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Abstract
The damage mechanism of a nanosecond extreme ultraviolet (EUV) laser with solid targets is complex and involves thermal and nonthermal effects. In this study, the interaction process of a nanosecond 46.9 nm laser with copper was investigated. A Faraday cup was used to measure the electron signals induced by the laser irradiation. The photo-ionization and thermal effects in the interaction process are discussed according to the results.
© 2022 Optica Publishing Group under the terms of the Optica Open Access Publishing Agreement
1. Introduction:
Laser surface processing can be employed to create micro/nanostructures on certain materials. Thus, the laser–matter interaction has been widely studied to gain an in-depth understanding of the damage mechanism for different lasers with various materials [1,2]. The interaction process can be studied in various ways. Laser-induced plasma is regarded as an accurate method for analyzing the laser damage mechanism and can probe sub-processes in the interaction [3,4]. Diagnostic techniques for laboratory plasmas generally include probe and optical methods. Among the probe methods, the Langmuir probe and Faraday cup can be used as intrusion probes [5–8]. With the volt-ampere characteristics, the Langmuir probe could evaluate the plasma parameters, i.e., the electron density and electron temperature. A Faraday cup can be used as a charge collector to detect the kinetic distribution of charged particles, for example, to detect the plasma instability inside a solenoid [9].
To enhance the resolution of surface processing, the laser wavelength is expected to be shorter to reduce the diffraction limit. Therefore, extreme ultraviolet (EUV) radiation is expected to be an improved light source for creating small-scale structures. Among short-wavelength radiation sources, nanosecond EUV lasers have unique advantages for surface processing because they have a long duration with a short wavelength [10]. The first report on the interaction of a nanosecond capillary discharge EUV laser with matters was the demonstration of the ablation on copper induced by 46.9 nm laser [11]. The laser with a peak intensity of 1011W/cm2 was used to damage the target and clear patterns varied with the position of the target along the beam axis were detected. Using a Fresnel zone plate, the nanosecond 46.9 nm laser was focused and created nanoscale holes on the PMMA layer [12]. The results demonstrated the feasibility of directly writing sub-100 nm structures using the nanosecond EUV laser. With the improvement of the capillary discharge EUV laser, various materials were utilized to investigate the interaction of nanosecond EUV laser with matters [13–17]. According to the research, the surface behavior to the nanosecond EUV laser ablation strongly depended on the type of the material and the parameters of the radiation (shot number, fluence, et al.). Moreover, self-formed structures whose scales is not limited by diffraction limitation could be detected under specific ablation condition [18,19]. Based on the simulation of POLLUX code, photo-ionization and heat conduction process are both involved during the ablation [20]. It is reasonable since the photon energy of EUV laser is quite high which causes photo-ionization process. Meanwhile, the duration of the laser is long enough to bring heat transport. The specific laser characteristics introduce both thermal and nonthermal effects in the laser–matter interaction, and the various surface behavior due to the irradiation of nanosecond EUV laser is resulted from this complex process. That makes the damage mechanism of nanosecond EUV laser valuable to be studied. Plasma diagnosis is an effective way to investigate the interaction. Pira et al. used a Langmuir probe to diagnose the Bi plasma induced by a nanosecond 46.9 nm laser with a repetition frequency of 3 Hz [21]. The electron temperature and electron density were determined to be 1–3 eV and 1013–1014 m−3, respectively. This was a direct observation of nanosecond-EUV-laser-induced plasma, providing valuable experimental data and a demonstration of the damage mechanism. However, the research space for this subject remains large because the radiation sources are rare, the characteristics of the laser bring the difficulties to probe the laser induced plasma (for example, electromagnetic interference during the lasing) and the damage mechanism for various materials is complex. In this study, a Faraday cup was used to detect the copper plasma induced by a nanosecond capillary discharge EUV laser at 46.9 nm. The thermal and nonthermal effects involved in the interaction were analyzed and discussed.
Experimental setup
The light source was a nanosecond capillary discharge EUV laser with a pulse width of 1.6 ns. The laser was produced from Ne-like Ar plasma excited with a fast current pulse and could output lasers at 46.9 nm and 69.8 nm [22–25]. In this experiment, the laser was operated at 46.9 nm. Ar gas was filled into a 35-cm-long Al2O3 capillary with a 3.2 mm inner diameter. A prepulse with a low current was used to ionize Ar into an initial plasma column. Then the main current came with a certain delay time, which was generated by the Marx generator and the Blumlein line to pinch the initial plasma column. Under the specific condition, the plasma column is suitable for lasing. A schematic of the laser ablation and plasma detection system is shown in Fig. 1. The entire experiment was performed in a vacuum chamber with a base pressure of ∼10−5 Pa. The laser was focused using a toroidal mirror at a grazing incidence angle of 7°. The focused laser spot was fan-shaped with a peak fluence of 1.6 J/cm2. A Faraday cup was used as a charge collector to detect the plasma induced by the nanosecond EUV laser. A copper target with a polished single surface was located parallel to the detection plane of the Faraday cup at a distance of 2 cm and was at a small angle from the beam axis. In the experiment, the Faraday cup was positively biased to collect electrons induced by the 46.9 nm laser ablation. The electron currents were measured by acquiring the voltage signal across a load resistor using a 1 GHz digital phosphor oscilloscope.
In the experiment, the laser and plasma signal could not be detected simultaneously. To measure the time taken by the electrons to fly into the Faraday cup from the copper surface, the main current that was used to excite the Ar plasma in the capillary was used to determine the time at which the laser irradiated the target. An X-ray diode (XRD) was used to detect the laser signal at the same position as the target. Figure 2 shows typical signals of the main current and 46.9 nm laser. The laser was always generated at the peak of the main current and left a dip on the current curve, as shown in the image. Thus, the time at which the laser irradiated the copper surface could be confirmed by detecting the main current during the ablation.
2. Results and discussion
Figure 3 shows a typical signal of the Faraday cup with the related main current signal. The cup was biased by +70 V. Therefore, the Faraday cup measured the temporal kinetic profile of the electrons induced by laser ablation. Because of the strong electromagnetic interference generated during the laser output, a noise signal with a high frequency was observed before the electron signal. Subsequently, the electron signal exhibited two major peaks. Peak 2 corresponds to electron emission from the core plasma. Peak 1, which presented a group of electrons with a higher average velocity, was considered to be caused by a different mechanism from the electrons emitted from the core plasma. From Fig. 3, the delay between the times at which the laser irradiated the target and the major electrons arrived at the Faraday cup was ∼1.05 µs.
The copper target was irradiated at the same position by multiple laser shots, and the electron signal was measured using a Faraday cup after each laser irradiation. Figures 4 and 5 show the electron signals of Peak 1 and Peak 2 induced by the 1st to the 8th laser shots, respectively. From Fig. 4, the electron signals of Peak 1 started immediately after the 46.9 nm laser was irradiated according to the corresponding position of the main current, which is marked by a dashed line. The duration of the signals of Peak 1 was longer than the duration of the 46.9 nm laser, which indicated a certain kinetic distribution of these electrons. It was evident that the signals of Peak 1 induced by multiple laser shots maintained a stable temporal profile, even though the surface morphology of copper after each laser shot irradiation changed. This was different from the variation of Peak 2 signals measured after each laser shot irradiation in Fig. 5, which presents the varied temporal profile of the electron signals from the core plasma.
The electrons of Peak 1 had a much higher kinetic energy from the 46.9 nm laser irradiation than the electrons of Peak 2. The velocity of the electrons was higher than 106 m/s, corresponding to a kinetic energy of ∼ 3 eV. Based on the high kinetic energy, the electrons of Peak 1 were assumed to be caused by the photo-ionization of copper. The photon energy of the 46.9 nm laser is 26.5 eV, which is sufficiently high to ionize any atoms. Based on Fig. 4, fast electrons were emitted before the core plasma was formed. This indicated that, even though the ablation area of copper presented melting and evaporation traces, photo-ionization was the first stage in the interaction process of the 46.9 nm laser with copper.
The electrons of Peak 2, i.e., those from the core plasma, showed a different behavior from the electrons of Peak 1. First, the electrons of Peak 2 had much lower velocities (lower than 2×104 m/s) from the laser irradiation, which indicated an indirect energy exchange between the laser photons and the electrons. Moreover, when the first laser shot was irradiated, most of the electrons had similar velocities. After the subsequent laser shots were irradiated, the average velocity of the emitted electrons was reduced, and the distribution of the electron velocities became wider.
The interaction process of the 46.9 nm laser with copper is complex. The photon energy of the 46.9 nm laser is sufficiently high to ionize any atoms. Therefore, when the leading part of the laser pulse interacts with the target surface, the first process is expected to be photo-ionization. The photons of the 46.9 nm laser exchanged their energy with the work function of the inner electrons of the copper atom, imparting a high kinetic energy to the emitted electrons. This was proved by the existence of the fast electrons in Fig. 4, which were emitted immediately after the laser irradiation. As copper contains plenty of free electrons at room temperature, laser energy could also be absorbed directly by these electrons and be exchanged into heat energy, thereby increasing the temperature of the copper surface. Therefore, the subsequent process of the interaction is heat conduction, leading to the melting or evaporation of the copper surface. Based on the above analysis, the interaction process consists of two parts: ionization and heat conduction. As shown in Fig. 5, there were some small peaks in Peak 2, which indicated several bunches of electrons with certain average velocities. This was different from the electron signal induced by laser irradiation with longer wavelengths [26]. The small peaks indicate the complexity of the interaction process, which indicates that the electrons could obtain energy in various ways. This indicates that the ionization and heat conduction processes were mixed and contributed to the surface damage. According to the Faraday cup measurement, the formation of the electrons in Peak 1 was induced by the photo-ionization effect because of the signal duration and delay time of the laser irradiation. However, for Peak 2, it was not apparent if the electrons were simply caused by surface melting and evaporation.
Additional measurements were performed to understand the mechanism. Figure 6 shows the variation in the number of electrons in Peak 2 induced by irradiation from 1 to 25 laser shots. The number of electrons decreased after the first several laser shots irradiation and then started to fluctuate. This reflects the process of plasma formation and the exchange of photon energy. The decrease in the electron emission in the case of irradiation with the first several laser shots represented a smaller scale of the laser-induced plasma. Therefore, more laser energy was deposited into the internal copper target to change the surface morphology rather than being exchanged with the kinetic energy of the electrons and ions to form the plasma. When the laser was irradiated at the same position, the surface damage became severe, and the physical characteristics of the damaged area were unstable. This was considered to be the reason for the fluctuation after irradiation with 10 laser shots, as shown in Fig. 6.
The delay of Peak 2 from the time of laser irradiation is shown in Fig. 7. Unlike the variation in the number of electrons in Fig. 6, the delay time of Peak 2 remained stable at ∼ 2.7 µs after the 6th laser shot irradiation. The delay time of Peak 2 represents the major velocity of the electrons. According to Fig. 7, the electrons induced by the first laser shot irradiation were the fastest. Then, the major electron velocities decreased after the subsequent laser irradiation and became stable after the 6th laser shot irradiation.
The ablation patterns induced by multiple laser shots were detected by Scanning Electron Microscope (SEM) and shown in Fig. 8. From the left to the right, the surface was irradiated by 1, 2, 3, 6, 12, 25 shots. The white area clearly presented in the ablation area induced by 6, 12 and 25 laser shots, which is enlarged and shown in the small image at right, indicates the melted trace. The top of the ablation patterns was detected by SEM and the details in the ablation area are shown in Fig. 9. From the results, the surface of the copper target is evidently damaged by the laser irradiation and the edge of the ablation area is quite sharp. The area of the melted part becomes evident after 6 laser shots irradiation. In the ablation area induced by 1, 2 and 3 laser shots, the melted trace is not quite clear. However, the electrons from Peak 2 in Fig. 5 indicate that the surface irradiated by 1, 2 and 3 laser shots was partly melted or at least partly modified by the thermal effect. The melted trace detected in the ablation area is supposed to be a distinct expression of thermal effect in the ablation process, which caused ejected electrons with slower average velocity. On the other hand, besides the fast electrons in Peak 1, a portion of fast electrons induced by photo-ionization is supposed to make contribution to forming Peak 2 together with the slower electrons ejected from the thermal effect. The photo-ionization induced damage was quite gentle and hardly detected by SEM. However, the contribution of photo-ionization and thermal effect could be presented by analyzing the average velocity of the ejected electrons. With the increasing of the irradiated laser shot number, the average velocity of the electrons from Peak 2 decreases, resulting in the delay time variation shown in Fig. 7. It indicates the contribution of photo-ionization effect decreases while the influence of the thermal effect increases. After 6 laser shots irradiation, a large area of the ablation pattern was melted. Under this condition, most electrons ejected from the surface were induced by the thermal effect, which is supposed to cause the saturation of the delay time in Fig. 7.
3. Summary
In summary, the thermal and photo-ionization effects in the interaction of the nanosecond 46.9 nm laser with copper were analyzed. According to the results, photo-ionization was the first stage in the interaction process. Then, the thermal effect dominated the formation of the core plasma with bunches of fast electrons accelerated by high-energy laser photons. In the case of irradiation with multiple laser shots, the electrons from the core plasma were considered to be induced by thermal and nonthermal effects together during the irradiation with the first several laser shots, and then mostly induced by the thermal effect during the subsequent laser shot irradiation. This study will be helpful to understand the interaction of high-energy photons with metal and help in controlling the interaction process of nanosecond EUV lasers with solid targets, which is expected to be a promising method to create self-formed nanostructures on the surface of certain materials.
Funding
National Natural Science Foundation of China ( 62005066, 61875045).
Acknowledgments
The authors thank the National Natural Science Foundation of China for their support.
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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