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  • 1.
    Ashraf, Shakeel
    Mid Sweden University, Faculty of Science, Technology and Media, Department of Electronics Design.
    Design and Integration of Infrared Absorber Structures into Polymer Membranes based Thermal Detectors2015Licentiate thesis, comprehensive summary (Other academic)
  • 2.
    Ashraf, Shakeel
    Mid Sweden University, Faculty of Science, Technology and Media, Department of Electronics Design.
    Thermal detector with integrated absorber structure for mid-IR gas detection2018Doctoral thesis, comprehensive summary (Other academic)
    Abstract [en]

    Measurement of the concentration of greenhouse gases, such as carbon dioxide(CO 2 ) and methane (CH 4 ), in the atmosphere has received significant attention in the last few decades. This work focusses on the development of high-performance thermopile detectors for use with the non-dispersive infrared (NDIR) measurements of such gases. The performance of the thermopile detectors could effectively be increased by selecting membrane materials with a low thermal conductivity value and an efficient infrared (IR) absorbing material and by selecting the materials with high Seebeck coefficient values. Graphite black paint can be used as a radiation absorber, as it has high absorption (80—93%) for a wide spectral range (2.5 μm — 20 μm). By using spray paint or a paint brush, the application of the absorber is simple and fast. However, the control over the processing process suffers with these simple methods. The thermal capacitance of the detector will rapidly increase due to uneven distribution and unknown thickness of the absorber, although the response of the thermopile detector will be maximum due to high absorption; however, the response time (τ th )for the detector will be longer.In order to improve the performance, IR absorbers have been designed to utilise the membrane (SU-8 epoxy) of the detector as an active part of the IR absorber. This utilisation of the SU-8 epoxy membrane will result in a maximum detector sensitivity and a minimum increase in both the thermal capacitance and thermal conductance of the thermopile detector. Absorber structures, based on SU-8 epoxy, with a narrow absorption band at 4.26 µm and a wider multi-layered absorption band at 3-6 µm, were designed, simulated, and fabricated, and their integration into the membrane of thermopile detectors have been presented. The response of the thermopile detector could also be increased by using materials with high Seebeck coefficient [1] values such as semiconductor materials, as they have higher Seebeckvalues compared to the metals. In the thesis, molybdenum disulfide (MoS 2 ) flakes were characterised, and Seebeck values were estimated through a measurement setup as a function of temperature difference (ΔT). The fabricated thermopile detectors were characterised, and the response time(τ th ) of a thermocouple with a multi-layered absorber structure has been estimated to be 21 ms. The detector has shown high responsivity value in the wavelength range of 3 µm – 4.5 µm, which is used for CO 2 and CH 4 detection. The thermopile detector was evaluated for CO 2 gas through a long-path-length NDIR platform. The results show that the evaluated thermopile could be used for the measurement of gas concentration down to levels of a few parts per million (ppm) by using the long-path-length NDIR platform.

  • 3.
    Ashraf, Shakeel
    et al.
    Mid Sweden University, Faculty of Science, Technology and Media, Department of Electronics Design.
    Forsberg, Viviane
    Mid Sweden University, Faculty of Science, Technology and Media, Department of Natural Sciences. KTH, Wallenberg Wood Science Center, Stockholm.
    Mattsson, Claes G.
    Mid Sweden University, Faculty of Science, Technology and Media, Department of Electronics Design.
    Thungström, Göran
    Mid Sweden University, Faculty of Science, Technology and Media, Department of Electronics Design.
    Thermoelectric properties of n-type molybdenum disulfide (MoS2) thin film by using a simple measurement method2019In: Materials, ISSN 1996-1944, E-ISSN 1996-1944, Vol. 12, no 21, article id 3521Article in journal (Refereed)
    Abstract [en]

    In this paper, a micrometre thin film of molybdenum disulfide (MoS2) is characterized for thermoelectric properties. The sample was prepared through mechanical exfoliation of a molybdenite crystal. The Seebeck coefficient measurement was performed by generating a temperature gradient across the sample and recording the induced electrical voltage, and for this purpose a simple measurement setup was developed. In the measurement, platinum was utilized as reference material in the electrodes. The Seebeck value of MoS2 was estimated to be approximately -600 μV/K at a temperature difference of 40 °C. The negative sign indicates that the polarity of the material is n-type. For measurement of the thermal conductivity, the sample was sandwiched between the heat source and the heat sink, and a steady-state power of 1.42Wwas provided while monitoring the temperature difference across the sample. Based on Fourier's law of conduction, the thermal conductivity of the sample was estimated to be approximately 0.26 Wm-1 K-. The electrical resistivity was estimated to be 29 W cm. The figure of merit of MoS2 was estimated to be 1.99 × 10-4. 

  • 4.
    Ashraf, Shakeel
    et al.
    Mid Sweden University, Faculty of Science, Technology and Media, Department of Electronics Design.
    Mattsson, Claes
    Mid Sweden University, Faculty of Science, Technology and Media, Department of Electronics Design.
    Fondell, Mattis
    Helmholtz Zentrum, Inst Methods & Instrumentat Synchrotron Radiat Re, Berlin, Germany.
    Lindblad, Andreas
    Uppsala Univ, Dept Chem, Angstrom Lab, Uppsala, Sweden.
    Thungström, Göran
    Mid Sweden University, Faculty of Science, Technology and Media, Department of Electronics Design.
    Surface modification of SU-8 for metal/SU-8 adhesion using RF plasma treatment for application in thermopile detectors2015In: Materials Research Express, ISSN 2053-1591, Vol. 2, no 8, article id 086501Article in journal (Refereed)
    Abstract [en]

    This article reports on plasma treatment of SU-8 epoxy in order to enhance adhesive strength for metals. Its samples were fabricated on standard silicon wafers and treated with (O2 & Ar) RF plasma at a power of 25W at a low pressure of (3×10-3 torr) for different time spans (10 sec – 70 sec). The sample surfaces were characterized in terms of contact angle, surface (roughness and chemistry) and using a tape test. During the contact angle measurement, it was observed that the contact angle was reduced from 73° to 5° (almost wet) and 23° for (O2 & Ar) treated samples, respectively. The RMS surface roughness was significantly increased by 21.5% and 37.2% for (O2 & Ar) treatment, respectively. A pattern of metal squares was formed on the samples using photolithography for a tape test. An adhesive tape was applied to the samples and peeled off at 180o. The maximum adhesion results, more than 90%, were achieved for the O2-treated samples, whereas the Ar-treated samples showed no change. The XPS study shows the formation of new species in the O2-treated sample compared to the Ar-treated samples. The high adhesive results were due to the formation of hydrophilic groups and new O2 species in the O2-treated samples, which were absent in Ar-treated samples.

  • 5.
    Ashraf, Shakeel
    et al.
    Mid Sweden University, Faculty of Science, Technology and Media, Department of Electronics Design.
    Mattsson, Claes
    Mid Sweden University, Faculty of Science, Technology and Media, Department of Electronics Design.
    Thungström, Göran
    Mid Sweden University, Faculty of Science, Technology and Media, Department of Electronics Design.
    Fabrication and characterization of a SU8-epoxy membrane based thermopile detector with an integrated multilayered absorber structure for the mid-IR region2019In: IEEE Sensors Journal, ISSN 1530-437X, E-ISSN 1558-1748, Vol. 19, no 11, p. 4000-4007Article in journal (Refereed)
    Abstract [en]

    This paper reports on the fabrication and characterization of a thermopile detector with an integrated mid-infrared absorber structure. The fabricated absorber structure has shown an absorption of more than 95% in the wavelength range of 3.2 – 5.47 μm. The detector was fabricated with standard cleanroom process techniques and equipment. The serial resistance was measured at about 315 kΩ at room temperature. The photosensitivity of the detector was characterized for a signle wavelength (4.26 µm) and a band of wavelength ranging from 2.5 –5.5 µm through two different measurement setups. In the first measurement setup, the photosensitivity was estimated at 57.5 V·mm2·W-1 through a MEMS-based infrared radiation source and with an optical band-pass filter of wavelength 4.26µm. The following characterization was performed to characterise the photosensitivity of the detector in a broader wavelength range. This measurement was taken using a monochromator setup utilizing a reference photodetector for calculations of the optical power of the infrared source. The photosensitivity and the specific detectivity (D*) of the fabricated detector were measured to values of 30-92 V·W-1 and 8.0×107-2.4×108 cm·Hz1/2·W-1, respectively, in the wavelength range of 2.8 – 5 µm. The time constant was estimated to around 21 ms

  • 6.
    Ashraf, Shakeel
    et al.
    Mid Sweden University, Faculty of Science, Technology and Media, Department of Electronics Design.
    Mattsson, Claes
    Mid Sweden University, Faculty of Science, Technology and Media, Department of Electronics Design.
    Thungström, Göran
    Mid Sweden University, Faculty of Science, Technology and Media, Department of Electronics Design.
    Fabrication of a mid-Ir sensitive thermopile detector2016In: Proceedings of IEEE Sensors, IEEE, 2016, article id 7808481Conference paper (Refereed)
    Abstract [en]

    This paper reports on the integration of a multilayered mid-infrared absorber structure into a SU-8 epoxy membrane-based thermopile detector. The absorber structure was designed and simulated using transfer matrix theory. The fabricated absorber structures were characterized through Fourier transform infrared spectroscopy. The structure shows an absorption of more than 95% in the wavelength range of 3.30pm–5pm for simulations, and 3.2pm–5.47pm for FTIR measurements. The complete fabrication process of a thermopile detector including the integration of a multilayered absorber structure has been presented. A MEMS based infrared emitter was used to characterize the fabricated detector. The serial resistance was measured to 315 kΩ and the responsivity was calculated to 57.5 Vmm2W−1 at a wavelength of 4.26pm. The time constant for the fabricated detector was estimated to around 21ms.

  • 7.
    Ashraf, Shakeel
    et al.
    Mid Sweden University, Faculty of Science, Technology and Media, Department of Electronics Design.
    Mattsson, Claes
    Mid Sweden University, Faculty of Science, Technology and Media, Department of Electronics Design.
    Thungström, Göran
    Mid Sweden University, Faculty of Science, Technology and Media, Department of Electronics Design.
    Gaynullin, B.
    Research and Development Department, SenseAir AB, Delsbo.
    Rödjegård, H.
    Research and Development Department, SenseAir AB, Delsbo.
    Evaluation of a CO2 sensitive thermopile with an integrated multilayered infrared absorber by using a long path length NDIR platform2018In: I2MTC 2018 - 2018 IEEE International Instrumentation and Measurement Technology Conference: Discovering New Horizons in Instrumentation and Measurement, Proceedings, IEEE, 2018, p. 1-6Conference paper (Refereed)
    Abstract [en]

    In this work, a mid-infrared-sensitive thermopile detector was evaluated for carbon dioxide gas. The thermopile has an integrated absorber structure that has more than 95% absorption for the wavelength range of 3.3-5.5 μm. For the measurement of carbon dioxide, the detector was initially assembled on a small printed circuit board holder and then installed in a long path length non-dispersive infrared platform. The measurement setup was calibrated by exposing the detector to carbon dioxide of known concentrations (from 50 ppm to 1,300 ppm) and measuring the corresponding voltage response of the detector. The measured voltage values allowed us to calculate the calibration constant for this detector. This was followed by a verification process using a concentration of 500 ppm (which was not used in the calibration) and a known value of 1,300 ppm. The verification results matched with real values, demonstrating that the sensor can be used for highly sensitive and accurate carbon dioxide measurements at concentrations from a few ppm level up to 1,300 ppm. Above this level, additional calibration is needed. 

  • 8.
    Ashraf, Shakeel
    et al.
    Mid Sweden University, Faculty of Science, Technology and Media, Department of Electronics Design.
    Mattsson, Claes
    Mid Sweden University, Faculty of Science, Technology and Media, Department of Electronics Design.
    Thungström, Göran
    Mid Sweden University, Faculty of Science, Technology and Media, Department of Electronics Design.
    Rödjegard, Henrik
    Design of a multilayered absorber structure based on SU-8 epoxy for broad and efficient absorption inMid-IR sensitive thermal detectors2014In: Proceedings, 2014, p. 938-941Conference paper (Refereed)
    Abstract [en]

    This paper reports on design, simulation and fabrication of a multilayered interferometric absorption structure with a broad absorption in the mid-infrared band. This region is used for IR based CH4 and CO2 detection. The structure consists of five layers of different thickness. The structure consists of one mirror layer of aluminium, two SU-8 epoxy layers and two thin titanium layers. This structure has been fabricated on a silicon substrate and verified for its absorption properties through Fourier transform infrared spectroscopy. The fabricated structure has been compared with simulations are performed using transfer matrix theory. The structure shows more than 90% absorption in the wavelength range of 3.20μm - 5.35μm for simulations and 3.13μm - 5.47μm for FT-IR measurements. The transmission and reflection of SU-8 epoxy was measured using FT-IR (that), resulting in a calculated absorption between 10 - 20% in the area of interest (3μm - 6μm). The use of SU-8 epoxy as dielectric medium, allows for direct integration of the structure into the membrane of a SU-8 membrane based thermopile. The integration results in minimum increase of the thermal capacitance and conductance, which results in maximum detector sensitivity and minimum time constant.

  • 9.
    Ashraf, Shakeel
    et al.
    Mid Sweden University, Faculty of Science, Technology and Media, Department of Electronics Design.
    Mattsson, Claes
    Mid Sweden University, Faculty of Science, Technology and Media, Department of Electronics Design.
    Thungström, Göran
    Mid Sweden University, Faculty of Science, Technology and Media, Department of Electronics Design.
    Rödjegard, Henrik
    SenseAir AB, Delsbo, Sweden.
    Integration of an interferometric IR absorber into an epoxy membrane based CO2 detector2014In: Journal of Instrumentation, ISSN 1748-0221, E-ISSN 1748-0221, Vol. 9, no 5, p. Art. no. C05035-Article in journal (Refereed)
    Abstract [en]

    Measurements of carbon dioxide levels in the environment are commonly performedby using non-dispersive infrared technology (NDIR). Thermopile detectors are often used in NDIRsystems because of their non-cooling advantages. The infrared absorber has a major influence onthe detector responsivity. In this paper, the fabrication of a SU-8 epoxy membrane based Al/Bithermopile detector and the integration of an interferometric infrared absorber structure of wavelength around 4 µ m into the detector is reported. The membrane of thermopile detector has beenutilized as a dielectric medium in an interferometric absorption structure. By doing so, a reduction in both thermal conductance and capacitance is achieved. In the fabrication of the thermopile,metal evaporation and lift off process had been used for the deposition of serially interconnectedAl/Bi thermocouples. Serial resistance of fabricated thermopile was measured as 220 kΩ. Theresponse of fabricated thermopile detector was measured using a visible to infrared source of radiation flux 3.23 mW mm−2. The radiation incident on the detector was limited using a band passfilter of wavelength 4.26 µ m in front of the detector. A responsivity of 27.86 V mm2W−1at roomtemperature was achieved using this setup. The fabricated detector was compared to a referencedetector with a broad band absorber. From the comparison it was concluded that the integratedinterferometric absorber is functioning correctly.

  • 10.
    Ashraf, Shakeel
    et al.
    Mid Sweden University, Faculty of Science, Technology and Media, Department of Electronics Design.
    Niskanen, Ilpo
    Mid Sweden University, Faculty of Science, Technology and Media, Department of Electronics Design. University of Oulu, Finland.
    Kanyathare, Boniphace
    Electronics and Telecommunications Department, Dar es salaam Institute of Technology, Tanzania.
    Vartiainen, Erik
    LUT School of Engineering Science, Lappeenranta University of Technology, Finland.
    Mattsson, Claes
    Mid Sweden University, Faculty of Science, Technology and Media, Department of Electronics Design.
    Heikkilä, Rauno
    Faculty of Technology, Structures and Construction Technology, University of Oulu, Finland.
    Thungström, Göran
    Mid Sweden University, Faculty of Science, Technology and Media, Department of Electronics Design.
    Determination of complex refractive index of SU-8 by Kramers-Kronig dispersion relation method at the wavelength range 2.5 – 22.0 μm2019In: Journal of Quantitative Spectroscopy and Radiative Transfer, ISSN 0022-4073, E-ISSN 1879-1352, Vol. 224, p. 309-311Article in journal (Refereed)
    Abstract [en]

    Accurate determination of the complex refractive index of SU-8 epoxy has significant for the wide variety of applications in optical sensor technology at IR range. The complex refractive index of SU-8 is determined by recording the transmission of light spectra for the wavelength range of 2.5 – 22.0 μm.  The data analysis is based on the Kramers-Kronig dispersion relation method. The method has several merits, such as ease of operation, non-contact technique, measurement accuracy, and rapid measurement. The present method is not restricted to the case of SU-8 but it is also proposed to be applicable across a broad range of applications, such as assessment of the optical properties of paints and biomedical samples.

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