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Design of a Micromachined Thermopile Infrared Sensor with a Self-Supported SiO2/SU-8 Membrane
Mid Sweden University, Faculty of Science, Technology and Media, Department of Information Technology and Media. (Electronics design division, STC)
Mid Sweden University, Faculty of Science, Technology and Media, Department of Information Technology and Media. (Electronics design division)
Mid Sweden University, Faculty of Science, Technology and Media, Department of Information Technology and Media. (Electronics design division)
Mid Sweden University, Faculty of Science, Technology and Media, Department of Information Technology and Media. (Electronics design division)ORCID iD: 0000-0002-3790-0729
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2008 (English)In: IEEE Sensors Journal, ISSN 1530-437X, E-ISSN 1558-1748, Vol. 8, no 12, 2044-2052 p.Article in journal (Refereed) Published
Abstract [en]

In the infrared region of the spectrum thermoelectric detectors such as the thermopile, are extensively used. These detectors rely on the well-known Seebeck effect, in which there is a direct conversion of thermoelectric differentials into electrical voltage. The temperature difference over thermocouple junctions is in general, created by forming a thin membrane connected to the silicon bulk. In many existing thermopiles, materials such as Si and Si3N4 have been used as membrane. These materials suffer from relatively high thermal conductivity, which lowers the membrane temperature and reduces the sensitivity of the detector. A material such as SU-8 2002 has a much lower thermal conductivity and is applied using standard photolithographic processing steps. This work presents thermal simulations regarding the use of SU-8 2002 as a thermal insulating membrane as compared to Si and Si3N4. The simulation results presented show that the temperature increase in a 5 µm SiO2/SU-8 membrane is about 9% higher than in a 1 µm Si3N4 membrane, despite the membrane thickness being increased by a factor of 5. A thermopile consisting of 196 serially interconnected Ti/Ni thermocouples positioned on a 5 µm SiO2/SU-8 2002 membrane has been fabricated. The sensitivity of the fabricated device has been evaluated in the infrared region, using a 1.56 µm IR laser and a xenon arc lamp together with a monochromator. The measurement results show a sensitivity of approximately 5 V/W over the wavelength range between 900 - 2200 nm. Measurements performed in a vacuum chamber show that the sensitivity of the detector could be increased by more than a factor of 3 by mounting the detector in a vacuum sealed capsule.

Place, publisher, year, edition, pages
Piscataway, USA: IEEE , 2008. Vol. 8, no 12, 2044-2052 p.
Keyword [en]
Infrared sensor, self-supported membrane, SU-8, thermopile
National Category
Other Electrical Engineering, Electronic Engineering, Information Engineering
Identifiers
URN: urn:nbn:se:miun:diva-6971DOI: 10.1109/JSEN.2008.2007679ISI: 000261545600037Scopus ID: 2-s2.0-85008020303OAI: oai:DiVA.org:miun-6971DiVA: diva2:114275
Projects
STC - Sensible Things That Communicate
Note

VR-Ecology

Available from: 2008-11-16 Created: 2008-11-11 Last updated: 2017-07-04Bibliographically approved
In thesis
1. Design, Fabrication and Optimization of Thermal Radiation Detectors Based on Thin Polymer Membranes
Open this publication in new window or tab >>Design, Fabrication and Optimization of Thermal Radiation Detectors Based on Thin Polymer Membranes
2009 (English)Doctoral thesis, comprehensive summary (Other academic)
Abstract [en]

The number of applications in which infrared radiation sensors are used is increasing. In some applications, the cost of the sensor itself is an issue, and simple solutions are thus required. In this thesis, the investigations have related to the use of thin polymer membranes in thermal infrared sensors, such as bolometers and thermopiles.

Infrared sensors are usually subcategorized into photonic sensors and thermal sensors. For detection of infrared radiation using a photodetector, there is a requirement for low band-gap material. The need of cooling makes infrared photodetectors rather expensive, and not an alternative for low-cost applications. In thermal sensors, the heat generated from the incident infrared radiation is converted into an electrical output by means of a heat sensitive element. Thermal sensors operate at room temperature, which makes them a low-cost alternative. The basic structure of thermal sensors consists of a temperature sensitive element connected to a heat sink through a structure with low thermal conductance. It is common to use thin membranes of Silicon or Silicon Nitride as thermal insulation between the heat sink and the sensitive element. In comparison, polymers have a thermal conductance that is lower than in these materials, and this increases the generated temperature in the sensitive element. A polymer such as SU-8 has a low thermal conductivity and is applied using a spin coater. This reduces the number of complex processing steps. This thesis presents a new application of SU-8 as a closed membrane in a thermal sensor.

The concept was initially demonstrated by fabricating a nickel bolometer and titanium/nickel thermopile structure with a 5 µm SU-8 / SiO2 membrane. However, for the sensor responsivity to be able to compete with commercial thermal sensors the structures, some optimization was required. Since the thermopile generates its own voltage output and requires no external bias, the optimizations were focused on this structure. There exist a number available software tools for thermal simulation of components. However, to the author’s best knowledge, there exist no tool for design optimization of thermopiles with closed membranes. An optimization tool using iterative thermal simulations was developed and evaluated. A new thermopile structure, based on the optimization results, was both fabricated and characterized. Using an infrared laser with a small spot, the measured responsivity of the manufactured thermopile was higher than that of a commercial sensor. In the case of a defocused spot and for longer wavelengths, the infrared absorption in the absorption layer reduces and degrades the responsivity.

The thermopile was further evaluated as a sensor in a carbon dioxide meter application based on the NDIR principle. An increase in the CO2 concentration demonstrated a clear decrease in the thermopile voltage response, as was expected. By normalizing the voltage response and comparing it with a commercial sensor, this showed that the SU-8 based thermopile is relatively more sensitive to changes in the CO2 concentration.

Place, publisher, year, edition, pages
Sundsvall: Kopieringen Mittuniversitetet, 2009. 64 p.
Series
Mid Sweden University doctoral thesis, ISSN 1652-893X ; 73
Keyword
Thermal detector, Polymer, Membrane, SU-8
National Category
Other Electrical Engineering, Electronic Engineering, Information Engineering
Identifiers
urn:nbn:se:miun:diva-9579 (URN)978-91-86073-46-6 (ISBN)
Public defence
2009-09-19, O111, Holmgatan 10, Sundsvall, 13:15 (English)
Opponent
Supervisors
Projects
STC
Available from: 2009-08-28 Created: 2009-08-27 Last updated: 2011-02-06Bibliographically approved

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Mattsson, ClaesThungström, GöranBertilsson, KentNilsson, Hans-Erik
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