Photolithographic Processing of Hybrid Glasses for Microoptics

614 JOURNAL OF LIGHTWAVE TECHNOLOGY, VOL. 21, NO. 3, MARCH 2003

Photolithographic Processing of Hybrid Glassesfor Microoptics

Ari H. O. Kärkkäinen, Juha T. Rantala, John M. Tamkin, and Michael R. Descour

Abstract—Hybrid glass materials are used in the photolitho-graphic fabrication of optical and optomechanical structures. Twodifferent photolithographic hybrid glass processing methods aredescribed. The first one is referred as photolithographic patterningand the second as direct photolithographic deforming of hybridglass materials. No etch transfer of the photoimaged structures isneeded. In the latter method even the chemical development stepcan be excluded from the fabrication. Fabrication of lens-arrays,gratings and other binary structures is presented. The synthesizedhybrid glass materials feature minimum optical transmission of97% at wavelengths ranging from 450 nm to 1600 nm and refrac-tive index of, e.g., 1.53 at 632.8 nm. The photolithographic pat-terning resulted in structure heights in excess of 180 m with rmssurface roughness values ranging from 10 to 45 nm. The direct pho-tolithographic deforming resulted in structure heights in excess of27 m with rms surface roughness values ranging from 1 and 15nm.

Index Terms—Hybrid glass materials, microoptics, photolithog-raphy, sol-gel.

I. INTRODUCTION

THE GROWING demand toward miniaturization of opticalsystems has created a challenge for material scientists

to develop novel materials and manufacturing techniquessuitable for applications, such as, miniaturized sensing andimaging systems, microoptical elements and microfluidicdevices. Furthermore, integration of disparate materials andcomponents into functional devices also imposes challengingrequirements on the material properties and their processing.Hybrid sol-gel synthesis method [1] has shown its potential forcontrol and optimization of material properties, and featuringattractive processing methods to overcome difficulties in themicrooptical structure fabrication and integration. Variety oftechniques have been introduced and studied for microopticsfabrication. Among these methods are: 1) conventional wetand dry etch transfer technologies including patterning of themasking photoresist films, [2]–[4] 2) replication technologiessuch as injection molding, hot embossing and ultraviolet (UV)molding including various techniques for mold fabrication, [2],[5]–[11], and 3) nonlithographic technologies such as ink-jetprinting and reel-to-reel printing [12]–[15].

Manuscript received March 28, 2002; revised September 18, 2002. This workwas supported by the National Technology Agency of Finland (TEKES), in partby U.S. National Science Foundation Grant BES-0086736 (M.R.D.), and theEmil Aaltonen Foundation and Tauno Tönning Foundation.

A. H. O. Kärkkäinen is with VTT Electronics, Oulu 90571, Finland, and alsowith the Optical Sciences Center, University of Arizona, Tucson, AZ 85721USA (e-mail: [email protected]).

J. T. Rantala is with GuideOptics Oy, FIN-90101, Finland.J. M. Tamkin and M. R. Descour are with the Optical Sciences Center, Uni-

versity of Arizona, Tucson, AZ 85721 USA.Digital Object Identifier 10.1109/JLT.2003.809581

Our specific application focus is the development of simple,potentially very low-cost, miniature imaging devices [16], [17].The miniature devices are constructed from active and passiveoptical elements and assembled using a bulk micromachined sil-icon microoptical table (MOT) [18]. Diverse optical elementscan be fabricated separately and embedded in the MOT usingmounting slots designed for the elements. The optical elementsconsist of microoptical structures, i.e., refractive lenslets and op-tomechanical positioning structures. This method of assemblingminiaturized optical systems relies on novel hybrid glass mate-rials that are suitable for photolithographic fabrication of opticalelements, such as diffractive and refractive lenses and mirrors,and that are suitable for the simultaneous fabrication of simpleoptomechanical structures necessary to accurately position eachoptical element. The refractive lenslets required by miniatureimaging devices need to exhibit sags, i.e., the height differencebetween the center of the lenslet aperture and the lenslet-aper-ture edge, in excess of 100 m in order to achieve short focallengths (under 2.5 mm) and to achieve a high numerical aperture(NA) in the microoptical system. Thus, thick and mechanicallystable hybrid glass structures must be created.

In this paper, we describe the fabrication of microopticalstructures, such as, lens arrays and gratings based on pho-tolithographic processing of hybrid glass materials. Hybridglass materials have the unique potential for simultaneousfabrication of optical and optomechanical structures by UVpatterning in a single lithography step [18]–[22]. Synthesis ofthese materials is based on hybrid sol-gel method [1]. Hybridglass materials combine functionalities from both organic andinorganic materials classes. The inorganic component providesthe material with good optical, mechanical and thermal prop-erties, whereas the organic component provides the materialwith photopolymerizability. The hybrid sol-gel method offersthe flexibility for tuning and optimization of the material’soptical properties such as refractive index and optical loss aswell as mechanical properties such as the coefficient of thermalexpansion (CTE) and viscosity. In addition, it is possible toincorporate chemical species into the hybrid glasses to provideoptically or electrically active properties.

Two different hybrid glass processing methods for struc-ture fabrication are introduced. The first one is referred asphotolithographic patterning [21], [22]. When using pho-tolithographic patterning of hybrid glasses no etch transfer ofpatterned hybrid glass structures is required. The elimination ofthe etch transfer step potentially improves the surface qualityof the finished microoptical elements and speeds up the fabri-cation process even when very thick structures are fabricatedresulting in a low cost process. The second one is referred

0733-8724/03$17.00 © 2003 IEEE

KÄRKKÄINEN: PHOTOLITHOGRAPHIC PROCESSING OF HYBRID GLASSES FOR MICROOPTICS 615

as direct photolithographic deforming of hybrid glasses [23],[24]. In this method even the structure development step can beexcluded from the fabrication and high quality optical surfacescan be obtained. These two fabrication methods, the usedhybrid glass materials, and results obtained in fabrication aredescribed in detail in the following sections.

Photolithographic patterning of lens arrays with lens sags inexcess of 100 m and optomechanical structures with thicknessin excess of 180 m are demonstrated, using negative litho-graphic tone hybrid glass materials and grayscale and binaryphotomasks, respectively. Direct photolithographic deformingof hybrid glasses is applied in fabrication of lens arrays andsinusoidal gratings. Deformation heights up to 27 m aredemonstrated with mercury UV-lamp exposure using a binaryand grayscale photomask. The UV-laser exposure experimentsresulted in deformation heights up to 6.8 m.

II. HYBRID GLASS MATERIALS FOR MICROOPTICS

A. Material Synthesis and Processing

1) Photolithographic Patterning: The negative tone hy-brid glass material used in the photolithographic patterningexperiments was synthesized the following way: the materialcontains an inorganic base matrix, which was prepared by hy-drolysis and condensation of phenyltrimethoxysilane (PTMS).[3-(methacryloyloxy)propyl]trimethoxysilane (MATMS) wasintroduced to the silica prepolymer solution providing thematerial with photopolymerizability. MATMS can react withboth inorganic and organic monomers and covalent linkagesbetween the inorganic and organic matrices can be formed.Terminal carbon double bonds in the MATMS molecules allowthe radical photopolymerization of the material during the ex-posure. Dilute aqueous solution of hydrochloric acid was usedto catalyze the hydrolysis of the methoxy groups of the PTMSand MATMS. In addition, acrylate monomers are added to thematerial to increase the crosslinking density of the prepolymersolution. In this case we used trimethylolpropane trimethacry-late (TMPTMA) as a three-functional crosslinking monomer.The organic matrix was prepolymerized during the synthesisby using benzoylperoxide as a thermal radical initiator. Theviscosity of the prepolymer solution can be controlled by theprepolymerization extent of the organic and inorganic matricesand by varying the amount of the crosslinking agent in thesolution. Tetrahydrofuran (THF) was used as a solvent in thereaction. The solution was refluxed at elevated temperaturesduring the synthesis. Finally, the volatile solvent THF andbyproducts including HCl and water were removed by usingvacuum distillation. The resulting clear and homogenoussolution was highly viscous. Before the film deposition smallamounts of phenyl bis(2,4,6-trimethylbenzoyl)phosphine oxideand 1-hydroxy-cyclohexyl-phenyl-ketone were dissolved asphotoinitiators to the solution. The photoinitiators initiate theradical polymerization under the UV exposure.

The film deposition was executed by a phased single stepspin-coating procedure: First the solution was spread on a glasssubstrate at 300 rpm for 30 s and then instantly spun on at 800rpm for 30 s. After the spin coating, the samples were prebakedfor few minutes at 90 C. Deposited films were then exposedusing binary and grayscale photomasks and mercury UV lamp.

The mercury UV lamp used in the exposure step had irradianceof 10.6 mW/cm at 365 nm. An exposure time of 2 seconds wasused (UV exposure dose of 21.2 mJ/cm ). The exposures wereperformed using proximity printing (Karl Suss MJB-3, UV-400optics). The exposed samples were developed in methyl isobutylketone/2-propanol solution (1 : 5 ratio). After the development,the films were postbaked at 145 C for two hours. No etchtransfer step was used in the structure fabrication.

2) Direct Photolithographic Deforming: One of the hybridglassmaterialsused indirectphotolithographicdeformingexper-imentswaspreparedbyusing3-(glycidoxypropyl)-trimethoxysi-lane (GPTMS), silicon tetrachloride (SiCl ), [3-(methacryloy-loxy)propyl]trimethoxysilane (MATMS), methanol (MeOH)and dilute hydrochloric acid aqueous solution in the synthesis.First the GPTMS and SiCl were mixed and the solution wasrefluxed. The SiCl reacts as a Lewis acid and catalyzes theepoxy ring opening in GPTMS, which results in formation ofpolyethylene oxide matrix. After the solution was cooled down,MATMS and MeOH were added to the solution and refluxed.Again, the solution was cooled down to room temperature anddilute hydrochloric acid was added to the solution under vigorousstirring.Thesolutionwas thenagedat roomtemperatureallowingthe hydrolysis and partial condensation of the methoxy groupsin GPTMS and MATMS to take place. To increase viscosity andto reduce shrinkage of the material upon baking, the volatilesolvent MeOH and byproducts including HCl and water wereremoved by distillation. Before the film deposition, a smallamount of phenylbis(2,4,6-trimethylbenzoyl)phosphine oxidewas dissolved to the siloxane prepolymer as photoinitiator.

The film deposition was executed by a phased single stepspin-coating procedure: First the solution was spread on a glasssubstrate at 500 rpm for 30 seconds and then instantly spun on at1000 rpm for 60 s. After the spin coating, the samples were pre-baked for few minutes at 90 C. The structure fabrication wasperformed by using a mercury UV-lamp (binary and grayscalephotomasks) and UV-laser exposures (no photomask applied).The used mercury UV-lamp had irradiance of 10.6 mW/cm at365 nm. Exposure doses of 21.2 mJ/cm and 10.6 mJ/cm wereapplied. Binary and grayscale photomask exposures were per-formed using proximity printing (Karl Suss MJB-3, UV-400 op-tics). Structure fabrication was also performed with masklessUV-laser exposure using a raster Laser Direct Imaging (LDI)phototool (Etec PDI 2000). Multiple patterns were written usingexposure dose values from 6 mJ/cm to 30 mJ/cm at 365 nm.After exposure the films were baked at 130 C for one hour. Nochemical development or etch transfer steps were utilized in thefabrication of the structures.

B. Material and Structure Characterization

The refractive index of the material was measured usinga prism coupling technique (Metricon, Model 2010). Thetransmission values of the solid films were measured usinga spectrophotometer (Cary 5G UV-VIS-NIR Spectropho-tometer). Samples for index and transmission measurementswere prepared by spin coating the hybrid glass solutions onglass substrates. The spin coated films were UV exposed andbaked. An uncoated glass substrate was used as a reference inthe transmission measurements.


Fig. 1. Optical transmission of the solid hybrid glass film samples as a functionof the wavelength. The inset table lists extinction coefficients [� (�m )] atsix representative wavelengths for the two hybrid glass materials. MaterialsA and B were used in the photolithographic patterning and photolithographicdeforming experiments, respectively. Material A had a film thickness of 150�m and material B had a film thickness of 19.1 �m. See text for measurementdetails.

Hybrid glass material used in the photolithographic pat-terning experiments exhibited: an index of refraction of 1.53at 632.8 nm and transmission of a 150 m thick film of thehybrid glass materials was measured to be greater than 97% atwavelengths ranging from 450 nm to 1600 nm. An uncoatedglass substrate was used as a reference in the measurement.Fig. 1 (graph A) shows the transmission spectra at wavelengthsranging from 250 nm to 2500 nm.

Hybrid glass material used in the photolithographic de-forming experiments exhibited: an index of refraction of 1.52at 632.8 nm and transmission of a 19.1 m thick film of thehybrid glass material was measured to be greater than 98% atwavelengths ranging from 450 nm to 2200 nm. An uncoatedglass substrate was used as a reference in the measurement.Fig. 1 (graph B) shows the transmission spectra at wavelengthsranging from 250 nm to 2500 nm.

The surface profiles and the rms surface roughness values ofthe fabricated structures were characterized with an optical pro-filer (NT3300), courtesy of Veeco Metrology Group. The fab-ricated structures were also characterized by using a scanningelectron microscope (SEM) (JMS-6400). Thermal and chemicalstability of the fabricated structures was studied by using ele-vated temperature aging and chemical shock tests. In thermalaging tests no optical or mechanical degradation occurred incomponents treated at 150 C over several hours. In chemicalshock tests the components were soaked in aggressive bathsfor several minutes. Common organic solvents (2-propanol andacetone) and mild (1.0 M) hydrochloric acid and potassium hy-droxide base solutions did not create any optical or mechanicaldegradation to the fabricated structures.

III. OPTICAL STRUCTURE FABRICATION

A. Photolithographic Patterning

When using photolithographic patterning, the fabrication ofthe structures is performed in three steps [Fig. 2(a)]: 1) deposi-

(a)

(b)

Fig. 2. Photolithographic patterning of hybrid glass materials. Part (a)shows the processing steps for the structure fabrication. Part (b) shows amicromechanical test element patterned in hybrid glass material to an averagethickness of 180 �m.

tion of the hybrid glass material (single step spin-coating), 2)exposure with a photomask (binary or grayscale) and 3) de-velopment with an organic solvent(s). A post baking step fol-lows the exposure step. No chemical or dry etching transfer ofthe structures to the underlying substrate is needed. The hybridglass material functions as a negative tone photosensitive ma-terial which means that the exposed regions of the film poly-merize. Only the nonpolymerized regions of the film, i.e., thoseregions that do not reach a critical UV exposure dose to be-come crosslinked, are dissolved in the solvent(s) used duringthe development step. Fabrication of the optical and optome-chanical structures by using photolithographic patterning canbe performed by: 1) binary and grayscale photomasks using amercury UV-lamp exposure and also by 2) maskless UV-laserimaging. Basically, all exposure sources that are able to initiateradical polymerization in the hybrid glass material can be ap-plied in the structure fabrication.

1) Binary Photomask Fabrication: Fig. 2 represents thephotolithographic patterning of two-level hybrid glass struc-tures by using a binary photomask. A binary photomaskconsists of areas that have 100% transmission and areas that areopaque for the used exposure wavelength. In the 100% trans-mission areas the hybrid glass film is fully photocrosslinkedand in the opaque areas the film is not crosslinked at all. Due tocrosslinking the material becomes less solvable to the chemicaldeveloper, i.e., IPA : MIBK (isopropanol : methyl isobutylketone) solution. The development of the exposed hybrid glassfilm results in binary structures. Fig. 2(b) shows a test elementthat was fabricated by using a binary photomask and a 2-smercury UV lamp exposure. The element shown in Fig. 2(b)contains the both optical (not shown) and optomechanical


structures (positioning features, shown) that are patternedsimultaneously in a single lithography step. This result repre-sents that we can use the same photomask for simultaneousphotolithographic patterning of the optical and optomechanicalstructures. The simultaneous photolithographic patterningof the optomechanical structures simplifies the fabrication,gives additional freedom in the design of optical systems andresults in accurate positioning ( 2.0 m) relative to the opticalstructures. The optomechanical structures allow an accuratealignment of the fabricated elements during the assemblyof miniaturized optical systems. The heights obtained in thebinary photomask imaging experiments resulted in thicknessesin excess of 180 m.

2) Grayscale Photomask Fabrication: The photolitho-graphic patterning of optical structures with varying thickness,e.g., lenslets, cannot be performed using a binary photomask.A grayscale photomask has to be used in such 3D-structurefabrication. A grayscale photomask consists of zones that havedifferent transmission values or equivalently, different opticaldensities. In our case, when fabricating convex lenses usingnegative tone photosensitive material, the grayscale photomaskexhibits the highest transmission value (lowest optical density)in the center of a lens pattern and lowest transmission value(highest optical density) at the edge of the lens aperture. Thegrayscale photomask used in fabrication of the lenslets shownin Fig. 3(a) featured optical density increase in 32 steps betweenthe center and the edge of the 480- m on-a-side lens aperture.The resulting spatial distribution of the exposure dose leadsto decreasing polymerization extent of the material from thelens center toward the edges of the lens aperture. A decreasein the polymerization extent results in a decreased thicknessof the developed structure and a convex lens shape is formed[see Fig. 3(a) and (b)]. Fig. 3(a) shows a SEM image of a lensarray photolithographically patterned in hybrid glass using agrayscale photomask. Fig. 3(b) presents three lens profiles.The profiles A, B, and C have sag heights of 102, 75, and 60

m, respectively. All these lens profiles were fabricated byusing the same grayscale photomask. Different hybrid glassmaterials were used in these cases. The different sag heightswere obtained by modifying the precursor concentrations ofthe hybrid glass material and the synthesis conditions.

In addition to sag, the departure in the rotational symmetryof the fabricated lenslets was characterized. The surface of thelenslet corresponding to the Profile A in Fig. 3(b) was fittedwith a rotationally symmetric model consisting of four Zernikepolynomials, i.e., , , , and [25]. The magnitude ofsurface irregularity was computed from the difference betweenthe measured topography data and the corresponding, rotation-ally symmetric surface model, after removal of tilt from mea-sured data. The 102- m sag lenslet was evaluated within a cir-cular sub-aperture with radius of 125 m. The radius was lim-ited by missing topography data [see profile gaps in Fig. 3(b)].The maximum departures from rotational symmetry occurredat boundaries between adjacent lenslets. The rms surface irreg-ularity was 320 nm. The rms surface roughness of the fabricatedstructures was measured to be in the range of 10 nm to 45 nmwithin a 50 m diameter circular region in the center of the lens,after removal of curvature and tilt from the surface.

(a)

(b)

Fig. 3. Microlens array photolithographically patterned in hybrid glassmaterial using a grayscale photomask. Part (a) shows an image (SEM image)of the microlens array (sag height 102 �m). Each lenslet aperture is a squaremeasuring 480 �m on a side. Part (b) shows three different diagonal lens arrayprofiles (Profile measured by using Veeco NT 3300). The Profile A shown inPart (b) is measured from the lens array shown in Part (a). Missing data in Part(b) occur due to regions of high slope on the patterned surface. See text fordetails.

The 102 m maximum patterned thickness of the hybrid glassmaterial can be interpreted in terms of a refractive or a reflectivemicrooptical element. With a 480- m clear aperture, a refractivelenslet can be printed with an effective focal length of

m (at 632.8 nm) and a numerical aperture of [22].The corresponding surface radius of curvature is 310 m. With a480- m clear aperture, a concave parabolic micromirror can beprinted with m and . The correspondingsurface base radius of curvature is 288 m.

B. Photolithographic Deforming

When using photolithographic deforming, fabrication of thestructures is performed in two steps [Fig. 4(a)]: 1) depositionof the hybrid glass material (single step spin-coating) and 2)exposure with a photomask (binary or grayscale). No develop-ment step in structure fabrication is needed. A postbaking stepfollows the exposure step. During the baking the residual hy-droxyl, alkoxy and epoxy groups react further leading finallyto the formation of solid hybrid glass. When a deposited filmof the hybrid glass material is exposed, the illuminated areas


(a)

(b)

Fig. 4. Direct photolithographic deforming of hybrid glass materials. Part (a)shows the processing steps for the structure fabrication. Part (b) shows a gratingstructure fabricated in hybrid glass material to a peak to valley height of 4.46�m with period of 70 �m.

of the film undergo a significant positive growth in volume, es-pecially in height, that leads to the formation of the structures.The surface-relief deformation of the hybrid glass film is causedby diffusion reactions and physical expansion and/or reorienta-tions of the monomer and oligomer constituents in the film. Ifthe exposure is performed, e.g., using a patterned photomask[see Fig. 4(a)], the polymerization takes place only in the illu-minated areas leading to formation of concentration and densitygradients in the film. This selective exposure results in diffusionof the constituents from the nonilluminated regions to the illu-minated regions of the film and surface deformation in the hy-brid glass film is produced [see Fig. 4(b)]. Fabrication of grat-ings, isolated lenslets, lens arrays and other structures can beperformed by photolithographic deforming with: 1) binary andgrayscale photomasks using a mercury UV-lamp exposure andalso 2) maskless UV-laser imaging.

1) Binary Photomask Fabrication: Figs. 4 and 5 representthe photolithographic deforming of hybrid glass structures byusing a binary photomask. Fig. 4(b) shows a surface-topographymeasurement of a grating fabricated by photolithographicdeforming using a binary photomask that had clear rectangularareas arranged periodically with line-width of 35 m and periodof 70 m. Exposure dose of 10.6 mJ/cm was used. The resultingsinusoidal grating had a full width at half-maximum (FWHM)line-width of 42.8 m and peak-to-valley height of 4.46 m. Therms surface roughness was measured to be 4.4 nm within a 10

m 150 m rectangular region, after removal of cylinder andtilt from the surface. Fig. 5 shows SEM images of another gratingfabricated by photolithographic deforming. The used binaryphotomask had clear rectangular areas arranged periodicallywith line-width of 41 m and period of 82 m. Exposure dose

(a)

(b)

Fig. 5. Direct photolithographic deforming of hybrid glass materials. Part (a)shows a SEM image of the fabricated grating structure (82.0 �m period, 27.6�m peak-to-valley height). Part (b) shows a SEM image of the grating profilemeasured perpendicular to the grating lines.

of 21.2 mJ/cm was used. The resulting sinusoidal grating had afull-width at half-maximum (FWHM) line-width of 43.4 m andpeak-to-valley height of 27.6 m. The rms surface roughness wasmeasured to be 12.6 nm within a 10 m 150 m rectangularregion, after removal of cylinder and tilt from the surface.

2) Grayscale Photomask Fabrication: Figs. 6–8 representthe photolithographic deforming of hybrid glass structures byusing a grayscale photomask. Fig. 6 shows a convex lens arraystructure that was fabricated using a grayscale photomask. Expo-sure dose of 10.6 mJ/cm was used. The resultant peak-to-valleyheight of the fabricated structure was 1.57 m and the size ofeach lens was 480 m 480 m. The grayscale photomask usedin the fabrication was designed to result in a convex lens arraypattern when negative tone resist is used. Consequently the pho-tomask has its lowest optical density in the center of single-lenspattern and the highest optical density at the edge of single-lenspattern. The photomask featured optical density increase in 32steps between the center and the edge of the 480- m on-a-sidelens aperture. The resultant profile in Fig. 6 represents that thespatial distribution of the grayscale photomask optical density,and therefore the exposure dose, leads to increasing deformationheight of the hybrid glass material toward the center of the lenspattern and formation of a convex lens shape is produced.


(a)

(b)

Fig. 6. Convex microlens array fabricated by direct photolithographicdeforming of hybrid glass material using a grayscale photomask. Part (a) showsSEM image of the microlens array (sag height 1.57 �m). Each lenslet apertureis a square measuring 480 �m on a side. Part (b) shows a diagonal profile ofthe fabricated lenslet.

The surface shown in Fig. 6 was fitted with a model consistingof a weighted sum of the first 22 standard Zernike polynomials[24], [25]. The lenslet was evaluated within a circular sub-aper-ture with radius of 240 m. The standard deviation of the fitresidual was 14 nm. The rms surface irregularity (as definedabove) was 30 nm. The rms surface roughness was measured tobe 1.46 nm within a 50 m diameter circular region in the centerof the lens, after removal of curvature and tilt from the surface.Using an optical design program [26] and the Zernike-polyno-mial surface model, the effective focal length of the 480 mdiameter lenslets was estimated to be mm at 632.8nm [24]The corresponding maximum numerical aperture is es-timated to be .

Fig. 7 shows a concave lens array structure that was fabricatedusing a grayscale photomask. Exposure dose of 10.6 mJ/cmwas used. The resultant peak-to-valley depth of the fabricatedstructure was 1.56 m and the size of each lens was 260 m260 m. The grayscale photomask used in the fabrication wasdesigned to form a concave lens array pattern in a negative tonematerial. The design of such a photomask dictates that the maskhas its highest optical density in the center of single-lens patternand the lowest optical density at the lens-aperture edges. Thephotomask featured optical density decrease in 32 steps betweenthe center and the edge of the 260- m on-a-side lens aperture.The resultant profile in Fig. 7 represents that the spatial distri-bution of the grayscale photomask optical density, and therefore

(a)

(b)

Fig. 7. Concave microlens array fabricated by direct photolithographicdeforming of hybrid glass material using a grayscale photomask. Part (a) showsSEM image of the microlens array (concave sag height 1.56 �m). Each lensletaperture is a square measuring 260 �m on a side. Part (b) shows a diagonalprofile of the fabricated lenslet.

the exposure dose, leads to increasing deformation height of thehybrid glass material toward the edges of the lens pattern andformation of a concave lens shape is produced.

The surface showed in Fig. 7was fitted with a model con-sisting of a weighted sum of the first 22 standard Zernike poly-nomials [24], [25]. The lenslet was evaluated within a circularsubaperture with radius of 130 m. The standard deviation ofthe fit residual is 15 nm. The rms surface irregularity was 29 nm.The rms surface roughness was measured to be 2.5 nm withina 50 m diameter circular region in the center of the lens, afterremoval of curvature and tilt from the surface. Using an opticaldesign program [26] and the Zernike-polynomial surface model,the effective focal length of the 260 m diameter lenslets wasestimated to be mm at 632.8 nm [24].

Fig. 8(a) shows a surface-topography measurement of an-other convex lens fabricated by photolithographic deformingusing the same grayscale photomask (480- m on-a-side lensaperture) used in fabrication of the lens array shown in Fig. 6.Fig. 8(b) shows a diagonal profile of the lenslet. An exposuredose of 21.2 mJ/cm was used. The achieved deformation heightin this case was 24 m. The rms surface roughness was mea-sured to be 15 nm within a 50 m diameter circular region inthe center of the lens, after removal of curvature and tilt fromthe surface.

The departure from rotational symmetry of the lenslet shownin Fig. 8 was characterized. The surface of the lenslet shown in


(a)

(b)

Fig. 8. Part (a) and (b) show a segment of a lenslet array fabricated bydirect photolithographic deforming of hybrid glass material using a grayscalephotomask. Part (a) shows a perspective view of the printed surface-reliefstructure. Part (b) shows a diagonal profile of the surface. Lens sag wasmeasured to be 24 �m.

Fig. 8(a) was fitted with a rotationally symmetric model con-sisting of four Zernike polynomials, i.e., , , , and[25]. The fit was applied after removal of tilt from the surface inFig. 8(a). The surface irregularity was computed from the differ-ence between the measured topography data (tilt removed) andthe corresponding, rotationally symmetric surface model. Thelenslet in Fig. 8(a) was evaluated within a circular sub-aperturewith radius of 125 m. The radius was chosen to be consistentwith the radius used in the analysis of the 102- m-sag lensletshown in Fig. 3. The maximum departures from rotational sym-metry occurred at boundaries between adjacent lenslets. Therms surface irregularity was 50 nm.

The surface shown in Fig. 8 was also fitted with a modelconsisting of a weighted sum of the first 45 standard Zernikepolynomials [25]. The lenslet’s surface was fitted within a cir-cular aperture with radius of 240 m. Table I lists the coeffi-cients associated with the fit. Only the 30 largest-magnitude co-efficients are listed, excluding the coefficient associated with

, the “piston” term. The listed coefficients and the associatedpolynomials result in a standard deviation of the fit residual of50 nm.

Using an optical design program [26], an index of refractionof 1.52, and the Zernike-polynomial surface model, the effectivefocal length of the 480 m diameter lenslets was estimated tobe mm at 632.8 nm. The corresponding maximumnumerical aperture is estimated to be .

3) Laser Direct Imaging: Fig. 9 represent the photolitho-graphic deforming of hybrid glass structures by using anUV-laser exposure. We investigated the material’s behaviorwith UV-laser exposure using a raster Laser Direct Imaging

(LDI) phototool (Etec PDI 2000). By using the LDI phototoolwe can directly image, i.e., write, structures on hybrid glassmaterial without the need for binary or grayscale photomasks.The LDI phototool can be programmed to write various pat-terns, e.g., grating lines, lenslets, and waveguides. The gratingsurface segment shown in Fig. 9 is a result of laser writinga bitmap consisting of equal-width lines and spaces in thehybrid glass material. The written feature size consisting offour pixels has a width of approximately 50 m. The period ofthe written grating was 100 m. At a UV-laser exposure doseof 10 mJ/cm , the structure resulting from this exposure patternexhibited a FWHM line-width of 61.7 m. The peak-to-valleyheight of the structure was 1.93 m. The rms surface rough-ness was measured to be 3.9 nm within a 10 m 150 mrectangular region, after removal of cylinder and tilt from thesurface. Exposure dose values were varied from 6 mJ/cm to30 mJ/cm . The peak-to-valley height increased with exposuredose from 1.2 m at 6 mJ/cm to 6.8 m at 30 mJ/cm .

IV. DISCUSSION

As can be seen from the experiments above the synthesizedphotosensitive hybrid glass materials can be applied in both bi-nary and grayscale mask photolithography. Fabrication of bothrefractive and diffractive optical microstructures is possible. Theuse of grayscale photolithography enables the fabrication ofnearly arbitrary structures with varying thickness, e.g., aspher-ical, spherical, positive (convex), or negative (concave) lenses,allowing for the best performance achievable. A wide selectionof surface types offers additional freedom in the design of op-tical systems that are no longer constrained to consist entirelyof spherical surfaces. Desired surface profiles can be fabricatedby appropriately choosing the optical densities of zones on thegrayscale photomask.

Fabrication of the optical structures can be performed onvarious substrate materials with high alignment accuracy.Patterning of optomechanical structures can be performedduring the same fabrication step. Furthermore, the lens andoptomechanical structure fabrication can be performed on bothsides of the substrate allowing the realization of, e.g., biconvexand biconcave lens structures. The fabricated optical surfacescan be finished by anti-reflection coatings, producing opticswith superior transmission.

The structure heights (in excess of 100 m) and surfaceroughness values (in the range of 10–45 nm) obtained bythe photolithographic patterning experiments indicate thesignificant potential of these kind of materials and methodsfor fabrication of microoptical systems and structures. Theflexibility in the materials synthesis by hybrid sol-gel methodprovides endless possibilities for modification and optimizationof the material properties. In addition to the high opticalquality and thermal stability, we have recently demonstratedthe synthesis of organic dye molecule doped photoimageablehybrid glass materials that can be patterned to result in similarlens shapes and quality that are presented in Fig. 3. This willenable the fabrication of multi-functional optical structures,such as, filtering lenslets. Because of mild synthesis conditionsand low temperature processing of the material, the organic dye


TABLE IRESULTS OF FITTING THE SURFACE SHOWN IN FIG. 8 WITH A STANDARD ZERNIKE-POLYNOMIAL MODEL. THE COEFFICIENT UNITS ARE NANOMETERS.

THE POLYNOMIALS ARE NUMBERED ACCORDING TO NOLL’S NOTATION THROUGH � . THE REMAINING POLYNOMIALS ARE NUMBERED

ACCORDING TO THE NOTATION � AND � , WHERE � INDICATES A SINUSOIDAL DEPENDENCE ON THE ANGULAR VARIABLE

AND � INDICATES A COSINUSOIDAL DEPENDENCE ON THE ANGULAR VARIABLE [25]

(a)

(b)

Fig. 9. Surface-topography measurement of a segment of UV-laser writtendiffraction grating. The grating parameters are 100 �m period, FWHMline-width 61.7 �m, space-width 42.2 �m, average peak-to-valley height 1.93�m and exposure dose 10 mJ/cm . Part (a) shows a perspective image and Part(b) shows a profile of the printed structure.

molecules can be easily incorporated by doping or covalentbonding into the hybrid glass matrix [27]. Our focus is alsodirected on the synthesis of photoimageable hybrid glass mate-rials with a range of indexes. An extensive family of materialswith broad selection of material properties will give excellentcapabilities for microoptics design and development.

Directed photolithographic deforming experiments resultedalso in very attractive structure dimension (heights up to 27 m)and quality (surface roughness 1–15 nm). The use of a grayscalephotomask (Figs. 6–8) in the exposure step result in more con-

trolled profiles than those achievable with binary photomasks.Figs. 4, 5and 9 demonstrate that we can fabricate grating struc-tures with sinusoidal shapes (see Figs. 4 and 9) and also gratingstructures close to ideal “stair step” shape (Fig. 5) by using bi-nary photomasks. However, all the structures fabricated in thispaper using direct photolithographic deforming exhibit in someextent “rounded” edges. This is caused by the diffusion of themonomeric and oligomeric species in the material. Therefore,this method is most suitable for fabrication of, e.g., gratings,lenses and lens arrays (concave and convex) and all other struc-tures where rounded surfaces are acceptable. However, we claimthat by using grayscale photomasks also in the grating fabri-cation it is possible to optimize the structure shapes to resultsin steeper sidewalls. In addition, structural improvement can bealso achieved by incorporating radical diffusion quenchers intothe hybrid glass, which can eliminate or limit diffusion effects.Because of the diffusion processes, there is also a possibilitythat the base matrix and diffusing constituents are not homo-geneously distributed in the fabricated structures. The nonho-mogenous distribution of the material components may lead,e.g., to refractive index variations inside the film, when thereis an index difference between used material components. Thepossible index change inside the structure has to be controlled toresult in desired optical properties. On the other hand, the con-trol of the index change inside fabricated structures gives veryattractive possibilities for optical component fabrication. Thediffusing components can also have other optical and/or elec-trical properties. The diffusing component(s) can be high-indexor low-index materials, light absorbing materials, light emit-ting material, lasing materials, liquid-crystals, etc. This methodgives us the possibility to fabricate optical structures such as dy-namic microlenses and miniaturized adaptive optics. Basicallythe base matrix of the material can be chosen to be any silanebased material that is provided with terminal carbon doublebond containing side chains, which can then participate to thepolymerization reactions initiated by the photoinitiator radicals.

In both of the fabrication methods the material’s response ontheexposureandotherprocessingstepshas tobecarefullystudied


to result in desired heights, shapes and high quality of the struc-tures. When using grayscale photomasks in the fabrication the re-sponse of the material to varying exposure dose has to be investi-gated. This can be done using a grayscale calibration photomaskpreferably designed to create shape and dimensions close to thefinal component. By using the obtained material-response cali-bration data, we can design a grayscale photomask layout that isneeded for the fabrication of specific optical elements.

V. CONCLUSION

By using negative tone hybrid glass materials, photolitho-graphic patterning of various optical structures, such asgratings, waveguides, lens arrays (see Fig. 3), and individuallenslets is possible. No chemical or dry etching transfer isneeded. In addition to optical structures, alignment-aidingoptomechanical structures can be patterned simultaneouslyin the hybrid glass used to fabricate microoptical elements.Such elements can be used in, e.g., microoptical table (MOT)systems. Grayscale lithography using photomasks or an UVlaser permits the controlled patterning of nearly arbitraryhigh-quality optical surfaces, e.g., conics and aspheric surfaces.The direct photolithographic deforming of hybrid glasses canbe also applied to fabricate variety of microoptical structures.No chemical structure development step is needed in thefabrication. The fabrication of, e.g., lens arrays (see Figs. 6–8)and gratings (Figs. 4, 5 and 9) can be performed.

Both fabrication methods result in useful structure heights (inexcess of 180 m) and rms surface roughness (1–45 nm). Thepresented materials and methods have a significant potential forsimple and low cost fabrication of optical surfaces for appli-cations including waveguides, gratings, microoptical elements,diffractive and refractive microoptics, MEMS technology andmicrofluidic technology. In our continuing work we are devel-oping wavefront-metrology techniques to evaluate the opticalcharacteristics of the microoptical elements. The reproducibilityand logn-time stability of the material are also going to be sub-ject of the future research.

ACKNOWLEDGMENT

The authors are grateful to the Veeco Metrology Group,WaveFront Sciences and Etec Systems, Inc., an AppliedMaterials Company, for their assistance with this research.

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Ari H. O. Kärkkäinen received the M.S. degree in organic chemistry (with anemphasis in synthesis and fabrication of organic light-emitting materials anddevices) in 2000 from the University of Oulu, Finland, where he is currentlyworking toward the Ph.D. degree in chemistry.

He joined the VTT Electronics Manufacturing Technology Group, Oulu, in1998. His research interests are in microoptics (fabrication methods, materialsynthesis, and fabrication of microoptical systems), organic thin-film lightsources (material synthesis and development of structures and fabricationmethods), and sol-gel chemistry. Currently, he is developing novel photoim-ageable hybrid glass materials for integrated optical systems.


Juha T. Rantala received the M.Sc. and Ph.D. degrees in chemistry from theUniversity of Oulu, Finland.

He is a Chief Technology Officer of GuideOptics, Inc. Prior to joiningGuideOptics, he was with VTT Electronics, Finland, since 1996. During 1997and 1998, he was a Visiting Scholar at the Optical Sciences Center of theUniversity of Arizona. Since 1999, he has been a Senior Research Scientist ofVTT Electronics and has headed activities in advanced materials developmentfor optical and wireless communication applications. He is also a docent andan adjunct faculty member of the Optical Sciences Center of the University ofArizona. In recent years, his research has mostly focused on the development ofadvanced materials and components for planar optics as well as for electronicsand microsystems. Furthermore, his R&D interests include manufacturingand packaging methods and technologies in the areas of modern micro andelectrooptical devices.

John M. Tamkin, photograph and biography not available at the time of publi-cation.

Michael R. Descour received the B.S. degree from the University of Colorado,Boulder, in 1990 and the M.S. and Ph.D. degrees from the Optical SciencesCenter, University of Arizona, Tucson, in 1992 and 1994, respectively.

He is currently an Associate Professor at the Optical Sciences Center. Hewas previously with Ball Aerospace Systems Group, Boulder, CO, and SandiaNational Laboratories, Albuquerque, NM, as a Postdoctoral appointee. His re-search group is developing miniature and microoptical imaging and nonimagingsensors aimed at applications in the health sciences.

Photolithographic Processing of Hybrid Glasses for Microoptics

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