Microfabrication Techniques for Chemical/Biosensors · other materials include BeO and AlN. The chemical in-ertness, biocompatibility, and mechanical stability make ceramics a very

Microfabrication Techniques forChemical/Biosensors

ANDREAS HIERLEMANN, OLIVER BRAND, CHRISTOPH HAGLEITNER,AND

HENRY BALTES, FELLOW, IEEE

Invited Paper

Microfabrication processes for chemical and biochemicalsensors are reviewed. Standard processing steps originatingfrom semiconductor technology are detailed, and specific mi-cromachining steps to fabricate three-dimensional mechanicalstructures are described. Fundamental chemical sensor principlesare briefly abstracted and corresponding state-of-the-art examplesof microfabricated chemical sensors and biosensors are given.The advantages and disadvantages of either fabricating devicesin IC fabrication technology with additional microfabricationsteps, or of using custom-designed nonstandard microfabricationprocess flows are debated. Finally, monolithic integrated chemicaland biological microsensor systems are presented, which includetransducer structures and operation circuitry on a single chip.

Keywords—Chemical sensors, CMOS, IC technology, integratedsensors, microelectromechanical systems (MEMS), microfabrica-tion.

I. INTRODUCTION

Microfabrication processes are used to produce deviceswith dimensions in the micrometer to millimeter range. Mi-crofabrication processes can be effectively applied to yielda single device or thousands of devices. The so-called batchprocessing, i.e., the fabrication of many devices in parallel,does not only lead to a tremendous cost reduction, but alsoenables the production of array structures or large deviceseries with minute fabrication tolerances. Microfabricationprocesses hence significantly differ from conventional, se-quential machining processes, such as drilling or milling withmechanical tools. IC fabrication processes are the most im-portant microfabrication processes [1]–[3]. The success of

Manuscript received September 5, 2002; revised February 3, 2003.A. Hierlemann, C. Hagleitner, and H. Baltes are with the Physical Elec-

tronics Laboratory, ETH Zurich, 8093 Zurich, Switzerland (e-mail: [email protected]; [email protected]; [email protected]).

O. Brand was with the Physical Electronics Laboratory, ETH Zurich,8093 Zurich, Switzerland. He is now with the School of Electrical andComputer Engineering, Georgia Institute of Technology, Atlanta, GA30332-0250 USA (e-mail: [email protected]).

Digital Object Identifier 10.1109/JPROC.2003.813583

CMOS technology, which is one of the enabling technologiesof the information age, clearly demonstrates the efficiency ofmicrofabrication technologies.

Standard processing steps originating from semiconductortechnology (see Section II) can be used in combinationwith dedicated micromachining steps (see Section III) tofabricate three-dimensional (3-D) mechanical structures,which form the basis for the chemical and biosensorsreviewed in this paper. A sequence of such microfabricationsteps, some of which are adapted from semiconductor fab-rication technology, and some of which include dedicatedmicromachining techniques, is arranged in a process flow.This sequence or process flow has to be detailed and defined(chronology of the steps) for fabricating a specific device.Key advantages of microfabricated chemical and biosensorsinclude small device size and sampling volume, the pos-sibility of batch processing, the optional cointegration ofelectronics, and the reproducibility of sensor characteristicsdue to the precise geometric control in the fabrication steps.Microfabrication techniques also can be used to eithersignificantly improve sensor characteristics (with respect toconventionally fabricated devices) or to develop devices withnew functionality, which cannot be realized in conventionalfabrication technology. Microsensor success stories, such asmicromachined pressure sensors and accelerometers, showthat microfabrication techniques are especially suitable forhigh-volume applications in, e.g., automotive industry. Inhigh-volume production, the advantage of batch processingis paramount, and the high development and setup costsamortize.

This review paper is organized as follows. Standard pro-cessing steps originating from semiconductor technology aredetailed in Section II, dedicated micromachining steps tofabricate 3-D mechanical structures are described in Sec-tion III. In Section IV, basic chemical sensor principles areabstracted and corresponding examples for microfabricatedchemical and biosensors are given. These exemplary devices

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Fig. 1. Flow diagram of an IC fabrication process using the fourbasic microfabrication techniques: deposition, photolithography,etching, and doping. Adapted from [1].

are either fabricated in available IC fabrication technologywith additional microfabrication steps or by using custom-designed nonstandard microfabrication process flows. Theadvantages and disadvantages of these two different fabri-cation approaches are discussed in Section V. Finally, Sec-tion VI presents monolithic integrated chemical and biolog-ical microsensor systems, which include sensing structuresand operation circuitry (sensor biasing, signal amplificationand conditioning, interfacing) on a single chip.

II. BASIC MICROFABRICATION PROCESSES

The four basic microfabrication techniques for chem-ical/biosensors are identical with those used in IC fabri-cation: deposition, patterning, doping and etching. Fig. 1illustrates how these techniques are combined to build upa device layer by layer. A thin layer, such as an insulatingsilicon dioxide film, is deposited on a substrate. A light-sen-sitive photoresist layer is then deposited on top and patternedusing photolithography. Finally, the pattern is transferredfrom the photoresist layer to the silicon dioxide layer by anetching process. After removing the remaining photoresist,the next layer is deposited and structured, and so on. Dopingof a semiconductor material by ion implantation can bedone directly after photolithography or after patterning animplantation mask (e.g., a patterned (sacrificial) silicondioxide layer).

In the following, a brief overview on common substratematerials and the four basic microfabrication steps will begiven. More details on fundamental microfabrication stepscan be found in books on semiconductor processing, e.g.[1]–[3].

A. Substrate Materials

Siliconis the standard substrate material for IC fabricationand thus the most common substrate material in microfabri-

cation. It is supplied as single crystal wafers with diametersfrom 100 to 300 mm. The use of silicon substrate materialenables the cointegration of transducers and circuitry, an ad-vantage which is explored, e.g., in CMOS-based microsys-tems [4]. Besides its favorable electrical properties, singlecrystal silicon also has excellent mechanical properties [5],which enable the design of micromechanical structures (seeSection III). A large number of micromachining techniqueshave been developed to structure silicon substrates [6]–[10].Consequently, silicon is also the most common substrate ma-terial for microfabricated chemical and biosensors.

Glassesexhibit attractive dielectric and optical properties.A number of micromachining techniques, such as isotropicwet etching or anisotropic dry etching, have been developedto structure glass. Glass is also supplied in wafer form in dif-ferent compositions (e.g., quartz, fused silica, and borosili-cate glass) and diameters. Since glass is transparent for vis-ible light, it is particularly suited for devices with optical de-tection principles. Single crystal quartz with its hexagonallattice structure is piezoelectric and is therefore used, e.g., assubstrate material for acoustic wave devices [11]. Last but notleast, glasses are chemically inert and suitable for high-tem-perature applications.

Ceramicshave been used extensively as substrate for hy-brid microelectronics and are common in microelectronicspackaging [12]. The standard material is alumina (AlO );other materials include BeO and AlN. The chemical in-ertness, biocompatibility, and mechanical stability makeceramics a very interesting material for microsystems. Mostmicrofabrication techniques for ceramic materials have beenadapted from microelectronics packaging processes. Anexample for the use of ceramics in microsensor applicationsis the wireless ceramic pressure sensor developed at theGeorgia Institute of Technology for use in high-temperatureapplications [13], which is currently further developed byCardioMEMS for biomedical applications [14].

Over the last years,polymershave been more and moreexplored as an inexpensive substrate material. Special pro-cesses, such as hot embossing, injection molding, laser ma-chining, or stereolithography, have been developed to struc-ture polymer materials even in the micrometer range. Due tothe cost advantage, disposable devices, such as microfluidicarrays or microstructured immuno or DNA assays, are oftenbased on polymers. An example includes the polymer-basedLabCard developed by ACLARA BioSciences [15].

B. Thin-Film Deposition

The two most common thin-film deposition methods inmicrofabrication arechemical vapor deposition(CVD),performed at low pressure (LPCVD), atmospheric pressure(APCVD) or plasma-enhanced (PECVD), andphysicalvapor deposition(PVD), such as sputtering and evaporating.Typical CVD and PVD film thicknesses are tens of nanome-ters up to a few micrometers. Other techniques includeelectroplating of metal films and spin- or spray-coatingof polymeric films such as photoresist. Both processescan yield film thicknesses from less than 1m to severalhundred micrometers.

840 PROCEEDINGS OF THE IEEE, VOL. 91, NO. 6, JUNE 2003

Dielectric layers, predominantly silicon dioxide and sil-icon nitride, are used as insulating material, as mask mate-rial, and for passivation. Silicon dioxide is either thermallygrown on top of a silicon surface (thermal oxide) at high tem-peratures in an oxidation furnace (900C to 1200 C), or it isdeposited in a CVD system (CVD oxide). CVD oxides canbe deposited at temperatures between 300C and 900 C,with the high temperature deposition usually yielding betterfilm properties. Silicon nitride layers are commonly used fordevice passivation and are deposited by CVD processes.

Metal layers are used, e.g., for electrical interconnects, aselectrode material, for resistive temperature sensors (ther-mistors), or as mirror surfaces. Metals, which are widelyused in the microelectronics industry, such as aluminum,titanium, and tungsten, are routinely deposited by sput-tering or by electron-beam evaporation. Depending on theapplication, a large number of other metals, including gold,palladium, platinum, silver, or alloys, can be deposited withPVD methods. A number of metals and metal compounds,such as Cu, WSi, TiSi , TiN, and W, can be depositedby CVD. Metal CVD processes are less common, but canprovide improved step coverage or local deposition ofmetals [3]. Whereas aluminum has been the standard met-allization in IC fabrication for many years, state-of-the-artsub-0.25 m CMOS technologies often feature copper asinterconnect material, due to its lower resistivity and higherelectromigration resistance as compared to aluminum.An example are IBM’s interconnect metallizations basedon the so-called damascene process [16], which employcopper films electroplated in a dielectric mold. After eachmetallization step, planarization is achieved with a chem-ical-mechanical polishing (CMP) step.

Highly doped polycrystalline silicon (polysilicon) isused as gate material for MOSFETs, as electrode andresistor material, for piezoresistive sensing structures, asthermoelectric material, and for thermistors. Polysiliconmicrostructures released by sacrificial layer etching arealso widely used in sensor applications (see Section III).Polysilicon is usually deposited in a LPCVD process usingsilane (SiH) as gaseous precursor.

Polymerssuch as photoresist are commonly deposited byspin- or spray-coating. Similar techniques are used to coat,e.g., chemical sensors with sensitive polymer films [17]. Al-ternatively, chemical or biological films can be deposited bydispensing or microcontact printing.

C. Patterning

Photolithography is the standard process to transfer a pat-tern, which has been designed with a computer-assisted de-sign (CAD) program, onto a certain material. The process se-quence is illustrated in Fig. 2. A mask with the desired patternis created. The mask is a glass plate with a patterned opaquelayer (typically chromium) on the surface. Electron-beamlithography is then used to write the mask pattern from theCAD data. In the photolithographic process, a photoresistlayer (photostructurable polymer) is spin-coated onto the ma-terial to be patterned. Next, the photoresist layer is exposed

Fig. 2. Schematic of a photolithographic process sequence forstructuring a thin-film layer.

to UV light through the mask. This step is done in a maskaligner, in which mask and wafer are aligned before the sub-sequent exposure step is performed. Depending on the maskaligner generation, mask and substrate are brought either incontact or close proximity (contact and proximity printing),or the image of the mask is projected (projection printing)onto the photoresist-coated substrate. Depending on whetherpositive or negative photoresist was used, the exposed or theunexposed photoresist areas are removed during the resist de-velopment process. The remaining photoresist acts as a pro-tective mask during the etching process, which transfers thepattern onto the underlying material. Alternatively, the pat-terned photoresist can be used as a mask for a subsequent ionimplantation. After the etching or ion implantation step, theremaining photoresist is removed, and the next layer can bedeposited and patterned.

The so-calledliftoff technique is a way to structure athin-film material, which would be difficult to etch. Here,the thin-film material is deposited on top of the patternedphotoresist layer. In order to avoid a continuous film, thethickness of the deposited film must be less than the resistthickness. By removing the underneath photoresist, thethin-film material on top is also removed by “lifting it off,”leaving a structured thin film on the substrate.

Thick photostructurable polymer layers, such as SU-8[18], can be used as a mold for electroplating metal struc-tures. A thick polymer layer is deposited on top of a metallicseed layer and photostructured. During the subsequentelectroplating process, the metal is only deposited in theareas where the seed layer is exposed to the plating solution,i.e., the polymer layer acts as a plating mold.

Recently, microcontact printing or soft lithography [19]has been introduced as an additional method for patterntransfer. A soft polymeric stamp is used to reproduce a de-sired pattern directly on a substrate. Routinely, feature sizeson the order of 1 m can be achieved with this technique. Thepolymer stamp, often made from poly(dimethylsiloxane)

HIERLEMANN et al.: MICROFABRICATION TECHNIQUES FOR CHEMICAL/BIOSENSORS 841

Fig. 3. Schematic of isotropic and anisotropic thin-film etching.

(PDMS), is formed by a molding process using a masterfabricated with conventional microfabrication techniques.After “inking” the stamp with the material to be printed,the stamp is brought in contact with the substrate material,and the pattern of the stamp is reproduced. Surface prop-erties of the substrate thus can be modified to, e.g., locallypromote or prevent molecule adhesion. Soft lithographyhas been specifically developed for biological applicationssuch as patterning cells or proteins with the help of, e.g.,self-assembled monolayers (SAM) [19].

D. Etching

The two different categories of etching processes in-clude wet etching using liquid chemicals and dry etchingusing gas-phase chemistry. Both methods can be eitherisotropic, i.e., provide the same etch rate in all directions,or anisotropic, i.e., provide different etch rates in differentdirections (see Fig. 3). The important criteria for selectinga particular etching process encompass the material etchrate, the selectivity to the material to be etched versusother materials, and the isotropy/anisotropy of the etchingprocess. An overview on various etching chemistries used inmicrofabrication can be found in [20].

Wet etching is usually isotropic with the important ex-ception of anisotropic silicon wet etching in, e.g., alkalinesolution, such as potassium hydroxide (see Section III).Moreover, wet etching typically provides a better etchselectivity for the material to be etched in comparison toaccompanying other materials. An example includes wetetching of silicon dioxide using hydrofluoric-acid-basedchemistry. SiO is isotropically etched in diluted hy-drofluoric acid (HF:HO) or buffered oxide etch (BOE)(HF:NH F). Typical etch rates for high-quality (thermallygrown) silicon dioxide films are 0.1m/min in BOE.

Dry etching, however, is often anisotropic, resulting ina better pattern transfer, as mask underetching is avoided(see Fig. 3). Therefore, anisotropic dry etching processes,such asreactive ion etching(RIE), of thin-film materials arevery common in the microelectronics industry. In an RIEsystem, reactive ions are generated in a plasma and are accel-erated toward the surface to be etched, thus providing direc-tional etching characteristics. Higher ion energies typically

result in more anisotropic etching characteristics, but alsolead to reduced etching selectivity. Whereas a large numberof dry etching recipes exist, mainly fluorine- or chlorine-based etching chemistry is used [2], [3].

E. Doping

Doping is used to modify the electrical conductivity ofsemiconducting materials such as silicon or gallium arsenide.It is hence the key process step for fabricating semiconductordevices such as diodes and transistors. In the case of sil-icon, doping with phosphorus or arsenic yields-type sil-icon, whereas -type silicon results from boron doping. Byvarying the dopant concentration of-type silicon fromto cm , the resistivity at room temperature can betuned from approximately 40cm to cm. Dopingis also used to realize, e.g., piezoresistive silicon resistors foroscillation detection in resonant chemical sensors [22].

Dopant atoms are introduced by either ion implantation ordiffusion from a gaseous, liquid, or solid source. Ion implan-tation has become the key process to introduce precisely de-fined quantities of dopants in the microelectronics industry.The substrate material, i.e., a silicon wafer, is bombardedwith accelerated ionized dopant atoms in an ion implanter.The result is approximately a Gaussian distribution of thedopant atoms in the substrate wafer with a mean penetrationdepth controlled by the acceleration voltage. A high-tem-perature diffusion process can then be used to additionally“drive-in” the dopant until a desired doping profile has beenachieved.

III. M ICROMACHINING FOR(BIO)CHEMICAL SENSORS

The basic microfabrication processes described previ-ously are often combined with special micromachining stepsto produce static and movable (3-D) microstructures, suchas cantilevers, bridges, and membranes. Microfabricateddevices exhibiting such micromachined structures areoften termed microelectromechanical systems(MEMS).Especially membrane and cantilever structures are widelyused in bio(chemical) sensors. Membranes provide, e.g.,the thermal isolation required by thermal chemical sensors[21], whereas cantilevers can be used as resonant structuresfor mass-sensitive chemical sensors [22]. In the following,the fundamental micromachining techniques are reviewed.More details on micromachining techniques can be foundin dedicated books on microsystem technology [6]–[9]. Arecent review on microfabrication in biology and medicinecan be found in [10].

The micromachining techniques are categorized into bulkmicromachining [23] and surface micromachining processes[24] (see Fig. 4). In the case of bulk micromachining, the mi-crostructure is formed by machining the relatively thick bulksubstrate material, whereas in the case of surface microma-chining, the microstructure comprises thin-film layers, whichare deposited on top of the substrate and selectively removedin a defined sequence to finally arrive at the MEMS struc-ture.


(a)

(b)

Fig. 4. Schematic of (a) bulk and (b) surface micromachining.

Table 1Examples of Etching Techniques for Machining the SiliconSubstrate

A. Bulk Micromachining

Bulk micromachining techniques [23], i.e., etching tech-niques to machine the (silicon) substrate, can be classifiedinto isotropic and anisotropic techniques (structure geom-etry), or into wet and dry etching techniques (reactant phase:liquid or gaseous) as can seen in Table 1.

The most commonisotropic wetsilicon etchant is HNA,a mixture of hydrofluoric acid (HF), nitric acid (HNO),and acetic acid (CHCOOH): nitric acid oxidizes the siliconsurface, and hydrofluoric acid etches the grown silicondioxide layer. The acetic acid controls the dissociation ofHNO , which provides the oxidation of the silicon. The etchrates and the resulting surface quality strongly depend on thechemical composition [8]. Glass wafers can be isotropicallyetched with, e.g., diluted hydrofluoric acid (HF:HO).

Anisotropic wet etchingof silicon is the most commonmicromachining technique and is used to release, e.g.,membranes and cantilevers for chemical and biosensors.Anisotropic silicon etchants etch single-crystal silicon withdifferent etch rates along different crystal directions. Theetch grooves are limited by crystal planes, along whichetching proceeds at slowest speed, i.e., the (111) planes ofsilicon. In case of (100) silicon wafers, the (111) planes

are intersecting the wafer surface at an angle of 54.7,yielding the typical pyramid-shape etch grooves shown inFig. 5. Masking materials for anisotropic silicon etchantsare silicon dioxide and silicon nitride. It is important to note,that “convex” corners of the etch mask (as shown in Fig. 5)are underetched in case of (100) silicon substrates, leadingto, e.g., completely underetched cantilever structures. Theetch rates in preferentially etched crystal directions suchas the 100 and the 110 direction, and the ratio of theetching rates in different crystal directions strongly dependson the exact chemical composition of the etching solutionand the process temperature [25].

The most common anisotropic silicon etching solutionis potassium hydroxide, KOH. As an example, a six-molarKOH solution at 95 C provides a 100 etch rate of150 m/h and an anisotropy between100 and 111direction etching of 30–100:1 [26]. Since the etch rate ofsilicon dioxide in KOH solution is rather high (for thermaloxide, approximately 1 m/h in six-molar KOH solution[8]), silicon nitride films are often used as etching mask.KOH solution is very stable, yields reproducible etchingresults, and is relatively inexpensive. KOH is therefore themost common anisotropic wet etching chemical in industrialmanufacturing. The disadvantages of KOH include therelatively high SiO- and Al-etch rates, which require aprotection of, e.g., IC structures during etching. Etchingwith KOH is typically performed from the back of the wafer,with the front side protected by a mechanical cover and/or aprotective film [26]. Another issue is the detrimental impactof alkali ions on the characteristics of MOSFET structuresin the gate region. Investigation of MOSFET characteristicsafter KOH etching from the back of CMOS wafers, however,did not reveal any etching-related damage [26].

Alternative silicon etchants are ammonium hydroxidecompounds, such as tetramethyl ammonium hydroxide(TMAH), and ethylene diamine/pyrochatechol (EDP) solu-tions. Some EDP formulations, such as EDP type S, exhibitrelatively low Al- and SiO-etch rates, which render themsuitable for releasing microstructures on the front side ofCMOS wafers [21]. However, EDP solutions age rapidly,are potentially carcinogenic, and are very difficult to disposeof. TMAH solutions exhibit similar etching characteristicsas EDP, but are easier to handle. By controlling the pHby, e.g., dissolving silicon in the etching solution, the etchrate for aluminum metallizations can be reduced [23],rendering TMAH also a candidate etchant for releasingmicrostructures on the front side of CMOS wafers. Moredetailed discussions of wet etching of silicon can be found,e.g., in [7], [8].

Reliable etch stop techniques are very important forachieving reproducible etching results. As already men-tioned, wet anisotropic silicon etchants “stop” etching, i.e.,the etch rate is reduced by at least one to two orders ofmagnitude, as soon as a (111) silicon plane or a silicondioxide/nitride layer is reached. In addition, the etch rate isgreatly reduced in highly boron doped regions (doping con-centration cm ). The etching can also be stoppedat a p–n junction using a so-called electrochemical etch


Fig. 5. (a) Schematic of a cantilever beam released by anisotropic silicon etching from the frontside of the wafer. The etching mask defines the cantilever shape. The underetching of the cantileverstructure starts at the convex corners. (b) SEM photograph showing two bulk-micromachined thermalconverters cointegrated with CMOS circuitry. The devices are released from the front side of aCMOS wafer by combining anisotropic silicon etching using TMAH with an electrochemicaletch-stop technique (reprinted from [23] with permission).

Fig. 6. Micrograph of an anisotropically etched cavity of thecapacitive chemical microsystem shown in Fig. 17. At the bottomof the cavity, the n-well island structure carrying the thermallystabilized capacitive sensor [168] is visible. The n well is suspendedby a membrane consisting of the CMOS dielectric layers (theembedded metal interconnects connecting the sensor are clearlyvisible). Reprinted from [169] with permission.

stop technique (ECE) [6]. This method has been extensivelyused to release silicon membranes and n-well structures (seeFig. 6). ECE relies on the passivation of silicon surfaceswhen an anodic potential is applied that is sufficiently highwith respect to the potential of the etching solution.

Isotropic dry etchingof silicon is done with xenon di-fluoride, XeF . This vapor-phase etching method exhibitsexcellent etch selectivity with respect to aluminum, silicondioxide, silicon nitride, and photoresist, all of which can beused as etch masks. However, the resulting etched siliconsurfaces are quite rough. The XeFsilicon etch rates de-pend on the loading (size of the overall silicon surface ex-posed to the etchant) with typical values of approximately1 m/min. XeF etching systems are commercially availablefrom XACTIX [37].

Anisotropic dry etchingof silicon is usually performed byRIE. By controlling the process parameters, such as processgases and process pressure, the etching can be rendered eitherisotropic or anisotropic. The dry-etching anisotropy origi-nates from experimental parameters such as the direction of

ion bombardment, and is therefore independent of the crystalorientation of the substrate material. Most bulk etching of sil-icon is accomplished using fluorine free radicals with SFasa typical process gas. Adding chlorofluorocarbons results inpolymer deposition in parallel with etching, which leads toenhanced anisotropy.

Very high aspect ratio microstructures can be achievedwith deep (D)RIE, a method which has gained importanceduring the last years. Deep RIE systems rely on high-densityplasma sources and an alternation of etching and polymer-as-sisted sidewall protection steps. In a process known as theBosch process [40], a mixture of trifluoromethane and argonis used for polymer deposition. Due to the ion bombard-ment, the polymer deposition on the horizontal surfacescan almost be prevented, while the sidewalls are passivatedwith a Teflon-like polymer. In the second process step, anSF -based etching chemistry provides silicon etching in thenonpassivated regions, i.e., the horizontal surfaces. Bothprocess steps are alternated, resulting in typical silicon etchrates of 2–3 m/min with an anisotropy on the order of 30 : 1[23]. Silicon dioxide and photoresist layers can be used asetch masks. Even though an exceptional anisotropy can beachieved, which is independent of the crystal orientation,one should keep in mind that deep RIE systems are by farmore expensive than a simple wet-etching setup, and thatonly one wafer is processed at a time. Commercial etchersof this type are available, e.g., from Surface TechnologySystems (STS) [38] and Unaxis Semiconductor [39].

The characteristics of the most common bulk siliconetchants are summarized in Table 2. In addition to the de-scribed “basic” micromachining processes, a large numberof specific silicon-based micromachining processes havebeen developed. Examples are the RIE-based, CMOS-com-patible micromachining process developed at CarnegieMellon [27] University, Pittsburgh, PA, used to releasedielectric and single-crystalline silicon microstructures,or the SCREAM (Single Crystal silicon Reactive EtchingAnd Metallization) process developed at Cornell University


Table 2Characteristics of Common Bulk Silicon Etchants

Typical etch rates are given. The individually achievableetch rates highly depend on the process parameters. (Sourcesof etching rates and selectivities: HNA [5], KOH [8], TMAH[29], [7], XeF [37], deep RIE [7], SiNx etch rate in HNAis smaller than SiO etch rate. Ratio of SiO :Si etch ratesand SiN :Si etch rates are given instead of numerical etchrate values).

[28], which is used to release single-crystalline siliconmicrostructures. In addition to silicon micromachining, anincreasing number of nonsilicon micromachining processeshas been developed. An example is the LIGA process(Lithographie, Galvanoformung, Abformung) [6], a processsequence consisting of X-ray lithography with synchrotronradiation, electroplating of metals, and molding of plasticsto fabricate high-aspect ratio microstructures. The X-raylithography is used to structure a poly(methylmethacrylate)(PMMA) layer, which is up to several hundred microm-eters thick. The PMMA layer is then used as a mold forelectroplating the metal microstructures. After removal ofthe remaining resist, the metal structures can be used as amaster in a subsequent plastic molding or hot-embossingprocess. LIGA technology has been employed to fabricate,e.g., a near-infrared microspectrometer system [6].

B. Surface Micromachining

The most commonly used surface micromachiningprocess is sacrificial-layer etching [24]. In this process,a microstructure, such as a cantilever beam, is releasedby removing a sacrificial thin-film material, which waspreviously deposited underneath the microstructure. As anexample, Fig. 7 shows a metal-oxide gas sensor consistingof a free-standing hotplate structure released by removing a2- m-thick sacrificial polysilicon layer using TMAH [30].The 120- m by 120- m hotplate structure consists of asandwich of silicon oxide and silicon nitride films with anembedded polysilicon heater and a polysilicon temperaturesensor (thermistor). The metal oxide film is contactedvia gold pads, which were deposited after the tin-dioxidegas-sensitive film had been sputtered and sintered.

The release of polysilicon microstructures by removing asacrificial silicon dioxide film is the most popular surface mi-cromachining technique [24]. Sacrificial aluminum etching(SALE) has been developed to release dielectric microstruc-tures with embedded metal layers [31]. Metallic microstruc-

Fig. 7. Micrograph of a surface-micromachined metal-oxide gassensor with a hotplate structure released by polysilicon sacrificiallayer etching. Reprinted from [30] with permission.

tures can also be released by sacrificial polymer etching. Aprominent example fabricated by sacrificial polymer etchingis the digital micromirror device (DMD) developed by TexasInstruments [32].

C. Wafer Bonding

While the majority of microstructures are fabricated froma single substrate or wafer, several wafers can be joined bywafer bonding [33]. Complex (3-D) microstructures, such asvalves, pumps, or even microturbines [34], can be fabricated.The substrates/wafers are bonded onto each other either di-rectly or via an intermediate layer.

Direct bonding techniques include silicon fusion bondingand anodic bonding. Silicon fusion bonding involves twosilicon wafers, which are bonded to each other at high tem-peratures ( 1000 C). Low-temperature ( 400 C)fusion bonding has been demonstrated by implementingspecial cleaning procedures, but exhibits reduced bondingstrength. Anodic bonding of a sodium-rich glass wafer ontoa silicon wafer is accomplished by applying an electric fieldacross the bonding interface at moderate bonding tempera-tures ( 300 C–400 C). Glass materials with a thermalexpansion coefficient similar to that of silicon (e.g., Pyrexglass 7740) are used for anodic bonding in order to minimizethermomechanical stress. Anodic bonding and silicon fusionbonding require very clean and smooth wafer surfaces toachieve void-free bonding. The surface quality and rough-ness is less important if an intermediate layer is used forwafer bonding. Possible intermediate bonding layers includeadhesives, low-melting-temperature glasses, solder films, ormetallic films such as gold, which is used in gold-siliconeutectic bonding. An example for using wafer bonding tofabricate chemical sensors is the absorption-based opticalCO -sensor schematically shown in Fig. 8 [35]. The sensormeasures the COconcentration in respiratory gases byanalyzing the IR absorption bands at 4.23m wavelength.The dual-beam IR sensor system consists of an IR source,a CO -filter chip providing a known absorptivity in thereference beam path, and an IR detector. The filter chip (seeFig. 8) is composed of three fusion-bonded silicon wafers


Fig. 8. Micromachined multichip optical CO-sensor: IR-source,wafer-bonded COfilter chip, IR-bandpass filter, and IR-detector.Redrawn from [35].

joined with a Pyrex glass wafer by anodic bonding. The finalanodic bonding is performed at a CO-overpressure to fillthe filter cavity with CO. The CO -filled cavity acts as anabsorption reference [35] during measurements. The ratiobetween sample and reference detector signal correlateswith the probed sample-CO-concentration.

D. Deposition of Chemically/Biologically Sensitive Layers

The set of microfabrication processes used for chem-ical/biosensors is completed by various deposition tech-niques for chemically or biologically sensitive layers (seeSection IV). Chemically sensitive polymer layers used forthe detection of volatile organics in air are deposited bydispensing or spray coating, metal-oxide films for detectionof, e.g., carbon monoxide and nitrogen oxides are depositedeither by a sol/gel process, by drop coating, or by sputtering.Proteins can be patterned onto surfaces by, e.g., microcontactprinting of self-assembled monolayers.

E. Process Integration

The various microfabrication processes can be com-bined in custom-designed process flows to produce desiredmicrostructures. Process flows that combine MEMS fabri-cation with IC fabrication, often termed integrated MEMSor CMOS-MEMS, are particularly promising: Circuitrycan be cointegrated with sensor structures, and establishedprocessing facilities for the fabrication of ICs can be usedto manufacture at least parts of the microsystem. Examplesinclude CMOS-based microsystems, for the fabrication ofwhich CMOS-compatible microfabrication steps are com-bined with a conventional CMOS process [4]. Additionalmicrofabrication/micromachining steps can be performedeither before (preprocessing), in-between, or after (post-processing) the regular CMOS process sequence. Thecomplexity of the additional process steps varies dependingon the specific sensor structure. Capacitive chemical sen-sors, e.g., [17] can be completely fabricated within a CMOS

Fig. 9. Micromachining steps necessary to fabricate an integratedCMOS resonant cantilever with on-chip circuitry. (a) Chip aftercompletion of the CMOS process and subsequent patterning of thesilicon nitride etching mask. The micromachining steps include (b)anisotropic silicon etching using KOH from the back side of thewafer to release an n-well membrane and (c) front-side reactive ionetching to release the cantilever.

process sequence and require only a depostion step of thechemically sensitive layer. CMOS calorimetric sensors[17] require an additional anistropic wet etching step withelectrochemical etch-stop technique. The advantages/dis-advantages of customized MEMS and integrated MEMSprocesses are further discussed in Section V.

A simple fabrication sequence of mass-sensitive chemicalcantilevers is outlined here and serves as an example forprocess integration [22]. Basic elements of the microsystemare already generated during a 0.8-m double-metal CMOSprocess, which is provided by, e.g., austriamicrosystems(Unterpremstaetten, Austria). The source/drain implantationis used on the one hand to form the heating resistors, whichexcite the cantilever in transverse vibrations, and on the otherhand to form the piezoresistors, which detect the cantilevervibrations. To obtain maximum sensitivity, the piezoresistorsare located close to the clamped end of the cantilever, i.e.,at the location of maximum mechanical stress. The depth ofthe CMOS n well predefines the cantilever thickness. Aftercompletion of the CMOS process, the backside of the waferis lapped and polished to ensure proper adhesion of theetching mask [see Fig. 9(a)]. A silicon nitride etching maskis deposited and patterned. A double-side mask aligner isused, which allows for the alignment of the etching maskwith regard to structures on the front side of the wafer.Then, n-well membranes are released using anisotropicsilicon etching in a KOH solution with an electrochemicaletch-stop technique [see Fig. 9(b)]. To this end, the wafer ismounted in a custom-designed wafer holder, which protectsthe wafer front with the CMOS circuitry from the etchantand supplies the required ECE etching potential to largecontact pads on the wafer surface [36]. From these contactpads, the etching potential is routed to the various n-welland p-substrate contacts on the wafer through a specialmetal network implemented in the scribe lines between thereticles. The etching process stops either upon reaching


a positively biased n well or upon reaching the thermaloxide of the CMOS process. The cantilevers are afterwardsreleased by front-side RIE etching of the dielectric layersand of the n-well membrane surrounding the cantilever [seeFig. 9(c)]. Finally, the chemically sensitive polymer layer isdeposited onto the cantilever.

IV. M ICROFABRICATED STRUCTURES FOR

CHEMICAL/BIOSENSORS

Chemical sensors usually consist of a sensitive layer orcoating and a transducer [41], [42]. Upon interaction witha chemical species (absorption, chemical reaction, chargetransfer, etc.), the physicochemical properties of the coating,such as its mass, volume, optical properties, or resistance,etc. reversibly change. These changes in the sensitive layerare detected by the respective transducer and translated intoan electrical signal such as frequency, current, or voltage,which is then read out and subjected to further data treatmentand processing.

Various inorganic and organic materials serve as chemi-cally sensitive layers that can be coated onto the differenttransducers [41]–[43]. Typical inorganic materials includeelectron-conducting oxides like tin dioxide (SnO) for mon-itoring reducing gases such as hydrogen, carbon monoxide,and nitrogen oxides. Ion-conducting oxides like zirconiumdioxide (ZrO ) are applied to determine oxygen, but also ni-trogen oxide and ammonia. Organic layers mostly consistingof conducting or nonconducting polymers such as polysilox-anes, polyurethanes, or polyaniline are used for monitoringhydrocarbons, halogenated compounds, and other kinds oftoxic volatile organics [41]–[43]. The different sensitive ma-terials and their operation conditions, such as elevated tem-perature, impose certain requirements on the transducer de-sign. Specific chemically sensitive materials will hence bementioned in the context of the corresponding transducerstructures in the following sections of this section.

A variety of transducers based on different physical princi-ples have been devised. Following the suggestion of Janata,chemical sensors can be classified into four principal cate-gories according to their transduction principles [42], [43]:

1) chemomechanical or mass-sensitive sensors (e.g.,mass changes upon absorption);

2) thermal sensors (e.g., temperature changes throughchemical interaction);

3) optical sensors (e.g., changes of light intensity by ab-sorption);

4) electrochemical sensors (e.g., changes of potential orresistance through charge transfer).

We will use the previously described classification scheme togive an overview on microfabricated bio/chemosensor struc-tures in the following sections of this section. We will brieflyintroduce each of those four sensor categories and detail therespective transducer requirements. We then will present ex-emplary microstructured transducers with typical sensitivematerials and corresponding applications.

A. Chemomechanical or Mass-Sensitive Sensors

Chemomechanical sensors are in the simplest case, e.g.,gravimetric sensors responding to the mass of species ac-cumulated in a sensing layer [11], [44], [45]. Some of thesensor devices additionally respond to changes in a varietyof other mechanical properties of solid or fluid media in con-tact with their surface such as polymer elastic moduli, liquiddensity, and viscosity [11], [44], [45], which will not be dis-cussed here. The responsiveness to a variety of mechanicalparameters, however, is the reason why “chemomechanicalsensors” is a more appropriate term than “mass-sensitive sen-sors.” Any species that can be immobilized on the sensor can,in principle, be sensed. As with most of the chemical sensors,the measurements are taken at a thermodynamic equilibriumstate.

Mass changes can be monitored by either deflectinga micromechanical structure due to stress changes ormass-loading (static measurements) or by assessing thefrequency changes of a resonating structure or a travelingacoustic wave upon mass loading. Both deflection andresonance frequency change in proportion to stress changesor mass loading on the device.

The most common chemomechanical devices are thethickness shear mode resonator (TSMR) or quartz mi-crobalance (QMB), and the Rayleigh surface acousticwave (SAW) device, both based on piezoelectric quartzsubstrates [11], [44], [45]. Shear horizontal acoustic platemode (SH-APM) devices, shear transverse wave (STW),and Love wave devices also require piezoelectric substratessuch as quartz, lithium niobate, or lithium tantalate [11],[44], [45]. For more details on those devices, see the paperon acoustic-wave chemical microsensors in this issue. Inthe following, we will describe in more detail two semi-conductor-technology-based micromechanical structuresfor mass-sensitive devices: 1) flexural-plate-wave devices(FPWs) and 2) micromachined cantilevers.

1) Flexural-Plate-Wave or Lamb-Wave Devices:Thechief advantage of FPWs is their high sensitivity to addedmass at a low operating frequency (typically 3–10 MHz)[46]. FPW devices feature plates that are only a fraction ofan acoustic wavelength thick (typically 2–3m). The platesare composite structures (see Fig. 10) consisting of a siliconnitride layer, an aluminum ground plane, a sputtered zincoxide piezoelectric layer, all supported by a silicon substrate[11], [44]–[49].

The interdigital transducers (IDTs) on these devices gen-erate flexural waves (Lamb waves; see Fig. 10) with ret-rograde elliptical particle motions as in SAW devices. TheLamb waves give rise to a series of plate modes, one of whichhas a frequency that is much lower than those of the otherpossible modes. The velocity of this unique wave decreaseswith decreasing plate thickness. The entire thickness of theplate is set in motion like the ripples in a flag [11], [44]–[49].The confinement of acoustic energy in the thin membrane re-sults in a very high mass sensitivity. Since the Lamb wavecauses an elliptical particle movement at the transducer sur-face (see Fig. 10), the sensitive films are deformed. The sen-


Fig. 10. Schematic of an FPW device. The side view showsthe different layers and the membrane movement. Interdigitatedelectrodes are used for actuation.

sitive layer can be deposited on either side of the membrane.Deposition on the backside (nonprocessed side of the wafer)has the advantage that on-chip-circuitry will not be exposedto chemicals [46]–[49]. These are the fabrication steps [47],[48], [50]:

1) deposition of Al (sputtering) and Si-nitride (LPCVD);2) bulk micromachining: back-side etching (KOH) to

achieve a membrane structure;3) zinc oxide deposition;4) IDT processing: Evaporation or sputtering of alu-

minum or gold;5) spin or spray coating of polymers, deposition of bio-

logical entities.

IC process-compatible fabrication sequences for monolithicintegration of the Lamb device with electronics are detailedin [48], [50].

An alternative to piezoelectric excitation is magnetic FPWactuation based on Lorentz forces: a metal meander-line ispatterned on the membrane surface; ac is flowing in the me-ander transducer and interacts with a static in-plane magneticfield to generate time-varying Lorentz forces, which deformthe membrane. Magnetic actuation requires an externally ap-plied magnetic field, but eliminates the need for a piezoelec-tric layer [51]. Such layers, apart from being difficult to de-posit, frequently contain elements such as Zn, etc., that posecontamination problems in IC fabrication [51].

Typical FPW applications include the detection of dif-ferent organic volatiles in the gas phase by using polymericlayers [48]–[50], or the use of an FPW-based immunoassayfor the detection of breast cancer antigens [52].

2) Resonating Cantilevers:A mass-sensitive cantileverusually is a layered structure (see Fig. 9) composed of, e.g.,silicon, silicon oxide/nitride, and, eventually, metallizations[17], [22]. The cantilever base is firmly attached to a siliconsupport (chip). The free-standing cantilever end is coatedwith a sensitive layer.

There are two fundamentally different operation methods:1) static mode: measurement of the cantilever deflectionupon stress changes or mass loading by means of, e.g.,a laser via beam reflection from the cantilever [53]–[58]and 2) dynamic mode: excitation of the cantilever in itsfundamental mode and measurement of the change in reso-

nance frequency upon mass loading [17], [22], [59], [60] inanalogy to other mass-sensitive transducers. Method 1) re-quires long and soft cantilevers to achieve large deflections,whereas method 2) requires short and stiff cantilevers toachieve high operation frequencies. Method 2) is preferablewith regard to simplicity of the setup [17], [22], [59], [60];method 1) can also be used in liquid media [57], [59]. Theexcitation of the cantilever is usually performed by applyingpiezoelectric materials (ZnO) [61] or by making use of thedifferent thermal expansion coefficients of the cantileverlayer materials (bimorph effect) [17], [22]. The differentmaterials give rise to a cantilever deflection upon heating.Cantilever deflection or resonance frequency changes canbe detected by embedding piezoresistors in the cantileverbase [17], [22], [59], [60], or by using optical detection bymeans of a laser [55]–[58]. The absolute mass resolution ofthe cantilevers is in the range of a few picograms [17], [22],[53]–[58]. This high mass-sensitivity does not necessarilyimply an exceptionally high gas sensor sensitivity sincethe area coated with the sensitive layer (on the order of100 150 m), i.e., the overall sorption matrix volume, andconsequently the achievable mass change, is very small.

These are the fabrication steps (see Fig. 9) [22]:

1) bulk micromachining: backside etching (KOH) toachieve a membrane structure;

2) release of the cantilevers by front-side RIE;3) spray or drop coating of sensitive layers, and deposi-

tion of biological entities.

CMOS-based cantilevers monolithically integrated withfeedback electronics are detailed in [17], [22], [59].

Typical applications include the detection of organicvolatiles or humidity in the gas phase by using polymericlayers [17], [22], and [53]–[55], [58], [60], or biosensing inliquids such as the hybridization and detection of comple-mentary strands of oligonucleotides (DNA fragments) [57].

B. Thermal Sensors

Calorimetric or thermal sensors rely on determining thepresence or concentration of a chemical by measurementof an enthalpy change produced by the chemical to be de-tected [41], [42], [62]. Any chemical reaction or even absorp-tion/desorption process releases to or absorbs from its sur-roundings a certain quantity of heat. Reactions liberating heatare termedexothermic; reactions abstracting heat are termedendothermic. This thermal effect shows a transient behavior:continuous liberation/abstraction occurs only as long as thereaction proceeds. However, there will be no heat productionand, hence, no measurable signal at thermodynamic equilib-rium 0 in contrast to mass-sensitive, optical, or elec-trochemical sensors. Conflicting constraints are imposed onthe design of a thermal sensor: the sensor has to interact withthe chemical species, but at the same time the sensing areashould be thermally as isolated as possible to enhance sensi-tivity.

The liberation or abstraction of heat can be convenientlymeasured as a change in temperature, which then is trans-duced into an electrical signal. The various types of calori-


Fig. 11. (a) Cross section of side-by-side microhotplates composed of a dielectric membrane on anetched silicon wafer, and platinum resistors/heaters. The device on the left has a deposited catalystmaking it the active element [65]. (b) Micrograph (SEM) of two meandered polysilicon microbridges[66]. The lower meandered bridge is coated with a thin (approximately 0.1-�m) layer of platinum(CVD). In a differential gas sensing mode, the upper, uncoated, filament acts to compensate changesin the ambient temperature, thermal conductivity, and flow rate, while the lower filament, is used tocalorimetrically detect combustible gases. Reprinted from [66] with permission.

metric sensors differ in the way that the heat evolved is trans-duced [41], [42], [62].

The catalytic sensor (often denoted “pellistor”) employsplatinum resistance thermometry [63]–[71], while the ther-moelectric sensor is based on the Seebeck effect [72]–[79].Both sensor types benefit from micromachined transducersand will be detailed in the following sections.

1) Catalytic Thermal Sensors (Pellistors):The catalyticthermal sensor measures the heat evolved during the con-trolled combustion of flammable gaseous compounds in am-bient air on the surface of a hot catalyst by means of a re-sistance thermometer in proximity with the catalyst. Thismethod is therefore calorimetric [64]. The heated catalysthere permits oxidation of the gas at reduced temperatures andat concentrations below the lower explosive limit (LEL). Theterm “pellistor” originally refers to a device consisting of asmall platinum coil embedded in a ceramic bead, which wasimpregnated with a noble metal catalyst [63], [64]. The ce-ramic bead was used since the rate of reaction (and thus thesensor signal) is directly proportional to the active surfacearea.

Fig. 11 shows two different micromachined designs torealize a catalytic calorimetric sensor: 1) a meander structureon a micromachined membrane [65] and 2) a freestanding,Pt-coated polysilicon microfilament (10m wide, 2 mthick) separated from the substrate by a 2-m air gap [66],[67]. Heat losses to the silicon frame are minimized inthese designs. By passing an electric current through themeander, the membrane/microbridge is heated to a tempera-ture sufficient for the Pt surface to catalytically oxidize thecombustible mixture; the heat of oxidation is then measuredas a resistance variation in the Pt. The combustion of, e.g.,methane generates 800-kJ/mol heat, which translates into acorresponding temperature change.

These are the fabrication steps:

1) bulk micromachining: backside etching (KOH) toachieve a membrane structure [65], [68], [69];

2) surface micromachining: sacrificial layer etching (HF)for the bridges [66], [67];

3) Pt or catalyst deposition: sputtering [70], evaporation[68], [69], and LPCVD [66], [67].

A detailed processing sequence for microbridges is given in[71].

Typical applications include monitoring and detection offlammable gas hazards such as methane [68], [71], hydrogen[65]–[68], propane [65], or carbon monoxide [65], [69] inindustrial, commercial, and domestic environments at con-centrations below the LEL. The LEL is the concentration ofgas in air, below which it cannot be ignited.

2) Thermoelectric or Seebeck-Effect-Based Sen-sors: This type of sensor relies on the thermoelectricor Seebeck effect [72]. When two different semiconductorsor metals are connected at a hot junction and a temperaturedifference is maintained between this hot junction and acolder point, then an open circuit voltage is developedbetween the different leads at the cold point. This ther-movoltage is proportional to the difference of the Fermilevels of the two materials at the two temperatures and thusproportional to the temperature difference itself [72]. Thiseffect can be used to develop a thermal sensor by placingthe hot junction on a thermally isolated structure like amembrane, bridge, etc., and the cold junction on the bulkchip with the thermally highly conducting silicon under-neath [17], [73], [74]. To achieve a higher thermoelectricvoltage, several thermocouples are connected in series toform a thermopile. The membrane structure supporting thehot junctions is covered with a sensitive or chemically activelayer liberating or abstracting heat upon interaction with ananalyte. The resulting temperature gradient between hot andcold junctions then generates a thermovoltage, which can bemeasured.

Fig. 12 displays the schematic of a CMOS-basedthermopile consisting of a dielectric membrane with polysil-icon/aluminum thermocouples (Seebeck coefficient: 111

V/K) [17], [75].These are the fabrication steps:

1) CMOS IC fabrication process;2) bulk micromachining: back-side etching (KOH) to

achieve a membrane structure [17], [75], [76];3) deposition of the sensitive layer by airbrush [17], [75],

dispensing, spin coating, or enzyme immobilizationmethods [77], [78].


Fig. 12. Schematic of a thermoelectric sensor. Polysilicon/aluminum thermopiles are used (hotjunctions on the membrane, cold junctions on the bulk chip) to record temperature variationscaused by analyte sorption in the polymer.

Processing sequences for the integration of thermoelectricsensors with circuitry in a CMOS standard process are de-tailed in [17], [76].

Typical applications include the detection of differentkinds of organic volatiles in the gas phase by using poly-meric layers [17], [75], [79], and the biosensing of glucose,urea, and penicillin in the liquid phase by using suitableenzymes [74], [77], [78].

C. Optical Sensors

Optical techniques offer a great deal of selectivity alreadyinherent in the various transduction mechanisms in compar-ison to other chemical sensing methods. Characteristic prop-erties of the electromagnetic waves such as amplitude, fre-quency, phase, and/or state of polarization can be used toadvantage [80]–[82]. Geometric effects (scattering) can pro-vide additional information. The wavelength of the radia-tion, e.g., can be tuned to specifically match the energy ofa desired resonance or absorption process. In addition, op-tical sensors like any other chemical sensor can capitalize onall the selectivity effects originating from the use of a sensi-tive layer. When a sample is irradiated with visible light orelectromagnetic waves, the radiation can be absorbed (inten-sity decrease), scattered (direction randomization, possiblyfrequency changes), refracted, or reflected (metallic reflec-tion, internal reflection mediated by evanescent waves) at theinterface(s), or can produce phosphorescence/fluorescence(absorption-emission process) or chemiluminescence (con-version of chemical energy into light) effects [80]–[82].

In the following, we will focus exclusively on semi-conductor- and microoptoelectromechanical (MOEMS[83])-based microfabricated transducers. Examples include:1) a Mach–Zehnder integrated optical device and 2) amicromachined integrated Fabry–Pérot spectrometer. Forinformation on optical sensors and fiber-optical techniques,we recommend reviews [80], [81], [84], and the paper onfiber optic sensors in this issue.

1) Integrated Mach–Zehnder Interferometer:Integratedoptical (IO) sensors make use of guided waves or modes in

planar optical waveguides [85]–[87]. The waveguide mate-rials usually include high refractivity silicon dioxide or ti-tanium dioxide and silicon nitride films on oxidized siliconwafer substrates. The guided waves or modes in planar op-tical waveguides include the transverse electric (TE), or s po-larized, surface-normal, and the transverse magnetic (TM),or p polarized, surface-parallel) modes. Changes in the effec-tive refractive index of a guided mode are induced by changesof the refractive index distribution in the immediate vicinityof the waveguide surface, i.e., within the penetration depth(some hundred nanometers) of the evanescent field in thesample [see Fig. 13(a)], [82], [85]. The evanescent field de-cays exponentially with increasing distance from the wave-guide surface. Changes in the effective refractive index canbe induced by absorption of an adlayer onto the surface ofthe waveguide from gas or liquid phase [85]–[87] or by in-teraction of an analyte molecule with a recognition structureimmobilized on the waveguide surface [85].

Mach–Zehnder IO devices [see Fig. 13(b)] are monomodechannel waveguides (TE or TM mode) and allow fora straightforward implementation of an interferometerstructure [85], [88]–[91]. A waveguide is split into anopen measurement path and a protected reference pathand recombined after some distance. The phase differenceintroduced by analyte interaction (refractive index change)in the sensing path is detected by interference effects.


1) patterning of silicon nitride as waveguide (LPCVD,RIE, lithography) [85], [88]–[91];

2) deposition of an silicon oxide cladding layer (PECVD)[85], [88]–[91];

3) deposition of the chemically sensitive layer or immo-bilization of biological entities [88]–[91].

Typical applications include the detection of different or-ganic solvents in liquid phase, and biotin/streptavidin-medi-ated immunosensing involving antibody-antigen binding ex-periments [85]–[91].

2) Fabry–Pérot-Based Microspectrometer:A Fabry–Pérot interferometer (FPI) is an optical element consisting of


(a) (b)

Fig. 13. (a) Schematic of an evanescent wave traveling in an optical waveguide. The traveling lightin the waveguide is influenced by analyte-induced changes of the refraction index in the adjacentsensitive layer. (b) Schematic of a conventional Mach–Zehnder interferometer. The cross sectionshows the separate sensor (left side, open) and reference (right side, covered) branches.

Fig. 14. Schematic of a tunable Fabry–Pérot interferometer. Thewavelength-defining gap width can be varied by applying dc to thecontrol electrodes. Aluminum is used as optical coating material.Adapted from [92] with permission.

two partially reflecting, low-loss, parallel mirrors separatedby a gap [92]–[95]. The optical transmission characteristicsthrough the mirrors consist of a series of sharp resonanttransmission peaks occurring when the gap is equal tomultiples of half a wavelength of the incident light. Thesetransmission peaks are caused by multiple reflections ofthe light in the cavity. By using highly reflective mirrors,small changes in the gap (width, absorptivity) can producelarge changes in the transmission response. Even thoughtwo reflective mirrors are used, transmission through theelement at the peak wavelengths approaches unity. Thetransmission is a function of both the gap spacing and theradiation wavelength. The devices hence can be used aswavelength selector or monochromator by adjusting the gapwidth to achieve the desired wavelength [92]–[95].

Fig. 14 shows a tunable device with a gap width variableby electrostatic actuation using electrodes on movable micro-machined parts [92]. Such devices operate preferably in thenear infrared region at wavelengths larger than 1m, wheresilicon substrates become transparent [92]–[95]. The siliconsurfaces on which the optical coatings are deposited must beequivalent in surface finish to unprocessed silicon (polishedwafer), which precludes the use of an etched silicon surface.Therefore a three-wafer solution was chosen [92] with a fu-

sion-bonded sacrificial wafer, which is first etched to the de-sired gap thickness. Then, the sacrificial wafer is patternedand etched again to form the depressions, which set the gapbetween the control electrodes, and in a final step it is etchedall the way through until the oxide of the bonded wafer isreached. The oxide-protected surface of the bonded wafer isacceptably flat and smooth enough to support optical qualitycoatings [92].

These are the fabrication steps [92]:

1) wafer fusion bonding;2) etching sequence to achieve the movable mesa;3) etching sequence to achieve the gap and control elec-

trode spacing;4) deposition of multilayer dielectric mirror layers, and

control electrodes.Surface micromachining [94], [96] can also be used toachieve the FPI cavities. A complete fabrication sequencefor a monolithic CMOS visible light (VIS) spectrometerwith Fabry–Pérot etalons realized with different thicknessesof PECVD silicon oxide is given in [95].

Typical applications include gas sensors to monitorcarbon monoxide (characteristic absorption wavelength: 4.7

m), carbon dioxide (4.2 m), and hydrocarbons (3.3m)[93]–[97]. The radiation source in most cases is a light bulbor light-emitting diode (LED).

D. Electrochemical Sensors

Electrochemical sensors constitute the largest and oldestgroup of chemical sensors. Many of them are commerciallyavailable. Electrochemical sensors make use of electrochem-ical or charge-transfer reactions. Electrochemistry includescharge transfer from an electrode to a solid or liquid samplephase or vice versa. Chemical changes take place at the elec-trodes or in the probed sample volume, and the resultingcharge or current is measured. Electrode reactions and chargetransport in the sample are both subject to changes by chem-ical processes and hence are at the base of electrochemicalsensing mechanisms [41]–[43].

An electrochemical sensor is always composed of at leasttwo electrodes with two electrical connections: one through


the probed sample, the other via transducer and measuringequipment. The charge transport in the sample can be ionic,electronic, or mixed, while that in the transducer branch isalways electronic [41]–[43].

Electrochemical sensors are usually classified accordingto their electro-analytical principles [42], [43].Voltammetricsensors are based on the measurement of the current-voltagerelationship. A potential is applied to the sensor, and a currentproportional to the concentration of the electroactive speciesof interest is measured (amperometry is a special case ofvoltammetry, where the potential is kept constant).Poten-tiometric sensors are based on the measurement of the po-tential at an electrode at equilibrium state, i.e., no current isallowed to flow during the measurement. The measured po-tential is proportional to the logarithm of the concentration ofthe electro-active species.Conductometricsensors are basedon the measurement of a conductance by applying an ac po-tential with a small amplitude to a pair of electrodes in orderto prevent polarization. The presence of charge carriers de-termines the sample conductance.

Another method of classification is according to electroniccomponents [41]–[43]. There are chemoresistors, chemoca-pacitors, and chemotransistors. We will use the componentclassification scheme here and present one example of a mi-crofabricated chemoresistor and a chemotransistor. An inte-grated capacitor system will be presented in the followingsection. Again, we will restrict the discussion to microfab-ricated semiconductor-based systems and omit a wealth ofliterature on other sensor designs.

1) Chemoresistors (Conductometry):Chemoresistorsrely on changes in the electric conductivity of a film orbulk material upon interaction with an analyte. Chemore-sistors are usually arranged in a metal electrode 1/sensitivelayer/metal electrode 2 configuration [42], [43]. The re-sistivity/conductance measurement is done either via aWheatstone bridge arrangement or by recording the currentat an applied voltage in a dc mode or in a low-amplitude,low-frequency ac mode to avoid electrode polarization.The contact resistance should be much lower than thesample resistance and should be minimized, so that the bulkcontribution dominates the measured overall conductance.

There are two major classes of chemoresistors: 1)high-temperature chemoresistors (200C–600 C) withsemiconductor metal oxide coatings and 2) low-temperaturechemoresistors (room temperature) with polymeric andorganic sensitive coatings. We will focus on the technolog-ically more challenging high-temperature sensors and onlybriefly mention the other type with the applications.

The sensitive materials used with high-temperaturechemoresistors include wide-bandgap semiconductingoxides such as tin dioxide, gallium oxide, indium oxide, orzinc oxide, all of which can only be operated as sensingmaterials at high temperature (200 C) [42], [43], [98],[99]. In general, gaseous electron donors (hydrogen) oracceptors (nitrogen oxide) adsorb on the metal oxides andform surface states, which, only at high temperature, canexchange electrons with the semiconductor. An acceptormolecule will extract electrons from the semiconductor and

Fig. 15. Scanning electron micrograph of a microhotplate array.The suspended plate exhibits a polysilicon heater, an aluminumplane for homogenous heat distribution and aluminum electrodesfor measuring the resistance of a semiconductor metal oxide.Reprinted from [103] and [104] with permission.

thus decrease its conductivity. The opposite holds true foran electron-donating surface state. A space charge layerwill thus be formed. By changing the surface concentrationof donors/acceptors, the conductivity of the space chargeregion is modulated [42], [43], [98]–[102]. The reactionbetween gases and oxide surface depends on the sensortemperature, the gas involved, and the sensor material [42],[43], [98]–[102].

Consequently, the device requirements for a microfabri-cated high-temperature chemoresistor include a thermallywell isolated stage such as a suspended membrane, whichallows for keeping the sensing materials at high temperaturewithout heating the bulk chip (power consumption), anintegrated heater, electrodes, and a temperature sensor[103]–[107]. An example is given in Fig. 15 [104].

Semiconductor metal oxide sensors usually are notvery selective, but respond to almost any analyte (carbonmonoxide, nitrogen oxide, hydrogen, hydrocarbons). Onemethod to modify the selectivity pattern includes surfacedoping of the metal oxide with catalytic metals such asplatinum, palladium, gold, and iridium [98], [99], [102].

Since microhotplates have a very low thermal mass,they allow for applying temperature-programmed operationmodes, which enhance the gas detection capability [108].


1) deposition/patterning of metal electrodes using liftoffand evaporation, or sputtering [103], [106], [109];

2) backside (KOH) [105]–[110], or front-side (RIE,EDP) [103] etching for membrane formation;

3) deposition of metal oxide materials by LPCVD,sol-gel process, sputtering, and screen printing[103]–[110];

4) sintering of the metal oxides (annealing) at elevatedtemperatures [103]–[109].

The fabrication of hotplates on CMOS substrates is describedin [103] and [104]. Concepts of silicon-on-insulator (SOI)-based hotplate structures are reported on in [111]. Completehotplate processing sequences are detailed in [105].

Typical applications include the detection of inorganicgases such as hydrogen [103], oxygen [103], nitrogen oxide


[108], [106], carbon monoxide [108], [105], and a varietyof organic volatiles [42], [43], [98], [109], [112] usingpredominantly tin dioxide as sensitive layer.

Several classes of predominantly organic materials areused for application with chemoresistors at room temper-ature (electrode spacing typically 5 to 100m, appliedvoltage 1–5 V). Conducting polymers such as polypyrroles,polyaniline, and polythiophene are used to monitor a varietyof polar organic volatiles like ethanol, methanol, and com-ponents of aromas [113]–[116]. Conducting carbon black isdispersed in nonconducting polymers so that if the polymerabsorbs vapor molecules and swells, the particles are, onaverage, further apart, and the conductivity of the film isreduced (conductivity by particle-to-particle charge perco-lation) [117]. Applications also include organic solventssuch as hydrocarbons, chlorinated compounds, and alcohols[118]–[120].

2) Chemotransistors (Potentiometry):The most impor-tant potentiometric sensors are based on the field-effect tran-sistor (FET). Both FET-based chemical microsensors pre-sented here do not require micromachining but only the pat-terning of metal films, oxide materials, or polymers in thetransistor gate area (fewm ). We include them anyway inthis overview, since they are among the most intensively in-vestigated devices since 1970 [121], and are produced usingIC microfabrication technology such as CMOS [41]–[43].

Field-effect-based transistors on modern IC logic chipsrely on modulation of the charge carrier density in the semi-conductor surface space-charge region through an electricfield perpendicular to the device surface: The source-draincurrent is controlled by an isolated gate electrode.

The metal–oxide–semiconductor field-effect transistor(MOSFET) as used for chemical gas sensing has a p-typesilicon substrate (bulk) with two n-type diffusion regions(source and drain). The structure is covered with a silicondioxide insulating layer, on top of which a noble metalgate electrode (instead of the polysilicon gate of the ICtransistors) is deposited [122]–[124].

When a positive voltage (with respect to the silicon) is ap-plied to the gate electrode, electrons, which are the minoritycarriers in the substrate, are attracted to the surface of thesemiconductor. Consequently, a conducting channel is cre-ated between source and drain near the silicon dioxide inter-face. The conductivity of this channel can be modulated byadjusting the strength of the electrical field between the gateelectrode and the silicon, perpendicular to the substrate sur-face [122]–[124]. Palladium (Pd) gate FET structures weredemonstrated to function as hydrogen sensors by Lundström[122]. Hydrogen molecules readily absorb on the gate metal(platinum, iridium, palladium) and dissociate into hydrogenatoms. These H-atoms can diffuse rapidly through the Pdand absorb at the metal/silicon oxide interface partly on themetal, partly on the oxide side of the interface [123], [124].Due to the absorbed species and the resulting polarizationphenomena at the interface, the drain current is alteredand the threshold voltage is shifted. The voltage shiftis proportional to the concentration or coverage of hydrogenat the oxide/metal interface [122]–[124]. Sensitivity and se-

Fig. 16. Schematic representation of (a) a MOSFET and (b) anISFET structure as used for chemical sensing.U denotes thegate voltage,U the drain voltage. By replacing the metal gateof the MOSFET with an ionic solution and a reference electrodeimmersed into this solution, the ISFET has been developed.

lectivity patterns of gas-sensitive FET devices hence dependon the type and thickness of the catalytic metal, the chemicalreactions at the metal surface, and the device operation tem-perature [41]–[43], [122]–[124].

FET sensor applications also include the detection of am-monia [125], [126], amines, and any kind of molecule thatgives rise to polarization in a thin metal film (hydrogen sul-fide, ethene, etc.) or causes charges/dipoles on the insulatorsurface [122]–[124].

In the case of the ion-selective field-effect transistor(ISFET), the gate metal electrode of the MOSFET isreplaced by an electrolyte solution which is contacted bythe reference electrode: The gate oxide is directly exposedto an aqueous electrolyte solution (see Fig. 16) [121]. Anexternal reference electrode is required for a stable operationof an ISFET [41]–[43], [127]–[129]. The source-draincurrent is influenced by the potential at the oxide/aqueoussolution interface. ISFET amplifiers with feedback keep thesource-drain current constant by compensating solution-in-duced changes in the gate oxide potential by modulationof the gate voltage, , applied to the reference electrode.The gate-source potential is then determined by the surfacepotential at the insulator/electrolyte interface. Mechanisticstudies of the processes occurring at the solution/gate oxideinterface (site binding model [130]) and the oxide semicon-ductor interface can be found in the literature [41]–[43],[129]–[131]. The insulator/solution interface is assumed torepresent in most cases a polarizable interface, i.e., therewill be charge accumulation across the structure but no netcharge passing through.

Classic ISFET applications include pH-sensing (acidityor basicity) with an exposed-gate-oxide FET [121]. Thesurface of the gate oxide contains OH-functionalities,which can be protonated and deprotonated; thus, whenthe gate oxide contacts an aqueous solution, a change ofpH will change the silicon oxide surface potential [132],[133]. Typical pH-sensitivities measured with silicon oxideISFETs are 37–40 mV/pH unit [133]. Gate materials suchas silicon nitride [134], [135], oxynitride [136], or alumina[137] have better properties than silicon oxide with regardto pH-response, hysteresis and drift. In practice, these layersare deposited on top of the silicon oxide by means of CVD.

ISFETs can also be covered with organic ion-selectivemembranes like polyurethane, silicone rubber, polystyrene,and polyacrylates containing ionophores to detect potassium


Table 3Overview of Common Micromachining Techniques for Hybrid and Monolithic Microsensor Design

The techniques are compared with regard to their availability for fabless design centers and their use in the field of chemical sensors. Exemplary serviceproviders (e.g. MEMS or IC-foundries) are listed as well. A more complete list of MEMS foundries can be found at http://www.memsnet.org/links/fabs.html.

[138], [139], sodium [140], and silver ions [141]. Suchdevices are termed “chemical FETs” or CHEMFETs.

These are the MOSFET and ISFET fabrication steps:

1) deposition/patterning of metal electrodes by evapora-tion, sputtering, liftoff, for MOSFETs [122]–[126];

2) deposition of additional metal oxides/nitrides byLPCVD (alumina, silicon nitride) for ISFETs[134]–[137];

3) optional membrane formation by anisotropic etchingfor temperature stabilization [142];

4) deposition of electroactive polymers, membrane mate-rials, and hydrogels by spin-casting, spraying, screenprinting for CHEMFETs [138]–[143].

The fabrication of field-effect-based electrochemical sensorsintegrated with CMOS circuitry is described in [134], [136],and [144].

V. CUSTOM-DESIGNEDSENSORS ANDSENSORFABRICATION

PROCESSESVERSUSIC-TECHNOLOGY-BASED APPROACHES

Key features of sensors in consumer-product applicationsinclude low costs, low power consumption, small size,on-chip calibration possibility and device robustness. Thistypically requires a large amount of control and signal pro-cessing functions to be integrated together with the sensorelements. In some cases, auxiliary sensors such as temper-ature sensors are needed to deal with cross sensitivities ofthe sensing device. Since the first papers on micromachinedsensors appeared in the 1970s, the question whether it is

more advantageous to use monolithic systems combiningCMOS circuitry and sensor structure on the same chip(CMOS-MEMS) or to use hybrid designs with optimizedsensor processes and external electronics has been intenselydiscussed. Table 3 gives an overview on the technologicaloptions for hybrid and monolithic designs. For hybriddesigns, most of the micromachining techniques describedin Sections II and III are available through inexpensivemultiproject-wafer (MPW) prototyping services. However,only a limited selection of MEMS techniques is available formonolithic designs. For fabless design-houses, which relyon MPW services for prototyping, this selection is furtherreduced to a small number of pre-CMOS and post-CMOSmicromachining options.

Micromachined chemical sensors are not yet establishedon the market. For a comparison of the commercial successof hybrid versus monolithic design, we hence analyze the sit-uation for well-established acceleration and pressure sensors.The commercially available products on the market provideno clear answer, which approach offers more competitive ad-vantages: while the products of Infineon [155], and Motorola[156] are based on monolithic integration, the sensor systemsoffered by SensoNor [157], Intersema [158], Delphi [159],and NovaSensors [160] use hybrid designs. Some companiessuch as Bosch [161] offer monolithic and hybrid products. Atrend toward monolithic solutions can be identified with re-gard to larger production volumes and more severe cost re-strictments.


For chemical sensor arrays, a monolithic solution offersseveral advantages despite the high initial costs and the re-strictions in the choice of the micromachining technique.

1) The full performance of capacitive, resonant, andcalorimetric sensors is only exploited in monolithicimplementations [162], [163] because, e.g., the influ-ence of parasitic capacitances and crosstalk-effectscan be significantly reduced.

2) The number of electrical connections prominentlycontributes to the overall system costs. The monolithicimplementation of a single-chip gas sensor (see Sec-tion VI-C) with three micromachined sensors requiresonly seven connections (three for supply voltages,one for a clock signal, one for reset, and two for theserial interface). Up to 16 chips can be connectedwithout adding any additional communication linesby implementation of a digital bus interface. A hybridapproach would require at least 30 pads for the threesensors and a total of 480 connections, if 16 sensorsof each type would be combined.

3) Hybrid implementations require complex packagesto reduce sensor interference, to minimize electriccrosstalk, and to optimize the critical connections.This further complicates the already difficult task ofchemical sensor packaging.

4) The use of a standardized digital bus interface makesthe system more scalable, because an additional sensorchip can be added without changing the system archi-tecture or the packaging scheme.

5) The typical development time until full commercial-ization of hybrid micromachined sensors is more than20 years (first concept to off-the-shelf product). Alarge part of this time is consumed by establishinga dedicated and reliable fabrication technology. Theproduction equipment and cleanrooms require largeinvestments. CMOS-MEMS technology relies on es-tablished and qualified industrial CMOS technologyto fabricate the read-out circuitry and the basic sensorstructures. The technology development is reduced toa few postprocessing steps that are characteristic forthe sensor. This reduces development time and enablesthe business model of fabless MEMS companies.

6) The response time of, e.g., a gas sensor is in mostcases determined by the volume of the measurementchamber and the flow rate (other relevant processes in-clude also, e.g., diffusion or dissociation). Using themonolithic approach and a suitable packaging tech-nique (e.g., flip-chip packaging), the volume of themeasurement chamber can be kept very small.

7) Auxiliary sensors such as temperature or flow sensors,which are often needed with chemical sensors, can becointegrated. Calibration and self-test features also canbe realized on-chip.

The main disadvantage of a monolithic CMOS-MEMSsolution is the restriction to CMOS-compatible materialsand micromachining processes. For post-CMOS microma-chining, processing steps requiring, e.g., high temperatures

Fig. 17. Micrograph of a capacitive microsystem including asensing and reference capacitor, and the�� readout circuitry.Both capacitors can be placed on membranes by anisotropic siliconetching (electrochemical etch-stop) [168].

( 400 C) cannot be used, because the aluminum metalliza-tion is destroyed, and the transistor parameters are altereddue to changes in the thermal budget of the CMOS process.

VI. I NTEGRATED MICROSENSORSYSTEMS

After the discussion on custom-designed sensors versusIC-technology-based fabrication approaches in the previoussection, we will now present case studies of monolithicallyintegrated chemical sensor systems in this section. Micro-electronics and micromechanics (MEMS-structures) havebeen realized on a single chip allowing for electronic on-chipcontrol and monitoring of the mechanical functions as wellas for data preprocessing such as signal amplification, signalconditioning, and data reduction. Additional system featuresinclude A/D and D/A conversion and the implementation ofstandard interfaces on the sensor system chip.

A. Integrated Capacitive Sensor System

Chemocapacitors (dielectrometers) rely on changes in thedielectric properties of a sensing material upon analyte ex-posure. Interdigitated electrode structures are predominantlyused [164]–[166]. The capacitances usually are measured atfrequencies between a few kHz and a few MHz.

The interdigitated capacitors described here [17], [167],[168] exhibit an electrode width and spacing of 1.6m.The total footprint of a capacitor is 800800 m . Sincethe nominal capacitance of such a microstructure is onthe order of 1 pF (parasitic capacitances of approximately10 pF), and since the expected capacitance changes (sensorsignals) are in the range of some attoFarads, an integratedsolution with on-chip circuitry is required. It is almostimpossible to transfer such minute analog signals via bondwires and cables to external desktop instruments. The inte-grated solution includes two capacitors, a polymer-coatedsensing capacitor, and a silicon-nitride-passivated referencecapacitor in a switched-capacitor scheme (see Fig. 17)[168]. The sensor response is read out as a differentialsignal between the polymer-coated sensing and a passivatedreference capacitor. The integrator is the first stage of a1-b second-order modulator. For static measurements,a simple 20-b counter is cointegrated on-chip to decimatethe digital bitstream at the output of the modulator. For


Fig. 18. Frequency responses of a capacitive chemical sensorwith switched-capacitor readout upon exposure to different analytesat 30 C. For a thick layer of PEUT (4.3�m), toluene (dielectricconstant lower than that of PEUT) causes negative frequency shifts,ethanol (dielectric constant higher than that of PEUT) causespositive frequency shifts [17].

dynamic measurements, the bitstream is decimated using anexternal digital filter (third-order comb-filterFIR-filter).The detailed implementation of the modulator can befound in [171]. A micrograph of a capacitive chemicalmicrosystem chip is shown in Fig. 17.

Two effects change the capacitance of a polymeric sensi-tive layer upon absorption of an analyte: 1) swelling and 2)change of the dielectric constant due to incorporation of theanalyte molecules into the polymer matrix [167]. The capac-itance change of a polymer layer with a thickness of approxi-mately half the periodicity of the electrodes is determined bythe ratio of the dielectric constants of analyte and polymer.If the dielectric constant of the polymer is lower than that ofthe analyte, the capacitance will be increased. Conversely, ifthe polymer dielectric constant is larger, the capacitance willbe decreased (see Fig. 18). These effects have been furtherdiscussed and supported by simulations in [170].

Sensor signals obtained with a 4.3-m-thick polymer layer[poly(etherurethane)] are shown in Fig. 18. The ratio of thedielectric constants of polymer (2.9) and analytes (toluene:2.4, ethanol: 24.5) controls the signs of the signals. The ca-pacitance changes are recorded as frequency changes as aconsequence of the on-chip circuitry (switched capacitors,

conversion); for details, see [17], [167], and [168].For conducting measurements at defined temperatures,

sensor and reference capacitors can be placed on thermallyisolated membrane structures (membranes) [168].


1) unaltered 15-mask commercial CMOS process pro-vided by austriamicrosystems, Unterpremstätten, Aus-tria;

2) optional anisotropic silicon etching, membrane forma-tion for temperature stabilization [168];

3) polymer coating of the sensing capacitor using an air-brush method.

The fabrication of capacitors integrated with CMOS circuitrycomponents is described in [17], [165], [167], [168], [172],and [173].

Typical applications include humidity sensing with poly-imide films [165], [166], [172], [173], since water has a highdielectric constant of 78.5 (liquid state) at 298 K leadingto large capacitance changes. CMOS-based integrated ca-pacitive humidity sensors are commercially available from,e.g., Sensirion AG, Switzerland [174]. More recent applica-tions also include the detection of organic volatiles in thegas phase using polymeric layers [17], [165], [167], [168],and the detection of nitrogen oxide [175], or carbon dioxide[176], [177] using ceramic materials.

B. CMOS Integrated Voltammetric/Amperometric Sensor

Two different electrode configurations are commonlyused for voltammetric/amperometric measurements. Thetwo-electrode configuration [41]–[43] consists of a refer-ence electrode (RE) and a working electrode (WE). Thedisadvantage of this method is that the RE carries currentand may become polarized if it is less than 100 times thesize of the WE. Material consumption due to the currentin the RE is another problem. A better approach is hencethe use of a three-electrode-system [41]–[43] in a potentio-static configuration. An additional auxiliary electrode (AE,sometimes denoted counterelectrode CE) is introduced forcurrent injection in the analyte [42], [178]. The referenceelectrode is now a true RE with a well-defined potential,since no current is flowing through the RE. The potentiostatcontrols the current at the auxiliary electrode as a functionof the applied potential. This is realized in practice with anoperational amplifier [178].

The measured current at any given potential differencedepends on the material properties, the composition andgeometry of the electrodes, the concentration of the elec-troactive species (presumably the target analyte) and themass transport mechanisms in the analyte phase [41]–[43],[178]. Among those aremigration, the movement of chargedparticles in an electric field;convection, the movement ofmaterial by forced means like stirring or from density ortemperature gradients; anddiffusion, the movement of ma-terial from high-concentration regions to low-concentrationregions. The electrochemical reactions at the electrodes arenormally fast in comparison to the transport and supplymechanisms. Since convection in the electrode vicinity isavoided, and migration is suppressed by, e.g., a large excessof electroinactive salts (at the respective potential), diffusionis normally regarded to be the dominant mechanism. Thereare two components to the measured current, a capacitivecomponent resulting from redistribution of charged andpolar particles in the electrode vicinity and a componentresulting from the electron exchange between the electrodeand the redox species (analyte) termed Faradaic current[41]–[43], [178]. The Faradaic component is the importantmeasurand and is, in the case of diffusion-limited condi-tions, directly and linearly proportional to the target analyteconcentration [178].

A picture and a schematic of the CMOS-based three-elec-trode voltammetric/amperometric sensor is shown in Fig. 19[179]. The monolithic device includes the electrochemicalsensor, a temperature sensor, and interface circuitry. The


Fig. 19. (a) Micrograph and (b) layout of a CMOS-based three-electrode amperometric sensor.Reprinted from [179] with permission.

circuitry contains an operational amplifier as potentiostat,a switched-capacitor current-to-voltage converter and aclock generator. Interface circuitry and temperature sensorare realized in 3-m CMOS-technology [179]. The circuitoperates at a supply voltage of2.5 V, can apply voltagesfrom V to V to the sensor and handles current rangesfrom 30 nA full scale to 1 A full scale. The output voltageof the temperature sensors is proportional to the absolutetemperature and has a sensitivity of 125V/K. The totalsensor dimensions are 0.75 by 5 mm.


1) CMOS IC process provided by INVOMEC MPC Ser-vice, IMEC, Leuven, Belgium [179];

2) additional silicon nitride as protective coating;3) patterning of metal electrodes (liftoff, thermal evapo-

ration, sputtering) [178]–[183];4) deposition of membrane materials (liftoff, pho-

tolithography, screen printing) [179]–[183];5) realization of the reference electrodes: silver/silver

chloride

Sensor processing sequences are given in [178]–[181].The reference electrodes in liquid phase are in most casessilver/silver chloride elements.

Typical applications include chemical analysis in gas orliquid phase. Typical target analytes in the gas phase includeoxygen [181]–[183], and carbon dioxide using liquid elec-trolytes [180]. Target analytes in liquid phase comprise dis-solved oxygen [179], [184]; glucose [179], [185], [186]; orhydrogen peroxide [180], [187]. If the target analyte is notan electroactive species like the glucose molecules moni-tored with the integrated system here, polymer electrolytes orenzymes (glucose oxidase) producing analyte-related ionicspecies are used in the sensitive electrode coatings.

C. CMOS Monolithically Integrated Multisensor System

The third case study is a CMOS single-chip chemicalmicrosensor system, which combines three different trans-ducers, a mass-sensitive cantilever, a capacitive sensor, and acalorimetric sensor, all of which rely on polymeric coatings

Fig. 20. Micrograph of the gas microsensor system chip (size: 7by 7 mm) [188]. The different components include: 1) flip-chipframe; 2) reference capacitor; 3) sensing capacitor; 4) calorimetricsensor and reference; 5) temperature sensor; 6) mass-sensitiveresonant cantilever; and 7) digital interface.

as sensitive layers to detect airborne volatile organic com-pounds (VOCs) [188], [189]. The three transducers respondto fundamentally different analyte molecule properties. Oneof them responds to the mass of sorbed molecules, anotherresponds to the heat of absorption, and the third responds tothe dielectric properties of the absorbates. The monolithicsystem such simultaneously provides three different (“or-thogonal”) sensor responses, which are used to classify orquantify analytes in the gas phase [188], [189].

The capacitive sensor has already been described in thepreceding section and again is integrated with a fully differ-ential second-order Sigma–Delta-modulator circuitry and acounter to decimate the output bit stream [167], [168].

The second type of transducer is a micromachinedcantilever, the operation principle of which has also been de-scribed previously in the context of mass-sensitive chemicalsensors [17], [22], [53]–[55]. The 150-m-long cantileverconsists of mainly silicon as well as CVD-deposited andthermal oxide. The different thermal expansion coefficientsof the silicon and the oxide (bimorph effect) are used to


Fig. 21. Sensor signals simultaneously recorded from all three polymer-coated transducers uponexposure to 1200 and 3000 ppm of ethanol and 1000 and 3000 ppm of toluene at 30C [188].(a) Frequency shifts (Sigma–Delta converter output) of the capacitor. (b) Frequency shifts of theresonating cantilever. (c) Thermovoltage transients of the calorimetric sensor. The close-up shows thedevelopment of the calorimetric transient within 6 s.

initiate a cantilever vibration by periodically applyingelectric pulses to heating resistors embedded at the can-tilever base [22]. The cantilever resonates at its fundamentalmechanical frequency of 380 kHz with a quality factorof approximately 1000 in air. The cantilever vibration isdetected by embedded piezoresistors in a Wheatstone-bridgeconfiguration [22]. An on-chip low-noise fully differentialdifference amplifier (DDA) with a gain of 30 dB amplifiesthe output signal of the Wheatstone bridge. The cantileveracts as the frequency-determining element in a feedbackoscillation circuit, which is entirely integrated on the chipwith a counter. For more details on the electronics, see [191].

The third transducer is a thermoelectric calorimeter basedon the Seebeck effect (see Section IV-B and [72]–[79]).Here, 256 polysilicon/aluminum thermopiles connected inseries are sandwiched between the dielectric membranelayers to record sorption-induced temperature variations.The generated thermovoltage is proportional to the temper-ature difference (mK-range). The sensor system includestwo 500- m by 500- m dielectric membranes; one of themis coated with the gas-sensitive polymer, the uncoated oneserves as a reference [75], [190]. The sensing and referencethermopiles are connected in parallel to the input stage ofa low-noise chopper-stabilized instrumentation amplifieron-chip (gain: 8000, bandwidth: 500 Hz) to record thetemperature differences between the two membranes. Thethermovoltage is translated into a digital signal on chip usinga Sigma–Delta analog/digital converter and a decimationfilter.

The overall system chip additionally includes a tempera-ture sensor since volatile absorption in polymers is stronglytemperature-dependent. The temperature sensor relies on thelinear temperature dependence of a bipolar transistor avail-able in the CMOS process. The voltage is converted to a dig-ital signal using a Sigma–Delta converter. After calibration,the temperature sensor exhibits an accuracy of 0.1 K at op-eration temperatures between 223 and 383 K.

A micrograph of the overall system is displayed in Fig. 20[188]. The overall chip size is 7 by 7 mm. The chip ex-hibits all the sensor-specific driving circuitry and signal-con-ditioning circuitry, which has been described in the context

of the different transducers [189]. The analog/digital conver-sion is done on-chip, which gives a favorable signal-to-noiseratio since noisy connections are avoided and a robust dig-ital signal is generated on chip and then transmitted to anoff-chip data port via an I2C digital interface.1 The I2C businterface offers the additional advantage of having only veryfew signal lines (essentially two) for bidirectional commu-nication and also allows for operating multiple chips on thesame bus system [189]. An on-chip digital controller man-ages the sensor timing and the chip power budget. The sen-sors are located in the center of a metal frame, which is usedto apply a flip-chip packaging technique [192].


1) unaltered 15-mask commercial CMOS process pro-vided by austriamicrosystems, Unterpremstätten, Aus-tria;

2) anisotropic potassium hydroxide etching of the mem-brane structures (calorimetric sensor and cantilever);

3) front-side RIE to release the cantilever;4) polymer coating of all sensors using an airbrush

method.

Fig. 21 displays simultaneously recorded sensor signals ofall three transducers upon exposure to 1200 and 3000 ppmof ethanol, and 1000 and 3000 ppm of toluene at 30C[188]. The sensors were alternately exposed to analyte gasand pure carrier gas. The polymeric coating consisted ofpoly(etherurethane), PEUT [17], at a thickness of approx-imately 4 m. Fig. 21(a) shows the measured frequencysignals (Sigma–Delta converter output) of the capacitor.Ethanol exhibiting a dielectric constant of 24.5, which islarger than that of PEUT (2.9), causes a capacitance increaseand hence positive frequency shifts; toluene with a dielectricconstant of 2.4 causes a capacitance decrease and negativefrequency shifts.

Fig. 21(b) displays the cantilever response. The resonancefrequency decreases with increasing oscillating mass as aconsequence of volatile absorption. Ethanol shows rather lowsignals as compared to toluene due to its lower molecular

1I2C is a communication standard developed by Philips, Eindhoven, TheNetherlands.


mass and due to its lower enrichment (partitioning) in thepolymeric phase. The extent of physisorption is in a first-order approach proportional to the analyte boiling tempera-ture (ethanol: 351 K, toluene: 360 K): the lower the boilingtemperature, the less enrichment in the polymer [193].

The calorimetric sensor response in Fig. 21(c) representsa superposition of partitioning and heat budget change uponanalyte ab/desorption. The absorbing analyte liberates heat(heat of condensation) causing a positive transient signal(positive peak), whereas the desorbing analyte abstractsvaporization heat from the environment generating a nega-tive transient signal [negative peak upon purging; see Fig.21(c)]. The close-up in the upper left corner shows thetime-resolved response upon 3000 ppm of toluene within thefirst 6 s. The condensation/vaporization heat of ethanol islarger (ethanol: 42.3 kJ/mol, toluene: 38.0 kJ/mol at 298 K)due to directional interaction in the liquid phase (compare,e.g., water), whereas polymer partitioning of toluene isstronger owing to its higher boiling point.

The signals of all three transducers linearly correlate withanalyte concentration at low concentrations (less than 3% ofsaturation vapor pressure at the operating temperature) [193].To further improve analyte identification/quantification, anarray of microsystem chips coated with different polymerscan be used.

The monolithic CMOS gas microsystem is targeted atidentifying organic solvents in transport containers or pro-viding workplace safety in, e.g., chemical industry. It willform part of a handheld or credit-card-size detection unit.

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Andreas Hierlemann received the Diploma de-gree in chemistry and the Ph.D. degree in phys-ical chemistry from the University of Tübingen,Tübingen, Germany in 1992 and 1996, respec-tively.

In 1997, he was a Postdoc at Texas A&MUniversity, College Station. In 1998, he wasa Postdoc at Sandia National Laboratories,Albuquerque, NM. He is currently a member ofthe technical staff at the ETH Zurich, Zurich,Switzerland. The focus of his research activities

is on chemical sensors and interfacial design.

Oliver Brand received the Diploma degree inphysics from Technical University Karlsruhe,Karlsruhe, Germany, in 1990 and the D.Sc.degree from ETH Zurich (Swiss Federal Instituteof Technology), Zurich, Switzerland, in 1994.

From 1995 to 1997, he was a PostdoctoralFellow with the Georgia Institute of Technology,Atlanta. In 1997, he joined the Physical Elec-tronics Laboratory of ETH Zurich as ProjectLeader, Group Leader, and Lecturer. In 2003he was appointed Associate Professor at the

School of Electrical and Computer Engineering of the Georgia Institute ofTechnology. His research interest focuses on the development and pack-aging of microsensors and microactuators using industrial IC processes incombination with postprocessing micromachining steps.

Christoph Hagleitner received the Diploma de-gree in electrical engineering and the Ph.D. de-gree from the ETH Zurich, Zurich, Switzerland,in 1997.

From 1997 to 2002, he was a ResearchAssistant at the Physical Electronics Lab, ETHZurich. Since February 2003, he is workingwith IBM, Rüschlikon, Switzerland.The focusof his research activities was on system designand interface circuitry development for smartchemical sensors.

Henry Baltes (Fellow, IEEE) received the D.Sc.degree from ETH Zurich, Zurich, Switzerland, in1971.

From 1974 to 1982 he was with Landis &Gyr (now Siemens), Zug, Switzerland, where hedirected the solid-state device laboratory. From1983 to 1988, he held the Henry Marshall ToryChair at the University of Alberta, Edmonton,Canada. In this position, he investigated sil-icon-based microsensors and served as ActingPresident of the Alberta Microelectronic Centre

and Director of LSI Logic Corporation of Canada. Since 1988, he has beenProfessor of Physical Electronics at ETH Zurich and directs the PhysicalElectronics Laboratory active in integrated micro and nano systemsincluding chemical sensors and bioelectronic devices. From 1992 to 1996,he served as Program Director of the Swiss Federal Priority ProgramLESIT. In 1996, he was Visiting Professor at Stanford University, Stanford,CA, and the University of Waterloo. He is also a Cofounder of the spinoffcompany Sensirion, which produces CMOS-based sensors. He serves asco-editor of the Wiley-VCH book seriesSensors Updateand editor of theSpringer book seriesMicrotechnology and MEMS. His publication listincludes 500 papers, 15 patents, and six books.

In 1998, Dr. Baltes received the European Science Award of the KoerberFoundation, in 1999 the Wilhelm Exner Medal of the Austrian Trade Asso-ciation, and in 2000 the degree of Doctor of Engineering (honoris causa) ofthe University of Waterloo. In 2001. he co-chaired the 14th IEEE Interna-tional Conference on MEMS. In 2002 he was elected Fellow of the IEEE forcontributions to the development and commercialization of CMOS-basedMEMS. In 2002–2003 he was Visiting Scientist at Ritsumeikan University,the University of Bologna, Bologna, Italy, and the University of Freiburg.He is a member of the Swiss Academy of Technical Sciences.


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