1. Pentti 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 project
Pentti Karioja, VTT Technical Research Centre of Finland

2. Consortium VTT (Finland) UTU (Finland) Gasera (Finland) Ioffe (Russia)
SELEX (Italy)
Dräger (Germany)
Doble (Norway) 2

3. Enabling technologies - Technology platforms
Process INSTR
Optical Comm. &
processing
Lighting
&
Displays
Life Sciences
Safety
&
Security
Energy
&
Environment
Application
Optical measurement & sensor technologies: spectroscopy, machine vision, & imaging, interferometry etc.
Design & characterization: • 3D design & integration • Optics • Thermal Mgt • Electronics • Integrated optics
System / module
Precision mechanics • 3D mechanical design & construction • 3D optics design
Enabling technologies - Technology platforms
3D System-in-Package & Integration: • Substrates • Assembly • Hermetic sealing • Thermal management
Polymer Integration: • Multi-layer lamination • Assembled foil over-molding • Nanoimprint
Si technology: MEMS/MOEMS • SOI waveguides
Component / device
Printing technologies: R2R, UV imprinting, printing processes, materials, devices
11/16/2009

4. Background: Photoacoustic spectroscopy
Photoacoustic effect was discovered in 1880 by Alexander Graham Bell
The theoretical limitations of this technology are far from what has been achieved with any technology today
The 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.

5. Background: Cantilever sensor with optical readout
Finnish SME Gasera has developed a novel MEMS cantilever approach where the displacement is 100 that of microphone membrane [1],
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.
[1] J. Kauppinen, K. Wilcken, I. Kauppinen, and V. Koskinen, “High sensitivity in gas analysis with photoacoustic detection,” Microchem. J. 76, (2004). 5

6. Background: 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.

7. Benefits 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.

8. MINIGAS targets
Compact, rugged gas sensor providing significant improvement in sensitivity:
Sensor volume target: 5 cm3
Analysis response time  100 ms
Dynamic range >10 000
Temperature range: C to + 500C
Cost of goods: €100 in high volume production
Sensors 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

9. MINIGAS technology roadmap
Commercialized
gas sensor
Commercial gas analyzer
EU-project
19” rack mount analyzer
volume < 5 cm3
integrated optic chip 9

10. Technology portfolio of sensor subsystems
IR LEDs with customized wavelengths and performance
Silicon MEMS cantilever pressure sensor
Spatial read-out interferometer
Low Temperature co-fired Ceramics (LTCC) photoacoustic measurement cell
On-board drive, detection and readout electronics

11. Infrared 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

12. Silicon MEMS cantilever
mask layout 12

13. Spatial 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

14. LTCC 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.
Cavities
Coating
Hermetic sealing
Photoacoustic
cell chamber dimensions:
2.5  2.5  10.0 mm 14

15. Predicted performance15

16. First experiments using sensor platform
Portable methane sensor demonstrator based on LTCC differential photo acoustic cell and silicon cantilever
The 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.

17. Mr. Juha Palve (VTT) juha.palve@vtt.fi +358 400 488 414 www.minigas.eu17