LED package design for high optical efficiency and low viewing angle

In response to this limitation, we design a low viewing angle and high extraction package based on total internal reflection HEP-TIR lens.. The HEP-TIR package produces the light output

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

1 Email address: ntran3000@yahoo.com

2

Email address: fgshi@uci.edu

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

(HEP-TIR) 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

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 RESULTSAND 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

FIG.1 Cross-sectional view of the InGaN/GaN LED

a) b)

c)

FIG.2 Light propagation in the cup of different tilted angles: a) 0

degree; b) 37 degree; c) 55 degree

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 2 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 3 and Fig 4 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

0 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

varying the escape surface curvature, this one-parameter

optimization is not enough to achieve the maximum LEE It is

evident from Fig 3 and 4 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

62 67 72 77 82 87 92

h/r

80 Angle (degree) Diffuse cup

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

65 70 75 80 85 90 95

h/r

Specular Cup

Angle (degree)

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

65 70 75 80 85 90 95

Cup Ange (degree)

Specular cup Diffuse cup

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,

LED chip

Convex lens Reflector cup

Heat sink Tilted angle

r

0.05

0.1

0.15

0.2

0.25

Dispersion Angle (degree)

0 0.1 0.2 0.3 0.4 0.5 0.6

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 1 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

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 6 Fig 6

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 7 The HEP-TIR

package eliminates the light absorption by the cup while it

provides directional light output with high LEE as shown in Fig

8 The HEP-TIR package is around 3 times smaller in height and

3 times smaller in diameter compared to Luxeon Collimator of

Lumileds Fig 8 presents angular radiation distribution of

HEP-TIR package and of a conventional LED without a secondary

collimator lens The graph and Table 1 show that the HEP-TIR

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

FIG.7 HEP-TIR package

0 20 40 60 80 100

Angular Distribution (degree)

Conventional package, 50 degree cup HEP-TIR

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

1

1.1

1.2

1.3

1.4

h/r

Simulation results Experimental results

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

best conventional package The total light output of HEP-TIR within 100 degree is 287% higher than that of the conventional package

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

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