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Sensors and Actuators A: Physical
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high frequency cMUT probe for ultrasound imaging of fingerprints
icola Lamberti a,∗, Giosuè Calianob, Antonio Iulac, Alessandro Stuart Savoiab
Dipartimento d‘Ingegneria dell‘Informazione ed Ingegneria Elettrica Università di Salerno, Via Ponte Don Melillo, 84084 Fisciano (SA), ItalyDipartimento di Ingegneria Elettronica Università Roma Tre, Via della Vasca Navale, 84, 00146 Roma, ItalyDIFA, Università della Basilicata Viale dell‘Ateneo Lucano, 10, 85100 Potenza, Italy
r t i c l e i n f o
rticle history:eceived 10 March 2011eceived in revised form 28 July 2011ccepted 29 September 2011vailable online 6 October 2011
a b s t r a c t
Fingerprint imaging using ultrasound has been investigated for several years and has shown to be avalid alternative to optical scanners. Capacitive micro-machined ultrasound transducers (cMUT) is anemerging MEMS technology that can be profitably exploited in biometric applications, like fingerprint orpalm print.
eywords:MUTltrasound imagingiometricsingerprint
In this work a FEM model of a cMUT is proposed and used to design a 192-element array with a fre-quency band centered at 12.5 MHz. A cMUT array was manufactured by means of a proprietary fabricationprocess and an ultrasound probe was assembled and experimentally characterized. Comparison betweensimulation and experimental results has shown a quite good agreement. The cMUT probe was operatedat a frequency of 12 MHz and used to produce images of fingerprints, the quality of which was quitesatisfactory.
. Introduction
Biometrics is the science and technology of measuring and ana-yzing biological data. In information technology, biometrics referso technologies that measure and analyze human body character-stics, such as palm prints, fingerprints, eye retinas and irises, voiceatterns, facial patterns and hand measurements, for authentica-ion purposes [1–7]. Authentication by biometric verification isecoming increasingly common in corporate and public securityystems, consumer electronics and point of sale (POS) applications.
Among numerous biometric technologies, fingerprint authen-ication has been in use for the longest time and bears moredvantages than other biometric technologies. Fingerprint real-ime image acquisition is roughly classified into optical andon-optical method. Optical method relies on the total reflectionhenomenon on the surface of glass or reinforced plastic wherehe fingertip is in contact. The sensor normally consists of an opticalens and a CCD module or CMOS image sensor. In contrast, semicon-uctor sensors, as a typical example of non-optical sensors, exploitlectrical characteristics of a fingertip such as capacitance [8]. Suchon-optical sensors are said to be relatively more suitable for mas-ive production and size reduction such as in the integration with
obile devices. Ultrasonic waves, heat [9], and pressure are also
tilized to obtain images with the non-optical fingerprint sensors.
∗ Corresponding author..E-mail address: [email protected] (N. Lamberti).
Fingerprint imaging using ultrasound has been investigated forseveral years. Ultrasound scan systems capable to acquire biomet-ric data of the fingertip surface and to represent the correspondingfingerprint pattern have shown to be a valid alternative as com-pared to optical scanners. Several scan methods, based on differentdata acquisition and processing approaches, have been evaluated.Acoustic impedance [10] and pulse-echo [11] measurements of thefingertip surface, using ultrasound transducers in conjunction toappropriate processing, have been shown to be suitable for a robustextraction and representation of fingerprints, as a binary pattern ofthe ridges and valleys.
Moreover, the use of ultrasonic waves lead to some intrinsicadvantages with respect to other methods. In fact, ultrasonic wavesare not sensitive to the surface contaminations, such as stain, dirt,oil, or skin wornness and, more important, they provide informa-tion not only of the skin surface but also the volume under theinvestigated skin region. Furthermore, ultrasonic waves detect lifeand therefore can easily detect fakes.
A convenient way to scan a volume is to mechanically shift asingle element transducer along two orthogonal directions, whileperforming a pulse-echo measurement for each step [12]. Theresulting vertical scan lines (A-SCAN) are combined into a volumethat can be horizontally sectioned, in order to represent 2D images(C-SCAN) of the fingertip surface or any other parallel surface.
The scanning acquisition time can be reduced using an
ultrasonic linear array transducer that moves in a direction per-pendicular to the electronic scanning plane. This technique is a 3Dultrasound imaging modality with many possible biomedical appli-cations [13] that can be extended to biometrics as well. In fact, it
as been recently experimented to acquire biometric characteris-ics of the hand like the hand geometry. In that case, a commercialiezoelectric probe was employed [14,15].
In some recent papers, first palm print and fingerprint sam-les, obtained with the same technique, but exploiting capacitiveicro-fabricated transducers (cMUT), which is an emerging MEMS
echnology able to compete with piezoelectric transducers, haveeen presented [16,17]. These prototypes were fabricated by meansf the proprietary reverse fabrication process developed by somef the authors and previously reported [18].
In the present work, a FEM model of a cMUT transducer, specif-cally designed for fingerprints, is presented and validated by aomparison with experimental results. The transducer has beenxperimentally characterized and a sample of 3D ultrasound imagef fingerprints is also shown.
. A novel MEMS device: the cMUT
After a rapid progress during the last 15–20 years, ultrasoundransducer (UT) performance indices are now tending towardssymptotic values, due to the low evolution rate of current man-facturing processes that are principally based on the matureiezoelectric technology.
At the same time, and by contrast, ultrasound imaging electronicystems are showing constant progress in terms of sophistica-ion, integration-level and cost reduction. Hence, the transducers becoming the limiting factor in the global performance of mod-rn high-end imaging systems, and the major “cost driver” in theevelopment of hand-held ultrasound scanners.
Micro electro-mechanical system (MEMS) technologies haveeen intensively explored over the same time period. Many MEMSabrication processes have been developed in research laboratoriesnd some of them are fully industrialized [19–23].
Recently, MEMS technology has begun to be applied to micro-achined ultrasonic transducers (MUTs), with several innovative
outes being explored, in particular in the area of electrostaticevices. The MUT concept appears to be the first real opportu-ity for a technological breakthrough in the area of UT working inhe 1–30 MHz range. It will lead to innovative and affordable solu-ions, especially in the upper part and above this frequency rangehigh frequency arrays), where the existing technology encountersonsiderable difficulties.
Among MUTs, capacitive micromachined ultrasound transduc-rs (cMUTs) have demonstrated to be the most promising deviceshat fully exploit the benefits of MEMS technology. These devicesre based on the electrostatic attraction between the backplatend the metalized membrane (Fig. 1). In transmission, the mem-rane is forced into flexural vibration by a time varying voltage; ineception, when the membrane is put into vibration by an inci-ent pressure wave, the capacitance modulation caused by the
embrane displacement is used to detect the signal. Therefore,
DC bias voltage must be used in reception for practical sig-al detection, whereas it is required in transmission for linearperation. In fact, because the electrostatic force depends on the
Fig. 1. Basic structure of a cMUT electrostatic cell.
ators A 172 (2011) 561– 569
square of the voltage, a sinusoidal voltage applied to the trans-ducer without polarization would drive the membrane to oscillateat double frequency. The alternating voltage VAC can be super-imposed to the bias voltage VDC through a simple polarizationcircuit.
In 1994, Haller et al. presented an ultrasonic transducer basedon the electrostatic effect [24], operating in air, and made of siliconby means of a micromachining technique. The researchers used theplanar silicon micromachining technology, currently employed forMEMS, to manufacture electrostatic cells of very small dimensions.
The transducer consists of an array of these miniaturized elec-trostatic cells, electrically connected in parallel. For operation in themegahertz range, the lateral dimension of each cell is in the rangeof tens of microns and, in order to achieve sufficient sensitivity,the number of cells must be in the order of thousands. Because ofthe working principle and the technology used, this transducer wascalled cMUT (capacitive micromachined ultrasonic transducer).
Basically, the conceptual steps of the fabrication process are thefollowing: a sacrificial layer is grown onto a silicon substrate; itsthickness defines the gap height of the cells. A second structurallayer (e.g. silicon nitride) is deposited over the sacrificial layer anda small hole is made to create via to remove the underlying sacri-ficial layer. A selective etchant solution is used in order to removeonly the sacrificial layer; depending on the etching time, a wide cir-cular cavity is fabricated under the structural layer, which remainssuspended above the cavity. Finally, the etching holes are sealedusing a second thin layer.
All sacrificial release processes have in common the presence ofmicro-holes on the surface of the transducer, necessary to evacuatethe cavities under the membranes. The presence of etching holeshas a negative effect for both the uniformity of the membranes andthe sealing of the cavities underneath. Moreover, the holes stronglyaffect the filling factor, especially in the case of high frequencytransducers, limiting the possibility to achieve large bandwidth andgood sensitivity. In view of these considerations, a new fabricationconcept was proposed in [25]. The fabrication of the capacitive cellis not performed, like in the standard process, growing successivelayers onto the silicon wafer, up to the silicon nitride structurallayer of the micro membranes, but inverting the sequence of thelayers. The device is built using a standard process on the top ofan LPCVD silicon nitride film, which is grown over a silicon wafer;finally, once the cMUT has been fabricated, the bulk silicon waferis completely removed, allowing the silicon nitride membranes tofreely vibrate. By working on the back of the device, there is no needto make holes in the structural silicon nitride layer to evacuate thecavities.
The device is fabricated using a commercial silicon wafer with alow-stress LPCVD silicon nitride layer and the temperatures of thewhole process are not higher than 350 ◦C, so that a wide range ofmaterials can be used.
Initially, a first aluminum electrode is deposited over the LPCVDlayer. Following, a PECVD silicon nitride passivation layer is grown.A chromium layer, acting as the sacrificial layer is then deposited.The sacrificial chromium layer is patterned in order to define thesmall circular regions that will be evacuated using wet etching toform the cavities of the cMUT. A further PECVD silicon nitride layer,acting as the backplate, is grown. At this point, an aluminum layeris sputtered and patterned, in order to define a second electrode.A passivation layer of PECVD silicon nitride is deposited, to pro-tect the electrode surface. The etching holes are opened in orderto access the sacrificial layer and to evacuate the cavities under themembrane; the cavities are then sealed using a PECVD nitride layer.
A further patterning step assures the opening of the contact pads.Electrical connection pads are available on the back side of the die,ready to be soldered, avoiding deep trenches or interface circuits[18,25].
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Fig. 2. Photo of a portion of the 192-element cMUT array.
The promising results together with the high flexibility offeredy the planar silicon technology have stimulated many authorso develop new manufacturing processes and adequate models tobtain a deeper physical insight and new design tools [23,26–29].
. Design of the CMUT array
An ultrasound transducer array specialized for fingerprint visu-lization was designed. The transducer has to generate a sufficientlyarrow acoustic beam that makes it possible to visualize, with aeasonable resolution and over the entire surface, the structuresf the fingertip, such as the “ridges” and the “valleys”. The typicalimensions of such structures are in the order of 0.5 mm, whilehe average width of a fingertip is about 20 mm. Moreover, theharacteristics of the ultrasound system used for image gener-tion, in terms of number of channels and maximum operatingrequency, must be taken into account to define the specifications ofhe transducer array. In particular, in this paper, the imaging resultsre obtained with an Esaote Technos ultrasound system (Esaote.p.A., Genova, Italy), which can perform linear B-scan imagingsing a 192-element probe at a maximum transmission frequencyf 12.5 MHz.
The cMUT array was designed to operate in a frequency bandentered at 12.5 MHz. A finite element analysis (FEM) design haseen used to define the geometrical parameters of the cMUT mem-ranes, such as the membrane and electrode diameter and theembrane air-gap thickness; the purpose was to achieve broad-
and (100% fractional bandwidth) in-water operation. Fig. 2 showshe membrane layout that was chosen with the aim of maximizinghe filling factor, while satisfying the dimensional constraints giveny the reverse fabrication process (RFP) described in the previousection.
Given the number of channels available in the Technos systemnd considering the maximum necessary field of view, a small pitchf 112 �m was chosen. In this way the acoustic beam shape, alonghe azimuth direction, is optimized while focusing at small dis-ances (3–10 mm). The array element is rectangular and its widths 112 �m. In fact, with this technology is possible to choose anlement-to-element separation (kerf) identical to the membrane-o-membrane separation, leading to the definition of a practicallyerfless array. The number of the array elements is 192, leading to a
aximum field of view of 21.5 mm, which is sufficient to visualizeost of the human fingertips.The height of the array elements was chosen in relation to the
esired beam width in the elevation direction, considering that the
Fig. 3. Photo of the cMUT probe head: packed device after silicon etching (on theleft) and final device with the silicone rubber lens (on the right).
beam is statically focused by means of an acoustic lens. For the spe-cific application, in which the lateral resolution is more importantthan the penetration capability, a narrow beam was chosen at theexpense of the depth of field. Hence, an array elevation of 3 mmwas selected together with a 6 mm fixed focus depth, in order toobtain a −3 dB beam width at 12.5 MHz given by
Beam width−3 dB ∼= �F# = 0.3 mm (1)
where � is the wavelength at 12.5 MHz and F# is the focal numberof the focused aperture in the elevation direction, defined as theratio of the focal depth to the aperture size.
A cMUT ultrasound probe head was obtained by electricallyinterconnecting the fabricated array to the flexible part of a rigid-flex PCB. A pre-shaped support was fixed to the assembly, allowingthe casting onto the cMUT chip of a custom acoustical and mechani-cal backing. The rigid parts of the PCB were then fixed to the support.Further, the silicon substrate of the chip was totally removed byusing a HNA wet etch process. The resulting probe head, in whichthe active surface of the cMUT array is exposed, is illustrated on theleft side of Fig. 3. In order to focus in the elevation direction at a focaldepth of 6 mm, a cylindrical acoustic lens made of silicone rubberwas needed. RTV60 Silicone (Momentive Performance MaterialsInc., Huntersville, NC, USA) was chosen for the lens fabrication forits low sound propagation speed characteristics. In order to matchthe acoustic impedance of water, the silicone rubber was dopedwith tungsten powder (13% of the silicone weight). The density andthe sound speed of the compound were measured resulting equalto 1720 kg/m3 and 860 m/s, respectively. A 4.5 mm curvature radiuswas chosen for the cylindrical lens, leading to a maximum rubberthickness of approximately 0.4 mm over the transducer surface. Theacoustic attenuation of the RTV60 silicone rubber was also char-acterized resulting to be ˛ = 9.2 dB/mm at 5 MHz and 18.4 dB/mmat 10 MHz. The silicone lens was applied to the cMUT probe headusing a conveniently shaped mould. The assembled probe head isillustrated in the right side of Fig. 3.
The cMUT probe head was finally connected to a multi polarcable for medical ultrasound probes via a 192-channel analog fron-tend electronic circuit. The single channel electronic circuit is ahigh input impedance unity-gain voltage buffer useful to providethe electrical current necessary to drive the cable impedance. Itwas chosen to use the MAX4203 (Maxim Integrated Products, Sun-
nyvale, CA, USA) for its low power requirement (2.2 mA – ±5 V),low noise (2.1 nV/
√Hz – 0.8 pA/
√Hz), and excellent capacitive-
load-driving capability, and for the availability of a 2 channel smallpackage (�MAX/8). A diode network is used to protect the input
564 N. Lamberti et al. / Sensors and Actu
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ig. 4. Photo of the cMUT probe prototype complete with connection cable andluminum housing.
nd output of the voltage buffer and to duplex the single channeln order to be used for both transmission and detection. The probeead and the electronic boards were housed in an aluminum case,s shown in Fig. 4.
. Finite element modeling of the cMUT array
The FEM analysis of the cMUT transducer was carried out byeans of a custom developed Ansys Parametric Design Language
APDL) code (ANSYS Inc., Canonsburg, PA, USA). We developed aodel able to compute the electrical input impedance, the water
oupled pulse-echo response and the pressure emitted by a cMUT.rom a geometrical point of view, the FEM model is representedy a quarter of a rectangular parallelepiped comprising the cMUT,he backing applied on the transducer back face, the silicone rubberens applied on the front face in order to focus in elevation the emit-ed beam, and the propagating medium. The FE model is shown inig. 5.
Due to the symmetry boundary conditions applied to the paral-elepiped lateral surfaces, this model describes the behavior of a 2D
rray composed by identical electrostatic cells, infinitely extendedn the plane orthogonal to the propagation direction. Due to theMUT array transducer element dimensions, this approximation
ig. 5. ANSYS FEM model of the single cMUT array element. The cMUT die compo-ents, the backing and the front face lens are evidenced.
ators A 172 (2011) 561– 569
is valid only in the elevation direction (several membrane rowsfor each element), while it is not valid in the azimuth direction(few membranes columns for each element). The response in thepulse-echo mode of the single array element emitting in waterand without focusing lens, computed by the FEM model with thesesymmetry boundary conditions, is only a first degree order approx-imation of the element behavior; in fact, the model is not able topredict second order effects like, for example, the acoustic interac-tions between the membranes. On the other hand, the presence ofthe silicone rubber lens has the effect to reduce the acoustic inter-actions between the membranes in the lateral (azimuth) directionand therefore the proposed simple model is able to predict both theelectrical input impedance and the in-water pulse-echo responseof the transducer element.
The described model was firstly used to define the cell geom-etry. As the mechanical resonance frequency of the single arrayelement depends on the cMUT membrane radius and thickness,we performed several modal analyses by varying the cell geometryin order to accurately design the single cell dimensions.
In order to describe the electro-mechanical coupling in eacharray element cell, several ANSYS TRANS126 elements are insertedin the cell cavity; such elements are able to take the electrostaticlinear or non linear effects into account in static, harmonic and tran-sient analyses. Due to the fact that the intended application doesnot involve imaging modalities that require high linearity, i.e., con-trast agents and tissue harmonic imaging, we did not investigate,in this work, the effects of cMUT non-linearity. Consequently, nonlinear effects were not taken into account in the dynamic analysesperformed.
As a first step, we used the FEM model to compute the cMUTcollapse voltage: we performed several static ANSYS analyses,increasing the applied DC voltage, until the distance between thebackplate and the metalized membrane becomes zero.
After that we computed the electrical input impedance (Zi) ofa single array element, by means of an ANSYS harmonic analysis.As the cMUT must be polarized by a DC voltage, harmonic analy-ses are performed considering a pre-stress condition, obtained as aresult of a previous ANSYS static analysis of the cMUT supplied bya DC polarization voltage. The single cell electrical capacitance iscomputed by summing the capacitance of each TRANS126 elementinserted in the FEM model of the cell cavity; the active capacitanceof the single array element, Ca, is obtained by multiplying the cellcapacitance for the number of the cells, electrically connected inparallel that compose the array element. In the FEM model is alsopossible to take into account a parasitic capacitance, Cp, and to addit to the simulated active capacitance; the total static capacitancefor each array element is therefore C0 = Cp + Ca.
In order to simulate the transducer behavior in operating con-ditions, we computed the water coupled pulse-echo response ofthe cMUT array element; the calculation was made by means of atransient analysis and by using the described FEM model. In sucha model a water volume, terminated by a completely reflectingplane, is used and the sound absorption is neglected. In order tosimplify the FEM model and to reduce the computation time, thecurvature of the lens used to focus the emitted acoustic beam inthe elevation direction is not taken into account: a flat lens layerwas used with a thickness equal to the maximum thickness of thelenses used in the transducer prototypes. The acoustic losses in therubber material were modeled by means of the Rayleigh dampingcapabilities available in ANSYS. Using the acoustic attenuation val-ues of the materials, we conveniently extracted the APLHAD andBETAD attenuation parameters and we computed the pulse-echo
response, including the acoustic damping.
The transmission transfer function (TTF) of the single trans-ducer element was computed by means of a transient analysis;the emitted pressure was computed at the interface between the
N. Lamberti et al. / Sensors and Actuators A 172 (2011) 561– 569 565
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Fig. 7. Representation of simulated (red line) and experimental (black line) voltagesignals used to excite the transducer element in the time domain (solid lines) andin the frequency domain (dashed lines). The frequency domain signals’ amplitude
ig. 6. Real (solid line) and imaginary (dashed line) parts of the electrical inputmpedance (Zi) measured (in red) for a central cMUT array element compared withhe result of the ANSYS analysis obtained with Cp = 5.5 pF and Rs = 15 �.
coustic lens and the propagating medium, averaging the values onhe nodes of the interface surface. Finally, the response in the fre-uency domain was computed by performing the FFT of the averageressure value.
. Comparison between FEM and experimental results
As already said in Section 3, we want to design a cMUT thathows a broadband pulse-echo response, in immersion opera-ion, with a frequency band centered at 12.5 MHz and 12.5 MHzide (100% fractional bandwidth); such behavior can be achieved
f the frequency of the first natural vibration mode of the in-acuum cMUT membranes is set to 20.5 MHz. As a result of aarametric set of simulations, in which we varied the diame-er and the thickness of the cMUT membranes, we obtained thathis specification is satisfied with a cell diameter of 29 �m and
total membrane thickness of 1.55 �m. Consequently, to satisfyhe specifications on the array element width (112 �m) and height3 mm), each array element must be composed by 4 cells alonghe width and 110 cells along the length, resulting in 440 cells perlement.
Some cMUT probe head prototypes were assembled followinghe design rules described in Section 3. The manufactured trans-ucers were electrically and acoustically characterized.
.1. Collapse voltage and electrical impedance
The collapse voltage was determined by means of impedanceeasurements observing the variation of the capacitance with the
pplied bias voltage. The collapse voltage was estimated to bepproximately equal to 250 V. This value is lower than that com-uted by ANSYS (310 V). This difference can be attributed to theact that the membrane thickness resulted to be lower than theesign specification. In fact, during the last step of the cMUT fabri-ation process, in which the silicon bulk is chemically etched, theilicon nitride membrane layer is thinned because of the not totalelectivity of the HNA etchant.
Fig. 6 shows the comparisons between computed and measuredlectrical impedance Zi. Measurements were carried out by meansf a HP 4194-A impedance analyzer (Hewlett-Packard, Palo Alto,A), biasing the transducer with a voltage VDC = 40 V. Simulation
esults were obtained by setting Cp = 5.5 pF and Rs = 15 � with aias of VDC = 40 V as well. With these values we obtained the bestt with experimental data. The element active capacitance Ca com-uted by FEM was equal to 7.5 pF and, consequently, C0 = 13 pF. As
is represented in dB, normalized to their maximum value. (For interpretation of thereferences to color in this figure legend, the reader is referred to the web version ofthe article.)
can be seen, the measured resonance frequency is lower than thatcomputed by FEM. Also this discrepancy can be justified consider-ing that the thickness of the membrane is thinner that the wantedvalue.
5.2. Pulse-echo response
The pulse-echo response was calculated using the FE approachdescribed in Section 4. In order to verify the results, we performed inwater pulse-echo measurements on the focused cMUT probe headdescribed in Section 3.
The set-up used for this measurement is based on a tank filledwith de-ionized water. A steel disk with a diameter of 100 mmand 17 mm thick is placed on the bottom of the tank. This disk(reflector) has the purpose to completely reflect the acoustic sig-nal emitted by the transducer. The distance between the cMUTand the reflector was the same as the focal depth of the acous-tic lens (6 mm from the transducer). The cMUT was excited bya Panametrics pulser/receiver (Pulser/Receiver 5800PR, Panamet-rics Inc., USA). The signal used in the FEM analysis to excite thetransducer element is a −150 V raised cosine pulse (RCP) with thesame amplitude and duration of the pulser-receiver signal used inthe experiments; the comparison between the two waveforms isshown in Fig. 7 together with the amplitude of their FFT. As can beseen, the two signals are very similar in the time domain, while theRCP signal used in simulations has a slightly narrower band.
Fig. 8 shows the (a) measured and (b) simulated pulse-echoresponse for a central cMUT array element in the time and in the fre-quency domain. The applied bias voltage was 220 V in both cases.A good agreement between simulation and measurement can beobserved.
It must be noted that, due to the strong acoustic attenuationof the acoustic lens that increases with frequency, the center fre-quency in both cases is significantly lower than the value defined inthe cMUT design phase of this work. In fact, as already said in Sec-tion 3, in order to focus a 3 mm elevation aperture at a distance of6 mm, a low speed lens material was needed. Moreover, a specificacoustic impedance close to water was also needed for minimizing
the artifacts due to the water–lens interface. For this reason, thesilicone elastomer proposed in this paper has been quite heavilydoped with tungsten powder obtaining good impedance match. Asa drawback, the acoustic attenuation was very high.
566 N. Lamberti et al. / Sensors and Actuators A 172 (2011) 561– 569
Fig. 8. (a) Pulse-echo response of a central cMUT array element measured in the time(black line) and frequency (red line) domain. (b) Pulse-echo response computedin the time (black line) and frequency (red line) domain by the FEM model for atov
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Fig. 9. Comparison between (a) the pressure detected by the hydrophone in thetime (black line) and the frequency (red line) domain when a central element of thecMUT array is excited and (b) the FEM results obtained with the damped model. (For
ransducer element. The losses in the focusing lens are accounted. (For interpretationf the references to color in this figure legend, the reader is referred to the webersion of the article.)
.3. Emitted pressure
The experimental set-up for the emitted pressure measure-ent is based on a tank filled with de-ionized water. To measure
he pressure emitted by a cMUT element, a hydrophone (MHA9-50, Force Technology, Denmark), is placed, using a specialxture, in front of the cMUT. The distance between the trans-ucer and the hydrophone is equal to the focal depth of thecoustic lens (6 mm from the transducer). The transducer istill excited by the −150 V negative broadband pulse signalFig. 7 in black) generated by a 5800PR Panametrics pulser/eceiver.
Fig. 9a shows the signal detected by the hydrophone, in theime and in the frequency domain, when a central element ofhe cMUT array is excited; also in this case the amplitude of theias voltage is 220 V. The pressure measured in the focus is esti-ated to be 32 kPa peak-to-peak by dividing the electrical 16 mV
eak-to-peak by the hydrophone nominal sensitivity (0.5 �V/Pa).n Fig. 9b the result computed by FEM, taking into account
he acoustic lens losses, is reported. As can be seen, the mea-ured central frequency (9.45 MHz) is comparable with the FEMesult (10 MHz), and the −6 dB bandwidths have the same value122%).
interpretation of the references to color in this figure legend, the reader is referredto the web version of the article.)
6. Biometric acquisitions
The manufactured cMUT probe prototype was used for theacquisition of a fingertip image. The acquisition was performed byfixing the finger onto a special fixture in a water tank. The finger-tip is facing upwards while being totally immersed in water. Theultrasound probe head is partially immersed, in order to be facedand acoustically coupled to the fingertip.
In order to acquire 3D ultrasound imaging data of a volume,the probe is mechanically shifted along the elevation direction bymeans of a micro metric motorized stepper stage. Fig. 10 shows thedescribed set-up. During the time interval between two consequentsteps, a B-scan is acquired and stored using an Esaote Technos com-mercial ultrasound imaging system. Successively, the whole set ofB-scans is processed and rendered using a custom software writ-ten in Matlab code (The MathWorks, Inc., MA, USA). The softwarecan provide simultaneous orthogonal B and C-scans of any part ofthe volume data as well as on-line measurements of lengths anddistances.
In order to estimate the focal depth and the lateral resolutionof the ultrasound imaging system along the elevation direction,
point spread functions (PSF) at various distances have been mea-sured, using the described set-up, by scanning a custom phantommade of 20 parallel and equally spaced copper wires (40 �m
N. Lamberti et al. / Sensors and Actuators A 172 (2011) 561– 569 567
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Note that fingertip sweat pores, which are not easily detectablewith optical techniques, are visible along the pattern. The verticalline artifacts visible in both fingerprint pictures are due to the nonperfect mechanical stability of the finger during the scan process.
Fig. 10. Ultrasound fingertip scan set-up.
iameter). The phantom has been scanned by shifting the cMUTrobe along the direction perpendicular to the wires and by using
12.5 MHz 1-cycle square wave pulse excitation. A B-SCAN inhe elevation direction, parallel to the mechanical scan directionnd perpendicular to the electronic scan direction, is extractedrom the acquired volume image data using the aforementionedoftware and is reported in Fig. 11, representing the PSFs with a0 dB dynamic range. Detail (−3 dB) and contrast (−20 dB) lateralesolutions are estimated by measuring the minimum PSF widths,esulting to be respectively equal to 0.3 mm at 5.6 mm depth and.5 mm at 5.9 mm depth. Hence, the focus distance is estimated toe approximately between 5.5 mm and 6 mm.
Finger scans have been performed placing the probe head at aistance of 6 mm, in order to achieve the maximum lateral res-lution on the fingertip surface. The acquisition and processingoftware is able to operate 3D voxel gray scale intensity represen-ations of the acquired volume, making it also possible to arbitrarilyssign a transparency value to a subset of the volume itself.
In Fig. 12 such kind of representation is reported showing 13 mm × 13 mm × 2 mm portion of the scanned volume. Theater–finger interface is detected by the software and the voxels
orresponding to water are set to be fully transparent, in order toisualize the fingertip surface. As can be seen, the ridges and val-eys of the fingertip surface are visible as well as the structure ofhe underlying tissues.
The software can also perform 2D representations of arbitraryon planar sections of the volume. Such capability makes it possibleo visualize the fingertip surface as a 2D image and to conveniently
xplore the underlying tissues using the same 2D representation.he selection of the section surface starts form the detection of theater–finger interface surface. The resulting surface is then spa-
ially filtered. By vertically shifting the position of the surface it
Fig. 11. PSF at various depths.
is possible to visualize the ridges and valleys pattern at variousdepths. Fig. 13 shows the extracted fingerprint at a 0.2 mm depth.
Fig. 12. 3D voxel representation of a fingertip.
568 N. Lamberti et al. / Sensors and Actu
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[
[
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Fig. 13. 2D ultrasound fingerprint.
. Conclusions
In this paper a high frequency cMUT probe for ultrasound imag-ng of fingerprints was modeled, by means of a FEM approach, andxperimentally evaluated for fingerprint recognition applications.he model takes into account the cMUT die, the backing applied onhe transducer back face and the silicone rubber lens; it is able toompute the electrical input impedance, the in-water pulse-echond the pressure emitted by the single array element.
The model was used to design a cMUT with a frequency bandentered at 12.5 MHz. A 192-element cMUT array was designednd fabricated by means of the reverse fabrication process, and aully engineered ultrasound probe prototype was assembled andonnected to a commercial ultrasound imaging system.
Both the cMUT array and the whole probe head were experi-entally characterized in order to validate the FE model.A relevant downshift of the band center frequency, caused by the
trong acoustic attenuation of the lens material, was highlightednd discussed.
Finally, the transducer was operated at a frequency of 12 MHznd used to produce images of fingerprints. The acquired ultra-ound images are of sufficient quality in order to identify thengerprint patterns. These images have been also rendered in a 3Doxel gray scale intensity representation, in order to highlight theain advantages of the ultrasonic technique in fingerprint recog-
ition.
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Biographies
Nicola Lamberti is associated professor in Electronic at the University of Salerno,Italy. He received the Dr. Sc. degree in Electrical Engineering from the Universityof Calabria, Cosenza, Italy, in 1985. He was leader of several national researchprograms. Nicola Lamberti worked mainly on the field of ultrasonic transducer
modeling for medical and underwater applications and for non destructive testing.He is currently engaged in research in transducer modeling, piezoelectric devices,capacitive transducers, and piezoelectric motors and actuators. Nicola Lamberti isauthor of about 100 papers on these fields, published on international magazinesand conferences proceedings, and of 3 international patents.
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ntonio Iula is an associate professor of electronics at the University of Basilicata,otenza, Italy. He received the Dr. Sc. degree from the University of Salerno, Italy, andhe PhD degree from the University Roma Tre, Rome, Italy, in electronic engineeringn 1992 and 1999, respectively. Prof. Iula works mainly on piezoelectric transducer
odeling and characterization for ultrasound applications in biomedical and indus-rial fields and on piezoelectric motors and actuators. He is author of more than 90apers in these fields that have been published in international journals and confer-nces proceedings, and he has been involved in several national and internationalesearch programs.
iosuè Caliano received the Dr. Sc. degree (MS) in Electronic Engineering at the
niversity of Salerno in 1993. In 1995 he joined at Pirelli-FOS as Industrial Automa-
ion Engineer. Since 1997 he works at the ACULAB, Dept. of Electronics of theniversity of Roma Tre as responsible of the laboratory and as researcher. He is
nvolved in design and characterization of micromachining ultrasonic transducerscMUT). In this position, he developed many typologies of cMUT transducers from
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single-element transducers to 192-element probe for echographic imaging system.He has obtained first echographic images using cMUT probe in conjunction witha commercial echographic system (2003). He is author of more than 80 papers onthese fields published on international magazines and conferences proceedings, and10 international patents.
Alessandro Savoia was born in Edinburgh, Scotland on November 9, 1978. Hereceived a M.S. degree in electronics engineering from the Università degli Studi“Roma Tre” in May 2003 and a PhD degree in electronics from the Università degliStudi “Roma Tre” in April 2007. He currently holds a post-doctoral research contractat the Dipartimento di Ingegneria Elettronica of the Università degli Studi “Roma
Tre”. His main research interests are in the field of ultrasound transducers and med-ical ultrasound imaging systems. He is currently working on the development ofcapacitive micromachined ultrasonic transducers (cMUTs). He has contributed 25publications in international journals and conference proceedings, and he holds 3patents in the field of cMUTs.
