| dc.description.abstract | Currently, the concept of multipurpose spacecrafts is being transformed into many small spacecrafts each of them performing specific tasks and thus leading to the realization of pico and nano satellites. No matter what is the application or size, demand for more number of IR channels for earth observation is ever increasing which necessitates significant reduction in the mass, power requirement and cost of the IR detectors. In this scenario, several order of magnitude mass and power savings associated with uncooled IR arrays are advantageous compared to cooled photon detectors. However the poor speed of response of uncooled microbolometer array devices obstruct the total replacement of cooled detectors in thermal imaging applications. This is especially true when the mission requires 50 m to 100 m ground resolution, in which even the "fastest" micro bolometer arrays turns "too slow" to follow the ground trace when looked from low earth orbit (LEO). Hence there is a great and unfulfilled requirement of faster uncooled detector arrays for meeting the demand for future micro and mini satellite projects for advanced missions. The present thesis describes the systematic studies carried out in development of high performance IR and THz detectors for space applications.
Ge-Si-O thin films are prepared by ion beam sputtering technique with argon (Ar) alone and argon and oxygen as sputtering species, using sputtering targets of different compositions of Ge and SiO2. The deposited thin films are amorphous in nature and have chemical compositions close to that of the target. The study of electrical properties has shown that the activation energy and hence the thermistor constant (β) and electrical resistivity (ρ) are sensitive to oxygen flow rate, and they are the least for thin films prepared with Ar alone as the sputtering species. Different thermal isolation structures (TIS), consisting of silicon nitride (Si3N4) membrane of different thicknesses, Ge-Si-O thin film and, chromium coating on the rear side of the membrane, are prepared by bulk micro-machining technique, whose thermal conductance (Gth) properties are evaluated from the experimentally determined current-voltage (I-V) characteristics. Gth shows non-linear dependence with respect to raise in temperature of thin film thermistor due to Joule heating. The infrared micro-bolometer detectors, fabricated using one of the TIS structures have shown responsivity (<v) close to 115 V W−1 at a bias voltage of 1.5 V and chopping frequency of 10 Hz, thermal time constant (τth) of 2.5 ms and noise voltage of 255 nV Hz−1⁄2 against the corresponding thermal properties of Gth and thermal capacitance Cth equal to 9.0 × 10−5 W K−1 and 1.95 × 10−7 J K−1 respectively. The detectors are found to have uniform spectral response in the infrared region from 2 µm to 20 µm, and NEDT in the range from 108 mK to 574 mK when used with an F/1 optical system. The detector, in an infrared earth sensor system, is tested before an extended black body which simulates the earth disc in the laboratory and the results are discussed.
As an extension of the single element detector to array device, design of a microbolometer array for earth sensor dispensing of scanning mechanisms is presented. It makes use of four microbolometer arrays with in-line staggered configuration that stare at the earth horizons, perceiving IR radiation in the spectral band of 14 µm to 16 µm. Design of the microbolometer has been carried out keeping in mind low power, lightweight, without compromising on the performance. An array configuration of 16 × 2 pixels is designed and developed for this purpose. Finite elemental analysis is carried out for design optimization to yield best thermal properties and thus high performance of the detectors. Suitable optical design configuration was arrived to image the earth horizon on to array. Using this optimum design, prototype arrays have been fabricated, packaged and tested in front of the black body radiation source and found to have Responsivity, NEP, and D∗ of 120 V W−1, 5.0 W Hz−1⁄2, 1.10 × 107 cm Hz1⁄2 W−1 respectively. The pixels show a uniform response within a spread of ±6 % and the pixel resistances are within a range of ±5 %.
Optically Immersed Bolometer IR detectors are fabricated using electron beam evaporated Vanadium Oxide as the sensing material. Spin coated polyimide is used as medium to optically immerse the sensing element to the flat surface of a hemispherical germanium lens. This optical immersion layer also serves as the thermal impedance control layer and decides the performance of the devices in terms of responsivity and noise parameters. The devices have been packaged in suitable electro-optical packages and the detector parameters are studied in detail. Thermal time constant varies from 0.57 ms to 6.1 ms and responsivity from 75VW−1 to 757VW−1 corresponding to polyimide thickness in the range 2.0 μm to 70 μm for a detector bias of 9V. Highest D obtained was 1.28 × 108 cm Hz1⁄2W−1. Noise Equivalent Temperature Difference (NETD) of 20mK is achieved for devices with polyimide thickness of 32 μm, whereas the NETD × th product is the lowest for devices with moderate thickness of thermal impedance layer.
Bolometric THz detectors were fabricated using V2O5 as sensing element immersed
onto germanium hemispherical lens using polyimide as immersion media. These
detectors were characterized for their efficiency in detection of THz radiation in
the range 10 THz to 35 THz emitted by a black body radiator. The responsivity of
the devices determined in four different frequency bands covering the spectrum of
interest and a maximum responsivity of 398VW−1 was observed. A variation in the
responsivity is observed which is due to the characteristics absorption of polyimide
in the THz region of interest and can be avoided by replacing with HDPE which
has less attenuation. NEP of 6.8 × 10−10WHz−1⁄2 was observed which is very close
to the state of art in the case of uncooled detectors which entitles the detectors for
spectroscopic applications. Specific Detectivity D* was observed to be much higher
than the conventional detectors thanks to the benefits of immersion. NETD of 26mK
was observed which is advantageous of application of these detectors in imaging
applications
These studies have lead to development of a new technology for fabrication of high
performance IR and THz detectors which can be used for spectroscopic and imaging
applications. Further, this technology can be scaled for development of linear and area
arrays finding applications where the speed of respnose as well as sensitivity are of
equal importance. from 0.57 ms to 6.1 ms and responsivity from 75 V W−1 to 757 V W−1 corresponding to polyimide thickness in the range 2.0 µm to 70 µm for a detector bias of 9 V. Highest D∗ obtained was 1.28 × 108 cm Hz1⁄2 W−1. Noise Equivalent Temperature Difference (NETD) of 20 mK is achieved for devices with polyimide thickness of 32 µm, whereas the NETD × τth product is the lowest for devices with moderate thickness of thermal impedance layer.
Bolometric THz detectors were fabricated using V2O5 as sensing element immersed onto germanium hemispherical lens using polyimide as immersion media. These detectors were characterized for their efficiency in detection of THz radiation in the range 10 THz to 35 THz emitted by a black body radiator. The responsivity of the devices determined in four different frequency bands covering the spectrum of interest and a maximum responsivity of 398 V W−1 was observed. A variation in the responsivity is observed which is due to the characteristics absorption of polyimide in the THz region of interest and can be avoided by replacing with HDPE which has less attenuation. NEP of 6.8 × 10−10 W Hz−1⁄2 was observed which is very close to the state of art in the case of uncooled detectors which entitles the detectors for spectroscopic applications. Specific Detectivity D* was observed to be much higher than the conventional detectors thanks to the benefits of immersion. NETD of 26 mK was observed which is advantageous of application of these detectors in imaging applications
These studies have lead to development of a new technology for fabrication of high performance IR and THz detectors which can be used for spectroscopic and imaging applications. Further, this technology can be scaled for development of linear and area arrays finding applications where the speed of respnose as well as sensitivity are of equal importance. | en_US |
