We fabricated a phosphor-conversion white light emitting diode (PC-WLED) using a thin-film flip-chip GaN LED with a roughened u-GaN surface (TFFC-SR-LED) that emits blue light at 450 nm wavelength with a conformal phosphor coating that converts the blue light into yellow light. It was found that the TFFC-SR-LED with the thin-film substrate removal process and surface roughening exhibits a power enhancement of 16.1% when compared with the TFFC-LED without a sapphire substrate. When a TFFC-SR-LED with phosphors on a Cu-metal packaging-base (TFFC-SR-Cu-WLED) was operated at a forward-bias current of 350 mA, luminous flux and luminous efficacy were increased by 17.8 and 11.9%, compared to a TFFC-SR-LED on a Cup-shaped packaging-base (TFFC-SR-Cup-WLED). The angular correlated color temperature (CCT) deviation of a TFFC-SR-Cu-WLED reaches 77 K in the range of −70° to + 70° when the average CCT of white LEDs is around 4300 K. Consequently, the TFFC-SR-LED in a conformal coating phosphor structure on a Cu packaging-base could not only increase the luminous flux output, but also improve the angular-dependent CCT uniformity, thereby reducing the yellow ring effect.
© 2014 Optical Society of America
High-brightness white light-emitting diodes (WLEDs) have been recognized as a promising next-generation solid-state lighting (SSL) source, which has offered great potential to replace general lighting and devices owing to the advantages of high luminous efficacy, high color rendering index (CRI) and low correlated color temperature (CCT) [1,2]. White LEDs are widely used in backlighting for liquid crystal displays (LCD), headlamps for automobiles, road lamps, flashlights and even general indoor lighting fixtures. However, continuous improvements are still needed in light extraction and thermal conduction from the high-brightness WLED packages. Generally, the white LED is composed of a short wavelength (blue or ultraviolet) LED chip and a phosphor to obtain white emission, which is generated by mixing blue light emitted from the LED chip and exciting yellow light emission from the yellow phosphor converted blue light. The complementary colors LED concept is the so-called phosphor-converted white LED (PC-WLED) [3–5]. The LED chip and phosphor are considered to be one of the most important factors in white LED packaging. The LED chip determines the initial optical power, which affects the final maximum luminous efficacy of white LEDs. The phosphor emission spectrum determines the luminous flux according to the visual sensitivity function. Color indices such as CCT, CRI and angular color uniformity (ACU) also depend on the phosphor characteristics [4, 6]. There are three phosphor coating approaches to accomplish PC-WLEDs. The first is the conventional phosphor dispensing coating. The second is the conformal phosphor coating, and another is remote phosphor coating. Among these three methods, the first one has been widely applied to form phosphor layers in white LED packaging due to its low manufacturing cost and simplicity. The geometry of the phosphor layer in this method is the spherical cap. However, this method will often produce yellow lights surrounding the periphery of the white light in the radiation pattern, resulting in poor ACU [7,8]. To overcome the yellow ring effect a second coating method in which the phosphor layer is uniformly distributed around the LED chip was proposed. This method means that a phosphor layer with uniform thickness replicates the LED chip surface to obtain high ACU performance [7,9–11]. However, using the conformal phosphor coating method creates 60% backscattering light from the phosphor layer reflected to the package base. This backscattering light is susceptible to reabsorption through the blue LED chip, and seriously decreases the luminous efficacy . The third method effectively overcomes the conformal phosphor coating drawback. The remote phosphor coating method indicates separates the LED chip from the phosphor layer to improve luminous efficacy by reducing the phosphor layer backscattering light reabsorbed by the blue LED chip [3,7,12,13]. In a previous study the conformal phosphor coating method was focused more on improving the ACU of a white LED. The remote phosphor coating method focuses on enhancing white LED luminous efficacy .
There is still a great need to improve the light output power, thereby increasing white LED luminous efficacy. Further improvement of the LED light extraction efficiency is recognized as a characteristic feature of current research involving chip shaping, surface texturing/ roughening, photonic crystals and flip-chip packaging (FC) [15–18]. These methods improve photon generation within the LEDs, producing multiple opportunities to find the escape cone and enhance the light extraction efficiency. In particular, the FC-LED configuration is effective in enhancing light extraction and heat dissipation, and has been extensively used in the fabrication of high power, high efficiency LEDs for high injection current applications . The light extraction efficiency of conventional FC-LEDs is still low because the LEDs have a total internal reflection effect between the sapphire substrate and air, reducing the transparent window layer extraction efficiency. The direct light is absorbed and lost from the phosphor layer and sapphire substrate in conventional phosphor-converted white FC-LEDs, leading to low efficiency, the yellow ring effect and color variation . In our previous study  thin-film FC-LEDs (TFFC-LEDs) were demonstrated by combining laser lift off (LLO) and surface roughening techniques, producing devices that exhibit excellent light extraction and thermal dissipation capabilities. Increasing the luminous efficacy and uniform angular CCT performance are crucial objectives in PC-WLED packaging. In this study TFFC-LEDs with roughened u-GaN surfaces were employed to enhance the light extraction efficiency and solve the light leaking issue that occurs from the sapphire substrate. White TFFC-LEDs (TFFC-WLEDs) with a uniform angular CCT were prepared using a spray coating technique, an alternative technique used mainly for phosphor conformal coating to produce a highly structured surface . The produced LEDs can emit collimating lights so that the phosphor can be made of a planar plate or film to give each light an equal path length in the phosphor. The characteristics of the fabricated TFFC-WLEDs will be discussed in this letter.
InGaN/GaN LED epiwafers were grown onto patterned sapphire substrates (PSSs) using metal-organic chemical vapor deposition. After epitaxial growth the wafers were fabricated using standard photolithography, dry etching and flip-chip (FC) bonding techniques to form FC-LEDs with a chip size of 45 × 45 mil2 and peak emission wavelength of 450 nm. Cr/Au/Sn (5 nm/1 μm/ 2 μm) metals were deposited onto the Al2O3–Si sub-mount for the FC circuit anode and cathode. Hence, the LED chips were flip-chip bonded onto the Al2O3–Si sub-mount using ultrasonic FC-bonder. The LLO process was performed using a UV laser. The laser light was irradiated from the back surface of the sapphire substrate and GaN was locally heated close to the sapphire/GaN interface. After the entire LED wafer was scanned by the laser beam, the sapphire substrate was separated from the LED structure. In order to enhance light extraction, the exposed GaN surface was roughed by dipping the sample into a NaOH solution (4M) at 80 °C for 6 min to achieve a maximum light output. Three types of FC-LEDs, as shown in Fig. 1, were fabricated for detailed comparison: conventional sapphire-based FC-LEDs (FC-LEDs), FC-LEDs without a sapphire substrate (TFFC-LEDs), and TFFC-LEDs with roughened u-GaN surface (TFFC-SR-LEDs). The current-voltage (I-V) characteristic and light output power of these FC-LEDs were measured using an Agilent 4155B semiconductor parameter analyzer at room temperature and integration sphere detector (CAS 140B, Instrument Systems), respectively.
After the thin-film LLO and surface roughening FC-LED processes the TFFC-SR-LEDs were placed in a cup-shaped packaging-base (TFFC-SR-Cup-WLED) and a Cu-metal packaging-base (TFFC-SR-Cu-WLED) using silver paste and wire-bonding, respectively. Figure 2(a) shows schematic diagram of the TFFC-SR-LED with conformal phosphor coating. The phosphor powder used in this experiment was YAG-432 phosphor. The phosphor slurry was uniformly mixed with KER-2500A/B silicone binder and n-Heptane solvent at a ratio of 1.5: 1: 1.5. The mixture was then filled onto the different packaging-bases to obtain the TFFC-SR-Cup-WLED (Fig. 2(b)) and TFFC-SR-Cu-WLED (Fig. 2(c)) with the same CCT of approximately 4300 K using the pulsed spray coating (PSC) technique. Because the phosphor covering ratio of the two packaging structures was different the pulse spray phosphor slurry coating can be uniformly controlled to achieve the same CCT. The angular dependent CCT of two TFFC-WLEDs were measured using a Radiant Zemax SIG-400 source imaging goniometer (SIG-400) for viewing angle changes from −90° to + 90°. For these three types LEDs, all measured data were the average from 50 packaged samples.
3. Results and discussion
Figure 3 presents the current-voltage (I-V) characteristics of FC-LED, TFFC-LED and TFFC-SR-LED. The inset of Fig. 3 presents the leakage behavior for these samples. It was found that the I-V curves of the three FC-LED types exhibited a close and normal p-n diode electrical behavior with forward voltages (at 350 mA) of about 3.73 V, indicating that the LLO and surface roughening processes do not appear to adversely affect the I-V characteristics of these devices. The leakage currents of the FC-LED, TFFC-LED, and TFFC-SR-LED at the reverse voltage of 5 V were 0.17, 0.86, and 0.69 μA, respectively. The leakage currents of TFFC-LED and TFFC-SR-LED were larger than that of FC-LED. This infers that the GaN layer processed using LLO and surface roughening leads to slight laser damage on the GaN layer surface for TFFC-LED and TFFC-SR-LED. Nevertheless, these results imply that the leakage current is still within the reasonable range.
Light output power and light extraction efficiency of FC-LED, TFFC-LED, and TFFC-SR-LED before encapsulation as a function of the injection current as shown in Fig. 4. The inset pictures (a) and (b) of Fig. 4 present the n-GaN surface of TFFC-LED before and after surface roughening, respectively. It is worthy to mention that because the flip-chip LEDs were used to study the uniform color temperature technologies, the light output power of LEDs before the laser lift-off and surface roughening was selected almost the same. Thus the comparing measured data before and after processing is meaningful. For these three types LEDs, all measured data were the average from 50 packaged samples. The variation in light output is less than 1%. The light output powers of FC-LED, TFFC-LED, and TFFC-SR-LED were 172, 161, and 187 mW at the injection current of 350 mA, respectively. The corresponding light extraction efficiencies were estimated to be 25.5, 23.8, and 27.7%. It was found that the light output power measured at 350 mA injection current for TFFC-SR-LED was about 8.7 and 16.1% higher than that of the FC-LED and TFFC-LED. This indicates that the roughed n-GaN surface has superior capability for enhancing the light extraction, allowing the light to escape from the n-GaN/air interface because of the change in light path caused by surface scattering due to the surface roughening, as shown in the inset of Fig. 4(b). The light output power of the TFFC-LED was decreased by 6.4% at 350 mA of injection current compared with that of FC-LEDs without LLO. The power decrement could be attributed mainly to reduced light escape, which is the result of refractive index (n) contrast change and has been discussed in detail elsewhere . It is worthy to mention that we combine the patterned sapphire substrate (PSS) and the thin-film techniques to fabricate the GaN-based FC-LEDs in this study. The n-GaN surface roughening of TFFC-LED was etching by wet-chemical etching technique, which can simultaneously satisfy thermal management and enhance light extraction. Although the vertical structure of commercial Cree EZBright series LED has a pyramidal structure, which can increase light extraction. But the vertical structure of Cree EZBright series LED has major weakness compared with a TFFC design. The patterned n-contact reduces the chip’s effective emitting area, while the wire bonds obstruct light emission. These wire bonds are particularly irksome in the tightly packed chip arrays used in projection displays and some illumination systems, as they increase the distance from the surface of the LED to the primary optic. Therefore, the TFFC-LEDs with n-GaN surface roughening structure provides an effective approach to solve the electrode shading light problems and omit the wire-bonding process, thereby enhancing light output power.
Because the TFFC-SR-LEDs presented the best bare chip performance they were used to study the package effect on the conformal coating properties. Figure 5(a) shows the CCT as a functions of the injection current for the TFFC-SR-Cup-WLED and TFFC-SR-Cu-WLED. The CCT of both types of WLEDs initially increases linearly with the injection current. As the injection current increased from 50 to 350 mA, the CCT shifts in the TFFC-SR-Cup-WLED and TFFC-SR-Cu-WLED were 59 and 70K, respectively, which indicates the CCT difference in the TFFC-SR-Cup-WLED is better than that for the TFFC-SR-Cu-WLED. With the increase in driving currents the light efficiency in the white LED dropped rapidly, which is responsible for the decrease in Yellow/Blue ratio and the apparent increase in CCT. Therefore, the heat generated inside the WLEDs also increased when the driving current increased, resulting in higher phosphor materials and chip temperatures. The angular-dependent CCTs in the TFFC-SR-Cup-WLED and TFFC-SR-Cu-WLED in the −70° to + 70° range are shown in Fig. 5(b). The angular CCT deviations in the TFFC-SR-Cup-WLED and TFFC-SR-Cu-WLED were 111 and 77K when the average CCT was around 4300K. It is obvious that the TFFC-SR-Cu-WLEDs can almost vertically enter the phosphor layer and combine to produce uniform white light. The angular CCT distribution of the TFFC-SR-Cu-WLEDs is more uniform than that of the TFFC-SR-Cup-WLED. The larger angular CCT deviation would lead to the yellow ring phenomenon and generate a non-uniform white color at the different angle, which resulted in higher CCT output.
The luminous flux and the luminous efficacy of both TFFC-SR-Cup-WLED and TFFC-SR-Cu-WLED measured using a calibrated integrating sphere are plotted in Fig. 6(a) as a function of injection currents ranging from 50 to 350 mA. With the increase in injection current the differences in luminous flux and luminous efficacy between the TFFC-SR-Cup-WLEDs and TFFC-SR-Cu-WLEDs became larger. The luminous flux and luminous efficacy of TFFC-SR-Cup-WLED were 62.8 lm and 49.7 lm/W, respectively. The luminous flux and luminous efficacy of TFFC-SR-Cu-WLED were 74 lm and 55.6 lm/W, respectively. It is clear that the TFFC-SR-Cu-WLED exhibited 17.8 and 11.9% higher luminous flux and luminous efficacy than that the TFFC-SR-Cup-WLED with the same CCT at an injection current of 350 mA. The reason for the increment was using the Cu-metal packaging-base structure could obtain improved heat dissipation and increased total luminous flux, which indicates this structure is more stable at high blue light excitation. At an injection current of 350 mA the emission spectra of the TFFC-SR-Cup-WLED and TFFC-SR-Cu-WLED at the same CCT at approximately 4300 K are shown in Fig. 6(b). The TFFC-SR-Cu-WLED structure produced a higher intensity in blue and yellow components and yielded a higher radiation flux than the TFFC-SR-Cup-WLED structure.
CCT uniformity light spot and schematic diagram comparisons for the TFFC-Cup- and TFFC-Cu- white LED (@350 mA) are shown in Fig. 7(a)-7(d). The cup-shaped packaging-base structure is conical and the depth of the circular truncated cone is 1.3 mm and its bottom-end radius is 3.4 mm. For the Cu-metal packaging-base structure the depth is 0.7 mm, and the bottom-end radius is 5.5 mm. It can be seen that the light spot image of the TFFC-SR-Cup-WLED, there is an obvious boundary between the white spot in the center and the surrounding yellow/ blue spots. This indicates that a yellow ring exists around the central white zone, resulting in higher angular CCT distribution and lower total luminous flux. In the light spot image of the TFFC-SR-Cu-WLED, the uniformity of the light spot becomes better, the boundary becomes vague, the surrounding yellow/ blue spots become white and the whole spot becomes a uniform white light spot. Because this packaging-base structure can converge the side rays to central angle the radiation patterns of blue light out of the chip and yellow light from the phosphor layer are both similar to Lambertian. Therefore, most of the rays could directly emit out without being internally reflected, which induces higher color uniformity. Although the Cu-metal packaging-base structure would slightly influence the CCT deviation, but it still has better performance and higher color uniformity than the cup-shaped packaging-base structure.
This study demonstrated that a thin-film flip-chip LED using laser lift-off and surface roughening process, using a conformal phosphor structure enhances the luminous efficacy of WLEDs. With 350 mA current injection the TFFC-SR-LED exhibited 8.7 and 16.1% improvement in light output power due to n-GaN layer surface roughening, compared with the FC-LED and TFFC-LED. The luminous flux and luminous efficacy of the TFFC-SR-Cu-WLED were increased by 17.8 and 11.9% at a driving current 350 mA than the TFFC-SR-Cup-WLED. The angular CCT deviation of the TFFC-SR-Cu-WLED was improved to 77K between the −70° to 70° range when the average CCT is 4300K. It was found that the CCT deviations could be improved by 44% using a Cu-metal packaging base compared to the cup-shaped packaging base. Therefore, the TFFC-SR-LED structure with a Cu metal packaging base could not only improve the uniformity of angular-dependent CCT, but also increase the luminous flux.
The authors would like to thank the Ministry of Economic Affairs under Grant No.102-E0605, and Ministry of Science and Technology, Taiwan under Grant Nos. NSC 100-2221-E-006-197-MY3, NSC 102-2221-E-005-071-MY3, NSC 103-2218-E-005-004, and Industrial Technology Research Institute under contract No.103-B-04. We also thank the Hermes-Epitek Corp. Ltd. for the phosphor spray coating support.
References and links
2. S. Pimputkar, J. S. Speck, S. P. DenBaars, and S. Nakamura, “Prospects for LED lighting,” Nat. Photonics 3(4), 180–182 (2009). [CrossRef]
3. 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(S4Suppl 4), A930–A936 (2011). [CrossRef] [PubMed]
4. R. Y. Yu, S. Z. Jin, S. Y. Cen, and P. Liang, “Effect of the phosphor geometry on the luminous flux of phosphor-converted light-emitting diodes,” IEEE Photon. Technol. Lett. 22(23), 1765–1767 (2010). [CrossRef]
5. H. C. Chen, K. J. Chen, C. H. Wang, C. C. Lin, C. C. Yeh, H. H. Tsai, M. H. Shih, H. C. Kuo, and T. C. Lu, “A novel randomly textured phosphor structure for highly efficient white light-emitting diodes,” Nanoscale Res. Lett. 7(188), 1–8 (2012). [CrossRef] [PubMed]
6. C. Sommer, P. Hartmann, P. Pachler, M. Schweighart, S. Tasch, G. Leising, and F. P. Wenzl, “A detailed study on the requirements for angular homogeneity of phosphor converted high power white LED light sources,” Opt. Mater. 31(6), 837–848 (2009). [CrossRef]
7. Z. Y. Liu, S. Liu, K. Wang, and X. B. Luo, “Optical analysis of color distribution in white LEDs with various packaging methods,” IEEE Photon. Technol. Lett. 20(24), 2027–2029 (2008). [CrossRef]
8. R. Hu, X. B. Luo, and S. Liu, “Study on the optical properties of conformal coating light-emitting diode by Monte Carlo simulation,” IEEE Photon. Technol. Lett. 23(22), 1673–1675 (2011). [CrossRef]
9. N. Narendran, Y. Gu, J. P. Freyssinier-Nova, and Y. Zhu, “Extracting phosphor-scattered photons to improve white LED efficiency,” Phys. Status Solidi A 202(6), R60–R62 (2005). [CrossRef]
10. H. Zheng, X. B. Luo, R. Hu, B. Cao, X. Fu, Y. M. Wang, and S. Liu, “Conformal phosphor coating using capillary microchannel for controlling color deviation of phosphor-converted white light-emitting diodes,” Opt. Express 20(5), 5092–5098 (2012). [CrossRef] [PubMed]
12. H. Luo, J. K. Kim, E. F. Schubert, J. Cho, C. Sone, and Y. Park, “Analysis of high-power packages for phosphor-based white-light-emitting diodes,” Appl. Phys. Lett. 86(24), 243505 (2005). [CrossRef]
13. M. T. Lin, S. P. Ying, M. Y. Lin, K. Y. Tai, S. C. Tai, C. H. Liu, J. C. Chen, and C. C. Sun, “Ring remote phosphor structure for phosphor-converted white LEDs,” IEEE Photon. Technol. Lett. 22(8), 574–576 (2010). [CrossRef]
14. 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]
15. S. H. Huang, R. H. Horng, K. S. Wen, Y. F. Lin, K. W. Yen, and D. S. Wuu, “Improved Light extraction of nitride-based flip-chip light-emitting diodes via sapphire shaping and texturing,” IEEE Photon. Technol. Lett. 18(24), 2623–2625 (2006). [CrossRef]
16. H. W. Huang, J. T. Chu, C. C. Kao, T. H. Hseuh, T. C. Lu, H. C. Kuo, S. C. Wang, and C. C. Yu, “Enhanced light output of an InGaN/GaN light emitting diode with a nano-roughened p-GaN surface,” Nanotechnology 16(9), 1844–1848 (2005). [CrossRef]
17. J. Lee, D. H. Kim, J. Kim, and H. Jeon, “GaN-based light-emitting diodes directly grown on sapphire substrate with holographically generated two-dimensional photonic crystal patterns,” Curr. Appl. Phys. 9(3), 633–635 (2009). [CrossRef]
18. R. H. Horng, S. H. Chuang, C. H. Tien, S. C. Lin, and D. S. Wuu, “High performance GaN-based flip-chip LEDs with different electrode patterns,” Opt. Express 22(S3Suppl 3), A941–A946 (2014). [CrossRef] [PubMed]
19. J. J. Wierer, D. A. Steigerwald, M. R. Krames, J. J. O’Shea, M. J. Ludowise, G. Christenson, Y. C. Shen, C. Lowery, P. S. Martin, S. Subramanya, W. Gotz, N. F. Gardner, R. S. Kern, and S. A. Stockman, “High-power AlGaInN flip-chip light-emitting diodes,” Appl. Phys. Lett. 78(22), 3379 (2001). [CrossRef]
21. R. H. Horng, H. L. Hu, M. T. Chu, Y. L. Tsai, Y. J. Tsai, C. P. Hsu, and D. S. Wuu, “Performance of flip-chip thin-film GaN light-emitting diodes with and without patterned sapphires,” IEEE Photon. Technol. Lett. 22(8), 550–552 (2010). [CrossRef]