Laser-Induced Failure Mechanisms in Silicon CMOS Image Sensors: Insights from 1550 nm Nanosecond Pulse Irradiation

Cameras, LiDAR, and radar are technologies designed to interpret the surrounding environment. They can capture data that helps in identifying objects and obstacles in the proximity of the autonomous system. For example, cameras can provide rich color and texture information, LiDAR can deliver precise distance measurements and high-resolution 3D imaging, while radar is excellent device in detecting moving objects under adverse weather conditions. Combining data from these diverse sensors enhances further the robustness and accuracy of autonomous systems, and allow them to make more informed decisions in real-time. However, the increasing use of high-powered LiDAR systems can be challenging specially the 1550 nm wavelength which is commonly used due to its superior beam quality and smaller beam spot because it can risk damaging silicon-based cameras that lack appropriate IR filters. This risk extends beyond automotive applications to include cameras in consumer electronics and security systems, where similar damage mechanisms could compromise functionality. In light of these challenges, new study published in Optics Express by Wanjun Bi, Ying Meng, Yunfeng Wang, Yingbiao Liu, Hui Yin, Hui Wu, and Han Liu from Hesai Technology investigated the failure mechanisms of silicon-based CMOS Image Sensors (CIS) when exposed to 1550 nm nanosecond laser pulses. The study identified and categorized the types of damage incurred, established the thresholds for damage under varying irradiation conditions, and analyzed the internal structural changes within the CIS using advanced imaging techniques such as focused ion beam (FIB) and scanning electron microscope (SEM).

The researchers used a backside-illuminated CIS from Sony with a resolution of 3864 x 2176 pixels and a pixel size of 1.45 x 1.45 µm. They employed a pulsed laser with a wavelength of 1550 nm, adjustable optical power, pulse width, and repetition rate. The laser beam was focused onto the CIS on a three-dimensional translation table, allowing precise control of the laser’s focal point. They varied irradiation time, pulse width, and repetition rate in their experiments. For each condition, laser power was increased until defects appeared on the CIS read-out images. These defects were categorized as point damage, line damage, and cross damage based on their appearance. The damage threshold was determined by dividing the single pulse energy by the measured spot area.

The authors characterized point damage by isolated spots of damage on the CIS surface. This type of damage occurred at lower laser energies and was the first observable damage as the laser power increased. Line damage appeared as continuous lines on the CIS read-out images, resulting from the connectivity of pixels in horizontal and vertical lines. This type of damage required higher laser energies than point damage and indicated a progression in the severity of the damage. Cross damage, the most severe type, involved intersecting lines forming cross patterns. This occurred at the highest laser energies and indicated significant structural damage to the CIS. The researchers used optical microscopy, FIB and SEM techniques to analyze the damaged CIS at the microscopic level. The authors observed at lower laser intensities, the Bayer filter layer under the micro-lens was ablated, causing point damage while at higher laser intensities, both the micro-lens and the Bayer filter were ablated, leading to more extensive damage, including line and cross damage. Moreover, the damaged CIS exhibited clear structural changes, with the thin dielectric layer between the photodiode and metal wiring layer being particularly affected.

The authors observed that the damage threshold decreased with increasing repetition rates above 100 kHz, due to reduced time intervals between pulses, leading to less heat dissipation and more heat accumulation. For repetition rates of 50 kHz and 100 kHz, the thresholds were similar, indicating sufficient time for heat dissipation at these rates. The damage threshold for a 4 ns pulse width was also significantly lower than that for a 10 ns pulse width. This suggests that peak power, rather than total energy, plays a critical role in causing damage. Moreover, longer irradiation times resulted in lower damage thresholds due to increased heat accumulation. However, the influence of repetition rate on the damage threshold was more pronounced than that of irradiation time. Additionally, the researchers observed that the change in damage threshold with varying laser parameters resembled the incubation effect observed in laser ablation. With consecutive pulses, surface defects and roughness increased, enhancing absorption and ablation. This effect suggested that heat accumulation facilitated damage, lowering the threshold with more pulses. They conducted simulations to explore heat accumulation and found that higher repetition rates led to less efficient heat dissipation, and caused temperature rise at the irradiated surface.

The findings of Wanjun Bi and colleagues enhance understanding of the damage thresholds and mechanisms of CIS under high-power laser exposure, enabling manufacturers to develop more resilient camera systems. This ensures that cameras in autonomous vehicles remain functional when exposed to potentially damaging laser sources, such as LiDAR systems. The study also provides guidelines for designing cameras with improved resistance to laser damage, including incorporating IR filters, optimizing sensor materials, and enhancing protective coatings. The successful identification of the specific damage mechanisms, such as the impact on the Bayer filter and micro-lens, manufacturers can now refine the materials and structures used in CIS to enhance their durability and performance under adverse conditions.

The impact on consumer electronics and security is beyond automotive applications, for instance it can also be applicable to cameras in smartphones, surveillance systems, and other consumer electronics. Understanding how 1550 nm lasers can damage these devices helps in designing protective measures to prevent accidental or intentional damage. Moreover, in security and surveillance applications, can ensure that cameras are resistant to high-power laser exposure which is critical for maintaining continuous and reliable monitoring. In conclusion, the systematic investigation of silicon-based CIS under 1550 nm nanosecond laser irradiation provided valuable insights into the failure mechanisms and damage thresholds of these sensors. The findings contribute to the existing database of laser damage mechanisms and thresholds, which will guide for better design and reinforcement of cameras and optoelectronic devices.