ISSN 10637850, Technical Physics Letters, 2013, Vol. 39, No. 11, pp. 990–993. © Pleiades Publishing, Ltd., 2013. Original Russian Text © A.V. Chernienko, E.V. Astrova, Yu.A. Zharova, 2013, published in Pis’ma v Zhurnal Tekhnicheskoi Fiziki, 2013, Vol. 39, No. 22, pp. 17–24.
Zigzag Structures Obtained by Anisotropic Etching of Macroporous Silicon A. V. Chernienko*, E. V. Astrova, and Yu. A. Zharova Ioffe Physical Technical Institute, Russian Academy of Sciences, St. Petersburg, 194021 Russia *email:
[email protected] Received May 23, 2013
Abstract—Highaspectratio structures with thin corrugated walls have been obtained by processing of macroporous silicon with a trigonal lattice in anisotropic etchants. For this purpose, a pattern of seeding etch pits with a certain orientation relative to crystallographic axes is created prior to electrochemical etching and a solution of definite composition for treating macropores is used after anodizing. The possibility of using zig zag structures as anodes in lithiumion batteries is discussed. DOI: 10.1134/S1063785013110175
Highaspect silicon structures with a depthto width ratio on the order of several hundred can be obtained either in nSi by photoelectrochemical etch ing in aqueous hydrofluoricacid solutions or in pSi by using organic electrolytes [1, 2]. For this purpose, a pattern of seeding etch pits (or grooves) with a certain orientation is created on the (100)oriented substrate surface. These pits serve as nucleation centers for the formation of regular cylindrical macropores or trenches with vertical walls [3–5]. The variety of obtained structures can be significantly increased by additional processing of macroporous silicon after anodizing. In order to increase porosity and obtain columns, it is a common practice to use isotropic pro cessing techniques, e.g., multiply repeated thermal oxidation and oxidedissolution cycles [6]. By means of anisotropic etching in various alkaline solutions, it is possible to obtain macropores with square cross sec tions. Matthias et al. [7] used anisotropic etching to obtain threedimensional photonic crystals based on macroporous silicon with depthmodulated diameter of channels. Lehmann [8] showed for the first time that pores with square cross sections and different ori entation of lateral sides can be obtained by postanod izing treatment of structures with cylindrical pores in different anisotropic etchants. In both cases [7, 8], ini tial macropores have been organized in square lattices. This Letter reports on highaspectratio silicon structures with thin corrugated walls, which have been obtained by anisotropic etching of macroporous sili con with a trigonal lattice. As is known, the rate of etching for a Si(111) plane in alkaline solutions is two orders of magnitude lower than that for other planes. The relation between the etching rates of the (100) and (110) planes, V(100) and V(110), can be inverted depending on the etchant composition [9]. For example, V(110) > V(100) in
aqueous potassiumhydroxide (KOH) solution. Upon adding isopropyl alcohol (IPA) to KOH solution, the rate of etching of the (110) plane decreases, while that of the (100) plane remains unchanged [10]. As a result, the relation between etching rates can be reversed, V(100) > V(110). The etching of concave surfaces leads to their faceting with slowly etched crystal faces, which are (100) in the case of KOH and (110) in KOH + IPA (Fig. 1). The trigonal lattice of macropores has two main mutually perpendicular directions, which are denoted by vectors Γ–K and Γ–M (Fig. 2a). If the Γ–K vector is parallel to the crystallographic 〈110〉 axis, then etch ing in a KOH + IPA solution will lead to the formation of square pores with cross section sides oriented along the Γ–K direction (Fig. 2b). The etching in KOH will rotate these squares by 45° (Fig. 2d). The same results can be obtained by simultaneously changing both the etchant and orientation of seeding etch pits relative to crystallographic axes. If the initial direction Γ–K is oriented parallel to 〈100〉 axis, then the sides of square pores will be oriented along the seeding rows upon
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Fig. 1. Transformation of a round pore cross section (bright field) into a square (dark field) during silicon etch ing in aqueous KOH and KOH + IPA solutions, respec tively.
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Fig. 2. Schematic diagrams of structures based on a trigonal lattice of macroporous silicon, obtained (a) upon anodizing and (b⎯e) after additional anisotropic etching: (b, c) Γ–K || 〈110〉, KOH + IPA or Γ–K || 〈100〉, KOH; (d, e) Γ–K || 〈110〉, KOH or Γ–K || 〈100〉, KOH + IPA. Silicon walls formed with increasing pore size are indicated in gray.
etching in KOH (Fig. 2b) and will be rotated upon etching in KOH + IPA (Fig. 2d). For both structures with the configurations pre sented in Figs. 2b and 2d, the porosity of a structure with square pores is p = 1.15b2/a2, where a is the lattice period and b is the square side length. In the former structure (Fig. 2b), an increase in the pore size leads to the formation of silicon walls of various thicknesses. In relative units, the wall thickness along Γ–K is tK/a = 1 – b/a and that along Γ–M is tM/a = 0.87 – b/a. The coalescence of pores with increasing square size pro ceeds in the Γ–M direction at b/a = 0.87 (p = 87%). This results in the formation of separate standing sili con walls of rectangular cross section with a thickness of t = 0.13a (Fig. 2c). In the case of a structure presented in Fig. 2d, the coalescence of pores takes place along the Γ–K direc tion at b/a = 0.707 (p = 57.5%) and results in the for mation of zigzag silicon walls with a thickness of t = 0.261a (Fig. 2e). A further increase in the pore size leads to a decrease in the wall thickness, which can be calculated as t = 0.968a – b. It should be noted that, as soon as the structure acquires a zigzag shape (i.e., the connectivity between rows of corrugated walls is lost), there appear both convex and concave surfaces, which may lead to faceting with rapidly etched planes. This implies that subsequent transformation of the struc ture (Fig. 2e) will lead to both a reduction in the wall width and truncation of the external corners. The experiments have been performed with ini tial nSi(100) single crystal wafers with a resistivity of 15 Ω cm. The wafers were cut into 30 × 30mm squares with lateral sides oriented along crystallo graphic directions 〈110〉. Then, the n+ contact was formed by phosphorus implantation into the rear side TECHNICAL PHYSICS LETTERS
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of wafers. The seeding etch pits were formed using a photomask with a pattern of trigonal lattice of 8 μm period. The mask was oriented so that rows of the seeding pits (Γ–K direction) was either parallel to crystallographic axis 〈110〉 (making angle 0° relative to the sample edge) or rotated by 45° (Γ–K direction par allel to crystallographic axis 〈100〉). The seeding pits were formed by anisotropic etching of silicon via win dows in a mask of plasmachemical SiO2 film. Deep cylindrical macropores were formed by photoelectro chemical etching with illumination from the rear aside [1]. The etching was performed at 25°C in a 4% aque ous HF solution containing 5% ethyl alcohol. The etching regime was selected so as to ensure constant pore diameter in depth. The initial current density was j0 = 6 mA/cm2. The anodized samples had a relatively low porosity of p ≈ 13% and possessed a rather high mechanical strength. Macroporous membranes have been prepared prior to anisotropic etching of samples in alkaline solutions. For this purpose, the substrate was removed from some samples by dissolution in a 30% aqueous KOH at 70°C (during this, the walls of porous silicon were protected by forming a layer of thermal oxide). In other samples, the substrate was removed by mechanical grinding and polishing. The membrane thickness was 340 μm. Macroporous membranes prepared from the sam ples with various orientations of seeding etch pits rela tive to the sample edge were treated in different aniso tropic etchants. The samples with 0° orientation were etched in 40% aqueous KOH, while the samples with 45° orientation were processed in 12% aqueous KOH with addition of IPA (2 : 3 v/v). All etching processes were carried out at room temperature for 50 and 95 min, respectively. 2013
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Fig. 3. SEM micrographs of zigzag structures obtained by anisotropic etching of macroporous silicon: (a) rearside view of a sam ple with seeding pits (and pore rows) oriented at 0° relative to crystallographic axis 〈110〉 (Γ–K || 〈110〉); (b) rear side viewed at an angle of 20° for a sample with seeding pits oriented at 45° (Γ–K || 〈100〉); (c) cross section of the macroporous membrane pre sented in (a), cleaved along zigzag walls; (d) magnified image of the wall area indicated by a rectangle in (c).
Figure 3 presents images of the zigzag structures obtained by scanning electron microscopy (SEM). The image in Fig. 3a shows the rear side of a zigzag structure obtained for the Γ–K direction parallel to the sample edge, while Fig. 3b presents the case of a struc ture rotated by 45°. The SEM image of a cross section (Fig. 3c) demonstrates the formation of a highaspect configuration. Greater magnification (Fig. 3d) reveals external and internal edges (bright and dark vertical lines) and side surfaces of the walls. The obtained structures have walls with a thickness of about 1 μm. For some applications of highaspect structures, the thickness of silicon walls is an important parameter [11–15]. In particular, macroporous silicon is a prom ising material for the anodes of lithium ion batteries, but practical implementation requires optimization of the material structure. The stability of the porous structure under conditions of cyclic lithium intercala tion/deintercalation depends on the thickness of sili con walls [16], since thin layers can better withstand mechanical stresses that arise during an increase in volume in the course of lithium intercalation. Investi gation and development of porous silicon anodes requires the wall thickness to be reliably controlled. Of the two variants of structures with monodisperse walls (Figs. 2c and 2e), the latter variant with corrugated walls is preferred for three reasons. First, the structure retains connectivity in one more direction and, hence, the current can pass not only in the vertical direction, but also in the horizontal direction along corrugated stripes. This factor improves the electric conductivity of the structure as compared to the case of isolated walls shaped as separate standing columns (Fig. 2c).
Second, in the configuration of corrugated walls, monodisperse walls are formed at a lower porosity. This circumstance not only increases the mechanical strength of initial anode, but also ensures a higher content of silicon per unit of flat anode surface, which determines the electric charge and discharge capacity QS. Estimates show that, upon reaching a theoretical capacity of silicon (Q = 3200 mA h/g), the anode made of macroporous silicon with a zigzag structure at a porosity of 57.5% and 200μmthick walls will have a capacity of QS = 63 mA h/cm2. Third, it can be expected that a zigzag structure will provide for the amortization of stresses that arise during pore volume increase in the course of lithium intercalation. The zigzag structures can also be of interest as twodimen sional photonic crystals with a specific shape and ori entation of voids in the dielectric lattice. Acknowledgments. This study was supported by the Russian Foundation for Basic Research (project no. 12030031) and the Presidential Program of Support for Leading Scientific Schools in Russia (project no. NSh3008.2012.2). REFERENCES 1. V. Lehmann, Electrochemistry of Silicon (WileyVCH, Weinheim, 2002). 2. H. Foll, M. Christophersen, J. Carstensen, and G. Haase, Mater. Sci. Eng. R39, 93 (2002). 3. G. Barillaro, P. Bruschi, A. Diligenti, and A. Nannini, Phys. Status Solidi C 2, 3198 (2005). 4. E. V. Astrova and G. V. Fedulova, J. Micromech. Microeng. 19, 095009 (2009).
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