A silicon IR-source and CO2-chamber for CO2 measurements

For the first time a silicon IR-source and CO₂-chamber system for measurement of CO₂ concentration is presented. This new miniaturized infrared sensor is specially designed for the measurement of respiratory gases present in patient airways during anaesthesia or intensive care. The IR-sensor assembly consists of an IR-source chip with two pairs of diagonally arranged IR-sources for the generation of two switched “sample” and reference beams. The gas filter chip is arranged with two CO₂ chambers directly beneath the two reference sources. These chambers “pre-absorb” the reference beam at the CO₂ wavelength band around 4.26 μm. The reference beam is needed for long term stability and for compensation against cuvette window contamination. The electrically modulated IR-sources consist of incandescent polysilicon filaments coated with silicon nitride across a 220 μm deep cavity. The CO₂-chambers with a length of 1 mm are fabricated by silicon fusion and anodic bonding at a chosen CO₂ pressure for optimal filter characteristics. Test measurements of the infrared sensor system show high CO₂-sensitivity meaning that the stringent requirements for this respirator application can be reached