ISSN 1063-7850, Technical Physics Letters, 2008, Vol. 34, No. 12, pp. 1030–1033. © Pleiades Publishing, Ltd., 2008. Original Russian Text © P.N. Brunkov, V.G. Melekhin, V.V. Goncharov, A.A. Lipovskii, M.I. Petrov, 2008, published in Pis’ma v Zhurnal Tekhnicheskoœ Fiziki, 2008, Vol. 34, No. 23, pp. 73–79.
Submicron-Resolved Relief Formation in Poled Glasses and Glass–Metal Nanocomposites P. N. Brunkov, V. G. Melekhin*, V. V. Goncharov, A. A. Lipovskii, and M. I. Petrov Ioffe Physico-Technical Institute, Russian Academy of Sciences, St. Petersburg, 194021 Russia St. Petersburg State Polytechnical University, St. Petersburg, Russia *e-mail:
[email protected] Received May 14, 2008
Abstract—The formation of a spatial relief reproducing that of the anode as a result of the thermal poling of glasses and glass–metal nanocomposites in a strong electric field has been studied by atomic force microscopy. The anode surface patterns exhibited either a square grid or a rectangular grating with a depth of 120 nm, a strip width of 0.5 µm, and a period of 1 µm manufactured using electron-beam lithography and ion etching on the surface of an n-type single crystal silicon wafer. The relief depth formed on the surface of poled samples varied within 5–15 nm, depending on the experimental conditions. The mechanism of relief formation in this system is discussed. PACS numbers: 66.30.hh, 66.30.Qa, 68.35.bj, 68.35.Ct, 68.37.Ps DOI: 10.1134/S1063785008120122
The process of thermostimulated polarization of glass in a strong electric field is called thermal poling. Glasses with a strong built-in field are of interest in nonlinear optics as promising materials for generating optical harmonics [1–3]. The electric-field-induced modification of the near-surface region composition as a result of thermal poling [4–6] is used for the creation of optical waveguides [7–9]. Recently, Takagi et al. [10] established that a spatial relief is formed on the surface of glass as a result of thermal poling using an electrode with a patterned surface and demonstrated that relief structures with submicron resolution can be formed on the poled glass surface. The electric-fieldassisted transfer of the relief pattern from electrode onto glass or a glass-metal nanocomposite is of interest for possible applications in diffraction optics and submicron lithography. The experiments were performed with commercial soda-lime glasses with a composition of 72SiO2– 15Na2O–6.8CaO–4MgO–1.6Al2O3–0.2Fe2O3–0.4SO3 (wt %). We have also used glasses of the same composition, additionally doped with silver nanoparticles in the near-surface region. This glass–metal nanocomposite was obtained by means of ion exchange in a salt mixture melt with a composition of 88NaNO3– Cd(NO3)2–0.005AgNO3 (molar fractions) at 320°C for 1 h, followed by heat treatment in a reducing hydrogen atmosphere at 330°C for 1 h. The sample thickness was 1 mm. The average size of metal nanoparticles according to the data of electron microscopy was 10 nm, and the filling factor (defined as the ratio of the metal phase volume to the total volume) in 200-nm-thick surface layer was evaluated at f ~ 0.1. The role of the profiled
anode was played by either a square grid or a rectangular grating with a depth of 120 nm, a strip width of 0.5 µm, and a period of 1 µm manufactured using electron-beam lithography and ion etching on the surface of an n-type single crystal silicon wafer. The electrode surface was covered with a 10-nm-thick chromium film. The spatial relief appearing on the glass surface as a result of thermal poling was studied using atomic force microscopy (AFM). The measurements were performed in air on a Dimension 3100 (Veeco) instrument operating in a tapping mode using NSG01 (NT-MDT Company) probes with a point curvature radius of about 10 nm. Figure 1 shows a three-dimensional (3D) AFM image of a surface region of the anode with a square grid relief (a), its spatial profile (b), a 3D image of the poled glass surface (c), and its spatial profile (d); the glass sample was studied after 20-min thermal poling at 300°C and an interelectrode voltage of 750 V. As can be seen, protrusions of the electrode relief correspond to depressions on the glass surface. The relief depth on the glass surface is about 15 nm, which is about eight times smaller than the relief depth on the electrode. The relief shapes on the electrode and glass are also different. Figure 2 presents a 3D AFM image of a surface region of the anode with a relief pattern of parallel stripes (a), its spatial profile (b), a 3D image of the poled glass–metal nanocomposite surface (c), and its spatial profile (d); the composite sample was also studied after thermal poling under the same conditions as those used for the glass (temperature, 300°C; interelectrode voltage, 750 V; duration, 20 min). As can be seen,
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Fig. 1. 3D AFM images and their spatial profiles, respectively, of (a, b) the anode with a square grid and (c, d) the glass surface upon thermal poling. The profiles were measured along the probe trajectories indicated by white horizontal lines on the corresponding image projections.
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Fig. 2. 3D AFM images and their spatial profiles, respectively, of (a, b) the anode with a relief pattern of parallel stripes and (c, d) the glass–metal nanocomposite surface upon thermal poling under the same conditions as those for the glass. The profiles were measured along the probe trajectories indicated by white horizontal lines on the corresponding image projections.
the relief depth on the glass–metal composite surface is about 15 nm (same as on the glass). The transferred relief depth depended on the experimental conditions. For both glass and the composite, a decrease of the voltage to 400 V and of the poling time to 5 min led to a decrease in the relief height to 5–6 nm. TECHNICAL PHYSICS LETTERS
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Thus, the relief depth in glass and glass–metal nanocomposite under otherwise identical conditions is virtually the same. In [10], which is the only available article devoted to the formation of spatial relief on the glass surface as a result of thermal poling against a patterned anode, a
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possible mechanism of this phenomenon was proposed. According to this, the glass is deformed under the action of electrostatic attractive forces between the electrode and the glass surface, which create the surface relief. However, we believe that the deformation of glass is caused by the internal mechanical stresses that arise in the glass matrix in the course of displacement of alkali metal ions under the action of the applied field, rather than by the electrostatic attraction between electrode and the sample surface. The profile of internal stresses is determined by the profile of concentration of the appearing cationic vacancies, which, in turn, is induced by the spatial distribution of the electric field strength on the surface and in the subsurface region. The development of stresses leads to local changes in the volume occupied by glass in the near-surface region and this results in the formation of a surface relief. The phenomenon of glass volume variation was previously observed in investigations of the ion exchange via a mask and the electrodiffusion from a metal film deposited onto a glass substrate [11–15]. The essence of this phenomenon consists in that the replacement of a small ion by of an ion of greater radius leads to a local increase in the volume of glass in the region of this substitution. In the case of the opposite ratio of the radii of mutually substituted ions, the local volume of glass decreases. In contrast to the cases of ion exchange and electrodiffusion, the process of thermal poling leads to displacement of the alkali metal cations from the nearsurface region inward the glass volume to a depth of up to 10 µm [5] rather than to their replacement by other species; only a fraction of the vacant positions is occupied by hydrogen ions from the environment. Since the volume fraction occupied by alkali metal cations in the glass is large, the field-induced displacement of alkali metal cations (as well as divalent cations entering in the glass composition) leads to a significant change in composition of the near-surface region. As a result, only the framework of the glass structure is retained, which is formed by silicon–oxygen tetrahedra bound by oxygen bridges. In the framework of glass deprived of the modifier (filler), internal mechanical stresses arise, which cause its deformation (i.e., the partial relaxation of the occupied volume). A decrease in the volume is more pronounced in the regions of electric contact between the glass sample and electrode (where the electric field is stronger and, hence, the concentration of cationic vacancies upon the thermal poling is greater). The local deformation of the volume proceeds primarily in the direction perpendicular to the free surface (because the bending deformation along the surface for a thick sample is insignificant), which accounts for the formation of a surface relief. The magnitude of mechanical stresses that arise at the glass surface and lead to the deformation of a silicate framework, is significantly greater than the value of electrostatic pressure developed in the course of thermal poling. For example, the value of mechanical stress at the glass surface, which was calculated from data on
the birefringence in a waveguide created by replacing sodium ions with monovalent copper ions possessing close ion radii (0.095 and 0.096 nm, respectively) amounted to 400–800 MPa [14]. In the course of poling, sodium ions are either replaced by hydrogen or form vacancies, so that mechanical stresses are unlikely to be lower. The electric field strength at the anode during poling does not exceed 1 V/nm [5] (higher field strengths lead to destruction of the material). For this reason, the electrostatic pressure (which is equal to the volume energy density of the electric field) near the glass surface can be estimated as p = 0.5ε0εE2 ≈ 20 MPa, where ε0 is the permittivity of vacuum, ε ≈ 5.5 is the relative permittivity of glass, and E = 1 V/nm is the maximum electric field strength for thermal poling. As can be seen from this estimation, the mechanical stresses related to the displacement of cations from the near-anode region exceed the electrostatic pressure by at least one order of magnitude. It should be noted that, in the case of a glass–metal nanocomposite, thermal poling is accompanied by the dissolution of metal nanoparticles in the near-surface region [16, 17], which can significantly influence the magnitude of volume relaxation. However, the experimental data indicate that the depth of the relief in the composite is comparable to that in the glass, so that the dissolution process can be ignored. This is probably explained by a relatively small total volume of metal nanoparticles in the given nanocomposite (where these particles are concentrated in a thin (~200 nm thick) near-surface layer) as compared to the free volume left by displaced cations (or replaced by hydrogen) as a result of thermal poling (depleted layer thickness amounts to several microns). Thus, the formation of a relief on the surface of glass or a glass–metal nanocomposite as a result of thermal poling is related to local deformation of the glass matrix under the action of internal mechanical stresses that arise in the near-surface region as a result of the fieldinduced displacement of alkali-metal cations. The depth and shape of the relief are determined by various factors, the main of which are the distribution of the electric field strength near the poled sample surface, the concentration of cations and their mobility, and the process parameters (duration, temperature). The electrostatic forces of attraction between the electrode and the sample surface are relatively small and do not significantly contribute to the relief formation. We believe that the formation of a spatial relief during thermal poling can take place not only on the surface of alkaline glasses and related glass-metal nanocomposites, but also on the surface of other solids possessing ion conductivity as well. Acknowledgments. This study was performed within the framework of the project no. 08-02-00522-a supported in part by the Russian Foundation for Basic Research, with the use of equipment of the regional
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Translated by P. Pozdeev
