1. INTRODUCTION
Intense pulsed ion beams (IPIB) have been intensively studied in the past three decades, primarily for the inertial confinement fusion and materials processing application (Humphries, Reference Humphries1980; Pogrebnyak, Reference Pogrebnyak1990). Due to the high compression of energy on time and space, during the process of IPIB irradiation, the material surfaces will endure superfast heating, melting, evaporation, and ablation. As a kind of flash heat source, IPIB can be used for surface treatment, film deposition, nanophase powder synthesis, and so on (Rej et al., Reference Rej, Davis, Olson, Remnev, Zakoutaev, Ryzhkov, Struts, Isakov, Shulov, Nochevnaya, Stinnett, Neau, Yatsui and Jiang1997; Remnev et al., Reference Remnev, Isakov, Opekounov, Matvienkoa, Ryzhkova, Strutsa, Grushina, Zakoutayeva, Potyomkina, Tarbokova, Pushkaryova, Kutuzovb and Ovsyannikovb1999; Zhao et al., Reference Zhao, Remnev, Yan, Opekounov, Le, Matvienko, Han, Xue and Wang2000).
The ablation effect has important influence on interaction between IPIB and material. When IPIB ablate the metal target, ablation plasma with high-temperature and high-pressure will be generated and expand in the perpendicular direction to the target surface. As a result of ablation plasma eruption, the target will lose mass, and shock wave will be formed in the material. If the ablation plasma touches a substrate, it will be cooled by the surface of substrate and deposit a thin film on it. Many kinds of thin films, such as ZnS, BaTiO3, B4C, YBCO, and FeTi, have been successfully prepared by using the ion beam ablation plasma deposition (Shimotori et al., Reference Shimotori, Yokoyama, Isobe, Harada, Masugata and Yatsui1988; Meli et al., Reference Meli, Grabowski, Hinshelwood, Stephanakis, Rej and Waganaar1995; Sonegawa et al., Reference Sonegawa, Grigoriu, Masugata, Yatsui, Shimotori, Furuuchi and Yamamoto1996; Suematsu et al., Reference Suematsu, Kitajima, Suzuki, Jiang, Yatsuil, Kurashima and Bando2002a , Reference Suematsu, Sengiku, Katom, Mitome, Kimoto, Matsui, Jiang and Yatsui b ; Saikusa et al., Reference Saikusa, Suzuki, Suematu, Jiang and Yatsui2003; Prasad et al., Reference Prasad, Renk, Kotula and Debroy2011). The thin films can be obtained on both the front and the back surfaces of the substrate. The thin film on the front surface has very high deposition rate, and on the back surface the film has very good surface quality and physical property (Sonegawa et al., Reference Sonegawa, Grigoriu, Masugata, Yatsui, Shimotori, Furuuchi and Yamamoto1996; Jiang et al., Reference Jiang, Ohtomo, Igarashi and Yatsui1998). Although numerous works about IPIB film deposition have been carried out, the characteristics of ablation products for each pulse are still not clear. Since the ablation product is a direct observable in the experiment, it is a good probe for investigating the ablation process and mechanism, which are of great significance to IPIB application. In order to analyze the influence of the IPIB energy density on the ablation products, zinc which has low boiling point and can be ablated easily was chosen as the target material. In this work, zinc targets were irradiated and ablated by IPIB. In the ablation process, the ablation products were collected by monocrystalline silicon substrates. By analyzing the ablation products, the surface morphology and the spatial distribution of different size ablation products were studied. The results are useful for understanding the ablation process and optimizing IPIB film deposition.
2. EXPERIMENT
IPIB irradiation experiments were performed on the BIPPAB-450 accelerator at Beihang University. Ion beam were produced by magnetically insulated diode, the main ion species of IPIB are about 70% hydrogen ion and 30% carbon ion. The diode voltage and current density profile of BIPPAB-450 were shown in Figure 1, which were measured by voltage divider and Faraday cup, respectively. The typically peak value of accelerating voltage, beam current density, and the pulse duration FWHM (full-width at half-maximum) are 450 kV, 150 A/cm2, and 80 ns. The diameter of IPIB is around 5 cm, and the IPIB energy densities are from 0 to 3 J/cm2.
Fig. 1. Diode voltage and beam current density of BIPPAB-450 accelerator.
The polished pure zinc targets with the dimension of 15 mm × 15 mm × 1 mm were irradiated by IPIB under two different energy densities in the ablation products collecting platform separately. The energy density of IPIB was measured by infrared (IR) image diagnostic system (Isakova, Reference Isakova2011; Isakova & Pushkarev, Reference Isakova and Pushkarev2013). The experiment platform, the IR diagnostic results of IPIB energy density distribution, and the position of zinc targets were demonstrated in Figures 2 and 3. Because the included angle between the zinc target and the IR target which placed vertically was 45°, the beam energy densities of different irradiation experiments on the zinc targets were 0.8–1 and 1.6–2 J/cm2, respectively. Single-crystal silicon wafer with 5 cm diameter was 3 cm below the target to receive the ablation products of zinc. Before and after IPIB irradiation, the masses of the samples were measured by Mettler XP205 analytical balance. The scanning electron microscope (SEM) and energy-dispersive spectrometer (EDS) analysis were conducted on FEI Nova NanoSEM 430.
Fig. 2. Ablation products collection platform.
Fig. 3. IR diagnostic results of two different energy density distribution (J/cm2) and the position of the zinc target projection on the IR target.
3. RESULTS AND DISCUSSION
3.1. Influence of beam energy density on ablation products
Because the ablation energy density threshold of zinc on BIPPAB-450 accelerator is 0.8 J/cm2 (Zhang et al., Reference Zhang, Yu, Zhong, Wei, Qu, Sha, Zhang, Zhang and Le2015), the zinc targets would melt, evaporate, and be ablated after IPIB irradiation with beam energy density higher than 0.8 J/cm2. The mass loss of zinc, the mass increase of silicon, and the SEM images of zinc surface before and after IPIB irradiation for one pulse were presented in Table 1 and Figure 4. From Table 1 it can be concluded that the zinc targets were ablated, and the silicon substrate received part of ablation products. After 1 J/cm2 IPIB bombarding melting traces appeared on the surface of zinc, and the polishing scratches disappeared (Fig. 4b). When the IPIB energy density reached 2 J/cm2, ablation mass increased, and obvious undulating surface morphology was formed on the zinc surface (Fig. 4c). Therefore, with higher IPIB energy density, the ablation effect became more intense, and the ablation products also increased.
Fig. 4. SEM images: (a) Zn surface before irradiation; (b) Zn surface after 0.8–1 J/cm2 IPIB irradiation; (c) Zn surface after 1.6–2 J/cm2 IPIB irradiation.
Table 1. Mass change after IPIB irradiation
It was shown in Figure 5 that there was no particle on the surface of the silicon substrate before IPIB irradiation zinc target. After the irradiation, some particles of different sizes appeared on the silicon substrate, and their elemental composition contained zinc, which meant that all these particles were the ablation products of zinc. The diameters of the ablation particles on silicon substrate in the lower-energy density experiment were from 50 to 200 nm; and the number of the collected ablation particles was very less (Fig. 5b). When 1.6–2 J/cm2 IPIB bombarded zinc target, the diameters of the ablation particles reached up to 1 µm, and the number of the collected ablation particles was much more than the results of low-energy density irradiation (Fig. 5c). These ablation particles are partly derived from the zinc ablation plasma plume. At the same time, due to the recoil force from the vaporization zinc gas, some molten zinc droplets also splash out of the surface of the zinc target and then fall onto the silicon substrate. Because when the zinc plasma plume and droplets leave the target, the temperature and recoil force of zinc are not the same, the sizes and shapes of the ablation products are different. In accordance with the increase of the IPIB beam energy density, the zinc target produces more vaporization zinc gas and plasma at the same time, and the recoil force to the zinc target surface becomes stronger. Therefore, the silicon substrate of 1.6–2 J/cm2 IPIB irradiation zinc can receive bigger and more ablation products.
Fig. 5. SEM images and EDS results: (a) Si substrate before irradiation; (b) Si substrate of 0.8–1 J/cm2 IPIB irradiation Zn; (c) Si substrate of 1.6–2 J/cm2 IPIB irradiation Zn.
3.2. Distribution of ablation products
In order to obtain the distributions of ablation products in the near-surface space, after 1.6–2 J/cm2 IPIB irradiating the zinc target for one pulse, the densities and sizes of zinc ablation products in different positions of the silicon were analyzed. Figures 6 and 7 illustrated the coordinate position and the SEM images of the different places in the silicon substrate, respectively. The numbers of the particles were not the same on the different positions of the silicon substrate (Fig. 7). The amounts of ablation products in different areas were counted, and the results were presented in Figure 8. In the vertical direction of the ion beam, the number of the ablation particles got the maximum on the position No. 8 (y = 0) in Figure 6, and reduced symmetrically along the positive and negative y-directions. In the parallel direction of ion beam, the maximum of the ablation particles number was arisen on the position No. 3 (x = 0) in Figure 6. But the decrease of numbers along the ablation products eruption direction (positive x-direction) was slower than the negative x-direction. Therefore, the zinc target normal extension region on the silicon possessed the maximum number density of ablation products.
Fig. 6. Surface appearance of Si after Zn irradiation by 1.6–2 J/cm2 IPIB and the coordinate position.
Fig. 7. SEM images of Si substrate for 1.6–2 J/cm2 IPIB irradiation Zn (first row, parallel direction of ion beam; second row, vertical direction of ion beam).
Fig. 8. Ablation products distribution in Si substrate.
In Figure 9, the SEM images with larger magnification revealed that the ablation products with numbers of different size particles in different areas were also not same. The amounts of different size ablation products in different areas were illustrated in Figure 10. The results demonstrated that the small size (diameter < 100 nm) ablation products were more than the large size ablation products, and the distribution of small size particles along the ablation products eruption direction was more uniform than the large size particles. It meant that the large size particles concentrated in the target normal extension area and small size ablation products can deposit on further areas. Because of the velocity of the different size ablation products are not same. These small size particles with faster speed can move further away. Consequently, in the IPIB front-side film deposition process, substrate can obtain larger film deposition rate on target normal extension area with non-uniform size ablation particles. On further areas, the film can be formed by small and more uniform size ablation particles, but the deposition rate is lower.
Fig. 9. SEM images of the Si substrate of 1.6–2 J/cm2 IPIB irradiation zinc in parallel direction of ion beam.
Fig. 10. Distributions of different size ablation products in the Si substrate.
4. CONCLUSION
Base on analyzing the ablation products of IPIB irradiation zinc, the influence of the IPIB parameter on the ablation effect and the distribution of the ablation products were emphatically studied in this work. The main conclusions are summarized as following: Firstly, the ablation effect will be more intense under the higher IPIB energy density. And the size and number of the zinc ablation products increase with the beam energy density increases. Secondly, the maximum of ablation product number density is in the target normal extension area. Thirdly, the distributions of different size ablation products are not same. The ablation particles with diameter <100 nm are more uniform along the ablation products eruption direction, and the bigger particles concentrate in the target normal extension area.
ACKNOWLEDGMENTS
This work is supported by the National Natural Science Foundation of China (Grant no. 11175012) and National Magnetic Confinement Fusion Program (Grant no. 2013GB109004).
