- Email: [email protected]
Buffer layer influence on guiding properties of oxidized porous silicon waveguides
Available online at www.sciencedirect.com
Physica E 16 (2003) 574 – 579 www.elsevier.com/locate/physe
Bu"er layer in$uence on guiding properties of oxidized porous silicon waveguides M. Balucania;∗ , V. Bondarenkob , N. Vorozovb , A. Ferraria a Electronics
Department, Engineering Faculty, INFM UNIT E6 University of Rome “LA SAPIENZA”, Rome, Italy b Belarussian State University of Informatics & Radioelectronics, Minsk, Belarus
Abstract We studied the in$uence of the thickness and porosity of the bu"er layer on the guiding properties of oxidized porous silicon waveguides (OPSWG). It is demonstrated how a modi7ed anodization process acts on the porosity of the 7nal oxidized porous silicon. In this way, it is possible to control the refractive index jump between the core of OPSWG made of compact silicon dioxide and the bottom bu"er layer made of porous silicon dioxide. The adoption of a double-step anodization process decreases the propagation losses to 0:5 dB=cm against the 8 dB=cm measured for the waveguide realized using a single-step anodization. The main reason seems not to be the increase of the di"erence of refractive index values but the more homogeneous bu"er layer obtained along the core of the waveguide. This homogeneous layer permits a better lateral con7nement of the light as demonstrated by spatial refractive index pro7le measurement. ? 2002 Elsevier Science B.V. All rights reserved. Keywords: Porous silicon; Waveguides; Optoelectronics
1. Introduction Porous silicon (PS) was 7rst discovered in 1956 by Uhlir and until Canham showed its photoluminescence proprieties [1], PS was mainly used to form thick SiO2 and Si3 N4 7lms [2,3]. Only in the 1990s the PS gained attention as a material to develop a complete silicon-based optoelectronic system [4]. In the optoelectronics application, PS structures are also used as guiding material in the infrared and visible
∗ Corresponding author. Tel.: +39-06-44-585-46; fax: +39-0644-585-409. E-mail address: [email protected] (M. Balucani).
range [5,6]. Obviously, the more interesting properties of PS waveguides are in the visible range where silicon does not have guiding proprieties. In this scenario, the oxidized porous silicon waveguide (OPSWG) represents a promising opportunity for the fabrication of CMOS compatible waveguides for visible light [7]. By modulating the anodization process during the formation of porous silicon it is possible to modify the porosity of the 7nal oxidized porous silicon. In this way, it is possible to control the thickness and the refractive index jump between the core of the OPSWG made of dense silicon dioxide (i.e. refractive index = 1:46) and the top and the bottom bu"er layer made of porous silicon dioxide (i.e. refractive index lower than 1.46).
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2. Process description The single-step OPSWG was realized on (1 0 0) silicon wafers of 0:01 Ohm cm-resistivity heavily doped with antimony. After standard chemical cleaning in the RCA solution, a 0:24-m-thick silicon nitride layer was deposited directly on the surface of the wafers by CVD. This layer served as a mask in the course of anodization which was performed to convert exposed regions into porous silicon. Using conventional photolithography, the silicon nitride layer was patterned to de7ne the layout of optical waveguides. Plasma etching the exposed regions of the silicon nitride layer was used to form openings in the mask. The photoresist was removed and wafers were chemically cleaned in the RCA solution. Selective anodization of silicon was performed in the electrolyte consisting of hydro$uoric acid and ethylene glycol. The anodization process was realized in galvanostatic regime in the dark by stirring the electrolyte during the process. Once anodization was performed, the silicon nitride mask layer was removed by etching in 40% HF aqueous solution. Then wafers were rinsed in deionized water for 20 min. Next, thermal oxidation was carried out. A three-step regime was used for thermal oxidation: (i) 300◦ C for 60 min in dry O2 , (ii) 1000◦ C for 25 min in wet O2 , and (iii) 1150◦ C for 60 min in dry O2 . In the course of the oxidation process, 0:56-m-thick silica was grown on the surface of the silicon wafers. The resulting structure of the single-step anodization OPSWG is structured in three regions as illustrated by the SEM picture reported in Fig. 1. The second kind of OPSWG, reported in Fig. 2, was realized by a two-step anodization process. The 7rst anodization step was performed at a current density of 40 mA=cm2 for 225 s providing a waveguide with 7–8 m porous silicon and in a further oxidation step this layer will form the dense oxidized porous silicon for the core waveguiding layer (region B). Then the current density was raised to 80 mA=cm2 without stopping the process and the samples were anodized for additional 45 s providing a waveguide periphery with 1 m porous silicon of much higher porosity in comparison with the 7rst porous layer. On further oxidation this layer will form the oxidized porous silicon that is the peripheral bu"er (cladding) layer (region C).
Fig. 1. SEM cross section of single-step anodization OPSWG. A, B and C are, respectively, the cladding, the core and the bu"er (cladding) layer.
Fig. 2. SEM cross section of two-step anodization OPSWG. A, B and C are, respectively, the cladding, the core and the bu"er (cladding) layer.
3. Results and discussion By looking at Figs. 1 and 2 it is possible to recognize the top and the bottom cladding region (letter A and C) made of porous material, and a core region (letter B) made of dense material: SiO2 . The porosity and the structure of each of these regions (A–C)
576
M. Balucani et al. / Physica E 16 (2003) 574 – 579
Fig. 3. Expected OPSWG refractive index pro7le.
Fig. 4. (a) Spatial distribution of the refractive index of single-step OPSWG, (b) Single-step OPSWG refractive index along X direction for di"erent Y cuts, and (c) Single-step OPSWG refractive index along Y direction for di"erent X deep cuts.
M. Balucani et al. / Physica E 16 (2003) 574 – 579
577
Fig. 4. continued.
depends mainly on the anodization regimes used during PS formation and the oxidation parameters. The expected refractive index pro7le is presented in Fig. 3 where up to X1 is region A. Region B is between X2 and X1 and region C extends from X2 to X3 , after that there is the silicon substrate. Refractive near 7eld (RNF) measurements have been carried out and the RNF measurements done on the OPSWG can be seen, roughly, as silica on silicon structures [8]. The RNF measurement were carried out with a wavelength of 670 nm and the refractive index of silicon is well above the maximum allowed by the RNF equipment (1.62) and appears saturated. Fig. 4a reports the spatial distribution of the refractive index on the cross section of a OPSWG realized by single-step anodization process and with di"erent width of the opening mask. The darker the color, the higher is the refractive index. The quantitative analysis of the refractive index along X direction of the waveguide cross section is
plotted in Fig. 4b. It is evident how a good refractive index jump (Ln = 0:05) for values of Y corresponding to the middle of the OPSWG (Y = 20 m; w = 9 m; Y =22 m; w=7 m; Y =21 m; w=5 m; Y = 23 m; w = 3 m) is obtained between the core and the bottom cladding region. However, this di"erence is present only in the lower part of the WG, as can be seen in Fig. 4c in which the refractive index values along Y directions is plotted. The Ln between the lower cladding layer and the core region disappears in proximity of the wafer surface (line sketched wide). These measurements demonstrate how a bad isolation of the core against the substrate is obtained using a single-step anodization process during PS formation. This e"ect is especially evident near the wafer surface causing the bad guiding properties of the structure. For this kind of OPSWG propagation losses of 5 –8 dB=cm have been measured at wavelengths of 632, 830 and 980 nm. To decrease the losses to the
578
M. Balucani et al. / Physica E 16 (2003) 574 – 579
Fig. 4. continued.
substrates the double-step anodization process was used to form PS, obtaining a more homogeneous lower bu"er (cladding) layer with a high porosity. It is visible, from Fig. 2, how the thickness of the lower bu"er region (C) is constant along all the periphery of the WG core realizing a good vertical and lateral con7nement. These results are con7rmed by the refractive index pro7le measurements. The refractive index jump has a value of 0.03 and this value is maintained constant also in wafer surface proximity. The refractive index pro7le in proximity of OPSWG surface is presented in Fig. 5a and b for di"erent widths of the opening in the anodization mask. The e"ect of the lateral con7nement induced by the adoption of the double-step anodization process is demonstrated by the propagation loss measurement, which gave values
of 0:41 dB=cm exciting the OPSWG with an Argon laser. 4. Conclusions The introduction of the double-step anodization process permits to obtain WG which shows a better light con7nement and lower propagation losses due to the realization of a lower cladding layer with constant porosity and thickness along all the periphery of the WG and especially in the proximity of the wafer surface. This result was impossible to obtain using a single-step approach. Furthermore, by changing the process parameter during the second anodization process it is possible to modify the thickness and the
M. Balucani et al. / Physica E 16 (2003) 574 – 579
579
Fig. 5. Double-step OPSWG refractive index along Y direction for di"erent mask opening at the center of the waveguide, and (b) Zoom of (a).
porosity of the bu"er layer in order to improve the guiding performance of the OPSWG.
[3] [4]
Acknowledgements This work was fully supported by the European Community Project OLSI No. 28934 (Optical Links in silicon). References [1] L.T. Canham, Appl. Phys. Lett. 57 (1990) 1046. [2] G. Bomchil, R. Herino, K. Barla, in: V.T. Nguyen, A. Cullis (Eds.), Energy Bea-Solid Interactions and Transient
[5] [6] [7]
[8]
Thermal Processes, Les Editions de Physique, Les Ulis, 1986, pp. 463. R.L. Smith, S.D. Collins, Sensor Actuators A A23 (1990) 830. M. Balucani, G. Lamedica, L. Franchina, L. Dolgyi, V. Yakovtseva, N. Vorozov, N. Kazuchits, V. Bondarenko, A. Ferrari, J. Porous Mater. 7 (2000) 215. H.F. Arrand, T.M. Benson, P. Sewell, A. Loni, R.J. Bozeat, R. Arens-Fisher, M. KrRuger, M. ThRonissen, IEEE J. Sel. Top. Quantum Electron. 4 (6) (1998) 975. G. Maiello, S. La Monica, A. Ferrari, G. Masini, V.P. Bondarenko, A.M. Dorofeev, N.M. Kazuchits, Thin Solid Films 297 (1997) 311. M. Balucani, V. Bondarenko, E. Fazio, G. La medica, A. Ricciarelli, E. Viarengo, H.F. Bulbuis, F.M. van der Vliet, J.F. Broquin, G. Vitran, IEEE LEOS 99, Vol. 2, 1999, pp. 643– 644. P. Oberson, B. Gisin, B. Huttner, N. Gisin, Appl. Opt. 37 (1998) 7268.
Physica E 16 (2003) 574 – 579 www.elsevier.com/locate/physe
Bu"er layer in$uence on guiding properties of oxidized porous silicon waveguides M. Balucania;∗ , V. Bondarenkob , N. Vorozovb , A. Ferraria a Electronics
Department, Engineering Faculty, INFM UNIT E6 University of Rome “LA SAPIENZA”, Rome, Italy b Belarussian State University of Informatics & Radioelectronics, Minsk, Belarus
Abstract We studied the in$uence of the thickness and porosity of the bu"er layer on the guiding properties of oxidized porous silicon waveguides (OPSWG). It is demonstrated how a modi7ed anodization process acts on the porosity of the 7nal oxidized porous silicon. In this way, it is possible to control the refractive index jump between the core of OPSWG made of compact silicon dioxide and the bottom bu"er layer made of porous silicon dioxide. The adoption of a double-step anodization process decreases the propagation losses to 0:5 dB=cm against the 8 dB=cm measured for the waveguide realized using a single-step anodization. The main reason seems not to be the increase of the di"erence of refractive index values but the more homogeneous bu"er layer obtained along the core of the waveguide. This homogeneous layer permits a better lateral con7nement of the light as demonstrated by spatial refractive index pro7le measurement. ? 2002 Elsevier Science B.V. All rights reserved. Keywords: Porous silicon; Waveguides; Optoelectronics
1. Introduction Porous silicon (PS) was 7rst discovered in 1956 by Uhlir and until Canham showed its photoluminescence proprieties [1], PS was mainly used to form thick SiO2 and Si3 N4 7lms [2,3]. Only in the 1990s the PS gained attention as a material to develop a complete silicon-based optoelectronic system [4]. In the optoelectronics application, PS structures are also used as guiding material in the infrared and visible
∗ Corresponding author. Tel.: +39-06-44-585-46; fax: +39-0644-585-409. E-mail address: [email protected] (M. Balucani).
range [5,6]. Obviously, the more interesting properties of PS waveguides are in the visible range where silicon does not have guiding proprieties. In this scenario, the oxidized porous silicon waveguide (OPSWG) represents a promising opportunity for the fabrication of CMOS compatible waveguides for visible light [7]. By modulating the anodization process during the formation of porous silicon it is possible to modify the porosity of the 7nal oxidized porous silicon. In this way, it is possible to control the thickness and the refractive index jump between the core of the OPSWG made of dense silicon dioxide (i.e. refractive index = 1:46) and the top and the bottom bu"er layer made of porous silicon dioxide (i.e. refractive index lower than 1.46).
1386-9477/03/$ - see front matter ? 2002 Elsevier Science B.V. All rights reserved. PII: S 1 3 8 6 - 9 4 7 7 ( 0 2 ) 0 0 6 4 9 - 5
M. Balucani et al. / Physica E 16 (2003) 574 – 579
575
2. Process description The single-step OPSWG was realized on (1 0 0) silicon wafers of 0:01 Ohm cm-resistivity heavily doped with antimony. After standard chemical cleaning in the RCA solution, a 0:24-m-thick silicon nitride layer was deposited directly on the surface of the wafers by CVD. This layer served as a mask in the course of anodization which was performed to convert exposed regions into porous silicon. Using conventional photolithography, the silicon nitride layer was patterned to de7ne the layout of optical waveguides. Plasma etching the exposed regions of the silicon nitride layer was used to form openings in the mask. The photoresist was removed and wafers were chemically cleaned in the RCA solution. Selective anodization of silicon was performed in the electrolyte consisting of hydro$uoric acid and ethylene glycol. The anodization process was realized in galvanostatic regime in the dark by stirring the electrolyte during the process. Once anodization was performed, the silicon nitride mask layer was removed by etching in 40% HF aqueous solution. Then wafers were rinsed in deionized water for 20 min. Next, thermal oxidation was carried out. A three-step regime was used for thermal oxidation: (i) 300◦ C for 60 min in dry O2 , (ii) 1000◦ C for 25 min in wet O2 , and (iii) 1150◦ C for 60 min in dry O2 . In the course of the oxidation process, 0:56-m-thick silica was grown on the surface of the silicon wafers. The resulting structure of the single-step anodization OPSWG is structured in three regions as illustrated by the SEM picture reported in Fig. 1. The second kind of OPSWG, reported in Fig. 2, was realized by a two-step anodization process. The 7rst anodization step was performed at a current density of 40 mA=cm2 for 225 s providing a waveguide with 7–8 m porous silicon and in a further oxidation step this layer will form the dense oxidized porous silicon for the core waveguiding layer (region B). Then the current density was raised to 80 mA=cm2 without stopping the process and the samples were anodized for additional 45 s providing a waveguide periphery with 1 m porous silicon of much higher porosity in comparison with the 7rst porous layer. On further oxidation this layer will form the oxidized porous silicon that is the peripheral bu"er (cladding) layer (region C).
Fig. 1. SEM cross section of single-step anodization OPSWG. A, B and C are, respectively, the cladding, the core and the bu"er (cladding) layer.
Fig. 2. SEM cross section of two-step anodization OPSWG. A, B and C are, respectively, the cladding, the core and the bu"er (cladding) layer.
3. Results and discussion By looking at Figs. 1 and 2 it is possible to recognize the top and the bottom cladding region (letter A and C) made of porous material, and a core region (letter B) made of dense material: SiO2 . The porosity and the structure of each of these regions (A–C)
576
M. Balucani et al. / Physica E 16 (2003) 574 – 579
Fig. 3. Expected OPSWG refractive index pro7le.
Fig. 4. (a) Spatial distribution of the refractive index of single-step OPSWG, (b) Single-step OPSWG refractive index along X direction for di"erent Y cuts, and (c) Single-step OPSWG refractive index along Y direction for di"erent X deep cuts.
M. Balucani et al. / Physica E 16 (2003) 574 – 579
577
Fig. 4. continued.
depends mainly on the anodization regimes used during PS formation and the oxidation parameters. The expected refractive index pro7le is presented in Fig. 3 where up to X1 is region A. Region B is between X2 and X1 and region C extends from X2 to X3 , after that there is the silicon substrate. Refractive near 7eld (RNF) measurements have been carried out and the RNF measurements done on the OPSWG can be seen, roughly, as silica on silicon structures [8]. The RNF measurement were carried out with a wavelength of 670 nm and the refractive index of silicon is well above the maximum allowed by the RNF equipment (1.62) and appears saturated. Fig. 4a reports the spatial distribution of the refractive index on the cross section of a OPSWG realized by single-step anodization process and with di"erent width of the opening mask. The darker the color, the higher is the refractive index. The quantitative analysis of the refractive index along X direction of the waveguide cross section is
plotted in Fig. 4b. It is evident how a good refractive index jump (Ln = 0:05) for values of Y corresponding to the middle of the OPSWG (Y = 20 m; w = 9 m; Y =22 m; w=7 m; Y =21 m; w=5 m; Y = 23 m; w = 3 m) is obtained between the core and the bottom cladding region. However, this di"erence is present only in the lower part of the WG, as can be seen in Fig. 4c in which the refractive index values along Y directions is plotted. The Ln between the lower cladding layer and the core region disappears in proximity of the wafer surface (line sketched wide). These measurements demonstrate how a bad isolation of the core against the substrate is obtained using a single-step anodization process during PS formation. This e"ect is especially evident near the wafer surface causing the bad guiding properties of the structure. For this kind of OPSWG propagation losses of 5 –8 dB=cm have been measured at wavelengths of 632, 830 and 980 nm. To decrease the losses to the
578
M. Balucani et al. / Physica E 16 (2003) 574 – 579
Fig. 4. continued.
substrates the double-step anodization process was used to form PS, obtaining a more homogeneous lower bu"er (cladding) layer with a high porosity. It is visible, from Fig. 2, how the thickness of the lower bu"er region (C) is constant along all the periphery of the WG core realizing a good vertical and lateral con7nement. These results are con7rmed by the refractive index pro7le measurements. The refractive index jump has a value of 0.03 and this value is maintained constant also in wafer surface proximity. The refractive index pro7le in proximity of OPSWG surface is presented in Fig. 5a and b for di"erent widths of the opening in the anodization mask. The e"ect of the lateral con7nement induced by the adoption of the double-step anodization process is demonstrated by the propagation loss measurement, which gave values
of 0:41 dB=cm exciting the OPSWG with an Argon laser. 4. Conclusions The introduction of the double-step anodization process permits to obtain WG which shows a better light con7nement and lower propagation losses due to the realization of a lower cladding layer with constant porosity and thickness along all the periphery of the WG and especially in the proximity of the wafer surface. This result was impossible to obtain using a single-step approach. Furthermore, by changing the process parameter during the second anodization process it is possible to modify the thickness and the
M. Balucani et al. / Physica E 16 (2003) 574 – 579
579
Fig. 5. Double-step OPSWG refractive index along Y direction for di"erent mask opening at the center of the waveguide, and (b) Zoom of (a).
porosity of the bu"er layer in order to improve the guiding performance of the OPSWG.
[3] [4]
Acknowledgements This work was fully supported by the European Community Project OLSI No. 28934 (Optical Links in silicon). References [1] L.T. Canham, Appl. Phys. Lett. 57 (1990) 1046. [2] G. Bomchil, R. Herino, K. Barla, in: V.T. Nguyen, A. Cullis (Eds.), Energy Bea-Solid Interactions and Transient
[5] [6] [7]
[8]
Thermal Processes, Les Editions de Physique, Les Ulis, 1986, pp. 463. R.L. Smith, S.D. Collins, Sensor Actuators A A23 (1990) 830. M. Balucani, G. Lamedica, L. Franchina, L. Dolgyi, V. Yakovtseva, N. Vorozov, N. Kazuchits, V. Bondarenko, A. Ferrari, J. Porous Mater. 7 (2000) 215. H.F. Arrand, T.M. Benson, P. Sewell, A. Loni, R.J. Bozeat, R. Arens-Fisher, M. KrRuger, M. ThRonissen, IEEE J. Sel. Top. Quantum Electron. 4 (6) (1998) 975. G. Maiello, S. La Monica, A. Ferrari, G. Masini, V.P. Bondarenko, A.M. Dorofeev, N.M. Kazuchits, Thin Solid Films 297 (1997) 311. M. Balucani, V. Bondarenko, E. Fazio, G. La medica, A. Ricciarelli, E. Viarengo, H.F. Bulbuis, F.M. van der Vliet, J.F. Broquin, G. Vitran, IEEE LEOS 99, Vol. 2, 1999, pp. 643– 644. P. Oberson, B. Gisin, B. Huttner, N. Gisin, Appl. Opt. 37 (1998) 7268.