Design of a Micromachined Thermopile Infrared Sensor With a Self-Supported ${\rm SiO}_{2}/{\rm SU}{-}8$ Membrane

Abstract

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 Si 3 N 4 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 Si 3 N 4 . The simulation results presented show that the temperature increase in a 5 m SiO 2 SU 8 membrane is about 9% higher than in a 1 m Si 3 N 4 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 m SiO 2 SU 8 2002 membrane has been fabricated. The sensitivity of the fabricated device has been evaluated in the infrared region, using a 1.56 m 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.

Overview of the MEMS Pirani Sensors

“…A thermocouple-based Pirani sensor works based on the Seebeck effect [82], when the hot ends are heated, at the cold ends there forms a voltage, as is shown in Figure 5a. This type of Pirani sensors is usually composed of several pairs of thermocouple strips [83][84][85][86][87][88].…”

Fabrication and Characterization of a SU-8 Epoxy Membrane-Based Thermopile Detector With an Integrated Multilayered Absorber Structure for the Mid-IR Region

“…Absorption in the structure was calculated by using 5, where, Top = transmittance, Aop = absorption, Rop = reflectance, and reflectance and transmittance coefficients were calculated by using the transfer matrix theory of multilayer structures [12]. The transfer matrix for each layer in the structure is described by (6).…”

“…Furthermore, SU-8 is chemically resistant to most solvents and acids, and it is thermally stable up to 300 °C. The fabrication of an Al/Bithermopile detector using SU-8 epoxy as a membrane material has been reported in [6].…”

Development of a thermopile infrared sensor using stacked double polycrystalline silicon layers based on the CMOS process

“…Polymers, e.g. SU-8, are emerging as attractive materials for this purpose [33][34][35], due to their relative low thermal conductance, flexibility [36] and potential on integration as a biocompatible layer for bio-chips [37]. We focus on CMOS materials in this paper, therefore silicon-di-oxide (SiO 2 ) was used as the supporting material, because of its low thermal conductance and high thermal and mechanical stability [38,39].…”