LED package design for high optical efficiency and low viewing angle

LED Package Design for High Optical Efficiency and Low Viewing Angle Nguyen T Tran1 and Frank G Shi2 Optoelectronics Packaging & Materials Labs The Henry Samueli School of Engineering, University of California, Irvine, CA 92697-2575 ABSTRACT Light extraction efficiency of an LED package can be improved by optimizing the cup angle and lens curvature However, this conventional package is not appropriate for many applications that require low viewing angle and distance view if secondary collimator optics is not used The use of secondary collimator lens can reduce power output more than 10% In response to this limitation, we design a low viewing angle and high extraction package based on total internal reflection (HEPTIR) lens The HEP-TIR package produces the light output with low dispersion angle without the use of secondary optics and has smaller size than secondary optics while its extraction efficiency is as high as the best conventional package The total light output of HEP-TIR within 100 degree is 287% higher than that of the conventional package I INTRODUCTION Because of the potential of high luminous efficiency of high brightness light emitting diodes (HB-LEDs), HB-LEDs are expected to be used not only for large size backlighting for LCD displays and TVs, for automobile lighting, but also eventually for general lighting The DOE estimates in 2002 indicate that a replacement of lighting by white LEDs with an efficiency of 150lm/W will reduce the US electricity consumption by 50%, and also will greatly reduce CO2 emission as well as the reduction of mercury use Although the optical efficiency for white LEDs can be as high as 300lm/W, the typical efficiency of various state-of-the-art white LED products in the market is quite low This is due to low extraction efficiency and internal quantum efficiency not reaching maximum Thus there is a strong need to increase the internal quantum efficiency and the light extraction efficiency of LEDs [1] The extraction efficiency of light emitted by the LED chip can be improved with different chip shapes such as truncated-inverted pyramid geometry [2-4], surface texturing [4,5] and photonic crystals [6]; with different encapsulant geometry [7] and reflector cup geometry; and with materials of high refractive index and low light absorption [8] For white LED (WLED), the luminous efficacy depends not only on the correlated color temperature but also on the extraction efficiency of each individual color LED or whole package (for RGB technology), or on the phosphor geometries and placement (for phosphor white LED technology) Recently, researchers are interested in discrete remote phosphor With this WLED technology, the luminous efficacy greatly depends on the light extraction efficiency from saturated color LED packages (blue or UV LEDs) Enhancing the external efficiency of saturated color LED packages, therefore, is paramount important Email address: ntran3000@yahoo.com Email address: fgshi@uci.edu 978-1-4244-1637-0 In many applications such as medical treatment (skin treatment using light), material curing, and sensing or security camera, the light output is required to have low dispersion angle besides the extraction efficiency Commercial LED packages require a secondary optics to focus the extracted light to adapt to these applications However, the use of secondary optics lens lowers the efficiency more than 10% [9] and makes the package bulkier In this study, we present the analysis and optimization of the light extraction efficiency and dispersion angle of surface mount LED device (SMD) based on cup geometry and encapsulant geometry by using ray tracing software The simulation is verified with key experiments In response to the disadvantage of the secondary collimator optics, we design a high brightness and high extraction package (HEP) based on total internal reflection (TIR) The HEP-TIR produces the light output with low dispersion angle without the use of secondary optics and has smaller size than secondary optics while its extraction efficiency is as high as the best conventional package The total light output of HEP-TIR within 100 degree is 287% higher than that of the conventional package II RESULTS AND DISCUSSION In this study, an InGaN/GaN die emitting at 460nm with the exact same size of the commercial InGaN/GaN die used in our experiments is placed at the center of the cup bottom surface (e.g Fig.1) We take into account every detail of a LED package including bonding wire, chip location, cup angle, epoxy lens, and photon scattering The dimension of the circular bonding wire is 25 micron in diameter with the length of 1.6 mm The entire chip and bonding wire are encapsulated in an epoxy material of the refractive index of 1.6048 and the absorption coefficient of 0.078 cm-1 Using Light Tools software, we have calculated the light extraction efficiency of blue LEDs with different encapsulant lens curvatures (in terms of h/r ratios) and different tilted cup angles for specular and diffuse reflector cups Here h and r represent the lens height and the lens bottom radius, respectively The reflector surface of both cups has 93% reflectance The light extraction efficiency used in this study is defined as the ratio of the number of photons emitted into free space per second to the number of photons emitted from active region per second Throughout the entire report except for experimental results, the normalized light extraction efficiency (LEE) that is normalized with respect to the maximum light extraction (light extraction is the amount of light emitted into the encapsulant for the first time) from the chip, will be used Convex lens Reflector cup r LED chip Flat h Heat sink varying the escape surface curvature, this one-parameter optimization is not enough to achieve the maximum LEE It is evident from Fig and that the LED package with a relatively higher cup angle usually has higher maximum LEE and relatively high LEE within a broader range of the h/r ratio 92 Diffuse cup Tilted angle FIG.1 Cross-sectional view of the InGaN/GaN LED a) b) Normalized LEE 87 82 77 Angle (degree) 20 40 60 80 72 67 10 30 50 70 62 0.2 0.4 95 Specular Cup 90 Normalized LEE In order to enhance the LEE, the package has to have low photon absorption and reflection loss which can be achieved by reducing the propagation pathlength of the emitted photon and the incident angle at the escape surface In the LED package with the reflector cup, the propagation of light is greatly influenced by the reflector cup as shown in Fig At a low tilted cup angle, the emitted photons impinge the side surface of the reflector cup several times before they reach the escape surface (Fig.2A) These photons have high propagation angle relative to the vertical axis of the package This means that the photons are difficult to be extracted with flat escape surface As the cup angle increases up to a certain level, photon traveling time becomes shorter (Fig.2B & 2C) and photons reach the flat escape surface at a lower incident angle Therefore, the photon absorption by the package materials such as cup, encapsulant and chip, is reduced, and more photons are extracted out of the package A common way to promote light extraction is to introduce a convex escape surface to lower the incident angle at the escape surface The curvature of the escape surface depends on the cup geometry As it is seen in Fig 2A, 2B and 2C, the escape surface for high light extraction should be curvier in the package of lower cup-angle The dependence of the LEE on the cup angle and the escape surface curvature is studied for two different types of cup surface roughness: diffuse reflector cup and smooth/specular reflector cup Fig and Fig show the LEE for the diffuse and specular reflector cups respectively, as the function of h/r ratio and cup angle The LEE greatly increases with increasing h/r value from to around 0.5 and slightly increases at the h/r value greater than 0.5 This trend of the LEE is similar for different cup angles of the diffuse reflector cup but it is quite different for the specular reflector cup For both the diffuse and specular reflector cups, the maximum LEE is achieved at the lower h/r ratio with the increase of cup angle This is because with the smaller cup angle the photons incident on the escape surface have larger propagation angle relative to vertical axis of the LED package Therefore, larger surface curvature (higher h/r ratio) is required for the LED package of smaller cup angle to reduce the incident angle at the escape surface and to facilitate the escape of photon from the LED package Several commercial products such as XLamp LED series use a vertical cup with convex lenses of high curvature to improve LEE Although LEE can be enhanced by 0.8 FIG.3 Normalized LEE of the diffuse cup LED as the function of the h/r ratio and cup angle in The cup height and base radius are 0.8mm and 0.17mm, respectively 85 80 Angle (degree) 75 30 50 70 70 65 0.2 0.4 20 40 60 80 0.6 0.8 h/r FIG.4 Normalized LEE of the specular cup LED as the function of the h/r ratio and cup angle The cup height and base radius are 0.8mm and 0.17mm, respectively 95 Normalized LEE (-) FIG.2 Light propagation in the cup of different tilted angles: a) degree; b) 37 degree; c) 55 degree 0.6 h/r c) 90 85 Specular cup 80 Diffuse cup 75 70 65 20 40 60 80 Cup Ange (degree) FIG.5 LEE of the specular and diffuse cup LED: the solid lines are for the highest LEE and the dash lines are for the LEE at flat surface Besides the cup angle and lens curvature, the roughness of the reflector cup surface also affects the LEE of the LED package The highest LEE of the specular cup, as shown in Fig 5, is always higher than that of the diffuse cup with the same surface reflectance This is because the diffuse reflector cup scatters light in different direction and thus it increases the probability of light being absorbed by other absorbing surface or materials, especially by the LED chip In contrast to the diffuse cup LED that has relatively high LEE obtained only in the h/r ratio range from 0.5 to 1, the specular cup LED with high cup angle (50, 60, 70 and 80 degrees) has relatively high LEE obtained at low h/r value and at the h/r value between 0.5 and For the specular cup package with high tilted-cup angle (50 to 80 degree), the LEE at the flat surface or low h/r ratio can reach up to 94% of the highest achieved LEE while it is only 87% for the diffuse cup High surface curvature or h/r ratio usually requires additional manufacturing step such as attaching the pre-made lens to the package and thus increases manufacturing cost An LED package with the specular reflector cup, therefore, can provide relatively high LEE at lower manufacturing cost compared to the diffuse reflector cup 10 60 50 30 70 FIG.7 HEP-TIR package 0.5 0.2 Intensity (a.u.) 0.6 0.4 0.15 0.3 0.1 0.2 0.05 100 0.1 0 20 40 60 Conventional package, 50 degree cup Relative Intensity (%) 40 80 Intensity (a.u.) 0.25 package produces much more useful light than the other packages Within 10-degree half solid angle, the HEP-TIR package provides power output of 287% higher than the 50-degree cup package HEP-TIR 80 60 40 20 80 Dispersion Angle (degree) FIG.6 Intensity versus dispersion angle of a specular reflector LED package with different cup angles In many applications such as medical treatment (skin treatment using light), material curing, and sensing or security camera, the light output is required to have low dispersion angle and distance view besides the extraction efficiency Light with high dispersion angle is considered as non-useful or waste Commercial LED packages require a secondary optics to focus the extracted light to adapt to these applications However, the use of secondary optics lens lowers the efficiency more than 10% and makes the package bulkier Similar to the commercial LED products, the presented package up to this point still needs a secondary optics to provide direction light output because its light output has large viewing angle as shown in Fig Fig shows that a package with a cup angle of 50 degree produces light output with more directional than other conventional packages However, the dispersion angle of this package is high, and a secondary optics is required to provide low dispersion angle light To improve the performance of the LED package for these applications, we designed a high brightness and high extraction package with directional light output based on total internal reflection (HEP-TIR) lens as shown Fig The HEP-TIR package eliminates the light absorption by the cup while it provides directional light output with high LEE as shown in Fig The HEP-TIR package is around times smaller in height and times smaller in diameter compared to Luxeon Collimator of Lumileds Fig presents angular radiation distribution of HEPTIR package and of a conventional LED without a secondary collimator lens The graph and Table show that the HEP-TIR 0 10 20 30 40 50 60 Angular Distribution (degree) FIG.8 Spatial radiation patterns of conventional LED with secondary collimator optics and that of HEP-TIR Table 1: Power ratio of HEP-TIR to a conventional package with 50-degree cup angle distributing within different solid angle best conventional package The total light output of HEP-TIR within 100 degree is 287% higher than that of the conventional package Normalized LEE (-) 1.4 1.3 ACKNOWLEDGEMENT 1.2 Thanks to Yongzhi He, Yuan-Chang Lin and J.P You for performing some of the experiments, and we are also grateful to them for useful technical insights 1.1 Simulation results REFERENCES Experimental results 0.9 0.2 0.4 0.6 0.8 h/r FIG.9 Experimental and simulated results for a single-chip LED package with the reflector cup height and base radius of 0.8mm and 0.21mm, respectively The experimental and simulation results are normalized to their results obtained from the package with the flat epoxy surface In order to validate our key simulation results, we conducted some critical experiments for different values of h/r using a commercial LED chip with the size of 0.3mm-by-0.3mm and our cup The chip was placed at the center of the cup bottom surface and encapsulated with epoxy The refractive index and the absorption coefficient of the epoxy were 1.605 and 0.078/cm respectively, at the wavelength of 460nm The lens curvature h/r was controlled by adding small amount of high viscosity epoxy under the microscope view That way we were able to make different devices of similar height The devices with the lens height (h) that is within 50 micron of the mean value and with the corresponding error of less than 5% were assigned into one group We used a silicone mold for the lens height of 2.3mm The optical power output of the LED at different h/r values was measured with an integrating sphere The supplied current and voltage were recorded The current was kept constant at 20mA In this study, the simulation results are merely the extraction efficiency while the experimental results are the wall-plug efficiency The wall-plug efficiency was calculated by taking the ratio of the optical power obtained from the integrating sphere to the electrical power (the product of the measured current and voltage) supplied to the LED It is also defined as the product of internal, extraction, and other (due to circuit resistance) efficiencies Therefore, in order to have a good comparison between the experiment and simulation results without making any assumption of internal and other efficiencies, the experimental or simulation results were normalized to the measured or simulation results obtained at the flat epoxy surface The simulation results, presented in Fig 9, are found to be supported by our experimental results III CONCLUSION LEE can be improved by optimizing the cup angle and lens curvature However, this conventional package has large viewing angle and thus requires secondary collimator lens The use of secondary collimator reduces the output power more than 10% An LED package of low viewing angle and high efficiency based on total internal reflection lens is presented The HEP-TIR package produces the light output with low dispersion angle without the use of secondary optics and has smaller size than secondary optics while its extraction efficiency is as high as the [1] E.F Schubert, APS March Meeting, Baltimore MD, March 2006 Achieved on Apr 24th, 2006 from: http://www.aps.org/meet/MAR06/loader.cfm?url=/commonspot /security/getfile.cfm&PageID=72704 [2] A R Franklin and R Newman, J Appl Phys 35, 1153 (1964) [3] M.R Krames, et al., Appl Phys Lett 75, 2365 (1999) [4] N.T Tran and F.G Shi, Microsystems, Packaging, Assembly Conference Taiwan, 2006 International Grand Formosa Regent, Taipei, Taiwan, Oct 2006 10.1109/IMPACT.2006.312209 page(s):1 – [5] I Schnitzer and E Yablonovitch, Appl Phys Lett 63, 2174– 2176 (1993) [6] M Boroditsky, T F Krauss, R Coccioli, R Vrijen, R Bhat, & E Yablonovitch, Appl Phys Lett 75, 1036 (1999) [7] V.S Abramov, A.E Puysha, A.V Shishow, N.V Scherbakov, and I.P Poliakava US Patent no 20060044806 A1 [8] Y.C Lin, N.T Tran, Y Zhou, Y He, and F.G Shi Microsystems, Packaging, Assembly Conference Taiwan, 2006 International Grand Formosa Regent, Taipei, Taiwan, Oct 2006 312173 p.1 – [9] http://www.lumileds.com/pdfs/DS26.PDF ... cup LED with high cup angle (50, 60, 70 and 80 degrees) has relatively high LEE obtained at low h/r value and at the h/r value between 0.5 and For the specular cup package with high tilted-cup angle. .. package is high, and a secondary optics is required to provide low dispersion angle light To improve the performance of the LED package for these applications, we designed a high brightness and. .. than 10% An LED package of low viewing angle and high efficiency based on total internal reflection lens is presented The HEP-TIR package produces the light output with low dispersion angle without