Microbolometer Focal Plane Array Research Articles

Design of MIR Dispersive Spectrograph System with Uncooled Microbolometer.

To make the mid-infrared (MIR) dispersive spectrograph a practical tool in industrial food processing lines, we designed a dispersive spectrograph system with an uncooled microbolometer focal plane array (FPA) detector for MIR spectral acquisition. To precisely regulate the angle of a rotatable grating to acquire the MIR spectrum, the spectral resolution and spatial resolution of the system were rigorously controlled to improve system performance. In the reflectance operation mode of the MIR dispersive spectrograph, the uncooled microbolometer FPA detector offered a maximum spectral resolution of 12 nm for the MIR, when a 300 grooves/mm blazed grating was used. Utilizing an optical parametric oscillator (OPO) pulse laser source, the wavelengths of the first-order diffraction were validated, and the system's spectral resolution limit was determined. As a line-scanning source, a Globar broadband source was installed, and the USAF 1951 Resolution Calculator was used to establish the spatial resolution of the imaging spectrograph. Using NI LabView, the logical operational technique for controlling the MIR dispersive spectrograph was encoded into system firmware. The GUI and test results are thoroughly described.

Improved Responsivity of an a-Si-based Micro-bolometer Focal Plane Array with a SiNx Membrane Layer

Improved Responsivity of an a-Si-based Micro-bolometer Focal Plane Array with a SiNx Membrane Layer

Failure analysis of fabrication process in hermetic wafer-level packaging for microbolometer focal plane arrays

Wafer-level hermetic packaging is attractive for microbolometer-based uncooled infrared cameras as it enables low manufacturing cost and high-volume production of the sensors. A large package is designed to accommodate highly sensitive microbolometers with a high pixel count. This paper focuses on the study of failure mechanisms related to fabricating such large hermetic packages at wafer-level. We bonded wafers containing ~12 mm × 12 mm dies using CuSn solid-liquid interdiffusion (SLID) bonding and subsequently analysed the possible causes for failures. A time-dependent cap deflection study was carried out on the successfully bonded dies to evaluate the vacuum condition inside the package. The main causes for failures determined by this study include insufficient bonding pressure, non-uniform electroplating, voids in the sealing frame, and local cracks on the cap wafer with cavity. These factors contribute to a low dicing yield and loss of vacuum. A high dicing yield were obtained by Cu electroplating with 5 mA/cm2 current density, ≥4 MPa bonding pressure, and ≥200 μm cap thickness. In the cap deflection study, 7 out of 19 dies in a wafer maintained high vacuum after 178 days as indicated by limited decline in the cap deflection (~1–6 %), which shows the potential of this wafer-level packaging for obtaining long-term reliability.

Overcoming the Challenges of Thermal Infrared Orthomosaics Using a Swath-Based Approach to Correct for Dynamic Temperature and Wind Effects

The miniaturization of thermal infrared sensors suitable for integration with unmanned aerial vehicles (UAVs) has provided new opportunities to observe surface temperature at ultra-high spatial and temporal resolutions. In parallel, there has been a rapid development of software capable of streamlining the generation of orthomosaics. However, these approaches were developed to process optical and multi-spectral image data and were not designed to account for the often rapidly changing surface characteristics inherent in the collection and processing of thermal data. Although radiometric calibration and shutter correction of uncooled sensors have improved, the processing of thermal image data remains difficult due to (1) vignetting effects on the uncooled microbolometer focal plane array; (2) inconsistencies between images relative to in-flight effects (wind-speed and direction); (3) unsuitable methods for thermal infrared orthomosaic generation. Here, we use thermal infrared UAV data collected with a FLIR-based TeAx camera over an agricultural field at different times of the day to assess inconsistencies in orthophotos and their impact on UAV-based thermal infrared orthomosaics. Depending on the wind direction and speed, we found a significant difference in UAV-based surface temperature (up to 2 °C) within overlapping areas of neighboring flight lines, with orthophotos collected with tail wind being systematically cooler than those with head wind. To address these issues, we introduce a new swath-based mosaicking approach, which was compared to three standard blending modes for orthomosaic generation. The swath-based mosaicking approach improves the ability to identify rapid changes of surface temperature during data acquisition, corrects for the influence of flight direction relative to the wind orientation, and provides uncertainty (pixel-based standard deviation) maps to accompany the orthomosaic of surface temperature. It also produced more accurate temperature retrievals than the other three standard orthomosaicking methods, with a root mean square error of 1.2 °C when assessed against in situ measurements. As importantly, our findings demonstrate that thermal infrared data require appropriate processing to reduce inconsistencies between observations, and thus, improve the accuracy and utility of orthomosaics.

Microbolometer parameters optimization for high-performance focal plane array

For high-performance microbolometric focal plane array, a pair of mutually restricted microbolometer performance parameters are analyzed and discussed. The main parameters optimization contradiction is between the microbolometric infrared signal to noise ratio and signal bandwidth, and these two performance characteristics parameters are decisive to the target-temperature resolution and imaging speed of the microbolometric focal plane array respectively. The microbolometric output signal to noise ratio of unit-power infrared signal and signal bandwidth are theoretically derived from the thermal balance equation of the typical microbolometer, and some vital factors to cause the optimization contradiction between them are revealed through theoretical analyses and numerical calculations. The microbolometer parameters optimization method based on these theoretical analyses and numerical calculations is pointed out, which can be guidable to design, fabricate and operate microbolometric focal plane arrays with high-performance characteristic parameters such as high temperature resolution and infrared image speed.

A low-cost chopping system and uncooled microbolometer array for ground-based astronomy

Mid-Infrared imaging is vital for the study of a wide variety of astronomical phenomena, including evolved stars, exoplanets, and dust enshrouded processes such as star formation in galaxies. However, infrared detectors have traditionally been expensive and it is difficult to achieve the sensitivity needed to see beyond the overwhelming mid-infrared background. Here we describe the upgrade and commissioning of a simple prototype, low-cost 10 μ m imaging instrument. The system was built using commercially available components including an uncooled microbolometer focal plane array and chopping system. The system was deployed for a week on the 1.52 m Carlos Sanchez Telescope and used to observe several very bright mid-infrared sources with catalogue fluxes down to sim 600 Jy. We report a sensitivity improvement of sim 4 mag over our previous unchopped observations, in line with our earlier predictions.

Single-shot terahertz spectrometer using a microbolometer camera

We demonstrate a single-shot terahertz spectrometer consisting of a modified Mach–Zehnder interferometer and a microbolometer focal plane array. The spectrometer is simple to use and can measure terahertz field autocorrelations and spectral power with no moving parts and no ultrashort-pulsed laser. It can effectively detect radiation at 10–40 THz when tested with a thermal source. It can also be used to measure the complex refractive index of a sample material. In principle, it can characterize both laser-based and non-laser-based terahertz sources and potentially cover 1–10 THz with specially designed terahertz microbolometers.

Bias voltage optimization for high signal-to-noise ratio microbolometric focal plane array

To increase the output signal-to-noise ratio of microbolometric focal plane array, the microbolometric bias optimization is crucial. The bias influence on the microbolometric responsivity is theoretically analyzed and numerically calculated, and the microbolometric responsivity can be optimized according to these theoretical analysis and calculated results. Some kinds of microbolometric noises are also theoretically discussed, from which the microbolometric noise can be theoretically derived to be related to the microbolometric bias parameter through numerical calculation. The bias optimization for microbolometric focal plane array can be obtained through the integrative effect of the microbolometric responsivity and current noise. These analysis and numerical calculation for the microbolometric bias optimization are theoretically strict and algorithmically reasonable, which has been experimentally verified in our laboratory. The bias parameter optimization method in this paper has guidable significance for high signal-to-noise ratio microbolometric focal plane array to detect low infrared radiation target.

New CMOS readout integrated circuits for future high-performance microbolometric focal plane array

For future high-performance microbolometric focal plane array, a new kind of CMOS readout integrated circuits is proposed in this paper. The whole readout integrated circuits consist of several or dozens of readout modules working in parallel and all the output signals of these readout modules can be output one by one by a common high-speed HCDS output circuit. The microbolometer in each readout module is biased with a pulse current, which can be adjusted with the substrate temperature fluctuation to achieve a uniform microbolometric infrared responsivity, and its voltage response signal can be directly amplified by a differential amplifier. Because there is no need of microbolometric response current integration time and capacitor in the readout module as those in traditional readout integrated circuits, the pixel-readout-speed of the readout module can easily reach up to about 100 MHz and the pixel-size is no longer restricted with the integration capacitor area. The common high-speed HCDS output circuit can effectively suppress the output noise of each readout module, and form a high-speed infrared image data stream from these output signals of all the readout modules. Furthermore, the adjustable pulse current can compensate the responsivity changes caused by the substrate temperature fluctuation. This kind of proposed CMOS readout integrated circuits can be utilized for future high-performance microbolometric focal plane array with low-power, high-speed, small-pixel, small-volume, and high-stability.

Substrate temperature compensation for microbolometric focal plane array without TEC

For a normal microbolometric focal plane array, the microbolometric infrared responsivity varies with its substrate temperature fluctuations because of the semiconductor characteristics of the microbolometer array. The relationship between the microbolometric infrared responsivity and substrate temperature is theoretically discussed in this paper. Above the discussion, a new substrate temperature compensation method is proposed for microbolometric focal plane array without TEC. The microbolometric focal plane array is based on a kind of high-speed CMOS readout integrated circuit, of which the microbolometric infrared response voltage signal can be directly read out without infrared response current integration. The bias current for the microbolometer with the proposed substrate temperature compensation method can be adjusted according to the substrate temperature change to achieve a steady microbolometric infrared responsivity. This kind of substrate temperature compensation method is theoretically strict and has been experimentally verified in our laboratory, which is potential for future high-speed, lower-power and small-pixel microbolometric focal plane array.

Spectral Characterization of a Microbolometer Focal Plane Array at Terahertz Frequencies

We have developed a method to characterize the spectral response of an uncooled microbolometer focal plane array at a broad range of terahertz (THz) frequencies (4–50 THz). This is achieved by using a spectrum-shaped blackbody radiator as a broadband THz source and measuring its spectral power with a Fourier transform infrared interferometer. With an additional measurement with a pyroelectric detector as a reference, the spectral response of the microbolometer relative to the pyroelectric reference is obtained with a signal-to-noise ratio of 100 over a $>$ 50-THz bandwidth.

Microbolometric theoretical responsivity analysis for focal plane array under pulse current bias

Based on the physical thermal balance equation of the microbolometer under pulse current bias, the microbolometric theoretical infrared responsivity is given out in this paper. The operating temperature of the microbolometer during the pulse current bias period is theoretically discussed and analyzed, from which the microbolometric theoretical infrared responsivity expression is derived for high-speed microbolometric focal plane array. The microbolometric theoretical infrared responsivity expression reveals some key factors to decide the infrared sensitivity of the microbolometric focal plane array and has been successfully verified through an experimental microbolometric focal plane array. The microbolometric theoretical infrared responsivity expression is theoretically strict and can be utilized for future microbolometric focal plane arrays with some high-performance characteristics parameters such as high-speed and small-pixel.

Theoretical analysis on signal bandwidth for microbolometric focal plane array

Based on the discussion of infrared signal readout structure for microbolometric focal plane array, the microbolometric infrared response signal bandwidth is theoretically analyzed in this paper. The infrared response signal channel in microbolometric focal plane array normally consists of several major parts and every one of these different parts has its own signal bandwidth. The signal bandwidth expressions for these parts are theoretically deduced on their physical thermal model or integrated circuit structures, and some key parameters to decide the microbolometric signal bandwidth are revealed in these expressions. These signal bandwidth expressions are theoretically strict and can be utilized to optimize the final infrared response signal bandwidth for any practical microbolometric focal plane array. These theoretical expressions have been successfully applied to guiding the microbolometer structure and readout integrated circuit design in our laboratory, and are useful for a microbolometric focal plane array to have an optimized signal bandwidth and achieve excellent signal-to-noise performance as well.

Uncooled Infrared Micro-Bolometer FPA for Multiple Digital Correlated Double Sampling

This letter presents the design, simulation, and fabrication of an uncooled infrared micro-bolometer focal plane array (MBFPA) for multiple digital correlated double sampling (MD-CDS) to reduce a large amount of fixed pattern noise (FPN). A hydrogenated amorphous silicon-based MBFPA is fabricated with $80\times 60$ active cell array and $80\times 9$ proposed warm cell array with a 35- $\mu \text{m}$ pitch. The proposed warm cell can be formed simply by using additional thermal paths on the active cell and it closely matches the properties of the active cell, including thermal conductance and capacitance, resistance, and structure. Averaged reference signals generated by the warm cell array during readout have the same FPN as the active cell without reaction from incident infrared. Experimental results show that FPN is 23% reduced by using the proposed warm cell array and the measured noise equivalent temperature difference referred to $f$ /1.0 is 103 mK after the MD-CDS.

Temperature compensation readout integrated circuit for microbolometric focal plane array

Based on the theoretical analysis of microbolometer thermoelectric characteristics and temperature response, a new kind of temperature compensation readout integrated circuit for microbolometric focal plane array is proposed. The proposed readout integrated circuit provides different bias voltages for different microbolometers to control their bias currents and allow the whole microbolometric focal plane array have a wide range of operating ambient temperature. This kind of readout integrated circuit can compensate not only the operating temperature fluctuation but also the thermoelectric-parameter nonuniformity for microbolometer array. Experimental results in our laboratory show that the microbolometric focal plane array on the basis of the proposed readout circuit has well uniform voltage responses of the microbolometers in the focal plane array at different ambient temperatures to the same microbolometric temperature change. The proposed temperature compensation readout integrated circuit is potential for the design of practical low-consumption and high-resolution microbolometric focal plane arrays without thermoelectric coolers.

Spectrally resolved far-fields of terahertz quantum cascade lasers.

We demonstrate a convenient and fast method to measure the spectrally resolved far-fields of multimode terahertz quantum cascade lasers by combining a microbolometer focal plane array with an FTIR spectrometer. Far-fields of fundamental TM0 and higher lateral order TM1 modes of multimode Fabry-Pérot type lasers have been distinguished, which very well fit to the results obtained by a 3D finite-element simulation. Furthermore, multimode random laser cavities have been investigated, analyzing the contribution of each single laser mode to the total far-field. The presented method is thus an important tool to gain in-depth knowledge of the emission properties of multimode laser cavities at terahertz frequencies, which become increasingly important for future sensing applications.

A new temperature compensation method for microbolometric focal plane array

Based on the analysis of the physical thermal model for a microbolometer under pulse bias, a new temperature compensation method for microbolometric focal plane array is proposed to compensate the substrate temperature fluctuation in microbolometric array. The self-heating and infrared responding voltage expressions of the microbolometer under pulse current bias are derived from the heat balance equation of the physical thermal model for the microbolometer, and these two expressions indicate that the microbolometer self-heating and infrared voltage responsivity expressions are dependent on the substrate temperature of the microbolometer with some other constant characteristic parameters. Through the obtained expressions, different pulse bias currents are provided to the microbolometer and its corresponding blind element at different substrate temperatures to get the same voltage responsivity of the microbolometer at different ambient temperatures. The proposed temperature compensation method can be utilized for a microbolometric focal plane array with low-power consumption and high temperature-resolution.

Shutter-less calibration of uncooled infrared cameras

Abstract. Infrared (IR) cameras based on microbolometer focal plane arrays (FPAs) are the most widely used cameras in thermography. New fields of applications like handheld devices and small distributed sensors benefit from the latest sensor improvements in terms of cost and size reduction. In order to compensate for disturbing influences derived from changing ambient conditions, radiometric cameras use an optical shutter for online recalibration purposes, partially also together with sensor temperature stabilization. For these new applications, IR cameras should consist only of infrared optics, a sensor array, and digital signal processing (DSP). For acceptable measurement uncertainty values without using an optical shutter (shutter-less), the disturbing influences of changing thermal conditions have to be treated based on temperature measurements of the camera interior. We propose a compensation approach based on calibration measurements under controlled ambient conditions. All correction parameters are determined during the calibration process. Without sensor temperature stabilization (TEC-less), the pixel responsivity is also affected by the camera temperature changes and has to be considered separately. This paper presents the details of the compensation procedure and discusses relevant aspects to gain low temperature measurement uncertainty. The residual measurement uncertainty values are compared to the shutter-based compensation approach.

High-speed CMOS readout integrated circuit for small-pixel-size microbolometric focal plane array

Abstract A novel kind of high-speed CMOS readout integrated circuit for large-scale microbolometric focal plane array is proposed in this paper. The proposed readout integrated circuit can directly amplify the voltage signals of all microbolometric pixels in focal plane array response to infrared irradiation and then output the amplified voltage signals column by column and row by row. Because there is no need of photo-generated current integration capacitor for microbolometric voltage response, the readout integrated circuit can save a lot of readout circuit area for small-pixel-size. In addition, the proposed CMOS readout integrated circuit can produce an output of the sensing voltage signals of microbolometer array at very high speed for which there is no integration time consumption when the microbolometric focal plane array operates.

Wavelength-Selective Infrared Metasurface Absorber for Multispectral Thermal Detection

A wavelength-selective metasurface absorber suitable for use in multispectral microbolometer focal plane arrays in the long-wavelength infrared (LWIR) region is theoretically and experimentally investigated. We show that a thin metal metasurface, which is characterized by effective surface impedance, as an absorbing layer can be integrated with an asymmetric Fabry–Perot cavity to construct a wavelength-selective perfect absorber. The absorbed infrared energy is mainly dissipated in a thin metal metasurface, in contrast to a multilayer metamaterial absorber. The calculated and experimental spectral responses of the metasurface absorbers show excellent wavelength-selective narrowband absorption to allow multispectral imaging in LWIR.