Chemical Modiﬁcation of Solid Surfaces at Nano-level by Water Cluster Ion Irradiation

We have investigated impact-process of water cluster ions on solid surfaces. Various kinds of substrates such as Si(100), SiO 2 and PMMA substrates were irradiated by adjusting the acceleration voltage and the ion dose. The sputtered depth of these substrates increased with increase of the acceleration voltage, and it was 37 nm for Si, 49 nm for SiO 2 , and 2.9 µ m for PMMA substrates, respectively at an acceleration voltage of 9 kV and an ion dose of 1.0 £ 10 16 ions/cm 2 . The sputtering yield calculated was 19 atoms per ion for Si and 13 molecules per ion for SiO 2 , which was approximately ten times larger than that for Ar monomer ion irradiation. The XPS measurement showed that the Si substrate surfaces sputtered at an ion dose of 1.0 £ 10 16 ions/cm 2 had an oxide layer such as SiO x . The oxide layer thickness increased with increase of the acceleration voltage, and it was approximately 10 nm at an acceleration voltage of 6 kV. On the other hand, at lower ion doses such as 1.0 £ 10 14 ions/cm 2 , the ratio of the sputtered depth of Si surface to SiO 2 surface became approximately 10. This indicated that the chemical erosion such as silicon hydride occurred, resulting in enhancement of chemical sputtering of Si substrates. Furthermore, for the case of PMMA substrates, the chemical erosion of the substrate surfaces occurred probably through the exchange between CH 3 and H radicals, and the chemical sputtering by the ejection of methacrylic acid molecule was enhanced. The surface roughness of PMMA substrates irradiated was less than 2 nm, and the smooth surface at the nano-level was obtained. In addition, micro-patterning was demonstrated with the water cluster ion beams on the PMMA substrates. [DOI: 10.1380/ejssnt.2011.163]


I. INTRODUCTION
Water is well-known liquid material which exists on earth and in human body, and it has an important role in maintaining clean environment and safe life. It is also important solvent which is used in cleaning process for various kinds of devices fabricated in semiconductor industries. On the other hand, ion beam process is one of the basic technologies in nanostructure fabrications [1,2]. It represents several features in material processing, one of which is that it can transfer charge, energy and mass employed for material surface treatment. Another feature is that various kinds of species such as atomic, molecular and cluster ions are available [3,4]. In particular, polyatomic ions such as water and alcohol ions contain several kinds of radicals such as hydroxyl and alkyl radicals, and these radicals have an important role in surface modification and chemical erosion of the surface [5,6]. Furthermore, in polyatomic cluster ions, their irradiation effects such as high density irradiation and low energy irradiation effects can be applied effectively to the surface treatment. Another feature is that local heating of impact region by the cluster ion irradiation can enhance chemical reaction on the surface even at room temperature [7,8].
We have developed several kinds of polyatomic cluster ion sources, and have investigated the interactions of cluster ions with solid surface atoms [9,10]. When polyatomic clusters such as water clusters are ionized and accelerated toward a substrate, impact of the cluster ions on the surface represents high-chemical reactivity of radicals such as hydroxyl radical and excited hydrogen atom. Furthermore, accelerating energy of water cluster ions enhances the chemical reactivity due to the high density irradiation effect, which is not obtained by monomer ion beam process. In this article, water cluster ion irradiation on various kinds of substrates such as Si(100), SiO 2 and poly-methyl-methacrylate (PMMA) is performed by changing the acceleration voltage and the ion dose, and the sputtered depth and the surface morphology are investigated. Furthermore, the X-ray photoelectron spectroscopy (XPS) measurement for the irradiated surfaces is performed, and the chemical modification of irradiated surfaces is discussed. On the basis of these results, micropatterning is demonstrated with the water cluster ion beams on the PMMA substrates.

II. EXPERIMENTAL PROCEDURE
The details of experimental apparatus were described elsewhere [11]. Water was introduced into the cluster source, and it was heated up to 150 • C by a wire heater attached around the source. When water vapors without helium gas were ejected through a nozzle into a vacuum region, water clusters were produced at vapor pressures larger than 0.1 MPa. The cluster size measured by the time-of-flight (TOF) method was distributed between a few hundreds and a few tens of thousands, and the peak size was approximately 4000 molecules per cluster. The peak size as well as the intensity of water clusters increased with increase of the vapor pressure. The water clusters produced passed through a collimator and entered an ionizer. In the ionizer, neutral clusters were ionized by an electron bombardment method. The electron voltage for ionization (V e ) was adjusted between 0 V and 300 V, and the electron current for ionization (I e ) was adjusted between 0 mA and 250 mA. The peak size decreased slightly with increase of the electron current for ionization, although it did not change with respect to the electron energy for ionization. The cluster ions were size-separated by a retarding potential method. The minimum size of cluster ions was controlled by adjusting the retarding voltage, and it was adjusted between 300 molecules per cluster and 1,000 molecules per cluster. The size-separated cluster ions were accelerated toward a substrate, which was set on a substrate holder. The acceleration voltage (V a ) was adjusted between 0 kV and 10 kV. The substrates used were Si(100), SiO 2 and PMMA. The substrate temperature was at room temperature. The substrate surfaces irradiated with water cluster ions were measured by using an atomic force microscope (AFM), and the composition and the chemical bond state were measured by the X-ray photoelectron spectroscopy (XPS). Furthermore, mask patterns demonstrated on the PMMA substrates with water cluster ion beams were observed by the laser microscope.

III. RESULTS AND DISCUSSION
In order to investigate the dissociation of water cluster ions, we measured a mass spectrum for the vacuum chamber by using the quadrapole mass-spectrometry (Q-Mass). Figure 1 shows the mass spectra (a) after impact of water cluster ions beams on Si(100) surface and (b) at water vapor atmosphere. The acceleration voltage was 9 kV. The peaks appeared at mass numbers 1, 2, 17 and 18 correspond to H, H 2 , OH and H 2 O. Compared with the mass spectra at evaporated water atmosphere, H 2 peak is stronger for the water cluster ion irradiation. After bombardment of water cluster ions on the Si surfaces, the cluster is broken up into the molecules. Furthermore, some of the water molecules are dissociated as follows; Therefore, some of H atoms produced react with another H atom, which result in the formation of H 2 molecule. Figure 2 shows the dependence of sputtered depth for Si(100), SiO 2 and PMMA surfaces on acceleration voltage for water cluster ions. The ion dose was 1.0×10 16 ions/cm 2 , and the cluster size used was larger than 300 molecules per cluster. As shown in the figure, the sput- tered depth increases with increase of the acceleration voltage, and it is 37 nm for Si, 49 nm for SiO 2 , and 2.9 µm for PMMA substrates, respectively at an acceleration voltage of 9 kV. The sputtering yield calculated is 19 atoms per ion for Si and 13 molecules per ion for SiO 2 , which was approximately ten times larger than that for Ar (mass number: 40) or Ne (mass number: 20) monomer ion irradiation [12,13]. For the Si surfaces, OH radicals or oxygen atoms produced have important roles in the oxidation due to implantation and/or diffusion processes, and the oxidation depends on both the kinetic energy and the reactivity of the bombarding species. Since the silicon oxide layer has a higher surface binding energy than the Si surface [14], chemical sputtering of the oxide layer decrease. Instead, the kinetic energy of another cluster ion bombarded is used for the physical sputtering of the oxide layer. As a result, the sputtered depth of Si surfaces is similar to that of SiO 2 surfaces.
Furthermore, for the PMMA substrates, the chemical erosion of the substrate surfaces occurs through the exchange of CH 3 radical in COOCH 3 with H atom of the water cluster or the exchange of OCH 3 radical with OH radical. As a result, the PMMA surface changes to polymethacrylic acid surface, which represents the low melting point less than room temperature and is dissolved by water. The impact of water cluster ions following-up enhances the ejection of methacrylic acid molecule at the monomer state from the surface. Thus, the high rate sputtering of PMMA surfaces is achieved by both the chemical erosion of the surface and the momentum transfer of the incident energy, which is different from the sputtering of PMMA surfaces reported elsewhere [15][16][17]. Figure 3 shows the dependence of sputtered depth for Si(100) and SiO 2 surfaces on the ion dose. The acceleration voltage was 6 kV, and the cluster size used was larger than 300 molecules per cluster. As shown in the figure, the sputtered depth increases with increase of the dose. However, the sputtered depth for the Si surface does not increase linearly with increase of the dose, and it increases gradually toward a given depth at an ion dose of 1.0×10 16 ions/cm 2 . At a lower ion dose of 1.0×10 14 ions/cm 2 , the ratio of the sputtered depth of Si surface to SiO 2 surface becomes approximately 10, which indi- cates that the chemical erosion such as silicon hydride occurs, resulting in enhancement of chemical sputtering of Si surface. When water cluster ions are irradiated on the Si surface, some of water molecules are dissociated into OH radical and hydrogen atom which is described in Eq. (1). Therefore, both oxide and hydride reactions on the Si surface occur at the initial stage of irradiation, and silicon oxide formation as well as chemical sputtering of Si surface is performed. However, the oxide reaction rather than the hydride reaction increases with increase of the ion dose, and the Si surface is oxidized, which results in decrease of the chemical sputtering by hydride reaction. As a result, the sputtered depth of Si(100) surface at the ion dose of 1.0×10 16 ions/cm 2 is similar to that of SiO 2 . It is noted that the sputtered depth for PMMA was much larger than those of Si and SiO 2 , as shown in Fig. 1, and it increased linearly with increase of the ion dose. Figure 4 shows the surface roughness of Si(100), SiO 2 and PMMA surfaces irradiated at different acceleration voltages for the water cluster ions. The ion dose was 1.0×10 15 ions/cm 2 . As shown in the figure, the sur- face roughness increases with increase of the acceleration voltage, and it is 0.89 nm for Si, 0.44 nm for SiO 2 and 1.59 nm for PMMA, respectively at an acceleration voltage of 9 kV. It is noted that smooth surface with a roughness less than 2 nm is obtained even for PMMA surfaces at a sputtered depth of 2.9 µm.
The Si(100) surfaces irradiated with water cluster ions were investigated by XPS measurement. Figure 5 shows the depth profiles of XPS peak intensities for (a) Si 2p spectra and (b) O 1s spectra. The acceleration voltage was 6 kV, and the ion dose was 1.0×10 16 ions/cm 2 . As shown in the figure, Si 2p peak on the surface is split into two peaks, one of which is shifted to the higher value of binding energy corresponding to the peak for SiO 2 . Another peak corresponds to the peak for Si. This indicates that the silicon oxide layer such as SiO x (x < 2) is formed near the surface. Also, Si 2p peak for SiO 2 as well as the O 1s peak decreases with increase of the depth, and it disappears at a depth of 10.5 nm. These indicate that the silicon oxide layer is formed by the water cluster ion irradiation, and the oxide layer thickness is approximately 10 nm at an acceleration voltage of 6 kV. Figure 6 shows C 1s peaks for the PMMA surfaces (a) unirradiated and irradiated at acceleration voltages of (b) 6 kV and (c) 9 kV. The ion dose was 1.0×10 16 ions/cm 2 , and the cluster size used was larger than 300 molecules per cluster. As shown in Fig. 6(a), the PMMA unirradiated substrate exhibits peaks assigned to -CH at 285.0 eV, -C< around 285.7 eV, -O-(C=O) around 286.8 eV and -(C=O)-O around 289.0 eV, respectively [18]. Compared with unirradiated surface, the PMMA surface irradiated at an acceleration voltage of 6 kV exhibits the decrease of the peak assigned to -O-(C=O) around 286.8 eV. This indicates that R-COOCH 3 is changed to R-COOH, in which R is the alkyl radical. Namely, OCH 3 radical or CH 3 radical in COOCH 3 is removed, and it might be exchanged by another radical such as OH radical or H atom. Furthermore, the C 1s peak for the PMMA surface irradiated at an acceleration voltage of 9 kV decreases, and the peaks assigned to -C< around 285.7 eV and -(C=O)-O around 289.0 eV becomes smaller than the peak around 286.8 eV. This indicates that the bond scissions occur, which results in the ejection of PMMA monomers as well as COOCH 3 radicals from the surface irradiated with the water clus- ter ion beams. Thus, the sputtering of PMMA surfaces by the water cluster ion irradiation represents unique features, which are different from the ablation and scission of PMMA surfaces during monomer ion and photon deposition induced surface modification [19][20][21]. Figure 7 shows the micro-patterning of PMMA substrate demonstrated with water cluster ion beams. The acceleration voltage was 9 kV, and the ion dose was 3.0×10 16 ions/cm 2 . A mask-pattern representing the letters "KYOTO" is prepared on the PMMA substrate, and the sputtered depth of the substrate is approximately 10 µm. Also, the width of a letter prepared is larger than the original one, that is 50 µm, and the increase of the width is ascribed to the lateral sputtering effect of the PMMA surface under the mask by the water cluster ion irradiation. If the sharp-edged pattern is performed, the contact of the mask on the substrate should be improved.

IV. CONCLUSION
We investigated interactions of water cluster ions with solid surfaces such as Si(100), SiO 2 and PMMA surfaces. The sputtered depth of these surfaces increased with increase of the acceleration voltage, and it was 37 nm for Si, 49 nm for SiO 2 , and 2.9 µm for PMMA substrates, respectively at an acceleration voltage of 9 kV and an ion dose of 1.0×10 16 ions/cm 2 . The sputtering yield calculated was 19 atoms per ion for Si and 13 molecules per ion for SiO 2 , which was approximately ten times larger than that for Ar monomer ion irradiation. The XPS measurement showed that Si surfaces sputtered at an ion dose of 1.0×10 16 ions/cm 2 had an oxide layer such as SiO x . This was due to the oxidation of Si surfaces by hydroxyl radicals. The oxide layer thickness increased with increase of the acceleration voltage, and it was approximately 10 nm at an acceleration voltage of 6 kV. Furthermore, it was noted that the hydride reaction of Si surface occurred at a lower dose of 1.0×10 14 ions/cm 2 , and it resulted in enhancement of chemical sputtering of Si surface. The hydrogen atom in water molecule, which was dissociated during impact of the water cluster ions on the Si surface, had an important role in the chemical sputtering.
With regard to the high-rate sputtering of PMMA surfaces, the XPS analysis indicated that the chemical ero-sion of the substrate surfaces occurred through the exchange between CH 3 and H radicals, and the chemical sputtering by the ejection of methacrylic acid molecule was enhanced. The AFM analysis showed that the surface roughness of Si, SiO 2 and PMMA substrates irradiated was less than 2 nm, and the smooth surface at the nano-level was obtained. On the basis of these results, the micro-patterning of PMMA substrate surfaces was demonstrated with water cluster ion beams, and the mask patterns such as KYOTO were performed on the substrate.