journal of microelectromechanical systems, vol. …a 4-in silicon wafer by drop-on-demand...

JOURNAL OF MICROELECTROMECHANICAL SYSTEMS, VOL. 15, NO. 4, AUGUST 2006 957

Acoustic Picoliter Droplets for Emerging Applicationsin Semiconductor Industry and Biotechnology

Utkan Demirci

Abstract—This paper presents the theory of operation, fabrica-tion, and experimental results obtained with a new acousticallyactuated two-dimensional (2-D) micromachined microdropletejector array. Direct droplet based deposition of chemicals used inIC manufacturing such as photoresist and other spin-on materials,low- and high- dielectrics by ejector arrays is demonstrated toreduce waste contributing to environmentally benign fabricationand lower production cost. These ejectors are chemically compat-ible with the materials used in IC manufacturing and do not harmfluids that are heat or pressure sensitive. A focused acoustic beamovercomes the surface tension and releases droplets in air in everyactuation cycle. The ejectors were operated most efficiently at 34.7MHz and generated 28 m diameter droplets in drop-on-demandand continuous modes of operation as predicted by the finiteelement analysis (FEA). Photoresist, water, isopropanol, ethylalcohol, and acetone were ejected from a 4 4 2-D microma-chined ejector array. Single photoresist droplets were printedonto a silicon wafer by drop-on-demand and continuous modesof operation. Parallel photoresist lines were drawn and a 4-inwafer was coated by Shipley 3612 photoresist by using acousticallyactuated 2-D micromachined microdroplet ejector arrays. [1513]

Index Terms—Acoustic radiation pressure, deposition, dropletejection, finite element analysis (FEA), inkjet, microfluidic chan-nels.

I. INTRODUCTION

THE invention of the printing press in the 15th century isa major force in the Renaissance and Reform movements

[1]. It burst the chains that fettered knowledge and revolution-ized society advancing civilization toward the freedom of mindfrom which humanity benefits today [2]. The capability to con-trollably eject ink droplets onto a white sheet of paper is a de-scendant technology of this very old and beneficial concept ofthe printing press. Droplet generation techniques have becomeeven more valuable and significant as numerous new applica-tions are posed by today’s technology. This concept will beelaborated in the following paragraphs focusing on the semi-conductor and biotechnology applications.

Biotechnology applies enabling technologies from other dis-ciplines to biology facilitating significant discoveries. The directimpact of these achievements on the human-beings makes thisfield a leading research area. Almost all of the experiments in

Manuscript received January 26, 2005; revised November 8, 2005. This workwas supported by NSF/SRC Engineering Research Center for EnvironmentallyBenign Semiconductor Manufacturing. Subject Editor C. H. Mastrangelo.

The author was with the E. L. Ginzton Laboratory, Stanford University,Stanford, CA 94305 USA. He is currently with Harvard-MIT Health Sciencesand Technology, Harvard Medical School, Boston, MA USA 02129 (e-mail:[email protected]).

Digital Object Identifier 10.1109/JMEMS.2006.878879

biotechnology are conducted in fluidic environments. Thus, ca-pability to generate droplets of various fluids has emerged as anattractive technology enabling various biological applications.For instance, bioarrays for drug testing could easily utilize con-trolled droplet generation techniques; two-dimensional (2-D)arrays of cells laid on a surface can be tested with small amountsof drugs delivered by microdroplet ejectors [3]. Writing DNAarrays by drop-on-demand ejection is another example [3].

The semiconductor industry is one of today’s largest existingmarkets, for example wafer fabrication equipment market was$18 billion market, flat display coating was $22 billion marketin 2003 [4]. The business model of semiconductor companiesrelies on Moore’s Law, which can be summarized as scaling,which enables packing more and more devices per unit area, de-creasing cost and increasing functionality and speed. Since suc-cess in scaling depends on success in lithography, droplet gen-eration applications that could improve or revolutionize lithog-raphy are of major interest. The deposition of photoresist thinfilm is the first step in lithography, followed by masking anddeveloping steps. In 2003, photoresist consumption worldwidewas $735 million [4]. Most of this photoresist is deposited bytracker systems that utilize spin coating technology [5], [6].However, this technology wastes up to 95% of the disposed re-sist. When the cost of hazardous waste disposal is included, thecost associated with photoresist consumption increases to ap-proximately $1 billion.

A droplet ejection methodology that prints photoresist ontowafer surface drop-by-drop has the potential to minimize pho-toresist waste [7]. This ejection technology relies on surface ten-sion of the fluid to provide thin film quality and uniform thick-ness of the deposited photoresist without spinning [5]–[7]. Re-ducing operational cost associated with lithography would be asignificant improvement, and this work will specifically targetthis application.

Although there are various droplet generation techniques cur-rently available, the semiconductor and biotech applications ofmicrodroplet generation require certain design parameters forsuccessful delivery of fluids. One of the most important factorsis avoiding damage to pressure and heat sensitive fluids. This re-quirement eliminates inkjet technology, where the temperatureand pressure in the ejection fluid can rise. Human and manyother living cells do not survive at high temperatures or pres-sures.

Second, the directionality of ejected droplets is a primary con-cern. The ejection directionality of traditional inkjet devices islimited by nozzle geometry. This limit may pose a problem onink printing applications, since the human eye is sensitive up toa resolution limit [8]. The current applications that are being tar-geted require knowing exactly where the droplet is going to land

1057-7157/$20.00 © 2006 IEEE

958 JOURNAL OF MICROELECTROMECHANICAL SYSTEMS, VOL. 15, NO. 4, AUGUST 2006

Fig. 1. Geometry of a 2-D ejector array. Signal and ground pads are used toactuate the interdigitated transducers.

on a surface. Another significant parameter is the reliability ofejection for drop-on-demand actuation. An unreliable ejectionmethod would cause non-uniform film thickness and non-uni-form film quality, if it were used for photoresist deposition ontoa wafer [9].

There has been previous work on droplet ejection by utilizingan acoustic focus [10]. These approaches use an acoustic lensor a fresnel lens approach to form a focal point, from whichdroplets are released. Huang and Kim demonstrated a focusedejector that utilizes the direct coupling of acoustic waves to afluid in order to form a focus [11]. This method could sufferfrom fabrication complexity such as alignment of multiplelayers and nonuniformity of piezoelectric film deposition,which may cause variation from one array element to another,satellite droplets, and problems with ejection stability. Acous-tically Focused 2-D Micromachined Microdroplet EjectorArrays that are demonstrated in this paper do not utilize anacoustic lens approach. The formation of the focal point isby surface acoustic waves which leak into the fluid mediumand interfere to form an acoustic focus. They are easily andrepeatably fabricated. The substrate uniformity and fabricationease ensures identical and stable operation of ejector array.This type of acoustic wave is previously employed by Farnellet al. in order to build a planer acoustic microscope [12]. Wedemonstrate controlled droplet generation by utilizing leakysurface acoustic waves.

This paper demonstrates a new micromachined acousticallyactuated 2-D microdroplet ejector array and initial coating ofa 4-in silicon wafer by drop-on-demand photoresist ejection.Since ejection takes place from an open-pool, nozzleless reser-voir, droplet directionality does not depend on the nozzle geom-etry, and is easily controllable. Furthermore, the device releasesacoustic waves at every cycle of actuation, which assures reli-ability of ejection. These novel ejector arrays fulfill all designparameters for sensitive fluid ejection applications.

II. THEORY

A. Theory of Device Operation

The basic building block of an acoustically actuated micro-droplet 2-D ejector array is an interdigital ring transducer asshown in Fig. 1. The rings are periodically spaced on a piezo-electric substrate. This unit cell can be repeated in a 2-D geom-etry as shown in Fig. 1.

Fig. 2. Schematic of the physical operation of an ejector array. Leaky surfaceacoustic waves form an acoustic focus and droplets are ejected from an openpool of a microfluidic channel etched into a silicon spacer.

The interdigital ring transducers launch surface acousticwaves as explained in [13] and [14]. If another medium isplaced on the piezoelectric substrate, such as a fluidic environ-ment as shown in Fig. 2, the launched surface acoustic waveswill leak into this medium. The angle of leakage follows Snell’sLaw, based on the ratios of two media indices (or velocitiesin acoustics) [13]. As waves travel through fluid, they reacha point where they interfere constructively forming a focus.If this focal point is right at the surface of the fluid and thereis enough force exerted by acoustic radiation at that point toovercome the surface tension forces of the fluid, then a dropletwill be ejected as shown in Fig. 2.

Chu et al. gives the Langevin radiation pressure by the meanenergy density of an acoustic beam at the surface of air liquidboundary [15], [16]. The time averaged energy density can becomputed [14]–[16]. Hunter et al. gives the average intensityof an incident wave as in (1) and the Langevin radiationpressure ’ as in (2).

(1)

(2)

where is the pressure amplitude of an incident acousticwave, is the bulk density of the liquid, and is the velocity.

The radiation pressure acts for a time, which generates an ini-tial momentum per unit area. We assume that the initial mo-mentum is given to a cylinder of fluid and is uniformly dis-tributed in that cylinder such that the full width at half max-imum (FWHM) of the acoustic focus is at the midpoint of thefocal fluid cylinder. This momentum results in formation of afluid droplet.

B. Explanation of the Finite Element Analysis (FEA)

The FEA of the device is performed using ANSYS 5.7(ANSYS Inc., PA). The initial objective of this analysis is to de-termine the location, width and height of the focal point, and thebest frequency of operation for the device, where the acousticpressure at that focal point is maximized per unit voltage input.The secondary objective of the FEA is to investigate effect ofthe number of interdigitated finger pairs on device performance,in order to finalize the device design and determine how manymetal finger pairs are needed for successful droplet ejectionor for a certain focal point geometry. The third objective is todetermine the evolution of acoustic wave as it travels from thedevice surface to the focal point, facilitating design of better

DEMIRCI: ACOUSTIC PICOLITER DROPLETS FOR EMERGING APPLICATIONS 959

Fig. 3. Schematic for FEA of ejection. The figure is axisymmetric around they axis. One of the four interdigitated finger pairs is marked on the figure. Thepressure distribution from point ‘A’ to ‘B’ determines the theoretical focal pointand droplet size generated by the device can be estimated by the lateral pressuredistribution at the focal point.

devices and microfluidic channel holding silicon spacers thatdo not interfere with device performance.

A harmonic analysis of the 2-D axisymmetric structuresshown in Fig. 3 was performed. The mesh had at least ten nodesper wavelength in the structure. The device was simulated byloading one side of a piezoelectric substrate with an infinitelylong fluid space, eliminating undesired reflections (FLUID129 for absorbing boundary conditions). The structure/fluidinteraction was taken into account by solid/fluid interfaceelements (FLUID 29). The characteristic piezoelectric materialvalues of the fabricated device and distilled water were usedfor simulations [14].

C. Real and Imaginary Impedances of the Device

The resonance frequency is inherent to the device geometryand the substrate and depends on the periodicity of the rings.Once the resonance frequency is known by simulations under acertain fluid loading, location of the focal point by simulationsunder that fluid loading can be directly determined. We simu-lated the impedance of a single ring device with various fluidloadings. The simulations can be run with various fluids for thesame device geometry. When the fluid loading is changed, thevelocity of acoustic wave and the location of the focal pointchange. It is important to run simulations with various fluidloadings to be able to predict the device behavior.

We searched for frequencies where the real part of impedanceis maximized under fluid loading in order to determine the op-timum frequency of operation, i.e., resonance. Another way todetermine the resonance frequency is to sweep frequency ina range of 1–50 MHz and obtain plots for pressure distribu-tion over the line from point A to point B (as shown in Fig. 3)for each frequency point. The highest pressure levels would beobtained on the line when device is operated at its resonance.Also, the same curve would show where the focal point is lo-cated. Moreover, comparison of simulated and experimental realparts of the impedance indicates presence of an 8– series re-sistance due to interconnect metal strips guiding the signal and

Fig. 4. (a) Real part of the impedance, theory, and experiment in air. (b) Imag-inary part of the impedance, theory, and experiment in air.

Fig. 5. Pressure on the axisymmetry line from ‘A’ to ‘B,’ as shown in Fig. 3,in the fluid medium for 4- and 8-pair ejector devices at 34.7 MHz.

ground pads to the ring ejector. The comparison of simulatedand experimental imaginary parts of the impedance indicatesthe presence of a parasitic capacitance on the device. This par-asitic capacitance should be minimized to achieve maximumpower delivery efficiency to interdigitated rings of the ejector.The simulation and experimental results for real and imaginary


Fig. 6. Theoretical lateral pressure distributions from the center of to the device to 1.5 mm radius outside the device are shown at various heights away from thepiezoelectric substrate showing the evolution of the focal point for a 4-finger pair device and leakage pathway of the acoustic waves into the fluid medium.

parts of the impedance of a 4–finger pair device in air are shownin Fig. 4(a) and (b). There is agreement between the simulationresults of the FEA and the experimental results.

D. Focal Point and Operation Frequency of the Device

FEA provides the pressure distribution at any point in thefluid. The focal point location is defined as the highest pressurepoint in the fluid space. As a sinusoidal signal of 1 V ampli-tude was applied on metal signal lines at varying frequencies,the pressure distribution in the fluid medium from the piezoelec-tric substrate surface into the fluid on a vertical path from A toB of Fig. 3 was monitored as a function of frequency. The sim-ulations predicted that devices generated the highest acousticpressure at the focal point at 34.7 MHz.

The number of finger pairs per ejector element is another cru-cial design parameter. The pressure distribution in the fluid on avertical path from A to B as shown in Fig. 3 was monitored fordevices with 4- and 8-finger pairs (Fig. 5). The pressure is max-imal at the focal point, which is 575 away from the surfaceof the piezoelectric substrate for a device with 4-finger pairs.The device can achieve efficient droplet ejection when the fluidsurface height is located between 550 and 585 , as demon-strated by Fig. 5. This point is located above the center of metalrings. The focal point of an 8-finger pair device can also be ob-tained from Fig. 5, and is 1500 above the piezoelectric sub-strate surface.

The lateral pressure distribution at the focal point is signifi-cant, since the focal point geometry determines the droplet size.The lateral pressure distribution at the optimal operating fre-quency of 34.7 MHz at 575 away from the piezoelectricsubstrate surface for a 4-finger pair ejector device is shown inFig. 6 for distilled water. At the focal point of 575 pres-sure reaches a maximum of 0.75 MPa as shown in Fig. 6. Thesimulation predicts the pressure amplitude to be half of its peakvalue of 0.75 MPa at 15 away from the focal point. This im-plies an effective droplet diameter of 30 for distilled water.Moreover, the FEA predicts a droplet diameter of 26 for iso-propanol loading of the device.

E. Theory of Leaky Waves

The evolution of acoustic waves as they travel in the fluid toform a focal point is significant, since it is a key to design betterdevices and microfluidic channel spacers that do not interferewith device performance. The lateral pressure distribution fromthe center of the device to 1.5 mm distance at various heightsof 10, 50, 250, and 575 above the substrate is shown inFig. 6. This figure demonstrates the evolution of the focal pointfrom the piezoelectric substrate to the focal point as acousticwaves leak into fluid. The outer ring of a 4-finger pair devicehas a radius of 755 . The acoustic pressure increases bothinside and outside the rings due to interference, but a focus isformed only inside, due to the circular geometry. Outside thering, waves travel away from the device.

F. Simulation of Coverage

The coverage of a wafer has to satisfy important parametersof throughput and uniformity to compete with the current ex-isting ejection technologies. The time required to cover a wafersurface with photoresist is calculated by a simple program as-suming that the droplet size on wafer is four times the dropletdiameter in air; uniform coverage is achieved by surface tensionforces spreading the droplets uniformly when the edges of theejected droplets barely touch; the wafer is moved beneath theejector, while a single ejector is ejecting droplets. Various algo-rithms for a wafer moving under an ejector array or an ejectorarray moving over a wafer can be developed for achieving uni-form coverage, since there is reliable control over the dropletsize and directionality in an open pool ejection, such as in thisdevice. The emphasis is on how fast an ejector array can gen-erate necessary number of droplets to cover a wafer. In the cal-culations, one drop is ejected per spot; however, by ejecting mul-tiple drops to each location, the same system can be used to ob-tain various film thicknesses. On the other hand, conventionalspin coating depends on various photoresist formulations withvarying viscosities to achieve various film thicknesses. As the2-D ejector array size increases, time to generate the necessarynumber of droplets for coating decreases. The predicted results


TABLE IPREDICTED COVERAGE RESULTS FOR AN N � N EJECTOR ARRAY COATING A 4-in WAFER

for the time required, fluid consumed, and number of dropletsrequired for a 4 wafer being covered by an ejector is shownin Table I considering three droplet sizes of 15, 24, and 30 .This model can be improved by accounting for photoresist sol-vent drying. These numbers are directly proportional with areaand will be approximately nine times larger for 12 wafers.

III. FABRICATION

A. Device Design

The basic building block of an acoustically actuated picoliter2–D ejector array is an interdigital ring transducer as shownin Fig. 1 and Fig. 7(a). The rings are periodically spaced on apiezoelectric substrate. This unit cell can be repeated in a 2–Dgeometry as shown in Fig. 7(a).

As shown in Figs. 2 and 7(b) the microfluidic channels havethe function of stabilizing the fluid surface for a stabilized fluidfocal point. The channels prevent the fluid from leaking by sur-face tension forces such that we could eject in all angles. More-over, the microfluidic channel openings vary from 50 to 300and are wider than the focal point diameter. This still enablesopen pool ejection and ensures directionality and size unifor-mity of droplets independent of nozzle geometry.

B. Device Fabrication

The device is fabricated using integrated circuit (IC) manu-facturing techniques. First, a piezoelectric substrate is coveredwith photoresist and resist is patterned so that circular activeareas are inscribed on a substrate. In this step, the substrate iscovered with photoresist everywhere except at locations wherethe gold circular lines will be deposited. Second, 3000 A of thingold film is deposited on to this patterned substrate by evapora-tion. A standard lift-off process etches the photoresist in acetoneand leaves behind the circular gold rings on a piezoelectric sub-strate surface. A single ring ejector and a 4 4 ejector array areshown in Fig. 7(a).

The top and bottom spacers are fabricated by using potas-sium hydroxide (KOH) etching of silicon substrate through thecrystal planes to achieve the desired opening on the other side.A 200- -thick and 300- -wide top spacer is bonded to a350- -thick and 1.5 mm-wide bottom spacer to form a fluidicchannel that stabilizes the fluid surface at a height of 550from the interdigitated ring array surface as shown in Fig. 7(b).

Fig. 7. (a) Fabricated 4� 4 ejector arrays, and a unit cell of the array is shown.The yellow lines are the deposited 17-�m-wide interdigitated gold lines on thesurface of the piezoelectric substrate and are separated by 17 �m. (b) 1.5-mm-wide microfluidic channels are etched to a 350-�m-thick bottom silicon spacer,and 50–300 �m wide microfluidic channels are etched to a 200-�m-thick topsilicon spacer. The two silicon spacer pieces which have the microfluidic chan-nels defined are directly bonded together.

This total spacer height is designed to match the fluid surfaceheight to the focal point location of the device, and enablesdroplet ejection from a focal point. The bonding of two mi-crofluidic spacers is achieved by bonding the two by an initialsilicon to silicon surface touch, which achieves an initial bondbetween the two pieces. This bond is strengthened by placingultraviolet light curable epoxy around the two silicon microflu-idic spacers.

IV. EXPERIMENTAL METHODS AND RESULTS

A. Capacitance Measurements

The impedance of a single ejector device in air were pre-sented above in Section II. The ejector device is then loaded withvarious solvents in order to understand its characteristics. Thecapacitance of the device changes with fluid loading, since the


TABLE IIDIELECTRIC CONSTANTS OF VARIOUS FLUIDS AND CAPACITANCES MEASURED BY A 1 � 3 ARRAY

Fig. 8. Ratio of “(area/distance)” in meters of the same device under variousfluid loadings.

capacitance of finger pairs can be considered as a parallel com-bination of the piezoelectric plate and the loading medium ca-pacitances. The dielectric constants of loading fluids and capac-itance measured by a network analyzer (HP 8752A, CA) for a 1

3 array are shown in Table II. In order to understand the vari-ation of capacitance with fluid medium, a simple capacitancecalculation approach “ (area/distance)”is utilized. The ratio of “(area/distance)” is observed to be con-stant over the same device under various fluid loadings as shownin Fig. 8. The mean and variance of “(area/distance)” are calcu-lated to be 0.071 m, , respectively. The rapid changein impedance and high sensitivity to dielectric constant and con-ductivity changes in fluid medium make these devices attractivefor biosensor applications such as viral, bacterial, or cellular de-tection and quantification.

Once the impedance and the optimum operating frequencyof the device are determined, it is possible to design a matchingnetwork to maximize the power delivered to the ejector. Thedevice impedance changes with various fluid loadings as can beseen from Table II, which requires the matching network to betuned for a specific fluid ejection. In our experiments, it was ob-served that the ejector ejected various solvents and photoresistwithout a matching network. However, a simple power reflec-tion calculation, which takes into consideration the impedanceof the ejector as measured before and a 50- input to the ejectorindicates that on average 70% of initial power delivered is beingreflected from the ejector array. Therefore, a matching networkshould be implemented for efficient use of power.

B. Pitch-Catch Measurements

The focal point measurements were obtained by placing asingle 4-finger ejector array element in water facing another,forming the angle of a square. The top ejector transmitting, the

Fig. 9. Schematic of the setup for pitch-catch measurements. The vertical andlateral distance can be controlled down to 0.1 �m by a micrometer stage.

bottom receiving the acoustic signals through the fluid volume,in which due form the pitch catch measurements were per-formed. The normal through the center of the receiving bottomejector should intersect with the center of the transmitting topejector and ejectors have to be facing parallel to each other inorder to receive maximal signal amplitude as shown in Fig. 9.The bottom ejector is placed on a controllable x, y, z, m stageand distance is incremented, as the acoustic signal receivedis recorded. Movement in the z direction gives the focal pointlocation as demonstrated in Fig. 10(a). The bottom ejector isbrought to the focal point, where the pressure is maximizedand then the ejector is moved in x and y directions to obtainthe lateral pressure distribution at the focal point as shownin Fig. 10(b). The simulated pressure amplitude is half of itspeak value at 15 away from the focal point. This impliesan effective droplet diameter of 30 for distilled water.The simulation of the focal point location overlaps with theexperimental results. Moreover, the insertion loss of a singleejector element was measured to be 11 dB.

C. Imaging and Wafer Coating

Stroboscopic imaging techniques were used to view ejecteddroplets. Light emitting diodes that shine light on ejecteddroplets were turned on and off by a periodic square pulsewaveform, which was synchronized with the drive signalto ejectors. Using this stroboscopic imaging technique, ajet of droplets could be viewed by an LCD camera (Sony,SSC-CD33V, Japan) with a microscopic lens (5 , NA 0.13,Olympus, Japan) on the monitor screen as shown in Fig. 11.

The input signal to a ring ejector array is a 10- -long34.7-MHz tone burst. The burst repetition rate determinesthe drop ejection rate. The ring array ejected various fluidssuch as photoresist, water, and solvents (acetone, isopropanol,methanol, and ethanol), at a drop ejection rate of 1 kHz to0.1 MHz, such as Fig. 12 shows 28 in diameter isopropanoldroplets ejected upward into the air at 1 kHz which are gener-ated at different periods of the driving signal. The size of a pixelon screen is determined by imaging a known sized object by the


Fig. 10. (a) Theoretical and experimental results for pressure on the axisym-metry line from “A” to “B” in the fluid medium for various frequencies fordistilled water. The maximum pressure is achieved around 34.7 MHz. (b) Theo-retical and experimental results for lateral pressure distribution at the focal pointof 575 �m at 34.7 MHz for a four-finger pair device.

Fig. 11. The silicon spacer consisting of the microfluidic channels are alignedon top of the 4� 4 ejector array and the stroboscopic imaging setup is demon-strated.

imaging setup. The number of pixels that a droplet covers onscreen reveals the droplet size. Moreover, current microscopescan automatically calculate size of an object on a screen byusing imbedded calibration data for a known objective size.

In order to achieve ejection not only upward but in alldirections (sideways, downwards), and control focus locationwithout being effected by evaporation or the device tilt, sil-icon microfluidic channel spacers were designed so that the

Fig. 12. 28 �m in diameter isopropanol droplets are ejected upward from anopen pool of fluid. The imaging system had a large viewing area and each dropletis generated at a different period of the driving signal.

Fig. 13. Photoresist solvent droplets are ejected downward from a100-�m-wide spacer opening. The first generated droplet has traveled inair when the second droplet generated by the second driving signal is followingthe same path. The ejector generated droplets on drop-on-demand withoutsatellite droplets.

fluid could continuously fill the microfluidic channel ejectionopenings through the fluidic channels, keeping the fluid levelconstant. Downward ejection of photoresist solvent dropletsthrough a microfluidic channel opening of 100 is shown inFig. 13. The ejection of a droplet through a neck of acousticallyraised fluid focus followed by a second droplet is also seenin Fig. 13. The ejector generated droplets on drop-on-demandwithout satellite droplets.

Various viscosity and surface tension fluids such as water (1Centistoke at 20 C), isopropanol, ethyl alcohol, ethylene glycol(18 Centistokes at 20 C), and acetone are ejected from 4 42-D micromachined ejector arrays [17]. Shipley 3612 photore-sist was also ejected through microfluidic channel openings ontothe surface of a silicon wafer at varying rates from 1 to 10 KHz.The setup for coating a wafer with photoresist was similar to theimaging setup that was demonstrated in Fig. 11. A wafer on topof a controllable micrometer stage was placed under an ejector,1 cm away and parallel to the ejector surface. The wafer wasmoved fast enough under the ejector so that single photoresistdroplets could land on the wafer surface as shown in Fig. 14(a).The profile through the center of a single droplet is shown inFig. 15. This profile is observed to be the same for all droplets.By changing ejection frequency or decreasing wafer movement


Fig. 14. (a) Ejected single photoresist droplets in air cover 165 �m in diameterarea on the silicon wafer surface. (b) Single photoresist droplets ejected every50 �m overlap and form a 200-�m-wide line by surface tension forces.

Fig. 15. Single photoresist froplet and a line profile on wafer surface.

speed the location where droplets land can be exactly deter-mined. Moreover, a line can be formed by overlapping singledroplets and achieving a uniform thickness with the help of sur-face tension forces as shown in Fig. 14(b). Lateral profile ofthe printed photoresist line is shown in Fig. 15. The wafer wasmoved at a speed of 50 000 at an ejection rate of 1 KHz,which corresponds to a single droplet ejected onto the wafer sur-face every 50 during the photoresist line printing. This alsoindicates that the wafer coverage can be performed rapidly.

We demonstrated that the photoresist lines can be repeatedmany times side by side and a 4-in diameter silicon wafer canbe coated with photoresist as shown in Fig. 16. The wafer ismoved at a speed of 20 000 as the photoresist ejection isperformed at 1 KHz. This corresponds to one droplet per 20separation for a single line. Photoresist lines were written in par-allel to perform full coverage. The separation between two lineswas set to 80, 100, and 120 at three regions on the wafer bythe automatic controlled x-y stage. This resulted into photoresistthicknesses of 2.4, 2.9, and 3.6 on wafer. The separation be-tween droplets and lines can be modified, which eventually de-termines the overall photoresist thickness. The photoresist thinfilm thickness uniformity was measured to vary peak-to-peak

at the most uniform coated areas of the wafer and

Fig. 16. Selective photoresist coverage of a 4-in wafer with three thicknessvalues by varying the overlap of printed individual photoresist lines. Three re-gions are shown with thicknesses of 3.6, 2.9, and 2.4 �m from left to right.

at the areas with the worst uniformity in all three re-gions. There was not any spinning involved in the process andcoverage experiments were done in a dry laboratory environ-ment. The surface roughness was measured to vary from 14to 600 A over the wafer. These measurements are taken by aDektak profilometer (Veeco Inc., Woodbury, NY). The thick-ness of the photoresist film can be decreased by decreasing thenumber of droplets per location, or by decreasing the overlapbetween two photoresist lines drawn side by side. We coatedsurfaces with photoresist drop-by-drop and results were repeat-able given the same ejection rate and wafer movement speeds.

V. DISCUSSIONS

Finite element theory indicates that the number of rings af-fects the location and width of focal point. A device with a largernumber of rings will be able to maintain wider vertical areas ofhigh pressure as can be seen in Fig. 5. In a sense, the number ofrings lowers the ejection sensitivity to the height of the fluid sur-face. An 8-finger pair device has a wider range of fluid heightsthat it can eject at. Since controlling fluid height is not a seriouschallenge and can be easily handled, increasing the number offinger pairs does not offer a significant advantage. Moreover,pressure amplitude at the focal point of an 8-finger pair ejectoris comparable to the 4-finger pair ejector. Furthermore, 8-fingerpair ejector does not provide a different lateral pressure distri-bution at focus from 4-finger pair ejector. The droplet sizes gen-erated by 4- and 8-finger pair devices are also comparable, sincedroplet size is determined by the acoustic wavelength and the in-terference geometry at the focal point. As the number of fingerpairs in the design does not play a significant role for currentapplications, 4-finger pairs are chosen as the design, which ac-cording to the FEA results, can generate a focal point and ejectdroplets.

The droplet diameter depends on the wavelength in fluid andfocal point geometry as described above. The theoretical focalpoint diameter was found to be 30 and 26 for distilled waterand isopropanol, respectively. The speed of sound in water is1480 m/s, and 1100 m/s in isopropanol. Therefore, the acousticwavelength in isopropanol is smaller resulting into smallerdroplets. Moreover, the theoretical 26 focal point diameteragrees with the experimental value of 28 in diameter forejected isopropanol droplets.

The capability to eject in all directions, control of the focuslocation without being affected by evaporation, and nozzleindependent open-pool ejection were desired objectives. Inorder to achieve these goals, the microfluidic channel spacers


were designed to continuously fill the microfluidic channelopenings through the microfluidic channels by capillary actionand maintain a constant fluid level. The height of the fluidlevel and the focal point which coincide with the microfluidicchannel opening was stabilized and rendered independent ofthe device tilt by the surface tension forces. Moreover, thisopen-pool ejection ensures nozzle independence. The nozzledependent ejection methods such as inkjet may suffer fromclogging. The droplet size and directionality in inkjet dependson the nozzle, where as in open pool ejection, the ejection poolcan be as wide as 300 m and clogging is not an issue. Wehave not observed any clogging for our ejectors. Any residue inthe microfluidic channels can be removed by some photoresistsolvent or by acetone flow through the channels, or if a cleaningstep is intended before loading a new type of a fluid.

The micromachined ejector arrays were able to eject photore-sist. However, since the experiments were carried out in a drylaboratory environment at room temperature ( 20 C), evapora-tion of the photoresist solvent and a rapid increase in viscosity ofthe photoresist took place. This evaporation degraded thicknessuniformity of the deposition. It should be possible to achievebetter thickness uniformity in a solvent saturated environment.Moreover, presence of 15–40 diameter dust particles in theexperimental environment also degraded the thickness and filmuniformity. This would not be a problem when experiments areperformed in a clean room. Furthermore, post deposition spin-ning was not performed in order to avoid any waste of photore-sist.

The presented FEA results were performed with a signal inputat 34.7 MHz. The simulation for the focal point location over-laps with the experimental result. However, a wider band ofejection is observed with the experimental results than simu-lation results. This could be explained by the fact that the par-allelism between the transmitting and the receiving ejectors ishard to control. An angle variation from horizontal could causevariation on the received signal. Moreover, the closer the trans-mitter and receiver get to each other the harder it gets to sep-arate the acoustic signal from the electromagnetic signal feed-through. This causes an interference of electromagnetic wavesand acoustic waves and the signal amplitude varies, which re-sults in a change in measured bandwidth.

Outside the metal rings, waves travel away from the device asshown in Fig. 6. These waves also leak into the fluid at a criticalangle, however without focusing and with attenuation. In thepresence of an attenuating environment these outside travelingwaves are quickly attenuated. They do not reach the next arrayelement or cause any undesired crosstalk effect that could im-pede the focal point formation and deter simultaneous dropletgeneration by all array elements. Moreover, the capability todetermine the pathway for leaky acoustic waves facilitates thedesign of the microfluidic channel spacers. The acoustic wavestraveling into the fluid medium should not be intercepted by thepresence of these microfluidic spacers.

The reliable control over the droplet size and directionalityin an open pool ejection, such as in this device enables var-ious deposition algorithms for a wafer moving under an ejectorarray or an ejector array moving over a wafer. These algorithmscan be developed for achieving uniform coverage, but these arebeyond the scope of this paper. The main emphasis is on howfast the ejector array can generate necessary number of droplets

to cover a wafer. As the number of ejectors of a 2-D ejectorarray increases, the time it takes to generate necessary number ofdroplets decreases. The capability to eject from many ejector ar-rays simultaneously could enable linear coverage algorithms asutilized for printing of single photoresist droplets demonstratedin Section IV. In theory, hundreds of these ejectors could be ina linear array or 2-D array format, printing a single line wideenough to cover a whole wafer by a single pass over the wafer.The wafer movement speed and the droplet ejection rate can beincreased. There is potential to coat a wafer in less than 5 s giventhat there are not other problems due to wafer speed. This wouldremarkably increase the throughput of coating. The possible in-crease in the throughput is clear, when we consider that there isno spinning or edge bead removal processes necessary after thecoating step, which take about a minute to complete in today’stracker systems by using spin coating method.

The surface tension and viscosity of the ejection fluid are im-portant parameters, since they affect the droplet size and thick-ness of the deposited resist. The droplet size is affected since theacoustic wavelength changes with fluid as explained earlier inthe theory. A fast evaporating fluid would have changing prop-erties, which makes a solvent saturated environment very im-portant. The droplet size eventually influences thickness of thephotoresist film. In theory, a device could place as many dropletsas possible to the same location for thicker resist coating. Weobserved that a single droplet can spread on a surface such thatthe thickness can be as small as 300 nm with Shipley 3612.This indicates that a wafer could be coated at nanometer scaleheights given that the droplets could be placed such that theyjust barely touch each other. This could be possible with thepresented droplet ejector, since the ejection directionality is uni-form. We observed thicknesses as low as 300 nm for singledroplets on wafer surface and as high as 8 by increasingthe overlap of the droplets or lines.

The temperature increase in the fluid reservoirs is not desired,since it could increase evaporation rate and affect photoresistuniformity, fluid properties, and droplet sizes. We did not expe-rience any significant fluid temperature increase or temperaturerelated problems in our experiments during ejection at 1 KHz.Heat generated on the device surface would be more readily ab-sorbed by the substrate and surrounding silicon and metal be-fore it influences the fluid reservoir. We also observed dropletsizes to be constant in size over 10 min of ejection. Our goalwas to demonstrate coating of a wafer with photoresist by usingmicromachined ejector arrays, which did not require elaboratetemperature control initially.

We made simple assumptions and calculations for a worstcase situation considering the interconnect resistor of 8 ohmsin order to get a quantitative feel of temperature generated perdroplet in a fluid reservoir. A more detailed model consideringthe acoustic waves could be developed. Here, the power dissi-pated on the resistor can be calculated by ‘ ’. Themaximum voltage on the whole ejector device was 17 V duringa droplet generation cycle, which lasts 10 . The tempera-ture raise in the reservoir can be approximately calculated by‘ ’, where ‘ ’ is the rise in temperature, ‘m’ isthe mass of fluid in kg and ‘c’ is the specific heat given as 1000calories/kg for water. The mass of the fluid in the fluid reservoircan be calculated from the microfluidic channel dimensions. Ifwe assume that all the heat is uniformly given to the reservoir, it


is not necessary to concentrate on the distribution of heat wavestraveling in the fluid reservoir.

These simple calculations indicate that the temperature offluid will increase per ejected droplet in the worstcase situation, if all the heat generated by the 8 ohm resistor isgiven to the reservoir at once. It is not a correct approach tomultiply this energy per droplet temperature value with 1000in order to obtain a total temperature increase per second foran ejector operating at 1 KHz. If the ejector is operating at 1KHz, this means that the ejector will generate one droplet every1000 . The length of the droplet generation signal is 10 . So,the fluid reservoir is left to cool for 990 after one droplet isejected. This could explain why temperature is not building upexperimentally when 1000 droplets per second are ejected. The

increase, which is very small, is already beingdissipated during the relaxation time of 990 . At 10 KHz, therelaxation time is 90 which is still 9 times the device actua-tion time.

The temperature could be an issue, if we had a much largerejector array or operated at continuous mode of ejection athigher frequencies for a long time. We initially consideredways such as an electronically controlled cooling pad at theback of the ejector chip or having a bottom layer fluid thatflows close to the device surface that would take any possibleheat away. These would complicate the device design and wasnot necessary for the ejector array we used. If this device wasto become a commercial product, an electronically controlledcooling pad could be placed at the back of the device to keepthe temperature constant in the fluid reservoir. These coolingdevices are available for temperature control. Hence, there aremany simple ways to control the temperature.

These ejectors have several major advantages: the directionof ejection is not dependent on nozzle geometry; the acousticejector is simple, repeatable and performs reliably; and dropletejectors do not harm the heat and pressure sensitive fluids.Moreover, these ejectors can be packed uniformly as arraysand easily addressed individually. Finally, these devices couldeject a broad range of fluids, polymers, cell and protein sus-pensions, which allows them to target various applications,in semiconductor and biotechnology fields, which predict apromising future for acoustically actuated 2-D micromachinedmicrodroplet ejector arrays.

VI. CONCLUSION

The theory of operation, fabrication and experimentalresults obtained with a novel acoustically actuated 2-D mi-cromachined picoliter droplet ejector array is demonstrated.Photoresist, water, isopropanol, ethyl alcohol, and acetoneare ejected from 4 4 2-D micromachined ejector arrays.The ejector operation at 34.7 MHz and generation of 28-diameter droplets in drop-on-demand and continuous modesof operation at 1–10 KHz are demonstrated. Single printedphotoresist droplets and lines drawn onto a silicon wafer aredemonstrated and characterized. A 4- in wafer was fully coatedwith photoresist. Acoustic picoliter droplets have great poten-tial to impact the bioengineering field. Current research focus ison interesting biological applications in tissue engineering fieldfor regenerative medicine, such as cell-by-cell 3-D vascularized

tissue printing and drug testing, cell sorting for cancer and HIV,and improving the device performance by analytical and FEAmethods.

ACKNOWLEDGMENT

The author would like to thank Prof. K. Saraswat, Prof.F. Shadman, Prof. G. Kovacs, Prof. E. Haegstrom, Dr. G. Percin,Prof. R. Reis, and Prof. M. Toner for their invaluable support.

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[4] Freeman, D. Johnson, R. Stromsburg, M. Rinnen, and Klaus, Midyear2003 Semicond. Manufact. Market: Wafer Fab. Equipment. NewYork: Gartner, Aug. 2003.

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[6] U. Demirci, G. Yaralioglu, E. Hæggström, G. Percin, S. Ergun, andB. T. Khuri-Yakub, “Acoustically actuated flextensional Si N andsingle crystal silicon 2D micromachined ejector arrays,” IEEE Trans.Semicond. Manuf., vol. 17, no. 4, pp. 517–524, Nov. 2004.

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Utkan Demirci received the B.S. degree in electrical engineering as a James B.Angell Scholar (Summa Cum Laude) from University of Michigan, Ann Arbor,in 1999 and the M.S. degree in electrical engineering, in 2001, the M.S. degree inmanagement science and engineering, in 2005, and the Ph.D. degree in electricalengineering, in 2005, all from Stanford University.

He is an Assistant Professor at Harvard-MIT Health Sciences and Technologyand Harvard Medical School, Boston, MA. He spent two years at MassachusettsGeneral Hospital, Harvard Medical School, as a Research Fellow. His craftstands on 2 pillars of truth and relief, and follows philanthropy. His researchinterests involve biological applications of microelectromechanical systems(MEMS) and acoustics, especially microfluidics for low cost CD4 counts forHIV in resource-limited-settings for global health, acoustic picoliter dropletsfor cell-by-cell 3D tissue generation and semiconductor applications, andcapacitive micromachined ultrasonic arrays (CMUTS) for medical imagingapplications.

Dr. Demirci is a Member of Phi Kappa Phi National Honor Society. He is oneof the few recipients of the prestigious Full Presidential Fellowship given bythe Turkish Ministry of Education. He is a corecipient of the 2002 OutstandingPaper Award of the IEEE Ultrasonics, Ferroelectrics and Frequency Control So-ciety. He is the winner of Stanford University Entrepreneur’s Challenge Com-petition in 2004 and Global Start-up Competition in Singapore in 2004.

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