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Proceedings Paper

LTCC-based differential photo acoustic cell for ppm gas sensing
Author(s): P. Karioja; K. Keränen; K. Kautio; J. Ollila; M. Heikkinen; I. Kauppinen; T. Kuusela; B. Matveev; M. E. McNie; R. M Jenkins; J. Palve
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Paper Abstract

Silicon MEMS cantilever-based photoacoustic technology allows for the sensing of ultra low gas concentrations with very wide dynamic range. The sensitivity enhancement is achieved with a cantilever microphone system in which the cantilever displacement is probed with an optical interferometer providing a pico-meter resolution. In the gas sensor, the silicon cantilever microphone is placed in a two-chamber differential gas cell. By monitoring differential pressure changes between the two chambers, the differential cell operates as a differential infra-red detector for optical absorption signals through a measurement and reference path. The differential pressure signal is proportional to gas concentration in the optical measurement path. We have designed, implemented and tested a differential photo acoustic gas cell based on Low Temperature Co-fired Ceramic (LTCC) multilayer substrate technology. Standard LTCC technology enables implementation of 2.5D structures including holes, cavities and channels into the electronic substrate. The implemented differential photoacoustic gas cell structure includes two 10 mm long cylindrical cells, diameter of 2.4 mm. Reflectance measurements of the cell showed that reflectivity of the substrate material can be improved by a factor 15 - 90 in the 3 - 8 μm spectral region using gold or silver paste coatings. A transparent window is required in the differential gas cell structure in order to probe the displacement of the silicon cantilever. The transparent sapphire window was sealed to the LTCC substrate using two methods: screen printed Au80/Sn20 solder paste and pre-attached glass solder paste (Diemat DM2700P/H848). Both methods were shown to provide hermetic sealing of sapphire windows to LTCC substrate. The measured He-leak rate for the 10 sealed test samples implemented using glass paste were less than 2.0 ×10-9 atm×cm3/s, which meets the requirement for the leak rate according to MIL-STD 883. The achieved hermetic level suggests that the proof-of-principle packaging demonstrator paves the way for implementing a novel differential photoacoustic gas cell for a future miniature gas sensor module. The future module consisting of a sample gas cell and immersion lens IR-LEDs together with interferometric probing of the cantilever microphone is expected to be capable of measuring ultra low concentrations of a wide range of gases with their fundamental absorption bands at 3 - 7 μm wavelength, such as CO, CO2 and CH4.

Paper Details

Date Published: 13 May 2010
PDF: 13 pages
Proc. SPIE 7726, Optical Sensing and Detection, 77260H (13 May 2010); doi: 10.1117/12.851854
Show Author Affiliations
P. Karioja, VTT Technical Research Ctr. of Finland (Finland)
K. Keränen, VTT Technical Research Ctr. of Finland (Finland)
K. Kautio, VTT Technical Research Ctr. of Finland (Finland)
J. Ollila, VTT Technical Research Ctr. of Finland (Finland)
M. Heikkinen, VTT Technical Research Ctr. of Finland (Finland)
I. Kauppinen, Gasera Ltd. (Finland)
T. Kuusela, Univ. of Turku (Finland)
B. Matveev, Ioffe Physical-Technical Institute (Russian Federation)
M. E. McNie, QinetiQ Ltd. (United Kingdom)
R. M Jenkins, QinetiQ Ltd. (United Kingdom)
J. Palve, VTT Technical Research Ctr. of Finland (Finland)

Published in SPIE Proceedings Vol. 7726:
Optical Sensing and Detection
Francis Berghmans; Anna Grazia Mignani; Chris A. van Hoof, Editor(s)

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