High Speed and High Precision Fs-laser Writing Using a Scanner with Large Numerical Aperture

Size: px

Start display at page:

Download "High Speed and High Precision Fs-laser Writing Using a Scanner with Large Numerical Aperture"

Rosamund Cook
6 years ago
Views:

1 High Speed ad High Precisio Fs-laser Writig Usig a Scaer with Large Numerical Aperture Jes GOTTMANN, Mare HÖRSTMANN-JUNGEMANN, Marti HERMANS, Deis BECKMANN Lehrstuhl für Lasertechik, RWTH Aache Uiversity, Steibachstraße 15, Aache, Germay jes.gottma@ilt.frauhofer.de Curretly available high speed scaig systems based o galvo mirrors are equipped with objectives with small umerical apertures (NA < 0.2) which are ot sufficiet for most of the ivolume micro structurig processes. Whereas high precisio air bearig traslatio stages are used with microscope objectives the speed is limited to approximately 100 mm/s. I order to overcome these limitatios we itegrated a galvo scaer ito a system with large maximum scaig velocity ( mm/s), large umerical aperture (NA = ), small focus size ( μm), high precisio ( m) ad computer cotrolled pre-compesatio of spherical aberratios for ivolume focusig. Furthermore, we combied the scaig system with a computer cotrolled three axis traslatio stage to process large flat work pieces such as wafers up to 2 mm i thickess. As applicatio of the scaer high speed direct writig of waveguides, markigs, ao structures iside trasparet materials as well as the high speed fabricatio of 3D micro structures by i-volume selective laser etchig are demostrated. Keywords: Scaig system, fs-laser direct writig, i-volume modificatio, fs-laser micro structurig, i-volume selective laser etchig 1. Itroductio Micro structurig i the volume of trasparet materials is eabled by ultrafast laser radiatio. By laser radiatio with pulse duratios i the fs ad ps regime multi photo processes are efficietly iduced resultig i high resolutio < 1 µm 3, very low heat iput ad high writig flexibility i three dimesios. High trasparet materials such as glasses ad sapphire are locally modified i-volume to chage the refractive idex for optical applicatios [1] or to selectively icrease the corrodibility locally for the maufacturig of micro chaels ad micro structured parts by i-volume selective laser iduced etchig for use i micro system ad medical techology [2, 3, 4]. Exploitig ultrafast laser radiatio with repetitio rates up to several MHz (e.g. from FCPA fs lasers) high maufacturig velocities are possible, if either the work piece or the laser beam could be moved fast eough [5]. Curretly available high speed scaig systems based o galvo mirrors are equipped with objectives with small umerical apertures (NA<0.2). Therefore, the threshold for self focusig i the volume of trasparet materials is reached before the threshold for dielectric breakdow, ofte resultig i the formatio of cracks istead of the desired homogeous modificatio of the refractive idex or the corrodibility [6]. Three axes micro positioig stages exhibit high precisio ad the laser radiatio ca be focused usig microscope objectives with large umerical apertures. However, because large masses have to be accelerated, the dyamics of the motio is limited resultig i low productivity of producig 3D shaped micro structures. Therefore, to exploit the productivity of high repetitio rate fs-lasers high dyamic galvo scaers have to be combied with microscope objectives with large umerical apertures for i-volume structurig. A simple combiatio of a galvo mirrors with microscope objectives results i very small scaig fields ad distortio, because the exit pupil of the les is typically several mm iside the objective. Therefore, we use a 1:1 telescope to image the galvo mirror ito the exit pupil of the microscope objective. 2. Pre-compesatio of spherical aberratios A plaar plate with a refractive idex differet from the air or the immersio liquid located betwee a focusig les ad the focal poit results i spherical aberratios. Therefore, spherical aberratios must be pre-compesated durig focusig i the volume of trasparet materials at variable depth e.g. usig the correctio collar of microscope objectives. Durig focusig with umerical aperture NA i the depth d of a material with refractive idex surrouded by a medium with refractive idex 0 the spherical aberratio results i a icreased focal diameter ad a elogatio of the legth of the focus L: NA L = d (1) NA Equatio (1) is directly derived from simple geometrical cosideratios: The maximum agle of the icidet rays is α i the surroudig medium ad α i the medium ad the umerical aperture is NA= 0 siα = siα. Followig Sells law the refracted rays with the maximum agle α hit the optical axis i the glass at the deepest poit z max : siα cosα 0 NA z = = max d d (2) 2 2 cosα siα 0 NA The elogatio of the legth of the focus L is the differece betwee the deepest itersectio with the optical axis 192

z max ad the itersectio of rays with small agles with the optical axis at d, resultig directly i equatio (1).

For example durig focusig by a les i air with a umerical aperture of NA=0.5 iside glass the spherical aberratio becomes sigificat for u-compesated glass thickess larger tha 30 µm (Figure 1).

larger before effects of spherical aberratios become remarkable. Nevertheless, for 3D micro structurig i the volume of glass several 100 µm i thickess with umerical apertures >0.

2 z max ad the itersectio of rays with small agles with the optical axis at d, resultig directly i equatio (1). The spherical aberratio becomes sigificat, if the elogatio L is icreased to the value of the legth of the focus without aberratios 2z R = 2ω0 / NA, ω = 0 λ / πna. For example durig focusig by a les i air with a umerical aperture of NA=0.5 iside glass the spherical aberratio becomes sigificat for u-compesated glass thickess larger tha 30 µm (Figure 1). Because other oliear effects durig the propagatio of ultrafast laser radiatio i trasparet materials also result i a elogatio of the legth of the focus, the u-compesated glass thickess may be somewhat larger before effects of spherical aberratios become remarkable. Nevertheless, for 3D micro structurig i the volume of glass several 100 µm i thickess with umerical apertures >0.4 the spherical aberratios have to be pre-compesated. Therefore, we have equipped the correctio collar of corrected microscope objectives [7] with computer cotrolled motor to allow dyamic pre-compesatio. A commercial galvo scaer (hurryscan 10 by ScaLab) is used for scaig. The galvo mirrors are imaged to the plae of the exit pupil of the telecetric microscope objective usig a 1:1 telescope desiged for miimal aberratios (Figure 2). The correctio collar of the microscope objective is equipped with a piio drive by a computer cotrolled motor. The itegrated system (Figure 3) is combied with a three axes traslatio stage (microstep by Kugler) for steppig i three dimesios. The galvo mirrors, the correctio collar ad the traslatio stage are cotrolled by CAD/CAM software (modified versios of sca2d ad sca3d by Frauhofer ILT). Figure 2: Optics scheme of the set-up, icludig the scaig system cosistig of galvo mirrors, telescope ad microscope objective 12 Rayleigh legth 2z R Elogatio by spherical aberratio L 11 Elogatio by spherical aberratio L [µm] Ucompesated glass thickess: 500 µm 100 µm 30 µm 0 0,0 0,1 0,2 0,3 0,4 0,5 0,6 0,7 0,8 0,9 1,0 Numerical Aperture NA [1] Figure 1: Elogatio of legth of focus L by spherical aberratio due to a plae glass plate 3. Scaer system ad experimetal 10 µm 2 µm Figure 3: Scaig system cosistig of galvo mirrors, telescope ad microscope objective with a motorized correctio collar for pre-compesatio of spherical aberratios Table 1 Objective Parameters of the scaig system with some of the microscope objectives used Focus Ø Precisio Mark velocity Jump velocity 20x/ µm 430 m 103mm/s 360mm/s 40x/ µm 210 m 51mm/s 180mm/s 40x/1.2 W 0.55 µm 210 m 51mm/s 180mm/s 100x/1.3 Oil 0.51 µm 86 m 21mm/s 72mm/s Depedig o the umerical aperture ad the magificatio of the used microscope objective the focus diameter, the precisio, the maximum costat velocity ad the maximum jump velocity result (Table 1). The usable sca- 193

ig field diameter is 25 mm divided by the magificatio of the respective microscope objective.

- 5 MHz. The maximum average power of P av = 1.5 W is adjustable usig a half wavelegth retardatio plate ad a liear polarizer.

The laser beam is trasferred ito the scaig system ad focused by microscope objectives usig a umerical aperture of 0.3 (20x/0.4) ad 0.6 (40x/0.6).

The materials used are sapphire with the crystallie orietatio of (0001), fused silica ad boro-silicate glass (D263 from Schott) with a sample size of 10 mm x 10 mm x 1 mm.

2 Markigs iside trasparet materials Micro markigs i fused silica are produced by direct laser writig 200 µm below the polished surface usig a effective umerical aperture of NA=0.3 (objective 20x/0.

3 ig field diameter is 25 mm divided by the magificatio of the respective microscope objective. The irradiatio is carried out with a Yb-glass fiber chirped pulse amplifier (FCPA) IMRA µjewel with a cetral wavelegth of λ= 1045 m, a pulse duratio τ= 450 fs ad a variable repetitio rate f= 100 khz - 5 MHz. The maximum average power of P av = 1.5 W is adjustable usig a half wavelegth retardatio plate ad a liear polarizer. The laser radiatio is modulated by a acousto-optical modulator (PM 106 from IMRA) triggered by the scaer to gate the laser radiatio o ad off. The laser beam is trasferred ito the scaig system ad focused by microscope objectives usig a umerical aperture of 0.3 (20x/0.4) ad 0.6 (40x/0.6). The sample is mouted o a programmable three axis traslatio stage (accuracy Δx= 100 m, maximum speed v max = 2 mm/s). The materials used are sapphire with the crystallie orietatio of (0001), fused silica ad boro-silicate glass (D263 from Schott) with a sample size of 10 mm x 10 mm x 1 mm. Top ad bottom surfaces are polished to optical quality. The irradiatio is carried out i ambiet air. 4.2 Markigs iside trasparet materials Micro markigs i fused silica are produced by direct laser writig 200 µm below the polished surface usig a effective umerical aperture of NA=0.3 (objective 20x/0.4, beam diameter 5 mm). The spherical aberratios caused by 200 µm glass are pre-compesated usig the correctio collar. Usig a repetitio rate of 500 khz, a average power of 320 mw a a scaig velocity of 100 mm/s a logo is writte i less tha 1 secod (Figure 5). No distortio of the writte grid is observed i the scaig field 1.3 mm i diameter (Figure 5). 4. Examples for high speed scaig 4.1 Waveguide writig Direct laser writig of waveguides i boro-silicate glass (D263 from Schott) is doe usig a microscope objective with umerical aperture of NA=0.6, a repetitio rate of 1 MHz ad a pulse eergy of E P =225 J. The umerical aperture of the resultig waveguides is measured usig far field measuremets ad the refractive idex modificatio is aalyzed usig iterferece microscopy [8]. Straight lies are writte with a scaig velocity of 100 mm/s resultig i homogeeous crack-free waveguides (Figure 4, left). Curved waveguides with radii of µm are writte usig a scaig velocity of 50 mm/s. The curved waveguides exhibit i-homogeeities caused by the vectorizatio of the circles (Figure 4, right). To prevet the ihomogeeities the scaig software has to be modified to allow for costat velocity durig scaig a curve ad the developmet of a adapted processig strategy is ecessary i future work. Figure 5: Markig iside fused silica by direct laser writig: Logo (top) ad detail (bottom) Figure 6: Colored markig iside sapphire by direct laser writig: Logo (left, color by diffractio) ad detail (right) Figure 4: Straight (left) ad curved waveguides (right) i D263 glass by direct fs-laser writig Micro markigs i sapphire are produced by direct laser writig 250 µm below the polished surface usig a effective umerical aperture of NA=0.3. The correctio collar for pre-compesatio of the spherical aberratios is set to 290 m to accout for the differet refractive idex of sapphire. The repetitio rate has bee 500 khz, the average power 320 mw ad the scaig velocity 100 mm/s. A logo 0.8x0.8 mm 2 i size has bee writte cosistig of parallel lies with a offset of 5 µm i about 3 secods (Figure 6). As measured by iterferece microscopy the modified lies exhibit a reduced refractive idex (Δ=- 0.03) ad o cracks are observed (Figure 6, right). Due to 194

$diffractio of light at the laser writte micro grid ad iterferece i the observers eye colors are observed varyig with the agle (Figure 6, left). 4.$ Additioally the sample is moved i perpedicular to the beam propagatio axes (z-sca) usig the micro positioig stage.

Additioally the sample is moved i perpedicular to the beam propagatio axes (z-sca) usig the micro positioig stage.

Figure 7: Micro parts i 1 mm sapphire Usig subsequet wet chemical etchig e.g. i aqueous solutio of hydrofluoric acid the modified volume ca be removed ad the part as well as the shaped hole may be obtaied [9].

aostructures (coheret i-volume sub-wavelegth ripples, aoplaes with a period of 340-350 m [10]), which would prevet the releasig of the etched part. 4.

4 diffractio of light at the laser writte micro grid ad iterferece i the observers eye colors are observed varyig with the agle (Figure 6, left). 4.3 I-volume selective laser etchig 3D micro structures are writte i sapphire 1 mm i thickess usig the same parameters as for direct laser writig of markigs i sapphire described above. Additioally the sample is moved i perpedicular to the beam propagatio axes (z-sca) usig the micro positioig stage. I this way structures 1 mm i diameter ad 1 mm i height (from the bottom to the top of the sample) have bee produced i secods (Figure 7). Figure 7: Micro parts i 1 mm sapphire Usig subsequet wet chemical etchig e.g. i aqueous solutio of hydrofluoric acid the modified volume ca be removed ad the part as well as the shaped hole may be obtaied [9]. Further developmet of the processig strategy is ecessary to prevet the formatio of cracks durig the i-volume selective laser etchig of complex 3D shaped parts ad to prevet the formatio of periodical aostructures (coheret i-volume sub-wavelegth ripples, aoplaes with a period of m [10]), which would prevet the releasig of the etched part. 4.4 Periodical aostructures Micro markigs o the surface of sapphire are produced by direct laser writig usig a umerical aperture of NA=0.6. Usig a repetitio rate of 500 khz, a average power of 350 mw a a scaig velocity of 50 mm/s a picture is writte i 15 secods (Figure 8, top). Usig scaig electro microscopy after cleaig the sample i a ultrasoic bath periodical aostructures (coheretly cotiued sub-wavelegth ripples) are observed i the laser writte lies with a period of 340 m, which is the same period as observed i the volume of sapphire durig ivolume selective laser etchig ad which is also obtaied usig writig velocities of 1 mm/s or less usig the same repetitio rate [10]. At itersectios of perpedicular lies ad at sharp turs a loss of coherece is observed while i rouded curves ad straight lies the sub-wavelegth ripples are coheretly cotiued (Figure 8, bottom). Figure 8: Periodical aostructures by high speed writig i the surface of sapphire: Structure (top) ad detail with coheret sub-wavelegth ripples (bottom) 5. Coclusios A scaig system cosistig of commercial galvo mirrors, imagig optics, microscope objective ad computer cotrolled pre-compesatio of spherical aberratios has bee build to eable high speed micro structurig i the volume of trasparet materials. A umerical aperture of results i small focal volume (< 1 µm 3 ), high precisio (up to less tha 100 m), high writig velocities (up to 360 mm/s) ad high dyamics. Usig existig CAD/CAM software allows for rapid maufacturig of micro structured parts, waveguides, i-volume micro markigs ad ao structures fully exploitig the potetial productivity of today s high repetitio rate FCPA fs-lasers. Straight ad curved waveguides have bee writte i boro-silicate glass usig scaig velocities of mm/s. Micro markigs i the volume of fused silica have bee produced i less tha 1 secod. Crack-free modificatios iside sapphire have bee produced ad a colored markig has bee writte i 3 secods exploitig diffrac- 195

5 tio ad iterferece i the observers eye. 3D structures have bee produced i sapphire usig i-volume selective laser etchig ad coheretly cotiued sub-wavelegth ripples have bee produced with scaig velocity of 50 mm/s exhibitig the same period of 340 m as obtaied by much slower scaig previously. Further work will iclude the developmet of processig strategies for complex shaped 3D micro parts. The trasfer of the process as well as the scaig system to idustrial applicatios is the ext step to icrease the productivity ad precisio exploitig today s reliable efficiet fs-lasers like FCPA lasers. Ackowledgmets ad Appedixes (1) We would like to thak IMRA America for the possibility to use the µjewel laser ad the acousto-optical modulator i the framework of the Premier Applicatio Lab at LLT. (2) We are grateful to Frauhofer ILT for foudig the hardware used to build the scaig system. Refereces [1] K.M. Davis, K. Miura, N. Sugiomto, K. Hirao: Writig waveguides i glass with a femtosecod laser, Optics Letters, 21, (1996), p [2] C. Hatovsky, R.S. Taylor, E. Simova, P.R. Rajeev, D.M. Rayer, V.R. Bhardwaj, P.B. Corkum: Fabricatio of microchaels i glass usig focused femtosecod laser radiatio ad selective chemical etchig, Appl. Phys. A 84, (2006), p. 47 [3] Y. Bellouard, A. Said, M. Duga, P. Bado: Fabricatio of high-aspect ratio micro fluidic chaels ad tuels usig femtosecod laser pulses ad chemical etchig, Optics Express 12, (2004), p [4] K. Sugioka, Y. Haada, K. Midorikawa: 3D Microstructurig of glass by femtosecod laser direct writig ad applicatio to biophotoic microchips, Progress i Electromagetics Research Letters 1, (2008), p [5] S.M. Eato, H. Zhag, P.R. Herma: Heat accumulatio effects i femtosecod laserwritte waveguides with variable repetitio rate. Optical Society of America 13, (2005), p [6] C. B. A. Schaffer, A. Brodeur, E. Mazur: Measuremet Sciece ad Techology, 12, (2001), p [7] Uited States Patet No , (1990) [8] D. Esser, D. Mahlma, D. Wortma, J. Gottma: Iterferece microscopy of femtosecod laser writte waveguides i phosphate glass, Appl. Phys. B 96, (2009) p [9] M. Hörstma-Jugema, J. Gottma, M. Keggehoff: 3D micro structurig of sapphire usig fs-laser irradiatio ad selective etchig, Submitted for publicatio i this joural. [10] J. Gottma, D. Wortma, M. Hörstma- Jugema: Fabricatio of sub-wavelegth surface ripples ad i-volume aostructures by fs-laser iduced selective etchig, Applied Surface Sciece, 255, (2009), p (Received: July 10, 2009, Accepted: November 17, 2009) 196

Geometrical Optics Fiber optics

Phys 322 Lecture 15 Chapter 5 Geometrical Optics Fiber optics First optical commuicatio Alexader Graham Bell 1847-1922 1880: photophoe 4 years after ivetig a telephoe! Fiberoptics: first lightguide 1870: