Uncooled Bolometer Research Articles

Investigation of electron beam lithography effects on metal–insulator transition behavior of vanadium dioxide

Vanadium dioxide (VO2) shows metal–insulator phase transition at nearly 68 °C. This metal–insulator transition (MIT) in VO2 leads to a significant change in near-infrared transmittance and an abrupt change in the resistivity of VO2. Due to these characteristics, VO2 plays an important role on optic and electronic devices, such as thermochromic windows, meta-materials with tunable frequency, uncooled bolometers and switching devices. In this work, VO2 thin films were fabricated by reactive direct current magnetron sputtering in O2/Ar atmosphere on sapphire substrates without any further post annealing processes. The effect of sputtering parameters on optical characteristics and structural properties of grown thin films was investigated by SEM, XRD, Raman and UV/VIS spectrophotometer measurements. Patterning process of VO2 thin films was realized by e-beam lithography technique to monitor the temperature dependent electrical characterization. Electrical properties of VO2 samples were characterized using microprobe station in a vacuum system. MIT with hysteresis behavior was observed for the unpatterned square samples at around 68 °C. By four orders of magnitude of resistivity change was measured for the deposited VO2 thin films at transition temperature. After e-beam lithography process, substantial results in patterned VO2 thin films were observed. In this stage, for patterned VO2 thin films as stripes, the change in resistivity of VO2 was reduced by a factor of 10. As a consequence of electrical resistivity measurements, MIT temperature was shifted from 68 °C to 50 °C. The influence of e-beam process on the properties of VO2 thin films and the mechanism of the effects are discussed. The presented results contribute to the achievement of VO2 based thermochromic windows and bolometer applications.

Comparative study of annealing and gold dopant effect on DC sputtered vanadium oxide films for bolometer applications

Vanadium oxide (VOx) thin film has been widely used for IR detectors and it is one of the promising materials for THz detectors due to its high temperature coefficient of resistance (TCR) values. VOx films with proper TCR values have also high resistance and it restricts bolometer performance especially for uncooled bolometers. To overcome this problem, deposition at elevated temperatures or annealing approach has been accepted and used but gold co-deposition approach has been proposed recently. In this study, vanadium oxide films were fabricated on high resistivity silicon substrates by reactive direct current magnetron sputtering in different O2/Ar atmosphere at room temperature. We investigated influence of oxygen partial pressure during deposition process and fabricated VOx thin films with sufficient TCR values for bolometer applications. In order to decrease resistivity of the deposited films, post annealing and gold doping approaches were performed separately. Effect of both post annealing process and gold doping process on structural and electrical properties of VOx thin films deposited at room temperature were investigated and detailed comparison between these methods were presented. We obtained the best possible approach to obtain optimum conditions for the highly reproducible VOx thin films which have the best resistivity and suitable TCR value for bolometer applications.

A 128x128, 34μm pitch, 8.9mW, 190mK NETD, TECless Uncooled IR bolometer image sensor with column-wise processing

In this paper, a 128x128, 34μm pixel-pitch, room temperature infrared image sensor and processor is presented. With a measured power consumption of 8.9mW (540μW / pixel) in full operating mode (image acquisition and data processing), the sensor exhibit a Noise Equivalent Temperature Difference (NETD) of 190mK at room temperature and doesn't require a Thermo-Electric Cooler (TEC). The circuit also features a novel ∑Δ Analogue to Digital Conversion architecture, 12 frames of Built-in SRAM and 128 column-wise full-custom processors that target a broad range of applications such as 2-points corrected IR camera or feature extraction for privacy-compliant presence detection, localization and counting. Built-in analogue and digital pixel-level offset pre-correction improves operability and manufacturing yields thus pushing bolometers IR technology one step forward towards high-end applications for consumer market.

Algorithm for Estimating the Efficiency of Uncooled Bolometers Based on Metals and Their Alloys

Algorithm for Estimating the Efficiency of Uncooled Bolometers Based on Metals and Their Alloys

Terahertz Imaging Progress at Capital Normal University

The progress on terahertz imaging at Capital Normal University in Beijing is presented. Our works on Terahertz Imaging include the active and passive imaging. For the active terahertz imaging, the pulse and continue wave terahertz imaging are studied, respectively. The active terahertz pulse imaging is based on the terahertz time-domain spectroscopy (THz-TDS). A practical focus-plane imaging system is built up based on the THz-TDS with the probe-beam-expanded electro-optical sampling and an infrared CCD imaging. The active terahertz continuous wave imaging is based on a CO₂-laser-pumped terahertz coherent source and an uncooled bolometer array of terahertz camera. The active terahertz polarization imaging and wave front imaging is developed. For the passive terahertz imaging, the low frequency of radiometers are used to detect the point-to-point beam-scanned terahertz signal by the optical focusing and scanning system so that the passive THz imaging is realized for the security inspection of human body. The related components and image processing methods are studied and used for the improvement of imaging speed and resolution. The research on THz imaging technology at Capital Normal University is opening up a potential application in the field of non-destructive testing and security inspection for the terahertz technology.

Tuning room temperature Tp and MR of La1-y(Cay-xSrx)MnO3 polycrystalline ceramics by Sr doping

Polycrystalline Sr−doped La1-y(Cay-xSrx)MnO3 (LCSMO, y=3/8) ceramics with x=0, y/25, 2y/25, 3y/25, 4y/25, y/3, y/2, and 2y/3 were prepared by sol−gel methods. X-ray diffraction patterns results indicated that all the bulk samples were single phase without any impurities. The structure of the samples transformed from orthorhombic phase (space group of Pnma) to rhombohedral phase (space group of R3̅c) when the Sr content (x) was equal to or greater than y/2. With the increasing of x content, the temperature coefficient of resistance (TCR) and magnetic resistance (MR) decreased gradually. However, the metal to insulator transition temperature (Tp) and the MR reached at 300.4K and 12% at 300K for the LCSMO (x=4y/25) sample, respectively, which meant the LCSMO material could be used as a candidate for photoelectric (uncooled bolometer or infrared detectors, etc.) or magnetic devices (uncooled magnetic sensors, etc.) at room temperature. Visibly, the results indicated that the room temperature modulation of Tp and MR was observed when the site of the Ca2+ ion was substituted by the Sr2+ ion in LCSMO (y=3/8) polycrystalline ceramics. This gave rise to the increasing size of the A−site average cationic radius <rA> and improving of the Mn4+ ion concentration through double exchange between the Mn3+ and Mn4+ ions, leading to enhance the electrical and magnetic properties of LCSMO ceramics.

Effects of plasmonic absorption of terahertz radiation in silica nanocomposite doped with erbium silicate

This paper studies terahertz transmission spectra of silicon and erbium silicides which can be used to create uncooled bolometers for terahertz frequency domain.

Perspective Material and Applications of Uncooled Metal Bolometers

A new method is proposed to compare the sensitivity of uncooled metal bolometers. This method allows us to conduct a comparative analysis for the sensitivity of uncooled bolometers with metal and metal alloy resistive elements. The experiments showed a twofold superiority in the sensitivity of the TiNi alloy-based bolometer with a resistive element less than $10~\Omega $ in resistance over the identical bolometers based on technologically applicable metals due to improved electrophysical properties. The Ti50.5Ni49.5 bolometer developed to measure the total solar radiation has a higher sensitivity, a higher measuring efficiency, a smaller weight, and size compared with a serial CMP-11 pyranometer, as well as the best environmental protection. The highly sensitive uncooled metal bolometers are shown to be used as an integral part of the optical absorption gas analyzer to provide the control of background mercury vapors in the atmosphere.

A sub-μs thermal time constant electrically driven Pt nanoheater: thermo-dynamic design and frequency characterization

Metal nanowires can emit coherent polarized thermal radiation, work as uncooled bolometers, and provide localized heating. In this paper, we engineer the temperature dynamics of electrically driven Pt nanoheaters on a silicon-on-insulator substrate. We present three designs and we electrically characterize and model their thermal impedance in the frequency range from 3 Hz to 3 MHz. Finally, we show a temperature modulation of 300 K while consuming less than 5 mW of power, up to a frequency of 1.3 MHz. This result can lead to significant advancements in thermography and absorption spectroscopy.

Chromium–niobium co-doped vanadium dioxide films: Large temperature coefficient of resistance and practically no thermal hysteresis of the metal–insulator transition

We investigated the effects of chromium (Cr) and niobium (Nb) co-doping on the temperature coefficient of resistance (TCR) and the thermal hysteresis of the metal–insulator transition of vanadium dioxide (VO2) films. We determined the TCR and thermal-hysteresis-width diagram of the V1−x−yCrxNbyO2 films by electrical-transport measurements and we found that the doping conditions x ≳ y and x + y ≥ 0.1 are appropriate for simultaneously realizing a large TCR value and an absence of thermal hysteresis in the films. By using these findings, we developed a V0.90Cr0.06Nb0.04O2 film grown on a TiO2-buffered SiO2/Si substrate that showed practically no thermal hysteresis while retaining a large TCR of 11.9%/K. This study has potential applications in the development of VO2-based uncooled bolometers.

Electrical conductivity of vacuum deposited films and crystals of redox-isomeric о-semiquinonato cobalt complexes

A study of the temperature dependences of electrical conductivity of crystalline and thin-film samples of о-semiquinonato complexes of cobalt (2,2′-bpy)Co(3,6-DBSQ)2 (complex 1) and (1,10-phen)Co(3,6-DBSQ)2 (complex 2) is reported; where 3,6-DBSQ is 3,6-di-tert-butyl-benzoquinone-1,2. It was shown that the electrical conductivity of samples having amorphous, polycrystalline and monocrystalline structure depends on temperature in the range 150–400 K in a different way. The abrupt drop of conductivity (up to 10% per a degree) was observed in crystalline samples in the region of phase transition (270–310 K for complex 1 and 140–230 K for complex 2). It makes such materials promising for design of uncooled matrix bolometers.

Study on the fabrication and performance of Mn1.56Co0.96Ni0.48O4 film optically immersed infrared detector

Mn1.56Co0.96Ni0.48O4 (MCN) thin films with cubic spinel phase were prepared by chemical solution deposition. The variable temperature electrical properties showed that the temperature coefficient of resistance reached − 4% K− 1 at 295 K. The immersed and non-immersed infrared (IR) detectors based on MCN films were fabricated and characterised. The experiment results indicated that the performance of immersed detector was better than the non-immersed one. It exhibited a responsivity of 4.4 × 103 V W− 1, detectivity of approximately 5 × 108 cm Hz1/2 W− 1, and thermal time constant of 18 ms at 295 K and 10 Hz. These results provide a possible avenue to develop a high-performance uncooled IR bolometer based on the thermistor materials.

Measurement of thermal conductance of La0.7Sr0.3MnO3 thin films deposited on SrTiO3 and MgO substrates

We present measurements of the thermal conductance of thin-film-on-substrate structures that could serve as thin film uncooled bolometers. Studied samples were 75nm thick epitaxial La0.7Sr0.3MnO3 thin films deposited on SrTiO3 (001) and MgO (001) substrates patterned in square geometries of areas ranging from 50μm×50μm to 200μm×200μm. The model allows estimating thermal boundary conductance values at the interface between film and substrate of 0.28±0.08×106WK−1m−2 for LSMO/STO (001) and 5.8±3.0×106WK−1m−2 for LSMO/MgO (001) from measurements performed in the static regime. Analytical expressions of thermal conductance and thermal capacitance versus modulation frequency are compared to measurements of the elevation temperature due to absorbed incoming optical power. The overall good agreement found between measurements and model finally provides the possibility to calculate the bolometric response of thin film bolometers, thus predicting their frequency response for various geometries.

Uncooled bolometer based on Mn1.56Co0.96Ni0.48O4 thin films for infrared detection and thermal imaging

Polycrystalline films of Mn1.56Co0.96Ni0.48O4 (MCN) were evaluated for uncooled bolometric applications grown by chemical solution deposition on amorphous Al2O3 substrate. The microstructural characterizations showed that the films were of excellent crystallization and compact surface morphology. Electrical results showed that the temperature coefficient of resistance reached −3.8%/K at 295 K. Low excess noise (normalized Hooge parameter αH/n of 7.6 × 10−28 m3) was achieved owing to the good epitaxial quality of the prepared films. Infrared bolometers were fabricated to evaluate the performance on infrared detection. It exhibited a noise equivalent temperature as low as 2.1 × 10−7 K/Hz1/2, responsivity of 330 V/W, detectivity of 0.6 × 108 cm Hz1/2/W, and noise equivalent power of 3.7 × 10−10 W/Hz1/2 at 30 Hz. The feasibility of the MCN films was demonstrated to be used for uncooled bolometric applications by thermal imaging. One can expect to get a responsivity of about 1 × 103 V/W and detectivity higher than 6 × 108 cm Hz1/2/W at 30 Hz for thermally isolated MCN film bolometer. The results of MCN showed its great potentiality for future room-temperature detection.

SpaceWire-based thermal-infrared imager system for asteroid sample return mission HAYABUSA2

A thermal-infrared (TIR) imager system is developed for HAYABUSA2, which is planned to be launched in 2014 and aims at sample-return from a C-class near-Earth asteroid 162173 (1999JU3) considered to contain organic or hydrated materials. The system consists of a TIR imager and digital electronics, which are used not only for the scientific investigation of physical properties of the asteroid surface, but also for the assessment of landing site selection and safe descent operation onto the asteroid surface with in situ measurement. TIR adopts an uncooled bolometer. Image operations such as multiple images summation, dark image subtraction, and the compensation of dead pixels are processed onboard. A processing module is connected to sensor interfaces through SpaceWire in order to provide deterministic processing time. Data compression is also provided to reduce the restriction of transmission time, which provides the equivalent compression ratio as JPEG2000 in 1/30 processing time in average. A high-speed data recorder is connected through SpaceWire in order to record TIR data in parallel with other sensor data. The modularity of SpaceWire enables us to use these as built devices for TIR and inherits the same design as the long-wavelength infrared imager developed for the Venus climate orbiter Akatsuki.

A heat balanced sigma–delta uncooled bolometer

This paper describes a heat balanced bolometer with an integrated CMOS sigma–delta interface. The sigma–delta interface accomplishes both the analogue-to-digital conversion and the heat feedback and therefore no other digital core is required. The heat feedback is implemented using a capacitively coupled electrical substitution technique with a digitally modulated signal that enables feedback linearization. Proof of concept is experimentally demonstrated with a polymer-type resistive bolometer and infrared optical source.

Very large scale heterogeneous integration (VLSHI) and wafer-level vacuum packaging for infrared bolometer focal plane arrays

Imaging in the long wavelength infrared (LWIR) range from 8 to 14μm is an extremely useful tool for non-contact measurement and imaging of temperature in many industrial, automotive and security applications. However, the cost of the infrared (IR) imaging components has to be significantly reduced to make IR imaging a viable technology for many cost-sensitive applications. This paper demonstrates new and improved fabrication and packaging technologies for next-generation IR imaging detectors based on uncooled IR bolometer focal plane arrays. The proposed technologies include very large scale heterogeneous integration for combining high-performance, SiGe quantum-well bolometers with electronic integrated read-out circuits and CMOS compatible wafer-level vacuum packing. The fabrication and characterization of bolometers with a pitch of 25μm×25μm that are arranged on read-out-wafers in arrays with 320×240pixels are presented. The bolometers contain a multi-layer quantum well SiGe thermistor with a temperature coefficient of resistance of −3.0%/K. The proposed CMOS compatible wafer-level vacuum packaging technology uses Cu–Sn solid–liquid interdiffusion (SLID) bonding. The presented technologies are suitable for implementation in cost-efficient fabless business models with the potential to bring about the cost reduction needed to enable low-cost IR imaging products for industrial, security and automotive applications.

La0.7Sr0.3MnO3 suspended microbridges for uncooled bolometers made using reactive ion etching of the silicon substrates

Suspended La0.7Sr0.3MnO3 (LSMO) microbridges were fabricated using standard silicon micromachining techniques. First epitaxial LSMO thin films were deposited on SrTiO3 (STO) buffered Si (0 0 1) substrates by molecular-beam epitaxy. A simple two photolithography step process using the reactive ion etching of the silicon substrate to release the suspended microbridges was developed. The electrical resistivity as a function of temperature of 4 μm wide 50–200 μm long and 75 nm thick LSMO/STO fully processed suspended microbridges was very close to the characteristics of the initial LSMO thin films, demonstrating that the fabrication process did not degrade the quality of the LSMO. The thermal conductance of the processed bolometers was very low (of the order of 10−7 W K−1) at 300 K. These structures are promising for uncooled bolometer applications and other micro-electromechanical systems based on LSMO or other epitaxial functional oxides.

Graphene terahertz uncooled bolometers

We propose the concept of a terahertz (THz) uncooled bolometer based on n-type and p-type graphene layers (GLs), constituting the absorbing regions, connected by an array of undoped graphene nanoribbons (GNRs). The GLs absorb the THz radiation with the GNR array playing the role of the barrier region (resulting in nGL-GNR-pGL bolometer). The absorption of the incident THz radiation in the GL n- and p- regions leads to variations of the effective temperature of electrons and holes and of their Fermi energy resulting in the variation of the current through the GNRs. Using the proposed device model, we calculate the dark current and the bolometer responsivity as functions of the GNR energy gap, applied voltage and the THz frequency. We demonstrate that the proposed bolometer can surpass the hot-electron bolometers using traditional semiconductor heterostructures.

Discrete frequency infrared microspectroscopy and imaging with a tunable quantum cascade laser.

Fourier-transform infrared (FT-IR) imaging is a well-established modality but requires the acquisition of a spectrum over a large bandwidth, even in cases where only a few spectral features may be of interest. Discrete frequency infrared (DF-IR) methods are now emerging in which a small number of measurements may provide all the analytical information needed. The DF-IR approach is enabled by the development of new sources integrating frequency selection, in particular of tunable, narrow-bandwidth sources with enough power at each wavelength to successfully make absorption measurements. Here, we describe a DF-IR imaging microscope that uses an external cavity quantum cascade laser (QCL) as a source. We present two configurations, one with an uncooled bolometer as a detector and another with a liquid nitrogen cooled mercury cadmium telluride (MCT) detector and compare their performance to a commercial FT-IR imaging instrument. We examine the consequences of the coherent properties of the beam with respect to imaging and compare these observations to simulations. Additionally, we demonstrate that the use of a tunable laser source represents a distinct advantage over broadband sources when using a small aperture (narrower than the wavelength of light) to perform high-quality point mapping. The two advances highlight the potential application areas for these emerging sources in IR microscopy and imaging.