Low-cost electronics and thin film technology for sol–gel titania lambda probes

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Sensors and Actuators B 128 (2008) 359–365

Low-cost electronics and thin film technology for sol–gel titania lambda probes L. Francioso ∗ , M. Prato, P. Siciliano CNR-IMM Institute for Microelectronics and Microsystems, Via Monteroni, University Campus, 73100 Lecce, Italy Received 7 March 2007; received in revised form 20 June 2007; accepted 21 June 2007 Available online 26 June 2007

Abstract The introduction and clear benefits of silicon technology into automotive application were characterized by fast growth during last years. In effect, the technological innovation of microelectronics carried out a fast revolution into automotive fields, considering physical sensors, gyroscopes, accelerometers and combustion monitoring and air quality control devices. The present paper describes a typical application of the microsystem technology gas sensor products into automotive fields. In particular, the fabrication enhancements of a sol–gel Pd-doped TiO2 sensor will be investigated. Main advantages of the realized sensors are the reduced dimensions, the low power consumption, and the cheap fabrication process. Some advancements regarding silicon wafer batch production are presented, together with a custom electronic board realized for proportional–integral–derivative (PID) temperature control and digital signal conditioning of sensors signals. © 2007 Elsevier B.V. All rights reserved. Keywords: Silicon technology; Automotive gas sensors; Metal oxide patterning; PID loop temperature control

1. Introduction Over the last decades, pressing and dutiful regulations on polluting emissions reduction induced mandatory technological development of engines, which was provided nowadays by electronic regulation of injection and fuel metering, working under optimal conditions in terms of stoichiometric mixture combustion. In relation to the technological development of Otto cycle engines, the scientific research on sensors, which are able to measure the residual oxygen concentrations in the exhaust products of the combustion and assure the stoichiometric working points, started from 1970s under incentive of worldwide engine manufacturers. Microelectronics technologies represent in this field the only possibility of an advanced and intelligent management of different sensors and transducers available onboard of modern vehicles. In effect, the gas sensor scientific community also provided and experimented with different solutions with chemical sensors in order to approach the automotive scenarios [1–4]. The main objective of this paper has been the optimization of deposition techniques by a cheap chemical route like sol–gel

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Corresponding author. E-mail address: [email protected] (L. Francioso).

0925-4005/$ – see front matter © 2007 Elsevier B.V. All rights reserved. doi:10.1016/j.snb.2007.06.022

with an aim to allow highly reproducible deposition of a Pddoped TiO2 thin film at a wafer level on a 3 in. (1 0 0)-oriented DSP (double side polished) silicon wafer; the repeatability of this process was successfully implemented for the fabrication of cheap thin film oxygen sensors for lambda measurements in spark ignition engines, considering also the authors’ expertises in thin film micromachining processing [5,6]. The actual cost of a Bosch commercial lambda probe (Model LSF 4.2) with an electronic control unit is about $240 in Italy, while the potential cost is about $120 for a batch production level thin film sensor with an electronics board presented in this work. About the sensitive films, over a total thickness of about 100 nm, it was easy to obtain a thickness spread not larger than 10% over a wafer, considering moreover the possibility to produce approximately 700 devices for each wafer. Optimization phases of physical and interaction properties of thin films with gaseous analytes have been carried out on cheap ceramic alumina substrates, having total surface dimensions of about 4 mm2 ; moreover a systematic activity of electrical characterization was carried out in controlled environment and with complex gas mixtures, in order to select materials with better performances for the final validation phase on engine bench scale. All experimental tests in laboratory or on engine benches have been carried out at different working temperatures. The main part of the paper was devoted to a deep engineering of

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sensors in terms of fabrication process and the development of an electronic board that allowed output signal acquisition/digital conversion, and a micro-controller embedded PID algorithm was presented for real-time temperature regulation. 2. Experimental, results and discussion A sensitive thin film of titania was deposited by a sol–gel method on an alumina substrate (34 mm × 34 mm) composed by a matrix of 225 single devices of 2 mm × 2 mm each and 350 ␮m thick. The production process was carried out on an entire alumina wafer after preliminary fabrication of backside platinum heaters; finally a UV (ultraviolet) lithography step was performed to obtain interdigitated electrodes onto the sensitive films. Pure TiO2 sols were prepared in a glove-box with <1 ppm H2 O. Titanium butoxide (2.13 ml) was dissolved in 10 ml of butanol and chelated with 0.64 ml of acetylacetone. After 30 min since the addition of acetylacetone, 0.45 ml of water were added dropwise, obtaining a yellow, clear sol; for the preparation of Pd-doped TiO2 sols, to a pure sol prepared as above Pd (II) bisacetylacetonate was added after dissolution in chloroform. The amount of the Pd precursor was such to get a Pd/Ti atomic ratio of 0.05. Thin films were deposited from the previous sols by spin coating onto alumina substrates in a low-moisture atmosphere (about 30% RH), then heat-treating the films at 500 ◦ C in a tubular oven. Two layers were deposited of each material. The sol thus prepared can be employed for deposition of metal oxide sensitive films through the spin-coating technique. The entire synthesis process was carried out in a low humidity atmosphere, in order to avoid fast reactions with the humidity of the atmosphere and subsequent opaque aspect of the resulting films. After the spin coating, the film was dried in air at 60 ◦ C and thereafter calcinated at 500 ◦ C in air for 1 h, to promote removal of residual organic and crystallization of the structure. A detailed schematic of the fabrication process is represented in Fig. 1, described below from left to right, top to bottom of Fig. 1 graphics: 1. Cleaning of an alumina substrate and realization of the first lithography process in order to define the embedded platinum heaters. A positive resist (Shipley S1813, 1.3 ␮m thickness

Fig. 1. Fabrication process flow chart for thin film titania lambda sensors.

@ 4000 rpm) was spun onto a substrate at 4000 rpm for 30 s with static dispension. Then a soft baking step was performed on a hotplate at 115 ◦ C for 120 s in order to allow evaporation of residual solvent. At this point we performed the first lithography process to define the platinum heaters by liftoff technique. After development of the resist, an oxygen plasma process was carried out to remove residuals of resist over opened areas of the resist layer. Soon after, the sample was inserted in a sputtering system to perform the platinum deposition, 400 nm thick, in a high vacuum chamber; after the deposition the sample was immersed in an acetone ultrasonic bath to remove all resist layers and leave the platinum layers only on the areas defined by the mask design. 2. Spinning of theTiO2 precursor onto an alumina substrate at 2000 rpm for 3 s. After the first spinning another layer was deposited to increase the thickness of the sensitive film and reduce the resistivity. Pd-doped titania film sols were spun onto the clean side of an alumina wafer, dried at 70 ◦ C in dry air and then annealed at 800 ◦ C for 1 h in dry air to obtain complete crystallization of structure into a rutile phase revealed by XRD (X-ray diffraction) analysis; the TiO2 thin film measured about 100 nm of thickness after calcination [7]. The palladium doping procedure modified, optimizing them, the properties of the pure materials, as found for Pt decoration [8]. 3. Second lithography step to define the electrical contacts onto the sensitive film, 50 ␮m width, and 50 ␮m spaced, and overall active area of 1400 ␮m × 1400 ␮m. The procedure was similar to point 1, while previous gold electrodes implemented in this type of combustion sensor were replaced by platinum electrodes that were more stable at higher temperatures than gold (gold diffusion into a sensitive film observed and lack of film’s integrity at high temperatures). After these fabrication steps were performed on a single alumina wafer, all single substrates were separated from each other in order to obtain 255 single sensors ready to be packaged onto TO-39 socket. Fig. 2 depicts the structure of a single sensor after the package step (for better clarity of the picture the structures presented have been realized onto a silicon substrate); the alumina die was bonded onto the socket suspended in air by means of four gold wires that acted as bridges. The structure of the interdigitated electrodes deposited onto a sensitive film, made of platinum, was 50 ␮m width and 50 ␮m spaced; the picture detail shows also the dicing saw markers (crosses), necessary for alignment of cutting patterns. The backside of a single device was provided by a platinum heater shaped like a meander to heat the device at working temperature. Finally, the complete sensor was ready for electrical characterizations in controlled environments and on a real spark ignition engine, as depicted in Fig. 3. Some devices, for experimental purposes, were provided by a steel cap with a topside window to reduce turbulence effects. An experimental bench was realized in order to perform contemporary data acquisition of a Bosch lambda probe (Model LSF 4.2) and our thin film sensors [9]. The internal combustion

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Fig. 2. Final package of the device onto TO-39 socket (left) and an optical microscope image of adopted layouts for interdigitated contacts and heater.

engine of 220 cm3 of displacement was mounted on the bench with a modified pipe supplied with two 18 mm threaded holes for probe and sensor insertion into pipes. A special brass adapter was realized with electrical feedthroughs to perform sensor heating and signal acquisition; an electrometer with a scanner card allowed to switch acquisition from the sensor to the lambda probe in 0.1 s and to acquire voltage signals from the commercial probe and dc currents from thin film sensor. The initial warm-up time for sensor was 2 min, under an exhaust flow and contemporary heating performed by the platinum heater embedded on an alumina die; the temperature of gas from engines ranged from 250 to 270 ◦ C, measured with a K-type thermocouple, acquired also from the scanner card of the electrometer, in order to discriminate the sensor drift related to modified exhaust temperature. In any case temperature variations related to different regimes of engine had no effect on the baseline signal of sensor, also during rich–lean large current shift. In order to realize reproducible experimental tests about performance of thin film titania sensors, an experimental protocol was defined by fixing the experimental parameters of sensor polarization and operative temperature and also the operative regimes of the spark ignition engine during the acquisition. The experimental protocol set the acquisition runs on constant rotation of engine with an acquisition frequency of 10 Hz, alternatively for both transducers. It was defined in order to have

10 min of warm-up time after engine ignition; after this period, engine rotation was regulated to 3000 rpm and data acquisition of both sensors started. The experimental protocol defined an acquisition rate of 10 samples/s (both transducers alternatively), voltage acquisition for the Bosch probe, and current acquisition for the metal oxide sensor. The engine rotation speed was set to constant 3000 rpm without applied torque, while the warm-up time before starting acquisition was 3 min and each run collected data for 15 min; the bias on the metal oxide sensor contact was 2.0 V, and the operative temperature was set to 720 ◦ C, considering previous investigations on sensitive materials and classification properties. Fig. 4 shows the dynamic response to engine air/fuel ratio transients at a constant rotation speed for the Pd-doped TiO2 thin film, in comparison with the commercial lambda probe. About the recorded current shifts, it is evident that the current variation is wider by more than two orders of magnitude and also a lower resistivity of the device was observed. When an engine is running in rich regimes, the current across the device rises up to a few mA, which is a good current value for a lowcost electronic board interface. Dashed lines in the figure mark the voltage level outputs for lambda probe, respectively at 0.85 and 1.02 values of lambda index, which are referred to the right scale. The solid line in the center area of the graph represents the output voltage for the lambda probe under stoichiometric

Fig. 3. Sensors ready for gas sensing characterization (left) and a sensor packaged with a steel custom protective cap (right).

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Fig. 4. Dynamic response to variations in air/fuel ratio at a constant rotation speed under engine combustion conditions.

conditions. The behaviour of the thin film device is similar to that of the commercial sensor, and the large current variations (more than two orders of magnitude) would be helpful when a discrimination of intermediate values of lambda is required. As found by the present authors for similar Pt-doped materials, higher temperatures result in faster response times also for Pd-doped materials; the 720 ◦ C operative temperature shows the best results in terms of fast saturation times and low influence of temperature gradients in large exhaust flows. The experimental results about the response times were calculated at 90% of saturation currents (for metal oxide sensor) and 90% of saturation voltages (for commercial probe), considering different rich-to-lean and lean-to-rich shifts (Fig. 5); the thin film sensors exhibited comparable response times at 720 ◦ C. Typical times at 90% of saturation currents are about 1.5 s. Checked the working properties, a further aim of the present work was the experimental validation of a standard technology, compatible with common CMOS processes, which allowed a perfect patterning of the titania metal oxide thin film; in effect the capability to pattern the sensitive film allowed to open a wider scenario in terms of mass production, sensor performance control, repeatability and downscaling of devices. The fabrication of silicon devices

Fig. 5. T90 response times in comparison between the commercial probe and the thin film sensor.

in a front–back configuration, that is the sensitive film on one side and the heater on the backside, was carried out by following the same receipt of the alumina devices. The only difference during the process was that the etching of the titania sensitive film was carried out by a RIE (reactive ion etching) dry process, as described in detail below and characterized by higher resolution. After the definition of the platinum heater by lift-off technique on the backside of the wafer, the sensitive film of doped TiO2 was deposited and annealed at 500 ◦ C for 1 h. The film is now ready for the next photolithographic step in order to define the mask for etching. Etching of the sensitive film after the annealing step was performed with a dry etching recipe, obtained with a SF6 and Ar mixture in an Oxford Plasmalab 80 RIE reactor, with a radio frequency (rf) power density of 0.5 W/cm2 , 25 sccm of SF6 /5 sccm of Ar for the total flow, and 50 mTorr of the process pressure. The typical etching time was about 6 min to completely remove the sensitive film from unwanted areas. A typical result on the wafer after the etching is shown in Fig. 6, left picture.

Fig. 6. Patterned areas of a TiO2 thin film with RIE process; bigger squares measure 1.4 mm × 1.4 mm (left); complete silicon substrate sensors after final dicing (right).

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Fig. 7. Schematic of developed electronic board circuit for gas sensor signal conditioning.

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The last fabrication step was a further lithography level for definition of the electrical interdigitated contact 50 ␮m spaced and 200 nm thick on the front of devices. The result obtained is shown in Fig. 6 (left picture); the bright square areas of the silicon wafer where the sensitive film is located are clearly visible. The sensors, presented above as potential low-cost lambda probes for combustion monitoring, suffer from fast flow variation and exhaust turbulence that may cause a lack of operative temperature stability; so, authors designed a specific low-cost interface based on a MicrochipTM 16F819 microcontroller. The aim of the activity was the realization of an electronic board (Fig. 7) capable to read the output signal of the fabricated sensor, to convert this signal into a digital output, to generate a reference stable voltage for electrical contact polarization and to provide a good proportional–derivative–integrative (PID) temperature control, supplying a pulse with modulation (PWM) wave to the heater and contemporary reading of the effective temperature by means of an embedded thermometer. Main specifications of the realized board are: • 100 kHz PWM heater driver; • conditioning and amplification of RTD (resistance temperature detector) signals (Pt thermometer); • PIC 16F819 ␮-controller based; • temperature compensated reference voltage generation (−2.5 V); • current to voltage sensor signal conversion; • 10 bit resolution for sensor signals and thermometer ADCs (analog-to-digital converter); • digital RS-232 output. The electronic board may be analyzed by locating different sections: (a) the temperature sensor reading circuit, (b) the transconductance amplifier of gas sensor signals, (c) the digital section for temperature control, and finally (d) the power supply and reference voltage generation section. Fig. 7 shows the electrical circuit schematics described in this section. The temperature reading will be realized by injecting a constant current into the RTD thermometer of approximately 5 mA and reading voltage variations at the ends of the resistance itself. The RTD sensor is connected as a reaction net on the U3A operational amplifier and the current that crosses it is controlled from the reference voltage applied to the not-inverting input and from

the R13 resistance. With chosen values of 5 V and 5 k, an excitation current of 1 mA will be generated. The two following operational amplifiers, U3B and U4A, serve, respectively to cut-off the offset voltage related to the intrinsic resistance of the sensor and to amplify the signal in order to adapt the scale of the A/D converter of the digital section described below. The reference voltage is obtained from the temperature stable reference U5 and can be easily tuned within a margin of ±5%. The same voltage polarizes the U4B inverting buffer, that drives the gas sensor with an equal voltage of the reference but with an inverted sign. The next section, U6A, is a transconductance amplifier that converts outgoing currents from the sensor in voltages that can be read with an external multimeter; it supplies moreover a signal to an amplifier driven from the digital section, capable to work with different gains in order to better adapt the signal scale of the sensor to the A/D converter of the microcontroller. This happens through the analogic switch U7 that operates when a resistor is necessary in the reaction net of the U6B amplifier. The digital section was designed with a PIC16F819 Microchip microcontroller that drives through embedded software the heater temperature through the RTD thermometer reading with PID control (proportional + integral + derivative). The power signal that drives the heater is a PWM type with a frequency of approximately 100 kHz and permits to perform optimal trade-off between temperature regulation speed and well-controlled setpoints. The microcontroller may be connected with the external world through a serial port, in order to be able to set up and subsequently read the temperature of the heater and the signal of the gas sensor. In order to enlarge the reading span of the ADC microcontroller internal converter, a solid-state digital switch has been inserted, that allows insertion of a different resistance on a gain net of U6B. With this approach the ADC reading range may work up to three decades of sensor resistance variation. The power supply section is composed from an already described reference to 5 V, an integrated stabilizer for the digital section and a DC–DC converter that makes available a dual voltage for the analog section, galvanically separating the grounds of the board in order to avoid possible signal loops. Fig. 8 shows the realized prototype of the interface described above. Work is in progress about experimental characterizations of the realized board, that represents a cheap solution for signal conditioning and temperature control of metal oxide gas sensors.

Fig. 8. CAD layout of electronic board (left); realized electronic board circuit for gas sensor (right).

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3. Conclusions The work presented a specific-target modified microfabrication technology for sol–gel deposited metal oxide film integration into chemical gas sensors for automotive application. In particular a detailed fabrication process of low-cost Pd-doped TiO2 devices onto ceramic substrates has been presented, and an experimental characterization of alumina substrate devices was carried out for combustion conditions discrimination properties; t90 response time has been calculated for a commercial lambda probe and the thin film sensor. A further engineering of the fabrication process for a batch fabrication run of sensors has been developed for a 3 in. (1 0 0)-oriented DSP silicon wafer, with a dry etching of the sensitive film that allows a high resolution patterning of metal oxide films. In the last section of the paper, a microcontroller-based custom electronic interface was presented and characterized with specific features for chemical gas sensor applications in harsh environment, like PID temperature control, AD conversion of sensor signals, and RS232 connection. Different patterning processes for various metal oxide materials are actually under development. Acknowledgments Acknowledgments to Mrs. C. Martucci for her technical support during sensor fabrication. Authors thank Prof. A. Ficarella from University of Lecce for preliminary investigation of devices on a spark ignition bench.

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Biographies Luca Francioso received the degree in Physics in April 2001 at the University of Lecce and he took in the same university the engineering PhD degree in March 2006 with a thesis on silicon technologies applications to automotive gas sensors. Since June 2001 he works in the Institute for Microelectronic and Microsystems of the National Council of Research (C.N.R.-I.M.M.) in Lecce (Italy) in the field of silicon micromachined systems and thin film gas sensor, devoted to fabrication processes. Since February 2003 he is in the position of researcher, devoted to silicon technology fabrication and integration of sol–gel process into silicon devices. At present he works in the field of combustion control sensors with implementation of thin film-based gas sensors and development of micromachining processes for metal oxide layers.

References

Mario Prato works since 2001 as technical staff at the Institute for Microelectronic and Microsystems of the National Council of Research (C.N.R.-I.M.M.) in Lecce (Italy) in the field of electronic interfaces development for gas sensors, low-cost A/D board, portable measurement systems and network administration.

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Pietro Siciliano, physicist, senior researcher, received his degree in physics in 1985 from the University of Lecce. He took his PhD in Physics in 1989 at the University of Bari. During the first years of activities he was involved in research in the field of electrical characterisation of semiconductors devices. He is currently a senior member of the National Council of Research in Lecce, where he has been working for many years in the field of preparation and characterisation of thin film for gas sensor and multisensing systems, being in charge of the Sensors and Microsystems Group. At the moment he is Director of IMM-CNR in the Department of Lecce.