32 Optics & Photonics News � June 20011047-6938/01/06/0032/4-$0015.00 © Optical Society of America

Micro-OpticsFabrication

by Ink-Jet Printing

W. ROYALL COX,TING CHEN, and DONALD J. HAYES

W ith hand assembly currentlyconstituting a major portion ofthe costs of manufacturing

many optoelectronic components, devicesand systems, process automation and in-tegration will become increasingly impor-tant factors in the competition for world-wide market share. The ink-jet printingmethod of micro-optics fabrication ad-dresses these factors because it is a fullyautomated, data-driven process withwhich micro-optics may be written direct-ly onto optical substrates and componentssuch as diode lasers, optical fibers andwaveguides.1,2 As a low-cost, additiveprocess capable of dispensing volumes aslow as 20 picoliters, the ink-jet printingmethod is also being employed in otheroptical applications such as fabrication oforganic LED displays3 and optical fiberbiochemical sensors.

In the direct-writing of micro-opticswe have used the piezoelectric-actuator-based, “drop-on-demand” (DOD) mode4

of ink-jet printing, in which a volumetricchange in the fluid within a printing de-vice is induced by the application of a volt-age pulse to a transducer coupled to thefluid. This volumetric change causes pres-sure/velocity transients to occur in the flu-id, which are directed to produce a dropletfrom the orifice of the device. Plano-con-vex microlenses of spherical profile may beprinted onto a substrate by dispensingdroplets of optical material at target sitesat frequencies up to 8 kHz, as indicatedschematically in Fig. 1. The printing maybe accomplished using either a step-and-print or print-on-fly process. In the print-on-fly process, droplets are dispensed con-tinually as the substrate rastors back andforth under the print head.

The materials best suited for micro-op-tics fabrication are typically UV-curing,100%-solids solutions of optical mono-mers and prepolymers, which are heatedin the print head to the temperature re-quired to reduce their viscosity below the -40 centipoise level required for dropletformation by this method. Droplet diame-ter is controlled by the diameter of the ori-fice of the dispensing device and is volu-metrically stable over time to better than2%. One of the characteristics of ink-jetprinting technology that makes it generallyattractive as a precision fluid micro-dis-pensing method is the repeatability of theprocess. The temporal, spatial and volu-metric stability of the DOD printingprocess is illustrated in the 1-s photo-

graphic exposure in Fig. 2 by the clarity ofthe superimposed images of 2000 droplets,using stroboscopic illumination at thedroplet generation frequency.

Printed micro-optical elementsArrays of refractive microlenses with cir-cular substrate footprint are fabricated byprinting the number of droplets requiredfor the lenslet volume at predeterminedlocations on a target substrate. Micro-lenses ranging in diameter from 20 �m to 5 mm have been fabricated by thismethod. Substrate surface treatmentprocesses enable control of microlensspacing and diameter within an array tothe 1-µm level, of focal lengths to 1%, andof sphericity to a quarter wavelength at1550 nm. Microlens speeds may be variedat a given diameter over a wide range(f/1.5 – f/10), even within the same array.

Microjet-printed arrays of plano-con-vex microlenses are now being used forfree-space optical interconnects in mas-sively-parallel, smart-pixel-based photon-ic switches, which are under developmentin conjunction with the DARPA (DefenseAdvanced Research Project Agency) VI-VACE (VCSEL-based Interconnects inVLSI Architectures for Computational En-hancement) program.5 These switches arebased on hybrid integration of VCSEL/photodetector arrays, Si ASICs, and mi-crolens arrays.6 A smart pixel consists of aVCSEL and an adjacent detector with twomicrolenses above for collimating the laseroutput beam and focusing a return beaminto the detector element. Photographs ofthis switch module and of the printed mi-crolens array incorporated in it are shownin Fig. 3. These arrays are printed in 32chip patterns onto 3-inch diameter, thinquartz wafers, in which each chip consistsof two each 16 x 16 identical arrays of 250-�m-diameter, 60-�m-sag microlenses,giving a total of 16,384 microlenses. Themicrolenses in each array are printed on 500-�m centers, and the two arrays areoffset from each other by 250 �m along adiagonal. Printed microlenses were select-ed for this application based on the greatercoupling efficiency and the wavelength in-dependence of refractive lenslets com-pared to diffractive ones. Another factorwas the greater thermal durability of opti-cal epoxy compared to the photoresistused in photo-lithographically fabricatedrefractive lenslets, which enables a solderre-flow step at 200°C after assembly of the modules.

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June 2001 � Optics & Photonics News 33

Figure 1. Schematic of drop-on-demand ink-jet printing of microlens array.

Figure 2. 50-�m droplets of fluid being emit-ted from a drop-on-demand printing device at 2000 Hz.

Figure 3. Smart-pixel array module (top) andprinted 250-�m diameter microlens array inmodule with VCSELs under 6 lenslets turned on(bottom). (Photos courtesy of Y. Liu & A. Cox ofHoneywell.)

Multimode ridge waveguidesof spherical cross-section may beprinted in arbitrary patterns fromdata files by depositing, along aline, droplets of optical materialhaving higher index of refractionthan the underlying substrate. Toprint a line of constant width, thespacing between droplets must besmall enough to provide coales-cence of adjacent deposits but notso large as to create discontinuitiesin the printed lines. An example ofa waveguide structure printedfrom data onto a glass substratewith optical thermoplastic havinga higher index than the glass isgiven in the photo in Fig. 4. By ex-tending to two dimensions theprocess of printing adjacentdroplets of material, anamorphicmicrolenses having non-circularsubstrate footprints may be fabri-cated by direct writing. Examplesinclude square, rectangular/cylin-drical and elliptical footprints.7

Hemi-elliptical microlenses are ofspecial interest, because they canbe printed to have a specified de-gree of difference in focal length intwo orthogonal directions. For ex-ample, the four hemi-ellipticalmicrolenses in Fig. 5, formed byprinting six each 35-µm diameterdroplets of optical thermoplasticon 30-µm centers, bring collimat-ed light from below the glass sub-strate initially to a line focus at 218µm above the surface by the cur-vature along the minor axis, thento an orthogonal line focus at 921µm by the curvature along themajor axis. The difference be-tween these “fast” and “slow” focallengths may be fine-tuned by ad-justing droplet spacing and thedifference in temperature betweenthe printing device and the sub-strate. One application for suchprinted hemi-elliptical microlens-es would be in circularization, col-limation and astigmatism correc-tion of anisotropically diverginglight from sources such as edge-emitting diode lasers.

Printing ontooptical componentsA commonly used approach forcoupling power from an array of

edge-emitting diode lasers intoan optical fiber is to circularizeand collimate the beam fromeach emitter using, for example,a prefabricated microlens arrayor crossed cylindrical lenses, andthen to focus all of the collimat-ed beams into the fiber with amacrolens.8 The cost of per-forming the beam-shaping stepcould potentially be significantlyimproved by ink-jet printing ar-rays of microlenses in situ abovethe emitter facets of the diodelaser bars; the lenslets could bemade hemi-elliptical to assistwith collimation or astigmatismcorrection.

A question that arises in us-ing printed optical epoxy lensletsfor this application is the degreeto which they can withstand sus-tained operation at high powerlevels. To find out, we printed200-�m-diameter hemisphericallenslets with an optical epoxyformulation at the 24 emitterpositions of a 20 W, 784-nmdiode laser bar, as pictured inFig. 6, then cured them by UV il-lumination and baking at 110°C.The microlenses were printed di-rectly above the emitters onto acover-glass slide which had beenepoxied onto the face of the de-vice with the same material, inorder to provide the offset need-ed to put the back focal points ofthe lenslets at the emitting facetplane. In this initial experiment,alignment of the microlenses tothe emitter positions was donepassively by indexing from theedge of the bar by the specifieddistance and assuming the de-sign pitch, but in the future, ac-tive alignment to individualemitters could be used with anIR vision system. The thermaldurability of the printed mi-crolenses was demonstrated byrunning the laser bar in the con-tinuous mode at 20 W and ob-serving a drop in total outputpower of only 0.3 W after 45 h. Itwas found that near collimation(3° divergence) was achieved inthe perpendicular plane, andthat the existence of this residualdivergence was due to the mi-

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34 Optics & Photonics News � June 2001

Figure 6. 200-�m-diameter spherical microlenses print-ed above emitter positions of 20 W diode laser bar, shownfrom above (top) and in profile (bottom).

Figure 5. Hemi-elliptical microlenses, 284-�m long,shown (left to right) in substrate, and in "fast" and "slow"focal planes.

Figure 4. 1x16 waveguide splitter with 100-�m widebranches, printed from data file.

Figure 7. 70-�m-diameter microlens printedonto tip of single-mode optical fiber for in-creasing numerical aperture.

crolens focal points being about30 �m behind the emitting facets.

It has been shown that form-ing a spherical microlens on thetip of an optical fiber can signifi-cantly increase the efficiency ofcoupling of light from an edge-emitting laser diode, thereby po-tentially reducing laser-fiberalignment time while increasingoverall transmission efficiency.Here the coupling efficiencyvaries inversely with both the ra-dius of curvature of the microlensand the magnitude of its mis-alignment to the core of the fiber.9

An example of a hemisphericalmicrolens microjet printed ontothe end of a single mode telecomfiber is shown in Fig. 7; here,the lenslet diameter of 70 �m was achieved by depositing and UV-curing one 50-�m-diameterdroplet of optical epoxy, after ap-plying a de-wetting coating to thetip of the fiber.

Another optical application ofmicrojet technology is the fabri-cation of multifunctional fiberoptic biochemical sensors, withpotential use in clinical diagnosis,manufacturing process control,and environmental monitoring,for example. If the UV-curing op-tical epoxies are adjusted to pro-vide enhanced porosity and aredoped with biochemical indica-tors, they may be printed into patterns ofsensor elements onto the tips of imagingfiber bundles, providing a sensor configu-ration as shown in the photographs in Fig.8. Biochemical fiber optic sensors in usetoday typically consist of one indicatorchemistry attached to a single fiber, wherethe indicator chemistry is designed tochange its optical properties (e.g., fluores-cence or absorption) quantitatively in re-sponse to a target ligand under illumina-tion of a suitable wavelength. The uniqueopportunity provided by microjet print-ing technology in this arena is the low-cost, reproducible fabrication of largenumbers of uniformly sized sensor ele-ments consisting of different indicatorchemistries on the same optical fiber bun-dle, in order to provide multifunctionalityin a single sensor. Signals from differentindicator elements are distinguished byspatial and spectral filtering in the detectorsystem. Initial characterization of fluoresc-

ing intensities from the seven elementsprinted with the same material on fibers inthe configuration of Fig. 8 have shown anelement-to-element variation of less than5%.10 Using multiple print heads with dif-fering indicator chemistries, such multi-functional fiber-optical array sensorscould be manufactured with microjetprinting technology at very high through-put rates and low materials costs.

Future development of optics-jet technologyIt may be concluded from the advancesachieved to date in the development of op-tics-jet technology that the microjet print-ing method of micro-optics fabricationprovides the potential for reducing costsand enhancing assembly process integra-tion in a number of optoelectronics man-ufacturing applications. Such applicationsinclude: microlens arrays for VLSI smart-pixel and micro-opto-electrical-mechani-

cal switches; fiber and fiber-rib-bon collimators; telecom trans-ceivers; and high-power, diode-laser-array fiber-delivery andamplifier-pumping systems. Fu-ture development of the tech-nology will focus on build-ing prototype microjet-printedcomponents and assemblies fortesting in these applications and,in the process, on demonstrat-ing the capabilities for precisionand reproducibility required toincorporate micro-optics print-ing, alongside solder-bumpprinting,11 into optoelectronicscomponent and system manu-facturing lines.

AcknowledgmentsThis work has been supportedin part by the U.S. Air Force Re-search Labs and by DARPA viathe U.S. Army Research Office,the U.S. Army Aviation and Mis-sile Defense Command, and theWright Patterson Air Force Basethrough subcontract to Honey-well. Outside collaborators in-clude: Allen Cox, Yue Liu andKlein Johnson of HoneywellTechnology Center, as well asBill Coleston and Steve Brownof Lawrence Livermore NationalLaboratories. Contributors inMicroFab include Hans-JochenTrost, Michael Grove and Rick

Hoenigman. To all we express thanks.

References1. W.R. Cox et al., IMAPS International Journal of Mi-

crocircuits & Electronic Packaging, 20 (2) 89-95,(1997).

2. Y. Ishii, S. Koike,Y.Arai, and Y.Ando, Proc. SPIE3952, 364-74, (2000).

3. T.R. Hebner et al., Appl. Phys. Lett. 72 (5) 519- 21,(1998).

4. D.B.Wallace, ASME Publication 89-WA/FE-4, (De-cember 1989).

5. Y. Liu, et al., SPIE Photonics West Symposia, paperCR76-05, (January 25, 2000).

6. Y. Liu, OSA Topical Meeting on Spatial Light Modula-tors,Technical Digest, Paper StuB1, pp. 64,April 13-16, Snowmass, Colorado, (1999).

7. W.R. Cox et al., Proc. SPIE 2687, 89-98, (1996).8. W. Chen, C.S. Roychoudhuri and C.N. Banas, Opt.

Eng. 33 (11) 3662-9, (1994).9. S. Gangopadhyay, S.N. Sarkar, J. Opt. Commun., 19

(6) 217-21, (1998).10. B.W. Coleston et al., Proc. SPIE, 3911, 2-9, (2000).11. D.B.Wallace and D.J. Hayes, IMAPS International

Journal of Microcircuits and Electronic Packaging,21 (1), 1998.

W. Royall Cox, Ting Chen, and Donald J. Hayes arewith MicroFab Technologies, Inc. W. Royall Cox can becontacted via e-mail at rcox@microfab.com.

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June 2001 � Optics & Photonics News 35

Figure 8. Array of 80-�m-diameter hemispheri-cal indicator elements printed onto 480-�m di-ameter fiber-optic bundle, shown from above (top)and in profile (bottom). (Fiber bundle courtesy of B.Coleston and S. Brown of Lawrence Livermore Nation-al Laboratories.)