In this paper, we propose a novel, miniaturized non-dispersive infrared (NDIR) CO2 sensor implemented on a silicon chip. The sensor has a simple structure, consisting of a hollow metallic cylindrical cavity along with access waveguides. A detailed analysis of the proposed sensor is presented. Simulation with 3D ray tracing shows that an integrating cylinder with 4 mm diameter gives an equivalent optical path length of     3 . 5     cm. The sensor is fabricated using Deep Reactive Ion Etching (DRIE) and wafer bonding. The fabricated sensor was evaluated by performing a CO2 concentration measurement, showing a limit of detection of ∼100 ppm. The response time of the sensor is only ∼2.8 s, due to its small footprint. The use of DRIE-based waveguide structures enables mass fabrication, as well as the potential co-integration of flip-chip integrated midIR light-emitting diodes (LEDs) and photodetectors, resulting in a compact, low-power, and low-cost NDIR CO2 sensor

Baets, Roel

Jia, Xiaoning

Roelkens, Günther

Roels, Joris

English

Ghent University Academic Bibliography

On-chip non-dispersive infrared CO2 sensor based on an integrating cylinder

A miniaturized non-dispersive infrared CO2 sensor based on a 2D integrating cylinderXiaoning Jia1,2*, Gunther Roelkens1,2, Roel Baets1,2, Juris Roels31 Photonics Rearch Group, INTEC, Ghent University-imec, Teclmologiepark 126, 9052 Belgium 2 Center for Nano- and Biophotonics, Ghent University, Belgium 3 Melexis Technologies NV, Transportstraat I, 3980 Tessenderto, Belgium xiaoning.jia @ ugent.beAbstract: We propose a novel, miniaturized NDIR CO2 sensor based on a 2D integrating cylinder, realized using deep RIE and wafer bonding on silicon. The sensor has a detection limit down to ~ 250ppm, with a small footprint of only ~ 5mm x 5mm. © 2019 The Author(s)OCIS codes: 130.6010, 230.3120, 280.47881. IntroductionCO2 sensors are widely applied, such as for air quality monitoring, greenhouse farming and industrial process control. Existing CO2 sensors are either electrochemical or non-dispersive infrared (NDIR) sensors. Electrochem­ical sensors suffer from short-term stability, low durability and cross-response to other gases (eg. water vapour). In contrast, NDIR sensors offer long-term stability, high accuracy and high gas specificity [1]. However, NDIR sensors tend to be bulky as a long (several cm) interaction length is required to achieve ppm level detection [2], Several efforts have been made to miniaturize NDIR sensors [3-5]. In this work we demonstrate a NDIR CO2 sensor based on a 2D integrating cylinder implemented on a silicon substrate with ~ 250ppm detection limit. The use of an integrating cylinder allows for a miniaturized sensor with a long interaction length.2. Sensor fabrication and experimental setupFigure 1(a) shows the schematic of the integrating cylinder. The structure consists of two gold coated and bonded Si substrates. The bottom substrate consists of a cylindrical cavity with one input waveguide and two output waveguides (all 300 n>n wide) defined by 300 nm Deep Reactive Ion Etching(DRIE), as shown in Fig 1(b). A thin layer of gold(~ 300nm) was then sputtered on the bottom substrate as well as on the (planar-) top substrate. After gold deposition, the two substrates undergo Ar plasma, treatment and are bonded using gold-to-gold direct bonding (at 300°C, IMPa for 30 minutes). The fabricated sensor is shown in Fig. 1(c).Fig. 1: (a) Sensor schematic, (b) DRIE etched cylindrical cavity with one input waveguide and two output waveg- uides(etch depth = 300jii/?!), the input waveguide is perpendicular to the output waveguides, this bottom substrate contains two sensors, (c) Fabricated sensor with a EURO 10 cent coin, the sensor tested in this work has a radius of R = 1.6mm and the length and width of the access waveguides are \mm and 300jim, respectively, (d) Experimental setup. L: lens MMF: multimode fiber, PD: photodiode, TIA: trans-impedance amplifier.The experimental setup is shown in Fig. 1(d). A stabilized broadband light source(Thorlabs SLS202) is used for the NDIR sensing experiment. The emitted light is focused by a lens onto multi-mode fiber. The fiber is butt- coupled to the input waveguide of the sensor (fiber to input waveguide distance ~ SO/im). At the output waveguideof the sensing arm, light is collimated and re-focused onto a photodiode by two identical lensesf/ = 1.8mm), with a band pass filtcr(Ao = 4.25/im, FWHM = 500nm) in between. The lens-filter-lens system is scaled and isolated from the ambient. A customized transimpedance amplifier(TIA) is used to amplify the signal. The reference arm only differs in the optical filter, with pass band centered at Aq = 3.8/Tm (FWHM=500nm), the two arms are otherwise identical. During the measurement, the signal at both arms are simultaneously acquired by two lock-in amplifiers(Standord SR830) with identical settings.3. Experimental results(a) (b) (c)Fig. 2: (a) Allan plot of the sensing and reference arm, as well as the normalized signal(in pure N2 environment), (b) Step response of the sensor, (c) Normalized absorbance calculated according to the equation in inset of (b).The experimental results are shown in Fig. 2(a-c). Figure 2(a) shows the Allan deviation plot. One can see that the optimal averaging time (when the Allan deviation of the normalized signal (|^) is at its minimum, where Iqa and Iqr are the sensing signal and reference signal at zero CO2) is approximately 0.7s, and the minimum change in transmission that can be measured is 0.0007 (1 a). In all following measurements an integration time of 0.3s was used. Figure 2(b) shows the sensor step response when various CO2 concentrations were applied. It can be seen that the reference signal stays relatively stable while the sensing signal responds to CO2. The normalized absorbance was calculated according to the equation in the inset of Fig.2(b) and plotted in Fig.2(c). The limit of detection is approximately 250ppm. The response time of the sensor was measured to be ~ 3s, by purging with 5% CO2 and then abruptly shutting the CO2 flow.4. ConclusionIn summary, a miniaturized integrating-cylinder-based CO2 sensor is demonstrated in this work. The sensor has a small footprint(~5mm x 5mm) and is able to achieve a ~250ppm CO2 detection limit, with a response time of only 3 seconds. The use of DRIB based waveguide structures enables mass fabrication, as well as the co-integration of flip-chip integrated midIR LEDs and photodetectors.The authors acknowledge Melexis and VLAIO for their financial support.References1. S. Neethirajan, D.S. Jayas, and S. Sadistap, “Carbon dioxide (CO2) sensors for the agri-food industry-a review” Food and Bioprocess Technology 2.2, 115-121 (2009).2. A. Wilk, J.C. Carter, M. Chrisp, A.M. Manuel, R Mirkarimi, J.B. Alameda, and B. Mizaikoff, “Substrate- integrated hollow waveguides: a new level of integration in mid-infrared gas sensing” Analytical chemistry 85.23, 11205-11210(2013).3. J. Hodgkinson, R. Smith, W. O. Hah, JR. Saffell, and R.P. Tatam, “ Non-dispersive infra-red (NDIR) measurement of carbon dioxide at A.ljim in a compact and optically efficient sensor” Sensors and Ac­tuators B: Chemical 186, 580-588 (2013).4. L. Scholz, A. Ortiz Perez, B. Bierer, P. Eaksen, J. Wllenstein, and S. Palzer, “ Miniature low-cost carbon dioxide sensor for mobile devices” IEEE Sensors. 17, 2889-2895 (2017).5. S. Moumen, 1.1. Raible, A. Krau, and J. Wllenstein, “Infrared investigation of C02 sorption by amine based materials for the development of a NDIR C02 sensor” Sensors and Actuators B: Chemical 236, 1083-1090 (2016).

A miniaturized non-dispersive infrared CO2 sensor based on a 2D integrating cylinder

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On-chip non-dispersive infrared CO2 sensor based on an integrating cylinder

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Ghent University Academic Bibliography

Ghent University Academic Bibliography