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774 IEEE SENSORS JOURNAL, VOL. 5, NO. 4, AUGUST 2005Moores Law in Homeland Defense: An IntegratedSensor Platform Based on Silicon MicrocantileversLal A. Pinnaduwage, Member, IEEE, Hai-Feng Ji, and Thomas ThundatAbstractAn urgent need exists for the development of inex-pensive, highly selective, and extremely sensitive sensors to helpcombat terrorism. If such sensors can be made miniature, theycould be deployed in virtually any situation. Terrorists have a widevariety of potential agents and delivery means to choose from forchemical, biological, radiological, or explosive attacks. Detectingterrorist weapons has become a complex and expensive endeavor,because a multitude of sensor platforms is currently needed to de-tect the various types of threats. The ability to mass produce andcost effectively deploy a single type of sensor that can detect a widerange of threats is essential in winning the war on terrorism. Sil-icon-based microelectromechanical sensors (MEMS) represent anideal sensor platform for combating terrorism because these minia-ture sensors are inexpensive and can be deployed almost anywhere.Recently, the high sensitivity of MEMS-based microcantilever sen-sors has been demonstrated in the detection of a variety of threats.Therefore, the critical requirements for a single, miniature sensorplatform have been met and the realization of an integrated, widelydeployable MEMS sensor could be near.Index TermsChemical, biological, radiological, or explosive(CBRE) detection, homeland defense, microcantilever, microelec-tromechanical sensors (MEMS) sensor, terrorism.I. INTRODUCTIONTERRORISTS have a huge economic advantage over law en-forcement because it is, many times, more expensive to de-tect terrorist threats than it is to deploy terrorist threats. For ex-ample, a crude explosive device can destroy an airplane in flight.On the other hand, current explosive detection technologies de-ployed at airports are expensive and require constant operator at-tention.Achemicalorbiologicalattack,whichcanalsobecarriedout with nominal cost and effort, might even go unnoticed untilinjured people start turning up at hospitals. On the nuclear side, a“dirty bomb,” which uses radioactive material that will be spreadusing a conventional bomb, is another likely threat.Even though sensitive detection of individual threats may becurrently possible, such techniques/sensor systems are bulky,expensive, and require time-consuming procedures. Also,Manuscript received February 3, 2004; revised September 2, 2004. This workwas supported in part by the Bureau of Alcohol, Tobacco, and Firearms (ATF),in part by the National Safe Skies Alliance, in part by the Department of Home-land Security, in part by the Department of Energys NA-22 program, in part bythe Environmental Management Science Program, and in part by the Office ofBiological and Environmental Research Program. The associate editor coordi-nating the review of this paper and approving it for publication was Dr. TimothySwager.L. A. Pinnaduwage and T. Thundat are with the Life Sciences Division, OakRidge National Laboratory, Oak Ridge, TN 37831-6122 USA, and also with theDepartment of Physics, University of Tennessee, Knoxville, TN 37996-1200USA (e-mail: pinnaduwagle).H.-F. Ji is with the Institute for Micromanufacturing, Louisiana Tech Univer-sity, Ruston, LA 71272 USA.Digital Object Identifier 10.1109/JSEN.2005.845517detection of multiple threats requires the use of a variety ofspecialized instruments based on different technologies/sensorplatforms. Therefore, a paradigm change in sensor technology isrequired for combating the war on terrorism. Ideally, homelanddefense requires a sensor system with the following features: 1) asingle-sensor platform that can detect multiple threats simulta-neously and rapidly; 2) an inexpensive, miniature, and robustsensor system that can be deployed almost anywhere; and (3)built-in telemetry for data transmission and networking. None ofthe available technologies satisfy these conditions. However, theemerging sensor technology based on MEMS has the promise ofsatisfying these conditions. Such miniature sensors could be de-ployed anywhereairports, seaports, public buildings, strategiclocations in waterwaysproviding omnipresent protection.The technical advances in a variety of fields, including com-puting, interphase chemistry, and telemetry, have maturedenough to be incorporated into MEMS sensor technology. Thus,the MEMS sensor platform is poised for such a revolution insensor technology. For example, present sensor technologycan be compared with the status of computer use in the 1960swhen any serious computing was restricted to giant computersinstalled in a handful of institutions. But the “silicon revolution,”expressedsuccinctlyinMooresLaw1,hasenabledwidespreadcomputer use. Currently, expensive and bulky detection systemsare sparsely deployed at strategic locations, such as airports. Ananalogous revolution in sensor technology may be possible witha sensor platform based on MEMS that will allow deploymentof intelligent, miniature sensors by the millions. Besides thewar against terrorism, such a sensor platform would be usefulin medical diagnostics, law enforcement, landmine detection,environmental monitoring, and many other applications.Therefore, the primary issue can be stated as follows: A needexists for rapid detection of trace quantities of a wide varietyof threat agents present in complex mixtures using miniature,inexpensive sensors. Here we discuss the current status of re-search on achieving this goal with sensor arrays and point outthat microcantilever MEMS sensors provide a suitable platform.It must be noted that this paper is not intended to be a compre-hensive review paper. We will briefly review the current statusof sensor arrays and refer to selected papers on microcantilverand other sensor technologies. Our intention is to point out thepossibility of achieving a miniature sensor platform for home-land defense based on microcantilever sensors.II. SIMULTANEOUS AND RAPID DETECTIONOF MULTIPLE ANALYTESFor just over two decades, research has been conducted ondevelopment of an “electronic nose” based on sensor arrays1530-437X/$20.00 2005 IEEEntsPINNADUWAGE et al.: MOORES LAW IN HOMELAND DEFENSE 77526. In 1982, Persaud and Dodd 7 published the first paperon a modern electronic nose that attempts to mimic the olfac-tory system: They pointed out 7 that the mammalian olfactorysystem is based on broadly tuned receptor cells, and that the dis-crimination properties of the olfactory system are a property ofthe system as a whole. The olfactory receptors are not highly se-lective toward specific odorants; each receptor responds to mul-tiple odorants, and many receptors respond to any given odorant8. Pattern recognition methods are thought to be a dominantmode of olfactory signal processing. The electronic nose tech-nologies are based on the same concept of using broadly tunedmultiple sensors. An advantage of this approach is the ability todetect a variety of analytes simultaneously.Several possible sensor array platforms have been studied upto now 2, 9 as candidates for an electronic nose, includingthose based on metal oxide, MOSFET, conductive polymer,fiber-optic, electrochemical, and acoustic wave sensors. Thesesensor technologies are described in detail by Gardner andBartlett 6. A wide variety of statistical and neural networktechniques have been employed to process the data originatingfrom the sensor arrays 6.It is not surprising that one of the first applications of elec-tronic noses based on sensor arrays has been for the evalua-tion of odors and for industrial process control 6. Sensor ar-rays have been shown to be successful in these applications inwhich the primary interest is in qualitative analyzes that relyon changes in the sensor-array response patterns. Commercialinstruments based on sensor arrays are available for these ap-plications 6. The commonly used sensor technologies in theseapplications are metal-oxide, conducting polymer, and acousticsurface acoustic wave (SAW) and QCM sensors 6. Metaloxide sensors are bulky, but they have fairly good sensitivitiesof sub-parts-per-million (ppm) levels; conducting polymer sen-sors are small with low-power consumption, but the sensitivitiesare generally an order of magnitude lower compared with metaloxide sensors; SAWs sensors derive their vapor-detecting capa-bility from sorbent coatings and can detect the mass of the vaporadsorbed, which in some cases can be distorted by the changesof the viscoelastic properties of the coating material 8.However, the detection of trace amounts of vapors in mix-tures has not yet been successfully achieved with sensor arrays6, 10. Almost all sensor array studies conducted up to nowhave used sensors with detection limits at or near 1 ppm leveland a maximum of about 12 sensors in an array 6, 10. Betterdetection sensitivities and higher numbers of sensors per arraymay be needed to achieve trace detection in complex mixtures.A recent study 11 has concluded that increasing the number ofsensors in an array did not improve performance significantlyfor mixture analysis; a maximum of six sensors were used inthe array, and the detection limits of the SAW sensors used inthe array for the component vapors were at low ppm levels 11.Moreover, it has been noted that there are over 1000 olfactorygenes in humans and over 100 million olfactory cells in a ca-nines nose 2.It is likely that MEMS sensors may provide both high sensi-tivity and the ability to use a much higher number of individualsensors in an array, thus enabling the detection of trace vaporsin complex mixtures. The high sensitivity for MEMS sensorsoriginates in the inherently large surface-to-volume ratio of themicroscopic objects. Thus, a MEMS sensor based on surface in-teractions for signal transduction can be expected to provide anenormous amplification in sensitivity.The rapid development of the integrated circuit (IC) tech-nology during the past decade has initiated the fabrication ofchemical sensors on silicon or complementary metal oxidesemiconductor (CMOS) 1214. The largely two-dimen-sional integrated circuit and chemical sensor structures pro-cessed by combining lithographic, thin-film, etching, diffusive,and oxidative steps have been recently extended into the thirddimension using micromachining or MEMS technologiesacombination of special etchants, etch stops, and sacrificiallayers 12. Therefore, MEMS technology provides an excel-lent means to meet other key criteria of chemical sensors, suchas miniaturization of the devices, low-power consumption, andbatch fabrication at low cost as well.Currently, two CMOS-technology-based MEMS sensors arebeing studied, flexural plate wave (FPW) sensors and micro-cantilever sensors. The FPW sensor is similar in many ways tothe more common SAW and QCM sensors, and it can monitorthe mass absorbed on a coating deposited on the sensor 15,16. It has shown detection sensitivities of high parts-per-tril-lion (ppt) levels at the highest measured sensitivity level 15.On the other hand, the microcantilever sensors have additionaldetection modes 17, including the highly sensitive bendingmode that makes use of the high surface-to-volume ratio of theMEMS sensor element and often displays sub-ppt detection sen-sitivities as discussed in Section III.III. MICROCANTILEVER SENSORSIn 1994, researchers observed that the microcantilevers usedin atomic force microscopy (AFM) were sensitive to externalphysical and chemical influences. Thundat et al. 1821pointed out the possible use of bending and frequency shift inmicrocantilevers for chemical sensing, and Gimzewski et al.2224 pointed out applications in thermal calorimetry. Sincethen, researchers all over the world have been reporting the useof microcantilevers for detecting various physical, chemical,biological, and radiological influences 17, 25.Microcantilevers are miniature diving boards that are micro-machined from silicon or other materials. The length of thesecantilevers is often in the range of 100200 m, whereas thethickness ranges from 0.3 to 1 m. (It is interesting to note thatin the human olfactory system, the site of interaction of the cellwith odorous molecules occurs at hairlike cilia, which are upto 200- m long and provide an increased surface area for odorsensing 6.) The key to the high sensitivity of the microcan-tilevers is the enormous surface-to-volume ratio, which leads toamplified surface stress, as discussed below. Fig. 1 shows a setof microcantilevers together with a human hair for comparison.Microcantilevers have two main signal transduction methods,bending and mass-loading. In the mass-loading mode, micro-cantilevers behave just like other gravimetric sensors such asQCM, SAW, and FPW transducers: Their resonance frequenciesdecrease due to the adsorbed mass. In the “gravimetric mode,”nts776 IEEE SENSORS JOURNAL, VOL. 5, NO. 4, AUGUST 2005Fig. 1. Scanning electron microscope image of a set of cantilevers of different sizes and shapes. For comparison, a human hair is also shown.the microcantilever detection sensitivity seems to be compa-rable with that of the other gravimetric sensors 26.The other signal transduction method, that is, the bendingresponse, is unique to the microcantilever; for example, ifa differential surface stress is achieved by preferentially ad-sorbing target molecules to one of its broad surfaces (by usinga chemical coating on that surface), the microcantilever willbend. Since a differential surface stress is required for bending,only one broad surface should be coated for bending-modeoperation. Therefore, the mass-loading information is used onlyas a bonus. In the bending mode, microcantilever detectionsensitivity is at least an order of magnitude higher than otherminiature sensors such as SAW and QCM that are also beinginvestigated as chemical sensors. Even though it is difficultto accurately compare the detection sensitivities for differentsensors, the low- or below-ppt detection sensitivities routinelyachieved with microcantilever sensors 2732 have not beenmatched by any other sensor. The closest comparison wouldbe the detection of dinitrotoluene (DNT) using SAW 33and microcantilever 30 sensors, where both sensors usedthe same polymer coating SXFA-poly(1-(4-hydroxy-4-tri-fluoromethyl-5,5,5-trifluoro)pent-1-enyl)methylsiloxane. Anestimated 400-ppt detection sensitivity is achieved for 200-sexposure of DNT to the SAW sensor (33, Fig. 10), whereas300-ppt detection sensitivity was achieved with the microcan-tilever in a few seconds 30.Since the microcantilever bending signal originates in surfacestress, diffusion of large amounts of vapor to a thick coating isnot necessary. Thus, even though one-monolayer-thick self-as-sembled monolayer (SAM) coatings may not be appropriate forgravimetric sensors 34, they are ideally suited for microcan-tilever sensors. Because diffusion of the analyte vapor to thebulk of the coating is avoided, the response and relaxation of amicrocantilever coated with a SAM can be fast. Fig. 2 shows theresponse of a microcantilever coated with a SAM of 4-mercap-tobenzoic acid to a vapor stream of the plastic explosive pen-taerythritol tetranitrate (PETN) at a concentration of 1.4 ppb29. The rapid and sensitive response of the bending signal aswell as the superiority of the bending signal compared with themass (frequency) measurement is clear.Fig. 2. Response of a 4-mercaptobenzoic acid (4-MBA)-coated siliconcantilever to the periodic turning on (10 s) and off (60 s) of a PETN streamof 1.4-ppb concentration in ambient air. The solid curve depicts the bendingresponse, and the dots connected by dashed lines depict the resonance frequencyof the cantilever 29.It must be emphasized that the bending of the microcantileveris not caused by the weight of the deposited material. A 40-ngmicrocantilever bends about 1 nm due to its own weight, whichis just above the noise level for a cantilever-bending signal.Therefore, the microcantilever bending caused by the weight ofthe deposited material of picogram levels is insignificant. Onthe other hand, for micron-size objects like microcantilevers,the surface-to-volume ratio is large, and the surface effects areenormously magnified. Thus, adsorption-induced surface forcescan be extremely large. The adsorption-induced force can be at-tributed to the change in surface free energy due to adsorption.Free energy density (millijoule/square meter) is the same as sur-face stress (newton/meter). This surface stress is analogous tosurface tension in a liquid. Incidentally, surface stress has theunits of a spring constant of a cantilever. Therefore, if the sur-face free-energy density change is comparable with the springntsPINNADUWAGE et al.: MOORES LAW IN HOMELAND DEFENSE 777constant of a cantilever, the cantilever will bend. When probemolecules bind to their targets, steric hindrance and electrostaticrepulsions cause the bound complexes to move apart. Becausethey are tethered at one end and because the surface area is fi-nite, they exert a force on the surface.Another advantage of the microcantilever sensor is that itworks with ease in air and in liquid. Both resonance frequencyand bending modes can be used in liquid. Because of thesmall mass of microcantilevers, they execute thermal motion(Brownian motion) in air and liquid. Therefore, no externalexcitation technique is needed for exciting cantilevers into res-onance; the degraded quality factor in liquid can be improvedby a feedback mechanism 35, 36. However, as in the vaporphase, the microcantilever bending signal is mainly used forsensing in liquid (see, for example, 3739).Despite its high sensitivity, the cantilever platform offers nointrinsic chemical selectivity just like other chemical sensorssuch as SAW and QCM. One surface
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