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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 under 2.0 ×10-9 atm×cm3/s,
which meets the requirement for the leak rate according to MIL-STD 883. The achieved hermeticity 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.
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