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Abstract
In this study, AlGaN-based deep-ultraviolet light-emitting diodes (DUV-LEDs) processed via standard laser dicing (SLD) and multifocal laser stealth dicing (MFLSD) were investigated. Adopting the MFLSD technology would generate a roughing surface rather than the V-shaped grooves on the sidewall of 508 × 508 µm2 DUV-LEDs, which would reduce the forward operating voltage and increase the wall-plug efficiency, light output power, and far-field radiation patterns of these devices. In addition, the wavelength shift, far-field patterns, and light-tracing simulation results of the DUV-LEDs processed with SLD and MFLSD were clearly demonstrated and analyzed. Accordingly, it was observed that the MFLSD process provided more possibilities for photon escape to increase the light extraction efficiency (LEE) of DUV-LEDs, thus decreased the wavelength-redshift and junction temperature in DUV-LEDs. These results provide a reference for advanced nano-processing practices implemented during the fabrication of semiconductor devices.
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1. Introduction
AlGaN-based deep-ultraviolet light-emitting diodes (DUV-LEDs) have various advantages, such as mercury-free, wavelength adjustable, high-temperature resistant, radiation resistant, and long lifetime characteristics. Therefore, they have significant potential application prospects in various fields, such as sterilization, medical treatment, storage, and communication [1,2]. However, the limited external quantum efficiency (EQE) and light-out power (LOP) of DUV-LEDs hinder their use in high-power applications [3]. The key factors affecting the performance of DUV-LEDs can be divided into two parts: (1) The large lattice mismatch-induced defects that seriously influence their internal quantum efficiency (IQE). (2) The limited light extraction efficiency (LEE) due to the absorption loss and reflection of ultraviolet (UV) light. Accordingly, related research on relieving compressive stress and improving the LEE of DUV-LEDs has attracted significant attention [4,5]. To enhance the EQE and LOP of DUV-LEDs, Zhou et al. used the laser lift-off and surface roughening technology, achieving a 2.5-time light extraction gain for UV-LEDs [6]. Moreover, Nishida et al. have improved the IQE of UV-LEDs by introducing thick bulk GaN as a substrate [7]. Jiang et al. have proposed a high-temperature-annealed AlN/Sapphire template and n-AlGaN epilayer growth to fabricate UV-LEDs, which successfully released the strong compressive stress, improving the crystal quality and interface roughness of UV-LEDs [8].
During the fabrication processes of LEDs, dicing is a necessary process used for separating the on-wafer semiconductor devices such as mosfets, micro-LEDs, and DUV-LEDs etc. [9–12]. However, traditional diamond dicing could only achieve the low-accuracy, and the plasma dicing faces the challenges of generating harmful gases and arcs during the dicing processes, thus the laser dicing technology which has the advantages of high-accuracy, high speed, low-cost and no debris and pollutants during process procedure has attracted people’s attention. For example, Guo et al. proposed a picosecond laser dicing technology to roughen the layers on the sidewall of a sapphire substrate, which improved the luminous efficiency of LEDs by 13.2% [13]. Lin et al. proposed a laser scribing technology to generate a rough-patterned back surface on the n-face GaN, which improved the LOP of LEDs by 47% [14]. However, having a laser beam with ultra-high energy density is dangerous. Moreover, the radiated area of the DUV-LEDs using standard laser dicing (SLD) would generate V-shaped grooves [15,16]. Therefore, because the multifocal laser stealth dicing (MFLSD) could distribute energy across multiple focal points, thereby reducing the energy density at each focus (minimized thermal damage) and increasing the affected area (further roughed the sidewall), thus it is more suitable for chip dicing [17].
In this study, we employed SLD and MFLSD technologies during the processing of DUV-LEDs. According to the measured performances of the SLD-DUV-LEDs and MFLSD-DUV-LEDs, a lower forward operating voltage was observed in the MFLSD-DUV-LEDs compared to that observed in the latter. Moreover, the LOP and EQE of the MFLSD-DUV-LEDs were improved, and the LOP of SLD-DUV-LEDs would be more quickly declined due to more heat generated in the device. In addition, the wavelength shift of SLD-DUV-LEDs is higher than that of MFLSD-DUV-LEDs owing to their differences in thermal effects. The far-field patterns and light-tracing simulation results also underline the mechanisms of the improvement observed in the LEE values of MFLSD-DUV-LEDs. Moreover, the lower junction temperature is also found in the MFLSD-DUV-LEDs.
2. Experiments
The epitaxial structure of 508 × 508 µm2 DUV-LEDs were grown on c-plane patterned sapphire substrates via metal-organic chemical vapor deposition (MOCVD), which consists n-AlN, 2.0 µm-thick Si-doped Al0.45Ga0.55N, 20 pairs of Si-doped Al0.4Ga0.6N/Al0.6Ga0.4N multiple quantum wells (MQWs), 30 nm-thick p-Al0.75Ga0.25N layer, Mg-doped p-GaN and 5 nm-thick Mg-doped p + -GaN (ohmic contact layer). Then, the mesa was processed with inductively coupled plasma reactive ion etching (ICP-RIE), and Ti/Al/Ni/Au (15/100/50/250 nm) metal layers were deposited as n-pads using E-beam evaporation and subsequently annealed at 950 °C for 30 s. The SiO2 passivation layer was deposited by plasma-enhanced chemical vapor deposition (PECVD) to passivate the sidewalls of the DUV-LEDs. The specific structure of a DUV-LED is shown in Fig. 1(a).
The on-wafer DUV-LEDs were split using SLD and MFLSD technologies, and are denoted as SLD-DUV-LEDs and MFLSD-DUV-LEDs, respectively. To avoid damaging the epitaxial structure of the devices, the penetration depth of the laser beams was controlled during the slicing process. The specific diagram is shown in Fig. 1(b): the SLD method used a high-power-density laser to dice the on-wafer DUV-LEDs, while the MFLSD used an optical lens to form a multifocal laser beam on the sapphire of the on-wafer DUV-LEDs. Finally, the DUV-LEDs were separated after laser dicing. In addition, the current-voltage (I-V) characteristics of the DUV-LEDs were measured at room temperature using an Agilent 4155 B semiconductor parameter analyzer, Moreover, the electroluminance performances of the packaged DUV-LEDs were measured using a calibrated integrating sphere (INSTRUMENT SYSTEMS) and TERALED and T3Ster combined system.
3. Results and discussion
To evaluate the effect of the SLD and MFLSD technologies on sapphire, side views of the SLD-DUV-LED and MFLSD-DUV-LED were obtained using a scanning electron microscope (SEM), as shown in Fig. 2. By using SLD technology, a rough surface morphology was generated on the syncrystalline orientation of sapphire (easy-to-crack surface) and V-shaped grooves on the unsyncrystalline orientation of sapphire (hard-to-crack surface) on SLD-DUV-LEDs. By using MFLSD technology, a rough surface morphology was generated on both the easy-to-crack and hard-to-crack surfaces of the sapphire on MFLSD-DUV-LEDs. In addition, the root mean square (RMS) roughness of the sidewall of the MFLSD-DUV-LEDs is larger than that of SLD-DUV-LEDs as shown in Fig. 2, that is because there is larger acting area on the sidewall of MFLSD-DUV-LEDs than that of SLD-DUV-LEDs.
The forward voltage-current (V-I) characteristics of the SLD-DUV-LEDs and MFLSD-DUV-LEDs are shown in Fig. 3(a). The dynamic resistances of the DUV-LEDs are shown in Fig. 3(b). In particular, the resistance of the MFLSD-DUV-LED was always larger than that of the SLD-DUV-LED at room temperature, that is because more light would be reflected and absorbed in the SLD-DUV-LEDs which with lower LEE, thus there would be more heat generated in SLD-DUV-LEDs. For the epitaxial structure of SLD-DUV-LEDs and MFLSD-DUV-LEDs is consistent, thus the difference of resistance in these DUV-LEDs should be attributed to the thermal-induced shrinkage of bandgap (corresponding to the severer red-shift of wavelength of SLD-DUV-LED than that of MFLSD-DUV-LED).
In addition, the LOP and EQE of these DUV-LEDs were studied; the EQE value was calculated using the following equation [18,19]:
To further investigate the origin of the differences in EQE, the wavelength shifts and far-field patterns of the SLD-DUV-LEDs and MFLSD-DUV-LEDs were measured. As shown in Fig. 5(a), the red shifts in the wavelengths of the SLD-DUV-LEDs and MFLSD-DUV-LEDs within a current range of 0–1040 mA are 1.46% and 1.36%, respectively. This phenomenon could be attributed to the differences in the light emitted from the sidewall of the sapphire, where much light is reflected or absorbed in the SLD-DUV-LEDs, and light escapes from the roughened surface of the MFLSD-DUV-LEDs. These lead to the differences in the thermal effect of the DUV-LEDs in question. The far-field patterns of the DUV-LEDs are shown in Fig. 5(b). Owing to the improved LEE characteristics caused by the roughed morphology on the sidewall of sapphire, more light would escape from the sidewalls and intensity of the light emitted by MFLSD-DUV-LEDs is higher than that of SLD-DUV-LEDs in the angle range of -90–90°. These results also validate the advantages of the MFLSD technology in enhancing the LEE characteristics of DUV-LEDs.
Fig. 5. (a) Wavelength-shift of the DUV-LEDs. (b) Far-field patterns of the DUV-LEDs at the injection current of 100 mA.
Using Trace-pro software, the Candela maps and light-tracing of the DUV-LEDs were simulated, as shown in Fig. 6. These results indicated that the number of photons escaping from the MFLSD-DUV-LEDs was more than those escaping from the SLD-DUV-LEDs. The refractive indices of GaN in the SLD-DUV-LED and MFLSD-DUV-LED are the same, which leads to the same transmittance. Therefore, the observed difference should be mainly attributed to the difference in the chip sidewall roughness of the DUV LEDs. For the rougher surface of the sidewall could improve the LEE of DUV-LEDs, thus there would be more photons escaped from the semiconductor to the air in MFLSD-DUV-LEDs than that of SLD-DUV-LEDs [20].
Figure 7 shows the junction temperature of SLD-DUV-LEDs and MFLSD-DUV-LED. For the higher LEE and lower heat-accumulation effect in the DUV-LEDs by using the technology of MFLSD, the measured junction temperature in the MFLSD-DUV-LEDs is lower than that of SLD-DUV-LEDs. In addition, it should be noted that this result is well-correspond to the wavelength-shift of these devices shown in Fig. 5. Moreover, the lower junction temperature of MFLSD-DUV-LEDs also reveal the reliability of DUV-LEDs could be enhanced with the MFLSD technology [21].
4. Conclusion
In this study, the light output performances of DUV-LEDs with laser stealth slicing and multifocal laser stealth dicing were investigated. We systematically analyzed the impact of the MFLSD technology on the device sidewalls and confirmed the experimental results using optical simulation software. These results indicated that the MFLSD-DUV-LEDs exhibited a lower forward operating voltage, a bit higher LOP and EQE (1 ∼ 2%) under low injected current due to the improvement of LEE, and because the heat is more easily generated in the SLD-DUV-LEDs, their LOP would be more quickly declined. In addition, the red-shift in the emission wavelength of the SLD-DUV-LED was slightly higher than that of the MFLSD-DUV-LED owing to the differences in their thermal effects. From the simulated far-field patterns and light-tracing results, the more roughed sidewall of MFLSD-DUV-LEDs also provided more possibilities for photon escape, increasing the LEE value of the LED, thus result in the lower junction temperature in the devices. These results proved the advantages of MFLSD during the process of chip dicing.
Funding
National Natural Science Foundation of China (62274138); Compound Semiconductor Technology Collaborative Innovation Platform Project of FuXiaQuan National Independent Innovation Demonstration Zone (3502ZCQXT2022005); Fundamental Research Funds for the Central Universities (20720230029); Science and Technology Projects of Fujian Province in 2022 (3502ZCQXT2022005); Science and Technology Plan Project in Fujian Province of China (2021H0011); Natural Science Foundation of Fujian Province (2023J06012).
Disclosures
The authors declare no conflicts of interest.
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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