Proceedings of the 2009 4th IEEE InternationalConference on Nano/Micro Engineered and Molecular Systems

January 5-8, 2009, Shenzhen, China

Direct Bonding SOl Wafer Based MEMS Cantilever Resonator forTrace Gas Sensor Applicaiton

Dong Ying, Gao Wei, You Zheng

Department ofPrecision Instruments and Mechanology, Tsinghua University, China

II. RESONATOR CONFIGURATION

A. Cantilever Dimension

The operation principle of the resonant gas sensor is basedon the following model [4]:

B. Transducers Arrangement

Thermal driving and piezoresistive sensing mode is adoptedby the resonator. The heating resistors and the piezoresistorsare integrated with the cantilever to construct a MEMSresonator. After the resonator is fabricated and packaged into achip, the gas sensing material will be deposited onto thecantilever to construct a gas sensor. The position of heatingresistors, piezoresistors and gas sensing layer on the surface ofthe cantilever affects the performance of the resonator and thesensor indeed. Trade-offs must be made in the consideration ofthe transducers arrangement.

where m and f are the mass and resonant frequency of thecantilever respectively, 11m is the additional mass loading onthe cantilever that is caused by the specific gas molecules beingtrapped in the sensing layer attached on the surface of thecantilever, and 3f is the changes offdue to the mass loading.Therefore, through monitoring the frequency change 3j, massloading 11m can be measured, and accordingly informationabout the gas concentration can be obtained.

It can be seen from the model that higher resonantfrequency and less mass of the cantilever resonator providebetter sensitivity for the sensor. Therefore the design principleof the cantilever dimension is that under the precondition ofproviding enough space to arrange the vibration driving andsensing elements on the cantilever surface, to get higherfrequency and less mass. Taking the fabrication factors intoconsideration, the dimension of the cantilever with rectanglecross section is 1000f.!m long, 400f.!m wide and 10f.!m thick.The density of silicon is 2.23x 10-15kg/f.!m3, then the mass of thecantilever is about 9f.!g. Calculation using classical formula [4]shows that the fundamental frequency of the cantilever is about14kHz. With a frequency resolution of 1Hz which can be easilyobtained by an ordinary counter, a mass resolution of 1~2ngcan be obtained according to Eq. (1). Therefore trace gasdetection can be expected with this cantilever dimension.

1 ~m----

2 m8ff

Abstract - A thermal driving and piezoresistive sensing MEMScantilever resonator has been proposed and developed toconstruct trace gas detection sensors. The problem of integratingvibration structure, transducers and electric elements is the mainconcern in the design and fabrication of the resonator. In thispaper, the parameters and the configuration of the resonator arediscussed, the fabrication process and the test results arepresented. Finite Element Analysis (FEA) has been carried out tooptimize the configuration of the resonator to obtain highsensitivity and efficiency with a uniform temperature distributionthat is propitious to the function of the gas sensing material. Thefabrication process is based on direct bonding silicon-on-insulator (SOl) wafer and inductive coupled plasma (ICP)etching technology, which conciliate the semiconductor processesand the micromaching processes, and provide precise control ofthe resonator parameters. The experimental test results of thefabricated resonator agreed well with the calculation andsimulation results and demonstrated that the proposed resonatorwas qualified to construct trace gas detection sensors.

Keywords -cantilever, gas sensor, resonator, SOl wafer

I. INTRODUCTION

Micromachined cantilevers have been proposed for avariety of sensor applications [1,2]. With a suitable drivingmechanism cantilever can operate in resonant vibrating modewhere causes of the resonant frequency shift can be detectedwith a frequency tracking mechanism. Several combinations ofvibration driving and frequency tracking mechanisms can beapplied to the micromachined cantilevers [3, 4]. For portablesensor application, it is necessary to integrate the driving andtracking transducers into the cantilever to form a MEMSresonator. Micromachined monocrystalline silicon cantilevershave inherently high quality factor (Q), providing the resonatorwith high frequency resolution. It has been demonstrated thatmicromachined resonant cantilever sensors can achievenanogram-level to picogram-Iever mass sensitivity, whichattracts intensive interests for bio-chemical applications [5].However, fabricating cantilever structure, transducers andelectric paths and pads from a single crystal wafer involvescomplicated processes. Efforts have to be made to compromisethe feasibility of the processes and the performances of thedevice [6]. We have developed a MEMS cantilever resonatorfor on-site trace gas detection application. Thermal driving andpiezoresistive sensing combination was used. A fabricationprocess based on direct bonding SOl wafer was proposed tosolve conflicts in different processes and to improve controlprecision of the device fabrication. In this paper the design,fabrication and test results of the MEMS cantilever resonatorare presented.

This project isfunded by the National Nature Science Foundation ofChina (NSFC) (Project code: 50605040).

*Contact author: Dong YingDept. Precision Instruments and Mechanology, Tsinghua UniversityBeijing 100084, ChinaPhone: 8610-62776000-803Email: dongy@tsinghua.edu.cn

978-1-4244-4630-8/09/$25.00 ©2009 IEEE 134

It is well known that stress increases to the most at the rootof deflected cantilever, therefore to obtain the maximal signalto noise ratio the piezoresistors should be positioned in the areaclose to the root. Another common knowledge is that thehighest efficiency can be obtained if the cantilever is driven atits free end. Also it has been understood that the resonantfrequency has the most sensitivity to the mass loadingoccurring at the tip of the cantilever [7]. Meanwhile thetemperature elevation and distribution on the cantilever due toheating has impact on the performance of the gas sensingmaterial. Generally to say, proper temperature with a uniformdistribution is propitious to the function of the gas sensingmaterial.

FEA simulation has been carried out in order to optimizethe transducers arrangement. The option we made is, twoidentic slits divide the cantilever into three areas and from thetip to the root of the cantilever there are gas sensing layer,heating resistors and piezoresistors ordinally in each of thethree areas. The slits obstruct the heat conduction and produceuniform temperature elevation in the gas sensing area.

c. Heating ResistorsThe thermal driving element is composed of four shunt-

wound resistor strips stretching along the width of thecantilever, as sketched in Fig.1 where dashed lines indicateelectrical connection. Using shunt-wound resistors instead of asingle resistor can avoid local heating due to the nonuniformdoping of the resistor strip. Each of the strips is 100J.! long and25J.!m wide and has 15J.!m space in between. Given the sheetresistance of the doping process 80'"100nJO, the overallresistance of the thermal driving element is about 80~ lOOn.Through electrical-thermal-mechanical couple simulation byANSYS, near 1J.!m deflection at the tip of the cantilever can beproduced under the Ie standard voltage 5V.

.......•....•...•....•

.......•....•....•....•

Figure 1. Sketch of the thermal driving element

D. Piezoresistors

The piezoresistors to construct the Wheatstone bridge areusually 1 ~ 5 kn for pressure or stress measurementapplications. The resistor strips will be too long to bepositioned within the cantilever if the same doping process asthe heating resistors is used to fabricate the piezoresistors.Therefore the resistor strips must be made into zigzag shape

just like those in pressure sensors. For the convenience of theconnection and the compactness of the arrangement, threeparallel resistor strips are connected in serial. Each of the stripsis 150J.!m long and 15J.!m wide and has 15J.!m space in between,as sketched in Fig.2. Because the connection path are verticalto the strips, reverse piezoresistive effect will be arosed. Toeliminate the reverse effect, the resistance of the connectionpaths must be much smaller than the strips, which means theymust be heavy doped. With this strategy the overall resistanceof a piezoresistor is approximately three times the resistance ofthe resistor strip which is about 800~ 1000n if the samedoping process as the heating resistors is used, and therefore2.4~3kn can be obtained.

....-------IV'.~..,.'...'..'..•.•...,..•.•:,..'..•.~.'.........• Connection pathConnectionpail--------~Figure 2. Sketch of the piezoresistor

Four piezoresistors are connected to construct a Wheatstonebridge in the way shown in Fig.3 where the dashed linesindicate electrical connection and PI"'P5 indicate electrodes.P5 is the grounding, P2 is the input port, PI and P4 are outputports, and an adjustable resistor will be connected between P2and P3 to eliminate the unbalance of the bridge due to the non-uniform resistance of the piezoresistors, which is unavoidablein doping process.

Pl ':

:~ :::::::::::::::·~ · ·il I Ir!I :...,..:

r·ir-----"-Ill: I,... '-------~~I

P4 _ 1P5 ; :

Figure 3. The construction of wheatstone bridge from the piezoersistors

E. Simulation Results

The configuration of the cantilever resonator is shown inFig.4. Space is reserved to deposit gas sensing material later.

FEA simulation has been carried out to evaluate thisconfiguration. The fundamental frequency obtained is about14.2kHz. Fig.5 shows the FEA simulation result of thetemperature distribution on the cantilever with 100mW drivingpower. It can be seen that uniform temperature elevation isachieved in the gas sensing area. The stress produced at theroot of the cantilever is about 3MPa which can provide sizablepiezoresistive signal.

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Figure 4. Skech of the cantilever resonator configuration

o 37.2'i4 ?i .487 111. 731 148.97516.622 55.6615 93.109 130.353 1157.5517

Figure 5. FEA simulation of temperature distribution on the cantilever withIOOmW driving power

III. FABRICATION

sal wafer was used as the start material for the fabricationof the resonator. The advantages of sal to MEMS aresummarized in reference [8]. In this project, the sal wafer isdeveloped through directly bonding technique: an oxidatedsilicon wafer is bonded onto a non-oxidated silicon wafer andthen grinded and polished to form a crystal-oxide-crystalsandwich structure, as show in Fig.6 (a). The thin crystal is asthe front of the wafer and usually called device silicon, whilethe thick crystal is as the back of the wafer and called thesubstrate silicon.

The resistors, conduct paths and electrodes were fabricatedon the device silicon. Boron implantation technology was usedto produce resistors. A light doping process of lOOQ/D sheetresistance and a heavy doping process of lOQ/D sheetresistance were used. Aluminium thin film was deposited byelectron beam evaporation technology to make the conductpaths and electrodes. To obtain ideal doping distribution andreal Ohm contact, anneal was performed in the processes.

The micromachining of the cantilever structure wasaccomplished by inductive coupled plasma (ICP) etchingtechnology. The sal wafer was etched from the front to formthe cantilever dimension and subsequently from the back toprovide the hanging height for the cantilever. The oxide

between the device silicon and the substrate silicon acting asstop layer of silicon etching was etched in the end to release thecantilever structure. Actually the cantilever was developedthrough ICP etching successively the nitride film, the oxidefilm and the device silicon from the front of the wafer, andafterwards the substrate silicon and the oxide layer from theback of the wafer. The thickness of the cantilever was preciselyguaranteed by the thickness of the device silicon.

The resistors, contacts, paths, pads and cantilever structurewere patterned through lithography. The whole fabricationprocess included six lithographic steps defining the lightdoping areas, heavy doping areas, Ohm contacting areas, shapeof electric paths and pads, front etching areas and back etchingareas respectively. Therefore six lithographic masks (Ml~M6)had to be developed. The fabrication processes of the resonatorare schematicly presented in Fig.6.

(a) The direct bonding sal wafer

(b) Lightly doping Boron (Ml)

(c) Heavily doping Boron (M2)

(d) Depositing and developing oxide film (M3)

(e) Depositing and developing AI film (M4)

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(f) Depositing nitride film

Figure 8. The packaged chip of the MEMS cantilever resonator

(h) ICP etching from the back (silicon, oxide) (M6)

(g) ICP etching from the front (nitride, oxide, silicon) (M5)IV. TEST RESULTS

The amplitude-frequency characteristics of the cantileverresonator at different levels of excitation have beeninvestigated. Fig.9 shows the amplitude-frequencycharacteristic of the resonator in ambient air under 3V drivingvoltage. The resonant frequency measured is approximate14.92 kHz and the quality factor (Q) is approximate 200.According to the tested parameters of the resonator, nanogram-level mass sensitivity can be expected, which provides thefeasibility for trace gas detection. The cantilever resonator alsoshows advantages of high efficiency and stability in the testexperiments.••

Figure 7. The SEM picture of the cantilever resonator

Figure 6. The schematilc fabrication processes of the resonator

Figure 9. Tested amplitude-frequency characteristic of the resonator

14.92kHz0.1' , , , , ,", , , , I

14.4 14.5 14.6 14.7 14.8 14.9 15 15.1 15.2 15.3 15.4

0.2

V. CONCLUSIONS

The significant advantage of thermal driving andpiezoresistive sensing mode over other operation modes ofMEMS cantilever resonator is comparative testing signal canbe produced with much lower driving voltage (below 5V in thiscase), which permits practical application and providesopportunity of IC integration. The designed configuration forthe MEMS cantilever resonator optimizes the sensitivity andthe efficiency of the resonator and can create uniform

••

Silicon Nitride

Silicon Oxide

Heavy-doped Boron

D

D

Aluminum

Light-doped Boron

Monocrystalline Silicon

Fig.7 is the SEM picture of the resonator fabricated fromthe SOl wafer. The developed wafer was diced, sifted andpackage into chips. Fig.8 is the picture of a resonator chip.

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temperature distribution for the function of the gas sensingmaterial. SOl wafer provides a solution to the integratedfabrication of microstructure, transducers and electronics, andthe precise control of the resonator parameters. The test resultsof the fabricated resonator agreed well with the theoreticalcalculation and FEA prediction. The resonator showedrelatively high quality factor (approximate 200 in the air) in theexperiment. According to the results, the proposed resonator isqualified to construct high sensitivity gas sensor.

With proper gas sensing material and close-loop vibrationcontrol circuit, the MEMS cantilever resonator will be madeinto specific trace gas sensors.

ACKNOWLEDGMENT

This project is supported by the National Nature ScienceFoundation of China (NSFC, No.50405060). The fabrication ofthe resonator was accomplished by the Institute ofSemiconductors, Chinese Science Academy (CSA). We aregrateful to Professor Ji An and Dr. Ning Jin for their advicesand contributions.

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[3] G. Stemme, "Resonant silicon sensors," J. Micromech. Microeng., vol. 1,pp. 113-125, 1991

[4] O. Brand and H. Baltes, "Micromachined resonant sensors- anoverview," Sensors update, vol. 4, pp. 3-51, 1999

[5] H. P. Lang and C. Gerber, "Mechanical Sensors for the Nanoworld,"Bionanotechnology Workshop and Meeting, Oxford University, UK,April 2002

[6] O. Brand, "CMOS-based resonant sensors," IEEE. sensors,Calffonda,USA, April 2005.

[7] Zhou Qin, "Study on MEMS Chemical Gas Sensor Based on ResonatingMicrocantilever", master's degree dissertation, Tsinghua University,2006

[8] A.Y.Usenko, W.N.Carr, "Silicon-on-insulator technology formicroelectromechanical applications", Semiconductor Physics, QuantumElectronics and Optoelectronics, vol. 1, pp. 93-97, 1999

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