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Mechanical properties of porous silicon and oxidized porous silicon by nanoindentation technique
Materials Science & Engineering A 711 (2018) 470–475
Contents lists available at ScienceDirect
Materials Science & Engineering A journal homepage: www.elsevier.com/locate/msea
Mechanical properties of porous silicon and oxidized porous silicon by nanoindentation technique
T
⁎
Souheyla Fakiria,c, , Alex Montagneb, Khadidja Rahmounc, Alain Iostb, Katir Ziouchea a
Univ. Lille, CNRS, UMR 8520 - IEMN, F-59000 Lille, France Arts et Métiers ParisTech, campus de Lille, MSMP, 8 Boulevard Louis XIV, CS 50008-59046 Lille Cedex, France c Unité de Recherche Matériaux et Energies Renouvelables, URMER, Université de Tlemcen Abou Bekr Belkaid, B.P. 119, 13000 Tlemcen, Algérie b
A R T I C L E I N F O
A B S T R A C T
Keywords: Mesoporous silicon Oxidation Young's modulus Hardness Porosity Micro cracks
A study of mechanical properties of mesoporous silicon (PS) is presented in this article. PS was prepared by electrochemical etching of a heavily doped P++ silicon wafer in a hydrofluoric acid electrolyte. The mechanical properties of PS and oxidized PS obtained by thermal treatment, were characterized by the nanoindentation technique associated to the continuous stiffness measurement option. The morphology of PS and oxidized PS were both characterized by scanning electron microscope. It is shown that the Young's modulus and hardness are related to the PS preparing conditions and decrease with increasing porosity. In particular, oxidation improves the mechanical properties of the mesoporous silicon. Surprisingly, modulus and hardness decrease with penetration depth, whereas a compaction could be expected resulting in a rising modulus and hardness. These results are mainly attributed to micro cracks formation, highlighted by focused ion beam cross section.
1. Introduction Due to its interesting physical properties, porous silicon (PS) is one of materials that attracts more attention since its discovery. Numerous studies have been proposed to characterize it [1–3]. This material has found applications in many devices such as electromagnetic interference filtering radiofrequency (EMIFRF) circuits [4], nanoelectromechanical systems (NEMS) [5], emitters [1], micro-hotplates [2], rechargeable Li-ion battery anode [6] and photonic crystal sensors [7]. PS can be prepared as thin layers or multilayers systems [8–11]. The thermal conductivity of PS is lower than that of mono-crystalline silicon and can be used as thermal insulator in microsystems [12]. This characteristic can find an application in a micro flow meter, where a large periodic thermal conductivity to the substrate surface is needed [3–14]. Monocrystalline PS is mainly classified according to the pore size as nanoporous (≤ 2 nm), mesoporous (2–50 nm) and macroporous silicon (> 50 nm). Thermal properties are primarily related to the porosity [15]; in addition, the dielectric constant of PS can be reduced with increasing porosity [16]. However porosity considerably weakens the structures. Consequently, it becomes essential to characterize precisely the mechanical behaviour of such structures to optimize the manufacturing process in order to reach mechanical requirements. The first studies on the mechanical characteristics of PS were performed in 1997 [17,18]. Numerous researches have been proposed to improve
⁎
understanding of the influence of morphology and structure of PS on its mechanical properties, such as micro indentation [19]. These authors examine numerous issues related to the mechanical properties of the mesoporous silicon, such that, the variation in hardness of aged porous silicon low dielectric constant. Nanoindentation has been used to show influence of both PS microstructure and porosity on the mechanical properties [20,21]. In the present study, mechanical properties are investigated using the nanoindentation technique associated to the continuous stiffness measurement method (CSM) [22]. This method has shown its reliability for determining the mechanical properties of materials at the microand nanoscale. This article analyzes the evolutions of hardness and Young's modulus of PS and oxidized PS regarding the compaction of PS and the rupture of the sponge-like structure. 2. Experimental 2.1. Sample preparation The mesoporous silicon is obtained by electrochemical anodization of heavily doped P++ type silicon (resistivity ρ = 0.009 ± 0.01 Ω cm, thickness about 380 ± 25 µm and {100} crystal orientation) in a double-tank cell (AMMT®). The concentration of the electrolyte (by volume) is 27% of hydrofluoric acid, 35% of pure ethanol C2H5OH and
Corresponding author at: Unité de Recherche Matériaux et Energies Renouvelables, URMER, Université de Tlemcen Abou Bekr Belkaid, B.P. 119, 13000 Tlemcen, Algérie E-mail addresses: [email protected] (S. Fakiri), [email protected] (K. Ziouche).
https://doi.org/10.1016/j.msea.2017.11.013 Received 1 August 2017; Received in revised form 4 November 2017; Accepted 6 November 2017 Available online 08 November 2017 0921-5093/ © 2017 Published by Elsevier B.V.
Materials Science & Engineering A 711 (2018) 470–475
S. Fakiri et al.
Table 1 Manufacturing parameters and description of PS and oxidized PS. Sample As-prepared PS
Oxidized PS
PS1 PS2 PS3 PS4 PS5 PS6 PS7 PS8 PS1O PS2O PS3O PS4O
Current density [mA/cm2]
Anodizing time [s]
Porosity [%]
Thickness [µm]
Oxidation temperature [°C]
20 40 60 80 20 20 40 40 20 40 60 80
5400 5400 3600 3600 1800 3600 1800 3600 5400 5400 3600 3600
44.44 58.17 59.66 68.00 38.16 41.86 45.71 52.55
110 190 170 206 41.17 78.7 57.07 113
Unoxidized Unoxidized Unoxidized Unoxidized Unoxidized Unoxidized Unoxidized Unoxidized 550 550 550 550
Fig. 1. Evolution of thickness and porosity of PS as a function of (a) time for two anodizing current density: 20 mA/cm2 and 40 mA/cm2 and, of (b) current density for a constant anodizing time of 60 min.
Fig. 2. SEM photographs of PS surface and PS in trench: (a, b) sample PS6; (c, d) sample PS4.
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porosity are shown in Table 1. Fig. 1.a shows both the linear evolution of the porosity and the thickness of the PS as a function of etching time for two anodization current densities (20 mA/cm2 and 40 mA/cm2). We note, as expected, that both thickness and porosity increase linearly as a function of time. In the same way, we observe that they also both increase in function of the current density for a same anodization time set to 60 min (see Fig. 1.b). 2.2. Morphological characterizations To understand the mechanisms involved in the mechanical properties of PS, we studied accurately the structuration of the as-prepared and oxidized mesoporous silicon. Figs. 2 and 4 show the surface and the cross section morphologies of the PS. We can note that the surfaces of the silicon are perforated with non-circular holes. The depth structuring in the trenches are columnar with ramification like sponges. This is explained by the parallels current lines passing through the substrate. As examples, Fig. 2 shows SEM images of different PS whose fabrication is described in Table 1 (a and b correspond to PS6 sample and c and d to PS4 sample). It can be seen that the pore shapes are oval on the surface as shown on Fig. 2.a and Fig. 2.c. The size of the pores for the PS6 sample is smaller than those of PS4 sample (R1 ~7 nm and R2 ~10 nm for PS6 sample and R1 ~8 nm and R2 ~17 nm for PS4 sample, where R1 and R2 are the semi-minor and semi-major axis respectively). The sizes of the pores evolve with the current density but also with the anodizing time (Fig. 1). At a constant current density the growth rate of pores is in the order of ~ 1 nm / 10 min. Although the etching of porous silicon mainly follows the current lines, the anodization time causes a lateral etching resulting in an increased pore size. Fig. 3 shows the relationship between the porosity and the pore diameter. The following figures (Fig. 4) show the morphologies of the surface
Fig. 3. Evolution of the pore diameter as function of the porosity.
38% of deionized water. The dispositive allows to realize uniform circular cells of 2.4 in. diameter in the center of the 3 in. diameter silicon substrate. Several samples of different porosities were made by changing anodizing conditions (courant density varying between 20 to 80 mA/cm2 and etching time increasing between 30 to 90 min). Anodization is performed at room temperature in a clean and controlled environment. The determination of the porosity was carried out by m −m gravimetric method [23,24]. The porosity is given by P = m1 − m2 , where 1 3 m1 and m2 are respectively the masses of the silicon sample before and after the anodization and m3 is the mass after etching of the porous layer in a sodium hydroxide solution. The masses of samples (about 0.25 in. square surface) are measured in the microgram range using a Mettler Toledo® microbalance with extreme accuracy (0.1 µg). Both surface morphology and volume structure as well as the thickness of PS and oxidized PS were characterized by scanning electron microscopy (SEM). For each sample of PS, anodizing conditions, thickness and
Fig. 4. SEM photographs of oxidized PS surface and in trench: (a, b) sample PS1O; (c, d) sample PS4O.
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Fig. 5. Hardness and Young's elastic modulus as a function displacement into surface of PS (a, b) PS6; (c, d) PS3; (e, f) PS4.
Fig. 6. Hardness and Young's elastic modulus of PS as a function of porosity.
measurement mode (CSM). The tip area function is calibrated by means of indentation in a reference material of known modulus (fused quartz, E = 72 GPa). The harmonic displacement was set to 2 nm with a 45 Hz frequency. The maximum penetration depth was set to 2000 nm conducted with a constant strain rate of 0.05 s−1. Thus the penetration depth did not exceed a tenth of the total thickness of the film (1/10 Bückle rule of thumb [25]). Doing this, it reduced the influence of the substrate (bulk silicon) on the mechanical properties of the porous silicon. In order to reduce discrepancy, 9 indentation tests were performed for each sample. Fig. 5 illustrates the evolution of hardness (H) and Young's modulus (E) as a function of penetration depth for as prepared PS. As shown in Fig. 5.a, 5.c and 5.e, it is observed that the hardness is lower than that of the porous silicon (12 GPa). Thus, for low depths of penetration the hardness is the same regardless of the porosity. As the penetration depth continues to increase, hardness slightly decreases and tends to
and in the trench for oxidized PS (a, b to PS1O and c, d to PS4O, see Table 1). Oxidation occurs along all the height of columnar porosities using annealing at 550 °C in atmosphere controlled (N2 and air). The rate of oxidation obtained is about 20% (determined by Energy Dispersive X-ray Spectroscopy into the SEM). A higher rate causes swelling of the porous structure and can damage it. Fig. 2.c and Fig. 4.c, corresponding to the same sample without oxidation (PS4) and with oxidation (PS4O) respectively, show that the size of the pores is reduced by the effect of the annealing treatment. This is explained by the growth of silicon oxide (SiO2) from the silicon nanocrystallites. 2.3. Nano indentation measurements The mechanical properties of PS were investigated by nanoindentation using a Nano indenter XP (MTS System®) equipped with a Berkovich diamond indenter, using the continuous stiffness 473
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of the modulus by a half for the sample PS4 and a poor overlapping. Thus, it is observed that for porosities above 60%, the structures of the samples become unstable and the measurements are not reproducible. The decrease of hardness and modulus with the penetration depth is quite different from what is classically reported for porous materials [26]. Indeed, for porous silicon, hardness and Young's modulus should increase with penetration due to the compaction effect. In our case, we do not observe this phenomenon. To obtain a better overview on this unexpected behaviour, we performed a cross-section using a focused ion beam (FIB) through an imprint (Fig. 7). Fig. 7 shows SEM images after FIB milling of an imprint performed in the PS3 sample using a Berkovich indenter and a 650 mN indentation load. For the sample PS3, it is observed that the Young's modulus and the hardness slightly increase at the beginning of the penetration of the tip. This increase can be explained by the compaction observed at the beginning of the penetration. Thereafter, from about 200 nm of penetration, there is a slight decrease in Young's modulus and hardness. This can be explained by the formation of cracks at the extremity of the tip indenter and also at the outer limit of the imprint (Fig. 7.c and Fig. 7.d). A compaction is observed at these points of rupture. The nucleation and growth of these micro-cracks during the indentation release the stress generated by the contact of the indenter, and thus apparent hardness and apparent Young's modulus decrease. Fig. 8 shows SEM photographs of an imprint in the sample PS4 performed using a Berkovich indenter and a charge of 200 mN. After the removal of the indenter, several significant cracks are observed. Comparing Fig. 7 and Fig. 8, we note more cracks and larger crack size for PS4 sample. From Fig. 5, the decrease of hardness and modulus is more pronounced for sample PS4. That is in accordance with a stress relaxation due to cracks nucleation and growth. Moreover, rupture mechanisms are more chaotic than plastic one, explaining the higher discrepancy in the case of sample PS4. To consolidate the PS4 sample, an oxidation was carried out with thermal treatment made by annealing at 550 °C in ambient environment (PS4O). The oxidization rate was measured less than 20%. A higher rate will have destroyed the porous silicon by bulging. The oxidation improves the mechanical properties of the PS4 layer, as shown on Fig. 9, by an increase of hardness and modulus compared to the non-oxidized sample PS4. In addition, hardness remains almost constant for all penetration depth and the evolution of modulus is more stable than in the non-oxidized case. Indeed, oxidation reduces cracks formation and growth. As shown Fig. 9.a, we observe that the value of the hardness of the PS4O sample starts approximately from the value of 2 GPa. Penetration continues until the maximum value around 4.5 GPa is reached for about 200 nm penetration. However, as the penetration depth continues to increase, the hardness reaches an asymptotic value of about 4 GPa.
Fig. 7. FIB photographs of sample PS3: (a) indentation, (b) cut off slice of PS, (c) observed crack on the top ridge of indentation and (d) observed crack on the depth ridge of indentation.
attain an asymptotic value in the range of approximately 4.5 GPa, 3 GPa and 2.5 GPa (respectively for PS6, PS3 and PS4). Figs. 5.b, 5.d and 5.f show similar evolution of Young's modulus in either case of PS, with an asymptotic value of approximately 57 GPa for PS6, 40 GPa for PS3 and 20 GPa for PS4. These results show that, as expected, hardness and Young's modulus decrease with increasing porosity. However, beyond a porosity of 60%, these values do not vary much anymore and it is observed that the porous layers become mechanically unstable. Fig. 6 shows a decrease in the hardness and Young's modulus as a function of the porosity explained by the fact that the porous structures become brittle. The porosity has a large influence on the mechanical properties of porous materials. Nevertheless, as shown by Fig. 5, the discrepancy is getting higher with higher porosity. 3. Results and discussion Fig. 5.a, 5.c and 5.e show the evolution of the hardness as a function of the penetration depth for several PS prepared at different current densities. As can be seen from Fig. 5.c, the hardness of sample PS3 remains constant, whereas the hardness of samples PS6 and PS4 decrease slightly (Fig. 5.a and Fig. 5.e). Furthermore, the Fig. 5.b, 5.d and 5.f show the variation of the Young's modulus as a function of the penetration depth for the same samples. Whereas the measured characteristics remain constant for the samples PS3 and PS6, we note a decrease
Fig. 8. SEM photographs of: (a) indentation print and (b) cracks on the top ridge of PS4 sample.
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Fig. 9. Hardness and Young's elastic modulus as a function displacement of oxidized porous silicon PS4O.
Fig. 9.b shows the variation of Young's modulus as a function of displacement for PS40. We observe that the asymptotic value is reached around 35 GPa for a penetration about 200 nm. At the same time, it is found that the hardness and the Young's modulus measured from the oxidized PS sample (PS4O) are considerably increased and the data obtained are more stable compared to the non-oxidized PS sample (PS4). This improved behaviour is mainly attributed to the pore size reduction. Indeed, during the oxidization process, a layer of SiO2 grows at the columnar surface of PS, resulting in a densification of the whole structure, i.e. encapsulation of the nanocrystallites which form the columnar structure. Therefore, the porosity is reduced and the interconnected microstructure is strengthened. Consequently, both hardness and Young's modulus are increased in this material configuration. Porosities in silicon considerably reduce the thermal conductivity and the dielectric permittivity. Nevertheless, the mechanical stability of the structure decreases and becomes fragile. We proposed a solution consisting in a thermal oxidation of the PS; the formation of oxide improves the mechanical stability of the structures while maintaining good thermal and dielectric properties.
[3] A. Drost, P. Steiner, H. Moser, W. Lang, Thermal conductivity of porous silicon, Sens. Mater. 7 (1995) 11–120. [4] M. Capelle, J. Billoue, J. Conord, P. Poveda, G. Gautier, Monolithic integration of low-pass filters with ESD protections on P+ silicon/porous silicon substrates, SolidState Electron. 116 (2016) 12–14. [5] S. Wang, R. Shen, Ch Yang, Y. Ye, Y. Hu, C. Li, Fabrication, characterization, and application in nanoenergetic materials of uncracked nano porous silicon thick films, Appl. Surf. Sci. 265 (2013) 4–9. [6] K. Nishio, S. Tagawa, T. Fukushima, H. Masuda, Highly ordered nanoporous Si for negative electrode of rechargeable lithium-ion battery, Electrochem. Solid-State Lett. 15 (2012) A41–A44. [7] C. Pacholski, Photonic crystal sensors based on porous silicon, Sensors 13 (2013) 4694–4713. [8] L. Pavesi, Porous silicon dielectric multilayers and microcavities, La Riv. Del. Nuovo Cim. 20 (1978–1999) (1997) 1–76. [9] E. Lorenzo, C.J. Oton, N.E. Capuj, M. Ghulinyan, D. Navarro-Urrios, Z. Gaburro, L. Pavesi, Porous silicon-based rugate filters, Appl. Opt. 44 (2005) 5415–5421. [10] V. Torres-Costa, F. Agullo-Rueda, R.J. Martin-Palma, J.M. Martinez-Duart, Porous silicon optical devices for sensing applications, Opt. Mater. 27 (2005) 1084–1087. [11] N.H. Maniya, S.R. Patel, Z.V.P. Murthy, Simulation and fabrication study of porous silicon photonic crystal, Optik 125 (2014) 828–831. [12] A.G. Nassiopoulou, G. Kaltsas, Porous silicon as an effective material for thermal isolation on bulk crystalline silicon, Phys. Status Solidi A 182 (2002) 307–312. [13] V. Lysenko, S. Périchon, B. Remaki, D. Barbier, Thermal isolation in microsystems with porous silicon, Sens. Actuators A 99 (2002) 13–24. [14] L.T. Canham, Silicon quantum wire array fabrication by electrochemical and chemical dissolution of wafers, Appl. Phys. Lett. 57 (1990) 1046–1048. [15] K. Ziouche, P. Godts, Z. Bougrioua, C. Sion, T. Lasri, D. Leclercq, Quasi-monolilthic heat flux microsensor based on porous silicon boxes, Sens. Actuators A: Phys. 164 (2010) 35–40. [16] K. Rahmoun, S. Fakiri, Modeling to predict effective dielectric constant of porous silicon low-dielectric-constant thin films, Spectrosc. Lett. 47 (5) (2014) 348–355. [17] D. Bellet, L.T. Canham (Eds.), New Developments in Porous Silicon. Relation with other Nanostructured Porous Materials, EMIS Datareviews Series 18, INSPEC. The Institution of Electrical Engineers, London, 1997. [18] S.P. Duttagupta, M. Fauchet, L. Canham (Ed.), Properties of Porous Si, EMIS Datareview Series No 18, IEEE, London, 1997. [19] K. Rahmoun, A. Iost, V. Keryvin, G. Guillemot, N.C. Sari, A multilayer model for describing hardness variations of aged porous silicon low-dielectric-constant thin films, Thin Solid Films 518 (2009) 213–221. [20] Z.Q. Fang, M. Hu, W. Zhang, X.R. Zhang, H.B. Yang, Mechanical properties of porous silicon by depth-sensing nanoindentation techniques, Thin Solid Films 517 (2009) 2930–2935. [21] C.A. Charitidis, A. Skarmoutsou, A.G. Nassiopoulou, A. Dragoneas, Nanomechanical properties of thick porous silicon layers grown on p- and p+-type bulk crystalline Si, Mater. Sci. Eng. A 528 (2011) 8715–8722. [22] W.C. Oliver, G.M. Pharr, Measurement of hardness and elastic modulus by instrumented indentation: advances in understanding and refinements to methodology, J. Mater. Res. 19 (2004) 3–20. [23] G. Bhagavannarayana, S.N. Sharma, R.K. Sharma, S.T. Lakshmikumar, A comparison of the properties of porous silicon formed on polished and textured (1 0 0) Si: high resolution XRD and PL studies, Mater. Chem. Phys. 97 (2006) 442–447. [24] A. Wolf, R. Brendel, Thermal conductivity of sintered porous silicon films, Thin Solid Films 513 (2006) 385–390. [25] H. Bückle, The science of hardness testing and its applications, in: J.W. Westbrook, H. Conrad (Eds.), American Society for Metals, Metal Park OH, 1973, pp. 453–491. [26] Y. Xiang, X. Chen, T.Y. Tsui, J.I. Jang, J.J. Vlassak, Mechanical properties of porous and fully dense low-k dielectric thin films measured by means of nanoindentation and the plane-strain bulge test technique, J. Mater. Res. 21 (2006) 386–395.
4. Conclusion We investigated the mechanical properties of non-oxidized and oxidized porous silicon (PS) prepared by electrochemically anodizing highly doped P++ silicon wafers. The mechanical properties are determined by the nanoindentation technique and compared to that of oxidized samples. The porosity ratio is controlled by the current density and the oxidation time. The results for the hardness and the Young's modulus show a strong correlation with the porosity ratio. Hardness and modulus decrease as porosity increase. Moreover, pores promote cracks formation that has a direct consequence on mechanical properties: reducing again hardness and modulus. In order to modify the mechanical properties of PS samples, we propose a controlled oxidization. Results show a higher hardness and modulus for oxidized samples. This is mainly attributed to the formation of an inner-pores SiO2 layer. This layer has for consequence a reduction of the porosity. References [1] He Li, Zhang Xiaoning, Wang Wenjiang, Wei Haicheng, The influence of oxidation properties on the electron emission characteristics of porous silicon, Appl. Surf. Sci. 382 (2016) 323–330. [2] C. Tsamis, A.G. Nassiopoulou, A. Tserepi, Thermal properties of suspended porous silicon micro-hotplatess for sensor applications, Sens. Actuators B 95 (2003) 78–82.
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Contents lists available at ScienceDirect
Materials Science & Engineering A journal homepage: www.elsevier.com/locate/msea
Mechanical properties of porous silicon and oxidized porous silicon by nanoindentation technique
T
⁎
Souheyla Fakiria,c, , Alex Montagneb, Khadidja Rahmounc, Alain Iostb, Katir Ziouchea a
Univ. Lille, CNRS, UMR 8520 - IEMN, F-59000 Lille, France Arts et Métiers ParisTech, campus de Lille, MSMP, 8 Boulevard Louis XIV, CS 50008-59046 Lille Cedex, France c Unité de Recherche Matériaux et Energies Renouvelables, URMER, Université de Tlemcen Abou Bekr Belkaid, B.P. 119, 13000 Tlemcen, Algérie b
A R T I C L E I N F O
A B S T R A C T
Keywords: Mesoporous silicon Oxidation Young's modulus Hardness Porosity Micro cracks
A study of mechanical properties of mesoporous silicon (PS) is presented in this article. PS was prepared by electrochemical etching of a heavily doped P++ silicon wafer in a hydrofluoric acid electrolyte. The mechanical properties of PS and oxidized PS obtained by thermal treatment, were characterized by the nanoindentation technique associated to the continuous stiffness measurement option. The morphology of PS and oxidized PS were both characterized by scanning electron microscope. It is shown that the Young's modulus and hardness are related to the PS preparing conditions and decrease with increasing porosity. In particular, oxidation improves the mechanical properties of the mesoporous silicon. Surprisingly, modulus and hardness decrease with penetration depth, whereas a compaction could be expected resulting in a rising modulus and hardness. These results are mainly attributed to micro cracks formation, highlighted by focused ion beam cross section.
1. Introduction Due to its interesting physical properties, porous silicon (PS) is one of materials that attracts more attention since its discovery. Numerous studies have been proposed to characterize it [1–3]. This material has found applications in many devices such as electromagnetic interference filtering radiofrequency (EMIFRF) circuits [4], nanoelectromechanical systems (NEMS) [5], emitters [1], micro-hotplates [2], rechargeable Li-ion battery anode [6] and photonic crystal sensors [7]. PS can be prepared as thin layers or multilayers systems [8–11]. The thermal conductivity of PS is lower than that of mono-crystalline silicon and can be used as thermal insulator in microsystems [12]. This characteristic can find an application in a micro flow meter, where a large periodic thermal conductivity to the substrate surface is needed [3–14]. Monocrystalline PS is mainly classified according to the pore size as nanoporous (≤ 2 nm), mesoporous (2–50 nm) and macroporous silicon (> 50 nm). Thermal properties are primarily related to the porosity [15]; in addition, the dielectric constant of PS can be reduced with increasing porosity [16]. However porosity considerably weakens the structures. Consequently, it becomes essential to characterize precisely the mechanical behaviour of such structures to optimize the manufacturing process in order to reach mechanical requirements. The first studies on the mechanical characteristics of PS were performed in 1997 [17,18]. Numerous researches have been proposed to improve
⁎
understanding of the influence of morphology and structure of PS on its mechanical properties, such as micro indentation [19]. These authors examine numerous issues related to the mechanical properties of the mesoporous silicon, such that, the variation in hardness of aged porous silicon low dielectric constant. Nanoindentation has been used to show influence of both PS microstructure and porosity on the mechanical properties [20,21]. In the present study, mechanical properties are investigated using the nanoindentation technique associated to the continuous stiffness measurement method (CSM) [22]. This method has shown its reliability for determining the mechanical properties of materials at the microand nanoscale. This article analyzes the evolutions of hardness and Young's modulus of PS and oxidized PS regarding the compaction of PS and the rupture of the sponge-like structure. 2. Experimental 2.1. Sample preparation The mesoporous silicon is obtained by electrochemical anodization of heavily doped P++ type silicon (resistivity ρ = 0.009 ± 0.01 Ω cm, thickness about 380 ± 25 µm and {100} crystal orientation) in a double-tank cell (AMMT®). The concentration of the electrolyte (by volume) is 27% of hydrofluoric acid, 35% of pure ethanol C2H5OH and
Corresponding author at: Unité de Recherche Matériaux et Energies Renouvelables, URMER, Université de Tlemcen Abou Bekr Belkaid, B.P. 119, 13000 Tlemcen, Algérie E-mail addresses: [email protected] (S. Fakiri), [email protected] (K. Ziouche).
https://doi.org/10.1016/j.msea.2017.11.013 Received 1 August 2017; Received in revised form 4 November 2017; Accepted 6 November 2017 Available online 08 November 2017 0921-5093/ © 2017 Published by Elsevier B.V.
Materials Science & Engineering A 711 (2018) 470–475
S. Fakiri et al.
Table 1 Manufacturing parameters and description of PS and oxidized PS. Sample As-prepared PS
Oxidized PS
PS1 PS2 PS3 PS4 PS5 PS6 PS7 PS8 PS1O PS2O PS3O PS4O
Current density [mA/cm2]
Anodizing time [s]
Porosity [%]
Thickness [µm]
Oxidation temperature [°C]
20 40 60 80 20 20 40 40 20 40 60 80
5400 5400 3600 3600 1800 3600 1800 3600 5400 5400 3600 3600
44.44 58.17 59.66 68.00 38.16 41.86 45.71 52.55
110 190 170 206 41.17 78.7 57.07 113
Unoxidized Unoxidized Unoxidized Unoxidized Unoxidized Unoxidized Unoxidized Unoxidized 550 550 550 550
Fig. 1. Evolution of thickness and porosity of PS as a function of (a) time for two anodizing current density: 20 mA/cm2 and 40 mA/cm2 and, of (b) current density for a constant anodizing time of 60 min.
Fig. 2. SEM photographs of PS surface and PS in trench: (a, b) sample PS6; (c, d) sample PS4.
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porosity are shown in Table 1. Fig. 1.a shows both the linear evolution of the porosity and the thickness of the PS as a function of etching time for two anodization current densities (20 mA/cm2 and 40 mA/cm2). We note, as expected, that both thickness and porosity increase linearly as a function of time. In the same way, we observe that they also both increase in function of the current density for a same anodization time set to 60 min (see Fig. 1.b). 2.2. Morphological characterizations To understand the mechanisms involved in the mechanical properties of PS, we studied accurately the structuration of the as-prepared and oxidized mesoporous silicon. Figs. 2 and 4 show the surface and the cross section morphologies of the PS. We can note that the surfaces of the silicon are perforated with non-circular holes. The depth structuring in the trenches are columnar with ramification like sponges. This is explained by the parallels current lines passing through the substrate. As examples, Fig. 2 shows SEM images of different PS whose fabrication is described in Table 1 (a and b correspond to PS6 sample and c and d to PS4 sample). It can be seen that the pore shapes are oval on the surface as shown on Fig. 2.a and Fig. 2.c. The size of the pores for the PS6 sample is smaller than those of PS4 sample (R1 ~7 nm and R2 ~10 nm for PS6 sample and R1 ~8 nm and R2 ~17 nm for PS4 sample, where R1 and R2 are the semi-minor and semi-major axis respectively). The sizes of the pores evolve with the current density but also with the anodizing time (Fig. 1). At a constant current density the growth rate of pores is in the order of ~ 1 nm / 10 min. Although the etching of porous silicon mainly follows the current lines, the anodization time causes a lateral etching resulting in an increased pore size. Fig. 3 shows the relationship between the porosity and the pore diameter. The following figures (Fig. 4) show the morphologies of the surface
Fig. 3. Evolution of the pore diameter as function of the porosity.
38% of deionized water. The dispositive allows to realize uniform circular cells of 2.4 in. diameter in the center of the 3 in. diameter silicon substrate. Several samples of different porosities were made by changing anodizing conditions (courant density varying between 20 to 80 mA/cm2 and etching time increasing between 30 to 90 min). Anodization is performed at room temperature in a clean and controlled environment. The determination of the porosity was carried out by m −m gravimetric method [23,24]. The porosity is given by P = m1 − m2 , where 1 3 m1 and m2 are respectively the masses of the silicon sample before and after the anodization and m3 is the mass after etching of the porous layer in a sodium hydroxide solution. The masses of samples (about 0.25 in. square surface) are measured in the microgram range using a Mettler Toledo® microbalance with extreme accuracy (0.1 µg). Both surface morphology and volume structure as well as the thickness of PS and oxidized PS were characterized by scanning electron microscopy (SEM). For each sample of PS, anodizing conditions, thickness and
Fig. 4. SEM photographs of oxidized PS surface and in trench: (a, b) sample PS1O; (c, d) sample PS4O.
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Fig. 5. Hardness and Young's elastic modulus as a function displacement into surface of PS (a, b) PS6; (c, d) PS3; (e, f) PS4.
Fig. 6. Hardness and Young's elastic modulus of PS as a function of porosity.
measurement mode (CSM). The tip area function is calibrated by means of indentation in a reference material of known modulus (fused quartz, E = 72 GPa). The harmonic displacement was set to 2 nm with a 45 Hz frequency. The maximum penetration depth was set to 2000 nm conducted with a constant strain rate of 0.05 s−1. Thus the penetration depth did not exceed a tenth of the total thickness of the film (1/10 Bückle rule of thumb [25]). Doing this, it reduced the influence of the substrate (bulk silicon) on the mechanical properties of the porous silicon. In order to reduce discrepancy, 9 indentation tests were performed for each sample. Fig. 5 illustrates the evolution of hardness (H) and Young's modulus (E) as a function of penetration depth for as prepared PS. As shown in Fig. 5.a, 5.c and 5.e, it is observed that the hardness is lower than that of the porous silicon (12 GPa). Thus, for low depths of penetration the hardness is the same regardless of the porosity. As the penetration depth continues to increase, hardness slightly decreases and tends to
and in the trench for oxidized PS (a, b to PS1O and c, d to PS4O, see Table 1). Oxidation occurs along all the height of columnar porosities using annealing at 550 °C in atmosphere controlled (N2 and air). The rate of oxidation obtained is about 20% (determined by Energy Dispersive X-ray Spectroscopy into the SEM). A higher rate causes swelling of the porous structure and can damage it. Fig. 2.c and Fig. 4.c, corresponding to the same sample without oxidation (PS4) and with oxidation (PS4O) respectively, show that the size of the pores is reduced by the effect of the annealing treatment. This is explained by the growth of silicon oxide (SiO2) from the silicon nanocrystallites. 2.3. Nano indentation measurements The mechanical properties of PS were investigated by nanoindentation using a Nano indenter XP (MTS System®) equipped with a Berkovich diamond indenter, using the continuous stiffness 473
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of the modulus by a half for the sample PS4 and a poor overlapping. Thus, it is observed that for porosities above 60%, the structures of the samples become unstable and the measurements are not reproducible. The decrease of hardness and modulus with the penetration depth is quite different from what is classically reported for porous materials [26]. Indeed, for porous silicon, hardness and Young's modulus should increase with penetration due to the compaction effect. In our case, we do not observe this phenomenon. To obtain a better overview on this unexpected behaviour, we performed a cross-section using a focused ion beam (FIB) through an imprint (Fig. 7). Fig. 7 shows SEM images after FIB milling of an imprint performed in the PS3 sample using a Berkovich indenter and a 650 mN indentation load. For the sample PS3, it is observed that the Young's modulus and the hardness slightly increase at the beginning of the penetration of the tip. This increase can be explained by the compaction observed at the beginning of the penetration. Thereafter, from about 200 nm of penetration, there is a slight decrease in Young's modulus and hardness. This can be explained by the formation of cracks at the extremity of the tip indenter and also at the outer limit of the imprint (Fig. 7.c and Fig. 7.d). A compaction is observed at these points of rupture. The nucleation and growth of these micro-cracks during the indentation release the stress generated by the contact of the indenter, and thus apparent hardness and apparent Young's modulus decrease. Fig. 8 shows SEM photographs of an imprint in the sample PS4 performed using a Berkovich indenter and a charge of 200 mN. After the removal of the indenter, several significant cracks are observed. Comparing Fig. 7 and Fig. 8, we note more cracks and larger crack size for PS4 sample. From Fig. 5, the decrease of hardness and modulus is more pronounced for sample PS4. That is in accordance with a stress relaxation due to cracks nucleation and growth. Moreover, rupture mechanisms are more chaotic than plastic one, explaining the higher discrepancy in the case of sample PS4. To consolidate the PS4 sample, an oxidation was carried out with thermal treatment made by annealing at 550 °C in ambient environment (PS4O). The oxidization rate was measured less than 20%. A higher rate will have destroyed the porous silicon by bulging. The oxidation improves the mechanical properties of the PS4 layer, as shown on Fig. 9, by an increase of hardness and modulus compared to the non-oxidized sample PS4. In addition, hardness remains almost constant for all penetration depth and the evolution of modulus is more stable than in the non-oxidized case. Indeed, oxidation reduces cracks formation and growth. As shown Fig. 9.a, we observe that the value of the hardness of the PS4O sample starts approximately from the value of 2 GPa. Penetration continues until the maximum value around 4.5 GPa is reached for about 200 nm penetration. However, as the penetration depth continues to increase, the hardness reaches an asymptotic value of about 4 GPa.
Fig. 7. FIB photographs of sample PS3: (a) indentation, (b) cut off slice of PS, (c) observed crack on the top ridge of indentation and (d) observed crack on the depth ridge of indentation.
attain an asymptotic value in the range of approximately 4.5 GPa, 3 GPa and 2.5 GPa (respectively for PS6, PS3 and PS4). Figs. 5.b, 5.d and 5.f show similar evolution of Young's modulus in either case of PS, with an asymptotic value of approximately 57 GPa for PS6, 40 GPa for PS3 and 20 GPa for PS4. These results show that, as expected, hardness and Young's modulus decrease with increasing porosity. However, beyond a porosity of 60%, these values do not vary much anymore and it is observed that the porous layers become mechanically unstable. Fig. 6 shows a decrease in the hardness and Young's modulus as a function of the porosity explained by the fact that the porous structures become brittle. The porosity has a large influence on the mechanical properties of porous materials. Nevertheless, as shown by Fig. 5, the discrepancy is getting higher with higher porosity. 3. Results and discussion Fig. 5.a, 5.c and 5.e show the evolution of the hardness as a function of the penetration depth for several PS prepared at different current densities. As can be seen from Fig. 5.c, the hardness of sample PS3 remains constant, whereas the hardness of samples PS6 and PS4 decrease slightly (Fig. 5.a and Fig. 5.e). Furthermore, the Fig. 5.b, 5.d and 5.f show the variation of the Young's modulus as a function of the penetration depth for the same samples. Whereas the measured characteristics remain constant for the samples PS3 and PS6, we note a decrease
Fig. 8. SEM photographs of: (a) indentation print and (b) cracks on the top ridge of PS4 sample.
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Fig. 9. Hardness and Young's elastic modulus as a function displacement of oxidized porous silicon PS4O.
Fig. 9.b shows the variation of Young's modulus as a function of displacement for PS40. We observe that the asymptotic value is reached around 35 GPa for a penetration about 200 nm. At the same time, it is found that the hardness and the Young's modulus measured from the oxidized PS sample (PS4O) are considerably increased and the data obtained are more stable compared to the non-oxidized PS sample (PS4). This improved behaviour is mainly attributed to the pore size reduction. Indeed, during the oxidization process, a layer of SiO2 grows at the columnar surface of PS, resulting in a densification of the whole structure, i.e. encapsulation of the nanocrystallites which form the columnar structure. Therefore, the porosity is reduced and the interconnected microstructure is strengthened. Consequently, both hardness and Young's modulus are increased in this material configuration. Porosities in silicon considerably reduce the thermal conductivity and the dielectric permittivity. Nevertheless, the mechanical stability of the structure decreases and becomes fragile. We proposed a solution consisting in a thermal oxidation of the PS; the formation of oxide improves the mechanical stability of the structures while maintaining good thermal and dielectric properties.
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4. Conclusion We investigated the mechanical properties of non-oxidized and oxidized porous silicon (PS) prepared by electrochemically anodizing highly doped P++ silicon wafers. The mechanical properties are determined by the nanoindentation technique and compared to that of oxidized samples. The porosity ratio is controlled by the current density and the oxidation time. The results for the hardness and the Young's modulus show a strong correlation with the porosity ratio. Hardness and modulus decrease as porosity increase. Moreover, pores promote cracks formation that has a direct consequence on mechanical properties: reducing again hardness and modulus. In order to modify the mechanical properties of PS samples, we propose a controlled oxidization. Results show a higher hardness and modulus for oxidized samples. This is mainly attributed to the formation of an inner-pores SiO2 layer. This layer has for consequence a reduction of the porosity. References [1] He Li, Zhang Xiaoning, Wang Wenjiang, Wei Haicheng, The influence of oxidation properties on the electron emission characteristics of porous silicon, Appl. Surf. Sci. 382 (2016) 323–330. [2] C. Tsamis, A.G. Nassiopoulou, A. Tserepi, Thermal properties of suspended porous silicon micro-hotplatess for sensor applications, Sens. Actuators B 95 (2003) 78–82.
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