Presentation on theme: "Pentti Karioja, VTT Technical Research Centre of Finland"— Presentation transcript:
1Pentti Karioja, VTT Technical Research Centre of Finland EU FP7 Programme Gasera, University of Turku and VTT developing a MEMS based gas sensor in an EU projectPentti Karioja, VTT Technical Research Centre of Finland
4Background: Photoacoustic spectroscopy Photoacoustic effect was discovered in 1880 by Alexander Graham BellThe theoretical limitations of this technology are far from what has been achieved with any technology todayThe full potential has not been reached due to the use of conventional microphones in sensing the pressure waves.Photoacoustic spectroscopy is based on the absorption of light leading to the local warming of the absorbing volume element. The subsequent expansion of the volume element generates a pressure wave proportional to the absorbed energy, which can be detected via a pressure detector.
5Background: Cantilever sensor with optical readout Finnish SME Gasera has developed a novel MEMS cantilever approach where the displacement is 100 that of microphone membrane ,Cantilever sensor is coupled with interferometric measurement of the displacement,Below picometer (10-12) displacement can be detected with the optical readout,Theoretical predictions indicate cantilever based PA cell can be miniaturized with sensitivity up to three orders of magnitude over the prior art. J. Kauppinen, K. Wilcken, I. Kauppinen, and V. Koskinen, “High sensitivity in gas analysis with photoacoustic detection,” Microchem. J. 76, (2004).5
6Background: Differential photoacoustic measurement Silicon cantilever microphone is placed in a two-chamber differential gas cell.The differential PA cell operates as a differential IR detector for optical absorption signals propagating through the measurement and reference path.Pressure difference modulation between the chambers is monitored by probing the cantilever movement with an optical interferometer.Allows for open-path and flow-through detection of gases.
7Benefits of the cantilever enhanced differential PA sensor The detection limit is independent on the size of the gas cell which gives potential for miniaturization.The novel cantilever microphone provides high sensitivity from short absorption path length and highly linear concentration response over a wide dynamic measurement range.Low detection limits can be reached with low power light source,The gas inside the differential cell acts like an optical filter (so-called gas correlation method) providing good selectivity without optical filters or spectrograph.
8MINIGAS targetsCompact, rugged gas sensor providing significant improvement in sensitivity:Sensor volume target: 5 cm3Analysis response time 100 msDynamic range >10 000Temperature range: C to + 500CCost of goods: €100 in high volume productionSensors modular structure allows it to be applicable to a wide range of gases including: CH4, CO2, CO, NH3, …Applications including: leak detection, safety, homeland security and air quality.8
9MINIGAS technology roadmap Commercializedgas sensorCommercial gas analyzerEU-project19” rack mount analyzervolume < 5 cm3integrated optic chip9
10Technology portfolio of sensor subsystems IR LEDs with customized wavelengths and performanceSilicon MEMS cantilever pressure sensorSpatial read-out interferometerLow Temperature co-fired Ceramics (LTCC) photoacoustic measurement cellOn-board drive, detection and readout electronics
11Infrared Light Emitting Diodes InAs and InAsSb based diode structures have been processed into flip-chip devices with active areas sized by 300 μm and reflective contacts by a multistage wet photolithography method.LEDs are equipped with Si lenses with shape close to hyperhemisphere attached to the contact free surface through the use of high reflective index glue.Indium Arsenide Antimonide (InAsSb)11
13Spatial Interferometer for cantilever readout Based on creating interference by wave-front splitting in order to avoid expensive beam splitting optics.High temperature stability is achieved by using the cantilever frame as the reference mirror.Initial tolerance analysis estimates 0.1 mm positioning accuracy and 0.5 degrees for angular alignment accuracy as the tightest requirement.Performance goal: 1.0 pm displacement sensitivity.13
14LTCC photoacoustic measurement cell The vertical cavity is laminated using a silicone insert,Cells are drilled at the laminated stage before the co-fire,Reflective metal coating by thick film coating,Sapphire windows are sealed with solder glass paste,The He-leak rate for the sealed modules was <2.0 ×10-9 atm×cm3/s, which fulfills the requirements for the leak rate according MIL- STD 883.CavitiesCoatingHermetic sealingPhotoacousticcell chamber dimensions:2.5 2.5 10.0 mm14
16First experiments using sensor platform Portable methane sensor demonstrator based on LTCC differential photo acoustic cell and silicon cantileverThe assembled sensor fitted in a volume of 40 mm x 40 mm x 35 mm. The achieved differential pressure signal was proportional to gas concentration in the open measurement path of gas flow.The sensitivity of the first prototype was 300 ppm for methane with 1 s response time. Sensitivity is increased to 30 ppm, when response time of 100 s is used.