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Improvement in surface properties of Si wafer by mechanical surface treatment
Journal of Physics and Chemistry of Solids xxx (xxxx) xxx
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Improvement in surface properties of Si wafer by mechanical surface treatment A. Amanov Department of Mechanical Engineering, Sun Moon University, Asan, 31460, South Korea
A R T I C L E I N F O
A B S T R A C T
Keywords: Si wafer Surface hardness Wear resistance UNSM
In the present study, a monocrystalline silicon (Si) wafer was treated by ultrasonic nanocrystal surface modifi cation (UNSM) to enhance the efficiency and service life of Si-based devices by reducing reflectivity and improving surface properties, respectively. Microstructure, surface reflection, surface free energy and dry tribological characteristics of Si wafer before and after UNSM were systematically investigated. A silicon nitride (Si3N4) ball with a diameter of 5 mm was used as a counterface. The wear resistance was enhanced by 23% after UNSM in comparison with that of the as-received Si wafer. The reduction in reflectivity is due to roughened surface, while the improvement in tribological characteristics due to the strengthening. Thus, it is recommended that a UNSM can be potentially used as an alternative mechanical surface treatment for thin Si wafers.
1. Introduction The main applications of silicon (Si) wafer are usually in solar battery and MEMS because of its unique properties [1,2]. However, there are variety of drawbacks of Si wafer related to the low efficiency of solar cells or poor tribological characteristics of NEMS/MEMS. An effort has been made to increase the efficiency and to improve the tribological characteristics of Si wafer, but unsolved issues are still remained [3–5]. Silicon slicing or processing induced surface defects are detrimental to the surface properties of Si wafer [6]. Optical properties and tribological characteristics of Si wafer depend on the surface quality [7,8], where the efficiency of solar cells and NEMS/MEMS can be controlled by surface roughness, surface hardness, etc. [9,10]. The application of various surface texturing technologies such as laser etching, wet chemical etching, anti-reflective coating deposition, mechanical engraving, etc. on Si wafer has already demonstrated the beneficial effects to increase the efficiency of cell by reducing the reflectivity of light on the surface due to the formation of texturing/ patterning (pyramidal structure) or alteration of surface roughness [11, 12]. In addition, nanotechnologies that create different types of nano structures such as periodic, self-assembled, etc. have been implemented to improve the efficiency of cell thanks to the reduction in reflectivity [13,14]. It has been reported earlier that etching is one of the widely used methods to form a pyramidal structure, and can reduce the reflectivity by about 25% for monocrystalline p-type Si wafer [15–17], but complex, high cost and time-consuming surface texturing
technologies are not the best option and their applications are less desirable. In terms of application of Si wafer for NEMS/MEMS, numerous different technologies including organic coating deposition have been applied to enhance the wear resistance of Si wafer by the presence of surface texturing/pattering, and to increase strength and reliving stiction [18]. The fabrication costs of surface texturing/patter ing through above mentioned technologies are much higher and there is always need to find alternative way of production with low cost and high quality. In addition, there is no mechanical surface treatment that can be applicable for Si wafer due to its thin thickness and brittleness. Apart from above mentioned technologies, the modification of Si wafer surface has also been attempted by a nondestructive alternative ultrasonic nanocrystal surface modification (UNSM). It is a cold-forging process that form a texturing/pattering by introducing a severe plastic deformation (SPD) providing an opportunity to achieve a material with high strength, and high and/or low surface roughness. This technology realized the fabrication of modified surface over a wide range of mate rials such as metals, alloys, ceramics, and coatings [19–22]. Recently, the application of UNSM to a Si wafer led to reduction in reflectivity and increase in nano-hardness [23]. However, a comprehensive mechanism of anti-reflection property, minority carrier lifetime (MCLT) and wear resistance of Si wafer before and after UNSM was limited, thus, the role of UNSM on surface properties, reflectivity, MCLT, surface energy and tribological characteristics of Si wafer before and after UNSM were systematically investigated and discussed in detail. In this work, I propose an alternative mechanical surface
E-mail address: [email protected]. https://doi.org/10.1016/j.jpcs.2019.109272 Received 5 September 2019; Received in revised form 12 November 2019; Accepted 18 November 2019 Available online 20 November 2019 0022-3697/© 2019 Elsevier Ltd. All rights reserved.
Please cite this article as: A. Amanov, Journal of Physics and Chemistry of Solids, https://doi.org/10.1016/j.jpcs.2019.109272
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modification technology which is a simple and cost-effective method to produce a nanostructured Si wafer with low reflectivity and high wear resistance. The objective of the present paper is to assess the influence of UNSM on the surface properties and tribological characteristics of Si wafer in order to evaluate the possibility of implementing into industry. The influence of UNSM in terms of changes in surface-related mechan ical, physical and tribological properties was investigated and discussed. A brief overview of the sample preparation and UNSM are provided below.
40 to 170 nm. The initial surface roughness of the as-received Si wafer is too smooth as shown in Figs. 3(a) and 4(a), so that the formation of pattering by UNSM as shown in Figs. 3(b) and 4(b) increased the surface roughness by more than 400%. The increase in surface roughness thanks to the formation of pyramidal structure by various surface texturing or etching technologies reduced the reflectivity of Si wafer [25,26]. Parmar and Shin [27] demonstrated that a globular microstructure formed by femtosecond laser texturing improved anti-reflection property of Si wafer. Meanwhile, the presence of texturing/pattering on the surface provided a low friction coefficient by the presence of a room for wear debris that is generated at the contact interface. Wu et al. [28] applied a coating to Si wafer to enhance the wear resistance of Si wafer by covering the surface defects. The surface hardness may be unimportant property and has no consequence of significance, which is far less significant than surface roughness, but it has a crucial effect on tribological characteristics of Si wafer for NEMS/MEMS [29]. Fig. 5(a and b) demonstrate the indents formed on the surface of the as-received and UNSM-treated Si wafers. The cross-sectional profiles provided information about the dimensions of indents, where the actual depth of the UNSM-treated Si wafer was found to be about 1.58 μm, while the actual depth of the as-received Si wafer was found to be about 1.87 μm, corresponding to a 14% shallower depth by UNSM. Hence, the UNSM-treated Si wafer demonstrated a higher resistance to plastic deformation, thus having a higher hardness compared to that of the as-received Si wafer by about 14% as shown in Fig. 5(c). It is well documented that the increase in surface hardness is associated with microstructure of a material, namely with grain size, precipitations, secondary or new phases, etc. depending on the material type and nature, but in case of monocrystalline Si wafer the increase in surface hardness associate with the work hardening effect and plasticity that alluded to the dislocation dynamic model [23,30,31].
2. Materials and methods 2.1. Sample preparation A monocrystalline Si wafer (100) purchased from WaferPro Co., Ltd. (CA, USA) was used as a specimen. Si wafer (thickness - 279 � 25 μm, diameter - 50.8 � 0.38 mm was manufactured by Czochralski (CZ) process. Si wafer properties and some details of manufacturing process can be found elsewhere [24]. The UNSM was applied to the as-received Si wafer under the optimized treatment parameters (see Table 1). Fig. 1 (a) shows the schematic configuration of UNSM system [19–23]. Fig. 1 (b) shows a photography of the UNSM-treated Si wafer. In UNSM, a ball made of silicon nitride (Si3N4) with a diameter of 6.0 mm was used. 2.2. Reflectivity and minority carrier lifetime (MCLT) The surface reflection of Si wafer was measured by double-beam spectrophotometer (V-670, Jasco, Japan) and it was analyzed using a Spectra Manager™ II software. MCLT was measured using a standard ized wafer lifetime unit (MDPspot – V1.2, Freiberg Instruments GmbH, Germany) as shown in Fig. 2. The measurement is based on the con tactless destruction free electrical characterization.
3.2. XRD and Raman spectroscopy
2.3. Surface free energy
The XRD intensity and FWHM of the as-received and UNSM-treated Si wafers were found to be 16.88 and 14.15, and 0.40 and 0.53, corre sponding to a 17% reduction in intensity and 25% broadening in FWHM, respectively (see http://www.sciencedirect.com/science/article/pii/ S0169433214011064 Fig. 6(a)). Lattice dislocation, increased lattice micro-strain and grain size reduction (see Fig. 7) are responsible for the change in intensity and FWHM. The position of the diffraction peaks shifted towards a higher angle due to the change in lattice parameters by UNSM resulting in a new phase at the 2θ of 28.620. This new peak is believed to be an oxygen, which was formed during the UNSM treat ment. UNSM was carried in ambient temperature in open air, where the process of oxidation of silicon dioxide (SiO2) is inevitable, but its contribution to the overall spectrum turns out to be extremely small and it is not possible to isolate it at the noise level. The oxide layer was evenly distributed over the UNSM-treated Si wafer as can be in Fig. 6(b). Depending on the thickness of oxide layer, the presence of it may serve as a solid lubricant that beneficial in improvement the tribological properties, but it has no any effect on surface hardness as it cannot bear an applied load due to its very thin thickness within a few tens of nanometers. The mean grain size calculated by Scherrer’s equation [32] of the UNSM-treated Si wafer was about 31.18 nm. The Raman intensity was significantly reduced after UNSM (see http://www.sciencedirect.com/science/article/pii/S0169433 214011064 Fig. 7), which is attributed to the grain size refinement as shown in Fig. 8. It is obvious that the UNSM refined the coarse-grain into nano-grain with an average size of 50 nm as shown in Fig. 8(a). Also, the corresponding diffraction pattern of the UNSM-treated Si wafer (Fig. 8 (b)) exhibited many rings, which are indication of the nano-grains. The intense and rather narrow peak at the line ~517 cm 1 corresponds to nanocrystals of Si [33]. It is possible to estimate the dimensions of nanocrystals using the relationship between the difference in positions of the Raman peaks of nanocrystalline and bulk Si wafer due to
Contact angle of Si wafer was measured using an optical contact angle measurement equipment, which has a charge-coupled device (CCD) camera together with an optical microscope (OM) (SmartDrop Lab2.28, Femtofab Co. Ltd., Korea) by the sessile deionized (DI) water drop (5 μl) method at ambient temperature to investigate the surface free energy, which was calculated by Young’s equation below: cosθCA ¼ γSL
γ SV=γ
(1)
LV
where: γLV, γSV and γSL are the interfacial tensions. θCA is the contact angle. 2.4. Wear resistance measurement Tribological characteristics were evaluated by a tribo tester (Anton Paar, Austria) following the conditions listed in Table 2. A counterface was a Si3N4 ball with a diameter of 5.0 mm. 3. Results & discussion 3.1. Surface roughness and surface hardness 3D LSM images of the as-received and UNSM-treated Si wafers are shown in Fig. 3. The surface roughness (Ra) after UNSM increased from Table 1 UNSM parameters. Frequency, kHz 20.0
Amplitude, μm 10.0
Static load, N 5.0
Rotating speed, rpm 60
Feed-rate, mm/rev 0.03
2
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Fig. 1. Schematic of UNSM (a) and photography of the UNSM-treated Si wafer (b) (dashed yellow circle).
3.3. Reflectivity The reflectivity of Si wafer was reduced at throughout wavelengths after UNSM (see Fig. 9), where the reflectivity reduced more essentially in ultra-violet (UV) spectral range, which means the expansion of photosensitivity of solar cell in the short-wave party of a spectrum. It is very important how the part of radiation getting into volume of a Si wafer, where it is absorbed by the semiconductor raises and more no equilibrium carriers of a charge are generated. An increase surface roughness thanks to the presence of texturing (see Fig. 4(b)), which caused the incident light to be reflected at different depth was respon sible for the reduction in reflectivity. Suppress of reflection coverts more incident light to electric energy that increases the efficiency of solar cell. A comprehensive discussion needs to be made to shed light on the mechanism of reduction and further improvement in reflectivity by UNSM [35]. Laser surface texturing reduced the reflectivity by trapping or transmission of light [36]. The UNSM-treated Si wafer demonstrated antireflection property in a wide spectrum range from 200 to 1150 nm [37]. The application of UNSM might find an application in solar cells. 3.4. MCLT An increase in MCLT from 0.88 μm to 1.25 μm, which is corre sponding to a 29.6% after UNSM was observed that can promote improvement of efficiency of solar cells. High-sensitivity photodetectors can be created after UNSM. Mechanism of such a change is the anom alous photovoltaic effect has been proposed earlier [38]. The change in microstructural parameters such as the decrease in lattice plane spacing and increase in lattice strain after UNSM leads to a polarization of the near-surface layer [39]. In addition, polarization of the UNSM-treated Si wafer is also associated with the presence of electronegativity of oxygen vacancies at the surface (see Fig. 6(a)), which creates a Si–O bond. The presence of oxygen vacancies at the surface seems to be responsible for the stability of polarization. In this case, the deformation index or displacement of a positively charged nucleus becomes non-zero (ΔR > 0). No deformation takes place in the as-received Si wafer, which has no asymmetry in the atom, crystal or layer (strain index ΔR ¼ 0). Thus, residual deformation may lead to increase in the effective time of charge carriers, which can provide an increase in the efficiency of solar cells [40,41]. Furthermore, Xie et al. [42] investigated the polarization of textured Si wafer by chemical etching. They stated that the texturing of Si wafer led to a polarization due to the electronegativity of oxygen.
Fig. 2. Photography of MCLT measurement device. Table 2 Tribological test conditions. Applied normal load, N 5.0
Reciprocating speed, cm/s
2.51
Total sliding distance, m 100
Stroke, mm
3.0
Ball diameter, mm 5.0
Contact stress, GPa 0.62
scattering by transverse optical phonon and the mean size of the nano crystals can be estimated by the following equation [34]: � Ay ¼ a a = d Y (2) where: a ¼ 0.543 nm; The mean size of the nanocrystalline grains is calculated as 29.98 nm, which corresponds to Raman intensity reduction at 517 cm 1 [33]. Reduction in intensity of the peak after UNSM can be evidence of a significant refinement of nanocrystals in size.
3.5. Surface free energy It is of interest to produce a surface with low energy by controlling it through contact angle because the wettability is a very important issue in Si surface. The contact angle is a meaningful measurement to estimate 3
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Fig. 3. 3D LSM images showing the surface topography and roughness of the as-received (a) and UNSM-treated (b) Si wafers.
the surface energy. To determine the surface energy, a contact angle with a surface tension of DI water was measured. As shown in Fig. 10, the contact angle of the as-received and UNSM-treated Si wafers was found to be 32.90 and 63.60, respectively. Hence, the surface energy was calculated by Young-Dupre expression below: γð1 þ cosθÞ ¼ W
(5)
where: θ is the contact angle, γ is the liquid surface tension and W is the adhesion energy of the liquid. DI water was used as a liquid which has a surface tension of 0.072 N/m. It could be seen from Fig. 10 shows that a low contact angle (θ) means good wetting and a high contact angle means poor wetting [43]. As shown in Fig. 10, surface energies of the
Fig. 4. SEM images showing the surface pattering of the as-received (a) and UNSM-treated (b) Si wafers.
Fig. 5. 3D LSM images along with cross-sectional profiles showing the indent generated on the surface of the as-received (a) and UNSM-treated (b) Si wafers. Comparison in surface hardness before and after UNSM (c). 4
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Fig. 6. XRD peaks (a) of the as-received and UNSM-treated Si wafers along with EDX mapping (b) of the UNSM-treated Si wafer.
Fig. 9. Reflectivity of the as-received and UNSM-treated Si wafers.
Fig. 7. Raman peaks of the as-received and UNSM-treated Si wafers.
improvement in wettability. The mechanism for the wettability improvement of the UNSM-treated Si wafer was considered to be due to the increase in surface roughness with different morphology and the formation of pattering (see Fig. 4(b)) on the surface and also the accu mulation of a certain amount of oxide onto the surface during UNSM treatment, resulting in a reduction of the surface energy. In addition, the existence of trapped air between pattering and underlying water droplet is another proposed mechanism. Laser texturing can also enhance the surface hydrophobicity of Si surface significantly due to the laser irradiation-induced change resulting in a low surface energy [44]. 3.6. Tribological characteristics Beneficial effects of UNSM on the tribological properties of Si wafer have already been proved [23]. The as-received and UNSM-treated Si wafers had a very stable friction behavior with a friction coefficient of 0.54 and 0.39, respectively (see Fig. 11(a)). The reduction in friction coefficient was owing to the formation of texturing served as a room for wear debris that cannot be interact at the contact interface and also controlled the contact pressure due to the number of asperities. Fig. 11 (b) shows the wear rate of the Si wafers. Wear track dimensions of the UNSM-treated Si wafer was narrower than those of the as-received one
Fig. 8. TEM image of the UNSM-treated Si wafer.
as-received and UNSM-treated Si wafers were 39.1 and 30.6 mJ/m2, respectively. As a result, the contact angle and surface energy of the UNSM-treated Si wafer were found to be higher by about 49.3% and lower by about 21.7% than those of the as-received one, respectively. The formation of pattering by UNSM (see Fig. 4(b)) is responsible for the reduction in surface energy and the increase in contact angle, thus the 5
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Fig. 10. Contact angle of the as-received (a) and UNSM-treated (b) Si wafers representing the surface energy.
Fig. 11. Friction coefficient with respect to sliding distance (a) and wear rate (b) of the as-received and UNSM-treated Si wafers.
Fig. 12. 3D LSM images along with cross-sectional profiles of the wear tracks formed on the surface of the as-received (a) and UNSM-treated (b) Si wafers rep resenting the wear resistance. 6
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(see http://www.sciencedirect.com/science/article/pii/S0301679 X16303620 Fig. 12). The wear resistance of the as-received Si wafer was enhanced after UNSM by about 26% owing to the increase in surface hardness, which in turn depends on refined grain size [23,45]. Friction and wear related issues, which are of critical importance in design and application, are a main disadvantage for many micro components in NEMS/MEMS. For example, friction and wear cause any kind of failures of NEMS/MEMS due to the contact interface in relative motion. There fore, possible wear mechanisms of Si wafer has been widely discussed earlier [46,47]. Si wafer is the most promising material for NEM S/MEMS, but it’s high friction coefficient and wear rate of Si wafer are the major obstacle. Wear mechanisms of the as-received and UNSM-treated Si wafer were investigated by scanning electron micro scope and energy dispersive X-ray spectroscopy (SEM-EDX). Fig. 13 shows the SEM images along with O distribution over the wear track generated during sliding against Si3N4 ball. No wear debris was found beyond the wear track as shown in high-magnification SEM images of wear track (see Fig. 14), where similar abrasive and oxidative wear mechanisms took place [48]. The oxidation process and its thickness affect the tribological characteristics of Si wafer [49]. Fig. 11(a1 and b1) show the O distribution over the wear track generated during sliding against Si3N4 ball. The oxidation layer was formed at the contact interface between both the Si wafers and Si3N4 counter surface, where the oxidation level was higher for the as-received Si wafer and Si3N4 counter surface than that of the UNSM-treated one and Si3N4 counter surface as shown in Fig. 13(a2 and b2). Hence, the wear mechanisms were essentially composed of abrasive and oxidative for both Si wafers as they were partially covered by SiO2. The activation energy of the formed SiO2 is the result of the bond energy Si–O [50]. The formation of SiO2 can improve the friction-reducing performance and anti-wear ability [51]. Fig. 14 shows the high-magnification SEM images of the wear tracks. It was observed that wear debris was seen within the wear track (see Fig. 14(a1 and b1)). Furthermore, spalling pits were observed inside the wear track of both the Si wafers, which were caused by micro-cracks as shown in Fig. 14(a1 and b1). The proposed wear mechanisms were found to be similar to the results of our previous study [23].
4. Conclusions The following conclusions are drawn based on the experimental work: - After UNSM treatment, the surface roughness and surface hardness were increased by about 400% and 14%, respectively. The increase in hardness may be attributed to the nano-grains with an average size of 50 nm. - The intensity and FWHM of the as-received and UNSM-treated Si wafers were 16.88 and 14.15, and 0.40 and 0.53, corresponding to a 17% reduction in intensity and 25% broadening in FWHM, respectively. - The Raman intensity was significantly reduced with no Raman shift after UNSM. - The reflectivity of Si wafer was reduced at throughout wavelengths after UNSM, where the reflectivity reduced more essentially in ultraviolet (UV) spectral range, which means that the expansion of photosensitivity of solar cell in the short-wave party of a spectrum that promotes increase of efficiency of solar cells. - MCLT of the as-received Si wafer was increased from 0.88 μm to 1.25 μm after UNSM, which is corresponding to a 29.6%. - The contact angle and surface free energy of the UNSM-treated Si wafer were found to be higher by about 49.3% and lower by about 21.7% than those of the as-received one. - The friction coefficient was reduced from 0.54 to 0.39, while the wear resistance was enhanced by about 26% after UNSM. - The wear mechanisms were essentially composed of abrasive, oxidative and spalling for both Si wafers. - As a main conclusion, UNSM is the process of roughening the Si wafer surface so as to minimize front surface reflection and enhance light trapping, to boost the solar cell efficiency. Hence, it can be considered to be potentially used as an alternative mechanical sur face treatment for improving the efficiency and service life of both solar cell and NEMS/MEMS made of Si wafer.
Fig. 13. SEM images along with O distribution over the wear tracks formed on the surface of the as-received (a) and UNSM-treated (b) Si wafers. 7
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Fig. 14. High-magnification SEM images of the wear tracks formed on the surface of the as-received (a and a1) and UNSM-treated (b and b1) Si wafers.
Author Statement
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A.Amanov: Conceptualization, Data curation, Formal analysis, Investigation, Methodology, Writing - original draft, Writing - review & editing. Declaration of competing interest The author declares that there is no conflict of interest regarding the publication of this manuscript. Acknowledgement This study was also supported by the Industrial Technology Inno vation Development Project of the Ministry of Commerce, Industry and Energy, Rep. Korea (No. 10067485). Appendix A. Supplementary data Supplementary data to this article can be found online at https://doi. org/10.1016/j.jpcs.2019.109272. References [1] T. Markvart, L. Castaner, Solar Cells: Materials, Manufacture and Operation, Elsevier, Oxford, UK, 2005. [2] B. Hoex, W. Zhang, A.G. Aberle, Advanced characterization of Si wafer solar cells, Energy Proceed 15 (2012) 147–154. [3] S.W. Glunz, R. Preu, D. Biro, Crystalline Silicon Solar Cells – State of the Art and Future Developments, Comprehensive Renewable Energy, Elsevier, Amsterdam, the Netherlands, 2012. [4] M.A. Green, Silicon solar cells: state of the art, Phil. Trans. R Soc. A 371 (2013) 20110413. [5] C. del Ca~ nizo, G. del Coso, W.C. Sinke, Crystalline silicon solar module technology: towards the 1 € per watt-peak goal, Prog. Photovolt. Res. Appl. 17 (2009) 199–209.
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Journal of Physics and Chemistry of Solids journal homepage: http://www.elsevier.com/locate/jpcs
Improvement in surface properties of Si wafer by mechanical surface treatment A. Amanov Department of Mechanical Engineering, Sun Moon University, Asan, 31460, South Korea
A R T I C L E I N F O
A B S T R A C T
Keywords: Si wafer Surface hardness Wear resistance UNSM
In the present study, a monocrystalline silicon (Si) wafer was treated by ultrasonic nanocrystal surface modifi cation (UNSM) to enhance the efficiency and service life of Si-based devices by reducing reflectivity and improving surface properties, respectively. Microstructure, surface reflection, surface free energy and dry tribological characteristics of Si wafer before and after UNSM were systematically investigated. A silicon nitride (Si3N4) ball with a diameter of 5 mm was used as a counterface. The wear resistance was enhanced by 23% after UNSM in comparison with that of the as-received Si wafer. The reduction in reflectivity is due to roughened surface, while the improvement in tribological characteristics due to the strengthening. Thus, it is recommended that a UNSM can be potentially used as an alternative mechanical surface treatment for thin Si wafers.
1. Introduction The main applications of silicon (Si) wafer are usually in solar battery and MEMS because of its unique properties [1,2]. However, there are variety of drawbacks of Si wafer related to the low efficiency of solar cells or poor tribological characteristics of NEMS/MEMS. An effort has been made to increase the efficiency and to improve the tribological characteristics of Si wafer, but unsolved issues are still remained [3–5]. Silicon slicing or processing induced surface defects are detrimental to the surface properties of Si wafer [6]. Optical properties and tribological characteristics of Si wafer depend on the surface quality [7,8], where the efficiency of solar cells and NEMS/MEMS can be controlled by surface roughness, surface hardness, etc. [9,10]. The application of various surface texturing technologies such as laser etching, wet chemical etching, anti-reflective coating deposition, mechanical engraving, etc. on Si wafer has already demonstrated the beneficial effects to increase the efficiency of cell by reducing the reflectivity of light on the surface due to the formation of texturing/ patterning (pyramidal structure) or alteration of surface roughness [11, 12]. In addition, nanotechnologies that create different types of nano structures such as periodic, self-assembled, etc. have been implemented to improve the efficiency of cell thanks to the reduction in reflectivity [13,14]. It has been reported earlier that etching is one of the widely used methods to form a pyramidal structure, and can reduce the reflectivity by about 25% for monocrystalline p-type Si wafer [15–17], but complex, high cost and time-consuming surface texturing
technologies are not the best option and their applications are less desirable. In terms of application of Si wafer for NEMS/MEMS, numerous different technologies including organic coating deposition have been applied to enhance the wear resistance of Si wafer by the presence of surface texturing/pattering, and to increase strength and reliving stiction [18]. The fabrication costs of surface texturing/patter ing through above mentioned technologies are much higher and there is always need to find alternative way of production with low cost and high quality. In addition, there is no mechanical surface treatment that can be applicable for Si wafer due to its thin thickness and brittleness. Apart from above mentioned technologies, the modification of Si wafer surface has also been attempted by a nondestructive alternative ultrasonic nanocrystal surface modification (UNSM). It is a cold-forging process that form a texturing/pattering by introducing a severe plastic deformation (SPD) providing an opportunity to achieve a material with high strength, and high and/or low surface roughness. This technology realized the fabrication of modified surface over a wide range of mate rials such as metals, alloys, ceramics, and coatings [19–22]. Recently, the application of UNSM to a Si wafer led to reduction in reflectivity and increase in nano-hardness [23]. However, a comprehensive mechanism of anti-reflection property, minority carrier lifetime (MCLT) and wear resistance of Si wafer before and after UNSM was limited, thus, the role of UNSM on surface properties, reflectivity, MCLT, surface energy and tribological characteristics of Si wafer before and after UNSM were systematically investigated and discussed in detail. In this work, I propose an alternative mechanical surface
E-mail address: [email protected]. https://doi.org/10.1016/j.jpcs.2019.109272 Received 5 September 2019; Received in revised form 12 November 2019; Accepted 18 November 2019 Available online 20 November 2019 0022-3697/© 2019 Elsevier Ltd. All rights reserved.
Please cite this article as: A. Amanov, Journal of Physics and Chemistry of Solids, https://doi.org/10.1016/j.jpcs.2019.109272
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modification technology which is a simple and cost-effective method to produce a nanostructured Si wafer with low reflectivity and high wear resistance. The objective of the present paper is to assess the influence of UNSM on the surface properties and tribological characteristics of Si wafer in order to evaluate the possibility of implementing into industry. The influence of UNSM in terms of changes in surface-related mechan ical, physical and tribological properties was investigated and discussed. A brief overview of the sample preparation and UNSM are provided below.
40 to 170 nm. The initial surface roughness of the as-received Si wafer is too smooth as shown in Figs. 3(a) and 4(a), so that the formation of pattering by UNSM as shown in Figs. 3(b) and 4(b) increased the surface roughness by more than 400%. The increase in surface roughness thanks to the formation of pyramidal structure by various surface texturing or etching technologies reduced the reflectivity of Si wafer [25,26]. Parmar and Shin [27] demonstrated that a globular microstructure formed by femtosecond laser texturing improved anti-reflection property of Si wafer. Meanwhile, the presence of texturing/pattering on the surface provided a low friction coefficient by the presence of a room for wear debris that is generated at the contact interface. Wu et al. [28] applied a coating to Si wafer to enhance the wear resistance of Si wafer by covering the surface defects. The surface hardness may be unimportant property and has no consequence of significance, which is far less significant than surface roughness, but it has a crucial effect on tribological characteristics of Si wafer for NEMS/MEMS [29]. Fig. 5(a and b) demonstrate the indents formed on the surface of the as-received and UNSM-treated Si wafers. The cross-sectional profiles provided information about the dimensions of indents, where the actual depth of the UNSM-treated Si wafer was found to be about 1.58 μm, while the actual depth of the as-received Si wafer was found to be about 1.87 μm, corresponding to a 14% shallower depth by UNSM. Hence, the UNSM-treated Si wafer demonstrated a higher resistance to plastic deformation, thus having a higher hardness compared to that of the as-received Si wafer by about 14% as shown in Fig. 5(c). It is well documented that the increase in surface hardness is associated with microstructure of a material, namely with grain size, precipitations, secondary or new phases, etc. depending on the material type and nature, but in case of monocrystalline Si wafer the increase in surface hardness associate with the work hardening effect and plasticity that alluded to the dislocation dynamic model [23,30,31].
2. Materials and methods 2.1. Sample preparation A monocrystalline Si wafer (100) purchased from WaferPro Co., Ltd. (CA, USA) was used as a specimen. Si wafer (thickness - 279 � 25 μm, diameter - 50.8 � 0.38 mm was manufactured by Czochralski (CZ) process. Si wafer properties and some details of manufacturing process can be found elsewhere [24]. The UNSM was applied to the as-received Si wafer under the optimized treatment parameters (see Table 1). Fig. 1 (a) shows the schematic configuration of UNSM system [19–23]. Fig. 1 (b) shows a photography of the UNSM-treated Si wafer. In UNSM, a ball made of silicon nitride (Si3N4) with a diameter of 6.0 mm was used. 2.2. Reflectivity and minority carrier lifetime (MCLT) The surface reflection of Si wafer was measured by double-beam spectrophotometer (V-670, Jasco, Japan) and it was analyzed using a Spectra Manager™ II software. MCLT was measured using a standard ized wafer lifetime unit (MDPspot – V1.2, Freiberg Instruments GmbH, Germany) as shown in Fig. 2. The measurement is based on the con tactless destruction free electrical characterization.
3.2. XRD and Raman spectroscopy
2.3. Surface free energy
The XRD intensity and FWHM of the as-received and UNSM-treated Si wafers were found to be 16.88 and 14.15, and 0.40 and 0.53, corre sponding to a 17% reduction in intensity and 25% broadening in FWHM, respectively (see http://www.sciencedirect.com/science/article/pii/ S0169433214011064 Fig. 6(a)). Lattice dislocation, increased lattice micro-strain and grain size reduction (see Fig. 7) are responsible for the change in intensity and FWHM. The position of the diffraction peaks shifted towards a higher angle due to the change in lattice parameters by UNSM resulting in a new phase at the 2θ of 28.620. This new peak is believed to be an oxygen, which was formed during the UNSM treat ment. UNSM was carried in ambient temperature in open air, where the process of oxidation of silicon dioxide (SiO2) is inevitable, but its contribution to the overall spectrum turns out to be extremely small and it is not possible to isolate it at the noise level. The oxide layer was evenly distributed over the UNSM-treated Si wafer as can be in Fig. 6(b). Depending on the thickness of oxide layer, the presence of it may serve as a solid lubricant that beneficial in improvement the tribological properties, but it has no any effect on surface hardness as it cannot bear an applied load due to its very thin thickness within a few tens of nanometers. The mean grain size calculated by Scherrer’s equation [32] of the UNSM-treated Si wafer was about 31.18 nm. The Raman intensity was significantly reduced after UNSM (see http://www.sciencedirect.com/science/article/pii/S0169433 214011064 Fig. 7), which is attributed to the grain size refinement as shown in Fig. 8. It is obvious that the UNSM refined the coarse-grain into nano-grain with an average size of 50 nm as shown in Fig. 8(a). Also, the corresponding diffraction pattern of the UNSM-treated Si wafer (Fig. 8 (b)) exhibited many rings, which are indication of the nano-grains. The intense and rather narrow peak at the line ~517 cm 1 corresponds to nanocrystals of Si [33]. It is possible to estimate the dimensions of nanocrystals using the relationship between the difference in positions of the Raman peaks of nanocrystalline and bulk Si wafer due to
Contact angle of Si wafer was measured using an optical contact angle measurement equipment, which has a charge-coupled device (CCD) camera together with an optical microscope (OM) (SmartDrop Lab2.28, Femtofab Co. Ltd., Korea) by the sessile deionized (DI) water drop (5 μl) method at ambient temperature to investigate the surface free energy, which was calculated by Young’s equation below: cosθCA ¼ γSL
γ SV=γ
(1)
LV
where: γLV, γSV and γSL are the interfacial tensions. θCA is the contact angle. 2.4. Wear resistance measurement Tribological characteristics were evaluated by a tribo tester (Anton Paar, Austria) following the conditions listed in Table 2. A counterface was a Si3N4 ball with a diameter of 5.0 mm. 3. Results & discussion 3.1. Surface roughness and surface hardness 3D LSM images of the as-received and UNSM-treated Si wafers are shown in Fig. 3. The surface roughness (Ra) after UNSM increased from Table 1 UNSM parameters. Frequency, kHz 20.0
Amplitude, μm 10.0
Static load, N 5.0
Rotating speed, rpm 60
Feed-rate, mm/rev 0.03
2
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Fig. 1. Schematic of UNSM (a) and photography of the UNSM-treated Si wafer (b) (dashed yellow circle).
3.3. Reflectivity The reflectivity of Si wafer was reduced at throughout wavelengths after UNSM (see Fig. 9), where the reflectivity reduced more essentially in ultra-violet (UV) spectral range, which means the expansion of photosensitivity of solar cell in the short-wave party of a spectrum. It is very important how the part of radiation getting into volume of a Si wafer, where it is absorbed by the semiconductor raises and more no equilibrium carriers of a charge are generated. An increase surface roughness thanks to the presence of texturing (see Fig. 4(b)), which caused the incident light to be reflected at different depth was respon sible for the reduction in reflectivity. Suppress of reflection coverts more incident light to electric energy that increases the efficiency of solar cell. A comprehensive discussion needs to be made to shed light on the mechanism of reduction and further improvement in reflectivity by UNSM [35]. Laser surface texturing reduced the reflectivity by trapping or transmission of light [36]. The UNSM-treated Si wafer demonstrated antireflection property in a wide spectrum range from 200 to 1150 nm [37]. The application of UNSM might find an application in solar cells. 3.4. MCLT An increase in MCLT from 0.88 μm to 1.25 μm, which is corre sponding to a 29.6% after UNSM was observed that can promote improvement of efficiency of solar cells. High-sensitivity photodetectors can be created after UNSM. Mechanism of such a change is the anom alous photovoltaic effect has been proposed earlier [38]. The change in microstructural parameters such as the decrease in lattice plane spacing and increase in lattice strain after UNSM leads to a polarization of the near-surface layer [39]. In addition, polarization of the UNSM-treated Si wafer is also associated with the presence of electronegativity of oxygen vacancies at the surface (see Fig. 6(a)), which creates a Si–O bond. The presence of oxygen vacancies at the surface seems to be responsible for the stability of polarization. In this case, the deformation index or displacement of a positively charged nucleus becomes non-zero (ΔR > 0). No deformation takes place in the as-received Si wafer, which has no asymmetry in the atom, crystal or layer (strain index ΔR ¼ 0). Thus, residual deformation may lead to increase in the effective time of charge carriers, which can provide an increase in the efficiency of solar cells [40,41]. Furthermore, Xie et al. [42] investigated the polarization of textured Si wafer by chemical etching. They stated that the texturing of Si wafer led to a polarization due to the electronegativity of oxygen.
Fig. 2. Photography of MCLT measurement device. Table 2 Tribological test conditions. Applied normal load, N 5.0
Reciprocating speed, cm/s
2.51
Total sliding distance, m 100
Stroke, mm
3.0
Ball diameter, mm 5.0
Contact stress, GPa 0.62
scattering by transverse optical phonon and the mean size of the nano crystals can be estimated by the following equation [34]: � Ay ¼ a a = d Y (2) where: a ¼ 0.543 nm; The mean size of the nanocrystalline grains is calculated as 29.98 nm, which corresponds to Raman intensity reduction at 517 cm 1 [33]. Reduction in intensity of the peak after UNSM can be evidence of a significant refinement of nanocrystals in size.
3.5. Surface free energy It is of interest to produce a surface with low energy by controlling it through contact angle because the wettability is a very important issue in Si surface. The contact angle is a meaningful measurement to estimate 3
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Fig. 3. 3D LSM images showing the surface topography and roughness of the as-received (a) and UNSM-treated (b) Si wafers.
the surface energy. To determine the surface energy, a contact angle with a surface tension of DI water was measured. As shown in Fig. 10, the contact angle of the as-received and UNSM-treated Si wafers was found to be 32.90 and 63.60, respectively. Hence, the surface energy was calculated by Young-Dupre expression below: γð1 þ cosθÞ ¼ W
(5)
where: θ is the contact angle, γ is the liquid surface tension and W is the adhesion energy of the liquid. DI water was used as a liquid which has a surface tension of 0.072 N/m. It could be seen from Fig. 10 shows that a low contact angle (θ) means good wetting and a high contact angle means poor wetting [43]. As shown in Fig. 10, surface energies of the
Fig. 4. SEM images showing the surface pattering of the as-received (a) and UNSM-treated (b) Si wafers.
Fig. 5. 3D LSM images along with cross-sectional profiles showing the indent generated on the surface of the as-received (a) and UNSM-treated (b) Si wafers. Comparison in surface hardness before and after UNSM (c). 4
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Fig. 6. XRD peaks (a) of the as-received and UNSM-treated Si wafers along with EDX mapping (b) of the UNSM-treated Si wafer.
Fig. 9. Reflectivity of the as-received and UNSM-treated Si wafers.
Fig. 7. Raman peaks of the as-received and UNSM-treated Si wafers.
improvement in wettability. The mechanism for the wettability improvement of the UNSM-treated Si wafer was considered to be due to the increase in surface roughness with different morphology and the formation of pattering (see Fig. 4(b)) on the surface and also the accu mulation of a certain amount of oxide onto the surface during UNSM treatment, resulting in a reduction of the surface energy. In addition, the existence of trapped air between pattering and underlying water droplet is another proposed mechanism. Laser texturing can also enhance the surface hydrophobicity of Si surface significantly due to the laser irradiation-induced change resulting in a low surface energy [44]. 3.6. Tribological characteristics Beneficial effects of UNSM on the tribological properties of Si wafer have already been proved [23]. The as-received and UNSM-treated Si wafers had a very stable friction behavior with a friction coefficient of 0.54 and 0.39, respectively (see Fig. 11(a)). The reduction in friction coefficient was owing to the formation of texturing served as a room for wear debris that cannot be interact at the contact interface and also controlled the contact pressure due to the number of asperities. Fig. 11 (b) shows the wear rate of the Si wafers. Wear track dimensions of the UNSM-treated Si wafer was narrower than those of the as-received one
Fig. 8. TEM image of the UNSM-treated Si wafer.
as-received and UNSM-treated Si wafers were 39.1 and 30.6 mJ/m2, respectively. As a result, the contact angle and surface energy of the UNSM-treated Si wafer were found to be higher by about 49.3% and lower by about 21.7% than those of the as-received one, respectively. The formation of pattering by UNSM (see Fig. 4(b)) is responsible for the reduction in surface energy and the increase in contact angle, thus the 5
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Fig. 10. Contact angle of the as-received (a) and UNSM-treated (b) Si wafers representing the surface energy.
Fig. 11. Friction coefficient with respect to sliding distance (a) and wear rate (b) of the as-received and UNSM-treated Si wafers.
Fig. 12. 3D LSM images along with cross-sectional profiles of the wear tracks formed on the surface of the as-received (a) and UNSM-treated (b) Si wafers rep resenting the wear resistance. 6
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(see http://www.sciencedirect.com/science/article/pii/S0301679 X16303620 Fig. 12). The wear resistance of the as-received Si wafer was enhanced after UNSM by about 26% owing to the increase in surface hardness, which in turn depends on refined grain size [23,45]. Friction and wear related issues, which are of critical importance in design and application, are a main disadvantage for many micro components in NEMS/MEMS. For example, friction and wear cause any kind of failures of NEMS/MEMS due to the contact interface in relative motion. There fore, possible wear mechanisms of Si wafer has been widely discussed earlier [46,47]. Si wafer is the most promising material for NEM S/MEMS, but it’s high friction coefficient and wear rate of Si wafer are the major obstacle. Wear mechanisms of the as-received and UNSM-treated Si wafer were investigated by scanning electron micro scope and energy dispersive X-ray spectroscopy (SEM-EDX). Fig. 13 shows the SEM images along with O distribution over the wear track generated during sliding against Si3N4 ball. No wear debris was found beyond the wear track as shown in high-magnification SEM images of wear track (see Fig. 14), where similar abrasive and oxidative wear mechanisms took place [48]. The oxidation process and its thickness affect the tribological characteristics of Si wafer [49]. Fig. 11(a1 and b1) show the O distribution over the wear track generated during sliding against Si3N4 ball. The oxidation layer was formed at the contact interface between both the Si wafers and Si3N4 counter surface, where the oxidation level was higher for the as-received Si wafer and Si3N4 counter surface than that of the UNSM-treated one and Si3N4 counter surface as shown in Fig. 13(a2 and b2). Hence, the wear mechanisms were essentially composed of abrasive and oxidative for both Si wafers as they were partially covered by SiO2. The activation energy of the formed SiO2 is the result of the bond energy Si–O [50]. The formation of SiO2 can improve the friction-reducing performance and anti-wear ability [51]. Fig. 14 shows the high-magnification SEM images of the wear tracks. It was observed that wear debris was seen within the wear track (see Fig. 14(a1 and b1)). Furthermore, spalling pits were observed inside the wear track of both the Si wafers, which were caused by micro-cracks as shown in Fig. 14(a1 and b1). The proposed wear mechanisms were found to be similar to the results of our previous study [23].
4. Conclusions The following conclusions are drawn based on the experimental work: - After UNSM treatment, the surface roughness and surface hardness were increased by about 400% and 14%, respectively. The increase in hardness may be attributed to the nano-grains with an average size of 50 nm. - The intensity and FWHM of the as-received and UNSM-treated Si wafers were 16.88 and 14.15, and 0.40 and 0.53, corresponding to a 17% reduction in intensity and 25% broadening in FWHM, respectively. - The Raman intensity was significantly reduced with no Raman shift after UNSM. - The reflectivity of Si wafer was reduced at throughout wavelengths after UNSM, where the reflectivity reduced more essentially in ultraviolet (UV) spectral range, which means that the expansion of photosensitivity of solar cell in the short-wave party of a spectrum that promotes increase of efficiency of solar cells. - MCLT of the as-received Si wafer was increased from 0.88 μm to 1.25 μm after UNSM, which is corresponding to a 29.6%. - The contact angle and surface free energy of the UNSM-treated Si wafer were found to be higher by about 49.3% and lower by about 21.7% than those of the as-received one. - The friction coefficient was reduced from 0.54 to 0.39, while the wear resistance was enhanced by about 26% after UNSM. - The wear mechanisms were essentially composed of abrasive, oxidative and spalling for both Si wafers. - As a main conclusion, UNSM is the process of roughening the Si wafer surface so as to minimize front surface reflection and enhance light trapping, to boost the solar cell efficiency. Hence, it can be considered to be potentially used as an alternative mechanical sur face treatment for improving the efficiency and service life of both solar cell and NEMS/MEMS made of Si wafer.
Fig. 13. SEM images along with O distribution over the wear tracks formed on the surface of the as-received (a) and UNSM-treated (b) Si wafers. 7
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Fig. 14. High-magnification SEM images of the wear tracks formed on the surface of the as-received (a and a1) and UNSM-treated (b and b1) Si wafers.
Author Statement
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