Noise Equivalent Temperature Difference Values Research Articles

Physics-based thermal noise effect reduction in sonic IR crack length estimation

ABSTRACT Using heat diffusion models in sonic infrared (IR) inspection technology that captures arbitrary frictional heat generation function along a crack lends itself to estimation of crack length using thermal images alone. This study introduces novel physics-based approach that relies on the governing heat diffusion model linearity and superposition of point heat sources along crack to reduce thermal noise effect in crack length estimation. It utilises thermal images and their timestamps in sonic IR inspection to deliver spatiotemporal average representation of the aforementioned point heat sources to reduce thermal noise effect in crack length estimation. The proposed methodology is tested against two-dimensional and three-dimensional finite element simulations with an arbitrary frictional heat generation function along crack. Virtual random thermal noise levels ranging between 50 and 200 mK, which are higher than IR cameras Noise Equivalent Temperature Difference values, are added to simulated results to reflect realistic and drastic thermal noise levels. Spatiotemporal averaging using combined sets of different thermal images captured at various times in single sonic IR inspection delivered about 16% total uncertainty in the estimation of a crack length at 95% confidence level based on 60 experiments. Similar uncertainty level is obtained using independently reported sonic IR experimental data.

Self calibrated cooler-less microbolometer readout architecture

This study describes a new method to compensate bias heating effects for microbolometer readout circuits using a finely adjustable CMOS resistance as a reference. The proposed self calibrated cooler-less structure dynamically modifies CMOS resistance during integration time. This compensation structure also includes a feedback loop, which forces the oscillators to work in the linear region. The proposed self calibration circuit is designed and simulated using a 65-nm process node, and achieves 40 mK NETD (Noise Equivalent Temperature Difference) value for one percent resistance non-uniformity of pixel resistance values across one row. The layout area of this circuit occupies approximately one third of the layout area used by classical readout circuits. The circuit dissipates one third of power compared with classical microbolometer readout approaches.

The system design based on long-wave uncooled infrared detector

According to the requirement of high-definition surveillance of the status of space targets in low illuminance, a high SNR (Signal to Noise Ratio) uncooled long-wave infrared system is designed. Based on the detector of UL04371 with 640 × 480 array, the system scheme, acquisition system and real-time image processing algorithms are realized. On the basis of system implementation, the paper mainly focuses on how to reduce the noises from the aspect of hardware circuits and how to realize the real-time processing in the way of specified infrared image algorithms. Finally, the paper gives the test result which shows that the system has achieved a relatively good SNR characteristic with the average NETD (Noise Equivalent Temperature Difference) value of 220mK under the condition of the integration time of 64us and the radiation of 300K black body.

High operating temperature InAsSb-based mid-infrared focal plane array with a band-aligned compound barrier

In this work, by utilizing a band-aligned AlAs0.08Sb0.92/AlSb-based unipolar compound barrier design, a high-operability (∼99.7%) InAsSb bulk absorber-based mid-wavelength infrared 640 × 512 focal plane array (with a 50% cut-off wavelength at 4.1 μm at 150 K) exhibiting distinct infrared images from 150 K up to 205 K has been achieved, which suggests great potential for high operation temperature detection applications. At 150 K and −400 mV bias, the photodetectors exhibit a low dark current density of ∼3.9 × 10−6 A∕cm2, a quantum efficiency of 65.1% at peak responsivity (∼3.8 μm), and a specific detectivity of 1.73 × 1012 Jones. From 150 to 185 K, the focal plane array exhibits ∼30.2 mK and ∼69.5 mK noise equivalent temperature difference values by using f/2.0 optics and 6.45 ms and 0.61 ms integration times, respectively.

Fabrication and characterization of InAs/GaSb type-II superlattice long-wavelength infrared detectors aiming high temperature sensitivity

Noise equivalent temperature difference (NETD) is an important figure of merit for infrared detectors. Lower NETD means that the detector can realize smaller temperature resolution and longer range recognition. In this work, we investigate the device performance, especially the NETD characteristics of a long-wavelength infrared (LWIR) photodetector based on InAs/GaSb type-II superlattice (T2SL). To achieve mK-level temperature-sensitivity, the effect of detector's intrinsic and extrinsic factors on NETD performance has been discussed. Through a series of efforts, a NETD value of 7 mK was realized for a T2SL detector with a 100% cutoff wavelength of 11.4 μm at 77 K.

Non-uniformity correction in a Long Wave Infrared Focal Plane Array as a calibration temperature function

Despite of the manufactory process, all detector or pixel in a Focal Plane Array (FPA) has a different responsivity and offset when evaluated. These variations result in a specific kind of noise, called Fixed Pattern Noise (FPN) or spatial noise. During the image processing, those pixels who has a large deviation in responsivity or in their noise are considered bad pixels, and have their influence removed during the exhibition of a scene. Knowing the answer of all those pixels are the first test procedure in an FPA characterization. However, during the Non-uniformity correction and bad pixel replacement it is necessary to choose two reference temperatures, calibration points, and those temperature affects important parameters such as Uniformity, Noise Equivalent Temperature Difference (NETD) and Signal Transfer Function (SiTF). This work will study the calibration temperature influence on the Non-Uniformity Correction (NUC) quality by comparing uniformity, bad pixel number, NETD and SiTF values during characterizations.

The Future of Infrared; III–Vs or HgCdTe?

For reasons associated with size, weight, power consumption, and cost, the future of infrared systems for all spectral bands is being driven towards megapixel formats operating under diffraction- and background-limited conditions with ever-smaller pixel pitches and ever-higher operating temperatures. The performance requirements of such systems with regard to both optical and detector limitations are examined for the materials technologies and device architectures that are in vogue today. At elevated operating temperatures, available noise equivalent temperature difference values for diffraction-limited operation are found to be strongly dependent on the available pixel pitch, optimizing at values ∼λ/4, where λ is the operating wavelength. The possibility for extending the operation of mid- and long-wavelength focal plane arrays to room temperature with diffraction- and background-limited performance is discussed, together with the potential issues that must be addressed in order to achieve this ultimate goal.

Uncooled Microbolometer Arrays With High Responsivity Using Meshed Leg Structure

In this letter, a meshed leg structure is proposed for the first time to control the thermal conductivity in the uncooled microbolometer arrays without changing the fill factor. An amorphous silicon-based microbolometer was fabricated with 64 × 64 arrays with a 35 μm pixel pitch to verify the proposed design. The hole in the meshed leg was 0.7 × 0.7 μm2. The uniformity of the pixel resistance within the 64 × 64 arrays was below 2%. Self-heating and responsivity under infrared (IR) irradiation were investigated in three leg types: 1) type 1-normal leg structure; 2) type 2-meshed leg structure; and 3) type 3-metal extended leg structure. Comparison of the IR irradiation current showed the responsivity of the proposed structure (type 2) that was improved by four times over type 3. For the thermal noise equivalent temperature difference value, type 2 is 194 mK, which is much better than 384 mK of type 3.

A holistic approach performance analysis of substrate-free focal plane array

Based on a bi-material micro-cantilever thermal deformation principle, an uncooled infrared focal plane array (FPA) with optical readout has been developed from a substrate-based structure to a substrate-free structure. Infrared imaging of the substrate-free FPA indicates that this structure does not satisfy temperature-constant substrate conditions when the FPA unit size decreases from 200 μm to 60 μm. To evaluate the performance of the FPA, this paper puts forward an analytical model of heat transfer in the substrate-free FPA, by using a holistic approach and an electrical circuit analogy. The analytical model provides a fast and convenient way to calculate the temperature gray response and the response time of the substrate-free FPA. A substrate-free FPA with a unit size of 50 μm was fabricated. Infrared imaging experiments validate the model and indicate a noise equivalent temperature difference value of 170 mK has been achieved.

320×256 HgCdTe IR FPA with a built-in shortwave cut-off filter

A photovoltaic detector design based on the graded band gap HgCdTe MBE structure with high conductivity layer (HCL) at interface, which provides photodiodes series resistance and a shortwave cut.off filter is developed. The optimal HCL parameters giving high quantum efficiency and minimal noise equivalent temperature difference were determined by calculations and experimentally confirmed. The hybrid 320×256 IR FPA operating in 8–12 μm spectral range was fabricated. The threshold power responsivity and minimal noise equivalent temperature difference values at wavelength maximum were 1.02×10−7 W/cm2, 4.1×108 V/W and 27 mK, respectively.

Temperature-sensitivity limits of third-generation thermal viewers

This paper presents an analysis of the limiting attainable values of temperature sensitivity—the threshold value of the noise-equivalent temperature difference (NETD) of staring thermal viewers. Calculational formulas are obtained that take into account how the sensitivity is affected by photon noise, the capacitance of the signal accumulator, and the spatial inhomogeneity of the staring-array response. The limiting NETD values are quantitatively determined for the mid-wavelength and long-wavelength windows of transparency of the atmosphere, along with the conditions under which they can be achieved.

320×256 HgCdTe IR FPA with a built-in shortwave cut-off filter

AbstractA photovoltaic detector design based on the graded band gap HgCdTe MBE structure with high conductivity layer (HCL) at interface, which provides photodiodes series resistance and a shortwave cut.off filter is developed. The optimal HCL parameters giving high quantum efficiency and minimal noise equivalent temperature difference were determined by calculations and experimentally confirmed. The hybrid 320×256 IR FPA operating in 8–12 μm spectral range was fabricated. The threshold power responsivity and minimal noise equivalent temperature difference values at wavelength maximum were 1.02×10−7 W/cm2, 4.1×108 V/W and 27 mK, respectively.

Thermal and mechanical characterizations of a substrate-free focal plane array

We propose a substrate-free focal plane array (FPA) in this paper. The solid substrate is completely removed, and the microcantilevers extend from a supporting frame. Using finite element analysis, the thermal and mechanical characterizations of the substrate-free FPA are presented. Because of the large decrease in thermal conductance, the supporting frame is temperature dependent, which brings out a unique feature: the lower the thermal conductance of the supporting frame is, the higher the energy conversion efficiency in the substrate-free FPA will be. The results from the finite element analyses are consistent with our measurements: two types of substrate-free FPAs with pixel sizes of 200×200 and 60×60 μm2 are implemented in the proposed infrared detector. The noise equivalent temperature difference (NETD) values are experimentally measured to be 520 and 300 mK respectively. Further refinements are considered in various aspects, and the substrate-free FPA with a pixel size of 30×30 μm2 has a potential of achieving an NETD value of 10 mK.

Performance of an optimized substrate-free focal plane array for optical readout uncooled infrared detector

This note presents an optimized substrate-free focal plane array (FPA), which is implemented in an optical readout uncooled infrared (IR) detector. The supporting frame of such FPA is a temperature-variable one due to the large decreases in both the heat capacity and the thermal conductance. This brings a unique thermal characteristic: the supporting frame functions as a “thermal isolation” frame which reduces the thermal conductance and therefore increases the temperature change and also functions as a “thermal diffusion” frame which certainly results in the temperature prechange in the ones not absorbing IR radiation. This characterization could significantly increase the temperature change of microcantilevers and therefore improve the performance of the substrate-free FPA. In the proposed IR detector, the fabricated 160×160 FPA has an average noise equivalent temperature difference (NETD) and a response time of 330 mK and 16 ms, respectively. The performance of the IR detector theoretically increases by about 5.5 times compared with the one using a substrate FPA. Here, the geometry of the substrate FPA is just the same as the fabricated FPA, but the supporting frame is assumed to be a temperature-constant one. If the optical readout sensitivity can be increased enough using an enhanced IR absorber, the fabricated FPA thus has the potential to achieve a NETD value of 70 mK.

A Passive Heterodyne Hot Electron Bolometer Imager Operating at 850 GHz

We report on the development and characterization of a passive heterodyne hot electron bolometer (HEB) imager operating at 850 GHz. HEB detectors provide unprecedented sensitivity and spectral resolution at terahertz frequencies covering the frequency range of 0.5-5 THz. Terahertz imagers based on HEB technology have sufficient sensitivity to operate in a passive imaging mode, thus eliminating the need for active illumination. We demonstrated a fully automated passive imaging system based on our HEB technology. The front-end heterodyne detector includes a quasi-optically coupled HEB device in close proximity to a monolithic microwave integrated circuit low-noise IF amplifier. We integrated a compact commercial harmonic multiplier local oscillator (LO) source to provide the LO biasing for the HEB detector. Our high spectral resolution terahertz imager has a noise equivalent temperature difference value of 0.4 K and a spatial resolution of approximately 4 mm.

A Low-Cost 128 $\times$ 128 Uncooled Infrared Detector Array in CMOS Process

This paper discusses the implementation of a low-cost 128 times 128 uncooled infrared microbolometer detector array together with its integrated readout circuit (ROC) using a standard 0.35 mum n-well CMOS and post-CMOS MEMS processes. The detector array can be created with simple bulk-micromachining processes after the CMOS fabrication, without the need for any complicated lithography or deposition steps. The array detectors are based on suspended p+-active/n-well diode microbolometers with a pixel size of 40 mum times 40 mum and a fill factor of 44%. The p+-active/n-well diode detector has a measured dc responsivity (R) of 4970 V/W and a thermal time constant of 36 ms at 50 mtorr vacuum level. The total measured rms noise of the diode type detector is 0.69 muV for an 8 kHz bandwidth, resulting in a detectivity (D*) of 9.7 times 108 cm ldr Hz1/2/W. The array is scanned by an integrated 32-channel parallel ROC including low-noise differential preamplifiers with an electrical bandwidth of 8 kHz. The 128 times 128 focal plane array (FPA) has one row of infrared-blind reference detectors that reduces the effect of FPA fixed pattern noise and variations in the operating temperature relaxing the requirements for the temperature stabilization. Including the noise of the reference and array detectors together with the ROC noise, the fabricated 128 times 128 FPA has an expected noise equivalent temperature difference (NETD) value of 1 K for f/1 optics at 30 frames/s (fps) scanning rate. This NETD value can be decreased to 350 mK by improving the post-CMOS fabrication steps and increasing the number of readout channels.

Uncooled infrared imaging using bimaterial microcantilever arrays

We report on fabrication and characterization of arrays of bimaterial microcantilevers and discuss their performance as uncooled infrared imagers. An optical readout was used to simultaneously measure deflections of all microcantilevers in the array. The fabricated arrays had an average noise equivalent temperature difference (NETD) and a response time of 1.5K and 6ms, respectively. Some microcantilevers in the array exhibited NETD values below 500mK, approaching our theoretical prediction of 151mK. A unique and valuable feature of the implemented approach is its straightforward scalability to higher resolution arrays, without progressively growing complexity and cost.

High responsivity InP-InGaAs quantum-well infrared photodetectors: characteristics and focal plane array performance

We report the detailed characteristics of long-wavelength infrared InP-In/sub 0.53/Ga/sub 0.47/As quantum-well infrared photodetectors (QWIPs) and 640/spl times/512 focal plane array (FPA) grown by molecular beam epitaxy. For reliable assessment of the detector performance, characterization was performed on test detectors of the same size and structure with the FPA pixels. Al/sub 0.27/Ga/sub 0.73/As-GaAs QWIPs with similar spectral response (/spl lambda//sub p/=/spl sim/7.8 /spl mu/m) were also fabricated and characterized for comparison. InP-InGaAs QWIPs (20-period) yielded quantum efficiency-gain product as high as 0.46 under -3-V bias with a 77-K peak detectivity above 1/spl times/10/sup 10/ cm/spl middot/Hz/sup 1/2//W. At 70 K, the detector performance is background limited with f/2 aperture up to /spl sim/ 3-V bias where the peak responsivity (2.9 A/W) is an order of magnitude higher than that of the AlGaAs-GaAs QWIP. The results show that impact ionization in similar InP-InGaAs QWIPs does not start until the average electric-field reaches /spl sim/25 kV/cm, and the detectivity remains high under moderately large bias, which yields high responsivity due to large photoconductive gain. The InP-InGaAs QWIP FPA offers reasonably low noise equivalent temperature difference (NETD) even with very short integration times (/spl tau/).70 K NETD values of the FPA with f/1.5 optics are 36 and 64 mK under bias voltages of -0.5 V (/spl tau/=11 ms) and -2 V (/spl tau/=650 /spl mu/s), respectively. The results clearly show the potential of InP-InGaAs QWIPs for thermal imaging applications requiring high responsivity and short integration times.

256×256 focal plane array midwavelength infrared camera based on InAs/GaSb short-period superlattices

An infrared camera based on a 256×256 focal plane array (FPA) for the second atmospheric window (3–5 µm) has been realized for the first time with InAs/GaSb short period superlattices (SLs). The SL detector structure with a broken gap type-II band alignment was grown by molecular beam epitaxy on GaSb substrates. Effective bandgap and strain in the superlattice were adjusted by varying the thickness of the InAs and GaSb layers and the controlled formation of InSb-like bonds at the interfaces. The FPAs were processed in a full wafer process using optical lithography, chemical-assisted ion beam etching, and conventional metallization technology. The FPAs were flip-chip bonded using indium solder bumps with a read-out integrated circuit and mounted into an integrated detector cooler assembly. The FPAs with a cut-off wavelength of 5.4 µm exhibit quantum efficiencies of 30% and detectivity values exceeding 1013 Jones at T=77 K. A noise equivalent temperature difference (NETD) of 11.1 mK was measured for an integration time of 5 ms using f/2 optics. The NETD scales inversely proportional to the square root of the integration time between 5 ms and 1 ms, revealing background limited performance. Excellent thermal images with low NETD values and a very good modulation transfer function demonstrate the high potential of this material system for the fabrication of future thermal imaging systems.