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Abstract
Cameras, LiDAR, and radars are indispensable for accurate perception of the surrounding environment and autonomous driving. Failure mechanisms of silicon-based CMOS image sensor (CIS) irradiated by 1550 nm nanosecond laser were investigated systematically in this paper. The damages of CIS were divided into point damage, line damage, and cross damage according to different damage performances. The damage thresholds under different irradiation conditions (different repetition rates, pulse widths, and irradiation times) were explored. Large repetition rates and long irradiation times would induce more heat accumulation, more temperature increase, and a low point damage threshold. The damage threshold for a pulse with a narrow pulse width is lower than that for a pulse with a long pulse width. The damaged CIS was analyzed further by focused ion beam (FIB) and scanning electron microscope (SEM). The damage location in the internal CIS structure was analyzed and the overall failure process was summarized. The results we get could enrich the database of laser damage mechanisms and laser damage thresholds of CIS, which will provide meaningful guidance for the camera design technology and anti-laser reinforcement technology of optoelectronic devices.
© 2024 Optica Publishing Group under the terms of the Optica Open Access Publishing Agreement
1. Introduction
Accurate perception of the surrounding environment is a prerequisite for autonomous driving. A complete environmental perception technology solution requires the coordination of information from multiple sensors such as cameras, LiDAR, and radar. The data and information acquired by multiple sensors are gathered together for comprehensive analysis to more accurately and reliably describe the external environment, to improve the correctness of systematical decisions. The camera could measure color and light intensity. The captured images have rich texture and color information. The camera is low-cost and easy to install on the car. However, its distance perception ability is weak, and its nighttime detection ability is low. LiDAR has accurate range sensing and can produce 3D imaging of surrounding objects both day and night with high resolution, but it is costly and its performance is greatly affected by weather. Millimeter wave radar is not susceptible to adverse weather conditions and can perceive speed and distance accurately, making it suitable for moving object detection. However, its resolution is low and its ability to perceive stationary objects is weak. The fusion of multiple sensors of the same kind or different kinds is essential to obtain complementary information of different parts and categories. Camera, LiDAR, and radar, none is dispensable.
LiDAR is a device to detect objects by sending a laser and collecting the return light of the object. To perceive objects far enough, LiDAR needs to emit more power, which may cause damage to cameras. This question may lead to problems in fusion perception. Considering the spectrum of solar radiation, the absorption of the atmosphere, and the manufacturing technology of laser and detector, the commonly used wavelengths for LiDAR are 905 nm and 1550 nm. A laser with a wavelength of 905 nm is usually a semiconductor laser while a laser with a wavelength of 1550 nm is usually a fiber laser. Due to the better beam quality and small beam spot and the lack of a 1550 nm IR filter in the camera, the 1550 nm laser is more dangerous to cameras, not only for cameras used for autonomous driving but also for cameras in telephones, cameras in security, etc.
The camera mainly consists of a lens module and an image sensor. Image sensors are devices that convert photons that fall on the surface of a pixel during integration time into photoelectrons through the photoelectric effect. Image sensors are the key components of cameras. Usually, these image sensors are silicon-based and are used in visible wavelength range. With a band gap of silicon of 1.1 eV, the largest wavelength that can excite electrons from the valence to the conduction band is roughly 1100 nm. Generally, image sensors can be divided into two categories: CCD (Charge-Coupled Device) and CIS (CMOS Image Sensor). Although there are performance differences between CCD and CIS [1–5], the pinned photodiode is the primary photodetector structure used in both [6].
CCD initially emerged for space imaging applications and is used more often in hostile environments where high levels of radiation are encountered. Early damage research on CCD is focused on high-energy radiation, such as X-ray [7,8]. With the development of technology, CCD enters the consumer market. Some research works have been carried out on camera damage by laser radiation. Although CIS emerged later than CCD, owing to the intrinsic advantages of CIS like low power consumption, low cost, high-speed imaging, integration capability, radiation hardness, etc [9], CIS has broader prospects in the consumer market in the future. At present, more and more devices use CIS cameras. The reported inevitable laser damage research works of CIS or CCD are summarized in Table. 1. It is obvious that most of these research works are concentrated in the visible band like 532 nm or an infrared band like 1064 nm in the response wavelength band of silicon. Besides, there is a wide span of damage threshold in Table. 1, which is mainly due to different experimental scenarios (eg. 10 s irradiation time in Ref. [10] or single shot in most of the other references) and different criteria for the damage threshold.
Table 1. Summary of reported damage thresholds of CIS or CCD by laser irradiation
Theoretically, the silicon-based camera is considered to have no response to 1550 nm. However, there have been reported cases of 1550 nm LiDAR burning out cameras. In 2019, a man attending the International Consumer Electronics Show (CES) show in Las Vegas said that a lidar had permanently damaged the sensor on his Sony camera [18]. 1550 nm is a key wavelength for LiDAR. However, there has been little research on the laser irradiation effect of CIS out of its working wavelength band [19,20], and there is almost no research on CIS damage by a 1550 nm laser up to now.
In this paper, we investigated the failure mechanisms of CIS by 1550 nm nanosecond laser systematically. The overall failure process was figured out and summarized. The damage performances of CIS could be divided into point damage, line damage, and cross damage [15,20,21]. The damage thresholds under different irradiate conditions (different repetition rates, pulse widths, and irradiation times) were explored. Large repetition rate and large irradiation time could induce more heat accumulation, more temperature increase, and a low point damage threshold. The damage threshold for a pulse with a narrow pulse width is lower than that for a pulse with a long pulse width. The damaged CIS was analyzed by focused ion beam (FIB) and scanning electron microscope (SEM). The results we get could enrich the database of laser damage mechanisms and laser damage thresholds of CIS, which will provide meaningful guidance for the camera design technology and is very beneficial to autonomous driving.
2. Setup and experiment
A commonly used Backside illumination CIS manufactured by Sony was the subject used for investigation. The preliminary specifications are shown in Table 2. The number of effective pixels is 3864 (horizontal) × 2176 (vertical) and the pixel size is 1.45 (horizontal) × 1.45 µm (vertical).
Table 2. Preliminary specifications of CIS
The experimental schematic is shown in Fig. 1. A pulsed laser with a wavelength of 1550 nm was adopted. The optical power, pulse width, and repetition rate are adjustable. The output laser was divided into two paths by the beam splitter 1. One-fifth of the optical power entered the power meter to monitor power stability. The power variation was less than 3% after one hour’s warm-up. The left four-fifths of the optical power was divided into two paths by beam splitter 2. Similarly, One-fifth of the optical power is transmitted into a detector and measured by an oscilloscope. The left four-fifths of the optical power was focused by a converging lens. A CIS affixed on the flat surface of the three-dimensional translation table was positioned in the focal plane of the converging lens. The spot size on the focal plane was 30 µm approximately. There was a mechanical shutter between beam splitter 1 and beam splitter 2. The opening and closing time could be controlled by the mechanical shutter. The time when the mechanical shutter opens is called “irradiation time” in the following. The irradiation time is monitored by the oscilloscope.
Firstly, to find the focal plane of the converging lens, the Z-axis of the three-dimensional translation table was shifted to find the minimal laser spot size on the CIS. Theoretically, the silicon-based CIS should have no response to wavelength of 1550 nm. However, we could see the focused laser spot during the experiment. There are investigations show that impurities in the silicon provide the energy levels in the band gap, from which electrons can be excited either thermally or by absorption of a photon. It is these impurities that contribute to the infrared response [19,22]. Secondly, the X-axis and Y-axis were controlled to shift CIS. Ideally, the laser-exposed sites will form a square lattice array on CIS (see right figure in Fig. 1), and laser interacted only once on each lattice site. The side length of the nearest two lattice sites was about 1 mm to ensure no mutual influence between laser-exposed sites. The irradiation time, pulse width, and repetition rate were varied. For each experimental condition, the power of the laser varied from low to high. After each irradiation, the CIS was examined online by capturing images under both dark and bright environments. If there was a defect on the read-out image, then the average power was recorded. Then, the average power is divided by the repetition rate to obtain a single pulse energy. The damage threshold was obtained by dividing the single pulse energy by the measured spot area on the focal plane. If line damage is formed (see Fig. 2), the first damage power is recorded and the following will not be recorded because pixels are connected by horizontal and vertical lines and will have a mutual influence between adjacent lattices.
By increasing laser energy, the 1550 nm laser could induce inevitable CIS damage. Figure 2 summarizes inevitable CIS damages observed during the experiment. The images captured by damaged CIS under both bright and dark environments were shown. These performances could be divided into three categories: point damage, line damage, and cross damage. From top to bottom and from left to right, the laser energy increases roughly but not absolutely. The meaning of ‘roughly but not absolutely’ is as follows. With the same experimental condition in the point damage region, the dark dot in the bright image may appear first in one CIS while the white spot in the bright image may appear first in another CIS. In the same experimental condition in the line damage region, the horizontal line damage may appear first in one CIS while the vertical line damage may appear first in another CIS. This may be attributed to the divergence of different CISs and the power variation. Although there are uncertainties, the overall trend is clear. The threshold of point damage is less than that of line damage. With the increase of laser energy, the damaged spot size became large. As pixels are connected by horizontal and vertical lines. Once line damage appears, the CIS becomes more vulnerable to be damaged. With the increase of laser energy, the damaged line became bold.
When severe cross damage formed, the damaged micro-morphology could be observed by optical microscope, which was shown in Fig. 3. It could be seen from Fig. 3(a) that the paler zone corresponds to the laser focus where the intensity was sufficiently high to ablate the Bayer layer under the micro-lens (see Fig. 8 for more details). In Fig. 3(b), the laser intensity was increased by a factor of 1.3, and the micro-lens above the Bayer layer was also ablated, exposing the hole formed in CIS. Around this hole where the laser intensity is not as high as that in the laser spot center, the damaged micro-morphology is much similar to that in Fig. 3(a).
Fig. 3. Surface morphology of the CIS observed by the optical microscope (500 x magnification). The experiment condition is irradiation time of 50 ms, repetition rate of 400 kHz, and pulse width of 4 ns. The single pulse energy in (a) is around 48 mJ/cm2, and the single pulse energy in (b) is around 62 mJ/cm2. The damage of (a) is less severe than that of (b). The inserts in both (a) and (b) are the corresponding raw images captured in the bright environment by damaged CIS.
3. Data analysis and results
In the following, we have summarized systematical experimental results to investigate the variation of damage threshold. Figure 4 was the point damage threshold for different repetition rates with irradiation time fixed at 50 ms and pulse width fixed at 4 ns. Four CISs were used. For different CISs, the value of the damage threshold was slightly different. However, the tendency is almost the same. For a repetition rate larger than 100 kHz, the point damage threshold drops with the increase in repetition rate, which may be due to the reduced time interval between two adjacent pulses. The time interval is about 1.25 µs for 800 kHz and the time interval is about 10 µs for 100 kHz. The smaller the time interval, the less heat dissipation, and the more heat accumulation. Thus, the damage threshold decreased. For the repetition rate of 50 kHz, the time interval is 20 µs, which is also considered large and leads to less heat accumulation. The damage threshold for 50 kHz is nearly the same as that for 100 kHz. The small variation is considered within a reasonable range of error. As the pulse width was fixed here, the influence of peak power is equivalent to energy density.
The experimental data confirms that for a growing number of incident laser pulses, heat accumulation is responsible for the decrease of the multi-pulse threshold. We further do some simulations to explore the heat accumulation and what is the time interval for sufficient heat dissipation. The structure of CIS is complex and the composed material is different. For simplification, we use silicon as a typical material because silicon is the basic material of photodetector in CIS. The simulation model is simplified as the thermal process of laser on silicon materials. The silicon is regarded as a cylinder. As shown in Fig. 5(a), the thickness of silicon is 150 µm, and the radius is 2.25 mm. The incident laser beam is aligned perpendicularly to the surface center and assumed to be Gaussian distribution. A surface heat source is applied to analyze. A three-dimensional finite element model is used to solve the thermal conduction equation. The details about the thermal conduction equation can be found in Ref. [23]. The thermophysical properties of silicon are shown in Table 3.
Fig. 5. (a) the simulation model of laser interaction with silicon. (b) the simulated results for different repetition rates in irradiated time of 50 µs.
Table 3. Thermophysical properties for silicon
The laser parameters used in the simulation are listed as follows: the pulse width is 4 ns, the peak power is 60 W, and the spot size is 30 µm. Assuming that the initial sample temperature is the room temperature of 300 K. The simulated results of temperature increase for different repetition rates from 50 kHz to 800 kHz are shown in Fig. 5(b). For the convenience of exploring the trend of temperature changes, the irradiation time is modified to 50 µs (1000× lower than 50 ms used in the experiment). As a result, the temperature rise is relatively small. However, it is obvious to see that the temperature increase is roughly the same for 50 kHz and 100 kHz at the end of irradiation time. When the repetition rate is larger than 200 kHz, the temperature increase becomes larger because a decrease in the time interval between consecutive pulses does not allow an efficient heat dissipation into the bulk material thus causing a temperature rise of the irradiated surface.
Figure 6 was the experimental point damage threshold for different irradiation times and different pulse widths with a repetition rate fixed at 400 kHz. Seven CISs were used in the experiment. For the same irradiation time of 50 ms, the damage threshold for 4 ns is 17-25 mJ/cm2, and the damage threshold for 10 ns is 37-43 mJ/cm2. The damage threshold for 10 ns is about two times higher than that for 4 ns. As the irradiation time increases, the difference in damage thresholds between 10 ns and 4 ns decreases. For pulse width of 10 ns and 4 ns, the difference between pulse width is about 2.5×, and the difference between peak power is less than 1.25×. From this data, it seems it is the peak power contributes to the damage. The damage threshold dropped with the increase in irradiation time due to more heat accumulation. Compared with Fig. 4 and Fig. 6, we could find that the influence of repetition rate was larger than irradiation time. The damage threshold drops about 8 mJ/cm2 from 50 kHz to 800 kHz (repetition rate increases ∼16×), while the damage threshold drops about 7 mJ/cm2 from 50 ms to 5000 ms (irradiation time increases ∼100×).
The change in damage threshold is kind of like the incubation effect during laser ablation in laser micromachining [24]. For many materials, it has been experimentally observed that by irradiating their surfaces with bursts of N consecutive pulses, their ablation threshold, defined as the minimum laser fluence to start the ablation process, is lowered. This effect is known as incubation. The most likely hypothesis on the origin of incubation is an increase of surface roughness after multi-pulse irradiation, due to ripples formation or accumulation of surface defects. Such defects, generated by the first impinging pulses, facilitate absorption of the next coming laser pulses, thus enhancing ablation [25]. For damage with repetitive laser pulses in this paper, when the decrease of the time interval between consecutive pulses is small enough and does not allow an efficient heat dissipation into the bulk material, the temperature rise of the irradiated surface will occur. It seems that the heat accumulation mechanism will facilitate the incubation effect in the same way. The damage threshold lowered with a larger number of pulses. There is an incubation model that can relate the multi-pulse threshold to the single pulse ablation threshold [24,25]. It will be significant and instructive if we can come up with an empirical incubation model to accurately predict the threshold under different numbers of pulses. Unfortunately, the multi-pulse threshold we got in Fig. 6 is for pulse numbers from 2 × 104 to 2 × 106. Thus, it is reasonable to consider that a saturation of the incubation effect has occurred. Besides, the minimum obtainable number of pulses is 2500 for this setup due to the minimal opening time of the mechanical shutter used is limited to 50 ms and the minimal frequency rate of the laser is limited to 50 kHz. The threshold for a few pulses can't be obtained.
Figure 7 shows the difference between the line damage threshold and point damage threshold (D-value) of the same CIS. The repetition rate was fixed at 400 kHz, the irradiation time was fixed at 50 ms, and the pulse width was fixed at 4 ns. Four CISs were used and the average D-value was 9.5 mJ/cm2. The D-value for CIS-2 was approximately 7 mJ/cm2, and the D-value for CIS-3 was approximately 14 mJ/cm2. It seems that the D-value variation was a little large. This may be because of the divergence of different CISs and the power variation as mentioned in part 2.
The damaged CIS were analyzed further by FIB and SEM. The results are shown in Fig. 8. The experiment condition is irradiation time of 50 ms, repetition rate of 400 kHz, and pulse width of 4 ns. Figure 8(a) and Fig. 8(b) were for the same damage position using a laser intensity of around 25 mJ/cm2 while Fig. 8(c) and Fig. 8(d) were for the same damage position using a laser intensity of around 50 mJ/cm2. Figure 8(b) and Fig. 8(d) were the cross section by FIB slicing from the slice position of Fig. 8(a) and Fig. 8(c), respectively. From Fig. 8(b), we can see the CIS structure. On the top is micro-lens used to collect more light. Under micro-lens, there is a layer of Bayer filter used to distinguish different colors. Under the Bayer filter is the photodiode used to convert light to photoelectron. In the photodiode region, there is deep trench isolation (DTI) used to eliminate crosstalk between pixels. Under the photodiode region, there is a thin dielectric layer to isolate photodiodes and metal wiring layers. The bottom is a metal wiring layer used to provide power supply, clock signal, and row and column selection function.
A few micro-lens were ablated in the surface morphology in Fig. 8(a). From the cross section in Fig. 8(b), we can see that in the area where the micro-lens were intact, the beneath Bayer filter has been damaged, which was consistent with the test result in Fig. 3(a). More micro-lens was ablated in Fig. 8(c) and the top of the DTI could be seen, which means the Bayer filter has been ablated absolutely. From the cross section in Fig. 8(d), we could see that the micro-lens and Bayer filter were ablated. Besides, in the zoom-in figure in Fig. 8(d), it could be seen more clearly that the thin dielectric layer between the photodiode and metal wiring layer was damaged. The same location will be damaged whether CIS is in working status or not in working status when the laser acts on the CIS, which means this is optical damage.
Overall, the damage process by the 1550 nm laser could be summarized as follows: light irradiated on the CIS was absorbed by the Bayer filter first. The temperature of the Bayer filter increases. When the absorbed laser was sufficient, the Bayer filter reached the boiling point and splashing, part of the micro-lens and the Bayer filter was missing due to thermal pressure. This process is consistent with that described in Ref. [26]. In this stage, the point damage shown in Fig. 2 formed. With the increase of laser fluence, the thin dielectric layer between the photodiode and metal wiring layer was destroyed. There seems to be no abnormal area in the photodiode in Fig. 8(b) and Fig. 8(d). Laser pulses on the order of nanoseconds can cause optical breakdown damage due to the dense plasma produced by the high laser electric field intensity and the short duration of the laser pulse effects. During such an optical breakdown mechanism, the generated plasma expands and the produced shock wave generates mechanical damages while the plasma recombination causes thermal damages [15,27]. Once the dielectric layer was breakdown, signal interruption caused by short circuits or open circuits formed line damage in the read-out image of the CIS. Cross damage is formed by further increasing laser energy. This process is different from the damage process with 1064 nm laser described in Ref. [26]. When the laser intensity is high enough, the photodiode melts and the channels were damaged.
4. Conclusions
In summary, we have investigated the failure mechanisms of silicon-based CIS by 1550 nm nanosecond laser systematically. The damage performances of CIS could be divided into point damage, line damage, and cross damage. The damage thresholds under different irradiation conditions were explored. The point damage threshold dropped with the increase of repetition rate, which may be due to the reduced time interval between two adjacent pulses. The smaller the time interval, the less heat dissipation, and the more heat accumulation. The damage threshold for pulse width of 10 ns was higher than that of pulse width of 4 ns. The damage threshold dropped with the increase of irradiation time due to more heat accumulation. The influence of repetition rate was larger than irradiation time. The damage threshold drops about 8 mJ/cm2 from 50 kHz to 800 kHz (repetition rate increases ∼16×) while the damage threshold drops about 7 mJ/cm2 from 50 ms to 5000 ms (irradiation time increases ∼100×). The difference between line damage threshold and point damage threshold of the same CIS for repetition rate of 400 kHz, irradiation time of 50 ms, and pulse width of 4 ns was 9.5 mJ/cm2. The damaged CIS were further analyzed by FIB and SEM. The damage location was figured out and the overall failure process was summarized.
The results we get could enrich the database of laser damage mechanisms and laser damage thresholds under different conditions. At the same time, the failure mechanism results will provide meaningful guidance for the camera design technology and are very beneficial to autonomous driving. For example, add an infrared filter that has low transmittance in the 1550 nm band before CIS to prevent infrared light entry in CIS.
Funding
Shanghai Hesai Technology Co., Ltd.
Acknowledgments
We thank Kewei You and Wenfeng Liu in Shanghai Institute of Optics and Fine Mechanics, Chinese Academy of Sciences for help for this work.
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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