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Stripes of 2D photonic crystal obtained from macroporous silicon
Optical Materials 27 (2005) 827–830 www.elsevier.com/locate/optmat
Stripes of 2D photonic crystal obtained from macroporous silicon E.V. Astrova
a,*
, T. Borovinskaya a, V.A. Tolmachev a, T.S. Perova b, R.A. Moore a
b
Ioffe Physico-Technical Institute, 194021 St.-Petersburg, Russia b University of Dublin, Trinity College, Dublin 2, Ireland Available online 12 October 2004
Abstract A novel technological process has been developed to pattern macroporous silicon. In contrast to the existing process, the technique proposed here does not require pore filling and planarisation of the surface, thanks to the use of photolithography on the backside of the wafer. Stripes with vertical walls of 2D photonic crystals were prepared from macroporous silicon and characterized by FTIR microscopy. Interesting by-products of the developed process are silica microtubes, which are organized in a periodic ‘‘lattice’’ and possess strong photoluminescence. 2004 Elsevier B.V. All rights reserved.
1. Introduction A technique for the fabrication of an ordered ‘‘lattice’’ of deep channels with vertical walls was first suggested in [1]. The process involves opening windows in the oxide mask on the surface of n-Si(1 0 0) by means of photolithography, alkaline etching of pits through the mask, and anodization in dilute HF under illumination from the back side of the wafer. The resulting layer of macroporous silicon (ma-Si) contains cylindrical channels (macropores) from 1 to 10 lm in diameter and up to 300 lm deep. A precondition for the formation of these channels is a uniform distribution of the nucleation centers over the entire anodization area and an appropriate relationship between the substrate resistivity and the lattice period of the photomask [2]. A rather wide transition layer is formed at the boundary between the macroporous region and the part of the wafer not subjected to anodization. Within this layer, the regular structure is distorted with pores branching and having different depths, etc.
*
Corresponding author. E-mail address: [email protected]ffe.ru (E.V. Astrova).
0925-3467/$ - see front matter 2004 Elsevier B.V. All rights reserved. doi:10.1016/j.optmat.2004.08.018
Various applications of ma-Si [3] frequently require that the separate regions of macroporous silicon with sharp edges and vertical walls should be created on a substrate. In particular, in order to input light into 2D photonic crystal requires fabrication of narrow bars oriented in a certain way with respect to the ‘‘lattice’’ of the initial ma-Si. The first study dealing with this problem [4] demonstrated that the attempts to create a pattern in ma-Si with the use of a positive or negative photoresist failed. The authors [4] then developed a complicated process that includes the deposition of silicon nitride onto the inner surface of channels, followed by pore filling with polycrystalline silicon or aluminium and the formation of a pattern on the front side of the wafer. This complex technique, named the Ottow process, allowed selective removal of certain parts of the ma-Si layer in order to obtain narrow bars with vertical walls on the substrate. Structures of this kind have been used for the study of the optical properties of photonic crystals [5,6]. This communication reports on a simpler process for creating ma-Si regions with vertical walls, which is based on pore masking with a thermal oxide only together with pattern formation from the back side of the wafer. In contrast to the Ottow method, the structures thus formed have no common substrate, which in turn is also structured.
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E.V. Astrova et al. / Optical Materials 27 (2005) 827–830
2. Experimental procedure The starting material, n-type Si wafers with resistivity 5 X cm and (1 0 0) orientation, was cut into 20 · 20 mm2 squares, which were thinned to 200 lm and polished from both sides. Further operations can be seen from the schematic process, shown in Fig. 1. After the thermal oxidation, a through alkaline etching of the alignment marks was performed (stage 2). Following this a triangular ‘‘lattice’’ of seed pits with a period A = 4 lm was formed and photo electrochemical etching was carried out to a depth of 170–180 lm (stages 3, 4). The pore pits were removed by polishing the front side of the wafer, and the sample was then oxidized in water vapor at 1200 C for 70 min. Photolithography was used to open square windows in the oxide on the backside of the
Fig. 2. Optical microscope image of the back side of the wafer in the initial stage of pore opening. A small region with exposed pore bottoms can be seen.
wafer. The sides of these square windows were oriented along the rows of macropores (stage 5). A minor bending of the oxidized wafer was elastic and could be eliminated by suction of the sample to the table in the mask aligner. Anisotropic alkaline etching was performed through these windows and this provided local substrate removal to the depth that was necessary for the porous layer to be reached (stage 6). The thermal oxide protected the inner surface of pores during etching in KOH. The initial stage of pore opening is illustrated in Fig. 2. Following this the oxide from the inner surface of the pores was removed via the squares opened on the backside of the wafer (stage 7) and the subsequent dissolution of thin silicon walls between the pores in the window area was performed (stage 8). In the final stage, the oxide was removed from the pores in HF solution. If higher porosity was required, additional oxidation with subsequent dissolution of the SiO2 layers was used. In the oxidation process, narrow stripes of ma-Si did not undergo any noticeable deformation, in contrast to the unstructured porous layer [7]. As a result, square windows separated by crosses of macroporous layer on the substrate, were obtained from the wafer. SEM image of a stripe is shown in Fig. 3. It can be seen that the window has vertical walls and is aligned with the rows of macropores. The width of the stripes is about 90 lm and comprises 22–26 rows, depending on direction in the lattice, and the height is 200 lm. This consists of a 180 lm porous layer and a 20 lm substrate.
3. Photonic crystal spectra
Fig. 1. Schematic of the process for patterning macroporous silicon.
The ma-Si stripes are oriented in perpendicular directions that correspond to the C–K and C–M directions in the reciprocal lattice of the 2D triangular lattice of ma-Si [8]. These stripes were used for optical characterization of the photonic crystal. The ma-Si bars were
E.V. Astrova et al. / Optical Materials 27 (2005) 827–830
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The spectra contain broad regions of high reflection (up to 60%), centered at around 11 lm, which correspond to the lowest photonic band gap of this crystal. The band gaps for light polarized perpendicularly to the channels (H-polarization) are somewhat wider than those for light polarized parallel to the channels (Epolarization), which is in agreement with the results of calculations for a triangular lattice of macroporous silicon [9]. 4. Silica microtubes Fig. 3. SEM image of macroporous silicon bars with vertical walls.
oxidized three times, followed by oxide dissolution, in order to increase the channel diameter from 1.7 to 3.4 lm. The ratio between the channel radius and the lattice constant r/A became as high as 0.425. According to [9], this value corresponds to the formation of a complete photonic band gap. Reflection spectra were measured on a FTIR Bio-Rad 6000 spectrometer equipped with a UMA-500 IR microscope, with an aperture of 100 · 100 lm2. The incident light was focused using the microscope onto the lateral side of a ma-Si bar, with the wave vector of light directed perpendicularly to the axis of the channels. IR measurements were performed with a spectral resolution of 8 cm 1 and the number of scans was 64. To obtain the spectra with different polarization, a polarizer was placed in front of the input window of photodetector. Fig. 4 shows reflection spectra recorded for different directions in the photonic crystal corresponding to two orthogonal polarizations of light.
During alkaline etching of the back side of the wafer (see stage 6 in Fig. 1), the bottom parts of the oxidized pores were uncovered, whereas their upper parts remained embedded in the ma-Si matrix. This resulted in an ordered array of vertically standing silica microtubes with closed ends (Fig. 5). Upon the deep penetration into the porous layer from the substrate side, a reddish ‘‘wool’’ appeared in many cases in addition to the vertically standing tubes. It was composed of long tangled microtubes. As is the case with the separate microtubes, this ‘‘wool’’ exhibits strong photoluminescence (PL) [10]. Usually, silicon-based light-emitting material is obtained by electrochemical or chemical etching in HF solution to produce nanoporous silicon [11] or by the introduction of nanoscaled silicon inclusions into a SiO2 matrix [12]. In the case of silica microtubes or ‘‘glass wool,’’ the luminescence appears upon treating the oxidized ma-Si with a hot alkaline solution. The reddish coloration of the ‘‘glass wool’’ suggests that the oxide contains an excess of silicon. The main advantages of these tubes are: (i) arrangement in a regular lattice; (ii) transparency in the visible spectral range, and (iii) chip compatibility. Objects of this kind may find applications as microreactors for combinatorial chemistry or for biochips, etc.
Fig. 4. Reflection spectra for two directions in the photonic crystal, C– K and C–M, and two polarizations of light: perpendicular (H) and parallel (E) to the channel axis.
Fig. 5. Silica microtubes formed under the alkaline etching of the wafer back side. SEM image.
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E.V. Astrova et al. / Optical Materials 27 (2005) 827–830
5. Conclusion An experimental procedure to pattern macroporous silicon has been developed. The structures obtained using this technique, have deep vertical walls and, can be used to fabricate components for micromechanics and microphotonics. Possibilities for the technique have been demonstrated by the formation of PC stripes, suitable for IR spectral characterization. Furthermore, the method can be employed to obtain silica microtubes.
Acknowledgements The authors are grateful to A. Nashchekin and Yu. Pilyugina for help in obtaining SEM images and in preparing the illustrative material. The work was supported by INTAS (grant no. 01-0642), Program ‘‘Physics of Solid-State Nanostructures’’ of the Ministry of Science and Technology of the Russian Federation, Program ‘‘Optics and Laser Physics’’ of the Russian Academy of Sciences, and Presidential Program for
Support of Scientific Schools (NSh 758-2003.2), and HEA, Ireland. References [1] V. Lehmann, H. Foll, J. Electrochem. Soc. 137 (1990) 653. [2] V. Lehmann, U. Gruning, Thin Solid Films 297 (1997) 13. [3] H. Foll, M. Christophersen, J. Carstensen, G. Haase, Mater. Sci. Eng. R39 (2002) 93. [4] S. Ottow, V. Lehmann, H. Foll, J. Electrochem. Soc. 143 (1996) 385. [5] U. Gruning, V. Lehmann, S. Ottow, K. Busch, Appl. Phys. Lett. 68 (1996) 747. [6] S.W. Leonard, H.M. van Driel, K. Busch, S. John, A. Birner, A.P. Li, F. Muller, U. Gosele, V. Lehmann, Appl. Phys. Lett. 75 (1999) 3063. [7] E.V. Astrova, V.V. Ratnikov, A.D. Remenyuk, I.L. Shulpina, Phys. Status Solidi A 197 (2003) 16. [8] J.D. Joannopoulos, R.D. Meade, R.D. Winn, Photonic Crystals. Molding the Flow of Light, Princeton University Press, 1995, Appendix C, p.125. [9] C. Jamois, R.B. Wehrspohn, L.C. Andreani, C. Hermann, O. Hess, U. Gosele, Photonics and Nanostructures—Fundamentals and Applications 1 (2003) 1. [10] E.V. Astrova, T.N. Borovinskaya, T.S. Perova, M.V. Zamoryanskaya, Semiconductors 38 (2004) 1084. [11] L.T. Canham, Appl. Phys. Lett. 57 (1990) 1046. [12] O. Bisi, S. Ossicini, L. Pavesi, Surf. Sci. Rep. 38 (2000) 1.
Stripes of 2D photonic crystal obtained from macroporous silicon E.V. Astrova
a,*
, T. Borovinskaya a, V.A. Tolmachev a, T.S. Perova b, R.A. Moore a
b
Ioffe Physico-Technical Institute, 194021 St.-Petersburg, Russia b University of Dublin, Trinity College, Dublin 2, Ireland Available online 12 October 2004
Abstract A novel technological process has been developed to pattern macroporous silicon. In contrast to the existing process, the technique proposed here does not require pore filling and planarisation of the surface, thanks to the use of photolithography on the backside of the wafer. Stripes with vertical walls of 2D photonic crystals were prepared from macroporous silicon and characterized by FTIR microscopy. Interesting by-products of the developed process are silica microtubes, which are organized in a periodic ‘‘lattice’’ and possess strong photoluminescence. 2004 Elsevier B.V. All rights reserved.
1. Introduction A technique for the fabrication of an ordered ‘‘lattice’’ of deep channels with vertical walls was first suggested in [1]. The process involves opening windows in the oxide mask on the surface of n-Si(1 0 0) by means of photolithography, alkaline etching of pits through the mask, and anodization in dilute HF under illumination from the back side of the wafer. The resulting layer of macroporous silicon (ma-Si) contains cylindrical channels (macropores) from 1 to 10 lm in diameter and up to 300 lm deep. A precondition for the formation of these channels is a uniform distribution of the nucleation centers over the entire anodization area and an appropriate relationship between the substrate resistivity and the lattice period of the photomask [2]. A rather wide transition layer is formed at the boundary between the macroporous region and the part of the wafer not subjected to anodization. Within this layer, the regular structure is distorted with pores branching and having different depths, etc.
*
Corresponding author. E-mail address: [email protected]ffe.ru (E.V. Astrova).
0925-3467/$ - see front matter 2004 Elsevier B.V. All rights reserved. doi:10.1016/j.optmat.2004.08.018
Various applications of ma-Si [3] frequently require that the separate regions of macroporous silicon with sharp edges and vertical walls should be created on a substrate. In particular, in order to input light into 2D photonic crystal requires fabrication of narrow bars oriented in a certain way with respect to the ‘‘lattice’’ of the initial ma-Si. The first study dealing with this problem [4] demonstrated that the attempts to create a pattern in ma-Si with the use of a positive or negative photoresist failed. The authors [4] then developed a complicated process that includes the deposition of silicon nitride onto the inner surface of channels, followed by pore filling with polycrystalline silicon or aluminium and the formation of a pattern on the front side of the wafer. This complex technique, named the Ottow process, allowed selective removal of certain parts of the ma-Si layer in order to obtain narrow bars with vertical walls on the substrate. Structures of this kind have been used for the study of the optical properties of photonic crystals [5,6]. This communication reports on a simpler process for creating ma-Si regions with vertical walls, which is based on pore masking with a thermal oxide only together with pattern formation from the back side of the wafer. In contrast to the Ottow method, the structures thus formed have no common substrate, which in turn is also structured.
828
E.V. Astrova et al. / Optical Materials 27 (2005) 827–830
2. Experimental procedure The starting material, n-type Si wafers with resistivity 5 X cm and (1 0 0) orientation, was cut into 20 · 20 mm2 squares, which were thinned to 200 lm and polished from both sides. Further operations can be seen from the schematic process, shown in Fig. 1. After the thermal oxidation, a through alkaline etching of the alignment marks was performed (stage 2). Following this a triangular ‘‘lattice’’ of seed pits with a period A = 4 lm was formed and photo electrochemical etching was carried out to a depth of 170–180 lm (stages 3, 4). The pore pits were removed by polishing the front side of the wafer, and the sample was then oxidized in water vapor at 1200 C for 70 min. Photolithography was used to open square windows in the oxide on the backside of the
Fig. 2. Optical microscope image of the back side of the wafer in the initial stage of pore opening. A small region with exposed pore bottoms can be seen.
wafer. The sides of these square windows were oriented along the rows of macropores (stage 5). A minor bending of the oxidized wafer was elastic and could be eliminated by suction of the sample to the table in the mask aligner. Anisotropic alkaline etching was performed through these windows and this provided local substrate removal to the depth that was necessary for the porous layer to be reached (stage 6). The thermal oxide protected the inner surface of pores during etching in KOH. The initial stage of pore opening is illustrated in Fig. 2. Following this the oxide from the inner surface of the pores was removed via the squares opened on the backside of the wafer (stage 7) and the subsequent dissolution of thin silicon walls between the pores in the window area was performed (stage 8). In the final stage, the oxide was removed from the pores in HF solution. If higher porosity was required, additional oxidation with subsequent dissolution of the SiO2 layers was used. In the oxidation process, narrow stripes of ma-Si did not undergo any noticeable deformation, in contrast to the unstructured porous layer [7]. As a result, square windows separated by crosses of macroporous layer on the substrate, were obtained from the wafer. SEM image of a stripe is shown in Fig. 3. It can be seen that the window has vertical walls and is aligned with the rows of macropores. The width of the stripes is about 90 lm and comprises 22–26 rows, depending on direction in the lattice, and the height is 200 lm. This consists of a 180 lm porous layer and a 20 lm substrate.
3. Photonic crystal spectra
Fig. 1. Schematic of the process for patterning macroporous silicon.
The ma-Si stripes are oriented in perpendicular directions that correspond to the C–K and C–M directions in the reciprocal lattice of the 2D triangular lattice of ma-Si [8]. These stripes were used for optical characterization of the photonic crystal. The ma-Si bars were
E.V. Astrova et al. / Optical Materials 27 (2005) 827–830
829
The spectra contain broad regions of high reflection (up to 60%), centered at around 11 lm, which correspond to the lowest photonic band gap of this crystal. The band gaps for light polarized perpendicularly to the channels (H-polarization) are somewhat wider than those for light polarized parallel to the channels (Epolarization), which is in agreement with the results of calculations for a triangular lattice of macroporous silicon [9]. 4. Silica microtubes Fig. 3. SEM image of macroporous silicon bars with vertical walls.
oxidized three times, followed by oxide dissolution, in order to increase the channel diameter from 1.7 to 3.4 lm. The ratio between the channel radius and the lattice constant r/A became as high as 0.425. According to [9], this value corresponds to the formation of a complete photonic band gap. Reflection spectra were measured on a FTIR Bio-Rad 6000 spectrometer equipped with a UMA-500 IR microscope, with an aperture of 100 · 100 lm2. The incident light was focused using the microscope onto the lateral side of a ma-Si bar, with the wave vector of light directed perpendicularly to the axis of the channels. IR measurements were performed with a spectral resolution of 8 cm 1 and the number of scans was 64. To obtain the spectra with different polarization, a polarizer was placed in front of the input window of photodetector. Fig. 4 shows reflection spectra recorded for different directions in the photonic crystal corresponding to two orthogonal polarizations of light.
During alkaline etching of the back side of the wafer (see stage 6 in Fig. 1), the bottom parts of the oxidized pores were uncovered, whereas their upper parts remained embedded in the ma-Si matrix. This resulted in an ordered array of vertically standing silica microtubes with closed ends (Fig. 5). Upon the deep penetration into the porous layer from the substrate side, a reddish ‘‘wool’’ appeared in many cases in addition to the vertically standing tubes. It was composed of long tangled microtubes. As is the case with the separate microtubes, this ‘‘wool’’ exhibits strong photoluminescence (PL) [10]. Usually, silicon-based light-emitting material is obtained by electrochemical or chemical etching in HF solution to produce nanoporous silicon [11] or by the introduction of nanoscaled silicon inclusions into a SiO2 matrix [12]. In the case of silica microtubes or ‘‘glass wool,’’ the luminescence appears upon treating the oxidized ma-Si with a hot alkaline solution. The reddish coloration of the ‘‘glass wool’’ suggests that the oxide contains an excess of silicon. The main advantages of these tubes are: (i) arrangement in a regular lattice; (ii) transparency in the visible spectral range, and (iii) chip compatibility. Objects of this kind may find applications as microreactors for combinatorial chemistry or for biochips, etc.
Fig. 4. Reflection spectra for two directions in the photonic crystal, C– K and C–M, and two polarizations of light: perpendicular (H) and parallel (E) to the channel axis.
Fig. 5. Silica microtubes formed under the alkaline etching of the wafer back side. SEM image.
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E.V. Astrova et al. / Optical Materials 27 (2005) 827–830
5. Conclusion An experimental procedure to pattern macroporous silicon has been developed. The structures obtained using this technique, have deep vertical walls and, can be used to fabricate components for micromechanics and microphotonics. Possibilities for the technique have been demonstrated by the formation of PC stripes, suitable for IR spectral characterization. Furthermore, the method can be employed to obtain silica microtubes.
Acknowledgements The authors are grateful to A. Nashchekin and Yu. Pilyugina for help in obtaining SEM images and in preparing the illustrative material. The work was supported by INTAS (grant no. 01-0642), Program ‘‘Physics of Solid-State Nanostructures’’ of the Ministry of Science and Technology of the Russian Federation, Program ‘‘Optics and Laser Physics’’ of the Russian Academy of Sciences, and Presidential Program for
Support of Scientific Schools (NSh 758-2003.2), and HEA, Ireland. References [1] V. Lehmann, H. Foll, J. Electrochem. Soc. 137 (1990) 653. [2] V. Lehmann, U. Gruning, Thin Solid Films 297 (1997) 13. [3] H. Foll, M. Christophersen, J. Carstensen, G. Haase, Mater. Sci. Eng. R39 (2002) 93. [4] S. Ottow, V. Lehmann, H. Foll, J. Electrochem. Soc. 143 (1996) 385. [5] U. Gruning, V. Lehmann, S. Ottow, K. Busch, Appl. Phys. Lett. 68 (1996) 747. [6] S.W. Leonard, H.M. van Driel, K. Busch, S. John, A. Birner, A.P. Li, F. Muller, U. Gosele, V. Lehmann, Appl. Phys. Lett. 75 (1999) 3063. [7] E.V. Astrova, V.V. Ratnikov, A.D. Remenyuk, I.L. Shulpina, Phys. Status Solidi A 197 (2003) 16. [8] J.D. Joannopoulos, R.D. Meade, R.D. Winn, Photonic Crystals. Molding the Flow of Light, Princeton University Press, 1995, Appendix C, p.125. [9] C. Jamois, R.B. Wehrspohn, L.C. Andreani, C. Hermann, O. Hess, U. Gosele, Photonics and Nanostructures—Fundamentals and Applications 1 (2003) 1. [10] E.V. Astrova, T.N. Borovinskaya, T.S. Perova, M.V. Zamoryanskaya, Semiconductors 38 (2004) 1084. [11] L.T. Canham, Appl. Phys. Lett. 57 (1990) 1046. [12] O. Bisi, S. Ossicini, L. Pavesi, Surf. Sci. Rep. 38 (2000) 1.