Microfluidic Injector Simulation With FSAW Sensor for 3 D Integration

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 continuo

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 INTRODUCTION

INKJET technology has been applied for various devices

such as printers and applications in life sciences

(diagno-sis, analy(diagno-sis, tissue synthe(diagno-sis, and drug discovery) [1], [2]

Inkjet printers are feasible tools for printing texts and images

because of their low cost and high resolution within

accept-able 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: hangbt@vnu.edu.vn).

T Bui Duc and T Chuc Duc are with the Department of

MicroElectro-Mechanical Systems and Microsystems, Faculty of Electronics and

Telecom-munications, University of Engineering and Technology, Vietnam National

University, Hanoi, Vietnam (e-mail: trinhcd@vnu.edu.vn).

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 trans-ducers (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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850 IEEE TRANSACTIONS ON INSTRUMENTATION AND MEASUREMENT, VOL 64, NO 4, APRIL 2015

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

multiple-segment IDTs, and simulation results corresponding to

each droplet state at the nozzle Finally, the conclusion is

summarized in Section V

II MATHEMATICALMODEL

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

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 amountT i 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

where

F 1s = [(T11+ T11)A1− T11A1]

+ [(T12+ T12)A2− T12A2]

+ [(T13+ T13)A3− T13A3] and

F 1 p= −1

ρ

∂ P

∂x xyz.

Here, A i = j ,k δ i j k x j x k (i = j = k) is the area of

a face with a normal component in the x i -direction and T i is the elastic constitutive relation From (1) and Newton’s law, the equation of motion for a solid is sought in the form

3



j=1

∂T i j

∂x j −ρ1

3



i=1

∂ P

∂x i = ρ ∂2u i

∂t2 . (2) 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

c i j kl ∂2u l

∂x k ∂x j +

3



j k=1

e i j k ∂2φ

∂x k ∂x j − 1

ρ

3



i=1

∂ P

∂x i = ρ ∂2u i

∂t2 (3)

where c i j kl , e i j k,ρ, and φ are the elastic stiffness constants,

piezoelectric stress constants, piezoelectric density, and

elec-trical 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 and k are the

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

angleφ with the x-axis Both FIDT structures have the same

degree of aperture D a 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

ψ(x, y)=

N



i=1

1

2π

 ∞

−∞ y )exp[− j{xk x (k y )+yk y}]dky (4) where [kx (k y )]2 = [k(φ)]2− k2

y and ¯ y ) is the amplitude

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(ky ) =

 ∞

−∞ψ(x i, y)exp( j yk y )dy. (5)

If ψ(x

i , y) is the SAW beam function of the straight

IDT, those of the conventional FIDT, two-segment FIDT,

Fig 3 Novel position of the SAW sensor in the injector.

and three-segment FIDT are

ψ(x

i , y) = ψ(x

i , y)exp[ jk0x] (6)

ψ(x i, y) = ψ(x i, y)exp

⎡

⎣jk0

⎛

⎝

x(2R − x) cot2( D a

4 )

⎞

⎠

⎤

⎦ (7)

ψ(x i, y) =

⎧

⎪

⎪

⎨

⎪

⎪

⎩

ψ(x i, y)exp[ jk0x∗], for x ≤ x∗

ψ(x

i , y)exp



j k0(x∗

+ tan D a

3



x(2R − x) − R sin D a

6



(8) wherex∗= 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

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

TABLE I

D ESIGN P ARAMETERS OF IDT

Fig 4 Inlet velocity is excited by one pulse within the first 14μs.

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 SYSTEMCONFIGURATION

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

and (b) t= 14 μs.

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 mod-els 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

Fig 7 Effect of the piezoelectric substrate on the liquid.

Fig 8 Sensitivity of the IDT device and two-segment FIDT device.

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 2μs

v i (x, y, t) = 4.5



x2+ y2+ 0.1[mm]

0.2[mm]



×



1−



x2+ y2+ 0.1[mm]

0.2[mm]



· v(t)(mm/s).

(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 2μs Therefore, the ink velocity

at the nozzle throat is sought in the following form:

v n (x, y) = v i (xy) R21

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

Fig 10 Total displacement measured at a point after the inkjet nozzle.

Fig 11 Mechanical attenuation of SAWs after propagating through the inkjet

nozzle.

The surface tension of the ink generates a capillary

pres-sure 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

IV RESULTS ANDDISCUSSION

The proposed simulation methodology has been

implemented using finite element method and COMSOL

Multiphysics 4.2a

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 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 200μs 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 25μs to determine the correlation between

the droplet generation and the output signal variation

B Working Mechanism of the FSAW Device

Pressure produced by the piezoelectric substrate insignifi-cantly 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

Fig 13 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

struc-ture 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 25μs 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) 3-straight-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

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

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 CONCLUSION 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.

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Thu Hang Bui received the B.Eng degree in

elec-tronics and telecommunications from the Hanoi Uni-versity 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 Uni-versity of Engineering and Technology, VNU Her current research interests include microfluidic sensor, actuator, and piezoelec-tric technology.

Tung Bui Duc received the B.S degree in

elec-tronics 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 micro-electromechanical systems with a focus on piezo-electric 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 electri-cal 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 con-cerned 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 Nanoelec-tronics, 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.