Bove, T.

Hindrichsen, Christian Carstensen

Lau-Moeller, R.

Thomsen, Erik Vilain

English

C. C. Hindrichsen

E. V Thomsen

R. Lou-Moller

T. Bove

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MEMS Accelerometer with Screen Printed Piezoelectric Thick Film

A bulk-micromachined piezoelectric MEMS accelerometer with screen printed piezoelectric Pb(ZrxTil )O3(PZT) thick film (TF) as the sensing material has been fabricated and characterized. The accelerometer has a four beam structure with a central seismic mass (3600x3600x500 pm3) and a total chip size of 10x1O mm2. The resonance frequency is found to be 17.5 kHz and the sensitivity 4.15 mV/g for a single beam

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IEEE SENSORS 2006, EXCO, Daegu, Korea / October 22-25, 2006MEMS Accelerometerwith Screen Printed Piezoelectric Thick FilmC. C. Hindrichsen andE. V ThomsenDepartment ofMicro andNanotechnology (MIC)Technical University ofDenmark (DTU)Lyngby, Denmarkcchgmic.dtu.dkR. Lou-M0llerFerroperm Piezoceramics A/SKvistgaard, DenmarkT. BoveInsensor A/SKvistgaard, DenmarkAbstract- A bulk-micromachined piezoelectric MEMSaccelerometer with screen printed piezoelectric Pb(ZrxTil )O3(PZT) thick film (TF) as the sensing material has beenfabricated and characterized. The accelerometer has a fourbeam structure with a central seismic mass (3600x3600x500pm3) and a total chip size of 10x1O mm2. The resonancefrequency is found to be 17.5 kHz and the sensitivity 4.15 mV/gfor a single beam.I. INTRODUCTIONPiezoelectric materials as PZT have the ability to convertmechanical forces to an electrical signal and vice versa.Screen printed TF PZT integrated with MEMS technologyis a way to get smaller, more integrated and cheaper sensorswith increased functionality. The high coupling betweenmechanical and electrical energy is well suited for state-of-the-art accelerometers sensing dynamic accelerations in ahigh bandwidth.A MEMS uniaxial accelerometer with TF PZT has beenpresented by Wang [1] with a 7 ptm thick PZT element and asensitivity ofup to 7.86 mV/g. The design in [1] consists ofan anular membrane and can not be used for tri axialmeasurements. In [2] an accelerometer with four beams anda central seismic mass is presented but with a low sensitivityof0.1 mV/g and a resonance frequency of7.55 kHz.The sensitivity is proportional to the thickness of thePZT element. The design and process flow presented here isoptimized for uni-axial accelerations but can be used for thedevelopment of a tri-axial accelerometer. The thickness ofthe PZT element is 60 ptm allowing higher sensitivities thanpossible with thin film technology. A four beam structureallows for measurements of rotations of the seismic massdue to in-plane accelerations.II. DESIGNSilicon is used as the substrate materialmechanical properties and the advancedprocesses available today. The accelerometerdue to itsfabricationconsist of aFigure 1. Concept drawing ofthe accelerometer with one centralseismic mass (1) and four beams (2) with TF PZT (3) and Au topelectrodes (4) on. The bottom electrode is covering the whole surface.Note that one fourth ofthe accelerometer is removed.central seismic mass (1), -16 mg, fixed by four beams (2)and with the sensing PZT layer (3) screen printed on top ofeach beam as shown in Fig. 1. Vertical acceleration willcause a deflection of the seismic mass and bend the fourbeams in a S-shape as shown in Fig. 2. The bending willinduce an electrical field between the top (4) and the bottomelectrode covering the whole surface. Each beam will givethe same output signal at vertical accelerations. In-planeacceleration causing a rotation ofthe seismic mass will giveinverse output signals for two opposite beams. The design isoptimized for vertical acceleration but the concept with fourbeams allows for tri-axial acceleration measurements infuture designs. FEM modeling is used to optimize thedimensions of the design in order to have a high enoughresonance frequency.The resonance frequency and the sensitivity aredetermined by the dimensions ofthe four supporting beamsand the size ofthe seismic mass. The in-plane shape oftheseismic mass is optimized in order to occupy as much area1-4244-0376-6/06/$20.00 }2006 IEEE 1477Authorized licensed use limited to: Danmarks Tekniske Informationscenter. Downloaded on November 11, 2009 at 05:27 from IEEE Xplore.  Restrictions apply. IEEE SENSORS 2006, EXCO, Daegu, Korea / October 22-25, 2006Figure 2. The S-shape ofthe deflection will cause a non uniform E field inthe PZT layer. The overlap ofthe top and bottom electrode, lP, should notbe longer than halfthe beam length, 1. The E field on the right halfofthebeam would only occur ifthere were an top electrode on the right halfofthe beam.as possible. FEM modeling is used to optimize thedimensions ofthe design.To maximize the sensitivity, it is vital that the activePZT layer does not extend all the way out to the seismicmass, since there will be a negative stress contribution, ax,and opposite electric field, E, close to the seismic mass. Fig.2 shows the shape of the deflection and the E field in thePZT layer. Only the part ofthe PZT layer between the topand bottom electrode will be the active part ofthe PZT. Thelength ofthe top electrode, lp, is halfthe length ofthe beam,1.The ratio between the thickness of the PZT layer andthe silicon beam is optimized in order to have the neutralplane just in between the two layers. The neutral planecoordinate, z, is determined by the geometry of the PZTlayer and the silicon beam, as the ratio ofthe widths, a, theratio of Young's modulus for PZT and silicon, r, and theratio ofthe thicknesses, P.z +ra,2 ( +1) t(1)1+ ra,82)3)4)s5)SiResistSiO2 .TiPt 0PZTAuFigure 3. The process flow for the accelerometer uses three photolithographic masks and two masks for the screen printing process. AASE process is used to etch out the seismic mass and the four beams.III. FABRICATIONThe accelerometer is fabricated with standard MEMSprocessing and for the TF PZT layer a screen printingtechnique is used. The clean room process flow (see Fig. 3)has been developed, aiming to eliminate post processingLimited resolution in the screen printing technique setsan upper limit for, P. All dimensions of the accelerometerare listed in Table 1.TABLE I. The most important dimensions ofthe accelerometer.Figure 4. Shematic drawing ofthe screen printing process. A thick PZTpaste is squeezed through a metal mask patterning the PZT elements. Thesame process is used for the Au top electrode.1-4244-0376-6/06/$20.00 }2006 IEEE1)0- IDimensions [mm]Width ofSi beams 1200Width ofPZT layer 800Length ofSi beams (1) 800Length oftop electrode on beam(lP) 300Width ofseismic mass 3600Thickness ofSi beam [ 37Thickness ofPZT layer [ 601478Authorized licensed use limited to: Danmarks Tekniske Informationscenter. Downloaded on November 11, 2009 at 05:27 from IEEE Xplore.  Restrictions apply. IEEE SENSORS 2006, EXCO, Daegu, Korea / October 22-25, 2006AmplifierCable +Accelerometer ReaderFigure 5. Accelerometer with the central seismic mass and the four beamson a finger tip. Signal read out is done with wire bonding connected to thefour circular bonding pads.after the TF deposition. 500 ptm thick double-side polished(100) 4" silicon wafers are used as substrate material. Threephotolithographic masks and two additional masks for thescreen printing process are used in the fabrication process.An Advanced Silicon Etch (ASE) process is used forthinning down the beams from the backside to get thedesired thickness of the silicon beams, t (step 1-2). Thebeam thickness, t, is set by controlling the etch time.Standard positive photo resist is used as the maskingmaterial. A dedicated diffusion barrier layer and the bottomelectrode layers are electron beam deposited on the frontside. jTijPtSiO2j has proven to be good stack of barrierlayers [2]. The metal layer stack is covering the whole waferexcept where the second ASE process is utilized from thefront side. A lift offprocess ofthe negative photo resist isused to define the pattern ofthe metal layers (step 3). Mask2 is used in a negative lithographic process and mask 3 isthe inverse ofmask 2 and is used in a positive lithographicprocess. For releasing the seismic mass and the beams thesecond ASE process is used from the front side (step 4). AFigure 7. A charge amplifier is used to transformthe high impedancesignal fromthe accelerometer into a low impedance signal. Cable lengthand input impedance ofthe read out equipment is not affecting the signal.handle wafer is bonded to the backside enabling a through-etch ofthe wafer. Standard positive photo resist is used asmasking material.Following the conventional MEMS processing, a 60ptmTF PZT (INSENSOR TF2100) and the 5pm Au topelectrode are screen printed through metal masks on thefront side by InSensor A/S. During the screen printingprocess a certain pressure is applied onto the thin siliconbeams. It proves possible to successfully screen print onsilicon beams with thicknesses down to 20 ptm withoutbreaking them. Fig. 4 illustrates the screen printing process.The final accelerometer is 10lOxmm2 including fourcircular bonding pads. Chip size can easily be reduced infuture design as Fig. 5 shows.lo'1010510Figure 6. The Au top electrode (5ptm) on the PZT layer (60ptm). Theblack areas are the pattern from the ASE through-etch. Athin Al wireis attached to the bonding pad signal read out.2,lxlo01 2xlolo 1,05xlo- 1,lxlO- 1,15xlo-f/ [Hz]10 10f Iz]Figure 8. Measured impedances as a function offrequency for four beamson the same accelerometer show a linear relation in a double logaritmicplot. The slope equals -1 proving capacitor like behavior and thecapacitance is found to be 79.1 pF.1-4244-0376-6/06/$20.00 }2006 IEEE 1479Authorized licensed use limited to: Danmarks Tekniske Informationscenter. Downloaded on November 11, 2009 at 05:27 from IEEE Xplore.  Restrictions apply. IEEE SENSORS 2006, EXCO, Daegu, Korea / October 22-25, 200660 -50 -5 40-30 -20 -10-- 12.0 g-- 6.6 g-4.2 g--- 3.4 g-- 1.9g5 10f[kHz]15Figure 9. The RMS value ofthe output voltage is measured for frequenciesranging from I kHzto 20 kHz. Five different accelerations have beenmeasured ranging from 1.9 g to 12.0 g. A connstant signal is measure from10 kHzto the resonance peak at 17.5 kHz.IV. CHARACTERIZATIONThe accelerometer is wire bonded for signal read out (seeFig. 6). The read out signal from the accelerometer is ahighimpedance signal and a charge amplifier is used totransform the signal into a low impedance signal. Theaccelerometer can electrically be described as a chargesource, a resistor, RA, and a capacitor, CA, in parallel (seeFig. 7). The capacitor CF is equal to the capacitance oftheaccelerometer in order to measure the correct outputvoltage.Contrary to the impedance of the resistor theimpedance ofthe capacitor is frequency dependant. At lowfrequency all current will run through the resistor and theaccelerometer will not be operational. The lower end ofthebandwidth is determined by this frequency. Fig. 8 shows themeasured impedance as a function of frequency rangingfrom 0.1 kHz to 10 kHz. A slope of -I in a doublelogarithmic plot proves a capacitor like behavior. Theaverage capacitance for the four beams on the accelerometeris 79.1 pF. The slight difference in capacitance is probablydue to small difference of the four top electrode areas orlocal difference in PZT film thickness. The lower end ofthebandwidth is therefore proven to be below 0(.1 kHz.A shaker (Briuel & Kjaer 4810) setup is used to measurethe output as a function of frequency. The accelerationinduced by the shaker is frequency dependent up to around5 kHz and the upper operational frequency ofthe shaker is20 kHz. Fig. 9 shows the measured RMS value for theoutput voltage as a function of frequency ranging from 1kHz to 20 kHz. The frequency sweep is performed for fivedifferent accelerations ranging from 1.9 g to 12.0 g. Theacceleration of the vibration is measured with a referenceaccelerometer. In the range from 5 kHz to 10 kHz anoscillating signal is observed. This is believed to be due toresonance frequencies ofthe chip holder placed between theshaker and the accelerometer chip. From around 10 kHz to* Ouput at 15 kHzLinear Fit: Vout =4.15 * g2 4 6 8 10 12Acceleration [g]Figure 10. Output signal as a function ofacceleration shows a linearrelation at 15 kHz. The slope ofthe linear fit equals the sensitivity and isfound to be 4.15 mV/g.the resonance peak at 17.5 kHz the signal is constant for allfive accelerations.Fig. 10 shows the measured RMS values ofthe outputsignal in the constant region at 15 kHz. Five accelerationsranging from 1.9 g to 12.0 g are measured and shows alinear relation. The slope equals the sensitivity and is foundto be 4.15 mv/g. This sensitivity is for a single beam whereall four beams on the accelerometer coupled in parallelresults in a sensitivity of 16.60 mV/g. The capacitance for asingle beam is found to be 79.1 pF which results in a chargesensitivity of0.33 pC/g.V. CONCLUSIONIn conclusion, MEMS and TF screen printing technologieshave successfully been combined to fabricate a low costaccelerometer with a simple fabrication process only usingthree lithographic masks and two for the screen printingprocess. The characterization has shown promising resultsso far. The sensitivity at 15 kHz is measured to be 4.15mV/g and the resonance peak is found to be at 17.5 kHz. Aconstant sensing region is found from 10 kHz to 16 kHz.The lower limit is believed to be much lower but has not yetbeen proven. The good results motivates for furtherinvestigation leading to a tri-axial accelerometer.ACKNOWLEDGMENTThe project is a collaboration with InSensor A/S andFerroperm Piezoceramics A/S and is supported by "TheDanish National Advanced Technology Foundation".REFERENCES[1] Li-Peng Wang, Richard A. WolfJr., Yu Wang, Ken K Deng, LichunZou, Robert J. Davis, Susan Troiler-McKinstry, Journal ofMEMS,Vol. 12, no. 4, 2003.0[2] S. P. Beeby, J. N. Ross, N. M. White; Journal of Micromech.Michroeng., Vol. 10, p. 322-328, 2000.1-4244-0376-6/06/$20.00 }2006 IEEE0V21480Authorized licensed use limited to: Danmarks Tekniske Informationscenter. Downloaded on November 11, 2009 at 05:27 from IEEE Xplore.  Restrictions apply. 

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MEMS Accelerometer with Screen Printed Piezoelectric Thick Film

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