Dry Tin Dioxide Hollow Microshells and Extreme Ultraviolet Radiation

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Dry Tin Dioxide Hollow Microshells and Extreme Ultraviolet Radiation Induced by CO2 Laser Illumination Liqin Ge,†,‡ Keiji Nagai,*,† ZhongZe Gu,‡ Yoshinori Shimada,§ Hiroaki Nishimura,† Noriaki Miyanaga,† Yasukazu Izawa,† Kunioki Mima,† and Takayoshi Norimatsu† Institute of Laser Engineering, and Institute for Laser Technology, Osaka UniVersity, 2-6 Yamada-oka, Suita, Osaka 565-0871, Japan, and State Key Laboratory of Bioelectronics, Biological Science and Medical Engineering Department, Southeast UniVersity, Nanjing 210096, P. R. China ReceiVed March 11, 2008. ReVised Manuscript ReceiVed June 30, 2008 Low-density tin dioxide (SnO2) is required for radiating monochromatic extreme ultraviolet (EUV) light with low debris and high conversion efficiency from a laser. In this paper, tin dioxide nanoparticle hollow microcapsules were successfully fabricated by a layer-by-layer template technique. The obtained capsules have a rougher surface (30 nm in rms) compared to the freshly prepared polyelectrolyte capsules. Based on the X-ray diffraction (XRD) results, the tin dioxide nanoparticles well maintained their size after they were assembled on the capsules’ surfaces. In order to remove the polymer template, a heat treatment was introduced, and after the heat treatment the capsule sizes shrank about 71% (the average size was from 4.9 to 3.5 µm), and the obtained capsules maintained their round shape after water evaporation. The narrowest bandwidth at the 13.5 nm emission in the EUV region was observed when the capsules were irradiated by a CO2 laser with an intensity of 2.9 × 1010 W/cm2.

Introduction Extreme ultraviolet lithography (EUVL) is a promising technology for the volume production of future integrated circuits, whose node size is less than 30 nm.1 For projection lithography at EUV, the wavelength of 13.5 nm was chosen because Mo/Si multilayer coated mirrors have the highest reflectivity of about 70% during normal injection around this wavelength. The development of a high-power, highly efficient, and long-running EUV light source is one of the most significant technical challenges in the development of the EUVL system. The EUVL system requires, at the source point, more than a 300 W power of 13.5 nm light within a 2% bandwidth (BW) and 1 × 1011 shots of lifetime under a 10 kHz operation. Laser-produced plasma is an attractive light source for EUV lithography in terms of its brightness and compactness. Various materials, for example, Li, Xe, and Sn, were investigated in order to obtain a sufficient EUV conversion efficiency (CE) and power. Laser-produced Sn plasmas,2 especially low density Sn plasmas,3 are an attractive 13.5 nm light source due to their compactness and high emissivity, and they have a highly intense emission peak at 13.5 nm; thus, much effort has been devoted to the development of the Sn* To whom correspondence should be addressed. Telephone/Fax: +81(6)-6879-8778. E-mail: [email protected]. † Institute of Laser Engineering, Osaka University. ‡ Southeast University. § Institute for Laser Technology, Osaka University. (1) Bakshi, V. Ed. EUV source for lithography; SPIE Press: Bellingham, WA, 2006. (2) Shimada, Y.; Nishimura, H.; Nakai, M.; Hashimoto, K.; Yamaura, M.; Tao, Y.; Shigemori, K.; Okuno, T.; Nishihara, K.; Kawamura, T.; Sunahara, A.; Nishikawa, T.; Sasaki, A.; Nagai, K.; Norimatsu, T.; Fujioka, S.; Uchida, S.; Miyanaga, N.; Izawa, Y.; Yamanaka, C. Appl. Phys. Lett. 2005, 86, 051501. (3) (a) Gu, Q. C.; Nagai, K.; Norimatsu, T.; Fujioka, S.; Nishimura, H.; Nishihara, K.; Miyanaga, N.; Izawa, Y. Chem. Mater. 2005, 17, 1115–1122. (b) Okuno, T.; Fujioka, S.; Nishimura, H.; Tao, Y.; Nagai, K.; Gu, Q.; Ueda, N.; Ando, T.; Nishihara, K.; Norimatsu, T.; Miyanaga, N.; Izawa, Y.; Mima, K.; Sunahara, A.; Furukawa, H.; Sasaki, A. Appl. Phys. Lett. 2006, 88, 161501. (c) Nagai, K.; Gu, Q. C.; Gu, Z. Z.; Okuno, T.; Fujioka, S.; Nishimura, H.; Tao, Y. Z.; Yasuda, Y.; Nakai, M.; Norimatsu, T.; Shimada, Y.; Yamaura, M.; Yoshida, H.; Nakatsuka, M.; Miyanaga, N.; Nishihara, K.; Izawa, Y. Appl. Phys. Lett. 2006, 88, 094102. (d) Pan, C.; Gu, Z. Z.; Nagai, K.; Norimatsu, T.; Birou, T.; Hashimoto, K.; Shimada, Y. J. Appl. Phys. 2006, 100, 016104. (e) Nagai, K.; Wada, D.; Nakai, M.; Norimatsu, T. Fusion Sci. Technol. 2006, 49, 686–690.

based EUV light source. However, debris emitted from the Sn plasma damages and contaminates the first EUV collection mirror, and degrades the mirror reflectivity.4 The most severe debris to remove is from the neutral tin; therefore, all target materials should be ionized by laser irradiation. In order to reduce debris while keeping the CE, 1014-1015 number of tin atoms is required for one laser irradiation target.4a Until now, there is no practical EUV light source system that satisfies the requested specifications for the high-volume production of semiconductors due to several technological difficulties. By considering etendue, which concerns the point source and laser power, a laser spot size of 500 µm is preferable. Tin atoms reaching to 1015 atm/((500)2 µm) are required for the EUV emission target material fabrication without a substrate.5 The layer-by-layer (LBL) technique based on electrostatic forces was first introduced by Decher and Kunitake et al., and it provided a controllable approach for surface coating with a designable layer structure,6 defined wall thickness, and size. Mo¨hwald et al. at the Max-Planck Institute for Colloids and Interfaces introduced the LBL technique to 3D colloidal particles to obtain multilayered polyelectrolyte (PE) microcapsules by selectively removing the template.7 By using melamine formaldehyde (MF) particles as the template, the LBL 3D particles were more easily handled and stable against chemical and (4) (a) Fujioka, S.; Nishimura, H.; Nishihara, K.; Murakami, M.; Kang, Y.-G.; Gu, Q. C.; Nagai, K.; Norimatsu, T.; Miyanaga, N.; Izawa, Y.; Mima, K.; Shimada, Y.; Sunahara, A.; Furukawa, H. Appl. Phys. Lett. 2005, 87, 241503. (b) Namba, S.; Fujioka, S.; Nishimura, H.; Yasuda, Y.; Nagai, K.; Miyanaga, N.; Izawa, Y.; Mima, K.; Takiyama, K. Appl. Phys. Lett. 2006, 88, 171503. (5) The specification of the EUV source is described in ref 1. The estimation of the minimum mass of tin was based on the experiments, for example, as shown in ref 4. More information about low density tin for EUV light is described in the following: Nanomaterials to generate extreme ultraviolet (EUV) light. In Encyclopedia of Nanoscience and Nanotechnology, 2nd ed.; Nalwa, E. H., Ed.; American Science Publishers: Stevenson Ranch, CA, 2008; in press. (6) (a) Decher, G. Science 1997, 277, 1232–1237. (b) Lvov, Y.; Ariga, K.; Ichinose, I.; Kunitake, T. J. Am. Chem. Soc. 1995, 117, 6117–6123. (7) Caruso, F.; Caruso, R.; Mo¨hwald, H. Science 1998, 282, 1111–1114.

10.1021/la800766q CCC: $40.75  2008 American Chemical Society Published on Web 08/15/2008

Fabrication of TiO2 Microcapsules

Figure 1. SEM image (a) and AFM image (b) of the starting nanoparticles of SnO2 on a carbon adhesive tape.

physical influences.8 With this method, a variety of polyions including synthetic and natural materials have been used to construct polyelectrolyte multilayer microcapsules, which provide capsules with special functions, such as metal nanoparticles,9 magnetic particles, 10 lipid and biosensing molecules,11 combining other polymers, 12 and so forth. The combination of poly(allylamine hydrochloride) (PAH) and poly(styrenesulfonate) sodium salt (PSS) layers exhibited an interesting heat response to shrunk and thicker layers.13 However, all these applications were based on the wet capsules. We employed MF particles coated with a multilayer polyelectrolyte as the template, and then tin dioxide nanoparticles were assembled on the outermost capsules’ surfaces. Removal of the MF and heat treatment were used to produce hollow tin dioxide dry capsules without a substrate. Such capsules will be used as EUV sources in the future. Tin dioxide nanoparticles are negatively charged when the solution pH is about 8-9 and can be assembled on the capsules if the capsules’ outmost layer is positively charged. Xray diffraction (XRD), atomic force microscopy (AFM), and scanning electronic microscopy (SEM) were used to study the tin dioxide containing capsules.

Materials and Methods 2.1. Materials. The sources of the chemicals were as follows: Poly(styrenesulfonate) sodium salt (PSS, Mw 70 000) and poly(allylamine hydrochloride) (PAH, Mw 70 000) were from Aldrich. The tin dioxide powder (average size is 250 nm as a catalog value) was obtained from Hosokana Micron (Hirakata, Japan). The water used in all experiments was prepared by a three-stage Millipore Milli-Q Plus 185 purification system and had a resistivity higher than 18.2 MΩ/cm. Positively charged melamine formaldehyde particles (MF particles) with a diameter of around 4.34 ( 0.08 µm were obtained from Microparticles GmbH, Berlin. 2.2. Preparation of Tin Dioxide Monodispersed Solution. The tin dioxide nanoparticles were dispersed into pH 8.3 solutions and ultrasonicated for 30 min. They were then stored at room temperature (8) (a) Donath, E.; Sukhorukov, G. B.; Caruso, F.; Davis, S. A.; Mo¨hwald, H. Angew. Chem., Int. Ed. 1998, 37, 2202–2205. (b) Radziuk, D.; Shchukin, D. G.; Skirtach, A.; Mo¨hwald, H.; Sukhorukov, G. Langmuir 2007, 23, 4612–4647. (c) Gao, C. Y.; Leporatti, S.; Moya, S.; Donath, E.; Mo¨hwald, H. Langmuir 2001, 17, 3491–3495. (d) Ko¨hler, K.; Shchukin, D. G.; Mo¨hwald, H.; Sukhorukov, G. B. J. Phys. Chem. B 2005, 109, 18250–18259. (9) (a) Radziuk, D.; Shchukin, D. G.; Skirtach, A.; Mo¨hwald, H.; Sukhorukov, G. Langmuir 2007, 23, 4612–4617. (b) Lee, D.; Rubner, M. F.; Cohen, R. E. Chem. Mater. 2005, 17, 1099–1105. (10) Koo, H. Y.; Chang, S. T.; Choi, W. S.; Park, J.-H.; Kim, D.-Y.; Velev, O. D. Chem. Mater. 2006, 18, 3308–3313. (11) (a) Zhu, H.; Stein, E. W.; Lu, Z.; Lvov, Y. M.; McShane, M. Chem. Mater. 2005, 17, 2323–2328. (b) Ge, L. Q.; Mo¨hwald, H.; Li, J. B. Chem.sEur. J. 2003, 9, 2589–2594. (c) Ge, L. Q.; Mo¨hwald, H.; Li, J. B. ChemPhysChem 2003, 4, 1351–1355. (12) Andreava, D. V.; Gorin, D. A.; Mo¨hwald, H.; Sukhoroukov, G. B. Langmuir 2007, 23, 9031–9036. (13) Koehler, K.; Shchukin, D. G.; Sukhorukov, G. B.; Mo¨hwald, H. Macromolecules 2004, 37, 9546–9550.

Langmuir, Vol. 24, No. 18, 2008 10403 for 1 night and used for the experiment. Figure 1 shows the SEM and AFM images of the nanoparticles and their size distribution. 2.3. Polyelectrolyte Shells Prepared through Layer-by-Layer Adsorption. The multilayer assembly was accomplished by adsorption of the polyelectrolytes at a concentration of 1.0 mg/mL in 0.5 mol/L NaCl aqueous solutions. Oppositely charged polyelectrolyte species were subsequently added to the suspension followed by repeated centrifugation cycles. After the intended number of layers was adsorbed, 0.1 mol/L HCl was used to remove the core (MF particles) and hollow polyelectrolyte shells were then obtained. PSS is used to form the first layer and the outermost layer is PAH, which has a positive charge in order to bind the negatively charged tin dioxide nanoparticles in the next step. 2.4. Heat Treatment of the Obtained Multilayered Polyelectrolyte Capsules and Tin Dioxide Containing Capsules. The obtained capsules were placed on aluminum foil and treated at 120 °C for 20 min and at 150 °C for 30 min in an oven. 2.5. Scanning Electronic Microscopy (SEM). SEM images were obtained using a JEOL JSM-7400F field-emission scanning electron microscope operated at an accelerating voltage of 2 kV in the gentlebeam mode (ultralow accelerating voltage and high resolution). The PS sample was coated with platinum to improve the conductivity when observed by SEM. 2.6. Wide-Angle X-ray Diffraction (WAXD) Measurements. WAXD measurements were carried out using a Rigaku RINT2500HF+ WAXD spectrometer with Cu KR radiation at 1.54 Å. The average grain size of the tin dioxide was calculated on the basis of the Scherrer equation as follows

L ) kλ ⁄ (β cos θ)

(1)

where k is the Scherrer shape factor that equals 0.94, λ is the X-ray wavelength of 1.54 Å, β is the peak full width at half-maximum in radians, and θ is the diffraction peak position. Jade 6 XRD processing software was used to analyze the crystalline phase and grain size. The grain size was calculated by fitting the 〈211〉 diffraction peak of SnO2. 2.7. Atomic Force Microscopy (AFM) Measurements. AFM images were recorded at room temperature using a Nanoscope III multimode atomic force microscope (Digital Instruments, Inc., JEOLJSPM 5200s). Silicon tips (Olympus and Nanotips, DI) with a resonance frequency of ≈300 kHz and a spring constant of ≈40 N/m were employed to obtain the AFM images in the tapping mode. The contact force between the tip and the sample was kept as low as possible (≈10 nN), so that the images were acquired in the constant force mode (height mode) at a scan rate of 0.5 ( 1 Hz. The samples were prepared by applying a drop of the capsule solution onto freshly cleaved mica. The images were processed and analyzed using Nanoscope software and a spatial autocorrelation function. 2.8. ζ-Potential Measurements. The zeta-potential of the capsules was measured by using an ELS 8000 instrument (Japan). The mobility µ was converted into a zeta (ζ)-potential by using the Smoluchowski relation, ζ ) µη/ε, where η and ε are the viscosity and permeability of the solution, respectively. 2.9. Laser Irradiation of the Targets. The EUV emission from the capsules was generated by a CO2 laser irradiation. As a photon detector, a liquid nitrogen cooled charge coupled device (CCD) camera was installed in the EUV spectrometer, and the emission spectrum was simultaneously accumulated. The laser energy was 260 mJ, the pulse was 28 ns, and the laser intensity was 2.9 × 1010 W/cm2.

Results and Discussion In order to characterize the starting SnO2 nanoparticles, their SEM and AFM images are shown in Figure 1. In the SEM image, most of them are 30 nm and smaller than the catalog value of 250 nm. For the AFM image, we can see a small number of particles around 250 nm, while a major portion is similar to the SEM image.

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Figure 2. SEM images of (a) bare MF particles, (b) MF particles after (PSS/PAH)4 coating, and (c) MF particles after coating of (PSS/PAH)4 layers and SnO2 nanoparticles.

Ge et al.

Figure 4. (a) AFM image of (PSS/PAH)4/SnO2 capsules after removal of the MF particles, (b) expanded image of the squared area of image (a), and (c) cross-sectional view of the line in image (a).

Such creases and folds have been seen in a previous (PAH/ PSS)n.13 In the present case, a flat basin was clearly shown, and the average roughness was estimated using the following equation for the square in Figure 3a (expanded in Figure 3b):

Ra ) 1 ⁄ S0

Figure 3. (a) AFM image of (PSS/PAH)4 capsules after the removal of MF particle, (b) expanded image of the squared area of image (a), and (c) cross-sectional view of the line in image (a).

Figure 2 shows the SEM images of MF particles (a), four bilayer PSS/PAH coated MF particles (b) (i.e., multilayered polyelectrolyte coated MF particles), and tin dioxide nanoparticles coated multilayered polyelectrolyte MF particles (c). From the images, one can observe that the surface of the MF bare particles is smooth, and also after coating by (PSS/PAH)4. The difference in the size of the particles between Figure 2a and b is negligible due to the very thin membrane coating. After the surface was covered by the tin dioxide nanoparticles, the surface was not smooth due to random aggregation and looked like a porous layer of tin dioxide nanoparticles. Comparing Figure 2c with Figure 2a and b, the diameter increased to about 500 nm for (PSS/PAH)4/SnO2 due to the coating. According to a previous publication,7 the thickness of the four bilayer PSS/PAH is about 12-16 nm; therefore, the thickness for the SnO2 layer would be ∼250 nm. This value is consistent with the catalog value of ∼250 nm; however, the morphology shown in Figure 2c implies that the tin dioxide particles consist of smaller aggregated particles. Figure 3 shows an AFM image of the multilayered (PSS/ PAH)4 and its surface structure. There is a flat basin at the center, and the surrounding belt area is similar to rising pleated layers.

∫0X ∫0Y max

max

| f(x, y) - z0|dx dy

(2)

where f (x,y) and z0 are the height at point (x,y) and average height, respectively, and S0 ) XmaxYmax. The Ra value for the flat basin at the center was 4 ( 2 nm, which is much smaller than the one reported earlier (∼13 nm for 3.8 µm diameter),14 and might be the intact structure for the flat surface of (PAH/PSS)n. Figure 3c shows a cross-sectional view of the image along the line shown in Figure 3a. In the surroundings, stepwise structures appear and these thicknesses are close to twice (∼55 nm) and three times (∼82 nm) that of the basin area (∼27 nm), implying a pleated layer structure again. Figure 4 shows an AFM image of the tin dioxide particle containing capsules and their surface structure. Compared to Figure 3, one can find that tin dioxide particles were assembled on the capsules; the capsules become much thicker and rougher. This morphology is consistent with the SEM image shown in Figure 2c. In the image of Figure 4a, the higher surrounding area was observed although the pleated layer structure is not clearly shown in comparison to Figure 3a. The difference could be due to the thicker tin dioxide, which had a loose thin skin property and became like a thick rubber ball without air inside. Regarding the roughness, the Ra value for the basin at the center (square of Figure 4a) was 31 ( 2 nm. This value is similar to that estimated from the SEM and AFM images as shown in Figure 1. The wavelength of the roughness for the area shown in Figure 3b was estimated to be ∼70 nm, which is the same order as the Ra value. The tin dioxide nanoparticles can be assembled on the capsules’ surface of PAH. The surface ζ-potential after the tin dioxide coating was estimated to be -6.61 mV, while the PSS and PAH layers were -40 mV and +45 mV, respectively. The negative ζ-potential for the tin dioxide layer shows the electrostatic contact on the outermost PAH layer of (PSS/PAH)4, which was positive. (14) Gao, C.; Leporatti, S.; Donath, E.; Mo¨hwald, H. J. Phys. Chem. B 2000, 104, 7144–7149.

Fabrication of TiO2 Microcapsules

Figure 5. XRD results of (a) SnO2 particles as the starting material, (b) MF(PSS/PAH)4/SnO2 particles, and (c) SnO2 after removal of the MF particles and heat treatment.

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Figure 7. SEM images for (a) (PSS/PAH)4 capsules after removal of the MF particles, (b) products obtained after the (PSS/PAH)4 capsules were heated at 120 °C for 20 min, and (c) products obtained after the (PSS/PAH)4 capsules were heated at 150° for 30 min.

Figure 6. SEM images for (a) (PSS/PAH)4 and (b) (PSS/PAH)4/SnO2 after removal of the MF particles.

Figure 8. (a) SEM image for the products on an aluminum foil obtained from (PSS/PAH)4/SnO2 heat treatment at 150° for 30 min after removal of the MF particles and (b) SEM image of an imperfect hollow shell after heat treatment.

WAXD measurements were carried out to investigate the crystallinity of the tin dioxide nanoparticles before and after assembly on the capsules, the results of which are shown in Figure 5. The diffraction peaks situated at 2θ ) 26, 33, 37, and 51° are assigned to the diffractions of 〈110〉, 〈101〉, 〈200〉, and 〈211〉 of the cassiterite, respectively. The grain size of the tin dioxide is estimated to be 12 nm from the broadening of the 〈211〉 diffraction peak using the Scherrer equation (eq 1). This value is smaller than that estimated from the SEM and AFM images as shown in Figure 1. The thickness of the SnO2 layer, which was estimated from SEM results, implies aggregation of the 12 nm SnO2 particles, and it agrees with the roughness shown in the AFM image of Figure 4b, where a >12 nm particle aggregation appears. From the XRD results, one can find that the two curves before and after the coating are almost the same, which means that the tin dioxide characteristics were maintained well after they were assembled on the capsules’ surfaces. It is consistent with the previous results that the layer-by-layer technique does not change the nanoparticle properties.8a As for heat treatment at 150 °C, the diffraction peaks became sharp and the crystal size estimated by the Scherrer equation was 30 nm. It is similar to the particle size of the initial SnO2 and suggests a crystallization of the aggregates into single crystals during heating. To remove the polymer supports, which can cause debris during the laser irradiation, a heat treatment was used. SEM was employed to study the morphology change by the heat treatment, the results of which are shown in Figures 6-8. Figure 6a shows the naked (PSS/PAH)4 capsules after the removal of the MF core. It becomes not round due to water evaporation, which means that the shell is not hard enough to withstand water evaporation. The flat basin also appears at the center, and the surrounding higher belt-area-like pleated layers as seen in Figure 3a. For the tin dioxide nanoparticles, the SEM image after removal of the MF core is shown in Figure 6b, where the morphology is obviously different when compared with Figure 6a, similar the difference seen in Figures 3 and 4. Next, these capsules were heated. In the case of naked (PSS/ PAH)4 capsules to be heated at 120 °C for 20 min first and then

studied by SEM, the size shrunk to about 72% (the average size is from 4.7 to 3.4 µm) and the capsules are still collapsed due to the water evaporation. We must point out that this result is different from the reported one by Mo¨hwald’s group.13 According to his publication, the obtained capsules have not collapsed after the heat treatment. Such a difference is regarded to be caused by the detailed difference during the heat treatment; that is, the capsules are floating in the aqueous solution, and the whole process including heating and cooling is about 2 h. In our study, the capsule solution is kept on the aluminum foil and the heating time is 20 min. If the capsules were treated at 150 °C for 30 min and then studied by SEM, one can obtain a round ball, and such capsules with an even number of layers exhibited a pronounced shrinking at elevated temperature, resulting in a transition to a dense sphere as discussed in ref 8. It is explained that shells with an even number of layers have a balanced ratio between the oppositely charged polyions, so that the temperature-dependent behavior is controlled by the different interactions between the polyelectrolytes and the bulk water. Similar ballooning happened for a tin dioxide coated capsule (Figure 8), where the dimple was not observed as seen in Figure 7c for the naked (PSS/PAH)4. After the tin dioxide nanoparticles were assembled on the capsule’s surface and then heated at 150 °C for 30 min, one can find that the capsule’s size shrunk to about 71% (the average size is from 4.9 to 3.5 µm) to form the uncollapsed capsules. Figure 8a shows the particles covered with a layer of material. Such a difference may be caused by not using the aluminum foil. Figure 8b shows that the sphere is a hollow capsule of SnO2 and indicates that the thickness of the SnO2 is ∼100 nm, implying a not fully dense structure. The lower shrinkage in comparison to the naked (PSS/PAH)4 implies a stiffness of the tin dioxide nanoparticles during heating. Such hollow capsules are the expected target materials in the EUV system, although the shrinkage happened. The mass of SnO2 and the number of tin are roughly estimated from the capsule size (4.9 µm) and thickness (2 × 102 nm) before heat treatment to be 4 × 10-12 g and 1 × 1011, respectively. Therefore 1 × 104 microcapsules are necessary to satisfy the number of minimum mass of tin of 1015 atoms. The amount of microcapsules can be

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for a tin cavity20). Although such a narrow band has been observed for low density tin using the 1.06 µm laser due to reabsorption of EUV by the corona plasma,5 it has been not expected for the 10.6 µm laser due to its narrow spectrum.

Conclusions

Figure 9. EUV emission spectrum from the microcapsule targets irradiated by CO2 laser whose pulse duration was 40 ns with a peak intensity of 2.9 × 1010 W/cm2.

supplied by combining with an electromagnetic force, 15 gas jet,16 double pulse, 17 droplet,18 and so forth in order to generate the EUV light. Figure 9 shows the emission spectra of the asprepared capsule irradiated by a laser at an intensity of 2.9 × 1010 W/cm2, which is typical to provide a highest conversion efficiency from laser to the EUV light. A peak exists at 13.5 nm which is close to the previously reported wavelength.2 Unfortunately, we cannot determine the conversion efficiency from the laser to EUV light because angular distribution measurements are necessary to estimate it, although the spectrum has a significant efficient intensity. Furthermore, the present EUV emission has a 0.5 nm half-width which is narrower than the previous data for the 10.6 µm irradiation (1.5-2.0 nm for tin film19,20 and 1 nm (15) Extreme ultraviolet light source. JP Patent 297737, 2003. (16) Laser plasma X-ray sources and semiconductor exposure apparatus. JP Patent 345698, 1999. (17) X-ray generation method, X-ray generator. JP Patent 85940, 2006. (18) Higashiguchi, T.; Hamada, M.; Kubodera, S. ReV. Sci. Instrum. 2007, 78, 036106. (19) Tanaka, H.; Matsumoto, A.; Akinaga, K.; Takahashi, A.; Okada, T. Appl. Phys. Lett. 2005, 87, 041503.

Tin dioxide containing multilayer capsules were successfully fabricated by a layer-by-layer technique. The thickness of the tin dioxide layer is about 2 × 102 nm which is calculated from the SEM results. The tin dioxide used in the experiment is about 30 nm which is based on the SEM and AFM images. The obtained capsules have a rougher surface compared to the freshly prepared polyelectrolyte capsules; the mean roughness of the surface is about 30 nm. The characteristics of the tin dioxide remained after they were assembled on the capsules’ surfaces. In order to reduce the mass of the polymer, a heat treatment was introduced, and after heat treatment, the capsules’ size shrank to about 71% (the average size changed from 4.9 to 3.5 µm). The obtained capsules maintained their round shape after water evaporation. A narrow 13.5 nm emission was observed when the capsules were irradiated by a CO2 laser with an intensity of 2.9 × 1010 W/cm2. Such a surface localized tin material will be used as a laser target for EUV light sources. Acknowledgment. This work was performed under the auspices of the MEXT (Ministry of Education, Culture, Science and Technology, Japan) with the contract subhect “Leading Project for EUV lithography source development” and was partly supported by the JSPS-CAS Core-University Program in the field of “Plasma and Nuclear Fusion”. L.G. thanks the National Natural Science Foundation of China (NNSFC20603006) for the financial support. LA800766Q (20) Ueno, Y.; Soumagne, G.; Sumitani, A.; Endo, A.; Higashiguchi, T. Appl. Phys. Lett. 2007, 91, 231501.