Active Packing Method for Blue Light-emitting Diodes

8/14/2019 Active Packing Method for Blue Light-emitting Diodes

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Active packing method for blue light-emittingdiodes with photosensitive polymerization:

formation of self-focusing encapsulates

Hao Wang

1, 2 *

, Jae-Hyoung Ryu

3

, Kyu-Seung Lee

2

, Chun Hua Tan

1

, Lihua Jin

2

,Songmei Li2, Chang-Hee Hong

3, Yong-Hoon Cho

2, Songhao Liu

1

1 School for Information and Optoelectronic Science and Engineering, South China Normal University.Guangzhou 510631, China

2 Nano-Bio-Photonics Laboratory, Department of Physics and Institute for Basic Science Research, ChungbukNational University, Cheongju 361-763, Korea

3Opto Electronics Laboratory, Department of Semiconductor Science and Technology, Chonbuk National University,Jeonju, 561-756, Korea

[email protected]

Abstract: A novel light-emitting diode (LED) packaging method, named theactive packaging (AP) method, is presented in this paper. In this method,during the LED packaging process, the light emitted from a GaN LED chipitself is employed to package the LED encapsulant, thereby eliminating theneed to utilize a mold. Current injection into a bare LED chip, triggers a

photosensitive epoxy to polymerize, leading to the formation of mushroomlamp cap on the LED chip. The emission properties of LEDs fabricated bythis method, including their emission beam profiles and light outputs, werecharacterized. The results showed that a self-focusing effect happened withthe addition of an epoxy on the chip. The simulation demonstrated that thegeometry the encapsulant controlled the beam pattern of emission. Further,the self-focusing effect was believed to be caused by the combination of thethreshold energy of epoxy polymerization, the beam pattern and the poweroutput of the LED chip.

2008 Optical Society of America

OCIS codes: (250.0250 ) Optoelectronics; (220.4610) Optical fabrication.

References and links

1. D.A. Steigerwald, J.C. Bhat, D. Collins, R.M. Fletcher, M. O. Holcomb, M. J. Ludowise, P. S. Martin, S. L.Rudaz, Illumination with solid state lighting technology, IEEE J. Selected Topics Quantum Electron. 8, 310-320 (2002).

2. R.J.M. Zwiers, H.J.L. Bressers, B. Ouwehand, D. Baumann, Development of a new low-stress hyperred LEDencapsulant, Components, Hybrids, and Manufacturing Technology, IEEE Transactions on 12, 387-392(1989).

3. R. N. Kumar, L. Y. Keem, N. C. Mang, A. Abubakar, Ultraviolet Radiation Curable Epoxy Resin Encapsulantfor Light Emitting Diodes, J. Appl. Polym. Sci. 100, 1048-1056 (2006).

4. J. N. Tey, A. M. Soutar, S. G. Mhaisalkar, H. Yu, K. M. Hew, Mechanical properties of UV-curablepolyurethane acrylate used in packaging of MEMS devices, Thin Solid Films 504, 384-390 (2006).

5. A. Hancock, L. Lin, Challenges of UV curable ink-jet printing inks a formulator's perspective, Pigment &Resin Technology 33, 280-286 ( 2004).

6. J. Serbin, A. Ovsianikov, B. Chichkov, Fabrication of woodpile structures by two-photon polymerization andinvestigation of their optical properties, Opt. Express 12, 5221-5228 (2004).

7. D. Therriault, S. White, J. Lewis, Chaotic mixing in three-dimensional microvascular networks fabricated bydirect-write assembly, Nature Materials 2, 265 - 271 (2003).

8. H. Wang, K. Lee, S. Li, L. Jin, S. Lee, Y. Wu,Y. Cho, J. Cai, Fabrication of CdSe-ZnS nanocrystal-based localfluorescent aperture probes by active polymerization of photosensitive epoxy, accepted by Opt. Comm. (2008)http://dx.doi.org/10.1016/j.optcom.2007.11.086 .

9. R. Bachelot, C. Ecoffet, D. Deloeil, P. Royer, and D. J. Lougnot, Appl. Opt. 40, 5860 (2001).

#89802 - $15.00 USD Received 16 Nov 2007; revised 28 Jan 2008; accepted 29 Jan 2008; published 5 Mar 2008

(C) 2008 OSA 17 March 2008 / Vol. 16, No. 6 / OPTICS EXPRESS 3680


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1. Introduction

The interest for using light-emitting diodes (LEDs) for display and illumination applicationshas been growing steadily over the past few years. The potential for long life and reducedconsumption of energy are the two key attributes of this rapidly evolving technology, whichhas generated tremendous interest for use in the abovementioned applications. Generally, anLED is fabricated by first bonding an LED chip to metal frame post and then capping it with a

molded epoxy encapsulant. Often, epoxy is utilized to mold an LED in the shape of a smallbullet: this structure is called a bullet head [1]. In general, LED encapsulation is a pottingprocess, frequently requiring the use of thermosetting or ultraviolet (UV) curable resins [2, 3].

A free radical polymerization reaction of the photosensitive polymer could be triggeredwhen the photosensitive polymer is exposed to visible light or UV light, and this could resultin rapid curing at room temperature. Such photon-induced curing has a number of uniqueadvantages over conventional curing technologies, thereby attracting extensive attentions invarious research disciplines and industrial sectors, such as packaging of LEDs [3] and MEMSdevices [4], application of printing inks [5], and fabrication of photonic crystal structure [6]and microvascular networks [7].

However, irrespective of the type of packaging method used--- thermal curing or the UVcurable packaging---the packaging procedures are nearly the same. The encapsulant, which isemployed to protect and control LED emission properties, is formed with the involvement of amold. One of the important aspects of this packaging method is that it does not consider

individual LED chip emission properties: therefore, it is termed a passive packaging (PP)method in our study. Our method, as mentioned below, utilizes the light emitted from theLED chip itself to package the LED with a photosensitive epoxy: we name this technique theactive packaging (AP) method. This photon-induced polymerization method, which leads tothe formation of specific encapsulant shapes and characteristics, eliminates the need to utilizea mold. By controlling the packaging parameters, for example, the polymerization time (PT)and the polymerization current (PC), the emission properties of LEDs can be controlled. Inthis report, a photosensitive epoxy was employed to fabricate LED encapsulants by the APmethod, and the relevant characteristics of the encapsulants were investigated also. Inaddition, we observed a self-focusing effect, which was generated during the experiment. Thesimulation of the LED emission beam profile confirmed that this kind of encapsulant couldfocus the light emitted from an LED chip.

2. Experiment

2.1 materials

The products utilized in this experiment----photoinitiator (Ciba Irgacure 819), difunctionalmonomer HDDA (1, 6-hexanediol diacrylate) and Ebecryl resin 80 (highly reactiveoligomeric polyether tetraacrylate), were made by Ciba Specialty Chemical Inc (SouthKorea). The epoxy resin system was composed of photoinitiator/HDDA/Ebecryl resin 80(0.2/25/74.5 wt.%). The sensitive wavelength of the photoinitiator ranged from 460 nm toshorter wavelengths, as shown in Fig. 1(a). The emission peak of the To-Can-type LED wasobserved at 455 nm with an FWHM of 15 nm under 1 mA current injection (Fig. 1(a)). Theoverlap of the spectrum between the electroluminescence (EL) of the To-can-type GaN LEDand the absorbance of the photoinitiator was utilized for the experiment. The emission beamprofiles of the LED chip under the corresponding work currents are given in Fig. 1(b). Fromthis figure, it is clear that the emission beam profile of the LED chip is almost the same withrespective light output intensities. The proposed threshold energy for triggering epoxy

polymerization with 455 nm is indicated by the dotted line in Fig. 1(b).

2.2 Encapsulation process

As shown in Fig. 1(c), the bare LED chip was immersed in the photosensitive epoxy, and thenthe PC was injected into the LED chip. The photoinitiator was triggered by the illumination ofthe LED chip and consequently, the photosensitive epoxy was polymerized. This resulted in




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an encapsulant cap forming on the LED chip. The samples were after removed from the epoxyliquid and washed with acetone to remove any non-polymerized epoxy. The typicalencapsulant was a mushroom-shaped lamp, as shown in Fig. 1(e) inset. The samplesfabricated with the same current (1 mA) at different polymerization times (PT) (1, 2, 3, and 4s) were named PT1, PT2, PT3, and PT4, respectively, as shown in Fig. 2(a). The samples thatcurved in 1 s with different PCs (1, 10, 20, and 30 mA) were named PC1, PC2, PC3, and PC4,respectively, and are presented in Fig. 2(b).

Fig. 1. The manufacturing process of the active packaging (AP) method. A To-Can-type LEDwas utilized to perform the polymerization. (a) The comparison of absorbance of the epoxy andthe electroluminescence (EL) of the To-Can-type LED at 1 mA injection. (b) The light outputprofile of the LED chip under different work currents (The working curve at 1 mA wasmultiplied with a factor of 5). The dashed grey line indicates the proposed threshold energy ofthe epoxy polymerization. (c) The experiment process of the AP method. The LED chip wasimmersed in the photosensitive epoxy resins and then a polymerization current (PC) wasinjected into the LED chip so that it emitted blue light, thereby triggering the polymerization ofthe photosensitive epoxy, and resulting in a mushroom-shaped encapsulant. (d) Top view of the

bare LED chip (inset shows the size of the chip: the scale bar is 0.1 mm). The yellow arrowsindicate the measurement direction of the beam pattern (Fig. 3). (e) Top view of a LED (PC3)fabricated by our proposed process. Inset shows the side view of the encapsulant.




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2.3 Measurement

The images of samples were captured using a CCD camera. Their geometries were sketchedfrom the side view of the samples. The beam profiles were measured under a 50 mA current.The light outputs and quantum efficiencies of the samples were characterized using an OL770 Multi-channel Spectroradiometer (Optronic laboratories, Inc.) under work currents of 1,10, 20, 30, 40, and 50 mA. The absorbance of the epoxy was characterized by an ultraviolet

(UV)visible absorption spectrometer (Thermo Spectronic, Genesys 6 spectrophotometer)[8].2.4 Simulation

The simulation was performed by LightTools (Version 5.4). The parameters for the simulationwere set similar to those of the samples. The GaN LED chip was set as a Lambertian source,

with an area of 0.60 mm 0.60 mm and a height of 0.08 mm. The profiles of the encapsulants

were sketched based on the samples. The reflective index of the chip and the encapsulant was2.4 and 1.55, respectively. The total number of rays for the simulation was set to 1,000,000.

Fig. 2. The geometry of LED lamps fabricated by the AP method. PT samples are shown in (a).

The manufacturing parameters are as follows. PT1: 1 mA 1 s, PT2: 1 mA 2 s, PT3: 1 mA 3 s, and PT4: 1 mA 4 s. PT samples shown in (b). The manufacturing parameters are asfollows. PC1: 1 mA 1 s, PC2: 10 mA 1 s, PC3: 20 mA 1 s, and PC4: 30 mA 1 s. (c) and(d) are the geometries for the PT and PC cases, respectively. (e) and (f) are the sizes for the PTand PC cases; the height, the diameter, and their ratio are also shown.

3. Results and discussion

The geometries of the LED encapsulants were characterized and presented in Fig 2(c), 2(d)and 2(e), 2(f) for the PT and PC cases, respectively. The figures show that the height of PTencapsulants increased from 2.0 mm to 2.2 mm to 2.9 mm to 3.4 mm as the diameter

increased from 1.3 mm to 1.9 mm to 2.2 mm to 2.4 mm, respectively. This caused the height-to-diameter ratio fluctuate from 0.67 mm to 0.79 mm to 0.84 mm to 0.78 mm, respectively.On the other hand, the height of the PC encapsulants increased from 1.3 mm to 1.9 mm to 2.1mm to 2.2 mm for PC1 to PC4 as the diameter increased from 2.0 mm to 3.4 mm to 3.5 mm to3.9 mm, respectively. This resulted in the height-to-diameter ratio varied from 0.67 to 0.59 to0.64 to 0.58, respectively. It could be concluded from the geometry and the size of the LED




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encapsulants that the encapsulant volume increases with an increase in both PT and PC.However, it is interesting to note that the encapsulant geometries are different in the PT andPC cases. In the PT case, the height-to-diameter ratio increases with the encapsulant volumeincrease while in the PC case, the height-to-diameter ratio decreases with the encapsulantvolume increase. Such distinctive morphological development variations with respectivepackaging parameters prove that the packaging parameters, for example, PC or PT, could beemployed to control the encapsulant morphology with good reproducibility.

In the PT case, the beam profile of the LED chip was defined; more polymerizationoccurred with the passage of time. This resulted in the encapsulant height and diameterincreasing. Since the emitted light output of the LED chip at center was higher than that atedge, therefore, the polymerization rate is greater at the center than at the edge, resulting inthe height increase being higher than the diameter increase; consequently, the height-to-diameter ratio decreased. The LED chip exhibits a curve shape similar to that of the beamprofiles under corresponding work currents. However, for a certain light output, such as theproposed threshold energy for triggering epoxy polymerization in Fig. 1(c), the higher thework current, the wider was the emission angle of the chip at this energy. This caused theencapsulant diameter to increase, since epoxy polymerization occurs only when the lightoutput of the chip is higher than the threshold energy triggering epoxy polymerization [9]. Onthe other hand, the height increase would be less than the diameter increase, since the epoxypolymerization rate above the threshold energy did not increase significantly as compared tochange in the polymerization rate from below to above the threshold energy. For this reason,the widening of the threshold energy emission angle caused the diameter to increase fasterthan the height with the PC increasing, resulting in the height-to-diameter ratio decrease.

Fig. 3. The beam profiles of (a) PT and (c) PC samples (driven at 50 mA). The orientation of

the LED lamps is along 0 (direction shown by arrows in Fig. 1(c)). (b) and (d) are thesimulated light output of the samples corresponding to (a) and (c).

Figure 3(a) shows the beam profiles of samples PT1 to PT4 under 20 mA current injection:

here, the emission angles decrease from 132 for the bare chip to 130, 97, 91, and 85 forPT1 to PT4, respectively. These decreases are accompanied with an increase in the peakintensities from 2.2 to 4.0 to 4.6 to 5.3 to 6.2, respectively. Both the narrowing of the emissionprofiles and the increase in the peak intensities demonstrate that the light emitted from thechip was focused. Figure 3(b) shows the simulation result obtained from the respectiveencapsulant profiles by using LightTools. The simulation shows that the emission angles




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decreased from 140 for a bare LED to 125, 96, 92, and 80 for the four samples,respectively. The corresponding peak emission intensities varied as 2.2, 4.0, 4.6, 5.3, and 6.2,respectively. The simulation result was in good agreement with the experiment result, whichindicates that the respective encapsulant profiles for the PT case resulted in the respectivebeam profiles for the samples.

The beam profiles of samples PC1 to PC4 (in Fig. 3(b)) illustrate that with an increase inPT, the emission angles of the samples decrease from 132 for the bare LED to 128, 105,

95, and 82 for the four samples, respectively. In addition, the peak emission intensitiescorrespondingly increased as follows: 2.1 (a. u.), 3.2, 4.2, 4.4, and to 5.2. The simulationresults revealed that the emission angles decreased from 140 for the bare LED to 124, 118,103 and 88 for the four samples, respectively. In addition, the peak emission intensitiescorrespondingly increased as follows: 2.2, 3.2, 4.0, 4.4, and 5.4. The simulation result was ingood agreement with the PC experiment result. The encapsulant has several functions. First, itserves as a transparent body that holds and protects the device; second, it behaves as a lensthat focuses the light in the desired way and improves the light output of the LED die [3].Currently, encapsulants are controlled by molds that form required shapes. From our results,we concluded that encapsulants formed by the AP method, without utilizing a mold, cancontrol the light emitted from chips. Irrespective of whether the PC case or PT case isconsidered, the beam patterns of the samples become narrow with an increase in PC or PT.The focusing effect is exceedingly apparent as the thickness of the continuous epoxy layer onthe LED chip increases. In fact, as the encapsulant volume on bare chips increased, more lightwas focused (in Fig. 2(e) and 2(f)). This clearly proved that the self-focusing effect originatedfrom the utilization of the AP process. The shape of the encausulant is close to a convex lens.The encapsulant magnified the LED chip 1.5 times (from 0.6 x 0.6 mm to 0.9 x 0.9 mm); theresulting shape appeared like a convex lens. These results demonstrate that the encapsulantfabricated by our method effectively focused the light emitted from an LED chip (Fig. 1(d)and 1(e) ).

4. Conclusions

We reported a novel LED packaging method---the active packaging (AP) method, anddiscussed the characteristics of the proposed method. In the AP method, during the LEDpackaging, the light emitted from the GaN LED itself was employed to obtain the LEDencapsulant. With current injection into the bare LED chip, mushroom encapsulant cap formson the LED chip. It was observed that the emission angles of LEDs changed with the type of

the encapsulant used, which means that the emission angles could be controlled by both PTsand PCs without the need to utilize a mold. Moreover, as the PC or PT increased, the emissionprofiles became narrower and could be reduced to 80 in our experiment. This proved that theself-focusing effect was generated. Lastly, the emitted peak intensities of the LEDs increasedwith the encapsulants volume increase

Acknowledgments

This work was supported by the Natural Fund of Guangdong Province, China (5300077), theKorea Research Foundation Grant funded by the Korean Goverment (MOEHRD)(KRF-2005-005-J07501) and the Regional Technology Innovation Program (Grant No. RTI04-03-06) ofMOCIE of the Korean Government and by the Korean Science and Engineering Foundationthrough the National Research Laboratory Program and through QSRC at DonggukUniversity.



Active Packing Method for Blue Light-emitting Diodes

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