2005 International Symposium on Electronics Materialsand Packaging (EMAP2005), December 11-14, 2005,Tokyo Institute of Trechnology, Tokyo, Japan

SAW chemical sensors based on AlGaN/GaN piezoelectric material system: acoustic design andpackaging considerations

L. Rufer', A. Torres', S. Mir', M. 0. Alam2, T. Lalinsky3, and Y. C. Chan2

'TIMA Laboratory, 46 Av. Felix Viallet, 38 031 Grenoble, France,2Department of Electronic Engineering, City University ofHong Kong, 83, Tat Chee Avenue, Kowloon Tong. Hong Kong,

3Institute of Electrical Engineering, Slovak Academy of Sciences, Bratislava, Slovak Republic

*Corresponding Author: Prof. Y. C. Chan, Fax: (852)-27887579,e-mail: "Prof. Y. C. Chan" EEYCCHANU"a citvu.edu.lik

AbstractIn this paper, we present the modeling of the mechanical

part of a MEMS (Micro-Electro-Mechanical Systems)-basedsensor for identifying enviromnental contaminants andchemical or biological agents in large applications scale. Themechanical part involves the structure for the generation andreception of the surface acoustic wave likewise the packagingand housing structure of the sensor.

Sensor detection mechanism is based on the changes of theSurface Acoustic Wave (SAW) propagation along thesubstrate. By using various coatings on the surface of theSAW device, various cells, chemicals, gases and bio materialscan be detected due to changes of the velocity or phase of apropagating acoustic wave induced by the outer environment.

The AlGaN/GaN material system preferentially grown onboth silicon and sapphire (A1203) substrates by Metal OrganicVapor Phase Epitaxy (MOCVD) or by Molecular BeamExpitaxy (MBE) is a promising platform for fabrication of anew generation of wireless SAW sensor devices. This willimplicate the development of High Electron MobilityTrnsistor (HEMT) structure integrated in a single chip withthe SAW sensor and thus creating a unique acoustic velocitytuning device with low acoustic loss and high frequency.1. Introduction

There is a renewed emphasis on development of robustsolid-state sensors capable of reliable operation in harshenvironments. These sensors should be capable of detectingchemical, gas, or biological releases as well as sending signalsto cental monitoring locations [1]. However, current methodsare costly and time intensive and limitations in sanpling andanalytical techniques exist. A need exists for accurate,inexpensive, real-time, in-situ analyses using robust sensorsthat can be remotely operated.

Microfabrication leading to Micro-Electro-MechanicalStructures (MEMS) can be utilized to provide a niniaturizeddevice that provides a fast response with an ability to utilizemultiple analysis channels for enhanced versatility andchemical discriminationL The MEMS based device improvesthe sensitivity and selectivity to individual chemicals by usinga cascaded approach where each channel includes achemically selective Surface Acoustic Wave (SAW) detector.Detection of the analyte is achieved using an array of SAWsensors acting as sensitive mass detectors. By coating theSAW devices with chemically distinct thin film materials, a

unique pattern of responses for different chemical analytes canbe used to provide chemically selective detection.

Surface Acoustic Wave Sensors (SAWS) are smallminiature sensors used to detect Volatile Organic Compounds(VOCs). A SAW device consists of an input transducer, achemical adsorbant film, and an output transducer on apiezoelectric substrate [2]. The piezoelectric substrate istypically quartz but other materials showing piezoelectriceffect can be considered. The input transducer launches anacoustic wave, which travels through the chemical film and isdetected by the output transducer. The device runs at a veryhigh frequency of hundreds of MHz. The velocity andattenuation of the signal are sensitive to the viscoelasticity aswell as the mass of the thin film, which can allow for theidentification of the contaninant. Heating elements under thechemical film can also be used to desotb chemicals from thedevice. A signal pattern recognition system that uses aclustering technique is needed to identify various chemicals.Chemical sensing materials consisting of pure or mixed noblemetal catalytic thin films, binary oxide thin films (zirconia,titania, tin dioxide) with and without metal ion doping, andtransition metal ion activated surfactant-templated mesoporousmetal oxide films. SAWS have been able to distinguishorganophosphates, chloinated hydrocarbons, ketones,alcohols, aromatic hydrocarbons, saturated hydrocarbons, andwater.

The field of new Micro-Electro-Mechanical structures wasup to now practically dominated by the silicon (Si). Thereason for that was the well-established CMOS technology ofmicroelectronic devices and circuits which in combinationwith extraordinary good mechanical properties of Si determinethe development of complicated nuiltifntional Micro-Electro-Mechanical Systems - MEMS based on supportedstatic as well as dynmic micromechanical structures [3].

In relation to this, III-V and III-N semiconductor materialswith their main representatives such as GaAs, AlAs (GalliumArsenide, Aluminum Arsenide) and GaN, AIN (GalliumNitride, Aluminum Nitride) offer better material from severalpoints of view and technological properties usable in thedesign of new MEEMS. Attractive are predominantly theirphysical properties: piezoelectric and piezoresistive effect,direct and wider energy band gap, lower temperatureconductivity and a higher mobility and saturation velocity ofelectrons. An important advantage of these materials is thelevel of epitaial growth of related heterostructure material

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systems (GaAs/AlGaAs, GaN/AlGaN) with the preciselydefined thickness, composition and doping concentration level.For the growh, the MBE orMOCVD methods are being used.This high level of technology enables using techniques ofsurface and bulk micro-(nano-) patterning of mechanicalstructures (cantilevers, bridges, membranes and islands) withprecisely controlled thickness and homogeneity on the level ofone or two monoatomic layers 14].

Nowadays, AlGaN/GaN based HEMT structures representthe most convenient devices for high power applications notondy in the microwave but also in the millimeter wavefrequency band. The charge carriers in two-dimensional (2D)channel of AlGaN/GaN HEMT are induced by thespontaneous and piezoelectric polarization and are inequilibrium with the positive charges at the surface. Hence anycharge changes at the non-passivated surface cause extremelysensitive changes in the concentration of the 2DEG localizedat the AlGaN/GaN heterointerface. This effect may be used inthe development of the new generation MEMS sensor andactuator devices with a high sensitivity, chemical stability anda short time response and an ability to function at hightemperatures (500 - 800 'C). This was confimned by the firstresults of the study of gateless AIGaN/GaN HEMT structuresin which extremely sensitive changes of the drain current wereobserved as a function of polarity and concentration ofpolymer solutions [51 as well as external mechanical stress [6].

AIN and GaN based semiconductor materials are excellentcandidates mainly for surface-acoustic-wave (SAW) sensordevices due to their large electromechanical (piezoelectric)coupling coefficients and a high SAW velocity (7].Conventional piezoelectric materials, such as quartz, lithiumniobate and lithium tantale, offer limited performance due totheir relatively low SAW velocities of (- 3 000 - 4 000 ms-').Comparing to this, AMN seems to be almost an ideal SAWmaterial (SAW velocity - 5 600 ms').

The detection mechanism of the sensor devices utilizes thechanges to the velocity or phase of an acoustic wavepropagating along the substrate due to changes to thecharacteristics of the propagation path. This is due to the factthat the energy of the surface acoustic wave is stored only inthe top region (< 1.5 times the wavelength) of the surface ofthe material in which this wave propagates. Therefore, thesurface of a SAW sensor device is extremely sensitive to theambient environment. Thus even slight changes in theenvironment on the surface can be detected.

AlGaN/GaN based HEMTs can be integrated with theSAW sensor devices to create a unique acoustic velocitytuning device with low acoustic loss, high frequency and lowloss RF performance. The 2DEG in the HEMTs interacts withthe lateral electric field resulting in Ohmic loss, whichattenuates and slows the surface acoustic wave. Thismechanism can be used to tune the acoustic velocity.

The mentioned SAW sensor devices are not only relativelycheap, very sensitive and reliable, but also do not need a dc-power supply for certain operations, which makes them perfectfor wireless applications. SAW devices operating in the kHz toGHz range can be designed and integrated with wirelessremote sensing applications.

2. Sensor ArchitectureThe schematic view of a two-port SAW device used as a

gas-sensitive part of the chemical sensor is shown in Figure 1.The generation of acoustic waves on the substrate is carriedout by means of Interdigital Transducers (IDTs). An IDT is afmigerlike periodic pattern of parallel in-plane metallicelectrodes, usually aluminum, used to build up the capacitanceassociated with the electric field that penetrates into thematerial.

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FingeirOverlap

Figure 1: Schematic view ofa two-polt SAW device.

An IDT is fabrcated by deposition and photolithographictechniques onto the substrate and allows the conversion ofhigh frequency (MHz-GHz) electrc fields in acoustic wavesand vice-versa. By changing the length, width, position, andthickness of the electrodes as well as the number of electrodesand the pattern shape of the IDT, process called apodization,the performance of the transducer can be optimized [8,9]. Inour case, the sensor has been designed for the workingfrequency of 250 MHz. This frequency is the result of thedemands put on the substrate size and sensor sensitivity. Thedimensions of the acoustic part of the sensor are thus 8 x 5.5mm, the sensitive coating covers the area of 3 x l-5 mm.

The sensor detection of an analyte is based on thesensitivity of a device to the SAW velocity perturbations givenby a mass loading induced by an exposure to a specificchemical environment. There were several detection systemsproposed earlier in the literature. These include oscillator loopconfiguration, phase detection configuration, and Phase-Locked Loop (PLL) based detection system. The latterconfiguration was used in our design. It offers advantages overthe other two systens by combining their best quality asfrequency output and stability. A PLL based detection systemis shown schematically in Figure 2.

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Figure 2: SAW PLL-based detection system.

The system consists in a Voltage Controlled Oscillator(VCO), a phase detector, a SAW device and a frequency

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counter. The phase detector output voltage is used in a closedloop to nmintain a constant phase shift across the SAW sensorby controlling the VCO frequency. The VCO generates a fixedamplitude sinusoidal voltage with a frequency that variesaccording to a control voltage. The changes of the VCOfrequency contain the information about changes of SAWvelocity. In our case, the characteristic parameters are thecentral frequency (fQ) and the sensitivity (Sm.) expressed inHz/V. Analog phase detector is composed of a mixer and alow-pass filter. Its response is generally non-linear butachieves a linear behavior when the phase difference of theinputs is close to ir/2. Despite the non-linear response of thephase detector, the global response results to be linear in arange of phase differences up to 2x.

3. Behavioral Modeling of the Acoustical PartThere are several ways of describing the function and

modeling an interdigital transducer. As we aimed at analyzingthe complete model of the acoustical part along with theelectronic circuitry necessary for the operation of the sensor,we have adopted an approach using equivalent circuits. Suchan approach facilitates the application of any typical softwarefor the simulation of electric circuits. Among the variety ofmodels used for piezoelectric devices, we have chosen theBallato model that has shown a good consistency withanalytical results of frequency response for a SAW delay line110]. It is common also for other types of models forpiezoelectric devices (e.g. Mason, KLM, Redwood) torepresent the acoustic aspects of the device by transmissionlines. A pair of interdigital electrodes can thus be seen as fourdelay lines modelling the area between the edges of theelectrodes (see Figure 3) [11]. The different conditionsexisting under and between the electrodes are consideredaccording to the parameters of each delay line.

V,.

The velocity in the metallized surface depends on thethickness of the deposited metal film and can be calculated bya second-order function of the film-thickness ratio (h/A). Thisfunction can be reduced to a sum of coefficients, which areinferred experimentally, resulting in the self-couplingcoefficient k,l, which is often given in the frequency-normalized form as k' 1:

(2)k, - -k" = Ikllp +krn+ klwhere kD is the wave vector (i.e., phase constant 60) atresonance frequency, (dv/) is the velocity shift of SAW aMdthe subindexes p, m and s refers to the origin of everycoefficient, being respectively the fractional velocity shift dueto the "shorting" of substrate piezoelectric field, to the mass-loading and to the change of the effective "stiffness" of thepropagating surface.

In order to model the reflections of interelectrode acousticwaves, different impedances are assigned to the delay lines.For this modeL the impedance Z0 is assumed to be theequivalent electrical characteristic impedance of the substrate,derived from its mechanical impedance for uniform acousticwave propagation [2]. This impedance is calculated as

ZO= 2C1°k2Cgwhere k2 is electromechanical coupling coefficient, fo is IDTcenter frequency (Hz) and C, is static capacitance of oneperiodic section; C. may also be expressed as C,= Co W, whereCo is capacitance of finger pair per unit length (pF/mm) and Wis fuiger apodization overlap or aperture (mm).

The characteristic impedance of the delay linecorresponding to the area under the electrodes may beexpressed by the following expression [11]:

zi = ZoVVf(4)

Figure 3: Equivalent cicuit for a pair of interdigital electrodes.

The different time delays for the lines result from thechange of propagation velocity due to the metal coating of thesurface caused by the presence of an electrode. For non-apodized IDTs, the metallization ratio ri is defined as hW(h+g),where h is the electrode width and g is the gap betweenconsecutive electrodes. For a uniform IDT with q = 0.5, thedelay time corresponding to the area between the electrodes toand the delay time corresponding to the area under theelectrodes, f, are calculated as follows:

P Pro 4V'r= 4V

f 4V (1)

where Vf and V. are the SAW velocity in the case of a freepiezoelectric surface and a metallized one, respectively.

The capacitor C,, appearing at the electrical port of thecircuit, represents the static capacitance of the electrodes.

Including this model in a simulation environment as anautonomous circuit allows the user to create a repetitive circuitlike the periodic pattem of an IDT. This is achieved byconnectng the acoustical ports of the electrode pairs incascade whereas the electrical ports are connected in parallel.

Once each IDT has been defined, an entire two-port SAWdevice can be modeled as shown in Figure 4.

r~ 77s | ^ We' 7-1i- !V-7., |

Figure 4: Equivalent circuit of an entire SAW device.

The acoustic delay path existing between the IDTs consistsin a delay line with the characteristic impedance Z. and itsmatching resistors at the input and the output of the latter. In

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(3)

order to match the lines properly, the resistors must have thefollowing values:

Ri Zo-Za R =Z (5)Z. -Z. R11where the characteristic impedance of the IDT models issupposed, for the sake of simplicity, to be equal to Z0. Sincethese devices are reciprocal in nature, signal voltages can beapplied to either IDT with the same final result.

In order to validate the model of the acoustic part of thesensor, the electrical equivalent circuit of the device wascomposed through the Schematic Editor of Cadence° andsimulated with Spectre". The simulations consisted in the ACanalysis, which allows observing the variations of signalparameters over a range of input source frequencies. Thebehavior of the electrical models regarding the frequencyresponse was determined by obtaining their Bode plot.

The study case is based on a SAW device consisting in athin film of GaN grown on (001) sapphire substrate. Thephysical properties of the layered configuration have beendeduced from 112, 13] for a kh value of 0,45. The term kh isthe GaN film thickness, h to SAW wavelength ratio sincek= 2,/A. In this case N= 50, h= 3,72 pm, AO= 21,2 pm,C0= 0.2 pF/mm, W= 1 mm and LD= 3 mm The value of theelectromechanical coupling coefficient k2 has been supposedto be 0.01. The expression relating the SAW velocity underthe metallized sulface, Vm with that on the free surface, Vf andthe coupling coefficient k2 has been approximated by theexpression [14]:

( f =2 (6)Supposing the velocity Vf = 5300 ms~', the centre

frequency isfo= Vf/Ao = 5300 / 21,2-106 - 250 MHz, and thevelocity V. = 5273.5 msU'.

The simulated frequency response of the device for thecase ofk2 = 0,01 is shown in Figure 5.

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Id245 Z"

VWY tMHl4255 2S

Figure 5: Magnitude and phase of the simulated transmissionresponse of the SAW devices based on GaN-on-sapphiretechnology.

In order to obtain the sensitivity of the device, expressed inthe frequency shift relative to the gas concentration, thecomplete measurement circuit must be analyzed. This kind ofsimulation performed in time domain using the modeldescribed previously is rather time consuming. For this reasonthe simulations we are currently realizing are based on aVerilog-A block, which models the transient response of theSAW device according to the transient response of theelectrical model. This allows proceeding more efficientlywhen adding more components that interact with the SAWdevice.

4. Packaging DesignPackaging design of the SAW chemical sensors was found

to be the most critical step in real application scale. Thepackages were considered as functional interfaces between theSAW sensor and the environment [15-21]. It must enable theSAW devices to interact with the environment, rather thansimply provide an impermeable barrier between the device andthe environment.

We have developed some SAW chemical sensor packagingapproaches that not only encase the microfabricatedcomponent, but also provide microfluidic pathway with allelectrical interconnects and thermal management features.

Mismatch of thermal expansion co-efficient (TEC) amongmaterials and moisture invasion induced crack anddelamination were considered during the packaging design.Some other factors such as the repeated absorption anddesorption of reaction species that affect the stability of thesubstrate and the quality of sensor surface, may changeresponse behavior that need to be evaluated by standardreliability tests. In addition, the susceptibility of sensorpoisoning by interferents was considered cautiously. However,it is believed that detailed experimental evaluation should beconducted before finalizing the design.

5. ConclusionsThis paper presents the study of a SAW chemical sensor.

In the first part, the model of the acoustic part is introduced,and in the second parll the packaging design is descnbed. Theequivalent circuit of the acoustic part is based on the Ballatomodel describing the behavior of a single IDT electrode. Thismodel offers an intuitive and satisfactory approach to thebehavior of these devices in the frequency domain. However,simulations in the time domain are relatively time consumingbecause of a large number of components the simulator mustdeal with. This was the reason why simulations were ealizedwith a Verilog-A block, which models the transient responseof the SAW device according to the transient response of theelectrical model. This allows proceeding more efficientlywhen adding more components that interact with the SAWdevice. Success of the packaging design will be verified afterreliability testing.

6. AcknowledgmentsThis work is partially funded by the Procore project of

French Ministry of Foreign Affaires and French Ministry of

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Education. Two (MOA & YCC) of the authors would like toacknowledge France/HK Joint Research Scheme (FR/HKJRS)funded project, "Reliable Packaging of Integrated Micro-Electro-Mechanical Sensors" (CityU internal ref. 9050178).

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