Portable Methane Sensor Demonstrator based on LTCC Differential Photo Acoustic Cell and Silicon Cantilever

Abstract

Abstract A novel portable methane sensor demonstrator based on Low Temperature Co-fired Ceramic (LTCC) differential Photo Acoustic (PA) cell, silicon cantilever and spatial interferometer was demonstrated. Silicon Micro-Electro-Mechanical-System (MEMS) cantilever-based PA technology allows sensing of extremely low gas concentrations with wide dynamic measuring range. The sensitivity enhancement is achieved with a cantilever microphone system in which the cantilever displacement was probed with an optical interferometer providing a pico-meter resolution. In the demonstrated gas sensor structure, the silicon cantilever microphone was placed in a two-chamber differential gas cell so that the achieved differential pressure signal was proportional to gas concentration in the open measurement path for gas flow. The pulsed optical power was produced by two Mid Infra-Red (MIR) Light Emitting Diodes (LEDs). The differential PA gas cell structure included two 8 mm cylindrical cells, diameter 2.4 mm, for reference and measurement detection portions coated with a silver paste. A transparent sapphire window was hermetically sealed on top of the differential gas cell structure in order to probe the displacement of the silicon cantilever inside the sealed differential cell. The sealed methane gas produced selectivity against other possible gases in the measurement path. The first sensor prototype sensitivity was 300 ppm with 1 s response time for the methane gas. Sensitivity is increased to be 30 ppm, when response time of 100 s is used. The selectivity in the demonstrated sensor is possible to tune simply by filling the differential cell with specific gas in focus and selecting corresponding LED with proper emission spectrum. Sensor concept provides possibility to measure extremely low gas concentrations of a wide range of gases having fundamental absorption bands at 3 - 7 μm wavelength range including CO, CO2 and CH4.