Microfluidic Injector Simulation With FSAW Sensor for 3 ... - IEEE Xplore

IEEE TRANSACTIONS ON INSTRUMENTATION AND MEASUREMENT, VOL. 64, NO. 4, APRIL 2015

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Microfluidic Injector Simulation With FSAW Sensor for 3-D Integration Thu Hang Bui, Tung Bui Duc, and Trinh Chu Duc

Abstract— This paper presents a possible creation of the optimized liquid sensors for the inkjet nozzles. The proposed focused surface acoustic wave (FSAW) device utilizing aluminum nitride (AlN) single crystal as the piezoelectric substrate is based on the pressure variation due to the continuous droplet ejector. The design, specification, and numerical simulation results are described. Comparisons between the output response of the conventional and concentric structures indicate a more efficient operation of the multiple-segment focused interdigital transducer (FIDT) structure. According to the angular spectrum of the plane wave theory, the amplitude field of FIDTs is calculated through that of straight interdigital transducers. The 3-D integrated model of the FSAW device has a number of advantages, such as the enhancement of the surface displacement amplitudes and an easier fabrication. It is able to detect the breakup appearance of the liquid in the droplet formation process. For the piezoelectric substrate AlN, it is compatible with the CMOS fabrication technology, leading to an inexpensive and reliable system. Moreover, for the proposed FIDTs with multiple straight segments, the acoustic energy is more optimized and focused near the center of the inkjet nozzle. The droplet generation process begins at an output voltage of roughly 0.074 V within 0.25 µs, and the background level of the attenuation of both the mechanical and electrical energy. Index Terms— Focused interdigital transducer (FIDT) device, level set method, liquid sensor, microfluidic injector, piezoelectric technology, surface acoustic wave (SAW) devices.

I. I NTRODUCTION

I

NKJET technology has been applied for various devices such as printers and applications in life sciences (diagnosis, analysis, tissue synthesis, and drug discovery) [1], [2]. Inkjet printers are feasible tools for printing texts and images because of their low cost and high resolution within acceptable droplet speed and volume. In inkjet technology, factors including the ink droplet volume, head alignment, jet blockage, and resolution may affect the photo-quality image [3], [4]. Manuscript received May 31, 2014; revised July 25, 2014; accepted October 12, 2014. Date of current version March 6, 2015. This work was supported by the Vietnam National Foundation for Science and Technology Development through the Nafosted Project under Grant 103.99-2012.24. The Associate Editor coordinating the review process was Dr. Deniz Gurkan. T. H. Bui is with the Delft Institute for Microelectronics and Submicron Technology, Delft University of Technology, Delft 2628 CN, The Netherlands; and also with the Department of MicroElectroMechanical Systems and Microsystems, Faculty of Electronics and Telecommunications, University of Engineering and Technology, Vietnam National University, Hanoi, Vietnam (e-mail: [email protected]). T. Bui Duc and T. Chuc Duc are with the Department of MicroElectroMechanical Systems and Microsystems, Faculty of Electronics and Telecommunications, University of Engineering and Technology, Vietnam National University, Hanoi, Vietnam (e-mail: [email protected]). Color versions of one or more of the figures in this paper are available online at http://ieeexplore.ieee.org. Digital Object Identifier 10.1109/TIM.2014.2366975

Failure, however, often occurs in the droplet ejection process. When the ink droplet volume and its movement are not controlled properly, bleed and blur might occur at regular break-off intervals in color [5]. This causes visual disturbances due to dark blue lines or blurred solids. Therefore, advanced technologies, such as inkjet systems with the closed-loop controls that quantify and monitor ejected droplet volumes at the orifice in real time, are needed. In other words, the negative feedback of the closed-loop systems may increase accuracy performance of inkjet printer. In industry, the control methods for measuring and detecting the state of the ink movement at the nozzle need to be simplified. Expected sensors may account for the break-off time properly to assess the accuracy of generated ink-drops. To achieve this, there are several sensing methods, such as membranes, cantilevers, cameras, and pressure sensors, that were proposed to be able to sense the droplet generation process [6]–[9]. While some approaches are directly based on the vibration excited by the flow rate, others work at the bending level of the material. Moreover, the principles of pressure sensors such as piezo resistive, capacitive, and resonant sensing are based on the pressure variation at the orifice or gas reservoir. However, the operation mechanism of several existing sensors is able to obstruct the flow rate at the nozzle. In our previous work, a surface acoustic wave (SAW) device was proposed for detecting the pressure state at the nozzle [9]. For SAW devices with straight interdigital transducers (IDTs), when SAWs uniformly spread on the whole piezoelectric substrate, the dissipated SAW energy may affect most points on the propagation path [10]–[12]. Therefore, the SAW streaming and velocity fields throughout the delay path influence the whole nozzle because of the uniform fingers. It may have more loss for environment and unwanted noise such as reflected waves from the edges. For small fixed sensing areas like the nozzle, the specialized IDT structures need to provide SAW beams with high intensity and large beamwidth compression ratio. In other words, for the determined sensing positions like the nozzle, the power generated by the focused IDTs (FIDTs) is mostly concentric on the local propagation path, and it decreases the energy loss to the medium [13]–[16]. According to the conventional curve FIDT structure, the SAW beam may have a close effect on the narrower arc of the ink nozzle. Hence, as the reflection phenomenon of SAWs at edges and the power dissipation are limited, the performance of the concentric IDTs is better than the conventional structure. However, it is not easy to fabricate various FIDTs with circular arcs. Therefore, substituting curve fingers, FIDTs with multiple straight segments are presented.

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on the substrate due to the piezoelectric mechanism and the liquid motion caused by the jetting. Consequently, the output electric signal at the receiver FIDTs is also altered. It is assumed that the ink movement inside the nozzle is driven through the initial velocity at the inlet feed. The ink pressure P at the nozzle includes steady and unsteady inertia, the viscous forces, and forces resulting from the surface tension of the ink [17]. To generate a droplet, the required inlet velocity for firing a drop has to overcome them. Therefore, force exerted on each face of the piezoelectric substrate is the product of the stress component F1s indicated times the area over which the stress acts and the pressure gradient force F1 p . The stresses that exert forces in the x-direction have changed a small amount Ti across the elemental lengths x ,  y , and z [12]. The summation of all forces along the x-direction acting on the piezoelectric cube is thus F1 = F1s + F1 p

(1)

where F1s = [(T11 + T 11 )A1 − T11 A1 ] + [(T12 + T 12 )A2 − T12 A2 ] + [(T13 + T 13 )A3 − T13 A3 ] and F1 p = − Fig. 1. Geometry of the FSAW sensor with the well in the middle of the propagation path. (a) Two straight segments. (b) Three straight segments.

The rest of the paper is organized as follows. Section II shows a detailed analysis and design of the proposed FSAW using angular spectrum of plane waves and the relation between the ink pressure and the piezoelectric wave parameters. In addition, the model of the integrated injector system is described. In Section III, the simulation parameters of the 3-D integrated inkjet system are presented. Section IV shows comparisons between conventional and concentric structures including straight, curve, and multiplesegment IDTs, and simulation results corresponding to each droplet state at the nozzle. Finally, the conclusion is summarized in Section V. II. M ATHEMATICAL M ODEL A. Relation Between the Ink Pressure and the Piezoelectric Wave Equation The device is composed of FIDTs with multiple straight segments, as shown in Fig. 1. Mechanical waves generated by the electrical energy of the applied voltage at the transmitter FIDTs include shear horizontal waves and Rayleigh waves traveling through the surface. The microfluidic channel as the nozzle orifice size is etched through wafer at the middle of the focal line and is perpendicular to the SAW propagation path between the transmitter and receiver FIDT. If there is an internal jetting phenomenon in the active area of the well, the well throat is affected by both the mechanical wave motion

1 ∂P xyz. ρ ∂x

 j k ijk Here, Ai = j,k δ x x (i  = j  = k) is the area of a face with a normal component in the x i -direction and Ti is the elastic constitutive relation. From (1) and Newton’s law, the equation of motion for a solid is sought in the form 3  ∂T ij j =1

∂x j

−

3 1  ∂P ∂ 2ui =ρ 2 . ρ ∂xi ∂t

(2)

i=1

For the piezoelectric medium, as the elastic constitutive relation comprises electric field and strain, equation of motion is rewritten as 3  j,k,l=1

3 3  1  ∂P ∂ 2ui ∂ 2ul ∂ 2φ ci j kl + ei j k − =ρ 2 ∂xk∂x j ∂xk∂x j ρ ∂xi ∂t j.k=1

i=1

(3) where ci j kl , ei j k , ρ, and φ are the elastic stiffness constants, piezoelectric stress constants, piezoelectric density, and electrical potential, respectively, and u i represents the particle displacement in one direction. Equation (3) indicates the effect of the ink pressure at the nozzle on the electromagnetic components of the piezoelectric plane waves. Consequently, the ink flowing inside the nozzle has an effect on electrical potential at the output FIDTs. B. Angular Spectrum of Plane Wave Theory for FIDT Structure For the analysis on the surface X −Y plane, the total surface displacement u(x, y) is represented by the scalar ψ(x, y), omitting components z and t. In Fig. 2, k x and k y are the

T. H. BUI et al.: MICROFLUIDIC INJECTOR SIMULATION WITH FSAW SENSOR

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Fig. 3.

Novel position of the SAW sensor in the injector.

and three-segment FIDT are 



(6) ψ  (x i , y ) = ψ(x i , y  )exp[ j k0 x ] ⎡ ⎛ ⎞⎤

 

x (2R − x )  ⎠⎦ (7) ψ  (x i , y ) = ψ(x i , y  )exp ⎣j k0 ⎝ cot 2 ( D4a ) 

Fig. 2. Concentric FIDTs with the shape as (a) circular arc and (b) three straight segments.

 x and y components of the wave vector k(ϕ) that makes an angle φ with the x-axis. Both FIDT structures have the same degree of aperture Da . According to the angular spectrum of plane wave theory [18], [19], with the number of the IDT fingers N, the total displacement distribution of both conventional and concentric IDTs is evaluated by  N  1 ∞ ¯ y )exp[− j {xk x (k y )+ yk y }]dk y (4)

(k ψ(x, y) = 2π −∞ i=1

¯ y ) is the amplitude where [k x (k y )]2 = [k(φ)]2 − k 2y and (k distribution of the component waves, x = l, l–p, l–2p,…,l–Np and l is the path between the first finger and the ink nozzle. For the component wave generated by the i th finger, it is the inverse Fourier transform of the acoustic source function ψ(x i , y  ) in the following when it is set x = x i :  ∞ ¯ ψ(x i , y  )exp( j y k y )d y  . (5) ψ(k y ) = −∞

If ψ(x i , y  ) is the SAW beam function of the straight IDT, those of the conventional FIDT, two-segment FIDT,

ψ  (x i , y ) = ⎧    ψ(x i , y )exp[j k0x ∗ ], for x ≤ x ∗ ⎪ ⎪ ⎪ ⎪ ⎪ ⎪ ⎨ψ(x i , y  )exp j k0 (x ∗    ⎪ Da ⎪ + tan x (2R − x ) − R sin ⎪ ⎪ 3 ⎪ ⎪ ⎩

 Da 6

(8) 

where x ∗ = R− R cos(Da /6), and x is the path difference between the real aperture and the equivalent aperture of the first input FIDT finger. When the number of straight segments increases, the path difference decreases. Moreover, FIDTs with multiple segments still have properties similar to concentric circular arc FIDTs. C. Integrated Injector System Fig. 3 shows the geometry of the integrated injector. During the formation of a droplet, a generated fluidic pressure forces the nozzle wall. The sensor is positioned at the nozzle, consisting of the transmitter and receiver FIDTs to detect the change of the liquid pressure by the deformation of the output response and thereby detect the droplet formation process. By detecting the amplitude and attenuation of the mechanical and electrical output signal, the necessary information about

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TABLE I D ESIGN PARAMETERS OF IDT

Fig. 5. Position of the air/ink interface and velocity field at (a) t = 13 μs and (b) t = 14 μs.

Fig. 4.

Inlet velocity is excited by one pulse within the first 14 μs.

the droplet generating process inside the ink channel can be extracted. To reduce the leaky SAW effect into the ink motion, which may cause jetting failure, the applied electrical energy at transmitter FIDTs should be enough to still receive the output potential. For FIDTs, the high-intensity and narrow SAW beam is mostly focused on a part of the nozzle. This also improves the sensitivity of the SAW device as the charge distribution on FIDTs is caused by the change of efficiency for SAW detection as well as excitation. III. S YSTEM C ONFIGURATION A. FSAW Configuration In this section, we focus on the SAW sensor configuration on the Aluminum Nitride single crystal, which is integrated into the injector. To build a 3-D model of the integrated sensing of the droplet volumes during generation, the size of the 3-D domain of piezoelectric substrate is the rectangle of 500 × 300 μm and the substrate thickness equals the nozzle size of 25 μm. The size of the piezoelectric substrate is excessive to decrease wave reflection occurring at the edges. FIDTs made of Al film are deposited on the surface. The microfluidic channel plays a role at the nozzle of the injector. When the number of fingers

Fig. 6. Positions of ink droplet at various times. (a) t = 1 μs. (b) t = 3 μs. (c) t = 5 μs. (d) t = 9 μs. (e) t = 11 μs. (f) t = 13 μs. (g) t = 14 μs. (h) t = 25 μs.

increases, the focusing properties become unstable. Therefore, to investigate in steady environment, the model is designed by three pairs of fingers. The other design parameters in Table I are as follows. The piezoelectric substrate and inkjet of the developed models were meshed adaptively to adjust the scaling of the fields manually and reduce the computation time. These parameters provided a much denser mesh at the nozzle boundary of the model, which is essential to achieve a high accuracy in simulations. A sinusoidal voltage of frequency 1430 MHz is applied to the input FIDTs to generate the needed SAWs. An input voltage of 0.1 V is applied to the receiver FIDTs. Moreover, this also avoids receiving very small changes at the receiver because the influence of the liquid pressure that is compared with the input signal needs to be significant. The output voltages in all cases are acquired at the alternating fingers of the output IDT. Due to the vibration coming from the driving signal and the droplet formation signal, a cross-talk effect including electrical, direct, and pressure-induced crosstalk occurs when

T. H. BUI et al.: MICROFLUIDIC INJECTOR SIMULATION WITH FSAW SENSOR

Fig. 7.

Effect of the piezoelectric substrate on the liquid.

Fig. 8.

Sensitivity of the IDT device and two-segment FIDT device.

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the frequencies of these signals are close to each other. In experiments, for piezoelectric actuators, passive devices are used to reduce the effective piezoelectric substrates. Another way is to use thin foil, external electrodes for the ground and inner electrodes for voltage [20]. It is possible to apply these methods for the piezoelectric sensors in experiments. In addition, the cross-talk effect of the piezoelectric sensor is much smaller because its frequency is much more than that of the droplet formation signal. Moreover, in simulations the use of few IDT fingers and low energy reduces the cross talk. B. Input Parameters of Ink at the Nozzle The inlet velocity in the z-direction increases from 0 to the parabolic profile during the first 2 μs  2  x + y 2 + 0.1[mm] vi (x, y, t) = 4.5 0.2[mm]    x 2 + y 2 + 0.1[mm] · v(t)(mm/s). × 1− 0.2[mm] (9) Here, (t) = u(t − 1 · 10−6 ) − u(t − 13 · 10−6 ), as shown in Fig. 4, and u(t) is the unit function. Hence, the pulse

Fig. 9. Total amplitude fields of IDTs with the conventional and concentric shapes on the surface. (a) Conventional IDTs. (b) FIDTs with circular arcs. (c) FIDTs with two straight segments. (d) FIDTs with three straight segments.

frequency of the droplet formation process is about 20 KHz. The velocity is then v(x, y) within 10 μs and finally falls down to zero within another 2 μs. Therefore, the ink velocity at the nozzle throat is sought in the following form: v n (x, y) = v i (x y)

R12 R22

.

(10)

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Fig. 10.

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Total displacement measured at a point after the inkjet nozzle.

Fig. 12. Spectral content of the mechanical wave motion of the FSAW devices with (a) curve fingers, (b) two-straight-segment fingers, and (c) three-straight-segment fingers.

A. Droplet States

Fig. 11. Mechanical attenuation of SAWs after propagating through the inkjet nozzle.

The surface tension of the ink generates a capillary pressure that is ignored due to its insignificant influence. To cut off the droplet, the pressure at the entrance of the nozzle has to overcome steady and unsteady inertia and forces resulting from the surface tension of the ink [20]. Therefore, pressure includes the positive and negative excitation pressure. After the negative excitation, the ink deformation at the nozzle happens to separate the droplet from the liquid reservoir.

Fig. 5 shows the ink surface and the velocity field at t = 13 μs when the velocity magnitude of ink is still focused at the nozzle. After 14 μs, the breakup phenomenon of the droplet generation occurs. Fig. 6 shows the time evolution of the ink jetting from the nozzle. To move to the outlet of the target, the jetted droplet from the inlet needs 200 μs. During the first 13 μs, ink at the nozzle throat is extensively forced [Fig. 6(a)–(f)]. After the second actuation pressure, the breakup point occurs, as shown in [Fig. 6(g) and (h)]. In other words, the potential energy becomes strong enough to cut off the droplet. Therefore, to detect the initial period of the droplet generation, the running time of the simulation only needs to be carried out within 25 μs to determine the correlation between the droplet generation and the output signal variation. B. Working Mechanism of the FSAW Device

IV. R ESULTS AND D ISCUSSION The proposed simulation methodology has been implemented using finite element method and COMSOL Multiphysics 4.2a.

Pressure produced by the piezoelectric substrate insignificantly affects the liquid (Fig. 7). Moreover, it also indicates that due to the uniform IDT fingers of the conventional, the liquid is influenced more at the region far from the local line.

T. H. BUI et al.: MICROFLUIDIC INJECTOR SIMULATION WITH FSAW SENSOR

Fig. 13.

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Output potential at the receiver FIDT of the SAW sensors.

The sensitivity S is defined as the relative change of the output signal per unit of the applied pressure and the input voltage [21]. Fig. 8 shows that that of the two-segment FIDT structure is better than that of the conventional IDT structure. Fig. 9 shows that the total displacement fields of FIDTs have a narrow concentric SAW beam. When the number of straight segments of the proposed structure increases, its SAW beam resembles that of curve FIDTs. Moreover, the total displacement magnitude of FIDTs with multiple segments is close to that of FIDTs with circular arcs in Fig. 10. In Fig. 11, the attenuation of the mechanical waves is almost due to the leaky wave phenomenon and the ink pressure. Simulation results for four structures also show that the mechanical energy of the FIDTs is lower. In other words, the FSAW devices organize more efficiently than the conventional devices. The mechanical waves of the FIDT structure are observed in frequency-time domain in Fig. 12. The spectrum of the mechanical motion at the output fingers in all focused structure cases illustrates that the total mechanical energy mostly focuses at 5.5 μs and it has other subharmonics. Hence, the performance of the proposed multiple-segment FIDTs is similar to that of FIDTs with circular arcs. In Fig. 13, the contour plot illustrates the output signals of the FSAW devices at times ranging from 0 to 25 μs. The output signal of the FIDT structures is larger than that of the conventional IDT structure. When it is excited by the first actuation pressure, the maximum voltage value still achieves 0.128 V. After 13 μs, its velocity is able to overcome the surface tension force and becomes strong enough to cut off the droplet. The breakup point may occur at around 0.074 V in this duration of 0.25 μs window (ranging from 13.2 to 15.7 μs). Hence, the alteration of the electrical signal at different

Fig. 14. Insertion loss of the output signal of the conventional and focused SAW devices with (a) conventional fingers, (b) curve fingers, and (c) 3straight-segment fingers.

generated pressures positioned at the nozzle wall and throat depends on the ink state. For each droplet formation period, the attenuation responses of conventional and concentric fingers are shown in Fig. 14. When all attenuation results of the electrical energy reach the background level, the separation process begins. The separated droplet process keeps on moving due to inertia although the excitation impact does not exist. After generating the droplet, as inertia oscillates, the significant attenuation continues and reduces gradually until the liquid surface tension returns to its resting state. As the power of the conventional structure is

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dissipated around the medium and more absorption happens at the edges, the energy loss is highest. Consequently, it is proved that the proposed FSAW devices do not only keep the advantageous properties of circular arcs, but like conventional IDTs, they are also quite sensitive to the actuation pressures of the inkjet nozzle. V. C ONCLUSION This paper presented a novel sensor for discovering the pressure variation at the inkjet nozzle. The relation between the ink pressure at the nozzle and the wave motion was found in the equation of motion for the piezoelectric medium. Based on the voltage, output power, and attenuation response of the electrical and mechanical signal, it is able to detect the droplet formation at the inkjet orifice. For the proposed FIDTs with multiple straight segments, the SAW beam is similar to that of the FSAW device with circular arcs. The greater the number of straight segments they get, the more their properties resemble circular arc FSAW devices. In addition, it influences insignificantly the flow rate at the nozzle due to the narrow SAW beam focused mostly on small arcs of the inkjet nozzle. Moreover, because of its straight shape, the proposed device is easier to fabricate. For the proposed FIDTs with multiple straight segments, based on the saturation state of the attenuation response of the electrical signal, it is still able to monitor the injected droplet process, such as estimating the beginning of the droplet generation process. The output signal may achieve up to 128 mV for the positive excitation pressure and down to approximately 74 mV for the negative excitation pressure. The breakup point keeps the potential value of 74 mV within 0.25 μs. R EFERENCES [1] W. Siqun and C. Qiong, “Microarray analysis in drug discovery and clinical applications,” in Bioinformatics and Drug Discovery, vol. 316. New York, NY, USA: Humana Press, 2006, pp. 49–65. [2] G. Wiederrecht, Handbook of Nanofabrication. Amsterdam, The Netherlands: Elsevier, 2010. [3] W. S. Rone and P. Ben-Tzvi, “MEMS-based microdroplet generation with integrated sensing,” in Proc. COMSOL Conf., 2011. [4] M. Elewenspoek and R. Wiegerink, Mechanical Microsensors. New York, NY, USA: Springer, 2001. [5] A. Hladnik, T. Muck, and G. Novak, “Quality evaluation of ink-jet paper with principal components analysis,” Int. J. Syst. Sci., vol. 33, no. 8, pp. 677–687, 2002. [6] J. Wei, Silicon MEMS for Detection of Liquid and Solid Fronts. Zutphen, The Netherlands: Wöhrmann Print Service, 2010. [7] J. Wei, P. M. Sarro, and C. D. Trinh, “A piezoresistive sensor for pressure monitoring at inkjet nozzle,” in Proc. IEEE Sens. Conf., Nov. 2010, pp. 2093–2096. [8] H. C. Wu and H. J. Lin, “Effects of actuating pressure waveforms on the droplet behavior in a piezoelectric inkjet,” Mater. Trans., vol. 51, no. 12, pp. 2269–2276, 2010. [9] T.H. Bui, T. Bui Duc and T. Chu Duc, “Microfluidic injector simulation with SAW sensor for 3D integration,” in Proc. IEEE Sens. Appl. Symp., Feb. 2014, pp. 213–218. [10] S. Shiokawa and J. Kondoh, “Surface acoustic wave sensors,” Jpn. J. Appl. Phys., vol. 43, no. 5B, pp. 2799–2802, 2004. [11] T.H. Bui and T. Chu Duc, “Multilayer SAW device for flow rate sensing in a microfluidic channel,” in Proc. IEEE Sens. Conf., Baltimore, MD, USA, Nov. 2013, pp. 487–490. [12] D. S. Ballantine et al., Acoustic Wave Sensors—Theory, Design and Physico—Chemical Applications. New York, NY, USA: Academic, 1997.

[13] R. Singh and V. R. Bhethanabotla, “Design of mutually interacting multi-directional transducer configurations on a surface acoustic wave device for enhanced biosensing,” in Proc. IEEE Sens. Conf., Oct. 2009, pp. 1044–1047. [14] M. Kirci and E. Akcakaya, “Analysis of focused surface wave transducers,” IEE Proc. G Circuits, Devices, Syst., vol. 137, no. 6, pp. 467–469, Dec. 1990. [15] P. Marechal, N. Felix, F. Levassort, L.-P. Tran-Huu-Hue, and M. Lethiecq, “P3P-7 modeling of lens focused piezoelectric transducer for medical imaging,” in Proc. IEEE Ultrason. Symp., Oct. 2006, pp. 2341–2344. [16] T.H. Bui, T. Bui Duc and T. Chu Duc, “An optimization of IDTs for surface acoustic wave sensors,” in Proc. IWNA, 2013, pp. 159–162. [17] H. Wijshoff, Structure- and Fluid-Dynamics in Piezo Inkjet Printheads. Paris, France: Hermann & Cie, 2008. [18] T.-T. Wu, H.-T. Tang, Y.-Y. Chen, and P.-L. Liu, “Analysis and design of focused interdigital transducers,” IEEE Trans. Ultrason., Ferroelectr., Freq. Control, vol. 52, no. 8, pp. 1384–1392, Aug. 2005. [19] D. P. Morgan, Surface-Wave Devices for Signal Processing. New York, NY, USA: Elsevier, 1985, pp. 129–155. [20] H. Wijshoff, “The dynamics of the piezo inkjet printhead operation,” Phys. Rep., vol. 491, nos. 4–5, pp. 77–177, 2010. [21] M.-H. Bao, Micro Mechanical Transducers—Pressure Sensors, Accelerometers and Gyroscopes. Amsterdam, The Netherlands: Elsevier, 2000.

Thu Hang Bui received the B.Eng. degree in electronics and telecommunications from the Hanoi University of Science and Technology, Hanoi, Vietnam, in 2010, and the Master’s (Hons.) degree from the Department of Electronics and Telecommunications, University of Engineering and Technology, Vietnam National University (VNU), Hanoi, in 2013. She is currently pursuing the Ph.D. degree from the Delft University of Technology, Delft, The Netherlands. She has been an Assistant Lecturer with the University of Engineering and Technology, VNU. Her current research interests include microfluidic sensor, actuator, and piezoelectric technology. Tung Bui Duc received the B.S. degree in electronics and telecommunications from the University of Engineering and Technology, Vietnam National University, Hanoi, Vietnam, in 2013, where he is currently pursuing the M.Sc. degree in microelectromechanical systems with a focus on piezoelectric and piezoresistive sensors, and microsystem technology.

Trinh Chu Duc received the B.S. degree in physics from the Hanoi University of Science, Hanoi, Vietnam, in 1998; the M.Sc. degree in electrical engineering from Vietnam National University (VNU), Hanoi, in 2002; and the Ph.D. degree from the Delft University of Technology, Delft, The Netherlands, in 2007. His doctoral research concerned piezoresistive sensors, polymeric actuators, sensing microgrippers for microparticle handling, and microsystems technology. He is currently an Associate Professor with the Faculty of Electronics and Telecommunications, University of Engineering and Technology, VNU. Since 2008, he has been the Vice Dean of the Faculty of Electronics and Telecommunications. Since 2011, he has been the Chair of the Department of MicroElectroMechanical Systems and Microsystems. He has authored or co-authored over 70 journal and conference papers and patents. Dr. Chu Duc was the recipient of the VNU Young Scientific Award in 2010 at the 20th Anniversary of the Delft Institute of Microsystems and Nanoelectronics, the Delft University of Technology Best Poster Award in 2007, and the 17th European Workshop on Micromechanics Best Poster Award in 2006. He was a Guest Editor of the Special Issue of the MicroElectroMechanical Systems, Vietnam Journal of Mechanics, in 2012.