Portable Methane Sensor Demonstrator based on LTCC Differential Photo Acoustic Cell and Silicon Cantilever Academic research paper on "Materials engineering"

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Procedía Engineering

ELSEVIER

Procedía Engineering 47 (2012) 1438 - 1441

www.elsevier.com/locate/procedia

Proc. Eurosensors XXVI, September 9-12, 2012, Krakow, Poland

Portable methane sensor demonstrator based on LTCC differential photo acoustic cell and silicon cantilever

K. Keränena*, J. Ollilaa, H. Saloniemib, B. Matveevc, J. Raittilad, A. Helled, I. Kauppinend, T. Kuuselae, L. Piernof, P. Kariojaa, M. Karppinena aVTT Technical Research Centre of Finland, 90570 Oulu, FINLAND bVTT Technical Research Centre of Finland, 02044 VTT, FINLAND cIoffe Physical-Technical Institute, St Petersburg 194021, RUSSIAN FEDERATION

dGasera Ltd., 20520 Turku, FINLAND eUniversity of Turku, Department of Physics and Astronomy, 20014 UTU, FINLAND

fSelex SI, 00131 Rome, ITALY

A novel portable methane sensor demonstrator based on Low Temperature Co-fired Ceramic (LTCC) differential Photo Acoustic (PA) cell, silicon cantilever and spatial interferometer was demonstrated. Silicon Micro-ElectroMechanical-System (MEMS) cantilever-based PA technology allows sensing of extremely low gas concentrations with wide dynamic measuring range. The sensitivity enhancement is achieved with a cantilever microphone system in which the cantilever displacement was probed with an optical interferometer providing a pico-meter resolution. In the demonstrated gas sensor structure, the silicon cantilever microphone was placed in a two-chamber differential gas cell so that the achieved differential pressure signal was proportional to gas concentration in the open measurement path for gas flow. The pulsed optical power was produced by two Mid Infra-Red (MIR) Light Emitting Diodes (LEDs). The differential PA gas cell structure included two 8 mm cylindrical cells, diameter 2.4 mm, for reference and measurement detection portions coated with a silver paste. A transparent sapphire window was hermetically sealed on top of the differential gas cell structure in order to probe the displacement of the silicon cantilever inside the sealed differential cell. The sealed methane gas produced selectivity against other possible gases in the measurement path. The first sensor prototype sensitivity was 300 ppm with 1 s response time for the methane gas. Sensitivity is increased to be 30 ppm, when response time of 100 s is used. The selectivity in the demonstrated sensor is possible to tune simply by filling the differential cell with specific gas in focus and selecting corresponding LED with proper emission spectrum. Sensor concept provides possibility to measure extremely low gas concentrations of a wide range of gases having fundamental absorption bands at 3 - 7 ^m wavelength range including CO, CO2 and CH4.

* Corresponding author. Tel.: +358-20-722-2272; fax: +358-20-722-2320. E-mail address: kimmo.keranen@vtt.fi

Abstract

1877-7058 © 2012 The Authors. Published by Elsevier Ltd. Selection and/or peer-review under responsibility of the Symposium Cracoviense Sp. z.o.o.

doi:10.1016/j.proeng.2012.09.428

© 2012 TheAuthors.PublishedbyElsevier Ltd. Selection and/or peer-review under responsibility of the Symposium Cracoviense Sp. z.o.o.

Keywords: differential LTCC gas cell, silicon cantilever, photo acoustic gas sensing, hermetic sealing, micro immersion lens, MIR LEDs, differential infra-red detector, interferometric probing

1. Introduction

Globally, the gas sensor market is $500M/year with sensor systems worth >$2500M and further growth driven by legislative requirements and emerging economies is expected. There is a wide range of industrial, environmental and safety applications in which sensitive trace gas detection is needed, for example in process control, environmental monitoring, combustion analysis and national security [1]. There are both non-spectroscopic and spectroscopic methods used for these purposes. In long-path absorption spectroscopy, the beam penetrates through the sample gas and absorption at two or more wavelength bands is compared. The sensitivity depends mainly on the absorption path length, radiation power and the response of the detector while the selectivity is limited by the resolution of the spectrum analyser. In practice, these requirements create great challenges when aiming for a portable instrument.

In the differential PA sensor, it was possible to combine the advantages of long-path absorption spectroscopy and photo acoustic spectroscopy [2]. In [2] differential photo acoustic cell equipped with an ultrasensitive optical microphone was used as a selective infrared detector similarly as in long-path absorption spectroscopy. The above technology allows a miniaturized, but still very sensitive, gas sensor with silicon cantilever microphone [3], high efficiency IR LEDs [4, 5] and high sensitivity interferometric read-out system [3].

In the PA sensor, a miniaturized differential gas cell is needed to obtain miniaturization targets required by the applications. Obviously, the differential gas cell including selective sample gas has to be hermetic in order to maintain the proper gas content inside the chamber in harsh environmental conditions [6]. The hermetic differential gas cell with required 2.5D geometry is possible to fabricate by using Low Temperature Co-fired Ceramics (LTCC) substrate technology [7]. The key asset for reduced cost of ownership (COO) of the sensor is the use of high-volume 3D integration and manufacturing technologies already matured in electronics industry. In addition, it is a key issue that the fundamental signal information is supported with required additional information about operating environment and the final measurement result is processed according to the available versatile information in an advanced information processing system [8]. In this paper, we introduce a novel portable methane sensor demonstrator based on LTCC differential photo acoustic cell, MID IR LEDs, silicon cantilever and a spatial interferometer.

2. Sensor concept

The initial sensor concept is shown in Fig. 1. Four different gas chambers are included in the system, namely, the sample cell, reference cell and differential PA cell comprising of two parts: the sample beam chamber (SBC) on top and reference beam chamber (RBC) at the bottom. In Fig. 1. the sample cell is labeled as the open-path cell. The cantilever microphone is located between the SBC and RBC both been filled with the target gas similar to the gas that is under interest in the sample cell. The sensor, therefore, is sensitive to radiation whose optical wavelengths correspond to the absorption lines of the target gas -principle used in correlation spectroscopy method. The reference chamber can contain vacuum or a non-absorbing gas like N2. In the implemented demonstrator, however, two LEDs were used instead of one LED and beam splitter was eliminated. In addition, the reference cell was eliminated by assembling

reference LED in close contact to RBC window. The required cantilever balance in zero concentration of monitored gas is achieved simply by tuning electrically modulated optical radiations from the LEDs to produce equal pressure increments from SBC and RBC to the silicon cantilever.

photoacoustlc dual cell

Fig. 1. Initial sensor system concept. 3. Demonstrator implementation and testing

Sensor optical design was performed and sensor optomechanics based on aluminium frame was designed and tooled using 5-axis CNC machine. Frame contained structures for MID IR LEDs, interferometer VCSEL source and CMOS detector PCBs. In addition, a spherical mirror for collecting optical power at the open path cell and coupling LED power to the SBC was aligned and assembled. The heart of the sensing system was the differential LTCC cell containing monitored gas - pure methane in slight over pressure (101 kPa < P < 152 kPa). The implemented differential PA gas cell structure included two 8 mm cylindrical cells, diameter 2.4 mm, for reference and measurement detection portions coated with a silver paste. A sapphire window was hermetically sealed on top of the differential gas cell structure in order to probe the displacement of the silicon cantilever tip inside the sealed differential cell. The sealed methane gas in differential cell produces selectivity against other possible gases at the measurement path. The differential LTCC cell included also a MEMS cantilever with chip dimensions of 3 mm x 4 mm x 0.6 mm active area dimensions of 1 mm x 2.2 mm x 0.01 mm. The dimensions of the differential cell were 14 mm x 8 mm x 4 mm. In Fig. 2. assembled differential LTCC cell is shown.

Fig. 2. Differential LTCC cell.

The assembled sensor fitted in a volume of 40 mm x 40 mm x 35 mm. The assembled sensor was placed in a chamber, which methane concentration was possible to adjust in a large range. The balancing of the optical power in SBC and RBC was achieved in zero methane concentration by adjusting LEDs driving currents. The operation of the spatial interferometer was checked and verified to produce 1 pm resolution, when probing the silicon cantilever tip position. The achieved differential pressure signal was proportional to gas concentration in the open measurement path for gas flow. The sensitivity of the first prototype implementation was 300 ppm for methane with 1 s response time. Sensitivity is increased to be 30 ppm, when response time of 100 s is used. The selectivity in the demonstrated sensor is possible to tune simply by filling the differential cell with specific gas under interest and selecting corresponding MIR LED with proper emission spectrum that cover the selected absorption peak of the monitored gas. Demonstrated sensor concept provides possibility to measure extremely low gas concentrations of a wide range of gases having fundamental absorption bands at 3 - 7 ^m wavelength range including CO, CO2 and CH4.

Acknowledgements

The EU FP7 grant n:o 224625 for MINIGAS project is acknowledged.

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