Photostructurable glasses are important materials for applications in microsystems. They enable structures with high aspect ratios and a high dependability of mechanical, optical and chemical properties in a large range of temperatures. The exposure of photostructurable glasses to UV laser radiation, as a rapid prototyping technique, is an alternative method to the exposure by a mask aligner.
Α photostructurable glass (FS21) was exposed to UV laser radiation of the wavelengths 248, 308 and 355 nm. Investigated was the influenee of the exposure parameters wavelength of laser radiation and energy density on structuring results such as crystallization depth, lateral geometry of crystallized areas, structure of crystallized areas and etch angle for single pulse exposure

Brokmann, Ulrike

Ertel-Ingrisch, Werner

Harnisch, Alf

Hülsenberg, Dagmar

English

Repositorium für Naturwissenschaften und Technik

Original PaperUV laser radiation for microstructuring of photostructurable glassesUlrike Brokmann, Alf Harnisch, Werner Ertel-Ingrisch and Dagmar HülsenbergTechnische Universität Ilmenau, Ilmenau (Germany)Photostructurable glasses are important materials for applieations in mierosystems. They enable structures with high aspect ratiosand a high dependability of meehanical, optieal and ehemieal properties in a large ränge of temperatures. The exposure of photo-structurable glasses to UV laser radiation, as a rapid prototyping technique, is an alternative method to the exposure by a maskaligner.Α photostructurable glass (FS21) was exposed to UV laser radiation of the wavelengths 248, 308 and 355 nm. Investigated wasthe influenee of the exposure parameters wavelength of laser radiation and energy density on strueturing results sueh as crystalliza-tion depth, lateral geometry of crystallized areas, strueture of crystallized areas and eteh angle for single pulse exposure.1. IntroductionGlasses are important materials for engineering of mierosys-tems in many fields. Main reasons are the transparency ofthe material in a large ränge of wavelength, high thermalstrength and extremely high mechanical strength of miniat-urized glass structures. Some applications are mechanicalelements like micromechanical grippers and Springs, components for microfluidic or microreaction technique andmicro-optical elements [1 and 2]. The strueturing technol-ogies are multifaceted and their exertion depends on defi-nite requirements.The strueturing process is divided into three steps, asshown in figure 1. Α photoehemical modification of the exposed areas takes place during the first step [3]. Due to theUV exposure of the glass, Ce-^^ ions will be stimulated todonate one photoelectron for reduction of Ag^ ions. In a second step, during the thermal treatment, in these exposedareas, silver atoms agglomerate to Clusters and build a nucleus for the crystallization of lithium metasilicate. The microstructuring of the glass in the proper meaning of theWord takes place in the third step, the wet chemical etchingin diluted hydrofluoric acid.Different technologies of exposure of photostructurableglasses are possible. State of the art is the exposure as a batch process by using a mask aligner. Due to the transparency in the UV ränge, a mask of fused siliea with a struc-tured chromium layer is necessary. For manufacturingprototypes, this has some drawbacks regarding the highReceived 24 November 2003.Presented in German at IT^ Annual Meeting of the GermanSociety of Glass Teehnology in Feipzig (Germany) on 28 May2003.costs of such masks. Exposure techniques that use UV laserradiation are more flexible. For instance writing techniquesand dynamic mask projection are possible. Therefore, theUV laser exposure of photostructurable glasses belongs tothe so-called rapid prototyping techniques (figure 1).R ' S tepT' Step3" StepFigure 1. Microstructuring proeess of photostructurable glasswith UV laser exposure; step: UV laser exposure, 2 ^ step:thermal treatment, 3'̂ '̂  step: wet chemical etching.-----" Ulrike Brokmann; Alf Harnisch; Werner Ertel-Ingrisch; Dagmar Hülsenberg:Λ L  248 NNN Λ L  308 nm λι  355 nm400—•200 250 300 350Wavelength Λ L in nm Figure 2. Speetral transmission of the glass FS21 in the UVränge. In addition the wavelengths of the later used laser radi-ations are marked.300Φ• ΣCΟΝ"CD(ΑC1 2 3 4 • Energy density E L in J/cm^-Figure 3. Crystallization depth hc after single pulse exposurewith 2L 248 nm and thermal treatment dependent on energydensity  ^L-2. ExperimentalT H E P H O T O S T R U C T U R A B L E GLASS FS21 ( C O M P O S I T I O N I N M O L % )69.3 S I 0 2 , 4.0 AI2O3, 21.8 F I 0 2 , 2.5 N A 2 0 , 2.5 K 2 O I S D O P E DW I T H A G 2 0 , C E 2 0 , S B 2 0 3 A N D S N O TO A C H I E V E P H O T O S E N S I T I V EP R O P E R T I E S . T H I S GLASS W A S M E L T E D B Y A LABORATORY M E T H O D I NA N ELECTRICAL F U R N A C E I N P L A T I N U M CRUCIBLES. A F T E R T H E P R O C E S SSteps M O L D I N G , C U T T I N G , G R I N D I N G A N D P O L I S H I N G , T H E S E M I -F M I S H E D P R O D U C T S H A V E  A D I A M E T E R O F 7.4 C M A N D A T H I C K N E S SO F A B O U T 700 P M .FS21 GLASS H A S A N A B S O R P T I O N B A N D AT Α  310 N M ,C A U S E D B Y C E ^ ^ I O N S ( F I G U R E 2). I N F IGURE  2 ARE ALSO M A R K E DT H E W A V E L E N G T H S LATER U S E D FOR T H E UV LASER E X P O S U R E . T H EE X P O S U R E T O O K P L A C E W I T H E X E I M E R LASER R A D I A T I O N  (AL  248A N D 308 N M ) A N D ( 3 i ü ) N D : Y A G SOLID-STATE LASER R A D I A T I O N(/IL  355 N M ) .F O L L O W I N G LASER P A R A M E T E R S ARE I M P O R T A N T I N O R D E R TOO B T A I N T H E D E S I R E D STRUCTURES: W A V E L E N G T H O F T H E LASER R A D I A T I O N  AL I N N M , E N E R G Y D E N S I T Y I N J / C M ^ , N U M B E R O F P U L S E S Λ̂ , P U L S E D U R A T I O N Τ I N N S , D I A M E T E R O F T H E LASER B E A M AT T H E I M A G I N G SURFACEI N P M , FOCAL P O S I T I O N R E G A R D I N G T H E E X P O S U R E T O P S I D E , LOCAL B E A M PROFILE.Η 200 pm200 pmFigures 4a and b. Crystallized structures after single pulse laserexposure £L  1 J/em-, with a)  AL 248 nm, dL  92 pm andthermal treatment, and b)  AL  355 nm, d]^  80 pm and ther-mal treatment.The thermal treatment took place at 590 C for 1 h. Exposed glass samples were positioned on a coated stainlesssteel plate in an electrical furnace. After this process, theexposed areas were partially crystallized to lithium metasil-icate and were brown colored.The crystallized areas were analyzed under  a light op-tical microscope to quantify the diameter of the partiallycrystallized structures at the topside of exposure. The erys-tallization depth was quantified by means of cross sections.Structural investigation of the crystallized areas tookplace using  a scanning electron microscope (SEM). Previously they were etched in 0 . 5 % hydrofluoric acid for5 min.The wet chemical etching process for strueturingtrenches took place in hydrofluoric acid (10 %) at the tem-perature 30 C in an ultrasonic bath for 15 min at themost.3. Results and discussion3.1 Crystallization depthThe crystallization depth depends on the optical penetration<5L of U V laser radiation in the glass FS21. is > 1000 pm =  = =-= =  = =  =° ­=-= == ° -------a)Figures 5a to e. SEM pietures after exposure with £L  5 J/em^ for different  AL and eonstant thermal treatment; a)  AL 248 nm,b)  AL  308 nm, e)  AL 355 nm.for laser radiation with AL ^ 308 nm. This has the conse-quence that all Substrates exposed to laser radiation in sucha wavelength ränge were crystallized in the total thickness.For laser exposure with a wavelength in the ränge of highabsorption, the crystallization depth was analyzed for singlepulse exposure dependent on Ε^. The results for exposurewith  AL  248 nm are shown in figure 3.The highest erystallization depth HC  (338 ± 10) pmwas found for the single pulse exposure.3.2 Geometry of crystallized areas at the topsideof exposureThere are differences in lateral geometry of structures afterthermal treatment between the exposure to exeimer laserradiation and (3ω)Nd:YAG solid State laser radiation. Figures 4a and b show crystallized structures exposed withAL  248 nm (figure 4a) and with  AL  355 nm (figure 4b).Exeimer laser radiation has a so-called top hat beamprofile. Α part of the laser beam was decoupled and imagedover an optical system on the Substrate surface by means ofa hole aperture. The steep slope of the edge leads to a nearlyhomogeneous intensity distribution of the local beam pro-file. Therefore, the widths of erystallized structures are inthe ränge of the diameter of the laser beam at the imagesurface. In contrast to this the widths of crystallized struc-tures after exposure to the solid State laser radiation werealways smaller than the diameter of the laser beam at theimage surface. In addition it was found that the contrastbetween unlighted glass and crystallized areas is inferiorcompared to the exposure to the exeimer laser radiation.The reason is the slight slope of the edge of the Gaussianlaser beam profile of the (3ω) Nd:YAG laser. It was alsofound that the widths of crystallized structures increasedwith increasing energy density at exposure. The reason forthis is the local overtravel of threshold energy densitiesnecessary in the beam profile of one pulse for the photo-ehemical reaction.3.3 Structural investigation of crystallized areasFor some samples the size of the lithium metasilicate crys-tals was quantified by means of SEM analysis. The crystalsare orderless and interlocked in the structure. So it is quite308 # Figure 6. Size of lithium metasilicate erystals Ί/^Κ in the texturedepending on  AL and S]^ after eonstant thermal treatment.difficult to examine the size of single crystals. Structures ofcrystallized areas after exposure with different AL but con-stant thermal treatment are shown in figures 5a to c. Figure6 shows the crystal size depending on AL and Ε^. Α depen-dence of the crystal size on Ε^ was found only for the expo-sure with AL ^ 355 nm. The crystal size decreases with anincreasing of Ε^.The structure of the crystallized areas will be specifiedby number and size of crystals. It is clear that the fineststructure is possible if many and small crystals are grownduring the thermal treatment. The finest structure wasfound for exposure to XeCl exeimer laser radiation (AL = 308 nm). In this case the crystals are obviously smaller(size  300 nm) than after exposure with  AL  248 nm andAL  355 nm.Α fine structure of crystallized areas is important for thegeometrical microstructuring process. It has a positive effecton the geometrical resolution of microstructures, the surfaceroughness in microstructures and not least on the etch ratiobetween unlighted glass and partially crystallized areas.Furthermore, the dependence of the crystal size on Ε^,detected for exposure with  AL  355 nm, offers the possibil-ity of tuning the crystals to specific sizes for instance fordifferent surface roughness in geometrical microstructures. =  = =  = ==-=  = = ~  = = =Ulrike Brokmann et al.: UV laser radiation for microstructuring of photostructurable glassesEtched TrenchGlass SubstrateCrystallized structureFigure 7. Geometrieal ratios of a photostruetured treneh,manufaetured by means of XeCl exeimer laser radiation; notations: d\ Subs t ra te thiekness, bQ. width of the treneh at thetopside, bQ\ width of the treneh at ground, /z^: eteh depth, ycangle of the erystallized strueture, y^: eteh angle, yw  wall angleof the etehed strueture.tailed information about the strueturing is published in [4].In these experiments an etch angle  (5 ± 1)  wasdetermined. That means an etch rate ratio of 1:10. 4. SummaryBesides the exposure of the photostructurable glass FS21 toUV laser radiation near the absorption maximum of Ce^^(AL  308 nm), the exposure in the ränge of high absorption(AL  248 nm) and also in the ränge of high transmission(AL  355 nm) of the glass is possible. The crystallizationdepth hc depends on the optical density  i^L. Α crystallizationdepth at the most of hc  (338 ± 10) pm for an exposurewith  AL  248 nm was found. For exposure to XeCl exeimerlaser radiation (AL  308 nm) the smallest lithium metasilic-ate crystals were found with a size of  300 nm. Photostrue-tured trenches were manufactured by using XeCl exeimerlaser radiation (AL  308 nm). Therefore, an etch rate ratioof 1:10 between unlighted glass and crystallized area wasfound.3.4 Investigations of geometrically defined micro-structuresBy means of XeCl exeimer laser radiation (AL  308 nm) theinfluence of the laser parameters and on the geometryof photostruetured trenches was investigated. Α scheme ofthe geometrical ratios is shown in figure 7.Differences between the width of crystallized structureson the topside and on the underside of exposure result inan angle of the crystallized structure yc- The wall angle ofthe trenches yw is the sum of an angle of the crystallizedstructure yc and an etch angle y^, caused by the etch rateratio between unexposed glass and crystallized area. It ispossible to form the angle of the crystallized structure bymeans of changing the local focus position, beam shapingand beam delivery. Here is a high potential for various indentation free structures for further investigations. More deThe authors thank the RWTH Aachen, chair of laser teehnol-ogy, for the realization of the UV-laser exposure experiments.5. References[1] Hülsenberg, D.: Glasses for mierosystems teehnology. Mie-roeleetron. J. 28 (1997) pp. 419-432.[2] Harnisch, Α.: Beitrag zur Entwicklung von Herstellungs-teehnologien für komplexe Bauteile aus mikrostrukturier-tem Glas. Technische Universität Ilmenau, dissertation1998.[3] Stookey, S. D.: Photosensitive glass. Ind. Eng. Chem. 41(1949) no. 4, pp. 856-861 .[4] Brokmann, U.; Harnisch, Α.; Hülsenberg, D.: Einfluss vonUV-Faserparametern auf die geometrische Strukturierungvon fotosensitivem Glas. Materialwiss. Werkstoffteeh. 34(2003) no. 7, pp. 666-670.• E 5 0 4 P 0 0 6Contaet:Ulrike BrokmannTechnische Universität IlmenauPf 10 05 65D-98684 IlmenauE-mail: Ulrike.Brokmann@Tu-Ilmenau.de = ° = = == = =~ =---=--

UV laser radiation for microstructuring of photostructurable glasses

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UV laser radiation for microstructuring of photostructurable glasses

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