Photoacoustic (PA) spectroscopy is one of the most sensitive technique used to monitor chemical emission or detect gas traces. Coupled to quantum cascade lasers, this system is widely used in a large number of application fields from industrial control to health monitoring. Mass production for a large dissemination of such systems requires however further development for both decreasing their footprint and manufacturing cost. Since the last 6 years CEA-LETI has developed different versions of miniaturized photoacoustic cells. We have already demonstrated the detection of gas traces with a tiny silicon based-PA cell. Nevertheless, this first result was obtained with commercial MEMS microphones. Even if these components are reliable and enough performant they are not dedicated to photoacoustic gas detection and cannot be easily integrated into a fabrication process flow. To cope with these issues we suggest using both the M&NEMS technology and the MIR photonics. The new PAdetector termed microPA is built by stacking two 200 mm wafers: a sensor wafer, which includes the microphone (MEMS mechanical diaphragm and NEMS piezoresistive gauges), capillaries and fluidic ports, and a cap wafer, which includes the PA cell, the expansion volume, SiGe waveguides guiding the light into the PA cell, metal routing and electric contacts. Frequency response measurements as well as PA gas detection have been carried out. The system shows a mechanical resonance of the diaphragm at the frequency of 6500 Hz, in good agreement with the simulation. First CO2 and CH4 tests in laboratory condition demonstrates a limit of detection in the ppm range and a NNEA of 10-8 W.cm-1.Hz-1/2.
Photoacoustic (PA) spectroscopy is among the most sensitive techniques used to monitor chemical emission or detect gas traces. In the mid-infrared, where most of gases of interest have their strongest absorption lines, this technique takes advantage of the high optical power and room temperature operation of quantum cascade lasers (QCL). We have recently demonstrated that centimeter-size PA cells can compete, with bulky commercial systems for gas sensing without any compromises on performances. We demonstrate a new step towards cost reduction, extreme integration, and mass deployment of such PA sensors with a miniaturized silicon PA-cell fabricated on standard CMOS tools. The design, fabrication and characterizations of this new sub-centimeter PA cell built on a silicon platform are presented. First, the component has been designed using a detailed physical model, accounting for viscous and thermal losses, and metamodel-based optimization techniques. Second, it has been fabricated on our 200 mm CMOS pilot line. Several wafers have been released and diced. Single chips have then been assembled with commercial capacitive microphones and finally characterized on our reference gas bench. The photoacoustic simulations and the acoustics experiments are in a good agreement. The tiny PA cell exhibits a sensitivity down to the ppm level for CO2 at 2300 cm-1, as well as for CH4 at 3057 cm-1 even in a gas flow. Taking advantage of the integration of QCLs on Si and photonic circuitry, the silicon PA cell concept is currently being extended towards a fully integrated multigas detector.
Remote leak detection of gases such as the homonuclear molecules (N2, H2, etc.) and noble gases (He, Ar etc.) is still an issue for tunable laser spectroscopy (TLS) because these gases do not have infrared absorption bands. In order to detect a leak in air, the gas displacement of the ambient air is used as an indirect indication of the leak. So, the unique idea is to measure the reduced oxygen concentration by a standoff laser spectrometer at an emission wavelength of 761 nm. The advantage of oxygen as indicator gas is the stable concentration level with respect to low spatial and temporal fluctuations. The challenge of the standoff detection is to analyze the small relative transmission change for weak light intensity scattered by the background. Furthermore, a remote measurement technique for high-level oxygen concentration on ppm level resolution is demonstrated. Here the combination of a high performance distributed feedback laser at 761 nm and high end sophisticated electronics for driver and data acquisition is required and designed. With the direct absorption spectroscopy, the concentration change of 2000 ppm within a 1 cm plume size (10 ml/min flow, ambient room conditions) corresponds to a transmission change in order of 2E-4 has been resolved on a low absolute power level of few micro watts in 1m distance. The detection limit corresponds to a nitrogen leakage rate of 0.1mbar·l/s which is comparable to ordinary remote detection systems for methane leakages.