An optical IR-source and CO sub(2)-chamber system for CO sub(2 ) measurements

A new silicon IR source and CO sub(2)-chamber system for measurement of CO sub(2) concentration is presented. The micromachined IR-source, which consists of four groups of polysilicon filaments coated with silicon nitride suspended across a KOH-etched cavity, is used to generate two switched "sample" and "reference" beams. The electrically modulated sources present a modulation time of 10 ms and a power consumption of 1 W. The CO sub(2) chambers, placed beneath the reference sources, are used to produce a reference beam for long term stability and compensation against cuvette window contamination. They consist of 1-mm-deep micromachined cavities in which CO sub(2) is encapsulated during an overpressure anodic bonding procedure. Silicon dioxide is used as an antireflective coating to optimize the filter's optical characteristics. Numerical simulations of the filters are presented and show good agreement with the measurements. The IR sources and the CO sub(2) filters are both fabricated at wafer level. Using the presented IR-sensor system, measurements with a 9-mm-wide test channel show high CO sub(2)-sensitivity for CO sub(2) concentrations between 0 and 10% confirming that the stringent requirements for this respirator application can be fulfilled