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Abstract
Ultraviolet B light-emitting diodes (UVB LEDs) hold promise in medical and agricultural applications. The commonly used sapphire substrate for their epitaxy growth possesses a high refractive index and excellent UV light absorption characteristics. However, this high refractive index can induce total internal reflection (TIR) within the substrate, leading to decreased Light Extraction Efficiency (LEE) due to light absorption within the material. In this study, UVB LED chips were detached from the sub-mount substrate and directly affixed onto an aluminum nitride (AlN) substrate with superior heat dissipation using a eutectic process. This was undertaken to diminish packaged thermal resistance (PTR). Simultaneously, optimization of the UVB LED packaging structure was employed to alleviate LEE losses caused by the TIR phenomenon, with the overarching goal of enhancin external quantum efficiency (EQE). The final experimental findings suggest that optimal LEE is achieved with packaging dimensions, including a length (ELL) of 2 mm, a width (ELW) of 1.62 mm, and a height (ELH) of 0.52 mm. At an input current of 200 mA, the output power reaches 50 mW, resulting in an EQE of 6.3%. Furthermore, the packaged thermal resistance from the chip to the substrate surface can be reduced to 4.615 K/W.
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1. Introduction
UVB-LEDs exhibit various characteristics when applied as light sources, such as long lifespan, robustness, low energy consumption, high environmental friendliness, and mercury-free composition. AlGaN belongs to the wide-bandgap nitride semiconductor, considered as a new generation of semiconductors [1]. Due to its tunable direct bandgap, high breakdown field, and excellent chemical and thermal stability, AlGaN has played a crucial role in high-efficiency deep ultraviolet illumination and detection, as well as in high-frequency and high-power electronic devices [2]. Ultraviolet (UV) light encompasses electromagnetic waves with wavelengths ranging from 100 nm to 400 nm, which are shorter than visible light. It can be further classified into UV-A, UV-B, and UV-C based on different wavelength bands [3–4]. Current research indicates that the UVB LEDs wavelength band can be applied in the treatment of vitiligo, and narrowband 311 nm UVB LEDs (NB-UVB LEDs) emission therapy is particularly effective [5–6]. Studies have also demonstrated the effectiveness of UVB phototherapy in alleviating symptoms of psoriasis, with good inhibitory effects on moderate to severe psoriasis that may be less responsive to conventional medications [7–8].
UV-LEDs have a wide range of potential applications, including optical communication, chemical decomposition, disinfection, air and water purification, as well as biological detection systems [9–10]. The Minamata Convention on Mercury, established by the United Nations, mandates that member countries completely prohibit the production and import/export of mercury-containing products by the year 2020 [11]. Light-emitting diodes (LEDs) containing aluminum gallium nitride AlGaN components, particularly ultraviolet LEDs (UV-LEDs), currently represent the most environmentally friendly light source that complies with the Minamata Convention's regulations to replace mercury lamps by 2020. It remains challenging to manufacture UV-LEDs with high epitaxial crystal quality, as the quantum structures of AlGaN and single-crystal III-nitride films are typically deposited on heterogeneous epitaxial substrates. Nevertheless, in heterogeneous epitaxy, the mismatch in thermal expansion coefficients and substantial lattice mismatch can lead to a significant decrease in the internal quantum efficiency (IQE) of UV-LEDs [12–14]. The method to enhance IQE is to epitaxially grow on substrates with low defect density to minimize the impact of lattice mismatch [15–17]. Enhancing the LEE of deep-UV LEDs by Optimizing tilt of the aluminum sidewall [18]. The common packaging method for UV-LEDs currently involves encapsulating LED chips by bonding them with transparent organic resin [19–20]. However, the sapphire material in the packaging structure strongly absorbs UVB light, leading to a decrease in LEE [21].
On the other hand, UV-LEDs typically use p-GaN as the contact layer, and its strong absorption of UVB light results in low LEE, and high contact resistance, which in turn reduces long-term reliability, increases forward voltage, and yields very low EQE [22–24]. To improve LEE, various methods have been studied to reduce absorption losses in the p-GaN contact layer or p-AlGaN covering layer [25–27]. Many researchers have also explored the use of high-reflectivity metals as electrode materials and structures instead of traditional electrodes to reduce optical losses [28–30]. Additionally, enhancing LEE involves increasing the surface texture of the sapphire substrate and employing patterning processes to avoid light losses caused by TIR [31–33].
High temperatures in LED packaging can adversely affect chip performance and cause aging of packaging materials, resulting in reduced reliability. Therefore, effective thermal management is crucial in LED packaging to ensure chip reliability, performance, maximum optical power output, luminous efficiency, and overall lifespan [34–36]. The thermal resistance values for traditional EMC packaging typically range from 32 K/W for low-power LEDs to 9.9 K/W for high-power LEDs. These high thermal resistance values are primarily attributed to differences in vertical thermal conductivity and high interface resistance [37–38].
In this study, we employed a laser welding system operating at a wavelength of approximately 1064 nm for laser lift-off, facilitating the removal of the UVB chip sub-mount. Subsequently, we utilized an eutectic process to directly bond the UVB LED chip with the AlN substrate, aiming to reduce packaged thermal resistance. Simultaneously, through optimization of the UVB LED packaging structure, we aimed to minimize TIR induced LEE losses. We chose silicone gel as the packaging material to narrow the refractive index difference between the sapphire substrate and the exit interface. By adjusting the thickness of the encapsulation layer, we optimized LEE to enhance EQE while analyzing the uniformity of the emitted light. The final experimental results indicate that optimal LEE is achieved when the dimensions of the packaging body, including length (ELL) of 2 mm, width (ELW) of 1.62 mm, and height (ELH) of 0.52 mm. At an input current of 200 mA, the output power reaches 50 mW, resulting in an EQE of 6.3%. Additionally, the packaged thermal resistance from the chip to the substrate surface can be reduced to 4.615 K/W.
2. Methods
The epitaxial structure of 310 nm UVB LEDs involves the growth of an AlN buffer layer on a sapphire substrate through metal-organic chemical vapor deposition (MOCVD). Subsequently, a strain-relief layer is grown on the AlN buffer layer, consisting of a high-period AlN/AlGaN superlattice. Following this, an undoped AlGaN layer is grown as the superlattice intermediate layer, followed by a Si-doped n-AlGaN layer serving as the n-contact layer. The active region of the multiple quantum wells (MQWs) comprises AlGaN quantum wells (QWs) and quantum barrier layers (QBs), followed by a magnesium-doped p-AlGaN structure as the electron blocking layer (EBL). Finally, a magnesium-doped p-GaN layer is deposited as the p-contact layer. The 3D epitaxial structure is illustrated in Fig. 1, where Fig. 1(a) represents a schematic diagram of the epitaxial structure, and Fig. 1(b) depicts the chip structure.
Fig. 1. UVB LEDs Device Diagram, (a) Schematic diagram of epitaxial structure, (b) Chip structure diagram. (c) Illustration of the encapsulant layer TIR phenomenon and 2D ray tracing diagram.
To enhance LEE, we optimized the thickness of the encapsulation layer, using silicone as the material. This intermediate medium serves to protect the UVB LEDs chip and reduces the refractive index difference between the chip and the air, thereby minimizing LEE losses. The TIR phenomenon occurs when light enters different media, resulting in a change in refractive index. When the angle of incidence is less than the critical angle, the light is divided into two parts: one part undergoes TIR, and the other part undergoes refraction. Conversely, when the angle of incidence is greater than the critical angle, all light undergoes internal total reflection without refraction. The critical angle θx of the encapsulation layer, where the refractive index of the internal medium is nA and the refractive index of the external medium is nB, can be calculated using Eq. (1).
When nA is the refractive index of sapphire, which is 1.78, the critical angle θx for TIR is 34.13°. If nA is replaced with the refractive index of silicone, which is 1.56, the critical angle θx for TIR becomes 39.86°, indicating that fewer angles of light will be consumed by TIR. When the length (L) and width (W) of the encapsulation layer are fixed, the thickness (ELH) of the encapsulation layer will affect the TIR region. The substrate used for UVB LED is sapphire, which exhibits strong absorption characteristics in the UVB spectrum. Therefore, reducing internal total reflection can enhance light extraction efficiency.
As shown in Fig. 1(c), light enters the encapsulation layer from the UVB LEDs chip, so TIR does not occur in the yellow region. If the angle of incidence exceeds this region, TIR will occur in the cyan area. The width of this region can be defined as TX, as represented by Eq. (2).
The sub-mount is commonly employed for safeguarding exposed chips; however, it introduces an additional level of thermal resistance. To address this, we utilize a Laser Welding System operating at a wavelength of approximately 1064 nm for laser lift-off, facilitating the removal of the UVB chip sub-mount, as shown in Fig. 2(a). The UVB LEDs packaging process is illustrated in Fig. 2(b)-(f). First, holes are drilled through the AlN substrate to the backside of the substrate, with a circuit layout for the electrode, where the electrode is made of Au, as shown in Fig. 2(b). Next, tin paste is applied to the surface of the AlN circuit board, as depicted in Fig. 2(c). Subsequently, the UVB LEDs chip is bonded to the AlN substrate, as schematically shown in Fig. 2(d). The Compression molded package process is then carried out, using silicone for encapsulation, as illustrated in Fig. 2(e). After allowing the silicone to cure (150°C for 2 hours), the final step involves the X-Y direction cutting process, as shown in Fig. 2(f).
Fig. 2. UVB LEDs Component and Packaging Process. (a) Facilitating the removal of the UVB chip sub-mount (b) AlN PCB process (c) Brush solder paste process (d) UVB LEDs chip bonding process (e) Compression molded package process, and (f) X-Y direction cutting process.
The 3D representation of the UVB LEDs chip is shown in Fig. 3, where CW, CL, and CH represent the width, length, and height of the UVB chip, and ELW, ELL, and ELH represent the width, length, and height of the encapsulant layer. In this study, Ansys SPEOS 2020 software was used for the simulation of the UVB LEDs packaging structure. Through the optimization of the encapsulation layer thickness (ELH), the principle is to utilize the refractive index difference between Silicone and sapphire. Compared to direct emission into the air, the difference is smaller, reducing Fresnel loss and providing protection to the UVB LEDs chip. However, a thicker encapsulation layer will lead to stronger material absorption and TIR, causing a decrease in light efficiency. The parameters are set as follows: input power of 1 W, central wavelength of 310 nm, encapsulation layer material is Silicone with a refractive index of 1.56, the substrate material is Al2O3 with a refractive index of 1.78, and the thermally conductive substrate material is AlN with a refractive index of 2.26. The number of rays is set at 5,000,000.
3. Results and discussion
Firstly, a simulation analysis is conducted to examine the impact of encapsulation layer thickness (ELH) on LEE. Therefore, the results for different encapsulation layer thicknesses need to be analyzed and compared. Figures 4(a)-(f) show the intensity distribution of LEE for different encapsulation layer thicknesses. When the encapsulation layer thickness is 0.52 mm, the LEE is 93.89%, as depicted in Fig. 4(b). This parameter exhibits a relatively optimal value.
Fig. 4. Simulated light efficiency intensity distribution diagram of different encapsulant thicknesses, (a) 0.47 mm, (b) 0.52 mm, (c) 0.58 mm, (d) 0.62 mm, (e) 0.72 mm, and (f) 0.82 mm.
The calculation of uniformity is shown in Eq. (3). This study analyzed the uniformity (UM) distribution at a distance of 3 mm.
L is the light output power, A is a constant, I is the injection current, where m represents the luminous efficiency and its calculation method is as shown in Eq. (4). EQE is an important indicator for evaluating the luminous efficiency of UVB LED. The calculation method can be calculated by Eq. (5).
The simulation results show that when the encapsulant thickness is 0.52 mm, the uniformity is 83.9%, which is the best value for all optimized thicknesses. The uniformity can reach 83.9%, as shown in Fig. 5(b). Compared with other parameters, better results can be obtained. Uniform distribution.
Fig. 5. Simulated illuminance distribution at an encapsulation distance of 3 mm, (a) 0.47 mm, (b) 0.52 mm, (c) 0.58 mm, (d) 0.62 mm, (e) 0.72 mm, (f) 0.82 mm.
Figure 6. Illustrates the simulated light distribution for different Encapsulant thicknesses of UVB LEDs, with ELH of 0.52 mm, the full width at half maximum (FWHM) is 148 degrees.
Following this study, to enhance the thermal performance of UVB LEDs, it is proposed to directly bond the inverted chip sub-mount of UVB LEDs to the AlN substrate after Laser Lift-Off (LLO). A layer of encapsulating glue is applied to the surface of the chip. The actual sample of UVB LEDs with an inverted chip structure bonded directly to the AlN substrate is depicted in the locally enlarged optical microscope (OM) image in Fig. 7(a)-(d). The AlN substrate dimensions of 2000µm (AlNL), 1620 µm (AlNW), and 360 µm (AlNH) in length, width, and height, respectively. The transparent encapsulant dimensions are 2000µm (ELL), 1620 µm (ELW), and 520 µm (ELH) in length, width, and height, respectively. The chip size is 700 µm (CL) in length, 700 µm (Cw) in width, and 420 µm (CH) in height. The UVB LED chip utilizes an inverted structure and is directly bonded to the AlN substrate. Figure 7(a) presents the top view, Fig. 7(b) illustrates the side view, Fig. 7(c) displays the electrode layout on the wafer, and Fig. 7(d) shows the side view of the chip with encapsulation glue.
Fig. 7. OM diagram of UVB LEDs chip, (a) top view of the chip, (b) side view, (c) chip electrode layout, (d) side view of the chip with encapsulant.
The peak wavelength of the emitted UVB LED is 310 nm. At currents of 30 mA and 200 mA, the peak wavelengths are 309.5 nm and 310.25 nm, respectively. As the current increases, the maximum peak wavelength shift is only 0.75 nm, demonstrating the high wavelength stability and reliability of the chip. The current-dependent peak wavelength is shown in Fig. 8(a). Figure 8(b) illustrates the electrical characteristics curve of the UVB LEDs. In operational mode, at currents of 10 mA and 200 mA, the voltages are 4.989 V and 6.024 V, respectively. The voltage increases exponentially with the injection current, while the output power shows a linear increase. At currents of 10 mA and 200 mA, the chip's output power is 2.921 mW and 0.511 mW, respectively. Figure 8(c) shows the curve of CIE x-y coordinates concerning current. At currents of 10 mA and 200 mA, the x-y coordinates are (0.2508, 0.2385) and (0.2592, 0.2396), respectively. Even with an increase in injection current from 0 to 200 mA, the change in CIE x-y coordinates remains minimal, indicating the chip's main wavelength color point is quite stable, with only a slight deviation occurring. Figure 8(d) presents the current-dependent curves of EQE and WPE for UVB LEDs. Under operational conditions, at an injection current of 10 mA, the EQE and WPE are 6.88% and 5.52%, respectively, while at 200 mA, they are 6.3% and 4.17%.
Fig. 8. (a) UVB LEDs current versus peak wavelength. (b) UVB LEDs L-I-V curve. (c) UVB LEDs current versus CIE-x-y plot. (d) UVB LEDs current vs. EQE and WPE curves.
This is a comparison chart of the light distribution curve of the actual sample UVB LED and lambertian 120 degrees. The peak light intensity angle of the chip used in this study is about ±14° and the center light intensity drops to 93.53%, as shown in Fig. 9.
Through the eutectic process, reducing the thermal interface resistance is crucial for bonding UVB LED chips onto aluminum nitride substrates to enhance performance. The measurement of thermal resistance utilizes the T3 Ster thermal transient tester. The thermal resistance measurement results for UVB LED packaging are illustrated in Fig. 10. Under an input voltage of 6.024 V and a current of 200 mA, the packaged thermal resistance from the chip to the substrate surface can be reduced to 4.615 K/W. The purple area indicates the total thermal resistance from the AlN substrate to the bottom circuit board, which is 21.390 K/W. Through the sub-mount removal process, we have successfully demonstrated a reduction in packaging thermal resistance values.
4. Conclusions
The study proposes an optimized packaged structure to enhance Light Extraction Efficiency (LEE) and reduce thermal resistance in Ultraviolet B (UVB) LEDs. We employ a Laser Welding System operating at approximately 1064 nm for laser lift-off, facilitating the removal of the UVB chip sub-mount. Subsequently, we utilize eutectic process to directly bond the UVB LED chips to an AlN substrate, aiming to decrease packaged thermal resistance. Simultaneously, by optimizing the packaging structure of UVB LEDs, we aim to reduce total internal reflection (TIR) and mitigate LEE losses. Experimental results demonstrate that optimal LEE is achieved with packaging dimensions of 2 mm in ELL, 1.62 mm in ELW, and 0.52 mm in ELH. At an input current of 200 mA, the output power reaches 50 mW, with an external quantum efficiency of 6.3%. The packaged thermal resistance from the chip to the substrate surface can be reduced to 4.615 K/W. This packaging method for UVB LEDs is suitable for applications like polymer curing and phototherapy, potentially replacing traditional mercury lamps. Furthermore, it shows promise for advancements in medical diagnostics, plant growth lighting, sensors, and other fields.
Funding
National Science and Technology Council (112-2622-E-194-004).
Acknowledgments
This work was supported by the National Science and Technology Council of Taiwan (NTSC 112-2622-E-194-004).
Disclosures
The authors have no conflicts to disclose
Authorship contribution statement. Zhi Ting Ye and Chun Nien Liu are responsible for the structure and conception of the entire article. Zhi Ting Ye, Yang Jun Zheng, and Chia Chun Hu is responsible for the simulation data of the article. Zhi Ting Ye, and Yang Jun Zheng is responsible for the initial writing of the manuscript.
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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