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Abstract
The light distribution of light-emitting diodes (LEDs) generally resembles that of a Lambertian light source. When used as large-area light sources, the light distribution angle of LEDs must be modified through secondary optics design to achieve uniformity and minimize the number of light sources. However, secondary optical components pose several challenges such as demanding alignment accuracy, material aging, detachment, and lower reliability. Therefore, this paper proposes a primary optical design approach to achieve full-angle emission in LEDs without the need for lenses. The design employs a flip-chip as the light source and incorporates a V-shaped packaged structure, including a white wall layer, optical structure layers, and a V-shaped diffuse structure. With this design, the LEDs achieve full-angle emission without relying on lenses. Our experimental results demonstrated a peak intensity angle of 77.7°, a 20.3% decrease in the intensity of the central point ratio, and a full width at half maximum (FWHM) of the light distribution of 175.5°. This design is particularly suitable for thin, large-area, and flexible backlight light sources. Moreover, the absence of secondary optical components allows for a thinner light source module.
© 2024 Optica Publishing Group under the terms of the Optica Open Access Publishing Agreement
1. Introduction
With the rapid advancement of display technology, displays have become an integral part of our daily lives, encompassing devices such as laptops, smartphones, tablets, televisions, and holographic displays [1–4]. Liquid crystal displays (LCDs) have emerged as the dominant technology in the modern display industry. In the era of artificial intelligence (AI) and the Internet of Things (IoT), wearable electronics are heading towards incorporating chip-scale packages of mini LEDs as essential light sources [5].
However, given that liquid crystal (LC) materials themselves do not emit light, a backlight unit (BLU) is necessary to serve as the display's light source [6,7]. Light-emitting diodes (LEDs) are widely used in LCD backlight modules (BLMs) due to their high efficiency, long lifespan, environmental friendliness, and energy efficiency [8–10]. BLM designs can be broadly categorized into direct-type and edge-lit types. Edge-lit methods utilize secondary optical techniques, such as light guide plates (LGPs), to achieve thin, large-area planar light sources. However, the weight of the light guide plate can contribute to the overall module's bulkiness [11,12]. On the other hand, direct-type designs are better suited for achieving high brightness and light extraction efficiency in large-area planar light sources compared to edge-lit designs. However, direct-type light sources face challenges such as long mixing distances (OD) and excessively dense pitch, resulting in increased module thickness and heat accumulation between LEDs [13,14]. Backlight modules incorporating mini-LEDs can leverage local dimming technology to mitigate dark zone leakage, achieve high dynamic range (HDR), and deliver improved contrast and brightness [15,16].
Previous studies have extensively investigated backlight modules to enhance panel uniformity. Researchers have proposed various improved designs for light guide plates [17,18], including tilted light coupling structures (TLCS), hollow light guide plates, and silk-screen printing techniques. These approaches aim to reduce the thickness of light guide plates while achieving high uniformity and light extraction efficiency [19–21].
Specialized techniques such as flip-chip (FC) and chip-scale packaging (CSP) have been introduced to enhance the internal quantum efficiency of LEDs for lighting and display applications. These methods offer benefits such as low thermal resistance and high current density [22–24]. Some examples of this approach include the use of prism structures and patterned n-GaAs epitaxial layers on the flip-chip to achieve higher light extraction efficiency and smaller divergence angles [25,26].
Moreover, numerous studies have proposed the use of secondary optical components to modify the light output angle and enhance uniformity. Secondary lens designs include freeform surface lenses, total internal reflection (TIR) lenses, and reflective lenses. Reflective lenses employ metal reflection to broaden the light angle and improve uniformity [27,12]. Other research has suggested designs such as freeform surface chip-scale packaging, Cartesian distributed freeform lens array designs, TIR lens designs, semi-transparent thin-film GaN mini-LEDs, reflective point-slit mini-LED arrays, and AlGaInP LEDs with photonic crystals (PC) to achieve enhanced light extraction efficiency, uniformity, and light output efficiency [28–34]. Previous studies have demonstrated that different material characteristics can be leveraged to enhance the optical scattering properties of certain materials. Relevant research includes the utilization of 3D flower-shaped hollow Mg-Al layered double hydroxide (LDH) microspheres, gradient refractive index layered double-shell nanoparticle optical films, organic silicone beads (OSB) in polycarbonate (PC)/styrene-co-acrylonitrile (SAN) mixtures, polymer-dispersed liquid crystal (PDLC) films, and potassium sodium niobite (KNN) films to achieve excellent light scattering performance and high optical transparency [35–39]. Polycarbonate (PC) or polymethyl methacrylate (PMMA) are commonly used materials for secondary lenses. However, in large outdoor displays, prolonged exposure to sunlight can cause yellowing and delamination of the lenses due to high temperatures and ultraviolet radiation [40,41]. To address this issue, related studies have explored the use of dual-functional microcapsules (MPCMs/TiO2) composed of n-octadecane as the core material and poly methyl-methacrylate (PMMA) with doped titanium dioxide nanoparticles as the shell material to ensure good thermal reliability and UV shielding [42]. Heart-shaped mini-LEDs, wide in shape and lacking a secondary lens, serve as a broad, ultra-bright, and flat light source [43]. Additionally, some studies have analyzed the chemical properties to quantify the impact of temperature and UV radiation on the aging of LED encapsulation materials, employing techniques such as a liquid temperature-controlled irradiation system (MLTIS) [44,45]. Secondary optical components require precise assembly and carry the risk of lens detachment, while also increasing the thickness of the light source module. Research has focused on large-area planar and lightweight surface light sources, including designs incorporating mini-LEDs with microlens arrays and wide-angle mini-LEDs with planar light guide layers [46,47]. Mini-LED backlight with zero optical distance and microstructure lenses for light guidance enables the creation of extra-thin, large-area notebook LCDs [48].
In summary, previous research efforts have mainly concentrated on secondary optical design, which involves manipulating light pathways using optical films to improve the emission angle, as well as analyzing the impact of different coating materials on light distribution. However, relatively fewer studies have focused on optimizing designs using primary optical packaging. Therefore, our study proposes a V-shaped packaged structure that achieves a wide emission angle without the need for secondary optical elements, while simultaneously reducing the thickness of the backlight module. The optimal light intensity peak angle is achieved at 77.7° with a V-shaped design of 25°, and the optimal center light intensity ratio was 20.3% with a V-shaped design of 30°. The advantages of this design lie in its slim profile and its applicability to large-area and flexible light sources.
2. Methods
To obtain full-angle LEDs, a diffuse reflection layer is incorporated into the optical packaging structure. The primary function of this layer is to redirect the majority of the light toward the optical structure layers. However, multiple reflections within these layers can result in reduced light extraction efficiency. To overcome this challenge, a V-shaped packaged structure is proposed to minimize optical losses. The V-shaped diffuse structure is designed to facilitate lateral light emission through the optical structure layers, ensuring a wider emission angle while maintaining high light extraction efficiency. The key parameters influencing the light extraction efficiency of full-angle chip-level LEDs in the optical packaging structure are the V-shape angle (θV) and the thickness (DVH) of the diffuse structure. The refractive index of the optical structure layers is represented by nx, whereas the refractive index of the medium through which light exits is represented by ny. When the width of the optical structure layers is fixed, the thickness (LH) of these layers determines the extent of the total internal reflection. In this paper, the width of the total internal reflection zone (TS), defined as the region where total internal reflection takes place, can be determined using Eq. (1).
The optical structure layers play a crucial role in controlling the emission pattern by minimizing material absorption resulting from multiple reflections within these layers.
This study focuses on the V-shape packaged structure of GaN-based flip-chip (FC) epitaxial wafers with a wavelength of 450 nm. Figure 1 illustrates the packaging structure, which includes FC mini-LEDs, a white wall layer, optical structure layers, and the V-shape diffuse structure. The white wall layer prevents lateral light leakage from the chip, whereas the optical structure layers guide the light to achieve lateral emission, thereby controlling the light extraction efficiency and the desired emission pattern. The V-shape diffuse structure is designed to control the proportion of transmitted and reflected light, thus further optimizing the emission pattern
Fig. 1. 3D schematic of the light trajectory within the V-shape packaged structure of the mini-LEDs.
In the V-shape packaged structure of the mini-LEDs, the package size is denoted as CS. The substrate thickness is represented by SH, whereas WH refers to the combined thickness of the white wall layer and FC mini-LEDs. LH corresponds to the thickness of the optical structure layers. DH indicates the thickness above the V-shape diffuse structure, and DVH represents the thickness of the V-shape diffuse structure itself. The V-shape angle is denoted as θv and can be adjusted to achieve different optical peak angles. The apex of the V-shape is denoted as θa, the angle of incidence as θi, and the point of incidence P(x, y) undergoes a reflection off the inclined side of the V-shape with an angle relative to the normal (N) as θout. Using Eq. (2) and Eq. (3), we can calculate the relationship between θi and θout.
The Solidworks 2022 software was used to design the V-shape packaged structure for the mini-LEDs. The thickness (DH) above the V-shape diffuse structure was set to 400 µm. The V-shape diffuse structure was designed with eight different angles (θv) for subsequent simulation analysis: 15°, 17.5°, 20°, 22°, 25°, 30°, 35°, and 40°. The thickness (LH) of the optical structure layers was set to 600 µm, except for the V-shape 35° and 40° designs, where their thicknesses were increased to 700 µm and 840 µm, respectively, due to angle constraints. The bare chip size of the FC mini-LEDs was 1016 µm × 1016 µm, and the surrounding molding white wall layer had a thickness (WH) of 120 µm. The substrate size was 2000 µm × 2000 µm, with a thickness (SH) of 250 µm.
The Ansys SPEOS 2020 software was used for simulating the V-shape packaged structure. The simulation parameters included an input power of 1 W, a center wavelength of 450 nm, a refractive index of 1.55 for the silicone material, and a total of 50,000,000 rays. Figure 2(a) illustrates the intensity distribution of the normalized packaged efficiency (NPE) for different V-shape angles (θv). As the θv increases, the occurrence of total internal reflection (TIR) increases, resulting in improved optical light extraction efficiency. The tested V-shape angles were 15°, 17.5°, 20°, 22°, 25°, 30°, 35°, and 40°.
Fig. 2. (a) Simulated normalized packaging efficiency intensity distribution of the V-shape packaged structure of the mini-LEDs. (b) 2D simulation of the light distribution curve.
The emission angle of the different designs was analyzed by simulating the light distribution curve of the V-shape full-angle CSP mini-LEDs, as shown in Fig. 2(b). The key parameters primarily include the peak angles of the radiation patterns, the full width at half maximum (FWHM) of the light distribution, and packaging efficiency. The V-shape diffuse structure design effectively enlarges the emission angle. The V-shape 25°, 30°, and 35° designs exhibited better emission angles, with peak intensities at 77°, 76°, and 70°, respectively. The V-shape 22° design had a peak intensity angle of 65.5°, whereas the V-shape 15°, 17.5°, 20°, and 40° designs had peak intensity angles of 52.5°, 50.5°, 54.5°, and 56.5°, respectively. Despite having a better emission angle, the V-shape 35° design required a thicker optical structure layer due to angle limitations. Based on the consideration of the peak intensity angle, the V-shape 22°, 25°, and 30° designs were selected for actual sample fabrication.
The packaging process of the V-shape packaged structure of the mini-LEDs was conducted in three steps. The first was a pre-processing step, which included adhesive adjustment, colloidal mixing, filling and sealing, and bubble removal. The second step was the molding process, which consisted of pre-baking treatment, injection molding, and material cooling. The third step consisted of post-processing procedures, which encompassed the removal of excess adhesive around the substrate and thermal curing via baking.
Figure 3 illustrates the fabrication process of the V-shape packaged structure of mini-LEDs. Following molding, the FC mini-LEDs were arranged in a 5 cm × 5 cm matrix wafer, which undergoes additional processing to produce the white wall layer, optical structure layers, and V-shape diffuse structure. Finally, the wafer is cut along the X-Y direction to separate and finalize the fabrication of the V-shape packaged structure of mini-LEDs.
Fig. 3. Illustrates the fabrication process of the V-shape packaged structure of full-angle mini-LEDs.
The packaging of the white wall layer, optical structure layers, and V-shape diffuse structure is accomplished through the molding process. The process begins with the pre-processing step, which includes adhesive adjustment. Anti-settling starch and silicone were mixed thoroughly and the colloidal mixture was poured into a 30 cc syringe. Dow Corning OE-7662, a transparent adhesive with a refractive index of 1.55 and a Shore hardness of D66, was used as the adhesive. AEROSIL R 202, an anti-settling starch, was added to prevent LED emission color non-uniformity caused by settling. To eliminate the bubbles generated during the filling process, the colloidal mixture was subjected to debubbling at 5400 rpm for 30 minutes. The second step is the molding process. The mold is preheated at 80 °C for 5 minutes, followed by injection molding for 300 s. The upper mold temperature is set to 130 °C, whereas the lower mold temperature was set to 128 °C. After molding, the wafer is cooled down to 80 °C and then removed after 5 minutes. The final step is the post-processing process, which involves removing excess adhesive around the substrate and performing thermal curing baking at 150 °C for 4 hours. The three optical layers, namely the white wall layer, optical structure layers, and V-shape diffuse structure, are sequentially molded onto the chip surface using different colloidal formulations. The white wall layer consisted of a mixture of Dow Corning OE-7662 and TiO2 (10%–30%) with a powder-to-gel ratio ranging from 20:100 to 40:100. The optical structure layers were composed of Dow Corning OE-7662 transparent adhesive. The V-shape diffuse structure consisted of a mixture of Dow Corning OE-7662 and TiO2 (10%–25%).
3. Results and discussion
To validate the optical simulation results, we fabricated samples with three different angles. Figure 4 presents side views of the V-shape diffuse structure for the V-shape packaged structures of full-angle mini-LEDs with V-shape angles (θV) of (a) 22°, (b) 25°, and (c) 30°, as observed using an optical microscope (OM). The V-shape diffuse structure, which is placed on top of the optical structure layer, controls the light distribution. The respective thicknesses above the V-shape diffuse structure (DH) were 424 µm, 405 µm, and 217 µm. The packaged size (CS) for the V-shape 22° design was 1938 µm, whereas, for the 25° and 30° designs, it was 1928 µm. The design thicknesses (DVH) for the three V-shape diffuse structures were 326 µm, 459 µm, and 567 µm, respectively.
Fig. 4. Side view optical microscopy images of the V-shape packaged structure and V-shape diffuse structure of full-angle mini-LEDs. The different V-shape angles shown in the images are (a) 22° (b) 25° (c) 30°.
Measurements were conducted on the actual samples to analyze the optoelectronic characteristics of the full-angle mini-LED with V-shaped package structure shows in Fig. 5. Figure 5(a) illustrates the relationship between the driving current and spectral intensity. Based on the measurement results, the spectral electroluminescence (EL) intensity increased with higher driving currents at the center wavelength of 450 nm. Figure 5(b) displays the measurement results of the driving current and output radiant flux for the three V-shape designs. The V-shape 30° design achieved a radiant flux of 0.137 W at a driving current of 0.1 A and 0.42 W at a driving current of 0.35 A. The V-shape angle θV of 30° exhibited a higher output radiant flux. The I-V curve of the three angle designs of V-shape packaged structures of mini-LEDs is presented in Fig. 5(c), illustrating the relationship between the driving current and forward voltage. At a driving current of 0.1 and 0.35 A, the forward voltages were 2.8 and 3 V, respectively. The different angles of V-shaped packaging do not affect voltage variation, but in practice, there may be slight differences in voltage between different bare chips, which become more noticeable with higher driving currents. The reason for the voltage difference is the inherent variation in the bare chips themselves. Furthermore, Fig. 5(d) shows the radiation pattern curves of Lambertian 120° LEDs and V-shape packaged structures of full-angle mini-LEDs. The black curve represents the radiation pattern of the Lambertian 120° light source, whereas the blue, red, and green curves represent the radiation patterns of the V-shape designs with θV angles of 22°, 25°, and 30°, respectively. The peak angles of the radiation patterns were 67.5°, 77.7°, and 73.6°, and the full width at half maximum (FWHM) of the light distribution were 171.5°, 175.5°, and 174°, respectively. Among them, the V-shape with θV of 25° exhibited the optimal emission angle. The calculation of the center light intensity ratio, as shown in Eq. (4), yielded center light intensity ratios of 21.9%, 22.4%, and 20.3% for the V-shape designs, with the lowest intensity of the center point ratio observed for θV of 30°.
Fig. 5. Optoelectronic characteristics of the full-angle mini-LED with V-shaped package structure. (a) Relative intensity spectrum. (b) L-I (light-current) curve. (c) The I-V (current-voltage) characteristics curve. (d) Demonstrates the 2D light distribution curve.
The intensity of the center point ratio calculation Eq. (4) is as follows:
To gain further insights into the impact of different angles in the V-shape design on light output radiant flux, we conducted measurements on the V-shape packaged structure of the full-angle mini-LEDs.
At a driving current of 0.331 A and a forward voltage of 3 V, the radiant flux values for the V-shape angles of 22°, 25°, and 30° were 0.351, 0.385, and 0.398 W, respectively. Among them, the V-shape design with an angle of 30° exhibited the highest light output radiant flux at the same driving current. The measured results align with the optical simulation results of the peak intensity angle and center light intensity ratio, thus confirming the viability of the V-shape packaged structure of the mini-LED design. Compared to Lambertian 120° LEDs, the V-shape design expands the emission angle and FWHM, thus enabling a reduction in the thickness of backlight modules. Therefore, this design can be effectively implemented in ultra-thin large-area light sources.
4. Conclusions
In our study, we propose a chip-level V-shape packaged structure for mini-LEDs that enables full-angle emission without the use of a lens. The structure includes a white wall layer, optical structure layers, and a V-shape diffuse structure. The key parameters primarily include the peak angles of the radiation patterns, the full width at half maximum (FWHM) of the light distribution, and packaging efficiency. Our experimental results demonstrated that at a driving current of 0.331 A and a forward voltage of 3 V, the measured radiative fluxes for V-shape angles of 22°, 25°, and 30° were 0.351, 0.385, and 0.398 W, respectively. Among these designs, the optimized parameters for evaluating this article include the peak angles, full width at half maximum (FWHM), center light intensity ratios, and radiative fluxes. The optimal design, achieved with a 25° V-shape angle, yields the following values at maximum peak angles and FWHM are 77.7°, 175.5°, 22.4%, and 0.385W, respectively. The proposed chip-level V-shape packaged structure of the full-angle mini-LEDs offers several advantages. Moreover, the absence of secondary optical components allows for a thinner light source module, making it suitable for ultra-thin, large-area, and flexible light sources. This study demonstrates the superior performance and potential applications of the chip-level V-shape packaged structure in the field of full-angle emission. In special applications like outdoor advertising billboards or industrial displays as backlight light source. Furthermore, the secondary optical lens has a height of approximately 6 to 8 mm, which cannot be used in applications requiring an extremely thin module thickness (<5 mm). To reduce the number of light sources used, designing the light distribution to be wide-angle is helpful in significantly decreasing the quantity of light sources required. The contribution of this paper lies in proposing a change in the packaging structure, without the need for secondary optical lenses, by introducing a V-shape packaging structure to increase the light emission angle and reduce the central intensity in the normal.
Funding
National Science and Technology Council (NSTC 112-2622-E-194-004).
Acknowledgments
This work was supported by the National Science and Technology Council of Taiwan NSTC 112-2622-E-194-004.
Disclosures
The authors have no conflicts to disclose.
Authorship contribution statement. Zhi Ting Ye and Chia Chun Hu are responsible for the structure and conception of the entire article. Zhi Ting Ye, Chia Chun Hu and Yang Jun Zheng are responsible for the simulation data of the article. Zhi Ting Ye and Yang Jun Zheng are responsible for the initial writing of the manuscript. Zhi Ting Ye and Chun-Nien Liu are responsible for final proofreading.
Data availability
Data underlying the results presented in this paper are not publicly available at this time but may be obtained from the authors upon reasonable request.
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