- Email: [email protected]
Surface wet-ability modification of thin PECVD silicon nitride layers by 40 keV argon ion treatments
Radiation Physics and Chemistry 115 (2015) 49–54
Contents lists available at ScienceDirect
Radiation Physics and Chemistry journal homepage: www.elsevier.com/locate/radphyschem
Surface wet-ability modification of thin PECVD silicon nitride layers by 40 keV argon ion treatments F. Caridi a,n, A. Picciotto b, L. Vanzetti b, E. Iacob b, C. Scolaro a a b
Università di Messina, Dipartimento di Fisica e di Scienze della Terra, Viale F. Stagno d’Alcontres, 31-98166 Messina, Italy Micro Nano Facility, Centro Materiali e Microsistemi, Fondazione Bruno Kessler, Via Sommarive 18, 38123 Povo, Trento, Italy
H I G H L I G H T S
Measurements of wet-ability of liquid drops on a 30 nm Si3N4 film were performed. Chemical composition was investigated by X-ray Photoelectron Spectroscopy (XPS). Surface morphology was tested by Atomic Force Microscopy (AFM). Ar þ bombardment increases the contact angle, oxygen content and surface roughness.
art ic l e i nf o
a b s t r a c t
Article history: Received 17 December 2014 Received in revised form 20 April 2015 Accepted 5 June 2015 Available online 9 June 2015
Measurements of wet-ability of liquid drops have been performed on a 30 nm silicon nitride (Si3N4) film deposited by a PECVD reactor on a silicon wafer and implanted by 40 keV argon ions at different doses. Surface treatments by using Ar ion beams have been employed to modify the wet-ability. The chemical composition of the first Si3N4 monolayer was investigated by means of X-ray Photoelectron Spectroscopy (XPS). The surface morphology was tested by Atomic Force Microscopy (AFM). Results put in evidence the best implantation conditions for silicon nitride to increase or to reduce the wet-ability of the biological liquid. This permits to improve the biocompatibility and functionality of Si3N4. In particular experimental results show that argon ion bombardment increases the contact angle, enhances the oxygen content and increases the surface roughness. & 2015 Elsevier Ltd. All rights reserved.
Keywords: Thin film deposition Ion implantation Surface wet-ability Surface roughness
1. Introduction Silicon nitride (Si3N4) is a dielectric ceramic material that is rarely observed in nature, but may occur naturally, since it has been found in particles of meteorite rock (Riley, 2000). Its material properties have led to speculation that it may have a role in biomedical fields since it is biocompatible (Howlett et al., 1989; Neumann et al., 2004). Because of a unique combination of material properties, Si3N4 has been used in spinal fusion implants and it has been developed for bearing components of prosthetic hip and knee joints. Cervical spacers and spinal fusion devices made of Si3N4 composites are presently in use, with successful short-term clinical results (Taylor et al., 2010). Surgical screws and plates made of Si3N4 as the source material, as well as bearings for spine disk surgery and prosthetic hip and knee joints have also been developed and tested (Neumann et al., 2006a, 2006b; Bal et al., 2008). n
Corresponding author. E-mail address: [email protected] (F. Caridi).
http://dx.doi.org/10.1016/j.radphyschem.2015.06.009 0969-806X/& 2015 Elsevier Ltd. All rights reserved.
Surface properties of silicon nitride are important for its biocompatibility (Ratner et al., 2004). The cell–material interaction is strongly influenced not only by the surface topography of the biocompatible material but also from the chemical properties of the surface, including the wet-ability (measured in terms of contact angle) and the roughness. Because the substrate of a material which must be implanted in the human body is not a passive anchoring surface but is able to stimulate the adhesion and cell proliferation, it is important to know both its hydrophilicity that its surface roughness in order to increase the ability of cell migration. A large number of studies indicate that cells tend to fixate on hydrophilic surfaces than on hydrophobic ones; that the diffusion and the formation of the continuous layer of cells are higher on a rough surface than on a smooth one (Cooper et al., 2010). The aim of this work is to analyze variations of the contact angle and roughness of silicon nitride after ion bombardment in order to evaluate how argon ions implantation modifies the surface of the material. In particular its roughness and wettability changes were investigated.
50
F. Caridi et al. / Radiation Physics and Chemistry 115 (2015) 49–54
2. Materials and methods A plasma enhanced chemical vapor deposition (PECVD) reactor coupled to a planar parallel electrode system (Surface Technology Systems Ltd.) was used to fabricate and to deposit a Si3N4 film (30 nm thickness) on a 6 in. silicon wafers with a thickness of 500 μm. The deposition chamber is cylindrical with a diameter of about 1 m. The electrode diameter is 24 cm. The system is equipped with a high frequency (HF) generator at 13.56 MHz with a maximum power of 500 W and with a low frequency (LF) generator at 308 kHz with a maximum power of 600 W. The temperature of the upper electrode (shower head) of the machine was 250 °C while the temperature of the lower electrode (the chuck) in which the wafer is placed during the deposition process was 300 °C. A scheme of PECVD system for the Si3N4 deposition is shown in Fig. 1a. After the deposition, samples were treated by a 40 KeV argon ion implantation. The ion implant angle was maintained at 60° with respect to the sample normal direction to be sure that only the first layers of the silicon nitride were affected from the Ar þ ions irradiation. The implantation dose was selected to be 5 1013 ions/cm2, 5 1014 ions/cm2 and 1 1015 ions/cm2. Moreover, a Montecarlo simulation by using the TRIM software was performed to evidence the Ar þ ions projected range inside the silicon nitride layer. The Ar ions distribution is shown in Fig. 2. As is possible to see, the projected range of the Ar þ ions is enclosed inside the 30 nm of the silicon nitride thickness. The peak is centered at depth of around 180 nm from the surface. In addition the simulation takes also into account the presence of a few nanometers of native silicon oxide always present and naturally grown because of the environment contact atmosphere. To evaluate the wet-ability of materials the technique of the sessile drop has been used (Cutroneo et al., 2013). As test liquid
Fig. 2. The TRIM simulation performed to evidence the Ar þ ions projected range inside the silicon nitride layer.
Fig. 3. The contact angle vs. the ion dose for the investigated samples.
distilled water was used. The drop placed on the surface of the sample will tend to widen when the surface is wettable (hydrophilic, θ o90°) or to assume a spherical shape when the surface is not-wettable (hydrophobic, θ 490°). For very small droplets the effect of distortion of gravity is minimal. The contact angle θ (degree) was calculated from the height h (mm) and the base diameter d (mm) of the droplet itself (Zisman, 1964)
θ = 2arctg for
2h d
θ o90° and
⎛ 4hd ⎞ ⎟ θ = 90° + cos−1⎜ ⎝ 4h2 + d2 ⎠
(2)
θ 4 90°. The determination of contact angles for larger droplets is more complicated because the effect of gravity is not negligible and the drops are not spherical. for
Fig. 1. A scheme of PECVD system for the Si3N4 deposition (a) and of the sessile drop technique (b).
(1)
F. Caridi et al. / Radiation Physics and Chemistry 115 (2015) 49–54
51
Fig. 4. The XPS atomic concentrations of nitrogen and oxygen in the different samples (a–d) and the concentration of O and N at the various ion implantation doses (e).
The technique of the sessile drop is well shown in Fig. 1b. There have been two theories to explain the effect of surface roughness on wet-ability. The first is described by Wenzel (Zisman, 1964)
assumes that the liquid substrate bathrooms completely wrinkled. The expression proposed to describe this situation is given by the following equation (Zisman, 1964):
cos θW = r cos θY
cos θCB = r f f cos θY + f − 1
(3)
where θW is the angle of effective contact of a rough surface real, or angle of Wenzel, r is the ratio between the actual area of the surface of the solid and the nominal one, and θY is the contact angle of equilibrium of Young observed on a ideally smooth surface. Since r is always greater than 1, this model provides an increase hydrophilicity (θW o θY) to hydrophilic surfaces (θY o90°), and an increase in hydrophobicity (θW 4 θY) to hydrophobic surfaces (θY 4 90°). The second theory was derived by Cassie and Baxter, and
(4)
where θCB is the angle of contact of Cassie–Baxter, f is the fraction of solid surface in contact with the liquid, and rf is the ratio of the surfaces of the wet area and nominal. When f¼ 1 and rf ¼r the Cassie–Baxter equation is identical with the equation of Wenzel. Since f and rf are always smaller than 1, the model of Cassie–Baxter expected that the increase of roughness of a surface face always increase the contact angle. The chemical composition of the first Si3N4 monolayer was investigated by means of non-invasive X-ray Photoelectron
52
F. Caridi et al. / Radiation Physics and Chemistry 115 (2015) 49–54
Fig. 5. The analyses of the PECVD-prepared Si3N4 films by atomic force (a–d) and the roughness vs. the ion dose for the investigated samples (e).
Spectroscopy (XPS), acquiring spectra by means of a Scienta Esca200 system equipped with a monochromatized Al Kα (1486.6 eV) source. An overall energy resolution of 0.4 eV is routinely used for core levels. Measurements were performed at 90° emission angle (maximum sampling depth). For each sample a survey (0– 1200 eV) was acquired with a lower energy resolution to identify elements present on the surface. Subsequently Si 2p, O 1s, N 1s and C 1s core levels were collected. All core level peak energies were referenced to the saturated hydrocarbon in C 1s at 285.0 eV (Caridi et al., 2013). The surface morphology of the investigated samples was tested by an atomic force microscope model Unisolver P47 from NT-MDT (Cicchi et al., 2010). Analyses were performed in semi-contact mode with a silicon tip with a nominal radius of less than 10 nm. Scan area was 5 5 mm2. Root mean square roughness (Sq) was estimated for each scanned areas. Related formula is reported below (Vorburger and Raja, 1990)
Sq – Root mean square roughness, ISO 4287/1[nm]: Sq =
1 Nx N y
Nx
Ny
2
∑ ∑ (zij ) i=1 j=1
(5)
where zij is the vertical distance from the mean line to the ith and jth data point. SEM-EDX measurements were collected by an ESEM-FEI Inspect-S electron microscope coupled with an Oxford INCA PentaFETx3 EDX spectrometer, a Si(Li) detector equipped by a ultra-thin window ATW2, by using a resolution of 137 eV at 5.9 keV (Mn Kα1). The spectral data were acquired in ESEM (Environmental Scanning Electron Microscope) condition at working distance of 10 mm with an acceleration voltage of 20 kV, counting times of 60 s, count for second approximately 3000 cps with dead time below 30%. Results were processed by INCA software Energy. This software uses the XPP matrix correction scheme developed by
F. Caridi et al. / Radiation Physics and Chemistry 115 (2015) 49–54
53
Fig. 6. A SEM-EDX analysis of the small localized bumps on the surface of the investigated samples.
Fig. 7. The linear increase of the contact angle as a function of the Si3N4 surface roughness.
Pouchou and Pichoir (1988).
3. Results and discussion The estimation of the contact angle was made with the sessile drop technique and gave the following results: 39.8°, 46°, 43.7° and 44.8° for Si3N4 not implanted, Si3N4 implanted with 5 1013 Ar þ /cm2, 5 1014 Ar þ /cm2 and 1 1015 Ar þ /cm2, respectively. These results are plotted in Fig. 3, showing a fast increase of the contact angle for low ion doses (until 5 1013 Ar þ /cm2) followed by a nearly constant value for higher quantity of implanted argon ions. X-ray Photoelectron Spectroscopy (XPS) spectra are reported in Fig. 4 for Si3N4 not implanted (a), Si3N4 implanted with 5 1013 Ar þ /cm2 (b), 5 1014 Ar þ /cm2 (c) and 1 1015 Ar þ /cm2 (d), showing that a great amount carbon and oxygen was detected in all spectra, in addition to silicon and nitrogen (the matrix elements). From the core level we have calculated the atomic concentration of all elements present on the samples surface, using the atomic sensitive factors. In particular for oxygen and nitrogen we
obtain: 10.1% and 31.9%, respectively, for Si3N4 not implanted (a); 19.9% and 24.2%, respectively, for Si3N4 implanted with 5 1013 Ar þ /cm2 (b); 15.9% and 27.6%, respectively, for Si3N4 implanted with 5 1014 Ar þ /cm2 (c); 18.5% and 25.2%, respectively, for Si3N4 implanted with 1 1015 Ar þ /cm2 (d). Fig. 4e shows the atomic concentration of O and N at the various ion implantation doses. We can notice that a decrease in the amount of nitrogen corresponds to an increase of the oxygen and vice versa. This may be due to a breakage of chemical bonds between silicon and nitrogen in the Si3N4 film after the ion implantation, with a subsequent formation of chemical bonds between silicon and oxygen, that leads to a decrease of wet-ability, as reported in Fig. 3. Analyses of the PECVD-prepared Si3N4 films by Atomic Force Microscopy are reported in Fig. 5 for Si3N4 not implanted (a), Si3N4 implanted with 5 1013 Ar þ /cm2 (b), 5 1014 Ar þ /cm2 (c) and 1 1015 Ar þ /cm2 (d). The root mean square roughness estimation was of about 0.24 nm, 0.52 nm, 0.42 nm and 0.5 nm, respectively, showing a fast increase for the lower ion dose followed by a nearly constant value for higher quantity of argon ions implanted, as reported in Fig. 5e. Small localized bumps, with heights of a few nanometers observable in the maps could be due to mass inhomogeneities during the deposition process. Such inconvenient might also be eliminated by means of some technical shrewdness such as the chopping of the material flux during the deposition in order to drastically reduce the presence of massive particles moving towards the substrate with lower kinetic energies. A SEM-EDX analysis of these bumps is shown in Fig. 6 for the investigated samples. The SEM image of the analyzed area is reported as inset. The EDX result is that bumps are made of silicon in all cases, with traces of carbon and oxygen and contaminants such as chlorine. Fig. 7 shows the linear increase of the contact angle as a function of the Si3N4 surface roughness. The ion implantation changes the thin film surface, making it more wrinkled and less wettable with the increase of the dose.
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4. Conclusions
References
Ceramics have remarkable properties that have fueled excitement about their potential applications in the biomedical field. Oxide ceramics such as Al2O3 and stabilized ZrO2 have a lengthy history in prosthetic hip and knee replacements; and the favorable outcomes and the limitations of these ceramics are well known. Si3N4 ceramics have mechanical properties that are superior to the Al2O3-based ceramics and composites currently used as bearings for total hip and knee joint replacement. Unlike metals, Si3N4 is semitransparent to X-rays and being non-magnetic, it enables MRI of soft tissues proximal to Si3N4 implants. Si3N4 is biocompatible, and porous Si3N4 has been shown to support bone ingrowths at rates comparable with those reported for porous Ti surfaces when implanted in a large animal model. Silicon nitride has been used to promote bone fusion in spinal surgery, and to develop bearings that can improve the wear and longevity of prosthetic hip and knee joints (Otten et al., 2010; Oduwole et al., 2010). In this work it was shown that ion implantation can modify the properties of adhesion and surface wet-ability of silicon nitride films. It was shown that changes in the surface wet-ability may be due to changes in surface roughness, created by treatments with accelerated ions, that leads to modify the surface morphology. In particular an increase of the contact angle with the surface roughness was reported, according to the Cassie–Baxter theory (Zisman, 1964). This angle is lower than 90° also after the ion bombardment, so the silicon nitride surface remains hydrophilic, according to Cooper et al. (2010).
Bal, B.S., Khandkar, A., Lakshminarayanan, R., Clarke, I., Hoffman, A.A., Rahaman, M. N., 2008. J. Biomed. Mater. Res. B 87, 447–454. Caridi, F., Torrisi, L., Cutroneo, M., Barreca, F., Gentile, C., Serafino, T., Castrizio, E.D., 2013. Appl. Surf. Sci. 272, 82–87. Cicchi, S., Ghini, G., Lascialfari, L., Brandi, A., Betti, F., Berti, D., Baglioni, P., Di Bari, L., Pescitelli, G., Mannini, M., Caneschi, A., 2010. Soft Matter 6, 1655–1661. Cooper, H.J., Ranawat, A.S., Potter, H.G., Foo, L.F., Koob, T.W., Ranawat, C.S., 2010. Clin. Orthop. Relat. Res. 468, 3278–3285. Cutroneo, M., Torrisi, L., Caridi, F., Sayed, R., Gentile, C., Mondio, G., Serafino, T., Castrizio, E.D., 2013. Appl. Surf. Sci. 272, 25–29. Howlett, C.R., McCartney, E., Ching, W., 1989. Clin. Orthop. Relat. Res. 244, 293–304. Neumann, A., Unkel, C., Werry, C., et al., 2006a. HNO 54, 937–942. Neumann, A., Unkel, C., Werry, C., et al., 2006b. Otolaryngol. – Head Neck Surg. 134, 923–930. Neumann, A., Jahnke, K., Maier, H.R., Rago, C., 2004. Laryngorhinootologie 83, 845–851. Oduwole, K.O., Molony, D.C., Walls, R.J., Bashir, S.P., Mulhall, K.J., 2010. Knee Surg. Sport Traumatol. Arthrosc. 18, 945–948. Otten, R., Van Roermund, P.M., Picavet, H.S.J., 2010. Ned. Tijdschr. Geneeskd. 154, A1534. Pouchou, J.L., Pichoir, F., 1988. Determination of mass absorption coefficients for soft X-rays by use of the electron microprobe. In: Newbury, D.E. (Ed.), Microbeam Analysis. San Francisco Press, San Francisco. Ratner, B.D., et al., 2004. Biomaterials Science: An Introduction to Materials in Medicine, 2nd edition. Academic Press, London, San Diego, CA. Riley, F.L., 2000. J. Am. Ceram. Soc. 83, 245–265. Taylor, R.M., Bernero, J.P., Patel, A.A., Brodke, D.S., Khandkar, A.C., 2010. J. Bone Joint Surg. Br. 92 (1), 133. Vorburger, T.V., Raja, J., 1990. Surface Finish Metrology Tutorial, pp. 1–162. Zisman, W.A., 1964. Am. Chem. Soc. 43, 1–51.
Contents lists available at ScienceDirect
Radiation Physics and Chemistry journal homepage: www.elsevier.com/locate/radphyschem
Surface wet-ability modification of thin PECVD silicon nitride layers by 40 keV argon ion treatments F. Caridi a,n, A. Picciotto b, L. Vanzetti b, E. Iacob b, C. Scolaro a a b
Università di Messina, Dipartimento di Fisica e di Scienze della Terra, Viale F. Stagno d’Alcontres, 31-98166 Messina, Italy Micro Nano Facility, Centro Materiali e Microsistemi, Fondazione Bruno Kessler, Via Sommarive 18, 38123 Povo, Trento, Italy
H I G H L I G H T S
Measurements of wet-ability of liquid drops on a 30 nm Si3N4 film were performed. Chemical composition was investigated by X-ray Photoelectron Spectroscopy (XPS). Surface morphology was tested by Atomic Force Microscopy (AFM). Ar þ bombardment increases the contact angle, oxygen content and surface roughness.
art ic l e i nf o
a b s t r a c t
Article history: Received 17 December 2014 Received in revised form 20 April 2015 Accepted 5 June 2015 Available online 9 June 2015
Measurements of wet-ability of liquid drops have been performed on a 30 nm silicon nitride (Si3N4) film deposited by a PECVD reactor on a silicon wafer and implanted by 40 keV argon ions at different doses. Surface treatments by using Ar ion beams have been employed to modify the wet-ability. The chemical composition of the first Si3N4 monolayer was investigated by means of X-ray Photoelectron Spectroscopy (XPS). The surface morphology was tested by Atomic Force Microscopy (AFM). Results put in evidence the best implantation conditions for silicon nitride to increase or to reduce the wet-ability of the biological liquid. This permits to improve the biocompatibility and functionality of Si3N4. In particular experimental results show that argon ion bombardment increases the contact angle, enhances the oxygen content and increases the surface roughness. & 2015 Elsevier Ltd. All rights reserved.
Keywords: Thin film deposition Ion implantation Surface wet-ability Surface roughness
1. Introduction Silicon nitride (Si3N4) is a dielectric ceramic material that is rarely observed in nature, but may occur naturally, since it has been found in particles of meteorite rock (Riley, 2000). Its material properties have led to speculation that it may have a role in biomedical fields since it is biocompatible (Howlett et al., 1989; Neumann et al., 2004). Because of a unique combination of material properties, Si3N4 has been used in spinal fusion implants and it has been developed for bearing components of prosthetic hip and knee joints. Cervical spacers and spinal fusion devices made of Si3N4 composites are presently in use, with successful short-term clinical results (Taylor et al., 2010). Surgical screws and plates made of Si3N4 as the source material, as well as bearings for spine disk surgery and prosthetic hip and knee joints have also been developed and tested (Neumann et al., 2006a, 2006b; Bal et al., 2008). n
Corresponding author. E-mail address: [email protected] (F. Caridi).
http://dx.doi.org/10.1016/j.radphyschem.2015.06.009 0969-806X/& 2015 Elsevier Ltd. All rights reserved.
Surface properties of silicon nitride are important for its biocompatibility (Ratner et al., 2004). The cell–material interaction is strongly influenced not only by the surface topography of the biocompatible material but also from the chemical properties of the surface, including the wet-ability (measured in terms of contact angle) and the roughness. Because the substrate of a material which must be implanted in the human body is not a passive anchoring surface but is able to stimulate the adhesion and cell proliferation, it is important to know both its hydrophilicity that its surface roughness in order to increase the ability of cell migration. A large number of studies indicate that cells tend to fixate on hydrophilic surfaces than on hydrophobic ones; that the diffusion and the formation of the continuous layer of cells are higher on a rough surface than on a smooth one (Cooper et al., 2010). The aim of this work is to analyze variations of the contact angle and roughness of silicon nitride after ion bombardment in order to evaluate how argon ions implantation modifies the surface of the material. In particular its roughness and wettability changes were investigated.
50
F. Caridi et al. / Radiation Physics and Chemistry 115 (2015) 49–54
2. Materials and methods A plasma enhanced chemical vapor deposition (PECVD) reactor coupled to a planar parallel electrode system (Surface Technology Systems Ltd.) was used to fabricate and to deposit a Si3N4 film (30 nm thickness) on a 6 in. silicon wafers with a thickness of 500 μm. The deposition chamber is cylindrical with a diameter of about 1 m. The electrode diameter is 24 cm. The system is equipped with a high frequency (HF) generator at 13.56 MHz with a maximum power of 500 W and with a low frequency (LF) generator at 308 kHz with a maximum power of 600 W. The temperature of the upper electrode (shower head) of the machine was 250 °C while the temperature of the lower electrode (the chuck) in which the wafer is placed during the deposition process was 300 °C. A scheme of PECVD system for the Si3N4 deposition is shown in Fig. 1a. After the deposition, samples were treated by a 40 KeV argon ion implantation. The ion implant angle was maintained at 60° with respect to the sample normal direction to be sure that only the first layers of the silicon nitride were affected from the Ar þ ions irradiation. The implantation dose was selected to be 5 1013 ions/cm2, 5 1014 ions/cm2 and 1 1015 ions/cm2. Moreover, a Montecarlo simulation by using the TRIM software was performed to evidence the Ar þ ions projected range inside the silicon nitride layer. The Ar ions distribution is shown in Fig. 2. As is possible to see, the projected range of the Ar þ ions is enclosed inside the 30 nm of the silicon nitride thickness. The peak is centered at depth of around 180 nm from the surface. In addition the simulation takes also into account the presence of a few nanometers of native silicon oxide always present and naturally grown because of the environment contact atmosphere. To evaluate the wet-ability of materials the technique of the sessile drop has been used (Cutroneo et al., 2013). As test liquid
Fig. 2. The TRIM simulation performed to evidence the Ar þ ions projected range inside the silicon nitride layer.
Fig. 3. The contact angle vs. the ion dose for the investigated samples.
distilled water was used. The drop placed on the surface of the sample will tend to widen when the surface is wettable (hydrophilic, θ o90°) or to assume a spherical shape when the surface is not-wettable (hydrophobic, θ 490°). For very small droplets the effect of distortion of gravity is minimal. The contact angle θ (degree) was calculated from the height h (mm) and the base diameter d (mm) of the droplet itself (Zisman, 1964)
θ = 2arctg for
2h d
θ o90° and
⎛ 4hd ⎞ ⎟ θ = 90° + cos−1⎜ ⎝ 4h2 + d2 ⎠
(2)
θ 4 90°. The determination of contact angles for larger droplets is more complicated because the effect of gravity is not negligible and the drops are not spherical. for
Fig. 1. A scheme of PECVD system for the Si3N4 deposition (a) and of the sessile drop technique (b).
(1)
F. Caridi et al. / Radiation Physics and Chemistry 115 (2015) 49–54
51
Fig. 4. The XPS atomic concentrations of nitrogen and oxygen in the different samples (a–d) and the concentration of O and N at the various ion implantation doses (e).
The technique of the sessile drop is well shown in Fig. 1b. There have been two theories to explain the effect of surface roughness on wet-ability. The first is described by Wenzel (Zisman, 1964)
assumes that the liquid substrate bathrooms completely wrinkled. The expression proposed to describe this situation is given by the following equation (Zisman, 1964):
cos θW = r cos θY
cos θCB = r f f cos θY + f − 1
(3)
where θW is the angle of effective contact of a rough surface real, or angle of Wenzel, r is the ratio between the actual area of the surface of the solid and the nominal one, and θY is the contact angle of equilibrium of Young observed on a ideally smooth surface. Since r is always greater than 1, this model provides an increase hydrophilicity (θW o θY) to hydrophilic surfaces (θY o90°), and an increase in hydrophobicity (θW 4 θY) to hydrophobic surfaces (θY 4 90°). The second theory was derived by Cassie and Baxter, and
(4)
where θCB is the angle of contact of Cassie–Baxter, f is the fraction of solid surface in contact with the liquid, and rf is the ratio of the surfaces of the wet area and nominal. When f¼ 1 and rf ¼r the Cassie–Baxter equation is identical with the equation of Wenzel. Since f and rf are always smaller than 1, the model of Cassie–Baxter expected that the increase of roughness of a surface face always increase the contact angle. The chemical composition of the first Si3N4 monolayer was investigated by means of non-invasive X-ray Photoelectron
52
F. Caridi et al. / Radiation Physics and Chemistry 115 (2015) 49–54
Fig. 5. The analyses of the PECVD-prepared Si3N4 films by atomic force (a–d) and the roughness vs. the ion dose for the investigated samples (e).
Spectroscopy (XPS), acquiring spectra by means of a Scienta Esca200 system equipped with a monochromatized Al Kα (1486.6 eV) source. An overall energy resolution of 0.4 eV is routinely used for core levels. Measurements were performed at 90° emission angle (maximum sampling depth). For each sample a survey (0– 1200 eV) was acquired with a lower energy resolution to identify elements present on the surface. Subsequently Si 2p, O 1s, N 1s and C 1s core levels were collected. All core level peak energies were referenced to the saturated hydrocarbon in C 1s at 285.0 eV (Caridi et al., 2013). The surface morphology of the investigated samples was tested by an atomic force microscope model Unisolver P47 from NT-MDT (Cicchi et al., 2010). Analyses were performed in semi-contact mode with a silicon tip with a nominal radius of less than 10 nm. Scan area was 5 5 mm2. Root mean square roughness (Sq) was estimated for each scanned areas. Related formula is reported below (Vorburger and Raja, 1990)
Sq – Root mean square roughness, ISO 4287/1[nm]: Sq =
1 Nx N y
Nx
Ny
2
∑ ∑ (zij ) i=1 j=1
(5)
where zij is the vertical distance from the mean line to the ith and jth data point. SEM-EDX measurements were collected by an ESEM-FEI Inspect-S electron microscope coupled with an Oxford INCA PentaFETx3 EDX spectrometer, a Si(Li) detector equipped by a ultra-thin window ATW2, by using a resolution of 137 eV at 5.9 keV (Mn Kα1). The spectral data were acquired in ESEM (Environmental Scanning Electron Microscope) condition at working distance of 10 mm with an acceleration voltage of 20 kV, counting times of 60 s, count for second approximately 3000 cps with dead time below 30%. Results were processed by INCA software Energy. This software uses the XPP matrix correction scheme developed by
F. Caridi et al. / Radiation Physics and Chemistry 115 (2015) 49–54
53
Fig. 6. A SEM-EDX analysis of the small localized bumps on the surface of the investigated samples.
Fig. 7. The linear increase of the contact angle as a function of the Si3N4 surface roughness.
Pouchou and Pichoir (1988).
3. Results and discussion The estimation of the contact angle was made with the sessile drop technique and gave the following results: 39.8°, 46°, 43.7° and 44.8° for Si3N4 not implanted, Si3N4 implanted with 5 1013 Ar þ /cm2, 5 1014 Ar þ /cm2 and 1 1015 Ar þ /cm2, respectively. These results are plotted in Fig. 3, showing a fast increase of the contact angle for low ion doses (until 5 1013 Ar þ /cm2) followed by a nearly constant value for higher quantity of implanted argon ions. X-ray Photoelectron Spectroscopy (XPS) spectra are reported in Fig. 4 for Si3N4 not implanted (a), Si3N4 implanted with 5 1013 Ar þ /cm2 (b), 5 1014 Ar þ /cm2 (c) and 1 1015 Ar þ /cm2 (d), showing that a great amount carbon and oxygen was detected in all spectra, in addition to silicon and nitrogen (the matrix elements). From the core level we have calculated the atomic concentration of all elements present on the samples surface, using the atomic sensitive factors. In particular for oxygen and nitrogen we
obtain: 10.1% and 31.9%, respectively, for Si3N4 not implanted (a); 19.9% and 24.2%, respectively, for Si3N4 implanted with 5 1013 Ar þ /cm2 (b); 15.9% and 27.6%, respectively, for Si3N4 implanted with 5 1014 Ar þ /cm2 (c); 18.5% and 25.2%, respectively, for Si3N4 implanted with 1 1015 Ar þ /cm2 (d). Fig. 4e shows the atomic concentration of O and N at the various ion implantation doses. We can notice that a decrease in the amount of nitrogen corresponds to an increase of the oxygen and vice versa. This may be due to a breakage of chemical bonds between silicon and nitrogen in the Si3N4 film after the ion implantation, with a subsequent formation of chemical bonds between silicon and oxygen, that leads to a decrease of wet-ability, as reported in Fig. 3. Analyses of the PECVD-prepared Si3N4 films by Atomic Force Microscopy are reported in Fig. 5 for Si3N4 not implanted (a), Si3N4 implanted with 5 1013 Ar þ /cm2 (b), 5 1014 Ar þ /cm2 (c) and 1 1015 Ar þ /cm2 (d). The root mean square roughness estimation was of about 0.24 nm, 0.52 nm, 0.42 nm and 0.5 nm, respectively, showing a fast increase for the lower ion dose followed by a nearly constant value for higher quantity of argon ions implanted, as reported in Fig. 5e. Small localized bumps, with heights of a few nanometers observable in the maps could be due to mass inhomogeneities during the deposition process. Such inconvenient might also be eliminated by means of some technical shrewdness such as the chopping of the material flux during the deposition in order to drastically reduce the presence of massive particles moving towards the substrate with lower kinetic energies. A SEM-EDX analysis of these bumps is shown in Fig. 6 for the investigated samples. The SEM image of the analyzed area is reported as inset. The EDX result is that bumps are made of silicon in all cases, with traces of carbon and oxygen and contaminants such as chlorine. Fig. 7 shows the linear increase of the contact angle as a function of the Si3N4 surface roughness. The ion implantation changes the thin film surface, making it more wrinkled and less wettable with the increase of the dose.
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4. Conclusions
References
Ceramics have remarkable properties that have fueled excitement about their potential applications in the biomedical field. Oxide ceramics such as Al2O3 and stabilized ZrO2 have a lengthy history in prosthetic hip and knee replacements; and the favorable outcomes and the limitations of these ceramics are well known. Si3N4 ceramics have mechanical properties that are superior to the Al2O3-based ceramics and composites currently used as bearings for total hip and knee joint replacement. Unlike metals, Si3N4 is semitransparent to X-rays and being non-magnetic, it enables MRI of soft tissues proximal to Si3N4 implants. Si3N4 is biocompatible, and porous Si3N4 has been shown to support bone ingrowths at rates comparable with those reported for porous Ti surfaces when implanted in a large animal model. Silicon nitride has been used to promote bone fusion in spinal surgery, and to develop bearings that can improve the wear and longevity of prosthetic hip and knee joints (Otten et al., 2010; Oduwole et al., 2010). In this work it was shown that ion implantation can modify the properties of adhesion and surface wet-ability of silicon nitride films. It was shown that changes in the surface wet-ability may be due to changes in surface roughness, created by treatments with accelerated ions, that leads to modify the surface morphology. In particular an increase of the contact angle with the surface roughness was reported, according to the Cassie–Baxter theory (Zisman, 1964). This angle is lower than 90° also after the ion bombardment, so the silicon nitride surface remains hydrophilic, according to Cooper et al. (2010).
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