Surface Texturing of High-Power Flip-Chip LEDs by Femtosecond Laser Direct Structuring

1250 IEEE JOURNAL OF SELECTED TOPICS IN QUANTUM ELECTRONICS, VOL. 15, NO. 4, JULY/AUGUST 2009

Surface Texturing of High-Power Flip-Chip LEDs byFemtosecond Laser Direct Structuring

Ladislav Kuna, Anja Haase, Frank Reil, Christian Sommer, Joachim R. Krenn, Paul Hartmann, Peter Pachler,Stefan Tasch, and Franz Peter Wenzl

Abstract—We report on the femtosecond laser direct structuringof the sapphire flipside surfaces of high-power flip-chip LEDs. It isfound that diameter and depth of the created submicrometer-sizedholes can be manipulated by varying the laser power at a constantnumber of laser pulses. This method enables the control of thestructure sizes with high precision. Our study shows that the lightextraction from such LEDs increases strongly with increasing holesizes.

Index Terms—LEDs, solid-state lighting.

I. INTRODUCTION

S INCE the invention of efficient blue LEDs [1], [2], solid-state lighting has been attracting increasing interest due

to its huge potential as the illumination technology of the fu-ture [3]–[5]. The advantages of solid-state lighting sources aremanifold, such as lower energy consumption and the possibil-ities of entirely new lighting effects. Generally, there are twobasic methods for the generation of white light by such de-vices [6]–[8]: 1) the red–green–blue (RGB) concept in whichthe light emitted from red, green, and blue monochromatic LEDsis mixed and 2) the color conversion concept, where part of thelight emitted from a blue LED is transformed into yellow lightby a phosphor layer, resulting in a whitish color in combinationwith the transmitted blue light. Generally, for both concepts, itis essential to further improve the radiative output of the solid-state lighting sources in order to successfully compete in themarket.

Among the different device designs that have been introducedover the past years in order to improve the external quantum ef-ficiency of a LED die, the flip-chip technology is one of the mostimportant. This technique not only benefits from improved lightextraction efficiency but also from a better thermal management

Manuscript received December 1, 2008; revised January 15, 2009. Firstpublished May 12, 2009; current version published August 5, 2009.

L. Kuna, A. Haase, F. Reil, C. Sommer, and F. P. Wenzl are withthe Institute of Nanostructured Materials and Photonics, Joanneum Re-search Forschungsges.mbH, Weiz A-8160, Austria (e-mail: [email protected]; [email protected]; [email protected]; [email protected]; [email protected]).

J. R. Krenn is with the University of Graz, A-8010 Graz, Austria, and alsowith the Institute of Nanostructured Materials and Photonics, Joanneum Re-search Forschungsges.mbH, Weiz A-8160, Austria (e-mail: [email protected]).

P. Hartmann, P. Pachler, and S. Tasch are with the Ledon Lighting Jen-nersdorf GmbH, A-8380 Jennersdorf, Austria (e-mail: [email protected]; [email protected]; [email protected]).

Color versions of one or more of the figures in this paper are available onlineat http://ieeexplore.ieee.org.

Digital Object Identifier 10.1109/JSTQE.2009.2018132

due to the close connection between the active layer and thepackage heat sink [4].

But, the rather large refractive index contrast between the sap-phire substrate (n = 1.766) and the ambient (n = 1) limits thelight extraction efficiency even of flip-chip LEDs. However, thetexturing of the flipside of the sapphire substrate can be an ef-ficient way to remedy this shortcoming [9]–[11]. Its underlyingeffect is based on angular randomization [12], which enables alarger part of the otherwise trapped photons to escape from thedie.

One of the biggest challenges in the surface texturing of asapphire surface is posed by its material properties: sapphire ischemically inert and highly resistive to acids at room tempera-ture [13]. Therefore, wet and dry structuring processes [13]–[15]are very challenging, especially for double-side-structured flip-chip LEDs [11].

From this point of view, there is a strong need for alterna-tive structuring techniques. Promising methods in this regardare based on the direct writing of a given structure into thesapphire. While the focused ion beam (FIB) technique, whichhas been applied for the inscription of microlens arrays [16]and, more recently, even more complex objects, such as a Fres-nel lens [17] and 2-D structures [18] into the sapphire surfaceof flip-chip LEDs, is known to be time-consuming and expen-sive, femtosecond (fs) laser direct structuring provides a fasterand more cost-efficient alternative. This technique, which de-scribes the microprocessing of various materials by means ofirradiation with fs laser pulses, is an aspiring technology inpresent-day materials sciences [19]. In addition to the local ma-nipulation of the bulk material with respect to its absorptionproperties or its refractive index [20]–[22] as well as the forma-tion of voids in it [23], material ablation by means of an fs laserhas become a valuable tool in modern day material process-ing, especially for sapphire substrates [24]–[27]. A fs laser pro-vides the highest inscription accuracy, compared to other lasertypes [28].

In this context, we have recently reported on the benefit of fs-laser structuring for the fabrication of continuous groove lineson the sapphire substrate flipside surfaces of flip-chip LEDs [29]for the purpose of light extraction enhancement. In the presentpaper, we show that with an alternative procedure, the varietyof possible inscription patterns can be largely expanded.

In particular, individual holes with sizes that can be varied ona submicrometer scale can be easily fabricated on the flipsidesubstrate surfaces of high-power flip-chip LEDs. Such structuresmay act as building blocks for future photonic applications withsubwavelength dimensions.

1077-260X/$25.00 © 2009 IEEE

KUNA et al.: SURFACE TEXTURING OF HIGH-POWER FLIP-CHIP LEDS BY FEMTOSECOND LASER DIRECT STRUCTURING 1251

Fig. 1. Schematic for the fs-laser structuring of the flipside surface of a flip-chip LED with point arrays.

II. EXPERIMENTAL DETAILS

Fig. 1 schematically shows the process scheme for structur-ing the sapphire’s flipside surface. The laser system has, as itscore element, a commercial 1-kHz fs Ti:Sapphire laser amplifier(Spitfire, Spectra Physics) operating at a wavelength of 800 nmand delivering pulse widths of ∼150 fs. A high-precision stage(Aerotech) with the LED fixed on a XY platform and the mi-croscope objective mounted on a vertical Z-stage is used for thepositional control during the structuring process. The fs laserpulses are focused onto the sapphire’s surface using a 60× ob-jective lens with a numerical aperture (NA) of 0.85 at a workingdistance of 0.280 mm. A detailed description of the laser setupand the alignment of the sapphire surface with respect to theoptical axis can be found in [29].

The LEDs used in this study are commercially available blue-emitting (peak wavelength about 460 nm) high-power GaN-based LED dice in flip-chip configuration that are mounted bychip-on-board technology on a printed circuit board. The lateraldimensions of the sapphire substrate are on the order of 1 mm× 1 mm, and the height is about 81 µm.

As reported previously [29], the fs laser direct structuringtechnique enables the surface modification within selected areasof the sapphire surface [30]. Using this technique, three patternsfabricated with different processing parameters were inscribedinto the surface of one LED, as schematically shown in Fig. 2.

Each of these patterns (see gray squares in Fig. 2) comprisesa 100 µm × 100 µm array consisting of individual points witha pitch of 2 µm both in x and y directions. It was ensuredthat the individual patterns were fabricated in locations witha homogeneous light emission, i.e., inside the regions of theelectrode structures. The white rectangles next to the patternedareas in Fig. 2 represent nonstructured areas that were used asa reference for the determination of the modifications of lightextraction.

In order to fabricate the arrays, the design of the pattern wasfirst generated as a bitmap file. This file contains pixels having

Fig. 2. Optical microscope image of the flip-chip LED (top view). The graysquares mark the structured areas. The white rectangles near the gray squaresmark the reference areas that are used for the determination of the enhancementof light extraction.

logical values that were assigned the values of 0, which refers to“laser off,” and 1, which refers to “laser on,” for a given position.These values and the information of the bit position withinthe pattern were then transferred into the computer numericalcontrol (CNC) code controlling the fs structuring procedure. Inthis point-by-point algorithm, the 1-kHz repetition rate of thelaser system at an exposure time of 5 ms/point corresponds to anumber of five laser pulses per point. In the present study, thelaser power was varied from 70 to 120 µW with an increment of10 µW, while the exposure time was kept constant at five pulsesper point.

Since sapphire particles become airborne during the ablationprocess, it is necessary to flood the area with N2 in order tolargely prevent ablated particles from accumulating on the mi-croscope objective, which is only 280 µm away from its focalspot; otherwise, the incident laser power and the beam profilewould degrade over time. In order to vastly remove residualablated particles, which accumulated on top of the sapphiresubstrate, the LEDs were blown off with a nitrogen source (7bar) after the structuring process.

In order to determine the enhancement of light extraction ofthe individual surface areas, the light intensity distribution wasrecorded by a charge-coupled device (CCD) camera (PulnixTM1010) in combination with an optical microscope (OlympusModel BX51, objective 100×, NA = 0.95) at a fixed LEDcurrent of 10 mA. The recorded light intensity distribution wasconverted into a color scale ranging from red (maximum) toblue (minimum) by means of image processing (software: LBA-400PC from SPIRICON, Inc.).

Assuming that the light intensity within the marked areasin Fig. 2 is, to a good degree, constant, the enhancement oflight extraction can be determined as the difference betweenthe intensities on the patterned surface I1 and the referencesurface I2 , normalized to the reference surface I2 : ηeff = (I1 −I2)/I2 × 100%.

In order to assess the quality of the fabricated holes,height profiles of the structured areas were measured using a


Fig. 3. SEM image of a point array fabricated with a laser power of 100 µW(five pulses per point).

commercial atomic force microscope (AFM) in tapping mode(Dimension 3100, Digital Instruments, Inc.; AFM software:NanoScope 6.13r1, Digital Instruments, Inc.). These measure-ments were complimented by scanning electron microscopic(SEM) investigations with a “RAITH 100-2” SEM, which re-quired the deposition of a 20-nm-thick gold on top of the sam-ples surfaces in order to ensure electrical conduction. Finally,the performance of a LED structured with 90 µW was deter-mined at a current of 100 mA before and after structuring, usinga goniophotometer (LEGDON 100, Instrument Systems).

III. RESULTS AND DISCUSSIONS

Laser ablation of sapphire allows the fabrication of a wealthof structures, with sizes ranging from far below the dimensionsof the laser spot up to tens of micrometer. The shape and thesizes of the structures mainly depend on the applied laser power,the number of laser pulses, the pulse duration, and the pulse rep-etition rate [31]. From all of these options, we have chosen thelaser power as the processing parameter that is varied throughoutthis study in order to evaluate to what extent the light extrac-tion from the flipside surface of the sapphire substrates can bemodified. For that purpose, the laser power was varied with anincrement of 10 µW between 70 and 120 µW, and the pulseexposure was kept constant at five pulses per point. This rangeof laser powers was in particular chosen since no changes of theI–V characteristics of the LEDs before and after laser structuringwere measured. Higher powers were avoided since they werefound to be prone to damage the LEDs, especially with regard tothe sapphire substrate itself (occurrence of micro-cracks). Fig. 3shows a SEM image of a point array inscribed with a laser powerof 100 µW.

Fig. 4 shows high-resolution SEM images of the patternsfabricated with laser powers of 80, 100, and 120 µW. As canbe seen, the laser pulses create a circular ablation crater on thesapphire surface. From Fig. 4, it is obvious that the holes growin depth and to a lower extent also in diameter with increasinglaser power. Also, the amount of ablation debris around and inbetween the craters increases, in spite of the nitrogen stream. Thetopologies of two arrays that were inscribed with laser powers of80 µW and 100 µW are shown in the AFM scans in Fig. 5(a) and

Fig. 4. Detailed SEM images of point arrays that were fabricated with laserpowers of (a) 80 µW, (b) 100 µW, and (c) 120 µW, at five pulses per point. In(d), a single hole fabricated with a laser power of 100 µW is shown.

(b), respectively. The measurements confirm the observations bythe SEM that the inscribed structures become deeper and widerwith increasing laser power. In order to quantify this behavior,Fig. 6 shows AFM height profiles of the structures fabricatedwith laser powers of 70 (a), 80 (b), 90 (c), 100 (d), and 110 µW(e). For 120 µW, the depth–width aspect ratio of the holes turnedout to be too high in order to permit reliable AFM measurements.The depth profile of the holes is to a good degree of Gaussiantype, which is due to the Gaussian intensity profile of the focalspot.

The measured depth profiles show a clear correlation betweenthe applied laser power (70, 80, 90, 100, and 110 µW) andthe corresponding depths of the holes (261, 408, 545, 750, and880 nm), the depths increase with increasing laser power. On theother hand, the hole widths are affected comparably less (950,1020, 1180, 1190, and 1260 nm, respectively, as determined at


Fig. 5. AFM area scans of point arrays fabricated with laser powers of (a)80 µW and (b) 100 µW, five pulses per point, on the flipside surface of aflip-chip LED.

Fig. 6. AFM height profiles of the holes fabricated with laser powers of (a)70 µW, (b) 80 µW, (c) 90 µW, (d) 100 µW, and (e) 110 µW, five pulses perpoint.

Fig. 7. XY light intensity distribution from a portion of the point arrayfabricated with a laser power of 120 µW and the nearby area that is used as areference. The light intensity distribution is determined with the help of a CCDcamera in combination with an optical microscope.

the flip-side sapphire surface level). The values given for thewidths and depths are mean values that were determined fromthe analysis of 20 individual holes from each of the individualpatterns. It has to be noted that for an identical laser power,some height variations among the individual holes occur that, inits most extreme case, may reach up to about 10 % of the meanvalue. Similar variations were also observed for the widths.

The present study clearly highlights the potential of fs directlaser writing for the structuring of flipside surfaces of flip-chipLEDs on a submicrometer scale, and shows that the sizes ofthe individual points can be controlled with high precision byan appropriate choice of the processing conditions, in particularthe laser power.

In order to correlate the hole sizes with the resulting modi-fication of light extraction, the latter was determined by mea-suring the XY light intensity distribution of some portion ofthe structured areas and taking the nearby nonstructured areasas a reference. As an example, Fig. 7 shows the colour-codedlight intensity distribution obtained from a section of the arraythat was inscribed with a laser power of 120 µW and the nearbynonstructured area, which are defined by two cursors, 1 and 2,respectively.

Here, blue corresponds to the lowest and red to the highestlight intensity. Clearly, the structured region shows an enhance-ment of the light extraction. Note that some of these holes showa larger enhancement of light extraction than others. This mightbe due to the fact that some holes are both deeper and widerthan the others or due to some microstructure within the light-emitting area of the LED. In addition, the debris (which all havean individual roughness superposed) surrounding the individualholes may also have some local impact on the light extraction.The light extraction enhancement of all remaining patterns wasdetermined likewise, and the results are summarized in Table Iand graphically shown in Fig. 8.


TABLE IHOLE DEPTHS, PROFILE AREAS, AND ENHANCEMENT OF LIGHT EXTRACTION

FOR THE HOLES FABRICATED AT THE INDIVIDUAL LASER POWERS. THE

VALUES FOR THE HOLE DEPTHS AND THE PROFILE AREAS ARE MEAN VALUES

AS DETERMINED FROM 20 HOLES FOR EACH PATTERN

Fig. 8. Enhancement of light extraction from the flipside surface of the sap-phire substrate and profile areas of the corresponding holes versus the laserpower.

As is evident from this figure, the enhancement of light extrac-tion increases almost linearly with the laser power the individualpatterns were fabricated with. An enhancement of the light ex-traction by 10% is measured for the pattern inscribed with a laserpower of 70 µW while the enhancement of the light extractionreaches 45.6% in case of the pattern that was inscribed with alaser power of 120 µW.

In order to correlate the hole dimensions with the enhance-ment of light extraction, the profile area for the individual holeswas determined from the height profiles by calculating the areaenclosed by a single-hole profile (Origin 8.0 software) using thesurface of the sapphire substrate as the upper level. Fig. 8 showsthat the profile area also increases almost linearly with the laserpower the individual patterns were fabricated with. Again, thevalues given for the profile areas were determined by averagingthe values of the profile areas of those 20 holes that were used todetermine the mean hole depths and widths. These observationsare in agreement with those reported by Han et al. [9], whoconcluded that the light extraction increases with the effectivesurface area of the texture on the sapphire.

Fig. 9. Radiation patterns of a flip-chip LED before and after structuring ofthe sapphire substrate’s flipside surface. The hole square array with a pitch of2 µm covered the entire flipside surface (1 mm × 1 mm). The inscription wasperformed at a laser power of 90 µW and five pulses per point.

As discussed before, the values for the enhancement of thelight extraction were determined by a comparison of the lightintensities of the structured and the nonstructured areas on theflipside surface of the sapphire substrate. In order to get animpression of the enhancement of light extraction of the wholedevice at usual working conditions, the whole flipside surfacearea of one LED (1 × 1mm2) was identically structured in thesame way (square point array with a pitch of 2 µm, five laserpulses per point) at a laser power of 90 µW. The output power ofthe LED was measured with a goniophotometer before and afterthe structuring process at a current of 100 mA. The radiationpatterns before and after laser ablation, respectively, are shownin Fig. 9. The obtained enhancement of the light extraction asmeasured in this way is 22%.

It should be noted that this enhancement is of the same orderas that obtained in other studies on flipside surface structuringof the sapphire substrates using similar structures that were fab-ricated by wet and dry chemical etching techniques (albeit it hasto be considered that the total enhancement of light extractionalso depends on the spatial density of the structures (number ofstructures and/or structured area within a given area).

For example, Han et al. [9] reported a 40.2% extraction en-hancement from the flipside surface (as determined by a pho-todetector) of the sapphire substrate patterned with a mesh-typestructure having a depth of 0.4 µm and a pitch of 13 µm. Shenet al. [11] reported on an enhancement of the output power(from 98.1 to 121.5 mW) comparing flip-chip LEDs fabricatedon a patterned sapphire substrate without and with additionalflipside patterning of the sapphire substrate (double-side pat-terned sapphire) using an integrated sphere detector from top ofthe devices. The structure consisted of a hole pattern with holeetching depth, hole diameter, and spacing of 1, 3, and 3 µm.This corresponds to an extraction enhancement of 23.8% usingthe calculation method as discussed before.


IV. CONCLUSION

It has been shown that fs-laser structuring has high potentialfor the structuring of the flipside surfaces of flip-chip LEDssince it avoids complex wet or dry chemical etching processesand is time-efficient in comparison with other direct structuringmethods like FIB. Furthermore, the variation of the processparameters and the process algorithms enables the fabricationof a wealth of diverse patterns, which can be used to controland to enhance the light extraction. In particular, it was foundthat the variation of the laser power affects the sizes of theinscribed holes on a submicrometer scale, and that the lightextraction increases with the hole sizes. Although in this studyonly the dependence of the hole geometry on the laser powerhas been investigated, it is obvious that a variation of the otherprocessing parameters, such as the number of laser pulses, thepulse duration, and the pulse repetition rate, have a strong impacton the hole geometry, and will facilitate the fabrication of largevariety of individual structures. This could be used to fine-tuneindividual values for the enhancement of light extraction, since ahigher number of laser pulses at a lower laser power may induceholes with similar depth but with somewhat different widths,and therefore, different profile areas.

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Ladislav Kuna was born in Piestany, Czechoslovakia, in 1972. He received theDiploma in microelectronics and laser technology from the Military Academy,Liptovsky Mikulas, Slovakia, and the Ph.D. degree in electrical engineeringfrom the Slovak University of Technology, Bratislava, Slovakia in 1995 and2002, respectively.

From 1997 to 2001, he was a Research Scientist at the International LaserCenter, Bratislava. From 2001 to 2003, he was a member of the Ultrafast Dy-namics Group, Institute of Physical Chemistry, University of Vienna, Austria,


where he was engaged in femtosecond laser spectroscopy of polymer materials.He is currently a Research Scientist at the Institute of Nanostructured Materialsand Photonics, JOANNEUM RESEARCH Forschungsges.mbH, Weiz, Austria,where he is involved in the fields of femtosecond laser micromachining andsolid-state lighting.

Anja Haase received the Diploma and the Ph.D. degree in chemistry from GrazUniversity of Technology, Graz, Austria, in 1996 and 1999, respectively.

She joined the Institute of Nanostructured Materials and Photonics, JOAN-NEUM RESEARCH Forschungsges.mbH, Weiz, Austria, where she is currentlya group leader in organic sensors and nanoimprinting technology.

Frank Reil received the M.A. and Ph.D. degrees in physics from Duke Univer-sity, Durham, NC, in 1999 and 2003, respectively.

Until 2008, he was with the University of Munster, Germany, and the Karl-Franzens-University Graz, Austria, where he was engaged in surface–plasmonicinteractions between metallic nanostructures and fluorescent molecules. In 2008,he joined the Institute of Nanostructured Materials and Photonics, JOANNEUMRESEARCH Forschungsges.mbH, Weiz, Austria, where he is currently engagedin the research of white LEDs.

Christian Sommer received the Diploma and the Ph.D degree in optics from theInstitute of Theoretical Physics, Graz University of Technology, Graz, Austria,in 2002 and 2004, respectively.

He is currently with the Institute of Nanostructured Materials and Photonics,JOANNEUM RESEARCH Forschungsges.mbH, Weiz, Austria. His currentresearch interests include optical simulations, especially in the fields of solid-state lighting and white light conversion.

Joachim R. Krenn received the Ph.D. degree from the University of Graz,Graz, Austria, in 1996.

He was a Postdoctoral Fellow at the University of Dijon. He is currently anAssociate Professor in the University of Graz. He is also the Director of the In-stitute of Nanostructured Materials and Photonics, JOANNEUM RESEARCHForschungsges mbH, Weiz, Austria. He was an Invited Professor in the Uni-versity of Dijon in 2002 and University of Paris 7 in 2003. He has authoredor coauthored more than 100 peer-reviewed papers in international journals.He is the Editor of the Springer journals: Applied Physics A and Plasmonics.His current research interests include optoelectronic properties of organic andorganic–inorganic materials, the optics of nanoscale systems, surface plasmons,surface-enhanced spectroscopy, and optical near-field microscopy.

Paul Hartmann was born in Austria in 1966. He received the Diploma degreein physics from Graz University of Technology, Graz, Austria, and the Ph.D.degree in experimental physics from Karl-Franzens University, Graz.

For ten years, he was with AVL Medical Instruments, Graz, where he wasengaged in research on optical chemical sensors. He was a Project Managerin the Product Development Team, Roche Diagnostics GmbH, Graz, wherehe was involved in the new concepts for critical care analyzers. In 2005, hejoined TridonicAtco Optoelectronics GmbH, Jennersdorf, Austria, as the Headof the R&D Department, where he is currently the Head of the Research andTechnology Department of the newly formed Zumtobel LED Division.

Peter Pachler was born in Austria in 1970. He received the Graduate degree inphysics from Graz University of Technology, Graz, Austria.

He joined TridonicAtco Optoelectronics GmbH, Jennersdorf, Austria, wherehe is currently a Group Leader in the Research and Technology Department ofthe newly formed Zumtobel LED Division, and is engaged in the research oninorganic LED technology.

Stefan Tasch was born in Austria in 1968. He received the Diploma degree inphysics from Graz University of Technology, Graz, Austria, and the Ph.D. degreein technical sciences for his work on organic polymers for electronic applicationsfrom the Institute of Solid State Physics, Graz University of Technology, in 1995.

He was an Assistant at the Institute of Solid State Physics. He was the Founderof Lumitech GmbH, St. Martin/Raab, Austria, where he was the chief executiveofficer (CEO) of the joint venture of Lumitech and TridonicAtco, TridonicAtcoOptoelectronics GmbH, in 2001, and was engaged in the built up of a successfultechnology and business around chip-on-board LED modules for signage andautomotive applications. He was one of the key drivers for generation of thenew Zumtobel LED Division, where he is currently the CEO of Ledon LightingJennerdorf GmbH and the Head of Innovation.

Franz Peter Wenzl received the Diploma degree in physics from Graz Univer-sity of Technology, Graz, Austria, and the Ph.D. degree in technical sciencesfor his work on organic LEDs and light-emitting electrochemical cells from theInstitute of Solid State Physics, Graz University of Technology, in 2004.

He joined the Institute of Nanostructured Materials and Photonics, JOAN-NEUM RESEARCH Forschungsges mbH, Weiz, Austria, where he is currentlyengaged in inorganic LED technology and white light conversion.

Surface Texturing of High-Power Flip-Chip LEDs by Femtosecond Laser Direct Structuring

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