Uncooled IR Detectors Research Articles

Low temperature fabrication of VOx thin films for uncooled IR detectors by direct current reactive magnetron sputtering method

Vanadium oxide films have been fabricated on Si3N4-film-coated silicon substrates by direct current reactive magnetron sputtering method. Conditions of deposition are optimized making use of parameters such as sputtering time, dc power, oxygen partial pressure and substrate temperature. X-ray diffraction indicates that the film is a mixture of VO2, V2O3, and V3O5. Four-probe measurement shows that the VOx thin film owns high temperature coefficient of resistance (TCR ∼−2.05%/°C) and suitable square resistance 18.40kΩ/□ (measured at 25°C), indicating it is a well candidate material for uncooled IR detectors. In addition, IR absorption in the wavelength of 2–16μm has been characterized. It is worth noting that the films are sputtered at a relatively low temperature of 210°C in a controlled Ar/O2 atmosphere. Compared to traditional craft, this method needs no post-anneal at high temperature (400–500°C).

Characterization of area arrays of microbolometer-based un-cooled IR detectors without using ROIC

We present the characterization of microbolometer area arrays being developed for IR imaging applications. Here, we propose a technique that does not use readout integrated circuit for accessing detector array elements for characterization, which is useful for initial detector technology development. The technique is based on a connection scheme that uses two levels of metal lines in a row–column fashion, thereby, reducing the number of pads and therefore the fan-out complexity from N × M to N + M for an array of N × M detectors. The resulting networked structure has been analyzed and the technique has been developed for extracting back the actual signal and noise values from the measured data. We have implemented this technique on a set of 16 × 16 arrays of Ti-microbolometers developed at our laboratory. However, it may be extended to other types as well. The measurement on these arrays and extraction of various performance parameters are discussed here. The results of detailed characterization show that the average detectivity is about 1.44 × 10 8 Jones, TCR is 0.27%, thermal time constant is 9.91 ms and thermal conductivity is 0.16 μW/K in these arrays. These values are very close to the design values.

Polycrystalline lead selenide: the resurgence of an old infrared detector

AbstractThe existing technology for uncooled MWIR photon detectors based on polycrystalline lead salts is stigmatized for being a 50-year-old technology. It has been traditionally relegated to single-element detectors and relatively small linear arrays due to the limitations imposed by its standard manufacture process based on a chemical bath deposition technique (CBD) developed more than 40 years ago. Recently, an innovative method for processing detectors, based on a vapour phase deposition (VPD) technique, has allowed manufacturing the first 2D array of polycrystalline PbSe with good electro optical characteristics. The new method of processing PbSe is an all silicon technology and it is compatible with standard CMOS circuitry. In addition to its affordability, VPD PbSe constitutes a perfect candidate to fill the existing gap in the photonic and uncooled IR imaging detectors sensitive to the MWIR photons. The perspectives opened are numerous and very important, converting the old PbSe detector in a serious alternative to others uncooled technologies in the low cost IR detection market. The number of potential applications is huge, some of them with high commercial impact such as personal IR imagers, enhanced vision systems for automotive applications and other not less important in the security/defence domain such as sensors for active protection systems (APS) or low cost seekers.Despite the fact, unanimously accepted, that uncooled will dominate the majority of the future IR detection applications, today, thermal detectors are the unique plausible alternative. There is plenty of room for photonic uncooled and complementary alternatives are needed. This work allocates polycrystalline PbSe in the current panorama of the uncooled IR detectors, underlining its potentiality in two areas of interest, i.e., very low cost imaging IR detectors and MWIR fast uncooled detectors for security and defence applications. The new method of processing again converts PbSe into an emerging technology.

Uncooled microbolometer detector: Recent developments at Ulis

Uncooled infrared focal plane arrays are being developed for a wide range of thermal imaging applications. Therefore, to answer these markets, a 35 μm pixel-pitch uncooled IR detector technology has been developed enabling high performance 160 × 120 and 384 × 288 arrays production. Besides a wide-band version from uncooled 320 × 240/45 μm array has been also developed in order to address process control and more precisely industrial furnaces control. The ULIS amorphous silicon technology is well adapted to manufacture low cost detector in mass production. After some brief microbolometer technological background, we present the characterization of 35 μm pixel-pitch detector as well as the wide-band 320 × 240 infrared focal plane arrays with a pixel pitch of 45 μm. Information on the new 640 × 480 array with a pixel pitch of 25 μm is also presented.

A novel uncooled substrate-free optical-readable infrared detector: design, fabrication and performance

A novel substrate-free uncooled IR detector based on an optical-readable method is presented and fabricated successfully. The detector is composed of a bi-material (BM) cantilever array, without a silicon substrate, which is eliminated in the fabrication process. Compared with the generally used sacrificial layer cantilever, the loss of incident IR energy caused by the reflection from and absorption by the silicon substrate is eliminated completely in the substrate-free structure. The IR radiation reaching the IR detector surface increases by over 80% in the case of the novel substrate-free detector array structure, compared to the sacrificial layer structure. Moreover, the substrate-free structure has less heat loss than the sacrificial layer structure. The results of thermal imaging of the human body show the detector is able to sense objects at room temperature. The experimental NETD was estimated to be 200 mK.

Simulation and experimental studies of an uncooled MEMS capacitive infrared detector for thermal imaging

This paper presents the finite element analysis (FEA) simulation results and electrostatic characterizations of an uncooled MEMS capacitive IR detector. Thermal, thermalstructural and electrostatic-structural behaviors were simulated. The peak displacement sensitivities were calculated as 0.109nm/pW·µm−2 and 0.029nm/pW·µm−2 for 100µm and 50µm plates, respectively. Electrostatic-structural pull-in behaviors of the devices were tested and the results agreed with simulation well when second order mode pull-in was considered in the FEA model.

Uncooled microbolometer detector: recent developments at ULIS

AbstractUncooled infrared focal plane arrays are being developed for a wide range of thermal imaging applications. Fire-fighting, predictive maintenance, process control and thermography are a few of the industrial applications which could take benefit from uncooled infrared detector. Therefore, to answer these markets, a 35-μm pixel-pitch uncooled IR detector technology has been developed enabling high performance 160×120 and 384×288 arrays production. Besides a wide-band version from uncooled 320×240/45 μm array has been also developed in order to address process control and more precisely industrial furnaces control. The ULIS amorphous silicon technology is well adapted to manufacture low cost detector in mass production. After some brief microbolometer technological background, we present the characterization of 35 μm pixel-pitch detector as well as the wide-band 320×240 infrared focal plane arrays with a pixel pitch of 45 μm.

Optical System Design for Thermal Target Recognition by Spiral Scanning [TRSS

Various kinds of systems, that can do target recognition and position detection simultaneously by using infrared sensing detectors, have been developed. In this paper, the detection system TRSS (Thermal target Recognition by Spiral Scanning) adopts linear array shaped uncooled IR detector and uses spiral type fast scanning method for relative position detection of target objects, which radiate an IR region wavelength spectrum. It can detect thermal energy radiating from a 9 m-size target object as far as 200 m distance. And the maximum field of a detector is fully filled with the same size of target object at the minimum approaching distance 50 m. We investigate two types of lens systems. One is a singlet lens and the other is a doublet lens system. Every system includes one aspheric surface and free positioned aperture stop. Many designs of F/1.5 system with <TEX>${\pm}5.2^{\circ}$</TEX> field at the Efl=20, 30 mm conditions for single element and double elements lens system respectively are compared in their resolution performance [MTF] according to the aspheric surface and stop position changing on their optimization process. Optimum design is established including mechanical boundary conditions and manufacturing considerations.

Preparation and Characterization of Ba1-xSrxTiO3 Based Thin Films for Pyroelectric Applications

ABSTRACTThe use of pyroelectric thin films in uncooled IR detectors has many advantages over the present IR detector technology, which requires extensive cooling for operation. These advantages include reduced weight, reduced footprint, reduced complexity, increased reliability, and decreased maintenance. Ba1-xSrxTiO3 (BST), based thin films are ideal candidates for use in these devices due to their tailorable materials properties. These properties include a high dielectric constant, low dielectric loss, high electrical resistivity, as well as a high pyroelectric constant. BST thin films were doped with Mg in varying amounts, from 0 to 20mol%. The thin films were deposited via metalorganic solution deposition (MOSD) on Pt/Ti/SiO2/Si substrates. Annealing temperatures ranged from 500 to 750 °C. The films were characterized for structural, microstructural, compositional, surface morphological, dielectric and insulating properties. Glancing angle x-ray diffraction (GAXRD) was used to determine crystallinity, phase formation and film orientation. Field emission scanning electron microscopy (FESEM) and cross sectional transmission electron microscopy (TEM) were employed to access surface morphology and microstructure. The Materials Detectivity Figure of Merit (FOM), D*, [D*=pi/(CV(ε0εrtanδ)1/2] was used to evaluate the film's detectivity response. Our results show that undoped Ba60Sr40TiO3, with a value D*= 0.08 (cm3/J)1/2, appears to out perform Pb based pyroelectric thin films, thus making it a viable candidate for IR pyroelectric detector applications.

High Performance PST Thin Films on Polysilicon Sacrificial Layers for Uncooled IR Detectors

High Performance PST Thin Films on Polysilicon Sacrificial Layers for Uncooled IR Detectors

High Performance PST Thin Films on Polysilicon Sacrificial Layers for Uncooled IR Detectors

Materials merit figures nearly 4 times that of PZT have been reported for sputtered lead scandium tantalate thin films. To achieve the required PST ferroelectric crystal phase, post-deposition annealing to temperatures above that allowable for silicon readout circuit survival is required. This paper describes the deposition of thin film lead scandium tantalate (Pb 2 ScTaO 6 ) over a polysilicon sacrificial layer, compatible with high temperature annealing, for use with a 'composite' wafer approach. A Tantalum oxide barrier layer is used to prevent PbO diffusion into the polysilicon during the high temperature annealing stage. Materials are characterised by X-ray diffraction and transmission electron microscopy. Some annealed films have shown B-site ordering. Induced pyroelectric coefficients over 1.2 2 10 m 3 Cm m 2 K m 1 have been measured in the annealed films, with materials merit figure exceeding 8 2 10 m 5 Pa m 1/2 at 9.3V/ w m.

Development of porous silica thick films by a new base-catalyzed sol–gel route

SiO 2 aerogel particles with a size of 2 nm were successfully dispersed into the SiO 2 gel solution, which was prepared by a modified base-catalyzed sol–gel process. These fine SiO 2 aerogel particles play an important role in releasing internal stress within the porous films and avoiding cracking of the porous thick films. As a result, the porous SiO 2 film with the thickness of 22 μm could be obtained on silicon substrates by spin coating the SiO 2 gel precursor solution with 0.02 wt.% SiO 2 aerogel particles while this film still retained the high porosity of 48% after annealing at 550°C. The effect of the SiO 2 aereogel particles on the roughness of the films was also discussed. This porous SiO 2 film can be applied in large-scale uncooled IR detectors as a thermal-insulating layer.

High temperature processing of ferroelectric thin films using interconnect wafer technology

There is interest in ferroelectric thin films for uncooled IR detector applications. Currently the processing of these devices takes a fully integrated approach where the thin films are deposited directly onto underlying CMOS readout circuitry, thereby imposing severe limits on the thermal budget available for the crystallisation of the ferroelectric material. This is incommensurate with obtaining the best ferroelectric properties from materials such as lead scandium tantalate (PST) which requires elevated temperature processing to attain the highest merit figures for IR detection. In this paper thin film PST processed within the CMOS survivability envelope will be compared to that processed at temperatures up to 850°C. A novel interconnect wafer technology will be outlined which enables processing to be extended to such temperatures. It will be shown that elevated temperature processing of the PST film can result in dramatic improvement of the materials merit figure for IR detection

Update: a protein microbolometer for focal plane arrays

The current infrared microbolometer technology using vanadium oxide material is not quite as sensitive as a cooled platinum silicide or cooled mercury cadmium telluride or indium antimonide array. The performance factor of the present IR bolometer needs to be improved by at least an order of magnitude to make a bolometric IR device comparable to the cooled detector arrays. I recently reported [1] that thin films of globular proteins such as cytochrome c exhibit a high temperature coefficient of resistance (TCR) value of about 10% as compared with a value of 2.0 to 2.5% normally observed in VOx films. The measurements that were initially reported were taken two weeks after the thin films had been prepared. The measurement of the TCR values presented in this report were taken within three days of the films being prepared. These TCR values are very high, approximately 35% for temperatures between 25 and 60°C, and they have a sheet resistance around 1 to 2 Ω/cm2 with a current of 1 mA. The values of TCR were very uniform over the wafer surface, but the values varied slightly from wafer to wafer. It appears that a cytochrome c monolayer on metal oxide semiconductor (CMOS) electronics could generate miniaturized, uncooled IR detector arrays that would be suitable for military and commercial applications that require very high performance and low-power drain. With these high TCR values, biological uncooled devices will have the potential for long-range thermal imaging applications comparable to cooled detectors.I have also examined the sheet resistance of the protein layer with an increase in the bias current near ambient temperature. I found that the sheet resistance of cytochrome c rapidly goes towards the negative values as the bias current is increased. On the other hand, TCR characteristics remained unchanged with an increase in bias current. This is a very interesting phenomenon that has never been reported in biomolecules. I am making further explorations into the electrical properties of many proteins for possible device applications.

Influence of Interface Structure on Chemical Etching Process for Air Gap of Microelectromechanical System Based on Surface Micromachining

This paper analyses the problems posed by the interface structure during chemical etching by Hydro-fluoric (HF) acid for creating air gaps in microelectromechnical system (MEMS) devices using PZT(53/47) films and surface micromachining techniques. In order to investigate the influence of interface structure on the HF chemical etching process, Pt/PZT/Pt/Ti/TiO2/polysilicon/Si3N4/PSG/Si (Samples A and C) and Pt/PZT/RuO2/Ru/Si3N4/PSG/Si (Sample B) structures were fabricated. These structures are selected for a microcantilever beam and/or an uncooled IR detectors fabricated with PZT piezoelectric/pyroelectric films based on the surface micromachining technique. Both need etching for the removal of phosphor silicate glass (PSG) to create an air gap. If the devices had a poor interface structure, they would fail during the HF chemical etching process because the poor interface structure would act as a kind of penetration path for etching acid leading to unwanted etching. Therefore, it is very important to investigate the interface structure to fabricate efficient MEMS devices. In this study two different solutions have been suggested to improve the interface structure. The first is post thermal annealing at 900°C for 30 min. after deposition of polycrystalline silicon for sample A. Secondly, a RuO2/Ru hybrid electrode was deposited on Si3N4 directly instead of on the Pt/Ti/TiO2/Polysilicon electrode, which has Pt/PZT/RuO2/Ru/Si3N4/PSG/Si as the device structure. These two solutions suggest that a dense interface structure increases enhances of success of the chemical etching process of MEMS devices fabricated using PZT films and surface micromachining techniques.

Pyroelectric, ferroelectric and dielectric properties of Mn and Sb-doped PZT thin films for uncooled IR detectors

Mn and Sb-doped Pb(Zr,Ti)O 3 (PMSZT)/YBa 2Cu 3O 7− y (YBCO) heterostructures grown on Si substrates have been investigated for IR detector arrays. The epitaxial PMSZT films deposited on c-oriented YBCO conducting electrodes show “self polarization” behavior and high IR responsivity even without poling. An asymmetric hysteresis loop and C– V curve show that the original polarization state of the capacitors is pointing towards the top electrode in the PMSZT film, and perpendicular to the YBCO thin film. Doping with Mn and Sb has been shown to enhance the pyroelectric and dielectric properties of PZT for enhanced IR sensitivity. The PMSZT IR detectors were examined for their pyroelectric coefficient and their detectivity D*. The PMSZT thin films have relatively high pyroelectric coefficient of 45 nC cm −2 K at 25°C and 148 nC cm −2 K at 90°C. The PMSZT detector shows detectivity D* values from 2.5×10 8 to 6.0×10 8 cmHz 1/2 W −1 in the 2.5–19.5 μm wavelength range.

Microscan in infrared staring systems

Sampling-limited resolution in a two-dimensional infrared focal plane array is discussed in terms of the reciprocal (inverse) lattice in the frequency domain. It is shown how the bandwidth of the signal that can be reconstructed without error can be increased by a microscan. Microscanning means translating the image over the detector array along a short distance and then resampling the image, thus increasing the spatial sampling rate by decreasing the distance between samples. Two current microscan modes are compared: the rectangular bidirectional mode, in which the image is translated by half the array’s pitch along the two main perpendicular axes, and the diagonal mode, in which the image is shifted by half the diagonal of a rectangle defined by four adjacent pixels. It is shown that the diagonal mode implies the same frequency limit in accordance with the Nyquist criterion, in the horizontal and vertical directions, as the bidirectional mode. The temporal MTF that is a result of the microscan is calculated for the case where the temporal impulse response is a one-sided exponential pulse, like the thermal response of uncooled IR detectors. A simple algorithm for the reconstruction of the diagonal-microscan sampled data is suggested, and its MTF is derived.

Pulsed laser deposition of electronic ceramics

High quality thin films of electronic ceramics are currently being deposited by pulsed laser deposition (PLD) for a variety of applications. Examples of these applications include ferroelectric thin films for active microwave electronics and rare-earth doped manganite thin films for uncooled infrared focal plane arrays (IRFPAs). Single phase and oriented Sr x Ba (1− x TiO 3 films have been deposited by PLD onto (100) substrates of LaAlO 3 for active microwave devices. The dielectric properties of these films has been measured using gold interdigital capacitors deposited on top of the film. A 2 : 1 change in the capacitance can be achieved using a 40 V bias across a 5–10 μm capacitor gap. The dissipation factor (measured at 1 MHz) depends on film composition and temperature. Rare-earth manganite (RE x M 1− x MnO δ) thin films have also been deposited by PLD for use in uncooled IRFPAs. The temperature dependence of the resistivity has been measured for La 0.67Ca 0.33MnO 3 and La 0.67Sr 0.33MnO 3 thin films deposited onto LaAlO 3 substrates. Film properties depend strongly on composition and post-deposition processing conditions to vary the oxygen stoichiometry. The temperature coefficient of resistance (TCR) has been measured to be between 2 and 15%. Based on preliminary measurements, these films will increase the sensitivity of the uncooled IR detector to be comparable or greater than a cooled detector based on HgCdTe.

Application of interferometric enhancement to self-absorbing thin film thermal IR detectors

Uncooled thermal IR detectors require a suitable absorbing mechanism in order to achieve efficient radiation capture. For bulk detector materials such as ferroelectric ceramics this mechanism may be a broad-band absorber in the form of a metallic black layer, or a thin-film optical interference filter tuned for maximum absorption at the desired wavelength, deposited onto the surface of the detector. A thin metal film having a sheet resistance of 189 Ω per square can absorb 50% of incident radiation, and is employed in the metal film resistance bolometer detector and Golay cell. In this paper an interferometric technique for thin film thermal detectors is described, whereby a thermally sensitive material in the form of a semiconductor or dielectric layer becomes an integral component of a 3-layer absorber stack. The theory of this absorber structure is reviewed and compared with experimental data. It is shown that an effective absorption of 90% can be achieved over the waveband 8–13μm for a blackbody radiation source at 300 K temperature.