Abstract– Excimer laser ablation is a flexible microfabrication

technique, suitable for surface structuring of polymers because oftheir high UV absorption and non-thermal ablation behaviour.Due to the non-contact and direct-write nature of the process,laser ablation is a very attractive technology allowing realizationof micro-optical functionality on top of opto-electroniccomponents in a late phase of a heterogeneous assembly.

In this paper we propose a flexible fabrication technique formicrolenses with excimer laser ablation and report on the presentstatus of experimental results on microlenses ablated inpolycarbonate. Based on scanning a polymer surface with anexcimer beam along well-chosen multiple concentric contours, amicrolens of arbitrary shape can be realized. The choice ofablation parameters and the selection of scanning paths arediscussed. Optical performance and lens surface quality areevaluated by imaging experiments, scanning electron microscopyand profilometer measurements.

Keywords– laser ablation, excimer laser, microlens

I. INTRODUCTION

One of the main trends in current opto-electronic devicefabrication, is a continuous miniaturization of the componentsin order to minimize manufacturing costs and maximizeoperation speed. This tendency has been the main drive todevelop micro-optics into a fully fledged discipline handlingmanipulation, imaging and/or focussing of (arrays of) lightbeams where the insertion of macro-optics - generally due to alack of space, is not allowed.

In literature, excimer laser ablation has been reported aviable technology to realize micro-optical components, com-petiting with other fabrication methods like embossing,injection molding, casting, lithography, micro-jet printing andlaser- or e-beam writing. Generally, these technologies sufferfrom low throughput, little flexibility in choice of material orare unsuitable for direct-writing of microstructures onto opto-electronic heterogeneous assemblies, thus requiring furtheralignment and preventing full integration of micro-optics andmicroelectronics.

Excimer laser ablation is a surface structuring techniquebased on the interaction of intense excimer laser pulses with amaterial. These lasers operate in the UV where polymerstypically show high absorption. This results in electronicexcitations which on their turn initiate both thermal and non-thermal processes leading to a dissociation of the polymer andsubsequently local ejection of material. The presence of non-thermal reactions are responsible for a clean, well-definedgeometry of the ablated zone. The lateral dimensions of the

K. Naessens, P. Van Daele and R. Baets are with the Department of

Information Technology, Ghent University (RUG), Gent, Belgium. E-mail:kris.naessens@intec.rug.ac.be .

removed polymer are determined by the laser beam size, whilethe depth of the hole depends on the laser intensity. Typicallythe lateral resolution is limited to a few micron while theablation rate can be as low as a few tens of nanometer. Theseproperties make polymers a very suitable substrate material forexcimer laser ablation.

Fig. 1. Excimer laser ablated hole in polycarbonate.

The fabrication approach we propose here is based onablation with a single circular excimer laser beam for shapingan arbitrary microlens surface. All laser parameters remaininvariant during fabrication, no complex and/or moveablemasks are used and no additional diffusion or thermalprocesses are involved, in contrary to previous reportedablation based methods1,2,3,4,5. After the lens shape has beenrealized, additional laser pulses using a larger beam diameterare applied to enhance the surface finish to optical quality.

II. MICROLENS FABRICATION

In this section we describe the fabrication procedure tomachine microlenses in a polymer substrate. In essence, thetechnique is based on ablating several concentric circulartrenches with scanning ablation. Each groove is realized bymoving the sample stage according to a circular path withdiameter D while the excimer laser is firing pulses at aconstant pulse frequency and energy (Fig. 2). Every time atrench is finished, the laser stops firing, and the stage is movedin order to ablate a new one with a different diameter. Thestart position for ablation of a trench is chosen randomly alongthe contour.

Each trench is determined by its depth, which is propor-tional to the number of overlapping pulses on every position inthe groove, and diameter which will determine the exactprofile of the trench as explained below. By ablating several ofthese concentric trenches with well-chosen depth anddiameter, one is able to fabricate an arbitrary, pre-determinedcircular-symmetric shape. In this paper we will focus on theseshapes which realize optical functions: microlenses.

Flexible Fabrication of Microlenses inPolymers with Excimer Laser Ablation

Kris Naessens, Peter Van Daele, Roel Baets

translation stage with polymersubstrate

maskprojectionlens

beamaperture

excimer beam

1 path 7 paths3 paths

TOP VIEW

Fig. 2. Lens fabrication procedure.

A. Theoretical analysis

We assume that the beam diameter ρ is homogeneous andthat ablation results in a hole with equal depth over its entirearea. In the case of scanning ablation, the depth of ablation ata certain position on the sample will be determined by thenumber of pulses which overlap on that location. Assumingthat the scanning speed and ablation depth per pulse issufficiently low, a smooth ablated surface avoiding a staircasepattern can be achieved. Both assumptions allow an analyticexpression for the ablated trenches. For a typical beamdiameter of 100 µm, an etch depth of 0.1 µm per pulse and acontour velocity of 10 µm/s for the sample stage, the resultingprofile is illustrated in Fig. 3 for 3 values of D: 50, 100 and200 µm, respectively D < ρ, D = ρ and D > ρ.

-90

-75

-60

-45

-30

-15

0

-200 -150 -100 -50 0 50 100 150 200

radial coordinate [um]

tren

ch p

rofi

le [

um

]

D = ρ

D > ρ

D < ρ

Fig. 3. Trench profiles.

Next we assume that a well-considered combination oftrenches can result in the fabrication of an arbitrary pre-determined circular-symmetric microlens. Given a number ofconcentric circular channels, we require an optimization oftheir contour diameters which determine the overlap betweenthe neighbouring channels and can be chosen to achieve asmooth surface in accordance with the intended microlensshape. The scan velocities, which determine the depth perchannel, will not be optimized since this would most probablyresult in values which might lead to staircase or ripple patternson the ablated surface due to respectively too high or too lowvelocities6.

The optimization is performed by a simulated annealingalgorithm, calculating the optimum contour diameters in orderto approximate the desired microlens shape according to aleast squares law.

B. Experimental results

The experiments are carried out with an excimer ArF laser(λ = 193 nm) by means of an optical set-up given in Fig. 4.The substrate is a polycarbonate film of 250 µm thickness.The ablated profiles have been measured by means of

Scanning Electron Microscopy and confocal microscopy (3Dprofilometry) as illustrated in Fig. 5.

mirror

mask

substrate on high-accuracytranslation table

off-axiscamera

mirror

mirror

mirror

on-axiscamera

variableattenuator

projection lens (NA=0.105)

mirror

LED based maskillumination

pyro -electric detector

beam splitter

computer control unit fortranslation stage, focussing,demagnification selectionand attenuator

ArF 193 nmKrF 248 nmCO2 9.3 um

Fig. 4. Excimer laser ablation set-up

After ablation of the microlens, a large excimer beam isused to ablate a thin layer away from the whole lens area atonce. In this way, we accomplish an ultimate removal ofdebris - ejected particles which did not evaporate andremained on the surface in or close to the ablated region - andreduce ripples which might have been introduced duringablation of the lens shape due to non-perfect overlap ofneighboring trenches.

Fig. 4. SEM image of a microlens array (left), 3D profilometermeasurement of a single microlens (right).

CONCLUSIONS

We developed a method to ablate microlenses using asimple laser ablation set-up. The measured surface finish andimaging properties of the ablated lenses are very encouragingin term of roughness and optical quality.

Further research is now focussed on fabrication of (arraysof) microlenses, integrated in an opto-electronic assembly.

ACKNOWLEDGEMENTS

This work has been performed within the framework of theFlemish IWT project ITA-II “VLSI photonics” and theBelgian IUAP-13 project.

Additional thanks goes to the flemish IWT for financialsupport of our laser ablation facilities.

REFERENCES

[1] X. Wang, J.R. Leger and R.H. Prediker, Appl. Opt. Vol. 36 No. 20,p. 4660 - 4665, 1997

[2] R. Matz, H. Weber, G. Weimann, Appl. Phys. A Vol. 65, p. 349 -353, 1997

[3] Stephen Mihailov and Sylvain Lazare, Appl. Opt. Vol. 32 No. 31, p.6211 - 6218, 1993

[4] M. Wakaki, Y. Komachi and G. Kanai, Appl. Opt. Vol. 37 No. 4, p.627 - 631, 1998

[5] F. Beinhorn , J. Ihlemann , K. Luther and J. Troe , Vol. 68 No. 6, p.709 – 713

[6] K. Naessens, P. Van Daele and R. Baets, Proceedings of LaserPrecision Microfabrication Conference, Singapore, 2001