1128 IEEE TRANSACTIONS ON ELECTRON DEVICES, VOL. 63, NO. 3, MARCH 2016

3-D Microlens Phosphor With CurvaturesManufactured by Imprinting for

Chip-on-Board Light-Emitting DiodesHua Xiao, Tien-Mo Shih, Zi-Quan Guo, Yue Lin, Yi-Jun Lu, and Zhong Chen

Abstract— High-power light-emitting diodes (LEDs) assembledin 3-D microlens-phosphor structures are investigated. For theproposed design, the phosphor–air interface with arrayed high-valued curvatures replaces the conventional flat counterpart,enabling abundant randomized lights to exit. Monte Carlo algo-rithm ray tracing and finite-element method were utilized in thispaper to analyze the light propagation and thermal migration,simulatively indicating that the redirecting regulation and heatdispassion way of the proposed architecture are considerablydifferent. Experimentally, in comparison with the conventionalflat chip-on-board (COB) LEDs, the proposed single-layeredstructure is capable of improving the luminous efficacy ofradiation by 38.1%. For multilayered structures, within whichthe light transmits first through red phosphor layer and thenyellow phosphor layer molded with microlens, a 7% increasein quantum efficiency and an 8.7 increase in color renderingindex have been achieved. The proposed structure can guide LEDmanufacturing communities to significantly improve the opticalperformance and designs of COB packaging.

Index Terms— Chip-on-board (COB) packaging, light-emittingdiodes (LEDs), microlens structure.

I. INTRODUCTION

IN DECADES, light-emitting diodes (LEDs) have becomeone of the most typical and attractive solid-state light-

ing (SSL) devices with great potential to replace the traditionalincandescent and florescent lamps due to their promisingcharacteristics, such as high efficiency, superior reliability,volume compactness, and ecofriendliness [1], [2]. In general,white light can be achieved by blending blue lights from LEDs

Manuscript received November 24, 2015; revised January 14, 2016;accepted January 15, 2016. Date of publication January 28, 2016; date ofcurrent version February 23, 2016. This work was supported in part by the863 Project of China under Grant 2013 AA03A107, in part by the MajorScience and Technology Project between University-Industry Cooperation inFujian Province under Grant 2013 H6024, in part by the National NaturalScience Foundation of China under Grant 61504112, and in part by theInstitute for Complex Adaptive Matter, University of California at Davis,Davis, CA, USA, under Grant ICAM-UCD13-08291. The review of this paperwas arranged by Editor C. Surya.

H. Xiao, Z.-Q. Guo, Y. Lin, Y.-J. Lu, and Z. Chen are with theFujian Engineering Research Center for Solid-State Lighting and theCollaborative Innovation Center for Optoelectronic Semiconductors andEfficient Devices, Department of Electronic Science, Xiamen University,Xiamen 361005, China (e-mail: changsixie@sina.com; zqguo@xmu.edu.cn;yue.lin@xmu.edu.cn; yjlu@xmu.edu.cn; chenz@xmu.edu.cn).

T.-M. Shih is with the Department of Physics, Xiamen University,Xiamen 361005, China, and also with the Institute for Complex AdaptiveMatter, University of California at Davis, Davis, CA 95616 USA (e-mail:tmshih@xmu.edu.cn).

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

Digital Object Identifier 10.1109/TED.2016.2519682

chips with yellow/red lights from phosphors [3]. In comparisonwith transparent ceramics [4], quantum dots [5], and phosphorglasses [6], silicone-combined phosphors have received con-siderable attention from the LED industry because of theirexcellent optical properties and strong operabilities [7], [8].With the development of high-brightness SSL, there has beena growing zest in efficient optically designed LED modules.Interesting architectures, such as domed-phosphor layer [9],cymbal-shaped-phosphor structure [10], and photonic-crystal-assisted ceramic phosphor [11], help to improve the opticaland chromatic performance of LED devices and broaden thediversity of LED modeling.

Huh et al. [12] reported an enhancement in light emissionand electrical efficiency of an Si nanocrystal LED by employ-ing indium-tin-oxide nanowires. According to his report,light output power and wall-plug efficiency were improvedby 45% and 38%, respectively. Furthermore, Chen et al. [13]improved the correlated color temperature (CCT) uniformityand the luminous flux by integrating ZrO2 nanoparticles intowhite LEDs, providing a 12% enhancement in the luminousflux over the conventional dispersing structures, and reducingthe CCT deviation from 522 to 7 K in a range of −70°–70°.These significant research and development progresses pri-marily focus on light outputs and packaging-structure-relatedcharacteristics, and are proven to be effective approachesto ameliorate the LED optical and chromatic performance.However, new phosphor architectures with simple manufac-turability, as well as excellent optical and thermal performance,are highly expected.

In this paper, a 3-D microlens-phosphor structure for chip-on-board (COB) LEDs is proposed with main attention paid toimprovements on quantum efficiency (QE), luminous efficacyof radiation (LER), color rendering index (CRI), and phosphor-layer cooling. Due to the installment of the hemisphericalmicrolens array at the phosphor up layer, photons can mostlyavoid total internal reflection (TIR) and can escape throughthe lens surfaces of high curvatures. Optical characteristics ofQE, LER, CRI, and cooling have been considerably improvedin comparison with the conventional counterpart.

II. EXPERIMENTS

The blue-light chip array was ultrasonically bonded witha circular COB substrate (diameter = 18 mm) to serveas the light source. Transparent silicone is utilized for bothprotection and gap-filling for the chips. Ninety six blue-light

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XIAO et al.: 3-D MICROLENS PHOSPHOR WITH CURVATURES MANUFACTURED BY IMPRINTING FOR COB LEDs 1129

Fig. 1. (a) Flat COB. (b) Single-layered microlens. (c) Red–yellow microlens.(d) Yellow–red microlens. (e) Tilted-top view of microlens with 50×. (f) Topview of microlens with 200×.

chips (200 × 300 μm2 each) provided a wavelength centeringat 460 nm and an output power of 5 W at 150 mA. TheYAG:Ce3+ phosphor powder and the nitride-based Eu2+ iondoping phosphor powder were used to yield yellow and redhumps in the spectra. The latter powder, which plays essentialroles in achieving high CRI and low CCT value for generallighting applications, has widely been used to yield broad-banded red emission with decent color purity. Silicone gel wasmixed with the phosphor powder to form desired shapes withthe aid of a macromolecular-compounded molding machines.The temperature is maintained at 120 °C during the moldingprocess for the purpose of rapid solidification. Afterward,mixtures were imprinted on LED light source using the gelcombined with mechanical forces. We controlled concentra-tions of both the yellow and red phosphor layers at 12 wt.%.In Fig. 1(c) and (d), the thickness and diameter of the singlephosphor layer were controlled at 500 μm and 18 mm, respec-tively, and the whole thickness of the multilayer phosphorbecame 1 mm. For the microlens, the diameter measured500 μm, and the filling coefficient of this array was 100%,indicating that the lens are located closely to one anotherwithout gaps, as shown in Fig. 1(e) and (f). Regarding theconventional flat COB module [Fig. 1(a)], the phosphor layerwas imprinted directly and smoothly on the top of the LEDlight source. In Fig. 1(b), we proposed the single-layered 3-Dmicrolens structure on the basis of the structure in Fig. 1(a),and defined it as Y module. Furthermore, the multilayeredmicrolens phosphor modules were shown in Fig. 1(c) and (d).In Fig. 1(c), the blue light transmitted first through the redphosphor layer and then the yellow layer, justifying namesas red–yellow microlens module (RY module). Similarly, themodule in Fig. 1(d) was named yellow–red microlens module(YR module). Tilted-top and top views of the microlensstructure were taken by a microscope with the amplificationfactor of 50× [Fig. 1(e)] and 200× [Fig. 1(f)], respectively.

III. RESULTS AND DISCUSSION

Fig. 2(a) and (b) shows the graphical result of the combi-nation of Monte Carlo algorithm and ray tracing method [14]

Fig. 2. Graphical result of the combination of Monte Carlo algorithm andray tracing method for (a) flat COB and (b) microlens-phosphor modules.(c) Light propagation in flat COB and microlens-phosphor module.

for both the flat-COB and microlens-phosphor modules. Thesimulation input data are taken to be the same as V (λ) thoseused in the experiment. Through simulation, we discoveredthat with the utilization of microlens-phosphor module, thelight output increased from the conventional 3.56–3.87 W(input optical power of blue lights equals 5 W), improvedby 8.7%. The light spatial distribution changed as well forthe proposed module, and will be discussed in the latterpart. Moreover, light propagations in flat COB and microlens-phosphor modules are shown in Fig. 2(c). Inside the formermodule, a portion of rays meets the criterion of TIR atthe interface between the phosphor and the air, and thus isreflected backward. By contrast, with hemispherical bumpsreplacing the flat surface, curvatures will play a critical role inreducing incident angles of exiting light beams. For example,the rightmost beam intersects with the flat surface at θ1incident angle (θ1 ≈ 38° ∼ 45° [15]). When the flat surface isreplaced with a surface having high curvature, the same direc-tion light beam will form a much smaller incident angle θ2,as clearly shown. The internal angle, which between lightbeams emerged from the phosphor and vertical direction,becomes smaller with the microlens curvatures, showing itsability of redirecting rays. Collectively and statistically, thesemicrolens bumps greatly reduce the probability of photonsundergoing TIR and being trapped inside the phosphor layer.

The spectral power distributions (SPDs) of four structuresand the CIE photopic spectral luminous efficiency functionV (λ) are shown in Fig. 3. In particular, the Y module providesa spectrum that coincides satisfactorily with V (λ) in yellow-light waveband and, therefore, possesses a relatively highlevel of radiation response to eyesight. Next, because of thepresence of Eu2+ ion in the nitride phosphor powder ofthe multilayered microlens-phosphor modules, the spectra ofthese modules shift toward long wavelengths relatively to theY module. Therefore, in comparison with the monochromaticmodules (conventional-COB and Y modules), stokes energyshifts in polychromatic modules (RY and YR modules) that areinduced during the florescent process reduce a certain amount

1130 IEEE TRANSACTIONS ON ELECTRON DEVICES, VOL. 63, NO. 3, MARCH 2016

Fig. 3. Normalized SPDs of four modules and V (λ) versus wavelength.

of generated light intensities. This provide explanation why thelight intensity of Y module is higher than that of RY module,as shown in Fig. 2(b).

To measure the device optical properties, we adopt def-initions of QE and LER. The QE defined as the ratio ofthe emission-photon number divided by the excitation-photonnumber

QE = Nem

Nex=

∫ λ2λ1

φem(λem)/( hc

λem

)dλem

∫ λ′2

λ′1

φex(λex)/( hc

λex

)dλex

=∫ λ2λ1

λemφem(λem)dλem∫ λ′

2λ′

1λexφex(λex)dλex

.

The LER defined as the ratio of the banded radiation perceivedby human eyes to the total incoming electromagnetic radiation

LER = �V

�ex= 683 · ∫ λ2

λ1φem(λem) · V (λem)dλem

∫ λ′2

λ′1

φex(λex)dλex

where Nem stands for the number of emission photons;Nex is the number of excitation photons; �V is the emit-ting optical power of the spectra perceived by human eyes;�ex is the total excitation optical power, V (λ) is theCIE photopic spectral luminous efficiency function, and λex isthe excitation wavelength; λem is the emission wavelength;φex(λex) and φem(λem) are the phosphor power distribution ofexcitation and emission spectra, respectively; λ1 and λ2 arethe spectral range of emission lights; and λ′

1 and λ′2 are the

spectral range of incidence lights.The QE and LER of the conventional flat COB module and

the proposed microlens modules parametrized in drive currentsunder 25 ± 0.05 °C are shown in Fig. 4(a) and (b). Resultsuggests that the QE and LER of microlens modules tend to behigher than those of the conventional flat COB module undervarious drive currents. Furthermore, the RY module possessesthe highest QE (83.9%), a relatively low LER (210.6 lm/W),and the highest CRI (81.5) among these four structures undera drive current of 100 mA. This QE result is higher than

Fig. 4. (a) QE of four modules versus drive current. (b) LER versus drivecurrent for four modules. Temperature is maintained at 25 ± 0.05 °C.

that of the conventional-COB module by 7%, signifying thatit is difficult to achieve high QE and LER simultaneously.The following paragraph displays possible explanations for theaforementioned phenomenon.

Theoretically, if the excitation wavelength λex and inputpower distribution φex were given, QE values only dependon the emission wavelength λem and output power distri-bution φem. For RY module, although the yellow/red peakis lower than that of Y module, the wavelength is longer.Comprehensive considering the performance of λem and φem,RY module performs a higher QE value than that of Y module.Meanwhile, a high LER value can be obtained when λemmatches V (λ) (Fig. 3) that explains why the LER valueof Y module is the highest.

The YR module possesses a moderate performance amongmicrolens modules due to the overlapping of the yellowemitting spectra and excitation spectra of red phosphor [16].Photons lying in this overlapped area may undesirablydown-convert again into red-wavelength ones, resulting inan additional energy loss. Note that QEs of all structurestend to exhibit small decreases after the current reaches100 mA because of the saturation of phosphor energy levels.In reference to Fig. 4(b), the yellow microlens module(Y module, CRI = 72.8) appears to enjoy among others thehighest LER, which peaks at 287.3 lm/W under the drivecurrent of 100 mA, showing a 38.1% improvements to theconventional flat COB module.

XIAO et al.: 3-D MICROLENS PHOSPHOR WITH CURVATURES MANUFACTURED BY IMPRINTING FOR COB LEDs 1131

Fig. 5. (a) Spatial intensity distribution of four modules versus phosphor-layer radius ranging from −8 to 8 mm. (b) Simulative results of polarcandela distribution plots for the conventional-COB module and the microlens-phosphor module.

To further understand module’s optical characteristics, wemeasured light-intensity distributions at the drive current of100 mA [Fig. 5(a)]. The light output of Y module is higherthan that of others as expected. The deviations of four mod-ules, which is the differences between the largest and smallestvalues in the spectrum, are 1.7, 1.8, 1.6, and 2.2, respectively,indicating that microlens structures are characterized by higherdistribution uniformity (lower deviation value) than the con-ventional packaged COB structure. In addition, the intensitydistribution of the flat COB module tends to flatten nearthe module center. These differences in intensity-distributionshapes suggest that the proposed microlens modules are capa-ble of redirecting the propagation of light orientation. Thesimulative result of light-intensity polar distributions with theconventional-COB module and microlens-phosphor moduleis shown in Fig. 5(b). The result is considerably coincidentwith the simulation result in Fig. 2(a) and (b), as well asthe experimental result in Fig. 5(a). Because of the domedstructure of microlens, the light-beam direction tends to becloser to the vertical direction, thus reduces spatial-light-distribution angles, and increases the light intensity towardthe perpendicular line.

To understand the thermal performance of these four pack-ages, modules were mounted on a temperature-controlledplatform with a stable temperature maintained at 25 °C, and athermocouple was utilized to monitor the produced joule heat

Fig. 6. (a) Temperature versus drive current parametrized in four modules.Thermal simulation of (b) conventional flat COB module and (c) microlens-installed module.

of the phosphor surface. In Fig. 6(a), temperature measuredon the top surfaces of four modules versus drive current isshown. Among four modules, the flat COB module suffersfrom the highest temperature at 110.8 °C under 200 mA.For microlens modules, the domed structure enlarges the areaof the phosphor–air interface, thus facilitating the coolingprocess. Finally, the phosphor is subjected to self-heatingeffect, especially for red phosphor [17]. For the YR module[Fig. 1(d)], the emitted yellow light is prone to transforminginto red light, accompanied with thermal energy produced,and hence it exhibits a higher temperature value than othermicrolens modules due to the thermal aggregation. Comparingbetween the RY (102.1 °C) and Y modules (98.3 °C), the for-mer intensifies the self-heating effect, whereas the latter, whichlacks red phosphor, is associated with a lower temperature.The same simulative modules utilized in optical simulationwere used in thermal simulation to verify the experiment resultin Fig. 6(a).

As shown in Fig. 6(b) and (c), the thermal simulation by thefinite-element method [18] of the conventional-COB moduleand microlens-installed module shows similar result, as shownin Fig. 6(a). For the flat COB module, thermal energy isprone to accumulate in the middle of the COB device dueto relatively lower cooling ability. With the domed structureof microlens, the thermal migration between the air and thephosphor becomes more appearance, which contributes to arelatively lower temperature in the middle of the microlensmodule in comparison with the flat COB module.

IV. CONCLUSION

This paper proposes a 3-D microlens-phosphor module,which takes advantage of bumped geometries to increaseinterface curvatures and thus assist the augmentation oflight extraction. To investigate the mechanism, the proposed

1132 IEEE TRANSACTIONS ON ELECTRON DEVICES, VOL. 63, NO. 3, MARCH 2016

phosphor architecture was simulated by the Monte Carloalgorithm ray tracing and finite-element method. Meanwhile,the light propagation and thermal conducting process wereanalyzed. In particular, the proposed domed module wasverified to offer large areas of the phosphor–air interface, totune light propagation directions, and to reduce the self-heatingeffect in phosphor layers. Results suggested that, comparedwith the conventional counterpart, the device LER is capableto increase by 38.1% utilizing the Y module; QE and CRIare able to increase by 7% and 8.7, respectively, utilizing theRY module. Furthermore, we find that such a microlens struc-ture also provides a method to redirect the light orientationand change the light spatial-angular distribution. Consequently,this structure accomplishes enhancements of QE, LER, CRI,and phosphor-layer cooling for COB packaged LED devices,representing a new concept for LED COB packaging design,and paving for the foundation for future studies in the designsof surface curvatures.

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Hua Xiao received the M.S. degree in condensedmatter physics from Xiamen University, Xiamen,China, in 2014.

She is currently a Research Assistant with theDepartment of Electronics Science, Xiamen Uni-versity. Her current research interests include light-emitting diode optical design, electronic-productreliability, and advanced electronics packaging andassemblies.

Tien-Mo Shih received the Ph.D. degree fromthe University of California at Berkeley, Berkeley,CA, USA.

He held the Post-Doctoral Fellowship withHarvard University, Cambridge, MA, USA, in 1978.He is currently a Professor with Xiamen University,Xiamen, China, and the University of Maryland,College Park, MD, USA.

Zi-Quan Guo received the Ph.D. degree fromXiamen University, Xiamen, China, in 2014.

He is currently a Professional Engineer with theFujian Engineering Research Center for Solid-StateLighting, Xiamen University. His current researchinterests include solid-state lighting technology andapplications.

Yue Lin received the B.S. degree in applied physicsfrom Southeast University, Nanjing, China, in 2007,and the Ph.D. degree in wireless physics fromXiamen University, Xiamen, China, in 2012.

He currently serves as an Assistant Professorwith the Department of Electronics Science, XiamenUniversity.

XIAO et al.: 3-D MICROLENS PHOSPHOR WITH CURVATURES MANUFACTURED BY IMPRINTING FOR COB LEDs 1133

Yi-Jun Lu received the B.S. and Ph.D. degrees incondensed matter physics from Xiamen University,Xiamen, China, in 1995 and 2000, respectively.

He is currently a Professor with the Fujian Engi-neering Research Center for Solid-State Lighting,Xiamen University.

Zhong Chen received the M.Sc. and Ph.D. degreesfrom Xiamen University, Xiamen, China, in 1988and 1993, respectively.

He has been a Full Professor with Xiamen Univer-sity since 2000, where he is currently the Vice Deanof the Physical Science and Technology Collegeand the Director of the Fujian Engineering ResearchCenter for Solid-State Lighting.