高功率衬底转移VCSEL阵列失效分析与早期筛选

Failure Analysis and Early Screening of High Power Substrate-Transferred VCSEL Arrays

高功率垂直腔面发射激光器(VCSEL)阵列在激光医疗和激光传感等领域有着广泛应用,不同的应用场景对高功率VCSEL阵列的可靠性、成本、功率密度等也有着相应的要求。本文针对长脉冲应用场景的5 mm×5 mm大尺寸高功率808 nm VCSEL阵列器件,开展了可靠性研究,并探讨了衬底转移、封装等因素对器件带来的影响。可靠性实验在电流为50 A、脉宽为30 ms、频率为10 Hz的注入水平下进行,利用共聚焦显微镜、X射线透视仪、电荷耦合器件(CCD)显微镜等进行数据收集。实验结果表明,对于衬底转移的大尺寸高功率808 nm器件,表面不规律凸起、焊接空洞、暗点数量等均会导致器件早期失效,尤其是20μm以上的不规律凸起高度、15%以上的不规律凸起面积、单个空洞比大于1.5%的焊接空洞和1%以上的暗点数量会直接带来致命损伤。本文的可靠性失效分析结果为高功率VCSEL阵列在工业应用中的早期失效筛选提供了一种简便可靠的方法。

Objective Vertical-cavity surface-emitting lasers(VCSELs) are characterized by low divergence angle, circular output beam, low threshold current, low temperature dependence(approximately 0.07 nm/K), high efficiency, and an extended lifespan. Because the distributed Bragg reflector(DBR) mirrors of VCSELs and the laser emission are perpendicular to the wafer epitaxial surface, they facilitate the realization of two-dimensional dense integration for high-power arrays. The 808 nm wavelength is crucial in high-power laser applications, serving as an optimal pump source for Nd∶YAG or Nd∶YVO4 crystals to generate 1064 nm wavelength lasers. Its suitability for medical aesthetics, owing to melanin absorption and deep skin penetration, extends its applicability to the military and industrial domains. Therefore, conducting reliability studies and analyses of high-power 808 nm VCSEL arrays is of significant practical importance. However, the operation of high-power VCSEL arrays results in substantial heat power density. The most direct approach to address the thermal management of 808 nm high-power VCSEL arrays is substrate transfer, which involves the migration of light-emitting units from a GaAs substrate to a high-thermal-conductivity substrate. Paradoxically, a reduction in the lifespan of substrate-transfer devices has been observed despite the concurrent enhancement in the optoelectronic conversion efficiency.Consequently, it is imperative to conduct reliability studies and analyses that specifically target substrate-transfer devices. This study addresses the challenges introduced during substrate-transfer packaging such as irregular surface protrusions, high solder void rates,and an increased number of failure-emitting points(dark spots under CCD microscopy). Through the implementation of detection methods, we selectively identify the devices exhibiting these issues, categorize them, and subject them to aging experiments. By comparing the power decay rates and analyzing the locations of the newly emerged failure-emitting points, we identify the key factors influencing the reliability of substrate-transfer devices. Based on our findings, we propose a straightforward and reliable approach for early screening of high-power VCSEL arrays.Methods In this study, devices exhibiting irregular surface protrusions, elevated solder void rates, and a significant number of failure-emitting points(dark spots under CCD microscopy) were selectively grouped for aging experiments. The aging process involved the systematic recording of power levels at regular intervals with the prompt removal of failed devices from the aging platform. Subsequently, CCD microscopy was employed to inspect the locations of failure-emitting points on the devices. In addition,an X-ray transmission instrument was used to observe the distribution of solder voids in the bonding layers. By meticulously comparing the data collected before and after the aging process, we identified the locations of the newly emerged failure-emitting points and assessed the changes in the solder void distribution. This comprehensive analysis aimed to elucidate the factors influencing the reliability of substrate-transfer devices.Results and Discussions Devices with irregular protrusion heights exceeding 20 μm and irregular protrusion area ratios exceeding 15% exhibit early failures(Table 2). Higher irregular protrusion heights result in the destruction of the vertical cavity structure of the device, causing a loss of normal light emission. The associated thermal accumulation affects both the irregular protrusion area and the surrounding regions, leading to early failures(Figs. 4 and 6). Devices with larger irregular protrusion areas have dark spots localized within the irregular protrusion area without diffusion. After failure, the solder layer in this region does not exhibit extensive voids,indicating a weaker thermal accumulation that affects only the light-emitting points within the irregular protrusion area without significantly affecting the solder layer and surrounding regions(Fig. 6). Early failures occur in devices with circular solder voids and individual void ratios exceeding 1.5%, with dark spots appearing in proximity to these voids(Fig. 7). Analysis of the remaining devices with void ratios exceeding 1.5% reveals individual void ratios below 1%, confirming that individual voids with higher ratios are the cause of early failures. Devices with dark spot counts exceeding 1% experience early failures(Table 4), and the dark spot area expands after failure, indicating an increased thermal dissipation rate in malfunctioning devices(Fig. 8). An extrapolation of the lifespan for the remaining normally aged devices suggests a minimum expected lifespan of 66.02 million cycles, which meets the requirements for applications in extended pulse scenarios(Fig. 10).Conclusions This study focuses on aging tests and analysis of the failure mechanisms in devices that, as a result of substratetransfer packaging, present issues such as elevated surface irregular protrusions, high solder void ratios, and an increased number of malfunctioning light-emitting points. The findings reveal that higher irregular protrusion heights directly damage the vertical cavity structure of the VCSEL, rendering it incapable of emitting light. Moreover, a direct correlation is observed between the increased irregular protrusion height and intensified thermal accumulation, affecting the surrounding light-emitting points and solder layers, and leading to early device failures. Larger irregular protrusion areas also contribute to early failures, albeit confined to the irregular protrusion regions. In high-power devices, severe solder voids adversely affect heat dissipation, causing early failures when the individual void ratio of a single solder void exceeds 1.5%. Furthermore, the thermal dissipation power of malfunctioning light-emitting points increases, and a count of dark spots exceeding 1% results in additional heat accumulation and early device failure.While substrate-transfer devices enhance electro-optical conversion efficiency, they impose stringent requirements on subsequent packaging processes. In long-pulse applications, substrate-transfer devices exhibit a shortened lifespan compared to that of their nontransferred counterparts, primarily because of the elevated thermal power density resulting from substrate transfer and subsequent packaging. Therefore, in the substrate transfer and packaging processes of high-power VCSEL arrays, process optimization is crucial for mitigating issues such as irregular surface protrusions, solder voids, and mechanical damage, thereby enhancing the reliability of high-power substrate transfer devices. The early screening of high-power VCSEL arrays in industrial production currently involves accelerated aging, which requires post-packaging electrical testing. However, this method is cumbersome and expensive. To address this, we propose an early screening method utilizing morphology detection to eliminate devices with irregular protrusion heights exceeding 20 μm and irregular protrusion areas exceeding 15%, solder void detection to remove devices with a void ratio exceeding 1.5% for a single solder void, and dark spot detection to exclude devices with dark spot quantities exceeding 1%. Compared with traditional accelerated aging methods, this approach is operationally simpler and more cost-effective. The devices selected through this screening process demonstrate, through extrapolated lifespan results, suitability for long-pulse applications.