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Determination of Vickers microhardness on porous silicon surfaces
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Thin Solid Films 516 (2008) 1961 – 1963 www.elsevier.com/locate/tsf
Determination of Vickers microhardness on porous silicon surfaces M. Morales-Masis a , A. Ramírez-Porras b,⁎ a
b
Department of Physics, Wright State University, Dayton OH 45435, United States Centro de Investigación en Ciencia e Ingeniería de Materiales (CICIMA) and Escuela de Física, Universidad de Costa Rica, San Pedro de Montes de Oca 2060, Costa Rica
Received 5 March 2007; received in revised form 27 August 2007; accepted 26 September 2007 Available online 4 October 2007
Abstract The Vickers microhardness values of two different sets of porous silicon layers were determined at applied load of 98 mN. The sets consisted of Boron-doped substrates anodized at diverse current densities for two different amounts of hydrofluoric acid (HF) in the etching solution. We found that the microhardness of the samples with lower content of HF at the anodization process showed higher values, whereas the Vickers parameter diminishes consistently for higher current densities. A possible explanation of this behavior is proposed. © 2007 Elsevier B.V. All rights reserved. Keywords: Vickers indentation; Microhardness; Porous silicon
1. Introduction The production of nanostructured silicon surfaces, in a variant named “porous silicon”, has attracted strong interest for more than a decade because of the possibility to develop optoelectronic devices such as light-emitting diodes [1]. In recent years the interest has turned to the production of sensingbased devices for chemical species in gas or liquid phase or for biomedical applications [2–7]. Nevertheless, a key point that presently has not been deeply studied is the question of mechanical stability [8,9]. Because the production of this particular material usually requires an electrochemical etching process, the control of characteristics such as morphology and porosity is hard to achieve in a fully predictable way. Some information related to this particular problem is provided in this work. 2. Experimental details Nanostructured silicon surfaces had been produced from commercially available crystalline boron-doped silicon with a resistivity in the 1.40–1.45 Ω·cm range and with a (100) ⁎ Corresponding author. Fax: +506 225 5511. E-mail address: [email protected] (A. Ramírez-Porras). 0040-6090/$ - see front matter © 2007 Elsevier B.V. All rights reserved. doi:10.1016/j.tsf.2007.09.048
crystallographic surface orientation. A back contact of aluminium was deposited by sputtering means and heated thereafter for 20 min at 400 °C in an nitrogen atmosphere to form an ohmic contact. The electrochemical etching process was performed in the following way: a Teflon cell with a circular piece (0.8 cm in diameter) of crystalline silicon at the bottom in contact with a solution of ethanoic hydrofluoric acid (HF) at room temperature in two distinct proportions: 12.5% and 25% acid content per volume. A rectangular piece of Pt foil immersed in the solutions constitutes the cathode of the setup, whereas the anode is placed to the back side of the semiconductor. The power was provided by a E3641A Agilent source. The current density was set to constant values ranging from 12.5 to 100 mA/cm2 for 20 min in all cases. After etching, the samples were rinsed with ethanol and deionised water and let to dry in the atmosphere at room temperature. The obtained porous region had diameters of around 4 mm. Microhardness has been determined by a Buehler Micromet 2001 tester, with load charge of 10 gf (corresponding to 98 mN) and armed with a Vickers indenter. The load application time was 15 s. 3. Results and discussion The first question that had to be answered was whether the indentation measurements are accurate enough to represent the
1962
M. Morales-Masis, A. Ramírez-Porras / Thin Solid Films 516 (2008) 1961–1963
Fig. 1. Typical imprint of a Vickers indenter onto a porous silicon surface (current density: 12.5 mA/cm2, acid concentration: 25% HF, load time: 15 s). The features in the upper right are foreign material attached to the surface (possibly dust from the environment).
hardness of the porous films, excluding the possible effect of the crystalline substrate. As stated elsewhere [8], the indenter imprint depth D and the thickness e of the porous layer must satisfy: D be
ð1Þ
For a Vickers indenter, D≈d/7 where d is the mean value of the imprint diagonals measured along the surface [10]. Fig. 1 shows a microphotograph taken with SEM of a typical imprint of a Vickers indenter onto a porous silicon surface. The preparation parameters were 25% HF and 12.5 mA/cm2. In this case, d ≈ 30 μm, and therefore D ≈ 4.5 μm. After exposing the cross-section of this
Fig. 2. Vickers hardness profile across the surface of two different samples with preparation conditions sketched in the figure. Hardness of typical diverse materials is included as horizontal lines for comparison.
Fig. 3. Mean Vickers hardness as a function of current density for two cases acid content, as marked in the figure. The straight lines are only a guide for the eye.
sample and determine the porous layer thickness, it turned out that e ≈ 18 μm, so that Eq. (1) is satisfied. This condition was fulfilled for all samples prepared. Fig. 2 shows the hardness profile obtained along the surface diameter of two specimens: one with 12.5% HF at 25 mA/cm 2 (dashed line), and the other with 25% HF at 12.5 mA/cm 2 (solid line). As a way of comparison, standard Vickers hardness levels (at the same load conditions) for different materials are also shown as horizontal bars: crystalline silicon, ISO 8037-7 slide glass, 304 austenitic stainless steel and high purity copper (oxygen free electronic grade, or OFE copper). For both curves, Vickers hardness coincides with crystalline silicon value near the edges of the samples, as expected, and
Fig. 4. Top view of a typical porous sample. Notice the presence of platelets all across the surface which arise from the drying process. The inset shows a higher magnification of the platelets.
M. Morales-Masis, A. Ramírez-Porras / Thin Solid Films 516 (2008) 1961–1963
drops strongly towards the centre of the porous region, where it remains in a fairly constant value (this will be called the “Mean Vickers Hardness”, MVH). In all cases, the MVH was lower than the OFE copper hardness, which is considered as a “soft” material. The MVH value is different for the two cases depicted in Fig. 2, which indicates a difference in their mechanical properties. Fig. 3 shows the values of the MVH as a function of current density for acid content of 12.5% (upper points) and 25% (lower points). The curves represent potential fittings of the form: MVH ¼ cJ a
ð2Þ
where J is the current density, c and α are fitting parameters. In these cases, α ≈ 0.5. Notice that layers grown with lower content of HF exhibit hardness values four times stronger than those with higher acid content. Some other studies on porous silicon [8,11] and on porous stainless steel [12] claim that there is a direct relation between hardness and porosity (which in turn is directly related with the current density in p-type layers). Nevertheless, the present samples show an opposite behavior. A possible explanation has been proposed by Mason and coworkers [13]. In their work, they classify the p-type porous layers in two regimes: the “low” and the “high” current densities, where the distinction between the two terms is dependent on the etching parameters (wafer resistivity, acid content and etching time). In the low current density regime, the dissolution of the silicon matrix is performed longitudinally along the electric field lines (“2D” structure, characterized by the presence of straight pillars), whereas in the high current density picture, the dissolution progresses both longitudinally and perpendicularly to those lines (“3D” structure, characterized by a morphology that resembles a coral). As a consequence, the 3D structure exhibits weaker stability to the drying process in the atmosphere, and a network of cracks and platelets arise. Fig. 4 shows that indeed this is the case in our samples. We therefore propose that our porous layers belong to the 3D structure, where a coral-like structure is present within the layers. The more acid content in the etching solution is used, the stronger the dissolution of the silicon matrix takes place and the lesser the value of the hardness. This explains why there are two distinct curves in Fig. 3. The negative slope can be understood in the following way: if the current density increases, so does the porosity [11] and therefore the hardness decreases, as is actually observed.
1963
4. Conclusions There are substantial differences on the hardness of porous silicon surfaces depending on both the current density and the acid content in the etching solution. We have found that the porous surfaces exhibit hardness values lower than those of high purity copper, and that the higher content of HF or stronger current density weakens the films on the application of mechanic loads. These results are important for eventual designing and manufacturing of sensor devices made up of this material. Acknowledgements The authors fully acknowledge the collaboration of the Vicerrectoría de Investigación of the Universidad de Costa Rica and the partial support of the Comisión Nacional de Investigaciones Científicas y Tecnológicas (CONICIT) providing founding from the FORINVES program. The collaboration of the Centro de Investigación en Estructuras Microscópicas (CIEMIC) is also acknowledged. References [1] N. Koshida, H. Koyama, Appl. Phys. Lett. 60 (1992) 347. [2] T. Taliercio, M. Dilhan, E. Massone, A.M. Gue, B. Fraisse, A. Foucaran, Thin Solid Films 255 (1995) 310. [3] C. Baratto, G. Faglia, G. Sberveglieri, L. Boarino, A.M. Rossi, G. Amato, Thin Solid Films 391 (2001) 261. [4] L. Pancheri, C.J. Oton, Z. Gaburro, G. Soncini, L. Pavesi, Sens. Actuators B, Chem. 89 (2003) 237. [5] L. De Stefano, L. Moretti, I. Rendina, A.M. Rossi, Sens. Actuators A, Phys. 104 (2003) 179. [6] R. Ravi Kumar Reddy, A. Chadha, E. Bhattacharya, Biosens. Bioelectron. 16 (2001) 313. [7] R. Ravi Kumar Reddy, I. Basu, E. Bhattacharya, A. Chadha, Current Appl. Phys. 3 (2003) 155. [8] S.P. Duttagupta, X.L. Chen, S.A. Jenekhe, P.M. Fauchet, Solid State Commun. 101 (1997) 33. [9] M. Morales-Masis, L. Segura, A. Ramirez-Porras, Appl. Surf. Sci. 253 (2007) 7188. [10] O. Vingsbo, S. Hogmark, B. Jönsson, A. Ingemarsson, in: P. Blau, B. Lawn (Eds.), Microindentation Techniques in Materials Science and Engineering, ASTM Spec. Tech. Publ., vol. 889, ASTM, 1986, p. 257. [11] O. Bisi, S. Ossicini, L. Pavesi, Surf. Sci. Rep. 38 (2000) 1. [12] F. Tancret, F. Osterstock, Philos. Mag. 83 (2003) 125. [13] M.D. Mason, D.J. Sirbuly, S.K. Burato, Thin Solid Films 406 (2002) 151.
Thin Solid Films 516 (2008) 1961 – 1963 www.elsevier.com/locate/tsf
Determination of Vickers microhardness on porous silicon surfaces M. Morales-Masis a , A. Ramírez-Porras b,⁎ a
b
Department of Physics, Wright State University, Dayton OH 45435, United States Centro de Investigación en Ciencia e Ingeniería de Materiales (CICIMA) and Escuela de Física, Universidad de Costa Rica, San Pedro de Montes de Oca 2060, Costa Rica
Received 5 March 2007; received in revised form 27 August 2007; accepted 26 September 2007 Available online 4 October 2007
Abstract The Vickers microhardness values of two different sets of porous silicon layers were determined at applied load of 98 mN. The sets consisted of Boron-doped substrates anodized at diverse current densities for two different amounts of hydrofluoric acid (HF) in the etching solution. We found that the microhardness of the samples with lower content of HF at the anodization process showed higher values, whereas the Vickers parameter diminishes consistently for higher current densities. A possible explanation of this behavior is proposed. © 2007 Elsevier B.V. All rights reserved. Keywords: Vickers indentation; Microhardness; Porous silicon
1. Introduction The production of nanostructured silicon surfaces, in a variant named “porous silicon”, has attracted strong interest for more than a decade because of the possibility to develop optoelectronic devices such as light-emitting diodes [1]. In recent years the interest has turned to the production of sensingbased devices for chemical species in gas or liquid phase or for biomedical applications [2–7]. Nevertheless, a key point that presently has not been deeply studied is the question of mechanical stability [8,9]. Because the production of this particular material usually requires an electrochemical etching process, the control of characteristics such as morphology and porosity is hard to achieve in a fully predictable way. Some information related to this particular problem is provided in this work. 2. Experimental details Nanostructured silicon surfaces had been produced from commercially available crystalline boron-doped silicon with a resistivity in the 1.40–1.45 Ω·cm range and with a (100) ⁎ Corresponding author. Fax: +506 225 5511. E-mail address: [email protected] (A. Ramírez-Porras). 0040-6090/$ - see front matter © 2007 Elsevier B.V. All rights reserved. doi:10.1016/j.tsf.2007.09.048
crystallographic surface orientation. A back contact of aluminium was deposited by sputtering means and heated thereafter for 20 min at 400 °C in an nitrogen atmosphere to form an ohmic contact. The electrochemical etching process was performed in the following way: a Teflon cell with a circular piece (0.8 cm in diameter) of crystalline silicon at the bottom in contact with a solution of ethanoic hydrofluoric acid (HF) at room temperature in two distinct proportions: 12.5% and 25% acid content per volume. A rectangular piece of Pt foil immersed in the solutions constitutes the cathode of the setup, whereas the anode is placed to the back side of the semiconductor. The power was provided by a E3641A Agilent source. The current density was set to constant values ranging from 12.5 to 100 mA/cm2 for 20 min in all cases. After etching, the samples were rinsed with ethanol and deionised water and let to dry in the atmosphere at room temperature. The obtained porous region had diameters of around 4 mm. Microhardness has been determined by a Buehler Micromet 2001 tester, with load charge of 10 gf (corresponding to 98 mN) and armed with a Vickers indenter. The load application time was 15 s. 3. Results and discussion The first question that had to be answered was whether the indentation measurements are accurate enough to represent the
1962
M. Morales-Masis, A. Ramírez-Porras / Thin Solid Films 516 (2008) 1961–1963
Fig. 1. Typical imprint of a Vickers indenter onto a porous silicon surface (current density: 12.5 mA/cm2, acid concentration: 25% HF, load time: 15 s). The features in the upper right are foreign material attached to the surface (possibly dust from the environment).
hardness of the porous films, excluding the possible effect of the crystalline substrate. As stated elsewhere [8], the indenter imprint depth D and the thickness e of the porous layer must satisfy: D be
ð1Þ
For a Vickers indenter, D≈d/7 where d is the mean value of the imprint diagonals measured along the surface [10]. Fig. 1 shows a microphotograph taken with SEM of a typical imprint of a Vickers indenter onto a porous silicon surface. The preparation parameters were 25% HF and 12.5 mA/cm2. In this case, d ≈ 30 μm, and therefore D ≈ 4.5 μm. After exposing the cross-section of this
Fig. 2. Vickers hardness profile across the surface of two different samples with preparation conditions sketched in the figure. Hardness of typical diverse materials is included as horizontal lines for comparison.
Fig. 3. Mean Vickers hardness as a function of current density for two cases acid content, as marked in the figure. The straight lines are only a guide for the eye.
sample and determine the porous layer thickness, it turned out that e ≈ 18 μm, so that Eq. (1) is satisfied. This condition was fulfilled for all samples prepared. Fig. 2 shows the hardness profile obtained along the surface diameter of two specimens: one with 12.5% HF at 25 mA/cm 2 (dashed line), and the other with 25% HF at 12.5 mA/cm 2 (solid line). As a way of comparison, standard Vickers hardness levels (at the same load conditions) for different materials are also shown as horizontal bars: crystalline silicon, ISO 8037-7 slide glass, 304 austenitic stainless steel and high purity copper (oxygen free electronic grade, or OFE copper). For both curves, Vickers hardness coincides with crystalline silicon value near the edges of the samples, as expected, and
Fig. 4. Top view of a typical porous sample. Notice the presence of platelets all across the surface which arise from the drying process. The inset shows a higher magnification of the platelets.
M. Morales-Masis, A. Ramírez-Porras / Thin Solid Films 516 (2008) 1961–1963
drops strongly towards the centre of the porous region, where it remains in a fairly constant value (this will be called the “Mean Vickers Hardness”, MVH). In all cases, the MVH was lower than the OFE copper hardness, which is considered as a “soft” material. The MVH value is different for the two cases depicted in Fig. 2, which indicates a difference in their mechanical properties. Fig. 3 shows the values of the MVH as a function of current density for acid content of 12.5% (upper points) and 25% (lower points). The curves represent potential fittings of the form: MVH ¼ cJ a
ð2Þ
where J is the current density, c and α are fitting parameters. In these cases, α ≈ 0.5. Notice that layers grown with lower content of HF exhibit hardness values four times stronger than those with higher acid content. Some other studies on porous silicon [8,11] and on porous stainless steel [12] claim that there is a direct relation between hardness and porosity (which in turn is directly related with the current density in p-type layers). Nevertheless, the present samples show an opposite behavior. A possible explanation has been proposed by Mason and coworkers [13]. In their work, they classify the p-type porous layers in two regimes: the “low” and the “high” current densities, where the distinction between the two terms is dependent on the etching parameters (wafer resistivity, acid content and etching time). In the low current density regime, the dissolution of the silicon matrix is performed longitudinally along the electric field lines (“2D” structure, characterized by the presence of straight pillars), whereas in the high current density picture, the dissolution progresses both longitudinally and perpendicularly to those lines (“3D” structure, characterized by a morphology that resembles a coral). As a consequence, the 3D structure exhibits weaker stability to the drying process in the atmosphere, and a network of cracks and platelets arise. Fig. 4 shows that indeed this is the case in our samples. We therefore propose that our porous layers belong to the 3D structure, where a coral-like structure is present within the layers. The more acid content in the etching solution is used, the stronger the dissolution of the silicon matrix takes place and the lesser the value of the hardness. This explains why there are two distinct curves in Fig. 3. The negative slope can be understood in the following way: if the current density increases, so does the porosity [11] and therefore the hardness decreases, as is actually observed.
1963
4. Conclusions There are substantial differences on the hardness of porous silicon surfaces depending on both the current density and the acid content in the etching solution. We have found that the porous surfaces exhibit hardness values lower than those of high purity copper, and that the higher content of HF or stronger current density weakens the films on the application of mechanic loads. These results are important for eventual designing and manufacturing of sensor devices made up of this material. Acknowledgements The authors fully acknowledge the collaboration of the Vicerrectoría de Investigación of the Universidad de Costa Rica and the partial support of the Comisión Nacional de Investigaciones Científicas y Tecnológicas (CONICIT) providing founding from the FORINVES program. The collaboration of the Centro de Investigación en Estructuras Microscópicas (CIEMIC) is also acknowledged. References [1] N. Koshida, H. Koyama, Appl. Phys. Lett. 60 (1992) 347. [2] T. Taliercio, M. Dilhan, E. Massone, A.M. Gue, B. Fraisse, A. Foucaran, Thin Solid Films 255 (1995) 310. [3] C. Baratto, G. Faglia, G. Sberveglieri, L. Boarino, A.M. Rossi, G. Amato, Thin Solid Films 391 (2001) 261. [4] L. Pancheri, C.J. Oton, Z. Gaburro, G. Soncini, L. Pavesi, Sens. Actuators B, Chem. 89 (2003) 237. [5] L. De Stefano, L. Moretti, I. Rendina, A.M. Rossi, Sens. Actuators A, Phys. 104 (2003) 179. [6] R. Ravi Kumar Reddy, A. Chadha, E. Bhattacharya, Biosens. Bioelectron. 16 (2001) 313. [7] R. Ravi Kumar Reddy, I. Basu, E. Bhattacharya, A. Chadha, Current Appl. Phys. 3 (2003) 155. [8] S.P. Duttagupta, X.L. Chen, S.A. Jenekhe, P.M. Fauchet, Solid State Commun. 101 (1997) 33. [9] M. Morales-Masis, L. Segura, A. Ramirez-Porras, Appl. Surf. Sci. 253 (2007) 7188. [10] O. Vingsbo, S. Hogmark, B. Jönsson, A. Ingemarsson, in: P. Blau, B. Lawn (Eds.), Microindentation Techniques in Materials Science and Engineering, ASTM Spec. Tech. Publ., vol. 889, ASTM, 1986, p. 257. [11] O. Bisi, S. Ossicini, L. Pavesi, Surf. Sci. Rep. 38 (2000) 1. [12] F. Tancret, F. Osterstock, Philos. Mag. 83 (2003) 125. [13] M.D. Mason, D.J. Sirbuly, S.K. Burato, Thin Solid Films 406 (2002) 151.