The Micro Damage Mechanism Of Charge Coupled Device Under Laser Irradiation

Charge-coupled device(CCD) is a semiconductor imaging device, which has the advantages of small volume, light weight, low power consumption, high sensitivity, wide spectral response and wide dynamic range. As the central part of the imaging and detecting system, they are widely used in industrial productions, daily lives, research and military defense fields. Laser has the ability to cause controllable damage to materials and devices owing to the strong direction, monochrome and energy concentration properties. Therefore, the study of laser induced damage to CCD is crucial both for electro-optical countermeasure in imaging system and for improvement of sensor robustness. Nowadays, the researchers have met two problems in this regime, one is that the structures and materials by which the CCD is constructed is not clear; the other is that there are not enough testing equipments to provide high precision examination after the CCD is damaged. Those two problems make the promoted mechanisms as hypotheses, without conclusive evidences. To change the situation, a typical front-lighting area CCD is selected as an object and its construction and working principles are quite clear after experimental dissection and theoretical analyze. The nanosecond laser, picosecond laser and femtosecond laser are used to damage CCD in single pulse regime. By combining the CCD structure and the irradiating laser, the mechanisms of dot damage, line damage and total damage are illustrated. The details are shown in below:Firstly, the CCD is dissected layer by layer and measured by AFM and SEM. The CCD is constructed by micro lens, silica optical layer, W shield, silicon electrode, silica insulating layer and the bulk silicon substrate. Precise time control of HF etching the silica optical layer is get in the experiment, which helps to reduce the damage to insulating layer. By using a focused ion beam to cut the CCD, the cross-sectional thickness of the insulating layer is obtained. The morphologies of every structure of the CCD are quite clear due to the high resolution of the AFM and SEM, which help us to understand the CCD’s construction deeply. It’s working principle is illustrated briefly by semiconductive theory, which lays a foundation for the analyze of the output images of the damaged CCD.Secondly, different CCDs of same kind are subjected to nanosecond laser, picosecond laser and femtosecond laser respectively. The experiments are all operated in single pulse mode, which helps to ignore the accumulation effects by multi-pulses. This different damage stages are decided by the output images, which also helps to determine the corresponding energy fluence thresholds. The energy of nanosecond laser is not stable, so that the energy fluences and the output images are quite different. While those of picosecond laser and femtosecond laser have smaller differences due to the stable energies.Thirdly, the damage mechanisms are illustrated for three different lasers. The energy distribution on the silicon surface is simulated, as the focus properties of the micro lens. The simulation results show that the energy of incident light is focus on the light sensing area with a dumbbell-shaped distribution. On this basis, the primary damage mechanism for femtosecond laser, the electric induced oxide breakdown, is determined by the ablation theory. The decrease of the read-out gate accounts for the line damage by the working principle and the output images of the CCD. An explanation in laser matter interaction regime is given out for the hardness to totally damage the CCD for single pulse femtosecond laser. Thermal damage is dominant in damaging the CCD for nanosecond laser and picosecond laser. The incident light energy is focus on the silicon surface, which is considered to be the thermal source for the thermal conduction model, different from the traditional model with a light shield thermal source. The results of the thermal conduction model of nanosecond laser and picosecond laser show that the temperature later diffusion distance is greater than longitudinal distance, which helps to determine the thermal damage to insulating layer as the mechanism for CCD from dot damage to total damage.In conclusion, the analysis of the damage mechanism of CCD under laser irradiation is limited in one pixel and the results are concluded by combining laser matter interaction theory, the structures and the working principle of the CCD. The high resolution images of structures and damaged areas of CCD are provided, especially, the images of material localized damages in one pixel, which provide the powerful demonstration to the damage mechanism presented in this paper.