Computer-generated holograms ~CGH’s!, diffractiveoptical elements, and other micro-optics are currentlyattracting increasing interest in their wider use in op-toelectronics.1 Current research is not only associ-ted with hologram design optimization but also withhe development of new fabrication methods aimed atchieving greater simplicity and flexibility and lowerabrication costs. The well-established microfabrica-ion methods combining a number of processing stepsuch as lithography and reactive ion etching,2 or ion
exchange3 and thin-film deposition,4 are inherentlyomplex and expensive. In several cases involving
When this research was done, the authors were with the Laserand Applications Division, Institute of Electronic Structure andLaser, Foundation for Research and Technology-Hellas, P.O. Box1527, Heraklion 71110, Greece. S. Mailis is now with the Opto-electronics Research Centre, University of Southampton, High-field, Southampton SO17 1BJ, UK. I. Zergioti is now with theMax Planck Institut fur Biophysikalische Chemie, P.O. Box 2841,37018 Gottingen, Germany. P. Papakonstantinou is now withNorthern Ireland Bio-engineering Research Centre, School of Elec-trical and Mechanical Engineering, University of Ulster, New-townabbey, Co. Antrim BT37 OQB, Northern Ireland, UK. N. A.Vainos’s e-mail address is [email protected]
Received 22 June 1998; revised manuscript received 2 November1998.
nonplanar, high-curvature segmented-surface or high-strength material’s processing, these conventional butwell-established schemes have been unable to producethe required results. Furthermore, in the context ofmicro-electro-mechanics,5 the introduction of novel op-tical concepts would certainly lead to further techno-logical advances.
Direct laser-based methods for the fabrication ofmicrostructures provide an alternative, relatively sim-ple, and cost-effective approach. New developmentsin optoelectronics and micro-electro-mechanics andthe prospective mass-production needs justify an in-creasing effort toward the investigation of such newfabrication methods aimed at versatile, cost-effective,high-quality structure development. On the onehand, high-intensity UV excimer-laser radiation hasbeen utilized to demonstrate the direct etching ofsurface-relief diffractive microstructures by selectiveablation of materials6 and also later by waveguide ex-cimer lasers with high pulse repetition rates.7 On theother hand, the potential for obtaining the selectivemicrodeposition of relatively large metallic structuresonto a surface has also been demonstrated8,9 by thedirect writing of 50-mm-wide Cu lines with nanosecondexcimer and Nd:YAG laser pulses under high vacuum~1026 mbar!. The deposition of 100-mm-wide pat-terns of superconducting thin films with nanosecondArF excimer and Nd:YAG lasers has also been report-ed.10,11
Both methods may be applied either in a direct
10 April 1999 y Vol. 38, No. 11 y APPLIED OPTICS 2301
2
mask-projection pattern-transfer mode or in a pixel-by-pixel step-and-repeat operation. In the lattercase a slit that is usually rectangular is projected inraster across the surface of the receiving substrate toproduce the required pattern.
Laser ablation by nanosecond excimer-laser pulsesis seen to exhibit significant thermal effects,12,13 nat-urally depending on the physical properties of thetarget material. Comparative studies14,15 concern-ing laser-pulse duration verify that subpicosecond la-ser radiation may be more suitable for suchapplications. Ultrashort UV laser pulses minimizethermal effects,16,17 because absorption and thermal-diffusion lengths are kept to a minimum. In thisregime, multiphoton electron ionization and electron-lattice energy coupling dominate, whereas the ava-lanche breakdown process initiated by single ionizedelectrons plays a secondary role. Depending alwayson the thermophysical properties of the materials,the melting zone front is greatly reduced with a cor-responding increase in the vaporization zone, even atnear-IR laser wavelengths.18
The effects outlined above lower the ablationthreshold values and further reduce their statisticalspread. Geometric spatial-filtering methods havethus produced high-quality etching of even submi-crometer regular periodic grating structures.19 Fur-thermore the selective microdeposition technique hasbeen advanced further, and very recently submi-crometer features have been successfully depositedby ultrashort excimer-laser pulses in low-vacuumconditions.20
In this research we extend our discussion on theapplication of the microablation methods of ultra-short pulsed lasers and demonstrate two reliablesingle-step microfabrication modes of operation. Byrelying on purely physical processes, one removesmaterial selectively from the substrate surface, leav-ing behind a precisely etched surface-relief pattern.In the second and complementary operation the ma-
Fig. 1. Experimental layout for direct excimer-laser microfabricaoptical fibers.
302 APPLIED OPTICS y Vol. 38, No. 11 y 10 April 1999
terial ablated from a thin-film source is transferredprecisely onto the substrate, producing again asurface-relief printed pattern with submicrometerresolution.
Diffractive optics are successfully produced, first,by microetching and, second, by the complementarymicroprinting operation. The high-definition, high-aspect-ratio, surface-relief patterns produced are theresult of very precise noncontact removal of materialsand transfer processes. CGH’s and Bragg-type mi-crostructures are fabricated onto planar, segmented,and high-curvature surfaces by high-intensity 500-fsUV excimer-laser pulses at a wavelength of 248 nm.The selective microablation process embraces a com-bination of advantageous features of high-resolutionimaging, threshold reduction, and stabilization aswell as minimal thermal effects. Furthermore themicroprinting method has been used for the first timeto our knowledge for fabricating submicrometerstructures onto highly curved surfaces of radii of lessthan 20 mm, addressing here optical-fiber-based sys-tems. This unique operation, particularly relevantto sensor applications, constitutes a major step for-ward and verifies the versatility and the potential ofthe methods presented.
2. Experimental Procedures
The experimental configuration is depicted schemat-ically in Fig. 1. The optical system is based on theinverse microscope principle, and it is configuredwith a hybrid distributed-feedback dye-laseryexcimer-laser femtosecond system delivering 500-fspulses and 5–20 mJ of energy per pulse at 248 nmwith a 10-Hz maximum pulse repetition rate. A spe-cially designed laser-beam delivery system is alsoused, incorporating the spatial modulation of thebeam by a chrome-on-quartz mask or a single slit.The demagnification projection ratio is approxi-mately 30:1 and may be conveniently amended. Notemporal pulse dispersion is observed. The device is
Inset: ~a! microetching, ~b! microprinting, ~c! microprinting on
tion.
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equipped with a pair of high-precision X–Y piezoelec-ric translation stages offering a 50-nm translationesolution determined by optical encoders across a 25m 3 25 mm area. Both laser and translation
tages are computer controlled, and the working areas viewed through the microscope ocular lenses and aCD camera. By utilizing the raster-scanning fab-
ication mode, the construction of masks is facilitatedy the same system, eliminating therefore the needor complicated lithographic methods. The masksroduced are subsequently used for fabricating dif-ractive structures. Multiperiodic masks can thuse constructed, and the process can be optimized tochieve a cost-effective operation.In the microetching mode the substrate is placed
ight under the microscope objective as shown in Fig., inset ~a!. Selective ablation of the material is per-ormed, achieving surface-relief patterning with highpatial and depth resolution. The substrate isranslated on the X–Y plane by the piezoelectricranslation stages to form multiperiodic structures instep-and-repeat fashion. Microetching in the ras-
er mode was mainly applied for the construction ofhe master CGH masks.
In the microprinting operation a receiver substrates placed near contact to a fused silica target sourcey use of an especially developed miniature cell.he miniature cell operates in a partial vacuum at aressure of 1021 Torr. The transparent targetource comprises a thin film of the material to beeposited. In these experiments evaporated Cr thinlms as well as pulsed-laser-deposited InOx thin
films were used. The UV laser beam was focusedthrough the fused silica plate onto the thin-film sur-face as shown in Fig. 1, inset ~b!. The selectivelyablated material is ejected from the source codirec-tionally to the laser beam and deposited onto thereceiving substrate. Selective deposition of the ma-terial in this forward-transfer mode is thus per-formed, establishing a microprinting operation. Avariety of receiving optical surfaces including glass,silicon, and zinc selenide have been used. For per-forming the operation onto optical-fiber surfaces aspecial attachment has been developed to achieveclose contact of the fiber and the planar source @Fig. 1,nset ~c!#. The optical fibers have been chemicallytched to access the core area, and radii of curvatures small as 18 mm have been used. An even smalleristance to the fiber core would be appropriate toacilitate interaction between the guided wave andhe microstructure or to provide evanescent waveoupling. Further research in this direction involv-ng asymmetric etching is in progress.
Single-pixel or single-period master masks weresed for the spatial modulation of the UV beam.elatively large arrays of this single-period patternould be produced in a consecutive step-and-repeatanner. Typical laser-fluence values for microetch-
ng and microprinting were in the range of 30–650Jycm2. A scanning electron microscope ~SEM!
nd an atomic force microscope ~AFM! were appliedo assess the quality of the patterns. Stylus pro-
lometry is not applicable in our case owing to themall features involved.
3. Results and Discussion
A. Ultrashort Laser-Pulse Microetching
In the microetching mode the excimer-laser beamdirectly illuminates the optical surface. For energyfluence above the ablation threshold ~a thresholdstrongly dependent on material properties! materialremoval occurs with high precision. The short ab-sorption length and the negligible thermal diffusionfor ultrashort pulsed radiation result in rapid plasmaformation from the solid. The liquid zone is usuallyof negligible extent, naturally depending on the prop-erties of the materials. Several materials were usedin the direct etching mode including a range of poly-mers, such as poly ~methyl methacrylate! ~PMMA!,polycarbonate, polyimide, photoresist, as well as met-als and metal alloys such as copper, aluminum, andstainless steel. In most cases a very limited ther-mally affected zone is observed, and well-defined,good-aspect-ratio etching results are obtained. Fur-thermore debris accumulation observed with nano-second pulses is also seen to be limited. Etchingquality deteriorates only at high fluence levels ~.1
ycm2! and very deep ~several microns! etching, bothof which are inappropriate for the present applica-tion.
Preliminary studies of the etching characteristicswere carried out for determining threshold EDth ~mJycm2! and etching rate ~nanometer per pulse! valuesfor the materials used. Such a procedure is of ex-treme importance here because it defines the range ofoperational parameters for achieving high-qualityetching and consequently the appropriate phasesteps for the diffractive structures. Although theaim of the present research is not to provide a de-tailed comparison between nanosecond and subpico-second operation, the dependence of the etching rateson the laser fluence has been investigated compara-tively to highlight differences and provide guidancefor subsequent experiments.
Denotative etching results obtained here forPMMA, polycarbonate, aluminum, and stainlesssteel are plotted in Fig. 2. Data from using 20-nspulses at 248 nm and exactly the same experimentalconfiguration are included for comparison. In allcases a well-stabilized, almost linear etch rate ~nmpulse21! is observed. Note the steeper slopes andthe absence of saturation for the fluence levelsreached. A dramatic decrease in the single-pulseetching-threshold value that is also observed in mostcases must be underlined. This value representsthe fluence level at the target required for ablativeremoval of material to be induced from a perfectlynew target in a single-pulse experiment. In contrastthe etching-threshold values obtained in multipulseexperiments are naturally lower, because gradual ra-diation deterioration of the material’s surface en-hances the ablation process. Such effects areresponsible for the apparent higher etching rates ob-
10 April 1999 y Vol. 38, No. 11 y APPLIED OPTICS 2303
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served in polymers for nanosecond pulses with, how-ever, a dramatic decrease in etching quality.
It has been proved, generally, difficult to achieve inpractice the optimum etch depth for a p-phase shiftby looking up a predefined table. This difficulty ismainly due to the instability of the energy level andthe profile of the laser beam. For a reliable perfor-mance an on-line system for monitoring diffractionefficiency6 can be incorporated for optimum opera-tion. Nevertheless the most emphasis here is to theproduction of high-quality clean etching without ap-plying additional processing or laser-beam shaping.Therefore the energy density on the target was keptrelatively high to alleviate such laser-output insta-bilities. Optimization procedures are not includedin this research and represent a further step.
The computer-generated holographic structurespresented have been fabricated here mainly by directprojection and replication of a single-period mastermask. Multiperiodic structures were etched formaximum diffraction efficiency and holographic fidel-ity. This arrangement was found to be convenientfor reducing respective fabrication time and costs,since a large part of the pattern is transferred to theoptical surface in a single step. As we have empha-sized above, the pixelated ~raster! mode has beenapplied only to the construction of masks.
A SEM picture of a fanout CGH etched on PMMAis depicted in Fig. 3~a!. This pattern of 4 3 4 repli-cations of the master hologram period was fabricatedin a step-and-repeat manner. In this example theminimum feature lateral size ~pixel! of the etched
attern is &2 mm. The etching-laser energy densityas 395 mJycm2, and six pulses were arbitrarily used
to provide a second phase level. In Fig. 3~b! an AFMimage of part of the hologram is shown. The rect-
304 APPLIED OPTICS y Vol. 38, No. 11 y 10 April 1999
angular pixel geometry is apparent, and the mea-sured rms etch roughness is ;200 nm. The etchaspect ratio is in the range of 3:1–5:1, depending onthe hologram area. The etch depth estimated bySEM image-projection methods is ;2.1 6 0.2 mm andcorresponds to a phase step of ;3.26p for 633-nmradiation. Detailed AFM analysis reveals a 2.3-mmaverage depth with a 10% uniformity over a 60 mm 30 mm area. This analysis represents an averagehase step of ;3.5p. Although this improper phasetep is produced, an acceptable hologram reconstruc-ion of the etched CGH is obtained as shown inig. 3~c!. The measured diffraction efficiency is rel-tively low at ;10% ~uncorrected for Fresnel andcattering losses! but is in line with rigorous coupled-ave analysis results indicating a maximum theoret-
cal value of ;20%. The reconstructed beamletniformity @defined as $1 2 s~I!y^I&%# is ;75%. Note
that for a large-area etching the spatial laser-beaminhomogeneity is an additional factor enhancing sur-face roughness. No beam homogenization or surfaceposttreatment has been applied here.
Other types of specialized optics have also beenfabricated including Fresnel-type fanout CGH ele-ments and holograms in alloys of high mechanicalstrength such as steel. In addition generic CGH pat-terns have been produced. The example in Fig. 4~a!is a Fourier CGH etched on polyimide. Hologramreconstruction by a HeNe laser beam produces thealphanumeric sentence THIS IS NOT A HOLO-GRAM 0123456789 in Fig. 4~b!. Such an operationmay be useful in holographic security applications.Holograms can be directly ~or indirectly by replica-tion! produced on the surface of the item of interest,whereas subsequent readout provides the required
Fig. 2. Etching rate versus excimer-laser fluence, ED, for ~a!, PMMA; ~b!, polycarbonate; ~c!, aluminum; ~d!, stainless steel, for 500-fs~solid circles! and 20-ns ~open circles! laser pulses at 248 nm. Arrows indicate the single-pulse threshold fluence: solid arrow, 500-fspulses; dot–dash arrow, 20-ns pulses.
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pass operation. Further optical encryption may alsobe applied.
As a general comment, radiation from the femto-second pulsed laser facilitates the production of mi-croetching of higher quality than in the case of
Fig. 3. Fanout element etched in PMMA: ~a! SEM picture, ~b!FM image, ~c! hologram reconstruction at 633 nm.
nanosecond pulses at the same wavelength of 248nm.6 Features are well defined, owing to the shortabsorption length ~additionally enhanced by means ofnonlinear processes! and minimal thermal diffusion,which results in a cleaner cut of the material’s sur-face. The well-defined microetched features areformed with crater walls exhibiting a greater aspectratio. Short absorption lengths may be well meteven when near-IR femtosecond pulses ~produced, forexample, by a Ti:sapphire laser! are used, owing tohe multiphoton absorption processes involved. Inhis latter case, however, the imaging resolution isnferior, owing to the 33 longer wavelength and the
higher degree of coherence attained. In addition,some thermal effects would be apparent in that case,caused by the greater penetration of the relatively low-intensity wings of the laser beamypattern. Becausethe processes involved are heavily dependent on thematerial’s nature, the particular application soughtdefines the range of the operational parameters.
Fig. 4. ~a! Generic-pattern CGH structure etched on polyimideand ~b! CGH reconstruction of an alphanumeric sentence at 633nm.
10 April 1999 y Vol. 38, No. 11 y APPLIED OPTICS 2305
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B. Microprinting onto Planar Surfaces
In the microprinting operation the target material isdeposited on fused silica substrates and placed nearcontact to the receiving optical surface. The maskilluminated by the femtosecond excimer laser is pro-jected demagnified onto the surface of the thin filmthrough the supporting substrate. The illuminatedpart of the thin film is selectively ablated in the for-ward direction and deposited onto the optical surface,as shown in Fig. 1, insets ~b! and ~c!. The micro-structures deposited exhibit exceptional adherence inmost cases.
Two types of source films have been used in thepresent experiments. Sputtered Cr films of 400–800 Å exhibit an absorption depth of ;75 Å. InOxfilms of 450–2200-Å thickness have been prepared byreactive pulsed excimer-laser deposition in oxygenatmosphere21 and exhibited an absorption depth of;400 Å. It is clear that in both cases most of theenergy is absorbed near the substrate–film interface.Thermal diffusion is negligible compared with thetime scale of the pulse duration, ensuring a rapidexpansion of the ablated front from this interface.We note here that nonlinear absorption further re-duces penetration depth. The complex nature ofthis process is currently under investigation.20
Fig. 5. ~a! Raster microprinted Cr on a glass CGH pattern and ~b!GH reconstruction at 633 nm.
306 APPLIED OPTICS y Vol. 38, No. 11 y 10 April 1999
However, depending on the projected spot energy pro-file, only a limited area may be above threshold andtherefore becomes ablated. In effect the ejected ma-terial can be of limited extent, producingsubmicrometer-size deposited features in a quite re-producible fashion.
A SEM photograph of a microdeposited CGH struc-ture of Cr deposited on glass is shown in Fig. 5~a!.
his diffractive structure is deposited pixel by pixelith a pixel size of 4 mm 3 4 mm. Upon illuminationy HeNe laser radiation, FORTH is reproduced ashown in Fig. 5~b!. This CGH is a purely absorptivelement, which, however, operates well with the ex-eption of the expected strong zero-order ~dc! scatter-ng component. Owing to loss, the diffractionfficiency is relatively low with typical values mea-ured here in the range of 5%.Microprinting diffractive structures by single-
eriod master-mask projection is also demonstrated.his operation is cost-effective because a large part ofhe pattern is produced in a single laser shot. Step-nd-repeat operation is also performed, and the ad-erence of features is also excellent. Various opticalurfaces including silicon and zinc–selenide sub-trates have also been used with good results.Indium oxide ~InOx! exhibits interesting electrical
nd optical properties including the demonstration ofolographic recording in amorphous and polycrystal-
ine material.21 Indium oxide patterns were grown ina forward-transfer mode by use of film sources of 45–220-nm thickness, as discussed above. Figure 6 de-picts a CGH pattern produced by microprinting from a200-nm-thick polycrystalline InOx target source onglass at an energy density of 150 mJycm2 with a pixelsize of 4 mm 3 4 mm. In other experiments the pro-duction of microdots having the same crystalline prop-erties as the source material has been achieved. Thepotential for obtaining submicrometer-size opticallyactive elements will be discussed elsewhere.
Fig. 6. Computer-generated holographic pattern of amorphousypolycrystalline InOx on glass.
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C. Microstructures on Optical Fibers
Microprinting micrometer and submicrometer struc-tures onto nonplanar surfaces, including segmented,discontinuous, or high-curvature surfaces, provides aunique means for developing miniature passive andpotentially active structures. In this research wehave addressed this operation in the context of opticalfiber-based sensor devices. Fibers can be etched orpolished so that the deposited material can be veryclose ~a few microns! to the core, thus interacting withhe guided field producing feedback and evanescentave coupling.Microscopic structures of chromium were deposited
n the outer surface of etched fibers by 500-fs, 248-nmulses. Pulse energy density was again in the rangeelow 100 mJycm2. The chromium targets were50–800-Å-thick thin films formed on fused silicaubstrate as above. The structure was deposited inpoint-to-point manner by accurate alignment of theber axis along the motion axis.Figures 7~a! and 7~b! depict closeup views of a
chirped gratinglike structure comprising submi-crometer features as deposited on the flat surface ofAndrew D-type fiber with a 125-mm outer diameter.In Fig. 7~c! an example appears of a periodic structure
eveloped on an etched fiber having an outer diame-er of ;35 mm. The size of the spots deposited inhis case is ;2 mm. The structure is used in thengoing investigation of guided wave coupling effects.t is clear that for single-mode fibers more complexsymmetric etching procedures must be applied topproach the well-confined field in the fiber core.The results above, however, demonstrate the
nique potential of the method for fabricating micro-tructures onto nonplanar and high-curvature sur-aces. Such operations would be proved significantn the further development of optical-fiber sensorsnd optical telecommunications devices, including al-ernative Bragg fiber gratings, rocking filters, fiberolarizes, and evanescent-wave fiber sensors.
4. Conclusions
The use of ultrashort excimer-laser radiation hasproved advantageous in the direct development ofcomputer-generated holograms and other diffractiveoptical microstructures. Ultrashort laser pulsesproduce a better etching quality, thus improving thealready established method of excimer-laser mi-croetching. The reduced thermal effects provide ahigh spatial resolution and a relatively large etchingaspect ratio. Furthermore the potential of micro-printing metals and oxide structures onto optical sur-faces has been demonstrated by the fabrication ofmetal diffractive structures onto planar and nonpla-nar surfaces. The growth of InOx diffractive struc-tures represents a further advance verifying thepotential for the development of active microstruc-tures. In addition, the unique scheme for directfabrication of microstructures onto nonplanar sur-faces has been demonstrated by the production of
submicrometer features onto etched optical-fibersurfaces.
The complementary etching and printing methodspresented rely on purely physical noncontact pro-
Fig. 7. ~a!, ~b! Closeup views of gratinglike structure developed on-type optical fiber revealing submicrometer features and ~c! 2-mm
eatures periodically deposited on a 35-mm-diameter optical fiber.
10 April 1999 y Vol. 38, No. 11 y APPLIED OPTICS 2307
9. J. Bohandy, B. F. Kim, F. J. Adrian, and A. N. Jette, “Metal
2
cesses requiring no additional processing procedures.They represent single-step alternative microfabrica-tion approaches, alleviating the limitations of well-established lithographic methods. Their highversatility and unique potential may lead to noveldevelopments. Further effort is currently being puton the growth of optically active microstructures.
Research supported by the European Unionthrough the Large Installations Plan, “UltravioletLaser Facility,” at the Foundation for Research andTechnology-Hellas, under European Union contractERBFMGECT950021 and the Hellenic Ministry ofDevelopment, in the framework of the nationalproject National Optoelectronic Vision Systems aswell as Regional Development Program and HumanResources support projects. Two CGH designs werekindly supplied by Trevor J. Hall.
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