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 <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3</sub> N <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">4</sub> 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 <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3</sub> N <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">4</sub> . The simulation results presented show that the temperature increase in a 5 mum SiO <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> /SU-8 membrane is about 9% higher than in a 1 mum Si <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3</sub> N <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">4</sub> 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 mum SiO <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> /SU-8 2002 membrane has been fabricated. The sensitivity of the fabricated device has been evaluated in the infrared region, using a 1.56 mum 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.