The MEMS Handbook MEMS Applications (2nd Ed) - M. Gad el Hak Episode 2 Part 3 ppt

FIGURE 10.19 Droplet ejection without satellite droplets, courtesy Tseng 1998a.Nozzle Liquid puddle Droplet FIGURE 10.20 Liquid puddle formation outside the micronozzles, courtesy Tseng

FIGURE 10.18 A printed vertical line smeared by satellite droplets, courtesy Tseng (1998c).

One way to eliminate puddle formation is to coat the chamber’s outer surface with a nonwetting material (Theinner surface of the chamber needs to remain hydrophilic for liquid refill.) However, even with this coating,there is still no guarantee that puddle will not form More research is underway to fully understand themechanism in the puddle formation process

FIGURE 10.19 Droplet ejection without satellite droplets, courtesy Tseng (1998a).

Nozzle

Liquid puddle Droplet

FIGURE 10.20 Liquid puddle formation outside the micronozzles, courtesy Tseng (1998d).

In selecting chamber materials, metals such as nickel and stainless steel have been widely employed fordifferent microdroplet generators due to their ease of fabrication (such as by wet etching or electroplating),high mechanical strength, resistance to erosion by certain bases or acids, imperviousness to solvents, andhigh durability in cycling operations, etc However, owing to cost and fabrication-precision considerations,the chamber materials of many commercial ink-jets have been changed to polymers, such as polyimide orPMMA in recent years As a result, chamber formation by bonding a polymer thin plate on the actuatorsubstrate and nozzle-array formation by laser drilling on those polymer thin plates are now very common

in the ink-jet industry Nevertheless, the bonding and laser drilling processes may encounter issues such asbonding nonuniformity, time consuming serial laser drilling processes, and alignment limitations Therefore,various polymer-MEMS processes for fabricating multiple embedded chambers integrated with nozzleshave been developed; these not only simplify the fabrication process but also improve the accuracy of align-ment and nozzle fabrication on the chamber structures Employing double exposures (one full power, andthe other partial power) on a single SU-8 resist layer with antireflection coating in the resist–substrate inter-face, Chuang, et al (2003) demonstrated that embedded microchannel structures can be fabricated easily,

as shown in Figure 10.21 The thickness of chamber roof can be controlled from 14 to 60 µm in a 2 µmresolution

Durability, stress, and erosion issues are the major problems concerning operation Due to the cyclingnature of the droplet generation process, the materials chosen for actuation face challenges not only fromstress but also from fatigue The HP Corporation reported that possible reasons for failure of the heater pas-sivation material are cavitation and thermal stress [Bhaskar et al., 1985] Silicon, low-stress silicon nitride, sil-icon carbide, silicon dioxide, and some metals, are usually used to overcome these problems In addition

to selecting proper materials, reducing sharp corners in the design is an important key to eliminating stressconcentration points and thus preventing material from cracking The working fluid’s erosion of structuralmaterials is another serious issue Lee et al (1999) reported the erosion of the spacer material in a com-mercial ink-jet head while using diesel fuel as working fluid In contrast, materials including silicon and siliconnitride used by Tseng et al (1998c, 1998d) and Lee et al (1999) in the microinjector are free of this problemand can also be used with a wide variety of fluids including solvents and chemicals Selecting materialswisely, ordering them correctly in the process, and properly designing the materials in the microstructuresare the three primary measures for reducing material issues

The structures in microdroplet generators commonly include a manifold for storing liquid, microchannelsfor transporting liquid, microchambers for holding liquid, nozzles for defining droplet size and direction, andactuation mechanisms for generating droplets Occasionally, droplet generators may not have nozzles but gen-erate droplets locally by energy focusing means, such as acoustic wave droplet generators [Zhu et al., 1996].Before micromachining processes became widely used, most processes for fabricating microdroplet generators

followed the same general method: nozzle plates, fluid handling plates, and actuation plates are manufacturedseparately and then integrated into a single device However, as the nozzle resolution becomes finer, bond-ing processes pose severe alignment, yield, and material problems as well as IC compatibility issues Onthe other hand, the interconnection lines may not have enough space to fan out from each chamber whennozzle resolution is higher than 600 dpi As a result, monolithic methods for fabricating high resolution ICintegrated droplet generators have become very important The following sections introduce examples ofdifferent fabrication techniques.

10.4.1 Multiple Pieces

Figure 10.22 schematically shows the traditional method of fabricating microdroplet generators by bondingseparately fabricated pieces [Tseng, 1998d] In this process, actuation plates are fabricated separately fromthe nozzle plates In the thermal bubble jet, heaters are usually sputtered or evaporated and then patternedwith an IC circuit on the bottom plate; piezo, thermal buckling, electrostatic, and inertial actuators consist

of more complex structures, such as piezo disks, thin plate structures, or cantilever beams Nozzles are cated by electroforming [Ta et al., 1988], molding, or laser drilling [Keefe et al., 1997] These separatelyprocessed pieces are assembled either by using intermediate layers of polymer spacer material [Siewell et al.,1985; Askeland et al., 1988; Hirata et al., 1996; Keefe et al., 1997] or directly adhering several piecesthrough anodic bonding [Kamisuki et al., 1998, 2000], fusion bonding [Gruhler et al., 1999], eutectic bond-ing, or low temperature chemical bonding However, most of the bonding methods are chip-level rather thanwafer-level processes and face challenges of alignment, bonding quality, and material–process compati-bility As the nozzle resolution becomes higher than 600 dpi, alignment accuracy approaching 4 µm (10% ofthe nozzle pitch) becomes difficult to attain Higher alignment accuracy significantly increases the fabricationcost, especially for the chip-level process Bonding quality is another important issue affecting the fabricationyield of large array and high-resolution devices Additionally, the bonding materials (mostly polymers)

chosen must be suitable for the application environments and working fluids Finally, bonding processesinvolving heat, pressure, high voltage, or chemical situations restrict IC integration with the droplet gen-erators, and the IC integration is essential for large-array and high resolution applications.

10.4.2 Monolithic Fabrication

To address the problems inherent in using multiple pieces, monolithic processes utilizing ing technology have been widely employed since the early 1990s Two primary monolithic methods havebeen introduced: one combines bulk and surface micromachining and the other uses bulk microma-chining and the deep UV lithography associated with electroforming (or UV lithography only)

micromachin-For example, Tseng et al (1998d) combined surface and bulk micromachining to fabricate a droplet generator array with potential nozzle resolution up to 1200 dpi (printing resolution can be 2400dpi or higher) This design used double bulk micromachining processes to fabricate the fluid handlingsystem, including the manifold, microchannels, and microchambers Surface micromachining, on theother hand, was used for fabricating heaters, interconnection lines, and nozzles The whole process wasfinished on (100) crystal orientation silicon wafers.Figures 10.23 and 10.24 show the three-dimensionalstructure of the microinjectors and the monolithic fabrication process respectively The ejection of 0.9 pldroplets has also been demonstrated by Tseng et al (2001b) using the high-resolution microinjectors Thestructural materials used in the microinjector are silicon, silicon nitride, and silicon oxide, which aredurable in high temperature and suitable for various liquids (even some harsh chemicals) Using thisdevice, ICs can be easily integrated on the same silicon substrate

micro-a1: Metal mold fabrication

a2: Sacrificial film pattern

a3: Nozzle plate plating

a4: Nozzle plate de-mold

Nozzle plate and actuator plate bonding

The second primary method can be found in Lee’s (1999) work This process used multiexposure and nal development (MESD) lithography to define microchannel and microchamber structures (photoresist

sig-as sacrificial layer) and constructed the physical structures with electroformed metal The manifold wsig-asmanufactured from the wafer’s backside by electrochemical methods [Lee et al., 1995] This device alsodemonstrated a capacity for very high-resolution arrays and compatibility with the IC process Anothermethod, using photoresist as sacrificial layer and polyimide as structure layer, was introduced by Chen et al.(1998) for high resolution and IC compatible applications

Droplet trajectory, volume, ejection direction, and ejection sequence/velocity are four important tative measures of the ejection quality of microdroplet generators The following sections briefly introducethe basic methods for testing droplet generation

quanti-10.5.1 Droplet Trajectory

Droplet trajectory can be visualized by directing a flashing light on the ejection stream, as shown in

Figure 10.25 [Tseng et al., 1998a] The white dots in Figure 10.26 show the visualized droplet stream Thevisualized droplet trajectory follows an exponential curve that is very different from the parabolic curveexpected for normal sized objects with a similar initial horizontal velocity Tseng et al (1998a) also esti-mated droplet trajectory by solving a set of ordinary differential equations from the balance of horizon-tal and vertical forces on a single droplet flying through air

From this analysis, the vertical position Y and horizontal position X of the droplet can be expressed by

the following equations:

where g is the acceleration due to gravity, t is the time, m is the mass and r0is the radius of the droplet,

µis the viscosity of air, U v ∞⫽ᎏ

veloc-Xmax⫽ ⫽ (U H0 r2

2ρliquidᎏ9µair

U H0 m

ᎏ

6πµr0

Liquid entrance

Manifold

Narrow heater

Wide heater Nozzle

Chamber

Common line Electrode

Liquid

FIGURE 10.23 Schematic three-dimensional structure view of microinjectors, after Tseng (1998c).

when t⬃ ∞ Here the maximum distance is proportional to the droplet velocity and droplet radius

to the second power For different droplet sizes with the same initial velocity, the maximum flyingdistance of smaller droplets decreases very fast To obtain 1 mm flying distance, the droplet with

10 m/s initial velocity needs a minimum radius of 2.7 µm From the above estimation, droplet sizeshould be maintained above a certain value to ensure enough flying distance for printing Printingwith very fine droplets (diameter smaller than a couple of micrometers) requires either increasingthe droplets’ initial velocity or printing in a special vacuum environment to overcome air drag

(d)

(e)

Nozzle formation & pad open

KOH etch manifold, PSG remove

(b)

Heater & interconnection formation

KOH etch enlarge chamber depth

FIGURE 10.24 Fabrication process flow of monolithic microinjectors, after Tseng (1998c).

CCD camera

Flashlight Microinjector

Main droplets

Calculated curve

Calculated curve

cm

7 3

10.5.3 Ejection Sequence/Velocity and Droplet Volume

To characterize the detailed droplet ejection sequence, a visualization system [P.-H Chen et al., 1997b, Tseng

et al., 1998c] as shown in Figure 10.27 has been widely used In this system, an LED was placed under the

droplet generator to back-illuminate the droplet stream Two signals synchronized with adjustable timedelay were sent to a microinjector and an LED respectively Droplets were ejected continuously from adroplet generator, and the droplet images were frozen by the LED’s flashing light at specified time delays,

as shown in Figure 10.17 Droplet volume can be determined from the images by assuming the droplet isaxi-symmetric, or from weighing certain numbers of droplets Droplet velocity can be estimated by meas-uring the difference in flying distance of the droplet fronts in two succeeding images

10.5.4 Flow Field Visualization

Flow field visualization is one of the most direct and effective tools to better understand flow propertiessuch as cross talk, actuation sequence, liquid refill, and droplet formation inside microdroplet generators.Flow visualization in small scale presents some difficulties that do not occur in large scale, such as lim-ited viewing angles, impossible to generate light sheet, reflection from the particles trapped on the wall,short response time, and small spatial scale Meinhart et al (2000) adopted a micrometer resolution par-ticle image velocimetry system to measure instantaneous velocity fields in an electrostatically actuatedink-jet head The system introduced 700 nm diameter fluorescent particles for flow tracing; the spatial aswell as temporal resolutions of the image velocimetry are 5–10 µm and 2–5 µs respectively The four pri-mary phases of ink-jet operation — infusion, inversion, ejection, and relaxation — were clearly capturedand quantitatively analyzed

More than a hundred applications for microdroplet generators have been explored This section marizes some of them

sum-CCD Camera

10.6.1 Ink-jet Printing

Ink-jet printing, which involves arranging small droplets on a printing medium to form texts, figures, orimages, is the most well-known microdroplet application The smaller and cleaner the droplets are, thesharper the printing is However, smaller droplets cover a smaller printing area and thus require moreprinting time Therefore in printing, high-speed microdroplet generation with stable and clean micro sizeddroplets is desired for fast and high quality printing The printing media can be paper, textile, skin, cans orother surfaces that can adsorb or absorb printing solutions Ink-jet printing generated revenues of morethan $10 billion worldwide in 2000 and will keep growing in the future

10.6.2 Biomedical and Chemical Sample Handling

The application of microdroplet generators in biomedical sample handling is an emerging field that hasdrawn much attention in the past few years Many research efforts have focused on droplet volume control,droplet size miniaturization, compatibility issues, the variety of samples, and high-throughput parallelmethods

Luginbuhl et al (1999), Miliotis et al (2000), and Wang et al (1999) developed piezo- and type droplet injectors respectively for mass spectrometry Figure 10.28 schematically shows the design ofthe injectors, which generate submicron to micron sized bioreagent droplets for sample separation and analy-sis in a mass spectrometer, as shown in Figure 10.29 Luginbuhl et al (1999) employed silicon bulk micro-machining to fabricate silicon nozzle plates and Pyrex glass actuation plates, while Wang et al (1999)employed a combination of surface and bulk micromachining to fabricate the droplet generator Theseinjectors are part of the lab-on-the-chip system for incorporating microchips with macroinstruments.Microdroplet generators were also used by Koide et al (2000), Nilsson et al (2000), Goldmann et al.(2000), and Szita et al (2000) for the accurate dispensing of biological solutions Piezo- and thermal-typeinjectors were used in those investigations for protein, peptide, enzyme, or DNA dispensing With an oper-ation principle similar to ink-jet printing, the devices provided for precisely dispensing and depositing a

pneumatic-Pressure chamber Nozzle Liquid path

Droplet generators

Droplets

FIGURE 10.29 Operation principle of mass spectrometry using microdroplet generators.

single biological droplet onto a desired medium, and they also could dispense droplet arrays The arrayedbioreagents can be bioprocessed further for high-throughput analysis.

Continuous jet-type droplet generators were reported by Asano et al (1995) to effectively focus and sortparticles by using the electrostatic force The experimental setup is shown in Figure 10.30 A syringe pumppressurizes the sample fluid to pass through a nitrogen sheath flow for focusing, and then the sample isejected from a piezoelectric transducer disturbed nozzle to form droplets Droplets containing the desiredparticles were charged at the breakup point and deflected into collectors The reported separation prob-ability for 5, 10, and 15 µm particles can be as high as 99% However, the inner jet diameter limits the par-ticle size for separation Other than solid particle separation, this method potentially can be applied tocell sorting for biomedical applications

In addition to biomedical reagent handling, microdroplet generators were widely used in chemical handling.For example, Shah et al (1999) used an ink-jet system to print catalyst patterns for electroless metal deposition.This system used a commercial ink-jet printhead to ejecte a Pt solution as a seed layer for Cu electroless plating.The lines produced by this method were reported to be 100 µm wide and 0.2–2 µm high

10.6.3 Fuel Injection and Mixing Control

Microdroplet generators used for fuel injection can dispense controllable and uniform droplets, whichare important for mixing and combustion applications

Collectors

Droplets with particles

FIGURE 10.30 Particle sorting using droplet generators, after Asano (1995).

Combustion efficiency depends on the mixing rate of the reactants The reactants in a shear flow are firstentrained by large vortical structures (Brown and Roshko, 1974) and then mixed by fine scale eddies Theentrainment can be greatly enhanced by controlling the evolution of large-scale vortices either actively (Ho

et al., 1982) or passively (Ho et al., 1987) The effectiveness of controlling large-scale vortical structures forincreasing combustion efficiency has also been experimentally demonstrated (Shadow et al., 1987) Althoughmuch work has been done on improving the mixing efficiency in combustion chambers, improving the smallscale mixing and reducing the evaporation time of liquid fuel remain great challenges in combustionresearch

Traditional injectors with nozzle diameters of around tens to hundreds of µms can neither supply form microdroplets for reducing evaporation time and fine scale mixing nor eject droplets that can becontrolled individually to modulate vortex structure [Lee et al., 1999] To overcome those limitations,Tseng et al (1996) proposed a microdroplet injector array fabricated by the micromachining technolo-gies used for fuel injection The droplets ejected from microinjectors are uniform and the diameter can

uni-be from µm to tens of µms, which is close to the micro scale of small turbulence eddies The fine scalemixing can be carried out by the reaction of the small turbulence eddies directly with the microdroplets.The smaller, more uniform droplets increase the overall size of the evaporation surface and thus greatlyreduce evaporation time In addition, an appropriate arrangement of the microinjectors around the noz-zle of a dump combustor (Figure 10.31) provides spatially coherent perturbations to control the largevortices Two types of coherent structures, spanwise and streamwise vortices, can be influenced by impos-ing subharmonics of the air jet’s most unstable instability frequency Control of the spanwise vortices can

be accomplished by applying temporal amplitude modulation on injection If the ejecting phases of themicrodroplets along the azimuthal direction are the same, the mode zero instability (Brown et al., 1974)

is enhanced Imposing a certain defined phase lag on these microinjectors generates higher mode bility (Brown et al., 1974) waves, which are usually beneficial for mass transfer enhancement Since about

insta-1000 injectors are placed around the nozzle, the spatial modulation in the azimuthal direction can turb the streamwise vortices The interaction of streamwise and spanwise vortices by microinjectors cre-ates fine scale mixing

per-Controller Fuel

Air

Acoustic driver

Coherent flow structure

Micro injectors

Fuel droplet injections

FIGURE 10.31 Control of mixing and fuel injection by microdroplet generators, after Tseng (1996).

10.6.4 Direct Writing and Packaging

Micro droplet generators offer an alternative to lithography for electronics and opto-electronics ing This approach has the advantages of precise volume control of dispensed materials, data driven flex-ibility, low cost, high speed, and low environmental impact, as described by Hayes and Cox (1998) Thematerials have been demonstrated for the application to manufacturing processes including adhesive forcomponent bonding, filled polymer systems for direct resistor writing and oxide deposition, and solder forsolder bumping of flip-chips, BGAs (ball grid arrays), PCBs (printed circuit boards), and CSPs (chip-scale-packages) [Hays et al., 1999; Teng et al., 1988] In those printings, the desired temperature is 100⬃200°C,and the viscosity of the fluids needs to be around 40 cps; in some cases, an inert process environment, such

manufactur-as nitrogen flow, is required to protect the materials from oxidation

Through direct writing by ink-jet printing, the difficulty of fabrication from photolithography or printing processes for solar cell metallization and LEP (light emitting polymer) deposition of LEPD (lightemitting polymer displays) can be easily eliminated In solar cell metallization, metalo-organic decomposition(MOD) silver ink was used to ink-jet print directly onto solar cell surfaces and thus avoid p-n junction degra-dation in the traditional screen printing method requiring firing temperatures of 600–800°C Ink-jet printingalso provides for the formation of a uniform line film on rough solar cell surfaces [Tang et al., 1988a, 1988b;Somberg et al., 1990], which is not easy to achieve using traditional photolithography

screen-On the other hand, organic light emitting devices requiring the deposition of multiple organic layers toperform full color operation present similar problems Due to the organic layers’ solubility in many solventsand aqueous solutions, conventional methods, such as photolithography, screen printing and evaporation,that require a wet patterning process are not compatible with them on the same substrate [Hebner et al., 1998;Shimoda et al., 1999; Kobayashi et al., 2000] Thus direct writing of organic materials by ink-jet printingbecomes one of the promising solutions for providing a safe patternable process without wet etching.However, owing to the pinholes that appear on the patterned materials, high quality polymer devices maynot be easily ink-jet printed Yang’s group proposed combining an ink-jet printed layer with a uniform spincoated polymer layer to overcome this problem [Bharathan et al., 1998] In such a system, the uniformlayer serves as a buffer layer to seal the pinholes, and the ink-jet-printing layer contains the desired patterns[Bharathan et al., 1998]

10.6.5 Optical Component Fabrication and Integration

Integrated microoptics has become a revolutionary concept in the optics field because it provides theadvantages of low cost, miniaturization, improved spatial resolution and time response, and a reducedassembly process that is not possible by traditional means As a result, fabricating and integrating minia-turized optical components with performance similar to or better than traditional components are criti-cal issues in integrated microoptics systems Standard bulk or surface micromachining provides variousways to fabricate active/passive micromirrors, wave-guides, and Fresnel lenses, but making a refractivelens with curved surfaces is not easy Compared to photolithography, which utilizes a patterned andmelted photoresist column as the lens, the ink-jet printing method allows more flexibility in processdesign, material choices, and system integration Cox et al employed ink-jet printing technology to ejectheated polymer material in fabricating a micro lenslet array [Cox et al., 1994; Hays et al., 1998] The shape

of the lens was controlled by the viscosity of the droplets at the impact point, the substrate wetting dition, and the cooling/curing rate of the droplets [Hays et al., 1998] A 70–150 µm diameter lens has beensuccessfully fabricated with a density greater than 15,000/cm2and focal lengths between 50 and 150 µm.Wave-guides using ink-jet technology also have been demonstrated by Cox et al (1994)

con-Since optical components with varying properties can be selectively deposited onto the desired region,integration of those components with fabricated ICs or other devices is possible and efficient

10.6.6 Solid Free Forming

Not only two-dimensional patterns but also three-dimensional solid structures can be generated bymicrodroplet generators Orme et al (1993) and Marusak et al (1993) reported the application of molten

metal drops for solid free form fabrication Evans’ group demonstrated the application of continuous anddrop-on-demand ink-jets for ceramic printing to fabricate 3-D structures as well as functionally gradedmaterials [Blasdell et al., 2000; Mott et al., 1999] Yamaguchi et al (2000) used metal jets to print func-tional three-dimensional microstructures; Figure 10.32 shows the operation principle Yamaguchi et al.(2000) proposed employing multijets for structure and sacrificial material deposition to print an over-hanging structure, while Fuller et al (2000) used laminated PMMA film as the supporting material forthe ejection of metal cantilever beams The fabrication principle is shown in Figure 10.33.

(a) PMMA deposition

Draw down bar PMMA

The operation principle is shown in Figure 10.34 A continuous jet printer was used for droplet formationfrom ceramic solution The ceramic stream was delivered into the hottest part of the plasma jet and thensprayed onto the working piece Splats from the plasma spray are claimed to be similar in morphology tothose produced using conventional plasma spraying of a coarse powers, but they are significantly smaller,which may impart unique characteristics such as extension of solid state solubility, refinement of grainsize, formation of metastable phases, and high concentration of point defects [Blazdell et al., 2000].

10.6.8 IC Cooling

Conventionally, blowing fans and fins are widely used for cooling IC chips, especially in CPUs Recently, asheating power has increased greatly with increasing CPU size, more advanced methods, such as heat pipes,CPL, and impinging air jets, have been introduced for quick heat removal However, no matter how thedesigns improve, the upper limit of of those devices’ heat removal ability is in the order of tens of W/cm2 Inaddition, as chips become larger, detecting hot spots and selectively removing the heat only from hot regions

FIGURE 10.34 Operation principle of plasma spraying by microdroplet generators, after Blazdell (2000).

2 cm

2 cm

Micro droplet generation chip

IC chip with integrated surface temperature sensors

Micro injectors

Signal and energy transport bus

Parallel packaging

Droplet ejection for cooling

FIGURE 10.35 Conceptual design of IC cooling by microdroplet injections.