Monolithic CMOS multi-transducer gas sensor microsystemfor organic and inorganic analytes
Y. Li, C. Vancura, D. Barrettino, M. Graf, C. Hagleitner, A. Kummer,M. Zimmermann, K.-U. Kirstein, A. Hierlemann ∗
ETH Zurich, Physical Electronics Laboratory, Wolfgang-Pauli-Strasse 16, 8093 Zurich, Switzerland
Received 12 December 2006; received in revised form 23 March 2007; accepted 24 March 2007Available online 5 April 2007
bstract
A monolithically integrated multi-transducer microsystem to detect organic and inorganic gases is presented. The system comprises two polymer-ased sensor arrays based on capacitive and gravimetric transducers, a metal-oxide-based sensor array, the respective driving and signal processinglectronics and a digital communication interface (see the first figure). The chip has been fabricated in industrial 0.8-�m, complementary-metal-xide-semiconductor (CMOS) technology with subsequent post-CMOS micromachining. The simultaneous detection of organic and inorganic
arget analytes with the single chip multi-transducer system has been demonstrated. The system is very flexible and can provide different informationf interest: the capacitive sensors can, e.g., act as humidity sensors to deal with the cross-sensitivity of the metal-oxide-based sensors to water, or theapacitive sensors can be coated with differently thick polymer layers to detect organic volatiles even in a background of water. The multi-transducerpproach provides a wealth of information that can be used to improve the system discrimination capability and performance in gas detection.
2007 Elsevier B.V. All rights reserved.
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eywords: CMOS; Multi-transducer system; Gas sensor; Cantilever; Capacitor
. Introduction
Gas sensors are widely employed for a variety of applica-ions, such as environmental monitoring and air quality control1–7]. In recent years, hand-held devices for gas detectionre getting more and more popular. This entails increasingesearch activities to develop gas sensors featuring small size,ow power consumption and low costs. The monolithic inte-ration of CMOS gas sensors is a promising approach thatas been fueled by the rapid development in integrated-circuitnd MEMS technology [7–12]. The aim in utilizing microfab-ication techniques and, in particular, CMOS technology forealizing chemical sensors is to devise more intelligent, moreutonomous, more integrated, and more reliable gas sensor sys-ems at low costs in a generic approach.
Since the sensor market is strongly fragmented, i.e., therexists a large variety of applications with different needs andensor requirements, a modular approach or “toolbox strategy”
elying on a platform technology was identified as the mostromising attempt to achieve major progress [13,14]. Oncehe platform technology has been chosen, the components ofhe toolbox such as transducers, sensor modules, and circuit
odules can be developed, some of which afterwards can bessembled into a customized system that meets the respectivepplications needs. A multitude of development activities areecessary to obtain all the modules needed for such a CMOStoolbox”: (a) the design and miniaturization of transducers andirectly related electronic components (potentiostats, heaters,mplifiers, etc.), (b) the development of digital-to-analog andnalog-to-digital conversion units, interface and communicationnits, (c) the development of additional and auxiliary functions,hich are pivotal for the system performance (e.g., tempera-
ure control, temperature sensors, humidity sensors), and (d)he development of dedicated microsystem packaging solutions,hich are suitable for chemical or gas analysis [13,14]. It is
mportant to note that the package has to be thought of already
n the initial conception phase of a microsystem, since the designnd architecture of a microsystem heavily depend on the envis-ged packaging concept, as will become evident later in thisaper (see system description and layout).
432 Y. Li et al. / Sensors and Actuators B 126 (2007) 431–440
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The main disadvantages of a monolithic CMOS–MEMSolution include the restriction to CMOS-compatible materialsnd the limited choice of micromachining processes. However,he use of CMOS–MEMS offers, on the other hand, unprece-ented advantages over hybrid designs, especially with regardo signal quality, device performance, increased functionalitynd available standard packaging solutions. These advantageslearly outweigh the drawbacks and limitations. In the casef well-established physical sensors such as acceleration andressure sensors, a trend towards monolithic solutions can bedentified for large production volumes and severe cost restric-ions [15–18].
The monolithic integration of sensor and circuitry allowsor on-chip control and monitoring of the mechanical functionsnd data processing. The reduction of the number of electricalonnections through the use of standard interface units on chipignificantly contributes to the reduction of the overall system
osts and improves its reliability. At the same time, the process-ng of the sensor signals at the signal source helps to improve theensor signal quality, as the influence of external interferencesan be reduced.
tmoe
Fig. 2. Schematic representation
thic multi-transducer chip.
The selectivity of individual gas sensors or systems still posesmajor problem. Many sensitive materials, such as metal oxidesr polymers, respond to a variety of inorganic gases or volatilerganic compounds (VOCs). The use of a set of identical trans-ucers coated with different materials along with software tools,uch as multi-component analysis algorithms [19–25], or thentegration of different transducers, which respond to distinctnalyte properties, with dedicated circuitry in a microsystem26–29] can help to overcome the problems associated with poorelectivity and drift of individual gas or liquid-phase sensors.
In the following a monolithically integrated, CMOS-basedulti-transducer microsystem to detect organic and inorganic
ases will be presented.
. System description
The monolithic multi-transducer chip (Fig. 1) comprises two
ypes of polymer-coated transducers (two cantilevers and two
easuring capacitors), which are predominantly sensitive torganic volatiles, two microhotplates, which respond prefer-ntially to inorganic gases, the needed driving electronics, the
of the system architecture.
ctuators B 126 (2007) 431–440 433
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eadout electronics, an on-chip biasing, and a digital commu-ication interface. The different transducers with the associatedircuitry units will be detailed below. The cantilevers and theapacitive sensors rely on bulk physisorption of organic volatilesn polymers, which is strongly temperature-dependent. There-ore, a temperature sensor relying on the linear temperatureependency of the base-emitter-voltage of the vertical bipolarransistor, which is available in the CMOS process, has beenmplemented to monitor the chip temperature. A block diagramf the system and its integrated signal processing capabilitiess shown in Fig. 2. The analog part of the system includes theensor driving circuitry, the sensor signal read-out circuitry androvides bias voltages and bias currents to set the operatingoints of the operational amplifiers and filter stages. The digitalart of the system includes a configuration register to control theystem, which enables to select a sensor element from the array,nd to set the hotplate temperatures and the cantilever feedbackarameters. Additionally, the digital part hosts the communi-ation interface, an I2C bus-interface, which features a robustommunication protocol and requires only two connection lineso that the overall number of connection lines (wire bonds) isery low.
.1. Resonant cantilever and feedback circuitry
The resonant cantilevers are 150 �m long and 140 �m wide,xhibit a quality factor of approximately 1000 in air at 400 kHzesonance frequency and feature electromagnetic excitation andiezoresistive read-out. The electromagnetic excitation is basedn the Lorentz force and requires a small permanent magnetn the package underneath the cantilever; for details, see Fig. 6nd Refs. [30,31]. The vibration of the magnetically excitedantilever is detected by a set of four stress-sensitive MOS tran-istors (two transistor gates parallel to the cantilever axis, whichre severely deformed, and two gate regions perpendicular tohe cantilever axis, which are hardly deformed) in a Wheat-tone bridge configuration located at the cantilever base, whichre biased in a linear region. Compared to another widely usedxcitation method, namely thermal excitation, this method fea-ures the advantage of low power dissipation (1.3 mW), whicheads to a significantly lower temperature (temperature increasenly 1–2 ◦C above ambient temperature) on the cantilever asell as in the sensitive polymer layer. Since, the quantity of
bsorbed analyte in the polymer is inversely proportional to theemperature, a reduction of the power dissipation on the can-ilever yields an enhanced chemical sensitivity of the device31].
Upon analyte absorption in the chemically sensitive polymern the silicon cantilever, the oscillating mass increases. As aonsequence the resonance frequency of the system decreases.his causes a negative frequency shift, �f [Hz]. The sensitivityof a polymer-coated cantilever is given by:
= �f
�cA= GCanthKcMA (1)
ere, f denotes the mechanical resonance frequency of the can-ilever and cA is the analyte concentration in the gas phase. Eq.
dcif
Fig. 3. Block diagram of the cantilever feedback circuitry.
1) includes a summary term for the mechanical properties of theantilever, GCant; see also [32]. The sensitivity is proportionalo the polymer layer thickness, h (for h < 5 �m) and the partitionoefficient, Kc, as well as to the molecular mass of the absorbednalyte, MA. Swelling effects and analyte-induced changes inhe elastic modulus of the polymer have been neglected sincenly very low analyte concentrations have been applied [32].
The cantilever mechanical resonator and the associated feed-ack circuitry form an oscillator. The high quality factor of theantilever in the range of 1000 in air relaxes the specificationsor the feedback circuitry, since the cantilever acts as a band-passlter with an extremely narrow pass band. However, amplitudeondition and phase condition still have to be met to achieve atable oscillation, which is the most important issue in designingantilever oscillator circuits.
The block diagram (see Fig. 3) illustrates the architecture ofhe feedback circuitry and the comparator, which converts theinusoidal oscillation signal into a square wave that serves ashe input of the digital counter. The variable-gain amplifier [33],hich is based on a differential difference amplifier (DDA) [34],rovides a tunable gain between 30 and 45 dB by changing onef the bias currents. The possibility to adjust the loop gain is verymportant because the variation in cantilever properties, such asesonance frequency, quality factor, and peak gain, can reach aaximum of 10% as a consequence of the fabrication spread.n all-pass filter that acts as a phase shifter adjusts the total loophase. Due to the high resonator Q-factor, the system can benefitrom the natural steep slope of the phase response so that a phaseuning with a step size of 10◦ is enough to achieve the requiredrequency stability of 0.1 Hz. Moreover, the narrow pass-bandlter function of the cantilever helps to remove noise so that andditional band-pass filter noise reduction is not necessary here.he oscillation amplitude is regulated by the nonlinear transcon-uctance to achieve a stable operation, the gain of which is aunction of the input signal amplitude. At a defined input levelhe gain decreases with increasing input signal amplitude, whichntails that the amplitude of oscillation builds up to the point, athich this nonlinear block decreases the loop gain to unity. Theonlinear transconductance is followed by a class-AB buffer to
rive the low-resistance coil. The frequency readout circuitryonsists of the comparator and a 24-bit digital counter includedn the digital part of the system. An analog multiplexer allowsor sharing the feedback loop and the frequency read-out mod-
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34 Y. Li et al. / Sensors and A
le for the two cantilevers, which reduces chip area and poweronsumption. The respective cantilever is activated by connect-ng it to the feedback circuitry through the multiplexer, then,he oscillation frequency is measured. This process is sequen-ially applied to the two cantilevers. The design is a trade-offetween intended low chip area and power consumption and thechievable sensor response time.
.2. Capacitive sensor array and readout circuitry
The capacitive sensor is based on two sets of interdigitatedlectrode structures, which correspond to the two plates of atandard capacitor (Fig. 4). The sensor monitors changes in theielectric coefficient of the polymer upon analyte absorption.he capacitors are fabricated using exclusively layers and mate-
ials available in a standard CMOS process. One of the electrodess made from the first CMOS metal layer, and the other is realizeds a stack of the first and second metal layer. The dimensions ofhe capacitor are 814 �m × 824 �m, and it includes 128 fingerairs. The electrode width and spacing are 1.6 �m.
The nominal capacitance of the interdigitated capacitor isfew picoFarad, whereas capacitance changes upon analyte
bsorption are in the range of a few attoFarad. Thus, a ded-cated on-chip measurement configuration and specific signalonditioning circuitry is needed. The sensor response is readut as a differential signal between a passivated reference andpolymer-coated sensing capacitor. A digital output signal is
hen generated by comparing the minute loading currents of bothapacitors using a fully differential second-order Sigma–Delta-odulator circuitry [35]. The modulator provides a pulse densityodulated output that can be decimated by using a frequency
ounter. Thus, the output signal is a frequency change, whichs linearly proportional to the capacitance change upon analytebsorption in the polymer (see Eq. (2) below).
For thin polymer layers the swelling of the polymer uponnalyte absorption always results in a capacitance increase
egardless of the dielectric constant of the absorbed analyte.his is due to the increased polymer/analyte volume within theeld line region exhibiting a larger dielectric constant than thatf the substituted air [36,37]. Thin polymer layers include layer
flTt
Fig. 4. Block diagram of the capac
rs B 126 (2007) 431–440
hicknesses of less than half the periodicity of the electrodes.n the other hand, the capacitance change for a polymer layer
hickness larger than half the periodicity of the electrodes isetermined by the ratio of the dielectric constants of analyte andolymer. If the dielectric constant of the polymer is lower thanhat of the analyte, the capacitance will be increased, and, if theolymer dielectric constant is larger than that of the analyte, theapacitance will be decreased. This effect has been previouslyetailed and supported by simulations [36–38]. For thick poly-er layers the sensitivity, S, is the change in capacitance, �C,
n dependence of the change in the analyte concentration, �cA,s given by:
= �C
�cA= GcapKc �ε (2)
here Gcap includes the capacitor geometry. The partition coef-cient, Kc, includes the polymer/analyte interactions, and �ε is
he change in the dielectric properties of the polymeric matrixpon analyte absorption. More details on capacitive sensing andq. (2) can be found in Refs. [35–38].
.3. Microhotplate sensor and circuitry
Microhotplates usually feature metal-oxide-based coatingshat have to be operated at temperatures between 200 and00 ◦C [39–42]. Resistance changes of the sensitive materialpon gas exposure produce the sensor signal. The microhotplates located on a micromachined membrane to thermally isolatet from the rest of the chip. Each hotplate features a resistiveolysilicon ring-heater, which provides symmetric heat genera-ion and dissipation [43]. The polysilicon temperature sensor onhe membrane is used to monitor the hotplate temperature androvides the feedback signal for the temperature control loop43,44].
Two microhotplate-based sensors and the necessary drivingnd signal-conditioning circuitry are integrated on the chip. They
eature platinum electrodes and are coated with a SnO2 sensitiveayer, which is operated at temperatures between 200 and 350 ◦C.he on-chip temperature controller regulates the temperature of
he membrane up to 350 ◦C with an accuracy of ±2 ◦C, whereat
itive sensor readout scheme.
Y. Li et al. / Sensors and Actuators B 126 (2007) 431–440 435
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he deviations at room temperature are relatively small and devi-tions of up to 2% occur in the range between 280 and 350 ◦Cue to the nonlinearity of the polysilicon temperature resistor.he two hotplates can be independently heated and controlled
o achieve the best possible sensitivity to a variety of analytes,.e., two temperature controllers are implemented on the chip.esides sufficient control accuracy the controller should con-
ume as little chip area as possible and may not interfere withther integrated transducers or circuitry units. The analog pro-ortional temperature controller that has been implemented onhe chip using an operational amplifier meets these requirements.he control voltage for setting the hotplate temperature can berogrammed through the digital interface and on-chip 10-bit-igital analog converters as shown in Fig. 5. The signal of theolysilicon temperature sensor can be also read out via the digi-al interface so that the hotplate temperature can be continuously
onitored. A more precise temperature control can be realizedsing an additional control loop.
The resistance of the SnO2 sensitive layer and the gas-inducedesistance changes can vary over a wide range between 1 k�
nd 10 M� (four orders of magnitude). Therefore, the resis-ance is measured using a logarithmic converter (Fig. 5), whichs implemented with a voltage-to-current converter and a pairf diode-connected vertical PNP transistors. The sensor signals multiplexed to the input of a single 10-bit analog-to-digitalonverter to save chip area and to reduce the power consumption.
. Experimental
.1. System fabrication
During completion of the industrial CMOS process sequence,he standard CMOS-passivation in the microhotplate area islready opened to establish contact between the platinumlectrode metallization and the CMOS aluminum. A pho-olithography step is used to define the size of the Pt electrodes,hen 50 nm Ti/W and, afterwards, 100 nm Pt are sputtered onto
he wafer through a shadow mask to ensure locally defined metaleposition. The electrodes are then fabricated using a lift-off pro-ess. The n-well-membrane of the mass-sensitive cantilevers andhe thermally insulated island structure of the metal-oxide based
Mtsa
ture control and resistance readout circuitry.
ensors are released simultaneously. This is done by anisotropicilicon etching with KOH (potassium hydroxide) from the backf the wafer with an electrochemical etch stop technique thattops at the n-well of the CMOS process. Then, the cantileversre released by two front-side reactive-ion-etching (RIE) steps,hich are used to release the cantilevers, i.e., to remove theielectric layers of the CMOS process and the fraction of theilicon n-well, which does not form a part of the cantilever.he resulting cantilevers are 8.7 �m thick and are composedf the dielectric layers (silicon oxide 2.2 �m, silicon nitride.0 �m) of the CMOS process on top of the silicon n-well layer5.5 �m). They are rather stiff and exhibit a force constant of00 N/m. The wafers are then diced using a protective foil overhe microstructures.
.2. Sensor packaging and coating
A package including a small permanent magnet has beeneveloped in order to perform chemical measurements with thelectromagnetically actuated cantilever sensors. The permanentagnet was placed underneath the chip, so that the polymer-
oated front side of the cantilever can be exposed to differentnalytes. A standard ceramic dual-in-line package was modifiedn a way that the bottom part was replaced with an aluminumlock. After placing the permanent magnet into a cavity in thisluminum block, the chip has been glued on the aluminum blockith the cantilever right on top of the rare-earth magnet. A cross-
ection is shown in Fig. 6. An important aspect of the packagingesign includes the selection of a magnetic material that gener-tes a strong magnetic field and the placement of this magnet aslose as possible to the cantilevers. Other issues that have beenonsidered in designing the system floor plan (sensor-packageo-design, see Fig. 7a) concern the gas flow direction over heatedhotplates) and nonheated (capacitor, cantilever) transducers andhe openings for gas exposure. Here the gas flow is first over theonheated area and then the heated area to avoid that temperatureuctuations influence or upset the polymer sorption processes.
oreover, all sensors and transducers are placed on one side of
he chip, whereas the electronics are placed on the opposite sideo that the electronics can be covered and are not exposed tonalytes.
436 Y. Li et al. / Sensors and Actuato
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ig. 6. Cross-sectional schematic of the chip system and the permanent magnetn the package.
The electrical interconnections have been made by wire bond-ng. To protect the bond wires and the circuitry, the packagedhip has been then partially covered with a glob-top epoxyncapsulant as illustrated in Fig. 7b. First, a dam of low-viscositypoxy has been placed by means of a dispenser along metal lineeatures on the chip, and, then, the circuitry part of the chip haseen covered using a higher-viscosity epoxy. Partial coveragef the chips with epoxy provides a good protection of the elec-ronics, while still enabling free access of the analyte gas to theensitive area of the sensors.
After completion of the post-CMOS micromachining stepsnd after packaging, the chips and transducers have been coatedith the sensitive layers. Polymers are widely used as a sen-
itive layer for the detection of VOCs. Standard polymersuch as the slightly polar poly(etherurethane) (PEUT) and theonpolar poly(dimethylsiloxane) (PDMS), both available fromluka, Buchs, Switzerland, have been used here and have beeneposited on the cantilevers and capacitive sensors by spray-oating using an airbrush method and shadow masks. For the
icrohotplates, the nanocrystalline SnO2 doped with 0.2 wt%d was deposited onto the hotplates using a drop-coating method45,46]. The minute Pd-content entails a large sensitivity toarbon monoxide.
stt
ig. 7. (a) System layout considerations (system-package co-design): magnet and mab) Micrograph of the packaged system chip.
rs B 126 (2007) 431–440
.3. Gas manifold
For gas tests, the CMOS chips were mounted on dual-in-lineackages and then loaded into the measurement chamber of aomputer-controlled gas manifold featuring a cross-over flowrchitecture. This cross-over flow architecture has two input gasines, one supplying a pure carrier gas and the other supplyingcarrier gas with defined doses of the volatile analyte, and twoutput gas lines, one leading to the measurement chamber andhe other leading directly to the exhaust. This architecture offershe advantage that both input flows and both output flows areontinuously flowing and the build-up time of a certain analyteoncentration does not influence the dynamic sensor responses.he overall gas volume between the valve and the sensors waspproximately 1.6 ml, which entails a time span of approxi-ately 0.5 s after switching the valve until the gas reaches the
ensors at the applied flow rate of 200 ml/min. The analyteapors were generated from specifically developed temperature-ontrolled (T = 223–293 K) vaporizers using synthetic air as aarrier gas, and then diluted as desired using computer-drivenass-flow controllers. The internal volume of these vaporizers,hich distribute the liquid over a large-area packed-bed type
upport to maximize the surface-to-volume ratio, was dramati-ally smaller than that of typical gas-washing bottles (bubblers)47]. The vapor-phase concentrations at the respective temper-tures were calculated following the Antoine equation [48]. Ahotoacoustic detector (infrared light for excitation, 1314 Pho-oacoustic Multi-gas Monitor, Innova Airtec Systems, Denmark)s used as an independent reference to assess the actual analyteas-phase concentrations. The sensor measurements were per-ormed in a thermo-regulated chamber at a temperature of 303 K.oth gas streams (pure carrier gas and carrier gas with analyte)ere thermostabilized at the measurement chamber temperatureefore entering the chamber. The response time of the sensors
econds. Typical experiments consisted of alternating exposureso pure synthetic air and analyte-loaded synthetic air. Exposureimes of 10–15 min to analyte-loaded gas (to reach thermody-
gnetic field for the cantilevers; gas and thermal flow, and gas exposure opening.
Y. Li et al. / Sensors and Actuators B 126 (2007) 431–440 437
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ig. 8. Sensor responses of the cantilevers (bottom part) and capacitive sensornalyte concentrations included 500–2500 ppm, up and down. The cantilever heen coated with a 1.4-�m-thick PEUT-layer.
amic equilibrium) were followed by 10–15 min purging thehamber with pure synthetic air.
The selected analytes included standard organic solventsnd used as purchased from Fluka, Buchs, Switzerland withouturther purification (n-octane, toluene, ethanol). The inorganicases like carbon monoxide were dosed from gas cylinders ofefined analyte gas concentration.
.4. Measurement results
As already mentioned in the introduction, the system is veryersatile and can provide a wealth of information that can besed for a certain detection problem at hand. In the followinge will try to give an idea on how the system can effectively besed in different application scenarios.
The first example is the detection of simple organic volatiles:thanol and toluene. The two cantilevers are coated with differ-nt polymers, the nonpolar PDMS and the slightly polar PEUT.he capacitors featuring interdigitated electrodes include one
eference capacitor protected by a passivation layer and twoensing capacitors; see Fig. 4. The two sensing capacitors areoated with PEUT layers of different thickness, 1.4 �m and�m. By connecting two of these three capacitors to the inputf the Sigma–Delta modulator, a differential measurement cane realized. For the organic-volatile-detection task, we connecthe thick-layer PEUT capacitor C1 and the reference capacitor
ref to the Sigma–Delta converter. Some of the sensor results areisplayed in Fig. 8a–d. The frequency responses of the PDMS-oated cantilever (0.5 �m thickness) upon exposure to variousoncentrations of toluene and ethanol are shown in Fig. 8c and
. Since, the molecular weight of toluene is larger than that ofthanol, the sensor signal upon exposure to toluene is higherhan upon exposure to ethanol. However, it is not possible toistinguish low concentrations of toluene from high concentra-
sc
h
er part) upon exposure to various concentrations of ethanol and toluene. Theen coated with a PDMS layer of 0.5 �m thickness. The capacitive sensor has
ions of ethanol. Here, the results of the capacitive sensor willelp. The frequency responses of the thick-layer capacitive sen-or upon exposure to these two analytes are shown in Fig. 8a and. Ethanol with a dielectric constant of 24.3, which is larger thanhat of the polymer PEUT (4.8), causes a capacitance increasend, hence, a positive frequency shift, whereas toluene with aielectric constant of 2.36 causes a capacitance decrease andnegative frequency shift. Hence, toluene and ethanol can be
asily differentiated using the multi-transducer chip. Of course,he signals of the microhotplates (not shown) can be additionallysed as input for pattern recognition or multicomponent analysisools.
A second example concerns the detection of carbon monoxideCO) on a background of changing humidity. For this scenario,e use the microhotplates and the capacitive sensor, which acts
n this case as a humidity sensor. The microhotplate covered withhe Pd-doped nanocrystalline SnO2 (0.2 wt% Pd) was heatedo 275 ◦C, and the sensors were exposed to different analyteoncentrations. Metal-oxide based sensors are highly sensitiveo inorganic gases such as CO, but exhibit a significant cross-ensitivity to humidity [39,41]. The measurements were carriedut with varying relative humidity as shown in Fig. 9. The respec-ive SnO2 sensor response amplitudes increase with increasingumidity, and the sensor baseline also shifts. Fig. 9b shows theignal as recorded from the capacitive sensor at the same time.he CO is too volatile to be enriched in a polymeric layer so that
he capacitive sensor exclusively monitors the changing humid-ty. The co-integration of a capacitive sensor, which is highlyensitive to humidity (water has a dielectric coefficient as highs 78), allows for taking into account the humidity influence
o that the cross-sensitivity of the hotplate to humidity can beompensated for in the subsequent data processing procedure.
Humidity will, due to its high dielectric coefficient, alsoave a major impact on any capacitive organic-volatile mea-
438 Y. Li et al. / Sensors and Actuators B 126 (2007) 431–440
Fig. 9. Sensor responses upon dosage of different CO concentrations at different humidity levels (10, 20 and 40% relative humidity): (a) microhotplate responses; (b)capacitive sensor responses. The hotplate has been coated with nanocrystalline SnO2 containing 0.2 wt% Pd, the operating temperature was 275 ◦C. The capacitivesensor has been coated with a 1.4-�m-thick PEUT layer.
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ig. 10. Frequency shifts of capacitive sensors exposed to relative humidity (100, 450 and 600 ppm). (a) The left curve displays the difference signal of one signal of two sensor capacitors coated with the same polymer at different thickn
urement. Therefore, we want to demonstrate in a third exampleow organic volatiles can be measured even on a backgroundf humidity or changing relative humidity. The capacitive sen-or exhibits high sensitivity to humidity. However, its sensitivityowards analytes with a high dielectric constant shows a sensitiv-ty maximum at a relatively low polymer layer thickness around–1.5 �m, whereas for analytes with lower dielectric constanthis sensitivity maximum varies considerably out to greater layerhickness (3–3.5 �m, for details see [36]). Hence, the signal dif-erence of two capacitors with different layer thicknesses in theange of 1.5–5 �m is almost insensitive to water but retains sensi-ivity to low-dielectric-constant analytes like toluene or n-octane36]. This is evident from Fig. 10, which shows the results aseasured in the standard configuration with C1 and the ref-
rence capacitor Cref connected to the Sigma–Delta converterFig. 4): large positive humidity signals and comparatively low-evel negative organic volatile signals. In Fig. 10b, the signalschieved with C1 and C2 connected to the Sigma–Delta con-erter (Fig. 4) are displayed. The humidity signal amplitudesre drastically reduced in the differential signal whilst the tworganic volatiles still show distinct and clear capacitive signals.
Finally we will briefly discuss the reproducibility of theesults and the devices. The device-to-device repeatability con-
erning the CMOS fabrication and micromachining is veryood. Within a wafer run, the devices are almost identical, theantilever frequencies, e.g., vary from 380 to 405 kHz, withhe center frequencies being at 395 kHz, i.e., the production
boaL
30, 40 and 50%), toluene (400, 800, 1200 and 1600 ppm), and n-octane (150,capacitor and the reference capacitor. (b) The right curve shows the differential
pread is ± 3%. Similar considerations hold for the polysili-on temperature sensor and the microhotplates. Nevertheless aevice-by-device calibration is necessary as it is common alsoor, e.g., commercially available humidity sensors. The mainontribution to variations in the sensor characteristics is due tohe sensitive layers. The reproducibility and long-term stabilitys, in general, better for the polymer-based sensors as comparedo the metal oxide sensors. For both sensitive materials, the trial-o trial reproducibility of the same device within a week shows a
aximum variation of 5%. For the polymer-based sensors, long-erm measurements evidenced a signal variation of ±10% overyear, while for metal-oxide-based sensors, the initial material
esistance can widely vary, and signal variations up to ±30%ver a year have been observed.
. Conclusion
In summary a very versatile and easy-to-use single chip sys-em has been presented that can be applied to a multitude ofetection tasks and scenarios, only a few of which have beenemonstrated here. The simultaneous detection of organic andnorganic gases, even in more complex mixtures, can be achievedy the combination of different polymer-based and metal-oxide-
ased sensors. The multi-transducer system provides a wealthf information that can be used to improve gas identificationnd quantification. The end-user can access the system via aabviewTM interface and can program all relevant parameters,
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Y. Li et al. / Sensors and A
.e., the complexity of the system is shifted to the system devel-pment level and is not really visible for the end-user. Theow-power design of the system enables its application in hand-eld devices.
cknowledgments
The authors thank Professor Henry Baltes for sharing labora-ory resources and for his ongoing interest in their work. Theoating of the microhotplates with the sensitive metal-oxideayers was performed by AppliedSensor GmbH, Reutlingen,ermany, Dr. Stefan Raible and Dr. Jurgen Kappler. This projectas been financially supported by the Commission of Technol-gy and Innovation, Bern, Switzerland under contract numberTI 5670.1.
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iographies
ue Li received her MS and PhD in electrical engineering from ETH Zurich in001 and 2005, respectively. Her research interests included analog integratedircuit design, and the design of microsensor systems. Currently, she is workingn circuit designs for image sensing systems.
yril Vancura studied physics at the University of Kaiserslautern, Germanynd the Swiss Federal Institute of Technology (ETH) in Zurich, Switzerland andeceived his diploma from the ETH Zurich in 2001. After his diploma, he joinedhe Physical Electronics Laboratory at ETH as a PhD student and received hishD in 2005. Since 2006, he is working at the Research and Technology Center ofobert Bosch LLC, Palo Alto, CA. His research interests include microstructures
or chemical and biological sensing.
iego Barrettino received the diploma in electronic engineering from the Uni-ersity of Buenos Aires, Argentina, in 1997 and the PhD degree in electricalngineering from the Swiss Federal Institute of Technology (ETH) Zurich,n 2004. From 2004 to 2005, he was a postdoctoral research associate in the
icroscale Life Sciences Center, Department of Electrical Engineering, Uni-ersity of Washington, Seattle, USA. From 2005 to 2006, he was a tenure-trackssistant professor in the Department of Electrical Engineering at the Universityf Hawaii, Honolulu, USA. He joined the Integrated Systems Laboratory, Swissederal Institute of Technology (EPF) Lausanne as senior research scientist ineptember 2006 where he is working on lab-on-a-chip microsystems for canceresearch.
arkus Graf received the degree in physics from the University of Konstanz,onstanz, Germany, in 1999. He was a research assistant at the Department oficro-and Nanotechnology (MIC), Lyngby, Denmark, and received the PhD
rom the Swiss Federal Institute of Technology, Zurich, Switzerland, in 2004.
TiZm
rs B 126 (2007) 431–440
ince 2005, he is with Sensirion AG, Staefa, Switzerland, where he heads the&D group for humidity sensors. His research is focused on CMOS-based
ensor systems and related microfabrication technologies.
hristoph Hagleitner obtained a diploma degree and a PhD degree in Electricalngineering from the Swiss Federal Institute of Technology (ETH), Zurich in997 and 2002, respectively. During his PhD work, he specialized in interfaceircuitry and system aspects of CMOS integrated micro- and nanosystems. Aftereceiving the PhD degree in 2002 with a thesis on a CMOS single-chip gasetection system he headed the circuit-design group of the Physical Electronicsaboratory at the ETH Zurich. In 2003, he joined the IBM Zurich Researchaboratory in Ruschlikon, Switzerland, where he works on the analog-frontendesign and system aspects of a novel probe storage device.
drian Kummer graduated 1999 in physics at ETH Zurich, Switzerland. Heeceived his PhD in applied physics, also from ETH Zurich in 2004. The topicf his thesis was capacitive chemical microsensors for the detection of volatilerganic compounds. Currently, he is employed in the Research & Technologyepartment of Kistler Instrumente AG, Winterthur, Switzerland. His present
esearch includes pressure sensors and optical sensors for harsh environments,specially for extreme temperatures and pressure ranges.
artin Zimmermann received the diploma degree in electrical engineeringrom the University of Applied Sciences, Rapperswil, Switzerland, 1996. From996 to 2005, he was involved in the development of mixed-signal circuitry forMOS sensor systems at the Physical Electronics Laboratory of ETH Zurich,witzerland. In 2006, he joined the Camille Bauer AG, Wohlen, Switzerland,here his current activities are focused on capacitive angular displacement
ensors.
ay-Uwe Kirstein received the diploma in electrical engineering from the Uni-ersity of Technology Hamburg-Harburg (TUHH), Germany, in 1997 and thehD degree from the University of Duisburg, Germany, in 2001. He was aesearch Associate with the Fraunhofer Institut of Microelectronic Circuits andystems in Dresden and worked in the analog circuit design group at MicronasmbH, Freiburg, Germany. Afterwards he has been team leader of the circuitesign group at the Physical Electronics Laboratory, ETH Zurich and is nowith Miromico AG, Zurich, Switzerland.
ndreas Hierlemann received his diploma in chemistry in 1992 and the PhDegree in physical chemistry in 1996 from the University of Tubingen, Ger-
X (1997), and Sandia National Laboratories, Albuquerque, NM (1998), hes currently associate professor at the Physical Electronics Laboratory at ETHurich in Switzerland. The focus of his research activities is on CMOS-basedicrosensors and on interfacing CMOS electronics with electrogenic cells.
