A new pixel structure with twice-bent beams and eaves structure, suitable for high-resolution uncooled infrared (IR) focal plane arrays (FPAs), is proposed. In comparison with previous results (FPA of 37-µm pixel pitch), the thermal conductance of the test device with the proposed pixel structure of 23.5-µm pitch is reduced about 2.5 times. The eaves structure, which is adopted to increase the fill factor of pixels, improves the responsivity by a factor of 1.3. A 640×480 bolometer-type uncooled IRFPA is demonstrated by utilizing the new pixel structure, with supplementary modification to improve thermal conductance and thermal time constant. It shows a noise equivalent temperature difference (NETD) of 50 mK for F/1.0 optics at 30 frames/sec, a thermal conductance of 0.03 µW/K, and a thermal time constant of 16 msec.
This paper proposes a new thermally isolated pixel structure, having a twice-bent beam structure and eaves structure, suitable for high-resolution uncooled infrared (IR) focal-plane arrays (FPAs). It also describes the properties of test devices, fabricated to verify the effect of the new pixel structure. Although the pixel size of the test devices is 23.5 μm × 23.5 μm, which represents a smaller area by a factor of about 2.5 than the 37 μm × 37 μm pixel size for the 320 × 240 bolometer-type uncooled IRFPA, previously developed by the authors, the test devices have beams with almost the same length as in the previous IRFPA by utilizing the new beam structure. In addition, the cross-sectional area of the beam is reduced. Accordingly, the thermal conductance of the test devices can be reduced by a factor of about 2.5. The eaves structure, which is adopted to increase the fill factor of pixels, improves the responsivity by a factor of 1.3, which is consistent with our calculations. By utilizing the new thermally isolated pixel structure, the test devices with 23.5 μm pixels enable us to achieve thermal sensitivity equivalent to the previous 37 μm pixels.
The performance of a 320 x 240 bolometer-type uncooled infrared (IR) Focal Plane Array (FPA) is described. Vanadium oxide thin film is adopted for the bolometer material, having a sheet resistance of approximately 10 kohms/square. It is patterned such that the bolometer resistance is by a factor of 10 larger than the sheet resistance. On-chip readout integrated circuit (ROIC) is designed to reduce signal drift, extend dynamic range for object temperature and extend ambient temperature range in which operates non-uniformity correction is carried out with about 1/10 fewer frequency than the former ROIC.The 320 x 240 FPA consists of pixels sensitive to IR radiation and optical black (OB) pixels covered with plate which shuts out IR radiation. Drift is reduced by current mirror circuit, using the OB pixels and digital compensation circuit based on voltage change of OB pixels resulting from change in operation temperature. Both the dynamic range and the ambient temperature range are extended by decreasing integration gain and developing low-noise, low-power and large swing operational amplifier(OP-AMP). Since decrease in integration gain degrades noise equivalent temperature difference (NETD), bias voltage for bolometer is increased by factor of 2 and bandwidth is reduced by route half. Finally, IR image was obtained with prototype camera and NETD value was found to be smaller than 0.1K for F/1 optics at 60Hz frame rate and thermal time constant was measured to be 12 msec.
The relationships between the figure of merit R0A representing the junction property and deep levels representing electric properties of semiconductors have been investigated. R0A can be estimated by current-voltage (I- V) measurements. Deep levels can be estimated using spectral analysis of deep level transient spectroscopy (SADLTS). It has been confirmed that values of activation energies concentrate around 30 meV with the increase of R0A. This suggests that the influence from the inherent deep levels in the HgCdTe device becomes strong due to the increase of R0A, resulting in the improvement of the diode characteristics.
This paper discusses the design and performance of a 256 X 256 bolometer-type uncooled infrared detector. First, model calculations are carried out to clarify the relations of the noise equivalent temperature difference (NETD) to the electrical properties of the bolometer material. The properties are mainly resistivity, the temperature coefficient of resistance (TCR) and 1/f noise. To obtain real-time images with NETD values smaller than 0.15 K for F/1 optics, vanadium oxide thin film was developed as the bolometer material, having a sheet resistance range of 5 - 50 k(Omega) /square and a TCR value of -2%/K. This material did not exhibit thermochroism like VO2(A), because it was identified as VO2(B). The bolometer-array was statistically evaluated and put into the infrared camera. Finally, a thermal image with a NETD of 0.15 K was obtained.
A process for fabricating a monolithic 256 X 256 bolometer-type uncooled IR detector array is presented that utilizes surface micromachining technology. Each pixel of the device is composed of two parts, a silicon readout integrated circuit in the lower part and suspended microbridge structures in the upper part. The device is based on vanadium oxide bolometer films, which typically exhibit a temperature coefficient of resistance of -2 percent K. The vanadium oxide film is subject to damage, especially during wet etching of the sacrificial layer. Hence, the material and deposition process of the passivation layer for vanadium oxide film were investigated toward attaining a damage-free and flat microbridge structure. This was achieved by adjusting both the thickness of the passivation layer and the stresses in the electrode and passivation layers. The stiction problem of the microbridge structure was solved, by investigating drying conditions after etching of the sacrificial layer. Since each pixel has a cavity structure of (lambda) /4 to absorb IR radiation of the wavelength (lambda) , the spectral response of the pixel was measured in the wavelength range of 2 to 12 micrometers . The interference characteristics can clearly be seen. From responsivity measurements both in vacuum and at one atmosphere, the thermal time constant, thermal mass, and thermal conductance were estimated.
We have developed hybrid 256 by 256 focal plane arrays (FPAs) using MBE grown HgCdTe(MCT) layers on Si substrates for 10 micrometer-wavelength band detection and successfully demonstrated infrared images for the first time. The characteristics of MCT-on-Si-substrate FPAs have been compared with those for MCT-on-GaAs-substrate FPAs. MCT epilayers grown on 3-inch Si substrates used in FPAs were found to have almost the same characteristics as MCT epilayers on GaAs, including etch pit density of 1 - 2 X 106cm-2 and p-type carrier concentration of 1 - 2 X 1016 cm-3. The 256 by 256 photodiode array consists of n+-on-p junctions formed by boron-ion implantation and ZnS films for surface passivation. It was hybridized on a silicon readout circuit with an indium bump array. The mean value of ROA for the diode array was measured and found to be 80 (Omega) cm2 with a cutoff wavelength of 8.7 micrometer at 77 K; this is comparable to the typical value for a diode array using MCT grown on GaAs substrates. A diode array with 95% operability was placed in a camera system with which infrared images were taken, and high image sensitivity was found to be obtained.
Highly sensitive HgCdTe infrared photoconductive detectors have been developed for detecting 5 - 8 micrometer wavelength band. HgCdTe crystals were grown with the solid state recrystallization method and Cd composition was adjusted to 23.5%. The detectors possess an optical mask and asymmetric electrodes. The responsivity of the detectors depends on bias current direction. Higher responsivity was obtained with bias current flowing from a wide electrode to a narrow one. This responsivity value is 5 to 10 times larger than that of a standard detector which has symmetric electrodes and no optical mask. One of the new highly sensitive detectors was installed in a thermal imaging system and was found to be applicable to non-destructive diagnoses of buildings.
A hybrid HgCdTe 256 by 256 FPA for LWIR detection was fabricated and an infrared image was demonstrated. MCT epilayers were grown on GaAs substrates by MBE and annealed to p- type. The n+ on p photodiodes were formed by boron ion implantation. The mean value of zero bias differential resistance for the diode array was measured to be 8.0 M(Omega) with a cutoff wavelength of 9.5 micrometer. The effective quantum efficiency was estimated to be 0.55, and the optical cross talk was estimated to be 8.2%. A multiline parallel integration readout circuit designed especially for this 256 by 256 LWIR FPA, had 8.3 X 107 electron capacity, a 190 microsecond integration time, and a single output. This work shows that the MBE growth method on GaAs substrates, pn junction formation process, the MLPI circuit design, and the hybridization technique are useful technologies.
Utilizing either the gallium or indium free substrate mounting technique is desirable for producing large area and high quality HgCdTe epitaxial layers. This paper reports that a conventional substrate holder was modified to handle radiational heating. This modification enables substrate rotation to obtain better layer uniformity and realizes reduction in the amount of growth process time. This paper also describes substrate temperature behavior during HgCdTe epitaxy. From the growth initiation to about 2 micrometers -thick HgCdTe growth, the temperature increase was confirmed as being due to absorption of thermal radiation from heated cells and the substrate heater. For further growth, radiation cooling occurred as well. The latter behavior was corrected by the infrared pyrometer. Crystallinity of the epilayer grown by radiational heating was comparable to that of the epilayer grown by conventional thermal conductance heating. Using this technique, both the reduction in process time and the epilayer uniformity of 0.5%((Delta) x/x) over a 2 inch wafer were achieved.
The HgCdTe (MCT) 64 X 64 focal plane array (FPA) for long wavelength infrared (LWIR) detection was developed, using MCT epilayers grown by molecular beam epitaxy (MBE). The n-on-p photodiode array has a cutoff wavelength of 10.7 micrometers . The readout circuit, with off focal plane integration capacitance, was designed for 77 K operation. These components were fabricated independently and were hybridized. The 97.8% operability was obtained. Photodiode characteristics for each pixel were measured directly. Mean R$o)A value of 1.9 (Omega) (DOT) cm2 and quantum efficiency of 0.3 were obtained. Using an infrared camera system with nonuniformity correction function, the infrared image was successfully demonstrated. An NETD (noise equivalent temperature difference) value of 0.117 K was attained with an F/2.5 optical lens under the 300 K background condition.
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