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ACOUSTIC INK PRINTING

B. Hadimioglu, S.A. Elrod, D. L. Steinmetz, M. Lim, J .C. Zesch,B. T. Khuri-Yakub*, E. G. Rawson, andC. F. QuateXerox Palo Alto Research Center3333 Coyote Hill RoadPalo Alto, CA. 94304

ABSTRACTWe have used an acoustic beam focused on a freeliquid surface to eject discrete ink droplets ofcontrolled diameter. Nozzles are not emplo ed. Theof asuitable focusing element, is excited with a burst ofacoustic energy. Spherical PZT shells, acousticmicroscope lenses, spherical lenses etched in silicon,and Fresnel acoustic lenses have been used successfullyto eject droplets. Droplet diameter scales directly withthe focal spot size, and hence inversely with theacoustic frequency. Droplet formation has beenexperimentally demonstrated over the frequencyrange of 5 to 300 MHz, with corresponding dropletdiameters from 300 to 5 microns.This droplet ejection process has been successfullyutilized for printin application by using ink as theliquid medium. In %is report we describe Acoustic InkPrinting with a single lens and with an array of lenses.The size of the printed spot can be changed bychanging the droplet size or by placing multipledroplets of ink on the same pixel. Our results showthat Acoustic Ink Printing is a promising technology forhigh resolution, high quality printing.

liquid surface, adjusted to be a t the focal pane

INTRODUCTIONHigh resolution printing requires the ability to marksmall spots on paper with high precision. Modernprinters have capabilities to print a t resolutionsas highas 600spots per inch (SPI) which requires a spot size ofonly 42.5 pm. Several print technologies are in usetoda for electronic publishing. Laser printers arecapagle of producing very high resolution and gooduality images. Color laser printing, however, is costly!e to color registration problems. Thermal transferprinters produce high quality color prints but they areexpensive and slow. Bubble and piezo type ink jetprinters are good candidates for low cost printing butthey require droplet defining nozzles which are proneto clogging.It has been shown that focused, high-intensity soundbeams can be used for ejecting droplets from freeliquid surfaces.1 This process is capable of producingdrops as small as a few microns without the need fornozzles. The drops are stable in size and directionality.Here we describe the use of this process for highresolution printing applications. This new printingconcept appears to be more favorable over other*E. L. Ginzton Laboratory, Stanford University,Stanford, CA.94305

1051-0117/92/0000-09291.000 1992 IEEE

printing technologies because of the capability of goodquality printing a t a relatively high speed. Italso holdspromise for high reliability, long lifetime and highdegree of integration for low-cost manufacturing.ULTRASONIC DROPLET EJ ECTION WITH FOCUSED

SOUND BEAMSWe have previously shown1 that a burst of acousticenergy focused to a diffraction-limited spot a t a liquidsurface can result in droplet ejection from the surface.The geometry of interest is shown in Fig. 1. A

iezoelectric transducer is attached to one end of aEuffer rod. On the other end of the rod, a sphericalcavity filled with a liquid is located and this cavityserves as the lens element. When the transducer isexcited with a tone burst of RF energy, it generatessound waves which propa ate in the buffer rodtowards the liquid surface which is adjusted to be atthe focal plane of the converging beam. Impact of thesound burst will cause a mound of liquid to rise fromthe surface due to the radiation pressure of theacoustic waves. If the energy of the incident soundbeam is high enou h, Rayleigh-Taylor instability2 willdroplet breaks free. The droplet I S expelled away fromthe surface a t a velocit of several meters per second.generated near the mound propagate radiallyoutwards. The ejected droplets have been found to bevery stable in size, velocity and directionality.

towards the lens. The lens 9cuses the sound waves

cause the top of tRe mound to neck down until aAfter ejection, the surY ce relaxes as capillary waves3

0

P U L S ENPUT r fUFig. 1. A single acoustic ejector

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It should be noted here that the geometry used fordrop ejection is very similar to that used in acousticmicroscopy,4 with the exception of the liquidisampleinterface replaced by the liquid/air interface. In factmost of the single ejector experiments in this studywere done using acoustic microscope lenses. Typicallythese lenses are fabricated by grinding and polishing aspherical cavity in the buffer rod. Isotropically etchedspherical lenses in silicon,5 spherical PZT shells andFresnel acoustic lenses6 have also been used forfocusing the sound waves. PZT shells are very efficientfor enerating sound waves but they are mostly limitedto frequencies of 20 MHz and below due to thefabrication difficulties with thinner devices.Isotropically etched spherical lenses in silicon andplasma etched Fresnel lenses can be fabricated usingbatch processing. Therefore, they are suitable forapplications that require the fabrication of an array oflensesa t low cost.The time evolution of dro le t formation is shown inFig.2 for an R F frequencyor5MHz. a pulse width of 20s, and a pulse energy of 58 pJ . Water is used as thedata are referenced to the arrival of the midpoint ofthe acoustic pulse a t the liquid surface. Observationswere made by stroboscopica ly i u m nating the processfrom behind and viewing through a microscope. Eachphotograph representsa superposition of 30 successivedroplets, thus the image sharpness attests to thestability of drop formation process. For lenses with F-number o f 1, the diameter of the droplet is found toscale inversely with the acoustic frequency, f. We findthe process is qualitatively similar over the entirefrequenc range from 5 to 300 MHz with

rquid medium. Times quoted for the experimental

correspond'ng dropdiametersfrom 300 pm to 5 pm.

120 ps0

550 ps

40 ps

280 ps0

~

690 ps

THE PHYSICS OF ULTRASONIC DROPLET EJECTIONThe pressure field for the sound beam propagating inthe liquid can be written as

The first term in Eqn. 1 is the acoustic pressure and thesecond is theDC radiation resistance given by2 1Q =- C

where I is the intensity of the acoustic beam and c isthe velocity of sound. This radiation pressure impartsto the liquid in the focal region an initial momentumMjnjt=RT, where T is the pulse duration. It is thismomentum which acts to overcome the restrainingforce of surface tension and expel the droplet. For the5 MHz experiment described above, the thresholdenergyE th to create a free droplet having zero velocityis measured to be 50 pJ , This implies that the intensityof the acoustic beam is 3.5 kWicm2 assuming that theacoustic power i s concentrated in an areaapproximately one wavelength in diameter (A = 300pm). The radiation pressure a t this intensity level isabout 0.4 atm as compared to the acoustic pressure ofnearly 70 atm. The energy required to break thesurface free is on the order of 05 where a is the surfacetension and 5 is the surface area of the drop.7 Forwater, with surface tension 74x10-3 N/m, we find thesurface energy to be 20 nJ. This result s consistent withthe fact that most of the acoustic energy is reflectedfrom the liquid surface.The droplet ejection process was also analyzed usingnumerical simulation.l.8 Fig. 3 shows the timeevolution of droplet ejection at 5 MHz obtained fromsimulations. The results of the computation compares

1 Omm4 1 A20ps 6 0 ~ sL120ps

38Ops

A240ps

Fig.2. Photos of ejectionat 5 MHz Fig. 3. Simulation of ejectiona t 5 MHz

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favorably with the experimental observations in Fig. 2.A more detailed account of the physics of dropletejection and modeling of this process can be found inRef. 1.ACOUSTIC INK PRINTING

We have used the droplet ejection process describedabove for printing applications. Ink was used as theliquid medium and the recordin medium, typicallyapproximately 1 mm from the ink surface. Uponimpacting the paper, the ink droplets spread out tocover an area approximatel twice the cross sectionalarea of the droplets. Thererore, changing the dropletsize will change the spot size on paper and theresolution of the printer. Table 1 shows the scaling ofthe printer resolution in SPI as a function of the RFfrequency. It can be seen from Table 1 that AcousticInk P rinting is capable of producing very highresolution images.Table 2 makes a comparison of energy required formarking a 100 m diameter spot for various directmarking technokgies. The low and high numbers inTable 2 for Acoustic Ink P rinting correspond totransducer to lens focal point efficiency of 10 and 20des, respectively. It can be seen that Acoustic InkPrinting compares favorably against other markingtechniques. It should also be noted that the energyrequired for ejection1 in Acoustic Ink Printing varies asf-2.3. Therefore, we can expect the energy requirementto be even lower a t higher frequencies for higherresolution pri n ng

silica coated paper, is scanne1 a t a distance of

70300

3 I ',":42 I50 30 420 T y p i c a l l a s er p r i n t e r

30 0 T y p i c a l in k j e t r e s o l u t i o n

20 600 B e st l as e r p r i n t e r5 2500 Photo q u a l i t y

Impact printerThermal transfer

Print technology I R,ef. II10-2 910-3 10

Bubble etAcoustic Ink PrintingPiezo ink jet printer

10-4 - 10-5 1110-5 10-6 110-5 10-7 12

Acoustic Ink Printing does not have a requirement toboil the ink as needed in bubble jet printing. Hence, awider range of admissible ink properties can beexpected for Acoustic Ink Printing. Furthermore, theabsence of drop defining nozzles suggests reliableoperation.SI NGLE EJECTOR ACOUSTIC INK PRINTING

Initial demonstrations of Acoustic Ink Printing wereaccomplished using a single ejector. As shown in Fig.4, the ejector consisted of a quartz buffer rod with aspherical depression a t one end, and a laminatedpiezoelectric transducer a t the other. For theoperating frequencies of 50 and 150 MHz, the systemgenerated 30 pm and 10 prn droplets, respectively,yielding printed resolutions of 420and 1250SPI.The size and stability of the ejected droplets dependcritically upon the height of the liquid surface abovethe acoustic lens. In this experiment, the lens wasimmersed in an open pool of ink. In order tocompensate for evaporation, and for depletion of inkby the printing process, the liquid was controlledusing the feedback mechanism shown in Fihelium-neon laser beam was reflected off t e poolof ink onto a split photodetector. The photodetectoroutput, suitably amplified, was fed back to amotorized syringe that controlled the liquid inkvolume. This degree of control was found to beadequate for printing applications up to 150 MHz,where the usable depth of focus of the acoustic beam i slessthan 10 pm.Droplets were ejected onto a rotating drum, as shownin Fig. 4. The paper was stepped an appropriatedistance along the drum axis after each rotation. Thepaper used in these experiments was a silica-coated inkjet paper, while the ink was a water-based solutionhaving a viscosity of 2.6 centipoise. The drum waspositioned 1-2 mm above the ink surface. Dropletswere ejected with velocities of approximately 2 m/s.

t 4 .

MICROMETER(FLUID LEVEL

. I PIEZOELECTRICTRANSDUCER IM I C R O M E T E R

Fig.4. Single-ejector Acoustic Ink Printer

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Fig. 5 shows printed text generated using 10 microndroplets placed on the paper a t a pitch of 1250 perinch. Fig. 5(a) shows the original size of the image,while 5(b) is a 15X enlargement. As can be seen in Fig.5, droplet placement is very accurate.Accurate droplet placement allows multiple droplets tobe fired a t the same location on the paper. Thispermits modulation of the diameter of the printedspot, hence, achieving more gray levels than binaryprinting. Fig. 6 shows how the spot diameter varies asthe number of droplets fired on the spot i s changedfrom 1 to 9. Achieving the capability of multiple graylevels is very important for printing gray scale images.Fig.7was produced in this manner. Up to 16 dropletswere fired in successionat pixels having a pitch of 420per inch. A halftone cell was comprised of 2x2 pixels togive a total of 65 gray levels per halftone cell a t 210halftone cells per inch. Shown in Fig. 7(a) is the printedimage a t the original size, while 7(b) shows a 2.6Xenlargement.High quality color images have also been printed bychanging the ink between three consecutive passes ofthe paper. The quality of these single-ejector printsamples highlights the stability and precision that canbe obtained with nozzleless acoustic ink jettechnology.

Dr.Calvi-Ink J et -- Paul Tu

I. (a). Text printed with 10 prn droplets at 1250SPIA 15X enlargement of part of the text shown in Fig.4

Fig 6. Spot diameter modulation as the number ofdrops fired on a spot is varied from 1 (top) to 9(bottom)

Fig. 7 (a). Halftone image with 2x2 pixel halftone celland 16 droplets (b). A 2.6X enlargement of the imageshown in Fig.7 (a)

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DESIGN OF AN EXPERIMENTAL PRINT HEAD WITH A

A prototype print head with a large number of ejectorshas been fabricated. The design parameters of theexperimental print head are summarized in Table 3. Itshould be noted that the ejector pitch is 3 4 0 pmwhereas the spot size is 4 2 .5 pm. Therefore, a linearrow of ejectors would not print a solid area; eightstaggered rows of ejectors are needed to be able toprint a continuous region without making multiplepasses.A cross sectional view of an individual ejector is shownin Fig. 8. The ejectors consist of two substrates. Thebottom substrate is a 1.25 mm-thick lass plate withback and front surfaces, respectively. A secondsubstrate is attached to the glass plate to supply inkand maintain the ink height at the focal plane of theacoustic lenses. The key components of the arrayprinter are discussed below.

900-EJECTOR ARRAY

ZnO transducers and Fresnel lenses fa%ricated on the

Resolution I 600SP I 1Pixel size 4 2 .5 pmPrinter widthNumber of ejectors

300 pmI Lens focal length I 300pm ITable3.Design parameters of array printer

-aper

bondedinterfacebondedinterface Fresnel lens

bottomelectrodetop electrode

Fig. 8. Cross section of an ejector in the array printhead

ZnO piezoelectric transducersThe acoustic transducer is one of the key componentsof Acoustic Ink Printing. Efficient generation ofultrasonicsignals is important to minimize the RF signalrequired, thereby reducing the power input to theprint head. Lower R F power requirement also reducesthe cost associated with high power RF signalgeneration. Thin film ZnO transducers have beenwidely used for efficient sound wave generation atfrequencies of 1 0 0 MHz and above.13.14 ZnO offers arelatively high electromechanical coupling betweenelectrical and acoustical fields and is reasonablycompatible with integrated circuit processingtechniques.An operating frequency of 16 5 MHz requires a ZnOfilm thickness of 17 pm to achieve maximum transducerefficiency. For vacuum deposited films with such arelatively large thickness, the stresses developed withinthe film is typically large. This stress can cause thesubstrate to bend significantly and, in the extremecase, to fracture. Therefore, the deposition process hasto be controlled precisely to achieve films with lowstress and with good crystallographic orientation forhigh electromechanical coupling.We have recently reported a process to deposit low-stress ZnO films with a high piezoelectric factor.15These films are grown by reactive QC magnetronsputtering from a zinc target. A 1 5 0 0 A thick layer ofgold with a titanium adhesion layer are evaporated at3OOOC onto the substrates prior to ZnO deposition toserve as back electrodes. The ZnO films are grown at3OOOC at a rate of approximately 2 pm/hour. Thetransducer deposition is completed by depositing a 1pm layerof aluminum to form the second electrode ofthe transducer. The ZnO films t pically have low-stresssubstrates. The films also have electromechanicalcoupling constants very close to that of ideal, single-crystal ZnO.

that results in relatively smalY deformations of the

Fresnel acoustic lens arraysFigure9shows a photo of a binary acoustic Fresnel lensarray as used in the experimental print head. Fresnellenses offer the advantages of planar geometry andrelative ease of fabrication over other forms of lensmaking techniques.6 The lens array is fabricated usingstandard photolithographic techniques. The thicknessof the lens elements, about 5 pm, is chosen tointroduce an phase shift between sound propagatingthrough the solid and liquid materials. The lensesfocal length is 300pm.Liquid level control plateThe acoustic waves used in the print head are sharplyfocused. At the 16 5 MHz operation frequency and forthe lenses with an F-number of 1, the usable depth offocus of the lenses is less than 1 0 pm. Hence, the inklevel has to be kept to within +5 pm of the focal planeof each lens for uniform droplet ejection. To achievethe location of the ink surface a t the focal plane ofeach lens with such precision, a second substrate i sattached to the glass plate. This substrate is a siliconwafer with narrow channels defined by anisotropicchemical etching techniques.16 The etchant removessilicon preferen ially in cprtain crystal ograp hicorientations resulting in the trapezoidal cross section

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array Fig. 10. Photograph of 900ejector print headshown in Fig.8. The bottom opening of each channel i s540 pm wide, larger than the size of the Fresnellenses. The width of the sl it on the top side of thesilicon wafer is approximately 100 pm. When thechannels are filled with ink, the surface tension of theink across this narrow opening holds the liquid levelnear the top surface of the silicon. The thickness of thesilicon wafer, therefore, is chosen to match the focallength of the acoustic lenses. The level of the liquidcan also be adjusted slightly with the application of anexternal back pressure. It should also be noted that theslit opening (100 pm) is much larger than the diameterof the ejected drops (10 pm). Hence, these slits have nosignificant impact on the size and. directionality of thedroplets.Assem b yThe glass/silicon assembl is mounted on a 12.5 cmsquare printed circuit io ard (PCB) to bring theelectrical signals into the print head. A photograph ofa finished print head assembly is shown in Fig. 10. Theink is supplied to the channels through holes a t bothends of the PCB. The paper is mounted on a flatcomputer-controlled translation stage assembly andscanned unidirectionallya t a distance of approximately1 mm from the print head.

PRINTING WITH AN ARRAY OF EJECTORSA typical print and a 2.5X enlargement are shown inFig. 11. The ink and paper are the same as those usedin the Xerox 4020 ink et printer. Gray scale in theima e is achieved by defining halftone cells with 4pixeys each, arranged as a 2x2 array, each of the 4 ixelslevels is 33, and the resolution is 300 halftone cells perinch. Most of the defects in the print are due to slightnon-uniformities in the array and particles on the lenssurfaces. The print clearly shows uniform dropletejection from over 90% of the ejectors in the array.

receiving up to 8 droplets. Thus the number oP gray

CONCLUSIONWe have described a novel printing technology calledAcoustic Ink Printing. Acoustic Ink Printing has manyadvantages including: 1) the absence of a droplet-defining nozzle; 2) the absence of a requirement toboil the ink to achieve ejection (which implies a widerlatitude of admissible ink properties); 3) the precisionwith which uniform small droplets of ink can beejected; and 4) the potential for a high degree ofintegration in the print head structure, including theuse of multiple colors in a single print head. Giventhese advantages, Acoustic Ink P rinting has a potentialfor high quality, high resolution printing.

ACKNOWLEDGEMENTSThe authors wish to thank R . Lujan, W. Meuli, S .Akamine, J . Mikkelsen and J . Ho for their contributionsto this work

REFERENCES1. S. A. Elrod, B. Hadimioglu, B. T. Khuri-Yakub, E. G.Rawson, E. Richley, C. F. Quate, N. N. Mansour, andT. S. Lundgren, Nozzleless droplet formation withfocused acoustic beams, J . Appl. Phys., 65 , 3441(1989).2. J . W. S. Rayleigh, The Theory of Sound, 2nd Ed., Ch.20, Dover Publications, New York (1945).3, W. Eisenmenger, Acustica 9, 327 (1 959).4. C. F. Quate, A. Atalar and H. K. Wickramasinghe,Proc. IEEE 67 , 1092. (1979).5. H. Yamamoto, S. Tanaka and K . Sato, SiliconAcoustic Lens for Scanning Acoustic Microscope(SAM), 1991 International Conference on Solid-State Sensors and Actuators, Digest of TechnicalPapers, 853 (1991).

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Fig. 11. (a) Image printed with a 900 ejector arrayAcoustic Ink Printer print head (b) A 2.5X enlargementof the print shown in Fig. 1 1 (a)

6. K. Yamada, H. Shimuzi and M. Minakata, "Planar-Structure Focusing Lens for Operation a t 200 MHzand I ts Application to the Reflection-Mode AcousticMicroscope," Proc. 1986 IEEE Ultrason. Symp., 745(1986).7. H. Rouse, "Elementary Mechanics of Fluids,"Chapter 10, Dover Publications, New York (1946).8. T. 5 . Lundgren and N. N. Mansour, J . Fluid Mech.

194,479 (1988).9. J. S. Craven, "Dot Matrix Print-Bar Design," Hewlett-Packard J ournal, 36(5), 6 (1985).10.K. S . Pennington and W. Crooks, "Resistive RibbonThermal Transfer Printing," IBM J our. of Res. andDev.29,449 (1985).l l . N . J . Nielsen, "History of Thinkjet Print HeadDevelopment," Hewlett-Packard J ournal, 36(6), 4(1985).12.J . Heinzl and C. H. Hertz, "Ink-J et Printing," inAdvances in Electronics and Electron Physics, P. W.Hawkes and B. Kazan eds., vol. 65, 91, AcademicPress, New York (1985).13.F. 5 . Hickernell, "Zinc Oxide Films fo rAcoustoelectric Device Applications," IEEE Trans.Sonics and Ultrason. SU-32,634 (1985).14.6. T. KhurilYakub, J . G . Smits, and T. Barbee,"Reactive Magnetron Sputtering of ZnO," J . Appl.Phys., 52,4772 (1981).15.J. C. Zesch, B. Hadimioglu, B. T . Khuri-Yakub, M.Lim, R. Lujan, J. Ho, S. Akamine, D. Steinmetz, C. F.Quate and E. G. Rawson, "Deposition of HighlyOriented Low-stress ZnO Films," Proc. 1991 I E E EUltrason. Symp.445 (1991).16.K. E. Peterson, "Silicon as a Mechanical Material,"Proc. of IEEE, 70,420 (1982).

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