springs. The motion of the piston expels liquid through acircular nozzle cut in the top-most layer of polysilicon.
Numerical Simulation of the Drop EjectorPrior to fabrication of the drop ejector, numerical mod-eling of the drop ejection process was conducted. Sev-eral levels of analysis were conducted before buildingthe devices: (i) an initial estimate of field strength re-quirements for drop ejection and for ink dielectric break-down indicated that the ejector concept was feasible;(ii) a 1-D dynamic model was used to investigate themagnitude of the forces involved; and (ii i) anaxisymmetric finite element model was used to providedetailed information for design, to answer specific de-sign questions, e.g., side walls or no side walls, and toverify that the device would work. Numerical simula-tion was used to design the devices before fabrication.Device testing showed excellent correlation with modelpredictions. This process of early detailed modeling ledto a very predictive design.
Operation of the drop ejector was simulated with thefinite-element code, GOMA.2 This analysis software,developed during the past several years at Sandia Na-tional Laboratories, possesses the necessary attributesto properly model drop creation by the ejector, viz., fluid–
IntroductionMEMS-fabricated fluid ejection systems have a wide va-riety of applications including ink jet printing,nanostructure fabrication and micro-dosing of drugs.Microfluidic drop ejectors fabricated using the SUMMiTTM
process accurately control the volume and velocity of liq-uid dispensed at a very high firing rate. For the dropejector to function properly, the fluid must be contami-nation free, compatible with the MEMS components, com-patible with the electrostatic requirements andcompatible with materials used to build the package.Drop ejectors fabricated from the SUMMiTTM-V processconsist of five layers of polysilicon separated by layers ofsacrificial oxide (SiO2). The fabricated polysilicon deviceis “released” by the removal of the sacrificial oxide withhydrofluoric acid (HF). The drop ejector operates usingan electrostatically driven piston supported by polysilicon
Design of a MEMS Ejector for Printing Applications
A. GoorayEastman Kodak Company, Rochester, New York
G. RollerXerox Corporation, Webster, New York
P. Galambos, K. Zavadil, R. Givler, F. PeterSandia National Laboratories, Albuquerque, New Mexico
J. Crowley▲
Electrostatic Applications, Morgan Hill, California
Applications for drop ejectors range from ink jet printing to drug delivery systems. MEMS (Micro-Electro-Mechanical Systems)fabrication techniques, particularly surface micromachining, allow production of small monolithic structures that can be adaptedto many applications. We report on the design, fabrication, and testing of a surface micromachined MEMS liquid ejection systemfor printing applications. The ejectors were fabricated using the SUMMiTTM process,1 a surface micromachining technique. Theonly assembly required is electrical connection and attachment of a fluid reservoir. The process features three, four or five layersof structural polysilicon, separated by layers of sacrificial silicon dioxide. The final step of the fabrication process is the removalof the oxide to release the “machined” structure. The ejector produces small volume (2 – 4 picoliters), satellite-free drops travel-ing at 5 – 10 m/s. To eject a drop a piston is drawn rapidly towards a plate containing a nozzle through which the drop is ejected.The piston is electrostatically actuated. Since the electric field acts across the liquid bath, device operation is sensitive to thedielectric strength, breakdown voltage and conductivity of the fluid.
Journal of Imaging Science and Technology 46: 415–421 (2002)
416 Journal of Imaging Science and Technology® Gooray, et al.
solid interaction at the moving piston boundary, andinvolves implementation of an ALE (ArbitraryLagrangian–Eulerian) technique to move the mesh inthe deforming domain. Field equations (Navier–Stokesin the fluid regions and finite elasticity in the solid re-gions) are solved with the Galerkin finite elementmethod using a full-Newton iterative scheme. Thesetechniques, in conjunction with a semi-automatedremeshing/remapping procedure allow one to follow dropevolution to the “break-off ’ point and in some cases be-yond. The remeshing/remapping schemes are neededwhen the grid becomes sufficiently distorted that accu-racy is lost during numerical integration. For the classof problems simulated in this work, usually four or fiveremeshing steps were needed to advance the simula-tion to the drop “break-off” point.
The essential features of the drop ejector model aredepicted in Color Plate 1 (p. 429). A thin polysiliconpiston (nominally 1 – 2 µm thick and 50 – 75 µm in di-ameter), immersed in a liquid bath, is pulled towardthe nozzle cover plate (2 – 10 µm thick) by a strong,fast-acting electrostatic force which is generated froma voltage difference between the piston and the nozzlecover plate. Hence, the electric field is energized acrossa liquid layer with the ink acting as a dielectric me-dium. In our MEMS-based ejector the piston is sup-ported mechanically and charged electrically via the leafsprings. The result of the piston motion is to force liq-uid through the nozzle (15 – 20 µm diameter), therebycreating a high velocity drop. A portion of the liquid ini-tially found between the piston and the nozzle coverplate is squeezed out radially from beneath the pistonand rejoins the fluid in the reservoir during drop ejec-tion. Typically, the initial gap between the piston andthe nozzle cover plate ranges from 4 – 6 µm dependingupon the particular micromachine design. As the pistonis drawn to the nozzle cover plate, a linear leaf springis deflected giving rise to a restoring force which acts toreturn the piston to its rest position when the activa-tion voltage subsides. Equivalent rate constants, k, forthe spring member ranged between 10 – 100 K dynes/cm.
Operating features of the ejector and constraints onthe ink flow manifest themselves in the numerical modelin the form of boundary conditions. The electrostaticforce divided by its area of application can be definedas an electrostatic pressure, p, and represented by theexpression
p E= − 1
2 02ε κ
r, (1)
where ε0 is a constant = 8.85 × 10–7 dynes/volt2, κ is thedielectric constant for the ink and
rE is the electric field
strength. The traction given by Eq. 1 is applied to theleading surface of the piston as shown in Color Plate 1(p. 429). Note that the electrostatic pressure is appliedonly to a portion of the piston surface (that portion ofthe piston that overlaps the nozzle cover plate); we haveneglected the effects from
rE fringe fields. Electrostatic
activation of the piston typically lasts for several mi-croseconds (µs).
In a similar manner, the restoring force generated bythe leaf spring, fsp, can be written as
fsp = –k(z – z0), (2)
where the deflection z – zo is measured from the initialrest position, zo, of the piston. This force is applied to a
few elements on the backside of the piston. Ink flowsfreely across the two clearly marked open-flow bound-aries and sticks to all no-slip surfaces (piston, nozzlecover plate and spring mount wall). A symmetry condi-tion is applied to the edge of the model along r = 0 whichasserts that no normal components of the velocity (fluid)or displacement (solid) can occur along this edge. Fi-nally, a kinematic condition is applied to the mesh atthe nozzle to ensure that mass conservation is preservedacross this free boundary.
The insert of Color Plate 1 (p. 429) illustrates theresults from a typical drop ejector simulation; this snap-shot of a developing drop was taken 15 µs after electro-static activation. The parameter values correspondingto this “baseline” case are summarized in Table I. Fromthe illustration one can see the form of the emergingdrop; the ejected mass possesses a bulb-like head witha tapered tail. We were not able to simulate the actualdrop separation event, but most likely the drop willbreak off at the base of the tail (location of rapid neck-ing). This approximation was made in order to performthe refill calculations that are discussed later. The to-tal drop volume (head + tail), Vdrop, was calculated to be2.5 pL and the average speed of the drop bulb was 4.5m/s. The peak pressure, Pmax, developed by the ejectorfor this case (stagnation pressure under the piston) was3.1 atm. The piston does not make material contact withthe nozzle cover plate during
rE field activation because
of the relatively large viscous forces that arise whenattempting to bring two parallel surfaces together. Thissqueeze flow resistance works to our advantage in pro-viding electrical isolation during the forward stroke buthinders the return stroke by the opposite mechanism.
The drop for the “baseline” case was produced with apiston stroke of 3.6 µm. This information can be used tocompute an efficiency rating, ζ, for the drop ejector, viz.,
ζ = =
V
Vdrop
disp
0 28. , (3)
where Vdisp is the volume of ink displaced by the piston.Eq. (3) indicates that only 28% of the ink displaced bythe piston is ejected through the nozzle. The remainderof the displaced ink is squeezed radially outward frombeneath the piston and does not contribute to drop for-mation. In an effort to circumvent this loss, it was de-cided to surround the piston with a flow-confining wall(see Color Plate 1 p. 429); the intent of this designmodification was to increase ejector efficiency. While thepresence of such a side wall did improve the ejector effi-ciency, its introduction had deleterious effects on drop
TABLE I. Material Property Data and Model Parameters
property symbol value
piston density ρs 2.3 g/ccink density ρf 1.0 g/ccink viscosity µ 2.5 cPink surface tension γ 30 dynes/cmYoung’s modulus E 0.155 N/µm2
shear modulus G 0.0772 N/µm2
piston diameter d 56 µmnozzle diameter a 20 µmpeak
rE field
rE 25 V/µm
spring rate k 10 Kdynes/cm
rE dwell time td 4.4 µs
Design of a MEMS Ejector for Printing Applications Vol. 46, No. 5, September/October 2002 417
quality; Table II summarizes ejector performance withchanges in side wall position.
The parameter used to differentiate between the casesin Table II was the clearance gap, defined as the distancebetween the piston edge and the confining side wall. Thedata clearly indicate that the presence of a side wall in-troduces additional flow resistance around the piston and,thereby, degrades both piston and drop velocities when aconstant voltage is supplied to the piston (60 V for 2.5µs). Furthermore, the data in Table II indicates that thepiston stroke is diminished as the side-wall gap decreases.Surprisingly the drop volume does not seem to be affectedby the decrease in piston stroke. We surmise that thiseffect is offset by the increase in ejector efficiency pro-duced by the presence of the side wall. It does appearthat the effect of the side wall begins to diminish (dropand piston velocities recover) as the wall is positionedgreater than 5 µm from the piston edge. Because wemaintained an interest in producing high speed dropswe decided not to pursue the implementation of an op-tional side wall for our ejector design.
The performance of the drop ejector can be improvedby increasing piston stroke. Color Plate 2 (p. 429) il-lustrates the improvement in both drop velocity and sizewhen the initial distance between the piston and thenozzle cover plate is increased from 4.5 to 5.5 µm. Be-cause of the inherent limitations with the surfacemicromachining process, it is difficult to build structureswhich offer a piston stroke greater than 5.5 µm. Hence,this value represents an upper bound on current MEMS-based fabrication techniques. Color fringe patterns de-marcate the velocity distribution within the drop—unitsof velocity corresponding to the color legend are cm/s.The maximum velocity within the elongated drop occursat the base of the bulb where the liquid pressure is lowbecause of the concave curvature of the drop surface. Aschematic of the ejector model is shown in the upperleft corner of Color Plate 2 (p. 429).
Liquid viscosity of the ink can affect the quality ofthe ejected drops. Color Plate 3 (p. 430) illustratespredicted drop characteristics for different-viscosity inksranging between 3.0 and 4.0 cP. The data plotted hereindicate that drop velocity will decrease linearly withan increase in ink viscosity. This fact is a manifestationof the relationship between the squeeze force that de-velops in the gap between the piston and the nozzle coverplate and the piston velocity—increased ink viscosity willdiminish piston velocity for fixed
rE . Intuitively, it fol-
lows that if the piston motion is retarded so to will thespeed of the emerging drop; this is the trend plotted inColor Plate 3 (p. 430). On a related note, drop volumeappears to be largely unaffected with increasing ink vis-cosity. The drop still develops to a size of ~ 4 pL forthicker inks; it just takes longer to do so. The error barsthat accompany the data points illustrate the uncer-tainty in estimating drop volume; we do not, yet, simu-late the complete drop pinch-off process.
Another parameter that was demonstrated to have aprofound effect on the performance of the drop ejector
is the stiffness of the restoring springs. Spring stiffnessaffects both quality of ejected drops and refill time. Inthe first instance, stiffer restoring springs will furtherretard piston motion producing drops with lesser depart-ing velocities. During refill the leaf springs act to re-turn the piston to its “ready” position for the next dropcycle; the total time needed to “reprime” the ejector dic-tates sustainable drop-ejection frequency.
Color Plate 4 (p. 430) illustrates emerging dropsfrom an ejector for three cases in which the stiffness ofthe restoring springs varies—10K, 25K and 50K dynes/cm, respectively from top to bottom. Increasing springstiffness tends to reduce departing drop velocities, asshown. Units for the velocity legends are in cm/s. Alsoapparent from Color Plate 4 (p. 430) is the associa-tion of a shortening of the drop tail with a reduction indrop velocity In each case, the piston is driven with aconstant
rE of 25 V/µm for a duration of 4.4 µs.
Color Plate 5 (p. 430) illustrates recovering menis-cus shapes for springs of differing strength. The initialstart for each simulation was a stress-free meniscuswhose shape corresponds to that just after drop “break-off”. Moreover, we have assumed for these simulationsthat the meniscus remains attached to the leading edgeof the nozzle cover plate at all times. This approxima-tion was made for expediency; it is simply not known toany level of detail how the meniscus behaves at the con-tact point. We can summarize the results presented inColor Plate 5 (p. 430) with the following comments:(i) In general, stiffer springs lead to shorter recoverytimes for the menisci. The small numerals within eachsubplot indicate elapsed time after “break-up” (in mi-croseconds). (ii) Drop size is mostly unaffected by springstiffness provided the electrostatic pressure dominatesthe spring force. (iii) It is probably not efficient to waituntil the meniscus fully recovers before initiating thenext ejector cycle. Much time is consumed in waitingfor the meniscus to recover to a completely flat profile.In fact, both forces (spring force and surface tensionforces generated by the meniscus curvature) that gov-ern the final moments of meniscus recovery become ex-ceedingly small. Thus, to achieve high drop productionrates the next drop cycle should start before the menis-cus is fully recovered. Judging from the recovery timesillustrated in Color Plate 5 (p. 430) the model pre-dicts that the drop ejector could be cycled at the follow-ing operating frequencies: 10 kHz (k = 10K dynes/cm)or 20 kHz (k = 50K dynes/cm).
In the case when the spring stiffness equals 50k dynes/cm, it is clear from Color Plate 5 (p. 430) that themeniscus becomes severely distorted, especially in theregion under the lip of the nozzle cover plate (see sub-plot corresponding to 15 µs in top row of Color Plate 5(p. 430)). This evidence suggests that, in general, themeniscus may not remain fixed at the orifice lip or, moreproblematic, an air bubble might be ingested by theejector during refill. In either instance further study iswarranted.
A two-dimensional planar simulation of ejector refillwas performed using springs of unequal stiffness on op-posite piston edges. The motivation for doing so was todisrupt the symmetric nature of the suction flow as thepiston is pulled away from the cover plate during refill.With springs on one side of the piston pulling harderthan those on the opposite side, the piston tilts slightlywhile returning to its rest position. The model predictedthat this design modification did, indeed, reduce refilltimes; unfortunately, this effect is limited by the amountof tilt that can be achieved given the small aspect ratio
TABLE II. Ejector Performance with a Confining Side Wall
gap Pmax drop velocity piston velocity stroke(µm) (atm) (m/s) (m/s) (µm)
418 Journal of Imaging Science and Technology® Gooray, et al.
of the ejector. Despite the limited effort that we devotedto this concept it seems like an idea worth pursuing.
A final demonstration of the simulation softwareinvestigated the fluid/elastic dynamic response of thedrop eject when a filter is placed upstream from the pis-ton. In theory a filter element placed at this locationwould prevent particulate contamination generated in theink reservoir from migrating to the nozzle region of theejector, where, such suspended particles might eventu-ally foul the ejector. The filter itself was intended to bemanufactured as part of the surface micromachine usingone of the layers of polysilicon in the SUMMiTTM-V pro-cess. Thus, by limitation, the filter element will be thin.This, in turn, dictates that it will be flexible as it mustspan the entire opening leading to the ink reservoir. Thebottom half of Color Plate 6 (p. 431) illustrates thelocation of the filter within the ejector; it measures 1µm thick and resembles a cross-wire screen in the span-wise direction. The purpose of this simulation was topredict both the elastic deformation of the filter and theflow of ink through it.
The filter was modeled as a poro-elastic medium; thematerial properties of the membrane were taken to bethose of polysilicon (see Table I). The model was com-pleted by choosing values for the porosity (φ = 0.25) andpermeability (λ = 1 × 10–9 cm2) of the filter. The resultsfrom a comparative study of ejector performance are il-lustrated in Color Plate 6 (p. 431). Drop developmentat 4 µs after piston activation is shown for two cases:drop ejector simulations with (bottom) and without (top)a filter element. One can conclude from this compari-son that a filter element (with the material propertiespreviously stated) does not alter the quality (size andspeed) of the ejected drops. In addition, one should no-tice the slight bending of the filter in response to theflow-induced piston motion. Both simulations used aconstant
rE of 35 V/µm to drive the piston; other rel-
evant problem data, that depart from that listed in TableI, is given in the color plate. Despite subtle differences(nozzle-end geometry, piston thickness, piston end-gap,spring stiffness) between the two cases featured inColor Plate 6 (p. 431), none of these is believed to af-fect our assessment of the filter on ejected drop quality.However, if the permeability of the filter is reduced bya factor of ten (λ = 1 × 0–10 cm2), flow resistance throughthe filter is increased enough to have visible effects ondrop quality. In this reduced permeability scenario, inkcannot pass through the filter rapidly enough to supplythe region on the back side of the piston—essentiallystarving this region. Also, a lower permeability leads toa higher pressure drop across the filter and, conse-quently, larger filter deformations. The deformations canbecome so large, in fact, that the filter is actually pulledinto contact with the piston during its forward stroke—a situation that would probably fracture the filter. Theanalysis has helped us to conclude that it is possible toimplement a filter in the design of the drop ejector, solong as, it is endowed with a narrow set of acceptableflow characteristics.
Electric Design of the Drop EjectorThe crucial requirement for a MEMS-based ink jet
printer is the ability to eject a droplet using the pres-sure generated by the electric field in the device. Whilethe exact magnitude of the electrostatic pressure de-pends on details of construction, the magnitude of thepressure at any location in a fluid is given quite simplyby Eq. 1.
This pressure is opposed by surface tension, the in-ertia of the fluid and piston, and the viscous drag in-duced by the motion of the ink in the confined region ofthe piston and orifice. An estimate of the electric fieldstrength needed to overcome the various mechanicalforces for a typical ink drop ejector is shown in Fig. 1.The figure shows the field requirement as a function ofdrop size for each opposing force, and for the sum of allthe forces. The total field requirement reaches a mini-mum for a drop diameter in the range from 10 to 20 µm;this corresponds to a volume of a few pL. The lowestfield needed for this example is approximately 10 V/µm,and of course it may become higher under other circum-stances. Fields this large pose a number of difficultiesin the design of the ejector and its driving circuits. Themost important ones are electrical breakdown, electroly-sis, heating of the ink and driving voltage waveformshaping.
The most fundamental problem is electric breakdownof the ink. The literature value for the breakdown ofwater is approximately 15 V/µm, only slightly above therequirement of Fig. 1. Thus, it is required that the inkwithstand the required high field necessary for drop ejec-tion. An empirically determined, practical field limit forH2O breakdown in macroscopic gaps (> 1 mm) for shortpulses (< 1 µs) is nominally 30 V/µm.3–5 Srebrov6 dem-onstrated a higher breakdown field for H2O that in-creased from 40 to 80 V/µm for a decreasing gap distancefrom 8 to 1 µm using 5 µs pulses. Our computationalsimulation and prototype device testing show the re-quired field for drop ejection is 20–30 V/µm for an aque-ous fluid, so dielectric breakdown is not expected in ourejectors. We confirm this expected absence of breakdownby the data shown in Fig. 2 for a typical candidate aque-ous ink. These measurements were made using MEMSmodules that had sufficiently short springs which pro-duced a rigidly held piston. Two different gap dimen-sions where produced by fabricating the piston in eitherthe first or the second polysilicon layer. We found thatfields of 70 and 170 V/µm were stable for 2 µs for 4.5and 2 µm gaps, respectively. Deionized water producedsimilar results with the exception of measurable attenu-ation of the pulse generator output due to increasedcurrent draw. This greater apparent stability at highfield is most likely the result of limited charge trans-port through the native oxide on the polysilicon surfaces.
Electrochemistry at the piston and nozzle plate sur-faces results in power dissipation and decreases the
Figure 1. Electric field (volts/micron) needed to overcomemechanical forces in droplet ejection.
Ele
ctric
fiel
d (
volts
/mic
ron
)
Drop volume (pL)
Design of a MEMS Ejector for Printing Applications Vol. 46, No. 5, September/October 2002 419
device efficiency. Gas bubble formation, due to electroly-sis, can produce cavitation during bubble nucleation thatcan be destructive to the piston. In addition, bubbles inthe ink create a more compressible mixture further de-creasing device efficiency and impeding fluid transport.Measurable current does flow when a high field is ap-plied across the ejector. We measure current densitieson the order of 102 A/cm2 for deionized H2O at 100 V.Visible O2 and/or H2 bubbles start to appear at 300 A/cm2. This current is a quasi-steady state faradaic cur-rent that is due to electrolysis of H2O as a result of largeinterfacial potential drops. A field of 22 V/µm requiresa potential of 100 V for a 4.5 µm initial inter-electrodegap distance. Even at the intrinsic conductivity of H2O,a significant fraction of this potential will appear acrossthe Si/SiO22/H2O interface allowing for the reductionand oxidation of H2O at the opposing electrode surfaces.The use of a bi-polar pulse train can aid in minimizingthe electrolysis rate. The bi-polar pulse allows for a frac-tion of the gas generated at one potential extreme to bereduced or oxidized at the other potential extreme. Theefficiency for this conversion will depend on relativereaction kinetics and diffusion rates. Tailoring the inkcomposition can reduce electrolysis rates. The additionof 10 weight percent of ethylene glycol or diethylene gly-col, desirable ink additives, decreases this current den-sity by a factor of 30, sufficiently lowering it below anapparent bubble nucleation threshold. It is likely thatthese diols form surface complexes that limit electrontransport to and from H2O.
We find evidence of oxide growth in our ejectors asmanifest in a significant decrease in device current withrepetitive pulsing at 100 V using deionized water.7 Thiseffect is less pronounced with diol-H2O mixtures andinks. Decreasing the duration of an individual pulse,e.g., to 200 ns, is expected to reduce the effects of oxidegrowth because of the limited mobility of the OH anionin the oxide.
Another design challenge with rE fields this high is
Joule heating of the liquid during the piston activation.The electric power density, ψ, in the liquid is given by
ψ =
rER
2
, (4)
where R is the electrical resistivity of the ink. Withwater-based inks, the resistivity can be quite low, lead-
ing to rapid heating of the ink during ejector operation.The time needed to raise the ink by a given tempera-ture can be estimated from the thermal balance equa-tions, and is shown in Fig. 3. As expected, less time isneeded to heat the ink when the field is high, or whenthe ink resistivity is low. In practice, this side effectrequired a different ink formulation from that normallyused in ink jet printers.
The piston moves during the ejection pulse so thatthe gap decreases from its initial value of 5 µm to a muchsmaller value between 1–2 µm. If the electrical input isset up to provide an
rE field close to the breakdown
strength at the beginning of the ejection cycle, it willsupply a much stronger field when the piston hasreached full stroke unless the voltage is reduced. Re-duction of the voltage during piston travel can beachieved by a number of methods, including driving thepiston with a current source or by causing the voltagetrain to decay during the pulse cycle. The latter methodwas used in the device described herein, with the volt-age decay dictated by modeling results for piston mo-tion during an activated cycle.
MEMS Ejector Design and FabricationThe ejector array was designed for and fabricated viathe SUMMiTTM process.1 This fabrication technique isa batch MEMS process that can be adapted to inex-pensive mass production methods, not unlike those ofIntegrated Circuit (IC) fabrication. It is currently be-ing used as a prototyping process. Up to eight differ-ent prototype design modules are incorporated on asingle silicon wafer. An individual lot of wafers (typi-cally 6–10 wafers per lot) is fabricated with standardIC tools adapted for MEMS processes. The prototypedesigns can then be tested. The SUMMiTTM process isa surface micromachining process that util izespolysilicon as the structural material and silicon diox-ide as the sacrificial material. The SUMMiTTM-V pro-cess has five layers of polysilicon (see Color Plate 7(p. 431)). Structural and sacrificial layers alternate ina stack of thin films on top of a silicon wafer. Cut-outregions in the sacrificial oxide layers allow connectionbetween adjacent polysilicon layers for anchoring struc-tures or making electrical connections. The final stepin the fabrication process is a releasing etch. An HF/HCl acid bath removes the oxide layers, leaving thepolysilicon structure intact. The bottom layer is a ther-mally grown oxide covered by silicon nitride, and pro-vides electrical isolation between a MEMS device andthe silicon substrate.
Figure 2. Application of a stable rE field across aqueous ink-
filled ejectors possessing either a 2 or 4.5 µm gap.
Figure 3. Time required to raise the temperature of the inkby 10°C.
Hea
ting
time
(µs
ec)
Conductivity (s/m)
420 Journal of Imaging Science and Technology® Gooray, et al.
The ejectors were designed with AutoCAD (ACAD).8
A top view of an ACAD drawing for an array of ejectorsis shown in Color Plate 8 (p. 432). Each color corre-sponds to a different layer in the SUMMiTTM-V process.One or more drawing layers are needed to create a mask;these masks are used to photolithographically patternphotoresist on top of a SUMMiTTM-V layer. As such, themasked surfaces define the shapes of the polysiliconlayers that comprise the MEMS topology. Sandia Na-tional Laboratories has incorporated a solid model andcross-sectional viewer with ACAD, version 2000. Thisvisualization software is used to simulate the fabrica-tion process and validate the MEMS design. Processinformation pertaining to thin film thickness and etch-ing characteristics is also incorporated into the software.The visualization tools allow one to accurately predictwhat the MEMS structure will look like before fabrica-tion. Previewing the design for possible errors is espe-cially important since the batch fabrication processrequires about 4 months to complete. An example of theACAD software with viewing capability is illustratedin Fig. 4; a single ejector piston is shown in a 3D cut-away view. The piston is approximately (50 × 50) µm2
and is supported on opposite sides by thin membersprings. The overall stroke of the piston is approximately5 µm.
A scanning electron photomicrograph (SEM) of the as-fabricated ejector array is illustrated in Fig. 5. The toppolysilicon layers containing the ejection nozzles havebeen cut away in this view to reveal the piston struc-ture beneath. The “waffle-iron” pattern on the surfaceof the pistons is due to a “dimple” cut that was used tostiffen the piston. A dimple cut is performed with a par-tial etch of an oxide layer, instead of entirely throughit, as would be the case for a corresponding anchor cut.The succeeding layer of polysilicon is deposited on theunderlying oxide that remains on the surface of the pis-
ton. The conformal deposition process accounts for theso-called “waffle-iron” pattern. The grooves in the pis-ton coincide with the dimple cut stiffener locations. Thegrooves do not show up in the visualization of Fig. 4because an idealized vertical-wall-deposition process isassumed in the viewing software. Otherwise, the ACADvisualization software accurately reflects the as-builtejectors (Fig. 5). As evidence, the nitride cut under thepiston that causes the piston to sit in a conformal re-cess is shown in both figures.
The nitride cut is used to remove the nitride and ox-ide layers from the area directly beneath each piston. ABosch etch process9 is used to cut a vertical-sided chan-nel through the wafer underneath each piston. TheBosch etch is highly selective to silicon and stops at thefirst layer of oxide it encounters. In this case, the ni-tride cut ensures that the Bosch etch stops at the sacri-ficial oxide layer underneath each piston. A two-stepBosch process was used to join each ejector to a com-mon ink reservoir; separate etching steps were neededbecause the aspect ratio of the thru-hole was too largeto complete effectively with one step. After the Boschetch was completed the wafers were diced and packaged.The prototype ejector arrays were then ready for pack-aging and testing. A typical packaging design is shownin Color Plate 9 (p. 432).
Experimental ResultsSpecific procedures for testing the ejector arrays havebeen described in a previous publication.10 The voltagesignal sent to the ejector is a bi-polar constant field sig-nal. Peak voltage is adjustable, so as to accommodatedifferent inks and their properties, and can be as largeas 100 V. Among the quantities measured were indi-vidual drop velocity and volume, drop trajectories andfiring frequency. Inks used as test liquids were typicalaqueous-based inks common to thermal ink jet applica-tions. With an electrostatically activated ejector, it isimportant for the ink to have a large dielectric coeffi-cient and small electrical conductivity. Hence, we choseinks which exhibited κ = 70 and σ = 0.001 S/m. Table III
Figure 4. 3D visualization of a single ejector. One half of thenozzle cover plate (poly3) is cut away to reveal the piston be-neath. The piston (gray – poly1) sits in a conformal recess cre-ated by the nitride etch and is supprted by thin-member springson opposite sides. One of the springs is shown and continuesbeyond the left edge of the figure. Support posts (poly2) stiffenthe nozzle cover plate.
Figure 5. Scanning electron micrograph of the as-built ejec-tor array. Part of the nozzle cover plate has been cut away toreveal the pistons beneath. Some of the pistons are also re-moved for viewing. The Bosch through holes were not etchedin this lot. The recessed well created by the nitride cut is clearlyvisible as is the “waffle-iron” pattern on each piston.
Design of a MEMS Ejector for Printing Applications Vol. 46, No. 5, September/October 2002 421
summarizes the results of the experimental character-ization for our MEMS-based drop ejectors. An image oftypical ejected drops is shown in Fig. 6; drop formationwith elapsed time for the conditions given in Table IIIis illustrated in Fig. 7.
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3. R. R. Allen, J. D. Meyer and W. R. Knight, Thermodynamics and Hy-drodynamics of Thermal Ink Jets, Hewlett Packard Journal, 36, 21–27 (1985).
4. B. De Heij, G. van der Schoot, B. Hu, J. Hess, and N. F. de Rooij,Characterization of Fluid Droplet Generator for Inhalation DrugTherapy, Sensors and Actuators A, 85, 430–434 (2000).
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6. B. A. Srebrov, L. P. Dishkova and F. I. Kuzmanova, Electrical Break-down of a Small Gap Filled with Distilled Water, Sov. Tech. Phys.Letters. 16(l), 70–71 (1990).
7. K. Ghowsi and R. J. Gale, Theoretical Model of the Anodic OxidationGrowth Kinetics of Si at Constant Voltage, J. Electrochem. Soc. 136(3),867–871 (1989).
8. C. Jorgenson and V. Yarberry, A 3D Geometry Modeler for theSUMMiTTM-V MEMS Designer, Proc. Modeling and SimulationMicrosystems, 594–597 (2001).
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10. P. Galambos, K. Zavadil, R. Givler, F. Peter, A. Gooray, G. Roller, andJ. Crowley, Surface Micromachine Electrostatic Drop Ejector, 11thInternational Conference on Solid- State Sensors and Actuators,Springer-Verlag, Berlin, 2001, pp 906–909.
Figure 6. Image of ejected drops.
Figure 7. Drop formation with elapsed time.
TABLE III. Measured Performance of the Electrostatic DropEjector
req. energy per drop κ = 68; σ = 0.001 S/m < 0.003 µJ/pl
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Color Plate 1. Axisymmetric, finite element model of the drop ejector. In this case both the piston (solid) and surrounding inkbath (fluid) are discretized. (Gooray, et al. pp. 415–421)
Color Plate 2. Comparison of drop size and velocity for the MEMS ink ejector (lower figure) with a micron of additional stroke.In both cases, the piston is driven with a constant field of 25 V/µm for a duration of 4.4 µs. Drops are shown at incipientseparation. (Gooray, et al. pp. 415–421)
430 Journal of Imaging Science and Technology® Color Plates
Color Plate 3. Predicted characteristics of the ejected drop for different viscosity inks: 3.0 cP, 3.5 cP and 4.0 cP. (Gooray, et al.pp. 415–421)
Color Plate 4. Emerging drops from the L50D20K(*) MEMS ink ejector device for three cases in which the stiffness of therestoring springs varies. (Gooray, et al. pp. 415–421)
numerals in each plot of the refill sequences indicate time in microseconds
Color Plate 5. Snapshot sequences for recovering menisci after drop “break-off”. From top to bottom sequence spring rates are50K, 25K and 10K dynes/cm, respectively. (Gooray, et al. pp. 415–421)
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Color Plate 6. Simulating the effect of a particulate filter on drop ejector performance. (Gooray, et al. pp. 415–421)
Color Plate 7. SUMMiTTM layer stack. Five layers of polysilicon alternate with four layers of sacrificial dioxide. The bottomlayer of polysilicon is used for electrical connections and the top four polylayers are devoted to building MEMS structures. Thefirst layer of thermal oxide covered with silicon nitride provides electrical isolation from the silicon substrate. (Gooray, et al. pp.415–421)
432 Journal of Imaging Science and Technology® Color Plates
Color Plate 9. Print head packaging design. (Gooray, et al. pp. 415–421)
Color Plate 8. AutoCAD design of the ejector array. Ejectors (13 in each array) are individually addressable from 13 differentbond pads. Ejector pistons are supported on opposite sides by thin member springs. (Gooray, et al. pp. 415–421)
![Electrostatic Discharge/Electrical Overstress Susceptibility in .../67531/metadc721298/...MEMS devices such as optical mirrors [6]or printheads [7] are structurally-isolated structures](https://static.documents.pub/doc/80x56/612f12171ecc5158694335c8/electrostatic-dischargeelectrical-overstress-susceptibility-in-67531metadc721298.jpg)
