1 overview of inkjet-based micromanufacturing...now dominate the low-end printer market (hp’s...

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1Overview of Inkjet-Based MicromanufacturingDavid Wallace

1.1Introduction

Inkjet technology has come to prominence in the past decade as the domi-nant printer technology in the combined home and small office/home office(SOHO) markets, and inkjet is now familiar to the general public. Also, inthe past decade, inkjet technology has become recognized in the technicalcommunity as a highly capable tool for manufacturing, particularly microman-ufacturing [1]. As an introduction to the more detailed discussions in the chaptersto follow, this chapter will, briefly, give a background on inkjet technology;discuss fluid requirements and pattern formation fundamentals; discuss thecharacteristics of microfabrication and the potential of inkjet as a tool; discussthe breadth of applications being addressed with inkjet technology, includinga few specific examples; and discuss issues, challenges, and possible futureapplications.

1.2Inkjet Technology

Inkjet technology is not a single technology, but a family of very different tech-nologies that have a similar function: the precise generation of free-flying fluiddroplets. Precise refers to the volume of the droplet, the time at which it is pro-duced, the velocity with which it travels, and the direction of travel. The rangeof diameter, velocity, and frequency of generation of the droplet obtained, alongwith the precision, will vary considerably depending on the specific technologyemployed. A complete discussion of the different inkjet technologies and theirrelationship to each other is beyond the scope of this chapter and only a briefoverview is given below. Excellent overviews of the physics [2, 3] and practi-cal applications [4] of inkjet are available for those interested in more detailedreviews.

Inkjet-based Micromanufacturing, First Edition. Edited by J. G. Korvink, P. J. Smith, and D.-Y. Shin. 2012 Wiley-VCH Verlag GmbH & Co. KGaA. Published 2012 by Wiley-VCH Verlag GmbH & Co. KGaA.

2 1 Overview of Inkjet-Based Micromanufacturing

1.2.1Continuous Mode Inkjet (CIJ) Technology

Repeatable drop formation from a cylinder of liquid was noted as early as 1833by Savart [5] and described mathematically by Lord Rayleigh for inviscid jets [6,7], and by Weber [8] for viscous jets. In continuous mode inkjet (CIJ) technology,fluid under pressure is forced through an orifice, typically 50–80 µm in diameter,and breaks up into uniform drops under the action of surface tension, by theamplification of diameter perturbations [9, 10] or surface tension perturbations [11].Piezoelectric actuators have been used as the source of diameter perturbations, butmicroheaters have been used as the source of surface tension perturbations [12].A drop breaks off from the jet in the presence of a varying electric field, and thusacquires a charge. The charged drops are directed to their desired location, eitherthe catcher or one of several locations on the substrate, by a static electric field [13].

This type of system is generally referred to as ‘‘continuous’’ because drops arecontinuously produced and their trajectories are varied by the amount of chargeapplied. CIJ printing systems produce droplets that are approximately twice theorifice diameter of the droplet generator, which is an important limitation in theuse of CIJ in micromanufacturing. Droplet generation rates for CIJ systems areusually in the 80–100 kHz range, but systems with operating frequencies up to1 MHz have been commercialized [14]. Droplet sizes can be as small as 20 µm in acontinuous system, but 150 µm is typical.

CIJ systems are currently in widespread use in product labeling of food andmedicines. They have high throughput capabilities, and are best suited for ap-plications where they are in continual use. Very few CIJ systems are multicolor(multifluid), but two-color systems are in use. CIJ systems require unused drops tobe recirculated or wasted, another limitation in using CIJ for micromanufacturing.Applications where CIJ is used include Massachusetts Institute of Technology(MIT’s) 3D printing rapid prototyping technology [15], metal jetting technology[16], and medical diagnostic test strip production [17].

Figure 1.1 illustrates a schematic of a continuous inkjet printer and a jet of waterbreaking up into 120 µm droplets at 20 kHz.

1.2.2Demand Mode Inkjet Technology

In demand mode inkjet technology, a piezoelectric transducer, or heated resistorthat produces a bubble [18], creates pressure/velocity waves in a fluid, which startsat essentially atmospheric pressure, and these waves propagate to produce a dropat an orifice [19–21] or free surface [22]. Since a drop is created only when desired,these types of systems are referred to as drop-on-demand or demand mode. Demandmode systems are conceptually far less complex than continuous mode systems.On the other hand, demand mode droplet generation requires the transducer todeliver three or more orders of magnitude greater energy to produce a droplet,compared to the continuous mode. Driven by the need for multiple orifices to

1.3 Fluid Requirements 3

Piezoelectrictransducer

Chargingelectrode

Deflection plates+V

Orifice

Pump

Transducerdriver

Inkreservoir

Data

Chargedriver Gutter

Substrate

Figure 1.1 Schematic of a continuous inkjet printer and ajet of water breaking up into 120 µm droplets at 20 kHz.

meet the throughput requirement in printing applications, and constrained by thetransducer energy requirements, there are many ‘‘elegant’’ (i.e., complex) arraydemand mode printhead designs [23–26].

Demand mode inkjet systems have been used primarily in desktop printers, andnow dominate the low-end printer market (HP’s DeskJets, Cannon’s Bubble Jets,and Epson’s Stylus). Demand mode inkjet systems have no fluid recirculation re-quirement, and this makes their use as a general fluid microdispensing technologymore straightforward than continuous mode technology. Almost all of the resultsdiscussed in this book have been obtained with demand mode systems. Figure 1.2illustrates the schematic of a demand mode system and drops of an organic solventbeing formed at 4 kHz.

1.3Fluid Requirements

The fluid property requirements for demand mode inkjet dispensing are viscosity<∼40 cP, and surface tension >∼20 dyn/cm. Low viscosities usually lead to satelliteformation (formation of multiple drops when one is desired) and low damping ofpost-drop oscillations, limiting the upper operating frequency. If the fluid is heatedor cooled, the critical properties are those at the operating temperature of the office,


Piezoelectrictransducer

Piezodriver

Data

SubstrateInk~atmospheric

pressure

Orifice

Figure 1.2 Schematic of a demand mode system and dropsof an organic solvent being formed at 4 kHz.

not room temperature. Higher viscosities can be tolerated in the fluid deliverysystem if this does not create a pressure drop that limits the desired maximumfrequency due to restriction of the flow into the printhead. For high density fluids,such as molten metals [27] (e.g., solders, tin, mercury, rubidium, and lithium), thefluid properties should be converted to kinematic values to determine if the fluidproperties are acceptable.

Newtonian behavior is not strictly required, but non-Newtonian effects arenever beneficial. Viscoelastic behavior causes significant performance problems byincreasing the amount of deformation the fluid can withstand without breaking offto form a drop [28].

Particle suspensions, such as pigmented inks, are acceptable as long as theparticle/agglomerate size and density do not cause the suspension to depart fromthe fluid properties range given above. Particles that are >5% of the orifice diameter(e.g., cells; [29]) will cause at least some instability in drop generation behavior, butstill may be acceptable in low concentrations.

The ‘‘window’’ of fluids and suspensions that can be dispensed using inkjettechnology has been enlarged by heating, cooling, stirring, wiping, purging,preoscillating, diluting, and other methods. However, this window is unavoidablynarrowed as orifice diameter decreases, frequency increases, and the number ofjets in an array increases.

The diversity of fluids that have been dispensed using inkjet technology isimpressive, given the fluid property restrictions described above. Inks by themselvesrepresent a broad class of materials. Dye and pigment aqueous dispersions are themost commonly used in conventional printing, volatile, and low-volatility solventinks are all in common use, as are UV-curing inks. Other materials that have beendispensed using inkjet technology are shown in Table 1.1.

1.4 Pattern Formation: Fluid/Substrate Interaction 5

Table 1.1 Materials that have been deposited using inkjet technology.

Electronic/opticalmaterials

Biological fluids Organic solvents Particlesuspensions

Other

Liquid metals DNA Alcohols Pigments Sol-gelsFluxes Nucleic acids Keytones Cells ThermoplasticsPhotoresists Amino acid Aliphatics Latex spheres ThermosetsEpoxies Cells Aromatics Metals AcrylicsPolyimides Proteins Dipolar solvents Teflon Photographic

developerElectroactivepolymers

Lipids Phosphors Fuels

Organometallics Biosorbablepolymers

Ferrites Aqueousadhesives

Zeolyts Odorants

1.4Pattern Formation: Fluid/Substrate Interaction

Except for the applications where inkjet technology is used to meter fluid, as in filinga well or a region bounded by a nonwetting coating, inkjet deposition processes areused to produce a pattern of material on a substrate. The interaction between thefluid properties, jetting parameters (drop size, velocity, and frequency), substrateproperties, printing grid (dots per inch), and printing sequence (interleave, over-printing, sequence of fluid, etc.) is a multivariate optimization in the developmentof all inkjet printing systems/applications. For the conventional inkjet case of liquidink on paper, the porosity of the paper and the low viscosity of the ink representeda major challenge in the initial development of inkjet printers. Rapid spreading ofliquid ink through the fibers can cause the spot size to become much larger thanthe drop size, decreasing the optical density of the spot and resulting in irregularspots that degrade the quality of characters, lines, and so on. Ink formulations thatproduce good print quality on a wide range of papers have been a cornerstone inthe wide acceptance of inkjet printers in the marketplace.

Most micromanufacturing applications of inkjet technology deposit liquid ontononporous substrates, similar to printing an overhead transparency. Control ofthe spreading is essential if the desired resolution is to be obtained. Phase changeinks were developed for conventional inkjet printers for precisely this reason, sincethey solidify quickly after impact. In micromanufacturing applications, solders forelectronic manufacturing [30] and thermoplastics for free-form fabrication [31] areexamples of phase change materials. The control of spreading by solidification isa beneficial aspect of phase change materials if the goal is to limit spreading andobtain the smallest spot for a given drop size. However, if the goal is deposition ofa uniform layer, solidification into a bump is a problem, not a feature.


Many organic liquids, such as isopropanol, acetone, and acetonitrile, are of verylow viscosity, low contact angle/surface tension, and volatile. Their ability to wetmost surfaces and low viscosity allows these fluids to spread rapidly. As with aphase change material’s lack of spreading, this is either a feature or a problem,depending on the application. If one is trying to write a small conductive line usingan organometallic ink, or create a pixel in a light-emitting polymer display, it isdefinitely a problem. In many cases, surface features, such as the wells commonlyused in flat panel displays, provide a barrier to spreading and help physically definethe feature size. In other cases, surface treatments, such as plasma cleaning orapplication of a nonwetting coating, are used to control spreading [32].

Volatility of a solution with dissolved or suspended solids can cause operationalissues, and ink drying in the orifice is one of the most common failure modes withoffice inkjet printers. In addition, volatility can cause nonuniform distribution ofthe solid on the substrate after drying [33]. Solutions to this problem have beenmany and diverse: reactive substrate, covalent binding of the solid to the substrate,cosolvents that are lower volatility, UV or thermal cross-linking, and so on.

Pattern or image formation in its simplest implementation can be just theselection of pixel (picture element) size and spacing, then using inkjet dispensingto fill the desired pixels, ratering out the image as is done in a conventionalinkjet printer. However, even the lowest cost inkjet printers have complicated printmodes that are used to increase print quality. Rows of spots are interleaved to hidecoherent errors from a single jet, colors are printed so as not to bleed together inwet state, multiple passes are made over an area to increase color saturation of theprinted area, and operating frequency is decreased for high-quality printing. All ofthese methods, and more, have applicability to micromanufacturing applicationsof inkjet.

1.5Micromanufacturing

1.5.1Introduction

Micromanufacturing today has evolved from microelectromechanical systems(MEMS) fabrication technology, developed initially in the 1980s with the goalof integrating electromechanical sensors and actuators with their conditioningelectronics. By adding silicon micromachining and deposition of metals and oxidesto silicon-based analog integrated circuit (IC) fabrication technology, very smallelectromechanical sensors and actuators could be fabricated at very low cost and inhigh volume [34, 35]. In addition, these MEMS-based devices were more robust andreliable than their larger conventional counterparts. MEMS-based accelerometersare the most widely used MEMS products. They have enabled the use of airbagsin automobiles and can currently be found in cell phones, computers, cameras,golf clubs, skis, and, of course, video games. MEMS-based pressure transducers

1.5 Micromanufacturing 7

have also been in widespread use for decades, primarily in automobiles, airplanes,process control equipment, and disposable biomedical sensors.

In most applications, the benefits of a high degree of system integration are read-ily apparent. The success of MEMS-based accelerometers and pressure sensors,along with the general drive toward small, low-cost, high-volume products has ledto an explosion of process technologies that can be used in MEMS fabrication andapplications targeted by these process technologies. Inclusion of digital electronicswas a fairly obvious step. Expansion to optical emitters, detectors, and switcheshas resulted in micro-optical electro-optical mechanical systems (MOEMS) [36].Examples include micromirror devices that are used in televisions and projectors,and solid-state laser devices that have enabled the growth of the telecommunicationand data communication industries [37]. Inclusion of biological functions has re-sulted in Bio-MEMS devices [38]. Lab-on-a-chip devices for point-of-care diagnosticsare one of the few examples of available Bio-MEMS products [39, 40], although alarge number of applications are being developed, including implantable sensors(e.g., glucose) [41] and microneedle-based transdermal drug delivery devices [42].Finally, by including fluid flow with thermal or piezoelectric actuators, many inkjetprintheads fall into the category of MEMS devices and inkjet printers have longbeen ‘‘adopted’’ by analysts when reporting MEMS industry revenue figures [43].

Many optical devices now included in the MOEMS category do not have a me-chanical function, being more properly categorized as electro-optics (EO) devices.Other devices cited above have similar issues when it comes to categorization. Thisillustrates the difficulty in using the term MEMS in any strict sense at this point intime, since it has been broadened and generalized to refer to micromanufacturingin general, partly as a result of its successes and familiarity. In this chapter, theterm ‘‘MEMS’’ is used in the loosest sense, referring to miniaturized devices andsystems that must be micromanufactured.

1.5.2Limitations and Opportunities in Micromanufacturing

MEMS device fabrication methods grew out of the silicon-based semiconductorindustry, so most rely on photolithography. Photolithographic processes are par-ticularly well suited for high-volume manufacturing of devices with high featuredensity and low diversity of fabrication processes and fabricated features [44, 45].The prime example is a dynamic random access memory (DRAM) device with rep-etition of the same features millions of times using a limited number of fabricationprocesses. MEMS fabrication has successfully built on the huge microelectronicsequipment and technology base, adding the feature diversity required to create a‘‘system’’ through a limited number of additional compatible processes. However,photolithography and other ‘‘IC-like’’ fabrication processes are severely limited inthe types of materials that can be used. In addition, there are technical and costlimitations that limit the number of ‘‘layers’’ that can be created in fabricating anMEMS device.


Materials limitations in conventional MEMS fabrication fall into two categories,compatibility and cost. First, materials must be compatible with photolithography,which means that they must survive the application, patterning, and removingof photosensitive masking materials, and, in general, must be compatible withthe creation of one or more additional layers. Functional materials such as onesthat are biologically active, chemically active/receptive, or optically active (emittersand receivers) are typically difficult or impossible to use in photolithographic orother ‘‘IC-like’’ fabrication methods. Yet it is these types of materials that couldenable a broad range of new small integrated devices for medicine, security,communications, and so on. Some of the most interesting materials are rare orexpensive and thus would be cost prohibitive to use in the current subtractiveprocesses employed in MEMS fabrication. Many biologically active materials fallinto this category.

Finally, there is an inherent conflict between the desire to increase the number offunctions in MEMS devices and the cost of each ‘‘layer’’ in a photolithographic orother ‘‘IC-like’’ fabrication method. Even if the functional materials are compatiblewith multiple layers of photolithography and are not particularly expensive, eachlayer has a substantial total cost (equipment, labor, facility, materials, yield, testing,etc.) associated with it. Thus, additional functions always have a strong costcounterweight in MEMS device design based on current manufacturing methods.

1.5.3Benefits of Inkjet in Microfabrication

Inkjet printing technology has a number of attributes that can overcome some ofthe inherent limitations of photolithographically based microfabrication methods.Since it is an additive method, material is only deposited where it is desired.Usage of rare or expensive materials can thus be conserved. The net savingscan be significant for even moderately priced materials if the amount of arearequired to be covered is small compared to the total area of the substrate (i.e.,low feature density). In addition to cost savings because of low materials usage,additive methods have little or no waste, so they are much more environmentallyfriendly. In contrast, subtractive methods waste large amounts of the functionalmaterial, plus the photosensitive masking material (if different) and the cleaningand etching solvents.

Since inkjet is noncontact, interaction between different materials when they aredeposited is eliminated or greatly reduced, as is the requirement to consider eachmaterial to be deposited as an expensive additional ‘‘layer.’’ Deposition can occur onnonplanar surfaces, eliminating planarization steps that are sometimes requiredin photolithography. Even very fragile or sensitive surfaces such as released layers(thin structures not supported by any solid material underneath them) can beprinted onto using inkjet because of the extremely low inertial force exerted bya deposited drop. Active layers (detector, emitter, biological, reactive, etc.) can bedeposited onto without degrading their function. Thick films can be created by

1.6 Examples of Inkjet in Micromanufacturing 9

overprinting the same location multiple times, and locally layered structures canbe created without having to process the entire substrate area.

Lastly, since inkjet is a fundamentally digital, data-driven process, the cost andtime associated with producing the masks required by photolithographically basedmicrofabrication methods is eliminated. This removes both the cost and turnaroundtime associated with fixed tooling. The ability of inkjet to deposit onto individual dyeand partial wafers, and to perform parametric process development experimentsunder digital control, can accelerate the development time for a microfabricateddevice.

Several examples of the use of inkjet in micromanufacturing are discussed belowto illustrate the characteristics and issues of inkjet deposition discussed earlier.

1.6Examples of Inkjet in Micromanufacturing

1.6.1Chemical Sensors

Chemical sensors represent a fairly new and broad area of research and developmentfor MEMS devices, driven by the need for large numbers of low-cost sensors forexplosive, chemical warfare agents, drugs of abuse, industrial gases, residentialgases, and many others. A majority of these sensors use materials that are electricallyor photonically active, or more simply have surfaces that cause the molecules ofinterest to temporarily adhere to them. Not surprisingly, most of these sensingmaterials are ‘‘sensitive,’’ meaning delicate, and cannot be photolithographicallyprocessed. Also, because they are sensitive, they are applied in the last or nearlylast fabrication process; typically, this is onto nonplanar, feature-rich surfaces thatcan be very fragile. All these factors make MEMS chemical sensor manufacturingan area that is broadly exploring the use of inkjet fabrication technology.

Chemoresistive materials, ones that change resistance when exposed to specificmolecules of interest, are one of the oldest and most broadly used sensing materialsclasses used in MEMS sensor devices [46]. Recent developments in nanomaterialsand MEMS structures have expanded the number of materials and sensor structuresbeing developed [47]. An example of an MEMS chemoresistive sensor is one that isbeing developed at Carnegie Mellon University to detect volatile organic compounds(VOCs) in respirators, indicating end of life [48]. The basic sensor structure, shownin Figure 1.3a, is a pair of spiral electrodes in a 250 µm circle that is in a 350 µmdiameter SU-8 well. Multiple sensing and reference elements, which, in general,could contain multiple sensing materials, are incorporated on a 2.65 mm die thatalso contains all the required control electronics, as shown in Figure 1.3b. The dieis assembled into a TO-5 package (Figure 1.3c), which is commonly used for opticaldevices.

The sensing material, gold-thiolate nanoparticles, are suspended in a carrierfluid (5–10 mg ml−1) and deposited onto the sensing area. Although not visible


(a) (b) (c) (d)

Figure 1.3 Carnegie Mellon chemoresistivesensor: (a) sensing element configurationshowing 250 µm diameter dual electrode spi-ral in a 350 µm SU-8 well; (b) multiple sens-ing and reference elements on a 2.65 mm

die; (c) sensor device in a TO-5 package;and (d) sensor element printed with 225nominally 30 pl drops of solution containinggold-thiolate nanoparticles. (Images courtesyCarnegie Mellon University.)

in Figure 1.3a, 15 drops of nominally 30 pl volume have been deposited ontothe sensor using an inkjet device. Figure 1.3d shows the sensing area after 225nominally 30 pl drops of solution have been deposited, producing an average filmthickness of 1.5 µm. It is interesting to note the use of two wetting ‘‘stops’’ in thesensing area. The SU-8 well contains the initial fluid volume dispensed, preventingundesired wetting onto other areas of the die. In addition, the fluid dewets fromthe outer portion of the well during drying so that all of the particles are depositedonto the electrode region. This self-centering behavior results in an impedancevariation of <10%.

The dispensing of the sensing material occurs not only on the singulated die butalso after it is assembled into the package. This effectively limits the sensing materialdeposition method to additive dispensing methods, and the requirement to printmultiple sensors held in a fixture in a product would require a data-driven methodunless the fixture is of high precision (i.e., expensive). If a contact dispensingmethod is used, throughput would be limited by the requirement for the dispenserto make a vertical movement for each dispense.

Resonant MEMS structures detect a change in resonant frequency associatedwith a change in the mass of the resonating structure due to the adsorption ofmolecules of interest. Detection of changes in resonance can be accomplished togreat accuracy with well-known electronic circuitry that can be implemented intointegrated circuitry on a MEMS device. MEMS fabrication techniques can produceextremely low-mass, high-Q resonant structures that allow detection of very lowconcentrations of the molecules of interest [49].

1.6.2Optical MEMS Devices

Refractive optical components in MEMS devices present a number of difficulties.Refractive lens and waveguide features are highly three-dimensional, have highspatial resolution requirements (location, diameter, and curvature), must be highlytransmissive, and, in general, should be able to tolerate solder reflow temperatures.Diffractive optical components have been widely employed in MEMS devicesbecause their planar structure allows them to be fabricated using traditional


Figure 1.4 SU-8 posts, 100 µm high and 125 µm in diam-eter, over VCSELs on a gallium arsenide wafer. The postsare on 225 µm center-to-center spacing with polymer lensesprinted onto the posts, 22 × 18 pl drops per post. (Waferwith posts courtesy Vixar Corp.)

MEMS processes. However, refractive optics have a higher performance thandiffractive optics, making their use in MEMS optical devices highly desirable. Byusing prepolymer solutions that are cross-linked after deposition, it is possibleto formulate jettable polymeric solutions that have no solvents, cure into hard,durable microlenses, and can tolerate reflow temperatures.

Fabrication of lenses onto MEMS devices at the wafer level has advantagesin both cost and throughput. Figure 1.4 shows a portion of a gallium arsenidewafer (from Vixar Corporation) containing 125 µm diameter SU-8 posts on 225 µmcenter-to-center spacing with polymer lenses printed onto the posts (22 drop at18 pl each). Divergence angle measurements for vertical-cavity surface emittinglasers (VCSELs) with and without lenses printed onto SU-8 indicated a decreasein divergence angle from 11 to 4◦. In addition to meeting optical performancerequirements, the lens and post-manufacturing processes must not degrade theperformance of the VCSEL in terms of threshold voltage and output power. Previousmeasurements of these for VCSELs with lenses postfabricated using inkjet haveshown no degradation in these performance parameters [50]. The capability to printvariable lens height, by varying the number of drops, can be used to experimentallydetermine the optimal lens height very rapidly during development, and can beused in production to create lenses of different height on a single wafer and/ordevice.

Released structures in MEMS are fabricated onto a sacrificial layer that issubsequently removed, leaving the structure suspended over air (or vacuum).They are frequently employed to allow the structure to move with relatively largedeflections. They are very fragile, making application of thick film materials directlyto released structures very difficult. Figure 1.5 shows a 100 µm diameter releasedstructure (oscillating micromirror), suspended by 10 µm width/thick supports,onto which a polymer lens has been deposited without breaking or deforming the


1.0 kV 10.6 mm ×450 SE(M) 3/2/04 66.7µm

(a) (b)

Figure 1.5 A 100 µm diameter released structure (oscillat-ing micromirror), suspended by 10 µm width/thick supports,onto which a polymer lens has been deposited.

released structure. A nominally 50 pl (46 µm diameter) drop at 2 m s−1 was usedto form this lens, making the impact momentum ∼0.1 µN · s.

1.6.3Bio-MEMS Devices

MEMS devices that are implantable in humans have both great potential andsignificant challenges. Continuous monitoring and adjustment of biological func-tions has great potential for more effective monitoring and treatment regimens.However, implantable MEMS devices must deal with biocompatibility and biofoul-ing issues [51]. The implantable microelectrode from the University of Kentuckyshown in Figure 1.6 has four 20 × 60 µm electrochemical measurement sites on

Figure 1.6 Implantable (brain) microelectrode with four20 × 60 µm electrochemical measurement sites on a ce-ramic substrate. All four sites have been coated with a gluta-mate oxidase enzyme and overcoated with gluteraldyhyde, afixative, both using inkjet printing. (Images courtesy the Uni-versity of Kentucky Center for Microelectrode Technology.)


a ceramic substrate that are used to measure brain activity [52]. All four sites inFigure 1.6 have been coated with a glutamate oxidase enzyme and overcoated withgluteraldyhyde, a fixative, both using inkjet printing. Since the enzyme coating isthin and transparent, it is not visible in the figure. Currently, this type of probeis used for research into neurological function and diseases such as Parkinson’s.Future variants of this type of device could be used as an indwelling sensor inhumans, which would monitor and report brain function as part of an overalltreatment plan, similar to blood glucose measurement and insulin injection.

Delivery of drugs by implantable devices [53] is of as much interest as are im-plantable sensors and current research includes drug delivery for diabetes, cancers,cardiac disease, and neurological disorders. The most widely used implantable drugdelivery device today is the drug-eluding stent, used to keep coronary arteries openafter an angioplasty procedure. The complex structure of a metal stent must allowthe device to collapse to travel through the blood vessels, then deploy (i.e., increasein diameter) and lock at the desired location. To prevent tissue from growing overthe stent and blocking the artery again, drugs are embedded into the stent. Thisrequires placing the drug onto one side (only) of complex features 50–150 µm inwidth. A number of companies are using inkjet for this process [54, 55]. Futurecoated stents may contain multiple drugs and/or varying concentration with posi-tion, for example, placing more drug at the ends, where restenosis (reclosing of theartery) occurs.

1.6.4Assembly and Packaging

Assembly of MEMS devices presents challenges that are different from and, in gen-eral, more difficult than encountered in standard IC packaging. Difficulties include

(a) (b)

Figure 1.7 (a) Top view of signal con-ditioning IC and flex circuit of disk driveread head assembly. (b) Side view show-ing 80 µm electrical interconnect pads onIC edge that interfaces with the flex circuit

(60 µm traces); solder has been dispensedand partially reflowed into the corner formedby the IC and flex using piezoelectric de-mand mode inkjet technology to dispensedroplets of molten solder.


the inherent three-dimensionality of MEMS devices, fragile structures, hermeticsealing, and so on. An example of a three-dimensional electrical interconnect is theassembly of the signal conditioning IC to the flex circuit containing the read-writehead for a hard disk drive. Figure 1.7a shows a top view of the IC and flex circuitand Figure 1.7b the side view. The IC has 80 µm electrical interconnect padson the edge that interfaces with the flex circuit, which has 60 µm traces. Solderhas been dispensed and partially reflowed into the corner formed by the IC andflex piezoelectric demand mode inkjet technology to dispense droplets of moltensolder [56].

Most of the recent developments in printed electronics have focused on low-cost,high-volume macroscale devices such as radiofrequency identification (RFID)tags [57]. However, inkjet printing processes for conductors, dielectrics, resistors,capacitors, inductors, antennae, organic transistors, batteries, fuels cells, and otherfunctional elements have been demonstrated and may have application to bothMEMS device manufacturing and packaging.

1.7Conclusions

Inkjet printing technology has been described, including its use in several mi-cromanufacturing applications. Inkjet methods have the potential to drasticallyincrease the range of functions in micromanufactured devices because of the addi-tive, noncontact, data-driven nature of inkjet technology. These same characteristicsalso can result in lower production cost and in less environmentally unfriendlywaste.

Acknowledgments

The author thanks the following for their valuable contributions: Donald Hayes,Ting Chen, Mike Boldman, Virang Shah, David Silva, Bogdan Antohe, Royall Cox,Rick Hoenigman, Lee Weiss, Peter Huettl, and Pooja Talauliker.

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1 overview of inkjet-based micromanufacturing...now dominate the low-end printer market (hp’s...

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