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Optics Express

Optics Express

  • Editor: Andrew M. Weiner
  • Vol. 22, Iss. 4 — Feb. 24, 2014
  • pp: 4516–4522
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Improvement of emission uniformity by using micro-cone patterned PDMS film

Che-Yu Liu, Kuo-Ju Chen, Da-Wei Lin, Chia-Yu Lee, Chien-Chung Lin, Shih-Hsuan Chien, Min-Hsiung Shih, Gou-Chung Chi, Chun-Yen Chang, and Hao-Chung Kuo  »View Author Affiliations


Optics Express, Vol. 22, Issue 4, pp. 4516-4522 (2014)
http://dx.doi.org/10.1364/OE.22.004516


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Abstract

Micro-patterned PDMS film was fabricated and combined with LED chip on board (COB) package to improve the emission uniformity of LED chip. The micro scale patterned sapphire substrate (PSS) was used as a mold to fabricate micro-cone patterned PDMS (MC-PDMS) film. A strong scattering effect from this MC-PDMS film can be verified by the high haze ratio and the Bi-directional Transmission effect. The angle dependent color temperature measurement system was used to measure the ΔCCT of COB with and without MC-PDMS. The measurement results indicate that the ΔCCT was reduced from 1025K to 428K. This improvement can effectively eliminate the yellow ring effect of LED chip. This technology can be thus considered as a cost-effective way for the next generation of light source packages.

© 2014 Optical Society of America

1. Introduction

In this study, we combined MC-PDMS film with COB package to fabricate WLEDs with highly uniform color temperature. This approach provides excellent scattering capability and improves the blue light emission of COB at large angle. The COB with uniform CCT could be achieved so the yellow ring effect was reduced. Furthermore, the cost of this method is not expensive as compared with general mold fabrication and can be used in mass production.

2. Experiments

Figure 2(a)
Fig. 2 (a) Fabrication flow of MC-PDMS chip on board sample and (b) Illustrations of patterned up and patterned down MC-PDMS samples.
illustrates the fabrication flow of MC-PDMS chip on board sample. A blue LED chip (450nm emission peak wavelength) with the size of 45 mils square was bonded on the top of the chip on board package. The radiant flux of COB with four bare blue LEDs was 1.735 W at 350 mA. The Y3Al5O12 (YAG) phosphor used in this experiment has an emission peak at 560 nm and the particle size is about 12μm. To form a phosphor-suspension, the phosphor powders was mixed in to the transparent silicone. The phosphor-suspension was then dispensed to fill up the package and then followed by baking at 150þC for 2 hours to solidify the phosphor glue. Finally, the prepared MC-PDMS film was covered on the top of the COB package, and the MC-PDMS chip on board sample is ready for testing. The patterned up MC-PDMS, patterned down MC-PDMS COB samples and the COB sample without PDMS have been prepared for comparison as shown in Fig. 2(b).

3. Results and discussion

In general, the angular correlated color temperature (CCT) uniformity could be evaluated by ΔCCT which is defined by the maximum CCT minus minimum CCT in the range of −70þ~70þ. Figure 3
Fig. 3 (a) The angular-dependent CCT and (b) ΔCCT of COB without PDMS (reference), with patterned up MC-PDMS and the patterned down MC-PDMS covering on the top of COB package.
shows the angular-dependent CCT for the three cases mentioned previously, COB without PDMS film, COB with patterned up MC-PDMS and patterned down MC-PDMS. It can be seen that the color temperature of COB sample with patterned down MC-PDMS film was more uniform than others samples. As shown in Fig. 3(b), the ΔCCT of patterned down MC-PDMS samples improve from the 1025 K to 428 K as compared with reference sample. The reason for COB package suffering from the large ΔCCT issue could be attributed to the different emission light patterns of LED chips and yellow phosphor, resulting in the lower blue light intensity at the large angles. This phenomenon leads to the color temperature of LED near normal direction is higher than the oblique direction. For the case of MC-PDMS covering on the top of COB, the better ΔCCT was achieved due to the stronger scattering at large angle brought by patterned PDMS and a better mixed blue/yellow light can be produced.

To further discuss the emission light color temperature of these three samples in different angles, the angular-dependent intensity ratio of yellow to blue rays (Iyellow/Iblue) was measured, as shown in Fig. 4
Fig. 4 The yellow/blue light ratio of COB with and without MC-PDMS films on the top of the COB package.
. The blue light of COB with patterned down MC-PDMS film is more uniform than the reference sample. This indicates that the patterned down MC-PDMS film can scatter the blue light from normal direction to large angle. Besides, the MC-PDMS can increase the light path of blue light. Thus, the yellow light was also enhanced slightly.

For the LED package, the reason for the CCT non-uniformity usually comes from the anisotropic (or wavelength-dependent) diffraction of the various optical layers in the structure. When there are two or more types of photons (for example, blue and yellow photons), this dispersion in light emission can cause the spatial non-uniformity of the photon distribution and eventually lead to the uneven color rendering in the detected LED emission. This phenomenon is most known the major cause of yellow ring in LED, and such condition is not favorable for high quality solid state lighting. To overcome this, it is conceivable to place an extra layer which can diffuse the photons omni-directionally. To evaluate this capability numerically, a haze value is frequently used. It is defined by the ratio of non-specular photons over the all diffracted photons. The higher the value is, the better light scatter it becomes, and the haze percentage was calculated by using the following equation [5

5. H. C. Chen, K. J. Chen, C. C. Lin, C. H. Wang, H. V. Han, H. H. Tsai, H. T. Kuo, S. H. Chien, M. H. Shih, and H. C. Kuo, “Improvement in uniformity of emission by ZrO₂ nano-particles for white LEDs,” Nanotechnology 23(26), 265201 (2012). [CrossRef] [PubMed]

, 19

19. C. C. Lin, W. L. Liu, and C. Y. Hsieh, “Scalar scattering model of highly textured transparent conducting oxide,” J. Appl. Phys. 109(1), 014508 (2011). [CrossRef]

]:
Haze(%)=Tdiffraction/Ttotal ×100%
(1)
where Tdiffraction is the diffractive transmittance (total transmittance − specular diffraction) and Ttotal is the total transmittance. By the definition of Eq. (1), the wavelength-dependent haze of these three samples are presented, as shown in Fig. 5(a)
Fig. 5 (a) The measured wavelength-dependent haze intensity of patterned up MC-PDMS, patterned down MC-PDMS and the flat PDMS. (b) the sketches of patterned up and patterned down.
. Figure 5(b) shows the sketches of patterned up and patterned down samples in haze measurement. It can be clearly seen that the MC-PDMS film scatters more 50% of the lights from 300 to 800nm while the flat PDMS film is almost zero. This indicates that the MC-PDMS film has higher scattering ability of normal incident light and thus can be a very efficient diffuser.

4. Conclusion

In conclusion, the micro-patterned PDMS film was applied on the top of chip on board package by using a simple, low cost, hand-made process. From the angular correlated color temperature uniformity measurement, the ΔCCT of COB covered by patterned down MC-PDMS film is improved from 1025K to 428K, as compared to standard COB package. The haze measurement shows the great scattering ability of patterned down MC-PDMS film. The BTDF measurement of different incident angles of 460 nm Lambertian light source further proved why PDMS film can improve theΔCCT. Finally, this work provides a simple and cost-effective method to greatly improve the ΔCCT, whose magnitude is crucial to determine the light quality of a LED chip on board package. Based on this study, we believe this technology is promising for the future LED packaging development.

Acknowledgments

The authors express their gratitude to EPISTAR Corporation and Helio Opto. Corporation for their technical support. This work was supported in part by the National Science Council in Taiwan under grant number, NSC102-3113-P-009-007-CC2, NSC101-2221-E-009-046-MY3, and NSC-102-2221-E-009-131-MY3.

References and links

1.

E. F. Schubert and J. K. Kim, “Solid-state light sources getting smart,” Science 308(5726), 1274–1278 (2005). [CrossRef] [PubMed]

2.

M. R. Krames, O. B. Shchekin, R. Mueller-Mach, G. O. Mueller, L. Zhou, G. Harbers, and M. G. Craford, “Status and future of high-power light-emitting diodes for solid-state lighting,” J. Disp. Technol 3(2), 160–175 (2007). [CrossRef]

3.

N. Narendran, N. Maliyagoda, L. Deng, and R. Pysar, “Characterizing LEDs for general illumination applications: Mixed-color and phosphor-based white sources,” in Solid State Lighting and Displays, I. T. Ferguson, Y. S. Park, N. Narendran, and S. P. DenBaars, eds. (SPIE, 2001), pp. 137–147.

4.

H. C. Kuo, C. W. Hung, H. C. Chen, K. J. Chen, C. H. Wang, C. W. Sher, C. C. Yeh, C. C. Lin, C. H. Chen, and Y. J. Cheng, “Patterned structure of Remote Phosphor for phosphor-converted white LEDs,” Opt. Express 19(Suppl 4), A930–A936 (2011). [CrossRef] [PubMed]

5.

H. C. Chen, K. J. Chen, C. C. Lin, C. H. Wang, H. V. Han, H. H. Tsai, H. T. Kuo, S. H. Chien, M. H. Shih, and H. C. Kuo, “Improvement in uniformity of emission by ZrO₂ nano-particles for white LEDs,” Nanotechnology 23(26), 265201 (2012). [CrossRef] [PubMed]

6.

Z. Liu, K. Wang, X. Luo, and S. Liu, “Realization of High Spatial Color Uniformity for White light-Emitting Diodes by Remote Hemispherical YAG: Ce Phosphor Film,” in 2010 Proceedings 60th Electronic Components and Technology Conference(2010), pp. 1703–1707. [CrossRef]

7.

J. Lau, Chip On Board Technologies for Multichip Modules (Kluwer Academic Publishers, 1994).

8.

Z. Liu, S. Liu, K. Wang, and X. Luo, “Optical Analysis of Color Distribution in White LEDs With Various Packaging Methods,” IEEE. Photonic. Tech. L 20(24), 2027–2029 (2008). [CrossRef]

9.

H. Y. Lin, Y. J. Chen, C. C. Chang, X. F. Li, S. C. Hsu, and C. Y. Liu, “Pattern-Coverage Effect on Light Extraction Efficiency of GaN LED on Patterned-Sapphire Substrate,” Electrochem. Solid. St 15(3), H72–H74 (2012). [CrossRef]

10.

T. V. Cuong, H. S. Cheong, H. G. Kim, H. Y. Kim, C. H. Hong, E. K. Suh, H. K. Cho, and B. H. Kong, “Enhanced light output from aligned micropit InGaN-based light emitting diodes using wet-etch sapphire patterning,” Appl. Phys. Lett. 90(13), 131107 (2007). [CrossRef]

11.

L. Ya-Ju, C. Ching-Hua, K. Chih Chun, L. Po Chun, L. Tien-Chang, K. Hao-Chung, and W. Shing-Chung, “Study of the excitation power dependent internal quantum efficiency in InGaN/GaN LEDs grown on patterned sapphire substrate,” IEEE. J. Sel. Top. Quant 15(4), 1137–1143 (2009). [CrossRef]

12.

Y. K. Ee, J. M. Biser, W. Cao, H. M. Chan, R. P. Vinci, and N. Tansu, “Metalorganic Vapor Phase Epitaxy of III-Nitride Light-Emitting Diodes on Nanopatterned AGOG Sapphire Substrate by Abbreviated Growth Mode,” IEEE J. Sel. Top. Quant. 15(4), 1066–1072 (2009). [CrossRef]

13.

Y. K. Ee, X. H. Li, J. Biser, W. Cao, H. M. Chan, R. P. Vinci, and N. Tansu, “Abbreviated MOVPE nucleation of III-nitride light-emitting diodes on nano-patterned sapphire,” J. Cryst. Growth 312(8), 1311–1315 (2010). [CrossRef]

14.

Y. Li, S. You, M. Zhu, L. Zhao, W. Hou, T. Detchprohm, Y. Taniguchi, N. Tamura, S. Tanaka, and C. Wetzel, “Defect-reduced green GaInN/GaN light-emitting diode on nanopatterned sapphire,” Appl. Phys. Lett. 98(15), 151102 (2011). [CrossRef]

15.

Y. K. Ee, P. Kumnorkaew, R. A. Arif, H. Tong, J. F. Gilchrist, and N. Tansu, “Light extraction efficiency enhancement of InGaN quantum wells light-emitting diodes with polydimethylsiloxane concave microstructures,” Opt. Express 17(16), 13747–13757 (2009). [CrossRef] [PubMed]

16.

W. H. Koo, W. Youn, P. Zhu, X.-H. Li, N. Tansu, and F. So, “Light Extraction of Organic Light Emitting Diodes by Defective Hexagonal-Close-Packed Array,” Adv. Funct. Mater. 22(16), 3454–3459 (2012). [CrossRef]

17.

Y. K. Ee, R. A. Arif, N. Tansu, P. Kumnorkaew, and J. F. Gilchrist, “Enhancement of light extraction efficiency of InGaN quantum wells light emitting diodes using SiO(2)/polystyrene microlens arrays,” Appl. Phys. Lett. 91(22), 221107 (2007). [CrossRef]

18.

X. H. Li, R. Song, Y. K. Ee, P. Kumnorkaew, J. F. Gilchrist, and N. Tansu, “Light Extraction Efficiency and Radiation Patterns of III-Nitride Light-Emitting Diodes With Colloidal Microlens Arrays With Various Aspect Ratios,” IEEE. Photonics. J. 3(3), 489–499 (2011). [CrossRef]

19.

C. C. Lin, W. L. Liu, and C. Y. Hsieh, “Scalar scattering model of highly textured transparent conducting oxide,” J. Appl. Phys. 109(1), 014508 (2011). [CrossRef]

20.

K. J. Chen, H. C. Chen, C. C. Lin, C. H. Wang, C. C. Yeh, H. H. Tsai, S. H. Chien, M. H. Shih, and H. C. Kuo, “An Investigation of the Optical Analysis in White Light-Emitting Diodes With Conformal and Remote Phosphor Structure,” J. Disp. Technol 9(11), 915–920 (2013). [CrossRef]

21.

C. H. Hung and C. H. Tien, “Phosphor-converted LED modeling by bidirectional photometric data,” Opt. Express 18(Suppl 3), A261–A271 (2010). [CrossRef] [PubMed]

22.

K. J. Chen, H. V. Han, B. C. Lin, H. C. Chen, M. H. Shih, S. H. Chien, K. Y. Wang, H. H. Tsai, P. Yu, P. T. Lee, C. C. Lin, and H. C. Kuo, “Improving the Angular Color Uniformity of Hybrid Phosphor Structures in White Light-Emitting Diodes,” IEEE Electron Device Lett. 34(10), 1280–1282 (2013). [CrossRef]

23.

X. H. Lee, I. Moreno, and C.-C. Sun, “High-performance LED street lighting using microlens arrays,” Opt. Express 21(9), 10612–10621 (2013). [CrossRef] [PubMed]

OCIS Codes
(230.2090) Optical devices : Electro-optical devices
(230.3670) Optical devices : Light-emitting diodes
(250.0250) Optoelectronics : Optoelectronics
(290.1483) Scattering : BSDF, BRDF, and BTDF

ToC Category:
Diffraction and Gratings

History
Original Manuscript: October 22, 2013
Revised Manuscript: January 28, 2014
Manuscript Accepted: February 4, 2014
Published: February 20, 2014

Citation
Che-Yu Liu, Kuo-Ju Chen, Da-Wei Lin, Chia-Yu Lee, Chien-Chung Lin, Shih-Hsuan Chien, Min-Hsiung Shih, Gou-Chung Chi, Chun-Yen Chang, and Hao-Chung Kuo, "Improvement of emission uniformity by using micro-cone patterned PDMS film," Opt. Express 22, 4516-4522 (2014)
http://www.opticsinfobase.org/oe/abstract.cfm?URI=oe-22-4-4516


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References

  1. E. F. Schubert, J. K. Kim, “Solid-state light sources getting smart,” Science 308(5726), 1274–1278 (2005). [CrossRef] [PubMed]
  2. M. R. Krames, O. B. Shchekin, R. Mueller-Mach, G. O. Mueller, L. Zhou, G. Harbers, M. G. Craford, “Status and future of high-power light-emitting diodes for solid-state lighting,” J. Disp. Technol 3(2), 160–175 (2007). [CrossRef]
  3. N. Narendran, N. Maliyagoda, L. Deng, and R. Pysar, “Characterizing LEDs for general illumination applications: Mixed-color and phosphor-based white sources,” in Solid State Lighting and Displays, I. T. Ferguson, Y. S. Park, N. Narendran, and S. P. DenBaars, eds. (SPIE, 2001), pp. 137–147.
  4. H. C. Kuo, C. W. Hung, H. C. Chen, K. J. Chen, C. H. Wang, C. W. Sher, C. C. Yeh, C. C. Lin, C. H. Chen, Y. J. Cheng, “Patterned structure of Remote Phosphor for phosphor-converted white LEDs,” Opt. Express 19(Suppl 4), A930–A936 (2011). [CrossRef] [PubMed]
  5. H. C. Chen, K. J. Chen, C. C. Lin, C. H. Wang, H. V. Han, H. H. Tsai, H. T. Kuo, S. H. Chien, M. H. Shih, H. C. Kuo, “Improvement in uniformity of emission by ZrO₂ nano-particles for white LEDs,” Nanotechnology 23(26), 265201 (2012). [CrossRef] [PubMed]
  6. Z. Liu, K. Wang, X. Luo, and S. Liu, “Realization of High Spatial Color Uniformity for White light-Emitting Diodes by Remote Hemispherical YAG: Ce Phosphor Film,” in 2010 Proceedings 60th Electronic Components and Technology Conference(2010), pp. 1703–1707. [CrossRef]
  7. J. Lau, Chip On Board Technologies for Multichip Modules (Kluwer Academic Publishers, 1994).
  8. Z. Liu, S. Liu, K. Wang, X. Luo, “Optical Analysis of Color Distribution in White LEDs With Various Packaging Methods,” IEEE. Photonic. Tech. L 20(24), 2027–2029 (2008). [CrossRef]
  9. H. Y. Lin, Y. J. Chen, C. C. Chang, X. F. Li, S. C. Hsu, C. Y. Liu, “Pattern-Coverage Effect on Light Extraction Efficiency of GaN LED on Patterned-Sapphire Substrate,” Electrochem. Solid. St 15(3), H72–H74 (2012). [CrossRef]
  10. T. V. Cuong, H. S. Cheong, H. G. Kim, H. Y. Kim, C. H. Hong, E. K. Suh, H. K. Cho, B. H. Kong, “Enhanced light output from aligned micropit InGaN-based light emitting diodes using wet-etch sapphire patterning,” Appl. Phys. Lett. 90(13), 131107 (2007). [CrossRef]
  11. L. Ya-Ju, C. Ching-Hua, K. Chih Chun, L. Po Chun, L. Tien-Chang, K. Hao-Chung, W. Shing-Chung, “Study of the excitation power dependent internal quantum efficiency in InGaN/GaN LEDs grown on patterned sapphire substrate,” IEEE. J. Sel. Top. Quant 15(4), 1137–1143 (2009). [CrossRef]
  12. Y. K. Ee, J. M. Biser, W. Cao, H. M. Chan, R. P. Vinci, N. Tansu, “Metalorganic Vapor Phase Epitaxy of III-Nitride Light-Emitting Diodes on Nanopatterned AGOG Sapphire Substrate by Abbreviated Growth Mode,” IEEE J. Sel. Top. Quant. 15(4), 1066–1072 (2009). [CrossRef]
  13. Y. K. Ee, X. H. Li, J. Biser, W. Cao, H. M. Chan, R. P. Vinci, N. Tansu, “Abbreviated MOVPE nucleation of III-nitride light-emitting diodes on nano-patterned sapphire,” J. Cryst. Growth 312(8), 1311–1315 (2010). [CrossRef]
  14. Y. Li, S. You, M. Zhu, L. Zhao, W. Hou, T. Detchprohm, Y. Taniguchi, N. Tamura, S. Tanaka, C. Wetzel, “Defect-reduced green GaInN/GaN light-emitting diode on nanopatterned sapphire,” Appl. Phys. Lett. 98(15), 151102 (2011). [CrossRef]
  15. Y. K. Ee, P. Kumnorkaew, R. A. Arif, H. Tong, J. F. Gilchrist, N. Tansu, “Light extraction efficiency enhancement of InGaN quantum wells light-emitting diodes with polydimethylsiloxane concave microstructures,” Opt. Express 17(16), 13747–13757 (2009). [CrossRef] [PubMed]
  16. W. H. Koo, W. Youn, P. Zhu, X.-H. Li, N. Tansu, F. So, “Light Extraction of Organic Light Emitting Diodes by Defective Hexagonal-Close-Packed Array,” Adv. Funct. Mater. 22(16), 3454–3459 (2012). [CrossRef]
  17. Y. K. Ee, R. A. Arif, N. Tansu, P. Kumnorkaew, J. F. Gilchrist, “Enhancement of light extraction efficiency of InGaN quantum wells light emitting diodes using SiO(2)/polystyrene microlens arrays,” Appl. Phys. Lett. 91(22), 221107 (2007). [CrossRef]
  18. X. H. Li, R. Song, Y. K. Ee, P. Kumnorkaew, J. F. Gilchrist, N. Tansu, “Light Extraction Efficiency and Radiation Patterns of III-Nitride Light-Emitting Diodes With Colloidal Microlens Arrays With Various Aspect Ratios,” IEEE. Photonics. J. 3(3), 489–499 (2011). [CrossRef]
  19. C. C. Lin, W. L. Liu, C. Y. Hsieh, “Scalar scattering model of highly textured transparent conducting oxide,” J. Appl. Phys. 109(1), 014508 (2011). [CrossRef]
  20. K. J. Chen, H. C. Chen, C. C. Lin, C. H. Wang, C. C. Yeh, H. H. Tsai, S. H. Chien, M. H. Shih, H. C. Kuo, “An Investigation of the Optical Analysis in White Light-Emitting Diodes With Conformal and Remote Phosphor Structure,” J. Disp. Technol 9(11), 915–920 (2013). [CrossRef]
  21. C. H. Hung, C. H. Tien, “Phosphor-converted LED modeling by bidirectional photometric data,” Opt. Express 18(Suppl 3), A261–A271 (2010). [CrossRef] [PubMed]
  22. K. J. Chen, H. V. Han, B. C. Lin, H. C. Chen, M. H. Shih, S. H. Chien, K. Y. Wang, H. H. Tsai, P. Yu, P. T. Lee, C. C. Lin, H. C. Kuo, “Improving the Angular Color Uniformity of Hybrid Phosphor Structures in White Light-Emitting Diodes,” IEEE Electron Device Lett. 34(10), 1280–1282 (2013). [CrossRef]
  23. X. H. Lee, I. Moreno, C.-C. Sun, “High-performance LED street lighting using microlens arrays,” Opt. Express 21(9), 10612–10621 (2013). [CrossRef] [PubMed]

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