- Email: [email protected]
Mid-IR source based on a free-standing microhotplate for autonomous CO2 sensing in indoor applications
Sensors and Actuators A 172 (2011) 379–385
Contents lists available at SciVerse ScienceDirect
Sensors and Actuators A: Physical journal homepage: www.elsevier.com/locate/sna
Mid-IR source based on a free-standing microhotplate for autonomous CO2 sensing in indoor applications Pierre Barritault, Mickael Brun, Serge Gidon, Sergio Nicoletti ∗ Département Optronique, CEA, LETI, MINATEC Campus, 17 Rue des Martyrs, 38054 Grenoble Cedex 9, France
a r t i c l e
i n f o
Article history: Received 9 February 2011 Received in revised form 22 August 2011 Accepted 21 September 2011 Available online 10 October 2011 Keywords: Mid-IR source Microhotplate CO2 sensing
a b s t r a c t The measurement of CO2 by Non-Dispersive InfraRed absorption (NDIR) is often used as a tracer of human occupancy in confined living spaces. The major constraint of commercial sensors comes from the power consumption of the IR source, which makes them unsuitable for autonomous operation. This paper reports the fabrication and the characterization of a black-body IR source based on a microhotplate micromachined in Si and suitable to work above 650 ◦ C. The use of state-of-the-art MEMS technologies allows to lower the power consumption below 50 mW while ensuring a lifetime well beyond 10 years. The radiance of the microhotplate in the spectral range where CO2 adsorption takes place indicated that the device works as a quasi-perfect blackbody source providing enough power to drive an autonomous NDIR system for CO2 detection. © 2011 Elsevier B.V. All rights reserved.
1. Introduction Heating and venting air conditioning (HVAC) systems can significantly reduce the overall power consumption when on-demand ventilation is optimized on the basis of pollutants monitoring. Upto-date building control systems usually address the measurement of three or four parameters to drive demand-controlled ventilation: temperature, hygrometry, carbon dioxide and pressure. In this context, the quantification of CO2 is used as a tracer of human occupancy in confined living spaces. This approach is inadequate to monitor toxic compounds released by building materials and furnishings and some classes of VOC detectors at ppb level with reasonable cost are unlikely to emerge soon due to the unavailability of cost effective and reliable components. However, optimization of the ventilation strategies allows keeping comfortable living conditions while lowering the overall power budget [1]. CO2 is an inert gas which poorly interacts with the surrounding environment. If the detection mechanism is based on chemical interactions, the low reactivity may represent a significant drawback to build a reliable and reproducible monitoring device. A large selection of CO2 sensors have been proposed in literature where almost every gas detecting technology has been explored [2]. In spite of some encouraging results, a large majority of laboratory devices suffer of poor selectivity and/or poor sensitivity and only very few sensors haves reached the commercialization.
∗ Corresponding author. Tel.: +33 438 78 0289. E-mail address: [email protected] (S. Nicoletti). 0924-4247/$ – see front matter © 2011 Elsevier B.V. All rights reserved. doi:10.1016/j.sna.2011.09.027
For simple molecules, such as CO2 , optical detection offers the unique advantage of being highly selective since a spectrally unique light–gas interaction can be chosen by tuning the light wavelength. Therefore, in optical gas detection the issue of the selectivity is transferred to the selection or the fabrication of a monochromatic light source or a filtered infrared (IR) detector working in a suitable wavelength range—for CO2 ∼4.2–4.3 m. This working principle, based on the Beer–Lambert absorption law, is used in several commercial sensors where the source has been associated with different types of detectors to measure the light absorption. For CO2 monitoring in public and domestic buildings, commercial sensors typically include an IR source such as a hot filament, a pyroelectric or a bolometric detector and an IR pass band filter to detect concentrations ranging from 300 to 2000 ppm with an accuracy of about 100 ppm and resolution of about 20 ppm [3]. The key figure for these devices is the price, which should not exceed few tens of Euros. However, another major constraint comes from the severe limitation in power consumption, since typically a sensor unit might need to work autonomously for more than a year. It is worth mentioning that, while a variety of commercial sensors can provide the required performance in terms of accuracy and resolution, they often fail to meet the power consumption constraints. In autonomous devices, an average power consumption of 1 W can sustain AA commercially available battery operation for a year or longer. Similar amounts of energy can be harvested with a photovoltaic module used in conjunction with the sensor. If CO2 concentration should be measured every 2 min, as seems to be acceptable in drive on-demand ventilation, a total amount of 120 J can be made available for each measurement. This implies that, if
380
P. Barritault et al. / Sensors and Actuators A 172 (2011) 379–385
the power consumption of the source is 50 mW, each measurement cycle should be performed in less than 24 ms. This simple calculation shows that, to comply with energy consumption requirements of indoor operated sensors, the microheater should be very agile in terms of thermal mass so that the operating temperature can be reached in few ms consuming a very low amount of energy. These goals can be reached by miniaturizing the source in Si by MEMS technologies to realize what is usually called a microhotplate. This type of devices has been first developed by Semancik and co-workers in the early’90 for gas sensing applications [4]. Since then, a number of different device layouts were designed, realized and tested and some devices are now commercially available [5]. However only few of them were used as a thermal IR sources because of their mechanical and electrical stability when the temperature is raised above 500 ◦ C. In this paper we report the realization of a novel IR source based on a free-standing microhotplate fabricated in MEMS technology and suitable to work at an operating temperature of 650 ◦ C or higher. The goal is to develop a source suitable for providing enough IR power in [4.2–4.3 m] wavelength range to work in conjunction with state-of-the-art room-temperature IR detectors whist maintaining the power consumption well below 50 mW.
2. Modeling of the microhotplate IR source In blackbody emission spectra, it can be observed that, for a given wavelength band, e.g., within the mid-IR range, the spectral radiance increases with the source temperature. Therefore, even if the maximum for 4.25 m emission can be reached at around 682 K, the higher the operating temperature of the source the higher the spectral radiance in the CO2 adsorption band. On the other hand, the maximum operating temperature should be set to guarantee mechanical and/or electrical stability of the device, which allows long term operation – typically more than 10 years – with negligible drift. The key issue to preserve the device stability is the choice of the heater material. As discussed in [6], a vast literature reporting microhotplate devices fabricated using C-MOS compatible and non-compatible materials is now available. However, for many of them reliability tests at high temperature are not reported. Sarro and co-workers [7] have recently reported smooth operation of a TiN based microhotplate well above 600 ◦ C, showing that this material can be very stable at high temperature. TiN is often used in microelectronics as conductive barrier to avoid diffusion and electromigration of metallic species in IC devices. In spite of the superior stability in a wide range of operating conditions, TiN is quite resistive and driving a resistor fabricated from this material might require voltages not currently available with a single battery or a small photovoltaic module. Fig. 1 shows a schematic of the hot filament IR source. According to the results discussed above, we designed the microheater as a free-standing Si3 N4 circular membrane, embedding a metallic conductor encapsulated in SiO2 and fabricated on a sacrificial Si substrate. Two arms support the entire structure, providing mechanical and the electrical connections with the substrate. The heater was realized by patterning a TiN/Pt/TiN stack, which combines the stability of TiN and the favorable electrical properties of Pt. This stack was already proposed in [8,9] showing excellent properties in terms of mechanical and electrical stability. A full 3D model of the microheater has been developed using COMSOL MultiphysicTM Ver. 3.5a Finite Element Modeling package. This model describes conductive and radiative thermal exchanges with the surrounding environment where energy transfer takes place by heat diffusion via the arms supporting the microheater,
Fig. 1. Schematic of the microhotplate. For symmetry reasons, the model domain can be limited to a simpler structure which includes part of the arm and part of the hotplate.
by IR radiative emission and by thermal dissipation with the air, neglecting all other possible heat dissipation phenomena. As discussed in [9,10], the assumption is made that the behavior is purely conductive so that the thermal conductivity of the air can be expressed as a function of the temperature gradient. The output provides the overall IR radiation power emitted and a map of the temperature distribution of the device as a function of the applied voltage, the number of heaters, their size and their position on the microhotplate. To design the device two boundary conditions dictated by technological constrains were taken into account: to comply with the ultimate resolution of the photolithographic tool used within our MEMS pilot line the width of the smallest resistor was set to 1 m while, to ensure mechanical and electrical stability to Si3 N4 embedding the heaters, the gap between the outmost resistor and the edge of the hotplate was set to 5 m. Furthermore, the thickness of the Pt layer was adjusted to allow operation at about 3 V, i.e., a resistance of about 200 . A detailed description of the model can be found in [11]. Since the device is symmetric with respect to the hotplate center, the modeling domain was restricted as shown in Fig. 1. To make the design procedure automatic, the model has been programmed to arbitrarily set the temperature of a given location in the hotplate to 650 ◦ C. According to [8,10], this value has been chosen as the highest average temperature allowed for the device, making an acceptable compromise between optical emissivity, power consumption and device robustness. Furthermore, to optimize the performances while still limiting mechanical and electrical drift, the assumption was made that the temperature should be maintained uniform within ±20 ◦ C around the target value. A typical plot of the temperature distribution within the model is shown in Fig. 2a. A preliminary set of modeling runs evidenced that the target uniformity could not be reached by simply distributing a number of resistors over the hotplate. For this reason, the interspaces between the resistors were filled with a metallic spacer to be fabricated with the same metal stack used for the resistor itself. Since the thermal conductivity is much higher in metals than in dielectric layers, these arc-shaped fillers act as temperature homogenizers. A comparison of the temperature distribution across the hotplate with and without filler is shown in Fig. 2b while a sketch of the final device layout is given in Fig. 2c. Temperature distribution optimization can be achieved by adjusting position and size of the conducting tracks. While within Comsol 3.5, parameterization of sizes and 3D geometries is difficult using and modeling requires high computational resources, optimization of parametric 2D structures can be handled efficiently. To simplify the 3D model to a 2D version, an equivalent thermal and electrical conductivity of the overall layers has been introduced
P. Barritault et al. / Sensors and Actuators A 172 (2011) 379–385
381
680
T (°C)
640
600
560 2D Model 3D Model
520 0
10
20
30
40
50
Radius (μm) Fig. 3. Comparison of the temperature profiles for 2D (red) and 3D (blue) models. The cross sections were taken perpendicular to the harms at the center of the microheater and the arrow indicates the setpoint. (For interpretation of the references to color in this figure caption, the reader is referred to the web version of the article.)
690
T (°C)
660
630
600 0
10
20
30
40
50
Radius (μm) Fig. 4. Temperature profiles homogeneity before (red line) and after (green line) 2D optimization. (For interpretation of the references to color in this figure caption, the reader is referred to the web version of the article.)
3. Device fabrication
Fig. 2. a) Modeling results of the temperature distribution at the surface of the microheater—the cross indicates the reference temperature checkpoint, b) comparison of the temperature distribution across the hotplate with and without filler and c) sketch of the device layout including the temperature homogenizer.
by considering them as being in parallel. As shown in Fig. 3, where the calculated temperature profiles across the hotplate are showed, the two models give comparable results in terms of size and position of the resistors. Nevertheless, it is important to note that 3D profile is smoother and that the difference is related to SiO2 cap layer, whose role of heat diffuser cannot be accurately modeled in 2D. The optimization was carried out by using 3 tracks in parallel, the optimization procedure provides very good homogeneity temperature profiles restricted to ±10 ◦ C, as shown in Fig. 4. A sketch of the final device layout with size and position of the tracks in the hotplate is shown in Fig. 5.
The microhotplates were fabricated on a freestanding membrane by standard MEMS technologies in order to minimize the thermal mass and to reduce power consumption. The membrane realization is based on two 100 nm thick Si3 N4 layers embedding the metallic resistors and supporting the entire hotplate. The suspended hotplate structure is then released from the substrate by anisotropic Si etching. The complete process flow is described in Fig. 6. First, a Si3 N4 layer was deposited on the 200 mm Si wafer by LP-CVD. Then, a TiN(10 nm)/Pt(30 nm)/TiN(10 nm) metallic tri-layer stack was deposited and ion beam etched to form the resistors and the heat homogenizers structures. Pt was chosen because it is mechanically, electrically and chemically stable even at high temperature. However, it is considered as a contaminant in standard microelectronic facilities. The TiN layers therefore acted as non-contaminant conducting barriers during the fabrication while additionally providing mechanical and electrical robustness to the final fabricated device. The resistors were then encapsulated in 100 nm of SiO2 deposited at 400 ◦ C and the whole structure was annealed at 800 ◦ C
Fig. 5. Sketch of the device layout with size and position of the tracks in the hotplate.
382
P. Barritault et al. / Sensors and Actuators A 172 (2011) 379–385
4. Electrical characterization The characterization carried out on the prototypes was driven by the use of the microheater as IR source. The temperature coefficient of the resistors (TCR) was initially measured and, then, we investigated the long term stability. It is well known that, for conductors, the temperature dependence of resistance is linear over a wide range of temperatures and can be described by the law: R = R0 × [1 + ˛(T − T0 )]
Fig. 6. Process flow for microhotplate fabrication.
for 1 h, to stabilize the entire stack at a temperature well above the operating temperature. The entire stack was then capped with a 100 nm thick Si3 N4 layer, which allows to compensate the stress mismatch between the different layers. To realize the contact to the heaters, the Si3 N4 /SiO2 layers were removed down to the TiN over a 100 m × 100 m area. To form the contact pads, the samples then deposited and etched with TiN (10 nm)/Au (200 nm) bi-layer. The final step was the release of the freestanding hotplates geometry. The Si3 N4 /SiO2 layers were etched down to the substrate and the Si underneath was removed by front-side TMAH wet etching. As shown in Fig. 7, the chosen geometry consists of disks being 50, 100 and 150 m in diameter, respectively, connected to the substrate by 150 m long, 30 m large arms (Fig. 7a). As TMAH etching reveals 1 1 1 planes of 1 0 0 oriented Si wafers, the arms were intentionally tilted by 30◦ in the heater design to ensure a complete Si etch beneath the membranes. A picture of the final device is shown in Fig. 7b.
where R0 is the resistance at T0 = 25 ◦ C, and ˛ is the TCR. To measure the TCR, the chip was placed on a heated chuck and the temperature was gradually increased from 25 ◦ C to 200 ◦ C in steps of 25 ◦ C. For each steps, the resistivity of a specific serpentine device patterned on the same stack used for the microheater (Fig. 7a), was measures. The serpentine was not released from the substrate which insured that its temperature was very close to that of the chuck. The experimental curve R(T) obtained shows no hysteresis and can be linearly fitted with ˛ = 9 ± 0.5 × 10−4 K−1 . This relationship was used to evaluate the temperature of the microheater from a measurement of its resistance. Assuming that ˛ is constant with temperature, when the microheater is self heated by a current, one can deduce the temperature by measuring its resistance. It is interesting to note that this TCR value (obtained for the stack TiN/Pt/TiN) is unexpectedly lower than values reported in the literature for Pt—˛Pt-Bulk = 3.9 × 10−3 K−1 [12]. On the other hand, in a separate experiment we measured for TiN ˛TiN = 2 × 10−4 K−1 , which is in good agreement with [7] for low stress layers deposited at high pressure. Taking into account that the Pt conductivity is 3 orders of magnitude higher than TiN, the overall electrical behavior should be governed by the properties of this material shunting the current flow in the stack. As discussed in [7] for TiN, the electrical properties of thin films can change with the residual stress, so that the stress induced by the Si3 N4 layers, which is typically tensile, might be responsible of the TCR lowering. To support this hypothesis, a set of new devices realized on Si3 N4 layers with different residual stress on are now under investigation. The evolution of the microheater resistance with respect to the dissipated power was measured using a Keithley 2400 sourcemeter in current-source four wires mode. As the resistance of the microheater (without the contribution of the arms) was the only parameter of interest, specific contact pads (in Fig. 7a the two extra pads for the right-most microheater) connected directly to the microheater were used. Fig. 8 shows a plot of the evolution of the microheater’s resistivity versus the current, as well as a plot of the temperature obtained
Fig. 7. a) Schematic of the hotplate arrangement within each die showing 4 freestanding hotplates and b) SEM image of a single hotplate.
P. Barritault et al. / Sensors and Actuators A 172 (2011) 379–385
383
source comfortably exceeds that lifetime with a drift of resistance within 10% of the initial value. On the other hand, failure occurs quickly when the drift goes beyond 20% and it can be associated with the appearance of hot spots on the heater. Since the behavior is highly reproducible and independent of the operating conditions, it can be used to handle scheduled maintenance well before failure occurs. According to Fig. 8, when operated at 44 mW, the microhotplate works at a temperature close to 700 ◦ C with an acceptable drift. Even if the error associated with temperature estimation can be as high as ±40 ◦ C, the initial target of 10 years lifetime with an operational temperature exceeding 650 ◦ C has been fulfilled.
5. Optical characterization
Fig. 8. Microheater’s resistivity and temperature of the hotplate versus current (a) and (b) temperature of the hotplate estimated from the heater resistance. Measurements were carried out in 4-probes modes across the hotplate region, only.
from equation T = T0 + ˛ − 1(R/R0 − 1), with R0 = 202 at T0 = 25 ◦ C versus the dissipated electrical power. On this figure, it can be seen that, for a current of 8 mA (which corresponds to P = 40 mW), the temperature of the microheater is T = 730 ± 40 ◦ C. The error associated to T is mainly due to the precision in the evaluation of the TCR (5%) which could be greatly improved if we could increase the temperature of the heating chuck to values higher than 200 ◦ C. To measure the drift of the IR source with the operating time, the microheater was electrically driven using a Keithley 2400 sourcemeter in two-wire current-source mode. Pulses of 20 ms duration every 60 ms were cyclically applied to the heaters. The electrical power consumption (Pelec ) was measured simultaneously by the source-meter and the current was adjusted in real time via a PID feedback loop to keep the dissipated power constant within the device, independently of the drift. The results of this accelerated aging test are shown in Fig. 9 where the resistances of devices operated at different powers are also shown. The solid line at 6.105 cycles corresponds to a lifetime of 10 years for a duty cycle of 2 min as required for autonomous sensing applications. As illustrated, when the operating power is kept below 46 mW, the lifetime of the
Fig. 9. Resistance drift of devices operated at different powers as a function of the number of cycles. The solid bar at 2.63 × 106 cycles represents a 10 years lifetime for a devices cycles every 2 m.
The emission spectrum of the microhotplate was measured using a Fourier Transform IR (FTIR) spectrometer equipped with a LN2-cooled HeCdTe photodetector in the photoluminescence mode. The FTIR was operated very close to the lowest detection limit and the obtained spectra – not reported here – were quite noisy because of the very low radiance of the source. However, a typical blackbody behavior was clearly identifiable. In order to measure the optical power emitted at = 4.26 m, an optically immersed photodiode PD42Sr from Ioffe [13] was placed in front of the microheater at a distance of 11 mm. In between them, an optical filter (C = 4.26 m, = 200 nm) from LaserComponents was introduced to limit the measurement only to the specific bandwidth of CO2 . A photocurrent of IPD = 0.07 ± 0.02 A (error is mainly due to the poor stability of the photodiode) was measured, in photovoltaic mode (i.e., no bias voltage is applied) using a Keithley 2000 multimeter. To convert this photocurrent in an optical power impinging the photodiode, the current-sensitivity of the photodiode at = 4.26 m was measured using a reference blackbody emitter. Since the sensitivity measured is Si = 0.2 A/W, the optical power impinging the photodiode is Pexp = 0.35 ± 0.1 W. From this measure, an interesting exercise was undertaken to compare this experimental value with the theoretical estimation calculated. According to photometric laws [14], the optical power P impinging a detector of area ADet located at a distance d (d2 > > ADet ) from a blackbody-source of radiance LS and area AS is: P = LS AS AD TAtm TFilter /d2 , where TFilter is the transmission of the optical filter and TAtm is the transmission of atmosphere. Assuming the microheater is a uniform blackbody of temperature 650 ◦ C, its radiance given by Planck’s law [7] is 2250 W m−2 sr−1 m−1 at 4.26 ± 0.1 m. The numerical application (with TAtm ≈ 1, TFilter = 0.8, AS = 752 m2 , AD = 1.62 mm2 and d = 11 mm) gives a theoretical value of Ptheo = 0.42 W for the optical power impinging the photodiode, which is close to the measured one: Pexp = 0.35 ± 0.1 W. This result although not rigorously complete, provides confidence that the effective temperature of the microheater is close to the expected value. Indeed, according to our model, a variation of optical power of ±0.1 W corresponds to a variation of ±60 ◦ C. Another important parameter to measure is the temperature uniformity of the microheater. This was done using an IR microscope composed of a reflecting objective (Ealing ×25) and an InSb cooled camera (FLIR Titanium 650 M + 50 mm objective). In the optical path, an optical filter (C = 4.26 m, = 200 nm) from LaserComponents was introduced to limit the measurement only to the specific bandwidth of CO2 . Unfortunately, the camera was not calibrated for this bandwidth and it was not possible to obtain straightforward “temperature images” of the microheater. As a consequence, the image shown in Fig. 10a is a relative optical power cartography of a device working at 40 mW, only. Nevertheless, since our optical system has a very small field of view (<2◦ ) and the microheater is considered as a blackbody (constant angular radiance), one can demonstrate [14] that an optical power cartography
384
P. Barritault et al. / Sensors and Actuators A 172 (2011) 379–385
Fig. 10. Experimental temperature cartography of a device powered at 40 mW and temperature profiles on a cross section taken perpendicularly to the harms at the center of the microheater: comparison between measured and to simulated data.
in the image plane is proportional to a radiance cartography in the object plane. Therefore the images obtained, using this optical setup are representative of the relative radiance of the microheater. Assuming that the microheater behaves as a blackbody of average temperature 650 ◦ C, using Planck’s law it is possible to determine a temperature image of the microheater. Fig. 10b compares a cross section of the experimental cartography with a temperature cross section issued from finite element modeling. From this figure, one can see that the temperature falls most quickly when approaching the outmost resistor, indicating that in our model thermal exchanges were underestimated close to the border of the hotplate. The overall temperature uniformity is therefore lower then expected (Texp = 15%; Tsimu = 5%), the result on the device behavior being a slightly higher power consumption for the same optical power emitted. In the prospect of using the microheater in pulsed operation, the rise time of the device has been measured. The hotplate was powered with 20 ms, 44 mW pulses and a duty cycle of 50%. The resistance of the hotplate was calculated by measuring the voltage drop across the microheater in 4-probe mode and the corresponding temperature was estimated according to the calibration curve (Fig. 8). As shown in Fig. 11, where the time behavior of the source is shown, the rise time is about 0.75 ms (10–90% of the overall resistance rise). A direct measurement of the recovery time cannot be easily performed without a second resistor integrated onto the heater but, as reported in [15], an estimation was possible by assuming that the time to reach room temperature is comparable with the rise time.
To further support these results the cut-off frequency was also measured. The experimental setup used was the following: the microheater was driven with a square wave current-function (duty cycle: 50%, POn = 44 mW, POff = 0 mW). The IR signal emitted was measured by a photodiode placed directly above the microheater. The photodiode was connected to a Stanford Research Systems SR830 DSP Lock-In amplifier. The output photodiode voltage was monitored while varying the frequency of the square wave current function. Two different photodiodes for this measurement were used: the Ioffe PD42Sr photodiode (C = 4.26 m, = 1 m) and a Newport 918D-IR Germanium photodiode (C = 1.3 m, = 800 nm). The resulting time dependency curves are presented in Fig. 12. These experimental results clearly show that the microheater can be used in pulsed operations up to 100 Hz. Another interesting result is the difference in cut-off frequencies with respect to the detector used: f−3db = 230 Hz and 550 Hz for Ge (C = 1.3 m) and Ioffe (C = 4.26 m) respectively. This can be explained by the difference in wavelength sensitivity. Indeed, as the frequency of the driving current was increased from 0 to 10 kHz (fcut ), the microheater progressively stabilizes to a constant temperature corresponding to a continuous driving power POn /2, which corresponds to a temperature much lower than the one reached for low frequency operation. Since, the microheater behaves as a blackbody, one can infer that the light intensity at C = 1.3 m decreases faster with increasing driving frequency, than the light intensity at C = 4.26 m.
800 0.75 ms
T (°C)
600
400
200
0 0,0
1,0
2,0
Time (ms) Fig. 11. Time behavior of the microheater for 44 mW power pulses applied to the device.
Fig. 12. Photodiode voltage versus frequency while the microheater is driven with a square wave current-function.
P. Barritault et al. / Sensors and Actuators A 172 (2011) 379–385
6. Conclusions In this paper the fabrication and the characterization of a blackbody IR source working at 650 ◦ C to be used for CO2 detection in an NDIR-type gas sensor, has been reported. To minimize the thermal mass and to reduce power consumption well below 50 mW, the micro hotplates have been fabricated on a freestanding Si3 N4 membrane by standard MEMS technologies. The overall heater design was optimized to keep the temperature spread within the microhotplate below 5%. This goal was reached by filling the interspace between the resistors with a metallic spacer fabricated with the same metal stack used for the resistor itself. In spite of the lower uniformity in the final device (∼15%), when the source is operated at 44 mW, the optical power emitted in the range (C = 4.26 m, = 200 nm) is Pexp = 0.35 ± 0.1 W. Assuming that the microhotplate behaves as a perfect blackbody, this result demonstrates that, as required by design, it is possible to reach an effective temperature above 650 ◦ C while consuming less than 50 mW. On the other hand, it has also been shown that the hotplate can reach a steady state temperature in less than a ms and that the device can be operated with an acceptable drift for more than 10 years with a duty cycle of 2 min (∼6 × 105 cycles), making this device an ideal IR source for autonomous NDIR detection of CO2 to be used in conjunction with heating, venting air conditioning systems. Acknowledgments The authors equally contribute to this paper and to the work needed for its preparation. References [1] ASHRAE Standard 62-2001: Ventilation for Acceptable Indoor Air Quality [ASHRAE 2001]. [2] See for example “Carbon dioxide (CO2 ) sensors for the agri-food industry—a review, Food Bioprocess Technol. 2 (2009) 115–121. [3] “IRidum® 100” datasheet on www.citytech.com. [4] J.S. Suehle, R.E. Cavicchi, M. Gaitan, S. Semancik, Tin oxide gas sensor fabricated using CMOS micro-hotplates and in situ processing, IEEE Electron. Dev. Lett. 14 (3) (1993). [5] KMHP-100 MEMS Micro-Hotplate datasheet on http://www.kebaili.com/. [6] S.Z. Ali, F. Udrea, W.I. Milne, J.W. Gardner, Tungsten-based SOI microhotplates for smart gas sensors, IEEE J. Microelectromechan. Syst. 17 (2008) 1408–1417. [7] J.F. Creemer, D. Briand, H.W. Zandbergen, W. Van der Vlist, C.R. de Boer, N.F. de Rooij, P.M. Sarro, Microhotplates with TiN heaters, Sens. Actuators A 148 (2008) 416–421.
385
[8] I. Elmi, S. Zampolli, E. Cozzani, M. Passini, G.C. Cardinali, M. Severi, Development of ultra low power consumption hotplates for gas sensing applications, Proc. IEEE Sensors (Oct 2006) 243–246. [9] E. Cozzani, A. Roncaglia, S. Zampolli, I. Elmi, F. Mancarella, F. Tamarri, G.C. Cardinali, Material properties measurement and numerical simulation for characterization of ultra-low-power consumption hotplates, in: Transducers & EuroSensors’07, 14th International Conference on Solid-State Sensors, Actuators and Microsystems, Lyon, France, June 10–14, 2007, 2007. [10] E. Cozzani, C. Summonte, L. Belsito, G.C. Cardinali, A. Roncaglia, Design study of micromachined thermal emitters for NDIR gas sensing in the 9–12 m wavelength range, in: IEEE SENSORS 2007 Conference, 2007. [11] S. Gidon, M. Brun, S. Nicoletti, Optimisation of filament geometry for gas sensor application, in: Proceedings of the COMSOL Conference 2010, Paris, 2010. [12] Platinum RTDs datasheet on content.honeywell.com/. [13] http://www.mirdog.spb.ru/products.htm. [14] J.-L. Meyzonnette, T. Lépine, Bases de radiométrie optique, Cépadués editions (2001). [15] I. Elmi, S. Nicoletti, G.C. Cardinali, L. Dori, S. Zampolli, Fast pulsed temperature mode operation: a viable way to improve the sensitivity and the selectivity in metal oxide gas sensors, in: Proceedings of the 14th European Conference on Solid-state Transducers, Copenhagen, Denmark, August 27–30, 2000.
Biographies Pierre Barritault is an engineer in photonics in the Commissariat à l’Energie Atomique (CEA). He graduated from SupOptique (a French engineer school specialised in optics) in 1998. From 1999 to 2002, he prepared a thesis in a laboratory specialised in optical thin films coating (CEA Grenoble (France)). This thesis, which was defended in October 2002, dealt with the theoretical and experimental study of the fluorescence of surface-bound molecules. He now works at CEA-LETI MINATEC on the design and development of NDIR gas sensors. Mickael Brun got his PhD at University Joseph Fourier (Grenoble I) in 2002 developing low temperature (4K) scanning near field optical microscopy for single quantum dot spectroscopic studies. He joints CEA-LETI in 2003 as post doc where he uses STM and AFM near field characterization techniques to probe structural and electrical properties of organic semi-conductors. He obtains a permanent position in CEA-LETI as research engineer in 2005 dealing with technological integration on silicon and move to the Optronic Department in 2007. He is now involved in various projects integrating optics and opto-electronic functions on silicon for various applications ranging from optical data storage, infrared photonics or gas sensing applications. Serge Gidon received the undergraduate degree in electro-technical engineering at the National College of Grenoble and a post-graduate diploma in optical instrumentation at the University of Grenoble. After 12 years working on large optical facilities at CEA, he joined LETI (one of the MINATEC labs) and for the last 16 years has participated in projects involving optical microsystems, micro lasers, bolometer images, data-storage discs and more recently optical sensors such as gas detectors. Sergio Nicoletti got his PhD at the University J. Fourier of Grenoble (Fr) discussing a thesis on HTc superconducting devices. Since late 1996, he was working as project leader at CNR-IMM in Bologna (It) from 1997 developing Smart Sensors/Systems for indoor and outdoor air quality monitoring. In 2004 he took visiting scientist position at Hitachi Global Storage Tech. in San Jose (USA) working on magnetic sensors for data storage. From 2006 he has been working as project leader at CEALETI MINATEC on micro and nanodevice fabrication and nanoobjects integration for electrical, optical and electro-optical applications. Since late 2008 he is managing “Sensors and optical architectures for gas detection” group at CEA-Leti.
Contents lists available at SciVerse ScienceDirect
Sensors and Actuators A: Physical journal homepage: www.elsevier.com/locate/sna
Mid-IR source based on a free-standing microhotplate for autonomous CO2 sensing in indoor applications Pierre Barritault, Mickael Brun, Serge Gidon, Sergio Nicoletti ∗ Département Optronique, CEA, LETI, MINATEC Campus, 17 Rue des Martyrs, 38054 Grenoble Cedex 9, France
a r t i c l e
i n f o
Article history: Received 9 February 2011 Received in revised form 22 August 2011 Accepted 21 September 2011 Available online 10 October 2011 Keywords: Mid-IR source Microhotplate CO2 sensing
a b s t r a c t The measurement of CO2 by Non-Dispersive InfraRed absorption (NDIR) is often used as a tracer of human occupancy in confined living spaces. The major constraint of commercial sensors comes from the power consumption of the IR source, which makes them unsuitable for autonomous operation. This paper reports the fabrication and the characterization of a black-body IR source based on a microhotplate micromachined in Si and suitable to work above 650 ◦ C. The use of state-of-the-art MEMS technologies allows to lower the power consumption below 50 mW while ensuring a lifetime well beyond 10 years. The radiance of the microhotplate in the spectral range where CO2 adsorption takes place indicated that the device works as a quasi-perfect blackbody source providing enough power to drive an autonomous NDIR system for CO2 detection. © 2011 Elsevier B.V. All rights reserved.
1. Introduction Heating and venting air conditioning (HVAC) systems can significantly reduce the overall power consumption when on-demand ventilation is optimized on the basis of pollutants monitoring. Upto-date building control systems usually address the measurement of three or four parameters to drive demand-controlled ventilation: temperature, hygrometry, carbon dioxide and pressure. In this context, the quantification of CO2 is used as a tracer of human occupancy in confined living spaces. This approach is inadequate to monitor toxic compounds released by building materials and furnishings and some classes of VOC detectors at ppb level with reasonable cost are unlikely to emerge soon due to the unavailability of cost effective and reliable components. However, optimization of the ventilation strategies allows keeping comfortable living conditions while lowering the overall power budget [1]. CO2 is an inert gas which poorly interacts with the surrounding environment. If the detection mechanism is based on chemical interactions, the low reactivity may represent a significant drawback to build a reliable and reproducible monitoring device. A large selection of CO2 sensors have been proposed in literature where almost every gas detecting technology has been explored [2]. In spite of some encouraging results, a large majority of laboratory devices suffer of poor selectivity and/or poor sensitivity and only very few sensors haves reached the commercialization.
∗ Corresponding author. Tel.: +33 438 78 0289. E-mail address: [email protected] (S. Nicoletti). 0924-4247/$ – see front matter © 2011 Elsevier B.V. All rights reserved. doi:10.1016/j.sna.2011.09.027
For simple molecules, such as CO2 , optical detection offers the unique advantage of being highly selective since a spectrally unique light–gas interaction can be chosen by tuning the light wavelength. Therefore, in optical gas detection the issue of the selectivity is transferred to the selection or the fabrication of a monochromatic light source or a filtered infrared (IR) detector working in a suitable wavelength range—for CO2 ∼4.2–4.3 m. This working principle, based on the Beer–Lambert absorption law, is used in several commercial sensors where the source has been associated with different types of detectors to measure the light absorption. For CO2 monitoring in public and domestic buildings, commercial sensors typically include an IR source such as a hot filament, a pyroelectric or a bolometric detector and an IR pass band filter to detect concentrations ranging from 300 to 2000 ppm with an accuracy of about 100 ppm and resolution of about 20 ppm [3]. The key figure for these devices is the price, which should not exceed few tens of Euros. However, another major constraint comes from the severe limitation in power consumption, since typically a sensor unit might need to work autonomously for more than a year. It is worth mentioning that, while a variety of commercial sensors can provide the required performance in terms of accuracy and resolution, they often fail to meet the power consumption constraints. In autonomous devices, an average power consumption of 1 W can sustain AA commercially available battery operation for a year or longer. Similar amounts of energy can be harvested with a photovoltaic module used in conjunction with the sensor. If CO2 concentration should be measured every 2 min, as seems to be acceptable in drive on-demand ventilation, a total amount of 120 J can be made available for each measurement. This implies that, if
380
P. Barritault et al. / Sensors and Actuators A 172 (2011) 379–385
the power consumption of the source is 50 mW, each measurement cycle should be performed in less than 24 ms. This simple calculation shows that, to comply with energy consumption requirements of indoor operated sensors, the microheater should be very agile in terms of thermal mass so that the operating temperature can be reached in few ms consuming a very low amount of energy. These goals can be reached by miniaturizing the source in Si by MEMS technologies to realize what is usually called a microhotplate. This type of devices has been first developed by Semancik and co-workers in the early’90 for gas sensing applications [4]. Since then, a number of different device layouts were designed, realized and tested and some devices are now commercially available [5]. However only few of them were used as a thermal IR sources because of their mechanical and electrical stability when the temperature is raised above 500 ◦ C. In this paper we report the realization of a novel IR source based on a free-standing microhotplate fabricated in MEMS technology and suitable to work at an operating temperature of 650 ◦ C or higher. The goal is to develop a source suitable for providing enough IR power in [4.2–4.3 m] wavelength range to work in conjunction with state-of-the-art room-temperature IR detectors whist maintaining the power consumption well below 50 mW.
2. Modeling of the microhotplate IR source In blackbody emission spectra, it can be observed that, for a given wavelength band, e.g., within the mid-IR range, the spectral radiance increases with the source temperature. Therefore, even if the maximum for 4.25 m emission can be reached at around 682 K, the higher the operating temperature of the source the higher the spectral radiance in the CO2 adsorption band. On the other hand, the maximum operating temperature should be set to guarantee mechanical and/or electrical stability of the device, which allows long term operation – typically more than 10 years – with negligible drift. The key issue to preserve the device stability is the choice of the heater material. As discussed in [6], a vast literature reporting microhotplate devices fabricated using C-MOS compatible and non-compatible materials is now available. However, for many of them reliability tests at high temperature are not reported. Sarro and co-workers [7] have recently reported smooth operation of a TiN based microhotplate well above 600 ◦ C, showing that this material can be very stable at high temperature. TiN is often used in microelectronics as conductive barrier to avoid diffusion and electromigration of metallic species in IC devices. In spite of the superior stability in a wide range of operating conditions, TiN is quite resistive and driving a resistor fabricated from this material might require voltages not currently available with a single battery or a small photovoltaic module. Fig. 1 shows a schematic of the hot filament IR source. According to the results discussed above, we designed the microheater as a free-standing Si3 N4 circular membrane, embedding a metallic conductor encapsulated in SiO2 and fabricated on a sacrificial Si substrate. Two arms support the entire structure, providing mechanical and the electrical connections with the substrate. The heater was realized by patterning a TiN/Pt/TiN stack, which combines the stability of TiN and the favorable electrical properties of Pt. This stack was already proposed in [8,9] showing excellent properties in terms of mechanical and electrical stability. A full 3D model of the microheater has been developed using COMSOL MultiphysicTM Ver. 3.5a Finite Element Modeling package. This model describes conductive and radiative thermal exchanges with the surrounding environment where energy transfer takes place by heat diffusion via the arms supporting the microheater,
Fig. 1. Schematic of the microhotplate. For symmetry reasons, the model domain can be limited to a simpler structure which includes part of the arm and part of the hotplate.
by IR radiative emission and by thermal dissipation with the air, neglecting all other possible heat dissipation phenomena. As discussed in [9,10], the assumption is made that the behavior is purely conductive so that the thermal conductivity of the air can be expressed as a function of the temperature gradient. The output provides the overall IR radiation power emitted and a map of the temperature distribution of the device as a function of the applied voltage, the number of heaters, their size and their position on the microhotplate. To design the device two boundary conditions dictated by technological constrains were taken into account: to comply with the ultimate resolution of the photolithographic tool used within our MEMS pilot line the width of the smallest resistor was set to 1 m while, to ensure mechanical and electrical stability to Si3 N4 embedding the heaters, the gap between the outmost resistor and the edge of the hotplate was set to 5 m. Furthermore, the thickness of the Pt layer was adjusted to allow operation at about 3 V, i.e., a resistance of about 200 . A detailed description of the model can be found in [11]. Since the device is symmetric with respect to the hotplate center, the modeling domain was restricted as shown in Fig. 1. To make the design procedure automatic, the model has been programmed to arbitrarily set the temperature of a given location in the hotplate to 650 ◦ C. According to [8,10], this value has been chosen as the highest average temperature allowed for the device, making an acceptable compromise between optical emissivity, power consumption and device robustness. Furthermore, to optimize the performances while still limiting mechanical and electrical drift, the assumption was made that the temperature should be maintained uniform within ±20 ◦ C around the target value. A typical plot of the temperature distribution within the model is shown in Fig. 2a. A preliminary set of modeling runs evidenced that the target uniformity could not be reached by simply distributing a number of resistors over the hotplate. For this reason, the interspaces between the resistors were filled with a metallic spacer to be fabricated with the same metal stack used for the resistor itself. Since the thermal conductivity is much higher in metals than in dielectric layers, these arc-shaped fillers act as temperature homogenizers. A comparison of the temperature distribution across the hotplate with and without filler is shown in Fig. 2b while a sketch of the final device layout is given in Fig. 2c. Temperature distribution optimization can be achieved by adjusting position and size of the conducting tracks. While within Comsol 3.5, parameterization of sizes and 3D geometries is difficult using and modeling requires high computational resources, optimization of parametric 2D structures can be handled efficiently. To simplify the 3D model to a 2D version, an equivalent thermal and electrical conductivity of the overall layers has been introduced
P. Barritault et al. / Sensors and Actuators A 172 (2011) 379–385
381
680
T (°C)
640
600
560 2D Model 3D Model
520 0
10
20
30
40
50
Radius (μm) Fig. 3. Comparison of the temperature profiles for 2D (red) and 3D (blue) models. The cross sections were taken perpendicular to the harms at the center of the microheater and the arrow indicates the setpoint. (For interpretation of the references to color in this figure caption, the reader is referred to the web version of the article.)
690
T (°C)
660
630
600 0
10
20
30
40
50
Radius (μm) Fig. 4. Temperature profiles homogeneity before (red line) and after (green line) 2D optimization. (For interpretation of the references to color in this figure caption, the reader is referred to the web version of the article.)
3. Device fabrication
Fig. 2. a) Modeling results of the temperature distribution at the surface of the microheater—the cross indicates the reference temperature checkpoint, b) comparison of the temperature distribution across the hotplate with and without filler and c) sketch of the device layout including the temperature homogenizer.
by considering them as being in parallel. As shown in Fig. 3, where the calculated temperature profiles across the hotplate are showed, the two models give comparable results in terms of size and position of the resistors. Nevertheless, it is important to note that 3D profile is smoother and that the difference is related to SiO2 cap layer, whose role of heat diffuser cannot be accurately modeled in 2D. The optimization was carried out by using 3 tracks in parallel, the optimization procedure provides very good homogeneity temperature profiles restricted to ±10 ◦ C, as shown in Fig. 4. A sketch of the final device layout with size and position of the tracks in the hotplate is shown in Fig. 5.
The microhotplates were fabricated on a freestanding membrane by standard MEMS technologies in order to minimize the thermal mass and to reduce power consumption. The membrane realization is based on two 100 nm thick Si3 N4 layers embedding the metallic resistors and supporting the entire hotplate. The suspended hotplate structure is then released from the substrate by anisotropic Si etching. The complete process flow is described in Fig. 6. First, a Si3 N4 layer was deposited on the 200 mm Si wafer by LP-CVD. Then, a TiN(10 nm)/Pt(30 nm)/TiN(10 nm) metallic tri-layer stack was deposited and ion beam etched to form the resistors and the heat homogenizers structures. Pt was chosen because it is mechanically, electrically and chemically stable even at high temperature. However, it is considered as a contaminant in standard microelectronic facilities. The TiN layers therefore acted as non-contaminant conducting barriers during the fabrication while additionally providing mechanical and electrical robustness to the final fabricated device. The resistors were then encapsulated in 100 nm of SiO2 deposited at 400 ◦ C and the whole structure was annealed at 800 ◦ C
Fig. 5. Sketch of the device layout with size and position of the tracks in the hotplate.
382
P. Barritault et al. / Sensors and Actuators A 172 (2011) 379–385
4. Electrical characterization The characterization carried out on the prototypes was driven by the use of the microheater as IR source. The temperature coefficient of the resistors (TCR) was initially measured and, then, we investigated the long term stability. It is well known that, for conductors, the temperature dependence of resistance is linear over a wide range of temperatures and can be described by the law: R = R0 × [1 + ˛(T − T0 )]
Fig. 6. Process flow for microhotplate fabrication.
for 1 h, to stabilize the entire stack at a temperature well above the operating temperature. The entire stack was then capped with a 100 nm thick Si3 N4 layer, which allows to compensate the stress mismatch between the different layers. To realize the contact to the heaters, the Si3 N4 /SiO2 layers were removed down to the TiN over a 100 m × 100 m area. To form the contact pads, the samples then deposited and etched with TiN (10 nm)/Au (200 nm) bi-layer. The final step was the release of the freestanding hotplates geometry. The Si3 N4 /SiO2 layers were etched down to the substrate and the Si underneath was removed by front-side TMAH wet etching. As shown in Fig. 7, the chosen geometry consists of disks being 50, 100 and 150 m in diameter, respectively, connected to the substrate by 150 m long, 30 m large arms (Fig. 7a). As TMAH etching reveals 1 1 1 planes of 1 0 0 oriented Si wafers, the arms were intentionally tilted by 30◦ in the heater design to ensure a complete Si etch beneath the membranes. A picture of the final device is shown in Fig. 7b.
where R0 is the resistance at T0 = 25 ◦ C, and ˛ is the TCR. To measure the TCR, the chip was placed on a heated chuck and the temperature was gradually increased from 25 ◦ C to 200 ◦ C in steps of 25 ◦ C. For each steps, the resistivity of a specific serpentine device patterned on the same stack used for the microheater (Fig. 7a), was measures. The serpentine was not released from the substrate which insured that its temperature was very close to that of the chuck. The experimental curve R(T) obtained shows no hysteresis and can be linearly fitted with ˛ = 9 ± 0.5 × 10−4 K−1 . This relationship was used to evaluate the temperature of the microheater from a measurement of its resistance. Assuming that ˛ is constant with temperature, when the microheater is self heated by a current, one can deduce the temperature by measuring its resistance. It is interesting to note that this TCR value (obtained for the stack TiN/Pt/TiN) is unexpectedly lower than values reported in the literature for Pt—˛Pt-Bulk = 3.9 × 10−3 K−1 [12]. On the other hand, in a separate experiment we measured for TiN ˛TiN = 2 × 10−4 K−1 , which is in good agreement with [7] for low stress layers deposited at high pressure. Taking into account that the Pt conductivity is 3 orders of magnitude higher than TiN, the overall electrical behavior should be governed by the properties of this material shunting the current flow in the stack. As discussed in [7] for TiN, the electrical properties of thin films can change with the residual stress, so that the stress induced by the Si3 N4 layers, which is typically tensile, might be responsible of the TCR lowering. To support this hypothesis, a set of new devices realized on Si3 N4 layers with different residual stress on are now under investigation. The evolution of the microheater resistance with respect to the dissipated power was measured using a Keithley 2400 sourcemeter in current-source four wires mode. As the resistance of the microheater (without the contribution of the arms) was the only parameter of interest, specific contact pads (in Fig. 7a the two extra pads for the right-most microheater) connected directly to the microheater were used. Fig. 8 shows a plot of the evolution of the microheater’s resistivity versus the current, as well as a plot of the temperature obtained
Fig. 7. a) Schematic of the hotplate arrangement within each die showing 4 freestanding hotplates and b) SEM image of a single hotplate.
P. Barritault et al. / Sensors and Actuators A 172 (2011) 379–385
383
source comfortably exceeds that lifetime with a drift of resistance within 10% of the initial value. On the other hand, failure occurs quickly when the drift goes beyond 20% and it can be associated with the appearance of hot spots on the heater. Since the behavior is highly reproducible and independent of the operating conditions, it can be used to handle scheduled maintenance well before failure occurs. According to Fig. 8, when operated at 44 mW, the microhotplate works at a temperature close to 700 ◦ C with an acceptable drift. Even if the error associated with temperature estimation can be as high as ±40 ◦ C, the initial target of 10 years lifetime with an operational temperature exceeding 650 ◦ C has been fulfilled.
5. Optical characterization
Fig. 8. Microheater’s resistivity and temperature of the hotplate versus current (a) and (b) temperature of the hotplate estimated from the heater resistance. Measurements were carried out in 4-probes modes across the hotplate region, only.
from equation T = T0 + ˛ − 1(R/R0 − 1), with R0 = 202 at T0 = 25 ◦ C versus the dissipated electrical power. On this figure, it can be seen that, for a current of 8 mA (which corresponds to P = 40 mW), the temperature of the microheater is T = 730 ± 40 ◦ C. The error associated to T is mainly due to the precision in the evaluation of the TCR (5%) which could be greatly improved if we could increase the temperature of the heating chuck to values higher than 200 ◦ C. To measure the drift of the IR source with the operating time, the microheater was electrically driven using a Keithley 2400 sourcemeter in two-wire current-source mode. Pulses of 20 ms duration every 60 ms were cyclically applied to the heaters. The electrical power consumption (Pelec ) was measured simultaneously by the source-meter and the current was adjusted in real time via a PID feedback loop to keep the dissipated power constant within the device, independently of the drift. The results of this accelerated aging test are shown in Fig. 9 where the resistances of devices operated at different powers are also shown. The solid line at 6.105 cycles corresponds to a lifetime of 10 years for a duty cycle of 2 min as required for autonomous sensing applications. As illustrated, when the operating power is kept below 46 mW, the lifetime of the
Fig. 9. Resistance drift of devices operated at different powers as a function of the number of cycles. The solid bar at 2.63 × 106 cycles represents a 10 years lifetime for a devices cycles every 2 m.
The emission spectrum of the microhotplate was measured using a Fourier Transform IR (FTIR) spectrometer equipped with a LN2-cooled HeCdTe photodetector in the photoluminescence mode. The FTIR was operated very close to the lowest detection limit and the obtained spectra – not reported here – were quite noisy because of the very low radiance of the source. However, a typical blackbody behavior was clearly identifiable. In order to measure the optical power emitted at = 4.26 m, an optically immersed photodiode PD42Sr from Ioffe [13] was placed in front of the microheater at a distance of 11 mm. In between them, an optical filter (C = 4.26 m, = 200 nm) from LaserComponents was introduced to limit the measurement only to the specific bandwidth of CO2 . A photocurrent of IPD = 0.07 ± 0.02 A (error is mainly due to the poor stability of the photodiode) was measured, in photovoltaic mode (i.e., no bias voltage is applied) using a Keithley 2000 multimeter. To convert this photocurrent in an optical power impinging the photodiode, the current-sensitivity of the photodiode at = 4.26 m was measured using a reference blackbody emitter. Since the sensitivity measured is Si = 0.2 A/W, the optical power impinging the photodiode is Pexp = 0.35 ± 0.1 W. From this measure, an interesting exercise was undertaken to compare this experimental value with the theoretical estimation calculated. According to photometric laws [14], the optical power P impinging a detector of area ADet located at a distance d (d2 > > ADet ) from a blackbody-source of radiance LS and area AS is: P = LS AS AD TAtm TFilter /d2 , where TFilter is the transmission of the optical filter and TAtm is the transmission of atmosphere. Assuming the microheater is a uniform blackbody of temperature 650 ◦ C, its radiance given by Planck’s law [7] is 2250 W m−2 sr−1 m−1 at 4.26 ± 0.1 m. The numerical application (with TAtm ≈ 1, TFilter = 0.8, AS = 752 m2 , AD = 1.62 mm2 and d = 11 mm) gives a theoretical value of Ptheo = 0.42 W for the optical power impinging the photodiode, which is close to the measured one: Pexp = 0.35 ± 0.1 W. This result although not rigorously complete, provides confidence that the effective temperature of the microheater is close to the expected value. Indeed, according to our model, a variation of optical power of ±0.1 W corresponds to a variation of ±60 ◦ C. Another important parameter to measure is the temperature uniformity of the microheater. This was done using an IR microscope composed of a reflecting objective (Ealing ×25) and an InSb cooled camera (FLIR Titanium 650 M + 50 mm objective). In the optical path, an optical filter (C = 4.26 m, = 200 nm) from LaserComponents was introduced to limit the measurement only to the specific bandwidth of CO2 . Unfortunately, the camera was not calibrated for this bandwidth and it was not possible to obtain straightforward “temperature images” of the microheater. As a consequence, the image shown in Fig. 10a is a relative optical power cartography of a device working at 40 mW, only. Nevertheless, since our optical system has a very small field of view (<2◦ ) and the microheater is considered as a blackbody (constant angular radiance), one can demonstrate [14] that an optical power cartography
384
P. Barritault et al. / Sensors and Actuators A 172 (2011) 379–385
Fig. 10. Experimental temperature cartography of a device powered at 40 mW and temperature profiles on a cross section taken perpendicularly to the harms at the center of the microheater: comparison between measured and to simulated data.
in the image plane is proportional to a radiance cartography in the object plane. Therefore the images obtained, using this optical setup are representative of the relative radiance of the microheater. Assuming that the microheater behaves as a blackbody of average temperature 650 ◦ C, using Planck’s law it is possible to determine a temperature image of the microheater. Fig. 10b compares a cross section of the experimental cartography with a temperature cross section issued from finite element modeling. From this figure, one can see that the temperature falls most quickly when approaching the outmost resistor, indicating that in our model thermal exchanges were underestimated close to the border of the hotplate. The overall temperature uniformity is therefore lower then expected (Texp = 15%; Tsimu = 5%), the result on the device behavior being a slightly higher power consumption for the same optical power emitted. In the prospect of using the microheater in pulsed operation, the rise time of the device has been measured. The hotplate was powered with 20 ms, 44 mW pulses and a duty cycle of 50%. The resistance of the hotplate was calculated by measuring the voltage drop across the microheater in 4-probe mode and the corresponding temperature was estimated according to the calibration curve (Fig. 8). As shown in Fig. 11, where the time behavior of the source is shown, the rise time is about 0.75 ms (10–90% of the overall resistance rise). A direct measurement of the recovery time cannot be easily performed without a second resistor integrated onto the heater but, as reported in [15], an estimation was possible by assuming that the time to reach room temperature is comparable with the rise time.
To further support these results the cut-off frequency was also measured. The experimental setup used was the following: the microheater was driven with a square wave current-function (duty cycle: 50%, POn = 44 mW, POff = 0 mW). The IR signal emitted was measured by a photodiode placed directly above the microheater. The photodiode was connected to a Stanford Research Systems SR830 DSP Lock-In amplifier. The output photodiode voltage was monitored while varying the frequency of the square wave current function. Two different photodiodes for this measurement were used: the Ioffe PD42Sr photodiode (C = 4.26 m, = 1 m) and a Newport 918D-IR Germanium photodiode (C = 1.3 m, = 800 nm). The resulting time dependency curves are presented in Fig. 12. These experimental results clearly show that the microheater can be used in pulsed operations up to 100 Hz. Another interesting result is the difference in cut-off frequencies with respect to the detector used: f−3db = 230 Hz and 550 Hz for Ge (C = 1.3 m) and Ioffe (C = 4.26 m) respectively. This can be explained by the difference in wavelength sensitivity. Indeed, as the frequency of the driving current was increased from 0 to 10 kHz (fcut ), the microheater progressively stabilizes to a constant temperature corresponding to a continuous driving power POn /2, which corresponds to a temperature much lower than the one reached for low frequency operation. Since, the microheater behaves as a blackbody, one can infer that the light intensity at C = 1.3 m decreases faster with increasing driving frequency, than the light intensity at C = 4.26 m.
800 0.75 ms
T (°C)
600
400
200
0 0,0
1,0
2,0
Time (ms) Fig. 11. Time behavior of the microheater for 44 mW power pulses applied to the device.
Fig. 12. Photodiode voltage versus frequency while the microheater is driven with a square wave current-function.
P. Barritault et al. / Sensors and Actuators A 172 (2011) 379–385
6. Conclusions In this paper the fabrication and the characterization of a blackbody IR source working at 650 ◦ C to be used for CO2 detection in an NDIR-type gas sensor, has been reported. To minimize the thermal mass and to reduce power consumption well below 50 mW, the micro hotplates have been fabricated on a freestanding Si3 N4 membrane by standard MEMS technologies. The overall heater design was optimized to keep the temperature spread within the microhotplate below 5%. This goal was reached by filling the interspace between the resistors with a metallic spacer fabricated with the same metal stack used for the resistor itself. In spite of the lower uniformity in the final device (∼15%), when the source is operated at 44 mW, the optical power emitted in the range (C = 4.26 m, = 200 nm) is Pexp = 0.35 ± 0.1 W. Assuming that the microhotplate behaves as a perfect blackbody, this result demonstrates that, as required by design, it is possible to reach an effective temperature above 650 ◦ C while consuming less than 50 mW. On the other hand, it has also been shown that the hotplate can reach a steady state temperature in less than a ms and that the device can be operated with an acceptable drift for more than 10 years with a duty cycle of 2 min (∼6 × 105 cycles), making this device an ideal IR source for autonomous NDIR detection of CO2 to be used in conjunction with heating, venting air conditioning systems. Acknowledgments The authors equally contribute to this paper and to the work needed for its preparation. References [1] ASHRAE Standard 62-2001: Ventilation for Acceptable Indoor Air Quality [ASHRAE 2001]. [2] See for example “Carbon dioxide (CO2 ) sensors for the agri-food industry—a review, Food Bioprocess Technol. 2 (2009) 115–121. [3] “IRidum® 100” datasheet on www.citytech.com. [4] J.S. Suehle, R.E. Cavicchi, M. Gaitan, S. Semancik, Tin oxide gas sensor fabricated using CMOS micro-hotplates and in situ processing, IEEE Electron. Dev. Lett. 14 (3) (1993). [5] KMHP-100 MEMS Micro-Hotplate datasheet on http://www.kebaili.com/. [6] S.Z. Ali, F. Udrea, W.I. Milne, J.W. Gardner, Tungsten-based SOI microhotplates for smart gas sensors, IEEE J. Microelectromechan. Syst. 17 (2008) 1408–1417. [7] J.F. Creemer, D. Briand, H.W. Zandbergen, W. Van der Vlist, C.R. de Boer, N.F. de Rooij, P.M. Sarro, Microhotplates with TiN heaters, Sens. Actuators A 148 (2008) 416–421.
385
[8] I. Elmi, S. Zampolli, E. Cozzani, M. Passini, G.C. Cardinali, M. Severi, Development of ultra low power consumption hotplates for gas sensing applications, Proc. IEEE Sensors (Oct 2006) 243–246. [9] E. Cozzani, A. Roncaglia, S. Zampolli, I. Elmi, F. Mancarella, F. Tamarri, G.C. Cardinali, Material properties measurement and numerical simulation for characterization of ultra-low-power consumption hotplates, in: Transducers & EuroSensors’07, 14th International Conference on Solid-State Sensors, Actuators and Microsystems, Lyon, France, June 10–14, 2007, 2007. [10] E. Cozzani, C. Summonte, L. Belsito, G.C. Cardinali, A. Roncaglia, Design study of micromachined thermal emitters for NDIR gas sensing in the 9–12 m wavelength range, in: IEEE SENSORS 2007 Conference, 2007. [11] S. Gidon, M. Brun, S. Nicoletti, Optimisation of filament geometry for gas sensor application, in: Proceedings of the COMSOL Conference 2010, Paris, 2010. [12] Platinum RTDs datasheet on content.honeywell.com/. [13] http://www.mirdog.spb.ru/products.htm. [14] J.-L. Meyzonnette, T. Lépine, Bases de radiométrie optique, Cépadués editions (2001). [15] I. Elmi, S. Nicoletti, G.C. Cardinali, L. Dori, S. Zampolli, Fast pulsed temperature mode operation: a viable way to improve the sensitivity and the selectivity in metal oxide gas sensors, in: Proceedings of the 14th European Conference on Solid-state Transducers, Copenhagen, Denmark, August 27–30, 2000.
Biographies Pierre Barritault is an engineer in photonics in the Commissariat à l’Energie Atomique (CEA). He graduated from SupOptique (a French engineer school specialised in optics) in 1998. From 1999 to 2002, he prepared a thesis in a laboratory specialised in optical thin films coating (CEA Grenoble (France)). This thesis, which was defended in October 2002, dealt with the theoretical and experimental study of the fluorescence of surface-bound molecules. He now works at CEA-LETI MINATEC on the design and development of NDIR gas sensors. Mickael Brun got his PhD at University Joseph Fourier (Grenoble I) in 2002 developing low temperature (4K) scanning near field optical microscopy for single quantum dot spectroscopic studies. He joints CEA-LETI in 2003 as post doc where he uses STM and AFM near field characterization techniques to probe structural and electrical properties of organic semi-conductors. He obtains a permanent position in CEA-LETI as research engineer in 2005 dealing with technological integration on silicon and move to the Optronic Department in 2007. He is now involved in various projects integrating optics and opto-electronic functions on silicon for various applications ranging from optical data storage, infrared photonics or gas sensing applications. Serge Gidon received the undergraduate degree in electro-technical engineering at the National College of Grenoble and a post-graduate diploma in optical instrumentation at the University of Grenoble. After 12 years working on large optical facilities at CEA, he joined LETI (one of the MINATEC labs) and for the last 16 years has participated in projects involving optical microsystems, micro lasers, bolometer images, data-storage discs and more recently optical sensors such as gas detectors. Sergio Nicoletti got his PhD at the University J. Fourier of Grenoble (Fr) discussing a thesis on HTc superconducting devices. Since late 1996, he was working as project leader at CNR-IMM in Bologna (It) from 1997 developing Smart Sensors/Systems for indoor and outdoor air quality monitoring. In 2004 he took visiting scientist position at Hitachi Global Storage Tech. in San Jose (USA) working on magnetic sensors for data storage. From 2006 he has been working as project leader at CEALETI MINATEC on micro and nanodevice fabrication and nanoobjects integration for electrical, optical and electro-optical applications. Since late 2008 he is managing “Sensors and optical architectures for gas detection” group at CEA-Leti.