Collimated Aerosol Beam Deposition: Sub 5-jm Resolution of Printed Actives and Passives

D. L. Schulzac*, J. M. Hoeyac, D. Thompsonc, 0. F. Swensonb C, S. Hanc, J. Lovaasenc, X. Daic, C. Braunc,K. Kellerc, I. S. Akhatova'C

a Department of Mechanical Engineering, North Dakota State University, Fargo, ND 58105, USAb Department of Physics, North Dakota State University, Fargo, ND 58105, USA

c Center for Nanoscale Science and Engineering, North Dakota State University, Fargo, ND 58105, USAdoug.schulzgndsu.edu, (701)231-5275

AbstractMaterials deposition based upon directed aerosol flow

has the potential of finding application in the field offlexible electronics where a low-temperature route toprinted transistors with high mobilities remains elusive.NDSU has been actively engaged in addressing thisopportunity from the following two perspectives: (1)developing an appreciation of the basic physics thatdominate aerosol beam deposition toward engineering arobust method that allows the realization of depositedfeatures with sub-5 ptm resolution; and, (2) developing anunderstanding of the mechanistic transformations of silane-based precursor inks toward the formation of electronicmaterials at atmospheric-pressure. In this paper, we willbriefly discuss the genesis of a new a materials depositionmethod termed collimated aerosol beam direct-write (CAB-DW) where precision linewidth control has been realizedusing a combined theoretical/ experimental approach. Next,we will discuss progress using Si6H12 (cyclohexasilane - aliquid silane) as a precursor for solution-processed diodesand transistors. Finally, we demonstrate the ability to CAB-DW Si6H12-based precursor inks for printing Si-basedsemiconductors.

KeywordsAerosol beam, aerodynamic focusing, printed

electronics, liquid silane

1. IntroductionWhile it is known that micron-sized particles flowing

through a converging micro-capillary tend toward thecenter line, classical aerodynamic focusing methodologiesthat consider only particle inertia and the Stokes force ofgas-particle interaction do not adequately describe thenature of this phenomenon. Recently, the influence ofSaffman force upon the fluid dynamics of focused aerosolflows through micro-capillaries has been established by ourgroup through a combined theoretical/experimentalapproach.[1,2] A new mathematical model for aerosol flowthrough a micro-capillary accounting for complicatedinteractions between particles and carrier gas wasdeveloped and the model was confirmed experimentally viaparticle imaging microscopy of the aerosol beam. It wasfurther shown that it is possible to design a micro-capillarysystem capable of generating a collimated aerosol beam(CAB) in which aerosol particles stay very close to thecapillary center line. The approach is based on the fact thata relatively high velocity (-100 m/s) aerosol micro-flow

through a capillary with an inner diameter -100 ptm and alength of 1 cm are subject to Saffman forces.[1]

These results were used in the rational design of amicro-capillary "aerosol gun" capable of generating afocused collimated beam where aerosol particles stay veryclose to a capillary center line. The feasibility ofCAB-DWwas established theoretically and experimentally and theutility of such an aerosol gun was demonstrated by printingsilver nanoparticle-based ink. [2,3]

The results of our initial investigations usingcyclohexasilane (Si6H12) as a liquid precursor to amorphoussilicon were recently presented.[4] Spin-coating of Si6H12-based inks with subsequent UV light and/or thermaltreatment yielded amorphous silicon films as determined byvibronic studies (i.e., FTIR, Raman) as well as chemicalanalysis (i.e., XPS).[4] Doping strategies for the formationof n-type and p-type a-Si layers led to heterojunctionstructures (i.e., thin a-Si films on heavily-doped Si wafersubstrates) that exhibited the field effect. All-solutionprocessed diodes were realized by spin coating theseSi6H12-based inks onto transparent conducting oxide -coated glass. [4] Preliminary investigations indicate thepossibility of generating 10 micron wide lines of a-Simaterials by CAB-DW of Si6H12based inks. In this paperwe briefly summarize the concepts of the two abovementioned aerosol dynamics papers and present our newresults that illustrate the utility ofCAB-DW for printingactive and passive transistor components.

This paper begins with a discussion of aerosol beamsflowing through micro-capillaries where the concept ofaerodynamic focusing is presented. Section 2 continueswith a review of the Saffman force and a description of ourmathematical model for aerosol flow in a micro-capillary.The concept of a focused collimated aerosol beam (CAB)for direct-write fabrication is presented in Section 3. Thedesign of a new micro-capillary direct-write system (i.e.,CAB-DW) that can be used to generate and deposit acollimated aerosol beam is presented in Section 4. Finally,efforts to utilize CAB-DW for printing Si6H12-based inkstoward field-effect transistors are presented in Section 5.

2. Aerosol Beam Flow within MicrocapillariesClassic aerodynamic focusing studies have typically

employed thin plate orifices or abruptly converging nozzlesfor the generation of an aerosol beam. Our approachemploys a relatively long nozzle with an exit radius of-100 ptm, a length of 1 cm and a slowly varying innerdiameter. We define such a nozzle as a micro-capillary andshow that the migration of aerosol particles perpendicularto the center line of the capillary is significantly affected by

978-1-4244-2054-4/08/$25.00 2008 IEEE

the Saffman force. In this manner, we have transitioned therealm of understanding beyond aerodynamic focusingmethodology where only particle inertia and Stokes forcehave been used to describe gas-particle interactions.

2.1. Aerodynamic FocusingIt is well established that the particles comprising an

aerosol flow are susceptible to inertial effects therebyobtaining a radially-inward motion during accelerationthrough a converging nozzle.[5-7] The radial motion isretained even as the rapidly expanding carrier gas divergesradially-outward resulting in focusing of the aerosol beamdownstream of the nozzle. This experimental observationwas later supported theoretically with the motion of aerosolparticles suspended in a carrier gas described in terms ofparticle inertia and Stokes drag using Newton's equation ofmotion.[8] These findings served as a basis forexperimental and numerical studies of aerodynamicfocusing of aerosol particles. [9,10 and references therein]

2.2. Saffman ForcesAcademic interest in particle-in-liquid dispersions

flowing through straight capillaries was initially motivatedby Poiseuille who observed that red blood cells tend toavoid vessel walls when flowing through capillaries.[1 1]This observation led to research interest in small particlemigration in shear flows where a force perpendicular to thedirection of flow was first described by Saffman et al. for a

small sphere in an unbounded shear flow.[12,13] It was

shown that the direction of this Saffman force depends on

the product (u - up)(culcy) where u and u,p are thevelocities of the fluid and the particle respectively in theaxial direction. Toward that end, the Saffman force pushesparticles to the region of higher fluid velocity when theyare dragged by the fluid (u- u,p > 0) while the same

Saffman force pushes the particle to the region where thefluid velocity is lower when the fluid is dragged by theparticles (u- up, < 0). This effect is shown schematically inFigure 1.

We recently presented the results of a combinedtheoretical/experimental study of the impact of the Saffmanforce on aerosol micro-flows where a mathematical modelthat accounts for complicated interactions between aerosolparticles and carrier gas flowing through a micro-capillarywas established.[ 1,2]

A beam of Ag-based aerosol ink particles with d-0.5ptm was visualized using an optical microscope via laserscattering. The beamwidth full-width at half-maximum(FWHM) data were compared to theoretical predictions.These results served as the basis of a new concept forcontrolling and optimizing focused aerosol beams throughlong micro-capillaries.

I

,lfb I

I_

~~mb

I"

fmmmol.t~~

1Fk,a

UP

Figure 1. Schematic of a particle dragged by a shear flow.

2.3. Mathematical Model for Aerosol Flow in a Micro-capillary

In this section we briefly present our mathematicalmodel for aerosol flow in a micro-capillary that we

previously discussed in detail and verifiedexperimentally.[1,2] In general, the following forces are

used to describe the interaction between a particle and a

carrier gas flow (Table 1): (1) the Stokes force, whichrepresents a steady viscous drag force; (2) the Basset force,which represents a non-steady viscous drag force; (3)virtual mass force, which represents the inertia of the fluidaround a particle added to that particle; (4) fluid pressure

gradient force; (5) gravity force; (6) the Magnus force,which represents a steady lift force of a viscous fluid on a

rotating particle; and, (7) the Saffman force, whichrepresents a steady lift force induced by the local shearflow of a viscous fluid. In our model, only the Stokes andSaffman forces are considered since the other forces are

negligible under the conditions of interest.

Force DescriptionStokes Steady viscous dragBasset Non-steady viscous dragMagnus Viscous fluid steady lift force on

rotating particleSaffman Viscous fluid steady lift force via

local shear flowVirtual mass Inertia of fluid around a particleFluid pressure Fluid pressure differential applied togradient the particleGravity Constant body force

Table 1: Forces used to describe particle flow in fluids.

The equation used to describe the Stokes force in thisstudy follows (Eq. 1):

Fst = 6Tap(v - vp ) CstF ( CKn

(1)

where a - particle radius; ,u - fluid viscosity; v, vp - vectorvelocities of the fluid and particle; C, Ckn are correctionfactors that describe the deviation of particle drag fromStokes law (i.e., Fst=6;Tapu(v-vp)).

Saffman provided an expression for the steady lift forceinduced by the local shear flow of a viscous fluid on asphere.[11,12] It was systematically derived that a sphericalparticle of radius a moving along the x-axis such that vp(up, 0, 0) in a fluid shear flow field v = (u(y), 0, 0)experiences a force directed along the y - axis which can becalculated as follows:

FSa = 6.46 a2(u up aSign auey (2)where

ay ay8)ey is a unit vector directed along the y - axis (Fig. 1) and pis the density of the fluid. It has previously beendetermined[ 1,2] that relatively fast aerosol flows throughmicro-capillaries may be treated as a steady laminarPoiseuille flow when the capillary length is much greaterthan the entrance length which is specifically -0.2 cm. Inthe parameter range considered in this study, gascompressibility essentially does not affect the aerosol flowand so the incompressible gas approximation is employed.Thus, the model is represented as aerosol particles movingin a steady laminar Poiseuille incompressible flow througha slowly converging or diverging micro-capillary. For theseestimates, the micro-capillary is constrained such that theradius changes linearly with length according to Equation 3as follows:

R(x) = R1 -(R1 - R2)x L (3)where R1 and R2 are the inner radii at the beginning and theend of the micro-capillary, respectively. Given the gasvelocity distribution, v = ue, + ver, one may calculate theparticle trajectories in such a flow where x is a coordinatealong the capillary, and r is a radial coordinate. Theequation of motion of a single particle follows (Eq. 4):

4Ta3p- dv = FSt +FSa (4)

where Fst and Fsa are the Stokes and the Saffman forcespresented in the Eqs (1) and (2), respectively. Furtherderivation (for details, see [1,2]) results in the equations ofparticle motion as shown in Eq. 5. This model is useful forestimating the trajectories of aerosol particles in slowlyvarying micro-capillaries of any shape.

To begin modeling particle trajectories, the internalgeometry of the alumina micro-capillaries (Gaiser Tool,Ventura, CA) were measured by x-ray imaging. An internalradius that changes linearly with length was confirmed.Next, the trajectories of particles with and without theSaffman force acting on them were calculated (Eq. 5) usingthe following parameters: R1 400 pim;R2= 50 ptm; nitrogen carrier gas flow Q = 40 cm3/min;

d3& drFEp _ pdt dt- =v

di CKfI dtp

dt CKf ((

(5)

RI -R2 I TktUmax (0)R(xP) = L

-Lxp'EL

IS' =R

aerosol particle density p. = 2000 kg/m3; and, particleradius a = 0.25 ptm (Fig. 2). It was found that all particlesof this size migrate towards the center line forming a focalspot immediately following the capillary exit. Thismigration is caused by both the geometrical convergence ofthe capillary and the Saffman force.

200

100ci

ci0

1 000

4a

-wan-10 -5 0 5 10

distance from end of micro-capillary (mm)

Figure 2. Trajectories of particles with different initialpositions with and without the Saffman force acting on

them: solid lines - with the Saffman force, dotted lines -

without the Saffman force; R1 = 400 ptm and R2 = 50 ptm; pp= 1600 kg/m3; t= 1.67x10-5 Ns/m2; p = 1.16 kg/m3(nitrogen); umax = -100 m/s; a = 0.25 ptm; and, Q= 40cm3/min.

A geometrical convergence is clearly observed whenthe particle trajectory calculations employ only inertia andthe Stokes drag (i.e., red dotted lines in Fig. 2). In thisinstance, the particles flow to a fine focal spot about 3 mmbeyond the capillary orifice. The contribution of theSaffman force noticeably changed the position of the focalspot (i.e., blue/solid lines in Fig. 2) which now resides just1.7 mm past the exit orifice of the nozzle.

These theoretical results were compared toexperimental observations of an aerosol beam exiting a

micro-capillary. While the details of the experimental setupand procedure can be found in Akhatov et al,[1] theexperimentally determined aerosol beamwidth as a functionof distance from the capillary exit is compared to thetheoretical model predictions in Fig. 3. It is apparent thatwhile the theoretical model that uses only the Stokes forcedoes not provide a strong correlation, the model thatconsiders both Stokes and Saffman forces closely relates tothe experimental data.

\capillary wall

IK-

40 -

35-

E 0125 3

~20E.10

105

1 1.5 2 2.5 3 3.5

distance from micro-capillary exit (mm)

Figure 3. Plots of beamwidth versus distance using (1)experimental data, (2) theoretical data with the Saffmanforce, and (3) theoretical data without the Saffman force.The following nozzle parameters are pertinent: L = 19.05mm; total gas flowrate Q = 40 cm3/min; pp = 1600 kg/m3;t= 1.67x10-5 N S/m2; p = 1.16 kg/m3 (nitrogen); particleradius a = 0.25 ptm (theoretical); and, nozzle exitdiameter = 100 ptm.

3. Collimated Aerosol Beam - Theoretical ConceptGiven the differences in curves 2 and 3 of Figure 3, it is

apparent that optimization of an aerosolfocusing apparatusmust take into account the Saffman force. However, anaerosol collimating apparatus may offer significantadvantages in the area of materials deposition. Forcollimation, we consider a narrow aerosol beam that is notfocused at some point next to the capillary tip, butcollimated such that all the particles have their velocityvectors parallel to the capillary axis. In an ideal system, allparticles would by traveling immediately adjacent andparallel to the capillary axis line. For such a collimatedaerosol beam (CAB), the width of a deposited line does notdepend critically on the distance between the capillary tipand the substrate. In addition, the line spreading due toparticle deflection in the radial shear flow in the vicinity ofthe substrate should be minimized. While the concept ofcollimating aerosol beams has been previously presented,[14-17] these reports did not employ micro-capillaries butinstead used "aerodynamic lens" systems - series of thinplates with about millimeter size pinholes.

These initial results that utilized a single nozzle servedas the basis for further consideration of how a multiple-nozzle system may allow the realization of beamcollimation. Several different scenarios were consideredusing the previously discussed mathematical model. Uponcompletion of this exercise, it was found that a three-nozzlesystem should serve to promote collimation as is nowdescribed.

To preclude focusing of the aerosol beam immediatelyafter exiting the capillary and to collimate the beam, alinearly converging micro-capillary (MC1) is connected inseries with a linearly diverging micro-capillary (MC2) suchthat radius of the end of MC1 matches with the radius ofthe beginning of MC2: (R2(MC]) = R1(MC2)). Collimation ofthe aerosol is promoted due to the following two reasons:

firstly, gas flow diverging in MC2 drags aerosol particlesaway from the center line due to the Stokes force; and,secondly, the gas flow decelerating in MC2 will push theparticles to the region where the fluid velocity is lower (i.e.,away from the center line) due to the Saffman force. If thegeometrical parameters of MC1 and MC2 are selectedproperly, one can expect that the aerosol beam will becomealmost perfectly collimated inside MC2. However, thevelocity of the aerosol beam at the MC2 exit will be so lowthat the aerosol beam will easily change its direction whileapproaching the substrate. To prevent the aerosol beamdeflection upon exiting MC2, a third micro-capillary(MC3), linearly converging, is connected to the linearlydiverging micro-capillary (MC2) such that the radius of theend of MC2 matches with the radius of the beginning ofMC3: (R2(Mc2) = R(Mc3)). In the converging MC3, the gasflow velocity increases and accelerates the aerosol particlesalong the capillary center line.

3.1. Collimated Aerosol Beam - ExperimentalThe efficacy of the proposed three-nozzle system was

demonstrated experimentally using aerosol particlescomposed of silver ink acquired from Harima [18] andNano-Size Inc.[19] diluted to 5000 with an ethylene glycol /water 50:50 mixture. The nano-inks were atomized via amodified Sonaer Sonozap 241PG atomizer operating at 2.4MHz. A glass ink vial was partially submersed in a 25 Ctemperature-controlled water bath with nitrogen gas (4N5)passing through the vial containing about 1 ml of ink toproduce an aerosol-rich flow.

A sheath flow was inserted annularly around the aerosolflow with the merged stream directed through the three-nozzle CAB system. The sheath flow prevents capillaryclogging by minimizing the initial aerosol beamwidth andby focusing the aerosol particles. The particles exit theCAB system with velocity -100 m/s. Coupling the CABdeposition system with a heated x-y stage allows depositionof lines and patterns as shown schematically in Figure 4.

Figithat utilizes a focused aerosol beam.

The same theoretical model that was developed todescribe the aerosol flow through a single MC was thenapplied for the three-nozzle system. The experimentalmethod for collecting beamwidth data was nearly identicalto that employed for the single MC with detail presentedpreviously. [1,2] The following geometrical parameterswere set as limits in this CAB study:

R(MCl) =4OO m Rid) =4R00 ) = 75pm,1 t

R(MC2) = R(MC3) =4O m, R( = 5O0m,2 2(M3

L(McI) - L(MC2) = L(Mc3) - 19.05 mm.

The theoretically calculated beam width is shown inFigure 5 with the particle radius set to 0.25 ptm. Accordingto the simulation (Fig. 5), the particles are focused to about500 of the total diameter by the time they reach the thirdnozzle. It can also be seen that the experimental beamwidthexiting the CAB system is indeed quite narrow.

400 D=1 50pm A D=1 OOpjm1-1300 ~1Ci MC2 MC3

2100

1 00kE-400

-50_ -403 2 -0

:200CL-300

-400-50 -40 -30 -20 -10 0 10distance from end of micro-capillary (mm)

Figure 5. The theoretically calculated beam width of anaerosol flowing through the CAB system where total gasflowrate Q = 40 cm3/min, pp = 1600 kg/m3; particle radiusa = 0.25 ptm, t= 1.67x10-5 N s/m2; p =1.16 kg/m3 N2 (g).

When comparing the modeled data for the three-nozzleand single micro-capillary systems, the beamwidth leavingthe CAB nozzle is thinner and more collimated with a

minimum beamwidth of 1.9 pm at 2.0 mm past the CABsystem exit orifice compared to 5.0 ptm at 1.8 mm past thesingle MC exit. In CAB, the beamwidth remains small even

to 5 mm past the nozzle exit where a width of only 12 pmis observed.

An experiment to compare the direct-write performanceof the single MC design and the CAB system was devisedwhereby lines were printed on a substrate with Harima ink,an atomizer flow of 25 cm3/min, a sheath flow of 15cm3/min, a stage velocity of 30 mm/s, and a stand-offdistance of 2 mm. The linewidths produced by the singleMC design and the CAB system are compared in Figure 6.The single MC design created lines approximately 31 ptmwide and the CAB system produced lines approximately 11

ptm wide [Note: flow parameters were not fully optimizedto produce the thinnest lines possible; rather they were setto a commonly used lab standard].

Ea1~15i 10

P 52

00

ConventionalAeroso SprayJ

CAB

51 2 3 4Distance from micro-capillary exit (mm)

Figure 6. Experimental results illustrating the beamwidthof conventional aerosol spray (i.e., single MC design) andCAB (i.e., three-nozzle design).

Experimental results from the linewidth comparison(Fig. 6) provide convincing evidence that the beamwidth ofthe CAB system is thinner than that of the single MCdesign. The lines written by the CAB system wereapproximately 1/3 the width of those written by the singleMC design under identical processing conditions.Additional improvements to the design of the CAB systemare anticipated after the effects of aerosol particle sizedistribution, particle density, and velocity field exiting themicro-capillary are better characterized and understood.

4. Direct-write FabricationDirect-write film deposition encompasses processes

whereby no additional processing steps (e.g.,photolithography) are required to produce functionalelectronic materials such as interconnects, diodes andtransistors. Direct-write includes conventional approaches(e.g., ink-jet printing, airbrush spray deposition) as well asmore esoteric methods such as matrix-assisted pulsed laserevaporation direct-write (MAPLE-DW). Direct-writefabrication of electronic components, especially sensors, isan area that has been active for years.[20] MasklessMesoscale Material Deposition (M3DTM) utilizes a focusedaerosol beam (aka aerosol jet) deposition system that isnow being commercialized by Optomec.[21] Thistechnology allows the deposition of 25 ptm-wide lines butsuffers from materials utilization and overspraylimitations.[21] The aforementioned process improvements(i.e., CAB) are applicable to direct-write as is nowexplained.

4.1. Collimated Aerosol Beam-Direct WriteCAB-DW is realized by simply placing a substrate

under the nozzle exit orifice of a CAB system. Thedynamics of CAB-DW printing are similar to otherprocesses wherein a careful balance is struck betweenstandoff distance, growth rate, surface wetting, solventevaporation and deposition temperature in order to realize adeposit of uniform composition and morphology.

Upon appropriate development of the precursor inks,CAB-DW will be enabling for the manufacture of flexiblemicroelectronics such as radio frequency identification(RFID), disposable wireless sensors and large-areadisplays/signs. The successful development of adequatetheoretical models and numerical codes for aerosol

I

dynamics at the micro-scale will have a significant impacton the progress of these technologies.

It is interesting to note that for the case described above(Section 2.3), the Saffman force has a deleterious effect ondirect-write deposition. If one uses a micro-capillary todeposit small features on a substrate (see Fig. 2), theSaffman force causes the particle trajectories to focuscloser to the capillary exit and diverge much faster after thefocal point than they would in the case of simplegeometrical focusing due to the Stokes force only.Consequently, precise control of the tip-to-substratedistance is required to assure the beam focal pointintersects the substrate surface to give a continuous line ofthe desired width. In practice, this represents a formidableengineering task and provides a basis for which CAB-DWmay provide marked benefits (vide infra).

4.2. CAB-Direct Write - Printing Ag over StepsThe beamwidth data presented in Section 2.5 (vide

supra) was an experimental demonstration of theadvantages that are realized when the effects of theSaffman force are appropriately considered. To betechnologically-relevant to the field of printed electronics,however, the phenomenon must be reproduced in thin filmform. Toward that end, an experiment was designedwhereby both the single-MC and CAB-DW systems wereused to print silver ink onto a substrate. This side-by-sidecomparison utilized identical processing conditions (i.e.,silver ink composition, deposition temperature, etc.) with

the only difference being the nozzle system where one andthree nozzles were employed for the single-MC and CAB-DW methods, respectively. The substrates employed inthese studies were Kapton that was cleaned with IPAimmediately prior to deposition. In addition, each substratecontained vertical relief in the form of a one mm step.

The results of this study are shown in Figure 7. Thegeneral experimental concept is illustrated in the inset(upper left) where a single-micro-capillary head is showntraversing the substrate step. Optical microscopy for boththe single-MC head (Figs. 7a, 7b) and the three-nozzleCAB-DW head (Figs. 7c, 7d) where nozzle-to-substratedistance was either 2.0 mm at the top of the step (Figs. 7a,7c) or 3.0 mm below the step (Figs. 7b, 7d). Theimprovements afforded by CAB-DW system are clearlyillustrated where linewidths of 11 and 16 ptm are observedbefore and after the step in the substrate respectively. Byway of comparison, the single-MC system produces Aglines that are 30 and 47 ptm wide before and after thesubstrate step. It is appropriate to recapitulate that identicalprinting parameters were used in the execution of theexperiments used in this study.

4.3. CAB-Direct Write - Printing SiliconTransistors provide the basis for the active components

in printed electronics such as inverters, ring oscillators andcharge pumps. Low-temperature atmospheric-pressuredeposition routes to semiconductors with good electricalproperties will be required to best utilize the advantages of

b. Cl.Figure 7. Optical microscopy data for printed Ag lines produced via aerosol deposition of nanoparticle inks using the followinghardware: a single-micro-capillary deposition head with a standoff distance of (a) 2.0 mm and (b) 3.0 mm; and, a three-nozzleCAB-DW head with a standoff distance of (c) 2.0 mm and (d) 3.0 mm. The average linewidth of the deposited features areshown in blue while the white arrows indicate the traverse direction of the deposition heads. Each technique used the sameprinting parameters. The inset (top left) is a cartoon that shows the nozzle orientation with respect to the substrate.

roll-to-roll technology where inexpensive, polymericsubstrates would be used. While polymers such aspentacene and poly-3hexylthiophene (P3HT) have beenused as the active layer in flexible electronics, printed

routes to inorganic semiconductors offers the potential forhigher carrier mobility which manifests value in the formof lower power usage and longer operational life fordisposable devices. We have recently reported initial

results using cyclohexasilane (Si6H12) as a liquid precursorto amorphous silicon.[4] Functioning transistors and diodesbased upon amorphous silicon (a-Si:H) were prepared byspin-coating Si6H12 followed by various light and/orthermal treatments. While present ink chemistries producea-Si:H with a high resistivity (i.e., > 106 Q.cm), severaldoping strategies are under development at NDSU.Assuming silicon-based materials with good electricalproperties will be developed, there may be significant costadvantages associated with the ability to controllablydeposit the semiconductor in a metered fashion.

Toward that end, we report here the initial results forCAB-DW of a liquid silane ink. Figure 8 shows opticalmicrographs of the resultant solids when a Si6H12-based inkis deposited by CAB-DW. The CAB-DW head used in thisstudy had three nozzles with inner diameters of 100 ptm,125 pm and 100 pm for the converging (MCI), diverging(MC2) and converging (MC3) nozzles

respectively. Control of the substrate translation is aplanned system upgrade, with the stage moved by hand forthis trial, with linear motion intented to be at a constantvelocity. Carrier and sheath gas flow rates were set to 15

cm3/s and 20 cm3/s, respectively. A microscope glass slidewas used as the substrate. The CAB-DW system was setupto produce lines with minimum linewidth and multipleparallel lines of Si6H12-based ink spaced at 20 ptm and 30ptm. [Note: Si6H12-based inks are pyrophoric and should behandled in an inert gas glovebox with an appropriateconsideration of the hazards involved.] Immediately afterprinting, the lines were exposed to UV light for 20 mingiving translucent deposits with non-uniform edges (Fig.8a). After the initial optical microscopy characterization,the films were placed into an inert gas (N2) tube furnaceand then subjected to the following thermal treatment:ramp to 350 °C in 30 min; soak at 350 °C for 30 min; and,quenched to room temperature by removal from the tube.Figure 8b shows the printed silicon lines after the thermaltreatment. Additional characterization in the short-term willinclude profilometry, vibronic analysis (i.e., FTIR) andphase identification (i.e., Raman). Future efforts willinclude printing ring oscillators where the semiconductor aswell as the metallic contacts will be deposited by CAB-DW.

[

Figure 8. Optical microscopy photographs of printed features produced by CAB-DW of a Si6H12-based ink after (a) UVtreatment (20 min) and (b) UV treatment (20 min) plus thermal treatment (350 °C/30 min).

5. ConclusionsAerosol flow through a capillary of diameter -100 ptm

and length -1 cm at a relatively high velocity of 100 m/swas shown to be markedly affected by the Saffman force.This force causes the aerosol particles to migrateperpendicular to the center line of the capillary. It wasdemonstrated both theoretically and experimentally that asystem consisting of a sequence of three slowlyconverging/diverging/converging micro-capillaries affordsa collimated aerosol beam in which aerosol particlesremain close to the capillary centerline. When locatedadjacent to a surface, this "aerosol gun" can be used forprinting electronic materials and the approach is termed

CAB-DW. The present CAB-DW system has not beenoptimized and it is anticipated that tuning the parameters ofthe slowly converging/diverging micro-capillaries will leadto improved collimation and reduced beamwidths.

The ability to print liquid silane-based features withlinewidths <10 pm by CAB-DW is encouraging. Futuremodifications to the system will include computerizedmotion control to improve edge uniformity. Silicon-basedelectronic materials (e.g., a-Si:H, recrystallized Si) areanticipated upon optimization of Si6H12 ink chemistries.

AcknowledgmentsThis material is based on research sponsored by the

Defense Microelectronics Activity under agreementnumber H94003-06-2-0601. The United States Governmentis authorized to reproduce and distribute reprints forgovernment purposes, notwithstanding any copyrightnotation thereon.

References and Footnotes1. Akhatov, I. S., Hoey, J. M., Swenson, 0. F., D. L.

Schulz, "Aerosol focusing in micro-capillaries: theory

7

and experiment", Journal of Aerosol Science (submittedAugust 23, 2007).

2. Akhatov, I. S., Hoey, J. M., Swenson, 0. F., D. L.Schulz, "Aerosol flow through a long micro-capillary:collimated aerosol beam", Microfluidics andNanofluidics (accepted for publication October 29,2007).

3. Hoey J. M., Akhatov I. S., Swenson 0. F., Schulz D. L.(2007) Focusing of Aerosol Particles. U.S. ProvisionalPatent Application # 60/956,493.

4. Han, S., Dai., X., Loy, P., Lovaasen, J., Huether, J.,Hoey, J. M., Wagner, A., Sandstrom, J., Bunzow, D.,Swenson, 0. F., Akhatov, I. S., Schulz, D.L.,"PrintedSilicon as Diode and FET Materials - PreliminaryResults" Journal of Non-crystalline Solids (accepted forpublication October 10, 2007).

5. Israel, G. W., S. K. Friedlander, "High-speed beams ofsmall particles", Journal of Colloid Interface Science,Vol. 24, pp. 330-337, 1967.

6. Dahneke, B., S.K. Friedlander, "Velocity characteristicsof beams of spherical polystyrene particles". Journal ofAerosol Science, Vol. 1, pp. 325-339, 1970.

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