Electric-induced oxide breakdown of a charge-coupled device under femtosecond laser irradiation Liuzheng Gao,1 Zhiwu Zhu,2 Zhengzheng Shao,1 Xiang’ai Cheng,2 and Shengli Chang1,* 1
College of Science, National University of Defense Technology, Changsha 410073, China 2
College of Photoelectric Science and Engineering, National University of Defense Technology, Changsha 410073, China *Corresponding author: [email protected]
Received 12 September 2013; revised 13 October 2013; accepted 14 October 2013; posted 14 October 2013 (Doc. ID 197495); published 25 October 2013
A femtosecond laser provides an ideal source to investigate the laser-induced damage of a charge-coupled device (CCD) owing to its thermal-free and localized damage properties. For conventional damage mechanisms in the nanosecond laser regime, a leakage current and degradation of a point spread function or modulation transfer function of the CCD are caused by the thermal damages to the oxide and adjacent electrodes. However, the damage mechanisms are quite different for a femtosecond laser. In this paper, an area CCD was subjected to Ti: sapphire laser irradiation at 800 nm by 100 fs single pulses. Electric-induced oxide breakdown is considered to be the primary mechanism to cause a leakage current, and the injured oxide is between the gate and source in the metal-oxide semiconductor field-effect transistor (MOSFET) structure for one CCD pixel. Optical microscopy and scanning electron microscopy are used to investigate the damaged areas and the results show that the electrodes and the oxide underneath are not directly affected by the femtosecond laser, which helps to get rid of the conventional damage mechanisms. For the primary damage mechanism, direct damage by hot carriers, anode hole injection, and an enlarged electric field in the insulating layer are three possible ways to cause oxide breakdown. The leakage current is proved by the decrease of the resistance of electrodes to the substrate. The output saturated images and the dynamics of an area CCD indicate that the leakage current is from an electrode to a light sensing area (or gate to source for a MOSFET), which proves the oxide breakdown mechanism. © 2013 Optical Society of America OCIS codes: (140.3538) Lasers, pulsed; (140.3330) Laser damage; (040.1520) CCD, charge-coupled device. http://dx.doi.org/10.1364/AO.52.007524
1. Introduction
The study of laser-induced damage to charge-coupled devices (CCDs) is of great significance both for optoelectronic countermeasures in imaging systems and for improvement of sensor robustness. In the functional damage mechanisms, damage to the isolation oxide between adjacent polysilicon clock lines and damage to metal-oxide semiconductor field-effect 1559-128X/13/317524-06$15.00/0 © 2013 Optical Society of America 7524
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transistors (MOSFETs) are the primary sources for leakage current in nanosecond lasers [1–4]. The damage comes from melting of the electrodes caused by laser irradiation. However, with new developments in CCD function and structure, those mechanisms may no longer be sufficient to explain the damage behaviors, especially in the latest developing femtosecond laser regime. Few studies have been done to investigate damage to CCDs in the femtosecond laser regime and the damage mechanism is not yet clear [5,6]. Because of its thermal-free properties, femtosecond lasers can cause localized damage to material [7],
which helps to limit the damage scale. According to femtosecond ablation theory [8], electrons are excited into the conduction band or higher by single-photon or multiphoton absorption to become hot electrons. When the electrons get to a crucial number, material damage occurs. Hot carriers and ablation of material are two main factors that could cause dysfunction in a CCD. The former could bring soft breakdown or hard breakdown to the oxide in the metal-oxide semiconductor (MOS) structures [9], while the latter destroys the materials of which the CCD is made [10–12]. To investigate reasonable damage mechanisms, we choose a typical front-illumination area CCD designed in recent years and irradiated it with a femtosecond laser in single pulse mode. During the experiment, different damage stages and the corresponding energy fluences were recorded. Stages of “dot damage” and “line damage” appeared in the experiment, both of which had been observed in previous work [4,13]. After irradiation, the CCD sample was examined by optical microscopy and scanning electron microscopy (SEM) layer by layer and the damaged areas were quite clear in the optical and SEM photos. By comparing the results completely, we find that the electric-induced oxide breakdown of the MOS structures [14,15] is the cause of the leakage current, which is quite different from the traditional damage mechanisms. Traditional mechanisms hold that leakage currents in gate to drain, gate to source, or drain to source come from the melting of insulating oxides caused by direct laser irradiation [1–3]. However, in the structure of the CCD we studied, the microlens and the material covering the electrode keep the light away from the electrode and the insulating layer underneath, so that direct damage to them is impossible for a thermal-free and localized femtosecond laser. However, hot electrons and holes are generated by the femtosecond laser in the source/drain area. Anode hole injection (AHI) [16,17] or direct damage by the hot carriers may account for the oxide breakdown between the gate and source/drain. The voltage applied on the electrodes and substrate might also promote the damage process. The damage is electric induced, and the femtosecond laser provides only hot carriers. 2. CCD Sample and Experiment A.
Structure of the CCD
The CCD we tested is a Sony ICX405AL. It has a size of 1∕3 in., is four vertical clock driven, and displays time integration. The effective pixels are 500H× 582V, and the pixel size is 9.8 μmH × 6.3 μmV. The CCD is a multilayer device, consisting of a microlens, a silica optical layer, a W-shield, a polysilicon electrode, a silica insulating layer, and silicon doped by P type or N type ions. The sketch cross section of the CCD is shown in Fig. 1 [18]. For normal incident light, most of the light is focused into the silicon. Very little light should irradiate the W-shield, and no light should get to the covered polysilicon electrodes.
Fig. 1. Cross-sectional view of a CCD.
Its working process can be seen in the literature [18,19]. The electrons are transferred from the light sensing area to the transfer channel or, in other words, from source to drain in a likely MOSFET structure. The N area and the N area under the electrode are the source and drain, respectively, while the electrode is the gate. B. Experiment
The experiment was carried out using a Ti:sapphire laser (λ 800 nm) with 100 fs full width at halfmaximum pulse duration. The output laser was linearly polarized, and the maximum single pulse energy was 0.25 mJ. The multipulse frequency can be moderated to 2 Hz. With a half-wave plate and a polarization beam splitter, we could vary the pulse energy continually. After that, the beam was split into equal parts through a calibrated neutral density splitter lens. One part was measured by an energy meter, and the other was focused onto the CCD surface by a biconvex lens with a 300 mm focus length. The damage situation was finally present on the screen of the image acquisition system. The CCD was working during the laser irradiation. The single pulse mode was performed in the experiment with assistance from an electric drive shutter. The laser energy was gradually increased to reveal the whole damage process. After irradiation, optical microscopy and SEM were used to investigate the characteristics of the damaged areas; meanwhile, the CCD sample was treated with physical and chemical methods to uncover different material layers. The microlenses were removed from the silica optical layer by plastic tweezers. Hydrogen fluoride was used to corrode the silica optical layer from the silicon and the W-shield by strictly controlling time. To make the damage area more clear, we use 30% H2 O2 liquid to corrode the W for about 4 h. 3. Results A. Damage Processes and the Corresponding Energy Fluences
The laser has a Gaussian spatial beam profile and the spot the on CCD is elliptical with a minor axis 1 November 2013 / Vol. 52, No. 31 / APPLIED OPTICS
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the results at 268 mJ∕cm2 . Figure 3(b) shows the damage crater clearly and the sharp damage edge is consistent with the profile of the femtosecond laser. Figures 3(c)–3(f) are SEM images of the area shown in Fig. 3(b) after sample preparation. The dashed circle surrounds the crater and the solid circle represents the laser irradiation boundary. Comparing Figs. 3(a)–3(f), we can draw several conclusions.
Fig. 2. Output images with corresponding energy fluences.
ωx 51.31 μm (1∕e2 beam horizontal radius) and a major axis ωy 63.53 μm (1∕e2 beam vertical radius). Therefore, the laser irradiating area is S πωx ωy 1.02 × 10−4 cm2 . The average energy fluence F E∕S, where E is the attenuated pulse energy. By calculating the energy fluences and labeling on every shot, as shown in Fig. 2, we can distinguish the different damage stages. Figure 2 gives the output images of the damaged areas while the CCD is still working. When the laser energy fluence is 2.0 mJ∕cm2 , the gray level of the white dot is 177 (the gray level of saturated output is 255), which means dot damage appears but it is not saturated. When the energy fluence reaches 2.5 mJ∕cm2 , the gray level of the white dot is 255, so that we define this energy fluence as the threshold of dot damage. With the increase of laser energy fluence, the white dot enlarges and evolves into a shuttle-shaped white area, with a white line going through the entire vertical line, which is called “line damage,” when the fluence is above 330 mJ∕cm2 . The white line at an energy fluence of 216 mJ∕cm2 is so unique that we choose to ignore it to avoid uncertainty. As a rough estimate, the shuttle-shaped image involves about 100 vertical pixels maximum at energy fluence of 330 mJ∕cm2. All damage is permanent and does not recover when the laser is removed. B.
Characteristics of Damage Areas
The damaged areas are investigated by optical microscopy and SEM. The optical image of the dot damage area indicates that the material’s color slightly changes, as shown in Fig. 3(a), but it cannot provide more detail. In the line damage areas, optical images show that the central part is elliptically ablated, leaving behind a crater. Outside the crater, there is a white corona, which has the same generation principle as the dot damage area due to the low laser energy fluence. SEM images give more information owing to larger amplification and higher resolution. Because there are no differences in the SEM images for the damaged areas from 216 mJ∕cm2 to 588 mJ∕cm2 except for the scale, we present only 7526
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(1) In the crater, microlenses are elliptically ablated [see Fig. 3(b)], which is well consistent with the laser profile, and the silicon under the microlens is severely damaged [see Figs. 3(c), 3(e), and 3(f)]. (2) Although the microlenses are fine in the corona area [see Fig. 3(b)], the silicon underneath is slightly damaged by the laser [see Fig. 3(d)] and that explains the color change in Fig. 3(a). (3) The damage area will expand if the laser energy fluence is larger by comparing the damage situations between the edge and the central area [see Figs. 3(d) and 3(f)]. (4) Even in the damaged center, the W-shield and electrodes under it are not affected by the laser [see Figs. 3(e) and 3(f)] and that means that materials only in the light sensing area are damaged. 4. Discussion
The optical microscopy and SEM images indicate that when the femtosecond laser irradiates the CCD, materials only in the light sensing area are damaged, while the functional transfer channel is not affected by the laser. Comparing Fig. 2 with Fig. 3(a), we can see that the number of saturated pixels on the screen is much larger than the number irradiated by the laser (100 saturated pixels versus 30 damaged pixels vertically). As we know, only pixels in the laser irradiating area are damaged; the additional saturated vertical pixels outside the irradiating area are not laser damaged. Therefore, the formation of saturated pixels comes from vertical overflow electrons. Dark current and leakage current are two possible sources of overflow electrons. The former comes from the defects of the damaged silicon crystal lattices and will express the shape of the damaged area on the screen for low electrons supplement. The latter is from gate to source/drain and will increase with more severe damage to the insulating oxide. Thus, the leakage current is dominant in generating the overflow electrons. However, there are still two possible ways for the leakage current to leak from the electrode to bulk silicon through the silica insulating layer, as expressed in Fig. 4. One is the transfer channel, which is unrealizable, because there would always be the vertical saturated lines on the screen when damage occurs and that does not fit with Fig. 2. The other is to the light sensing area, which is convincing because the most photoemission carriers accumulate in this area during laser irradiation. When the laser irradiates the CCD, abundant hot carriers generate in the light sensing area on a femtosecond timescale. There are three possible ways for those electrons to damage the insulating layer
Fig. 3. Optical microscopy and SEM images of damaged areas. (a) Optical image of three different areas, (b) SEM image of damaged area at the energy fluence of 268 mJ∕cm2, (c) SEM image of (b) after corroding the W-shield, (d) the enlarged area of (c), (e) the enlarged central area of (c) after removing the fused silica, and (f) the enlarged central area of (c).
between the gate and source/drain. First, the diffusion of the hot carriers will cause damage to the insulating layer for the voltages applied on the electrode. Second, the AHI model might also make a contribution to damaging the insulating oxide. When the hot electrons are injected into the anode through the oxide, the holes generated at the anode will get trapped in the oxide. The holes trapped in the oxide allow for increased current density due to holeinduced trap generation [20]. There will be increased current density when the damage occurs and more electrons create more hot holes. Thus, the positive feedback will continue until the oxide breaks down [21]. The efficiency of AHI might be promoted owing to the large numbers of hot electrons generated by the femtosecond laser. Third, if the electrons fill the N area (see Fig. 4) and diffuse to the substrate, the transient current will result in the electrode
voltage and substrate voltage being straightforwardly applied on each side of the insulating layer. The electric field in the insulating layer will be
Fig. 4. Doping distribution in bulk silicon. 1 November 2013 / Vol. 52, No. 31 / APPLIED OPTICS
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Table 1.
Resistance of Vertical Electrodes to Substrate under Different Stages (Unit: MΩ)
Normal Dot damage Line damage
RΦ1-sub
RΦ2-sub
RΦ3-sub
RΦ4-sub
4.73 4.2 0.79
4.92 4.7 0.83
4.72 4.4 0.86
4.94 4.7 0.87
enlarged, even more sharply due to the point effects for the distinctive structure between the electrode and the light sensing area [22]. To sum up, hot carriers and enlarged electric fields are two main factors that can cause damage to the insulating layer and both of them are electric induced. To examine the leakage current, we measure the resistance of the vertical electrodes to the substrate at three different stages. The results are shown in Table 1. When the CCD is dot damaged, the resistance between vertical electrodes and substrate is slightly smaller than in the normal stage. This means that the leakage current appears but is not sufficient enough to overflow into the surrounding vertical pixels and leave behind a white dot on the output screen (see Fig. 2). At the time when the white line presents itself, those resistances apparently move to a much lower level, which means more severe damage to the insulating layer and more leakage current to create overflow electrons. As we know, for a Gaussian beam, the central irradiating area has the greatest energy fluence, and, consequently, the severest damage to the insulating layer, and the most plentiful overflow electrons will emerge in this area. With a vertical barrier (P area in Fig. 4), the electrons cannot overflow horizontally but easily overflow to vertical adjacent light sensing areas and eventually evolve into a shuttle shape. But it is a difficult matter to establish a precise oxide breakdown model for a CCD damaged by a femtosecond laser. First, the MOS structure of the CCD is complicated both for the doping distributions in the bulk silicon and for the setting of vertical electrodes. Second, because the hot carriers are generated on a femtosecond timescale, the excursion, diffusion, and ionization of the carries should all be considered, which make the solution difficult. As stated, the circuit in which we measure the resistance is from electrode to substrate through the light sensing area. The electrons are not directly leaked into the transfer channel, but come from the light sensing area through the read-out gate (see Fig. 4). If the laser damaged area expands to the readout gate area (see Figs. 3(d) and 3(f)), the threshold of the gate will decrease. That will allow the electrons in the light sensing area to leak into the transfer channel and make every vertical pixel passing by carry the leaking electrons. Those leaking electrons will be regarded as information electrons by the measurement end of the CCD and will finally show on the screen as a white line. The change of saturation level of the white lines with the increase of laser energy fluence also proves the above mechanism. 7528
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5. Conclusion
In this paper, we have investigated the damage processes and mechanism of a typical area CCD by a femtosecond laser. Dot damage and line damage are observed on the output images with threshold energy fluences of 2.5 mJ∕cm2 and 330 mJ∕cm2 , respectively. Optical microscopy and SEM images of the damaged area show that only materials in the light sensing area are damaged by the laser. Materials including the W-shield, electrode, and the silicon underneath are not affected. Those results help us to get rid of the conventional damage mechanisms of laser direct irradiation. The lengthening saturated figures on the output image and the special structure of the CCD lead us to find the electric-induced oxide breakdown between gate and source, which is proved by the changes of resistance of the vertical electrodes to the substrate. Although hot carriers and an enlarged electric field make contributions to the oxide breakdown, the precise damage model is difficult to establish. As a conclusion, the damage mechanisms in a CCD depend on its structure and the irradiating laser. References 1. C. Z. Zhang, S. E. Watkins, R. M. Walser, and M. F. Becher, “Laser-induced damage to silicon charge-coupled imaging devices,” Opt. Eng. 30, 651–657 (1991). 2. C. Z. Zhang, S. E. Watkins, R. M. Walser, and M. F. Becher, “Mechanisms for laser-induced functional damage to silicon charge-coupled imaging sensors,” Appl. Opt. 32, 5201–5210 (1993). 3. G. Li, H. B. Shen, L. Li, C. Zhang, S. J. Mao, and Y. B. Wang, “Laser-induced damages to charge coupled device detector using a high-repetition-rate and high-peak-power laser,” Opt. Laser Technol. 47, 221–227 (2013). 4. D. D. Qiu, Z. Zheng, R. Wang, T. Jiang, and X. A. Cheng, “Mechanism research of pulsed-laser induced damage to CCD imaging devices,” Acta Opt. Sinica 31, 0214006 (2011). 5. J. J. Jiang, F. Luo, and J. G. Chen, “Research on femtosecond laser induced damage to CCD,” High Power Laser and Particle Beams 17, 515–517 (2005). 6. S. Y. Huang, Y. S. Zhang, B. Q. Tang, Y. Zhang, Z. J. Wang, and Z. G. Xiao, “Damage effect on CCD detector irradiated by 500 fs laser pulse,” High Power Laser and Particle Beams 17, 1445–1448 (2005). 7. Q. Z. Zhao, J. R. Qiu, X. W. Jiang, E. W. Dai, C. H. Zhou, and C. S. Zhu, “Direct writing computer-generated holograms on metal film by an infrared femtosecond laser,” Opt. Express 13, 2089–2092 (2005). 8. E. G. Gamaly, A. V. Rode, B. L. Davies, and V. T. Tikhonchuk, “Ablation of solids by femtosecond lasers: ablation mechanism and ablation thresholds for metals and dielectrics,” Phys. Plasmas 9, 949–957 (2002). 9. C. H. Dai, T. C. Chang, A. K. Chu, Y. J. Kuo, S. H. Ho, T. Y. Hsieh, W. H. Lo, C. E. Chen, J. M. Shih, W. L. Chung, B. S. Dai, H. M. Chen, G. R. Xia, O. Cheng, and C. T. Huang, “Hot carrier effect on gate-induced drain leakage current in high-k/metal gate n-channel metal-oxide-semiconductor field-effect transistors,” Appl. Phys. Lett. 99, 012106 (2011). 10. S. Baudach, J. Bonse, and W. Kautek, “Ablation experiments on polyimide with femtosecond laser pulses,” Appl. Phys. A 69, S395–S398 (1999). 11. B. Chimier, O. Utéza, N. Sanner, M. Sentis, T. Itina, P. Lassonde, F. Légaré, F. Vidal, and J. C. Kieffer, “Damage and ablation thresholds of fused-silica in femtosecond regime,” Phys. Rev. B 84, 094104 (2011).
12. J. Bonse, S. Baudach, J. Krüger, W. Kautek, and M. Lenzner, “Femtosecond laser ablation of silicon-modification thresholds and morphology,” Appl. Phys. A 74, 19–25 (2002). 13. H. B. Shen, X. J. Shen, B. Zhou, S. J. Mao, N. Jiang, and G. Li, “Experimental study of 532 nm pulsed laser irradiating CCD,” High Power Laser and Particle Beams 21, 1449–1454 (2009). 14. S. I. Takagi and M. Takayanagi, “Carrier transport properties of thin gate oxides after soft and hard breakdown,” Microelectron. Eng. 59, 5–15 (2001). 15. A. Cester, “Wear-out and breakdown of ultra-thin gate oxide after irradiation,” Electron. Lett. 38, 1137–1139 (2002). 16. T. Tomita, H. Utsunomiya, Y. Kamakura, and K. Taniguchi, “Hot hole induced breakdown of thin silicon dioxide films,” Appl. Phys. Lett. 71, 3664–3666 (1997).
17. Y. C. Yeo, Q. Liu, and C. M. Hu, “MOSFET gate oxide reliability: anode hole injection model and its applications,” Int. J. High Speed Electron. Syst. 11, 849–886 (2001). 18. H. Abe, “Solid state image sensor,” U.S. patent 6,521,920 B2 (18 February 2003). 19. Z. Zhen, “Laser-induced dazzling phenomena in visible light CCD and their mechanisms,” Ph.D. dissertation (National University of Defense Technology, 2010). 20. K. F. Schuegraf and C. M. Hu, “Hole injection SiO2 breakdown model for very low voltage lifetime extrapolation,” IEEE Trans. Electron Devices 41, 761–767 (1994). 21. C. M. Hu and Q. Lu, “A unified gate oxide reliability model,” in IEEE International Reliability Physics Symposium (1999), pp. 47–51. 22. D. A. Neamen, Semiconductor Physics and Devices: Basic Principles, 3rd ed. (McGraw-Hill, 2011), pp. 380–384.
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College of Science, National University of Defense Technology, Changsha 410073, China 2
College of Photoelectric Science and Engineering, National University of Defense Technology, Changsha 410073, China *Corresponding author: [email protected]
Received 12 September 2013; revised 13 October 2013; accepted 14 October 2013; posted 14 October 2013 (Doc. ID 197495); published 25 October 2013
A femtosecond laser provides an ideal source to investigate the laser-induced damage of a charge-coupled device (CCD) owing to its thermal-free and localized damage properties. For conventional damage mechanisms in the nanosecond laser regime, a leakage current and degradation of a point spread function or modulation transfer function of the CCD are caused by the thermal damages to the oxide and adjacent electrodes. However, the damage mechanisms are quite different for a femtosecond laser. In this paper, an area CCD was subjected to Ti: sapphire laser irradiation at 800 nm by 100 fs single pulses. Electric-induced oxide breakdown is considered to be the primary mechanism to cause a leakage current, and the injured oxide is between the gate and source in the metal-oxide semiconductor field-effect transistor (MOSFET) structure for one CCD pixel. Optical microscopy and scanning electron microscopy are used to investigate the damaged areas and the results show that the electrodes and the oxide underneath are not directly affected by the femtosecond laser, which helps to get rid of the conventional damage mechanisms. For the primary damage mechanism, direct damage by hot carriers, anode hole injection, and an enlarged electric field in the insulating layer are three possible ways to cause oxide breakdown. The leakage current is proved by the decrease of the resistance of electrodes to the substrate. The output saturated images and the dynamics of an area CCD indicate that the leakage current is from an electrode to a light sensing area (or gate to source for a MOSFET), which proves the oxide breakdown mechanism. © 2013 Optical Society of America OCIS codes: (140.3538) Lasers, pulsed; (140.3330) Laser damage; (040.1520) CCD, charge-coupled device. http://dx.doi.org/10.1364/AO.52.007524
1. Introduction
The study of laser-induced damage to charge-coupled devices (CCDs) is of great significance both for optoelectronic countermeasures in imaging systems and for improvement of sensor robustness. In the functional damage mechanisms, damage to the isolation oxide between adjacent polysilicon clock lines and damage to metal-oxide semiconductor field-effect 1559-128X/13/317524-06$15.00/0 © 2013 Optical Society of America 7524
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transistors (MOSFETs) are the primary sources for leakage current in nanosecond lasers [1–4]. The damage comes from melting of the electrodes caused by laser irradiation. However, with new developments in CCD function and structure, those mechanisms may no longer be sufficient to explain the damage behaviors, especially in the latest developing femtosecond laser regime. Few studies have been done to investigate damage to CCDs in the femtosecond laser regime and the damage mechanism is not yet clear [5,6]. Because of its thermal-free properties, femtosecond lasers can cause localized damage to material [7],
which helps to limit the damage scale. According to femtosecond ablation theory [8], electrons are excited into the conduction band or higher by single-photon or multiphoton absorption to become hot electrons. When the electrons get to a crucial number, material damage occurs. Hot carriers and ablation of material are two main factors that could cause dysfunction in a CCD. The former could bring soft breakdown or hard breakdown to the oxide in the metal-oxide semiconductor (MOS) structures [9], while the latter destroys the materials of which the CCD is made [10–12]. To investigate reasonable damage mechanisms, we choose a typical front-illumination area CCD designed in recent years and irradiated it with a femtosecond laser in single pulse mode. During the experiment, different damage stages and the corresponding energy fluences were recorded. Stages of “dot damage” and “line damage” appeared in the experiment, both of which had been observed in previous work [4,13]. After irradiation, the CCD sample was examined by optical microscopy and scanning electron microscopy (SEM) layer by layer and the damaged areas were quite clear in the optical and SEM photos. By comparing the results completely, we find that the electric-induced oxide breakdown of the MOS structures [14,15] is the cause of the leakage current, which is quite different from the traditional damage mechanisms. Traditional mechanisms hold that leakage currents in gate to drain, gate to source, or drain to source come from the melting of insulating oxides caused by direct laser irradiation [1–3]. However, in the structure of the CCD we studied, the microlens and the material covering the electrode keep the light away from the electrode and the insulating layer underneath, so that direct damage to them is impossible for a thermal-free and localized femtosecond laser. However, hot electrons and holes are generated by the femtosecond laser in the source/drain area. Anode hole injection (AHI) [16,17] or direct damage by the hot carriers may account for the oxide breakdown between the gate and source/drain. The voltage applied on the electrodes and substrate might also promote the damage process. The damage is electric induced, and the femtosecond laser provides only hot carriers. 2. CCD Sample and Experiment A.
Structure of the CCD
The CCD we tested is a Sony ICX405AL. It has a size of 1∕3 in., is four vertical clock driven, and displays time integration. The effective pixels are 500H× 582V, and the pixel size is 9.8 μmH × 6.3 μmV. The CCD is a multilayer device, consisting of a microlens, a silica optical layer, a W-shield, a polysilicon electrode, a silica insulating layer, and silicon doped by P type or N type ions. The sketch cross section of the CCD is shown in Fig. 1 [18]. For normal incident light, most of the light is focused into the silicon. Very little light should irradiate the W-shield, and no light should get to the covered polysilicon electrodes.
Fig. 1. Cross-sectional view of a CCD.
Its working process can be seen in the literature [18,19]. The electrons are transferred from the light sensing area to the transfer channel or, in other words, from source to drain in a likely MOSFET structure. The N area and the N area under the electrode are the source and drain, respectively, while the electrode is the gate. B. Experiment
The experiment was carried out using a Ti:sapphire laser (λ 800 nm) with 100 fs full width at halfmaximum pulse duration. The output laser was linearly polarized, and the maximum single pulse energy was 0.25 mJ. The multipulse frequency can be moderated to 2 Hz. With a half-wave plate and a polarization beam splitter, we could vary the pulse energy continually. After that, the beam was split into equal parts through a calibrated neutral density splitter lens. One part was measured by an energy meter, and the other was focused onto the CCD surface by a biconvex lens with a 300 mm focus length. The damage situation was finally present on the screen of the image acquisition system. The CCD was working during the laser irradiation. The single pulse mode was performed in the experiment with assistance from an electric drive shutter. The laser energy was gradually increased to reveal the whole damage process. After irradiation, optical microscopy and SEM were used to investigate the characteristics of the damaged areas; meanwhile, the CCD sample was treated with physical and chemical methods to uncover different material layers. The microlenses were removed from the silica optical layer by plastic tweezers. Hydrogen fluoride was used to corrode the silica optical layer from the silicon and the W-shield by strictly controlling time. To make the damage area more clear, we use 30% H2 O2 liquid to corrode the W for about 4 h. 3. Results A. Damage Processes and the Corresponding Energy Fluences
The laser has a Gaussian spatial beam profile and the spot the on CCD is elliptical with a minor axis 1 November 2013 / Vol. 52, No. 31 / APPLIED OPTICS
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the results at 268 mJ∕cm2 . Figure 3(b) shows the damage crater clearly and the sharp damage edge is consistent with the profile of the femtosecond laser. Figures 3(c)–3(f) are SEM images of the area shown in Fig. 3(b) after sample preparation. The dashed circle surrounds the crater and the solid circle represents the laser irradiation boundary. Comparing Figs. 3(a)–3(f), we can draw several conclusions.
Fig. 2. Output images with corresponding energy fluences.
ωx 51.31 μm (1∕e2 beam horizontal radius) and a major axis ωy 63.53 μm (1∕e2 beam vertical radius). Therefore, the laser irradiating area is S πωx ωy 1.02 × 10−4 cm2 . The average energy fluence F E∕S, where E is the attenuated pulse energy. By calculating the energy fluences and labeling on every shot, as shown in Fig. 2, we can distinguish the different damage stages. Figure 2 gives the output images of the damaged areas while the CCD is still working. When the laser energy fluence is 2.0 mJ∕cm2 , the gray level of the white dot is 177 (the gray level of saturated output is 255), which means dot damage appears but it is not saturated. When the energy fluence reaches 2.5 mJ∕cm2 , the gray level of the white dot is 255, so that we define this energy fluence as the threshold of dot damage. With the increase of laser energy fluence, the white dot enlarges and evolves into a shuttle-shaped white area, with a white line going through the entire vertical line, which is called “line damage,” when the fluence is above 330 mJ∕cm2 . The white line at an energy fluence of 216 mJ∕cm2 is so unique that we choose to ignore it to avoid uncertainty. As a rough estimate, the shuttle-shaped image involves about 100 vertical pixels maximum at energy fluence of 330 mJ∕cm2. All damage is permanent and does not recover when the laser is removed. B.
Characteristics of Damage Areas
The damaged areas are investigated by optical microscopy and SEM. The optical image of the dot damage area indicates that the material’s color slightly changes, as shown in Fig. 3(a), but it cannot provide more detail. In the line damage areas, optical images show that the central part is elliptically ablated, leaving behind a crater. Outside the crater, there is a white corona, which has the same generation principle as the dot damage area due to the low laser energy fluence. SEM images give more information owing to larger amplification and higher resolution. Because there are no differences in the SEM images for the damaged areas from 216 mJ∕cm2 to 588 mJ∕cm2 except for the scale, we present only 7526
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(1) In the crater, microlenses are elliptically ablated [see Fig. 3(b)], which is well consistent with the laser profile, and the silicon under the microlens is severely damaged [see Figs. 3(c), 3(e), and 3(f)]. (2) Although the microlenses are fine in the corona area [see Fig. 3(b)], the silicon underneath is slightly damaged by the laser [see Fig. 3(d)] and that explains the color change in Fig. 3(a). (3) The damage area will expand if the laser energy fluence is larger by comparing the damage situations between the edge and the central area [see Figs. 3(d) and 3(f)]. (4) Even in the damaged center, the W-shield and electrodes under it are not affected by the laser [see Figs. 3(e) and 3(f)] and that means that materials only in the light sensing area are damaged. 4. Discussion
The optical microscopy and SEM images indicate that when the femtosecond laser irradiates the CCD, materials only in the light sensing area are damaged, while the functional transfer channel is not affected by the laser. Comparing Fig. 2 with Fig. 3(a), we can see that the number of saturated pixels on the screen is much larger than the number irradiated by the laser (100 saturated pixels versus 30 damaged pixels vertically). As we know, only pixels in the laser irradiating area are damaged; the additional saturated vertical pixels outside the irradiating area are not laser damaged. Therefore, the formation of saturated pixels comes from vertical overflow electrons. Dark current and leakage current are two possible sources of overflow electrons. The former comes from the defects of the damaged silicon crystal lattices and will express the shape of the damaged area on the screen for low electrons supplement. The latter is from gate to source/drain and will increase with more severe damage to the insulating oxide. Thus, the leakage current is dominant in generating the overflow electrons. However, there are still two possible ways for the leakage current to leak from the electrode to bulk silicon through the silica insulating layer, as expressed in Fig. 4. One is the transfer channel, which is unrealizable, because there would always be the vertical saturated lines on the screen when damage occurs and that does not fit with Fig. 2. The other is to the light sensing area, which is convincing because the most photoemission carriers accumulate in this area during laser irradiation. When the laser irradiates the CCD, abundant hot carriers generate in the light sensing area on a femtosecond timescale. There are three possible ways for those electrons to damage the insulating layer
Fig. 3. Optical microscopy and SEM images of damaged areas. (a) Optical image of three different areas, (b) SEM image of damaged area at the energy fluence of 268 mJ∕cm2, (c) SEM image of (b) after corroding the W-shield, (d) the enlarged area of (c), (e) the enlarged central area of (c) after removing the fused silica, and (f) the enlarged central area of (c).
between the gate and source/drain. First, the diffusion of the hot carriers will cause damage to the insulating layer for the voltages applied on the electrode. Second, the AHI model might also make a contribution to damaging the insulating oxide. When the hot electrons are injected into the anode through the oxide, the holes generated at the anode will get trapped in the oxide. The holes trapped in the oxide allow for increased current density due to holeinduced trap generation [20]. There will be increased current density when the damage occurs and more electrons create more hot holes. Thus, the positive feedback will continue until the oxide breaks down [21]. The efficiency of AHI might be promoted owing to the large numbers of hot electrons generated by the femtosecond laser. Third, if the electrons fill the N area (see Fig. 4) and diffuse to the substrate, the transient current will result in the electrode
voltage and substrate voltage being straightforwardly applied on each side of the insulating layer. The electric field in the insulating layer will be
Fig. 4. Doping distribution in bulk silicon. 1 November 2013 / Vol. 52, No. 31 / APPLIED OPTICS
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Table 1.
Resistance of Vertical Electrodes to Substrate under Different Stages (Unit: MΩ)
Normal Dot damage Line damage
RΦ1-sub
RΦ2-sub
RΦ3-sub
RΦ4-sub
4.73 4.2 0.79
4.92 4.7 0.83
4.72 4.4 0.86
4.94 4.7 0.87
enlarged, even more sharply due to the point effects for the distinctive structure between the electrode and the light sensing area [22]. To sum up, hot carriers and enlarged electric fields are two main factors that can cause damage to the insulating layer and both of them are electric induced. To examine the leakage current, we measure the resistance of the vertical electrodes to the substrate at three different stages. The results are shown in Table 1. When the CCD is dot damaged, the resistance between vertical electrodes and substrate is slightly smaller than in the normal stage. This means that the leakage current appears but is not sufficient enough to overflow into the surrounding vertical pixels and leave behind a white dot on the output screen (see Fig. 2). At the time when the white line presents itself, those resistances apparently move to a much lower level, which means more severe damage to the insulating layer and more leakage current to create overflow electrons. As we know, for a Gaussian beam, the central irradiating area has the greatest energy fluence, and, consequently, the severest damage to the insulating layer, and the most plentiful overflow electrons will emerge in this area. With a vertical barrier (P area in Fig. 4), the electrons cannot overflow horizontally but easily overflow to vertical adjacent light sensing areas and eventually evolve into a shuttle shape. But it is a difficult matter to establish a precise oxide breakdown model for a CCD damaged by a femtosecond laser. First, the MOS structure of the CCD is complicated both for the doping distributions in the bulk silicon and for the setting of vertical electrodes. Second, because the hot carriers are generated on a femtosecond timescale, the excursion, diffusion, and ionization of the carries should all be considered, which make the solution difficult. As stated, the circuit in which we measure the resistance is from electrode to substrate through the light sensing area. The electrons are not directly leaked into the transfer channel, but come from the light sensing area through the read-out gate (see Fig. 4). If the laser damaged area expands to the readout gate area (see Figs. 3(d) and 3(f)), the threshold of the gate will decrease. That will allow the electrons in the light sensing area to leak into the transfer channel and make every vertical pixel passing by carry the leaking electrons. Those leaking electrons will be regarded as information electrons by the measurement end of the CCD and will finally show on the screen as a white line. The change of saturation level of the white lines with the increase of laser energy fluence also proves the above mechanism. 7528
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5. Conclusion
In this paper, we have investigated the damage processes and mechanism of a typical area CCD by a femtosecond laser. Dot damage and line damage are observed on the output images with threshold energy fluences of 2.5 mJ∕cm2 and 330 mJ∕cm2 , respectively. Optical microscopy and SEM images of the damaged area show that only materials in the light sensing area are damaged by the laser. Materials including the W-shield, electrode, and the silicon underneath are not affected. Those results help us to get rid of the conventional damage mechanisms of laser direct irradiation. The lengthening saturated figures on the output image and the special structure of the CCD lead us to find the electric-induced oxide breakdown between gate and source, which is proved by the changes of resistance of the vertical electrodes to the substrate. Although hot carriers and an enlarged electric field make contributions to the oxide breakdown, the precise damage model is difficult to establish. As a conclusion, the damage mechanisms in a CCD depend on its structure and the irradiating laser. References 1. C. Z. Zhang, S. E. Watkins, R. M. Walser, and M. F. Becher, “Laser-induced damage to silicon charge-coupled imaging devices,” Opt. Eng. 30, 651–657 (1991). 2. C. Z. Zhang, S. E. Watkins, R. M. Walser, and M. F. Becher, “Mechanisms for laser-induced functional damage to silicon charge-coupled imaging sensors,” Appl. Opt. 32, 5201–5210 (1993). 3. G. Li, H. B. Shen, L. Li, C. Zhang, S. J. Mao, and Y. B. Wang, “Laser-induced damages to charge coupled device detector using a high-repetition-rate and high-peak-power laser,” Opt. Laser Technol. 47, 221–227 (2013). 4. D. D. Qiu, Z. Zheng, R. Wang, T. Jiang, and X. A. Cheng, “Mechanism research of pulsed-laser induced damage to CCD imaging devices,” Acta Opt. Sinica 31, 0214006 (2011). 5. J. J. Jiang, F. Luo, and J. G. Chen, “Research on femtosecond laser induced damage to CCD,” High Power Laser and Particle Beams 17, 515–517 (2005). 6. S. Y. Huang, Y. S. Zhang, B. Q. Tang, Y. Zhang, Z. J. Wang, and Z. G. Xiao, “Damage effect on CCD detector irradiated by 500 fs laser pulse,” High Power Laser and Particle Beams 17, 1445–1448 (2005). 7. Q. Z. Zhao, J. R. Qiu, X. W. Jiang, E. W. Dai, C. H. Zhou, and C. S. Zhu, “Direct writing computer-generated holograms on metal film by an infrared femtosecond laser,” Opt. Express 13, 2089–2092 (2005). 8. E. G. Gamaly, A. V. Rode, B. L. Davies, and V. T. Tikhonchuk, “Ablation of solids by femtosecond lasers: ablation mechanism and ablation thresholds for metals and dielectrics,” Phys. Plasmas 9, 949–957 (2002). 9. C. H. Dai, T. C. Chang, A. K. Chu, Y. J. Kuo, S. H. Ho, T. Y. Hsieh, W. H. Lo, C. E. Chen, J. M. Shih, W. L. Chung, B. S. Dai, H. M. Chen, G. R. Xia, O. Cheng, and C. T. Huang, “Hot carrier effect on gate-induced drain leakage current in high-k/metal gate n-channel metal-oxide-semiconductor field-effect transistors,” Appl. Phys. Lett. 99, 012106 (2011). 10. S. Baudach, J. Bonse, and W. Kautek, “Ablation experiments on polyimide with femtosecond laser pulses,” Appl. Phys. A 69, S395–S398 (1999). 11. B. Chimier, O. Utéza, N. Sanner, M. Sentis, T. Itina, P. Lassonde, F. Légaré, F. Vidal, and J. C. Kieffer, “Damage and ablation thresholds of fused-silica in femtosecond regime,” Phys. Rev. B 84, 094104 (2011).
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