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  • Titill er á ensku Sub-wavelength platinum wires as bolometers: Fabrication and Optimization
  • Doktors
  • Útdráttur er á ensku

    This thesis presents results on nano- to microscopic bolometers. They are made of polycrystalline metallic thin- films shaped into narrow wires that sense infrared radiation in the near- to mid-infrared (IR) range. Using state-of-the-art e-beam lithography
    and metal deposition, the wires were fabricated on Si/ SiO2 wafers. The central part has lateral dimensions ranging from hundreds of nanometers to several micrometers, while the metallic thin- film has a thickness between 2 and 80 nm. By illuminating the bolometers with light from a ceramic blackbody infrared source (1060 K, peak wavelength at 2.74 u m), we monitor the change in resistance R in the structures. The resistance change is a direct result of infrared heating and the corresponding temperature change T can be characterized thanks to the approximately linear relationship between resistance and temperature characterized by alpha, the temperature coe fficient of resistance (TCR) of platinum wire. Based on the extracted temperature change of the bolometers, we calculate two figures of merit to characterize the sensitivity of our devices: the responsivity and the specific detectivity.
    The performance of our bolometers has been optimized through multiple experiments carried out for several years. We studied the in fluence on performance of physical and operating parameters such as oxide thickness, bolometer dimensions, platinum thickness or drive current. Depending on sample dimension and surroundings, a responsivity of 10^4 to 10^5 V/W was achieved, as well as a detectivity of 10^7 to 10^8 cmHz^(1/2)/W, which is comparable to the latest literature results in the field of thermal detectors.
    At the size range of our bolometers, changes in absorption properties due to sample dimensions were observed. Antenna-like resonances in IR absorption and enhanced responsivity for narrow wires have been revealed and discussed. It is also suggested
    that as the width is reduced, the wires become selective to the polarization of light they absorb. Bolometers thinner than 40 nm present altered electronic properties, such as a lower TCR value and a higher resistivity. Detectivity is maximized for thicknesses
    equal or larger than 40 nm. A silicon dioxide thickness of 350 nm or 1 um leads to an improved infrared sensitivity. For the latter thickness value, interference effects of infrared light reflected at the interface between silicon dioxide and silicon contribute to the sensitivity.
    Finally, the time response of our thin- film bolometers has been calculated to be less than 4 ns, which is a great improvement compared to the general time response of thermal detectors. This value can be further reduced by reducing the metallic film thickness. Similarly, the sensitivity can be improved by reducing the 1/f noise component.

  • 25.3.2014

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